Elettra-Sincrotrone Trieste S.C.p.A. website uses session cookies which are required for users to navigate appropriately and safely. Session cookies created by the Elettra-Sincrotrone Trieste S.C.p.A. website navigation do not affect users' privacy during their browsing experience on our website, as they do not entail processing their personal identification data. Session cookies are not permanently stored and indeed are cancelled when the connection to the Elettra-Sincrotrone Trieste S.C.p.A. website is terminated.
More info
OK

Nanobubbles at GPa pressure under graphene

Owing to the exceptional strength and flexibility of sp2-carbon, graphene is able to trap mesoscopic volumes of liquid or gas, resulting in the formation of nanobubbles (NB) at the interface between film and substrate. These structures recently came in the spotlight in view of potential applications such as gas storage and high-pressure chemistry. Since NBs can induce large lattice deformations in graphene, they may be used to strain-engineer the electronic, magnetic and optical properties of this intriguing carbon allotrope, fostering fruition in a plethora of novel devices.

To date, only few studies have addressed the basic structural properties of NB under graphene using a surface science approach. In the present study we have investigated the morphology and spatial distribution of Ar under micron-sized graphene flakes supported on Ir(100), specifically addressing the dynamics of NB formation and their thermal stability. Low energy ion irradiation was used to implant Ar under graphene, a method which causes minimal damage to the lattice while ensuring a uniform lateral distribution of the intercalated species.

Indeed, the study of buried species poses great experimental challenges, mainly arising from the limited bulk sensitivity of most experimental probes. Low energy electron microscopy (LEEM) and synchrotron-based photoemission electron microscopy (XPEEM) measurements at the Nanospectroscopy beamline of Elettra enabled us to monitor the evolution of the interface structure and stoichiometry, yet maintaining sufficiently high sensitivity to subsurface species. The film morphology was quantitatively characterized in complementary STM measurements, carried out at IOM-CNR TASC laboratory, Trieste.

Figure 1 shows room-temperature LEEM and PEEM images of a graphene flake after 0.1 keV Ar+ irradiation and subsequent thermal treatment to 1050 °C. In (a), the bright and neutral grey regions correspond to graphene, the dark patch to the bare Ir surface, respectively. The small black dots, highlighted by red circles, correspond to NBs under graphene. In order to assess the interface stoichiometry, we performed X-ray absorption spectromicroscopy (XAS-PEEM) at the Ar L edge. In the XAS-PEEM image shown in (b), the image intensity is proportional to the local Ar concentration. Spectra obtained from inside and outside the red circles are shown in (c). Only the NB spectra display resonances at about 245, 248, and 250 eV, a clear fingerprint of their Ar elemental composition. The remarkable inversion of contrast between the Ar L3 XAS-PEEM (d) and Ir 4f7/2 XPEEM (e) images indicates that the substrate core level emission is screened by Ar. The NBs are thus located at the graphene/Ir interface, at variance with softer substrates where Ar segregates into bubbles buried in the metal. Notably, the larger aggregates display a lateral size up to few tens of nanometers. STM measurements indicate that their height may reach several atomic layers, thus including up to few thousand Ar atoms.

In order to unravel the physics governing the ripening of intercalated Ar, ab initio calculations were carried out at ICTP, Trieste, for Ar single atom and mini-clusters (dimer, trimer, tetramer) at the graphene/Ir interface, see Figure 2 (a-g). By calculating the formation energy of Ar mini-clusters under graphene (see e), we demonstrated that there is a drastic net energy gain when two or more protrusions merge into a single, larger one, compared to the energy gain for Ar cluster formation in vacuum. Since the energy gain alone is not sufficient to explain the ripening process, we also verified that intercalated atoms have sufficient mobility. The calculated energy barriers for diffusion on Ir under graphene vary from 40 to 120 meV, which permits Ar aggregation already near room temperature, as confirmed by STM.

   

Figure 1:Room temperature LEEM image (Vstart = 12 V) of a graphene flake after Ar+ irradiation (0.1 kV, 150 s at 1.5 × 10–5 mbar Ar) and subsequent annealing to 1050 °C; the black dots correspond to protrusions in the film. (b) Ar L3 PEEM of the same region, obtained after subtracting PEEM images acquired at 243 eV (baseline intensity) from images acquired at 247.7 eV (Ar L3 edge). The intensity of the resulting image is proportional to the Ar concentration, with the bright regions corresponding to Ar clusters. Red circles have been added to facilitate comparison with a. (c) Average XAS-PEEM spectra from regions of interest inside and outside of the red circles. (d) Ar L3 XAS-PEEM image of another flake along with (e) Ir 4f7/2 and (f) C 1s XPEEM images; the Ar clusters show up as bright spots in d; the inversion of contrast in (e) is due to screening of the substrate emission by the clusters.

Most interestingly, the calculations allowed us estimating the pressure to which Ar is subject to. The pressure exerted by the graphene membrane on Ar dimers is large and non-isotropic, nearing 3-8 GPa from the sides and 70-75 GPa from the top. These pressures are expected to decrease with increasing cluster size. Careful observation of STM and LEEM data reveals that the NBs display a polygonal shape near room-temperature. Owing to the high flexibility of graphene, such configurations must reflect the actual shape of underlying clusters, suggesting that Ar may form solid aggregates.

   

Figure 2. :  Top view of the unit cell used in our ab initio calculations showing graphene (blue spheres) and the upper Ir layer (grey spheres). Side view of the optimized atomic geometries for (b) a single intercalated Ar (red sphere); (c) vertical and (d) horizontal Ar dimers; (f) horizontal trimer and (g) tetramer. The comparison between the energy gain (-∆E)when forming Ar clusters from single Ar atoms at the graphene/Ir interface and in vacuum is shown in (e).

We expect that ripening of intercalated noble gases can also occur in other graphene/metal systems showing comparable adhesion strength, where it might be fruitfully exploited to strain-engineer the local chemical properties of graphene. Our study also fosters the synchrotron-based investigation of van der Waals solids under extreme pressure and temperature conditions. Rather than using molecular beams to obtain condensation, our approach exploits ripening of Ar implanted under graphene, the cluster size being controlled by a simple annealing process

This research was conducted by the following research team:

G. Zamborlini1,2, M. Imam3, L.L. Patera1,4, T.O. Menteş5, N. Stojić3,6, C. Africh4, A. Sala5, N. Binggeli3,6, G. Comelli1,4, and A. Locatelli5
1Department of Physics, University of Trieste, Via Valerio 2, I-34127 Trieste, Italy
2Peter Grünberg Institute (PGI-6), Research Centre Jülich, 52425 Jülich, Germany
3Abdus Salam International Centre for Theoretical Physics, Strada Costiera 11, Trieste I-34151, Italy
4IOM-CNR Laboratorio TASC, S.S. 14 km 163.5 in AREA Science Park, Basovizza, I-34149 Trieste, Italy
5Elettra - Sincrotrone Trieste, S.S. 14 km 163.5 in AREA Science Park, Basovizza, I-34149 Trieste, Italy
6IOM-CNR Democritos, Trieste I-34151, Italy

Contact persons:

Andrea Locatelli, e-mail: andrea.locatelli@elettra.eu;
Nataša Stojić, e-mail: nstojic@ictp.it.

Reference

G. Zamborlini, M. Imam, L.L. Patera, T.O. Menteş, N. Stojić, C. Africh, A. Sala, N. Binggeli, G. Comelli, and A. Locatelli; “Nanobubbles at GPa pressure under graphene”, Nano Lett. 15 (9), 6162–6169 (2015); DOI: 10.1021/acs.nanolett.5b02475

 

Last Updated on Thursday, 07 January 2016 23:08