Nanometer-sized Regions Research Articles

Exploring the Links between Photoluminescence and Microstructure in Cs2InBr5·H2O Samples Doped with Pb2+

Precipitation of Cs2InBr5·H2O from HBr(aq) solutions containing Pb2+ ions results in powders that exhibit narrowband green photoluminescence (λmax = 521 nm, fwhm = 89.9 meV). Synchrotron powder X-ray diffraction reveals trace amounts of Cs4PbBr6 and CsPbBr3 that cannot be detected by laboratory powder diffraction measurements. Broadening of the CsPbBr3 diffraction peaks suggests crystals that are tens of nanometers in size. Evidence for the presence of CsPbBr3 can also be seen in diffuse reflectance spectra. Cathodoluminescence imaging shows that luminescence originates from small nanometer-sized regions. Taken together, these observations point to CsPbBr3 nanocrystal inclusions as the source of photoluminescence. Heating these samples to temperatures at or above 80 °C triggers a reversible dehydration process that leads to an irreversible change in the photoluminescence from green to blue (λmax ≈ 480 nm, fwhm = 278 meV), accompanied by significant changes in the microstructure. Cathodoluminescence imaging indicates that the blue emission occurs over much larger micron-sized regions of the sample. The position of blue PL is similar to other hybrid lead bromide compounds where the emission has been assigned to 3P1 → 1S0 transitions on [PbBr4]2– ions. Based on the emission wavelength and cathodoluminescence imaging, the blue emission is assigned to isolated [PbBr4]2– ions that substitute for [InBr5·H2O]2– ions in the parent hydrate phase. This work provides new insight on the spontaneous formation of halide perovskite nanocrystals in an inert matrix, one that does not rely on the use of organic solvents and is stable in ambient atmospheres.

The limits of near field immersion microwave microscopy evaluated by imaging bilayer graphene moir\xe9 patterns

Near field scanning Microwave Impedance Microscopy can resolve structures as small as 1 nm using radiation with wavelengths of 0.1 m. Combining liquid immersion microscopy concepts with exquisite force control exerted on nanoscale water menisci, concentration of electromagnetic fields in nanometer-size regions was achieved. As a test material we use twisted bilayer graphene, because it provides a sample where the modulation of the moiré superstructure pattern can be systematically tuned from Ångstroms up to tens of nanometers. Here we demonstrate that a probe-to-pattern resolution of 108 can be obtained by analyzing and adjusting the tip-sample distance influence on the dynamics of water meniscus formation and stability.

Stochastic Analysis of Electron Transfer and Mass Transport in Confined Solid/Liquid Interfaces

Molecular-level understanding of electrified solid/liquid interfaces has recently been enabled thanks to the development of novel in situ/operando spectroscopic tools. Among those, ambient pressure photoelectron spectroscopy performed in the tender/hard X-ray region and coupled with the “dip and pull” method makes it possible to simultaneously interrogate the chemical composition of the interface and built-in electrical potentials. On the other hand, only thin liquid films (on the order of tens of nanometers at most) can be investigated, since the photo-emitted electrons must travel through the electrolyte layer to reach the photoelectron analyzer. Due to the challenging control and stability of nm-thick liquid films, a detailed experimental electrochemical investigation of such thin electrolyte layers is still lacking. This work therefore aims at characterizing the electrochemical behavior of solid/liquid interfaces when confined in nanometer-sized regions using a stochastic simulation approach. The investigation was performed by modeling (i) the electron transfer between a solid surface and a one-electron redox couple and (ii) its diffusion in solution. Our findings show that the well-known thin-layer voltammetry theory elaborated by Hubbard can be successfully applied to describe the voltammetric behavior of such nanometer-sized interfaces. We also provide an estimation of the current densities developed in these confined interfaces, resulting in values on the order of few hundreds of nA·cm−2. We believe that our results can contribute to the comprehension of the physical/chemical properties of nano-interfaces, thereby aiding to a better understanding of the capabilities and limitations of the “dip and pull” method.

Unidirectional and Selective Proton Transport in Artificial Heterostructured Nanochannels with Nano-to-Subnano Confined Water Clusters.

The construction of biological proton channel analogues has attracted substantial interest owing to their wide potential in separation of ions, sensing, and energy conversion. Here, metal-organic framework (MOF)/polymer heterogeneous nanochannels are presented, in which water molecules are confined to disordered clusters in the nanometer-sized polymer regions and to ordered chains with unique molecular configurations in the 1D sub-1-nm porous MOF regions, to realize unidirectional, fast, and selective proton transport properties, analogous to natural proton channels. Given the nano-to-subnano confined water junctions, experimental proton conductivities in the polymer-to-MOF direction of the channels are much higher than those in the opposite direction, showing a high rectification up to 500 and one to two orders of magnitude enhancement compared to the conductivity of proton transport in bulk water. The channels also show a good proton selectivity over other cations. Theoretical simulations further reveal that the preferential and fast proton conduction in the nano-to-subnano channel direction is attributed to extremely low energy barriers for proton transport from disordered to ordered water clusters. This study opens a novel approach to regulate ion permeability and selectivity of artificial ion channels.

Analysis of I​G​Z​O​​​ Crystalline Structure and Its Stability by First-Principles Calculations

This study discusses the effect of the IGZO crystalline regions on the basis of the theoretical analysis by first-principles calculations. We created two models by quantum molecular dynamics simulation: a crystalline IGZO model having a nanometer-sized crystalline region and an amorphous region, and an amorphous IGZO model having no crystalline region. According to energy comparison between the two models, the total energy of the crystalline IGZO model is lower than that of the amorphous IGZO model. This suggests that the crystalline region contributes to a stable structure in the bulk of an IGZO film. Furthermore, formation energies of HO defects, which are regarded as carrier sources, were calculated. Formation energies were high in the following order: the center of the crystalline region, the edge of the crystalline region, and the amorphous region. The results suggest that HO defects are less likely to be generated in the region with high crystallinity.

Quantum dot plasmonics: from weak to strong coupling

Abstract The complementary optical properties of surface plasmon excitations of metal nanostructures and long-lived excitations of semiconductor quantum dots (QDs) make them excellent candidates for studies of optical coupling at the nanoscale level. Plasmonic devices confine light to nanometer-sized regions of space, which turns them into effective cavities for quantum emitters. QDs possess large oscillator strengths and high photostability, making them useful for studies down to the single-particle level. Depending on structure and energy scales, QD excitons and surface plasmons (SPs) can couple either weakly or strongly, resulting in different unique optical properties. While in the weak coupling regime plasmonic cavities (PCs) mostly enhance the radiative rate of an emitter, in the strong coupling regime the energy level of the two systems mix together, forming coupled matter-light states. The interaction of QD excitons with PCs has been widely investigated experimentally as well as theoretically, with an eye on potential applications ranging from sensing to quantum information technology. In this review we provide a comprehensive introduction to this exciting field of current research, and an overview of studies of QD-plasmon systems in the weak and strong coupling regimes.

Microstructural Investigation of CoCrFeMnNi High Entropy Alloy Oxynitride Films Prepared by Sputtering Using an Air Gas

Microstructural properties of as-grown and annealed CoCrFeMnNi high entropy alloy (HEA) oxynitride thin films were investigated. The CoCrFeMnNi HEA oxynitride thin film was grown by magnetron sputtering method using an air gas, and annealed under the argon plus air flow for 5 h at 800 °C. The as-grown film was homogeneous and uniform composed of nanometer-sized crystalline regions mixed with amorphous-like phase. The crystalline phase in the as-grown film was face centered cubic structure with the lattice constant of 0.4242 nm. Significant microstructural changes were observed after the annealing process. First, it was fully recrystallized and grain growth happened. Second, Ni-rich region was observed in nanometer-scale range. Third, phase change happened and it was determined to be Fe3O4 spinel structure with the lattice constant of 0.8326 nm. Hardness and Young’s modulus of the as-grown film were 4.1 and 150.5 GPa, while those were 9.4 and 156.4 GPa for the annealed film, respectively.

Thermal stability of the Ti-Zr-Cu-Pd nano-glassy thin films

Nano-glassy thin films consist of nanometer-sized glassy regions (clusters) with a structure corresponding to melt-quenched glasses and amorphous interfacial regions characterized by a reduced density, or locally enhanced free volume, and in many cases a modified chemical composition. Ti-Zr-Cu-Pd nano-glass thin films were synthesized by using direct current (DC) magnetron sputtering on Si substrates. The influence of the sputtering conditions (sputtering power and Ar gas pressure) on the nano- and microstructure of deposited films and on their thermal behavior was analyzed. The thin films were noted to exhibit a homogeneous amorphous structure when sputtered at low Ar pressure (0.2 Pa) and a nano-glass structure with various sizes of the glassy cluster at higher Ar pressures (0.5–0.8 Pa). By raising the Ar pressure, the average size of the glassy clusters was noted to decrease corresponding to an increase of the volume fraction of the interfacial regions. The glass transition and crystallization temperatures were determined by using differential scanning calorimetry (DSC) at heating rates up to 40 Kmin−1. The thermal stability of the thin films was studied by annealing at temperatures above the glass transition and crystallization temperatures. The microstructure of the thin films was studied by means of X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy. The results obtained indicate that the nano-glassy thin films exhibit higher thermal stability than homogeneous glassy thin films. The fact that an increase of the volume fraction of the interfacial component in nano-glassy thin films results in an ultra-stable thermal behavior is indicating that the presence of nano-glassy interfaces impedes crystallization.

Zinc blende–wurtzite polytypism in nanocrystalline ZnO films

ZnO is commonly observed and assumed to crystallize fully in a wurtzite structure. Indeed, while polytypism between a wurtzite and a zinc blende phase is often seen in some II-VI compounds, it has only rarely been observed in ZnO. Here, by means of automated crystal orientation mapping and high-resolution imaging in transmission electron microscopy, we identify nanometer-sized regions of zinc blende forming during the growth of compact nanocrystalline metal–organic chemical vapor deposited ZnO films. We analyze a c-fiber-textured wurtzite ZnO film, where the columnar grain growth, typical for polycrystalline films, is interrupted by the formation of these zinc blende regions. These regions in turn provide surfaces for the heteroepitaxial renucleation of newly oriented wurtzite grains. In contrast to the columnar grains, which grow fastest along their c-axis, these new grains are identified to grow fastest along their basal plane. By means of convergent beam electron diffraction, we show that this difference in fast growth direction relates to the polarity of exposed facets. A growth model is proposed, showing how this polytypism affects the microstructure evolution of the film in ways that were not predicted by existing film growth models.

Data bit stored on single atom

In the drive to cram ever more information into handy data-storage devices, researchers have reduced the size of a bit of data to the ultimate limit—a single atom (Nature 2017, DOI: 10.1038/nature21371). The study may lead to ways of increasing the data-storage density of devices such as computer hard drives, in which information is encoded in magnetic materials. To store data on a computer hard drive, a device known as a read-write head rapidly magnetizes nanometer-sized regions of the hard disk. That process sets the magnetic polarities of these domains, or bits, in one of two states, corresponding to the zeros and ones of digital data. The head reads the data by sensing the magnetic state of the domains. For decades, hard-disk manufacturers have been increasing their devices’ data-storage densities by gradually shrinking the size of the magnetic domains in which data are stored. One challenge has been ensuring that

Reducing low-frequency noise during reversible fluctuations

The noise from most materials exhibits a power-spectral density that tends to diverge as S(f) ∝ 1/f at low frequencies, f. A fundamental mechanism for this 1/f noise comes from the thermodynamics of small systems applied to reversible fluctuations of nanometer-sized regions inside bulk samples. Here this “nanothermodynamics” is used to derive a nonlinear correction to Boltzmann’s factor. Specifically: Boltzmann’s factor comes from the first-order (linear) derivative of entropy with respect to energy, whereas the nonlinear correction comes from higher-order terms. The nonlinear correction is applied to Monte Carlo simulations of small regions in the Ising model, yielding a low-frequency crossover to white noise that keeps the power-spectral density finite as f → 0. It is shown that the low-frequency noise in the model is reduced by reducing the size of the regions.

Progress of nanostructured metallic glasses

Today's technologies are primarily based on crystalline materials (metals, semiconductors, etc.), for their properties can be controlled by changing their chemical and/or defect microstructures. This is not possible in today's glasses. The new features of nanostructured glasses consisting of nanometer-sized glassy regions connected by interfaces are that their properties may be controlled by changing their chemical and/or defect microstructures, and that their interfaces each have a new kind of non-crystalline structure. In this paper we mainly discuss the research progress of nanostructured metallic glasses, including their preparation methods, structure characteristics and new properties. By utilizing these new features, an era of new technologies based on non-crystalline materials (a “glass age”) can be opened up.

Nanophotoactivity of Porphyrin Functionalized Polycrystalline ZnO Films.

Kelvin probe force microscopy in darkness and under illumination is reported to provide nanoscale-resolved surface photovoltage maps of hybrid materials. In particular, nanoscale charge injection and charge recombination mechanisms occurring in ZnO polycrystalline surfaces functionalized with Protoporphyrin IX (H2PPIX) are analyzed. Local surface potential and surface photovoltage maps not only reveal that upon molecular adsorption the bare ZnO work function increases, but also they allow study of its local dependence. Nanometer-sized regions not correlated with apparent topographic features were identified, presenting values significantly different from the average work function. Depending on the region, the response to the light excitation is different, distinguishing two relaxation processes, one faster than the other. This behavior can be explained in terms of electrons trapped closed to the molecule-semiconductor interface or electrons pushed into the ZnO bulk, respectively. Moreover, the origin of these differences is correlated with the H2PPIX-ZnO bonding and molecules configuration and aggregation. The chenodeoxycholic acid (CDCA) coadsorption leads to a more homogeneous surface potential distribution, confirming the antiaggregate effect of this additive, while the surface photovoltage is mostly dominated by the slow relaxation component. This work reveals the complexity of real device architectures with ill-defined surfaces even in a relatively simple system with only one type of dye molecule and hightlights the importance of nanoscale characterization with appropriate tools.

Gap-Plasmon-Enhanced Nanofocusing Near-Field Microscopy

We report the observation of coherent light scattering from nanometer-sized gap regions in a nanofocusing scanning near-field optical microscope. When approaching a nanofocusing gold taper to the surface of a thin semitransparent gold film and detecting in transmission, we find a steep increase in scattering intensity over the last 5 nm in a near-field signal selected in k-space. This is confirmed as a signature of highly confined gap plasmons by detailed comparisons to finite element method simulations. The simulations reveal that the confinement is adjustable via the underlying probe–sample distance control scheme even to levels well below the taper apex radius. This controlled experimental realization of gap plasmons and the extraction of their signature in a scanning probe microscope pave the way toward broadband spectroscopy at and below single-nanometer length scales, using parallel detection at multiple wavelengths, for instance, in transient absorption or two-dimensional spectroscopy.

Nanoglasses: A New Kind of Noncrystalline Material and the Way to an Age of New Technologies?

Today's technologies are primarily based on crystalline materials (metals, semiconductors, etc.), as their properties can be controlled by varying their chemical and/or defect microstructures. This is not possible in today's glasses. The new features of nanoglasses--consisting of nanometer-sized glassy regions connected by interfaces--are that their properties may be controlled by varying their chemical and/or defect microstructures, and that their interfaces have a new kind of non-crystalline structure. By utilizing these new features, an age of new technologies based on non-crystalline materials (a "glass age") may be initiated.

Polycrystallinity of green rust minerals and their synthetic analogs: Implications for particle formation and reactivity in complex systems

We demonstrate in this study that natural green rust nanoparticles and their synthetic analogs can be complex polycrystalline phases composed of crystallites only a few nanometers in size, and they often include nanometer-sized regions of amorphous material. The natural green rusts are Zn-bearing pseudo-hexagonal platelets previously identified by us in the contaminated mine drainage of the former Ronneburg uranium mine in Germany (Johnson et al. 2014). We also identified Ni- and Cu-bearing green rust platelets in the sediment underlying the drainage outflow 20 m downstream, and, using dark-field transmission electron microscopy (DF-TEM), found that these natural green rusts are not usually structurally coherent single crystals. Synthetic sulfate green rusts are also polycrystalline and composed of crystallites of only a few nanometers in size, though different synthesis conditions produced different patterns of polycrystallinity. While pseudo-hexagonal platelets are the typical morphology of green rust, we also synthesized green rust nanorods, which have not previously been reported. In addition to the known characteristics of green rusts (including a very large aspect ratio and surface area to volume ratio, and the redox properties allowed by the structural mixture of Fe 2+ and Fe 3+ ), these polycrystalline platelets exhibit a high abundance of defect sites and likely a rough surface topography. The combination of these characteristics has important implications for the reactivity of green rust with biogeochemical interfaces in natural and anthropogenic systems.

Tracing temperature in a nanometer size region in a picosecond time period.

Irradiation of materials with either swift heavy ions or slow highly charged ions leads to ultrafast heating on a timescale of several picosecond in a region of several nanometer. This ultrafast local heating result in formation of nanostructures, which provide a number of potential applications in nanotechnologies. These nanostructures are believed to be formed when the local temperature rises beyond the melting or boiling point of the material. Conventional techniques, however, are not applicable to measure temperature in such a localized region in a short time period. Here, we propose a novel method for tracing temperature in a nanometer region in a picosecond time period by utilizing desorption of gold nanoparticles around the ion impact position. The feasibility is examined by comparing with the temperature evolution predicted by a theoretical model.

Ultrafast Electron Emission from a Sharp Metal Nanotaper Driven by Adiabatic Nanofocusing of Surface Plasmons.

We report photoelectron emission from the apex of a sharp gold nanotaper illuminated via grating coupling at a distance of 50 μm from the emission site with few-cycle near-infrared laser pulses. We find a fifty-fold increase in electron yield over that for direct apex illumination. Spatial localization of the electron emission to a nanometer-sized region is demonstrated by point-projection microscopic imaging of a silver nanowire. Our results reveal negligible plasmon-induced electron emission from the taper shaft and thus efficient nanofocusing of few-cycle plasmon wavepackets. This novel, remotely driven emission scheme offers a particularly compact source of ultrashort electron pulses of immediate interest for miniaturized electron microscopy and diffraction schemes with ultrahigh time resolution.

Tailoring crystallinity and configuration of silica nanotubes by electron irradiation

SiO2 nanotubes show potential in applications such as nanoscale electronic and optical devices, bioseparation, biocatalysis, and nanomedicine. As-grown SiO2 nanotubes in the previous studies always have an amorphous wall, and here we demonstrate the successful synthesis of single-crystal nanotubes for the first time by the heat treatment of SiC nanotubes at 1300°C for 10h under low-vacuum conditions. According to TEM observations, the single-crystal SiO2 was α-cristobalite. We also demonstrate that single-crystal SiO2 nanotubes can be transformed into amorphous SiO2 nanotubes by electron beam irradiation. Moreover, we synthesized a crystalline/amorphous SiO2 composite nanotube, in which crystalline and amorphous SiO2 coexisted in different localized regions. In addition, for biomedical applications such as drug delivery systems, controlling the configuration of the open end, the diameter, and capsulation of SiO2 nanotubes is crucial. We can also obturate, capsulate, and cut a SiO2 nanotube, as well as modify the inner diameter of the nanotube at a specific, nanometer-sized region using the focused electron beam irradiation technique.

Low-temperature thermal properties of the aqueous solutions of simple aminoalcohols: Finding of peculiarities in 1-amino-2-propanol solutions

Low-temperature heat capacity (Cp) and glass-transition temperature (Tg) values of the aqueous solutions of three simple aminoalcohols (hereafter abbreviated as AA); i.e., 2-aminoethanol (2AE), 3-amino-1-propanol (3A1P) and 1-amino-2-propanol (1A2P), were examined by differential scanning calorimetry (DSC). Both the 2AE and 3A1P aqueous solutions revealed gradual decrease of Cp values on cooling in an AA molar-fraction (xAA) in the range of 0.2–0.7, while displaying no crystallization. On the other hand, the 1A2P aqueous solutions revealed a Cp hump at the temperature slightly above Tg on both cooling and heating curves when the xAA approaches 0.19. The Tg values of the 1A2P solutions simultaneously displayed a maximum as a function of the xAA, which suggests formation of an ordered aggregate structure with development of a certain hydrogen-bond network. We propose, as the origin of the anomalies, formation of a molecular-aggregate structure of nanometer-sized regions of two kinds; one region is composed of the hydrophilic groups of AA molecules and water molecules and the other of the hydrophobic groups of AA molecules.