THESIS
2014
xviii, 119 pages : illustrations (some color) ; 30 cm
Abstract
This thesis mainly focuses on the mesoscopic transport properties of nanostructured graphene,
in particular, the antidot graphene, quantum dot graphene and substitutionally boron-doped
graphene.
For antidot graphene, the most important result is the discovery of the very large dephasing
length—up to 10 microns at 2 K. Owing to the large dephasing length, two dimensional (2D)
strong Anderson localization of electrons in graphene was observed for the first time.
Anderson localization is one of the most important physical phenomena caused by the wave
nature of quantum particles. It was originally proposed for the electronic system, but never
clearly observed because the wave nature of electrons is usually only manifest at extremely
small distances, denoted the dephasing length, an...[
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This thesis mainly focuses on the mesoscopic transport properties of nanostructured graphene,
in particular, the antidot graphene, quantum dot graphene and substitutionally boron-doped
graphene.
For antidot graphene, the most important result is the discovery of the very large dephasing
length—up to 10 microns at 2 K. Owing to the large dephasing length, two dimensional (2D)
strong Anderson localization of electrons in graphene was observed for the first time.
Anderson localization is one of the most important physical phenomena caused by the wave
nature of quantum particles. It was originally proposed for the electronic system, but never
clearly observed because the wave nature of electrons is usually only manifest at extremely
small distances, denoted the dephasing length, and therefore making its observation very difficult. Here through exponential sample size scaling of conductance, the strong electron
localization was demonstrated in three sets of nanostructured antidot graphene samples. The
localization lengths observed are 1.1, 2.0, and 3.4 μm. The localization length was observed
to increase with applied magnetic field, in accurate agreement with the theoretical prediction.
The large-scale mesoscopic transport is manifest as a parallel conduction channel to 2D
variable range hopping (VRH), with a Coulomb quasigap centered around the Fermi level.
The opening of the correlation quasigap, observable below 25 K through the temperature
dependence of hopping conductance, makes possible the exponential suppression of inelastic
scatterings and thereby leads to an observed dephasing length of 10 μm.
The transport characteristics in graphene quantum dot are described in the second part of
thesis, with the focus on the temperature and bias voltage dependence of conductance. For a
single graphene quantum dot, the conductance displays a periodic peak-valley behavior as a
function of the gate voltage. While the valley conductance exhibits the fluctuation-induced
tunneling conduction behavior; a clear power-law behavior was found for the peak
conductance, as a function of both temperature and bias voltage. The latter is in agreement
with the Luttinger-liquid behavior.
The last part of the thesis details the characterization of the substitutionally boron-doped
graphene, in which the Raman spectroscopy and electronic transport measurement are
combined to study the effect of the doping with boron atoms. The two approaches give
consistent results on how the electronic transport properties of graphene can be modified by
the doping of boron. The doping of boron atoms leads to the increase of the sheet resistance
together with the appearance of negative magneto-resistance at low temperature. In all cases,
the sheet resistance of boron-doped graphene shows a pronounced semiconducting
temperature dependence. In particular, 2D VRH is found to give a good account of the
temperature dependence. In bilayer boron-doped graphene, which has the largest boron
concentration, a transition from the 2D VRH to Efros-Shklovskii VRH was observed,
indicating the opening of a Coulomb gap.
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