The magnetic hysteresis property during the spin transition of spin-crossover (SCO) materials holds great promise for their applications in spin electronics, information storage, thermochromic, and nanophotonic devices. Existing studies often measured the averaged property of a bulk sample consisting of lots of individuals. When considering the significant heterogeneity among different individuals and the inevitable interparticle interactions, ensemble measurement not only blurred the structure−property relationship but also compromised the intrinsic hysteresis property and cyclability. Herein, we employed a recently developed surface plasmon resonance microscopy (SPRM) method to measure the thermal hysteresis curve of single isolated SCO nanoparticles. The thermal-induced spin transition was found to alter the optical contrast of single SCO nanoparticles, which was optically readout using SPRM in a quantitative, nonintrusive, and high-throughput manner. Single nanoparticle measurements revealed an intrinsic transition temperature that was independent of the temperature scan rate and superior stability after over 11 000 cycles of single SCO nanoparticles. Correlations between the hysteresis and the size and morphology of the same individuals further uncovered the significant nanoparticle-to-nanoparticle heterogeneity with implications for the size−property relationship and rational design of SCO materials with improved performance.

In a typical bipolar electrochemistry (BPE) configuration, voltage applied between the two driving electrodes induced a potential drop through solution filled in the microchannel, resulting in an interfacial potential difference between solution and BPE varied along the BPE. In the present work, we employed a recently developed plasmonic imaging technique to map the distribution of surface potential of bipolar electrodes with various geometries including round, triangle, hexagon, star, and rhombus shapes under the nonfaradaic charging process, from which the line of zero potential (LZP) was visualized and determined. We further investigated the dependence of LZP on electrode geometry and the distribution of external electric field and explained the experimental results with a charge balance mechanism. The triangular and star-shaped BPEs show quite different LZP features from the other ones with symmetrical geometry. These experimentally obtained potential distributions are all in good agreement with electromagnetic simulations. Finally, the line of zero overpotential (LZO) of the triangular-shaped BPE under faradaic reactions were investigated. The results confirm the shift of LZO when faradaic reactions occurred at the corresponding ends of BPE. The present work demonstrates the first experimental capability to map the potential distribution of BPE with arbitrary geometry under an arbitrary driving field. It is anticipated to help the design and optimization on the geometry of electrodes and microchannels with implications for boosting their applications in chemical sensing and materials synthesis.

The inevitable microstructural defects, including cracks, grain boundaries and cavities, make a portion of the material inaccessible to electrons and ions, becoming the incentives for electrochemically inactive zones in single...

An ultrathin surface layer with extraordinary molecular mobility has been discovered and intensively investigated on thin-film polymer materials for decades. However, because of the lack of suitable characterization techniques, it remains largely unexplored whether such a surface mobile layer also exists on individual polymeric nanospheres. Here, we propose a thermal-optical imaging technique to determine the glass transition (T g) and rubber-fluid transition (T f) temperatures of single isolated polystyrene nanospheres (PSNS) in a high-throughput and nonintrusive manner for the first time. Two distinct steps, corresponding to the glass transition and rubber-fluid transition, respectively, were clearly observed in the optical trace of single PSNS during temperature ramping. Because the transition temperature and size of the same individuals were both determined, single nanoparticle measurements revealed the reduced apparent T f and increased T g of single PSNS on the gold substrate with a decreasing radius from 130 to 70 nm. Further experiments revealed that the substrate effect played an important role in the increased T g. More importantly, a gradual decrease in the optical signal was detected prior to the glass transition, which was consistent with a surface layer with enhanced molecular mobility. Quantitative analysis further revealed the thickness of this layer to be ∼8 nm. This work not only uncovered the existence and thickness of a surface mobile layer in single isolated nanospheres but also demonstrated a general bottom-up strategy to investigate the structure–property relationship of polymeric nanomaterials by correlating the thermal property (T g and T f) and structural features (size) at single nanoparticle level.

Electrochemical etching is a powerful and popular method for fabricating micropatterns on metal substrates for use in electronic devices, electrochemical sensors, and plasmonic substrates. In order to achieve micropatterning, either a prepatterned insulating layer (mask) or a scanning microelectrode is often required to selectively trigger electrochemical etching at the desired locations. In the present work, we employed a well-focused light beam to enable the photoassisted electrochemical etching of gold film with a spatial resolution close to the optical diffraction limit (∼300 nm). It was found that the simultaneous application of light irradiation and appropriate potential were critical for the oxidative dissolution (i.e., etching) of gold to occur. Superior controllability of light beam allowed for the direct-write micropatterning without the need of mask or probe. Etching kinetics and mechanism were also studied by monitoring the dynamic evolution of optical transparency with a conventional transmission bright-field microscope, together with characterizations on the as-obtained patterns with atomic force microscopy and electron microscopy. This study is anticipated to contribute a feasible method for the micropatterning of gold film with implications for nanoelectronics and electrochemical sensors.