Graphene became one of the most investigated system in the last decade, being considered by the scientific community the wonder material of this century. It is formed from a single layer of carbon atoms packed in a honeycomb lattice with a series of exotic and unique properties which make this system very convenient for engineering new electronic applications. However, besides the outstanding mechanical and electronic properties, graphene has also some serious limitations due to the difficulty in the synthesis of high-quality monolayers at a large scale, or in creating of an electronic band gap in the material, necessary for on-off switching operations in the transistors technology. To overcame this problem, one solution comes from the interaction between epitaxial graphene and transition metal substrates, but this will results in a hardly controlled opening of an energy band gap at a Fermi level. Another solution arises from the possibility to grow a second layer of graphene which will employ a stable band gap. At present, only the high temperature Chemical Vapour Deposition (CVD) can supply large flakes of free standing graphene to an industrial level, but unfortunately this process is self-limiting to one single graphene layer. Therefore, a different approach is needed for the growth of high quality single- and multi-layer on a large scale to overcome the aforementioned limitations. My research activity during these three years was focused in developing a new method of growing graphene, beyond the standard CVD. Specifically, we were interested in the molecular beam epitaxy (MBE) growth, where C atoms are directly delivered on the surface. To this purpose, a new solid carbon source was designed and constructed, which is based on the electron bombardment principle to achieve carbon sublimation from a hot graphite rod. This was used on different substrates like Ir(111), where the CVD technique is well established, Ag(111) and PbZr0.2Ti0.8O3(PZT(001)), a ferroelectric oxide where CVD is precluded. The first part is dedicated to the study of the growth mechanism of graphene on Ir(111) using the carbon (MBE) a very low substrate temperature (T=80 K) to reduce surface diffusion. This allowed us to assimilate the origin of different species of carbon atoms present in the incipient stage of graphene formation on the Ir(111). We also exploit the possibility to grow a second layer of graphene using the C source, at a high substrate temperature which was impossible to achieve with standard CVD method. The second part is dedicated to the growth of graphene on substrates where, the dissociation of the hydrocarbon molecules used in the CVD is precluded. To this purpose we involved carbon MBE to grow single and double layers on Ag(111) by keeping the substrate at elevated temperature. The last part of the thesis is dedicated to the graphene ferroelectric oxides interfaces, which can be considered the forward step in the integration of graphene in the new era of electronics Specifically we were interested in the synthesis of graphene on top of PZT(001 using the solid carbon source, because on this kind of substrate the hydrocarbon used in CVD does not dissociate. A special cleaning procedure was defined to promote carbon adsorption on the substrate. A key role is played also by the temperature deposition which not may exceed a certain value in order that carbon to adsorb.

Different approaches for the growth of graphene monolayers

TACHE, CRISTIAN ALEXANDRU
2017-04-05

Abstract

Graphene became one of the most investigated system in the last decade, being considered by the scientific community the wonder material of this century. It is formed from a single layer of carbon atoms packed in a honeycomb lattice with a series of exotic and unique properties which make this system very convenient for engineering new electronic applications. However, besides the outstanding mechanical and electronic properties, graphene has also some serious limitations due to the difficulty in the synthesis of high-quality monolayers at a large scale, or in creating of an electronic band gap in the material, necessary for on-off switching operations in the transistors technology. To overcame this problem, one solution comes from the interaction between epitaxial graphene and transition metal substrates, but this will results in a hardly controlled opening of an energy band gap at a Fermi level. Another solution arises from the possibility to grow a second layer of graphene which will employ a stable band gap. At present, only the high temperature Chemical Vapour Deposition (CVD) can supply large flakes of free standing graphene to an industrial level, but unfortunately this process is self-limiting to one single graphene layer. Therefore, a different approach is needed for the growth of high quality single- and multi-layer on a large scale to overcome the aforementioned limitations. My research activity during these three years was focused in developing a new method of growing graphene, beyond the standard CVD. Specifically, we were interested in the molecular beam epitaxy (MBE) growth, where C atoms are directly delivered on the surface. To this purpose, a new solid carbon source was designed and constructed, which is based on the electron bombardment principle to achieve carbon sublimation from a hot graphite rod. This was used on different substrates like Ir(111), where the CVD technique is well established, Ag(111) and PbZr0.2Ti0.8O3(PZT(001)), a ferroelectric oxide where CVD is precluded. The first part is dedicated to the study of the growth mechanism of graphene on Ir(111) using the carbon (MBE) a very low substrate temperature (T=80 K) to reduce surface diffusion. This allowed us to assimilate the origin of different species of carbon atoms present in the incipient stage of graphene formation on the Ir(111). We also exploit the possibility to grow a second layer of graphene using the C source, at a high substrate temperature which was impossible to achieve with standard CVD method. The second part is dedicated to the growth of graphene on substrates where, the dissociation of the hydrocarbon molecules used in the CVD is precluded. To this purpose we involved carbon MBE to grow single and double layers on Ag(111) by keeping the substrate at elevated temperature. The last part of the thesis is dedicated to the graphene ferroelectric oxides interfaces, which can be considered the forward step in the integration of graphene in the new era of electronics Specifically we were interested in the synthesis of graphene on top of PZT(001 using the solid carbon source, because on this kind of substrate the hydrocarbon used in CVD does not dissociate. A special cleaning procedure was defined to promote carbon adsorption on the substrate. A key role is played also by the temperature deposition which not may exceed a certain value in order that carbon to adsorb.
BARALDI, Alessandro
29
2015/2016
Settore FIS/03 - Fisica della Materia
Università degli Studi di Trieste
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11368/2908125
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