E. H. Huisman

Interpretation of transition voltage spectroscopy.

The promise of transition voltage spectroscopy (TVS) is that molecular level positions can be determined in molecular devices without applying extreme voltages. Here, we consider the physics behind TVS in more detail. Remarkably, we find that the Simmons model employed thus far is inconsistent with experimental data. However, a coherent molecular transport model does justify TVS as a spectroscopic tool. Moreover, TVS may become a critical test to distinguish molecular junctions from vacuum tunnel junctions.

Single atom adhesion in optimized gold nanojunctions

We study the interaction between single apex atoms in a metallic contact, using the break junction geometry. By carefully training our samples, we create stable junctions in which no further atomic reorganization takes place. This allows us to study the relation between the so-called jump out of contact (from contact to tunneling regime) and jump to contact (from tunneling to contact regime) in detail. Our data can be fully understood within a relatively simple elastic model, where the elasticity k of the electrodes is the only free parameter. We find 5<k<32 N/m. Furthermore, the interaction between the two apex atoms on both electrodes, observed as a change of slope in the tunneling regime, is accounted for by the same model.

Stabilizing Single Atom Contacts by Molecular Bridge Formation

Gold-molecule-gold junctions can be formed by carefully breaking a gold wire in a solution containing dithiolated molecules. Surprisingly, there is little understanding on the mechanical details of the bridge formation process and specifically on the role that the dithiol molecules play themselves. We propose that alkanedithiol molecules have already formed bridges between the gold electrodes before the atomic gold-gold junction is broken. This leads to stabilization of the single atomic gold junction, as observed experimentally. Our data can be understood within a simple spring model.

High Gain Hybrid Graphene–Organic Semiconductor Phototransistors

Hybrid phototransistors of graphene and the organic semiconductor poly(3-hexylthiophene-2,5-diyl) (P3HT) are presented. Two types of phototransistors are demonstrated with a charge carrier transit time that differs by more than 6 orders of magnitude. High transit time devices are fabricated using a photoresist-free recipe to create large-area graphene transistors made out of graphene grown by chemical vapor deposition. Low transit time devices are fabricated out of mechanically exfoliated graphene on top of mechanically exfoliated hexagonal boron nitride using standard e-beam lithography. Responsivities exceeding 10(5) A/W are obtained for the low transit time devices.

Electrical characteristics of conjugated self-assembled monolayers in large-area molecular junctions

We have studied the electrical characteristics of close-packed monolayers of conjugated para-phenylene oligomers as a function of molecular length in large-area molecular junctions. An exponential increase in resistance with molecular length is observed, Rexp (βL), with β=0.26±0.04 A-1 and β=0.20±0.06 A-1 for dithiol and monothiol derivatives, respectively. The decay coefficients are lower than previously determined experimentally using scanning probe or breakjunction techniques. We tentatively explain the low values by the forced planer geometry of the self-assembled molecules.

Supramolecular Chemistry on Graphene Field‐Effect Transistors

Graphene, a one-atom thick two-dimensional honeycomb carbon lattice, has shown extraordinary properties and potential applications in numerous fi elds. [ 1 ] Since all atoms in graphene are surface atoms, a natural approach to tune graphene’s electronic properties is to use surface chemistry . [ 2–4 ] Recently, a number of studies on self-assembly of organic molecules on graphene have been reported, [ 5–13 ]

Localized States Influence Spin Transport in Epitaxial Graphene

We developed a spin transport model for a diffusive channel with coupled localized states that result in an effective increase of spin precession frequencies and a reduction of spin relaxation times in the system. We apply this model to Hanle spin precession measurements obtained on monolayer epitaxial graphene on SiC(0001). Combined with newly performed measurements on quasi-free-standing monolayer epitaxial graphene on SiC(0001) our analysis shows that the different values for the diffusion coefficient measured in charge and spin transport measurements on monolayer epitaxial graphene on SiC(0001) and the high values for the spin relaxation time can be explained by the influence of localized states arising from the buffer layer at the interface between the graphene and the SiC surface.

The mechanical response of lithographically defined break junctions

We present an experimental study on the mechanical response of lithographically defined break junctions by measuring atomic chain formation, tunneling traces and Gundlach oscillations. The calibration factor, i.e., the ratio between the electrode movement and the bending of the substrate, is found to be 2.5 times larger than expected from a simple mechanical model. This result is consistent with previous finite-element calculations. Comparing different samples, the mechanical response is found to be similar for electrode separations >4 A. However, for smaller electrode separations significant sample-to-sample variations appear. These variations are ascribed to differences in the shape of the two electrodes on the atomic scale which cannot be controlled by the fabrication process.

Spin relaxation in graphene with self-assembled cobalt porphyrin molecules

In graphene spintronics, interaction of localized magnetic moments with the electron spins paves a new way to explore the underlying spin-relaxation mechanism. A self-assembled layer of organic cobalt porphyrin (CoPP) molecules on graphene provides a desired platform for such studies via the magnetic moments of porphyrin-bound cobalt atoms. In this work a study of spin-transport properties of graphene spin-valve devices functionalized with such CoPP molecules as a function of temperature via nonlocal spin-valve and Hanle spin-precession measurements is reported. For the functionalized (molecular) devices, we observe a decrease in the spin-relaxation time tau(s) even up to 50%, which could be an indication of enhanced spin-flip scattering of the electron spins in graphene in the presence of the molecular magnetic moments. The effect of the molecular layer is masked for low-quality samples (low mobility), possibly due to dominance of Elliot-Yafet-type spin relaxation mechanisms.