Computational Study of Gas Molecules Adsorption on Defective Graphene

Nanoribbon

Zuriana Auzar, Zaharah Johari, Sakina S.H and N. Ezaila AliasDepartment of Computer & Electronic Engineering

Faculty of Electrical EngineeringUniversiti Teknologi Malaysia

81310 UTM Johor Bahru, Johor, Malaysiazaharahj@utm.my

Abstract—This paper reported on the structural and elec-tronics properties of armchair graphene nanoribbon (AGNR)distorted with single vacancy (SV) defect when adsorbing var-ious gas molecules including oxygen (O2), nitrogen (N2) andammonia (NH3). The main focus is to investigate using self-consistent Extended-Huckel the adsorption geometry of the gasmolecules located at on, near and far from the SV defect. Bothmolecular and atomic gas molecules configuration are consideredin determining the adsorption energy. Through simulation, itis demonstrated the gas molecules adsorption energy greatlyinfluenced by its position towards the SV defects. MolecularO2, N2 and NH3 adsorption at near position achieved highestadsorption energy compared to other positions. Other gases havelittle effect on modifying the electronic structure. The calculatedcharge transfer shows that molecular NH3 acts as a donor whileatomic NH3, O2 and N2 acts as acceptor. We show that thedefective AGNR are sensitive to the adsorption of molecular NH3.

Index Terms—graphene nanoribbon, adsorption, gasmolecules.

I. INTRODUCTION

Sensing exploit semiconductor materials and become thefundamental building block in the design of sensitive gassensor. Workers studying gas sensor recognized the limita-tion such as low binding energy, low sensitivity and poorselectivity. As an alternative, considerable interest has gen-erated in carbon based materials like graphene to help boostsensing device performance. Since GNR and graphene haveflat structure and larger possible surface area applying as gassensor is more appropriate than other carbon based materials.Gas molecules detection has a great significant in many areassuch as environmental and industrial inspection. For exampleNH3 is a toxic gas which present in atmosphere naturally. Thisgas with higher concentration are found in chemical industryand medical application. Therefore sensor to detect possibleleakage for early detection is important. It is reported thatusing CNT can detect small concentration of NH3 and O2

at room temperature with high sensitivity [1]. Researchersstudying graphene since it was isolated in 2004 [2] recognizedit potential application in detecting individuals gas molecules[3] since it properties is similar to CNT. Graphene havebeen shown to posses large surface to volume ratio and highcarrier density leading to maximize the interaction betweenits surface and adsorbates. The tunable bandgap and excellentelectrical conductivity in graphene is beneficial for conducting

molecules and the electrode surface. Furthermore, graphenehave a low degree of crystal defects to withstand thermalswitching [4].

There is extensive literature on the investigation of graphenein detecting various gas and bioactive molecules [3], [5]. Toachieve a high figure of merit for such application requiresa materials with high reactivity. As such, graphene has beentreated by many ways in order to enhance the reactivity withadsorbates allowing for control over the subsequent properties.GNR are more attainable to doping, chemical modificationand sensitive to structural defects and impurities. This is dueto the present of long and reactive edges [1]. Considerableroles played by the graphene edges in improving gas sensingproperties has also been explored by Tan et al. [6] due tothe fact that imperfection at the edges provide more danglingbond for molecules attachment. These studies suggest thatthrough the application of defect, it is possible to alter theelectronic properties of graphene that leads to changes in theirchemical reactivity. In other study, first principle calculationreveals that by introducing dopant and defect on the graphenesurface could improved the sensitivity of the sensor [1], [7].Further study by Hajati et al. [8] support experimentally bydistorting bilayer graphene using Ga+ ion bormbardment.Their work proved that imperfection happen on the graphenesurface has improved the sensitivity of sensing NO2 moleculesby 3 times. Sensing mechanism is often studied based onchanges in charge carrier concentration which is induced bygas molecule which is absord on GNR surface either acts asdonor or acceptor [9]. For example, Khadije et al. [10] showedthat H2O and NO2 behave as acceptor while NH3 and CO actas donor [10]. In addition,the adsorption of the gas moleculesis expected to be dependent on the orientation of the adsorbateswith respect to the graphene surface [5].

Understanding the interaction of graphene surface and ad-sorbates molecules are paramount important in developmenthighly sensitive sensing devices. Although previous work haveshown that gas molecules bind more strongly on defectivegraphene surface compared to perfect counterpart, the ad-sorption of the molecules toward the defect deserve a closerlook. Therefore in this study, we used computer simulationto investigate the adsorbates gas onto defective AGNR withseveral position and configuration. The molecules of O2, N2

2016 First International Conference on Micro and Nano Technologies, Modelling and Simulation

978-1-5090-2406-3/16 $31.00 © 2016 IEEE

DOI 10.1109/MNTMSim.2016.16

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2016 First International Conference on Micro and Nano Technologies, Modelling and Simulation

978-1-5090-2406-3/16 $31.00 © 2016 IEEE

DOI 10.1109/MNTMSim.2016.16

3

and NH3 were located on, near and far from the defect asillustrated in Fig 1. The purpose is to examine the positionof the adsorbed gas on the AGNR suface with respect tothe SV defect that result in high adsorption energy. Fromthe result obtained, it is showed that the adsorption energyis high for molecular O2 adsorption at position on and nearwhile molecular NH3 adsorption located at far form defectexhibit strongest binding compare to other molecules. Thecalculated charge transfer shows that molecular NH3 acts asa donor while atomic NH3, O2 and N2 acts as acceptor. Inthe following section, the details of the computational methodused are described.

II. COMPUTATIONAL DETAILS.

The simulations were carried out using ATK from QuantumWise version 13.8.1. The electronic properties calculationswere performed using self-consistent Extended-Huckel andNon-Equilibrium Green Function (NEGF). EHT is used as areference point for generalized gradient approximation (GGA)in density-function theory (DFT). For numerical accuracy andefficiency, density of mesh cut off energy is assumed at20 Hatree and the Brillouin zone is sampled with k-pointsampling 1 x 1 x 100. The average Fermi level of energyzero is used. Structural optimization was carried out on allconfiguration with ReaxFF-CHO_2008/2009 until the residualforces on all ions were converged to 0.05 eV/Å.

(a) (b)

(c)

Fig. 1. a) N2 molecule at on b) N2 molecule at near c) N2 molecule at farposition.

To understand the interaction between the SV defect onthe AGNR surface with the position of the gas molecules,two adsorption configuration namely molecular and atomicare considered. Molecular adsorption is evaluated when thegas molecules are placed at certain distance from the AGNRsurface while atomic adsorption is when each of the gasmolecules are separately bounded in the same hexagon in theAGNR structure [11]. The illustration of this can be found inFig 2.

(a) Molecular O2 (b) Molecular N2 (c) Molecular NH3

(d) Atomic O2 (e) Atomic N2 (f) Atomic NH3

Fig. 2. Top and side view for molecular and atomic configuration for O2,N2 and NH3 on defective AGNR

The adsorption energy for molecular gas configuration, X2

were calculated using equation is taken from reference [11].

EX2 = E(GNR+SV+X2) − [E(GNR+SV ) + EX2 ] (1)

where E(GNR+SV ) is the energy for AGNR with SV defecton the middle of the surface while E(X2) is denoted as energyfor the system with X2 molecule adsorbed on the graphene[11]. For the case atomic gas configuration, m represent num-ber of atoms chemically bonded on the surface of grapheneand the adsorption energy was defined as,

EmX = E(GNR+SV+mEX) − [E(GNR+SV ) +mEX ]/m (2)

where E(GNR+SV+mEX) is the energy for defective AGNRwith number of atom bounded on it [11]. While, the chargetransfer was defined using basic set from Mulliken population,

N =∑

DijSji (3)

where D is the density matrix, S the overlap matrix, and thesum is over all orbitals in the system.

III. RESULTS AND DISCUSSION

Different possible position of O2, N2 and NH3 moleculesonto the AGNR surface with respect to the SV defect asshown in Fig 1 are considered. In each of the position, thesegas molecules are initially placed about 2Å for molecularadsorption from the AGNR surface. The molecular O2, N2

and NH3 adsorption at on, near and far are simulated and theresulted bond length are displayed in Table I. The bond lengthbefore adsorption for O-O, N-N and N-H are 1.21 Å, 1.10Å and 1.01 Å. It can be observed that molecular adsorption

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for all investigated gas molecules do not have dissociativeadsorption. This is evident by the slight changes in bond lengthof each molecules before and after adsorption. The same trendof observation is also achieved in previous study by Rad etal. [12]. The top view of the most stable configuration of O2,N2 and NH3 for location on, near and far are showed in Fig3. It can be observed that the distance between O2 at positionon has lowest distance compare to position near and far. Asapposed to N2 adsorption, This result suggest the dependenciesof SV defect on the AGNR surface towards the position of theadsorbed molecules.

For atomic adsorption, each of the gas molecules is at-tached to different carbon atom on the same hexagonal. Itcan be found from Table II that different gas moleculesprefer different geometries in the adsorption. For exampleO2 lies at defective AGNR surface at location on, the bondlength obtained for C-O are 1.48 Å and 1.39 Å and theangle C-C-O are 77.71o and 128.87o respectively. Infinitesimalchanges is observed in bond length and C-O-O angle whenthe O2 molecules adsorbed near and far from the SV defect.Meanwhile, atomic adsorption of N2 molecules exhibit fewerchanged in bond length and the C-N angle when it is attachednear and far the SV defect. On the contrary, the C-N moleculeson the SV defect have noticable changes in its bond length andangle. Different observation is achieved for NH3. For all the 3positions investigated to placed the NH3 molecules, the bondlength for C-N are greater than the C-H. In addition, the angleC-C-N for on, near and far from th defect do no have distictdifferent compare to C-C-H angle.

TABLE IOPTIMIZATION GEOMETRIES OF GAS MOLECULES ON DEFECTIVE AGNR

IN MOLECULAR ADSORPTION CONFIGURATION

Position Case Gas Bond Bond Length, ÅMolecular

Ona) O2 O-O 1.28b) N2 N-N 1.17c) NH3 N-H 1.00

Neard) O2 O-O 1.29e) N2 N-N 1.12f) NH3 N-H 0.95

Farg) O2 O-O 1.28h) N2 N-N 1.12i) NH3 N-H 0.95

The calculated adsorption energy for both molecular andatomic configuration of O2, N2, and NH3 on the AGNRsurface for various position with the SV defect are listedin Table III. It can be seen that by exposing O2, N2, andNH3 molecules on the SV defect introduced the endothermicprocess since all the adsorption gas values are positive. Thevalues of the adsorption energy achieved indicates strongchemisorption and weak physisorption due to the values arelarger than 1. Furthermore, the atomic O2 adsorption inAGNR on the SV defect exhibit is between chemisorptionweak and strong physisorption. In general, larger absolutevalue of the adsorption energy, Ea means stronger bindingbetween gas molecule and graphene [13]. Fig 3 shows the

(a) O2 at on (b) N2 at on (c) NH3 at on

(d) O2 at near (e) N2 at near (f) NH3 at near

(g) O2 at far (h) N2 at far (i) NH3 at far

Fig. 3. Side view of the most stable configuration of O2, N2 and NH3 locatedat the on, near and far from SV defect.

TABLE IIOPTIMIZATION GEOMETRIES OF GAS MOLECULES AND DEFECTIVE AGNR

IN ATOMIC ADSORPTION CONFIGURATION

Position Gas Bond Bond Length, Å θ

On

O2

C1-O1 1.48 77.71oC2-O2 1.39 128.87o

Near C1-O1 1.58 98.42oC2-O2 1.60 101.92o

Far C1-O1 1.52 93.51oC2-O2 1.46 93.32o

On

N2

C1-N1 1.47 94.42oC2-N2 1.66 72.79o

Near C1-N1 1.64 80.34oC2-N2 1.58 89.22o

Far C1-N1 1.62 67.14oC2-N2 1.61 74.42o

On

NH3

C-N 1.33 91.35oC1-H1 1.16 73.93oC2-H2 1.14 126.45oC3-H3 1.15 81.54o

Near

C-N 1.39 92.54oC1-H1 1.08 102.31oC2-H2 1.03 104.24oC3-H3 1.10 81.25o

Far

C-N 1.47 85.06oC1-H1 1.20 57.52oC2-H2 1.07 67.00oC3-H3 1.12 130.12o

most stable configurations near the SV defect which revealall gas molecules achieved high adsorption energy among theother two positions. For example, O2 molecule, the most stableconfiguration is O2 adsorbed on C-C bond site of the carbonring with Ea= 37.66 eV and the molecular distance of 2.17Å. Similarly for N2 molecule, the most stable configurationis on the C-C bond site of the carbon ring with Ea= 24.91eV and the molecular distance of 1.74 Å. While, for NH3

molecule, the most stable configuration is NH3 adsorbed on

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TABLE IIIVALUES OF GAS ADSORPTION, Ea (EV) FOR O2 , N2 , AND NH3 IN THE

THREE POSITIONS OF GAS IN THE MOLECULAR AND ATOMIC ADSORBEDIN DEFECTIVE AGNR.

Position Gas Configurations O2 N2 NH3

On Molecular -0.53 -0.86 -8.44Atomic -7.66 -6.77 -68.72

Near Molecular 37.66 24.91 27.94Atomic -2.12 2.24 8.03

Far Molecular 12.00 19.42 69.20Atomic 2.52 2.8 7.06

C-C bond site of the carbon ring with Ea= 27.94 eV andthe molecular distance of 1.79 Å. From the presented result,it can be conclude that the strength of the adsorption of thegas molecules is dependent on the position its attached on theAGNR surface with the SV defect. For example O2 moleculesachieved the highest adsorption energy when adsorbed onand near the SV defect. In contrast, NH3 molecule exhibitgreater energy when it is far from the SV defect with Ea=69.20 eV and the molecular distance is 1.46 Å. It is worthmentioning that the adsorption energy of NH3 obtained inthis study is higher than those discovered by O. Leenaerts etal. [14] suggesting the role played by the adsorption positionwith the SV defect. On the other hand, the atomic interactionbetween O2, N2, and NH3 and AGNR with SV defect in allpositions investigated show unfavorable value of adsorptionenergy. The same kind of observation is also achieved byJunkermeier et al. [15] where they noted that NH3 adsorbednear to the primary defect at certain site of graphene. As aresult, molecular configuration can influence the interactionbetween gas molecule and defective AGNR especially NH3

molecule. Consequently, AGNR with SV defect is favorablein detecting NH3 gas.

In order to investigate the charge transfer between O2, N2

and NH3 molecules on the AGNR surface with SV defect, theMulliken population analysis of charge distribution was usedto determine the interaction between the gas molecules andAGNR with the SV defect. The capability of the adsorbedmolecules as a basis of relative electron-withdrawing or -donating can be observed based on the trend of charge transferare listed in Table IV. Molecular NH3 adsorption in on, nearand far positions exhibit high value of charges (0.84 e to0.90 e) that were transferred from molecules to the graphenesurface. The results suggest the molecular NH3 acts as a donorwith relative strong electron-donating molecule (N-type). Thisis consistent with previous theoritical and experimental workdemonstrated earlier [13], [14], [15], [16]. However, there wasa slight charge transfer of 0.09 e to 0.37 e from graphene toNH3 in atomic configuration which reveal that NH3 in thisconfiguration can acts as an acceptor to graphene. Addition-ally, atomic N2 in position near the SV defect has capability ofadsorbing molecules in strong electron-withdrawing (P-type)due to the high generated charge (2.28 e) that were transferfrom graphene surface to the molecules. Similar behavior isobserved for O2 molecules where the charge transfer value

achieved is comparable to N2 molecules. Therefore, it canconclude that NH3 act as donor while O2 and N2 as acceptor.

TABLE IVVALUES OF CHARGE TRANSFER, 4Q (E) FOR O2 , N2 , AND NH3 IN THETHREE POSITIONS OF GAS IN THE MOLECULAR AND ATOMIC ADSORBED

IN DEFECTIVE AGNR.

Position Gas Configurations O2 N2 NH3

On Molecular -0.98 -0.84 +0.88Atomic -1.39 -1.48 -0.09

Near Molecular -1.86 -1.61 +0.90Atomic -1.64 -2.28 -0.37

Far Molecular -0.59 -0.86 +0.84Atomic -0.94 -1.88 -0.09

IV. CONCLUSIONS

The interaction of O2, N2 and NH3 gas molecules ondefective AGNR for molecular and atomic adsorption wasstudied. The strength of the gas molecules adsorb towardsthe SV defect on AGNR is determined by calculating itsadsorption energy after optimized the position of adsorbatedmolecules on graphene. After full relaxation, it is found thathighest energy is achieved for atomic O2 adsorption locatedon the SV defect. Meanwhile, molecular O2 adsorption ishighest when located near the SV defect while molecularNH3 adsorption is stronger when the molecule is placed farfrom SV defect. The calculated charge transfer shows thatmolecular NH3 acts as a donor while atomic NH3, O2 andN2 acts as acceptor. This study demonstrated that moleculesNH3 are ideal candidate for molecular doping for grapheneand its became N-type semiconductor after NH3 moleculesadsorption.

ACKNOWLEDGMENT

The authors would like to acknowledge the financial supportfrom Ministry of Higher Education (MOHE) under projectR.J130000.7823.4F620. Also thanks to the Research Man-agement Centre (RMC) of Universiti Teknologi Malaysia(UTM) for providing excellent research environment in orderto complete this work.

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