Inorganic semiconductor nanostructures are ideal systems for exploring a large number of novel phenomena at the nanoscale and investigating the size and dimensionality dependence of their properties for potential applications. The use of such nanostructures with tailored geometries as building blocks is also expected to play crucial roles in future nanodevices. Since the discovery of carbon nanotubes much attention has been paid to exploring the usage of inorganic semiconductor nanostructures as field-emitters due to their low work functions, high aspect ratios and mechanical stabilities, and high electrical and thermal conductivities. This article provides a comprehensive review of the state-of-the-art research activities in the field. It mainly focuses on the most widely studied inorganic nanostructures, such as ZnO, ZnS, Si, WO3, AlN, SiC, and their field-emission properties. We begin with a survey of inorganic semiconductor nanostructures and the field-emission principle, and then discuss the recent progresses on several kinds of important nanostructures and their field-emission characteristics in detail and overview some additional inorganic semiconducting nanomaterials in short. Finally, we conclude this review with some perspectives and outlook on the future developments in this area.

Inorganic semiconductor nanostructures and their field-emission applications

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

https://www.econstor.eu/bitstream/10419/244006/1/1693663112.pdf

Enhanced sensing performance of ZnO nanostructures-based gas sensors: A review

Semiconductor-based photodetectors (PDs) convert light signals into electrical signals via the photoelectric effect, which involves the generation, separation, and transportation of the photoinduced charge carriers, as well as the extraction of these charge carriers to external circuits. Because of their specific electronic and optoelectronic properties, metal oxide semiconductors are widely used building blocks in photoelectric devices. However, the compromise between enhancing the photoresponse and reducing the rise/ decay times limits the practical applications of PDs based on metal oxide semiconductors. As the behaviors of the charge carriers play important roles in the photoelectric conversion process of these PDs, researchers have proposed several strategies, including modification of light absorption, design of novel PD heterostructures, construction of specific geometries, and adoption of specific electrode configurations to modulate the charge-carrier behaviors and improve the photoelectric performance of related PDs. This review aims to introduce and summarize the latest researches on enhancing the photoelectric performance of PDs based on metal oxide semiconductors via chargecarrier engineering, and proposes possible opportunities and directions for the future developments of these PDs in the last section.

Enhancing the Photoelectric Performance of Photodetectors Based on Metal Oxide Semiconductors by Charge‐Carrier Engineering

In the present work, impact of Er3+ doping on the response and selectivity of SnO2 based gas sensor has been investigated in detail. X-ray diffraction (XRD) results confirmed formation of a tetragonal rutile structure of undoped and erbium doped SnO2 nanoparticles. It has been observed that specific surface area of nanoparticles has increased with increase in dopant concentration. The oxidation states and presence of erbium in SnO2 lattice has been confirmed by X-ray photoelectron spectroscopy (XPS). Photoluminescence (PL) analysis revealed that concentration of oxygen vacancies increases with increase in dopant incorporation. It has been observed that 3% Er-doped SnO2 sensor exhibited enhanced sensor response and temperature dependent selectivity towards ethanol and hydrogen at 240 and 360 ℃ respectively. The enhanced sensor response of the fabricated sensor has been ascribed to large surface area, enormous oxygen vacancies and elevated surface basicity of doped nanoparticles used. The tunable dual selectivity of 3% doped sensor towards ethanol and hydrogen makes it a perfect candidate for ethanol-hydrogen sensing for ethanol steam reforming systems combined to fuel cells.

Highly sensitive gas sensor based on Er-doped SnO2 nanostructures and its temperature dependent selectivity towards hydrogen and ethanol

We report here excellent H 2 gas response property of Co-doped ZnO nanorods (Co:ZnO NRs). Co:ZnO NRs have been synthesized by hydrothermal method by varying the Co content from 0 to 10 mol%. It is found that Co:ZnO NRs with 8 mol% of Co exhibits the highest and faster H 2 gas response ability compared to the undoped ZnO resulting in a ∼5 fold enhancement in the gas response in the presence of air at 150 °C. While the gas response value (S (%) = [(I g -I a )/I a ] × 100, where I a is the current of the sensors in the presence of air only and I g is the current in the presence of certain H 2 concentration) to 3000 ppm H 2 for undoped ZnO NRs’sensor is 11%, an unprecedented high value of 53.7% is obtained for the Co:ZnO with 8 mol% of Co. Based on the electrical current and photoluminescence results, a mechanism for the enhanced H 2 response of Co:ZnO is proposed that involves donor-related oxygen vacancies (V O ) introduced by the Co dopants in the ZnO lattice.

Highly enhanced H2 gas sensing characteristics of Co:ZnO nanorods and its mechanism

Though persistent photoconductivity and spectral response characteristics corresponding to sub-band gap energies of ZnO as a result of different defects are certainly problematic, study on this is quite useful in the sense that it provides information about the defects levels. A developed understanding of the energetic distributions of sub-band gap electron and hole traps in ZnO nanorods is pre-requisite for device applications and needs to be acquired. Through a study on the simple successive photocurrent transients of as-grown and annealed aqueous chemically grown ZnO nanorods, we have revealed the sub-band gap electron and hole traps responsible for their visible photoresponse. The evolutions in the room temperature photoluminescence properties of ZnO nanorods with annealing temperature have also been investigated critically to correlate the corresponding photocurrent results. As-grown samples show fewer excess O related electron trap states with energy corresponds to (Ec − 1.77 eV). A greater amount of O vacancy related hole traps, as well as fewer O interstitial related electron traps with energy corresponds to (Ec − 2.34 eV) were detected in the as-grown as compared to the annealed samples. These findings also provide general guidelines for probing sub-band gap traps, and engineering the optoelectronic properties of similar oxide semiconducting nanoparticles.

A successive photocurrent transient study to probe the sub-band gap electron and hole traps in ZnO nanorods

The point defects in ZnO nanorods (NRs) play very crucial role in its photoconductivity (PC) properties and thus it is essential to understand the sub-band gap carrier dynamics in order to have efficient ultraviolet (UV) photodetection. In order to understand the role of a dominant point defect, Zn vacancy (VZn) which is prevalent on the surface of the NRs, we employ a high temperature annealing step in air and also an excess hydration step for one set of annealed NRs, each followed by a final surface passivation step by poly-vinyl butyral (PVB). A comprehensive study on the photocurrent spectra, photocurrent transients under different sub-band gap excitations and power dependence of photocurrent of aqueous chemically grown ZnO NRs treated under various conditions have been carried out and demonstrates the superiority (extraordinarily high and fast UV response) of the point defect rich but surface passivated NRs as compared to ones with absence or less VZn defects. Further a good support on the major role...

Toward Understanding the Role of VZn Defect on the Photoconductivity of Surface-Passivated ZnO NRs

Surface-adsorbed species play an important role in the photophysical properties of one-dimensional (1D) ZnO providing a scope to comprehend the change in these properties with growth techniques as well as parameters. We have shown an unprecedented change in the photoluminescence properties of aqueous chemically grown (ACG) ZnO nanorods (NRs) by positioning the substrate at different precursor column heights by exploiting a simple phenomenon of different amounts of dissolved O2 along the depth of a precursor solution with its surface exposed to air. As anticipated, the NRs grown at the bottom of the reaction vessel show more OH-related defects and/or adsorbed O2 than those grown nearer to the liquid–air interface, resulting in better crystalline quality, ultraviolet (UV)/visible (Vis) photoluminescence intensity and UV photocurrent properties in the latter. Therefore, one can easily tune the photophysical properties of similar 1D oxide NRs simply by varying the substrate position in a columnar reaction vessel used in aqueous chemical growth.

Tuning the luminescence and UV photosensing properties of ZnO nanorods by strategic aqueous chemical growth

The effect of surface capping with poly-vinyl butyral (PVB) on the ultraviolet (UV) photosensing properties of ZnO nanorods (NRs) has been investigated The PVB capped ZnO NRs show enhanced UV photocurrent and photoresponse due to surface modification as compared to the as-grown NRs The photocurrent dependence on light intensity indicates that there is improvement in the space-charge–limited photocurrent in the capped NRs as compared to the as-grown NRs

Dipanwita Sett

Papers

Highly enhanced H2 gas sensing characteristics of Co:ZnO nanorods and its mechanism

A successive photocurrent transient study to probe the sub-band gap electron and hole traps in ZnO nanorods

Toward Understanding the Role of VZn Defect on the Photoconductivity of Surface-Passivated ZnO NRs

Tuning the luminescence and UV photosensing properties of ZnO nanorods by strategic aqueous chemical growth

Effects of surface capping with Poly-vinyl butyral (PVB) on the ultraviolet photosensing properties of ZnO nanorods