The development of
spectroscopic approaches to study molecules
at interfaces is important as the molecular properties often differ
from those in the bulk. Implementation of surface-enhanced two-dimensional
infrared (SE 2DIR) spectroscopy using lithographically fabricated
plasmonic nanoarrays is demonstrated for nanometer thick films. The
sample, 4-azidobutyrate-N-hydroxysuccinimide ester
(azNHS), dispersed in polystyrene was deposited onto the nanoarray.
Raw enhancements in the SE 2DIR spectra exceeding 5 × 104 and 1.3 × 103 fold were achieved for the
CO and NN peaks, respectively. The field enhancement provided by the
nanoarray was sufficient to record cross-peaks in 1 nm thick samples
under dilute conditions for azNHS (∼0.1 M). Note that the cross-peaks
were recorded for vibrational modes frequency separated by ∼350
cm–1 with the enhancement factor of 4.1 × 104. The effective electric field enhancement factors, measured
for NN and CO modes via linear and two nonlinear IR techniques, have
similar sample-thickness dependences, which permit using linear spectroscopy
for enhancement evaluation. High-quality cross-peak waiting-time dependences
were recorded for samples as thin as 1 nm involving several IR reporters
demonstrating the applicability of an arsenal of 2DIR approaches,
including spectral diffusion, chemical exchange, and relaxation-assisted
2DIR, to interrogate samples in nanometer thick films. The study opens
new opportunities in analyzing structures and dynamics of molecules
at interfaces.
A novel dual-frequency two-dimensional infrared instrument is designed and built that permits three-pulse heterodyned echo measurements of any cross-peak within a spectral range from 800 to 4000 cm(-1) to be performed in a fully automated fashion. The superior sensitivity of the instrument is achieved by a combination of spectral interferometry, phase cycling, and closed-loop phase stabilization accurate to ~70 as. The anharmonicity of smaller than 10(-4) cm(-1) was recorded for strong carbonyl stretching modes using 800 laser shot accumulations. The novel design of the phase stabilization scheme permits tuning polarizations of the mid-infrared (m-IR) pulses, thus supporting measurements of the angles between vibrational transition dipoles. The automatic frequency tuning is achieved by implementing beam direction stabilization schemes for each m-IR beam, providing better than 50 μrad beam stability, and novel scheme for setting the phase-matching geometry for the m-IR beams at the sample. The errors in the cross-peak amplitudes associated with imperfect phase matching conditions and alignment are found to be at the level of 20%. The instrument can be used by non-specialists in ultrafast spectroscopy.
Developing
interfacial probes of ligand–nanocluster interactions
is crucial for understanding and tailoring the optoelectronic properties
of these emerging nanomaterials. Using transient IR spectroscopy,
we demonstrate that ligand vibrational modes of oleate-capped 1.3
nm InP nanoclusters report on the photogenerated exciton. The exciton
induces an intensity change in the asymmetric carboxylate stretching
mode by 57% while generating no appreciable shift in frequency. Thus,
the observed difference signal is attributed to an exciton-induced
change in the dipole magnitude of the asymmetric carboxylate stretching
mode. Additionally, the transient IR data reveal that the infrared
dipole change is dependent on the geometry of the ligand bound to
the nanocluster. The experimental results are interpreted using TDDFT
calculations, which identify how the spatial dependence of an exciton-induced
electron density shift affects the vibrational motion of the carboxylate
anchors. More broadly, this work demonstrates transient IR spectroscopy
as a useful method for characterizing ligand–nanocluster coupling
interactions.
A compact laser beam direction stabilization scheme is developed that provides the angular stability of better than 50 μrad over a wide range of frequencies from 800 to 4000 cm-1. The schematic is fully automated and features a single MCT quadrant detector. The schematic was tested to stabilize directions of the two IR beams used for dual-frequency two-dimensional infrared (2DIR) measurements and showed excellent results: automatic tuning of the beam direction allowed achieving the alignment quality within 10% of the optimal alignment obtained manually. The schematic can be easily implemented to any nonlinear spectroscopic measurements in the mid-IR spectral region.
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