Narrowing Of Optical Band Gap Research Articles

The photoluminescence and Judd-Ofelt investigations of UV, near-UV and blue excited highly pure red emitting BaWO4: Eu3+ phosphor for solid state lighting

The article explores the photoluminescence properties of UV, near-UV and blue excited BaWO4: Eu3+ red phosphors. The optical band gap narrowing is observed after adding Eu3+ ions. The excitation spectra exhibit a broad peak centred at 255 nm and several narrow peaks of Eu3+ ions at 362, 382, 395, 415 and 465 nm. The three peaks located at 255 (UV), 395 (near-UV) and 465 nm (blue) are the most intense and suitable for the excitation of the sample. The spectral profile of the emission spectra irradiated at 255, 395 and 465 nm are identical. The brightest emission is realized at 18 mol% doping of Eu3+ ions at 255 and 465 nm excitations. However, the optimum concentration is 16 mol% at 395 nm excitation. The phosphors emit highly pure red lights with color purity 98.192%, 97.794% and 98.693% for the optimised concentration under UV, near-UV and blue excitations of 255, 395 and 465 nm, respectively. The quantum efficiency of BaWO4: 16 mol% Eu3+ phosphor is calculated as∼48.234% at 395 nm excitation. The emission intensity parameters are calculated in view of the Judd-Ofelt theory. The spectroscopic parameters like radiative transition probability, radiative lifetime, branching ratio and stimulated emission cross-section are further evaluated from the three emission spectra recorded at 255, 395 and 465 nm excitations. The derived results advocate the use of synthesized material as a red phosphor in white-LED, laser and display devices.

Rapid thermal annealing induced engineering of surface and photoluminescence properties of (K,Na)NbO3 thin films for optoelectronic applications

The present study is focused on understanding the mechanism of luminescence behavior and how it gets affected with post annealing temperature via rapid thermal annealing of (K,Na)NbO3 thin films. The crystalline KNN perovskite structure with preferred (001)-orientation is observed upon annealing at 750 and 800 °C. After annealing, the evolution kinetics of surface features is resulted from the simultaneous evaporation-recondensation and volume diffusion of particles. The volatilization of alkali species after annealing and, in contrast, the lattice oxygen and Nb5+ chemical state of niobium related to KNN is enhanced. Narrowing of optical band gap after annealing is attributed to the generation of shallow defects deep below the conduction band owing to order–disorder and/or distortion in NbO6 octahedra. Photoluminescence (PL) quenching behavior is observed after RTA with 290 nm excitation wavelength. A strong blue emission is observed at 800 °C after exciting with 266 nm wavelength. The kinetics of decay lifetime from time-resolved PL results revealed the enhancement of transitions more likely via radiative states. The RTA induced results specify that KNN can be one of the promising candidates for use in LEDs, smart glass windows, optical switches/sensors, and optoelectronic devices.

Tuning optical and electrical properties of TixSn1−xO2 alloy thin films with dipole-forbidden transition via band gap and defect engineering

We investigated the optical, electrical properties and electronic band structures of band gap tunable TixSn1−xO2 alloy with dipole-forbidden transition. TixSn1−xO2 alloy thin films with various Ti concentrations (0 ≤ x ≤ 0.36) were fabricated on quartz substrate by sol-gel method. As Ti is alloyed into SnO2, the narrowing of optical band gap of TixSn1−xO2 alloy was observed in the optical absorption spectra and was supported by first-principles calculations, which differs from the widening of fundamental band gap. Hall effect measurements indicate that the electrical resistivity of the TixSn1−xO2 alloy thin films increases with the increase of Ti concentration. Comparing with the pure SnO2, the TixSn1−xO2 alloy thin films exhibit a strong deep-level emission associated to oxygen vacancy (VO) defects in the photoluminescence spectra. First-principles calculation results suggest that the formation energy of TiSn-VO complex defect in a TixSn1−xO2 alloy is lower than that of the single VO in a perfect SnO2 under both oxygen-rich and oxygen-poor conditions. Especially, under the oxygen-poor limit, the TiSn-VO complex defect has the lowest formation energy among several major point defects of SnO2. Our results suggest that the band gap engineering of TixSn1−xO2 can not only control the band gap, but also modulate the defects, thus tuning the electrical and optical properties, which is of great significance for the understanding of the nature of electronic band structure of TixSn1−xO2 alloys.

Band Gap Narrowing in Silane-Grafted ZnO Nanocrystals. A Comprehensive Study by Wide-Angle X-ray Total Scattering Methods

Zinc oxide (ZnO) is a largely investigated semiconducting nanomaterial for photocatalytic applications and is an excellent active layer candidate in photovoltaics. Among native defects, having a primary role in ZnO optoelectronic properties, the influence of nearly ubiquitous planar faults of wurtzite sequences in ultrasmall (≤5 nm) nanocrystals (NCs) remains poorly understood. Here, we present a thorough study of ZnO NCs prepared under morphological control of covalently grafted vinyltrimethoxysilane (VTMS) and exhibiting either narrowing or widening of the band gap upon NCs downsizing, depending on the NC growth rate. By using synchrotron X-ray total scattering data, atomistic models and the Debye Scattering Equation (DSE) method, complemented by spectroscopic (FTIR and UV–vis) investigations, we provide a comprehensive quantitative picture in which effects from planar defects are disentangled from those due to NC size, morphology, and lattice strain (here controlled by preferential binding of VTMS on the ZnO basal faces). When faults occur in high concentration (linear density up to 1.6 × 10⁶ cm–¹), NCs exhibit optical band gap narrowing (3.27 eV vs 3.37 eV in bulk ZnO), whereas gap widening (3.52 eV) is observed at a lower density (0.8 × 10⁶ cm–¹), at which quantum-size confinement effects prevail. Supported by photoluminescence and photodegradation experiments, surface defect passivation by VTMS, affecting visible emissions and photocatalytic properties of ZnO, is also discussed in relation to silane coating and fault-driven bandgap. This work sheds light on the complex interplay among planar defects, quantum size effect, and surface modifications in ultrasmall ZnO NCs and on the importance of advanced X-ray total scattering methods toward atomically precise control of defects in nanostructures.

Tailoring the NIR range optical absorption, band-gap narrowing and ferromagnetic response in defect modulated TiO2 nanocrystals by varying the annealing conditions

In this investigation, we report the influence of oxygen vacancy and Ti3+ related defects in the visible and IR range absorption as well as on the bandgap and magnetism of TiO2 nanocrystals annealed in different atmospheres. The X-ray diffraction patterns and Raman spectra indicate anatase phase formation and TEM analysis confirm the highly crystalline nature of TiO2 nanocrystals with a particle size of around ~ 9–12 nm. Significant absorption in the visible and IR region along with much-reduced bandgap is observed in argon, hydrogen and vacuum annealed samples. These samples exhibit strong visible photoluminescence associated mostly with oxygen vacancies. The room temperature ferromagnetism observed in argon annealed TiO2 nanocrystals is explained by taking into consideration the concept of bound magnetic polarons. Hence, this study reports the formation of TiO2 nanocrystals with extended absorption in visible as well as in IR region and ferromagnetism at room temperature which could have potential applications for future technologies.

Enhanced red shift in optical absorption edge and photoelectrochemical performance of N-incorporated gallium oxide nanostructures

This research paper reports enhanced red shift in the optical bandgap of β-Ga2O3 films. Intrinsic and N-incorporated β-Ga2O3 nanostructures were deposited on Si substrates by the hydrogen reducing-ambient chemical vapor deposition method. The effects of N addition on the structural, morphological, optical and photoelectrochemical properties were studied appropriately. X-ray diffraction analysis showed decreasing crystallite size from 165.2 to 38.2 nm with increasing NH3 flow rate. Microstructural observations by field-emission scanning electron microscope exhibited morphology transformation from nanobelts to full nanoclumps. Energy dispersive X-ray analysis revealed N concentration from 0 up to 17.1 at.% with rising ammonia flow rate. Optical reflectance measurements by UV–vis–NIR spectrophotometer showed enhanced narrowing of optical bandgap from 4.87 to 3.54 eV attributed to rising N content. Photoelectrochemical performance of the doped β-Ga2O3 films, measured in 0.1 M HCl electrolyte showed significant photoresponse with photocurrent density up to 0.347 mA/cm2 and ~ 0.1% photon-to-current conversion efficiency.

Controlled synthesis of CuO decorated defect enriched ZnO nanoflakes for improved sunlight-induced photocatalytic degradation of organic pollutants

We report synthesis and application of a unique heterojunction nanocomposite formed between copper nanoparticles (CNPs) and defect rich ZnO nanoflakes for application towards water remediation. Systemic variation in CNPs functionalization was seen to modulate the optical, structural and photocatalytic properties of the heterojunction, which were found to be well in accordance with the results from TEM, UV-DRS and EPR. The remediation ability of the heterojunction measured in terms of its photocatalytic ability was studied by degrading three different organic dye pollutants Rhodamine 6G (R6G), Methylene Blue (MB) and Methyl Orange (MO) using solar light (850 W/cm2) as the renewable excitation source. CNPs loading density was seen to follow a direct linear relationship to optical absorbance and band gap narrowing. The initial increase in photolytic efficiency is associated with improved visible light response and reduced band gap due to attachment of CNPs while reduction in activity was found to be associated with generation of self-recombination centres in CuO functionalization. Optimized CNP modified ZnO nanoflakes was able to decompose 10 µM of R6G, MB and MO dye solutions in 80, 40 and 60 min respectively.

Utilization of Coal Fly Ash and Rice Hull Ash as Geopolymer Matrix-cum-Metal Dopant Applied to Visible-Light-Active Nanotitania Photocatalyst System for Degradation of Dye in Wastewater

Geopolymer (GP) spheres made from coal fly ash (FA) and rice hull ash (RHA) waste products are utilized as both support matrix and dopant applied to titania (TiO2) photocatalyst for organic dye degradation in wastewater. Processing of FA and RHA via suspension-solidification method resulted in GP spheres with nanoporous morphology. The nanocrevices enabled low-energy sol-gel TiO2 coating technique because they served as anchoring sites on the geopolymer surface that favored rigidity and larger surface area. The GP-TiO2 system has been characterized by infrared spectroscopy, X-ray diffraction and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy. Diffuse reflectance spectroscopy revealed a narrowing of the GP-TiO2 system optical band gap due to the interaction of metal dopants contained in RHA and FA with TiO2, thus making the GP-TiO2 system a visible-light-active photocatalyst, as confirmed by methylene blue dye degradation measured through UV-Vis spectroscopy.

Bifunctional Au–TiO2 thin films with enhanced photocatalytic activity and SERS based multiplexed detection of organic pollutant

In the current study, Au–TiO2 thin films were prepared on glass substrates through the combined approach of spin-coating thermal evaporation method. The decoration of Au nanoparticles on to TiO2 surface has been achieved by the thermal annealing process under Ar atmosphere. Au–TiO2 thin films with improved optical absorption and effective bandgap narrowing exhibits outstanding photodegradation activity and ultra-sensitivity towards surface-enhanced Raman spectroscopy (SERS) based detection of the organic molecules. The existence of Au nanoparticles on TiO2 nanostructures efficiently controls the rate of recombination consequently Au–TiO2 thin films show significantly improved sun-light induced photodegradation efficiency for methylene blue (MB) dye. Au–TiO2 thin films decomposed 5 µM MB dye solution in 40 min under sun light exposure (850 W/cm2). Au–TiO2 thin films also exhibit efficient detection capabilities for the two organic molecules rhodamine 6G (R6G) and methylene blue (MB) with the Raman intensity enhancement factors of the order of ~ 107. The observed excellent SERS sensitivity of Au–TiO2 thin films towards the pollutant molecules are ascribed to the contribution of charge transfer mechanism among TiO2 and dye molecules. Furthermore, the fabricated Au–TiO2 thin films were also evaluated for the multiplexed detection by simultaneously detecting the two analytes (MB and R6G) from their mixture with superior sensitivity. Au–TiO2 nanohybrids thin films with these tremendous applications can be further employed for different applications like solar cell, gas sensing, energy production, and water splitting.

Phase transformations, vacancy formation and variations of optical and photocatalytic properties in TiO2-ZnO composites by high-pressure torsion

TiO2 and ZnO, two semiconductors with promising optical properties, are considered as potential candidates for solar and photocatalytic applications. Although chemical methods have been primarily used to enhance the optical properties of these oxides, the current authors recently reported enhanced photocatalytic performance of pure TiO2 and ZnO by plastic straining due to the generation of high-pressure phases and oxygen vacancies. In this study, to improve the optical properties further, large fractions of ZnO/TiO2 interphase boundaries are also introduced by application of high-pressure torsion (HPT) straining to a mixture of anatase-TiO2 and wurtzite-ZnO powders. It was found that the amounts of oxygen vacancies and nanograined high-pressure TiO2-II and rocksalt-ZnO phases increase with increasing plastic strain. Moreover, due to the plastic strain effect, the rutile-TiO2 phase is formed at room temperature, which is at least 600 K below the reported anatase-to-rutile transition temperature. These structural features, together with the formation of large fraction of interphase boundaries, lead to electron spin resonance, optical bandgap narrowing, diminishing of the band-to-band photoluminescence and thus, improvement of photocatalytic hydrogen generation. Despite improvements in the photocatalytic activity of TiO2-ZnO composites after large straining, photocatalytic activity becomes poor by processing at ultra-large strains due to the significant reduction in crystallinity.

Atomic-Level Insight into the Postsynthesis Band Gap Engineering of a Lewis Base Polymer Using Lewis Acid Tris(pentafluorophenyl)borane

In this report, we investigate the binding properties of the Lewis acid tris(pentafluorophenyl)borane with a Lewis base semiconducting polymer, PFPT, and the subsequent mechanism of band gap reduction. Experiments and quantum chemical calculations confirm that the formation of a Lewis acid adduct is energetically favorable (ΔG° < −0.2 eV), with preferential binding at the pyridyl nitrogen in the polymer backbone over other Lewis base sites. Upon adduct formation, ultraviolet photoelectron spectroscopy indicates only a slight decrease in the HOMO energy, implying that a larger reduction in the LUMO energy is primarily responsible for the observed optical band gap narrowing (ΔEopt = 0.3 eV). Herein, we also provide the first spatially resolved picture of how Lewis acid adducts form in heterogeneous, disordered polymer/tris(pentafluorophenyl)borane thin films via one- (1D) and two-dimensional (2D) solid-state nuclear magnetic resonance. Notably, solid-state 1D 11B, 13C{1H}, and 13C{19F} cross-polarization ma...

Effect of hydrogen peroxide on photoelectric properties of high-transmittance FTO films prepared by spray pyrolysis

The FTO film prepared by spray pyrolysis has low efficiency and most of the precursors are discharged in the form of waste steam. According to the high oxidation property of H2O2, it is attempted to improve the film formation rate by changing the concentration of H2O2. Fluorine-doped SnO2 (FTO) thin films with high transmittance were prepared by spray pyrolysis using monobutyltin trichloride as the tin source and ammonium fluoride as the fluorine source. Different concentrations of hydrogen peroxide (0–0.08 M) are added to the precursor solution. In this paper, we studied the effect of hydrogen peroxide on the structure, surface morphology and photoelectric properties of FTO thin films. The results show that the growth rate of the FTO films increased from 6.04 nm/s to 8.36 nm/s with the increase of H2O2 concentration from 0 to 0.08 M. The optimum preparation process is H2O2 concentration controlled at 0.04 M, and FTO thin films suitable for solar cells are prepared. It has excellent performance parameters; carrier concentration:2.74 ∗ 1021 cm−3; carrier mobility:55.92 cm2 V−1 s−1; photoelectric quality factor:2.66 × 10−3·Ω−1and the average transmittance of visible light: 79.87%. At the same time, increasing H2O2 concentration leads to narrowing of optical band gap. Adding appropriate hydrogen peroxide concentration can improve the film production rate and obtain excellent quality films.

First-principles study of electronic and optical properties of sulfur doped tin monoxide: A potential applicant for optoelectronic devices

Recently, Tin Monoxide (SnO) has attained a considerable interest due to its striking electronic, optical, thermoelectric and gas sensing properties in various advanced technological applications. In this study, we focus on the first principles calculations to examine the structural, electronic and optical properties of the pristine, 1S, 2S, and 3S doped SnO. The plane-wave ultra-soft pseudopotential method is used under the GGA-PBE exchange-correlation energy with supercell approach. Bandgap reduces with the increase in the concentration of doping from 0.460 eV to 0.330 eV, 0.064 eV for 1S, 2S respectively and shows metallic behavior for 3S doped SnO. Formation and cohesive energies decrease in order of Sn15S1O16 < Sn14S2O16 < Sn13S3O16, which shows that the most suitable and favorable configuration is of Sn15S1O16. The static dielectric constant ε1(0) and refractive index n (0) are 7.71, 7.82, 8.48, 9.42 eV and 2.76, 2.80, 2.91, 3.07 respectively, which are showing increasing trends. The absorption spectrum and optical conductivity curves of S doping show a significant blueshift towards ultraviolet spectrum. The optical properties and bandgap narrowing effect suggest that the sulfur-doped SnO can be a promising new semiconductor in the field of optoelectronics.

Observation of Optical Band-Gap Narrowing and Enhanced Magnetic Moment in Co-Doped Sol–Gel-Derived Anatase TiO2 Nanocrystals

The magnetic behavior of TiO2 and doped TiO2 nanocrystals has been a challenge due to the unambiguous nature of defects present in oxide semiconductors. Here, a simple, low-temperature sol–gel method is developed for the synthesis of low-dimensional and highly efficient stable anatase TiO2 nanocrystals. The X-ray powder diffraction pattern and Raman spectra confirm the formation of a single-phase anatase structure of TiO2. High-resolution transmission electron microscopy studies reveal the crystalline nature of the sol–gel-derived nanocrystals. The increase in lattice parameters together with the shifting and broadening of the most intense Eg(1) mode in micro-Raman spectra of Co-doped TiO2 nanocrystals indicate the incorporation of Co in TiO2. Shifting of the absorption edge to the visible region in UV–visible spectra indicates narrowing of the band gap due to Co incorporation in TiO2. X-ray photoelectron spectra confirm the presence of Co2+ and Co3+ in Co-doped TiO2 samples. Oxygen vacancy defects lead t...

Tuning the optical band Gap of pure TiO2 via photon induced method

Titanium dioxide, a well known photocatalyst will become more useful if it utilizes visible light. Many efforts have been made to achieve the utilization of visible light by TiO2 for photocatalytic activity by changing the preparation methods or by doping various transition metals. Herein, a novel Photon Induced Method (PIM) is being proposed to produce oxygen-rich TiO2 with modified optical band gap and high stability. The prepared TiO2 nanoparticles have been subjected to various characterizations such as XRD, HRSEM, HRTEM, FTIR, UV–vis and PL. XRD results reveal that as the calcination temperature is increased, the crystallite size of the TiO2 nanoparticles is seen to increase. Also, the XRD results reveal an increase in pure anatase phase stability upto 750 °C when compared to earlier reports of pure anatase phase stability upto 650 °C. HRTEM analysis shows the formation of nanosized bullet like structure. A narrowing of optical band gap to 3.09 eV is evident through UV–vis absorption studies. This tuning of band gap is expected to facilitate the absorption of visible light. The photocatalytic activity of the prepared TiO2 nanoparticles has also been studied. The results reveal that TiO2 nanoparticles prepared by this novel method exhibit an increased photocatalytic activity when compared to the standard Degussa P25 sample reported earlier.

First-principles calculations of nitrogen-doped antimony triselenide: A prospective material for solar cells and infrared optoelectronic devices

This study is focused on calculation of the electronic structure and optical properties of non-metal doped Sb2Se3 using the first-principles method. One and two N atoms are introduced to Sb and Se sites in a Sb2Se3 crystal. When one and two N atoms are introduced into the Sb2Se3 lattice at Sb sites, the electronic structure shows that the doping significantly modifies the bandgap of Sb2Se3 from 1.11 eV to 0.787 and 0.685 eV, respectively. When N atoms are introduced to Se sites, the material shows a metallic behavior. The static dielectric constants ϵ1(0) for Sb16Se24, Sb15N1Se24, Sb14N2Se24, Sb16Se23N1, and Sb16Se22N2 are 14.84, 15.54, 15.02, 18.9, and 39.29, respectively. The calculated values of the refractive index n(0) for Sb16Se24, Sb15N1Se24, Sb14N2Se24, Sb16Se23N1, and Sb16Se22N2 are 3.83, 3.92, 3.86, 4.33, and 6.21, respectively. The optical absorbance and optical conductivity curves of the crystal for N-doping at Sb sites show a significant redshift towards the short-wave infrared spectral region as compared to N-doping at Se sites. The modulation of the static refractive index and static dielectric constant is mainly dependent on the doping level. The optical properties and bandgap narrowing effect suggest that the N-doped Sb2Se3is a promising new semiconductor and can be a replacement for GaSb due to its very similar bandgap and low cost.

Air–water interface solar heating using titanium gauze coated with reduced TiO2 nanotubes

Using method of electrochemical anodization and subsequent reduction, titanium gauze with reduced TiO2 nanotubes on the surface (reduced TiO2 nanotubes/Ti gauze) was prepared and used for air–water interface solar heating. The electrochemical reduction method can generate Ti3+ and causes the narrowing of optical band gap of TiO2 (ca. 2.91 eV). Combining with the nanotubular structure, reduced TiO2 nanotubes/Ti gauze demonstrated higher absorption ability of visible light than other types of titanium gauzes (reduced P25 TiO2 nanoparticles/Ti gauze, TiO2 nanotubes/Ti gauze and P25 TiO2 nanoparticles/Ti gauze). For evaluating the property of air–water interface solar heating, solar water evaporation test was conducted. The results demonstrated that the reduced TiO2 nanotubes/Ti gauze can efficiently accelerate water evaporation. The water evaporation rate and solar thermal conversion efficiency were 1.41 kg m−2 h−1 and 44.2%, respectively, under solar light irradiation with intensity of 2 kW m−2, which are higher than that of reduced P25 TiO2 nanoparticles/Ti gauze, TiO2 nanotubes/Ti gauze, P25 TiO2 nanoparticles/Ti gauze and pristine Ti gauze. It was further found that the solar thermal conversion efficiency of reduced TiO2 nanotubes/Ti gauze attained 84.2% when solar light intensity increased to 5.6 kW m−2. This work may provide a new route to design more advanced photothermal materials for industrial applications such as waste water treatment, salt production and solar desalination.

Acceptor-modulated optical enhancements and band-gap narrowing in ZnO thin films

Fermi-Dirac distribution for doped semiconductors and Burstein-Moss effect have been correlated first time to figure out the conductivity type of ZnO. Hall Effect in the Van der Pauw configuration has been applied to reconcile our theoretical estimations which evince our assumption. Band-gap narrowing has been found in all p-type samples, whereas blue Burstein-Moss shift has been recorded in the n-type films. Atomic Force Microscopic (AFM) analysis shows that both p-type and n-type films have almost same granular-like structure with minor change in average grain size (∼ 6 nm to 10 nm) and surface roughness rms value 3 nm for thickness ∼315 nm which points that grain size and surface roughness did not play any significant role in order to modulate the conductivity type of ZnO. X-ray diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Photoelectron Spectroscopy (XPS) have been employed to perform the structural, chemical and elemental analysis. Hexagonal wurtzite structure has been observed in all samples. The introduction of nitrogen reduces the crystallinity of host lattice. 97% transmittance in the visible range with 1.4 × 107 Ω-1cm-1 optical conductivity have been detected. High absorption value in the ultra-violet (UV) region reveals that NZOs thin films can be used to fabricate next-generation high-performance UV detectors.

Variation of structure and band gap for N doped Cu2O films deposited with ceramic target

With ceramic target, N doped Cu2O films were deposited by Radio-Frequency magnetron sputtering. The structural, optical and electrical properties were studied. Along with the increase of nitrogen flow rate, the preferred orientation of Cu2O films varied from (111) plane to (200) plane. In addition, may be due to the inhibition of oxygen vacancy by using Cu2O target, an obvious optical band gap narrowing was observed. Hall results indicated that the resistivity decreased sharply with the increase of carrier concentration. And the lowest value 2.1 Ωcm was obtained when the nitrogen flow rate was increased to 20 sccm. However, due to the defects which induced by N doping, the hall mobility is decreased slightly. X-ray photoelectron spectra results further demonstrated the formation of Cu2O films without other phases.

Defect-related optical bandgap narrowing and visible photoluminescence of hydrothermal-derived SnO2 nanoparticles

Tin oxide (SnO2) nanoparticles are fabricated by a novel hydrothermal route using H2O2 as oxidizer without adding any corrosive acids or bases. The morphology, composition, and microstructure are characterized by transmission electron microscopy, X-ray diffraction (XRD) and Raman spectra. XRD analysis reveals the formation of single phase rutile type tetragonal structure of all samples which is further supported by Raman studies. The average crystallite size is observed to vary from 2.8 to 4.2 nm as the reacting temperature increases from 120 to 180 °C, suggesting the promotion of crystal growth with increasing temperature. Raman spectroscopy measurement presents the existence of the defect modes, of which the mode at about 565 cm−1 is identified to relate to oxygen vacancies as this mode becomes more prominent in the as-grown sample. Unusual optical band gap narrowing and the excitation wavelength dependent visible luminescence are observed, and both are supposed to be induced by the intrinsic defects. The photoluminescence (PL) spectra of all samples consist of two defect-related subbands, i.e., orange and blue luminescence bands. The two subbands are attributed to optical transitions in oxygen vacancies and some intrinsic surface states, respectively. Spectral analysis suggests that the excitation wavelength dependences of the two subbands are due to a distribution of the band gap of the nanoparticles induced by the internal strain, defect concentration as well as the quantum confinement effect. This work provides a feasible route to modulate the band gap and defect emissions and a good understanding of the PL behavior in the present SnO2 nanoparticles.