Search

Intercalation of metal atoms at the SiC(0001)-graphene (Gr) interface can provide confined 2D metal layers with interesting properties. The intercalated Pb monolayer (ML) has shown the coexistence of the Gr(10x10)-moire and a stripe phase, which still lacks understanding. Using density functional theory calculation and thermal annealing with ab initio molecular dynamics, we have studied the formation energy of SiC(0001)/Pb/Gr for different coverages of intercalated Pb. Near the coverage of a Pb(111)-like ML mimicking the (10x10)-moire, we find a slightly more stable stripe structure, where one half of the structure has compressive strain with Pb occupying the Si-top sites and the other half has tensile strain with Pb off the Si-top sites. This stripe structure along the Gr zigzag direction has a periodicity of 2.3 nm across the [1-210] direction agreeing with the previous observations using scanning tunneling microscopy. Analysis with electron density difference and density of states show the tensile region has a more metallic character than the compressive region, while both are dominated by the charge transfer from the Pb ML to SiC(0001). The small energy difference between the stripe and Pb(111)-like structures means the two phases are almost degenerate and can coexist, which explains the experimental observations., Comment: 25 pages, 8 figures

We report a nano-infrared (IR) imaging study of trilayer graphene (TLG) with both ABA (Bernal) and ABC (rhombohedral) stacking orders using the scattering-type scanning near-field optical microscope (s-SNOM). With s-SNOM operating in the mid-IR region, we mapped in real space the surface plasmon polaritons (SPPs) of ABA-TLG and ABC-TLG, which are tunable with electrical gating. Through quantitative modeling of the plasmonic imaging data, we found that the plasmon wavelength of ABA-TLG is significantly larger than that of ABC-TLG, resulting in a sizable impedance mismatch and hence a strong plasmon reflection at the ABA/ABC lateral junction. Further analysis indicates that the different plasmonic responses of the two types of TLG are directly linked to their electronic structures and carrier properties. Our work uncovers the physics behind the stacking-dependent plasmonic responses of TLG and sheds light on future applications of TLG and the ABA/ABC junctions in IR plasmonics and planar nano-optics., Comment: 11 pages

A general method was developed to intercalate metals under layered materials through a controlled density of sputtered defects. The method has been already applied to study a range of metals intercalated under graphite and different types of morphologies were realized. In the current work, we extend the method to the study of intercalation under MoS2 noting that work on this system is rather limited. We use Cu as the prototype metal for comparison with Cu intercalation under graphite. Although the growth conditions needed for intercalation under graphite and MoS2 are similar, the type of intercalated phases is very different. Each Cu island which nucleates on top of MoS2 during Cu deposition provides material that is transferred below MoS2, through sputtered defects under the island base; this transfer results in a uniform intercalated Cu "carpet" morphology that extends over the mesoscale. On the contrary, Cu intercalation under graphite results in well separated, compact islands formed by monomer detachment from small Cu islands on top and transfer below through defects far from the islands. The structural techniques (scanning electron microscopy and atomic force microscopy) and spectroscopic techniques (x-ray photoelectron spectroscopy and energy dispersive spectroscopy) are used for the characterization of the intercalated Cu layer.

Previous experimental observations for H intercalation under graphene on SiC surfaces motivate clarification of configuration stabilities and kinetic processes related to intercalation. From first-principles density-functional-theory (DFT) calculations, we analyze H adsorption and intercalation for graphene on a 6H-SiC(0001) surface, where the system includes two single-atom-thick graphene layers: the top-layer graphene (TLG) and the underling buffer-layer graphene (BLG) above the terminal Si layer. Our chemical potential analysis shows that, in the low-H coverage regime (described by a single H atom within a sufficiently large supercell), intercalation into the gallery between TLG and BLG, or into the gallery underneath BLG, is more favorable thermodynamically than adsorption on top of TLG. However, intercalation into the gallery between TLG and BLG is most favorable. We obtain energy barriers of about 1.3 eV and 2.3 eV for a H atom diffusing on and under TLG, respectively. From an additional analysis of the energy landscape in the vicinity of a step on the TLG, we assess how readily one guest H atom on the TLG terrace can directly penetrate the TLG into the gallery between TLG and BLG versus crossing a TLG step to access the gallery. We also perform DFT calculations for higher H coverages revealing a shift in favorability to intercalation of H underneath BLG, as well as characterizing the variation with H coverage in interlayer spacings.

Functional manipulation of graphene is an important topic in view of both fundamental researches and practical applications. In this study, we show that intercalation of 5$d$ transition metals in epitaxial graphene on SiC is a promising approach to realize topologically nontrivial phases with a finite band gap in graphene. Using first-principles calculations based on density functional theory, we show that the Re- and Ta-intercalated graphene become two-dimensional topological insulators which exhibit linear Dirac cones and quadratic bands with topological band gaps, respectively. The appearance of the topological states is attributed to the strong spin-orbit coupling strength of the intercalants. We find that topological edge states exist within the finite bulk band gap in accordance with the bulk-boundary correspondence. We also discuss the spin splitting of the band structure due to the inversion symmetry breaking and the spin-orbit coupling. Our results demonstrate that intercalation of graphene is an effective and viable method to manipulate the band gap and the topological character of graphene. Such intercalated graphene systems are potentially useful for spintronics and quantum computing applications., Comment: 11 pages, 4 figures

X-ray magnetic circular dichroism (XMCD) reveal the magnetic properties of intercalated europium metal under graphene on SiC(0001). Intercalation of Eu nano-clusters (average size 2.5 nm) between graphene and SiC substate are formed by deposition of Eu on epitaxially grown graphene that is subsequently annealed at various temperatures while keeping the integrity of the graphene layer. Using sum-rules analysis of the XMCD of Eu M$_{4,5}$ edges at $T = 15$ K, our samples show paramagnetic-like behavior with distinct anomaly at T $\approx$ 90 K which may be related to the N{\`e}el transition, T$_N$ = 91 K, of bulk metal Eu. We find no evidence of ferromagnetism due to EuO or antiferromagnetism due to Eu$_2$O$_3$ indicating that the graphene layer protects the intercalated metallic Eu against oxidation over months of exposure to atmospheric environment., Comment: 6 pages, 5 figures

We study structural and electronic properties of graphene grown on SiC substrate using scanning tunneling microscope (STM), spot-profile-analysis low energy electron diffraction (SPA-LEED) and angle resolved photoemission spectroscopy (ARPES). We find several new replicas of Dirac cones in the Brillouin zone (BZ). Their locations can be understood in terms of combination of basis vectors linked to SiC 6x6 and graphene 6xsqrt(3) x 6sqrt(3) reconstruction. Therefore these new features originate from the Moie caused by the lattice mismatch between SiC and graphene. More specifically, Dirac cones replicas are caused by underlying weak modulation of the ionic potential by the substrate that is then experienced by the electrons in the graphene. We also demonstrate that this effect is equally strong in single and tri-layer graphene, therefore the additional Dirac cones are intrinsic features rather than result of photoelectron diffraction. These new features in the electronic structure are very important for the interpretation of recent transport measurements and can assist in tuning the properties of graphene for practical applications., Comment: 6 pages, 2 figures

We have determined the magnetic properties of epitaxially grown Dy islands on graphene/SiC(0001) that are passivated by a gold film (deposited in the ultra-high vacuum growth chamber) for {\it ex-situ} X-ray magnetic circular dichroism (XMCD). Our sum-rule analysis of the Dy $M_{4,5}$ XMCD spectra at low temperatures ($T=15$ K) as a function of magnetic field assuming Dy$^{3+}$ (spin configuration $^6H_{15/2}$) indicate that the projection of the magnetic moment along an applied magnetic field of 5 T is 3.5(3) $\mu_B$. Temperature dependence of the magnetic moment (extracted from the $M_5$ XMCD spectra) shows an onset of a change in magnetic moment at about 175 K in proximity of the transition from paramagnetic to helical magnetic structure at $T_{\rm H} =179$ K in bulk Dy. No feature at the vicinity of the ferromagnetic transition of hcp bulk Dy at $T_{\rm c}$ = 88 K is observed. However, below $\sim$130 K, the inverse magnetic moment (extracted from the XMCD) is linear in temperature as commonly expected from a paramagnetic system suggesting different behavior of Dy nano-island than bulk Dy.

X-ray magnetic circular dichroism (XMCD) measurements of iron nano-islands grown on graphene and covered with a Au film for passivation reveal that the oxidation through defects in the Au film spontaneously leads to the formation of magnetite nano-particles (i.e, $Fe_3$$O_4$). The Fe nano-islands (20 and 75 monolayers; MLs) are grown on epitaxial graphene formed by thermally annealing 6H-SiC(0001) and subsequently covered, in the growth chamber, with nominal 20 layers of Au. Our X-ray absorption spectroscopy and XMCD measurements at applied magnetic fields show that the thin film (20 ML) is totally converted to magnetite whereas the thicker film (75 ML) exhibits properties of magnetite but also those of pure metallic iron. Temperature dependence of the XMCD signal (of both samples) shows a clear transition at $T_{\rm V}\approx 120$ K consistent with the Verwey transition of bulk magnetite. These results have implications on the synthesis of magnetite nano-crystals and also on their regular arrangements on functional substrates such as graphene., Comment: 5 pages, 6 figures