Michael Giersig

Silica encapsulation of quantum dots and metal clusters

The use of nanometre thick silica shells as a means to stabilize metal clusters and semiconductor particles is discussed, and its potential advantages over conventional organic capping agents are presented. Shell deposition depends on control of the double layer potential, and requires priming of the core particle surface. Chemical reactions are possible within the core, via diffusion of reactants through the shell layer. Quantum dots can be stabilized against photochemical degradation through silica deposition, whilst retaining strong fluorescence quantum yields and their size dependent optical properties. Ordered 3D and 2D arrays of a macroscopic size with uniform particle spacing can be created. Thin colloid films can also be created with well-defined interparticle spacing, allowing controlled coupling of exciton and surface plasmon modes to be investigated. A number of future core–shell nanocomposite structures are postulated, including quantum bubbles and single electron capacitors based on Au@SiO2.

Stabilization of CdS semiconductor nanoparticles against photodegradation by a silica coating procedure

Abstract Nanometre-sized CdS semiconductor particles were synthesized in the presence of sodium citrate, and subsequently surrounded by a homogeneous silica shell. The coating procedure makes use of 3-(mercaptopropyl) trimethoxy silane (MPS) as a surface primer to deposit a thin silica shell in water. The dispersion is then transferred into ethanol, where thicker shells can be grown. The citrate-stabilized particles are slowly degraded through photochemical oxidation in the presence of dissolved oxygen. This destabilizing process is suppressed when a homogeneous, microporous silica shell is built up around the particles, through a limited access of O 2 molecules to the CdS surface.

Chemical bath deposition of indium sulphide thin films : preparation and characterization

Abstract Indium sulphide (In2S3) thin films have been successfully deposited on different substrates under varying deposition conditions using chemical bath deposition technique. The deposition mechanism of In2S3 thin films from thioacetamide deposition bath has been proposed. Films have been characterized with respect to their crystalline structure, composition, optical and electrical properties by means of X-ray diffraction, TEM, EDAX, optical absorption, TRMC (time resolved microwave conductivity) and RBS. Films on glass substrates were amorphous and on FTO (flourine doped tin oxide coated) glass substrates were polycrystalline (∈ phase). The optical band gap of In2S3 thin film was estimated to be 2.75 eV. The as-deposited films were photoactive as evidenced by TRMC studies. The presence of oxygen in the film was detected by RBS analysis.

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.

Uniform self-forming metallic network as a high-performance transparent conductive electrode.

A transparent, conductive, and flexible electrode is demonstrated. It is based on an inexpensive and easily manufacturable metallic network formed by depositing metals onto a template film. This electrode shows excellent electro-optical properties, with the figure of merit ranging from 300 to 700, and transmittance from 82% (~4.3 Ω sq(-1) ) to 45% (~0.5 Ω sq(-1) ).

Growth of large periodic arrays of carbon nanotubes

Large periodic arrays of carbon nanotubes have been grown by plasma-enhanced hot filament chemical vapor deposition on periodic arrays of nickel dots that were prepared by polystyrene nanosphere lithography. A single layer of self-assembled polystyrene spheres was first uniformly deposited on a silicon wafer as a mask, and then electron beam vaporization was used to deposit a nickel layer through the mask. The size of and spacing between the nickel dots are tunable by varying the diameter of the polystyrene spheres, which consequently determines the diameter and site density of carbon nanotubes. The technique can be scaled up at much lower cost than electron beam lithography.

Layer-By-Layer Assembly of Core-Shell Magnetite Nanoparticles: Effect of Silica Coating on Interparticle Interactions and Magnetic Properties

Monodisperse PS spheres were synthesized and packed into colloidal crystals by centrifugation as described previously [6]. Sphere diameters in various batches ranged from 420 ‐ 50 nm to 660 ‐ 30 nm. To prepare macroporous nickel oxide, nickel(II) acetate (2 g) was dissolved in either acetic acid (5 mL) or a mixture of H2O (5 mL)and ethanol (5 mL) at 60 C. After cooling to room temperature, any undissolved solid was removed by filtration. Centimeter-scale, close-packed, colloidal polystyrene crystals (1 g) were soaked in this solution for 3‐5 min. Excess solution was removed from the impregnated colloidal crystals by vacuum filtration. The samples were dried at 60 C for 2 h. The dried composites were soaked in 10 mL of a saturated solution of oxalic acid in ethanol (ca. 25‐30 g in 100 mL) for 3‐5 min. After additional vacuum filtration and drying, the PS spheres were removed by calcination in flowing air at 360‐575 C for 7‐10 h (heating rate: 1 C/ min). Macroporous Ni was prepared by heating macroporous NiO in flowing H2 at 300 C for 2‐5 h (heating rate: 1 C/min) or by heating the nickel oxalate/PS composite in flowing nitrogen at 450‐500 C for 10 h. Preparation of partially reduced samples containing both NiO and Ni was possible by decreasing the reduction time or temperature, or by carrying out a fast calcination, for example at 360 C for 1 h in fast flowing air. Electrochemical tests were performed in two electrode cells using 1 M aqueous KOH as the electrolyte and nickel pellets as both electrodes. Pellets were made by pressing a mixture of 10 wt.-% poly(vinylidene fluoride) binder with 90 wt.-% nickel to 1500 psi for 30 s. An SEM of the material indicated that a porous structure was still maintained. No carbon powder was needed to enhance the pellet conductivity. Impedance analysis was performed using a Solartron 1260 Impedance/Gain-Phase Analyzer along with a Solartron 1287 Electrochemical Interface. Other product characterization was carried out as described previously [6].

Evidence of an aggregative mechanism during the formation of silver nanowires in N,N-dimethylformamide

A detailed high-resolution electron microscopy study of the formation of Ag nanowires by solvent reduction in N,N-dimethylformamide (DMF), in the presence of poly(vinylpyrrolidone) (PVP) at high temperature provided clear evidence that the nanowires grow through the oriented aggregation of precursor nanoparticles. The HRTEM images demonstrate that initially, icosahedral and cuboctahedral Ag nanoparticles are formed, which then self-assemble to form aligned stripes, and eventually fuse with each other to yield single crystalline nanowires. The nanoparticle aggregation only takes place through preferred facets of the original nanocrystals, and therefore intermediate twisted wires are observed.

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.

Periodic Large‐Area Metallic Split‐Ring Resonator Metamaterial Fabrication Based on Shadow Nanosphere Lithography

Metamaterials have gained substantial attention because of their potential for negative permeability as well as negative refractive index in the optical frequency range[1,2]. Due to their unique electromagnetic properties, these nanostructures show promise for numerous applications such as perfect lenses and optical cloaking devices.