Thin tellurium (Te) has been predicted as a potential two dimensional system exhibiting superior thermoelectric and electrical properties. Here, we report the synthesis of high quality ultrathin Te nanostructures and the study of their electrical properties at room temperature. High quality ultrathin Te nanostructures are obtained by high temperature vapor phase deposition on c-plane sapphire substrates. The obtained nanostructures are as thin as 3 nm and exhibit α-Te phase with trigonal crystal structure. Room temperature electrical measurements show significantly higher electrical conductivity compared to prior reports of Te in bulk form or in nanostructure form synthesized by low temperature vapor deposition or wet chemical methods. Additionally, these nanostructures exhibit high field effect hole mobility comparable to black-phosphorous measured previously under similar conditions.
Grain boundary (GB) solute segregation has been proposed as a new mechanism to stabilize nanocrystalline (NC) metals. In this study, we investigate the thermal stability and GB solute segregation in a noble metal alloy system (Pt–Au). Thermal stability of the Pt.90Au.10 alloy system was evaluated by annealing a thin film (∼20 nm in thickness) at 500 °C and 700 °C as well as a thick film (∼2 µm in thickness) at a temperature range from 200 °C to 700 °C. The remarkable stability of the Pt.90Au.10 alloy system was demonstrated by comparing its thermal stability to that of pure Pt films processed under identical conditions. Although presence of voids in the GBs may contribute to thermal stability, the enhanced thermal stability of the Pt.90Au.10 alloy is mainly attributed to preferential Au segregation to GBs in the alloy film, which is revealed by aberration-corrected scanning transmission electron microscopy. Our results show that Au segregation to GBs is heterogeneous, with variation in solute content between different GBs as well as non-uniformity along individual GBs. The heterogeneity is dependent on the annealing temperature and is less pronounced at a higher processing temperatures (e.g., 700 °C). By using the noble Pt–Au system, which avoids oxidation and impurities, this study validates the mechanism of GB solute segregation and provides further understanding of the thermodynamics and kinetics underlying NC stabilization.
Vertically aligned nanocomposite (VAN) films have self-assembled pillar-matrix nanostructures. Owing to their large area-to-volume ratios, interfaces in VAN films are expected to play key roles in inducing functional properties, but our understanding is hindered by limited knowledge about their structures. Motivated by the lack of definitive explanation for the experimentally found enhanced ionic conductivity in Sm-doped-CeO2/SrTiO3 VAN films, we determine the structure at vertical interfaces using random structure searching and explore how it can affect ionic conduction. Interatomic potentials are used to perform the initial searching, followed by first-principles calculations for refinement. Previously unknown structures are found, with lower energy than that of an optimized hand-built model. We find a strongly distorted oxygen sublattice which gives a complex landscape of vacancy energies. The cation lattice remains similar to the bulk phase, but has a localized strain field. The excess energy of the interface is similar to that of high angle grain boundaries in SrTiO3.
Tunable plasmonic structure at the nanometer scale presents enormous opportunities for various photonic devices. In this work, we present a hybrid plasmonic thin film platform: i.e., a vertically aligned Au nanopillar array grown inside a TiN matrix with controllable Au pillar density. Compared to single phase plasmonic materials, the presented tunable hybrid nanostructures attain optical flexibility including gradual tuning and anisotropic behavior of the complex dielectric function, resonant peak shifting and change of surface plasmon resonances (SPRs) in the UV-visible range, all confirmed by numerical simulations. The tailorable hybrid platform also demonstrates enhanced surface plasmon Raman response for Fourier-transform infrared spectroscopy (FTIR) and photoluminescence (PL) measurements, and presents great potentials as designable hybrid platforms for tunable optical-based chemical sensing applications.
A long-term goal for superconductors is to increase the superconducting transition temperature, TC. In cuprates, TC depends strongly on the out-of-plane Cu-apical oxygen distance and the in-plane Cu-O distance, but there has been little attention paid to tuning them independently. Here, in simply grown, self-assembled, vertically aligned nanocomposite thin films of La2CuO4+δ + LaCuO3, by strongly increasing out-of-plane distances without reducing in-plane distances (three-dimensional strain engineering), we achieve superconductivity up to 50 K in the vertical interface regions, spaced ∼50 nm apart. No additional process to supply excess oxygen, e.g., by ozone or high-pressure oxygen annealing, was required, as is normally the case for plain La2CuO4+δ films. Our proof-of-concept work represents an entirely new approach to increasing TC in cuprates or other superconductors.
We discuss chemical, structural, and ellipsometry characterization of low temperature epitaxial Si. While low temperature growth is not ideal, we are still able to prepare crystalline Si to cap functional atomic precision devices.
Nanocrystalline metals offer significant improvements in structural performance over conventional alloys. However, their performance is limited by grain boundary instability and limited ductility. Solute segregation has been proposed as a stabilization mechanism, however the solute atoms can embrittle grain boundaries and further degrade the toughness. In the present study, we confirm the embrittling effect of solute segregation in Pt–Au alloys. However, more importantly, we show that inhomogeneous chemical segregation to the grain boundary can lead to a new toughening mechanism termed compositional crack arrest. Energy dissipation is facilitated by the formation of nanocrack networks formed when cracks arrested at regions of the grain boundaries that were starved in the embrittling element. This mechanism, in concert with triple junction crack arrest, provides pathways to optimize both thermal stability and energy dissipation. A combination of in situ tensile deformation experiments and molecular dynamics simulations elucidate both the embrittling and toughening processes that can occur as a function of solute content.
Magnetic property enhancement of alnico, a rare-earth free permanent magnet, is highly dependent on both the initial microstructure and the evolution of the spinodal decomposition (SD) inside each grain during the heat treatment process. The size, shape and distribution of the magnetic FeCo-rich (α1) phase embedded in continuous non-magnetic AlNi-rich (α2) phase as well as a minor Cu-enriched phase residing in between are shown to be crucial in controlling coercivity. Phase and magnetic domain morphology in a commercial alnico 9 alloy was studied using a combination of structural characterization techniques, including scanning electron microscopy, electron backscatter diffraction, aberration-corrected scanning transmission electron microscopy and Lorentz microscopy. Our results showed that casting created structural nonuniformity and defects, such as porosity, TiS2 precipitates and grain misorientation, are heterogeneously distributed, with the center section having the best crystallographic orientation and minimal defects. The optimal spinodal is a “mosaic structure”, composed of rod-shape α1 phase with {110} or {100} planar faceting and diameter of ~30–45nm. There is also a Cu-enriched phase residing at the corners of two < 110 > facets of the α1 phase. Furthermore, it was observed that grain boundary phase reverse magnetization direction at lower external magnetic field compared to the SD region inside the grain. Improving grain orientation uniformity, reducing detrimental grain boundary phase volume fraction, and the branching of the α1 rods, as well as their diameter, are promising routes to improve energy product of alnico.
Quantum-size-controlled photoelectrochemical (QSC-PEC) etching, which uses quantum confinement effects to control size, can potentially enable the fabrication of epitaxial quantum nanostructures with unprecedented accuracy and precision across a wide range of materials systems. However, many open questions remain about this new technique, including its limitations and broader applicability. In this project, using an integrated experimental and theoretical modeling approach, we pursue a greater understanding of the time-dependent QSC-PEC etch process and to uncover the underlying mechanisms that determine its ultimate accuracy and precision. We also seek to broaden our understanding of the scope of its ultimate applicability in emerging nanostructures and nanodevices.
Recent work suggests that thermally stable nanocrystallinity in metals is achievable in several binary alloys by modifying grain boundary energies via solute segregation. The remarkable thermal stability of these alloys has been demonstrated in recent reports, with many alloys exhibiting negligible grain growth during prolonged exposure to near-melting temperatures. Pt–Au, a proposed stable alloy consisting of two noble metals, is shown to exhibit extraordinary resistance to wear. Ultralow wear rates, less than a monolayer of material removed per sliding pass, are measured for Pt–Au thin films at a maximum Hertz contact stress of up to 1.1 GPa. This is the first instance of an all-metallic material exhibiting a specific wear rate on the order of 10−9 mm3 N−1 m−1, comparable to diamond-like carbon (DLC) and sapphire. Remarkably, the wear rate of sapphire and silicon nitride probes used in wear experiments are either higher or comparable to that of the Pt–Au alloy, despite the substantially higher hardness of the ceramic probe materials. High-resolution microscopy shows negligible surface microstructural evolution in the wear tracks after 100k sliding passes. Mitigation of fatigue-driven delamination enables a transition to wear by atomic attrition, a regime previously limited to highly wear-resistant materials such as DLC.
For this study, diamond-like carbon (DLC) films were tribochemically formed from ambient hydrocarbons on the surface of a highly stable nanocrystalline Pt-Au alloy. A sliding contact between an alumina sphere and Pt-Au coated steel exhibited friction coefficients as low as μ = 0.01 after dry sliding in environments containing trace (ppb) organics. Ex situ analysis indicated that the change in friction coefficient was due to the formation of amorphous carbon films, and Raman spectroscopy and elastic recoil analysis showed that these films consist of sp2/sp3 amorphous carbon with as much as 20% hydrogen. Transmission electron microscopy indicated these films had thicknesses exceeding 100 nm, and were enhanced by the incorporation of worn Pt-Au nanoparticles. The result was highly wear-resistant, low-friction DLC/Pt-Au nanocomposites. Atomistic simulations of hydrocarbons under shear between rigid Pt slabs using a reactive force field showed stress-induced changes in bonding through chain scission, a likely route towards the formation of these coatings. This novel demonstration of in situ tribochemical formation of self-lubricating films has significant impact potential in a wide range of engineering applications.