Search

Harnessing electronic excitations involving coherent coupling to bosonic modes is essential for the design and control of emergent phenomena in quantum materials. In situations where charge carriers induce a lattice distortion due to the electron-phonon interaction, the conducting states get "dressed", which leads to the formation of polaronic quasiparticles. The exploration of polaronic effects on low-energy excitations is in its infancy in two-dimensional materials. Here, we present the discovery of an interlayer plasmon polaron in heterostructures composed of graphene on top of single-layer WS2. By using micro-focused angle-resolved photoemission spectroscopy during in situ doping of the top graphene layer, we observe a strong quasiparticle peak accompanied by several carrier density-dependent shake-off replicas around the single-layer WS2 conduction band minimum. Our results are explained by an effective many-body model in terms of a coupling between single-layer WS2 conduction electrons and an interlayer plasmon mode. It is important to take into account the presence of such interlayer collective modes, as they have profound consequences for the electronic and optical properties of heterostructures that are routinely explored in many device architectures involving 2D transition metal dichalcogenides. Here, the authors report the observation of an interlayer plasmon polaron in heterostructures composed of graphene and monolayer WS2. This is manifested in the ARPES spectra as a strong quasiparticle peak accompanied by several carrier density-dependent shake-off replicas around the WS2 conduction band minimum. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) materials offer unique opportunities in engineering the ultrafast spatiotemporal response of composite nanomechanical structures. In this work, we report on high frequency, high quality factor (Q) 2D acoustic cavities operating in the 50–600 GHz frequency (f) range with f × Q up to 1 × 1014. Monolayer steps and material interfaces expand cavity functionality, as demonstrated by building adjacent cavities that are isolated or strongly-coupled, as well as a frequency comb generator in MoS2/h-BN systems. Energy dissipation measurements in 2D cavities are compared with attenuation derived from phonon-phonon scattering rates calculated using a fully microscopic ab initio approach. Phonon lifetime calculations extended to low frequencies (<1 THz) and combined with sound propagation analysis in ultrathin plates provide a framework for designing acoustic cavities that approach their fundamental performance limit. These results provide a pathway for developing platforms employing phonon-based signal processing and for exploring the quantum nature of phonons. Here, authors report on acoustic cavities in 2D materials operating in the 50–600 GHz range and show that quality factors approach the limit set by lattice anharmonicity. Functionality expanded by heterogeneities (steps and interfaces) is demonstrated through coupled cavities and frequency comb generation. [ABSTRACT FROM AUTHOR]

2D materials play a special role in the race to make smaller and smaller devices. Their unique and strong in-plane bonding makes them impervious to diffusion into other layers and provides excellent thickness control. Their van der Waal's bonding with other monolayers or substrates allows for heterostructures unattainable by any other technique. This is reflected by the abundant popularity of research into graphene and other 2D materials. In this review article, we will describe the out-of-plane properties of graphene and functionalized graphene. We will use three specific examples to illustrate how these out-of-plane properties can be used in spintronic devices, in section "Graphene as a Tunnel Barrier" we will describe a magnetic tunnel junction (MTJ) based on graphene. Section "Graphene Based MTJs" will describe the spin injecting properties of a graphene tunnel barrier on silicon. Section "Graphene in Semiconductor Spintronic Devices" describes how you can use functionalized graphene to make a homoepitaxial graphene device. The second part of this article reviews monolayer transition-metal dichalcogenides (TMDs). First, we will show how TMDs are grown and specifically how we can grow large-area TMDs by chemical vapor deposition. Secondly, we will describe the optical properties of several TMDs and compare the results from several authors. Finally, we choose a chemical sensor as a specific example to show how TMDs can be used in a device. [ABSTRACT FROM AUTHOR]

Heterostructures composed of multiple layers of different atomically thin materials are of interest due to their unique properties and potential for new device functionality. MoS2-graphene heterostructures have shown promise as photodetectors and vertical tunnel transistors. However, progress is limited by the typically micrometer-scale devices and by the multiple alignments required for fabrication when utilizing mechanically exfoliated material. Here, the synthesis of large-area, continuous, and uniform MoS2 monolayers directly on graphene by chemical vapor deposition is reported, resulting in heterostructure samples on the centimeter scale with the possibility for even larger lateral dimensions. Atomic force microscopy, photoluminescence, X-ray photoelectron, and Raman spectroscopies demonstrate uniform single-layer growth of stoichiometric MoS2. The ability to reproducibly generate large-area heterostructures is highly advantageous for both fundamental investigations and technological applications. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editor discusses various reports within the issue on topics including two dimensional heterostructure material research, transition metal dichalcogenides, and graphene heterostructures and its mechanical properties.

The mechanisms leading to spin relaxation in graphene and its heterostructures continue to be debated. Control of the spin relaxation in graphene-based structures is necessary to achieve the envisioned utility of graphene in future spintronic devices beyond Moore's law. Proximity induced spin relaxation caused by contact to a high spin-orbit material, such as WS 2 , offers a promising avenue to manipulate the spin lifetime. We demonstrate the operation of WS 2 /graphene/fluorographene non-local spin valves and extract the spin lifetimes for a range of carrier concentrations by Hanle effect measurements. Four-terminal charge transport measurements allow us to calculate the momentum relaxation time as a function of carrier concentration and compare it to the spin lifetime. These data show that the D'yakonov-Perel’ mechanism is the dominant spin relaxation mechanism for WS 2 /graphene/fluorographene devices, while, for reference graphene/fluorographene devices, linear scaling between the spin and momentum lifetimes points to spin-flip scattering during strong elastic scattering events where the scattering event is strongly coupled to the electron spin. We attribute the change in spin relaxation type in part with the inclusion of WS 2 as a substrate to proximity induced spin-orbit coupling due to the adjacent WS 2 layer, and we compare our data to the literature. [ABSTRACT FROM AUTHOR]

Heterogeneous engineering of two-dimensionallayered materials,including metallic graphene and semiconducting transition metal dichalcogenides,presents an exciting opportunity to produce highly tunable electronicand optoelectronic systems. In order to engineer pristine layers andtheir interfaces, epitaxial growth of such heterostructures is required.We report the direct growth of crystalline, monolayer tungsten diselenide(WSe2) on epitaxial graphene (EG) grown from silicon carbide.Raman spectroscopy, photoluminescence, and scanning tunneling microscopyconfirm high-quality WSe2monolayers, whereas transmissionelectron microscopy shows an atomically sharp interface, and low energyelectron diffraction confirms near perfect orientation between WSe2and EG. Vertical transport measurements across the WSe2/EG heterostructure provides evidence that an additional barrierto carrier transport beyond the expected WSe2/EG band offsetexists due to the interlayer gap, which is supported by theoreticallocal density of states (LDOS) calculations using self-consistentdensity functional theory (DFT) and nonequilibrium Green’sfunction (NEGF). [ABSTRACT FROM AUTHOR]