Adsorption of gas molecules on monolayer MoS2 and effect of applied electric field
© Yue et al.; licensee Springer. 2013
Received: 19 July 2013
Accepted: 2 October 2013
Published: 17 October 2013
Using first-principles calculations, we investigate the adsorption of various gas molecules (H2, O2, H2O, NH3, NO, NO2, and CO) on monolayer MoS2. The most stable adsorption configuration, adsorption energy, and charge transfer are obtained. It is shown that all the molecules are weakly adsorbed on the monolayer MoS2 surface and act as charge acceptors for the monolayer, except NH3 which is found to be a charge donor. Furthermore, we show that charge transfer between the adsorbed molecule and MoS2 can be significantly modulated by a perpendicular electric field. Our theoretical results are consistent with the recent experiments and suggest MoS2 as a potential material for gas sensing application.
KeywordsMonolayer MoS2 Molecular adsorption Electric field First-principles calculations 68.43.-h 73.20.Hb 73.22.-f
Sensing gas molecules, especially toxic gas, is critical in environmental pollution monitoring and agricultural and medical applications. For this reason, sensitive solid-state sensors with low noise and low power consumption are highly demanded. While sensors made from semiconducting metal oxide nanowires[2, 3], carbon nanotubes[4, 5], etc. have been widely studied for gas detection for some time, graphene as a novel sensing material has further stimulated strong interests in the research community since Schedin et al. demonstrated that a micrometer-sized graphene transistor can be used to detect the ultimate concentration of molecules at room temperature, presenting a pronounced sensitivity many orders of magnitude higher than that of earlier sensors. The graphene-based sensor is actualized by monitoring the change in resistivity due to the adsorption or desorption of molecules, which act as charge acceptors or donors[7–9]. It is shown that sensitivity of this sensor can be further improved through introduction of the dopant or defect in graphene[10–13]. Despite these achievements, researchers continue to seek for novel sensitive sensors similar to or even more fascinating than graphene gas sensors.
Recently, two-dimensional monolayer MoS2, a kind of transition metal dichalcogenide, has attracted increasing attention because of its versatile and tunable properties for application in transistor, flexible optoelectronic device, photodetector, and so on[14–19]. Unlike graphene which lacks a band gap and needs to be engineered to open the gap for practical application, pristine monolayer MoS2 has a direct band gap of 1.9 eV and can be readily used to fabricate an interband tunnel field-effect transistor (FET)[21–26]. In this context, Radisavljevic and co-workers first reported a top-gated FET on the basis of monolayer MoS2, which possesses a room-temperature current on/off ratio exceeding 108 and mobility of 200 cm2 V-1 s-1. At the same time, the success of graphene-FET sensors also greatly inspires the intensive exploration of MoS2 as a sensing material. Since monolayer MoS2 holds a high surface-to-volume ratio comparable to graphene, a MoS2-based gas sensor is expected to have excellent sensing performance as well. More recently, FET sensors made from mechanically cleaved monolayer and multilayer MoS2 have been demonstrated, which exhibit high sensitivity for NO gas with a detection limit down to 0.8 ppm. The superior sensitivity for NO2 has been observed in a flexible FET sensor array on a polyethylene terephthalate (PET) substrate based on a MoS2 channel and reduced graphene oxide (rGO) electrodes. Compared to the rGO-FET sensor, this novel sensor array displays much higher sensitivity, which can even be enhanced by up to three times via functionalization of MoS2 with Pt nanoparticles.
Although the MoS2-FET sensor for nitride oxide has been experimentally realized, the underlying mechanisms regarding how NO x molecules interact with the MoS2 surface and affect the electronic properties are not clear. Moreover, the response of MoS2 upon exposure to other gas molecules like H2, O2, H2O, NH3, CO, etc. remains to be examined either. In order to fully exploit the possibilities of a MoS2-based gas sensor, a systematic study on the adsorption of gas molecules on a MoS2 surface is thus desired from a theoretical point of view. In this work, using first-principles calculations, we first determine the most stable configuration for gas molecules adsorbed on monolayer MoS2, as well as the corresponding charge transfer between them. Modification of the electronic properties of host monolayer MoS2 due to the molecule adsorption is then examined. Furthermore, the effect of an external electric field on the charge transfer is also discussed. To the best of our knowledge, no prior theoretical work has been conducted on these issues.
First-principles calculations are performed using the Vienna ab initio simulation package (VASP)[29, 30] on the basis of density functional theory (DFT). The exchange-correlation interaction is treated by local spin density approximation (LSDA). Spin-polarized calculations are also carried out with generalized gradient approximation (GGA) in some specific cases. A cutoff energy of 400 eV for the plane-wave basis set and a Monkhorst-Pack mesh of 5 × 5 × 1 for the Brillouin zone integration are employed. In order to eliminate the interaction between two adjacent monolayer MoS2, a vacuum layer larger than 15 Å is adopted in the calculations. All the structures are fully relaxed by using the conjugate gradient method until the maximum Hellmann-Feynman forces acting on each atom is less than 0.02 eV/Å. By means of Bader analysis, charge transfer between the monolayer substrate and the adsorbate is obtained. The electric field in VASP is actualized by adding an artificial dipole sheet at the center of the simulation cell.
Results and discussion
We consider the absorption of H2, O2, H2O, NH3, NO, NO2, and CO on two-dimensional monolayer MoS2. A 4 × 4 supercell of monolayer MoS2, with a single gas molecule adsorbed to it, is chosen as the computational model. The optimized lattice constant of monolayer MoS2 is 3.12 Å, and consequently, the distance between two neighboring gas molecules is larger than 12 Å. The monolayer MoS2 consists of a monatomic Mo-layer between two monatomic S-layers like a sandwich structure, in which Mo and S atoms are alternately located at the corners of a hexagon. In order to determine the favorable adsorption configuration, four adsorption sites are considered, namely, H site (on top of a hexagon), TM (on top of a Mo atom), TS (on top of a S atom), and B site (on top of a Mo-S bond). The gas molecule is initially placed with its center of mass exactly located at these sites. For each site, configurations with different molecular orientations are then examined. Take NO as an example, three initial molecular orientations are involved, one with NO axis parallel to the monolayer and two with NO axis perpendicular to it, with O atom above N atom and O atom below N atom [see Additional file1 for more detailed adsorption configurations]. The adsorption energy is calculated as, where is the total energy of MoS2 with an absorbed molecule and and Emolecule are the total energies of pristine MoS2 and isolated molecule, respectively. A negative value of E a indicates that the adsorption is exothermic.
Results for gas molecules on monolayer MoS 2 calculated by LDA functional
Results for gas molecules on monolayer MoS 2 calculated by PW91 and PBE functionals
Next, Bader analysis is performed to predict the charge transfer value. It is found that most molecules studied except NH3 are charge acceptors with 0.004 ∼ 0.1e obtained from monolayer MoS2, whereas NH3 behaves as a charge donor, providing 0.069e to the monolayer. The charge transfer values for O2 and H2O are in good agreement with recently reported values (approximately 0.04e for O2 and 0.01e for H2O) by Tongay et al.. Note that our results are somewhat similar to the previous reports on the adsorption of gas molecules on graphene and carbon nanotube, where the gas molecules also behave as either charge acceptors or donors. We need to point out that although different methods besides Bader analysis may give rise to different values in determining the electronic charge transfer, the direction and order of magnitude should be the same. The mechanism of the MoS2-FET gas sensor for NO can then be understood. Before NO adsorption, the mechanically cleaved MoS2 channel is an n-type semiconductor in the experiment, implying that some electrons have already existed in the conduction band. After NO adsorption, electron charge is transferred to the NO molecule, inducing a p-doping effect on the MoS2 channel. As a result, the channel resistance increases and current decreases. The similar behavior, which has been previously reported for MoS2-FET devices in an O2 environment[35, 36], is probably due to the adsorption of O2 on the MoS2 surface, which traps electrons and sequentially reduces the current of the MoS2-FET.
In this work, we present a first-principles study on the structural and electronic properties of monolayer MoS2 upon adsorption of gas molecules. Various adsorption sites and molecule orientations are involved to determine the most stable configurations. We find that all molecules are physisorbed on monolayer MoS2 with small charge transfer, acting as either charge acceptors or donors. Band structure calculations indicate that the valence and conduction bands of monolayer MoS2 is not significantly altered upon the molecule adsorption, though certain molecules such as O2, NO, and NO2 introduce adsorbate states in the band gap of the host monolayer. In addition, we demonstrate that the application of a perpendicular electric field can consistently modify the charge transfer between the adsorbed molecule and the MoS2 substrate. Our theoretical findings show that MoS2 holds great promise for fabricating gas sensors.
J.L. gratefully acknowledges the financial support from the National Science Fund for Distinguished Young Scholar (grant no. 60925016). This work is supported by the National Natural Science Foundation of China (NSFC; grant no. 51302316).
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