Abstract
Mid to far-infrared (terahertz) spectroscopy is a valuable tool for probing and characterizing macromolecular structures and motions of complex molecules, including low frequency vibrational and phonon modes in condensed phases. We describe here an improved and readily implemented method for performing terahertz spectroscopic measurements by using a nanoporous silicon substrate to capture and concentrate the substance to be analyzed. We compare the results to conventional sampling methods, including dissolution and crystallization on a flat silicon surface and dispersing crystallites in compressed polyethylene pellets, and show that the use of a transparent, nanoporous substrate provides both increased sensitivity and yields sharper spectral features than conventional solid-state sampling approaches. FTIR measurements are reported over the spectral range from 50–2000 cm−1 (1.5–60 THz), for salicylic acid, dicyanobenzene, glycine, and aspartame.
Highlights
Far infrared (IR) or terahertz spectroscopy is an important diagnostic tool that can provide insight into low frequency vibrational and phonon modes of complex molecules and crystals
We present a novel method for terahertz spectroscopy that uses nanoporous silicon as a host substrate for capturing and collecting the analyte
The porous silicon contains at most 1 mg of salicylic acid, compared to 7 mg in the pressed pellet of comparable diameter, we observe a significant increase in the spectral absorption feature peak intensities in the porous template
Summary
Far infrared (IR) or terahertz spectroscopy is an important diagnostic tool that can provide insight into low frequency vibrational and phonon modes of complex molecules and crystals. Waveguide-based spectroscopy methods have been demonstrated, in which a thin layer of analyte is precipitated onto one plate of a metallic waveguide or parallel plate transmission line [7, 10,11,12] This leads to a stronger absorption spectrum compared to what would be obtained by transmission through a single layer, and a sharpening of features in comparison to results obtained in a pressed pellet. One cubic centimeter of porous silicon can have as much internal surface area as a large tennis court Such a nanoporous matrix will allow a large quantity of surface-bound molecules to be distributed throughout an interaction volume that could be thicker than the wavelength - a tremendous advantage when conducting terahertz spectroscopy. We compare the absorption spectrum measured using a porous silicon substrate to conventional pressed-pellet and surface evaporation/crystallization methods for four different molecules: salicylic acid, dicyanobenzene, glycine, and aspartame. We find that in cases where the molecules can be efficiently dissolved and deposited inside of the pores, smaller quantities of analytes are used and the absorption spectrum can be both sharper and more intense (all by as much as an order of magnitude, as is found for the waveguide technique) than for the pressed-pellet or
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