Interlayer Carrier Dynamics in Two-Dimensional Perovskites Determined by the Length of Conjugated Organic Cations.

Unlocking the restricted interlayer carrier transfer in a two-dimensional perovskite is a crucial means to achieve the harmonization of efficiency and stability in perovskite solar cells. In this work, the effects of conjugated organic molecules on the interlayer carrier dynamics of 2D perovskites were investigated through nonadiabatic molecular dynamics simulations. We found that elongated conjugated organic cations contributed significantly to the accelerated interlayer carrier dynamics, originating from lowered transport barrier and boosted π-p coupling between organic and inorganic layers. Utilizing conjugated molecules of moderate length as spacer cations can yield both superior efficiency and exceptional stability simultaneously. However, conjugated chains that are too long lead to structural instability and stronger carrier recombination. The potential of conjugated chain-like molecules as spacer cations in 2D perovskites has been demonstrated in our work, offering valuable insights for the development of high-performance perovskite solar cells.

Functionalizing Janus-structured Ti2B2 unveils exceptional capacity and performance in lithium-ion battery anodes.

With the ever-growing demand for high-capacity energy storage technologies, lithium-ion batteries (LIBs) have drawn increasing attention. Ti2B2, a typical two-dimensional MBenes material, has been considered as a strong contender for anode materials of LIBs with significant performance. However, the limited Li storage capacity of MBenes has hindered its wide applications. To address this issue, we have functionalized Janus-structured MBenes, denoted as Ti2B2XaXb (Xa/Xb = N, O, S, Se). Employing first-principles simulations based on density functional theory, we have investigated the geometric characteristics and electrochemical properties of Ti2B2XaXb. Our results reveal that Ti2B2NO exhibits an exceptionally large theoretical specific capacity of 1091.17 mAh·g-1, improved by 2.4 times compared with the pristine Ti2B2 (456 mAh·g-1). Li atoms on the O side of Ti2B2NO possess a low diffusion barrier of 0.33 eV, which is conducive to the rapid charging and discharging of the battery. Moreover, the open-circuit voltage of Ti2B2NO within the safe voltage range of 0-1 V ensures the safety of battery operation. Overall, our study sheds light on understanding the underlying mechanism of surface functionalization on the Li storage properties of Janus-structured MBenes from atomic-scale, laying the groundwork for future design of high-performance anode materials.

Selective Interfacial Excited-State Carrier Dynamics and Efficient Charge Separation in Borophene-Based Heterostructures.

Borophene-based van der Waals heterostructures have demonstrated enormous potential in the realm of optoelectronic and photovoltaic devices, which has sparked a wide range of interest. However, a thorough understanding of the microscopic excited-state electronic dynamics at interfaces is lacking, which is essential for determining the macroscopic optoelectronic and photovoltaic performance of borophene-based devices. In this study, photoexcited carrier dynamics of ß12 , χ3 , and α΄ borophene/MoS2 heterostructures are systematically studied based on time-domain nonadiabatic molecular dynamics simulations. Different Schottky contacts are found in borophene/semiconductor heterostructures. The interplay between Schottky barriers, electronic coupling, and the involvement of different phonon modes collectively contribute to the unique carrier dynamics in borophene-based heterostructures. The diverse borophene allotropes within the heterostructures exhibit distinct and selective carrier transfer behaviors on an ultrafast timescale: electrons tunnel into α΄ borophene with an ultrafast transfer rate (≈29 fs) in α΄/MoS2 heterostructures, whereas ß12 borophene only allows holes to migrate with a lifetime of 176 fs. The feature enables efficient charge separation and offers promising avenues for applications in optoelectronic and photovoltaic devices. This study provides insight into the interfacial carrier dynamics in borophene-based heterostructures, which is helpful in further design of advanced 2D boron-based optoelectronic and photovoltaic devices.

Graphdiyne and its Composites for Lithium-Ion and Hydrogen Storage.

Graphynes (GYs) are a novel type of carbon allotrope composed of sp and sp2 hybridized carbon atoms, boasting both a planar conjugated structure akin to graphene and a pore-like configuration in three-dimensional space. Graphdiyne (GDY), the first successfully synthesized member of GYs family, has gained much interest due to its fascinating electrochemical properties including a greater theoretical capacity, high charge mobility and advanced electronic transport properties, making it a promising material for energy storage applications for lithium-ion and hydrogen storage. Various methods, including heteroatom substitution, embedding, strain, and nanomorphology control, have been employed to further enhance the energy storage performance of GDY. Despite the potential of GDY in energy storage applications, there are still challenges to overcome in scaling up mass production. This review summarizes recent progress in the synthesis and application of GDY in lithium-ion and hydrogen storage, highlighting the obstacles faced in large-scale commercial application of GDY-based energy storage devices. Suggestions on possible solutions to overcome these hurdles have also been provided. Overall, the unique properties of GDY make it a promising material for energy storage applications in lithium-ion and hydrogen storage devices. The findings presented here will inspire further development of energy storage devices utilizing GDY.

Substrate-Mediated Borophane Polymorphs through Hydrogenation of Two-Dimensional Boron Sheets.

The two-dimensional boron monolayer (borophene) stands out from the two-dimensional atomic layered materials due to its structural flexibility and tunable electronic and mechanical properties from a large number of allotropic materials. The stability of pristine borophene polymorphs could possibly be improved via hydrogenation with atomic hydrogen (referred to as borophane). However, the precise adsorption structures and the underlying mechanism are still elusive. Employing first-principles calculations, we demonstrate the optimal configurations of freestanding borophanes and the ones grown on metallic substrates. For freestanding borophenes, the energetically favored hydrogen adsorption sites are sensitive to the polymorphs and corresponding coordination numbers of boron atoms. With various metal substrates, the hydrogenation configurations of borophenes are modulated significantly, attributed to the overlap between B pz and H s orbitals. These findings provide a deep insight into the hydrogenating borophenes and facilitate the stabilization of two-dimensional boron polymorphs by engineering hydrogen adsorption sites and concentrations.

In situ preparation of an anatase/rutile-TiO2/Ti3C2T x hybrid electrode for durable sodium ion batteries.

Herein, a facile one-step method is developed to in situ prepare crystalline anatase and rutile TiO2 nanocrystals on Ti3C2T x by regulating the metastable Ti ions. The combination of TiO2 nanocrystals and Ti3C2T x not only introduces extensive accessible sites for Na+ storage, but also promotes the charge transport by efficiently relieving the collapse of Ti3C2T x . Compared with pristine Ti3C2T x , the optimized TiO2/Ti3C2T x hybrid electrode (anatase/rutile-TiO2/Ti3C2T x , A/R-TiO2/Ti3C2T x ) exhibits a desirable specific surface area (22.5 m2 g-1), an ultralow charge transfer resistance (42.46 Ω) and excellent ion diffusion (4.01 × 10-14). Remarkably, rich oxygen vacancies are produced on TiO2/Ti3C2T x which is beneficial to enhance the insertion/de-insertion of Na+ during the charge/discharge process. As a result, the A/R-TiO2/Ti3C2T x delivers a high average capacity of 205.4 mA h g-1 at 100 mA g-1 and a desirable capacitance retention rate of 84.7% can be achieved after 600 cycles at 500 mA g-1.