Patrick Odenthal

2-Dimensional Transition Metal Dichalcogenides with Tunable Direct Band Gaps: MoS2(1–x)Se2x Monolayers

MoS2(1-x) Se2x single-layer films are prepared using a mixture of organic selenium and sulfur precursors as well as a solid molybdenum source. The direct bandgaps are found to scale nearly linearly with composition in the range of 1.87 eV (pure single-layer MoS2 ) to 1.55 eV (pure single-layer MoSe2 ) permitting straightforward bandgap engineering.

Control of Schottky Barriers in Single Layer MoS2 Transistors with Ferromagnetic Contacts

MoS2 and related metal dichalcogenides (MoSe2, WS2, WSe2) are layered two-dimensional materials that are promising for nanoelectronics and spintronics. For instance, large spin-orbit coupling and spin splitting in the valence band of single layer (SL) MoS2 could lead to enhanced spin lifetimes and large spin Hall angles. Understanding the nature of the contacts is a critical first step for realizing spin injection and spin transport in MoS2. Here, we have investigated Co contacts to SL MoS2 and find that the Schottky barrier height can be significantly decreased with the addition of a thin oxide barrier (MgO). Further, we show that the barrier height can be reduced to zero by tuning the carrier density with back gate. Therefore, the MgO could simultaneously provide a tunnel barrier to alleviate conductance mismatch while minimizing carrier depletion near the contacts. Such control over the barrier height should allow for careful engineering of the contacts to realize spin injection in these materials.

Integration of the Ferromagnetic Insulator EuO onto Graphene

We have demonstrated the deposition of EuO films on graphene by reactive molecular beam epitaxy in a special adsorption-controlled and oxygen-limited regime, which is a critical advance toward the realization of the exchange proximity interaction (EPI). It has been predicted that when the ferromagnetic insulator (FMI) EuO is brought into contact with graphene, an overlap of electronic wave functions at the FMI/graphene interface can induce a large spin splitting inside the graphene. Experimental realization of this effect could lead to new routes for spin manipulation, which is a necessary requirement for a functional spin transistor. Furthermore, EPI could lead to novel spintronic behavior such as controllable magnetoresistance, gate tunable exchange bias, and quantized anomalous Hall effect. However, experimentally, EuO has not yet been integrated onto graphene. Here we report the successful growth of high-quality crystalline EuO on highly oriented pyrolytic graphite and single-layer graphene. The epitaxial EuO layers have (001) orientation and do not induce an observable D peak (defect) in the Raman spectra. Magneto-optic measurements indicate ferromagnetism with a Curie temperature of 69 K, which is the value for bulk EuO. Transport measurements on exfoliated graphene before and after EuO deposition indicate only a slight decrease in mobility.

Controlled argon beam-induced desulfurization of monolayer molybdenum disulfide

Sputtering of MoS2 films of single-layer thickness by low-energy argon ions selectively reduces the sulfur content of the material without significant depletion of molybdenum. X-ray photoelectron spectroscopy shows little modification of the Mo 3d states during this process, suggesting the absence of significant reorganization or damage to the overall structure of the MoS2 film. Accompanying ab initio molecular dynamics simulations find clusters of sulfur vacancies in the top plane of single-layer MoS2 to be structurally stable. Measurements of the photoluminescence at temperatures between 175 and 300 K show quenching of almost 80% for an ~10% decrease in sulfur content.

Postgrowth Tuning of the Bandgap of Single-Layer Molybdenum Disulfide Films by Sulfur/Selenium Exchange

We demonstrate bandgap tuning of a single-layer MoS2 film on SiO2/Si via substitution of its sulfur atoms by selenium through a process of gentle sputtering, exposure to a selenium precursor, and annealing. We characterize the substitution process both for S/S and S/Se replacement. Photoluminescence and, in the latter case, X-ray photoelectron spectroscopy provide direct evidence of optical band gap shift and selenium incorporation, respectively. We discuss our experimental observations, including the limit of the achievable bandgap shift, in terms of the role of stress in the film as elucidated by computational studies, based on density functional theory. The resultant films are stable in vacuum, but deteriorate under optical excitation in air.

Large area epitaxial germanane for electronic devices

We report the synthesis and transfer of epitaxial germanane (GeH) onto arbitrary substrates by electrochemical delamination and investigate its optoelectronic properties. GeH films with thickness ranging from 1 to 600 nm (2–1000 layers) and areas up to ~1 cm2 have been reliably transferred and characterized by photoluminescence, x-ray diffraction, and energy-dispersive x-ray spectroscopy. Wavelength dependent photoconductivity measurements on few-layer GeH exhibit an absorption edge and provide a sensitive characterization tool for ultrathin germanane materials. The transfer process also enables the possibility of integrating germanane into vertically stacked heterostructures.

Epitaxial Co-Deposition Growth of CaGe

Two-dimensional crystals are an important class of materials for novel physics, chemistry, and engineering. Germanane (GeH), the germanium-based analogue of graphane (CH), is of particular interest due to its direct band gap and spin-orbit coupling. Here, we report the successful co-deposition growth of CaGe2 films on Ge(111) substrates by molecular beam epitaxy (MBE) and their subsequent conversion to germanane by immersion in hydrochloric acid. We find that the growth of CaGe2 occurs within an adsorption-limited growth regime, which ensures stoichiometry of the film. We utilize in situ reflection high energy electron diffraction (RHEED) to explore the growth temperature window and find the best RHEED patterns at 750 {\deg}C. Finally, the CaGe2 films are immersed in hydrochloric acid to convert the films to germanane. Auger electron spectroscopy of the resulting film indicates the removal of Ca and RHEED patterns indicate a single-crystal film with in-plane orientation dictated by the underlying Ge(111) substrate. X-ray diffraction and Raman spectroscopy indicate that the resulting films are indeed germanane. Ex situ atomic force microscopy (AFM) shows that the grain size of the germanane is on the order of a few micrometers, being primarily limited by terraces induced by the miscut of the Ge substrate. Thus, optimization of the substrate could lead to the long-term goal of large area germanane films.

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Magnetism in graphene is an emerging field that has received much theoretical attention. In particular, there have been exciting predictions for induced magnetism through proximity to a ferromagnetic insulator as well as through localized dopants and defects. Here, the authors discuss their experimental work using molecular beam epitaxy to modify the surface of graphene and induce novel spin-dependent phenomena. First, they investigate the epitaxial growth of the ferromagnetic insulator EuO on graphene and discuss possible scenarios for realizing exchange splitting and exchange fields by ferromagnetic insulators. Second, they investigate the properties of magnetic moments in graphene originating from localized pz -orbital defects (i.e., adsorbed hydrogen atoms). The behavior of these magnetic moments is studied using nonlocal spin transport to directly probe the spin-degree of freedom of the defect-induced states. They also report the presence of enhanced electron g-factors caused by the exchange fields present in the system. Importantly, the exchange field is found to be highly gate dependent, with decreasing g-factors with increasing carrier densities.

Films by Molecular Beam Epitaxy for Large Area Germanane

We have systematically introduced charged impurity scatterers in the form of Mg adsorbates to exfoliated single-layer graphene and observe little variation of the spin relaxation times despite pronounced changes in the charge transport behavior. All measurements are performed on nonlocal graphene tunneling spin valves exposed in situ to Mg adatoms, thus systematically introducing atomic-scale charged impurity scattering. While charge transport properties exhibit decreased mobility and decreased momentum scattering times, the observed spin lifetimes are not significantly affected, indicating that charged impurity scattering is inconsequential in the present regime of spin relaxation times (

Integrating MBE materials with graphene to induce novel spin-based phenomena

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