Increased Formation of Trions and Charged Biexcitons by Above-Gap Excitation in Single-layer WSe.

Two-dimensional semiconductors exhibit pronounced many-body effects and intense optical responses due to strong Coulombic interactions. Consequently, subtle differences in photoexcitation conditions can strongly influence how the material dissipates energy during thermalization. Here, using multiple excitation spectroscopies, we show that a distinct thermalization pathway emerges at elevated excitation energies, enhancing the formation of trions and charged biexcitons in single-layer WSe by up to 2× and 5× , respectively.

Quantifying noise effects in optical measures of excited state transport.

Time-resolved microscopy is a widely used approach for imaging and quantifying charge and energy transport in functional materials. While it is generally recognized that resolving small diffusion lengths is limited by measurement noise, the impacts of noise have not been systematically assessed or quantified. This article reports modeling efforts to elucidate the impact of noise on optical probes of transport.

Rapid Exciton Transport and Structural Defects in Individual Porphyrinic Metal Organic Framework Microcrystals.

To date, spectroscopic characterization of porphyrin-based metal organic frameworks (MOFs) has relied almost exclusively on ensemble techniques, which provide only structurally averaged insight into the functional properties of these promising photochemical platforms. This work employs time-resolved pump-probe microscopy to probe ultrafast dynamics in PCN-222 MOF single crystals. The simultaneous high spatial and temporal resolution of the technique enables the correlation of spectroscopic observables to both inter- and intracrystal structural heterogeneity.

Ultrafast Charge Injection in Silver-Modified Graphitic Carbon Nitride.

Graphitic carbon nitride (gCN) is a promising organic platform for driving light-activated charge-transfer reactions in a number of valuable photocatalytic cycles. A primary limitation of gCN as a photocatalyst is its short excited-state lifetime, which is mediated by a high density of trap and defect sites that result in rapid excited-state decay and low photocatalytic efficiency. To enhance the catalytic activity, gCN is often functionalized with a metal co-catalyst; however, the mechanism by which metal co-catalysts enhance the reactivity has not been clearly established.

Photocatalytic Reduction of Aqueous Nitrate with Hybrid Ag/g-CN under Ultraviolet and Visible Light.

The concentration of nitrate in natural surface waters by agricultural runoff remains a challenging problem in environmental chemistry. One promising denitrification strategy is to utilize photocatalysts, whose light-driven excited states are capable of reducing nitrate to nitrogen gas. We have synthesized and characterized pristine and silver-loaded graphitic carbon nitrides and assessed their activity for photocatalytic nitrate reduction at neutral pH.

Direct Correlation of Charge Carrier Transport to Local Crystal Quality in Lead Halide Perovskites.

Although solution processing methods provide an attractive route toward development of low-cost functional materials, these accessible fabrication approaches can engender high concentrations of microscopic structural defects that are detrimental to performance. In lead halide perovskites, structural disorder derived from solution processing has been implicated as an important determiner of photophysical properties. However, a direct correlation between the functional properties of these materials and the local crystal structure in which non-equilibrium states evolve has remained elusive, in part because structural heterogeneities occur on length scales that defy conventional characterization techniques.

Spatiotemporal coupling of excited state dynamics in time-resolved microscopies.

In the high-density excitation limit, as is often probed with ultrafast spectroscopies, spatial and temporal evolution of photogenerated excited states are strongly coupled, giving rise to artifacts that influence experimentally-determined material parameters. The interplay between spatial and temporal degrees of freedom is especially pronounced in pump-probe microscopy, where small laser spot sizes amplify the effects of spatiotemporal coupling on spectroscopic observables. To quantitatively model these effects, a continuum model is developed that accounts for laser spot size as well as nonlinear excited state decay and diffusion.

High-Repetition Rate Broadband Pump-Probe Microscopy.

Pump-probe microscopy has recently emerged as an important tool for characterizing the effects of nanoscale chemical and compositional heterogeneity on the optoelectronic properties of material systems. This article describes the development of broadband pump-probe microscopy, which utilizes a high-speed line camera and high repetition rate amplified fiber laser to collect full transient spectra at 30+ kHz and with sub 100 fs temporal resolution. The broadband imaging and spectroscopic capabilities of the technique are demonstrated on individual micron-sized lead halide perovskite domains.

Q-FADD: A Mechanistic Approach for Modeling the Accumulation of Proteins at Sites of DNA Damage.

The repair of DNA damage requires the ordered recruitment of many different proteins that are responsible for signaling and subsequent repair. A powerful and widely used tool for studying the orchestrated accumulation of these proteins at damage sites is laser microirradiation in live cells, followed by monitoring the accumulation of the fluorescently labeled protein in question. Despite the widespread use of this approach, there exists no rigorous method for characterizing the recruitment process quantitatively.

Perovskite Carrier Transport: Disentangling the Impacts of Effective Mass and Scattering Time Through Microscopic Optical Detection.

While carrier mobility is a practical and commonly cited measure of transport, it conflates the effects of two more fundamental material properties: the effective mass and mean scattering time of charge carriers. This Letter describes the correlation of two ultrafast imaging techniques to disentangle the effect of each on carrier transport in lead halide perovskites. Two materials are compared: methylammonium lead tri-iodide (MAPbI) and cesium lead bromide diiodide (CsPbBrI).

Probing Intrawire, Interwire, and Diameter-Dependent Variations in Silicon Nanowire Surface Trap Density with Pump-Probe Microscopy.

Surface trap density in silicon nanowires (NWs) plays a key role in the performance of many semiconductor NW-based devices. We use pump-probe microscopy to characterize the surface recombination dynamics on a point-by-point basis in 301 silicon NWs grown using the vapor-liquid-solid (VLS) method. The surface recombination velocity (S), a metric of the surface quality that is directly proportional to trap density, is determined by the relationship S = d/4τ from measurements of the recombination lifetime (τ) and NW diameter (d) at distinct spatial locations in individual NWs.

Screened Charge Carrier Transport in Methylammonium Lead Iodide Perovskite Thin Films.

While organometal halide perovskites are promising for a variety of optoelectronic applications, the morphological and compositional defects introduced by solution processing techniques have hindered efforts at understanding their fundamental properties. To provide a detailed picture of the intrinsic carrier transport properties of methylammonium lead iodide without contributions from defects such as grain boundaries, we utilized pump-probe microscopy to measure diffusion in individual crystalline domains of a thin film. Direct imaging of carrier transport in 25 individual domains yields diffusivities between 0.

Imaging theory of structured pump-probe microscopy.

With sub-micron spatial resolution and femtosecond temporal resolution, pump probe microscopy provides a powerful spectroscopic probe of complex electronic environments in bulk and nanoscale materials. However, the electronic structure of many materials systems are governed by compositional and morphological heterogeneities on length scales that lie below the diffraction limit. We have recently demonstrated Structured Pump Probe Microscopy (SPPM), which employs a patterned pump excitation field to provide spectroscopic interrogation of sub-diffraction limited sample volumes.

Disentangling the Physical Processes Responsible for the Kinetic Complexity in Interfacial Electron Transfer of Excited Ru(II) Polypyridyl Dyes on TiO2.

Interfacial electron transfer at titanium dioxide (TiO2) is investigated for a series of surface bound ruthenium-polypyridyl dyes whose metal-to-ligand charge-transfer state (MLCT) energetics are tuned through chemical modification. The 12 complexes are of the form Ru(II)(bpy-A)(L)2(2+), where bpy-A is a bipyridine ligand functionalized with phosphonate groups for surface attachment to TiO2. Functionalization of ancillary bipyridine ligands (L) enables the potential of the excited state Ru(III/)* couple, E(+/)*, in 0.

Ultrafast, Light-Induced Electron Transfer in a Perylene Diimide Chromophore-Donor Assembly on TiO2.

Surface-bound, perylenediimide (PDI)-based molecular assemblies have been synthesized on nanocrystalline TiO2 by reaction of a dianhydride with a surface-bound aniline and succinimide bonding. In a second step, the Fe(II) polypyridyl complex [Fe(II)(tpy-PhNH2)2](2+) was added to the outside of the film, also by succinimide bonding. Ultrafast transient absorption measurements in 0.

Reversible strain-induced electron-hole recombination in silicon nanowires observed with femtosecond pump-probe microscopy.

Strain-induced changes to the electronic structure of nanoscale materials provide a promising avenue for expanding the optoelectronic functionality of semiconductor nanostructures in device applications. Here we use pump-probe microscopy with femtosecond temporal resolution and submicron spatial resolution to characterize charge-carrier recombination and transport dynamics in silicon nanowires (NWs) locally strained by bending deformation. The electron-hole recombination rate increases with strain for values above a threshold of ∼1% and, in highly strained (∼5%) regions of the NW, increases 6-fold.

Imaging charge separation and carrier recombination in nanowire p-i-n junctions using ultrafast microscopy.

Silicon nanowires incorporating p-type/n-type (p-n) junctions have been introduced as basic building blocks for future nanoscale electronic components. Controlling charge flow through these doped nanostructures is central to their function, yet our understanding of this process is inferred from measurements that average over entire structures or integrate over long times. Here, we have used femtosecond pump-probe microscopy to directly image the dynamics of photogenerated charge carriers in silicon nanowires encoded with p-n junctions along the growth axis.

Light-harvesting polymers: ultrafast energy transfer in polystyrene-based arrays of π-conjugated chromophores.

Energy transfer along a nonconjugated polymer chain is studied with a polystyrene-based copolymer of oligo(phenylene-ethynylene) (OPE) donor and thiophene-benzothiadiazole (TBT) acceptor pendants. The graft copolymers are prepared from reversible addition-fragmentation transfer polymerization (RAFT) and copper(I)-catalyzed azide-alkyne "click" reaction. The singlet energy transfer from donor to accept is studied via fluorescence emission and ultrafast transient absorption spectroscopy.

Synthetically encoding 10 nm morphology in silicon nanowires.

Si nanowires (NWs) have been widely explored as a platform for photonic and electronic technologies. Here, we report a bottom-up method to break the conventional "wire" symmetry and synthetically encode a high-resolution array of arbitrary shapes, including nanorods, sinusoids, bowties, tapers, nanogaps, and gratings, along the NW growth axis. Rapid modulation of phosphorus doping combined with selective wet-chemical etching enabled morphological features as small as 10 nm to be patterned over wires more than 50 μm in length.

Frequency modulated femtosecond stimulated Raman spectroscopy of ultrafast energy transfer in a donor-acceptor copolymer.

A Raman-pump frequency modulation scheme and an automated signal-processing algorithm are developed for improved collection of time-resolved femtosecond stimulated Raman spectra. Together, these two advancements remove the broad background signals endemic to FSRS measurements and retrieve signals with high sensitivity. We apply this frequency-modulated femtosecond stimulated Raman spectroscopy (FM-FSRS) to the characterization of ultrafast energy transport in a copolymer comprised of polystyrene linked oligo(phenylene-ethynylene) donor and thiophene-benzothiadiazole acceptor chromophores.

Direct imaging of free carrier and trap carrier motion in silicon nanowires by spatially-separated femtosecond pump-probe microscopy.

We have developed a pump-probe microscope capable of exciting a single semiconductor nanostructure in one location and probing it in another with both high spatial and temporal resolution. Experiments performed on Si nanowires enable a direct visualization of the charge cloud produced by photoexcitation at a localized spot as it spreads along the nanowire axis. The time-resolved images show clear evidence of rapid diffusional spreading and recombination of the free carriers, which is consistent with ambipolar diffusion and a surface recombination velocity of ∼10(4) cm/s.

Modeling and correction of distorted two-dimensional Fourier transform spectra from pixelated pulse shaping devices.

Two-dimensional Fourier transform spectra of a three level model system are simulated using a non-perturbative density matrix formalism. The electric field distortions resultant from using pixelated pulse shaping devices to produce phase-locked pulse pairs are modeled and the effects on the recovered spectra are examined. To minimize spectral distortions, a temporal filtering scheme is employed which eliminates contributions from spurious sample polarizations.

Competition between Ultrafast Energy Flow and Electron Transfer in a Ru(II)-Loaded Polyfluorene Light-Harvesting Polymer.

This Letter describes the synthesis and photophysical characterization of a Ru(II) assembly consisting of metal polypyridyl complexes linked together by a polyfluorene scaffold. Unlike many scaffolds incorporating saturated linkages, the conjugated polymer in this system acts as a functional light-harvesting component. Conformational disorder breaks the conjugation in the polymer backbone, resulting in a chain composed of many chromophore units, whose relative energies depend on the segment lengths.

Enhanced triplet formation in polycrystalline tetracene films by femtosecond optical-pulse shaping.

Polycrystalline tetracene films have been explored using weak ∼ 30 fs visible laser pulses that excite the lowest singlet exciton as well as coherent vibrational motion. Transient difference spectra show a triplet absorption which arises following singlet fission (SF) and persists for 1.6 ns without decay.

Effects of (18)O isotopic substitution on the rotational spectra and potential splitting in the OH-OH2 complex: improved measurements for (16)OH-(16)OH2 and (18)OH-(18)OH2, new measurements for the mixed isotopic forms, and ab initio calculations of the (2)A'-(2)A" energy separation.

Rotational spectra have been observed for (16)OH-(16)OH(2), (16)OH-(18)OH(2), (18)OH-(16)OH(2), and (18)OH-(18)OH(2) with complete resolution of the nuclear magnetic hyperfine structure from the OH and water protons. Transition frequencies have been analyzed for each isotopic form using the model of Marshall and Lester [J. Chem.