Disorder and Halide Distributions in Cesium Lead Halide Nanocrystals as Seen by Colloidal 133Cs Nuclear Magnetic Resonance Spectroscopy.

Colloidal nuclear magnetic resonance (cNMR) spectroscopy on inorganic cesium lead halide nanocrystals (CsPbX3 NCs) is found to serve for noninvasive characterization and quantification of disorder within these structurally soft and labile particles. In particular, we show that 133Cs cNMR is highly responsive to size variations from 3 to 11 nm or to altering the capping ligands on the surfaces of CsPbX3 NCs. Distinct 133Cs signals are attributed to the surface and core NC regions. Increased heterogeneous broadening of 133Cs signals, observed for smaller NCs as well as for long-chain zwitterionic capping ligands (phosphocholines, phosphoethanol(propanol)amine, and sulfobetaines), can be attributed to more significant surface disorder and multifaceted surfaces (truncated cubes). On the contrary, capping with dimethyldidodecylammonium bromide (DDAB) successfully reduces signal broadening owing to better surface passivation and sharper (001)-bound cuboid shape. DFT calculations on various sizes of NCs corroborate the notion that the surface disorder propagates over several octahedral layers. 133Cs NMR is a sensitive probe for studying halide gradients in mixed Br/Cl NCs, indicating bromide-rich surfaces and chloride-rich cores. On the contrary, mixed Br/I NCs exhibit homogeneous halide distributions.

Unravelling the amorphous structure and crystallization mechanism of GeTe phase change memory materials.

The reversible phase transitions in phase-change memory devices can switch on the order of nanoseconds, suggesting a close structural resemblance between the amorphous and crystalline phases. Despite this, the link between crystalline and amorphous tellurides is not fully understood nor quantified. Here we use in-situ high-temperature x-ray absorption spectroscopy (XAS) and theoretical calculations to quantify the amorphous structure of bulk and nanoscale GeTe. Based on XAS experiments, we develop a theoretical model of the amorphous GeTe structure, consisting of a disordered fcc-type Te sublattice and randomly arranged chains of Ge atoms in a tetrahedral coordination. Strikingly, our intuitive and scalable model provides an accurate description of the structural dynamics in phase-change memory materials, observed experimentally. Specifically, we present a detailed crystallization mechanism through the formation of an intermediate, partially stable 'ideal glass' state and demonstrate differences between bulk and nanoscale GeTe leading to size-dependent crystallization temperature.

Coupling to octahedral tilts in halide perovskite nanocrystals induces phonon-mediated attractive interactions between excitons.

Understanding the origin of electron-phonon coupling in lead halide perovskites is key to interpreting and leveraging their optical and electronic properties. Here we show that photoexcitation drives a reduction of the lead-halide-lead bond angles, a result of deformation potential coupling to low-energy optical phonons. We accomplish this by performing femtosecond-resolved, optical-pump-electron-diffraction-probe measurements to quantify the lattice reorganization occurring as a result of photoexcitation in nanocrystals of FAPbBr3. Our results indicate a stronger coupling in FAPbBr3 than CsPbBr3. We attribute the enhanced coupling in FAPbBr3 to its disordered crystal structure, which persists down to cryogenic temperatures. We find the reorganizations induced by each exciton in a multi-excitonic state constructively interfere, giving rise to a coupling strength that scales quadratically with the exciton number. This superlinear scaling induces phonon-mediated attractive interactions between excitations in lead halide perovskites.

Phase-Change Memory from Molecular Tellurides.

Phase-change memory (PCM) is an emerging memory technology based on the resistance contrast between the crystalline and amorphous states of a material. Further development and realization of PCM as a mainstream memory technology rely on innovative materials and inexpensive fabrication methods. Here, we propose a generalizable and scalable solution-processing approach to synthesize phase-change telluride inks in order to meet demands for high-throughput material screening, increased energy efficiency, and advanced device architectures. Bulk tellurides, such as Sb2Te3, GeTe, Sc2Te3, and TiTe2, are dissolved and purified to obtain inks of molecular metal telluride complexes. This allowed us to unlock a wide range of solution-processed ternary tellurides by the simple mixing of binary inks. We demonstrate accurate and quantitative composition control, including prototype materials (Ge-Sb-Te) and emerging rare-earth-metal telluride-doped materials (Sc-Sb-Te). Spin-coating and annealing convert ink formulations into high-quality, phase-pure telluride films with preferred orientation along the (00l) direction. Deposition engineering of liquid tellurides enables thickness-tunable films, infilling of nanoscale vias, and film preparation on flexible substrates. Finally, we demonstrate cyclable and non-volatile prototype memory devices, achieving performance indicators such as resistance contrast and low reset energy on par with state-of-the-art sputtered PCM layers.

Sol-Gel Barium Titanate Nanohole Array as a Nonlinear Metasurface and a Photonic Crystal.

The quest of a nonlinear optical material that can be easily nanostructured over a large surface area is still ongoing. Here, we demonstrate a nanoimprinted nonlinear barium titanate 2D nanohole array that shows the optical properties of a 2D photonic crystal and a metasurface, depending on the direction of the optical axis. The challenge of nanostructuring the inert metal-oxide is resolved by direct soft nanoimprint lithography with sol-gel derived barium titanate enabling critical dimensions of 120 nm with aspect ratios of five. The nanohole array exhibits a photonic bandgap in the infrared range when probed along the slab axis, while lattice resonant states are observed in out-of-plane transmission configuration. The enhanced light-matter interaction from the resonant structure enables to increase in the second-harmonic generation in the near-ultraviolet by a factor of 18 illustrating the potential in the flexible fabrication technique for barium titanate photonic devices.

Synthesis and Electronic Structure of Mid-Infrared Absorbing Cu3SbSe4 and CuxSbSe4 Nanocrystals.

Aliovalent I-V-VI semiconductor nanocrystals are promising candidates for thermoelectric and optoelectronic applications. Famatinite Cu3SbSe4 stands out due to its high absorption coefficient and narrow band gap in the mid-infrared spectral range. This paper combines experiment and theory to investigate the synthesis and electronic structure of colloidal CuxSbSe4 nanocrystals. We achieve predictive composition control of size-uniform CuxSbSe4 (x = 1.9-3.4) nanocrystals. Density functional theory (DFT)-parametrized tight-binding simulations on nanocrystals show that the more the Cu-vacancies, the wider the band gap of CuxSbSe4 nanocrystals, a trend which we also confirm experimentally via FTIR spectroscopy. We show that SbCu antisite defects can create mid-gap states, which may give rise to sub-bandgap absorption. This work provides a detailed study of CuxSbSe4 nanocrystals and highlights the potential opportunities as well as challenges for their application in infrared devices.

Colloidal Ternary Telluride Quantum Dots for Tunable Phase Change Optics in the Visible and Near-Infrared.

A structural change between amorphous and crystalline phase provides a basis for reliable and modular photonic and electronic devices, such as nonvolatile memory, beam steerers, solid-state reflective displays, or mid-IR antennas. In this paper, we leverage the benefits of liquid-based synthesis to access phase-change memory tellurides in the form of colloidally stable quantum dots. We report a library of ternary MxGe1-xTe colloids (where M is Sn, Bi, Pb, In, Co, Ag) and then showcase the phase, composition, and size tunability for Sn-Ge-Te quantum dots. Full chemical control of Sn-Ge-Te quantum dots permits a systematic study of structural and optical properties of this phase-change nanomaterial. Specifically, we report composition-dependent crystallization temperature for Sn-Ge-Te quantum dots, which is notably higher compared to bulk thin films. This gives the synergistic benefit of tailoring dopant and material dimension to combine the superior aging properties and ultrafast crystallization kinetics of bulk Sn-Ge-Te, while improving memory data retention due to nanoscale size effects. Furthermore, we discover a large reflectivity contrast between amorphous and crystalline Sn-Ge-Te thin films, exceeding 0.7 in the near-IR spectrum region. We utilize these excellent phase-change optical properties of Sn-Ge-Te quantum dots along with liquid-based processability for nonvolatile multicolor images and electro-optical phase-change devices. Our colloidal approach for phase-change applications offers higher customizability of materials, simpler fabrication, and further miniaturization to the sub-10 nm phase-change devices.

Highly Responsive Mid-Infrared Metamaterial Enhanced Heterostructure Photodetector Formed out of Sintered PbSe/PbS Colloidal Quantum Dots.

Efficient and simple-to-fabricate light detectors in the mid infrared (MIR) spectral range are of great importance for various applications in existing and emerging technologies. Here, we demonstrate compact and efficient photodetectors operating at room temperature in a wavelength range of 2710-4250 nm with responsivities as high as 375 and 4 A/W. Key to the high performance is the combination of a sintered colloidal quantum dot (CQD) lead selenide (PbSe) and lead sulfide (PbS) heterojunction photoconductor with a metallic metasurface perfect absorber. The combination of this photoconductor stack with the metallic metasurface perfect absorber provides an overall ∼20-fold increase of the responsivity compared against reference sintered PbSe photoconductors. More precisely, the introduction of a PbSe/PbS heterojunction increases the responsivity by a factor of ∼2 and the metallic metasurface enhances the responsivity by an order of magnitude. The metasurface not only enhances the light-matter interaction but also acts as an electrode to the detector. Furthermore, fabrication of our devices relies on simple and inexpensive methods. This is in contrast to most of the currently available (state-of-the-art) MIR photodetectors that rely on rather expensive as well as nontrivial fabrication technologies that often require cooling for efficient operation.

Nanocrystal phononics.

Colloidal nanocrystals are successfully used as nanoscale building blocks for creating hierarchical solids with structures that range from amorphous networks to sophisticated periodic superlattices. Recently, it has been observed that these superlattices exhibit collective vibrations, which stem from the correlated motion of the nanocrystals, with their surface-bound ligands acting as molecular linkers. In this Perspective, we describe the work so far on collective vibrations in nanocrystal solids and their as-of-yet untapped potential for phononic applications. With the ability to engineer vibrations in the hypersonic regime through the choice of nanocrystal and linker composition, as well as by controlling their size, shape and chemical interactions, such superstructures offer new opportunities for phononic crystals, acoustic metamaterials and optomechanical systems.

Ligand Tuning of Localized Surface Plasmon Resonances in Antimony-Doped Tin Oxide Nanocrystals.

Aliovalent-doped metal oxide nanocrystals exhibiting localized surface plasmons (LSPRs) are applied in systems that require reflection/scattering/absorption in infrared and optical transparency in visible. Indium tin oxide (ITO) is currently leading the field, but indium resources are known to be very restricted. Antimony-doped tin oxide (ATO) is a cheap candidate to substitute the ITO, but it exhibits less advantageous electronic properties and limited control of the LSPRs. To date, LSPR tuning in ATO NCs has been achieved electrochemically and by aliovalent doping, with a significant decrease in doping efficiency with an increasing doping level. Here, we synthesize plasmonic ATO nanocrystals (NCs) via a solvothermal route and demonstrate ligand exchange to tune the LSPR energies. Attachment of ligands acting as Lewis acids and bases results in LSPR peak shifts with a doping efficiency overcoming those by aliovalent doping. Thus, this strategy is of potential interest for plasmon implementations, which are of potential interest for infrared upconversion, smart glazing, heat absorbers, or thermal barriers.

On the nanoscale structural evolution of solid discharge products in lithium-sulfur batteries using operando scattering.

The inadequate understanding of the mechanisms that reversibly convert molecular sulfur (S) into lithium sulfide (Li2S) via soluble polysulfides (PSs) formation impedes the development of high-performance lithium-sulfur (Li-S) batteries with non-aqueous electrolyte solutions. Here, we use operando small and wide angle X-ray scattering and operando small angle neutron scattering (SANS) measurements to track the nucleation, growth and dissolution of solid deposits from atomic to sub-micron scales during real-time Li-S cell operation. In particular, stochastic modelling based on the SANS data allows quantifying the nanoscale phase evolution during battery cycling. We show that next to nano-crystalline Li2S the deposit comprises solid short-chain PSs particles. The analysis of the experimental data suggests that initially, Li2S2 precipitates from the solution and then is partially converted via solid-state electroreduction to Li2S. We further demonstrate that mass transport, rather than electron transport through a thin passivating film, limits the discharge capacity and rate performance in Li-S cells.

Nonequilibrium Lattice Dynamics in Photoexcited 2D Perovskites.

Interplay between structural and photophysical properties of metal halide perovskites is critical to their utility in optoelectronics, but there is limited understanding of lattice response upon photoexcitation. Here, 2D perovskites butylammonium lead iodide, (BA)2 PbI4 , and phenethylammonium lead iodide, (PEA)2 PbI4 , are investigated using ultrafast transient X-ray diffraction as a function of optical excitation fluence to discern structural dynamics. Both powder X-ray diffraction and time-resolved photoluminescence linewidths narrow over 1 ns following optical excitation for the fluence range studied, concurrent with slight redshifting of the optical bandgaps. These observations are attributed to transient relaxation and ordering of distorted lead iodide octahedra stimulated mainly by electron-hole pair creation. The c axis expands up to 0.37% over hundreds of picoseconds; reflections sampling the a and b axes undergo one tenth of this expansion with the same timescale. Post-photoexcitation appearance of the (110) reflection in (BA)2 PbI4 would suggest a transient phase transition, however, through new single-crystal XRD, reflections are found that violate glide plane conditions in the reported Pbca structure. The static structure space group is reassigned as P21 21 21 . With this, a nonequilibrium phase transition is ruled out. These findings offer increased understanding of remarkable lattice response in 2D perovskites upon excitation.

Toward a European carbon footprint rule for batteries.

Focus on upstream production, not downstream use.

Engineering of Oxide Protected Gold Nanoparticles.

Gold nanoparticles that are partially or fully covered by metal oxide shells provide superior functionality and stability for catalytic and plasmonic applications. Yet, facile methods for controlled fabrication of thin oxide layers on metal nanoparticles are lacking. Here, we report an easy method to reliably engineer thin Ga2O3 shells on Au nanoparticles, based on liquid-phase chemical oxidation of Au-Ga alloy nanoparticles. We demonstrate that, with this technique, laminar and ultrathin Ga2O3 shells can be grown with ranging thickness from sub- to several monolayers. We show how the localized surface plasmon resonance can be used to understand the reaction process and quantitatively monitor the Ga2O3 shell growth. Finally, we demonstrate that the Ga2O3 coating prevents sintering of the Au nanoparticles, providing thermal stability to at least 250 °C. This approach, building on dealloying of bimetallic nanoparticles by the solution-phase oxidation, promises a general technique for achieving controlled metal/oxide core/shell nanoparticles.

Ultra-narrow room-temperature emission from single CsPbBr3 perovskite quantum dots.

Semiconductor quantum dots have long been considered artificial atoms, but despite the overarching analogies in the strong energy-level quantization and the single-photon emission capability, their emission spectrum is far broader than typical atomic emission lines. Here, by using ab-initio molecular dynamics for simulating exciton-surface-phonon interactions in structurally dynamic CsPbBr3 quantum dots, followed by single quantum dot optical spectroscopy, we demonstrate that emission line-broadening in these quantum dots is primarily governed by the coupling of excitons to low-energy surface phonons. Mild adjustments of the surface chemical composition allow for attaining much smaller emission linewidths of 35-65 meV (vs. initial values of 70-120 meV), which are on par with the best values known for structurally rigid, colloidal II-VI quantum dots (20-60 meV). Ultra-narrow emission at room-temperature is desired for conventional light-emitting devices and paramount for emerging quantum light sources.

Ligand Dynamics in Nanocrystal Solids Studied with Quasi-Elastic Neutron Scattering.

Nanocrystal surfaces are commonly populated by organic ligands, which play a determining role in the optical, electronic, thermal, and catalytic properties of the individual nanocrystals and their assemblies. Understanding the bonding of ligands to nanocrystal surfaces and their dynamics is therefore important for the optimization of nanocrystals for different applications. In this study, we use temperature-dependent, quasi-elastic neutron scattering (QENS) to investigate the dynamics of different surface bound alkanethiols in lead sulfide nanocrystal solids. We select alkanethiols with mono- and dithiol terminations, as well as different backbone types and lengths. QENS spectra are collected both on a time-of-flight spectrometer and on a backscattering spectrometer, allowing us to investigate ligand dynamics in a time range from a few picoseconds to nanoseconds. Through model-based analysis of the QENS data, we find that ligands can either (1) precess around a central axis, while simultaneously rotating around their own molecular axis, or (2) only undergo uniaxial rotation with no precession. We establish the percentage of ligands undergoing each type of motion, the average relaxation times, and activation energies for these motions. We determine, for example, that dithiols which link facets of neighboring nanocrystals only exhibit uniaxial rotation and that longer ligands have higher activation energies and show smaller opening angles of precession due to stronger ligand-ligand interactions. Generally, this work provides insight into the arrangement and dynamics of ligands in nanocrystal solids, which is key to understanding their mechanical and thermal properties, and, more generally, highlights the potential of QENS for studying ligand behavior.

Deep learning-based segmentation of lithium-ion battery microstructures enhanced by artificially generated electrodes.

Accurate 3D representations of lithium-ion battery electrodes, in which the active particles, binder and pore phases are distinguished and labeled, can assist in understanding and ultimately improving battery performance. Here, we demonstrate a methodology for using deep-learning tools to achieve reliable segmentations of volumetric images of electrodes on which standard segmentation approaches fail due to insufficient contrast. We implement the 3D U-Net architecture for segmentation, and, to overcome the limitations of training data obtained experimentally through imaging, we show how synthetic learning data, consisting of realistic artificial electrode structures and their tomographic reconstructions, can be generated and used to enhance network performance. We apply our method to segment x-ray tomographic microscopy images of graphite-silicon composite electrodes and show it is accurate across standard metrics. We then apply it to obtain a statistically meaningful analysis of the microstructural evolution of the carbon-black and binder domain during battery operation.

Size- and composition-controlled intermetallic nanocrystals via amalgamation seeded growth.

Intermetallic nanocrystals are a large family of emerging materials with extensive applications in many fields. Yet, a generalized synthetic method for intermetallic nanocrystals is lacking. Here, we report the development of a colloidal synthesis method based on amalgamation of monometallic nanocrystal seeds with low-melting point metals. We use this approach to achieve crystalline and compositionally uniform intermetallic nanocrystals of Au-Ga, Ag-Ga, Cu-Ga, Ni-Ga, Pd-Ga, Pd-In, and Pd-Zn compounds. We demonstrate both compositional tunability across the phase spaces (e.g., AuGa2, AuGa, Au7Ga2, and Ga-doped Au), size tunability (e.g., 14.0-, 7.6-, and 3.8-nm AuGa2), and size uniformity (e.g., 5.4% size deviations). This approach makes it possible to systematically achieve size- and composition-controlled intermetallic nanocrystals, opening up a multitude of possibilities for these materials.

Nanocrystal Quantum Dot Devices: How the Lead Sulfide (PbS) System Teaches Us the Importance of Surfaces.

Semiconducting thin films made from nanocrystals hold potential as composite hybrid materials with new functionalities. With nanocrystal syntheses, composition can be controlled at the sub-nanometer level, and, by tuning size, shape, and surface termination of the nanocrystals as well as their packing, it is possible to select the electronic, phononic, and photonic properties of the resulting thin films. While the ability to tune the properties of a semiconductor from the atomistic- to macro-scale using solution-based techniques presents unique opportunities, it also introduces challenges for process control and reproducibility. In this review, we use the example of well-studied lead sulfide (PbS) nanocrystals and describe the key advances in nanocrystal synthesis and thin-film fabrication that have enabled improvement in performance of photovoltaic devices. While research moves forward with novel nanocrystal materials, it is important to consider what decades of work on PbS nanocrystals has taught us and how we can apply these learnings to realize the full potential of nanocrystal solids as highly flexible materials systems for functional semiconductor thin-film devices. One key lesson is the importance of controlling and manipulating surfaces.

Dynamic lattice distortions driven by surface trapping in semiconductor nanocrystals.

Nonradiative processes limit optoelectronic functionality of nanocrystals and curb their device performance. Nevertheless, the dynamic structural origins of nonradiative relaxations in such materials are not understood. Here, femtosecond electron diffraction measurements corroborated by atomistic simulations uncover transient lattice deformations accompanying radiationless electronic processes in colloidal semiconductor nanocrystals. Investigation of the excitation energy dependence in a core/shell system shows that hot carriers created by a photon energy considerably larger than the bandgap induce structural distortions at nanocrystal surfaces on few picosecond timescales associated with the localization of trapped holes. On the other hand, carriers created by a photon energy close to the bandgap of the core in the same system result in transient lattice heating that occurs on a much longer 200 picosecond timescale, dominated by an Auger heating mechanism. Elucidation of the structural deformations associated with the surface trapping of hot holes provides atomic-scale insights into the mechanisms deteriorating optoelectronic performance and a pathway towards minimizing these losses in nanocrystal devices.