Highly Monodisperse, Size Tunable Glucosamine Conjugated CdSe Quantum Dots for Enhanced Cellular Uptake and Bioimaging.

Semiconductor quantum dots (QDs) have been used in a variety of applications ranging from optoelectronics to biodiagnostic fields, primarily due to their size dependent fluorescent nature. CdSe nanocrystals (NCs) are generally synthesized via a hot injection method in an organic solvent. However, such NCs are insoluble in water and therefore preclude the direct usage toward biological systems. Thus, the preparation of more biocompatible water-soluble QDs with a high photoluminescent quantum yield (PLQY) is extremely important for imaging applications. Although previous literature has detailed on the synthesis of CdSe NCs in water, they suffer from poor size distribution and very low PLQY. The complex formation mechanism of CdSe NCs in an aqueous environment adversely affects the quality of NCs due to the presence of OH-, H+, and H2O moieties. Here in this article, we have presented the facile hydrothermal approach to obtain size tunable (2.9-5.1 nm), aqueous CdSe NCs with a narrow emission profile having ∼40 nm fwhm with 56% PLQY. Physicochemical properties of the synthesized water-soluble CdSe NCs were studied with the help of UV-vis, PL, XRD, FTIR, XPS, and HR-TEM analysis. Furthermore, the surface of the synthesized CdSe NCs was modified with d-glucosamine via EDC and NHS coupling to obtain a stable, biocompatible bioimaging probe. Furthermore, we demonstrated that their successful bioconjugation with glucosamine could facilitate effective internalization into the cellular matrix.

Improved Charge Transport across Bovine Serum Albumin-Au Nanoclusters' Hybrid Molecular Junction.

Proteins, a highly complex substance, have been an essential element in living organisms, and various applications are envisioned due to their biocompatible nature. Apart from proteins' biological functions, contemporary research mainly focuses on their evolving potential associated with nanoscale electronics. Here, we report one chemical doping process in model protein molecules (BSA) to modulate their electrical conductivity by incorporating metal (gold) nanoclusters on the surface or within them. The as-synthesized Au NCs incorporated inside the BSA (Au 1 to Au 6) were optically well characterized with UV-vis, time-resolved photoluminescence (TRPL), X-ray photon spectroscopy, and high-resolution transmission electron microscopy techniques. The PL quantum yield for Au 1 is 6.8%, whereas that for Au 6 is 0.03%. In addition, the electrical measurements showed ∼10-fold enhancement of conductivity in Au 6 (8.78 × 10-3 S/cm), where maximum loading of Au NCs was predicted inside the protein matrix. We observed a dynamic behavior in the electrical conduction of such protein-nanocluster films, which could have real-time applications in preparing biocompatible electronic devices.

Strongly Improving the Sensitivity of Phosphorescence-Based Optical Oxygen Sensors by Exploiting Nano-Porous Substrates.

Sensitivity is one of the crucial factors in determining the quality of a fluorescence/phosphorescence-based gas sensor, and is estimated from the measurement of responses (I0/I, where I0 and I refer to the measured optical intensity of a sensor in absence and presence of analyte molecules) at various concentrations of analytes. In this work, we demonstrate phosphorescence-based optical oxygen sensors fabricated on highly porous anodic aluminum oxide (AAO) membranes showing dramatically high response. These sensors exploit the enormous surface area of the AAO to facilitate the effective interaction between the sensing molecules and the analytes. We spin-coat an AAO membrane (200 nm pore diameter) with a platinum-based oxygen sensing porphyrin dye, platinum(II) meso-tetrakis (pentafluorophenyl) porphyrin (PtTFPP), to fabricate a sensor exhibiting I0/I ~400 at 100% oxygen atmosphere. To address the generality of the AAO membrane, we fabricate a separate sensor with another porphyrin dye, platinum octaethylporphyrin (PtOEP), which exhibits an even higher I0/I of ~500. Both of these sensors offer the highest responses as an optical oxygen sensor hitherto reported. SEM and EDS analysis are performed to realize the effect of the increased surface area of the AAO membrane on the enhanced sensitivity.

Nanoengineered Advanced Materials for Enabling Hydrogen Economy: Functionalized Graphene-Incorporated Cupric Oxide Catalyst for Efficient Solar Hydrogen Production.

Cupric oxide (CuO) is a promising candidate as a photocathode for visible-light-driven photo-electrochemical (PEC) water splitting. However, the stability of the CuO photocathode against photo-corrosion is crucial for developing CuO-based PEC cells. This study demonstrates a stable and efficient photocathode through the introduction of graphene into CuO film (CuO:G). The CuO:G composite electrodes are prepared using graphene-incorporated CuO sol-gel solution via spin-coating techniques. The graphene is modified with two different types of functional groups, such as amine (-NH2) and carboxylic acid (-COOH). The -COOH-functionalized graphene incorporation into CuO photocathode exhibits better stability and also improves the photocurrent generation compare to control CuO electrode. In addition, -COOH-functionalized graphene reduces the conversion of CuO phase into cuprous oxide (Cu2O) during photo-electrochemical reaction due to effective charge transfer and leads to a more stable photocathode. The reduction of CuO to Cu2O phase is significantly lesser in CuO:G-COOH as compared to CuO and CuO:G-NH2 photocathodes. The photocatalytic degradation of methylene blue (MB) by CuO, CuO:G-NH2 and CuO:G-COOH is also investigated. By integrating CuO:G-COOH photocathode with a sol-gel-deposited TiO2 protecting layer and Au-Pd nanostructure, stable and efficient photocathode are developed for solar hydrogen generation.

Stable and Efficient CuO Based Photocathode through Oxygen-Rich Composition and Au-Pd Nanostructure Incorporation for Solar-Hydrogen Production.

Enhancing stability against photocorrosion and improving photocurrent response are the main challenges toward the development of cupric oxide (CuO) based photocathodes for solar-driven hydrogen production. In this paper, stable and efficient CuO-photocathodes have been developed using in situ materials engineering and through gold-palladium (Au-Pd) nanoparticles deposition on the CuO surface. The CuO photocathode exhibits a photocurrent generation of ∼3 mA/cm2 at 0 V v/s RHE. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis and X-ray spectroscopy (XPS) confirm the formation of oxygen-rich (O-rich) CuO film which demonstrates a highly stable photocathode with retained photocurrent of ∼90% for 20 min. The influence of chemical composition on the photocathode performance and stability has been discussed in detail. In addition, O-rich CuO photocathodes deposited with Au-Pd nanostructures have shown enhanced photoelectrochemical performance. Linear scan voltammetry characteristic shows ∼25% enhancement in photocurrent after Au-Pd deposition and reaches ∼4 mA/cm2 at "0" V v/s RHE. Hydrogen evolution rate significantly depends on the elemental composition of CuO and metal nanostructure. The present work has demonstrated a stable photocathode with high photocurrent for visible-light-driven water splitting and hydrogen production.

Impact of molybdenum out diffusion and interface quality on the performance of sputter grown CZTS based solar cells.

We have investigated the impact of Cu2ZnSnS4-Molybdenum (Mo) interface quality on the performance of sputter-grown Cu2ZnSnS4 (CZTS) solar cell. Thin film CZTS was deposited by sputter deposition technique using stoichiometry quaternary CZTS target. Formation of molybdenum sulphide (MoSx) interfacial layer is observed in sputter grown CZTS films after sulphurization. Thickness of MoSx layer is found ~142 nm when CZTS layer (550 nm thick) is sulphurized at 600 °C. Thickness of MoSx layer significantly increased to ~240 nm in case of thicker CZTS layer (650 nm) under similar sulphurization condition. We also observe that high temperature (600 °C) annealing suppress the elemental impurities (Cu, Zn, Sn) at interfacial layer. The amount of out-diffused Mo significantly varies with the change in sulphurization temperature. The out-diffused Mo into CZTS layer and reconstructed interfacial layer remarkably decreases series resistance and increases shunt resistance of the solar cell. The overall efficiency of the solar cell is improved by nearly five times when 600 °C sulphurized CZTS layer is applied in place of 500 °C sulphurized layer. Molybdenum and sulphur diffusion reconstruct the interface layer during heat treatment and play the major role in charge carrier dynamics of a photovoltaic device.

Color tunable low cost transparent heat reflector using copper and titanium oxide for energy saving application.

Multilayer coating structure comprising a copper (Cu) layer sandwiched between titanium dioxide (TiO2) were demonstrated as a transparent heat reflecting (THR) coating on glass for energy-saving window application. The main highlight is the utilization of Cu, a low-cost material, in-lieu of silver which is widely used in current commercial heat reflecting coating on glass. Color tunable transparent heat reflecting coating was realized through the design of multilayer structure and process optimization. The impact of thermal treatment on the overall performance of sputter deposited TiO2/Cu/TiO2 multilayer thin film on glass substrate is investigated in detail. Significant enhancement of transmittance in the visible range and reflectance in the infra-red (IR) region has been observed after thermal treatment of TiO2/Cu/TiO2 multilayer thin film at 500 °C due to the improvement of crystal quality of TiO2. Highest visible transmittance of 90% and IR reflectance of 85% at a wavelength of 1200 nm are demonstrated for the TiO2/Cu/TiO2 multilayer thin film after annealing at 500 °C. Performance of TiO2/Cu/TiO2 heat reflector coating decreases after thermal treatment at 600 °C. The wear performance of the TiO2/Cu/TiO2 multilayer structure has been evaluated through scratch hardness test. The present work shows promising characteristics of Cu-based THR coating for energy-saving building industry.

Nanocrystal Engineering of Sputter-Grown CuO Photocathode for Visible-Light-Driven Electrochemical Water Splitting.

Cupric oxide (CuO) thin film was sputtered onto fluorine-doped tin oxide (FTO) coated glass substrate and incorporated into a photoelectrochemical (PEC) cell as a photocathode. Through in situ nanocrystal engineering, sputtered CuO film shows an improvement in its stability and photocurrent generation capability. For the same CuO film thickness (150 nm), films deposited at a sputtering power of 300 W exhibit a photocurrent of ∼0.92 mAcm(-2) (0 V vs RHE), which is significantly higher than those deposited at 30 W (∼0.58 mAcm(-2)). By increasing the film thickness to 500 nm, the photocurrent is further enhanced to 2.5 mAcm(-2), which represents a photocurrent conversion efficiency of 3.1%. Systematic characterization using Raman, XRD, and HR-TEM reveals that the high sputtering power results in an improvement in CuO film crystallinity, which enhances its charge transport property and, hence, its photocurrent generation capabilities.

Impact of Al passivation and cosputter on the structural property of ß-FeSi2 for Al-doped ß-FeSi2/n-Si(100) based solar cells application.

The aluminum (Al) doped polycrystalline p-type ß-phase iron disilicide (p-ß-FeSi2) is grown by thermal diffusion of Al from Al-passivated n-type Si(100) surface into FeSi2 during crystallization of amorphous FeSi2 to form a p-type ß-FeSi2/n-Si(100) heterostructure solar cell. The structural and photovoltaic properties of p-type ß-FeSi2/n-type c-Si structures is then investigated in detail by using X-ray diffraction, Raman spectroscopy, transmission electron microscopy analysis, and electrical characterization. The results are compared with Al-doped p-ß-FeSi2 prepared by using cosputtering of Al and FeSi2 layers on Al-passivated n-Si(100) substrates. A significant improvement in the maximum open-circuit voltage (Voc) from 120 to 320 mV is achieved upon the introduction of Al doping through cosputtering of Al and amorphous FeSi2 layer. The improvement in Voc is attributed to better structural quality of Al-doped FeSi2 film through Al doping and to the formation of high quality crystalline interface between Al-doped ß-FeSi2 and n-type c-Si. The effects of Al-out diffusion on the performance of heterostructure solar cells have been investigated and discussed in detail.

Characterization of epitaxial GaAs MOS capacitors using atomic layer-deposited TiO2/Al2O3 gate stack: study of Ge auto-doping and p-type Zn doping.

Electrical and physical properties of a metal-oxide-semiconductor [MOS] structure using atomic layer-deposited high-k dielectrics (TiO2/Al2O3) and epitaxial GaAs [epi-GaAs] grown on Ge(100) substrates have been investigated. The epi-GaAs, either undoped or Zn-doped, was grown using metal-organic chemical vapor deposition method at 620°C to 650°C. The diffusion of Ge atoms into epi-GaAs resulted in auto-doping, and therefore, an n-MOS behavior was observed for undoped and Zn-doped epi-GaAs with the doping concentration up to approximately 1017 cm-3. This is attributed to the diffusion of a significant amount of Ge atoms from the Ge substrate as confirmed by the simulation using SILVACO software and also from the secondary ion mass spectrometry analyses. The Zn-doped epi-GaAs with a doping concentration of approximately 1018 cm-3 converts the epi-GaAs layer into p-type since the Zn doping is relatively higher than the out-diffused Ge concentration. The capacitance-voltage characteristics show similar frequency dispersion and leakage current for n-type and p-type epi-GaAs layers with very low hysteresis voltage (approximately 10 mV).PACS: 81.15.Gh.