2D semiconductor nanomaterials and heterostructures : controlled synthesis and functional applications

Two-dimensional (2D) semiconductors beyond graphene represent the thinnest stable known nanomaterials. Rapid growth of their family and applications during the last decade of the twenty-first century have brought unprecedented opportunities to the advanced nano- and opto-electronic technologies. In this article, we review the latest progress in findings on the developed 2D nanomaterials. Advanced synthesis techniques of these 2D nanomaterials and heterostructures were summarized and their novel applications were discussed. The fabrication techniques include the state-of-the-art developments of the vapor-phase-based deposition methods and novel van der Waals (vdW) exfoliation approaches for fabrication both amorphous and crystalline 2D nanomaterials with a particular focus on the chemical vapor deposition (CVD), atomic layer deposition (ALD) of 2D semiconductors and their heterostructures as well as on vdW exfoliation of 2D surface oxide films of liquid metals

Photoassisted Nanoscale Memory Resistors

Memristors or memory resistors are promising two-terminal devices, which have the potential to revolutionize current electronic memory technologies. Memristors have been extensively investigated and reported to be practical devices, although they still suffer from poor stability, low retention time, and laborious fabrication processes. The primary aim of this project was to achieve a device structure of quantum dots or thin films to address a fundamental challenge of unstable resistive switching behavior in memristors. Moreover, we aimed to investigate the effects of light illumination in terms of intensity and wavelength on the performance of the fabricated memristor. The parameters such as power consumption, retention time, endurance, and stability were investigated to determine the overall performance of the device. The experiment was designed and divided into three steps. First, a memristor was designed, fabricated, and characterized to explore the resistive switching mechanism in the device. Second, the same material used in the first step was incorporated into a photodetector, which was characterized to investigate the device photosensitivity, detectivity, responsivity, and photocurrent to dark current ratio. Finally, a new device was designed, fabricated, and characterized, which showed both memristivity and photodetectivity properties. The device is called a photomemristor since it has both functions of a memristor and a photodetector. The “bottom-up” approach was used for fabricating the proposed memristor. In bottom-up methodology, nanostructures are synthesized and then assembled onto the substrate by stacking crystal planes onto each other. The fabricated memristors demonstrated bipolar resistive switching behavior with a low working voltage, efficient power consumption, and high endurance. We suggested the resistive switching mechanism of the device is related to the formation and rupture of conducting filaments inside the switching layer of the memristor. Moreover, the conduction mechanism and electron transport in the switching layer of the device during the resistive switching process were analyzed. In addition, the effect of light illumination on the performance of the device was investigated and the SET voltage of the memristor was reduced as the light intensity increased. A gold-coated probe tip was used as the top electrode to confine the conductive filaments growth. The obtained results demonstrate significant improvement in the resistive switching behavior in terms of stability and uniformity compared to similar devices with larger electrode surface area. This work provides new insights and suggests a measurement setup to further understand the resistive switching behavior in metal oxide and perovskite thin films for future applications of optoelectronic memristors in logic circuits, digital data storage, the internet of things, and neuromorphic computing

Resistive switching in FTO/CuO-Cu2O/Au memory devices

Memristors are considered to be next-generation non-volatile memory devices owing to their fast switching and low power consumption. Metal oxide memristors have been extensively investigated and reported to be promising devices, although they still suffer from poor stability and laborious fabrication process. Herein, we report a stable and power-efficient memristor with novel heterogenous electrodes structure and facile fabrication based on CuO-Cu2O complex thin films. The proposed structure of the memristor contains an active complex layer of cupric oxide (CuO) and cuprous oxide (Cu2O) sandwiched between fluorine-doped tin oxide (FTO) and gold (Au) electrodes. The fabricated memristors demonstrate bipolar resistive switching (RS) behavior with a low working voltage (~1 V), efficient power consumption, and high endurance over 100 switching cycles. We suggest the RS mechanism of the proposed device is related to the formation and rupture of conducting filaments inside the memristor. Moreover, we analyze the conduction mechanism and electron transport in the active layer of the device during the RS process. Such a facile fabricated device has a promising potential for future memristive applications

An account of Natural material based Non Volatile Memory Device

The development in electronic sector has brought a remarkable change in the life style of mankind. At the same time this technological advancement results adverse effect on environment due to the use of toxic and non degradable materials in various electronic devices. With the emergence of environmental problems, the green, reprogrammable, biodegradable, sustainable and environmental-friendly electronic devices have become one of the best solutions for protecting our environment from hazardous materials without compromising the growth of the electronic industry. Natural material has emerged as the promising candidate for the next generation electronic devices due to its easy processing, transparency, flexibility, abundant resources, sustainability, recyclability, and simple extraction. This review targets the characteristics, advancements, role, limitations, and prospects of using natural materials as the functional layer of a resistive switching memory device with a primary focus on the switching/memory properties. Among the available memory devices, resistive random access memory (RRAM), write once read many (WORM) unipolar memory etc. devices have a huge potential to become the non-volatile memory of the next generation owing to their simple structure, high scalability, and low power consumption. The motivation behind this work is to promote the use of natural materials in electronic devices and attract researchers towards a green solution of hazardous problems associated with the electronic devices.Comment: 32 pages, 8 figures, 2 table

Energetically deposited tin oxide: characterization and device applications

Semiconductor oxides are promising materials that have made impressive progress in recent years, challenging the dominance of silicon not only in conventional devices including field-effect transistors but being amenable to next-generation electronic devices such as memristors. Although a variety of oxides have been explored, tin oxide has been an interesting material for researchers when offering p-type characteristics of tin monoxide SnO and n-type characteristics in tin dioxide SnO2. While SnO2 is easy to grow and well suited for a wide range of applications, it is difficult to form p-type SnO due to its metastability where it forms into the more stable phase SnO2. The work presented in this Doctoral Dissertation focus on exploring the characteristics and applications of energetically deposited tin oxide thin films. The tin oxide film deposited using high-power impulse magnetron sputtering was found to be mixed-phase nanocrystalline SnO and SnO2 in which SnO2 is dominant. The high resistivity, low carrier concentration and low mobility in the as-deposited and annealed samples hindered the application of the high-power impulse magnetron sputtering (HiPIMS) SnOx in thin film transistors, however, suggested suitability for these films as a memristive material. A small but quantifiable variation in film stoichiometry (Sn:O) resulting from the off-axis deposition led to the formation of two different types of memristive devices, namely filamentary and nanoparticle network memristors. Both devices exhibited stable volatile bidirectional resistive switching with a ratio between high resistance and low resistance of more than two orders of magnitude. However, their underlying resistive switching mechanisms and device characteristics were significantly different. Synaptic-like behaviours were observed on both filamentary devices (FDs) and nanoparticle network devices (NNDs), highlighting their potential for information processing in neuromorphic computing systems. While a FD can become only an individual cell in reservoir computing circuits, an NND can be implemented as a reservoir due to their available inter-connectivity which is required for reservoir computing

Tuning and engineering of ZnO and CuxO for sensor, solar cells and memory devices

In this thesis, the PhD candidate pursued the development of model sensors, solar cells and memory devices based on transition metal oxides, which are tuned and engineered in order to obtain enhanced properties. The author made informed choices regarding the incorporation of zinc oxide (ZnO) and copper oxides (CuxO) (cuprous oxide (Cu2O) and cupric oxide (CuO)) as the model transition metal oxides to achieve the goals of this PhD research. ZnO and CuxO are well investigated metal oxides, and a broad range of information regarding their fundamental properties, synthesis methods and applications is available. Their complimentary electronic nature is also required for the proposed studies in this dissertation: ZnO and CuxO are intrinsically n- and p-type semiconductors, respectively. This PhD research focuses on the engineering and tuning the morphology, crystallinity and stoichiometry of transition metal oxides in order to investigate and devise scenarios that result in the highest efficiencies for the abovementioned model devices. The author of this thesis thoroughly reviewed the physical and chemical properties, as well as methods of synthesis of CuxO and ZnO. He studied factors that have been previously employed for enhancing the targeted materials functionalities. This included tuning the synthesis’ parameters such as changes in temperature and pressure, the incorporation of seed layers or templates and nanostructuring. In summary, this PhD thesis provides the readers with an in-depth knowledge of the capabilities that tuning and engineering transition metal oxides provide in enhancing the performance of such materials for specific applications

Über die Entwicklung von Memsensoren

Since the postulation of the experimental realization of memristive devices in 2008, a broad variety of concepts for the fabrication of memristive devices has been pursued and the underlying switching mechanisms have been studied in detail. The unique electronic properties of memristive devices inspire applications that go beyond conventional electronics, such as using memristive devices as programmable interconnects, to realize logics for in-array-computing or in neuromorphic engineering. A particularly interesting aspect of biological neural networks is the close connection between signal detection and processing at the neuron level, which is an essential contribution to their outstanding efficiency. This work evolves around the concept of memsensors, which unify the characteristic features of memristive devices and sensor devices and as such appear as promising candidates to realize a close connection between signal detection and processing on the device level. Memsensors are a highly interdisciplinary topic, bridging research in the fields of material science and electrical engineering and relating to insights from biology and medicine through neuromorphic engineering. The major objective of this thesis is to provide tools and building blocks and showcase pathways to incorporate memristive and sensitive properties into memsensor devices. For this purpose, motivated by an experimental point of view, a nanoparticle-based memristive device with diffusive memristive switching characteristics was developed and characterised in detail and sensors relying on semiconducting metal oxide thin films and nanostructures were thoroughly studied. In addition, in terms of modelling of memsensor circuits, emerging features such as amplitude adaptation are discussed, showcasing the particular eligibility of memsensors in the context of neuromorphic engineering

Low-power emerging memristive designs towards secure hardware systems for applications in internet of things

Emerging memristive devices offer enormous advantages for applications such as non-volatile memories and in-memory computing (IMC), but there is a rising interest in using memristive technologies for security applications in the era of internet of things (IoT). In this review article, for achieving secure hardware systems in IoT, low-power design techniques based on emerging memristive technology for hardware security primitives/systems are presented. By reviewing the state-of-the-art in three highlighted memristive application areas, i.e. memristive non-volatile memory, memristive reconfigurable logic computing and memristive artificial intelligent computing, their application-level impacts on the novel implementations of secret key generation, crypto functions and machine learning attacks are explored, respectively. For the low-power security applications in IoT, it is essential to understand how to best realize cryptographic circuitry using memristive circuitries, and to assess the implications of memristive crypto implementations on security and to develop novel computing paradigms that will enhance their security. This review article aims to help researchers to explore security solutions, to analyze new possible threats and to develop corresponding protections for the secure hardware systems based on low-cost memristive circuit designs

2D MoO3 synthesis and its application in electronic devices

Two-dimensional (2D) materials have significant technological importance due to their exceptional electronic and mechanical properties, which stem from the quantum confinement of charge carriers along a single plane. Their thin atomic nature and large surface-to-volume ratio offer an opportunity to tailor their properties, making them suitable candidates for next-generation electronic devices. Molybdenum trioxide (MoO3) is a wide bandgap and high dielectric material that can be obtained in 2D structure. The bandgap of the material can be readily tuned using ion intercalation method. Consequently, carrier mobility can be enhanced by increasing the charge carriers density near the Fermi level. As such, reliable production of few atoms thick 2D material is essential for translating their properties into electronic applications. However, obtaining the desired thickness of uniform 2D MoO3 crystal is challenging, as the existing exfoliation technique do not produce crystals of uniform thickness efficiently. A new chemical route has been developed to thin down bulk crystals of MoO3 in order to obtain them in 2D form. The viability and reliability of the etching process has been established via detail characterisation of the material pre- and post-etching. The electrical characterisation of the 2D MoO3 crystals based field effect transistors show high switching ratios. Non-volatile resistive memory devices are theorised to be the most promising pathway towards analogue memory and neuromorphic computing. Metal oxides are widely used as channel material in such memory devices. High dielectric constant and thermal stability of MoO3 renders it ideal for resistive memory applications as high dielectric nature suppresses the undesirable parasitic effects during resistive switching performance. The reversible and non-volatile resistive switching behaviour of planar MoO3 crystals has been investigated. The room temperature memory retention shows high on/off ratio of >103 for 104 s duration and endurance of > 6,000 cycles, and low power consumption. This study demonstrates the viability of MoO3 as a resistive memory element and paves the way for future 2D resistive memory technologies. Furthermore, conductometric gas sensors have been developed based on the 2D crystals of non-stoichiometric MoO3. Thermodynamically stable MoO3 shows excellent electron affinity towards various gaseous elements. In addition, 2D structure endows them with an ultrahigh surface area that contains an extremely large proportion of surface atoms. These surface atoms serve as active sites to effectively react with gas molecules for gas sensing applications. Detail characterisations of the sensors show excellent selectivity and high sensitivity towards toxic and health hazard gases such as, H2S and NO2. The cyclic repeatability shows a negligible variation in sensitivity that establishes the viability of a high responsive gas sensor based on 2D MoO3. Hence, thermally stable and high dielectric 2D MoO3 has the potential to offer a new-generation of nano-electronic applications with excellent performance

A PUF based Lightweight Hardware Security Architecture for IoT

With an increasing number of hand-held electronics, gadgets, and other smart devices, data is present in a large number of platforms, thereby increasing the risk of security, privacy, and safety breach than ever before. Due to the extreme lightweight nature of these devices, commonly referred to as IoT or `Internet of Things\u27, providing any kind of security is prohibitive due to high overhead associated with any traditional and mathematically robust cryptographic techniques. Therefore, researchers have searched for alternative intuitive solutions for such devices. Hardware security, unlike traditional cryptography, can provide unique device-specific security solutions with little overhead, address vulnerability in hardware and, therefore, are attractive in this domain. As Moore\u27s law is almost at its end, different emerging devices are being explored more by researchers as they present opportunities to build better application-specific devices along with their challenges compared to CMOS technology. In this work, we have proposed emerging nanotechnology-based hardware security as a security solution for resource constrained IoT domain. Specifically, we have built two hardware security primitives i.e. physical unclonable function (PUF) and true random number generator (TRNG) and used these components as part of a security protocol proposed in this work as well. Both PUF and TRNG are built from metal-oxide memristors, an emerging nanoscale device and are generally lightweight compared to their CMOS counterparts in terms of area, power, and delay. Design challenges associated with designing these hardware security primitives and with memristive devices are properly addressed. Finally, a complete security protocol is proposed where all of these different pieces come together to provide a practical, robust, and device-specific security for resource-limited IoT systems