The uplift processes of the Qilian Shan (northern Tibetan Plateau) play a central role in our understanding of the dynamics of outward and upward growth of Tibet due to sustained convergence of the Indian and Asian plates. We employ apatite fission track chronology and geological mapping to reveal the time and pattern of the deformation along the Qilian Shan. Our results indicate that the emergence of the Tuolai Shan in the central Qilian Shan occurred at 17-14 Ma, that northern Qilian Shan thrusting began at 10-8 Ma, and that the Laojunmiao anticline formed ca. 3.6 Ma. Together with previous results that show that uplift of the southern Qilian Shan began in the Oligocene, we suggest that the Qilian Shan has undergone progressively northward expansion in the Cenozoic due to significant crustal shortening driven by Qilian Shan thrust fault systems.

Mechanical exfoliation from bulk layered crystal is widely used for preparing two-dimensional (2D) layered materials, which involves not only out-of-plane interlayer cleavage but also in-plane fracture. Through a statistical analysis on the exfoliated 2D flakes, we reveal the in-plane cleavage behaviors of six representative layered materials, including graphene, h-BN, 2H phase MoS2, 1T phase PtS2, FePS3, and black phosphorus. In addition to the well-known interlayer cleavage, these 2D layered materials show a distinctive tendency to fracture along certain in-plane crystallography orientations. With theoretical modeling and analysis, these distinct in-plane cleavage behaviors can be understood as a result of the competition between the release of the elastic energy and the increase of the surface energy during the fracture process. More importantly, these in-plane cleavage behaviors provide a fast and noninvasive method using optical microscopy to identify the lattice direction of mechanical exfoliated 2D layered materials.

Conducting bridge random access memory (CBRAM) is one of the most promising candidates for future nonvolatile memories. It is important to understand the scalability and retention of CBRAM cells to realize better memory performance. Here, we directly observe the switching dynamics of Cu tip/SiO/W cells with various active electrode sizes using in situ transmission electron microscopy. Conducting filaments (CFs) grow from the active electrode (Cu tip) to inert electrode (W) during the SET operations. The size of the Cu tip affects the electric-field distribution, the amount of the cation injection into electrolyte, and the dimension of the CF. This study provides helpful understanding on the relationship between power consumption and retention of CBRAM cells. We also construct a theoretical model to explain the electrode-size-dependent CF growth in SET operations, showing good agreement with our experimental results.

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High-power transistors suffer greatly from inefficient heat dissipation of the hotspots, which elevate the local temperature and significantly degrade the performance and reliability of the high-power devices. Although various thermal management methods at package-level have been demonstrated, the heat dissipation from non-uniform hotspots at micro/nanoscale still persist in the high power transistors. Here, we develop a method for local thermal management using thermally conductive and electrical insulating few-layer hexagonal boron nitride (h-BN) as heat spreaders and thick counterpart as heat sinks. The electrically insulating characteristic of h-BN nanosheet allows it to be intimately contacted with the hotspot region that is located at the gate electrode edge near the drain side of a high-electron-mobility transistor (HEMT). The high thermal conductivity of h-BN nanosheet, which is quantitatively measured by Raman thermography, reduces the temperature of the hotspot by introducing an additional heat transporting pathway. Our DC and radio-frequency characterizations of the HEMT show the improvement of saturation current, cut-off frequency and maximum oscillation frequency. The finite element simulations show a temperature decrease of ∼32 °C at the hotspot with the use of h-BN nanosheet. This method can be further extended for the micro/nanoscale thermal management of other high-power devices.