Reversible regeneration of molybdenum carbonyl species on an alumina thin film

The thin alumina film-supported metallic molybdenum model catalyst was prepared by thermal decomposition of MO(CO)6, and CO chemisorption on the catalyst was investigated in-situ by thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). The results showed that a molybdenum-carbonyl-like species was formed on the alumina surface at low temperature by high coordination of CO with the surface metallic molybdenum nanoparticles, indicating a reversible regeneration of molybdenum carbonyl on the alumina surface. CO chemisorption on the model catalyst surface caused the Mo 3d XPS peak to shift toward higher binding energy. The formed molybdenum carbonyl species appeared at about 240 K in the TDS. The supported metallic molybdenum nanoparticles were quite different from the bulk molybdenum in chemical properties, which indicated a prominent particle-size effect of the clusters.

高迁移率有机半导体材料及有机薄膜晶体管

有机薄膜晶体管中以并五苯和齐聚唾酚为代表的几种有机半导体材料材料多晶薄膜的迁移率已经达到1cm2V-1s-l以上，已经非常接近这些材料单晶的迁移率，也是目前有机薄膜晶体管所使用的材料中迁移率最高的。但是这几种材料都具有合成困难，价格昂贵、稳定性较差的缺点。金属酞警是一种具有较好热和化学的稳定性、价格便宜、可以在商业上直接购得大量高纯度产品的有机半导体材料。它的单晶迁移率很高，但是现在它多晶薄膜的迁移率比单晶的迁移率低两个量级。因此尽快提高这种金属酞普多晶薄膜的迁移率是目前有机薄膜晶体管研究的一个迫切要求。在对并五苯材料的研究中，认为采用提高有机半导体薄膜的有序性、有机薄膜晶体管中的载流子注入的方法可以提高其多晶薄膜中的迁移率，深入的机理有待更进一步的研究。因此本论文的工作主要集中在以下三个方面：(1)以酞普铜为研究对象，利用经典的薄膜生长理论解释了生长条件对薄膜形态结构的影响。然后在薄膜生一氏理论的指导下，制备出大尺寸，高有序，连续的酞普铜薄膜。在这个基础上获得了基于酞普铜薄膜的有机薄膜晶体管目前最高的迁移率。并且成功的将这种方法扩展到了平面型单酞警、全氟代酞普铜、并五苯等多个有机半导体材料上。〔2〕发明了一种新有机薄膜晶体管构型，夹心型有机薄膜晶体管。通过提高器件中的载流子注入，将基于金属酞普薄膜的有机薄膜晶体管的迁移率提高了一个量级，接近了金属酞瞥单晶中的迁移率，达到了目前平板显示中的大量使用的非晶硅薄膜晶体管的水平。并．目．成功的将这一构型扩展到了更广阔的有机半导体材料上。〔3〕利用两种单金属酞背共晶复合得到了比单一组分具有更高迁移率的酞普共晶复合材料的同时，发明了一种利用两种有机半导体材料复合来获得高迁移率有机半导体材料的物理方法。利用多种表征手段对单金属酞蓄共晶材料进行了表征，寻找到了单金属酞普共晶复合材料高迁移率的原因。

功能性有机薄膜晶体管的研究

有机薄膜晶体管由于具有操作温度低、可以大面积加工和兼容柔性基底等优点，在有源矩阵显示、集成电路和化学传感器等方面显示出广泛的应用潜力。基于这个原因，有关有机半导体材料和器件的研究已经成为当前的热点。从实际应用的角度考虑，当前有机薄膜晶体管的器件性能和单一的工作模式还不能充分的满足实际的需要。因此如何提高有机薄膜晶体管的器件性能、开发多工作模式的器件成为有机电子学领域研究人员的重要挑战。本论文的工作以有机薄膜晶体管器件为主要研究对象。在对二种栅绝缘层的有机薄膜晶体管的研究基础上，提出了有机／无机双介电层有机薄膜晶体管的概念，解决器件的闭值电压和栅源漏电的问题；通过的对有机薄膜晶体管中电荷注入的研究的基础上，提出了含有缓冲层的有机薄膜晶体管，显著的改善了电荷注入效率，提高了器件性能。在对异质结型器件研究的基础上，获得了耗尽型和双极型有机薄膜晶体管，满足实际应用对多种工作模式有机薄膜晶体管的需求。论文的主体工作主要分为基于不同栅绝缘层的有机薄膜晶体管的研究，有机薄膜晶体管中接触效应的研究以及异质结型有机晶体管三个部分其具体内容如下：（l）基于不同栅绝缘层有机薄膜晶体管的研究。通过对有机晶体管中常用的高介电和低介电绝缘层对器件性能的影响进行了系统的研究，在有机晶体管构建中提出了有机／无机双介电层的概念。这一概念包含两个方面意义：一方面采用的双绝缘层分别为一层有机材料和一层无机材料的双介电层；另一方面无机介电层使用的是高介电常数的材料，有机部分为低介电常数材料。有机半导体沉积在这种方法构建的栅绝缘层基底上，改善了有机半导体的薄膜形态，提高了有机半导体层和无机基底的乳附力，从而达到了优化了器件电子性质的目的。与此同时解决了在当前含有高介电绝缘栅的有机晶体管中存在的高漏电的问题。通过这种方法构建的有机薄膜晶体管具有低阂值电压、低漏电、高迁移率等优点。由于我们在有机绝缘层的加工中采用了化学修饰的方法，该方法有加工简便、性质稳定好所以具有很好的实际应用前景。（2）有机薄膜晶体管中接触效应的研究。通过实验中采用TML（Transfer－Line一Method）的方法系统的研究了酞著铜薄膜晶体管中的接触效应，首次获得了酞著铜和金电极接触电阻定量化的结果。通过改变不同的电极材料，研究了不同的源漏电极对有机晶体管器件性能的影响。在此基础上为了能减小有机晶体管中的接触电阻，改善电荷注入效率，我们采用了在源漏电极和有机半导体之间插入一层高电导率的有机材料的方法，有效的降低了金属和有机半导体的注入势垒，增大了电荷隧穿几率，从而显著的改善了有机晶体管的器件性质。（3）异质结型有机晶体管。为了推动有机晶体管在集成互补电路中的应用，多工作模式的有机晶体管是需要的。在这里，我们把有机的异质结引入到有机晶体管的制备中，分别获得了高性能的耗尽型和双极型有机晶体管。通过了一系列的设计实验，我们提出了双沟道的载流子输运模型，为异质结型晶体管中双载流子的输运机制提供了理论上的解释。此外，我们通过有机异质结厚度的调节，进而改变沟道区的载流子密度，从而可以有效的调节了器件的阂值电压。当前的研究结果的意义在于利用的有机的异质结效应可以获得不同阂值电压的全塑型有机晶体管，为其实际中的广泛应用开辟了道路。此外我们还在同一基底上构建了基于一个常开型（耗尽）和一个常关型（累积）两个有机晶体管的反向器电路，从而表明异质结型有机晶体管在集成电路方面具有广泛的应用潜力。综上所述，本论文对目前有机薄膜晶体管器件中存在的问题进行了深入的研究。在此基础上，针对不同的问题分别提出了三种功能性的有机晶体管，优化了器件的性能。当前的研究也将为加速有机薄膜晶体管的在实际中的应用做出贡献。

Behavior of Pentacene Initial Nucleation on Various Dielectrics and Its Effect on Carrier Transport in Organic Field-Effect Transistor

The influence of dielectric surface energy on the initial nucleation and the growth of pentacene films as well as the electrical properties of the pentacene-based field-effect transistors are investigated. We have examined a range of organic and inorganic dielectrics with different surface energies, such as polycarbonate/SiO2, polystyrene/SiO2, and PMMA/SiO2 bi-layered dielectrics and also the bare SiO2 dielectric. Atomic force microscopy measurements of sub-monolayer and thick pentacene films indicated that the growth of pentacene film was in Stranski-Kranstanow growth mode on all the dielectrics. However, the initial nucleation density and the size of the first-layered pentacene islands deposited on different dielectrics are drastically influenced by the dielectric surface energy. With the increasing of the surface energy, the nucleation density increased and thus the average size of pentacene islands for the first mono-layer deposition decreased. The performance of fabricated pentacene-based thin film transistors was found to be highly related to nucleation density and the island size of deposited Pentacene film, and it had no relationship to the final particle size of the thick pentacene film. The field effect mobility of the thin film transistor could be achieved as high as 1.38 cm(2)/Vs with on/off ratio over 3 x 10(7) on the PS/SiO2 where the lowest surface energy existed among all the dielectrics. For comparison, the values of mobility and on/off ratio were 0.42 cm(2)/Vs and 1 x 10(6) for thin film transistor deposited directly on bare SiO2 having the highest surface energy.

Tin (IV) phthalocyanine oxide: An air-stable semiconductor with high electron mobility

Air-stable n-type field effect transistors were fabricated with an axially oxygen substituted metal phthalocyanine, tin (IV) phthalocyanine oxide (SnOPc), as active layers. The SnOPc thin films showed highly crystallinity on modified dielectric layer, and the electron field-effect mobility reached 0.44 cm(2) V-1 s(-1). After storage in air for 32 days, the mobility and on/off ratio did not obviously change. The above results also indicated that it is an effective approach of seeking n-type semiconductor by incorporating the appropriate metal connected with electron-withdrawing group into pi-pi conjugated system.

Pyrazine-containing Acene-Type Molecular Ribbons with up to 16 Rectilinearly Arranged Fused Aromatic Rings

A series of acene-type conjugated molecules(1-5) containing 2-6 pyrazine units and up to 16 rectilinearly arranged fused aromatic rings were synthesized by condensation coupling of 1,2-diamines and 1,2-diketones. The energy gap of the molecules estimated from absorption edge decreases with an increase in molecular length, indicating the well-delocalized nature of the molecules. The cyclic voltarnmetry measurements suggest that the n-type properties of these ribbonlike pyrazine derivatives are dependent on the molecular length and the number of the pyrazine units.

Morphology and crystalline transition of poly (3-butylthiophene) associated with its polymorphic modifications

Poly (3-butylthiophene) (P3BT) is a much less studied conjugated polymer despite its high crystallizability and thus excellent electrical property. In this work, morphology of P3BT at different crystalline polymorphs and solvent/thermal induced phase transition between form I and U modifications have been intensively investigated by using optical microscopy, electron microscopy, differential scanning calorimetry, and X-ray diffraction. It is shown that a direct deposition from carbon disulfide (CS2) at fast evaporation results in P3BT crystals in form I modification, giving typical whiskerlike morphology. In contrast, low evaporation rate from CS, leads to formation of form II crystals with spherulitic morphology, which is so far scarcely observed in polythiophene.

Phthalocyanato tin(IV) dichloride: An air-stable, high-performance, n-type organic semiconductor with a high field-effect electron mobility

Phthalocyanato tin(IV) dichloride, an axially dichloriniated MPc, is an air-stable high performance n-type organic semiconductor with a field-effect electron mobility of up to 0.30 cm(2) V-1 s(-1). This high mobility together with good device stability and commercial availability makes it a most suitable n-type material for future organic thin-film transistor applications.

Surface potential images of polycrystalline organic semiconductors obtained by Kelvin probe force microscopy

P-type copper phthalocyanine (CuPc) and n-type hexadecafluorophthalocyanina-tocopper (F16CuPc) polycrystalline films were investigated by Kelvin probe force microscopy (KPFM). Topographic and corresponding surface potential images are obtained simultaneously. Surface potential images are related with the local work function of crystalline facets and potential barriers at the grain boundaries (GBs) in organic semiconductors. Based on the spatial distribution of surface potential at GBs, donor- and acceptor-like trapping states in the grain boundaries (GBs) of p-CuPc and n-F16CuPc films are confirmed respectively.

Poly(oligothiophene-alt-benzothiadiazole)s: Tuning the Structures of Oligothlophene Units toward High-Mobility "Black" Conjugated Polymers

A series of donor-acceptor low-bandgap conjugated polymers, i.e., PTnBT (n = 2-6), composed of alternating oligothiophene (OTh) and 2,1,3-benzothiadiazole (BT) units were synthesized by Stille cross-coupling polymerization. The number of thiophene rings in OTh units, that is n, was tuned from 2 to 6. All these polymers display two absorption bands in both solutions and films with absorption maxima depending on n. From solution to film, absorption spectra of the polymers exhibit a noticeable red shift. Both high- and low-energy absorption bands or P'F5BT and PT6BT films locate in the visible region, which are at 468 and 662 nm for PT5BT and 494 and 657 nm for PT6BT.

Micropatterning of polymeric semiconductor by selective lift-off method using epoxy mold

A simple and efficient method for patterning polymeric semiconductors for applications in the field of organic electronics is proposed. The entire polymer layer, except for the desired pattern, is selectively lifted off from a flat poly(dimethylsiloxane) (PDMS) stamp surface by an epoxy mold with a relief pattern. This is advantageous because the elastic deformation of the PDMS stamp around protrusions of a patterned stamp under pressure can assist the plastic deformation of a polymer film along the pattern edges, yielding large area and high quality patterns, and the PDMS surface has low surface energy, which allows the easy removal of the polymer film.

Ambipolar permeable metal-base transistor based on NPB/C-60 heterojunction

In this paper, we report the fabrication of permeable metal-base organic transistors based on N,N'-diphenyl-N,N'-bis(1-naphthylphenyl)-1,1'-biphenyl-4,4'-diamine (NPB)/C-60 heterojunction as both emitter and collector. By applying different polarities of voltage bias to the collector and the base, and input current to the emitter, the ambipolar behavior can be observed. The device demonstrates excellent common-base characteristics both in P-type and N-type modes with common-base current gains of 0.998 and 0.999, respectively.

Study on the Single Crystals of Poly(3-octylthiophene) Induced by Solvent-Vapor Annealing

Needle-like single crystals of poly(3-octylthiophene) (P3OT) have been prepared by tetrahydrofuran-vapor annealing. The morphology and structure of the crystals were characterized with optical microscopy, scanning electron microscopy, atomic force microscopy, transmission electron microscopy, and wide-angle X-ray diffraction. It is observed that the P3OT molecules are packed with the backbones parallel to the length axis of the crystal and the alkyl side chains perpendicular to the substrate. The field effect transistor based on the P3OT single crystal exhibited a charge carrier mobility of 1.54 x 10(-4) cm(2)/(Vs) and on/off current ratio of 37, and the molecular orientation of the crystal is ascribed to account for the device performance. The time-dependent morphological evolution demonstrated that the crystals underwent Ostwald ripening when annealed.

Enhanced electrical conductivity of highly crystalline polythiophene/insulating-polymer composite

Poly(3-butylthiophene) (P3BT)/insulating-polymer composites with high electrical conductivity have been prepared directly from the solution. These composites exhibit much higher conductivity compared to pure P3BT with the same preparation method provided that P3BT content is higher than 10 wt %. Morphological studies on both the pure P3BT and the composites with insulating polymer show that P3BT highly crystallizes and develops into whisker-like crystals. These nanowires are homogeneously distributed within the insulating polymer matrix and form conductive networks, which provide both extremely large interface area between conjugated polymer and insulating polymer matrix and highly efficient conductive channels through out the whole composite. In contrast, the conductivity enhancement of P3HT/PS composite is not so obvious and drops down immediately with increased PS content due mainly to the absence of highly crystalline whisker-like crystals and much larger scale phase separation between the components. The results presented here could further illuminate the origin of conductivity formation in organic semiconducting composites and promote applications of these polymer semiconductor/insulator composites in the fields of organic (opto-)electronics, electromagnetic shielding, and antistatic materials.

Naphthyl and thionaphthyl end-capped oligothiophenes as organic semiconductors: Effect of chain length and end-capping groups

Two series of oligothiophenes (OThs), NaTn and TNTn (n = 2-6 represents the number of thiophene rings), end-capped with naphthyl and thionaphthyl units have been synthesized by means of Stille coupling. Their thermal properties, optical properties, single crystal structures, and organic field-effect transistor performance have been characterized. All oligomers display great thermal stability and crystallinity. ne crystallographic structures of NaT2, NaT3, TNT2, and TNT3 have been determined. The crystals of NaT2 and NaT3 are monoclinic with space group P2(1)/C, while those of TNT2 and TNT3 are triclinic and orthorhombic with space groups P-1(-) and P2(1)2(1)2(1), respectively. All oligomers adopt the well-known herringbone packing-mode in crystals with packing parameters dependent on the structure of the end-capping units and the number of thiophene rings. The shorter intermolecular distance in NaT3 compared to NaT2 indicates that the intermolecular interaction principally increases with increasing molecular length. X-ray diffraction and atomic force microscopy (AFM) characterization indicate that the NaTn oligomers can form films with better morphology and high molecular order than TNTn oligomers with the same number of thiophene rings. The NaTn oligomers exhibit mobilities that are much higher than those for TNTn oligomers (0.028-0.39 cm(2) V-1 s(-1) versus 0.010-0.055 cm(2) V-1 s(-1), respectively).