Sforzini J, Nemec L, Denig T, Stadtmüller B, Lee TL, Kumpf C, Soubatch S, Starke U, Rinke P, Blum V, Bocquet FC, Tautz FS. Approaching Truly Freestanding Graphene: The Structure of Hydrogen-Intercalated Graphene on 6H-SiC(0001). Physical Review Letters 114, doi:10.1103/PhysRevLett.114.106804 (2015).
Graphene; X-ray standing wave; SiC(0001).
Graphene is a single layer hexagonal lattice form of carbon known for its remarkable electronic and structural properties. Applications include transistors in nanoscale devices, fuel cells, lithium-ion batteries, and high strength composite materials. Graphene in electronics is almost always supported by another material and to preserve its electronic properties, the interaction with the associated substrate has to be as small as possible. The precise material properties of graphene depend on the growth conditions on the substrate and its interaction with the substrate. Here, a supported form of graphene termed ‘hydrogen intercalated quasi free standing monolayer graphene’ (QFMLG) was grown on a hydrogen-saturated silicon carbide (SiC) surface (6H-SiC(0001)). The hydrogen rich SiC surface was expected to provide a low interaction with graphene according to theoretical calculations involving density functional theory and van der Waals correction. Normal Incidence X-ray Standing Wave (NIXSW) spectroscopy combining both X-ray diffraction and X-ray photoelectron spectroscopy was used to determine the vertical distance between graphene and SiC layers.
Using the Surface and Interface Structural Analysis beamline (I09) at Diamond Light Source, NIXSW measurements showed that the predicted graphene-Si distance of 4.16 Å was in excellent agreement with the theoretical findings. This indicates that QFMLG is effectively decoupled from the SiC surface hydrogen intercalation and will have superior electronic properties akin to pure graphene. Results also show that NIXSW Spectroscopy to measure adsorption distance is a very sensitive and suitable technique to assess the quality of graphene for electronic applications.
During the past decade, graphene attracted broad interest for its structural and electronic properties, which makes it a promising material for a wide range of applications, e.g. transistors in nanoscale devices1 and energy storage2. The precise material properties of graphene depend on the growth conditions on a given substrate and its interaction with the substrate. In order to be able to preserve its unique electronic properties, it is paramount to understand the coupling between the graphene layer and the substrate, in particular in terms of covalent and non-covalent bonding, residual corrugation, and doping. To characterise and compare the quality of graphene layers grown on different substrates, Angle Resolved Photoemission (ARPES) is commonly used. It measures the valence band of graphene, and most importantly the Dirac cone. The doping level can then easily be extracted, and the full width at half maximum (FWHM) of the measured π-band gives an indication of the relative interaction strength with the substrate. However, in the interesting case of very low interaction, for which the FWHM of the π-band is small, one reaches the instrumental limits of ARPES. Using a different approach is thus necessary.
Figure 1: Comparison of the overlap Δ for different epitaxial graphene systems. Δ is calculated by subtracting zG−sub from the sum of graphene and substrate vdW radii. The empty and the filled squares correspond to DFT and measured values, respectively. The experimental point for QFMLG stems from the present work. The inset presents a graphical illustration of Δ. Adapted from Phys. Rev. Lett. 114 (2015) 106804.
In this work, a qualitative analysis in terms of overlapping van der Waals (vdW) radii is proposed. The overlap is defined by Δ = rG vdW + rsub vdW – zG-sub. Here rG vdW is the vdW radius of graphene, rsub vdW is the vdW radius of the topmost atoms of the substrate, and zG-sub is the measured distance between graphene and the substrate. The definition of the overlap Δ is illustrated in the inset of Fig. 1. Δ > 0 means that the vdW radii of the graphene and the substrate overlap, indicating some degree of chemical interaction. On the other hand, for Δ ≲ 0, the graphene-substrate interaction is expected to be very weak. To test this approach, a sample expected to exhibit one of the lowest substrate interactions is measured, namely hydrogen intercalated quasi freestanding monolayer graphene (QFMLG) grown on 6H-SiC(0001)3. It consists of a graphene layer in contact with a hydrogen passivated SiC(0001) substrate, as shown in Fig. 2.
Figure 2: a) Summary of vertical distances measured by NIXSW on QFMLG. The position of the Bragg planes around the surface are indicated by blue lines;b) DFT PBE+vdW calculated geometry for QFMLG on 6H-SiC(0001). All values are given in Å. Adapted from Phys. Rev. Lett. 114 (2015) 106804.
The vertical distances z between the different atomic species present at the surface have been measured by NIXSW. The NIXSW experiments were performed in an ultra-high vacuum end station at beamline I09 at Diamond equipped with a VG Scienta EW4000 hemispherical electron analyser (acceptance angle of 60°) perpendicular to the incident beamdirection. All data sets were recorded at room temperature and in a normal incidence geometry. A photon energy of approximately 2463 eV was used to reach the 6H-SiC(0006) reflection, which corresponds to a Bragg plane spacing of 2.517 Å. The NIXSW method, combining dynamical X-ray diffraction and photoelectron spectroscopy, is a powerful tool for determining the vertical adsorption distances at surfaces with sub-Å accuracy and high chemical sensitivity. In addition, ARPES using monochromatised He Iα radiation and low energy electron diffraction were used to check the electronic and structural properties.
The C atoms present in the SiC surface (CSiC surf) and in graphene (G) give rise to components at binding energies of 283.1 and 284.7 eV, respectively (Fig. 3a) in the C 1s core level spectrum. The Si 2s core level is found at 152.2 eV (Fig. 3b) for the Si atoms in the SiC surface (SiSiC surf). The photoelectron yield of each chemical species is deduced from the peak area that was determined by a line-shape analysis of the core-level spectrum. This is repeated for 30 photon energies within a 2 eV range around the Bragg energy (EBragg ) for all three species. Following a well-established procedure, the reflectivity of the X-ray beam and photoelectron yield curves are fit within the framework of dynamical diffraction theory to determine the heights of the three different species with respect to the bulk-extrapolated SiC(0006) atomic plane. The CSiC surf atoms are located at 0.61±0.04 Å below the bulk-extrapolated silicon plane, while the SiSiC surf atoms are found 0.05±0.04 Å above this plane. Thus, an experimental Si-C distance of 0.66±0.06 Å is obtained, in agreement with the SiC crystal structure. In the same way, the adsorption height of the graphene layer is determined with respect to the topmost Si layer as 4.22±0.06 Å, as shown in Fig. 2a. This height is approximately equal to the sum of the vdW radii of carbon and hydrogen (plus the Si-H distance of approximately 1.50 Å), and thus indicates the absence of interactions besides vdW. Density Functional Theory (DFT) simulations, on the large experimentally observed unit cell and including state-of-the-art van der Waals corrections (PBE+vdW), predicted a graphene-Si distance of 4.16 Å, in excellent agreement with the experimental findings.
Figure 3: NIXSW data measured for QFMLG on 6H-SiC(0001). a) C 1s core level, fitted with two asymmetric Lorentzians. G and CSiC surf correspond to the graphene and the surface carbon atoms of SiC, respectively;b) Si 2s core level fitted with a pseudo-Voigt function;c) experimental photoelectron yield curves (black dots) versus photon energy relative to the (0006) Bragg energy. The coloured lines are the best fits to the yield curves. The absolute distances for each component are given with respect to the topmost silicon atoms. The X-ray reflectivity R is plotted with black diamonds and its best fit in red. Adapted from Phys. Rev. Lett. 114 (2015) 106804.
In order to check the ability of the overlap approach to compare graphene/substrate systems with very low interaction, the present result for QFMLG is plotted in Fig. 1 together with other results taken from the literature for which the graphene/substrate distance have been measured or calculated. Besides QFMLG, both epitaxial monolayer graphene (EMLG) and graphene grown on Ir(111) also exhibit a low overlap Δ. EMLG consists of a buffer-layer (a layer composed of a carbon honeycomb lattice covalently bound to the non-passivated SiC substrate) on top of which a single graphene layer is epitaxially grown. Note that these two systems also present a low band broadening in ARPES. However, the present overlap analysis reveals that QFMLG on SiC exhibits by far the lowest value.
In conclusion, NIXSW measurements show that QFMLG is the system having the largest adsorption distance among the studied graphene–substrate systems; in particular, the overlap vanishes, suggesting a very effective decoupling of the graphene layer from its substrate. These experimental findings are supported by DFT PBE+vdW calculations. This significant difference between QFMLG and EMLG, for example, translates into a dramatic improvement of transistors after hydrogen intercalation4. It suggests that the adsorption distance is a valid parameter to assess the ideality of graphene.
F. C. B. acknowledges financial support from the Initiative and Networking Fund of the Helmholtz Association, Postdoc Programme VH-PD-025.
Dr François C. Bocquet, Peter Grünberg Institut (PGI- 3) and Jülich Aachen Research Alliance (JARA), firstname.lastname@example.org.
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