By means of ab initio metadynamics runs we explored the lower-pressure region of the phase diagram of germanium. A monoclinic germanium phase with four-membered rings, less dense than diamond and compressible into β-tin phase (tI4) was found. A metallic bct-5 phase, mechanically stable down to room conditions appeared between diamond and tI4. mC16 is a narrow-gap semiconductor, while bct-5 is metallic and potentially still superconducting in the very low pressure range. This finding may help resolving outstanding experimental issues.
Monochalcogenides of germanium (or tin) are considered as stable isoelectronic and isostructural analogue of black phosphorous. Their two-dimensional (2D) forms have been just predicted to shown strong thickness-dependent physical properties, and even indirect to direct band gap crossover at the monolayer limit. Here, we demonstrate the synthesis of atomically thin GeSe by direct sonication-assisted exfoliation of bulk microcrystalline powders in solvents. The exfoliated few-layer GeSe sheets characterize high crystallinity with lateral sizes over 100 nm and, importantly, strong resistance aga…
Reversible non-equilibrium phase transformation in amorphous germanium
University of Science and Technology of China（中国科学技术大学）；Suzhou Institute for Advanced Research, University of Science and Technology of China（中国科学技术大学苏州高等研究院）；Institute of Physics, Slovak Academy of Sciences（斯洛伐克科学院物理研究所）；Carnegie Mellon University（卡内基梅隆大学）
The response to compression of the clathrate type-II structure Ge(cF136) is investigated by means of \textit{ab initio} small-cell metadynamics at different temperatures and pressures. At lower pressure $p$=2.5 GPa the metastable metallic bct-5 phase competes against $β$-Sn Ge(tI4), which forms at higher pressures. On lowering temperature, the presence of amorphous intermediates obtained from Ge$_{136}$ is more pronounced and is instrumental to the formation of denser structural motifs, from which metallic bct-5 can form. Therein, anisotropic box fluctuations promote phase formation. The metad…
Controlled chemical transformation of nanostructures is a promising technique to obtain precisely designed novel materials which are difficult to synthesize otherwise. We report high-temperature vapor phase anion exchange reactions to chemically transform II-VI semiconductor nanostructures (100-300 nm length scale) while retaining the single crystallinity, crystal structure, morphology and even defect distribution of the parent material via atomic templating. The concept of atomic templating is employed to obtain kinetically controlled, thermodynamically metastable structural phases such as zi…
Chemical exfoliation of MoS$_2$ via Li-intercalation route has led to many desirable properties and spectacular applications due to the presence of a metastable state in addition to the stable H phase. However, the nature of the specific metastable phase formed, and its basic charge conduction properties have remained controversial. Using spatially resolved Raman spectroscopy (~1 micrometer resolution) and photoelectron spectroscopy (~120 nm resolution), we probe such chemically exfoliated MoS$_2$ samples in comparison to a mechanically exfoliated H phase sample and confirm that the dominant m…
Monolayer (ML) transition-metal dichalcogenides exist in different phases, such as the hexagonal (2H) and distorted octahedral or monoclinic (1T') phases. The different structures show vastly different properties. For example, the 2H MoTe2 ML is a direct-gap semiconductor while 1T' MoTe2 is a semi-metal. It has been suggested that the formation energies between 2H and 1T' MoTe2 differ very little, so there is a high chance to tune the structures of MoTe2 and thereby to bring in new applications such as phase-change electronics. In this work, we report growth of both 2H and 1T' MoTe2 ML by mole…
Resistivity of metastable amorphous Ge2Sb2Te5 (GST) measured at device level show an exponential decline with temperature matching with the steady-state thin-film resistivity measured at 858 K (melting temperature). This suggests that the free carrier activation mechanisms form a continuum in a large temperature scale (300 K - 858 K) and the metastable amorphous phase can be treated as a super-cooled liquid. The effective activation energy calculated using the resistivity versus temperature data follow a parabolic behavior, with a room temperature value of 333 meV, peaking to ~377 meV at ~465 …
Searches for low-energy tetrahedral polymorphs of carbon and silicon have been performed using density functional theory computations and the ab initio random structure searching (AIRSS) ap- proach. Several of the hypothetical phases obtained in our searches have enthalpies that are lower or comparable to those of other polymorphs of group 14 elements that have either been experimentally synthesized or recently proposed as the structure of unknown phases obtained in experiments, and should thus be considered as particularly interesting candidates. A structure of P bam symmetry with 24 atoms in…
Integrated Systems Laboratory, ETH Zurich（苏黎世联邦理工学院集成系统实验室）；Binnig & Rohrer Nanotechnology Center (BRNC), ETH Zurich（苏黎世联邦理工学院宾尼格与罗雷尔纳米技术中心）；DeepNano Group, James Watt School of Engineering, University of Glasgow（格拉斯哥大学詹姆斯瓦特工程学院深纳小组）
A practical Laser-Heated Diamond Anvil Cell synthesis technique and recovery workflow for metastable MnSb2 and YbZn2 phases
Ames National Laboratory, US DOE, Iowa State University（艾姆斯国家实验室，美国能源部，爱荷华州立大学）；Department of Physics and Astronomy, Iowa State University（爱荷华州立大学物理与天文系）；Institute of Physics, University of São Paulo（圣保罗大学物理研究所）；Center for Advanced Radiation Sources, The University of Chicago（芝加哥大学高级辐射源中心）
Novel Transformations of PbTiO3 with Pressure and Temperature
University of Illinois Chicago（伊利诺伊大学芝加哥分校）；Deutsches Elektronen-Synchrotron (DESY)（德国电子同步加速器）；State University of New York Buffalo（纽约州立大学布法罗分校）；University of Cambridge（剑桥大学）；European Synchrotron Radiation Facility（欧洲同步辐射装置）；National Institute of Technology, Agartala（阿加塔拉国立技术学院）；Universitat Politècnica de València（瓦伦西亚理工大学）；Argonne National Laboratory（阿贡国家实验室）；Carnegie Science（卡内基科学研究所）
Density functional theory (DFT) was employed to study the stress-strain behavior, elastic instabilities, and metallization during a solid-solid phase transformation (PT) between semiconducting Si I (cubic A4) and metallic Si II (tetragonal A5 structure) when subjected to a general stress tensor. With normal stresses ($σ_1$, $σ_2$, $σ_3$) acting along $\left<110 \right>$, $\left<1\bar{1}0 \right>$, and $\left<001\right>$, respectively, dictating the simulation cell, we determine combinations of 6 independent stresses that drive a lattice instability for the Si I$\rightarrow$Si II PT, and a semi…
