Daniele Stradi

Probing the site-dependent Kondo response of nanostructured graphene with organic molecules.

TCNQ molecules are used as a sensitive probe for the Kondo response of the electron gas of a nanostructured graphene grown on Ru(0001) presenting a moiré pattern. All adsorbed molecules acquired an extra electron by charge transfer from the substrate, but only those adsorbed in the FCC-Top areas of the moiré show magnetic moment and Kondo resonance in the STS spectra. DFT calculations trace back this behavior to the existence of a surface resonance in the low areas of the graphene moiré, whose density distribution strongly depends on the stacking sequence of the moiré area and effectively quenches the magnetic moment for HCP-Top sites.

Organic Covalent Patterning of Nanostructured Graphene with Selectivity at the Atomic Level.

Organic covalent functionalization of graphene with long-range periodicity is highly desirable-it is anticipated to provide control over its electronic, optical, or magnetic properties-and remarkably challenging. In this work we describe a method for the covalent modification of graphene with strict spatial periodicity at the nanometer scale. The periodic landscape is provided by a single monolayer of graphene grown on Ru(0001) that presents a moiré pattern due to the mismatch between the carbon and ruthenium hexagonal lattices. The moiré contains periodically arranged areas where the graphene-ruthenium interaction is enhanced and shows higher chemical reactivity. This phenomenon is demonstrated by the attachment of cyanomethyl radicals (CH2CN(•)) produced by homolytic breaking of acetonitrile (CH3CN), which is shown to present a nearly complete selectivity (>98%) binding covalently to graphene on specific atomic sites. This method can be extended to other organic nitriles, paving the way for the attachment of functional molecules.

Elastic Response of Graphene Nanodomes

The mechanical behavior of a periodically buckled graphene membrane has been investigated by noncontact atomic force microscopy in ultrahigh vacuum. When a graphene monolayer is grown on Ru(0001), a regular arrangement of 0.075 nm high nanodomes forming a honeycomb lattice with 3 nm periodicity forms spontaneously. This structure responds in a perfectly reversible way to relative normal displacements up to 0.12 nm. Indeed, the elasticity of the nanodomes is proven by realistic DFT calculations, with an estimated normal stiffness k∼40 N/m. Our observations extend previous results on macroscopic graphene samples and confirm that the elastic behavior of this material is maintained down to nanometer length scales, which is important for the development of new high-frequency (terahertz) electromechanical devices.

Manipulating the voltage drop in graphene nanojunctions using a gate potential

Graphene is an attractive electrode material to contact nanostructures down to the molecular scale since it can be gated electrostatically. Gating can be used to control the doping and the energy level alignment in the nanojunction, thereby influencing its conductance. Here we investigate the impact of electrostatic gating in nanojunctions between graphene electrodes operating at finite bias. Using quantum transport simulations based on density functional theory, we show that the voltage drop across symmetric junctions changes dramatically and controllably in gated systems compared to non-gated junctions. In particular, for p-type(n-type) carriers the voltage drop is located close to the electrode with positive(negative) polarity, the potential of the junction is pinned to the negative(positive) electrode. We trace this behaviour back to the vanishing density of states of graphene in the proximity of the Dirac point. Due to the electrostatic gating, each electrode exposes different density of states in the bias window between the two different electrode Fermi energies, thereby leading to a non-symmetry in the voltage drop across the device. This selective pinning is found to be independent of device length when carriers are induced either by the gate or dopant atoms, indicating a general effect for electronic circuitry based on graphene electrodes. We envision this could be used to control the spatial distribution of Joule heating in graphene nanostructures, and possibly the chemical reaction rate around high potential gradients.

Proximity Band Structure and Spin Textures on Both Sides of Topological-Insulator/Ferromagnetic-Metal Interface and Their Charge Transport Probes

The control of recently observed spintronic effects in topological-insulator/ferromagnetic-metal (TI/FM) heterostructures is thwarted by the lack of understanding of band structure and spin textures around their interfaces. Here we combine density functional theory with Greens function techniques to obtain the spectral function at any plane passing through atoms of Bi2Se3 and Co or Cu layers comprising the interface. Instead of naively assumed Dirac cone gapped by the proximity exchange field spectral function, we find that the Rashba ferromagnetic model describes the spectral function on the surface of Bi2Se3 in contact with Co near the Fermi level EF0, where circular and snowflake-like constant energy contours coexist around which spin locks to momentum. The remnant of the Dirac cone is hybridized with evanescent wave functions from metallic layers and pushed, due to charge transfer from Co or Cu layers, a few tenths of an electron-volt below EF0 for both Bi2Se3/Co and Bi2Se3/Cu interfaces while hosting distorted helical spin texture wounding around a single circle. These features explain recent observation of sensitivity of spin-to-charge conversion signal at TI/Cu interface to tuning of EF0. Crucially for spin-orbit torque in TI/FM heterostructures, few monolayers of Co adjacent to Bi2Se3 host spectral functions very different from the bulk metal, as well as in-plane spin textures (despite Co magnetization being out-of-plane) due to proximity spin-orbit coupling in Co induced by Bi2Se3. We predict that out-of-plane tunneling anisotropic magnetoresistance in Cu/Bi2Se3/Co vertical heterostructure can serve as a sensitive probe of the type of spin texture residing at EF0.

Electron-phonon scattering from Green’s function transport combined with molecular dynamics: Applications to mobility predictions

We present a conceptually simple method for treating electron-phonon scattering and phonon limited mobilities. By combining Greens function based transport calculations and molecular dynamics, we obtain a temperature dependent transmission from which we evaluate the mobility. We validate our approach by comparing to mobilities and conductivities obtained by the Boltzmann transport equation for different bulk and one-dimensional systems. For bulk silicon and gold we compare against experimental values. We discuss limitations and advantages of each of the computational approaches.

Field Effect in Graphene-Based van der Waals Heterostructures: Stacking Sequence Matters

Stacked van der Waals (vdW) heterostructures where semiconducting two-dimensional (2D) materials are contacted by overlaid graphene electrodes enable atomically thin, flexible electronics. We use first-principles quantum transport simulations of graphene-contacted MoS2 devices to show how the transistor effect critically depends on the stacking configuration relative to the gate electrode. We can trace this behavior to the stacking-dependent response of the contact region to the capacitive electric field induced by the gate. The contact resistance is a central parameter and our observation establishes an important design rule for ultrathin devices based on 2D atomic crystals.

Field effect in stacked van der Waals heterostructures: Stacking sequence matters

Stacked van der Waals (vdW) heterostructures where semiconducting two-dimensional (2D) materials are contacted by overlaid graphene electrodes enable atomically thin, flexible electronics. We use first-principles quantum transport simulations of graphene-contacted MoS2 devices to show how the transistor effect critically depends on the stacking configuration relative to the gate electrode. We can trace this behavior to the stacking-dependent response of the contact region to the capacitive electric field induced by the gate. The contact resistance is a central parameter and our observation establishes an important design rule for ultrathin devices based on 2D atomic crystals.

Understanding the self-assembly of TCNQ on Cu(111): a combined study based on scanning tunnelling microscopy experiments and density functional theory simulations

The structure of self-assembled monolayers of 7,7′,8,8′-tetracyano-p-quinodimethane (TCNQ) adsorbed on Cu(111) has been studied using a combination of scanning tunnelling microscopy (STM) experiments and density functional theory (DFT) calculations. We show that the polymorphism of the self-assembled molecular layer can be controlled by tuning of the experimental conditions under which the deposition is carried out. When the Cu(111) substrate is held above room temperature (TCu(111) = 350 K) during deposition, a structure is formed in which the two molecules in the unit cell are oriented one perpendicular to the other. Conversely, when the substrate is held at room temperature during deposition and slightly annealed afterwards, a more complex structure with five molecules per unit cell is formed. DFT calculations complement the experimental results by revealing that the building blocks of the two superstructures are two mutually orthogonal adsorption configurations of the molecule. The relative stability between the two observed polymorphs is reproduced by models of the two superstructures based on these two adsorption configurations.

Method for determining optimal supercell representation of interfaces

The geometry and structure of an interface ultimately determines the behavior of devices at the nanoscale. We present a generic method to determine the possible lattice matches between two arbitrary surfaces and to calculate the strain of the corresponding matched interface. We apply this method to explore two relevant classes of interfaces for which accurate structural measurements of the interface are available: (i) the interface between pentacene crystals and the (1 1 1) surface of gold, and (ii) the interface between the semiconductor indium-arsenide and aluminum. For both systems, we demonstrate that the presented method predicts interface geometries in good agreement with those measured experimentally, which present nontrivial matching characteristics and would be difficult to guess without relying on automated structure-searching methods.