New Nanoparticle-based Materials and Devices Produced by Advanced Gas Evaporation, Reactive Sputtering and Microorganisms

Efficient photothermal conversion of solar energy requires spectrally selective surfaces. The purpose of this work is the development of new economically and ecologically benign coating materials by biological means and the evaluation of their optical properties. Our approach to produce wavelength selective composite materials structures relies on the use of metal accumulating microorganisms to produce metallic and metal ion containing nano-particles. Crystalline silver particles with distinct shapes are formed in the periplasmic space of the silver accumulating bacterial strain Pseudomonas stutzeri AG259. A non-metallic carbon host matrix for the silver particles is provided by the organic biomass of the bacteria. The innovation and relevance of our work lies in the biotechnological approach to materials science and in the relatively low investment and operating costs to produce the coating material. Through different heat treatment procedures adjustable optical properties are obtained. Optical spectroscopic measurements were carried out in the UV/VIS/NIR and IR in order to characterise the material. The performance of the material and strategies to optimise the spectral selectivity of the coatings are discussed in terms of effective medium theories.

Metal micro-/nano-particles with suitable chemical modification can be organized into new ceramic–metal (cermet) or organic–metal (orgmet) composites or structured materials. These materials are attracting significant attention because of their unique structures and highly optimized properties. However, the synthesis of composite materials with inhomogeneities on the nanometer or sub-micrometer scale is a continuing challenge in materials science. Many industrial physical and chemical surface-coating processes using conventional techniques are both energy and cost inefficient and require sophisticated instrumentation. In the future, biology might offer a superior option.

Tandem absorbers are often used in the design of solar absorbers for photo thermal conversion. They consist of a thin coating, selectively absorbing in the wavelength range of the solar spectrum, on a metal substrate. The optical performance of a tandem absorber depends on the optical constants and thickness of the absorbing coating and also on the reflectivity of the underlying metal. A very high solar absorptance is achieved when the coating has a non-uniform composition in the sense that the refractive index is highest closest to the metal substrate and then gradually decreases towards the front surface. This type of composition suppresses coating interference and gives a low front surface reflection if the refractive index at the front surface is low. We report on optical analysis of a solar absorber with a graded index coating of sputtered nickel–nickel oxide deposited on aluminium. The optical constants have been determined from reflectance, transmittance and ellipsometry data by fitting the data to a two-layer model of the coating. The optical constants of the two layers can be regarded as effective optical constants for the lower and upper part of the graded index coating respectively. It is found that the effective refractive index of the top layer is somewhat lower than for the base layer. The extinction coefficient is higher in the lower part of the coating. Both effective refractive index and extinction coefficient of the base layer increase monotonically with increasing wavelength as for metallic materials.

Metal-containing bacteria as precursors for thin films is the concept behind the production of these ceramic–metal composites (cermets) that consist of an organic carbon matrix with embedded crystalline metallic silver particles of various morphologies and sizes (see Figure). It is demonstrated that the optical properties of the films can be tailored by adjustment of temperature and metal volume fraction.

Silver is accumulated to high concentrations in certain microbial strains. Here a bomb digestion method is proposed, using HNO3 and HCl, for the extraction and digestion of silver and silver compounds from the organic matrix. The method is applicable for the quantitative determination of silver by inductively coupled plasma atomic emission spectroscopy.

Silver-dielectric composite films were produced by a biomimetic technique, using the bacterial strain Pseudomonas stutzeri AG259 as a precursor for sol-gel-type deposition. Heat treated films backed by a metal displayed pronounced spectral selectivity of a kind that makes them interesting for photothermal conversion of solar energy. The optical properties could be reconciled with the Bruggeman effective medium theory.

One mechanism of silver resistance in microorganisms is accumulation of the metal ions in the cell. Here, we report on the phenomenon of biosynthesis of silver-based single crystals with well-defined compositions and shapes, such as equilateral triangles and hexagons, in Pseudomonas stutzeri AG259. The crystals were up to 200 nm in size and were often located at the cell poles. Transmission electron microscopy, quantitative energy-dispersive x-ray analysis, and electron diffraction established that the crystals comprise at least three different types, found both in whole cells and thin sections. These Ag-containing crystals are embedded in the organic matrix of the bacteria. Their possible potential as organic-metal composites in thin film and surface coating technology is discussed.

Inhomogeneous media such as ceramic–metal composites are used in a wide range of applications as functional optical coatings e.g. in selective solar absorber coatings. Their optical properties can be calculated by the use of effective medium theories. These theories need the optical constants of the components as input parameters. Especially for the metallic component, these parameters depend strongly on the deposition conditions and on the particle sizes. The limitation of the mean free path of the conduction electrons can be explained by the well-known Drude theory. It was found that additional size effects occur due to a size- dependent modified band structure of the metal particles. The size dependence of the band structure and the resulting effects on the optical constants are discussed for the model system of sputtered small gold particles embedded in an amorphous carbon matrix produced simultaneously by hydrogenated, plasma- activated, chemical vapour deposition. Changes in the band structure were analysed by ultraviolet and X-ray photoelectron spectroscopy (UPS/XPS). The optical constants of the cermets were determined from reflectance and transmittance measurements as well as from spectral ellipsometry. Changes in the optical constants of the gold particles were found from comparison of these measurements with effective medium theories. The size of the metal particles and the surface structure of the cermets were determined from TEM images and atomic force microscopy. The size-dependent changes of the optical constants of the metal particles were correlated with the UPS spectra.

A combination of ellipsometric and photometric measurements provides a convenient and accurate method for the determination of the optical properties of thick polymer films. Of course, a reasonably good surface and layer quality is necessary. Multiple reflections in the thick film and thus incoherent superposition causes partial depolarization of the reflected or transmitted light. Therefore, the Mueller matrix elements of these layers are measured and compared to calculated ones. A relationship between Jones and Mueller matrices originally given for random media is used to derive the formulas for the Mueller matrix elements of these anisotropic thick films. This relationship can also be applied to other incoherent effects in spectroscopic ellipsometry, such as depolarization upon reflection by a sample with a varying film thickness or by a layer with fluctuating optical constants.

A relationship between Jones and Mueller matrices originally derived for random media is applied to incoherence effects in photometric ellipsometry. Such effects are for example depolarization after reflection from a sample with varying film thickness or from a layer which is thicker than the coherence length of the incident light. The main task is to calculate the expectation value of a statistical ensemble. For the important case of thick layers, this expectation value is derived in a symbolic form. Results calculated with this method for transmission ellipsometry, ellipsometry at the back surface of the substrate and the determination of the optical constants at the substrate—layer interface are compared with measurements.

Reflections from the back surface of a transparent substrate influence the evaluation of optical constants of thin films from ellipsometric measurements. If the thickness of the substrate is large compared with the coherence length of the light, the relative phase between the p and s mode, which commonly is measured by ellipsometry, cannot be defined properly. We show how the reflections from the back surface of the substrate are taken into account in ellipsometric measurements by calculating the intensities of reflections for arbitrary angles of polarization. Applications of the new method, such as transmittance ellipsometry, ellipsometry at the back surface of the substrate, and the determination of the optical constants at the substrate-layer interface, are compared with measurements.

The optical constants of thin sputtered molybdenum layers, embedded in a ceramic-metal composite produced by a batch sputtering deposition system were analyzed. This was accomplished by assuming a multilayer system for a tin oxide-molybdenum cermet and calculating the optical constants from angular and polarization dependent reflection and transmission spectra. These optical constants differ strongly from those for sputtered bulk material obtained ellipsometrically. A good agreement between measured effective refractive indices for cermets and effective medium calculation was found, if these optical constants for molybdenum were used in the effective medium calculations. Differences to the optical constants of the cermet determined ellipsometrically were explained by the birefringence of the cermet. The size and the shape of the embedded particles were investigated with an atomic force microscope.

The optical constants of thin molybdenum layers and molybdenum/tin oxide cermets, produced by a batch sputtering deposition system, were determined by spectroscopic ellipsometry. The geometrical structure of the layers and the embedded particles was analysed with an atomic force microscope. The results were used for the investigation of a suitable effective medium theory to describe the optical constants of cermets with relatively high filling factors.

Tungsten and chromium containing hydrogenated amorphous carbon films were deposited in a process which combines plasma activated chemical vapor deposition of methane and r.f.- sputtering of a metallic target. The metal content of the deposits can be adjusted by the ratio of gas flow between argon and methane and was determined by X-ray Photoelectron Spectroscopy (XPS). For the a-C:H/W composites the XPS data are presented in detail and supply information about the chemical state of carbon and tungsten. The presence of W in carbidic state could be proven. Furthermore the optical constants n and k of a-C:H/W were obtained in the wavelength range between 0.4 and 2.6micrometers and for W concentrations of up to 33 at.%. The morphology of the deposits, determined by atomic force microscopy, ranges from very porous to compact and can be controlled by the substrate bias voltage. Accelerated ageing investigations were performed in air in order to characterize the deterioration mechanisms of a-C:H/W and a-C:H/Cr films deposited on different substrate materials. The significance of the morphology of the coating and the roughness of the substrate for the ageing mechanisms could be shown. Very promising results concerning the film stability were obtained for a-C:H/Cr.

The optical constants of sputtered molybdenum layers and molybdenum/tin oxide cermets in the wavelength range 0.4–7 μm were determined by spectroscopic ellipsometry. The results were used for the evaluation of a suitable effective medium theory to describe cermets produced by a batch sputtering deposition system. In a following step, application of the effective medium theory to the refractive index of a cermet with low metal content allowed estimation of the optical constants of the embedded molybdenum particles.