纳米材料和纳米结构的制造？ Fabrication of nanomaterials and nanostructures Kannan M. Krishnan's research group, University of Washington, WA, USA The basic introduction to the fabrication of nanomaterials and nanostructures is given first, including the information of scanning probe devices, top-down and bottom-up methods. Then the formation of nanoparticles by homogeneous nucleation and self-assembly is analyzed in detail as a promising bottom-up method. Several specific fabrication techniques of magnetic nanomaterials and nanostructures from low-dimensional systems to complex bulk structures are introduced at last. After the idea of “nano” was brought out by the classic and visionary talk of Richard Feynman at the 1959 meeting of the American Physical Society at Caltech, which describes nanotechnology as an important field for future scientific investigation , people began to strive for the novel and fascinating “nano” world depicted in his exciting lecture. Not until 1981, with the birth of the revolutionary invention for observing substances at an atomic scale — the scanning tunneling microscope (STM), which led to the later development of other scanning probe devices such as the atomic force microscope (AFM) and the magnetic force microscope (MFM), did the dream of nanometer-scale fabrication gradually come true. In recent years, scientists have learned lots of techniques for building nanoscale structures. In general, these techniques can be sorted into so-called “top-down” and “bottom-up” methods.1 Both approaches play very important roles in modern industry and most likely in nanotechnology as well. There are advantages and disadvantages in both approaches. Most top-down methods begin with a pattern generated on a large scale and reduce its lateral dimensions (usually 10 times smaller) before carving out nanostructures. This strategy is required in fabricating electronic devices such as microchips, whose functions depend more on their patterns than on their dimensions. But none of the top-down methods can conveniently, cheaply and quickly make nanostructures of any material. By contrast, researchers have shown a growing interest in bottom-up methods, which start with atoms or molecules and build up to nanostructures. As opposed to the top-down methods, the bottom-up methods are able to easily make the smallest nanostructures (2~10 nm) with much lower cost. However, the resulting nanostructures are usually fashioned as simple particles in suspensions or on surfaces, rather than as designed, interconnected patterns.1 One of the most promising bottom-up methods in nanofabrication is the formation of nanoparticles by homogeneous nucleation. In this method, a supersaturation (a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances) of growth species must be created at first. A reduction in temperature of an equilibrium mixture, such as a saturated solution would lead to supersaturation. Formation of metal quantum dots (a semiconductor nanostructure that confines the motion of conduction band electrons, valence band holes or pairs of conduction band electrons and valence band holes in all three spatial directions) in glass matrix by annealing at moderate temperature is a good example of this approach. Another method is to generate a supersaturation through in situ (in the reaction mixture but cannot be isolated on their own) chemical reactions by converting highly soluble chemicals into less soluble chemicals. Nanoparticles can be synthesized through homogeneous nucleation in three mediums: liquid, gas and solid. However, the fundamentals of nucleation and subsequent growth processes are essentially the same. After the nanoparticles are fabricated, they may be applied as nanodevices through self-assembly. Self-assembly is the spontaneous organization of molecules or molecular clusters into 2D arrays and 3D networks. Why will nanoparticles arrange themselves in an ordered state without any external driven force? Is there any internal motivation? In fact, this “magic” assembly process stems from a series of weak (usually comparable to thermal energies) and noncovalent interactions such as van der Waals and Coulombic interactions, hydrophobic interactions and hydrogen bonds.3,4 The process occurs when molecules interact with one another through a balance of attractive and repulsive forces, and eventually attain equilibrium, i.e. the ordered state with lowest potential energy. In addition, since the strength of the bonds between the components is generally weak and comparable to forces caused by thermal motion, this association process is reversible: it allows the components to adjust their positions within an aggregate once the structure has formed. Magnetic nanostructure fabrication techniques can be characterized by a fascinating diversity of geometries, ranging from a broad variety of low-dimensional systems to complex bulk structures. At low dimension, electron-beam lithography is usually used to produce periodic arrays of 0D nanoscale magnetic particles. Compared with the conventional lithography that utilizes a beam of light to carve patterns, electron-beam lithography employs a beam of electrons instead. Since the electron beam does not diffract at atomic scales, it does not cause blurring of the features on patterns, and therefore enables nanometer-scale resolution. For example, an array of Nickel pillars on silicon that have a uniform diameter of 35 nm, a height of 120 nm, and a density of 65 Gbits/in2 can be achieved using this technique.5 At the same time, there are some chemical methods to synthesize nanoparticles, such as through the reduction of metal salts or by the thermal decomposition of metal complexes (a structure consisting of a central atom or molecule weakly connected to surrounding atoms or molecules). On the other hand, 1D wire arrays can be produced via electrodeposition, which is a technique to deposit a dissolved or suspended substance on an electrode by electrolysis. According to Pankhurst et al., by electrodeposition into porous anodic alumina it is now possible to produce Iron, Cobalt and Nickel wires with diameters ranging from 4 to 200 nm, depending on the anodization conditions, and lengths up to about 1 μm.6 For the 2D magnetic thin film structure, a Langmuir-Blodgett technique can be applied in fabrication, particularly to assemble nanoparticles with shape anisotropy (the property of being directionally dependent). Langmuir-Blodgett films are monolayers and multilayers of amphiphilic molecules (with one end that is hydrophilic, and therefore is preferentially immersed in the water and the other that is hydrophobic and preferentially resides in the air or in the nonpolar solvent) transferred from the liquid-gas interface (commonly water-air interface) onto a solid substrate and the process is generally referred to as Langmuir-Blodgett technique. There are two commonly used methods to achieve the transferring: vertical deposition and horizontal lifting. The interparticle distance and the final superstructures can be finely tuned through control of the compression pressure.7 When it comes to 3D magnetic nanostructures, chemical reactions concerned with self-assembly are found capable to produce complex nanocomposites such as exchange-spring magnets — composites composed of magnetically hard and soft phases that interact by magnetic exchange coupling. A notable example of chemical synthesis has been devised by Zeng et al.8 According to their work, FePt and Ferrite (Fe3O4) nanoparticles of similar sizes (about 4 nm) were mixed under ultrasonic agitation, and three-dimensional binary assemblies were induced by either evaporation of the hexane or addition of ethanol. Then subsequent annealing converted the assembly into FePt–Fe3Pt 5nm-scale homogeneous nanocomposites, where FePt is a magnetically hard tetragonal phase and Fe3Pt a high-magnetization soft phase. Here the outcome was found to be a three-dimensional magnet with high-energy product and excellent magnetic properties. In conclusion, various types of techniques have been discovered for the fabrication of nanomaterials and nanostructures today, and these methods can generally be divided into two categories: top-down methods, which mostly carve out nanostructures, and bottom-up methods, which assemble atoms or molecules into nanostructures. Nanofabrication will play a significant role in the future development of nanotechnology. One of the most promising bottom-up methods is the formation of nanoparticles by homogeneous nucleation and self-assembly, especially in the fabrication of 3D magnetic nanocomposites. Magnetic nanostructure fabrication techniques can be applied in systems of different dimensions. The nanometer length scale (1~100nm) will play a significant role in the future application of magnetism. Compared with macroscopic magnetic materials, the nanoscale magnetism may have a profound influence on the extrinsic properties. Structuring from this length scale with magnetic materials may give rise to a series of scientifically interesting and technologically important novel functional devices. However, to fully realize the range of magnetic nanostructures and their potential for exploring new applications remains a great challenge for future research. References: 1 George M. Whitesides and J. Christopher Love, The art of building small , Scientific American, Sep. 39-47(2001) 2 S. W. Chung, J. Yu, and J. R. Heath, Appl. Phys. Lett. 76, 2068 (2000) 3 George M. Whitesides and Bartosz Grzybowski , Self-assembly at all scale, Science 295( 200 2) 4 M. Ratner, D. Ratner, Tools to make nanostructure , Ch4, p43-61 5 S. Y. Chou, M. Wei, P. R. Krauss and P. B. Fisher, 1994 J. Vac. Sci. Technol B 12 3695 6 R. Skomski, Nanomagnetics , J. Phys. Condens. Matter 15 (2003) R841–R896 7 O. B. Shchekin and D. G. Deppe, Appl. Phys. Lett. 80, 3277 (2002) 8 H. Zeng, J. Li, J. P. Liu, Z. Wang and L. Sun, Exchange-coupled nanocomposite magnets by nanoparticle self-assembly ,S. Nature 420, 395–398 (2002). 查看更多0个回答 . 1人已关注
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