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Departmental Oral Examination (Thesis Title: “Tuning Graphene’s Electronic and Transport Properties via Adatom Deposition”)
Event Date and Time:Wed, 2017-02-22 12:00 - 15:00
Local Contact:Physics and Astronomy, UBC
This thesis investigates the effect of adatom deposition, especially alkali and heavy adatoms on graphene’s electronic and transport properties. 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 ultra high vacuum (UHV) conditions. After adatom deposition, continuous measurements of their resistance at cryogenic temperature are made with applying variable perpendicular magnetic field.
The doping efficiency of lithium deposited at cryogenic temperatures on epitaxial and CVD monolayer graphene has been investigated under UHV conditions. Change of charge carrier density was monitored by gate voltage shift of the Dirac point and Hall measurements, in low and high doping regimes. It was found that pre-annealing the graphene greatly enhanced the maximum levels of doping that could be achieved: doping saturated at Dn = 2x10^13 cm^-2 without annealing, independent of sample type or previous processing; after a 900 K anneal, the saturated doping rose one order of magnitude to Dn = 2x10^14 cm^-2. For the samples which were annealed to 700 K/900 K prior to cryogenic Li deposition, long-range Coulomb interaction due to Li adatoms shows itself in change of mobility while its short-range interaction caused dramatic decline of the intervalley length, a result that contradicts the naive expectation that alkali adatoms on graphene cannot cause short-range scattering. This report is the first experimental evidence of short-range scattering due to alkali adatoms in graphene and its effect on intervalley scattering. In addition to intervalley scattering, the theoretically predicted superconductivity of Li doped graphene has been investigated. No signatures of superconductivity were observed down to 3 K.
In addition to lithium as a light adatom, the effect of two heavy adtoms, indium and copper, on doping and affecting spin-orbit interaction in graphene was probed.