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Three-dimensional (3D) imaging of thin, extended specimens at nanometer resolution is critical for applications in biology, materials science, advanced synthesis, and manufacturing. One route to 3D imaging is tomography, which requires a tilt series of a local region. However, capturing images at higher tilt angles is infeasible for such thin, extended specimens. Here, we explore a suitable alternative to reconstruct the 3D volume using a single, energy-filtered, bright-field coherent image. We show that when our specimen is homogeneous and amorphous, simultaneously inferring local depth and thickness for 3D imaging is possible in the near-field limit. We demonstrated this technique with a transmission electron microscope to fill a glaring gap for rapid, accessible 3D nanometrology. This technique is applicable, in general, to any coherent bright field imaging with electrons, photons, or any other wavelike particles. The authors experimentally demonstrate with a transmission electron microscope that single-shot 3D imaging is possible in the near-field limit, by simultaneously inferring local depth and thickness. The proposed reconstruction method uses priors from the homogenously amorphous specimen, and it can be extended for imaging multi-layered samples. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are considered to be promising building blocks for the next generation electronic and optoelectronic devices. Various doping schemes and work function engineering techniques have been explored to overcome the intrinsic performance limits of 2D TMDs. However, a reliable and long-time air stable doping scheme is still lacking in this field. In this work, we utilize keV ion beams of H2+ to irradiate layered WSe2 crystals and obtain efficient n-type doping effect for all irradiated crystals within a fluence of 1 × 1014 protons·cm−2 (1e14). Moreover, the irradiated WSe2 remains an n-type semiconductor even after it is exposed to ambient conditions for a year. Localized ion irradiation with a focused beam can directly pattern on the sample to make high performance homogenous p-n junction diodes. Raman and photoluminescence (PL) spectra demonstrate that the WSe2 crystal lattice stays intact after irradiation within 1e14. We attribute the reliable electron-doping to the significant increase in Se vacancies after the proton irradiation, which is confirmed by our scanning transmission electron microscope (STEM) results. Our work demonstrates a reliable and long-term air stable n-type doping scheme to realize high-performance electronic TMD devices, which is also suitable for further integration with other 2D devices. [ABSTRACT FROM AUTHOR]

Thermoelectrics enable direct heat-to-electricity transformation, but their performance has so far been restricted by the closely coupled carrier and phonon transport. Here, we demonstrate that the quantum gaps, a class of planar defects characterized by nano-sized potential wells, can decouple carrier and phonon transport by selectively scattering phonons while allowing carriers to pass effectively. We choose the van der Waals gap in GeTe-based materials as a representative example of the quantum gap to illustrate the decoupling mechanism. The nano-sized potential well of the quantum gap in GeTe-based materials is directly visualized by in situ electron holography. Moreover, a more diffused distribution of quantum gaps results in further reduction of lattice thermal conductivity, which leads to a peak ZT of 2.6 at 673 K and an average ZT of 1.6 (323–723 K) in a GeTe system. The quantum gap can also be engineered into other thermoelectrics, which provides a general method for boosting their thermoelectric performance. Defects are believed to always scatter carriers. Here, the authors find that the quantum gaps in GeTe-based materials do not scatter carriers, which decouple the carriers and phonons transport leading to high thermoelectric performance. [ABSTRACT FROM AUTHOR]

Chalcogenide phase change materials (PCMs) have been extensively applied in data storage, and they are now being proposed for high resolution displays, holographic displays, reprogrammable photonics, and all-optical neural networks. These wide-ranging applications all exploit the radical property contrast between the PCMs' different structural phases, extremely fast switching speed, long-term stability, and low energy consumption. Designing PCM photonic devices requires an accurate model to predict the response of the device during phase transitions. Here, we describe an approach that accurately predicts the microstructure and optical response of phase change materials during laser induced heating. The framework couples the Gillespie Cellular Automata approach for modelling phase transitions with effective medium theory and Fresnel equations. The accuracy of the approach is verified by comparing the PCM's optical response and microstructure evolution with the results of nanosecond laser switching experiments. We anticipate that this approach to simulating the switching response of PCMs will become an important component for designing and simulating programmable photonics devices. The method is particularly important for predicting the multi-level optical response of PCMs, which is important for all-optical neural networks and PCM-programmable perceptrons. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have been rapidly established as promising building blocks for versatile atomic scale circuits and multifunctional devices. However, the high contact resistance in TMDs based transistors seriously hinders their applications in complementary electronics. In this work, we show that an Ohmic homojunction n-type tungsten diselenide (WSe2) transistor is realized through spatially controlling cesium (Cs) doping region near the contacts. We find that the remarkable electron doping effect of Cs stimulates a semiconductor to metal (2H to 1T') phase transition in WSe2, and hence the formation of 2H-1T' hetero-phase contact. Our method significantly optimizes the WSe2 transport behavior with a perfect low subthreshold swing of ∼ 61 mV/dec and ultrahigh current on/off ratio exceeding ∼ 109. Meanwhile, the electron mobility is enhanced by nearly 50 times. We elucidate that the ideal n-type behavior originates from the negligible Schottky barrier height of ∼ 19 meV and low contact resistance of ∼ 0.9 kΩ·µm in the 2H-1T' homojunction device. Moreover, based on the Ohmic hetero-phase configuration, a WSe2 inverter is achieved with a high gain of ∼ 270 and low power consumption of ∼ 28 pW. Our findings envision Cs functionalization as an effective method to realize ideal Ohmic contact in 2D WSe2 transistors towards high performance complementary electronic devices. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) van der Waals transition metal dichalcogenides (TMDs) are a new class of electronic materials offering tremendous opportunities for advanced technologies and fundamental studies. Similar to conventional semiconductors, substitutional doping is key to tailoring their electronic properties and enabling their device applications. Here, we review recent progress in doping methods and understanding of doping effects in group 6 TMDs (MX2, M = Mo, W; X = S, Se, Te), which are the most widely studied model 2D semiconductor system. Experimental and theoretical studies have shown that a number of different elements can substitute either M or X atoms in these materials and act as n- or p-type dopants. This review will survey the impact of substitutional doping on the electrical and optical properties of these materials, discuss open questions, and provide an outlook for further studies. [ABSTRACT FROM AUTHOR]

The crystal phase-based heterostructures of noble metal nanomaterials are of great research interest for various applications, such as plasmonics and catalysis. However, the synthesis of unusual crystal phases of noble metals still remains a great challenge, making the construction of heterophase noble metal nanostructures difficult. Here, we report a one-pot wet-chemical synthesis of well-defined heterophase fcc-2H-fcc gold nanorods (fcc: face-centred cubic; 2H: hexagonal close-packed with stacking sequence of "AB") at mild conditions. Single particle-level experiments and theoretical investigations reveal that the heterophase gold nanorods demonstrate a distinct optical property compared to that of the conventional fcc gold nanorods. Moreover, the heterophase gold nanorods possess superior electrocatalytic activity for the carbon dioxide reduction reaction over their fcc counterparts under ambient conditions. First-principles calculations suggest that the boosted catalytic performance stems from the energetically favourable adsorption of reaction intermediates, endowed by the unique heterophase characteristic of gold nanorods. The crystal phase-based heterostructures of noble metal nanomaterials are of interest for various applications. Here, the authors report the wet-chemical synthesis of gold nanorods with a well-defined fcc-2H-fcc heterophase, which possess unique optical and catalytic properties. [ABSTRACT FROM AUTHOR]

Galvanic replacement (GR) is a simple and widely used approach to synthesize hollow nanostructures for applications in catalysis, plasmonics, and biomedical research. The reaction is driven by the difference in electrochemical potential between two metals in a solution. However, transient stages of this reaction are not fully understood. Here, we show using liquid cell transmission electron microscopy that silver (Ag) nanocubes become hollow via the nucleation, growth, and coalescence of voids inside the nanocubes, as they undergo GR with gold (Au) ions at different temperatures. These direct in situ observations indicate that void formation due to the nanoscale Kirkendall effect occurs in conjunction with GR. Although this mechanism has been suggested before, it has not been verified experimentally until now. These experiments can inform future strategies for deriving such nanostructures by providing insights into the structural transformations as a function of Au ion concentration, oxidation state of Au, and temperature. [ABSTRACT FROM AUTHOR]

Harnessing the optical properties of noble metals down to the nanometre scale is a key step towards fast and low-dissipative information processing. At the 10-nm length scale, metal crystallinity and patterning as well as probing of surface plasmon properties must be controlled with a challenging high level of precision. Here, we demonstrate that ultimate lateral confinement and delocalization of surface plasmon modes are simultaneously achieved in extended self-assembled networks comprising linear chains of partially fused gold nanoparticles. The spectral and spatial distributions of the surface plasmon modes associated with the colloidal superstructures are evidenced by performing monochromated electron energy-loss spectroscopy with a nanometre-sized electron probe. We prepare the metallic bead strings by electron-beam-induced interparticle fusion of nanoparticle networks. The fused superstructures retain the native morphology and crystallinity but develop very low-energy surface plasmon modes that are capable of supporting long-range and spectrally tunable propagation in nanoscale waveguides. [ABSTRACT FROM AUTHOR]

Lithography provides the precision to pattern large arrays of metallic nanostructures with varying geometries, enabling systematic studies and discoveries of new phenomena in plasmonics. However, surface plasmon resonances experience more damping in lithographically–defined structures than in chemically–synthesized nanoparticles of comparable geometries. Grain boundaries, surface roughness, substrate effects, and adhesion layers have been reported as causes of plasmon damping, but it is difficult to isolate these effects. Using monochromated electron energy–loss spectroscopy (EELS) and numerical analysis, we demonstrate an experimental technique that allows the study of these effects individually, to significantly reduce the plasmon damping in lithographically–defined structures. We introduce a method of encapsulated annealing that preserves the shape of polycrystalline gold nanostructures, while their grain-boundary density is reduced. We demonstrate enhanced Q–factors in lithographically–defined nanostructures, with intrinsic damping that matches the theoretical Drude damping limit. [ABSTRACT FROM AUTHOR]