Distinguishing Different Stackings in Layered Materials via Luminescence Spectroscopy.

Despite its simple crystal structure, layered boron nitride features a surprisingly complex variety of phonon-assisted luminescence peaks. We present a combined experimental and theoretical study on ultraviolet-light emission in hexagonal and rhombohedral bulk boron nitride crystals. Emission spectra of high-quality samples are measured via cathodoluminescence spectroscopy, displaying characteristic differences between the two polytypes.

Structural Size Effect in Capped Metallic Nanoparticles.

The surfactant used during a colloidal synthesis is known to control the size and shape of metallic nanoparticles. However, its influence on the nanoparticle (NP) structure is still not well understood. In this study, we show that the surfactant can significantly modify the lattice parameter of a crystalline particle.

Sublattice Dependence and Gate Tunability of Midgap and Resonant States Induced by Native Dopants in Bernal-Stacked Bilayer Graphene.

The properties of semiconductors can be crucially impacted by midgap states induced by dopants, which can be native or intentionally incorporated in the crystal lattice. For Bernal-stacked bilayer graphene (BLG), which has a tunable band gap, the existence of midgap states induced by dopants or adatoms has been investigated theoretically and observed indirectly in electron transport experiments. Here, we characterize BLG midgap states in real space, with atomic-scale resolution with scanning tunneling microscopy and spectroscopy.

Momentum-Resolved Dielectric Response of Free-Standing Mono-, Bi-, and Trilayer Black Phosphorus.

Black phosphorus (BP), a 2D semiconducting material of interest in electronics and photonics, exhibits physical properties characterized by strong anisotropy and band gap energy that scales with reducing layer number. However, the investigation of its intrinsic properties is challenging because thin-layer BP is photo-oxidized under ambient conditions and the energy of its electronic states shifts in different dielectric environments. We prepared free-standing samples of few-layer BP under glovebox conditions and probed the dielectric response in a vacuum using scanning transmission electron microscopy and electron energy loss spectroscopy (STEM-EELS).

Bright Luminescence from Indirect and Strongly Bound Excitons in h-BN.

A quantitative analysis of the excitonic luminescence efficiency in hexagonal boron nitride (h-BN) is carried out by cathodoluminescence in the ultraviolet range and compared with zinc oxide and diamond single crystals. A high quantum yield value of ∼50% is found for h-BN at 10 K comparable to that of direct band-gap semiconductors. This bright luminescence at 215 nm remains stable up to room temperature, evidencing the strongly bound character of excitons in bulk h-BN.

Entropy-driven stability of chiral single-walled carbon nanotubes.

Single-walled carbon nanotubes are hollow cylinders that can grow centimeters long via carbon incorporation at the interface with a catalyst. They display semiconducting or metallic characteristics, depending on their helicity, which is determined during their growth. To support the quest for a selective synthesis, we develop a thermodynamic model that relates the tube-catalyst interfacial energies, temperature, and the resulting tube chirality.

Dimensionality effects on the luminescence properties of hBN.

Cathodoluminescence (CL) experiments at low temperature have been undertaken on various bulk and exfoliated hexagonal boron nitride (hBN) samples. Different bulk crystals grown from different synthesis methods have been studied. All of them present the same so-called S series in the 5.

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.

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.

Interdependency of subsurface carbon distribution and graphene-catalyst interaction.

The dynamics of the graphene-catalyst interaction during chemical vapor deposition are investigated using in situ, time- and depth-resolved X-ray photoelectron spectroscopy, and complementary grand canonical Monte Carlo simulations coupled to a tight-binding model. We thereby reveal the interdependency of the distribution of carbon close to the catalyst surface and the strength of the graphene-catalyst interaction. The strong interaction of epitaxial graphene with Ni(111) causes a depletion of dissolved carbon close to the catalyst surface, which prevents additional layer formation leading to a self-limiting graphene growth behavior for low exposure pressures (10(-6)-10(-3) mbar).

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.

Nickel-assisted healing of defective graphene.

The healing of graphene grown from a metallic substrate is investigated using tight-binding Monte Carlo simulations. At temperatures (ranging from 1000 to 2500 K), an isolated graphene sheet can anneal a large number of defects suggesting that their healings are thermally activated. We show that in the presence of a nickel substrate we obtain a perfect graphene layer.

Early stages in the nucleation process of carbon nanotubes.

The early stages of carbon nanotube nucleation are investigated using field ion/electron microscopy along with in situ local chemical probing of a single nanosized nickel crystal. To go beyond experiments, tight-binding Monte Carlo simulations are performed on oriented Ni slabs. Real-time field electron imaging demonstrates a carbon-induced increase of the number density of steps in the truncated vertices of a polyhedral Ni nanoparticle.