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Open Access Publications from the University of California

Scholarly Works (12 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2013)

Understanding electron and phonon transport in silicon nanostructures is essential for developing advanced electronic and thermoelectric systems. As opposed to electrons that contribute to conduction only near the Fermi surface, phonons show broadband spectrum when they transport heat in silicon. By controlling the dimension and morphology and accounting for characteristic length scale of electrons and phonons, we can engineer their transport properties. In this work, the transport of electrons and phonons in silicon nanostructures was investigated to improve future designs of thermoelectrics as well as nanoelectronics and phononics.

Phonon scattering at boundaries in solid is considered diffusive or specular depending on the wavelength of phonons and the length scales of surface roughness. Thermal conductivity will be at minimum when the phonon scattering is purely diffusive, which is known as the Casimir limit. However, in rough silicon nanowires, the thermal conductivity was found to break the limit. Hence, the role of the roughness parameters (rms, correlation length and p, high frequency roughness factor) of etched VLS nanowires was systematically addressed for the suppression of thermal conductivity. The surface roughness, characterized by amplitude (rms) and lateral length scale of roughness (correlation length), is demonstrated to dictate phonon transport in such nanowires down to 5W/mK. More interestingly, high frequency spectrum window of surface roughness close to dominant phonon wavelength (1 - 10 nm) at room temperature was chosen to correlate to thermal conductivity and it showed better the trend with the high frequency roughness factor. Electroless etched (EE) nanowires regardless of their unique irregular morphologies also further have been shown to follow the trend, which indicates sub-diffusive regime of phonon is mainly governed by high frequency surface roughness.

The measurement platforms where both electrical and thermal properties can be characterized for the same holy silicon (HS) ribbon were first developed. It was unclear how the morphology, the doping concentration, and the dimensions affect the transport properties of nanostructures mainly for thermoelectric applications. Hence we developed this platform for HS ribbons known for enhancing thermoelectric performance. Electron and phonon transport was independently controlled by tuning the neck size, which is the limiting distance between adjacent holes. Owing to accurate thermal conductivity characterization by the superior device design, the necking effect was quantified with a fixed pitch distance. The thermal conductivity in this structure was found below the incoherent phonon scattering regime due to the periodicity in nano-hole array, which possibly opens the new realm of coherent phonon scattering.

Finally, the first demonstration of ballistic phonon transport in the cross-plane direction of silicon has been shown with HS structures. We showed that long wavelength phonons can propagate ballistically up to a few hundred nanometers despite the lateral dimension of 20nm. The fundamental understanding in thermal conduction has direct relevance for heat management in silicon based electronics and thermoelectrics. Furthermore, phononic devices could potentially be realized via ballistic phonon components in silicon nanostructures.

    Cover page: Electron and phonon transport in silicon nanostructures
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2015)
    When the size of semiconductors is smaller than the phonon mean free path, phonons can carry heat with no internal scattering. Ballistic phonon transport has received attention for both theoretical and practical aspects because Fourier's law of heat conduction breaks down and the heat dissipation in nanoscale transistors becomes unpredictable in the ballistic regime. While recent experiments demonstrate room-temperature evidence of ballistic phonon transport in various nanomaterials, the thermal conductivity data for silicon in the length scale of 10-100 nm is still not available due to experimental challenges. Here we show ballistic phonon transport prevails in the cross-plane direction of holey silicon from 35 to 200 nm. The thermal conductivity scales linearly with the length (thickness) even though the lateral dimension (neck) is as narrow as 20 nm. We assess the impact of long-wavelength phonons and predict a transition from ballistic to diffusive regime using scaling models. Our results support strong persistence of long-wavelength phonons in nanostructures and are useful for controlling phonon transport for thermoelectrics and potential phononic applications.
      Cover page: Ballistic Phonon Transport in Holey Silicon
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2014)
      Studying bacteria-nanostructure interactions is crucial to gaining controllable interfacing of biotic and abiotic components in advanced biotechnologies. For bioelectrochemical systems, tunable cell-electrode architectures offer a path toward improving performance and discovering emergent properties. As such, Sporomusa ovata cells cultured on vertical silicon nanowire arrays formed filamentous cells and aligned parallel to the nanowires when grown in increasing ionic concentrations. Here, we propose a model describing the kinetic and the thermodynamic driving forces of bacteria-nanowire interactions.
        Cover page: Salt-Induced Self-Assembly of Bacteria on Nanowire Arrays
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2014)
          Cover page: Salt-induced self-assembly of bacteria on nanowire arrays
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2018)
          Future solar-to-chemical production will rely upon a deep understanding of the material-microorganism interface. Hybrid technologies, which combine inorganic semiconductor light harvesters with biological catalysis to transform light, air, and water into chemicals, already demonstrate a wide product scope and energy efficiencies surpassing that of natural photosynthesis. But optimization to economic competitiveness and fundamental curiosity beg for answers to two basic questions: (1) how do materials transfer energy and charge to microorganisms, and (2) how do we design for bio- and chemocompatibility between these seemingly unnatural partners? This Perspective highlights the state-of-the-art and outlines future research paths to inform the cadre of spectroscopists, electrochemists, bioinorganic chemists, material scientists, and biologists who will ultimately solve these mysteries.
            Cover page: Physical Biology of the Materials–Microorganism Interface
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2016)
            Thermal transport in silicon nanowires has captured the attention of scientists for understanding phonon transport at the nanoscale, and the thermoelectric figure-of-merit (ZT) reported in rough nanowires has inspired engineers to develop cost-effective waste heat recovery systems. Thermoelectric generators composed of silicon target high-temperature applications due to improved efficiency beyond 550 K. However, there have been no studies of thermal transport in silicon nanowires beyond room temperature. High-temperature measurements also enable studies of unanswered questions regarding the impact of surface boundaries and varying mode contributions as the highest vibrational modes are activated (Debye temperature of silicon is 645 K). Here, we develop a technique to investigate thermal transport in nanowires up to 700 K. Our thermal conductivity measurements on smooth silicon nanowires show the classical diameter dependence from 40 to 120 nm. In conjunction with Boltzmann transport equation, we also probe an increasing contribution of high-frequency phonons (optical phonons) in smooth silicon nanowires as the diameter decreases and the temperature increases. Thermal conductivity of rough silicon nanowires is significantly reduced throughout the temperature range, demonstrating a potential for efficient thermoelectric generation (e.g., ZT = 1 at 700 K).
              Cover page: Thermal Transport in Silicon Nanowires at High Temperature up to 700 K
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2014)
              Semiconductor nanowires (NWs) have been studied extensively for over two decades for their novel electronic, photonic, thermal, electrochemical and mechanical properties. This comprehensive review article summarizes major advances in the synthesis, characterization, and application of these materials in the past decade. Developments in the understanding of the fundamental principles of "bottom-up" growth mechanisms are presented, with an emphasis on rational control of the morphology, stoichiometry, and crystal structure of the materials. This is followed by a discussion of the application of nanowires in i) electronic, ii) sensor, iii) photonic, iv) thermoelectric, v) photovoltaic, vi) photoelectrochemical, vii) battery, viii) mechanical, and ix) biological applications. Throughout the discussion, a detailed explanation of the unique properties associated with the one-dimensional nanowire geometry will be presented, and the benefits of these properties for the various applications will be highlighted. The review concludes with a brief perspective on future research directions, and remaining barriers which must be overcome for the successful commercial application of these technologies.
                Cover page: 25th Anniversary Article: Semiconductor Nanowires – Synthesis, Characterization, and Applications
                • Article
                • Peer Reviewed
                UC Merced Previously Published Works (2020)
                In recent years, numerous effective multi-object tracking (MOT) methods are developed because of the wide range of applications. Existing performance evaluations of MOT methods usually separate the object tracking step from the object detection step by using the same fixed object detection results for comparisons. In this work, we perform a comprehensive quantitative study on the effects of object detection accuracy to the overall MOT performance, using the new large-scale University at Albany DETection and tRACking (UA-DETRAC) benchmark dataset. The UA-DETRAC benchmark dataset consists of 100 challenging video sequences captured from real-world traffic scenes (over 140,000 frames with rich annotations, including occlusion, weather, vehicle category, truncation, and vehicle bounding boxes) for object detection, object tracking and MOT system. We evaluate complete MOT systems constructed from combinations of state-of-the-art object detection and object tracking methods. Our analysis shows the complex effects of object detection accuracy on MOT system performance. Based on these observations, we propose new evaluation tools and metrics for MOT systems that consider both object detection and object tracking for comprehensive analysis.
                  Cover page: UA-DETRAC: A new benchmark and protocol for multi-object detection and tracking
                  • Article
                  • Peer Reviewed
                  LBL Publications (2015)
                    Cover page: Electrode Lithiation: Effects of Particle Size, Electronic Connectivity, and Incoherent Nanoscale Domains on the Sequence of Lithiation in LiFePO4 Porous Electrodes (Adv. Mater. 42/2015)
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2016)
                    Block copolymer patterned holey silicon (HS) was successfully integrated into a microdevice for simultaneous measurements of Seebeck coefficient, electrical conductivity, and thermal conductivity of the same HS microribbon. These fully integrated HS microdevices provided excellent platforms for the systematic investigation of thermoelectric transport properties tailored by the dimensions of the periodic hole array, that is, neck and pitch size, and the doping concentrations. Specifically, thermoelectric transport properties of HS with a neck size in the range of 16-34 nm and a fixed pitch size of 60 nm were characterized, and a clear neck size dependency was shown in the doping range of 3.1 × 10(18) to 6.5 × 10(19) cm(-3). At 300 K, thermal conductivity as low as 1.8 ± 0.2 W/mK was found in HS with a neck size of 16 nm, while optimized zT values were shown in HS with a neck size of 24 nm. The controllable effects of holey array dimensions and doping concentrations on HS thermoelectric performance could aid in improving the understanding of the phonon scattering process in a holey structure and also in facilitating the development of silicon-based thermoelectric devices.
                      Cover page: Simultaneous Thermoelectric Property Measurement and Incoherent Phonon Transport in Holey Silicon
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