identified for Si wafers. This identification was substantiated by the isotope shift
introduced by substitution of hydrogen with deuterium. These results are in accord
with atomically resolved STM images of SiNWs described in a later section. Annealing of the SiNWs results in the weakening and disappearance of the trihydride,
whereas the monohydride peaks remain strong. The hydrogen is completely removed only at 850 K. Hydrogen is observed on a SiNW surface even 26 days after
exposure to air. SiaO bands are however detected 17 h after exposure to air and
increase with time while the SiH bands decrease. It is likely that this is a superposition of the incorporation of O in the large-diameter SiNWs while the smallerdiameter (and probably more stable) SiNWs maintain the SiaH signal. It is obvious that the stability of H-terminated SiNW surfaces in water is much lower
than in air, and SiaO bands are apparent after immersion in water for 15 min.
These results indicate that it is possible to remove the oxide layer from the SiNW
surfaces and terminate them by H by immersion in HF, similar to Si wafers. The
H-terminated surfaces seem to be stable in air for at least a day, whereas the stability of small-diameter SiNWs, as determined by single-wire STM measurements,
seems to be substantially better than that of Si wafers.
10.6.2
Reduction of Metals in Liquid Solutions
The reductive deposition of silver and copper ions on (oxygen-removed and
hydrogen-terminated) SiNW surfaces in a solution was investigated [76] as an
alternative method to ion implantation. The SiNWs surface is indeed capable of
reducing silver and cooper ions to metal aggregates of various morphologies at
room temperature.
Laser ablation was used [22] to produce SiNWs @ 20 nm in diameter with a
polycrystalline silicon core in a thin silicon oxide sheath with 1/4–1/3 of the nominal diameter and 1/3 of the weight of the SiNW. The oxide layer (which makes the
SiNWs surfaces inert) was removed by a 5% HF dip for 5 min resulting in smooth,
stable, H-terminated SiNW surfaces [77]. The etched SiNWs were immersed into
solutions of silver nitrate and copper sulfate of different concentrations. Silver and
copper ions were reduced to metallic aggregates deposited onto the surface of
SiNWs. The TEM image of the sample treated with a 10
À4 M silver nitrate solution
(Figure 10.28) shows dark, round silver particles 5–50 nm in diameter. The HFetched SiNWs treated with 1:0 Â 10
À3 M copper sulfate show much smaller (a few
nm) particles (Figure 10.29) identified by EELS as Cu particles.
The silver metal deposition on SiNWs in different concentrations of silver solution (10
À6 to 0.1 M) was studied in detail with SEM, EDS, and XPS. The SEM images show that the morphology of the deposited silver depends on the concentration of silver nitrate. At high concentrations the redox reaction is controlled by the
concentration of Ag ions (mass action), so that large quantities of Ag ions in the
vicinity of SiNWs are reduced and aggregated as dendrites. At low concentrations,
the conditions required for dendrite formation are not reached and silver is de10.6 Chemical Properties of SiNWs 343
introduced by substitution of hydrogen with deuterium. These results are in accord
with atomically resolved STM images of SiNWs described in a later section. Annealing of the SiNWs results in the weakening and disappearance of the trihydride,
whereas the monohydride peaks remain strong. The hydrogen is completely removed only at 850 K. Hydrogen is observed on a SiNW surface even 26 days after
exposure to air. SiaO bands are however detected 17 h after exposure to air and
increase with time while the SiH bands decrease. It is likely that this is a superposition of the incorporation of O in the large-diameter SiNWs while the smallerdiameter (and probably more stable) SiNWs maintain the SiaH signal. It is obvious that the stability of H-terminated SiNW surfaces in water is much lower
than in air, and SiaO bands are apparent after immersion in water for 15 min.
These results indicate that it is possible to remove the oxide layer from the SiNW
surfaces and terminate them by H by immersion in HF, similar to Si wafers. The
H-terminated surfaces seem to be stable in air for at least a day, whereas the stability of small-diameter SiNWs, as determined by single-wire STM measurements,
seems to be substantially better than that of Si wafers.
10.6.2
Reduction of Metals in Liquid Solutions
The reductive deposition of silver and copper ions on (oxygen-removed and
hydrogen-terminated) SiNW surfaces in a solution was investigated [76] as an
alternative method to ion implantation. The SiNWs surface is indeed capable of
reducing silver and cooper ions to metal aggregates of various morphologies at
room temperature.
Laser ablation was used [22] to produce SiNWs @ 20 nm in diameter with a
polycrystalline silicon core in a thin silicon oxide sheath with 1/4–1/3 of the nominal diameter and 1/3 of the weight of the SiNW. The oxide layer (which makes the
SiNWs surfaces inert) was removed by a 5% HF dip for 5 min resulting in smooth,
stable, H-terminated SiNW surfaces [77]. The etched SiNWs were immersed into
solutions of silver nitrate and copper sulfate of different concentrations. Silver and
copper ions were reduced to metallic aggregates deposited onto the surface of
SiNWs. The TEM image of the sample treated with a 10
À4 M silver nitrate solution
(Figure 10.28) shows dark, round silver particles 5–50 nm in diameter. The HFetched SiNWs treated with 1:0 Â 10
À3 M copper sulfate show much smaller (a few
nm) particles (Figure 10.29) identified by EELS as Cu particles.
The silver metal deposition on SiNWs in different concentrations of silver solution (10
À6 to 0.1 M) was studied in detail with SEM, EDS, and XPS. The SEM images show that the morphology of the deposited silver depends on the concentration of silver nitrate. At high concentrations the redox reaction is controlled by the
concentration of Ag ions (mass action), so that large quantities of Ag ions in the
vicinity of SiNWs are reduced and aggregated as dendrites. At low concentrations,
the conditions required for dendrite formation are not reached and silver is de10.6 Chemical Properties of SiNWs 343