Thermoluminescence measurements of liquid crystal azomethines and poly(azomethines) with different shapes as thermo-detectors

We have explored the thermoluminescence (TL) properties of a few series of liquid crystal unsymmetrical and symmetrical azomethines and poly(azomethines) with different shapes. Rod-, bent- and star-shaped compounds were investigated as thermoluminescent detectors. All of the investigated compounds, except ASB3, AG2 and PAZ2, exhibited mesomorphic behavior. The thermoluminescence studies have shown that molecular structure of the compound along with shape influenced the TL properties of azomethines and poly(azomethines). Thermoluminescence intensity (I), temperature of maximum TL (T), activation energy values (E) and frequency factor (s) were detected. For the first time, to the best of our knowledge, thermoluminescence properties of liquid crystalline azomethines and polyazomethines were described and compared in this article.     

Synthesis and mesomorphism of 2,5-bis(3,4-bis(n-alkoxy)phenyl)thiazolo[5,4-d]thiazole tetracatenar liquid crystals

A series of new symmetrical 2,5-bis(3,4-bis(n-alkoxy)phenyl)thiazolo[5,4-d]thiazoles (TTn) prepared via condensation of dithiooxamide with six different 3,4-di-n-alkoxybenzaldehydes were characterized by ¹H NMR, FTIR, UV-Vis, and photoluminescence spectroscopy. Moreover, one compound lack of aliphatic chains, i.e., 2,5-diphenylthiazolo[5,4-d]thiazole (TT) was synthesized and characterized. The investigated TTn compounds emitted blue light. The effect of length of n-alkoxy (i.e., OC _n H_{2 n +1}, n?=?8, 10, 12, 14, 16, and 18) peripheral groups on thermal and mesomorphic behavior was investigated by means of differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. Variable heating and cooling rates were used to study the liquid-crystalline properties of TTn. All compounds exhibited liquid-crystalline properties which self-organize into one or two smectic mesophases (SmB and SmE).     

Study of Preferential Solvation in Binary Mixtures by Means of Frequency-Domain Fluorescence Spectroscopy

The preferential solvation of 8-N,N-(dimethylamino)-11H-indeno[2,1-a]pyrene, Py(S)DMA, in its transient charge transfer (CT) state in binary solvents such as toluene/DMSO liquid mixtures was studied by means of frequency-domain fluorometry. The data obtained were considered within the following kinetic scheme: the preferential solvation was described by the system of consecutive reversible reactions of which each step is associated with the absorption of one DMSO molecule in the first solvation shell of the fluoresent Py(S)DMA dipolar CT molecule. The rate constants of the first two reversible elementary processes (i.e., the decay of solvation complexes of Py(S)DMA with one and two polar molecules, k _–1 = 1.1 10⁹ s^–1 and k _–2 = 1.4 10⁹ s^–1) were determined.     

Anomalous photoluminescence and raman scattering behavior in heavily Mg+ ion-implanted InP

Mg⁺ ions were implanted into highly pure InP grown by the liquid encapsulated Czochralski (LEC) method in which the Mg concentration [Mg] was varied between 1×10¹⁵ cm^–3 and 3×10²⁰ cm^–3. Two annealing methods were used: furnace annealing (FA) up to 740° C and flash lamp annealing (rapid thermal annealing, RTA) up to 900° C. For characterization, photoluminescence (PL) spectra were measured between 2K and room temperature together with Raman scattering measurements at room temperature. An emission designated by g, which was attributed to a novel energy state of an isolated acceptor, was found to be produced for a rather low value of [Mg]. In addition, a broad emission denoted by [g–g], which was ascribed to acceptor-acceptor pairs, was observed below bound exciton emissions for moderate values of [Mg]. These features were quite similar to those previously observed in acceptor-doped GaAs when the background concentration of donors is extremely low. Two additional novel emissions located far below the band-to-acceptor emission were also obtained, and each showed a remarkable energy shift towards lower energy with increasing [Mg]. The binding energies of these emissions were estimated from the temperature dependence of PL spectra and the results suggest that they are complex-type radiative recombination centers, presumably donor-acceptor-type centers. A strong broad emission centered near the band-to-acceptor emission was observed for [Mg]=3×10²⁰ cm^–3. This observation indicates a formation of a new material between In, P and Mg, which was also attested by the appearance of a new TO-like Raman signal for [Mg] greater than 1×10¹⁹ cm^–3. A substantial difference of PL and Raman spectra was revealed for the two annealing methods, suggesting that the annealing behaviour of ion-implanted InP should be investigated more extensively in order to establish reliable annealing procedures.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Generation of cyclotron radiation by light holes in germanium

The properties of the light hole cyclotron resonance laser are studied in detail. The emission spectrum and the tuning characteristics are analysed. Two regions of continuously tunable stimulated emission between 30 to 50 cm^–1 and 70 to 90 cm^–1 are found. Besides the time behaviour of the emission the spectral gain is measured for the first time with the help of a secondp-Ge laser, A gain value of 0.2 cm^–1 is found, which is significantly larger than the value found for the light to heavy hole laser. An application of the tunable laser is demonstrated by measuring the central cell splitting the photoconductivity signal of shallow donors inn-GaAs.     

Synthesis and characterization of wide range mesogenic esters based on asymmetrical 2,5-disubstituted 1,3,4-thiadiazole

A homologous series of new 13 esters, 4-(5-(p-tolyl)-1,3,4-thiadiazol-2-yl)-phenyl-4-alkoxybenzoate, (IV_n), based on 1,3,4-thiadiazole core has been synthesized. The structures of these esters were confirmed by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance and mass techniques. Their mesophases behavior was investigated with hot-stage polarizing optical microscope and differential scanning calorimetry. The thermal stability for most of these derivatives was measured by thermal gravimetric analysis. All the target esters showed enantiotropic mesomorphic behaviors with nematic and nematic/smectic C phases. The phase transition temperatures and liquid crystalline properties were affected by the nature of heterocyclic ring and the length of the alkoxy chain. Only the nematic phase was observed in the first 10 derivatives, (n = 1–10), while the last 3, (n = 12, 16 and 18) showed nematic and smectic C phases. These compounds demonstrated high liquid crystalline ranges, both in heating and cooling cycles. The mesomorphic results obtained were compared with the reported analogs of similar constituents.     

Porous silicon with β-cyclodextrin modified surface for photoluminescence sensing of organic molecules in gas and liquid phase

Porous silicon surface was modified by photochemically activated hydrosilylation reaction with permethyl-6^I-alkenoylamino-6^I-deoxy-β-cyclodextrins terminated with linear alkenoyl spacers of various lengths. As compared to unmodified surface, derivatized surfaces revealed modified photoluminescence response in the presence of controlled amounts of various organic molecules in gas and liquid phase. For the selected set of analytes we observed most significant modification of photoluminescence response for aromatic compounds what corresponds to optimum molecular size for strong host–guest interaction with β-cyclodextrin cavity. Aliphatic compounds quenched photoluminescence from both unmodified and surface modified porous silicon. For low gas phase concentrations of aromatic analytes β-cyclodextrin modified porous silicon revealed photoluminescence enhancement, at higher concentrations common photoluminescence quenching was observed. The size-dependent host–guest interaction between β-cyclodextrin cavity and detected molecule was observed in photoluminescence quenching in the presence of aliphatic molecules in liquid phase. The role of the strength of host–guest interactions between detected analytes and β-cyclodextrin cavity on photoluminescence sensor response is discussed.     

Effect of electron irradiation on the properties of germanium-doped gallium arsenide

The Hall effect, electrical conductivity (77–370 K), and photoluminescence spectra (77 K) are studied in single-crystals of nuclearly doped GaAs (NDG) and GaAs doped with Ge by the metallurgical method after irradiation by electrons (E= 1 MeV, D=1.1·10¹⁵–3.8·10¹⁸ cm^–2). Initial electron concentrations were n= 1.7·10¹⁷ cm^–3 and n₀=2.6·10¹⁷ cm^–3 respectively. In the GaAs doped during crystal growth by the Czochralski method the degree of compensation related to the amphoteric impurity Ge is higher (K=0.8) than in the NDG (K=0.4) for identical initial electron concentration. It was established that the rate of charge carrier removal in GaAs is lower than in NDG, while radiation defects are more thermostable in NDG. The energy spectrum of radiation defects and radiating recombination centers, and the basic steps in reestablishment of electrophysical and optical properties in GaAs and NDG are similar, i.e., they do not depend on the method of germanium doping.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 82–86, April, 1991.     

Time-resolved emission spectroscopy of gadolinium vanadate ceramics (GdVO4:Bi3+)

The preparation of GdVO₄:Bi³⁺ ceramics is indicated. Bismuth shows a strong tendency to evaporate during the sintering process. Time-resolved emission spectroscopy shows for sufficiently low Bi³⁺ concentrations subsequently: blue VO ₄ ^3– emission with a decay time corresponding to the transfer rate (10⁶ s^–1), yellow VO ₄ ^3– –Bi³⁺ emission, rare-earth impurity emission and VO ₄ ^3– –Bi³⁺ afterglow.     

Luminescence of CsPbBr3-like quantum dots in CsBr single crystals

Luminescence and decay kinetics of the Pb²⁺ aggregates in CsBr host crystals were measured in the 4–300 K temperature interval and in 10⁻¹⁰–10⁻³ time scale. Their emission properties are similar to those of CsPbBr₃ bulk crystal showing a subnanosecond free exciton emission in the 520–540 nm spectral region and slower trapped exciton emission in the 530–580 nm spectral region. An efficient energy exchange between the free and trapped exciton states is shown by the temperature dependencies of emission spectra. The quantum size effect is demonstrated in the high energy shift and broadening of the absorption and emission spectra and an estimate of the size of the CsPbBr₃-like aggregates is provided. Independent evidence of the presence of the CsPbBr₃ and Cs₄PbBr₆ aggregated phases in the CsBr host was obtained by X-ray structural studies.     

Synthesis, characterization and low field emission of CNx nanotubes

Aligned CN_x nanotubes were fabricated by pyrolyzing ethylenediamine on p-type Si(1 1 1) substrates using iron as the catalyst. Scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectrum (XPS) and Raman spectroscopy were used to characterize the CN_x nanotubes. The CN_x nanotubes with the average length of 20 μm and diameters in the range of 50–100 nm have the “bamboo-like” structure and worse crystalline order. The low-field emission measurements of the CN_x nanotubes indicated that 20 μA/cm² current densities were observed at an electric field of 1.4 V/μm and 1.280 mA/cm² were obtained at 2.54 V/μm. The CN_x nanotubes exhibit better field emission properties than the carbon nanotubes and the BCN nanotubes. The emission mechanism of CN_x nanotubes is also discussed.     

Energy distributions and emission coefficients of secondary ions ejected from AIIIBV group semiconductors

The energy distributions of Ga⁺ and Ga₂ ⁺ secondary ions sputtered from the surface of single crystals of GaAs and GaP by Ar⁺ (E = 18 keV, j = 1·10^–6 A/cm²) ions were studied. Integral emission coefficients for secondary ions from four semiconductors (GaAs, GaP, InAs, and A1_0.35Ga_0.65As) bombarded by Ar⁺ ions (E = 2 keV, j₀ = 1·10^–6 A/cm²) were also measured. It is shown that the energy distribution of the Ga⁺ secondary ions are broad and qualitatively similar to distributions of secondary ions sputtered from metal oxide surfaces. Very high integral emission coefficients (up to 20–30%) for secondary ions are also observed, exceeding analogous coefficients for clean metal surfaces. These facts (the significant number of low energy secondary electrons and high secondary ion emission coefficients) are due to the lower density of free electrons in comparison with metals, which ensures a high degree of survival of sputtered ions, and the partially ionic character of the bonds in the semiconductors under study.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 88–92, November, 1990.     

Tunnett

Conclusion The p⁺–n⁺–i(v)–n⁺ GaAs Tunnett diodes have been fabricated with control of the growing diffusion technique in TDM CVP liquid phase epitaxial growth. The threshold current density (J_th) of 2.8×10⁴ A/cm², the threshold voltage (V_th) of 5.3V for the oscillation and the 1% duty cycle operation have been succeeded. The more reduction of J_th and V_th will be able to realize the CW operation.     

Electrical properties of doped 3-tetradecylpolypyrrole/metal devices

The electrical properties of devices made of doped 3-tetradecylpolypyrrole (PPy-C₁₄) thin films sandwiched between indium-tin-oxyde (ITO) and gold metal electrodes are reported. The current density–voltage (J–V) curves are asymmetric and nonlinear implying a non Ohmic rectifying contact. Using standard thermionic emission theory (Schottky) J–V characteristics were satisfactorily fitted with a saturation current of J₀=1.5×10^-5 A cm^-2, a barrier height of ϕ_b=0.7 eV, and an ideality factor of n=5.3. Characteristics from the plot of J–V versus 1/T show that the activation energy of the thermionic emission process is higher below the glass transition temperature of PPy-C₁₄ (T_g=45 °C) than above, which seems to indicate that the hopping conduction process is enhanced at T>T_g. The carrier concentration has been calculated from capacitance–voltage (C-V) measurements (N=1.9×10¹⁷ cm^-3) allowing estimation of the carrier mobility μ=2.6×10^-2 cm² V^-1 s^-1. PACS 73.61.Ph; 73.40.Sx; 73.30.+y     

Synthesis and field emission of diamond-like carbon nanorods on TiO2/Ti nanotube arrays

Highly ordered TiO₂/Ti nanotube arrays were fabricated by anodic oxidation method in 0.5 wt% HF. Using prepared TiO₂/Ti nanotube arrays deposited Ni nanoparticles as substrate, high quality diamond-like carbon nanorods (DLCNRs) were synthesized by a conventional method of chemical vapor deposition at 750 °C in nitrogen atmosphere. DLCNRs were analyzed by filed emission scanning electron microscopy and Raman spectrometer. It is very interesting that DLCNRs possess pagoda shape with the length of 3–10 μm. Raman spectra show two strong peaks about 1332 cm⁻¹ and 1598 cm⁻¹, indicating the formation of diamond-like carbon. The field emission measurements suggest that DLCNRs/TiO₂/Ti has excellent field emission properties, a low turn-on field about 3.0 V/μm, no evident decay at 3.4 mA/cm² in 480 min.     

R branch tuning characteristics of the13CH3F Raman FIR laser

Summary We described a¹³CH₃F Raman laser pumped by a grating tuned 20 atmospheres CO₂ laser. The emission characteristics of the¹³CH₃F laser extends from 14 cm^–1–35 cm^–1 and from 49 cm^–1–72 cm^–1; about 65% of these frequency ranges can be covered with tunable radiation. The characteristics shows a strong dependence on the rotaional quantum numbers of the states involved in the Raman laser transitions and, within each tuning interval, on the frequency offset with respect to the frequencies of resonant transitions. We obtained, at 51 cm^–1, a maximum FIR laser pulse energy of about 800 J (at a pump energy of 200 mJ), corresponding to a photon conversion of about 8%. In some cases we have observed simultaneous emission at a Raman and a cascade frequency. In addition, FIR emission power dependence on¹³CH₃F gas pressure and pump pulse power were investigated for different J quantum numbers.     

Fabrication and properties of ZnO:Cu and ZnO:Ag thin films

Thin films of ZnS and ZnO:Cu were grown by an original metal–organic chemical vapour deposition (MOCVD) method under atmospheric pressure onto glass substrates. Pulse photo-assisted rapid thermal annealing of ZnO:Cu films in ambient air and at the temperature of 700–800 C was used instead of the common long-duration annealing in a vacuum furnace. ZnO:Ag thin films were prepared by oxidation and Ag doping of ZnS films. At first a closed space sublimation technique was used for Ag doping of ZnO films. The oxidation and Ag doping were carried out by a new non-vacuum method at a temperature >500 C. Crystal quality and optical properties were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), and photoluminescence (PL). It was found that the doped films have a higher degree of crystallinity than undoped films. The spectra of as-deposited ZnO:Cu films contained the bands typical for copper, i.e. the green band and the yellow band. After pulse annealing at high temperature the 410 and 435 nm photoluminescent peaks were observed. This allows changing of the emission colour from blue to white. Flat-top ZnO:Ag films were obtained with the surface roughness of 7 nm. These samples show a strong ultraviolet (UV) emission at room temperature. The 385 nm photoluminescent peak obtained is assigned to the exciton–exciton emission.     

Evolution of the Electroluminescence Spectra and the Acoustic Emission of the Epitaxial Structures of GaAsP

The dependence of the spectral position of the electroluminescence bands of epitaxial light-emitting diode n ⁺–n–p structures (GaAs_0.15P_0.85) on the density of a direct heterojunction current at different successive instants of time — before an acoustic emission and after it — has been revealed. The shifts of the electroluminescence bands accompanied by acoustic emission can be divided into three types according to the density of the current: (1) short-term shifts due to relaxation of the defect structure of a sample — at relatively low currents, (2) the magnitude of the reverse shift being determined by the current density — at large currents, and (3) formation of an IR band (1.5–1.1 eV) with irreversible degradation changing in the red (1.75 eV) and green (2.19 eV) bands of the electroluminescence spectrum — at ultrahigh currents (100–200 A/cm²).     

Effect of annealing in N2 atmosphere on net acceptor concentration in ZnSe:N grown by MOCVD

Annealing effect on net acceptor concentration in ZnSe:N is investigated. ZnSe:N homo-epitaxial layer was grown at 823 K by MOCVD using ammonia (NH₃) as a dopant source. Photoluminescence (PL) spectra measured on as-grown layer exhibited the strong deep donor–acceptor pair (D^dAP) emission and the weak I₁^N emission line. In order to enhance the activation of nitrogen in ZnSe epitaxial layer, sample was annealed at the 823 K in nitrogen (N₂) and hydrogen (H₂) atmosphere. Only the annealing in nitrogen atmosphere increased I₁^N emission intensity indicate the activation of nitrogen acceptor. And net acceptor concentration was estimated to be 3 × 10¹⁷cm⁻³ by C–V measurements. This activation mechanism is interpreted as hydrogen is released from N–H bonds during annealing in nitrogen atmosphere.     

Nickel bolometers

A modified method of the production of nickel bolometers and their properties are described. A typical example is a bolometer of dimensions 005×2 mm and resistance 14.9 , the NEP (noise equivalent power) of which is 6.23×10^–11 Wc^–1/2 in air and 1.9×10^–11 Wc^–1/2 in vacuo. The paper also describes the influence of cooling the surroundings with liquid nitrogen on the properties of the bolometer. The NEP of a bolometer that works in vacuo with cooled surroundings is 5 times lower. A method is given for correcting the characteristics of two similar bolometers for use in a d.c. bridge which makes the equilibrium of the bridge independent of bridge-current fluctuations in a large range of current.