Long-Range Propagation of Exciton-Polaritons in Large-Area 2D Semiconductor Monolayers.

Atomically thin transition metal dichalcogenides (TMDs), a subclass of two-dimensional (2D) layered materials, have numerous fascinating properties that make them a promising platform for photonic and optoelectronic devices. In particular, excited state transport by TMDs is important in energy harvesting and photonic switching; however, long-range transport in TMDs is challenging due to the lack of availability of large area films. Whereas most previous studies have focused on small, exfoliated monolayer flakes, in this work we demonstrate metal-organic chemical vapor deposition grown centimeter-scale monolayers of WS2 that support polariton propagation lengths of up to 60 µm. The polaritons form through the strong coupling of excitons with Bloch surface waves (BSWs) supported by all-dielectric photonic structures. We observe that the propagation length increases with the number of dielectric pairs due to the increased quality factor of the supporting distributed Bragg reflector. Furthermore, a longer propagation length is observed as the guided or BSW content of the polariton is increased. Our results provide a practical approach for the systematic engineering of long-range energy transport mediated by exciton-polaritons in TMD layers. Along with the accessibility of large area TMDs, our work enables applications for practical TMD-based polaritonic devices that operate at room temperature.

Near- and Far-Field Observation of Phonon Polaritons in Wafer-Scale Multilayer Hexagonal Boron Nitride Prepared by Chemical Vapor Deposition.

Polaritons in layered materials (LMs) are a promising platform to manipulate and control light at the nanometer scale. Thus, the observation of polaritons in wafer-scale LMs is critically important for the development of industrially relevant nanophotonics and optoelectronics applications. In this work, phonon polaritons (PhPs) in wafer-scale multilayer hexagonal boron nitride (hBN) grown by chemical vapor deposition are reported. By infrared nanoimaging, the PhPs are visualized, and PhP lifetimes of ≈0.6 ps are measured, comparable to that of micromechanically exfoliated multilayer hBN. Further, PhP nanoresonators are demonstrated. Their quality factors of ≈50 are about 0.7 times that of state-of-the-art devices based on exfoliated hBN. These results can enable PhP-based surface-enhanced infrared spectroscopy (e.g., for gas sensing) and infrared photodetector applications.

Selective Growth of van der Waals Heterostructures Enabled by Electron-Beam Irradiation.

Van der Waals heterostructures (vdWHSs) enable the fabrication of complex electronic devices based on two-dimensional (2D) materials. Ideally, these vdWHSs should be fabricated in a scalable and repeatable way and only in the specific areas of the substrate to lower the number of technological operations inducing defects and impurities. Here, we present a method of selective fabrication of vdWHSs via chemical vapor deposition by electron-beam (EB) irradiation. We distinguish two growth modes: positive (2D materials nucleate on the irradiated regions) on graphene and tungsten disulfide (WS2) substrates, and negative (2D materials do not nucleate on the irradiated regions) on the graphene substrate. The growth mode is controlled by limiting the air exposure of the irradiated substrate and the time between irradiation and growth. We conducted Raman mapping, Kelvin-probe force microscopy, X-ray photoelectron spectroscopy, and density-functional theory modeling studies to investigate the selective growth mechanism. We conclude that the selective growth is explained by the competition of three effects: EB-induced defects, adsorption of carbon species, and electrostatic interaction. The method here is a critical step toward the industry-scale fabrication of 2D-materials-based devices.

Role of Surface Adsorbates on the Photoresponse of (MO)CVD-Grown Graphene-MoS2 Heterostructure Photodetectors.

A promising strategy toward ultrathin, sensitive photodetectors is the combination of a photoactive semiconducting transition-metal dichalcogenide (TMDC) monolayer like MoS2 with highly conductive graphene. Such devices often exhibit a complex and contradictory photoresponse as incident light can trigger both photoconductivity and photoinduced desorption of molecules from the surface. Here, we use metal-organic chemical vapor deposition (MOCVD) to directly grow MoS2 on top of graphene that is deposited on a sapphire wafer via chemical vapor deposition (CVD) for realizing graphene-MoS2 photodetectors. Two-color optical pump-electrical probe experiments allow for separation of light-induced carrier transfer across the graphene-MoS2 heterointerface from adsorbate-induced effects. We demonstrate that adsorbates strongly modify both magnitude and sign of the photoconductivity. This is attributed to a change of the graphene doping from p- to n-type in case adsorbates are being desorbed, while in either case, photogenerated electrons are transferred from MoS2 to graphene. This nondestructive probing method sheds light on the charge carrier transfer mechanisms and the role of adsorbates in two-dimensional (2D) heterostructure photodetectors.

Defect-Free Metal Deposition on 2D Materials via Inkjet Printing Technology.

2D materials have many outstanding properties that make them attractive for the fabrication of electronic devices, such as high conductivity, flexibility, and transparency. However, integrating 2D materials in commercial devices and circuits is challenging because their structure and properties can be damaged during the fabrication process. Recent studies have demonstrated that standard metal deposition techniques (like electron beam evaporation and sputtering) significantly damage the atomic structure of 2D materials. Here it is shown that the deposition of metal via inkjet printing technology does not produce any observable damage in the atomic structure of ultrathin 2D materials, and it can keep a sharp interface. These conclusions are supported by abundant data obtained via atomistic simulations, transmission electron microscopy, nanochemical metrology, and device characterization in a probe station. The results are important for the understanding of inkjet printing technology applied to 2D materials, and they could contribute to the better design and optimization of electronic devices and circuits.

Light-matter coupling in large-area van der Waals superlattices.

Two-dimensional (2D) crystals have renewed opportunities in design and assembly of artificial lattices without the constraints of epitaxy. However, the lack of thickness control in exfoliated van der Waals (vdW) layers prevents realization of repeat units with high fidelity. Recent availability of uniform, wafer-scale samples permits engineering of both electronic and optical dispersions in stacks of disparate 2D layers with multiple repeating units. Here we present optical dispersion engineering in a superlattice structure comprising alternating layers of 2D excitonic chalcogenides and dielectric insulators. By carefully designing the unit cell parameters, we demonstrate greater than 90% narrow band absorption in less than 4 nm of active layer excitonic absorber medium at room temperature, concurrently with enhanced photoluminescence in square-centimetre samples. These superlattices show evidence of strong light-matter coupling and exciton-polariton formation with geometry-tuneable coupling constants. Our results demonstrate proof of concept structures with engineered optical properties and pave the way for a broad class of scalable, designer optical metamaterials from atomically thin layers.

Substrate-Induced Variances in Morphological and Structural Properties of MoS2 Grown by Chemical Vapor Deposition on Epitaxial Graphene and SiO2.

In this work, we report the impact of substrate type on the morphological and structural properties of molybdenum disulfide (MoS2) grown by chemical vapor deposition (CVD). MoS2 synthesized on a three-dimensional (3D) substrate, that is, SiO2, in response to the change of the thermodynamic conditions yielded different grain morphologies, including triangles, truncated triangles, and circles. Simultaneously, MoS2 on graphene is highly immune to the modifications of the growth conditions, forming triangular crystals only. We explain the differences between MoS2 on SiO2 and graphene by the different surface diffusion mechanisms, namely, hopping and gas-molecule-collision-like mechanisms, respectively. As a result, we observe the formation of thermodynamically favorable nuclei shapes on graphene, while on SiO2, a full spectrum of domain shapes can be achieved. Additionally, graphene withstands the growth process well, with only slight changes in strain and doping. Furthermore, by the application of graphene as a growth substrate, we realize van der Waals epitaxy and achieve strain-free growth, as suggested by the photoluminescence (PL) studies. We indicate that PL, contrary to Raman spectroscopy, enables us to arbitrarily determine the strain levels in MoS2.

Wafer-Scale Synthesis of Graphene on Sapphire: Toward Fab-Compatible Graphene.

The adoption of graphene in electronics, optoelectronics, and photonics is hindered by the difficulty in obtaining high-quality material on technologically relevant substrates, over wafer-scale sizes, and with metal contamination levels compatible with industrial requirements. To date, the direct growth of graphene on insulating substrates has proved to be challenging, usually requiring metal-catalysts or yielding defective graphene. In this work, a metal-free approach implemented in commercially available reactors to obtain high-quality monolayer graphene on c-plane sapphire substrates via chemical vapor deposition is demonstrated. Low energy electron diffraction, low energy electron microscopy, and scanning tunneling microscopy measurements identify the Al-rich reconstruction 31 × 31 R ± 9 ° of sapphire to be crucial for obtaining epitaxial graphene. Raman spectroscopy and electrical transport measurements reveal high-quality graphene with mobilities consistently above 2000 cm2 V-1 s-1 . The process is scaled up to 4 and 6 in. wafers sizes and metal contamination levels are retrieved to be within the limits for back-end-of-line integration. The growth process introduced here establishes a method for the synthesis of wafer-scale graphene films on a technologically viable basis.