**2.1 Main features of NSL**

To obtain desired nanostructures by NSL, one monolayer of self-assembled nanospheres is always obtained first and served as a mask for the subsequent fabrication of nanostructures. The material of the nanosphere can be nanoscale polystyrene (PS), SiO2, or polydimethylsiloxane (PDMS) (Choi et al, 2009), and etc. The shape of the resulting pattern is most often spherical. Using PDMS, the shape and the feature size of the pattern can be modulated by changing the stretching axis and ratio of the PDMS replica. The nonspherical shaped patterns, such as rectangular or elongated hexagonal shaped patterns, can then be obtained (Choi et al, 2009). An additional noteworthy feature of the PDMS is that different pattern can be produced from a single PDMS replica mold (Choi et al, 2009). In addition, binary nanospheres composed of two different-size colloidal particles can be self-assembled both in hexagonal lattices via a two-step process (Kim et al., 2009), forming binary colloid crystals (BCCs). Such BCCs may have potential applications in the fabrication of photonic crystal structures, theoretical models of phase transition, and templates of inverse structure (Kim et al., 2009). The arrangement of selfassembled nanospheres can be close-packed or non-close-packed (Vogel et al., 2011). In general, the self-assembled nanospheres are arranged in a hexagonal lattice. Using more sophisticated processes, squarely ordered array of nanospheres can also be realized, which is speculated to be metastable structures between more stable hexagonal structures (Sun et al., 2009).

In the simplest NSL, a monolayer of close-packed nanospheres in a hexagonal lattice is first obtained on the substrate, which can be served as a mask for the subsequent deposition of desired materials. For generally vertical deposition, the three-fold interstices allow deposited material to reach the substrate, giving rise to an array of triangularly shaped nanoparticles with P6mm symmetry (Haynes & Van Duyne, 2001). The perpendicular bisector of the triangular nanoparticles, *a*, and the interparticle spacing, *dip*, are proportional to the nanosphere diameter, *D*, which can be simply calculated by,

Enormous efforts have been devoted to investigate alternative nanolithography approaches. One of promising methods is nanosphere lithography (NSL) (Fuhrmann et al., 2005; Haynes & Van Duyne, 2001; Hulteen & Van Duyne, 1995; Kosiorek et al., 2004; Sinitskii et al., 2007), which is a highly accessible, low cost, parallel fabrication process capable of producing nanostructured surfaces over large areas and with high resolution. In NSL, self-assembled nanospheres can be served directly as ordered nanostructures (Park et al., 1998) or a mask for the subsequent fabrication of nanostructures, which can be realized by deposition of desired materials, or by etching on desired substrates. A rich varieties of ordered nanostructures have been achieved by NSL, such as triangular structures (Winzer et al., 1996), metallic rings (Boneberg et al., 1997), nanopillars (Weeks et al., 2004), and multilayer with modified topography (Albrecht et al., 2005), nanodots (Chen et al., 2009; Weekes et al., 2007), 3D nanostructure (Zhang et al., 2007), discs (Hanarp et al., 2003) and nanoscale crescents (Gwinner et al., 2009; Retsch et al., 2009; Vogel et al, 2011). The shape, the size and the arrangement of ordered nanostructures can be readily controlled in combination of NSL and the subsequent deposition of desired materials (Haynes & Van Duyne, 2001; Zhang et

To obtain desired nanostructures by NSL, one monolayer of self-assembled nanospheres is always obtained first and served as a mask for the subsequent fabrication of nanostructures. The material of the nanosphere can be nanoscale polystyrene (PS), SiO2, or polydimethylsiloxane (PDMS) (Choi et al, 2009), and etc. The shape of the resulting pattern is most often spherical. Using PDMS, the shape and the feature size of the pattern can be modulated by changing the stretching axis and ratio of the PDMS replica. The nonspherical shaped patterns, such as rectangular or elongated hexagonal shaped patterns, can then be obtained (Choi et al, 2009). An additional noteworthy feature of the PDMS is that different pattern can be produced from a single PDMS replica mold (Choi et al, 2009). In addition, binary nanospheres composed of two different-size colloidal particles can be self-assembled both in hexagonal lattices via a two-step process (Kim et al., 2009), forming binary colloid crystals (BCCs). Such BCCs may have potential applications in the fabrication of photonic crystal structures, theoretical models of phase transition, and templates of inverse structure (Kim et al., 2009). The arrangement of selfassembled nanospheres can be close-packed or non-close-packed (Vogel et al., 2011). In general, the self-assembled nanospheres are arranged in a hexagonal lattice. Using more sophisticated processes, squarely ordered array of nanospheres can also be realized, which is speculated to be metastable structures between more stable hexagonal structures

In the simplest NSL, a monolayer of close-packed nanospheres in a hexagonal lattice is first obtained on the substrate, which can be served as a mask for the subsequent deposition of desired materials. For generally vertical deposition, the three-fold interstices allow deposited material to reach the substrate, giving rise to an array of triangularly shaped nanoparticles with P6mm symmetry (Haynes & Van Duyne, 2001). The perpendicular bisector of the triangular nanoparticles, *a*, and the interparticle spacing, *dip*, are proportional to the nanosphere diameter, *D*, which can be simply

**2. Nanosphere lithography (NSL)** 

al., 2007; Vogel et al, 2011).

**2.1 Main features of NSL** 

(Sun et al., 2009).

calculated by,

$$a = \frac{3}{2}(\sqrt{3} - 1 - \frac{1}{\sqrt{3}})D, \quad d\_{\circ p} = \frac{1}{\sqrt{3}}D^2$$

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

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.

#### **2.2 NSL based on transferal coating**

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

A Feasible Routine for Large-Scale Nanopatterning via Nanosphere Lithography 537

the interaction among PS spheres. It has been found that the ordering of self-assembled PS spheres at interface can be considerably improved by adding suitable acid (H2SO4) in water (Sirotkin et al., 2010), as shown in Fig. 2. Given that the charges on PS spheres are related to the diameter of PS spheres,the suitable amount of acid or other electrolyte is dependent on the size of PS spheres。In addition, the temperature of the water to some degree also affect the self-assembly of the PS spheres at the interface. It was found that ordering of PS was improved on the water of ~ 4 oC. Such an improvement may be related to the increase of water surface tension and the suppression of the Brownian motion of the PS spheres and

Fig. 2. Self-assembled monolayer of PS spheres (5 x 5 m2) at the interface of air and the solution of de-ionized water and H2SO4 with PH value of, (a) ~7, (b) ~ 5.3. The diameter of PS sphere is 240 nm. The H2SO4 can provide some additional ions around PS spheres, which can effectively change the interaction among PS spheres. Under certain PH value of the

The self-assembled monolayer of PS spheres can be finally transferred to varieties of smooth substrates underneath the water by draining off the water. This process can be affected by some charges on the substrates. In addition, some cracks may appear once the monolayer was disturbed by movements of the water during draining. The PS spheres nearby the cracks slightly displaced from the ideal sites of a hexagonal lattice. In this case, the longrange ordering of the subsequent structure is degraded. The monolayer of ordered PS spheres on substrates can then serve as a mask for the subsequent fabrication of ordered

Periodic pit-pattern can be obtained in combination of NSL and selective chemical etching (Chen et al., 2009). The processes mainly involves three steps: (i) self-assembling monolayer of PS spheres on hydrogenated Si surface; (ii) forming a novel net-like Au-Oxide mask via Au catalyzed oxidation; (iii) resulting in periodic pits by selective chemical etching of Si in

solution, considerable improvement of ordering of PS spheres can be made.

nanostructures by deposition of varieties of materials or by etching.

**2.3 Periodic pit-pattern obtained by NSL and chemical etching** 

dust clusters in the water.

KOH solution.

by electrostatic stresses at interface. Both of these two forces were associated with an electric dipolar field, which resulted from an asymmetric charge distribution on particles at the interface due to mismatch in dielectric constant of adjacent fluids. Such a creation of the attractive capillary force is crucial because spheres with diameters of less than 5 m generally do not have sufficient weight to deform the liquid meniscus by means of gravity (Kralchevsky & Nagayama, 2000).

Considering the importance of the electrical field in self-assembling PS spheres at interface, it is natural to find ways to change the electric field to control the arrangement of PS spheres. One way is to apply an external electric field during the self-assembly of PS spheres at the interface (Aubry & Singh, 2008; Boneva et al., 2009; Nikolaides et al., 2002; Trau et al., 1996). This external electrical field will change the charge distribution on the surface of PS Spheres, leading to the change of the electric dipole field around PS spheres. It may also exert an additional electric force on the negatively charged PS spheres, which will affect the deformation of the surface. In a word, the external electric field will affect both the attraction force and the repulsion force mentioned above. It has been found that the inter-particle distance can be remarkably changed by the electric field (Nikolaides et al., 2002). As a result, the ordering of self-assembled PS spheres can be improved considerably, as shown in Fig. 1. In addition, the effect of an external electric field on the arrangement of PS spheres is more pronounced for smaller spheres.

Fig. 1. Self-assembled monolayer of PS spheres (5 x 5 m2) with an external electric field of, (a) 0 v/m, (b) 5 x 104 v/m. The diameter of PS sphere is 200 nm. The electric field is upward and perpendicular to the interface of water and air. The external electric field can efficiently reduce defects in the regular arrangement of PS spheres, remarkably improving the ordering of the self-assembled monolayer of PS spheres.

Another way to modify the balance between the attraction force and the repulsion force among PS spheres is to change the surface chemistry of PS spheres or the electrostatic environment of the water-air interface (Sirotkin et al., 2010). By adding some electrolyte, e.g. acid (H2SO4) and NaCl, into the water, effective surface charge density of PS spheres and /or effect of electric screen of PS spheres can be changed, which give rise to the change of

by electrostatic stresses at interface. Both of these two forces were associated with an electric dipolar field, which resulted from an asymmetric charge distribution on particles at the interface due to mismatch in dielectric constant of adjacent fluids. Such a creation of the attractive capillary force is crucial because spheres with diameters of less than 5 m generally do not have sufficient weight to deform the liquid meniscus by means of gravity

Considering the importance of the electrical field in self-assembling PS spheres at interface, it is natural to find ways to change the electric field to control the arrangement of PS spheres. One way is to apply an external electric field during the self-assembly of PS spheres at the interface (Aubry & Singh, 2008; Boneva et al., 2009; Nikolaides et al., 2002; Trau et al., 1996). This external electrical field will change the charge distribution on the surface of PS Spheres, leading to the change of the electric dipole field around PS spheres. It may also exert an additional electric force on the negatively charged PS spheres, which will affect the deformation of the surface. In a word, the external electric field will affect both the attraction force and the repulsion force mentioned above. It has been found that the inter-particle distance can be remarkably changed by the electric field (Nikolaides et al., 2002). As a result, the ordering of self-assembled PS spheres can be improved considerably, as shown in Fig. 1. In addition, the effect of an external electric field on the arrangement of PS spheres is more

Fig. 1. Self-assembled monolayer of PS spheres (5 x 5 m2) with an external electric field of, (a) 0 v/m, (b) 5 x 104 v/m. The diameter of PS sphere is 200 nm. The electric field is upward and perpendicular to the interface of water and air. The external electric field can efficiently

Another way to modify the balance between the attraction force and the repulsion force among PS spheres is to change the surface chemistry of PS spheres or the electrostatic environment of the water-air interface (Sirotkin et al., 2010). By adding some electrolyte, e.g. acid (H2SO4) and NaCl, into the water, effective surface charge density of PS spheres and /or effect of electric screen of PS spheres can be changed, which give rise to the change of

reduce defects in the regular arrangement of PS spheres, remarkably improving the

ordering of the self-assembled monolayer of PS spheres.

(Kralchevsky & Nagayama, 2000).

pronounced for smaller spheres.

the interaction among PS spheres. It has been found that the ordering of self-assembled PS spheres at interface can be considerably improved by adding suitable acid (H2SO4) in water (Sirotkin et al., 2010), as shown in Fig. 2. Given that the charges on PS spheres are related to the diameter of PS spheres,the suitable amount of acid or other electrolyte is dependent on the size of PS spheres。In addition, the temperature of the water to some degree also affect the self-assembly of the PS spheres at the interface. It was found that ordering of PS was improved on the water of ~ 4 oC. Such an improvement may be related to the increase of water surface tension and the suppression of the Brownian motion of the PS spheres and dust clusters in the water.

Fig. 2. Self-assembled monolayer of PS spheres (5 x 5 m2) at the interface of air and the solution of de-ionized water and H2SO4 with PH value of, (a) ~7, (b) ~ 5.3. The diameter of PS sphere is 240 nm. The H2SO4 can provide some additional ions around PS spheres, which can effectively change the interaction among PS spheres. Under certain PH value of the solution, considerable improvement of ordering of PS spheres can be made.

The self-assembled monolayer of PS spheres can be finally transferred to varieties of smooth substrates underneath the water by draining off the water. This process can be affected by some charges on the substrates. In addition, some cracks may appear once the monolayer was disturbed by movements of the water during draining. The PS spheres nearby the cracks slightly displaced from the ideal sites of a hexagonal lattice. In this case, the longrange ordering of the subsequent structure is degraded. The monolayer of ordered PS spheres on substrates can then serve as a mask for the subsequent fabrication of ordered nanostructures by deposition of varieties of materials or by etching.
