Effect of intrinsic vacancy defects on the electronic properties of monoclinic Ag2S

doi:10.1016/j.matchemphys.2020.122961

Materials Chemistry and Physics

Volume 249, 15 July 2020, 122961

https://doi.org/10.1016/j.matchemphys.2020.122961 Get rights and content

Highlights

•
Formation energy of V_Ag defect is lower than that of V_S defect.
•
V_Ag defect is intrinsic defect in Ag₂S crystal.
•
Non-zero N(E_F) leads to higher conductivity for V_Ag defects.
•
Effect masses of holes are lighter than those of electrons.
•
Ag₂S with V_Ag defect is a p-type conductive material.

Abstract

The formation energy and electronic properties of intrinsic defects in Ag₂S are studied using first-principle calculations. Three possible intrinsic defect configurations are considered, including V_{Ag_I}, V_{Ag_II} and V_S. The calculation results show that V_Ag defects are more energetic favorable than V_S defects. It is found that electrical neutral V_{Ag_I} is energetically favorable over the negative charged V_{Ag_I} below E_F = 0.335eV. We find electronic conductivity could be enhanced by introducing intrinsic V_Ag defects into crystalline Ag₂S. Our electronic band structures show that the curvatures of conduction band minimum of Ag₂S are much flatter than the curvatures of valence band maximum, suggesting that the effect masses of holes are much lighter than those of electrons. Compared to pristine Ag₂S, the effective masses of holes get larger by introducing vacancy into crystal. Fermi level is passing through the valence band, suggesting Ag₂S with V_Ag defect would be a better conductivity. Non-vanishing N(E_F) as well as light effect masses of holes make Ag₂S suitable for construction of highly conductive p-type materials.

Introduction

Point defects are traditionally considered to play a major role in influencing on the properties of semiconductors. It is well known that lattice defects, especially for intrinsic point defects are inevitable at finite temperature. These intrinsic point defects could influence the geometric, mechanical, optical and electronic properties of the semiconductors. In recent years, considerable theoretical and experimental efforts are devoted to the fundamental understanding of the role of point defects on the electronic and optical properties of semiconductors. The conductive properties of semiconductors such as SnSe [1], MoSe₂ [2] and Ag₂S [3] could be significantly affected by intrinsic point defects.

The monoclinic Ag₂S (α-Ag₂S) is a narrow band gap semiconductor with the band gap values of 1.1 eV [[4], [5], [6], [7], [8]]. Extensive researches showed that Ag₂S would be potential candidate materials in photo conducting cells [9], IR detectors [10], solar selective coating [11], photovoltaic [12], and photochemical cells [13]. It is found that the electronic and ionic conductivity of Ag₂S increases three and two orders of magnitude, respectively, during structural phase transition from monoclinic structure to the disordered cubic structure [[14], [15], [16]]. Due to its superior ionic characteristics, Ag₂S is considered potential materials for solid electrolytes. The existence of intrinsic point vacancy defects in monoclinic Ag₂S would reduce the hopping barrier between lattices and improve the ionic conductivity. Point vacancy defects not only modify the geometry of Ag₂S but also alter its electronic properties. Alekberov et al. [3] reported that cationic vacancy defects in Ag₂S are acceptor-like defects.

Nanoparticles, quantum dots and core-shell nanostructures of non-stoichiometric Ag_1.93-1.98S as well as nanoheterostructures Ag₂S/Ag were synthesized through hydrochemical bath deposition [[17], [18], [19], [20], [21], [22], [23]]. Various structures, including hexagonal structure, quantum dots, Ag₂S/Ag composite were prepared by template synthesis, thermal decomposition, etc. [[24], [25], [26], [27], [28], [29], [30]] Mixed nanostructures of Ag_2-δS/Ag were shown to exhibit better photocatalytic performance. By changing the composition of non-stoichiometric Ag_2-δS (δ > 0), Ag vacancy also become an active site of charge recombination, which can control the optical properties of the composite and improve its photocatalytic activity under irradiation conditions [19]. Different non-stoichiometric Ag_2-δS, include Ag_1.1S, Ag_1.7S, Ag_1.93S, Ag_1.95S, Ag_1.98S were reported by experiments. It was found that the occupancy of the metal sublattice by Ag atoms is smaller than 1, suggesting Ag₂S nanoparticles are non-stoichiometric and contain vacant sites in the metal sublattice [31]. However, the electronic properties of monoclinic Ag_2-δS at low temperature are still unknown. A systematic theoretical study on the effect of the intrinsic vacancy defects on the electronic properties, correlation between defect concentration and electronic conductivity, and formation energy of these vacancy defects is highly desirable. In this paper, we aim to elucidate the effects of Ag vacancy defects on the electronic properties of Ag₂S by using different compositions near the stoichiometric composition.

Formation energy and electronic properties of monoclinic Ag₂S with intrinsic vacancy defects are investigated by first-principles calculations. We find V_Ag defects are intrinsic defects. V_{Ag_I} defects with electrical neutral states are stable below E_F = 0.335 eV and V_{Ag_I} defects negative charged are energetic favorable above E_F = 0.335 eV. The density-of-state at Fermi level gradually increases along with the concentration of the V_Ag defects increases. The valence band holes are much lighter than those of conductive band electrons, indicating Ag₂S with V_Ag defects is a p-type conductive material.

Section snippets

Computational methods

Ab initio density functional theory (DFT) calculations are performed by using the VASP [[32], [33], [34]] code, and the projector augmented wave (PAW) [35,36] method is applied to describe for electron-ion interaction. The generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) functional [37] is adopted for electron exchange-correlation. 4d¹⁰ and 5s¹ electrons of Ag as well as 3s² and 3p⁴ electrons of S are treated as valence electrons in the calculations. The wave functions

Structure and bond length

First, we examine the geometry structures of Ag₂S with intrinsic vacancy defects compared to that of defect free Ag₂S. Local structures of pristine Ag₂S, Ag₂S with V_{Ag_I}, V_{Ag_II} and V_S are shown in Fig. 2. The dashed circles represent the location of the intrinsic Ag or S vacancy defects in Fig. 2(b)~(d). The bond lengths between Ag_I and S atoms as well as Ag_II and S atoms are 2.53 Å and 2.42 Å, respectively in pure Ag₂S as we can see from Fig. 2(a). The results show that the bond length

Conclusions

We have performed first-principles calculations to study formation energy and electronic properties of Ag₂S with intrinsic vacancy defects. The geometry of our non-stoichiometric Ag₂S is similar to the case of the experimental studies. We find the formation energy of V_Ag is lower than that of V_S, suggesting that V_Ag defects are intrinsic defects. The transition energy level of $V_{Ag}^{0} / V_{Ag}^{1 -}$ occurs at E_F = 0.335 for V_{Ag_I} defects. V_Ag defects are shallow acceptor-like defects. Electronic

CRediT authorship contribution statement

Chunyan Du: Software, Investigation, Writing - original draft, Formal analysis, Visualization, Validation, Visualization, Data curation, Writing - review & editing. Jiayuan Tian: Formal analysis, Software. Xiaojie Liu: Resources, Writing - review & editing, Supervision.

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.

Acknowledgements

The authors acknowledge the support by the National Natural Science Foundation of China under Grant Nos. 11574044, 11774048 and Open Project of Key Laboratory for UV-Emitting Materials and Technology of Ministry of Education (No. 130028608).

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View full text

Effect of intrinsic vacancy defects on the electronic properties of monoclinic Ag2S

Highlights

Abstract

Introduction

Section snippets

Computational methods

Structure and bond length

Conclusions

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgements

Thin Solid Films

Biomaterials

Synthetic Met.

Solid State Ionics

Vacuum

Thin Solid Films

Solid State Ionics

Superlattice. Microst.

Chem. Phys. Lett.

Int. J. Hydrogen Energy

Mater. Lett.

Mater. Lett.

Synth. Met.

Mater. Lett.

Comput. Mater. Sci.

Comput. Mater. Sci.

Solid State Ionics

RSC Adv.

AIP Adv.

Phys. Status Solidi C

RSC Adv.

Nanotechnology

J. Am. Chem. Soc.

Bull. Electrochem.

Effect of intrinsic vacancy defects on the electronic properties of monoclinic Ag₂S