Physical Chemistry Chemical Physics c4cp00128a

Enriching Q1 Q2physisorption of H2S and NH3 gases on agraphane sheet by doping with Li adatoms

Tanveer Hussain,* Puspamitra Panigrahi* andRajeev Ahuja

We have used density functional Q4theory to investigate theadsorption efficiency of a hydrogenated graphene(graphane) sheet for H2S and NH3 gases.

Please check this proof carefully. Our staff will not read it in detail after you have returned it.

Translation errors between word-processor files and typesetting systems can occur so the whole proof needs to be read.Please pay particular attention to: tabulated material; equations; numerical data; figures and graphics; and references. If youhave not already indicated the corresponding author(s) please mark their name(s) with an asterisk. Please e-mail a list ofcorrections or the PDF with electronic notes attached – do not change the text within the PDF file or send a revisedmanuscript. Corrections at this stage should be minor and not involve extensive changes. All corrections must be sent at thesame time.

Please bear in mind that minor layout improvements, e.g. in line breaking, table widths and graphic placement, areroutinely applied to the final version.

Please note that, in the typefaces we use, an italic vee looks like this: n, and a Greek nu looks like this: n.

We will publish articles on the web as soon as possible after receiving your corrections; no late corrections will be made.

Please return your final corrections, where possible within 48 hours of receipt, by e-mail to: pccp@rsc.org

Queries for the attention of the authors

Journal: PCCP

Paper: c4cp00128a

Title: Enriching physisorption of H2S and NH3 gases on a graphane sheet by doping with Li adatoms

Editor’s queries are marked on your proof like this Q1, Q2, etc. and for your convenience line numbers areindicated like this 5, 10, 15, ...

Please ensure that all queries are answered when returning your proof corrections so that publication of yourarticle is not delayed.

Queryreference Query Remarks

Q1 For your information: You can cite this article before youreceive notification of the page numbers by using thefollowing format: (authors), Phys. Chem. Chem. Phys.,(year), DOI: 10.1039/c4cp00128a.

Q2 Please carefully check the spelling of all author names.This is important for the correct indexing and futurecitation of your article. No late corrections can be made.

Q3 Do you wish to add e-mail addresses for thecorresponding author(s)?

Q4 Please check that the inserted GA image and text aresuitable.

Q5 ‘‘Ying’’ is not cited as an author of ref. 4. Please indicateany changes that are required here.

Q6 The sentence beginning "Among many, H2S is a toxicgas..." has been altered for clarity. Please check that thescientific meaning has not been changed.

Q7 ‘‘Tomas’’ is not cited as an author of ref. 35. Please indicateany changes that are required here.

Q8 Please check that changes to the English in this sectionhave not affected the meaning.

Q9 The meaning of "to far enough" is not clear in thesentence beginning "The work function is the minimum..."- please provide an alternative phrase.

Q10 The sentence beginning "Thus, the change in..." has beenaltered for clarity. Please check that the scientific meaninghas not been changed.

Q11 Ref. 32: Please check that the authors names have beendisplayed correctly.

Enriching Q1 Q2physisorption of H2S and NH3 gases ona graphane sheet by doping with Li adatoms

Tanveer Hussain,*a Puspamitra Panigrahi*a and Rajeev Ahujaab

We have used density functional theory to investigate the adsorption efficiency of a hydrogenated

graphene (graphane) sheet for H2S and NH3 gases. We find that neither the pristine graphane sheet nor

the sheet defected by removing a few surface H atoms have sufficient affinity for either H2S or NH3 gas

molecules. However, a graphane sheet doped with Li adatoms shows a strong sensing affinity for both

the mentioned gas molecules. We have calculated the absorption energies with one [referred to as half

coverage] molecule and two molecules [referred to as full coverage] for both gases with the Li-doped

graphane sheet. We find that for both the gases, the calculated absorption energies are adequate

enough to decide that the Li-doped graphane sheet is suitable for sensing H2S and NH3 gases. The Li-

doped sheet shows a higher affinity for the NH3 gas compared to the H2S gas molecules due to a

stronger Li(s)–N(p) hybridization compared to that of Li(s)–S(p). However, while going from the half

coverage effect to the full coverage effect, the calculated binding energies show a decreasing trend for

both the gases. The calculated work function of the Li-doped graphane sheet decreases while bringing

the gas molecules within its vicinity, which explains the affinity of the sheet towards both the gas

molecules.

1 Introduction

Graphene is an experimentally isolated single atomic layer ofcarbon atoms arranged in a honeycomb lattice structure,forming the thinnest free standing material to date. The Diraccharge carriers in graphene and its large surface to volume ratioincreases the probability of a chemical reaction between thesurface atoms and various absorbates. When an adsorbate(donors or acceptors) is exposed to the graphene surface, itchanges the charge concentration and ultimately alters theelectrical properties of the graphene surface, thus openingthe possibility for graphene as a future gas sensor.1 In recentyears, there have been a handful of studies reporting theadsorption of selected gas molecules, like CO, NO, NO2, N2,CO2, H2O, CO, NO and NH3, on various graphene structures,2,3

which predict graphene as an efficient sensor and having thepotential to replace conventional solid-state sensors.2,3

The introduction of heteroatoms to an ideal graphene sheet,entirely composed of C atoms, further changes its electronic aswell as its electrochemical properties to bring about a slightchange in its sensing aptitude. Recently, Ying et al. havereportedQ5 the change in the electron density of a graphene sheet

during nitrogen doping, which makes it an excellent candidatefor glucose biosensing.4

However, compared to heteroatomic substitution, completehydrogenation of graphene rather results in a graphane sheet(CH),5,6 which looks like a graphene sheet painted with hydro-gen on both sides. The resulting graphane sheet retains thethinness, strength and flexibility but has very different electro-nic properties from that of the parent graphene. The C–Hbonding ties down the electrons, which leads to insulatingbehavior for graphane7 and makes it an activated precursor forsuitable chemical modifications.8 Besides the robust transportproperties, comparatively larger adsorption energies with dif-ferent adsorbates and the tunable band gap properties revealgraphane as a superior candidate for future gas sensing pur-poses.9–14

Though carbon-based nanostructures like graphene, andgraphane show very little interaction with the adsorbents, adrastic improvement in the binding energies of some gasmolecules while introducing defects and dopants to the surfaceof pristine graphene and graphane structures have already beenreported elsewhere.15–18 In particular, various alkali metals arereported to adhere to a graphene sheet without destroying itsbasic electronic properties while creating a surface chargeredistribution and further changing the work function of thesheets.19–22 In this context, a stable structure of a lithium-doped graphane (CHLi) sheet has already been reported and

1

5

10

15

20

25

30

35

40

45

50

55

1

5

10

15

20

25

30

35

40

45

50

55

Cite this:DOI: 10.1039/c4cp00128a

a Condensed Matter Theory GroupQ3 , Department of Physics and Astronomy, Box 516,

Uppsala University, S-75120, Uppsala, Swedenb Department of Materials and Engineering, Royal Institute of Technology (KTH) S-

100 44, Stockholm, Sweden

Received 10th January 2014,Accepted 5th February 2014

DOI: 10.1039/c4cp00128a

www.rsc.org/pccp

This journal is c the Owner Societies 2014 Phys. Chem. Chem. Phys., 2014, 00, 1�6 | 1

PCCP

PAPER

is found to be a prospective candidate for future hydrogenstorage.23–25

However, very few theoretical calculations have so far beenreported on modulated graphane sheets, predicting their prop-erties as efficient gas sensors.

Among many, H2S is a toxic gas which is extremely harmfulto human beings as well as for the atmosphere and it is foundto be fatal if the concentration goes beyond 250 ppmQ6 .26 There-fore, the detection, sensing and monitoring of H2S gas indifferent circumstances is very important for us and ourenvironment. On the other hand, though NH3 gas is useful invarious fields like in agriculture, industry, and medical uses, itis also considered to be a toxic gas when used at high concen-trations.27 Thus, it is very necessary to have an efficient sensorto avoid the dangerous consequences of the presence of suchNH3 and H2S gases when needed.

In this work, by using density functional theory we havestudied a graphane sheet (CH) containing defects from intro-ducing hydrogen vacancies. We find that the absorption ener-gies of the hydrogen deficient graphane sheet with NH3 andH2S gases are not adequate enough for sensing purposes.Furthermore, we have doped the graphane sheet with two Liatoms at the hydrogen sites. The Li atoms are kept far apart toavoid the clustering effect. We find a stable structure for this Li-doped graphene structure (CHLi), which has ample absorptionenergies for sensing NH3 and H2S gases. We have calculated theabsorption energies with one molecule and two molecules ofboth the gases with the CHLi sheet. During this physisorptionprocess, the band gap of the CHLi structure shows a decreasingtrend. However, while going from one molecule to two mole-cules, the absorption energies show a decreasing trend for boththe gases.

2 Computational methods

We have performed first principles DFT calculations using theprojector augmented wave method (PAW) as implementedwithin the VASP package.28–31 The approximation for theexchange and correlation potential is executed using a Per-dew–Burke–Ernzerhof functional.32 The graphane (CH) sheet ismodeled as a 4 � 4 � 1 supercell having 64 atoms (C32H32) forthis study. In order to have a Li-doped graphane sheet(C32H30Li2), the two Li atoms are substituted at the H sites, asshown in Fig. 1(a and b). During the periodic calculation, avacuum space of 15 Å has been inserted along the z direction tominimize the interaction between the periodic images of thementioned sheets.

To investigate the sensing mechanism for H2S and NH3 onthe Li-doped graphane sheet, we have exposed the C32H30Li2sheet first to one formula unit of H2S and NH3 moleculesseparately, as shown in Fig. 2(a and b) and Fig. 3(a and b)respectively. In the next step, we have introduced the Li-dopedgraphane sheet to two formula units of H2S and NH3 moleculesin a similar fashion, as shown in Fig. 2(c and d) and Fig. 3(c andd).

For sampling the Brillouin zone, we have used a dense (5 � 5� 3) k-point mesh in Monkhorst–Pack33 schemes. To obtain theground state of all the above mentioned structures, we havekept the criterion of the forces acting on each ion to be lessthan 0.005 eV Å�1. A very accurate converged energy cut-off of450 eV has been used for the plane wave basis set. In ourconventional DFT calculations, the approximation of theexchange correlation potential within the local density

1

5

10

15

20

25

30

35

40

45

50

55

1

5

10

15

20

25

30

35

40

45

50

55

Fig. 1 Side view (a) and top view (b) of the optimized structure of theC32H30Li2 sheet; the blue, green and red balls represent C, H and Li atomsrespectively.

Fig. 2 Side view (a, c) and top view (b, d) of the optimized structure of theC32H30Li2 sheet physisorbed with H2S gas molecules; the blue, green, redand magenta balls represent C, H, Li and S atoms respectively. Here, (a, b)are half coverage and (c, d) present the full coverage effect respectively.

2 | Phys. Chem. Chem. Phys., 2014, 00, 1�6 This journal is c the Owner Societies 2014

Paper PCCP

approximation (LDA) overestimates the binding energies butperforms better in the case of weakly interacting systems,whereas the generalized gradient approximation (GGA) usuallyunderestimates the binding energies. To obtain a betterdescription of H2S and NH3 physisorption over CHLi, we haveused LDA approximations along with the van der Waals correc-tion, as implemented in the DFT-D2 approach by Grimme34

within the VASP package and benchmarkedQ7 by Tomas et al.35

Here, a semi-empirical dispersion potential is added to ourconventional Kohn–Sham DFT energy.

To gain a better understanding of the adsorption effect ofH2S and NH3 on a C32H30Li2 graphane sheet containing defects,we have calculated the work function, f, of the bare andphysisorbed sheets as:

f = V(f) � Ef (1)

where, f, V(f) and Ef are the work function, electrostaticpotential at the vacuum level and the Fermi energy of the Li-doped graphane sheet respectively.

3 Results and discussion

Here onwards, we have divided our results into three differentparts. In the first part, we will discuss the structural propertiesof the Li-doped graphane sheet. In the second part, we willconsider the absorption mechanism of H2S on the graphanesheet at different concentrations and the third part deals withthat of NH3 on the graphane sheet.

3.1 Structural properties of the Li-doped graphane

Complete hydrogenation turns the metallic graphene to wideband gap (B5 eV) graphane (CH). Our optimized structure of a64 atom pristine graphane sheet doped with Li as a C32H30Li2graphane sheet is shown in Fig. 1(a and b). In the Li-dopedoptimized graphane sheet, the C–C, C–H and C–Li bondlengths are found to be 1.54 Å, 1.10 Å and 1.98 Å respectively.

However, for efficient utilization of a metal functionalizedgraphane sheet, the metal-to-graphane binding should alwaysbe high enough to prevent the metal–metal clustering effect,which could halt the performance of a designed sensor. Thus, itis important to maintain a reasonably high adatom–adatomdistance (low doping concentration) on a graphane sheet con-taining defects to avoid the clustering effect.25 Here, in themodeled Li-doped graphane sheet, we have maintained adistance of 8.8 Å between the two Li adatoms. The bindingenergy is estimated to be �2.3 eV per Li atom. A significantdecreasing trend in the binding energies of the Li-dopedgraphane sheet on increasing the Li doping concentration from50% to 3.12% has already been reported in previous stu-dies.36,37 At a higher doping concentration, the possibility ofclustering between the Li adatoms ultimately lowers the bind-ing energies of the Li-doped graphane sheet and thus, a 3.12%Li doping concentration is carefully maintained for this currentstudy. The band gap of the C32H30 graphane sheet containingdefects (two H atoms are removed) is calculated to be B1.5 eVand when doped with Li, the band gap of the C32H30Li2graphane sheet further reduces to 0.96 eV (Table 1), which isalso evident from the density of states (DOS) plot shown inFig. 4(a and b). Very similar metallic behavior for the graphanesheet, having a higher doping concentration of different metaladatoms like Li, Na, K, Be, Mg, and Ca, have also been reportedelsewhere.18,20,25 Other than that, compared to other adatoms,the Li-to-graphane sheet binding energy is found to be higher.The strong CH–Li binding allows uniform distribution of Liover the sheet and also nullifies the possibility of Li–Li clusterformation, which is highly desirable to preserve the reversibilityof the designed system.

From the DOS plot, we see that the valence band region nearthe Fermi energy is mostly dominated by Li(s) and the band gapof the C32H30 graphane sheet containing defects was reducedon introducing Li to the sheet. Near the Fermi energy we findC(p)–Li(s) hybridization, which leads to an increase in the

1

5

10

15

20

25

30

35

40

45

50

55

1

5

10

15

20

25

30

35

40

45

50

55

Fig. 3 Side view (a, c) and top view (b, d) of the optimized structure of theC32H30Li2 sheet physisorbed with NH3 gas molecules; the blue, green, redand orange balls represent C, H, Li and N atoms respectively. Here, (a, b)are half coverage and (c, d) present the full coverage effect respectively.

Table 1 The adsorption energies, Eads, in eV calculated with the LDA andvan der Waals (Vdw) approaches. CHLi represents the Li-doped graphanesheet. CHLi–H2S and CHLi–NH3 represent the CHLi sheet exposed to H2Sand NH3 gases

System CoverageLDA-Eads (eVper molecule)

Vdw-Eads (eVper molecule)

Bandgap(eV)

Work func-tion (f) (eV)

CHLi — — — 0.96 2.76CHLi–H2S

Half �0.92 �0.43 1.10 2.68Full �0.76 �0.36 0.67 2.42

CHLi–NH3

Half �1.35 �0.83 0.76 2.44Full �1.10 �0.76 0.43 2.11

This journal is c the Owner Societies 2014 Phys. Chem. Chem. Phys., 2014, 00, 1�6 | 3

PCCP Paper

binding energy per Li atom for the doped C32H30Li2 sheet of 2.5eV. Henceforth, the graphane sheet doped with Li which resultsin a gain in the binding energy leads to a comparatively stablerC32H30Li2 graphane sheet and will be further discussed for thesensing of H2S and NH3 gases.

3.2 Adsorption energy of H2S

Our rigorous computation leads to the preferred structure forthe H2S molecule over the C32H30Li2 sheet, as shown in Fig. 2(a,b) (c, d), for one [referred to as half coverage] and two H2Smolecules [referred to as full coverage] respectively. We havecalculated the adsorption energies as well as the band gap ofthe resulting structures in terms of the coverage effect. Theaverage distance of the H2S molecule from the C32H30Li2 sheetis found to be 2.47 Å for both coverage effects. With the halfcoverage effect, the estimated binding is �0.44 eV and that ofthe full coverage effect is found to be �0.36 eV. The negativesign indicates the affinity of the Li-doped graphene sheettowards the H2S molecule. Thus, with an increase in thecoverage effect, the affinity shows a decreasing trend. On theother hand, the estimated binding energies of H2S moleculeson a C32H30 graphane sheet containing defects without Lidoping are found to be positive, which shows that the affinityof the graphane sheet containing defects towards H2S isinadequate to serve the purpose of a gas sensor. To increasethe confidence in our results, we have performed all thecalculations using the LDA approach as well as the van derWaals correction approach, as implemented in the DFT-D2.Here, the binding energies of the C32H30Li2 sheet physisorbedwith H2S estimated from both methods are summarized inTable 1. We have found a similar decreasing trend in theadsorption energy with the half coverage effect to the fullcoverage effect in both methods (Table 1). The estimated bandgap also shows a decreasing trend from 1.1 eV to 0.67 eV ongoing from the half to full coverage effect (Table 1). To under-stand the electronic structure of the physisorbed system, wehave plotted the partial density of states (PDOS) of C(p), Li(s),

S(p) and H(s), as shown in Fig. 5(a). We find that S(p) shows astrong hybridization with C(p) as well as with Li(s) near theFermi energy (Fig. 5(a)), which explains the affinity of the Li-doped graphane sheet towards the H2S gas molecules. How-ever, with the full coverage effect, the Li(s) bands are broadenednear the Fermi energy compared to the half coverage effect.With increasing S concentration, the extra S(p) bands are rathermore spread into the conduction band region, which do notparticipate strongly with the parent C32H30Li2 sheet. Thus, theaffinity decreases on increasing the concentration of gasmolecules Q8.

3.3 Adsorption energy of NH3

We have performed our calculation for NH3 gas over theC32H30Li2 sheet in a similar fashion to that of H2S molecule.The preferred optimized configurations of NH3 over theC32H30Li2 sheet with half coverage and full coverage are shownin Fig. 3(a, b) and (c, d) respectively. The average bond distancefrom NH3 to the C32H30Li2 sheet is found to be B2.03 Å forboth coverage effects. However, unlike H2S, the NH3 moleculeundergoes a structural relaxation in the presence of theC32H30Li2 sheet. The N–H bond distance, which is 1.017 Å inits isolated configuration, becomes 1.023 Å and 1.025 Å with thehalf and full coverage effect respectively. Also, the initial H–N–H bond angle changes from 107.801 to 107.6301. For NH3 gasmolecules, the binding energies are estimated to be �0.83 eVand �0.76 eV per molecule for half coverage and full coveragerespectively, which are higher compared to that of H2S. Thedetails of the binding energy calculation with the van der Waalscorrection are give in Table 1. Here, the band gap also shows asimilar decreasing trend from 0.76 eV to 0.43 eV, like theprevious one (Table 1). We have also plotted the partial densityof states (PDOS) of C(p), Li(s), N(p) and H(s), as shown inFig. 5(b). We find that there is strong hybridization between

1

5

10

15

20

25

30

35

40

45

50

55

1

5

10

15

20

25

30

35

40

45

50

55

Fig. 4 (a) The partial DOS plot of C(p), and H(s) of the graphane sheetcontaing defects. (b) The C(p), H(s), and Li(s) in the C32H30Li2 sheet.

Fig. 5 (a) The partial DOS plot of C(p), H(s), Li(s) and S(p) of the C32H30Li2sheet physisorbed with H2S gas molecules whereas (b) shows that of theC(p), H(s), Li(s) and N(p) of the C32H30Li2 sheet physisorbed with NH3 gas.The solid line represents the half coverage effect whereas the dotted lineshows the full coverage effect for both gases.

4 | Phys. Chem. Chem. Phys., 2014, 00, 1�6 This journal is c the Owner Societies 2014

Paper PCCP

C(p) and N(p) here compared to that of C(p) and S(p) in the H2Sphysisorbed graphane sheet (Fig. 5(a)). C(p) and N(p) also havestrong hybridization with Li(s) near the Fermi energy (Fig. 5(b)),which leads to a higher affinity of the Li-doped graphane sheettowards NH3 gas molecules compared to that for H2S gasmolecules (Table 1). With the full coverage effect of NH3, asimilar decreasing effect occurs, like that of the H2S gasmolecules.

3.4 Change in the work function (/)

The surface reactivity of the doped graphane sheet towards thegas molecules will mostly depend upon the surface potentialvariation, which could be perceived by the change in workfunction, f. The work function is the minimum amount ofenergy required to remove an electron from the Fermi level tofar enoughQ9 . While the electron moves through the surfaceregion, f becomes sensitive towards any changes at the surfacein the presence of other gas molecules. Thus, the change in fcould determine the operating conditions of the Li-dopedgraphane sheet as a gas sensor since the gas molecules intro-duce a net dipole moment during the physisorption processdue to charge transfer between the surface and the gasmoleculeQ10 . Thus, there will be a shift in the work function,which could further change the sensing properties of thesystem.38–41 Here, we have calculated the work function asper eqn (1) for the bare C32H30Li2 sheet and while the sheetis physisorbed with H2S and NH3 at different coverage effects,as summarized in Table 1. We find that both the H2S, and NH3

molecules show a decreasing trend in the work function for thephysisorbed sheet compared to the bare sheet and the resultingwork functions become even smaller when we go from the fullcoverage to the half coverage effect (Table 1). In general, fdepends upon the energy difference between the Fermi leveland conduction band. We find from our PDOS plots, as shownin Fig. 5(a and b), that for both the gas molecules on going fromthe half coverage effect to the full coverage effect, the energydifference between the Fermi level and the conduction banddecreases, as do the work functions (Table 1). From our Badercharge analysis,42 we find that in the case of the pristineC32H30Li2 sheet, as mentioned before, two H atoms have beenreplaced with Li atoms. Due to the higher electronegativity of Cthan Li, there is a charge transfer from Li to C. Our analysisreveals that 0.98 e of charge have been transferred from both Liadatoms and have accumulated at the C sites, which arebonded directly to the Li atoms. However, a small fraction ofthese charges will be acquired by those H atoms lying in thevicinity of the C and Li atoms, which leads to a state where theLi atoms have a partial positive charge. In the case of H2Sadsorption for both coverage effects, there is hardly any trans-fer of charge from H2S to the C32H30Li2 sheet. There is aninterchange of electric charge within the H2S molecules and theinteraction of incident molecules with the sheet is due to aweak van der Waals type of interaction. However, in the case ofNH3 molecules, the H atoms transfer their charge to the N atomwithin the molecule, resulting in a net negative charge on Nand a positive charge on the H atoms. During the exposure of

NH3 molecules to the C32H30Li2 sheet, precisely 0.06 e and 0.01e unit of charge have been transferred to the N atoms from theLi atoms in the case of the half and full coverage effectrespectively.

4 Summary

In summary, the absorption of H2S and NH3 gas molecules on aLi-doped graphane sheet is investigated using density func-tional theory. We find that both the molecules are very wellphysisorbed onto the Li-doped graphane sheet. However, thesheet has a higher affinity for the NH3 gas molecules comparedto the H2S gas molecules. The C(p)–N(p) hybridization in thecase of NH3 physisorption is more efficient compared to that ofC(p)–S(p) hybridization during H2S physisorption. However, onincreasing the concentration of both gases while exposing themto the graphane sheet, the affinity shows a decreasing trend.The work function (f), which determines the surface reactivity,also shows a decreasing trend for the Li-doped graphane sheetduring the physisorption of both the gas molecules. A verysimilar trend is observed for the calculated binding energieswhile using the LDA potential as well as with the van der Waalscorrection, as implemented in the DFT-D2 approach, whichgives us confidence for the robustness of our approach.

Acknowledgements

Thanveer Hussen thanks the higher education commission ofPakistan for his doctoral fellowship. Puspamitra Panigrahi isgrateful to the Swedish Institute for her postdoctoral Fellow-ship. SNIC and UPPMAX are acknowledged for providingcomputing resources.

References

1 F. Schedin, A. K. Geim, S. V. Morozov, E. W. Hill, P. Blake,M. I. Katsnelson and K. S. Novoselov, Nat. Mater., 2007,6, 652.

2 B. Huang, Z. Li, Z. Liu, G. Zhou, S. Hao, J. Wu, B.-L. Gu andW. H. Duan, J. Phys. Chem. C, 2008, 112, 13442.

3 O. Leenaerts, B. Partoens and F. M. Peeters, Phys. Rev. B:Condens. Matter Mater. Phys., 2008, 77, 125416.

4 Y. Wang, Y. Shao, D. W. Matson, J. Li and Y. Lin, ACS Nano,2010, 4, 1790.

5 J. O. Sofo, A. S. Chaudhari and G. D. Barber, Phys. Rev. B:Condens. Matter Mater. Phys., 2007, 75, 153401.

6 D. C. Elias, R. R. Nair, T. M. G. Mohiunddin, S. V. Morozov,P. Blake, M. P. Halsall, A. C. Ferrari, D. W. Boukhvalov,A. K. Geim and K. Novoselov, Science, 2009, 323, 610.

7 S. Lebegue, M. Klintenberg, O. Eriksson andM. I. Katsnelson, Phys. Rev. B: Condens. Matter Mater. Phys.,2009, 79, 245117.

8 Z. Sun, C. L. Pint, D. C. Marcano, C. Zhang, J. Yao, G. Ruan,Z. Yan, Y. Zhu, R. H. Hauge and J. M. Tour, Nat. Commun.,2011, 2, 559.

1

5

10

15

20

25

30

35

40

45

50

55

1

5

10

15

20

25

30

35

40

45

50

55

This journal is c the Owner Societies 2014 Phys. Chem. Chem. Phys., 2014, 00, 1�6 | 5

PCCP Paper

9 C. K. Yang, Appl. Phys. Lett., 2009, 94, 163115.10 H. Sahin, C. Ataca and S. Ciraci, Phys. Rev. B: Condens.

Matter Mater. Phys., 2010, 81, 205417.11 J. Berashevich and T. Chakraborty, Phys. Rev. B: Condens.

Matter Mater. Phys., 2010, 82, 134415.12 L. F. Huang, X. H. Zheng, G. R. Zhang, L. Li and Z. J. Zeng,

J. Phys. Chem. C, 2011, 115, 21088.13 T. Hussain, B. Pathak, M. Ramzan, T. A. Maark and

R. Ahuja, Appl. Phys. Lett., 2012, 100, 183902.14 M. Nava, D. E. Galli, M. W. Cole and L. Reatto, Phys. Rev. B:

Condens. Matter Mater. Phys., 2012, 86, 174509.15 Y.-H. Zhang, Y.-B. Chen, K.-G. Zhou, C.-H. Liu, J. Zeng,

H. L. Zhang and Y. Peng, Nanotechnology, 2009, 20, 185504.16 M. Gautam and A. H. Jayatissa, J. Appl. Phys., 2012,

112, 114326.17 H. Sahin, C. Ataca and S. Ciraci, Appl. Phys. Lett., 2009,

95, 222510.18 T. Hussain, B. Pathak, T. A. Maark, M. Ramzan and

R. Ahuja, Europhys. Lett., 2012, 99, 47004.19 A. Politano and G. Chiarello, J. Chem. Phys., 2013,

138, 044703.20 P. Medeiros, F. Mota, A. Mascarenhas and C. Castilho,

Nanotechnology, 2010, 21, 115701.21 D. Boukhvalov and C. Virojanadara, Nanoscale, 2012,

4, 1749.22 K. Jin, S. Choi and S. Jhi, Phys. Rev. B: Condens. Matter Mater.

Phys., 2010, 82, 033414.23 M. Khanzai, M. S. Bahramy, N.S. Venkataramanan,

H. Mizuseki and Y. Kawazoe, J. Appl. Phys., 2009,106, 094303.

24 T. Hussain, A. D. Sarkar and R. Ahuja, Appl. Phys. Lett., 2012,101, 103907.

25 L. Antipina, P. Avramov, S. Sakai, H. Naramoto, M. Ohtomo,S. Entani, Y. Matsumoto and P. Sorokin, Phys. Rev. B:Condens. Matter Mater. Phys., 2012, 86, 085435.

26 W. H. Tao and C. H. Tsai, Sens. Actuators, B, 2002, 81, 237.27 Z. Wu, X. Chen, S. Zhu, Z. Zhou, Y. Yao, W. Quan and B. Liu,

Sens. Actuators, B, 2013, 178, 485.28 P. E. Blochl, Phys. Rev. B: Condens. Matter, 1994, 50, 17953.29 G. Kresse and J. Hafner, Phys. Rev. B: Condens. Matter, 1993,

47, 558.30 G. Kresse and J. Hafner, Phys. Rev. B: Condens. Matter, 1994,

49, 14251.31 G. Kresse and D. Joubert, Phys. Rev. B: Condens. Matter

Mater. Phys., 1999, 59, 1758.32 J. P. Perdew, J. A. Chevary, S. H. Vosko, K. A. Jackson,

M. R. Pederson, D. J. Singh and C. Fiolhais, Phys. Rev. B:Condens. Matter, 1992, 46, 6671.

33 Q11H. J. Monkhorst and J. D. Pack, Phys. Rev. B: Solid State,1976, 13, 5188.

34 J. J. Grimme, J. Comput. Chem., 2006, 27, 1787.35 T. Bucko, J. Hafner, S. Lebegue and J. G. Angyan, J. Phys.

Chem. A, 2010, 114, 11814.36 T. Hussain, B. Pathak, T. A. Maark, M. Ramzan and

R. Ahuja, Europhys. Lett., 2012, 99, 47004.37 T. Hussain, B. Pathak, T. A. Maark, C. Moyses, R. Scheicher

and R. Ahuja, Europhys. Lett., 2011, 96, 27013.38 Y. Paska, T. Stelzner, S. Christiansen and H. Haick, ACS

Nano, 2011, 5, 5620.39 G. Heimel, L. Romaner, J. Bredas and E. Zojer, Phys. Rev.

Lett., 2006, 96, 196806.40 A. Natan, L. Kronik, H. Haick and R. Tung, Adv. Mater.,

2007, 19, 4103.41 N. Barsan, D. Koziej and U. Weimar, Sens. Actuators, B, 2007,

121, 18235.42 G. Henkelman, A. Arnaldsson and H. Jnsson, Comput.

Mater. Sci., 2006, 36, 254.

1

5

10

15

20

25

30

35

40

45

50

55

1

5

10

15

20

25

30

35

40

45

50

55

6 | Phys. Chem. Chem. Phys., 2014, 00, 1�6 This journal is c the Owner Societies 2014

Paper PCCP