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2D Dirac Materials: From Graphene to Topological Insulators

  • Author(s): Teweldebrhan, Desalegne Bekuretsion
  • Advisor(s): Balandin, Alexander A
  • et al.

Silicon has been reaching physical limits as the semiconductor industry moves to smaller device feature sizes, increased integration densities and faster operation speeds. There is a strong need to engineer alternative materials, which can become foundation of new computational paradigms or lead to other applications such as efficient solid-state energy conversion. Recently discovered Dirac materials, which are characterized by the liner electron dispersion, are examples of such alternative materials. In this dissertation, I investigate two representatives of Dirac materials - graphene and topological insulators. Specifically, I focus on the (i) effects of electron beam irradiation on graphene properties and (ii) electronic and thermal characteristics of exfoliated films of Bi2Te3-family of topological insulators. I carried out Raman investigation of changes in graphene crystal lattice induced by the low and medium energy electron-beam irradiation (5-20 keV). It was found that radiation exposures result in appearance of the disorder D band around 1345 cm−1. The dependence of the ratio of the intensities of D and G peaks, I(D)/I(G), on the irradiation dose is non-monotonic suggesting graphene's transformation to polycrystalline and then to disordered state. By controlling the irradiation dose one can change the carrier mobility and increase the resistance at the minimum conduction point. The obtained results may lead to new methods of defect engineering of graphene properties. They also have important implications for fabrication of graphene nanodevices, which involve electron beams. Bismuth telluride and related compounds are the best thermoelectric materials known today. Recently, it was determined that they reveal the topological insulator properties. We succeeded in the first "graphene-like" exfoliation of large-area crystalline films and ribbons of Bi2Te3 with the thickness going down to a single quintuple. The presence of van der Waals gaps allowed us to disassemble Bi2Te3 crystal into the five mono-atomic sheets consisting of Te(1)-Bi-Te(2)-Bi-Te(1). The exfoliated films had extremely low thermal conductivity and electrical resistance in the range required for thermoelectric applications. The obtained results may pave the way for producing Bi2Te3 films and stacked superlattices with strong quantum confinement of charge carriers and predominantly surface transport, and allow one to obtain theoretically predicted order-of-magnitude higher thermoelectric figure-of-merit.

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