Conformational heterogeneity of molecules physisorbed on a gold surface at room temperature

A quantitative single-molecule tip-enhanced Raman spectroscopy (TERS) study at room temperature remained a challenge due to the rapid structural dynamics of molecules exposed to air. Here, we demonstrate the hyperspectral TERS imaging of single or a few brilliant cresyl blue (BCB) molecules at room temperature, along with quantitative spectral analyses. Robust chemical imaging is enabled by the freeze-frame approach using a thin Al2O3 capping layer, which suppresses spectral diffusions and inhibits chemical reactions and contamination in air. For the molecules resolved spatially in the TERS image, a clear Raman peak variation up to 7.5 cm(-1) is observed, which cannot be found in molecular ensembles. From density functional theory-based quantitative analyses of the varied TERS peaks, we reveal the conformational heterogeneity at the single-molecule level. This work provides a facile way to investigate the single-molecule properties in interacting media, expanding the scope of single-molecule vibrational spectroscopy studies. Tip-enhanced vibrational spectroscopy at room temperature is complicated by molecular conformational dynamics, photobleaching, contaminations, and chemical reactions in air. This study demonstrates that a sub-nm protective layer of Al2O3 provides robust conditions for probing single-molecule conformations

First-Principle Study of Heterojunction Electronic Structure for Optoelectronic Applications

Department of Chemistryclos

Impact of fluorination on the energy level alignment of an FnZnPc/MAPbI(3) interface

We have studied interactions at an interface between a Methylammonium Lead Iodide (MAPbI(3)) surface and zinc-phthalocyanine molecules with F substituting peripheral H (FnZnPc; n = 4, 8, 12, and 16) by employing hybrid density functional theory (DFT) based simulations. These calculations show that FnZnPc molecules form a stable interface with MAPbI(3), whose binding strength is comparable to that of the un-substituted (ZnPc) case. As a consequence of fluorination, an increase in the ionization potential/electron affinity (i.e., a systematic lowering of molecular energy levels), as well as interfacial charge transfer, is observed whose magnitude depends upon the degree of fluorination. In contrast to the common belief of unfavorable hole transfer for excessive fluorination, our work unveils that the valence band offset remains favorable for all ranges of substitution (n); thus, hole transfer from MAPbI(3) to FnZnPc is facilitated while the electron transfer process is suppressed. This unusual behavior originates from the intermolecular interaction and substrate-to-molecule electron transfer at the heterojunction, which gradually suppresses the downward shift of FnZnPc energy levels by increasing the value of n. Given the beneficial impacts of fluorination, such as hydrophobicity, our work provides valuable insight for exploiting stable FnZnPc towards high-efficiency perovskite solar cells

Stoichiometric Engineering of Cs2AgBiBr6 for Photomultiplication- Type Photodetectors

Photomultiplication (PM)-type photodetectors with a high external quantum efficiency (EQE) can be realized through adequately engineered trap states and trap-assisted charge injection. By strategically introducing slightly rich Bi and highly rich Br stoichiometric conditions, efficient trap states are realized for holes in lead-free Cs1.98AgBi1.15Br7.9 double perovskite (DP). With the diode structure of ITO/SnO2/Cs1.98AgBi1.15Br7.9/poly(3-hexylthiophene) (P3HT)/MoOx/Ag, where SnO2 and P3HT layers are used as the hole-and electron-blocking layers, respectively, successful realization of the selective hole trap and the resulting band bending/electron injection at the anode interface is demonstrated. As a result, a high EQE of similar to 16,000%, responsivity of similar to 50 A W-1, and specific detectivity of over 1012 Jones at -3 V are demonstrated. The origin of the suggested PM mechanism is discussed using photophysical and optoelectronic measurements and theoretical studies. This work ensures the successful demonstration of PM-type photodetectors using lead-free Cs2AgBiBr6 DP through strategic trap engineering

Probing single-molecule conformational heterogeneity at room temperature via hyperspectral tip-enhanced Raman imaging

Cryogenic tip-enhanced Raman spectroscopy (TERS) studies have revealed single-molecule dynamics; however, understanding the nature of single molecules in ambient conditions has remained challenging. Here, we demonstrate the hyperspectral TERS imaging of single brilliant cresyl blue (BCB) molecules, along with quantitative spectral analyses, revealing their conformational heterogeneity in ambient conditions. Robust single-molecule imaging is enabled by encapsulating the molecules with a thin Al$_{2}$O$_{3}$ film, which suppresses spectral diffusions and inhibits chemical reactions and contaminations in air. For the single molecules resolved spatially in the TERS image, a clear Raman peak variation up to ~7.5 cm$^{-1}$ is observed, which cannot be found in molecular ensembles. From density functional theory-based quantitative analyses of the varied TERS peaks of single molecules, we reveal the single-molecule conformational heterogeneity at room temperature. This work provides a facile way to investigate the single-molecule properties in interacting media, expanding the scope of single-molecule vibrational spectroscopy studies.Comment: 23 pages, 5 figure

A Multifunctional Self-Assembled Monolayer for Highly Luminescent Pure-Blue Quasi-2D Perovskite Light-Emitting Diodes

Quasi-2D perovskite materials have promise to unlock the full potential of blue perovskite light-emitting diodes (PeLEDs). However, the efficiency of blue emissive PeLEDs still lags behind the green- and red-emitting counterparts. Here, a multifunctional passivating molecule of (2-(3,6-dichloro-9H-carbazol-9-yl)ethyl)phosphonic acid (36ClCzEPA) that can form a self-assembled monolayer (SAM) on the indium tin oxide (ITO) electrode is reported. The 36ClCzEPA SAM facilitates hole injection by increasing the work function of ITO through the strong interfacial dipole layer formation at the interface between the perovskite emitter and the ITO electrode. Moreover, it allows a pure-blue emission and reduces the exciton quenching of luminescence in the perovskite emitter considerably because of its neutral nature, compared to the commonly used acidic PEDOT:PSS. Furthermore, chlorine atoms in the 36ClCzEPA promote well-ordered crystalline 2D perovskite phases and decrease interfacial trap-assisted deactivation channels by interfacial passivation. These beneficial characteristics of the 36ClCzEPA SAM yield the excellent luminescence property of PeLEDs with a maximum luminance of 1253 cd m(-2) and a peak external quantum efficiency of 4.80% at 473 nm. This work demonstrates that a well-designed molecule forming an interfacial SAM can be an important component for enhancing the luminescence property of pure-blue PeLEDs