The aim of this paper is the development of a qualitative understanding of thermal hysteresis, namely the difference between the melting Tm and solidification Ts temperatures of nanoparticles as a function of the particle size. In contrast to the melting temperature, the determination of the absolute value of the solidification temperature for nanoparticles is generally more difficult and subjected to significant uncertainties. In this study, we implemented a very generic approach based on classical nucleation theory and define the thermal hysteresis for a nanoparticle relative to its value for a much larger 'reference' particle made of the same material. The obtained thermal hysteresis is found to vanish when decreasing the nanoparticle size. The approach is illustrated using the examples of gold, bismuth, and platinum nanoparticles.

This paper deals with the excitons formed by electrons and holes located in different, closely placed semiconducting nanowires (spatially indirect excitons). We calculated the charge densities and the binding energies of the excitons for different nanowire diameters D and distances h between the nanowires. Together with the estimated exciton lifetimes, these results suggest that at certain h and D, the spatially indirect excitons in the nanowire arrays may have the potential to serve as information-processing units. Possible ways of exciton generation in the nanowire arrays are discussed.

Well-oriented PbBr2 microwires with a length-to-width ratio of up to 5000 were grown using a concentration gradient in co-crystallization with perovskite. Planar-integrated microwires showed a response to X-ray photons.

We consider the development of ligand-assisted growth processes for generating shape-anisotropic nanomaterials. Using statistical mechanics, we analyze the conditions under which ligand-assisted growth of shape-anisotropic crystalline nanomaterials from solution can take place. Depending on ligand-facet interaction energy and crystal facet area, molecular ligands can form compact layers on some facets leaving other facets free. The growth process is then restricted to free facets and may result in significant anisotropy in crystal shape. Our study uncovers the conditions for ligand-assisted growth of nanoplatelets and nanowires from isotropic or anisotropic seed nanocrystals of cuboid shape. We show that in contrast to nanoplatelets, ligand-assisted growth of nanowires requires certain anisotropy in the ligand-facet interaction energy.

We analyze nucleation-controlled nanocrystal growth in a solution containing surface-binding molecular ligands, which can also nucleate compact layers on the crystal surfaces. We show that, if the critical nucleus size for ligands is larger and the nucleation barrier is lower than those for crystal atoms, the ligands nucleate faster than the atoms on relatively wide crystal facets but much slower, if at all, on narrow facets. Such competitive nucleation of ligands and atoms results in ligands covering predominantly wider facets, thus excluding them from the growth process, and acts as a selection mechanism for the growth of crystals with narrower facets, the so-called nanoplatelets. The theory is confirmed by Monte Carlo simulations and validated experimentally for CsPbBr3 nanoplatelets grown from solution. We find that the anisotropic crystal growth is controlled by the growth temperature and the strength of surface bonding for the passivating molecular ligands.

The minimization of surface area, as a result of the minimization of (positive) surface energy, is a well-known driving force behind the spontaneous broadening of (nano) particle size distribution. We show that surfactant molecules binding to particle surfaces effectively decrease the surface energy and may change its sign. In this case, contrary to the expected broadening behavior, a minimum of free energy is achieved at the maximum surface area for all particles, i.e., when the particles are identical. Numerical simulations based on the classical Lifshitz–Slyozov–Wagner theory with surfactant-induced surface energy renormalization confirm the collapse of the particle size distribution. As the particle size evolution is much slower than particle nucleation and growth, the manipulation of surface energy with in-situ replacement of surfactant molecules provides a method for controlling particle size distribution with great potential for creating mono-disperse nanoparticles, a key goal of nanotechnology.

Herein, we describe the selective mapping of surface charge carriers, achieved using scanning ultrafast electron microscopy, for visualization of light-triggered carrier dynamics on the nanometer scale, diffusing on the surface of CdTe, a leading semiconductor in commercial photovoltaics. We reveal a novel diffusion model of charge carriers on surfaces. The nature of charge diffusion varies significantly, from superior diffusion to virtually trapped, depending on the crystal orientation. For instance, carriers on the (110) crystal orientation displayed a diffusion coefficient of , similar to 40,000 cm(2).s(-1) (i.e., similar to 10(4) times larger than measured in the bulk). In contrast, the (211) polar facets formed oxidative layers that, in turn, formed mid-surface-trap states that nearly arrested the charge diffusion completely. Complementary techniques and theoretical modeling explain the acute sensitivity of charge carriers to surface orientation and termination. Our findings reveal the enormous untapped potential for improved CdTe-based devices and optoelectronic materials in general through facet management.

Introducing dopants into InGaN NWs is known to significantly improve their device performances through a variety of mechanisms. However, to further optimize device operation under the influence of large specific surfaces, thorough knowledge of ultrafast dynamical processes at the surface and interface of these NWs is imperative. Here, we describe the development of four-dimensional scanning ultrafast electron microscopy (4D S-UEM) as an extremely surface-sensitive method to directly visualize in space and time the enormous impact of silicon doping on the surface-carrier dynamics of InGaN NWs. Two time regimes of surface dynamics are identified for the first time in a 4D S-UEM experiment: an early time behavior (within 200 ps) associated with the deferred evolution of secondary electrons due to the presence of localized trap states that decrease the electron escape rate and a longer time scale behavior (several ns) marked by accelerated charge carrier recombination. The results are further corroborated by conductivity studies carried out in the dark and under illumination.

Understanding defect chemistry, particularly ion migration, and its significant effect on the surface's optical and electronic properties is one of the major challenges impeding the development of hybrid perovskite-based devices. Here, using both experimental and theoretical approaches, we demonstrated that the surface layers of the perovskite crystals may acquire a high concentration of positively charged vacancies with the complementary negatively charged halide ions pushed to the surface. This charge separation near the surface generates an electric field that can induce an increase of optical band gap in the surface layers relative to the bulk. We found that the charge separation, electric field, and the amplitude of shift in the bandgap strongly depend on the halides and organic moieties of perovskite crystals. Our findings reveal the peculiarity of surface effects that are currently limiting the applications of perovskite crystals and more importantly explain their origins, thus enabling viable surface passivation strategies to remediate them.

A key problem in many emerging nanotechnologies is to create a narrow distribution of particles. Here, we prove the existence of a fundamental lower bound for the width of a particle size distribution (PSD) at equilibrium. This thermodynamic limit arises from the decrease of entropy for an ideal monodisperse distribution and is determined by the effective surface energy and temperature. The existence of this limit, provided that it is energetically favourable, implies that it is not possible to generate an arbitrarily narrow PSD through any process that reaches a thermodynamic equilibrium. We show that if a narrow PSD is generated it will tend to broaden in time until it reaches an equilibrium PSD of Gaussian shape. Conversely, if the initial PSD is broad and free to evolve to its equilibrium shape, it will eventually acquire the same Gaussian shape and width. The effect of PSD broadening can be significant for functional materials built from nanoparticulate arrays. Our study provides the framework for understanding and describing this phenomenon. Copyright (C) EPLA, 2017

Metal halide perovskite thin films can be crystallized via a broad range of solution-based routes. However, the quality of the final films is strongly dependent upon small changes in solution composition and processing parameters. Here, this study demonstrates that a fractional substitution of PbCl2 with PbI2 in the 3CH3 NH3 I:PbCl2 mixed-halide starting solution has a profound influence upon the ensuing thin-film crystallization. The presence of PbI2 in the precursor induces a uniform distribution of regular quadrilateral-shaped CH3 NH3 PbI3 perovskite crystals in as-cast films, which subsequently grow to form pinhole-free perovskite films with highly crystalline domains. With this new formulation of 3CH3 NH3 I:0.98PbCl2 :0.02PbI2 , this study achieves a 19.1% current-voltage measured power conversion efficiency and a 17.2% stabilized power output in regular planar heterojunction solar cells.

Continuous flow chemistry is used to trap the nucleation and growth stages of platinum-nickel nano-octahedra with second time resolution and high throughputs to probe their properties ex situ. The growth starts from poorly crystalline particles (nucleation) at 5 seconds, to crystalline 1.5 nm particles bounded by the {111}-facets at 7.5 seconds, followed by truncation and further growth to octahedral nanoparticles at 20 seconds.

The exciting intrinsic properties discovered in single crystals of metal halide perovskites still await their translation into optoelectronic devices. The poor understanding and control of the crystallization process of these materials are current bottlenecks retarding the shift toward single-crystal-based optoelectronics. Here we theoretically and experimentally elucidate the role of surface tension in the rapid synthesis of perovskite single crystals by inverse temperature crystallization. Understanding the nucleation and growth mechanisms enabled us to exploit surface tension to direct the growth of monocrystalline films of perovskites (AMX(3), where A = CH3NH3+ or MA; M = Pb2+, Sn2+; X = Br-, I- on the solution surface. We achieve up to 1 cm(2)-sized monocrystalline films with thickness on the order of the charge carrier diffusion length (similar to 5-10 mu m). Our work paves the way to control the crystallization process of perovskites, including thin-film deposition, which is essential to advance the performance benchmarks of perovskite optoelectronics.

High-quality perovskite monocrystalline films are successfully grown through cavitation-triggered asymmetric crystallization. These films enable a simple cell structure, ITO/CH3 NH3 PbBr3 /Au, with near 100% internal quantum efficiency, promising power conversion efficiencies (PCEs) >5%, and superior stability for prototype cells. Furthermore, the monocrystalline devices using a hole-transporter-free structure yield PCEs ≈6.5%, the highest among other similar-structured CH3 NH3 PbBr3 solar cells to date.

A breakthrough in the development of 4D scanning ultrafast electron microscopy is described for real-time and space imaging of secondary electron energy loss and carrier diffusion on the surface of an array of nanowires as a model system, providing access to a territory that is beyond the reach of either static electron imaging or any time-resolved laser spectroscopy.

Controlling crystal orientations and macroscopic morphology is vital to develop the electronic properties of hybrid perovskites. Here we show that a large-area, orientationally pure crystalline (OPC) methylammonium lead iodide (MAPbI 3 ) hybrid perovskite film can be fabricated using a thermal-gradient-assisted directional crystallization method that relies on the sharp liquid-to-solid transition of MAPbI 3 from ionic liquid solution. We find that the OPC films spontaneously form periodic microarrays that are distinguishable from general polycrystalline perovskite materials in terms of their crystal orientation, film morphology and electronic properties. X-ray diffraction patterns reveal that the film is strongly oriented in the (112) and (200) planes parallel to the substrate. This film is structurally confined by directional crystal growth, inducing intense anisotropy in charge transport. In addition, the low trap-state density (7.9 × 10 13 cm −3 ) leads to strong amplified stimulated emission. This ability to control crystal orientation and morphology could be widely adopted in optoelectronic devices.

Hybrid organic-inorganic perovskite crystals have recently become one of the most important classes of photoactive materials in the solar cell and optoelectronic communities. Albeit improvements have focused on state-of-the-art technology including various fabrication methods, device architectures, and surface passivation, progress is yet to be made in understanding the actual operational temperature on the electronic properties and the device performances. Therefore, the substantial effect of temperature on the optoelectronic properties, charge separation, charge recombination dynamics, and photoconversion efficiency are explored. The results clearly demonstrated a significant enhancement in the carrier mobility, photocurrent, charge carrier lifetime, and solar cell performance in the 60 ± 5 °C temperature range. In this temperature range, perovskite crystal exhibits a highly symmetrical relaxed cubic structure with well-aligned domains that are perpendicular to a principal axis, thereby remarkably improving the device operation. This finding provides a new key variable component and paves the way toward using perovskite crystals in highly efficient photovoltaic cells.

Solution-processed metal halide perovskite semiconductors, such as CH 3 NH 3 PbI 3 , have exhibited remarkable performance in solar cells, despite having non-negligible density of defect states. A likely candidate is halide vacancies within the perovskite crystals, or the presence of metallic lead, both generated due to the imbalanced I/Pb stoichiometry which could evolve during crystallization. Herein, we show that the addition of hypophosphorous acid (HPA) in the precursor solution can significantly improve the film quality, both electronically and topologically, and enhance the photoluminescence intensity, which leads to more efficient and reproducible photovoltaic devices. We demonstrate that the HPA can reduce the oxidized I 2 back into I − , and our results indicate that this facilitates an improved stoichiometry in the perovskite crystal and a reduced density of metallic lead.