We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.

Theory of the linear and nonlinear optical properties of semiconductor microcrystallites

This review article highlights the application of beneficial physico-chemical properties of ZnO nanostructures for the detection of wide range of biological compounds. As the medical diagnostics require accurate, fast and inexpensive biosensors, the advantages inherent optical methods of detection are considered. The crucial points of the immobilization process, responsible for biosensor performance (biomolecule adsorption, surface properties, surface defects role, surface functionalization etc.) along with the interaction mechanism between biomolecules and ZnO are disclosed. The latest achievements in surface plasmon resonance (SPR), surface enhanced Raman spectroscopy (SERS) and photoluminescence based biosensors along with novel trends in the development of ZnO biosensor platform are presented.

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Optical biosensors based on ZnO nanostructures: advantages and perspectives. A review

This review describes the most popular architectures and transduction strategies that have been proposed for the development of electrochemical immunosensors. Relative published work has been classified into four main categories: i) enzyme-labelled immunosensors, ii) metal nanoparticle- and quantum dot-labelled immunosensors, iii) capacitive and (faradic) impedimetric immunosensors, and iv) magnetoimmunosensors. The principle of operation, analytical features, various signal amplification strategies as well as the adaptability of the afore-mentioned types of immunosensors to multiplexed formats, (micro) fluidic platforms and paper-based approaches are critically discussed. Perspectives in point-of-care analysis and commercialization opportunities are also discussed.

Electrochemical immunosensors: Critical survey of different architectures and transduction strategies

Growth, characterization and studying of sol–gel derived CdS nanoscrystalline thin films incorporated in polyethyleneglycol: Effects of post-heat treatment

Cupric oxide (CuO) thin films prepared by reactive d.c. magnetron sputtering technique for photovoltaic application

Determination of optical constants of thin films from transmittance trace

Modulation of residual stress in diamond like carbon films with incorporation of nanocrystalline gold

Vertically well-aligned n-ZnO nanowire (NW) thin films were deposited onto p-Si substrates by a two-step wet chemical technique to form a p–n heterojunction diode. The morphological and structural characteristics of the ZnO NW performed by scanning electron microscopy (SEM) and x-ray diffraction (XRD) revealed well-aligned h-ZnO NW with a wurtzite structure. A direct optical band gap of 3.30 eV was calculated from the transmittance trace obtained using a UV–VIS–NIR spectrophotometer. The electrical characteristics of the heterojunction diode were studied by capacitance–voltage (C–V) measurement at room temperature, and current–voltage–temperature (I–V–T) measurements performed in the 300–400 K range. The C–V measurements yield a carrier concentration of 1.3 × 1016 c.c.−1 for the ZnO NW thin film. The ideality factor (n) was found to decrease, while the barrier height ( b0) increased with the increase in temperature, when calculated using a thermionic emission model from the non-linear I–V–T plots. The series resistance (R s) calculated by the Cheung–Cheung method decreased with the increase in temperature. The mean barrier height (0.718 eV) and modified Richardson constant (28.4 A cm−2 K−2) calculated using a Gaussian distribution of barrier heights (considering barrier height inhomogeneity) were closer to the theoretical value than those calculated from the linear approximation of the ln(I s/T 2) versus 1000/T plot. The variation of the density of interface states with interface state energy was also studied. The n-ZnO NW/p-Si heterojunction diode performed very good half wave rectification in the frequency range 50 Hz–10 kHz, when a sinusoidal ac voltage of amplitude 4.5 V was applied across it.

https://iopscience.iop.org/article/10.1088/0022-3727/49/11/115102/pdf

Electrical characteristics and rectification performance of wet chemically synthesized vertically aligned n-ZnO nanowire/p-Si heterojunction

ZnO nanorod structures were deposited on micrometer interdigitated Au electrodes to function as three-dimensional matrixes for the immobilization of antibodies in a capacitive immunosensor format. As a proof of concept, anti-horseradish peroxidase (anti-HRP) antibodies were immobilized on the ZnO nanostructured surface by a crosslinking process. The ZnO nanorod layer allows distribution of antibodies across the entire region probed by the measuring electric field applied to the microelectrodes. This is an alternative approach to the use of more expensive nanometer electrodes necessary in the detection of smaller layers of antibodies. The new micrometer interdigitated capacitive immunosensor was able to discriminate between HRP antigen in buffer; a non-specific antigen in buffer; or buffer alone, as proven by capacitance measurements. Maximum response of the sensor was achieved in the 5–6 kHz frequency range, opening the possibility for a simplified single frequency detection system for direct antigen detection in complex biological samples.

ZnO nanorods as immobilization layers for interdigitated capacitive immunosensors

In this communication, we report on the synthesis of vertically aligned aluminium (Al)-doped ZnO (ZnO:Al) nanowire (NW) thin films on FTO-coated glass substrates and their use as photoanode in dye-sensitized solar cells (DSSC). Very thin Al layers (∼3 nm, ∼6 nm and ∼10 nm) were deposited onto chemically synthesized ZnO nanowire film by electron-beam evaporation. The films were then subjected to rapid thermal annealing to incorporate different amounts of Al (∼0.98 at.%, 1.94 at.% and ∼2.89 at.%) into the ZnO nanowires. Optical, microstructural and compositional study of the films confirmed the growth of highly transparent and well-aligned ZnO:Al nanowires with a hexagonal crystal structure. The basic DSSC structure was fabricated using both undoped ZnO nanowire and ZnO:Al nanowire thin films as photoanode. In both cases, commercially available N3 dye was used as a photosensitizer, iodide/tri-iodide solution as electrolyte and FTO-coated glass as counter electrode. A significant increase in short-circuit current was observed, from 1.3 mA cm−2 for the pristine ZnO nanowire film-based DSSC to 4.4 mA cm−2 for the ZnO:Al (2.89 at.%) nanowire film-based DSSC. The overall power conversion efficiency (PCE) was also found to increase from 0.13% (for pristine ZnO nanowire thin film) to 0.49% for the ZnO:Al thin film-based DSSC.

S. R. Bhattacharyya

Papers

Determination of optical constants of thin films from transmittance trace

Modulation of residual stress in diamond like carbon films with incorporation of nanocrystalline gold

Electrical characteristics and rectification performance of wet chemically synthesized vertically aligned n-ZnO nanowire/p-Si heterojunction

ZnO nanorods as immobilization layers for interdigitated capacitive immunosensors

Vertically Aligned Al-Doped ZnO Nanowire Arrays as Efficient Photoanode for Dye-Sensitized Solar Cells