Transparent Conductive Films Based on Polymer-Encapsulated Graphene Oxide Sheets

Transparent conductive films (TCFs) play a key role in number of devices, including solar panels, LCD/OLED displays and touchscreens. Graphene has emerged as a promising material in this area due to its unique mechanical and electrical properties. Despite noteworthy progress in the fabrication of large-area graphene sheet-like nanomaterials, the vapor-based processing still requires sophisticated equipment and a multistage handling of the material. An alternative approach to manufacturing functional graphene-based films includes the employment of graphene oxide (GO) micron-scale sheets as precursors. However, search for a scalable manufacturing technique for the production of high-quality GO nanoscale films with high uniformity and high electrical conductivity is still continuing. The study presented in this dissertation is dedicated to the fabrication and characterization of electrically conductive films made of reduced graphene oxide sheets (rGO) deposited on both rigid and flexible substrates. Here we show that conventional dip-coating technique can offer fabrication of high quality mono- and bilayered films made of GO sheets. The method is based on our recent discovery that encapsulating individual GO sheets in a nanometer-thick copolymer layer poly(Oligo Ethylene Glycol methyl ether Methacrylate [OEGMA]- Glycidyl Methacrylate [GMA]) allows for the nearly perfect formation of the GO layers on hydrophilic substrates. By thermal reduction at 1000 ⁰C the bilayers (cemented by a carbon-forming polymer linker) are converted into highly conductive and transparent reduced GO films with a high conductivity up to 10000 S/cm and optical transparency on the level of 90%. The value is the highest electrical conductivity reported for thermally reduced nanoscale GO films and is close to the conductivity of indium tin oxide (ITO) currently in use for transparent electronic devices, thus making these layers intriguing candidates for replacement of ITO films. To facilitate the deposition of GO sheets on rigid and flexible hydrophobic substrates, the amphiphilic copolymer poly(Oligo Ethylene Glycol methyl ether Methacrylate [OEGMA]- Glycidyl Methacrylate [GMA]- Lauryl Methacrylate [LMA]) with additional hydrophobic block was used. The results show that the obtained GO layers had well-defined and uniform structure. Thus, it leads to enhanced hydrophobic-hydrophobic (van der Waals) interaction between the hydrophobic substrate and GO. To this end, the morphology, opto-electrical properties and electro-mechanical stability of chemically reduced GO layers are also investigated. Finally, we demonstrate the excellent stability of rGO on polymeric substrates with no delamination or significant loss in conductivity even after 50000 bending cycle

Applications of Kelvin Probe Force Microscopy in the characterization of 2D materials and their composites

Kelvin Probe Force Microscopy (KPFM), since its relatively recent introduction in 1991, has become a widely used technique to assess surface charge distribution and work function of metal/semiconductor interfaces in electronic devices at the nanoscale. Today, this characterization technique is employed in many nanotechnology-related applications, with 2D layered material being a notable example due to the necessity of characterizing various electrical properties with high spatial resolution to better understand how these parameters scale or change relatively with synthesis, deposition and environment conditions. In this thesis I will discuss the main applications of KPFM characterization that I have encountered during my three years of doctorate, showing its usefulness in various fields, its main limitations and some practical considerations about the best practices I developed to optimize sample preparation. Regarding the study of fundamental electronic properties I will present my results on the correlation of work function of 2D materials like MoS2 and their thickness (Chapter 3), and how laser induced 3D structures can locally tune the work function of the graphene basal plane for possible future localized functionalization (Chapter 4). As an example of the use of KPFM characterization of 2D composites, I will present the analyses of a metal/graphene interface, more specifically the silver nanowire (AgNW)/graphene interface, selected as it represents a very promising type of hybrid for devices requiring highly conductive transparent electrodes (Chapter 5). Finally, I will show how KPFM can be employed even in a macroscopic conductive 2D material/polymer composite to elucidate the structure or to verify the presence of the conductive elements inside the insulating polymeric matrix (Chapter 6). At the start of this work, I will present a theory background on the origin and principles of the technique, its evolution and main limitations (Chapter 1) followed by a brief introduction of the main other characterization techniques employed and my personal consideration on the best deposition techniques (Chapter 2)

Tunable Functionality of Pure Nano Cu- and Cu-based Oxide Flexible Conductive Thin Film with Superior Surface Modification

Flexible and soft conductive thin film using pure Cu and Cu-based oxide nanostructures equally benefit from the versatility of their assembling individual materials and robustness of device design components. Their small-scale soft conductive thin film made of curved elastomeric bilayers driven by the responsive forces acting by the embedded printed liquid of pure Cu and/or Cu-based oxide nanostructures channels carrying alternating currents of those compact integrated circuits. As such, the localised oxide growth of those complex multiphase thin film architectures is the empirical knowledge that guides to further understanding of many interrelated factors of their intrinsic multiscale physical-electro-chemical interactions characteristics. Although not much literatures have been reported on the soft, flexible pure Cu and Cu-based oxide nanostructured thin films, still, the compelling unusual shapes/forms/construct of such nanostructures in the preparation of those superior functionalities thin film using various curvilinear shapes would seem to establish a predominant foundation in technologically important MEMS/NEMS devices. Herein, this article attempts to summarise the recent advances, challenges, and prospects of employing pure Cu and Cu-based oxide nanostructures in both fundamental and applied tunable functionality of varying dimensionality. Also, special emphasis on the emerging related critical issues and outlook of technical challenges that pave to research improvement opportunities are included

Feature Papers in Electronic Materials Section

This book entitled "Feature Papers in Electronic Materials Section" is a collection of selected papers recently published on the journal Materials, focusing on the latest advances in electronic materials and devices in different fields (e.g., power- and high-frequency electronics, optoelectronic devices, detectors, etc.). In the first part of the book, many articles are dedicated to wide band gap semiconductors (e.g., SiC, GaN, Ga2O3, diamond), focusing on the current relevant materials and devices technology issues. The second part of the book is a miscellaneous of other electronics materials for various applications, including two-dimensional materials for optoelectronic and high-frequency devices. Finally, some recent advances in materials and flexible sensors for bioelectronics and medical applications are presented at the end of the book

Local Characterization of Resistance Switching Phenomena in Transition Metal Oxides

The development of neuromorphic computing systems that emulate the analog charge states and plasticity of the brain’s neuron-synapse architecture has been a major driver of resistance switching materials exploration. Materials that demonstrate changes in conductance with tunable ratios and volatility of resistance states within a single layer are highly desirable. Although excellent resistance switching device performance has been demonstrated in a range of transition metal oxides, a lack of understanding of the fundamental microscale evolution of a material during resistance switching presents a key limitation to controlling switching parameters. Here, we examine the role of materials defects on local resistance switching structures in two representative transition metal oxide materials: HfOv2 thin films and hydrothermally synthesized VOv2 single crystals. In each material, we seek to clarify the structure of resistance switching domains and the kinetics of domain formation resulting from intentional defect introduction. This thesis is therefore divided into two main parts concerning (1) the introduction of planar defects in HfOv2 filamentary resistance switching devices, and (2) the impact of introduction of point defects on the metal-insulator transition in VOv2 single crystals. Part I (Sections 2 – 3) details investigation of Cu ion migration rates in Cu/HfOv2/p+Si and Cu/HfOv2/TiN devices in which oxide microstructure varies between amorphous, polycrystalline, and oriented polycrystalline. Ion migration across the oxide layer is shown to be rate limiting and faster in polycrystalline layers than in amorphous HfO2 layers at equivalent electric field. Moreover, the 3D shape of conductive filaments is investigated by a scribing atomic force microscopy experiment in Cu/HfOv2/p+Si devices and reveals a broad range of filament shapes under identical electrical stress conditions. Thermal dissipation is interpreted as the principal determinant of filament area, while oxide microstructure is shown to direct the location of filaments within the device. In part II (Sections 4 – 5), the hysteresis of the metal-insulator transition (switching volatility) in VOv2 is shown to intrinsically derive from nucleation limited transformations in individual particles. Here, hysteresis is a strong function of particle size, but may be increased or decreased by synthesis techniques that affect the concentration and potency of intrinsic point defects. Upon chemical doping with boron at interstitial lattice sites, a unique kinetic effect on the hysteresis of the current driven metal-insulator transition in two terminal BxVOv2 devices is observed. Dependence of the critical switching current on thermal relaxation time and temperature is characterized and recommendations for further kinetic testing are made. Finally, a few experimental extensions of the work presented in this thesis are made in Section 6

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

Large-area flexible electronics based on low-temperature solution-processed oxide semiconductors

Due to their high charge carrier mobility, optical transparency and mechanical flexibility, thin-film transistors (TFTs) based on metal oxide semiconductors represent an emerging technology that offers the potential to revolutionise the next-generations of large-area electronics. This thesis focuses on the development of high-performance TFTs based on low-temperature, solution-processed metal oxide semiconductors that are compatible with inexpensive flexible plastic substrates. The first part of the dissertation describes an ultraviolet light assisted processing method suitable for room-temperature activation of ZnO nanoparticles and their application as semiconducting channels in TFTs. The impact of the semiconductor/dielectric interface on electrical performance is studied using different device configurations and dielectric surface-passivation methods. Furthermore, a nanocomposite concept is proposed in order to assist electron transport between different crystalline domains. Using this approach, TFTs with electron mobilities of ~3 cm2/Vs are demonstrated. The second part of this work explores a carbon-free, aqueous-based Zn-ammine complex route for the synthesis of polycrystalline ZnO thin-films at low temperature and their subsequent use in TFTs. Concurrently, the development of a complementary high-κ oxide dielectric system enables the demonstration of high-performance ZnO TFTs with electron mobilities >10 cm2/Vs and operation voltage down to ~1.2 V. This low-temperature aqueous chemistry is further explored using a facile n-type doping approach. Enhancement in electrical performance is attributed to the different crystallographic properties of the Al-doped ZnO layers. The final part of the thesis introduces a novel TFT concept that exploits the enhanced electron transport properties of low-dimensional polycrystalline quasi-superlattices (QSLs), consisting of sequentially spin-cast layers of In2O3, Ga2O3 and ZnO deposited at temperatures 40 cm2/Vs - an order of magnitude higher than devices based on single binary oxide layers. Based on temperature dependent electron transport and capacitance-voltage measurements, it is reasoned that the enhanced electrical performance arises from the presence of quasi two-dimensional electron gas-like systems formed at the carefully engineered oxide heterointerfaces buried within the QSLs.Open Acces

Mechanically flexible, transparent conductors based on ultrathin metallic layers

Transparent Conductors are essential components in many opto-electronic devices. Ultrathin Metal Films (UTMFs) represent an effective alternative to the ITO state-of-art. Their potential was demonstrated in organic solar cells with efficiencies comparable to those with ITO

Flexible and Stretchable Electronics

Flexible and stretchable electronics are receiving tremendous attention as future electronics due to their flexibility and light weight, especially as applications in wearable electronics. Flexible electronics are usually fabricated on heat sensitive flexible substrates such as plastic, fabric or even paper, while stretchable electronics are usually fabricated from an elastomeric substrate to survive large deformation in their practical application. Therefore, successful fabrication of flexible electronics needs low temperature processable novel materials and a particular processing development because traditional materials and processes are not compatible with flexible/stretchable electronics. Huge technical challenges and opportunities surround these dramatic changes from the perspective of new material design and processing, new fabrication techniques, large deformation mechanics, new application development and so on. Here, we invited talented researchers to join us in this new vital field that holds the potential to reshape our future life, by contributing their words of wisdom from their particular perspective

Additive Manufacturing of Graphene-based Patterns

The focus of this dissertation is on the deployment and characterization of a micro-scale aerosol-jet additive manufacturing technology to print highly conductive and flexible graphene-based patterns. For this purpose, a highly concentrated graphene ink with a viscosity of 21 cP and 3.1 mg/ml graphene flakes with the lateral size below 200 nm was developed and adopted for the aerosol-jet printing process to make a reliable and repeatable graphene deposition on the treated Si/SiO2 wafers. To this end, the influence of the most significant process parameters, including the atomizer power, the atomizer flow rate, and the number of the printed layers, on the size and properties of graphene patterns was studied. Results showed that the aerosol-jet printing process is capable of printing micro-scale graphene pattern with variable widths in the range of 10 to 90 micron. These patterns, as the finest printed graphene patterns, with resistivity as low as 0.018 Ω.cm and a sheet resistance of 1.64 kΩ/□ may ease the development of miniaturized printed electronic applications of graphene. In this work, a laser processing protocol for the heat treatment of the printed graphene patterns was also developed, and the results were compared with the counterpart results obtained by the conventional heat treatment process carried out in a furnace. A continuous-wave Erbium fiber laser was used to enhance electrical properties of the aerosol-jet printed graphene patterns through removing solvents and a stabilizer polymer. The laser power and the process speed were optimized to effectively treat the printed patterns without compromising the quality of the graphene flakes. Furthermore, a heat transfer model was developed, and its results were utilized to optimize the laser treatment process. It was found that the laser heat treatment process with a laser speed of 0.03 mm/s, a laser beam diameter ~50 µm, and a laser power of 10 W results in pure graphene patterns with no excessive components. The results suggested that the laser processing has the capability of removing stabilizer polymers and solvents through a localized moving heat source, which is preferable for flexible electronics with low working temperature substrates. This dissertation also addresses the deployment of a graphene/silver nanoparticle (Ag NP) ink in an aerosol-jet additive manufacturing system in order to print highly conductive and flexible graphene/Ag patterns for flexible printed electronics. A graphene/Ag NP ink was developed using stabilized graphene powder, viscose Ag NP ink, and solvents compatible with the printing system. Printing with this ink produced a uniform microstructure and crack-free printed interconnects. With a mean resistivity of 1.07×〖10〗^(-4) Ω.cm, these interconnects are about 100 times more conductive than graphene and three times more conductive than Ag NP interconnects printed with the same printing system. With their high degree of conductivity and a level of flexibility identical to that of graphene printed patterns, concluded from bending test results, graphene/Ag aerosol-jet printed patterns may therefore be considered as an efficient candidate compared to either graphene or Ag NP printed patterns for flexible electronics