Elements Of X Ray Diffraction

High-precision gas sensors operated at room temperature are attractive for various real-time gas monitoring applications, with advantages including low energy consumption, cost effectiveness and device miniaturization/flexibility. Studies on sensing materials, which play a key role in good gas sensing performance, are currently focused extensively on semiconducting metal oxide nanostructures (SMONs) used in the conventional resistance type gas sensors. This topical review highlights the designs and mechanisms of different SMONs with various patterns (e.g. nanoparticles, nanowires, nanosheets, nanorods, nanotubes, nanofilms, etc.) for gas sensors to detect various hazardous gases at room temperature. The key topics include (1) single phase SMONs including both n-type and p-type ones; (2) noble metal nanoparticle and metal ion modified SMONs; (3) composite oxides of SMONs; (4) composites of SMONs with carbon nanomaterials. Enhancement of the sensing performance of SMONs at room temperature can also be realized using a photo-activation effect such as ultraviolet light. SMON based mechanically flexible and wearable room temperature gas sensors are also discussed. Various mechanisms have been discussed for the enhanced sensing performance, which include redox reactions, heterojunction generation, formation of metal sulfides and the spillover effect. Finally, major challenges and prospects for the SMON based room temperature gas sensors are highlighted.

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Advances in designs and mechanisms of semiconducting metal oxide nanostructures for high-precision gas sensors operated at room temperature

Micro- and nano-sensors lie at the heart of critical innovation in fields ranging from medical to environmental sciences. In recent years, there has been a significant improvement in sensor design along with the advances in micro- and nano-fabrication technology and the use of newly designed materials, leading to the development of high-performance gas sensors. Advanced micro- and nano-fabrication technology enables miniaturization of these sensors into micro-sized gas sensor arrays while maintaining the sensing performance. These capabilities facilitate the development of miniaturized integrated gas sensor arrays that enhance both sensor sensitivity and selectivity towards various analytes. In the past, several micro- and nano-gas sensors have been proposed and investigated where each type of sensor exhibits various advantages and limitations in sensing resolution, operating power, response, and recovery time. This paper presents an overview of the recent progress made in a wide range of gas-sensing technology. The sensing functionalizing materials, the advanced micro-machining fabrication methods, as well as their constraints on the sensor design, are discussed. The sensors’ working mechanisms and their structures and configurations are reviewed. Finally, the future development outlook and the potential applications made feasible by each category of the sensors are discussed.

Advanced Micro- and Nano-Gas Sensor Technology: A Review

Nucleic Acid Biosensors: Recent Advances and Perspectives

Graphene as a novel material has laid a foundation for its applications in optical fiber sensors, due to its unique properties, especially the optical properties. On the other hand, optical fiber sensors have received world-wide attention due to their high sensitivity, small size, good anti-electromagnetism disturbance ability and other potential advantages. In this paper, the developments of graphene in the applications of optical fiber sensors were reviewed from four aspects. Firstly, the common preparation methods of graphene were introduced. Next, the optical properties of graphene have been concluded. And then, some typical optical fiber chemical and biological sensors based on graphene, such as temperature sensors, biological sensors and gas sensors, were reviewed. It was shown that graphene had a great potential in the optical fiber sensing technology. Furthermore, the deficiencies and challenges of the graphene in the applications of optical fiber sensors were analyzed. In a whole, the unique advantages of graphene have present their versatility and importance in the application fields of optical fiber sensors.

Review on the graphene based optical fiber chemical and biological sensors

Optical gas sensors provide an alternative over the conventional conductometric gas sensors Carbon monoxide (CO) gas is generally regarded as one of the most dangerous air pollutants, hence, a room temperature operated CO gas sensor using ZnO sensing film deposited on Au coated prisms has been developed using an indigenously developed surface plasmon resonance (SPR) measurement setup This system is found to be highly sensitive with very fast response towards a wide concentration range (05–100 ppm) of CO gas at room temperature and hence pave way for commercial application of an efficient optical CO gas sensor

Carbon monoxide (CO) optical gas sensor based on ZnO thin films

Indigenously assembled surface plasmon resonance (SPR) technique has been exploited to study the thickness dependent dielectric properties of WO3 thin films. WO3 thin films (80 nm to 200 nm) have been deposited onto gold (Au) coated glass prism by sputtering technique. The structural, optical properties and surface morphology of the deposited WO3 thin films were studied using X-ray diffraction, UV-visible spectrophotometer, Raman spectroscopy, and Scanning electron microscopy (SEM). XRD analysis shows that all the deposited WO3 thin films are exhibiting preferred (020) orientation and Raman data indicates that the films possess single phase monoclinic structure. SEM images reveal the variation in grain size with increase in thickness. The SPR reflectance curves of the WO3/Au/prism structure were utilized to estimate the dielectric properties of WO3 thin films at optical frequency (λ = 633 nm). As the thickness of WO3 thin film increases from 80 nm to 200 nm, the dielectric constant is seen to be decreasing from 5.76 to 3.42, while the dielectric loss reduces from 0.098 to 0.01. The estimated value of refractive index of WO3 film is in agreement to that obtained from UV-visible spectroscopy studies. The strong dispersion in refractive index is observed with wavelength of incident laser light.

Optical properties of WO3 thin films using surface plasmon resonance technique

Present study focuses on determination of complex dielectric constant of biomolecules as function of frequency by means of surface plasmon resonance (SPR) technique without losing their biofunctionality. Surface plasmon modes have been excited in Kretschmann configuration at interface of ZnO-Au thin films. Various biomolecules (glucose oxidase, cholesterol oxidase, urease, and uricase) have been immobilized successfully on surface of ZnO thin film by electrostatic interaction. SPR reflectance curves for all biomolecules were recorded separately at different wavelengths (407–635 nm). Complex dielectric constant was determined by fitting the experimental SPR data with Fresnel's equations. Dielectric constant of all biomolecules shows frequency dispersion and attributed to ionic polarization.

Complex dielectric constant of various biomolecules as a function of wavelength using surface plasmon resonance

A gas sensing measurement set up has been indigenously developed using a low cost table top surface plasmon resonance (SPR) system. RF magnetron sputtered WO3 thin film has been incorporated with gold coated prism (BK-7 glass) to excite the surface plasmon modes in Kretschmann configuration (WO3/Au/prim). The structural and morphological properties of WO3 thin film influence the SPR mode significantly. WO3 thin film deposited at a sputtering pressure of 50 mT exhibits a very sharp SPR curve with low value of reflectance (0.21) at the resonance angle of 46.6°. The optimized system (WO3 (50 mT)/Au/prim) has been utilized for detection of NO2 gas of varying concentration (0.5 to 50 ppm) at room temperature. SPR curves obtained on interaction with low concentration (1 ppm) of NO2 gas under static configuration show a significant shift in resonance angle from 46.6 to 50.6° with very fast response time (0.8 s). The WO3/Au/prism SPR system has been found suitable for efficient and selective detection of low concentration of NO2 gas (minimum concentration = 0.5 ppm) with high sensitivity at room temperature, thus paving a way for the development of commercial optical sensors for NO2 gas detection.

Room temperature detection of NO2 gas using optical sensor based on surface plasmon resonance technique

An ultraviolet (UV) photodetector exhibiting enhanced response characteristics has been realized successfully after integrating various metal nanoparticles (NPs) such as silver (Ag), gold (Au) and platinum (Pt) with sol–gel derived ZnO thin film (NPs–ZnO). The metal NP based photodetector (Ag, Au, Pt-NPs–ZnO) exhibits a relatively high photoresponse in comparison to the bare ZnO based UV photodetector and gives a maximum value of about 4.27 × 103. The combined effect of the lowering of dark current due to the formation of a Schottky barrier at the interface of the metal NPs with the ZnO thin film and the photocurrent upon UV illumination due to the plasmonic effect of loaded NPs results in an enhanced photoresponse of the prepared metal NP–ZnO photodetector. The trapping of incident UV radiation mainly through the enhanced optical absorption by loaded metal NPs due to the plasmonic effect and subsequent coupling of harvesting photons into underlying optical modes of the surface of photoconducting ZnO thin films lead to a significant increase in photocurrent. The observed results provide an indication that the plasmonic assisted UV response of the novel metal NP–ZnO photodetector might provide a breakthrough for the development of next generation photodetectors.

http://iopscience.iop.org/article/10.1088/0022-3727/47/42/425102/pdf

Ayushi Paliwal

Papers

Carbon monoxide (CO) optical gas sensor based on ZnO thin films

Optical properties of WO3 thin films using surface plasmon resonance technique

Complex dielectric constant of various biomolecules as a function of wavelength using surface plasmon resonance

Room temperature detection of NO2 gas using optical sensor based on surface plasmon resonance technique

Plasmonic assisted enhanced photoresponse of metal nanoparticle loaded ZnO thin film ultraviolet photodetectors