Physical principles of memory and logic devices based on nanostructured Dirac materials

During the las decades, the silicon-based semiconductor industry has enabled higher performance per cost of integrated circuits due to the ability of nearly doubling the amount of transistors per chip every two years, however, this has resulted in overheating issues and fundamental manufacturing problems that are very di¿cult to solve. Therefore, Dirac materials (DMs), such as graphene and topological insulators (TIs), are being extensively investigated as possible candidates for replacing silicon-channel devices in the next-generation integrated circuits, due to their attractive ultrahigh carrier mobility and possibility of quantum e¿ects that may be useful for electronic applications. This requires to study the physical principles of such nanostructures to e¿ectivelypredictthequantumtransportbehaviorofpossibledevices. Theaimofthis work is to explore the physical properties of Dirac material-based nanostructures that could be used for novel memory and logic devices, by using tight-binding (TB) and density function theory (DFT) methods combined with the non-equilibrium function (NEGF) formulationDoctoradoDOCTOR(A) EN INGENIERÍA ELECTRICA Y ELECTRÓNIC

Metal and semiconductor nanostructures for energy conversion applications

Department of Energy EngineeringEnergy crisis is one of the serious concerns, which needs to be addressed for a better future. Energy is majorly involved in our daily lives through electricity and transportation purposes. In present scenario, major part of electricity generation and transportation needs is fulfilled by fossil fuels. However due to the growing energy demand and lack of fossil fuel resources, alternatively renewable energy based technologies are being actively explored. Solar energy is one of the safe, clean, renewable energy which provides light and heat on a large scale. The solar light (photons) can be used to generate electricity/fuel via solar photovoltaics, solar water splitting, and artificial photosynthesis. Solar water splitting /Photo electrochemical cell (PEC) consists of semiconductor materials (as photo anodes/photo cathodes) which utilize solar light to generate oxygen/hydrogen from water. Hydrogen is a safe and clean fuel which has future prospects in transportation sector. Thermoelectric technology is another attractive technology, which can transform temperature into electricity or vice versa. This dissertation focuses on nanomaterials based novel strategies for improving the photo anodes performance in PEC cell and synthesize an eco-friendly material to enhance thermoelectric performance. With regard to design a better photo anodes, many metal nanoparticles and semiconductor nanowires are synthesized and their optical properties are carefully studied. Nanostructured materials have better optical, electrical, thermal properties over the bulk materials. Metal nanoparticles (NPs) have visible light absorption which can be advantageous in making hybrid nanostructures of metal NP and semiconductor nanowire. A hierarchically patterned metal/semiconductor (gold (Au) NPs/pat-zinc oxide (ZnO) nanowires) was fabricated via interference lithography (IL). The PEC performance of photo anodes (Au/pat-ZnO nanowires) displays a stable and increased photocurrent than the non-patterned samples due to efficient light trapping structures and plasmon enhanced water splitting. This hybrid nanostructure can also be used in solar cell, light emitting diodes, and as SERS substrates. The most-efficient thermoelectric materials till date are telluride/selenide based devices which are toxic and hazardous to the environment. Graphene is considered as potential alternative to the current state-of-art thermoelectric materials; provided the issues in graphene are properly addressed. Graphene has high electrical and thermal conductivity, which degrades the overall thermoelectric performance. The introduction of pores in graphene, allows better control over the thermoelectric performance. The effect of pore size and the structure-property relationships are studied in detail. The porous graphene displays promising thermoelectric performance that can be used for electricity generation. The usage of nanomaterials based energy conversion technologies shows promising outcomes, which holds the key for future energy generation.ope

Inorganic micro/nanostructures-based high-performance flexible electronics for electronic skin application

Electronics in the future will be printed on diverse substrates, benefiting several emerging applications such as electronic skin (e-skin) for robotics/prosthetics, flexible displays, flexible/conformable biosensors, large area electronics, and implantable devices. For such applications, electronics based on inorganic micro/nanostructures (IMNSs) from high mobility materials such as single crystal silicon and compound semiconductors in the form of ultrathin chips, membranes, nanoribbons (NRs), nanowires (NWs) etc., offer promising high-performance solutions compared to conventional organic materials. This thesis presents an investigation of the various forms of IMNSs for high-performance electronics. Active components (from Silicon) and sensor components (from indium tin oxide (ITO), vanadium pentaoxide (V2O5), and zinc oxide (ZnO)) were realised based on the IMNS for application in artificial tactile skin for prosthetics/robotics. Inspired by human tactile sensing, a capacitive-piezoelectric tandem architecture was realised with indium tin oxide (ITO) on a flexible polymer sheet for achieving static (upto 0.25 kPa-1 sensitivity) and dynamic (2.28 kPa-1 sensitivity) tactile sensing. These passive tactile sensors were interfaced in extended gate mode with flexible high-performance metal oxide semiconductor field effect transistors (MOSFETs) fabricated through a scalable process. The developed process enabled wafer scale transfer of ultrathin chips (UTCs) of silicon with various devices (ultrathin chip resistive samples, metal oxide semiconductor (MOS) capacitors and n‐channel MOSFETs) on flexible substrates up to 4″ diameter. The devices were capable of bending upto 1.437 mm radius of curvature and exhibited surface mobility above 330 cm2/V-s, on-to-off current ratios above 4.32 decades, and a subthreshold slope above 0.98 V/decade, under various bending conditions. While UTCs are useful for realizing high-density high-performance micro-electronics on small areas, high-performance electronics on large area flexible substrates along with low-cost fabrication techniques are also important for realizing e-skin. In this regard, two other IMNS forms are investigated in this thesis, namely, NWs and NRs. The controlled selective source/drain doping needed to obtain transistors from such structure remains a bottleneck during post transfer printing. An attractive solution to address this challenge based on junctionless FETs (JLFETs), is investigated in this thesis via technology computer-aided design (TCAD) simulation and practical fabrication. The TCAD optimization implies a current of 3.36 mA for a 15 μm channel length, 40 μm channel width with an on-to-off ratio of 4.02x 107. Similar to the NRs, NWs are also suitable for realizing high performance e-skin. NWs of various sizes, distribution and length have been fabricated using various nano-patterning methods followed by metal assisted chemical etching (MACE). Synthesis of Si NWs of diameter as low as 10 nm and of aspect ratio more than 200:1 was achieved. Apart from Si NWs, V2O5 and ZnO NWs were also explored for sensor applications. Two approaches were investigated for printing NWs on flexible substrates namely (i) contact printing and (ii) large-area dielectrophoresis (DEP) assisted transfer printing. Both approaches were used to realize electronic layers with high NW density. The former approach resulted in 7 NWs/μm for bottom-up ZnO and 3 NWs/μm for top-down Si NWs while the latter approach resulted in 7 NWs/μm with simultaneous assembly on 30x30 electrode patterns in a 3 cm x 3 cm area. The contact-printing system was used to fabricate ZnO and Si NW-based ultraviolet (UV) photodetectors (PDs) with a Wheatstone bridge (WB) configuration. The assembled V2O5 NWs were used to realize temperature sensors with sensitivity of 0.03% /K. The sensor arrays are suitable for tactile e-skin application. While the above focuses on realizing conventional sensing and addressing elements for e-skin, processing of a large amount of data from e-skin has remained a challenge, especially in the case of large area skin. A Neural NW Field Effect Transistors (υ-NWFETs) based hardware-implementable neural network (HNN) approach for tactile data processing in e-skin is presented in the final part of this thesis. The concept is evaluated by interfacing with a fabricated kirigami-inspired e-skin. Apart from e-skin for prosthetics and robotics, the presented research will also be useful for obtaining high performance flexible circuits needed in many futuristic flexible electronics applications such as smart surgical tools, biosensors, implantable electronics/electroceuticals and flexible mobile phones

Development of Supported 1D Nanomaterials by Vacuum and Plasma Technologies: From Sensors to Nanogenerators

Memoria presentada para optar al grado de Doctor por la Universidad de Sevilla Sevilla, September 2015Peer reviewe