Organic Semiconducting Thin Films on Au(111) and Au(111)–benzenethiolate Surface : Growth, Structure, and Transistor Application

Autor: 呂健瑋
Rok vydání: 2014
Druh dokumentu: 學位論文 ; thesis
Popis: 102
Recent years have seen an increasing number of studies on the utilization of fused thiophene-based materials for organic filed effect transistor (OFET) applications due to their strong intermolecular S-S interaction, large ionization potential energy, and better ambient stability. In OFETs, the interface between the electrodes and the organic semiconductor plays a critical role in affecting the device performance. In our work, the 2-phenylbenzo [d,d’]thieno-[3,2-b;4,5-b’] dithiophene (P-BTDT) and its derivative, monofluorine-substituted 2–phenylbenzo [d,d_]thieno [3,2-b;4,5-b_]–dithiophene (m-FP-BTDT), were grown on the Au (111) and modified Au (111) substrate as the organic semiconductor thin films, respectively. The growth mechanisms, the molecular orientations and the electronic structure of the thin films were analyzed via XPS, NEXAFS and UPS. In addition, the above results were combined with the XRD and AFM data to reach a thorough understanding of the performances of various OFETs. The thermal desorption data show that the chemisorbed P-BTDT layer remains thermally stable up to 750 K on Au(111). However, P-BTDT desorption on Au-BT substrate is found to derive exclusively from physisorption state, and chemisorption desorption is entirely suppressed. Based on the change of XPS intensity with the amount of P-BTDT deposited, it is concluded that the growth of P-BTDT film on Au(111) follows the Stranski-Krastanov mechansim, i.e., a completion of one monolayer (2D growth) followed by a 3D crystallite growth. In comparison, for the growth of P-BTDT on Au(111)-benzenethiolate, a pseudo 2D-like growth is observed. From the results of UPS, the change of work function and valence band spectrum with the film-thickness is in accord with this growth behavior. The results obtained by XPS are also in agreement with UPS observations. The molecules of the multilayer grown on Au(111)-BT have their aromatic rings inclined toward the surface by an average 66°, as determined by NEXAFS data. For the thick film grown on Au(111), out-of-plane x-ray diffraction reveals the presence of several crystallite alignment schemes. In contrast, the XRD data of the thicker layer grown on Au(111)-BT show (002) diffraction peaks only, indicating that the crystal orientation of the thin film is (002) preferred. Therefore, we can change the crystal structure and control the growth behavior of the thin film through different modification of substrates, leading to an improved performance of the OFETs. The results show that P-BTDT thin film that is grown on BT-Au can yield a better OFET mobility up to 3.0 × 10-2 cm2/Vs with thickness of 120 nm. In order to shift energy level of LUMO toward lower value, we also prepared m-FP-BTDT organic semiconducting thin films and investigated their properties. XPS intensity analysis shows that on Au(111), m-FP-BTDT film grows according to Stranski–Krastanov (SK) mode. It is interesting to note an abrupt increase of Au 4f signal at around one ML of m-FP-BTDT, which is explained by the production of excess Au adatoms, accompanied by the lifting of the herringbone reconstruction of Au(111). In comparison, the initial growth of m-FP-BTDT on Au-BT proceeds via a pseudo layer-by-layer growth mechanism. NEXAFS data show that m-FP-BTDT molecules on Au-BT adopt a more erected configuration, the angle between the molecule and substrate is 62.5°, resulting in a better cofacial π-stacking. The XRD pattern reveals that the crystal growth orientation of m-FP-BTD thin film is along c axis. Work function for the thick m-FP-BTDT film on Au-BT is determined with UPS as 4.62 eV and the hole injection barrier as 0.94 eV. According to above results the thin film of m-FP-BTDT grown on Au(111)-BT is expected to produce better carrier transport phenomenon.
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