Fluorescence and phosphorescence behavior of TPD doped and TPD neat films

Photoluminescence spectra of N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)-benzidine (TPD) are studied in a temperature range from 10 K to room temperature using a neat TPD film, and 5 wt.% TPD doped films with polystyrene (PS), 4,4′-N,N′-dicarbazole-biphenyl (CBP), and polycarbonate (PC) as hosts. The photo-excitation occurred in the singlet absorption region of TPD. The blue fluorescence and green phosphorescence quantum yields, _F and _P, of TPD are determined from their quantum distributions, E_F(λ) and E_P(λ). The yields are found to be _F = 0.39 and _P = 0.012 for neat films at room temperature, while _F = 0.78 and _P = 0.026 are measured for a TPD doped PS film. The lower luminescence quantum yield in the neat film is caused by self-quenching.     

Coherent Brillouin scattering in noncentrosymmetric semiconductors: bound and free charge carriers contribution

With the aid of a hydrodynamic model for semiconductor plasmas, an analytical investigation of coherent Brillouin scattering (CBS) has been made in noncentrosymmetric (NCS) semiconductor plasmas both under the presence and in the absence of an externally applied magnetic field. Using the coupled mode approach, the nonlinear induced polarization and third-order nonlinear optical susceptibility, due to bound and free charge carrier nonlinearity, are obtained. The analysis further deals with the qualitative behavior of the threshold pump electric field E _T for onset of CBS and the resulting gain coefficient (steady-state as well as transient, [g _B]_SS,TR) in NCS semiconductor plasmas. Numerical estimates are made for InSb crystal at 77 K duly irradiated by a pulsed 10.6 μm CO₂ laser. The effects of piezoelectricity, doping concentration and magnetic field on both the E _T and [g _B]_SS,TR have been studied in detail. E _T required for onset of the CBS process is found to be lower when piezoelectricity is present and the doping level of the semiconductor is moderate than in other conditions. It is found that when the magnetic field is applied, the coherent backward Stokes wave can be amplified by a factor of 10² in NCS semiconducting crystals. The analysis also suggests the idea of pulse compression and the possibility of observing the phase conjugation reflection coefficient ～10⁶ which proves its potential for the fabrication of CBS-based phase conjugate mirrors.     

Molecularly ordered aluminum tris-(8-hydroxyquinoline) thin films grown by hot-wall deposition

We report on the growth and microstructural analysis of molecularly ordered thin film layers of aluminum tris-(8-hydroxyquinoline) (Alq₃) by hot-wall deposition onto amorphous glass substrates. Using transmission electron microscopy (TEM), ordering on a scale of 100 nm was observed. Raman measurements of these films indicated that they corresponded to the α-polymorph of crystalline Alq₃, and photoluminescence measurements exhibited a single broad peak centered at 500 nm, which is also consistent with the α-form. As a comparison, we deposited films of Alq3 using organic molecular beam deposition (OMBD), which exhibited no molecular ordering from the TEM studies. For these films, strong point-to-point variations in the Raman spectrum, and the existence of a double peak in the photoluminescence at 500 and 522 nm were observed. These measurements indicate that the OMBD films possess a mixture of both α and amorphous phases.     

Influence of thermal annealing on photoluminescence and structural properties of N,N′-diphenyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) organic thin films

Photoluminescence (PL) spectra and intensities of thin N,N′-diphenyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) films have been measured at room temperature, during sample heating with various rates and annealing times at constant temperatures, and after annealing. It was found that the temperature of T = 80 °C being considerably lower than the glass transition temperature of α-NPD is sufficient to cause substantial irreversible changes in PL and PL excitation characteristics. A PL efficiency increase up to 10 times, an emission spectrum short-wavelength shift up to 130 meV and a spectral narrowing from 69 to 39 nm are reached using annealing. The surface roughness of the films annealed at the moderate temperature of 80 °C does not undergo observable changes contrary to films annealed at higher temperatures.     

真空高压气淬炉中淬火气体压力、流速和类型对工件冷却性能影响的数值模拟

本文运用FLUENT软件,通过大量的计算机模拟,研究了真空高压气淬炉中淬火气体压力、进口速度、气体类型对工件冷却性能的影响.通过对比氮气在0.45 MPa、0.6 MPa、1.0 MPa和1.5 MPa淬火压力下工件的冷却速度,量化了淬火压力对工件冷却速度的影响程度.氮气在0.6 MPa下,将气体速度由40 m/s增至60m/s,工件冷却速度提高27％,但风机功率增加3.4倍.由于气体体积流量一定的情况下,淬火气体比热和密度的协同影响了换热系数的大小,通过计算机模拟了四种淬火气体氢气、氦气、氮气和氩气对工件冷却速度的影响,得出在相同气体压力和流量下,四种气体的冷却能力是:氮气＞氢气＞氦气＞氩气;在消耗相同的风机功率下,密度小比热大的气体冷却能力高,四种气体的冷却能力依次是氢气＞氦气＞氮气＞氩气.     

Post-processing of spin-coated organic thin films in solvent vapors: Vapor pressure monitoring by infrared absorption and the effect of electric fields

We have examined the effect of the post-processing of spin-coated tetracene and DAST (4′-dimethylamino-N-methyl-4-stilbazolium tosylate) films in their solvent vapor to examine the possibility of precisely controlling the vapor pressure and studying the effect of applying electric fields to their structural properties. It was demonstrated that the solvent concentration in the vapor phase can be monitored by infrared spectroscopy. The thickness uniformity of the tetracene films was improved by the formation of a mixture of the tetracene crystals with the saturated solution (“solution ultrathin film”) on the surface. Crystals with an ordered orientation were obtained by additionally applying an electric field during the vapor exposure of the DAST. The present technique for monitoring the solvent vapor pressure is also applicable for controlling the drying speed during the inkjet printing of functional organic materials.     

Diffusion Coefficients Estimated by Dynamic Fluorescence Quenching at High Pressure: Pyrene, 9,10-Dimethylanthracene, and Oxygen in n-Hexane

The fluorescence quenching of pyrene (PY) by carbon tetrabromide (CBr₄) at pressures of up to 400 MPa in n-hexane was investigated. It was found that the fluorescence quenching is not fully, but nearly, diffusion-controlled. From the pressure-induced solvent viscosity dependence, the quenching rate constant, k _q , was separated into the contributions of the bimolecular rate constant in the solvent cage, k _bim, and that for diffusion, k _diff. Using the values of k _diff separated, together with those of the diffusion coefficient of CBr₄, the diffusion coefficients of PY were successfully estimated. This analysis was applied to the quenching systems of 9,10-dimethylanthracene (DMEA)/CBr₄ and of PY/O₂ and DMEA/O₂ that were studied previously. Using the values of k _diff for these systems, together with those of the corresponding diffusion coefficients of the fluorophore or quencher, the diffusion coefficients of DMEA and O₂ were also evaluated. Based on the results, the pressure-induced solvent viscosity, , dependence on the diffusion coefficients is discussed.     

Impedance spectroscopy measurements of charge carrier mobility in 4,4'-N,N'-dicarbazole-biphenyl thin films doped with tris(2-phenylpyridine) iridium

The charge carrier mobility of green phosphorescent emissive layers, tris(2-phenylpyridine) iridium [Ir(ppy)₃]-doped 4,4'-N,N'-dicarbazole-biphenyl (CBP) thin films, has been determined using impedance spectroscopy (IS) measurements. The theoretical basis of mobility measurement by IS rests on a theory for single-injection space-charge limited current. The hole mobilities of the Ir(ppy)₃-doped CBP thin films were measured to be 10^− 10–10^− 8 cm²V^− 1 s^− 1 in the 2–7 wt.% Ir(ppy)₃-doped CBP from the frequency dependence of both conductance and capacitance. These hole mobility values are much lower than those of the undoped CBP thin films (~ 10^− 3 cm²V^− 1 s^− 1) because the Ir(ppy)₃ molecules act as trapping centers in the CBP host matrix. These mobility measurements in the Ir(ppy)₃-doped CBP thin films provide insight into the hole injection process.     

Microstructure evolution and age hardening in (Ti,Si)(C,N) thin films deposited by cathodic arc evaporation

Ti_1 − xSi_xC_yN_1 − y films have been deposited by reactive cathodic arc evaporation onto cemented carbide substrates. The films were characterized by X-ray diffraction, elastic recoil detection analysis, transmission electron microscopy, energy-dispersive X-ray spectroscopy, electron-energy loss spectroscopy and nanoindentation. Reactive arc evaporation in a mixed CH₄ and N₂ gas gave films with 0 ≤ x ≤ 0.13 and 0 ≤ y ≤ 0.27. All films had the NaCl-structure with a dense columnar microstructure, containing a featherlike pattern of nanocrystalline grains for high Si and C contents. The film hardness was 32-40 GPa. Films with x > 0 and y > 0 exhibited age-hardening up to 35-44 GPa when isothermally annealed up to 900 °C. The temperature threshold for over-ageing was decreased to 700 °C with increasing C and Si content, due to migration of Co, W and Cr from the substrate to the film, and loss of Si. The diffusion pathway was tied to grain boundaries provided by the featherlike substructure.     

Hysteresis,structural and composition characteristics of deposited thin films by reactively sputtered titanium target in Ar/CH4/N2 gas mixture

Abstract

Hysteresis, crystal structure and chemical composition of thin films deposited through reactive sputtering of titanium metal target in Ar/CH₄/N₂ gas mixture have been investigated. The transition from metallic to compound sputtering mode was clearly seen as the reactive gases (CH₄ and N₂) flowrate concentration first increased and subsequently decreased. Abrupt cathode current drop from 273 mA to reach a minimum value of 195 mA was observed upon addition of nitrogen gas from 0 to 10% flowrate concentration to the Ar/CH₄ gas mixture. This was also accompanied by an abrupt change in reactive gas partial pressure. Exploration of the deposition rate and film thickness showed that it decreased from 4·5 to 1·5 nm min^?1 and from 140 to 40 nm as the N₂ flowrate concentration increased from 1·5 to 7·5% at 5·5%CH₄ flowrate concentration respectively. X-ray diffraction and X-ray photoelectron spectroscopy analyses of the deposited films confirmed the formation of titanium carbide and carbonitride phases as the methane and nitrogen gas concentrations in the sputtering gas were increased.