Influence of SnO2 coating and annealing on the luminescence properties of porous silicon nanowires

Porous silicon (PS)-core/SnO₂-shell nanowires (NWs) were synthesized by a two step process: electrochemical anodization of silicon followed by atomic layer deposition of SnO₂. The photoluminescence spectrum of the PS nanowires showed a broad blue green emission band centered at approximately 510 nm. PL measurement also showed that the blue green emission was enhanced by SnO₂ coating and enhanced further by thermal annealing. It appeared that annealing in a reducing atmosphere was more efficient in increasing the blue green emission intensity than annealing in an oxidizing atmosphere. Energy-dispersive X-ray spectroscopy revealed that the enhancement in the blue green emission by annealing in a reducing atmosphere was attributed to the formation of Sn interstitials in the PS cores due to the dissociation of the SnO₂ shells followed by the diffusion of the Sn atoms, generated as a result of the dissociation of SnO₂, into the PS cores during the annealing process.     

Temperature dependent growth and optical properties of SnO<Subscript>2</Subscript> nanowires and nanobelts

SnO₂ nanowires and nanobelts have been grown by the thermal evaporation of Sn powders. The growth of nanowires and nanobelts has been investigated at different temperatures (750–1000°C). The field emission scanning electron microscopic and transmission electron microscopic studies revealed the growth of nanowires and nano-belts at different growth temperatures. The growth mechanisms of the formation of the nanostructures have also been discussed. X-ray diffraction patterns showed that the nanowires and nanobelts are highly crystalline with tetragonal rutile phase. UV-visible absorption spectrum showed the bulk bandgap value (∼ 3–6 eV) of SnO₂. Photoluminescence spectra demonstrated a Stokes-shifted emission in the wavelength range 558–588 nm. The Raman and Fourier transform infrared spectra revealed the formation of stoichiometric SnO₂ at different growth temperatures.     

Cu-doping effect on the structural, optical and photoluminescence properties of Sn0.98Cr0.02O2 nanoparticles by co-precipitation method

Sn_0.98-xCr_0.02Cu_xO₂ nanoparticles were prepared by co-precipitation method with different Cu concentrations from x = 0 to 0.05 and annealed at 600°C for 2 h in air atmosphere. The prepared particle were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectra, UV–visible spectrophotometer, and Fourier transform infrared spectroscopy. The XRD measurement reveals that the prepared nanoparticles have different microstructure without changing a tetragonal structure. The calculated average crystalline size decreased from 12.2 to 10.5 nm for x = 0–0.02 then gradually increased up to 16.8 nm for x = 0.05 which were confirmed by Scanning electron microscope. The optical studies were done by UV–visible spectrometer and the energy band gap values were calculated from absorption spectra. A small red shift from 3.52 eV (Cu = 0) to 3.49 eV (Cu = 0.02, ΔE_g = 0.03 eV) at lower Cu concentrations and a remarkable blue shift from 3.49 eV (Cu = 0.02) to 3.71 eV (Cu = 0.05, ΔE_g = 0.22 eV) at higher Cu concentrations is due to Cu-doping in Cr–SnO₂ matrix. The presence of functional groups and the chemical bonding is confirmed by Fourier transforms infrared spectra. PL spectra of Sn_0.98-xCr_0.02Cu_xO₂ nanoparticle described the shift in UV emission from 366 to 381 nm (red shift) and a shift in blue band emission from 458 to 482 nm (red shift) which confirms the Cu-doping in Cr–SnO₂ matrix. The doping of Cu in the present system is useful to tune the emission wavelength and hence is appreciable for the fabrication of nano-optoelectronic devices and photo-catalytic applications.     

A novel method for massive synthesis of SnO2 nanowires

This paper reports a simple, inexpensive and fast method to prepare SnO₂ nanowires. A large amount of ultra-long high purity single-crystalline SnO₂ nanowires with rutile structure, that is over hundreds of micrometers in length and 20–100 nm in diameter, have been synthesized through a one-step typical thermite reaction at 200 °C in O₂ atmosphere, with a gas pressure of 0·9 atm. These SnO₂ nanowires do not grow in one direction as those synthesized by other methods do, and are perfect single crystals without any dislocation or point defects detected in TEM images. The optoelectronic properties of these smooth and uniform nanowires have been characterized by means of X-ray photoelectron spectra, laser Raman spectrum and Fourier transform infrared spectrum. The result of X-ray photoelectron spectra analysis shows that some oxygen vacancies exist in these SnO₂ nanowires. In addition, possible growth mechanism of the SnO₂ nanowires has been described in detail by the studies of comparative experiments, which is quite different from that of SnO₂ nanowires synthesized by some other methods.     

Investigations on structural, optical and electrochemical properties of blue luminescence SnO2 nanoparticles

A simple solution approach has been developed to synthesis SnO₂ nanoparticles using polyethylene glycol as stabilizer. X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), UV–Vis absorption spectroscopy, photoluminescence (PL) emission spectroscopy, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to characterize the nanoparticles. XRD, HR-TEM and AFM indicate that the SnO₂ nanoparticles correspond to tetragonal crystal structure with size ranges below Bohr’s exciton radius. UV–Vis absorption spectrum showed band gap exhibiting a 1.3 eV shift from that of bulk SnO₂ structures. The blue emission owing to transition of an electron from conduction band to deeply trapped hole in SnO₂ nanoparticles was analyzed using PL spectroscopy. The charge transfer capability had been investigated using CV and EIS in different electrolytes. A detailed exploration on confinement effect that occurred in SnO₂ nanoparticles, mechanism behind visible emission and electron transfer mechanism in different electrolyte was discussed.     

Structure and photoluminescence properties of ZnS nanowires sheathed with SnO2 by atomic layer deposition

Influence of the thermal annealing atmosphere on the photoluminescence properties of ZnS-core/SnO₂-shell coaxial nanowires was investigated. ZnS nanowires were synthesized by a two-step process: the thermal evaporation of ZnS powders and the atomic layer deposition of SnO₂. Transmission electron microscopy and X-ray diffraction analyses reveal that two crystalline ZnS phases: one with a zinc blende structure and the other with an wurtzite structure coexist in the cores whereas the SnO₂ cores in the as-prepared coaxial nanowires are amorphous. The SnO₂ shells are found to be crystallized by thermal annealing. Photoluminescence (PL) measurements at room temperature show that the green emission of the ZnS/SnO₂ coaxial nanowires is enhanced in intensity by thermal annealing regardless of the annealing atmosphere. The PL emission is more significantly enhanced in intensity by annealing in a reducing atmosphere than in an oxidative atmosphere since Au_Zn ⁻ is more easily generated in the ZnS cores in the former atmosphere.     

Synthesis of Ga-riched zinc gallate nanowires by reactive evaporation and the cathodoluminescence properties of individual nanowires

A facile Au-catalytic reactive evaporation method was developed to synthesize cubic zinc gallate (ZnGa₂O₄) nanowires through the reaction of Ga₂O₃ film and ZnO vapor at high temperature. The ZnGa₂O₄ nanowires are well crystalline and the length is up to tens of micrometers. The growth process follows typical vapor-liquid-solid (VLS) mechanism. All the cathodoluminescence (CL) spectra of individual nanowires reveal a strong, broad and asymmetric blue emission band centered at ca. 460 nm. It is thought that the excess Ga³⁺ is the main reason for the unusual blue emission properties.     

Regulation of electro-optical activities of nanostructured SnO2:Al powders by using a micro drop fluidized reactor

Nanostructured SnO₂:Al powders were prepared and their electro-optical activities were regulated by using a micro drop fluidized reactor (MDFR), which were conducted continuously in a simple one-step process without additional calcination, annealing, and drying steps. The crystallite size and BET surface area (up to a 2.6 times) increased but bandgap energy of SnO₂:Al powders decreased, respectively, by providing the unique pyro-hydraulic reaction conditions during the formation of powders. The micro shear force and strain generated due to the fluidization of micro drops could enhance the doping effects and surface activities of SnO₂:Al powders. The analyzes of room-temperature photoluminescence (PL) spectra, UV–vis photospectroscope and X-ray photoelectron spectra (XPS) exhibited that the surface activities of SnO₂:Al powders were enhanced considerably by regulating the excitation and separation of electrons and holes and increasing the oxygen vacancies at the surface of SnO₂:Al powders by means of U_MB control. The optimum amount of dopant of Al³⁺ ions (C_Al) and flow rate of micro bubbles (U_MB) for effective surface activities were 2.0at% and 1.0 L/min, respectively. The scheme of photocatalytic mechanism of SnO₂:Al powders to degrade the methylene blue (MB) was discussed by means of surface charge transfer and morphology of the powders.     

Sub-2 nm SnO2 nanocrystals: A reduction/oxidation chemical reaction synthesis and optical properties

A simple reduction/oxidation chemical solution approach at room temperature has been developed to synthesize ultrafine SnO₂ nanocrystals, in which NaBH₄ is used as a reducing agent instead of mineralizers such as sodium hydroxide, ammonia, and alcohol. The morphology, structure, and optical property of the ultrafine SnO₂ nanocrystals have been characterized by high-resolution transmission electron microscopy (HRTEM), X-ray powder diffraction (XRD), differential scanning calorimetry and thermogravimetric analysis (DSC-TGA), X-ray photoelectron spectroscopy (XPS) and UV-vis absorption spectroscopy. It is indicated that the uniform tetragonal ultrafine SnO₂ nanocrystals with the size below 2 nm have been fabricated at room temperature. The band gap of the ultrafine SnO₂ nanocrystals is about 4.1 eV, exhibiting 0.5 eV blue shift from that of the bulk SnO₂ (3.6 eV). Furthermore, the mechanism for the reduction/oxidation chemical reaction synthesis of the ultrafine SnO₂ nanocrystals has been preliminary presented.     

Influence of Al doping on the structural,morphological and opto-electrical properties of spray deposited lead sulfide thin films

This paper reports the synthesis of Al-doped PbS (PbS:Al) thin films by spray pyrolysis technique on glass substrates. Al doping concentration is varied as 0, 2, 4, 6 and 8 at.% in undoped PbS. Undoped and doped films exhibit cubic crystal structure with a (2 0 0) preferential orientation. The 2θ value of the doped films shifts towards higher Bragg angles confirming a contraction in their unit cell volume. The crystallite size values determined using the Scherrer formula decreased from 27.88 to 25.79 nm with increase in Al doping concentration. EDX spectra confirmed the presence of Al in the doped films. Increased transparency and blue shift in the optical band gap is observed with Al doping. The resistivity range of all the films were found to be in the order of 10² Ω-cm. Increased transparency, widened band gap and decreased resistivity observed make PbS:Al films suitable for tandem solar cells which uses multilayered pn junctions.     

A simple route to the synthesis of single crystalline copper metagermanate nanowires

Single crystalline copper metagermanate (CuGeO₃) nanowires with the diameter of 30–300 nm and length of longer than 100 µm have been prepared by a simple hydrothermal deposition route. X-ray diffraction (XRD), selected area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM) and Raman analyses confirm that the nanowires are orthorhombic single crystals with a main growth direction along <101>. Room temperature photoluminescence (PL) measurement shows a strong blue emission peak at 442 nm with a broad emission band. The blue emission may be ascribed to radiative recombination of oxygen vacancies and oxygen–germanium vacancies. The formation process of CuGeO₃ nanowires is also discussed.     

Microstructure and luminescence properties of Co-doped SnO2 nanoparticles synthesized by hydrothermal method

Co-doped SnO₂ nanoparticles were synthesized by a simple hydrothermal method, and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Transmission electron microscopy (TEM), UV–Vis diffuse reflectance spectra (DRS) and Photoluminescence spectroscopy (PL). It is found that the SnO₂ crystallites with the tetragonal rutile structure formed directly during the hydrothermal process without calcination. The Co-doped SnO₂ nanoparticles were spheric and well-dispersed with narrow size distribution. The crystalline size of SnO₂ decreased from 5.98 to 2.22 nm when the Co content increased from 0% to 20%. A considerable red shift in the absorbing band edge was observed with increasing of Co dopant.     

Effect of Sb doping on the opto-electronic properties of SnO2 nanowires

Antimony (Sb) doping of SnO₂ nanowires (NWs) was investigated for its optical and electrical effects. The low-temperature photoluminescence spectra of SnO₂ NWs varied significantly with increasing Sb content, where the temperature-dependence of the visible emission at ca. 400 nm was distinctive with Sb-doping, indicating different defect states, such as neutral and positively charged oxygen vacancies. Field effect transistors (FETs) with low-level Sb-doped SnO₂ NW channels exhibited higher mobility, charge concentration, and faster response and recovery to UV light than intrinsic SnO₂ NW FETs.     

Visible light induced photocatalytic performance of Mn-SnO2@ZnO nanocomposite for high efficient cationic dye degradation

In this work, we have synthesized Mn-doped SnO₂@ZnO nanocomposite for photo degradation of Methylene blue and Rhodamine B dyes upon visible light irradiation. The crystal structure, functional group, optical absorption, defect related emission, morphology, purity and binding energy state of synthesized samples were identified by using various analytical tools. The crystal structure revealed the rutile tetragonal, hexagonal wurtzite for SnO₂ and ZnO samples and the average crystal sizes were found in the range of 23.3 nm to 16.7 nm for the synthesized samples. The optical absorption peaks were shifted to higher wavelength side and optical band gap values were found between 3.52 eV and 2.77 eV which confirm the formation of hetero-junction of SnO₂@ZnO composites. The field emission scanning electron spectroscopy (FESEM) revealed the spherical grain morphology for pure and composite samples. The energy dispersive spectra (EDS) and element mapping confirms the purity of the synthesized samples. The X-ray photoelectron spectroscopy (XPS) revealed that the composition and energy state of Mn⁴⁺, Sn⁴⁺ and Zn²⁺ for composite samples. The photocatalytic degradation results clearly indicate that the Mn-doped SnO₂@ZnO nanocomposite has higher degradation efficiency of 98% and 92% for the Methylene blue and Rhodamine B dyes, respectively and is higher than the other synthesized samples. The present study reveals a low cost and highly efficient photo-catalyst which works up on visible light irradiation for the purification of waste water from industries.

     

Self-catalytic formation and characterization of Zn2SnO4 nanowires

Mass production of transparent semiconducting ternary oxide Zn₂SnO₄ nanowires is successfully synthesized by the thermal evaporation method without any catalyst. The as-synthesized products are characterized with field-emission scanning electron microscope (FE-SEM), X-ray powder diffraction (XRD), energy-dispersive spectroscopy (EDS), high-resolution transmission electron microscope (HR-TEM) and selected area electron diffraction (SEAD). A formation of Zn₂SnO₄ nanowires based on a self-catalytic VLS growth mechanism is discussed. The photoluminescence spectrum (PL) of the nanowires shows a broad blue-green emission around the 300-600 nm wavelengths with a maximum center at 580 nm under room temperature.     

A facile one step synthesis of SnO<Subscript>2</Subscript>/CuO and CuO/SnO<Subscript>2</Subscript> nanocomposites: photocatalytic application

Here novel photocatalysts, SnO₂/CuO and CuO/SnO₂ nanocomposites were successfully synthesized by chemical method at room temperature. X-ray Diffraction (XRD), transmission electron microscopy (TEM), Fourier transform Infrared (FT-IR), UV–Visible (UV–Vis) and photoluminescence (PL) spectroscopy were utilized for characterization of the nanocomposites. The photocatalytic activity of the nanocomposites was investigated. The hybrid nanocomposites exhibited high photocatalytic activity as evident from the degradation of methylene blue (MB) dye. The result revealed substantial degradation of the MB dye (92 and 69.5% degradation of SnO₂/CuO and CuO/SnO₂, respectively) under visible light illumination with short period of 30 min. Their large conduction band potential difference and the inner electrostatic field formed in the p–n heterojunction provide a strong driving force for the photogenerated electrons to move from Cu₂O to SnO₂ under visible light illumination. The excellent photodegradation of methylene blue suggested that the heterostructured SnO₂/CuO nanocomposite possessed higher charge separation and photodegradation abilities than CuO/SnO₂ nanocomposite under visible light irradiation.     

Synthesis of SnO2 nano-sheets by a template-free hydrothermal method

Square SnO₂ nano-sheets have been successfully synthesized by a template-free hydrothermal method basing on the reaction between SnCl₂ and NaOH in ethanol/water solution. The products have been characterized by X-ray diffraction, scanning electron microscope, and transmission electron microscope. The size and morphology of nano-sheets could be controlled by changing the ratio of ethanol to water. Compared experiments showed that the addition of NaOH was crucial to the formation of SnO₂ nano-sheets, and the addition of glycol could induce the formation of cabbage-like structure. The nano-sheets show a strong absorption peak around 330 nm, corresponding to a band gap value of 3.75 eV. This blue shift of the band gap energy was possibly induced by the spatial confinement of excitons in the nano-sheet structures.     

Ultraviolet photodetectors made from SnO2 nanowires

SnO₂ nanowires can be synthesized on alumina substrates and formed into an ultraviolet (UV) photodetector. The photoelectric current of the SnO₂ nanowires exhibited a rapid photo-response as a UV lamp was switched on and off. The ratio of UV-exposed current to dark current has been investigated. The SnO₂ nanowires were synthesized by a vapor-liquid-solid process at a temperature of 900 °C. It was found that the nanowires were around 70-100 nm in diameter and several hundred microns in length. High-resolution transmission electron microscopy (HRTEM) image indicated that the nanowires grew along the [200] axis as a single crystallinity. Cathodoluminescence (CL), thin-film X-ray diffractometry, and X-ray photoelectron spectroscopy (XPS) were used to characterize the as-synthesized nanowires.     

Blue shift in optical band gap of sol–gel derived Sn1−xZnxO2 polycrystalline thin films

Undoped and Zn doped SnO₂ thin films are deposited by sol–gel spin coating on glass substrate. XRD spectra with prominent peaks along (110) and (101) planes shows the polycrystalline nature of thin films. The particle size lies between 9.30 and 42.09 nm as estimated by Debye–Scherer method. SEM micrographs of the films contain pebble like structures spread throughout whose diameter decreases with increase in dopant concentration. Surface topology of the films is studied by atomic force microscopy. Transmission spectra show that all the films are highly transparent in the visible and IR spectral region (80–90 %) and a sharp absorption occurs near 300 nm. Approximately a change of 4 % is observed in the optical band gap by Zn doping. The optical band gap is tunable between 3.55 and 3.68 eV for 0 ≤ x ≤ 0.15 in nanocrystalline Sn_1?xZn_xO₂. Broad UV emission at 395 nm is observed in photoluminescence spectra of the films along with a blue emission. Emission intensity decreases as amount of Zn incorporated into SnO₂ increases.     

Growth and characterization of thermally evaporated ATO nanowires

The Sb-doped SnO₂ (ATO) nanowires have been synthesized on an alumina substrate using thermal evaporation with various growth durations of 1, 1.5 and 2 h. The morphology and structure of Sb-doped SnO₂ nanowires were characterized by a field emission scanning electron microscope (FESEM), an X-ray diffraction (XRD) spectrometer and a transmission electron microscope (TEM). Chemical composition and bonding were investigated by X-ray photoelectron spectroscopy (XPS), which shows that the Sb concentration of the nanowires increases with increasing growth durations. It is found that the electrical conductance of a single ATO nanowire- and nanowire films-based devices both increase with growth durations. Additionally, the photon-sensing measurement shows that the photon-sensing properties are improved with increasing growth durations, which provides a practicable method for the fabrication of ATO nanowire-based photodetectors.