Final PhD Oral Examination (Thesis Title: “Tuning Graphene’s Electronic and Transport Properties via Adatom Deposition”)

Speaker: 
ALI KHADEMI
Event Date and Time: 
Wed, 2017-07-26 12:30 - 14:30
Location: 
Room 202, Anthropology and Sociology Building
Local Contact: 
Physics and Astronomy, UBC
Intended Audience: 
Public

Abstract:
This thesis investigates the effect of adatom deposition, especially alkali and heavy adatoms, on graphene’s electronic and transport properties. While there are many theoretical predictions for tuning graphene’s properties via adatom deposition, only a few of them have been observed. Solving this enigma of inconsistency between theory and experiment raises the need for deeper experimental investigation of this matter. To achieve this goal, an experimental set up was built which enables us to evaporate different metal adatoms on graphene samples while they are at cryogenic temperatures and under ultra-high vacuum (UHV) conditions.
The critical role of in situ high-temperature annealing in creating reliable interactions between adatoms and graphene is observed. This contradicts the commonly accepted assumption in the transport community that placing a graphene sample in UHV and performing an in situ bakeout at 400-500 K is enough to provide a reliable adatom-graphene interaction. Even charge doping by alkali atoms (Li), which is arguably the simplest of all adatom effects, cannot be achieved completely without in situ 900 K annealing. This observation may explain the difficulty many groups have faced in inducing superconductivity, spin-orbit interaction, or similar electronic modifications to graphene by adatom deposition, and it points toward a straightforward, if experimentally challenging, solution.
The first experimental evidence of short-range scattering due to alkali adatoms in graphene is presented in my Ph.D. thesis, a result that contradicts the naive expectation that alkali adatoms on graphene only cause long-range Coulomb scattering. The induced short-range scattering by Li caused decline of intervalley time and length (i.e., enhancement of intervalley scattering). No signatures of theoretically predicted superconductivity of Li doped graphene were observed down to 3 K.
Cryogenic deposition of copper increased the dephasing rate of graphene. This increase in dephasing rate is either a sign of inducing Kane-Mele spin-orbit interaction or magnetic moments by copper. No similar effect was observed for indium.

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