8.4.2.3 Solution-Based Growth of Nanowires
This synthetic strategy for nanowires makes use of anisotropic growth dictated by
the crystallographic structure of a solid material; or confined and directed by templates; or kinetically controlled by supersaturation; or by the use of appropriate
capping agents.
Highly Anisotropic Crystal Structures by Non-Template Methods Many solid materials such as polysulphur nitride, (SN) x , grow into 1D nanostructures and this
habit is determined by the highly anisotropic bonding in the crystallographic
structure [305a,b]. Other materials such as selenium [306, 307b], tellurium [308]
and molybdenum chalcogenides [309a,b], are also easily obtained as nanowires
due to the anisotropic bonding, which makes the crystallization occur along the
c-axis, favoring the stronger covalent bonds over the relatively weak van der Waals
forces between the chains. Molybdenum chalcogenides, with the general formula
M 2 Mo 6 X 6 (M ¼ Li, Na; X ¼ Se, Te) contain hexagonally close packed linear chains
of formula Mo 6 X 6 . When dissolved in a highly polar solvent such as dimethylsulfoxide or N-methylformamide, they mainly exist as chains of @2 nm diameter.
Some chains may aggregate into bundles or fibres with cross sections of @1 mm
diameter and lengths up to @1 mm. Yang and co-workers [309a,b] studied the selforganization of these molecular wires (Li 2 Mo 6 Se 6 ) into mesoscopic bundles in the
presence of organic surfactants of opposite charges. By changing the length of
the surfactant molecule, the spacing between these inorganic nanowires could be
varied in the range 2–4 nm. Xia et al. [307] synthesized a spherical colloidal
suspension/dispersion of amorphous (a-) selenium with diameters of @300 nm by
refluxing selenious acid and hydrazine at elevated temperatures. After cooling the
suspension to room temperature a small amount of selenium dissolved in the solution precipitates out as nanocrystallites of triagonal Se (t-Se). During aging of this
dispersion in the dark, the a-Se dissolves slowly in the solution and subsequently
crystallizes out slowly on a t-Se seed. The intrinsic anisotropic nature of t-Se
building blocks, that is extended, helical chains of Se atoms (Figure 8.33(a)) in the
Fig. 8.32. (A) Synthesis of core–shell
nanowires by chemical vapor deposition.
(a) Gaseous reactants (red) catalytically
decompose on the surface of a gold nanocluster leading to nucleation and directed nanowire
growth. (b) One-dimensional growth is maintained as reactant decomposition on the gold
catalyst is strongly preferred. (c) Synthetic
conditions are altered to induce homogeneous reactant decomposition on the nanowire surface, leading to a thin, uniform shell
(blue). (d) Multiple shells are grown by
repeated modulation of reactants. (B) SiaSi
homoepitaxial core–shell nanowires. (a), (b)
Diffraction contrast and high-resolution TEM
images, respectively, of an unannealed intrinsic
silicon core and p-type silicon shell nanowire
grown at 450
C. Crystal facets in the highresolution TEM image designated by arrows
indicate initially epitaxial shell growth at low
temperature. Scale bars are 50 nm and 5 nm,
respectively. (c), (d) TEM images (analogous
to (a) and (b)) of an aSi/paSi core–shell
nanowire annealed at 600
C for 30 min after
core–shell growth at 50
C. Inset, twodimensional Fourier transforms of the image
depicting the [111] zone axis of the single
crystal nanowire. The 1/3{422} reflections,
although forbidden in bulk silicon, arise as a
result of the finite thickness of the nanowire.
Scale bar is 50 nm. Reproduced from ref. [304],
with permission.
8.4 Nanowires 265
This synthetic strategy for nanowires makes use of anisotropic growth dictated by
the crystallographic structure of a solid material; or confined and directed by templates; or kinetically controlled by supersaturation; or by the use of appropriate
capping agents.
Highly Anisotropic Crystal Structures by Non-Template Methods Many solid materials such as polysulphur nitride, (SN) x , grow into 1D nanostructures and this
habit is determined by the highly anisotropic bonding in the crystallographic
structure [305a,b]. Other materials such as selenium [306, 307b], tellurium [308]
and molybdenum chalcogenides [309a,b], are also easily obtained as nanowires
due to the anisotropic bonding, which makes the crystallization occur along the
c-axis, favoring the stronger covalent bonds over the relatively weak van der Waals
forces between the chains. Molybdenum chalcogenides, with the general formula
M 2 Mo 6 X 6 (M ¼ Li, Na; X ¼ Se, Te) contain hexagonally close packed linear chains
of formula Mo 6 X 6 . When dissolved in a highly polar solvent such as dimethylsulfoxide or N-methylformamide, they mainly exist as chains of @2 nm diameter.
Some chains may aggregate into bundles or fibres with cross sections of @1 mm
diameter and lengths up to @1 mm. Yang and co-workers [309a,b] studied the selforganization of these molecular wires (Li 2 Mo 6 Se 6 ) into mesoscopic bundles in the
presence of organic surfactants of opposite charges. By changing the length of
the surfactant molecule, the spacing between these inorganic nanowires could be
varied in the range 2–4 nm. Xia et al. [307] synthesized a spherical colloidal
suspension/dispersion of amorphous (a-) selenium with diameters of @300 nm by
refluxing selenious acid and hydrazine at elevated temperatures. After cooling the
suspension to room temperature a small amount of selenium dissolved in the solution precipitates out as nanocrystallites of triagonal Se (t-Se). During aging of this
dispersion in the dark, the a-Se dissolves slowly in the solution and subsequently
crystallizes out slowly on a t-Se seed. The intrinsic anisotropic nature of t-Se
building blocks, that is extended, helical chains of Se atoms (Figure 8.33(a)) in the
Fig. 8.32. (A) Synthesis of core–shell
nanowires by chemical vapor deposition.
(a) Gaseous reactants (red) catalytically
decompose on the surface of a gold nanocluster leading to nucleation and directed nanowire
growth. (b) One-dimensional growth is maintained as reactant decomposition on the gold
catalyst is strongly preferred. (c) Synthetic
conditions are altered to induce homogeneous reactant decomposition on the nanowire surface, leading to a thin, uniform shell
(blue). (d) Multiple shells are grown by
repeated modulation of reactants. (B) SiaSi
homoepitaxial core–shell nanowires. (a), (b)
Diffraction contrast and high-resolution TEM
images, respectively, of an unannealed intrinsic
silicon core and p-type silicon shell nanowire
grown at 450
C. Crystal facets in the highresolution TEM image designated by arrows
indicate initially epitaxial shell growth at low
temperature. Scale bars are 50 nm and 5 nm,
respectively. (c), (d) TEM images (analogous
to (a) and (b)) of an aSi/paSi core–shell
nanowire annealed at 600
C for 30 min after
core–shell growth at 50
C. Inset, twodimensional Fourier transforms of the image
depicting the [111] zone axis of the single
crystal nanowire. The 1/3{422} reflections,
although forbidden in bulk silicon, arise as a
result of the finite thickness of the nanowire.
Scale bar is 50 nm. Reproduced from ref. [304],
with permission.
8.4 Nanowires 265