Modeling and Fundamental Design Considerations for Portable, Wearable and Implantable Electronic Biosensors

Chronic diseases such as cancer, diabetes, acquired immune deficiency syndrome (AIDS), etc. are leading causes of mortality all over the world. Portable, wearable and implantable biosensors can go a long way in preventing these premature deaths by frequent or continuous self-monitoring of vital health parameters

Opto-Electro-Thermal Approach to Modeling Photovoltaic Performance and Reliability from Cell to Module

Thanks to technology advancement in recent decades, the levelized cost of electricity (LCOE) of solar photovoltaics (PV) has finally been driven down close to that of traditional fossil fuels. Still, PV only provides approximately 0.5% of the total electricity consumption in the United States. To make PV more competitive with other energy resources, we must continuously reduce the LCOE of PV through improving their performance and reliability. As PV efficiencies approach the theoretical limit, however, further improvements are difficult. Meanwhile, solar modules in the field regularly fail prematurely before the manufacturers 25-year warranty. Therefore, future PV research needs innovative approaches and inventive solutions to continuously drive LCOE down. In this work, we present a novel approach to PV system design and analysis. The approach, comprised of three components: multiscale, multiphysics, and time, aims at systemically and collaboratively improving the performance and reliability of PV. First, we establish a simulation framework for translating the cell-level characteristics to the module level (multiscale). This framework has been demonstrated to reduce the cell-to-module efficiency gap. The framework also enables the investigation of module-level reliability. Physics-based compact models -the building blocks for this multiscale framework are, however, still missing or underdeveloped for promising materials such as perovskites and CIGS. Hence, we have developed compact models for these two technologies, which analytically describe salient features of their operation as a function of illumination and temperature. The models are also suitable for integration into a large-scale circuit network to simulate a solar module. In the second aspect of the approach, we study the fundamental physics underlying the notorious self-heating effects for PV and examine their detrimental influence on the electrical performance (multiphysics). After ascertaining the sources of self-heating, we propose novel optics-based self-cooling methodologies to reduce the operating temperature. The cooling technique developed in this work has been predicted to substantially enhance the efficiency and durability of commercial Si solar modules. In the third and last aspect of the approach, we have established a simulation framework that can forward predict the future energy yield for PV systems for financial scrutiny and inversely mine the historical field data to diagnose the pathology of degraded solar modules (time). The framework, which physically accounts for environmental factors (e.g., irradiance, temperature), can generate accurate projection and insightful analysis of the geographic-and technology-specific performance and reliability of solar modules. For the forward modeling, we simulate the optimization and predict the performance of bifacial solar modules to rigorously evaluate this emerging technology in a global context. For the inverse modeling, we apply this framework to physically mine the 20-year field data for a nearly worn-out silicon PV system and successfully pin down the primary degradation pathways, something that is beyond the capability of conventional methods. This framework can be applied to solar farms installed globally (an abundant yet unexploited testbed) to establish a rich database of these geographic-and technology-dependent degradation processes, a knowledge prerequisite for the next-generation reliability-aware design of PV systems. Finally, we note that the research paradigm for PV developed in this work can also be applied to other applications, e.g., battery and electronics, which share similar technical challenges for performance and reliability

Addressing and tailoring the electronic properties of semiconductor nanostructures: nanowires and transition metal dichalcogenides

Semiconductor materials played and still play a pivotal role in the technological development of modern life. From personal computer to data storage (e.g. solid-state disk-drives), from solar cells and cell phones to LEDs and biological sensors, there has always been a new system to study, a novel application to develop, and a solution to an otherwise unsolvable problem in which semiconductors play an essential role. All those technological goals have been achieved thanks to the synergic works carried out by basic researches in materials science, in particular in the semiconductor field. In the last decades, tremendous efforts have been made to miniaturize semiconductor devices at a nanometer scale, aiming at obtaining more compact devices with optimized speed and reduced power consumption. Unfortunately, or fortunately, the physical properties of any material dramatically change when the material dimensions are reduced to nanometer-lengths. Therefore, many efforts are required to understand the properties of any desired nanostructure, if they have to play a central role in technological applications. In recent years, a great interest has grown in the investigation and applications of nanowires (NWs). NWs are several micron-long filamentary-crystals whose diameters range from few to hundreds of nanometers. Their dimensions make NWs suitable to bridge the gap between the microscopic and the nanoscopic world in both research and technology fields. Although several types of materials can be grown in a NW form, e.g. metals, insulators, and semiconductors, the latter are the most interesting and promising materials. As a matter of fact, owing to their peculiar shape and dimensions, semiconductor NWs are valuable candidates for novel nanoscale devices, in which they act as both functionalized components and interconnects. Moreover, semiconductor NWs represent nanostructured systems for which some key parameters in device engineering, e.g. chemical composition, size, and crystal phase, are well controlled nowadays. This is mainly due to the technique used to grow NWs. NWs are usually fabricated via the vapor-liquid-solid (VLS) technique, in which metal nanoparticles are used as catalyst seeds to induce a one-dimensional crystal growth. This well-controlled process allows for the synthesis of a wide range of semiconductor systems in the NW form, ranging from IV-IV to II-VI, with a high degree of manageability of both the chemical composition and morphology. In addition, under suitable VLS conditions, non-nitride III-V NWs can crystallize in the hexagonal wurtzite (WZ) structure in materials that, instead, are notoriously stable in the cubic zinc-blende (ZB) structure. The opportunity to controllably grow NWs in different crystal phases, namely, the polytypism, adds a new degree of freedom in device engineering. The presence of a WZ crystal phase in many III-V NWs offers also the opportunity to address the electronic band structure of this poorly known structure, vii viii whose presence itself is a subject of fundamental interest in materials science and chemistry. As an example, there is no experimental information concerning the variation of the spin and transport properties, i.e. gyromagnetic factors and carrier effective-masses, respectively, when the phase transition from ZB to WZ occurs. Even the fundamental band-gap value of some WZ semiconductor materials have not been determined, yet. Therefore, a comprehensive study aimed at the investigation of the correlation between the NW electronic properties and NW crystal structure is mandatory nowadays. A great interest has grown also in the field of layered materials. Since the discovery of graphene in 2004, it has been understood the great potential of layered systems for advanced-technological applications. As a matter of fact, layered materials thinned to their physical limits -and usually referred to as two-dimensional (2D) materials- exhibit properties quite different from those of their bulk counterparts. A very wide spectrum of 2D materials has been then investigated. The most studied material is graphene because of its exceptional electronic and mechanical properties. Group VI transition metal dichalcogenides (TMDs) have also attracted the attention of researchers involved in the semiconductor field. TMDs have a crystal structure similar to that of graphite. Their layered structure, X-M-X, where M is the transition metal and X is the chalcogen atom, is characterized by weak interlayer van der Waals bonds and strong intralayer covalent bonds. That structure allows for an easy mechanical exfoliation, as in the graphene case, which is a major advantage of 2D materials, together with their synthesis techniques, cheap and easy as compared to the molecular-beam-epitaxy or metal-organic chemical-vapor-deposition techniques used for the fabrication of other nanostructured systems. The most surprising feature observed in 2D TMDs is the transition from an indirect band-gap in the infrared region to a direct band-gap in the visible region when they are thinned to the mono- layer limit. That feature, coupled with the TMD extremely high flexibility, elasticity, and resistance, makes TMDs suitable in the field of low-dimensional optoelectronic devices. In addition, the TMD high surface-to-volume ratio is valuable in biological fields, as they can be used as highly reactive sensors. Besides, the TMD unique properties in the single-layer limit of valley-valley coupling and valley-spin coupling render TMDs the suitable candidates for novel technologies based on valleytronic and spintronic. However, almost all these aforementioned properties are at the early stage of investigation and systematic studies are necessary before TMDs could be exploited in future applications. In this thesis, the electronic properties of InP NWs and MX2 TMDs, with M=Mo or W and X=S or Se, are thoroughly investigated mainly by means of optical spectroscopy, in particular photoluminescence (PL) in combination with external perturbations, e.g. high magnetic fields. The response of semiconductor TMDs to hydrogen irradiation is studied, too. The thesis is therefore structured in two parts, the first one, from chap. 1 to chap. 3, is devoted to InP NWs, the second one, from chap. 4 to chap. 6, is devoted to 2D TMDs. • In the first chapter, the high degree of freedom achieved in NW fabrication is presented and accounted for by the VLS technique, which is also discussed in details together with its recent development: the selective-area-epitaxy technique. Then, the differences between the structural, electronic, and optical ix properties of WZ and ZB crystal phases are discussed. The striking variation induced in the band structure by the crystal phase-transition is highlighted, too. Moreover, the different optical anisotropies of the two crystal phases are summarized. The chapter is concluded by a review of the technological applications of semiconductor NWs in the fields of optoelectronic, energy conversion, biosensoring, and as probes of elusive quantum effects. • The second chapter comprehends a systematic investigation of InP NWs in both the ZB and WZ crystal-phases. The morphological characteristics of the investigated samples as accessed through scanning-electron-microscopy, transmission-electron-microscopy, and selective-area-diffraction patterning are also presented. The basic optical properties of InP in both crystal phases are assessed by either PL or μ-PL experiments as a function of lattice temperature and power excitation. Polarization-resolved measurements are shown, too. The three lowest-energy critical points of the WZ band-structure are investigated by PL excitation (PLE) as a function of lattice temperature. A quantitative reproduction of those spectra allows for establishing the temperature depen- dence of the A, B, and C inter-band transitions. A comparison with ZB results is made, too. Finally, the hot-carrier effect in NWs is found and its dependence on NW morphology is investigated. • In the third chapter, the transport and spin properties of WZ InP are assessed by PL spectroscopy under high magnetic fields (up to 28 T ). A brief review of the effects that a magnetic field has on the energy and symmetry of exciton recombinations and of free-electron-to-acceptor and donor-to-acceptor transi- tions in WZ crystal is presented. Both diamagnetic shift and Zeeman splitting depend on the magnetic-field direction with respect to the NW symmetry-axis, namely the WZ cˆ-axis. That dependence has been investigated by applying the magnetic field either parallel or orthogonal to the NW axis. The obtained results are compared with the literature of both theoretical models of WZ InP and experimental results in other WZ compounds, such as GaN, InN, and ZnO. Finally, the non-linearity observed in the Zeeman splitting for magnetic fields above 10T and parallel to the NW axis is compared to a theoretical prediction. • In the fourth chapter, the lattice, electronic, and vibrational properties of 2D TMDs are described. In particular, the lattice structures of several polytypes are shown, with special emphasis on the 2H polytype, whose electronic and vibrational properties are investigated and its different properties in the bulk and single-layer regimes highlighted. Then, several methods aimed at reaching the mono-layer limit are presented and top-down exfoliations from bulk mate- rials are singled out from bottom-up syntheses. The chapter ends with a brief review of the technological applications of semiconductor 2D TMDs in the fields of optoelectronic, energy conversion and storage, and molecular sensing. • The fifth chapter comprehends a systematic investigation of the effects of hydrogen irradiation on the emission properties of single- and bi-layer TMDs, such as MoSe2 and WSe2. Firstly, a wide variety of experimental results con- x cerning MX2 optical band-gaps and vibrational mode-energies are summarized. A brief description of the investigated samples is presented, too. The optical properties of pristine samples are assessed by means of either μ-Raman or μ-PL experiments whose room- and low-temperature results agree well with the existing literature. Then, the pristine flakes are irradiated with progressively increasing doses of hydrogen and the results thus obtained are reported. In the single-layer regime, a worsening of the material optical quality is observed together with the appearances of very sharp peaks below the band-gap energy. Conversely, a small improvement in the PL efficiency is obtained in the bi-layer regime. Finally, a solution to the worsening of the optical quality observed in hydrogenated single-layer flakes is provided. • In the sixth chapter, the effects of hydrogen irradiation on the morphological and optical properties of multi-layer TMDs are discussed. Surprisingly, hy- drogenation favors unique conditions for the production and accumulation of molecular hydrogen just one or few layers beneath the crystal surface of all the multi-layer MX2 compounds investigated. That turns into the creation of atomically-thin domes filled with hydrogen molecules. The results of an atomic-force-microscopy and optical investigation of these new fascinating nanostructures are discussed. Finally, the possibility to tailor the dome posi- tion, size, and density is demonstrated, which provides a tool to manage the mechanical and electronic structure of 2D materials. • The main results obtained in this work are summarized in the conclusive remarks. • In the appendix, the theoretical basis of the optical-spectroscopy techniques here used, such as PL, PLE, magneto-PL, and Raman spectroscopy, are provided. PL and PLE are complementary techniques that enable a complete characterization of the electronic states of any optically-efficient material. Indeed, PL is an extremely sensitive probe of low-density electronic states, such as impurities or defects, while PLE can address the full density of states, i.e, it mimics absorption measurements, at least under certain approximations. On the other hand, PL spectroscopy under magnetic field allows for the determination of carrier effective-masses and g-factors, while Raman allows for getting information about the lattice properties of solids. A description of all the used experimental setups is also given. Finally, a description of the experimental apparatus used for hydrogen irradiation and atomic-force- microscopy measurements is provided. • Finally, a list of the publications to which the author of this thesis has contributed is provided, along with a list of poster/oral contributions to international conferences given by the author of this thesis during his PhD studies

Fundamentals and Recent Advances in Epitaxial Graphene on SiC

This book is a compilation of recent studies by recognized experts in the field of epitaxial graphene working towards a deep comprehension of growth mechanisms, property engineering, and device processing. The results of investigations published within this book develop cumulative knowledge on matters related to device-quality epaxial graphene on SiC, bringing this material closer to realistic applications

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

Molecular Layer Functionalized Neuroelectronic Interfaces: From Sub-Nanometer Molecular Surface Functionalization to Improved Mechanical and Electronic Cell-Chip Coupling

The interface between electronic components and biological objects plays a crucial role for the success of bioelectronic devices. Since the electronics typically include different elements such as an insulating substrate in combination with conducting electrodes, an important issue of bioelectronics involves tailoring and optimizing the interface for any envisioned application. In this work, we present a method of functionalizing insulating substrates (SiO2) and metallic electrodes (Pt) simultaneously with a stable monolayer of organic molecules ((3-aminopropyl)triethoxysilane (APTES)). This monolayer is characterized by various techniques like atomic force microscope (AFM), ellipsometry, time-of-flight secondary ion mass spectrometry (ToF-SIMS), surface plasmon resonance (SPR), and streaming potential measurements. The molecule layers of APTES on both substrates, Pt and SiO2, show a high molecule density, a coverage of ~ 50 %, a long-term stability (at least one year), a positive surface net charge, and the characteristics of a self-assembled monolayer (SAM). In the electronical characterization of the functionalized Pt electrodes via impedance spectroscopy measurements, the static properties of the electronic double layer could be separated from the diffusive part using a specially developed model. It could be demonstrated that compared to cleaned Pt electrodes the double layer capacitance is increased by an APTES coating and the charge transfer resistance is reduced, which leads to a total increase of the electronic signal transfer of ~13 %. In the final cell culture measurements, it could be shown that an APTES coating facilitates a conversion of bio-unfriendly Pt surfaces into biocompatible surfaces which allows cell growth (neurons) on both functionalized components (SiO2 and Pt) comparable to that of reference samples coated with poly-L-lysine. Furthermore, APTES coating leads to an improved mechanical coupling, which increases the sealing resistance and reduces losses. These increases were finally confirmed by electronic measurements on neurons, which showed action potential signals in the mV regime compared to signals of typically 200 – 400 µV obtained for reference measurements on PLL coated samples. Therefore, the functionalization with APTES molecules seems to be able to greatly improve the electronic cell-chip coupling (here by ~1 500 %). This significant increase of the electronic and mechanical cell-chip coupling might represent an important step for the improvement of neuroelectronic sensor and actuator devices

Report / Institute für Physik

The 2015 Report of the Physics Institutes of the Universität Leipzig presents an interesting overview of our research activities in the past year. It is also testimony of our scientific interaction with colleagues and partners worldwide

NASA Thesaurus. Volume 2: Access vocabulary

The NASA Thesaurus -- Volume 2, Access Vocabulary -- contains an alphabetical listing of all Thesaurus terms (postable and nonpostable) and permutations of all multiword and pseudo-multiword terms. Also included are Other Words (non-Thesaurus terms) consisting of abbreviations, chemical symbols, etc. The permutations and Other Words provide 'access' to the appropriate postable entries in the Thesaurus

Physical properties of vanadium dioxide nanoparticles: application as 1-d nanobelts room temperature for hydrogen gas sensing

Philosophiae Doctor - PhDTransition metal oxides magneli phases present crystallographic shear structure which is of great interest in multiple applications because of their wide range of valence, which is exhibited by the transition metals. The latter affect chemical and physical properties of the oxides. Amongst them we have nanostructures VO2 system of V and O components which are studied including chemical and physical reactions based on non-equilibrium thermodynamics. Due to their structural classes of corundum, rocksalt, wurtzite, spinel, perovskite, rutile, and layer structure, these oxides are generally used as catalytic materials which are prepared by common methods under mild conditions presenting distortion or defects in the case of VO2. Existence of an intermediate phase is proved using an x-ray thermodiffraction experiment providing structural information as the nanoparticles are heated. Potential application as gas sensing device has been the first time obtained due to the high surface to volume ratio, and good crystallinity, purity of the material and presence of suitable nucleating defects sites due to its n-type semiconductor behavior. In addition, annealing effect on nanostructures VO2 nanobelts shows a preferential gas reductant of Ar comparing to the N2 gas. Also, the hysteresis loop shows that there is strong size dependence to annealing treatment on our samples. This is of great interest in the need of obtaining high stable and durable material for Mott insulator transistor and Gas sensor device at room temperature