H. Hattab

In?situ observation of stress relaxation in epitaxial graphene

Upon cooling, branched line defects develop in epitaxial graphene grown at high temperature on Pt(111) and Ir(111). Using atomically resolved scanning tunneling microscopy we demonstrate that these defects are wrinkles in the graphene layer, i.e. stripes of partially delaminated graphene. With low energy electron microscopy (LEEM) we investigate the wrinkling phenomenon in situ. Upon temperature cycling we observe hysteresis in the appearance and disappearance of the wrinkles. Simultaneously with wrinkle formation a change in bright field imaging intensity of adjacent areas and a shift in the moire spot positions for micro diffraction of such areas takes place. The stress relieved by wrinkle formation results from the mismatch in thermal expansion coefficients of graphene and the substrate. A simple one-dimensional model taking into account the energies related to strain, delamination and bending of graphene is in qualitative agreement with our observations.

Interplay of Wrinkles, Strain, and Lattice Parameter in Graphene on Iridium

Following graphene growth by thermal decomposition of ethylene on Ir(111) at high temperatures we analyzed the strain state and the wrinkle formation kinetics as function of temperature. Using the moiré spot separation in a low energy electron diffraction pattern as a magnifying mechanism for the difference in the lattice parameters between Ir and graphene, we achieved an unrivaled relative precision of ±0.1 pm for the graphene lattice parameter. Our data reveals a characteristic hysteresis of the graphene lattice parameter that is explained by the interplay of reversible wrinkle formation and film strain. We show that graphene on Ir(111) always exhibits residual compressive strain at room temperature. Our results provide important guidelines for strategies to avoid wrinkling.

Growth temperature dependent graphene alignment on Ir(111)

The morphology of graphene monolayers on Ir(111) prepared by thermal decomposition of ethylene between 1000 and 1530 K was studied with high resolution low energy electron diffraction. In addition to a well-oriented epitaxial phase, randomly oriented domains are observed for growth temperatures between 1255 and 1460 K. For rotational angles of ±3° around 30° these domains lock-in in a 30° oriented epitaxial phase. Below 1200 K the graphene layer exhibits high disorder and structural disintegrity. Above 1500 K the clear moire spots reflect graphene in a single orientation epitaxial incommensurate phase.

Lost in reciprocal space? Determination of the scattering condition in spot profile analysis low-energy electron diffraction

The precise knowledge of the diffraction condition, i.e., the angle of incidence and electron energy, is crucial for the study of surface morphology through spot profile analysis low-energy electron diffraction (LEED). We demonstrate four different procedures to determine the diffraction condition: employing the distortion of the LEED pattern under large angles of incidence, the layer-by-layer growth oscillations during homoepitaxial growth, a G(S) analysis of a rough surface, and the intersection of facet rods with 3D Bragg conditions.

Lattice-matching periodic array of misfit dislocations: Heteroepitaxy of Bi(111) on Si(001)

In spite of the large lattice mismatch between Bi and Si, it is possible to grow expitaxial Bi(111) films on Si(001) substrates, which are atomically smooth and almost free of defects. The remaining lattice mismatch of 2.3% is accommodated by the formation of a periodic array of edge-type dislocations confined to the interface. The strain fields surrounding each dislocation cause a weak periodic surface undulation, which results in the splitting of all spots in low-energy electron diffraction (LEED). From a high resolution spot profile analyzing LEED study, an amplitude of 0.66 A and a separation of 200 A were derived. Comparison with elasticity theory gives a full lattice spacing of the Si surface as a Burgers vector b-vector=(1/2)[110] of the misfit dislocation array. With increasing thickness, the Bi film relaxes toward its bulk lattice constant.

Erratum: Epitaxial Growth of Bi(111) on Si(001) [e-J. Surf. Sci. Nanotech. Vol. 7, pp. 441-447 (2009)]

In this article [1], one of the coauthors, B. Krenzer, is missing in the author list by mistake. The correct author list is the same as the one shown in this erratum.