【作者】 陶永升；

【导师】 何丕模；

【作者基本信息】 浙江大学 ， 凝聚态物理， 2015， 博士

【摘要】 过去几十年,由于有机半导体材料合成的多样化,材料的高产率和低成本,并能够根据功能需要进行准确的控制等优势,在有机发光二极管(OLEDs)、有机光伏电池(OPVs)、气体传感器、异质结结构、超快光开关等有机元器件中显示了巨大的应用前景。有机半导体器件通常为层状结构,包含有多个有机/有机,有机/金属(电极)界面。已有研究结果表明,有机薄膜的晶体取向及其有序化对有机电子器件的性能有着巨大的影响。因此对有机半导体在金属衬底上的吸附行为和电子性质的深入研究,将有助于我们对有机分子的生长机理以及有机半导体的器件行为加深理解。在本论文中,我们运用表面科学技术方法紫外光电子能谱(UPS),并结合基于密度泛函理论(DFT)的第一性原理计算研究了有机分子在相关电极表面的吸附属性和电子性质。首先,我们利用UPS表面分析技术和基于DFT的第一性原理的计算研究了有机分子tetracene吸附在Ag(110)的表面的行为。UPS测量得出源于有机tetracene分子的五个特征峰α、β、γ、δ和ε,分别位于费米能级以下3.4、5.7、6.3、7.2和8.8eV。特征峰a由tetracene分子HOMO(最高占据分子)轨道形成,具有b1g对称性的π轨道特征,主要源于C 2p轨道中的电子贡献。随着有机覆盖度增加,相应的特征峰位置几乎没有发生位移,说明tetracene分子和Ag(110)衬底的相互作用相对较弱。表面功函数的减少表明tetracene分子在Ag(110)表面存在极化。理论计算揭示了有机分子tetracece吸附在Ag(110)表面的最稳定吸附位置。有机分子tetracene长轴沿Ag(110)表面的[001]方向,分子平躺于Ag表面两列Ag原子行之间,有机分子tetracene第二个苯环的中心位于Ag(110)表面第二层Ag原子的正上方。有机tetracene分子吸附前后的态密度(DOS)计算结果显示,来自于有机tetracene分子的s和p轨道的DOS强度几乎没有变化,DOS特征峰位置向低能端移动了1.15eV,表明了衬底d电子和有机分子的p轨道的耦合相互作用较弱。然后,我们利用UPS和基于DFT的第一性原理计算,对CuPc在石墨烯(graphene)表面的吸附结构和电子性质进行了研究。在UPS谱中,出现了五个与CuPc分子有关的特征峰α、β、γ、δ和ε,它们分别位于费米能级下1.48、3.66、4.98、6.90和9.04eV处。LUMO轨道主要是Cu 3d轨道的贡献,HOMO轨道来自于C 2p轨道的贡献。随着CuPc分子覆盖度的增加,CuPc分子的最高占据分子轨道(HOMO)向高结合能方向发生了移动,这一移动源于CuPc薄膜在界面处的能带弯曲。当CuPc薄膜厚度达到10nm时,HOMO位置沿着高结合能方向移动了0.14eV。在刚开始沉淀CuPc薄膜过程中,功函数发生了略有增加,且这一功函数的增加可以归结为Cu2+3d电子与石墨烯2p电子的相互作用,导致界面处出现弱的极化。DFT理论计算表明CuPc在graphene表面吸附最稳定结构是在顶位(T)位置,吸附分子CuPc离衬底表面的高度大约为3.47A。比较CuPc在石墨烯表面吸附前后的态密度(DOS)变化,吸附后CuPc的最低未占据分子轨道(LUMO)略有占据,石墨烯的狄拉克点(Dirac point)向高能量方向发生了移动,表明在这一吸附体系中,石墨烯衬底向有机分子CuPc的产生了电荷转移。这些结果有助于加深理解有机半导体和石墨烯界面的形成以及界面电子性质。更多还原

【Abstract】 In the past decades, because of the compatibilities of high-throughput, low-cost solution processable and the capabilities of precisely functionalization for desired performance by different synthesis methods, organic semiconductors have shown great potentials in the applications of organic electronic devices, such as organic light-emitting diodes (OLEDs), organic photovoltaic cells, gas sensors, heterojunctions and ultrafast optical switches. Both the properties of organic materials and organic/electrode interface play important roles in determining devices performance. More importantly, various organic/metal interfaces are involved in such devices, and hence the interfacial properties are of key importance. It has been demonstrated that the molecular orientation of crystalline organic thin films have great impact on device performance of organic electronic devices. Therefore, it is critical to have a thoroughly understanding about the electronic properties and the adsorption geometries at the interface between organic materials and substrate. In this thesis, the properties at the interface between the active material and electrode were studied base on the ultraviolet photoemission spectroscopy (UPS) together with density functional theory (DFT) calculations.First, the adsorption of tetracene on the Ag(110) surface has been studied using UPS and first-principles DFT calculations. The five emission features originating from the organic molecules locate at 3.4,5.7,6.3,7.2, and 8.8 eV below the Fermi level, respectively. There is no apparent binding energy shift of these features with the increasing coverage of tetracene, indicating a relatively weak interaction between tetracene and the Ag(110) substrate. Moreover, our DFT calculation shows that tetracene molecules prefer to adsorb between two Ag atomic rows of the first layer with its molecular axis along the [001] direction of the substrate, and the center of its second benzene ring locates at top site of Ag atoms of the second layer. With this optimized adsorption configuration, the π-d interactions between tetracene and Ag(110) are maximized. Comparing with the densities of states (DOS) before and after tetracene adsorption, no apparent intensity change can be observed for the DOS from p-orbital of tetracene, suggesting that the coupling between the substrate d-band and the filled p-orbitals of tetracene are relatively weak-Then the electronic and structural properties at the copper phthalocyanine (CuPc)/graphene interface have been studied using UPS and first-principles DFT calculations. The five emission features α, β, γ,δ and ε originating from the CuPc molecules locate at 1.48,3.66,4.98,6.90, and 9.04 eV, respectively. These features shift in binding energy with the increasing CuPc coverage. The LUMO orbital of CuPc is originated from Cu 3 d orbital and the HOMO orbital is originated from C 2p orbital. Further theoretical calculation indicates that the adsorption of CuPc on the top site is the most favorable configuration, and the separation between the adsorbate and graphene is about 3.47 A. According to the density of states before and after CuPc adsorption, the LUMO of CuPc is slightly occupied, while the Dirac point of graphene slightly shift towards higher energy, suggesting that a small amount of electron transfer from graphene to CuPc upon contact.更多还原