Over the last decade, since the introduction of graphene in 2004, 2D materials have become a very hot topic. Excellent chemical and mechanical stability along with incredible transport carrier properties, graphene has sparked interest in other 2D materials. Graphene has a portfolio of applications but, lack of a band gap hinders its potential in semiconductor applications which has pushed researcher to look at more exotic 2D materials with semiconductor properties. This new class of 2D semiconductors under the form of MX2 which is composed or a transition metal and a chalcogen atom. Mobility is good for low energy loss transmission of electrons from one point to another and is an important aspect in electronics and optoelectronics which apply to conductivity and light absorption. In conductors usually we have an overlap of the valence and conduction band where electrons move freely on the other side of the spectrum we have insulators where the gap between the valence and conduction band are too large to feasible transition between to allow for conductivity. Semiconductor are located in between these two extremes with enough gap where electrons only require a small amount of energy for them to move from one side to another. This concept is very important when talking about light interactions and absorption. In order for these devices to be feasible we need band gap to exist in the spectrum of visible light. Semiconductors have a high and low resistance states which has an on/off ration making a semiconductor of interest in many applications such as transistors and photodetectors. Direct bang gap in photodetector rely on transfer of photons and we can achieve this by using monolayer semiconductor materials. In order to be able to obtain all these properties we would have to be able to synthesize all these new 2D materials in the first place. In this work synthesis methods were studied to try to understand these materials and achieve large area growth. First we start with powered growth of novel semiconductor materials and move to a scalable liquid technique that is potentially able achieve wafer scale growth.

Lithium-ion batteries (LIBs) is believed to be one of the most promising alternatives to non-renewable fossil fuels for energy crisis, environmental protection and sustainable development. To further increase the energy and power densities of LIBs, nanostructured silicon and its nanocomposites were synthesized to achieve high-capacity, environmentally benign, and highly scalable candidates for the next generations of Li-Ion anodes. Herein, four distinct silicon-based nanostructures were synthesized and thoroughly characterized including silicon-carbon fabrics, nano-silicon from unrecycled glass bottles (gSi), conducting hydrogel coated silicon nanocomposites, and graphene wrapped conducting polymer-silicon hybrid structure. The electrochemical properties of silicon based nanomaterials were characterized in terms of cyclic voltammetry (CV), long-term capacity stability, rate capability and potentiostatic electrochemical impdedance spectroscopy (PEIS), and galvanostatic charge-discharge curves.

Highly flexible and free-standing silicon-carbon (Si/C) fabrics are synthesized via simultaneous double-nozzle electrospinning. The binderless and current collector-free fabrics are used as Li-ion anodes with an overall capacity of ~1000 mAh g-1. 92 % capacity retention after 100 cycles coupled with the excellent rate capability up to 9.3 A g-1 demonstrate its superior flexibility over the conventional electrodes.

A conversion from potential glass waste into high-purity nano-silicon is synthesized by a surface protected magnesiothermic reduction. Carbon-coated glass derived-silicon (gSi@C) anodes demonstrate excellent electrochemical performance with a capacity of ~1420 mAh g-1 at C/2 after 400 cycles. Full cells are assembled using gSi@C anodes and LiCoO2 cathodes, and achieve good cycling stability with high energy density.

Conducting hydrogels are formed to coat Si surfaces via an in-situ polymerization process. Functional groups from hydrogels chemically improve the adhesion of conducting coatings on Si surface, rendering the Si-hydrogel hybrid structure without resistive binders and carbon black. It is observed that the degree of enhancing the cycling stability and rate capability of the conductive hydrogels decrease in the order of polypyrrole (PPy) > polyaniline (PANI) > poly(acrylic acid) (PAA).

Silicon nanoparticles (SiNPs) are coated with polypyrrole-hydrogel and wrapped with reduced graphene oxide sheets (rGO) via an environmentally benign and scalable sol-gel process. The PPy/SiNPs/rGO electrodes can produce highly reversible capacities of 1312, 1285 and 1066 mAh g-1 at 100, 250 and 500 cycles at a high current density of 2.1 A g-1, respectively.

Abstract

Inch-Scale High Throughput Metrology of Graphene

and Patterned Graphene Oxide

by

Dennis Pleskot

Master of Science, Graduate Program in Materials Science

and Engineering

University of California, Riverside, June 2013

Dr. Cengiz Ozkan, Chairperson

In order to fully utilize the unique properties of graphene, large-area sheets of the material must be produced. As the demand for large, continuous sheets of graphene increases, the need to quickly and effectively characterize such a material correspondingly increases. In tandem, the use of patterned graphene oxide in practical applications has also expanded at a rapid pace, leading to a greater need to characterize this material as well. In this study, fluorescence quenching microscopy was examined as a means of analyzing 4 in^{2 sheets of graphene and patterned graphene oxide in a fast and efficient manner. It was determined that fluorescence microscopy offers a number of advantages in imaging these materials over conventional techniques such as Raman spectroscopy, SEM, and TEM. Fluorescence microscopy proved to be a highly scalable technique that is able to image a graphitic material in substantially less time than would be possible in these other methods while still maintaining adequate resolution. Unlike SEM and TEM, fluorescence microscopy is also a non-destructive characterization technique, allowing the samples that were actually characterized to be used in practical applications. Additionally, a clear contrast between graphene oxide and pristine graphene could be observed. Analysis of the fluorescence contrast histogram shows that graphene oxide displays a unique intensity peak and can thus not only be distinguished from pristine graphene but also defects in the sample, the presence of multiple layers, and changes in the uniformity of the graphene surface. These observations and results demonstrate the expanding usefulness of fluorescence microscopy as a high-throughput characterization technique.}

Recently, semiconductor nanostructures have generated a continuously growing interest owing to their intriguing physical properties and potential for future technology development. Quasi-one-dimensional nanowires (NWs), usually defined as round or polyhedral shaped cross-sectional structures with a high aspect ratio of 1000 or more, is one of the most active research areas for nanoelectronics, optoelectronics and sensors. Precise controlled growth of NWs, including dimensions, growth rate, morphology, growth direction, and composition, is essential from the viewpoints of fundamental materials fabrication and their performance in future applications. However, there are still many challenges in the fabrication of NWs with controlled dimensions and properties. This dissertation focuses on investigating the fundamental synthesis aspects of epitaxially grown III-V NWs by chemical vapor deposition (CVD).

First, gold (Au) catalyst - assisted growth of InSb NWs directly on lattice mismatched (~7%) InAs (100) substrates have been studied. The influences of NW diameter and growth temperature on the NW growth rate and morphology have been investigated. The results indicate that NW growth is limited by the growth species direct impingement and the diffusion of surface adatoms. Next, the dependence of morphology of epitaxial InSb NWs on CVD growth parameters over InSb (100) substrates has been investigated. NW length and tapering factors correlated to morphological variations are determined as a function of growth parameters including growth temperature (300°C-480°C), powder source, and duration of growth. Results showed that NW morphology is influenced by axial and radial growth modes, reflecting the competition of various growth mechanisms under different growth conditions. We discovered that Indium droplets may also promote the growth of NWs with a similar role to that of catalytic Au nanoparticles. A potential growth model is accordingly described and discussed.

An optimum growth condition for straight and minimally tapered InSb NWs has been established. The as-grown NWs are investigated using Transmission Electron Microscopy (TEM), indicating a dominant <110> growth direction with the zincblende crystal structure. Current-voltage (I-V) measurements of single nanowire field effect transistors (NW-FET), suggest that NWs are of n-type.

A study of the effect of growth temperature on the growth direction of InSb NWs has been studied. A uniform growth direction of InSb NWs is achieved by optimizing the growth temperature profile. Results demonstrate that growth temperature plays an important role when controlling the NW growth direction similar to research on other types of III-V semiconductor NWs. We also investigated the large assembly of NW arrays aligned on the substrate using both in-situ controlled growth method and ex-situ contact printing method.

Finally, accurate electrical characterization of NWs and their ultimate implementation in electronics devices fabrication of metal contacts to semiconductor NWs with good ohmic behavior and low contact resistance has been studied. NW-FETs have been fabricated and I-V characteristic was measured to study the NW electrical properties and performance of devices. The improvements of the contact quality and other properties are discussed.

Our findings provide an in-depth understanding of semiconductor NW growth via chemical vapor deposition, and the first basis for further investigations on how to achieve controlled growth of individual and arrayed NWs, and novel device applications for switching and computation based on future development of nanoscale specific integrated circuits - NASICs.

From the standpoint of mainstream IC manufacturing, newly introduced two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), represented by molybdenum disulfide (MoS2), may turn out to be the game changer in the relay of MOSFET downscaling process. However, Schottky barrier formed at metal-semiconductor interface makes MoS2 transistors operate way below their capability. In the first part of this study, by depositing two kinds of contact metal combinations on the same MoS2 sample, the effect of Schottky and Ohmic contacts on the extrinsic field-effect mobility has been studied. The result indicates that non-optimal contacts can become the “bottleneck’ that hinders carrier transport, making transistors operate way below their intrinsic performance limit. A highly transparent M-S interface should be regarded as a prerequisite for stabilizing transistors in deep triode region and further mobility extraction. In the second part, a “passivation first, metallization second” technique is developed for fabricating edge contacts to MoS2 in two heterostructures - Al2O3/MoS2/SiO2 and h-BN/MoS2/h-BN. Electrostatic gating effect has been characterized through the configuration of back-gated FETs. A plasma etching step with volatile product, and subsequent smooth side wall profiles are found related to more efficient Ohmic-like channel conduction. This technique is applicable to both exfoliated and synthesized TMDs, and it presents a useful route for preserving the pristine quality of 2D semiconductor from material preparation to device characterization. In the end, strain is exerted to MoS2 channel by depositing a silicon nitride stress capping layer that covers the entire transistor. Current on/off ratio and other transistor performance metrics are measured as the transistor evolves from back-gate, to top-gate and finally, strain-gate configurations. A 58% increase in electron mobility and 46 % increase in on-current magnitude are observed in strain-gated, compared with top-gated transistors. This is the first study that directly links the strain effect to device performance of MoS2 top-gated transistors.

This dissertation focuses on investigating the synthesis of graphene and its hybrid nanostructures by chemical vapor deposition (CVD) process, as well as their applications in nanoelectronics and energy conversion/storage. The substantial understanding of interaction of graphene layers between the bio-molecules and gas molecules will enable the improvement in the applications of graphene in bio-sensor and chemical sensor. To achieve these objectives, graphene field effect transistors are fabricated to study the interactions of graphene between single stranded Deoxyribonucleic Acids (ssDNA) and gas molecules. ssDNA is found to act as negative potential gating agent that increases the hole density in single layer graphene (SLG). The molecular photodesorption and absorption from pristine and functionalized graphene are studied. The photodesorption induced current decrease in functionalized graphene by concentrated HNO3 becomes less significant than pristine graphene layers. We suggest this is due to the passivation of oxygen-bearing functionalities to CVD grown graphene structure defects via HNO3 functionalization, which prevents the further absorption of gas molecules.

The advance of synthesis of graphene and its hybrid nanostructures by CVD promotes their applications in the industrial level. We synthesize the graphene both on nickel thin film and copper foils by CVD, and investigate the grow kinetics, such as the effect of growth pressure on the uniformity and quality of Cu-grown graphene. By controlling the growth pressure we achieve uniform single layer graphene sheets and suggest the feasible methods of synthesizing uniform few-layer graphene. To decrease the sheet resistance, we stack the single layer graphene using layer-layer transfer technique. Highly concentrated HNO3 is employed to improve the conductivity and surface wettability of graphene layers. Four-layer graphene films with optical transmittance of 90% after HNO3 treatment are applied in organic solar cell as anode. Further, we report the fabrication of highly conductive, large surface-area 3D pillar graphene nanostructures (PGN) films from assembly of vertically aligned CNT pillars on flexible copper foils and directly employed for the application in electrochemical double layer capacitor (EDLC). The PGN films with MWCNTs on graphene layers, which were one-step synthesized on flexible copper foil (25 um) by CVD process, exhibit high conductivity with sheet resistance as low as 1.6 Ohm per square and high mechanical flexibility. The fabricated EDLC supercapacitor based on high surface-area PGN electrodes (563m2/g) showed high performance with high specific capacitance of 330F/g.

We demonstrate the synthesis and characterization of three dimensional heterostructures graphene hybrid nanostructures (HGN) comprising continuous large area graphene layers and ZnO nanostructures, fabricated via two-step CVD process. The material characterizations exhibit the highly crystalline ZnO nanostructures synthesized on few-layer graphene film. Ultraviolet spectroscopy and photoluminescence spectroscopy indicate that the CVD grown HGN layers has excellent optical properties. The electrical study on the graphene/ZnO/graphene devices suggests that graphene readily form Ohmic contact to ZnO, although rarely Shockley may exist. The electron transport properties highly depend on the temperature. Devices show photoconductive effect under UV illumination. A combination of electrical and optical properties of graphene and ZnO building blocks in ZnO based HGN provides unique characteristics for opportunities in future optoelectronic devices.

Nanostructured synthetic carbon allotropes are considered good candidates for applications in electronics and energy storage devices due to the unique combination of their physical and chemical properties. In this thesis, synthesis of nanostructured carbon materials including pillared CNT and graphene nanostructures (PGN), cone-shape CNT clusters (CCC), and graphene and CNT (GM) hybrid foam have been developed. Potential applications in electrochemical energy storage devices including supercapacitors (SCs) and lithium ion batteries (LIBs) have been demonstrated.

The seamlessly connected graphene and CNT pillars in PGN provide a relatively strong active material-current collector connection integrity, which facilitates conduction of electrons in the system. LIB based PGN electrode shows a reversible capacity of 900 mAh g-1. Moreover, with a subsequent inductively coupled plasma (ICP) treatment on PGN, an CCC structure is achieved. The cone-shape nature can be advantageous due to a decrease in homogeneity and in surface area of the array which provides interpenetrating channels and minimizes the effects of solid electrolyte interface (SEI) formation during lithiation and delithiation. LIBs based on Si-CCC architecture demonstrates high reversible capacity of 1954 mAh g-1.

Furthermore, a three-dimensional (3D) GM foam is grown on industrial grade metal foam foils and applied as electrodes for SCs. SC based on GM foam demonstrates a high specific capacitance of 286 F g-1 which leads to an energy density of 39.72 Wh kg-1 and a superior power density of up to 154.67 kW kg-1. Improved functionality of carbon materials via a non-invasive, high-throughput, and inexpensive UV generated ozone (UV-ozone) treatment has been developed and systematically studied. This process allows precise and continous tuning of the transition of GM foam from ultrahydrophobic to hydrophilicwithin 60 sec. Hydrothermally synthesized α-MnO2 nanowires are integrated onto the GM foam by a simple bath deposition which yields a monographical graphene foam conformally covered with intertwined, densely packed CNT/MnO2 nanowire nanocomposite network. SC based on GMM show an extended operational voltage window of 1.6 V in aqueous electrolyte. A high specific capacitance (1108.79 F g-1) is also achieved. The SC based 3D chrysanthemum-like carbon nanofiber (CNF) foam architecture demonstrates a high areal capacitance of 1.37 F/cm2 (gravimetric specific capacitance: 23.83 Fg-1), which leads to superior values for per-area energy density (0.19 Wh cm-2) and power density (141.77 W cm-2).

Research on two-dimensional nanomaterials has become a topic of considerable interest since the pioneering work experimentally introducing the two-dimensional carbon allotrope of graphene in 2004. The atomically thin hexagonally arranged carbon crystal structure has offered the opportunity for numerous studies in condensed matter physics and materials science, revealing new phenomenon and remarkable properties. Graphene has excellent chemical and mechanical stability, allowing researchers to probe the properties of graphene in a wide variety of applications and in contact with a wide variety of materials. Ballistic transport of graphene at room temperature suggests that graphene would be poised to enter in to a wide variety of microelectronic application;

vii

however, synthesis methods and surface effects have so far limited the widespread use of graphene. Additionally, the absence of electronic band gap in graphene, classifying it as a "semi-metal", limits the use of graphene to areas other than logic applications.

In this work, fabrication methods for the improved synthesis graphene and selected two-dimensional transition metal dichalcogenides, molybdenum disulfide and tungsten disulfide, are presented for thin film transistor applications. First, the introduction of thin film zwitterionic polymer interlayers in graphene devices is outlined as a means to reduce the contact resistance between metal contacts and the underlying graphene layer. Second, a self-assembly nanoscale lithography process utilizing diblock copolymer templates as an etching mask directly on the surface of graphene is shown as a method to introduce a band gap in graphene due to quantum confinement effects. The third chapter applies to another class of two-dimensional materials, transition metal dichalcogenides, which, unlike graphene, can exhibit suitable electronic band structures for logic applications. When the thickness of these transition metal dichalcogenides is reduced to a single atomic layer, electronic band states transition from an indirect band gap to a direct band gap. A wafer-scale method for the synthesis of atomically thin transition metal dichalcogenides by the thermolysis of spin coated precursors is introduced which holds promise for next-generation low-power consumption applications.

Over the past two decades, there has been extensive development in the field of renewable energy and energy storage technologies in an effort to keep up with the pace of continuously rising global energy demand as the world abroad continues to develop into technology and energy driven societies. The race for renewables especially is driven by the ever-looming threat of climate change and rising global temperatures caused by greenhouse gas emissions from fossil fuels and natural gas power sources. Renewable energy generation infrastructure and technology has grown dramatically since the early 2000s to the point that we are getting close to potential overgeneration for energy grids without the sufficient energy storage technologies to back up any excess power generation during the peak generation times of the day for solar and wind technologies. While renewable energy technology has advanced drastically, the same pace has not been kept with energy storage technologies especially in the battery field with the last major development being the Li-ion battery cells. As the demand has continued to grow with new consumer technologies such as phones, computers, and electric vehicles, we have continued to deplete the natural resources required for creating these batteries such as lithium and cobalt and need to start looking elsewhere to alternatives. 2D materials have been a promising avenue for many different technology sectors and batteries are no exception. MXenes are a new class of 2D material that shows promise as a new candidate for energy storage technologies that has the potential to revolutionize this field, but suffers from a lack of scalable synthesis methods to make the material viable on a large industrial scale. In this study, we will be evaluating a novel sol-gel method to synthesize MXenes that has the potential to be scalable with an emphasis on being a renewable and environmentally friendly process. While there is still much work to be done to make this a reality, this will lay the groundwork and foundation to bring this idea to fruition.

As the demand for higher capacity, longer lasting lithium ion battery rises, finding a new material system that can replace the current commercial lithium ion battery system has become the necessity. Out of all possible candidates, nickel oxide, silicon shows potential for next generation anode material, and sulfur promises great improvement if used as the cathode material. In this work, new lithium ion battery systems utilizing nickel oxide, silicon, and sulfur were developed and studied using both physical and electrochemical characterization techniques. The free-standing nickel oxide nanofiber cloth anode shows a high capacity of 1054 mAh/g cycling at 20 minuets per charge. It also shows a cycle life of over 1500 cycles. The novel silicon sulfur full cell architecture presents a functioning silicon sulfur system that does not require prelithiation and shows a energy density of 350 Wh/kg for 250 cycles. The novel plateau targeted conditioning method for sulfur half cells shows a 10% increase in battery capacity and great increase in battery stability, as well as proof of stable sei formation on both the anode and cathode.