硅烯与锗烯英文文献报告

合集下载

graphene综述

Cite this:DOI:10.1039/c1cs15193b Chemistry and physics of a single atomic layer:strategies and challenges for functionalization of graphene and graphene-based materialsLiang Yan,ab Yue Bing Zheng,c Feng Zhao,ab Shoujian Li,d Xingfa Gao,ab Bingqian Xu,e Paul S.Weiss*c and Yuliang Zhao*abdReceived 18th July 2011DOI:10.1039/c1cs15193bGraphene has attracted great interest for its superior physical,chemical,mechanical,and electrical properties that enable a wide range of applications from electronics tonanoelectromechanical systems.Functionalization is among the signiﬁcant vectors that drive graphene towards technological applications.While the physical properties of graphene have been at the center of attention,we still lack the knowledge framework for targeted graphenefunctionalization.In this critical review ,we describe some of the important chemical and physical processes for graphene functionalization.We also identify six major challenges in graphene research and give perspectives and practical strategies for both fundamental studies and applications of graphene (315references).1.IntroductionSince its conception,1–18graphene has attracted enormous interest due to its unique physical properties,such as novel magneto transport,2electromechanical modulation,5extremely high carrier mobility,6,7tunable band gap,8quantum Hall eﬀect,2,9–11,19Klein tunneling nature,12and electron conﬁnement eﬀects.14These properties make graphene a promising candidate for a broad range of applications in next-generation nanotechnologies where current materials are limited in functionality.For example,current chemical separation materials cannot eﬃciently remove residual actinide fuels from radioactive ﬁssion fragments.20This costs more than $2billion per year for treating the large amount of radioactive wastes generated from nuclear power facilities.21–23Graphene exhibits superior properties as separation materials due to its selective adsorption of heavy metals (e.g.,lanthanides and actinides)with super loading and in situ rebirth capacity.For example,we may use graphene-nanoparticle multilayers for separation column materials in nuclear fuel recycling.Other ﬁelds in which graphene may have potential applications include electronics,photonics,optoelectronics,and mechanics (Fig.1).1–12,24,25Despite the wide range of possible applications,there are still many challenges for graphene to reach its full potential.For example,graphene has an intrinsic zero band-gap energy.We have to manage to open up the band gap for semiconductor applications,possibly even as a successor of silicon in the post-Moore’s law electronics era,26,27as a single-molecule gas or bio-sensor,28–30and as a stretchable transparent electrode or electron devices.31–38In addition,graphene is insoluble in organic solvents and susceptible to aggregation in aqueous solu-tions.Even for relatively simple uses of graphene,e.g.,ﬁller in aChinese Academy of Sciences Key Laboratory for Biomedical Eﬀects of Nanomaterials and Nanosafety,National Center for Nanoscience and Technology of China,Beijing 100190,China.E-mail:zhaoyuliang@ bInstitute of High Energy Physics,Chinese Academy of Sciences,Beijing 100049,China cCalifornia NanoSystems Institute,and Departments of Chemistry &Biochemistry and Materials Science &Engineering,University of California,Los Angeles,Los Angeles,California 90095,USA.E-mail:psw@ dThe College of Chemistry,Sichuan University,Chengdu 610064,China eFaculty of Engineering &Nanoscale Science and Engineering Center,University of Georgia,Athens,Georgia 30602,USALiang Yan is a graduate student of Chinese Academy of Sciences Key Laboratory for Biomedical Eﬀects of Nanomaterials &Nanosafety.He received his BSc in Chemistry from Sichuan University in 2009.He is now studying on the chemical function-alization of graphenes as well their biomedical eﬀects.Yue Bing Zheng is a postdoctoral scholar with Prof.Weiss at the University of California,Los Angeles.He received his PhD in Engineering Science and Mechanics from The Pennsylvania State University in 2010,his MS in Physics from the National University of Singapore in 2004,and his BS in Physics from Nankai University,China,in 2001.From 2004to 2006,he was a Research Fellow in the Institute of Materials Research and Engineering,Singapore.His research centers on designing,measuring,and controlling molecules and light at the nanoscale.Chem Soc RevDynamic Article Links/csrCRITICAL REVIEWD o w n l o a d e d b y H e n a n N o r m a l U n i v e r s i t y o n 23 N o v e m b e r 2011P u b l i s h e d o n 16 N o v e m b e r 2011 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 1C S 15193BView Online / Journal Homepagepolymer matrices,the surface properties of graphene sheets must be altered through further functionalization to obtain stable dispersions in solvents.Most recently,it was found that a supercapacitor fabricated by chemical activation of graphene with KOH possessed high values of gravimetric capacitance and energy density,39higher than the supercapacitor produced by the chemically reduced graphene.40Therefore,functionalization approaches that can modify the structural,electronic,and chemical properties of graphene are critical for applications.41–47In principle,graphene can be functionalized at two classes of locations:the basal plane and the edges.On the basal plane,sp 2hybridization of carbon leads to a strong covalent bonding,as well as to delocalization of the p electrons.The interaction of the basal plane with guest atoms or molecules leads to modi-ﬁcation of the p –p conjugation and thus the electron density distribution and the physical and chemical properties.The dangling bonds at edge sites of graphene are highly reactive to guest atoms or molecules.Typically,functionalization approaches used for fullerenes and carbon nanotubes (CNTs)can be applied to graphene.There are signiﬁcant chemical diﬀerences due to the single atomic layer of graphene sheets.Much of this chemistry remains to be explored.The rehybridization from sp 2to sp 3via covalent reaction occurs both at the edges and on the basal plane.Before discussing functionalization methods,we sum-marize six critical issues to consider regarding functionaliza-tion of graphene and related materials.1.1.Multiple methods for graphene productionSo far,several methods have been successfully established for graphene preparation,such as peeling-oﬀgraphite,1liquid-phase exfoliation,48–50chemical vapor deposition,39,51–53graphiti-zation of silicon carbide,26,54templating,55,56reduction of graphene oxide,57,58unzipping carbon nanotubes,59,60organic synthesis,61,62and anodic bonding.63These diﬀerent methods produce graphene with diﬀerent size,shape,chemical composition,and environ-ment (Fig.2),all of which have diﬀerent requirements for functionalization.1.2.DefectsAll current methods cannot produce structurally and morpho-logically perfect graphene sheets because of the defects created unintentionally and unavoidably.64,65These defects,including structural imperfections and chemical impurities,complicate graphene functionalization.1.3.EdgesCombining the two basic types of edge conﬁgurations (i.e.,armchair and zigzag)leads to a variety of edges in graphene.This makes it diﬃcult to characterize the structural and electronic properties and thus to control the functionalization processes at the edges.Feng Zhao is currently a research fellow at the Chinese Academy of Sciences Key Laboratory for Biomedical Eﬀects of Nano-materials and Nanosafety,Institute of High Energy Physics,Chinese Academy of Sciences.She received her BSc in Chemistry from the College of Chemical Engineering,Tsinghua University in 2002,and MSc in Biology from the College of Biology,Tsinghua University in 2005.Her current research interests include nanotoxicology,and studies on nanoparticles and their interactions with cells or biomacromolecules.Shoujian Li received his PhD in Radiochemistry at Sichuan University.He is the head of the research group Functional Materials for Radionuclides Separation of the Chemistry College of Sichuan University.His research interests include the design and synthesis of new functional carbonaceous materials,advanced materials and technologies for spent fuel reprocessing,and treat-ment and disposal of nuclear wastes,including the use of carbon nanomaterials.Xingfa Gao received his PhD from the Institute of High Energy Physics,Chinese Academy of Sciences in 2006.After working as a postdoctoral fellow (2006–2011)at the Institute for Mole-cular Science (Japan)and Rensselaer Polytechnic Institute (USA),he returned to the Institute of High Energy Physics as a professor.His research interests lie in applying computa-tional methods to p -conjugated systems including fullerenes,carbon nanotubes,graphene and their hybrids.He has great interest in developing eﬃcient theoretical methods to investigate novel p -conjugated molecules and reactions through close inter-play between theoretical predictions and experimental tests.Bingqian Xu received his PhD degree in Materials Science and Engineering from Arizona State University (ASU),Tempe,in 2004.Then,he was with the Electrical Engineering Department,ASU,as a Faculty Research Associate in molecular electronics.In 2006,he moved to the University of Georgia (UGA),Athens,and now is an Associate Professor,directing the Molecular Nanoelectronics Laboratory.His research interests include molecular electronics and single-molecule studies of biomolecular assemblies and systems.Dr Xu is a member of the American Chemical Society,Materials Research Society,and American Physical Society.Paul S.Weiss received his PhD in Chemistry in 1986from UC Berkeley.He was a postdoctoral fellow at AT&T Bell Laboratories and IBM Almaden Research Center.He began his academic career at Penn State,becoming Distinguished Professor of Chemistry and Physics before moving to UCLA in 2009.At UCLA,he is the Fred Kavli Chair in NanoSystems Sciences,the Director of the California NanoSystems Institute,and Distinguished Professor of Chemistry &Biochemistry and Materials Science &Engineering.He is the founding Editor-in-Chief of ACS Nano.His research interests are in single-molecule/assembly function,chemical pattern-ing,self-assembly,and nanoscale analyses.Yuliang Zhao’s research interests include Nanotoxicological Chemistry (nanotoxicology,cancer nanotechnology,and nano-chemistry),Nanobioanalytical Sciences,and Molecular Dynamics Simulations of biochemical processes at the nano/bio interface.He serves as editorial board member for eight international journals in the United States and Europe.He is Professor and Director,Chinese Academy of Sciences Key Laboratory for Biomedical Eﬀects of Nanomaterials &Nanosafety,and also serves as Deputy Director-General of National Center for Nanoscience and Technology of China,and a member of National Steering Council for Nanosciences and Technology of China.D o w n l o a d e d b y H e n a n N o r m a l U n i v e r s i t y o n 23 N o v e m b e r 2011P u b l i s h e d o n 16 N o v e m b e r 2011 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 1C S 15193B1.4.StoichiometryThe functionalized graphene is non-stoichiometric in its chemical composition.This makes it diﬃcult to control its properties.1.5.Low reactivityGraphene has a much lower chemical reactivity than fullerenes and CNTs.66,67The low reactivity limits the eﬀective approaches for functionalizing graphene.411.6.Low solubilityGraphene has low solubility in both aqueous and organic solutions.This makes it diﬃcult to manipulate graphene in solutions in which many functionalization processes occur.Thus,pre-treatment is sometimes required to increase the solubility of graphene for further functionalization.The electronic structure of graphene underlies its chemical properties.Ideal graphene is an inﬁnite-scale two-dimensional sheet without edges and basal plane ﬂposed of sp 2carbon,ideal graphene is chemically unsaturated (Fig.3A).Carbon atoms can form an extra covalent bond,which converts sp 2to sp 3hybridization and makes carbon reach its saturated state.Thus,this ‘‘unsaturation’’is regarded as the origin of graphene’s reactivity in covalent addition reactions.However,graphene is chemically inert (or stable)because all its p z atomic orbitals are strongly coupled and stabilized in a giant,deloca-lized p bonding system (Fig.3A).On one hand,this p system usually precludes graphene from covalent addition.On the other hand,as a kind of p ligand,it renders versatile com-plexation reactions for graphene, e.g.,with organic com-pounds and transition metals through p –p ,H–p and metal–p interactions.The associated anti-bonding p *molecular orbitals can accommodate electrons,which facilitates favorable adsorp-tion between graphene and electron-rich particles such as ions and alkali metals.In contrast to ideal graphene,practical graphene unavoidably contains edges,basal plane ﬂuctuations,vacancies,andotherFig.1Application examples of functionalized graphene in electronics and single-molecule gas sensors:(a)schematic exploded illustration of a graphene mixer circuit.The critical design aspects include a top-gated graphene transistor and two inductors connected to the gate and the drain of the graphene ﬁeld-eﬀect transistor (GFET).24(b)Concentration,D n ,of chemically induced charge carriers in a single-layer graphene exposed to diﬀerent concentrations,C ,of NO 2.Upper inset:scanning electron micro-graph of this device.Lower inset:characterization of the graphene device by using the electric-ﬁeld eﬀect.28Reproduced from ref.24.Copyright 2011American Association for the Advancement of Science.Reproduced from ref.28.Copyright 2007Nature PublishingGroup.Fig.2Graphene prepared by diﬀerent methods:(a)Large graphene crystal prepared on an oxidized Si wafer by the scotch-tape technique.(b)Left panel:suspension of microcrystals obtained by ultrasound cleavage of graphite in chloroform.Right panel:such suspensions can be printed on various substrates.(c)The ﬁrst graphene wafers are now available as polycrystalline one-to ﬁve-layer ﬁlms grown on Ni and transferred onto a Si wafer.(d)State-of-the-art SiC wafer with atomic terraces covered by a graphitic monolayer (indicated by ‘‘1’’).Double and triple layers (‘‘2’’and ‘‘3’’)grow at the steps.7(e)Representation of the gradual unzipping of one wall of a carbon nanotube to form a nanoribbon.60Reproduced from ref.7.Copyright 2009American Association for the Advancement of Science.Reproduced from ref.60.Copyright 2009Nature Publishing Group.D o w n l o a d e d b y H e n a n N o r m a l U n i v e r s i t y o n 23 N o v e m b e r 2011P u b l i s h e d o n 16 N o v e m b e r 2011 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 1C S 15193Bchemical impurities.By altering the electronic structure,these structural ‘‘imperfections’’can alter the chemical properties and reactions of graphene.During covalent addition,inner carbons that are strictly constrained in the basal plane have to protrude outward from the plane to adopt a tetrahedral sp 3geometry,causing strain in the plane.Edge carbon atoms are usually terminated by chemical groups such as hydrogen.Unlike inner carbon atoms,edge carbon atoms can adopt tetrahedral geo-metries more freely without causing extra strain.Therefore,edge carbons are preferred in covalent addition.Because fron-tier molecular orbitals are mainly localized at zigzag edges,68,69zigzag edges are particularly reactive in graphene (Fig.3B).67For the same reason,vacancies like edges created inside the graphene basal plane,are also very reactive (Fig.3C).Basal plane ﬂuctuations cause curvature of graphene sheets.The curvature reduces the overlap of the p z atomic orbital of one carbon with p z orbitals of the three surrounding carbons (Fig.3D).Thus,the curvature can lead to localized states with higher energies,which enhances the reactivity of the carbon.70This paper reviews recent progress on graphene functiona-lization based on the diﬀerent types of reactivity discussed above.We discuss the functionalization of graphene by basal plane covalent addition (Section 2).Because edges are parti-cularly reactive sites in graphene,the covalent functionaliza-tion of graphene edges is discussed separately (Section 3).Unlike covalent addition,both complexation and charge-transfer adsorption do not destroy graphene’s p bonding network and are discussed in Section 4.Finally,we review perspectives and challenges for graphene functionalization (Section 5).It is worth noting that practical reactions of graphene are usually compli-cated.Diﬀerent types of reactivity may be involved simulta-neously or at diﬀerent stages of reactions.It is usually diﬃcult toattribute a reaction solely to one or two origins of reactivity.Therefore,the above classiﬁcation of the origins of reactivity,as well as the settings of Sections 2through 4,are only attempted to emphasize the main driving force for the reactions.2.Functionalization of graphene targeting basal plane covalent additionComposed of sp 2carbon,graphene is chemically unsaturated.Intrinsically,it is possible to undergo covalent addition to change the carbons from sp 2to sp 3hydridization.However,carbon atoms in the graphene basal plane are protected by their p -conjugation system,whose motion is constrained by surrounding carbon atoms.Therefore,basal plane covalent addition usually encounters large energy barriers,and reactive chemical groups,such as atomic hydrogen,ﬂuorine,and pre-cursors of other chemical radicals,are usually needed as the reactants.So far,the chemical modiﬁcation of graphene cannot be fully controlled.Therefore,most of the reactions discussed in this section can also take place on graphene edges.2.1.Hydrogenation and dehydrogenationHydrogenation of the free-standing graphene and of graphene located on top of oxidized Si substrates has been investigated both experimentally and theoretically.66,71–75The supported graphene displays diﬀerent structures and electronic properties before and after hydrogenation.Hydrogenation changes the hybridization of carbon atoms from sp 2to sp 3,resulting in elongated C–C bonds in the H-modiﬁed graphene.Hydrogen atoms tend to react with both surfaces of the plane of pristine graphene (Fig.4).If only one side is hydrogenated,it can then be rolled to form CNTs because of the unbalanced external stress.76Fig.3Origin of chemical reactivity of graphene.(A)Intrinsic reactivity arising from the delocalized p -bonding system.(B)Zigzag and armchair edges.(C)Monovacancy.(D)Local structure of a curved graphene sheet.In (A)and (D),the p z atomic orbitals are shown;the dashed lines represent overlap between p z orbitals.In (C),the dangling s bonds are shown.D o w n l o a d e d b y H e n a n N o r m a l U n i v e r s i t y o n 23 N o v e m b e r 2011P u b l i s h e d o n 16 N o v e m b e r 2011 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 1C S 15193BThe semi-hydrogenated graphene possesses ferromagnetic semi-conductor properties because the partial hydrogenation can destroy the delocalized p bonding network of graphene.77The fully hydrogenated graphene is called ‘‘graphane’’,which is prepared under hydrogen plasma atmosphere and has also been the subject of a number of studies.75In addition,it is worth mentioning that plasma activation supplies an alternative way to prepare the precursors of graphene derivatives,such as graphenol,graphenoic acid,and graphenamine.Alternatively,several research groups also studied the chemi-sorptions of hydrogen on the graphene surface,in which case only small subareas of the graphene basal plane are hydro-genated unintentionally.78–81There are four types of conﬁgura-tions that hydrogen pairs form on the basal plane from thermal graphitization of SiC (0001)after exposing to a 1600K D-atom beam.79The adsorbate forms and morphologies depend on various conditions,such as temperature,pressure,and hydrogen coverage.And hydrogens can be desorbed by annealing without inﬂuencing the electronic properties of graphene.Moreover,an energy band-gap can be induced via patterned hydrogen chemisorption.78The dehydrogenation of graphene can proceed via an annealing process through which the properties of graphene can be partially restored.82The reactivity of annealed graphene is higher than that of pristine graphene 66largely because defects form during annealing.Hydrogenation and dehydrogenation are useful processes for graphene functionalization,particularly by which ferromagnetism can be introduced,the band gap can be opened,and the p -conjugated skeleton of graphene can be altered.Additionally,the dehydrogenated graphene displays higher chemical reactivity than pristine graphene.This opens up new means for generating multifunctional graphene-based materials.Hydrogenation also oﬀers a practical means to control electro-nic structure,which is the basis for fabricating novel devices.In addition,graphane,(CH)n ,if produced,may be an excellent new material for hydrogen storage.722.2.Fluorination reactionsAb initio pseudopotential calculations indicate that the electronicstructure of ﬂuorinated graphene sheets has a ﬃﬃﬃ3p Âﬃﬃﬃ3p periodic structure in charge distribution near the Fermi energy.83It is known that ﬂuorine on the sidewall of ﬂuoro-CNTs can be replaced by alkyl groups in the presence of alkyl lithium,84alkylidene amino,85or Grignard reagents.86The replacement reactions can be extended to ﬂuoro-graphene functionaliza-tion.The ﬂuorination of graphene is also useful for further substitution and functionalization.For example,ﬂuorinated graphene sheets successfully reacted in situ with butylamine 87to obtain the alkylated graphene.Alkylated graphenes are important graphene-based materials because they are readily dispersed in common organic solvents such as dichlorobenzene,dichloromethane,and THF.Moreover,they can be completely dealkylated by annealing to recover the original properties of pristine graphene sheets.88The ﬂuorinated graphene has been prepared by plasma treat-ment of chemically converted graphene (CCG)at room tempera-ture followed by subsequent reaction with butylamine.87If the initial graphene sheets are prepared by thermally exfoliating graphene oxide or CCG,they have some residual oxygen-containing species on the basal plane.The presence of oxygen-containing species may prevent ﬂuorine atoms from attach-ing to the graphene sheet in the ﬂuorination process,which undoubtedly reduces the degree of ﬂuorination.2.3.Oxidation reactionsThe oxidized graphene sheet is one of the most important forms of graphene,in which graphene is heavily oxygenated with a wide variety of oxygen species such as carbonyl,carboxyl,and hydroxyl groups.89Because of the existence of oxygen-containing functional groups,graphene oxides react easily with soluble moieties.This enables changing the hydrophilicity,hydrophobicity,or organophilicity of graphene,as required for many applications.For example,many modiﬁed graphene sheets are readily dispersed in organic solvents for further functionalization,or for mixing with organic matrices to form new nanocomposite materials.Oxidation can also generate a monotype of oxygen-containing functional group,such as graphene with only hydroxyls (graphenol),or only carboxyls (graphenic acid)on the basal plane.Three experimental chemical routes have been developed for graphene oxidation.The ﬁrst is a one-step process,and is achieved through direct oxidization of graphene with strong oxidants such as concentrated sulfuric acid,concentrated nitric acid,or potassium permanganate.90The second is a two-step process,in which graphite is oxidized through Hummers’,91Brodies’,92Staudenmaiers’,93or modiﬁed Hummer’s methods,94or electrochemic oxidation,95,96followed by exfoliating or thermally expanding the graphene oxide obtained.97,98The third isaFig.4The most favorable conformations of graphene after hydro-genation,the chair (a)and boat (b)conformations.Carbon is shown as black and hydrogen is shown as red.In functionalization,hydrogen tends to attach to both sides of the graphene plane.D o w n l o a d e d b y H e n a n N o r m a l U n i v e r s i t y o n 23 N o v e m b e r 2011P u b l i s h e d o n 16 N o v e m b e r 2011 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 1C S 15193Bphysicochemical process:graphene oxide nanoribbons are created through lengthwise cutting and unraveling the side walls of multiwalled carbon nanotubes (MWCNTs)by oxida-tive processes.59,60During the oxidation process,graphitic structures break down into smaller fragments.The formation mechanism,electronic properties,and conformations of these fragments were studied both experimentally and theoretically.99–102It is worth noting that direct oxidization is not usually used to prepare graphene oxide but has the potential to control the size,shape,number of layers,and electronic properties of graphene.For example,experimentally,thermal oxidation of few-layer graphene with O 2can remove 2–3layers of graphene and eventually yield a single layer,half-metallic graphene.1032.4.DiazotizationGraphene has an electron-rich surface due to its p electrons.When electron-accepting moieties such as aryl diazonium salts react with graphene,electrons can transfer from the basal plane to the reactant.Because of the increase of pyramidalization of the deformed plane,diazonium salts easily react with graphene.Diazotization could be used to regulate electrical conductivity of the graphene since it can modulate the surface potential of graphene via regiofunctionalization.104–108Both CCG and epitaxial graphene (EG)have been success-fully modiﬁed by diazotization at room temperature.98,109When nitrophenyl groups were attached to the EG surface,the resultant diazonium-functionalized graphene sheets were easily dispersed in polar aprotic solvents such as dimethylformamide (DMF),dimethylacetamide (DMAc),and N -methylpyrrolidone (NMP).Nitro groups on the surface of diazotized graphene can be reduced further to amine.The amine groups make sub-sequent graphene functionalization possible because amine can react with many other groups,such as hydroxyl radicals,carboxyl groups,and acyl chlorides.Interestingly,nonvolatile memory devices were fabricated by attachment of gold nanoparticles (AuNPs)-4-mercapto-benzenediazonium tetraﬂuoroborate salt (MBDT)conjugates (AuNPs-MBDTs)through diazotization reactions onto the p -conjugated skeleton of CCG.110Diﬀerent graphene preparation methods lead to graphene with diﬀerent properties after diazotization.For EG on a sub-strate,there is a preferential adjustment of the density distribu-tion of electrons in response to the regiofunctionalization of the surface.The diazotization process can form a defect-free and oxygen-free surface for EG that can be used to fabricate sensors,detectors,and other electronic devices.For CCG,both surfaces of the basal plane can be diazotized,but with defects and oxygen-ated groups.As a result,the conductivity of the diazotized CCG is signiﬁcantly lower than that of the diazotized EG.2.5.Other cycloaddition reactionsThe thermal functionalization of fullerenes 111and carbon nanotubes 112,113by nitrenes via [2+1]cycloaddition reactions has been studied extensively.These reactions have also been applied to graphene to form aziridine adducts and to immo-bilize graphene on silicon wafers.114The ﬁrst step of the reaction protocol is the thermal or photochemical decomposition of per-ﬂuorophenylazides (PFPA)via nitrogen elimination,giving rise to the highly reactive single perﬂuorophenylnitrene.The second stepconsists of [2+1]cycloaddition of the nitrene to the surface of graphene.Alkyne-terminated molecules have been coupled to graphene sheets via ‘‘click’’reactions to form stable hetero-cyclic linkages under mild reaction conditions.115Cyclo-addition reactions of fullerenes and CNTs such as the Bingel [2+1]cyclopropanation reaction,116can also be applied to graphene functionalization.And 1,3-dipolar cycloaddition has been successfully carried out not just at the edge but also at the internal C Q C bonds of graphene.117Claisen rearrangement was also used to functionalize graphene oxide,yielding a water-soluble derivative.118The modular zwitterion-mediated transformations that have been used to functionalize CNTs and fullerenes 119can also be applied to graphene.These processes introduce a variety of functional groups for further reaction of graphene to expand the library of graphene-based materials for practical applications.For example,if graphene is coupled with chelating ligands,it can complex metal ions,and hence can be used to detect and to remove toxic heavy metal ions from the environment,or can be used as a novel ﬁller to fabricate the columns for the eﬃcient separation/treatment of radioactive wastes of nuclear power stations.2.6.Reverse reactions of covalent additionChemical and thermal reduction of graphene oxide provide alternative approaches for the production of CCG 120,121and graphene-based composites.122The oxygen-containing func-tional groups on the basal plane of graphene can be removed by thermal treatment or chemical reaction with reductants such as hydroquinone,reducing sugar,L -glutathione,N 2H 4,and their derivatives.57–58,121–134Heating graphene oxide in ultrahigh vacuum is eﬀective in controlling the percentage of oxygen-containing groups being removed.124Solvothermal reduction of graphene oxide occurs at a much lower reaction temperature than pyrolysis and provides a means to reduce the defects in graphene.135–137Photothermal heating has also been used to reduce graphene oxide.In contrast to chemical and thermal treatments,a photothermal heating process is rapid and does not require other reactants.In addition,heated atomic force microscope (AFM)tips can be used to reduce graphene oxide locally.138However,none of these methods are capable of achieving complete reduction of graphene oxide.As a result,the reduced graphene oxide exhibits non-metallic behavior due to defects related to residual oxygen functional groups.57,58,124,129,130A combination of several diﬀerent reduction methods may enhance the degree of reduction and improve the quality of graphene.139For example,hydrogen reduction,combined with thermal treatment,can remove oxygen species eﬀectively with little contamination on the basal plane.Nonetheless,the reduction process makes graphene hydrophobic and easy to aggregate.140One way to prevent aggregation is to add poly-mers into the graphene oxide solution during the reduction process.However,the polymers also introduce extra impurities into the graphene,which are undesirable for some applications because the impurities can modify both surface and electronic properties.An alternative solution to the aggregation issue is to graft a charged unit such as sulfonate (–SO 3H)to the graphene plane so that it can maintain the stability of grapheneD o w n l o a d e d b y H e n a n N o r m a l U n i v e r s i t y o n 23 N o v e m b e r 2011P u b l i s h e d o n 16 N o v e m b e r 2011 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 1C S 15193B。

14族杂环戊二烯分子(硅、锗、锡)的电子结构与光谱性质(英文)

ＡｂｔａｔＧｒｕ一４ｍｅａｌｌｓｐｓｅｓｎｅｅｔｇｏｔａｒｐｒｅｎｒｒｍｉｉｇｍｏｅｕｅｒｉｈ－ｍｉｉｇｓｒｃ：ｏｐ１ｔｏｅｏｓｓｔｒｓｎｐｉｌｏｅｔｓａｄａｅｐｏｓｎｌｃｌｓｆｇｔｌｉｉｃｐｉｏｌｅｔｎｔ
ｓｕｔｒｓｏｅｒｕｄｓｔ（ｏａｄｔｅｒｔｉｌｃｔａｅ（１ｗｒａｕａｄｕｉｅｓｙｆｎｔｎｅｒｔｃｅｆｒｏｎａＳｎｓｓｇｅｅｉｄｓｔＳ）ｅｅｃｌｌｅｓｇｄｎｉｃｏａｔｏｒｕｔｇｈｔｅ）ｈｆｎｔｘｅｔｉｃｔｎｔｕｉｌｈｙ（ｒａｄｔｅｅｅｄｎｄｎｉｎｔｎｌｅｒ（ＤＤｒ，ｓｅｔｅ．ｈｐｃｌｂｏｐｉｎｄｍｓｏｅｔＤＦ）ｍ－ｐｎｅｔｅｓｆｃｉａｔｏＴ－Ｆ）ｅｐｃｖｌＴｅｔａａｓｒｔｓｎｓｃａｎｉｄｙｔｕｏｙｈｒｉｙｏｉｏａｅｎｉｉｐｒ
中图分类号：Ｏ４６１
ＥｌｃｒｎｉｔｕｃｕｅｎｄＳｃｒｓＯｉｏｒｉｓｏｏｐ－４ｅｔｏｃＳｒｔｒｓａｐｅｔ０ｃｐｃＰｒｐｅｔｅｆＧｒｕ１
ＭｅａｌｌｓＭＰｈ（＝ｉＧｅＳ）ｔｌｏｅ６ＭＳ，，ｎ
吸收光谱和发射光谱，特别是发射光谱的半峰宽与现有的实验值吻合很好．通过分析结构和光谱性质的关系，指出光谱的性质主要取决于苯环转动对应的低频振动模式和中心环ｃ —Ｃ键的伸缩振动对应的高频振动模式．关键词：密度泛函理论；１族杂环戊二烯；振动关联函数；光吸收；光发射４

类石墨烯锗烯研究进展秦志辉Recentprogressof-物理学报

硅烯中受电场调控的体能隙和朗道能级 Electric field controlled energy gap and Landau levels in silicene 物理学报.2015, 64(8): 087302 /10.7498/aps.64.087302
一维石墨烯超晶格上的氢吸附 Hydrogen adsorption on one-dimensional graphene superlattices 物理学报.2014, 63(19): 197301 /10.7498/aps.63.197301
2.1 翘曲结构与电子结构
自从理论预言存在稳定的自由锗烯 [12] 以来,
研究人员采用基于密度泛函理论维也纳从头计算
模拟软件包 (VASP), 运用投影缀加波法和广义梯
度近似等, 理论研究了有关锗烯的几何与电子结构
性质 [14,19−21].
Si 与 Ge 和 C 同属 IV 主族, 具有相似的电子组
早在 1994 年, Takeda 和 Shiraishi [9] 基于密度泛函理论研究提出与碳同 IV 族的硅和锗可形成翘曲的蜂窝结构, 但并未引起研究人员的关注. 近几年来, 伴随石墨烯研究的深入开展, 硅和锗的类石墨烯的单质二维原子晶体也逐渐成为研究前沿 [10]. 2007 年, Guzmán-Verri 和 Lew Yan Voon [11] 采用
墨烯中 pz 轨道形成 π 键, s, px, py 轨道形成 σ 键, π 键与 σ 键垂直而互相不耦合. 相比石墨烯, 锗烯中
由于 Ge—Ge 原子之间较大的成键间距削弱了其 π
电子交叠, (π/π*) 成键极大地减弱 [19], π-π 相互作
用比石墨烯中弱. 而非平面结构中的 pz 轨道与 sp2

英文文献汇报

Hollow nanostructures的合成困境
• The template-free or self-templating bottom-up approaches：hardly extended to HGN synthesis； • A template-involving top-down strategy：no catalytic capability to regularly manage the arrangement of carbon atoms； • Thus, controllable synthesis of HGNs with an engineered hollow cavity, predetermined layer number, small size, and highly crystalline fewlayer graphene shells is rarely achieved
Results and Discussion
The in situ catalytic self-limited assembly of HGNs
Morphology and structure of α-Ni(OH)2-DS
Morphology and structure of HGNs Electrochemical performance of HGN-S Morphology of cycled HGN-S electrodes
The in situ catalytic self-limited assembly of HGNs
Morphology and structure of α-Ni(OH)2-DS
Morphology and structure of HGNs

SiO2 graphene composite for highly selective adsorption of Pb(II) ion

SiO 2/graphene composite for highly selective adsorption of Pb(II)ionLiying Hao,Hongjie Song,Lichun Zhang,Xiangyu Wan,Yurong Tang,Yi Lv ⇑Key Laboratory of Green Chemistry &Technology,Ministry of Education,College of Chemistry,Sichuan University,Chengdu,Sichuan 610064,Chinaa r t i c l e i n f o Article history:Received 24October 2011Accepted 8December 2011Available online 16December 2011Keywords:Pb(II)ionSiO 2/graphene composite Adsorptiona b s t r a c tSiO 2/graphene composite was prepared through a simple two-step reaction,including the preparation of SiO 2/graphene oxide and the reduction of graphene oxide (GO).The composite was characterized by UV–Vis spectroscopy,Fourier transform infrared spectroscopy,scanning electron microscope,and X-ray pho-toelectron spectroscopy,and what is more,the adsorption behavior of as-synthesized SiO 2/graphene composite was investigated.It was interestingly found that the composite shows high efﬁciency and high selectivity toward Pb(II)ion.The maximum adsorption capacity of SiO 2/graphene composite for Pb(II)ion was found to be 113.6mg g À1,which was much higher than that of bare SiO 2nanoparticles.The results indicated that SiO 2/graphene composite with high adsorption efﬁciency and fast adsorption equilibrium can be used as a practical adsorbent for Pb(II)ion.Ó2011Elsevier Inc.All rights reserved.1.IntroductionGraphene (G),discovered in 2004[1],has been attempted in many applications due to its excellent characteristics,such as mobil-ity of charge carriers,mechanical ﬂexibility,thermal and chemical stability,and large surface area [2–4].Signiﬁcantly,graphene,as ideal two-dimensional ultrathin material with large surface area,is a promising building block material for composites [5];further-more,decoration of the graphene nanosheets with metal/metal oxide/nonmetallic oxide nanomaterials can bring about an impor-tant kind of graphene-based composites [6–10].The decoration of nanomaterials onto graphene nanosheets is also helpful to over-come the aggregation of individual graphene nanosheets [11]and nanomaterials themselves.Besides,the composites with larger sur-face area show superior properties,compared with bare nanomate-rials [12],due to the synergistic effect between graphene nanosheets and nanomaterials.Therefore,in recent years,many endeavors have been poured on the synthesis of graphene-based nanocomposites,e.g.,graphene/metal oxide and graphene/metal composites,and these composite materials have been explored as adsorbents [13,14],catalysts [15],and lithium ion batteries [16]along with an excellent application potential.Considering the inexpensive cost,innocuity,reliable and chemical stability,biocompatibility,and ver-satility of SiO 2[17],graphene/silica composite would be one of the greatly popular and interest topics in the ﬁeld of nanomaterial and nanotechnology.On the other hand,there has been a long-time concern on the pollution of heavy metals to the aquatic environment because oftheir toxicity and detriment to living species including humans.Among all of the heavy metal ions,lead ion,which commonly exists in industrial and agricultural wastewater and in acidic leach-ate from landﬁll sites [18],is ubiquitous in the environment and severely hazardous to human and living things.Long-term drink-ing water containing high level of lead ion would cause serious dis-orders,such as anemia,kidney disease,nausea,convulsions,coma,renal failure,and cancer,along with subtly negative effects on metabolism and intelligence [19].Up to now,many techniques have been applied to remove Pb(II)ion from waste water,such as ion exchange [20],cloud point extraction [21],coprecipitation [22],ﬂocculation [23],membrane ﬁltration [24],reverse osmosis [25],adsorption [26],and so forth.Among these methods,adsorption-based methodology is greatly popular thanks to its high efﬁciency,cost-effectiveness,simple operation,and environmental friendliness [27].Especially,adsorptive removal of aqueous Pb(II)ion has been widely investigated by using various materials,such as activated carbon,ash,zeolites,metal oxides,chitosan,and agri-cultural by-products [28].It is also worth mentioning here that graphene/nanomaterials composites are also considered to be a highly effective adsorbent due to the peculiar properties and large surface area.Particularly,the research about the application of graphene/nanomaterials composites in the adsorption of heavy metal ions is important for environment and human.In this work,SiO 2/graphene composite was prepared via a two-step procedure route that contains the preparation of silica nanoparticles in the presence of graphene oxide solution and the reduction of graphene oxide in the presence of silica nanoparticles.Then,the resulting composite was chosen as an adsorbent toward Pb(II)ion and the adsorption behaviors were investigated in de-tails.Meanwhile,the inﬂuence of experimental conditions,includ-ing pH value,ionic strength and contact time,adsorbability,and0021-9797/$-see front matter Ó2011Elsevier Inc.All rights reserved.doi:10.1016/j.jcis.2011.12.023Corresponding author.Fax:+862885412798.E-mail address:lvy@ (Y.Lv).adsorption capacity,was also discussed.Interestingly,the SiO 2/graphene composite was found to be highly effective adsorbent with high selectivity and fast adsorption equilibrium toward Pb(II)ion.2.Materials and methods2.1.Chemical reagents and materialsGraphite powder was of SpecPure grade and was purchased from Tianjin Guangfu Fine Chemical Research Institute.Other reagents were of analytical grade and were used without further puriﬁcation.Deionized (DI)water from ULUPURE Water Puriﬁcation System (Chengdu,China)was used to prepare all solutions.Lead nitrate (Pb(NO 3)2,P 99.0%),ethanol,sodium hydroxide (NaOH),hydrazine hydrate (H 2NNH 2ÁH 2O,P 50.06%),and hydrochloric acid (HCl,P 36.46%)were obtained from Chengdu Kelong Chemical Reagent Company (China).Stock standard solution of lead (1000mg L À1)was prepared from analytical grade lead nitrate.2.2.Preparation and characterization of SiO 2/graphene composite The soluble graphene oxide–based sheets were produced by complete exfoliation of graphite oxide as an entry into SiO 2/graph-ene composite.Graphite oxide was synthesized according to the Hummers method through the oxidation of natural graphite pow-der [12].After that,graphite oxide (100mg)was exfoliated in 400mL of distilled alcohol–water (7:1,v/v)solution by ultrasonic treatment for 2h to form a colloidal suspension approximately.Then,the collected colloidal suspension was separated by centrifu-gation at 4000rpm,and the supernatant was obtained in order to prepare the followed composite.The well-known hydrolysis of tet-ramethyl orthosilicate (TEOS)was used for the fabrication of the composite.Brieﬂy,the pH of the reaction mixture was adjusted to 9.00with ammonia solution and then added TEOS (2.1mL)into this dispersion,resulting graphene oxide–containing sol [29].The obtained mixture was stirred magnetically and reduced with hydrazine hydrate (P 50.06%)at 95°C for 24h.was collected through 0.45l m ﬁlter and water to remove the excess hydrazine hydrate,thesized composite was dried at 323K overnight 2.3.Characterization and apparatusThe UV–Vis spectra of GO and SiO 2/GO 200–500nm were recorded by U-2910UV–Vis The surface properties and composition of silica nanoparticles were investigated by Fourier IR)spectroscopy using Thermo Nicolet IS10FT-IR KBr pellets in the range 500–4000cm À1.X-ray troscopy (XPS)was performed with a XSAM 800eter (Kratos)using medium resolution and radiation to analyze the surface composition and of products.The binding energies were calibrated tainment carbon (284.8eV).Also,the wide-angle 35mA)powder X-ray diffraction (XRD)using a X’Pert Pro X-ray diffractometer (Philips)tion (k =1.5406Å).The surface morphology of the examined by SEM (Hitachi,S3400).The (BET)surface area and the pore size distribution were measured using N 2adsorption and desorption SI,Quantachrome,USA)at 77K over a relative 0.0955to 0.993.2.4.Batch adsorption experimentBatch adsorption tests were carried out at room temperature (25°C)and used to investigate the effects of various parameters on the adsorption of Pb(II)ion by SiO 2/graphene.For adsorption experiments,3mg of adsorbents was dispersed into a 20mg L À1Pb(II)ion solution (10mL)and was shaken with a magnetic stirrer for 60min to reach equilibrium except kinetic experiments.The SiO 2/graphene solution mixtures were ﬁltered with a 0.45l m ﬁl-ter,and the equilibrium concentrations of Pb(II)ion in the solution were quantiﬁed by ﬂame atomic absorption spectroscopy (FAAS,Zeeman GGX-6,China).According to the above procedure,the impact of the pH value,ionic strength,and contact time on adsorp-tion was investigated.The adsorption capacity (q e ,mg g À1)and the adsorption efﬁciency (E ,%)were calculated according to Eqs.(1)and (2):q e ¼ðC 0ÀC e ÞÁVWð1ÞE ¼C 0ÀC eC 0Â100%ð2Þwhere C 0and C e (mg L À1)are the initial and equilibrium concentra-tions of Pb(II)ion in aqueous phase,and V is the volume of the solu-tion (L ),and W is the mass of dry adsorbent used (g ),respectively.3.Results and discussion 3.1.Characterizations of compositeUV–Vis spectrogram (Fig.S1)shows that GO nanosheets pres-ent a clear characteristic absorption in aqueous solution with a maximum wavelength at 228nm.On the other hand,SiO 2/GO composite exhibits a weak absorption at 226nm,which is due to the assembly of the GO nanosheets.The phenomenon of blueshift of the maximum wavelength is attributed to the change of the environment around the GO nanosheets,which preliminarily indi-cates that SiO 2/GO composite is successfully prepared.Similar 382L.Hao et al./Journal of Colloid and Interface Science 369(2012)381–387the A OH bending vibration of the adsorbed water molecules[29]. This suggests that SiO2nanoparticles are successfully prepared through the above pared with SiO2nanoparticles, the minor and weak peaks are observed at2980and2930cmÀ1, which are attributed to the C A H stretching vibration[33],which indicate the effective attachment of graphene and the successful preparation of SiO2/graphene composite.The surface composition and the element characterization of the composite were analyzed using XPS spectra of composite, which was conducted in the region of0–1100eV.As shown in Fig.2a,there are three elements in the XPS spectra of the compos-ite,namely carbon,oxygen,and silicon,without other elements. The spectra of XPS(Fig.2a)exist the characteristic peaks of Si2s (150eV),Si2p(104.5eV),which is indicative of the formation of the SiO2phase in composite.Moreover,the presence of SiO2can be further conﬁrmed by the O1s XPS peak at532.8eV(Fig.2c), which is regarded as the oxygen species in the SiO2[34,35].In addition,there are at least three types of oxygen species about the O1s peak(Fig.2c),that is,the contribution of the anionic oxy-gen in SiO2at about532.8eV,the oxygen-containing functional groups at around532.3eV,and water at higher binding energies. The C1s XPS spectra,as shown in Fig.2b,contain four components corresponding to carbon atoms in different oxygen-containing functional groups[36]:(a)the non-oxygenated ring C at 284.8eV,(b)the carbon in C A O at285.9eV,(c)the carbonyl car-bon(C@O)at287.0eV,and(d)the carboxylate carbon(O A C@O) at288.2eV.The C1s spectrum of SiO2/graphene shows mainly the nonoxygenated carbon(284.8eV)and the carbon in C A O (285.9eV).Moreover,nonoxygenated carbon is more than the car-bon in C A O,which indicates that deoxygenation has appeared. Meanwhile,XRD was used to further verify the deoxygenation (Supplementary data,Fig.S2).The peak at10.4°corresponding to the diffraction peak of GO was disappeared and the newly obtuse peak at23.0°was observed,which conﬁrm that GO was reduced with hydrazine hydrate and amorphous SiO2nanoparticles were formed[29,32].However,small amount residual oxygenated groups are still left,which are veriﬁed by the O1s XPS peaks at 532.3eV(C@O)and533.6eV(C A O)and also indicated that GO has not been completely reduced by hydrazine hydrate.Besides,silica nanoparticles to form a composite in nanoscale.In the images of SiO2/graphene(Fig.3c and d),the layer structure of graphene is well observed at high magniﬁcation and SiO2nanoparticles are tightly covered by the corrugated grapheneﬂakes,which is differ-ent from previous report[30]that graphene nanosheets are immo-bilized onto SiO2nanoparticles through surface assembly.In addition,N2adsorption–desorption isotherms were also em-ployed to investigate the speciﬁc surface area and the pore struc-tures of prepared samples(the chemical analysis reveals that the weight percentage of graphene is about12.5wt.%)(Fig.4).The BET surface area and pore volume estimated from Barret–Joyner–Halenda(BJH)analysis of the isotherms were determined to be 252.5m2gÀ1and0.3771cm3gÀ1,respectively.Also,the average of the pore size distribution is2.987nm,which was calculated from the absorption branch by the BJH method.As Fig.4shows, a slight adsorption is observed in the low pressure region(<0.6P/ P0),followed by a sharp adsorption at0.8P/P0,which suggests that this adsorption step occurs on its surface and the interlayer of restacking graphene layers[37].Also,a hysteresis loop can be seen in desorption branch.The shape of adsorption isotherms may be considered to be reversible type V isotherms,which is considered that there is weak interaction between materials and nitrogen. The shape of desorption branch is a typical H3type,indicating that the slit holes in the composite may be formed by the aggregation of various platelike particles.Thus,SiO2/graphene could be a good candidate as a kind of adsorption material.3.2.Adsorption performance3.2.1.Effect of pH on the adsorptionThe pH of the solution usually exerts a great effect on the adsorption of metal ions.According to the solubility-product con-stant of Pb(OH)2(Ksp=1.43Â10À15)and the initial concentration of Pb(II)ion of20mg LÀ1,the pH value of appearance of metal ion hydroxides precipitation is calculated as8.59.In order to investi-gate the effect of pH on the adsorption of Pb(II)ion onto SiO2/ graphene,10mL Pb(II)ion solution with the concentration of 20mg LÀ1was adjusted to a pH range of2.00–7.00with different concentrations of NaOH and HCl solutions,during which noL.Hao et al./Journal of Colloid and Interface Science369(2012)381–387383results in low adsorption.As the pH increased,more binding sites were released and there were less competition of active sites between hydrogen ion and lead(II)ion,resulting in better adsorp-tion behavior.In addition,the surface charge of SiO2/graphene with more negative charge density at higher pH causes more electro-static attractions of Pb(II)ion,which serves as another reason for the better adsorption behavior.3.2.2.Effect of ionic strength on the adsorptionThe different ionic strengths,such as0.001M,0.005M,0.01M, 0.05M,0.1M KNO3,and without KNO3,were chosen to investigate their effect on Pb(II)ion adsorption by SiO2/graphene.Fig.5b shows that Pb(II)ion adsorption decreases with increasing ionic strength.This phenomenon could be attributed to following reasons:(1)the Pb(II)ion forms outer-spherethe adsorbent sites,which favor the adsorptiontration of the competing salt is decreased.adsorption between the adsorbent andmainly of ionic interaction nature;(2)ionicinﬂuences the activity coefﬁcient of metaltransfer to the composite surfaces[38].3.2.3.Effect of contact time on the adsorptionTime course of Pb(II)ion adsorption ontoexecuted under Pb(II)ion solution with concentrationat pH=6.00and I=0.001M KNO3.Fig.6contact time on the adsorption of Pb(II)composite.It can be seen that the adsorptionsharply,with about95%of total Pb(II)ion10min,then the adsorption reaches equilibriumfast adsorption rate is attributed to the laminatedlarge external surface of SiO2/graphene.Furthertime does not enhance the adsorption percentage2value industrial applications.The kinetics of Pb(II)ion adsorption was determined in order to understand the adsorption behavior of the SiO2/graphene compos-ite.The adsorption data of Pb(II)ion at different time intervals are ﬁt for a pseudo-second-order kinetic model.The calculated curve corresponding to Pb(II)ion sorption was plotted in Fig.6(inset). The kinetic rate equation is expressed asdqtdt¼k2Áðq eÀq tÞ2ð3ÞBy integrating Eq.(3)with the boundary conditions of q t=0at t=0and q t=q t at t=t,the following linear equation can be obtained:tt¼12eþteð4ÞFig.3.SEM image of SiO2(a)and the different magniﬁcation of SiO2/graphene composite(b–d). (black)–desorption(red)isotherms and pore sizeV0¼k2Áq2eð5Þwhere q t and q e are the amounts of Pb(II)ion adsorbed at time t and at equilibrium(mg gÀ1),respectively.The k2(g mgÀ1minÀ1)repre-sents the pseudo-second-order rate constant for the kinetic model, which can be obtained by a plot of t/q t against t.V0(mg gÀ1minÀ1) is the initial sorption rate.As shown in Table S1,the comparison be-tween the experimental adsorption capacity(q exp)value and the calculated adsorption capacity(q cal)value shows that q cal value is very close to q exp value for the pseudo-second-order kinetics. Moreover,the adsorbent system can be well described by pseudo-second-order kinetic model,which also is conﬁrmed according to the correlation coefﬁcient value for pseudo-second-order model, equal to1.000,higher than that of pseudo-ﬁrst-order,suggesting that the adsorption may be the rate-limiting step involving valence forces through sharing or exchange of electrons between the adsor-bent and the adsorbate.3.3.Adsorption isothermsIn addition to adsorption kinetics,we measured the absorption isotherms of Pb(II)ion onto SiO2/graphene to explore the adsorp-tion mechanism much deeply.As shown in Fig.7,at low initial Pb(II)ion concentration,the composite exhibits high adsorption percentage as98.82%.Although the adsorptivity decreases with increasing initial Pb(II)ion concentration,Pb(II)ion adsorption capacity steadily rises.The Langmuir and Freundlich models are the most frequently used models among the abundant isothermal models.The Lang-muir isotherm,which assumes monolayer coverage on adsorbent [39]and no subsequent interaction among adsorbed molecules, is expressed as[40]:1qe¼1qmþ1K LÁq mÁC eThe Freundlich isotherm is derived to model multilayer adsorp-tion on adsorbent.It can be described as[40]:ln qe¼ln K Fþ1nÁln C ewhere q e and C e are the adsorption capacity(mg gÀ1)and the equi-librium concentration of the adsorbate(mg LÀ1),respectively.K L is the constant related to the free energy of adsorption(L mgÀ1),and q m is the maximum adsorption capacity(mg gÀ1).K F and n are the Freundlich constants,which represent the adsorption capacity (mg gÀ1)and the adsorption strength,respectively.The values of q m and K L are calculated from the slope and intercept of the linear plot of1/q e against1/C e.ln K F and1/n can be obtained from the intercept and the slope of the linear plot of ln q e versus ln C e.The adsorption isotherms of Pb(II)ion on the SiO2/graphene com-posite as a function of Pb(II)concentration(pH=6.00,30min adsorption time)are shown in Fig.7(inset),and the Langmuir and Freundlich constants are presented in Table1.The adsorption data(a)and ionic strength(b)for the adsorption percentage and capacity of Pb(II)ion at room temperature(25°C):adsorption time, concentration,20mg LÀ1;and ionic strength,0.001M KNO3for a;pH,6.00for b.areﬁt for Langmuir model,and it shows the maximum adsorption capacity of113.6mg gÀ1for the SiO2/graphene composite.Also, the higher correlation coefﬁcients indicate that the Langmuir model ﬁts the adsorption data better than the Freundlich model.In other words,this adsorption process took place by monolayer on the homogeneous sites of the surface of SiO2/graphene.The adsorption capacities of other absorbents toward Pb(II)ion are listed in Table S2,and the comparative results show that the adsorption capacity of SiO2/graphene is higher.Therefore,it can be concluded that SiO2/graphene has much superior adsorption capacity for removing Pb(II)ion.3.4.Selective adsorption experimentThere are mainly six different heavy metals in the waste water: Cu2+,Pb2+,Ni2+,Co2+,Cd2+,and Cr3+.We chose a mixed solution of metal ions,which was prepared by diluting1000mg LÀ1of Cu2+, Pb2+,Ni2+,Cd2+,Co2+,and Cr3+to20mg LÀ1in25mLﬂask volumet-ric for a selective adsorption experiment;6.0mg adsorbent was dispersed in20mL of solution and the mixture was stirred for 30min at room temperature.In order to avoid to produce Cu(OH)2 (pH=5.92)and Cr(OH)3(pH=5.07)at the optimum condition (pH=6.00),the pH value of solution was chosen as4.80,at which the adsorption efﬁciency of Pb(II)ion would be little lower.Under this condition,the uptake of Pb(II)ion from this mixed metal ion solution on the SiO2/graphene composite is as high as84.23%, while other ions show only slight/negligible adsorption.The exper-iment data demonstrate highly selective adsorption of Pb(II)ion on the SiO2/graphene composite.3.5.Adsorption mechanismGenerally speaking,the adsorption of metal ions is based on the three adsorption mechanisms:electrostatic interactions,ion ex-change,and complex formation[41].In our study,the pH value of the solution increased after adsorption of Pb(II)ion(Table2), and the adsorption efﬁciency of Pb(II)ion increased with increas-ing the pH value until the optimum pH,which is in accordance with the related literature[27].As we all know,graphene sheets, containing delocalized p electrons,and lead ion/hydrogen ion act as electron donor and acceptor,respectively,which can form the electron donor–acceptor complexes.In this system,the complex is formed by a coordination bond(or dative bond or dipolar bond) between the unshared electron pair of the composites and an elec-tron-deﬁcient atom of lead ion and hydrogen ion.So,it suggests that lead ion and hydrogen ion simultaneously adsorbed onto graphene and form composites,and results in an increase in the pH value.This phenomenon indicates that ion exchange is not the main cause.For our study,the surface charge is regarded as negative at high pH,which provides the ability of binding cations through electrostatic interaction.Besides,according to the previ-ous report,the basic sites as C p electrons on graphene sheets are considered as the important adsorption sites[42].Conse-quently,electrostatic interaction between Pb(II)cations and nega-tive surface charge and/or C p electrons of the composite is regarded as the main interaction for the adsorption of Pb(II)ion onto the composite.In addition,the speciﬁc surface area (252.5m2gÀ1)according to BET measure is another course,which provides more active sites for Pb(II)ion adsorption.However,re-search is needed for the clear mechanism in further investigations.4.ConclusionsIn summary,the SiO2/graphene composite was synthesized via a facile,fast,and low-cost process and further was developed to be highly efﬁcient adsorbent for Pb(II)ion in aqueous solution.The SiO2/graphene composite reduces the serious stacking of graphene sheets and prevents the agglomeration of SiO2nanoparticles,and also produces a high surface area,which enables the composite to show high binding capability and excellent adsorption proper-ties for Pb(II)ion.This adsorbent is stable,low-cost,and environ-mentally friendly and shows potential application in the removal of Pb(II)ion from agricultural and industrial waste water.In addi-tion,successful preparation of SiO2/graphene composite was very helpful to understand the fundamental properties of graphene-based composites and some practical applications.AcknowledgmentsThis work was supported by the National Nature Science Foun-dation of China(21075084)and the Sichuan Youth Science&Tech-nology Foundation(No.2009-18-409).The authors also would like to show gratitude for Dr.Jiqiu Wen and Dr.Hong Chen of Analytical &Testing Center at Sichuan University for their assistance in the XRD and XPS analysis.Appendix A.Supplementary materialSupplementary data associated with this article can be found,in the online version,at doi:10.1016/j.jcis.2011.12.023.References[1]K.S.Novoselov,A.K.Geim,S.V.Morozov,D.Jiang,Y.Zhang,S.V.Dubonos,I.V.Grigorieva,A.A.Firsov,Science306(2004)666.[2]A.H.Castro Neto,F.Guinea,N.M.R.Peres,K.S.Novoselov,A.K.Geim,Rev.Mod.Phys.81(2009)109.[3]C.N.R.Rao,A.K.Sood,K.S.Subrahmanyam,indaraj,Angew.Chem.Int.Ed.48(2009)7752.[4]A.K.Geim,K.S.Novoselov,Nat.Mater.6(2007)183.[5]D.A.D.Sasha Stankovich,Geoffrey H.B.Dommett,Kevin M.Kohlhaas,Eric J.Zimney,Nature442(2006)282.[6]C.Xu,X.Wang,J.Zhu,J.Phys.Chem.C112(2008)19841.[7]J.Shen,Y.Hu,M.Shi,N.Li,H.Ma,M.Ye,J.Phys.Chem.C114(2010)1498.[8]T.S.Sreeprasad,S.M.Maliyekkal,K.P.Lisha,T.Pradeep,J.Hazard.Mater.186(2011)921.[9]R.Muszynski,B.Seger,P.V.Kamat,J.Phys.Chem.C112(2008)5263.[10]Y.Si,E.T.Samulski,Chem.Mater.20(2008)6792.[11]G.Williams,B.Seger,P.V.Kamat,ACS Nano2(2008)1487.[12]H.Song,L.Zhang,C.He,Y.Qu,Y.Tian,Y.Lv,J.Mater.Chem.21(2011)5972.[13]K.Zhang,V.Dwivedi,C.Chi,J.Wu,J.Hazard.Mater.182(2010)162.[14]V.Chandra,J.Park,Y.Chun,J.W.Lee,I.C.Hwang,K.S.Kim,ACS Nano4(2010)3979.Table1Parameters of Langmuir and Freundlich isotherm models for Pb(II)ion on SiO2/graphene composite.Langmuir FreundlichAdsorbent q m(mg gÀ1)K L(L mgÀ1)R2K F(mg gÀ1)n R2SiO2/graphene113.6 3.3860.992070.39 3.4820.9682Table2Changes in the pH value after adsorption of Pb(II)ion.pH b 2.00 3.00 4.00 5.00 5.50 6.007.00pH a 2.68 4.04 5.60 5.81 6.16 6.38 6.67pH b:the pH value before adsorption of Pb(II)ion;pH a:the pH value after adsorption of Pb(II)ion.386L.Hao et al./Journal of Colloid and Interface Science369(2012)381–387[15]C.Xu,X.Wang,J.Zhu,X.Yang,L.Lu,J.Mater.Chem.18(2008)5625.[16]S.Yang,X.Feng,S.Ivanovici,K.Müllen,Angew.Chem.Int.Ed.49(2010)8408.[17]F.G.H.H.Yuan,Z.G.Zhang,L.D.Miao,R.H.Yu,H.L.Zhao,n,J.Health Sci.56(2010)632.[18]S.W.Yan Hui Li,J.Q.Wei,X.F.Zhang,C.L.Xu,Z.K.Luan,D.H.Wu,B.Q.Wei,Chem.Phys.Lett.357(2002)263.[19]Y.H.Li,Z.Di,J.Ding,D.Wu,Z.Luan,Y.Zhu,Water Res.39(2005)605.[20]G.P.Rao,C.Lu,F.Su,Sep.Pur.Technol.58(2007)224.[21]M.Ghaedi,A.Shokrollahi,K.Niknam,E.Niknam,A.Najibi,M.Soylak,J.Hazard.Mater.168(2009)1022.[22]O.D.Uluozlu,M.Tuzen,D.Mendil,M.Soylak,J.Hazard.Mater.176(2010)1032.[23]A.R.Karbassi,S.Nadjafpour,Environ.Pollut.93(1996)257.[24]M.Soylak,Y.E.Unsal,N.Kizil,A.Aydin,Food Chem.Toxicol.48(2010)517.[25]M.M.Rao,D.K.Ramana,K.Seshaiah,M.C.Wang,S.W.C.Chien,J.Hazard.Mater.166(2009)1006.[26]V.K.Gupta,I.Ali,J.Colloid Interface Sci.271(2004)321.[27]Z.H.Huang,X.Zheng,W.Lv,M.Wang,Q.H.Yang,F.Kang,Langmuir27(2011)7558.[28]A.H.Chen,S.C.Liu,C.Y.Chen,C.Y.Chen,J.Hazard.Mater.154(2008)184.[29]L.Kou,C.Gao,Nanoscale3(2011)519.[30]J.W.Liu,Q.Zhang,X.W.Chen,J.H.Wang,Chem.Eur.J.17(2011)4864.[31]Y.Yang,S.Qiu,W.Cui,Q.Zhao,X.Cheng,R.Li,X.Xie,Y.W.Mai,J.Mater.Sci.44(2009)4539.[32]Q.Liu,J.T.Sun,Thanh Wang,L.X.Zeng,G.B.Jiang,Angew.Chem.123(2011)6035.[33]X.G.Li,H.Feng,M.R.Huang,Chem.Eur.J.16(2010)10113.[34]D.S.f,Amauri J.Paula,Antonio G.Souza Filho,Yoong Ahm Kim,MorinobuEndo,Oswaldo L.Alves,Chem.Eur.J.17(2011)3228.[35]C.Y.Kim,S.H.Kim,R.Navamathavan,C.K.Choi,W.Y.Jeung,Thin Solid Films516(2007)340.[36]S.Stankovich,R.D.Piner,X.Chen,N.Wu,S.T.Nguyen,R.S.Ruoff,J.Mater.Chem.16(2006)155.[37]X.Deng,L.Lü,H.Li,F.Luo,J.Hazard.Mater.183(2010)923.[38]C.Chen,X.Wang,Ind.Eng.Chem.Res.45(2006)9144.[39]ngmuir,J.Am.Chem.Soc.38(1916)2221.[40]Y.Li,P.Zhang,Q.Du,X.Peng,T.Liu,Z.Wang,Y.Xia,W.Zhang,K.Wang,H.Zhu,D.Wu,J.Colloid Interface Sci.363(2011)348.[41]C.Moreno-Castilla,M.A.Álvarez-Merino,L.M.Pastrana-Martínez,M.V.López-Ramón,J.Colloid Interface Sci.345(2010)461.[42]M.Machida,T.Mochimaru,H.Tatsumoto,Carbon44(2006)2681.L.Hao et al./Journal of Colloid and Interface Science369(2012)381–387387。

Observation of long-lived interlayer excitons in monolayer MoSe2–WSe2 heterostructures

ARTICLEReceived1Apr2014|Accepted9Jan2015|Published24Feb2015Observation of long-lived interlayer excitonsin monolayer MoSe2–WSe2heterostructuresPasqual Rivera1,John R.Schaibley1,Aaron M.Jones1,Jason S.Ross2,Sanfeng Wu1,Grant Aivazian1,Philip Klement1,Kyle Seyler1,Genevieve Clark2,Nirmal J.Ghimire3,4,Jiaqiang Yan4,5,D.G.Mandrus3,4,5, Wang Yao6&Xiaodong Xu1,2Van der Waals bound heterostructures constructed with two-dimensional materials,such asgraphene,boron nitride and transition metal dichalcogenides,have sparked wide interest indevice physics and technologies at the two-dimensional limit.One highly coveted hetero-structure is that of differing monolayer transition metal dichalcogenides with type-II bandalignment,with bound electrons and holes localized in individual monolayers,that is,interlayer excitons.Here,we report the observation of interlayer excitons in monolayerMoSe2–WSe2heterostructures by photoluminescence and photoluminescence excitationspectroscopy.Weﬁnd that their energy and luminescence intensity are highly tunable by anapplied vertical gate voltage.Moreover,we measure an interlayer exciton lifetime of B1.8ns,an order of magnitude longer than intralayer excitons in monolayers.Our work demonstratesoptical pumping of interlayer electric polarization,which may provoke further explorationof interlayer exciton condensation,as well as new applications in two-dimensional lasers,light-emitting diodes and photovoltaic devices.1Department of Physics,University of Washington,Seattle,Washington98195,USA.2Department of Materials Science and Engineering,University of Washington,Seattle,Washington98195,USA.3Department of Physics and Astronomy,University of T ennessee,Knoxville,T ennessee37996,USA.4Materials Science and T echnology Division,Oak Ridge National Laboratory,Oak Ridge,T ennessee37831,USA.5Department of Materials Science and Engineering,University of T ennessee,Knoxville,T ennessee37996,USA.6Department of Physics and Center of Theoretical and Computational Physics, University of Hong Kong,Hong Kong,China.Correspondence and requests for materials should be addressed to P.R.(email:pasqual@)or to X.X. (email:xuxd@).T he recently developed ability to vertically assemble different two-dimensional(2D)materials heralds a newrealm of device physics based on van der Waals heterostructures(HSs)1.The most successful example to date is the vertical integration of graphene on boron nitride.Such novel HSs not only markedly enhance graphene’s electronic properties2, but also give rise to superlattice structures demonstrating exotic physical phenomena3–5.A fascinating counterpart to gapless graphene is a class of monolayer direct bandgap semiconductors, namely transition metal dichalcogenides(TMDs)6–8.Due to the large binding energy in these2D semiconductors,excitons dominate the optical response,exhibiting strong light–matter interactions that are electrically tunable9,10.The discovery of excitonic valley physics11–15and strongly coupled spin and pseudospin physics16,17in2D TMDs opens up new possibilities for device concepts not possible in other material systems. Monolayer TMDs have the chemical formula MX2where the M is tungsten(W)or molybdenum(Mo),and the X is sulfur(S) or selenium(Se).Although these TMDs share the same crystalline structure,their physical properties,such as bandgap,exciton resonance and spin–orbit coupling strength,can vary signiﬁ-cantly.Therefore,an intriguing possibility is to stack different TMD monolayers on top of one another to form2D HSs.First-principle calculations show that heterojunctions formed between monolayer tungsten and molybdenum dichalcogenides have type-II band alignment18–20.Recently,this has been conﬁrmed by X-ray photoelectron spectroscopy and scanning tunnelling spectroscopy21.Since the Coulomb binding energy in2D TMDs is much stronger than in conventional semiconductors, it is possible to realize interlayer excitonic states in van der Waals bound heterobilayers,that is,bound electrons and holes that are localized in different layers.Such interlayer excitons have been intensely pursued in bilayer graphene for possible exciton condensation22,but direct optical observation demonstrating the existence of such excitons is challenging owing to the lack of a sizable bandgap in graphene.Monolayer TMDs with bandgaps in the visible range provide the opportunity to optically pump interlayer excitons,which can be directly observed through photoluminescence(PL)measurements.In this report,we present direct observation of interlayer excitons in vertically stacked monolayer MoSe2–WSe2HSs.We show that interlayer exciton PL is enhanced under optical excitation resonant with the intralayer excitons in isolated monolayers,consistent with the interlayer charge transfer resulting from the underlying type-II band structure.We demonstrate the tuning of the interlayer exciton energy by applying a vertical gate voltage,which is consistent with the permanent out-of-plane electric dipole nature of interlayer excitons.Moreover,weﬁnd a blue shift in PL energy at increasing excitation power,a hallmark of repulsive dipole–dipole interac-tions between spatially indirect excitons.Finally,time-resolved PL measurements yield a lifetime of1.8ns,which is at least an order of magnitude longer than that of intralayer excitons.Our work shows that monolayer semiconducting HSs are a promising platform for exploring new optoelectronic phenomena.ResultsMoSe2–WSe2HS photoluminescence.HSs are prepared by standard polymethyl methacrylate(PMMA)transfer techniques using mechanically exfoliated monolayers of WSe2and MoSe2(see Methods).Since there is no effort made to match the crystal lattices of the two monolayers,the obtained HSs are considered incom-mensurate.An idealized depiction of the vertical MoSe2–WSe2HS is shown in Fig.1a.We have fabricated six devices that all show similar results as those reported below.The data presented here are from two independent MoSe2–WSe2HSs,labelled device1and device2.Figure1b shows an optical micrograph of device1,which has individual monolayers,as well as a large area of vertically stacked HS.This device architecture allows for the comparison of the excitonic spectrum of individual monolayers with that of the HS region,allowing for a controlled identiﬁcation of spectral changes resulting from interlayer coupling.We characterize the MoSe2–WSe2monolayers and HS using PL measurements.Inspection of the PL from the HS at room temperature reveals three dominant spectral features(Fig.1c). The emission at1.65and1.57eV corresponds to the excitonic states from monolayer WSe2and MoSe2(refs10,15),respectively. PL from the HS region,outlined by the dashed white line in Fig.1a,reveals a distinct spectral feature at1.35eV(X I).Two-dimensional mapping of the spectrally integrated PL from X I shows that it is isolated entirely to the HS region(inset,Fig.1c), with highly uniform peak intensity and spectral position (Supplementary Materials1).Low-temperature characterization of the HS is performed with 1.88eV laser excitation at20K.PL from individual monolayer WSe2(top),MoSe2(bottom)and the HS area(middle)are shown with the same scale in Fig.1d.At low temperature,the intralayer neutral(X M o)and charged(X MÀ)excitons are resolved10,15,where M labels either W or parison of the three spectra shows that both intralayer X M o and X MÀexist in the HS with emission at the same energy as from isolated monolayers,demonstrating the preservation of intralayer excitons in the HS region.PL from X I becomes more pronounced and is comparable to the intralayer excitons at low temperature.We note that the X I energy position has variation across the pool of HS samples we have studied (Supplementary Fig.1),which we attribute to differences in the interlayer separation,possibly due to imperfect transfer and a different twisting angle between monolayers.We further perform PL excitation(PLE)spectroscopy to investigate the correlation between X I and intralayer excitons.A narrow bandwidth(o50kHz)frequency tunable laser is swept across the energy resonances of intralayer excitons(from1.6to 1.75eV)while monitoring X I PL response.Figure2a shows an intensity plot of X I emission as a function of photoexcitation energy from device2.We clearly observe the enhancement of X I emission when the excitation energy is resonant with intralayer exciton states(Fig.2b).Now we discuss the origin of X I.Since X I has never been observed in our exfoliated monolayer and bilayer samples,if its origin were related to defects,they must be introduced by the fabrication process.This would result in sample-dependent X I properties with non-uniform spatial dependence.However,our data show that key physical properties of X I,such as the resonance energy and intensity,are spatially uniform and isolated to the HS region(inset of Fig.1c and Supplementary Fig.2).In addition,X I has not been observed in WSe2–WSe2homo-structures constructed from exfoliated or physical vapor deposi-tion(PVD)grown monolayers(Supplementary Fig.3).All these facts suggest that X I is not a defect-related exciton.Instead,the experimental results support the observation of an interlayer exciton.Due to the type-II band alignment of the MoSe2–WSe2HS18–20,as shown in Fig.2c,photoexcited electrons and holes will relax(dashed lines)to the conduction band edge of MoSe2and the valence band edge of WSe2,respectively.The Coulomb attraction between electrons in the MoSe2and holes in the WSe2gives rise to an interlayer exciton,X I,analogous to spatially indirect excitons in coupled quantum wells.The interlayer coupling yields the lowest energy bright exciton in the HS,which is consistent with the temperature dependence of X I PL,that is,it increases as temperature decreases (Supplementary Fig.4).From the intralayer and interlayer exciton spectral positions,we can infer the band offsets between the WSe 2and MoSe 2monolayers (Fig.2c).The energy difference between X W and X I at room temperature is 310meV.Considering the smaller binding energy of interlayer than intralayer excitons,this sets a lower bound on the conduction band offset.The energy difference between X M and X I then provides a lower bound on the valence band offset of 230meV.This value is consistent with the valence band offset of 228meV found in MoS 2–WSe 2HSs by micro X-ray photoelectron spectroscopy and scanning tunnelling spectro-scopy measurements 21.This experimental evidence strongly corroborates X I as an interlayer exciton.The observation of bright interlayer excitons in monolayer semiconducting HSs is of central importance,and the remainder of this paper will focus on their physical properties resulting from their spatially indirect nature and the underlying type-II band alignment.WSe 2HSMoSe 2W M SeIn te n s i t y (a .u .)1.31.51.7Energy (eV)MoSe 2HeterostructureWSe 2W0WX X X X −0MoMo−e hehe h1.3 1.41.51.6 1.7I n t e n s i t y (a .u .)Energy (eV)5μm 0123×104Y (μm )246X (μm)0246Figure 1|Intralayer and interlayer excitons of a monolayer MoSe 2–WSe 2vertical heterostructure.(a )Cartoon depiction of a MoSe 2–WSe 2heterostructure (HS).(b )Microscope image of a MoSe 2–WSe 2HS (device 1)with a white dashed line outlining the HS region.(c )Room-temperature photoluminescence of the heterostructure under 20m W laser excitation at 2.33eV.Inset:spatial map of integrated PL intensity from the low-energy peak (1.273–1.400eV),which is only appreciable in the heterostructure area,outlined by the dashed black line.(d )Photoluminescence of individual monolayers and the HS at 20K under 20m W excitation at 1.88eV (plotted on the samescale).Energy (eV)WSe MoSe PL energy (eV)E x c i t a t i o n e n e r g y (e V )1.28 1.3 1.32 1.34 1.36 1.381.61.651.71.754,0006,0008,00010,000IntensityFigure 2|Photoluminescence excitation spectroscopy of the interlayer exciton at 20K.(a )PLE intensity plot of the heterostructure region with an excitation power of 30m W and 5s charge-coupled device CCD integration time.(b )Spectrally integrated PLE response (red dots)overlaid on PL (black line)with 100m W excitation at 1.88eV.(c )Type-II semiconductor band alignment diagram for the 2D MoSe 2–WSe 2heterojunction.interlayer exciton .Applying vertical energy of Figure 3a contact stacked insu-Electrostatic contact shows the 100to about analogue of reversed,varied expected for from reduces device 2,conduction 3b,c.of the in the on top band-offset at X I PL energy of basis of would should have X I PL This effect,intensity.further Power dependence and lifetime of interlayer exciton PL .The interlayer exciton PLE spectrum as a function of laser power with excitation energy in resonance with X W o reveals several properties of the X I .Inspection of the normalized PLE intensity (Fig.4a)shows the evolution of a doublet in the interlayer excitonspectrum,highlighted by the red and Both peaks of the doublet display a consistent increased laser intensity,shown by the dashed which are included as a guide to the eye.intensity of X I also exhibits a strong saturation laser power,as shown in Fig.4b (absolute Supplementary Fig.6).The sublinear power excitation powers above 0.5m W is distinctly the intralayer excitons in isolated monolayers,saturation power threshold of about Fig.7).The low power saturation of X I PL lifetime than that of intralayer excitons.the intralayer exciton is substantially reduced interlayer charge hopping 23,which is quenching of intralayer exciton PL (Fig.Fig.8).Moreover,the lifetime of the interlayer because it is the lowest energy conﬁguration indirect nature leads to a reduced optical long lifetime is conﬁrmed by time-resolved Fig.4c.A ﬁt to a single exponential decay exciton lifetime of 1.8±0.3ns.This timescale the intralayer exciton lifetime,which is ps 24–27.By modelling the saturation behaviour three-level diagram,the calculated saturation interlayer exciton is about 180times (Supplementary Fig.7;Supplementary with our observation of low saturation intensity DiscussionWe attribute the observed doublet feature splitting of the monolayer MoSe 2conduction assignment is mainly based on the fact difference between the doublet is B 25with MoSe 2conduction band splitting predicted calculations 28.This explanation is also supported by the evolution of the relative strength of the two peaks with increasing excitation power,as shown in Fig.4a (similar results in device 1with 1.88eV excitation shown in Supplementary Fig.9).At low power,the lowest energy conﬁguration of interlayer excitons,with the electron in the lower spin-split band of MoSe 2,is populated ﬁrst.Due to phase space ﬁlling effects,the interlayer excitonSiO 2n + Si2MoSe 2e –h +e –h +P Ee –h +V g < 0WSe 2MoSe 2WSe 2MoSe 2h ωV g = 0Photon energy (eV)1.321.361.41.444080e –h +h +PL intensity (a.u.) -hω’-the interlayer exciton and band alignment.(a )Device 2geometry.The interlayer exciton has a out-of-plane electric polarization.(b )Electrostatic control of the band alignment and the interlayer exciton photoluminescence as a function of applied gate voltage under 70m W excitation at 1.744eV,1s integrationconﬁguration with the electron in the higher energy spin-split band starts to be ﬁlled at higher laser power.Consequently,the higher energy peak of the doublet becomes more prominent at higher excitation powers.The observed blue shift of X I as the excitation power increases,indicated by the dashed arrows in Fig.4a,is a signature of the repulsive interaction between the dipole-aligned interlayer excitons (cf.Fig.3a).This is a hallmark of spatially indirect excitons in gallium arsenide (GaAs)coupled quantum wells,which have been intensely studied for exciton Bose-Einstein condensation (BEC)phenomena 29.The observation of spatially indirect interlayer excitons in a type-II semiconducting 2D HS provides an intriguing platform to explore exciton BEC,where the observed extended lifetimes and repulsive interactions are two key ingredients towards the realization of this exotic state of matter.Moreover,the extraordinarily high binding energy for excitons in this truly 2D system may provide for degenerate exciton gases at elevated temperatures compared with other material systems 30.The long-lived interlayer exciton may also lead to new optoelectronic applications,such as photovoltaics 31–34and 2D HS nanolasers.MethodsDevice fabrication .Monolayers of MoSe 2are mechanically exfoliated onto 300nm SiO 2on heavily doped Si wafers and monolayers of WSe 2onto a layer of PMMA atop polyvinyl alcohol on Si.Both monolayers are identiﬁed with an opticalmicroscope and conﬁrmed by their PL spectra.Polyvinyl alcohol is dissolved in H 2O and the PMMA layer is then placed on a transfer loop or thin layer of poly-dimethylsiloxane (PDMS).The top monolayer is then placed in contact with the bottom monolayer with the aid of an optical microscope and micromanipulators.The substrate is then heated to cause the PMMA layer to release from the transfer media.The PMMA is subsequently dissolved in acetone for B 30min and then rinsed with isopropyl alcohol.Low-temperature PL measurements .Low-temperature measurements are con-ducted in a temperature-controlled Janis cold ﬁnger cryostat (sample in vacuum)with a diffraction-limited excitation beam diameter of B 1m m.PL is spectrally ﬁltered through a 0.5-m monochromator (Andor–Shamrock)and detected on a charge-coupled device (Andor—Newton).Spatial PL mapping is performed using a Mad City Labs Nano-T555nanopositioning system.For PLE measurements,a continuous wave Ti:sapphire laser (MSquared—SolsTiS)is used for excitation and ﬁltered from the PL signal using an 815-nm-long pass optical ﬁlter (Semrock).Electrostatic doping is accomplished with an indium drain contact deposited onto the monolayer WSe 2region of device 2and using the heavily doped Si as a tunable backgate.Time-resolved PL measurements .For interlayer lifetime measurements,we excite the sample with a o 200-fs pulsed Ti:sapphire laser (Coherent—MIRA).Interlayer PL is spectrally ﬁltered through a 0.5-m monochromator (Princeton—Acton 2500),and detected with a fast time-correlated single-photon counting system composed of a fast (o 30ps full width at half maximum)single-photon avalanche detector (Micro Photon Devices—PDM series)and a picosecond event timer (PicoQuant—PicoHarp 300).References1.Geim,A.K.&Grigorieva,I.V.Van der Waals heterostructures.Nature 499,419–425(2013).2.Dean,C.R.et al.Boron nitride substrates for high-quality graphene electronics.Nat.Nanotechnol.5,722–726(2010).3.Hunt,B.et al.Massive Dirac fermions and Hofstadter butterﬂy in a van derWaals heterostructure.Science 340,1427–1430(2013).4.Dean,C.R.et al.Hofstadter’s butterﬂy and the fractal quantum Hall effect inmoire superlattices.Nature 497,598–602(2013).5.Ponomarenko,L.A.et al.Cloning of Dirac fermions in graphene superlattices.Nature 497,594–597(2013).6.Novoselov,K.S.et al.Two-dimensional atomic crystals.Proc.Natl Acad.SciUSA 102,10451–10453(2005).7.Mak,K.F.,Lee,C.,Hone,J.,Shan,J.&Heinz,T.F.Atomically thin MoS 2:anew direct-gap semiconductor.Phys.Rev.Lett.105,136805(2010).8.Splendiani,A.et al.Emerging photoluminescence in monolayer MoS 2.NanoLett.10,1271–1275(2010).MoSe2WSe 2dPhoton energy (eV)Laser power (μW)Time (ns)I n t e n s i t y (a .u .)N o r m a l i z e d P L E i n t e n s i t y (c o u n t s (μW s )–1)Power (μW)I n t e g r a t e d P L E i n t e n s i t y (C o u n t s (μW s )–1)2060404Figure 4|Power-dependent photoluminescence of interlayer exciton and its lifetime at 20K.(a )Power dependence of the interlayer exciton for 1.722eV excitation with a bi-Lorentzian ﬁt to the 5and 100m W plots,normalized for power and charge-coupled device (CCD)integration time.(b )Spectrally integrated intensity of the interlayer exciton emission as a function of excitation power shows the saturation effect.(c )Time-resolvedphotoluminescence of the interlayer exciton (1.35eV)shows a lifetime of about 1.8ns.The dashed curve is the instrument response to the excitation laser pulse.(d )Illustration of the heterojunction band diagram,including the spin levels of the MoSe 2conduction band.The X I doublet has energy splitting equal to (o 0I Ào I )E 25meV.9.Mak,K.F.et al.Tightly bound trions in monolayer MoS2.Nat.Mater.12,207–211(2013).10.Ross,J.S.et al.Electrical control of neutral and charged excitons in amonolayer mun.4,1474(2013).11.Xiao,D.,Liu,G.-B.,Feng,W.,Xu,X.&Yao,W.Coupled spin and valleyphysics in monolayers of MoS2and other group-VI dichalcogenides.Phys.Rev.Lett.108,196802(2012).12.Cao,T.et al.Valley-selective circular dichroism of monolayer molybdenummun.3,887(2012).13.Zeng,H.,Dai,J.,Yao,W.,Xiao,D.&Cui,X.Valley polarization inMoS2monolayers by optical pumping.Nat.Nanotechnol.7,490–493(2012).14.Mak,K.F.,He,K.,Shan,J.&Heinz,T.F.Control of valley polarization inmonolayer MoS2by optical helicity.Nat.Nanotechnol.7,494–498(2012). 15.Jones,A.M.et al.Optical generation of excitonic valley coherence inmonolayer WSe2.Nat.Nanotechnol.8,634–638(2013).16.Gong,Z.et al.Magnetoelectric effects and valley-controlled spin quantumgates in transition metal dichalcogenide mun.4,2053(2013).17.Jones,A.M.et al.Spin-layer locking effects in optical orientation of excitonspin in bilayer WSe2.Nat.Phys.10,130–134(2014).18.Kang,J.,Tongay,S.,Zhou,J.,Li,J.&Wu,J.Band offsets and heterostructuresof two-dimensional semiconductors.Appl.Phys.Lett.102,012111–012114 (2013).19.Kos´mider,K.&Ferna´ndez-Rossier,J.Electronic properties of the MoS2-WS2heterojunction.Phys.Rev.B87,075451(2013).20.Terrones,H.,Lopez-Urias,F.&Terrones,M.Novel hetero-layered materialswith tunable direct band gaps by sandwiching different metal disulﬁdes and diselenides.Sci.Rep.3,1549(2013).21.Chiu,M.-H.et al.Determination of band alignment in transition metaldichalcogenide heterojunctions,Preprint at http://arXiv:1406.5137(2014). 22.Su,J.-J.&MacDonald,A.H.How to make a bilayer exciton condensateﬂow.Nat.Phys.4,799–802(2008).23.Hong,X.P.et al.Ultrafast charge transfer in atomically thin MoS2/WS2heterostructures.Nat.Nanotechnol.9,682–686(2014).24.Wang,G.et al.Valley dynamics probed through charged and neutral excitonemission in monolayer WSe2.Phys.Rev.B90,075413(2014).garde,D.et al.Carrier and polarization dynamics in monolayer MoS2.Phys.Rev.Lett.112,047401(2014).26.Mai,C.et al.Many-body effects in valleytronics:direct measurement of valleylifetimes in single-layer MoS2.Nano Lett.14,202–206(2013).27.Shi,H.et al.Exciton dynamics in suspended monolayer and few-layer MoS22Dcrystals.ACS Nano7,1072–1080(2012).28.Korma´nyos,A.,Zo´lyomi,V.,Drummond,N.D.&Burkard,G.Spin-orbitcoupling,quantum dots,and qubits in monolayer transition metaldichalcogenides.Phys.Rev.X4,011034(2014).29.Butov,L.V.,Lai,C.W.,Ivanov,A.L.,Gossard,A.C.&Chemla,D.S.TowardsBose-Einstein condensation of excitons in potential traps.Nature417,47–52 (2002).30.Fogler,M.M.,Butov,L.V.&Novoselov,K.S.High-temperature superﬂuiditywith indirect excitons in van der Waals mun.5,4555 (2014).31.Lee,C.H.et al.Atomically thin p-n junctions with van der Waalsheterointerfaces.Nat.Nanotechnol.9,676–681(2014).32.Furchi,M.M.,Pospischil,A.,Libisch,F.,Burgdorfer,J.&Mueller,T.Photovoltaic effect in an electrically tunable van der Waals heterojunction.Nano Lett.14,4785–4791(2014).33.Cheng,R.et al.Electroluminescence and photocurrent generation fromatomically sharp WSe2/MoS2heterojunction p-n diodes.Nano Lett.14,5590–5597(2014).34.Fang,H.et al.Strong interlayer coupling in van der Waals heterostructuresbuilt from single-layer chalcogenides.Proc.Natl A111,6198–6202 (2014).AcknowledgementsThis work is mainly supported by the US DoE,BES,Materials Sciences and Engineering Division(DE-SC0008145).N.J.G.,J.Y.and D.G.M.are supported by US DoE,BES, Materials Sciences and Engineering Division.W.Y.is supported by the Research Grant Council of Hong Kong(HKU17305914P,HKU9/CRF/13G),and the Croucher Foun-dation under the Croucher Innovation Award.X.X.thanks the support of the Cottrell Scholar Award.P.R.thanks the UW GO-MAP program for their support.A.M.J.is partially supported by the NSF(DGE-0718124).J.S.R.is partially supported by the NSF (DGE-1256082).S.W.and G.C.are partially supported by the State of Washington through the UW Clean Energy Institute.Device fabrication was performed at the Washington Nanofabrication Facility and NSF-funded Nanotech User Facility. Author contributionsX.X.and P.R.conceived the experiments.P.R.and P.K.fabricated the devices,assisted by J.S.R.P.R.performed the measurements,assisted by J.R.S.,A.M.J.,J.S.R.,S.W.and G.A. P.R.and X.X.performed data analysis,with input from W.Y.N.J.G.,J.Y.and D.G.M. synthesized and characterized the bulk WSe2crystals.X.X.,P.R.,J.R.S.and W.Y.wrote the paper.All authors discussed the results.Additional informationSupplementary Information accompanies this paper at / naturecommunicationsCompetingﬁnancial interests:The authors declare no competingﬁnancial interests. Reprints and permission information is available online at / reprintsandpermissions/How to cite this article:Rivera,P.et al.Observation of long-lived interlayer excitons in monolayer MoSe2–mun.6:6242doi:10.1038/ncomms7242(2015).。

不饱和类锗烯H2C=GeLiCl的DFT研究

不饱和类锗烯H 2C ‗GeLiCl 的DFT 研究李文佐1,*谭海娜1肖翠平1宫宝安1程建波1,2(1烟台大学化学生物理工学院,山东烟台264005;2吉林大学超分子结构与材料教育部重点实验室,长春130012)摘要:采用密度泛函理论方法,在B3LYP/6⁃311G(d,p )水平上研究了不饱和类锗烯H 2C ‗GeLiCl 的结构及异构化反应.结果表明,不饱和类锗烯H 2C ‗GeLiCl 有三种平衡构型,其中非平面的p ⁃配合物型构型能量最低,是其存在的主要构型.对平衡构型间异构化反应的过渡态进行了计算,求得了转化势垒.计算预言了最稳定构型的振动频率和红外吸收强度.关键词:不饱和类锗烯H 2C ‗GeLiCl ；DFT ；异构化中图分类号:O641.12DFT Study on the Unsaturated Germylenoid H 2C ‗GeLiClLI Wen ⁃Zuo 1,*TAN Hai ⁃Na 1XIAO Cui ⁃Ping 1GONG Bao ⁃An 1CHENG Jian ⁃Bo 1,2(1Science and Engineering College of Chemistry and Biology,Yantai University,Yantai 264005,Shandong Province,P.R.China;2KeyLaboratory for Supramolecular Structure and Materials of Ministry of Education,Jilin University,Changchun130012,P.R.China )Abstract:The unsaturated germylenoid H 2C ‗GeLiCl was studied by using the DFT method at the B3LYP/6⁃311G(d,p )level of theory.Geometry optimization calculations indicated that H 2C ‗GeLiCl had three equilibrium configurations,in which the non ⁃planar p ⁃complex was lowest in energy and was the most stable structure.The transition states for isomerization reactions of H 2C ‗GeLiCl were located and the energy barriers were calculated.For the most stable structure,the vibrational frequencies and infrared intensities had been predicted.Key Words:Unsaturated germylenoid H 2C ‗GeLiCl;DFT;Isomerization锗烯(germylene)与卡宾和硅烯类似,是一种重要的有机反应活性中间体[1-3].由于有机锗类化合物具有某些生物活性[4-10],对锗烯及其衍生物的研究日益增多[11-16].类锗烯(germylenoid)与类卡宾与类硅烯类似,可看作由锗烯与碱金属卤化物形成,用通式R 1R 2GeMX 来表示.自Gaspar 等[17]提出类锗烯是某些化学反应的中间体以来,对类锗烯的研究逐渐增多.Qiu 等[18]首次对最简单的类锗烯H 2GeLiF 进行了ab initio 计算;Tan 等[19,20]对H 2GeMF(M=Li,Na)与R —H (R=F,OH,NH 2,CH 3)的插入反应进行了研究;Ma 等[21]报道了对H 2GeLiCl 的理论研究结果.H 2C ‗GeMX 是一类与不饱和类碳烯H 2C ‗CMX [22]和不饱和类硅烯H 2C ‗SiMX [23]类似的不饱和类锗烯[24,25],对其结构、性质及稳定性的研究具有重要意义.为进一步丰富类锗烯的内容,深入了解不饱和类锗烯的结构和性质,本文采用密度泛函理论(DFT)方法对不饱和类锗烯H 2C ‗GeLiCl 进行了理论研究,以期为实验工作者提供更多的理论依据,并对进一步研究类锗烯有所裨益.1计算方法对不饱和类锗烯H 2C ‗GeLiCl 的平衡构型及构型间异构化反应的过渡态均采用DFT B3LYP [26,27]方法进行全参数优化,并进行振动频率分析及IRC [28]Received ：May 21,2007;Revised ：June 6,2007;Published on Web:August 6,2007.*Corresponding author.Email:liwenzuo@;Tel:+86535⁃6902063.烟台大学博士科研基金(HY05B30)资助项目ⒸEditorial office of Acta Physico ⁃Chimica Sinica[Note]物理化学学报(Wuli Huaxue Xuebao )Acta Phys.鄄Chim.Sin .,2007,23(11)：1811-1814November 1811Acta Phys.鄄Chim.Sin.,2007Vol.23计算,以确证过渡态.计算采用Gaussian 03[29]程序的6⁃311G(d,p )基组[30].2结果和讨论与不饱和卡宾H 2C ‗C:[22]和不饱和硅烯H 2C ‗Si:[23]类似,不饱和锗烯H 2C ‗Ge:的基态是单重态[31].在该分子中,Ge 原子采取sp 杂化,一个sp 杂化轨道与碳原子形成σ键,两个电子占据另一个sp 杂化轨道.两个相互垂直的p 轨道中,一个p 轨道与另一个碳原子的对应p 轨道形成π键,另一个p 轨道是不占电子的空轨道.不饱和锗烯H 2C ‗Ge:的结构如图1所示.不饱和锗烯中的p 轨道是空轨道,具有亲电性,sp 杂化轨道是占据轨道具有亲核性.这种具有双重性的H 2C ‗Ge:与强极性的LiCl 分子结合,从而构成不饱和类锗烯H 2C ‗GeLiCl 在构型上的复杂性.2.1平衡构型B3LYP/6⁃311G(d,p )构型优化与频率计算结果表明,H 2C ‗GeLiCl 有三种平衡构型,见图2中1-3所示.图2中4和5为H 2C ‗GeLiCl 异构化反应的过渡态构型.图2中还给出自然电荷布居结果.各构型的总能量、相对能量、偶极矩及虚频数目列于表1.图2中的构型1,是缺电子带正电荷的Li 端与Ge 原子占据两个电子的sp 杂化轨道结合,富电子带负电荷的Cl 端与Ge 原子的空p 轨道结合形成的三元环状结构.这一构型中,所有原子在同一平面内,属C s 点群.三元环中Li —Cl 的键长(0.2141nm)比LiCl 分子中Li —Cl 的键长(0.2025nm)长0.0116nm.1中C —Ge 键长(0.1759nm)比H 2C ‗Ge 中C —Ge 键长(0.1798nm)略短.通过比较构型1、H 2C ‗Ge 及LiCl 中Ge 、Li 和Cl 原子上的自然电荷布居大小变化(见图2),可以看出,计算得到构型1中存在Cl →Ge →Li →Cl 的电子流向,这应该是构型1能够稳定存在的主要原因.计算得到构型1的解离能(1→H 2C ‗Ge(1A 1)+LiCl)为74.44kJ ·mol -1.从表1可以看出,H 2C ‗GeLiCl 的构型1比能量最低构型2的能量高54.09kJ ·mol -1.构型2可看作带负电的Cl 端进攻Ge 的p 空图2H 2C ‗GeLiCl 的平衡构型及异构化反应的过渡态Fig.2The equilibrium configurations and the transition states for isomerization reactions of H 2C ‗GeLiClcalculated at B3LYP/6⁃311G(d,p )level;bond lengths are given in nm and angles in degrees,values in parentheses are the natural charges图1不饱和锗烯H 2C ‗Ge:的结构示意图Fig.1Schematic diagram of unsaturated germyleneH 2C ‗Ge:C 2vC 2v1812No.11李文佐等：不饱和类锗烯H 2C ‗GeLiCl 的DFT 研究轨道产生的构型,在该构型中,Cl 上的电子向Ge 的p 空轨道迁移,构型2可称之为p ⁃配合物型构型.p ⁃配合物型构型是非平面结构,可看作[H 2C —Ge —Cl]-Li +结构,其中Li 原子不仅与Cl 有相互作用,与C 和Ge 也有相互作用.2中C ‗Ge 键长(0.1860nm)比H 2C ‗Ge 中的C ‗Ge 键长(0.1798nm)长0.0062nm,Li —Cl 键长(0.2189nm)比LiCl 分子中的Li —Cl 键长长0.0164nm.在该结构中存在一个三原子(C 、Ge 、Cl)四电子的弯曲的离域π键,使得2的能量较低,从表1可以看出,构型2在三个平衡构型中能量最低.构型2的解离能(2→H 2C ‗Ge(1A 1)+LiCl)为128.53kJ ·mol -1.构型3可以称为σ⁃配合物型构型,它可看作LiCl 分子以电正性的Li 端接近Ge 原子的σ占据轨道形成Ge →Li 授受键而得到的一种配合物.结果表明构型3属C 2v 点群.构型3中,C ‗Ge 键长(0.1780nm)比H 2C ‗Ge 中的C ‗Ge 键长(0.1798nm)短,而Li —Cl 键长(0.2040nm)比LiCl 分子中Li —Cl 键长稍长.从表1可以看出,构型3是三个平衡构型中能量最高的构型.构型3的解离能(3→H 2C ‗Ge(1A 1)+LiCl)为28.86kJ ·mol -1.综上所述,不饱和类锗烯H 2C ‗GeLiCl 共有三种平衡构型,三种平衡构型均可视为不饱和锗烯H 2C ‗Ge:与LiCl 的加成物.从各平衡构型能量上分析(见表1),H 2C ‗GeLiCl 三种平衡构型的热力学稳定次序为:2>1>3.2.2构型间的异构化反应及动力学稳定性图2中构型4和5为H 2C ‗GeLiCl 势能面上的两个过渡态,由它们的反应矢量(能量二阶导数的唯一负本征值对应的本征矢)及IRC 计算结果分析可知,构型4为1与2异构化的过渡态,构型5为1与3异构化的过渡态.4和5的反应矢量(e )分别为e (4)=-0.003L CGe -0.002L ClGe -1.089θH1CGe +9.454θClGeC -2.521θLiClGe -2.578θClGeCH1+56.207θLiClGeC -3.323θH2CGeH1e (5)=0.002L LiGe +7.219θLiGeC -34.779θClLiGe式中,键长(L )和键角(θ)的单位分别为nm 和(°).键长系数小于0.0005和键角系数小于0.5的全部忽略.振动分析表明,构型4和5均存在唯一的虚频.在B3LYP/6⁃311G(d,p )水平上,4和5的虚频分别为84.4i 和27.1i,从而确证为真正的过渡态.为了确证过渡态与稳定几何构型的连接,以得到的过渡态为起始点,沿着反应途径分别向前和向后进行了IRC 计算,结果表明过渡态结构与稳定几何构型的连接正确.一般说来,各平衡构型的稳定性取决于它们自身能量的高低及相互异构化的活化能.由图3可以比较直观地看到,构型1异构为3的活化势垒为45.83kJ ·mol -1,而构型3异构为1的活化势垒仅为0.25kJ ·mol -1,所以构型3很容易异构为构型1.构型1异构为2的活化势垒为6.10kJ ·mol -1,而构型2异构为1的活化势垒为60.19kJ ·mol -1,所以构型1很容易异构为构型2.构型2应该是H 2C ‗GeLiCl 存在的主要结构.2.3最稳定构型的振动频率和红外吸收强度由上述计算结果可知,构型2是不饱和类锗烯H 2C ‗GeLiCl 存在的主要结构.为了给不饱和类锗烯H 2C ‗GeLiCl 的结构分析提供参考,将在B3LYP/6⁃311G(d,p )水平上计算得到的构型2的振动频率及红外吸收强度列于表2.图3H 2C ‗GeLiCl 的势能面沿构型异构化反应通道的剖面图Fig.3The potential profile for the isomerizationreactions of H 2C ‗GeLiClGeometriesE tot (a.u.)E rel /(kJ ·mol -1)1030μ/(C ·m)N imag 1-2584.1260019018.9902-2584.1466034-54.0913.0903-2584.108641945.5829.8104-2584.1236796 6.1017.7915-2584.108544545.8328.6816-2116.2645558-0.6907-467.8330933-23.94表1B3LYP/6⁃311G(d,p )水平上计算的H 2C ‗GeLiCl的总能量(E tot ),相对能量(E rel ),偶极矩(μ)及虚频个数(N imag )Table 1The B3LYP/6⁃311G(d,p )calculated total energies(E tot ),relative energies(E rel ),dipole moments (μ),and number of imaginary frequency (N imag )of thegeometries for H 2C ‗GeLiCl1813Acta Phys.⁃Chim.Sin.,2007Vol.233结论应用密度泛函理论DFT 方法研究了不饱和类锗烯H 2C ‗GeLiCl 的结构及异构化反应.B3LYP/6⁃311G(d,p )计算结果表明,不饱和类锗烯H 2C ‗GeLiCl 有3种可能的平衡构型,其中p ⁃配合物型构型能量最低,是其存在的主要构型.本文对进一步研究类锗烯提供了有用信息.References1Satge,J.;Massil,M.;Riviere,anomet.Chem.,1973,56:12Alexander,U.N.;Trout,N.A.;King,K.D.Chem.Phys.Lett.,1999,299:2913Su,M.D.;Chu,S.Y.Tetrahedron Lett.,1999,42:43714Satge,J.Pure Appl.Chem.,1984,56:1375Shoda,S.;Iwata,S.;Yajima,K.Tetrahedron,1997,53:152816Tokitoh,N.;Kishikawa,K.;Okazaki,R.Polyhedron,2002,21:5637Riviere,B.;Monique,anomet.Chem.,2000,595:1538Broclawik,E.;Bocho ⁃Janiszewska,A.J.Mol.Struct.⁃Theochem,2000,531:2419Iwamoto,T.;Masuda,H.;Ishida,anomet.Chem.,2004,689:133710Kassaee,M.Z.;Ghambarian,M.;Musavi,anomet.Chem.,2005,690:469211Neumann,W.P.Chem.Rev.,1991,91:31112Lu,X.H.;Wang,Y.X.;Liu,C.B.Acta Chimi.Sin.,1999,57:1343[卢秀慧,王沂轩,刘成卜.化学学报,1999,57:1343]13Tan,X.J.;Li,P.;Wang,D.S.Chinese J.Chem.Phys.,2005,18:913[谭晓军,李平,王党生.化学物理学报,2005,18:913]14Geng,Z.Y.;Wang,Y.C.;Wang,H.Q.Acta Phys.⁃Chim.Sin.,2004,20:1417[耿志远,王永成,汪汉卿.物理化学学报,2004,20:1417]15Fang,R.;Geng,Z.Y.;Wang,Y.C.;Zhang,X.H.;Wang,D.M.;Gao,L.G.;Chen,X.X.Acta Phys.⁃Chim.Sin.,2005,21:1331[方冉,耿志远,王永成,张兴辉,王冬梅,高立国,陈晓霞.物理化学学报,2005,21:1331]16Geng,Z.Y.;Jia,B.L.;Wang,Y.C.;Yao,K.;Fang,R.;Zhang,X.H.Acta Chimi.Sin.,2006,64:1974[耿志远,贾宝丽,王永成,姚琨,方冉,张兴辉.化学学报,2006,64:1974]17Lei,D.Q.;Gaspar,P.P.Polyhedron,1991,10:122118Qiu,H.Y.;Ma,W.Y.;Li,G.B.;Deng,C.H.Chinese Chem.Lett.,1999,10:51119Tan,X.;Li,P.;Wang,D.;Yang,X.J.Mol.Struct.⁃Theochem,2006,761:2720Tan,X.;Li,P.;Yang,X.;Wang,D.Int.J.Quan.Chem.,2006,106:190221Zhu,Y.F.;Fang,Y.Z.;Zhou,J.H.;Ma,W.Y.Chinese J.Struct.Chem.,2007,26:39522Liu,F.L.Acta Phys.⁃Chim.Sin.,2002,18:228[刘奉岭.物理化学学报,2002,18:228]23Li,W.Z.;Gong,B.A.;Cheng,J.B.Acta Phys.⁃Chim.Sin.,2006,22:653[李文佐,宫宝安,程建波.物理化学学报,2006,22:653]24Li,W.Z.;Cheng,J.B.;Gong,B.A.;Xiao,anomet.Chem.,2006,691:598425Li,W.Z.;Cheng,J.B.;Gong,B.A.;Xiao,C .P.Chinese J.Struct.Chem.,2007,26:61326Becke,A.D.J.Chem.Phys.,1993,98:564827Lee,C.;Yang,W.;Parr,R.G.Phys.Rev.,1988,B37:78528Gonzales,C.;Schlegel,H.B.J.Chem.Phys.,1991,95:585329Frisch,M.J.;Trucks,G.W.;Schlegel,H.B.;et al.Gaussian 03,Revision B.03.Pittsburgh,PA,200330Hehre,W.J.;Radom,L.;Schleyer,P.V.R;Pople,J.A.Ab initio molecular orbital theory.New York:Wiley,198631He,S.G.;Tackett,B.S.;Clouthier,D.J.J.Chem.Phys.,2004,121:257ν/cm-1I /(km ·mol -1)ν/cm -1I /(km ·mol -1)172.230.61625.725.35202.445.15678.852.18252.7 6.43744.7158.12442.960.781381.010.81451.068.843021.614.69566.421.663140.37.78表2图2中不饱和类锗烯H 2C ‗GeLiCl 的构型2的振动频率(ν)及红外吸收强度(I )Table 2Calculated vibrational frequencies(ν)and infrared intensities(I )of configuration 2in Fig.2forH 2C ‗GeLiCl1814。

半导体-毕业论文外文文献翻译

附录附录A:外文资料翻译—原文部分SemiconductorA semiconductor is a solid material that has electrical conductivity between those of a conductor and an insulator; it can vary over that wide range either permanently or dynamically.[1]Semiconductors are important in electronic technology. Semiconductor devices, electronic components made of semiconductor materials, are essential in modern consumer electronics, including computers, mobile phones, and digital audio players. Silicon is used to create most semiconductors commercially, but dozens of other materials are used.Bragg reflection in a diffuse latticeA second way starts with free electrons waves. When fading in an electrostatic potential due to the cores, due to Bragg reflection some waves are reflected and cannot penetrate the bulk, that is a band gap opens. In this description it is not clear, while the number of electrons fills up exactly all states below the gap.Energy level splitting due to spin state Pauli exclusionA third description starts with two atoms. The split states form a covalent bond where two electrons with spin up and spin down are mostly in between the two atoms. Adding more atoms now is supposed not to lead to splitting, but to more bonds. This is the way silicon is typically drawn. The band gap is now formed by lifting one electron from the lower electron level into the upper level. This level is known to be anti-bonding, but bulk silicon has not been seen to lose atoms as easy as electrons are wandering through it. Also this model is most unsuitable to explain how in graded hetero-junction the band gap can vary smoothly.Energy bands and electrical conductionLike in other solids, the electrons in semiconductors can have energies only within certain bands (ie. ranges of levels of energy) between the energy of the ground state, corresponding to electrons tightly bound to the atomic nuclei of the material, and the free electron energy, which is the energy required for an electron to escape entirely from the material. The energy bands each correspond to a large number of discrete quantum states of the electrons, and most of the states with low energy (closer to the nucleus) are full, up to a particular band called the valence band. Semiconductors and insulators are distinguished from metals because the valence band in the semiconductor materials is very nearly full under usual operating conditions, thus causing more electrons to be available in the conduction band.The ease with which electrons in a semiconductor can be excited from the valence band to the conduction band depends on the band gap between the bands, and it is the size of this energybandgap that serves as an arbitrary dividing line (roughly 4 eV) between semiconductors and insulators.In the picture of covalent bonds, an electron moves by hopping to a neighboring bond. Because of the Pauli exclusion principle it has to be lifted into the higher anti-bonding state of that bond. In the picture of delocalized states, for example in one dimension that is in a wire, for every energy there is a state with electrons flowing in one direction and one state for the electrons flowing in the other. For a net current to flow some more states for one direction than for the other direction have to be occupied and for this energy is needed. For a metal this can be a very small energy in the semiconductor the next higher states lie above the band gap. Often this is stated as: full bands do not contribute to the electrical conductivity. However, as the temperature of a semiconductor rises above absolute zero, there is more energy in the semiconductor to spend on lattice vibration and — more importantly for us — on lifting some electrons into an energy states of the conduction band, which is the band immediately above the valence band. The current-carrying electrons in the conduction band are known as "free electrons", although they are often simply called "electrons" if context allows this usage to be clear.Electrons excited to the conduction band also leave behind electron holes, or unoccupied states in the valence band. Both the conduction band electrons and the valence band holes contribute to electrical conductivity. The holes themselves don't actually move, but a neighboring electron can move to fill the hole, leaving a hole at the place it has just come from, and in this way the holes appear to move, and the holes behave as if they were actual positively charged particles.One covalent bond between neighboring atoms in the solid is ten times stronger than the binding of the single electron to the atom, so freeing the electron does not imply destruction of the crystal structure.Holes: electron absence as a charge carrierThe notion of holes, which was introduced for semiconductors, can also be applied to metals, where the Fermi level lies within the conduction band. With most metals the Hall effect reveals electrons to be the charge carriers, but some metals have a mostly filled conduction band, and the Hall effect reveals positive charge carriers, which are not the ion-cores, but holes. Contrast this to some conductors like solutions of salts, or plasma. In the case of a metal, only a small amount of energy is needed for the electrons to find other unoccupied states to move into, and hence for current to flow. Sometimes even in this case it may be said that a hole was left behind, to explain why the electron does not fall back to lower energies: It cannot find a hole. In the end in both materials electron-phonon scattering and defects are the dominant causes for resistance.Fermi-Dirac distribution. States with energy εbelow the Fermi energy, here μ, have higher probability n to be occupied, and those above are less likely to be occupied. Smearing of the distribution increases with temperature.The energy distribution of the electrons determines which of the states are filled and which are empty. This distribution is described by Fermi-Dirac statistics. The distribution is characterized bythe temperature of the electrons, and the Fermi energy or Fermi level. Under absolute zero conditions the Fermi energy can be thought of as the energy up to which available electron states are occupied. At higher temperatures, the Fermi energy is the energy at which the probability of a state being occupied has fallen to 0.5.The dependence of the electron energy distribution on temperature also explains why the conductivity of a semiconductor has a strong temperature dependency, as a semiconductor operating at lower temperatures will have fewer available free electrons and holes able to do the work.Energy–momentum dispersionIn the preceding description an important fact is ignored for the sake of simplicity: the dispersion of the energy. The reason that the energies of the states are broadened into a band is that the energy depends on the value of the wave vector, or k-vector, of the electron. The k-vector, in quantum mechanics, is the representation of the momentum of a particle.The dispersion relationship determines the effective mass, m* , of electrons or holes in the semiconductor, according to the formula:The effective mass is important as it affects many of the electrical properties of the semiconductor, such as the electron or hole mobility, which in turn influences the diffusivity of the charge carriers and the electrical conductivity of the semiconductor.Typically the effective mass of electrons and holes are different. This affects the relative performance of p-channel and n-channel IGFETs, for example (Muller & Kamins 1986:427).The top of the valence band and the bottom of the conduction band might not occur at that same value of k. Materials with this situation, such as silicon and germanium, are known as indirect bandgap materials. Materials in which the band extrema are aligned in k, for example gallium arsenide, are called direct bandgap semiconductors. Direct gap semiconductors are particularly important in optoelectronics because they are much more efficient as light emitters than indirect gap materials.Carrier generation and recombinationWhen ionizing radiation strikes a semiconductor, it may excite an electron out of its energy level and consequently leave a hole. This process is known as electron–hole pair generation.Electron-hole pairs are constantly generated from thermal energy as well, in the absence of any external energy source.Electron-hole pairs are also apt to recombine. Conservation of energy demands that these recombination events, in which an electron loses an amount of energy larger than the band gap, beaccompanied by the emission of thermal energy (in the form of phonons) or radiation (in the form of photons).In some states, the generation and recombination of electron–hole pairs are in equipoise. The number of electron-hole pairs in the steady state at a given temperature is determined by quantum statistical mechanics. The precise quantum mechanical mechanisms of generation and recombination are governed by conservation of energy and conservation of momentum.As the probability that electrons and holes meet together is proportional to the product of their amounts, the product is in steady state nearly constant at a given temperature, providing that there is no significant electric field (which might "flush" carriers of both types, or move them from neighbour regions containing more of them to meet together) or externally driven pair generation. The product is a function of the temperature, as the probability of getting enough thermal energy to produce a pair increases with temperature, being approximately 1×exp(−E G / kT), where k is Boltzmann's constant, T is absolute temperature and E G is band gap.The probability of meeting is increased by carrier traps – impurities or dislocations which can trap an electron or hole and hold it until a pair is completed. Such carrier traps are sometimes purposely added to reduce the time needed to reach the steady state.DopingThe property of semiconductors that makes them most useful for constructing electronic devices is that their conductivity may easily be modified by introducing impurities into their crystal lattice. The process of adding controlled impurities to a semiconductor is known as doping. The amount of impurity, or dopant, added to an intrinsic (pure) semiconductor varies its level of conductivity. Doped semiconductors are often referred to as extrinsic.DopantsThe materials chosen as suitable dopants depend on the atomic properties of both the dopant and the material to be doped. In general, dopants that produce the desired controlled changes are classified as either electron acceptors or donors. A donor atom that activates (that is, becomes incorporated into the crystal lattice) donates weakly-bound valence electrons to the material, creating excess negative charge carriers. These weakly-bound electrons can move about in the crystal lattice relatively freely and can facilitate conduction in the presence of an electric field. (The donor atoms introduce some states under, but very close to the conduction band edge. Electrons at these states can be easily excited to conduction band, becoming free electrons, at room temperature.) Conversely, an activated acceptor produces a hole. Semiconductors doped with donor impurities are called n-type, while those doped with acceptor impurities are known as p-type. The n and p type designations indicate which charge carrier acts as the material's majority carrier. The opposite carrier is called the minority carrier, which exists due to thermal excitation at a much lower concentration compared to the majority carrier.For example, the pure semiconductor silicon has four valence electrons. In silicon, the most common dopants are IUPAC group 13 (commonly known as group III) and group 15 (commonly known as group V) elements. Group 13 elements all contain three valence electrons, causing them to function as acceptors when used to dope silicon. Group 15 elements have five valence electrons, which allows them to act as a donor. Therefore, a silicon crystal doped with boron creates a p-type semiconductor whereas one doped with phosphorus results in ann-type material.Carrier concentrationThe concentration of dopant introduced to an intrinsic semiconductor determines its concentration and indirectly affects many of its electrical properties. The most important factor that doping directly affects is the material's carrier concentration. In an intrinsic semiconductor under thermal equilibrium, the concentration of electrons and holes is equivalent. That is,n = p = n iIf we have a non-intrinsic semiconductor in thermal equilibrium the relation becomes:n0 * p0 = (n i)2Where n is the concentration of conducting electrons, p is the electron hole concentration, and n i is the material's intrinsic carrier concentration. Intrinsic carrier concentration varies between materials and is dependent on temperature. Silicon's n i, for example, is roughly 1.6×1010 cm-3 at 300 kelvin (room temperature).In general, an increase in doping concentration affords an increase in conductivity due to the higher concentration of carriers available for conduction. Degenerately (very highly) doped semiconductors have conductivity levels comparable to metals and are often used in modern integrated circuits as a replacement for metal. Often superscript plus and minus symbols are used to denote relative doping concentration in semiconductors. For example, n+ denotes an n-type semiconductor with a high, often degenerate, doping concentration. Similarly, p−would indicate a very lightly doped p-type material. It is useful to note that even degenerate levels of doping imply low concentrations of impurities with respect to the base semiconductor. In crystalline intrinsic silicon, there are approximately 5×1022 atoms/cm³. Doping concentration for silicon semiconductors may range anywhere from 1013 cm-3 to 1018 cm-3. Doping concentration above about 1018 cm-3 is considered degenerate at room temperature. Degenerately doped silicon contains a proportion of impurity to silicon in the order of parts per thousand. This proportion may be reduced to parts per billion in very lightly doped silicon. Typical concentration values fall somewhere in this range and are tailored to produce the desired properties in the device that the semiconductor is intended for.Effect on band structureDoping a semiconductor crystal introduces allowed energy states within the band gap but very close to the energy band that corresponds with the dopant type. In other words, donor impurities create states near the conduction band while acceptors create states near the valence band. The gap between these energy states and the nearest energy band is usually referred to as dopant-sitebonding energy or E B and is relatively small. For example, the E B for boron in silicon bulk is0.045 eV, compared with silicon's band gap of about 1.12 eV. Because E B is so small, it takes little energy to ionize the dopant atoms and create free carriers in the conduction or valence bands. Usually the thermal energy available at room temperature is sufficient to ionize most of the dopant.Dopants also have the important effect of shifting the material's Fermi level towards the energy band that corresponds with the dopant with the greatest concentration. Since the Fermi level must remain constant in a system in thermodynamic equilibrium, stacking layers of materials with different properties leads to many useful electrical properties. For example, the p-n junction's properties are due to the energy band bending that happens as a result of lining up the Fermi levels in contacting regions of p-type and n-type material.This effect is shown in a band diagram. The band diagram typically indicates the variation in the valence band and conduction band edges versus some spatial dimension, often denoted x. The Fermi energy is also usually indicated in the diagram. Sometimes the intrinsic Fermi energy, E i, which is the Fermi level in the absence of doping, is shown. These diagrams are useful in explaining the operation of many kinds of semiconductor devices.Preparation of semiconductor materialsSemiconductors with predictable, reliable electronic properties are necessary for mass production. The level of chemical purity needed is extremely high because the presence of impurities even in very small proportions can have large effects on the properties of the material. A high degree of crystalline perfection is also required, since faults in crystal structure (such as dislocations, twins, and stacking faults) interfere with the semiconducting properties of the material. Crystalline faults are a major cause of defective semiconductor devices. The larger the crystal, the more difficult it is to achieve the necessary perfection. Current mass production processes use crystal ingots between four and twelve inches (300 mm) in diameter which are grown as cylinders and sliced into wafers.Because of the required level of chemical purity and the perfection of the crystal structure which are needed to make semiconductor devices, special methods have been developed to produce the initial semiconductor material. A technique for achieving high purity includes growing the crystal using the Czochralski process. An additional step that can be used to further increase purity is known as zone refining. In zone refining, part of a solid crystal is melted. The impurities tend to concentrate in the melted region, while the desired material recrystalizes leaving the solid material more pure and with fewer crystalline faults.In manufacturing semiconductor devices involving heterojunctions between different semiconductor materials, the lattice constant, which is the length of the repeating element of the crystal structure, is important for determining the compatibility of materials.附录B:外文资料翻译—译文部分半导体半导体是一种导电性能介于导体与绝缘体之间的固体材料。

1、下载文档前请自行甄别文档内容的完整性，平台不提供额外的编辑、内容补充、找答案等附加服务。
2、"仅部分预览"的文档,不可在线预览部分如存在完整性等问题,可反馈申请退款(可完整预览的文档不适用该条件!)。
3、如文档侵犯您的权益，请联系客服反馈,我们会尽快为您处理(人工客服工作时间：9:00-18:30)。

——Nano Lett. 2012, 12, 113–118
First-principles studies of the hydrogenation effects in silicene sheets
——P. Zhang et al. / Physics Letters A 376 (2012) 1230–1233
——PHYSICAL REVIEW B 80, 155453 2009
Structures and electronic properties of silicene clusters: a promising material for FET and hydrogen storage
——Phys. Chem. Chem. Phys., 2011, 13, 7304–7311
First-Principles Study of Ferromagnetism in Two- Dimensional Silicene with Hydrogenation
——J. Phys. Chem. C 2012, 116, 4163−4166
Tunable Bandgap in Silicene and Germanene
Graphene based core/shell quantum dots
——Appl. Phys. Lett. 99, 183102 (2011)
Meanings and applications
The electronic structures and hence properties and functionalities of Graphene-based quantum dots (GQDs) can be tailored by size and shape, the potential applications including spin qubits, single electron transistors, photovoltaics, and light-emitting diodes. Analogous to graphanes, silicanes are predicted to be interesting materials for hydrogen storage and for their band engineering properties.
A review on silicene — New candidate for electronics
——A. Kara et al. / Surface Science Reports 67 (2012) 1–18
It was shown that the self-aligned silicene nanoribbons deposited on Ag(110) substrate have honeycomb, graphene-like buckled structure. Another clear evidence of the buckling has been identified in silicene epitaxially grown on a close-packed silver surface Ag in (111) plane. There it was found a highly ordered silicon structure, arranged within a honeycomb lattice, consisting of two silicon sublattices occupying positions at different heights. The value of the sublattices displacement has been determined and is equal to 0.2 Å .
Even before the synthesis of isolated graphene, ab initio studies based on the minimization of the total energy has revealed that a buckled honeycomb st.
Silicene: Compelling Experimental Evidence for Graphenelike TwoDimensional Silicon
——PRL 108, 155501 (2012)
Silicene – the silicon-based counterpart of graphene – has a two dimensional structure that is responsible for the variety of potentially useful chemical and physical properties. The existence of silicene has been achieved recently owing to experiments involving epitaxial growth of silicon as stripes on Ag(001), ribbons on Ag(110), and sheets on Ag(111). Though the number of independent experimental investigations on silicene is limited, there is a clear indication that silicon may form a ‘‘quasi-2D’’ structure resembling that of graphene.
Combination of graphene and quantum dots
Hydrogenation of graphene to form graphane, changes the hybridization of carbon atoms from sp2 to sp3, and from a semimetal (bandgap = 0) to an insulator (DFT : bandgap = 3.9 eV). By embedding graphene islands into graphane matrix, the graphene-based quantum dots (GQDs) exhibit unique properties in connection with quantum confinement effects.
Two- and One-Dimensional Honeycomb Structures of Silicon and Germanium
——PRL 102, 236804 (2009)
Monolayer honeycomb structures of group-IV elements and IIIV binary compounds: First-principles calculations
Experimental Literatures
Silicene, the graphene equivalent for silicon, opening new perspectives for applications, especially due to its compatibility with Si-based electronics. Here we provide compelling evidence, from both structural and electronic properties, for the synthesis of epitaxial silicene sheets on a silver (111) substrate, through the combination of scanning tunneling microscopy and angular-resolved photoemission spectroscopy.
A Stable “Flat″ Form of Two-Dimensional Crystals: Could Graphene, Silicene, Germanene Be Minigap Semiconductors
——Nano Lett. 2012, 12, 1045−1052
Theoretical calculations
Silicene & Germanene
The modeled silicene and germanene, bandgap energy = 0 eV
Silicene @ H
Bandgap energy = 2.165 eV