【作者】 高平奇；

【导师】 贺德衍；

【作者基本信息】 兰州大学 ， 凝聚态物理， 2010， 博士

【摘要】 本论文研究了单壁碳纳米管(SWNTs)的拉曼光谱和电输运特性。首先,利用共振拉曼光谱研究了SWNTs对几种外界作用,包括应力、静电注入和纳米颗粒修饰等的响应。对较大直径超长碳管中反常拉曼光谱的研究发现,沿着碳管轴向特征拉曼光谱G+、G-和G’峰分别位于1553、1563和2597 cm-1,均明显低于文献报道的实验和理论值。显著的拉曼红移可归因于内建张应力作用,用红外激光退火可以使应力释放,拉曼峰向高波数移动。较大直径的碳纳米管具有相对较小的曲率效应,更容易受到内建应力的影响。均匀应力的存在为利用应力工程调制碳管能带结构提供了叮能。在SWNTs场效应管(FETs)工作状态下,分别用光子能量为1.96 eV和2.41eV的两种激光激发平均直径为1.4 nm的SWNTs束中金属性和半导体性碳管,原位研究了半导体性及金属性碳管拉曼光谱对外加电场的响应。发现金属性碳管中,G-拉曼模在负栅压下向高波数移动并伴随着峰宽的窄化,但对正偏压无响应。这一发现证明了在SWNTs束中存在Kohn反常现象,金属性碳管的LO声子模随着费米能级的变化而变化,为利用拉曼光谱原位监测FETs沟道中载流子浓度提供了可能。在源、漏偏压作用下的拉曼光谱测量发现,随着单位长度SWNTs上电功率消耗的增加,G峰向低波数平移,波形同时逐渐展宽。这一现象说明产生了热声子,并且在通常的SWNT-FETs中,大大低估了衬底对声子的散射。其次,用电子束蒸发和快速热退火工艺在SWNT-FETs表面自组装了金纳米颗粒,制备出新奇的SWNTs复合结构——SWNT@金纳米颗粒。拉曼光谱测量表明,表面金属纳米颗粒的库仑作用改变了SWNTs的电子结构。电输运测量发现,当温度低于150 K时,在长约2μm的导电沟道中观察到强烈的电流振荡。这种库仑阻塞引起的单电子效应可以用两种机理解释：(1)肖特基势垒高度调制——库仑阻塞效应主导着对金纳米颗粒的电注入过程,一旦在电极附近的金纳米颗粒俘获了电荷,其静电势能的改变会影响到接触附近肖特基势垒的高度,进而影响碳管沟道中的电流；(2)能带调制——俘获了电荷的金纳米颗粒充当散射中心,它们改变了沿碳管沟道的局部电势,缩短了电荷在碳管中传输的平均自由程,进而引起电流振荡。借助于纳米碳链@SWNT复合结构,第一次详细研究了sp杂化的一维碳原子链的拉曼谱和电学特性。发现纳米碳链@SWNT复合结构具有独特的量了线特性,呈现非线性电学特性,电导随温度的变化遵从幂指数关系,类似于Luttinger流体。低温下观察到均匀的库仑振荡峰,表明其可用于制作性能优良的单电子器件。再次,用传统光刻技术结合自对准的方法制备出间距约为5 nm的金属电极,为单分子电学特性的研究创造了条件。成功制备出大面积纳米间距电极阵列,利用交流介电泳技术成功实现了在整个硅片上碳管纳米器件的集成。这一简单且廉价的工艺有望应用于未来分子电路的构建。最后,用低功函数的金属(Sc,Sm,Al等)作为源、漏电极材料制备了性能稳定的n型SWNT-FETs。Sc和碳管具有较好的亲和性,费米能级与碳管的导带相匹配,显示出良好的欧姆接触特性。所制备的SWNT-FETs的开关电流比大于3个数量级,并且在约5μm的导电通道中呈现出近似弹道输运的性质,电导约为0.02 G0。更多还原

【Abstract】 This thesis presents the main findings in my PhD project on the Raman spectrum and the electron transport properties of single-walled carbon nanotubes (SWNTs). Resonance Raman spectroscopy is firstly used to characterize the structural properties of SWNTs under various external perturbations, including strain, nanoparticles decorated, electrostatic gating, electrical heating, et al. Afterwards, the electron transport mechanisms in two novel SWNTs hybrid structures, SWNT@Au-NCs (gold nanocrystals) and SWNT@Cn (carbon chains), are investigated. At last, stable n-type SWNT field effect transistors (FETs) are successfully fabricated. One conventional optical lithography compatible method is developed for sub-10 nm nanogap electrode arrays fabrication, which has been proven an excellent way for nano-sized devices integration.Abnormal Raman scattering from a large diameter ultralong SWNT is studied in detail. Along the SWNT, the Raman spectra show the frequencies of 1553,1563, and 2597 cm-1 for G+, G-, and G’peaks, respectively, much lower than the corresponding frequencies well reported both experimentally and theoretically. The significant downshifts in the peaks frequencies can be attributed to self-built tensile strain which is likely caused by carbon nanodots decorated on the tube. After infrared laser heating is performed to one point of it, all the Raman modes are found to shift to higher frequencies and approach to their conventional values. We suggest that the SWNTs with larger diameters easily possess such self-built strain compared to small-diameter SWNTs because of the weaker curvature effect for the larger ones.In situ Raman measurements have been carried out on a thin bundle of SWNTs in FET configuration at various gate voltages. Two excitation lasers with the photon energy of 1.96 eV and 2.41 eV are selected to excite the Raman scattering modes of metallic and semiconducting SWNTs in the bundle, respectively. For the metallic SWNTs, the G-Raman mode is found to shift to higher frequencies and narrow down its line shape at negative gate voltages, but be insensitive to positive gate voltages. These findings confirm that the Kohn anomaly exists in a thin SWNTs bundle and that the LO phonon mode changes along with the position of the Fermi level in the metallic SWNTs. In contrast, semiconducting SWNTs do not show any observable changes in the Raman spectra.Electrical bias voltages tuned Raman spectroscopic of SWNT-FET show that the G peaks shift gradually to lower frequency, become broader and decrease in intensity, along with the increasing of electrical power per unit length on the SWNT. This result indicates that the generation of hot phonons and the underestimation of the interaction between the SWNTs and the SiO2 substrate.Self-assembly gold nanocrystals (Au-NCs) are formed on the surface of SWNTs on a platform of FET by electron-beam evaporation and post rapid thermal annealing processes. Strong oscillations in the SWNTs channel (～2000 nm long) current occur below 150 K. The Raman scattering from the SWNTs suggests that the electronic structure of the SWNTs has been significantly altered by the strong Coulombic interaction with attached Au-NCs. Two possible mechanisms are presented to explain the observations:(1) the charging process of Au-NCs is dominated by the Coulomb blockade effect. Thus the electrostatic potential of charged Au-NCs modifies the Schottky barrier at the SWNT/Au electrode contacts and subsequently affects the SWNT channel current (or the Schottky barrier modulation) and (2) the charged Au-NCs serve as scattering centers, which modify the local potential along the SWNTs channel and then induce the oscillations in the current (i.e. energy band modulation).Pure sp-hybridized linear carbon chains (Cn) encapsulated within vertically aligned SWNTs (Cn@SWNT) is observed using Raman spectroscopy and electrical measurements. For the first time Cn@SWNT is shown to possess unique quantum wire properties, including nonlinear electrical property and conductance power law behaving characteristic of a Luttinger-liquid. Single electron tunneling (SET) and Coulomb blockade phenomena were clearly recorded at 40 K with a series of different bias voltages from 1 to 10 mV. The high bias voltage 10 mV indicates that SET can occur on Cn@SWNT at temperature up to 110 K, providing many potential applications in computer industry, single electron memory, highly sensitive electrometer among others.We have developed a fabrication method for nanogap electrodes without employing electron-beam lithography to measure the electrical characteristics of nanostructured molecules. This side layer-controlled method enables us to reproducibly fabricate down to～5 nm nanogap electrodes. The fully compatibility with traditional microfabrication make us easily fabricate nanogap electrode array for higher density integration. Combined with the AC dielectrophresis method, one example for short channel SWNT-FETs array is demonstrated.Owing to the unique 1-D structure of SWNTs with fully saturated surface bonds and no interface states, resulting in no Fermi-level pinning, stable n-type SWNT-FETs are fabricated using the low workfunction metals, Sc, Sm, or even Al as source/drain electrodes. The well matching between the Fermi level of Sc and the conductance band of SWNTs make the SWNT-FETs show nice ohmic contact property and with quite high Ion/Ioff ratio (more than 3 orders) and conductance (～0.02 Go in 5μm channel).更多还原