ABSTRACT
Since the first report by Iijima on finding a new type of finite carbon structures consisting of needlelike tubes, C nanotubes, in an arc-discharge-produced graphitic material, one-dimensional (1D) inorganic semiconductor nanostructures with various shapes and morphologies, such as nanotubes, nanowires, nanorods, nanobelts/nanoribbons, nanocables, and nanoheterostructures, have been attracting a great deal of research interest due to their unique properties and potential to revolutionize broad areas of nanotechnology [1-10]. For example, Lieber and coworkers reported the synthesis of solid carbide nanorods of TiC, NbC, Fe3C, SiC, and
Xiaosheng Fang,a,b Liang Li,b Ujjal K. Gautam,c Tianyou Zhai,b Yoshio Bando,b and Dmitri Golbergb a Department of Materials Science, Fudan University, Shanghai 200433, People’s Republic of China b International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan c New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore, India 560064 xshfang@fudan.edu.cn, Li.liang@nims.go.jp, ujjalgautam@jncasr.ac.in, and zhai.tianyou@gmail.com
BCx in high yields. The structures were produced by a reaction of C nanotubes with volatile metal or nonmetal complexes and had typical diameters between 2 and 30 nm and lengths of up to 20 µm [11]. Namatsu and coworkers proposed a process that relies on electron-cyclotron resonance plasma deposition of a SiO2 film through openings of a patterned resist film and fabrication of thin Si nanowires with a reduced electrical resistance. The authors named this technique a self-aligned thickness reduction technique. This enables one to reduce the nanowire thickness without reducing the thicknesses of the source and the drain regions [12]. Subsequently, Deng and coworkers employed a novel scheme to fabricate silicongermanium (SiGe) nanowires in a Si (100) substrate using pulsed ultraviolet (UV) laser-induced epitaxy. Defect-free SiGe nanowires/ Si with a cross section of ~25 × 95 nm2 were successfully fabricated using this technique with a thin LTO capping layer deposited on a Ge nanowires/Si sample [13]. Loiseau and Pascard successfully fabricated long carbon nanotubes filled with Se, S, Sb, and Ge by the arc method [14]. In the early 1990s, using ordered channel anodization alumina membrane (AAM) templates became one of the facile routes to fabricate 1D inorganic semiconductor nanostructure arrays. For examples, Martin synthesized alumina templates with pore diameters ranging from 12 to 250 nm. The method entails synthesis of the desired material within the pores of a nanoporous membrane. Because the membranes used contain cylindrical pores of uniform diameter, monodisperse nanocylinders of the desired material, whose dimensions can be carefully controlled, are obtained. This “template” method has been used to prepare polymers, metals, semiconductors, and other materials on a nanoscopic scale [15]. In 1995, Masuda and Fukuda first reported on the synthesized alumina membranes composed of highly ordered and hexagonally closepacked arrays. After that, they improved the synthesis technique to obtain an alumina template containing almost perfect and hexagonal channel arrays with long-range ordering (ordered channel AAM templates). Also a highly ordered metal nanohole array (Pt and Au) was fabricated by a two-step replication of the honeycomb structure of anodic porous alumina. The metal hole array of the film has a uniform, closely packed honeycomb structure approximately 70 nm in diameter and from 1 to 3 µm thick [16]. In 2000, Zhang and coworkers reported the synthesis of ordered semiconductor ZnO nanowire arrays embedded in AAM by generating alumina templates with nanochannels, electrodepositing Zn in them, and then oxidizing the
Zn nanowire arrays. The results showed that the polycrystalline ZnO nanowires with diameters ranging from 15 to 90 nm were uniformly assembled into the hexagonally ordered nanochannels of the AAM [17]. These pioneering researches exploit a new subfield within 1D inorganic semiconductor nanostructures, and ordered nanoarrays of Te, Bi, Bi2O3, Si, Co2O3, In2O3, CuO, SnO2, Fe2O3, TiO2, CdSe, CdTe, Bi2Te3, Bi2S3, GaN, PbS, ZnS, and CdS nanowires, nanotubes, and nanocables with controlled diameter, length, and microstructures were successfully fabricated using the AAM template method [5]. For example, Li and coworkers developed a new AAM templatebased heat treatment method to convert metal nanowire arrays into arrays of metal-metal oxide core-shell nanowires and singlecrystalline metal oxide nanotubes, which significantly extended the AAM template method. This process was reproduced very well by kinetically controlling the conversion of a Bi nanowire array to an array of Bi-Bi2O3 core-shell nanowires and Bi2O3 nanotubes [18]. In one of the latest perspectives, Peidong Yang provided a critical look at the research progress within the nanowire community for the past decade. From his viewpoint, it is easy to note that the field of semiconductor nanowires has become one of the most active research areas within the nanoscience community [19]. In chapter 1, we have presented the recent progresses on the seed-assisted synthesis of 1D nanostructures, showing their characteristics and advantages. Here, as closely relative to the topic of chapter 1, the development process of 1D inorganic semiconductor nanostructures using a gas phase reaction is briefly summarized. Generally four growth mechanisms are adopted to explain the gas phase-based process for 1D nanomaterials and nanostructures [20]. The first one is the well-accepted so-called vapor-liquid-solid (VLS) process of nanowire and nanowhisker growth proposed by Wagner and Ellis [21]. Morales and Lieber developed a new method combining laser ablation cluster formation and VLS growth to successfully fabricate crystalline semiconductor nanowires. In this process, laser ablation was used to prepare nanometer-diameter catalyst clusters that define the size of wires produced by VLS growth. This approach was used to prepare bulk quantities of uniform singlecrystalline Si and Ge nanowires with diameters of 6 to 20 and 3 to 9 nm, respectively, and lengths ranging from 1 to 30 µm. They pointed out that it should be possible to make nanowires of SiC, GaAs, Bi2Te3, and BN in this way and perhaps, in the presence of atomic hydrogen, even diamond nanowires [22]. In 2001, Wu and Yang reported the
first real-time observation of semiconductor nanowire growth at high temperature under transmission electron microscopy (TEM), which unambiguously demonstrates the validity of the VLS growth mechanism at the nanometer scale. They clearly demonstrated three well-defined stages during the process: metal alloying, crystal nucleation, and axial growth. By using monodispersed Au nanoclusters as catalysts, the authors successfully stimulated the growth of Si nanowires with different diameters [23]. Also Lieber and coworkers synthesized monodisperse Si nanowires by exploiting well-defined Au nanoclusters as catalysts for 1D growth via the VLS mechanism, and TEM studies of the materials grown from 5, 10, 20, and 30 nm nanocluster catalysts showed that the nanowires had mean diameters of 6, 12, 20, and 31 nm, respectively. This implies that nanowires with a narrow size distribution could be obtained by exploiting well-defined catalysts. The positions of 1D nanostructures can be controlled by the initial position of Au or other catalyst clusters or thin films [24]. The second one is the vapor-solid (VS) growth process. In this process, vapor is first generated by evaporation, chemical reduction, or gaseous reaction. The vapor is subsequently transported by a carrying gas and condensed onto a substrate. Most of the nanobelts have successfully been fabricated using this growth process. For example, Wang and coworkers successfully synthesized ultralong beltlike nanostructures (so-called nanobelts; they have a rectanglelike cross section with typical widths of 30 to 300 nm, width-tothickness ratios of 5 to 10, and lengths of up to a few millimeters) for semiconducting oxides of Zn, Sn, In, Cd, and Ga by simply evaporating the desired commercial metal oxide powders at high temperatures without the presence of a catalyst [25]. After this report, almost all of the inorganic semiconductors were fabricated into nanobelts with controlled width and width-to-thickness ratios. These have become a new materials family for a systematic experimental and theoretical understanding of the fundamentals of electrical, thermal, optical, and ionic transport processes and many potential applications [26-30]. The third one is oxide-assisted growth proposed by Lee and coworkers, in which oxides instead of metals play an important role in the nucleation and growth of nanowires. This method is capable of producing a series of nanowires [31]. The last method is a combination of the some templates and chemical vapor deposition (CVD). For example, by using the above-
discussed AAM template method, Zhang and coworkers synthesized single-crystalline GaN nanowires in an anodic alumina membrane on a large scale by a gas reaction of Ga2O vapors with a constant flowing ammonia atmosphere at 1,273 K [32]. In the initial decade, most of the research in the field of 1D inorganic semiconductor nanostructures mainly focused on the syntheses and characterizations of C nanotubes and Si nanowires. After 2000, 1D oxide and sulfide nanostructures have also become one of the most active research areas, especially keeping in mind three pioneering works on ZnO nanowire arrays, UV nanowire nanolasers, and nanobelts [17, 25, 33]. Up to now, 1D inorganic semiconductor nanostructures have been exploited for usages in photonic devices, sensors, lasers, photodetectors, field emitters, and energy applications (conversion and storage) and biorelative applications [34-40]. In this chapter, we will provide a brief account of the recent progress in the controlled growth and potential applications of 1D inorganic semiconductor nanostructures, which were synthesized by the gas phase-based process, and will introduce some representative examples.
