Thermal Evaporation Of ZnSe Powders Research Articles

Synthesis of nanograined ZnO nanorods functionalized with NiO nanoparticles and their enhanced hydrogen sensing properties

NiO nanoparticle (NP)-functionalized nanograined ZnO nanorods (NRs) were grown by using the thermal evaporation of ZnSe powders; this was followed by the thermal oxidation of the ZnSe NRs to synthesize ZnO NRs and a solvothermal process for NiO functionalization. The diameters of the grains in the synthesized nanograined ZnO NRs are in the range of 50–150 nm. The ZnO grains were not spherical, but they were shaped like rice grains. The diameters and the lengths of the ZnO NRs were in the range of 40–50 nm and 1–6 µm, respectively. The diameters of the NiO NPs were in the range of 40–80 nm. A multiple-networked chemiresistive sensor was fabricated by pouring the IPA solution containing NiO NP-decorated nanograined ZnO NRs onto SiO2/Si substrates with a patterned electrode. A pristine nanograined ZnO NR sensor was also prepared in a similar manner. The NiO NP-functionalized nanograined ZnO NR sensor exhibited a greater and faster response to H2, as compared to its pristine nanograined ZnO counterpart; it also showed higher selectivity toward H2 against other reducing gases, as compared to the pristine nanograined ZnO counterpart. Herein, the origin of the enhanced sensing performance of the NiO NP-functionalized nanograined ZnO sensor is discussed in detail.

Room temperature sensitivity of ZnSe nanolayers to ethanol vapours

Layers of ZnSe (50, 110 and 160 nm thick) are deposited at room substrate temperature by thermal evaporation of ZnSe powder in vacuum. As-deposited and relaxed layers are investigated by spectroscopic ellipsometry to get information about their optical properties, porosity and surface roughness. The obtained data indicate that all layers are nanocrystalline and the lattice relaxation causes a porosity decrease and surface roughness increase. Ethanol sensing experiments are carried out at room temperature in air and results show an increase of the layers sensitivity with decreasing thickness while the relaxation causes sensitivity decrease. The sensitivity behavior is discussed considering the changes in the films porosity and the ethanol related decrease of the potential barriers at the nanocrystallite interfaces.

Photoluminescence properties of Pt-functionalized ZnSe nanowires

ZnSe nanowires were synthesized by using the thermal evaporation of ZnSe powder, and the effects of Pt functionalization and thermal annealing on the photoluminescence (PL) properties of the nanostructures were examined. PL spectroscopy showed that the intensity of the near-band-edge (NBE) emission of the ZnSe nanowires was enhanced considerably by Pt coating and annealing in a H2 atmosphere. The intensity ratio of NBE emission to deep-level emission, INBE/IDL, of Pt-coated ZnSe nanowires after annealing in a H2 atmosphere was ∼11 times higher than that of the pristine (unannealed, uncoated) ZnSe nanowires. The increase in INBE/IDL might be due to a combination of carrier transfer from defect levels to the Fermi level of the Pt nanoparticles, surface plasmon resonance in the Pt nanoparticles and hydrogen passivation of deep-level defects.

Room temperature, ppb-level NO2 gas sensing of multiple-networked ZnSe nanowire sensors under UV illumination.

Reports of the gas sensing properties of ZnSe are few, presumably because of the decomposition and oxidation of ZnSe at high temperatures. In this study, ZnSe nanowires were synthesized by the thermal evaporation of ZnSe powders and the sensing performance of multiple-networked ZnSe nanowire sensors toward NO2 gas was examined. The results showed that ZnSe might be a promising gas sensor material if it is used at room temperature. The response of the ZnSe nanowires to 50 ppb–5 ppm NO2 at room temperature under dark and UV illumination conditions were 101–102% and 113–234%, respectively. The responses of the ZnSe nanowires to 5 ppm NO2 increased from 102 to 234% with increasing UV illumination intensity from 0 to 1.2 mW/cm2. The response of the ZnSe nanowires was stronger than or comparable to that of typical metal oxide semiconductors reported in the literature, which require higher NO2 concentrations and operate at higher temperatures. The origin of the enhanced response of the ZnSe nanowires towards NO2 under UV illumination is also discussed.

Effects of annealing on the photoluminescence of ZnSe nanorods coated with Au

Au-functionalized ZnSe nanorods were synthesized by the thermal evaporation of ZnSe powder followed by Au sputter-deposition and thermal annealing. Photoluminescence (PL) showed that the intensity of near-band edge (NBE) emission of ZnSe nanorods was enhanced remarkably by Au-coating and annealing in a H2 atmosphere. The intensity ratio of NBE emission to the deep level emission, INBE/IDL of Au-coated ZnSe nanorods after annealing in a H2 atmosphere was ∼68 times higher than that of the pristine (unannealed, uncoated) ZnSe nanorods. The increase in INBE/IDL might be due to a combination of carrier transfer from the defect level to the Fermi level of Au nanoparticles, surface plasmon resonance in Au nanoparticles and hydrogen passivated deep level defects.

Synthesis and Characterization of SiO2-Sheathed ZnSe Nanowires

ZnSe/ coaxial nanowires were synthesized by a two-step process: thermal evaporation of ZnSe powders and sputter-deposition of . Two different types of nanowires are observed: thin rod-like ones with a few to a few tens of nanometers in diameter and up to a few hundred of micrometers in length and wide belt-like ones with a few micrometers in width. Room-temperature photoluminescence (PL) measurement showed that ZnSe/ coaxial nanowires had an orange emission band centered at approximately 610 nm. The intensity of the orange emission from the -sheathed ZnSe nanowires was enhanced significantly by annealing in a reducing atmosphere whereas it was degraded by annealing in an oxidizing atmosphere. The origins of the PL changes by annealing are discussed based on the energy-dispersive X-ray spectroscopy analysis results.

Characterization of Luminescence Properties of V2O5-Capped ZnSe Nanowires

ZnSe-core/V2O5-shell nanowires were synthesized by the thermal evaporation of ZnSe powders on gold-coated Si (100) substrates followed by the sputter depositon of V2O5. Scanning electron microscopic images showed that the core-shell nanowires were a few tens to a few hundreds of nanometers in diameter and a few hundreds of micrometers in length. Transmission electron microscopy and X-ray diffraction analyses revealed that the core and shell of the core-shell nanowires were single crystal wurtzite-structured ZnSe and amorphous V2O5, respectively. Photoluminescence measurement showed that the core-shell nanowires as-synthesized or annealed in an oxidative atmosphere had a green emission band centered at around 520 nm whereas the as-synthesized ZnSe nanowires and the ZnSe-core/V2O5-shell nanowires annealed in a reducing atmosphere had a yellow emission band centered at around 590 nm. Our results also showed that V2O5 capping with an optimal thickness and subsequent annealing in a reducing atmosphere could significantly enhance the emission intensity of the ZnSe nanowires. In addition, the origins of the enhancement in intensity and the blue shift of the major emission by V2O5 capping are discussed.

SYNTHESIS AND PHOTOLUMINESCENCE PROPERTIES OF ZnSe-CORE/SnO2-SHELL NANORODS

ZnSe -core/ SnO2 -shell nanorods have been prepared by using a two-step process comprising the thermal evaporation of ZnSe powders and the atomic layer deposition of SnO2 . ZnSe nanorods have been grown by the thermal evaporation of ZnSe powders on gold-coated Al2O3 (0001) substrates. The ZnSe -core/ SnO2 -shell nanorods were 0.1–0.2 μm in diameter and up to a few tens of micrometers in length. Transmission electron microscopy and X-ray diffraction analysis results indicate that the cores and shells are single crystal zinc blende-type ZnSe and polycrystalline simple orthorhombic-type SnO2 , respectively. Photoluminescence measurements show that the ZnSe -core/ SnO2 -shell nanorods have a strong orange emission band centered at around 650 nm. The orange emission of the ZnSe nanorods is enhanced in intensity by coating them with SnO2 and further enhanced by annealing them. A reducing atmosphere is more efficient in enhancing the orange emission than an oxidative atmosphere. The highest emission intensity is obtained for the shell layer thickness corresponding to 1500 ALD cycles. The orange emission enhancement by annealing is mainly attributed to the formation of Sn interstitials in the ZnSe cores during the annealing process.

Influence of the annealing atmosphere on the photoluminescence of ZnSe-core/In2O3-shell nanowires

The influence of thermal annealing on the photoluminescence properties of the ZnSe-core/In 2 O 3 -shell nanowires prepared by a two-step process comprising the thermal evaporation of ZnSe powders and the sputter-deposition of In 2 O 3 was investigated. The ZnSe nanowires were a few tens to a few hundreds of nanometers in diameter and up to a few hundreds of micrometers in length. Photoluminescence measurements showed that ZnSe nanowires had an emission band centered at around 630 nm in the orange region. In contrast, ZnSe-core/In 2 O 3 -shell nanowires had a stronger emission band centered at around 560 nm in the yellow-green region. The major emission of the ZnSe nanowires was found to be blue-shifted and enhanced in intensity by coating them with In 2 O 3 . The major emission is shifted back to 630 nm and further enhanced by annealing in a reducing atmosphere, whereas it is shifted back to ~620 nm and degraded in intensity by annealing in an oxidative atmosphere. The PL enhancement by annealing in a reducing atmosphere is mainly attributed to the formation of In interstitials in the ZnSe cores during the annealing process.

The photoluminescence properties of TiO 2-sheathed ZnSe nanowires

ZnSe nanowires have been synthesized by thermal evaporation of ZnSe powders on gold-coated Al 2O 3(0 0 0 1) substrates and then sheathed with TiO 2 by sputtering. Our results show that sheathing Zn nanowires with thin TiO 2 layers can significantly enhance the photoluminescence (PL) emission intensity. XPS analysis results suggest that the PL enhancement is attributed to increases in the concentrations of deep levels such as oxygen and titanium interstitials as well as the density of interface states. The PL emission of ZnSe nanowires is also enhanced by thermal annealing. Annealing in an argon atmosphere is more efficient in enhancing the PL emission than annealing in an oxygen atmosphere.

Characterization of ZnSe Nanolayers by Spectroscopic Ellipsometry

Single layers of ZnSe with thicknesses of 30, 40, 50, 70 and 100 nm are deposited at room substrate temperature by thermal evaporation of ZnSe powder in vacuum. The layers surface morphology has been investigated by atomic force microscopy. Structural characterization by the Raman scattering measurement revealed the existence of randomly oriented crystalline ZnSe particles in all layers, and the presence of amorphous phase in layers thinner than 100 nm. The ellipsometric measurements were performed in the range from 1.5 to 5 eV at room temperature in air. To interpret the experimental results, the Bruggeman eective medium approximation of dielectric function of ZnSe layers has been used, representing the layers as dierent mixtures of crystalline ZnSe (c-ZnSe), amorphous ZnSe (a-ZnSe), and voids. The assumption of polycrystalline ZnSe layers modeled as mixture of porous c-ZnSe (with volume fraction of voids … 0:17) and a-ZnSe gives the best fit of ellipsometric experimental data. Single layer thicknesses similar to those expected from preparation conditions have been obtained by this fitting procedure. It has been also found that decrease in the layer thickness causes an increase of the volume fraction of a-ZnSe. Thus, c-ZnSe/a-ZnSe ratio, porosity and layer thickness obtained by spectroscopic ellipsometry, provides useful information about crystallinity and micro-/nanostructure of ZnSe nanolayers.

Enhanced Photoluminescence of ZnSe Nanowires with Branches

ZnSe nanowires have been grown by thermal evaporation of ZnSe powder on Si(100) and Al2O3(0001) substrates coated with gold (Au) thin films. ZnSe nanowires grown on Si(100) were thicker (∼2 μm in diameter) and have numerous fine branch nanowires (20 50 nm in diameter and 400 – 500 nm in length) while those grown on Al2O3 (0001) were thinner (0.1 – 0.2 μm in diameter) and do not have branch nanowires. The former (ZnSe nanowires with branches) shows far stronger deep level photoluminescence (PL) emission than the latter. This enhanced PL emission may be attributed to the abundant surface states at the surfaces of the extremely fine branch nanowires which have a high surface-to-volume ratio.

Raman Scattering from ZnSe Nanolayers

A series of ZnSe single layers having thickness between 30 nm and 1 μm was deposited on c-Si and glass substrates at room substrate temperature. Thermal evaporation of ZnSe powder in high vacuum has been applied. Moreover, SiOx/ZnSe periodic multilayers prepared by the same deposition technique and having ZnSe layer thickness of 2 and 4 nm have been studied. Raman spectra were measured at 295 K, using the 442 nm line of a He–Cd laser as well as different lines of the Ar or Ar/Kr lasers. The observed Raman features have been related to multiple optical phonon (1LO to 4LO) light scattering and connected with the existence of randomly oriented crystalline ZnSe grains in both ZnSe single layers and ZnSe layers of the multilayers. Relatively large line width (≈ 15 cm−1) of the 1LO band has been observed and related to lattice distortion in the crystalline grains and existence of amorphous phase in the layers thinner than 100 nm. The Raman spectra measured on both ZnSe single layers and SiOx/ZnSe multilayers using the 488 nm line with a gradually increased laser beam power indicate an increased crystallinity at high irradiation levels.

{113} Twinned ZnSe Bicrystal Nanobelts Filled with <111> Twinnings

Zinc selenide (ZnSe) bicrystal nanobelts with zinc blende structure were obtained via simple thermal evaporation of ZnSe powder and were further investigated by various transmission electron microscopy (TEM) techniques. The bicrystal nanobelts form by introducing {113} twinning. It is interesting to note that the commonly found <111> twinning lamellas, with width ranging from 5 to 30 nm, concurrently grow in each single plate of the bicrystal nanobelts. The crystallographic relations of the ZnSe bicrystal nanobelts are comprehensively characterized by using selected area electron diffraction analysis and high-resolution TEM. The polarity around the twin planes are determined by convergent beam electron diffraction technique, which indicates the absence of polarity reversal along <111> direction. By taking account of the polarity of zinc blende ZnSe, the atomic model and the growth mechanism are also discussed. This novel structure provides a model system for further study of its mechanical, electrical, and optical properties.

Triple-Crystal Zinc Selenide Nanobelts

One-dimensional ZnSe nanobelts with three-dimensional triple-crystal architecture have been fabricated on Au-coated Si substrates by thermal evaporation of ZnSe powders. The as-synthesized triple-crystal ZnSe nanobelts are a metastable 2H-wurtzite structure while the typical structure of ZnSe nanocrystal is a stable zinc blend. The triple-crystal nanobelts have a typical length of tens of micrometers and a thickness of 30−80 nm. The morphology and growth mechanisms of the triple-crystal nanobelts, which cannot be described by the commonly used octahedral multiple-twin growth model for similar nanostructures, are investigated and explained based on the {101̄3} twins and two fastest-growing directions of [0001] and [11̄00] of the belt. The thermodynamics of the formation of metastable wurtzite ZnSe nanostructures are also discussed in terms of the temperature and surface energy. The photoluminescence spectra show that the triple-crystal nanobelts possess high-quality crystalline structure.

Synthesis of wurtzite ZnSe nanorings by thermal evaporation

In this study, free standing crystalline ZnSe nanorings and nanowires have been fabricated on Au coated Si substrates by simple thermal evaporation of ZnSe powders. Ring- or wirelike morphology can be achieved in a controllable manner by using different reactor pressures during growth, while all the other conditions remain the same. Our results show that the ZnSe nanorings are wurtzite phase instead of the zinc-blende phase, observed in typical one-dimensional ZnSe nanostructures. The growth mechanism of the nanorings has been discussed, and the cathodoluminescence of the nanorings has been described.

ZnSe-Si Bi-coaxial Nanowire Heterostructures

We report on the fabrication, structural characterization, and luminescence properties of ZnSe/Si bi-coaxial nanowire heterostructures. Uniform ZnSe/Si bi-coaxial nanowire heterostructures are grown on silicon substrates by the simple one-step thermal evaporation of ZnSe powder in the presence of hydrogen. Both ZnSe and silicon are single-crystalline in the bi-coaxial nanowire heterostructures, and there is a sharp interface along the nanowire axial direction. Furthermore, secondary nanostructures of either ZnSe nanobrushes or a SiOx sheath are also grown on the primary bi-coaxial nanowires, depending on the ratio of the source materials. The experimental evidence strongly suggests that bi-coaxial nanowires are formed via a co-growth mechanism, that is, ZnSe terminates specific surfaces of silicon and leads to anisotropic, one-dimensional silicon growth, which simultaneously serves as preferential nucleation sites for ZnSe, resulting in the bi-coaxial nanowire heterostructures. In addition, the optical properties of ZnSe/Si nanowires are investigated using low-temperature photoluminescence spectroscopy.