Ferry Prins

Room‐Temperature Electrical Addressing of a Bistable Spin‐Crossover Molecular System

The design of switchable nanodevices based on magnetic molecules has therefore remained a theoretical topic. [ 10–12 ] Here, we report a switchable molecular device made by contacting individual nanoparticles based on spin-crossover molecules between nanometer-spaced electrodes. This nanoscale device exhibits switching and memory effects near room temperature as a consequence of the intrinsic bistability of the spin-crossover nanoparticle. Thus, a sharp increase in the conductance is observed upon heating above ca. 350 K, together with the presence of a thermal hysteresis as large as 30 K for a single-particle device, after which the conductance switches back to the original value. This is a long-sought-for result, as it confi rms the existence of hysteretic spin crossover effects in a single nanoobject. [ 13–17 ] Interestingly for molecular spintronics, the spin crossover in this molecular nanodevice can also be induced by applying a voltage, showing that its magnetic state is controllable electrically. Spin-crossover metal complexes are one of the paradigmatic examples of magnetic molecular materials showing switching and bistability at the molecular level. [ 18 ] In these systems lowspin to high-spin transitions can be triggered through a variety of external stimuli (temperature, illumination, or pressure) [ 19 ]

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.

Monodisperse, Air-Stable PbS Nanocrystals via Precursor Stoichiometry Control

Despite their technological importance, lead sulfide (PbS) nanocrystals have lagged behind nanocrystals of cadmium selenide (CdSe) and lead selenide (PbSe) in terms of size and energy homogeneity. Here, we show that the ratio of lead to sulfur precursor available during nucleation is a critical parameter affecting subsequent growth and monodispersity of PbS nanocrystal ensembles. Applying this knowledge, we synthesize highly monodisperse (size dispersity <5%) PbS nanocrystals over a wide range of sizes (exciton energies from 0.70 to 1.25 eV, or 1000-1800 nm) without the use of size-selective precipitations. This degree of monodispersity results in absorption peak half width at half max (HWHM) values as small as 20 meV, indicating an ensemble that is close to the homogeneous limit. Photoluminescence emission is correspondingly narrow and exhibits small Stokes shifts and quantum efficiencies of 30-60%. The nanocrystals readily self-assemble into ordered superlattices and exhibit exceptional air stability over several months.

Reduced Dielectric Screening and Enhanced Energy Transfer in Single- and Few-Layer MoS2

We report highly efficient nonradiative energy transfer from cadmium selenide (CdSe) quantum dots to monolayer and few-layer molybdenum disulfide (MoS2). The quenching of the donor quantum dot photoluminescence increases as the MoS2 flake thickness decreases with the highest efficiency (>95%) observed for monolayer MoS2. This counterintuitive result arises from reduced dielectric screening in thin layer semiconductors having unusually large permittivity and a strong in-plane transition dipole moment, as found in MoS2. Excitonic energy transfer between a zero-dimensional emitter and a two-dimensional absorber is fundamentally interesting and enables a wide range of applications including broadband optical down-conversion, optical detection, photovoltaic sensitization, and color shifting in light-emitting devices.

Subdiffusive Exciton Transport in Quantum Dot Solids

Colloidal quantum dots (QDs) are promising materials for use in solar cells, light-emitting diodes, lasers, and photodetectors, but the mechanism and length of exciton transport in QD materials is not well understood. We use time-resolved optical microscopy to spatially visualize exciton transport in CdSe/ZnCdS core/shell QD assemblies. We find that the exciton diffusion length, which exceeds 30 nm in some cases, can be tuned by adjusting the inorganic shell thickness and organic ligand length, offering a powerful strategy for controlling exciton movement. Moreover, we show experimentally and through kinetic Monte Carlo simulations that exciton diffusion in QD solids does not occur by a random-walk process; instead, energetic disorder within the inhomogeneously broadened ensemble causes the exciton diffusivity to decrease over time. These findings reveal new insights into exciton dynamics in disordered systems and demonstrate the flexibility of QD materials for photonic and optoelectronic applications.

Wedge Waveguides and Resonators for Quantum Plasmonics

Plasmonic structures can provide deep-subwavelength electromagnetic fields that are useful for enhancing light–matter interactions. However, because these localized modes are also dissipative, structures that offer the best compromise between field confinement and loss have been sought. Metallic wedge waveguides were initially identified as an ideal candidate but have been largely abandoned because to date their experimental performance has been limited. We combine state-of-the-art metallic wedges with integrated reflectors and precisely placed colloidal quantum dots (down to the single-emitter level) and demonstrate quantum-plasmonic waveguides and resonators with performance approaching theoretical limits. By exploiting a nearly 10-fold improvement in wedge-plasmon propagation (19 μm at a vacuum wavelength, λvac, of 630 nm), efficient reflectors (93%), and effective coupling (estimated to be >70%) to highly emissive (∼90%) quantum dots, we obtain Ag plasmonic resonators at visible wavelengths with quality factors approaching 200 (3.3 nm line widths). As our structures offer modal volumes down to ∼0.004λvac3 in an exposed single-mode waveguide–resonator geometry, they provide advantages over both traditional photonic microcavities and localized-plasmonic resonators for enhancing light–matter interactions. Our results confirm the promise of wedges for creating plasmonic devices and for studying coherent quantum-plasmonic effects such as long-distance plasmon-mediated entanglement and strong plasmon–matter coupling.

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.

Highly efficient, dual state emission from an organic semiconductor

We report highly efficient, simultaneous fluorescence and phosphorescence (74% yield) at room temperature from a single molecule ensemble of (BzP)PB [N,N′-bis(4-benzoyl-phenyl)-N,N′-diphenyl-benzidine] dispersed into a polymer host. The slow phosphorescence (208 ms lifetime) is very efficient (50%) at room temperature and only possible because the non-radiative rate for the triplet state is extremely low (2.4 × 100 s−1). The ability of an organic molecule to function as an efficient dual state emitter at room temperature is unusual and enables a wide range of applications including the use as broadband down-conversion emitters, optical sensors and attenuators, exciton probes, and spin-independent intermediates for Forster resonant energy transfer.

Room-temperature stability of Pt nanogaps formed by self-breaking

We present a method to make Pt nanometer-spaced electrodes that are free of metallic particles and stable at ambient conditions. The nanogaps are fabricated using feedback-controlled electromigration to form few-atom contacts. When performing this procedure at elevated temperatures (>420 K), the Pt contacts undergo self-breaking so that nanometer separated electrode pairs are formed. Once cooled down to lower temperatures, the nanogaps stabilize and can be characterized in detail. We find that current-voltage characteristics can be well fitted to a Simmons model for tunneling and gap-size fluctuations at room temperature determined from these fits stay within 0.6 A for at least 50 h.

Influence of the Chemical Structure on the Stability and Conductance of Porphyrin Single‐Molecule Junctions

The use of porphyrin molecules as building blocks of functional molecular devices has been widely investigated. The structural flexibility and well-developed synthetic chemistry of porphyrins allows their physical and chemical properties to be tailored by choosing from a wide library of macrocycle substituents and central metal atoms. Nature itself offers magnificent examples of processes that utilize porphyrin derivatives, such as the activation and the transport of molecular oxygen in mammals and the harvesting of sunlight in plant photosynthetic systems. In order to exploit the highly desirable functionality of porphyrins in artificial molecular devices, it is imperative to understand and control the interactions that occur at the molecule–substrate interface. Such interactions largely depend on the electronic and conformational structures of the adsorbed molecules, which can be studied using techniques such as scanning tunneling microscopy, UV photoemission spectroscopy, and X-ray photoemission spectroscopy, and on a theoretical level with density functional calculations. Recent studies on conjugated rod-like molecules have shown that molecular conductance measurements can be significantly affected by the binding geometry, coupling of the p orbitals to the leads, or p–p stacking between adjacent molecules. Herein, we present the results of a study of the interaction of laterally extended p-conjugated porphyrin molecules with electrodes by means of timeand stretching-dependent conductance measurements on molecular junctions. We further investigate strategies to reduce interactions of the molecular p electrons with the metal electrodes by modifying the chemical structure of the porphyrin molecules. We used the series of molecules represented in Figure 1a– c to examine the influence of the molecular structure on the formation of porphyrin single-molecule junctions. Since the thiol group is most commonly used to contact rod-like molecules to form straight molecular bridges, we first compared 5,10,15,20-tetraphenylporphyrin without thiol termination (H2-TPP; Figure 1a) to a nearly identical molecule with two thiol groups on opposite sides of the molecule (5,15di(p-thiophenyl)-10,20-di(p-tolyl)porphyrin (H2-TPPdT); Figure 1b). To investigate the influence of the molecular backbone geometry on the junction formation we further studied a thiol-terminated porphyrin molecule with two bulky pyridine axial groups attached through an octahedral Ru ion ([Ru{5,15-di(p-thiophenyl)-10,20-diphenylporphyrin}(py)2] (Ru-TPPdT); Figure 1c). As a consequence of steric hindrance, the pyridine groups in Ru-TPPdT reduce the direct interaction of the metal electrodes with the p face of the porphyrin. A similar strategy was used previously. Prior to electrical characterization, the molecules were deposited using self-assembly from solution. To study the conductance of these molecules we used lithographic mechanically controllable break junctions (MCBJs) in vacuum at room temperature. The layout of an MCBJ device in a threepoint bending mechanism is shown in Figure 1d. Details concerning the synthesis of the molecules and the experimental procedures are given in the Supporting Information. Sets of 1000 consecutive breaking traces from individual junctions were analyzed numerically to construct “trace histograms” of the conductance (log10G versus the electrode displacement d). This statistical method maps the breaking dynamics of the junctions beyond the point of rupture of the last monatomic gold contact (defined as d= 0), which has a conductance of one quantum unit G0= 2e h. Areas of high counts represent the most typical breaking behavior of the molecular junctions. Figure 2 presents trace histograms as well as examples of individual breaking traces for acetone as reference, H2-TPP, H2-TPPdT, and Ru-TPPdT. For all three porphyrin molecules as well as for the reference sample several junctions were measured (see the Supporting Information). Herein, we only show a typical set of junctions. In the junction which was exposed to pure acetone (Figure 2a), the Au bridge initially gets stretched until a plateau around the conductance quantum (G G0) is observed (only visible in the individual traces shown in [*] M. L. Perrin, F. Prins, Dr. C. A. Martin, Prof. Dr. H. S. J. van der Zant, Dr. D. Dulic Kavli Institute of Nanoscience, Delft University of Technology Lorentzweg 1, 2628 CJ Delft (The Netherlands) E-mail: d.dulic@tudelft.nl