Supplementary document for Wavelength-Tunable Ultrafast Two Arm Fiber Laser System for Transient Interferometric Scattering Microscopy on Nanoscopic Objects - 6795970.pdf

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Tip Enhancement of Upconversion Photoluminescence from Rare Earth Ion Doped Nanocrystals

We present tip-enhanced upconversion photoluminescence (PL) images of Er³⁺- and Yb³⁺-doped NaYF₄ nanocrystals on glass substrates with subdiffraction spatial resolution. Tip–sample distance dependent measurements clearly demonstrate the near-field origin of the image contrast. Time-resolved PL measurements show that the tip increases the spontaneous emission rate of the two emission channels of Er³⁺ in the visible region. Very efficient enhancement of upconversion PL is discussed in the context of the two-photon nature of the excitation process and homoenergy transfer between the ions within the nanocrystals. Comparison between different nanocrystals and tips shows a strong influence of the tip shape on the image contrast that becomes particularly relevant for the larger dimensions of the investigated nanocrystals

Microscopic View on the Ultrafast Photoluminescence from Photoexcited Graphene

We present a joint theory-experiment study on ultrafast photoluminescence from photoexcited graphene. On the basis of a microscopic theory, we reveal two distinct mechanisms behind the occurring photoluminescence: besides the well-known incoherent contribution driven by nonequilibrium carrier occupations, we found a coherent part that spectrally shifts with the excitation energy. In our experiments, we demonstrate for the first time the predicted appearance and spectral shift of the coherent photoluminescence

Contactless Visualization of Fast Charge Carrier Diffusion in Hybrid Halide Perovskite Thin Films

Organic–inorganic metal halide perovskite solar cells have recently attracted considerable attention with reported device efficiencies approaching those achieved in polycrystalline silicon. Key for an efficient extraction of photogenerated carriers is the combination of low nonradiative relaxation rates leading to long carrier lifetimes and rapid diffusive transport. The latter, however, is difficult to assess directly with reported values varying widely. Here, we present an experimental approach for a contactless visualization of the charge carrier diffusion length and velocity in thin films based on time-resolved confocal detection of photoluminescence at varying distances from the excitation position. Our measurements on chloride-treated methylammonium lead iodide thin films, the material for which the highest solar cell efficiencies have been reported, reveal a charge carrier diffusion length of 5.5–7.7 μm and a transport time of 100 ps for the first micrometer corresponding to a diffusion constant of about 5–10 cm² s^–1, similar to GaAs thin films

Antenna-Enhanced Photocurrent Microscopy on Single-Walled Carbon Nanotubes at 30 nm Resolution

We present the first photocurrent measurements along single carbon nanotube (CNT) devices with 30 nm resolution. Our technique is based on tip-enhanced near-field optical microscopy, exploiting the plasmonically enhanced absorption controlled by an optical nanoantenna. This allows for imaging of the zero-bias photocurrent caused by charge separation in local built-in electric fields at the contacts and close to charged particles that cannot be resolved using confocal microscopy. Simultaneously recorded Raman scattering images reveal the structural properties and the defect densities of the CNTs. Antenna-enhanced scanning photocurrent microscopy extends the available set of scanning-probe techniques by combining high-resolution photovoltaic and optical probing and could become a valuable tool for the characterization of nanoelectronic devices

Microscopic View on the Ultrafast Photoluminescence from Photoexcited Graphene

We present a joint theory-experiment study on ultrafast photoluminescence from photoexcited graphene. On the basis of a microscopic theory, we reveal two distinct mechanisms behind the occurring photoluminescence: besides the well-known incoherent contribution driven by nonequilibrium carrier occupations, we found a coherent part that spectrally shifts with the excitation energy. In our experiments, we demonstrate for the first time the predicted appearance and spectral shift of the coherent photoluminescence

Grain Boundaries Act as Solid Walls for Charge Carrier Diffusion in Large Crystal MAPI Thin Films

Micro- and nanocrystalline methylammonium lead iodide (MAPI)-based thin-film solar cells today reach power conversion efficiencies of over 20%. We investigate the impact of grain boundaries on charge carrier transport in large crystal MAPI thin films using time-resolved photoluminescence (PL) microscopy and numerical model calculations. Crystal sizes in the range of several tens of micrometers allow for the spatially and time resolved study of boundary effects. Whereas long-ranged diffusive charge carrier transport is observed within single crystals, no detectable diffusive transport occurs across grain boundaries. The observed PL transients are found to crucially depend on the microscopic geometry of the crystal and the point of observation. In particular, spatially restricted diffusion of charge carriers leads to slower PL decay near crystal edges as compared to the crystal center. In contrast to many reports in the literature, our experimental results show no quenching or additional loss channels due to grain boundaries for the studied material, which thus do not negatively affect the performance of the derived thin-film devices

Charge Transport Limitations in Perovskite Solar Cells: The Effect of Charge Extraction Layers

Understanding the charge transport characteristics and their limiting factors in organolead halide perovskites is of great importance for the development of competitive and economically advantageous photovoltaic systems derived from these materials. In the present work, we examine the charge carrier mobilities in CH₃NH₃PbI₃ (MAPI) thin films obtained from a one-step synthesis procedure and in planar n–i–p devices based on these films. By performing time-of-flight measurements, we find mobilities around 6 cm²/V s for electrons and holes in MAPI thin films, whereas in working solar cells, the respective effective mobility values are reduced by 3 orders of magnitude. From complementary experiments on devices with varying thicknesses of electron and hole transport layers, we identify the charge extraction layers and the associated interfaces rather than the perovskite material itself as the major limiting factors of the charge carrier transport time in working devices

Titanium Doping and Its Effect on the Morphology of Three-Dimensional Hierarchical Nb₃O₇(OH) Nanostructures for Enhanced Light-Induced Water Splitting

This study presents a simple method that allows us to modify the composition, morphological, and surface properties of three-dimensional hierarchical Nb₃O₇(OH) superstructures, resulting in strongly enhanced photocatalytic H₂ production. The superstructures consist of highly ordered nanowire networks and self-assemble under hydrothermal conditions. The presence of titanium affects the morphology of the superstructures, resulting in increased surface areas for higher doping levels. Up to 12 at. % titanium is incorporated into the Nb₃O₇(OH) crystal lattice via substitution of niobium at its octahedral lattice sites. Further titanium excess results in the formation of niobium-doped TiO₂ plates, which overgrow the surface of the Nb₃O₇(OH) superstructures. Photoluminescence spectroscopy indicates fewer charge recombination processes near the surface of the nanostructures with an increasing titanium concentration in the crystal lattice. The combination of larger surface areas with fewer quenching sites at the crystal surface yields higher H₂ evolution rates for the doped samples, with the rate being doubled by incorporation of 5.5 ± 0.7 at. % Ti