Johannes S. Seldenthuis

Room-Temperature Gating of Molecular Junctions Using Few-Layer Graphene Nanogap Electrodes

We report on a method to fabricate and measure gateable molecular junctions that are stable at room temperature. The devices are made by depositing molecules inside a few-layer graphene nanogap, formed by feedback controlled electroburning. The gaps have separations on the order of 1-2 nm as estimated from a Simmons model for tunneling. The molecular junctions display gateable I-V-characteristics at room temperature.

Signatures of Quantum Interference Effects on Charge Transport Through a Single Benzene Ring

Inthis description, the width and height of the energy barrier,and the electronic coupling between the molecule and theelectrodes are the main parameters that characterize theefficiency of charge transport. Electron transfer througha molecule then depends exponentially on the length of theconductance pathway and this has indeed been observed inmany experiments.

Large negative differential conductance in single-molecule break junctions

Molecular electronics aims at exploiting the internal structure and electronic orbitals of molecules to construct functional building blocks. To date, however, the overwhelming majority of experimentally realized single-molecule junctions can be described as single quantum dots, where transport is mainly determined by the alignment of the molecular orbital levels with respect to the Fermi energies of the electrodes and the electronic coupling with those electrodes. Particularly appealing exceptions include molecules in which two moieties are twisted with respect to each other and molecules in which quantum interference effects are possible. Here, we report the experimental observation of pronounced negative differential conductance in the current-voltage characteristics of a single molecule in break junctions. The molecule of interest consists of two conjugated arms, connected by a non-conjugated segment, resulting in two coupled sites. A voltage applied across the molecule pulls the energy of the sites apart, suppressing resonant transport through the molecule and causing the current to decrease. A generic theoretical model based on a two-site molecular orbital structure captures the experimental findings well, as confirmed by density functional theory with non-equilibrium Greens functions calculations that include the effect of the bias. Our results point towards a conductance mechanism mediated by the intrinsic molecular orbitals alignment of the molecule.

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.

An All-Electric Single-Molecule Motor

Many types of molecular motors have been proposed and synthesized in recent years, displaying different kinds of motion, and fueled by different driving forces such as light, heat, or chemical reactions. We propose a new type of molecular motor based on electric field actuation and electric current detection of the rotational motion of a molecular dipole embedded in a three-terminal single-molecule device. The key aspect of this all-electronic design is the conjugated backbone of the molecule, which simultaneously provides the potential landscape of the rotor orientation and a real-time measure of that orientation through the modulation of the conductivity. Using quantum chemistry calculations, we show that this approach provides full control over the speed and continuity of motion, thereby combining electrical and mechanical control at the molecular level over a wide range of temperatures. Moreover, chemistry can be used to change all key parameters of the device, enabling a variety of new experiments on molecular motors.

Vibrational excitations in weakly coupled single-molecule junctions: A computational analysis

In bulk systems, molecules are routinely identified by their vibrational spectrum using Raman or infrared spectroscopy. In recent years, vibrational excitation lines have been observed in low-temperature conductance measurements on single-molecule junctions, and they can provide a similar means of identification. We present a method to efficiently calculate these excitation lines in weakly coupled, gateable single-molecule junctions, using a combination of ab initio density functional theory and rate equations. Our method takes transitions from excited to excited vibrational state into account by evaluating the Franck-Condon factors for an arbitrary number of vibrational quanta and is therefore able to predict qualitatively different behavior from calculations limited to transitions from ground state to excited vibrational state. We find that the vibrational spectrum is sensitive to the molecular contact geometry and the charge state, and that it is generally necessary to take more than one vibrational quantum into account. Quantitative comparison to previously reported measurements on pi-conjugated molecules reveals that our method is able to characterize the vibrational excitations and can be used to identify single molecules in a junction. The method is computationally feasible on commodity hardware.

Quantum interference effects at room temperature in OPV-based single-molecule junctions

Interference effects on charge transport through an individual molecule can lead to a notable modulation and suppression on its conductance. In this letter, we report the observation of quantum interference effects occurring at room temperature in single-molecule junctions based on oligo(3)-phenylenevinylene (OPV3) derivatives, in which the central benzene ring is coupled to either para- or meta-positions. Using the break-junction technique, we find that the conductance for a single meta-OPV3 molecule wired between gold electrodes is one order of magnitude smaller than that of a para-OPV3 molecule. Theoretical calculations confirm the occurrence of constructive and destructive interference in the para- and meta-OPV3 molecules respectively, which arises from the phase difference of the transmission coefficients through the molecular orbitals.

Conductance Switching and Vibrational Fine Structure of a [2 × 2] CoII4 Gridlike Single Molecule Measured in a Three‐Terminal Device

The observation of electrical switching in molecular junctions has recently attracted a lot of attention and the operation of a 160 kB dynamic random access memory (DRAM) circuit based on monolayers of bistable rotaxane molecules as data storage elements has been described. Reports on conductance switching in the literature, however, remain controversial with regard to the nature of the mechanism itself: conformational changes, molecule-bond fluctuations, or reversible formation of metallic filaments through molecular layers are among the proposed mechanisms. Herein, we present a study of conductance switching in molecular junctions carried out using [Co4L4] (BF4)8 gridlike [2 2] molecules in threeterminal devices, where L is 4,6-bis(20,200-bipyrid-60-yl)-2phenylpyrimidine). Transport measurements at cryogenic temperatures reveal Coulomb blockade and exhibit excitation lines at low energy, which are attributed to vibrational modes of the molecule. When applying a bias above a threshold voltage, current–voltage (I–V) characteristics show switch events between bistable branches. The measurements hint at an electrostatic origin of this effect and we argue that subnanometer motion of electrostatically bound counter ions in the

Electroluminescence spectra in weakly coupled single-molecule junctions

We have combined ab initio quantum chemistry calculations with a rate-equation formalism to analyze electroluminescence spectra in single-molecule junctions, measured recently by several groups in scanning tunneling microscope setups. In our method, the entire vibrational spectrum is taken into account. Our method leads to good quantitative agreement with both the spectroscopic features of the measurements and their current and voltage dependence. Moreover, our method is able to explain several previously unexplained features. We show that in general, the quantum yield is expected to be suppressed at high bias, as is observed in one of the measurements. Additionally, we comment on the influence of the vibrational relaxation times on several features of the spectrum.