Functional Properties of Nanostructured Materials

This contribution provides a brief introduction to the topic of this NATO-ASI, functional properties of nanostructured materials. It addresses briefly some basic aspects of nanotechnology and nanoscaled/nanostructured materials, emphasizing especially those topics which are addressed by the contributions to this volume. The examples given stem to some extent from the work at the Institute for Microstructure Technologies and Analytics (IMA) at the University of Kassel.

This paper commences with a brief review of the conventional concepts of electronic transport in the solid state, for ideal single-crystal materials. It then describes the manner in which these must be modified to incorporate the effects of disorder, how parameters such as the carrier mobilities are influenced, and how some of the techniques used to determine these properties are affected. Additionally, it is shown that computer-based modelling is a powerful tool for predicting and understanding the experimental behaviour.

Computer-based modelling constitutes an extremely powerful tool for both, predicting and understanding carrier transport in disordered solids. This paper initially outlines the techniques that can be employed.

Investigation of the physical processes taking place in MIM systems as well as precise determination of the basic parameters of the semiconductor layer (dielectric permittivity and effective electron mass) and of the potential barrier (electron work function) is of great interest. Bulk samples from the Ge- Se-(B,Ga,Tl) ternary system were prepared by direct monotemperature synthesis in evacuated quartz ampoules. Thin films of the system were prepared by vacuum sublimation. The film structure has been investigated by X-ray and electron diffraction, their composition by Auger electron spectroscopy.

This survey paper is the substantially extended version of three lectures presented at the NATO Advanced Study Institute. It covers various investigations of our Cluster Matter Research Group. It is aimed to show different facets of the fascinating and often unique optical and electronic properties of metals in the lower nanometer size region, i.e. the region where the solid state is born. Topics, dealt with, are optical size effects in noble metal nanoparticles, optical surface/interface effects, chemical effects at the interface of embedded nanoparticles, silver nanowires. They are exemplified by presenting key experiments rather than to treat them systematically.

Nanoscaled/nanocomposite thin films with improved mechanical/ tribological properties are presented. First some basic effects determining these properties in the case of nanophase materials (Hall-Petch effect, Griffith mechanisms, grain boundary sliding) are introduced. After presenting briefly a classification of nanoscaled thin films, emphasis is given to multilayer and nanocomposite films. For both classes, examples are given, and the underlaying mechanisms are discussed in some detail.

The fabrication of nanostructures involves two main processes: lithography, which creates the desired pattern, and the etching process realizing the structure in a material. The paper gives an overview of lithographic and dry etching processes, showing a number of examples and some up-to-date top achievements of these techniques.

The present work concentrates on the synthesis of metal nanoparticles in dielectrics by low energy ion implantation and particle modification by excimer laser pulsed lasers. One of the main features of composite samples prepared by implantation is the growth of metal particles with a very wide size distribution in the depth from the irradiated substrate surface. Pulsed laser irradiation makes it possible to change this composite layer, improving the uniformity of the particle sizes.

The strategies for synthesizing nanoparticles of semiconductors or metals in oxide films derived from gels are overviewed. Thermochemical treatments of gels obtained from sols containing metallic salts in solid solution, sols mixed with colloids or mesoporous xerogels impregnated with salts may be used. Ion irradiation is particularly suitable to convert gels to fully dense oxides, due to the high sensitivity of these materials to electronic excitations. The interest of using ion irradiation to form nanoparticles with a narrow size distribution in thin films, for optical or magnetic applications, is emphasized.

The addition of transition metal oxides with high enough concentrations to traditional glass-forming systems can lead to semiconducting electrical properties.

Synthesis of immiscible borate glasses containing PbO, TeO₂ and Fe₂O₃ was performed by conventional melting technology and by lowtemperature sol-gel method. Thermal treatment was performed to obtain glasscrystalline materials. Incorporation techniques were used for the preparation of nanostructured composites based on an amorphous matrix and crystalline powder nanoparticles of PbMoO₄ and Fe₃O₄. Structural characterisation of nanoparticles and nanocomposites was performed by TEM and SAED.

During the last years wood filled polymers have demonstrated remarkable scientific and ecological progress. In order to improve the mixture of filler and polymer material, it is advisable to undertake a preliminary modification of the wood filler. The present investigation is focused on the microstructure of composites materials based on polypropylene and wood flour, modified with monochloracetic acid.

The influence of laser wavelength, fluence of the laser radiation, substrate temperature, laser operational frequency, reactive atmosphere parameters and deposited layer thickness on the physical properties of pulsed laser deposited hydroxyapatite layers is presented. The text also shows the impact of the substrate material on the parameters of the deposited layers.

This contribution is concerned with thin waveguiding films fabricated by the pulsed laser deposition (PLD) method. Active waveguides are represented by waveguiding lasers and passive by layers for gas sensing. An overview of applications of waveguiding films is presented also.

We review the successful deposition of functionalized thin films of a wide range of biomaterials: natural polysaccharides and derivates, chemoselective polymers, structural proteins, adhesive proteins analogs, and enzymes by Matrix Assisted Pulsed-Laser Evaporation (MAPLE), and active proteins and living cells by Direct Write Matrix Assisted Pulsed-Laser Evaporation (MAPLE DW). We demonstrate that the deposited thin films and structures are identical to the starting material, preserving their chemical structure and very likely their functionality and biologic activity.

This contribution addresses the formation of ultrathin films on silicon surfaces: Metallic films, oxides and nitrides, and aluminum oxide and nitride, and their properties.

We report a multi-element study by dynamic and quantitative secondary ion mass spectrometry (SIMS) of impurities in CdTe/CdS/TCO/glass solar cell structures. The data obtained for O, Cl, Pb, Si, In, Cu, Sb, Zn, Na, and Sn were compared and discussed.

Nanocomposites consisting of diamond nanocrystals of 3–5 nm diameter embedded in an amorphous carbon matrix have been deposited by microwave plasma chemical vapour deposition from CH₄/N₂ mixtures. A comprehensive study utilizing a variety of different, often complementary techniques was carried out to establish the basic properties of the films (morphology and structure, composition, crystallinity, bonding environment, etc.). This study gave also some insight into the mechanisms of nucleation and growth of nanocrystalline diamond films. Finally, the films have been characterized with respect to a number of properties which are relevant for future applications (mechanical, optical, electrical, biomedical).

Protective coatings for tools and components are nowadays required in an increasingly demanding and sophisticated profile with respect to the desired mechanical, tribological, physical or chemical properties. Frequently, the necessary requirements can only be realized by multi-functional coatings through a nano-scale thin film design and the selection of materials with outstanding properties.

A concept for the deposition of thick, well-adherent cubic boron nitride thin films is presented. It takes into account the pecularities of c-BN deposition, i.e. the unique nucleation and growth sequence, the well-defined parameter spaces for nucleation and growth, and the differences between the two steps. Our concept includes measures to improve the adhesion during the nucleation step, e.g. by gradient interlayers, and to reduce the stress during the subsequent growth by appropriate parameter selection. It results in the deposition of well-adherent, stable c-BN films of several hundred nm thickness.

Boron nitride thin layers were fabricated by the pulsed laser deposition technique using a KrF excimer laser. In order to improve stoichiometry, a radio frequency discharge generator was additionally applied.

Results on the Al- and Ni-induced crystallisation of a-Si deposited by RF magnetron sputtering under isothermal annealing in air, N₂, H₂+N₂, H₂ or vacuum are presented. The influences of different technological parameters - substrate temperature, metal concentration and annealing atmosphere - on the crystallinity of the poly-Si films are investigated. The results obtained are explained in terms of a metal induced crystallisation mechanism.

A review of the results of a study of the optical and structural properties of a-SiC:H films, subjected to Ge+ and Sn+ implantation in order to obtain optical contrast, is presented.

A review of the properties of porous silicon (PS), prepared by stain etching, and of light emitting and solar cell heterostructures utilising it is presented. The mechanisms of carrier transport in light emitting diode (LED) structures and of the electroluminescence are also discussed.

Poly-Si thin films were fabricated by Aluminium Induced Crystallization (AIC) of a-Si on ZnO:Al/Al coated glass substrates. ZnO:Al, Al and a-Si layers were deposited by rf magnetron sputtering. We have investigated the influence of different annealing atmospheres on the structural properties of the poly-Si films. The resulting polycrystalline Si films were analysed by microprobe Raman spectroscopy, optical microscopy and X-ray diffraction.

The properties of transition metal oxides are mainly determined by their specific band structure and electron distribution. Electrically activated materials are very interesting due to the variety of potential applications, including elements for information displays, antiglare rearview mirrors, sunroofs and smart windows.

Multilayered films with nanometer sized single layers are important as mirrors for extreme ultraviolet radiation or soft x-rays. Oxide systems are promising multilayers for the so-called water-window (between 2.4 and 4.4 nm wavelength). Oxygen is transparent for radiation at this wavelength, and oxides are known to form very smooth, stable interfaces without interdiffusion. In this paper we present tungsten oxide / silicon oxide multilayers, deposited by plasma assisted chemical vapor deposition (PECVD). We deposited multilayer systems consisting of up to 80 individual layers, with single layers of less than one nanometer. The roughness of the layer interfaces is low enough to avoid island formation.

The transient conductivity and polarization of nano-sized ZrO₂ thin films were investigated. RF magnetron sputtering was used for the deposition of the films with different thicknesses on poly-Si. The layers were annealed at 600 and 850°C in oxygen ambient. Capacitors fabricated with top Al contacts were studied by current-voltage and hysteresis loop measurements. The dependence of the polarization on thickness and annealing temperature was evaluated.

A multilayer coating on a glass pane, specifically designed for artwork protection, has been designed. The materials selected for the coating are SiO₂ (silicon oxide), TiO₂ (titanium oxide) and ITO (indium tin oxide). Prototype layers were deposited by thermal evaporation on glass substrates. Neutron reflectometry consists of measuring the amount of neutrons reflected by a specimen at angles close to total reflection and can be successfully used to evaluate thickness, surface and interfacial roughness, and material inhomogeneity of each layer in the coating. Neutron reflectometry has demonstrated that some layers are likely to be affected by inhomogeneity that should be controlled to optimize the final performance of the device.

We are reporting new features of metal oxide coatings for gas detection applications based upon the modification of the optical parameters in order to measure gas concentrations.

Thin films of erbium-doped yttrium aluminum garnet (Er:Y₃Al₅O₁₂) were grown by pulsed laser deposition (PLD) followed by high temperature annealing. Such films are very appropriate for use in planar waveguide lasers and can be used in medical and telecommunication applications as well.

The optical properties of as-deposited As₂S₃ films with amorphous and nanocrystalline structures and those illuminated to saturation are investigated.

The application of a recently developed spectrophotometric method for the determination of the complex refractive index ñ and the physical

This paper presents an overview over nanostructured carbon materials, starting with a classification of carbon nanostructures based on the chemical bonding of the carbon atoms, i.e. on the type of hybridization. Then a summary of the carbon nanoworld is given, addressing the stability and the typical geometrical sizes of its family members. Finally, the most common methods for the synthesis of nanostructured carbon materials and some application relevant properties of nanocrystalline diamond films, nanodiamond powders, carbon nanotubes and fullerenes are discussed.

Encapsulates containing magnetic crystals are produced by the arcdischarge method. The functionalised magnetic encapsulates could be used in targeted drug delivery systems and as NMR contrast agents.

Laser pyrolysis of gaseous mixtures based on hydrocarbons produces, depending on experimental parameters, carbon blacks with different nanostructures. Some heteroatoms play a crucial role in the formation of special structures with higher degree of order than that found in classical turbostratic carbon.

Single-crystalline zincsulfide nanowires of different diameters were synthesized according to the vapor liquid solid (VLS) mechanism. We investigated the crystal structure by means of transmission electron microscopy and X-ray diffraction, showing both, wurtzite and zincblende structure. The optical properties were studied by photoluminescence.

One particular group of materials exhibits a peculiar property called shape memory effect.

An overview is given on the fabrication of semiconductor nanostructures and their applications in fundamental physics and optoelectronic devices. With advanced epitaxial growth techniques for self-assembled quantum dots in combination with high resolution lithography and high aspect ratio dry etching techniques, a new class of miniaturized quantum laboratories based on the interaction of single electrons and photons could be realised. High quality quantum dot structures are used to fabricate lasers in the wavelength range from 1 - 2 μm. Examples are shown for telecom and high power applications with extraordinary properties based on specific quantum dot effects.

A brief overview of properties, synthesis routes, and possible applications of nanostructured materials is presented.

Urban pollution data assessment by real time monitoring using nanotechnology based sensors and satellite imagery is presented. A study of security risks and strategies of environmental information systems is introduced.

We summarize the current understanding of how stoichiometry affects structure, optical and electronic properties and the transition kinetics of phase change materials for rewritable optical and nonvolatile electronic storage.

In the past decades chalcogenide alloys have been successfully employed for rewritable optical data storage. Beside the optical contrast upon phase change from the amorphous to the crystalline state utilized in optical data storage devices, chalcogenide alloys show an even more pronounced change in their electronic properties, namely their resistivity.

Many metals and alloys are capable of reversibly absorbing large amounts of hydrogen. Charging can be done by molecular hydrogen gas or H atoms from electrolytes. This paper reviews the achievements in the fields of synthesis of nano-sized Mg alloys and their applications as hydrogen storages as well as precursors for negative electrodes in alkaline Ni/MH batteries.

Nanocrystalline and nanocrystalline/amorphous (nc/a) Mg based alloys with different compositions were obtained by ball milling. Microstructure and morphology of the alloy powders were determined by X-ray diffration (XRD) and scanning electron microscopy (SEM). The maximum electrochemical discharge capacity, measured with an electrode prepared from an MgNi alloy of in strong alkaline solution (6M KOH), was 382 mAhg-1.

Powder of α-Fe₂O₃ (hematite) with particles sizes of about 100 - 200 nm was synthesized by the low external temperature method (LETM) using a 1M solution of Fe(NO₃)₃ 9H₂O as a starting material.

Vanadium bronzes are attractive materials for rechargeable Libatteries. The electrochemical behaviour of bronzes strongly depends on the synthesis method due to the particle size of the material.

Transparent mesoporous TiO₂ and TiO₂/Ag coatings were prepared via the template assisted sol-gel method with following thermoinduced reduction of Ag+ ions to the Ag0 state. Uniform distribution of the Ag nanoparticles was confirmed by UV-Vis and SEM measurements. TiO₂/Ag films with low Ag contents exhibit a higher photocatalytic activity under UV irradiation than unmodified ones. Rhodamine B photooxidation in the presence of TiO₂ and TiO₂/Ag coatings occurs preferentially through chromophore ring degradation.

Three types of nanocomposite thin films, namely nanocrystalline diamond films, diamond-like carbon films containing diamond nanoparticles, and nanocrystalline TiC/a-C composites, have been investigated regarding their suitability for biomedical applications, especially for the coating of implants for the replacement of major joints. All three types turned out to be biocompatible and not cytotoxic. Nanocrystalline diamond films are bioinert in their as-grown state and possess in addition very promising mechanical and tribological properties. For the two other types of films, on the other hand, very good adhesion and spread of osteoblast(like) cells were observed on the surfaces.

Recently nanostructured materials are receiving considerable attention for their possible applications as biomaterials. In this work three groups of samples have been prepared for testing their bioactivity (i.e. growth of calcium phosphate layers).

A preliminary investigation to articulate the effect of dystrophin immobilized on nanostructured therapeutic templates on aging skeletal muscles to cure Duchenne Muscular Dystrophy (DMD) is presented.

We investigated the possibility of immobilization of RNA and protein biomolecules on the surface of nanocrystalline diamond films which are modified by plasma treatment, followed by chemical functionalization. The work aims at the preparation of a new type of biosensors relying on the specific interaction between the binding partners.