Article pubs.acs.org/JPCC
Energy Gap Tuning of Graphene Layers with Single Molecular F2 Adsorption Farzaneh Shayeganfar* Engineering Physics Department and Regroupement québécois sur les matériaux de pointe (RQMP), Polytechnique Montréal, Montréal, Québec H3C 3A7, Canada ABSTRACT: Structural and electronic properties of adsorption of single fluorine molecular (F2) on graphene and between bilayer graphene were investigated using firstprinciples DFT calculations. The broken symmetry of graphene layers because of charge transfer between adsorbate and substrate and existence of sp2 and sp3 orbitals of C−C and C−F bonds introduces the energy gap whose magnitude of gap opening depends strongly on its atomic arrangement of adsorbate relative to surface. We demonstrate that fluorine can be adsorbed on graphene with two types of chemisorption and physisorption mechanism with regard to its atomic configuration on substrate, both in-plane and out-of-plane molecular orientation. Significant property found for the adsorption of fluorine molecule between bilayer graphene is the existence of linear behavior between dipole moment and energy gap. Electronic properties of this adsorption suggest that with inducing and controlling of band gap of graphene layers the choice of functionalized graphene can be applied for future nanoelectronic devices.
I. INTRODUCTION Graphene and bilayer graphene (BLG) with zero band gaps are unsuitable for graphene-base nanoelectronic devices. Molecular doping and external electric field application1−5 are two strategies to induce electronic energy gap in graphene and BLG because of breaking inversion symmetry and creating of potential difference between graphene layers.2,6 Such perpendicular electric fields can be generated with an external gate potential in field effect transistors,1,7,8 with organic molecular adsorption,9,10 self-assembled monolayers,11,12 and subjecting of graphene layers with to different strains.13 Adsorption can lead to net charge transfer between adsorbate and substrate; for instance, the Fermi energy level of single graphene layer by adsorbing of potassium monolayers shifts more than 1 eV because of electron donation of adsorbate.14 Halogen molecules such as bromine molecule Br2 can create high hole doping at adsorption on single graphene layer by electron transfer to adsorbate.15 Chen and his coworkers in their recent paper,16 by using an encapsulated graphene single layer between hexagonal boron nitride (h-BN) layers, could be a monitor of the adsorption and charge transfer of Br2 on graphene. The strong seal of graphene and BN prevents the creation of significant hole doping on graphene and also diffusion of Br2 on both sites of graphene.16 Hence, with new design of BLG sandwiched between hexagonal boron nitride (h-BN/BLG/h-BN), one can experimentally study the adsorption and diffusing of halogen molecules between BLG. Recently, the fluorides have attracted a considerable research interest in field of energy conversion and storage, such as their applications in the transparent electrodes for solar cells,17 and in high energy lithium batteries.18−22 The novel electrochemical properties such as high transport and thermal stability of carbon fluorides and recently fluorinated BLG have extended © XXXX American Chemical Society
Figure 1. Different adsorption sites of F2 on graphene and between bilayer graphene, both in-plane and out-of-plane molecular orientation such as center, cross, top, and bridge.
new potential application for nanoelectronic graphene-based devices.23 In the present work, we try to understand profoundly the adsorption mechanism and electronic band structure properties of fluorine molecular F2 on single and between BLG by means of first-principles calculation based on density functional theory (DFT). To understand the adsorption and chargetransfer process, we compare the properties of F2/GE with isolated graphene and F2@BLG with the pristine BLG. The stability, electronic structure, and adsorption profile of this molecule adsorption with different configurations of in-plane and out-of-plane molecular orientation were analyzed in detail. We find that adsorption of F2@BLG results in asymmetric charge distribution between two layers (top and bottom). Received: February 15, 2015 Revised: April 21, 2015
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DOI: 10.1021/acs.jpcc.5b01560 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 2. Optimized configuration of F2/GE for in-plane (a) center, (b) cross, (c) top, (d) bridge and for out-of-plane (perpendicular) molecular orientation (e) center, (f) cross, and (g) top adsorption site.
Table 1. Structural Optimization Parameters for Adsorption of F2/GE at Different Molecular Orientations and Adsorption Sites in-plane Ead (eV) Pz (debye) Eg (meV) F2 net charge (|e|)
out-of-plane
center
cross
top
bridge
center
cross
top
−0.85 −1.57 100 −0.15
−2.33 −2.03 265 −0.31
−2.80 −1.76 72 −0.28
−0.87 −1.55 96 −0.13
−0.81 −1.51 71 −0.11
−0.80 −0.78 18 −0.08
−0.80 −1.42 66 −0.1
This charge transfer between molecule and graphene layers induces electrical field and as a conclusion introduces an energy gap.
II. COMPUTATIONAL DETAILS For investigation of structural stability and properties of F2 on graphene and between BLG, we have used DFT by SIESTA package based on periodic supercell method.24 The local density approximation (LDA) and 12 × 12 × 1 Monkhorst−Pack for sampling of Brillouin zone has been employed with considering the point that LDA approach reproduces graphene interlayer distance in good agreement with experimental studies.25 The convergence criterion for geometry optimization was
