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Precise control of light-matter interactions at the nanoscale lies at the heart of nanophotonics. Yet their experimental examination is challenging since the corresponding electromagnetic near-field is often confined within volumes below the resolution of conventional optical microscopy. In this work, we demonstrate that photoelectron emission microscopy (PEEM) can be used to image near-field optical fields of nanophotonic structures with a sub-monolayer potassium surface layer to lower the work function and enable two-photon photoemission using near-infrared illumination. We present concurrent spectroscopy and imaging of the near-field distribution of the resonance modes supported by broken-symmetry silicon metasurfaces. We find that the photoemission signal reveals the wavelength dependent, electric near-field distribution of the optical modes. In addition, the large field of view of PEEM allows us to examine the evolution of the collective modes by monitoring individual resonators near the boundary of the metasurface and deduce that coupling between eight resonators establishes the collective excitations. All told, the high-spatial resolution of this hyperspectral imaging approach, combined with an extended field of view, is valuable for the metrology of collective, non-local optical resonances in nanophotonic systems., 16 pages, 5 Figures, Supporting Information: 6 pages, 5 figures

Silylethyne-functionalized benzodithiophene serves as a universal crystal engineering core to yield stable, soluble, π-stacked arrays of aromatic chromophores., Herein, we describe the design and synthesis of a suite of molecules based on a benzodithiophene “universal crystal engineering core”. After computationally screening derivatives, a trialkylsilylethyne-based crystal engineering strategy was employed to tailor the crystal packing for use as the active material in an organic field-effect transistor. Electronic structure calculations were undertaken to reveal derivatives that exhibit exceptional potential for high-efficiency hole transport. The promising theoretical properties are reflected in the preliminary device results, with the computationally optimized material showing simple solution processing, enhanced stability, and a maximum hole mobility of 1.6 cm2 V–1 s–1.

Halide perovskites have generated tremendous interest as low-cost semiconductors for optoelectronics, such as photovoltaics, lasers, and light emitting diodes due to their extraordinary optical and transport properties. Perovskite photovoltaics in particular have demonstrated a meteoric rise in power conversion efficiencies and drawn considerable interest as a next-generation solar energy technology. The rapid development has centered around lead-based derivatives, and concerns regarding the toxicity of lead has sparked interest in low toxicity and more environmentally friendly perovskite derivatives. In this regime tin (Sn) is regarded as a prominent alternative owing to the ideal bandgap and reduced toxicity exhibited by Sn-halide perovskites. Not without its own shortcomings, tin perovskites photovoltaics currently suffer from lower efficiencies and stability than their lead counterparts. Here, interfaces within the device are of upmost importance as the surface chemistry and energetics significantly impact the performance and stability of photovoltaic devices. To investigate the interfacial energetics in tin perovskites low energy ultraviolet photoelectron spectroscopy (UPS) and low energy inverse photoelectron spectroscopy (IPES) systems were developed. For the former, a novel H Lyman-α vacuum ultraviolet photon source was introduced and characterized to show its ability to mitigate sample degradation and reduce background noise. Application of these systems to tin perovskites provided insight into the variation of reported ionization energies of neat films and revealed that both the inclusion of common additives and sustained air exposure, can play a considerable role on measured energetics. UPS, IPES, and X-ray photoelectron spectroscopy (XPS) were also employed to perform the first direct measurements of frontier electronic energy levels at the formamidinium tin iodide (FASnI3)/C60 interface. We observe band bending in both materials and transport gap widening in FASnI3 at the interface with C60. XPS results show that this is due, in part, to iodide migration from the perovskite into C60,which results in a distortion of the perovskite surface structure and n-doping of C60. To further probe surface/interfacial chemistry and energetics perovskite films were modified with small molecules. FASnI3 treated with carboxylic acids and bulky ammonium substituted surface ligands resulted in slight alterations of the interfacial energetics, which resulted in a more favorable energetic landscape and improved PV performance. Further, PV devices treated with a fluorinated carboxylic acid derivate showed greatly enhanced device stability. Finally, the ability of surface ligands to reduce iodide diffusion out of the perovskite layer was examined., © 2020 Alex Boehm