llustration of subsurface infrared nanoimaging (nanoGUNE).
Credit: Elhuyar Fundazioa
Researchers
from the Nanooptics Group at CIC nanoGUNE (San Sebastian) demonstrate that
nanoscale infrared imaging—which is established as a surface-sensitive
technique—can be employed for chemical nanoidentification of materials that are
located up to 100 nm below a surface. The results further show that the
infrared signatures of thin surface layers differ from that of subsurface
layers of the same material, which can be exploited to distinguish the two
cases. The findings, recently published in Nature Communications, push the
technique one important step further to quantitative chemometrics at the
nanoscale in three dimensions.
Optical
spectroscopy with infrared light, such as Fourier transform infrared (FTIR)
spectroscopy, allows for chemical identification of organic and inorganic
materials. The smallest objects which can be distinguished with conventional
FTIR microscopes have sizes on the micrometer-scale. Scientists at CIC nanoGUNE
(San Sebastian), however, employed nano-FTIR to resolve objects, which can be
as small as a few nanometres.
In
nano-FTIR (which is based on near-field optical microscopy), infrared light is
scattered at a sharp metallized tip of a scanning-probe microscope. The tip is
scanned across the surface of a sample of interest and the spectra of scattered
light are recorded using Fourier transform detection principles. Recording of
the tip-scattered light yields the sample's infrared spectral properties and
thus the chemical composition of an area located directly below the tip apex.
Because the tip is scanned across the sample surface, nano-FTIR is typically
considered to be a surface-characterization technique.
Importantly
though, the infrared light that is nano-focussed by the tip does not only probe
a nanometric area below the tip, but in fact probes a nanometric volume below
the tip. Now the researchers at CIC nanoGUNE showed that spectral signatures of
materials located below the sample surface can be detected and chemically
identified up to a depth of 100 nm. Furthermore, the researchers showed that
nano-FTIR signals from thin surface layers differ from that of subsurface
layers of the same material, which can be exploited for determination of the
materials distribution within the sample. Remarkably, surface layers and
subsurface layers can be distinguished directly from experimental data without
involving time-consuming modeling. The findings have recently been published in
Nature Communications.