Raman Active Modes Research Articles

Strain dependent properties of the Bi2Sr2CaCu2O8 superconductor: an ab initio study

Abstract Ab initio calculations were performed to investigate the effects of strain on the structural, electronic, and vibrational properties of the Bi2Sr2CaCu2O8 (Bi-2212) compound. To accurately represent the Bi-2212 ground state, a modulation correction was applied, generating a distorted structure with lower symmetry that better represents the incommensurate superstructure observed in this compound. Phonon spectra and electronic properties were calculated under various levels of c-axis strain, ranging from -2.0% to +2.0%. For the electronic properties, minor changes were observed in the electronic density of states and band structure. However, trends could be identified by analyzing the fine features of the band structure through a tight-binding model. The most significant changes were observed in the vibrational properties, where different trends emerged for the various Raman-active modes. The changes observed in the vibrational and electronic properties can be explained by examining the distances and overlap populations of the relevant bonds, as well as the reduced mass of certain modes. This work can serve as an input for analyzing experimental measurements, helping to distinguish structural effects from others.

Polaronic transport and magnetic field dependence of conduction parameters in rGO-CFO nanocomposites

Abstract In this paper we report the AC conductivity σ and magnetoconductivity Δσ of a series of reduced graphene oxide-cobalt ferrite (rGO-CFO) composites in which the mass ratio of rGO:CFO has been systematically varied over six compositions. The conductivity measured in the frequency range of 20 Hz to 3 MHz showed an anomalous increase with CFO content in the composite, even though the conductivity of CFO is at least three orders of magnitude smaller than that of rGO. Magnetoconductivity measured in fields upto 2000 Oe displayed both negative and positive values depending on the rGO:CFO ratio. Maximum magnetoconductivity of ∼ +14% was obtained at room temperature in a magnetic field of only 750 Oe. Samples having the largest σ and Δσ values also showed significant softening/hardening of the Raman active modes of CFO, indicating polaronic transport in these composites. Fits of σ(f) data to a power law σ(f) = σ o + Afn showed that parameters A and n depend strongly on the applied magnetic field. This dependence is qualitatively and quantitatively different for samples that display positive or negative magnetoconductivity. Although the temperature dependence of these conduction parameters has been extensively studied, we have not come across any reports of their magnetic field dependence. Based on these observations we present a surface polaron model in which hybridization of π-orbitals of graphene with surface states of CFO nanoparticles provides for a polaron conduction mechanism through a grid of CFO nanoparticles intercalated with rGO sheets.

Raman signatures of inversion symmetry breaking structural transition in quasi-1D compound, (TaSe4)3I

The breaking of inversion symmetry combined with spin-orbit coupling, can give rise to intriguing quantum phases and collective excitations. Here, we report systematic temperature dependent Raman scattering and theoretical calculations of phonon modes across the inversion symmetry-breaking structural transitions in a quasi-one-dimensional compound (TaSe4)3I. Our investigation revealed the emergence of three additional Raman-active modes in Raman spectra of the low-temperature non-centrosymmetric (NC) structure of the material. From polarization dependent Raman spectra and phonon mode symmetry analysis, we have identified the origin of these three newly appeared additional Raman-active modes. Notably, two of these modes become Raman active due to the loss of inversion symmetry, while the third mode is identified as a soft phonon mode, arising from the distinctive vibrational motion of tantalum (Ta) atoms along the -Ta-Ta- chains. Furthermore, the temperature evolution of self-energy parameters indicates significant changes in the characteristics of the Raman modes across the transition. Latent heat measurements near the phase transition using Differential Scanning Calorimetry confirm the first-order nature of the transition. Theoretical analysis, including group theory and modeling, reaffirms the displacive first-order nature of the structural transition. Our findings establish (TaSe4)3I as a model quasi-one-dimensional system with broken inversion symmetry facilitated through a displacive first-order structural transition.

Symmetry-Breaking Charge-Separation in a Subphthalocyanine Dimer Resolved by Two-Dimensional Electronic Spectroscopy.

Understanding the role of structural and environmental dynamics in the excited state properties of strongly coupled chromophores is of paramount importance in molecular photonics. Ultrafast, coherent, and multidimensional spectroscopies have been utilized to investigate such dynamics in the simplest model system, the molecular dimer. Here, we present a half-broadband two-dimensional electronic spectroscopy (HB2DES) study of the previously reported ultrafast symmetry-breaking charge separation (SB-CS) in the subphthalocyanine oxo-bridged homodimer μ-OSubPc2. Electronic structure calculations and 2D cross-peaks reveal the dimer's excitonic structure, while ultrafast evolution of the multidimensional spectra unveils subtle features of structural relaxation, solvation dynamics, and inhomogeneous broadening in the SB-CS. Analysis of coherently excited vibrational motions reveals dimer-specific low-frequency Raman active modes coupled to higher-frequency vibrations localized on the SubPc cores. Finally, beatmap amplitude distributions characteristic of excitonic dimers with multiple bright states are reported and analyzed.

Polaronic transport and magnetic field dependence of conduction parameters in rGO-CFO nanocomposites

Abstract In this paper we report the AC conductivity σ and magnetoconductivity Δσ% of a series of reduced graphene oxide-cobalt ferrite (rGO-CFO) composites in which the mass ratio of rGO:CFO has been systematically varied over six compositions. The conductivity measured in the frequency range of 20 Hz to 3 MHz showed an anomalous increase with the CFO content in the composite, even though the conductivity of CFO is at least three orders of magnitude smaller than that of rGO. Magnetoconductivity measured in fields upto 2000 Oe displayed both negative and positive values depending on the rGO:CFO ratio. Maximum magnetoconductivity of ~ +14% was obtained at room temperature in a magnetic field of only 750 Oe. Samples having the largest σ and Δσ% values also showed significant softening/hardening of the Raman active modes of CFO, indicating polaronic transport in these composites. Fits of σ(f) data to a power law σ(f) = σo + Afn showed that parameters A and n depend strongly on the applied magnetic field. This dependence is qualitatively and quantitatively different for samples that display positive or negative magnetoconductivity. Although the temperature dependence of these conduction parameters has been extensively studied, we have not come across any reports of their field dependence. Based on these observations we present a surface polaron model in which hybridization of π-orbitals of graphene with surface states of CFO nanoparticles provide for a polaron conduction mechanism through a grid of CFO nanoparticles intercalated with rGO sheets.

Coherent Anti-Stokes Hyper-Raman Spectroscopy

Coherent Raman scattering spectroscopies have been established as a powerful tool for investigating molecular systems with high chemical specificity. The existing coherent Raman scattering techniques detect only Raman active modes, which are a part of the whole molecular vibrations. Here, we report the first observation of coherent anti-Stokes hyper-Raman scattering (CAHRS) spectroscopy, which allows measuring hyper-Raman active vibrations at high speed. The CAHRS process relies on a fifth-order nonlinear process that combines hyper-Raman scattering with coherent Raman scattering. Observed signals are proven to come from the CAHRS process through various experiments concerning the dependences of the signals on incident laser powers, time-delay, polarizations, and selection rules of molecular vibrations. Comparisons of CAHRS signals with spontaneous hyper-Raman signals from para-nitroaniline solutions and benzene liquid manifest much higher signal-to-noise ratios of CAHRS signals than spontaneous hyper-Raman signals. This study illustrates that CAHRS spectroscopy can offer additional information on molecular vibrations unobtainable from the present coherent Raman techniques at a much higher speed than spontaneous hyper-Raman spectroscopy.

Angle-resolved Raman scattering study of anisotropic two-dimensional tellurium nanoflakes

As an elemental crystal, anisotropic two-dimensional (2D) tellurium (Te) flakes have recently garnered significant attention due to their exceptional chemical stability, tunable bandgap, low thermal conductivity, and high carrier mobility. To further investigate the anisotropic properties of Te nanoflakes, a rapid and effective method of determining their crystal axes is essential. In this study, it is demonstrated that the intensity of the Raman-active mode in Te nanoflakes exhibits a laser-polarization-dependence, varying periodically with the polarization angle. The crystal axis in two-dimensional Te nanoflakes can be identified using angle-resolved polarized Raman spectroscopy. Specifically, the Raman intensity of the A1 mode is the highest when the incident light is polarized along the [12¯10] direction and the lowest when polarized along the [0001] direction. This identification of the crystal axis via Raman spectroscopy is further verified by transmission electron microscopy measurements. In addition, theoretical simulations reveal that anisotropic Raman scattering is closely associated with the interference effect in a multilayer stacking system, as well as anisotropic absorption and anisotropic electron–phonon coupling in Te nanoflakes. This discovery not only provides a rapid method for locating the crystal axes in Te nanoflakes but also offers new insights into the scattering phenomenon in anisotropic materials beyond Te nanoflakes.

Intrinsic Temperature Dependence of Raman-Active Modes in Individual Isolated Single- and Double-Walled Carbon Nanotubes.

The intrinsic temperature dependence of Raman-active modes in carbon nanotubes (CNTs), particularly the radial breathing mode (RBM), has been a topic of a long-standing controversy. In this study, we prepared suspended individual CNTs to investigate how their Raman spectra depend on temperature and to understand the effects of environmental conditions on this dependency. We analyzed the intrinsic temperature dependence of the main Raman-active modes, including the RBM, the moiré-activated R feature, and the G-band in double-walled carbon nanotubes (DWCNT) and single-walled carbon nanotubes (SWCNTs) after complete desorption of air. The inner tube of the DWCNT, like the desorbed SWCNTs, was free from environmental influences, resulting in minimal temperature-induced RBM frequency shifts. We show that the larger RBM shift of SWCNTs upon initial heating is not intrinsic but is due to air desorption. The R feature, attributed to moiré-activated phonon scattering and nondispersive in nature, demonstrated a quasi-linear temperature dependence, akin to the G-band but with a lower temperature coefficient. The G-band, which was largely unaffected by environmental conditions, exhibited a consistent temperature coefficient across SWCNTs, DWCNTs, and small SWCNT bundles.

Raman intensity enhancement of Raman active and forbidden modes induced by naturally occurred hot spot at GaAs edge

Abstract Edge structures are ubiquitous in the processing and fabrication of various optoelectronic devices. Novel physical properties and enhanced light-matter interactions are anticipated to occur at crystal edges due to its broken spatial translational symmetry. However, the intensity of first-order Raman scattering at crystal edges has been rarely explored, although the mechanical stress and edge characteristics have been thoroughly studied by the Raman peak shift and the spectral features of the edge-related Raman modes. Here, by taking GaAs crystal with a well-defined edge as an example, we reveal the intensity enhancement of Raman-active modes and the emergence of Raman-forbidden modes under specific polarization Raman configurations at the edge. This is attributed to the presence of hot spot at the edge, due to the redistributed electromagnetic fields and electromagnetic wave propagations of incident laser and Raman signal near the edge, which are confirmed by the finite-difference time-domain simulations. Spatially-resolved Raman intensity of both Raman-active and Raman-forbidden modes near the edge is calculated based on the redistributed electromagnetic fields, which quantitatively reproduce the corresponding experimental result. These findings offer new insights into the intensity enhancement of Raman scattering at crystal edges and presents a new avenue to manipulate the light-matter interactions of crystal by manufacturing various types of edges and to characterize the edge structures in photonic and optoelectronic devices.

Efficient Ethanol Sensor Based on Vertically Aligned ZnO Nanorods on Radio Frequency Sputtered ZnO/Pt/SiO2/Si Substrate

The volatile organic compounds (VOCs) sensing device is developed by using hydrothermally grown vertically aligned 1D ZnO nanorods (ZnO NRs) on radio frequency sputtered ZnO/Pt/SiO2/Si substrate. Before the fabrication of the sensor device, the microstructure, morphology, and temperature stability of the NRs are characterized using x‐ray diffraction, absorption and emission spectrum, field emission scanning electron microscopy, and thermal gravimetric analysis. Observation shows the formation of well‐vertically aligned, temperature‐stable (up to 900 °C) ZnO NRs with a hexagonal wurzite phase. The NRs show enhanced defects, vacancies, and interstitial‐related green emission (503 nm) and red emission (655 nm) with a bandgap of ≈3.74 eV. The observed Raman active E1 and A1 modes are prominent for such NRs. The sensing device exhibits a selective response to ethanol as compared with other VOCs (e.g., methanol, ethanol, and 2‐propanol). The oxygen‐vacancy‐rich ZnO NRs show enhanced ethanol sensing with a speedy response time (87–97 s) and recovery characteristics time (66–169 s). The device shows a lower detection limit for ethanol ≈4.5 ppm. The ethanol sensing mechanism and the advantages of the as‐grown vertically aligned ZnO NRs structure as sensing material are also discussed. The device shows excellent long‐term stability (≈20 weeks) and reproducing capability for ethanol.

Hybrid magnetic nanocomposite NiFe2O4/TiO2 for Photocatalytic dye degradation and green hydrogen (H2) generation

The present paper focuses on designing and developing nanocomposites to modify their properties to increase the ability to generate hydrogen and degrade photocatalytic dyes. A nanocomposite of nickel ferrite (NiFe2O4)/Titanium dioxide (TiO2) was formulated using the sol-gel self-ignition method. X-ray diffraction (XRD) tool was adopted to analyze the structural behavior of pure nickel ferrite (NiFe2O4), pure titanium dioxide (TiO2), and nanocomposite of NiFe2O4/TiO2. XRD pattern of NiFe2O4 and TiO2 shows a monophasic nature whereas the XRD pattern of nanocomposites shows mixed phases of NiFe2O4 and TiO2. FTIR spectra confirm the formation of NiFe2O4, TiO2, and their composites with absorption bands near 400 cm-1 and 600 cm-1, 1500 cm-1. Raman spectra unveiled the presence of five active Raman modes in the case of NiFe2O4 whereas three active Raman modes are seen in TiO2. Field emission scanning electron microscopy (FE-SEM) images exposed the dense structure with spherical cluster-like morphology in the case of NiFe2O4. Energy-dispersive X-ray spectroscopy (EDX) analysis confirms the presence of all the desired elements of NiFe2O4, TiO2, and their composites in stoichiometric proportions. BET analysis of nickel ferrite NPs suggests a large surface area of the order of 31.54 m2/gm in comparison with TiO2 (19.23 m2/gm). The optical characterization was made through the UV-visible spectroscopic technique. TiO2 shows a maximum band gap of the order of 3.02 eV while NiFe2O4 shows a low band gap of the order of 1.74 eV. A typical M-H curve with saturation magnetization of the order of 27.54 emu/gm was observed for pure NiFe2O4. No magnetic properties were observed for TiO2 as it is a semiconductor. Enhancement in magnetic properties is observed with increases in NiFe2O4 content in a composite of NiFe2O4/TiO2. The photocatalytic activities were examined by degrading Methylene Blue (MB) dye under sunlight for pure nickel ferrite (NiFe2O4), pure titanium dioxide (TiO2), and a nanocomposite of NiFe2O4/TiO2. Photocatalytic hydrogen production reactions were carried out in a methanol/water solution under visible light. Consequently, the best configuration of the photocatalyst was determined as 25% wt% NiFe2O4/ TiO2 which had shown the maximum hydrogen (H2) production rate as 146.3 μmol/g/h after 8 h owing to its reduced bandgap energy and delayed e- / h+ recombination.

Influence of LaMnO3 Nanocrystallite Size on its Optical and Raman Spectra

: In the current study, nanocrystalline LaMnO3 perovskite was prepared by combustion method and annealed at different annealing temperatures. The X-ray diffraction (XRD) patterns provided evidence that the structure formed has a Rhombohedral structure with R3 ̅c space group. The remarkable growth in the crystallite size, reduction in microstrain, and dislocation density were observed with annealing temperature. Ultraviolet-visible spectroscopy was used to determine the optical band gap by the Tauc-plot method. The optical band gap was found to be 3.5 ± 0.4eV and 2.9 ± 0.5eV for 600°C and 1200°C annealed samples, respectively. The observed results were influenced by crystallite size. Raman spectra of the LaMnO3 nanocrystallites revealed five Raman-active modes, like out-of-phase rotation modes and bending mode of MnO6 octahedra. Moreover, the intensity of vibrational modes also varied significantly with annealing temperature.

Femtosecond Raman-induced Kerr effect spectroscopic study of the intermolecular dynamics in aqueous solutions of imidazolium hydrochloride, imidazole, sodium triazolide, and triazole: concentration dependence.

In this study, we employed femtosecond Raman-induced Kerr effect spectroscopy to analyze the concentration-dependent intermolecular dynamics in positively or negatively charged aromatics and their neutral analogous aromatics (imidazolium hydrochloride (ImHCl), imidazole (Im), sodium triazolide (NaTr), and triazole (Tr)) in aqueous solutions at 293 K. We also measured their liquid properties, such as density, viscosity, and surface tension, at 293 K, and compared them with their dynamic properties. Furthermore, we performed the quantum chemistry calculations of the target aromatics and some clusters to elucidate their optimized structures, interaction energies, charge populations, and Raman-active normal modes. We characterized the Kerr transients over 2ps using a triexponential function. The results revealed that the aqueous solutions' intermediate and slow relaxation time constants were linearly proportional to the viscosities. The slopes of the time constants to the viscosity of the aqueous ImHCl solutions were steeper than those of the aqueous Im solutions, whereas the slopes of the aqueous NaTr solutions were milder than those of the aqueous Tr solutions. These findings indicated that the charge of the aromatics in the aqueous solutions affected the coupling parameter between the solute and solvent in the orientational dynamics with different ways. The first moment (M1) of the low-frequency band (< 200 cm-1), coming from the intermolecular vibrations, in the difference spectra between the aqueous aromatic solutions and neat water shifted to the high-frequency region as the concentration increased. The M1 slope to the concentration for the aqueous ImHCl solutions was steeper than that for the aqueous Im solutions. Conversely, the concentration dependence of M1 for the aqueous NaTr solutions was similar to that for the aqueous Tr solutions. We used the local structures of the target aromatics based on the quantum chemistry calculations to rationally clarify their concentration-dependent intermolecular dynamics in the aqueous solutions.

Raman activity, piezoelectric response, and carrier mobility in two-dimensional Janus TiGe[formula omitted]H ([formula omitted] N, P, As) semiconductors: A first-principles prediction

In this article, we theoretically propose a series of TiGeZ3H (Z= N, P, As) monolayers and comprehensively investigate their structural, vibrational, piezoelectric, electronic, and transport properties using first-principles simulations. The structural stability of the suggested monolayers is verified by phonon dispersion analysis, ab-initio molecular dynamics calculations, and Born–Huang mechanical stability criteria. Based on the calculations for the mechanical response, it is shown that TiGeN3H is the stiffest material compared to the other two structures with Young’s modulus found to be 252.11 Nm−1. Besides, we also examine the vibrational characteristics of TiGeZ3H through the analysis of their phonon spectra and Raman active modes. Due to the broken vertical mirror symmetry, TiGeZ3H monolayers possess both out-of-plane and in-plane piezoelectric responses, in particular, the out-of-plane piezoelectric coefficient of TiGeAs3H is computed to be up to −0.42 pm/V. Janus TiGeZ3H monolayers are found to be indirect semiconductors with decreasing bandgap as Z changes from N to As. Particularly, the Rashba spin splitting is found in TiGeAs3H when the spin–orbit coupling is taken into account. The calculations for the transport features indicate that while TiGeN3H monolayer exhibits low electron mobility, both TiGeP3H and TiGeAs3H have electron mobility μx over 400 cm2V−1s−1, which is suitable for applications in electronics.

Structural modifications of BiOBr nanoplates by electron beam irradiation

Abstract In this study, BiOBr nanoplates are synthesized through the hydrothermal method, then samples have been irradiated by an electron beam of 10 MeV energy to deliver doses of 100 kGy and 500 kGy. XRD and Raman investigations corroborated the presence of a pure phase in all nanoplate samples. The sharp and narrow peaks in XRD indicate well crystalline in nature of the samples, which decreases with increasing the electron dose as confirmed by the decay in peak intensity. Conversely, the peak position shifts at lower angle with increasing the electron dose. Structural factors including lattice parameter, dislocation density, cell volume, microstrain, as well as stacking fault have been found to change due to electron beam irradiation. We employed the Debey-Scherrer formula (D-S), Size Strain Plot method (SSP), and Halder Wagner (H-W) methods to determine the crystal size and strain of purified and irradiated BiOBr nanoplates. It has been found that the size of the crystallites increases nonlinearly as the irradiation dose increases. All three of the aforementioned calculation methods have observed this trend. Strain and dislocation density exhibited the opposite trend of crystallite size, as they decreased as the irradiation dose increased. The dislocation density, strain, and crystallite size values are nearly identical for the SSP and H-W procedures, while the D-S method exhibits values with deviation. The tetragonal structure and the Raman active mode, which we have identified as closely resembling those in other literature of this composition, are discussed in the respective section. Because of their intriguing phase strength, the synthesized nanoplates may be suitable for the degradation of organic pollutants and the separation of water through photo-electrocatalysis.

Nontrivial Raman Characteristics in 2D Non-Van der Waals Mo5N6.

Resonant Raman spectra of a two-dimensional (2D) non-van der Waals (vdW) material, molybdenum nitride (Mo5N6), are measured across varying thicknesses, ranging from a few to tens of nanometers. Fifteen distinct Raman peaks are observed experimentally, and their assignments are made using first-principles calculations for the most stable AABB-stacking structure of Mo5N6. The assignments are further supported by angular-dependent Raman measurements for all peaks, except the most intense one at 215 cm-1. Calculations reveal that the 215 cm-1 peak does not appear for three-dimensional molybdenum nitrides and is not a first-order Raman-active mode. We further investigated the origin of the 215 cm-1 peak and assigned it as a defect-induced double-resonance peak. Moreover, thickness-dependent Raman measurements reveal that both the 215 and 540 cm-1 peaks─assigned to out-of-plane and in-plane modes, respectively─blue shift as thickness increases, reaching a plateau around 20 nm. This thickness-dependent Raman shift over a wide thickness range is nontrivial compared to other common vdW 2D materials and is attributed to the much stronger stacking interaction between the constituent layers in non-vdW materials. This finding highlights Raman spectroscopy as a valuable tool for characterizing the thickness of 2D non-vdW materials.

High-Pressure Monosulfide Solid Solution FexNi1−xS Phases: X-Ray Diffraction Analysis and Raman Spectroscopy

High-pressure high-temperature (HPHT) crystalline monosulfide solid solution (Mss) phases (FexNi1−xS, x = 0.90, 0.75, 0.50, 0.25), troilite (FeS I), and α-NiS of the Fe-Ni-S system were synthesized at 7 GPa and 900–1550 °C. The structural parameters of the obtained phases were refined by XRD using the Rietveld method. Factor group analysis revealed the number of active Raman modes for FeS I and α-NiS. Raman spectra of troilite, α-NiS, and Mss phases were obtained. It was shown that the Raman spectra of Mss phases and α-NiS have a similar topology. The Raman spectra of the experimental phases in the Fe-Ni-S system were analyzed with non-negative matrix factorization, which provided a meaningful concentration dependence of the spectral patterns. The spectral components were assigned to the FeS I and α-NiS structures. The structural and spectroscopic studies show linear dependencies of unit cell parameters and spectral components on composition and confirm the existence of a series of monosulfide solid solution FexNi1−xS.

Impact of the host lattice on crystallographic, Raman, optical, and luminescent properties on the terbium ion

AbstractA comparative study of the Gd2O3:Tb and Y2O3:Tb nanoparticles (NPs) was presented to examine the influence of the host lattices on the crystallographic phase, surface characteristics, optical properties, Raman effect, and photoluminescence properties. Cubic shapes Gd2O3:Tb and Y2O3:Tb NPs with an estimated crystalline size of 17 and 11 nm were recorded from the X‐ray diffraction pattern, respectively. Comparative absorption spectra of both oxides NPs were presented to examine the dispersibility, colloidal stability, and optical behavior of the NPs. Energy bandgap values were estimated to monitor the optical characteristics of the metal oxides in the UV/Vis region. Fourier transform infrared was recorded to confirm the surface‐attached water and organic moieties. The Raman spectra of metal oxides were recorded to examine the symmetrical dis‐order and polarizability of the vibrational bands. The ionic radius of the atoms distorts the bond structure resulting in it greatly influenced the vibrational bands of the materials. The emission spectra certified the effective doping of the luminescent Tb3+‐ion in the metal oxide host lattices. Raman spectra and emission spectra validated the crystal symmetry imperfections; subsequently, the efficiency of the Raman active modes and emission transitions was greatly influenced. In a comparative analysis, Y2O3:Tb NPs exhibited higher luminescence intensity in comparison with the Gd2O3:Tb NPs.

Study of Pure and Mixed-Phase TiO2 Nanomaterials Synthesized via a Modified Solvothermal Method and Their Optical Properties

The modified-solvothermal method has been used to synthesize pure anatase and anatase-rutile mixed-phase TiO2 nanomaterials using titanium isopropoxide, sodium hydroxide, and triethylamine as primary sources with different calcination temperatures (500 °C and 700 °C). The effect of the precipitating agent, capping agent, and calcination temperatures on the properties, such as structural, morphological, and optical of the resultant nanomaterials has been investigated through several techniques including X-ray diffraction (XRD), Raman spectroscopy, Transmission electron microscopy (TEM), UV–Vis absorption spectroscopy, Scanning electron microscopy (SEM), selected area electron diffraction (SAED), and high-resolution transmission electron microscopy (HRTEM). XRD and Raman spectra confirmed the pure anatase and anatase-rutile mixed structures of the prepared nanomaterials at 500 °C and 700 °C. Furthermore, the Raman active modes confirmed the formation of TiO2. It is noticed that the crystallite size of the nanomaterials increased with increasing calcination temperature of the XRD pattern. The morphologies of the prepared samples were displayed using SEM and TEM micrographs, which are almost the same in shape (cubic and cubic rod mixed). The particle size increased with an increase in calcination temperature and was found to be between 10 and 20 nm, which is similar to the crystallite size. The lattice fringes (d) from HRTEM images are assigned to the XRD plane (101) and (110) plane of anatase and anatase-rutile TiO2, respectively. The SAED exhibited well crystalline nature and confirmed the polycrystalline nature of the prepared nanomaterials by the bright points and the ring patterns. The optical band gap decreased with an increase in temperature and the optical properties are affected by calcination temperature and triethylamine.

Tuning of electrical properties and persistent photoconductivity of SnO2 thin films via La doping for optical memory applications

This work explored the potential of utilizing Lanthanum doped tin oxide Sn1−xLaxO2 (x = 0.01 to 0.1) based Metal-Semiconductor-Metal Ohmic photoconductors for optical memory applications making use of the persistent photoconductivity (PPC) property. The structural, optical, and electrical properties of Sn1−xLaxO2 thin films deposited on glass substrates using the spray pyrolysis method, with a focus on the impact of lanthanum concentration on the photoresponse characteristics was investigated. Raman spectroscopy confirmed the presence of oxygen vacancies and nanometric grain size in the films along with the typical Raman active modes of tin oxide. The Sn4+ and La3+ oxidation states in Sn1−xLaxO2 as well as the contributions from lattice oxygen and oxygen vacancies were identified using XPS. Photoluminescence studies revealed emissions in the UV, violet, blue, and yellow regions, corresponding to tin interstitials, oxygen vacancies, and other defects, with intensity variations based on La concentration. All films exhibited n-type conductivity, with La content influencing both resistivity and carrier concentration. Photoconductivity measurements demonstrated enhanced photocurrent under UV illumination, with La doping affecting energy levels and defect states. The Sn0.90La0.10O2 film possessed a photocurrent retention of nearly 64 % within a span of 104 s, showing that higher concentration of La favoured the enhancement of retention of photocurrent for a comparatively longer duration. The significant persistent photoconductivity requires the conditions like optically active materials, a built-in electric field to separate electron-hole pairs, and defect states to trap carriers, which are all met by the prepared Sn1-xLaxO2 photoconductor with higher La doping levels, confirming the suitability of these films for practical use as optical non-volatile memory elements.