仲云雷

The microscopic mechanism for nucleation and growth of nano-sized bubbles within 2D flakes is thoroughly revealed by in situ transmission electron microscopy observations. The nucleation is initiated by spinodal decomposition of solid precursor, which then leads to irregular voids before reorganizing into spherical bubbles. Afterward, the bubbles grow and merge each other via formation of either metastable or unstable necks.

Abstract

The nucleation and growth of bubbles within a solid matrix is a ubiquitous phenomenon that affects many natural and synthetic processes. However, such a bubbling process is almost “invisible” to common characterization methods because it has an intrinsically multiphased nature and occurs on very short time/length scales. Using in situ transmission electron microscopy to explore the decomposition of a solid precursor that emits gaseous byproducts, the direct observation of a complete nanoscale bubbling process confined in ultrathin 2D flakes is presented here. This result suggests a three-step pathway for bubble formation in the confined environment: void formation via spinodal decomposition, bubble nucleation from the spherization of voids, and bubble growth by coalescence. Furthermore, the systematic kinetics analysis based on COMSOL simulations shows that bubble growth is actually achieved by developing metastable or unstable necks between neighboring bubbles before coalescing into one. This thorough understanding of the bubbling mechanism in a confined geometry has implications for refining modern nucleation theories and controlling bubble-related processes in the fabrication of advanced materials (i.e., topological porous materials).

A fully bottom-up technique for fabricating a self-releasing ultrathin silicon wafer without sacrificing any of the substrate is presented, whereas conventional technologies waste large amounts of such material. A plasma-assisted epitaxially grown silicon seed layer with a self-organized nanogap is a key for the realization of the fully bottom-up process. The results represent a technological breakthrough in advanced silicon microelectronics and photovoltaics.

Abstract

The fabrication of ultrathin silicon wafers at low cost is crucial for advancing silicon electronics toward stretchability and flexibility. However, conventional fabrication techniques are inefficient because they sacrifice a large amount of substrate material. Thus, advanced silicon electronics that have been realized in laboratories cannot move forward to commercialization. Here, a fully bottom-up technique for producing a self-releasing ultrathin silicon wafer without sacrificing any of the substrate is presented. The key to this approach is a self-organized nanogap on the substrate fabricated by plasma-assisted epitaxial growth (plasma-epi) and subsequent hydrogen annealing. The wafer thickness can be independently controlled during the bulk growth after the formation of plasma-epi seed layer. In addition, semiconductor devices are realized using the ultrathin silicon wafer. Given the high scalability of plasma-epi and its compatibility with conventional semiconductor process, the proposed bottom-up wafer fabrication process will open a new route to developing advanced silicon electronics.

Air-stable large-area 2D-In _{x Ga 1−x} alloys with tunable composition and no evidence of phase segregation are realized by confinement heteroepitaxy. The optical and electronic properties directly correlate with alloy composition, wherein the dielectric function, band structure, superconductivity, and charge transfer from the metal to graphene are all controlled by the indium/gallium ratio in the 2D metal layer.

Abstract

Chemically stable quantum-confined 2D metals are of interest in next-generation nanoscale quantum devices. Bottom-up design and synthesis of such metals could enable the creation of materials with tailored, on-demand, electronic and optical properties for applications that utilize tunable plasmonic coupling, optical nonlinearity, epsilon-near-zero behavior, or wavelength-specific light trapping. In this work, it is demonstrated that the electronic, superconducting, and optical properties of air-stable 2D metals can be controllably tuned by the formation of alloys. Environmentally robust large-area 2D-In _x Ga_{1− x} alloys are synthesized byConfinement Heteroepitaxy (CHet). Near-complete solid solubility is achieved with no evidence of phase segregation, and the composition is tunable over the full range of x by changing the relative elemental composition of the precursor. The optical and electronic properties directly correlate with alloy composition, wherein the dielectric function, band structure, superconductivity, and charge transfer from the metal to graphene are all controlled by the indium/gallium ratio in the 2D metal layer.

Nature Nanotechnology, Published online: 02 September 2021; doi:10.1038/s41565-021-00963-8

Nature, Published online: 01 September 2021; doi:10.1038/s41586-021-03926-0

Electrostatically defined quantum confinement in large-area chemical vapor deposition-grown 2D WS₂ with HfO₂ dielectrics grown by atomic layer deposition is reported. This marks a key milestone in scalable approaches toward 2D-semiconductor-based quantum devices, which has hitherto only been demonstrated with micrometer-sized exfoliated flakes. The measurements show that low-temperature carrier mobility is not charge impurity limited as commonly thought, but is due to another important but commonly overlooked factor: interface roughness.

Abstract

Temperature-dependent transport measurements are performed on the same set of chemical vapor deposition (CVD)-grown WS₂ single- and bilayer devices before and after atomic layer deposition (ALD) of HfO₂. This isolates the influence of HfO₂ deposition on low-temperature carrier transport and shows that carrier mobility is not charge impurity limited as commonly thought, but due to another important but commonly overlooked factor: interface roughness. This finding is corroborated by circular dichroic photoluminescence spectroscopy, X-ray photoemission spectroscopy, cross-sectional scanning transmission electron microscopy, carrier-transport modeling, and density functional modeling. Finally, electrostatic gate-defined quantum confinement is demonstrated using a scalable approach of large-area CVD-grown bilayer WS₂ and ALD-grown HfO₂. The high dielectric constant and low leakage current enabled by HfO₂ allows an estimated quantum dot size as small as 58 nm. The ability to lithographically define increasingly smaller devices is especially important for transition metal dichalcogenides due to their large effective masses, and should pave the way toward their use in quantum information processing applications.

Saturated molecules are not preferred in the research of organic thermoelectrics because of intrinsically wide bandgaps and poor thermopower. Graphene electrodes are demonstrated to be able to enhance thermopower of monolayers of saturated compounds. A noncovalent amine anchor induces the creation of in-gap states and n-type doping of graphene, thereby leading to an increased Seebeck coefficient relative to analogous covalent gold–thiolate monolayers.

Abstract

Enhancing thermopower is a key goal in organic and molecular thermoelectrics. Herein, it is shown that introducing noncovalent contact with a single-layer graphene (SLG) electrode improves the thermopower of saturated molecules as compared to the traditional gold–thiolate covalent contact. Thermoelectric junction measurements with a liquid-metal technique reveal that the value of Seebeck coefficient in large-area junctions based on n-alkylamine self-assembled monolayers (SAMs) on SLG is increased up to fivefold compared to the analogous junction based on n-alkanethiolate SAMs on gold. Experiments with Raman spectroscopy and field-effect transistor analysis indicate that such enhancements benefit from the creation of new in-gap states and electron doping through noncovalent interaction between the amine anchor and the SLG electrode, which leads to a reduced energy offset between the Fermi level and the transport channel. This work demonstrates that control of interfacial bonding nature in molecular junctions improves the Seebeck effect in saturated molecules.

Nature Communications, Published online: 27 August 2021; doi:10.1038/s41467-021-25262-7

Nature Materials, Published online: 25 August 2021; doi:10.1038/s41563-021-01059-3

Nature Materials, Published online: 25 August 2021; doi:10.1038/s41563-021-01055-7

Nature Materials, Published online: 26 August 2021; doi:10.1038/s41563-021-01084-2