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Reversed Crystal Growth
摘要： In the last decade, a reversed growth route has been found in many crystal growth processes. In these systems, a single crystal does not develop from a single nucleus. The precursor molecules/ions or nanocrystallites aggregate into some large amorphous or polycrystalline particles. Multiple-nucleation on the surface of the amorphous particles or surface re-crystallization of the polycrystalline particles then takes place, forming a single crystal shell with a regular morphology. Finally, the crystallization extends from the surface to the core to form single crystals. This non-classical crystal growth route often results in some special morphologies, such as core-shell structures, hollow single crystals, sandwich structures, etc. This article gives a brief review of the research into reversed crystal growth and demonstrates that investigation of detailed mechanisms of crystal growth enables us to better understand the formation of many novel morphologies of the crystals. Some unsolved problems are also discussed.
关键词： nucleation,crystal growth,core-shell structure,crystal morphology,hollow crystal,electron microscopy
更新于2025-09-23 15:23:52
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Localized Deformation and Fracture Behaviors in InP Single Crystals by Indentation
摘要： The indentation-induced deformation mechanisms in InP(100) single crystals were investigated by using nanoindentation and cross-sectional transmission electron microscopy (XTEM) techniques. The results indicated that there were multiple “pop-in” events randomly distributed in the loading curves, which were conceived to arise primarily from the dislocation nucleation and propagation activities. An energetic estimation on the number of nanoindentation-induced dislocations associated with pop-in effects is discussed. Furthermore, the fracture patterns were performed by Vickers indentation. The fracture toughness and the fracture energy of InP(100) single crystals were calculated to be around 1.2 MPa·m1/2 and 14.1 J/m2, respectively.
关键词： InP(100) single crystal,nanoindentation,fracture toughness,transmission electron microscopy,Pop-in
更新于2025-09-23 15:23:52
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Strain-Induced Metastable Phase Stabilization in Ga2O3 Thin Films
摘要： It is well known that metastable and transient structures in bulk can be stabilized in thin films via epitaxial strain (heteroepitaxy) and appropriate growth conditions that are often far from equilibrium. However, the mechanism of heteroepitaxy, particularly how the nominally unstable or metastable phase gets stabilized, remains largely unclear. This is especially intriguing for thin film Ga2O3, where multiple crystal phases may exist under varied growth conditions with spatial and dimensional constraints. Herein, the development and distribution of epitaxial strain at the Ga2O3/Al2O3 film-substrate interfaces is revealed down to the atomic resolution along different orientations, with an aberration-corrected scanning transmission electron microscope (STEM). Just a few layers of metastable α-Ga2O3 structure were found to accommodate the misfit strain in direct contact with the substrate. Following an epitaxial α-Ga2O3 structure of about couple unit cells, several layers (4~5) of transient phase appear as the intermediate structure to release the misfit strain. Subsequent to this transient crystal phase, the nominally unstable κ-Ga2O3 phase is stabilized as the major thin film phase form. We show that the epitaxial strain is gracefully accommodated by rearrangement of the oxygen polyhedra. When the structure is under large compressive strain, Ga3+ ions occupy only the oxygen octahedral sites to form a dense structure. With gradual release of the compressive strain, more and more Ga3+ ions occupy the oxygen tetrahedral sites, leading to volumetric expansion and the phase transformation. The structure of the transition phase is identified by high resolution electron microscopy (HREM) observation, complemented by the density functional theory (DFT) calculations. This study provides insights from the atomic scale and their implications for the design of functional thin film materials using epitaxial engineering.
关键词： κ-Ga2O3,misfit strain,α-Ga2O3,metastable phase,aberration-corrected scanning transmission electron microscopy
更新于2025-09-23 15:23:52
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Interface Engineering of Au(111) for the Growth of 1T′-MoSe <sub/>2</sub>
摘要： Phase-controlled synthesis of two-dimensional transition metal dichalcogenides (TMDCs) is of great interest due to the distinct properties of the different phases. However, it is challenging to prepare metallic phase of group-VI TMDCs due to their metastability. At the monolayer level, interface engineering can be used to stabilize the metastable phase. Here, we demonstrate the selective growth of either single-layer 1H or 1T’-MoSe2 on Au(111) by molecular beam epitaxy; the two phases can be unambiguously distinguished using scanning tunnelling microscopy and spectroscopy. While the growth of 1H-MoSe2 is favourable on pristine Au(111), the growth of 1T’-MoSe2 is promoted by the pre-deposition of Se on Au(111). The selective growth of 1T’-MoSe2 phase on Se-pretreated Au(111) is attributed to Mo intercalation-induced stabilization of the 1T’ phase, which is supported by density functional theory calculations. In addition, 1T’ twin boundaries and 1H-1T’ heterojunctions were observed and found to exhibit enhanced tunnelling conductivity. The substrate pre-treatment approach for phase-controlled epitaxy should be applicable to other group-VI TMDCs grown on Au (111).
关键词： phase control,heterojunction,scanning tunnelling microscopy/spectroscopy,interface engineering,transition metal dichalcogenides,MoSe2
更新于2025-09-23 15:23:52
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Monitoring dynamic collagen reorganization during skin stretching with fast polarization-resolved SHG imaging
摘要： The mechanical properties of biological tissues are strongly correlated to the specific distribution of their collagen fibers. Monitoring the dynamic reorganization of the collagen network during mechanical stretching is however a technical challenge because it requires mapping orientation of collagen fibers in a thick and deforming sample. In this work, a fast polarization-resolved SHG microscope is implemented to map collagen orientation during mechanical assays. This system is based on line-to-line switching of polarization using an electro-optical modulator and works in epidetection geometry. After proper calibration, it successfully highlights the collagen dynamic alignment along the traction direction in ex vivo murine skin dermis. This microstructure reorganization is quantified by the entropy of the collagen orientation distribution as a function of the stretch ratio. It exhibits a linear behavior, whose slope is measured with a good accuracy. This approach can be generalized to probe a variety of dynamic processes in thick tissues.
关键词： biomechanics,multiphoton microscopy,skin,polarization,collagen
更新于2025-09-23 15:23:52
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Functional imaging of visual cortical layers and subplate in awake mice with optimized three-photon microscopy
摘要： Two-photon microscopy is used to image neuronal activity, but has severe limitations for studying deeper cortical layers. Here, we developed a custom three-photon microscope optimized to image a vertical column of the cerebral cortex > 1 mm in depth in awake mice with low (<20 mW) average laser power. Our measurements of physiological responses and tissue-damage thresholds define pulse parameters and safety limits for damage-free three-photon imaging. We image functional visual responses of neurons expressing GCaMP6s across all layers of the primary visual cortex (V1) and in the subplate. These recordings reveal diverse visual selectivity in deep layers: layer 5 neurons are more broadly tuned to visual stimuli, whereas mean orientation selectivity of layer 6 neurons is slightly sharper, compared to neurons in other layers. Subplate neurons, located in the white matter below cortical layer 6 and characterized here for the first time, show low visual responsivity and broad orientation selectivity.
关键词： subplate neurons,deep brain imaging,visual cortex,neuronal activity,three-photon microscopy,GCaMP6s
更新于2025-09-23 15:23:52
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Dispersion relations of elastic waves in three-dimensional cubical piezoelectric phononic crystal with initial stresses and mechanically and dielectrically imperfect interfaces
摘要： The shape of individual building blocks is an important parameter in bottom-up self-assembly of nano-structured materials. A simple shape change from sphere to spheroid can significantly affect the assembly process due to the modification to the orientational degrees of freedom. When a layer of spheres is placed upon a layer of spheroids, the strain at the interface can be minimized by the spheroid taking a special orientation. C70 fullerenes represent the smallest spheroids, and their interaction with a sphere-like C60 is investigated. We find that the orientation of the C70 within a close-packed C70 layer can be steered by contacting a layer of C60. This orientational steering phenomenon is potentially useful for epitaxial growth of multilayer van der Waals molecular heterostructures.
关键词： interface,self-assembly,epitaxy,scanning tunnelling microscopy,graphene,van der Waals heterostructures,fullerene
更新于2025-09-23 15:23:52
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Doping modulation of quasi-free-standing monolayer graphene formed on SiC(0001) through Sn1-Ge intercalation
摘要： In order to modulate the transfer doping of quasi-free-standing monolayer graphene (QFMLG) formed on SiC(0001), Ge atoms were intercalated additionally into QFMLG already formed by Sn intercalation between ZL and 6H-SiC(0001). By postannealing the Ge-deposited surface at 600 °C, the Sn1-xGex film with the 4 × √3 structure, composed of a bilayer and adatoms with dangling bonds under QFMLG, has been formed. It turns out that, in this Sn1-xGex film, Ge atoms preferentially occupy the bottom layer bound to the top Si atoms of the substrate, while Sn atoms occupy the top adatom sites. Strong correlation among the electrons localized at these adatom sites induces a semiconducting alloy film. As the postannealing temperature is increased up to 800 °C, the concentration of Ge in the intercalated film of the same 4 × √3 structure is gradually increased and the Dirac point also shifts gradually from ?0.16 eV to +0.20 eV relative to the Fermi level. Such a result confirms that the transfer doping of QFMLG on SiC(0001) has been modulated by varying the alloy composition of the Sn1-xGex interfacial film.
关键词： Scanning tunneling microscopy,Quasi-free-standing graphene,SnGe alloy intercalation,Hubbard band,Doping modulation,Photoemission spectroscopy,SiC(0001)
更新于2025-09-23 15:23:52
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One-dimensional integrals to calculate the two-dimensional Rayleigh–Sommerfeld diffraction integrals for non-rotationally symmetric functions and general polarizing illuminating fields
摘要： The recently reported (Alcalá Ochoa, 2018) approximation to the Rayleigh–Sommerfeld diffraction integrals (RSDI) is used to obtain a set of expressions to calculate the diffraction properties of non-rotationally symmetric optical systems illuminated with polarization fields. It is proved that the three two-dimensional integrals of RSDI can be expressed accurately from two to eight one-dimensional integrals depending of the type of polarization studied. It is proved also that previous developments based on Taylor’s approximations to the RSDI can be described by these new expressions.
关键词： Imaging optics,Diffraction,Fourier optics,Physical optics,Microscopy
更新于2025-09-23 15:23:52
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Molecular Identification, Bond Order Discrimination, and Apparent Intermolecular Features in Atomic Force Microscopy Studied with a Charge Density Based Method
摘要： We introduce an efficient method to simulate high-resolution atomic force microscopy (HR-AFM) images with CO probes. Our model explicitly takes into account the charge densities of the sample and the probe for the calculation of the short-range (SR) interaction and retains ab initio accuracy with only two parameters, that are essentially universal, independent of the number of chemical species and the complexity of the bonding topology. The application to molecular images shows a strong dependence on the stoichiometry and bonding configuration that precludes the chemical identification of individual atoms based on local force–distance curves. However, we have identified features in the 2D images and 3D force maps that reflect the highly anisotropic spatial decay of the molecular charge density and provide a way towards molecular identification. The model treats SR and electrostatics interactions on an equal footing and correctly pinpoints the Pauli repulsion as the underlying interaction responsible for the bond order discrimination in C60. Finally, we settle the controversy regarding the origin of the intermolecular features, discarding the effect of the charge redistribution associated with the H bonds, and linking them with the overlap of the wave functions of the atoms that constitute the bond. This overlap creates saddle regions in the potential energy landscape that are sensed by the probe.
关键词： tip functionalization,DFT,chemical identification,CO molecule,hydrogen bonds,non–contact atomic force microscopy,bond order
更新于2025-09-23 15:23:52