**1. Introduction**

504 Recent Advances in Nanofabrication Techniques and Applications

Saito, N.; Lee, SH.; Ishizaki, T.; Hieda, J.; Sugimura, H. & Takai, O. (2005). Surface-Potential

Sugimura, H.; Saito, N.; Lee, SH. & Takai, O. (2004). Reversible nanochemical conversion.

ISSN 0928-4931.

ISSN 1520-8567.

2005), pp. 11602-11605, ISSN 1089-5647.

*Materials Science and Engineering C*, Vol.27, No.5-8 (September 2007), pp. 1241-1246,

Reversibility of an Amino-Terminated Self-Assembled Monolayer Based on Nanoprobe Chemistry. *The Journal of Physical Chemistry B,* Vol.109, No.23 (May

*Journal of Vacuum Science & Technology B*, Vol.22, No.6 (November 2004), pp. L44-46,

Since ancient time, noble metal has been used to make ornaments, jewelry, high-value tableware, utensils, currency coins and medicines due to its brilliant metallic luster, stability in air and water and anti-bacteria and anti-fungi properties (Jain, Huang et al. 2007) (Erhardt 2003; Daniel and Astruc 2004; Brayner 2008; Maneerung, Tokura et al. 2008). In fact, noble metal is also valuable due to its unique physicochemical properties, the highest electrical and thermal conductivity, the lowest contact resistance, and the highest optical reflectivity (particularly in ultra-violet region) of all metals(Edwards and Petersen 1936 ; Hammond 2000). Its d-electron configuration endows them with active chemical properties, for example, 3 variable oxidation states for silver, the most common of which is the +1 state, as in AgNO3, the +2 state as in silver(II) fluoride AgF2, and the +3 state as in compounds such as potassium tetrafluoroargentate K[AgF4], and suitability as catalysts by losing one or two more 4d electrons (Dhar, Cao et al. 2007). Silver and gold have the stable face-centered cubic (fcc) crystal structures but readily absorbs free neutrons due to its massive nucleus, which make them good absorbers for nucleus raidation. These unique features have enabled them to be applied to diverse applications such as those mentioned above, medical and dental applications, photography, electronics, nuclear reactors, catalysts, clothing and foods (http://en.wikipedia.org/wiki/Silver).

The intrinsic features of noble metal also endow their nanoscale species with attractive physicochemical properties due to the size and shape effects, including unique optical properties (e.g. Localized Surface Plasmon Resonance: LSPR; Surface Enhanced Raman Scattering: SERS), catalytic/electric properties and bio-functions (Percival, Bowler et al. 2005; Jain, Huang et al. 2007; Schwartzberg and Zhang 2008; Zhou, Qian et al. 2008; Vo-Dinh, Wang et al. 2009). Although ancient people used some features of Ag or Au nanocolloids (e.g. optical property) in fabrication of ceramic glazes for lustrous or iridescent effect in ancient Persia, they did not realize that these effects were due to nanoscale effects from size, shape and surface morphology dependent physicochemical properties of silver materials (Erhardt 2003; Brayner 2008). As materials science has progressed down to nanoscale, the unique properties of nanoscaled noble metal materials are only now being recognized and realized intentionally. These properties have shown vast applications in

Controlled Fabrication of Noble Metal Nanomaterials

polystyrene nanospheres as the template (Song 2009).

via Nanosphere Lithography and Their Optical Properties 507

Ali 2010). The routine procedure for the production of triangular shaped nanoprisms, based on the nanosphere LIGA, is described in Figure 1 (a: cross-section view; b: top view) (Hulteen, Treichel et al. 1999; Haynes and van Duyne 2001; Song 2009). The hexagonal arranged nanosphere mono layer is first formed on the substrate by a coating process (e.g. dip-coating, rotating-coating or spinning-coating) (Step 1: a). The interstitials among any three adjacent nanospheres will form triangle shaped voids (Step 1: b) as templates. The desired noble metal (e.g. Ag) will then be deposited on the triangle shaped interstitials among the nanospheres to form triangle shaped Ag NPs (Step 2: a and b). After nanospheres are released by sonication or other methods, surface-confined triangular Ag nanoprisms can be obtained (Step 3: a). By this nanosphere LIGA process, uniform hexagonal-arrayed triangle nanoprisms can be fabricated on a variety of substrates (e.g. glass, mica, silica wafer, PMMA, etc.). Step 3-b is an Atomic Force Microscope (AFM) image of Ag triangular nanoprisms fabricated by our group using a self-assembled monolayer of 300 nm

The initial critical step in NSL is the formation of a uniform large scale nanosphere template. Both drop-coating or spin-coating can produce uniform templates on a glass, silica wafer or mica substrate. The uniformity of the nanosphere template produced by drop coating depends on the nanosphere type and concentration, the hydrophilic properties of the substrate, the environmental humidity and temperature, and the drying speed. A monolayer colloidal polystyrene nanosphere mask can be prepared by drop-coating of ~3.0-4.0 µL, 3-10 times diluted nanosphere solution (conc. 4.0 wt.%) onto the glass support and leaving them to dry overnight. A detailed procedure to fabricate the nanosphere mask using drop-coating is as follows. The glass substrates are cleaned by sonication with a mixture of sulfuric acid and hydrogen peroxide (3:1 = conc. H2SO4: 30% H2O2, Volume ratio) at 80 ºC for 30 min and washed using sufficient nanopure water. Then, the glass substrates are sonicated in a mixture of ammonia and hydrogen peroxide (5:1:1 = H2O: NH4OH (37%): 30% H2O2, volume ratio) to increase the hydrophilic property on the surface of the glass substrates. Finally, the glass substrates are washed using sufficient nanopure water again and stored in the nanopure water for future use. When drop coating is to be performed, the glass substrate is picked up from the nanopure water from one of its edges. The remaining water droplets on the glass substrate are removed by touching the opposite edge on filter paper. The substrate is then left flat in a clean Petri-dish with a tilt angle of ~3-5º. A 15 μL of PS nanosphere solution is added on the surface of the glass substrate using a droplet. The water spreads over the whole glass substrate to form a semi-ellipsoidal shaped water spot. The Petri-dish is left for enough time to allow the water to evaporate. During evaporation, the temperature is kept at 18±3 ºC and the humidity is kept ~50±5%. In our group, a nearuniform monolayer nanosphere template can be prepared on almost the whole glass substrate (18 mm diameter). Figure 2 shows one typical area of a near-uniform monolayer template over scale ~20 μm. From the magnified image, a selected area shown in the inset, no lattice defects can be observed. Using this template, uniform Ag nanoprisms can be fabricated by vapor deposition process. One typical area fabricated by my group is shown in Step 3-b in Figure 1, where these nanoprisms have very uniform edge length of 67 ± 4 nm

(STDEV% of 6%) and thickness of 20.0±1.0 nm (STDEV% of 5%) (Song 2009).

Recent progress in nanosphere lithography (NSL) has shown that it provides a good template for other shape (besides triangle) controlled fabrication of surface confined NPs by a combination of deposition angle tilting, multi-step deposition and different post treatment

microelectronics, photonic devices, optoelectric coupling, catalytic processes, biomedical engineering and medicines. As understanding that the intrinsic properties (e.g. optical, catalytic) of nanomaterials on the size, shape, surface spatial morphology and arrangement (Ahmadi, Wang et al. 1996; Jensen, Duval Malinsky et al. 2000; Mock, Barbic et al. 2002; Haynes, Mcfarland et al. 2003; Noguez 2007; Song 2009) has increased, fabrication of silver or gold nanoparticles (NPs) and their arrays with controlled three-dimensional (3D) morphologies, interspacing and orientation has become a very significant research stream in recent years. A variety of fabrication techniques, such as thermal decomposition, metal salt reduction, photo reduction/conversion, template assisted growth and deposition, γ-rayirridation, as well as microfluidic processes have been developed. As a result, significant progresses have been achieved in the tailoring of the 3-dimension (3D) morphologies (size, shape and surface morphology), crystal structures and spatial arrangement of noble metal nanomaterials as desired.

Template assisted (TA) lithography (LIGA) has developed to a powerful physical technique that enables the production of surface morphology confined NPs and NPs arrays with controlled shapes, sizes and interparticle spacing (Jensen, Duval Malinsky et al. 2000; Lee, Morrill et al. 2006; Zhang, Whitney et al. 2006; Lombardi, Cavallotti et al. 2007; Zhu, Li et al. 2008; Song 2009). Lots of templates have been developed for these purposes, such as porous polymers(Lombardi, Cavallotti et al. 2007), porous Al2O3 foils(Chong, Zheng et al. 2006; Lee, Morrill et al. 2006; Xu, Meng et al. 2009), or nanosphere arrays (polymers or ceramics)(Zhang, Whitney et al. 2006; Song 2009; Song and Elsayed-Ali 2010; Song , Zhang et al. 2011), resulting in varieties of template-assisted lithography, correspondingly as porous polymers LIGA (PP-LIGA), porous anodic Al2O3 LIGA (PAA-LIGA), or nanosphere-LIGA (NSL). Among them, the most popular and well-developed method may be NSL. In this chapter, recent progresses in NSL, for controlled producing noble metal nanomaterials will be summarized. The first discussion involves in this technique for size, shape and surface morphology controlled fabrication of noble metal nanoparticles (NPs) and nanoarrays. Then four distinct progresses in the development of NSL techniques: (1) Fabrication of hierarchically ordered nanowire arrays on substrates by combination of NSL and Porous anodic alumina (PAA); (2) Identification of single nanoparticles and nano-arrays by combination of NSL and multi-hierarchy arrayed micro windows; (3) Fabrication of biosensing system based on the combination of the noble metal nanoparticles and nanoarrays fabricated by NSL and microfluidic techniques; (4) Synthesis of solution-phased nanoparticles by the transfer of the surface confined NPs fabricated by NSL into solutions, will be discussed. In (2) and (3), the related 3D morphologies and arrangement dependent optical properties, and comparison between the numerical and experimental results, revealing their intrinsic quantum mechanism, such as LSPR will be analyzed. These researches are fundamental requirements for the discovery of novel properties and applications of noble metal NPs, as well as for paving the theory development. Finally, issues and perspectives in the controlled fabrication of noble metal nanomaterials by NSL, and investigation of their 3D morphologies and arrangement dependent optical properties for future potential applications will be highlighted and discussed in closing.

#### **2. Size and shape controlled fabrication of nanomateials via NSL**

Nanospheres have been used to form uniformly arranged layers as templates to produce perfect triangle nanoprisms on substrates (Haynes and van Duyne 2001; Song and Elsayed-

microelectronics, photonic devices, optoelectric coupling, catalytic processes, biomedical engineering and medicines. As understanding that the intrinsic properties (e.g. optical, catalytic) of nanomaterials on the size, shape, surface spatial morphology and arrangement (Ahmadi, Wang et al. 1996; Jensen, Duval Malinsky et al. 2000; Mock, Barbic et al. 2002; Haynes, Mcfarland et al. 2003; Noguez 2007; Song 2009) has increased, fabrication of silver or gold nanoparticles (NPs) and their arrays with controlled three-dimensional (3D) morphologies, interspacing and orientation has become a very significant research stream in recent years. A variety of fabrication techniques, such as thermal decomposition, metal salt reduction, photo reduction/conversion, template assisted growth and deposition, γ-rayirridation, as well as microfluidic processes have been developed. As a result, significant progresses have been achieved in the tailoring of the 3-dimension (3D) morphologies (size, shape and surface morphology), crystal structures and spatial arrangement of noble metal

Template assisted (TA) lithography (LIGA) has developed to a powerful physical technique that enables the production of surface morphology confined NPs and NPs arrays with controlled shapes, sizes and interparticle spacing (Jensen, Duval Malinsky et al. 2000; Lee, Morrill et al. 2006; Zhang, Whitney et al. 2006; Lombardi, Cavallotti et al. 2007; Zhu, Li et al. 2008; Song 2009). Lots of templates have been developed for these purposes, such as porous polymers(Lombardi, Cavallotti et al. 2007), porous Al2O3 foils(Chong, Zheng et al. 2006; Lee, Morrill et al. 2006; Xu, Meng et al. 2009), or nanosphere arrays (polymers or ceramics)(Zhang, Whitney et al. 2006; Song 2009; Song and Elsayed-Ali 2010; Song , Zhang et al. 2011), resulting in varieties of template-assisted lithography, correspondingly as porous polymers LIGA (PP-LIGA), porous anodic Al2O3 LIGA (PAA-LIGA), or nanosphere-LIGA (NSL). Among them, the most popular and well-developed method may be NSL. In this chapter, recent progresses in NSL, for controlled producing noble metal nanomaterials will be summarized. The first discussion involves in this technique for size, shape and surface morphology controlled fabrication of noble metal nanoparticles (NPs) and nanoarrays. Then four distinct progresses in the development of NSL techniques: (1) Fabrication of hierarchically ordered nanowire arrays on substrates by combination of NSL and Porous anodic alumina (PAA); (2) Identification of single nanoparticles and nano-arrays by combination of NSL and multi-hierarchy arrayed micro windows; (3) Fabrication of biosensing system based on the combination of the noble metal nanoparticles and nanoarrays fabricated by NSL and microfluidic techniques; (4) Synthesis of solution-phased nanoparticles by the transfer of the surface confined NPs fabricated by NSL into solutions, will be discussed. In (2) and (3), the related 3D morphologies and arrangement dependent optical properties, and comparison between the numerical and experimental results, revealing their intrinsic quantum mechanism, such as LSPR will be analyzed. These researches are fundamental requirements for the discovery of novel properties and applications of noble metal NPs, as well as for paving the theory development. Finally, issues and perspectives in the controlled fabrication of noble metal nanomaterials by NSL, and investigation of their 3D morphologies and arrangement dependent optical properties

for future potential applications will be highlighted and discussed in closing.

**2. Size and shape controlled fabrication of nanomateials via NSL** 

Nanospheres have been used to form uniformly arranged layers as templates to produce perfect triangle nanoprisms on substrates (Haynes and van Duyne 2001; Song and Elsayed-

nanomaterials as desired.

Ali 2010). The routine procedure for the production of triangular shaped nanoprisms, based on the nanosphere LIGA, is described in Figure 1 (a: cross-section view; b: top view) (Hulteen, Treichel et al. 1999; Haynes and van Duyne 2001; Song 2009). The hexagonal arranged nanosphere mono layer is first formed on the substrate by a coating process (e.g. dip-coating, rotating-coating or spinning-coating) (Step 1: a). The interstitials among any three adjacent nanospheres will form triangle shaped voids (Step 1: b) as templates. The desired noble metal (e.g. Ag) will then be deposited on the triangle shaped interstitials among the nanospheres to form triangle shaped Ag NPs (Step 2: a and b). After nanospheres are released by sonication or other methods, surface-confined triangular Ag nanoprisms can be obtained (Step 3: a). By this nanosphere LIGA process, uniform hexagonal-arrayed triangle nanoprisms can be fabricated on a variety of substrates (e.g. glass, mica, silica wafer, PMMA, etc.). Step 3-b is an Atomic Force Microscope (AFM) image of Ag triangular nanoprisms fabricated by our group using a self-assembled monolayer of 300 nm polystyrene nanospheres as the template (Song 2009).

The initial critical step in NSL is the formation of a uniform large scale nanosphere template. Both drop-coating or spin-coating can produce uniform templates on a glass, silica wafer or mica substrate. The uniformity of the nanosphere template produced by drop coating depends on the nanosphere type and concentration, the hydrophilic properties of the substrate, the environmental humidity and temperature, and the drying speed. A monolayer colloidal polystyrene nanosphere mask can be prepared by drop-coating of ~3.0-4.0 µL, 3-10 times diluted nanosphere solution (conc. 4.0 wt.%) onto the glass support and leaving them to dry overnight. A detailed procedure to fabricate the nanosphere mask using drop-coating is as follows. The glass substrates are cleaned by sonication with a mixture of sulfuric acid and hydrogen peroxide (3:1 = conc. H2SO4: 30% H2O2, Volume ratio) at 80 ºC for 30 min and washed using sufficient nanopure water. Then, the glass substrates are sonicated in a mixture of ammonia and hydrogen peroxide (5:1:1 = H2O: NH4OH (37%): 30% H2O2, volume ratio) to increase the hydrophilic property on the surface of the glass substrates. Finally, the glass substrates are washed using sufficient nanopure water again and stored in the nanopure water for future use. When drop coating is to be performed, the glass substrate is picked up from the nanopure water from one of its edges. The remaining water droplets on the glass substrate are removed by touching the opposite edge on filter paper. The substrate is then left flat in a clean Petri-dish with a tilt angle of ~3-5º. A 15 μL of PS nanosphere solution is added on the surface of the glass substrate using a droplet. The water spreads over the whole glass substrate to form a semi-ellipsoidal shaped water spot. The Petri-dish is left for enough time to allow the water to evaporate. During evaporation, the temperature is kept at 18±3 ºC and the humidity is kept ~50±5%. In our group, a nearuniform monolayer nanosphere template can be prepared on almost the whole glass substrate (18 mm diameter). Figure 2 shows one typical area of a near-uniform monolayer template over scale ~20 μm. From the magnified image, a selected area shown in the inset, no lattice defects can be observed. Using this template, uniform Ag nanoprisms can be fabricated by vapor deposition process. One typical area fabricated by my group is shown in Step 3-b in Figure 1, where these nanoprisms have very uniform edge length of 67 ± 4 nm (STDEV% of 6%) and thickness of 20.0±1.0 nm (STDEV% of 5%) (Song 2009).

Recent progress in nanosphere lithography (NSL) has shown that it provides a good template for other shape (besides triangle) controlled fabrication of surface confined NPs by a combination of deposition angle tilting, multi-step deposition and different post treatment

Controlled Fabrication of Noble Metal Nanomaterials

Figure 1. Copyright (2010) Elsevier.).

nanostructures.

blocking the substrate to line of sight deposition.

via Nanosphere Lithography and Their Optical Properties 509

Fig. 2**.** Nanosphere templates based on 290 nm spherical polystyrene nanospheres for Ag nanoparticle fabrication (Reprinted from Song et al., Appl. Surf. Sci. 2010 256, (20), 5961,

θdep, and accordingly, the deposited nanoparticles' shape and size are controlled directly by θdep and the diameter of nanosphere. Figure 3 schematically describes the effect of angleresolved deposition on the interstices of a NSL mask from the top view. (Figure 3). As a convention, θdep = 0o represents a substrate mounted normal to the evaporation beam (Figure 3A), and all variations of θdep are made by mounting the substrates on machined aluminum blocks. It is clear from this illustration that an increase in θdep causes the projections of the interstices onto the substrate to decrease and shift (Figure 3B and 3C). At high values of θdep (e.g. 45o, Figure 3C), the projections of the interstices close, completely

One very important consequence of AR NSL, beyond the increased flexibility in nanostructure architecture, lies in the decrease in nanoparticle size. Before AR NSL, the only way to fabricate nanoparticles in the 1-20 nm size range with NSL required self-assembly of nanospheres with diameters on the order of 5-100 nm. Not only synthesis of uniform nanospheres at this range is usually difficult, but self-assembly of such small nanospheres into well ordered 2D arrays is extremely challenging because of problems with greater polydispersity and the surface roughness of substrates. However, with AR NSL, increasing θdep from 0o to 20o will halve the in-plane dimension of nanosphere templates, leading to the success in small nanoparticle preparation by NSL. In addition, nano-overlapped, nanogapped and nano-chained structures can be addressed by multi-step AR NSL, which is fulfilled by depositing materials through a nanosphere mask mounted at different θdep below the overlap threshold value of θdep several times. Van Duyne et al have used two step AR NSL to fabricate over-lapped and gapped Ag nanostructures through a nanosphere mask with *D* = 542 nm onto mica substrates by a first deposition at θdep = 0o and a second deposition at an increased θdep. The importance in the fabrication of over-lapped nanoparticles theoretically exists in the enhanced optical properties due to their increased aspect ratio (in-plane width/out-of-plane height) nanoparticles(Kreibig and Vollmer 1995). Nano-overlapped structures can give an significantly increased sensitivity of optical response since they allow predictable aspect ratio to increase up to double of the original value (Haynes and van Duyne 2001). One of the interests for gapped nanostructures may exist in the investigation of the distance dependent LSPR coupling amomng gapped

methods(Haynes and van Duyne 2001; Song and Elsayed-Ali 2010). A new class of NSL structures has been fabricated by varying the deposition angle, θdep, between the nanosphere mask and the beam of material being deposited, which is hereafter referred to as angleresolved NSL (AR NSL)(Haynes and van Duyne 2001). The size and shape of the three-fold interstices of the nanosphere mask change relative to the deposition source as a function of

Fig. 1. The NSL process for triangular NPs fabrication. Step 1a: The hexagonal arranged nanosphere mono layer is first formed on the substrate by coating process; Step 1b: The interstitials among any three adjacent nanospheres will form triangle shaped voids as templates; Step 2a-b: the Ag metal will be deposited on the triangle shaped interstitials among the nanospheres to form triangle shaped Ag NPs; Step 3a: The nanospheres will be released by sonication or other methods, leaving the triangle shaped Ag nanoprisms on the substrates, by this nanosphere LIGA process, the hexagonal arrayed uniform triangle nanoprisms can be fabricated on variety of substrates (e.g. glass, mica, silica wafer, PMMA, etc.); Step 3b: The AFM image for Ag triangle nanoprisms fabricated by monolayer template from 290 nm polystyrene nanospheres in my group, these nanoprisms have very uniform edge length of 67 ± 4 nm (STDEV% of 6%) and thickness of 20.0 ± 1.0 nm (STDEV% of 5%). (a): cross-section view; (b) top 3D view. (Adapted in part from Song, Y. China Patent, CN200910085973.9; Haynes, C. L.; van Duyne, R. P., *J. Phys. Chem. B* 2001 105, 5599, Figure 2, Copyright (2001) American Chemical Society; and Hulteen, J. C.; et al., *J. Phys. Chem. B* **1999** 103, 3854, Figure 1, Copyright (1999) American Chemical Society.)

methods(Haynes and van Duyne 2001; Song and Elsayed-Ali 2010). A new class of NSL structures has been fabricated by varying the deposition angle, θdep, between the nanosphere mask and the beam of material being deposited, which is hereafter referred to as angleresolved NSL (AR NSL)(Haynes and van Duyne 2001). The size and shape of the three-fold interstices of the nanosphere mask change relative to the deposition source as a function of

Fig. 1. The NSL process for triangular NPs fabrication. Step 1a: The hexagonal arranged nanosphere mono layer is first formed on the substrate by coating process; Step 1b: The interstitials among any three adjacent nanospheres will form triangle shaped voids as templates; Step 2a-b: the Ag metal will be deposited on the triangle shaped interstitials among the nanospheres to form triangle shaped Ag NPs; Step 3a: The nanospheres will be released by sonication or other methods, leaving the triangle shaped Ag nanoprisms on the substrates, by this nanosphere LIGA process, the hexagonal arrayed uniform triangle nanoprisms can be fabricated on variety of substrates (e.g. glass, mica, silica wafer, PMMA, etc.); Step 3b: The AFM image for Ag triangle nanoprisms fabricated by monolayer template from 290 nm polystyrene nanospheres in my group, these nanoprisms have very uniform edge length of 67 ± 4 nm (STDEV% of 6%) and thickness of 20.0 ± 1.0 nm (STDEV% of 5%). (a): cross-section view; (b) top 3D view. (Adapted in part from Song, Y. China Patent, CN200910085973.9; Haynes, C. L.; van Duyne, R. P., *J. Phys. Chem. B* 2001 105, 5599, Figure 2, Copyright (2001) American Chemical Society; and Hulteen, J. C.; et al., *J. Phys. Chem. B* **1999**

(b)

103, 3854, Figure 1, Copyright (1999) American Chemical Society.)

Fig. 2**.** Nanosphere templates based on 290 nm spherical polystyrene nanospheres for Ag nanoparticle fabrication (Reprinted from Song et al., Appl. Surf. Sci. 2010 256, (20), 5961, Figure 1. Copyright (2010) Elsevier.).

θdep, and accordingly, the deposited nanoparticles' shape and size are controlled directly by θdep and the diameter of nanosphere. Figure 3 schematically describes the effect of angleresolved deposition on the interstices of a NSL mask from the top view. (Figure 3). As a convention, θdep = 0o represents a substrate mounted normal to the evaporation beam (Figure 3A), and all variations of θdep are made by mounting the substrates on machined aluminum blocks. It is clear from this illustration that an increase in θdep causes the projections of the interstices onto the substrate to decrease and shift (Figure 3B and 3C). At high values of θdep (e.g. 45o, Figure 3C), the projections of the interstices close, completely blocking the substrate to line of sight deposition.

One very important consequence of AR NSL, beyond the increased flexibility in nanostructure architecture, lies in the decrease in nanoparticle size. Before AR NSL, the only way to fabricate nanoparticles in the 1-20 nm size range with NSL required self-assembly of nanospheres with diameters on the order of 5-100 nm. Not only synthesis of uniform nanospheres at this range is usually difficult, but self-assembly of such small nanospheres into well ordered 2D arrays is extremely challenging because of problems with greater polydispersity and the surface roughness of substrates. However, with AR NSL, increasing θdep from 0o to 20o will halve the in-plane dimension of nanosphere templates, leading to the success in small nanoparticle preparation by NSL. In addition, nano-overlapped, nanogapped and nano-chained structures can be addressed by multi-step AR NSL, which is fulfilled by depositing materials through a nanosphere mask mounted at different θdep below the overlap threshold value of θdep several times. Van Duyne et al have used two step AR NSL to fabricate over-lapped and gapped Ag nanostructures through a nanosphere mask with *D* = 542 nm onto mica substrates by a first deposition at θdep = 0o and a second deposition at an increased θdep. The importance in the fabrication of over-lapped nanoparticles theoretically exists in the enhanced optical properties due to their increased aspect ratio (in-plane width/out-of-plane height) nanoparticles(Kreibig and Vollmer 1995). Nano-overlapped structures can give an significantly increased sensitivity of optical response since they allow predictable aspect ratio to increase up to double of the original value (Haynes and van Duyne 2001). One of the interests for gapped nanostructures may exist in the investigation of the distance dependent LSPR coupling amomng gapped nanostructures.

Controlled Fabrication of Noble Metal Nanomaterials

fabrication of nanowires.

via Nanosphere Lithography and Their Optical Properties 511

The overlap percent of nanoparticles can be adjusted by θdep at a certain mass deposition thickness (e.g. 20 nm) and nanosphere diameter (e.g. 542 nm). As shown in Figure 4A-D, the overlap percent decreases with the increase of θdep from 0o to 20o. The θdep at 20o is the threshold deposition angle since neither overlap nor gap is visible by AFM investigation at this point (Figure 4D). When the second deposition angle is more than the threshold angle (e.g. 20 o based on the fabrication condition using nanosphere mask with *D* = 542 nm and mass thickness dm = 20 nm(Haynes and van Duyne 2001)), nano-gapped structures can be formed. With the same experimental parameters defined above, the gap between nanoparticles increases as θdep is increased from 22o to higher values up to thecritical θdep value at which the interstitial projections are closed to line-of-sight deposition. Figure 4E-H shows the AFM images of the typical nano-gapped Ag structures with different gap distances when the second deposition angles change from 22o, to 23o, to 24o and to 26o. Another of the applications proposed by van Duyne is to use the nanogap architecture to measure the electrical conductivity of a single molecule or nanoparticle (Haynes and van Duyne 2001). If one side of the nanogap is insulated from a conductive substrate while the other side of the nanogap is in contact with a conductive substrate, the conductance of the

junction should be measurable with a scanning tunneling microscopy probe.

Fig. 5. (A) Schematic fabrication process and (B) contact mode AFM image for three

5599, Figure 9, Copyright (2001) Amercian Chemical Society.)

deposition nanochain structure on mica. 1.6 *μ*m - 1.6*μ*m area, *D* = 542 nm, *d*m = 10 nm, *θ*dep = +15°, 0°, and -15°. (Reprinted from Haynes, C. L.; van Duyne, R. P., *J. Phys. Chem. B* 2001 105,

Clearly, like the two depositions at different values of θdep, three or more depositions will further extend the range of nanoparticle architectures accessible by AR NSL. An endless number of nanostructures are possible when one combines the ability to vary θdep and to perform multiple material depositions. As an example, the nanochain motif with threeconnected-nanoparticle chains can be fabricated by three consecutive depositions. The first deposition is done at θdep = -150, whereas the second and third depositions will be further done at θdep = 0o (tilted forward) and θdep = 15o (tilted backward). An AFM image of the nanochain structure is shown in Figure 5 and gives a typical domain where the sample tilt axis is aligned with the triangular base of the nanoparticles. Possible applications of the nanochain architecture include sub-100 nm near-field optical waveguides, chemical and biological sensors based on the LSPR of these high aspect ratio nanoparticles, and the

Fig. 3. Scheme of the angle resolved deposition process. (A) Samples viewed at 0o. The interstices in the nanosphere mask are equally spaced and of equal size. (B) Sample viewed at 30o. The interstices in the nanosphere mask follow a pattern including two different interparticle spacing values, and the interstitial area is smaller. (C) Sample viewed at 45o. The interstices are now closed to line of sight deposition. (Adapted from Haynes, C. L.; van Duyne, R. P., *J. Phys. Chem. B* 2001 105, 5599, Figure 5, Copyright (2001) Amercian Chemical Society.)

Fig. 4. Contact mode AFM images of typical Ag nano-overlapped and nano-gapped structures fabricated on mica substrates using the single layer nanosphere template. (A) 300 nm - 300 nm image, *D* = 542 nm, *d*m = 20 nm, *θ*dep = 0 o and 6 o. (B) 250 nm - 250 nm image, *D*  = 542 nm, *d*m = 20 nm, *θ*dep = 0 o and 10 o. (C) 300 nm - 300 nm image, *D* = 542 nm, *d*m = 20 nm, *θ*dep = 0° and 15o. (D) 250 nm-250 nm image, *D* = 542 nm, *d*m = 20 nm, *θ*dep = 0° and 20 o. (E) 250 nm - 250 nm image, *D* = 542 nm, *d*m = 20 nm, *θ*dep = 0° and 22 o. (F) 250 nm - 250 nm image, *D* = 542 nm, *d*m = 20 nm, *θ*dep = 0° and 23 o. (G) 250 nm - 250 nm image, *D* = 542 nm, *d*m = 20 nm, *θ*dep = 0° and 24 o. (H) 250 nm - 250 nm image, *D* = 542 nm, *d*m = 20 nm, *θ*dep = 0° and 26 o. (Adapted from Haynes, C. L.; van Duyne, R. P., N *J. Phys. Chem. B* 2001 105, 5599, Figure 6 and Figure 7, Copyright (2001) Amercian Chemical Society.)

Fig. 3. Scheme of the angle resolved deposition process. (A) Samples viewed at 0o. The interstices in the nanosphere mask are equally spaced and of equal size. (B) Sample viewed at

interparticle spacing values, and the interstitial area is smaller. (C) Sample viewed at 45o. The interstices are now closed to line of sight deposition. (Adapted from Haynes, C. L.; van Duyne, R. P., *J. Phys. Chem. B* 2001 105, 5599, Figure 5, Copyright (2001) Amercian Chemical Society.)

30o. The interstices in the nanosphere mask follow a pattern including two different

Fig. 4. Contact mode AFM images of typical Ag nano-overlapped and nano-gapped

 **E F G H** 

Figure 6 and Figure 7, Copyright (2001) Amercian Chemical Society.)

structures fabricated on mica substrates using the single layer nanosphere template. (A) 300 nm - 300 nm image, *D* = 542 nm, *d*m = 20 nm, *θ*dep = 0 o and 6 o. (B) 250 nm - 250 nm image, *D*  = 542 nm, *d*m = 20 nm, *θ*dep = 0 o and 10 o. (C) 300 nm - 300 nm image, *D* = 542 nm, *d*m = 20 nm, *θ*dep = 0° and 15o. (D) 250 nm-250 nm image, *D* = 542 nm, *d*m = 20 nm, *θ*dep = 0° and 20 o. (E) 250 nm - 250 nm image, *D* = 542 nm, *d*m = 20 nm, *θ*dep = 0° and 22 o. (F) 250 nm - 250 nm image, *D* = 542 nm, *d*m = 20 nm, *θ*dep = 0° and 23 o. (G) 250 nm - 250 nm image, *D* = 542 nm, *d*m = 20 nm, *θ*dep = 0° and 24 o. (H) 250 nm - 250 nm image, *D* = 542 nm, *d*m = 20 nm, *θ*dep = 0° and 26 o. (Adapted from Haynes, C. L.; van Duyne, R. P., N *J. Phys. Chem. B* 2001 105, 5599,

The overlap percent of nanoparticles can be adjusted by θdep at a certain mass deposition thickness (e.g. 20 nm) and nanosphere diameter (e.g. 542 nm). As shown in Figure 4A-D, the overlap percent decreases with the increase of θdep from 0o to 20o. The θdep at 20o is the threshold deposition angle since neither overlap nor gap is visible by AFM investigation at this point (Figure 4D). When the second deposition angle is more than the threshold angle (e.g. 20 o based on the fabrication condition using nanosphere mask with *D* = 542 nm and mass thickness dm = 20 nm(Haynes and van Duyne 2001)), nano-gapped structures can be formed. With the same experimental parameters defined above, the gap between nanoparticles increases as θdep is increased from 22o to higher values up to thecritical θdep value at which the interstitial projections are closed to line-of-sight deposition. Figure 4E-H shows the AFM images of the typical nano-gapped Ag structures with different gap distances when the second deposition angles change from 22o, to 23o, to 24o and to 26o. Another of the applications proposed by van Duyne is to use the nanogap architecture to measure the electrical conductivity of a single molecule or nanoparticle (Haynes and van Duyne 2001). If one side of the nanogap is insulated from a conductive substrate while the other side of the nanogap is in contact with a conductive substrate, the conductance of the junction should be measurable with a scanning tunneling microscopy probe.

Clearly, like the two depositions at different values of θdep, three or more depositions will further extend the range of nanoparticle architectures accessible by AR NSL. An endless number of nanostructures are possible when one combines the ability to vary θdep and to perform multiple material depositions. As an example, the nanochain motif with threeconnected-nanoparticle chains can be fabricated by three consecutive depositions. The first deposition is done at θdep = -150, whereas the second and third depositions will be further done at θdep = 0o (tilted forward) and θdep = 15o (tilted backward). An AFM image of the nanochain structure is shown in Figure 5 and gives a typical domain where the sample tilt axis is aligned with the triangular base of the nanoparticles. Possible applications of the nanochain architecture include sub-100 nm near-field optical waveguides, chemical and biological sensors based on the LSPR of these high aspect ratio nanoparticles, and the fabrication of nanowires.

Fig. 5. (A) Schematic fabrication process and (B) contact mode AFM image for three deposition nanochain structure on mica. 1.6 *μ*m - 1.6*μ*m area, *D* = 542 nm, *d*m = 10 nm, *θ*dep = +15°, 0°, and -15°. (Reprinted from Haynes, C. L.; van Duyne, R. P., *J. Phys. Chem. B* 2001 105, 5599, Figure 9, Copyright (2001) Amercian Chemical Society.)

Controlled Fabrication of Noble Metal Nanomaterials

AFM images.

**windows** 

optics and nanoarrays.

via Nanosphere Lithography and Their Optical Properties 513

rounded edges and little surface defects. Alternatively, if we sonicate the NPs produced by NSL for ~30-45 s to remove a weak tip, anneal them at 200 ºC for 1- 4 hours, then wash them with 5% nitric acid, trapezoidal shaped NPs with rounded edges are formed, as shown in Figure 6(b-1) and 6(b-2). If the sonication time is increased to more than 2 min, the NPs lose their two sharp tips and form quadrilateral or pentagon shaped NPs. After thermal annealing for 1-4 hours and washing with 5% nitric acid, their edges and corners become rounded, as shown in Fig. 6(c-1), which show quadrilateral NPs (in dashed squares) or pentagon (in dashed circles). The 3D AFM image, Figure 6(c-2), shows that these NPs have rounded edges and corners. Clearly, even after thermal annealing, they are still showing prism shapes with increased thickness from their edges to centers according to their 3D

The work described above demonstrates that NSL, broadly defined to include AR NSL and some modified post treatment after deposition of the desired materials, is manifestly capable of creating far more than arrays of nanotriangles, nanodots as previously supposed. The progresses in NSL endow much potential in the size and shape controlled fabrication of nanoparticles and nanoarrays, which gives NSL a bright future since the ability of NSL to synthesize monodisperse, size- and shape- tunable nanoparticles can be exploited to precisely investigate the size- and shape- dependent physiochemical properties of nano-

**3. Investigation of optical properties of specific noble metal nanoparticles and nanoarrays by the combination of NSL and multi-hierarchy arrayed micro** 

The physicochemical properties of nanomaterials significantly depend on their threedimensional (3D) morphologies (sizes, shapes and surface topography), their surrounding media, and their spatial arrangement. Systematically and precisely correlating these parameters with the related physicochemical properties of specific single nanoparticles (NPs) or nanoarrays is a fundamental requirement for the discovery of their novel properties and applications, as well as for advancing the fundamental and practical knowledge required for the design and fabrication of new materials(Song , Zhang et al. 2011). The lack of effective means of fabricating recognizable 3D morphologies controlled NPs and nanoarrays and correlating their structure parameters with their physicochemical properties as observed by different characterization techniques represents an obstacle for studying the 3D morphology-dependent properties of individual NPs and nanoarrays(Song, Zhang et al. 2011). Most current studies investigate the physicochemical properties of the NP ensemble, but not of a single NP(Jin, Cao et al. 2001; Kelly, Coronado et al. 2003; Haes, Zou et al. 2004; Song , Zhang et al. 2011). The ensemble of NPs is typically heterogeneous, because the morphologies of individual NPs prepared by routine chemical synthesis or physical vapor fabrication methods are rarely identical at the nanometre or sub-nanometre scale (Song , Zhang et al. 2011). Effective methods for 3D morphology controlled fabrication of nanomaterials, and to correlate their 3D morphology of single NPs or nanoarrays with their physicochemical properties are also essential to address fundamental and practical

An important research area in nanoscale plasmonic optics is single NP identification and characterization of their 3D morphologies and space-orientation dependent physicochemical

questions related to the single NPs (Song , Zhang et al. 2011).

We recently developed a modified NSL process to fabricate Ag NPs with controlled shapes on substrates. The modification in NSL is performed by thermally annealing the triangular nanoprisms, and sonication to remove weak tips, followed by removing debris and small broken parts around the NPs on the substrates(Song and Elsayed-Ali 2010). The detailed process is shown in the following: (1) Releasing the nanospheres by immersing the cover slip into a 5% HCl solution for 30 minutes, then immersing the glass substrates into CH2Cl2 for 30 s, then sonication for ~20-60 s; (2) The fabricated Ag nanoprisms on the glass substrates are annealed at 100-300 ºC for 2-5 hours; (3) Then Ag nanoprisms are cleaned by immersing the glass cover slip into 5% HNO3 for 10-20 s to remove any surface contamination and dissolve debris around the NPs, and then washed by large amount of nanopure water. Comparing the AFM images in Step 3-b of Figure 1 showing the NPs without above post treatment, tip-rounded triangle nanoprisms, square-shaped and trapezoidal Ag NPs (Figure 6) can be obtained via one or two of the above treatment. We observe that thermal annealing results in much more uniform NP surfaces without the thin, weak tips and edges (Figure 6(a-1)). From the magnified AFM plane image in the inset of Fig. 6(a-1) and the 3D image in Figure 6(a-2), the NPs still show triangular prism shape with

Fig. 6. Surface-confined Ag NPs with controlled shapes fabricated by the modified NSL process. (a-1) AFM image of triangular prism Ag NPs with rounded tips after thermal annealing at 200 ºC for 4 hours, cleaning by 5% nitric acid, and washing by nanopure water. (a-2) The 3D image of the triangular prism Ag NPs with rounded tips. (b-1) Flat trapezoidal Ag NPs after sonication to remove one tip, thermal annealing, cleaning by 5% nitric acid, and washing by nanopure water. (b-2) The 3D image of the trapezoidal Ag NPs with one snipped tip. (c-1) The quadrilateral or pentagon shaped Ag NPs after sonication intensively to remove two tips, thermal annealing, cleaning by 5% nitric acid and washing by nanopure water. Dashed circles: pentagonal Ag NPs with one sharp tip left; dashed squares: quadrilateral Ag NPs. (c-2) is the 3D image of the quadrilateral and pentagon shaped Ag NPs. (Song Y.; Elsayed-Ali H. E., Appl. Surf. Sci. 2010 256, (20), 5961, Figure 3, Copyright (2010) Elsevier.)

We recently developed a modified NSL process to fabricate Ag NPs with controlled shapes on substrates. The modification in NSL is performed by thermally annealing the triangular nanoprisms, and sonication to remove weak tips, followed by removing debris and small broken parts around the NPs on the substrates(Song and Elsayed-Ali 2010). The detailed process is shown in the following: (1) Releasing the nanospheres by immersing the cover slip into a 5% HCl solution for 30 minutes, then immersing the glass substrates into CH2Cl2 for 30 s, then sonication for ~20-60 s; (2) The fabricated Ag nanoprisms on the glass substrates are annealed at 100-300 ºC for 2-5 hours; (3) Then Ag nanoprisms are cleaned by immersing the glass cover slip into 5% HNO3 for 10-20 s to remove any surface contamination and dissolve debris around the NPs, and then washed by large amount of nanopure water. Comparing the AFM images in Step 3-b of Figure 1 showing the NPs without above post treatment, tip-rounded triangle nanoprisms, square-shaped and trapezoidal Ag NPs (Figure 6) can be obtained via one or two of the above treatment. We observe that thermal annealing results in much more uniform NP surfaces without the thin, weak tips and edges (Figure 6(a-1)). From the magnified AFM plane image in the inset of Fig. 6(a-1) and the 3D image in Figure 6(a-2), the NPs still show triangular prism shape with

Fig. 6. Surface-confined Ag NPs with controlled shapes fabricated by the modified NSL process. (a-1) AFM image of triangular prism Ag NPs with rounded tips after thermal annealing at 200 ºC for 4 hours, cleaning by 5% nitric acid, and washing by nanopure water. (a-2) The 3D image of the triangular prism Ag NPs with rounded tips. (b-1) Flat trapezoidal Ag NPs after sonication to remove one tip, thermal annealing, cleaning by 5% nitric acid, and washing by nanopure water. (b-2) The 3D image of the trapezoidal Ag NPs with one snipped tip. (c-1) The quadrilateral or pentagon shaped Ag NPs after sonication intensively to remove two tips, thermal annealing, cleaning by 5% nitric acid and washing by nanopure

**200 nm** 

**200 nm** 

**(c-1)** 

**(b-1) E**

**B-2 (a-2) B (b-2) (b- (c-2)** 

water. Dashed circles: pentagonal Ag NPs with one sharp tip left; dashed squares: quadrilateral Ag NPs. (c-2) is the 3D image of the quadrilateral and pentagon shaped Ag NPs. (Song Y.; Elsayed-Ali H. E., Appl. Surf. Sci. 2010 256, (20), 5961, Figure 3, Copyright

(2010) Elsevier.)

200nm

**(a-1)** 

rounded edges and little surface defects. Alternatively, if we sonicate the NPs produced by NSL for ~30-45 s to remove a weak tip, anneal them at 200 ºC for 1- 4 hours, then wash them with 5% nitric acid, trapezoidal shaped NPs with rounded edges are formed, as shown in Figure 6(b-1) and 6(b-2). If the sonication time is increased to more than 2 min, the NPs lose their two sharp tips and form quadrilateral or pentagon shaped NPs. After thermal annealing for 1-4 hours and washing with 5% nitric acid, their edges and corners become rounded, as shown in Fig. 6(c-1), which show quadrilateral NPs (in dashed squares) or pentagon (in dashed circles). The 3D AFM image, Figure 6(c-2), shows that these NPs have rounded edges and corners. Clearly, even after thermal annealing, they are still showing prism shapes with increased thickness from their edges to centers according to their 3D AFM images.

The work described above demonstrates that NSL, broadly defined to include AR NSL and some modified post treatment after deposition of the desired materials, is manifestly capable of creating far more than arrays of nanotriangles, nanodots as previously supposed. The progresses in NSL endow much potential in the size and shape controlled fabrication of nanoparticles and nanoarrays, which gives NSL a bright future since the ability of NSL to synthesize monodisperse, size- and shape- tunable nanoparticles can be exploited to precisely investigate the size- and shape- dependent physiochemical properties of nanooptics and nanoarrays.
