Hot carrier extraction from 2D semiconductor photoelectrodes.

Hot carrier-based energy conversion systems could double the efficiency of conventional solar energy technology or drive photochemical reactions that would not be possible using fully thermalized, "cool" carriers, but current strategies require expensive multijunction architectures. Using an unprecedented combination of photoelectrochemical and in situ transient absorption spectroscopy measurements, we demonstrate ultrafast (<50 fs) hot exciton and free carrier extraction under applied bias in a proof-of-concept photoelectrochemical solar cell made from earth-abundant and potentially inexpensive monolayer (ML) MoS2. Our approach facilitates ultrathin 7 Å charge transport distances over 1 cm2 areas by intimately coupling ML-MoS2 to an electron-selective solid contact and a hole-selective electrolyte contact. Our theoretical investigations of the spatial distribution of exciton states suggest greater electronic coupling between hot exciton states located on peripheral S atoms and neighboring contacts likely facilitates ultrafast charge transfer. Our work delineates future two-dimensional (2D) semiconductor design strategies for practical implementation in ultrathin photovoltaic and solar fuel applications.

Breaking Barriers in Ultrafast Spectroscopy and Imaging Using 100 kHz Amplified Yb-Laser Systems.

ConspectusUltrafast spectroscopy and imaging have become tools utilized by a broad range of scientists involved in materials, energy, biological, and chemical sciences. Commercialization of ultrafast spectrometers including transient absorption spectrometers, vibrational sum frequency generation spectrometers, and even multidimensional spectrometers have put these advanced spectroscopy measurements into the hands of practitioners originally outside the field of ultrafast spectroscopy. There is now a technology shift occurring in ultrafast spectroscopy, made possible by new Yb-based lasers, that is opening exciting new experiments in the chemical and physical sciences. Amplified Yb-based lasers are not only more compact and efficient than their predecessors but also, most importantly, operate at many times the repetition rate with improved noise characteristics in comparison to the previous generation of Ti:sapphire amplifier technologies. Taken together, these attributes are enabling new experiments, generating improvements to long-standing techniques, and affording the transformation of spectroscopies to microscopies. This Account aims to show that the shift to 100 kHz lasers is a transformative step in nonlinear spectroscopy and imaging, much like the dramatic expansion that occurred with the commercialization of Ti:sapphire laser systems in the 1990s. The impact of this technology will be felt across a great swath of scientific communities. We first describe the technology landscape of amplified Yb-based laser systems used in conjunction with 100 kHz spectrometers operating with shot-to-shot pulse shaping and detection. We also identify the range of different parametric conversion and supercontinuum techniques which now provide a path to making pulses of light optimal for ultrafast spectroscopy. Second, we describe specific instances from our laboratories of how the amplified Yb-based light sources and spectrometers are transformative. For multiple probe time-resolved infrared and transient 2D IR spectroscopy, the gain in temporal span and signal-to-noise enables dynamical spectroscopy measurements from femtoseconds to seconds. These gains widen the applicability of time-resolved infrared techniques across a range of topics in photochemistry, photocatalysis, and photobiology as well as lower the technical barriers to implementation in a laboratory. For 2D visible spectroscopy and microscopy with white light, as well as 2D IR imaging, the high repetition rates of these new Yb-based light sources allow one to spatially map 2D spectra while maintaining high signal-to-noise in the data. To illustrate the gains, we provide examples of imaging applications in the study of photovoltaic materials and spectroelectrochemistry.

Trion Formation Resolves Observed Peak Shifts in the Optical Spectra of Transition-Metal Dichalcogenides.

Monolayer transition-metal dichalcogenides (ML-TMDs) have the potential to unlock novel photonic and chemical technologies if their optoelectronic properties can be understood and controlled. Yet, recent work has offered contradictory explanations for how TMD absorption spectra change with carrier concentration, fluence, and time. Here, we test our hypothesis that the large broadening and shifting of the strong band-edge features observed in optical spectra arise from the formation of negative trions. We do this by fitting an ab initio based, many-body model to our experimental electrochemical data. Our approach provides an excellent, global description of the potential-dependent linear absorption data. We further leverage our model to demonstrate that trion formation explains the nonmonotonic potential dependence of the transient absorption spectra, including through photoinduced derivative line shapes for the trion peak. Our results motivate the continued development of theoretical methods to describe cutting-edge experiments in a physically transparent way.

The N3/TiO2 interfacial structure is dependent on the pH conditions during sensitization.

The electronic structure of the N3/TiO2 interface can directly influence the performance of a dye sensitized solar cell (DSSC). Therefore, it is crucial to understand the parameters that control the dye's orientation on the semiconductor's surface. A typical step in DSSC fabrication is to submerge the nanoparticulate semiconductor film in a solution containing the dye, the sensitizing solution. The pH of the N3 sensitizing solution determines the distribution of the N3 protonation states that exist in solution. Altering the pH of the sensitizing solution changes the N3 protonation states that exist in solution and, subsequently, the N3 protonation states that anchor to the TiO2 substrate. We utilize the surface specific technique of heterodyne detected vibrational sum frequency generation spectroscopy to determine the binding geometry of N3 on a TiO2 surface as a function of the sensitizing solution pH conditions. It is determined that significant reorientation of the dye occurs in pH ≤2.0 conditions due to the lack of N3-dye carboxylate anchoring groups participating in adsorption to the TiO2 substrate. Consequently, the change in molecular geometry is met with a change in the interfacial electronic structure that can hinder electron transfer in DSSC architectures.

The pH-dependent orientation of N3 dye on a gold substrate is revealed using heterodyne-detected vibrational sum frequency generation spectroscopy.

We report on systematic changes to the adsorption geometry of the dye N3 {[cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato ruthenium(II)]} on a gold substrate as the pH of the deposition environment is altered. The protonation states of the four -COOH groups of the N3 dye change according to the modified pH conditions, thus affecting the number of -COOH and -NCS functional groups that participate in the adsorption to gold. Here, we use heterodyne detected vibrational sum frequency generation (HD-VSFG) spectroscopy to obtain surface specific vibrational information on both -COOH and -NCS groups as a function of pH of the deposition conditions. Polarization-dependent HD-VSFG yields sets of complex χ(2) spectra, enabling us to perform a simultaneous fitting procedure to the polarization-dependent real and imaginary components and thus extract detailed structural information of the N3/gold interface. Our results show that N3 preferentially adsorbs to gold either with two -COOH groups and one -NCS group in more acidic conditions or with one -COOH group and two -NCS groups in more basic conditions.

Ultrafast vibrational dynamics of a trigonal planar anionic probe in ionic liquids (ILs): A two-dimensional infrared (2DIR) spectroscopic investigation.

A major impediment limiting the widespread application of ionic liquids (ILs) is their high shear viscosity. Incorporation of a tricyanomethanide (TCM-) anion in ILs leads to low shear viscosity and improvement of several characteristics suitable for large scale applications. However, properties including interactions of TCM- with the local environment and dynamics of TCM- have not been thoroughly investigated. Herein, we have studied the ultrafast dynamics of TCM- in several imidazolium ILs using linear IR and two-dimensional infrared spectroscopy techniques. The spectral diffusion dynamics of the CN stretching modes of TCM- in all ILs exhibit a nonexponential behavior with a short time component of ∼2 ps and a long time component spanning ∼9 ps to 14 ps. The TCM- vibrational probe reports a significantly faster relaxation of ILs compared to those observed previously using linear vibrational probes, such as thiocyanate and selenocyanate. Our results indicate a rapid relaxation of the local ion-cage structure embedding the vibrational probe in the ILs. The faster relaxation suggests that the lifetime of the local ion-cage structure decreases in the presence of TCM- in the ILs. Linear IR spectroscopic results show that the hydrogen-bonding interaction between TCM- and imidazolium cations in ILs is much weaker. Shorter ion-cage lifetimes together with weaker hydrogen-bonding interactions account for the low shear viscosity of TCM- based ILs compared to commonly used ILs. In addition, this study demonstrates that TCM- can be used as a potential vibrational reporter to study the structure and dynamics of ILs and other molecular systems.

2D IR spectroscopy at 100 kHz utilizing a Mid-IR OPCPA laser source.

We present a 100 kHz 2D IR spectrometer. The system utilizes a ytterbium all normal dispersion fiber oscillator as a common source for the pump and seed beams of a MgO:PPLN OPCPA. The 1030 nm OPCPA pump is generated by amplification of the oscillator in cryocooled Yb:YAG amplifiers, while the 1.68 µm seed is generated in a OPO pumped by the oscillator. The OPCPA outputs are used in a ZGP DFG stage to generate 4.65 µm pulses. A mid-IR pulse shaper delivers pulse pairs to a 2D IR spectrometer allowing for data collection at 100 kHz.

Probing structural features of self-assembled violanthrone-79 using two dimensional infrared spectroscopy.

Two-dimensional infrared (2D IR) spectroscopy was used to characterize the structure of a self-assembled polycyclic aromatic hydrocarbon (PAH), violanthrone-79. A local mode basis was constructed using spectroscopic and computational results of anthrone and monomer violanthrone-79. The vibrational modes in the spectral region 1550-1700 cm(-1), carbonyl stretching and in-plane ring breathing, are used as vibrational probes. The local mode basis and an electrostatic coupling model were applied to three nanoaggregate structures: parallel, antiparallel, and a chiral configuration produced by a 28° rotation from parallel. Angular disorder within each nanoaggregate configuration was also explored. This investigation is a first approach to probe self-assembled PAHs with 2D IR spectroscopy. The experimental and calculated 2D IR spectra align best when the violanthrone-79 molecules are in an anti-parallel configuration within the nanoaggregate.

Polymeric infrared compatible microfluidic devices for spectrochemical analysis.

An innovative fabrication method is presented that affords the combination of polydimethyl-siloxane (PDMS) microfluidic technology with vibrational spectroscopy. PDMS devices are produced with uniform thicknesses ranging from 25 to 400 µm. The optical characteristics of the microfluidic devices in the mid-infrared are reported. The broad utility of this approach is demonstrated through IR imaging of flows in functional gradient generators and flow-focusing devices.

Dicyanamide Anion Reports on Water Induced Local Structural and Dynamic Heterogeneity in Ionic Liquid Mixtures.

The effects of limited amounts (under 21.6% χWater) of water on 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4) and 1-butyl-3-methylimidazolium dicyanamide (BmimDCA) room-temperature ionic liquid (RTIL) mixtures were characterized by tracking changes in the linear and two-dimensional infrared (2D IR) vibrational features of the dicyanamide anion (DCA). Peak shifts with increasing water suggest the formation of water-associated and nonwater-associated DCA populations. Further results showed clear differences in the dynamic behavior of these different populations of DCA at low (defined here as below 2.5% χWater), mid (defined here as between 2.5% χWater and 9.6% χWater), and high (defined here as between 11.6% χWater and 21.6% χWater) range water concentrations. Vibrational relaxation is accelerated with increasing water content for water-associated populations of DCA, indicating water facilitates population relaxation, possibly through the provision of additional bath modes. Conversely, spectral diffusion of water-associated populations slowed dramatically with increasing water, suggesting that water drives the formation of distinct and noninterchangeable or very slowly interchangeable local solvent environments.

Active Bragg angle compensation for shaping ultrafast mid-infrared pulses.

Active Bragg angle compensation is demonstrated for shaping ultrafast, mid-infrared pulses. The effects of angular dispersion introduced by the acousto-optic modulator on the temporal characteristics of the pulse are measured by autocorrelating the output from the pulse shaper. The time duration of the output pulses were measured to be thirty times shorter than pulses produced with a constant frequency amplitude waveform. This approach acts to mitigate angular dispersion in Bragg-regime acousto-optic devices, thus affording the ability to shape ultrafast pulses of light with broad bandwidths that are centered at mid-IR wavelengths and longer.

Functional patterning of PDMS microfluidic devices using integrated chemo-masks.

Microfluidic devices can be molded easily from PDMS using soft lithography. However, the softness of the resulting microchannels makes it difficult to photolithographically pattern their surface properties, as is needed for applications such as double emulsification. We introduce a new patterning method for PDMS devices, using integrated oxygen reservoirs fabricated simultaneously with the microfluidic channels, which serve as "chemo-masks". Oxygen diffuses through the PDMS to the nearby channel segments and there inhibits functional polymer growth; by placement of the chemo-masks, we thus control the polymerization pattern. This patterning method is simple, scalable, and compatible with a variety of surface chemistries.

Photoreactive coating for high-contrast spatial patterning of microfluidic device wettability.

For many applications in microfluidics, the wettability of the devices must be spatially controlled. We introduce a photoreactive sol-gel coating that enables high-contrast spatial patterning of microfluidic device wettability.

Evidence for coupling between nitrile groups using DNA templates: a promising new method for monitoring structures with infrared spectroscopy.

Infrared spectroscopy is a common method for monitoring biomolecular structures but suffers from spectral congestion. Non-natural vibrational probes provide a way to regain structural specificity because they provide a unique vibrational signature and can be incorporated into proteins or other biomolecules at specific locations. A popular probe is the nitrile group because its frequency is sensitive to the electrostatics of its environment. In this work, we show that pairs of nitrile groups can be used to directly probe distances and angles in dual labeled molecules. By labeling model DNA oligomers with pairs of nitrile tags, we demonstrate that the vibrational coupling between two nitrile groups is strong enough that Fourier transform infrared (FTIR) spectra can be used to probe relative nitrile distances >4.5 A. Our approach is similar in spirit to monitoring structures with fluorescence resonance energy transfer (FRET) using a pair of fluorescent labels or a pair of spin labels in electron spin resonance spectroscopy. The small sizes of nitrile groups make especially valuable probes of sterically confined regions like the inner cores of large biomolecules where other spectroscopic probes do not fit.

IR-Compatible PDMS microfluidic devices for monitoring of enzyme kinetics.

Coupling infrared (IR) spectroscopy to microfluidic devices provides a powerful tool for characterizing complex chemical and biochemical reactions. Examples of microfluidic devices coupled with infrared spectroscopy have been limited, however, largely due to the difficulties associated with fabricating systems in common infrared transparent materials like CaF2. Recent reports have shown that polydimethylsiloxane (PDMS) can be used as an IR transparent substrate when fabricated with thin layers. The use of soft lithography with PDMS expands the library of possible designs that can be achieved for IR measurements in microfluidics. In initial reports with thin PDMS, the target analytes were small molecules; however, IR spectroscopy offers a powerful tool to study protein structure and reactions. Here, a PDMS microfluidic device compatible with IR spectroscopy was fabricated by means of spin-coating of PDMS pre-polymer to obtain thin PDMS microfluidic features. The device was comprised of only PDMS and IR absorption of PDMS was significantly minimized due to the thickness (∼40 µm) of the PDMS layer. The use of thin PDMS allowed for measuring the amide I and II vibrational bands of proteins that have been difficult to measure in other microfluidic devices. To demonstrate the power of the system, the microfluidic device was successfully used to measure the enzyme kinetics as one class of important biochemical reactions with broad use in a variety of fields from medicine to biotechnology. As a model, the reaction of glucose oxidase with glucose was tracked by following the formation of gluconic acid. Michaelis-Menten kinetics from the device were compared with bulk solution measurements and found to be in good agreement.

Elucidating Structural Evolution of Perylene Diimide Aggregates Using Vibrational Spectroscopy and Molecular Dynamics Simulations.

Perylene diimides (PDIs) are a family of molecules that have potential applications to organic photovoltaics. These systems typically aggregate cofacially due to π-stacking interactions between the aromatic perylene cores. In this study, the structure and characteristics of aggregated N, N'-bis(2,6-diisopropylphenyl)-3,4,9,10-perylenetetracarboxylic diimide (common name lumogen orange), a perylene diimide (PDI) with sterically bulky imide functional groups, were investigated using both experimental vibrational spectroscopy and molecular dynamics (MD) simulations. Samples of lumogen orange dispersed in chloroform exhibited complex aggregation behavior, as evidenced by the evolution of the FTIR spectrum over a period of several hours. While for many PDI systems with less bulky imide functional groups aggregation is dominated by π-stacking interactions between perylene cores, MD simulations of lumogen orange dimers indicated a second, more energetically favorable aggregate structure mediated by "edge-to-edge" interactions between PDI units. Two-dimensional infrared spectroscopy together with orientational statistics obtained from MD simulations were employed to identify and rationalize aggregation-induced coupling between vibrational modes.

Phase Acrobatics: The Influence of Excitonic Resonance and Gold Nonresonant Background on Heterodyne-Detected Vibrational Sum Frequency Generation Emission.

We show how heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy can discriminate between the excitonic and monomeric properties of a helical, nanotube molecular aggregate by monitoring the phase of the VSFG emission associated with different polarization configurations. By keeping track of the "phase acrobatics" associated with the added phase of the nonresonant SFG emission of gold as well as that of the double-resonance conditions achieved when the SF frequency is resonant with an electronic exciton transition, we discover that for aggregates of tetra(sulfonatophenyl)porphyrin (TSPP) the PPP-polarized spectra exhibit double-resonance conditions while SSP-polarized spectra exhibit resonance only with the ground-state vibration. Along with observed shifts in the vibrational frequency, intensity differences, and sign flips in the imaginary second-order susceptibility, χs,Im(2), we conclude that PPP-polarized HD-VSFG spectra reflect the delocalized, excitonic nature of the molecular aggregate, while the SSP-polarized HD-VSFG spectra measure the localized, monomeric nature of the molecular subunits. It is implied from this study that HD-VSFG spectroscopy can be uniquely utilized to measure the excitonic and monomeric properties associated with molecular assemblies for a single sample.

Detection of high levels of recombination generated during PCR amplification of RNA templates.

Recombination during the PCR amplification of DNA templates can be a serious problem for those seeking to genotype heterogeneous populations, yet a boon to those seeking to enhance variation during in vitro evolution. Here, the extent to which PCR generates chimeric full-length products was estimated using a powerful restriction fragment-length polymorphism (RFLP) assay involving the use of fluorescently labeled PCR primers. Three different RNA-encoding DNA templates were assayed: (i) one for a group I ribozyme, (ii) one for a 16S ribosomal RNA (rRNA), and (iii) one for a messenger RNA (mRNA). In all cases, the observed frequency of chimeric PCR products exceeded 20%, and longer templates appear to produce more chimeric products. Although two of these templates have the potential to form secondary structures during the PCR, this tendency does not seem to heighten recombination frequency. These results corroborate previous studies that show that the production of chimeras can be best attenuated to a certain extent by varying the extension times in PCR.

DNA vibrational coupling revealed with two-dimensional infrared spectroscopy: insight into why vibrational spectroscopy is sensitive to DNA structure.

Two-dimensional infrared (2D IR) spectroscopy was used to study the carbonyl vibrational modes of guanine and cytosine bases in A- and B-form DNA. Located between 1600 and 1700 cm(-1), these modes are often used to monitor DNA secondary structure with traditional infrared spectroscopies such as FTIR, but traditional spectroscopies lack the necessary observables to unravel the coupling mechanisms that make these modes sensitive to secondary structure. By using 2D IR spectroscopy and electronic structure calculations on d(G(5)C(5)) and d(GC)(8) model nucleic acids, we find that hydrogen-bonded guanine/cytosine base pairs are primarily electrostatically coupled and that the coupling between these modes can be modeled with a transition dipole density approach. In comparison, electrostatics is insufficient to model stacked bases because of cooperative charge-sharing effects, but the coupling can be accurately calculated using a finite difference method. We find that the coupling is very strong for both hydrogen-bonded and stacked base geometries, creating vibrational modes that extend both across the base pairs and along the lengths of the helices. Our results provide a physical basis for understanding how strong coupling gives rise to the empirically established relationship between infrared spectroscopy and DNA/RNA secondary structure.

Interpreting DNA vibrational circular dichroism spectra using a coupling model from two-dimensional infrared spectroscopy.

Two-dimensional infrared spectroscopy was recently used to measure the vibrational couplings between carbonyl bonds located on DNA nucleobases (Krummel, A. T.; Mukherjee, P.; Zanni, M. T. J. Phys. Chem. B 2003, 107, 9165 and Krummel, A. T.; Zanni, M. T. J. Phys. Chem. B 2006, 110, 13991). Here, we extend the coupling model derived from these 2D IR experiments to simulate the vibrational absorption and vibrational circular dichroism (VCD) spectra of three double-stranded DNA oligomers: poly(dG)-poly(dC), poly(dG-dC), and dGGCC. Using this model, we determine that the VCD spectrum of A-form poly(dG)-poly(dC) is dominated by interactions between stacked bases, whereas the coupling between base pairs and stacked bases carries equal importance in the VCD spectrum of B-form poly(dG-dC). We also simulate the absorption and VCD spectra of dGGCC, which is a combination of A- and B-form configurations. These simulations give insight into the structural interpretation of VCD and absorption spectroscopies that have long been used to monitor DNA secondary structure and kinetics.