**3.3 Nanorods and nanodisc arrays from nanoimprint lithography from self-assembly derived high-resolution masters**

The uniform templates obtained using reverse micelle approach is well-suited as masks for nanolithography to produce Si nanopillar arrays. These nanopillar arrays are highly interesting for exploitation as high-resolution molds for nanoimprint lithography (NIL) [44]. NIL is a convenient top-down patterning tool that allows replication of surface relief structures down to sub-10 nm feature sizes present in a mold into the polymer substrate [24, 92]. Replication using the NIL process is achieved by pressing the mold against a molten polymer film, followed by solidifying the polymer either by cooling below its Tg or by cross-linking, before removing the mold. Among the several top-down techniques known, NIL has particularly recognized as manufacturing compatible, scalable and low-cost solution for fabricating nanoscale templates of high resolution. The use of the silicon nanopillar arrays as NIL molds offers a distinct advantage of producing multiple copies of templates with identical pitch and width as the original pillars. Since these templates are used to produce metal arrays in the next step, it is possible to attain asymmetric metal features, with reproducibility in optical properties within and between samples. In a specific example, full wafers consisting of Si nanopillar arrays with a height of 120nm were fabricated as described in Section 3.1. They were subsequently diced into smaller pieces and used as NIL molds to replicate the pillar arrays into a thin

**125**

**Figure 9.**

*Nanoplasmonic Arrays with High Spatial Resolutions, Quality, and Throughput…*

film of PMMA (100 nm thick) coated on Si substrate. Prior to imprinting, the pillar arrays are functionalized with a perfluorosilane layer to enable anti-stiction property. The NIL process results in a nanoporous PMMA film with geometric characteristics that match with the Si pillar array mold (**Figure 9**). The pores were found to be ~120 nm deep, with a residual layer of ~10 nm thickness present beneath the pores. The residual layer was removed using a controlled O2 plasma exposure and treated with HF in order to expose the bare Si substrate beneath. The porous template with through-holes was subsequently employed to guide the growth of metal to achieve gold nanorods from the surface through an electroless deposition process. The electroless deposition of gold was performed by galvanic displacement reaction where the oxidation of silicon substrate by HF provides electrons to reduce the Au(III) ions to Au(0), in a process earlier shown by Aizawa et al. [93, 94]. Exposure of the nanoporous template to an electroless plating bath consisting of 0.9% HF and 2.3 mM of HAuCl4 for a duration of 1 min was sufficient to grow the nanorods within the pores. The selectivity of the process to the Si substrate ensured the absence of any non-specific metal deposition in unintended areas. The nanoporous PMMA template was then removed using an O2 plasma exposure. The Au nanorod arrays were found to be 80 nm in height with a pitch identical with that of the NIL mold used. Upon annealing the nanorod array at 200°C for 2 h, a transformation in morphology into nanodiscs was observed. Such transformation that was observed to occur significantly below the melting point of bulk gold metal is likely due to compacting of nanoparticulate and porous gold features obtained upon electroless deposition. The nanoparticulate nature of nanorods is evident from the SEM measurements. The nanodiscs exhibit an ellipsoidal shape, with a diameter of 55.2 (± 4) nm and height of 35.3(± 5.4) nm, as measured using SEM and AFM. TEM cross section of the nanodisc arrays shows that the discs were present within depressions on the surface with a depth of ~5 nm. The depression below the nanodisc is presumably formed by substrate etching, due to the presence of HF in the electroless chemical bath. The transformation of the rod to disc morphology was found necessary in order to ensure mechanical stability of the metal arrays when exposed to solvents. It was found that the nanorod arrays disintegrated when dipped in aqueous solutions,

*Process steps involved in production of (a, b) high-resolution molds for NIL produced from self-assembled polymeric templates, (c, d) followed by replication onto PMMA thin film to form nanopore arrays and (e, f) selected area growth of gold nanorods within the pores and their subsequent transformation into nanodiscs [44].*

*DOI: http://dx.doi.org/10.5772/intechopen.89064*

*Nanoplasmonic Arrays with High Spatial Resolutions, Quality, and Throughput… DOI: http://dx.doi.org/10.5772/intechopen.89064*

film of PMMA (100 nm thick) coated on Si substrate. Prior to imprinting, the pillar arrays are functionalized with a perfluorosilane layer to enable anti-stiction property. The NIL process results in a nanoporous PMMA film with geometric characteristics that match with the Si pillar array mold (**Figure 9**). The pores were found to be ~120 nm deep, with a residual layer of ~10 nm thickness present beneath the pores. The residual layer was removed using a controlled O2 plasma exposure and treated with HF in order to expose the bare Si substrate beneath. The porous template with through-holes was subsequently employed to guide the growth of metal to achieve gold nanorods from the surface through an electroless deposition process. The electroless deposition of gold was performed by galvanic displacement reaction where the oxidation of silicon substrate by HF provides electrons to reduce the Au(III) ions to Au(0), in a process earlier shown by Aizawa et al. [93, 94]. Exposure of the nanoporous template to an electroless plating bath consisting of 0.9% HF and 2.3 mM of HAuCl4 for a duration of 1 min was sufficient to grow the nanorods within the pores. The selectivity of the process to the Si substrate ensured the absence of any non-specific metal deposition in unintended areas. The nanoporous PMMA template was then removed using an O2 plasma exposure. The Au nanorod arrays were found to be 80 nm in height with a pitch identical with that of the NIL mold used. Upon annealing the nanorod array at 200°C for 2 h, a transformation in morphology into nanodiscs was observed. Such transformation that was observed to occur significantly below the melting point of bulk gold metal is likely due to compacting of nanoparticulate and porous gold features obtained upon electroless deposition. The nanoparticulate nature of nanorods is evident from the SEM measurements. The nanodiscs exhibit an ellipsoidal shape, with a diameter of 55.2 (± 4) nm and height of 35.3(± 5.4) nm, as measured using SEM and AFM. TEM cross section of the nanodisc arrays shows that the discs were present within depressions on the surface with a depth of ~5 nm. The depression below the nanodisc is presumably formed by substrate etching, due to the presence of HF in the electroless chemical bath. The transformation of the rod to disc morphology was found necessary in order to ensure mechanical stability of the metal arrays when exposed to solvents. It was found that the nanorod arrays disintegrated when dipped in aqueous solutions,

#### **Figure 9.**

*Nanoplasmonics*

**Figure 8.**

the plasmonic sensor.

*with low detection limits [43].*

a different PS-b-P2VP (114 kDa, PDI - 1.1, fPS ~0.5) system. The Si pillar arrays are converted to plasmonic nanoarrays bycoating with thin layer of Cr, followed by systematically increasing thicknesses of gold by electron beam evaporation. The nanopillars coated with 5 nm Cr/120 nm of Au were found to result in gold pillars with separations below 10 nm. The optical modeling of the nanopillars proved EM hot spots with increasing intensity and with increasing metal thickness, as expected. This was correlated well with SERS experiments of 1-naphthalene thiol, where the evolution in SERS intensity was found to saturate at 100 nm. SERS-based plasmonic assays of naphthalene thiol show analytical sensitivity down to 74 ppb, low standard deviations in SERS intensities for different concentrations, and feasibility for quantification with large dynamic range. The approach established clear control over structure, property, and function to optimize the final performance of

*(Top) Silicon nanopillar arrays with increasing thickness of gold (top, insets) show evolution in electromagnetic field profiles (bottom, left) and SERS spectra of 1-naphthalene thiol as function of metal pillar geometry* 

 *showing quantitative detection* 

**3.3 Nanorods and nanodisc arrays from nanoimprint lithography from** 

The uniform templates obtained using reverse micelle approach is well-suited as masks for nanolithography to produce Si nanopillar arrays. These nanopillar arrays are highly interesting for exploitation as high-resolution molds for nanoimprint lithography (NIL) [44]. NIL is a convenient top-down patterning tool that allows replication of surface relief structures down to sub-10 nm feature sizes present in a mold into the polymer substrate [24, 92]. Replication using the NIL process is achieved by pressing the mold against a molten polymer film, followed by solidifying the polymer either by cooling below its Tg or by cross-linking, before removing the mold. Among the several top-down techniques known, NIL has particularly recognized as manufacturing compatible, scalable and low-cost solution for fabricating nanoscale templates of high resolution. The use of the silicon nanopillar arrays as NIL molds offers a distinct advantage of producing multiple copies of templates with identical pitch and width as the original pillars. Since these templates are used to produce metal arrays in the next step, it is possible to attain asymmetric metal features, with reproducibility in optical properties within and between samples. In a specific example, full wafers consisting of Si nanopillar arrays with a height of 120nm were fabricated as described in Section 3.1. They were subsequently diced into smaller pieces and used as NIL molds to replicate the pillar arrays into a thin

**self-assembly derived high-resolution masters**

*(bottom, right) concentration dependence of intensity of the peak at 1371 cm<sup>−</sup><sup>1</sup>*

**124**

*Process steps involved in production of (a, b) high-resolution molds for NIL produced from self-assembled polymeric templates, (c, d) followed by replication onto PMMA thin film to form nanopore arrays and (e, f) selected area growth of gold nanorods within the pores and their subsequent transformation into nanodiscs [44].*

while the nanodiscs remained undisturbed. Since our study primarily intended application of these arrays within a SERS-based sensor in liquid media, the nanorod arrays could not be considered further. Despite this instability, the nanorods could still find a use for plasmonic sensors functioning in the gas phase, or alternatively, if their mechanical stability can be improved through deposition of an overcoating of a dielectric, for example, alumina as shown earlier in literature [95]. The nanodisc arrays were evaluated for SERS performance against three different molecules, namely, naphthalene thiol that represented a covalently bound analyte, 2,2′-bipyridyl that represented non-covalently bound analyte, and crystal violet which represented the possibility of resonance Raman effect. The SERS performance was benchmarked against commercially available Klarite substrates and was proved better for all three molecules. The approach demonstrated an inherently reproducible approach to the fabrication of plasmonic arrays, as several copies can be derived starting from the same nanopillars NIL stamp. The performance of these arrays nevertheless has scope for improvements by further EM enhancements by reducing feature separations. The inability to deliver low feature separations is a limitation of the approach, which can be overcome by post-processing of the porous templates obtained after NIL, for instance, via pore-widening approaches.
