Sculptured thin films

Abstract

Thin films began to be produced in the mid 1800s following the development of vacuum systems and sources of electricity (Mattox 2003). The first recorded thin films, created by physical vapor deposition (PVD), were made by Grove (Grove 1852) in 1852 when he sputtered metallic films by applying a voltage between a wire and a silver substrate at a reduced pressure. Discovery of internal structure in films is attributed to Kundt (Kundt 1886) who noted the rotation of polarisation of light in transmission through thin films of iron, cobalt and nickel in 1884. Direct observation of physical structure, however, had to await the development of the transmission electron microscope (TEM), and was not made until 1966 by Niewenhuizen and Haanstra (Niewenhuizen and Haanstra 1966). Thin films made under suitable conditions were observed to be composed of columnar structures on the nanometer scale. Electron micrographs, which they painstakingly made by producing transparent replicas for the TEM, are now made much more easily and routinely with the scanning electron microscope (SEM). Within the last 10 years or so the technology has developed to control the morphology of the columnar structure to produce sculptured thin films (STFs) (Lakhtakia and Messier 2004). Rotating and tilting the substrate while depositing the film can engineer the columnar structures engineered to a wide variety of nanowire shapes. Thus sculptured thin films have entered the growing number of technologies in the exploding arena of nanotechnology (Lakhtakia and Messier 2004). STFs have a number of properties, both optical and mechanical, which make them suitable for exploitation. In fact, investigation into commercial applications of STF technology has already begun (Suzuki et al. 1999). Many other applications have been proposed and, with the intensity of research currently being carried out in this nascent technology, many more applications are sure to come. In some ways STFs are complementary to another class of self-assembling systems, liquid crystals (LCs). They share many properties. Both can have either nematic or chiral structure, and they have similar optical properties. While STFs and LCs are both self-assembling systems, STF technology offers self-assembly with control. Although LCs can be manipulated to some degree, the versatility of STFs is unmatched. STF nanowires can be fabricated to virtually any preconceived shape, simply by programming the rotation of a substrate during deposition of the film. In addition, STFs can be made of a wide variety of materials and offer both mechanical and thermal stability not available in LCs. On the other hand, the optical properties of LCs are easily manipulated electronically, and this has resulted in their ubiquitous use in numerical displays and computer screens seen today. In order to make STFs electronically addressable, the combination of STFs and LCs has been proposed (Robbie et al. 1999). This is made possible because of another important attribute of STFs, high porosity, which allows LCs and other fluids to be infused into them. Many of the proposed non-optical applications of STFs take advantage of the high degree of porosity in STFs. © Springer-Verlag Berlin Heidelberg 2006.