Effect of vacancy defects of SnS on gas adsorption and its potential for selective gas detection
Introduction
Since the discovery of graphene, the searches for new two-dimensional layered materials and their potential application in various fields have been a frontier hotspot [1]. Among them, group-IV monochalcogenides MX (M = Ge, Sn; X = S, Se) show potential advantages in photodetectors, photovoltaic devices, thermoelectric devices, piezoelectric devices, gas-sensitive devices, etc, due to their stability, environmental friendliness and excellent electronic properties [[2], [3], [4], [5], [6], [7], [8]]. As for the gas sensor applications, the monochalcogenides are expected due to the extremely large surface-to-volume ratios resulted from their ultrathin 2D architecture. Especially, the monochalcogenides MX processes unique 2D puckered layers similar to black phosphorus. It induces much larger active surface area beneficial for the enhanced adsorption of gas molecules.
SnS is a typical group-IV monochalcogenides. Up to date, many works concerning the photoelectric and thermoelectric properties and applications of SnS have been carried out thoroughly [[9], [10], [11]]. Comparatively, the investigations on the gas-sensitive properties of SnS are still limited. Several experimental results have revealed the gas-sensing potential of SnS. For instance, the heterojunction of SnS/SnS2 is ultra-sensitive to NO2 gas at room temperature and the heterojunction is also found to show high sensitivity to ethanol [12,13]. Further, high selectivity for ethanol is clarified by SnS nanosheets [14]. For any semiconductor material grown experimentally, including SnS, the occurrence of vacancy defects is inevitable. So far, research on vacancies in SnS has made some progress. Generally, the SnS semiconductor has p-type conductivity and the presence of Sn vacancies are the main reason [15,16]. The vacancy type and density existed in the defective SnS can be flexibly modulated in experiment. For example, the SnS grown in an S-rich environment generally possesses more Sn-vacancy. In this case, the occurrence of S-vacancy could be suppressed obviously [17]. This modulation approach has also been confirmed with theoretical calculations. In the Sn-rich (S-rich) environment, S-site (Sn-site) vacancies are confirmed to be formed much easier than Sn-site (S-site) vacancy [18]. For the gas-sensing application of SnS, the existed vacancy defects will be crucial; it could affect the gas-sensing performances of the material significantly due to the possible modulation to gas adsorption characteristic [[19], [20], [21]]. In related reports, the adsorption characteristics of perfect SnS towards gases like O3 and NO2 were investigated by first-principles calculations. It shows that O3 and NO2 adsorption cause charge transfer accompanied by large value of adsorption energy [22,23]. To clarify the role of vacancy defects in SnS clearly during gas-sensing, further calculation study on surface adsorption and sensing characteristics of the vacancy-defective SnS towards different gas molecules will be required and very meaningful.
In this work, we focus on the gas adsorption calculations on the intrinsic and defective stannous sulfide with S-vacancy or Sn-vacancy, with aim to reveal the potential of defective SnS on selective detection for certain gas theoretically. To this end, NO2, CH4, CH2O, CO, NH3 and H2S are chosen as target gases for surface adsorption. The vacancy models of defective stannous sulfide are constructed by removing of S atoms or Sn atoms from intrinsic surface. Based on the first-principles calculations about band structure, density of states (DOS), atomic Mulliken population and charge density difference, the adsorption and sensing characteristic of stannous sulfide and its defect configuration towards different gases were demonstrated. By clarifying the possible effects of vacancy defects on surface adsorption of stannous sulfide towards different gas molecules and the resultant electronic properties, this work reveals that the presence of S vacancy in SnS creates a preferring surface for NH3 adsorption, while the defective SnS with Sn vacancy is capable of enhanced adsorption and selective detection for NO2.
Section snippets
Modeling system
We chose a 3 × 3 supercell along the y and z directions to construct the structural model of 2D SnS monolayer [24]. Fig. 1 shows the optimized models of SnS and defective SnS. In the layer, each sulfur atom coordinates with adjacent three tin atoms. To eliminate the interaction between two neighbor images, a vacuum layer with thickness of 15 Å was set in the direction perpendicular to the 2D SnS monolayer [12,25,26].
In order to study the effects of S-vacancy and Sn-vacancy on gas selectivity of
Basic model research
The calculated energy gaps of SnS, [SnS + VSn] and [SnS + VS] are 1.219, 0.0045 and 0.673 eV, respectively, consistent with previous reports [18]. Fig. 3 shows the density of states of the pristine SnS and defective SnS. Compared with the pristine SnS, [SnS + VSn] shows a amount of shallow acceptors on the side close to the valence band, as shown in Fig. 3(b), which narrows the energy gap obviously and thus might induces evident enhancement in conductivity for [SnS + VSn]. In case of S-vacancy
Conclusions
In this work, first-principles calculation was employed to study the gas adsorption of SnS and the effect of vacancy defects. The gas-adsorption configurations were constructed based on the method of single-ended adsorption. The calculations on the different configurations indicate that the gas selectivity of SnS could be modulated by the vacancy defects in the lattice. The performance of gas adsorption and detection limit of SnS to NO2 will be improved with the introduction of Sn-vacancy,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the National Natural Science Foundation (NO.61971308) and Tianjin Natural Science Foundation (NO.19JCZDJC30900) of China.
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