A Quaternary ZnCdSeTe Nanotip Photodetector

The authors report the growth of needle-like high density quaternary Zn_0.87Cd_0.13Se_0.98Te_0.02 nanotips on oxidized Si(100) substrate. It was found that average length and average diameter of the nanotips were 1.3 μm and 91 nm, respectively. It was also found that the as-grown ZnCdSeTe nanotips exhibit mixture of cubic zinc-blende and hexagonal wurtzite structures. Furthermore, it was found that the operation speeds of the fabricated ZnCdSeTe nanotip photodetector were fast with turn-on and turn-off time constants both less than 2 s.     

Temperature effects on mechanically alloyed nanometric ZnSe powder

A ZnSe alloy was prepared by mechanical alloying and annealing at 520 °C for 6 h. Samples were analyzed using X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The as-milled alloy had a dark brown color, which turned to the yellow color characteristic of commercial ZnSe products after annealing. After 10 h of milling the alloy exhibited a zincblende structure. The DSC spectrum showed a broad exothermic band related to structural relaxation and an incomplete endothermic peak associated with the sublimation of nanometric ZnSe, corroborated by a second DSC scanning measurement which indicated the presence of amorphous Se as a product of sublimation. XRD and DSC measurements confirmed that the annealing process led to the structural relaxation and sublimation of nanometric ZnSe. As products of sublimation, hexagonal Se and zincblende ZnSe phases were observed.     

Synthesis of Fe:ZnSe nanopowders via the co-precipitation method for processing transparent ceramics

Fe:ZnSe nanopowders were synthesized via the co-precipitation method for fabricating transparent ceramics. Fe_xZn_1−xSe (0.00 ≤ x ≤ 0.06) powders that were calcined at 400°C yielded a single-phased cubic ZnSe, but when the calcination temperature was raised to 500-600°C, ZnO phase was created. Introduction of pressure could avoid appearance of ZnO. XRD Scherrer analysis revealed a monotonic increase in lattice parameter with increasing Fe²⁺ content. The average powder particle size increased with calcination temperature from several nanometers at 80°C to hundreds of nanometers at 600°C. Attempts to pressurelessly sinter ZnSe powders resulted in the partial decomposition of ZnSe, thus spark plasma sintering was employed to sinter Fe_0.01Zn_0.99Se transparent ceramics with pure ZnSe phase composition, which could be well sintered at 950°C for 30 minutes under an applied pressure of 60 MPa. SEM observations of the polished and thermally etched microstructure of the ceramic revealed a dense microstructure with average grain size of approximately 35 μm, and a few micropores were observed at the grain boundaries. The transparent ceramic exhibited good transmittance in the mid-far infrared range, with the highest transmittance 57% at 12 μm. This paper confirmed the scheme of synthesis of Fe:ZnSe nanopowders by liquid-phase co-precipitation method for sintering transparent ceramics.     

Microstructure development and optical properties of Fe:ZnSe transparent ceramics sintered by spark plasma sintering

Fe:ZnSe transparent ceramics were prepared by spark plasma sintering. Fe:ZnSe powders synthesized via co-precipitation yielded well-dispersed particles with an average particle size of 550 nm. These powders were in the cubic phase Fe:ZnSe, indicating the successful substitution of Fe²⁺ for Zn²⁺. The highest relative density, 99.4%, was obtained by increasing the pressure and sintering time. The effects of sintering temperature, pressure, and time on the microstructure of SPS prepared ceramics were presented by micrographs. With increasing sintering temperature, from 600°C to 900°C, the average grain size increased from < 1 to 10 μm. The intergranular fracture indicated no neck formation in the sintering process. High pressure was essential for the densification process. The average grain size deceased from approximately 10 to 5 μm when the pressure was increased. Increasing the sintering time from 10 to 120 minutes lead to a change in the microstructure, from inter- to transgranular fracture, and eliminated the micropores. The as-prepared Fe:ZnSe ceramics were composed of single-phased cubic ZnSe. The sample sintered at 900°C under a pressure of 90 MPa for 120 minutes yielded a transmittance of approximately 60% at 1.4 μm and 68% at 7.5 μm and had residual micropores as its main scattering source. There was a strong characteristic absorption peak of Fe²⁺ ions at around 3 μm, which was red-shifted compared to Fe:ZnS transparent ceramics. Fe:ZnSe transparent ceramics have a reddish-brown color and it could be a promising mid-infrared laser material.     

Hot-pressed Fe2+:ZnSe transparent ceramics with different doping concentrations

In this study, Fe²⁺:ZnSe transparent ceramics with different doping concentrations were prepared from annealed FeSe and ZnSe powders by hot pressing. The as-prepared ceramics consisted of a cubic ZnSe phase and compact microstructures. Doping concentrations of Fe²⁺ ions in the range 0.66–3.05 at.% were accurately realised, which could influence the absorption intensity of Fe²⁺:ZnSe transparent ceramics. An absorption peak was observed at ～3 μm, and its intensity could be controlled by the concentration of Fe²⁺ ions. Fe_xZn_1-xSe (0.0066 ≤ x ≤ 0.0305) ceramics with Fe²⁺ ions concentrations in the range 0.66–3.05 at.% exhibited significant absorption cross sections from 0.6676 × 10^?19 to 0.1075 × 10^?18 cm². The specimen doped with 1.55 at.% Fe²⁺ ions displayed the highest transmittance of 67% at a wavelength of 14 μm and a carbonate absorption peak at 9 μm. The proposed transparent ceramic technique appears promising for preparing Fe²⁺:ZnSe laser gain media because of its advantage of allowing control over Fe²⁺ concentration.     

Radial AlN nanotips on carbon fibers as flexible electron emitters

A facile catalyst-free approach using a simple thermal transport method has been developed to fabricate high-density AlN nanotips on flexible carbon cloth at large scales for use as field emission (FE) emitters. The AlN nanotips exhibit good performance as flexible cold-cathode electron emitters, with a very low turn-on electric field of 1.1–2.3 V μm⁻¹, a low threshold electric field of 1.5–2.5 V μm⁻¹, and a high emission current density. The excellent field emission properties of the AlN nanotips are attributed to the large field enhancement factor of 6895 as well as the combined effect of the tip profile of the AlN nanostructures and the excellent electron transport path of the conductive carbon cloth substrate.     

The effect of temperature on the mechanism of photoluminescence from plasma-nucleated,nitrogenated carbon nanotips

Nitrogenated carbon nanotips (NCNTPs) have been synthesized using customized plasma-enhanced hot filament chemical vapor deposition. The morphological, structural, and photoluminescent properties of the NCNTPs are investigated using scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and photoluminescence spectroscopy. The photoluminescence measurements show that the NCNTPs predominantly emit a green band at room temperature while strong blue emission is generated at 77 K. It is shown that these very different emission behaviors are related to the change of the optical band-gap and the concentration of the paramagnetic defects of the carbon nanotips. The studies shed light on the controversies on the photoluminescence mechanisms of carbon-based amorphous films measured at different temperatures. The relevance of the results to the use of nitrogenated carbon nanotips in light-emitting optoelectronic devices is discussed.     

Compositional Dependence of CdSe Quantum Dot Formation on Silicate Host Glass Composition

Silicate glasses were synthesized with additional CdO and ZnSe of varying content. When CdO was added up to 3 mol% with the amount of ZnSe fixed at 1 mol%, absorption due to Se₂^? color centers significantly decreased and the color of glasses was bleached without any evidence of CdSe quantum dots (QDs). However, characteristic absorption due to CdSe QDs appeared without additional heat treatment when the ZnSe content exceeded 1.5 mol%, whereas the CdO content was fixed at 1 mol%. Raman spectra and transmission electron microscopy verified the formation of CdSe QDs. The role of CdO and ZnSe in CdSe QD formation has been discussed in relation to their structural contribution within silicate glasses.     

Influence of Uniaxial Tensile Stress on the Mechanical and Piezoelectric Properties of Short-period Ferroelectric Superlattice

Tetragonal ferroelectric/ferroelectric BaTiO₃/PbTiO₃\hbox{BaTiO}_3/\hbox{PbTiO}_3 superlattice under uniaxial tensile stress along the c axis is investigated from first principles. We show that the calculated ideal tensile strength is 6.85 GPa and that the superlattice under the loading of uniaxial tensile stress becomes soft along the nonpolar axes. We also find that the appropriately applied uniaxial tensile stress can significantly enhance the piezoelectricity for the superlattice, with piezoelectric coefficient d ₃₃ increasing from the ground state value by a factor of about 8, reaching 678.42 pC/N. The underlying mechanism for the enhancement of piezoelectricity is discussed.     

Fabrication and electrochemical characterization of zinc selenide thin film by pulsed laser deposition

High-quality crystalline ZnSe thin film has been successfully fabricated by using pulsed laser ablation of mixed target of Zn and Se. The physical, electrochemical and spectroelectrochemical properties of the as-deposited thin film at different substrate temperatures have been investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), the charge/discharge, cyclic voltammetry (CV) and in situ absorbance spectra measurements. The thin film deposited at room temperature consisted of a mixture of Zn and Se. When the substrate temperature was elevated to 300 °C, the as-deposited thin film was composed of crystallized ZnSe. The cycle performance of ZnSe/Li cell is well than that of Zn-Se/Li cell. A couple of reduction and oxidation peaks at 0.51 and 1.4 V from CV curves of ZnSe/Li cell was found, indicating the reversible formation and decomposition of Li₂Se. Both classical alloying/dealloying processes and the partially selenidation/reduction of nano-sized metal zinc were proposed in lithium electrochemical reaction of ZnSe. The high degree of crystallinity in the zinc alloys and the slow kinetics should be responsible for a large irreversible capacity loss and poor cyclability.     

Synthesis of diamond films and nanotips through graphite etching

A hot filament CVD process based on hydrogen etching of graphite has been developed to synthesize diamond films and nanotips. The graphite sheet was placed close to the substrate and only hydrogen was supplied during deposition. No hydrocarbon feed gases are required for this process. High quality diamond films were synthesized with high growth rate on P-type (1 0 0)-oriented silicon wafers without discharge or bias. The diamond growth rate is approximately five times higher than that through conventional hot filament chemical vapor deposition using a gas mixture of methane and hydrogen (1 vol.% methane) under similar deposition conditions. The diamond films synthesized in this process exhibit smaller crystallites and contain smaller amount of non-diamond carbon phases. Synthesis of well-aligned diamond nanotips with various orientation angles was achieved on the CVD diamond-coated Si substrate when the substrate holder was negatively biased in a DC glow discharge. The nanotips grown at locations far enough from the sample edges are aligned vertically, while those around the sample edges are tilted and point away from the sample center. The alignment orientation of the nanotips appears to be determined by the direction of the local electric field lines on the sample surfaces.     

Domain morphology of newly designed lead-free antiferroelectric NaNbO3-SrSnO3 ceramics

Reversible antiferroelectric-ferroelectric phase transitions were recently observed in a series of SrSnO₃-modified NaNbO₃ lead-free antiferroelectric materials, exhibiting well-defined double polarization hysteresis loops at ambient conditions. Here, transmission electron microscopy was employed to investigate the crystallography and domain configuration of this newly designed system via electron diffraction and centered dark-field imaging. It was confirmed that antiferroelectricity is maintained in all compositions, manifested by the characteristic ¼ superlattice reflections in the electron-diffraction patterns. By investigating the antiferroelectric domains and domain boundaries in NaNbO₃, we demonstrate that antiphase boundaries are present and their irregular periodicity is responsible for the streaking features along the ¼ superlattice reflections in the electron-diffraction patterns. The signature domain blocks observed in pure NaNbO₃ are maintained in the SrSnO₃-modified ceramics, but disappear when the amount of SrSnO₃ reaches 7 mol.%. In particular, a well-defined and distinct domain configuration is observed in the NaNbO₃ sample modified with 5 mol.% SrSnO₃, which presents a parallelogram domain morphology.     

Aluminosilicate glasses for zinc selenide tunable fiber laser cladding

A set of novel potassium-calcium-aluminosilicate glasses are developed to serve as the cladding material for a doped ZnSe core infrared fiber laser. The compositions exhibit high glass transition temperatures between 753°C and 918°C and high thermal expansion coefficients between 5.75 ppm/°C and 8.21 ppm/°C. We demonstrate successful application of these glass compositions as a cladding for a ZnSe tunable fiber laser.     

Synthesis and characterization of ZnSe rose-like nanoflowers and microspheres by the hydrothermal method

ZnSe rose-like nanoflowers and microspheres were successfully grown on Zn foils by the hydrothermal method at 220 °C for 36 h. Scanning electron microscope (SEM), X-ray diffraction (XRD), energy dispersive spectrometer (EDS), ultraviolet–visible (UV–vis) absorption spectroscopy and photoluminescence (PL) spectroscopy were used to observe the morphologies, structures, chemical compositions and optical properties of the as-synthesized ZnSe samples. The XRD patterns revealed that as-synthesized ZnSe nanoflowers and microspheres have cubic zinc blende structure. The SEM observations showed that low concentration of EDTA was beneficial to obtain the ZnSe rose-like nanoflowers. With increase of EDTA concentration, the morphology of the as-synthesized samples transformed into microspheres. It was proved that EDTA played a significant role during the synthesis of ZnSe rose-like nanoflowers and microspheres. Room temperature photoluminescence (PL) spectroscopy of the samples showed that the spectra were wide band from blue light to orange light. Furthermore, a possible formation mechanism of ZnSe nanoflowers and microspheres was proposed and discussed.     

Synthesis and characterization of water soluble choline labeled cadmium selenide/zinc selenide/zinc sulfide luminescent quantum dots

Water soluble choline-labeled CdSe/ZnSe/ZnS quantum dot (QD) bioconjugates were synthesized by attaching a thiolated choline analogue to the core-shell QD surface. Characterization was conducted by absorption and luminescence spectroscopy and scanning electron microscopy. QDs with diameters of 5–6 nm resulted and exhibited a luminescence maximum at 663 nm in aqueous solution.     

Order–disorder behaviour in 0.9Ba([Zn0.60Co0.40]1/3Nb2/3)O3–0.1Ba(Ga0.5Ta0.5)O3 microwave dielectric resonators

0.9Ba([Zn_0.60Co_0.40]_1/3Nb_2/3)O₃–0.1Ba(Ga_0.5Ta_0.5)O₃ (BCZN–BGT) ceramic resonators (quality factor, Q=32,000 at the rate of 3.05 GHz, relative permittivity, ε_r=35 and temperature coefficient of the resonant, τ_f=0) have been fabricated which are suitable in terms of cost and performance for base stations supporting third generation architecture. The new compounds are perovskite structured (a=4.09 Å) but exhibit no superlattice reflections at any heat treatment temperature according to X-ray diffraction (XRD). However, annealing and quenching of samples followed by transmission electron microscopy and Raman spectroscopy revealed an order–disorder phase transition at ∼1200 °C. Annealing below this temperature (1100 °C) gave rise to discrete ±1/3{h k l}_p and diffuse 1/2{h k l}_p superlattice reflections in the same 〈1 1 0〉_p zone axis electron diffraction patterns and the presence of F_2g and A_1g modes in Raman spectra. It is proposed that ±1/3{h k l}_p reflections result from 1:2 long-range ordered domains of BCZN whereas the diffuse 1/2{h k l}_p reflections arise from short range fcc ordered BGT rich regions at the 1:2 domain boundaries. A short-range ordered fcc superlattice was observed in samples quenched from above the order–disorder phase transition (>1200 °C) which was accompanied by the presence of only the A_1g mode in Raman spectra.     

Microstructure of Lanthanum Magnesium Niobate at Elevated Temperature

Microstructural studies of the complex perovskite compound La(Mg_2/3Nb_1/3)O₃ (LMN) were conducted using transmission electron microscopy (TEM) and X-ray diffractometry (XRD) at elevated temperatures. 1:1 chemical ordering of B-site cations and tilting of oxygen octahedra were observed in LMN. Three types of superlattice reflections, [1—2]{111}, [1—2]{110}, and [1—2]{100} were observed at room temperature and at 800°C in electron diffraction patterns. In the XRD experiments, the [1—2]{210} and [1—2]{300} extra peaks disappeared at temperatures >1200°C. However, the intensity of the superlattice [1—2]{111} peak did not change with increased temperature up to 1400°C. These results strongly indicated that the origin of superlattice reflection [1—2]{111} was different from that of the other superlattice reflections. It was mainly caused by the 1:1 chemical ordering of magnesium and niobium atoms. The TEM image observed at 800°C showed the ordered domain structures separated by the antiphase boundaries.     

Study of nanostructure growth with nanoscale apex induced by femtosecond laser irradiation at megahertz repetition rate

Leaf-like nanostructures with nanoscale apex are induced on dielectric target surfaces by high-repetition-rate femtosecond laser irradiation in ambient conditions. We have recently developed this unique technique to grow leaf-like nanostructures with such interesting geometry without the use of any catalyst. It was found to be possible only in the presence of background nitrogen gas flow. In this synthesis method, the target serves as the source for building material as well as the substrate upon which these nanostructures can grow. In our investigation, it was found that there are three possible kinds of nanotips that can grow on target surfaces. In this report, we have presented the study of the growth mechanisms of such leaf-like nanostructures under various conditions such as different laser pulse widths, pulse repetition rates, dwell times, and laser polarizations. We observed a clear transformation in the kind of nanotips that grew for the given laser conditions.     

The optical spectra characterization of Cr2+:ZnSe polycrystalline synthesized by direct reaction of Zn–Cr alloy and element Se

Cr²⁺:ZnSe materials have attracted much attention as candidates for mid-infrared laser source either in the form of polycrystalline powders, or bulk ceramics, single crystals and nano-materials. In this work, a novel method for synthesizing Cr²⁺:ZnSe polycrystalline by direct reaction of Zn–Cr alloy and element Se (DRAE) was proposed. The zinc alloy containing 0.1 at% Cr was prepared by dissolving Cr in zinc liquid in a closed quartz ampoule. X-ray diffraction (XRD) results showed that the synthesized Cr²⁺:ZnSe polycrystalline was with a Zinc-blend structure. X-ray photoelectron spectroscopy (XPS) spectra showed that there was no un-reacted element of Zn, or Se. Cr²⁺ ions successfully and uniformly doped into ZnSe crystal lattice, which is confirmed by the diffuse reflectance spectrum, Raman spectrum and mid-infrared photoluminescence spectra. Furthermore, the sample showed excellent mid-infrared properties without luminescence quenching in the region 1800–3000 nm, and the decay-time was about 5 μs. The as-synthesized Cr²⁺:ZnSe polycrystalline meets the requirement for the preparation of mid-infrared ceramic or single crystals. These results indicate that the novel strategy of DRAE is valid for the synthesis of other transition metal doped ZnSe materials.     

Focused ion beam patterning of diamondlike carbon films

For applications of diamondlike carbon films in optics, microelectronics and other fields, it is in some cases necessary to form submicron size patterns. A finely focused beam of 50 keV Ga⁺ ions was used to mill various patterns in amorphous carbon films prepared by a pulsed cathodic arc discharge method. The trenches with width down to 30 nm and depth-to-width ratios up to 25 have been milled. The minimum trench width down to 20 nm has been achieved. The nanotips with a radius down to 35–40 nm were fabricated. The influence of the focused ion beam parameters on the film's surface modification as well as on the shape of fabricated pattern elements is discussed.