Table of contents

With its tunable band-gap and its unique optical and electronic properties black phosphorus (BP) opens exciting opportunities for optoelectronic nanotechnology. The band-gap extends from the visible to the mid-infrared spectral range, as a function of sample thickness and external parameters such as electric field and pressure. This, combined with the saturable absorption and in-plane anisotropic optical properties, makes BP a versatile platform for realizing polarization-sensitive photodetectors and absorbers. Although its near-equilibrium properties have been intensively studied, the development of efficient ultrafast optical devices requires detailed knowledge of the temporal dynamics of the photoexcited hot-carriers. Here we address the electronic response of BP to an ultrafast laser excitation, by means of time-and angle-resolved photoelectron spectroscopy. Following the optical excitation, we directly observe a shift of the valence band (VB) position, indicative of band-gap renormalization (BGR). Our data also show that the hole population in the VB relaxes with a characteristic time ps, while the lifetime of the electrons accumulated at the minimum of the conduction band is ps. The experimental results are well reproduced by ab initio calculations of the out-of-equilibrium electronic properties. Our study sets the reference for the ultrafast carrier dynamics in BP and demonstrates the material's ultrafast BGR, which is promising for optoelectronic switches.

Here we present a size selection model for liquid-exfoliated 2D nanosheets. The ability to consistently select exfoliated nanosheets with desired properties is important for development of applications in all areas. The model presented facilitates determination of centrifugation parameters for production of dispersions with controlled size and thickness for different materials, solvents and exfoliation processes. Importantly, after accounting for the influence of viscosity on exfoliation, comparisons of different solvents are shown to be well described by the surface tension and Hansen parameter matching. This suggests that previous analyses may have overestimated the relative performance of more viscous solvents. This understanding can be extended to develop a model based on the force balance of nanosheets falling under viscous drag during centrifugation. By considering the microscopic aspect ratio relationships, this model can be both calibrated for size selection of nanosheets and compare the exfoliation processes themselves.

We reveal that optical saturation of the low-energy states takes place in graphene for arbitrarily weak electromagnetic fields. This effect originates from the diverging field-induced interband coupling at the Dirac point. Using semiconductor Bloch equations to model the electronic dynamics of graphene, we argue that the charge carriers undergo ultrafast Rabi oscillations leading to the anomalous saturation effect. The theory is complemented by a many-body study of the carrier relaxations dynamics in graphene. It will be demonstrated that the carrier relaxation dynamics is slow around the Dirac point, which in turn leads to a more pronounced saturation. The implications of this effect for the nonlinear optics of graphene are then discussed. Our analysis shows that the conventional perturbative treatment of the nonlinear optics, i.e. expanding the polarization field in a Taylor series of the electric field, is problematic for graphene, in particular at small Fermi levels and large field amplitudes.

Rotational-symmetry-protected topological crystalline insulators (TCIs) are expected to host unique boundary modes, in that the surface normal to the rotational axis can feature surface states with 'unpinned' Dirac points, which are not constrained to lie on high symmetry points or lines, but can lie at any general k point in the Brillouin zone. Also, as a higher order bulk boundary correspondence is involved here, a three-dimensional (3D) TCI can support one-dimensional (1D) helical edge states. Using first-principles band structure calculations, we identify the van der Waals material -Bi₄Br₄ as a purely rotation symmetry protected TCI. We show that the surface of Bi₄Br₄ exhibits a pair of unpinned topological Dirac fermions which are related to the presence of a two-fold rotation axis. These unpinned Dirac fermions possess an exotic spin texture which will be highly favorable for spin transport, and a band structure that consists of van Hove singularities due to a Lifshitz transition. We also identify 1D topological hinge states along the edges of an -Bi₄Br₄ rod. We comment on how the predicted topological features in -Bi₄Br₄ could be accessed experimentally.

Knowing exact nature of broken-symmetry states around individual atomic defects of graphene is very important in understanding the electronic properties. Here, we realize measurement of valley-dependent spin splitting around atomic defects of graphene at nanoscale and single-electron level by using edge-free graphene quantum dots. Our experiments detect large valley-dependent spin splitting around atomic defects of graphene due to the coexistence of sublattice symmetry breaking, enhanced spin–orbit coupling and time reversal symmetry breaking. The spin splitting in the two distinct valleys has opposite direction and almost has the same amplitude. The strategy reported in this work is quite robust and could be extended to measure broken-symmetry states in other 2D systems.

We review recent experimental progresses on layered topological materials, mainly focusing on transitional metal dichalcogenides with various lattice types including 1T, T_d and 1T' structural phases. Their electronic quantum states are interestingly rich, and many appear to be topological nontrivial, such as Dirac/Weyl semimetallic phase in multilayers and quantum spin hall insulator phase in monolayers. The content covers recent major advances from material synthesis, basic characterizations, angle-resolved photoemission spectroscopy measurements, transport and optical responses. Following those, we outlook the exciting future possibilities enabled by the marriage of topological physics and two dimensional van der Waals layered heterostructures.

Two-dimensional (2D) nanomaterials (sheet-like materials with few-atoms thickness and lateral size above 100 nm) have always aroused scientists' interests since 2004 when Novoselov et al successfully exfoliated graphene from graphite using Scotch tape. It is some unique characters of 2D nanomaterials such as the confinement of electrons in two dimensions in the ultrathin region, strong in-plane covalent bond and atomic thickness, ultra-high specific surface area and exposed atoms that enable 2D nanomaterials to show excellent properties in electrics, catalysis and mechanics. Recently, amorphous materials (varied from crystal materials by atomic arrangement) have demonstrated high performance in mechanics, catalysis and magnetic owing to their unique long-range atomic disorder arrangements. Thus, the 2D amorphous nanomaterials inspire a new path to the study of high performance 2D materials. Herein, we summarize the recent progress in 2D amorphous nanomaterials, whose synthetic methods and potential applications in fields of catalysis, energy storage and mechanics discussed in details. The vital blocking mechanisms for synthesis of 2D amorphous nanomaterials and their performance-structure relationship are focused on. Finally, we conclude the review with our personal insights and provide a critical outlook for the development of 2D amorphous nanomaterials.

This paper reviews the recent progress on electronic and optoelectronic devices based on 2D black phosphorus (BP). First, the crystal structure, band structure, and optical properties of BP, as well as some currently-known passivation methods used for making BP stable in ambient conditions are briefly summarized. Device architectures and operating principles of the state-of-the-art few-layer BP based electronic and optoelectronic devices will then be discussed in detail, with a focus on field-effect transistors, heterojunction diodes, and photodetectors. Next, solution-based exfoliation methods aimed for addressing the scalability challenge faced by BP are briefly discussed, followed by their potential applications in gas sensors and biomedicine. By reviewing recent process and discussing remaining challenges faced by BP, this paper aims to provide perspectives on opportunities and future research directions for utilizing BP and moving towards practical applications.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have very versatile chemical, electrical and optical properties. In particular, they exhibit rich and highly tunable electronic properties, with a bandgap that spans from semi-metallic up to 2 eV depending on the crystal phase, material composition, number of layers and even external stimulus. This paper provides an overview of the electronic devices and circuits based on 2D TMDs, such as Esaki diodes, resonant tunneling diodes (RTDs), logic and RF transistors, tunneling field-effect transistors (TFETs), static random access memories (SRAMs), dynamic RAM (DRAMs), flash memory, ferroelectric memories, resistitive memories and phase-change memories. We address the basic device principles, the advantages and limitations of these 2D electronic devices, and our perspectives on future developments.

Understanding the physics underlying energy dissipation is necessary for the effective thermal management of devices based on two-dimensional (2D) materials and requires insights into the interplay between heat generation and diffusion in such materials. We review the microscopic mechanisms that govern Joule heating and energy dissipation processes in 2D materials such as graphene, black phosphorus and semiconducting transition metal dichalcogenides. We discuss the processes through which non-equilibrium charge carriers, created either transiently through photoexcitation or at steady state by a large electric field, undergo energy relaxation with the lattice and the substrate. We also discuss how these energy dissipation processes are affected by the device configuration (heterostructure, substrate material including hexagonal boron nitride, etc) as the use of different substrates, encapsulation, disorder, etc can introduce or remove scattering processes that change the energy relaxation pathways. Finally, we discuss how the unique carrier scattering dynamics in graphene-based vdW heterostructures can be exploited for optoelectronic applications in light emission and photodetection.

Molecules with helical structure have fascinating optical and electronic properties due to their inherently chiral non-planar structure. To template flexible polyethylene (PE) chain into helical structure rather than randomly insertion into carbon nanotube (CNT), graphene nanoribbon (GNR) has been induced to the CNT/PE system achieving confined hetero-double-helical nanostructures (HDHNSs) implemented by molecular dynamics (MD) simulations. The strong interaction between the CNT and GNR activate and guide the helical encapsulation of the PE chain by the van der Waals (vdWs) and π–π stacking interaction among the CNT, GNR and PE chain. Meanwhile, the GNR size (length, width), PE chain length and CNT size (length, diameter) significantly influences the self-assembly process. The chirality has a little effect on the final structure. In addition, multiple GNRs and PE chains confined inside CNTs and the spiral wrapping of the PE chain and the GNR onto the CNT are also researched. This study provides novel strategies for designing and fabricating confined HDHNSs inside CNT and eventually on their applications in extensive fields involving medicine, chemistry, biology, and even power source.

Nonvolatile charge trap memory is an important part of the continuous development of information technology. As a 2-dimensional (2D) material with fantastic physical characteristics, molybdenum disulfide (MoS₂) has been receiving extensive attention for its potential applications in electronic devices. However, while various attempts have been made to devise its charge-trap gate stack, it's still impossible to avoid a certain performance degradation. Here, a MoS₂-based nonvolatile charge trapping memory device with a charge-trap gate stack formed by implanting N ions into SiO₂ is reported. The fabricated N-implanted memory devices with the energy of 6.5 keV and the dose of 1  ×  10¹⁵ ions cm⁻² exhibit a high on/off current ratio up to 10⁷, a large memory window of 9.1 V, and a high program/erase speed of 10/100 µs. Moreover, the memory device shows an excellent cycling endurance of more than 10⁴ cycles. By combining the MoS₂ channel with the N-implanted charge-trap gate stack, this research opens up a fascinating field of nonvolatile charge trap memory devices.

We demonstrate a high-yield fabrication of non-local spin valve devices with room-temperature spin lifetimes of up to 3 ns and spin relaxation lengths as long as 9 µm in platinum-based chemical vapor deposition (Pt-CVD) synthesized single-layer graphene on SiO₂/Si substrates. The spin-lifetime systematically presents a marked minimum at the charge neutrality point, as typically observed in pristine exfoliated graphene. However, by studying the carrier density dependence beyond n ~ 5  ×  10¹² cm⁻², via electrostatic gating, it is found that the spin lifetime reaches a maximum and then starts decreasing, a behavior that is reminiscent of that predicted when the spin-relaxation is driven by spin–orbit interaction. The spin lifetimes and relaxation lengths compare well with state-of-the-art results using exfoliated graphene on SiO₂/Si, being a factor two-to-three larger than the best values reported at room temperature using the same substrate. As a result, the spin signal can be readily measured across 30 µm long graphene channels. These observations indicate that Pt-CVD graphene is a promising material for large-scale spin-based logic-in-memory applications.

Van der Waals heterojunctions based on atomically thin 2D materials have opened up new realms in modern semiconductor industry. However, it is still challenging to fabricate large-area ultrathin 2D films. Herein, we successfully fabricate wafer-size 2D SnSe films on Si substrate by magnetron sputtering technique, enabling the formation of SnSe/Si van der Waals (vdWs) heterojunction device. The high-resolution transmission electron microscopy is employed to character the structure of SnSe film and SnSe/Si heterojunction with ideal orthorhombic structure and atomically abrupt interface, respectively. The energy diagram of SnSe/Si heterojunction is constructed, exhibiting similar barrier heights for electron and hole carrier. The SnSe/Si heterojunction shows obvious diode behavior with rectification ratio of ~1.6  ×  10⁴, forward current of ~194.5 mA cm⁻² at  ±1.0 V. Furthermore, owing to the high crystalline orientation, specific energy-band alignment, as well as the strong built-in electrical field, the SnSe/Si heterojunction illustrates a broadband photodetecting properties with the wavelength ranging from ultraviolet to near-infrared light, showing a high detectivity of 4.4  ×  10¹² cmHz^1/2 W⁻¹, a high responsivity of 566.4 mA mW⁻¹ and an ultrafast response/recovery time of ~1.6/47.7 µs under zero external bias. This work provides a new strategy for fabrication of low cost 2D optoelectronic devices with high-performance.

Tailoring the properties of two-dimensional (2D) crystals is important for both understanding the material behavior and exploring new functionality. Here we demonstrate the alteration of MoS₂ and metal-MoS₂ interfaces using a convergent ion beam. Different beam energies, from 60 eV to 600 eV, are shown to have distinct effects on the optical and electrical properties of MoS₂. Defects and deformations created across different layers were investigated, revealing an unanticipated improvement in the Raman peak intensity of multilayer MoS₂ when exposed to a 60 eV Ar⁺ ion beam, and attenuation of the MoS₂ Raman peaks with a 200 eV ion beam. Using cross-sectional scanning transmission electron microscopy (STEM), alteration of the crystal structure after a 600 eV ion beam bombardment was observed, including generated defects and voids in the crystal. We show that the 60 eV ion beam yields improvement in the metal-MoS₂ interface by decreasing the contact resistance from 17.5 kΩ · µm to 6 kΩ · µm at a carrier concentration of n_2D  =  5.4  ×  10¹² cm⁻². These results advance the use of low-energy ion beams to modify 2D materials and interfaces for tuning and improving performance in applications of sensors, transistors, optoelectronics, and so forth.

The research in the area of graphene derivatives containing chalcogen elements is currently an exclusive domain of oxygen- and sulfur-containing functional groups. Here we report an effective introduction of selenium-based organic functional groups and propose a mechanism of their attachment to graphene and graphene-related materials. Fluorographene can be effectively functionalized by selenium-containing group, forming C-Se bonds without undesired formation of elemental selenium, using selenourea. A similar reaction on graphene oxide is accompanied by the formation of elemental selenium due to oxidation of selenourea by oxygen-containing functional groups of graphene oxide. The formation of C-Se bonds was confirmed by several spectroscopic methods. The selenium covalently-modified graphene represents the first example of graphene-selenium compound with potential application in gas sensing.

Silicene, the silicon analogue of graphene, consists of an atomically buckled honeycomb lattice of silicon atoms. Theory predicts exceptional electronic properties, including Dirac fermions and a topological spin Hall insulator phase. An important obstacle impeding exploration of such properties in electronic devices is the chemical sensitivity of silicene, hampering its incorporation in layer stacks. Here we show experimentally that epitaxial silicene and hexagonal boron nitride (h-BN) can be stacked without perturbing the electronic properties of silicene. Intercalated silicene underneath epitaxial h-BN on ZrB₂(0 0 0 1) substrate films is obtained by depositing Si atoms at room temperature. Using (angle resolved) photoelectron spectroscopy (ARPES, PES) and scanning tunneling microscopy (STM) we find that the intercalated silicene exhibits the same electronic properties as epitaxial silicene on ZrB₂, while it resists oxidation in air up to several hours. This is an essential step towards the development of layer stacks that allow for fabrication of devices.

We carry out a systematic study of the thermal conductivity of four single-layer transition metal dichalcogenides, MX₂ (M  =  Mo, W; X  =  S, Se) from first-principles by solving the Boltzmann transport equation (BTE). We compare three different theoretical frameworks to solve the BTE beyond the relaxation time approximation (RTA), using the same set of interatomic force constants computed within density functional theory (DFT), finding that the RTA severely underpredicts the thermal conductivity of MS₂ materials. Calculations of the different phonon scattering relaxation times of the main collision mechanisms and their corresponding mean free paths (MFP) allow evaluating the expected hydrodynamic behaviour in the heat transport of such monolayers. These calculations indicate that despite of their low thermal conductivity, the present TMDs can exhibit large hydrodynamic effects, being comparable to those of graphene, especially for WSe₂ at high temperatures.

Monolayers and bilayers of transition metal dichalcogenides (TMDCs) are currently being intensively scrutinized not least due to their rich opto-electronic properties which are governed by strongly bound excitons. Until now the main focus has been on excitons with zero momentum. In this study we employ ab initio many-body perturbation theory within the GW/BSE approximation to describe the entire Q-resolved exciton band structure for mono- and bilayers of the MX₂ (M  =  Mo, W and X  =  Se, S) TMDCs. We find that the strong excitonic effects, i.e. strong electron–hole interactions, are present throughout the entire Q-space. While the exciton binding energies of the lowest excitons do not vary significantly with Q, we find a strong variation in their coupling strength. In particular, the latter are strongly peaked for excitons at Q  =  0 and . For MoX₂ monolayers the K excitons constitutes the exciton ground state, while in WX₂ monolayers direct transitions at K are lowest in energy. Our calculations further show that the exciton landscape is highly sensitive to strain and interlayer hybridization. For all four bilayers the exciton ground state is shifted to or K transitions closely following the trends of the single-particle band structures.

By comparing the growth of Cu thin films on bare and graphene-covered Ru(0 0 0 1) surfaces, we demonstrate the role of graphene as a surfactant allowing the formation of flat Cu films. Low-energy electron microscopy, x-ray photoemission electron microscopy and x-ray absorption spectroscopy reveal that depositing Cu at 580 K leads to distinct behaviors on both types of surfaces. On bare Ru, a Stranski–Krastanov growth is observed, with first the formation of an atomically flat and monolayer-thick wetting layer, followed by the nucleation of three-dimensional islands. In sharp contrast, when Cu is deposited on a graphene-covered Ru surface under the very same conditions, Cu intercalates below graphene and grows in a step-flow manner: atomically-high growth fronts of intercalated Cu form at the graphene edges, and extend towards the center of the flakes. Our findings suggest potential routes in metal heteroepitaxy for the control of thin film morphology.

Ultrathin and light heterojunction bipolar transistors based on two-dimensional (2D) layered materials with flexible semiconducting properties have been considered for several electronic applications. In this paper, a van der Waals p-BP/n-MoS₂/p-BP BJT is demonstrated. It is fabricated using mechanical exfoliation, where a dry transfer technique is used to stack a vertical double heterojunction. The device structure includes nanoflakes of black phosphorus (BP) and MoS₂. The current–voltage characteristics of the common–emitter and common–base configurations are examined. These p-BP/n-MoS₂/p-BP bipolar transistors exhibit current–voltage characteristics similar to those of conventional p-n-p bipolar transistors. Devices with thin MoS₂ layers show good saturation current–voltage characteristics, and a maximum common–emitter current gain (β  =  I_C/I_B) of approximately 10.1 is obtained at room temperature (300 K). Furthermore, the thickness dependence of the base region (n-MoS₂) is investigated for the common–emitter output electrical characteristics (V_CE  −  I_C) of a double heterojunction bipolar transistor in which the emitter is grounded. The collector current decreases as the thickness of n-MoS₂ is increased. This study can pave the way for the application of 2D materials as controllable amplifiers in flexible electronics.

Defect centers in hexagonal boron nitride represent room-temperature single-photon sources in a layered van der Waals material. These light emitters appear with a wide range of transition energies ranging over the entire visible spectrum, which renders the identification of the underlying atomic structure challenging. In addition to their eminent properties as quantum light emitters, the coupling to phonons is remarkable. Their photoluminescence exhibits significant side band emission well separated from the zero phonon line (ZPL) and an asymmetric broadening of the ZPL itself. In this combined theoretical and experimental study we show that the phonon side bands can be well described in terms of the coupling to bulk longitudinal optical (LO) phonons. To describe the ZPL asymmetry we show that in addition to the coupling to longitudinal acoustic (LA) phonons also the coupling to local mode oscillations of the defect center with respect to the entire host crystal has to be considered. By studying the influence of the emitter's wave function dimensions on the phonon side bands we find reasonable values for the size of the wave function and the deformation potentials. We perform photoluminescence excitation measurements to demonstrate that the excitation of the emitters is most efficient by LO-phonon assisted absorption.

The use of graphene‐based electrodes is burgeoning in a wide range of applications, including solar cells, light emitting diodes, touch screens, field‐effect transistors, photodetectors, sensors and energy storage systems. The success of such electrodes strongly depends on the implementation of effective production and processing methods for graphene. In this work, we take advantage of two different graphene production methods to design an advanced, conductive oxide- and platinum-free, graphene-based counter electrode for dye-sensitized solar cells (DSSCs). In particular, we exploit the combination of a graphene film, produced by chemical vapor deposition (CVD) (CVD-graphene), with few-layer graphene (FLG) flakes, produced by liquid phase exfoliation. The CVD-graphene is used as charge collector, while the FLG flakes, deposited atop by spray coating, act as catalyst for the reduction of the electrolyte redox couple (i.e. - and Co^+2/+3). The as-produced counter electrodes are tested in both - and Co^+2/+3-based semitransparent DSSCs, showing power conversion efficiencies of 2.1% and 5.09%, respectively, under 1 SUN illumination. At 0.1 SUN, Co^+2/+3-based DSSCs achieve a power conversion efficiency as high as 6.87%. Our results demonstrate that the electrical, optical, chemical and catalytic properties of graphene-based dual films, designed by combining CVD-graphene and FLG flakes, are effective alternatives to FTO/Pt counter electrodes for DSSCs for both outdoor and indoor applications.

Compared with conventional bulk 3D metal-organic framework (MOF), the MOF nanosheets possess large surface area, ultrathin thickness, and well solution dispersion properties, which is suitable to be used as novel templates to synthesize the functional composites. However, up to now, it remains difficult to grow noble metal nanoplates on 2D MOF surface due to the complexity of synthetic process. Here, for the first time, we used MOF nanosheets (Cu-TCPP or Cu-TCPP(Fe)) as an template for the solution-phase growth of silver (Ag), gold (Au), and palladium (Pd) nanoplates at ambient conditions. The noble metal plates on the surface of 2D MOF are uniform and possess ultrathin structure, which are the ideal materials for catalysis due to accessible active sites, fast electron transfer, high stability, and good solution-dispersion. Notably, the synthesized Pd/Cu-TCPP(Fe) hybrid nanomaterials shows enzyme-mimic catalytic activity, which exhibits enhanced catalytic performance compared to Pd nanoplates, MOF nanosheets, and their mixture. Owing to their high catalytic activity, a simple, fast and highly sensitive fluorescence method was developed to detect explosive residues such as organic peroxides in trace amounts, which existed widely in the environment.

We measure the absolute absorbance of a single layer of seven atom wide armchair graphene nanoribbons and study the influence of laser-induced defects on the absorption spectrum of the ribbons. We find that the absorption spectrum shows a broad peak at approximately 2.4 eV that is attributed to excitonic transitions and a smaller peak at 1.77 eV. The low-energy peak is diminished when we induce defects in the material. Simultaneously the photoluminescence is significantly enhanced. We thus attribute the 1.77 eV spectral feature in the absorption spectrum to a quenching state, which energetically coincides with the emission. Our results clearly demonstrate the significance of this state in photoluminescence processes in the ribbons. We additionally measure the dependence of the generation of defects on the energy of the incident photons and the photoluminescence excitation spectrum. The photoluminescence excitation efficiency peaks at a higher photon energy than the maximum absorption, hinting at an efficient decay from higher energetic states to the emissive state.

The ionic component of the strong bond in hexagonal boron nitride (hBN) has been grossly disregarded in literature. Precisely this quantity is demonstrated to govern the shape of monolayer hBN islands grown at high temperatures. HBN zigzag edges are charged and energetically less favorable than the neutral armchair edges, in contrast to those of the purely covalent graphene. Nucleation of hBN islands occurs exclusively on either the inner or the outer corners of substrate steps. Taking into account the charge at edges of hBN islands offers a powerful framework to understand the nucleation of the islands and their orientation with respect the founding steps, as well as various equilibrium shapes, including prominently a right-angled trapezoid. BN dimers are identified as basic building blocks for hBN. A surprisingly strong interaction between hBN and the pre-existing steps on the moderately reactive Ir(1 1 1) substrate is uncovered. Localized charges are probably relevant for all 2D-materials lacking inversion symmetry.

PtSe₂, an emerging 2D group-10 transition metal dichalcogenide (TMD), has aroused significant attention recently due to its intriguing physical properties. Here, the optical properties of chemical vapor deposition-grown PtSe₂ films with different thicknesses were characterized with nondestructive spectroscopic ellipsometry and Fourier transform infrared spectroscopy. The polarized optical microscopy reveals the isotropic in-plane optical response of the continuous PtSe₂ films in a scale size of at least as small as 143  ×  108 µm². The electrical transport characterization and UV-mid infrared absorption spectra reveal the coexistence of both semiconducting and metallic contents in these PtSe₂ films, making PtSe₂ quite different among the 2D material family. The effective refractive index (n) and the extinction coefficient (k) over a spectra range of 360–1700 nm were obtained. In contrast to other TMDs, the values of n and k of PtSe₂ were found to have a strong dependence on the thickness and they decrease as the reduction of thickness. This work is conducive to provide vital parameters for further study on PtSe₂ and could facilitate its application in optoelectronic devices.

A method to grow multi layers graphene (MLG) just by thermal annealing in an inert atmosphere is reported. A molybdenum (Mo) catalyst layer is used in combination with a solid amorphous carbon (a-C) source on top or below the Mo layer.

The formation of MLG directly on top of the catalyst substrate surface is confirmed by Raman spectroscopy, atomic force microscopy, cross-section transmission electron microscopy, electron energy loss spectroscopy and x-ray photoelectron spectroscopy. Growth of MLG on top of the Mo catalyst is demonstrated both with a-C below and above the Mo layer. The growth mechanism is attributed to the diffusion of a-C through the Mo layer and precipitation into the graphene at the surface, similar to the growth by chemical vapour deposition (CVD) on a Ni catalyst. The role of the inert Ar/H₂ atmosphere, carbon thickness, catalyst thickness, anneal time and anneal temperature are reported. Fast growth of MLG (5 min) at 915 °C is demonstrated. The quality of MLG prepared by thermal annealing is at least as good as that of MLG synthesized by CVD. The relevant achievements presented in this study make the proposed technique a promising alternative to CVD based MLG.

We describe a detailed experimental investigation of the ultrafast nonlinear response of a voltage-controlled graphene-gold saturable absorber (VCG-gold-SA) by employing femtosecond pump probe spectroscopy. Visible and near-infrared continuum probe pulses covering the spectral range from 500 nm to 1600 nm were used. In the experiments, the saturation fluence, modulation depth, ultrafast relaxation times, and the saturable absorption bandwidth of the VCG-gold-SA were measured as a function of the applied bias. We observed both saturable absorption and multi-photon absorption regimes as the applied bias voltage was varied between 0 and 2 V. Measurements indicate that under bias voltages in the range of 0–2 V, it should be possible to adjust the insertion loss of the VCG-gold-SA and at the same time, maintain a sufficient amount of modulation depth as well as an attainable level of saturation fluence over an ultrabroad spectral bandwidth. In particular, at the bias voltage of 1 V, the VCG-gold-SA exhibited fast saturable absorber behavior with adjustable insertion loss from 630 nm to 1100 nm. These results clearly demonstrate that the VCG-gold-SA can operate as a versatile mode locker for femtosecond pulse generation from lasers operating in the visible and near-infrared wavelengths.

It is very challenging to effectively exfoliate and functionalize hexagonal boron nitride (h-BN). Here, an efficient exfoliation and functionalization of bulk h-BN was carried out by a ball-milling method using boric acid (BA) as a lubricant and modifier. A series of boric-acid-functionalized boron nitride nanosheets (BNNSs) was successfully produced using this approach. The obtained BNNS thermo-stable suspension can be easily condensed into a jelly-like dispersion with ultra-high concentration, up to 90 mg ml⁻¹. The eco-friendly BA was readily and easily recyclable and remarkably reusable during the BNNS exfoliation. Interestingly, by means of a differential-centrifugation technique, the BNNSs could be easily separated and screened with different sizes and thicknesses. These screened BNNS samples also exhibited different levels of functionalization. As a result, filtration membranes made of various well-screened BNNSs exhibited an obviously different rejection rate for pollutant in water. In addition, the different screened BNNS products show a variable ability to dielectric behavior due to their different-level functionalization. We believe that our created boric-acid-functionalized BNNSs, combined with the smartly screened separation by differential centrifugation, can broaden the future practical applications of BNNS materials.

We report a direct and fast synthesis route to grow boron–carbon–nitrogen layers based on microwave-assisted plasma enhanced chemical vapour deposition (PECVD) by using methylamine borane as a single source molecular precursor. This easy and inexpensive method allows controlled and reproducible growth of B–C–N layers onto thin Cu foils. Their morphological, structural, chemical, optical and transport properties have been thoroughly characterized by a number of different microscopies, transport and spectroscopic techniques. Though disorder and segregation into C-rich and h-BN-rich domains have been observed in ultrathin flat few layers, high doping levels have been reached, inducing strong modifications of the electronic, optical and transport properties of C-rich and h-BN-rich phases. This synthesis procedure can open new routes towards the achievement of homogeneous highly mixed ternary B–C–N phases.

The interaction of organic C₆₀ molecules with germanene grown on Al(1 1 1) is investigated by experimental tools and calculations. By mean of scanning tunneling microscopy, we show that a single but disordered C₆₀ layer is formed upon deposition in the monolayer range. Density functional theory calculations indicate that van der Waals and electrostatic interactions are present between the C₆₀ layer and the germanene, without formation of covalent bond. Moreover, the C₆₀ molecules are adsorbed without major crystallographic and charge modifications at the interface between germanene and the Al(1 1 1) substrate. The predicted charge transfer from the Al(1 1 1) substrate to the C₆₀ molecules is very small, which means that the germanene layer acts as an electronic buffer between the highly electron attractor C₆₀ molecules and the large electron reservoir Al(1 1 1). This leaves the C₆₀ layer quite uncharged, and so preserves its electron attractor properties for further layers grown on it, in the design of new molecular sensors.

2D semiconductors hosting strain-induced quantum emitters offer unique abilities to achieve scalable architectures for deterministic coupling to nanocavities and waveguides that are required to enable chip-based quantum information processing technologies. A severe drawback remains that exciton emission from quantum emitters in WSe₂ quenches beyond 30 K, which requires cryogenic cooling. Here we demonstrate an approach to increase the temperature survival of exciton quantum emitters in WSe₂ that is based on maximizing the emitter quantum yield. Utilizing optimized material growth that leads to reduced density of nonradiative defects as well as coupling of the exciton emission to plasmonic nanocavities modes, we achieve average quantum yields up to 44%, thermal activation energies up to 92 meV, and single photon emission signatures up to temperatures of 160 K. At these values non-cryogenic cooling with thermo-electric chips becomes already feasible, while quantitative analysis shows that room temperature operation is within reach through active strain engineering.

A novel concept of membrane process in a thermal-driven system is proposed for water desalination. By means of molecular dynamics simulations, we show fast water transport through graphene galleries at a temperature gradient. Water molecules are driven to migrate through nanometer-wide graphene channels from cold reservoir to hot reservoir by the effect of thermal creep flow. Reducing the interlayer spacing to 6.5 Å, an abrupt escalation occurs in water permeation between angstrom-distance graphene slabs. A change from disordered bulklike water to quasi-square structure has been found under this extremely confined condition. This leads to a transition to subcontinuum transport. Water molecules perform collective diffusion behaviors inside graphene channels. The special transport processes with structure change convert thermal energy into motion without dissipation, resulting in unexpectedly high water permeability. The thermal-driven system reaches maximum flowrate at temperature variance of 80 K, corresponding to the quantity at pressure difference up to 10⁵ bar in commercial reverse osmosis processes and 230 bar in pressure-driven slip flow. Our results also reveal the movement of saline ions influenced by thermophoretic effect, which complements the geometry limitation at greater layer spacing, enhancing the blockage of ions. This finding aims to provide an innovational idea of developing a high-efficiency desalination technology able to utilize various forms of energy.

Large-area growth of continuous transition metal dichalcogenides (TMDCs) layers is a prerequisite to transfer their exceptional electronic and optical properties into practical devices. It still represents an open issue nowadays. Electric and magnetic doping of TMDC layers to develop basic devices such as p-n junctions or diluted magnetic semiconductors for spintronic applications are also an important field of investigation. Here, we have developed two different techniques to grow MoSe₂ mono- and multi-layers on SiO₂/Si substrates over large areas. First, we co-deposited Mo and Se atoms on SiO₂/Si by molecular beam epitaxy in the van der Waals regime to obtain continuous MoSe₂ monolayers over 1 cm². To grow MoSe₂ multilayers, we then used the van der Waals solid phase epitaxy which consists in depositing an amorphous Se/Mo bilayer on top of a co-deposited MoSe₂ monolayer which serves as a van der Waals growth template. By annealing, we obtained continuous MoSe₂ multilayers over 1 cm². Moreover, by inserting a thin layer of Mn in the stack, we could demonstrate the incorporation of up to 10% of Mn in MoSe₂ bilayers.

Heteroatomic doping (such as N, B, S, P) is one of the most effective strategies to improve the electrochemical performance of carbon-based materials. Herein, 2D phosphorus doped graphdiyne (P-GDY) is prepared via a facile calcination method with phosphoric acid as phosphorus source, and the structure-function relationship of P doping and the electrochemical performance of P-GDY are also investigated using a method combining experiment and density functional theory (DFT) calculations. X-ray photoelectron spectroscopy (XPS) and fourier transform infrared spectroscopy (FTIR) verify that phosphorus (P) is doped in GDY framework via the forms of P–O, P  =  O and P–C bonds. Raman spectra and Brunauer–Emmett–Teller (BET) results reveal that the P doping creates numerous heteroatomic defects and active sites, causes more hierarchical micro-mesoporous, which provide more storage sites of Li and transmission paths for corresponding ions. Besides, DFT results imply that the most stable geometries can be obtained when the P-containing groups are doped at the benzene in GDY structure, and the doping P  =  O bonds are beneficial to Li storage. As a result, for P-GDY, enhanced electrochemical performances for lithium-ion batteries are obtained compared with pristine GDY, including higher reversible capacity, improved rate performance, and superior cycling stability.

Due to their outstanding electronic and physical properties, two-dimensional (2D) materials have attracted much interest for the fabrication of solid-state microelectronic devices. Among all methods to synthesize 2D materials, chemical vapor deposition (CVD) is the most attractive in the field of solid-state microelectronics because it can produce high quality 2D material in a scalable manner. However, the high temperatures (>900 °C) required during the CVD growth of the 2D materials impede their direct synthesis on metal-coated wafers due to prohibitive metal diffusion and de-wetting. This makes necessary carrying out the 2D materials CVD growth independently on metallic foils, and transfer them on the wafers using polymer scaffolds. However, this process is slower, more expensive, and can lead to abundant contamination and cracks in the 2D material. Here we present a facile method to allow the direct growth of multilayer hexagonal boron nitride (h-BN) on Ni-coated Si wafers, which consists on placing a protective cover 30 μm above the Ni surface. The resulting h-BN stacks have been used to fabricate Au/Ti/h-BN/Ni memristors with low cycle-to-cycle variability. This work contributes to the integration of 2D materials in solid-state micro- and nano-electronic technologies.

We present a method to study the interlayer interaction of 2D heterostructures by analyzing the rotational statistics of the as-grown twist angles, as well as in situ manipulation of their relative twist angles using an electron beam. We investigated this in a family of 2D heterostructures: 1–2 layers of Bi₂Se₃ grown on different monolayer transition metal dichalcogenides (TMDs), i.e. MoS₂, MoSe₂, WS₂, and an alloy MoSe_2(1−x)S_2x, which enabled us to compare the relative coupling strengths at junctions with not only similar and dissimilar 'nearest-layer' chalcogens, but also with 'next-nearest-layer' transition metals, as well as 'nearest-mixed-layer' chalcogens. We found that while higher e-beam current densities tend to 'disrupt', and lower values 'recrystallize' the crystal structure, TMD-specific intermediate-value-ranges can dynamically twist the layers with respect to each other. From their initial as-grown twist angle, as well as the ease with which they can be perturbed to twist in response to various electron beam current densities, we infer that Bi₂Se₃/MoSe₂ layers have the strongest interlayer strength, followed by Bi₂Se₃/MoS₂, Bi₂Se₃/WS₂, and Bi₂Se₃/MoSe_2(1−x)S_2x. Finally, the recipe can be tuned to induce the Bi₂Se₃ to form nanoparticles that emit a broad photoluminescence and alter the 2D heterostructure's perceived color. Our results reveal that interlayer interactions play a substantial role even in heterostructures of chemically and crystallographically dissimilar 2D materials, where they are traditionally expected to be 'weak'.

We report the isolation of thin flakes of cylindrite, a naturally occurring van der Waals superlattice, by means of mechanical and liquid phase exfoliation. We find that this material is a heavily doped p-type semiconductor with a narrow gap (<0.85 eV) with intrinsic magnetic interactions that are preserved even in the exfoliated nanosheets. Due to its environmental stability and high electrical conductivity, cylindrite can be an interesting alternative to the existing 2D magnetic materials.

Hydrodynamic behavior in electronic systems is commonly accepted to be associated with extremely clean samples such that electron–electron collisions dominate and total momentum is conserved. Contrary to this, we show that in monolayer graphene the presence of disorder is essential to enable an unconventional hydrodynamic regime which exists near the charge neutrality point and is characterized by a large enhancement of the Wiedemann–Franz ratio. Although the enhancement becomes more pronounced with decreasing disorder, the very possibility of observing the effect depends crucially on the presence of disorder. We calculate the maximum extrinsic carrier density n_c below which the effect becomes manifest, and show that n_c vanishes in the limit of zero disorder. For n  >  n_c we predict that the Wiedemann–Franz ratio actually decreases with decreasing disorder. We complete our analysis by presenting a transparent picture of the physical processes that are responsible for the crossover from conventional to disorder-enabled hydrodynamics. Recent experiments on monolayer graphene are discussed and shown to be consistent with this picture.

Two-dimensional (2D) semiconductors have attracted increasing attention due to their potential application in next generation optoelectronic devices. The chemical vapor deposition (CVD) grown monolayer MoS₂ is widely investigated as photodetector, but low photoresponsivity and slow response time hinder its practical application. In this work, 0D carbon quantum dots (CQDs) are applied to enhance photodetection performance of 2D MoS₂ as efficient and environmental photosensitizer for the first time. The photoresponsivity and detectivity of hybrid photodetector are 377 A W⁻¹ and 1.6  ×  10¹³ Jones under 360 nm illumination, respectively, which are 22 and 7 times higher than pristine MoS₂. The enhanced photoresponse of hybrid photodetector arises from synergetic absorption effect and interlayer excitonic transition of CQDs and MoS₂ layer. The response speed of hybrid photodetector is four times faster than pristine MoS₂ photodetector. Those results pave a way to hybrid 2D–0D MoS₂–CQDs application in high performance photodetector.

The recent success to synthesize an ordered array of pores in graphene by a bottom-up approach (Moreno et al 2018 Science360 199) yields a semiconducting nanoporous graphene with a bandgap of 0.6 eV. In this paper, we present calculations of the intrinsic carrier mobility in this new type of two-dimensional material. Using a fully atomistic approach, we show that carriers are mostly scattered by acoustic phonons, approximately like in semiconducting carbon nanotubes. The carrier mobility shows strong anisotropy and is as high as 800 cm² (Vs)⁻¹ at low carrier density. Such a high mobility, together with symmetric properties of electrons and holes, suggests that porous graphene is a promising candidate for next generations of complementary field-effect transistor technology.

The lithium ion migration dynamics and the fast lithiation pathways in electrode materials are quite critical for the rate capability of lithium ion batteries (LIBs). To reveal the relationship between lithiation behavior and crystallographic orientations in layered MoSe₂, its dynamic electrochemical reactions are investigated with in situ microscopy technique. We report the crystallographic-orientation dependent lithium ion migration and reaction in MoSe₂, where the preferential lithium-ion migration is observed along the orientations. These oriented-lithiation tips further assisted the surface to interior lithiation of layered MoSe₂. Meanwhile, a phase transition from pristine 2H-MoSe₂ to 1T-Li_xMoSe₂ and the following conversion reaction with the formation of Li₂Se and Mo are observed during the first lithiation process. The lithiation induced stress and orientation-dependent interfacial reaction rate might be the main factor leading to the orientation-dependent lithiation behavior. The experimentally revealed crystallographic-orientation dependent lithium ion migration and reaction in layered MoSe₂ will provide intuitive understanding of the structural evolution during the discharge/charge cycling process, give guidance for the design of anode materials and can broaden the promising application of MoSe₂ in solid electrochemical devices.

We demonstrate that microwave absorption experiments offer a route for efficient measurements of transport properties for fast and accurate quality control of graphene. This conctactless characterization method can be used to quickly evaluate transport properties over large areas without recourse to complex lithographic methods making it suitable as a probe of quality during wafer scale fabrication. In particular, we demonstrate that absorption measurement of transport properties is sensitive to inhomogeneities in sample transport properties. This is in contrast to traditional methods using electrical contacts which tend to overestimate transport properties due to the formation of preferential conducting channels between the electrodes. Here we compare Shubnikov–de Haas oscillations simultaneously detected by microwave absorption and by conventional contact Hall bar measurements in fields up to 15 T on quasi-free standing, large area (25 mm²) monolayer graphene. We find that although the evaluated charge carrier densities from both measurements are similar, the mobility differs considerably due to electronic transport inhomogeneity.

The recent discovery of intrinsic two-dimensional (2D) ferromagnetism has sparked growing interests in the search for new 2D magnets with diverse and tunable properties for both fundamental scientific advances and novel spintronic applications. Here we report on the synthesis of layered chromium sulfide (Cr₂S₃) nanoplates via a facile sulfurization approach and the studies of their highly tunable Raman and (magneto-)transport properties. Depending on the specific growth conditions, we have achieved both epitaxial (and hence strained) and non-epitaxial nanoplates of Cr₂S₃ on the c-cut sapphire substrates. Via Raman scattering and density functional theory (DFT) calculations, we determined both types of nanoplates to be a rhombohedral R3 phase whose bulk counterpart exhibits weak ferromagnetism below a metal–insulator transition (MIT) temperature of ~120 K. Compressive strain from the lattice-mismatched substrate yields a red-shift of up to 8 cm⁻¹ in Raman peaks in comparison to the strain-free nanoplates obtained from the non-epitaxial growth. The strain-free nanoplate shows a variable-range-hopping type of insulating behavior, while the strained nanoplates exhibit an enhanced MIT up to ~275 K in comparison to 120 K in bulk samples. The room temperature resistivity values of the two types of nanoplates differ by 2 to 3 orders of magnitude. The distinct transport properties can be understood qualitatively based on the electronic band structures calculated by DFT.

Heterostructures of low-dimensional semiconducting materials, such as transition metal dichalcogenides (MX₂), are promising building blocks for future electronic and optoelectronic devices. The patterning of one MX₂ material on top of another one is challenging due to their structural similarity. This prevents an intrinsic etch stop when conventional anisotropic dry etching processes are used. An alternative approach consist in a two-step process, where a sacrificial silicon layer is pre-patterned with a low damage plasma process, stopping on the underlying MoS₂ film. The pre-patterned layer is used as sacrificial template for the formation of the top WS₂ film. This study describes the optimization of a cyclic Ar/Cl₂ atomic layer etch process applied to etch silicon on top of MoS₂, with minimal damage, followed by a selective conversion of the patterned Si into WS₂. The impact of the Si atomic layer etch towards the MoS₂ is evaluated: in the ion energy range used for this study, MoS₂ removal occurs in the over-etch step over 1–2 layers, leading to the appearance of MoO_x but without significant lattice distortions to the remaining layers. The combination of Si atomic layer etch, on top of MoS₂, and subsequent Si-to-WS₂ selective conversion, allows to create a WS₂/MoS₂ heterostructure, with clear Raman signals and horizontal lattice alignment. These results demonstrate a scalable, transfer free method to achieve horizontally individually patterned heterostacks and open the route towards wafer-level processing of 2D materials.

Two-dimensional (2D) molybdenum trioxide has been attracting research interest due to its bandgap tunability and a wide variety of desirable electronic/optoelectronic properties. However, the lack of a reproducible synthesis process for obtaining large coverage 2D MoO₃ has limited the use of this material. Here we report the synthesis of large area 2D MoO_3−x via physical vapor deposition, using MoO₃ powder as the precursor. The as-grown layers are directly deposited on SiO₂/Si, eliminating the necessity for any transfer process. These as-grown MoO_3−x layers allow for the large-scale fabrication of planar device arrays. The applicability of 2D MoO_3−x in optoelectronics is established via the demonstration of low-power ultraviolet (UV) sensor arrays, with rapid response times (200 µs) and responsivity up to 54.4 A · W⁻¹. At a bias voltage of 0.1 V, they are at least 400 times more power efficient than their next best contender.

Due to their unique 2D nature, charge carriers in semiconducting transition metal dichalcogenides (TMDs) exhibit strong unscreened Coulomb interactions and sensitivity to defects and impurities. The versatility of van der Waals layer stacking allows spatially separating electrons and holes between different TMD layers with staggered band structure, yielding interlayer few-body excitonic complexes whose nature is still debated. Here we combine quantum Monte Carlo calculations with spectrally and temporally resolved photoluminescence (PL) measurements on a top- and bottom-gated MoSe₂/WSe₂ heterostructure, and identify the emitters as impurity-bound interlayer excitonic complexes. Using independent electrostatic control of doping and out-of-plane electric field, we demonstrate control of the relative populations of neutral and charged complexes, their emission energies on a scale larger than their linewidth, and an increase of their lifetime into the microsecond regime. This work unveils new physics of confined carriers and is key to the development of novel optoelectronics applications.

Tunable photo-response is highly desirable by photodiodes for future optoelectronic applications. As compared to bulk semiconducting materials, the atomically thin two-dimensional (2D) materials may be one of the potential candidates to fabricate such adaptive photodiodes, since they possess not only excellent but also widely tunable optoelectronic properties. The most extensively applied device structure for the 2D materials based photodiodes is the vertically aligned van der Waals heterostructure. However, fabricating the vertical 2D material heterostructures is usually complicated, involving manually stacking multiple 2D material flakes together, which is undesirable for industry applications. In this work, we developed a vertical MoO₃/MoS₂ heterojunction for photodetection and photovoltaic applications. The device used MoS₂ and its oxidation layer of MoO₃ as the n- and p-type regions, respectively, which can greatly simplify the fabrication process of 2D vertical heterojunctions. Moreover, the device exhibited prominent photo-response with photo-responsivity of 670 mA W⁻¹, detectivity of 4.77  ×  10¹⁰ Jones and power conversion efficiency (PCE) of 3.5% under 0 V bias. The device also presented efficient gate tunability on photocurrent with on/off ratio of 10³. This research provides an alternative way to fabricate 2D materials based vertical heterojunctions for optoelectronic applications with tunable photo-responses.

Photodetectors based on α-phase In₂Se₃ ultrathin films demonstrate unusually high photoresponsivity comparing to those based on other two-dimensional (2D) materials, such as MoS₂. To understand the underlying mechanism, we investigate the ultrafast dynamics of In₂Se₃ ultrathin films ranging from 11 nm to 40 nm on mica and Au substrates, respectively, analogous to the practical layout of a photodetector. Our results show that the carrier lifetime of α-phase In₂Se₃ on mica is nearly independent of thickness and comparable to that of MoS₂, and the efficient charge carrier separation occurs on Au substrate. Because all of the key parameters of In₂Se₃ nanoflakes that determine its photoresponsive behavior are of similar values to those of MoS₂, we suggest that the interface effect, i.e. photogating effect and contact resistance, should be responsible for the dramatic photoresponsivity reported for field-effect transistor-type optoelectronic devices.

In this study we present results on the AC admittance response of bilayer MoS₂ films grown using chemical vapor deposition. A new MOS capacitor design for ultra-thin body 2D materials is proposed. We show that along with the density of interface traps (D_it), a transverse electric field distribution in the semiconductor and parasitic capacitance also cause frequency dispersion in measured capacitance. D_it extracted using the conductance method in 40 devices indicates reliable measurements for channel length, L  <  10 m. For devices with L  >  10 m, an increase in D_it is an artifact of access resistance in the semiconductor. Temperature measurements show an increasing defect distribution from cm⁻² eV⁻¹ around mid-gap to cm⁻² eV⁻¹ close to the conduction band minimum.

Two-dimensional (2D) materials are increasingly being used as active components in nanoscale devices. Many interesting properties of 2D materials stem from the reduced and highly non-local electronic screening in two dimensions. While electronic screening within 2D materials has been studied extensively, the question still remains of how 2D substrates screen charge perturbations or electronic excitations adjacent to them. Thickness-dependent dielectric screening properties have recently been studied using electrostatic force microscopy (EFM) experiments. However, it was suggested that some of the thickness-dependent trends were due to extrinsic effects. Similarly, Kelvin probe measurements (KPM) indicate that charge fluctuations are reduced when BN slabs are placed on SiO₂, but it is unclear if this effect is due to intrinsic screening from BN. In this work, we use first principles calculations to study the fully non-local dielectric screening properties of 2D material substrates. Our simulations give results in good qualitative agreement with those from EFM experiments, for hexagonal boron nitride (BN), graphene and MoS₂, indicating that the experimentally observed thickness-dependent screening effects are intrinsic to the 2D materials. We further investigate explicitly the role of BN in lowering charge potential fluctuations arising from charge impurities on an underlying SiO₂ substrate, as observed in the KPM experiments. 2D material substrates can also dramatically change the HOMO-LUMO gaps of adsorbates, especially for small molecules, such as benzene. We propose a reliable and very quick method to predict the HOMO-LUMO gaps of small physisorbed molecules on 2D and 3D substrates, using only the band gap of the substrate and the gas phase gap of the molecule.

Platinum (Pt) is one of the most commonly used materials for neural interface owing to its excellent biocompatibility and good charge transfer characteristics. Although Pt is generally regarded to be a safe and inert material, it is known to undergo irreversible electrochemical dissolution during neurostimulation. The byproducts of these irreversible electrochemical reactions are known to be cytotoxic that can damage the surrounding neural substrate. With decreasing size of microelectrodes for more advanced high-density neural interfaces, there is a need for a more reliable, safe, and high-performance neurostimulating electrodes. In this work, we demonstrate that a monolayer of graphene can significantly suppress Pt dissolution while maintaining excellent electrochemical functionality. We microfabricated bare and graphene-coated Pt microelectrodes with circular and fractal designs and measured their Pt dissolution rate using inductively coupled plasma mass spectrometry. In addition, we measured changes in electrochemical characteristics of these microelectrodes before and after a prolonged stimulation period to quantify the effects of Pt dissolution and graphene protective layer. We confirm that fractal microelectrodes do have a better charge transfer performance than conventional circular designs but bare Pt fractal microelectrodes had significantly faster dissolution rate than the circular ones. When coated with monolayer of graphene, however, Pt dissolution was reduced  >97% for fractal microelectrodes while they retained the superior charge transfer characteristics. The results of our work suggest that graphene can serve as an excellent diffusion barrier that can ameliorate the concerns for Pt dissolution in chronically implanted neurostimulation microelectrodes.

The Kondo effect due to a magnetic atom adsorbed on graphene is investigated using two theoretical approaches, including the non-equilibrium Green's function method within the slave-boson mean-field approximation and a newly developed tight-binding (TB) model with an effectively infinite Hubbard U term. Both methods reveal the presence of a Kondo peak in the local density of state (LDOS) of graphene near the Fermi level. A sharp Kondo peak is only observed in the odd-neighboring C sites of the C atom directly below the magnetic atom placed in top position. The peak intensity is found to decay quickly with respect to the distance between the C site and the magnetic atom. A theoretical model of scanning tunneling microscopy (STM) of graphene is presented as a means to identify the manifestation of the Kondo effect via direct topographic STM measurements. STM simulations show that in the proximity of the magnetic atom, only the C atoms of one sublattice are visible at low bias potential and thus a triangular lattice can be seen, in agreement with recent experiments. Further away from the magnetic atom, the Kondo effect dies down and eventually both sublattices are visible, leading to the recovery of the hexagonal lattice. This work not only provides a new TB method to study the Kondo effect in graphene, but also presents both scanning tunneling spectroscopy and microscopy fingerprints of the Kondo effect to facilitate its experimental verification.

The paper reports operando study of ultra-thin (50 nm) graphene oxide membranes by grazing incidence x-ray scattering in air dehumidification experiments. Absorption of water vapors in GO layers follows a modified Kelvin equation revealing condensation in an elastic slit, while desorption of water is limited by a few outer GO layers providing bottleneck restrictions to water transport and resulting in classical H2-type isotherms. GO interlayer distances (d) vary in range from 7.2 Å to 11.5 Å depending on partial water pressures in a feed stream and permeate. The permeance of water vapor through GO decreases steadily with decreasing interlayer distance between GO sheets from ~80 000 l/(m² · atm · h) to ~30 000 l/(m² · atm · h) falling down to negligible values below d  ≈  9.2 Å. Water transport in GO is described by Poiseuille equation and hopping diffusion depending on the number of water layers between GO planes, and was modelled with semi-empirical methods. It is shown the performance of thin GO membranes is strongly governed by the interstitial water quantity as dictated by water partial pressure.

The development of extrinsic antibacterial agents using newly identified nanoscale 2D layered transition metal carbide (MXene) was established by different delamination and intercalation approaches to explore structure-bioproperty relations. Herein, we report the potential impact of the microbial inactivation of Escherichia coli (E. coli) by delaminating (water, dimethyl sulfoxide (DMSO), and isopropylamine) and intercalating (hydrazine monohydrate (HMH), sodium hydroxide, and potassium hydroxide) of MXene (Ti₃C₂T_x), and its bactericidal mechanism was interpreted. The delaminations and intercalations all significantly increased the colloidal stability and bactericidal effect of Ti₃C₂T_x suspensions via unpacking of stacked MXene layers, among which the HMH intercalation showed the best performance. Our experimental results reveal that the mechanism of bacterial killing was primarily because of surface wrapping followed by extracellular reactive oxygen species independent oxidative stress. This study also shows that the stacked layer separation along with the surface moiety is an essential and decisive factor that determines the lethal bacterial potency of Ti₃C₂T_x. The application of Ti₃C₂T_x-coated polyvinylidene fluoride (PVDF) membranes effectively inactivates E. coli; importantly, it prevents biofilm formation on the active membrane surfaces and thus has high potential for antibiofouling. This study provides useful guidelines for the future development of Ti₃C₂T_x-based antimicrobial surface coatings and increases their bioapplication potential.

In bulk samples and few layer flakes, the transition metal dichalcogenides NbS₂ and NbSe₂ assume the H polytype structure with trigonal prismatic coordination of the Nb atom. Recently, however, single and few layers of 1T-NbSe₂ with octahedral coordination around the transition metal ion were synthesized. Motivated by these experiments and by using first-principles calculations, we investigate the structural, electronic and dynamical properties of single layer 1T-NbS₂. We find that single-layer 1T-NbS₂ undergoes a star-of-David charge density wave. Within the generalized gradient approximation, the weak interaction between the stars leads to an ultraflat band that results to be located at the Fermi level and isolated from all other bands. The spin-polarized generalized gradient approximation stabilizes a total spin 1/2 magnetic state with a magnetic moment localized on the central Nb in the star and the opening of a 0.15 eV band gap. In the GGA  +  U framework, the magnetic moment on the central Nb is enhanced to and a larger gap occurs. Most important, this approximation gives a small energy difference between the 1T and 1H polytypes (only  +0.5 mRy/Nb), suggesting that the 1T-polytype can be synthesized in a similar way as done for single layer 1T-NbSe₂. Finally we compute first and second nearest neighbors magnetic inter-star exchange interactions finding J₁  =  9.5 K and J₂  =  0.4 K ferromagnetic coupling constants.

Next generation of spintronic devices aims to utilize the spin-polarized current injection and transport to control the magnetization dynamics in the spin logic and memory technology. However, the detailed evolution process of the frequently observed bias current-induced sign change phenomenon of the spin polarization has not been examined in details and the underlying microscopic mechanism is not well understood. Here, we report the observation of a systematic evolution of the sign change process of Hanle spin precession signal in the graphene nonlocal spintronic devices at room temperature. By tuning the interface tunnel resistances of the ferromagnetic contacts to graphene, different transformation processes of Hanle spin precession signal are probed in a controlled manner by tuning the injection bias current/voltage. Detailed analysis and first-principles calculations indicate a possible magnetic proximity and the energy dependent electronic structure of the ferromagnet-graphene interface can be responsible for the sign change process of the spin signal and open a new perspective to realize a spin-switch at very low bias-current or voltage.

The study of the nanomechanics of graphene—and other 2D materials—has led to the discovery of exciting new properties in 2D crystals, such as their remarkable in-plane stiffness and out of plane flexibility, as well as their unique frictional and wear properties at the nanoscale. Recently, nanomechanics of graphene has generated renovated interest for new findings on the pressure-induced chemical transformation of a few-layer thick epitaxial graphene into a new ultra-hard carbon phase, named diamene. In this work, by means of a machine learning technique, we provide a fast and efficient tool for identification of graphene domains (areas with a defined number of layers) in epitaxial and exfoliated films, by combining data from atomic force microscopy (AFM) topography and friction force microscopy (FFM). Through the analysis of the number of graphene layers and detailed Å-indentation experiments, we demonstrate that the formation of ultra-stiff diamene is exclusively found in 1-layer plus buffer layer epitaxial graphene on silicon carbide (SiC) and that an ultra-stiff phase is not observed in neither thicker epitaxial graphene (2-layer or more) nor exfoliated graphene films of any thickness on silicon oxide (SiO₂).

This corrigendum amends typing errors in one equation in the original manuscript. This correction does not make any difference to our results, plots, statements, or conclusions.