**5. Fabrication of hierarchically ordered nanowire arrays on substrates by combination of NSL and Porous anodic alumina (PAA)**

In some applications of nanomaterials, the NPs need to be arranged in some particular patterns, architectures or motifs with controlled interspacing, or conjugated with some other kinds of materials (e.g. polymers) (Chong, Zheng et al. 2006; Song , Zhang et al. 2011). The controlled arrangement and immobilization of Ag NPs on substrates will be very crucial to enable some fascinating and delicate applications, particularly in electronic circuit based electro-optical devices and long term functional composites for biological applications. Many methods have been explored for this purpose. Among them, template-assisted LIGA or structure controlled artificial fabrication methods (e.g. E-beam LIGA, NSL, PAA-LIGA) may be the most convenient techniques(Song , Zhang et al. 2011). In the NSL development, the suitability and powerful ability in the architecture and interspacing controlled fabrication of NPs and nanoarrays can be expanded extremely if the NSL can be combined with other template-assisted LIGA methods. Here we just show one example to fabricate hierarchically ordered nanowire arrays on substrates by the combination of NSL and porous anodic alumina (PAA) LIGA(Chong, Zheng et al. 2006).

Like NSL, porous anodic alumina (PAA) templates have attracted intense attention in nanodevice-oriented fabrication in recent years(Xu, Meng et al. 2009). As a welldeveloped template, PAA offers amazing simplicity and convenience for nanofabrication due to the capabilities of forming high-density, well-aligned, and hexagonally packed sub-100-nm pores, the ability to control the 3D pore structures by simply varying the anodization conditions, and the ease of selectively removing the template after fabrication(Chong, Zheng et al. 2006). As shown in Figure 12, one typical PAA-LIGA process includes(Lombardi, Cavallotti et al. 2007): (a) formation of a 300 nm thick PAA film on Al by a two step anodization process in 0.3M oxalic acid; (b) dissolution of unoxidized Al; (c) barrier layer etching in 5wt% phosphoric acid; (d) transfer of the PAA mask onto Au-coated Si followed by a thermal treatment to improve the adhesion of the films to the substrate; (e) Ag electrodeposition through the PAA pores; (f) PAA mask removal. By carefully controling the sizes and interpore spacing of the nanoholes, very uniform Ag nanorods with controlled interspacing can be fabricated by electroplating. The typical Ag nanorods prepared by this template assisted electrodeposition process can give a much uniform size and interparticle spacing distribution, with a standard size deviation less than 5% and a spacing deviation less than 7%.

Controlled Fabrication of Noble Metal Nanomaterials

1

2

3

via Nanosphere Lithography and Their Optical Properties 523

thickness of ~500 nm deposits onto the porous gold film. Prior to anodization, the aluminum film is subjected to an imprinting step, in which a free-standing PAA with a thickness of ~10 μm fabricated by the process in Figure 12 is used as a mold for imprinting. Finally, the imprinted aluminum film is anodized in 0.3*M* oxalic acid at 2 °C and the barrier layer at the bottom is removed in 5 wt % H3PO4 for 60 min. The gold nanowires can be deposited at −1.0 V versus standard calomel electrode (SCE) from a commercial bath (Orotemp 24, Technic) for varying amounts of time. Alumina templates can be removed in 1M KOH for some time (e.g.

Fig. 13. (A: 1-6) Schematic of method to create hierarchical nanowire arrays on substrates. (B) SEM images of hierarchical nanowire arrays on substrate:(a) Top view of the nanowire arrays with hexagonally organized microvoids over large areas; (b) High magnification SEM image of (a); (c) Side view of the cleaved sample from (a), the concaves caused by voids are clearly apparent; (d) Side view of Au/Ni/Au/Ni segmented nanowire arrays. The side view shows clear contrast; brighter segment is gold portion. Scale bars in (c) and (d): 500 nm. (Chong, M. A. S.; et al., Appl. Phys. Lett. 2006, 89, 233104. Figure 1 and Figure 3. Copyright

Figure 13B (a) shows the top view of one typical nanowire arrays with hexagonally organized microvoids (e.g. 500 nm in diameter) over large areas (Chong, Zheng et al. 2006). One of its high magnification images as shown in Figure 13B(b) clearly demonstrates the arranged patterns by individual gold nanowires. The nanowires with uniform size, replicated from the PAA, are hexagonally packed at the nanoscale. The hierarchical nanowire arrays standing on the substrate are further checked from a cleaved sample by side view SEM images (Figure 13B(c), suggesting the well aligned nanowires and the concave features from the nanoscale voids. In addition to fabricating pure gold nanowire arrays, this combinational template is also suitable for selectively electrodepositing other materials for functional device applications. In particular, the vertical structure/composition along the length of the nanowire is also tunable. For example, multilayer Au/Ni/Au/Ni nanowire arrays with in-plane hierarchical structure can be fabricated by the alternating deposition twice of nickel from a Watt's bath (300 g/L NiSO4·6H2O, 45 g/L each H3BO3 and NiCl2·6H2O) and of gold at −1.0 V versus SCE electrode. Figure 13B(d) shows the cross section of Au (bottom, brighter segment) /Ni (darker

(2006) from American Institute of Physics. Adapted with permission.)

segment)/Au/Ni (top) nanowire arrays.

10 min) to obtain hierarchically patterned free-standing nanowires or nanorods.

4

5

6

**(A) (B)** 

Progresses in PAA-LIGA have shown its abilities not only in the synthesis of the traditional high aspect ratio nanomaterials, such as nanorods (Pan, Zeng et al. 2000; Lombardi, Cavallotti et al. 2007; Xu, Meng et al. 2009)and nanowires (Hong, Bae et al. 2001; Xu, Zhang et al. 2005), but also some unique nanostructures, such as porous metallic nanorods by galvanic exchange reaction (Mohl, Kumar et al. 2010), Y-junction nanowires and multiply branched nanomaterials due to its flexibility in the pore structure control according to the principle 1 *Vs <sup>n</sup>* , where Vs is the anodizing voltage for stem pores and n is the number of branched pores from that stem(Meng, Jung et al. 2005; Xu, Meng et al. 2009).

Fig. 12. Fabrication scheme for Ag nanoparticle arrays: (a) formation of a PAA film on Al by a two step anodization process in 0.3Moxalic acid; (b) dissolution of unoxidized Al; (c) barrier layer etching in 5 wt.% phosphoric acid; (d) transfer of the PAA mask onto Aucoated Si followed by a thermal treatment to improve the adhesion of the films to the substrate; (e) Ag electrodeposition through the PAA pores; (f) PAA mask removal. (Lombardi I.; et al., Sensors and Actuators B: chemical 2007, 125, 353-356, Figure 1. Copyright (2007) Elsevier.)

The combination of NSL and PAA-LIGA has been further developed to create hierarchically ordered nanowire arrays, as schematically shown in Figure 13A(Chong, Zheng et al. 2006). A monolayer of self-assembled polystyrene nano or microspheres as masks is first used to deposit periodic porous gold films on silicon substrates (Figure 13A: i-ii). Next, PAA films are fabricated on top of the porous gold film/substrate (Figure 13A: iii). Nanowires are then selectively electrodeposited into the pores of the alumina using the porous gold film as a working electrode (Figure 13A: iv-vi). In detail, a drop of polystyrene sphere suspension (e.g. 1 μm in diameter, 10 wt % aqueous dispersion) is spin-coated onto a pretreated substrate (e.g. Si, glass or mica) to form close-packed microsphere monolayers. The size of the nano or microspheres can be tuned using O2 reactive ion etching (RIE) with a suitable O2 flow (e.g. 20 SCCM, SCCM denotes cubic centimeter per minute at STP) at a certain pressure (e.g. 15 mTorr) and a power density (e.g. 110 W) for 6–10 min. After RIE, isolated nano or microsphere monolayers with tunable spacing will be formed. Then, about 5 nm Ti (as an adhesion layer) and 40 nm gold films in turn deposit onto the substrate using the RIE reduced nano or microspheres as masks. After removal of the mask by sonicating in a solvent (e.g. toluene) for 3 min, a porous gold film will be formed on the substrate. After that, an aluminum film with a

Progresses in PAA-LIGA have shown its abilities not only in the synthesis of the traditional high aspect ratio nanomaterials, such as nanorods (Pan, Zeng et al. 2000; Lombardi, Cavallotti et al. 2007; Xu, Meng et al. 2009)and nanowires (Hong, Bae et al. 2001; Xu, Zhang et al. 2005), but also some unique nanostructures, such as porous metallic nanorods by galvanic exchange reaction (Mohl, Kumar et al. 2010), Y-junction nanowires and multiply branched nanomaterials due to its flexibility in the pore structure control according to the principle 1 *Vs <sup>n</sup>* , where Vs is the anodizing voltage for stem pores and n is the number of

Fig. 12. Fabrication scheme for Ag nanoparticle arrays: (a) formation of a PAA film on Al by a two step anodization process in 0.3Moxalic acid; (b) dissolution of unoxidized Al; (c) barrier layer etching in 5 wt.% phosphoric acid; (d) transfer of the PAA mask onto Aucoated Si followed by a thermal treatment to improve the adhesion of the films to the substrate; (e) Ag electrodeposition through the PAA pores; (f) PAA mask removal. (Lombardi I.; et al., Sensors and Actuators B: chemical 2007, 125, 353-356, Figure 1.

The combination of NSL and PAA-LIGA has been further developed to create hierarchically ordered nanowire arrays, as schematically shown in Figure 13A(Chong, Zheng et al. 2006). A monolayer of self-assembled polystyrene nano or microspheres as masks is first used to deposit periodic porous gold films on silicon substrates (Figure 13A: i-ii). Next, PAA films are fabricated on top of the porous gold film/substrate (Figure 13A: iii). Nanowires are then selectively electrodeposited into the pores of the alumina using the porous gold film as a working electrode (Figure 13A: iv-vi). In detail, a drop of polystyrene sphere suspension (e.g. 1 μm in diameter, 10 wt % aqueous dispersion) is spin-coated onto a pretreated substrate (e.g. Si, glass or mica) to form close-packed microsphere monolayers. The size of the nano or microspheres can be tuned using O2 reactive ion etching (RIE) with a suitable O2 flow (e.g. 20 SCCM, SCCM denotes cubic centimeter per minute at STP) at a certain pressure (e.g. 15 mTorr) and a power density (e.g. 110 W) for 6–10 min. After RIE, isolated nano or microsphere monolayers with tunable spacing will be formed. Then, about 5 nm Ti (as an adhesion layer) and 40 nm gold films in turn deposit onto the substrate using the RIE reduced nano or microspheres as masks. After removal of the mask by sonicating in a solvent (e.g. toluene) for 3 min, a porous gold film will be formed on the substrate. After that, an aluminum film with a

Copyright (2007) Elsevier.)

branched pores from that stem(Meng, Jung et al. 2005; Xu, Meng et al. 2009).

thickness of ~500 nm deposits onto the porous gold film. Prior to anodization, the aluminum film is subjected to an imprinting step, in which a free-standing PAA with a thickness of ~10 μm fabricated by the process in Figure 12 is used as a mold for imprinting. Finally, the imprinted aluminum film is anodized in 0.3*M* oxalic acid at 2 °C and the barrier layer at the bottom is removed in 5 wt % H3PO4 for 60 min. The gold nanowires can be deposited at −1.0 V versus standard calomel electrode (SCE) from a commercial bath (Orotemp 24, Technic) for varying amounts of time. Alumina templates can be removed in 1M KOH for some time (e.g. 10 min) to obtain hierarchically patterned free-standing nanowires or nanorods.

Fig. 13. (A: 1-6) Schematic of method to create hierarchical nanowire arrays on substrates. (B) SEM images of hierarchical nanowire arrays on substrate:(a) Top view of the nanowire arrays with hexagonally organized microvoids over large areas; (b) High magnification SEM image of (a); (c) Side view of the cleaved sample from (a), the concaves caused by voids are clearly apparent; (d) Side view of Au/Ni/Au/Ni segmented nanowire arrays. The side view shows clear contrast; brighter segment is gold portion. Scale bars in (c) and (d): 500 nm. (Chong, M. A. S.; et al., Appl. Phys. Lett. 2006, 89, 233104. Figure 1 and Figure 3. Copyright (2006) from American Institute of Physics. Adapted with permission.)

Figure 13B (a) shows the top view of one typical nanowire arrays with hexagonally organized microvoids (e.g. 500 nm in diameter) over large areas (Chong, Zheng et al. 2006). One of its high magnification images as shown in Figure 13B(b) clearly demonstrates the arranged patterns by individual gold nanowires. The nanowires with uniform size, replicated from the PAA, are hexagonally packed at the nanoscale. The hierarchical nanowire arrays standing on the substrate are further checked from a cleaved sample by side view SEM images (Figure 13B(c), suggesting the well aligned nanowires and the concave features from the nanoscale voids. In addition to fabricating pure gold nanowire arrays, this combinational template is also suitable for selectively electrodepositing other materials for functional device applications. In particular, the vertical structure/composition along the length of the nanowire is also tunable. For example, multilayer Au/Ni/Au/Ni nanowire arrays with in-plane hierarchical structure can be fabricated by the alternating deposition twice of nickel from a Watt's bath (300 g/L NiSO4·6H2O, 45 g/L each H3BO3 and NiCl2·6H2O) and of gold at −1.0 V versus SCE electrode. Figure 13B(d) shows the cross section of Au (bottom, brighter segment) /Ni (darker segment)/Au/Ni (top) nanowire arrays.

Controlled Fabrication of Noble Metal Nanomaterials

CN200910085973.9)

triangular nanoprisms to the substrate.

via Nanosphere Lithography and Their Optical Properties 525

**Releasing**

HOOCCOOH OH OH HOOCHOHOHOOH HO HOHO HO

HOCOOHCOOH HO HOCOOHHOOHOHOHCOO HO HO

HOOCCOOH OH OH HOOCHOHOHOOH HO HOHO HO

HOCOOHCOOH HO HOCOOHHOOHOHOHCOO HO HO

COOH OH OH OH

COOH OH OH OH

HOOCCOOH OH OH HOOCHOHOHOOH HO HOHO HO

HOCOOHCOOH HO HOCOOHHOOHOHOHCOO HO HO HOOCCOOH OH OH HOOCHOHOHOOH HO HOHO HO

HOCOOHCOOH HO HOCOOHHOOHOHOHCOO HO HO

COOH OH OH OH

HOOCCOOH OH OH HOOCHOHOHOOH HO HOHO HO

HOCOOHCOOH HO HOCOOHHOOHOHOHCOO HO HO HOOCCOOH OH OH HOOCHOHOHOOH HO HOHO HO

HOCOOHCOOH HO HOCOOHHOOHOHOHCOO HO HO

COOH OH OH OH

COOH OH OH OH

COOH OH OH OH

Fig. 14. (A) the surface-confined triangular Ag NPs are functionalized by chemicals with thiol groups and (B) can be further released into water or other solvents forming solution-

Comparing with those Ag NPs via the traditional dislodging method(Amanda, Zhao et al. 2005; Song and Elsayed-Ali 2010), the shape integrity of the heat-treated NPs after releasing them into water can be retained perfectly. Figure 15(a) shows TEM image of the Ag NPs after thermal annealing without pre-sonication. Most of those Ag NPs show triangular shapes with rounded tips (doted circles in Fig. 15(a)) and some with snipped tips (dashed circles in Fig. 15(a)). The inset is a magnified image of these NPs, clearly showing a triangular shape with rounded tips. The histogram for these Ag NPs (Fig. 15(b)) gives a mean size of 39.6 ± 4.9 nm with much narrower size distribution of STDEV % = 12.4 % than those obtained from surface-confined Ag NPs without any post annealing (STDEV % = 41.7 %)(Song and Elsayed-Ali 2010). Fig. 15(c) is a TEM image for Ag NPs that were thermally annealed after removing two tips by sonication, whose histogram gives a mean size of about 33.9 ± 6.8 nm (Fig. 15(d)), less than that for those triangular shaped NPs with rounded tips after post-annealing. Most of these NPs show quadrilateral shapes (dashed circles) or pentagon shapes as shown more clearly in the inset of Fig. 15(c). These NPs have a similar shape as those observed by AFM images in Figs. 6(c-1) and (c-2). From the TEM images in Fig. 15(c), some of the NPs give less contrast in their central parts (NPs labeled by dashed circles). We believe that the lighter centers in these NPs are from a thinner center resulting from adhesion of the center of these NPs to the glass substrate during annealing. AFM observation of the glass substrate after removal of the NPs show debris forming hexagonal shaped arrangements. This observation is consistent with adhesion of the central part of the

Variations in the shape, surface modification and surrounding environment of these NPs give significant shifts in their UV-vis absorption spectra for the surface confined NPs before and after tip rounding, after surface modification, and after dislodging into water, as shown in Figure 16. The absorption spectrum for the surface-confined Ag NPs fabricated by NSL without any tip rounding and surface modification has two distinct peaks at 476 nm and 672 nm (Figure 16a). The absorption peak at 476 nm is primarily from the higher-order mode surface plasmon resonance (e.g., quadrupole) of the NPs, and the peak at 672 nm is mainly from the dipole resonance of the NPs. We note that the higher-order resonance peak has almost the same intensity as that for the dipole resonance for all types of NPs, although the higher order modes are expected to be much weaker than the dipole resonance. Since the substrate is continuously covered by a hexagonally arranged array of Ag NPs with tip-tip distance less than 100 nm, we postulate that the particle-particle coupling will contribute to the LSPR spectrum. This particle-particle interaction effect could be responsible for the observed spectrum. When the tips in the Ag triangular nanoprisms are rounded,the tip-tip

phased nanocolloids. (Adapted from reference Y. Song, China Patent, Appl. No.

**(A) (B)**

### **6. Solution phased nanomaterials by the releasing of nanoparticles fabricated by NSL**

Recent progress in nanosphere lithography (NSL) has shown that this method provides a good template for shape-controlled fabrication of surface confined NPs(Zhang, Whitney et al. 2006; Song , Zhang et al. 2011), which also allows for flexible functionalization of these NPs on the clean surface (as cartooned in Figure 14A) using the routine functionalization process from equation 1 to 3. After the surface functionalization of surface confined nanoparticles fabricated by NSL, they can be dislodged into solution phase (as schemed in Figure 14B). This dislodging process provides a useful alternative to synthesis uniform solution phase NPs besides the wet chemical process. Van Duyne *et al.* have developed this process and used it to fabricate solution phase NPs in ethanol(Amanda, Zhao et al. 2005). However, their results indicated that most of the NPs in the solution have nonuniform surface morphologies with truncated tips in addition to the presence of debris and some of the NPs attached together on the glass substrate surface causing the agglomeration of the released NPs. In addition, aqueous phase NPs are expected to be more biocompatible than those in ethanol. Therefore, technology development to obtain aqueous-stable nanocolloids via surface modification and releasing of the surface-confined NPs fabricated by NSL into water solution are much desired.

Our group recently developed a modified NSL process to fabricate Ag NPs with controlled shapes on glass substrates and with the ability to release them into the aqueous solution without any obvious agglomeration(Song and Elsayed-Ali 2010). Three modifications of the standard procedure of nanosphere lithography were made in order to obtain stable NPs with different shapes. The modification to the process were 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 were annealed at 100-300 ºC for 2-5 hours then 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; (3) The glass substrates were immersed into 5-10wt.% HF and HCl acid mixture (HF: HCl = 1:1) for 30-60 s or 10% NaOH solution for 60-120 s to etch part of the glass substrate under the Ag NPs, and then the substrates were washed with sufficient amounts of nanopure water. Finally, the glass substrates with the Ag NPs were dried by inert gas flow and kept in desiccators. The surfaces of the Ag NPs can be modified by chemicals containing thiol groups (such as 1-OT, MUA, 6-MCH and Tiopronin (TP)) forming strong sulfur-silver covalent bonds. We used 1-OT and MUA as functional reagents. The functional solution was prepared by dissolving 0.049 g 1-OT and 0.073 g MUA into 100 mL pure ethanol in a 100 mL volume certificated flask to form 2mM 5:1 1-OT /11- MUA solution. The Ag NPs were once again cleaned using 5 % nitric acid and then immersed into the 2 mM 5:1 1-OT /11-MUA solution and left overnight. The releasing aqueous solution contains 5 V% of 2mM 5:1 1-OT /11-MUA in nanopure water. The glass substrates with surface modified Ag NPs were removed from the functional solution and immersed into the releasing solution. The NPs was then sonicated for 30-120 s to remove them from the substrates into the releasing solution. For a 2-4 mL releasing solution, 4-8 glass substrates were used in order to reach a NP concentration suitable for optical property measurements.

Recent progress in nanosphere lithography (NSL) has shown that this method provides a good template for shape-controlled fabrication of surface confined NPs(Zhang, Whitney et al. 2006; Song , Zhang et al. 2011), which also allows for flexible functionalization of these NPs on the clean surface (as cartooned in Figure 14A) using the routine functionalization process from equation 1 to 3. After the surface functionalization of surface confined nanoparticles fabricated by NSL, they can be dislodged into solution phase (as schemed in Figure 14B). This dislodging process provides a useful alternative to synthesis uniform solution phase NPs besides the wet chemical process. Van Duyne *et al.* have developed this process and used it to fabricate solution phase NPs in ethanol(Amanda, Zhao et al. 2005). However, their results indicated that most of the NPs in the solution have nonuniform surface morphologies with truncated tips in addition to the presence of debris and some of the NPs attached together on the glass substrate surface causing the agglomeration of the released NPs. In addition, aqueous phase NPs are expected to be more biocompatible than those in ethanol. Therefore, technology development to obtain aqueous-stable nanocolloids via surface modification and releasing of the surface-confined NPs fabricated by NSL into

Our group recently developed a modified NSL process to fabricate Ag NPs with controlled shapes on glass substrates and with the ability to release them into the aqueous solution without any obvious agglomeration(Song and Elsayed-Ali 2010). Three modifications of the standard procedure of nanosphere lithography were made in order to obtain stable NPs with different shapes. The modification to the process were 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 were annealed at 100-300 ºC for 2-5 hours then 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; (3) The glass substrates were immersed into 5-10wt.% HF and HCl acid mixture (HF: HCl = 1:1) for 30-60 s or 10% NaOH solution for 60-120 s to etch part of the glass substrate under the Ag NPs, and then the substrates were washed with sufficient amounts of nanopure water. Finally, the glass substrates with the Ag NPs were dried by inert gas flow and kept in desiccators. The surfaces of the Ag NPs can be modified by chemicals containing thiol groups (such as 1-OT, MUA, 6-MCH and Tiopronin (TP)) forming strong sulfur-silver covalent bonds. We used 1-OT and MUA as functional reagents. The functional solution was prepared by dissolving 0.049 g 1-OT and 0.073 g MUA into 100 mL pure ethanol in a 100 mL volume certificated flask to form 2mM 5:1 1-OT /11- MUA solution. The Ag NPs were once again cleaned using 5 % nitric acid and then immersed into the 2 mM 5:1 1-OT /11-MUA solution and left overnight. The releasing aqueous solution contains 5 V% of 2mM 5:1 1-OT /11-MUA in nanopure water. The glass substrates with surface modified Ag NPs were removed from the functional solution and immersed into the releasing solution. The NPs was then sonicated for 30-120 s to remove them from the substrates into the releasing solution. For a 2-4 mL releasing solution, 4-8 glass substrates were used in order to reach a NP concentration suitable for optical property

**6. Solution phased nanomaterials by the releasing of nanoparticles** 

**fabricated by NSL** 

water solution are much desired.

measurements.

Fig. 14. (A) the surface-confined triangular Ag NPs are functionalized by chemicals with thiol groups and (B) can be further released into water or other solvents forming solutionphased nanocolloids. (Adapted from reference Y. Song, China Patent, Appl. No. CN200910085973.9)

Comparing with those Ag NPs via the traditional dislodging method(Amanda, Zhao et al. 2005; Song and Elsayed-Ali 2010), the shape integrity of the heat-treated NPs after releasing them into water can be retained perfectly. Figure 15(a) shows TEM image of the Ag NPs after thermal annealing without pre-sonication. Most of those Ag NPs show triangular shapes with rounded tips (doted circles in Fig. 15(a)) and some with snipped tips (dashed circles in Fig. 15(a)). The inset is a magnified image of these NPs, clearly showing a triangular shape with rounded tips. The histogram for these Ag NPs (Fig. 15(b)) gives a mean size of 39.6 ± 4.9 nm with much narrower size distribution of STDEV % = 12.4 % than those obtained from surface-confined Ag NPs without any post annealing (STDEV % = 41.7 %)(Song and Elsayed-Ali 2010). Fig. 15(c) is a TEM image for Ag NPs that were thermally annealed after removing two tips by sonication, whose histogram gives a mean size of about 33.9 ± 6.8 nm (Fig. 15(d)), less than that for those triangular shaped NPs with rounded tips after post-annealing. Most of these NPs show quadrilateral shapes (dashed circles) or pentagon shapes as shown more clearly in the inset of Fig. 15(c). These NPs have a similar shape as those observed by AFM images in Figs. 6(c-1) and (c-2). From the TEM images in Fig. 15(c), some of the NPs give less contrast in their central parts (NPs labeled by dashed circles). We believe that the lighter centers in these NPs are from a thinner center resulting from adhesion of the center of these NPs to the glass substrate during annealing. AFM observation of the glass substrate after removal of the NPs show debris forming hexagonal shaped arrangements. This observation is consistent with adhesion of the central part of the triangular nanoprisms to the substrate.

Variations in the shape, surface modification and surrounding environment of these NPs give significant shifts in their UV-vis absorption spectra for the surface confined NPs before and after tip rounding, after surface modification, and after dislodging into water, as shown in Figure 16. The absorption spectrum for the surface-confined Ag NPs fabricated by NSL without any tip rounding and surface modification has two distinct peaks at 476 nm and 672 nm (Figure 16a). The absorption peak at 476 nm is primarily from the higher-order mode surface plasmon resonance (e.g., quadrupole) of the NPs, and the peak at 672 nm is mainly from the dipole resonance of the NPs. We note that the higher-order resonance peak has almost the same intensity as that for the dipole resonance for all types of NPs, although the higher order modes are expected to be much weaker than the dipole resonance. Since the substrate is continuously covered by a hexagonally arranged array of Ag NPs with tip-tip distance less than 100 nm, we postulate that the particle-particle coupling will contribute to the LSPR spectrum. This particle-particle interaction effect could be responsible for the observed spectrum. When the tips in the Ag triangular nanoprisms are rounded,the tip-tip

Controlled Fabrication of Noble Metal Nanomaterials

significantly by the modified NSL and releasing processes.

via Nanosphere Lithography and Their Optical Properties 527

aqueous phased Ag NPs produced by the modified NSL method show a main peak and another peak with very low intensity attributed mainly to small debris produced during the dislodging process. The noticeable reduction in the intensity of the short wavelength peak for the modified NSL method compared to the routine method is due to the significant reduction in Ag debris. TEM images show that the uniformity of Ag NPs can be improved

Fig. 15. TEM images of the aqueous phase Ag NPs after surface modification by thiol compounds and dislodging from the glass substrate. (a) Triangle Ag NPs with rounded tips. Dashed circles: Ag NPs with rounded tips; dotted circle: Ag NPs with slightly rounded tips. (b) Histogram of triangular shaped Ag NPs with rounded tips based on 45 NPs giving a mean size of 39.6±4.9 nm. (c) Quadrilateral and pentagon shaped Ag NPs. Dashed circles: some typical Ag NPs with quadrilateral shapes. (d) Histogram of quadrilateral and pentagon shaped Ag NPs based on 45 NPs giving a mean size of 33.9±6.8 nm. (Reprinted from Song et al., Appl. Surf. Sci. 2010 256, (20), 5961, Figure 4. Copyright (2010) Elsevier.)

LSPR coupling effects are alleviated, as indicated by the disappearance of the peak at 672 and the red-shift of the peak at 476 nm to 504 nm representing the higher-order surface plasmon resonance mode (Figure 16b). The absorption spectrum for the surface modified Ag NPs (Fig. 16c) shows a slight blue shift at the peak of 476 nm (to 470 nm) and a significant blue shift at 672 nm (to 626 nm) with reduced intensities. This spectrum was expected to give a red shift due to the increased dielectric constant from the adsorbed thiol compounds(Amanda, Zhao et al. 2005). We attribute this blue shift to shape variation (e.g., increased height, smooth surface topography) during surface modification by immersion that was similar to solvent annealing which results in blue-shift of LSPR since any solvent annealing has not been done on our NPs(Jensen, Duval Malinsky et al. 2000; Malinsky, Kelly et al. 2001). These variations have been observed by the slightly reduced NP size and rounded shapes observed in the TEM image of Figure 15 when compared with the AFM image of Figure 1 and Figure 6. In addition, when the Ag NPs are covered by thiol groups, the surface free electron density may be reduced, leading to weaker surface plasmon resonance in single NPs and surface plasmon resonance coupling among nanoparticle arrays(Kelly, Coronado et al. 2003). This will result in a blue shift of the LSPR peak and a reduced LSPR intensity.

The UV-vis absorption spectrum of the Ag NPs after release in water, shown in Fig. 16d, was compared to other surface confined NPs. The aqueous Ag NPs give a main peak at 532 nm and a very weak peak at 352 nm. The main peak at 532 nm appears to be from LSPR by the triangular nanoprisms with rounded tips and is blue shifted from that obtained for NPs with a LSPR peak at 605 nm fabricated by the routine NSL and released from the surface. This is attributed to the reduced size and rounded tips. The peak at 352 nm in Fig. 16d becomes much weaker and narrower than that for the aqueous Ag NPs released from the surface confined Ag NPs as fabricated by routine NSL, obviously due to the shape variation of NPs and almost no small spherical shaped debris observed in the aqueous Ag NPs released from the surface-confined Ag NPs fabricated by the modified NSL (Figure 16a). By comparing the TEM images for the two kinds of Ag NPs, it can be deduced that the peak at 352 nm in Fig. 16d is mainly from the out-of-plane quadrupole resonance of Ag nanoprisms with rounded tips according to the previous investigation(Jin, Cao et al. 2001; Amanda, Zhao et al. 2005; Zhang, Li et al. 2005). The peak intensity ratio between the main peak at 532 nm and the weak peak at 352 nm for these NPs is ~11.5:1 (after subtracting the background), which is much higher than that for the NPs obtained by the routine NSL and releasing process (1:3.6)(Song and Elsayed-Ali 2010). Clearly, the number of the small debris caused by the sonication is greatly reduced using the modified NSL and releasing process. The modified NSL process favors the formation of uniform Ag NPs with rounded tips with significant reduction in Ag debris, as shown in Fig. 15. In addition, 1-OT and 11-MUA can be substituted by the combination of 1-BT and TP, or MCH and MUA if more water-soluble NPs are desired.

Clearly, Ag NPs with controlled shapes and reduced defect density can be fabricated by a modified NSL process. Upon dislodging these NPs into a solution, they retain their shapes significantly better than NPs produced by routine NSL. Thus, aqueous phase Ag NPs with relatively uniform size and shape distribution can be fabricated. The UV-vis absorption spectra for surface confined NPs show two distinct absorption peaks (Figure 16a), comparing with those with rounded tips (Figure 16b). After surface modification, the central wavelengths of the two absorption peaks blue shifted and showed reduced intensities. The

LSPR coupling effects are alleviated, as indicated by the disappearance of the peak at 672 and the red-shift of the peak at 476 nm to 504 nm representing the higher-order surface plasmon resonance mode (Figure 16b). The absorption spectrum for the surface modified Ag NPs (Fig. 16c) shows a slight blue shift at the peak of 476 nm (to 470 nm) and a significant blue shift at 672 nm (to 626 nm) with reduced intensities. This spectrum was expected to give a red shift due to the increased dielectric constant from the adsorbed thiol compounds(Amanda, Zhao et al. 2005). We attribute this blue shift to shape variation (e.g., increased height, smooth surface topography) during surface modification by immersion that was similar to solvent annealing which results in blue-shift of LSPR since any solvent annealing has not been done on our NPs(Jensen, Duval Malinsky et al. 2000; Malinsky, Kelly et al. 2001). These variations have been observed by the slightly reduced NP size and rounded shapes observed in the TEM image of Figure 15 when compared with the AFM image of Figure 1 and Figure 6. In addition, when the Ag NPs are covered by thiol groups, the surface free electron density may be reduced, leading to weaker surface plasmon resonance in single NPs and surface plasmon resonance coupling among nanoparticle arrays(Kelly, Coronado et al. 2003). This will result in a blue shift of the LSPR peak and a

The UV-vis absorption spectrum of the Ag NPs after release in water, shown in Fig. 16d, was compared to other surface confined NPs. The aqueous Ag NPs give a main peak at 532 nm and a very weak peak at 352 nm. The main peak at 532 nm appears to be from LSPR by the triangular nanoprisms with rounded tips and is blue shifted from that obtained for NPs with a LSPR peak at 605 nm fabricated by the routine NSL and released from the surface. This is attributed to the reduced size and rounded tips. The peak at 352 nm in Fig. 16d becomes much weaker and narrower than that for the aqueous Ag NPs released from the surface confined Ag NPs as fabricated by routine NSL, obviously due to the shape variation of NPs and almost no small spherical shaped debris observed in the aqueous Ag NPs released from the surface-confined Ag NPs fabricated by the modified NSL (Figure 16a). By comparing the TEM images for the two kinds of Ag NPs, it can be deduced that the peak at 352 nm in Fig. 16d is mainly from the out-of-plane quadrupole resonance of Ag nanoprisms with rounded tips according to the previous investigation(Jin, Cao et al. 2001; Amanda, Zhao et al. 2005; Zhang, Li et al. 2005). The peak intensity ratio between the main peak at 532 nm and the weak peak at 352 nm for these NPs is ~11.5:1 (after subtracting the background), which is much higher than that for the NPs obtained by the routine NSL and releasing process (1:3.6)(Song and Elsayed-Ali 2010). Clearly, the number of the small debris caused by the sonication is greatly reduced using the modified NSL and releasing process. The modified NSL process favors the formation of uniform Ag NPs with rounded tips with significant reduction in Ag debris, as shown in Fig. 15. In addition, 1-OT and 11-MUA can be substituted by the combination of 1-BT and TP, or MCH and MUA if more water-soluble

Clearly, Ag NPs with controlled shapes and reduced defect density can be fabricated by a modified NSL process. Upon dislodging these NPs into a solution, they retain their shapes significantly better than NPs produced by routine NSL. Thus, aqueous phase Ag NPs with relatively uniform size and shape distribution can be fabricated. The UV-vis absorption spectra for surface confined NPs show two distinct absorption peaks (Figure 16a), comparing with those with rounded tips (Figure 16b). After surface modification, the central wavelengths of the two absorption peaks blue shifted and showed reduced intensities. The

reduced LSPR intensity.

NPs are desired.

aqueous phased Ag NPs produced by the modified NSL method show a main peak and another peak with very low intensity attributed mainly to small debris produced during the dislodging process. The noticeable reduction in the intensity of the short wavelength peak for the modified NSL method compared to the routine method is due to the significant reduction in Ag debris. TEM images show that the uniformity of Ag NPs can be improved significantly by the modified NSL and releasing processes.

Fig. 15. TEM images of the aqueous phase Ag NPs after surface modification by thiol compounds and dislodging from the glass substrate. (a) Triangle Ag NPs with rounded tips. Dashed circles: Ag NPs with rounded tips; dotted circle: Ag NPs with slightly rounded tips. (b) Histogram of triangular shaped Ag NPs with rounded tips based on 45 NPs giving a mean size of 39.6±4.9 nm. (c) Quadrilateral and pentagon shaped Ag NPs. Dashed circles: some typical Ag NPs with quadrilateral shapes. (d) Histogram of quadrilateral and pentagon shaped Ag NPs based on 45 NPs giving a mean size of 33.9±6.8 nm. (Reprinted from Song et al., Appl. Surf. Sci. 2010 256, (20), 5961, Figure 4. Copyright (2010) Elsevier.)

Controlled Fabrication of Noble Metal Nanomaterials

or nanorods will be realized conveniently.

**8. Acknowledgement** 

Rev. 9: 871.

510.

**9. References** 

via Nanosphere Lithography and Their Optical Properties 529

surfaces are modified by the traditional functionalization process. By combination of PAA-LIGA and NSL, the possibility for building hierarchically ordered multi-segment nanowires

Summarizing from the recent progresses and discussion on NSL presented in this chapter, four main researches thrust that includes several active and challenging topics may form the primary research focuses and directions in this particular field. One is the fabrication technique development for the formation of monolayer of nanospheres with uniform area as large as several centimeter squares, which founds the basis of NSL. Another is the convenient and practical process in the releasing of these surface confined nanomaterials into solvent with perfectly retained 3D morphologies, which is still challenging but a desired alternative to obtain the uniform nanomateirals besides the well-developed wet chemical process. The third is the advanced incorporation of NSL with other fabrication techniques besides LIGA processes for the building more complex 3D hierarchically ordered nanostructures, which will definitely make a breakthrough in the nanoscale device and assemble development. The fourth may be the fabrication of tunable hetero-structurecomposition nanocomposites, such as sandwich discs or multi-layer nanostructures, which will produce hetero-nanostructures with multi-functions (e.g. magntic, optical, electronic, etc). Consequently, outcomes of these challenging researches will result in the discovery of many exciting and versatile techniques for nanomaterials fabrication, and theoretical breakthrough in their novel physicochemical properties and for advanced applications.

The author appreciates the support from the basic research Vision Funds (YWF-11-03-Q-002)

Ahmadi, T. S.; Wang, Z. L.; et al. (1996). "Shape-Controlled Synthesis of Colloidal Platinum

Amanda, H. J.; Zhao, J.; et al. (2005). "Solution-Phase, Triangular Ag Nanotriangles Fabricated by Nanosphere Lithography." J. Phys. Chem. B 109(22): 11158-62. Brayner, R. (2008). "The toxiccological impact of nanoparticles." Nano Today 3(1-2): 48-55. Chong, M. A. S.; Zheng,Y. B.; et al. (2006). "Combinational template-assisted fabrication of

Daniel, M.-C. and Astruc, D. (2004). "Gold Nanoparticles: Assembly, Supramolecular

Dhar, P.; Cao, Y.; et al. (2007). "Autonomously Moving Local Nanoprobes in Heterogeneous

Edwards, H. W. and Petersen, R. P. (1936 ). "Reflectivity of evaporated silver films." Phys.

Erhardt, D. (2003). Materials conservation: Not-so-new technology Nature Materials 2: 509-

Catalysis, and Nanotechnology." Chem. Rev. 104: 293-346.

Magnetic Fields." J. Phys. Chem. C 111: 3607-3613.

hierarchically ordered nanowires arrays on substrates for device applications."

Chemistry, Quantum-Size-Related Properties, and Applications toward Biology,

and, Chinese Scholarship Council (File No. 2010307428) NSFC (Grant No. 50971010).

Nanoparticles." Science 272: 1924-1927.

Appl. Phys. Lett. 89: 233104-1-3.

Fig. 16. UV-vis optical absorption of Ag NPs. (a) Surface confined Ag NPs before tip rounding and surface modification. (b) Surface confined Ag NPs after tip rounding (c) Surface confined Ag NPs with rounded tips after surface modification with thiol. (d) Aqueous phase Ag NPs after releasing the surface confined NPs into water. (Adapted from Song et al., Appl. Surf. Sci. 2010 256, (20), 5961, Figure 5. Copyright (2010) Elsevier.)
