Search

Introduction: In the applications of room temperature detector for high-energy radiation, there are two critical requirements for the semiconducting material cadmium zinc telluride (CdZnTe): (1) high electrical resistivity to reduce the bulk leakage current and (2) low levels of structural defects which hinder the detectivity as trapping and recombination centers for the carriers. To enhance the performance of the detectors, an optimal single process has been developed in the melt growth of Cd0.8Zn0.2Te by directional solidification with controlled Cd overpressure to maximize the electrical resistivity as well as minimize the structural defects, including Te precipitates/inclusions of the grown CdZnTe crystals. Method: Using the phase diagram data of pressure-temperature-composition (P-T-X), melt growth of Cd0.8Zn0.2Te crystals by directional solidification from a starting melt at 1145oC has been performed with various Cd overpressures controlled by the temperature of a Cd reservoir. The grown crystals were sliced and were characterized by electrical resistivity measurements and chemical analysis of Glow Discharge Mass Spectroscopy (GDMS). The structural defects were studied by the infrared (IR) transmission images taken by an IR microscope. Result: By doping of In (4 – 6 ppm, atomic) and growing with a Cd reservoir in the range of 785 to 825oC, the electrical conductivity were consistently higher than 109cm and up to 2x1011cm. From the trend of the Te precipitates density observed by the IR micrographs, it was concluded that a Cd reservoir temperature of 820+10oC resulted in the lowest precipitate density. Conclusion: The employment of a Cd reservoir temperature of 820+5 oC during the growth process will provide the optimal Cd pressure over the melt at 1145oC to maximize the electrical resistivity as well as minimize the structural defects, including Te precipitates/inclusions of the grown Cd0.8Zn0.2Te crystals. Discussion: Since the solids of different compositions, x in the Cd1-xZnxTe system, have different liquidus/solidus temperatures as well as different homogeneity ranges, the procedure presented here for the Cd0.8Zn0.2Te solid may not applicable to other compositions.

In this study, the solidus curve of the Cd0.8Zn0.2Te homogeneity range was constructed from the partial pressure measurements by optical absorption measurements which provided the information of the melt-growth parameters to achieve crystals with the required electrical resistivity. The melt growth of Cd0.8Zn0.2Te crystals was then processed by directional solidification under controlled Cd overpressure. During the growth experiments, several procedures have been developed to improve the crystalline quality: (1) reducing the structural defects from wetting by HF etching of fused silica ampoule, (2) minimizing the contamination of impurities during homogenization, (3) promoting single-crystal growth by mechanical pulsed disturbance, and (4) maximizing electrical resistivity and minimizing Te precipitates by controlling Cd overpressure during growth and post-growth cooling. Additionally, the thermal conductivity, electrical conductivity, and Seebeck coefficient of a vapor-grown CdTe and two melt-grown Cd0.8Zn0.2Te crystals were measured between 190 °C and 780 °C to provide an in-depth understanding of the thermal and electrical conduction mechanisms of the crystals as well as the prospect of its thermoelectric applications.

ZnSe has been a great choice for the rare-earth and transition metal doping to develop lasers. It is an excellent material for variety of optical applications due to wide transparency range, good fabricability and very low optical absorption similar to other selenides. NASA Marshall Space Flight Center has developed large crystals using physical vapor deposition (PVD) doped with transition metals for lasing. GaAs based quasi-phase matched structures have a lot of limitations including difficulty of frequency conversion from available high-power lasers. We are developing Si- and GaAs- based templates and using microfabrication process to deposit ZnSe using physical vapor transport (PVT) method. Experimental results of the fabrication of templates and growth of ZnSe on templates will be presented.

During the past several decades physical vapor transport (PVT) method has been extensively used for developing laser and electronic and optical sensor materials especially for incongruent and high vapor materials. Extensive careful studies of the NASA Marshall Space Flight Center on ZnSe growth by PVT has demonstrated that both thermal and solutal convection play very important roles on the quality of crystals and can be controlled by microgravity experiments. In case, the growth is performed by sputtering or systems such as DENTON, it is very difficult to control fluid flow and both thermal and solutal convective flows. We have demonstrated that by controlling the transport path, temperature of substrate and source and using purified source micron size thick ness can be achieved. We will present the experimental results of pure and doped lead selenide (PbSe) which demonstrated various morphologies and bandgap based on size of particles based on growth condition.

Thermoelectric devices convert thermal energy, i.e. heat, into electric energy. With no moving parts, the thermoelectric generator has demonstrated its advantage of long-duration operational reliability. The IV–VI compound semiconductor PbTe-based materials have been widely adopted for the thermoelectric applications in the medium temperature range of 350–650 °C. In most of the reports, thermoelectric materials were manufactured by a hot pressing or quench and annealing method. The recent advancements in the converting efficiency of thermoelectrics, including PbTe-based materials, have been attributed to the modification on material inhomogeneity of microstructures by hot pressing or simply cooling the melt to reduce the thermal conductivity. On the other hand, due to its time-consuming preparation/processing and unnecessary good crystalline quality (for thermoelectric applications), the processing of thermoelectric materials by crystal growth resulted in very few investigations. In this report, the design and growth of the PbTe-based materials solidified from the melt for thermoelectric applications as well as the results of their thermoelectric characterizations will be reviewed. It shows that, besides its Figure of Merit comparable to other processing methods, the melt grown PbTe material has several additional capabilities, including the reproducibility, thermal stability and the functional gradient characteristics from the variation of properties along the growth length.

Doping of semiconductors is a process of intentionally incorporating impurity into the materials to adjust and optimize the electrical properties during the processing of semiconductors. The doping level has certain upper limit, which is usually corresponding to the solubility of the dopant in the host material under processing conditions. Sometimes, the maximum solubility level is still not high enough to provide the desired opto-electronic properties and a higher doping level is needed. Hence, enhancing the dopant level is one of the critical issues in the semiconductor industry, especially for those advanced devices made from compound semiconductors, including binary, ternary, as well as multi-component compounds. In this report, we designed a processing method, by simply varying a processing parameter during melt growth, to increase the doping level in the compound semiconductors well above the maximum values obtained under otherwise regular processing procedures and demonstrated it in the melt growth of Cl-doped PbTe.

The density of Te, HgTe, Hg0.9Cd0.1Te, Hg0.8Cd0.2Te, Hg0.9Zn0.1Te and Hg0.84Zn0.16Te melts were measured by a method similar to the pycnometric technique which measures the volume of a melt, with known mass, in a sealed fused silica ampoule positioning coaxially inside a transparent furnace. The volume was determined from the height of the meniscus, by in situ visual observation, and the calibration of volume vs. height for the ampoule which was performed by measuring the mass of water at different heights before the sample was loaded. The density measurements covered the temperature ranging from the corresponding melting points or liquidus temperatures to about 1130 K. The Te liquid showed the normal thermal expansion, i.e., its density decreases as temperature increases except at temperatures just above the melting point where it went through a maximum. Similar to the behavior of the Te melt, the density of HgTe, Hg0.9Cd0.1Te melts also went through a maximum as a function of temperature. It has been reported by other measurements, such as neutron scattering and nuclear magnetic resonance, that the melt consists of a mixture of two domains—one with low-coordination number and the other with high-coordination number structure which corresponding to low- and high-density melt, respectively. At temperature just above the melting point, the melt structure started to evolve with the low-coordination number domain changing into high-coordination number one which resulted in an increase in melt density. As temperature increasing to the density maximum, most of the structural changes has been completed and the normal thermal expansion took over and the density started to decrease as the temperature keeps on increasing. The measurements on the melts of Hg0.8Cd0.2Te, Hg0.9Zn0.1Te and Hg0.84Zn0.16Te did not show a maximum as functions of temperature probably because the liquidus of these melts are higher than the temperature where the density maximum occurred.

The viscosities of Te, HgTe, Hg0.9Cd0.1Te, Hg0.8Cd0.2Te, and Hg0.84Zn0.16Te melts were measured. Earlier in the research, the viscosities of HgTe, Hg0.8Cd0.2Te and Hg0.84Zn0.16Te melts have been measured by an oscillating-cup (OC) viscometer. However, the long duration for the measurements by OC, approximately an hour of continuous data collection, prevented the studies of systems with shorter relaxation time. Therefore, a novel transient torque viscometer (TTV) was developed to rapidly measure the viscosity by creating a rotating flow of the melt in the presence of a uniform rotating magnetic field (RMF). The measured transient rotation of the ampoule can be used to determine the viscosity of the melt. Because this transient process finishes in very short time, the required measurements can be completed within two minutes. More importantly, with the Lorentz force introduced by the RMF to the melt, the electrical conductivity can also be simultaneously measured as the torque that induced by the interaction between the RMF and the melt is a linear function of electrical conductivity of the melt. Then the viscosity of Te, HgTe, Hg0.9Cd0.1Te and Hg0.8Cd0.2Te melts and the electrical conductivity of the solids and melts of Te, HgTe, Hg0.9Cd0.1Te and Hg0.8Cd0.2Te were measured by TTV. The measured viscosity by TTV showed more consistency and less scattering comparing to the data obtained from the OC method. The theoretical analyses on the viscosity data of Te melt indicate a structural transition occurring in the liquid around 873 K, about 150 K above its melting point, which is consistent with the finding on the density of the liquid Te given in Chap. 4. The analyses of the measured viscosity on the melts of HgTe, Hg0.9Cd0.1Te and Hg0.8Cd0.2Te implies a structural transition approximately at 1078 K and 1066 K, respectively, for HgTe and Hg0.9Cd0.1Te and no structural transition in the measured temperature range for the Hg0.8Cd0.2Te melt. The measured electrical conductivity of the solids indicated a characteristic of metals or degenerate semiconductors. Above the melting point of HgTe and the solidus temperatures of Hg0.9Cd0.1Te and Hg0.8Cd0.2Te, the measured electrical conductivity increases with increasing temperature, indicating a semiconductor-like behavior. As the temperature increasing, the measured electrical conductivity started to reach saturated values, implying a metallic behavior. The time relaxation behavior was studied by monitoring viscosity and electrical conductivity after rapidly lowering the sample temperature after long time soaking at elevated temperature. Two samples out of these melts, Hg0.84Zn0.16Te and Hg0.8Cd0.2Te, showed relaxation behavior although their trends were different. During the relaxation, the viscosity of the Hg0.84Zn0.16Te melt continuously increased by about 50% to reach its steady state value after 120 h whereas the viscosity for the Hg0.8Cd0.2Te melt went through a maximum and then decreased 62% to its equilibrium value in about 60 h. At the same time, the measured electrical conductivity of Hg0.8Cd0.2Te melts initially decreased and reached a minimum at approximately 3.4 h then it increased 5% to the equilibrium value after 50 h.

Chromium doped ZnSe nanocrystals were grown at low temperature using zinc acetate and sodium selenite. A capping agent was used to avoid agglomeration of particles. It was observed that the addition of the capping agent before or after chemical synthesis of ZnSe played a very important role in controlling the size of nanoparticles and to avoid agglomeration. Size of nanoparticles of ZnSe was as small as 10 nm for both doped and undoped material. In a few cases, in spite of preventative measures, clustering of particles produced large agglomerates at room temperature. Cr-ZnSe nano particles showed fluorescence at different wavelengths compared to Cr-doped bulk crystals. Cr-doped nanocrystals showed higher bandgap than PVT grown bulk Cr-ZnSe crystals. The fluorescence intensity for Cr-ZnSe nanoparticles was significantly higher compared to undoped ZnSe nanoparticles.

During the thermal conductivity measurements of the HgTe-based ternary melts, because the samples need to be hermetically contained inside a sealed fused silica ampoule the transient measurement by laser flash method was adopted. In most of the reports, measurements were performed to obtain thermal diffusivity first and then, thermal conductivity was calculated from the data of density and heat capacity. The density of the HgTe-based melts has been reported in Chap. 4 and the thermodynamic and phase diagram data, including the heat capacity, for the Hg–Cd–Te and Hg–Zn–Te ternary systems were obtained by assuming an associated solution model for the liquid phase as illustrated in Chap. 3. The detailed sample ampoule preparation, experimental setup and procedures for the thermal diffusivity measurements by laser flash method are presented. Then the thermal diffusivity measurements have been performed on both solids and melts of the pseudo-binaries of Hg1-xCdxTe, x = 0, 0.05, 0.1, 0.2 and 0.3, and Hg1-xZnxTe, for x = 0.10, 0.16, and 0.30. The corresponding thermal conductivity were then calculated. The data show similar trends for both systems. The thermal conductivity of the solid decreased gradually as temperature increased and reach a minimum at solidus temperature. The data of the melts show an almost linear dependence on temperature after the samples were heated above the liquidus temperatures. At a fixed temperature, the measured values of thermal diffusivity and conductivity decreased monotonically with x, the content of CdTe. The alternate approach for the measurement of thermal conductivity basically used the same laser flash method but analyzed the data differently by including the heat transfer between the sample melt and the fused silica cell. Based on the assumption that the temperature response of the sample melt, including the rapid rise and the slow fall, was controlled by the thermal diffusivity and conductivity of the sample and the fused silica cell, both thermal diffusivity and thermal conductivity of the sample can be obtained by a computational fitting process. An analytical solution, using parametric values and graphical results from the numerical simulation of the transient heat transfer, was developed.

During phase transitions in various systems, when changes in observable quantities of the system occur, they are often accompanied by pre-transitional or pre-cursor phenomena and, sometimes, display behavioral anomalies on the thermophysical properties. The origin of the phenomena is the formation of nuclei, or molecular complexes, of different competing thermodynamic phases inside the host phase. The formations of these nuclei are called the hetero-phase fluctuations. As opposed to the equilibrium homo-phase fluctuations, which can be described by equilibrium thermodynamics or statistical mechanics, systems with hetero-phase fluctuations are in quasi-equilibrium, or local equilibrium. In those phase transitions that one or more first derivatives of the relevant thermodynamic potentials, as a function of their variables, exhibit discontinuities may be called first-order, or discontinuous transitions. Those transitions in which the first derivatives of the thermodynamic potentials at the transition points remain continuous while only higher-order derivatives become divergent or change discontinuously, may be termed continuous transitions. In a normal scientific sense, a transition should exist over a region, not at one point and the hetero-phase fluctuation around a phase-transition point can be considered as a phenomenon that smears a transition into a region. A great number of experiments describing the hetero-phase states during the nucleation process has been reported in various phase transitions, which included solid–liquid, liquid–vapor, glass-liquid, martensitic, ferroelectric, superconducting and magnetic transitions. As one type of structural phase transitions, there were also experimental studies of the semiconductor–metal transition on the Se–Te, Ga2Te3, In2Te3 and Al2Te3 liquid solutions as well as on the HgTe-based pseudo-binary melts by the measurements of density, viscosity, electrical conductivity, thermal conductivity, Hall coefficient and mobility, thermoelectric power, coefficient of volume expansion, isothermal compressibility, Knight shift, magnetic susceptibility, relaxation phenomenon and structure factor. The results have confirmed the proposed description of inhomogeneous hetero-phase that the liquids consist of two well-defined domains, which have the same composition but different structures and transport properties. Investigations of the thermophysical properties on various semiconducting compound melts, such as the electrical conductivity, magnetic susceptibility, viscosity, density, etc., have been conducted to understand and to develop the fundamentals of the growth processes. Some interesting experimental results have been observed for those systems with solid phase of narrow homogeneity ranges such as the Zn–Te, Cd–Te, Mg–Bi, Tl–Te, In–Te and Ga-Te systems. In other cases of self-seeded melt growth of II–VI compounds, namely, CdTe, CdZnTe and ZnSe, there were a large number of experimental facts showing the correlation between melt state and crystal quality. It is believed that the crystalline quality can be improved if the melt was either markedly superheated or long-time soaked before the growth is commenced. It was suggested that the thermal history of how the solid sample was solidified governs the structure of the molten state near its melting point. These results suggest that the liquid state and structure of these narrow homogeneity range II–VI semiconductors need an in-depth examination to enhance the fundamental knowledge of hetero-phase fluctuations phenomena in the melts so as to improve the melt growth processes of these semiconductors.

In the associated solution treatment of a three-element system such as the Hg-Cd-Te (or Hg-Zn-Te) system, the liquid is assumed to consist of five species: Hg, Cd and Te atomic species and HgTe and CdTe (or ZnTe) molecular species. The various thermodynamic quantities will be derived from the model in terms of the mole fractions of these species. Having chosen the species in the liquid phase, the thermodynamic characterization of the liquid will be completed by assuming an expression for the excess Gibbs energy of mixing, ∆G M x , of forming the solution from the liquid elements with interaction parameters between various species. To optimize the values of these parameters and to delineate the phase diagram of the ternary system, such as liquidus temperatures, a thermodynamic description for the solid solution (A1-uBu)2-yCy is also required and the experimental information from partial pressure measurements on the solid solution is consistent with a simple quasi-regular solution. The procedure of fitting the phase diagram and thermodynamic properties starts with selecting values from the experimental data on the constant pressure heat capacity for the elements and the binary compounds as well as the vapor pressures of the elements. Then, values for the standard enthalpy and entropy of formation at 298 K (25 °C), the melting point, and the enthalpy of fusion are chosen for each compound. Before the fitting, certain thermodynamic constraints and auxiliary conditions were imposed to reduce the number of interaction parameters and to provide thermodynamic consistence of the model. With such constraints applied, the remaining liquid model parameters for each binary telluride system are fixed by obtaining the closest fit possible to the liquidus line and the partial pressures along some portion of the three-phase curve of the compound. The remaining parameters of the liquid phase model for each ternary system are fixed by obtaining an optimal fit to the experimental pseudo-binary liquidus and solidus. The calculated values for any remaining data, such as ternary liquidus points or partial pressures along the solid solution three-phase curves, are obtained and a measure of fit calculated. Then, all the thermodynamic properties of the melt in the systems, such as heat of mixing, heat capacity and composition fluctuation factor, can be generated as functions of temperature and composition either for the comparison with other existing data or to extrapolate the knowledge where no experiments have been performed. The liquid model and the procedure described above have also been practiced on the In-Ga-Sb systems with satisfactory results. Using the same approach, the recent analysis on the Hg-Cd-Zn-Te quaternary systems is more exact in a thermodynamic sense than the previous one although both obtained quite similar fits for the Hg-Cd-Te system. The results on the ternary systems of Hg-Cd-Te and Hg-Zn-Te will be presented here.

The effect of sulfur, iron, and chromium doping on the electrical characteristics of ZnSe single crystals was studied. The crystals, grown by the physical vapor transport method (PVT) at NASA Marshall Space Flight Center, were characterized by measuring electrical resistivity, capacitance, and dielectric constant using LCR meter. The morphology was studied by scanning electron microscopy to determine the crystallinity and micro defects. The measured resistivity and dielectric constant showed tunability as the function of frequency in the range of 100 Hz to 100,000 Hz, indicating the suitability of doped material for tuning devices. Besides, for the range from 50 mV to 1000mV, there was no difference in values for the studied frequency range, indicating no degradation or breakdown in the material. All doped ZnSe crystals with sulfur, iron, and chromium showed a similar trend as the function of frequency. Cr-ZnSe showed very high resistivity and lower dielectric constant compared to S-ZnSe and Fe-ZnSe crystals.

The crystalline structural quality of the grown crystals were examined by synchrotron white beam X-ray topography (SWBXT), high resolution triple axis X-ray diffraction (HRTXD) analysis, cathodoluminescence (CL) imaging and chemical etching. Two modes of SWBXT have been performed: (1) the transmission mode when the images of X-ray transmitted through a thin sample slab and (2) the reflection mode from the sample surface. Detailed topographs and HRTXD analyses have been performed on self-seeded grown ZnTe and ZnSe crystals as well as seeded grown ZnSe crystals. The major structural defects revealed on these vapor grown II–VI semiconductors were twins, subgrain boundary and dislocations. Chemical etching and cathodoluminescence revealed the density and shape of the dislocations in grown crystals of ZnSe, CdTe and CdS. The concentrations of impurities and dopants in the grown crystals were examined by Secondary Ion Mass Spectroscopy (SIMS) and Glow Discharge Mass Spectroscopy (GDMS) whereas the distributions of impurities and native point defects were mapped by photoluminescence (PL). The compositional distributions in the grown crystals of ternary ZnSeTe crystals were studied by wavelength dispersive X-ray spectroscopy (WDS), optical transmission measurements and precision density measurements.

The temperature dependences of thermal, electrical conductivity and Seebeck coefficients provide the critical information on the thermoelectric properties of the materials. The recent advancements in the converting efficiency of thermoelectrics have been attributed to the modification on material inhomogeneity of microstructures by hot pressing or simply cooling the melt to reduce the thermal conductivity. On the other hand, due to its time-consuming preparation/processing and unnecessary good crystalline quality (for thermoelectric applications), the processing of thermoelectric materials by crystal growth resulted in very few investigations. In this section, the thermoelectric properties of CdTe, grown by PVT, will be presented and compared with CdZnTe grown by directional solidification from the melt. Two different types of CdS crystals were produced from the PVT process in our laboratory, i.e., the “clear” and the “dark”, even though the growth parameters were kept essentially the same. To investigate the cause of this optical difference, the studies were emphasized on the stoichiometry changes of the crystals through the formation of point defects and their complexes. In the study, the approach was to subject the CdS crystals to heat treatments in the presence of Cd, S, and Se vapor and then compared their low-temperature photoluminescence (PL) spectra complemented with the measurements of electrical properties.

The crystal growth of physical vapor transport (PVT) transforms the original source material powder into the final form of compound semiconducting crystal in a closed ampoule. The vapor species were transported from the source at one end of the ampoule to the crystal at the other end because of the applied temperature gradient between the source and the crystal. To understand the fundamentals of PVT, one of the most important parameters during PVT, the partial pressures of the vapor species in equilibrium with the compounds as a function of temperature with different stoichiometry, have been measured by optical absorption technique to establish the three-phase curve. Then using an associated solution model for the liquid phase, which is assumed to consist of certain atomic/molecular species, the Gibbs energy of mixing for the liquid can be expressed in terms of the interaction parameters between these species. After the establishment of the best-fit parameters, the complete phase diagram and thermodynamic properties of the system can be generated for the applications of crystal growth experiments. The thermodynamic analysis has been applied to binary, ternary and quaternary systems such as Hg–Te, Cd–Te and Hg–Cd–Te as well as In–Sb, Ga–Sb and In–Ga–Sb, Hg–Cd–Zn–Te, Zn–Se and Zn–Se–Te. Then, a one-dimensional diffusion model, which includes the vapor species in equilibrium with a binary compound and the residual inert gases, was established to identify the critical growth parameters such as the heat treatment conditions, the thermal field for the growth process, the composition of the grown (ternary) crystal as well as the growth rate. From the results of the one-dimensional diffusion analysis, four experimentally adjustable parameters: the source temperature, the deposition temperature, the partial pressure ratio over the source and the residual gas pressure, determine the diffusive mass flux in a PVT system. However, two of these four parameters, the partial pressure ratio over source and the residual gas pressure, are more critical than the others.

The vapor transport rates have been measured experimentally for the systems of CdS, CdTe and ZnSe under various experimental conditions. To enhance the transport rate, the optimum procedures in the preparations of fused silica ampoule and heat treatments of starting materials have been established experimentally. From the theoretical results, the heat treatments were aimed at reducing the residual gas as well as shifting the source material toward congruent sublimation. A hydrogen reduction treatment was established in reducing the oxygen-related residual gas species and a baking under dynamic vacuum procedure was effective in changing the stoichiometry of source materials. Hence, the hydrogen reduction followed by the dynamic vacuum treatment procedure was established for the source to maximize the mass flux. Several in-situ dynamic measurements have been employed to monitor the partial pressures and transport rates. The mass flux in the ternary systems of ZnSe1−xTex, ZnSe1−xSx and Zn1−xCdxSe have been calculated from the one-dimensional diffusion model for the ternary systems and experimentally measured on the systems of ZnSe1−xTex and ZnSe1−xSx.

Acoustic vibration is often used to improve the yield of crystals and nanoparticles growing from solutions and melts. As there is still a debate on how acoustic vibration actually works, we have examined the possibility that acoustic pressure can affect the rate of nucleation. Our method is based on an expansion of the free energy of the nucleus in powers of the acoustic pressure. With the assumption that the period of the sound wave is short as compared to the time scale for nucleation, we replace the powers of the acoustic pressure by their time averages, retaining the average of the square of the acoustic pressure as the leading term. By assuming a nucleus having spherical shape, we use the Young-Laplace equation to relate the pressure inside the nucleus to the ambient pressure. Without making further approximations not already standard in classical nucleation theory, we find that the proximate effect of acoustic pressure is to reduce both the size of the critical nucleus as well as the work required to form it from monomers. As the work serves as the activation energy, the ultimate effect of acoustic pressure is to increase the rate of nucleation. If we assume that the atomic structure of the nucleus is the same as that of an ordinary solid, however, we find the compressibility is too small for acoustic vibration effects to be noticeable. If on the other hand, we assume that the structure is similar to that of a loosely bound colloidal particle, then the effects of acoustic vibration become potentially observable.

Background: Candida albicans, a common fungal pathogen that can cause opportunistic infections, is regarded as an apparently asexual, diploid fungus. A parasexual cycle was previously found between homozygotes with opposite mating type-like loci (MTLa/α). Fluconazole-resistant strains had a higher proportion of MTL homozygotes, whereas MTL homozygous C. albicans was found in only about 3.2% of clinical strains. MTL heterozygotes had a low frequency (1.4 Ã 10â4) of whiteâopaque switching to MTL homozygotes in nature. Methods: Here, a reference C. albicans strain (SC5314) was used in a fluconazole-induced assay to obtain standard opaque MTL homozygous strains and first-generation daughter strains from the fluconazole inhibition zone. Further separation methods were employed to produce second- and third-generation daughter strains. Polymerase chain reaction analysis based on MTL genes was used to define MTL genotypes, and microscopic observations, a flow-cytometric assay, and an antifungal E-test were used to compare microbiological characteristics. Results: MTL homozygotes were found at a high frequency (17 of 35; 48.6%) in fluconazole-induced first-generation daughter strains, as were morphological polymorphisms, decreased DNA content, and modified antifungal drug susceptibility. High-frequency MTL homozygosity was identified inside the fluconazole inhibition zone within 24 hours. The DNA content of fluconazole-induced daughter strains was reduced compared with their progenitor SC5314 and standard MTL homozygous strains. Conclusion: Treatment with fluconazole, commonly used to treat invasive candidiasis, inhibited the growth of C. albicans and altered its microbiological characteristics. Our results suggest that fluconazole treatment induces the high frequency of loss of heterozygosity and microbiological polymorphism in C. albicans. Keywords: Candida albicans, fluconazole, loss of heterozygosity, mating type-like gene

Composite dressing composed of Rhizochitosan and Regenplex™ to promote wound healing were assessed. Rhizochitosan was fabricated by deacetylation of Rhizochitin, which obtained by simply depigmenting sporangium-free mycelial mattress produced from Rhizopus stolonifer F6. Physicochemical characterizations of Rhizochitosan demonstrated that it contained 13.5% chitosan with a water-absorption ability of 35-fold dry weight and exhibiting hydrogel nature after hydration. In a wound-healing study on SD rats with full-thickness injury, the composite dressing had a better healing effect than those for each individual components and control group and wound even healed as functional tissue instead of scar tissue. The underlying mechanism of the composite beneficial to wound remodeling is likely attributable to a more reduction level of matrix metalloproteinase (MMP)-9 expression in early stage and a higher MMP-2 expression level in a later stage of healing process. Conclusively, the composite dressing demonstrated to be highly beneficial to the healing of full-thickness injury wound.

As peri-implantitis is associated with biofilm development, the characteristics of titanium implants may influence biofilm formation, and thereby increase the risk for inflammation. The objective of this study was to evaluate the effect of titanium surface roughness induced by various debridement methods of peri-implants, such as the use of an ultrasonic scaler, rubber polishing cup, gallium–aluminum–arsenide laser, and chlorhexidine (CHX) rinse, on Porphyromonas (P.) gingivalis. Surface debridement was performed by immersing titanium discs in CHX rinse for 24 h or treatment with a laser, polishing cup, or ultrasonic scaler for 60 s. Surface topography was examined using a profilometer. For the bacterial assay, specimens were inoculated with P. gingivalis for 2 h and incubated for 6, 12, and 24 h. After incubation, bacterial adhesion on the discs was quantified via spectrophotometric evaluation. Moreover, scanning electron microscopy (SEM) images were analyzed to quantify P. gingivalis colonization on the titanium surfaces. Data were analyzed using one-way analysis of variance and Pearson’s correlation test (p

There is tremendous need for the 2D graphene-like single and multiple layered materials since they can be very useful as multifunctional materials for electronic, optical and energy generation and storage applications. We describe the preparation and morphology of selenide crystals and compared with wrinkled graphene which we had grown by annealing method. The characterization of observed standing fibrous film indicates behavior like graphene. Compositional analysis of graphene showed traces of oxygen impurities in some cases. Thermal analysis showed that material was stable up to 400C. However, some morphological changes were observed at high temperature. We studied an important multifunctional semiconductor material Gases for exploring 2D-characteristics and compare with graphene. The class of GaSe materials are highly anisotropic and in many cases, involve covalent and Van der Wall forces making processing very difficult. We will describe results of highly anisotropic gallium selenides, and thallium gallium selenide crystals which have been investigated for nonlinear optical and THz applications. GaSe has strong tendency of cleaving due to Van der Waal forces along –C direction making 2D materials possible. Very thin (smaller than 500 mm) GaSe was fabricated and optically characterized. The material was free from oxygen and water, and does not have any sign of hygroscopic nature indicating GaSe a very good alternate material. GaSe sheet showed very low absorption coefficient in their transparency region. Transmission and Raman studies near cut-off wavelength were performed to understand the defect in the material.

Although the thermoelectric performance of PbTe-based materials, processed by hot-pressing, has recently been improved, there are concerns about its thermal stability. On the other hand, during the process of melt crystal growth the material is formed in a quasi-equilibrium condition, which results in homogeneous solid structure in a lower-energy state and, consequently, more thermally stable and mechanically robust. To test the thermal stability of the melt-grown crystal, a Tl-doped PbTe 0.85 Se 0.15 ingot has been grown from the melt by directional solidification. Four adjacent disc-shape samples were sliced perpendicular to the growth axis from the grown boule. The thermoelectric characterizations together with microstructure examination and mechanical hardness measurements were performed on each of the three as-grown samples as well as on the fourth sample after it has been annealed at 455 °C for 570 h. The characterization results, showing that the annealing has essentially no effects on the thermoelectric properties, metallurgical microstructure as well as mechanical hardness of the annealed sample, have demonstrated that structurally homogeneous materials processed by melt growth are more suitable than those processed by other methods, such as hot press or quench-annealing, for long-term thermoelectric applications at elevated temperatures.