Size-dependent and tunable crystallization of GeSbTe phasechange nanoparticles

Chalcogenide-based nanostructured phase-change materials (PCMs) are considered promising building blocks for non-volatile memory due to their high write and read speeds, high data-storage density, and low power consumption. Top-down fabrication of PCM nanoparticles (NPs), however, often results in damage and deterioration of their useful properties. Gas-phase condensation based on magnetron sputtering offers an attractive and straightforward solution to continuously down-scale the PCMs into sub-lithographic sizes. Here we unprecedentedly present the size dependence of crystallization for Ge2Sb2Te5 (GST) NPs, whose production is currently highly challenging for chemical synthesis or top-down fabrication. Both amorphous and crystalline NPs have been produced with excellent size and composition control with average diameters varying between 8 and 17 nm. The size-dependent crystallization of these NPs was carefully analyzed through in-situ heating in a transmission electron microscope, where the crystallization temperatures (Tc) decrease when the NPs become smaller. Moreover, methane incorporation has been observed as an effective method to enhance the amorphous phase stability of the NPs. This work therefore elucidates that GST NPs synthesized by gas-phase condensation with tailored properties are promising alternatives in designing phase-change memories constrained by optical lithography limitations

The Impact of Stoichiometry on the Photophysical Properties of Ruddlesden-Popper Perovskites

2D Ruddlesden-Popper perovskites are interesting for a variety of applications owing to their tunable optical properties and their excellent ambient stability. As these materials are processable from solution, they hold the promise of procuring flexible and cost-effective films through large-scale fabrication techniques. However, such solution-based deposition techniques often induce large degrees of heterogeneity due to poorly controlled crystallization. The microscopic properties of films of (PEA)(2)PbI4 cast from precursor solutions of different stoichiometry are therefore investigated. The stoichiometry of the precursor solution is found to have a large impact on the crystallinity, morphology, and optical properties of the resulting thin films. Even for films cast from stoichiometric precursors, differences in photoluminescence intensities occur on a subgranular level. The heterogeneity in these films is found to be thermally activated with an activation energy of 0.4 eV for the emergence of local variations in nonradiative recombination rates. The spatial variation in the distribution of trap states is attributed to local fluctuations in the stoichiometry. In line with this, the surface can successfully be passivated by providing an excess of phenylethylammonium iodide (PEAI) to an as-cast film, enhancing the photoluminescence by as much as 85% without significantly altering the film's morphology.</p

Tailoring Growth Kinetics toward a Size-Dependent Work Function of Ge Nanocrystals Synthesized by Inert Gas Condensation

Understanding the size-dependent electronic properties of germanium nanocrystals (Ge NCs) is of fundamental importance for improving the efficiency of optoelectronic devices based on such NCs. Here, Ge NCs with a tunable size were synthesized by magnetron-sputtering cluster-beam deposition, where the size of the as-deposited Ge NCs can be finely controlled between 6 and 36 nm by helium gas flow rates and variable magnetic field configurations above the target surface. Because the size of the as-deposited Ge NCs highly depends on the nucleation process inside the plasma region, a detailed comparison between these two process parameters on the size control was formulated from the perspective of the growth kinetic mechanism. Furthermore, the local surface potential of different-sized Ge NCs deposited on n-type silicon substrates was measured by Kelvin probe force microscopy. The surface potential fluctuation of n-type Si covered by Ge NCs shows a strong size-dependent relationship with the size of the Ge NCs, whereas the surface potential fluctuation increases when their size reduced. Because the surface potential fluctuation between the intrinsic Ge NCs and the n-type silicon substrate tends to get smaller as the NCs' size decreases due to the quantum confinement effect, the number of charges transferred between the electronic bands will reduce as the size of Ge NCs decreases. The latter exactly explains the observed experimental results. Therefore, this work offers a perspective to understand the behavior of charge transfer, which plays an important role in the performance of optoelectronic devices

Schottky barrier formation at amorphous-crystalline interfaces of GeSb phase change materials

The electrical properties of amorphous-crystalline interfaces in phase change materials, which are important for rewritable optical data storage and for random access memory devices, have been investigated by surface scanning potential microscopy. Analysis of GeSb systems indicates that the surface potential of the crystalline phase is similar to 30-60 mV higher than that of the amorphous phase. This potential asymmetry is explained qualitatively by the presence of a Schottky barrier at the amorphous-crystalline interface and supported also by quantitative Schottky model calculations. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3691179

Investigation of the Nanoscale Morphology in Industrially Relevant Clearcoats of Waterborne Polymer Colloids by Means of Variable-Angle-Grazing Incidence Small-Angle X-ray Scattering

Soft polymer colloidal water suspensions are extremely important formulations for industrial applications such as water-based environmental-friendly coatings, paints, and adhesives. Homogeneity of the final coating at the micrometer and nanoscale is a crucial factor for optimal coating performance, such as barrier properties against solvent permeation. Here, we investigated the remnant nanostructure in slot-die-coated micrometer-sized thick clear coating films (clearcoats) of three different waterborne polymer colloids (pure soft, pure hard, and soft/hard multiphase), commonly utilized as primers in paint formulations [Mader et al. Prog. Org. Coat. 2011, 71, 123-135], using variable-angle grazing incidence small-angle X-ray scattering (GISAXS) complemented with cross-sectional atomic force microscopy (cs-AFM). After complete macroscopic drying, the coating films exhibit the presence of residual nanostructure with characteristic distance (d*) smaller than the original particle size and even smaller

Tunable wettability of polymer films by partial engulfment of nanoparticles

A series of poly(methyl methacrylate) (PMMA) surfaces decorated by Cu nanoparticles (NP) with gradually varied morphology were prepared by high-pressure CO2 treatment at various time spans. Combining the characterizations of transmission electron microscopy (TEM) and atomic force microscopy (AFM), an accurate three-dimensional view of the morphology of the surfaces was presented. Subsequently, the wettability of the surfaces decreases near linearly with the increase of the apparent height of the decorating NPs in both static (static contact angle) and dynamic (contact angle hysteresis) aspects. The observed tendency contradicts to the Wenzel or Cassie-Baxter model and is explained by the contribution of nanomeniscus formed between the decorating NP and the flat substrate. The capillary pressure from this meniscus is negative and results in the increase of the contact angle with the apparent height (H-N) of the Cu NPs decorating the PMMA surface. In addition, the effect of the coverage (C-N) by NPs on the wettability can be explained on the same basis. Our experiment demonstrates the important influence of the nanomeniscus on the wettability, which is usually not taken into account. The results in this work provide a comprehensive understanding of how nanostructure affects the wettability of the decorated surfaces and shed light on how to obtain certain wettability through nanostructuring of the surface morphology

Effect of the Device Architecture on the Performance of FA(0.85)MA(0.15)PbBr(0.45)I(2.55) Planar Perovskite Solar Cells

Hybrid perovskite solar cells have attracted an unprecedented research attention due to their skyrocketing record power conversion efficiency (PCE), which now exceeds 23% in less than a decade from the initial PCE of 3.8%. Besides the excellent optoelectronic properties of the perovskite absorbers, the high efficiencies are also dependent on preparation methods and advanced device engineering. In this study, the role of the device architecture (planar n-i-p vs inverted p-i-n structure) and of the charge-selective interlayer on the photophysical properties of the perovskite absorber and device performance are explored. FA(0.85)MA(0.15)PbBr(0.45)I(2.55) (MA = methylammonium, FA = formamidinium) as the perovskite absorber and chloride-capped TiO2 colloidal nanocrystals (TiO2-Cl) and poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as close-to-the-substrate layers in the conventional and inverted structures are employed, respectively. Extremely different device performances are demonstrated by the two structures. The device where the active layer is deposited on TiO2-Cl displays a champion PCE of 19.9%, while the one using PEDOT:PSS gives about 15.1% efficiency. The photophysical and electrical investigations indicate that the TiO2-Cl/perovskite interface has lower number of traps, underlining the importance of interfaces for achieving highly performing perovskite solar cells

Influence of the stoichiometry of tin-based 2D/3D perovskite active layers on solar cell performance

2D/3D mixed tin perovskites have the advantages of high crystallinity and preferential orientation compared to pure 3D tin perovskite. However, solar cells based on 2D/3D mixed tin perovskites are still limited by low power conversion efficiency (PCE) when compared to their lead-based counterparts. It is essential to gain deeper insight into the factors that limit the performance of these solar cells in order to further improve them. In this work, we demonstrate that the starting stoichiometry of 2D/3D (PEA(0.08)FA(x)SnI(3)) tin perovskite films influences their crystallization and photophysical properties as well as the solar cell performance. The reference 2D/3D film (x = 0.92, where x refers to the stoichiometry of the precursors) is highly crystalline with the 3D phase preferentially oriented and a small amount of 2D phase located at the bottom of the film. The reference solar cell delivers a PCE of about 8.0%. 2D/3D films with even higher FA concentration (x > 1.0) mainly consist of poorly crystalline and randomly oriented 3D phases, with much higher trap density compared to the reference film. The corresponding solar cells therefore suffer from severe trap-assisted charge recombination, and deliver a poor PCE o

Effect of Airborne Hydrocarbons on the Wettability of Phase Change Nanoparticle Decorated Surfaces

We present here a detailed study of the wettability of surfaces nanostructured with amorphous and crystalline nanoparticles (NPs) derived from the phase-change material Ge2Sb2Te5 (GST). Particular attention was devoted to the effect of airborne surface hydrocarbons on surface wetting. Our analysis illustrates that a reversible hydrophilic-hydrophobic wettability switch is revealed by combined ultraviolet-ozone (UV-O-3) treatments and exposure to hydrocarbon atmospheres. Indeed, the as prepared surfaces exhibited a hydrophilic state after thermal annealing or UV-O-3 treatment which can partially remove hydrocarbon contaminants, while a hydrophobic state was realized after exposure to hydrocarbon atmosphere. Using high-angle annular dark-field scanning transmission electron microscopy for the specially designed GST NP decorated graphene substrates, a network of hydrocarbon connecting GST NPs was observed. Our findings indicate that airborne hydrocarbons can significantly enhance the hydrophobicity of nanostructured surfaces. Finally, the experiments reveal that previously defined hydrophilic materials can be used for the design of hydrophobic surfaces even if the meniscus is highly adhered to a solid surface, which is in agreement with our qualitative model involving the contribution of the nanomeniscus formed between the substrate and a decorating NP