Lieb lattice has been predicted to host various exotic electronic properties due to its unusual Dirac-flat band structure. However, the realization of a Lieb lattice in a real material is still unachievable. Based on tight-binding modeling, we find that the lattice distortion can significantly determine the electronic and topological properties of a Lieb lattice. Importantly, based on first-principles calculations, we predict that the two existing covalent organic frameworks (COFs), i.e., sp²C-COF and sp²N-COF, are actually the first two material realizations of organic-ligand-based Lieb lattice. Interestingly, the sp²C-COF can experience the phase transitions from a paramagnetic state to a ferromagnetic one and then to a Néel antiferromagnetic one, as the carrier doping concentration increases. Our findings not only confirm the first material realization of Lieb lattice in COFs, but also offer a possible way to achieve tunable topology and magnetism in organic lattices.

Shrinking the size of the electronic synapse to molecular length-scale, for example, an artificial synapse directly fabricated by using individual or monolayer molecules, is important for maximizing the integration density, reducing the energy consumption, and enabling functionalities not easily achieved by other synaptic materials. Here, we show that the conductance of the self-assembled peptide molecule monolayer could be dynamically modulated by placing electrical biases, enabling us to implement basic synaptic functions. Both short-term plasticity (e.g., paired-pulse facilitation) and long-term plasticity (e.g., spike-timing-dependent plasticity) are demonstrated in a single molecular synapse. The dynamic current response is due to a combination of both chemical gating and coordination effects between Ag⁺ and hosting groups within peptides which adjusts the electron hopping rate through the molecular junction. In the end, based on the nonlinearity and short-term synaptic characteristics, the molecular synapses are utilized as reservoirs for waveform recognition with 100% accuracy at a small mask length.

Dichotomy between local spins and conjugated electrons spawns various exotic physical phenomena, however, it has mostly been reported in three-dimensional (3D) inorganic systems. We show, for the first time, that a rare 2D metal–organic framework exhibits intriguing dichotomy behavior, which can be directly identified through scanning tunneling microscopy/spectroscopy (STM/STS). In a newly synthesized Cu-hexaiminobenzene [Cu₃(HAB)₂], on the one hand, the Cu²⁺ ions form an ideal S − 1/2 antiferromagnetic (AFM) kagome lattice; on the other hand, the conjugated-electrons from the organic ligands produce a frustrated π_x,y model on a honeycomb lattice, giving rise to completely dispersionless energy bands around the Fermi level that favour the ferromagnetic (FM) state. Remarkably, the frustrated local spins and conjugated electrons interact through a strong FM Hund's coupling, giving rise to a wide range of intriguing quantum phases. Furthermore, we propose that this dichotomy can be directly characterized through STM/STS measurements due to its special 2D nature, which provides a unique exciting platform to investigate the dichotomy of frustrated spins and electrons in a single lattice.

We investigate the structural and electronic properties of SnX and SnX₂ (X = S, Se) monolayers and further explore the separation and migration of photo-generated electrons/holes and redox capabilities.

Semiconductor photocatalysts, using sunlight to stimulate various photocatalytic reactions, are promising materials for solving the energy crisis and environmental problems. However, the low photocatalytic efficiency and high cost pose major challenges for their widespread application. Mimicking the natural photosynthesis system, we propose a direct Z-scheme photocatalyst based on a Janus van der Waals heterostructure (vdWH) comprising SnC and Janus SeSnS monolayers. From first-principles calculations, the intrinsic built-in electric field of Janus SeSnS and the charge transfer from the SnC to the SeSnS layer give rise to a type-II band alignment. Such a band alignment benefits the formation of spatially separated reductive and oxidative active sites and the reduction of the global bandgap of the Janus vdWH. The proposed material increases the solar-to-hydrogen conversion efficiency to 60.8%. Besides, we also find that the light absorption coefficient is stacking configuration controllable and strain-tunable, e.g., the tensile strain promotes photocatalytic efficiency. Moreover, because Sn, S, and Se are environmentally benign and inexpensive elements, SnC/SeSnS vdWH is a promising noble-metal-free direct Z-scheme photocatalyst.

Alongside highly efficient photocatalysts, high photovoltaic performance is also a key element for efficiently harvesting solar energy. Developing bifunctional materials which satisfy concurrently these two demands is an appealing strategy for solving the current serious energy and environmental issues. Based on first-principles and quantum transport calculations, we designed this kind of novel bifunctional material: Janus GeC/SnSSe van der Waals heterostructure (vdWH). We demonstrate that it is a highly efficient direct Z-scheme photocatalyst. However, unlike traditional direct Z-scheme photocatalysts, the GeC/SnSSe vdWH possesses a small energy separation between the low conduction band located in SnSSe and the high valence band residing in the GeC layer, which significantly fosters the interlayer charge transfer. Hence, its solar-to-hydrogen conversion efficiency reaches as high as 68.37%. Moreover, we also find that tensile strain promotes an astonishing increase in photovoltaic performance, e.g., 4% tensile strain leads to an increase of the photocurrent by 40%.