Structure Of Amorphous Selenium Research Articles

Structure of Amorphous Selenium: Small Ring, Big Controversy.

Selenium (Se) discovered in 1817 belongs to the family of chalcogens. Surprisingly, despite the long history of over two centuries and the chemical simplicity of Se, the structure of amorphous Se (a-Se) remains controversial to date regarding the dominance of chains versus rings. Here, we find that vapor-deposited a-Se is composed of disordered rings rather than chains in melt-quenched a-Se. We further reveal that the main origin of this controversy is the facile transition of rings to chains arising from the inherent instability of rings. This transition can be inadvertently triggered by certain characterization techniques themselves containing above-bandgap illumination (above 2.1 eV) or heating (above 50 °C). We finally build a roadmap for obtaining accurate Raman spectra by using above-bandgap excitation lasers with low photon flux (below 1017 phs m-2 s-1) and below-bandgap excitation lasers measured at low temperatures (below -40 °C) to minimize the photoexcitation- and heat-induced ring-to-chain transitions.

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

Amorphous selenium, owing to its tremendous technological importance and perhaps to its chemical simplicity, has been studied for nearly a century and yet an unequivocal structural description of this material remains lacking to date. The primary controversy regarding the structure of amorphous Se relates to the relative fraction of Se atoms residing in ∞1Se chains versus in Se8 rings. Herein we present the results of a two-dimensional solid-state 77 Se nuclear magnetic resonance (NMR) spectroscopic study of the chain and ring crystalline allotropes of Se as well as of amorphous Se to unequivocally demonstrate that 1) the Se8 rings and the ∞1Se chains are characterized by their unique 77 Se NMR signatures and 2) the structure of amorphous Se consists exclusively of ∞1Se chains.

Molecular structure of vapor-deposited amorphous selenium

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.

A molecular dynamics study of amorphous selenium

The structure of amorphous selenium has been studied using molecular dynamics. Ten chains of selenium consisting of 30 atoms each have been generated by a new algorithm with glass density as a criterion. A covalent bond has been simulated using harmonic potential and non-bonded closed shell interactions have been treated using Lennard-Jones potentials both for intra-and inter-chain atoms. Good agreement of bond distances, bond angles and nearest neighbour coordinations with experimental data has been observed. Absence of any preferred dihedral angle has been noticed. Diffusion of selenium has been interpreted as a result of coupled motion of atoms in the chains whose overall dynamics is considered as jiggling. It has been shown that the activation barrier for this kind of a displacement is fairly low, although atomic displacements give a diffusion constant of around 2×10 −7 cm 2/s at 200 K.

Amorphous-crystalline transformation and structure in selenium

This paper reports the effects of cooling rate on the crystallization process, properties and structure of amorphous selenium by means of differential scanning calometer (DSC), X-ray diffractometry andpositron annihilation techniques. The experimental results show that the crystallization temperature, the formation of the metastable phase during the crystallization process, the crystallization activation energy and the structure of as-quenched amorphous selenium are controlled by the cooling rate.

Room temperature ageing of vapour deposited amorphous selenium thick films

Many investigations of the structure of amorphous selenium have been reported [1] but a unique structure has not yet been agreed upon. The amorphous phase is considered to be composed of hexagonal chains or monoclinic rings or a mixture of" both. To determine the nature of the condensed species, experiments on ageing have been carried out [ 2 4 ] where the sample has been either in the bulk or thin film ( 200 nm thick) form and has been aged under controlled experimental conditions (for example, the sample has been kept either in the dark or in an atmosphere saturated with water vapour, etc.). However, there has been no report of amorphous-crystal l ine transition of amorphous selenium thick films upon ageing under ordinary (i.e. uncontrolled) experimental conditions. As a part of our programme of structural studies of amorphous semiconductors, we have recently observed by X-ray diffraction the effects of room temperature ageing on amorphous selenium films in the range of thickness 30 to 913/~m having different deposition rates. The films were prepared by evaporation in wLcuum ( ~ 5 x 10 -s tort) onto cleaned glass substrates at room temperature at the deposition rates of 7, 1 t and 12/~m min -1 which are rather fast compared to the ones usually reported in most of the publications on amorphous selenium. The films were aged at room temperature (25 -+ 2 ° C) under normal atmospheric pressure for a period of nearly three months and no special method was adopted to age the films. Rather the films were exposed in the laboratory to ordinary day (room light only) and night cycles by keeping them in a glass covered petri dish. X-ray diffraction patterns 66 were obtained with a Philips PW 1050, 1051 dif43 fractometer coupled to a Philips 1130 generator 30 92 and with LiF monochromated Cu-radiation.

Atomic models of amorphous selenium

A critical review of the literature has been carried out on the structure of amorphous selenium. Two molecular models were then constructed to follow the conclusions derived from the survey and to also fit newly obtained radial distribution functions for bulk quenched amorphous selenium. These computer models followed four criteria: (1) the molecules were disordered chains; (2) the chains were packed to maximize Van der Waals attractions; (3) the close-packing was achieved by letting the magnitude of the dihedral angle vary slightly and by letting the phase of the dihedral angle vary randomly; (4) closed rings were assumed to be few in number and thus negligible in fitting the RDF. It was found that the percentage of cis and trans configurations in the molecules could vary widely and still be consistent with both the radial distribution function and with physical criteria. Thus the cis/trans ratio, or the phase of the dihedral angle, is not unique for amorphous selenium. The structure of a-Se may be described as consisting of coiled disordered chains which are packed in such a way as to minimize the Van der Waals bond lengths in a minimum cluster which has a radius of 11 Å.

Highly disordered amorphous selenium studied by ultraviolet photoemission spectroscopy

Highly disordered amorphous (HDA) selenium films were prepared by vapor deposition onto a cold substrate held at 77 K under ultrahigh vacuum. HeI photoemission spectra of these films were found to be considerably different from the previously observed spectrum of the amorphous film prepared at room temperature (RT film). Characteristic features in the spectra of the HDA films compared with those in the spectrum of the RT film are as follows: (1) the width of the $4p$ nonbonding band is remarkably narrower; (2) the intensity of the upper $4p$ bonding band is decreased, and the intensity ratio of the upper and lower $4p$ bonding bands is reversed; (3) the upper $4p$ bonding band is located at higher initial energy; and (4) a part of the $4s$ band is well resolved. The HDA films underwent thermal annealing effects and at room temperature gave a photoemission spectrum similar to that observed in earlier work. In order to interpret the experimental results, CNDO molecular-orbital calculations were made on a newly proposed model for amorphous selenium, in which distorted infinite chains consist of small spiral units including a few selenium atoms. On this model, dependence of the density of states on the following factors was studied: (1) the number of atoms in a unit cluster, (2) the dihedral angle for the bonds on the junction of the units, and (3) the bond angle. The results of the calculations well explained the spectrum of HDA selenium and its thermally induced changes. The importance of the above three factors for the structure of amorphous selenium was stressed.

Molecular dynamic computer experiments on the structure of amorphous selenium

This work demonstrates the feasibility of deriving intramolecular atomic interaction potentials by fitting the results of molecular dynamic model calculations to experimental data - specifically, to atomic radial pair distribution functions (diffraction data) and vibrational frequencies (optical data). An intermolecular atomic distribution function is also derived for amorphous selenium, on the basis of the model calculations and the presumption of predominantly ring-type molecular units.