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Optoelectronic devices are becoming increasingly important due to their compatibility with CMOS fabrication technology and their superior performance in all dimensions compared to currently available devices. Numerous modern applications are formulated based on various aspects of optoelectronic materials and devices, such as artificial intelligence, optical memory, optoelectronic synapses, humanoid-photodetectors, holography, solar cells, charge storage devices, bio-electronic devices, and so on. Persistent photoconductivity (PPC), an optoelectronic phenomenon that has piqued the scientific community's interest, is a novel approach to these modern applications. In this article, we highlighted the use of PPC in a variety of emerging optoelectronic applications. PPC is a light-induced mechanism that persists after light excitation is terminated, i.e., the response does not stop immediately but remains available for a period of time. In recent years, the time duration over which the response after turning off the illumination is available has been proposed for a variety of applications. PPC has primarily been explored from a theoretical point of view, with the application component being largely ignored. Very recently, the scientific community has started exploring the possible applications pertaining to PPC such as optoelectronic synapses, holography, optical memory, bioelectronics, and artificial intelligence. Depending on the nature of the material and the type of model used in the application, a variety of mechanisms can be used to modulate the charge trapping and de-trapping methodologies for a specific application. This topical review summarizes the origins of PPC, its control mechanism, and recent advances in a variety of materials such as metal oxides, superconductors, nanofibers, 2D-semiconductors, alloys, nitrides, organic materials, topological insulators, and so on. In addition, the paper has carefully explored the development of next-generation optoelectronic applications designed for industry 4.0 leveraging the PPC phenomenon.

The continuous depletion of fossil fuels and the increasing demand for eco-friendly and sustainable energy sources have prompted researchers to look for alternative energy sources. The loss of thermal energy in heat engines (100-350 ºC), coal-based thermal plants (150-700 ºC), heated water pumping in the geothermal process (150-700 ºC), and burning of petrol in the automobiles (150-250 ºC) in form of untapped waste-heat can be directly and/or reversibly converted into usable electricity by means of charge carriers (electrons or holes) as moving fluids using thermoelectric (TE) technology, which works based on typical Seebeck effect. The enhancement in TE conversion efficiency has been a key challenge because of the coupled relation between thermal and electrical transport of charge carriers in a given material. In this review, we have deliberated the physical concepts governing the materials to device performance as well as key challenges for enhancing the TE performance. Moreover, the role of crystal structure in the form of chemical bonding, crystal symmetry, order-disorder and phase transition on charge carrier transport in the material has been explored. Further, this review has also emphasized some insights on various approaches employed recently to improve the TE performance, such as, (i). carrier engineering via band engineering, low dimensional effects, and energy filtering effects and (ii). Phonon engineering via doping/alloying, nano-structuring, embedding secondary phases in the matrix and microstructural engineering. Wehave also briefed the importance of magnetic elements on thermoelectric properties of the selected materials and spin Seebeck effect. Furthermore, the design and fabrication of TE modules and their major challenges are also discussed. As, thermoelectric figure of merit, zT does not have any theoretical limitation, an ideal high performance thermoelectric device should consist of low-cost, eco-friendly, efficient, n- or p-type materials that operate at wide- temperature range and similar coefficients of thermal expansion, suitable contact materials, less electrical/ thermal losses and constant source of thermal energy. Overall, this review provides the recent physical concepts adopted and fabrication procedures of TE materials and device so as to improve the fundamental understanding and to develop a promising TE device.

Diamond due to its outstanding optical, electrical, mechanical and thermal properties finds an important place in electronic, opto-electronic and quantum technologies. Recent progresses showing superconductivity in diamond by boron doping has opened up many avenues including its applications in SQUID devices especially with polycrystalline diamond films. Granular boron doped diamond films find applications in quantum inductance devices where high surface inductance is required. Particularly important are the defect centers in diamond like nitrogen-vacancy (N-V), silicon vacancy (SiV) and other color centres which are ideal candidates for next generation quantum hardware systems. For efficient device applications, an indispensable need remains for a substitutional donor in diamond lattice that yields a lower thermal activation energy at room temperature. In this review, a comprehensive summary of research and the technological challenges has been reported including some of our latest results on nitrogen doping in polycrystalline diamond to understand the transport phenomenon emphasizing on its possible future applications.

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Here, we report the fabrication of ZnO/p-GaN based high responsivity ultraviolet (UV) photodetector. Hydrothermal technique has been followed to grow ZnO nanorods; where two different types of precursors (nitrate and chloride) were used for the growth of nanorods. Interestingly, it was found that the ZnO nanorods grown using nitrate precursor are less prone to defect emission in comparison to the ZnO nanorods prepared using chloride-based precursor which resulted in low dark current levels. The photo-responsivity and photo-detectivity values of the as-fabricated device were calculated to be 350 mA/W and 3.5x1011 Jones, respectively at 360 nm excitation wavelength and ~1.79 µW/cm2 excitation intensity. The demonstration of high responsivity UV detectors (at low excitation intensity values) using ZnO nanorods/GaN can pave the way towards the development of high-performance next generation UV photodetectors.

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'Without carbon, life cannot exist', the saying goes, and not only life. For technological development, carbon was the ultimate material of the 19th century. It allowed the beginnings of the industrial revolution, enabling the rise of the steel and chemical industries, it made the railways run, and it played a major role in the development of naval transportation. Silicon, another very interesting material which makes up a quarter of the earth's crust, became the material of the 20th century in its turn. It gave us the development of high performance electronics and photovoltaics with large fields of applications and played a pivotal role in the evolution of computer technology. The increased device performance of information and data processing systems is changing our lives on a daily basis, producing scientific innovations for a new industrial era. However, success breeds its own problems, and there is ever more data to be handled—which requires a nanoscience approach. This cluster aims to address various aspects, prospects and challenges in this area of great interest for all our futures. Carbon exists in various allotropic forms that are intensively investigated for their unusual and fascinating properties, from both fundamental and applied points of view. Among them, the sp 2 (fullerenes, nanotubes and graphene) and sp 3 (diamond) bonding configurations are of special interest since they have outstanding and, in some cases, unsurpassed properties compared to other materials. These properties include very high mechanical resistance, very high hardness, high resistance to radiation damage, high thermal conductivity, biocompatibility and superconductivity. Graphene, for example, possesses very uncommon electronic structure and a high carrier mobility, with charge carriers of zero mass moving at constant velocity, just like photons. All these characteristics have put carbon and carbon-related nanomaterials in the spotlight of science and technology research. The main challenges for future understanding include i) material growth, ii) fundamental properties, and iii) developing advanced applications. The reviews in this Cluster Issue of Journal of Physics D: Applied Physics cover carbon nanoparticles and nanotubes, graphene, nano-diamond and films. They address the most current aspects and issues related to their fundamental and outstanding properties, and describe various classes of high-tech applications based on these promising materials. Future prospects, difficulties and challenges are addressed. Important issues include growth, morphology, atomic and electronic structure, transport properties, superconductivity, doping, nanochemistry using hydrogen, chemical and bio-sensors, and bio-imaging, allowing readers to evaluate this very interesting topic and draw perspectives for the future.