Roman Viter

Evolution of microstructure and related optical properties of ZnO grown by atomic layer deposition

Summary A study of transmittance and photoluminescence spectra on the growth of oxygen-rich ultra-thin ZnO films prepared by atomic layer deposition is reported. The structural transition from an amorphous to a polycrystalline state is observed upon increasing the thickness. The unusual behavior of the energy gap with thickness reflected by optical properties is attributed to the improvement of the crystalline structure resulting from a decreasing concentration of point defects at the growth of grains. The spectra of UV and visible photoluminescence emissions correspond to transitions near the band-edge and defect-related transitions. Additional emissions were observed from band-tail states near the edge. A high oxygen ratio and variable optical properties could be attractive for an application of atomic layer deposition (ALD) deposited ultrathin ZnO films in optical sensors and biosensors.

Application of 2D Non-Graphene Materials and 2D Oxide Nanostructures for Biosensing Technology

The discovery of graphene and its unique properties has inspired researchers to try to invent other two-dimensional (2D) materials. After considerable research effort, a distinct “beyond graphene” domain has been established, comprising the library of non-graphene 2D materials. It is significant that some 2D non-graphene materials possess solid advantages over their predecessor, such as having a direct band gap, and therefore are highly promising for a number of applications. These applications are not limited to nano- and opto-electronics, but have a strong potential in biosensing technologies, as one example. However, since most of the 2D non-graphene materials have been newly discovered, most of the research efforts are concentrated on material synthesis and the investigation of the properties of the material. Applications of 2D non-graphene materials are still at the embryonic stage, and the integration of 2D non-graphene materials into devices is scarcely reported. However, in recent years, numerous reports have blossomed about 2D material-based biosensors, evidencing the growing potential of 2D non-graphene materials for biosensing applications. This review highlights the recent progress in research on the potential of using 2D non-graphene materials and similar oxide nanostructures for different types of biosensors (optical and electrochemical). A wide range of biological targets, such as glucose, dopamine, cortisol, DNA, IgG, bisphenol, ascorbic acid, cytochrome and estradiol, has been reported to be successfully detected by biosensors with transducers made of 2D non-graphene materials.

Tuning of ZnO 1D nanostructures by atomic layer deposition and electrospinning for optical gas sensor applications

We explored for the first time the ability of a three-dimensional polyacrylonitrile/ZnO material-prepared by a combination of electrospinning and atomic layer deposition (ALD) as a new material with a large surface area-to enhance the performance of optical sensors for volatile organic compound (VOC) detection. The photoluminescence (PL) peak intensity of these one-dimensional nanostructures has been enhanced by a factor of 2000 compared to a flat Si substrate. In addition, a phase transition of the ZnO ALD coating from amorphous to crystalline has been observed due to the properties of a polyacrylonitrile nanofiber template: surface strain, roughness, and an increased number of nucleation sites in comparison with a flat Si substrate. The greatly improved PL performance of these nanostructured surfaces could produce exciting materials for implantation in VOC optical sensor applications.

Application of Room Temperature Photoluminescence From ZnO Nanorods for Salmonella Detection

ZnO nanorods grown by gaseous-disperse synthesis are confirmed by XRD analysis to have the wurtzite crystal structure. The obtained crystallites, as found from SEM studies, are 57 ± 9 nm in diameter and 470 ± 30 nm long on the average. Two emission bands of photoluminescence from ZnO nanorods observed at room temperature are centered at 376 and 520 nm. A biosensitive layer is prepared by immobilization of anti-Salmonella antibodies from liquid solutions on the ZnO surface. Immobilization of the biosensitive layer onto ZnO nanorods is found to increase the intensity of PL. After further reaction with Salmonella antigens (Ags), the PL intensity is found to decrease proportional to Ag concentrations in the range of 102 - 105 cell/ml. The possible mechanism of biosensor response is suggested and discussed.

Application of Thin ZnO ALD Layers in Fiber-Optic Fabry-Pérot Sensing Interferometers

In this paper we investigated the response of a fiber-optic Fabry-Pérot sensing interferometer with thin ZnO layers deposited on the end faces of the optical fibers forming the cavity. Standard telecommunication single-mode optical fiber (SMF-28) segments were used with the thin ZnO layers deposited by Atomic Layer Deposition (ALD). Measurements were performed with the interferometer illuminated by two broadband sources operating at 1300 nm and 1550 nm. Reflected interference signal was acquired by an optical spectrum analyzer while the length of the air cavity was varied. Thickness of the ZnO layers used in the experiments was 50 nm, 100 nm, and 200 nm. Uncoated SMF-28 fiber was also used as a reference. Based on the results of measurements, the thickness of the ZnO layers and the length of the cavity were selected in order to achieve good visibility. Following, the interferometer was used to determine the refractive index of selected liquids.

A Novel Optochemical Sensor Based on

In this paper, a fiber optic sensing system, designed, and developed for the detection of ammonia in aqueous ambient at room temperature, is presented. The sensor is constituted by a standard silica optical fiber (SOF) coated by a tin dioxide sensitive layer. The SnO2 films have been transferred onto the distal end of the SOF by means of the simple and low-cost electrostatic-spray-pyrolysis deposition technique. The spectral characterization of the fabricated samples has been carried out in the wavelength range 400-1750 nm in order to estimate the thickness of the SnO2 fiber coatings. The morphology and the elemental composition of the deposited layers have also been investigated by means of scanning-electron-microscopy observation and energy-dispersive-spectrometer analysis, respectively. Single-wavelength reflectance measurements have been carried out to test the sensing performances of the realized sensors toward ammonia traces in water. A fiber-Bragg-grating temperature sensor has also been used for monitoring the temperature changes occurring inside the test ambient during the experimental measurements, in order to identify the effects of thermal drifts on the sensor response. The results here presented demonstrate that the developed refractometric chemical sensor is able to provide measurements of ammonia concentration in water and at room temperature with a high sensitivity, response times of few minutes, and a resolution as low as 2 ppm

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Optical and structural experiments have been carried out on Si/ZnO thin films modified with ultra-thin gold layers of different thicknesses. ZnO was produced via Atomic Layer Deposition (ALD) and Au via Physical Vapor Deposition (sputtering). The structural properties of nanostructures were studied by XRD and AFM. Optical characterization was performed by absorbance, photoluminescence (PL) and spectroscopic ellipsometry (SE). A transition from cluster-to-thin films with the increase of Au thickness has been revealed from an analysis of optical and structural parameters. The analysis of optical features of the system has shown that slight changes of the localized plasmon absorption peaks in spectra make a significant contribution to complex refractive index of gold film and, as a result, leads to a strong enhancement of UV PL peak in the ZnO layer. The mechanism of the tailoring of ZnO optical features changes by varying the Au layer thickness was discussed. Our studies have shown that through the changes of structural properties of thin gold layer between the Si substrate and the ZnO film, we can tune the optical dispersion of each layer and hence the control of ZnO PL spectra enhancement and quenching in UV-Vis wavelengths region is possible. In order to apply the hybrid structure under consideration in various optical applications, such as LED, the dispersion of the complex refractive index of the components should be optimized taking into account a particular target.

Sensitive Thin Film for ppm Ammonia Detection in Liquid Environment

High activity boron nitride/titanium dioxide (BN/TiO2) composite nanofiber photocatalysts were synthesized for the first time via the electrospinning technique. The as-spun nanofibers with a controlled ratio of boron nitride nanosheets (BN) were calcined under air at 500 °C for 4 hours. Their morphological, structural and optical properties were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), BET surface area, Fourier-transform infrared (FTIR), Raman spectroscopy, UV-Visible spectrophotometry and room temperature photoluminescence (PL). The effect of loading different BN sheet amounts on the photocatalytic degradation of methyl orange (MO) was investigated. The results indicated that the presence of BN sheets improved the separation of the photo-induced electron–hole pairs in TiO2 and increased the band gap energy and the specific surface area compared to pure TiO2 nanofibers. BN/TiO2 (10 wt%) composite nanofiber photocatalytic activity is enhanced to 99% compared to 60% and 65% for P25 and TiO2 nanofibers, respectively. Thus, the BN/TiO2 composites significantly increase the UV light photo-response and improve the separation of photo-induced electron–hole pairs of TiO2.

The influence of localized plasmons on the optical properties of Au/ZnO nanostructures

In this letter, experimental results on the capability of a tin dioxide (SnO2)-based silica optical fiber (SOF) sensor to detect sub-ppm ammonia concentrations in water environments, at room temperature, are presented. SnO2 sensitive layers have been deposited on the fiber end by using the simple and low cost electrostatic spray pyrolysis deposition technique. The surface morphology of the deposited SnO2 layers as well as its influence on the near field profile of the emergent electromagnetic field from the fiber coating have been investigated by means of atomic force microscopy and scanning near field optical microscopy. The room temperature adsorption measurements reveal the excellent sensor resolution of 80ppb, good recovery features, high repeatability, and fast response times (a few minutes). The results obtained demonstrate the strong potentiality of the proposed SnO2-based SOF sensor to be employed for water quality monitoring applications.

Enhanced photocatalytic performance of novel electrospun BN/TiO2 composite nanofibers

SnO2 nanofibers with various grain sizes ranging from 18.5 to 31.6 nm in diameter were fabricated by electrospinning a polymeric solution and subsequent controlled calcination of the as-spun fibers. The calcined fibers were polycrystalline and composed of densely packed nano-sized SnO2 grains. The effect of the nanograin size on the optical bandgap of SnO2 nanofibers was examined by ultraviolet-visible spectroscopy. The bandgap showed a strong dependence on the nanograin size. The bandgap decreased with increasing nanograin size. Some calculations were performed to understand the relationship between the experimentally obtained bandgaps of the SnO2 nanofibers and the theoretical ones. Quantum confinement and lattice strain of the SnO2 nanofibers are likely responsible for the bandgap shift. This suggests that optimization of the nanograin size is essential not only for achieving the required optical properties of oxide nanofibers, but also to secure superior working properties of electronic devices that are fabricated with electrospinning-synthesized oxide nanofibers.