Herzberg–Teller Effect in Single-Crystalline Hexacene at Finite Temperatures

The nuclear dependency of electronic wave function upon optical excitation is encoded in the line shape of light absorption spectrum. Experimentally, we measured the electronic spectra of single-crystalline hexacene at 77, 185, and 293 K using polarized visible–near -IR microscopy. Through a comparison with our simulated vibrationally resolved absorption spectrum, the Herzberg–Teller (HT) coupling was found to be the dominant contributor to all spectroscopic features, except for the lowest excitation peak when the incident photon is polarized along crystal axis b. With the aim to explore the evolution of the absorption spectrum upon temperature change, the quasi-harmonic approximation was first employed to calculate the thermal expansion coefficient of the hexacene unit cell before its volumes at 77, 185, and 293 K were determined by Helmholtz free energy minimization. Then, at each temperature, the unit cell’s phonon modes were ascertained at the density functional theory (DFT) level while its electronic transition dipole moments and their nuclear gradients were evaluated by quasi-particle GW method in conjunction with the Bethe–Salpeter equation (BSE) approach. Finally, the HT vibronic theory was utilized to afford the full absorption spectrum by considering the Franck–Condon factor (FC), FC/HT interference, and HT coupling. Our proposed computational framework was justified by the satisfactory agreement between our theoretical and experimental spectra, making it a reliable tool to investigate the delicate vibronic coupling by examining temperature-dependent light absorption spectrum

<i>In Situ</i> Detection of Chemical Compositions at Nanodroplet Surfaces and In-Nanodroplet Phases

Small-volume nanodroplets play an increasingly common role in chemistry and biology. Such nanodroplets are believed to have unique chemical and physical properties at the interface between a droplet and its surrounding medium, however, they are underexamined. In this study, we present the novel technique of vibrational sum frequency scattering (VSFS) spectroscopy as an interface-specific, high-performance method for the in situ investigation of nanodroplets with sub-micron radii; as well as the droplet bulk through simultaneous hyper-Raman scattering (HRS) spectroscopy. We use laboratory-generated nanodroplets from aqueous alcohol solutions to demonstrate this technique’s ability to separate the vibrational phenomena which take place at droplet surfaces from the underlying bulk phase. In addition, we systemically examine interfacial spectra of nanodroplets containing methanol, ethanol, 1-propanol, and 1-butanol through VSFS. Furthermore, we demonstrate interfacial differences between such nanodroplets and their analogous planar surfaces. The sensitivity of this technique to probe droplet surfaces with few-particle density at standard conditions validates VSFS as an analytical technique for the in situ investigation of small nanodroplets, providing breakthrough information about these species of ever-increasing relevance

Spectral Phase Measurements of Heterodyne Detection in Interfacial Broadband Electronic Spectroscopy

A spectral phase is vital to retrieve both the real and imaginary parts of both second-order vibrational and electronic sum frequency generation spectra. Despite recent efforts, phase measurements in electronic spectra at interfaces are lacking. In particular, references are not well established for frequency-dependent spectral phases in interfacial electronic spectroscopy. In this work, we present broadband heterodyne detection (HD) of interfacial electronic spectroscopy for spectral phases of references. Such a broadband HD method was based upon a broadband short-wave IR laser source from 1100 to 2200 nm. A collinear geometry method was adopted for passive phase stability. We further compared often-used reference samples, including gold (Au) thin films, silver (Ag) thin films, p-type GaP (100), p-type GaAs (100), as well as left-handed and right-handed z-cut α-quartz crystals. The phases of the second-order susceptibilities for Ag thin films, the GaP crystal, the GaAs crystal, and the quartz crystals are independent of frequency from 630 to 730 nm. More importantly, it was found that the often-used Au thin film in the literature is not a good phase reference for frequency-dependent phases

Surface States for Photoelectrodes of Gallium Phosphide (GaP) with Surface-Specific Electronic Spectra and Phase Measurements

Gallium phosphide (GaP) photoelectrodes have received tremendous attention owing to their applications in photocatalysis and photoelectrocatalytic reduction of CO2. Surface electronic states of GaP are important in such catalysis applications. However, knowledge of surface states of GaP under ambient conditions is lacking. Here, we combined azimuth-dependent electronic sum-frequency generation (ESFG) spectroscopy with phase measurements to investigate the surface states for n-type and p-type GaP(100) semiconductors. ESFG spectroscopic studies enabled us to identify three surface states of the GaP crystals under ambient conditions. These experiments have also shown that all of the spectral features come from surface contributions for both the n-type and p-type GaP(100) crystals and that both surface dipoles and surface charges were responsible for the electronic transitions of isotropic and anisotropic components. Combined with azimuth-dependent phase measurements, surface charges were found to account for the isotropic surface ESFG components: negative for n-type and positive for p-type GaP(100). Finally, we conducted a thorough theoretical analysis of surface and bulk contributions for azimuth-dependent ESFG responses. With these spectral and phase signatures, we have further quantified surface and bulk contributions along different orientations for the n-type and p-type GaP(100) crystals

Photoinduced Surface Electric Fields and Surface Population Dynamics of GaP(100) Photoelectrodes

Gallium phosphide (GaP) photoelectrodes have received remarkable focus due to their applications in photocatalysis and photoelectrocatalysis of CO2 reduction reactions. Understanding the dynamical mechanisms of surfaces of photoelectrodes is essential in improving their working efficiencies in any application. However, knowledge of photoinduced surface dynamics of these materials is lacking. Here, we investigate surface dynamics of n-type and p-type GaP(100) semiconductors by utilizing time-resolved electronic sum frequency generation (TR-ESFG). Transient ESFG spectra showed that four surface states in both n- and p-type GaP(100) were involved in subsequent kinetics. Transient spectral signatures of the surface states showed that photoexcited electrons move toward the surface regions for p-type GaP, while photoexcited holes move to the surface regions for n-type GaP. These carriers first build up surface electric fields, resulting in fluence-dependent band flattening. The buildup rates of the surface electric fields were found to be on the order of 2.86 ± 0.30 ps–1 for n-type and 2.50 ± 0.25 ps–1 for p-type. Subsequently, a relatively slow process occurs, being attributed to population dynamics of surface states dependent upon applied fluences. We found that surface population behaves as a bimolecular process with rates of 0.020 ± 0.002 cm2 s–1 for n-type and 0.035 ± 0.002 cm2 s–1 for p-type GaP. The four surface states, shallow and deep for both n- and p-type GaP(100), were found to be involved in both surface electric fields and surface carrier populations, contrary to previous hypotheses. Our time-resolved surface-specific approach provides unique information on surface dynamical behaviors of photoelectrodes under ambient conditions

Interface Catalysis of Nickel Molybdenum (NiMo) Alloys on Two-Dimensional (2D) MXene for Enhanced Hydrogen Electrochemistry

Development of efficient bifunctional nonprecious metallic electrocatalysts for hydrogen electrochemistry in alkaline solution is of importance to enable commercialization of a low-cost alkaline hydrogen fuel cell and water electrolyzer, but it is very challenging. Two-dimensional (2D) MXene-based electrocatalysts hold tremendous potential for the applications of hydrogen fuel cell and water electrolyzer. Here, we successfully immobilized transition-metal-based NiMo nanoparticles (NPs) on 2D Ti3C2Tx (Tx: surface terminations, such as O, OH, or F) surfaces by a wet chemical method. Our results demonstrate that the NiMo NPs are monodispersed on Ti3C2Tx with surface functionalization. These monodisperse NPs resulted in superior hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) activities in an alkaline media. The NiMo NPs/Ti3C2Tx in 1.0 M KOH yielded an HER current of −10 mA cm–2 at −0.044 V vs reversible hydrogen electrode (RHE), nearly 232 mV smaller than that of the parent NiMo NPs. The NiMo NPs/Ti3C2Tx produced an HOR current density of 1.5 mA cm–2 at 0.1 V vs RHE. Density functional theory (DFT) results further reveal that Ti3C2Tx support can facilitate the charge transfer to metallic NPs and tailor the electronic structure of catalytic sites, resulting in optimized adsorption free energies of H* species for hydrogen electrochemistry. This work provides a facile and universal strategy in the development of 2D Ti3C2Tx with nonprecious metals for low-cost bifunctional hydrogen electrocatalysts

Development of Two-Dimensional Electronic-Vibrational Sum Frequency Generation (2D-EVSFG) for Vibronic and Solvent Couplings of Molecules at Interfaces and Surfaces

Many photoinduced excited states’ relaxation processes and chemical reactions occur at interfaces and surfaces, including charge transfer, energy transfer, proton transfer, proton-coupled electron transfer, configurational dynamics, conical intersections, etc. Of them, interactions of electronic and vibrational motions, namely, vibronic couplings, are the main determining factors for the relaxation processes or reaction pathways. However, time-resolved electronic-vibrational spectroscopy for interfaces and surfaces is lacking. Here we develop interface/surface-specific two-dimensional electronic-vibrational sum frequency generation spectroscopy (2D-EVSFG) for time-dependent vibronic coupling of excited states at interfaces and surfaces. We further demonstrate the fourth-order technique by investigating vibronic coupling, solvent correlation, and time evolution of the coupling for photoexcited interface-active molecules, crystal violet (CV), at the air/water interface as an example. The two vibronic absorption peaks for CV molecules at the interface from the 2D-EVSFG experiments were found to be more prominent than their counterparts in bulk from 2D-EV. Quantitative analysis of the vibronic peaks in 2D-EVSFG suggested that a non-Condon process participates in the photoexcitation of CV at the interface. We further reveal vibrational solvent coupling for the zeroth level on the electronic state with respect to that on the ground state, which is directly related to the magnitude of its change in solvent reorganization energy. The change in the solvent reorganization energy at the interface is much smaller than that in bulk methanol. Time-dependent center line slopes (CLSs) of 2D-EVSFG also showed that kinetic behaviors of CV at the air/water interface are significantly different from those in bulk methanol. Our ultrafast 2D-EVSFG experiments not only offer vibrational information on both excited states and the ground state as compared with the traditional doubly resonant sum frequency generation and electronic-vibrational coupling but also provide vibronic coupling, dynamical solvent effects, and time evolution of vibronic coupling at interfaces

Temperature-Dependent Recombination of Triplet Biexcitons in Singlet Fission of Hexacene

Singlet fission is a spin-conserving process for the multiplication conversion of one singlet exciton into two individual triplet excitons by absorbing one photon. Such a multiplication is believed to circumvent the Shockley–Queisser thermodynamic limit for improving efficiency of solar energy conversion. A mechanistic understanding of generation and yields of triplet excitons from singlet fission materials is essential for efficient exploitation of solar energy. Here we employ temperature-dependent transient absorption spectroscopy to examine the dynamical nature of singlet fission and triplet excitons in hexacene. The generation and dissociation rates of the intermediate correlated biexciton, 1(TT), are independent of temperature from 77 K to the room temperature. On the other hand, the triplet excitons in spatially separated biexcitons, 1(T···T), relax via geminate and nongeminate recombination. The former was found to be temperature-dependent, whereas the latter is temperature-independent. Quantitative analyses of the temperate-dependent rates for the two recombination processes yield the energy difference between the 1(T···T) and 1(TT), which were further confirmed by our density functional theory (DFT) calculations