Dr. Alex Benderskii
Coherent Nonlinear Spectroscopic Studies of Molecular Structure and Dynamics at Interfaces and Nanostructures
Department of Chemistry
Many exciting emerging nanotechnology applications are made possible by organic molecules chemi- or physisorbed on surfaces of nanostructures which perform the desired function. This motif is common in applications such as chemical and bio-sensors, drug delivery and photodynamic therapy, heterogeneous nanocatalysis, nanocomposite energetic materials, molecular electronics, and hybrid organic-inorganic photovoltaic devices. Accordingly, a clear need is emerging for experimental characterization techniques capable of providing fundamental molecular-level understanding of the chemical structure, conformation, and reactivity at nanostructured surfaces. This lecture will survey our recent developments of the surface-selective coherent nonlinear spectroscopic techniques and their applications to the studies of molecular structure and dynamics at interfaces and nanostructures. In particular, we focus on the vibrational spectroscopy in the fingerprint mid-infrared (IR) region implemented using IR + visible sum frequency generation (SFG) spectroscopy. The sensitivity of the SFG spectroscopy to the molecular orientation and conformation will be demonstrated for metal-ligand Langmuir-Blodgett films of interest as molecularly organized organic electronic materials, and to molecules adsorbed at nanoparticles.
A new effect arises when the nanoparticle size approaches the molecular scale: the molecular conformation starts to depend on the geometry of the substrate. Using SFG spectroscopy, we demonstrate that the conformation of the dodecane chain of the ligand shows systematic variation with the particle diameter in the 1.8 – 25 nm range. More gauche defects are observed on smaller particles, judged from the relative intensities of the CH 2 and CH 3 stretch transitions in SFG spectra. Similar behavior observed for both gold and silver particles suggests a universal nanoscale geometric effect, whence more volume is available to the chain on a curved surface than a flat surface. Drying-mediated aggregation of gold nanoparticles results in an enhancement of the SFG signal, enabling vibrational SFG spectra of single sub-micron size aggregates to be obtained. These are different from spectra of isolated particles, indicating a vibrational mode-selective enhancement mechanism.
If time permits, new developments of the heterodyne-detected vibrational SFG spectroscopy will be discussed, which allow significant improvements in the detection limits of the technique, down to few percent of a single monolayer.
Biosketch
Location: Welcome Center at 2:30 p.m.
