image: Figure 1. SEM images of arsenic sulfide materials prepared by plasma discharge at different conditions: pole-like (a) and layered (b) structures. view more
Credit: Lobachevsky University
Since the discovery of graphene in 2004, there has been a rapidly growing interest among scientists in the study of 2D materials "beyond graphene". In the family of chalcogenide materials, 2D-layered transition-metal dichalcogenides demonstrate excellent electronic and optical properties, outstanding mechanical flexibility, and exceptional catalytic performance. At the same time, chalcogenides like As2S3, As2Se3, etc., have never been considered as materials capable of forming structures of this type.
It could be due to the limitations of the methods used for preparing those materials. A group of researchers (L. Mochalov, D. Dorosz, M. Kudryashov, A. Nezhdanov, D. Usanov, D. Gogova, S. Zelentsov, A. Boryakov, A. Mashin) from the laboratory of functional nanomaterials at the Lobachevsky State University of Nizhny Novgorod, the Alekseev Nizhny Novgorod State Technical University and the AGH University of Science and Technology (Krakow, Poland) in their paper "Infrared and Raman spectroscopy study of As-S chalcogenide films prepared by plasma-enhanced chemical vapor deposition" show the possibility of forming honeycomb structural fragments in plasma using the example of the well-known arsenic sulfide chalcogenide system.
For the first time, optical emission of two-dimensional "beyond graphene" arsenic sulfide prepared in plasma has been demonstrated. A strong structural photoluminescence exited by continuous wave operation lasers with a laser excitation wavelength of 473 nm and 632.8 nm has been observed. The influence of excitation parameters, chemical composition, structure, and annealing conditions on the intensity of photoluminescence of the chalcogenide materials has been established. Mass-spectrometry and Raman spectroscopy were coupled with quantum-chemical calculations to reveal the fragments which are the building blocks of the 2D As-S materials.
A plausible mechanism of formation and modification of the arsenic sulfide luminescent structural units has been proposed. The properties of 2D pole-structured and layered arsenic sulfide are suggested to be the key to advancing to 2D photosensitive devices.
Scanning Electron Microscopy (SEM) images typical of the arsenic sulfide with a composition As40S60 are illustrated in Figure 1 (a) and (b). The striking difference in the surface morphology and structure is due to very different conditions of plasma deposition.
Both pictures (1a and 1b) illustrate arsenic sulfide structures consisting of (As2S2)n-units (see below) formed in plasma by spherical structural fragments with a diameter of about 100 nm. Figure 1a shows a pole-structured material and Figure 1b depicts a 2D layered structure, the theoretical possibility of existence of which has been described recently. We have reported quantum-chemical estimations of these structures together with experimental results on the unusually broad transparency window of these materials (0.43 - 20 μm) in comparison with those of As2S3 (0.6 - 11 μm) prepared by traditional thermal methods.
Photoluminescence of the plasma prepared arsenic sulfide was measured by excitation at 473 nm and 632.8 nm employing continuous wave operation lasers at room temperature (RT, Figure 2).
The arsenic sulfide materials prepared in plasma were analyzed using the mass-spectrometry method to reveal and to gain insight into the main structural fragments affecting the luminescence intensity.
The mass-spectroscopy data presented clarify the main structural fragments affecting the PL intensity of the plasma prepared arsenic sulfide. According to the data obtained and taking into account the point of view formulated previously we may assume that the main reason for appearance and enhancing of the luminescence in arsenic sulfide materials prepared in plasma is the (As2S2)n cyclic structure unit playing the role of a "disk-like polarizability tensor".
Due to its properties, two-dimensional pole-structured and layered "beyond graphene" arsenic sulfide is a promising material for developing 2D photosensitive devices. These properties are also useful when creating field-effect transistors, highly sensitive photodetectors and gas sensors.
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