Growth and ordering of coherently strained Ge-rich islands in Ge/Si single layer and multilayer systems and the influence of island arrangements on the evolutio n of the surface morphology of Si cap layers during depo...Growth and ordering of coherently strained Ge-rich islands in Ge/Si single layer and multilayer systems and the influence of island arrangements on the evolutio n of the surface morphology of Si cap layers during deposition by low-pressure c hemical vapour deposition(LPCVD) on Si(001) substrates at 700℃ have been invest igated by TEM of cross-section and plan-view specimens. At distances between the Ge layers of 35-50nm, vertical order of GeSi islands is observed for Ge-Si bila yer systems and for Ge-Si multilayer systems consisting of 5 layer pairs whereas lateral ordering parallel to <100> substrate directions is observed for the lat ter case only. In agreement with earlier results the vertical ordering in the mu ltilayer system can be understood as result of the elastic interaction between i sland nuclei forming in the layers with close islands in a buried layer below. T he lateral ordering along <100> may be attributed to the anisotropy of the elast ic interaction. Characteristic for all Si surfaces are the spatial correlation b etween the presence of island-induced lattice strain and the appearance of array s of larger square-shaped pyramids with distinct faceting and facet edges along <110>. The results reflect the importance of the control of growth parameters an d of the island-induced strain state for the evolution of the Si top layer surfa ce morphology during LPCVD growth.展开更多
V defects in GaN layer grown on Si(111)using metalorganic chemical vapor deposition(MOCVD)were investigated by atomic force microscopy(AFM),plan-view transmission electron microscopy(TEM)and energy-dispersive X-ray sp...V defects in GaN layer grown on Si(111)using metalorganic chemical vapor deposition(MOCVD)were investigated by atomic force microscopy(AFM),plan-view transmission electron microscopy(TEM)and energy-dispersive X-ray spectrometer(EDS).Dislocations are the origination of V defects.Stress field around dislocations induce the concentration of C atoms,furthermore,slow growth rate on those{10-11}planes are suggested as being responsible for the initiation of V defects.The formation mechanism of V defects was discussed.展开更多
Two-dimensional material(2D)that possesses atomic thin geometry and remarkable properties is a star material for the fundamental researches and advanced applications.Defects in 2D materials are critical and fundamenta...Two-dimensional material(2D)that possesses atomic thin geometry and remarkable properties is a star material for the fundamental researches and advanced applications.Defects in 2D materials are critical and fundamental to understand the chemical,physical,and optical properties.Photoluminescence arises in 2D materials owing to various physical phenomena including activator/dopant-induced luminescence and defect-related emissions,and so forth.With the advanced transmission electron microscopy(TEM)technologies,such as aberration correction and low voltage technologies,the morphology,chemical compositions and electronic structures of defects in 2D material could be directly characterized at the atomic scale.In this review,we introduce the applications of state-of-the-art TEM technologies on the studies of the role of atomic defects in the photoluminescence characteristics in 2D material.The challenges in spatial and time resolution are also discussed.It is proved that TEM is a powerful tool to pinpoint the relationship between the defects and the photoluminescence characteristics.展开更多
Shapes, dimensions, arrangements and the microstructure of self-assembled island s fabricated by low-pressure chemical vapour deposition (LPCVD) of Ge at 700℃ o nto Si(110) substrates have been investigated for diffe...Shapes, dimensions, arrangements and the microstructure of self-assembled island s fabricated by low-pressure chemical vapour deposition (LPCVD) of Ge at 700℃ o nto Si(110) substrates have been investigated for different nominal Ge coverage by transmission electron microscopy (TEM) of plan-view and cross-section specime ns and have been compared with photoluminescence (PL) measurements of Si-capped layer samples. The transition from the 2-dimensional layer to the 3-dimensional island growth mode takes place for a Ge deposition of nominally less than 2 mono layers. Upon this transition, many coherent islands and few larger islands with extended defects are observed. The coherent islands possess a dome-like shape an d lateral sizes up to 130nm. Photoluminescence spectra show island-related peaks whose energy positions are shifted towards lower energy with higher Ge coverage .展开更多
The formation of complex hierarchical nanostructures has attracted a lot of attention from both the fundamental science and potential applications point of view.Spherulite structures with radial fibrillar branches hav...The formation of complex hierarchical nanostructures has attracted a lot of attention from both the fundamental science and potential applications point of view.Spherulite structures with radial fibrillar branches have been found in various solids;however,their growth mechanisms remain poorly understood.Here,we report real time imaging of the formation of two-dimensional(2D)iron oxide spherulite nanostructures in a liquid cell using transmission electron microscopy(TEM).By tracking the growth trajectories,we show the characteristics of the reaction front and growth kinetics.Our observations reveal that the tip of a growing branch splits as the width exceeds certain sizes(5.5–8.5 nm).The radius of a spherulite nanostructure increases linearly with time at the early stage,transitioning to nonlinear growth at the later stage.Furthermore,a thin layer of solid is accumulated at the tip and nanoparticles from secondary nucleation also appear at the growing front which later develop into fibrillar branches.The spherulite nanostructure is polycrystalline with the co-existence of ferrihydrite and Fe3O4 through-out the growth.A growth model is further established,which provides rational explanations on the linear growth at the early stage and the nonlinearity at the later stage of growth.展开更多
Heterogeneous gradient nanostructured metals have been shown to achieve the strength-ductility synergy, thus potentially possessing the enhanced tribological performance in comparison with their homogeneous nanograine...Heterogeneous gradient nanostructured metals have been shown to achieve the strength-ductility synergy, thus potentially possessing the enhanced tribological performance in comparison with their homogeneous nanograined counterparts. In this work, a facile laser surface remelting-based surface treatment technique is developed to fabricate a gradient nanostructured layer on a TiZrHfTaNb refractory highentropy alloy. The characterization of the microstructural evolution along the depth direction from the matrix to the topmost surface layer shows that the average grain size in the ~100 μm-thick gradient nanostructured layer is dramatically refined from the original ~200 μm to only ~8 nm in the top surface layer. The microhardness is therefore gradually increased from ~240 HV in matrix to ~650 HV in the topmost surface layer, approximately 2.7 times. Noticeably, the original coarse-grained single-phase bodycentered-cubic TiZrHfTaNb refractory high-entropy alloy is gradually decomposed into TiNb-rich bodycentered-cubic phase, TaNb-rich body-centered-cubic phase, ZrHf-rich hexagonal-close-packed phase and TiZr Hf-rich face-centered-cubic phase with gradient distribution in grain size along the depth direction during the gradient refinement process. As a result, the novel laser surface treatment-introduced gradient nanostructured TiZrHfTaNb refractory high-entropy alloy demonstrates the significantly improved wear resistance, with the wear rate reducing markedly by an order of magnitude, as compared with the as-cast one. The decomposed multi-phases and gradient nanostructures should account for the enhanced wear resistance. Our findings provide new insights into the refinement mechanisms of the laser-treated refractory high-entropy alloys and broaden their potential applications via heterogeneous gradient nanostructure engineering.展开更多
We report the systematic changes of nano-scaled features and optical properties in a promising transparent oxide material, namely, Alx (x = 0, 1, 2 and 5%) doped ZnO1-x (AZO). Electron microscopy investigations reveal...We report the systematic changes of nano-scaled features and optical properties in a promising transparent oxide material, namely, Alx (x = 0, 1, 2 and 5%) doped ZnO1-x (AZO). Electron microscopy investigations revealed the alterations at lattice scale depending on the presence of Al-content in ZnO nanostructures. Lattice spacings of 0.26 and 0.28 nm oriented along the (0002) and (10 0) planes, respectively, were attributed to euhedral-and facetted-structures of hexagonal-ZnO. The AZO samples were further characterized by XRD, SEM, UV-vis spectrophotometry, Raman spectroscopy and photoluminescence studies. It has been shown that at a dopant concentration of 2%Al in ZnO, an optimal balance could be achieved between microstructure and optical properties.展开更多
文摘Growth and ordering of coherently strained Ge-rich islands in Ge/Si single layer and multilayer systems and the influence of island arrangements on the evolutio n of the surface morphology of Si cap layers during deposition by low-pressure c hemical vapour deposition(LPCVD) on Si(001) substrates at 700℃ have been invest igated by TEM of cross-section and plan-view specimens. At distances between the Ge layers of 35-50nm, vertical order of GeSi islands is observed for Ge-Si bila yer systems and for Ge-Si multilayer systems consisting of 5 layer pairs whereas lateral ordering parallel to <100> substrate directions is observed for the lat ter case only. In agreement with earlier results the vertical ordering in the mu ltilayer system can be understood as result of the elastic interaction between i sland nuclei forming in the layers with close islands in a buried layer below. T he lateral ordering along <100> may be attributed to the anisotropy of the elast ic interaction. Characteristic for all Si surfaces are the spatial correlation b etween the presence of island-induced lattice strain and the appearance of array s of larger square-shaped pyramids with distinct faceting and facet edges along <110>. The results reflect the importance of the control of growth parameters an d of the island-induced strain state for the evolution of the Si top layer surfa ce morphology during LPCVD growth.
基金Project supported by Program for New Century Excellent Talents in University and Nature Science Foundation of Hebei Province(E2005000042)
文摘V defects in GaN layer grown on Si(111)using metalorganic chemical vapor deposition(MOCVD)were investigated by atomic force microscopy(AFM),plan-view transmission electron microscopy(TEM)and energy-dispersive X-ray spectrometer(EDS).Dislocations are the origination of V defects.Stress field around dislocations induce the concentration of C atoms,furthermore,slow growth rate on those{10-11}planes are suggested as being responsible for the initiation of V defects.The formation mechanism of V defects was discussed.
基金X.W.thanks the supported by the NSFC(61574060)the Projects of Science and Technology Commission of Shanghai Municipality(14DZ2260800)+2 种基金the Shanghai Rising-Star Program(17QA1401400)the Fundamental Research Funds for the Central Universities.Z.S.thanks the financial support from Australian Research Council through an ARC DECRA project(DE150100280)an ARC Future Fellow project(FT180100387).
文摘Two-dimensional material(2D)that possesses atomic thin geometry and remarkable properties is a star material for the fundamental researches and advanced applications.Defects in 2D materials are critical and fundamental to understand the chemical,physical,and optical properties.Photoluminescence arises in 2D materials owing to various physical phenomena including activator/dopant-induced luminescence and defect-related emissions,and so forth.With the advanced transmission electron microscopy(TEM)technologies,such as aberration correction and low voltage technologies,the morphology,chemical compositions and electronic structures of defects in 2D material could be directly characterized at the atomic scale.In this review,we introduce the applications of state-of-the-art TEM technologies on the studies of the role of atomic defects in the photoluminescence characteristics in 2D material.The challenges in spatial and time resolution are also discussed.It is proved that TEM is a powerful tool to pinpoint the relationship between the defects and the photoluminescence characteristics.
文摘Shapes, dimensions, arrangements and the microstructure of self-assembled island s fabricated by low-pressure chemical vapour deposition (LPCVD) of Ge at 700℃ o nto Si(110) substrates have been investigated for different nominal Ge coverage by transmission electron microscopy (TEM) of plan-view and cross-section specime ns and have been compared with photoluminescence (PL) measurements of Si-capped layer samples. The transition from the 2-dimensional layer to the 3-dimensional island growth mode takes place for a Ge deposition of nominally less than 2 mono layers. Upon this transition, many coherent islands and few larger islands with extended defects are observed. The coherent islands possess a dome-like shape an d lateral sizes up to 130nm. Photoluminescence spectra show island-related peaks whose energy positions are shifted towards lower energy with higher Ge coverage .
基金This project was supported by the U.S.Department of Energy(DOE),Office of Science,Office of Basic Energy Sciences(BES),Materials Sciences and Engineering Division under Contract No.DE-AC02-05-CH11231 within the in-situ TEM(KC22ZH)program.Work at the Molecular Foundry was supported by the Office of Science,Office of Basic Energy Sciences,of the U.S.Department of Energy under Contract No.DE-AC02-05CH11231.We acknowledge Gatan Inc.for the advanced K2 IS camera and Dr.Ming Pan and Dr.Cory Czarnik for their help with part of experimental set up in this work.W.J.Z.acknowledges the support from Tianjin University Graduate School International Academic Exchange Fund.M.R.H.was funded by KAUST project under H.M.Z.at UC Berkeley.
文摘The formation of complex hierarchical nanostructures has attracted a lot of attention from both the fundamental science and potential applications point of view.Spherulite structures with radial fibrillar branches have been found in various solids;however,their growth mechanisms remain poorly understood.Here,we report real time imaging of the formation of two-dimensional(2D)iron oxide spherulite nanostructures in a liquid cell using transmission electron microscopy(TEM).By tracking the growth trajectories,we show the characteristics of the reaction front and growth kinetics.Our observations reveal that the tip of a growing branch splits as the width exceeds certain sizes(5.5–8.5 nm).The radius of a spherulite nanostructure increases linearly with time at the early stage,transitioning to nonlinear growth at the later stage.Furthermore,a thin layer of solid is accumulated at the tip and nanoparticles from secondary nucleation also appear at the growing front which later develop into fibrillar branches.The spherulite nanostructure is polycrystalline with the co-existence of ferrihydrite and Fe3O4 through-out the growth.A growth model is further established,which provides rational explanations on the linear growth at the early stage and the nonlinearity at the later stage of growth.
基金supported by the joint Ph D project between the Hong Kong Polytechnic University and Southern University of Science and Technologythe grant from the Research Committee of Poly U under student account code RK2N+1 种基金supported by the National Natural Science Foundation of China Projects (Nos. 51701171 and 51971187)the Fundamental Research Program of Shenzhen (Grant No. JCYJ20170412153039309)。
文摘Heterogeneous gradient nanostructured metals have been shown to achieve the strength-ductility synergy, thus potentially possessing the enhanced tribological performance in comparison with their homogeneous nanograined counterparts. In this work, a facile laser surface remelting-based surface treatment technique is developed to fabricate a gradient nanostructured layer on a TiZrHfTaNb refractory highentropy alloy. The characterization of the microstructural evolution along the depth direction from the matrix to the topmost surface layer shows that the average grain size in the ~100 μm-thick gradient nanostructured layer is dramatically refined from the original ~200 μm to only ~8 nm in the top surface layer. The microhardness is therefore gradually increased from ~240 HV in matrix to ~650 HV in the topmost surface layer, approximately 2.7 times. Noticeably, the original coarse-grained single-phase bodycentered-cubic TiZrHfTaNb refractory high-entropy alloy is gradually decomposed into TiNb-rich bodycentered-cubic phase, TaNb-rich body-centered-cubic phase, ZrHf-rich hexagonal-close-packed phase and TiZr Hf-rich face-centered-cubic phase with gradient distribution in grain size along the depth direction during the gradient refinement process. As a result, the novel laser surface treatment-introduced gradient nanostructured TiZrHfTaNb refractory high-entropy alloy demonstrates the significantly improved wear resistance, with the wear rate reducing markedly by an order of magnitude, as compared with the as-cast one. The decomposed multi-phases and gradient nanostructures should account for the enhanced wear resistance. Our findings provide new insights into the refinement mechanisms of the laser-treated refractory high-entropy alloys and broaden their potential applications via heterogeneous gradient nanostructure engineering.
文摘We report the systematic changes of nano-scaled features and optical properties in a promising transparent oxide material, namely, Alx (x = 0, 1, 2 and 5%) doped ZnO1-x (AZO). Electron microscopy investigations revealed the alterations at lattice scale depending on the presence of Al-content in ZnO nanostructures. Lattice spacings of 0.26 and 0.28 nm oriented along the (0002) and (10 0) planes, respectively, were attributed to euhedral-and facetted-structures of hexagonal-ZnO. The AZO samples were further characterized by XRD, SEM, UV-vis spectrophotometry, Raman spectroscopy and photoluminescence studies. It has been shown that at a dopant concentration of 2%Al in ZnO, an optimal balance could be achieved between microstructure and optical properties.
