Sub-20 nm Core-Shell-Shell Nanoparticles for Bright Upconversion and Enhanced Förster Resonant Energy Transfer.

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.

Optically Robust and Biocompatible Mechanosensitive Upconverting Nanoparticles.

Upconverting nanoparticles (UCNPs) are promising tools for background-free imaging and sensing. However, their usefulness for applications depends on their biocompatibility, which we define by their optical performance in biological environments and their toxicity in living organisms. For UCNPs with a ratiometric color response to mechanical stress, consistent emission intensity and color are desired for the particles under nonmechanical stimuli.

Small Alkaline-Earth-based Core/Shell Nanoparticles for Efficient Upconversion.

The optical efficiency of lanthanide-based upconversion is intricately related to the crystalline host lattice. Different crystal fields interacting with the electron clouds of the lanthanides can significantly affect transition probabilities between the energy levels. Here, we investigate six distinct alkaline-earth rare-earth fluoride host materials (MLn F, MLnF) for infrared-to-visible upconversion, focusing on nanoparticles of CaYF, CaLuF, SrYF, SrLuF, BaYF, and BaLuF doped with Yb and Er.

Bright, Mechanosensitive Upconversion with Cubic-Phase Heteroepitaxial Core-Shell Nanoparticles.

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.

Upconverting Nanoparticles as Optical Sensors of Nano- to Micro-Newton Forces.

Mechanical forces affect a myriad of processes, from bone growth to material fracture to touch-responsive robotics. While nano- to micro-Newton forces are prevalent at the microscopic scale, few methods have the nanoscopic size and signal stability to measure them in vivo or in situ. Here, we develop an optical force-sensing platform based on sub-25 nm NaYF nanoparticles (NPs) doped with Yb, Er, and Mn.

Broadband 2D electronic spectrometer using white light and pulse shaping: noise and signal evaluation at 1 and 100 kHz.

We have developed a broad bandwidth two-dimensional electronic spectrometer that operates shot-to-shot at repetition rates up to 100 kHz using an acousto-optic pulse shaper. It is called a two-dimensional white-light (2D-WL) spectrometer because the input is white-light supercontinuum. Methods for 100 kHz data collection are studied to understand how laser noise is incorporated into 2D spectra during measurement.

Ultrafast Exciton Hopping Observed in Bare Semiconducting Carbon Nanotube Thin Films with Two-Dimensional White-Light Spectroscopy.

We observe ultrafast energy transfer between bare carbon nanotubes in a thin film using two-dimensional (2D) white-light spectroscopy. Using aqueous two-phase separation, semiconducting carbon nanotubes are purified from their metallic counterparts and condensed into a 10 nm thin film with no residual surfactant. Cross peak intensities put the time scale for energy transfer at <60 fs, and 2D anisotropy measurements determine that energy transfer is most efficient between parallel nanotubes, thus favoring directional energy flow.

Experimental Measurement of the Binding Configuration and Coverage of Chirality-Sorting Polyfluorenes on Carbon Nanotubes.

Poly(9,9-dioctylfluorene-2,7-diyl) (PFO) exhibits exceptional (n,m) chirality and electronic-type selectivity for near-armchair semiconducting carbon nanotubes. To better understand and control the factors governing this behavior, we experimentally determine the surface coverage and binding configuration of PFO on nanotubes in solution using photoluminescence energy transfer and anisotropy measurements. The coverage increases with PFO concentration in solution, following Langmuir-isotherm adsorption behavior with cooperativity.

Energy transfer pathways in semiconducting carbon nanotubes revealed using two-dimensional white-light spectroscopy.

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.

Photoexcitation dynamics of coupled semiconducting carbon nanotube thin films.

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.

Adding a dimension to the infrared spectra of interfaces using heterodyne detected 2D sum-frequency generation (HD 2D SFG) spectroscopy.

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.

Probing the charge transfer reaction coordinate of 4-(dimethylamino)benzonitrile with femtosecond stimulated Raman spectroscopy.

Femtosecond stimulated Raman spectroscopy (FSRS) and femtosecond transient absorption have been used to probe the photoinduced charge transfer (CT) dynamics of 4-(dimethylamino)benzonitrile in methanol and n-hexane. Through a combined analysis of temporal changes in the Raman modes and transient absorption kinetics, a more complete picture of the reaction coordinate of the intramolecular charge transfer process has been established. FSRS spectra of the phenyl C═C stretching mode (Wilson mode 8a) at 1607 cm(-1), which shifts to 1581 cm(-1) in the CT state, and transient absorption measurements ranging from 360 to 700 nm support internal conversion from the locally excited to the charge transfer state in 4-5 ps and then a subsequent vibrational relaxation within the CT state manifold on a 6-8 ps time scale.

Theoretical analysis of anharmonic coupling and cascading Raman signals observed with femtosecond stimulated Raman spectroscopy.

We present a classical theoretical treatment of a two-dimensional Raman spectroscopy based on the initiation of vibrational coherence with an impulsive Raman pump and subsequent probing by two-pulse femtosecond stimulated Raman spectroscopy (FSRS). The classical model offers an intuitive picture of the molecular dynamics initiated by each laser pulse and the generation of the signal field traveling along the probe wave vector. Previous reports have assigned the observed FSRS signals to anharmonic coupling between the impulsively driven vibration and the higher-frequency vibration observed with FSRS.

Two-dimensional femtosecond stimulated Raman spectroscopy: Observation of cascading Raman signals in acetonitrile.

A new methodology for two-dimensional Raman spectroscopy-termed two-dimensional femtosecond stimulated Raman spectroscopy (2D-FSRS)-is presented and experimental results for acetonitrile are discussed. 2D-FSRS can potentially observe molecular anharmonicity by measuring the modulation of the frequency of a probed Raman mode, at frequency omega(hi), by the coherent motion of an impulsively driven mode, at frequency omega(low). In acetonitrile, the signal is generated by driving the CCN bend (379 cm(-1)) and CC stretch (920 cm(-1)) into coherence via impulsive stimulated Raman scattering and subsequently probing the stimulated Raman spectrum of the CC stretch, the CN stretch (2250 cm(-1)) and the CH stretch (2942 cm(-1)).