The development of high-precision large-area optical coatings and devices comprising low-dimensional materials hinges on scalable solution-based manufacturability with control over exfoliation procedure-dependent effects. As such, it is critical to understand the influence of technique-induced transition metal dichalcogenide (TMDC) optical properties that impact the design, performance, and integration of advanced optical coatings and devices. Here, we examine the optical properties of semiconducting MoS films from the exfoliation formulations of four prominent approaches: solvent-mediated exfoliation, chemical exfoliation with phase reconversion, redox exfoliation, and native redox exfoliation. The resulting MoS films exhibit distinct refractive indices (), extinction coefficients (), dielectric functions (ε and ε), and absorption coefficients (α). For example, a large index contrast of Δ ≈ 2.3 is observed. These exfoliation procedures and related chemistries produce different exfoliated flake dimensions, chemical impurities, carrier doping, and lattice strain that influence the resulting optical properties. First-principles calculations further confirm the impact of lattice defects and doping characteristics on MoS optical properties. Overall, incomplete phase reconfiguration (from 1T to mixed crystalline 2H and amorphous phases), lattice vacancies, intraflake strain, and Mo oxidation largely contribute to the observed differences in the reported MoS optical properties. These findings highlight the need for controlled technique-induced effects as well as the opportunity for continued development of, and improvement to, liquid phase exfoliation methodologies. Such chemical and processing-induced effects present compelling routes to engineer exfoliated TMDC optical properties toward the development of next-generation high-performance mirrors, narrow bandpass filters, and wavelength-tailored absorbers.

This work demonstrates the fabrication and characterization of single-layer MoS field-effect transistors using biodegradable albumen (chicken eggwhite) as gate dielectric. By introducing albumen as an insulator for MoS transistors high carrier mobilities (up to ~90 cm V s) are observed, which is remarkably superior to that obtained with commonly used SiO dielectric which we attribute to ionic gating due to the formation of an electric double layer in the albumen MoS interface. In addition, the investigated devices are characterized upon illumination, observing responsivities of 4.5 AW (operated in photogating regime) and rise times as low as 52 ms (operated in photoconductivity regime). The presented study reveals the combination of albumen with van der Waals materials for prospective biodegradable and biocompatible optoelectronic device applications. Furthermore, the demonstrated universal fabrication process can be easily adopted to fabricate albumen-based devices with any other van der Waals material.

We present a tunable phononic crystal which can be switched from a mechanically insulating to a mechanically conductive (transmissive) state. Specifically, in our simulations for a phononic lattice under biaxial tension (  =   = 0.01 N m), we find a bandgap for out-of-plane phonons in the range of 48.8-56.4 MHz, which we can close by increasing the degree of tension uniaxiality ( / ) to 1.7. To manipulate the tension distribution, we design a realistic device of finite size, where / is tuned by applying a gate voltage to a phononic crystal made from suspended graphene. We show that the bandgap closing can be probed via acoustic transmission measurements and that the phononic bandgap persists even after the inclusion of surface contaminants and random tension variations present in realistic devices. The proposed system acts as a transistor for MHz-phonons with an on/off ratio of 10 (100 dB suppression) and is thus a valuable extension for phonon logic applications. In addition, the transition from conductive to isolating can be seen as a mechanical analogue to a metal-insulator transition and allows tunable coupling between mechanical entities (e.g. mechanical qubits).

The presence of metal atoms at the edges of graphene nanoribbons (GNRs) opens new possibilities toward tailoring their physical properties. We present here formation and high-resolution characterization of indium (In) chains on the edges of graphene-supported GNRs. The GNRs are formed when adsorbed hydrocarbon contamination crystallizes via laser heating into small ribbon-like patches of a second graphitic layer on a continuous graphene monolayer and onto which In is subsequently physical vapor deposited. Using aberration-corrected scanning transmission electron microscopy (STEM), we find that this leads to the preferential decoration of the edges of the overlying GNRs with multiple In atoms along their graphitic edges. Electron-beam irradiation during STEM induces migration of In atoms along the edges of the GNRs and triggers the formation of longer In atom chains during imaging. Density functional theory (DFT) calculations of GNRs similar to our experimentally observed structures indicate that both bare zigzag (ZZ) GNRs as well as In-terminated ZZ-GNRs have metallic character, whereas in contrast, In termination induces metallicity for otherwise semiconducting armchair (AC) GNRs. Our findings provide insights into the creation and properties of long linear metal atom chains at graphitic edges.

Nanopores in two-dimensional (2D) membranes hold immense potential in single-molecule sensing, osmotic power generation, and information storage. Recent advances in 2D nanopores, especially on single-layer MoS, focus on the scalable growth and manufacturing of nanopore devices. However, there still remains a bottleneck in controlling the nanopore stability in atomically thin membranes. Here, we evaluate the major factors responsible for the instability of the monolayer MoS nanopores. We identify chemical oxidation and delamination of monolayers from their underlying substrates as the major reasons for the instability of MoS nanopores. Surface modification of the substrate and reducing the oxygen from the measurement solution improves nanopore stability and dramatically increases their shelf-life. Understanding nanopore growth and stability can provide insights into controlling the pore size, shape and can enable long-term measurements with a high signal-to-noise ratio and engineering durable nanopore devices.

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 WSe, 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 ( ~ 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 WSe in novel optoelectronic and orbitronics applications.

We demonstrate Andreev pair injection across Nb-WS junction evident as Andreev reflection in differential conductivity spectra below Nb critical temperature . The superconducting- 2D semiconducting junction defined by a focused ion beam, shaped Nb pads, and semi-dry transfer of single layer CVD-grown WS crystals ensured the mechanical integrity of the 2D TMD film, reduced contamination and defects at Nb-WS junction, enabling the pristine study of the interface and facilitating Andreev pair injection. We observed enhanced conductivity in spectra for junction voltages smaller than the corresponding Nb superconducting gap, which vanishes as the device temperature is increased above the . 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.

Two-dimensional materials can be strongly influenced by their surroundings. A dielectric environment screens and reduces the Coulomb interaction between electrons in the two-dimensional material. Since in Mott materials the Coulomb interaction is responsible for the insulating state, manipulating the dielectric screening provides direct control over Mottness. Our many-body calculations reveal the spectroscopic fingerprints of such Coulomb engineering: we demonstrate eV-scale changes to the position of the Hubbard bands and show a Coulomb engineered insulator-to-metal transition. Based on our proof-of-principle calculations, we discuss the (feasible) conditions under which our scenario of Coulomb engineering of Mott materials can be realized experimentally.

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  = 1 (PEA)PbI RP films by pulsed laser deposition (PLD). In situ photoluminescence (PL) during deposition reveals the formation of the  = 1 phase at the early stages of growth. X-ray diffraction (XRD) and grazing-incidence wide-angle scattering (GIWAXS) confirm a single oriented  = 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)PbI films grown on strained epitaxial MAPbI 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)PbI RP films and stable 2D/3D heterostructures.

Thermoelectric materials are of great interest for heat energy harvesting applications. One such promising material is TlGaSe, a 2D-layered, -type semiconducting ternary chalcogenide. Recent reports show it can be processed as a thin film, opening the door for large-scale commercialization. However, TlGaSe is prone to stacking faults along the [001] stacking direction and their role in its thermoelectric properties has not yet been understood. Herein, TlGaSe 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. 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 ~10cm.

We propose a spin-charge qubit based on a bilayer graphene and WSe 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 WSe. Time-dependent investigations of the quantum device reveal rapid qubit gate operation in the order of picoseconds. Our simulations indicate that bilayer graphene and WSe heterostructures provide a promising platform for the processing of quantum information.

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.

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 Kelvin probe force microscopy to image electrostatic potential profiles during the operation of MoS 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.

Sunlight is widely seen as one of the most abundant forms of renewable energy, with photovoltaic cells based on pn junctions being the most commonly used platform attempting to harness it. Unlike in conventional photovoltaic cells, the bulk photovoltaic effect (BPVE) allows for the generation of photocurrent and photovoltage in a single material without the need to engineer a pn junction and create a built-in electric field, thus offering a solution that can potentially exceed the Shockley-Queisser efficiency limit. However, it requires a material with no inversion symmetry and is therefore absent in centrosymmetric materials. Here, we demonstrate that breaking the inversion symmetry by structural disorder can induce BPVE in ultrathin PtSe, a centrosymmetric semiconducting van der Waals material. Homogenous illumination of defective PtSe by linearly and circularly polarized light results in a photoresponse termed as linear photogalvanic effect (LPGE) and circular photogalvanic effect (CPGE), which is mostly absent in the pristine crystal. First-principles calculations reveal that LPGE originates from Se vacancies that act as asymmetric scattering centers for the photo-generated electron-hole pairs. Our work emphasizes the importance of defects to induce photovoltaic functionality in centrosymmetric materials and shows how the range of materials suitable for light sensing and energy-harvesting applications can be extended.

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 × 10S/m) at very low thickness (11 nm). Finally, we connect our high conductivities to low inter-nanosheet junction resistances (R), which we estimate at R ~ 1kΩ.

Current progress in two-dimensional (2D) materials explorations leads to constant specie enrichments of possible advanced materials down to two dimensions. The metal chalcogenide-based 2D materials are promising grounds where many adjacent territories are waiting to be explored. Here, a stable monolayer NiTeO (NTO) structure was computationally predicted and its stacked 2D nanosheets experimentally synthesized. Theoretical design undergoes featuring coordination of metalloid chalcogen, slicing the bulk structure, geometrical optimizations and stability study. The predicted layered NTO structure is realized in nanometer-thick nanosheets via a one-pot shape-controlled hydrothermal synthesis. Compared to the bulk, the 2D NTO own a lowered bandgap energy, more sensitive wavelength selectivity and an emerging photocatalytic hydrogen evolution ability under visible light. Beside a new 2D NTO with the optoelectrical and photocatalytic merits, its existing polar space group, structural specification, and design route are hoped to benefit 2D semiconductor innovations both in species enrichment and future applications.

Quantum emitters in transition metal dichalcogenides (TMDs) have recently emerged as a promising platform for generating single photons for optical quantum information processing. In this work, we present an approach for deterministically controlling the polarization of fabricated quantum emitters in a tungsten diselenide (WSe) monolayer. We employ novel nanopillar geometries with long and sharp tips to induce a controlled directional strain in the monolayer, and we report on fabricated WSe emitters producing single photons with a high degree of polarization (99 ± 4%) and high purity ( (0) = 0.030 ± 0.025). Our work paves the way for the deterministic integration of TMD-based quantum emitters for future photonic quantum technologies.

Optical nanoresonators are key building blocks in various nanotechnological applications (e.g., spectroscopy) due to their ability to effectively confine light at the nanoscale. Recently, nanoresonators based on phonon polaritons (PhPs)-light coupled to lattice vibrations-in polar crystals (e.g., SiC, or h-BN) have attracted much attention due to their strong field confinement, high quality factors, and their potential to enhance the photonic density of states at mid-infrared (mid-IR) frequencies, where numerous molecular vibrations reside. Here, we introduce a new class of mid-IR nanoresonators that not only exhibit the extraordinary properties previously reported, but also incorporate a new degree of freedom: twist tuning, i.e., the possibility of controlling their spectral response by simply rotating the constituent material. To achieve this result, we place a pristine slab of the van der Waals (vdW) α-MoO crystal, which supports in-plane hyperbolic PhPs, on an array of metallic ribbons. This sample design based on electromagnetic engineering, not only allows the definition of α-MoO nanoresonators with low losses (quality factors, Q, up to 200), but also enables a broad spectral tuning of the polaritonic resonances (up to 32 cm, i.e., up to ~6 times their full width at half maximum, FWHM ~5 cm) by a simple in-plane rotation of the same slab (from 0 to 45°). These results open the door to the development of tunable and low-loss IR nanotechnologies, fundamental requirements for their implementation in molecular sensing, emission or photodetection applications.

Although chromium trihalides are widely regarded as a promising class of two-dimensional magnets for next-generation devices, an accurate description of their electronic structure and magnetic interactions has proven challenging to achieve. Here, we quantify electronic excitations and spin interactions in Cr ( = Cl, Br, I) using embedded many-body wavefunction calculations and fully generalized spin Hamiltonians. We find that the three trihalides feature comparable -shell excitations, consisting of a high-spin ground state lying 1.5-1.7 eV below the first excited state ( ). CrCl exhibits a single-ion anisotropy  = - 0.02 meV, while the Cr spin-3/2 moments are ferromagnetically coupled through bilinear and biquadratic exchange interactions of  = - 0.97 meV and  = - 0.05 meV, respectively. The corresponding values for CrBr and CrI increase to  = -0.08 meV and = - 0.12 meV for the single-ion anisotropy,  = -1.21 meV,  = -0.05 meV and  = -1.38 meV,  = -0.06 meV for the exchange couplings, respectively. We find that the overall magnetic anisotropy is defined by the interplay between and due to magnetic dipole-dipole interaction that favors in-plane orientation of magnetic moments in ferromagnetic monolayers and bulk layered magnets. The competition between the two contributions sets CrCl and CrI as the easy-plane ( + >0) and easy-axis ( + <0) ferromagnets, respectively. The differences between the magnets trace back to the atomic radii of the halogen ligands and the magnitude of spin-orbit coupling. Our findings are in excellent agreement with recent experiments, thus providing reference values for the fundamental interactions in chromium trihalides.

Very recently, it has been reported that mixed transition metal oxide (TMO)/MXene catalysts show improved performance over TMO only catalysts for the oxygen evolution reaction (OER). However, the reasoning behind this observation is unknown. In this work mixed Co(OH)/TiCT were prepared and characterized for the OER using ex situ and operando spectroscopy techniques in order to initiate the understanding of why mixed TMO/MXene materials show better performances compared to TMO only catalysts. This work shows that the improved electrocatalysis for the composite material compared to the TMO only catalyst is due to the presence of higher Co oxide oxidation states at lower OER overpotentials for the mixed TMO/MXene catalysts. Furthermore, the presence of the MXene allows for a more mechanically robust film during OER, making the film more stable. Finally, our results show that small amounts of MXene are more advantageous for the OER during long-term stability measurements, which is linked to the formation of TiO. The sensitivity of MXene oxidation ultimately limits TMO/MXene composites under alkaline OER conditions, meaning mass fractions must be carefully considered when designing such a catalyst to minimize the residual TiO formed during its lifetime.

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 B and B isotopes by high-temperature molecular beam epitaxy (HT-MBE). Controlled nucleation, improved surface uniformity, and step-flow growth of an hBN epilayer were achieved by etching the hBN bulk crystals with molecular hydrogen. The alignment of the hBN epilayer and host hBN lattices was confirmed by lattice-resolved atomic force microscopy. Micro-Raman spectroscopy and scattering-type scanning near-field optical microscopy show that the bulk hBN and hBN 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.