Shengfu Yang

Predicting the lowest-energy arrangement of like charges on a sphere defines the Thomson problem, a long-standing challenge with analytical solutions only for a few small systems. We report a real-space experimental realization using superfluid helium nanodroplets, where surface-localized charges are tagged by singly charged gold nanoparticles and recorded as transmission electron microscopy imprints. The observed configurations reveal Thomson charge ordering from finite- N minima to curvature-frustrated Coulomb crystallization, establishing helium nanodroplets as a directly imageable platform for charge organization on spherical surfaces.

To develop highly thermally stable thermoset materials and achieve a deep understanding of the chemical structure–physical property relationship of polybenzoxazines, a series of chalcogen-containing bis-benzoxazine monomers (PH-O, PH-S, PH-Se, and PH-Te) were designed and synthesized in the current work. The chemical structures of chalcogen-containing bis-benzoxazines were characterized by Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance, and high-resolution mass spectrometry. In addition, their polymerization behaviors were systematically investigated using differential scanning calorimetry, in situ FT-IR spectroscopy, and computational calculations. The results revealed that benzoxazines incorporating heavy chalcogen atoms with lower electronegativity exhibit a pronounced tendency toward ring-opening polymerization at lower temperatures. Moreover, the thermal degradation and flame retardant characteristics of the resulting chalcogen-containing polybenzoxazine thermosets were evaluated using thermogravimetric analysis and microscale combustion calorimetry, respectively. Notably, the chalcogen-containing polybenzoxazines displayed significantly reduced flammability compared to those polybenzoxazines without chalcogen atoms. These findings provide valuable insights into the effect of chalcogen atoms on the polymerization mechanism and thermal properties of polybenzoxazines, presenting a promising strategy for the development of thermosets with low curing temperature as well as enhanced thermal stability and flame retardancy.

Nanoparticle-functionalized polymer surfaces must reconcile particle retention with interfacial accessibility, a balance central to catalytic and multifunctional surfaces. Existing architectures use binders, primers or grafting layers that bury active area or add chemical complexity. Here we show that mild annealing below the main bulk-softening regime partially embeds nanoparticles into cured polymer surfaces, forming a close-packed, one-particle-thick monolayer through two coupled self-limiting processes: particles above the polymer-contacting layer remain removable, while the retained layer approaches a finite embedding depth. On a commercial epoxy, CeO ₂ nanoparticles embed to approximately their radius, providing anchoring while remaining partially exposed. The monolayer withstands vortexing, ultrasonication and repeated tape peeling, retains reversible Ce ³⁺ /Ce ⁴⁺ cycling, and suppresses degradation during peroxide cycling, short-term outdoor exposure and artificial-seawater immersion. Density-functional theory supports a vacancy-mediated peroxide-decomposition pathway consistent with redox regeneration. Spray deposition and Ag embedding on low-density and high-density polyethylene support transferability across deposition routes, nanoparticle chemistries and polymer substrates. Contact-layer-selective, depth-limited thermal embedding thus establishes a geometry-based, grafting-free design principle that provides mechanical retention while preserving interfacial accessibility, with broader relevance to catalytic and other functional polymer interfaces.

Propargyl alcohol (HC≡C–C(H2)–OH) and its clusters have been isolated in helium nanodroplets and probed using infrared spectroscopy. The global energy minimum of the dimer, in which a strong hydrogen bond forms between the two OH groups, does not yield the most intense spectral features. Instead, bands from a higher energy structure are prominent and derive from dimers held together primarily by a single and far weaker –C≡C–H⋯O–H hydrogen bond. This suggests that helium nanodroplet encapsulation kinetically traps the dimers in a local minimum on account of the rapid cooling and low temperature. This provides access to an unusual part of the potential energy surface of the dimer and, more generally, allows the probing of weak hydrogen bonding interactions.

Semiconductor photonic nanowires are critical components for nanoscale light manipulation in integrated photonic and electronic devices. Optimizing their optical performance requires enhanced photon conversion efficiency, for which a promising solution is to combine semiconductors with noble metals, using the surface plasmon resonance of noble metals to enhance the photon absorption efficiency. Here, we report plasmon-enhanced light emission in a hybrid nanowire device composed of perovskite semiconductor nanowires and silver nanoparticles formed using superfluid helium droplets. A cesium lead halide perovskite-based four-layer structure (CsPbBr₃/PMMA/Ag/Si) effectively reduces the metal’s plasmonic losses while ensuring efficient surface plasmon–photon coupling at moderate power. Microphotoluminescence and time-resolved spectroscopy techniques are used to investigate the optical properties and emission dynamics of carriers and excitons within the hybrid device. Our results demonstrate an intensity enhancement factor of 29 compared with pure semiconductor structures at 4 K, along with enhanced carrier recombination dynamics due to plasmonic interactions between silver nanoparticles and perovskite nanowires. This work advances existing approaches for exciting photonic nanowires at low photon densities, with potential applications in optimizing single-photon excitations and emissions for quantum information processing.

Semiconductor photonic nanowires are critical components for nanoscale light manipulation in integrated photonic and electronic devices. Optimizing their optical performance requires enhanced photon conversion efficiency, for which a promising solution is to combine semiconductors with noble metals, using the surface plasmon resonance of noble metals to enhance the photon absorption efficiency. Here, we report plasmon-enhanced light emission in a hybrid nanowire device composed of perovskite semiconductor nanowires and silver nanoparticles formed using superfluid helium droplets. A cesium lead halide perovskite-based four-layer structure (CsPbBr3/PMMA/Ag/Si) effectively reduces the metal’s plasmonic losses while ensuring efficient surface plasmon-photon coupling at moderate power. Microphotoluminescence and time-resolved spectroscopy techniques are used to investigate the optical properties and emission dynamics of carriers and excitons within the hybrid device. Our results demonstrate an intensity enhancement factor of 30 compared to pure semiconductor structures at 4 K, along with enhanced carrier recombination dynamics due to plasmonic interactions between silver nanoparticles and the perovskite nanowires. This work advances existing approaches for exciting photonic nanowires at low photon densities, with potential applications in optimizing single-photon excitation and emission for quantum information processing.

Conformers of the dimer of ethylene glycol within a helium nanodroplet.

The ability to precisely engineer nanostructures underpins a wide range of applications in areas such as electronics, optics, and biomedical sciences. Here we present a novel approach for the growth of nanoparticle assemblies that leverages the unique properties of superfluid helium. Unlike viscous solvents at or near room temperature, superfluid helium provides an unperturbed and cold environment in which weak van der Waals interactions between molecular templates and metal atoms become significant and can define the spatial arrangement of nanoparticles. To demonstrate this concept, diol and porphyrin-based molecules are employed as templates to grow gold nanoparticle assemblies in superfluid helium droplets. After soft-landing on a solid surface to remove the helium, transmission electron microscopy (TEM) imaging shows the growth of gold nanoparticles at specific binding sites within the molecular templates where the interaction between gold atoms and the molecular template is at its strongest.