Charlotte Herbig

Ion impacts on graphene/Ir(111): interface channeling, vacancy funnels, and a nanomesh.

By combining ion beam experiments and atomistic simulations we study the production of defects in graphene on Ir(111) under grazing incidence of low energy Xe ions. We demonstrate that the ions are channeled in between graphene and the substrate, giving rise to chains of vacancy clusters with their edges bending down toward the substrate. These clusters self-organize to a graphene nanomesh via thermally activated diffusion as their formation energy varies within the graphene moiré supercell.

Interfacial Carbon Nanoplatelet Formation by Ion Irradiation of Graphene on Iridium(111)

We expose epitaxial graphene (Gr) on Ir(111) to low-energy noble gas ion irradiation and investigate by scanning tunneling microscopy and atomistic simulations the behavior of C atoms detached from Gr due to ion impacts. Consistent with our density functional theory calculations, upon annealing Gr nanoplatelets nucleate at the Gr/Ir(111) interface from trapped C atoms initially displaced with momentum toward the substrate. Making use of the nanoplatelet formation phenomenon, we measure the trapping yield as a function of ion energy and species and compare the values to those obtained using molecular dynamics simulations. Thereby, complementary to the sputtering yield, the trapping yield is established as a quantity characterizing the response of supported 2D materials to ion exposure. Our findings shed light on the microscopic mechanisms of defect production in supported 2D materials under ion irradiation and pave the way toward precise control of such systems by ion beam engineering.

Mechanical exfoliation of epitaxial graphene on Ir(111) enabled by Br2 intercalation

We show here that Br(2) intercalation is an efficient method to enable exfoliation of epitaxial graphene on metals by adhesive tape. We exemplify this method for high-quality graphene of macroscopic extension on Ir(111). The sample quality and the transfer process are monitored using low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), scanning electron microscopy (SEM) and Raman spectroscopy. The developed process provides an opportunity for preparing graphene of strictly monatomic thickness and well-defined orientation including the transfer to poly(ethylene terephthalate) (PET) foil.

Xe Irradiation of Graphene on Ir(111): From Trapping to Blistering

Using x-ray photoelectron spectroscopy, thermal desorption spectroscopy, and scanning tunneling microscopy, we show that upon keV Xe+ irradiation of graphene on Ir(111), Xe atoms are trapped under the graphene. Upon annealing, aggregation of Xe leads to graphene bulges and blisters. The efficient trapping is an unexpected and remarkable phenomenon given the absence of chemical binding of Xe to Ir and to graphene, the weak interaction of a perfect graphene layer with Ir(111), as well as the substantial damage to graphene due to irradiation. By combining molecular dynamics simulations and density functional theory calculations with our experiments, we uncover the mechanism of trapping. We describe ways to avoid blister formation during graphene growth, and also demonstrate how ion implantation can be used to intentionally create blisters without introducing damage to the graphene layer. Our approach may provide a pathway to synthesize new materials at a substrate-2D material interface or to enable confined reactions at high pressures and temperatures. (Less)

Core level shifts of intercalated graphene

Through intercalation of metals and gases the Dirac cone of graphene on Ir(111) can be shifted with respect to the Fermi level without becoming destroyed by strong hybridization. Here, we use x-ray photoelectron spectroscopy to measure the C 1s core level shift (CLS) of graphene in contact with a number of structurally well-defined intercalation layers (O, H, Eu, and Cs). By analysis of our own and additional literature data for decoupled graphene, the C 1s CLS is found to be a non-monotonic function of the doping level. For small doping levels the shifts are well described by a rigid band model. However, at larger doping levels, a second effect comes into play which is proportional to the transferred charge and counteracts the rigid band shift. Moreover, not only the position, but also the C 1s peak shape displays a unique evolution as a function of doping level. Our conclusions are supported by intercalation experiments with Li, with which, due to the absence of phase separation, the doping level of graphene can be continuously tuned.

From Permeation to Cluster Arrays: Graphene on Ir(111) Exposed to Carbon Vapor

Our scanning tunneling microscopy and X-ray photoelectron spectroscopy experiments along with first-principles calculations uncover the rich phenomenology and enable a coherent understanding of carbon vapor interaction with graphene on Ir(111). At high temperatures, carbon vapor not only permeates to the metal surface but also densifies the graphene cover. Thereby, in addition to underlayer graphene growth, upon cool down also severe wrinkling of the densified graphene cover is observed. In contrast, at low temperatures the adsorbed carbon largely remains on top and self-organizes into a regular array of fullerene-like, thermally highly stable clusters that are covalently bonded to the underlying graphene sheet. Thus, a new type of predominantly sp2-hybridized nanostructured and ultrathin carbon material emerges, which may be useful to encage or stably bind metal in finely dispersed form.

Graphene: the ultimately thin sputtering shield

Scanning tunneling microscopy methods are applied to investigate the potential of monolayer graphene as a sputtering shield for the underlying metal substrate. To visualize the effect, a bare and a graphene protected Ir(111) surface are irradiated with 500 eV Xe+, as well as 200 eV Xe+ and Ar+ ions, all at 1000 K. By quantitatively evaluating the sputtered material from the surface vacancy island area, we find a drastic decrease in metal sputtering for the graphene protected surface. It is demonstrated that efficient sputter protection relies on self-repair of the ion damage in graphene, which takes place efficiently in the temperature range of chemical vapor deposition growth. Based on the generality of the underlying principles of ion damage, graphene self-repair, and graphene growth, we speculate that efficient sputter protection is possible for a broad range of metals and alloys.

Comment on "interfacial carbon nanoplatelet formation by ion irradiation of graphene on iridium(111)".

’ In a recent article by Herbig et al., some of us reported the formation of bulges in a graphene (Gr) sheet on Ir(111) after conducting Xeþ irradiation (energy range of 0.1 5 keV) at 300 K and subsequent annealing to 1000 K. Additional X-ray photoelectron experiments after irradiation and annealing now invalidate the following assessment in the article: “We also rule out that the bulges are agglomerations of implanted noble gas atoms. Although noble gas is certainly implanted into the Ir crystal, trapped in bulk vacancies as well as bulk vacancy clusters, and partially released during annealing to 1000 K, the Gr cover will not protect it from desorption.” Our additional experiments show that indeed the Gr cover protects Xe efficiently from desorption. In a first experiment, we exposed the bare Ir(111) sample to 3 keV Xeþ at 300 K and conducted successive annealing to 1000 and 1300 K. The ion fluence selected was 0.1 MLE, where 1 MLE is 1.57 10 ions/m, that is, numerically identical to the surface atomic density of Ir(111). After irradiation at 300 K, the bottom spectrum in Figure 1a displays an Ir 4s core level peak together with the Xe 3d3/2/Xe 3d5/2 core level doublet. Since Xe only physisorbs on Ir(111), consistent with desorption around 100 K and a binding energy of 0.21 eV as obtained by our density function theory (DFT) calculations, the Xe 3d signal after irradiation must be attributed to Xe implanted into the Ir sample. Upon annealing to 1000 K and subsequently to 1300 K, the Xe signal diminishes (compare middle and top spectra in Figure 1a, as well as solid squares in the inset). We explain these changes as follows: due to thermal excitation, Xe is partially released from its trapping sites inside the crystal, diffuses to the surface, and desorbs to the vacuum, consistent with our statement in ref 1. The residual Xe signal after annealing to 1300 K is due to Xe aggregates trapped in bulk vacancy clusters, as found also in previous studies for similar systems. Conducting precisely the same irradiation experiment, but for Ir(111) covered by a complete monolayer of Gr, yields very different photoelectron spectra, as shown in Figure 1b. Already after ion exposure at 300 K (bottom spectrum of Figure 1b), the integrated Xe 3d intensity is higher by a factor of 2 compared to irradiation of bare Ir(111) (see also inset of Figure 1a). All of the additional intensity must be due to Xe trapped between the Gr cover and the Ir substrate. Annealing to 1000 and 1300 K leads to a slight increase and a substantial increase, respectively, of the integrated Xe 3d peak intensity. The interpretation is straightforward: Instead of being released to the vacuum, the Xe diffusing out from the Ir bulk becomes trapped under the Gr cover and thereby enhances the Xe 3d intensity (smaller photoelectron attenuation due to smaller depth). By comparison with the Xe 3d signal of a saturated Xe 3dmonolayer adsorbed to Ir(111), it is found that the amount of trapped Xe is on the order of 10%of a saturated Xe layer. The efficient trapping of Xe during room temperature irradiation and subsequent annealing is remarkable since (i) a Xe atom does not bind chemically to Ir(111) or to Gr/Ir(111) (DFT binding energy 0.17 eV) but instead causes a DFTcalculated energy penalty of 2.9 eV in the trapped state due to elastic deformation of the Gr on Ir(111); (ii) the Gr cover was heavily damaged and partially sputtered (about 15% of the Gr area is removed according to the sputtering yield obtained from our molecular dynamics simulation); and (iii) the perfect Gr layer adheres only weakly to the substrate with an average height of 3.4 Å. The latter two factors make the situation different from two previously reported cases, where trapping was observed in the limits of very small fluence and very low energy (marginal damage and sputtering) and for strong adhesion of the 2D material to the substrate [a monolayer of hexagonal boron nitride on Rh(111) and Gr on Ru(0001)]. Given the large amount of Xe trapped under Gr, it appears that the bulges observed in ref 1 are formed predominantly due to Xe aggregates rather than interfacial Gr nanoplatelets as was proposed by Herbig et al. In light of the experiments reported here, bulges observed by Herbig et al. after 0.3 keV Neþ or Arþ room temperature irradiation and 1000 K annealing are most likely due to trapped Ne or Ar.

Suppression of Quasiparticle Scattering Signals in Bilayer Graphene Due to Layer Polarization and Destructive Interference

We study chemically gated bilayer graphene using scanning tunneling microscopy and spectroscopy complemented by tight-binding calculations. Gating is achieved by intercalating Cs between bilayer graphene and Ir(111), thereby shifting the conduction band minima below the chemical potential. Scattering between electronic states (both intraband and interband) is detected via quasiparticle interference. However, not all expected processes are visible in our experiment. We uncover two general effects causing this suppression: first, intercalation leads to an asymmetrical distribution of the states within the two layers, which significantly reduces the scanning tunneling spectroscopy signal of standing waves mainly present in the lower layer; second, forward scattering processes, connecting points on the constant energy contours with parallel velocities, do not produce pronounced standing waves due to destructive interference. We present a theory to describe the interference signal for a general n-band material.

A Monolayer of Hexagonal Boron Nitride on Ir(111) as a Template for Cluster Superlattices

The moiré of a monolayer of hexagonal boron nitride on Ir(111) is found to be a template for Ir, C, and Au cluster superlattices. Using scanning tunneling microscopy, the cluster structure and epitaxial relation to the substrate, the cluster binding site, the role of defects, as well as the thermal stability of the cluster lattice are investigated. The Ir and C cluster superlattices display a high thermal stability, before they decay by intercalation and Smoluchowski ripening. Ab initio calculations explain the extraordinarily strong Ir cluster binding through selective sp3 rehybridization of boron nitride involving B-Ir cluster bonds and a strengthening of the nitrogen bonds to the Ir substrate in a specific, initially only chemisorbed valley area within the moiré.