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Open Access Publications from the University of California

Scholarly Works (3 results)

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

Graphene has become one of the most widely researched materials lately since its discovery in 2004. It has interested scientists due to the exceptional electronic properties of the relativistic Dirac fermions. In this thesis I will present the research I have done to study the charged impurity scattering mechanism in graphene by placing a device on a high dielectric material. An entire device with metal electrodes can be fabricated on SiO2 substrates and transferred to any arbitrary substrate using the device transfer method that I have developed. This method eliminates the need to locate single layer graphene with optical microscopy for alignment and patterning on substrates other than 300nm thick SiO2 which may be very difficult. The target substrate I used is 200 micron thick SrTiO3 (STO). The dielectric constant is two orders of magnitude higher than SiO2 at room temperature and increases to 5,000 to 10,000 at cryogenic temperatures. We expected to observe an associated increase in the mobility with the increased dielectric screening of charged impurities. The mobility is only affected around the Dirac point which is in agreement with theory. An unusual gate dependent hysteretic effect and time dependence is also observed and explained using a surface dipole model of STO. It is graphene that is sensitive to the surface states of the STO.

    Cover page: Electron Transport and Scattering in Graphene Devices Transferred to SrTiO3 Substrates
    • Article
    • Peer Reviewed
    UC Riverside Previously Published Works (2014)
    The electrical transport properties of graphene are greatly influenced by its environment. Owing to its high dielectric constant, strontium titanate (STO) is expected to suppress the long-range charged impurity scattering and consequently to enhance the mobility. However, the absence of such enhancement has caused some controversies regarding the scattering mechanism. In graphene devices transferred from SiO2 to STO using a newly developed technique, we observe a moderate mobility enhancement near the Dirac point, which is the point of charge neutrality achieved by adjusting the Fermi level. While bulk STO is not known as a ferroelectric material, its surface was previously reported to exhibit an outward displacement of oxygen atoms and ferroelectric-like dipole moment. When placed on STO, graphene shows strong and asymmetric hysteresis in resistivity, which is consistent with the dipole picture associated with the oxygen displacement. The hysteretic response of the surface dipole moment diminishes the polarizability, therefore weakens the ability of STO to screen the Coulomb potential of the impurities.
      Cover page: Ferroelectric-like SrTiO3 surface dipoles probed by graphene
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2018)
      Between 1982 and 1986, 126 patients who had undergone ACL reconstruction were followed in a prospective manner. One year follow-up statistics were reviewed for the presence of 13 different complications. The most prevalent complications were quadriceps weakness, flexion contracture, and patellofemoral pain. Quadriceps weakness (strength less than 80% of the normal side) was present in 65% of patients and correlated positively with flexion contracture, patellar irritability, and ACL reconstructions using patellar tendon grafts. Flexion contracture of 5° or more was present in 24% of patients and correlated positively with increased age and patellar irritability. Patellofemoral pain was present in 19% of patients and correlated positively with flexion contracture. Clinical relevance: The three most common complications of knee ligament surgery are shown to be strongly interrelated. It is likely that a causal relationship is present in which flexion contracture causes patellofemoral irritability, and that both of these factors, alone or in combination, result in quadriceps weakness. If this theory is correct, then it is crucial that postoperative rehabilitation programs place a major emphasis on the avoidance of flexion contracture.
        Cover page: Patellofemoral problems after anterior cruciate ligament reconstruction.
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