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.

Activation of O(2) is the most critical step in catalytic oxidation reactions involving gold and remains poorly understood. Here we report a systematic investigation of the interactions between O(2) and small gold cluster anions Au(n)(-) (n = 1-7) using photoelectron spectroscopy. Higher resolution photoelectron spectra are obtained for the molecularly chemisorbed even-sized Au(n)O(2)(-) (n = 2, 4, 6) complexes. Well-resolved vibrational structures due to O-O stretching are observed and can be readily distinguished from the Au-derived PES bands. The adiabatic detachment energies and O-O vibrational frequencies are measured to be 3.03 +/- 0.04, 3.53 +/- 0.05, and 3.17 +/- 0.05 eV, and 1360 +/- 80, 1360 +/- 80, and 1330 +/- 80 cm(-1) for n = 2, 4, 6, respectively. Physisorbed Au(n)(-)(O(2)) complexes for n = 1, 3, 5, 7 are observed for the first time, providing direct evidence for the inertness of the closed-shell odd-sized Au(n)(-) clusters toward O(2). Neutral even-sized Au(n) clusters are closed-shell and are expected to be inert toward O(2), which is not consistent with the reduced O-O vibrational frequencies observed in the photoelectron spectra relative to free O(2). It is suggested that the photodetachment transitions can only access excited states of the neutral even-sized Au(n)O(2) complexes; a double-well potential is proposed consisting of the ground-state van der Walls well at long Au(n)-O(2) distances and a higher energy deeper well at short Au(n)-O(2) distances derived from singlet O(2) ((1)Delta(g)). The current study provides further insight into O(2) interactions with small gold clusters, as well as accurate experimental data to benchmark theoretical investigations.

The structural evolution of negatively charged gold clusters (Au(n)(-)) in the medium size range for n = 27-35 has been investigated using photoelectron spectroscopy (PES) and theoretical calculations. New PES data are obtained using Ar-seeded He supersonic beams to achieve better cluster cooling, resulting in well-resolved spectra and revealing the presence of low-lying isomers in a number of systems. Density-functional theory calculations are used for global minimum searches. For each cluster anion, more than 200 low-lying isomers are generated using the basin-hopping global minimum search algorithm. The most viable structures and low-lying isomers are obtained using both the relative energies and comparisons between the simulated spectra and experimental PES data. The global minimum structures of Au(n)(-) (n = 27, 28, 30, and 32-35) are found to exhibit low-symmetry core-shell structures with the number of core atoms increasing with cluster size: Au(27)(-), Au(28)(-), and Au(30)(-) possess a one-atom core; Au(32)(-) features a three-atom triangular core; and Au(33)(-) to Au(35)(-) all contain a four-atom tetrahedral core. The global searches reveal that the tetrahedral core is a popular motif for low-lying structures of Au(33)(-) to Au(35)(-). The structural information forms the basis for future chemisorption studies to unravel the catalytic effects of gold nanoparticles.

Van der Waals-coupled two-dimensional (2D) heterostructures have attracted great attention recently due to their high potential in the next-generation photodetectors and solar cells. The understanding of charge-transfer process between adjacent atomic layers is the key to design optimal devices as it directly determines the fundamental response speed and photon-electron conversion efficiency. However, general belief and theoretical studies have shown that the charge transfer behavior depends sensitively on interlayer configurations, which is difficult to control accurately, bringing great uncertainties in device designing. Here we investigate the ultrafast dynamics of interlayer charge transfer in a prototype heterostructure, the MoS/WS bilayer with various stacking configurations, by optical two-color ultrafast pump-probe spectroscopy. Surprisingly, we found that the charge transfer is robust against varying interlayer twist angles and interlayer coupling strength, in time scale of ∼90 fs. Our observation, together with atomic-resolved transmission electron characterization and time-dependent density functional theory simulations, reveals that the robust ultrafast charge transfer is attributed to the heterogeneous interlayer stretching/sliding, which provides additional channels for efficient charge transfer previously unknown. Our results elucidate the origin of transfer rate robustness against interlayer stacking configurations in optical devices based on 2D heterostructures, facilitating their applications in ultrafast and high-efficient optoelectronic and photovoltaic devices in the near future.

Cubic inorganic perovskite CsPbI 3 is a direct bandgap semiconductor, which is promising for optoelectronic applications, such as solar cells, light emitting diodes, and lasers. The intrinsic defects in semiconductors play crucial roles in determining carrier conductivity, the efficiency of carrier recombination, and so on. However, the thermodynamic stability and intrinsic defect physics are still unclear for cubic CsPbI 3 . By using the first-principles calculations, we study the thermodynamic process and find out that the window for CsPbI 3 growth is quite narrow and the concentration of Cs is important for cubic CsPbI 3 growth. Under Pb-rich conditions, V Pb and V I can pin the Fermi energy in the middle of the bandgap, which results in a low carrier concentration. Under Pb-poor conditions, V Pb is the dominant defect and the material has a high concentration of hole carriers with a long lifetime. Our present work gives an insight view of the defect physics of cubic CsPbI 3 and will be beneficial for optoelectronic applications based on cubic CsPbI 3 and other analogous inorganic perovskites.

Semiconducting p–n heterojunctions, serving as the basic unit of modern electronic devices, such as photodetectors, solar-energy conversion devices, and light-emitting diodes (LEDs), have been extensively investigated in recent years. In this work, high performance self-powered broad-band photodetectors were fabricated based on vertically stacked p–n heterojunctions though combining p-type WSe2 with n-type Bi2Te3 via van der Waals (vdW) epitaxial growth. Devices based on the p–n heterojunction show obvious current rectification behaviors in the dark and superior photovoltaic characteristics under light irradiation. A maximum short circuit current of 18 nA and open circuit voltage of 0.25 V can be achieved with the illumination light of 633 nm (power density: 26.4 mW/cm2), which are among the highest values compared with the ever reported 2D vdW heterojunctions synthesized by chemical vapor deposition (CVD) method. Benefiting from the broad-band absorption of the heterostructures, the detection range can be expanded from the visible to near-infrared (375–1550 nm). Moreover, ascribing to the efficient carriers separation process at the junction interfaces, the devices can be further employed as self-powered photodetectors, where a fast response time (∼210 μs) and high responsivity (20.5 A/W at 633 nm and 27 mA/W at 1550 nm) are obtained under zero bias voltage. The WSe2/Bi2Te3 p–n heterojunction-based self-powered photodetectors with high photoresponsivity, fast photoresponse time, and broad spectral response will find potential applications in high speed and self-sufficient broad-band devices.