Impact of Substrate Steps and of Monolayer-Bilayer Junctions on the Electronic Transport in Epitaxial Graphene on 4H-SiC (0001)
Filippo GiannazzoIoannis DeretzisAntonino La MagnaSalvatore Di FrancoNicolò PilusoPatrick FiorenzaFabrizio RoccafortePatrick SchmidWilfried LerchRositza Yakimova
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Two dimensional maps of the electronic conductance in epitaxial graphene (EG) grown on SiC were obtained by conductive atomic force microscopy (CAFM). The correlation between morphological and electrical maps revealed the local conductance degradation in EG over the SiC substrate steps or at the junction between monolayer (1L) and bilayer (2L) graphene regions. The effect of steps strongly depends on the charge transfer phenomena between the step sidewall and graphene, whereas the resistance increase at 1L/2L junction is a purely quantum mechanical effect, due to the weak coupling between 1L and 2L electron wavefunctions.Keywords:
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The conductance through single-layer graphene (SLG) and AA/AB-stacked bilayer graphene (BLG) junctions is obtained by taking into account band gap and bias voltage terms. First, we consider gapped SLG, while in between, they are connected into pristine BLG. For Fermi energy larger than the interlayer hopping, the conductance as a function of the bilayer region length $d$ reveals two different models of anti-resonances with the same period. As a function of the band gap, with AA-BLG stacking, the results show that the conductance has the same minima whatever the value of $d$, and for AB-BLG, $d$ remains relevant such that the system creates a global energy gap. Second, we consider pristine SLG, and in between, they are connected to gapped-biased BLG. We observe the appearance of peaks in the conductance profile with different periods and shapes, and also the presence of Klein tunneling with zero conductance in contrast to the first configuration. When $ d $ is less than 10, $G(E)$ vanishes and exhibits anti-Klein tunneling as a function of the Fermi energy $E$. We also investigate the conductance as a function of the bias. For AA-BLG, the results show antiresonances and diminish for a large value of the bias, independently of the bilayer region of length. In contrast, the conductance in AB-BLG has distinct characteristics in that it begins conducting with maxima for small $E$ and with minima for large $E$.
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High quality sub-monolayer, monolayer, and bilayer graphene were grown on Ru(0001). For the sub-monolayer graphene, the size of graphene islands with zigzag edges can be controlled by the dose of ethylene exposure. By increasing the dose of ethylene to 100 Langmuir at a high substrate temperature(800℃), high quality single-crystalline monolayer graphene was synthesized on Ru(0001). High quality bilayer graphene was formed by further increasing the dose of ethylene while reducing the cooling rate to 5℃/min. Raman spectroscopy revealed the vibrational states of graphene, G and 2D peaks appeared only in the bilayer graphene, which demonstrates that it behaves as the intrinsic graphene. Our present work affords methods to produce high quality sub-monolayer, monolayer, and bilayer graphene, both for basic research and applications.
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There are quite a few types of equation of state and abundant π−a curves of various lipid monolayers at the air−water or the oil−water interface in the literature. However, it has been a problem to interpret mechanochemical properties of bilayer vesicles from the π−a information of the monolayer. In fact, even the bilayer surface pressure has not yet been well characterized although the monolayer surface pressure has already been traditionally defined as the lowering in the surface tension from the clean interfacial tension due to the presence of the monolayer. The monolayer−bilayer correspondence problem, therefore, could not be well defined and completely solved despite its importance in practice to apply the monolayer π−a data to elucidate bilayer vesicle properties. In the present analysis, we thus first define the bilayer leaflet pressure as the intrinsic pressure of the lipid layer−water substrate system. This intrinsic surface pressure should be the same function of the lipid density and the temperature for both the monolayer at an interface and a leaflet of bilayer vesicles. Therefore, the difference between a leaflet of a bilayer and the monolayer at an interface is merely that the latter, but not the former, exhibits a microscopic interfacial tension between the air or the oil and the lipid layer. We show that the value of this intrinsic pressure agrees with that of the traditional monolayer pressure if the macroscopic and microscopic hydrophobic effect assumes the same magnitude. We conclude that the equilibrium pressure between the monolayer and bilayer vesicles is equal in magnitude to the microscopic interfacial tension between the water and the monolayer, which, in first approximation, is equal to the macroscopic oil−water interfacial tension, i.e., ca. 49 mN/m. This conclusion agrees with that briefly derived by pioneers (Gruen and Wolfe, 1982; Nagle, 1976, 1986; Jähnig, 1984). We further develop from mechanics and thermodynamics of membranes a procedure to obtain either analytically from a theoretical or empirical equation of state, or graphically from the π−a curve of the monolayer at an interface, mechanochemical properties of this monolayer and bilayer vesicles. The method is exemplified for the monolayer and bilayer vesicles of dilauroyl phosphatidylethanolamine (DLPE).
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The electrostatic binding and metal coordination between metal ions and Langmuir monolayers or LB films are discussed, and their effects on the monolayer 2D structure and related phase behavior are analyzed. The interfacial recognition and sensing for metal ions by Langmuir monolayers are also shown. Langmuir monolayers and LB films as 2D template to induce the 2D-oriented crystal growth via metal/monolayer binding is especially demonstrated. The abnormal catalytic characteristics, the functions and devices of metal-incorporated Langmuir monolayers and LB films are displayed by some examples. The review also shows the application of metal-chelating lipid monolayers on the interfacial study of bio-macromolecules. The review suggests the great roles of metal/monolayer binding in alternating monolayer structures and the assembly of functional metal complexes.
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The molecular interactions of monolayers composed of cyclic and linear forms of surfactins (SFs) were evaluated through atomic force microscopy (AFM) together with a Langmuir monolayer technique. The surface pressure (π)-area per molecule (A) isotherm of a pure cyclic surfactin (CSF) monolayer exhibited a liquid expanded (Le) monolayer, while that of a pure linear surfactin (LSF) monolayer exhibited a liquid condensed (Lc) monolayer, demonstrating that the CSFs are in a rather loose molecular packing state owing to its bulky heptapeptide ring. The plots of the mean area per molecule of the CSF/LSF monolayers were well fitted to the ideal curves, suggesting that ideal mixing occurs, or that the two components are immiscible in a monolayer. The AFM images of the CSF/LSF monolayers transferred at 25 mN/m gave phase-separated microdomain structures, indicating that the CSFs and LSFs are almost immiscible and separated into a CSF-rich and LSF-rich phases, as suggested from the analysis of the mean area per molecule of the monolayers. Our results clearly demonstrated that the cleavage of the cyclic heptapeptide headgroup of CSF drastically changes its molecular packing state in a monolayer and that AFM observation combined with the Langmuir monolayer technique is quite useful to explore the manner of self-assembly of a binary system of microbial products such as CSFs and LSFs.
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Due to the unique bandgap tunability of bilayer graphene, the preparation of large‐sized bilayer graphene has attracted a wide range of attention. Herein, the preparation of bilayer graphene, from stacking order to growth mechanism, is reviewed, and the chemical vapor deposition (CVD) of AB‐stacked bilayer graphene on copper substrate is emphasized. Various methods and growth strategies to synthesize bilayer graphene and the corresponding growth mechanisms are discussed. Mechanisms of layer‐by‐layer growth, the hydrogen passivation of graphene edges for the formation of bilayers, and carbon atoms penetrating through a copper wall for bilayer growth are included and highlighted for a better understanding of controlling bilayer graphene uniformity and forming its stacking order. Finally, the remaining challenges and the potential development of CVD‐controlled growth of bilayer graphene are outlined.
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