D. Aubel

Surface intercalation of gold underneath a graphene monolayer on SiC(0001) studied by scanning tunneling microscopy and spectroscopy

The effects of gold deposition on monolayer graphene (MG) epitaxied on SiC (0001) substrate are examined via scanning tunneling microscopy and scanning tunneling spectroscopy (STS). Two types of surfaces with distinctive topography are demonstrated: (i) intercalated gold clusters having no interaction with graphene and (ii) 13×13-G reconstruction attributed to a Moire pattern arising from the intercalation of 1 ML of gold between a MG and the underlying SiC substrate. This surface also displays a 23×23R30-Au (111) surface reconstruction interpreted as surface corrugation. The STS curve shows a possible hole-doping effect in the latter case.

Ge growth mode modification on carbon-induced Si 001 -c 4 = 4 surfaces

Abstract Strong Ge morphological modifications were observed upon an ordered C-pre-covered Si(001)-c(4×4) reconstructed surface used as a template as compared to the growth on bare Si(001)-(2×1) substrates. While on bare substrates, the Ge wetting layer of the Stranski–Krastanov mode has a critical thickness of approximately 3–4 monolayers (ML), with the c-(4×4) template, island nucleation already occurs after 1 Ge ML, and growth proceeds in a Volmer–Weber mode. This suggests that the C-rich surface derm associated with the c-(4×4) reconstruction is able to strongly affect the Ge wetting.

The Ge Stranski-Krastanov growth mode on Si(001) (2 × 1) tested by X-ray photoelectron and Auger electron diffraction

X-ray photoelectron and Auger electron diffractions have been used here for the first time to identify growth morphology in the earliest stages (0–10 monolayers) of Ge epitaxy on Si(001)(2 × 1) surfaces held at room temperature (RT) and at 400°C. The Ge atomic arrangement in the (110) plane is examined by performing polar angle distribution of the Ge LMM intensities and by comparison with the corresponding Si2p substrate pattern. A detailed plot as a function of the Ge coverage of the forward scattering peak contrasts in the [111] and [001] directions, which correspond to the 1st and 3rd atomic neighbour rows, respectively, yields informations about the layer number distribution and the growth mode. Contrarily to the nearly two-dimensional (2D) growth taking place at RT, we obtain a 3D island formation at 400°C for a critical thickness exceeding 5 ML. Nevertheless, in the coverage domain between 2 and 5 ML for which layer-by-layer growth is normally expected, the observation of a significant up to 4 ML roughness across the surface prefigurates the islanding process and confirms very recent STM reports. Photoelectron scattering results are only consistent with pure 2D formation during the first 2 ML growth.

Site-selective local fluorination of graphene induced by focused ion beam irradiation.

The functionalization of graphene remains an important challenge for numerous applications expected by this fascinating material. To keep advantageous properties of graphene after modification or functionalization of its structure, local approaches are a promising road. A novel technique is reported here that allows precise site-selective fluorination of graphene. The basic idea of this approach consists in the local radicalization of graphene by focused ion beam (FIB) irradiation and simultaneous introduction of XeF2 gas. A systematic series of experiments were carried out to outline the relation between inserted defect creation and the fluorination process. Based on a subsequent X-ray photoelectron spectroscopy (XPS) analysis, a 6-fold increase of the fluorine concentration on graphene under simultaneous irradiation was observed when compared to fluorination under normal conditions. The fluorine atoms are predominately localized at the defects as indicated from scanning tunneling microscopy (STM). The experimental findings are confirmed by density functional theory which predicts a strong increase of the binding energy of fluorine atoms when bound to the defect sites. The developed technique allows for local fluorination of graphene without using resists and has potential to be a general enabler of site-selective functionalization of graphene using a wide range of gases.

Selective thermal — as opposed to non-selective plasma — nitridation of SiGe related materials examined by in situ photoemission techniques

Abstract NH 3 thermal and N 2 plasma reactivity with Si(0 0 1), Ge(0 0 1), Si 1- x Ge x (0 0 1) surfaces has been studied by means of in situ X-ray photoelectron spectroscopy (XPS) in a temperature domain ( T ∼ 600°C) compatible with the MBE growth of GeSi-based heterostructures. Si(0 0 1) surfaces present a strong initial thermal reactivity against NH 3 , contrary to Ge(0 0 1) which is totally inert. The selectivity against thermal nitridation, which may be anticipated for thermodynamical reasons, has been verified by nitrogen uptake measurements of N 1s core level intensities as a function of NH 3 exposure, both for Si(0 0 1) and Ge(0 0 1) surfaces. As a consequence of this strong reactivity difference, an exclusive Si 3 N 4 formation and Ge phase separation result from nitridation attempts of Si 1- x Ge x alloys. Thus, an important finding is the indispensable utilization of plasma-assisted nitridation methods in order to achieve low-temperature Ge nitridation, either on clean Ge(0 0 1) surfaces or simultaneously with Si on SiGe alloys. In this paper, the first results relevant to Ge and SiGe alloy nitridation by irradiation of these surfaces by electron cyclotron resonance (ECR) nitrogen (N 2 ) plasmas are presented. These alloys are thermally unstable as nitridation transfer from Ge to Si occurs after annealing, in accordance with thermally favored Si nitridation. In addition, Ge 3 N 4 (Si 3 N 4 ) thick layers were grown using ECR N 2 plasma treatment associated with a concomitant Ge(Si) atom-supply on the substrate, performed by Ge evaporation (SiH 4 reacting gas).

Covalent functionalization by cycloaddition reactions of pristine, defect-free graphene

Based on a low-temperature scanning tunneling microscopy study, we present a direct visualization of a cycloaddition reaction performed for some specific fluorinated maleimide molecules deposited on graphene. Up to now, it was widely admitted that such a cycloaddition reaction can not happen without pre-existing defects. However, our study shows that the cycloaddition reaction can be carried out on a defect-free basal graphene plane at room temperature. In the course of covalently grafting the molecules to graphene, the sp2 conjugation of carbon atoms was broken, and local sp3 bonds were created. The grafted molecules perturbed the graphene lattice, generating a standing-wave pattern with an anisotropy which was attributed to a (1,2) cycloaddition, as revealed by T-matrix approximation calculations. DFT calculations showed that while both (1,4) and (1,2) cycloadditions were possible on free-standing graphene, only the (1,2) cycloaddition could be obtained for graphene on SiC(0001). Globally averaging spectroscopic techniques, XPS and ARPES, were used to determine the modification in the elemental composition of the samples induced by the reaction, indicating an opening of an electronic gap in graphene.

Generating long supramolecular pathways with a continuous density of states by physically linking conjugated molecules via their end groups.

Self-assembly of conjugated 2,5-dialkoxy-phenylene-thienylene-based oligomers on epitaxial monolayer graphene was studied in ultrahigh vacuum by low-temperature scanning tunneling microscopy (STM). The formation of long one-dimensional (1D) supramolecular chain-like structures has been observed, associated to a physical linking of their ends which involved the rotation of the end thiophene rings in order to allow π-π stacking of these end-groups. dI/dV maps taken at an energy corresponding to the excited states showed a continuous electronic density of states, which tentatively suggests that within such molecular chains conjugation of electrons is preserved even across physically linked molecules. Thus, in a self-organization process conjugation may be extended by appropriately adapting conformations of neighboring molecules. Our STM results on such self-organized end-linked molecules potentially represent a direct visualization of J-aggregates.

A C 1s core level x-ray photoelectron diffraction characterization of substitutional carbon in epitaxial Si1−yCy alloys grown on Si(111) and Si(001)

Epitaxial strained growth of Si1−yCy alloys with rather high C concentrations (y∼1.5%) has been performed on Si(111) and Si(001) using molecular beam epitaxy (MBE) Si evaporation and thermal interaction of the growth surface with a low C2H4 pressure at 500 °C. Carbon contents, determined by secondary ion mass spectrometry, infrared (ir) spectrometry, in situ C 1s and Si 2p x-ray photoelectron spectroscopy measurements and x-ray diffraction (XRD), are being compared. Monocrystalline quality of the epilayers is checked by low energy electron diffraction and x-ray photoelectron diffraction (XPD). As indirectly ascertained by the ir local vibration mode (LVM) and a shifted partially strain induced epilayer diffraction line in the θ-2θ XRD analysis, carbon is accommodated in substitutional sites (Csub) whose local atomic order is investigated for the first time by XPD, C 1s polar angle distributions being measured in different azimuthal directions. As the data reveal, for a C emitter, next nearest neighbor bon...

X-ray photoelectron diffraction investigation of Ge segregation and film morphology during first stage heteroepitaxy of Si on Ge(001)

Abstract X-ray photoelectron diffraction (XPD) and its chemical selectivity have been used in conjunction with surface atom titration by ultra-violet photoelectron spectroscopy and conventional X-ray photoelectron spectroscopy to investigate the interplay between Ge segregation and growth morphology in the earliest stages (0–12 monolayers) of Si solid-source-epitaxy on Ge(001)-2 × 1 surfaces held at room temperature (RT) and at 400°C. A detailed examination, as a function of the Si coverage, of the forward scattering peak contrasts in the [111] and [001] directions of the Si 2p and Ge 3d polar angle distributions provides a diagnostic of whether or not a particular atom is in the top layer (segregation) and yields information about the stacking sequences (morphology). These investigations allowed us to conclude in favour of an initial Si bilayer patch-growth-mode exempt of segregation at RT and of a floating Ge surface at 400°C, the Si agglomeration staying rather mild in the first monolayers. Accessorily, the XPD-specific problems of electron defocusing effects through long atomic chains and strain-induced lattice expansion or contraction have also been addressed.

Carbon-associated Si(001)-c(4×4) reconstruction dosed with hydrogen

It has recently been recognized that initial carbon (C) accommodation on Si(001)-(2 × 1) surfaces (less than 1 monolayer, at 600°C and using various C precursors) causes a c(4 x 4) surface reconstruction. In this paper we intend to determine the C lattice sites associated to this reconstruction. C Is XPS signatures indicate a main contribution of C atoms dominantly located in substitutional subsurface sites and a weaker contribution, of 1.6 eV higher binding energy, attributed to C surface sites. C 1s X-ray photoelectron diffraction polar observations along azimuths ascertain C location within the first five Si layers and the C atoms at the origin of the observed angular modulations are necessarily located from the third to fifth subsurface layers. By c(4 x 4) surface exposure to atomic hydrogen at room temperature, a 1 x 1 LEED diagram is restored. As for the Si(001)-(2 x 1) reconstruction, it demonstrates a break of hydrogen-saturated surface dimers. The latter may be restored by annealing but with a kinetics different from that of a simple Si dihydride desorption. These results essentially support c(4 x 4) reconstruction models with surface dimers including C atoms and a subsurface C-rich Si n C alloy.