9 research outputs found
Spin-Orbit Dirac Fermions in 2D Systems
We propose a novel model for including spin-orbit interactions in buckled two
dimensional systems. Our results show that in such systems, intrinsic
spin-orbit coupling leads to a formation of Dirac cones, similar to Rashba
model. We explore the microscopic origins of this behaviour and confirm our
results using DFT calculations
Electronic properties of low-dimensional systems
This work deals with transport and general electronic phenomena in low-dimensional systems. The first chapter is dedicated to Variable Range Hopping. It starts with a brief review of the general hopping formalism, based on previous work. Next, new methods and results are presented and discussed. In particular, studies of both Ohmic and non-Ohmic regime are performed and the stark differences between the two are elucidated. In addition, apparent power law dependence of current on voltage in disordered one-dimensional materials is analyzed. The results obtained compare favorably with the experiments. Finally, the behavior of the conducting network in d dimensions is discussed using the percolation approach. The second chapter deals with plasmonic effects in graphene. After giving a short introduction to graphene and plasmonic behavior, current work is presented. Charge oscillations in graphene half-plane are discussed and compared with experimental results obtained from near-field microscopy. In addition, plasmonic oscillations in a "narrow-flake" geometry are analyzed analytically and numerically, showing good agreement between the two method
Unusual angular dependence of the Raman response in black phosphorus
10.1021/acsnano.5b00698ACS Nano944270-427
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Tuning and Persistent Switching of Graphene Plasmons on a Ferroelectric Substrate.
We characterized plasmon propagation in graphene on thin films of the high-κ dielectric PbZr0.3Ti0.7O3 (PZT). Significant modulation (up to ±75%) of the plasmon wavelength was achieved with application of ultrasmall voltages (< ±1 V) across PZT. Analysis of the observed plasmonic fringes at the graphene edge indicates that carriers in graphene on PZT behave as noninteracting Dirac Fermions approximated by a semiclassical Drude response, which may be attributed to strong dielectric screening at the graphene/PZT interface. Additionally, significant plasmon scattering occurs at the grain boundaries of PZT from topographic and/or polarization induced graphene conductivity variation in the interior of graphene, reducing the overall plasmon propagation length. Lastly, through application of 2 V across PZT, we demonstrate the capability to persistently modify the plasmonic response of graphene through transient voltage application
Colossal Ultraviolet Photoresponsivity of Few-Layer Black Phosphorus
Black phosphorus has an orthorhombic layered structure with a layer-dependent direct band gap from monolayer to bulk, making this material an emerging material for photodetection. Inspired by this and the recent excitement over this material, we studied the optoelectronics characteristics of high-quality, few-layer black phosphorus-based photodetectors over a wide spectrum ranging from near-ultraviolet (UV) to near-infrared (NIR). It is demonstrated for the first time that black phosphorus can be configured as an excellent UV photodetector with a specific detectivity ∼3 × 10<sup>13</sup> Jones. More critically, we found that the UV photoresponsivity can be significantly enhanced to ∼9 × 10<sup>4</sup> A W<sup>–1</sup> by applying a source-drain bias (<i>V</i><sub>SD</sub>) of 3 V, which is the highest ever measured in any 2D material and 10<sup>7</sup> times higher than the previously reported value for black phosphorus. We attribute such a colossal UV photoresponsivity to the resonant-interband transition between two specially nested valence and conduction bands. These nested bands provide an unusually high density of states for highly efficient UV absorption due to the singularity of their nature
Tuning and Persistent Switching of Graphene Plasmons on a Ferroelectric Substrate
We characterized plasmon propagation
in graphene on thin films of the high-κ dielectric PbZr<sub>0.3</sub>Ti<sub>0.7</sub>O<sub>3</sub> (PZT). Significant modulation
(up to ±75%) of the plasmon wavelength was achieved with application
of ultrasmall voltages (< ±1 V) across PZT. Analysis of the
observed plasmonic fringes at the graphene edge indicates that carriers
in graphene on PZT behave as noninteracting Dirac Fermions approximated
by a semiclassical Drude response, which may be attributed to strong
dielectric screening at the graphene/PZT interface. Additionally,
significant plasmon scattering occurs at the grain boundaries of PZT
from topographic and/or polarization induced graphene conductivity
variation in the interior of graphene, reducing the overall plasmon
propagation length. Lastly, through application of 2 V across PZT,
we demonstrate the capability to persistently modify the plasmonic
response of graphene through transient voltage application
Ultrafast and Nanoscale Plasmonic Phenomena in Exfoliated Graphene Revealed by Infrared Pump–Probe Nanoscopy
Pump–probe spectroscopy is
central for exploring ultrafast
dynamics of fundamental excitations, collective modes, and energy
transfer processes. Typically carried out using conventional diffraction-limited
optics, pump–probe experiments inherently average over local
chemical, compositional, and electronic inhomogeneities. Here, we
circumvent this deficiency and introduce pump–probe infrared
spectroscopy with ∼20 nm spatial resolution, far below the
diffraction limit, which is accomplished using a scattering scanning
near-field optical microscope (s-SNOM). This technique allows us to
investigate exfoliated graphene single-layers on SiO<sub>2</sub> at
technologically significant mid-infrared (MIR) frequencies where the
local optical conductivity becomes experimentally accessible through
the excitation of surface plasmons via the s-SNOM tip. Optical pumping
at near-infrared (NIR) frequencies prompts distinct changes in the
plasmonic behavior on 200 fs time scales. The origin of the pump-induced,
enhanced plasmonic response is identified as an increase in the effective
electron temperature up to several thousand Kelvin, as deduced directly
from the Drude weight associated with the plasmonic resonances