Controllable Nanofabrication of Aggregate-like Nanoparticle Substrates and Evaluation for Surface-Enhanced Raman Spectroscopy

Abstract

The development of new and better substrates is a major focus of research aimed at improving the analytical capabilities of surface-enhanced Raman spectroscopy (SERS). Perhaps the most common type of SERS substrate, one consistently exhibiting large enhancements, is simple colloidal gold or silver nanoparticles in the 10-150 nm size range. The colloidal systems that are used most for ultrasensitive detection are generally aggregated clusters that possess "hot spot(s)" within some of the aggregates. A significant limitation of these synthetic substrates is that the "hot" aggregates are extremely difficult to create consistently or predict. Electron beam lithography (EBL) along with combinatorial spectral mapping can be used to overcome this limitation. Our previous work, and that of other researchers, invokes the special capabilities of EBL to design and fabricate periodic, highly ordered nanoparticle arrays for SERS. Building on this work, EBL, in conjunction with ancillary fabrication steps, can be used to create complex patterns that mimic random aggregates. These aggregates, unlike those created by colloidal deposition methods, can be uniquely reproduced within the resolution limits of EBL. In the work reported herein, we use a unique approach to create substrates containing a large number of randomly generated cells with different morphologies that are arrayed on silicon wafers. Instead of isolated metal nanoparticles, these structures resemble the aggregates of colloid. By spectral mapping, we investigate the SERS activity of the combinatorial arrays of cells using probe analytes. Two general categories of shapes are randomly designed in different sizes and densities into several hundred different 5 mum square cells. Following fabrication, it is shown that a SERS performance contrast of more than a factor of 44 is achieved among these cells and that the best performing cells can be cloned into uniformly high performing macropatterns of lithographically defined nanoaggregates (LDNAs). In this manner, extended LDNA surfaces with uniform 5 x 10(8) enhancement factors are created. Furthermore, the LDNAs can be further dissected and studied in an effort to increase the SERS enhancement per unit geometric substrate area.

Stamping plasmonic nanoarrays on SERS‐supporting platforms

“…EBL can pattern arbitrary features with a resolution of approximately 10 nm. [2,3] Silicon stamps were then treated with fluorinated silane prior to the metal deposition. The presence of a fluorinated silane interface between the relief pillar and the metallic film weakens the adhesion between the latter two and facilitates the transfer of metallic disks.…”

“…The detailed procedure is explained elsewhere. [1,3,25] Briefly, 300 nm thick positive resist, ZEP 520 A (Zeon Chemicals, KY, USA), was spin-coated onto the silicon wafer at 6000 rpm for 45 s, baked at 180 • C for 2 min, and then placed under vacuum in the EBL system. The resist was patterned by EBL (Jeol JBX-9300, 100 keV) and then developed in xylene for 30 s and rinsed with isopropanol.…”

“…[23,24] Nevertheless, controlled nanostructured arrays having larger field enhancement and reproducible SERS signals are usually fabricated via the top-down approach. [3] Electron beam lithography (EBL) is a burgeoning top-down technique for SERS research. However, the access of EBL to general users is limited by the exceptionally high cost of EBL instrumentation and its limitation to specialized materials.…”

Surface-Enhanced Raman Scattering as an Emerging Characterization and Detection Technique

“…A growing number of promising results on SERS active substrates prepared using lithographyderived processing have been reported more recently [18,[47][48][49][50][51][52][53][54][55][56][57][58].…”

“…Compared to these latter techniques, advantages of EBL include its wider availability and applicability to patterning both periodic [40] and aperiodic patterns [18,50] of arbitrary shapes, a particularly useful feature at the research stage. A relatively low throughput and high cost of EBL tools makes them less attractive for scaled-up fabrication of SERS substrates.…”