Chapter 4 One-dimensional Nanostructures: Nanowires and Nanorods

Transcription

1 Chapter 4 One-dimensional Nanostructures: Nanowires and Nanorods Introduction 4.2 Spontaneous Growth Vapor-liquid-solid growth 4.3 Template-based Synthesis Electrochemical deposition Electrophoretic deposition Template filling Converting through chemical reactions 4.4 Electrospinning 4.5 Lithography Six Different Strategies for achieving 1D growth (a) dictation by the anisotropic crystallographic structure of a solid. (b) confinement by a liquid droplet as in the VLS process. (c) direction through the use of a template. (d) kinetic control provided by a capping reagent. (e) self assembly of 0D nanostructures. (f) size reduction of 1D microstructure.

2 4.2 Spontaneous Growth Spontaneous growth is a process driven by the reduction of Gibbs free energy or chemical potential. The reduction of Gibbs free energy is commonly realized by Phase transformation (Vapor-liquid-solid) Chemical reaction (unstable-stable, chemical potential; high-low) Release of stress (ordered-disordered) For the formation of nanowires or nanorods, anisotropic growth is required, i.e. the crystal grows along a certain orientation faster than other directions. Uniformly sized nanowires, i.e. the same diameter along the longitudinal direction of a given nanowire, can be obtained when crystal growth proceeds along one direction, whereas no growth along other directions Fundamentals of evaporation-condensation growth Evaporation-condensation = Vapor-solid growth = vapor phase growth 1D structure growth requires Anisotropic Growth wire/rod growth Ex. Different facets (selection of growth facet with fast growth rate) Presents of imperfections screw dislocations Preperantial accumulation of

3 Fundamentals of evaporation-condensation growth Process 1: Fast, not a rate limiting process. Process 2: Can be rate limiting if the supersaturation or concentration of growth species is low. Process 4: Will be the rate limiting process when a sufficient supersaturation or a high concentration of growth species is present. Process 3, 5 and 6: Not rate limiting processes in general Fundamentals of evaporation-condensation growth For most crystal growth, rate-limiting step is either adsorption-desorption of growth species on the growth surface (step 2) or surface growth (step 4). When step 2 is rate limiting, the growth rate is determined by condensation rate, Accommodation coefficient s is the fraction of impinging growth species that becomes accommodated on the growing surface, and is a surface specific property. A surface with a high s will have a high growth rate as compared with low s surfaces. A significant difference in accommodation coefficients in different facets would result in anisotropic growth. a: accommodation coefficient s=(p-p0)/p0, supersaturation T: temperature m: atomic weight of the growth species k: Boltzmann constant J ασp o = (atoms/cm 2 sec), 2πmkT

4 Fundamentals of evaporation-condensation growth When the concentration of the growth species is very low, the adsorption is more likely a rate-limiting step. The growth rate increases with the increase in the concentration of growth species. Further increase in the concentration of growth species would result in a change from an adsorption limited to surface growth limited process. When the surface growth becomes a rate-limiting step, the growth rate becomes independent of the concentration of the growth species Fundamentals of evaporation-condensation growth Residence time (τ s ) for a growth species on the growing surface τ s 1 = exp ν E des kt Diffusion coefficient of a growth species on the growing surface D s = 1 2 E a0ν exp kt s Mean diffusion distance of a growth species on the growing surface X = D τ = a 2 s s 0 E exp E kt des s

5 Accommodation coefficient If the mean diffusion distance is far longer than the distance between two growth sites such as kinks ledges, all adsorbed growth species will be incorporated in to the crystal structure and the accommodation coefficient would be unity. If the mean diffusion distance is far shorter than the distance between two growth sites, all adatoms will escape back to the vapor and the accommodation coefficient will be zero. The accommodation coefficient is dependent on desorption energy, activation energy of surface diffusion, and the density of growth sites Fundamentals of evaporation-condensation growth KSV theory: Classic step-growth theory for a flat surface, developed by Kossel, Stranski and Volmer, based on reorganization that a crystal surface is not smooth on the atomic scale. For a simple cubic crystal, each atom as a cube has a 6 coordination number (6 chemical bonds). An atom adsorbed (adatom) on a terrace would form 1 chemical bond between the atom and the surface. If adatom diffuses to a ledge site, it would form 2 chemical bonds. If an atom were incorporated to a ledgekink site, 3 chemical bonds would be form. An atom incorporated into a kink site would form 4 chemical bonds.

6 Fundamentals of evaporation-condensation growth Ledge, ledge-kink, kink sites are all considered as growth sites; incorporation of atoms into these sites is irreversible and results in growth. The growth is due to advancement of the steps (or ledges) and the growth rate will be dependent on the step density. A misoriented surface (vicinal surface) would result in an increased density of steps and consequently lead to a high growth rate. An increased step density would favor the irreversible incorporation of adatoms by reducing the surface diffusion distance between the impinging site and the growth site before adatoms escape back to the vapor phase. Limitation of KSV theory is how to regenerate growth site if all available growth sites are consumed Fundamentals of evaporation-condensation growth BCF theory: Developed by Burton, Cabrera and Frank in Propose screw dislocation as a continuous source to generate growth sites so that stepped growth would continue. The crystal growth proceeds in a spiral growth. The presence of screw dislocation ensure the continuing advancement of the growth surface and also enhance growth increased density of screw dislocations parallel to the growth direction. It is also known that different facets can have a significantly different ability to accommodate dislocations. The presence of dislocations on a certain facet can result in anisotropic growth.

7 Fundamentals of evaporation-condensation growth Fundamentals of evaporation-condensation growth PBC theory: Periodic Bond Chain theory, developed by Hartmann and Perdok in All crystal facets can be categorized in to three groups based on the number of broken periodic bond chains (simply understood as it means the number of broken bonds per atom on a given facet) on a given facets. Flat surface (F-face): {100} surfaces in a simple cubic Have 1 PBC Stepped surface (S-face): {110} surfaces in a simple cubic Have 2 PBCs. Kinked surface (K-face): {111} surfaces in a simple cubic Have 3 PBCs.

8 Evaporation-condensation growth ZnO nanobelts Characteristics of the ZnO nanobelts Typical thickness: nm. Width to thickness ratio: ~5 to 10. Two growth directions were observed: [0001] and [0110]. No screw dislocation was found through out the entire length of nanobelt, except a single stacking fault parallel to the growth axis in the nanobelts grown along [0110]. The surface of nanobelts are clean, atomically sharp and free of any sheathed amorphous phase Evaporation-condensation growth

9 Evaporation-condensation growth (a) and (b): CuO nanowires synthesized by heating a copper wire (0.1 mm diameter) in air to a temperature of 500 O C for 4h. Each CuO nanowires was bicrystal. [Nano Letters, vol 2, pp. 1333, 2002] Dissolution-condensation growth

10 4.2.2 Vapor-liquid-solid growth Fundamental aspects of VLS and SLS growth Liquid solution (catalyst, impurity) Distribution coefficient must be less than unity Evaporation pressure of catalyst must be very small Catalyst must be chemically inert Interfacial energy plays an important role For a compound nanowire, one of constituents can serve as the catalyst For controlled unidirectional growth solid-liquid interface must be well defined crystallographically Vapor-liquid-solid growth Fundamental aspects of VLS and SLS growth Growth procedures The growth species is evaporated first and then diffuses and dissolves into a liquid droplet. The surface of the liquid has a large accommodation coefficient, and is therefore a preferred site for deposition. Saturated growth species in the liquid droplet will diffuse to and precipitate at the interface between the substrate and the liquid. The precipitation will first follow nucleation and the crystal growth. Continued precipitation or growth will separate the substrate and the liquid droplet, resulting in a growth of nanowires.

11 4.2.2 Vapor-liquid-solid growth Fundamental aspects of VLS and SLS growth Vapor-liquid-solid growth Fundamental aspects of VLS and SLS growth

12 VLS growth of various nanowires VLS growth of various nanowires

13 Control of the size of nanowires Precursors and catalysts Ge ( s) 2GeI + GeI 2( g ) 2GaN ( g ) Ge 2GeI ( l) ZnO + C Zn + CO + x O + GeI 2( g ) 2( g ) 4( g ) Ga 2 O 3 + 2NO x( g )

14 SLS growth Stress induced recrystallization Strain/Stress

15 4.3 Template-based synthesis Electrochemical deposition Nerst Equation R T E = E + ln( a ) g 0 i nif 4.3 Template-based synthesis Electrochemical deposition

16 4.3 Template-based synthesis Electrochemical deposition Electrophoretic deposition

17 4.3.2 Electrophoretic deposition ξ = κ = 4πε r Q a ( 1+ κa) 2 e ni z ε ε kt r 0 2 i Electrophoretic deposition

18 4.3.2 Electrophoretic deposition Colloidal dispersion filling Electrophoretic deposition Melt and solution filling Chemical vapor deposition Deposition by centrifugation

19 4.3.4 Converting through chemical reactions 4.4 Electrospinning

20 4.5 Lithography 4.6 Summary