npj 2D Materials and Applications最新文献

We investigate the properties of momentum-dark excitons and trions formed in two-dimensional (2D) materials that exhibit an inverted Mexican hat-shaped-dispersion relation, taking as an example monolayer InSe. We employ variational techniques to obtain the momentum-dark ground state and bright state (non-zero and zero quasiparticle momenta, respectively). These states are particularly relevant due to their peaks in the quasiparticle density of states, where for the momentum-dark ground state, the contribution here is largest due to the presence of a van Hove singularity (VHS). The momentum-dark systems require a brightening procedure to provide the necessary momentum to become bright. We study the brightening through coupling to phonons and compute the photoluminescence spectrum. This work opens new avenues of research, such as exploiting dark excitons in solar cells and other semiconductor-based optoelectronic devices.

Two-dimensional (2D) Ruddlesden-Popper (RP) metal halides present unique and tunable properties. However, direct and oriented synthesis is challenging due to low formation energies that lead to rapid, uncontrolled growth during solution-based processing. Here, we report the solvent-free growth of oriented n = 1 (PEA)₂PbI₄ RP films by pulsed laser deposition (PLD). In situ photoluminescence (PL) during deposition reveals the formation of the n = 1 phase at the early stages of growth. X-ray diffraction (XRD) and grazing-incidence wide-angle scattering (GIWAXS) confirm a single oriented n = 1 phase, independent of the substrate. Co-localized spatially resolved PL and AFM further validate the conformal growth. While oriented growth is substrate-independent, film stability is not. (PEA)₂PbI₄ films grown on strained epitaxial MAPbI₃ remain stable for over 184 days without any sign of cation exchange. This work highlights the potential of PLD for direct, room-temperature synthesis of 2D (PEA)₂PbI₄ RP films and stable 2D/3D heterostructures.

Thin films fabricated from solution-processed graphene nanosheets are of considerable technological interest for a wide variety of applications, such as transparent conductors, supercapacitors, and memristors. However, very thin printed films tend to have low conductivity compared to thicker ones. In this work, we demonstrate a simple layer-by-layer deposition method which yields thin films of highly-aligned, electrochemically-exfoliated graphene which have low roughness and nanometer-scale thickness control. By optimising the deposition parameters, we demonstrate films with high conductivity (1.3 × 10⁵S/m) at very low thickness (11 nm). Finally, we connect our high conductivities to low inter-nanosheet junction resistances (R_J), which we estimate at R_J ~ 1kΩ.

Thermoelectric materials are of great interest for heat energy harvesting applications. One such promising material is TlGaSe₂, a 2D-layered, p-type semiconducting ternary chalcogenide. Recent reports show it can be processed as a thin film, opening the door for large-scale commercialization. However, TlGaSe₂ is prone to stacking faults along the [001] stacking direction and their role in its thermoelectric properties has not yet been understood. Herein, TlGaSe₂ is investigated via (scanning) transmission electron microscopy and first-principles calculations. Stacking faults are found to be present throughout the material, as density functional theory calculations reveal a low stacking fault energy of ~12 mJ m^-2. Electron transport calculations show an enhancement of thermoelectric power factors when stacking faults are present. This implies the presence of stacking faults is key to the material's excellent thermoelectric properties along the [001] stacking direction, which can be further enhanced by doping the material to carrier concentrations of ~10¹⁹cm^-3.

We propose a spin-charge qubit based on a bilayer graphene and WSe₂ van der Waals heterostructure that together form a quantum dot and demonstrate its functionality from first-principles simulations. Electron and hole confinement as well as electrically controllable spin-orbit coupling (SOC) are modeled by self-consistently solving the Schrödinger and Poisson equations with material parameters extracted from density functional theory as inputs. In both electron and hole quantum dots, we find a two orders of magnitude enhancement of SOC (1.8 meV) compared to intrinsic graphene, in the layer directly adjacent to WSe₂. Time-dependent investigations of the quantum device reveal rapid qubit gate operation in the order of picoseconds. Our simulations indicate that bilayer graphene and WSe₂ heterostructures provide a promising platform for the processing of quantum information.

Isotope-enriched bulk hexagonal boron nitride (hBN) crystals have enhanced properties that improve the performance of nanophotonic and quantum technologies. Developing methods to deposit epitaxial layers on such crystals enables the exciting prospects of producing isotope-engineered hBN layers and heterostructures. Here, we demonstrate the homoepitaxial growth of hBN with phase-separated ¹⁰B and ¹¹B isotopes by high-temperature molecular beam epitaxy (HT-MBE). Controlled nucleation, improved surface uniformity, and step-flow growth of an h¹⁰BN epilayer were achieved by etching the h¹¹BN bulk crystals with molecular hydrogen. The alignment of the h¹⁰BN epilayer and host h¹¹BN lattices was confirmed by lattice-resolved atomic force microscopy. Micro-Raman spectroscopy and scattering-type scanning near-field optical microscopy show that the bulk h¹¹BN and h¹⁰BN epilayer have distinct phonon energies, with no intermixing of the van der Waals layers, thus enabling the different boron isotopes to be spatially separated in the heterostructure. This work demonstrates the potential of HT-MBE to produce isotopic heterostructures of hBN to advance future nanophotonic and quantum technologies.

Single-layer semiconducting transition-metal dichalcogenides, lacking point inversion symmetry, provide an efficient platform for valleytronics, where the electronic, orbital, magnetic, valley, and lattice degrees of freedom can be selectively manipulated by using polarized light. This task is, however, thought to be impeded in parent bulk compounds where the point inversion symmetry is restored. Exploiting the underlying quantum physics in bulk materials is thus one of the biggest paradigmatic challenges. Here we show that a sizable optical Kerr rotation can be efficiently generated without breaking point-inversion symmetry in a wide energy range on ultrafast timescales in bulk WSe₂, by means of circularly-polarized light. We rationalize this finding as a result of the hidden spin/layer/orbital/valley order. The spectral analysis reveals distinct A-, B-, and C-exciton features, which we show to stem from a selective Pauli blocking effect on top of the hidden-order pseudospin order and of the spin Berry curvature. The Kerr response lifetime (τ ~ 500 fs), common to all the peaks, suggests that excitonic dynamics dominate over single-particle decay. The present report demonstrates that the hidden order at play in bulk centrosymmetric layered materials can stem out in macroscopical bulk features, opening the way for an effective exploitation of bulk WSe₂ in novel optoelectronic and orbitronics applications.

This study explores the challenges associated with translating electrical characteristics of individual two-dimensional semiconductor nanosheets into a network of partially overlapping sheets. Such systems typically suffer from high-energy barriers required to overcome the junctions formed between the adjacent nanosheets, and consequently quench the current passing through the network. We use in-operando Kelvin probe force microscopy to image electrostatic potential profiles during the operation of MoS₂ nanosheet network transistors. Direct imaging of the potential drops allows us to distinguish contributions from individual nanosheets and those from junctions, correlated by the junction-related potential drops with the network morphology. A diagram-based model is developed to describe the system numerically and to estimate the current path formation probabilities. Finally, a correlation with the integral electrical characteristics of the nanosheet-based transistors is made using a robust Y-function approach. It is shown that the total junction resistance is well estimated by the proposed equivalent circiut model.

We demonstrate Andreev pair injection across Nb-WS₂ junction evident as Andreev reflection in differential conductivity spectra below Nb critical temperature $T_c$ . The superconducting- 2D semiconducting junction defined by a focused ion beam, shaped Nb pads, and semi-dry transfer of single layer CVD-grown WS₂ crystals ensured the mechanical integrity of the 2D TMD film, reduced contamination and defects at Nb-WS₂ junction, enabling the pristine study of the interface and facilitating Andreev pair injection. We observed enhanced conductivity in $\mathrm{dI}/\mathrm{dV}$ spectra for junction voltages smaller than the corresponding Nb superconducting gap, which vanishes as the device temperature is increased above the $T_c$ . The position and the temperature dependence of the conductivity peaks suggest proximity effect-related phenomena explained by developed modified BTK theory. The presented results are crucial for the future implementation of proximity-based 2D hybrid devices including quantum light sources and superconducting field-effect transistors based on superconductor-semiconductor junctions.