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Open Access Publications from the University of California

Scholarly Works (60 results)

  • Article
  • Peer Reviewed
LBL Publications (2018)
Macroscale arrays of cobalt oxide-silica core-shell nanotubes with high aspect ratio and ultrathin walls of less than 20 nm have been fabricated. The silica shells feature embedded oligo-para(phenylenevinylene) molecules for charge transport across the insulating silica layer, which is tightly controlled by their electronic properties. The assembly is based on the use of a sacrificial Si nanorod array template combined with atomic layer deposition, covalent anchoring of organic wire molecules, and dry cryo-etching. High-resolution TEM imaging of samples prepared by microtome affords structural details of single core-shell nanotubes. The integrity of silica-embedded organic wire molecules exposed to atomic layer deposition, thermal treatment, and harsh etching procedures is demonstrated by grazing angle ATR FT-IR, FT-Raman, and XPS spectroscopy. The inorganic oxide-based core-shell nanotubes with ultrathin gas-impermeable, proton-conducting silica shells functionalized by molecular wires enable complete nanoscale photosynthetic units for CO2 reduction by H2O under membrane separation. Arrays of massive numbers of such core-shell nanotube units afford a design that extends the separation of the incompatible H2O oxidation and CO2 reduction catalysis environments across the continuum of length scales from nanometers to centimeters.
    Cover page: Fabrication of Core–Shell Nanotube Array for Artificial Photosynthesis Featuring an Ultrathin Composite Separation Membrane
    • Thesis
    • Peer Reviewed
    UC Berkeley Electronic Theses and Dissertations (2019)

    Nanoscale material systems are of great importance for answering open questions in funda- mental science and enabling advancements in next-generation technologies. This dissertation explores the structural, electronic, and optical properties of several nanomaterial systems at the atomic-, nano-, and micro- length scales. A collection of analytical characterization tools, non-trivial imaging methods, and advanced data analysis techniques are presented. This methodology is essential for finding and understanding new physics in nanomaterial systems; it is also an important tool for characterizing new nanomaterial systems and determining the relationship between synthetic parameters and relevant material properties. Applications of these techniques to a wide array of scientific questions are given throughout the thesis.

    A newly discovered source of highly localized light emission in hexagonal boron nitride (h-BN) is presented. This luminescent color center is closely related to unique impurities and native defects that occur exclusively in high-quality samples grown using a special high-pressure, high-temperature synthetic routine. An electron beam is used as a probe to characterize the luminescence, as well as activate and deactivate the light emission. The electron beam can also be used to deterministically create arbitrary nanoscale light emission patterns in h-BN synthesized using more conventional methods. A variety of h-BN polytypes are discussed and the effects of growth parameters, geometry, dimensionality, impurity concentration, and sample preparation on light emission are discussed.

    Transition-metal dichalcogenides (TMDs) are an exciting class of Van der Waals (VDW) materials that display a variety of unique phenomena in their 2D (monolayers) and 1D (nanotubes and nanoribbons) forms that are absent in the bulk material. A special synthesis method which allows for targeted growth of specific nanostructures is presented along with characterization of the TMD nanomaterials. A combination of special transmission electron microscopy (TEM) techniques and advanced data analysis methodology allows for rapid determination of the grain structure of continuous, nanocrystalline, monolayer TMD films.

    Gallium chalcogenides (GCs) are another class of VDW materials, closely related to TMDs, that are of significant interest to the nanoscience community. The type and concentration of chalcogen atoms in these materials can be varied, resulting in a alloys with tunable structural, electronic, and optical properties. A combination of TEM, angle-resolved photoemission spectroscopy (ARPES), and photoluminescence (PL) is used to fully characterize the quasiparticle band structure, optical bandgap, and crystalline phase of the alloys across their compositional range. Monolayers of these materials exhibit a distinct change in band structure, resulting in the material transitioning from direct bandgap in the bulk to indirect bandgap in the 2D limit.

    The characterization and analysis methods discussed here are broadly applicable to a wide variety of nanomaterial systems. Widespread adoption of machine learning and automation in the nanoscience community is a promising route toward high-throughput scientific inquiry.

      Cover page: Hyperspectral Imaging and Machine Learning for Nanomaterials Characterization
      • Article
      • Peer Reviewed
      LBL Publications (2019)
      As one of the highest mobility semiconductor materials, carbon nanotubes (CNTs) have been extensively studied for use in field effect transistors (FETs). To fabricate surround-gate FETs- which offer the best switching performance-deposition of conformal, weakly-interacting dielectric layers is necessary. This is challenging due to the chemically inert surface of CNTs and a lack of nucleation sites-especially for defect-free CNTs. As a result, a technique that enables integration of uniform high-k dielectrics, while preserving the CNT's exceptional properties is required. In this work, we show a method that enables conformal atomic layer deposition (ALD) of high-k dielectrics on defect-free CNTs. By depositing a thin Ti metal film, followed by oxidation to TiO2 under ambient conditions, a nucleation layer is formed for subsequent ALD deposition of Al2O3. The technique is easy to implement and is VLSI-compatible. We show that the ALD coatings are uniform, continuous and conformal, and Raman spectroscopy reveals that the technique does not induce defects in the CNT. The resulting bilayer TiO2/Al2O3 thin-film shows an improved dielectric constant of 21.7 and an equivalent oxide thickness of 2.7 nm. The electrical properties of back-gated and top-gated devices fabricated using this method are presented.
        Cover page: Conformal High-K Dielectric Coating of Suspended Single-Walled Carbon Nanotubes by Atomic Layer Deposition.
        • Article
        LBL Publications (2015)
          Cover page: Modular Synthesis of a Dual Metal-Dual Semiconductor Nano-Heterostructure
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2016)
          Clay swelling is a colloidal phenomenon that has a large influence on flow and solute migration in soils and sediments. While models for clay swelling have been proposed over many years, debate remains as to the interaction forces that combine to produce the observed swelling behavior. Using cryogenic transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering, we study the influence of salinity, in combination with layer charge, interlayer cation, and particle size, on montmorillonite swelling. We observe a decrease in swelling with increased layer charge, increased cation charge, and decreased cation hydration, each indicative of the critical influence of Coulombic attraction between the negatively charged layers and interlayer cations. Cryo-TEM images of individual montmorillonite particles also reveal that swelling is dependent upon the number of layers in a particle. Calculations of the van der Waals (vdW) interaction based on new measurements of Hamaker coefficients confirm that long-range vdW interactions extend beyond near-neighbor layer interactions and result in a decrease in layer spacing with a larger number of layers. This work clarifies the short- and long-range attractive interactions that govern clay structure and ultimately the stability and permeability of hydrated clays in the environment.
            Cover page: Short- and Long-Range Attractive Forces That Influence the Structure of Montmorillonite Osmotic Hydrates
            • Article
            • Peer Reviewed
            LBL Publications (2019)
            As one of the highest mobility semiconductor materials, carbon nanotubes (CNTs) have been extensively studied for use in field effect transistors (FETs). To fabricate surround-gate FETs- which offer the best switching performance-deposition of conformal, weakly-interacting dielectric layers is necessary. This is challenging due to the chemically inert surface of CNTs and a lack of nucleation sites-especially for defect-free CNTs. As a result, a technique that enables integration of uniform high-k dielectrics, while preserving the CNT's exceptional properties is required. In this work, we show a method that enables conformal atomic layer deposition (ALD) of high-k dielectrics on defect-free CNTs. By depositing a thin Ti metal film, followed by oxidation to TiO2 under ambient conditions, a nucleation layer is formed for subsequent ALD deposition of Al2O3. The technique is easy to implement and is VLSI-compatible. We show that the ALD coatings are uniform, continuous and conformal, and Raman spectroscopy reveals that the technique does not induce defects in the CNT. The resulting bilayer TiO2/Al2O3 thin-film shows an improved dielectric constant of 21.7 and an equivalent oxide thickness of 2.7 nm. The electrical properties of back-gated and top-gated devices fabricated using this method are presented.
              Cover page: Conformal High-K Dielectric Coating of Suspended Single-Walled Carbon Nanotubes by Atomic Layer Deposition
              • Article
              • Peer Reviewed
              UC Santa Cruz Previously Published Works (2016)
              Nature Communications 7:10804 Article number 10804 (2016); Published 23 February 2016; Updated 18 March 2016 The financial support for this Article was not fully acknowledged. The Acknowledgements should have included the following: The authors gratefully acknowledge research support from the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, under Contract No.
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2023)
                Carrier and phonon dynamics in a multilayer WSe2 film are captured by extreme ultraviolet (XUV) transient absorption (TA) spectroscopy at the W N6,7, W O2,3, and Se M4,5 edges (30-60 eV). After the broadband optical pump pulse, the XUV probe directly reports on occupations of optically excited holes and phonon-induced band renormalizations. When compared with density functional theory calculations, XUV transient absorption due to holes is identified below the W O3 edge whereas signals at the Se M4,5 edges are dominated by phonon dynamics. Therein, 0.4 ps hole relaxation time, 1.5 ps carrier recombination time, and 1.7 ps phonon heating time are extracted. The acquisition of hole and phonon-induced signals in a single experiment can facilitate the investigation of the correlations between electron and phonon dynamics. Furthermore, the simultaneous observation of signals from different elements can be further extended to explore photochemical processes in multilayers and alloys, thereby providing key information for their applications in electronics, photocatalysts, and spintronics.
                  Cover page: Carrier and Phonon Dynamics in Multilayer WSe2 Captured by Extreme Ultraviolet Transient Absorption SpectroscopyCreative Commons 'BY-NC' version 4.0 license
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2023)
                  Photoelectrochemical carbon-dioxide reduction (PEC CO2R) is a potentially attractive means for producing chemicals and fuels using sunlight, water, and carbon dioxide; however, this technology is in its infancy. To date, most studies of PEC CO2R have reported products containing one carbon atom (C1 products) but the production of valuable products containing two or more carbons (C2+ products), such as ethylene, ethanol, etc., is rarely demonstrated. Metal-semiconductor-insulator (MIS) photocathode/catalyst structures offer a promising approach for this purpose, since they integrate the functions of light absorption, charge separation, and catalysis. In this study, we have investigated a Cu/TiO2/p-Si photocathode/catalyst structure with the aim of establishing the effects of semiconductor-insulator interactions on the performance of the photocathode and the influence of the direction of illumination of the MIS structure on the total current density and the distribution of products formed by on the Cu catalyst. We have also examined the influence of ionomer coatings deposited on the Cu surface on the total current density and the distribution of products formed. A major finding is that for a fixed Cu potential the distribution of products formed by PEC CO2R are the same, irrespective of the direction of illumination, and are identical to those obtained by electrochemical reduction of CO2 (EC CO2R). Another important finding is that the total current density and the faradaic efficiency to ethylene are enhanced significantly by deposition of a thin bilayer of Sustainion/Nafion onto the surface of the Cu.
                    Cover page: Codesign of an integrated metal–insulator–semiconductor photocathode for photoelectrochemical reduction of CO 2 to ethyleneCreative Commons 'BY-NC' version 4.0 license
                    • Article
                    • Peer Reviewed
                    UCLA Previously Published Works (2015)
                    Modern microelectronic devices have nanoscale features that dissipate power nonuniformly, but fundamental physical limits frustrate efforts to detect the resulting temperature gradients. Contact thermometers disturb the temperature of a small system, while radiation thermometers struggle to beat the diffraction limit. Exploiting the same physics as Fahrenheit's glass-bulb thermometer, we mapped the thermal expansion of Joule-heated, 80-nanometer-thick aluminum wires by precisely measuring changes in density. With a scanning transmission electron microscope and electron energy loss spectroscopy, we quantified the local density via the energy of aluminum's bulk plasmon. Rescaling density to temperature yields maps with a statistical precision of 3 kelvin/hertz(-1/2), an accuracy of 10%, and nanometer-scale resolution. Many common metals and semiconductors have sufficiently sharp plasmon resonances to serve as their own thermometers.
                      Cover page: Thermal measurement. Nanoscale temperature mapping in operating microelectronic devices.
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