When conjugate molecules are self-assembled on the surface of semiconductors, emergent properties resulting from the electronic coupling between the conjugate moieties are of importance in the interfacial electron-transfer dynamics for photoelectrochemical and optoelectronics devices. In this work, we investigate the self-assembly of triphenylamine–oligothiophene–perylenemonoimide (PMI) molecules, denoted as BH4, on metal oxide surfaces via UV–vis absorption, photoluminescence, and transient near-infrared absorption spectroscopies and molecular dynamics simulations, and we report the excimer formation due to the π–π interaction of the PMI units between the neighboring dye molecules. To our best knowledge, this is the first experimental observation of intermolecular excimer formation when conjugate donor–acceptor molecules form a self-assembled monolayer. In addition, a long-lived (4.3 μs) intermolecular charge separation is observed, and a new excimer-mediated intermolecular charger-transfer mechanism is proposed. This work demonstrates that, through the design of dye molecules, the excited complexes or aggregates can provide a pathway to slow down the recombination rate in photoelectrodes that utilize donor–acceptor dyad molecules.

Nanowires possess unique strain relieving properties making them compatible with a wide variety of substrates ranging from single crystalline semiconductors, amorphous ceramics, and polycrystalline metals. Flexible metallic foils are particularly interesting substrates for nanowires for both flexible optoelectronics and high throughput manufacturing techniques. However, nanowires grown on polycrystalline metals exhibit grain-dependent morphologies. As an alternative route, the authors demonstrate the growth of highly uniform III-Nitride nanowires on bulk metallic glass (amorphous metal) and nanocrystalline Pt metal films using molecular beam epitaxy. Nanowire arrays on metallic glass substrates show uniformity over length scales >100 μm. The quality of these nanowires is explored by photoluminescence spectroscopy. The electrical characteristics of individual nanowires are measured via conductive atomic force microscopy, and mesoscale light-emitting diodes (LEDs) are fabricated. Nanowires grown on nanocrystalline Pt films showed an increase in output power by a factor of up to 32, and an increase in the overall LED efficiency by up to 13× compared with simultaneously grown nanowire LEDs on bare Si.

Van der Waals bonding relaxes the constraints of lattice matching, making two-dimensional (2D) transition metal dichalcogenides attractive in the field of epitaxy. Recently, molecular beam epitaxy (MBE) of MoSe2 was demonstrated on a variety of substrates. Here, the authors use MBE to investigate the early stages of 2D nucleation of MoSe2 grown on Si in pursuit of controlled monolayer island size. The 2D nucleation rate varies by a factor of >2 over a narrow substrate temperature range of 550–560 °C. Above 560 °C, the desorption rate of Se from the surface exceeds the nucleation rate leading to fully suppressed 2D monolayer nucleation. X-ray diffraction confirms (001) oriented MoSe2 on Si (111). Raman spectra are consistent with 1–3 monolayer-thick MoSe2, in agreement with atomic force microscopy measurements of the monolayer height of 2D islands.

Superlattices of antiferromagnetic μ-Fe2O3 and diamagnetic β-Ga2O3 are grown by plasma-assisted molecular beam epitaxy on (010) oriented β-Ga2O3 substrates in which ferromagnetism emerges above room temperature. To investigate the suspected interface origin of the ferromagnetic phase, identical superlattice structures are grown at various substrate temperatures and beam fluxes. Atomic-resolution scanning transmission electron microscopy images confirm the registry of μ-Fe2O3 to the β-Ga2O3 layers in these superlattices. Atomic force microscopy and high-resolution x-ray diffraction are used to examine the growth morphology and characterize the superlattice interface roughness. The saturation magnetization of the ferromagnetic phase is observed to increase strongly with the interface roughness. Conversely, smoother superlattices exhibit a weaker ferromagnetic response and a higher density of paramagnetic moments along with evidence of superparamagnetic clusters. These findings are consistent with the interface origin for the ferromagnetic response in these superlattices. The demonstration of an interface magnetic phase in nearly lattice-matched monoclinic Fe2O3/Ga2O3 opens the door to ultrawide bandgap heterostructure-engineered magnetoelectronic devices, where ferromagnetic switching of the interface phase can be incorporated into high-field devices.

Superlattices composed of either monoclinic μ-Fe2O3 or β-(AlxGa1−x)2O3 with β-Ga2O3 spacers are grown on (010) β-Ga2O3 substrates using plasma-assisted molecular beam epitaxy. High-resolution x-ray diffraction data are quantitatively fit using commercial dynamical x-ray diffraction software (LEPTOS) to obtain layer thicknesses, strain, and compositions. The strain state of β-(AlxGa1−x)2O3 and μ-Fe2O3 superlattices as characterized using reciprocal space maps in the symmetric (020) and asymmetric (420) diffraction conditions indicates coherent growths that are strained to the (010) β-Ga2O3 lattice. β-(AlxGa1−x)2O3 and μ-Fe2O3 superlattices grown at hotter substrate temperatures result in crystal structures with better coherency and reduced defects compared to colder growths. The growth rate of μ-Fe2O3 is ∼2.6 nm/min at Tsub = 700 °C and drops to ∼1.6 nm/min at Tsub = 800 °C due to increased Fe interdiffusion at hotter substrate temperatures. Scanning transmission electron microscopy data of a μ-Fe2O3 superlattice grown at Tsub = 700 °C confirm that there is significant diffusion of Fe atoms into β-Ga2O3 layers.