As stated above the only investigation on the synthesis of nanostructured Sn3
is that of single phase, cubic tin nitride nanoparticles grown via atmospheric pressure-halide vapour phase epitaxy by Nand et al.
]. In particular Nand et al.
O as a source of Sn and used 10 nm Au/Si(001) p-type substrates that were positioned at different distances from the SnCl4
O, along the reactor. The SnCl4
O was heated up to 500 °C under a flow of NH3
and the temperatures of the samples along the reactor were 400, 300 and 150 °C, respectively. However, Nand et al.
] did not obtain any NWs. Before discussing the synthesis of the Snx
NWs obtained here it is instructive to consider first the synthesis of SnO2
NWs on 0.5 nm Au/Si(111) that were previously obtained by heating up Sn in an inert gas flow of 100 sccm Ar at 30 °C/min up to 800 °C and then maintaining the flow of Ar at this temperature for a further 90 min before cool down [27
]. A typical SEM image of the SnO2
NWs obtained in this way is shown in Fig. a from which it is clear that a large yield of SnO2
NWs was obtained with an average diameter of 50 nm due to the reaction of Sn with residual O2
in the APCVD reactor. Performing the reaction under a direct flow of O2
leads to the formation of SnO2
around the molten Sn which limits the vapour pressure significantly and hence the growth of NWs. As a consequence the molten Sn upstream always had a grey like, non reflective appearance at the end of the process, while no droplets were observed among the SnO2
NWs. A similar process was also used recently for the growth of In2
NWs at 700 °C [26
Figure 1 SEM images ofaSnO2NWs with an average diameter of 50 nm grown on Au/Si(111) at 800 °C in an inert gas flow of 100 sccm Ar. No droplets exist among the NWsbSn droplets on Au/Si(111) deposited using the same temperature-time profile inabut in a (more ...)
At first sight it would seem that the synthesis of Snx
NWs by direct nitridation of Sn with NH3
is feasible by changing from Ar to NH3
since Sn, like In, has a low melting point [28
] and InN NWs have been obtained by direct nitridation of In with NH3
at a heater temperature of 600 °C [25
]. However, Snx
NWs were not obtained by the direct nitridation of Sn with NH3
. Instead many Sn droplets appeared on the Si(111) surface and a typical SEM image of such Sn droplets after the attempted nitridation of Sn with NH3
at 800 °C is shown in Fig. b. The Sn droplets cover the entire surface and have a density of ≈107
and diameters ≤5 μm. Furthermore the size of the Sn droplets decreased as the temperature was reduced to 600 °C and many of then became elongated as shown in Fig. c.
The formation of large droplets on the Au/Si(111) surface during the direct nitridation of Sn with NH3
is a direct consequence of the reducing action of NH3
which eliminates the background O2
in the APCVD reactor. This in turn prevents the formation of an oxide around the Sn and so the molten drop always had a highly reflective, metallic like surface. In contrast when Sn is heated up in a flow of pure, inert Ar, the surface is grey like and not reflective due to the O2
background which is responsible for the formation of SnO2
NWs that were grown optimally on 0.5 nm Au/Si(111) at 800 °C using the same temperature-time profile and Ar as opposed to NH3
Apart from droplets, no nanostructures were obtained via the attempted nitridation of Sn with NH3in the temperature range 600 °C <TG < 800 °C. Turning on the flow of NH3, after ramping up the temperature in an inert gas flow of Ar, did not lead to the growth of SnxNyNWs either but again resulted into the deposition of Sn droplets. However, there was some evidence of one-dimensional growth atTG = 500 °C. Literally a few NWs with diameters >500 nm’s and lengths up to 3 μm appeared at a few locations on the Si(111) surface, hence the yield was extremely poor. Nevertheless, a further reduction of the growth temperature down to 300 °C did not lead to any significant deposition on the Si, no NWs were obtained and moreover, the Sn upstream lost its metallic shine due to the build up of a black deposit on the molten Sn which limited the vapour transport. In addition, no differences were observed upon changing the flow rate of NH3during the growth while keeping everything else equal at all temperatures so the direct nitridation of Sn alone under a flow of NH3is not effective and leads to the deposition of Sn droplets on the Au/Si(111) surface which impedes one-dimensional growth over a wide temperature range i.e. 300–800 °C as shown below in Table .
The XRD spectrum of the Sn droplets deposited at 800 °C is shown in Fig. and is characterized by an intense peak corresponding to the (2 0 0) orientation of Sn and less intense but well resolved peaks corresponding to the (1 0 1), (3 0 1), (4 0 0) and (3 2 1) orientations. In addition to the Sn droplets the Al holder peaks have also been identified but no peaks associated with SnxNywere found.
XRD spectrum of Sn droplets deposited on Si(111) at 800 °C under a flow of 250 sccm NH3
These findings are in direct contrast with the case of InN where NWs can be grown by direct nitridation of In with NH3via a self-catalytic mechanism. The optimum heater temperature for the growth of InN NWs was found to be 600 °C where its vapour pressure is equal to 10−6 Torr. Large In droplets comparable in size to those in Fig b started appearing only at temperatures ≥800 °C in contrast to the Sn droplets whose density was large and persisted even down to 600 °C where its vapour pressure is <10−11 Torr. It appears therefore that the Sn droplets are born out from the melt upstream and are transferred to the Si(111) surface where they coalesce to form larger droplets.
The tendency for one-dimensional growth observed atTG = 500 °C during the direct nitridation of Sn with NH3was promoted by the addition of NH4Cl into the Sn at a ratio of 1:1 by weight. The reaction of NH4Cl with Sn was carried out under a flow of NH3keeping the flow rate, ramp rate and temperature profile identical to those used in the case of ‘direct nitridation’ of Sn with NH3. A typical SEM image of SnxNyNWs obtained by the reaction of Sn with NH4Cl under NH3at 450 °C is shown in Fig a. The SnxNyNWs have an average diameter of 200 nm’s and lengths up to 5 μm while the reaction of Sn with NH4Cl always lead to the deposition of a white powder downstream, near the cool end of the reactor, in contrast to the direct nitridation of Sn with NH3where no by products occurred.
aSnxNyNWs grown on Au/Si(111) at the optimum temperature of 450 °C using a Sn:NH4Cl mixture under a flow of 250 sccm’s NH3bhigh magnification SEM image of SnxNyNWs
The XRD spectrum of the Snx
NWs grown at 450 °C is shown in Fig. and is characterized by the (2 2 0), (3 1 1), (5 1 1) and (4 4 0) peaks, which can be indexed to the hexagonal structure of Sn3
]. The intense peak of Sn(200) observed in Fig. has disappeared and once more the Al peaks appearing in the XRD spectrum of Fig. due to the sample holder have been identified. Furthermore there are no peaks associated with SnO2
XRD spectrum of SnxNyNWs grown on Au/Si(111) at 450 °C via the reaction of Sn and NH4Cl
The promotion of one dimensional growth is attributed to the dissociation of NH4
Cl. Upon increasing the temperature NH4
Cl undergoes sublimation at 338 °C and therefore dissociates into NH3
and HCl according to the following reaction.
As explained by Chaiken et al.
] the sublimation rate of NH4
Cl increases by a factor of 104
when changing the temperature from T
= 100 to 600 °C and the typical sublimation weight loss of NH4
Cl is over 90% when heated for ≈60 min. It is interesting to point out here that this sublimation process is endothermic and the temperature is expected to be reduced only by a few tens °C in the case of NH4
]. The decomposition of NH4
Cl enhances the porosity of the Sn melt and more importantly acts as a dispersant increasing the amount of Sn that is transferred into the gas stream. In fact the sublimation of NH4
Cl and the generation of NH3
and HCl gasses which act to disperse the molten Sn occurs abruptly and leads to strong dispersion of the molten Sn inside the boat for the ramp rate used here i.e. 30 °C/min suggesting that lower ramp rates would be more suitable. In addition to acting as a dispersant, the sublimation of NH4
Cl yields HCl which reacts with Sn leading to the formation of SnCl2
according to the reaction,
Note that SnCl2
melts at 38 °C and decomposes above 600 °C. Subsequently the SnCl2
reacts with NH3
Consequently the role of the NH4Cl is two fold. First, it prevents the Sn from melting up into one single drop and second it supplies the necessary HCl for the formation of SnCl2. As stated above heating up Sn alone in NH3did not lead to the deposition of any products near the cool end of the reactor so the deposition that occurs from heating up Sn and NH4Cl in NH3is due to the reaction of Sn with HCl since XRD of the deposit showed no peaks related to NH4Cl.
The reaction outlined above is in a way similar to that put forward by Nand et al.
] whereby SnCl4
reacts with NH3
leading to the growth of Sn3
NPs on 10 nmAu/Si(111) that were placed at various positions along the reactor but also similar to the growth of Sn3
thin films by APCVD using halides, by Gordon et al.
and Takahashi et al.
A similar kind of reaction was also used to grow InN nanocrystals on Si(111) whereby the incorporation of NH4
Cl into the In lead to the complete elimination and transfer of the In into the gas stream where it formed primarily InCl which in turn reacted with the NH3
leading to the formation of InN nanocrystals with diameters of 300 nm [30
While the addition of NH4Cl in Sn did not result into its complete transfer in the gas stream like with In, it provided nonetheless the necessary HCl for the formation of SnCl2which subsequently reacts with NH3on the Au/Si(111) leading to the one dimensional growth of SnxNy. Interestingly the distance of the sample from the Sn:NH4Cl mixture was found to be critical and for distances >10 mm the reaction led to the formation of closely packed NPs with sizes <100 nm on the Au/Si(111) most probably due to the lower vapour pressure of the SnCl2.
Although the details of the growth mechanism are not understood thoroughly at present it is suggested that the Snx
NWs grow self catalytically from Snx
NPs although the role of the Au which appears to enhance the one dimensional growth still needs to be clarified [32
A first estimate of the band-gap of the Snx
nanowires grown on Si(111) was obtained from optical reflection measurements using a UV–IR spectrometer at near normal incidence on both the NW sample and the Si(111) substrate for comparison, shown in Fig. . Clearly evident is the distinct difference in the reflection spectra from the substrate and the NWs. Also evident is the band edge of the Snx
NWs which is estimated to be approximately 2.6 eV [31
Optical reflection from the SnxNyNWs (Left) and plainn-type Si(111) substrate (Right)
In addition to Snx
NWs that were obtained at TG
= 450 °C there is also evidence for the formation of more complex nanostructures obtained for TG
< 450 °C as shown in Fig. a and b. However, their density was smaller compared to that in Fig. a due to the lower growth temperature which limits the amount of Sn transferred over to the Si(111). The radial growth of NWs from the droplet shown in Fig a is very similar to the case of InN [25
] whereby nucleation centres form on the surface of droplets which then facilitate radial growth [32
]. Moreover, the circular arrangement of NWs shown in Fig. b is due to the formation of droplets that accumulate near the periphery of well defined circles similar to the growth of In2
nano pyramids that self assemble in the form of wreaths due to the reaction of In with NH4
Cl in a flow of N2
Nanostructures obtained atT = 400 °C on 0.7 nm Au/Si(111)aNWs emanating from a droplet andbNWs which have grown from droplets in a circular configuration