K.w. Kolasinski/ Current Opinion in Solid State and Materials Science 10(2006)182-191 Fig. 2 illustrates several other aspects of growth that Initiation Steady State Termination must be considered. Will the nanowire be produced from deposition I transport to drdt<0 root growth in which the catalyst particle is found at the nucleation base of the nanowire, Fig. 2a, or by float growth, in which the particle is located at the tip of the nanowire? will multi drdt>0 of sidewalls ple prong growth ensue, Fig. 2c, in which more than one dndt=o nanowire emanates from each particle or will single-prong growth occur, Fig. 2d? A rather widely reported misunderstanding [1, 66, 67] is that VLS growth occurs because the sticking coeffic Fig. 1. General considerations on the different regimes that occur during on a liquid is unity and must be higher than the sticking talytic growth of es and nanotubes coefficient on the solid. This has also been mistakenly repeated in other fields of structure formation involving growth [68]. For growth of SiNWs from silanes, which tor to show that catalytic particle are not involved in the have a much higher dissociative sticking coefficient on growth process the particle than on the substrate or sidewalls, the reason Once the catalytic particles are formed or deposited, for the greater rate of dissociation is because of the cata- they may still need to be primed for the growth of nano- lytic action of the metal in the particle not the fact that it wires. For instance, the pure metal catalytic particle might is liquid. There is no general evidence for the assertion not be that active for nanowire formation. Instead, an that the sticking coefficient must be larger on a liquid than admixture of the growth compound and the metal might a metal. Second, the assertion does not even apply gener be required to form an (unstable or stable)alloy, a true ally to VLS growth. The success of mBe in semiconduc- eutectic or some other solid/liquid solution. In this case, tor processing relies largely on the fact that the sticking saturation of the catalytic particle with the growth material coefficient of numerous evaporated semiconductor materi or the formation of the proper composition may lead to an als is virtually unity on a solid substrate regardless of its induction period before growth. Note that the incorpora- composition. VLS growth in a MBE configuration, in tion of a significant amount of growth material into the which the growth material is supplied by evaporation catalytic particle is expected to change the volume and, from a crucible unto the substrate, has been demonstrated therefore, the diameter of the particle from its initial value. for nanowires composed of, e.g. Si, SiGe and Ill-V com- Hence Ostwald ripening and incorporation of growth pounds. a sticking coefficient difference alone cannot material can both conspire to change the size of the cata- account for the formation of nanowires. Other factors lytic particles must also be considered that allow the nanowire/catalytic The growth of nanowires with a uniform radius is asso- particle interface to act as a sink for the incorporation of ciated with a steady state growth in which material is trans- new material into the nanowire at a greater rate than the ported to the particle/nanowire interface. If the nanowire growth of the sidewalls or the thickness of the substrate radius is related to the particle radius and the particle in between particle sites. For instance, the catalytic parti radius is constant, a natural explanation for a uniform cle can lower the barrier that is present for the incorpora nanowire diameter is that the particles have reached a tion of new material at the growth interface as compared steady state and their diameter is not evolving in this to the nucleation of an island on a sidewall or the region. If the incorporation of material is directed solely substrate. y the particle and incorporation directly into the sidewalls Fig. 2a and b also illustrate several dynamical process is suppressed, a constant particle diameter leads to a con- that can affect growth. Adsorption occurs from the fluid stant nanowire diameter. The particle might be able to (whether gaseous, liquid or supercritical) phase. Adsorp- affect the size of the nanowire either by direct matching tion might be molecular or dissociative and may either of the size of the nanowire to the size of the particle or else occur (vi) on the nanowire(viii) on the particle, (ix)on by some mechanism involving the curvature of the particle the substrate. A natural way for the catalytic particle to in which strain and lattice matching play a role. direct material to the growth interface is if the sticking Finally, a tapering to smaller diameters and cessation of coefficient(probability of adsorption) is higher on the par growth will occur if either the particle enters a phase in ticle and vanishingly small elsewhere. Diffusion of adatoms which it is consumed, if the growth material is no longer will occur (i)across the substrate(if the sticking probability supplied to the system, or if the temperature is reduced is not negligible), (ii) across the particle and (iii) along the below a critical value. The temperature will play a role in sidewalls. Diffusion across the substrate and along the side- umerous processes, e.g. dissociative adsorption, surface walls must be rapid and cannot lead to nucleation events diffusion, bulk diffusion through the particle, in determin- Nucleation of the nanowire anywhere other than on the ng the composition of the particle by affecting solubilities particle must be suppressed so that growth only occurs at and thermodynamic stability of certain phases as well as the particle/ nanowire interface and so that sidewalls do diffusion of metal atoms away from the particle not grow independently of the axial growth. There maytor to show that catalytic particle are not involved in the growth process. Once the catalytic particles are formed or deposited, they may still need to be primed for the growth of nano￾wires. For instance, the pure metal catalytic particle might not be that active for nanowire formation. Instead, an admixture of the growth compound and the metal might be required to form an (unstable or stable) alloy, a true eutectic or some other solid/liquid solution. In this case, saturation of the catalytic particle with the growth material or the formation of the proper composition may lead to an induction period before growth. Note that the incorpora￾tion of a significant amount of growth material into the catalytic particle is expected to change the volume and, therefore, the diameter of the particle from its initial value. Hence Ostwald ripening and incorporation of growth material can both conspire to change the size of the cata￾lytic particles. The growth of nanowires with a uniform radius is asso￾ciated with a steady state growth in which material is trans￾ported to the particle/nanowire interface. If the nanowire radius is related to the particle radius and the particle radius is constant, a natural explanation for a uniform nanowire diameter is that the particles have reached a steady state and their diameter is not evolving in this region. If the incorporation of material is directed solely by the particle and incorporation directly into the sidewalls is suppressed, a constant particle diameter leads to a con￾stant nanowire diameter. The particle might be able to affect the size of the nanowire either by direct matching of the size of the nanowire to the size of the particle or else by some mechanism involving the curvature of the particle in which strain and lattice matching play a role. Finally, a tapering to smaller diameters and cessation of growth will occur if either the particle enters a phase in which it is consumed, if the growth material is no longer supplied to the system, or if the temperature is reduced below a critical value. The temperature will play a role in numerous processes, e.g. dissociative adsorption, surface diffusion, bulk diffusion through the particle, in determin￾ing the composition of the particle by affecting solubilities and thermodynamic stability of certain phases as well as diffusion of metal atoms away from the particle. Fig. 2 illustrates several other aspects of growth that must be considered. Will the nanowire be produced from root growth in which the catalyst particle is found at the base of the nanowire, Fig. 2a, or by float growth, in which the particle is located at the tip of the nanowire? Will multi￾ple prong growth ensue, Fig. 2c, in which more than one nanowire emanates from each particle or will single-prong growth occur, Fig. 2d? A rather widely reported misunderstanding [1,66,67] is that VLS growth occurs because the sticking coefficient on a liquid is unity and must be higher than the sticking coefficient on the solid. This has also been mistakenly repeated in other fields of structure formation involving growth [68]. For growth of SiNWs from silanes, which have a much higher dissociative sticking coefficient on the particle than on the substrate or sidewalls, the reason for the greater rate of dissociation is because of the cata￾lytic action of the metal in the particle not the fact that it is liquid. There is no general evidence for the assertion that the sticking coefficient must be larger on a liquid than a metal. Second, the assertion does not even apply gener￾ally to VLS growth. The success of MBE in semiconduc￾tor processing relies largely on the fact that the sticking coefficient of numerous evaporated semiconductor materi￾als is virtually unity on a solid substrate regardless of its composition. VLS growth in a MBE configuration, in which the growth material is supplied by evaporation from a crucible unto the substrate, has been demonstrated for nanowires composed of, e.g. Si, SiGe and III–V com￾pounds. A sticking coefficient difference alone cannot account for the formation of nanowires. Other factors must also be considered that allow the nanowire/catalytic particle interface to act as a sink for the incorporation of new material into the nanowire at a greater rate than the growth of the sidewalls or the thickness of the substrate in between particle sites. For instance, the catalytic parti￾cle can lower the barrier that is present for the incorpora￾tion of new material at the growth interface as compared to the nucleation of an island on a sidewall or the substrate. Fig. 2a and b also illustrate several dynamical process that can affect growth. Adsorption occurs from the fluid (whether gaseous, liquid or supercritical) phase. Adsorp￾tion might be molecular or dissociative and may either occur (vii) on the nanowire (viii) on the particle, (ix) on the substrate. A natural way for the catalytic particle to direct material to the growth interface is if the sticking coefficient (probability of adsorption) is higher on the par￾ticle and vanishingly small elsewhere. Diffusion of adatoms will occur (i) across the substrate (if the sticking probability is not negligible), (ii) across the particle and (iii) along the sidewalls. Diffusion across the substrate and along the side￾walls must be rapid and cannot lead to nucleation events. Nucleation of the nanowire anywhere other than on the particle must be suppressed so that growth only occurs at the particle/nanowire interface and so that sidewalls do not grow independently of the axial growth. There may time Initiation deposition nucleation saturation dr/dt > 0 Steady State transport to growth interface passivation of sidewalls dr/dt = 0 Termination dr/dt < 0 Fig. 1. General considerations on the different regimes that occur during catalytic growth of nanowires and nanotubes. 186 K.W. Kolasinski / Current Opinion in Solid State and Materials Science 10 (2006) 182–191