Improvements in the Performance of a Visible-NIR Photodetector Using Horizontally Aligned TiS3 Nanoribbons.

We report the fabrication and characterization of visible and near-infrared-resistive photodetector using horizontally aligned titanium tri sulfide (TiS3) nanoribbons. The fabrication process employed micro-electromechanical system, photolithography and dielectrophoretic (DEP) methods. The interdigitated electrodes (IDE) fingers were fabricated using photolithography and thin-film metallization techniques onto the Si/SiO2 substrate, and then TiS3 nanoribbons were horizontally aligned in between IDE using DEP. The fabricated device was first characterized in absence of light and then, the photodetector-based characteristics were obtained by illuminating it with fiber-coupled laser beam. These characteristics were optimized by varying wavelength and power density of the laser beam. The present photodetector shows a maximum responsivity of 5.22 × 102 A/W, quantum efficiency of 6.08 × 102, and detectivity of 1.69 × 109 Jones. The switching times, i.e., response and recovery times were found to be 1.53 and 0.74 s, respectively, with 1064 nm wavelength and 3.4 mW/mm2 power density of the laser beam. Also, the effect of O2 adsorption on nanoribbons has been studied and it is found that adsorbed O2 acts as electron acceptor and decreases the conductivity of the photodetector. Experimentally, it is found that the photoresponse of the horizontally aligned TiS3 nanoribbons is better than that of a randomly oriented TiS3 nanoribbon-based photodetector. Finally, the performance of the present photodetector was compared to that of the previous ones that were found to outperform the reported ones. The additional advantages of the photodetector include excellent stability and portability from which it may be concluded that TiS3 nanoribbons can be a promising candidate for application in nanoscale electronic and optoelectronic devices.

Influence of growth temperature on titanium sulphide nanostructures: from trisulphide nanosheets and nanoribbons to disulphide nanodiscs.

We report the fabrication and characterization of titanium sulphide nanostructures using a chemical vapour transport (CVT) method. In CVT, reactions occur between titanium and sulphur powder in the vapor phase for TiS x nanostructure growth. Systematic studies on the effect of temperature, consequent structural evolution and optical properties were investigated by various characterization techniques. A series of experiments were performed by maintaining a fixed compositional ratio (1 : 3) of Ti and S within a temperature range from 400 °C to 650 °C. On increasing the temperature from 400 °C to 650 °C; a gradual change in morphology was obtained from nanosheets (NS) to mixed phase nanoribbons and nanosheets (NS: NR), nanoribbons (NR), and nanodiscs (ND) of titanium sulphide, which was confirmed using SEM/TEM analysis. Then, the composition of titanium sulphides was studied using XRD, EDX and Raman spectroscopic techniques and it is observed that NS, NR and NS: NR have the composition ratio of TiS3 whereas ND has a ratio of TiS2. The phenomenon of decomposition of TiS3 into TiS2 at elevated temperatures was explained using thermogravimetric analysis (TGA) and differential thermal analysis (DTA) along with pictorial representations. The optical properties of the prepared TiS3/TiS2 nanostructures were studied using UV-vis and photoluminescence spectroscopy. It is concluded that composition ratio of Ti and S as well as the temperature variation plays a crucial role in the formation of different Ti-S nanostructures with unique optical, electronic and thermal properties.

Xanthine oxidase functionalized Ta2O5 nanostructures as a novel scaffold for highly sensitive SPR based fiber optic xanthine sensor.

Fabrication and characterization of a surface plasmon resonance based fiber optic xanthine sensor using entrapment of xanthine oxidase (XO) enzyme in several nanostructures of tantalum (v) oxide (Ta2O5) have been reported. Chemical route was adopted for synthesizing Ta2O5 nanoparticles, nanorods, nanotubes and nanowires while Ta2O5 nanofibers were prepared by electrospinning technique. The synthesized Ta2O5 nanostructures were characterized by photoluminescence, scanning electron microscopy, UV-Visible spectra and X-ray diffraction pattern. The probes were fabricated by coating an unclad core of the fiber with silver layer followed by the deposition of XO entrapped Ta2O5 nanostructures. The crux of sensing mechanism relies on the modification of dielectric function of sensing layer upon exposure to xanthine solution of diverse concentrations, reflected in terms of shift in resonance wavelength. The sensing probe coated with XO entrapped Ta2O5 nanofibers has been turned out to possess maximum sensitivity amongst the synthesized nanostructures. The probe was optimized in terms of pH of the sample and the concentration of XO entrapped in Ta2O5 nanofibers. The optimized sensing probe possesses a remarkably good sensitivity of 26.2nm/µM in addition to linear range from 0 to 3µM with an invincible LOD value of 0.0127µM together with a response time of 1min. Furthermore, probe selectivity with real sample analysis ensure the usage of the sensor for practical scenario. The results reported open a novel perspective towards a sensitive, rapid, reliable and selective detection of xanthine.

Integrating nanohybrid membranes of reduced graphene oxide: chitosan: silica sol gel with fiber optic SPR for caffeine detection.

Caffeine is the most popular psychoactive drug consumed in the world for improving alertness and enhancing wakefulness. However, caffeine consumption beyond limits can result in lot of physiological complications in human beings. In this work, we report a novel detection scheme for caffeine integrating nanohybrid membranes of reduced graphene oxide (rGO) in chitosan modified silica sol gel (rGO: chitosan: silica sol gel) with fiber optic surface plasmon resonance. The chemically synthesized nanohybrid membrane forming the sensing route has been dip coated over silver coated unclad central portion of an optical fiber. The sensor works on the mechanism of modification of dielectric function of sensing layer on exposure to analyte solution which is manifested in terms of red shift in resonance wavelength. The concentration of rGO in polymer network of chitosan and silica sol gel and dipping time of the silver coated probe in the solution of nanohybrid membrane have been optimized to extricate the supreme performance of the sensor. The optimized sensing probe possesses a reasonably good sensitivity and follows an exponentially declining trend within the entire investigating range of caffeine concentration. The sensor boasts of an unparalleled limit of detection value of 1.994 nM and works well in concentration range of 0-500 nM with a response time of 16 s. The impeccable sensor methodology adopted in this work combining fiber optic SPR with nanotechnology furnishes a novel perspective for caffeine determination in commercial foodstuffs and biological fluids.

Simultaneous tuning of electric field intensity and structural properties of ZnO: Graphene nanostructures for FOSPR based nicotine sensor.

We report theoretical and experimental realization of a SPR based fiber optic nicotine sensor having coatings of silver and graphene doped ZnO nanostructure onto the unclad core of the optical fiber. The volume fraction (f) of graphene in ZnO was optimized using simulation of electric field intensity. Four types of graphene doped ZnO nanostructures viz. nanocomposites, nanoflowers, nanotubes and nanofibers were prepared using optimized value of f. The morphology, photoluminescence (PL) spectra and UV-vis spectra of these nanostructures were studied. The peak PL intensity was found to be highest for ZnO: graphene nanofibers. The optimized value of f in ZnO: graphene nanofiber was reconfirmed using UV-vis spectroscopy. The experiments were performed on the fiber optic probe fabricated with Ag/ZnO: graphene layer and optimized parameters for in-situ detection of nicotine. The interaction of nicotine with ZnO: graphene nanostructures alters the dielectric function of ZnO: graphene nanostructure which is manifested in terms of shift in resonance wavelength. From the sensing signal, the performance parameters were measured including sensitivity, limit of detection (LOD), limit of quantification (LOQ), stability, repeatability and selectivity. The real sample prepared using cigarette tobacco leaves and analyzed using the fabricated sensor makes it suitable for practical applications. The achieved values of LOD and LOQ are found to be unrivalled in comparison to the reported ones. The sensor possesses additional advantages such as, immunity to electromagnetic interference, low cost, capability of online monitoring, remote sensing.

Simultaneous estimation of vitamin K1 and heparin with low limit of detection using cascaded channels fiber optic surface plasmon resonance.

We report an approach for the simultaneous estimation of vitamin K1 (VK1) and heparin via cascaded channel multianalyte sensing probe employing fiber optic surface plasmon resonance technique. Cladding from two well separated portions of the fiber is removed and are respectively coated with thin films of silver (channel-1) and copper (channel-2). The nanohybrid of multiwalled carbon nanotube in chitosan is fabricated over silver layer for the sensing of VK1 whereas core shell nanostructure of polybrene@ZnO is coated over copper layer for the sensing of heparin. Spectral interrogation method is used for the characterization of the sensor. Analyte selectivity of both the channels is performed by carrying out experiments using independent solutions of VK1 and heparin. Experiments performed on the solution of the mixture of VK1 and heparin show red shifts in both the channels on changing the concentration of both the analytes in the mixture. The operating range of both VK1 and heparin is from 0 to 10(-3)g/l. The limit of detection of the sensor is 2.66×10(-4)µg/l and 2.88×10(-4)µg/l for VK1 and heparin respectively which are lower than the reported ones. The additional advantages of the present sensor are low cost, possibility of online monitoring and remote sensing.

Tuning the field distribution and fabrication of an Al@ZnO core-shell nanostructure for a SPR-based fiber optic phenyl hydrazine sensor.

We report the fabrication and characterization of a surface plasmon resonance (SPR)-based fiber optic sensor that uses coatings of silver and aluminum (Al)-zinc oxide (ZnO) core-shell nanostructure (Al@ZnO) for the detection of phenyl hydrazine (Ph-Hyd). To optimize the volume fraction (f) of Al in ZnO and the thickness of the core-shell nanostructure layer (d), the electric field intensity along the normal to the multilayer system is simulated using the two-dimensional multilayer matrix method. The Al@ZnO core-shell nanostructure is prepared using the laser ablation technique. Various probes are fabricated with different values of f and an optimized thickness of core-shell nanostructure for the characterization of the Ph-Hyd sensor. The performance of the Ph-Hyd sensor is evaluated in terms of sensitivity. It is found that the Ag/Al@ZnO nanostructure core-shell-coated SPR probe with f = 0.25 and d = 0.040 µm possesses the maximum sensitivity towards Ph-Hyd. These results are in agreement with the simulated ones obtained using electric field intensity. In addition, the performance of the proposed probe is compared with that of probes coated with (i) Al@ZnO nanocomposite, (ii) Al nanoparticles and (iii) ZnO nanoparticles. It is found that the probe coated with an Al@ZnO core-shell nanostructure shows the largest resonance wavelength shift. The detailed mechanism of the sensing (involving chemical reactions) is presented. The sensor also manifests optimum performance at pH 7.

Surface plasmon resonance-based fiber-optic hydrogen gas sensor utilizing palladium supported zinc oxide multilayers and their nanocomposite.

We analyze surface plasmon resonance-based fiber-optic sensor for sensing of small concentrations of hydrogen gas in the visible region of the electromagnetic spectrum. One of the two probes considered has multilayers of zinc oxide (ZnO) and palladium (Pd) while the other has layer of their composite over a silver coated unclad core of the fiber. The analysis is carried out for different volume fractions of palladium nanoparticles dispersed in zinc oxide host material in the nanocomposite layer. For the analysis, a Maxwell-Garnett model is adopted for calculating the dielectric function of a ZnO:Pd nanocomposite having nanoparticles of dimensions smaller than the wavelength of radiation used. The effects of the volume fraction of the nanoparticles in the nanocomposite and the thickness of the nanocomposite layer on the figure of merit of the sensor have been studied. The film thickness of the layer and the volume fraction of nanoparticles in the ZnO:Pd nanocomposite layer have been optimized to achieve the maximum value of the figure of merit of the sensor. It has been found that the figure of merit of the sensing probe coated with ZnO:Pd nanocomposite is more than twofold of the sensing probe coated with multilayers of Pd and ZnO over a silver coated unclad core of the fiber; hence, the sensor with a nanocomposite layer works better than that with multilayers of zinc oxide and palladium. The sensor can be used for online monitoring and remote sensing of hydrogen gas.

Surface plasmon resonance based fiber optic detection of chlorine utilizing polyvinylpyrolidone supported zinc oxide thin films.

A highly sensitive chlorine sensor for an aqueous medium is fabricated using an optical fiber surface plasmon resonance (OFSPR) system. An OFSPR-based chlorine sensor is designed with a multilayer-type platform by zinc oxide (ZnO) and polyvinylpyrollidone (PVP) film morphology manipulations. Among all the methodologies of transduction reported in the field of solid state chemical and biochemical sensing, our attention is focused on the Kretschmann configuration optical fiber sensing technique using the mechanism of surface plasmon resonance. The optical fiber surface plasmon resonance (SPR) chlorine sensor is developed using a multimode optical fiber with the PVP-supported ZnO film deposited over a silver-coated unclad core of the fiber. A spectral interrogation mode of operation is used to characterize the sensor. In an Ag/ZnO/PVP multilayer system, the absorption of chlorine in the vicinity of the sensing region is performed by the PVP layer and the zinc oxide layer enhances the shift in resonance wavelength. It is, experimentally, demonstrated that the SPR wavelength shifts nonlinearly towards the red side of the visible region with an increase in the chlorine concentration in an aqueous medium while the sensitivity of the sensor decreases linearly with an increase in the chlorine concentration. As the proposed sensor utilizes an optical fiber, it possesses the additional advantages of fiber such as less signal degradation, less susceptibility to electromagnetic interference, possibility of remote sensing, probe miniaturization, probe re-usability, online monitoring, small size, light weight and low cost.

Surface plasmon resonance-based fiber optic hydrogen sulphide gas sensor utilizing Cu-ZnO thin films.

We report an experimental study on a surface plasmon resonance (SPR)-based fiber optic hydrogen sulphide gas sensor with a thin metal oxide (zinc oxide (ZnO)) layer as the additional layer. This zinc oxide layer is grown over the copper layer to support surface plasmons at the metal-dielectric interface at room temperature. The wavelength interrogation mode of operation has been used to characterize the sensor. The thin film of zinc oxide over the copper film was deposited on the unclad portion of the fiber by the thermal evaporation technique. Experiments were performed for the detection of concentrations of hydrogen sulphide gas varying from 0 to 100 ppm around the probe. The unpolarized light from a polychromatic source is launched from one end of the fiber and the corresponding SPR spectrum is recorded at the other end. The recorded SPR spectrum shows a shift in the resonance wavelength on a change in the hydrogen sulphide gas concentration, which is considered as a detectable signal for the characterization of the sensor. Further, the optimization of the performance of the sensor was achieved by varying the thickness of the zinc oxide film. The sensor possesses a very fast response time and high sensitivity. Since the sensor utilizes optical fibers it has additional advantages of remote sensing, online monitoring, light weight and low cost.