77 research outputs found
Non-equilibrium dynamics of electron emission from cold and hot graphene under proton irradiation
Characteristic properties of secondary electrons emitted from irradiated
two-dimensional materials arise from multi-length and time-scale relaxation
processes that connect the initial non-equilibrium excited electron
distribution with their eventual emission. To understand these processes, which
are critical for using secondary electrons as high-resolution thermalization
probes, we combine first-principles real-time electron dynamics with modern
experiments. Our data for cold and hot proton-irradiated graphene shows
signatures of kinetic and potential emission and generally good agreement for
electron yields between experiment and theory. The duration of the emission
pulse is about 1.5 femtoseconds, indicating high time resolution when used as a
probe. Our newly developed method to predict kinetic energy spectra shows good
agreement with electron and ion irradiation experiments and prior models. We
find that lattice temperature significantly increases secondary electron
emission, whereas electron temperature has a negligible effect
Distance dependence of the phase signal in eddy current microscopy
Atomic force microscopy using a magnetic tip is a promising tool for
investigating conductivity on the nano-scale. By the oscillating magnetic tip
eddy currents are induced in the conducting parts of the sample which can be
detected in the phase signal of the cantilever. However, the origin of the
phase signal is still controversial because theoretical calculations using a
monopole appoximation for taking the electromagnetic forces acting on the tip
into account yield an effect which is too small by more than two orders of
magnitude. In order to determine the origin of the signal we used especially
prepared gold nano patterns embedded in a non-conducting polycarbonate matrix
and measured the distance dependence of the phase signal. Our data clearly
shows that the interacting forces are long ranged and therefore, are likely due
to the electromagnetic interaction between the magnetic tip and the conducting
parts of the surface. Due to the long range character of the interaction a
change in conductivity of m can be
detected far away from the surface without any interference from the
topography
Isolating the Nonlinear Optical Response of a MoS Monolayer under Extreme Screening of a Metal Substrate
Transition metal dichalcogenides (TMDCs) monolayers, as two-dimensional (2D)
direct bandgap semiconductors, hold promise for advanced optoelectronic and
photocatalytic devices. Interaction with three-dimensional (3D) metals, like
Au, profoundly affects their optical properties, posing challenges in
characterizing the monolayer's optical responses within the semiconductor-metal
junction. In this study, using precise polarization-controlled final-state sum
frequency generation (FS-SFG), we successfully isolated the optical responses
of a MoS monolayer from a MoS/Au junction. The resulting SFG spectra
exhibit a linear lineshape, devoid of A or B exciton features, attributed to
the strong dielectric screening and substrate induced doping. The linear
lineshape illustrates the expected constant density of states (DOS) at the band
edge of the 2D semiconductor, a feature often obscured by excitonic
interactions in week-screening conditions such as in a free-standing monolayer.
Extrapolation yields the onset of a direct quasiparticle bandgap of about
eV, indicating a strong bandgap renormalization. This study not
only enriches our understanding of the optical responses of a 2D semiconductor
in extreme screening conditions but also provides a critical reference for
advancing 2D semiconductor-based photocatalytic applications.Comment: 14 pages, 4 figures + supplemental materia
Characterization of the electric transport properties of black phosphorous back-gated field-effect transistors
We use thin layers of exfoliated black phosphorus to realize back-gated field-effect
transistors in which the Si/SiO2 substrate is exploited as gate electrode. To prevent the
detrimental effect of the air exposure the devices are protected by Poly(methyl methacrylate).
We report the observation of an improved contact resistance at the interface between the layered
material and the metal contact by electrical conditioning. We also demonstrate the existence of
a hysteresis in the transfer characteristics that improves by increasing the gate voltage sweep
range. Finally, we prove the suitability of such transistors as memory devices
Temperature dependence of the energy dissipation in dynamic force microscopy
The dissipation of energy in dynamic force microscopy is usually described in
terms of an adhesion hysteresis mechanism. This mechanism should become less
efficient with increasing temperature. To verify this prediction we have
measured topography and dissipation data with dynamic force microscopy in the
temperature range from 100 K up to 300 K. We used
3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) grown on KBr(001), both
materials exhibiting a strong dissipation signal at large frequency shifts. At
room temperature, the energy dissipated into the sample (or tip) is 1.9
eV/cycle for PTCDA and 2.7 eV/cycle for KBr, respectively, and is in good
agreement with an adhesion hysteresis mechanism. The energy dissipation over
the PTCDA surface decreases with increasing temperature yielding a negative
temperature coefficient. For the KBr substrate, we find the opposite behaviour:
an increase of dissipated energy with increasing temperature. While the
negative temperature coefficient in case of PTCDA agrees rather well with the
adhesion hysteresis model, the positive slope found for KBr points to a
hitherto unknown dissipation mechanism
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