12 research outputs found
Quantum Transport through a Single Conjugated Rigid Molecule, a Mechanical Break Junction Study
ConspectusThis Account provides an overview of our recent efforts to unravel
charge transport characteristics of a metalāmoleculeāmetal
junction containing an individual Ļ-conjugated molecule. The
model system of our choice is an oligoĀ(phenylene-ethynylene) consisting
of three rings, in short OPE3, which represents a paradigmatic model
system for molecular-scale electronics. Members of the OPE family
are among the most studied in the field thanks to their simple and
rigid structure, the possibility of chemically functionalizing them,
and their clear transport characteristics. When investigating charge
transport in molecular systems, two general directions can be distinguished:
one in which assemblies composed of many molecules contacted in parallel
are studied, while in the other a single molecule is investigated
at a time. In the former approach, moleculeāmolecule interactions
and ensemble-averaged quantities may play a role, thereby introducing
broadening of spectral features and hindering the study of the behavior
of individual molecules making it more difficult to deconvolute local
and intrinsic molecular effects from collective ones. In contrast,
single-molecule experiments directly probe individual molecular features
and, when they are repeated many times, allow build up of a statistical
representation of the changes introduced by, e.g., different junction
configurations. Especially in recent years, experimental techniques
have advanced such that now large sets of individual events can be
measured and analyzed with statistical tools. To study individual
single-molecule junctions, we use the break junction technique, in
which two sharp movable electrodes are formed by breaking a thin metallic
wire and used to contact a single or few molecules. By probing thousands
of single-molecule junctions in different conditions, we show that
their creation involves independent events justifying the statistical
tools that are used. By combining room- and low-temperature data,
we show that the dominant transport mechanism for electrons through
the OPE3 molecule is off-resonant tunneling. The simplest model capturing
transport details in this case is a single-level model characterized
by three parameters: the level alignment of the frontier orbital with
the Fermi energy of the leads and the electronic couplings to the
leads. Variations in these parameters give a broad distribution (1
order of magnitude) in the observed conductance values, indicating
that at the microscopic level both the hybridization with the metallic
electrodes and the molecular electronic configuration can fluctuate.
The low-temperature data show that these variations are due to abrupt
changes in the configuration of the molecule in the junction leading
to changes in either one of these parameters or both at the same time.
The complementary information gained from different experiments is
needed to build up a consistent and extended picture of the variability
of molecular configurations, omnipresent in single-molecule studies.
Knowledge of this variability can help one to better understand the
behavior of molecules at the atomic level and at the metalāmolecule
interface in particular
Quantum Dots at Room Temperature Carved out from Few-Layer Graphene
We present graphene quantum dots endowed with addition
energies
as large as 1.6 eV, fabricated by the controlled rupture of a graphene
sheet subjected to a large electron current in air. The size of the
quantum dot islands is estimated to be in the 1 nm range. The large
addition energies allow for Coulomb blockade at room temperature,
with possible application to single-electron devices
Electric-Field Control of Interfering Transport Pathways in a Single-Molecule Anthraquinone Transistor
It is understood that molecular conjugation
plays an important role in charge transport through single-molecule
junctions. Here, we investigate electron transport through an anthraquinone
based single-molecule three-terminal device. With the use of an electric-field
induced by a gate electrode, the molecule is reduced resulting into
a 10-fold increase in the off-resonant differential conductance. Theoretical
calculations link the change in differential conductance to a reduction-induced
change in conjugation, thereby lifting destructive interference of
transport pathways
On-chip Heaters for Tension Tuning of Graphene Nanodrums
For
the study and application of graphene membranes, it is essential
to have means to control their resonance frequency and temperature.
Here, we present an on-chip heater platform for local tuning of in-plane
tension in graphene mechanical resonators. By Joule heating of a metallic
suspension ring we show thermomechanical resonance frequency tuning
in a few-layer (FL) graphene nanodrum, which is accompanied by an
increase in its quality factor, which we attribute to the increase
of the in-plane tension. The in situ control of temperature, in-plane
tension, resonance frequency, and quality factor of suspended two-dimensional
(2D) nanodrums makes this device a unique platform for investigating
the origin of dissipation in these ultrathin structures and can be
of fundamental importance for studying the thermal properties of 2D
materials. Moreover, by simultaneously controlling the heater and
the backgate voltage, we can independently control the resonance frequency
and quality factor, which is of great importance for applications
in sensors and resonant mechanical filters
Large and Tunable Photothermoelectric Effect in Single-Layer MoS<sub>2</sub>
We study the photoresponse of single-layer MoS<sub>2</sub> field-effect
transistors by scanning photocurrent microscopy. We find that, unlike
in many other semiconductors, the photocurrent generation in single-layer
MoS<sub>2</sub> is dominated by the photothermoelectric effect and
not by the separation of photoexcited electronāhole pairs across
the Schottky barriers at the MoS<sub>2</sub>/electrode interfaces.
We observe a large value for the Seebeck coefficient for single-layer
MoS<sub>2</sub> that by an external electric field can be tuned between
ā4 Ć 10<sup>2</sup> and ā1 Ć 10<sup>5</sup> Ī¼V K<sup>ā1</sup>. This large and tunable Seebeck coefficient
of the single-layer MoS<sub>2</sub> paves the way to new applications
of this material such as on-chip thermopower generation and waste
thermal energy harvesting
Single-Molecule Resonant Tunneling Diode
Rectification
has been at the foundation of molecular electronics.
Most single-molecule diodes realized experimentally so far are based
on asymmetries in the coupling with the electrodes or using the donorāacceptor
principle. In general, however, their rectification ratios are usually
small (<10). Here, we propose a single-molecule diode based on
an orbital resonance while using the highest occupied molecular orbital
(HOMO) and HOMOā1 as transport channels. Our proposed diode
design is based on an asymmetric two-site model and analyzed with
DFT + NEGF calculations. We find high rectification ratios, even in
the case of symmetric coupling to the electrodes. In addition, we
show that diode parameters such as the operating voltage and rectification
ratio can be tuned by chemical design
Fast and Broadband Photoresponse of Few-Layer Black Phosphorus Field-Effect Transistors
Few-layer black phosphorus, a new
elemental two-dimensional (2D)
material recently isolated by mechanical exfoliation, is a high-mobility
layered semiconductor with a direct bandgap that is predicted to strongly
depend on the number of layers, from 0.35 eV (bulk) to 2.0 eV (single
layer). Therefore, black phosphorus is an appealing candidate for
tunable photodetection from the visible to the infrared part of the
spectrum. We study the photoresponse of field-effect transistors (FETs)
made of few-layer black phosphorus (3ā8 nm thick), as a function
of excitation wavelength, power, and frequency. In the dark state,
the black phosphorus FETs can be tuned both in hole and electron doping
regimes allowing for ambipolar operation. We measure mobilities in
the order of 100 cm<sup>2</sup>/V s and a current ON/OFF ratio larger
than 10<sup>3</sup>. Upon illumination, the black phosphorus transistors
show a response to excitation wavelengths from the visible region
up to 940 nm and a rise time of about 1 ms, demonstrating broadband
and fast detection. The responsivity reaches 4.8 mA/W, and it could
be drastically enhanced by engineering a detector based on a PN junction.
The ambipolar behavior coupled to the fast and broadband photodetection
make few-layer black phosphorus a promising 2D material for photodetection
across the visible and near-infrared part of the electromagnetic spectrum
Photovoltaic and Photothermoelectric Effect in a Double-Gated WSe<sub>2</sub> Device
Tungsten diselenide (WSe<sub>2</sub>), a semiconducting transition
metal dichalcogenide (TMDC), shows great potential as active material
in optoelectronic devices due to its ambipolarity and direct bandgap
in its single-layer form. Recently, different groups have exploited
the ambipolarity of WSe<sub>2</sub> to realize electrically tunable
PN junctions, demonstrating its potential for digital electronics
and solar cell applications. In this Letter, we focus on the different
photocurrent generation mechanisms in a double-gated WSe<sub>2</sub> device by measuring the photocurrent (and photovoltage) as the local
gate voltages are varied independently in combination with above-
and below-bandgap illumination. This enables us to distinguish between
two main photocurrent generation mechanisms, the photovoltaic and
photothermoelectric effect. We find that the dominant mechanism depends
on the defined gate configuration. In the PN and NP configurations,
photocurrent is mainly generated by the photovoltaic effect and the
device displays a maximum responsivity of 0.70 mA/W at 532 nm illumination
and rise and fall times close to 10 ms. Photocurrent generated by
the photothermoelectric effect emerges in the PP configuration and
is a factor of 2 larger than the current generated by the photovoltaic
effect (in PN and NP configurations). This demonstrates that the photothermoelectric
effect can play a significant role in devices based on WSe<sub>2</sub> where a region of strong optical absorption, caused by, for example,
an asymmetry in flake thickness or optical absorption of the electrodes,
generates a sizable thermal gradient upon illumination
Kondo Effect in a Neutral and Stable All Organic Radical Single Molecule Break Junction
Organic radicals are neutral, purely
organic molecules exhibiting an intrinsic magnetic moment due to the
presence of an unpaired electron in the molecule in its ground state.
This property, added to the low spināorbit coupling and weak
hyperfine interactions, make neutral organic radicals good candidates
for molecular spintronics insofar as the radical character is stable
in solid state electronic devices. Here we show that the paramagnetism
of the polychlorotriphenylmethyl radical molecule in the form of a
Kondo anomaly is preserved in two- and three-terminal solid-state
devices, regardless of mechanical and electrostatic changes. Indeed,
our results demonstrate that the Kondo anomaly is robust under electrodes
displacement and changes of the electrostatic environment, pointing
to a localized orbital in the radical as the source of magnetism.
Strong support to this picture is provided by density functional calculations
and measurements of the corresponding nonradical species. These results
pave the way toward the use of all-organic neutral radical molecules
in spintronics devices and open the door to further investigations
into Kondo physics
Fast and Efficient Photodetection in Nanoscale Quantum-Dot Junctions
We report on a photodetector in which colloidal quantum
dots directly
bridge nanometer-spaced electrodes. Unlike in conventional quantum-dot
thin film photodetectors, charge mobility no longer plays a role in
our quantum-dot junctions as charge extraction requires only two individual
tunnel events. We find an efficient photoconductive gain mechanism
with external quantum efficiencies of 38 electrons-per-photon in combination
with response times faster than 300 ns. This compact device-architecture
may open up new routes for improved photodetector performance in which
efficiency and bandwidth do not go at the cost of one another