Molecular structure of vapor-deposited amorphous selenium

Abstract

The structure of amorphous selenium is clouded with much uncertainty and contradictory results regarding the dominance of polymeric chains versus monomer rings. The analysis of the diffraction radial distribution functions are inconclusive because of the similarities between the crystalline allotropes of selenium in terms of the coordination number, bond length, bond angle, and dihedral angle. Here, we took a much different approach and probed the molecular symmetry of the thermodynamically unstable amorphous state via analysis of structural phase transformations. We verified the structure of the converted metastable and stable crystalline structures using scanning transmission electron microscopy. In addition, given that no experimental technique can tell us the exact three-dimensional atomic arrangements in glassy semiconductors, we performed molecular-dynamic simulations using a well-established empirical three-body interatomic potential. We developed a true vapor-deposited process for the deposition of selenium molecules onto a substrate using empirical molecular vapor compositions and densities. We prepared both vapor-deposited and melt-quenched samples and showed that the simulated radial distribution functions match very well to experiment. The combination of our experimental and molecular-dynamic analyses shows that the structures of vapor- and melt-quenched glassy/amorphous selenium are quite different, based primarily on rings and chains, respectively, reflecting the predominant structure of the parent phase in its thermodynamic equilibrium.

Phytogreen synthesis of multifunctional nano selenium with antibacterial and antioxidant implications

“…The pattern depicted a broad 2θ diffraction peak at 23.30°in accordance with the previouly documented literature [45,48,49]. Raman spectrum of nSe in the range of 150 to 600 cm −1 exhibited a prominent resonance peak at 253 cm −1 owing to Se-Se stretching vibrations, confirming the amorphous nature of nSe ( figure 3(f)) [46,[50][51][52]. Raman spectral analysis was in corroboration with the XRD and SAED results.…”

Structure of Amorphous Selenium by 2D 77Se NMR Spectroscopy: An End to the Dilemma of Chain versus Ring

“…This similarity is manifested in the radial distribution functions corresponding to the chain‐ and ring‐dominated structural models of g‐Se , which are practically indistinguishable . On the other hand, the interpretation of the Raman spectrum of g‐Se also remains equivocal . For example, the strongest band in the Raman spectrum of selenium arises from the A 1 symmetric stretching mode which is sensitive to the secondary or inter‐chain interactions.…”

“…Amorphous or glassy selenium ( g‐Se ) is a unique photoconductor material that has found wide ranging technological applications that involve detection and imaging. Consequently, the structure of g‐Se has been studied quite extensively in the past using a wide variety of diffraction, spectroscopic and molecular simulation techniques . Surprisingly enough, in spite of its chemical simplicity, the structure of g‐Se remains controversial to date.…”

“…The structure of the trigonal modification ( t‐Se ) is characterized by infinite helical chains while that of the monoclinic modifications (α‐ Se , β‐ Se , and γ‐ Se ) is characterized by Se 8 ring molecules in various packing geometries . On the other hand, g‐Se can be prepared via melt‐quenching, vapor deposition and mechanical attrition or ball‐milling and the ring:chain ratio in these structures remain unconstrained …”

Manipulation of Glassy State in Amorphous Selenium by Low-temperature Internal Friction Measurements

“…Amorphous selenium (a-Se) is a simple monatomic amorphous system that can be easily deposited as a thin film 15 . Its twofold covalence bonding enables it to form twisted chains of random lengths and monomer rings that are highly crystallization-resistant 16 . Its T g is conveniently ~ 10 K above room temperature so that its structure can be 17 .…”