ARTICLE pubs.acs.org/JPCC
Molecular Charge Transfer: A Simple and Effective Route To Engineer the Band Structures of BN Nanosheets and Nanoribbons Qing Tang,†,‡ Zhen Zhou,*,‡ and Zhongfang Chen*,† †
Department of Chemistry, Institute for Functional Nanomaterials, University of Puerto Rico, Rio Piedras Campus, San Juan, Puerto Rico 00931 ‡ Institute of New Energy Material Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, P. R. China ABSTRACT: The electronic properties of BN nanosheets and nanoribbons doped with organic molecules with strong electron-donating or accepting abilities were investigated by means of density functional theory computations. The interfacial charge transfer between BN nanosheets and the acceptor (tetracyanoquinodimethane, TCNQ) or donor (tetrathiafulvalene, TTF) molecule significantly reduces the intrinsic wide band gap of pristine BN nanosheets and consequently results in a p- or n-type semiconductor, respectively. Similar behavior is observed for both zigzag and armchair-edged BN nanoribbons. These findings suggest a simple and effective route to tune the electronic properties of BN materials in a wide range and also facilitate the design of BN-based molecular electronics.
’ INTRODUCTION Graphene and its derivatives have exhibited fascinating properties and splendid performances,1 and the same is expected for novel noncarbon two-dimensional (2D) materials.2,3 As a structural analogue to graphene, hexagonal BN, a honeycomb network with the C superlattice substituted by B and N atoms, has triggered tremendous research interests.4,5 The exceptional properties, such as extreme robustness, low density, high chemical stability, and superb thermal conductivity, manifest BN as a promising material in optics and optoelectronic devices.6 Compared with carbon-based counterparts, most studies on BN nanomaterials focus on BN nanotubes,7,8 while BN nanosheets and nanoribbons (BNNRs) have been less explored. This is mainly due to the large difficulty in producing welldefined BN sheets on large scale. However, the challenges confronted in experiments did not deter the enthusiasm of scientists, and impressive progress has been made in the fabrication of BN nanosheets and nanoribbons. Experimentally, 2D BN nanosheets with single or a few atomic layers (two to five layers) have been obtained based on micromechanical cleavage,9 chemical-derived route,10 sonication-centrifugation technique,11,12 bottom-up chemical synthesis,13 chemical vapor deposition,14 and liquid exfoliation,15 and chemical functionalization has been done on the defects.16 The mono- and few-layer BN nanoribbons were successfully produced by unzipping multiwalled BN nanotubes.17 The superb properties of BN nanosheets have also been demonstrated; for example, BN nanosheets can considerably improve the thermal and mechanical properties of polymeric composites.12 r 2011 American Chemical Society
The pristine BN nanosheets, as well as their other morphologies, such as nanotubes and nanoribbons, are intrinsically insulators or wide-band gap semiconductors, which are not ideal for electronic applications. Note that the wide band gap of BN nanomaterials is a severe block in processing BN-based electronics. To tackle this, significant efforts have been made to tailor the band structures of BN nanomaterials. Experimentally, it has been found that the band gap of BN nanotubes can be directly reduced by an electric field,18 bending deformation,19 and carbon substitution.20 Theoretically, various approaches have been proposed to modulate BN nanosheets/nanoribbons, such as C adatom adsorption,21 electric field application,22 defect introduction,23 and surface or edge modification.24 In particular, Chen et al.25 predicted that a semiconductor half metal metal transition can be realized by precisely controlling the hydrogenation ratio in the hydrogenated zigzag BN nanoribbons. A promising simple and effective, yet not well-explored method to tune the electronic structure of BN nanosystems, is to form a charge-transfer complex by molecular doping. Compared with other dopants, such as metals, light atoms, and inorganic compounds, organic molecules have several advantages as surface dopants: a large variety of such molecules are available, and new ones can be designed and synthesized; different functional groups can be incorporated into the molecules, which can provide control over the molecular dipole moments, light Received: July 14, 2011 Revised: August 14, 2011 Published: August 22, 2011 18531
dx.doi.org/10.1021/jp2067205 | J. Phys. Chem. C 2011, 115, 18531–18537
The Journal of Physical Chemistry C
ARTICLE
Figure 1. Top views of the molecular structures for TCNQ and TTF dopants (left) and the supercell of 8 8 1 BN sheet (right), with the possible doping sites specified by four circular symbols in different colors.
sensing properties, hydrophilic or hydrophobic characters, and so forth. So far, the surface doping by organic molecules has been used to modulate the electronic properties of graphene.26 29 For example, the Raman spectroscopy and electronic structure of graphene are markedly affected by the interaction with electron -donating and -accepting molecules such as aniline and nitrobenzene.28 Chen et al.29 revealed that the surface electron transfer from graphene to the adsorbed tetrafluorotetracyanoquinodimethane (F4-TCNQ) is responsible for the p-doping of epitaxial graphene grown on 6H-SiC (0001). The same effect is expected on BN and other noncarbon 2D nanomaterials. However, to our best knowledge, there is no theoretical or experimental report on the noncovalent surface doping via organic molecules on BN nanosheets/nanoribbons. In this work, by means of density functional theory (DFT) computations, we investigated the effects of doping BN nanosheets/nanoribbons with organic molecules of effective electron -donating and -withdrawing capabilities, namely, tetrathiafulvalene (TTF) and TCNQ. Our results indicate that the doping behaviors on the BN nanosheets and nanoribbons are almost the same: in both cases, they can be turned into p- or n-type semiconductors by adsorbing acceptor (TCNQ) or donor (TTF) molecules, respectively.
’ COMPUTATIONAL METHOD The selected dopants are TTF, an excellent electron donor, and TCNQ, a strong electron acceptor, both of which are widely used as building blocks of charge transfer salts and have drawn deep interest in the field of molecular electronics.30 All of the DFT computations were carried out by using the Vienna ab initio simulation package (VASP). The ion electron interaction is described with the projected augmented wave (PAW)31 method. The exchange-correlation energy is described by the functional of Perdew, Burke, and Ernzerhof (PBE)32 based on the generalized gradient approximation (GGA). In all computations, the electron wave functions were expanded in plane waves with an energy cutoff of 400 eV. The total energy computations were performed by using 8 8 1 and 1 1 5 Monkhorst-Pack k-point grid for BN nanosheets and nanoribbons, respectively. The geometry optimizations were performed by using the conjugated gradient method, and the convergence threshold was set to be 10 4 eV in energy and 10 3 eV/Å in force. The larger k-point grids (24 24 1 and 1 1 21 k points for BN nanosheets and nanoribbons, respectively) were used for band structure computations.
To study the adsorption of TCNQ and TTF molecule on the BN sheet, an 8 8 1 BN supercell containing 64 B and 64 N atoms was adopted. The nearest distance between two TCNQ (TTF) molecules in adjacent supercells is ∼10 Å, and a vacuum separation of 14 Å was used to ensure negligible interaction between the organic molecules and the BN sheet in the adjacent unit cell. For BNNRs, the 8-zigzag (with width of 16.0 Å) and 14armchair (with width of 16.2 Å) nanoribbons are chosen as the representative models, with the two bare edges terminated by H atoms. The model is composed of 1 1 6 and 1 1 4 supercell for 8-zigzag and 14-armchair BNNRs, respectively. This leads to the separation of at least 8 Å for the molecule and its image in adjacent supercells.
’ RESULTS AND DISCUSSION Charge-Transfer Complexes of BN Nanosheets. The doping molecules are initially placed parallel to the BN monolayer surface with a vertical distance of 3.1 Å, significantly larger than the sum of the corresponding atomic van der Waals radii. Four different adsorption sites are considered, with the molecular center lying directly atop the B or N atoms, the B N bridge center, and the central hollow ring (Figure 1). The energetically most favorable geometries for the complexes between BN sheet and TCNQ/TTF are illustrated in Figure 2 parts a and b, respectively. No chemical bonds form between these interactive species. In both complexes, the in-plane B N bonds or intramolecular bonds of TCNQ (TTF) change negligibly, and the bond lengths also differ very slightly (
