Two-Way Twisting of a Confined Monolayer: Orientational Ordering within the van der Waals Gap between Graphene and Its Crystalline Substrate.

Two-dimensional confinement of lattices produces a variety of order and disorder phenomena. When the confining walls have atomic granularity, unique structural phases are expected, of relevance in nanotribology, porous materials, or intercalation compounds where, e.g., electronic states can emerge accordingly. The interlayer's own order is frustrated by the competing interactions exerted by the two confining surfaces. We revisit the concept of orientational ordering, introduced by Novaco and McTague to describe the twist of incommensurate monolayers on crystalline surfaces. We predict a two-way twist of the monolayer as its density increases. We discover such a behavior in alkali atom monolayers (sodium, cesium) confined between graphene and an iridium surface, using scanning tunneling microscopy and electron diffraction.

Nano-sheets of two-dimensional polymers with dinuclear (arene)ruthenium nodes, synthesised at a liquid/liquid interface.

We developed a new class of mono- or few-layered two-dimensional polymers based on dinuclear (arene)ruthenium nodes, obtained by combining the imine condensation with an interfacial chemistry process, and use a modified Langmuir-Schaefer method to transfer them onto solid surfaces. Robust nano-sheets of two-dimensional polymers including dinuclear complexes of heavy ruthenium atoms as nodes were synthesised. These nano-sheets, whose thickness is of a few tens of nanometers, were suspended onto solid porous membranes. Then, they were thoroughly characterised with a combination of local probes, including Raman spectroscopy, Fourier transform infrared spectroscopy and transmission electron microscopy in imaging and diffraction mode.

Mechanical Exfoliation of Select MAX Phases and Mo4 Ce4 Al7 C3 Single Crystals to Produce MAXenes.

MXenes-2D carbides/nitrides derived from their bulk nanolamellar Mn +1 AXn phase (MAX) counterparts-are, for the most part, obtained by chemical etching. Despite the fact that the MA bonds in the MAX phases are not weak, in this work it is demonstrated that relatively large MAX single crystals can be mechanically exfoliated using the adhesive tape method to produce flakes whose thickness can be reduced down to half a unit cell. The exfoliated flakes, transferred onto SiO2 /Si substrates, are analyzed using electric force microscopy (EFM). No appreciable variation in EFM signal with flake thickness is found. EFM contrast between the flakes and SiO2 not only depends on the contact surface potential, but also on the local capacitance. The contribution of the latter can be used to show the metallic character-confirmed by four-contact resistivity measurements-of even the thinnest of flakes. Because the A-layers are preserved, strictly speaking MXenes are not dealt with in this work, but rather MAXenes. This is important in the case where the "A" layers contain magnetic elements such as Mo4 Ce4 Al7 C3 , whose structure is a derivative of the MAX structure.

Temperature-Controlled Rotational Epitaxy of Graphene.

When graphene is placed on a crystalline surface, the periodic structures within the layers superimpose and moiré superlattices form. Small lattice rotations between the two materials in contact strongly modify the moiré lattice parameter, upon which many electronic, vibrational, and chemical properties depend. While precise adjustment of the relative orientation in the degree- and sub-degree-range can be achieved via careful deterministic transfer of graphene, we report on the spontaneous reorientation of graphene on a metallic substrate, Ir(111). We find that selecting a substrate temperature between 1530 and 1000 K during the growth of graphene leads to distinct relative rotational angles of 0°, ± 0.6°, ±1.1°, and ±1.7°. When modeling the moiré superlattices as two-dimensional coincidence networks, we can ascribe the observed rotations to favorable low-strain graphene structures. The dissimilar thermal expansion of the substrate and graphene is regarded as an effective compressive biaxial pressure that is more easily accommodated in graphene by small rotations rather than by compression.

Size-Selective Carbon Clusters as Obstacles to Graphene Growth on a Metal.

Chemical vapor deposition (CVD) on metals is so far the best suited method to produce high-quality, large-area graphene. We discovered an unprecedentedly large family of small size-selective carbon clusters that form together with graphene during CVD. Using scanning tunneling microscopy (STM) and density functional theory (DFT), we unambiguously determine their atomic structure. For that purpose, we use grids based on a graphene moiré and a dilute atomic lattice that unambiguously reveal the binding geometry of the clusters. We find that the observed clusters bind in metastable configurations on the substrate, while the thermodynamically stable configurations are not observed. We argue that the clusters are formed under kinetic control and establish that the evolution of the smallest clusters is blocked. They are hence products of surface reactions in competition with graphene growth, rather than intermediary species to the formation of extended graphene, as often assumed in the literature. We expect such obstacles to the synthesis of perfect graphene to be ubiquitous on a variety of metallic surfaces.

Soluble Two-Dimensional Covalent Organometallic Polymers by (Arene)Ruthenium-Sulfur Chemistry.

A class of two-dimensional (2D) covalent organometallic polymers, with nanometer-scale crosslinking, was obtained by arene(ruthenium) sulfur chemistry. Their ambivalent nature, with positively charged crosslinks and lypophylic branches is the key to the often sought-for and usually hard-to-achieve solubility of 2D polymers in various kinds of solvents. Solubility is here controlled by the planarity of the polymer, which in turn controls Coulomb interactions between the polymer layers. High planarity is achieved for high symmetry crosslinks and short, rigid branches. Owing to their solubility, the polymers are easily processable, and can be handled as powder, deposited on surfaces by mere spin-coating, or suspended across membranes by drop-casting. The novel 2D materials are potential candidates as flexible membranes for catalysis, cancer therapy, and electronics.

Anatomy and Giant Enhancement of the Perpendicular Magnetic Anisotropy of Cobalt-Graphene Heterostructures.

We report strongly enhanced perpendicular magnetic anisotropy (PMA) of Co films by graphene coating from both first-principles and experiments. Our calculations show that graphene can dramatically boost the surface anisotropy of Co films up to twice the value of its pristine counterpart and can extend the out-of-plane effective anisotropy up to unprecedented thickness of 25 Å. These findings are supported by our experiments on graphene coating on Co films grown on Ir substrate. Furthermore, we report layer-resolved and orbital-hybridization-resolved anisotropy analysis, which help understanding of the physical mechanisms of PMA and more practically can help design structures with giant PMA. As an example, we propose superexchange stabilized Co-graphene heterostructures with a robust constant effective PMA and linearly increasing interfacial anisotropy as a function of film thickness. These findings point toward possibilities to engineer graphene/ferromagnetic metal heterostructures with giant magnetic anisotropy more than 20-times larger compared to conventional multilayers, which constitutes a hallmark for future graphene and traditional spintronic technologies.

Strain Relaxation in CVD Graphene: Wrinkling with Shear Lag.

We measure uniaxial strain fields in the vicinity of edges and wrinkles in graphene prepared by chemical vapor deposition (CVD), by combining microscopy techniques and local vibrational characterization. These strain fields have magnitudes of several tenths of a percent and extend across micrometer distances. The nonlinear shear-lag model remarkably captures these strain fields in terms of the graphene-substrate interaction and provides a complete understanding of strain-relieving wrinkles in graphene for any level of graphene-substrate coherency.

Functional hybrid systems based on large-area high-quality graphene.

The properties of sp² carbon allotropes can be tuned and enriched by their interaction with other materials. The large interface to the outside world in these forms of carbon is ideally suited for combining in an optimal manner several functionalities thanks to this interaction. A wide range of novel materials holding strong promise in energy, optoelectronics, microelectronics, mechanics, or medical applications have been designed accordingly. Graphene, the last representative of this family of sp² carbon materials, has already yielded a wealth of hybrid systems. A new class of these hybrids is emerging, which allows researchers to exploit the properties of truly single-layer graphene. These systems rely on high-quality graphene. In this Account, we describe our recent efforts to develop hybrid systems through various approaches and with various scopes. Depending on the interaction between graphene and molecules, metal clusters, layers, and substrates, either graphene may essentially preserve the electronic properties that make it a unique platform for electronic transport, or new organization and properties in the materials may arise due to the graphene contact at the expense of deep modification of graphene's properties. We prepare our graphene samples by both mechanical exfoliation of graphite and chemical vapor deposition on metals. We use this to study graphene in contact with various species, which either decorate graphene or are intercalated between it and its substrate. We first address the electronic and magnetic properties in systems where graphene is in epitaxy with a metal and discuss the potential to manipulate the properties of both materials, highlighting graphene's role as a protective capping layer in magnetic functional systems. We then present graphene/metal dot hybrids, which can utilize the two-dimensional gas properties of Dirac fermions in graphene. These hybrids allow one to tune the coupling between clusters hosting electronically ordered states such as superconductivity and explore quantum phase transitions controlled by electrostatic back gates. We finally discuss the optical properties of hybrids in which graphene is decorated with optically active molecules. Depending on how close these molecules are to the graphene's electromechanical systems, the interaction of the system with light can be changed. Fields such as spintronics and catalysis could benefit from high-quality graphene based hybrid systems, which have not been fully explored.

Strains induced by point defects in graphene on a metal.

Strains strongly affect the properties of low-dimensional materials, such as graphene. By combining in situ, in operando, reflection high-energy electron diffraction experiments with first-principles calculations, we show that large strains, above 2%, are present in graphene during its growth by chemical vapor deposition on Ir(111) and when it is subjected to oxygen etching and ion bombardment. Our results unravel the microscopic relationship between point defects and strains in epitaxial graphene and suggest new avenues for graphene nanostructuring and engineering its properties through introduction of defects and intercalation of atoms and molecules between graphene and its metal substrate.

Interplay of wrinkles, strain, and lattice parameter in graphene on iridium.

Following graphene growth by thermal decomposition of ethylene on Ir(111) at high temperatures we analyzed the strain state and the wrinkle formation kinetics as function of temperature. Using the moiré spot separation in a low energy electron diffraction pattern as a magnifying mechanism for the difference in the lattice parameters between Ir and graphene, we achieved an unrivaled relative precision of ±0.1 pm for the graphene lattice parameter. Our data reveals a characteristic hysteresis of the graphene lattice parameter that is explained by the interplay of reversible wrinkle formation and film strain. We show that graphene on Ir(111) always exhibits residual compressive strain at room temperature. Our results provide important guidelines for strategies to avoid wrinkling.

Superstructures, Commensurations, and Rotation of Single-Layer TaS2 on Au(111) Induced by Cs Intercalation/Deintercalation.

We use in situ synchrotron grazing incidence X-ray diffraction and X-ray reflectivity to investigate with high resolution the structure of a two-dimensional single layer of tantalum sulfide grown on a Au(111) surface and its evolution during intercalation by Cs atoms and deintercalation, which decouples and recouples the two materials, respectively. The grown single layer consists of a mixture of TaS2 and its S-depleted version, TaS, both aligned with gold, and forming moirés where 7 (respectively 13) lattice constants of the 2D layer almost perfectly match 8 (respectively 15) substrate lattice constants. Intercalation fully decouples the system by lifting the single layer by ∼370 pm and induces an increase of its lattice parameter by 1-2 picometers. The system gradually evolves, during cycles of intercalation/deintercalation assisted by an H2S atmosphere, toward a final coupled state consisting of the fully stoichiometric TaS2 dichalcogenide whose moiré is found very close to the 7/8 commensurability. The reactive H2S atmosphere appears necessary to achieve full deintercalation, presumably by preventing S depletion and the concomitant strong bonding with the intercalant. The structural quality of the layer improves during the cyclic treatment. In parallel, because they are decoupled from the substrate by the intercalation of cesium, some of the TaS2 flakes rotate by 30°. These produce two additional superlattices with characteristic diffraction patterns of different origins. The first is aligned with gold's high symmetry crystallographic directions and is a commensurate moiré ((6 × 6)-Au(111) coinciding with (3√3 × 3√3)R30°-TaS2). The second is incommensurate and corresponds to a near coincidence of (6 × 6) unit cells of 30°-rotated TaS2 with (4√3 × 4√3)Au(111) surface ones. This structure, which is less coupled to gold, might be related to the ∼(3× 3) charge density wave previously reported even at room temperature in TaS2 grown on noninteracting substrates. A (3 × 3) superstructure of 30°-rotated TaS2 islands is indeed revealed by complementary scanning tunneling microscopy.

Graphene on Ir(111): physisorption with chemical modulation.

The nonlocal van der Waals density functional approach is applied to calculate the binding of graphene to Ir(111). The precise agreement of the calculated mean height h = 3.41 Å of the C atoms with their mean height h = (3.38±0.04) Å as measured by the x-ray standing wave technique provides a benchmark for the applicability of the nonlocal functional. We find bonding of graphene to Ir(111) to be due to the van der Waals interaction with an antibonding average contribution from chemical interaction. Despite its globally repulsive character, in certain areas of the large graphene moiré unit cell charge accumulation between Ir substrate and graphene C atoms is observed, signaling a weak covalent bond formation.

Elementary processes governing V2AlC chemical etching in HF.

The literature on MXenes, an important class of 2D materials discovered in 2011, is now abundant. Yet, the lack of well-defined structures, with definite crystal orientations, has so far hindered our capability to identify some key aspects ruling MXene's chemical exfoliation from their parent MAX phase. Herein the chemical exfoliation of V2AlC is studied by using well-defined square pillars with lateral sizes from 7 µm up to 500 µm, processed from centimeter-sized V2AlC single crystals. The MXene conversion kinetics are assessed with µm spatial resolution by combining Raman spectroscopy with scanning electron and optical microscopies. HF penetration, and the loss of the Al species, take place through the edges. At room temperature, and on a reasonable time scale, no etching can takes place by HF penetration through the basal planes, viz. normal to the basal planes. In defect-free pillars, etching through the edges is isotropic. Initially the etching rate is linear with a rate of 2.2 ± 0.3 µm h-1 at 25 °C. At a distance of ≈45 µm, the etching rate is greatly diminished.

Copper-assisted oxidation of catechols into quinone derivatives.

Catechols are ubiquitous substances often acting as antioxidants, thus of importance in a variety of biological processes. The Fenton and Haber-Weiss processes are thought to transform these molecules into aggressive reactive oxygen species (ROS), a source of oxidative stress and possibly inducing degenerative diseases. Here, using model conditions (ultrahigh vacuum and single crystals), we unveil another process capable of converting catechols into ROSs, namely an intramolecular redox reaction catalysed by a Cu surface. We focus on a tri-catechol, the hexahydroxytriphenylene molecule, and show that this antioxidant is thereby transformed into a semiquinone, as an intermediate product, and then into an even stronger oxidant, a quinone, as final product. We argue that the transformations occur via two intramolecular redox reactions: since the Cu surface cannot oxidise the molecules, the starting catechol and the semiquinone forms each are, at the same time, self-oxidised and self-reduced. Thanks to these reactions, the quinone and semiquinone are able to interact with the substrate by readily accepting electrons donated by the substrate. Our combined experimental surface science and ab initio analysis highlights the key role played by metal nanoparticles in the development of degenerative diseases.

In-Plane Magnetic Domains and Néel-like Domain Walls in Thin Flakes of the Room Temperature CrTe2 Van der Waals Ferromagnet.

The recent discovery of magnetic van der Waals (vdW) materials triggered a wealth of investigations in materials science and now offers genuinely new prospects for both fundamental and applied research. Although the catalog of vdW ferromagnets is rapidly expanding, most of them have a Curie temperature below 300 K, a notable disadvantage for potential applications. Combining element-selective X-ray magnetic imaging and magnetic force microscopy, we resolve at room temperature the magnetic domains and domain walls in micron-sized flakes of the CrTe2 vdW ferromagnet. Flux-closure magnetic patterns suggesting an in-plane six-fold symmetry are observed. Upon annealing the material above its Curie point (315 K), the magnetic domains disappear. By cooling back the sample, a different magnetic domain distribution is obtained, indicating material stability and lack of magnetic memory upon thermal cycling. The domain walls presumably have Néel texture, are preferentially oriented along directions separated by 120°, and have a width of several tens of nanometers. Besides microscopic mapping of magnetic domains and domain walls, the coercivity of the material is found to be of a few millitesla only, showing that the CrTe2 compound is magnetically soft. The coercivity is found to increase as the volume of the material decreases.

Evolution of inter-layer coupling in artificially stacked bilayer MoS2.

In this paper, we show experimentally that for van der Waals heterostructures (vdWh) of atomically-thin materials, the hybridization of bands of adjacent layers is possible only for ultra-clean interfaces. This we achieve through a detailed experimental study of the effect of interfacial separation and adsorbate content on the photoluminescence emission and Raman spectra of ultra-thin vdWh. For vdWh with atomically-clean interfaces, we find the emergence of novel vibrational Raman-active modes whose optical signatures differ significantly from that of the constituent layers. Additionally, we find for such systems a significant modification of the photoluminescence emission spectra with the appearance of peaks whose strength and intensity directly correlate with the inter-layer coupling strength. Our ability to control the intensity of the photoluminescence emission led to the observation of detailed optical features like indirect-band peaks. Our study establishes that it is possible to engineer atomically-thin van der Waals heterostructures with desired optical properties by controlling the inter-layer spacing, and consequently the inter-layer coupling between the constituent layers.

Electronic Band Structure of Ultimately Thin Silicon Oxide on Ru(0001).

Silicon oxide can be formed in a crystalline form, when prepared on a metallic substrate. It is a candidate support catalyst and possibly the ultimately thin version of a dielectric host material for two-dimensional materials and heterostructures. We determine the atomic structure and chemical bonding of the ultimately thin version of the oxide, epitaxially grown on Ru(0001). In particular, we establish the existence of two sublattices defined by metal-oxygen-silicon bridges involving inequivalent substrate sites. We further discover four electronic bands below the Fermi level, at high binding energy, two of them having a linear dispersion at their crossing K point (Dirac cones) and two others forming semiflat bands. While the latter two correspond to hybridized states between the oxide and the metal, the former relate to the topmost silicon-oxygen plane, which is not directly coupled to the substrate. Our analysis is based on high-resolution X-ray photoelectron spectroscopy, angle-resolved photoemission spectroscopy, scanning tunneling microscopy, and density functional theory calculations.

Coherence and Density Dynamics of Excitons in a Single-Layer MoS2 Reaching the Homogeneous Limit.

We measure the coherent nonlinear response of excitons in a single layer of molybdenum disulfide embedded in hexagonal boron nitride, forming a h-BN/MoS2/ h-BN heterostructure. Using four-wave mixing microscopy and imaging, we correlate the exciton inhomogeneous broadening with the homogeneous one and population lifetime. We find that the exciton dynamics is governed by microscopic disorder on top of the ideal crystal properties. Analyzing the exciton ultrafast density dynamics using amplitude and phase of the response, we investigate the relaxation pathways of the resonantly driven exciton population. The surface protection via encapsulation provides stable monolayer samples with low disorder, avoiding surface contaminations and the resulting exciton broadening and modifications of the dynamics. We identify areas localized to a few microns where the optical response is totally dominated by homogeneous broadening. Across the sample of tens of micrometers, weak inhomogeneous broadening and strain effects are observed, attributed to the remaining interaction with the h-BN and imperfections in the encapsulation process.

Graphene as a Mechanically Active, Deformable Two-Dimensional Surfactant.

In crystal growth, surfactants are additive molecules used in dilute amount or as dense, permeable layers to control surface morphologies. We investigate the properties of a strikingly different surfactant: a 2D and covalent layer with close atomic packing, graphene. Using in situ, real-time electron microscopy, scanning tunneling microscopy, kinetic Monte Carlo simulations, and continuum mechanics calculations, we reveal why metallic atomic layers can grow in a 2D manner below an impermeable graphene membrane. Upon metal growth, graphene dynamically opens nanochannels called wrinkles, facilitating mass transport while at the same time storing and releasing elastic energy via lattice distortions. Graphene thus behaves as a mechanically active, deformable surfactant. The wrinkle-driven mass transport of the metallic layer intercalated between graphene and the substrate is observed for two graphene-based systems, characterized by different physicochemical interactions, between graphene and the substrate and between the intercalated material and graphene. The deformable surfactant character of graphene that we unveil should then apply to a broad variety of species, opening new avenues for using graphene as a 2D surfactant forcing the growth of flat films, nanostructures, and unconventional crystalline phases.