Atomic clusters often show unique, size-dependent properties and have become a fertile ground for the discovery of novel molecular structures and chemical bonding. Here we report an investigation of the B₁₉⁻ cluster, which shows chemical bonding reminiscent of that in [10]annulene (C₁₀H₁₀) and [6]circulene (C₂₄H₁₂). Photoelectron spectroscopy reveals a relatively simple spectrum for B₁₉⁻, with a high electron-binding energy. Theoretical calculations show that the global minimum of B₁₉⁻ is a nearly circular planar structure with a central B₆ pentagonal unit bonded to an outer B₁₃ ring. Chemical bonding analyses reveal that the B₁₉⁻ cluster possesses a unique double π-aromaticity in two concentric π-systems, with two π-electrons delocalized over the central pentagonal B₆ unit and another ten π-electrons responsible for the π-bonding between the central pentagonal unit and the outer ring. Such peculiar chemical bonding does not exist in organic compounds; it can only be found in atomic clusters.

The atomic structures of bare gold clusters provide the foundation to understand the enhanced catalytic properties of supported gold nanoparticles. However, the richness of diverse structures and the strong relativistic effects have posed considerable challenges for a systematic understanding of gold clusters with more than 20 atoms. We use photoelectron spectroscopy of size-selected anions, in combination with first principles calculations, to elucidate the structures of gold nanoclusters in a critical size regime from 55 to 64 atoms (1.1-1.3 nm in diameter). Au(55)(-) is found to be a nonicosahedral disordered cluster as a result of relativistic effects that induce strong surface contractions analogous to bulk surface reconstructions, whereas low-symmetry core-shell-type structures are found for Au(56)(-) to Au(64)(-). Au(58) exhibits a major electron-shell closing and is shown to possess a low-symmetry, but nearly spherical structure with a large energy gap. Clear spectroscopic and computational evidence has been observed, showing that Au(58)(-) is a highly robust cluster and additional atoms are simply added to its surface from Au(59)(-) to Au(64)(-) without inducing significant structural changes. The unique low-symmetry structures characteristic of gold nanoclusters due to the strong relativistic effects allow abundant surface defects sites, providing a key structure-function relationship to understand the catalytic capabilities of gold nanoparticles.

The controlled production of high-quality atomically thin III-VI semiconductors poses a challenge for practical applications in electronics, optoelectronics, and energy science. Here, we exploit a controlled synthesis of single- and few-layer In2Se3 flakes on different substrates, such as graphene and mica, by van der Waals epitaxy. The thickness, orientation, nucleation site, and crystal phase of In2Se3 flakes were well-controlled by tuning the growth condition. The obtained In2Se3 flakes exhibit either semiconducting or metallic behavior depending on the crystal structures. Meanwhile, field-effect transistors based on the semiconducting In2Se3 flakes showed an efficient photoresponse. The controlled growth of atomically thin In2Se3 flakes with diverse conductivity and efficient photoresponsivity could lead to new applications in photodetectors and phase change memory devices.