Oxygen-Mediated (0D) Cs4PbX6 Formation during Open-Air Thermal Processing Improves Inorganic Perovskite Solar Cell Performance.

The desire to commercialize perovskite solar cells continues to mount, motivating the development of scalable production. Evaluations of the impact of open-air processing have revealed a variety of physical changes in the fabricated devices─with few changes having the capacity to be functionalized. Here, we highlight the beneficial role of ambient oxygen during the open-air thermal processing of metastable γ-CsPbI3-based perovskite thin films and devices. Physiochemical-sensitive probes elucidate oxygen intercalation and the formation of Pb-O bonds in the CsPbI3 crystal, entering via iodine vacancies at the surface, creating superoxide (O2-) through electron transfer reactions with molecular oxygen, which drives the formation of a zero-dimensional Cs4PbI6 capping layer during annealing (>330 °C). The chemical conversion permanently alters the film structure, helping to shield the subsurface perovskite from moisture and introduces lattice anchoring sites, stabilizing otherwise unstable γ-CsPbI3 films. This functional modification is demonstrated in γ-CsPbI2Br perovskite solar cells, boosting the operational stability and photoconversion efficiency of champion devices from 12.7 to 15.4% when annealed in dry air. Such findings prompt a reconsideration of glovebox-based perovskite solar cell research and establish a scenario where device fabrication can in fact greatly benefit from ambient oxygen.

Pre-ablation regime light-induced optical changes in nanometer thick metal films.

We report on small optical reflection increases after illumination of nanometer thick gold and aluminum thin films on different substrates with single, femtosecond 400 nm wavelength pump laser pulses, in a pre-ablation fluence regime. In this fluence regime, small, irreversible and subtle morphological changes of the sample are observed. Dark-field, scanning electron, and atomic force microscopy images reveal subwavelength spallation features in the aluminum, and delamination in the gold layers in this pre-ablation regime. All of these morphological changes coincide with minute optical increases in the reflectivity, at the 0.1-2% level, as observed in-situ with a weak probe beam. From Liu-analysis, transfer-matrix, and two-temperature model calculations, we infer that in this pre-ablation regime, the aluminum layers already reach the melting temperature. Electron Backscatter Diffraction measurements show that the Al grains melt and resolidify into bigger grains. This suggests that for Al, resolidification into bigger grains is responsible for both the increased reflection, and the spallation in the pre-ablation regime. For gold, the optical change is most likely due to the etalon effect caused by delamination.

Directional quantum dot emission by soft-stamping on silicon Mie resonators.

We present a soft-stamping method to selectively print a homogenous layer of CdSeTe/ZnS core-shell quantum dots (QDs) on top of an array of Si nanocylinders with Mie-type resonant modes. Using this new method, we gain accurate control of the quantum dot's angular emission through engineered coupling of the QDs to these resonant modes. Using numerical simulations we show that the emission into or away from the Si substrate can be precisely controlled by the QD position on the nanocylinder. QDs centered on a 400 nm diameter nanocylinder surface show 98% emission directionality into the Si substrate. Alternatively, for homogenous ensembles placed over the nanocylinder top-surface, the upward emission is enhanced 10-fold for 150 nm diameter cylinders. Experimental PL intensity measurements corroborate the simulated trends with cylinder diameter. PL lifetime measurements reflect well the variations of the local density of states at the QD position due to coupling to the resonant cylinders. These results demonstrate that the soft imprint technique provides a unique manner to directly integrate optical emitters with a wide range of nanophotonic geometries, with potential applications in LEDs, luminescent solar concentrators, and up- and down-conversion schemes for improved photovoltaics.

Direct Patterning of CsPbBr3 Nanocrystals via Electron-Beam Lithography.

Lead-halide perovskite (LHP) nanocrystals have proven themselves as an interesting material platform due to their easy synthesis and compositional versatility, allowing for a tunable band gap, strong absorption, and high photoluminescence quantum yield (PLQY). This tunability and performance make LHP nanocrystals interesting for optoelectronic applications. Patterning active materials like these is a useful way to expand their tunability and applicability as it may allow more intricate designs that can improve efficiencies or increase functionality. Based on a technique for II-VI quantum dots, here we pattern colloidal LHP nanocrystals using electron-beam lithography (EBL). We create patterns of LHP nanocrystals on the order of 100s of nanometers to several microns and use these patterns to form intricate designs. The patterning mechanism is induced by ligand cross-linking, which binds adjacent nanocrystals together. We find that the luminescent properties are somewhat diminished after exposure, but that the structures are nonetheless still emissive. We believe that this is an interesting step toward patterning LHP nanocrystals at the nanoscale for device fabrication.

Ultrafast Thermal Imprinting of Plasmonic Hotspots.

Plasmonic photochemistry is driven by a rich collection of near-field, hot charge carrier, energy transfer, and thermal effects, most often accomplished by continuous wave illumination. Heat generation is usually considered undesirable, because noble metal nanoparticles heat up isotropically, losing the extreme energy confinement of the optical resonance. Here it is demonstrated through optical and heat-transfer modelling that the judicious choice of nanoreactor geometry and material enables the direct thermal imprint of plasmonic optical absorption hotspots onto the lattice with high fidelity. Transition metal nitrides (TMNs, e.g., TiN/HfN) embody the ideal material requirements, where ultrafast electron-phonon coupling prevents fast electronic heat dissipation and low thermal conductivity prolongs the heat confinement. The extreme energy confinement leads to unprecedented peak temperatures and internal heat gradients (>10 K nm-1 ) that cannot be achieved using noble metals or any current heating method. TMN nanoreactors consequently yield up to ten thousand times more product in pulsed photothermal chemical conversion compared with noble metals (Ag, Au, Cu). These findings open up a completely unexplored realm of nano-photochemistry, where adjacent reaction centers experience substantially different temperatures for hundreds of picoseconds, long enough for bond breaking to occur.

Energy-resolved plasmonic chemistry in individual nanoreactors.

Plasmonic resonances can concentrate light into exceptionally small volumes, which approach the molecular scale. The extreme light confinement provides an advantageous pathway to probe molecules at the surface of plasmonic nanostructures with highly sensitive spectroscopies, such as surface-enhanced Raman scattering. Unavoidable energy losses associated with metals, which are usually seen as a nuisance, carry invaluable information on energy transfer to the adsorbed molecules through the resonance linewidth. We measured a thousand single nanocavities with sharp gap plasmon resonances spanning the red to near-infrared spectral range and used changes in their linewidth, peak energy and surface-enhanced Raman scattering spectra to monitor energy transfer and plasmon-driven chemical reactions at their surface. Using methylene blue as a model system, we measured shifts in the absorption spectrum of molecules following surface adsorption and revealed a rich plasmon-driven reactivity landscape that consists of distinct reaction pathways that occur in separate resonance energy windows.

Unlocking Higher Power Efficiencies in Luminescent Solar Concentrators through Anisotropic Luminophore Emission.

The luminescent solar concentrator (LSC) offers a potential pathway for achieving low-cost, fixed-tilt light concentration. Despite decades of research, conversion efficiency for LSC modules has fallen far short of that achievable by geometric concentrators. However, recent advances in anisotropically emitting nanophotonic structures could enable a significant step forward in efficiency. Here, we employ Monte Carlo ray-trace modeling to evaluate the conversion efficiency for anisotropic luminophore emission as a function of photoluminescence quantum yield, waveguide concentration, and geometric gain. By spanning the full LSC parameter space, we define a roadmap toward high conversion efficiency. An analytical function is derived for the dark radiative current of an LSC to calculate the conversion efficiency from the ray-tracing results. We show that luminescent concentrator conversion efficiency can be increased from the current record value of 7.1-9.6% by incorporating anisotropy. We provide design parameters for optimized luminescent solar concentrators with practical geometrical gains of 10. Using luminophores with strongly anisotropic emission and high (99%) quantum yield, we conclude that conversion efficiencies beyond 28% are achievable. This analysis reveals that for high LSC performance, waveguide losses are as important as the luminophore quantum yield.

Shaping Tin Nanocomposites through Transient Local Conversion Reactions.

Shape-preserving conversion offers a promising strategy to transform self-assembled structures into advanced functional components with customizable composition and shape. Specifically, the assembly of barium carbonate nanocrystals and amorphous silica nanocomposites (BaCO3/SiO2) offers a plethora of programmable three-dimensional (3D) microscopic geometries, and the nanocrystals can subsequently be converted into functional chemical compositions, while preserving the original 3D geometry. Despite this progress, the scope of these conversion reactions has been limited by the requirement to form carbonate salts. Here, we overcome this limitation using a single-step cation/anion exchange that is driven by the temporal pH change at the converting nanocomposite. We demonstrate the proof of principle by converting BaCO3/SiO2 nanocomposites into tin-containing nanocomposites, a metal without a stable carbonate. We find that BaCO3/SiO2 nanocomposites convert in a single step into hydroromarchite nanocomposites (Sn3(OH)2O2/SiO2) with excellent preservation of the 3D geometry and fine features. We explore the versatility and tunability of these Sn3(OH)2O2/SiO2 nanocomposites as a precursor for functional compositions by developing shape-preserving conversion routes to two desirable compositions: tin perovskites (CH3NH3SnX3, with X = I or Br) with tunable photoluminescence (PL) and cassiterite (SnO2)-a widely used transparent conductor. Ultimately, these findings may enable integration of functional chemical compositions into advanced morphologies for next-generation optoelectronic devices.

Intermittency of CsPbBr3 Perovskite Quantum Dots Analyzed by an Unbiased Statistical Analysis.

We analyze intermittency in intensity and fluorescence lifetime of CsPbBr3 perovskite quantum dots by applying unbiased Bayesian inference analysis methods. We apply change-point analysis (CPA) and a Bayesian state clustering algorithm to determine the timing of switching events and the number of states between which switching occurs in a statistically unbiased manner, which we have benchmarked particularly to apply to highly multistate emitters. We conclude that perovskite quantum dots display a plethora of gray states in which brightness, broadly speaking, correlates inversely with decay rate, confirming the multiple recombination centers model. We leverage the CPA partitioning analysis to examine aging and memory effects. We find that dots tend to return to the bright state before jumping to a dim state and that when choosing a dim state, they tend to explore the entire set of states available.

Integrating Sphere Fourier Microscopy of Highly Directional Emission.

Accurately controlling light emission using nano- and microstructured lenses and antennas is an active field of research. Dielectrics are especially attractive lens materials due to their low optical losses over a broad bandwidth. In this work we measure highly directional light emission from patterned quantum dots (QDs) aligned underneath all-dielectric nanostructured microlenses. The lenses are designed with an evolutionary algorithm and have a theoretical directivity of 160. The fabricated structures demonstrate an experimental full directivity of 61 ± 3, three times higher than what has been estimated before, with a beaming half-angle of 2.6°. This high value compared to previous works is achieved via three mechanisms. First, direct electron beam patterning of QD emitters and alignment markers allowed for more localized emission and better emitter-lens alignment. Second, the lens fabrication was refined to minimize distortions between the designed shape and the final structure. Finally, a new measurement technique was developed that combines integrating sphere microscopy with Fourier microscopy. This enables complete directivity measurements, contrary to other reported values, which are typically only partial directivities or estimates of the full directivity that rely partly on simulations. The experimentally measured values of the complete directivity were higher than predicted by combining simulations with partial directivity measurements. High directivity was obtained from three different materials (cadmium-selenide-based QDs and two lead halide perovskite materials), emitting at 520, 620, and 700 nm, by scaling the lens size according to the emission wavelength.

Fano Lineshapes and Rabi Splittings: Can They Be Artificially Generated or Obscured by the Numerical Aperture?

Fano resonances and Rabi splittings are routinely reported in the scientific literature. Asymmetric resonance lineshapes are usually associated with Fano resonances, and two split peaks in the spectrum are often attributed to a Rabi splitting. True Fano resonances and Rabi splittings are unequivocal signatures of coherent coupling between subsystems. However, can the same spectral lineshapes characterizing Fano resonances and Rabi splittings arise from a purely incoherent sum of intensities? Here we answer this question through experiments with a tunable Fabry-Pérot cavity containing a CsPbBr3 perovskite crystal. By measuring the transmission and photoluminescence of this system using microscope objectives with different numerical aperture (NA), we find that even a modest NA = 0.4 can artificially generate Fano resonances and Rabi splittings. We furthermore show that this modest NA can obscure the anticrossing of a bona fide strongly coupled light-matter system. Through transfer matrix calculations we confirm that these spectral artifacts are due to the incoherent sum of transmitted intensities at different angles captured by the NA. Our results are relevant to the wide nanophotonics community, characterizing dispersive optical systems with high numerical aperture microscope objectives. We conclude with general guidelines to avoid pitfalls in the characterization of such optical systems.

Photonics for Photovoltaics: Advances and Opportunities.

Photovoltaic systems have reached impressive efficiencies, with records in the range of 20-30% for single-junction cells based on many different materials, yet the fundamental Shockley-Queisser efficiency limit of 34% is still out of reach. Improved photonic design can help approach the efficiency limit by eliminating losses from incomplete absorption or nonradiative recombination. This Perspective reviews nanopatterning methods and metasurfaces for increased light incoupling and light trapping in light absorbers and describes nanophotonics opportunities to reduce carrier recombination and utilize spectral conversion. Beyond the state-of-the-art single junction cells, photonic design plays a crucial role in the next generation of photovoltaics, including tandem and self-adaptive solar cells, and to extend the applicability of solar cells in many different ways. We address the exciting research opportunities and challenges in photonic design principles and fabrication that will accelerate the massive upscaling and (invisible) integration of photovoltaics into every available surface.

Self-inhibition effect of metal incorporation in nanoscaled semiconductors.

There has been a persistent effort to understand and control the incorporation of metal impurities in semiconductors at nanoscale, as it is important for semiconductor processing from growth, doping to making contact. Previously, the injection of metal atoms into nanoscaled semiconductor, with concentrations orders of magnitude higher than the equilibrium solid solubility, has been reported, which is often deemed to be detrimental. Here our theoretical exploration reveals that this colossal injection is because gold or aluminum atoms tend to substitute Si atoms and thus are not mobile in the lattice of Si. In contrast, the interstitial atoms in the Si lattice such as manganese (Mn) are expected to quickly diffuse out conveniently. Experimentally, we confirm the self-inhibition effect of Mn incorporation in nanoscaled silicon, as no metal atoms can be found in the body of silicon (below 1017 atoms per cm-3) by careful three-dimensional atomic mappings using highly focused ultraviolet-laser-assisted atom-probe tomography. As a result of self-inhibition effect of metal incorporation, the corresponding field-effect devices demonstrate superior transport properties. This finding of self-inhibition effect provides a missing piece for understanding the metal incorporation in semiconductor at nanoscale, which is critical not only for growing nanoscale building blocks, but also for designing and processing metal-semiconductor structures and fine-tuning their properties at nanoscale.

Nanocube Imprint Lithography.

In recent years, imprint lithography has emerged as a promising patterning technique capable of high-speed and volume production. In this work, we report highly reproducible one-step printing of metal nanocubes. A dried film of monocrystalline silver cubes serves as the resist, and a soft polydimethylsiloxane stamp directly imprints the final pattern. The use of atomically smooth and sharp faceted nanocubes facilitates the printing of high-resolution and well-defined patterns with face-to-face alignment between adjacent cubes. It also permits digital control over the line width of patterns such as straight lines, curves, and complex junctions over an area of several square millimeters. Single-particle lattices as well as three-dimensional nanopatterns are also demonstrated with an aspect ratio up to 5 in the vertical direction. The high-fidelity nanocube patterning combined with the previously demonstrated epitaxial overgrowth can enable curved (single) crystals from solution at room temperature or highly efficient transparent conductors.

Using Hot Electrons and Hot Holes for Simultaneous Cocatalyst Deposition on Plasmonic Nanostructures.

Hot electrons generated in metal nanoparticles can drive chemical reactions and selectively deposit cocatalyst materials on the plasmonic hotspots, the areas where the decay of plasmons takes place and the hot electrons are created. While hot electrons have been extensively used for nanomaterial formation, the utilization of hot holes for simultaneous cocatalyst deposition has not yet been explored. Herein, we demonstrate that hot holes can drive an oxidation reaction for the deposition of the manganese oxide (MnOx) cocatalyst on different plasmonic gold (Au) nanostructures on a thin titanium dioxide (TiO2) layer, excited at their surface plasmon resonance. An 80% correlation between the hot-hole deposition sites and the simulated plasmonic hotspot location is showed when considering the typical hot-hole diffusion length. Simultaneous deposition of more than one cocatalyst is also achieved on one of the investigated plasmonic systems (Au plasmonic nanoislands) through the hot-hole oxidation of a manganese salt and the hot-electron reduction of a platinum precursor in the same solution. These results add more flexibility to the use of hot carriers and open up the way for the design of complex photocatalytic nanostructures.

Controlling Optically Driven Atomic Migration Using Crystal-Facet Control in Plasmonic Nanocavities.

Plasmonic nanoconstructs are widely exploited to confine light for applications ranging from quantum emitters to medical imaging and biosensing. However, accessing extreme near-field confinement using the surfaces of metallic nanoparticles often induces permanent structural changes from light, even at low intensities. Here, we report a robust and simple technique to exploit crystal facets and their atomic boundaries to prevent the hopping of atoms along and between facet planes. Avoiding X-ray or electron microscopy techniques that perturb these atomic restructurings, we use elastic and inelastic light scattering to resolve the influence of crystal habit. A clear increase in stability is found for {100} facets with steep inter-facet angles, compared to multiple atomic steps and shallow facet curvature on spherical nanoparticles. Avoiding atomic hopping allows Raman scattering on molecules with low Raman cross-section while circumventing effects of charging and adatom binding, even over long measurement times. These nanoconstructs allow the optical probing of dynamic reconstruction in nanoscale surface science, photocatalysis, and molecular electronics.

Phase-Resolved Surface Plasmon Scattering Probed by Cathodoluminescence Holography.

High-energy (1-100 keV) electrons can coherently couple to plasmonic and dielectric nanostructures, creating cathodoluminescence (CL) of which the spectral features reveal details of the material's resonant modes at a deep-subwavelength spatial resolution. While CL provides fundamental insight in optical modes, detecting its phase has remained elusive. Here, we use Fourier-transform CL holography to determine the far-field phase distribution of fields scattered from plasmonic nanoholes, nanocubes, and helical nanoapertures and reconstruct the angle-resolved phase distributions. From the derived fields, we derive the relative strength and phase of induced scattering dipoles. Fourier-transform CL holography opens up a new world of coherent light scattering and surface wave studies with nanoscale spatial resolution.

Close-Packed Ultrasmooth Self-assembled Monolayer of CsPbBr3 Perovskite Nanocubes.

The use of colloidal self-assembly to form the complex multiscale patterns in many optoelectronic devices has been a long-standing dream of the nanoscience community. While great progress has been made using charged colloids in polar solvents, controlled assembly from nonpolar solvents is much more challenging. The major challenge is colloidal clustering caused by strong van der Waals (vdW) attraction between long-chain surface capping ligands passivating the surface of nanocrystals. Such clustering degrades ordering in packing during the self-assembly process. While ligand exchange to provide colloidal stability in polar phases is often an option, this is not the case for the exciting new class of halide perovskites due to the material's solubility in essentially all polar solvents. Here, we report surface-functionalized self-assembly of luminescent CsPbBr3 perovskite nanocubes by partially replacing long-chain oleyl groups (18 carbon chain) with short-chain thiocyanate (SCN-). This enables the fabrication of ultrasmooth monolayer thin films of nanocubes with a root-mean-square (RMS) roughness of around 4 Å. This ultrasmooth large area self-assembled layer could act as high-efficiency optoelectronic devices like solar cells, light-emitting diodes (LEDs), transistors, etc. We correlate our experimental results with simulations, providing detailed predictions for lattice constants with chain conformations showing reduced free energy for cubes grafted with short-chain thiocyanate compared to long-chain oleyl groups, thus facilitating better self-assembly.

Quantifying Strain and Dislocation Density at Nanocube Interfaces after Assembly and Epitaxy.

Nanoparticle self-assembly and epitaxy are utilized extensively to make 1D and 2D structures with complex shapes. High-resolution transmission electron microscopy (HRTEM) has shown that single-crystalline interfaces can form, but little is known about the strain and dislocations at these interfaces. Such information is critically important for applications: drastically reducing dislocation density was the key breakthrough enabling widespread implementation of light-emitting diodes, while strain engineering has been fundamental to modern high-performance transistors, solar cells, and thermoelectrics. In this work, the interfacial defect and strain formation after self-assembly and room temperature epitaxy of 7 nm Pd nanocubes capped with polyvinylpyrrolidone (PVP) is examined. It is observed that, during ligand removal, the cubes move over large distances on the substrate, leading to both spontaneous self-assembly and epitaxy to form single crystals. Subsequently, atomically resolved images are used to quantify the strain and dislocation density at the epitaxial interfaces between cubes with different lateral and angular misorientations. It is shown that dislocation- and strain-free interfaces form when the nanocubes align parallel to each other. Angular misalignment between adjacent cubes does not necessarily lead to grain boundaries but does cause dislocations, with higher densities associated with larger rotations.