Michael Hilgendorff

Size Effects in ZnO: The Cluster to Quantum Dot Transition

The use of tetraalkylammonium hydroxides to prepare ZnO colloids with diameters ranging from 1 to 6 nm is described. The position of the first excitonic transition has been measured by UV-vis spectrometry and correlated with the particle size, which has been measured using high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and ultracentrifugation (UC). The exciton transition is first visible at 265–270 nm corresponding to particle diameters around 1 nm; the exciton absorption band then becomes sharper and narrower, while the band red-shifts only slowly. Based on the sizing data from HRTEM, XRD, and UC, it is concluded that the quantum size effect at sizes less than the Bohr radius is significantly less than predicted from the Kayanuma equation. Based on the blue-shift in the trap emission as a function of nanocrystal size, the effective masses of the electron and hole (me, mh) remain constant in particles down to 1 nm in diameter, with a relative value given by me/(me+mh)=0.55 ± 0.04.

Synthesis of Flexible, Ultrathin Gold Nanowires in Organic Media

Gold nanoparticles are very interesting because of their potential applications in microelectronics, optical devices, analytical detection schemes, and biomedicine. Though shape control has been achieved in several polar solvents, the capability to prepare organosols containing elongated gold nanoparticles has been very limited. In this work we report a novel, simplified method to produce long, thin gold nanowires in an organic solvent (oleylamine), which can be readily redispersed into nonpolar organic solvents. These wires have a characteristic flexible, hairy morphology arising from a small thickness (<2 nm) and an enormous length (up to several micrometers), with the possibility of adjusting the dimensions through modification of the growth conditions, in particular, the gold salt concentration. Despite their extreme aspect ratio, the wires are stable in solution for long periods of time but easily break when irradiated with high-energy electron beams during transmission electron microscopy.

Magnetic and optical tunable microspheres with a magnetite/gold nanoparticle shell

Multifunctional core-shell microspheres consisting of a polystyrene 640 nm diameter core covered with a selectable number of layers of magnetite (12 nm) and silica-coated gold (15 nm) nanoparticles have been fabricated using the layer-by-layer (LbL) technique in aqueous solution. By varying the shell thickness and layer composition the magnetic and optical properties and the diameter of the colloids can be controlled independently. Optical spectra of the core-shell colloidal microspheres show a well-resolved Au surface plasmon peak in the visible which red-shifts with the number of adsorbed Au nanoparticle layers. The magnetic moment per sphere increases nearly linearly with the number of adsorbed magnetite nanoparticle layers, and the spheres can be assembled into chains of up to 1 mm length by deposition in a magnetic field.

A SIMPLE COLLOIDAL ROUTE TO NANOCRYSTALLINE ZNO/CUINS2 BILAYERS

Thin semiconductor CuInSe2 and CuInS2 films (CIS) with bandgap values (Eg) of around 1.04 eV (for selenide) and 1.5 eV (for sulfide) represent an important class of the currently developed light absorbers for solar energy harvesting. Conversion efficiencies of 12±13 % were achieved on large area modules, whereas close to 18 % was achieved with laboratory cells, indicating a large potential for CIS-derived photovoltaic materials. For their preparation, a broad range of physical and electrochemical deposition routes are available. Typically, CIS films are created via a rapid thermal sintering of elemental Cu, In and Se layers evaporated on Mo-coated glass substrates. The photovoltaic cell is then completed by overcoating the CIS-macrograins with a thin CdS buffer layer and a metal± organic chemical vapor deposition derived, transparent Al/ ZnO window electrode. In this contribution, we address a low cost colloidal route to nanocrystalline ZnO/CIS bilayers on indium tin oxide (ITO) glass. For the film deposition, concentrated coating colloids, with size-quantized CuInS2 particles were developed. It is well-established that size quantization in semiconductors (i.e. increasing bandgap energy with decreasing semiconductor dimension) takes place at particle dimensions smaller than the Wannier±Mott (WM) exciton of the corresponding macroscopic bulk phase. By knowledge of the high frequency dielectric constant, e¥, and the reduced effective exciton mass, m = 1/(m e + m ±1 h ), one can calculate the WM-exciton Bohr radius according to RB = (e¥/m) aB, with aB being the Bohr radius of the hydrogen atom. Taking the CIS bulk values of e¥ = 11, me = 0.16 and mh = 1.3, we calculated the WM-exciton size to be 8.1 nm, which predicts a blue shift in the optical absorption threshold (below 826 nm = 1240/1.5 eV) for CIS-particle sizes below 8 nm. Figure 1 shows changes in the optical absorption spectrum during the CIS condensation. Condensation was induced on addition of bis(trimethylsilyl)sulfide to a mixture of Cu(I)±P(OPh)3 and In(III)±P(OPh)3 complexes (Cu/ In = 1) in Ar saturated acetonitrile (for details see Experimental). At sulfide concentrations ~25 % (with respect to the present metals), the absorption spectrum exhibits a shoulder located at 370 nm that is strongly blue-shifted with respect to the bulk crystals (a gap energy difference of more than 2 eV). On further addition of the sulfide source (50 %), the absorption shoulder shifts from 370 nm to 400 nm, and the optical density rises due to increasing particle concentration. Under stoichiometric conditions (100 % S corresponds to the Cu:In:S stoichiometry of 1:1:2), a steep tail is observed with the absorption onset located near 580 nm. A remarkable dynamic color change accompanies this condensation process which can be seen with the naked eye. On each dropwise addition of the sulfide source, the color of the reacting solution rapidly changes from colorless to yellow to orange to red and becomes colorless or yellow again later on (build-up and decay of size-quantized cluster±cluster aggregates). There is a crossover concentration value around 50 % S, above which the sol does not self-bleach anymore, retaining a constant orange color. On the further allat-once addition of 50 % S, it takes at least 10 hours for the stoichiometric orange colloid to become deep red. Solvent removal from the above 0.05 M solutions on a rotary evaporator yields 0.5 M stable lacquers which can be used to produce single step coated, 1 mm thick compact CIS-layers with good adhesion to glass or nanocrystalline ZnO films. The preparation of nanocrystalline ZnO films from particulate colloids follows a recently published procedure (see also Experimental). Figure 2 shows the UV± Vis optical spectrum of a 2 mm thick (single step dip-coated, and at 400 C pre-sintered) ZnO film displaying the characteristic absorption edge at around 380 nm. After dip-withdrawing the ZnO film in CIS-colloid, and subsequent sintering in vacuum at 400 C for 20 minutes, a dark brown colored ZnO/CIS bilayer is formed. Its optical absorption exhibits a shoulder at 750 nm indicating that coalescence of small CIS particles to larger nanocrystals took

The preparation of ordered colloidal magnetic particles by magnetophoretic deposition

The preparation of ordered two-dimensional (2D) magnetic nanoparticles using a magnetophoretic technique is reported. The quality of the ordering can be observed by electron microscopy and the lattice constants determined by electron diffraction. Using image processing it can be shown that the cobalt particles condense into a hexagonal close packed array, and that the crystallographic axes of the individual cobalt particles are randomly oriented. Furthermore, the equilibrium distance between the 2D ordered particles decreases with increasing strength of the magnetic field while the minimum distance corresponds approximately to the size of the adsorbed stabilizers. The method is of general interest as a means of preparing monolayer films of nanosized magnetic particles such as cobalt or iron oxide.

Creation of 3-D Crystals from Single Cobalt Nanoparticles in External Magnetic Fields

Using monodisperse nanocrystalline cobalt (Co) particles in non-polar colloidal dispersions, large areas of symmetric multi-dimensional structures were created using magnetophoretic deposition (MPD). To overcome the van der Waals and magnetic dipole–dipole interactions, the particles were stabilized with hydrophobic amines, phosphines, carboxylates and/or polymers. Depending on the preparation parameters, our particles had either bcc or e-Co crystalline structures. Using MPD with magnetic fields up to 1 T, it was possible to create two-dimensional (2-D) arrays of near-perfect symmetry up to 1 m2 in size on various substrates, e.g. carbon-coated copper grids, silicon, or glass. Growth of the 2-D crystal was shown to be dependent on the direction of the applied external magnetic field. Three-dimensional (3-D) crystals could be created by increasing the magnetic field strength up to 6 T.

Ferromagnetic resonance of monodisperse Co particles

Two-dimensional arrays of monodisperse nanosized Co particles are prepared on carbon and glass substrates by a magnetophoretic deposition technique from colloidal suspensions. Transmission electron microscopy (TEM) reveals a complicated cubic crystalline structure of the particles and hexagonal ordering over several micrometers squared, if the colloidal suspension is dried in magnetic fields of up to 0.8 T. Angular-dependent ferromagnetic resonance (FMR) spectra of 4-, 5-, 9-, and 12-nm-diameter particles at 297 K show that the easy axis of magnetization is in plane and that only the 12 nm particles are measured below the blocking temperature estimated to be 656 K. The resonance linewidth is on the order of 0.1 T, indicating a much larger magnetic inhomogeneity of the particles than the small geometric and size distribution (<10%) observed by TEM suggests. Characteristic differences of the FMR spectra for different substrates and deposition parameters are observed.

Magnetic properties of arrays of interacting Co nanocrystals

Monodisperse Co FCC nanocrystals with 12 nm diameter were self-assembled into regular quasi-two-dimensional triangular periodic arrays on carbon substrates from a toluene-based colloidal suspension. At 300 K the regular arrays show a collective magnetic behaviour due to dipolar coupling. A remanent magnetization with an easy axis in the film-plane and an uniaxial in-plane anisotropy field of 0.037 T were determined by SQUID magnetometry and angular dependent ferromagnetic resonance.

Gold encapsulation of star-shaped FePt nanoparticles

We present a seeded-growth method for the encapsulation of star-like FePt nanoparticles with gold shells in aqueous solution, which allows not only further functionalization (such as silica coating) but also the organization of the synthesized core-shell nanoparticles into mesoscopic one-dimensional nanostructures under the influence of an externally applied magnetic field.

Self-assembly and magnetism in core-shell microspheres

We report on the fabrication and characterization of magnetic composite colloids with a defined shape, composition and multilayer shell thickness. They consist of a core of a polystyrene microsphere (640 nm diameter) coated with consecutive shells of Fe3O4 nanoparticles (12 nm diameter), polyelectrolytes and Au nanoparticles (15 nm). The homogeneity of the coating was confirmed by transmission electron microscopy. Composite core-shell microspheres were self-assembled into periodically ordered chains up to 2 mm in length by magnetophoretic deposition. The self-organization was visualized by optical and atomic force microscopy. Magnetic properties were determined by angular dependent ferromagnetic resonance (FMR) and superconducting quantum interference device magnetometry between 5 and 300 K. The FMR reveals long-range magnetic order at 300 K due to dipolar coupling and an easy axis in plane along the chains. We find a magnetic moment that is reduced in comparison with the magnetite bulk value.