Contents Nanoscience II: Nanowires Kai Nordlund 17.11.2010 Faculty of Science Department of Physics Division of Materials Physics 1. Introduction: nanowire concepts 2. Growth of nanowires 1. Spontaneous 2. Vapor-liquid-solid growth 3. Synthesis of nanowires 1. Templating 2. Tracks 3. Self-assembly templates 4. Templating against existing nanowires 4. Structure of nanowires 1. Crystalline and amorphous parts 2. Nanowire heterostructures 5. Mechanical properties 6. Electrical properties 7. Optical properties 8. Example: generating current from mechanical energy N.b. Carbon nanotubes are the topic of two separate lectures and not dealt with at all here. Key references: [Xia et al, One-dimensional Nanostructures: Synthesis, Characterization and Applications Advanced Materials 15 (2003) 353]; [Krasheninnikov and Nordlund, Ion and electron irradiation-induced effects in nanostructured materials, J. Appl. Phys. (Applied Physics Reviews) 107, 071301 (2010). ] 1. Nanowire concepts 2. Growth of nanowires A nanowire is a macroscopically long wire, which is only 1-100 nm thick in 2 dimensions y z, but can be arbitrarily long in x A 1D nanoobject Quantized density of states expected Several subconcepts exist: Whiskers: Old concept, often not on nano but micro scale Nanorods nanowires - Difference not clear, may mean mechanically stiff nanowires or nanowires where S x only 3-5 times larger than S y, S z Nanobelts, nanoribbons: S y >> S z Nanopillars = nanorods standing on a surface Nanotube = nanowire hollow inside Nanowires can be grown bottom-up in a multitude of different ways Historical note: Still in the 1970 s, studies of growth focused to a large extent on obtaining good-quality single crystals But some materials actually exhibit spontaneous growth of wires rather than 3D growth - This was considered a nuisance and great effort was put into avoiding the growth of nanowires! Growth of micrometer-scale whiskers is observed regularly when metals are placed in a high electric field Tends to cause short circuits in electrical transformers and the like The mechanism is actually not well understood, but appears to be caused by many different mechanisms [Pic from wikipedia: nanowire]
2.0 Whiskers images [reason.com] [www.efsot-europe.info/servlet/is/837/] [Pic from wikipedia: whisker] 2.1. Spontaneous growth of nanowires Some materials actually preferentially grow in nanowire form Typically ones which have a layered crystal structure The layers grow preferentially in some direction, leaving one or two dimensions in a non-growing mode => a wire results Examples: Asbestos(!), chrysolite LiMo 6 Se 6 (figure) [Xia et al, Advanced Materials 15 (2003) 353] 2.1. Spontaneous growth of nanowires The pure elements Te and Se tend to grow in wires because they have in their trigonal crystal structures straight helical chains During growth from a solution these drive the growth in a straight direction [Xia et al, Advanced Materials 15 (2003) 353] 2.2. Vapour-liquid-solid (VLS) growth The VLS growth method is one of the most used approaches to grow nanowires Contrary to spontaneous growth, it works for a very wide range of materials Key idea: use a liquid droplet of material B, into which material A is dissolved from the vapour phase (e.g. from CVD or PVD) to form an AB alloy Eventually liquid gets supersaturated in A If then T melt,a < T melt,ab (typically in an eutectic alloy), A will start precipitating out of B to form an A nanowire Size of droplet limits the size of nanowire, forcing 1D rather than 3D growth [http://www.nanowirephotonics.com/research-nanowires.html]
2.2. Vapour-liquid-solid (VLS) growth Size of droplet limits the size of nanowire, forcing 1D rather than 3D growth More specifically, after the growth has reached a steady-state condition (c), minimization of the Gibbs free interface energies (interface energies between: liquid particle and solid LS, vapor and liquid VL, solid and vapour VS ) in the system gives rise to the condition [Nebolsin, Inorganic Materials 39 (2003) 899] 2.2. Vapour-liquid-solid (VLS) growth Example: the original system was Au and Si (in 1964!) The grown wire is not cylindrical but has 211 and 110 side facets to minimize surface energy LS VL r 1 R from which it is possible to determine the radius of the wire r for a given nanocluster radius R One sees that growth not possible for all 2 2 R combinations, e.g. if LS > VL Also, the values may not be well known in nanosystems [Pic from wikipedia Vapor-liquid-solid method] [Wagner and Ellis, Appl. Phys. Lett. 4 (1964) 89] 3.1. Template methods Another, top-down (or mixed top-down+bottom-up) approach to nanowires is to use templates Something with a nm-narrow ridge, groove or the like onto which nanowires can be grown These can be fabricated with e.g. semiconductor lithography techniques such as etching Also features may be inherent, such as step edges These can be decorated with e.g. sputtering or deposition techniques to form nanowires 3.2 Growth in porous channels Nanowires can also be grown into nanoporous channels Pores can be fabricated e.g. using: Swift heavy ion irradiation that modified a crystal or amorphous structure in such a way that it can be etched away By anodic etching of alumina Once the pores have been formed, they can be filled completely or incompletely on the inside Finally the original material into the pore was made can be etched or dissolved away => only nanowires remain [Xia et al, Advanced Materials 15 (2003) 353]
3.3. Self-assembly templates Certain classes or organic molecules have the property that they tend to self-assemble into certain ordered structures More on this by Olli Ikkala and Robin Ras later during this course Some of these can form hollow or filled cylinders These cylinders can then be filled on the inside or outside by the nanowire material to grow nanowires or nanotubes 3.4. Templating against existing nanowires Existing nanowires can be used as templates for additional material, to form composite wires! Example 1: silica grown on top of Ag nanowires Example 2: Nanotape 3.4. Templating against existing nanowires TiO 2 grown on SnO 2 nanobelt: composite nanowire TEM image shows they are even epitaxial 3.4. Templating against existing nanowires A special case of templating is to use one kind of nanowire, a multiwalled carbon nanotube, as a mask against ion irradiation etching of an underlying thin film Using suitable irradiations and nanotube thickness, one can remove all of the thin film except that under the nanotube The nanotube gets damaged, and can then be etched away The minimum diameter thus achieved can be only a few nm [A. V. Krasheninnikov, K. Nordlund, and J. Keinonen,, Appl. Phys. Lett. 81, 1101 (2002)]
4 Structure of nanowires Nanowires come in all varieties of structures They can be amorphous, single crystalline, poly crystalline, a combination of these, or composites Example 1: single crystalline Au nanowire Note extremely good (actually perfect) crystal quality 4 Structure of nanowires Example 2: polycrystalline ZnO nanowire Note extremely good crystal quality within one grain 4.1. Crystalline and amorphous parts Semiconductor nanowires are often crystalline on the inside and amorpous on the outside E.g. Si nanowires are often completely surrounded by SiO 2 on the outside 4.2. Nanowire composites Nanowires can be composites both along the length and width directions TiO 2 SnO 2 [http://www-g.eng.cam.ac.uk/edm/old/research/nanowires/raman_nanowires.html]
5. Mechanical properties 5. Mechanical properties Single-crystalline whiskers and nanowires can be atomically perfect Bulk counterparts always contain some concentration of defects Hence they can be mechanically stronger than the corresponding bulk material Chain is as strong as its weakest point but now there are no weak points Whiskers have hence for a long time been used as reinforcements of composites Cu whiskers [http://www.tms.org/pubs/journals/ JOM/0903/gianola-0903.html] Standard example: SiC whisker reinforced carbon fiber composites Since whiskers/nanowires form atomically perfect single crystals, they are also of basic mechanical science interest as a prototype material to observe crystallographic slip [http://www.tms.org/pubs/journals/jom/0903/gianola-0903.html] 6. Electrical properties 6. Electrical properties Transistors with single nanowires as the current-carrying element have been fabricated Example: InP nanowires with clear transistor-like characteristics Even more advanced circuitry consisting of several nanowires have been made
7. Optical properties As in quantum dots, quantum confinement makes the optical properties of nanowires interesting On the other hand, if the nanowires are free-standing, a suitable combination of the refractive indices of the nanowire and surrounding material (e.g. air) can make them act as waveguides for light Finally, in some cases of even more suitable combinations of the refractive indices, the ends of the nanowire can act as mirrors Thus a single nanowire may act as a lasing element This has been demonstrated for ZnO nanowires on sapphire surrounded by air Example 7. Optical properties 8. Nanowire sensors Nanowires also have great potential to be used as sensors Like nanoclusters, they have high surface area/volume ratio But at the same time, they can also be used to carry current, i.e. transmit information Idea is that molecules get absorbed on a suitable nanowire material, and then change its electrical conductivity properties Since the wire is not much thicker than the molecule themselves, relatice effect can be large Moreover, because the molecules to be detected are absorbed only in small quantities (submonolayer may be enough), operation can be fast 8. Nanowire sensors Example: detection of NO 2 using SnO 2 nanobelts Belt was integrated over several electrodes to get information out Figure demonstrates how the current in the nanowire changes more than an order of magnitude when it is exposed alternatingly to pure air and 100 ppm NO 2 This was made possible by the fact that the conductivity of SnO 2 is strongly dependent on surface states
9. Example: generating electricity from ZnO A really exciting possible application stems from the observation that ZnO nanowires are piezoelectric, i.e. a mechanical strain can generate electricity in them This effect was shown [Wang et al, Science 312 (2006) 242] to enable electricity generation directly from mechanical force with efficiencies of 17 30% 9. Example: generating electricity from ZnO The reason this is exciting is that such nanowires could be inserted into really small devices, such as portable electronics, implantable medical devices or even the heels of a shoe to generate electrical power for other nano- and microscale devices No external power source needed?