**1. Introduction**

532 Recent Advances in Nanofabrication Techniques and Applications

Zhao, L. L.; Kelly, K. L.; et al. (2003). "The extinction spectra of Ag nanoparticle arrays:

Zhou, Q.; Qian, G.; et al. (2008). "Two-dimentional assembly of silver nanoparticles for

Zhu, S.; Li, F.; et al. (2008). "A localized surface plasmon resonance nanosensor based on rhombic Ag nanoparticle array." Sensors and Actuators B: Chemical 134: 193-198.

catalytic reduction of 4-nitroaniline." Thin Solid Films 516: 953-956.

Chem. B 107: 7343-7350.

influence of array structure on plasmon resonance wavelength and widths." J. Phys.

Due to the unique electronic, optical, catalytic and biological properties, well ordered nanostructures have attracted enormous interest. They have potential applications in photonic crystal devices (Yablonovitch, 1987), large-density magnetic recording devices (Chou et al., 1994), novel electronic devices (Schmidt & Eberl, 2001), synthesis of DNA electrophoresis mediate (Volkmuth & Austin, 1992),nanocontainers (Chen et al., 2008), surface-plasmon resonance biosensors (Brolo et al., 2004), antireflective coatings for solar cells (Yae et al., 2005), and etc. Such broad applications of nanostructures were intimately associated with their unique properties, which are sensitively dependent on their size and/or shape. It is well-established that magnetic (Shi et al., 1996; Zhu et al.,2004), optical (Aizpurua et al., 2003; Larsson et al. 2007), electrocatalytic (Bratlie et al., 2007; Narayanan & El-Sayed, 2004), optoelectronic (Chovin et al., 2004), data storage (Ma, 2008), thermodynamic (Volokitin et al., 1996; Wang et al., 1998) and electrical transport (Andres et al., 1996; Bezryadin et al., 1997) properties of the nanostructures are affected by the shape and the size, as well as the interfeature spacing.

In general, there are two approaches to realize ordered nanostructures with desired size, shape and arrangement. One is the "bottom up" approach on pre-patterned substrates (Zhong et al, 2007; Zhong et al., 2008). The other is the "top-down" approach (Ito & Okazaki, 2000). Both of these two approaches are always based on lithographic technology. In the first approach, lithographic techniques were employed to fabricate various patterned substrates, on which ordered nanostructures can then be realized by subsequent growth of desired materials. The main reason for this approach is to suppress defects in the nanostructures. In the second approach, ordered nanostructures can be directly fabricated by lithographic techniques. Several standard lithographic techniques are frequently exploited to fabricate desired surface nanostructures, including holographic lithography, electron-beam lithography (EBL) and ion-beam lithography (IBL) (Arshak et al., 2004;Ebbesen et al., 1998; Ito & Okazaki, 2000). Recently, a new extreme ultraviolet (EUV) lithography was developed, which is a potential candidate for achieving critical dimensions below 100 nm (Service, 2001). In addition, there are some other lithographic techniques applied in the fabrication of nanostructures (Haynes & Van Duyne, 2001). However, fabrication of nanostructures in a regular arrangement over large areas is still a major challenge in modern nanotechnologies. There is substantial interest in developing new technologies to facilitate pattern fabrication.

A Feasible Routine for Large-Scale Nanopatterning via Nanosphere Lithography 535

31 1 (3 1 ), <sup>2</sup> 3 3 *ip <sup>a</sup> Dd D*

If double layer of nanospheres are employed as the mask, both *a* and *dip* will be changed (Haynes & Van Duyne, 2001). In addition, circular shaped interstice particles are frequently obtained in the case of small nanospheres mainly because some materials are not perpendicularly deposited in the interstices, and the general hot materials can diffuse in some region. More interestingly, for angle resolved deposition, some particularly-shaped nanostructures can be realized, such as nanochain structures (Haynes & Van Duyne, 2001) and nanocrescents (Vogel et al., 2011). If the monolayer of nanospheres is non-close-packed, more complex nanostructures can be obtained by changing the incidence angle of the material vapor beam and the azimuth angle of the vapor beam with respect to the normal

The critical step of NSL is to form monolayer of ordered nanospheres on desired substrates. Several methods have been developed to form regularly arranged nanospheres on substrates, including transferal coating (Weekes et al., 2007), vertically dipping coating (Choi et al., 2009), spin coating (Hulteen & Van Duyne, 1995), drop coating (Hulteen et al., 1999), and thermoelectrically cooled angle coating (Micheletto et al., 1995). All of these formation methods are based on the ability of the nanospheres to freely diffuse to seek their lowest energy configuration. The diffusion processes and the interaction among nanospheres can be influenced by chemically modifying the nanosphere surface with a negatively charged functional group such as carboxylate or sulfate. Such a modification of the surface features of nanospheres can be easily realized for polystyrene (PS) spheres (Weekes et al., 2007). The self-assembled monolayer of nanosphere masks always include a variety of defects that arise as a result of nanosphere polydispersity, site randomness, point defects (vacancies), line defects (slip dislocations), and polycrystalline domains (Haynes & Van Duyne, 2001). These defects are always remained in the finally obtained nanostructures, which will degrade the properties of the ordered nanostructures. Therefore, it is important

to try to get rid of those defects in the monolayer of self-assembled nanospheres.

It was found that the transferal coating is much easier in operation to obtain ordered nanospheres in large areas than the other methods. The domain size of ordered PS spheres can be up to 1 cm2 (Weekes et al., 2007). The key step of the transferal coating is to selfassemble highly ordered monolayer of PS spheres at the interface between water and air (or oil). In general, the suspensions (1-10 wt%) of PS spheres in de-ionized (DI) water are diluted in a 1:1 ratio in some spreading agent, such as ethanol or methanol. Drops of the diluted suspension of PS sphere are then introduced into water surface via a titled glass from a pipet. On contact with the water, the PS spheres immediately form a momolayer and start to assemble. The inherent mechanism for the ordering of PS spheres at a liquid interface has been studied by several groups (Aubry & Singh, 2008; Boneva et al., 2009; Larsen & Grier, 1997; Nikolaides et al., 2002; Pieranski, 1980; Trau et al., 1996; Yeh et al., 1997). A reasonable model has been provided to account for the ordering of PS spheres at the interface (Nikolaides et al., 2002). It was suggested that the ordering arrangement of PS spheres resulted from the balance between an electrostatic repulsion and an additional capillary attraction among PS spheres. The former is originated from the negative charges on PS spheres (Weekes et al, 2007). The latter is due to the deformation of liquid meniscus

direction of the nanospheres mask (Zhang et al., 2007 ).

**2.2 NSL based on transferal coating** 
