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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 interest 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 controllable way has driven studies on graphene functionalization by metal deposition. Although the macroscopic effects of metal adsorption or intercalation in supported graphene can easily be accessed, the study of metal deposition on suspended graphene on an atomic scale, in real time and under well-defined deposition conditions remained a challenging task so far.The low-energy electron point source (LEEPS) microscopy is an investigation technique based on Gabor’s holography principle and represents a lensless transmission setup whereby 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 the sample, producing a hologram on a distant electron detector. The LEEPS microscope realized at the UZH operates with coherent electrons in the 50-250 eV energy range, corresponding to de Broglie wavelengths in the range of 0.17-0.08 nm. Graphene is highly transparent to low-energy electrons and has been successfully used as a substrate in several LEEPS investigations. Owing to the high sensitivity of low-energy electrons to electric and magnetic fields, the detection of even a fractional elementary charge has become possible. As alkali metals adsorbed on free-standing graphene are expected to transfer their outermost electron, which in turn gets delocalized in graphene, a positive ion remains, and single alkali atoms can thus be detected when adsorbed on free-standing graphene. The work presented in this thesis represents the first in-situ experimental investigation of the deposition of alkali and transition metals on free-standing single and bilayer graphene by means of the LEEPS microscope. In particular, the investigation has focused on the adsorption and nucleation processes of Li, K and Cs alkali metals and of Pd as one representative for a transition metal. LEEPS images of metal deposition on graphene under ultra-high vacuum conditions have been acquired in real time, respectively with 25 frames/second. A comparison between the acquired images for different alkali metals shows a very similar signature; namely a bright spot due to the positive charge for Cs and K and a much smaller one for Li. A further similarity between Cs and K has been observed once the deposition has been terminated; these two metals do not remain localised on the graphene, on the contrary to Li that forms localised charged entities. The analysis of alkali metal deposition on adjacent domains of single and bilayer graphene showed that they readily intercalate in between the bilayer domain. This finding allows to quantitatively analyse the particle density in the two graphene domains during the deposition and eventually also under equilibrium conditions. In particular, the particle density in the single layer domain has been found to be much lower than in the bilayer domain. Once an equilibrium distribution has been established, a quantitative estimate of the difference in the free energy of binding between the single and bilayer domains has been obtained for K. A control experiment performed with depositing Pd shows the formation of a similar distribution of clusters on both domains and no intercalation. The effect of the electron beam illumination on the Pd cluster growth has also been investigated. The graphene window 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 exhibit a high density of smaller clusters instead. Although the LEEPS technique does not provide any information on cluster thickness, from a comparison with TEM images it was inferred that such islands are thinner than 50 nm.

Abstract

Functionalizing graphene, the atomically thin carbon layer, has attracted considerable interest 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 controllable way has driven studies on graphene functionalization by metal deposition. Although the macroscopic effects of metal adsorption or intercalation in supported graphene can easily be accessed, the study of metal deposition on suspended graphene on an atomic scale, in real time and under well-defined deposition conditions remained a challenging task so far.The low-energy electron point source (LEEPS) microscopy is an investigation technique based on Gabor’s holography principle and represents a lensless transmission setup whereby 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 the sample, producing a hologram on a distant electron detector. The LEEPS microscope realized at the UZH operates with coherent electrons in the 50-250 eV energy range, corresponding to de Broglie wavelengths in the range of 0.17-0.08 nm. Graphene is highly transparent to low-energy electrons and has been successfully used as a substrate in several LEEPS investigations. Owing to the high sensitivity of low-energy electrons to electric and magnetic fields, the detection of even a fractional elementary charge has become possible. As alkali metals adsorbed on free-standing graphene are expected to transfer their outermost electron, which in turn gets delocalized in graphene, a positive ion remains, and single alkali atoms can thus be detected when adsorbed on free-standing graphene. The work presented in this thesis represents the first in-situ experimental investigation of the deposition of alkali and transition metals on free-standing single and bilayer graphene by means of the LEEPS microscope. In particular, the investigation has focused on the adsorption and nucleation processes of Li, K and Cs alkali metals and of Pd as one representative for a transition metal. LEEPS images of metal deposition on graphene under ultra-high vacuum conditions have been acquired in real time, respectively with 25 frames/second. A comparison between the acquired images for different alkali metals shows a very similar signature; namely a bright spot due to the positive charge for Cs and K and a much smaller one for Li. A further similarity between Cs and K has been observed once the deposition has been terminated; these two metals do not remain localised on the graphene, on the contrary to Li that forms localised charged entities. The analysis of alkali metal deposition on adjacent domains of single and bilayer graphene showed that they readily intercalate in between the bilayer domain. This finding allows to quantitatively analyse the particle density in the two graphene domains during the deposition and eventually also under equilibrium conditions. In particular, the particle density in the single layer domain has been found to be much lower than in the bilayer domain. Once an equilibrium distribution has been established, a quantitative estimate of the difference in the free energy of binding between the single and bilayer domains has been obtained for K. A control experiment performed with depositing Pd shows the formation of a similar distribution of clusters on both domains and no intercalation. The effect of the electron beam illumination on the Pd cluster growth has also been investigated. The graphene window 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 exhibit a high density of smaller clusters instead. Although the LEEPS technique does not provide any information on cluster thickness, from a comparison with TEM images it was inferred that such islands are thinner than 50 nm.

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Additional indexing

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:12 Aug 2021 06:36
OA Status:Green
  • Content: Published Version