Article Cite This: J. Phys. Chem. C 2019, 123, 13104−13109
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New Paradigm for Gas Sensing by Two-Dimensional Materials Vasudeo Babar, Sitansh Sharma, and Udo Schwingenschlögl*
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Physical Science and Engineering Division (PSE), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia ABSTRACT: The adsorption behavior and electronic transport properties of CO and NH3 molecules on para-C3Si and meta-C3Si monolayers are studied using first-principles calculations and the non-equilibrium Green’s function method. The adsorption sites are determined along with their adsorption energies. It turns out that CO and NH3 molecules physisorb on both monolayers. The current−voltage characteristics show that the para-C3Si monolayer can be used to sense CO and NH3 gases with high sensitivity. In contrast to other two-dimensional materials, the sensing mechanism is not based on charge transfer but on the presence of Dirac states and their susceptibility to symmetry-breaking structural distortions.
1. INTRODUCTION Toxic gases such as carbon monoxide (CO) and ammonia (NH3) pose increasing risks to environment and human health.1,2 Incomplete combustion of C-containing materials, such as petroleum products and biomass, is the major source of CO release. NH3 is emitted in agriculture (animals, fertilizers), industry, and traffic. Efficient sensing of these harmful gases is an important task, calling for chemical gas sensors with a short response time and low detection limit. Because of quantum effects, two-dimensional materials can possess outstanding electronic, mechanical, and chemical properties not present in their bulk counterparts. In addition, large surface to volume ratios combined with chemical stability make them promising candidates for gas sensing applications. Experimental and theoretical studies therefore have dealt with graphene,3−7 silicene,8 phosphorene,9 stanene,10 MoS2,11,12 WS2,13,14 and PtSe2,15 for example. While the gas sensing properties of graphene turn out to be not competitive with other technologies,16 some improvements can be achieved by suitable doping17 and utilization of defects.18 C3B and C3N monolayers, structural analogues of graphene, are known since several years and have been studied for various applications.19−21 The sensitivity of the C3N monolayer for sensing CO and NH3 gases, however, is very low,22 though it can be enhanced by B-doping.23 As another structural analogue of graphene, C3Si monolayers have been predicted to be thermally stable at 300 K both in para and meta phases.24 In addition, given the existence of C3B,19 C3N,20 and siligraphene (with different Si contents)25 monolayers, it is likely that application of the same pyrolysis technique with 1,4-disilylbenzene as a building block will result in experimental realization of C3Si monolayers in the near future. As most chemical sensors use charge transfer to/from the gas molecules to detect their presence, it appears to be interesting that Si (in contrast to B, C, and N) has 3p valence © 2019 American Chemical Society
states that are easy to access and therefore may promote charge transfer. In the present work, we show that this presumption is not correct. However, another mechanism is discovered that gives rise to even greater potential of C3Si monolayers as gas sensing materials. Specifically, we use first-principles calculations to understand the adsorption behavior of CO and NH3 molecules on C3Si monolayers. After identifying the preferential adsorption sites and molecular orientations, the modifications of the structure and electronic states are analyzed. The non-equilibrium Green’s function method is used to obtain the current−voltage characteristics before and after gas adsorption. Because of the structural anisotropy of C3Si monolayers, we discuss transport results for both the armchair and zigzag directions.
2. COMPUTATIONAL DETAILS We build 2 × 2 supercells of the hexagonal and rectangular unit cells of the para-C3Si and meta-C3Si monolayers, respectively. In each structure model, a vacuum layer of at least 15 Å thickness is added to the supercells in the out-ofplane direction to eliminate artificial interaction between periodic images. In order to find the preferential adsorption sites of the gas molecules, we study for the para-C3Si monolayer the hollow sites of the C−Si (H1) and C−C (H2) rings, the top sites of the Si (T1) and C (T2) atoms, and the bridge sites of the C−Si (B1) and C−C (B2) bonds. For the meta-C3Si monolayer, we consider the hollow sites of the C−Si rings with 1 (H1) and 2 (H2) Si atoms, the top sites of the Si (T1) and C atoms with 0/1/2 Si neighbors (T2/T3/ T4), and the bridge sites of the C−Si (B1) and C−C (B2) bonds. Initially, the center of mass of a molecule is placed at Received: February 9, 2019 Revised: April 25, 2019 Published: May 8, 2019 13104
DOI: 10.1021/acs.jpcc.9b01313 J. Phys. Chem. C 2019, 123, 13104−13109
Article
The Journal of Physical Chemistry C these sites in a height of 3 Å. For each site, we consider three orientations of the CO molecule (C or O pointing toward the monolayer; parallel to the monolayer) and two orientations of the NH3 molecule (N or H pointing toward the monolayer). We employ the Vienna Ab-initio Simulation Package,26 the generalized gradient approximation (Perdew−Burke−Ernzerhof parametrization), and the long-range dispersion correction of Grimme.27 The pseudopotentials comprise H 1s1, C 2s22p2, N 2s22p3, O 2s22p4, and Si 3s23p2 valence states. The cutoff energy of the plane wave basis is set to 500 eV and the Brillouin zone is integrated using the Monkhorst−Pack scheme with a 5 × 5 × 1 grid for the para-C3Si monolayer and a 4 × 5 × 1 grid for the meta-C3Si monolayer.28 Denser 7 × 7 × 1 and 5 × 6 × 1 grids, respectively, lead to minor changes in the total energy (
