Thin layers of copper, aluminum and chalcogen sequentially deposited by evaporation are annealed to synthesize CuAlX2 (X=Se, Te) films. The films crystallized in the chalcopyrite structure. For CuAlSe2, three characteristic energy gaps of 2.66(6), 2.78(2), and 2.91(5) eV were obtained from an analysis of the optical transmission spectra in the wavelength range 350–800 nm. The energies of the spin-orbit and the crystal field were found to be, respectively, 162 meV and −143 meV. Reflectivity measurements in the far infrared yield four mode frequencies for CuAlSe2 and CuAlTe2.

Tin diselenide single crystals were grown by direct synthesis using the Bridgman technique and were subsequently purified by successive recrystallizations. An X-ray powder diffraction study verified that the crystals obtained belong to the D 3d 3 — P¯3m1 space group with lattice parameters a=0.3811 nm and c=0.6137 nm. A stoichiometric analysis gave a maximum deviation of their nominal SnSe2 composition of ∼ 4%. Study of their morphology by scanning electron microscopy reveals the layered-type structure of the obtained material. Finally, the microhardness parallel and perpendicular to the layer planes was found to be strongly anisotropic, as expected from the layered structure of SnSe2.

The optical properties of the layered compound SnS2 have been studied. Reflectivity spectra were measured at room temperature, in the energy region 1.8–5.6 eV. Optical transmission measurements were carried out in the temperature range 13–300 K, in order to evaluate the energy gaps and their temperature dependences. The spectra of the optical constantsn, κ, ɛ1 and ɛ2 versus photon energyhν have also been presented.

SnS2 crystals were grown by the stoichiometric composition technique (total synthesis). After their stoichiometrical investigation, they were characterized by means of (i) X-ray diffraction, which gave the following results for their structure: space group D33d — P3m1, lattice parameters a = 3.646 Å and c = 5.879 Å; (ii) scanning electron microscopy, which revealed the layered structure of the crystals; and (iii) microhardness measurements, which were performed for the first time on this material. The obtained results are consistent with the reported values for the energy gap and melting point of this material.

Absorption spectra of thin layers of TlGaSe2 crystals are used to study the energy gap and the interband transitions of the compound in the energy region 1.5-3.8 eV and in the temperature range 12-290K. A peak of excitonic origin appears at 2.39 eV at 12K, 0.3 eV above the fundamental direct gap. It is followed by two sharp structures at 2.49 eV and 2.56 eV (12K). A broad structure appears at 2.64 eV. A room temperature reflection spectrum is also presented. The temperature dependence of the critical energies of all the observed structures is given. The dependence of the broadening parameter on temperature and the binding energy of the main exciton peak are also presented.Photoconductivity measurements at 290K and 90K resolve a number of structures that can be identified with several optical transitions. A peak associated with an acceptor level at 0.43 eV above the valence band maximum is also registered.

Crystals of different members of the family TlxIn1 − xSe (x = 0.0, 0.1, 0.2,…, 0.9, 1.0) were grown by direct synthesis of their constituent elements. The crystals are forming bundles of parallel rectangular fibers. Their stoichiometry was examined by means of X-ray microanalysis. Preliminary electrical measurements along the fibers show a strong dependence of the conductivity on x.

Single crystals of the layered compounds TlInS2, TlGaS2 and TlGaSe2 were grown by direct synthesis of their constituents. Their electrical conductivity was studied as a function of the temperature, perpendicularly and parallel to the layer planes and it proved to be highly anisotropic. Furthermore, a differential evaluation of measured lno vs. 103/T curves reveals the existence of an acceptor level at 0.237 eV, 0.370 eV and 0.207 eV above the valence band maximum of each compound, respectively. Optical absorption measurements were performed in the temperature range 15 to 300 K and the corresponding absorption coefficients were studied as a function of the temperature.

The I-U characteristics Of V2O5 single crystals show at high current densities an S-shaped negative-differential-resistance region. In this region chaotic voltage oscillations were registered. The maximum amplitude of them was in the range of 100 mV. An analysis with the Grassberger-Proccacia method led to the determination of the minimum embedding dimension and of the fractal dimension of the attractor describing these oscillations. The corresponding power spectrum and the Kolmogorov entropy, which was found to have a constant positive value, confirm the chaotic nature of the oscillations. The influence of external white noise and of the sampling rate used for the evaluation on the obtained results is also examined.

The I-U characteristics of ${\mathrm{V}}_{2}$${\mathrm{O}}_{5}$ single crystals show at high current densities an S-shaped negative-differential-resistance region. In this region chaotic voltage oscillations were registered. The maximum amplitude of them was in the range of 100 mV. An analysis with the Grassberger-Proccacia method led to the determination of the minimum embedding dimension and of the fractal dimension of the attractor describing these oscillations. The corresponding power spectrum and the Kolmogorov entropy, which was found to have a constant positive value, confirm the chaotic nature of the oscillations. The influence of external white noise and of the sampling rate used for the evaluation on the obtained results is also examined.

Electrical conductivity measurements were performed on TlInSe2 and TlInTe2 single crystals. The corresponding current-voltage (I-U) characteristics consist of two parts: An Ohmic part, at low current densities, and a nonlinear one of the S type, at moderate and higher current densities. In the latter part a well-formed negative-differential-resistance (NDR) region appears. Acceptor levels and their concentrations were determined from lnp versus 10(3)/T curves measured in the Ohmic region of the corresponding I-U curves. The nonlinear behavior of the I-U curves was studied at different ambient temperatures; the sample temperature and the threshold voltage of the NDR region were examined as a function of the current density and the ambient temperature, respectively.

The insertion of TlInSe2 chain crystals with lithium ions has been accomplished using an electrochemical method in non-acquous galvanic cells. Electrochemical properties have been studied during the first discharge and optical properties have been investigated on pure and Li-inserted samples by using FTIR spectroscopy. The EMF vs x(Li) in LixTlInSe2 curve contains a smooth voltage decrease and the inverse derivative voltage shows only peaks for small capacities. In the far-infrared absorption spectra of TllnX2 (with X=S,Se,Te) in the range 20–400 cm−1, optical modes and the combination of optical phonons are observed. The absorption spectrum of Li inserted TlInSe2 shows the original spectrum with the addition of a broad band attributed to Li vibration in the host structure.

Defects created by intercalation in MoS2 single crystals were studied by plane and cross-sectional electron microscopy. In the first stage of intercalation the effect is to create extensive dislocations. Further intercalation leads to a structural transformation of the type 2H → 1T, as is evident from the appearance of spots in the diffraction pattern belonging to the new phase. In addition, the transformation is accompanied by a 2a0 × 2a0 superstructure. This superstructure is unstable and disappears on heating, while the structural transformation is irreversible.

α-In 2 Se 3 compound and α-In 2 S 3− x Se x mixed crystals are grown by direct melting of stoichiometric mixtures of their components. Optical absorption measurements were made in the temperature range 15–330 K and the absorption coefficients were measured as a function of the wavelength of the incident light between 750 and 920 nm in the composition range 2.4 < x < 3. These crystals are layered and keep the α-In 2 Se 3 defect hexagonal wurtzite-like structure, with cationic lattice vacancies. For all compounds the energy gap is temperature dependent and the absorption edges shift to lower energy values with increasing temperature. An almost linear variation of the absorption edges with chemical concentration and shift to lower energy values is observed when the Se content is increased. The relation of the absorption coefficient to some power of ( hv − E g ) is not simple, possible mechanisms of the optical transitions are discussed, and an attempt is made to correlate them with the peculiarities of the defect structures of this system. Finally, no phase transitions were found down to 15 K for all samples.

Optical and electrical properties of the TlInX2 (X = S, Se, Te) were studied. The selenides and tellurides are of chain-like structure while the sulfides are layered. The measurements (I-U characteristics and optical absorption) seem to have a direct connection with the main structural features. The I-U-curves of the first two compounds are S-type with characteristic oscillations in the negative differential resistance (NDR) region while the sulfides show an Ohmic behavior. The optical absorption measurements in the temperature range 15–300 K for Se- and Te-compounds show a positive temperature coefficient of the gap while for the S-compound it is negative. Possible explanations are mentioned.

In2Se3 which crystallizes in layer structure has been studied at different post preparation temperatures. The presence of a large number if intrinsic defects affects strongly its electrical and optical properties. Our experiments aimed at measuring the influence of the post-preparation annealing on conductivity and anisotropy. From FIR spectroscopy the reflectivity spectrum is found to be influenced by crystal disorder. Photoluminescence measurements are also included and provide valuable information concerning the crystal degree of ordering.

Annealing treatments and doping effects in layered In2Se3 compounds have been investigated by means of conductivity and Hall mobility measurements in the temperature range 5–300 K. The presence of a large number of intrinsic defects strongly affects the electrical properties of these compounds, and the influence of the post-preparation annealing on conductivity and anisotropy is analysed.

The electrical properties of the layered compound Zn3In2S6, which is the third member of the system mZnS  In2S3 (m =3), were studied. The results obtained from the conductivity measurements indicate that a shallow electron trap at Ex = Ec − 0.54 eV with a concentration of about 1011 cm−3 is responsible for its electrical behavior. Optical absorption results at various temperatures indicate a direct energy gap. Its relatively high value at room temperature Eg = 2,77 eV is attributed to the specific structure of this compound. A stability of the values of the temperature coefficients in the mZnS  In2S3 compounds is also observed.