arXiv subjects

Herre S. J. van der Zant

Publications and source records attributed to Herre S. J. van der Zant.

At least 145 records · Page 8Linked to original sources

arXiv2013

Large and tunable photo-thermoelectric 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 photo-thermoelectric 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 -4x10^2 uV/K and -1x10^5 uV/K. 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.

cond-mat.mes-hall↗

arXiv2012

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.

cond-mat.mes-hall↗

arXiv2012

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.

cond-mat.mes-hall↗

arXiv2012

Probing the charge of a quantum dot with a nanomechanical resonator

We have used the mechanical motion of a carbon nanotube (CNT) as a probe of the average charge on a quantum dot. Variations of the resonance frequency and the quality factor are determined by the change in average charge on the quantum dot during a mechanical oscillation. The average charge, in turn, is influenced by the gate voltage, the bias voltage, and the tunnel rates of the barriers to the leads. At bias voltages that exceed the broadening due to tunnel coupling, the resonance frequency and quality factor show a double dip as a function of gate voltage. We find that increasing the current flowing through the CNT at the Coulomb peak does not increase the damping, but in fact decreases damping. Using a model with energy-dependent tunnel rates, we obtain quantitative agreement between the experimental observations and the model. We theoretically compare different contributions to the single-electron induced nonlinearity, and show that only one term is significant for both the Duffing parameter and the mode coupling parameter. We also present additional measurements which support the model we develop: Tuning the tunnel barriers of the quantum dot to the leads gives a 200-fold decrease of the quality factor. Single-electron tunneling through an excited state of the CNT quantum dot also changes the average charge on the quantum dot, bringing about a decrease in the resonance frequency. In the Fabry-Pérot regime, the absence of charge quantization results in a spring behaviour without resonance frequency dips, which could be used, for example, to probe the transition from quantized to continuous charge with a nanomechanical resonator.

cond-mat.mes-hall↗

arXiv2012

Mechanical properties of freely suspended atomically thin dielectric layers of mica

We have studied the elastic deformation of freely suspended atomically thin sheets of muscovite mica, a widely used electrical insulator in its bulk form. Using an atomic force microscope, we carried out bending test experiments to determine the Young's modulus and the initial pre-tension of mica nanosheets with thicknesses ranging from 14 layers down to just one bilayer. We found that their Young's modulus is high (190 GPa), in agreement with the bulk value, which indicates that the exfoliation procedure employed to fabricate these nanolayers does not introduce a noticeable amount of defects. Additionally, ultrathin mica shows low pre-strain and can withstand reversible deformations up to tens of nanometers without breaking. The low pre-tension and high Young's modulus and breaking force found in these ultrathin mica layers demonstrates their prospective use as a complement for graphene in applications requiring flexible insulating materials or as reinforcement in nanocomposites.

cond-mat.mtrl-sci↗

arXiv2012

Strong and tunable mode coupling in carbon nanotube resonators

The non-linear interaction between two mechanical resonances of the same freely suspended carbon nanotube resonator is studied. We find that in the Coulomb blockade regime, the non-linear modal interaction is dominated by single-electron-tunneling processes, and that the mode-coupling parameter can be tuned with the gate voltage, allowing both mode softening and mode stiffening behavior. This is in striking contrast to tension-induced mode coupling in strings, where the coupling parameter is positive and gives rise to a stiffening of the mode. The strength of the mode coupling in carbon nanotubes in the Coulomb blockade regime is observed to be six orders of magnitude larger than the mechanical mode coupling in micromechanical resonators.

cond-mat.mes-hall↗

arXiv2012

Laser-thinning of MoS2: on demand generation of a single-layer semiconductor

Single-layer MoS2 is an attractive semiconducting analogue of graphene that combines high mechanical flexibility with a large direct bandgap of 1.8 eV. On the other hand, bulk MoS2 is an indirect bandgap semiconductor similar to silicon, with a gap of 1.2 eV, and therefore deterministic preparation of single MoS2 layers is a crucial step towards exploiting the large direct bandgap of monolayer MoS2 in electronic, optoelectronic, and photovoltaic applications. Although mechanical and chemical exfoliation methods can be used to obtain high quality MoS2 single-layers, the lack of control in the thickness, shape, size, and position of the flakes limits their usefulness. Here we present a technique for controllably thinning multilayered MoS2 down to a single-layer two-dimensional crystal using a laser. We generate single layers in arbitrary shapes and patterns with feature sizes down to 200 nm, and show that the resulting two-dimensional crystals have optical and electronic properties comparable to that of pristine exfoliated MoS2 single layers.

cond-mat.mes-hall↗

arXiv2012

Carbon nanotubes: Nonlinear high-Q resonators with strong coupling to single-electron tunneling

Carbon nanotubes (CNTs) are nonlinear high-Q resonators with strong coupling to single-electron tunneling. We begin by describing several methods to detect the flexural motion of a CNT resonator. Next, we illustrate how single-electron tunneling in quantum dot CNT resonators leads to sharp dips in the mechanical resonance frequency and significant damping. We discuss four different contributions to the nonlinear oscillation of a CNT resonator: beam-like mechanical nonlinearity, nonlinearity due to gate-induced mechanical tension, electrostatic nonlinearity, and nonlinearity due to single-electron tunneling, and provide quantitative estimates of their strengths. Finally, we show how the large response of the resonance frequency of a CNT resonator to a change in gate voltage or tension makes CNT resonators ideally suited for parametric excitation and for studying the coupling between different mechanical modes.

cond-mat.mes-hall↗

arXiv2012

Mechanical properties of freely suspended semiconducting graphene-like layers based on MoS2

We fabricate freely suspended nanosheets of MoS2 which are characterized by quantitative optical microscopy and high resolution friction force microscopy. We study the elastic deformation of freely suspended nanosheets of MoS2 using an atomic force microscope. The Young's modulus and the initial pre-tension of the nanosheets are determined by performing a nanoscopic version of a bending test experiment. MoS2 sheets show high elasticity and an extremely high Young's modulus (0.30 TPa, 50% larger than steel). These results make them a potential alternative to graphene in applications requiring flexible semiconductor materials.

cond-mat.mes-hall↗

arXiv2012

Elastic properties of freely suspended MoS2 nanosheets

We study the elastic deformation of few layers (5 to 25) thick freely suspended MoS2 nanosheets by means of a nanoscopic version of a bending test experiment, carried out with the tip of an atomic force microscope. The Young's modulus of these nanosheets is extremely high (E = 0.33 TPa), comparable to that of graphene oxide, and the deflections are reversible up to tens of nanometers.

cond-mat.mes-hall↗

arXiv2012

Room-temperature gating of molecular junctions using few-layer graphene nanogap electrodes

We report on a method to fabricate and measure gateable molecular junctions which 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 IV-characteristics at room temperature.

cond-mat.mes-hall↗

arXiv2011

Mechanical systems in the quantum regime

Mechanical systems are ideal candidates for studying quantumbehavior of macroscopic objects. To this end, a mechanical resonator has to be cooled to its ground state and its position has to be measured with great accuracy. Currently, various routes to reach these goals are being explored. In this review, we discuss different techniques for sensitive position detection and we give an overview of the cooling techniques that are being employed. The latter include sideband cooling and active feedback cooling. The basic concepts that are important when measuring on mechanical systems with high accuracy and/or at very low temperatures, such as thermal and quantum noise, linear response theory, and backaction, are explained. From this, the quantum limit on linear position detection is obtained and the sensitivities that have been achieved in recent opto and nanoelectromechanical experiments are compared to this limit. The mechanical resonators that are used in the experiments range from meter-sized gravitational wave detectors to nanomechanical systems that can only be read out using mesoscopic devices such as single-electron transistors or superconducting quantum interference devices. A special class of nanomechanical systems are bottom-up fabricated carbon-based devices, which have very high frequencies and yet a large zero-point motion, making them ideal for reaching the quantum regime. The mechanics of some of the different mechanical systems at the nanoscale is studied. We conclude this review with an outlook of how state-of-the-art mechanical resonators can be improved to study quantum {\it mechanics}.

cond-mat.mes-hall↗

arXiv2011

Charge Transport in a Zn-Porphyrin single molecule junction

We have investigated charge transport in ZnTPPdT-Pyr molecular junctions using the lithographic MCBJ technique at room temperature and cryogenic temperature (6K). We combined low-bias statistical measurements with spectroscopy of the molecular levels using I(V) characteristics. This combination allows us to characterize the transport in a molecular junction in detail. This complex molecule can form different junction configurations, which is observed in trace histograms and in current-voltage (I(V)) measurements. Both methods show that multiple stable single-molecule junction configurations can be obtained by modulating the inter-electrode distance. In addition we demonstrate that different ZnTPPdT-Pyr junction configurations can lead to completely different spectroscopic features with the same conductance values. We show that statistical low-bias conductance measurements should be interpreted with care, and that the combination with I(V) spectroscopy represents an essential tool for a more detailed characterization of the charge transport in a single molecule.

cond-mat.mes-hall↗

arXiv2011

Influence of chemical structure on the stability and the conductance of porphyrin single-molecule junctions

Porphyrin molecules can form stable single molecule junctions without anchoring groups. Adding thiol end groups and pyridine axial groups yields more stable junctions with an increased spread in low-bias conductance. This is a result of different bridging geometries during breaking, the stability of which is demonstrated in time-dependent conductance measurements. This is in strong contrast with rod like molecules which show one preferential binding geometry.

cond-mat.mes-hall↗

arXiv2010

Mechanical stiffening, bistability, and bit operations in a microcantilever

We investigate the nonlinear dynamics of microcantilevers. We demonstrate mechanical stiffening of the frequency response at large amplitudes, originating from the geometric nonlinearity. At strong driving the cantilever amplitude is bistable. We map the bistable regime as a function of drive frequency and amplitude, and suggest several applications for the bistable microcantilever, of which a mechanical memory is demonstrated.

cond-mat.mes-hall↗

arXiv2010

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.

cond-mat.mes-hall↗

arXiv2010

Electric Field Controlled Magnetic Anisotropy in a Single Molecule

We have measured quantum transport through an individual Fe$_4$ single-molecule magnet embedded in a three-terminal device geometry. The characteristic zero-field splittings of adjacent charge states and their magnetic field evolution are observed in inelastic tunneling spectroscopy. We demonstrate that the molecule retains its magnetic properties, and moreover, that the magnetic anisotropy is significantly enhanced by reversible electron addition / subtraction controlled with the gate voltage. Single-molecule magnetism can thus be electrically controlled.

cond-mat.mes-hall↗

arXiv2010

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.

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