Jérôme Lagoute

We present the superstructure formed during phosphorus deposition on the reconstructed Au(100)-hexagonal surface together with its electronic properties. Scanning tunneling microscopy and spectroscopy combined with X-ray diffraction are compared to first-principles calculations. All these results consistently indicate a p(2×2) phosphorus structure with P atoms in hollow sites above a strained Au(100) surface. At intermediate coverage, Au(100)-hexagonal domains coexist with the p(2×2) superstructure. For full coverage, the reconstruction is completely lifted. Structure factors deduced from theoretical calculations and those measured by diffraction are in excellent agreement and demonstrate that P atoms induce significant Au strain which promotes a 0.2 Å outward relaxation associated with the quadrimerization of the underlying Au atoms. From the point of view of an electronic density perspective, a peak at 3.6 eV is observed in the unoccupied states, which is also supported by density functional theory. This is explained by a transfer of electrons from the Au surface to the P atoms.

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.

Due to its diverse allotropes and intriguing properties, 2D phosphorus, also known as phosphorene, is a material of great interest. Here, the successful growth of flat hexagonal 2D phosphorus on Au(111) is reported. Starting from phosphorus linear chains at low coverage, a porous network and finally an extended 2D flat hexagonal (HexP) layer while increasing phosphorus deposition is formed. Using scanning tunneling microscopy/spectroscopy combined with ab initio calculations, the structure and electronic properties of the as‐grown phosphorus structures are followed. The progressive formation of a phosphorus electronic band in the conduction band region is followed. More strikingly, a partial flatband that appears only on the HexP phase characterized by a sharp peak in the electronic spectrum is observed. These bands arise from the hybridization of phosphorus and gold atoms. This novel phosphorus‐based structure exhibits remarkable electronic properties due to gold mediated phosphorus–phosphorus electronic coupling. This work paves the way for new interface material developments with attractive electronic properties.

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.

Spin-crossover molecules present the unique property of having two spin states that can be controlled by light excitation at low temperature. Here, we report on the photoexcitation of [Fe^II((3, 5-(CH₃)₂Pz)₃BH)₂] (Pz = pyrazolyl) ultrathin films, with thicknesses ranging from 0.9 to 5.3 monolayers, adsorbed on Cu(111) substrate. Using X-ray absorption spectroscopy measurements, we confirm the anomalous light-induced spin-state switching observed for sub-monolayer coverage and demonstrate that it is confined to the first molecular layer in contact with the metallic substrate. For higher coverages, the well-known light-induced excited spin-state trapping effect is recovered. Combining continuous light excitation with thermal cycling, we demonstrate that at low temperature light-induced thermal hysteresis is measured for the thicker films, while for sub-monolayer coverage, the light enables extension of the thermal conversion over a large temperature range. Mechanoelastic simulations underline that, due to the intermolecular interactions, opposite behaviors are observed in the different layers composing the films.

Alkali atom doping is an efficient way to induce charge transfer and Fermi level tuning in layered materials through intercalation. However, there is a general lack of microscopic understanding of the effect of doping inhomogeneity in geometric and electronic aspects. Here, we report surface doping of a bulk VSe₂ crystal by sodium. Na atoms form intercalated subsurface islands that modify the electronic phase of the top layer of VSe₂. In addition to n‐doping, the charge density wave of the intercalated VSe₂ surface layer changes from the (4 × 4) bulk phase to the () known in monolayer phase of VSe₂. Surprisingly, an electronic state at the edges of Na‐intercalated area shift anomalously upward in energy as detected by scanning tunneling spectroscopy. This is explained by a local gating effect resulting from local dipoles at the edges. The study illustrates a clear example of intercalation effect that should be general in alkali‐intercalated bulk layered materials.

Controlling the spatial distribution of dopants in graphene is the gateway to the realization of graphene‐based electronic components. Here, it is shown that a submonolayer of self‐assembled physisorbed molecules can be used as a resist during a post‐synthesis nitrogen doping process to realize a nanopatterning of nitrogen dopants in graphene. The resulting formation of domains with different nitrogen concentrations allows obtaining n–n’ and p–n junctions in graphene. A scanning tunneling microscopy is used to measure the electronic properties of the junctions at the atomic scale and reveal their intrinsic width that is found to be ≈7 nm corresponding to a sharp junction regime.