Among the different graphene synthesis methods, chemical vapor deposition of graphene on low cost copper foil shows great promise for large scale applications. Here, we present growth experiments to ...obtain high quality graphene and its clean transfer onto any substrates. Bilayer-free monolayer graphene was obtained by a careful pre-annealing step and by optimizing the H sub(2) flow during growth. The as-grown graphene was transferred using an improved wet chemical graphene transfer process. Some major flaws in the conventional wet chemical, polymethyl methacrylate (PMMA) assisted, graphene transfer process are addressed. The transferred graphene on arbitrary substrates was found to be free of metallic contaminants, defects (cracks, holes or folds caused by water trapped beneath graphene) and PMMA residues. The high quality of the transferred graphene was further evidenced by angle resolved photoelectron spectroscopy studies, for which the linear dependency of the electronic band structure characteristic of graphene was measured at the Dirac point. This is the first Dirac cone observation on the CVD grown graphene transferred on some 3D bulk substrate.
In recent years there has been growing interest in the electronic properties of 'few layer' graphene films. Twisted layers, different stacking and register with the substrate result in remarkable ...unconventional couplings. These distinctive electronic behaviours have been attributed to structural differences, even if only a few structural determinations are available. Here we report the results of a structural study of bilayer graphene on the Si-terminated SiC(0001) surface, investigated using synchrotron radiation-based photoelectron diffraction and complemented by angle-resolved photoemission mapping of the electronic valence bands. Photoelectron diffraction angular distributions of the graphene C 1s component have been measured at different kinetic energies and compared with the results of multiple scattering simulations for model structures. The results confirm that bilayer graphene on SiC(0001) has a layer spacing of 3.48 Å and an AB (Bernal) stacking, with a distance between the C buffer layer and the first graphene layer of 3.24 Å. Our work generalises the use of a versatile and precise diffraction method capable to shed light on the structure of low-dimensional materials.
•N-polar InN epilayers grown by PA-MBE on GaN/AlN/Al2O3(0001).•Growth temperature effect on physical and mechanical features was analysed.•Optimal growth temperature leads to good physical/mechanical ...performances.
A set of N-polar InN epilayers has been grown at different temperatures by plasma–assisted molecular beam epitaxy (PA-MBE) on GaN/AlN/Al2O3(0001) templates. The purpose is to understand how the variation of crucial factor, such as the temperature, impacts the growth process and the resulting samples’ properties. The characterization of these InN samples using atomic force microscopy and scanning electron microscopy showed different island distributions and shapes by varying the growth temperature. High resolution-X-ray diffraction (HRXRD) enabled to identify a single crystalline phase (hexagonal wurtzite), whatever the growth temperature. Actually, the increase of growth temperature up to 560 °C has improved the crystalline quality; whereas for high temperature, the crystalline quality degrades. The dislocation density of the epilayer grown at this optimum temperature (around 560 °C) is about 1.9 × 1010 cm−2, which is determined using HRXRD spectra analysis. High compressive residual stress value of 0.54 GPa was derived using Raman spectroscopy. Room temperature photoluminescence (PL) displays a band gap energy around 0.69 eV. Besides, Burstein-Moss effect, the PL band gap energy measured at 10 K is dictated by the biaxial compressive residual stresses. Nanoindentation tests were carried out on InN epilayers. Only, the sample grown at 560 °C exhibited a pop-in event, for which the measured hardness and Young’s modulus are of 4.5 ± 0.5 GPa and of 171 ± 8 GPa, respectively. Accordingly, the growth temperature of InN epilayers influences the resulting physical and mechanical performances, thus a good compromise between physical and mechanical features permits to manufacture efficient devices.
The structural and electronic properties of twisted bilayer graphene (TBG) on SiC(000) grown by Si flux-assisted molecular beam epitaxy were investigated using scanning tunneling microscopy (STM) and ...angle-resolved photoelectron spectroscopy with nanometric spatial resolution. STM images revealed a wide distribution of twist angles between the two graphene layers. The electronic structure recorded in single TBG grains showed two closely-spaced Dirac π bands associated to the two stacked layers with respective twist angles in the range 1-3°. The renormalization of velocity predicted in previous theoretical calculations for small twist angles was not observed.
Solar neutrinos are produced in the core of the Sun in different nuclear reactions all based on the conversion of hydrogen into helium, releasing energy and making the Sun shine. Until now, the ...observation of solar neutrinos has demonstrated: a) the nuclear origin of the Sun's energy; b) that the ve produced were undergoing lepton flavor transformation into vμ or vτ, the neutrino oscillation mechanism. In the recent years, the Borexino experiment, in the Gran Sasso underground laboratory, has made significant contributions to the solar neutrino spectroscopy: first observation and precision measurement of the 7Be neutrinos, first observation of the pep reaction, stringent limit on CNO neutrinos, observation of 8B neutrinos with a 3 MeV threshold. These measurements reinforce the so-called LMA solution of the neutrino oscillation explaining the solar ve survival probability as a function of energy.
The direct elaboration of graphene by molecular beam epitaxy (MBE) has been studied, using a solid carbon source. Successful growth has been achieved on both the carbon and silicon terminated faces ...of silicon carbide substrates in the temperature range 1000–1100 °C, as confirmed by low energy electron diffraction (LEED) and X‐ray photoemission spectroscopy (XPS) analysis. Atomic force microscopy (AFM) observations show that the initial substrate structure, i.e. flat atomic terraces and half‐period high steps, remains almost unaffected during the growth, contrary to what is observed following the graphitization process.
The practical difficulties to use graphene in microelectronics and optoelectronics is that the available methods to grow graphene are not easily integrated in the mainstream technologies. A growth ...method that could overcome at least some of these problems is chemical vapour deposition (CVD) of graphene directly on semiconducting (Si or Ge) substrates. Here we report on the comparison of the CVD and molecular beam epitaxy (MBE) growth of graphene on the technologically relevant Ge(001)/Si(001) substrate from ethene (C2H4) precursor and describe the physical properties of the films as well as we discuss the surface reaction and diffusion processes that may be responsible for the observed behavior. Using nano angle resolved photoemission (nanoARPES) complemented by transport studies and Raman spectroscopy as well as density functional theory (DFT) calculations, we report the direct observation of massless Dirac particles in monolayer graphene, providing a comprehensive mapping of their low-hole doped Dirac electron bands. The micrometric graphene flakes are oriented along two predominant directions rotated by 30° with respect to each other. The growth mode is attributed to the mechanism when small graphene "molecules" nucleate on the Ge(001) surface and it is found that hydrogen plays a significant role in this process.
The static and dynamic characteristics of top-gated graphene nanoribbon-based field-effect transistors were investigated. Multilayer graphene was synthesized by thermal decomposition of Si-face ...silicon carbide. The impact of the number of graphene layers on device performance was explored. It was found that, with the reduction of the layer number from ten to five, a significant improvement of direct-current characteristics and high-frequency performance can be observed. A high intrinsic current-gain cutoff frequency of 60 GHz and a maximum oscillation frequency of 28 GHz are reported.
High-energy and k-space resolution angle-resolved photoemission spectroscopy experiments were achieved on nominally single and bilayer graphene grown by Si-flux assisted molecular beam epitaxy (MBB) ...on the C-face of SiC. This material shows the same structure as the graphene grown by standard high-temperature annealing of SiC, noticeably the rotational disorder and the very weak electronic coupling between stacked layers. The SiC substrate induces a strong doping by charge transfer, with a Dirac point located 320 meV below the Fermi level for monolayer graphene. The efficient screening by the successive graphene layers results in a reduction of this value to 190 meV for bilayer graphene. The opening of an energy band gap, whose width is inversely dependent on the thickness, is also reported. These measurements emphasize the potentialities of the Si-flux assisted MBB technique, more particularly for homogeneous low thickness graphene growth on the C-face of SiC.
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