Direct Vapour Transport Research Articles

Growth and Electrical Characterizations of Sn<sub>0.3</sub>Se<sub>0.7</sub> Crystals Grown by Direct Vapour Transport Technique

Tin monoselenide (Sn0.3Se0.7) crystals have been grown by direct vapour transport technique. These crystals were found exhibit orthorhombic structure with space group D2h16 (Pcmn). The electrical measurements such as thermoelectric power and resistivity were carried out at high temperature on these as-grown crystals. The parameters like See back co-efficient, Fermi energy and scattering co-efficient were calculated from experimental data of TEP. The primary studies show the semiconducting behaviour of these crystals. Activation energy was also calculated by employing Arrhenius equation for grown samples. These layered materials, due to different kinds of interactions exhibits anisotropic crystalline structure and this lead to an anisotropy in electrical transport. Anisotropy seems to be decrease with temperature for all grown crystals. A detailed study about electrical transport and anisotropy in all such crystals will be presented at the time of presentation.

Observation of near-band-edge photoluminescence and UV photoresponse in near-stoichiometric Zn2SnO4 nanowires

Single-phase, near-stoichiometric zinc stannate (Zn2SnO4) nanowires were synthesized by a direct vapor transport process on c-Al2O3 substrates under optimized growth conditions. The optimal growth temperature for Zn2SnO4 nanowires is above 700 °C. Structural characterization indicates that the nanowires had the single crystal cubic spinel structure and diameters of ∼90 nm and they grew in the [1 direction. Low-temperature photoluminescence (PL) shows a strong emission peak at 3.7 eV, attributed to the near-band-edge-emission of Zn2SnO4 nanowires. Temperature-dependent PL results are consistent with the Varshni equation, and the band gap is redshifted by ∼170 meV as the temperature is increased from 11 to 300 K. The obtained direct gap of Zn2SnO4 nanowires is 3.546 eV at 300 K. A UV photodetector based on as-grown Zn2SnO4 nanowires was fabricated by a simple and cost-effective process. The Zn2SnO4 nanowires exhibited UV photoconductivity, and good selectivity and decent response to UV. The efficient fabrication method, high chemical and thermal stability of the Zn2SnO4 nanowire UV photodetector made it very suitable for use in harsh environments.

Excitonic emission and absorption resonances in V0.25W0.75Se2 single crystals grown by direct vapour transport technique

A systematic study on emission and absorption spectra of vanadium mixed tungsten diselenide single crystals grown by direct vapour transport (DVT) technique is reported. The grown crystals were characterized by energy dispersive analysis of X-ray (EDAX), which gives the confirmation about the stoichiometry. The structural characterizations were accomplished by X-ray diffraction (XRD), surface morphology and transmission electron microscopy (TEM). These characterizations were indicating the growth of V0.25W0.75Se2 single crystal from vapour phase. The optical response of this material has been observed by combination of UV–vis-NIR spectroscopy and photo luminescence (PL) spectroscopy. A detailed study of excitonic emission and absorption resonances was carried out on grown crystals. The energy band gap was calculated for indirect allowed transition with absorbed and emitted phonon. Additionally, absorption tail for grown crystal is found to obey the Urbach׳s rule.

Study of structural, electrical, and optical properties of nickel-doped tin selenide crystals

Nickel-doped tin selenide crystals can be grown using the direct vapour transport technique. This paper includes structural, electrical, and optical characterizations of SnSeNi0.2 crystals done by X-ray diffraction (XRD), energy-dispersive analysis of X-rays (EDAX), surface microstructure topography, Hall effect measurement carried out to determine type of material, Seebeck coefficient and calculation of power factor has been carried out from the data of thermoelectric power measurements and the ultraviolet–visible (UV-Vis) spectroscopic technique, respectively. The optical absorption spectrum was obtained in an UV-Vis–near-infrared (NIR) spectrometer in the range of 200–2500 nm. The UV-Vis spectroscopic technique has been used to calculate band gap, and the value of the band gap was found to be 0.84 eV in the visible region. Calculation of various optical parameters has been carried out.

Alteration of the morphology and electrocatalytic activity of IrO2 nanowires upon reduction by hydrogen gas

This paper reports the facile synthesis of highly single-crystalline IrO2 nanowires grown on an Au microwire (IrO2NW-Au) and its reduced form (Ir/IrO2NW-Au); and their electrocatalytic activity for oxygen reduction reaction, H2O2 reduction/oxidation, and dopamine (DA) oxidation. IrO2NW-Au, prepared by direct vapor transport process under atmospheric pressure, was reduced by H2 gas flowing at 200°C. This additional modification resulted in the significant morphological changes from the smooth nanowire structures of IrO2NW-Au to substantially porous structures of Ir/IrO2NW-Au with sustaining the external nanowire frameworks. The compositions were also changed from mostly IrO2 in IrO2NW-Au to the mixture of IrO2 and Ir(0) metal in Ir/IrO2NW-Au. Ir/IrO2NW-Au showed highly enhanced and facilitated electrochemical reaction kinetics compared to IrO2NW-Au for ORR and H2O2 reduction/oxidation. The ORR limiting-like current at Ir/IrO2NW-Au was measured to be ∼19-fold greater than that of IrO2NW-Au. In addition, the amperometric responses to varying H2O2 concentration confirmed that Ir/IrO2NW-Au exhibited ∼8-fold (for H2O2 oxidation) and ∼750-fold (for H2O2 reduction) higher sensitivity than IrO2NW-Au. The observed enhanced activity of Ir/IrO2NW-Au could be attributed to the enlarged active surface area as well as the inherent electroactivity of Ir/IrO2NW material induced by co-existence of Ir oxide and metal. In contrast, more stable and decent anodic current responding to DA oxidation was measured at IrO2NW-Au than Ir/IrO2NW-Au, indicating IrO2 rather than Ir(0) has higher catalytic activity for DA oxidation.

English

Among the IV-VI semiconductor compounds, Tin sulphide (SnS), Tin selenide (SnSe), Germanium Sulphide (GeS) and Germanium Selenide (GeSe) have the layered orthorhombic structure with eight atoms per unit cell forming biplanar layers normal to the largest c axis [1-4]. In the present work, investigation on growth of single crystals of SnSeRex (x = 0, 0.1, 0.2, 0.3, 0.4) by direct vapour transport technique (DVT) using two zone horizontal furnace is reported. Confirmation of stoichiometric proportion of constituent elements and determination of crystal structure of grown crystals is done by Energy Dispersive Analysis of X-rays (EDAX).The structural behaviour was accomplished by X-Ray diffraction (XRD) studies. As a result, it seen to be orthorhombic structure and results are discussed.

Effect of indium and antimony doping in SnS single crystals

Single crystals of pure SnS, indium (In) doped SnS and antimony (Sb) doped SnS were grown by direct vapour transport (DVT) technique. Two doping concentrations of 5% and 15% each were employed for both In and Sb dopants. Thus in total five samples were studied viz., pure SnS (S1), 5% In doped SnS (S2), 15% In doped SnS (S3), 5% Sb doped SnS (S4) and 15% Sb doped SnS (S5). The grown single crystal samples were characterized by evaluating their surface microstructure, stoichiometric composition, crystal structure, Raman spectroscopy, optical and electrical transport properties using appropriate techniques. The d.c. electrical resistivity and thermoelectric power variations with temperature showed semiconducting and p-type nature of the as-grown single crystal samples. The room temperature Hall Effect measurements further substantiated the semiconducting and p-type nature of the as-grown single crystal samples. The obtained results are deliberated in detail.

Effect of Indium and Antimony Doping on SnS Photoelectrochemical Solar Cells

Single crystals of pure SnS, indium (In) and antimony (Sb) doped SnS are grown by the direct vapor transport technique. Two doping concentrations of 5at.% and 15 at.% are employed for both In and Sb dopants. In total, five samples are studied, i.e., pure SnS, 5at.% In-doped SnS, 15 at.% In-doped SnS, 5at.% Sb-doped SnS and 15 at.% Sb-doped SnS single crystals. The energy dispersive analysis of x-ray (EDAX) and x-ray diffraction (XRD) analysis show that all the five as-grown single crystal samples possess near perfect stoichiometry and orthorhombic structure, respectively. The doping of In and Sb in SnS is established from the EDAX data and from the shift in the peak positions in XRD. Photoelectrochemical (PEC) solar cells are fabricated by using the as-grown single crystal samples along with iodine/iodide electrolytes. Mott—Schottky plots for different compositions of iodine/iodide electrolytes show that 0.025M I2+1M NaI+2M Na2SO4+0.5M H2SO4 will be the most suitable electrolyte. Study of efficiency (η) and fill factor for different intensities of illuminations at room temperature is carried out for the five samples. The In-doped SnS single crystals show better PEC efficiency than the undoped and Sb-doped SnS single crystals.

Growth, Structural and High Pressure Study of GeS<sub>0.25</sub>Se<sub>0.75</sub> and GeS<sub>0.75</sub>Se<sub>0.25</sub> Single Crystals

The orthorhombic semi-conducting compound GeS0.25Se0.75 and GeS0.75Se0.25 possess interesting electrical properties and can been the subject of numerous investigations. The changes in solids under high pressure can reveal several new features of interatomic forces, which are responsible for their diverse physical properties. Authors have carried out growth of GeS0.25Se0.75 and GeS0.75Se0.25 crystals by Direct Vapor Transport (DVT) technique. For compositional confirmation energy dispersive analysis of X-ray (EDAX) has been used. EDAX results show that the grown crystals are nearly stoichiometrycally perfect. The grown crystals have been characterized by X-ray diffraction technique (using Philips X Pert MPD diffractometer) for structural characterization. These crystals are crystallized in orthorhombic structure. The values of lattice parameters, unit cell volume and X-ray density are calculated and presented. It is observed from lattice parameters, unit cell volume and X-ray density, that as the content of sulfur increases the value of all the lattice parameters decreases. High pressure study is also of great importance to visualize the mechanism governing the structural changes and to reveal solid state properties associated with different structure. For the room temperature measurement of resistance as a function of pressure, up to 7 GPa, the sample was set at the centre of the talc disc on the lower anvil. The pressure was generated by a hydraulic press on the Bridgman type tungsten carbide opposed anvil apparatus with in-situ Bismuth pressure calibration. The resistance was measured in several independent runs on these crystals as a function of pressure and was found to be reproducible. The results of variation of electrical resistance do not show presence of any phase transition up to 7 GPa. We investigate in GeS0.25Se0.75 and GeS0.75Se0.25 single crystals that as sulfur content increases, resistance of this compound increases.

DVT Grown GeSe Single Crystals and their Thermal Parameters in N<sub>2</sub>

Most of the applications of semiconductors involve either photon absorption to form free carriers (photodetectors or solar cells) or the formation of photons by free carrier recombination (light emittiing diodes or lasers). Both kinds of applications require high quality single crystals having desirable dimensions, devoid of defects, grain boundaries, or impurities that act as electron scatterers, traps and non radiative recombination centers. As a consequence of the requirement of high quality, fabrication and growth conditions for the semiconductors must be carefully controlled for most applications. Hence single crystals of GeSe were grown by Direct Vapor Transport (DVT) technique, in a two zone horizontal furnace with temperature difference of 50 K between growth and source zones. The material crystallizes in the form of shining gray and platelets like crystals at the end of growth cycles. Thermogravimetric analysis (TGA) has been used for many years to evaluate thermal stability of material as it will determine the range of stable operation for a device made up out of these materials under investigation. Thermal characteristics of GeSe crystals were studied employing thermoanalytical techniques, viz. TGA and DTA. Thermal analysis experiments were carried out with constant heating rate of 10 °C/ min in N2. The objective of this study is to determine activation energy and other kinetic parameters of GeSe crystals. Broido and Coats-Redfern (C-R) methods are used to evaluate different kinetic parameters of GeSe crystals viz. activation energy, entropy, enthalpy, Gibbs mean free energy etc.

Growth and Optical Characterization of DVT Grown SnSe<sub>0.5</sub>Te<sub>0.5</sub> Single Crystals

In present investigation, we report the growth of SnSe0.5Te0.5single crystals by direct vapor transport (DVT) technique. The main requirement of this technique is precise adjustment of the temperature gradient between two zones of a horizontal furnace to enhance the transport of material in vapour form. For structural characterization, X-ray diffraction technique is used and it is found that this material is crystallized in orthorhombic structure. The values of lattice parameters, unit cell volume and X-ray density are calculated and presented. For optical characterization, absorption spectra of thin flake like crystals have been taken by using UV-VIS-NIR Spectrophotometer in the wave number range 200 nm to 2000 nm at room temperature with the incident plane polarized light beam normal to the basal plane i. e. along the c-axis of the grown flakes of SnSe0.5Te0.5single crystals. Direct as well as indirect band gap has been calculated near the fundamental absorption edge. Optical parameters such as transmittance (T), reflectance (R), refractive index (η) and extinction coefficient (k) have been determined and represented here from the measured value of absorbance of this crystal within said wavelength range.

Structural and Optical Characterization of Tungsten Diselenide Crystals Grown by DVT Technique

WSe2 is a member of groupVI Transition Metal Dichalcogenides (TMDCs) and has been observed to be a highly stable semiconducting material. It has been grown in crystal form using a direct vapour transport technique in present case. The grown WSe2 crystals were characterized for the structural properties using X-ray diffraction technique (XRD). The hexagonal structure was confirmed through this analysis. Using the data of XRD, various parameters like crystalline size, lattice parameters, micro strain, dislocation density, unit cell volume, unit cell density, growth fault and deformation fault probability etc were found. It was seen that the micro strain, dislocation density and growth & deformation fault probabilities were found to be very low in WSe2 crystals. The grown WSe2 crystals were characterized for optical properties using UV-VIS-NIR spectroscopy. The absorption spectra of WSe2 grown in present case revealed the fact that WSe2 posses direct band gap around 1.38eV. Layered di-chalcogenides WSe2 (or MoSe2) are used for desired applications such as in photo-conversion devices.

Study of Photoelectrochemical Solar Cell Using WSe<sub>2</sub> Crystal

The solar cells have been used as most promising device to convert light energy into electrical energy. In this paper authors have attempted to fabricate Photoelectrochemical solar cell with semiconductor electrode using TMDCs. The Photoelectrochemical solar cells are the solar cells which convert the solar energy into electrical energy. The photoelectrochemical cells are clean and inexhaustible sources of energy. The photoelectrochemical solar cells are fabricated using WSe2crystal and electrolyte solution of 0.025M I2, 0.5M NaI, 0.5M Na2SO4. Here the WSe2crystals were grown by direct vapour transport technique. In our investigations the solar cell parameters like short circuit current (Isc) and Open circuit voltage (Voc) were measured and from that Fill factor (F.F.) and photoconversion efficiency (η) are investigated. The results obtained shows that the value of efficiency and fill factor of solar cell varies with the illumination intensities.

Direct vapor transport synthesis of ZnGa2O4 nanowires with superior photocatalytic activity

ZnGa2O4 nanowires have been synthesized by a direct vapor transport process on c-sapphire substrate without any template under low temperature. Their structural and optical properties were characterized by field-emission scanning electron microscopy (FE-SEM), energy-dispersive spectroscopy (EDS) X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), and photoluminescence (PL). Structural characterization revealed that the as-synthesized samples consist of cubic spinel ZnGa2O4 nanowires with 30–90nm in diameter and 3–5μm long. Room temperature photoluminescence show intense ultraviolet (375nm) and green (530nm) emissions originate from oxygen vacancies. Specifically, a near-band-edge emission centered at 283nm was observed. The photocatalytic activity of ZnGa2O4 nanowires was evaluated by the degradation of methyl blue (MB) under ultraviolet light illumination. The results show that ZnGa2O4 nanowires exhibit high photocatalytic activity and is attributed to the enhanced light absorption and strong redox ability of photo-generated electron–hole pairs.

Growth and Dielectric Properties of DVT Grown GeS<SUB>0.25</SUB>Se<SUB>0.75 </SUB>Single Crystals

The present paper reports the growth of GeS<SUB>0.25</SUB>Se<SUB>0.75</SUB><b> </b>single crystals by direct vapor transport technique (DVT) using two zone horizontal furnace. The precise adjustment of the temperature gradient between source and growth zone of a horizontal furnace that enhance the transport of material in vapor form could result in the growth of gray, shining thin flakes of GeS<SUB>0.25</SUB>Se<SUB>0.75 </SUB>single crystals. The average dimensions of grown crystals were 0.90 x 0.56 cm<SUP>2</SUP>. As grown GeS<SUB>0.25</SUB>Se<SUB>0.75</SUB> single crystals were used for dielectric measurement. The dielectric measurements were carried out by standard two electrode method using HP4284A LCR meter and a digital temperature controller DT4284. The entire measurement cycle was programmed by LABVIEW software. The dielectric properties i.e. capacitance (C) alternating current conductivity (σ<SUB>ac</SUB>) and dielectric constant (ε’) are measured in the frequency range 100 Hz to 1 MHz with different temperature from room temperature to 523 K, in the step of 50 K temperature using 4284A LCR meter. In this investigation, dielectric constant is observed to be high at lower frequency and it systematically decreases with increasing frequency up to 600 kHz and then after it nearly becomes frequency independent while dielectric loss tanδ decreases with temperature and frequency.

Stacking fault energy in some single crystals

The stacking fault energy of single crystals has been reported using the peak shift method. Presently studied all single crystals are grown by using a direct vapor transport (DVT) technique in the laboratory. The structural characterizations of these crystals are made by XRD. Considerable variations are shown in deformation (α) and growth (β) probabilities in single crystals due to off-stoichiometry, which possesses the stacking fault in the single crystal.

Effect of rhenium doping on various physical properties of single crystals of MoSe2

Effect of rhenium doping is examined in single crystals of MoSe2 viz. MoRe0.005Se1.995, MoRe0.001Se1.999 and Mo0.995Re0.005Se2, which is grown by using the direct vapor transport (DVT) technique. The grown crystals are structurally characterized by X-ray diffraction, by determining their lattice parameters a and c, and X-ray density. Also, the Hall effect and thermoelectric power (TEP) measurements show that the single crystals exhibit a p-type semiconducting nature. The direct and indirect band gap measurements are also undertaken on these semiconducting materials.

Studies on transport properties of copper doped tungsten diselenide single crystals

During recent years, transition metal dichalcogenides of groups IVB, VB and VIB have received considerable attention because of the great diversity in their transport properties. 2H-WSe 2 (Tungsten diselenide) is an interesting member of the transition metal dichalcogenide (TMDC's) family and known to be a semiconductor useful for photovoltaic and optoelectronic applications. The anisotropy usually observed in this diamagnetic semiconductor material is a result of the sandwich structure of Se–W–Se layers interacting with each other, loosely bonded by the weak Van der Waals forces. Recent efforts in studying the influence of the anisotropic electrical and optical properties of this layered-type transition metal dichalcogenides have been implemented by doping the samples with different alkali group elements. Unfortunately, little work is reported on doping of metals in WSe 2. Therefore, it is proposed in this work to carry out a systematic growth of single crystals of WSe 2 by doping it with copper in different proportions i.e. Cu x WSe 2 ( x=0, 0.5, 1.0) by direct vapour transport technique. Transport properties like low and high temperature resistivity measurements, high pressure resistivity, Seebeck coefficient measurements at low temperature and Hall Effect at room temperature were studied in detail on all these samples. These measurements show that tungsten diselenide single crystals are p-type whereas doped with copper makes it n-type in nature. The results obtained and their implications are discussed in this paper.

Growth, electrical transport properties and microtopographic studies of Mo0·3W0·7Se2 single crystals

Single crystals of mixed transition metal dichalcogenide Mo0·3W0·7Se2 were grown by direct vapour transport technique. The stoichiometric composition and the crystallographic lattice parameters of the grown crystals were determined by energy dispersive X-ray analysis and X-ray diffraction techniques respectively. The optical energy bandgap of the as grown single crystals was determined by optical absorption studies. The high temperature Hall effect measurement showed that the carrier concentration increases with temperature, asserting the specimen to be a semiconductor. The Hall coefficient RH showed that the as grown crystals are p type semiconductors. The semiconducting nature was further substantiated from the direct current resistivity versus temperature measurement. The room temperature direct current resistivity variation with pressure showed a linear decrease in resistivity with the increase in pressure. Detailed surface microtopographic studies of these crystals by means of optical microscopy showed that spiral and the layer mechanism are predominant during crystal growth.

Low temperature electrical transport properties in p-SnSe single crystals

The electronic transport properties of p-type tin selenide (SnSe) grown by direct vapor transport (DVT) technique were investigated via Hall effect in the temperature range 40 T 300 K. The temperature dependence of conductivity revealed the existence of impurity energy level in the band gap of the crystal. The temperature dependence of the carrier concentration was analyzed using the single-donor – single-acceptor model. The Hall mobility increases by decreasing temperature up to 120 K and then decreases along with temperature. The observed temperature dependant mobility in the temperature range 120 T 300 K and 40 T 120 K was found to be limited by homopolar and ionized impurity mode scatterings respectively.