Understanding the chemical vapor deposition (CVD) kinetics of graphene growth is important for advancing graphene processing and achieving better control of graphene thickness and properties. In the perspective of improving large area graphene quality, we have investigated in real-time the CVD kinetics using CH4–H2 precursors on both polycrystalline copper and nickel. We highlighted the role of hydrogen in differentiating the growth kinetics and thickness of graphene on copper and nickel. Specifically, the growth kinetics and mechanism is framed in the competitive dissociative chemisorption of H2 and dehydrogenating chemisorption of CH4, and in the competition of the in-diffusion of carbon and hydrogen, being hydrogen in-diffusion faster in copper than nickel, while carbon diffusion is faster in nickel than copper. It is shown that hydrogen acts as an inhibitor for the CH4 dehydrogenation on copper, contributing to suppress deposition onto the copper substrate, and degrades quality of graphene. Additionally, the evidence of the role of hydrogen in forming C–H out of plane defects in CVD graphene on Cu is also provided. Conversely, resurfacing recombination of hydrogen aids CH4 decomposition in the case of Ni. Understanding better and providing other elements to the kinetics of graphene growth is helpful to define the optimal CH4/H2 ratio, which ultimately can contribute to improve graphene layer thickness uniformity even on polycrystalline substrates.

Graphene CVD growth on copper and nickel: role of hydrogen in kinetics and structure

The electronic energy band structure, density of states (DOS) and charge density contour of KNbO3 in the paraelectric cubic phase have been studied using the full-potential linearized augmented plane wave method within the generalized gradient approximation for exchange and correlation. The band structure shows an indirect (R–Γ) band gap. From the DOS analysis as well as charge density studies, we find that the bonding between K and NbO3 is mainly ionic while that between Nb and O is covalent. We have also reported results on the pressure variation of the energy gap of this compound and found that the band gap increases with increasing pressure. In order to understand the optical properties of the perovskite, the real and imaginary parts of the dielectric function, reflectivity, absorption coefficient, optical conductivity, electron energy-loss function, refractive index and extinction coefficient were calculated. The general profiles of the optical spectra were analysed and origins of the structures discussed.

https://iopscience.iop.org/article/10.1088/0953-8984/15/35/304/pdf

Theoretical study of the electronic structure, chemical bonding and optical properties of KNbO3 in the paraelectric cubic phase

Reflection anisotropy spectroscopy (RAS) is a non-destructive optical probe of surfaces that is capable of operation within a wide range of environments. In this review we trace the development of RAS from its origins in the 1980s as a probe of semiconductor surfaces and semiconductor growth through to the present where it is emerging as a powerful addition to the wide range of existing ultra-high vacuum (UHV) surface science techniques. The principles, instrumentation and theoretical considerations of RAS are discussed. The recent progress in the application of RAS to investigate phenomena at metal surfaces is reviewed, and applications in fields including electrochemistry, molecular assembly, liquid crystal device fabrication and remote stress sensing are discussed. We show that the experimental study of relatively simple surfaces combined with continuing progress in the theoretical description of surface optics promises to unlock the full potential of RAS. This provides a firm foundation for the application of the technique to the challenging fields of ambient, high pressure and liquid environments. It is in these environments that RAS has a clear advantage over UHV-based probes for investigating surface phenomena, and its surface sensitivity, ability to monitor macroscopic areas and rapidity of response make it an ideal complement to scanning probe techniques which can also operate in such environments.

http://iopscience.iop.org/article/10.1088/0034-4885/68/6/R01/pdf

Reflection anisotropy spectroscopy

Characterization of plasmonic effects in thin films and metamaterials using spectroscopic ellipsometry

Using x-ray absorption spectroscopy recent progress is achieved all over in solid state physics. This review focuses on these advances, with particular emphasis on applications to surface physics and to magnetism of ultrathin 3d and 5d films that are made possible by the use of undulators in third generation synchrotron radiation sources: the unambiguous appearance of an atomic extended x-ray absorption fine structure for atomic adsorbates and of σ* resonances in near-edge x-ray absorption fine structure spectra of oriented molecules is demonstrated. The induced magnetism at the interfaces of 3d and 5d layers is studied by x-ray magnetic circular dichroism. Fundamental aspects of the spectroscopy are clarified for rare earth crystals. The determination of the ground state properties and the detailed understanding of the underlying mechanisms was obtained by comparison of the experimental data to state-of-the-art ab initio calculations.

https://qmlab.ubc.ca/ARPES/PEOPLE/Andrea/Teaching/PHYS555_2007/XASreview.pdf

Recent advances in x-ray absorption spectroscopy

The optical properties of copper and silver bulk crystals with atomically clean surfaces have been determined experimentally and interpreted in terms of ab initio band structure calculations. The dielectric functions are evaluated from spectroscopic ellipsometry data taken in ultrahigh vacuum (UHV) in the spectral range of 2.5--9.0 eV (at room temperature). The data are corrected for surface roughness using results from ex situ atomic force microscopy (AFM). Significant differences of detail in the amplitudes and line shape are attributed to the better surface quality of our samples. Density functional calculations of the dielectric functions of copper and silver are carried out, based on models of the valence bands deduced by fitting to experimental Fermi surface and quasiparticle mass data. Small energy shifts, which take into account many-body effects in the final states of the optical transitions in an extended scissors approximation, are needed to bring the calculated dielectric functions into good agreement with the experimental data. The interband transitions associated with individual features in the dielectric function are identified by comparing the energy derivatives of the measured and calculated dielectric functions.

Optical properties of copper and silver in the energy range 2.5-9.0 eV

The surface optical and electronical properties of Cu(110) surfaces were studied by using reflectance anisotropy spectroscopy (RAS) together with angle-resolved photoemission spectroscopy (ARUPS). On the clean surface, a structure in the optical spectra at 2.1 eV is assigned to transitions between occupied p-type and unoccupied s-type surface states occurring at the $\overline{Y}$ point of the surface Brillouin zone. Another structure at 4.2 eV is associated with a transition at the $\overline{X}$ point between a surface resonance (occupied) split off from the bulk d bands and a p-derived surface state (unoccupied). The oxygen-induced $(2\ifmmode\times\else\texttimes\fi{}1)$ surface exhibits a double peak in the reflectance anisotropy which can be explained partly by transitions from oxygen induced d- to empty p-derived states. Surface modified bulk d-states are responsible for parts of the features around 2 eV and features at higher energies.

Surface optical properties of clean Cu(110) and Cu(110)- ( 2 × 1 ) -O

GaAs (001) surfaces in both metalorganic vapour phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) are studied under As stabilisation as well as during growth by reflectance anisotropy spectroscopy (RAS). During MBE growth reflection high energy electron diffraction (RHEED) measurements are performed simultaneously. As a consequence of the oscillating density of steps during island growth, time resolved RAS measurements show oscillations with monolayer periodicity in both epitaxial systems. In order to separate morphology related and surface reconstruction domain related contributions to RAS oscillations an anisotropic effective medium model is applied. In both MBE and MOVPE domains of reduced As coverage close to the island boundaries are found to be responsible for RAS oscillations. In MBE this typically results in oscillations between spectra belonging to (2 x 4) reconstructions of different mean As dimer density. In MOVPE the surface oscillates between states with an increased and a decreased concentration of Ga dimers while c(4 x 4)-like As dimers are still present.

K. Stahrenberg

Papers

Optical properties of copper and silver in the energy range 2.5-9.0 eV

Surface optical properties of clean Cu(110) and Cu(110)- ( 2 × 1 ) -O

Reflectance anisotropy oscillations during MOCVD and MBE growth of GaAs (001)

The influence of surface steps on the optical and electronic anisotropy of Ag(110)

Surface processes responsible for reflectance-anisotropy oscillations during molecular beam epitaxy growth of GaAs(001)