Search

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. ISSN:1476-1122 ISSN:1476-4660

Understanding the impact of positional and energetic disorders in nanocrystal (NC) quantum dot thin films on charge transport is crucial to determine what to prioritize in terms of the synthesis and fabrication of these materials and to accelerate their development for electronics. Here, we computationally construct realistic NC thin films with different types of disorders and apply a density functional theory (DFT)-parameterized, kinetic Monte Carlo simulation to systematically study the effects of disorders on transport. We obtain statistics on the carrier transit pathways through the NC films and carrier residence times on individual NCs. This provides insights into the distribution of transit times across the thin films and the effective mobility. We conclude that the impact of positional disorders on charge transport depends on the type of disorder and how it affects the spacing between neighboring NCs. The formation of transport paths with short inter-NC distances can enhance mobility. Meanwhile, random packing (RP) of NCs and energetic disorders due to a distribution of NC sizes decreases mobility 2- to 4-fold. Because of the large reorganization energy of small NCs, increasing the electric field has little influence on the median residence time of a charge carrier on an NC; however, an electric field straightens the transport path of the charge carrier and reduces the average number of hops a carrier makes, which can slightly enhance mobility. Deep electronic trap states are especially detrimental to carrier mobility, particularly at low fields and when the films are otherwise highly ordered., ACS Applied Electronic Materials, 4 (2), ISSN:2637-6113

The influence of surfactant type and concentration on the crystal habit of LiFePO4 particles during hydrothermal synthesis.

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., ACS Nano, 17 (7), ISSN:1936-0851, ISSN:1936-086X

Among the alternatives for the successful family of Cd- and Pb-chalcogenide nanocrystals (NCs), I-III-VI NCs appear to be highly promising as they combine low toxicity with up-and-coming light-emitting and light-imaging applications. Much of the focus has been placed on the prototype Cu-In-S NC system, while Ag-based NCs are significantly less developed. Herein we report spectroscopic studies of robust Ag3In5Se9 NCs fabricated via an amide-promoted synthesis, using NC size as the structural varying parameter. Temperature-dependent photoluminescence, combined with Gaussian line-shape analysis, reveals the presence of three radiative channels in colloids and films of the Ag3In5Se9 NCs. On the basis of their energetics, dynamics, and size- and temperature-dependent behavior, we assign the radiative channels to optical transitions from the conduction-band minimum to (i) in-gap states of self-trapped holes, (ii) valence-band maximum, and (iii) lower-energy valence-band states. Furthermore, by combining pump-probe experiments with tight-binding band-structure calculations whose parameters were derived from first principles, we reveal the presence of two main interband absorption transitions associated with NC electronic states. The dynamics of such transitions are influenced by shallow surface states that act as sites in which fast exciton trapping and much slower detrapping occurs, whereas the trapping and detrapping rates are gauged by confinement and temperature. ISSN:2574-0970

Herein, a methodology to investigate aging of commercial cylindrical Li-ion cells is introduced. Distribution of relaxation time (DRT) method is applied to deconvolute electrochemical impedance spectroscopy (EIS) measurements and separate those polarization effects that are usually overlapped in the frequency domain by means of a peak-based representation. Half-cells are built at the beginning and end of life to link the electrochemical and aging processes occurring at anode and/or the cathode sides. Moreover, lab-made full-cells are exploited to verify the reproducibility when compared with cylindrical cells. The results of an extensive analysis of around 500 EIS spectra return an unambiguous attribution of different electrochemical processes to different time constants and ultimately to different DRT peaks. Digital imaging validates graphite degradation, mainly related to lithium plating. Scanning electron microscopy validates the degradation at NMC cathode, mainly attributed to particle cracking. It is concluded that DRT peaks allow to characterize cell aging and their tracking can help to develop more reliable state of health estimators., Energy Technology, 10 (10), ISSN:2194-4296, ISSN:2194-4288

Focus on upstream production, not downstream use

Insufficient understanding of the mechanism that reversibly converts sulfur into lithium sulfide (Li2S) via soluble polysulfides (PS) hampers the realization of high performance lithium-sulfur (Li-S) cells. Typically Li2S formation is explained by direct electroreduction of a PS to Li2S; however, this is not consistent with the size and shape of the insulating Li2S deposits. Here, we use operando small and wide angle X-ray scattering (SAXS/WAXS) and operando small angle neutron scattering (SANS) to track the growth and dissolution of solid deposits from atomic to sub-micron scales during charge and discharge. Stochastic modelling based on the SANS data allows quantification of the chemical phase evolution during discharge and charge. We show that the deposit is comprised of nanocrystalline Li2S and smaller, solid short-chain PS particles, which we argue are likely Li2S2. Our data are consistent with solid Li2S2 precipitating from solution and then being converted in the solid-state to Li2S, probably via direct electroreduction and chemical diffusion through the Li2S2 particle network. Solution-mediated Li2S2 precipitation, therefore, controls the Li2S/Li2S2 aggregate structure. Discharge capacity and rate capability in Li-S battery cathodes are limited by mass transport rather than electron transport through a thin passivating surface film. The found mechanism also explains why in practice not all S is converted to Li2S; a certain amount of Li2S2 remains as a second solid phase.

Water has now become the standard process solvent for graphite-based anodes, eliminating the use of toxic and costly N-Methyl-pyrrolidone (NMP) in anode manufacturing. Ideally, water could also become the standard for cathodes; however, water-based processing of NMC cathode materials induces lithium leaching, which reduces their specific capacity and leads to capacity fade. Here, we demonstrate that leached lithium ions can be exploited during aqueous slurry preparation to create a Li-containing polymer binder that enables cathode performance comparable to those fabricated using NMP. Specifically, we show that leached lithium ions from LiNi0.8Mn0.1Co0.1O2 (NMC 811) particles react with polyacrylic acid (PAA) to form a lithium polyacrylate (LPA) surface coating and binder. Because the resulting LPA binder is water soluble, aqueous-based recycling of the cathode particles is feasible and over 90% capacity retention is shown in recycled material after 100 cycles.

Insufficient understanding of the mechanism that reversibly converts sulfur into lithium sulfide (Li2S) via soluble polysulfides (PS) hampers the realization of high performance lithium-sulfur (Li-S) cells. Typically Li2S formation is explained by direct electroreduction of a PS to Li2S; however, this is not consistent with the size and shape of the insulating Li2S deposits. Here, we use operando small and wide angle X-ray scattering (SAXS/WAXS) and operando small angle neutron scattering (SANS) to track the growth and dissolution of solid deposits from atomic to sub-micron scales during charge and discharge. Stochastic modelling based on the SANS data allows quantification of the chemical phase evolution during discharge and charge. We show that the deposit is comprised of nanocrystalline Li2S and smaller, solid short-chain PS particles, which we argue are likely Li2S2. Our data are consistent with solid Li2S2 precipitating from solution and then being converted in the solid-state to Li2S, probably via direct electroreduction and chemical diffusion through the Li2S2 particle network. Solution-mediated Li2S2 precipitation, therefore, controls the Li2S/Li2S2 aggregate structure. Discharge capacity and rate capability in Li-S battery cathodes are limited by mass transport rather than electron transport through a thin passivating surface film. Capacity and rate capability are thus influenced by are species solvation, mobility, and applied current density. The found mechanism also explains why in practice not all S is converted to Li2S; a certain amount of Li2S2 remains as a second solid phase.

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, quasielastic 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-offlight 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., ACS Nano, 15 (12), ISSN:1936-0851, ISSN:1936-086X

Efficient photodetectors that can be easily engineered for a specific spectral window are of high interest. Here, we report on the design, fabrication, and characterization of metasurface-enhanced photodetectors and photodiodes using colloidal quantum dots. We fabricate photoconductors optimized for the wavelength range around 1550 nm featuring responsivities of up to 8000 A/W with low noise equivalent powers on the order of tens of pW/Hz. Further, we produce photodiodes with responsivities of ∼5 mA/W that offer faster responses (14 μs rise time). The high responsivities are due to the metasurface, which increases the absorption by a factor of 10 compared to that of a quantum dot thin film of the same thickness and a structure that enables a photo-gain. We introduce a metasurface to detect either of the two orthogonal polarizations. The fabricated photoconductors operate at low voltages (1-5 V), making them compatible with the complementary metal-oxide-semiconductor (CMOS) read-out circuitry. ISSN:2330-4022

Semiconductors assembled from colloidal nanocrystals (NCs) are often described in the same terms as their single-crystalline counterparts with references to conduction and valence band edges, doping densities, and electronic defects; however, how and why semiconductor properties manifest in these bottom-up fabricated thin films can be fundamentally different. In this Perspective, we describe the factors that determine the electronic structure in colloidal NC-based semiconductors, and comment on approaches for measuring or calculating this electronic structure. Finally, we discuss future directions for these semiconductors and highlight their potential to bridge the divide between localized quantum effects and long-range transport in thin films., The Journal of Physical Chemistry Letters, 11 (21), ISSN:1948-7185

Phase-change materials (PCMs), which are well-established in optical and random-access memories, are increasingly studied for emerging topics such as brain-inspired computing and active photonics. ...

Nonisovalent ternary and quaternary semiconductors (i.e., having two or more cations with different valence electrons) have unique structural and electronic properties that are leveraged in photovo...

We combine state-of-the-art ultrafast photoluminescence and absorption spectroscopy and nonadiabatic molecular dynamics simulations to investigate charge-carrier cooling in CsPbBr3 nanocrystals over a very broad size regime, from 0.8 to 12 nm. Contrary to the prevailing notion that polaron formation slows down charge-carrier cooling in lead-halide perovskites, no suppression of carrier cooling is observed in CsPbBr3 nanocrystals except for a slow cooling (over ∼10 ps) of "warm" electrons in the vicinity (within ∼0.1 eV) of the conduction band edge. At higher excess energies, electrons and holes cool with similar rates, on the order of 1 eV ps-1 carrier-1, increasing weakly with size. Our ab initio simulations suggest that cooling proceeds via fast phonon-mediated intraband transitions driven by strong and size-dependent electron-phonon coupling. The presented experimental and computational methods yield the spectrum of involved phonons and may guide the development of devices utilizing hot charge carriers.

The significant performance increase in nanocrystal (NC)-based solar cells over the last decade is very encouraging. However, many of these gains have been achieved by trial-and-error optimization, and a systematic understanding of what limits the device performance is lacking. In parallel, experimental and computational techniques provide increasing insights into the electronic properties of individual NCs and their assemblies in thin films. Here, we utilize these insights to parameterize drift–diffusion simulations of PbS NC solar cells, which enable us to track the distribution of charge carriers in the device and quantify recombination dynamics, which limit the device performance. We simulate both Schottky- and heterojunction-type devices and, through temperature-dependent measurements in the light and dark, experimentally validate the appropriateness of the parameterization. The results reveal that Schottky-type devices are limited by surface recombination between the PbS and aluminum contact, while heterojunction devices are currently limited by NC dopants and electronic defects in the PbS layer. The simulations highlight a number of opportunities for further performance enhancement, including the reduction of dopants in the nanocrystal active layer, the control over doping and electronic structure in electron- and hole-blocking layers (e.g., ZnO), and the optimization of the interfaces to improve the band alignment and reduce surface recombination. For example, reduction in the percentage of p-type NCs from the current 1–0.01% in the heterojunction device can lead to a 25% percent increase in the power conversion efficiency. ISSN:2637-6113

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., Nature Communications, 12 (1), ISSN:2041-1723

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 CsPbBr

Insufficient understanding of the mechanism that reversibly converts sulphur into lithium sulphide (Li2S) via soluble polysulphides (PS) hampers the realization of high performance lithium-sulphur cells. Typically Li2S formation is explained by direct electroreduction of a PS to Li2S; however, this is not consistent with the size of the insulating Li2S deposits. Here, we use in situ small and wide angle X-ray scattering (SAXS/WAXS) to track the growth and dissolution of crystalline and amorphous deposits from atomic to sub-micron scales during charge and discharge. Stochastic modelling based on the SAXS data allows quantification of the chemical phase evolution during discharge and charge. We show that Li2S deposits predominantly via disproportionation of transient, solid Li2S2 to form primary Li2S crystallites and solid Li2S4 particles. We further demonstrate that this process happens in reverse during charge. These findings show that the discharge capacity and rate capability in Li-S battery cathodes are therefore limited by mass transport through the increasingly tortuous network of Li2S / Li2S4 / carbon pores rather than electron transport through a passivating surface film.

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., Chimia, 75 (5), ISSN:0009-4293

Fine graphite micropowder was processed in a downstream tubular reactor to perform a fast and homogeneous plasma-enhanced chemical vapor deposition of an organosilicon-plasma coating onto the powder surface. As a single process run results in the deposition of a non-continuous coating, consisting of a nanoparticle distribution, on the powder surface, the powder was repeatedly reprocessed until a continuous coating was obtained. The coating was imaged with focused ion-beam scanning electron microscopy and chemically characterized with Raman spectroscopy and X-ray photoelectron spectroscopy. The assessment of the powder flowability was also performed to investigate the roughness of the coated surface. The chemical characterization indicated that the coating is composed of amorphous hydrogenated silicon carbide with a little oxygen contamination.

Lithium-ion batteries (LIBs) with liquid electrolytes and microporous polyolefin separator membranes are ubiquitous. Though not necessarily an active component in a cell, the separator plays a key role in ion transport and influences rate performance, cell life and safety. As our understanding of separator properties and the interactions between the separator and the electrolyte deepens, it becomes evident that there are opportunities for improving separators to help meet the greater demands that new applications place on LIB technology. Here, we review the impact of the separator structure and chemistry on LIB performance, assess characterization techniques relevant for understanding structure–performance relationships in separator membranes, and provide an outlook on next-generation separator technology. Insights from this Review indicate that LIB performance can be improved by taking into account the interplay of the separator with its surroundings and indicate that, in the future, separators will be designed to play a more active role in LIB operation. Current and emerging characterization techniques will play an important role in guiding this evolution in separator technology. Separators are an essential part of current lithium-ion batteries. Vanessa Wood and co-workers review the properties of separators, discuss their relationship with battery performance and survey the techniques for characterizing separators.

Electrochemical Impedance Spectroscopy (EIS) is a powerful non-invasive technique used to characterize Lithium-ion cells. Its application allows to identify and track the evolutions of cell degradation processes within a short testing time. This review collects all the available works which have used EIS spectra either to characterize Li-ion cell degradation or to develop Electric Circuit Models (ECMs). The objectives of this work are: (i) to highlight the influence of different aging test conditions on the EIS spectra; (ii) to find the correlations between EIS spectra variations and the underlying degradation mechanisms and (iii) to list the available options to formulate ECMs from EIS spectra of aged cells. After an exhaustive analysis of the state-of-the-art, a critical review is presented to discuss the existing links between degradation mechanisms and the most reliable solutions to model them., Journal of Power Sources, 505, ISSN:0378-7753, ISSN:1873-2755