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Observation of Metal Nucleation on Free-Standing Graphene by Means of Low-Energy Electron Holography


Lorenzo, Marianna. Observation of Metal Nucleation on Free-Standing Graphene by Means of Low-Energy Electron Holography. 2018, University of Zurich, Faculty of Science.

Abstract

Functionalizing graphene, the atomically thin carbon layer, has attracted considerable in-terest in view of possible technological applications in using graphene in electronic devices.The requirement of tuning the electronic properties of graphene in an efficient and control-lable way has driven studies on graphene functionalization by metal deposition. Althoughthe macroscopic effects of metal adsorption or intercalation in supported graphene caneasily be accessed, the study of metal deposition on suspended graphene on an atomicscale, in real time and under well-defined deposition conditions remained a challengingtask so far.The low-energy electron point source (LEEPS) microscopy is an investigation tech-nique based on Gabor’s holography principle and represents a lens-less transmission setupwhereby the divergent coherent electron beam is emitted by an ultra-sharp tungsten tip.The electron reference wave interferes with the object wave, elastically scattered off thesample, producing a hologram on a distant electron detector. The LEEPS microscoperealized at the UZH operates with coherent electrons in the 50-250 eV energy range, cor-responding to de Broglie wavelengths in the range of 0.17-0.08 nm. Graphene is highlytransparent to low-energy electrons and has been successfully used as a substrate in severalLEEPS investigations. Owing to the high sensitivity of low-energy electrons to electricand magnetic fields, the detection of even a fractional elementary charge has become pos-sible. As alkali metals adsorbed on free-standing graphene are expected to transfer theiroutermost electron, which in turn gets delocalized in graphene, a positive ion remains, andsingle alkali atoms can thus be detected when adsorbed on free-standing graphene.The work presented in this thesis represents the first in-situ experimental investiga-tion of the deposition of alkali and transition metals on free-standing single and bilayergraphene by means of the LEEPS microscope. In particular, the investigation has focusedon the adsorption and nucleation processes of Li, K and Cs alkali metals and of Pd asiiione representative for a transition metal. LEEPS images of metal deposition on grapheneunder ultra-high vacuum conditions have been acquired in real time, respectively with25 frames/second. A comparison between the acquired images for different alkali metalsshows a very similar signature; namely a bright spot due to the positive charge for Cs andK and a much smaller one for Li. A further similarity between Cs and K has been ob-served once the deposition has been terminated; these two metals do not remain localisedon the graphene, on the contrary to Li that forms localised charged entities. The analysisof alkali metal deposition on adjacent domains of single and bilayer graphene showed thatthey readily intercalate in between the bilayer domain. This finding allows to quantita-tively analyse the particle density in the two graphene domains during the deposition andeventually also under equilibrium conditions. In particular, the particle density in thesingle layer domain has been found to be much lower than in the bilayer domain. Once anequilibrium distribution has been established, a quantitative estimate of the difference inthe free energy of binding between the single and bilayer domains has been obtained forK. A control experiment performed with depositing Pd shows the formation of a similardistribution of clusters on both domains and no intercalation. The effect of the electronbeam illumination on the Pd cluster growth has also been investigated. The graphenewindow imaged continuously during the deposition shows the formation of large islands;while the adjacent windows imaged only before and after the end of the deposition ex-hibit a high density of smaller clusters instead. Although the LEEPS technique does notprovide any information on cluster thickness, from a comparison with TEM images it wasinferred that such islands are thinner than 50 nm.

Abstract

Functionalizing graphene, the atomically thin carbon layer, has attracted considerable in-terest in view of possible technological applications in using graphene in electronic devices.The requirement of tuning the electronic properties of graphene in an efficient and control-lable way has driven studies on graphene functionalization by metal deposition. Althoughthe macroscopic effects of metal adsorption or intercalation in supported graphene caneasily be accessed, the study of metal deposition on suspended graphene on an atomicscale, in real time and under well-defined deposition conditions remained a challengingtask so far.The low-energy electron point source (LEEPS) microscopy is an investigation tech-nique based on Gabor’s holography principle and represents a lens-less transmission setupwhereby the divergent coherent electron beam is emitted by an ultra-sharp tungsten tip.The electron reference wave interferes with the object wave, elastically scattered off thesample, producing a hologram on a distant electron detector. The LEEPS microscoperealized at the UZH operates with coherent electrons in the 50-250 eV energy range, cor-responding to de Broglie wavelengths in the range of 0.17-0.08 nm. Graphene is highlytransparent to low-energy electrons and has been successfully used as a substrate in severalLEEPS investigations. Owing to the high sensitivity of low-energy electrons to electricand magnetic fields, the detection of even a fractional elementary charge has become pos-sible. As alkali metals adsorbed on free-standing graphene are expected to transfer theiroutermost electron, which in turn gets delocalized in graphene, a positive ion remains, andsingle alkali atoms can thus be detected when adsorbed on free-standing graphene.The work presented in this thesis represents the first in-situ experimental investiga-tion of the deposition of alkali and transition metals on free-standing single and bilayergraphene by means of the LEEPS microscope. In particular, the investigation has focusedon the adsorption and nucleation processes of Li, K and Cs alkali metals and of Pd asiiione representative for a transition metal. LEEPS images of metal deposition on grapheneunder ultra-high vacuum conditions have been acquired in real time, respectively with25 frames/second. A comparison between the acquired images for different alkali metalsshows a very similar signature; namely a bright spot due to the positive charge for Cs andK and a much smaller one for Li. A further similarity between Cs and K has been ob-served once the deposition has been terminated; these two metals do not remain localisedon the graphene, on the contrary to Li that forms localised charged entities. The analysisof alkali metal deposition on adjacent domains of single and bilayer graphene showed thatthey readily intercalate in between the bilayer domain. This finding allows to quantita-tively analyse the particle density in the two graphene domains during the deposition andeventually also under equilibrium conditions. In particular, the particle density in thesingle layer domain has been found to be much lower than in the bilayer domain. Once anequilibrium distribution has been established, a quantitative estimate of the difference inthe free energy of binding between the single and bilayer domains has been obtained forK. A control experiment performed with depositing Pd shows the formation of a similardistribution of clusters on both domains and no intercalation. The effect of the electronbeam illumination on the Pd cluster growth has also been investigated. The graphenewindow imaged continuously during the deposition shows the formation of large islands;while the adjacent windows imaged only before and after the end of the deposition ex-hibit a high density of smaller clusters instead. Although the LEEPS technique does notprovide any information on cluster thickness, from a comparison with TEM images it wasinferred that such islands are thinner than 50 nm.

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Item Type:Dissertation (monographical)
Referees:Fink Hans-Werner, Osterwalder Jürg, Hommelhoff Peter, Morin Roger, Escher Conrad, Latychevskaia Tatiana
Communities & Collections:07 Faculty of Science > Physics Institute
UZH Dissertations
Dewey Decimal Classification:530 Physics
Language:English
Place of Publication:Zürich
Date:2018
Deposited On:19 Mar 2019 14:15
Last Modified:15 Apr 2021 15:05
OA Status:Green

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