【作者】 王炜华；

【导师】 侯建国； 王兵；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2012， 博士

【摘要】 扫描隧道显微镜(scanning tunneling microscope,STM)不仅能够对表面和表面纳米结构进行高分辨成像,还能进行原位扫描隧道谱(scanning tunneling spectroscopy, STS)测量,甚至对纳米结构进行操纵。二十多年来,STM作为一种非常重要的科学研究工具,已经在物理、化学、材料、生物等学科领域得到了广泛的应用。在本论文中,我们利用低温超高真空扫描隧道显微镜对纳米结构的电子输运性质进行了研究。在第一章中,首先介绍了扫描隧道显微学的基本理论,然后介绍STM的工作原理和工作模式,并对实验部分所使用的低温超高真空STM做了简要介绍,最后结合前人的工作,介绍STM对表面及表面吸附体系的各种分析方法和研究手段。在第二章中,我们用STM研究了双势垒隧道结中并联Au纳米颗粒的电子输运性质。当针尖位于两纳米颗粒之间时,有时dI/dV谱上会表现出增强的电导峰,其强度是普通电导峰强度的两倍以上,我们认为这是由电子在两纳米颗粒之间发生干涉造成的。我们还对单层和多层纳米颗粒二维体系进行了研究,并引入peak ratio来表征某一纳米颗粒与周围颗粒耦合的强弱,我们发现随着颗粒配位数增加及层数增多,其I-V谱中的库伦阻塞和库伦台阶变弱或者消失,其dI/dV谱上分立的电导峰也变得不明显,peak ratio变小,这说明颗粒与周围环境的耦合增强。在第三章中,我们用STM研究了低温下Si(111)-(?)3×(?)3-Ag表面的电子输运性质。低掺杂Si衬底上的Si(111)-(?)3×(?)3-Ag表面在78K以及重掺杂Si衬底上的Si(111)-(?)3×(?)3-Ag表面在5K下有相似的电子输运行为,但是低掺杂衬底上的Si(111)-(?)3×(?)3-Ag表面在5K下表现出独特的电子输运行为,这是由5K下低掺杂衬底中电子输运受到限制以及空间电荷层中能带弯曲状况发生变化引起的。通过光照,我们可以改变Si衬底中的载流子浓度,从而改变空间电荷层中的能带弯曲,并最终调控系统的电子输运性质。在第四章中,我们用STM在80K和5K下对Si(111)-(?)3×(?)3-Ag表面CoPc分子的电子输运性质进行了研究,发现CoPc分子中心表现出负微分电阻效应,并且该效应的出现与测量温度,以及衬底的掺杂类型或掺杂浓度无关。理论计算表明,该负微分电阻效应来源于在外加偏压下CoPc分子中Co2+离子的dz2轨道与Si(111)-(?)3×(?)3-Ag表面S1态发生了相对移动。5K下,由于受衬底中空间电荷层分压的影响,低掺杂衬底上Si(111)-(?)3×(?)3-Ag表面CoPc分子的负微分电阻效应出现在更高的负偏压,并且出现的偏压位置受做谱时设定点的影响较为显著。同时,我们可以通过光照来调制5K下低掺杂衬底上Si(111)-(?)3×(?)3-Ag表面CoPc分子的负微分电阻效应。我们的工作展示了可以利用Si(111)-(?)3×(?)3-Ag的表面态及具有特定电子结构的有机分子来构造单分子负微分电阻器件,这一发现为在Si表面构造单分子电子器件提供了新的思路。在第五章中,我们将STM用于研究Si(111)-(?)3×(?)3-Ag表面吸附的单个Dy@C82分子,发现在Dy@C82分子内部特定位置表现出NDR效应,而且对于不同吸附取向的分子,NDR效应可能出现在正偏压或负偏压。通过用针尖操纵分子,可以使分子发生NDR效应的偏压极性发生变化,说明分子发生NDR效应的偏压极性与分子的吸附取向有关。理论计算结果表明,Dy@C82分子上的NDR效应是由在外加电场作用下Dy原子在碳笼内运动,使分子的电子结构发生变化引起的。更多还原

【Abstract】 With the advent of the scanning tunneling microscope (STM), for the first time it has been possible to explore the atomic-realm of surfaces in real space. With the help of STM, we can not only resolve the surface or surface-adsorbed nanostructure in atomic scale, but also carry out in situ spectroscopic studies. Moreover, manipulation of single atoms, molecules and clusters has become possible. STM has been widely utilized in physical science, chemistry, material science and biology in recent decades. In this thesis, we studied the electron transport properties of surface-adsorbed nanostructures by taking advantage of UHV-LT-STM.In Chapter1, we briefly introduced the fundamental principles and methods of STM both in theoretical and experimental aspects. We also reviewed the recent progress in surface and surface-adsorbed nanostructures study with STM as well as the STM techniques employed.In Chapter2, we studied both parallel coupled gold nanoparticles and two dimensional gold nanoparticle assemblies in double barrier tunneling junction (DBTJ). We observed strengthened conductance peak in dIdV spectra when place the tip between the two nanoparticles. The strengthened peak has a magnitude more than twice of the other peaks, which is attributed to constructive electron interference between the two nanoparticles. We employ peak ratio to describe the coupling of a nanoparticle with its neighbors in gold nanoparticle assemblies. It is found that peak ratio decreases with increasing neighbors and the multilayer has a smaller peak ratio than the monolayer.In Chapter3, STM/STS experiments have been performed on Si(111)-(?)×(?)-Ag surface at low temperature. The surface on lightly doped wafer at78K and on heavily doped wafer at5K show similar electron transport properties, whereas the surface on lightly doped wafer at5K shows different electron transport behaviors, which are attributed to limited carrier transport in silicon substrate and band-bending change beneath the surface. Furthermore, we demonstrate that these electron transport behaviors can be tuned by light irradiation.In Chapter4, we have studied the electron transport behaviors of single CoPc molecules on Si(111)-(?)×(?)-Ag surface using STM/STS at5K and80K. The CoPc molecule shows NDR effect at its center, irrespective of measuring temperature and doping type and doping concentration of the silicon substrate. On the basis of our DFT calculations, the NDR observed at the CoPc center is attributed to the resonant tunneling between the surface-state band S1and the localized dz2orbital of central Co2+ion. The NDR position of the Co2+ion center of CoPc on Si(111)-(?×(?)-Ag surface with light doped silicon substrate locates at much high negative bias voltage at5K, which is explained by considering the voltage drop at the space-charge layer. Moreover, the NDR position can be tuned through light illumination with various light intensities. By taking advantage of the intrinsic surface-states of Si(111)-(?)×(?)-Ag surface and the proper molecular orbital of CoPc molecule, we demonstrate a way to build single-molecular NDR device on silicon, which may be used to fabricate hybrid silicon-molecular electronics.In Chapter5, we studied single Dy@Cg2molecule on Si(111)-(?)×(?)-Ag surface by using STM/STS. We find NDR effect in the Ⅰ-Ⅴ curves of the molecule and the NDR effect is sensitive to the intromolecular position and the molecular orientation. We demonstrated that the position of the NDR either in positive or negative bias voltage can be tuned by the STM tip manipulation. Theoretical studies reveal that the NDR effect is determined by the different moving route of Dy3+inside the carbon cage when the tip locates over the different sites of the molecule.更多还原