Ultrafast vibrational control of organohalide perovskite optoelectronic devices using vibrationally promoted electronic resonance

Vibrational control (VC) of photochemistry through the optical stimulation of structural dynamics is a nascent concept only recently demonstrated for model molecules in solution. Extending VC to state-of-the-art materials may lead to new applications and improved performance for optoelectronic devices. Metal halide perovskites are promising targets for VC due to their mechanical softness and the rich array of vibrational motions of both their inorganic and organic sublattices. Here, we demonstrate the ultrafast VC of FAPbBr3 perovskite solar cells via intramolecular vibrations of the formamidinium cation using spectroscopic techniques based on vibrationally promoted electronic resonance. The observed short (~300 fs) time window of VC highlights the fast dynamics of coupling between the cation and inorganic sublattice. First-principles modelling reveals that this coupling is mediated by hydrogen bonds that modulate both lead halide lattice and electronic states. Cation dynamics modulating this coupling may suppress non-radiative recombination in perovskites, leading to photovoltaics with reduced voltage losses

Enhanced luminescence from electron-hole droplets in silicon nanolayers

We have studied photoluminescence (PL) from the condensed phase in silicon-on-insulator samples with different Si layer thickness from 50 to 340 nm. Two major PL bands are observed at low temperatures, originating from free excitons (FE) and electron–hole droplets (EHD). It is found that with an increase of the excitation intensity the EHD PL shows a linear increase in the 50-nm-thick layer while a superlinear increase in the 340-nm-thick layer. The intensity ratio of the EHD PL to the FE PL in the 50-nm-thick layer is much larger than that in the 340-nm-thick layer under the same experimental conditions. The luminescence from the EHD is enhanced in thin Si nanolayers. These results suggest that highly dense electrons and holes are formed in the Si nanolayer and the interfaces act as the nucleation center of the EHD

Interfacial Water Structure at As-Prepared and UV-Induced Hydrophilic TiO2 Surfaces Studied by Sum Frequency Generation Spectroscopy and Quartz Crystal Microbalance

The interfacial water structures at surfaces of an as-prepared TiO2 and of the TiO2 after UV irradiation were investigated by sum frequency generation (SFG) spectroscopy and quartz crystal microbalance (QCM). It was shown that UV illumination led to an increase in the amount not only of adsorbed water as a whole but also of the ordered adsorbed water on the TiO2 surface, confirming the increase of hydrophilicity of the surface

BROADBAND SUM-FREQUENCY GENERATION SPECTROSCOPY OF HIGH-FREQUENCY VIBRATIONS OF WATER MOLECULES AT SILICA SURFACES

Author Institution: Temple University, Department of Chemistry, 1901 North 13th Street, Philadelphia, Pennsylvania, 19122 USABuilding on our discovery of a method to extend noncollinear optical parametric amplification to a broad class of materials, we developed one of the first sources generating ultrabroadband infrared pulses with bandwidths $\Delta\nu$ $>$ 2500 cm$^{-1}$ in the near-IR ($\lambda$ = 1.1-1.6 $\mu$m) nderline{\textbf{16}}(6), 3949-3954 March 2008.} and $\Delta\nu$ $>$ 1000 cm$^{-1}$ in the mid-IR ($\lambda$ = 1.7-3.5 $\mu$m; $\nu$ = 2800-6000 cm$^{-1}$) nderline{\textbf{20}}(1), 547-561 January 2012.}. The ultra-broadband IR source enabled surface-sensitive sum-frequency generation (SFG) vibrational spectroscopy of mineral-water interfaces crucial in many natural and man-made processes such as ion exchange in geochemical environments and oil extraction from tar sands. This novel ultrabroadband IR source allowed the acquisition of SFG spectra of water OH stretch (spanning ~3000-3800 cm$^{-1}$) from mineral surfaces without tuning the IR frequency, in 60 sec or less. The high signal-to-noise ratio of the broadband-IR SFG setup allowed the extension of SFG spectroscopy of interfacial hydroxyls at mineral/water surfaces to the low cross-section vibrational modes found in the high frequency range (4000-7000 cm$^{-1}$). We performed, what we believe to be, the first surface-specific vibrational SFG spectroscopic measurements of the stretch+bend combination band, $\nu_{comb}$ = $\nu_{OH}$+$\delta_{HOH}$ of liquid water at silica surfaces near 5200 cm$^{-1}$ ).}. SFG of the $\nu_{comb}$ mode allows in-situ probing of surface-bound, e.g., SiOH, and H-OH hydroxyls separately. This provides access to the interfacial water bending mode $\delta$ (near 1600 cm$^{-1}$), which has not been observed directly in SFG

Unified picture of vibrational relaxation of OH stretch at the air/water interface

Abstract The elucidation of the energy dissipation process is crucial for understanding various phenomena occurring in nature. Yet, the vibrational relaxation and its timescale at the water interface, where the hydrogen-bonding network is truncated, are not well understood and are still under debate. In the present study, we focus on the OH stretch of interfacial water at the air/water interface and investigate its vibrational relaxation by femtosecond time-resolved, heterodyne-detected vibrational sum-frequency generation (TR-HD-VSFG) spectroscopy. The temporal change of the vibrationally excited hydrogen-bonded (HB) OH stretch band (ν=1→2 transition) is measured, enabling us to determine reliable vibrational relaxation (T1) time. The T1 times obtained with direct excitations of HB OH stretch are 0.2-0.4 ps, which are similar to the T1 time in bulk water and do not noticeably change with the excitation frequency. It suggests that vibrational relaxation of the interfacial HB OH proceeds predominantly with the intramolecular relaxation mechanism as in the case of bulk water. The delayed rise and following decay of the excited-state HB OH band are observed with excitation of free OH stretch, indicating conversion from excited free OH to excited HB OH (~0.9 ps) followed by relaxation to low-frequency vibrations (~0.3 ps). This study provides a complete set of the T1 time of the interfacial OH stretch and presents a unified picture of its vibrational relaxation at the air/water interface

Reorientation-induced relaxation of free OH at the air/water interface revealed by ultrafast heterodyne-detected nonlinear spectroscopy

Water’s hydrogen-bond network is truncated at hydrophobic interfaces and the dynamics of the resulting free OH groups is not well understood. The authors experimentally show that the main vibrational relaxation mechanism for free OH at the air-water interface is a diffusive molecular reorientation, rather than intramolecular energy transfer