8 research outputs found
Nanoscale Patterning of Organosilane Molecular Thin Films from the Gas Phase and Its Applications: Fabrication of Multifunctional Surfaces and Large Area Molecular Templates for Site-Selective Material Deposition
A simple methodology to fabricate micrometer- and nanometer-scale
patterned surfaces with multiple chemical functionalities is presented.
Patterns with lateral dimensions down to 110 nm were made. The fabrication
process involves multistep gas-phase patterning of amine, thiol, alkyl,
and fluorinated alkyl-functional organosilane molecules using PDMS
molds as shadow masks. Also, a combination process of channel diffused
plasma etching of organosilane molecular thin films in combination
with masked gas-phase deposition to fabricate multilength scale, multifunctional
surfaces is demonstrated
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Atomic-scale characterization of contact interfaces between thermally self-assembled Au islands and few-layer MoS2 surfaces on SiO2
The interaction between metallic nanoparticles and transition metal chalcogenides (TMDs) can realize new functionalities in thriving technologies such as optoelectronics and nanoengineering. Here we have investigated the self-assembly of triangular-shaped crystalline Au nanoislands on MoS2 flakes mechanically exfoliated or grown by chemical vapor deposition (CVD). The density and size of the islands are determined by substrate temperature, deposition flux, and subsurface morphology. The thickness of the MoS2 layers is measured by Raman spectroscopy, which also enables the evaluation of the strain and doping distributions induced by the Au islands. Top and cross-sectional images of the Au-MoS2 interface are obtained by scanning electron microscopy (SEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Sub-nanometer resolution of the Au, Mo and S layers reveals that the MoS2 flakes follow the corrugation of the SiO2 substrate, with flattening and wrinkling effects induced by the growth of the Au islands on top
Influence of resonant plasmonic nanoparticles on optically accessing the valley degree of freedom in 2D semiconductors
The valley degree of freedom is one of the most intriguing properties of atomically thin transition metal dichalcogenides. Together with the possibility to address this degree of freedom by valley-contrasting optical selection rules, it has the potential to enable a completely new class of future electronic and optoelectronic devices. Resonant optical nanostructures emerge as promising tools for controlling the valley degree of freedom at the nanoscale. However, a critical understanding gap remains in how nanostructures and their nearfields affect the polarization properties of valley-selective chiral emission hindering further developments in this field. In order to address this issue, our study delves into the experimental investigation of a hybrid model system where valley-specific chiral emission from monolayer molybdenum disulfide is interacting with a resonant plasmonic nanosphere. Contrary to the intuition suggesting that a centrosymmetric nanoresonator preserves the degree of circular polarization in the farfield, our cryogenic photoluminescence microscopy reveals almost complete depolarization. We rigorously study the nature of this phenomenon numerically considering the monolayer-nanoparticle interaction at different levels including excitation and emission. We find that the farfield degree of polarization strongly reduces in the hybrid system when including excitons emitting from outside of the system's symmetry point, which in combination with depolarisation at the excitation level causes the observed effect. Our results highlight the importance of considering spatially distributed chiral emitters for precise predictions of polarization responses in these hybrid systems. This finding advances our fundamental knowledge of the light-valley interactions at the nanoscale but also unveils a serious impediment of the practical fabrication of resonant valleytronic nanostructures
An Atomically Layered InSe Avalanche Photodetector
Atomically thin photodetectors based
on 2D materials have attracted great interest due to their potential
as highly energy-efficient integrated devices. However, photoinduced
carrier generation in these media is relatively poor due to low optical
absorption, limiting device performance. Current methods for overcoming
this problem, such as reducing contact resistances or back gating,
tend to increase dark current and suffer slow response times. Here,
we realize the avalanche effect in a 2D material-based photodetector
and show that avalanche multiplication can greatly enhance the device
response of an ultrathin InSe-based photodetector. This is achieved
by exploiting the large Schottky barrier formed between InSe and Al
electrodes, enabling the application of a large bias voltage. Plasmonic
enhancement of the photosensitivity, achieved by patterning arrays
of Al nanodisks onto the InSe layer, further improves device efficiency.
With an external quantum efficiency approaching 866%, a dark current
in the picoamp range, and a fast response time of 87 μs, this
atomic layer device exhibits multiple significant advances in overall
performance for this class of devices
Few-cycle laser pulse characterization on-target using high-harmonic generation from nano-scale solids
We demonstrate high-harmonic generation for the time-domain observation of the electric field (HHG-TOE) and use it to measure the waveform of ultrashort mid-infrared (MIR) laser pulses interacting with ZnO thin-films or WS monolayers. The working principle relies on perturbing HHG in solids with a weak replica of the pump pulse. We measure the duration of few-cycle pulses at 3100\,nm, in reasonable agreement with the results of established pulse characterization techniques. Our method provides a straightforward approach to accurately characterize femtosecond laser pulses used for HHG experiments right at the point of interaction
Exciton Dynamics in MoS<sub>2</sub>‑Pentacene and WSe<sub>2</sub>‑Pentacene Heterojunctions
We measured the exciton dynamics in van der Waals heterojunctions
of transition metal dichalcogenides (TMDCs) and organic semiconductors
(OSs). TMDCs and OSs are semiconducting materials with rich and highly
diverse optical and electronic properties. Their heterostructures,
exhibiting van der Waals bonding at their interfaces, can be utilized
in the field of optoelectronics and photovoltaics. Two types of heterojunctions,
MoS2-pentacene and WSe2-pentacene, were prepared
by layer transfer of 20 nm pentacene thin films as well as MoS2 and WSe2 monolayer crystals onto Au surfaces.
The samples were studied by means of transient absorption spectroscopy
in the reflectance mode. We found that A-exciton decay by hole transfer
from MoS2 to pentacene occurs with a characteristic time
of 21 ± 3 ps. This is slow compared to previously reported hole
transfer times of 6.7 ps in MoS2-pentacene junctions formed
by vapor deposition of pentacene molecules onto MoS2 on
SiO2. The B-exciton decay in WSe2 shows faster
hole transfer rates for WSe2-pentacene heterojunctions,
with a characteristic time of 7 ± 1 ps. The A-exciton in WSe2 also decays faster due to the presence of a pentacene overlayer;
however, fitting the decay traces did not allow for the unambiguous
assignment of the associated decay time. Our work provides important
insights into excitonic dynamics in the growing field of TMDC-OS heterojunctions
Tailoring the Physical Properties of Molybdenum Disulfide Monolayers by Control of Interfacial Chemistry
We
demonstrate how substrate interfacial chemistry can be utilized
to tailor the physical properties of single-crystalline molybdenum
disulfide (MoS<sub>2</sub>) atomic-layers. Semiconducting, two-dimensional
MoS<sub>2</sub> possesses unique properties that are promising for
future optical and electrical applications for which the ability to
tune its physical properties is essential. We use self-assembled monolayers
with a variety of end termination chemistries to functionalize substrates
and systematically study their influence on the physical properties
of MoS<sub>2</sub>. Using electrical transport measurements, temperature-dependent
photoluminescence spectroscopy, and empirical and first-principles
calculations, we explore the possible mechanisms involved. Our data
shows that combined interface-related effects of charge transfer,
built-in molecular polarities, varied densities of defects, and remote
interfacial phonons strongly modify the electrical and optical properties
of MoS<sub>2</sub>. These findings can be used to effectively enhance
or modulate the conductivity, field-effect mobility, and photoluminescence
in MoS<sub>2</sub> monolayers, illustrating an approach for local
and universal property modulations in two-dimensional atomic-layers
Scalable Transfer of Suspended Two-Dimensional Single Crystals
Large-scale suspended architectures
of various two-dimensional (2D) materials (MoS<sub>2</sub>, MoSe<sub>2</sub>, WS<sub>2</sub>, and graphene) are demonstrated on nanoscale
patterned substrates with different physical and chemical surface
properties, such as flexible polymer substrates (polydimethylsiloxane),
rigid Si substrates, and rigid metal substrates (Au/Ag). This transfer
method represents a generic, fast, clean, and scalable technique to
suspend 2D atomic layers. The underlying principle behind this approach,
which employs a capillary-force-free wet-contact printing method,
was studied by characterizing the nanoscale solid–liquid–vapor
interface of 2D layers with respect to different substrates. As a
proof-of-concept, a photodetector of suspended MoS<sub>2</sub> has
been demonstrated with significantly improved photosensitivity. This
strategy could be extended to several other 2D material systems and
open the pathway toward better optoelectronic and nanoelectromechnical
systems