In the last ten years, two-dimensional infrared spectroscopy has become an important technique for studying molecular structures and dynamics. We report the implementation of heterodyne detected two-dimensional sum-frequency generation (HD 2D SFG) spectroscopy, which is the analog of 2D infrared (2D IR) spectroscopy, but is selective to noncentrosymmetric systems such as interfaces. We implement the technique using mid-IR pulse shaping, which enables rapid scanning, phase cycling, and automatic phasing. Absorptive spectra are obtained, that have the highest frequency resolution possible, from which we extract the rephasing and nonrephasing signals that are sometimes preferred. Using this technique, we measure the vibrational mode of CO adsorbed on a polycrystalline Pt surface. The 2D spectrum reveals a significant inhomogenous contribution to the spectral line shape, which is quantified by simulations. This observation indicates that the surface conformation and environment of CO molecules is more complicated than the simple "atop" configuration assumed in previous work. Our method can be straightforwardly incorporated into many existing SFG spectrometers. The technique enables one to quantify inhomogeneity, vibrational couplings, spectral diffusion, chemical exchange, and many other properties analogous to 2D IR spectroscopy, but specifically for interfaces. multidimensional spectroscopy | vibrational spectroscopy | surface-sensitive | Pt catalysis | CO monolayer M olecular spectroscopies are some of the best tools for studying structures and dynamics. Two particularly useful variants are sum-frequency generation (SFG) and two-dimensional infrared (2D IR) spectroscopy. SFG spectroscopy provides a vibrational spectrum of molecular systems that lack an inversion center (1), and so has become a valuable tool for probing interfaces because no signal arises from the bulk. SFG spectroscopy has helped reveal the surface structure of liquids, characterize the surfaces of materials, and probe membrane proteins, to name only a few applications (2-5). 2D IR spectroscopy is also a vibrational spectroscopy, although not interface specific. 2D IR spectroscopy spreads the infrared spectrum into a second coordinate so that coupled vibrational modes are correlated by cross peaks, vibrational dynamics quantified by 2D line shapes, and energy transfer or chemical exchange revealed from peak intensities (6-8), in addition to many other capabilities not possible with linear one-dimensional (1D) vibrational spectroscopies like SFG spectroscopy. 2D IR spectroscopy is now being used to study protein structure and dynamics, solvent dynamics, charge transfer in semiconductors, and many other processes (9-12). These two techniques might be considered the core technologies of modern infrared spectroscopy.In this article, we combine the surface sensitivity of SFG spectroscopy with the multidimensional capabilities of 2D IR spectroscopy in a technique that we call heterodyne detected (HD) 2D SFG spectroscopy. With this technique, one obtain...

Thin film networks of highly purified semiconducting carbon nanotubes (CNTs) are being explored for energy harvesting and optoelectronic devices because of their exceptional transport and optical properties. The nanotubes in these films are in close contact, which permits energy to flow through the films, although the pathways and mechanisms for energy transfer are largely unknown. Here we use a broadband continuum to collect femtosecond two-dimensional white-light spectra. The continuum spans 500 to 1,300 nm, resolving energy transfer between all combinations of bandgap (S 1 ) and higher (S 2 ) transitions. We observe ultrafast energy redistribution on the S 2 states, non-Förster energy transfer on the S 1 states and anti-correlated energy levels. The two-dimensional spectra reveal competing pathways for energy transfer, with S 2 excitons taking routes depending on the bandgap separation, whereas S 1 excitons relax independent of the bandgap. These observations provide a basis for understanding and ultimately controlling the photophysics of energy flow in CNT-based devices.

Carbon nanotubes are a promising means of capturing photons for use in solar cell devices. We time-resolved the photoexcitation dynamics of coupled, bandgap-selected, semiconducting carbon nanotubes in thin films tailored for photovoltaics. Using transient absorption spectroscopy and anisotropy measurements, we found that the photoexcitation evolves by two mechanisms with a fast and long-range component followed by a slow and short-range component. Within 300 fs of optical excitation, 20% of nanotubes transfer their photoexcitation over 5-10 nm into nearby nanotube fibers. After 3 ps, 70% of the photoexcitation resides on the smallest bandgap nanotubes. After this ultrafast process, the photoexcitation continues to transfer on a ~10 ps time scale but to predominantly aligned tubes. Ultimately the photoexcitation hops twice on average between fibers. These results are important for understanding the flow of energy and charge in coupled nanotube materials and light-harvesting devices.

Upconverting nanoparticles provide valuable benefits as optical probes for bioimaging and Förster resonant energy transfer (FRET) due to their high signal-to-noise ratio, photostability, and biocompatibility; yet making nanoparticles small yields a significant decay in brightness due to increased surface quenching. Approaches to improve the brightness of UCNPs exist but often require increased nanoparticle size. Here we present a unique core-shell-shell nanoparticle architecture for small (sub-20 nm), bright upconversion with several key features: 1) maximal sensitizer concentration in the core for high near-infrared absorption, 2) efficient energy transfer between core and interior shell for strong emission, and 3) emitter localization near the nanoparticle surface for efficient FRET. This architecture consists of β-NaYbF 4 (core) @NaY 0.8−x Er x Gd 0.2 F 4 (interior shell) @NaY 0.8 Gd 0.2 F 4 (exterior shell), where sensitizer and emitter ions are partitioned into core and interior shell, respectively. Emitter concentration is varied (x = 1, 2, 5, 10, 20, 50, and 80%) to investigate influence on single particle brightness, upconversion quantum yield, decay lifetimes, and FRET coupling. We compare these seven samples with the field-standard core-shell architecture of β-NaY 0.58 Gd 0.2 Yb 0.2 Er 0.02 F 4 (core) @NaY 0.8 Gd 0.2 F 4 (shell), with sensitizer and emitter ions codoped in the core. At a single particle level, the core-shell-shell design was up to 2-fold brighter than the standard core-shell design. Further, by coupling a fluorescent dye to the surface of the two different architectures, we demonstrated up to 8-fold improved emission enhancement with the core-shell-shell compared to the core-shell design. We show how, given proper consideration for emitter concentration, we can design a unique nanoparticle architecture to yield comparable or improved brightness and FRET coupling within a small volume.

Lanthanide-doped nanoparticles are an emerging class of optical sensors, exhibiting sharp emission peaks, high signal-to-noise ratio, photostability, and a ratiometric color response to stress. The same centrosymmetric crystal field environment that allows for high mechanosensitivity in the cubic-phase (α), however, contributes to low upconversion quantum yield (UCQY). In this work, we engineer brighter mechanosensitive upconverters using a core-shell geometry. Sub-25 nm α-NaYF:Yb,Er cores are shelled with an optically inert surface passivation layer of ∼4.5 nm thickness. Using different shell materials, including NaGdF, NaYF, and NaLuF, we study how compressive to tensile strain influences the nanoparticles' imaging and sensing properties. All core-shell nanoparticles exhibit enhanced UCQY, up to 0.14% at 150 W/cm, which rivals the efficiency of unshelled hexagonal-phase (β) nanoparticles. Additionally, strain at the core-shell interface can tune mechanosensitivity. In particular, the compressive Gd shell results in the largest color response from yellow-green to orange or, quantitatively, a change in the red to green ratio of 12.2 ± 1.2% per GPa. For all samples, the ratiometric readouts are consistent over three pressure cycles from ambient to 5 GPa. Therefore, heteroepitaxial shelling significantly improves signal brightness without compromising the core's mechano-sensing capabilities and further, promotes core-shell cubic-phase nanoparticles as upcoming in vivo and in situ optical sensors.