Thin films islanding

With advances in nanotechnology, quantum dots are ubiquitous in surfaces with myriad applications in electrochemistry. In this chapter, we discuss the numerical modelling of quantum dots both the spherically symmetric (metal or semiconductor) dots embedded in a matrix (usually a dielectric) and the hemispherical metal dots (a.k.a. nanoparticles or thin-film islands) supported by a substrate (again, usually a dielectric). We are particularly concerned about the optical properties of quantum dots, since the interaction of light with quantum dots is used widely for several applications in surface electrochemistry - from characterization of the thickness and quality of thin films, to the development of surface sensors - as well as in nanoelectronics and quantum computing. 

In modem sensing applications, nanoparticles are immobilized on a surface so they present the maximum detection surface to the analyte. The sensing signal is the optical absorption spectmm. This configuration is well known to researchers in the surface science community as surface quantum dots or supported thin-film islands, and their optical properties have been studied for a while. Specifically, the Marton-Schlesinger method and the Bedeaux Vlieger methods have provided both quantitative calculations of the optical properties of nanoparticles on a surface. One big advantage of the latter method is the effect of the substrate is naturally built into the formalism (see Fig. 1). A limitation of these methods is that the... 

Nanoparticle Formation by Ion Etching of Island Thin Films... 

Figure 5. Morphology and particle size distribution of an island silver thin film deposited on native oxide covered silicon (a) before ion bombardment and after (b) 0.5 keV Ar sputtering with 1.1 X 10, (c) 2.5 X 10, and (d) 3.9 x 10 ion/cm dose. Sputtering speed for silver was around 3-4ML/min. Total elapsed sputtering time is indicated on each size distribution graphs. (Reprinted from Ref [123], 2003, with permission from Springer.)...

Because of the possibility of focusing laser beams, thin films can be produced at precisely defined locations. Using a microscope train of lenses to focus a laser beam makes possible the production of microregions suitable for application in computer chip production. The photolytic process produces islands of product nuclei, which act as preferential nucleation sites for further deposition, and thus to some unevenness in the product film. This is because the substrate is relatively cool, and therefore the surface mobility of the deposited atoms is low. In pyrolytic decomposition, the region over which deposition occurs depends on the thermal conductivity of the substrate, being wider the lower the thermal conductivity. For example, the surface area of a deposit of silicon on silicon is narrower than the deposition of silicon on silica, or on a surface-oxidized silicon sample, using the same beam geometry. 

Unlike the bulk morphology, block copolymer thin films are often characterized by thickness-dependent highly oriented domains, as a result of surface and interfacial energy minimization [115,116]. For example, in the simplest composition-symmetric (ID lamellae) coil-coil thin films, the overall trend when t>Lo is for the lamellae to be oriented parallel to the plane of the film [115]. Under symmetric boundary conditions, frustration cannot be avoided if t is not commensurate with L0 in a confined film and the lamellar period deviates from the bulk value by compressing the chain conformation [117]. Under asymmetric boundary conditions, an incomplete top layer composed of islands and holes of height Lo forms as in the incommensurate case [118]. However, it has also been observed that microdomains can reorient such that they are perpendicular to the surface [ 119], or they can take mixed orientations to relieve the constraint [66]. 

A cross-sectional schematic of a monolithic gas sensor system featuring a microhotplate is shown in Fig. 2.2. Its fabrication relies on an industrial CMOS-process with subsequent micromachining steps. Diverse thin-film layers, which can be used for electrical insulation and passivation, are available in the CMOS-process. They are denoted dielectric layers and include several silicon-oxide layers such as the thermal field oxide, the contact oxide and the intermetal oxide as well as a silicon-nitride layer that serves as passivation. All these materials exhibit a characteristically low thermal conductivity, so that a membrane, which consists of only the dielectric layers, provides excellent thermal insulation between the bulk-silicon chip and a heated area. The heated area features a resistive heater, a temperature sensor, and the electrodes that contact the deposited sensitive metal oxide. An additional temperature sensor is integrated close to the circuitry on the bulk chip to monitor the overall chip temperature. The membrane is released by etching away the silicon underneath the dielectric layers. Depending on the micromachining procedure, it is possible to leave a silicon island underneath the heated area. Such an island can serve as a heat spreader and also mechanically stabihzes the membrane. The fabrication process will be explained in more detail in Chap 4. 

In conclusion, a simple KOH-etching process without ECE is applicable for future microhotplate designs, although the best temperature homogeneity is achieved with the silicon island heat spreader. The island remains an important design feature, especially for the use of thin-film sensitive layers, where the additional heat spreading effect of the sensor materials is small. 

N. Najafi, K.D. Wise, R. Mechant, and J.W Schwank. An integrated multi-element ultra-thin-film gas analy2er . Digest IEEE Solid State Sensor and Actuator Workshop, Hilton Head Island, SC, USA (1992), 19-22. 

Fig. 5.16 The key stages in fabrication of a transparent Ti02 nanotube array film (top) Sputter deposition of a high quality Ti thin film (middle) anodization of resulting film, and (bottom) heat treatment to oxidize remaining metallic islands.

Like other non-oxidic semiconductors in aqueous solutions, surface oxidized and photocorrosive InP is a poor photoelectrode for water decomposition [19,27,32,33], To enhance properties several efforts have focused on coupling of the semiconductor with discontinuous noble metal layers of island-like topology. For example, rhodium, ruthenium and platinum thin films, less than 10 nm in thickness, have been electrodeposited onto p-type InP followed by a brief etching treatment to achieve an island-like topology on the surface [27,28]. In combination with a Pt counter electrode, under AM 1.5 illumination of 87 mW/cm the metal (Pt, Rh, Ru) functionalized p-InP photocathodes [27] see a reduction in the threshold voltage for water electrolysis from 1.23 V to 0.64 V, and in aqueous HCl solution a photocurrent density of 24 mA/cm with a photoconversion efficiency of 12% [27]. 

Figure 1. Morphological evolution of unstressed thin-film regions, made ofinitially fourmonolayers, through thermal fluctuations. The early stages of islanding are examined on the left side, while the final equilibrium shapes are shown on the right for three different cases of substrate-vapor surface energy, y ...

Figure 4. Thin films with small misfit strains under the zero-torque condition. As the misfit strains are less than the critical value, 0.006, no islands are created out of the undulations. The periodic length is equal to 100 a and the film thickness is 30 ML.

When a film is very thin, it may not be continuous, and conduction is subject to the percolation effect, whereby charge migrates by hopping or tunneling between island sites [50,51]. Such a process is activation controlled, and such thin films do not obey Ohm s law. The activation energy can be decreased by the presence of an applied electric field, making development of a rigorous theory difficult. The resistivity can be expressed by the relationship [5]... 

The effect on structure of confining block copolymers in thin films has been examined, largely using neutron reflectivity and atomic force microscopy. A number of features that result from the constraint of reduced dimensionality have been reported, such as the observation of islands and holes at the surface... 

X-ray reflectivity, AFM and optical microscopy have been used to probe lateral structure in a thin film of a symmetric PS-PMMA diblock that formed islands at the surface (Cai et al. 1993). The step height obtained from beating of fringes in the X-ray reflectivity profile was found to be in good agreement with a direct... 

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