Stefan Kudera

End‐to‐End Assembly of Shape‐Controlled Nanocrystals via a Nanowelding Approach Mediated by Gold Domains

[*] Dr. A. Figuerola, I. R. Franchini, A. Fiore, Dr. S. Kudera, Prof. R. Cingolani, Dr. L. Manna National Nanotechnology Laboratory of CNR-INFM, Unita di Ricerca IIT Distretto Tecnologico ISUFI, via per Arnesano km 5, I-73100 Lecce (Italy) Fax: (þ39) 0832298237 E-mail: liberato.manna@unile.it Dr. A. Figuerola, A. Fiore, R. Mastria, Prof. R. Cingolani Scuola Superiore ISUFI; University of Salento Distretto Tecnologico ISUFI, via per Arnesano km 5, I-73100 Lecce (Italy)

Selective reactions on the tips of colloidal semiconductor nanorods

A strategy to access several types of Au-tipped dumbbell-like nanocrystal heterostructures is presented, which involves the selective oxidation of either PbSe or CdTe sacrificial domains, initially grown on CdSe and CdS nanorods, with a Au(III) ∶ surfactant complex. The formation of gold patches is supported by TEM, XRD and elemental analysis. This approach has allowed us to grow Au domains onto specific locations of anisotropically shaped nanocrystals for which direct metal deposition is unfeasible, as for the case of CdS nanorods. We believe that this strategy may be of general utility to create other types of complex colloidal nanoheterostructures, provided that a suitable sacrificial material can be grown on top of the starting nanocrystal seeds.

Blue light emitting diodes based on fluorescent CdSe∕ZnS nanocrystals

The authors report on the blue electroluminescence from CdSe∕ZnS core/shell nanocrystals prepared from ultrasmall, magic size CdSe clusters that have a diameter of less than 2nm. The light emitting device consists of an active layer of nanocrystals blended with 4,4′,N,N′- diphenylcarbazole and an evaporated electron transporting/hole blocking layer made of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline. A blue, stable electroluminescence at 485nm from the hybrid device was observed, in good agreement with the photoluminescence spectra of a solid film of the same nanocrystals used for the device.

Ligand exchange of CdSe nanocrystals probed by optical spectroscopy in the visible and mid-IR

Optical spectroscopy in the UV/VIS and mid-IR spectral range was applied for a quantitative analysis of ligand exchange in CdSe semiconductor nanocrystals. CdSe cores from a single synthesis run were used for the study in order to ensure identical starting conditions for the exchange of different ligand molecules. From the analysis of the mid-IR spectrum the binding of the ligand molecules was monitored and the ligand exchange rate was obtained. DFT calculations yielded the vibrational normal modes of the system CdSe/ligand and allowed the identification of specific marker bands for the binding of different ligands to the nanocrystal surface.

Growth of colloidal nanoparticles of group II–VI and IV–VI semiconductors on top of magnetic iron–platinum nanocrystals

Colloidal hybrid nanoparticles with blebs of II–VI and IV–VI particles on top of FePt particles were successfully grown. Whereas in the case of XE = CdS, ZnS, PbS and CdSe, FePt–XE dimer structures were successfully obtained, the growth failed for XE = ZnSe, PbSe. Structural, magnetic, and optical analysis of the structures revealed a moderate influence of the semiconductor domain on the magnetic properties of the FePt core particle, which can be ascribed to the effect of annealing conditions, atom diffusion and compositional variations during heterostructure formation. On the other hand, we report a significant fluorescence quenching of the semiconductor bleb due to the underlying FePt particle.

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.

Fabrication of porous silicon by metal-assisted etching using highly ordered gold nanoparticle arrays

A simple method for the fabrication of porous silicon (Si) by metal-assisted etching was developed using gold nanoparticles as catalytic sites. The etching masks were prepared by spin-coating of colloidal gold nanoparticles onto Si. An appropriate functionalization of the gold nanoparticle surface prior to the deposition step enabled the formation of quasi-hexagonally ordered arrays by self-assembly which were translated into an array of pores by subsequent etching in HF solution containing H2O2. The quality of the pattern transfer depended on the chosen preparation conditions for the gold nanoparticle etching mask. The influence of the Si surface properties was investigated by using either hydrophilic or hydrophobic Si substrates resulting from piranha solution or HF treatment, respectively. The polymer-coated gold nanoparticles had to be thermally treated in order to provide a direct contact at the metal/Si interface which is required for the following metal-assisted etching. Plasma treatment as well as flame annealing was successfully applied. The best results were obtained for Si substrates which were flame annealed in order to remove the polymer matrix - independent of the substrate surface properties prior to spin-coating (hydrophilic or hydrophobic). The presented method opens up new resources for the fabrication of porous silicon by metal-assisted etching. Here, a vast variety of metal nanoparticles accessible by well-established wet-chemical synthesis can be employed for the fabrication of the etching masks.

Confinement Effects on Optical Phonons in Polar Tetrapod Nanocrystals Detected by Resonant Inelastic Light Scattering

We investigated CdTe nanocrystal tetrapods of different sizes by resonant inelastic light scattering at room temperature and under cryogenic conditions. We observe a strongly resonant behavior of the phonon scattering with the excitonic structure of the tetrapods. Under resonant conditions we detect a set of phonon modes that can be understood as confined longitudinal-optical phonons, surface-optical phonons, and transverse-optical phonons in a nanowire picture.

Growth mechanism, shape and composition control of semiconductor nanocrystals

In the last few years the development of colloidal nanocrystals has been extended from the pure adjustment of the size of the particles to the control of more sophisticated properties as their shape and composition. The hope is that through these features different fields of applications might be opened for the use of nanocrystals. In this chapter we will introduce a concept for the preparation of colloidal nanocrystals with a narrow size distribution and then discuss the shape control of colloidal nanocrystals as well as a technique for composition control, i.e. the formation of hybrid materials.

Spatially resolved photoconductivity of thin films formed by colloidal octapod-shaped CdSe/CdS nanocrystals

We studied the optical absorption and photoconductive properties of thin films consisting of core-shell octapod-shaped nanocrystals, which consisted of CdS pods that branch out from a CdSe core. The current-voltage characteristics were measured at room and cryogenic temperatures and agreed well with a phenomenological exponential fitting model, from which we could extract the sheet resistance and the average voltage barrier for the charge tunneling between the octapods. The temperature dependence of the photocurrent showed temperature activated behavior above 220 K and a non-Arrhenius exponential (T/T(0))(n) dispersion below 220 K. Furthermore, we mapped the photocurrent generation within the octapod film via scanning photocurrent microscopy, which revealed photocurrent enhancement near micron-size voids and spatial shifts of the photocurrent maxima with bias voltage.