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 metal–molecule–metal junction containing an individual π-conjugated molecule. The model system of our choice is an oligo­(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, molecule–molecule 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 metal–molecule 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 MoS₂

We study the photoresponse of single-layer MoS₂ field-effect transistors by scanning photocurrent microscopy. We find that, unlike in many other semiconductors, the photocurrent generation in single-layer MoS₂ is dominated by the photothermoelectric effect and not by the separation of photoexcited electron–hole pairs across the Schottky barriers at the MoS₂/electrode interfaces. We observe a large value for the Seebeck coefficient for single-layer MoS₂ that by an external electric field can be tuned between −4 × 10² and −1 × 10⁵ μV K^–1. This large and tunable Seebeck coefficient of the single-layer MoS₂ 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 donor–acceptor 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 HOMO–1 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 (3–8 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 cm²/V s and a current ON/OFF ratio larger than 10³. 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 WSe₂ Device

Tungsten diselenide (WSe₂), 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 WSe₂ 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 WSe₂ 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 WSe₂ 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 spin–orbit 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