TY - THES

T1 - Growth morphologies of dihydro-tetraaza-pentacene and -heptacene on (0001) sapphire

AU - Cicek, Aydan

N1 - embargoed until null

PY - 2021

Y1 - 2021

N2 - Organic electronics drive the development of new materials; the main goals are to increase the performance of the devices, to decrease the manufacturing costs, and to enable new functionalities. One main trend in the advancement of organic electronics is the introduction of novel molecular building blocks. There are millions of thus far investigated organic compounds, and the number keeps rising. However, the development of new organic compounds is usually not straightforward and involves many challenges as purity, functionality, stability, scalability, environment-friendly processing, and many others before it can be considered for realistic technological applications. Oligoacene compounds are a novel and very promising group of organic semiconductors that is likely to be of great importance in the future development of organic electronics due to their promising properties such as their intermolecular interaction that favors charge transfer between the molecules and enables potentially high charge carrier mobilities. In this thesis, the thin film growth of sub-monolayers of dihydrotetraaza-pentacene (DHTA5) and -heptacene (DHTA7) molecules which display a similar dipolar momentum and H-bonding is investigated. Due to the different size of the conjugated backbones (five rings and seven rings in a sequence) their growth morphologies change significantly. The molecules are deposited by hot wall epitaxy (HWE) on a (0001) vicinal sapphire substrate with an average step distance of 50 nm and a step height of 0.2 nm. The morphology of the grown films is studied using ex-situ atomic force microscopy (AFM). At deposition temperatures (TD) ranging from 343 – 366 K and 310 – 420 K, the morphology of the DHTA5 and DHTA7 crystallites were analyzed on the nanometer scale. Mound formation is found in the multi-layer DHTA7 films. Furthermore, the surface coverage, nucleation density, surface compactness, and evolution of crystallite morphologies are examined as a function of TD. The results demonstrate that DHTA5 molecules mainly form needle-like crystallites representing a flat-lying configuration of the molecules on the substrate. DHTA7 films, on the other hand, are mainly forming island-like crystallites displaying an up-right configuration of the molecules on the substrate. Quasi layer-by-layer (LbL) growth (terraced mound shape) is favored in this case. Another observation was that unlike DHTA7 islands, DHTA5 islands are not stable for short-term exposure to ambient conditions. In an attempt to mechanically test the robustness of DHTA5 needles, AFM based manipulation has been used to push and move the needles along the surface. It was found that the needles cannot be pushed like it has been previously found for the nonpolar molecule parahexaphenyl but are rather cut into small fragments defined by the size of the tip. Very often it is observed that the AFM tip picked needle fragments up along the direction of the tip motion. These nano-manipulation experiments point towards either weak bonding within DHTA5 needles or towards the needles being riddled with defects and thus effectively not representing single crystals. Crosscheck experiments for pushing short DHTA7 needles grown on hBN are planned in the future.

AB - Organic electronics drive the development of new materials; the main goals are to increase the performance of the devices, to decrease the manufacturing costs, and to enable new functionalities. One main trend in the advancement of organic electronics is the introduction of novel molecular building blocks. There are millions of thus far investigated organic compounds, and the number keeps rising. However, the development of new organic compounds is usually not straightforward and involves many challenges as purity, functionality, stability, scalability, environment-friendly processing, and many others before it can be considered for realistic technological applications. Oligoacene compounds are a novel and very promising group of organic semiconductors that is likely to be of great importance in the future development of organic electronics due to their promising properties such as their intermolecular interaction that favors charge transfer between the molecules and enables potentially high charge carrier mobilities. In this thesis, the thin film growth of sub-monolayers of dihydrotetraaza-pentacene (DHTA5) and -heptacene (DHTA7) molecules which display a similar dipolar momentum and H-bonding is investigated. Due to the different size of the conjugated backbones (five rings and seven rings in a sequence) their growth morphologies change significantly. The molecules are deposited by hot wall epitaxy (HWE) on a (0001) vicinal sapphire substrate with an average step distance of 50 nm and a step height of 0.2 nm. The morphology of the grown films is studied using ex-situ atomic force microscopy (AFM). At deposition temperatures (TD) ranging from 343 – 366 K and 310 – 420 K, the morphology of the DHTA5 and DHTA7 crystallites were analyzed on the nanometer scale. Mound formation is found in the multi-layer DHTA7 films. Furthermore, the surface coverage, nucleation density, surface compactness, and evolution of crystallite morphologies are examined as a function of TD. The results demonstrate that DHTA5 molecules mainly form needle-like crystallites representing a flat-lying configuration of the molecules on the substrate. DHTA7 films, on the other hand, are mainly forming island-like crystallites displaying an up-right configuration of the molecules on the substrate. Quasi layer-by-layer (LbL) growth (terraced mound shape) is favored in this case. Another observation was that unlike DHTA7 islands, DHTA5 islands are not stable for short-term exposure to ambient conditions. In an attempt to mechanically test the robustness of DHTA5 needles, AFM based manipulation has been used to push and move the needles along the surface. It was found that the needles cannot be pushed like it has been previously found for the nonpolar molecule parahexaphenyl but are rather cut into small fragments defined by the size of the tip. Very often it is observed that the AFM tip picked needle fragments up along the direction of the tip motion. These nano-manipulation experiments point towards either weak bonding within DHTA5 needles or towards the needles being riddled with defects and thus effectively not representing single crystals. Crosscheck experiments for pushing short DHTA7 needles grown on hBN are planned in the future.

KW - organische Dünnfilme

KW - Rasterkraftmikroskopie

KW - Dihydro-Tetraaza-Acene

KW - DHTAP

KW - DHTA7

KW - Wasserstoffbrücken-Bindung

KW - Hot-Wall-Epitaxie

KW - organische Moleküle

KW - Nano-Manipulation

KW - organic thin films

KW - atomic force microscopy

KW - Dihydro-tetraaza-acenes

KW - DHTAP

KW - DHTA7

KW - H-bonding

KW - hot wall epitaxy

KW - organic molecules

KW - nano-manipulation

M3 - Master's Thesis

ER -