Search

The influence of morphology and compositional mixing on the electrochemical performances of Li-rich layered oxides (LLOs), specifically to address the high rate capability, is investigated. LLOs of composition xLi₂MnO₃·(1 – x)LiMn₀.₂₅Ni₀.₃₈Co₀.₃₇O₂ (LMNC, x = 0, 0.2, 0.4, and 0.6), lying in the plane NMC(640)–LCO–LMO, are synthesized in nanoplatelet morphology, and the results are compared to the same compounds prepared by a conventional solid-state reaction (SSR). Hexagonal-shaped thin (∼50 nm) flakes of transition metal oxide–hydroxide [TMO(OH)], prepared by the hydrothermal process, are reacted with Li carbonate to derive nanoplatelet morphology of LMNC by topotactic conversion. Structural and compositional evolutions of LLOs are analyzed with Rietveld refinement. The composite nature of LMNC comprising of monoclinic Li₂MnO₃ and rhombohedral LiMO₂ phases is evidenced. High-resolution transmission electron microscopy studies show the existence of a monoclinic Li₂MnO₃ phase embedded within the rhombohedral layered oxide phase. A uniform compositional distribution of all elements is discerned from EDS mapping, strongly suggesting that metal cations in both TMO/OH and LMNC are highly intermixed. Electrochemical properties become better with the larger fraction of the Li₂MnO₃ phase in LiMO₂. Among four compositions examined, LMNC (x = 0.6) shows the best electrochemical performance, with a capacity of ∼240 mAh g–¹ (∼173 mAh g–¹) at 0.1 C (1 C) current rate. Cycling stability studies, carried out at 1 C rate for 100 cycles, show a high capacity retention of 86%. Capacity at 3 C (5 C) is ∼140 mAh g–¹ (∼80 mAh g–¹) in LMNC (x = 0.6). LMNC (x = 0 and 0.6) prepared by SSR show inferior properties, suggesting that morphology and thorough intermixing of monoclinic Li₂MnO₃ and rhombohedral LiMO₂ phases are shown to play a significant role. Although enhanced performance is generally attributed to the extra capacity contribution from the Li₂MnO₃ phase, this study unequivocally brings out the influence of morphology on the electrochemical properties.

Designing solid-state perovskite oxide solar cells with large short circuit current (JSC) and open circuit voltage (VOC) has been a challenging problem. Epitaxial BiFeO3 (BFO) films are known to exhibit large VOC (>50 V). However, they exhibit low JSC (≪μA/cm2) under 1 Sun illumination. In this work, taking polycrystalline BiFeO3 thin films, we demonstrate that oxygen vacancies (VO) present within the lattice and at grain boundary (GB) can explicitly be controlled to achieve high JSC and VOC simultaneously. While aliovalent substitution (Ca2+ at Bi3+ site) is used to control the lattice VO, Ca and Ti cosubstitution is used to bring out only GB-VO. Fluorine-doped tin oxide (FTO)/Bi1-xCaxFe1-yTiyO3-δ/Au devices are tested for photovoltaic characteristics. Introducing VO increases the photocurrent by four orders (JSC ∼ 3 mA/cm2). On the contrary, VOC is found to be

Rutile TiO2 (TiO2-NR) and ZnO (ZnO-NR) in nanorod microstructural forms are synthesized by hydrothermal route. The oxides are grown directly on fluorine doped SnO2 coated glass, and annealed in air (AA) and hydrogen (HA) atmosphere for 3 h at 450 °C. A detailed structural, optical, and microstructural study confirms the phase formation in the AA and stability in HA samples. EPR signal with g-values of ~1.99 and ~1.95 from TiO2-NR and ZnO-NR photoanodes respectively confirms the presence of oxygen vacancy (OV) related defects. The EPR signals are stronger in TiO2-NR-HA compared to TiO2-NR-AA and is absent in ZnO-NR-AA suggesting increased defect concentration on hydrogenation. Further this leads to two order increase in photoconductivity for hydrogenated TiO2-NR-HA and ZnO-NR-HA photoanodes when measured under 1 Sun illumination compared to AA photoanodes. Photovoltaic power conversion efficiency for both dye and Sb2S3 sensitized solar cells are found to increase by 2 to 3 times in hydrogenated ZnO-NR. In contrast, TiO2-NR show two-fold decrease in efficiency on hydrogenation. The efficiency change is consistent with the change in interface impedance, which increases for dye or Sb2S3 sensitized TiO2-NR-HA photoanode, whereas, decreases for sensitized ZnO-NR-HA photoanode. These studies show that despite of one to two to three order increase in photoconductivity, the sensitizer/photoanode interface impedance play a significant role in deciding the efficiency.

Semiconductor quantum dot (QD)–graphene composites are promising materials for photovoltaics and photocatalysis because of efficient charge extraction and transport property of graphene. Analysis o...

One step electrodeposition method has been used to realize highly porous ZnO pin hole (ZP) and ZnO rosette sheets (ZS) nanostructure based photo-anodes for efficient photoelectrochemical (PEC) splitting of water. Electrodeposited ZP and ZS based photo-anodes exhibit enhanced photocurrent density of 0.62 mA/cm2 and 0.76 mA/cm2 respectively (at a bias of 0.75 V). Further on hydrothermal amination (A), these electrodeposited ZP and ZS (A-ZP and A-ZS) nanostructure based photo-anodes had shown enhanced photocurrent density of 1.02 mA/cm2 and 1.27 mA/cm2, respectively. Surface morphology, evolution and elemental study were done using FESEM and EDX. Raman spectra of aminated photo-anodes have peaks at ∼270 cm−1 and ∼511 cm−1 related to stretching vibration mode between Zn N and Zn O. The peaks at wave number ∼558 cm−1 and ∼571 cm−1 is due to formation of Zn C bonds and because of complex defects respectively. ZnO exhibits low PEC activity, but on nano-structuring in the form of ZP and ZS improves its light absorption capacity. Hydrothermal amination red shifts (∼25 nm) the absorption band at ∼ 425 nm. The N and C act as electron reservoirs in A-ZP and A-ZS photo-anodes and efficiently separate the photo-generated electron/hole pairs and restrain charge recombination to generate photo-reactive sites. Electrochemical impedance spectroscopy (EIS) revealed that A-ZP and A-ZS had low charge transfer resistance compared to their bare counterparts. This lead to considerably improved PEC performance. An unprecedented increase in IPCE values in A-ZP and A-ZS can be assigned to the decrease in band gap and thereby significant enhancement in photocurrent density. These result in to proper charge segregation and improved charge transportation. The maximum value of IPCE is 9.6% for A-ZS sample and it is also clear that ZP and ZS nanostructured film have higher IPCE values at ∼400 nm than traditional ZnO thin film. A-ZP and A-ZS based photo-anodes have exhibited enhanced PEC performance as evident from IPCE measurements and thus can be a prospective candidate for PEC and optoelectronic applications.

Bi-rich Bi2Te3 thin films are prepared at 300 K using e-beam evaporation technique. A source power of 45 W for e-beam is used. Post deposition, these as-deposited Bi-rich Bi2Te3 (Bi-BT-AD) films are annealed at 100 °C (Bi-BT-100), 200 °C (Bi-BT-200) and 300 °C (Bi-BT-300) for 1 h under a pressure of 3 × 10−4 Pa. X-ray diffraction measurements reveal the presence of Bi phase together with crystalline Bi2Te3 indicating the possible presence of Bi-rich Bi2Te3 phase in the Bi-BT-AD film. The broad peaks from Bi2Te3 (015) plane indicate nanocrystalline nature of particles. With annealing, no change in diffraction pattern is observed for Bi-BT-100. However, Bi-BT-200 and Bi-BT-300 films show the emergence of x-ray reflection from unknown phases around 2θ ~ 20° and 47°. This indicates Bi related secondary phase segregation and the thermodynamic instability for the presence of Bi in Bi2Te3 lattice. From Raman studies it is discerned that Bi secondary phase coexists along with the Bi-rich Bi2Te3 nanocrystalline grains. On vacuum annealing Bi-rich Bi2Te3 phase in thin films prevails as evidenced from the p-type electrical characteristics, while excess Bi disappears and converts into an unknown minor phase. The resistivity of all the annealed films are ~ 0.9 × 10−4 Ωcm. The Seebeck coefficients also do not show any change and remain around 33 to 36 μV/K. Thermoelectric properties of Bi-BT-100 exhibit high power factors when measured at different ΔT with a maximum of ~ 17.5 × 10−4 W/K2 m for ΔT = 100 °C. Thus, unlike the near-stoichiometric thin films, Bi-rich thin films require low temperature annealing (~100 °C) to achieve optimized parameters. Bi-rich Bi2Te3 thin films also show higher power factor compared to the near-stoichiometric thin films. Thus, favourable thermoelectric properties can be achieved at 300 K for temperature sensitive device fabrication using Bi-rich Bi 2Te3 thin films.

A Li-rich layered oxide (LLO) cathode with morphology-dependent electrochemical performance with the composition Li1.23Mn0.538Ni0.117Co0.114O2 in three different microstructural forms, namely, randomly shaped particles, platelets, and nanofibers, is synthesized through the solid-state reaction (SSR-LLO), hydrothermal method (HT-LLO), and electrospinning process (ES-LLO), respectively. Even though the cathodes possess different morphologies, structurally they are identical. The elemental dispersion studies using energy-dispersive X-ray spectroscopy mapping in scanning transmission electron microscopy show uniform distribution of elements. However, SSR-LLO and ES-LLO nanofibers show slight Co-rich regions. The electrochemical studies of LLO cathodes are evaluated in terms of charging/discharging, C-rate capability, and cyclic stability performances. A high reversible capacity of 275 mA h g-1 is achieved in the fibrous LLO cathode which also demonstrates good high-rate capability (80 mA h g-1 at 10 C-rate). These capacities and rate capabilities are superior to those of SSR-LLO [210.5 mA h g-1 (0.1 C-rate) and 4 mA h g-1 (3 C-rate)] and HT-LLO [242 mA h g-1 (0.1 C-rate) and 22 mA h g-1 (10 C-rate)] cathodes. The ES-LLO cathode exhibits 88% capacity retention after 100 cycles at 1 C-rate. A decrease in voltage on cycling is found to be common in all three cathodes; however, minimal voltage decay and capacity loss are observed in ES-LLO upon cycling. Well-connected small LLO particles constituting fibrous microstructural forms in ES-LLO provide an enhanced electrolyte/cathode interfacial area and reduced diffusion path length for Li+. This, in turn, facilitates superior electrochemical performance of the electrospun Co-low LLO cathode suitable for quick charge battery applications.

(i) Edge and planar defect-rich and (ii) defect-suppressed MoS2 nanosheets are fabricated by controlled annealing of wet-chemically processed precursors. Wrinkles, folds, bends, and tears lead to the introduction of severe defects in MoS2 nanosheets. These defects are suppressed and highly crystalline MoS2 nanosheets are obtained upon high-temperature annealing. The influence of defects on the electrochemical properties, particularly rate capability and cycling stability, in the Li intercalation regime (1 V to 3 V vs. Li/Li+) and conversion regime (10 mV to 3 V vs. Li/Li+) are investigated. In the intercalation regime, the initial Li intake (x in LixMoS2) for defect-rich nanosheets is larger (x ≈ 1.6) as compared to that in defect-suppressed MoS2 (x ≈ 1.2). Although the reversible initial capacity of all the anodes is nearly the same (x ≈ 0.9) at 0.05C rate, defect-rich MoS2 exhibits high rate capability (>40 mA h g-1 at 40C or 26.8 A g-1). When cycled at 10C (6.7 A g-1) for 1000 cycles, 75% capacity retention is observed. High rate capability can be attributed to the defect-rich nature of MoS2, providing faster access to lithium intercalation by a shortened diffusion length facilitated by Li adsorption at the defect sites. The defect-rich nanosheets exhibit a power density of ∼20% more than that of defect-suppressed nanosheets. For the first time, MoS2/Li cells with a high power density of 10-40 kW kg-1 in the intercalation regime have been realized. In the conversion regime, defect-rich and defect-suppressed MoS2 exhibit initial lithiation capacities of ∼1000 and ∼840 mA h g-1, respectively. Defect-rich MoS2 had a capacity of ∼800 mA h g-1 at 0.1C (67 mA g-1), whereas defect-suppressed MoS2 had a capacity of only ∼80 mA h g-1 at the same current rate. Capacity retention of 78% was observed for defect-rich MoS2 with a reversible capacity of 591 mA h g-1 when cycled at 0.1C (67 mA g-1) for 100 cycles. Despite having a lower energy density in the intercalation regime, the power density of defect-rich MoS2 in the intercalation regime is significantly larger (by three orders of magnitude) as compared to that of defect-suppressed MoS2 in the conversion regime. Defect-rich MoS2 nanosheets are promising for high-rate-capability applications when operated in the intercalation regime.

Nano-porous nano-silicon (npn-Si) anode for Li-ion battery is a great promise to mitigate problems associated with large volume expansion and pulverization during the charging-discharging cycle. Here we report the synthesis of highly porous Si from MCM-41, through a magnesiothermic reduction ((MR) method. Structural studies confirm the complete conversion of MCM-41 to crystalline silicon from the particle core to the surface. Morphological studies reveal the retention of porous network in the parent compound after reduction. The silicon anode is fabricated using two types of aqueous binders viz Na salt of carboxymethyl cellulose (Na-CMC), alginate (Na-Alg) demonstrates good cycling stability and rate performance. Among these, porous silicon with former binder shows a higher initial charge capacity of 2767 mAhg−1 even at a current rate of C/2 and retains 705 mAhg−1 capacity after 500 cycles. While, alginate binder showed much higher initial capacity of 3000 mAhg−1 and retained almost the same capacity as Na-CMC after 10 cycles and beyond. High capacity and reasonably good retention of nano-porous nano-silicon is attributed to (i) the accommodation of the drastic volume change in porous Si to effectively mitigate the mechanical stress, (ii) enhanced electrochemical kinetics and (iii) high Li flux in the porous structure.

Composite photoanode comprising mesoporous microspheres with a smooth spherical morphology exhibiting high light scattering due to optical whispering gallery modes (WGMs) is used to fabricate whisperonic solar cell (WSC) devices. The photoanode is sensitized with CdS–CuInS2 thin films (CdS–CIS-TF) of ∼5 nm thickness. CdS–CIS-TF-sensitized photoanodes exhibit strong coupling with WGM. These WSC devices show an average (ηavg) efficiency (η) of ≈3.2% in comparison with ηavg ≈ 1.9% for the nanoparticulate-based photoanode. The observed efficiency is the highest for CdS–CIS-TF-sensitized solar cells made using I–/I3– electrolyte. This remarkable increase in ηavg (∼60%) is attributed to increased photon absorption by the sensitizer films because of the presence of WGM scattering prevailing in smooth microspheres. Thus, WSC photoanode configuration is a promising approach to enhance the efficiency of TF-sensitized solar cells.

Carbon-coated single crystalline nanotubular (NT) and nanoparticular (NP) LiFe1-xMnxPO4 (x = 0, 0.2, and 0.5) cathodes are fabricated to study the effect of compositional and microstructural changes on Li+ diffusion and electrochemical properties. Insight in to the compositional effect on Li+ diffusion is obtained from DFT facilitated climbing image nudged elastic band (CI-NEB) simulations. NT cathodes exhibit exceptionally good discharge capacities ∼60 (∼165) mAhg−1, ∼32 (∼110) mAhg−1 and ∼22 (∼82) mAhg−1 at 25C (1C) rate for x = 0, 0.2, 0.5, respectively. NP cathodes show capacity

A synergetic approach of employing smooth mesoporous TiO2 microsphere (SμS-TiO2)–nanoparticulate TiO2 (np-TiO2) composite photoanode, and size and defect controlled CdSe quantum dots (QD) to achieve high efficiency (η) in a modified Grätzel solar cell, quantum dot sensitized whisperonic solar cells (QDSWSC), is reported. SμS-TiO2 exhibits whispering gallery modes (WGM) and assists in enhancing the light scattering. SμS-TiO2 and np-TiO2 provide conductive path for efficient photocurrent charge transport and sensitizer loading. The sensitizer strongly couples with the WGM and significantly enhances the photon absorption to electron conversion. The efficiency of QDSWSC is shown to strongly depend on the size and defect characteristics of CdSe QD. Detailed structural, optical, microstructural and Raman spectral studies on CdSe QD suggest that surface defects are prominent for size ~2.5 nm, while the QD with size > 4.5 nm are well crystalline with lower surface defects. QDSWSC devices exhibit an increase in η from ≈0.46% to η ≈ 2.74% with increasing CdSe QD size. The reported efficiency (2.74%) is the highest compared to other CdSe based QDSSC made using TiO2 photoanode and I−/I3− liquid electrolyte. The concept of using whispering gallery for enhanced scattering is very promising for sensitized whisperonic solar cells.

CuInS2 (CIS) based solar cell devices are fabricated by sensitizing TiO2 photoanodes with CIS quantum dots (CIS-QDs). Morphologically different TiO2, viz. Degussa P25 nanoparticles, smooth and fibrous microspheres (SμS and FμS respectively) are used to fabricate photoanodes. CIS-QDs are synthesized using dodecanethiol (DDT), CuI and In(OAc)3 precursors by solvothermal method. DDT surfactant present on the CIS QDs surface is replaced with 3-mercaptopropionic acid in a single phase one step procedure to enable efficient loading of QDs onto photoanode and as linker molecule for charge carrier extraction. The CIS QDs sensitized on SμS and FμS microsphere photoanode layers exhibit a photoconversion efficiency (η) of 3.2% and 1.6%, respectively, in comparison to η ≈ 2.1% for nanoparticulate TiO2 (Degussa P25). Further increase in efficiency is obtained (3.8% for SμS and 2.5% for FμS) when composite photoanode films made of porous microspheres filled with nanoparticulate P25 are used. A maximum efficiency of 3.8% (with JSC ≈ 6.2 mA, VOC ≈ 926 mV and FF ≈ 66 for cell area ≈ 0.25 cm2 and thickness ≈ 20 µm) is realized when 4.6 nm CIS QDs sensitized on composite photoanode (consisting of 80 wt. % SμS and 20 wt. % P25) is used. High VOC observed is unprecedented and is possible due to combined effect of SμS+P25 composite photoanode properties such as fewer defects, good connectivity between particles, effective light scattering, minimum recombination, and effective electron transport and size optimized CuInS2 QDs. Electrochemical impedance spectroscopy studies reveal a low interfacial resistance and longer electron life time in SμS+P25 composite photoanodes.

Mesoporous TiO2 nanoparticles are excellent photoanodes for sensitized solar cells (SSC). However, a significant loss in the photoconversion efficiency (PCE) occurs due to the inefficient light absorption. Large Mie scattering from mesoporous micron-sized spheres could be used to enhance the absorption and hence the PCE. Here, we show that a composite comprising a TiO2 mesoporous microsphere with smooth surface (SμS-TiO2) exhibiting whispering gallery modes (WGM) and a commercial TiO2 mesoporous nanoparticle (Degussa P25) with an optimum ratio (80:20 wt %) shows significant enhancement in PCE of DSSC and QDSSC devices. SμS-TiO2 exhibits strong WGM as evidenced from the photoluminescence studies carried out using a laser with an excitation wavelength λexc = 325 nm. These microspheres sensitized with N719 dye or CdSe/CuInS2 QDs are shown to exhibit emission characteristics strongly coupled to WGM resulting in an enhanced PCE in SSC. Increases in PCE of ∼24% in DSSC devices and 80–95% in QDSSC devices of 0.2...

We present a detailed study on the photovoltaic performance of dye and quantum dot (QD) sensitized solar cells fabricated using nanoporous TiO2 photoanodes prepared by a template approach. Nanoporous TiO2 particles (TNPO) are prepared by sol-gel method using dextran (TNPO-D) and glucose (TNPO-G) as templates. The decomposition of templating agents during calcination introduces nanopores (∼10 nm) in TiO2 nanoparticles as evidenced from the HRTEM studies. Oxygen vacancies are also introduced in TiO2 nanoparticles due to prevailing reducing ambient during decomposition. Oxygen vacancy (VO) related defects are evidenced from EPR spectra from a strong absorption signal observed at g-value of 1.994 related to Ti3+-VO-Ti3+ defect sites. The defect concentration estimated from EPR spectra is found to be ∼5 to 15 times larger for nanoporous TiO2 particles compared to Degussa P25-TiO2. TNPO samples also show enhanced defect level photoluminescence emission. The photovoltaic efficiency (η) and the IPCE of DSSC and QDSSC are strongly influenced by the defect concentration and seen to decrease with increasing VO in TiO2 nanoparticles. Twofold lower η is found in both DSSC (∼6.38% to 3.66%) and QDSSC (∼1.69% to 0.46%) devices made using nanoporous TiO2 compared to P25-TiO2 photoanode solar cells. P25-TiO2 also shows significant decrease in η when oxygen vacancies are introduced by hydrogen annealing process, which reveal detrimental effect of VO on the photovoltaic properties. EIS studies suggest increased interfacial resistance in solar cells made from nanoporous TiO2 photoanodes.

We report a facile and low-cost solution-based approach to synthesize various binary, ternary and quaternary metal chalcogenide thin films. Homogeneous metal chalcogenides single precursor is prepared by dissolving metal salt (metal chloride/acetate) in ethanolic/methanolic solution of 3-Mercaptopropionic acid (MPA) at room temperature. High-quality CdS, ZnS, Sb2S3, CuInS2 (CIS) and Cu2ZnSnS4 (CZTS) metal sulfide thin films are successfully synthesized by thermalizing these metal-MPA precursor solution at 350 °C in air for 15 min. The mechanism of metal sulfide formation is analyzed by FTIR and thermogravimetry coupled with mass spectrometry. This study highlights the formation of intermediate metal-MPA complex as a single precursor, which eventually decomposes into metal sulfide upon heating at 350 °C. The film thickness, morphology and composition of metal sulfide can be controlled by the concentration of reactants, deposition cycles and the choice of solvent. XRD, Raman, UV–vis spectroscopy, diffuse reflectance spectroscopy and energy dispersive analyses ascertain the phase purity of the films. Bandgap estimates, morphology, narrow Urbach width with low defect densities in the bandgap region, uniform thickness discerned from cross-section SEM, uniformity in the elemental maps and the photoconductivity studies further confirm high-quality of the films. Attaining molecular-level homogeneous precursor solution and inertness to the air ambient enables precise control on the stoichiometry and the enhanced uniformity of deposited films. This facile method will have potential application in large scale synthesis of different metal chalcogenide thin films by a thermolysis process and is highly suitable for controlling stoichiometry and spatial uniformity of deposited films sought in solar cell, optoelectronic and catalysis applications.

Anatase TiO2 microspheres with smooth and fibrous morphology (SμS and FμS) resembling laddu and dandelion are synthesized by solvothermal and hydrothermal methods, respectively. A detailed analysis of these two microstructures using XRD, UV–vis spectroscopy, electron microscopy and surface area measurement techniques are presented. Photoanodes fabricated using these microspheres and their composites with nanoparticles (Degussa P25) are tested for photovoltaic (PV) performance using a standard Gratzel-type dye-sensitized solar cell (DSSC) configuration. The DSSC made up of SμS and FμS microspheres exhibit an efficiency (η) of 8.4% and 6.4% respectively, in comparison to η≈7.1% for nanoparticulate P25. Further enhancement in η is realized in the composite photoanode films made of porous SμS/FμS microspheres filled with nanoparticulate P25 TiO2. High power conversion efficiency of 8.9% (for cell area of 0.5 cm2 and thickness of ~25 µm) was achieved in composite photoanode film consisting of 80 wt% SμS and 20 wt% P25. Electrochemical impedance spectroscopy studies reveal a low interfacial resistance in composite photoanodes, which is desired for efficient electron generation and transport. Composite microspheres filled with P25 nanoparticles in their voids show enhanced efficiency than the mesoporous TiO2 microspheres. Thus, SμS-nanoparticle composite TiO2 film possesses essential attributes necessary for an efficient photoanode viz. large surface area for dye adsorption, good connectivity between nanocrystallites for the efficient electron transport, and higher scattering properties for better light harvesting efficiency.

The influence of Ag plasmons and reduced graphene oxide (RGO) on ZnO nanorods (Z-NRs)-based photoanodes for photoelectrochemical splitting of water is the main focus of the present experimental study. Plasmonic layer of Ag is incorporated either as a base (Ag-Z-NRs) layer or as a top layer (Z-NRs-Ag) in an electrochemically deposited Z-NRs-based photoanodes. Z-NRs-Ag photoanodes exhibited better optical absorption as plasmonic layer stimulates charge transfer and restrain charge recombination. It had shown the photocurrent density of ~0.79 mA cm−2, at a bias of 1.4 V/RHE. A mediator layer of RGO when introduced in Z-NRs-Ag photoanodes synergistically with Ag plasmons enhances the photocurrent density to ~1.3 mA cm−2 at a bias of 1.4 V/RHE. Structure and surface morphology of the synthesized photoanodes was studied using x-ray diffraction and field emission scanning electron microscopy. Elemental analysis and optical characterization was done using energy-dispersive x-ray analysis, UV–Visible absorption spectroscopy and Raman spectroscopy. The current–voltage characteristics, electrochemical impedance spectroscopy, Mott–Schottky analysis, photoconversion efficiency and incident photon to current conversion efficiency measurements have been used to substantiate our observations of synthesized photoanodes.

Nanocrystalline thin films of a V-VI compound Bi 2 Te 3 are fabricated with uniform thickness by e-beam evaporation at room temperature. The as-deposited films are stoichiometric, monophasic, highly strained and polycrystalline. We studied the effect of vacuum annealing (at a pressure of ~ 3 × 10 − 6 mbar) on composition, structure, optical and electrical properties of these films. It is observed that, as the annealing temperature increases (from 100 °C to 300 °C), the crystallites grow with a preferential orientation along (110) planes with slight increase in the crystallite size from ~ 14 nm to 30 nm. This is associated with the breaking of quintuple layers and rearrangement of crystallographic planes in the crystallites with Te rich surface emerging on vacuum annealing as evidenced from the XRD, Raman and high-resolution TEM studies. The direct bandgap (0.116 eV) of as-deposited Bi 2 Te 3 changes from 0.092 eV to 0.113 eV on annealing at 100 °C to 300 °C, respectively. Interestingly, we observe a gradual change from a semiconductor to metallic behavior on annealing the samples from 100 °C to 300 °C. Such a transition from negative temperature coefficient (NTC) to positive temperature coefficient (PTC) is seen mainly due to the percolation of Te - rich crystallite surfaces, which evolve as the annealing temperature increases. While the films annealed at 200 °C and 250 °C shows a broad semiconductor to metallic transition at ~ 150 K and 200 K respectively, the thin films annealed at 300 °C are found to exhibit complete metallic behavior below room temperature. The electrical property and Seebeck coefficient studies with power factors in the range of ~ 4 to 12 × 10 − 4 W/K 2 m for films annealed above 200 °C suggest that the vacuum annealed Bi 2 Te 3 thin films are favorable for thermoelectric applications.

Environmental deterioration and depletion in conventional energy resources greatly demand the need for photovoltaic devices, which use solar radiation to meet future energy demands. Efficient light management plays a pivotal role in improving the performance of photovoltaic devices. Various avenues have been explored to address light management in solar cells. Employing whispering gallery mode (WGM) microresonators in solar cell device is one such strategy. Using resonating structures for light scattering is recently gaining momentum as they exhibit great potential to enhance the efficiency through light trapping. Functional material based microresonators further provide added advantage as they combine inherent optical resonance with the material properties suitable for photovoltaics like efficient charge separation and transport in one platform. “Whisperonic solar cell” is a broadly classified device in which resonating cavities are used in the cell architecture to effectively scatter the light, resulting in enhanced light absorption and thus efficiency. Recent studies reveal that WGM enabled optical microcavities can effectively get coupled to the light absorber in a sensitized solar cell (SSC) and improve the performance of SSC significantly. In this short review, we briefly present the idea of enhancing the efficiency of solar cell using whispering gallery modes. Several case studies available from the literature for realizing the concept of WGM for light trapping are highlighted. Particular focus is given to the quantum dot sensitized whisperonic solar cells. The concept is much more universal and will be useful both in thin film and sensitizer solar cells.

LiVPO4F (LVPF) is a promising high voltage (4.2 V vs. Li/Li+) and high energy density (655 Wh kg−1) cathode material for lithium ion batteries. The challenge is to prepare phase pure and conducting material suitable for battery application. In this work carbon coated LiVPO4F (LVPF/C) is synthesized by sol-gel method, and the effect of carbon coating and control on F content on the structure, morphology, electrochemical properties of LVPF/C are studied comprehensively. Li3V2(PO4)3 and Li9V3(P2O7)3(PO4)2 form as minor secondary phases when LVPF is prepared without carbon source. In contrast Li3V2(PO4)3 and V2O3 are found when lauric acid is used as carbon source. Using optimal concentration of polytetrafluoroethylene (35 wt. % PTFE) and lauric acid (27 wt. % LA) in the synthesis yields carbon coated LVPF (LVPF/C-35) with best electrochemical performance. Lattice parameters of LiVPO4F are consistent with the reported values. Electrochemical properties of the LVPF/C-X cathodes (X = 25, 35 and 45 wt. % PTFE) show the discharge capacities of ∼ 85.4 (∼ 7), ∼ 114.7 (∼ 84.1), ∼ 102.7 (∼ 14.2) mA h g−1 at 0.1C (10C) rates, when scanned in the voltage range of 3.0–4.5 V vs. Li/Li+. A flat potential profile at ∼ 4.2 V vs. Li/Li+ is seen during charging-discharging profiles of the LVPF/C-X samples. From CV studies, the diffusion coefficient is found to be of the order of 10−16 cm2s−1 during oxidation and reduction. BET surface area is more (∼ 75.95 m2 g−1) for LVPF/C-35 compared to other two samples. From Electrochemical impedance spectroscopy, the influence of charge-transfer resistance resulting from the cathode electrolyte interface layer on electrochemical properties is studied.

Phase pure carbon-coated high voltage cathode material LiVPO4F (LVPF/C) is synthesized by a facile sol-gel method and reacting the precursor powder with PTFE. Secondary phases such as Li3V2(PO4)3, Li9V3(P2O7)3(PO4)2, V2O3 and LiVP2O7 are shown to get mitigated by the addition of lauric acid, which also acts as carbon source. Phase pure LiVPO4F with good conducting properties are produced under the optimum use of oleic acid (OA; 1.31 mg, 0.84 wt. %) and lauric acid (LA; 0.25 mg, 0.16 wt. %) which are used to control particle size and carbon coating reagent, respectively. Polytetrafluoroethylene (PTFE, 35 wt.%) is reacted with the precursor during annealing to control fluorine loss. Rietveld refinement indicates the formation of phase pure LiVPO4F. The optimized cathode, LVPF/C (0.25 g LA), exhibit flat potential during the lithiation and dilithiation process with plateaus ~ 4.21 V and ~4.17 V at 0.1C-rate. Discharge capacities of 96.3 mAh g−1 (44.6 mAh g−1) are obtained at 0.1C(10C)-rate. Diffusion coefficient ~10−15 cm2s−1 is estimated from cyclic voltammetry studies. Carbon additive decreases the internal resistance and enables easy charge transfer resistance through the electrode-electrolyte interface. The cumulative effect of synthesizing phase pure compound, carbon coating, with optimum diffusion coefficient and charge transfer resistance provide the better electrochemical performance of the LVPF/C cathode.

Bandgap engineering in lead-free Cs2Ag1-xNaxBiCl6 (x = 0 to 1) double perovskite alloys synthesized through solution-based approach is investigated. The bandgap is shown to vary from 2.64 eV to 3.01 eV as Ag+ at B′ site gets replaced with Na+ cation. Despite a linear change in the lattice parameter according to Vegard's law, bandgap (Eg) changes in a nonlinear fashion for x = 0 to 1 with much lower Eg values observed than predicted by Vegard's rule. Further, we show the bandgap bowing effect in Cs2Ag1-xNaxBiCl6. Raman spectroscopic studies reveal that the changes in the vibrational mode positions arise due to the systematic variations in local distortions of [BiCl6]3– and [AgCl6]5– octahedra. The bandgap change, Raman mode frequency shift, Raman peak width, and the ratio of intensities of Raman modes all show a similar trend as a function of Na substitution concentration (x). The changes are minimal and linear for x from 0 to ∼0.6 and deviate sharply for higher Na concentration (x > 0.6). These observations strongly suggest that the sublattice distortion in the A2B′B″X6 lattice arises due to a mismatch in the octahedra. This imparts a nonlinear change in the bandgap. Thus, a strong interplay between the [Ag(Na)Cl6]5− and [BiCl6]3– octahedra is shown to have a significant influence on the deviation of bandgap from Vegard's rule and further enforces the bandgap bowing effect in Cs2Ag1-xNaxBiCl6.

Synthesis of phase pure Sb2S3 in open atmosphere has been a long-time challenging task due to the oxidation of metal. Here, we demonstrate a robust, one-step solution approach for synthesis of Sb2S3 thin films in open air atmosphere. One-step thermolysis of Sb-MPA complex obtained by mixing antimony chloride and 3-Mercaptopropionic acid (3-MPA) in polar and non-polar solvents yields phase pure thin films. Upon changing the concentration of sulphur in the solvent and polarity of solvent, different microstructures of Sb2S3 such as nanostripes, nanorods and tubular morphology evolve. Irrespective of sulphur concentration in the precursor solution, Sb2S3 thin films are always phase pure and formed in stoichiometric composition. The films are photoconductive in nature and morphology of Sb2S3 influences the photoconductivity. This novel and facile methodology will open new avenues for direct deposition of Sb2S3 thin films for wide range of applications including optoelectronics, photovoltaics and photocatalysis research.

Sb2S3 is a promising sensitizer material for solar cell devices. Growth and morphological characteristics of Sb2S3 nanoparticles made by chemical bath deposition on anatase TiO2 with various morphological attributes including mesoporous P25 nanoparticles, microsphere and its composite, anodized nanotubes, and nanorods are shown to widely differ. Sb2S3 coated on P25 photoanode for different time duration (45 min to 4 h) show that the deposition for 2 h is sufficient to obtain uniform coating with good crystallinity. Sb2S3 growth on TiO2 nanotube, nanorod and microspheres lead to conformal coverage, whereas, it hardly grows on glass/FTO. Photoconversion ability is studied using photoconductivity and photovoltaic measurements. Optimally sensitized Sb2S3 solar cells fabricated using I−/I3− liquid electrolyte exhibit photoconversion efficiency up to 0.35 % for cell area of 0.25 cm2. High photoconductivity found in Sb2S3 sensitized TiO2 microsphere-composite and nanorod compared to the P25-TiO2 photoanode is attributed to good connectivity between particles and effective electron transport.

Tellurium-rich Bi2Te3 thin films are deposited by electron−beam evaporation technique at 300 K. These as-deposited thin films are further annealed at 100 °C, 200 °C and 300 °C for 1 h at a pressure of 3 × 10−4 Pa. X-ray diffraction (XRD) patterns of as-deposited films distinctly show Te phase along with Bi2Te3. Peak intensity ratio suggests the polycrystalline nature of as-deposited Bi2Te3 films. On vacuum annealing Te-rich Bi2Te3 films exhibit improved crystallinity with a c-axis preferred orientation. In addition, structural features related to Te and Bi2Te3 composition change with Te fraction diminishing on annealing at 300 °C. From Raman spectral studies, the presence of distinct Te-rich regions, predominantly within the interlayers of Bi2Te3, are discerned. Te becomes structurally integrated within the quintuples of Bi2Te3 lattice as intergrown layers. Disordered planar structures, mostly concentrated on crystallite surfaces result from Te accumulations as evidenced in high-resolution transmission electron microscopy lattice images and energy dispersive X-ray spectroscopy mapping. These are consistent with the observations from XRD and Raman studies further confirming Te-rich Bi2Te3 characteristics. Electrical properties of Te-rich Bi2Te3 thin films exhibit n-type semiconductor behaviour. Seebeck coefficient for as-deposited film is ~ 32 μV/K, which increases to ~ 97 μV/K on 200 °C annealing. Resistivity increases from 1.39 × 10−4 Ωcm to 18.76 × 10−4 Ωcm and power factor changes from 7.4 × 10 − 4 W / K 2 m to 27.17 × 10 − 4 W / K 2 m going through a maximum at 200 °C upon systematic annealing. Te-rich Bi2Te3 thin films annealed at 200 °C exhibit high power factor ( ∼ 29 × 10 − 4 W / K 2 m ) for a wide range of temperature gradients ( ΔT from 30 °C to 165 °C).

Continuous capacity loss or gain in electrodes during extended cycling is very intriguing. We investigate influence of nanosheet thickness and defects on charge storage properties of MoS2 nanosheets in intercalation regime. Strong correlation with irreversible lithium intake during first cycle is observed. Defect-rich nanosheets (MoS2−5L; with an average 5-layer thickness) show sloping potential indicating single-phase lithium intercalation during first lithiation, with unprecedentedly large Li+-intake (x~1.9) of which only half is extractable. Defect-suppressed nanosheets (MoS2−15L; 15 layers thick) exhibit flat potential characteristics of a first-order phase transformation with Li+-intake of x~1.22 and a capacity loss x~0.3. MoS2−15L nanosheets have larger charge storage at high current rates. MoS2−5L nanosheets show an increase in charge storage fraction upon cycling at high current rates. Similar results are seen for crystallites of intermediate layer thickness. EIS studies suggest reorganization of excess lithium in electrodes during high current cycling. Irreversible lithium trapped in the MoS2−5L nanosheets during the first cycle is contemplated to shuttle around activating the electrode and enhancing the charge storage properties. Optimum performance with large charge storage and extended cycling stability at high C-rates are seen in MoS2 nanosheets with a mix of certain degree of disorder and crystallinity.

Surfactant aided sol–gel method is used to fabricate highly crystalline carbon-coated LiFePO4 (LFP) in two contrasting microstructural forms: (i) nanoparticulate crystallites (LFP-NP), and (ii). bamboo-like single crystalline nanotubes (LFP-NT) obtained using template-assisted method. LFP-NT exhibit a high specific capacity (∼165 mA h g−1 at 1C-rate) and superior rate capability with reversible capacity of ∼100 mA h g−1 (∼60 mA h g−1) at a current rate of 10C (25C) with almost 100% capacity retention after 1000 cycles, whereas, LFP-NP exhibit poor capacity retention even at low C-rates (

We present a detailed study on the microstructural, structural and optical properties of hydrogen treated (HT) diverse TiO2 nanostructures (TNS) in comparison to the as-prepared (AP) samples. The effect of oxygen vacancies (VO), introduced in the anatase TiO2 by hydrogenation, on the photovoltaic (PV) characteristics is discussed. Raman spectroscopy shows A1g second order scattering modes due to VO-influenced surface structural changes. EELS confirm the presence of Ti4+/Ti3+ mixed states and oxygen deficiency in all TNS-HT. Bandgap (Eg) of TNS can be tuned by controlling the temperature and/or duration of annealing. The indirect bandgap is found to be larger (~3.2–3.4 eV) for TNS derived from Ti-foil, whereas TNS prepared using wet-chemical methods exhibit Eg in the range of 3.1–3.3 eV. The introduction of VO red-shifts the band-edge by ~0.25 eV. Hydrogenation decreases the PV efficiency (η) by 2–4 times compared to η=6–7% observed in DSSCs of diverse nanostructures. Despite the reduction in interfacial resistance enhancing electron generation and transport in TNS-HT samples, EIS studies indicate that the drop in η (%) is mainly due to the recapture of conduction band electrons via defect states shortening the electron lifetime.

Spark plasma sintered Bi1−xCaxFe1−yTiyO3−δ (BCFTO) (x = y = 0.05 and 0.1) nanoparticle ceramics are studied for photoconductivity properties. As-prepared (AP) BCFTO hosts a large concentration of grain boundary (GB) oxygen vacancies (OV), whereas air annealed (AA) BCFTO have significantly suppressed GB OV. X-ray absorption near edge spectroscopy study confirms that Fe and Ti remain in 3+ and 4+ oxidation states, respectively. Thus, lattice OV created when only Ca2+ is substituted in BiFeO3 are charge compensated in Ca and Ti codoped BiFeO3. This ascertains that BCFTO is devoid of lattice OV. Photoconductivity studies show four orders of more photocurrent arising from GB OV contributions in BCFTO-AP compared to that in BCFTO-AA samples. A large increase in the activation energy for the AA samples (0.4 eV to 1.6 eV) compared to that for the AP samples (0.06 eV to 0.5 eV) is obtained from ln ω vs 1/T Arrhenius plots. This further substantiates the suppression of GB OV resulting in poor photoconductivity. Diffuse band edges observed in Kubelka-Munk plots of BCFTO-AP samples are a consequence of OV defect states occupying the bulk bandgap. In the absence of OV defect states, band edge becomes sharper. Density functional theory (DFT) calculations further support the experimental observations. DFT study shows that the presence of Ca and Ti does not enhance the photocurrent as these codopants do not produce mid-bandgap states. The mid-bandgap defect states are attributed only to the unsaturated bonds and OV at the GB in BCFTO. These studies manifest a critical role of OV residing at the GB in tuning the photoconductivity and, hence, the photoresponse of BCFTO.

BiFeO3 (BFO), a Pb-free perovskite oxide, is being explored for its potential use in a multitude of applications. We report on the oriented growth of BFO thin films using a facile metal-organic chemical solution deposition. Unlike the growth characteristics observed in Si/SiO2 and glass/FTO substrates, the solution growth process on sapphire (0001) is found to yield highly oriented thin films along (100)pc planes. Furthermore, annealing in air (BFO-A) and high-vacuum (BFO-V) ambients are done to explore the tunable limits of its physical properties. Temperature-dependent Raman studies highlight the high quality of thin films with sharp changes in Raman modes around transition temperatures. In addition, the films exhibit a hitherto unreported anomalous shift in A1(TO) and E(TO) modes around 450 K. The bandgap of BFO-V (Eg = 2 eV) is lower than that of BFO-A (Eg = 2.12 eV) and exhibits an increased defect photoluminescence emission. The magnetization (M) is twofold higher for BFO-V [M ≈ 42 (67) emu/cm3 at 300 K (5 K)]. In-plane and out-of-plane M vs H plots show larger anisotropy and hard hysteresis for BFO-A compared to BFO-V. Piezoelectric switching with d33 values of 5–10 pm/V is the characteristic of BFO ferroelectric materials. Photoconductivity measurements show a one order increase due to vacuum annealing. Carrier generation and recombination lifetimes are twofold faster in BFO-V as compared to BFO-A thin films. The controllable physical properties of oriented BiFeO3 thin films will be useful in magnetoelectrics and photoferroelectrics applications.

Cu-deficient graded-zinc Cu-In-Zn-S (CIZS) quantum dots (QDs) were synthesized by a two-step solvothermal method. These CIZS QDs exhibited size and composition tunable photoluminescence characteristics with emission color tunable from greenish-yellow to orange to red with a relatively high quantum yield between 45 and 60%. Novel white-light-emitting (WLE) hybrid composite is fabricated by integrating the blue-emissive 1,4-bis-2-(5-phenyl oxazolyl)-benzene (POPOP) organic fluorophore and quaternary CIZS inorganic QDs. Integrating CIZS QDs with POPOP fluorophore resulted in series of tunable emission colors with CIE coordinates lying in a straight line between the coordinates of the end member. WLE was shown for hybrid mixture comprising 0.5 nM of POPOP and 3 mg/mL of CIZS QDs with color coordinates (0.3312, 0.3324). Thin films of this hybrid mixture in PMMA matrix coated on UV-LED or on glass substrates with UV backlit light also showed broadband WLE with ideal CIE color coordinates of (0.34, 0.33), high color-rendering index value of 92, and correlated color temperature value of 5143 K. The hybrid composite exhibit Forster resonance energy transfer cascading from POPOP to CIZS which results in emission covering the entire visible spectral range. POPOP and CIZS QDs hybrid composite is a versatile material for WLED applications.

Based on experimental and density functional studies, we show that tailoring of oxygen vacancies (OV) leads to large scale enhancement of photoconductivity in BiFeO3 (BFO). The OV concentration is increased by substituting an aliovalent cation Ca2+ at Bi3+ sites in the BFO structure. Furthermore, the OV concentration at the disordered grain boundaries can be increased by reducing the particle size. Photoconductivity studies carried out on spark plasma sintered Bi1-xCaxFeO3-δ ceramics show four orders of enhancement for x = 0.1. Temperature dependent Nyquist plots depict a clear decrease in impedance with increasing Ca2+ concentration which signifies the role of OV. A significant reduction in photoconductivity by 2 to 4 orders and a large increase in impedance of the air-annealed (AA) nanocrystalline ceramics suggest that OV at the grain boundaries primarily control the photocurrent. In fact, activation energy for AA samples (0.5 to 1.4 eV) is larger than the as-prepared (AP) samples (0.1 to 0.5 eV). Therefore, the room temperature J-V characteristics under 1 sun illumination show 2–4 orders more current density for AP samples. Density-functional calculations reveal that, while the defect states due to bulk OV are nearly flat, degenerate, and discrete, the defect states due to surface OV are non-degenerate and interact with the surface dangling states to become dispersive. With large vacancy concentration, they form a defect band that enables a continuous transition of charge carriers leading to significant enhancement in the photoconductivity. These studies reveal the importance of tailoring the microstructural features as well as the composition-tailored properties to achieve large short circuit current in perovskite oxide based solar cells.Based on experimental and density functional studies, we show that tailoring of oxygen vacancies (OV) leads to large scale enhancement of photoconductivity in BiFeO3 (BFO). The OV concentration is increased by substituting an aliovalent cation Ca2+ at Bi3+ sites in the BFO structure. Furthermore, the OV concentration at the disordered grain boundaries can be increased by reducing the particle size. Photoconductivity studies carried out on spark plasma sintered Bi1-xCaxFeO3-δ ceramics show four orders of enhancement for x = 0.1. Temperature dependent Nyquist plots depict a clear decrease in impedance with increasing Ca2+ concentration which signifies the role of OV. A significant reduction in photoconductivity by 2 to 4 orders and a large increase in impedance of the air-annealed (AA) nanocrystalline ceramics suggest that OV at the grain boundaries primarily control the photocurrent. In fact, activation energy for AA samples (0.5 to 1.4 eV) is larger than the as-prepared (AP) samples (0.1 to 0.5 eV). There...

The magnetic properties of Bi1-xCaxFe1-yTiyO3-δ (BCFO: y = 0 and BCFTO: x = y) nanoparticles are studied across a wide range of temperatures (20 K to 960 K) for different Ca (and Ti) concentrations [x (= y) = 0, 0.025, 0.05, and 0.1]. X-ray diffraction and electron microscopy revealed the gradual emergence of the orthorhombic phase (Pnma) with an increase in the Ca2+ content in BCFO, contrary to the retention of parent rhombohedral symmetry (R3c) in Ca2+-Ti4+ co-doped BCFTO. XPS indicates the presence of 3+ valence states for Bi and Fe and under-coordinated defect peaks in O 1s spectra. The ordering of oxygen vacancies in BCFO affects the FeO6 octahedral alignment, resulting in a systematic shift of Fe-O Raman modes. Oxygen vacancies formed due to Ca2+ doping in BCFO and the non-magnetic Ti4+ ion at the Fe3+ site in BCFTO disrupt the spin-cycloid propagation in BiFeO3, largely influencing the magnetic properties. These substitutional changes, in addition to the large surface area, are the sources of net magnetization in these systems. Magnetic hysteresis and field dependent zero field cooled-field cooled curves indicate the combined presence of anti-ferromagnetic and ferromagnetic components in BCFO and BCFTO nanoparticles. High temperature magnetic studies present a clear bifurcation of magnetic Neel transition centered at ∼600 K associated with the structural variation in BCFO. A strong anomaly observed at 860 ± 40 K in all the samples suggests a Hopkinson-like effect arising due to sudden loss of anisotropy by the FM component.The magnetic properties of Bi1-xCaxFe1-yTiyO3-δ (BCFO: y = 0 and BCFTO: x = y) nanoparticles are studied across a wide range of temperatures (20 K to 960 K) for different Ca (and Ti) concentrations [x (= y) = 0, 0.025, 0.05, and 0.1]. X-ray diffraction and electron microscopy revealed the gradual emergence of the orthorhombic phase (Pnma) with an increase in the Ca2+ content in BCFO, contrary to the retention of parent rhombohedral symmetry (R3c) in Ca2+-Ti4+ co-doped BCFTO. XPS indicates the presence of 3+ valence states for Bi and Fe and under-coordinated defect peaks in O 1s spectra. The ordering of oxygen vacancies in BCFO affects the FeO6 octahedral alignment, resulting in a systematic shift of Fe-O Raman modes. Oxygen vacancies formed due to Ca2+ doping in BCFO and the non-magnetic Ti4+ ion at the Fe3+ site in BCFTO disrupt the spin-cycloid propagation in BiFeO3, largely influencing the magnetic properties. These substitutional changes, in addition to the large surface area, are the sources of net m...

Magnetization of antiferromagnetic nanoparticles is known to generally scale up inversely to their diameter (d) according to Neel's model. Here we report a deviation from this conventional linear 1/d dependence, altered significantly by the microstrain, in Ca and Ti substituted BiFeO3 nanoparticles. Magnetic properties of microstrain-controlled Bi1−x Ca x Fe1−y Ti y O3−δ (y = 0 and x = y) nanoparticles are analyzed as a function of their size ranging from 18 nm to 200 nm. A complex interdependence of doping concentration (x or y), annealing temperature (T), microstrain (e) and particle size (d) is established. X-ray diffraction studies reveal a linear variation of microstrain with inverse particle size, 1/d nm−1 (i.e. e d = 16.5 nm%). A rapid increase in the saturation magnetization below a critical size d c ~ 35 nm, exhibiting a (1/d) α (α ≈ 2.6) dependence, is attributed to the influence of microstrain. We propose an empirical formula M (1/d)e β (β ≈ 1.6) to highlight the contributions from both the size and microstrain towards the total magnetization in the doped systems. The magnetization observed in nanoparticles is thus, a result of the competing magnetic contribution from the terminated spin cycloid on the surface and counteracting microstrain present at a given size.

Photoconductivity studies on spark plasma sintered BiFeO3 samples with two contrasting morphologies, viz., nanoparticle-segregated grain boundary (BFO-AP) and clean grain boundary (BFO-AA), show that their photo-response is largely influenced by the grain boundary defects. Impedance analyses at 300 K and 573 K clearly demarcate the contributions from grain, grain-boundary, and the nanoparticle-segregated grain-boundary conductivities. I-V characteristics under 1 sun illumination show one order of higher conductivity for BFO-AP, whereas conductivity decreases for BFO-AA sample. Larger photocurrent in BFO-AP is attributed to the extra conduction path provided by oxygen vacancies on the nanoparticle surfaces residing at the grain boundaries. Creation of photo-induced traps under illumination and the absence of surface conduction channels in BFO-AA are surmised to result in a decreased conductivity on illumination.

We demonstrate white light emission (WLE) from (Cd,Zn)Se system, which is a composite of Zn alloyed CdSe quantum dot and ZnSe-amorphous (ZnSe-a) phase. Detailed structural and photoluminescence emission studies on pure CdSe and (Cd,Zn)Se show cubic zinc blende structure in the size range of 2.5 to 5 nm. (Cd,Zn)Se quantum dots (QDs) also have a significant fraction of ZnSe-a phase. The near-band-edge green-emission in crystalline CdSe and (Cd,Zn)Se is tunable between 500 to 600 nm. The (Cd,Zn)Se system also exhibits a broad, deep defect level (DL) red-emission in the range 600 to 750 nm and a sharp ZnSe near-band-edge blue-emission (ZS-NBE) between 445 to 465 nm. While DL and CdSe near-band-edge (CS-NBE) emissions significantly shift with the size of QD due to strong confinement effect, the ZS-NBE show minimal change in peak position indicating a weak confinement effect. The intensities of ZS-NBE and DL emissions also exhibit a strong dependence on the QD size. A gamut of emission colors is obtained by com...

We present evidence for spin-polarized charge carriers in ${\text{In}}_{2}{\text{O}}_{3}$ films. Both ${\text{In}}_{2}{\text{O}}_{3}$ and Cr doped ${\text{In}}_{2}{\text{O}}_{3}$ films exhibit room-temperature ferromagnetism after vacuum annealing, with a saturation moment reaching approximately $0.5\text{ }\text{emu}/{\text{cm}}^{3}$ for the Cr doped samples. We used point contact Andreev reflection measurements to directly determine the spin polarization, which was found to be approximately $50%\ifmmode\pm\else\textpm\fi{}5%$ for both compositions. These results are consistent with suggestions that the ferromagnetism observed in certain oxide semiconductors may be carrier mediated.