Transparent conductors as solar energy materials: A panoramic review

Physical vapor deposition under conditions of obliquely incident flux and limited adatom diffusion results in a film with a columnar microstructure. These columns will be oriented toward the vapor source and substrate rotation can be used to sculpt the columns into various morphologies. This is the basis for glancing angle deposition (GLAD), a technique for fabricating porous thin films with engineered structures. The origin of the columnar structure characteristic of GLAD films is discussed in terms of nucleation processes and structure zone models. As deposition continues, the columnar structures are influenced by atomic-scale ballistic shadowing and surface diffusion. Competitive growth is observed where the tallest columns grow at the expense of smaller features. The column shape evolves during growth, and power-law scaling behavior is observed as shown in both experimental results and theoretical simulations. Due to the porous nature of the films and the increased surface area, a variety of chemical applications and sensor device architectures are possible. Because the GLAD process provides precise nanoscale control over the film structure, characteristics such as the mechanical, magnetic, and optical properties of the deposited film may be engineered for various applications. Depositing onto prepatterned substrates forces the columns to adopt a planar ordering, an important requirement for photonic crystal applications.

Glancing angle deposition: Fabrication, properties, and applications of micro- and nanostructured thin films

Helical nanomaterials represent an emerging group of nanostructures with unique spiral geometry as well as multiple functionalities owing to their enriched physical and chemical properties. With the novel properties enabled by their nanoscale dimension and unique geometry, the helical nanostructures may open opportunities to develop our understanding of new physics, chemistry and biology, and enable new nanodevice design and fabrication. This review article presents a comprehensive and in-depth overview of the latest progress in helical nanostructures synthesis, properties and potential applications. Specific attention is concentrated on the crystal growth theory for helical nanostructures, summary of the helical nanomaterials obtained so far, and their fabrication techniques as well as typical physical properties that can be potentially utilized for various applications.

A review of helical nanostructures: growth theories, synthesis strategies and properties

Publisher Summary This chapter focuses on experimental aspects of the Glancing angle deposition (GLAD) technique – its methods and possibilities. It introduces the basic theory, and describes the canonical microstructures and apparatus required to produce GLAD films, describes advanced GLAD techniques, covering procedures, which occur before, during, and after a GLAD process. The chapter also discusses the properties of GLAD films at several length scales: single column properties, the properties of an ensemble of columns, and the ‘bulk’ properties of a GLAD film and reviews applications and devices made with GLAD films. GLAD is a highly flexible nanofabrication technique combining oblique angle deposition and precise manipulation of substrate position during deposition. By depositing at oblique angles, columnar structures arise owing to a combination of ballistic shadowing and limited surface diffusion. The resulting columns are tilted toward the incident flux direction, and can be sculpted into various morphologies: slanted and vertical posts, helices, chevrons, square spirals, and combinations thereof. GLAD can partially decouple the different length scales involved in thin film production, yielding novel materials with interesting properties. GLAD is compatible with a large number of materials, further increasing the combinatorial power of the GLAD technique. Advanced GLAD techniques may offer additional control or modification of the canonical GLAD structures. Such techniques include forced nucleation through seeding and advanced substrate motion control to combat film broadening. While GLAD has been used for many applications, the dominant use to date is in optics.

http://www.ece.ualberta.ca/~mtaschuk/documents/taschuk-2010-glad-book-chapter.pdf

Glancing Angle Deposition

Growth dynamics of thin-films involves both shadowing and re-emission effects. Shadowing can originate from obliquely incident atoms being preferentially deposited on hills of the surface, which leads to a long range geometrical effect, as well as from an atomic shadowing process that can occur even during normal angle deposition. Re-emission effect is a result of nonsticking atoms, which can bounce off from hills and deposit on valleys of the surface. In the case of an energetic incident flux, re-emission can also originate from a resputtering process that includes a surface atom being knocked off by an incident ion/atom followed by redeposition to another surface point. Due to their long-range nonlocal nature, both the shadowing effect (which tries to roughen the surface) and re-emission effect (which has a smoothening effect) have been shown to be more dominant over local effects such as surface diffusion, and have been proven to be critical processes in accurately determining the dynamic evolution of surface roughness. Recent Monte Carlo simulation methods that involve shadowing, re-emission, surface diffusion, and noise effects successfully predicted many experimentally relevant surface roughness evolution results reported in the literature. For example, root-mean-square surface roughness (ω) of Monte Carlo simulated thin-films have evolved with time t according to a power law behavior ω ∼ t β , with β values ranging from about 0 to 1 for a growth with strong re-emission effects (i.e., low sticking coefficients) and a growth with dominant shadowing effects (i.e., with high sticking coefficients), respectively. Potential future thin-film growth modeling studies are also discussed. These include advanced simulation approaches that can incorporate atomistic details of physical and chemical processes and a recently developed network growth model that can potentially capture some universal aspects of thin-film growth dynamics independent of the details of growth process. C 2011

http://www.ualr.edu/txkarabacak/wp-content/uploads/2010/01/Thin-film-growth-dynamics_J-of-Nanophotonics_2011.pdf

Thin-film growth dynamics with shadowing and re-emission effects

Growth front morphology of a thin film formed by physical vapor deposition is controlled by many factors including surface diffusion and shadowing effects. Instabilities can occur if shadowing is more dominant compared to other surface effects and can lead to many diverse physically self-assembled three-dimensional nano-size structures. In this article, we explore the fundamental nucleation and growth mechanisms of the nanostructures during oblique angle deposition. Monte Carlo simulations were carried out to predict the island density, island size distribution, and island–island correlation during the initial stages of growth. The results were compared to that obtained by the oblique angle sputter deposited tungsten films imaged by atomic force microscopy and scanning electron microscopy. Isolated islands with quasiperiodic distribution were formed as a natural consequence of the shadowing effect. Isolated columnar structures are shown to grow on these islands and the width W of the columns is predicted ...

/pdf/physical-self-assembly-and-the-nucleation-of-three-3su2ij5k5o.pdf

T.-M. Lu

Papers

Physical self-assembly and the nucleation of three-dimensional nanostructures by oblique angle deposition