VI2 Compounds Research Articles

When IV−VI Meets I−V−VI2: A Reinvigorating Thermoelectric Strategy for Tin Monochalcogenides

AbstractResembling the widely studied PbTe−AgSbTe2(LAST) and GeTe−AgSbTe2(TAGS) thermoelectric materials, alloying group I−V−VI2compounds to tin monochalcogenides (SnTe, SnSe, and SnS) was also initially studied in the 1960s. Nevertheless, this thermoelectric strategy has regained popularity only in recent years, and boosted performance is now generally expected in these alloys. Given its encouraging revival and the absence of a corresponding research summary, here we carefully review current advances in this system, which is categorized into three major subgroups, that is, I−Sb−VI2‐alloyed SnTe, I−Bi−VI2‐alloyed SnTe, and I−Sb−VI2‐alloyed SnSe. Emphasis is placed on exploring the underlying mechanisms of this system in addition to its thermoelectric properties. Some future directions of this reinvigorating strategy are also proposed in the hope that they can serve as a guide for its further exploration.

Synthesis of CuInS2/ZnS Nanocrystals with Thick Shell: The Effect of Temperature and Sulfur Precursor

As heavy-metal-free alternatives to Cd- and Pb-containing semiconductor nanocrystals (NCs), ternary I–III–VI2 compounds NCs have been actively studied, especially the CuInS2 NCs. Recently, it has been found that thick ZnS shelling can greatly improve the photochemical stability of such NCs, which undoubtedly enhances their application potential although it is still limited by the development of synthetic methods. This paper provides a facile method for preparation of thick-shell CuInS2/ZnS NCs. The effects of reaction temperatures ([Formula: see text]C and [Formula: see text]C) and sulfur precursors (dodecanethiol and sulfur powder) on the shell overgrowth are discussed in detail. When low reaction temperature ([Formula: see text]C) and inactive sulfur precursor are used, the overgrowth of ZnS shell is considerably slow and raising temperature have a limited impact on the particles’ size. On the contrary, high reaction temperature and reactive sulfur precursor can effectively improve the overgrowth rate of ZnS shell, and then thick-shell CuInS2/ZnS NCs can be received. Furthermore, a high-speed centrifugation method is used to screen out product NCs with a relatively uniform size.

Predicting a Novel Phase of 2D SiTe2.

Layered IV–VI2 compounds often exist in a CdI2 structure. Using the evolution algorithm and first-principles calculations, we predict a novel layered structure of silicon ditelluride (SiTe2) that is more stable than the CdI2 phase. The structure has a triclinic unit cell in its bulk form. The atomic arrangement indicates the competition between the Si atoms’ tendency to form tetrahedral bonds and the Te atoms’ tendency to form hexagonal close-packing. The electronic and vibrational properties of the predicted phase are investigated. The effective mass of an electron is small among two-dimensional (2D) semiconductors, which is beneficial for applications such as field-effect transistors. The vibrational Raman and IR spectra are calculated to facilitate future experimental investigations.

Model-free temperature scaling for heat capacity

In treating the experimental data on the heat capacity of solids, the essence of any model application is in the searching for the scaling factors (k i or 1/Θi) which transform a set of independent functions C P,i(T) for every substance into a function C P(T·k i) universal for the particular set of substances. DSC heat capacities of I–III–VI2 compounds at elevated temperatures exceed the upper limit of 12R (3R per mole of atoms) and make impossible application of any model. Nevertheless, the temperature scaling of heat capacity can be solved as a pure mathematical problem without any physical model (theory). The benefits of the model-free scaling are illustrated with the case of four isostructural chalcogenides (LiInS2, LiInSe2, LiGaS2, and LiGaSe2) measured recently with DSC in a temperature range from 180 to 460 K. The upper limit of C P(T·k i) functions was expanded up to 635 K. Low-temperature heat capacity of LiInSe2 published in 1995 made it possible to derive the thermodynamic functions (enthalpy and entropy) for LiInS2 (0–590 K), LiGaS2 (0–640 K), and LiGaSe2 (0–490 K) and expand those data for LiInSe2 from 300 to 460 K.

Structural, Optical and Photoelectrical Properties of AgInSe2 Films

The I–III–VI2 compounds are the ternary analogues of II–VI compounds. AgInSe2 is a ternary analogue of CdSe, which has been used for a number of electronic devices. AgInSe2 is a semiconductor with energy gap of 1.20 eV. They crystallize in the chalcopyrite structure, which is closely related to zinc blend structure. In the present work we employ the pulse electrodeposition technique for the first time to deposit AgInSe2 films and study their properties.AgInSe2 films were deposited by the pulse electrodepsoition technique at room temperature from a bath containing Analar grade 10 mM silversulphate, 50 mM Indium sulphate and 5 mM SeO2 at deposition potential of – 0.98V (SCE). Tin oxide coated glass substrates (5.0 ohms/ sq) were used. The duty cycle was varied in the range of 6 – 50 %. Thickness of the films measured by surface profilometer increased from 500 nm to 1000 nm as the duty cycle increased from 6 % to 50 %. Structural, optical and photoconductivity studies were made on the films.The films deposited at different duty cycles were characterized by the X-ray diffraction (XRD) pattern. All the reflections could be indexed as the tetragonal phase of AgInSe2 with the lattice parameters a = 6.09Å , c = 11:67Å , which were very close to the reported data (JCPDS Cards, 35– 1099). As the duty cycle increased, the intensity of the peaks increased. The height of the peak increased with duty cycle and the width of the peaks decreased with increase of duty cycle. No characteristic peaks of other impurities, such as Ag2Se or In2Se3, were detected in the pattern. The crystallite size was determined from Scherrer’s equation varied from 10 to 40 nm with increase of duty cycle.Energy dispersive X-ray (EDAX) spectrum gives the elemental composition of the as prepared samples. It contains 29.29% of Ag, 20.76% of In and 49.95% Se. This indicates that the as prepared films were indium deficient. The indium sites can be occupied by Ag atoms, acting as donors.The transmittance spectra at RT of AgInSe2 thin films deposited at different duty cycles exhibited interference fringes. Refractive index was calculated by envelope method and it increased from 1.25 to 1.80 with increase of wavelength from 500 nm to 1200 nm. The extinction coefficient 0.8 to 0.4 with increase of wavelength in the above range. This result is similar to that observed earlier. Photon energy vs. (αhν)2 plot of AgInSe2 thin films deposited at different duty cycles, indicated a band gap of 1.172 eVPhotoelectrochemical cell studies were made using 1M polysulphide (1M S, 1 M Na2S, 1 M NaOH) as the redox electrolyte. Graphite was used as the counter electrode. The as deposited films exhibited very poor photo output after post annealing in argon atmosphere photo output was observed. For a film deposited at 50 % duty cycle, an open circuit voltage of 0.45 V and a short circuit current density of 4.0 mA cm-2 at 60 mW cm-2 illumination was observed. Both photocurrent and photovoltage are found to increase up to 80 s photoetch, beyond which they begin to decrease. Photoetching leads to selective attack of surface states not accessible to chemical etchants. The power output characteristics after 80s photoetching indicates a Voc of 0.60V, Jsc of 7.50 mA cm-2, ff of 0.53 and h of 3.0% for 80 mWcm-2 illumination.The results of this investigation clearly points to the possibility of depositing single phase AgInSe2 films with grain size in the range of 10 – 40 nm which exhibit reasonable output when used as photoelectrodes.

Fabrication of hollow-sphere films of wurtzite CuInS2 on copper substrate

As important semiconductors, I–III–VI2 compounds have attracted wide attention, among which the wurtzite structured CuInS2 has been the research focus due to its metastable phase. In this paper, the wurtzite CuInS2 hollow-sphere films have been successfully prepared on copper substrate in a self-designed solvothermal detached system. The films of Cu(OH)2 one-dimensional nanostructure arrays and thioacetamide were used as the precursors and triethylene glycol was used as the solvent. Experiments showed that the amount of indium trichloride played a determinative role in the final morphology of the products. Meanwhile, the one-dimensional nanostructure arrays and the detached solvothermal system have great influences on the crystal shape as well. Based on the experimental results, a possible formation mechanism for the CuInS2 hollow spheres was also proposed. The UV–Vis absorption spectrum showed a broad absorption over the entire visible light and extending into the near-infrared region and presented the band gap of 1.53 eV for the as-prepared wurtzite CuInS2, which indicates the potential applications in solar cells.

THEORETICAL STUDIES ON BOND PROPERTIES AND HARDNESS OF ABC2(A = Zn, Cd, Cu, Ag; B = Si, Ge, Sn; C = P, As, S, Se, Te) SEMICONDUCTOR WITH CHALCOPYRITES STRUCTURE

The chemical bond parameters of II–IV–V2 and I–III–VI2 compounds have calculated using the chemical bond theory of complex crystals. Their hardnesses have been predicted by the chemical bond definition of hardness. The calculated results are in agreement with their experimental values. The calculation of hardness indicates that all of compounds possess good mechanical properties.

On the band structure lineup of ZnO heterostructures

The interface-induced gap states (IFIGS) are the fundamental mechanism which determines the band structure lineup at semiconductor interfaces. The valence-band offsets of semiconductor heterostructures are given by the difference of the respective IFIGS branch-point energies and electric-dipole terms which may be omitted for elemental group-IV semiconductors, SiC, as well as the III–V, II–VI, and I–III–VI2 compounds and alloys. The branch-point energy of ZnO is determined as 3.04±0.21eV from an analysis of experimental valence-band offsets reported for various ZnO heterostructures.

Strong enhancement of second-order response coefficients in tellurium containing Ag–III–VI2 compounds

First-principles calculations of the second-order optical response coefficients are reported for Ag–III–VI2 compounds with III=Ga, In and VI=S, Se, Te. While both the substitutions of In for Ga and Te for Se or S lower the band gap by similar amounts, the substitution of Se by Te is significantly more favorable for increasing χ(2). The enhancement of χ(2) by a factor 2 from AgGaSe2 to AgGaTe2 is surprising compared to the only modest enhancement obtained in II–VI compounds. The origin of these enhancements is analyzed in terms of the frequency-dependent response functions and the band structures.

Preparation and Properties of Ag(AlxGa1-x)S2 Crystals by Iodine Transport Method

The growth of chalcopyrite Ag(AlxGa1-x)S2 crystals is demonstrated in the entire alloy composition range using the conventional iodine transport method. The composition of the grown crystals corresponds to that of starting materials. With increasing x, lattice constants decrease, following Vegard's law. The vibration modes for Raman scattering lines are assigned on the basis of the reported AgGaS2 data. The compositional dependence of the A1 mode spectrum is discussed in comparison with that for Cu–III–VI2 compounds. Two bands observed in photoluminescence excitation spectra for a dominantly deep emission at 77 K correspond to band-to-band transitions, which are related to the uppermost valence band and the lower two closed valence bands. The changes in photon energies of these photoluminescence excitation bands indicate an increase in the energy gap from 2.7 eV to approximately 3.6 eV with x. The spectral broadening of the A1 mode line and the two photoluminescence excitation bands around x=0.5 is considered to be due to the clustering effect.

A phenomenological model for systematization and prediction of doping limits in II–VI and I–III–VI2 compounds

Semiconductors differ widely in their ability to be doped. As their band gap increases, it is usually possible to dope them either n or p type, but not both. This asymmetry is documented here, and explained phenomenologically in terms of the “doping pinning rule.”

Silver Gallium Sulfide by XPS

High quality single crystal AgGaS2 has been analyzed by x-ray photoelectron spectroscopy. This material is one of a group of I–III–VI2 compounds that crystallize in a tetragonal chalcopyrite structure and have been shown to have very promising nonlinear optical properties. The spectra presented here include survey and high resolution scans of the photoemission peaks and valence band spectra.

I–III–VI2 compound semiconductors for solar cell applications

I–III–VI2 compounds in general and CuInSe2 (CIS) in particular are important photovoltaic materials. Among the various thin film deposition techniques that have been employed for CIS film growth, so far only three-source evaporation and the two-stage method have yielded films that could be used for high efficiency solar cell fabrication. In this paper we briefly review some of these thin film deposition techniques and describe a two-stage method that gives high quality CIS films with uniform morphology and stoichiometry.

Temperature dependence of the tetragonal distortion in I–III–VI2 compounds

AbstractOn the basis of free energy considerations with anharmonic contributions included a model is developed for the temperature dependence of the tetragonal distortion Δ of the I–III–VI2 compounds. Agreement with experimental data can be only achieved if compounds with Δ < 0.05 and Δ > 0.05 are considered separately. It is suggested that this result is related to the structural order‐disorder behaviour of the I–III–VI2 compounds.

Electrical and optical properties of CuGaTe2

AbstractThe electrical and optical properties of CuGaTe2 single crystals were investigated by resistivity and Hall effect measurements in the temperatur range T = 77… 300 K and optical transmission measurements in the temperature range T = 20… 300 K at photon energies hν = 1.15…1.50 eV. All samples were p‐type conducting due to shallow acceptors with ionization energies EA1 ≈︁ 10−3 eV and concentrations NA1 ≈︁ p = (2…4). 1018 cm−3. The absorption spectra could be described by simultaneous consideration of acceptor ‐ to ‐ conduction band transitions with EA2 = 360 meV and NA2 ≈︁ 1022 cm−3 and valence band ‐ to ‐ conduction band transitions with EG = 1.24 eV at room temperaure. The temperature coefficient of the fundamental edge is dEG/dT = −4.0. 10−4 eV/K. The results are discussed with regard to some general trends found in the Cu‐III–VI2 compounds.

Preparation and dta of some A IB IIIC VI2 compounds

Single crystals of AgGaSe 2, AgGaTe 2, AgInSe 2 and AgInTe 2 were grown by a step-wise slow cooling technique. Phase transformations on the two Tellurides have been detected by DTA on powder samples.