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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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Selected mode mixing and interference visualized within a single optical nanoantenna
摘要： Interference-based directional antennas typically consist of multiple dipoles with properly set distances and phases, which cause constructive interferences towards certain directions in radiation or reception. For nano optical antennas, the directionality can be realized by superposition of multiple eigen modes in a single structure. Such mode mixing creates locally strong field enhancement, which should be properly controlled for energy-conversion or sensing applications. However, experimental verification of the nano optical field, or especially the hot-spots, created by interference of selected eigen modes is not trivial. We here visualize how optical fields are distributed when multiple modes interfere within a silver disk nano antenna. We use angle- and polarization-resolved cathodoluminescence based on scanning transmission electron microscopy to select specific modes and visualize the field distribution at the nanoscale. The interfered field distribution significantly changes depending on the detection angles even when the detection geometry is symmetric, which can be explained by the phase difference of the excited mode. The cathodoluminescence signals are also modeled as superpositions of analytical eigen mode functions consisting of multipoles in space and complex Lorentzians in frequency to reproduce the experimentally obtained photon maps.
关键词： Scanning Transmission Electron Microscopy,Surface Plasmon,Multipole,Cathodoluminescence,Nanodisk
更新于2025-09-23 15:21:21
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Plasmonics || Plasmonic Modes in Au and AuAg Nanowires and Nanowire Dimers Studied by Electron Energy Loss Spectroscopy
摘要： In this chapter, we review our recent work on the investigation of surface plasmon modes in metallic nanowires and nanowire dimers by means of electron energy loss spectroscopy combined with scanning transmission electron microscopy (STEM-EELS). Due to the very high spatial resolution, STEM-EELS is a powerful technique to visualize multipole order surface plasmon modes in nanowires and study the dependency of their resonance energies on different parameters such as nanowire dimensions or nanowire porosity. In addition, we investigate surface plasmon hybridization in nanowires separated by gaps of less than 10 nm or connected by small metallic bridges. In such structures new modes arise, which depend strongly on gap or bridge sizes. Experimental results are supported by finite element simulations. The investigated nanowires and dimers are fabricated by electrodeposition in etched ion-track templates, combined with a selective dissolution processes. The synthesis techniques and their advantages for the fabrication of plasmonic nanostructures are also discussed.
关键词： nanowire dimers,scanning transmission electron microscopy,ion-track technology,electron energy loss spectroscopy,nanogaps,nanowires,electrodeposition,plasmon hybridization,etched ion-track membranes,surface plasmons,gold
更新于2025-09-23 15:21:01
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Deep Learning Enabled Strain Mapping of Single-Atom Defects in 2D Transition Metal Dichalcogenides with Sub-picometer Precision
摘要： 2D materials offer an ideal platform to study the strain fields induced by individual atomic defects, yet challenges associated with radiation damage have so-far limited electron microscopy methods to probe these atomic-scale strain fields. Here, we demonstrate an approach to probe single-atom defects with sub-picometer precision in a monolayer 2D transition metal dichalcogenide, WSe2-2xTe2x. We utilize deep learning to mine large datasets of aberration-corrected scanning transmission electron microscopy images to locate and classify point defects. By combining hundreds of images of nominally identical defects, we generate high signal-to-noise class averages which allow us to measure 2D atomic spacings with up to 0.2 pm precision. Our methods reveal that Se vacancies introduce complex, oscillating strain fields in the WSe2-2xTe2x lattice that correspond to alternating rings of lattice expansion and contraction. These results indicate the potential impact of computer vision for the development of high-precision electron microscopy methods for beam-sensitive materials.
关键词： scanning transmission electron microscopy,strain mapping,single-atom defects,Deep learning,fully convolutional network (FCN),2D materials
更新于2025-09-23 15:21:01
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European Microscopy Congress 2016: Proceedings || Characterizing Localized Surface Plasmons using Electron Energy-Loss Spectroscopy
摘要： Localized surface plasmon resonances (LSPRs) are the coherent and collective oscillations of conduction band electrons at the surface of metallic nanoparticles. LSPRs are known to localize far-field light to a sub-diffraction-limited length scale, yielding an intense electric field at the particle surface. This effect has been harnessed to dramatically enhance light-matter interactions, leading to a variety of applications such as surface-enhanced Raman spectroscopy (SERS), photothermal cancer therapy and solar energy harvesting. Though a variety of near- and far-field optical methods are used to probe LSPRs, the spatial resolution of these methods is on the order of tens of nanometers, limiting their effectiveness. In contrast, electron energy loss spectroscopy (EELS) performed in a scanning transmission electron microscope (STEM) combines sub-nanometer resolving power with the capability to excite both optical-accessible and –inaccessible plasmon modes and therefore has emerged as one of the leading techniques (Figure 1). In this presentation, I will briefly introduce the STEM/EELS technique and demonstrate the power of STEM/EELS in the characterization of LSPRs. In addition to the traditional use of STEM/EELS for LSPR imaging, we have recently demonstrated that STEM/EELS can also be used to spatially map LSP-semiconductor energy transfer at the nanoscale. The future of STEM/EELS as a window into the nanoscopic world is especially promising, and we expect continued advances in the molecular, optical, materials, information, and energy sciences as a result.
关键词： Localized Surface Plasmon,Hot Electrons,Scanning Transmission Electron Microscopy,Energy Transfer,Electron Energy-Loss Spectroscopy
更新于2025-09-23 15:19:57
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In situ atomic level studies of thermally controlled interlayer stacking shifts in 2D transition metal dichalcogenide bilayers
摘要： We show interlayer stacking shifts occur in transition metal dichalcogenides (TMD) bilayers due to the strain introduced during sample heating, and attributed to rippling of one layer relative to the other. The atomic structure of the interlayer stacking is studied using annular dark field scanning transmission electron microscopy with an in situ heating holder. Before heating, bilayers show uniform interlayer stacking of AA9 and AB. When heated, contrast change is seen and associated with interlayer stacking changes at the atomic scale due to ripples. When cooled down to room temperature, these contrast features disappear, confirming it is a reversible process that is not related to defects or vacancies. Because the bottom layer is attached to the in situ heating chip made from Si3N4 and the top layer is in contact with the underlying TMD layer with weak van der Waals interaction, the two layers experience different forces during thermal expansion.
关键词： transition metal dichalcogenides,thermal expansion,scanning transmission electron microscopy,interlayer stacking,2D materials
更新于2025-09-23 15:19:57
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The maximum a posteriori probability rule for atom column detection from HAADF STEM images
摘要： Recently, the maximum a posteriori (MAP) probability rule has been proposed as an objective and quantitative method to detect atom columns and even single atoms from high-resolution high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) images. The method combines statistical parameter estimation and model-order selection using a Bayesian framework and has been shown to be especially useful for the analysis of the structure of beam-sensitive nanomaterials. In order to avoid beam damage, images of such materials are usually acquired using a limited incoming electron dose resulting in a low contrast-to-noise ratio (CNR) which makes visual inspection unreliable. This creates a need for an objective and quantitative approach. The present paper describes the methodology of the MAP probability rule, gives its step-by-step derivation and discusses its algorithmic implementation for atom column detection. In addition, simulation results are presented showing that the performance of the MAP probability rule to detect the correct number of atomic columns from HAADF STEM images is superior to that of other model-order selection criteria, including the Akaike Information Criterion (AIC) and the Bayesian Information Criterion (BIC). Moreover, the MAP probability rule is used as a tool to evaluate the relation between STEM image quality measures and atom detectability resulting in the introduction of the so-called integrated CNR (ICNR) as a new image quality measure that better correlates with atom detectability than conventional measures such as signal-to-noise ratio (SNR) and CNR.
关键词： Atom detection,Scanning transmission electron microscopy (STEM),Atom detectability,Model selection
更新于2025-09-19 17:15:36
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Plasmonic Pt Superstructures with Boosted Near‐Infrared Absorption and Photothermal Conversion Efficiency in the Second Biowindow for Cancer Therapy
摘要： Defects are commonly found in two-dimensional (2D) transition-metal dichalcogenide (TMD) materials. Such defects usually dictate the optical and electrical properties of TMDs. It is thus important to develop techniques to characterize the defects directly with good spatial resolution, specificity, and throughput. Herein, we demonstrate that Kelvin probe force microscopy (KPFM) is a versatile technique for this task. It is able to unveil defect heterogeneity of 2D materials with a spatial resolution of 10 nm and energy sensitivity better than 10 meV. KPFM mappings of monolayer WS2 exhibit interesting work function variances that are associated with defects distribution. This finding is verified by aberration-corrected scanning transmission electron microscopy and density functional theory calculations. In particular, a strong correlation among the work function, electrical and optical responses to the defects is revealed. Our findings demonstrate the potential of KPFM as an effective tool for exploring the intrinsic defects in TMDs.
关键词： transition-metal dichalcogenides,density functional theory,defects,Kelvin probe force microscopy,scanning transmission electron microscopy,work function,two-dimensional materials
更新于2025-09-19 17:13:59
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Elucidation of Donor:Acceptor Phase Separation in Nonfullerene Organic Solar Cells and Its Implications on Device Performance and Charge Carrier Mobility
摘要： In bulk-heterojunction solar cells, the device performance strongly depends on the donor and acceptor properties, the phase separation in the absorber layer, and the formation of a bicontinuous network. While this phase separation is well explored for polymer:fullerene solar cells, only little is known for polymer:nonfullerene acceptor solar cells. The main hurdle in this regard is often the chemical similarity of the conjugated polymer donor and the organic nonfullerene acceptor (NFA), which makes the analysis of the phase separation via atomic force microscopic (AFM) phase images or conventional transmission electron microscopy difficult. In this work, we use the donor polymer PTB7-Th and the small molecule acceptor O-IDTBR as the model system and visualized the phase separation in PTB7-Th:O-IDTBR bulk-heterojunctions with different donor:acceptor ratios via scanning transmission electron microscopy (STEM) high-angle annular dark-field (HAADF) images and electron energy loss spectroscopy (EELS) based elemental mapping, which resulted in a good contrast between the donor and the acceptor despite very low differences in the chemical composition. AFM as well as grazing-incidence wide-angle X-ray scattering (GIWAXS) investigations support the electron microscopic data. Furthermore, we elucidate the implications of the phase separation on the device performance as well as charge carrier mobilities in the bulk-heterojunction layers, and a high performance of the solar cells was found over a relatively broad range of polymer domain sizes. This can be related to the larger domain sizes of the acceptor phase with higher amounts of O-IDTBR in the blend, while the polymer donor phase still forms continuous pathways to the electrode, which keeps the hole mobility at a relatively constant level.
关键词： nanomorphology,organic photovoltaics,charge carrier mobility,bulk-heterojunction,scanning transmission electron microscopy
更新于2025-09-19 17:13:59
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Evidence of Low Temperature Joints in Silver Nanowire Based Transparent Conducting Layers for Solar cells
摘要： The primary stage of joint formation of silver nanowires (AgNWs) at 60 °C is investigated using rotary scanning transmission electron microscopy (STEM with tomographic reconstruction images), and super large-scale molecular dynamic (MD) simulation (2×106 atoms). This study proves to establish that silver nanowires do not require the conventional high temperature post treatment process at 200 °C to form fused contacts at the intersections. In fact, a low temperature annealing at 60 °C facilitates formation of highly conductive networks. The connection between the nanowires is made through a stage called thinning, shown in this report for the first time, which occurs before broadening of the nanowires and is caused due to simultaneous effects of loads from the top nanowires and the heating, as confirmed by STEM and MD result. The outcomes of our investigation significantly promote the application of AgNWs as a transparent conductive layer for solar cells with requirement of low temperature processing such as Kasterite, Perovskite and Organic solar cells.
关键词： Low temperature process,Scanning transmission electron microscopy,Molecular dynamic simulation,Junction resistivity,Transparent conductive layer,Silver nanowire
更新于2025-09-19 17:13:59