In this work, we present a method for growing highly
-axis oriented aluminum scandium nitride (AlScN) thin films on (100) silicon (Si), silicon dioxide (SiO
) and epitaxial polysilicon (poly-Si) ...substrates using a substrate independent approach. The presented method offers great advantages in applications such as piezoelectric thin-film-based surface acoustic wave devices where a metallic seed layer cannot be used. The approach relies on a thin AlN layer to establish a wurtzite nucleation layer for the growth of
-AlScN films. Both AlScN thin film and seed layer AlN are prepared by DC reactive magnetron sputtering process where a Sc concentration of 27% is used throughout this study. The crystal quality of (0002) orientation of Al
Sc
N films on all three substrates is significantly improved by introducing a 20 nm AlN seed layer. Although AlN has a smaller capacitance than AlScN, limiting the charge stored on the electrode plates, the combined piezoelectric coefficient d
with 500 nm AlScN is only slightly reduced by about 4.5% in the presence of the seed layer.
In recent years, semiconductor photocatalysts such as zinc oxide (ZnO) and titanium dioxide (TiO2) have been studied to utilize solar light for energy production and environmental remediation. ...However, their wide band gap energy and fast charge recombination limit their practical applications. So far, various approaches, such as noble metal incorporation, metal oxide coupling, and the design of Z-scheme photocatalyst systems, among others, have been explored to enhance the lifetime of photogenerated charge carriers in ZnO and TiO2. In this study, a novel TiO2-Ag-ZnO nanocomposite photocatalyst has been successfully synthesized by a combination of wet chemistry, photochemistry, and plasma-based processes. Simple spin coating of ZnO thin films was coupled with subsequent photodeposition of metallic Ag nanoparticles (NPs). Afterward, well-uniform TiO2 NPs (roughly 18 nm) were deposited on this unique Ag-ZnO surface via a gas cluster aggregation source (GAS) process. The results showed that the activity of the ZnO photocatalyst under mixed UV-visible light was significantly increased by the TiO2 and Ag contributions. A single mechanism to be responsible for the photocatalytic activity can be excluded. Instead, a critical assessment of the possible mechanisms corresponding to the different interfaces present at the TiO2-Ag-ZnO surface including a Z–scheme is provided.
Memristive devices are under intense development as non‐volatile memory elements for extending the computing capabilities of traditional silicon technology by enabling novel computing primitives. In ...this respect, interface‐based memristive devices are promising candidates to emulate synaptic functionalities in neuromorphic circuits aiming to replicate the information processing of nervous systems. A device composed of Nb/NbOx/Al2O3/HfO2/Au that shows promising features like analog switching, no electro‐forming, and high current‐voltage non‐linearity is reported. Synchrotron‐based X‐ray photoelectron spectroscopy and depth‐dependent hard X‐ray photoelectron spectroscopy are used to probe in situ different resistance states and thus the origin of memristive switching. Spectroscopic evidence for memristive switching based on the charge state of electron traps within HfO2 is found. Electron energy loss spectroscopy and transmission electron microscopy support the analysis. A device model is proposed that considers a two‐terminal metal–insulator–semiconductor structure in which traps within the insulator (HfO2/Al2O3) modulate the space charge region within the semiconductor (NbOx) and, thereby, the overall resistance. The experimental findings are in line with impedance spectroscopy data reported in the companion paper (Marquardt et al). Both works complement one another to derive a detailed device model, which helps to engineer device performance and integrate devices into silicon technology.
The switching mechanism of an HfO2‐based analog interface‐type memristive device is investigated by depth‐dependent hard X‐ray photoelectron spectroscopy on functional devices. Spectroscopic evidence for switching based on the charge state of electron traps within HfO2 is found. The findings coincide with impedance spectroscopy data reported in a companion paper. The proposed device model helps to engineer device performance and integrate devices into silicon technology.
Plasma‐electrochemical deposition: Metals can be deposited on the surface of an ionic liquid by electrochemical reduction with free electrons from a low‐temperature plasma (see picture). This unusual ...reaction represents a novel electrochemical process—leading to the reproducible growth of nanoscale products. The preparation of silver nanoparticles is presented as an experimental example.
The morphology of nanocomposites plays a pivotal role in understanding their functionality and determines their capabilities for applications. The use of nanocomposite coatings requires a study of ...the size effects on their functional properties. Noble metal nanoparticles are promising candidates for nanocomposite thin film applications due to their antibacterial, plasmonic and photocatalytic properties. In this contribution, the morphology of Ag-TiO2 and Au-TiO2 nanocomposite thin films has been investigated experimentally using electron tomography in transmission electron microscopy in combination with UV/vis spectroscopy. Based on the additional 3D information obtained from tomography, we propose a two-step model towards the observed bimodal particle size in these nanocomposite thin films prepared by co-sputtering from two different sources. Furthermore, we show that the optical properties exhibit a well-defined relation with the morphology of the nanocomposite thin films. The present investigations demonstrate the potential of electron tomography for revealing the complex structure and formation processes of functional nanocomposites.
Upon slow crystallization from silica‐containing solutions or gels at elevated pH, alkaline‐earth carbonates spontaneously self‐assemble into remarkable nanocrystalline ultrastructures. These ...so‐called silica biomorphs exhibit curved morphologies beyond crystallographic symmetry and ordered textures reminiscent of the hierarchical design found in many biominerals. The formation of these fascinating materials is thought to be driven by a dynamic coupling of the components’ speciations in solution, which causes concerted autocatalytic mineralization of silica‐stabilized nanocrystals over hours. In the present work, we have studied the precipitation kinetics of this unique system by determining growth rates of individual aggregates using video microscopy, and correlated the results with time‐dependent data on the concentration of metal ions and pH acquired online during crystallization. In this manner, insight to the evolution of chemical conditions during growth was gained. It is shown that crystallization proceeds linearly with time and is essentially reaction controlled, which fits well in the proposed morphogenetic scenario, and thus, indirectly supports it. Measurements of the silica concentration in solution, combined with analyses of crystal aggregates isolated at distinct stages of morphogenesis, further demonstrate that the fraction of silica coprecipitated with carbonate during active growth is rather small. We discuss our findings with respect to the role of silica in the formation of biomorphs, and moreover, prove that the external silica skins that occasionally sheath the aggregates—previously supposed to be involved in the growth mechanism— originate from secondary precipitation after growth is already terminated.
Even spread: Precipitation of barium carbonate in silica‐rich media at high pH yields hierarchically structured polycrystalline materials, termed silica–carbonate biomorphs, which exhibit complex curved shapes. Determination of microscopic growth rates and reagent concentration profiles (see figure) helped to elucidate the chemistry underlying the formation of these materials and provided insight into growth kinetics and morphogenetic aspects.
Here, we report on a new aero-material, called aero-ZnS, representing self-organized architectures made of ZnS hollow micro-tetrapod structures with nanoscale thin walls. The fabrication process is ...based on the hydride vapor phase epitaxy of CdS on sacrificial micro-tetrapods of ZnO with simultaneous or subsequent transformation of CdS into ZnS and removal of the sacrificial ZnO crystals. The nanostructure of the obtained ZnS hollow micro-tetrapods exhibits the polytypic intergrowth of wurtzite- and sphalerite-type phases perpendicular to their close packed planes. The inner surface of the micro-tetrapod walls preserves oxygen sites, as demonstrated by imaging based on electron energy-loss filtering. The self-organized aero-ZnS architecture proves to be hydrophilic under tension and hydrophobic when compressed against water. Self-propelled liquid marbles assembled using ZnS hollow micro-tetrapod structures are demonstrated.
The morphology and real structure of wurtzite‐type ZnO nanospikes grown by the recently introduced flame transport synthesis have been examined by means of advanced transmission electron microscopy ...(TEM). The rapid synthesis produces nanospikes showing a well defined texture which restricts TEM experiments to a preferred viewing direction of . Forced by the specific morphology, all of the observed nanospikes show a complicated superposition of twinned domains as an intrinsic real structural feature. The high‐resolution contrasts are characterized by lamellar fringes parallel to the () planes, and the quasi‐kinematic diffraction patterns contain satellite peaks based on multiple scattering. All these phenomena can be interpreted by comparison of experimental and simulated data relying on a supercell approach.
LHT-9, a layered hydrazinium titanate with an interlayer spacing of ∼9 Å, is a new nanohybrid compound combining the redox functionality of hydrazine, the ion-exchange properties of layered titanate, ...the large surface area of quasi-two-dimensional crystallites, surface Brønsted acidity, and the occurrence of surface titanyl bonds. LHT-9, ideally formulated as (N2H5)1/2Ti1.87O4, relates to a family of lepidocrocite-type titanates. It possesses a high uptake capacity of ∼50 elements of the periodic table. Irreversibility of reductive adsorption allows LHT-9 to be used for cumulative extraction of reducible moieties (noble metals, chromate, mercury, etc.) from industrial solutions and wastewaters. Unlike sodium titanates that do not tolerate an acidic environment, LHT-9 is capable of uptake of transition metals and lanthanides at pH > 3. Adsorption products loaded with the desired elements retain their layered structures and can be used as precursors for tailored titanium dioxide nanomaterials. In this respect, the uptake of metal ions by LHT-9 can be considered as a method complementary to electrostatic self-assembly deposition (ESD) and layer-by-layer self-assembly (LBL) techniques. LHT-9 is readily synthesized in one step by a mild fluoride route involving hydrazine-induced hydrolysis of hexafluorotitanic acid under near-ambient conditions.
Joints of three-dimensional (3D) rutile-type (r) tin dioxide (SnO
2
) nanowire networks, produced by the flame transport synthesis (FTS), are formed by coherent twin boundaries at (101)
r
serving for ...the interpenetration of the nanowires. Transmission electron microscopy (TEM) methods, i.e. high resolution and (precession) electron diffraction (PED), were utilized to collect information of the atomic interface structure along the edge-on zone axes 010
r
, 111
r
and superposition directions 001
r
, 101
r
. A model of the twin boundary is generated by a supercell approach, serving as base for simulations of all given real and reciprocal space data as for the elaboration of three-dimensional, i.e. relrod and higher order Laue zones (HOLZ), contributions to the intensity distribution of PED patterns. Confirmed by the comparison of simulated and experimental findings, details of the structural distortion at the twin boundary can be demonstrated.
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