4 research outputs found
Zener Tunneling and Photocurrent Generation in Quasi-Metallic Carbon Nanotube pn-Devices
We investigate the electronic and
optoelectronic properties of
quasi-metallic nanotube pn-devices, which have smaller band gaps than
most known bulk semiconductors. These carbon nanotube-based devices
deviate from conventional bulk semiconductor device behavior due to
their low-dimensional nature. We observe rectifying behavior based
on Zener tunneling of ballistic carriers instead of ideal diode behavior,
as limited by the diffusive transport of carriers. We observe substantial
photocurrents at room temperature, suggesting that these quasi-metallic
pn-devices may have a broader impact in optoelectronic devices. A
new technique based on photocurrent spectroscopy is presented to identify
the unique chirality of nanotubes in a functional device. This chirality
information is crucial in obtaining a theoretical understanding of
the underlying device physics that depends sensitively on nanotube
chirality, as is the case for quasi-metallic nanotube devices. A detailed
model is developed to fit the observed <i>I–V</i> characteristics, which enables us to verify the band gap from these
measurements as well as the dimensions of the insulating tunneling
barrier region
Indirect Band Gap Emission by Hot Electron Injection in Metal/MoS<sub>2</sub> and Metal/WSe<sub>2</sub> Heterojunctions
Transition metal dichalcogenides
(TMDCs), such as MoS<sub>2</sub> and WSe<sub>2</sub>, are free of
dangling bonds and therefore make more “ideal” Schottky
junctions than bulk semiconductors, which produce Fermi energy pinning
and recombination centers at the interface with bulk metals, inhibiting
charge transfer. Here, we observe a more than 10× enhancement
in the indirect band gap photoluminescence of transition metal dichalcogenides
(TMDCs) deposited on various metals (e.g., Cu, Au, Ag), while the
direct band gap emission remains unchanged. We believe the main mechanism
of light emission arises from photoexcited hot electrons in the metal
that are injected into the conduction band of MoS<sub>2</sub> and
WSe<sub>2</sub> and subsequently recombine radiatively with minority
holes in the TMDC. Since the conduction band at the K-point is 0.5
eV higher than at the Σ-point, a lower Schottky barrier exists
for the Σ-point band, making electron injection more favorable.
Also, the Σ band consists of the sulfur <i>p</i><sub><i>z</i></sub> orbital, which overlaps more significantly
with the electron wave functions in the metal. This enhancement in
the indirect emission only occurs for thick flakes of MoS<sub>2</sub> and WSe<sub>2</sub> (≥100 nm) and is completely absent in
monolayer and few-layer (∼10 nm) flakes. Here, the flake thickness
must exceed the depletion width of the Schottky junction, in order
for efficient radiative recombination to occur in the TMDC. The intensity
of this indirect peak decreases at low temperatures, which is consistent
with the hot electron injection model
Layer Control of WSe<sub>2</sub> <i>via</i> Selective Surface Layer Oxidation
We
report Raman and photoluminescence spectra of mono- and few-layer
WSe<sub>2</sub> and MoSe<sub>2</sub> taken before and after exposure
to a remote oxygen plasma. For bilayer and trilayer WSe<sub>2</sub>, we observe an increase in the photoluminescence intensity and a
blue shift of the photoluminescence peak positions after oxygen plasma
treatment. The photoluminescence spectra of trilayer WSe<sub>2</sub> exhibit features of a bilayer after oxygen plasma treatment. Bilayer
WSe<sub>2</sub> exhibits features of a monolayer, and the photoluminescence
of monolayer WSe<sub>2</sub> is completely absent after the oxygen
plasma treatment. These changes are observed consistently in more
than 20 flakes. The mechanism of the changes observed in the photoluminescence
spectra of WSe<sub>2</sub> is due to the selective oxidation of the
topmost layer. As a result, <i>N</i>-layer WSe<sub>2</sub> is reduced to <i>N</i>–1 layers. Raman spectra
and AFM images taken from the WSe<sub>2</sub> flakes before and after
the oxygen treatment corroborate these findings. Because of the low
kinetic energy of the oxygen radicals in the remote oxygen plasma,
the oxidation is self-limiting. By varying the process duration from
1 to 10 min, we confirmed that the oxidation will only affect the
topmost layer of the WSe<sub>2</sub> flakes. X-ray photoelectron spectroscopy
shows that the surface layer WO<sub><i>x</i></sub> of the
sample can be removed by a quick dip in KOH solution. Therefore, this
technique provides a promising way of controlling the thickness of
WSe<sub>2</sub> layer by layer
Clamping Instability and van der Waals Forces in Carbon Nanotube Mechanical Resonators
We investigate the role of weak clamping
forces, typically assumed
to be infinite, in carbon nanotube mechanical resonators. Due to these
forces, we observe a hysteretic clamping and unclamping of the nanotube
device that results in a discrete drop in the mechanical resonance
frequency on the order of 5–20 MHz, when the temperature is
cycled between 340 and 375 K. This instability in the resonant frequency
results from the nanotube unpinning from the electrode/trench sidewall
where it is bound weakly by van der Waals forces. Interestingly, this
unpinning does not affect the <i>Q</i>-factor of the resonance,
since the clamping is still governed by van der Waals forces above
and below the unpinning. For a 1 μm device, the drop observed
in resonance frequency corresponds to a change in nanotube length
of approximately 50–65 nm. On the basis of these findings,
we introduce a new model, which includes a finite tension around zero
gate voltage due to van der Waals forces and shows better agreement
with the experimental data than the perfect clamping model. From the
gate dependence of the mechanical resonance frequency, we extract
the van der Waals clamping force to be 1.8 pN. The mechanical resonance
frequency exhibits a striking temperature dependence below 200 K attributed
to a temperature-dependent slack arising from the competition between
the van der Waals force and the thermal fluctuations in the suspended
nanotube