Atomically thin Molybdenum disulfide, MoS2, a star member of the group VI transition metal dichalcogenide(TMDC) family has been attracting rising interests for its potential applications in emerging electronics and optoelectronics. Bulk MoS2is a semiconductor with an indirect gap located between the top of valence band at Γ points and the bottom of conduction band in mid of K and Γ points in its Brillouin zone. Atomically thin MoS2 films including monolayers and multilayers, being chemically inert, present a class of intrinsic 2D semiconductors which are widely regarded as a platform for ultimate electronics. As yet tremendous efforts focus on the optical properties and electric transport study. In this thesis, we report the experimental study of photocurrent measurements on MoS2thin films. The sample preparation, device fabrication, optical and electric characterizations are introduced. The experiments have been carried out on a field effect transistor (FET) structured MoS2 device. The photocurrent spectroscopy reveals the interband excitonic transitions at spin-split bands around K valleys. The results demonstrate that MoS2has potential applications in optoelectronics. / published_or_final_version / Physics / Master / Master of Philosophy

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儘管在提高有機薄膜晶體管（OTFTs）的性能方面已經取得了顯著地進步，但是由傳統二氧化矽介電層的低面電容密度引起的高驅動電壓一直是阻礙其在實際應用中發展的絆腳石。因此，開發具有低成本、高介電常數等特點的新型材料對於學術界和工業界都具有非常重要的意義。 / 本文首先介紹了一種簡單的溶液法在低溫下制備高介電常数的Al₂O{U+ABB7}/TiO{U+2093}（ATO）材料體系， 并詳細表徵和討論了它的介電性能。通過運用ATO作為介電層，我們成功地製備了低電壓銅酞菁（CuPc）基OTFT。有趣的是，該低電壓器件顯示出優異的性能，並且遠遠超過在二氧化矽上製備的器件性能。這個結果似乎和報道的結果相矛盾，因為高介電常數往往對器件性能造成不利影響。本文就此异常現象進行了詳細研究。基於初期生長的研究表明，在ATO表面上，CuPc分子組裝成有利於載流子輸運的棒狀晶體，并形成網狀結構。相反，在SiO₂表面上CuPc分子卻形成由無定形結構組成的孤立小島。此外，在ATO上還觀察到了更好的金屬/有機分子接觸，有利於載流子的注入。以上研究表明溶液法製備的ATO在实现高性能、低電壓的OTFT方面有著非常實用的前景。 / 此外，界面的性質對決定OTFT的電學性能非常關鍵。因此研究界面功能化對提高器件性能的作用也非常重要。在應用十八烷基磷酸（ODPA）和原位改性的Cu（M-Cu）分別對介電層/半導體、電極/半導體界面進行修飾后，并五苯（pentacene）基OTFT的電學性能得到大幅提高。此外，通過採用一薄層金覆蓋的M-Cu做電極（Au/M-Cu），器件性能得到進一步提升。本文就其詳細的機理進行了討論。 / 最后，由於具有低成本，可捲曲，可大面積加工等特點，柔性有機電子器件引起了廣汎關注。實現柔性OTFT的關鍵問題之一就是介電層同柔性襯底之間的結合。在此，我們成功地將ODPA和ATO集成到金覆蓋的柔性聚酰亞胺襯底上。通過使用Au/M-Cu做電極，柔性pentacene TFT顯現出優異的電學性能。另外，本文就器件的機械柔性及可靠性也做了詳細地探討，從而展示了一個實現低成本高性能柔性OTFT的有效途徑。 / Although impressive progress has been made in improving the performance of organic thin film transistors (OTFTs), the high operation voltage resulting from the low gate areal capacitance of traditional SiO₂ remains a severe limitation that hinders OTFTs’ development in practical applications. In this regard, developing new materials with high-k characteristics at low cost is of great scientific and technological importance in the area of both academia and industry. / In this thesis, we first describe a simple solution-based method to fabricate a high-k bilayer Al₂O{U+ABB7}/TiO{U+2093} (ATO) dielectric system at low temperature. Then the dielectric properties of the ATO are characterized and discussed in detail. Furthermore, by employing the high-k ATO as gate dielectric, low-voltage copper phthalocyanine (CuPc) based OTFTs are successfully developed. Interestingly, the obtained low-voltage CuPc TFT exhibits outstanding electrical performance, which is even higher than the device fabricated on traditional low-k SiO₂. The above results seem to be contradictory to the reported results due to the fact that high-k usually shows adverse effect on the device performance. This abnormal phenomenon is then studied in detail. Characterization on the initial growth shows that the CuPc molecules assemble in a “rod-like“ nano crystal with interconnected network on ATO, which probably promotes the charge carrier transport, whereas, they form isolated small islands with amorphous structure onSiO₂. In addition, a better metal/organic contact is observed on ATO, which benefits the charge carrier injection. Our studies suggest that the low-temperature, solution-processed high-k ATO is a promising candidate for fabrication of high-performance, low-voltage OTFTs. / Furthermore, it is well known that the properties of the dielectric/semiconductor and electrode/semiconductor interfaces are crucial in controlling the electrical properties of OTFTs. Hence, investigation the effects of interfaces engineering on improving the electrical characteristics of OTFTs is of great technological importance. For the dielectric/semiconductor interface, an octadecylphosphonic acid (ODPA) self-assembled monolayer (SAM) is used to modify the surface of ATO (ODPA/ATO). For the electrode/semiconductor interface, a simple in-situ modified Cu (M-Cu) is employed as source-drain (S/D) electrodes in stead of commonly used Au. The electrical characteristics of pentacene TFT are drastically enhanced upon interfaces modification. Moreover, by encapsulating the M-Cu with a thin layer of Au (Au/M-Cu), the device performance is further improved. The detailed mechanism is systematically explored. / Finally, organic electronic devices on flexible plastic substrates have attracted much attention due to their low-cost, rollability, large-area processability, and so on. One of the most critical issues in realization flexible OTFTs is the integration of gate dielectrics with flexible substrates. We have successfully incorporated the ODPA/ATO with Au coated flexible polyimide (PI) substrate. By using Au/M-Cu as S/D electrode, the flexible pentacene TFTs show outstanding electrical performance. In addition, the mechanical flexibility and reliability of the devices are studied in detail. Our approach demonstrates an effective way to realize low-cost, high-performance flexible OTFTs. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Su, Yaorong. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2013. / Includes bibliographical references. / Abstract also in Chinese. / Abstract --- p.i / Acknowledgement --- p.vi / Table of contents --- p.viii / List of Figure Captions --- p.xii / List of Table Captions --- p.xvi / Chapter Chapter 1 --- Introduction --- p.1 / References --- p.6 / Chapter Chapter 2 --- Background --- p.8 / Chapter 2.1 --- Intrinsic electronic structure of small molecule semiconductors --- p.8 / Chapter 2.2 --- Organic field-effect transistors --- p.13 / Chapter 2.2.1 --- Architecture of OTFTs --- p.13 / Chapter 2.2.2 --- Operation principles of OTFTs --- p.15 / Chapter 2.3 --- Charge transport mechanisms --- p.20 / Chapter 2.3.1 --- Band transport --- p.20 / Chapter 2.3.2 --- Polaron transport --- p.22 / Chapter 2.3.3 --- Hopping transport --- p.24 / Chapter 2.3.4 --- Multiple trapping and thermal release (MTR) model --- p.26 / Chapter 2.4 --- Parameters extraction --- p.29 / Chapter 2.4.1 --- Current-voltage (I-V) characteristics --- p.30 / Chapter 2.4.2 --- Field-effect mobility (μ) and threshold voltage (VT) --- p.31 / Chapter 2.4.3 --- On/off current ratio and subthershold swing (SS) --- p.33 / Chapter 2.4.4 --- Contact resistance (RC) --- p.35 / Chapter 2.1.1.1 --- Origin of contact resistance --- p.35 / Chapter 2.1.1.2 --- Extraction of contact resistance --- p.38 / Chapter 2.1.1.2.1 --- Transfer line method (TLM) --- p.38 / Chapter 2.1.1.2.2 --- Gated four-probe technique --- p.40 / Chapter 2.1.1.2.3 --- Kelvin probe force microscopy (KPFM) --- p.42 / Chapter 2.5 --- Gate dielectrics --- p.44 / References --- p.47 / Chapter Chapter 3 --- Materials and Experimental Techniques --- p.51 / Chapter 3.1 --- Materials --- p.51 / Chapter 3.2 --- Device fabrication procedures --- p.53 / Chapter 3.3 --- Characterization --- p.55 / Chapter 3.3.1 --- Electrical performance testing --- p.55 / Chapter 3.3.2 --- Atomic force microscope (AFM) --- p.56 / Chapter 3.3.3 --- Kelvin probe force microscopy (KPFM) --- p.58 / Chapter 3.3.4 --- X-ray diffraction (XRD) --- p.60 / Chapter 3.3.5 --- Grazing incidence X-ray diffraction (GIXD) --- p.63 / Chapter 3.3.6 --- X-ray photoelectron spectroscopy (XPS) --- p.66 / References --- p.71 / Chapter Chapter 4 --- Solution-processed High-k Gate Dielectric --- p.73 / Chapter 4.1 --- Introduction --- p.73 / Chapter 4.2 --- Experimental details --- p.75 / Chapter 4.3 --- Results and discussion --- p.77 / Chapter 4.3.1 --- Structure of dielectric film --- p.77 / Chapter 4.3.2 --- XPS characterization --- p.79 / Chapter 4.3.3 --- Leakage current and capacitance --- p.80 / Chapter 4.3.4 --- Low-voltage CuPc TFTs --- p.86 / Chapter 4.4 --- Conclusion --- p.88 / References --- p.88 / Chapter Chapter 5 --- Study of CuPc OTFT with High-k Gate Dielectric --- p.91 / Chapter 5.1 --- Introduction --- p.91 / Chapter 5.2 --- Experimental details --- p.93 / Chapter 5.3 --- Results and discussion --- p.95 / Chapter 5.3.1 --- Devices electrical characteristics --- p.95 / Chapter 5.3.2 --- Morphologies of dielectrics and CuPc fims --- p.99 / Chapter 5.3.3 --- Crystal structure of CuPc films --- p.101 / Chapter 5.3.4 --- Initial growth study --- p.102 / Chapter 5.3.5 --- Surface energy characterization --- p.104 / Chapter 5.3.6 --- In-plane structure of CuPc films --- p.105 / Chapter 5.3.7 --- XPS characterization --- p.107 / Chapter 5.3.8 --- KPFM study --- p.108 / Chapter 5.3.9 --- Extended application to other materials --- p.110 / Chapter 5.4 --- Conclusion --- p.113 / References --- p.114 / Chapter Chapter 6 --- Interface Engineering for High-performance Pentacene OTFTs --- p.117 / Chapter 6.1 --- Introduction --- p.117 / Chapter 6.2 --- Experimental details --- p.120 / Chapter 6.3 --- Results and discussion --- p.121 / Chapter 6.3.1 --- Leakage and capacitance of dielectric --- p.121 / Chapter 6.3.2 --- Devices electrical characteristics --- p.123 / Chapter 6.3.3 --- Contact resistance --- p.126 / Chapter 6.3.4 --- Electrode/pentacene interface structure --- p.127 / Chapter 6.3.5 --- XPS characterization --- p.129 / Chapter 6.3.6 --- Proposed band diagram --- p.132 / Chapter 6.3.7 --- Au encapsulation --- p.133 / Chapter 6.4 --- Conclution --- p.136 / References --- p.136 / Chapter Chapter 7 --- Flexible Pentacene OTFTs --- p.140 / Chapter 7.1 --- Introduction --- p.140 / Chapter 7.2 --- Experimental details --- p.143 / Chapter 7.3 --- Results and discussion --- p.146 / Chapter 7.3.1 --- Leakage and capacitance characterization --- p.146 / Chapter 7.3.2 --- Structure of pentacene thin film --- p.147 / Chapter 7.3.3 --- Electrical properties of flexible OTFTs --- p.149 / Chapter 7.3.4 --- Mechanical performance characterization --- p.152 / Chapter 7.3.5 --- Ambient stability study --- p.158 / Chapter 7.3.6 --- Study on the electrode structure --- p.160 / Chapter 7.3.7 --- Study on the operational stability and lifetime --- p.163 / Chapter 7.4 --- Conclusion --- p.166 / References --- p.167 / Chapter Chapter 8 --- Summary and Perspectives --- p.171 / Chapter 8.1 --- Summary --- p.171 / Chapter 8.2 --- Future work --- p.175 / References --- p.177 / Publications --- p.179

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