Studying local surface electric conductivity of a ZnSe/CdSe/ZnSe heterostructure by scanning tunneling microscopy in the field electron emission regime

The surface morphology and local electric conductivity of a ZnSe/CdSe/ZnSe nanoheterostructure have been studied by scanning tunneling microscopy (STM) in the field electron emission regime. The homogeneity of the local conductivity distribution in a near-surface layer has been evaluated. The main parameters of a potential barrier in the local field contact are determined. It is shown that the STM probe can be used for creating local regions with nonequilibrium carrier concentration in a semiconductor.     

Holographic diffractive collimators based on recording with homocentric diverging beams for diode lasers

A single collimating transmission off-axis blazed surface-relief holographic diffractive element that corrects the ellipticity and the astigmatism of a diode-laser beam was designed. The procedure was applied to a hypothetical laser operating at a wavelength of 633 nm that possesses extremely large astigmatism and relatively small ellipticity together with small diverging angles of the laser beam. The element was recorded with only homocentric diverging beams that had pure spherical wave fronts. Because of the off-axis nature of the element, the aberration of asymmetry had to be corrected. The element was tested in a reciprocal way when the incident wave was a collimated He-Ne laser beam. A diffraction-limited beam was obtained that was as large as approximately half of the designed aperture.     

Improve the binding force of PEEK coating with Mg surface by femtosecond lasers induced micro/nanostructures

Journal of Materials Science - A polymer coating is an effective approach to increase the surface wear and corrosion resistance of magnesium alloys, but the low bonding strength limits its further...     

Formation of dense electron beams in a gun with a plasma anode based on a reflecting discharge

A method of obtaining low-energy, high-current electron beams in a gun with a plasma anode based on a reflecting discharge is proposed and implemented for the first time. Pis’ma Zh. Tekh. Fiz. 23, 42–46 (May 26, 1997)     

Type-II CdSe/CdTe/ZnTe (core-shell-shell) quantum dots with cascade band edges: the separation of electron (at CdSe) and hole (at ZnTe) by the CdTe layer

The rational design and synthesis of CdSe/CdTe/ZnTe (core-shell-shell) type-II quantum dots are reported. Their photophysical properties are investigated via the interband CdSe-->ZnTe emission and its associated relaxation dynamics. In comparison to the strong CdSe (core only) emission (lambda(max) approximately 550 nm, Phi(f) approximately 0.28), a moderate CdSe-->CdTe emission (lambda(max) approximately 1026 nm, Phi(f) approximately 1.2 x 10(-3)) and rather weak CdSe-->ZnTe interband emission (lambda(max) approximately 1415 nm, Phi(f) approximately 1.1 x 10(-5)) are resolved for the CdSe/CdTe/ZnTe structure (3.4/1.8/1.3 nm). Capping CdSe/CdTe with ZnTe results in a distant electron-hole separation between CdSe (electron) and ZnTe (hole) via an intermediate CdTe layer. In the case of the CdSe/CdTe/ZnTe structure, a lifetime as long as 150 ns is observed for the CdSe-->ZnTe (1415 nm) emission. This result further indicates an enormously long radiative lifetime of approximately 10 ms. Upon excitation of the CdSe/CdTe/ZnTe structure, the long-lived charge separation may further serve as an excellent hole carrier for catalyzing the redox oxidation reaction.     

Semiconductor InGaAs/GaAs injection lasers with waveguides based on a single quantum well

Semiconductor InGaAs/GaAs injection lasers emitting at λ = 1065 nm have been created with waveguides based on a single InGaAs quantum well. It is found that internal optical losses are determined by the width of an undoped region confined between the n and p type emitters. The room-temperature total output optical power in lasers with 100 μm aperture amounted up to about 2 W at a radiation beam divergence of 15° in the plane perpendicular to the p-n junction.     

Mesastripe single-mode separately bounded lasers based on InGaAsP/InP heterostructures obtained by VPE of organometallic compounds

Mesastripe single-mode InGaAsP/InP laser diode heterostructures with an optical emission power of 200 mW in the continuous lasing mode at two wavelengths (1.3 and 1.55 μm) were obtained by metalorganic VPE. In the samples with a mesastripe contact width of W=5 μm, the single-mode lasing regime was observed in the entire range of pumping currents. For a cavity length of 1.0–2.5 mm, the threshold current densities varied within 450–600 A/cm². The differential quantum efficiency reached 30–40%. The internal optical losses in the mesastripe laser heterostructures are reduced to 7.7 cm⁻¹. The output power-pumping current characteristics of control samples remained unchanged upon testing for 1500 h at 50°C.     

Flexible electrochromic devices based on crystalline WO3 nanostructures produced with hot-wire chemical vapor deposition

Crystalline WO₃ nanoparticles are employed in the development of flexible electrochromic (EC) devices. The nanoparticles are synthesized at high-density with a hot-wire chemical vapor deposition process where the hot filament provides the source of the tungsten metal. Polyethylene terephthalate coated with indium tin oxide is employed as a transparent flexible substrate. A simple electrophoresis technique is employed to deposit the WO₃ nanoparticles on the polymer, resulting in a uniform thin film. The EC performance is optimized for WO₃ particles that were baked at ~ 300 °C for 2 h prior to electrode fabrication. The transmittance is modulated between ~ 94% and ~ 28% without degradation for 100 cycles.     

Microwave power amplifiers based on AlGaN/GaN transistors with a two-dimensional electron gas

A technique for synthesis of microwave power amplifiers based on transistors with a AlGaN/GaN heterojunction is discussed. Special focus is on the development of a technique for synthesis of transformation circuits of the power amplifier to increase efficiency with a retained high output power. The use of independent matching at the harmonic frequencies and fundamental frequency makes it possible to control the attainable efficiency in a wide frequency band along with the total suppression of harmonics beyond the operational band. Microwave power amplifiers for operation at 4 and 9 GHz have been developed and experimentally investigated.     

High color rendering index white LED based on nano-YAG:Ce3+ phosphor hybrid with CdSe/CdS/ZnS core/shell/shell quantum dots

To improve the poor color rendering index (CRI) of YAG:Ce-based white light-emitting diode (LED) due to the lack of red spectral component, core/shell/shell CdSe/CdS/ZnS quantum dots (QDs) were synthesized and blended into nano-YAG:Ce³⁺ phosphors. Prominent spectral evolution has been achieved by increasing the content of QDs. A white LED combining a blue LED with the blends of nano-YAG phosphors and orange- and red-emission QDs with a weight ratio of 1:1:1 was obtained. This kind of white LED showed excellent white light with luminescent efficiency, color coordinates, CRI and correlated color temperature (CCT) of 82.5?lm/W, (0.3264, 0.3255), 91 and 4580?K, respectively.     

Study on the optical property and crystallinity of Ge90Te10 films deposited by electron beam evaporation

Optical property and crystallinity of Ge₉₀Te₁₀ films prepared by electron beam evaporation have been studied. The films grown at different substrate temperatures (Ts) and deposition rates (R) have been characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and spectroscopic ellipsometry. The polycrystalline film was obtained at Ts = 300 °C, while the amorphous film was obtained when Ts ≦ 200 °C. However, the film showed the columnar structure when Ts ≦ 100 °C. It was found that Ts had the stronger effect on the crystallinity of the film rather than R. The optical constant in the infrared region was determined. All the film exhibited no absorption, but the refractive index was varied with the change of Ts and R. The relationship between optical constant, the film structure and the deposition parameters were also discussed. In addition, the optimum deposition condition of Ge₉₀Te₁₀ film was found.     

Fabrication of white light-emitting diodes based on UV light-emitting diodes with conjugated polymers-(CdSe/ZnS) quantum dots as hybrid phosphors

White light-emitting diodes (LEDs) were fabricated using GaN-based 380-nm UV LEDs precoated with the composite of blue-emitting polymer (poly[(9,9-dihexylfluorenyl-2,7-diyl)-alt-co-(2-methoxy-5-{2-ethylhexyloxy)-1 ,4-phenylene)]), yellow green-emitting polymer (poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1',3}-thiadiazole)]), and 605-nm red-emitting quantum dots (QDs). CdSe cores were obtained by solvothermal route using CdO, Se precursors and ZnS shells were synthesized by using diethylzinc, and hexamethyldisilathiane precursors. The optical properties of CdSe/ZnS QDs were characterized by UV-visible and photoluminescence (PL) spectra. The structural data and composition of the QDs were transmission electron microscopy (TEM), and EDX technique. The quantum yield and size of the QDs were 58.7% and about 6.7 nm, respectively. Three-band white light was generated by hybridizing blue (430 nm), green (535 nm), and red (605 nm) emission. The color-rendering index (CRI) of the device was extremely improved by introducing the QDs. The CIE-1931 chromaticity coordinate, color temperature, and CRI of a white LED at 20 mA were (0.379, 0.368), 3969 K, and 90, respectively.     

Determination of pyrimidine and purine bases by reversed-phase capillary liquid chromatography with at-line surface-enhanced Raman spectroscopic detection employing a novel SERS substrate based on ZnS/CdSe silver-quantum dots

We have developed a new SERS substrate based on the reduction of silver nitrate in the presence of ZnS-capped CdSe quantum dots. This substrate showed higher sensitivities as compared to a hydroxylamine-reduced silver sol. On the basis of this new substrate, at-line SERS detection was coupled with capillary liquid chromatography (cap-LC) for the separation and selective determination of pyrimidine and purine bases. For this purpose, wells of a dedicated microtiter plate were loaded with 20 μL of the SERS substrate and placed on an automated x,y translation stage. A flow-through microdispenser capable of ejecting 50 pL droplets, at a frequency 100 Hz, was used as the interface to connect the cap-LC system to the wells loaded with SERS substrate. A detailed study of the dependence of both the separation and the surface-enhanced Raman spectra of each base on the pH was performed to optimize the system for maximum sensitivity and selectivity. Highly satisfactory analytical figures of merit were obtained for the six investigated bases (cytosine, xanthine, hypoxanthine, guanine, thymine, and adenine) with detection limits ranging between 0.2 and 0.3 ng injected on the capillary LC column, and the precisions were in the range of 3.0-6.3%.     

Preparation and properties of nanocomposites based on PS-b-(PS/PB)-b-PS triblock copolymer by controlling the size of silica nanoparticles with electron beam irradiation

In this work, silica particles are synthesized in a pre-cross-linked triblock copolymer with styrene-b-(styrene-co-butadiene)-b-styrene (LN4) chain architecture by in situ sol–gel method. Using the proposed method, an easy access to control size and distribution of silica nanoparticles, generated inside the polymer matrix, was achieved by varying the cross-linking density of the polymer network with the aid of electron beam (EB) irradiation. The morphological investigations from atomic force microscopy and small angle X-ray scattering measurements reveal that this technique not only allows to control the size of silica particles but also is helpful in restoring the microphase separation in LN4. Dynamic–mechanical and stress–strain behavior also suggested that the reinforcement effect of the sol–gel silica in the cross-linked elastomers was increasing with decreasing the particle size.     

Photodiodes based on InAs/InAsSb/InAsSbP heterostructures with quantum efficiency increased by changing directions of reflected light fluxes

It is shown, using the example of InAs/InAsSb/InAsSbP heterostructures, that the formation of a curvilinear reflecting surface consisting of hemispherical etch pits on the rear side of a photodiode chip leads to an increase in the quantum efficiency of photodiodes by a factor of 1.5–1.7 in the entire mid-IR wave-length interval studied (λ = 3–5 μm). For the obtained photodiodes with a cutoff wavelength of 4.8 μm, a photosensitive area of 0.1 mm², and a chip area of 0.9 mm², a monochromatic responsivity at λ = 4.0 μm reached 0.6 A/W, while a dark current at a reverse bias voltage of 0.2 V was within 4–6 A/cm².     

Consideration of the band-gap tunability of BaSi2 by alloying with Ca or Sr based on the electronic structure calculations

The electronic structures and total energies of BaSi₂-SrSi₂ and BaSi₂-CaSi₂ systems have been calculated using the first-principle pseudopotential method to clarify the band gap tunability of BaSi₂ by alloying with Sr or Ca. From an energetic consideration of the compounds where all the Ba_I sites or all the Ba_II sites of the BaSi₂ lattice are preferentially replaced by Sr or Ca, it is expected that the Ba_I site will be preferentially replaced by Sr rather than the Ba_II sites. Compounds where all the Ba_II sites are replaced by Sr or all the Ba_II or all the Ba_I sites are replaced by Ca are energetically unfavorable compared to the undissolved system of BaSi₂ and SrSi₂ or CaSi₂. The effect of the addition of Sr or Ca into the BaSi₂ lattice on the gap value is different depending on the replaced sites of Ba. The replacement of Ba_I site by Sr will broaden the band gap of BaSi₂, which is consistent with the observed results.     

A microscale formaldehyde gas sensor based on Zn2SnO4/SnO2 and produced by combining hydrothermal synthesis with post-synthetic heat treatment

A silicon-based micro-structure gas sensor for detecting formaldehyde was successfully fabricated and the sensing material was synthesized via a method of combining the traditional hydrothermal synthesis with subsequent heat treatment. Finite element analysis software of ANSYS was used to analyze the temperature distribution on the SiO₂/Si substrate with heating electrodes and signal electrodes on the same plane with the aim of reducing the complexity of micro-machining process. Meanwhile, in order to obtain lower power consumption, two different structures of the SiO₂/Si substrates with and without back etched were simulated, respectively. The simulation results showed that in the same heat rate and convection conditions, the back-etched SiO₂/Si substrate had higher temperature, more uniform temperature distribution, and lower energy consumption. The Zn₂SnO₄/SnO₂ cubes were obtained by annealing the as-synthesized precursors of ZnSn(OH)₆ at 700 °C for 3 h. Thermal gravimetric and differential thermal analyzer, X-ray diffraction, Fourier transform infrared spectra, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were analyzed to characterize the phase structure, composition, morphology, and elemental atomic ratio of Zn₂SnO₄/SnO₂. The gas sensing properties of Zn₂SnO₄/SnO₂ were tested, which showed that the gas sensor based on Zn₂SnO₄/SnO₂ exhibited excellent formaldehyde sensing performance.