Science news — 15/03/2017

Intercalated epitaxial graphene

Our colleagues Marin Petrović and Marko Kralj together with collaborators from Germany, Spain, Sweden, and Israel have recently published two papers in journals with high impact factors on the topic of intercalated epitaxial graphene.

 

Core level shifts of intercalated graphene

Ulrike A. Schröder, Marin Petrović, Timm Gerber, Antonio J. Martinez-Galera, Elin Grånäs, Mohammad A. Arman, Charlotte Herbig, Joachim Schnadt, Marko Kralj, Jan Knudsen, Thomas Michely

2D Materials 4, 015013 (2017)

doi: 10.1088/2053-1583/4/1/015013

 

Graphene is an allotropic modification of carbon in the form of two-dimensional honeycomb lattice which is only one atom thick. Thanks to its high specific strength, electric conductivity, heat conductivity, and optical transparency, it is highly interesting for application as well as fundamental research. Together with colleagues from Germany, Sweden, and Spain, Marin Petrović and Marko Kralj placed graphene on iridium in order to determine how the energies of deep, core levels of carbon shift in the presence of different intercalated layers. Namely, intercalation is expected to lead to electron- or hole-doping of epitaxial graphene, an effect which should be seen in x-ray photoemission spectroscopy (XPS) as a shift of the carbon 1s level with respect to neutral graphene.

The position of the Dirac point of graphene was precisely determined from angle-resolved photoemission spectroscopy (ARPES) data which was used for comparison with XPS measurements in this work and from literature. Within a relatively wide doping range of +/- 0.5 eV, linear shifts of C deep levels and Dirac point of graphene are observed, which suggests a rigid band shift of the whole electron structure. However, higher doping reduces the deep level shift and eventually changes its sign, which is attributed to screening by a considerable amount of transferred charge. The observed behavior depends only on the amount of charge and not on the type of intercalated layers. Hence, the paper proposes that determining the C 1s level via XPS measurement is a universal probe of graphene doping and is even applicable for devices where the charge carrier concentrations are controlled by electric field, like in field-effect transistors.

Plot of the C 1s binding energy (BE) as a function of the Dirac point binding energy ED. Colored dots represent data measured by some of  the authors, red squares are data from literature. Additional open black circle in brackets corresponds to saturated Li intercalation. Straight black line: prediction of the rigid band model. Blue line: experimentally determined nonlinear dependence.

Plot of the C 1s binding energy (BE) as a function of the Dirac point binding energy ED. Colored dots represent data measured by some of the authors, red squares are data from literature. Additional open black circle in brackets corresponds to saturated Li intercalation. Straight black line: prediction of the rigid band model. Blue line: experimentally determined nonlinear dependence.

 

 

Energy dependent chirality effects in quasi free-standing graphene

Daniela Dombrowski, Wouter Jolie, Marin Petrović, Sven Runte, Fabian Craes, Jürgen Klinkhammer, Marko Kralj, Predrag Lazić, Eran Sela, Carsten Busse

Physical Review Letters 118, 116401 (2017).

doi: 10.1103/PhysRevLett.118.116401

 

In order to describe electron properties of graphene in full, one has to take into account the so-called chirality or “pseudospin” which roughly describes the connection between the direction of motion of electrons and the amplitude of their wave function. This second paper with colleagues from Germany and Israel is published in Physical Review Letters and analyzes the scattering of charge carriers on defects in doped epitaxial graphene by using the low-temperature scanning tunneling microscopy (STM) and spatial spectroscopy (scanning tunneling spectroscopy, STS). In spectroscopic maps, the Fourier transform (FT) of standing electron waves is directly related to maps of constant energy of Dirac electrons. The intensity of contours in the FT reflects the symmetry rules for charge carrier scattering of rules, ie. the chirality of graphene electrons. Detailed maps of constant energy were obtained which allowed reliable determination of Dirac dispersion, including the trigonal warping as well as adjusting the van Hove singularity (EVHS) near the M point of graphene.

 

Scattering patterns in strongly doped graphene. (a) Inverted low energy electron diffraction (LEED) pattern of graphene-cesium-indium at 143.5 eV, the intercalated Cs forms a (2 × 2)gr superstructure. (b) STM image and (c) simultaneously recorded STS map of the two different standing wave patterns at voltage U = -50 mV and current I = 500 pA. (d) Fourier transformation of image (c) reveals triangular and hexagonal scattering patterns. The dashed hexagon indicates the first Brillouin zone. The right side shows a magnification of the framed areas.

Scattering patterns in strongly doped graphene. (a) Inverted low energy electron diffraction (LEED) pattern of graphene-cesium-indium at 143.5 eV, the intercalated Cs forms a (2 × 2)gr superstructure. (b) STM image and (c) simultaneously recorded STS map of the two different standing wave patterns at voltage U = -50 mV and current I = 500 pA. (d) Fourier transformation of image (c) reveals triangular and hexagonal scattering patterns. The dashed hexagon indicates the first Brillouin zone. The right side shows a magnification of the framed areas.

 

The formation of standing waves reveals two scattering processes: one inside the Dirac cone (intervalley) and one between the two Dirac cones in the K and K’ points of the Brillouin zone (intravalley). The two processes are associated with two different wave vectors. The experiments shown in this paper confirm that the pseudospin is conserved only close to the area of perfect conical dispersion. Pseudospin conservation, ie. chirality of Dirac electrons, blocks the two scattering processes and results in a partially broken FT contour. At energies above 0.5EVHS with respect to the Dirac point, chirality disappears completely and all scattering channels are allowed. Modelling of scattering processes shows that the observed behavior is not a consequence of simple graphene symmetry breaking due to intercalation; in fact it is an intrinsic property of graphene and its electron structure which is potentially of great use in spintronics and chiralotronics.

IF Ⓒ 2017 Ndoc Deda