Spatially Extended Charge Density Wave Switching by Nanoscale Local Manipulation in a VTe2 Monolayer.

Monolayer transition metal dichalcogenide VTe2 exhibits multiple charge density wave (CDW) phases, mainly (4 × 4) and (4 × 1). Here we report facile dynamic and tens-of-nanometer scale switching between these CDW phases with gentle bias pulses in scanning tunneling microscopy. Bias pulses purposely stimulate a reversible random CDW symmetry change between the isotropic (4 × 4) and anisotropic (4 × 1) CDWs, as well as CDW phase slips and rotation. The switching threshold of ∼1.0 V is independent of bias polarity, and the switching rate varies linearly with the tunneling current. Density functional theory calculations indicate that a coherent CDW phase switching incurs an energy barrier of ∼2.0-3.0 eV per (4 × 4) unit cell. While there is a challenge in understanding the observed large-area CDW random fluttering, we provide some possible explanations. The ability to manipulate electronic CDW phases sheds new light on tailoring CDW properties on demand.

Combining nitrogen doping and vacancies for tunable resonant states in graphite.

We investigate the combination of nitrogen doping and vacancies in highly ordered pyrolytic graphite (HOPG), to engineer defect sites with adjustable electronic properties. We combine scanning tunneling microscopy and spectroscopy and density functional theory calculations to reveal the synergistic effects of nitrogen and vacancies in HOPG. Our findings reveal a remarkable shift of the vacancy-induced resonance peak from an unoccupied state in pristine HOPG to an occupied state in nitrogen-doped HOPG. This shift directly correlates with the shift of the charge neutrality point resulting from the n-doping induced by substitutional nitrogen. These results open new avenues for defect engineering in graphite or graphene and achieving novel functionalities for chemical activity or electronic properties.

Direct Observation of the Reduction of a Molecule on Nitrogen Pairs in Doped Graphene.

Incorporating functional atomic sites in graphene is essential for realizing advanced two-dimensional materials. Doping graphene with nitrogen offers the opportunity to tune its chemical activity with significant charge redistribution occurring between molecules and substrate. The necessary atomic scale understanding of how this depends on the spatial distribution of dopants, as well as their positions relative to the molecule, can be provided by scanning tunneling microscopy. Here we show that a noncovalently bonded molecule such as CoPc undergoes a variable charge transfer when placed on N-doped graphene; on a nitrogen pair, it undergoes a redox reaction with an integral charge transfer whereas a lower fractional charge transfer occurs over a single nitrogen. Thus, the charge state of molecules can be tuned by suitably tailoring the conformation of dopant atoms.

Anomalous Light-Induced Spin-State Switching for Iron(II) Spin-Crossover Molecules in Direct Contact with Metal Surfaces.

Light-induced spin-state switching is one of the most attractive properties of spin-crossover materials. In bulk, low-spin (LS) to high-spin (HS) conversion via the light-induced excited spin-state trapping (LIESST) effect may be achieved with a visible light, while the HS-to-LS one (reverse-LIESST) requires an excitation in the near-infrared range. Now, it is shown that those phenomena are strongly modified at the interface with a metal. Indeed, an anomalous spin conversion is presented from HS state to LS state under blue light illumination for FeII spin-crossover molecules that are in direct contact with metallic (111) single-crystal surfaces (copper, silver, and gold). To interpret this anomalous spin-state switching, a new mechanism is proposed for the spin conversion based on the light absorption by the substrate that can generate low energy valence photoelectrons promoting molecular vibrational excitations and subsequent spin-state switching at the molecule-metal interface.

Grain boundaries in graphene on SiC(0001Ì ) substrate.

Grain boundaries in epitaxial graphene on the SiC(0001̅) substrate are studied using scanning tunneling microscopy and spectroscopy. All investigated small-angle grain boundaries show pronounced out-of-plane buckling induced by the strain fields of constituent dislocations. The ensemble of observations determines the critical misorientation angle of buckling transition θc = 19 ± 2°. Periodic structures are found among the flat large-angle grain boundaries. In particular, the observed θ = 33 ± 2° highly ordered grain boundary is assigned to the previously proposed lowest formation energy structural motif composed of a continuous chain of edge-sharing alternating pentagons and heptagons. This periodic grain boundary defect is predicted to exhibit strong valley filtering of charge carriers thus promising the practical realization of all-electric valleytronic devices.

Quantum Transport in Large-Scale Patterned Nitrogen-Doped Graphene.

It has recently been demonstrated how the nitrogen dopant concentration in graphene can be controlled spatially on the nano-meter scale using a molecular mask. This technique may be used to create ballistic electron optics-like structures of high/low doping regions; for example, to focus electron beams, harnessing the quantum wave nature of the electronic propagation. Here, we employ large-scale Greens function transport calculations based on a tight-binding approach. We first benchmark different tight-binding models of nitrogen in graphene with parameters based on density functional theory (DFT) and the virtual crystal approximation (VCA). Then, we study theoretically how the random distribution within the masked regions and the discreteness of the nitrogen scattering centers impact the transport behavior of sharp n-p and n-n' interfaces formed by different, realistic nitrogen concentrations. We investigate how constrictions for the current can be realized by patterned high/low doping regions with experimentally feasible nitrogen concentrations. The constrictions can guide the electronic current, while the quantized conductance is significantly washed out due to the nitrogen scattering. The implications for device design is that a p-n junction with nitrogen corrugation should still be viable for current focusing. Furthermore, a guiding channel with less nitrogen in the conducting canal preserves more features of quantized conductance and, therefore, its low-noise regime.

Identification and Manipulation of Defects in Black Phosphorus.

We identify and manipulate commonly occurring defects in black phosphorus, combining scanning tunneling microscopy experiments with density functional theory calculations. A ubiquitous defect, imaged at negative bias as a bright dumbbell extending over several nanometers, is shown to arise from a substitutional Sn impurity in the second sublayer. Another frequently observed defect type is identified as arising from an interstitial Sn atom; this defect can be switched to a more stable configuration consisting of a Sn substitutional defect + P adatom, by application of an electrical pulse via the STM tip. DFT calculations show that this pulse-induced structural transition switches the system from a non-magnetic configuration to a magnetic one. We introduce States Projected Onto Individual Layers (SPOIL) quantities which provide information about atom-wise and orbital-wise contributions to bias-dependent features observed in STM images.

Intraconfigurational Transition due to Surface-Induced Symmetry Breaking in Noncovalently Bonded Molecules.

The interaction of molecules with surfaces plays a crucial role in the electronic and chemical properties of supported molecules and needs a comprehensive description of interfacial effects. Here, we unveil the effect of the substrate on the electronic configuration of iron porphyrin molecules on Au(111) and graphene, and we provide a physical picture of the molecule-surface interaction. We show that the frontier orbitals derive from different electronic states depending on the substrate. The origin of this difference comes from molecule-substrate orbital selective coupling caused by reduced symmetry and interaction with the substrate. The weak interaction on graphene keeps a ground state configuration close to the gas phase, while the stronger interaction on gold stabilizes another electronic solution. Our findings reveal the origin of the energy redistribution of molecular states for noncovalently bonded molecules on surfaces.

Controlling Hydrogen-Transfer Rate in Molecules on Graphene by Tunable Molecular Orbital Levels.

Molecular switches are building blocks of potential interest to store binary information, especially when they can be organized in periodic lattices. Among the variety of possible systems, switches based on hydrogen transfer are of special importance because they allow the switching operation to occur without severe conformational change that may interfere with neighboring molecular units. We have studied the excitation process of hydrogen transfer inside porphyrin molecules assembled on a graphene surface, using a low-temperature scanning tunneling microscope. We show that this hydrogen transfer is induced by an electronic resonant tunneling process through the molecular orbitals. Using nitrogen doping of graphene, we tune the rate of hydrogen transfer by shifting the molecular orbital energies owing to the charge transfer at nitrogen dopant sites in the graphene lattice. The control of the switching process allows the storage of information inside a molecular lattice, which is demonstrated by writing an artificial pattern inside a molecular island.

Importance of Epitaxial Strain at a Spin-Crossover Molecule-Metal Interface.

Spin-crossover molecules are very appealing for use in multifunctional spintronic devices because of their ability to switch between high-spin and low-spin states with external stimuli such as voltage and light. In actual devices, the molecules are deposited on a substrate, which can modify their properties. However, surprisingly little is known about such molecule-substrate effects. Here we show for the first time, by grazing incidence X-ray diffraction, that an FeII spin-crossover molecular layer displays a well-defined epitaxial relationship with a metal substrate. Then we show, by both density functional calculations and a mechanoelastic model, that the resulting epitaxial strain and the related internal pressure can induce a partial spin conversion at low temperatures, which has indeed been observed experimentally. Our results emphasize the importance of substrate-induced spin state transitions and raise the possibility of exploiting them.

Tuning the Electronic and Dynamical Properties of a Molecule by Atom Trapping Chemistry.

The ability to trap adatoms with an organic molecule on a surface has been used to obtain a range of molecular functionalities controlled by the choice of the molecular trapping site and local deprotonation. The tetraphenylporphyrin molecule used in this study contains three types of trapping sites: two carbon rings (phenyl and pyrrole) and the center of a macrocycle. Catching a gold adatom on the carbon rings leads to an electronic doping of the molecule, whereas trapping the adatom at the macrocycle center with single deprotonation leads to a molecular rotor and a second deprotonation leads to a molecular jumper. We call "atom trapping chemistry" the control of the structure, electronic, and dynamical properties of a molecule achieved by trapping metallic atoms with a molecule on a surface. In addition to the examples previously described, we show that more complex structures can be envisaged.

Control of Molecule-Metal Interaction by Hydrogen Manipulation in an Organic Molecule.

Free-base porphyrin molecules offer appealing options to tune the interaction with their environment via the manipulation of their inner hydrogen atoms and molecular conformation. Using scanning tunneling microscopy we show, through a systematic study, that the molecular conformation, electronic gap, wave function, and molecule-substrate interaction are modified by hydrogen switch or removal. Experimental results in combination with ab initio calculations provide an understanding of the underlying physical process.

Molecular adsorbates as probes of the local properties of doped graphene.

Graphene-based sensors are among the most promising of graphene's applications. The ability to signal the presence of molecular species adsorbed on this atomically thin substrate has been explored from electric measurements to light scattering. Here we show that the adsorbed molecules can be used to sense graphene properties. The interaction of porphyrin molecules with nitrogen-doped graphene has been investigated using scanning tunneling microscopy and ab initio calculations. Molecular manipulation was used to reveal the surface below the adsorbed molecules allowing to achieve an atomic-scale measure of the interaction of molecules with doped graphene. The adsorbate's frontier electronic states are downshifted in energy as the molecule approaches the doping site, with largest effect when the molecule sits over the nitrogen dopant. Theoretical calculations showed that, due to graphene's high polarizability, the adsorption of porphyrin induces a charge rearrangement on the substrate similar to the image charges on a metal. This charge polarization is enhanced around nitrogen site, leading to an increased interaction of molecules with their image charges on graphene. Consequently, the molecular states are stabilized and shift to lower energies. These findings reveal the local variation of polarizability induced by nitrogen dopant opening new routes towards the electronic tuning of graphene.

Molecular-scale dynamics of light-induced spin cross-over in a two-dimensional layer.

Spin cross-over molecules show the unique ability to switch between two spin states when submitted to external stimuli such as temperature, light or voltage. If controlled at the molecular scale, such switches would be of great interest for the development of genuine molecular devices in spintronics, sensing and for nanomechanics. Unfortunately, up to now, little is known on the behaviour of spin cross-over molecules organized in two dimensions and their ability to show cooperative transformation. Here we demonstrate that a combination of scanning tunnelling microscopy measurements and ab initio calculations allows discriminating unambiguously between both states by local vibrational spectroscopy. We also show that a single layer of spin cross-over molecules in contact with a metallic surface displays light-induced collective processes between two ordered mixed spin-state phases with two distinct timescale dynamics. These results open a way to molecular scale control of two-dimensional spin cross-over layers.

Electronic interaction between nitrogen atoms in doped graphene.

Many potential applications of graphene require either the possibility of tuning its electronic structure or the addition of reactive sites on its chemically inert basal plane. Among the various strategies proposed to reach these objectives, nitrogen doping, i.e., the incorporation of nitrogen atoms in the carbon lattice, leads in most cases to a globally n-doped material and to the presence of various types of point defects. In this context, the interactions between chemical dopants in graphene have important consequences on the electronic properties of the systems and cannot be neglected when interpreting spectroscopic data or setting up devices. In this report, the structural and electronic properties of complex doping sites in nitrogen-doped graphene have been investigated by means of scanning tunneling microscopy and spectroscopy, supported by density functional theory and tight-binding calculations. In particular, based on combined experimental and simulation works, we have systematically studied the electronic fingerprints of complex doping configurations made of pairs of substitutional nitrogen atoms. Localized bonding states are observed between the Dirac point and the Fermi level in contrast with the unoccupied state associated with single substitutional N atoms. For pyridinic nitrogen sites (i.e., the combination of N atoms with vacancies), a resonant state is observed close to the Dirac energy. This insight into the modifications of electronic structure induced by nitrogen doping in graphene provides us with a fair understanding of complex doping configurations in graphene, as it appears in real samples.

Charge transfer and electronic doping in nitrogen-doped graphene.

Understanding the modification of the graphene's electronic structure upon doping is crucial for enlarging its potential applications. We present a study of nitrogen-doped graphene samples on SiC(000) combining angle-resolved photoelectron spectroscopy, scanning tunneling microscopy and spectroscopy and X-ray photoelectron spectroscopy (XPS). The comparison between tunneling and angle-resolved photoelectron spectra reveals the spatial inhomogeneity of the Dirac energy shift and that a phonon correction has to be applied to the tunneling measurements. XPS data demonstrate the dependence of the N 1s binding energy of graphitic nitrogen on the nitrogen concentration. The measure of the Dirac energy for different nitrogen concentrations reveals that the ratio usually computed between the excess charge brought by the dopants and the dopants' concentration depends on the latter. This is supported by a tight-binding model considering different values for the potentials on the nitrogen site and on its first neighbors.

Electronic interaction between nitrogen-doped graphene and porphyrin molecules.

The chemical doping of graphene is a promising route to improve the performances of graphene-based devices through enhanced chemical reactivity, catalytic activity, or transport characteristics. Understanding the interaction of molecules with doped graphene at the atomic scale is therefore a leading challenge to be overcome for the development of graphene-based electronics and sensors. Here, we use scanning tunneling microscopy and spectroscopy to study the electronic interaction of pristine and nitrogen-doped graphene with self-assembled tetraphenylporphyrin molecules. We provide an extensive measurement of the electronic structure of single porphyrins on Au(111), thus revealing an electronic decoupling effect of the porphyrins adsorbed on graphene. A tip-induced switching of the inner hydrogen atoms of porphyrins, first identified on Au(111), is observed on graphene, allowing the identification of the molecular conformation of porphyrins in the self-assembled molecular layer. On nitrogen-doped graphene, a local modification of the charge transfer around the nitrogen sites is evidenced via a downshift of the energies of the molecular elecronic states. These data show how the presence of nitrogen atoms in the graphene network modifies the electronic interaction of organic molecules with graphene. These results provide a basic understanding for the exploitation of doped graphene in molecular sensors or nanoelectronics.

Identification of nitrogen dopants in single-walled carbon nanotubes by scanning tunneling microscopy.

Using scanning tunnelling microscopy and spectroscopy, we investigated the atomic and electronic structure of nitrogen-doped single walled carbon nanotubes synthesized by chemical vapor deposition. The insertion of nitrogen in the carbon lattice induces several types of point defects involving different atomic configurations. Spectroscopic measurements on semiconducting nanotubes reveal that these local structures can induce either extended shallow levels or more localized deep levels. In a metallic tube, a single doping site associated with a donor state was observed in the gap at an energy close to that of the first van Hove singularity. Density functional theory calculations reveal that this feature corresponds to a substitutional nitrogen atom in the carbon network.

Perpendicular magnetocrystalline anisotropy in tetragonally distorted Fe-Co alloys.

We report on the experimental realization of tetragonal Fe-Co alloys as a constituent of Fe0.36Co0.64/Pt superlattices with huge perpendicular magnetocrystalline anisotropy energy, reaching 210 microeV/atom, and a saturation magnetization of 2.5 microB/atom at 40 K, in qualitative agreement with theoretical predictions. At room temperature the corresponding values and are achieved. This suggests that Fe-Co alloys with carefully chosen combinations of composition and distortion are good candidates for high-density perpendicular storage materials.