Patterned tungsten disulfide/graphene heterostructures for efficient multifunctional optoelectronic devices

Epitaxy of GaSe Coupled to Graphene: From In Situ Band Engineering to Photon Sensing

2D semiconductors can drive advances in quantum science and technologies. However, they should be free of any contamination; also, the crystallographic ordering and coupling of adjacent layers and their electronic properties should be well-controlled, tunable, and scalable. Here, these challenges are addressed by a new approach, which combines molecular beam epitaxy and in situ band engineering in ultra-high vacuum of semiconducting gallium selenide (GaSe) on graphene. In situ studies by electron diffraction, scanning probe microscopy, and angle-resolved photoelectron spectroscopy reveal that atomically-thin layers of GaSe align in the layer plane with the underlying lattice of graphene. The GaSe/graphene heterostructure, referred to as 2semgraphene, features a centrosymmetric (group symmetry D3d) polymorph of GaSe, a charge dipole at the GaSe/graphene interface, and a band structure tunable by the layer thickness. The newly-developed, scalable 2semgraphene is used in optical sensors that exploit the photoactive GaSe layer and the built-in potential at its interface with the graphene channel. This proof of concept has the potential for further advances and device architectures that exploit 2semgraphene as a functional building block.

Strain and Spin-Orbit Coupling Engineering in Twisted WS2/Graphene Heterobilayer

The strain in hybrid van der Waals heterostructures, made of two distinct two-dimensional van der Waals materials, offers an interesting handle on their corresponding electronic band structure. Such strain can be engineered by changing the relative crystallographic orientation between the constitutive monolayers, notably, the angular misorientation, also known as the “twist angle”. By combining angle-resolved photoemission spectroscopy with density functional theory calculations, we investigate here the band structure of the WS₂/graphene heterobilayer for various twist angles. Despite the relatively weak coupling between WS₂ and graphene, we demonstrate that the resulting strain quantitatively affects many electronic features of the WS₂ monolayers, including the spin-orbit coupling strength. In particular, we show that the WS₂ spin-orbit splitting of the valence band maximum at K can be tuned from 430 to 460 meV. Our findings open perspectives in controlling the band dispersion of van der Waals materials.

Nano-optical Imaging of In-Plane Homojunctions in Graphene and MoS2 van der Waals Heterostructures on Talc and SiO2

Understanding the impact of doping variations on the physical properties of two-dimensional materials is important for their application in electronic and optoelectronic devices. Here we report a nano-optical study on graphene and MoS₂ homojunctions by placing these two materials partly on top of a layered talc substrate, partly on top of an SiO₂ substrate. By analyzing the nano-Raman scattering from graphene and the nanophotoluminescense emission from MoS₂, two different doping zones are evident with sub-100 nm wide charge oscillations. The oscillations occur abruptly at the homojuction and extend over longer distances away from the interface, indicating imperfect deposition of the two-dimensional layer on the substrate. These results evidence fine and unexpected details of the homojuctions, important to build better electronic and optoelectronic devices.

Effect of Chemical Vapor Deposition WS2 on Viability and Differentiation of SH-SY5Y Cells

In recent years, transition metal dichalcogenides have been attracting an increasing interest in the biomedical field, thus implying the need of a deeper understanding of their impact on cell behavior. In this study we investigate tungsten disulfide (WS2) grown via chemical vapor deposition (CVD) on a transparent substrate (sapphire) as a platform for neural-like cell culture. We culture SH-SY5Y human neuroblastoma cells on WS2, using graphene, sapphire and standard culture well as controls. The quality, thickness and homogeneity of the materials is analyzed using atomic force microscopy and Raman spectroscopy. The cytocompatibility of CVD WS2 is investigated for the first time by cell viability and differentiation assessment on SH-SY5Y cells. We find that cells differentiated on WS2, displaying a viability and neurite length comparable with the controls. These findings shine light on the possibility of using WS2 as a cytocompatible material for interfacing neural cells.

Synthesis and characterization of WS2/graphene/SiC van der Waals heterostructures via WO3-x thin film sulfurization

Van der Waals heterostructures of monolayer transition metal dichalcogenides (TMDs) and graphene have attracted keen scientific interest due to the complementary properties of the materials, which have wide reaching technological applications. Direct growth of uniform, large area TMDs on graphene substrates by chemical vapor deposition (CVD) is limited by slow lateral growth rates, which result in a tendency for non-uniform multilayer growth. In this work, monolayer and few-layer WS2 was grown on epitaxial graphene on SiC by sulfurization of WO3-x thin films deposited directly onto the substrate. Using this method, WS2 growth was achieved at temperatures as low as 700 °C - significantly less than the temperature required for conventional CVD. Achieving long-range uniformity remains a challenge, but this process could provide a route to synthesize a broad range of TMD/graphene van der Waals heterostructures with novel properties and functionality not accessible by conventional CVD growth.

Direct evidence for efficient ultrafast charge separation in epitaxial WS2/graphene heterostructures

We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to investigate ultrafast charge transfer in an epitaxial heterostructure made of monolayer WS₂ and graphene. This heterostructure combines the benefits of a direct-gap semiconductor with strong spin-orbit coupling and strong light-matter interaction with those of a semimetal hosting massless carriers with extremely high mobility and long spin lifetimes. We find that, after photoexcitation at resonance to the A-exciton in WS₂, the photoexcited holes rapidly transfer into the graphene layer while the photoexcited electrons remain in the WS₂ layer. The resulting charge-separated transient state is found to have a lifetime of ∼1 ps. We attribute our findings to differences in scattering phase space caused by the relative alignment of WS₂ and graphene bands as revealed by high-resolution ARPES. In combination with spin-selective optical excitation, the investigated WS₂/graphene heterostructure might provide a platform for efficient optical spin injection into graphene.

Local photodoping in monolayer MoS2

Inducing electrostatic doping in 2D materials by laser exposure (photodoping effect) is an exciting route to tune optoelectronic phenomena. However, there is a lack of investigation concerning in what respect the action of photodoping in optoelectronic devices is local. Here, we employ scanning photocurrent microscopy (SPCM) techniques to investigate how a permanent photodoping modulates the photocurrent generation in MoS₂ transistors locally. We claim that the photodoping fills the electronic states in MoS₂ conduction band, preventing the photon-absorption and the photocurrent generation by the MoS₂ sheet. Moreover, by comparing the persistent photocurrent (PPC) generation of MoS₂ on top of different substrates, we elucidate that the interface between the material used for the gate and the insulator (gate-insulator interface) is essential for the photodoping generation. Our work gives a step forward to the understanding of the photodoping effect in MoS₂ transistors and the implementation of such an effect in integrated devices.

Electrospun Janus-like pellicle displays coinstantaneous tri-function of aeolotropic conduction, magnetism and luminescence

A new Janus-like pellicle with top-bottom structure, functionalized by conductive aeolotropism, magnetism and luminescence (defined as a CML Janus-like pellicle), is conceived and constructed via electrospinning by combining microcosmic with macroscopic partitions. [PANI/PMMA]//[Eu(BA)₃phen/PMMA] and [Fe₃O₄/PMMA]//[Tb(BA)₃phen/PMMA] Janus-like microribbons are selected as building units to construct a conductive aeolotropism-luminescence layer (CL layer) and magnetism-luminescence layer (ML layer), and the two layers are combined to form a CML Janus-like pellicle. Macroscopic partition is achieved by designing the Janus-like structure of the pellicle, while Janus-like microribbons are used for the microcosmic partition by separating rare earth luminescent compounds from dark-colored magnetic Fe₃O₄ NPs and conductive PANI. The CML Janus-like pellicle has stronger luminescence compared to the contrast samples. The magnetism of the CML Janus-like pellicle can be adjusted by changing the doping amount of Fe₃O₄ NPs. The CML Janus-like pellicle can achieve a strong and variable conductive aeolotropism via changing the doping amount of PANI and the highest conductive aeolotropism ratio can reach ca. 10⁸ times when the PANI content is 70%. Microcosmic and macroscopic partitions are simultaneously integrated into the CML Janus-like pellicle, which results in almost no detrimental mutual influences between the two layers, and the overall performances of the CML Janus-like pellicle are greatly improved.

Janus-like pellicle with conductive aeolotropism, magnetism and luminescence is designed and constructed via electrospinning by combining microcosmic with macroscopic partitions.