Showing papers by "Chandran Sudakar published in 2019"

Enhanced high rate capability of Li intercalation in planar and edge defect-rich MoS2 nanosheets.

Abstract: (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.

Enhanced Charge Transport and Excited-State Charge-Transfer Dynamics in a Colloidal Mixture of CdTe and Graphene Quantum Dots

Abstract: 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...

Band engineering via grain boundary defect states for large scale tuning of photoconductivity in Bi1−xCaxFe1−yTiyO3−δ

Abstract: 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.

Optical Whispering Gallery-Enabled Enhanced Photovoltaic Efficiency of CdS–CuInS2 Thin Film-Sensitized Whisperonic Solar Cells

Abstract: 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.

Influence of chemical solution growth and vacuum annealing on the properties of (100) pseudocubic oriented BiFeO3 thin films

Abstract: 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.

Quantum Dot Sensitized Whisperonic Solar Cells—Improving Efficiency Through Whispering Gallery Modes

Abstract: 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.