Al-Milli, Zain and Lambert, Colin (2017) Electrical and mechanical properties of molecular junctions and nano surfaces. PhD thesis, Lancaster University.
ELECTRICAL_AND_MECHANICAL_PROPERTIES_OF_MOLECULAR_JUNCTIONS_AND_NANO_SURFACES.pdf - Published Version
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Abstract
The behaviour of two graphene based structures has been theoretically investigated and analysed by using one or more tools. This tool kit consists firstly of SIESTA, a density functional theory software package, secondly Gollum, a non-equilibrium Green’s function code, and finally the tight binding approach. The first project considers the variation of the thermoelectric properties of a graphene-graphene junction functionalised by the amino-silane molecule. The second project studies the mechanical properties of the interface between a silicon-carbide substrate and monolayer graphene. The results of these two projects are summarised in the next paragraphs. The calculation of the thermoelectric properties of a graphene-silane-graphene junction reveals a number of interesting results. The most important result is that silane hinders the cross-plane electron transmission and thermal conductance. Such properties have effective applications through controlling the heat flow in the electronic chip. Furthermore, the silane molecule enhances the figure of merit of the junction which refers to the ability to convert heat. To sum up, silane-functionalized graphene has an improved heat mediation over a non-functionalised junction. The second project analyses the mechanical properties of the silicon-carbide/graphene junction. The study of this junction focuses on the trends in terms of stiffness and work function as the hydrogen concentration intercalating the interface and the number of graphene sheets on top of the silicon-carbide substrate varies. As a result of this study I have found that the effect of increasing the number of penetrating hydrogen atoms is to reduce the stiffness and to enhance the work function. The same situation is found for the stiffness when the number of graphene layers is increased. However the work functions shows two completely opposing behaviours; the first one can be seen in the quasi-free standing graphene layer type 1 and type 2, where the work function has increased, while it has decreased for as-grown interface. An additional property can be deduced is that a certain amount of hydrogen atoms at the interface of approximately 33%, can dramatically change the characteristics of the interface. Another feature is that the junctions exhibit three distinct values of stiffness depending on the hydrogen concentration. The highest value is calculated for the directly attached graphene sheet to the silicon-carbide, while the softest junction is obtained when the concentration of hydrogen atoms passivates more than 50% of the surface silicon atoms. The last value has been shown for the graphene-graphene layer.