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ACS nano 15(6), 9638 - 9648 (2021). doi:10.1021/acsnano.0c10377, The synthesis of large, defect-free two-dimensional materials (2DMs) such as graphene is a major challenge toward industrial applications. Chemical vapor deposition (CVD) on liquid metal catalysts (LMCats) is a recently developed process for the fast synthesis of high-quality single crystals of 2DMs. However, up to now, the lack of in situ techniques enabling direct feedback on the growth has limited our understanding of the process dynamics and primarily led to empirical growth recipes. Thus, an in situ multiscale monitoring of the 2DMs structure, coupled with a real-time control of the growth parameters, is necessary for efficient synthesis. Here we report real-time monitoring of graphene growth on liquid copper (at 1370 K under atmospheric pressure CVD conditions) via four complementary in situ methods: synchrotron X-ray diffraction and reflectivity, Raman spectroscopy, and radiation-mode optical microscopy. This has allowed us to control graphene growth parameters such as shape, dispersion, and the hexagonal supra-organization with very high accuracy. Furthermore, the switch from continuous polycrystalline film to the growth of millimeter-sized defect-free single crystals could also be accomplished. The presented results have far-reaching consequences for studying and tailoring 2D material formation processes on LMCats under CVD growth conditions. Finally, the experimental observations are supported by multiscale modeling that has thrown light into the underlying mechanisms of graphene growth., Published by Soc., Washington, DC

The formation of an ultra-thin aluminum oxide film at Fe0.85Al0.15(110) surface (A2 random alloy) has been studied by a variety of surface sensitive techniques (X-ray photoemission, low-energy electron diffraction, surface X-ray diffraction and scanning tunneling microscopy) supplemented by ab initio atomistic simulations. Since iron is not oxidized in the used conditions, the study focused on the coupling between aluminum oxidation and segregation processes. Compared to the bare surface, whose average composition (Fe0.6Al0.4) is closer to the B2-CsCl structure over a ∼ 3 nm depth, the oxidation hardly affects the subsurface segregation of aluminum. All the structural and chemical fingerprints point to an oxide film similar to that found on NiAl(110). It is a bilayer (∼ 7.5A thick) with a composition close to Al10O13 and a large (18.8 × 10.7)A 2 nearly rectangular unit cell; an almost perfect match between substrate periodicity and the (1 × 2) oxide supercell is found. Nevertheless, microscopy reveals the presence of anti-phase domain boundaries. Measured Al 2p and O 1s core level shifts match calculated ones; their origin and the relative contributions of initial/final state effects are discussed. The ubiquity of the present oxide on different supports asks for the origin of its stability.

Intercalation of alkali atoms within the lamellar transition metal dichalcogenides is a possible route toward a new generation of batteries. It is also a way to induce structural phase transitions authorizing the realization of optical and electrical switches in this class of materials. The process of intercalation has been mostly studied in three-dimensional dichalcogenide films. Here, we address the case of a single-layer of molybdenum disulfide (MoS$_2$), deposited on a gold substrate, and intercalated with cesium (Cs) in ultra-clean conditions (ultrahigh vacuum). We show that intercalation decouples MoS$_2$ from its substrate. We reveal electron transfer from Cs to MoS$_2$, relative changes in the energy of the valence band maxima, and electronic disorder induced by structural disorder in the intercalated Cs layer. Besides, we find an abnormal lattice expansion of MoS$_2$, which we relate to immediate vicinity of Cs. Intercalation is thermally activated, and so is the reverse process of de-intercalation. Our work opens the route to a microscopic understanding of a process of relevance in several possible future technologies, and shows a way to manipulate the properties of two-dimensional dichalcogenides by "under-cover" functionalization., Comment: 6 figures

Single and few layers of the two-dimensional (2D) semimetal ZrTe2 are grown by molecular beam epitaxy on InAs(111)/Si(111) substrates. Excellent rotational commensurability, van der Waals gap at the interface and moire pattern are observed indicating good registry between the ZrTe2 epilayer and the substrate through weak van der Waals forces. The electronic band structure imaged by angle resolved photoelectron spectroscopy shows that valence and conduction bands cross at the Fermi level exhibiting abrupt linear dispersions. The latter indicates massless Dirac Fermions which are maintained down to the 2D limit suggesting that single-layer ZrTe2 could be considered as the electronic analogue of graphene.

Liquid metal catalysts (LMCats) (e.g., molten copper) can provide a new mass-production method for two-dimensional materials (2DMs) (e.g., graphene) with significantly higher quality and speed and lower energy and material consumption. To reach such technological excellence, the physicochemical properties of LMCats and the growth mechanisms of 2DMs on LMCats should be investigated. Here, we report the development of a chemical vapor deposition (CVD) reactor which allows the investigation of ongoing chemical reactions on the surface of a molten metal at elevated temperatures and under reactive conditions. The surface of the molten metal is monitored simultaneously using synchrotron x-ray scattering, Raman spectroscopy, and optical microscopy, thereby providing complementary information about the atomic structure and chemical state of the surface. To enable in situ characterization on a molten substrate at high temperatures (e.g., similar to 1370 K for copper), the optical and x-ray windows need to be protected from the evaporating LMCat, reaction products, and intense heat. This has been achieved by creating specific gas-flow patterns inside the reactor. The optimized design of the reactor has been achieved using multiphysics COMSOL simulations, which take into account the heat transfer, fluid dynamics, and transport of LMCat vapor inside the reactor. The setup has been successfully tested and is currently used to investigate the CVD growth of graphene on the surface of molten copper under pressures ranging from medium vacuum up to atmospheric pressure. Published under license by AIP Publishing.

We investigate by in situ X-ray diffraction the early stage of GaN nanowire growth on a single layer of polycrystalline graphene transferred onto a Si(100)/SiO2 carrier substrate. The experiment wa...

Epitaxial SnTe (111) is grown by molecular-beam epitaxy on $\mathrm{B}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{3}$ substrates. Structural evaluation indicates that SnTe deviates from cubic due to in-plane compressive strain, which induces significant changes in the electronic band structure. More specifically, a pair of gapless crossings between the two uppermost valence bands occurs in $k$ space along the out-of-plane \ensuremath{\Gamma}Z direction of the Brillouin zone, associated with a band inversion, thus defining topological three-dimensional Dirac nodes. Combined first-principles calculations and angle-resolved photoelectron spectroscopy reveal an overtilted Dirac cone indicating that the crossing is a topological type-III Dirac node at the borders between type-I and type-II Dirac nodes.

The moir\'e pattern created by the epitaxy of a graphene sheet on an iridium substrate can be used as a template for the growth of 2D atomic or cluster arrays. We observed for the first time a coherent organization of hard magnetic preformed FePt nanoparticles on the 2D lattice of graphene on Ir(111). Nanoparticles of 2 nm diameter have been mass selected in a gas phase and deposited with low energy on the hexagonal moir\'e pattern. Their morphology and organization have been investigated using grazing incidence small angle x-ray scattering, while their magnetic properties have been studied by x-ray magnetic circular dichroism, both pointing to a FePt cluster-graphene surface specific interaction. The spatial coherence of the nanoparticles is preserved upon annealing up to $700\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ where the hard magnetic phase of FePt is obtained.

International audience; Despite a large number of studies [2,3] over the years since the first discovery [7] and a couple of comprehensive reviews [8,9] the actual mechanism for PLD/CDW formation is still under debate. The most recent experimental [10-13] and theoretical [14] works focus on the large area growth of the CDW phase [13] the thickness dependence , and the possible unconventional behavior in the ultimate 2D limit of a single layer TiSe 2. [10-12,14] On the other hand, the other Ti dichalcogenides namely TiS 2 and TiTe 2 did not show any clear evidence until very recently when a CDW state was reported only for 1 monolayer (ML)-thin TiTe 2 at temperatures lower than 92 K. [15] It is surprising that the CDW in TiTe 2 was found to be totally suppressed for films thicker than 1 ML, [15] unlike the case of other TMDs where 1 ML and bulk-like films both make the transition to a CDW at nearly the same temperature. The interest about TiTe 2 is continuously increasing in view of theoretical predictions [16] and more recent experimental evidence [17] about pressure induced topological phase transitions in TiTe 2. The possibility to also manipulate superconduc-tivity by external pressure as predicted [18] and more recently evidenced [19] in bulk TiTe 2 creates the prospect to explore the emergence of topological superconductivity in this material. In the latter work [19] it has been shown that under nonhydro-static pressure, a CDW-like state with estimated transition temperature above room temperature (RT) appears in bulk TiTe 2 at around 0.5-1.8 GPa. These results call for a re-examination of the possibility to obtain a CDW in multilayer TiTe 2 and indeed at RT with good potential for real world applications utilizing the properties of the CDW state. These applications include a voltage-controlled oscillator device operating at room temperature , [20] fast electronic resistance switching for nonvolatile memories, [21,22] and field-effect transistor devices potentially suitable for implementation of non-Boolean logic. [23] In this paper it is shown that multilayer films (50 ML ≈ 32 nm), as well as single layer TiTe 2 epitaxially grown on InAs(111)/ Si(111) substrates by molecular beam epitaxy exhibit, in ambient pressure conditions, a CDW distortion at room temperature which is sustained up to higher temperatures, at least 400 °C, as evidenced by reflection high energy electron diffrac-tion (RHEED) (Figure S1, Supporting Information). The results are explained in terms of anisotropic strain imposed by the substrate. The group IVB 2D transition metal dichalcogenides are considered to be stable in the high symmetry trigonal octahedral structure due to the lack of unpaired d-electrons on the metal site. It is found that multilayer epitaxial TiTe 2 is an exception adopting a commensurate 2 × 2 × 2 charge density wave (CDW) structure at room temperature with an ABA type of stacking as evidenced by direct lattice imaging and reciprocal space mapping. The CDW is stabilized by highly anisotropic strain imposed by the substrate with an out-off-plane compression which reduces the interlayer van der Waals gap increasing the coupling between TiTe 2 layers.

Based on previous photoemission findings, the influence of newly observed aluminum surface segregation at the low (110), (100) and (111) index surfaces of the A 2 body-centered Fe 0.85 Al 0.15 random alloy is explored. Scanning Tun-neling Microscopy, Low-Energy Electron Diffraction and Grazing-Incidence X-Ray Diffraction are combined to explore surface structures and topogra-phies. The formation of a surface composition close to the B 2 Fe 0.5 Al 0.5 occurs with the appearance of (i) a long-range pseudo-hexagonal incommen-surate superstructure with a periodicity of ∼ 18A on the (110) surface, (ii) a (1 × 1) termination on the (100) surface and (iii) a intense vicinal faceting of the (111) surface in the form of triangular pits.

International audience; Van der Waals heterojunctions composed of graphene and transition metal dichalcogenides have gain much attention because of the possibility to control and tailor band structure, promising applications in two-dimensional optoelectronics and electronics. In this report, we characterized the van der Waals heterojunction MoSe$_2$/few-layer graphene with a high-quality interface using cutting-edge surface techniques scaling from atomic to microscopic range. These surface analyses gave us a complete picture of the atomic structure and electronic properties of the heterojunction. In particular, we found two important results: the commensurability between the MoSe$_2$ and few-layer graphene lattices and a band-gap opening in the few-layer graphene. The band gap is as large as 250 meV, and we ascribed it to an interface charge transfer that results in an electronic depletion in the few-layer graphene. This conclusion is well supported by electron spectroscopy data and density functional theory calculations. The commensurability between the MoSe$_2$ and graphene lattices as well as the band-gap opening clearly show that the interlayer interaction goes beyond the simple van der Waals interaction. Hence, stacking two-dimensional materials in van der Waals heterojunctions enables us to tailor the atomic and electronic properties of individual layers. It also permits the introduction of a band gap in few-layer graphene by interface charge transfer.

Van der Waals heterojunctions composed of graphene and transition metal dichalcogenides have gain much attention because of the possibility to control and tailor band structure, promising applications in two-dimensional optoelectronics and electronics. In this report, we characterized the van der Waals heterojunction MoSe

Single and few layers of the two-dimensional (2D) semimetal ZrTe

International audience; Author(s) of this paper may load this reprint on their own web site or institutional repository provided that this cover page is retained. Republication of this article or its storage in electronic databases other than as specified above is not permitted without prior permission in writing from the IUCr. For further information see http://journals.iucr.org/services/authorrights.html Many research topics in condensed matter research, materials science and the life sciences make use of crystallographic methods to study crystalline and non-crystalline matter with neutrons, X-rays and electrons. Articles published in the Journal of Applied Crystallography focus on these methods and their use in identifying structural and diffusion-controlled phase transformations, structure-property relationships, structural changes of defects, interfaces and surfaces, etc. Developments of instrumentation and crystallographic apparatus, theory and interpretation, numerical analysis and other related subjects are also covered. The journal is the primary place where crystallographic computer program information is published. This article proposes two integration methods to determine the structure factors along a surface diffraction rod measured with a two-dimensional detector. The first method applies the classic way of calculating integrated intensities in angular space. This is adapted to work efficiently with two-dimensional data. The second method is based on integration in reciprocal space. An intensity map is created by converting the detected intensity pixel by pixel to the reciprocal space. The integration is then performed directly on this map. A theoretical framework, as well as a comparison between the two integration methods, is provided.

La diffusion/diffraction et l’absorption des rayons X issus d’une source synchrotron sont des techniques tres puissantes d’etude de la structure, de la morphologie et de la composition de surfaces, d’interfaces et de nano-objets, en particulier in situ , durant leur elaboration, ou in operando .Nous les illustrons par l’exemple de la croissance de germanium sur des surfaces (001) de silicium monocristallin, ou apparaissent successivement une couche bidimensionnelle contrainte presentant un arrangement complexe des atomes de surface, puis differentes nanostructures facettees qui relaxent progressivement leur parametre de maille via une interdiffusion avec le substrat.

Supported platinum nanoparticles in the presence of carbon monoxide in the 10–6–103 Pa CO pressure range were studied by grazing incidence small-angle X-ray scattering. Disruption and aggregation of platinum nanoparticles upon CO exposure are shown to occur in parallel. Particles smaller than a critical size of 1 nm undergo scavenging after disruption in the form of carbonyls at CO pressure as low as 10–1 Pa. Mobile carbonyls then agglomerate to larger clusters. Upon annealing, a CO-driven diffusion and agglomeration of clusters of size ranging between the critical size and 2 nm is observed. These phenomena are discussed in relation to the CO–metal, metal–metal, and metal–substrate relative bond strengths. Similar mechanisms are suggested to hold for other supported metal catalysts.

The most accessible way to grow semiconductor nanowires remains so far the use of gold nanoclusters on a (1 1 1) semiconductor surface, for instance Si(1 1 1). Precise knowledge of the morphology of gold nanoparticles obtained by growth or dewetting on Si(1 1 1) is essential to better control the growth of nanowires. Both formation routes were investigated in situ by grazing incidence small-angle x-ray scattering. During the growth at 300 °C, Au atoms first nucleate on the wetting layer and give rise to islands having the shape of truncated spheres with an average contact angle of 38° for deposits of 1.4 and 2 monolayers. Dewetting is obtained by annealing of a 2 monolayer room temperature deposit, leading to the formation of particles. The average contact angle increases from 53° to 73° from 270 °C to 360 °C. Moreover, an analysis of the island height suggests that an AuSi alloy with a thickness of approximately 1.5 nm could be present at the top of the monocrystalline islands in the solid state before the eutectic temperature. The liquid–solid interface tension of the AuSi alloy droplet (1.0 J m−2) and the work of adhesion (1.1 J m−2) are deduced at the eutectic temperature.

The growth of Ge islands on a pit-patterned Si(001) template is investigated in situ, combining grazing incidence diffraction, multiple wavelength anomalous diffraction, and small angle scattering. This allows monitoring in situ the detailed structural and morphological evolutions of the pits, of the wetting-layer and of the nucleated islands on the pit-patterned Si(001) substrate. It is shown that after Si regrowth, the Si substrate displays {107} and {1 1 11} facets. During the very first stages of Ge growth, the preliminary facets of the Si substrate are energetically unfavourable, and the pit facets break up into a rather complex pattern of {10n} and {11m} facets with n > 7 and m > 11. At 5 and 6 ML, intensity rods from {105} and {113}-type facets appear in the GISAXS images revealing the formation of pyramids and domes, respectively. The degree of ordering, the shape, strain and composition of the islands are characterized during the growth process to provide a detailed evolution of their structure and morphology.

International audience; The direct observation at room temperature (RT) of the noncentrosym-metric orthorhombic topological Weyl semimetal phase in epitaxial thin films of MoTe 2 grown on InAs(111)/Si(111) substrates by molecular beam epitaxy (MBE) is reported. The orthorhombic phase is typically found at lower temperatures but its observation at RT in this work is attributed to the enlarged lattice parameters, influenced by the substrate, which stabilize an interlayer antibonding state compatible with the orthorhombic stacking. First-principles calculations predict eight type II Weyl nodes which are located below (but near) the Fermi energy making them accessible to charge transport and creating the prospect for practical applications exploiting the nontrivial topological properties. The orthorhombic phase coexists with an unconventional triclinic layer stacking which is different than the monoclinic or orthorhombic structures but it is centrosymmetric and topologically trivial.

A large number of 2D Transition Metal Dichalcogenide (TMD) materials on micron-sized flakes exfoliated from bulk have been studied extensively showing exciting new physical properties and good prospects to impact a wide range of applications [1]. On the other hand, large area growth on suitable substrates is urgently needed in order to make the transition from lab-based research to real world applications. Molecular beam epitaxy (MBE) is traditionally used to achieve van der Waals epitaxy as first conceptualized by Koma and co-workers[2]. The epitaxial growth of single or few layer 2D TMDs is very challenging because it depends critically on the substrate and surface preparation methodology. Although Si as one of the most technologically important substrates is preferred for TMD growth, the film quality is not adequate. Engineered substrates such as epi-AlN(0001)/Si and epi-InAs(111)/Si available in large wafer sizes (200 and 300 mm) could be good candidates in the effort to improve the thin film quality. In this work, we will first review our work on MoSe2 [3], HfSe2 [4] WSe2[5], SnSe2[5] and HfTe2 [6] and their van der Waals heterostructures [5,7] grown by MBE on AlN/Si focusing on the microstructure, thin film morphology and homogeneity. Then we will show more recent results on metallic or semimetallic telluride TMDs (MoTe2, TiTe2, ZrTe2, HfTe2) grown on InAs/Si substrates with much improved epitaxial quality. Using synchrotron XRD, high resolution STEM and in-situ STM, excellent rotational commensurability, quasi van der Waals gap at the interface and Moire pattern are observed indicating good registry between the 2D TMD epilayer and the substrate through weak van der Waals forces. We will show that MBE grown 1ML MoTe2 on InAs(111) adopts the metastable distorted 1T’ crystal structure which is metallic and predicted to be a 2D tolological insulator (or Quantum Spin Hall state). Using in-situ ARPES complemented by DFT calculations we show that the valence and conduction bands of few layer HfTe2 [6] and ZrTe2 cross at the Fermi level exhibiting abrupt linear dispersions at the zone center. The latter indicates massles Dirac fermions which are maintained down to the 2D limit suggesting that single layers of epitaxial HfTe2 and ZrTe2 could be considered as the electronic analogue of graphene. Finally, we show that in 1 ML TiTe2 it is possible to substitute the top Te row by Se to form the ordered Janus ternary 2D material Te-Ti-Se similar to S-Mo-Se material recently reported [8,9]. These materials are predicted to have exiting new physical properties and are expected to open a new field in the wide area of 2D materials and devices. Acknowledgements: This work is financially supported by the LANEF Chair of Excellence program of U. Grenoble Alpes and CEA (A.D.), the Greek State Scholarships Foundation (IKY) Program for the strengthening of postdoctoral research (P.T.), The FLAG-ERA project MELoDICA (A.D.) and the French state funds ANR-10-LABX-51-01 (Labex LANEF du Programme d'Investissements d'Avenir) and Equipex ANR-11-EQPX-0010 (G.R.) References: [1] W. Choi, et al., Materials Today, 20(3), 116-130, (2017); [2] A. Koma, J. Cryst. Growth, 201/202, 236-241 (1999); [3] E. Xenogiannopoulou, et al., Nanoscale, 7, 7896-7905, (2015); [4] K. E. Aretouli, et al., Appl. Phys. Lett. 106, 143105 (2015) ; [5] K. E. Aretouli et al., ACS Appl. Mater. Interfaces 8, 23222-23229 (2016); [6] S. A. Giamini et al., 2D Mater. 4, 015001 (2017); [7] D. Tsoutsou, et al., ACS Appl. Mater. Interfaces 8, 1836-1841 (2016).

We investigate the morphology of islands obtained by epitaxial growth of Ge on Si(0 0 1). During a low growth rate or during annealing and depending of the quench method, we observe the formation of a new type of superdomes, which exhibit additional {15 3 23} facets, an increase of their {0 0 1} and {1 0 5} facet area on their top and a smaller aspect ratio compared to known superdomes. This flat-top superdome shape results from the complex evolution of domes and superdomes: coalescence, intermixing and ripening. Dome or superdome coalescence which results in the newly observed shape appears as a dominant pathway towards dislocation nucleation for the investigated temperature range.