Xiaojun Li

Nonlinear optical materials have potential applications in optical signal processing, photovoltaic conversion, and optical information storage. As organic conjugated material, its high second-order nonlinear optical (NLO) properties can be produced due to the introduction of alkali metal species on porphyrin surface. In order to investigate the effects of alkali metal atoms on the second-order NLO properties of porphyrin derivative (PYD) complexes, a series of novel alkali metal porphyrin derivatives (M@PYD and M2F@PYD, M=Li, Na, K) are systematically studied using the density-functional theory calculations. Firstly, the stable structures of all the complexes are confirmed, and the high chemical stability of alkali metal porphyrin complexes is explored by the analysis of binding energy. The nonlinear optical properties of stable structures are analyzed to obtain the dipole moments, polarizabilities and the static first hyperpolarizabilities of the functionalized complexes. The calculated results show that the system has the maximum static first hyperpolarizability (β0=2457.573 a.u.) when the Na2F molecule is introduced onto the PYD surface, whereas M@PYD has relatively small static first hyperpolarizability. The present findings will provide the important reference for designing the high-performance porphyrin-based NLO materials.

The structural, electronic and nonlinear optical (NLO) properties of BN-substituted graphdiyne (BNGDY) with the doping of Li₃NM (M = Li, Na, and K) alkalides are systematically studied using density-functional theory calculations, in order to disclose the effect of the alkalides on molecular optical properties. The results show that the Li₃NM alkalides can stably adsorb on the surface of six planar BNGDY structures, of which the structures with small substitution ratios of BN-pairs on diacetylenic linkages exhibit strong chemical stabilities. Moreover, the Li₃NM alkalides have the slightly increased charge transfer with alkali metals from Li to K, and that is a decreasing trend with increasing BN substitution ratios (A → F) on GDY. More interestingly, the successive introduction of the alkalides on BN-substituted graphdiyne can effectively modulate the electronic properties and nonlinear optical responses of the systems. Importantly, the BNGDY structure (D) with the doping of Li₃NLi on triangular hole possesses the excellent nonlinear optical performance with much larger first hyperpolarizability values (e.g., 1.890 × 10⁶ a.u.), which can be further understood by the analysis of the first hyperpolarizability density and the two-level model from the excitation energy calculations. This study will inevitably provide an excellent strategy for designing the next generation high-performance NLO materials.

Identifying efficient and low-cost electrocatalysts for the hydrogen evolution reaction (HER) is essential for renewable energy applications. In recent years, single-atom catalysts (SACs) anchored on graphdiyne (GDY) have attracted great interest as promising HER electrocatalysts. However, the introduction of heteroatoms on the catalyst surface alters its catalytic performance. Herein, we rationally designed nickel SACs (Ni/nX-GDY, n = 1–2) anchored on graphdiyne doped with four types of heteroatoms (X = B, N, P, and S) and explored their HER catalytic activity using density functional theory calculations. By doping different heteroatoms, twelve different catalyst configurations were obtained with nickel anchored on the nX-GDY substrate, and compared with P/S doping, B/N doping resulted in superior thermodynamic stability. By evaluating hydrogen adsorption free energy (ΔG_H*), four optimal HER models (2B55, 1N3, 1P4, and 1S4) were identified; especially, the N-doped 1N3 model at the acetylenic carbon site exhibited the best ΔG_H* value, close to zero (−0.004 eV), demonstrating Ni/1N3-GDY as the most efficient HER catalyst in this series. Furthermore, it is concluded that at zero overpotential, the Volmer–Tafel mechanism with electrochemical desorption acts as the rate-limiting step in Ni/1N3-GDY, surpassing the Volmer–Heyrovsky mechanism. This study not only offers valuable insights into the rational design and discovery of highly efficient GDY-based HER electrocatalysts, but also promotes the development of hydrogen energy technologies.

Novel graphdiyne (GDY) materials with tunable electronic and nonlinear optical (NLO) properties are always desirable for material development. Herein, we conducted a comprehensive prediction of silagraphdiyne (SiGDY) with the adsorption of alkali metals (M) and superalkalis molecules (M₂F) onto its surface, and their geometric structures, electronic properties, charge transfer and nonlinear optical responses were systematically studied. Our calculations demonstrate that the M/M₂F molecules can be adsorbed stably on large delocalized SiGDY surfaces reflected by high binding energies. Notably, the SiGDY can accept almost an electron transferred from M/M₂F, forming strong intramolecular charge-transfer (CT) process, while the reduced vertical ionization potentials (VIP) create the necessary conditions for enhancing the NLO response. Furthermore, it is found that both the substitution of silicon atoms within GDY and the adsorption of M/M₂F onto SiGDY significantly modulate electronic properties, and narrow the HOMO–LUMO gaps of complexes. More interestingly, the adsorption of M/M₂F on SiGDY enhances remarkably the first hyperpolarizability (β_tot) of complexes, especially for M@SiGDY, of which Li@SiGDY possesses the exceptionally large β_tot value (~7.65 × 10⁵ a.u.), further explained by two-level model. The present findings show that the functionalized SiGDY materials can be considered as promising candidate of novel carbon-based NLO nanodevices for future applications.

Carbon family elements (Si, Ge, Sn, Pb) have attracted a lot of attention because of their unique structural features and potential applications in microelectronics industry. Here, the structure, chemical stability, electronic properties, and nonlinear optical properties of neutral and charged Au@X₂₀H₂₀ (X=Si, Ge, Sn, Pb) clusters have been systematically studied using density‐functional theory calculations. Structurally, the neutral/anionic Au@X₂₀H₂₀ (X=Si, Ge, Sn) as well as cationic Au@Pb₂₀H₂₀ possess Au‐endohedral (XH)₂₀ unit, forming fullerene‐like framework, whereas other species are Au‐doped structures with hydrogen‐bridged bond. Analysis of binding energy and HOMO‐LUMO energy gaps reveals that the charged clusters possess high chemical stabilities due to closed‐shell structures. The charge transfers from X₂₀ cage to Au atom, and the Au atom acts as electron acceptor. The Au atom and charged states play an important role in the structural stability, and can effectively modulate the electronic properties of clusters. Interestingly, the neutral Au@X₂₀H₂₀ (X=Si, Sn) clusters possess large first hyperpolarizabilities, especially for Au@Sn₂₀H₂₀, which has remarkably giant value (~5.65×10⁸ a. u.), and the enhanced NLO behaviors can be further explained by the TDDFT calculation. The work may provide a theoretical reference for further applications considered as novel cluster‐assembled nanomaterials.

Perfluorocubane is a fluorinated analogue of a polyhedral hydrocarbon observed recently in experiment (M. Sugiyama, M. Akiyama, Y. Yonezawa, K. Komaguchi, M. Higashi, K. Nozaki and T. Okazoe, Electron in a cube: Synthesis and characterization of perfluorocubane as an electron acceptor, Science, 2022, 377, 756–759). Inspired by its unique structure and electronic properties, we systematically explored its chemical stabilities, bonding character, electronic properties, and electronic transitions using density functional theory calculations. Analysis of the binding energies and Mayer bond orders reveals that the C₈F₈ molecule has high chemical stability with a special electronic structure, and the addition of an extra electron to the neutral molecule may generate a radical anion with a hosted electron inside the cubane frame. The induced ring currents under an external magnetic field reveal that perfluorocubane exhibits an overall small diamagnetic current supported by anisotropy of the current-induced density (ACID) and gauge-including magnetically induced currents (GIMIC). AdNDP analysis indicates that the localized two-center two-electron (2c–2e) σ-bonds are responsible for structural stabilization of the molecule. Further study into the electronic excitations shows that the main electronic transitions of perfluorocubane come from the surface of the cubic frame to its inner structure. Consequently, perfluorocubane can be recognized as a small electron acceptor material due to its special electron-carrying capacity, which will promote further applications of novel conducting and semiconducting materials.