Metal-assisted chemical etching (MacEtch) of silicon in oxidizing hydrofluoric acid (HF) solutions has emerged as a prominent top-down micro/nanofabrication approach for a wide variety of silicon micro/nanostructures. The popularity of the process is due to its simplicity, rapidity, versatility, and scalability. In recent years, there has been a surge of interest in developing MacEtch silicon micro/nanostructures for advanced energy conversion and storage applications, such as photovoltaic devices, thermoelectric devices, lithiumion rechargeable batteries, and supercapacitors. Particularly, MacEtch has emerged as a powerful surface micro/nanostructuring method for low-cost and scalable production of commercial black silicon (b-Si) with excellent light trapping properties. This review on MacEtch processing of silicon in oxidizing HF solutions provides a critical description of its history and origin including how it evolved into what it is today, the understanding of its mechanism and important technical advances in the field. As regards MacEtch-fabricated b-Si, its initial discovery and further improvements to its large-scale deployment in silicon photovoltaic industry are traced. Some fundamental challenges and perspectives in this exciting field are also discussed.

Incorporating metal nanocrystals with semiconductor photoanodes significantly enhances the efficiency of the energy conversion in the visible range during water splitting due to the excitation of hot electrons. While extensively studied on ensemble samples, hot electron response of metal nanocrystals in a photoelectrochemical cell remains unexploited at the single-particle level. Herein, we systematically investigate hot electron response of individual single-crystalline gold nanocrystals (AuNCs) on a TiO2 photoanode during water splitting. We directly correlate the morphology of the AuNC and its plasmonic property to the efficiencies involving hot electrons with the help of single-particle dark-field microscopy and photocurrent mapping. Our results show that the efficiencies of individual AuNCs are dependent on a variety of factors including interface condition, applied bias, excitation power, incident angle, and AuNC size. Our research may shed light on optimizing the light-harvesting capability of metal/semiconductor photoanodes by providing insights into the photocatalytic processes.

Dark field microspectroscopy is a powerful tool for studying plasmon resonances of noble metal nanoparticles and for developing their applications in sensing and imaging. Here we calibrate a dark field microspectrometer with measurements on gold nanospheres in a uniform dielectric medium to yield quantitative spectral scattering cross sections for elongated nanoparticle shapes. Gold bipyramids, 135 nm in length, were found to have a peak differential cross section of 1.2 × 10 -16 m 2 . Measurements of a small ensemble of gold nanorods, 13 nm in diameter and 45 nm in length on average, were found to have a peak differential cross section of only 1 × 10 -18 m 2 . For the smaller gold nanorods, approximate expressions for the total scattering cross section may be used to indicate their scattering signal in microscopy applications.

Cortical inhibition plays an important role in information processing in the brain. However, the mechanisms by which inhibition and excitation are coordinated to generate functions in the six layers of the cortex remain unclear. Here, we measured laminar-specific responses to stimulus orientations in primary visual cortex (V1) of awake monkeys (male, Macaca mulatta ). We distinguished inhibitory effects (suppression) from excitation, by taking advantage of the separability of excitation and inhibition in the orientation and time domains. We found two distinct types of suppression governing different layers. Fast suppression (FS) was strongest in input layers (4C and 6), and slow suppression (SS) was 3 times stronger in output layers (2/3 and 5). Interestingly, the two types of suppression were correlated with different functional properties measured with drifting gratings. FS was primarily correlated with orientation selectivity in input layers ( r = −0.65, p < 10 −9 ), whereas SS was primarily correlated with surround suppression in output layers ( r = 0.61, p < 10 −4 ). The earliest SS in layer 1 indicates the origin of cortical feedback for SS, in contrast to the feedforward/recurrent origin of FS. Our results reveal two V1 laminar subnetworks with different response suppression that may provide a general framework for laminar processing in other sensory cortices. SIGNIFICANCE STATEMENT This study sought to understand inhibitory effects (suppression) and their relationships with functional properties in the six different layers of the cortex. We found that the diversity of neural responses across layers in primary visual cortex (V1) could be fully explained by one excitatory and two suppressive components (fast and slow suppression). The distinct laminar distributions, origins, and functional roles of the two types of suppression provided a simplified representation of the differences between two V1 subnetworks (input network and output network). These results not only help to elucidate computational principles in macaque V1, but also provide a framework for general computation of cortical laminae in other sensory cortices.