The Rational Design and Lithographic Fabrication of Surface Enhanced Raman Spectroscopy Substrates

The analytical capabilities of surface-enhanced Raman spectroscopy (SERS) reside in the performance characteristics of the SERS-active substrate. Signal enhancement observed in SERS is attributable to the presence of noble metal nanostructures on substrate surfaces. The rational design and control of variables such as shape and size, and distribution, density, and spacing of these nanostructures can lead to substrates that have greater analytical sensitivity and yield more reproducible enhancement. Through systematic control of the morphology of our SERS substrates, we have created ordered periodic arrays as well as random aggregates of nanoscale particles using electron beam lithography (EBL). A unique aspect of these EBL-created substrates is that the morphology is known with great precision. Once fabricated, the arrays and/or aggregates are coated with a SERS-active noble metal through physical vapor deposition (PVD). Both the uniform and random lithographically produced nanopatterns are studied by surface enhanced Raman spectroscopy to examine the Stokes responses of various analytes, while scanning electron microscopy (SEM) is used to examine pattern surfaces post lithographic development and post noble metal deposition. In the case of the ordered structures, raw and normalized SERS data is seen to correlate with data from simple electrostatic calculations as well as the broad background continuum underlying each spectrum collected. Borrowing from the biological concepts of cloning and combinatorial chemistry, random morphology patterns are designed and spectrally mapped to locate “hot spots” within aggregates. Regardless of the type of substrate, ordered or random, by using EBL, the substrates can be reproducibly fabricated, yielding consistent analyte environments each time the substrate is created