Determination of surface oxygen vacancy position in SnO2 nanocrystals by Raman spectroscopy

Raman spectra acquired from SnO₂ nanocrystals with different sizes show a size-independent Raman mode at ∼574 cm⁻¹. The intensity increases as the nanocrystal size decreases and this tendency is contrary to that of the normal bulk Raman modes. By considering the existence of oxygen vacancies at the nanocrystal surface, we adopt the density functional theory to calculate the Raman spectra with different oxygen vacancy positions and concentrations. The results clearly demonstrate that the in-plane oxygen vacancy is responsible for the 574 cm⁻¹ mode and the intensity enhancement is a result of the higher in-plane oxygen vacancy concentration.     

Micro‐Raman investigation of tin dioxide nanostructured material based on annealing effect

Rutile‐structured nanocrystalline tin dioxide (SnO₂) powder was synthesized by the chemical precipitation method using the precursor SnCl₂• 5H₂O. The SnO₂ powder was annealed at different temperatures, namely, 600, 800 and 1000 °C. Micro‐Raman spectra were recorded for both the as‐grown and annealed SnO₂ nanocrystalline samples. Micro‐Raman spectral measurements on the SnO₂ nanoparticle show the first‐order Raman modes A_1g (633 cm⁻¹), E_1g (475 cm⁻¹) and B_2g (775 cm⁻¹), indicating that the grown SnO₂ belongs to the rutile structure. The first‐order A_1g mode is observed as an intense band, whereas the other two modes show low intensity. The full width at half‐maximum and band area of the Raman lines of SnO₂ nanoparticle annealed at various temperatures were calculated. The effect of high‐temperature annealing on the vibrational modes of SnO₂ was studied. The optical image of SnO₂ nanocrystalline material was used to understand the surface morphology effect. Copyright © 2011 John Wiley & Sons, Ltd.     

Invoking forbidden modes in SnO2 nanoparticles using tip enhanced Raman spectroscopy

Raman forbidden modes and surface defect‐related Raman features in SnO₂ nanostructures carry information about disorder and surface defects which strongly influence important technological applications like catalysis and sensing. Because of the weak intensities of these peaks, it is difficult to identify these features by using conventional Raman spectroscopy. Tip enhanced Raman spectroscopy (TERS) studies conducted on SnO₂ nanoparticles (NPs) of size 4 and 25 nm have offered significant insights of prevalent defects and disorders. Along with one order enhancement in symmetry allowed Raman modes, new peaks related to disorder and surface defects of SnO₂ NPs were found with significant intensity. Temperature‐dependent Raman studies were also carried out for these NPs and correlated with the TERS spectra. For quasi‐quantum dot sized 4‐nm NPs, the TERS study was found to be the best technique to probe the finite size‐related Raman forbidden modes. Copyright © 2015 John Wiley & Sons, Ltd.     

Sensitisation of erbium emission by silicon nanocrystals-doped SnO2

SnO₂ thin films undoped and doped with antimony (Sb), erbium (Er) and Si nanocrystals (Si-nc) have been grown on silicon (Si) substrate using sol-gel method. Room-temperature photoluminescence (PL) measurement of undoped SnO₂, under excitation at 280 nm, shows only one broad emission at 395 nm, which is related to oxygen vacancies. The PL of Er³⁺ ions was found to be enhanced after doping SnO₂ with Sb and Si-nc. The excitation process of Er is studied and discussed. The calculation of cross-section suggests a sensitisation of Er PL by Si-nc.     

Structural and spectral properties of SnO2 nanocrystal prepared by microemulsion technique

Extrafine SnO₂ nanocrystals as small as 2.4 nm were synthesized by the microemulsion method. The grain sizes and crystallization process were measured and investigated by X-ray diffraction. Two growth processes were proposed, and the activation energies of 4.3 and 23 KJ/mol were obtained for respective low-and fast-growth processes. TEM micrographs and the selected-area diffraction recorded their morphology and crystallization, well crystallized at about 773 K. All the IR modes measured by FT-IR spectrometer were assigned. The 616 cm^-1 mode after annealing at 673 K showed fine crystallization. The temperature dependence of the Raman spectra shows that increase in intensity and decrease in linewidth of the 636 cm^-1 mode with the increasing grain size indicate a phonon confinement effect. A new 330 cm^-1 Raman mode originally inactive in bulk, was observed in the SnO₂ nanocrystal by size effect. A low frequency mode at 76 cm^-1 shifts to 38 cm^-1 as the temperature goes up 873 K, which can be characterized to determine the SnO₂ grain sizes. PACS 61.46.+w; 61.10.-i; 78.30.-j; 81.20.-n     

Controlled SnO2 nanocrystal growth in SiO2–SnO2 glass‐ceramic monoliths

Crack‐free (100–x) SiO₂–x SnO₂ glass‐ceramic monoliths have been prepared by the sol–gel method obtaining for the first time SnO₂ concentrations of 20% with annealing at 1100 °C. Heat‐treatment resulted in the formation and growth of SnO₂ nanocrystals within the silica matrices. Combined use of Fourier transform–Raman spectroscopy and in situ high‐temperature X‐Ray diffraction shows that SnO₂ particles begin to crystallize in the cassiterite‐type phase at 80 °C and that their average apparent size remains around 7 nm, even after annealing at 1100 °C. Nanocrystal sizes and size distributions determined by low‐wavenumber Raman are in good agreement with those obtained from transmission electron microscopy measurements. Results indicate that the formation and the growth of SnO₂ nanocrystals impose a residual porosity in the silica matrix. Copyright © 2011 John Wiley & Sons, Ltd.     

Unravelling the effect of Ni doping on the structural,optical and dielectric properties of nanocrystalline SnO2

Nanocrystalline pristine and Ni doped (x= 0.05, 0.10 and 0.15) SnO₂ samples are synthesized via sol-gel process. An extensive microstructure, optical and dielectric studies are performed through several analytical techniques. Analysis of the x-ray diffraction (XRD) data through Rietveld refinement confirms tetragonal crystal structure with P4₂/mnm space group for all the samples without any impurity or secondary phase. Scanning electron microscopy (SEM) images exhibit smooth surface morphology with agglomeration of the particles and energy dispersive x-ray spectra (EDS) confirm elemental composition of the samples. The average crystallite/particles size of the samples is found to increase for 5% Ni doped SnO₂ but reduces on further increase in the Ni content. Fourier transform infrared (FTIR) spectra show vibrational modes of the functional groups present in the samples. UV-visible absorption spectra indicate a red shift on Ni doping in SnO_2, ascribed to the trapping of excitons by the oxygen vacancies and thereby creating extra energy states within the bandgap. The band gap is found to diminish from 3.81 eV for pristine SnO₂ to 3.57 eV for the sample with 15% Ni concentration. Frequency dependent dielectric measurements at room temperature reveal higher values of the dielectric constant at lower frequencies that can be described on the basis of Maxwell-Wagner theory. The results demonstrate that the oxygen vacancies play a crucial role and concentration of Ni has strong influence on the microstructure, optical and dielectric behaviour of SnO₂.     

Evolution of Raman and Mössbauer spectra at high pressures up to 75 GPa and a phase transition in CaSnO<Subscript>3</Subscript> perovskite

Raman and Mössbauer spectra from ¹¹⁹Sn nuclei in CaSnO₃ perovskite have been studied at high pressures up to 75 GPa. A linear increase in the frequency of the main Raman modes and a monotonic decrease in the isomer shift in Mössbauer spectra in the pressure range of 0–40 GPa are established. It is shown that the pressure-induced increase in Raman frequencies can be associated with the variation of the angle between the Sn–O–Sn bonds in chains of oxygen octahedra SnO₆ along the c axis. The sharp variation of the parameters of the Raman and Mössbauer spectra is observed in the pressure region of 40–55 GPa, indicating the structural phase transformations, which can be associated with the transition into the post-perovskite state. Raman spectra of CaSnO₃ samples with the ilmenite structure have been obtained for the first time.     

Evidences of the evolution from solid solution to surface segregation in Ni‐doped SnO2 nanoparticles using Raman spectroscopy

Ni‐doped SnO₂ nanoparticles, promising for gas‐sensing applications, have been synthesized by a polymer precursor method. X‐ray diffraction (XRD) and transmission electron microscopy (TEM) data analyses indicate the exclusive formation of nanosized particles with rutile‐type phase (tetragonal SnO₂) for Ni contents below 10 mol%. The mean crystallite size shows a progressive reduction with the Ni content. Room‐temperature Raman spectra of Ni‐doped SnO₂ nanoparticles show the presence of Raman active modes and modes activated by size effects. From the evolution of the A_1g mode with the Ni content, a solubility limit at ∼2 mol% was estimated. Below that content, Raman results are consistent with the occurrence of solid solution (ss) and surface segregation (seg.) of Ni ions. Above ∼2 mol% Ni, the redshift of A_1g mode suggests that the surface segregation of Ni ions takes place. Disorder‐activated bands were determined and their integrated intensity evolution with the Ni content suggest that the solid‐solution regime favors the increase of disorder; meanwhile, that disorder becomes weaker as the Ni content is increased. Copyright © 2010 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering of 4‐mercaptopyridine on sub‐monolayers of α‐Fe2O3 nanocrystals (sphere,spindle, cube)

We report surface‐enhanced Raman scattering (SERS) spectra from 4‐mercaptopyridine (4‐Mpy) adsorbed on sub‐monolayers of α‐Fe₂O₃ nanocrystals (sphere, spindle, cube). The maximum enhancement factor has been estimated to be about 10⁴ compared to that of 4‐Mpy in solution. A possible mechanism has been proposed that the charge transfer between the α‐Fe₂O₃ nanocrystals and the 4‐Mpy molecules is most likely responsible for the observed enhancement of Raman intensity of adsorbed 4‐Mpy molecules as surface plasmon resonances have not occurred. Copyright © 2009 John Wiley & Sons, Ltd.     

Size-dependent luminescence of Sm3+ doped SnO2 nano-particles dispersed in sol-gel silica glass

Sol-gel glasses with composition (100?x)SiO₂–xSnO₂ doped with 0.4 mol% of Sm³⁺, with x ranging from 1 to 10, have been successfully synthesized. Transparent doped nano-glass-ceramics were prepared by thermal treatment of the precursor glasses at 900°C during 4 hours, leading to nanocomposites comprising SnO₂ nanocrystals embedded into an amorphous SiO₂ phase. A structural analysis in terms of X-ray Diffraction and High Resolution Transmission Electron Microscopy confirms the precipitation of SnO₂ nanocrystals within the glassy matrix. The mean radius of the obtained SnO₂ nanocrystals, ranging from 2.1 to 4.7 nm calculated by the Scherrer and Brus equations, similar to the Bohr’s exciton radius, constitutes a wide band-gap semiconductor quantum-dot system. Energy transfer from SnO₂ nanocrystal host to Sm³⁺ ions is confirmed by luminescence spectra and analyzed as a function of SnO₂ concentration, showing an evolution that could be ascribed to selective excitation of nanocrystal sets with predetermined size. Besides, a study of the luminescence as a function of temperature helps to clarify the involved energy transfer mechanisms.     

Preparation and structural characterization of nanocrystalline SnO2 powders

Nanocrystalline SnO₂ powders have been prepared by solid–liquid reaction and solid-state thermal oxidizing techniques. The microstructures and phase compositions of the product were characterized by thermogravimetry analysis, X-ray diffraction, and the Raman spectrum. It is shown that at least two phases, SnO₂ and SnO_x, coexist at 450 °C. However, only the tetragonal rutile structure SnO₂ phase is detected after the Sn powders were annealed at 550 °C. The Raman peaks of the nanocrystalline SnO₂ powders reveal remarkable red shift and broadening, which could be attributed to the phonon confinement effect, oxygen vacancies, and the stress effect. PACS 81.07.Wx; 81.10.Jt; 78.30.-j     

Photoluminescence and Raman spectra of SnO<Subscript>x</Subscript> nanostructures doped with Sm ions

The photoluminescence (PL) and Raman scattering of SnO_x nanoparticles deposited from vapor phase have been studied. The PL spectra are characterized by a two-band structure. The high-energy band in the range from 300 to 350 nm is due to the exciton pair annihilation and may characterize the band gap of SnO_x nanocrystals as a function of their diameter. In the red spectral region (from 600 to 700 nm), a luminescence band due to defects in nanocrystals manifests itself. The existence of defects in SnO_x nanostructures is confirmed by Raman spectroscopy. Doping of SnO_x nanoparticles with rare earth (samarium) atoms leads to the appearance of strong luminescence lines in the red region of the PL spectrum.     

Growth mechanism and photoluminescence of the SnO2 nanotwists on thin film and the SnO2 short nanowires on nanorods

SnO2 nanotwists on thin film and SnO2 short nanowires on nanorods have been grown on single silicon substrates by using Au-Ag alloying catalyst assisted carbothermal evaporation of SnO2 and active carbon powders.The morphology and the structure of the prepared nanostructures are determined on the basis of field-emission scanning electron microscopy(FESEM),transmission electron microscopy(TEM),selected area electronic diffraction(SAED),high-resolution transmission electron microscopy(HRTEM),x-ray diffraction(XRD),Raman and photoluminescence(PL) spectra analysis.The new peaks at 356,450,and 489 nm in the measured PL spectra of two kinds of SnO2 nanostructures are observed,implying that more luminescence centres exist in these SnO2 nanostructures due to nanocrystals and defects.The growth mechanism of these nanostructures belongs to the vapour-liquid-solid(VLS) mechanism.     

Vibrational properties of Cs4W11O35 and Rb4W11O35 systems: high pressure and polarized Raman spectra

Cs₄ W₁₁O₃₅ (CW) and Rb₄ W₁₁O₃₅ (RW) belong to the class of hexagonal bronzes whose structure originates from the K_xWO₃ superconductor hexatungstate. Charge‐imbalanced tungsten bronzes are dielectric materials with rich polymorphism, ferroelectric properties and second‐harmonic generation. In this work, we report the polarized Raman spectra results for both CW and RW, as well as results of high‐pressure Raman scattering experiments (0.0–11.0 GPa) for the Cs₄ W₁₁O₃₅ system, in which we have observed two structural phase transitions at ∼4 and 7.5 GPa. We discuss these transformations and polarized Raman spectra on the basis of lattice dynamics calculation in the related system KNbW₂O₉. Polarized Raman spectra provide strong indication that the highest wavenumber modes observed in these systems originate from tungsten or oxygen vacancies. The observation of a soft‐like mode indicates that the observed phase transitions exhibit a displacive‐type behavior, thus further indicating that these transformations are likely related to reorientations of the octahedral units. The soft mode nature is discussed as well. PACS: 77.80.Bh; 78.30.Hv; 78.30.‐j. Copyright © 2010 John Wiley & Sons, Ltd.     

Influence of nanoenvironment on luminescence of Euactivated SnO2 nanocrystals

The emission intensity of the peak at 612 nm (⁵D₀→⁷F₂) of the Eu³⁺ ions activated SnO₂ nanocrystals (doped and coated) is found to be sensitive to the nanoenvironment. We have compared the luminescence efficiencies of the nanocrystals of SnO₂ doped by Eu₂O₃ with those of SnO₂ coated by Eu₂O₃ and we found that the intensities are significantly higher in coated nanocrystals. Furthermore, it is clear from luminescence intensity measurements that Eu³⁺ ions occupy low symmetry sites in the Eu₂O₃ coated SnO₂ nanocrystal. The analysis suggests that the radiative relaxation rate is higher in Eu₂O₃ coated SnO₂ nanocrystals than Eu₂O₃ doped SnO₂ nanocrystals due to the asymmetric environment of Eu³⁺ ions in coated samples.     

Oxygen vacancy‐induced microstructural changes of annealed CeO2−x nanocrystals

Nanocrystalline ceria (CeO₂) is known for its ionic conductivity and oxygen storage properties, which depend on the presence of oxygen ion vacancies. The vacancies cause several important changes in CeO₂ involving microstrain, electronic structure, magnetic properties, etc. In this article, we focus our attention to the microstructural changes of nanocrystalline CeO_2−x annealed at different temperatures in the range 200–500 °C. Structural and vibrational properties were investigated by X‐ray diffraction and Raman spectroscopy. It was observed that the content of oxygen vacancies changed significantly with increasing annealing temperature, which plays an important role in the observed microstructural changes of the annealed samples. We demonstrate that the observed microstrain changes, because of variable defect content, dominate over the crystallite size effect. This finding is opposite to the conclusions made by several other authors. A new mode, classified as a probable surface mode, was observed in the Raman spectra at ∼480 cm⁻¹, the appearance of which can be explained by the large defective structure and disorder in the ceria lattice. Copyright © 2011 John Wiley & Sons, Ltd.     

Raman study of structural disorder in ZnO nanopowders

Raman scattering spectroscopy has been used for the characterization of zinc oxide nanoparticles obtained by mechanical activation in a high‐energy vibro‐mill and planetary ball mill. Raman modes observed in spectra of nonactivated sample are assigned to Raman spectra of the ZnO monocrystal, while the spectra of mechanically activated samples point out to the structural and stoichiometric changes, depending on the milling time and the choice of equipment. Observed redshift and peak broadening of the E₂^high and E₁ (LO) first‐order Raman modes are attributed to increased disorder induced by mechanical milling, followed by the effects of phonon confinement due to correlation length decrease. The additional modes identified in Raman spectra of activated ZnO samples are related to the surface optical phonon modes, due to the intrinsic surface defects and presence of ZrO₂as extrinsic defects introduced by milling in zirconia vials. Copyright © 2009 John Wiley & Sons, Ltd.     

Correlation of size and oxygen bonding at the interface of Si nanocrystal in Si–SiO2 nanocomposite: A Raman mapping study

Raman spectroscopy/mapping is used to investigate the variation of Si phonon wavenumbers, i.e., lower wavenumber (LW ~ 495–510 cm⁻¹) and higher wavenumber (HW ~ 515–519 cm⁻¹) phonons, observed in Si–SiO₂ multilayer nanocomposite (NCp) grown using pulsed laser deposition. Sensitivity of Raman spectroscopy as a local probe to surface/interface is effectively used to show that LW and HW phonons originate at surface (Si–SiO₂ interface) and core of Si nanocrystals, respectively. The consistent picture of this understanding is developed using Raman spectroscopy monitored laser heating/annealing and cooling experiment at the site of the desired wavenumber, chosen with the help of Raman mapping. Raman spectra calculations for Si₄₁ cluster with oxygen and hydrogen termination show strong mode at 512 cm⁻¹ for oxygen terminated cluster corresponding to the vibration of surface Si atoms. This supports our attribution of LW phonons to be originating at the Si–SiO₂ surface/interface. These results along with XPS show that nature of interface (oxygen bonding) in turn depends on the size of nanocrystals and LW phonons originate at the surface of smaller Si nanocrystals. The understanding developed can conclude the ongoing debate on large variation in Si phonon wavenumbers of Si–SiO₂ NCps in the literature. Copyright © 2015 John Wiley & Sons, Ltd.     

Optical properties of SnO2 nanoparticles

In recent times, considerable research efforts have been focused on the exploration of novel optical properties of nanocrystalline SnO₂ particles such as photoluminescence (PL). In the present work, spherical SnO₂ nanoparticles are synthesized by solid state reaction at different temperatures using tin chloride dihydrate and sodium hydroxide flakes as precursors. Transmission electron micrograph shows uniform and spherical SnO₂ nanoparticles of diameter 30–40 nm. Micro-Raman spectra show an inverse relationship of line-width with SnO₂ particle size. The band gap of SnO₂ is calculated by plotting (αhν)² vs. hν and extrapolating the linear portion of it to α = 0 and found it to be 3.76–4.24 eV. Photoluminescence intensity increases with the increase of solid state reaction temperature up to 873 K. This is attributed to the generation of the oxygen ion vacancies in SnO₂.