Wafer-scale δ waveguides for integrated two-dimensional photonics.

The efficient, large-scale generation and control of photonic modes guided by van der Waals materials remains as a challenge despite their potential for on-chip photonic circuitry. We report three-atom-thick waveguides-δ waveguides-based on wafer-scale molybdenum disulfide (MoS2) monolayers that can guide visible and near-infrared light over millimeter-scale distances with low loss and an efficient in-coupling. The extreme thinness provides a light-trapping mechanism analogous to a δ-potential well in quantum mechanics and enables the guided waves that are essentially a plane wave freely propagating along the in-plane, but confined along the out-of-plane, direction of the waveguide. We further demonstrate key functionalities essential for two-dimensional photonics, including refraction, focusing, grating, interconnection, and intensity modulation, by integrating thin-film optical components with δ waveguides using microfabricated dielectric, metal, or patterned MoS2.

Imaging particulate matter exposed pine trees by vehicle exhaust experiment and hyperspectral analysis.

This study analyzed spectral variations of the particulate matter (PM hereafter)-exposed pine trees using a spectrometer and a hyperspectral imager to derive the most effective spectral indices to detect the pine needle exposure to PM emission. We found that the spectral variation in the near-infrared (NIR hereafter) bands systemically coincided with the variations in PM concentration, showing larger variations for the diesel group whereas larger dust particles showed spectral variations in both visible and NIR bands. It is because the PM adsorption on needles is the main source of NIR band variation, and the combination of visible and NIR spectra can detect PM absorption. Fourteen bands were selected to classify PM-exposed pine trees with an accuracy of 82% and a kappa coefficient of 0.61. Given that this index employed both visible and NIR bands, it would be able to detect PM adsorption. The findings can be transferred to real-world applications for monitoring air pollution in an urban area.

Two-Dimensional Mechanics of Atomically Thin Solids on Water.

Movement of a three-dimensional solid at an air-water interface is strongly influenced by the extrinsic interactions between the solid and the water. The finite thickness and volume of a moving solid causes capillary interactions and water-induced drag. In this Letter, we report the fabrication and dynamical imaging of freely floating MoS2 solids on water, which minimizes such extrinsic effects. For this, we delaminate a synthesized wafer-scale monolayer MoS2 onto a water surface, which shows negligible height difference across water and MoS2. Subsequently patterning by a laser generates arbitrarily shaped MoS2 with negligible in-plane strain. We introduce photoswitchable surfactants to exert a lateral force to floating MoS2 with a spatiotemporal control. Using this platform, we demonstrate a variety of two-dimensional mechanical systems that show reversible shape changes. Our experiment provides a versatile approach for designing and controlling a large array of atomically thin solids on water for intrinsically two-dimensional dynamics and mechanics.

Direct Heat-Induced Patterning of Inorganic Nanomaterials.

Patterning functional inorganic nanomaterials is an important process for advanced manufacturing of quantum dot (QD) electronic and optoelectronic devices. This is typically achieved by inkjet printing, microcontact printing, and photo- and e-beam lithography. Here, we investigate a different patterning approach that utilizes local heating, which can be generated by various sources, such as UV-, visible-, and IR-illumination, or by proximity heat transfer. This direct thermal lithography method, termed here heat-induced patterning of inorganic nanomaterials (HIPIN), uses colloidal nanomaterials with thermally unstable surface ligands. We designed several families of such ligands and investigated their chemical and physical transformations responsible for heat-induced changes of nanocrystal solubility. Compared to traditional photolithography using photochemical surface reactions, HIPIN extends the scope of direct optical lithography toward longer wavelengths of visible (532 nm) and infrared (10.6 µm) radiation, which is necessary for patterning optically thick layers (e.g., 1.2 µm) of light-absorbing nanomaterials. HIPIN enables patterning of features defined by the diffraction-limited beam size. Our approach can be used for direct patterning of metal, semiconductor, and dielectric nanomaterials. Patterned semiconductor QDs retain the majority of their as-synthesized photoluminescence quantum yield. This work demonstrates the generality of thermal patterning of nanomaterials and provides a new path for additive device manufacturing using diverse colloidal nanoscale building blocks.

Resist-Free Lithography for Monolayer Transition Metal Dichalcogenides.

Photolithography and electron-beam lithography are the most common methods for making nanoscale devices from semiconductors. While these methods are robust for bulk materials, they disturb the electrical properties of two-dimensional (2D) materials, which are highly sensitive to chemicals used during lithography processes. Here, we report a resist-free lithography method, based on direct laser patterning and resist-free electrode transfer, which avoids unintentional modification to the 2D materials throughout the process. We successfully fabricate large arrays of field-effect transistors using MoS2 and WSe2 monolayers, the performance of which reflects the properties of the pristine materials. Furthermore, using these pristine devices as a reference, we reveal that among the various stages of a conventional lithography process, exposure to a solvent like acetone changes the electrical conductivity of MoS2 the most. This new approach will enable a rational design of reproducible processes for making large-scale integrated circuits based on 2D materials and other surface-sensitive materials.

Designing the Bending Stiffness of 2D Material Heterostructures.

2D monolayers represent some of the most deformable inorganic materials, with bending stiffnesses approaching those of lipid bilayers. Achieving 2D heterostructures with similar properties would enable a new class of deformable devices orders of magnitude softer than conventional thin-film electronics. Here, by systematically introducing low-friction twisted or heterointerfaces, interfacial engineering is leveraged to tailor the bending stiffness of 2D heterostructures over several hundred percent. A bending model is developed and experimentally validated to predict and design the deformability of 2D heterostructures and how it evolves with the composition of the stack, the atomic arrangements at the interfaces, and the geometry of the structure. Notably, when each atomic layer is separated by heterointerfaces, the total bending stiffness reaches a theoretical minimum, equal to the sum of the constituent layers regardless of scale of deformation-lending the extreme deformability of 2D monolayers to device-compatible multilayers.

Brucite shows antibacterial activity via establishment of alkaline conditions.

Brucite, a mineral form of magnesium hydroxide, has been used industrially as a useful refractory insulator. However, its antibacterial activity remains largely unknown. This study investigated the antibacterial activity of brucite in two bacterial strains, Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive). Brucite, compared to other minerals, showed a strong antibacterial activity against both bacterial strains. The surface structure of brucite examined using scanning electron microscopy (SEM) had no implications in its antibacterial activity. Brucite in lysogeny broth (LB) medium maintained the basicity (pH 9) of the solution for a prolonged period of time. The basicity of the medium was responsible for the antibacterial activity of brucite. The study results identified brucite as a potent antibacterial agent that required no further processing and also revealed the underlying mechanism of action responsible for its antibacterial activity.

Tailoring Single- and Double-Sided Fluorination of Bilayer Graphene via Substrate Interactions.

While many technologies rely on multilayer heterostructures, most of the studies on chemical functionalization have been limited to monolayer graphene. In order to use functionalization in multilayer systems, we must first understand the interlayer interactions between functionalized and nonfunctionalized (intact) layers and how to selectively functionalize one layer at a time. Here, we demonstrate a method to fabricate single- or double-sided fluorinated bilayer graphene (FBG) by tailoring substrate interactions. Both the top and bottom surfaces of bilayer graphene on the rough silicon dioxide (SiO2) are fluorinated; meanwhile, only the top surface of graphene on hexagonal boron nitride (hBN) is fluorinated. The functionalization type affects electronic properties; double-sided FBG on SiO2 is insulating, whereas single-sided FBG on hBN maintains conducting, showing that the intact bottom layer becomes electrically decoupled from the fluorinated top insulating layer. Our results define a straightforward method to selectively functionalize the top and bottom surfaces of bilayer graphene.

Realizing Optoelectronic Devices from Crumpled Two-Dimensional Material Heterostructures.

Due to their high in-plane stiffness and low flexural rigidity, two-dimensional (2D) materials are excellent candidates for engineering three-dimensional (3D) nanostructures using crumpling. An important new direction is to integrate 2D materials into crumpled heterostructures, which can have much more complex device geometries. Here, we demonstrate phototransistors from crumpled 2D heterostructures formed from graphene contacts to a monolayer transition-metal dichalcogenide (MoS2, WSe2) channel and quantify the membrane morphology and optoelectronic performance. First, we examined the morphology of folds in the heterostructure and constituent monolayers under uniaxial compression. The 2D membranes relieve the stress by delaminating from the substrate and creating nearly periodic folds whose spacing depends on the membrane type. The matched mechanical stiffness of the constituting layers allows the 2D heterostructure to maintain a conformal interface through large deformations. Next, we examined the optoelectronic performance of a biaxially crumpled graphene-WSe2 phototransistor. Photoluminescence (PL) spectroscopy shows that the optical band gap of WSe2 shifts by less than 2 meV between flat and 15% biaxial crumpling, corresponding to a change in strain of less than 0.05%. The photoresponsivity scaled as P-0.38 and reached 20 A/W under an illumination power density of 4 µW/cm2 at 20 V bias, a performance comparable to flat photosensors. Using photocurrent microscopy, we observe that the photoresponsivity increases by only 20% after crumpling. Both the PL and photoresponse confirm that crumpling and delamination prevent the buildup of compressive strain leading to highly deformed materials and devices with similar performance to their flat analogs. These results set a foundation for crumpled all-2D heterostructure devices and circuitry for flexible and stretchable electronic applications.

Material-Dependent Evolution of Mechanical Folding Instabilities in Two-Dimensional Atomic Membranes.

Inducing and controlling three-dimensional deformations in monolayer two-dimensional materials is important for applications from stretchable electronics to origami nanoelectromechanical systems. For these applications, it is critical to understand how the properties of different materials influence the morphologies of two-dimensional atomic membranes under mechanical loading. Here, we systematically investigate the evolution of mechanical folding instabilities in uniaxially compressed monolayer graphene and MoS2 on a soft polydimethylsiloxane substrate. We examine the morphology of the compressed membranes using atomic force microscopy for compression from 0 to 33%. We find the membranes display roughly evenly spaced folds and observe two distinct stress release mechanisms under increasing compression. At low compression, the membranes delaminate to generate new folds. At higher compression, the membranes slip over the surface to enlarge existing folds. We observe a material-dependent transition between these two behaviors at a critical fold spacing of 1000 ± 250 nm for graphene and 550 ± 20 nm for MoS2. We establish a simple shear-lag model which attributes the transition to a competition between static friction and adhesion and gives the maximum interfacial static friction on polydimethylsiloxane of 3.8 ± 0.8 MPa for graphene and 7.7 ± 2.5 MPa for MoS2. Furthermore, in graphene, we observe an additional transition from standing folds to fallen folds at 8.5 ± 2.3 nm fold height. These results provide a framework to control the nanoscale fold structure of monolayer atomic membranes, which is a critical step in deterministically designing stretchable or foldable nanosystems based on two-dimensional materials.

Stochastic Stress Jumps Due to Soliton Dynamics in Two-Dimensional van der Waals Interfaces.

The creation and movement of dislocations determine the nonlinear mechanics of materials. At the nanoscale, the number of dislocations in structures become countable, and even single defects impact material properties. While the impact of solitons on electronic properties is well studied, the impact of solitons on mechanics is less understood. In this study, we construct nanoelectromechanical drumhead resonators from Bernal stacked bilayer graphene and observe stochastic jumps in frequency. Similar frequency jumps occur in few-layer but not twisted bilayer or monolayer graphene. Using atomistic simulations, we show that the measured shifts are a result of changes in stress due to the creation and annihilation of individual solitons. We develop a simple model relating the magnitude of the stress induced by soliton dynamics across length scales, ranging from <0.01 N/m for the measured 5 µm diameter to ∼1.2 N/m for the 38.7 nm simulations. These results demonstrate the sensitivity of 2D resonators are sufficient to probe the nonlinear mechanics of single dislocations in an atomic membrane and provide a model to understand the interfacial mechanics of different kinds of van der Waals structures under stress, which is important to many emerging applications such as engineering quantum states through electromechanical manipulation and mechanical devices like highly tunable nanoelectromechanical systems, stretchable electronics, and origami nanomachines.

Author Correction: Ultrasoft slip-mediated bending in few-layer graphene.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Ultrasoft slip-mediated bending in few-layer graphene.

Continuum scaling laws often break down when materials approach atomic length scales, reflecting changes in their underlying physics and the opportunities to access unconventional properties. These continuum limits are evident in two-dimensional materials, where there is no consensus on their bending stiffnesses or how they scale with thickness. Through combined computational and electron microscopy experiments, we measure the bending stiffness of graphene, obtaining 1.2-1.7 eV for a monolayer. Moreover, we find that the bending stiffness of few-layer graphene decreases sharply as a function of bending angle, tuning by almost 400% for trilayer graphene. This softening results from shear, slip and the onset of superlubricity between the atomic layers and corresponds with a gradual change in scaling power from cubic to linear. Our results provide a unified model for bending in two-dimensional materials and show that their multilayers can be orders of magnitude softer than previously thought, among the most flexible electronic materials currently known.

Author Correction: Atomically precise graphene etch stops for three dimensional integrated systems from two dimensional material heterostructures.

The original version of this Article contained an error in the second sentence of the second paragraph of the 'Electrical properties of fluorinated graphene contacts' section of the Results, which incorrectly read 'The mobility was calculated by the Drude model, µ = ne/σ where µ, n, e, and σ are the carrier mobility, carrier density, electron charge, and sheet conductivity, respectively'. The correct version states 'µ = σ/ne ' in place of 'µ = ne/σ '. This has been corrected in both the PDF and HTML versions of the Article.

Nano-electromechanical Drumhead Resonators from Two-Dimensional Material Bimorphs.

Atomic membranes of monolayer 2D materials represent the ultimate limit in the size of nano-electromechanical systems. However, new properties and new functionalities emerge by looking at the interface between layers in heterostructures of 2D materials. Here, we demonstrate the integration of 2D heterostructures as tunable nano-electromechanical systems, exploring the competition between the mechanics of the ultrathin membrane and the incommensurate van der Waals interface. We fabricate electrically contacted 5 or 6 µm circular drumheads of suspended heterostructure membranes of monolayer graphene on monolayer molybdenum disulfide (MoS2), which we call a 2D bimorph. We characterize the mechanical resonance through electrostatic actuation and laser interferometry detection. The 2D bimorphs have resonance frequencies of 5-20 MHz and quality factors of 50-700, comparable to resonators from monolayer or few-layer 2D materials. The frequencies and eigenmode shapes of the higher harmonics display split degenerate modes, showing that the 2D bimorphs behave as membranes with asymmetric tension. The devices display dynamic ranges of 44 dB, with an additional nonlinearity in the dissipation at small drive. Under electrostatic frequency tuning, devices display a small tuning of ∼20% compared with graphene resonators, which have >100%. In addition, the tuning shows a kink that deviates from the tensioned membrane model for atomic membranes and corresponds with a changing in stress of 14 mN/m. A model that accounts for this tuning behavior is the onset of interlayer slip in the heterostructure, allowing the tension in the membrane to relax. Using density functional theory simulations, we find that the change in stress at the kink is much larger than the predicted energy barrier for interlayer slip of 0.102 mN/m in an incommensurate 2D heterostructure but smaller than the energy barrier for an aligned graphene bilayer of 35 mN/m, suggesting a local pinning effect at ripples or folds in the heterostructure. Finally, we observe an asymmetry in tuning of the full width at half-maximum that does not exist in monolayer resonators. These findings demonstrate a new class of nano-electromechanical systems from 2D heterostructures and unravel the complex interaction of membrane morphology versus interlayer adhesion and slip on the mechanics of incommensurate van der Waals interfaces.

Atomically precise graphene etch stops for three dimensional integrated systems from two dimensional material heterostructures.

Atomically precise fabrication methods are critical for the development of next-generation technologies. For example, in nanoelectronics based on van der Waals heterostructures, where two-dimensional materials are stacked to form devices with nanometer thicknesses, a major challenge is patterning with atomic precision and individually addressing each molecular layer. Here we demonstrate an atomically thin graphene etch stop for patterning van der Waals heterostructures through the selective etch of two-dimensional materials with xenon difluoride gas. Graphene etch stops enable one-step patterning of sophisticated devices from heterostructures by accessing buried layers and forming one-dimensional contacts. Graphene transistors with fluorinated graphene contacts show a room temperature mobility of 40,000 cm2 V-1 s-1 at carrier density of 4 × 1012 cm-2 and contact resistivity of 80 Ω·µm. We demonstrate the versatility of graphene etch stops with three-dimensionally integrated nanoelectronics with multiple active layers and nanoelectromechanical devices with performance comparable to the state-of-the-art.

Towards a systematic assessment of errors in diffusion Monte Carlo calculations of semiconductors: Case study of zinc selenide and zinc oxide.

The fixed node diffusion Monte Carlo (DMC) method has attracted interest in recent years as a way to calculate properties of solid materials with high accuracy. However, the framework for the calculation of properties such as total energies, atomization energies, and excited state energies is not yet fully established. Several outstanding questions remain as to the effect of pseudopotentials, the magnitude of the fixed node error, and the size of supercell finite size effects. Here, we consider in detail the semiconductors ZnSe and ZnO and carry out systematic studies to assess the magnitude of the energy differences arising from controlled and uncontrolled approximations in DMC. The former include time step errors and supercell finite size effects for ground and optically excited states, and the latter include pseudopotentials, the pseudopotential localization approximation, and the fixed node approximation. We find that for these compounds, the errors can be controlled to good precision using modern computational resources and that quantum Monte Carlo calculations using Dirac-Fock pseudopotentials can offer good estimates of both cohesive energy and the gap of these systems. We do however observe differences in calculated optical gaps that arise when different pseudopotentials are used.