arXiv subjects

Mingjian Wu

Publications and source records attributed to Mingjian Wu.

10 recordsLinked to original sources

arXiv2026

Polarization-Dependent Raman Selection Rules in Sb$_2$S$_3$ from First Principles and Experiment

Antimony sulfide (Sb$_2$S$_3$) is a semiconductor composed of quasi-one-dimensional ribbon-like structural units, which give rise to pronounced structural anisotropy in the bulk crystal. Despite growing interest in Sb$_2$S$_3$, in particular Sb$_2$S$_3$ thin films, a detailed understanding of its symmetry-based lattice dynamics remains incomplete. Here, we present a combined experimental and theoretical study of polarization-dependent Raman scattering in Sb$_2$S$_3$ thin films. We derive the Raman selection rules from the crystal symmetry and calculate the zone-center phonon modes and corresponding Raman tensors using density functional theory. The calculated polarization dependencies are systematically compared with polarization-dependent Raman measurements performed on oriented crystalline domains of Sb$_2$S$_3$ thin films. This combined analysis enables reliable mode assignments, elucidates the anisotropic Raman response associated with the ribbon-like crystal structure, and demonstrates the sensitivity of polarized Raman spectroscopy to crystal orientation and structural order in antimony chalcogenide (Sb$_2$S$_3$, Sb$_2$Se$_3$) as well as isostructural Bi$_2$S$_3$ thin films.

cond-mat.mtrl-sci↗

arXiv2026

3D Electron Diffraction -- The Missing Slice for a Complete Nanoscale Analysis of Organic Solar Cells in TEM

Optimizing the performance of organic solar cells (OSCs) hinges on a comprehensive understanding of their nanostructures, yet traditional characterization methods often fall short, delivering incomplete structural snapshots. We introduce elastically filtered 3D Electron Diffraction (3D ED) as a ground-breaking technique bridging full reciprocal- and real-space structural analysis within a single transmission electron microscope (TEM). Using solvent-vapor annealed DRCN5T:PC71BM, 3D ED reproduces key structural parameters from GIWAXS including lattice spacings, coherence lengths, and mosaicity, while uniquely delivering true in-plane access and direct registration with high-resolution imaging, diffraction imaging and nano-spectroscopy on the same sample. A low-dose, distributed tilt strategy plus energy filtering yields high signal-to-background below damage thresholds. Extension to a second archetypal blend (P3HT:PC71BM; annealing evolution) demonstrates generality. Our findings underscore the transformative potential of 3D ED, particularly in analysing beam-sensitive organic thin films. This paves the way for new avenues in advanced correlative structural characterization of OSCs and holds potential for application to a multitude of other nanostructured materials.

cond-mat.mtrl-sci↗

arXiv2025

Gap-free Information Transfer in 4D-STEM via Fusion of Complementary Scattering Channels

Linear phase-contrast scanning transmission electron microscopy (STEM) techniques compatible with high-throughput 4D-STEM acquisition are widely used to enhance phase contrast in weakly scattering and beam-sensitive materials. In these modalities, contrast transfer is often suppressed at low spatial frequencies, resulting in a characteristic contrast gap that limits quantitative imaging. Approaches that retain low-frequency phase contrast exist but typically require substantially increased experimental complexity, restricting routine use. Dark-field STEM imaging captures this missing low-frequency information through electrons scattered outside the bright-field disk, but discards a large fraction of the scattered signal and is therefore dose-inefficient. Fused Full-field STEM (FF-STEM) is introduced as a 4D-STEM imaging modality that overcomes this limitation by combining ptychographic phase reconstruction with tilt-corrected dark-field imaging within a single acquisition. Bright-field data are used to estimate probe aberrations and reconstruct a high-resolution phase image, while dark-field data provide complementary low-frequency contrast. The two channels are optimally fused in Fourier space using minimum-variance weighting based on the spectral signal-to-noise ratio, yielding transfer-gap-free images with high contrast and quantitative fidelity. FF-STEM preserves the upsampling and depth-sectioning capabilities of ptychography, adds robust low-frequency contrast characteristic of dark-field imaging, and enables dose-efficient, near-real-time reconstruction.

physics.optics↗

arXiv2025

Understanding the effect of drying time in process-structure-performance relationships for PM6-Y6 organic solar cells

Making solution-cast organic solar cells industrially available generally comes at the cost of significant performance losses compared to device prototypes manufactured under laboratory conditions. Adjusting solvent evaporation kinetics is postulated to recover efficiency. Yet, a comprehensive characterization of their effect, independently of other property-defining parameters, is lacking. Thus, the present objective is to isolate the influence of the solvent drying rate on solution-deposited organic active layer nanomorphologies and performances. To this end, a specially designed gas quenching technique is employed to fabricate PM6:Y6 donor-acceptor films under systematic variations of evaporation conditions. Using an extensive investigation protocol that combines insights from numerical simulations and experimental measurements, process-structure-performance relationships are unraveled. It is found that higher drying rates imply finer and more dispersed nanomorphologies with increased fractions of amorphous material. This enhances electric charge generation, thereby improving short-circuit current density and overall cell performance. The open-circuit voltage is also boosted under accelerated evaporation due to changes in the aggregation mode of the Y6 small molecule that induce higher effective bandgaps. The results demonstrate that the developed gas-quenching technique is a valuable tool for optimizing the performance of upscaled organic photovoltaics, as it is readily compatible with high-throughput equipment, such as roll-to-roll coating machines.

cond-mat.mtrl-sci↗

arXiv2025

A Simultaneous Synergistic Protection Mechanism in Hybrid Perovskite-Organic Multi-junctions Enables Long-Term Stable and Efficient Tandem Solar Cells

Perovskite-organic tandem solar cells (P-O TSCs) hold great promise for next-generation thin-film photovoltaics, with steadily improving power conversion efficiency (PCE). However, the development of optimal interconnecting layers (ICLs) remains one major challenge for further efficiency gains, and progress in understanding the improved long-term stability of P-O tandem configuration has been lagging. In this study, we experimentally investigate the enhanced stability of p-i-n P-O TSCs employing a simplified C60/atomic-layer-deposition (ALD) SnOx/PEDOT: PSS ICL without an additional charge recombination layer (CRL), which achieve an averaged efficiency of 25.12% and a hero efficiency of 25.5%. Our finding discovers that the recrystallization of C60, a widely used electron transport layer in perovskite photovoltaics, leads to the formation of grain boundaries during operation, which act as migration channels for the interdiffusion of halide and Ag ions. Critically, we demonstrate for the first time that the tandem device architecture, incorporating organic semiconductor layers, effectively suppresses the bi-directional ion diffusion and mitigates electrode corrosion. Thus, the P-O TSC establishes a mutual protection system: the organic layers stabilize the perovskite sub-cell by suppressing ion diffusion-induced degradation, and the perovskite layer shields the organic sub-cell from spectrally induced degradation. The simultaneous synergistic protection mechanism enables P-O TSCs to achieve exceptional long-term operational stability, retaining over 91% of their initial efficiency after 1000 hours of continuous metal-halide lamp illumination, and to exhibit minimal fatigue after 86 cycles (2067 hours) of long-term diurnal (12/12-hour) testing. These results demonstrate that tandem cells significantly outperform their single-junction counterparts in both efficiency and stability.

physics.app-ph↗

arXiv2025

Correlative and in situ microscopy investigation of phase transformation, crystal growth and degradation of antimony sulfide thin films

Antimony sulfide (Sb$_2$S$_3$), a compound of earth-abundant elements with highly anisotropic, quasi-layered crystal structure, triggered growing interest as a solar absorber in photovoltaics and as a phase change material in memory devices, yet challenges remain in achieving high-quality thin films with controlled nucleation and growth for optimal performance. Here, we investigate the phase transformation, crystal structure and properties, growth and degradation of atomic layer deposited Sb$_2$S$_3$ thin films using in situ TEM and correlative ex situ analysis. The as-deposited amorphous films crystallized at 243°C, forming grains with an [100] out-of-plane texture and developed into tens to hundreds of micrometer, leaves-shaped grains. Introducing an ultra-thin ZnS interfacial layer increased nucleation density, and resulted in a few micrometer-sized, more uniform grains while retaining the overall [100] texture. In situ observations and subsequent crystal orientation analysis with cutting-edge 4D-STEM and EBSD revealed that the grains grew faster along the [010] ribbon direction and that the bare films underwent early-stage degradation, forming holes in amorphous regions during annealing. The ZnS interlayer mitigated degradation, stabilizing the films and improving their uniformity. These findings offer valuable insights for optimizing Sb$_2$S$_3$ thin films for applications both as solar cell materials and phase change materials.

cond-mat.mtrl-sci↗

arXiv2025

Improving electron tomography of mesoporous silica by Ga intrusion

Electron tomography (ET) offers nanoscale 3D characterization of mesoporous materials but is often limited by their low scattering contrast. Here, we introduce a gallium (Ga) intrusion strategy for mesoporous silica that dramatically improves imaging contrast - a key benefit that enables more accurate 3D reconstructions. By infiltrating Ga through a modified mercury intrusion porosimetry process, the high-angle annular dark-field (HAADF) STEM signal is enhanced by 5 times, resulting in a 34% improvement in reconstruction resolution and a 49% enhancement in interface sharpness. In addition, the increased sample conductivity facilitates focused ion beam (FIB) milling by minimizing charging effects and reducing drift. This approach enables precise segmentation and quantitative analysis of pore connectivity and size distribution, thereby extending the applicability of ET to light-element non-conductive materials and advancing structure-property characterization of complex porous systems.

physics.optics↗

arXiv2025

Identification of Polytypism and Their Dislocations in Bilayer MoS2 Using Correlative Transmission Electron Microscopy and Raman Spectroscopy

Stacking orders and topological defects substantially influence the physical properties of 2D van der Waals (vdW) materials. However, the inherent features of 2D materials challenge the effectiveness of single characterization techniques in identifying stacking sequences, necessitating correlative approaches. Using bilayer MoS2 as a benchmark, we differentiate its polytypism and specific dislocations through transmission electron microscopy (TEM) and Raman spectroscopy. Perfect and partial dislocations were revealed in TEM, which are closely linked to the stacking sequences, thus indirectly indicating the 2H and 3R polytypes. 3D electron diffraction reconstruction on relrods and low-frequency Raman spectroscopy further validated these polytypes owing to their reliance on crystal symmetry. Surprisingly, we unexpectedly resolved both polytypes despite starting with 2H bulk crystal, pointing to a possible phase transition during mechanical exfoliation. The correlative TEM-Raman approach can be extended to other 2D materials, paving the way for property alteration via stacking and defect engineering.

cond-mat.mtrl-sci↗

arXiv2021

Seeing Structural Evolution of Organic Molecular Nano-crystallites Using 4D Scanning Confocal Electron Diffraction

Direct observation of organic molecular nanocrystals and their evolution using electron microscopy is extremely challenging, due to their radiation sensitivity and complex structure. Here, we introduce 4D-scanning confocal electron diffraction (4D-SCED), which enables direct in situ observation of bulk heterojunction (BHJ) thin films. 4D-SCED combines confocal electron microscopy with a pixelated detector to record focused spot-like diffraction patterns with high angular resolution, using an order of magnitude lower dose than previous methods. We apply it to study an active layer in organic solar cells, namely DRCN5T:PC$_{71}$BM BHJ thin films. Structural details of DRCN5T nano-crystallites oriented both in- and out-of-plane are imaged at ~5 nm resolution and dose budget of ~5 e$^-$/A$^2$. We use in situ annealing to observe the growth of the donor crystals, evolution of the crystal orientation, and progressive enrichment of PC$_{71}$BM at interfaces. This highly dose-efficient method opens new possibilities for studying beam sensitive soft materials.

cond-mat.mtrl-sci↗

arXiv2019

Fabrication and structural characterization of diamond-coated tungsten tips

Coating metal nanotips with a negative electron affinity material like hydrogen-terminated diamond bears promise for a high brightness photocathode. We report a recipe on the fabrication of diamond coated tungsten tips. A tungsten wire is etched electrochemically to a nanometer sharp tip, dip-seeded in diamond suspension and subsequently overgrown with a diamond film by plasma-enhanced chemical vapor deposition. With dip-seeding only, the seeding density declines towards the tip apex due to seed migration during solvent evaporation. The migration of seeds can be counteracted by nitrogen gas flow towards the apex, which makes coating of the apex with nanometer-thin diamond possible. At moderate gas flow, diamond grows homogeneously at shaft and apex whereas at high flow diamond grows in the apex region only. With this technique, we achieve a thickness of a few tens of nanometers of diamond coating within less than 1 $μ$m away from the apex. Conventional transmission electron microscopy (TEM), electron diffraction and electron energy loss spectroscopy confirm that the coating is composed of dense nanocrystalline diamond with a typical grain size of 20 nm. High resolution TEM reveals graphitic paths between the diamond grains.

cond-mat.mtrl-sci↗