Article pubs.acs.org/JPCC
Mechanism of H2O‑Induced Conductance Changes in AuCl4‑Functionalized CNTs Altynbek Murat,† Ivan Rungger,‡ Stefano Sanvito,† and Udo Schwingenschlögl*,† †
PSE Division, KAUST, Thuwal 23955-6900, Kingdom of Saudi Arabia School of Physics, AMBER and CRANN Institute, Trinity College, Dublin 2, Ireland
‡
ABSTRACT: We employ ab initio self-interaction corrected density functional theory combined with the nonequilibrium Green’s function method to study the electronic and quantum transport properties of carbon nanotubes (CNTs) functionalized with AuCl4 molecules. In particular, we investigate the electronic structure and characterize the conductance for different concentrations and configurations of randomly distributed AuCl4 molecules with and without the adsorption of H2O. We thus propose a mechanism that explains the origin of the recently observed resistivity changes of AuCl4functionalized CNTs upon H2O adsorption. We find that water adsorption shifts the highest occupied Cl and Au states down in energy and thereby reduces the scattering of the electrons around the Fermi energy, hence enhancing the conductivity. Our results help in the development of highly sensitive nanoscale H2O vapor sensors based on AuCl4-functionalized CNTs.
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mechanism behind the p-type conductivity,10,11 it has been only recently revealed that it is due to the adsorption of AuCl4 molecules.12 During the functionalization, several ionic conformations of AuCl3 are possible when dissolved in a coordinating agent such as nitromethane. However, AuCl4 has been found to be the most stable conformation when adsorbed on CNTs because it has the highest binding energy.12 In addition, it is the only candidate that leads to the observed ptype behavior.9 This is an important insight because the identification of a correct conformation of the dopant molecule represents the first step in the process of elucidating the mechanism behind the conductance changes upon exposure to water. The role and effect of H2O adsorption needs to be investigated at the atomic level to have a microscopic understanding of the experimentally observed H2O sensing ability of AuCl4-functionalized CNTs (the key for the development of nanoscale water sensors). In this work, we perform self-interaction corrected density functional theory (DFT) calculations combined with the nonequilibrium Green’s function (NEGF) method for quantum transport. We investigate the role of H2O adsorption by analyzing the electronic structure and characterizing the transmittance for different concentrations and configurations of randomly distributed AuCl4 molecules on CNTs with and without adsorbed H2O. More importantly, we propose an explanation for the mechanism underlying the experimentally observed resistivity changes. We find that the AuCl4 functionalization
INTRODUCTION Carbon nanotubes (CNTs) are used in a wide variety of nanoscale applications due to their excellent electronic and quantum transport properties, for example, in transistors and spintronics devices.1−4 Because of their high surface-to-volume ratio and the fact that the properties are very sensitive to perturbations, they are considered to be promising candidates for extremely sensitive nanoscale gas sensors.5−8 However, because of the strong sp2 bonds of the hexagonal carbon network, characterized by a low chemical reactivity, pristine CNTs have a weak gas sensing response. Hence, to design CNT-based gas sensors with high selectivity and sensitivity, it is necessary to functionalize their surface. CNTs play a vital role in gas sensors because their Fermi energy can be shifted upon functionalization, resulting in a change of the carrier concentration. To achieve high conductance one needs first a high carrier concentration and second carriers of high mobility. Recently, semiconducting CNTs functionalized with a salt solution, such as AuCl3 , have been investigated both experimentally and theoretically.9−12 It has been observed that the system is selectively sensitive to H2O adsorption,9 such that a strong resistance change happens upon exposure to H2O, which is the electrical functional requirement of a H2O vapor sensor. However, the microscopic origin of the resistance change has not been identified and the role of the H2O adsorption on the electronic and transport properties has not been established. This calls for an in-depth systematic investigation. AuCl3 functionalization leads to p-type doping,9,13 such that there are no defect states in the CNT band gap and the Fermi energy is located just below the top of the valence band.13 Although several studies have been conducted to identify the © 2015 American Chemical Society
Received: January 2, 2015 Revised: March 29, 2015 Published: April 3, 2015 9568
DOI: 10.1021/acs.jpcc.5b00022 J. Phys. Chem. C 2015, 119, 9568−9573
Article
The Journal of Physical Chemistry C
Figure 1. Electronic band structure and PDOS of CNTs doped with one AuCl4 molecule for adsorption of different amounts of H2O molecules: (a) none, (b) five, (c) ten, and (d) twenty. The corresponding crystal structures are shown on the top.
provides charge carriers but of low mobility due to the AuCl4 induced scattering potential. However, upon adsorption of H2O the mobility is enhanced massively due to increased charge delocalization for low AuCl4 concentration and decreased scattering for high AuCl4 concentration. Our conclusions provide the basic understanding needed for the development of water vapor sensors based on AuCl4-functionalized CNTs as well as for the further investigation of their quantum transport properties.
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METHODS We consider an infinite system with periodic CNT as well as a two-terminal finite system with Au electrodes. For both we use a semiconducting (10,0) CNT where one unit cell of the CNT contains 40 C atoms and has a length of 4.25 Å. For the infinite system we use a periodic 17 Å CNT supercell along the z direction (see Figure 1), containing 160 C atoms with a cell size of 27 × 27 × 17 Å3. The investigated two-terminal device comprises a scattering region and a pair of semi-infinite Au(111) electrodes, attached at both ends, as illustrated in the device setups of Figures 2 and 3. For the scattering region we use a CNT supercell containing 1600 C atoms with a cell size of 30 × 26 × 170 Å3 (see Figures 2 and 3) and five Au layers on each side, which are then connected to the semi-infinite Au electrodes. First-principles electronic structure calculations are performed using the SIESTA implementation of the DFT formalism.14 Quantum transport calculations are performed using self-interaction corrected DFT combined with the NEGF method, as implemented in the linear scaling SMEAGOL ab initio electronic transport code, which uses SIESTA as electronic structure platform.15−18 A double-ζ polarized atomic orbital basis set for C, Cl, H, and O is employed in all simulations, together with the Troullier−Martins scheme for constructing norm-conserving pseudopotentials.19 For Au a
Figure 2. PDOS and zero-bias transmission coefficient as a function of the energy for a pristine 170 Å CNT, that is, without AuCl4 functionalization and H2O adsorption.
double-ζ polarized basis set is used for geometry relaxation and electronic structure calculations, while a single-ζ polarized (6s) basis set is used for the transport calculations to reduce the computational cost. The usage of a s-basis for Au has been shown to give reliable results for the transport characteristics, at least in the low bias regime.20,21 A plane-wave cutoff of 250 Ry is chosen for the real space grid, and the Brillouin zone is sampled with a 1 × 1 × 4 Monkhorst−Pack k-point grid for the infinite system (periodic CNT) and with a 1 × 1 × 1 k-point grid for the finite system (Au electrodes) in the transport calculations. Periodic boundary conditions are applied along x and y for the Au electrodes of the finite system (the transport is along the z direction) in a sufficiently large simulation cell to avoid interaction between periodic images of the CNT. During the structural relaxations all atoms are allowed to move until the atomic forces are
