TY - THES

T1 - Growth morphology of para-hexaphenyl on exfoliated hexagonal boron nitride flakes

AU - Genser, Jakob

N1 - embargoed until null

PY - 2016

Y1 - 2016

N2 - Organic semiconductors have been the subject of active research for about a quarter of a century, in the meantime with a number of relevant industrial applications, as energy-efficient lighting, organic and hybrid solar cells, as well as various sensing devices. The most crucial part of every organic field-effect device is the gate dielectric/organic semiconductor interface since it defines charge carrier transport. Interface engineering has been demonstrated to be an effective approach in increasing device quality and performance. Recently, two-dimensional materials have been suggested as a gate dielectric, on which epitaxial grown organic thin films form the active layer. They enable the fabrication of ultra-thin and flexible electronic devices with an atomically smooth and dangling bond free interface due to their van der Waals nature. Among those materials, hexagonal boron nitride (hBN) proved to be one of the most promising candidates for future large-scale industrial applications, since it is capable of sustaining high electric fields, is chemically inert, very flexible, optically transparent, and possible to be produced on a large scale. This thesis examines the epitaxial growth of the organic semiconductor para hexaphenyl (6P), serving as an example of rod-like conjugated molecules, on top of hBN substrates. High quality, atomically thin hBN flakes, obtained via micromechanical exfoliation, were employed, and the 6P molecules were deposited using hot wall epitaxy. The morphology of the 6P needle-like crystallites was analysed as a function of deposition temperature - ranging from 298-413 K - and hBN substrate thickness using atomic force microscopy. In the temperature range investigated, 6P forms needle-like crystallites, composed of flat-lying molecules, which evolve into networks of long needles only limited by the lateral size of the hBN substrate at elevated growth temperatures. At the same time, the needle density clearly decreases with increasing temperature. The hBN substrate thickness showed to have a major impact on the resulting 6P needle length and degree of ordering. It has been found that a minimal hBN flake thickness of 1.5 nm is necessary, in order to avoid negative effects on the morphology arising from the hBN/SiO2 interface. Furthermore, the epitaxial relation between the bulk 6P crystallites and the basal plane of hBN has been investigated. Atomic force microscopy measurement analysis revealed a total of six preferential orientations of 6P needles on top of the hBN substrate. To determine the exact adsorption site of 6P on hBN, the experimentally observed morphology has been compared with density functional theory calculations, which revealed the energetically most favourable adsorption site of individual 6P molecules to lie along the hBN armchair direction, with the centre of the 6P phenyl rings above the nitrogen atoms of the hBN substrate. Therefore, the 6P needle orientations are expected to be along the zigzag directions of the hBN substrate. The observed deviation of the 6P needle axis by ±5° from these zigzag directions can be explained by the formation of the (-629) contact plane of 6P, previously reported for 6P growth on Cu (110) substrates. The achieved results provide a deeper understanding of the growth and morphology of rod like organic molecules on the hBN interface.

AB - Organic semiconductors have been the subject of active research for about a quarter of a century, in the meantime with a number of relevant industrial applications, as energy-efficient lighting, organic and hybrid solar cells, as well as various sensing devices. The most crucial part of every organic field-effect device is the gate dielectric/organic semiconductor interface since it defines charge carrier transport. Interface engineering has been demonstrated to be an effective approach in increasing device quality and performance. Recently, two-dimensional materials have been suggested as a gate dielectric, on which epitaxial grown organic thin films form the active layer. They enable the fabrication of ultra-thin and flexible electronic devices with an atomically smooth and dangling bond free interface due to their van der Waals nature. Among those materials, hexagonal boron nitride (hBN) proved to be one of the most promising candidates for future large-scale industrial applications, since it is capable of sustaining high electric fields, is chemically inert, very flexible, optically transparent, and possible to be produced on a large scale. This thesis examines the epitaxial growth of the organic semiconductor para hexaphenyl (6P), serving as an example of rod-like conjugated molecules, on top of hBN substrates. High quality, atomically thin hBN flakes, obtained via micromechanical exfoliation, were employed, and the 6P molecules were deposited using hot wall epitaxy. The morphology of the 6P needle-like crystallites was analysed as a function of deposition temperature - ranging from 298-413 K - and hBN substrate thickness using atomic force microscopy. In the temperature range investigated, 6P forms needle-like crystallites, composed of flat-lying molecules, which evolve into networks of long needles only limited by the lateral size of the hBN substrate at elevated growth temperatures. At the same time, the needle density clearly decreases with increasing temperature. The hBN substrate thickness showed to have a major impact on the resulting 6P needle length and degree of ordering. It has been found that a minimal hBN flake thickness of 1.5 nm is necessary, in order to avoid negative effects on the morphology arising from the hBN/SiO2 interface. Furthermore, the epitaxial relation between the bulk 6P crystallites and the basal plane of hBN has been investigated. Atomic force microscopy measurement analysis revealed a total of six preferential orientations of 6P needles on top of the hBN substrate. To determine the exact adsorption site of 6P on hBN, the experimentally observed morphology has been compared with density functional theory calculations, which revealed the energetically most favourable adsorption site of individual 6P molecules to lie along the hBN armchair direction, with the centre of the 6P phenyl rings above the nitrogen atoms of the hBN substrate. Therefore, the 6P needle orientations are expected to be along the zigzag directions of the hBN substrate. The observed deviation of the 6P needle axis by ±5° from these zigzag directions can be explained by the formation of the (-629) contact plane of 6P, previously reported for 6P growth on Cu (110) substrates. The achieved results provide a deeper understanding of the growth and morphology of rod like organic molecules on the hBN interface.

KW - hexagonales Bornitrid

KW - hBN

KW - Para-Hexaphenyl

KW - 6P

KW - Rasterkraftmikroskopie

KW - AFM

KW - zweidimensionale Materialien

KW - van der Waals Materialien

KW - Hot-Wall-Epitaxy

KW - hexagonal boron nitride

KW - hBN

KW - para-hexaphenyl

KW - 6P

KW - atomic force microscopy

KW - AFM

KW - two-dimensional materials

KW - van der Waals materials

KW - hot wall epitaxy

M3 - Diploma Thesis

ER -