石墨烯吸附氨气-2009

Home Search Collections Journals About Contact us My IOPscienceAdsorption of ammonia on grapheneThis article has been downloaded from IOPscience. Please scroll down to see the full text article.2009 Nanotechnology 20 245501(/0957-4484/20/24/245501)View the table of contents for this issue, or go to the journal homepage for moreDownload details:IP Address: 218.197.201.89The article was downloaded on 02/03/2012 at 03:40Please note that terms and conditions apply.IOP P UBLISHING N ANOTECHNOLOGY Nanotechnology20(2009)245501(8pp)doi:10.1088/0957-4484/20/24/245501Adsorption of ammonia on grapheneHugo E Romero1,Prasoon Joshi2,Awnish K Gupta1,Humberto R Gutierrez1,Milton W Cole1,3,Srinivas A Tadigadapa2,3,4and Peter C Eklund1,3,41Department of Physics,Pennsylvania State University,University Park,PA16802,USA2Department of Electrical Engineering,Pennsylvania State University,University Park,PA16802,USA3Materials Research Institute,Pennsylvania State University,University Park,PA16802,USAE-mail:sat10@ and pce3@Received26January2009,infinal form28April2009Published26May2009Online at /Nano/20/245501AbstractWe report on experimental studies of NH3adsorption/desorption on graphene surfaces.Thestudy employs bottom-gated graphenefield effect transistors supported on Si/SiO2substrates.Detection of NH3occurs through the shift of the source–drain resistance maximum(‘Diracpeak’)with the gate voltage.The observed shift of the Dirac peak toward negative gate voltagesin response to NH3exposure is consistent with a small charge transfer(f∼0.068±0.004electrons per molecule at pristine sites)from NH3to graphene.The desorption kinetics involvesa very rapid loss of NH3from the top surface and a much slower removal from the bottomsurface at the interface with the SiO2that we identify with a Fickian diffusion process.(Somefigures in this article are in colour only in the electronic version)1.IntroductionGraphene is a singleflat atomic sheet of carbon with the atoms arranged in a two-dimensional(2D)honeycomb configuration.Recent progress in isolating graphene on an insulating substrate(e.g.,SiO2or SiC)now enable this exotic 2D system to be probed experimentally[1–3].It has been shown to be a promising building block for novel generation of high speed and sensitive electronic devices[4–12].Electron transport experiments on graphene have demonstrated,among other effects,unusual carrier-density-dependent conductiv-ity[1,13,14],anomalous quantum Hall effect[13–15], minimum quantum conductivity[13],and exceptionally high electron mobilities[16,17].These remarkable electronic properties stem from the unique band structure of graphene, which exhibits conduction and valence bands with near-linear dispersion that touch at the Brillouin zone corners to make a zero gap semiconductor.Similar to earlier experiments on carbon nanotubes[18], the transport properties of graphene have been shown to be sensitive to molecules adsorbed on the surface(e.g.NH3,NO2, H2O and CO)[10,19].The details of the strength and character of the adsorption(chemi versus physisorption),and the degree of charge transfer between the analyte and graphene is still 4Authors to whom any correspondence should be addressed.under debate.Geim and co-workers were thefirst to report that a graphene Hall effect sensor device is capable of detecting individual molecules of NO2[10].Charge transfer between the graphene and NO2is thought to be important in this particular case[19,20].Here,we report studies of the interaction of NH3with graphenefield effect transistors(FETs)supported on Si/SiO2 substrates in order to provide further insight into the nature of the molecule–graphene interaction.The SiO2is used as a gate dielectric and the heavily doped Si substrate as the bottom gate electrode.By sweeping the gate voltage,we can follow the time evolution of the peak in the drain-source resistance (known as the‘Dirac’peak)to monitor the change of the Fermi level in graphene in response to the adsorption and desorption of NH3.Presumably,this shift of the Dirac peak is dominated by charge transfer effects.The Dirac peak shift and the thermodynamic data for NH3on graphite are used to determine the effective charge transfer per NH3molecule(f) to the graphene.Our value for f will be compared to recent theoretical calculations for NH3bound to the surface[20]and to the edges[21]of graphene.2.Experimental detailsThe graphene FETs studied here were supported on Si/SiO2 substrates and bottom-gated using the SiO2(300nm thermalb)10m μ10οmI II IIIII(a)(b)IIIII III 210 μm Figure 1.(a)Schematic of a graphene device supported on SiO 2with underlying doped Si serving as the back gate (G);S and D refer to thesource and drain contacts.(b)Optical micrograph of a graphene device fabricated with TEM grids as a shadow mask.The graphene flake is ∼3μm wide in this device.The following areas can be identified on the optical image:(I)single graphene layer (also indicated with dashed lines),(II)Cr/Au electrodes,and (III)SiO 2dielectric.oxide)as the gate dielectric and the degenerately p +doped Si substrate (ρ∼3×10−3 cm)as the gate electrode.Our graphene films were produced by mechanical exfoliation of highly oriented pyrolytic graphite (HOPG Grade ZYH TM ,SPI Supplies,Inc.,West Chester,PA)using adhesive tape (Scotch Tape,3M,Inc.,Maplewood,MN).Before the transfer of graphene flakes,the Si/SiO 2substrates were first cleaned in isopropanol and later exposed to O 2-plasma.The FET was constructed by the physical vapor deposition of pads of Cr(5nm)followed by Au(100nm)for electrical contact at opposite ends of the graphene FET channel (i.e.,the ‘source’and ‘drain’).The Cr and Au were evaporated onto the graphene through square holes in a transmission electron microscopy (TEM)grid (SPI Supplies,Inc.,West Chester,PA)used as a shadow mask and consisted of a 10×10array of 115×115μm 2square holes separated by ∼10μm bars.By using a micromanipulator while viewing the area through an optical microscope,the TEM grid was carefully positioned on the graphene flake of interest (i.e.,natural flakes of approximately rectangular shape and more than 10μm long)so that one the TEM bars covers the graphene along its width,leaving its ends uncovered through the apertures of the mask.Then,the TEM grid is temporarily clamped to the substrate during contact evaporation.Our approach for fabricating electrical contacts avoids unnecessary complications that may stem from the exposure of the graphene to lithography chemicals.A schematic drawing of our bottom-gated field FET that is supported on a Si/SiO 2substrate is shown in figure 1together with an optical micrograph of the device.The optical image in figure 1shows a particular FET with two square Cr/Au contacts deposited over each end of a rectangular graphene flake.The number of layers (N )in a graphene film was determined by micro-Raman scattering using the shape of the 2D Raman band at ∼2700cm −1and the intensity of the Raman-active G-band scattering at ∼1585cm −1[22,23].The Raman characterization of N can be summarized as follows:[22](1)the G-band intensity is approximately linear in N ,(2)an N =1graphene film can be easily recognizedby the narrow Lorentzian shape of the 2D Raman band,(3)at least one N =1film must be present on the same substrate so that the relative G-band intensity associated with other films can be measured by a simple translation of the microscope stage while all optics remain fixed.Here,we present results on one representative sample that was demonstrated by Raman scattering to be a N =1graphene film of dimensions ∼10μm ×1.5μm.Each Si/SiO 2substrate supporting several graphene FETs was mounted on a ceramic chip carrier that was placed in a socket fixed inside a vacuum sealed stainless steel (SS)tube equipped with vacuum and gas (NH 3)connections.The SS tube was inserted into a tube furnace and evacuated to 5×10−7Torr using a turbo-molecular pump.Vacuum-annealing of the samples were performed in situ by heating to 200◦C at a constant rate of 2◦C min −1to drive out all gases.Once the vacuum-annealing procedure was ceased and graphene reached a fully degassed state,the samples were allowed to cool to room temperature.We assume that graphene is a fully degassed state when there is no further evolution of the Dirac peak shape and/or position with time at 200◦C.Samples could then be exposed to anhydrous NH 3(99.99%,H 2O <1ppm)diluted in ultra high purity He (99.999%).A type-K thermocouple was mounted on the chip carrier near the Si substrate to monitor the local temperature.Electrical measurements were made using a programmable voltage source and digital voltmeter/ammeter (2400General-Purpose SourceMeter,Keithley Instruments,Inc.,Cleveland,OH)interfaced to a computer via LabVIEW (National Instruments Co.,Austin,TX).The measurements consisted of the application of a constant source–drain voltage V ds =1mV to the device,while monitoring the resultant source–drain current (I ds )as a function of the applied gate bias (V g ).The source–drain resistance was computed as R ds =V ds /I ds .3.Results and discussionSince our device fabrication scheme involves shadow masking to make contacts to graphene films,it therefore has the-33530252015105(b)R ds )t (h )V g (V )(a)(k Ωrs )t (hours)V D i r a c (V )906030Figure 2.Time evolution of (a)the R ds versus V g curves at T =200◦C during the first 8h of vacuum-annealing of the graphene FET device shown schematically in figure 1.Initially,the Dirac peak is out of range (V g >90V);after ∼5h,the Dirac peak appears at negative V g and evolves slowly to the final position at V g ∼−47V .This particular sample was vacuum annealed for ∼72h,when the position of V Dirac stop evolving with time,i.e.,sample reached a saturation;(b)the gate voltage for the charge neutrality point,V Dirac ,during vacuum-annealing of a graphene FET at T ∼200◦C.advantage that it is a lithography-free process.Recent studies have shown that resist,such as poly(methyl methacrylate)resin,can be quite difficult to remove,requiring heat treatment at 400◦C in Ar/H 2flow [24].These adsorbed molecules may dope (i.e.,charge transfer with)the sample,or may block active adsorption sites for analytes and/or might otherwise interfere with the reproducibility of the FET electrical data.The electronic band structure of graphene near the Fermi level (E F )is somewhat unique.Near-mirror image valence and conduction bands touch each other at the six corners (K,K points)of a 2D Brillouin zone.These contact points are commonly called ‘Dirac points’[1].This nomenclature stems from the linear dispersion of the electronic energy (E )with 2D wavevector (k ),i.e.,E =±¯h v F k ,where v F is the Fermi velocity of the electrons.In pristine graphene,E F is located where the conduction and valence bands touch,i.e.,the Dirac point.A FET built from pristine graphene (i.e.,where E F is located near the Dirac point)should exhibit ambipolar conduction.That is,depending on the sign of the gate bias voltage (V g ),the gate induces E F to rise or fall in the band structure and as a result either electrons or holes dominate the source–drain current (I ds ).Defining the source–drain resistance as R ds =V ds /I ds ,where V ds is the applied potential difference between source and drain,it can easily be shown that the position (V Dirac )of the peak in R ds is related to n ,the 2D net carrier density [1].Thus,the ambipolar character of graphene FETs refers to the fact that V g can be used to control n (gate-induced doping).A transition between electron and hole conduction can be shown to occur when V g positions E F at the Dirac point (n =0),where the conduction and valence bands touch.Since this is the position in ‘intrinsic graphene’,this value of V g also achieves charge neutrality in the graphene.An approximate relationship between n (in number of electrons cm −2)and V g can be used to describethe device response to gate voltage [1],n ≈εε0(V g −V Dirac )/le ≡α(V g −V Dirac )(1)where εε0and l are,respectively,the gate dielectric permittivity and thickness,e is the charge of the electron and V Dirac is a constant.Equation (1)is an approximate relationship,in that thermal broadening of the Fermi–Dirac function is not included.Using the well known linear electronic π-band dispersion,it is easy to show that the carrier concentration (electrons orholes)∼E 2F [25].The accepted value for the Fermi velocity is v F ∼c /300,where c is the speed of light [13,14].Using equation (1),and the values l =300nm,and ε/ε0∼3.9,we can therefore obtain an approximate numerical relationship between E F and V Dirac for our graphene FETs,i.e.,E F ≈±31.7(meV /√V )V Dirac (2)where the positive and negative signs apply,respectively,to hole and electron conduction.This equation,however,ignores the details of the electrostatics in the FET and the SiO 2/graphene interface.When these details are included,the left (or right?)side of equation (1)contains an additional small term that is proportional to √n [25].Our graphene FETs were exposed to ambient conditions for several days before electrical characterization.We found that most of our devices exhibited a Dirac peak in the range +50V <V g <+100V ,i.e.,they are p-type.A positive shift of the Dirac peak of the same order of magnitude is not uncommon in graphene devices on Si/SiO 2substrates [1,26,27].However,all our samples’p-doping can be reversed by vacuum-annealing at 200◦C and ∼5×10−7Torr over ∼10–200h.Over this time,the peak passes through V g =0V and ends up in the range −10V <V g <−60V ,depending on the sample (cf figure 2).We have interpretedR d s (k Ω)-9-44321t (m in )V g (V)(a)t (min)5t (hours)V D i r a c (V )Figure 3.(a)Response of bottom-gated graphene FET (on Si/SiO 2)to a sudden overpressure of 1atm of 10%NH 3in He during the first exposure.The initial Dirac peak corresponds to the FET before NH 3exposure.Data were collected in the range −90V <V g <+90V to avoid breakdown of the gate dielectric.(b)Time dependence of the Dirac voltage V Dirac ,during NH 3desorption at T =25◦C,after two successive exposure–desorption cycles (1)and (2).The solid lines are fit of the data to a double exponential function.these results [25],to indicate that the p-doping occurs from ambient exposure to air and stems from the adsorption of O 2molecules that remove electrons from the graphene.A clean graphene sample supported on Si/SiO 2should be n-type [25].This conclusion stems from work function (W )calculations [25]for graphene and amorphous Si where we found that W (SiO 2)is ∼1eV higher than in graphene,as E F in SiO 2is pinned in a surface state band located just below the conduction band edge.In the present studies of the response of graphene FETs to NH 3,we start with the graphene FET in a fully degassed state,i.e.,it is initially n-type.We now discuss the time response of our n-type FETs to a sudden rise in NH 3pressure.We present all data from one device,but the same general behavior was observed in six devices.As described above,the typical device chosen for analysis here was first degassed in high vacuum at 200◦C for ∼72h until the Dirac peak remained fixed at a negative gate bias V g ∼−47V .The particular device for systematic study was then cooled in vacuum to T =25◦C and exposed for 30min to 1atm overpressure of 10%NH 3gas diluted in He.The time evolution of Dirac peak to this first NH 3exposure is shown in figure 3(a).The Dirac peak can be seen to shift rapidly to more negative V g ,eventually moving beyond the safe gate voltage limit for our FETs,i.e.,V g ∼−90V .The shift of the Dirac peak to more negative V g shows clearly that NH 3appears to be acting as an n-type dopant,upshifting the position of E F in the electron pockets at the Brillouin zone corners.All six graphene FETs showed this rapid shift of the Dirac peak to negative V g beyond our safe gate limit.Indeed,the electrical response to NH 3at T =25◦C is so rapid that we propose that it is determined by either adsorption onto the top surface of the graphene,or by top surface chemical reactions whereby the NH 3displaces strongly bound molecules remaining after degassing.We expect therefore that the diffusion of NH 3into the SiO 2/graphene interface is a much slower process.Next,the NH 3-dosed FET wasexposed to vacuum desorption conditions,i.e.,the NH 3supply is isolated and the sample chamber was rapidly evacuated in ∼10s,i.e.,the NH 3overpressure therefore drops very rapidly compared to the characteristic time for the ensuing changes in the FET resistance.The kinetics for the first desorption cycle at T =25◦C can only be followed once the Dirac peak recovers below our gate voltage limit of V g =−90V .As can be seen in the figure,the desorption kinetics at T =25◦C are considerably slower than estimated for the adsorption response (figure 3(b),curve 1);we observed that this particular device recovers during desorption from being very strongly n-type (i.e.,when the Dirac peak appears at gate voltages beyond −90V)to exhibit a new stationary value V Dirac ∼−52V in ∼5h which is somewhat more negative than the initial vacuum-degassed FET state (V Dirac =−47V).Each N =1device behaves a little differently in the details of the kinetics and final equilibrium values of the Dirac peak.The NH 3response of this particular device is typical of the general character of all devices measured.Next,the FET was further degassed in vacuum at T =100◦C until R ds (V g )no longer changed with time,after which it was cooled down to room temperature,and exposed to the same 10%NH 3/He mixture at p =1atm a second time.The Dirac peak was then observed to rapidly shift again toward more negative V g ,disappearing behind our (device-safe)gate dielectric limit of −90V .As can be seen in figure 3(b),the device response to a second desorption (degas)cycle (curve 2)at T =25◦C is similar to the first desorption (degas)cycle (curve 1).Curve 2shows the Dirac peak rapidly upshifting from below −90V to a new steady state value of −38V at T =25◦C in vacuum which is less negative than was observed after the first cycle.The solid lines in figure 3(b)represent fits to the desorption data by a double exponential function,V Dirac (t )=V ∞+V 1exp (−t /τ1)+V 2exp (−t /τ2)(3)where t is the time and τ1and τ2are effective time constants.The constant V ∞in equation (3)is the steady state position40302010V g (V)40302040302010R d s (k Ω)Figure 4.Room temperature source–drain resistance as a function of gate voltage for three successive NH 3exposure–desorption cycles (1)–(3).Three representative R ds (V g )curves are plotted:before NH 3exposure (solid lines),after NH 3desorption at T =25◦C (dotted lines),and after vacuum-degassing at T =100◦C (dashed lines).The arrows indicate the position of the Dirac peaks.of the Dirac peak after long-term desorption.As can be seen from the figure,equation (3)provides a good description of the experimental observation.Of course,for the double exponential fit to be meaningful,we expect to obtain two very different values for τ1and τ2.Indeed,the slow and fast components of the NH 3desorption process are (first desorption)τ1=2.1±0.2h and τ2=0.22±0.02h (i.e.,τ1/τ2=9±2);(second desorption)τ1=30±1min and τ2=4±1min (i.e.,τ1/τ2=8±3).The values returned for the prefactors are,respectively,(first desorption)V 1=−18.3±0.4V and V 2=−12.1±0.7V ,(second desorption)V 1=−30.2±0.8V and V 2=−13.5±0.5V .Our results indicate that the FET desorbs somewhat faster the second time.We are not sure what the physical or chemical implications are for this change in response times.The fact that the ratio τ1/τ2∼9is found to be the same for both desorption cycles is discussed below and appears to stem from Fickian diffusion.All samples showed the general behavior exhibited by V Dirac (t )with the kinetics described by a combination of fast and slow processes (equation (3)).In one other sample,we did a fitting of equation (3)to the data and again found the ratio τ1/τ2=9.5±0.6.In figure 4,we show the representative R ds (V g )curves corresponding to the first (bottom,curves 1),second (middle,curves 2),and third (top,curves 3)NH 3exposure–desorption cycles.The Dirac peaks are coded in the figure to indicate when they were collected:before NH 3exposure (solid curve),after NH 3desorption at T =25◦C (dotted curve)and after NH 3desorption at T =100◦C for ∼16h in vacuum (dashed curve).As shown in figure 4,the amplitude andposition of the Dirac peak were not observed to recover exactly to their original vacuum-degassed FET state values after a T =25◦C vacuum desorption.However,after a T =100◦C vacuum desorption,the amplitude,but not the position,of the Dirac peak recovers to the original (degassed state)value;the Dirac peak shift appears at a slightly less negative V g value after this treatment.We attribute this result to the possibility of displacement chemical reactions in the SiO 2/graphene interface whereby the NH 3molecules displace some species that weakly charge transfer,thereby partially neutralizing the ambient p-doped SiO 2surface.Despite much effort by several groups,the interaction of NH 3with SWNTs remains controversial.A few years ago,it was reported that individual semiconducting nanotubes (s-SWNTs)could be used to detect small concentrations of NH 3with high sensitivity by measuring changes of the nanotube conductance and the FET threshold voltage [18].The authors reported that the electrical conductance of a p-doped nanotube-FET decreased by approximately two orders of magnitude after exposure to a flow of 1%NH 3at room temperature,and they suggested that a charge transfer of electrons from NH 3to SWNTs was responsible for the change in properties of the SWNTs.On the other hand,theoretical calculations on isolated defect-free nanotubes predicted that NH 3molecules should physisorb to the tube wall without significant change of the band structure,although a very small charge transfer of f ∼0.03−0.04e per adsorbed NH 3molecule was predicted to occur [18,28–31].Therefore,from this work on SWNTs it appears that experimental and theoretical results are consistent with the NH 3acting as a weak n-type dopant,overriding or compensating the initial p-type behavior deduced from the sign of the gate voltage at threshold.Theoretical studies of gas adsorption on a pristine graphene surface have also been reported recently by that indicate that NH 3molecules can act as donors with a small charge transfer of f ∼0.03e [20].This is consistent with calculations on defect-free SWNTs [18,28–31].More recent theoretical investigations have explored the adsorption of NH 3on nanotubes near wall defects,e.g.,Stone-Wales defects [32].The results are possibly relevant to our study in graphene and can be summarized as follows:(i)chemisorption of NH 3at a wall defect is an exothermic process with a relatively low activation barrier;therefore chemisorption of NH 3near these defects should readily happen at room temperature;(ii)NH 3adsorption at defects is accompanied by a high charge transfer,i.e.,f =0.18e ;(iii)the presence of pre-dissociated oxygen near or at the defect enhances the NH 3chemisorption process which becomes nearly spontaneous.These significant electron transfers from chemisorbed NH 3at defects,rather than low charge transfer at more numerous pristine sites might also explain the large drops in electrical conductance of s-SWNTs observed experimentally [18,33].It is also relevant to our results here that recent temperature-programed-desorption (TPD)measurements of NH 3on SWNTs indicate that NH 3can be strongly bound to the SWNT surface (i.e.,chemisorbed)[34].Perhaps this conclusion is consistent with a weak charge transfer that enhances the binding energy of the NH 3to the tubewall,i.e.,over and above that associated with physisorption.In these TPD experiments,most of the NH 3gas desorbed only at elevated temperatures (500–700K).In addition,and in contrast to the previous work on NH 3-SWNTs,Bradley et al [35]found that vacuum-degassed nanotube-FETs were insensitive to NH 3.They suggested that for NH 3gas to be detected in the FET response,the NH 3must first dissolve in a H 2O monolayer which forms on the nanotube-FETs under ambient lab conditions.They proposed that the NH 3molecule in this environment can become a cation with the charge being compensated in the SWNTs,i.e.,act as an n-dopant.As a result,it was proposed that the nanotube-FETs respond to NH 3gas only when they are exposed in ambient (humid)conditions.Feng et al [32],on the other hand,reported experimental evidence of the extreme sensitivity of NH 3adsorption on thermal-annealing prehistory.Their results revealed a systematic decrease of NH 3adsorption on SWNTs after they were annealed to successively higher temperatures from 500up to 1400K.However,they did not identify the decrease in NH 3doping on a reduction in H 2O bound to the surface,but rather to large charge transfer associated with less numerous sites near functional groups (e.g.,chemisorbed O 2)and defects on the surface that can be removed by vacuum-annealing.Upon exposure of our degassed graphene FET to 0.1atm of NH 3,the Dirac peak rapidly shifted past our safe gate voltage limit and we were therefore only able to partially observe the R ds (V g )peak,i.e.,the peak itself and its negative voltage tail were beyond the gate limit.Since we did not observe any significant change in the shape of the R ds (V g )curves as they shift with adsorption/desorption,we then fit this partial R ds (V g )curve to obtain the peak position,i.e.,V Dirac ∼−140V .Before NH 3exposure,the degassed device exhibited a Dirac peak at −47V;we therefore observed a negative shift in the Dirac peak of V Dirac ∼90V due to equilibrium adsorption of NH 3under our experimental conditions.The same basic behavior was observed in five other graphene FETs.The shift V Dirac was detectable in three other samples where the tail of the Dirac peak was still visible within our gate voltage limit,as described above.The estimated values of V Dirac determined as described above by a rigid shift of a typical Dirac peak,are 100V ,89V ,and 90V ,respectively.In average, V Dirac =92±5V .We can interpret the ∼92V Dirac peak shift due to equilibrium coverage of NH 3under our experimental conditions in terms of the charge transfer of electrons between adsorbed NH 3and the graphene.For simplicity,we can begin to analyze this process by assuming a monolayer coverage of NH 3in a closed-packed hexagonal configuration [36],anduse the collision diameter for NH 3,i.e.,d ∼4.09˚Afor the effective molecular diameter [37].The effective graphene area per NH 3molecule is therefore A m =√3(√3+1)2d 2/14.The relationship between the charge transfer per molecule f ,A m and the excess charge per unit area on the graphene ( n )is given by f = n A m ,which by equation (1)can be expressed as f =α V Dirac A m .Using this equation,we can estimate f ∼0.01e for a monolayer coverage of NH 3on graphene.This value of f is approximately a factor of 3lower than that predicted by theory [20].However,based on thermodynamic data for NH 3on graphite [38,39],a surface coverage of much less than a monolayer is expected.Our estimate for f must be refined to take this lower coverage into account.The well known thermodynamic relationship between surface coverage and binding energy E b is given by [40]ρsurface =ρgas exp E bk B T (4)where ρsurface and ρgas are,respectively,the three-dimensional (3D)number density on the surface and in the gas phase.This relationship is derived assuming that only one molecule can occupy a site with this binding energy and there is no interaction among the adsorbed molecules.The fractional coverage is defined as θ=A m d ρsurface .Using the experimental thermodynamic value E b =0.18eV [38,39],we find θ=0.15for p =0.1atm (partial pressure of NH 3)and T =25◦C which is indeed much less than a monolayer.We then correct our estimation of the charge transfer f taking into account the fractional coverage θ=0.15.With this coverage we obtain f ∼0.068±0.004electrons per NH 3molecule.This value is in excellent agreement with the recent theoretical study of the physisorption of NH 3on defect-free graphene discussed above [20].It is important to notice that since our graphene was fully degassed in vacuum at 200◦C for long periods of time,we do not expect any significant interference from H 2O and/or O 2on the surface,as discussed above in relationship to Bradley et al ’s work with NH 3on s-SWNT FETs [35].To make sure the NH 3gas (stated to be <1ppm H 2O)did not bring significant H 2O into our experimental chamber,we carried out one additional experiment first introducing a KOH trap between the gas cylinder and the chamber.Essentially the same experimental results were obtained.Similar to SWNTs,recent theoretical calculations [21]showed that NH 3can strongly adsorb on defect sites located at a graphene’s edge and significantly influence the electronic and transport properties of narrow ribbons.The charge transfer at these edge sites is high,f =0.27e [21].Consequently,it is important to consider the possibility that defect sites might affect NH 3detection by graphene ing equation (1),we can write f =α V Dirac w/2λ,where w =1.5μm isthe average width of our graphene flake and λ=0.47˚A−1is the defect concentration on a graphene flake having two free armchair-shaped edges.Assuming full decoration of these sites with NH 3,we estimate that a charge transfer of f =0.27e corresponds to a Dirac peak shift of V Dirac ∼2V .Although this value is easily measurable;it is too small to explain the 90V shift we observe due to NH 3exposure.That large a shift must come from basal plane adsorption.We now discuss the adsorption/desorption kinetics of NH 3on graphene as observed from our experiments.To explain our results,we assume that for a particular hexagon or adsorption site of the graphene,the molecule can be adsorbed either on top (free surface)or on the bottom (next to the SiO 2substrate)of the graphene.It is not likely,however,that both top and bottom sites associated with the same hexagon can be simultaneously occupied,i.e.,the second adsorption would be expected to have。