6.2.3
Raman microscopy
Raman spectra were recorded for all films. Reference spectra of tin sulfides are given in Figure 2.6. These show that bands appear in tin(II) sulfide at 216-220, 185-189, 159- 163 and 94-96 c m '\ and at 313 and 214 cm'^ in tin(IY) sulfide. As tin selenide has the same structure as tin sulfide, it would be expected that the pattern observed is similar, although bands would move. Selenium is heavier than sulfur, so bands would be expected to appear at lower wavenumber. Figure 6.1 shows Raman spectra recorded for the films deposited from [Sn(SPh)n(SePh)4.n] (n = 0 - 4), at 300, 400 and 500 °C.
ac
300 350 400
0 50 100 150 200 250
Wavenumber I cm-1
Figure 6.1 Raman spectra of films deposited from lSn(SPh)n(SePh)4.„J with hydrogen sulfide at (a) 300 °C, (b) 400 °C and (c) 500 °C.
Figure 6.1 shows that at 300 °C tin(IV) sulfide is deposited, at 500 °C tin(II) sulfide is the product and a mixture of the two is achieved at 400 °C. No evidence for mixed valent tin sulfide is seen.
In the Raman spectra of the films deposited at 300 and 400 °C, the SnSi band occurs at 315 c m '\ The bands for SnS are noticed at 221, 182-191, 161-164 and 96-97 cm’* in
the spectra recorded for films deposited at 400 and 500 °C. These bands are all in roughly the same place as in other spectra recorded for tin sulfides,
Tlie spectra of the films deposited without hydrogen sulfide are given in Figure 6.2.
c
100 150 200 250 300 350 400
0 50
Wavenumber/ cm-1
Figure 6.2 Raman spectra of the films deposited from |Sn(SPh)„(SePh)4.„] without hydrogen sulfide at (a) 400 °C, (b) 450 "C and (c) 500 °C.
Figure 6.2 shows that at 400 °C tin(IV) sulfide is deposited, while at 450 °C a film with a Raman spectrum similar to that of tin(II) sulfide is deposited. The position of the SnSi band in Figure 6.2a is 315 c m '\ however the bands for SnS in Figure 6.2b are observed at 154, 130 and 78 c m '\ The band at 220 is not observed. The film deposited at 500 °C begins to show the band that should be at 185 c m '\
The shifting of the bands in the spectra o f tin(II) sulfide is significant. Spectra are calibrated to within 2 c m '\ and shifts o f ca. 20 cm'^ are experienced here. In the films deposited in the presence of hydrogen sulfide, a shift of 7 cm'^ occurs in one of the bands, wliile when no hydrogen sulfide is used shifts o f 20 cm'^ occur. These shifts may be due to some incorporation o f selenium into the films.
6.2.4
Energy dispersive analysis by X-rays.
All film s w ere analysed by EDAX. They w ere found to be thin, w ith less than 10 % of the excitation volum e being m ade up o f tin, sulfur or selenium. The substrate did not contain tin, how ever calcium found in the glass interferes w ith the tin signal. At these low am ounts of tin, the effect of calcium is significant. R atios are reported here for com parison with one another, and not as absolute values.
In the film s deposited from [Sn(SPh)n(SePh)4.n] with hydrogen sulfide, tin and sulfur
were observed in all films. Some films appeared to contain selenium , while others did not, but in all cases selenium content was so low that it could have been due to error.
O f the film s deposited w ithout H2S in the reaction m ixture, the film deposited at 450 °C
again show ed a selenium content w hich m ay be due to error in the instrum ent. The film deposited at 500 °C, how ever, showed a selenium content w hich w as above the error of the E D A X m achine. Ignoring the tin content, selenium accounted for ca. 30 % of the chalcogenide present.
If the tin content in these films is inspected, it is seen to be higher than observed in films deposited under the sam e conditions w ithout hydrogen sulfide. The total chalcogenide content is, how ever, similar. This high tin content is far higher than norm ally observed due to calcium in the underlying glass, so it m ay be that som e tin oxide is present in the films. O xygen is not detected by the E D A X system used in this work.
Table 6.1 gives the ratio of sulfur and selenium in all film s, although some selenium ratios are low er than 2 sigma, as calculated by the E D A X detector.
Table 6.1 Sulfur and selenium ratios in films deposited from [Sn(SPh)n(SePh)4.n].
D eposition tem perature / °C
H ydrogen sulfide flow rate / dm^min^ Selenium as percentage of total chalcogenide 300 0 . 2 1 0 * 350 0 . 2 0 400 0 . 2 9 * 450 0 . 2 0 500 0 . 2 1 0 * 550 0 . 2 8 * 450 0 2 0 * 500 0 30
* denotes film s w here selenium content is low er than instrum ent error.
A lthough error in m easurem ents on the ED A X m achine are high, due to the film being very thin, it is evident from Table 6.1 that presence o f H2S in the reaction m ixture leads
to low er selenium content in the films. This is to be expected.
6.2.5
Scanning electron microscopy
Im ages o f a representative sample of the films are shown in F igure 6.3.
The m orphologies observed in Figure 6.3 are not sim ilar to those observed for other film s deposited in this thesis. It is to be expected that som e sim ilarities w ould arise in the SEM s, as tin sulfide, tin selenide and all interm ediate com pounds have the same structure. In addition, as silica coated glass was used in both series o f depositions, no difference can arise due to the substrate. In Figure 6.3a the film deposited from [Sn(SPh)n(SePh)4.n] is shown. This shows sim ilar m orphology to film s deposited from
other precursors, how ever the particles are m uch sm aller and less well defined. In Figure 6.3b the film deposited from the sam e precursor at 450 °C is shown. Here, small round particles are observed on the surface of the glass, along w ith needles over the top. A t the higher tem peratures o f 500 and 550 °C, film s are m ade up of small regular particles. These are larger at the higher tem perature indicating that growth rates are higher.