The Brownian Castle is a new interface growth model that is a variation on the well-known ballistic deposition model that results in an entirely new universality class. We present numerical verification that the interface width for BC interfaces displays modified Family-Vicsek scaling properties up to finite size corrections. Specifically, we find a growth exponent of β=0.470 ± 0.012 and a roughness exponent of α=1.01 ± 0.018. The scaling is modified at short times with a size scaling exponent that described the early time dependence of interface width on length. The probability distribution of heights for the BC interface shows significant deviations from simple Gaussian behavior and the probability distribution of height changes shows a Cauchy-Lorentz form consistent with expectations for a process involving relatively large jumps.

Bulk heterojunction (BHJ) organic solar cells have made remarkable inroads toward 20% power conversion efficiency, yet non-radiative recombination losses (DVnr) remain high. Here, we spatially map the energetic landscape of BHJs and ascribe charge transfer (CT) states to each interface, revealing where non-radiative recombination losses occur. We do so by locally mapping the energy distributions of modern PM6-based BHJs using scanning tunneling microscopy (STM) in combination with sensitive external quantum efficiency (s-EQE) analysis. The non-radiative energy losses are dictated by a combination of the singlet (S1) to CT energy offset (DES1-CT) and the interfacial energetic disorder. PM6:Y6 achieves low DVnr by forming a sharp donor/acceptor (D/A) interface with low interfacial disorder that can be tuned by judicious formulation and processing of the BHJ. The emerging design rule for low DVnr in modern non-fullerene acceptors (NFAs) is to achieve sharp D/A interfaces with minimized DES1-CT and low interfacial electronic disorder of both D and A components.

The layered van der Waals material CrCl3 exhibits very strongly bound ligand field excitons that control optoelectronic applications and are connected with magnetic ordering by virtue of their d-orbital origin. Time-resolved photoluminescence of these exciton populations at room temperature shows that their relaxation is dominated by exciton-exciton annihilation and that the spontaneous decay lifetime is very long. These observations allow the rough quantification of the exciton annihilation rate constant and contextualization in light of a recent theory of universal scaling behavior of the annihilation process.

Understanding charge transport in conjugated polymers is crucial for the development of next-generation organic electronic applications. It is presumed that structural disorder in conjugated polymers originating from their semicrystallinity, processing, or polymorphism leads to a complex energetic landscape that influences charge carrier transport properties. However, the link between polymer order parameters and energetic landscape is not well established experimentally. In this work, we successfully link statistical surveys of the local polymer electronic structure with paracrystalline structural disorder, a measure of statistical fluctuations away from the ideal polymer packing structure. We use scanning tunneling microscopy/spectroscopy to measure spatial variability in electronic band edges in PM6 films, a high-performance conjugated polymer, and find that films with higher paracrystallinity exhibit greater electronic disorder, as expected. In addition, we show that macroscopic charge carrier mobility in field effect transistors and and trap influence in hole-only diode devices is positively correlated with these microscopic structural and electronic parameters.

The Mott insulating state is the progenitor of many interesting quantum phases of matter including the famous high-temperature superconductors and quantum spin liquids. A recent candidate for novel spin liquid phenomena is alpha-RuCl3, a layered honeycomb Mott insulator whose electronic structure has been a source of mystery. In particular, scanning tunneling spectroscopy has indicated a Mott gap in alpha-RuCl3 that is much lower than the 2-eV value observed in photoemission measurements. Here, we show that the origin of this discrepancy is a spreading resistance artifact associated with tunneling into highly resistive materials by comparing with prior experiments and numerical modeling. A similar phenomenon is also observed in a substitutional alloy, Ir0.5Ru0.5Cl3, that has a higher resistivity than the parent compound. While the tunneling measurements cannot be used to accurately measure the sample density of states for these materials, we can take advantage of the spreading resistance sensitivity to quantify the anisotropic resistivity of these layered materials and connect to previous macroscopic transport observations.

The Mott insulating state is the progenitor of many interesting quantum phases of matter including the famous high-temperature superconductors and quantum spin liquids. A recent candidate for novel spin liquid phenomena is $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$, a layered honeycomb Mott insulator whose electronic structure has been a source of mystery. In particular, scanning tunneling spectroscopy has indicated a Mott gap in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ that is much lower than the 2-eV value observed in photoemission measurements. Here, we show that the origin of this discrepancy is a spreading resistance artifact associated with tunneling into highly resistive materials by comparing with prior experiments and numerical modeling. A similar phenomenon is also observed in a substitutional alloy, ${\mathrm{Ir}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{Cl}}_{3}$, that has a higher resistivity than the parent compound. While the tunneling measurements cannot be used to accurately measure the sample density of states for these materials, we can take advantage of the spreading resistance sensitivity to quantify the anisotropic resistivity of these layered materials and connect to previous macroscopic transport observations.

The nonequilibrium properties of strongly correlated materials present a target in the search for new phases of matter. It is important to observe the types of excitations that exist in these materials and their associated relaxation dynamics. We have studied the photoexcitations in a spin-orbit assisted Mott insulator alpha-RuCl3 using time-resolved two-photon photoemission spectroscopy and transient reflection spectroscopy. We find that photoexcited carriers (doublons) in the upper Hubbard band rapidly relax to Mott-Hubbard excitons on a timescale of less than 200 fs. Subsequently, further relaxation of these lower-energy quasiparticles occurs with an energy-dependent time constant of that ranges from 370 to 600 fs due to exciton cooling. The population of Mott-Hubbard excitons persists for timescales up to several microseconds.

Perovskite light-emitting diodes (PeLEDs) have received great attention for their potential as next-generation display technology. While remarkable progress has been achieved in green, red, and near-infrared PeLEDs with external quantum efficiencies (EQEs) exceeding 20%, obtaining high performance blue PeLEDs remains a challenge. Poor charge balance due to large charge injection barriers in blue PeLEDs has been identified as one of the major roadblocks to achieve high efficiency. Here band edge control of perovskite emitting layers for blue PeLEDs with enhanced charge balance and device performance is reported. By using organic spacer cations with different dipole moments, that is, phenethyl ammonium (PEA), methoxy phenethyl ammonium (MePEA), and 4-fluoro phenethyl ammonium (4FPEA), the band edges of quasi-2D perovskites are tuned without affecting their band gaps. Detailed characterization and computational studies have confirmed the effect of dipole moment modification to be mostly electrostatic, resulting in changes in the ionization energies of approximate to 0.45 eV for MePEA and approximate to -0.65 eV for 4FPEA based thin films relative to PEA-based thin films. With improved charge balance, blue PeLEDs based on MePEA quasi-2D perovskites show twofold increase of the EQE as compared to the control PEA based devices.

Domain boundaries are a determining factor in the performance of organic electronic devices since they can trap mobile charge carriers. We point out the possibility of time-dependent motion of these boundaries and suggest that their thermal fluctuations can be a source of dynamic disorder in organic films. In particular, we study the C8-BTBT monolayer films with several different domain boundaries. After characterizing the crystallography and diversity of structures in the first layer of C8-BTBT on Au(111), we focus on quantifying the domain boundary fluctuations in the saturated monolayer. We find that the mean squared displacement of the boundary position grows linearly with time at early times but tends to saturate after about 7 s. This behavior is ascribed to confined diffusion of the interface position based on fits and numerical integration of a Langevin equation for the interface motion.

Memristor devices have history-dependent charge transport properties that are ideal for neuromorphic computing applications. We reveal a memristor material and mechanism in the layered Mott insulator α-RuCl3. The pinched hysteresis loops and S-shaped negative differential resistance in bulk crystals verify memristor behavior and are attributed to a nonlinear coupling between charge injection over a Schottky barrier at the electrical contacts and concurrent Joule heating. Direct simulations of this coupling can reproduce the device characteristics.

The final report summarizes advances in the use of spin polarized scanning tunneling microscopy to measure the electronic properties on individual organic molecules on magnetic substrates. These experiments led to a new and simple conceptual paradigm for understanding these magnet-molecule interfaces using ideas and models that have historically been applied to catalysis and surface chemistry. It further summarizes related advances in the scanning tunneling microscopy studies of molecules on topological insulator surfaces and the search for new spin liquid physics. These experiments were designed to push the boundaries of spintronics technology and quantum materials design.

Organic light-emitting diodes are important in display applications, but thin films used in these devices often exhibit complex and highly disordered structures. We have studied the adsorption of a typical hole transport material used in such devices, N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (α-NPD), on the Au(111) surface. Scanning tunneling microscopy images reveal the appearance of different conformations in the first monolayer with submolecular resolution. Scanning tunneling spectra identify the highest occupied molecular orbital on several different adsorption structures. We directly compare the statistical distribution of this orbital energy between an ordered monolayer structure and a disordered bilayer structure of α-NPD on Au(111). The disordered structure exhibits a very broad distribution that is consistent with inferences from prior organic device studies and that we propose arises from minor conformational variations.

Organic semiconductors are prone to strong disorder effects that often exhibit significant dynamic characteristics. In this study, static disorder and dynamic disorder of fullerene molecules at the interface of Au(111) are directly distinguished using Scanning Tunneling Microscopy (STM) and Scanning Tunneling Spectroscopy. We consider an STM image consisting of hundreds of different molecules, as an ensemble which exhibits conductance fluctuations due to both dynamic disorder and static disorder. On the other hand, local conductance measurement of single molecules over time reflects only dynamic disorder. We demonstrate that dynamic disorder is always smaller than static disorder at the C60/Au(111) interface due to structural constraints of molecules at the interface. Dynamic disorder in our experiment is due to small librations of individual molecules that are restricted by the metal surface, while static disorder is related to different bonding orientations of the C60 cage that are frozen-in at room temperature. Our experimental results can be modeled with direct simulations of differential tunneling conductance.

In organic electronic materials, charge carrier transport is often limited by disorder‐induced trap states very close in energy to the ideal band transport states. We directly view the location and impact of these “shallow” traps on an ultrathin transistor active layer using Kelvin Probe Force Microscopy. As the transistor turns on, dramatic fluctuations in the surface potential of the active channel suddenly arise due to charge trapping and release processes. Importantly, the spatial distribution of rapid fluctuations in surface potential is uniform throughout the active channel. These facts strongly constrain the microscopic origin of shallow charge traps, and associated efforts to optimize the mobility and noise performance baseline in device applications.

Adsorption of the acceptor material tetracyanoquinodimethane can control optoelectronic properties of MoS2 by accepting defect generated excess negative charge from the surface that would otherwise interfere with radiative decay processes. Angle Resolved Photoelectron Spectroscopy measurements show that the MoS2 band structure near the Gamma point shifts rigidly upward by similar to 0.2 eV for a complete surface coverage of acceptor species as expected for an upward Fermi level shift due to charge transfer to the TCNQ. The molecular adsorbate orbitals visible in photoemission are indicative of an anionic species, consistent with interfacial charge transfer but without evidence for hybrid states arising from covalent adsorbate-surface interactions. Thus, our interface studies support the notion that molecular adsorbates are a useful tool for controlling optoelectronic functionality in 2D materials without fundamentally modifying their favorable band structures.

The effect of growth conditions on the structural and electronic properties of the polar/non-polar LaCrO$_3$/SrTiO$_3$ (LCO/STO) interface has been investigated. The interface is either insulating or metallic depending on growth conditions. A high sheet carrier concentration of 2x10$^{16}$ cm$^{-2}$ and mobility of 30,000 cm$^2$/V s is reported for the metallic interfaces, which is similar to the quasi-two dimensional gas at the LaAlO$_{3}$/SrTiO$_{3}$ interface with similar growth conditions. High-resolution synchrotron X-ray-based structural determination of the atomic-scale structures of both metallic and insulating LCO/STO interfaces show chemical intermixing and an interfacial lattice expansion. Angle resolved photoemission spectroscopy of 2 and 4 uc metallic LCO/STO shows no intensity near the Fermi level indicating that the conducting region is occurring deep enough in the substrate to be inaccessible to photoemission spectroscopy. Post-growth annealing in flowing oxygen causes a reduction in the sheet carrier concentration and mobility for the metallic interface while annealing the insulating interface at high temperatures and low oxygen partial pressures results in metallicity. These results highlight the critical role of defects related to oxygen vacancies on the creation of mobile charge carriers at the LCO/STO heterointerface.