Roman Krahne

Measurement of the conductance of single conjugated molecules

Electrical conduction through molecules depends critically on the delocalization of the molecular electronic orbitals and their connection to the metallic contacts. Thiolated (- SH) conjugated organic molecules are therefore considered good candidates for molecular conductors: in such molecules, the orbitals are delocalized throughout the molecular backbone, with substantial weight on the sulphur–metal bonds. However, their relatively small size, typically ∼1 nm, calls for innovative approaches to realize a functioning single-molecule device. Here we report an approach for contacting a single molecule, and use it to study the effect of localizing groups within a conjugated molecule on the electrical conduction. Our method is based on synthesizing a dimer structure, consisting of two colloidal gold particles connected by a dithiolated short organic molecule, and electrostatically trapping it between two metal electrodes. We study the electrical conduction through three short organic molecules: 4,4′-biphenyldithiol (BPD), a fully conjugated molecule; bis-(4-mercaptophenyl)-ether (BPE), in which the conjugation is broken at the centre by an oxygen atom; and 1,4-benzenedimethanethiol (BDMT), in which the conjugation is broken near the contacts by a methylene group. We find that the oxygen in BPE and the methylene groups in BDMT both suppress the electrical conduction relative to that in BPD.

Continuous-wave biexciton lasing at room temperature using solution-processed quantum wells

Solution-processed inorganic and organic materials have been pursued for more than a decade as low-threshold, high-gain lasing media, motivated in large part by their tunable optoelectronic properties and ease of synthesis and processing. Although both have demonstrated stimulated emission and lasing, they have not yet approached the continuous-wave pumping regime. Two-dimensional CdSe colloidal nanosheets combine the advantage of solution synthesis with the optoelectronic properties of epitaxial two-dimensional quantum wells. Here, we show that these colloidal quantum wells possess large exciton and biexciton binding energies of 132 meV and 30 meV, respectively, giving rise to stimulated emission from biexcitons at room temperature. Under femtosecond pulsed excitation, close-packed thin films yield an ultralow stimulated emission threshold of 6 μJ cm(-2), sufficient to achieve continuous-wave pumped stimulated emission, and lasing when these layers are embedded in surface-emitting microcavities.

Phosphine-free synthesis of p-type copper(I) selenide nanocrystals in hot coordinating solvents.

We report a phosphine-free synthesis of p-type copper(I) selenide nanocrystals by a colloidal approach in a mixture of oleylamine and 1-octadecene. The nanocrystals had a cuboctahedral shape and cubic berzelianite phase. Films of these nonstoichiometric copper-deficient Cu(2-x)Se nanocrystals were highly conductive and showed high absorption coefficient in the near-infrared region. These nanocrystals could be used as hole-injection layers in optoelectronic devices.

3D nanostar dimers with a sub-10-nm gap for single-/few-molecule surface-enhanced raman scattering.

Plasmonic nanostar-dimers, decoupled from the substrate, have been fabricated by combining electron-beam lithography and reactive-ion etching techniques. The 3D architecture, the sharp tips of the nanostars and the sub-10 nm gap size promote the formation of giant electric-field in highly localized hot-spots. The single/few molecule detection capability of the 3D nanostar-dimers has been demonstrated by Surface-Enhanced Raman Scattering.

Blue-UV-Emitting ZnSe(Dot)/ZnS(Rod) Core/Shell Nanocrystals Prepared from CdSe/CdS Nanocrystals by Sequential Cation Exchange

Great control over size, shape and optical properties is now possible in colloidal Cd-based nanocrystals, which has paved the way for many fundamental studies and applications. One popular example of such class of nanocrystals is represented by CdSe(spherical core)/CdS(rod shell) nanorods. These can be nearly monodisperse in size and shape and have strong and stable photoluminescence that is tunable in the visible range (mainly by varying the size of the CdSe core). The corresponding Zn-based core/shell nanorods would be good candidates for tunable emission in the blue-UV region. However, while the synthesis of ZnS nanocrystals with elongated shapes has been demonstrated based on the oriented-attachment mechanism, elongated ZnS shells are difficult to fabricate because the more common cubic phase of ZnS has a highly symmetric crystal structure. We report here a procedure based on a sequence of two cation exchange reactions, namely, Cd(2+)⇒Cu(+) and then Cu(+)⇒Zn(2+), by which we transform colloidal CdSe(core)/CdS(shell) nanorods first into into Cu(2)Se/Cu(2)S nanorods, which are then converted into blue-UV fluorescent ZnSe(core)/ZnS(shell) nanorods. The procedure transfers the morphological and structural information of the initial Cd-based nanorods to the Zn-based nanorods. Therefore, the final nanoparticles are made by a ZnSe dot embedded in a rod-shaped shell of wurtzite ZnS. Since in the starting Cd-based nanorods the size of the CdSe core and the length of the CdS shell can be well controlled, the same holds for the final Zn-based rods. In the second step of the exchange reaction (Cu(+)⇒Zn(2+)), a large excess of Zn(2+) ions added over the Cu(+) ions present in the Cu(2)Se/Cu(2)S nanorods is the key requisite to obtain bright, band-edge emission (with quantum yields approaching 15%) with narrow line widths (approaching 75 meV). In these ZnSe/ZnS nanorods, photogenerated carriers appear to be more confined in the core region compared to their parent CdSe/CdS nanorods.

Polymer-Free Films of Inorganic Halide Perovskite Nanocrystals as UV-to-White Color-Conversion Layers in LEDs

Being considerably more efficient than traditional incandescent bulbs or fluorescent tubes, light-emitting diodes (LEDs) are becoming the main technology for general lighting applications.1,2 In the last years, a variety of white-LEDs have been demonstrated employing, for example, organic molecules (OLEDs)3−7 or inorganic quantum dots (QDs) of different composition.8 A common way of achieving a white-light emission is to couple a near-UV or blue LED with down-conversion materials such as phosphors.9 More recently, QDs have been incorporated into LEDs, replacing conventional phosphors to tune finely the emission spectrum.10 Among the different available QDs, lead halide-based perovskite nanocrystals have recently emerged as very promising candidates for many optoelectronic applications.11−15 Colloidal perovskite nanocrystals can be synthesized and/or transformed postsynthesis, so that samples emitting at different wavelengths throughout the whole visible spectrum, with high photoluminescence quantum yield (PLQY), can be prepared easily. This can be achieved either by changing the chemical composition (by anion-exchange for example)16,17 or the shape (cubes, platelets, sheets, wires).18−21 Also, electroluminescence has been observed from bulk perovskite films,22−25 blended perovskite-in-polymer films,26 as well as from nanocrystals-only films27,28 leading to the first perovskite-based LEDs.

Photoconduction Properties in Aligned Assemblies of Colloidal CdSe/CdS Nanorods

We report on photoconduction and optical properties of aligned assemblies of core-shell CdSe/CdS nanorods prepared by a seeded growth approach. We fabricate oriented layers of nanorods by drop casting the nanorods from a solution on substrates with prepatterned, micrometer-spaced electrodes and obtain nanorod alignment due to the coffee stain effect. The photoconductivity of the nanorod layers can be improved significantly by an annealing process under vacuum conditions. The spectral response of the photocurrent shows distinct features that can be assigned to the electronic level structure of the core-shell nanorods and that relate well to the spectra obtained by absorption measurements. We study assemblies of nanorods oriented parallel and perpendicular to the applied electric field by the combined use of photocurrent and photoluminescence spectroscopy. We obtain consistent results which show that charge carrier separation and transport are more efficient for nanorods oriented parallel to the electric field. We also investigate the light polarization sensitivity of the photocurrent for the oriented nanorod layers and observe higher conductivity in the case of perpendicular polarization with respect to the long axis of the nanorods.

Self-assembly of highly fluorescent semiconductor nanorods into large scale smectic liquid crystal structures by coffee stain evaporation dynamics

We deposit droplets of nanorods dispersed in solvents on substrate surfaces and let the solvent evaporate. We find that strong contact line pinning leads to dense nanorod deposition inside coffee stain fringes, where we observe large scale lateral ordering of the nanorods with the long axis of the rods oriented parallel to the contact line. We observe birefringence of these coffee stain fringes by polarized microscopy and we find the direction of the extraordinary refractive index parallel to the long axis of the nanorods.

Bimetallic 3D nanostar dimers in ring cavities: recyclable and robust surface-enhanced Raman scattering substrates for signal detection from few molecules.

Top-down fabrication of electron-beam lithography (EBL)-defined metallic nanostructures is a successful route to obtain extremely high electromagnetic field enhancement via plasmonic effects in well-defined regions. To this aim, various geometries have been introduced such as disks, triangles, dimers, rings, self-similar lenses, and more. In particular, metallic dimers are highly efficient for surface-enhanced Raman spectroscopy (SERS), and their decoupling from the substrate in a three-dimensional design has proven to further improve their performance. However, the large fabrication time and cost has hindered EBL-defined structures from playing a role in practical applications. Here we present three-dimensional nanostar dimer devices that can be recycled via maskless metal etching and deposition processes, due to conservation of the nanostructure pattern in the 3D geometry of the underlying Si substrate. Furthermore, our 3D-nanostar-dimer-in-ring structures (3D-NSDiRs) incorporate several advantageous aspects for SERS by enhancing the performance of plasmonic dimers via an external ring cavity, by efficient decoupling from the substrate through an elevated 3D design, and by bimetallic AuAg layers that exploit the increased performance of Ag while maintaining the biocompatibility of Au. We demonstrate SERS detection on rhodamine and adenine at extremely low density up to the limit of few molecules and analyze the field enhancement of the 3D-NSDiRs with respect to the exciting wavelength and metal composition.

Optical properties of tetrapod-shaped CdTe nanocrystals

We studied the carrier confinement in tetrapod-shaped colloidal CdTe nanocrystals by means of absorption, photoluminescence, and photoluminescence excitation spectroscopy at room and cryogenic temperatures. The spectra show features characteristic of the tetrapod shape together with a clear dependence on the dominant confinement parameter, i.e., the diameter of the tetrapod arm. Theoretical calculations based on an envelope-function approximation and using the exact tetrapod shape have been performed to assign the observed spectral features. Oscillator strength and size dependence of the transitions energy have been calculated showing a direct correlation between the oscillator strength and the nanocrystal shape.