Multilayer Thin Film Coatings Research Articles

Formation of Nanoparticle-Loaded Microcapsules Based on Hydrogen-Bonded Multilayers

We show that hydrogen-bonded multilayer thin film coatings assembled on colloidal particles can be used as templates for the in situ synthesis of nanoparticles. The concentration and size of nanoparticles grown within the coatings can be tuned by the number of loading cycles. The hydrogen-bonded multilayer thin films were coated on polystyrene micrometer-sized colloidal particles and subsequently cross-linked via water-soluble carbodiimide chemistry. We show that metal nanoparticles (e.g., Ag and Pd) can be synthesized via the nanoreactor scheme, where corresponding metal ions are loaded into the coating and then subsequently reduced to create nanoparticles dispersed within the coating. In addition, we demonstrate the multiple loading capability of the hydrogen-bonded multilayer coatings. UV−vis spectroscopy and direct observation by transmission electron microscopy confirm that the nanoparticles are well-dispersed within the thin film coating. Finally, the polystyrene particles can be extracted by treating the nanocomposite-coated particles with tetrahydrofuran or toluene, leaving the nanoparticles embedded within the resultant multilayer hollow microcapsule walls.

Markov-chain Monte Carlo approach to the design of multilayer thin-film optical coatings.

We reconsider the problem of locating the globally optimal solution of a multilayer-optical-coating design problem, within some predetermined space of parameters, with the aim of obtaining a robust technique that requires a minimum of user intervention. The approach we adopt centers on exploring the space of the parameters of interest by using a Markov-chain Monte Carlo sampling algorithm. This technique enables one to locate the global optimum automatically with high confidence and without the need for a good starting design. It also allows the trivial inclusion of prior constraints on the variables and provides a natural means for investigating the robustness of the optimal solution.

Plasma Absorption of Femtosecond Laser Pulses in Dielectrics

Dielectric (high bandgap) materials represent an important and diverse class of materials in micro and nanotechnology, including MEMS devices, biomedical and bioengineering systems, multilayer thin film coatings, fiber optics, etc. Micromachining dielectrics using ultrafast lasers is an exciting and promising new research area with many significant advantages, including precision material removal, negligible heating of the workpiece, micron and sub-micron-size feature fabrication, and high aspect ratio features. During ultrafast laser processing of dielectrics, the intense laser pulse ionizes the irradiated material and produces an optical breakdown region, or plasma, that is characterized by a high density of free electrons. These high-density electrons can efficiently absorb a large fraction of the laser irradiance energy, part of which will then be coupled into the bulk material, resulting in material removal through direct vaporization. The energy deposited into the material depends on the time and space-dependent breakdown region, the plasma rise time, and the plasma absorption coefficient. Higher coupling efficiency results in higher material removal rate; thus energy deposition is one of the most important issues for ultrafast laser materials processing, particularly for micron and sub-micron-scale laser materials processing. In the present work, a femtosecond breakdown model is developed to investigate energy deposition during ultrafast laser material interactions. One substantial contribution of the current work is that pulse propagation effects have been taken into account, which have been shown to become significant for pulse durations less than 10 ps. By accounting for the pulse propagation, the time and space-resolved plasma evolution can be characterized and used to determine the energy deposition through plasma absorption. With knowledge of the plasma absorption, changes in the pulse profile as it propagates in the focal region can be determined as well. Absorption of the laser pulse by plasma in water is compared with experimental data to validate the model, as water is a well characterized dielectric. The model, however, is also applicable to other transparent or moderately absorbing solid and liquid dielectric media during ultrafast laser-materials interactions.

Large-area production of solar absorbent multilayers by MF-pulsed plasma technology

Since solar absorbers ensure an extremely high efficiency in solar energy utilization, there is an increasing demand for high-quality, highly durable and cost-reasonable absorber units. The medium-frequency (MF) pulsed plasma technology opens unique possibilities to manufacture thin-film multilayer coatings that meet these demands. A new industrial coater for the deposition of a-C:H/Cr-based solar absorbent coatings onto a 1 m wide copper strip is presented. The essential process steps are heating, medium frequency (MF) ion-etching, reactive MF-dual magnetron sputtering of graded chromium–carbon layers and the magnetically enhanced MF-PECVD of an a-C : H cover layer. High-performance solar absorbers which meet the IEA performance Criterion are being manufactured.

Manufacture of complex optical multilayer filters using an automated deposition system

It is now possible to design complex thin film multilayer coatings that can achieve almost any desired spectral performance. Moreover, these coatings can consist of tens or hundreds of layers each of which may have a different thickness. It is a challenge to accurately manufacture such complex coatings. An automated deposition system (ADS) has been developed which is able to fabricate complex optical coatings on a routine basis. This deposition system uses a wideband optical monitor to accurately determine the deposited layer thickness and to re-optimize the remaining layer thicknesses, if required. One advantage of this process is that no operators are required during the manufacture of a filter. For this technique to work, however, an energetic deposition process such as sputtering is necessary in order to achieve thin films with a bulk-like refractive index with little or no porosity or inhomogeneity. As well, the optical constants of the thin film materials must be accurately known. A number of thin film filters have been manufactured using this ADS. These include all-dielectric coatings as well as metal/dielectric filters. In this paper, the ADS and the real-time process control algorithms will be described in more detail and several examples of manufactured complex thin film filters will be described for different applications.

All-dielectric high-efficiency reflection gratings made with multilayer thin-film coatings

We made a high-efficiency ref lection grating by coating a nine-layer quarter-wave system of ZnS/Na(3)AlF(6) directly onto a surface-relief grating in photoresist. Eff iciencies of 70% and 84% were measured at the firstorder Littrow mount with the laser beam incident from the air side and from the substrate side, respectively. The experimental results are in qualitative agreement with theoretical predictions. The key to further improvement in the diffraction eff iciency is to improve the contour conformation of the coated dielectric layers.

Shock fracture replicas for thin film microstructure investigation in a transmission electron microscope

Pt-C surface replicas were examined by transmission electron microscopy to study the structural details of the surfaces and fracture cross-sections of single films and multilayer thin film coatings. Manual fracturing caused artifacts in the surface and fracture face of multilayer systems. Single films are less sensitive to deformation by external mechanical forces in this fracturing process. Heat shock fracturing applied to multilayer systems and single films resulted in relatively smooth and reproducible cross-sectional areas. Most fracturing methods will reveal the characteristic film microstructures, but more or less artifactive fracture structures are always superimposed on them. The goal of this investigation was to demonstrate that the new heat shock fracturing method minimizes these disturbing artifacts and allows, in a unique way, the correlation of optical and mechanical film properties to the microstructure of the films.

Heat shock fracturing and replication for the electron microscopy of thin films

Pt-C surface replicas were examined by transmission electron microscopy (TEM) to study the structural details of the surfaces and fracture cross sections of single films and multilayer thin film coatings. Manual fracturing caused artifacts in the surface and fracture face of multilayer systems. Single films are less subject to deformation by external mechanical forces in this fracturing process. Heat shock fracturing applied to multilayer systems and single films resulted in relatively smooth and reproducible cross sectional areas. Most fracturing methods will reveal the characteristic film microstructures, but more or less artifactual structures are always superimposed on them. The goal of this investigation was to demonstrate that the new heat shock fracturing method minimises these disturbing artifacts.

Analysis of depth profiles of sol-gel derived multilayer coatings by Rutherford backscattering spectrometry and by cross-sectional transmission electron microscopy1

This paper describes the preparation and compositional analysis of multilayer thin-film coatings prepared using sol-gel techniques. Alternate layers were labeled with an iron tag, derived from hydrolyzed 1,1'-bis(triethoxysilyl)ferrocene. Iron-free layers were composed of SiO2 derived from hydrolyzed tetraethylorthosilicate (TEOS). Analyses of these systems were based on Rutherford Backscattering Spectrometry (RBS) and Cross-Sectional Transmission Electron Microscopy (XTEM). The depth profile of iron, measured by RBS, yielded thicknesses (1000–1600 Å) for the individual layers that could be verified independently by XTEM.

Transmission electron microscopy of Ni/Ti neutron mirrors

Transmission electron microscopy (TEM) is a revealing method to characterize the microstructure of multilayer thin film coatings. The performance of nickel mirrors, used for neutron spin polarization at grazing incident angles, may be enhanced by replacing the nickel coating with a Ni/Ti multilayer. Difficulties have historically arisen in fabricating defect-free multilayer structures. TEM of cross-sectioned multilayers provides a direct means to identify debilitating structural defects. During the sputter deposition of long-period Ni/Ti multilayers, an undesirable growth morphology has been prevalent, i.e. nodular. Modifications to sputter deposition processes have improved the quality of the multilayer coatings, hence their performance as efficient neutron mirrors should be improved as well. The origin of the nodular growth structure and the measures taken to curtail this morphology will be revealed in the TEM characterization of Ni/Ti neutron, multilayer mirrors.

Practical Thin-film Polarizing Beam-splitters

Polarizing beam-splitters of the cemented cube form are considered and the designs of a number of multilayer thin-film coatings of ZnS and cryolite giving polarizing efficiencies at the design wavelength of greater than 99·8 per cent are discussed. The refractive index of the cube is chosen to match that of the optical cement, so that wedge fringes due to the cement layer are eliminated.