Ultrafast spectroscopy of single molecules

We present a single-molecule study on femtosecond dynamics in multichromophoric systems, combining fs pump-probe, emission-spectra and fluorescence-lifetime analysis. At the single molecule level a wide range of exciton delocalisation lengths and energy redistribution times is revealed. Next, two color pump-probe experiments are presented as a step to addressing ultrafast energy transfer in individual complexes

Pair tunneling through single molecules

By a polaronic energy shift, the effective charging energy of molecules can become negative, favoring ground states with even numbers of electrons. Here, we show that charge transport through such molecules near ground-state degeneracies is dominated by tunneling of electron pairs which coexists with (featureless) single-electron cotunneling. Due to the restricted phase space for pair tunneling, the current-voltage characteristics exhibits striking differences from the conventional Coulomb blockade. In asymmetric junctions, pair tunneling can be used for gate-controlled current rectification and switching.Comment: 4+ pages, 4 figures; minor changes, version published in Phys. Rev. Let

Rotational Correlation Functions of Single Molecules

Single molecule rotational correlation functions are analyzed for several reorientation geometries. Even for the simplest model of isotropic rotational diffusion our findings predict non-exponential correlation functions to be observed by polarization sensitive single molecule fluorescence microscopy. This may have a deep impact on interpreting the results of molecular reorientation measurements in heterogeneous environments.Comment: 5 pages, 4 figure

Tuning the magnetic anisotropy of single molecules

The magnetism of single atoms and molecules is governed by the atomic scale environment. In general, the reduced symmetry of the surrounding splits the $d$ states and aligns the magnetic moment along certain favorable directions. Here, we show that we can reversibly modify the magnetocrystalline anisotropy by manipulating the environment of single iron(II) porphyrin molecules adsorbed on Pb(111) with the tip of a scanning tunneling microscope. When we decrease the tip--molecule distance, we first observe a small increase followed by an exponential decrease of the axial anisotropy on the molecules. This is in contrast to the monotonous increase observed earlier for the same molecule with an additional axial Cl ligand. We ascribe the changes in the anisotropy of both species to a deformation of the molecules in the presence of the attractive force of the tip, which leads to a change in the $d$ level alignment. These experiments demonstrate the feasibility of a precise tuning of the magnetic anisotropy of an individual molecule by mechanical control.Comment: 16 pages, 5 figures; online at Nano Letters (2015

Self-contained Kondo effect in single molecules

Kondo coupling of f and conduction electrons is a common feature of f-electron intermetallics. Similar effects should occur in carbon ring systems(metallocenes). Evidence for Kondo coupling in Ce(C8H8)2 (cerocene) and the ytterbocene Cp*2Yb(bipy) is reported from magnetic susceptibility and L_III-edge x-ray absorption spectroscopy. These well-defined systems provide a new way to study the Kondo effect on the nanoscale, should generate insight into the Anderson Lattice problem, and indicate the importance of this often-ignored contribution to bonding in organometallics.Comment: 4 pages, 5 figures (eps

Collecting single molecules with conventional optical tweezers

The size of particles which can be trapped in optical tweezers ranges from tens of nanometres to tens of micrometres. This size regime also includes large single molecules. Here we present experiments demonstrating that optical tweezers can be used to collect polyethylene oxide (PEO) molecules suspended in water. The molecules that accumulate in the focal volume do not aggregate and therefore represent a region of increased molecule concentration, which can be controlled by the trapping potential. We also present a model which relates the change in concentration to the trapping potential. Since many protein molecules have molecular weights for which this method is applicable the effect may be useful in assisting nucleation of protein crystals.Comment: 5 pages, 4 figure

Interaction of single molecules with metallic nanoparticles

We theoretically investigate the interaction between a single molecule and a metallic nanoparticle. We develop a general quantum mechanical description for the calculation of the enhancement of radiative and non-radiative decay channels for a molecule situated in the nearfield regime of the metallic nanoparticle. Using a boundary element method approach, we compute the scattering rates for several nanoparticle shapes. We also introduce an eigenmode expansion and quantization scheme for the surface plasmons, which allows us to analyze the scattering processes in simple physical terms. An intuitive explanation is given for the large quantum yield of quasi one- and two-dimensional nanostructures. Finally, we briefly discuss resonant Foerster energy transfer in presence of metallic nanoparticles.Comment: 12 pages, 4 figure

Rectification effects in coherent transport through single molecules

A minimal model for coherent transport through a donor/acceptor molecular junction is presented. The two donor and acceptor sites are described by single levels energetically separated by an intramolecular tunnel barrier. In the limit of strong coupling to the electrodes a current rectification for different bias voltage polarities occurs. Contacts with recent experiments of molecular rectification are also given.Comment: 10 pages, 4 figure

On-demand delivery of single DNA molecules using nanopipettes

Understanding the behavioral properties of single molecules or larger scale populations interacting with single molecules is currently a hotly pursued topic in nanotechnology. This arises from the potential such techniques have in relation to applications such as targeted drug delivery, early stage detection of disease, and drug screening. Although label and label-free single molecule detection strategies have existed for a number of years, currently lacking are efficient methods for the controllable delivery of single molecules in aqueous environments. In this article we show both experimentally and from simulations that nanopipets in conjunction with asymmetric voltage pulses can be used for label-free detection and delivery of single molecules through the tip of a nanopipet with “on-demand” timing resolution. This was demonstrated by controllable delivery of 5 kbp and 10 kbp DNA molecules from solutions with concentrations as low as 3 pM