Amorphous As2S3 Research Articles

Flexoelectric and local heating effects on CdSe nanocrystals in amorphous As2Se3 films

Raman and photoluminescence (PL) spectra of amorphous As2Se3:Cd films show that laser irradiation can result in the local formation of CdSe nanocrystals (NCs). Due to a pronounced laser-induced drastic drop of the film viscosity, a strong local enhancement of diffusion of atoms in the laser spot area results in a material transfer from the laser spot (evidenced by SEM and AFM data) and simultaneously facilitates the formation of CdSe NCs. The difference of the PL maximum positions measured at a high laser power density Pexc and subsequently (from the same spot) at a lower Pexc value can be explained either by local heating of the sample by the high-Pexc illumination, or by the flexoelectric effect (additional strain in the CdSe NCs induced by the electric field gradient caused by the charging of the NC surface). Based on the spectral positions of the PL maximum, with the account of the Stokes shift, the average radii of the CdSe NCs formed are estimated.

Structural and optical study of Zn-doped As2Se3 thin films: Evidence for photoinduced formation of ZnSe nanocrystallites

Amorphous Zn-doped As2Se3 films with a nominal zinc content x up to 10 at.% were prepared by thermal evaporation. Their structure is characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoemission spectroscopy (XPS), and Raman spectroscopy. The AFM data show a considerable increase of the film surface roughness for films with x > 5 at.%. A strong gradient of the Zn content decreasing into the film depth is confirmed by the EDX and XPS data. Heavily Zn-doped (above 7 at.%) As2Se3 films reveal photostructural changes in the course of the Raman measurements. New Raman features are attributed to TO and LO vibrations of ZnSe nanocrystallites formed in the film under laser illumination. Depending on the laser wavelength and power density, the ZnSe nanocrystallites can experience tensile strain in the film due to a non-thermal photoplastic effect in the As2Se3 film resulting in a partial removal of the material from the laser spot. The tensile strain value, estimated from the TO and LO phonon frequency shift, is shown to reach up to 2.9 GPa.

Formation Mechanism, Structural and Optoelectronic Properties of As2Se3/CdS Heterojunctions Prepared by Physical Vapor Deposition Technique

In the current study we discuss the formation mechanism and optical performance of cadmium sulfide-arsenic treseleinde heterojunctions. The cadmium sulfide-arsenic treseleinde interfaces are prepared by the physical vapor deposition technique. The deposited thin films of CdS onto amorphous As2Se3 substrates contain both cubic (65%) and hexagonal (35%) phases in their structure. Optically, while the As2Se3 exhibited an energy band gap value of 2.40 eV, the CdS exhibits two energy band gaps of values of 3.24 eV and 2.40 eV. The valence and conduction band offsets for the As2Se3/CdS heterojunction are found to be 2.97 eV and 2.13 eV, respectively. On the other hand, the spectral analysis of the dielectric constant and optical conductivity reveal interesting properties presented by drift mobility and plasmon frequency values in the ranges of 35.02–4.11 cm2/Vs and 0.13–5.90 GHz, respectively. The optical conductivity increases from 0.5 Ω cm−1 to 20 Ω cm−1 in the spectral range of 1.1–3.0 eV. These features are promising as they indicate the applicability of the As2Se3/CdS for the fabrication of metal–oxide–field effect transistors and photosensors as well.

In-doped As2Se3 thin films studied by Raman and X-ray photoelectron spectroscopies

Amorphous In-doped As2Se3 films with nominal indium contents x up to 7 at.% were prepared by thermal evaporation. Atomic force microscopy studies confirm the uniform film structure with a surface roughness near 5 nm, noticeably higher than for similarly prepared undoped As2Se3 film. X-ray photoelectron spectroscopy (XPS) studies enabled the chemical composition of the films to be examined. As follows from the XPS data, the In content in the film strongly decreases with the film depth. For films with x ≥ 2 at.%, Raman features attributed to transverse and longitudinal optical phonons of InAs are revealed in the Raman spectra as an evidence for the formation of InAs nanocrystallites in the As2Se3:In film under laser illumination.

Characteristics of Amorphous As2S3 Semiconductor Films Obtained via Spin Coating

Centrifugation is used in fabricating, e.g., films with large areas and/or thicknesses of several micrometers. However, it has yet to be widely employed for chalcogenide compounds, due to their relatively weak solubility in most solvents. Determining the optimum conditions for preparing solutions of chalcogenide compounds and obtaining films via centrifugation is therefore of great interest. Specific features of amorphous arsenic sulfide (As2S3) films prepared via the centrifugation of solutions in n-butylamine have been studied. These films were characterized by means of X-ray diffraction analysis, IR spectroscopy, atomic-force microscopy and Raman spectroscopy. It was shown that amorphous As2S3 films have a greater elasticity modulus than those of analogous composition produced via thermal evaporation in vacuum, or As2S3 glass. A structural model based on arsenic sulfide clusters whose surfaces are bound by negatively and positively charged ions is used to explain the experimental results obtained in this work. DC measurements show that the amorphous films exhibit semiconductor-type conductivity. Their room temperature conductivity is ~10−15 S/cm, which indicates good dielectric properties. The films are optically transparent starting from the yellow spectral range, making them promising functional materials for engineering applications in optics and photonics.

Raman evidence for surface oxidation of amorphous As2S3 thin films under ultraviolet irradiation

Amorphous As2S3 films with surface roughness near 0.5 nm were prepared by thermal evaporation. Raman measurements performed under ultraviolet (UV) laser excitation (325 nm) revealed a series of sharp intense peaks appearing at laser power density Pexc above 150 kW/cm2. No such effects were observed at Raman measurements under visible (514.7 or 632.8 nm) light excitation, even for much higher Pexc values. The new Raman peaks are identified as arsenolite (As2O3) features. UV light-assisted oxidation of the As2S3 surface is discussed.

Optical hysteresis in SPR structures with amorphous As2S3 film under low-power laser irradiation

Optical hysteresis is a fundamental phenomenon that can lead to optical bistability and high-speed signal processing. Here, we present a theoretical and experimental study of the optical hysteresis phenomenon in amorphous As2S3 chalcogenide based waveguide structures under surface plasmon resonance (SPR) conditions. The SPR structure is irradiated with low power CW Ar laser radiation at 514 nm wavelength, with photon energy near the optical band-gap of As2S3, in a Kretschmann–Raether configuration. First, we determined the incidence angle on the SPR structure for resonant coupling of the laser radiation within the waveguide structure. Subsequently, by setting the near resonance incidence angle, we analyzed the variation of the laser power reflected on the SPR structure with incident power. We demonstrated that, by setting the incidence angle at a value slightly smaller than the resonance angle, the increase followed by the decrease of the incident power lead to a wide (up to 60%) hysteresis loop of the reflected power. This behavior is related to the slow and persistent photo-induced modification of the complex refractive index of As2S3 under 514 nm laser irradiation. The experimental and theoretical results are in good agreement, demonstrating the validity of the theoretical model presented here.

Surface relief and refractive index gratings patterned in chalcogenide glasses and studied by off-axis digital holography.

Surface relief gratings and refractive index gratings are formed by direct holographic recording in amorphous chalcogenide nanomultilayer structures As2S3-Se and thin films As2S3. The evolution of the grating parameters, such as the modulation of refractive index and relief depth in dependence of the holographic exposure, is investigated. Off-axis digital holographic microscopy is applied for the measurement of the photoinduced phase gratings. For the high-accuracy reconstruction of the wavefront (amplitude and phase) transmitted by the fabricated gratings, we used a computational technique based on the sparse modeling of phase and amplitude. Both topography and refractive index maps of recorded gratings are revealed. Their separated contribution in diffraction efficiency is estimated.

Особенности характеристик аморфных полупроводниковых пленок As2S3, полученных методом центрифугирования раствора

Особенности характеристик аморфных полупроводниковых пленок As2S3, полученных методом центрифугирования раствора

Preparation of Pure Amorphous As2S3 Films of a Stoichiometric Composition

The results of studying the composition of arsenic trisulphide (As2S3) films obtained by adding finely dispersed copper or silver powders to the starting materials for sputtering or by using triple compositions As–S–As and As–S–Cu are presented. The issues of preserving the composition of the films obtained and the degree of their purity are discussed.

Analysis of pulsed laser deposited amorphous chalcogenide film thickness distribution: Plume deflection angle dependence

Pulsed laser deposition exploiting a KrF excimer laser was used to fabricate amorphous As-S thin films from bulk As2S3 glass target. Thickness profile of the film was extracted from variable angle spectroscopic ellipsometry data. The dependence of thickness distribution of prepared thin layer on laser beam plume deflection angle was evaluated and corresponding equations were suggested.

Microstructure characterization of multifunctional As4S4/Fe3O4 nanocomposites prepared by high-energy mechanical milling

Multifunctional As4S4/Fe3O4 nanocomposites prepared by high-energy mechanical milling are probed by complementary methods of positron annihilation lifetime (PAL) spectroscopy, supported by microstructure characterization using X-ray powder diffraction (XRPD) with analysis applied to the first sharp diffraction peak (FSDP), morphology studies by transmission electron microscopy (TEM) with energy-dispersive spectroscopy (EDS) and high-resolution TEM (HRTEM).These nanocomposites are shown to consist of Fe3O4 crystallites with particle sizes of 8–21nm, and far-separated β-As4S4 crystallites surrounded by amorphous As-S phase. In respect to PAL data, the effect of milling is identified as possible conversion from Ps traps to positron-trapping sites depending on preferential chemistry of atomic surrounding. So, the interfacial triple junctions at the intersections of Fe3O4 crystallites are identified as principal positron traps in As4S4/Fe3O4 nanocomposites with competitive influence from free-volume defects of amorphous As-S phase.

Formation of CdSe nanocrystals in Cd-doped thin arsenic selenide films under laser irradiation

Amorphous Cd-doped As2Se3 films with nominal Cd contents up to 12at.% were prepared by thermal evaporation. Atomic force microscopy studies confirm the uniform film structure with surface roughnesses below 1nm, independent of the Cd content. As shown by energy-dispersive X-ray spectroscopy, the Cd content in the film reveals a strong gradient and decreases with the film depth. For heavily Cd-doped (above 7at.%) As2Se3 films, Raman features attributed to CdSe longitudinal optical (LO) phonon and its overtone (2LO) are revealed in the Raman spectra as an evidence for the formation of CdSe nanocrystallites in the As2Se3:Cd film under above-bandgap or below-bandgap laser illumination. The CdSe nanocrystallites undergo tensile strain due to a photoplastic effect in the As2Se3 film, i. e. partial removal of the material from the laser spot. The tensile strain value, estimated from the LO phonon frequency shift, is shown to reach nearly 2GPa.

Processes of Nucleation of Amorphous As-S Films at Condensation on Carbon Substrates

Processes of Nucleation of Amorphous As-S Films at Condensation on Carbon Substrates

Reversible devitrification in amorphous As2Se3 under pressure

In pressure-induced structural transitions, the term reversible refers to the recovery of the virgin structure in a material upon complete decompression. Pressure-induced amorphous-to-crystalline transitions have been claimed to be reversible, but evidence that amorphous material recovers its virgin amorphous structure upon complete depressurization has been lacking. In amorphous As2Se3 (a-As2Se3) chalcogenide, however, we report a novel amorphous-to-crystalline transition that provides compelling experimental evidence that upon complete decompression, the recovered amorphous phase is structurally the same as that of the virgin (as-cast) amorphous phase. Combining the experimental results with ab initio molecular dynamics simulations, we elucidate that the amorphization is mediated by a surplus of total free energy in the high-pressure face-centered cubic phase as compared to the virgin amorphous phase and that the structural recovery to the virgin amorphous phase is a consequence of an enhancement in covalent bonding character over interlayer forces upon complete decompression. Furthermore, we observed a two-dimensional to three-dimensional network transition under compression and its reversibility upon decompression. © 2016 American Physical Society.

Breakdown of intermediate range order in AsSe chalcogenide glass

As-cast amorphous AsSe (a-AsSe) has been characterized by in-situ high pressure XRD and Raman spectroscopy up to the pressure of ∼30 GPa using diamond anvil cell together with ab-initio molecular dynamics simulations. A gradual densification has been observed under compression along with the breakdown of intermediate range ordering at ∼16 GPa. The whole transformation process can be divided into three relatively distinct pressure regimes from 1 bar to 7 GPa, from 7 to 16 GPa, and beyond 16 GPa. Our XRD results together with Raman spectroscopic studies confirm that in the a-AsSe pressure tuning results in network transformations only, without sudden jump in the density. The results obtained by high pressure ab-initio molecular dynamics simulations demonstrate the variations in the local structures associated with the experimentally observed transformations. The amorphous-to-amorphous network transformation is found to be reversible upon decompression.

Photoexpansion in amorphous As2S3: A new explanation

Thin amorphous As2S3 films show a giant photoexpansion of ~5% upon femtosecond laser illumination and this expansion remains after switching off the laser beam. To understand the structural modifications which occur during the photoexpansion process we assumed a molecular cluster structure of the amorphous As2S3. The focused femtosecond laser beam induces electrical charge redistribution on the sulfur atoms of each cluster, which increases the electrical repulsion between sulfur atoms, and thus induces an expansion of the clusters, by network reconfiguration, without breaking the bonds. An increase of the van der Waals distance between molecular clusters also takes place.

As2Se3기반 Resistive Random Access Memory의 채널 직선화를 통한 신뢰성 향상

Resistive random access memory (ReRAM) of metallic conduction channel mechanism is based on the electrochemical control of metal in solid electrolyte thin film. Amorphous chalcogenide materials have the solid electrolyte characteristic and optical reactivity at the same time. The optical reactivity has been used to improve the memory switching characteristics of the amorphous As2Se3-based ReRAM. This study focuses on the formation of holographic lattices patterns in the amorphous As2Se3 thin film for straight conductive channel. The optical parameters of amorphous As2Se3 thin film which is a refractive index and extinction coefficient was taken by n&k thin film analyzer. He-Cd laser (wavelength: 325 nm) was selected based on these basic optical parameters. The straighten conduction channel was formed by holographic lithography method using He-Cd laser. Ag + ions that photo-diffused periodically by holographic lithography method will be the role of straight channel patterns. The fabricated ReRAM operated more less voltage and indicated better reliability.

Correlation between photo-induced red shift of the optical band gap and the slope of Urbach edge in amorphous and glassy As2S3

Photo-induced changes in the Tauc gap and the slope of Urbach edge in As2S3 were found to be interrelated via simple linear relation. It is suggested that this correlation indicates a certain similarity between by disorder affected electronic transitions associated with the Tauc gap and the Urbach edge.

Field Effect Transistors Based on Semiconductive Microbially Synthesized Chalcogenide Nanofibers

Microbial redox activity offers a potentially transformative approach to the low-temperature synthesis of nanostructured inorganic materials. Diverse strains of the dissimilatory metal-reducing bacteria Shewanella are known to produce photoactive filamentous arsenic sulfide nanomaterials by reducing arsenate and thiosulfate in anaerobic culture conditions. We present in situ microscopic observations and measure the thermally activated (79 kJ/mol) precipitation kinetics of high yield (504 mg per liter of culture, 82% of theoretical maximum) extracellular As2S3 nanofibers produced by Shewanella sp. strain ANA-3, and demonstrate their potential in functional devices by constructing field effect transistors (FETs) based on individual nanofibers and photoelectrochemical cells based on macroscopic mats. The use of strain ANA-3, which possesses both respiratory and detoxification arsenic reductases, result in significantly faster nanofiber synthesis than other strains previously tested, mutants of ANA-3 deficient in arsenic reduction, and when compared to abiotic arsenic sulfide precipitation from As(III) and S2-. Detailed characterization by electron microscopy, energy-dispersive X-ray spectroscopy, electron probe micro-analysis, and Tauc analysis of UV-Vis spectrophotometry show the biogenic precipitate to consist primarily of amorphous As2S3 nanofibers with an indirect optical band gap of 2.37 eV. X-ray diffraction also reveal the presence of crystalline As8S9-x minerals that, until recently, were thought to form only at higher temperatures and hydrothermal conditions. The nanoscale FETs enable a detailed characterization of the charge mobility (∼10−5 cm2/V s) and gating behavior of the heterogeneously doped nanofibers. We also present a characterization of detected photocurrent. These studies indicate that the biotransformation of metalloids and chalcogens by bacteria enables fast, efficient, sustainable synthesis of technologically relevant chalcogenides for potential electronic and optoelectronic applications.