Bi-rich grow topological insulator Bi2Se3 nanodomains structures

Bi-rich grow topological insulator Bi 2Se 3

nanodomains

structures

Guozhi Jia a ,b ,?,Xionglong Wang c ,Qiang Li a ,Jianghong Yao c

a

Tianjin Chengjian University,26Jinjing Road,Tianjin 300384,PR China

b Institute of Semiconductors,Chinese Academy of Sciences,P.O.Box 912,Beijing 100083,PR China

c Key Laboratory of Weak-Light Nonlinear Photonics,Ministry of Education,School of Physics an

d TEDA Applied Physics School,Nankai University,Tianjin 300457,PR China a r t i c l

e i n

f o Article history:Received 28September 2013Accepted 25November 2013Available online 3December 2013Keywords:Bi 2Se 3Topological insulator

Nanodomains

a b s t r a c t

Bi 2Se 3hexagonal nanodomains structures have been synthesized

under the condition of Bi-rich without surfactant.Bi-rich growth

condition can result in the forming of quintuple-layer Bi 2Se 3nan-

odomains through forming surface Se vacancy.Bi 2Se 3nanodo-

mains structures possess high crystal quality and Se defects

without disrupting the host lattice.The forming mechanism of

defects is discussed in details.

ó2013Elsevier Ltd.All rights reserved.Bi 2Se 3nanomaterials are well-known candidates for applications as topological insulators (TIs)and spintronical applications because of their unique layered crystal structure and electronic properties

[1].Recently,the research activities focus on phase transitions between a TI phase and nontopological metals,the exotic properties of the topological surface states [2],or the emerging of topological states and converting TIs to superconductors by changing their morphology,structure,and doping Zhang et al.[3].Doping in the few layered TIs has attracted increasing attentions because of the enhancement of the surface to volume ratio by tuning the carrier’s density [4].Although doping can be used to tune the carrier’s density widely,a great deal of defects would be generated in Bi 2Se 3lattice due to the dif-ferent atom sizes and growth habits.Furthermore,intercalation of zero-valence metals can be formed in the van der Waals gap by controlling the suitable experiment condition,which can open a surface state gap in topological insulator Bi 2Se 3[5].Peng et al.have shown that lattice dislocations can be used to protect modes in topological insulators [6].The defects engineering in the topological insulators material play an important role in tuning and controlling the electrical properties [6–8].One remark-able feature of intrinsic doping TIs is the degree to which their electrical properties can be tuned by 0749-6036/$-see front matter ó2013Elsevier Ltd.All rights reserved.https://www.360docs.net/doc/7e5619297.html,/10.1016/j.spmi.2013.11.021

?Corresponding author at:Tianjin Chengjian University,26Jinjing Road,Tianjin 300384,PR China.Tel./fax:+862223085207.E-mail address:gzjia@https://www.360docs.net/doc/7e5619297.html, (G.Jia).

34G.Jia et al./Superlattices and Microstructures66(2014)33–38

changing of surface defects https://www.360docs.net/doc/7e5619297.html,pared with the guest atoms doping TIs,the host lattice can be seriously affected and result in the properties partly deteriorated.However,effectively control-ling the concentration of intrinsic defects and deeply understanding the mechanism of the intrinsic defect forming in Bi2Se3nano-structure is still lacking.

In the present work,Bi2Se3hexagonal nanodomains structures have been synthesized in solution without surfactant.Bi2Se3nanodomains structures possess high crystal quality and defects without disrupting the host lattice.We foresee that these structures can be used to complicate new physics device by tuning the wavefunction distribution of carriers in nanodomains structure with con?nement features.

The Bi2Se3and Bi-rich Bi2Se3nanocrystals were synthesized via modi?ed method published previ-ously with hydroxylamine triggering and without adding any surfactant[9].In brief,0.91g Bi(NO3)3-á5H2O and15mL ethylene glycol were added in a150mL three-neck round bottom?ask and stirred at room temperature for30min.Then,the Na2SeO3solutions with different molars were added into ?ask,and elevating temperature to180°C under nitrogen environment and keeping constant stirring. The composition in reaction solution was controlled by introducing the Bi and Se ion molar ratios2:3, 2:2.7,2:2.4,and2:1.5to obtain the different Se vacancies concentration samples.After the solution turned blue-white,2.4mL hydroxylamine was rapidly injected into the reaction solution.The reaction was stopped after30min and the solution was allowed to cool down to room temperature naturally. The black precipitated powder was collected and washed several times using acetone and water, respectively.The samples were?nally obtained after drying under a vacuum at50°C for several hours.

The microstructure characteristics of synthesized samples are analyzed by transmission electron microscopy(TEM).TEM and the high-resolution TEM(HRTEM)analysis on samples of stochiometric Bi2Se3are summarized in Fig1(A),respectively.The high-HRTEM image is taken along the[0001] direction,clearly revealing crystalline lattice fringes.Sharp diffraction spots in the selected area dif-fraction pattern(top-right inset)further con?rm the sample is a high-quality crystal.The?ake-like morphology results reveal the growth habit of Bi2Se3owing to the intrinsically anisotropic bonding nature.An EDS spectrum from pure Bi2Se3shows stoichiometry of Bi/Se(=2/3).Fig1(B)shows the de-tailed information of the samples prepared under the condition of Bi-rich and kept the molar ratio be-tween Bi3+and Se2àas2:2.4,not stochiometric2:3.We note that Bi-rich Bi2Se3nanodomain structures showing good crystalline quality are easily found in representative TEM imaging in Fig.1(B).In the middle-left of Fig.1(B),the corresponding fast Fourier transform(FFT)pattern shows6-fold symmetry [0001],in agreement with the hexagonal structure of Bi2Se3along its c-axis.The dashed lines show the boundary between crystalline grains.The high-resolution TEM image is taken along the[0001] direction,clearly revealing crystalline lattice fringes.Bi-rich grown Bi2Se3nano?akes typically exhibit hexagonal morphology with120°edge facets.The lattice spacing in HRTEM is0.21nm,well matching the expected spacing of(220)planes.The plane(220)is perpendicular to the growth axis of the sam-ples,indicating that the growth along the[110]direction(c-axis).It can clearly be seen that the quin-tuple-layer Bi2Se3were formed and isolated by Se vacancy,the width of nanodomains was only 2.5nm.The lattice distribution is further analyzed,shown in Fig1(E).It can be found although a great number of defects produced at the edge of nanodomains,the host lattice maintains high quality.The proper Bi-rich growth condition is important to assure enough Se(2)to combine with Bi ion,which result in forming the perfect host lattice while only keeping Se(1)vacancy in the vicinity of quintu-ple-layer Bi2Se3nanodomains.

Fig2(A)shows typical crystal structure performed using X-ray diffraction(XRD).Compared with JCPDS data card No.33-0214for Bi2Se3,the XRD pattern of all samples can be indexed exclusively as Bi2Se3with a rhombohedral structure.We found that the most intense diffraction peak(015)shift toward higher angles under the small Bi/Se molar ratio condition.The shift in diffraction peaks might be caused by vacancy and interstitial defects.The evolution of the hexagonal cell parameters of sam-ples are shown in Fig.2(C).We note that a decrease in the lattice parameters can be ascribed the form-ing of Se vacancy.The lattice parameters will tend to increase when Bi ions incorporate at interstitial sites.Bi2Se3is layered rhombohedral crystal structure(Fig.2(D))corresponds to space group(R3m/ D53d)with quintuple layers ordered in a Se1-Bi-Se2-Bi-Se1sequence.Se at the different sites are with different electrical characteristics,which can decide the forming way of defects.The forming mecha-

nism of the defects can be a key to understand physics and develop the new devices,which will be discussed in more detail later.

Raman spectroscopy has been an important investigation way as a high-resolution probe technol-ogy of crystal structures and the vibrational properties of materials [10,11].Bi 2Se 3has the rhombohe-dral crystal structure and ?ve atoms in a primitive unit cell,and each unit cell consisting of three quintuple layers stacked together by weak van der Waals forces [12,13].There are four Raman active modes:E g1,A1g1,E g2and A1g2corresponding to atomic displacements vibrations at the C point [13].Raman spectroscopy with a 532nm excitation laser was also performed to investigate the effect of S doping to the lattice vibrations and electron phonon interactions in Bi 2Se 3samples in a backscattering con?guration.The focused laser spot of less than 0.5mm is small enough to analyze the local proper-ties of samples.A low laser power of about 15mW was used during the Raman measurements to avoid heating effect on the characteristics of samples.Fig.3(A)shows typical Raman spectra obtained from as-prepared samples with different Bi ion concentration values.It can clearly be seen that three main Raman modes with E g ,A 1g and A 2g were observed in the stochiometric and Bi-rich Bi 2Se 3.As Bi/Se molar ratio increased from 2:3to 2:1.5,the spectrum is similar to that obtained for the stochiometric Bi 2Se 3sample,except for the small shift of the mode peak position to low wave (directed by the ver-tical dot line).The Raman shifts of the different vibrational modes with the increasing of Bi ion are summarized in Fig 3(B).All modes display a blue shift when x is increased generally according

to

the normal modes (Fig 3(B)inset),the vibrational modes is sensitive to defects because the interlayer van der Waals interactions in?uence the effective restoring forces acting on Bi and Se atoms [10].This effect can be more obvious on the out-of-plane mode than in-plane mode because that the defect can cause the changing of not only the restoring forces also the thickness [13–19].

In order to clearly understand the formation mechanism of intercalation doping in rhombohedral structure Bi 2Se 3,we analyzed Se vacancy defects based on the growth habit and electrical structure of Bi 2Se 3.It can clearly be seen that the lattice constant continues to decrease with increasing of the rel-ative Bi 3+concentration.This can be ascribed to Se(2)vacancy further forming [8].Based on the XRD results,Bi interstitial atoms can appear in the lattice under the higher concentration Bi 3+reaction con-dition.The ion of growth habit,being the most critical factor,decides the growth way.Triggering re-agent hydroxylamine results in the reaction rate’s accelerating sharply,the reaction can be completed in a very short time [9].According to the thermal dynamics characteristics of nanomaterials growth,Se ion can obtain enough energy to escape from metastable state Se(1)sites,and arrive at the steady-state Se(2).This process can be carried out more easily in the case of Bi-rich reaction.With the increas-ing of Bi ion concentration,more and more vacancies appear at the edge of nanodomains.This can be the main reason of the lattice constant decreasing.The further increasing of Bi ion concentration can result in emerging of Se(2)vacancy.When more Bi ions appear in solution,Bi interstitial atoms

can

38G.Jia et al./Superlattices and Microstructures66(2014)33–38

cause the deformation of lattice due to occurrence of Se(2)vacancies,which results in the lattice con-stant seriously increasing due to Bi atom’s large size.Moreover,as for the structure characteristics of Bi2Se3TIs,Bi-Se(1)chemical bonding belongs to pure covalent nature,which also is an important rea-son that Se(1)vacancy can?rst be formed under the condition of Bi-rich reaction,which results in that the atoms in Se(1)sites are easily absent between quintuple stacking layers combined by the van der Waals forces.In addition,Se(1)vacancy has very little impact on the host lattice.Thus,the formed nanocrystal mainly is the Bi2Se3-like orthorhombic structure for the different Bi3+concentration.Se vacancy defect did not disrupt the Bi2Se3nanodomains host lattice even if at the high Bi concentration. This result is very similar with one of the zero-valence metallic atoms doping into Bi2Se3nanoribbons [5].The forming mechanism is different from that of zero-valence metallic atoms intercalation layer in Bi2Se3layered materials.The results can provide ideas for preparing periodic nanodomains structures and studying the physics of topological insulators with Se vacancy con?nement at the edge.We fore-see that these structures can be used to complicate new physics device by tuning the wavefunction distribution of carriers in nanodomains structure with con?nement features.

In summary,we have studied Bi-rich Bi2Se3hexagonal nanodomains structures synthesized in solution without surfactant and investigated their dependence on the molar ration of Bi/Se.Bi-rich Bi2-Se3hexagonal nanodomains structures have been synthesized in solution without surfactant.Bi2Se3 nanodomains structures possess high crystal quality and defects without disrupting the host lattice. The forming mechanism of Se vacancy has been analyzed in details.

Acknowledgments

This work has been partly supported by the National Key Basic Research Program of China (2012CB934201),the National Natural Science Foundation of China(11147024,11247025,and 11074129).

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