Small transition metal cluster adsorbed on graphene and graphene nanoribbons: A density functional based tight binding molecular dynamics study

Accepted Manuscript Small transition metal cluster adsorbed on graphene and graphene nanoribbons: A density functional based tight binding molecular dynamics study Michael Rivera Mananghaya, Gil Nonato Santos, Dennis Yu PII:

S1566-1199(18)30496-8

DOI:

10.1016/j.orgel.2018.09.031

Reference:

ORGELE 4897

To appear in:

Organic Electronics

Received Date: 23 August 2018 Revised Date:

25 September 2018

Accepted Date: 25 September 2018

Please cite this article as: M.R. Mananghaya, G.N. Santos, D. Yu, Small transition metal cluster adsorbed on graphene and graphene nanoribbons: A density functional based tight binding molecular dynamics study, Organic Electronics (2018), doi: https://doi.org/10.1016/j.orgel.2018.09.031. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Small Transition Metal Cluster Adsorbed on Graphene: A density functional based tight binding molecular dynamics study Michael Rivera Mananghaya 1, * Gil Nonato Santos 2 Dennis Yu 3

School of Science and Engineering, Ateneo de Manila University, Katipunan Ave, Quezon City, 1108 Metro Manila, Philippines. 2 Physics Department, De La Salle University, 2401 Taft Avenue, 0922 Manila, Philippines. 3 Chemical Engineering Department, De La Salle University, 2401 Taft Avenue, 0922 Manila, Philippines. *Corresponding author. E-mail: [email protected], +632.915.326.1245

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Small Transition Metal Cluster Adsorbed on Graphene and Graphene Nanoribbons: A density functional based tight binding molecular dynamics study

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Michael Rivera Mananghaya 1, * Gil Nonato Santos 2 Dennis Yu 3 1 School of Science and Engineering, Ateneo de Manila University, Katipunan Ave, Quezon City, 1108 Metro Manila, Philippines. 2 Physics Department, De La Salle University, 2401 Taft Avenue, 0922 3 Chemical Engineering

Manila, Philippines. Department, De La Salle University, 2401 Taft Avenue, 0922 Manila, Philippines.

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*Corresponding author. E-mail: [email protected], +632.915.326.1245

the systemic study of the electronic properties of different transition metals (TMs-Sc, Ti, Fe, Co, Ni, Zn, Ag and Au) adsorbed in the surface of graphene was done with the aid of self-consistent charge density functional based tight binding method. Results show that the Silver metal adsorbed in the surface of graphene can open its gapless bandstructure. In addition, a single-gated field effect transistor based on Ag-adsorbed on zigzag graphene nanoribbon (zGNR) can act as a potential semiconductor for modern electronic applications. An important feature is that the Ag does not break the structure of zGNR on adsorption. Further, the resulting Ag/zGNR energy band gap is inversely proportional on the dimer lines across its width as predicted by tight-binding calculations.

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Abstract—

Index Terms— Binding Enthalpy, Density Functional Theory Tight Binding, Graphene nanoribbon, Transition Metals.

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1 INTRODUCTION

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Graphene can be considered as an infinitely large aromatic molecule. It has attracted much interest due to its many exceptional properties [1-23] for example in electronics; because of their rather wide spread characteristic, physical and chemical properties and high specific surface area. Graphene also has an abundant pore volume and its porous carbon is considered as a good adsorbent. For conventional graphene, there are nonetheless many factors to be considered first before it can be used practically in electronic applications. One such factor is to modify theoretically and experimentally its electronic properties such as opening its gapless bandstructure. Undoped graphene is fully metallic within its zigzag orientation. The zigzag orientation however, can be a semiconductor or a metal with varying electronic character that is dependent on the width. In general, graphene nanostructures reactivity and electronic properties may be tuned through immobilize species such as transition metal (TMs). Doping of graphene can be an efficient way achieve this, which are strongly dependent on the TMs absorbed [20]. The immobilization scheme is not only very promising but also the bandgap is quite dependent on the TM-graphene ratio [18, 20]. The incorporation of TM atoms into the honeycomb lattice leads not only to the potential opening of its gapless bandstructure around the Fermi level but also the chemical activation of its rather passive surface. In addition, upon adsorption of transition metal impurities, the binding strengths of TMs with graphene are quite comparable to its cohesive energy such that adsorption is relatively favorable. Although a competition may exist between covalent binding and strong metal-metal cohesion upon adsorption over the TM decorated graphene, the later accounts for small amount

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of transition metal clusters [20]. The TM-doped materials was studied recently, it is stressed out that the characterization of the electronic effects that doping has on graphene is difficult to qualitatively and quantitatively determine. These problems arise mainly because of sample impurities and cluster concentrations. The characterization comprises the direct probing of the dopants concentration in the graphene and the overall effects the dopants has on its properties. The changes incurred in the electronic structure of graphene due to the introduction of TM in the lattice can be identified using density functional theory (DFT). It is possible that the resulting TM-doped sheets are metallic with no apparent band gap or semiconducting depending on the TM adsorbed [515]. The incorporation of transition metals into graphene affords structures with the ability to participate as a semiconductor in most modern electronic applications.

The aim in this work is to investigate theoretically the structural geometry, energetic stabilities, and electronic properties of TMchemisorptions for graphene with different TMs (Sc, Ti, Fe, Co, Ni, Ag and Au) impurities in small cluster concentration using the density-functional theory tight binding (DFTB) method. The DFTB approach is an efficient approximate version of the DFT method. The unit cell that comprises the actual cell is typically in the order of hundreds of atoms to mimic true experimental conditions; this forbids the direct application of the primitive DFT methods. The DFTB however, offers fast and reliable results comparable to the native DFT and it accounts for the quantummechanical phenomena and dispersion energy.

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(a)

Fig.1. Optimized geometry of (a) coronene with Sc (highlighted in red) atom as a typical example for C and Sc parametrization and a (b) large graphene cell populated with Scfor DFTB analysis. Gray color depicts Carbon while red is Sc atoms and white is hydrogen.

D4TM (Å) 2.984 2.533 2.623 2.482 2.859 2.771 2.926 2.846

DTM-G (Å) 1.875 1.921 1.923 1.930 1.932 3.515 3.265 3.350

DC-C (Å) 1.425 1.425 1.428 1.428 1.427 1.421 1.421 1.421

Eb (eV) 2.256 2.234 0.811 1.101 1.112 0.223 0.296 0.321

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DTM-TM (Å) 2.273 1.966 1.991 2.105 2.111 3.148 2.583 2.488

CTM (e) 0.262 0.258 0.082 0.119 0.121 0.019 0.033 0.025

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TM (Å) Sc Ti Fe Co Ni Zn Ag Au

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Table 1. Average TM–TM distances (DTM–TM for diatomic and D4TM for a tetrahedron of TM), TM–graphene distances (DTM–G), carbon to carbon distance near the TM cluster (DC–C), binding enthalpy (Eb) of an individual TM on graphene, calculated charge transferred from TMs to the nearest C atom (CTM).

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nanoelectronic devices. Theoretically, the electronic structures of GNRs is deeply rooted on the width and crystallographic orientation. Specifically, the bandgaps of the zGNRs is quite sensitive not only on the TM adsorbed but also on the number of dimer lines denoted by N. The final aim of this work is the evaluations of the possibility that these TM decorated nanoribbons can be utilized as a semiconductor in electronics as a single-gated field effect transistor (FET). Specifically, to clarify the intricate roles of the width in Ag functionalized zGNR in its energetics and bandgap modifications for FET applications. Here, we study electronic structures of the Ag/zGNRs as a function of its width.

Two dimensional (2D) graphene sheets, when cut into rectangle slices, can become one-dimensional (1-D) semiconductors with unusual electron states called graphene nanoribbons (GNRs). Graphene nanoribbons (GNRs) commonly synthesized using bottom-up synthesis exhibit different electronic properties mainly localized at its edges. The approach was successful in producing seven dimer lines across its width as one of the first smallest nanoribbon. Ever since, GNRs with different widths, edge orientations and incorporated metal dopants has attracted numerous scientific investigations primarily due to its remarkable mechanical, electronic and magnetic properties. The GNRs are classified into two groups depending on their edge shape namely, armchair and zigzag nanoribbons (zGNR). The zGNR are predicted to be key nanomaterial in the development of modern

2 METHODOLOGY The atomic positions of graphene were optimized and a single TM atom is added such as in Fig. 1(a) for Scandium, the standard procedure of parameter generation was imposed [24-26]. The minimal atomic valence basis for all atoms including polarization functions when needed from GAMESS code [27] were applied in the parametrization scheme within the frozen-core approximation. The short-range repulsive pair-potential was fitted to the results from DFT gradient-corrected Perdew-Burke-Ernzerhof (PBE) functional [28] employing an empirical dispersion part with 6-311G (d, p) basis set. The importance of DFT+dispersion corrections was assessed against reference interaction energies. Specifically Grimme's method [29] provided sufficiently accurate results. In addition, Grimme's method is proven successful due to the incorporation of dumping function leading to an accurate binding at short distances and correct energetics. Structures were minimize until the force on each atom during relaxation was less than 0.03 eV/Å. Regardless of the initial position the same final optimized structure was obtained, the transition metal Sc prefers to sit in the hole site of the graphene structure. Next, the electronic properties were studied using the DFTB formalisms. The dispersion interactions are included by employing a nonlocal van der Waals (vdW) to address the cross interactions. Hubbard corrections are applied to the localized d electrons

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Charge (e)

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Binding enthalpies (eV)

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Fig.2. Average Binding enthalpies of a single TM atom, a small cluster of TM-TM pair and a TM4 on graphene as a function of average charge transferred.

(b) Ti

(c) Fe

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(a) Sc

(e) Ni

(f) Zn

(g) Ag

(h) Au

Fig.3. Density of States for (a) Sc (b) Titanium (c) Iron (d) Cobalt (e) Nickel (f) Zinc (g) Silver and (h) Gold on single layer graphene. The density is for a 1:1:1 adsorption ratio of a TM atom, a small cluster of a TM-TM pair and a TM4 on graphene. The partial Density of States is given in terms of d (green), p (red) and s (blue) orbital.

ACCEPTED MANUSCRIPT cially. Seven Sc atom, which corresponds to a 73 wt%, is evidently an excellent choice to mimic the actual adsorption as seen from Fig. S2(b). Energy optimization using periodic boundary conditions was carried out using the conjugate gradient method to locate a suitable configuration with a minimum potential energy. In total, the adsorption energetics were considered throughout the simulations, the binding enthalpies (Eb) between the TM and graphene is calculated using the equation,

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Eb = ( ∆ HTM + ∆ HGraphene - ∆ HGraphene+TM) / n

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(1)

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where ∆ HTM denotes the total enthalpy of the relaxed adsorbed TM, ∆ HGraphene is the total enthalpy of the graphene, ∆ HGraphene+TM is the total enthalpy of the TM adsorbed on the surface of the graphene system. The system is normalized with the number of n TM adsorbed in the graphene surface. With the given definition, an Eb > 0 denotes a thermodynamically stable adsorption. The Fermi levels of the density of states (DOS) were reset at the zero eV position. The Non-Equilibrium Green’s Functions formalism (NEGF) is the theory used here for simulating the electron transport through the Ag/zGNR with open boundary conditions. It is treated as a one-dimensional scattering phenomena in the channel region for electrons coming in from the source towards the drain. The selfconsistent Green’s function ( ) of the scattering region situated at the channel is obtained by ( − )( ) = where is the overlap matrix, is the Hamiltonian and is the identity matrix. The Hamiltonian is composed of the source lead, the channel region and the drain lead expressed as

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of the TMs. The DFTB [30] code is free for both non-commercial and commercial research purposes and made available from the DFTB.org website. The DFTB methods implemented in GAMESS involves Broyden charge mixing to relatively increase the convergence of self-consistent field calculations. The derivatives of the Hamiltonian and overlap matrix elements were calculated numerically in the full DFTB calculations. This gradient calculations of the Hamiltonian matrix are presented in the supplementary section S1, whereas other derivatives were computed analytically. The computational task of calculating the overlap matrix is minimized if the basis set is properly chosen. This is recommended to be as small as possible yet it should provide results which are in good agreement with values obtained directly with much larger basis sets. To this end, the minimum Slater type valence orbital basis set was applied for a typical C-Sc set developed for graphene with the first row transition metal element Scandium. The expansion coefficients of the respective Slater type orbitals is given in the parameter set “Sc-C” file in bold text. From supplementary Fig. S1(a) coronene uses delocalized valence electrons to form a bond with a TM such as Sc. Planar Polyaromatic hydrocarbons with only six-membered rings, such as coronene provide molecular models of graphene sheets [31]. The potential curves for the coronene dimers will aid the development of new force fields and potential energy functions that are computationally efficient and capable of modeling large graphene sheets. Apparently, the structures and interaction potentials of coronene can act as a good prototype for graphene material simulation. The methods to be used for computational materials design of graphene interactions with metals should be validated for smaller clusters such as coronene in order to obtain reliable prediction of interaction energies. Strong interaction exists between p orbitals of C atoms and the d orbitals of Sc due to their hybridization with each other. The bond is dominated by C(p)-Sc(d) σ (0.434 Å, -11.2eV) and π (0.677Å, -14.8eV) along with C(p)Sc(p) σ (2.15 Å, -6.75 eV) resulting to an average bond length of 1.875 Å for Sc-C. Noticeably no bond is detected for C(s)Sc(d) σ . In Fig. S1(b) the four Sc cluster has an interaction dominated by Sc(p)-Sc(p) σ at (2.8 Å,-10.8 eV). The DFTB simulations was carried out by placing the Sc in the rectangular box with the graphene having a dimension in x-, y- of 14.759 Å. The current parameters works quite well for geometries, for which the bond length differ from PBE/6-311G (d, p) at the accuracy of 0.1 Å while the bond angle differ by 10 degree and the corresponding energetics differ about 15 kcal/mol on the average. The Graphene was built by cleaving the graphite structure with P63/MMC space group. The graphene unit cell consisted of two carbon atoms having a lattice parameter of a=b =2.46 Å where a 6x6 supercell was created. A vacuum slab of length 30 Å was used along the z axis to prevent spurious interaction with its own periodic image. The size of the rectangular cell was determined by performing ordinary energy optimization schemes as depicted in supplementary Fig. S2(a). Three kinds of initial configurations were identified in graphene structure that possess high symmetry namely Hollow site (H), Bridge site (B) and atop site (T). The n in the n x n graphene sheet with a Sc metal were incrementally increased from n=1 to 10, wherein n=6 is the most stable. Consequently, the 6x6 sheet was populated by different concentrations of the TM (Sc) in wt%. The wt% is the abbreviation of percentage by mass which is often called percentage by weight commer-

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here H and S is the basis of non-orthogonal atomic orbitals. The three diagonal matrix blocks correspond to the H and S of individual parts that consists of the source lead, the channel region and the drain lead, while the off-diagonal blocks stand for the interaction between these regions. The size of these matrices is infinite because of the semi-infinite electrodes. However, the system can be transformed to a finite one by considering the electrodes to be periodic with the proper boundary conditions. The transmission function ( ) derived from the Green’s function ( ) and the coupling matrices Γdrain ( ) and Γsource ( ) as ( ) = [ ( )Γdrain ( ) ( )Γsource ( )]. The electrode temperature is set to 298 K. The electrodes are calculated with a Monkhorst–Pack 25 × 1 × 50 and the central region at 25 × 1 × 1 kpoint grid. 3

RESULTS AND DISCUSSIONS

3.1 Transition Metal-adsorbed on graphene.

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The binding enthalpies (Eb) of an individual TM on graphene as defined above is all greater than zero corresponding to a stable configuration and indicates bonding. In Table 1: the calculated charge transfer from TMs to the graphene range from 0.019 e (Zn) to 0.262 e (Sc). When the adsorbed TMs possess a smaller atomic radius, such as Sc the charge transfer density provided by per TM atom is generally big over the graphene surface. In fact, given a transition metal, the Eb and the charge transfer density have an approximate relation, as shown in Fig. 2(a). The 3d transition metals are adsorbed more strongly on graphene than Ag/Au, except for the case of Zn, due to the generally larger charge transfer from the 3d TMs to graphene surface. Partially cationic character of the TMs results due to the charge transfer, and thus it can facilitate the adsorption of foreign species such as small molecules [36-50] for gas storage applications. To understand the nature of binding between the metal atom and the graphene surface, the partial density of states (PDOS) is calculated. The calculated PDOS for Sc, Ti, Fe, Co, Ni, Ag and Au atom doped graphene are provided in Figure 3(a)-(h), respectively. The binding states just below the Fermi level E-Ef is mainly attributed to the TM d orbital are hybridized with p of carbon atom for Sc, Ti, Fe, Co, Ni. For the transition metal elements Fe, Co, Ni, the binding enthalpy increases with an increasing number of delectrons. The bond enthalpy here is increasing, but this state is due to localized non-bonding situated at the Fermi level with the corresponding DOS that is quite large. There is low bond enthalpy for Ag, Au due to the fact that all of its d-orbitals are occupied. Furthermore, the bond enthalpy is so small for Zn metal because the s-orbital is close enough to the d-orbital. In addition, there is negligible difference in the adsorption energy of the three adsorption H, B and T sites when the Eb is small. In the case that the bond distance between the TM and graphene is large, the binding enthalpy is reduced. In the Zn, Ag, Au case, the longer bond distance is evident, the TM cluster shows physical adsorption behavior. The band hybridization for Ag and Au adsorption with DTM–G > 1.932 Å is quite negligible. No bond is detected however for Zn. The band gap is 0.201 and 0.190 eV for Ag and Au adsorption, respectively. Whereas the bond distance is shorter for Sc, Ti, Fe, Co, Ni, the bond enthalpy tends to increase with features common to chemical adsorption. Strong band hybridization takes place between graphene and these adsorbed TM atoms with DTM– G < 1.932 Å, confirming the formation of a covalent bond. Apparently, silver has the strongest capability to open the band gap of graphene, followed by Au. The opened band gaps by Ag/Au adsorption is comparable to the theoretical band gap opened by subjecting the graphene surface in a uniform electric field [49, 51-58].

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From the prescribed number of Sc atoms mentioned in the methodology section, twenty-five types of TM doped graphene were considered each having the same wt%. Figure S3(a)-(e) enumerates five of the configurations with the lowest energy accompanying the functionalization of Sc on the graphene surface. It is notable that the system prefers to have a 1:1:1 ratio for adsorption namely, one Sc atom, one small cluster of Sc-Sc pair and a Sc4, which is a central cluster, consisted of a tetrahedron of Scatoms. The preferred coverage is highly dependent on the competition between the decreasing metal-graphene and the increasing metal-metal interactions with increasing coverage. It can be argued that there is a decreasing metal-graphene interaction with increasing TM coverage, attributed to the reduction of charge transfer. While the increasing metal-metal interaction with the increasing coverage is due to the formation of a metallic bond. These 1:1:1 combinations were found in a larger TM/graphene system simulated in Fig 1(b). The graphene units considered has a dimension of ~73.76 x 73.76 Å, enough to simulate true experimental conditions [30, 31]. To analyze the bond lengths and bond angles, all geometries were optimized at the same level of theory, including bare graphene as a reference. The bond lengths calculated by DFTB for the C-C in graphene are~1.421–1.428 Å tabulated in table 1. A typical (sp3)C—C(sp3) bond length in the graphene surface is 1.420 Å. Based on the comparison of the C-C bond of graphene, the structure with TM impurity has in general an almost the same C–C bond lengths. When a TM atom is adsorbed on graphene, initial configurations were considered: (1) The TM is located in the geometric center of six C atoms in the H site of the graphene surface, (2) the TM is attached to the T site above a Carbon atom and (3) in-between the C-C bond labeled as B site. Nearly irrespective of the coverage, all the transition metals favor to orient itself near the hollow site, which are the most common after optimization in Fig. S3(a)-(c) followed by small amounts of bridge site in Fig. S3(d) and Fig. S3(e) [31-35] instead of the T site as observed in multi layered graphene. Although the present study investigates graphene interacting with various TM, for conciseness only Sc interacting with graphene results are presented here as it shows all the essential tasks for parametrization scheme and optimization. For a typical example, by calculating the Scadsorption on graphene, a discussion of the transition metal–graphene junction is carried out, because the Sc, Ti, Fe, Co, Ni atom does not break the structure of graphene on adsorption. For example, in Fig. S4 shows that Sc disturbs the structure of graphene on adsorption at the B- and Tsite. However, no distortion of the graphene structure from the Hsite adsorption which corresponds to the most stable adsorption site. It is important for the growth of a small transition metal cluster on a semiconductor that the dopant does not cause significant distortions on the substrates surface. Interestingly, for Au and Ag adsorption the surface also remains undisturbed which is expected since the adsorption enthalpy is lower with less interaction with the surface of graphene, to be discussed shortly. The five most stable configurations of TMs adsorbed on graphene is illustrated in Fig. S3(a)-(e) and are characterized by forming multiple anchorage from the graphene surface. The average TM– graphene distances (DTM–G) range from 1.932 (Ni) to 3.515 (Zn) Å as shown in table 1. Although relatively few, the metal adsorption at the B- and T-site causes a slight buckling of the graphene

Surface adsorption of TMs in graphene is a key to open its band gap with associated electronic states. In Figures 3(a)–3(h), it can be seen from the PDOS of different TM doped-graphene that all exhibit metallic nature, except for Ag and Au. Upon adsorption of

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Here, the present study investigates zGNRs with a width of sixteen. In the two-probe measurement technique, the pure zigzag GNR act as electrodes to obtain the conductivity of the Ag/zGNR system. In order to effectively connect the source and the drain, the channel must be of sufficient length. The channel length is for pure zGNR device consists of nineteen primitive unit cells of ZGNR. In order to remove the effects of dangling bonds, hydrogen atoms that are not shown here are incorporated at the top and bottom edges of the zGNR. The zGNRs structure presented here has relatively stable U-type edge defects present. The zGNRs structure with U-type defects in its edge are relatively stable [48]. There is a negligible deformation of the carbon atoms after structural optimization. Specifically, the deformation of the U-type defected zGNRs in the process of structural optimization is relatively small compared to pure ZGNRs. There is zero gap for the zigzag nanoribbon simulation in Fig. 4(a) as expected. By contrast, in the transmission spectrum of the device in Fig. 4(b) for the Ag case a gap of 0.501 eV is established, which originates partially from the gap of the PDOS of the channel Ag/graphene, as show in Fig. 2(g). The transmission coefficient of the Ag/graphene device is ultimately connected with the PDOS of the channel. As discussed in elementary quantum mechanics the coefficient is a well-known property that is proportional to the PDOS of the channel. A greater band gap further emerges due to the reduced width brought by the sixteen-dimer line along the nanoribbon width. The zigzag nanoribbon exhibit no scattering in the incoming wave function detected and it is able to reach to the other end of the probe. By contrast, upon the introduction of Ag clusters, the transport gap is quite visible, the transmission eigenvalue vanishes upon Ag adsorption, and wave function from the left is scattered and unable to reach the right side to the probe. Experimentally, clustering of transition metal atoms such on graphene surface is frequently observed. Therefore, in the study of TM atom super lattice at Ag-Ag pair and a small Ag4 cluster, the band gap introduced is not significantly degraded even if the adsorbed TM atoms form clusters.

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the metal, certain impurity states are introduced into their band structures, rendering the electronic properties to change to different degrees that are strongly dependent on the adsorbed TMs, which is dependent on the d orbital hybridization of TM with the p orbital of carbon present in the surface of graphene. For example, when Sc, Ti, Fe, Co, Ni are adsorbed, metallic nanocomposites emerges as no band gap is detected. For Zn near the surface of the carbon based material a gapless density of state is also observed. The changes in DOS of the graphene upon adsorption of Au and Ag are also evident by the charge transfer between the TM and graphene. In Table 1, we list the calculated result of the population analysis, the charges transferred from TMs to graphene is 0.025 e (for Au) and 0.033 e (for Ag). Interestingly, for Ag and Au adsorptions the influences on electronic properties of the graphene are completely different: for the Ag adsorption, the composite changed into a semiconductor while Au has a smaller bandgap. This means that semiconducting devices with tuned electronic properties is realizable. This can be done by surface functionalization of Au or Ag. Pristine graphene has linear dispersion around the Fermi level and introduction of dopants breaks the symmetry in graphene. In Fig.3(a) to (e) the corresponding Density of states shows that the electronic states near the Fermi are mainly attributed to the d orbital (green) interacting strongly with the p (red) states in the carbon atoms of the graphene sheet in a well-coordinated fashion. However, for the Au/Ag case in Fig.3(g) and (h) the d and p orbitals do not interact appreciably near the Fermi. The resulting energy gap is established due to poor d orbital interaction of the Au/Ag at the K point. Intuitively, for the occupied Valence Band Maximum, the change is predominantly on the Au/Ag cluster, whereas for the Conduction Band Minimum, the charges is on the graphene sheet. Charge transfer density for Au and Ag to graphene causes a perpendicular built-in electric field and opens the band gap [50]. Further, this can be attributed to existence of an extra mechanism, namely, the breaking of bond symmetry, beside the well-known mechanism of the breaking of the inversion symmetry. The transport property of Ag doped nanoribbon follows in detail in the next section. 3.2 Transport properties of Ag doped graphene nanoribbon.

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The final objective is to calculate the rate at which electrons are transferred from the source to the drain by propagating through the channel region. The transport properties of GNR before and after Ag adsorption is simulated next. Theoretically, the electronic structures of graphene can be modified by cutting it to rectangle slices, a one-dimensional nanoribbon semiconductor with unusual electron states localized at its edges. Although it is universally recognized that all zigzag GNRs are metallic without an energy gap, the addition of Ag may significantly modify the gap as seen previously for Ag/graphene. Accordingly, based on the tight-binding investigations the band gap of Ag/zGNRs is just inversely proportional to the width. A FET device consisting of three main parts is shown in Figure 4(a); source, channel, and drain. The source and drain are modeled as ideal zGNRs representing the pristine case as depicted in Fig. S4. The number of atoms in the direction of the arrow gives the width of the zGNR.

It has been demonstrated that armchair GNRs can be divided into three groups, i.e., N = 3p, N = 3p + 1, and N = 3p + 2 (here p is an integer), according to their electronic structures, exhibiting three distinct behaviors [48]. The energy gaps for the three groups are quite different even with the same p. The

observed gaps as a function of N grouped into three categories predicted by DFTB calculations. In Fig. 5, it is quite interesting that here in this study the systems bandgap shows only two distinct behavior (N = 2l, 2m, l is even, m is odd). It is obvious that the proposed Ag/zGNRs opens the bandgap in two ways compared to the intrinsic zGNRs, which is gapless. As shown in Fig. 5 a systematic study of the band gap and energy level alignment of zGNRs is presented, starting from the smallest possible zGNR with N=8 to its next immediate wider structures. The result demonstrated explicitly that the energy gaps of these decoupling zigzag GNRs doped with Ag are about the expected gaps. The bandgap is proportional to N-1 for the two groups established and further confirms that the origin of the energy gaps for zGNRs is quantum confinement [54].

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Band gap in eV

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Fig.4. Two-probe model of (a) 8, 0 zigzag nanoribbon and (b) small Ag clusters adsorbed in the same zigzag nanoribbon. Gray ball: C; light blue ball: Ag with the corresponding transmission spectrum situated in the right side. The isovalue is 0.2 a.u.

Dimer width N Fig.5. The band gaps of Ag/zGNRs system as a function of number of dimer lines along the nanoribbon width grouped into two

categories.

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The electronic properties of graphene in the presence of TM impurities were studied using density functional theory thigh binding method. The chemical functionalization of TMs (Sc, Ti, Fe, Co, Ni, Ag and Au) is thermodynamically stable except for Zn. Finally, it is verified through DFTB that surface adsorption of small Ag cluster is able to effectively open a band gap in zigzag nanoribbon and the gap can be tuned by simply adding or cutting one dimer line along the ribbon width. Therefore, a proposed nanomaterial composed of Ag-adsorbed transition metal in a zigzag graphene nanoribbon qualifies as a single-gated field effect transistor device, which demands extensive experimental research in the future.

Acknowledgment

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Deepest appreciation to Dr. Raphael A. Guerrero for hiring Dr. Mananghaya as a faculty member of Ateneo de Manila University. Also to Dr. Emmanuel Rodulfo, Dr. Raymond Tan, Dr. Joseph Auresenia, Dr. Kathleen Aviso and Dr. Michael Angelo Promentilla for the entire valuable support and advise.

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ACCEPTED MANUSCRIPT Transition metals (TMs-Sc, Ti, Fe, Co, Ni, Zn, Ag and Au) adsorbed in the surface of graphene. Adsoprtion was done with the aid of density functional based tight binding method.

A single-gated field effect transistor based on Ag-adsorbed on zigzag graphene nanoribbon (zGNR)

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The resulting Ag/zGNR energy band gap is inversely proportional to its width.

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Silver metal adsorbed in the surface of graphene can open its gapless bandstructure.