Charge reorganization energy and small polaron binding energy of rubrene thin films by ultraviolet photoelectron spectroscopy

Interlayer Charge Transport in 2D Lead Halide Perovskites from First Principles

We report on the implementation of a versatile projection-operator diabatization approach to calculate electronic coupling integrals in layered periodic systems. The approach is applied to model charge transport across the saturated organic spacers in two-dimensional (2D) lead halide perovskites. The calculations yield out-of-plane charge transfer rates that decay exponentially with the increasing length of the alkyl chain, range from a few nanoseconds to milliseconds, and are supportive of a hopping mechanism. Most importantly, we show that the charge carriers strongly couple to distortions of the Pb-I framework and that accounting for the associated nonlocal dynamic disorder increases the thermally averaged interlayer rates by a few orders of magnitude compared to the frozen-ion 0 K-optimized structure. Our formalism provides the first comprehensive insight into the role of the organic spacer cation on vertical transport in 2D lead halide perovskites and can be readily extended to functional π-conjugated spacers, where we expect the improved energy alignment with the inorganic layout to speed up the charge transfer between the semiconducting layers.

Thermal and electrical cross-plane conductivity at the nanoscale in poly(3,4-ethylenedioxythiophene):trifluoromethanesulfonate thin films

Cross-plane electrical and thermal transport in thin films of a conducting polymer (poly(3,4-ethylenedioxythiophene), PEDOT) stabilized with trifluoromethanesulfonate (OTf) is investigated in this study. We explore their electrical properties by conductive atomic force microscopy (C-AFM), which reveals the presence of highly conductive nano-domains. Thermal conductivity in the cross-plane direction is measured by null-point scanning thermal microscopy (NP-SThM). PEDOT:OTf indeed demonstrates a non-negligible electronic contribution to the thermal transport. We further investigate the correlation between electrical and thermal conductivity by applying post-treatment: chemical reduction (de-doping) to lower charge carrier concentration and hence, electrical conductivity and acid treatment (over-doping) to increase the latter. From our measurements, we find a vibrational thermal conductivity of 0.34 ± 0.04 W m^-1 K^-1. From the linear dependence or the electronic contribution of thermal conductivity vs. the electronic conductivity (Wiedemann-Franz law), we infer a Lorenz number 6 times larger than the classical Sommerfeld value as also observed in many organic materials for in-plane thermal transport. By applying the recently proposed molecular Wiedemann-Franz law, we deduced a reorganization energy of 0.53 ± 0.06 eV.

Reorganization Energy upon Controlled Intermolecular Charge-Transfer Reactions in Monolithically Integrated Nanodevices

Intermolecular electron-transfer reactions are key processes in physics, chemistry, and biology. The electron-transfer rates depend primarily on the system reorganization energy, that is, the energetic cost to rearrange each reactant and its surrounding environment when a charge is transferred. Despite the evident impact of electron-transfer reactions on charge-carrier hopping, well-controlled electronic transport measurements using monolithically integrated electrochemical devices have not successfully measured the reorganization energies to this date. Here, it is shown that self-rolling nanomembrane devices with strain-engineered mechanical properties, on-a-chip monolithic integration, and multi-environment operation features can overcome this challenge. The ongoing advances in nanomembrane-origami technology allow to manufacture the nCap, a nanocapacitor platform, to perform molecular-level charge transport characterization. Thereby, employing nCap, the copper-phthalocyanine (CuPc) reorganization energy is probed, ≈0.93 eV, from temperature-dependent measurements of CuPc nanometer-thick films. Supporting the experimental findings, density functional theory calculations provide the atomistic picture of the measured CuPc charge-transfer reaction. The experimental strategy demonstrated here is a consistent route towards determining the reorganization energy of a system formed by molecules monolithically integrated into electrochemical nanodevices.

Polaron Diffusion in Pentathienoacene Crystals

Molecular crystals have been used as prototypes for studying the energetic and dynamic properties of charge carriers in organic electronics. The growing interest in oligoacenes and fused-ring oligothiophenes in the last two decades is due, in particular, to the success achieved in conceiving pentacene-based organic photovoltaic devices. In the present work, a one-dimensional Holstein-Peierls model is designed to study the temperature-dependent polaron transport in pentathienoacene (PTA) lattices. The tight-binding Hamiltonian employed here takes into account intra and intermolecular electron-lattice interactions. Results reveal that polarons in PTAs can be stable structures even at high temperatures, about 400 K. During the dynamical process, these charge carriers present a typical 1D random walk diffusive motion with a low activation energy of 13 meV and a room temperature diffusivity constant of 1.07 × 10⁻³ cm² s⁻¹. Importantly, these critical values for the polaron diffusion and activation energy are related to the choice of model parameters, which are adopted to describe pristine lattices.

Electronic and Crystallographic Examinations of the Homoepitaxially Grown Rubrene Single Crystals

Homoepitaxial growth of organic semiconductor single crystals is a promising methodology toward the establishment of doping technology for organic opto-electronic applications. In this study, both electronic and crystallographic properties of homoepitaxially grown single crystals of rubrene were accurately examined. Undistorted lattice structures of homoepitaxial rubrene were confirmed by high-resolution analyses of grazing-incidence X-ray diffraction (GIXD) using synchrotron radiation. Upon bulk doping of acceptor molecules into the homoepitaxial single crystals of rubrene, highly sensitive photoelectron yield spectroscopy (PYS) measurements unveiled a transition of the electronic states, from induction of hole states at the valence band maximum at an adequate doping ratio (10 ppm), to disturbance of the valence band itself for excessive ratios (≥ 1000 ppm), probably due to the lattice distortion.

Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?

The field of organic electronics has been prolific in the last couple of years, leading to the design and synthesis of several molecular semiconductors presenting a mobility in excess of 10 cm² V^-1 s^-1 . However, it is also started to recently falter, as a result of doubtful mobility extractions and reduced industrial interest. This critical review addresses the community of chemists and materials scientists to share with it a critical analysis of the best performing molecular semiconductors and of the inherent charge transport physics that takes place in them. The goal is to inspire chemists and materials scientists and to give them hope that the field of molecular semiconductors for logic operations is not engaged into a dead end. To the contrary, it offers plenty of research opportunities in materials chemistry.

The role of initial and final states in molecular spectroscopies

Interpreting experimental spectra of thin films of organic semiconductors is challenging, and understanding the relationship between experimental data obtained by different spectroscopic techniques requires a careful consideration of the initial and final states for each process.

Reorganization energy upon charging a single molecule on an insulator measured by atomic force microscopy

Intermolecular single-electron transfer on electrically insulating films is a key process in molecular electronics^1-4 and an important example of a redox reaction^5,6. Electron-transfer rates in molecular systems depend on a few fundamental parameters, such as interadsorbate distance, temperature and, in particular, the Marcus reorganization energy ⁷ . This crucial parameter is the energy gain that results from the distortion of the equilibrium nuclear geometry in the molecule and its environment on charging^8,9. The substrate, especially ionic films ¹⁰ , can have an important influence on the reorganization energy^11,12. Reorganization energies are measured in electrochemistry ¹³ as well as with optical^14,15 and photoemission spectroscopies^16,17, but not at the single-molecule limit and nor on insulating surfaces. Atomic force microscopy (AFM), with single-charge sensitivity^18-22, atomic-scale spatial resolution ²⁰ and operable on insulating films, overcomes these challenges. Here, we investigate redox reactions of single naphthalocyanine (NPc) molecules on multilayered NaCl films. Employing the atomic force microscope as an ultralow current meter allows us to measure the differential conductance related to transitions between two charge states in both directions. Thereby, the reorganization energy of NPc on NaCl is determined as (0.8 ± 0.2) eV, and density functional theory (DFT) calculations provide the atomistic picture of the nuclear relaxations on charging. Our approach presents a route to perform tunnelling spectroscopy of single adsorbates on insulating substrates and provides insight into single-electron intermolecular transport.

Enantiomer surface chemistry: conglomerate versus racemate formation on surfaces

Research on surface chirality is motivated by the need to develop functional chiral surfaces for enantiospecific applications. While molecular chirality in 3D has been the subject of study for almost two centuries, many aspects of 2D chiral surface chemistry have yet to be addressed. In 3D, racemic mixtures of chiral molecules tend to aggregate into racemate (molecularly heterochiral) crystals much more frequently than conglomerate (molecularly homochiral) crystals. Whether chiral adsorbates on surfaces preferentially aggregate into heterochiral rather than homochiral domains (2D crystals or clusters) is not known. In this review, we have made the first attempt to answer the following question based on available data: in 2D racemic mixtures adsorbed on surfaces, is there a clear preference for homochiral or heterochiral aggregation? The current hypothesis is that homochiral packing is preferred on surfaces; in contrast to 3D where heterochiral packing is more common. In this review, we present a simple hierarchical scheme to categorize the chirality of adsorbate-surface systems. We then review the body of work using scanning tunneling microscopy predominantly to study aggregation of racemic adsorbates. Our analysis of the existing literature suggests that there is no clear evidence of any preference for either homochiral or heterochiral aggregation at the molecular level by chiral and prochiral adsorbates on surfaces.

Hole-phonon coupling effect on the band dispersion of organic molecular semiconductors

The dynamic interaction between the traveling charges and the molecular vibrations is critical for the charge transport in organic semiconductors. However, a direct evidence of the expected impact of the charge-phonon coupling on the band dispersion of organic semiconductors is yet to be provided. Here, we report on the electronic properties of rubrene single crystal as investigated by angle resolved ultraviolet photoelectron spectroscopy. A gap opening and kink-like features in the rubrene electronic band dispersion are observed. In particular, the latter results in a large enhancement of the hole effective mass (> 1.4), well above the limit of the theoretical estimations. The results are consistent with the expected modifications of the band structures in organic semiconductors as introduced by hole-phonon coupling effects and represent an important experimental step toward the understanding of the charge localization phenomena in organic materials.The charge transport properties in organic semiconductors are affected by the impact of molecular vibrations, yet it has been challenging to quantify them to date. Here, Bussolotti et al. provide direct experimental evidence on the band dispersion modified by molecular vibrations in a rubrene single crystal.

Strong Metal-Phosphide Interactions in Core-Shell Geometry for Enhanced Electrocatalysis

Rational design of multicomponent material structures with strong interfacial interactions enabling enhanced electrocatalysis represents an attractive but underdeveloped paradigm for creating better catalysts for important electrochemical energy conversion reactions. In this work, we report metal-phosphide core-shell nanostructures as a new model electrocatalyst material system where the surface electronic states of the shell phosphide and its interactions with reaction intermediates can be effectively influenced by the core metal to achieve higher catalytic activity. The strategy is demonstrated by the design and synthesis of iron-iron phosphide (Fe@FeP) core-shell nanoparticles on carbon nanotubes (CNTs) where we find that the electronic interactions between the metal and the phosphide components increase the binding strength of hydrogen adatoms toward the optimum. As a consequence, the Fe@FeP/CNT material exhibits exceptional catalytic activity for the hydrogen evolution reaction, only requiring overpotentials of 53-110 mV to reach catalytic current densities of 10-100 mA cm^-2.

Role of molecular conformations in rubrene polycrystalline films growth from vacuum deposition at various substrate temperatures

We report on the optical and structural characterization of rubrene polycrystalline films fabricated from vacuum deposition with various substrate temperatures (Tsub). Depending on Tsub, the role of twisted and planar rubrene conformational isomers on the properties of rubrene films is focused. The temperature (T)-dependent inverse optical transmission (IOT) and photoluminescence (PL) spectra were performed on these rubrene films. The origins of these IOT and PL peaks are explained in terms of the features from twisted and planar rubrene molecules and of the band characteristics from rubrene molecular solid films. Here, two rarely reported weak-peaks at 2.431 and 2.605 eV were observed from IOT spectra, which are associated with planar rubrene. Besides, the T-dependence of optical bandgap deduced from IOT spectra is discussed with respect to Tsub. Together with IOT and PL spectra, for Tsub > 170 °C, the changes in surface morphology and unit cell volume were observed for the first time, and are attributed to the isomeric transformation from twisted to planar rubrenes during the deposition processes. Furthermore, a unified schematic diagram in terms of Frenkel exciton recombination is suggested to explain the origins of the dominant PL peaks performed on these rubrene films at 15 K.

Structure–property relationship of isomeric diphenylethenyl-disubstituted dimethoxycarbazoles

Isomeric 3,6-dimethoxy- and 2,7-dimethoxycarbazoles containing diphenylethenyl moieties were synthesized by condensation of the appropriate dimethoxycarbazoles with diphenylacetaldehyde.

Polaron stability in molecular semiconductors: theoretical insight into the impact of the temperature, electric field and the system dimensionality

A semi-empirical Holstein–Peierls model is used to study the temperature effects on the polaron stability in organic semiconductors at a molecular scale.

Accessing surface Brillouin zone and band structure of picene single crystals

We have experimentally revealed the band structure and the surface Brillouin zone of insulating picene single crystals (SCs), the mother organic system for a recently discovered aromatic superconductor, with ultraviolet photoelectron spectroscopy (UPS) and low-energy electron diffraction with a laser for photoconduction. A hole effective mass of 2.24m(0) and the hole mobility μ(h)≥9.0 cm(2)/V s (298 K) were deduced in the Γ-Y direction. We have further shown that some picene SCs did not show charging during UPS even without the laser, which indicates that pristine UPS works for high-quality organic SCs.