ESQUEMA DE PROTECCIÓN DEL ACTUAL SISTEMA DE TRANSMISIÓN DE

In general, little difference was observed in the film s deposited from the two different precursors, tin tetrachloride and tin tetrabrom ide. It should be m ade clear again here that the substrates film s were deposited onto w ere different for the reactions of tin

tetrachloride and tin tetrabrom ide. Film s deposited from tin tetrachloride were deposited onto glass with a pre-coating of carbon-doped silica, w hereas the substrate used in reactions involving tin tetrabrom ide w as glass w ith tw o coatings - tin oxide first and silica on top. A difference in m orphology was observed betw een films deposited from the different precursors, but this was attributed to the substrate and will be discussed in Section 3.6.

3.5.1

Visual appearance o f the films

Film s deposited under the sam e conditions from the two different precursors showed the same colours. The m ost significant difference in the appearance was that films deposited from tin tetrabrom ide with hydrogen sulfide w ere visibly thinner than those deposited from tin tetrachloride. Yellow film s could appear orange when deposited from the tetrachloride due to thickness. Silver film s deposited from the tetrabrom ide were alm ost invisible in som e cases.

3.5.2

X-ray diffraction

X -ray diffraction patterns of films deposited at the sam e tem perature appeared to be different. Som e peaks that are evident in film s produced from the tetrachloride do not appear in the X -ray diffraction patterns o f films deposited from the tetrabromide. Relative intensities also vary depending on the film. Figure 3.16 shows the diffraction patterns o f film s deposited from tin tetrabrom ide with hydrogen sulfide at 450, 500 and 600 °C. If these are com pared with Figures 3.2, 3.3 and 3.4, w hich show the diffraction patterns of film s deposited from tin tetrachloride at 300, 525 and 545 °C, it is seen that the film s are similar. Indexing the films gives the sam e lattice param eters, as seen in Table 3.7. It is im possible to calculate lattice param eters for the film deposited at 600 °C from tin tetrabrom ide with hydrogen sulfide, as only three peaks are evident. Changes in relative intensities could be due to deposition o f different polytypes, or preferred orientation on the different types of glass.

c 3 2 €n c 3 o 10 20 30 40 50 60 70 2 theta / degrees

Figure 3.16 X-ray diffraction patterns of films deposited from tin tetrabromide with hydrogen sulfide at (a) 450 °C, (h) 500 °C and (c) 600 “C.

Table 3.8 Comparison of the calculated lattice parameters o f films deposited from tin tetrachloride and films deposited from tin tetrabromide with existing literature values.

Film/reference a-parameter b-parameter c-parameter

Film from SnCU at 300 °C 3.642(7) 5.92(1)

Film from SnBr4 at 400 °C 3.638(5) 5.93(3)

Literature SnS2^'^ 3.6472(8) 5.8990(5)

Film from SnCU at 525 °C 8.83(1) 3.76(1) 14.03(1) Film from SnBr4 at 500 °C 8.85(1) 3.767(6) 13.99(1)

Literature SniSg^^ 8.878(2) 3.751(1) 14.020(3)

Film from SnCU at 545 °C 1 1.2 0(1) 3.99(1) 4.32(1)

3.5.3

Raman microscopy

Tlie Raman spectra o f films deposited from tin tetrachloride with hydrogen sulfide are shown in Figures 3.5, 3.6 and 3.7. Spectra collected from films deposited from the reactions of tin tetrabromide with hydrogen sulfide are given in Figure 3.17. It is clear from these that reactions at the same temperature from the two different precursors yield the same product.

M

C

0 50 100 150 200 250 300 350 40 0

Wavenumber I cm-1

Figure 3.17 Raman spectra of films deposited from tin tetrabromide with hydrogen sulfide at (a) 300 °C, (b) 525 °C and (c) 545 °C.

3.5.4

Energy dispersive analysis by X-rays

This technique would be useful to quantify the difference in thickness observed visually between films deposited from the two different precursors. However, since films deposited from reactions o f tin tetrabromide with hydrogen sulfide were deposited onto substrates containing tin, results from this analysis method can tell us only that the films ccntain sulfur. It would be possible to compare sulfur content of the films, but the error in EDAX is ca. 1 % at accelerating voltages of 20 kV and sulfur comprises 2 % o f the ej citation volume of some films deposited from the tetrabromide.

Film thicknesses were also determined from EDAX measurements. As reported in Section 3.3.4, the film deposited from tin tetrachloride and hydrogen sulfide were found to be 0.9 pm thick at a deposition temperature o f 300 °C, 1.3 pm at a deposition temperature of 525 °C and 0.8 pm thick at a deposition temperature of 545 °C. For film deposited from tin tetrabromide with hydrogen sulfide, thicknesses o ï ca. 0 . 1 pm were

recorded for films deposited at 400 and 525 °C, while a film 0.5 pm thick was formed at 550 “C. This quantitatively shows that which was observed in the visual appearance - that films deposited from the tetrabromide are thinner than those deposited from the tetrachloride under equivalent conditions.

3.5.5

Transmittance/reflectance spectroscopy

It was clear from transmittance/reflectance spectroscopy that the tetrabromide led to deposition o f thinner films, even though the deposition temperature, H2S flow rate and

vapour pressure in the bubbler were all identical. This is shown in Figure 3.18, which gives the transmittance and reflectance spectra for two films deposited at 400 °C from the reactions of the two precursors.

100 80 .6 0 40 20 100 40 1000 1100 1200 300 40 0 500 600 70 0 BOO Wavelength I nm 900

Figure 3.18 Transmittance(rcd or purplc)/Reflectance(blue or grey) spectra o f films deposited from (a) tin tetrachloride and (b) tin tetrabromide at 400 °C.

Transm ittance is m uch higher for films deposited from the tetrabrom ide. This is because the film s are m uch thinner. R eflectance is also higher in the film deposited from the tetrabrom ide. This suggests that the reflectance m easured is that from the glass substrate. It is higher in the case o f the film deposited from tin tetrabrom ide, because the thinner film perm its m ore transm ittance.

3.5.6

Discussion

The difference in volatility o f the precursors should not have had any effect on films deposited, as bubbler tem peratures w ere adjusted to suit. Figure 3.1 shows the vapour pressure curve for the tin tetrahalides. This shows that a bubbler tem perature of 70 °C for tin tetrachloride, w hich gives rise to a vapour pressure o f 200 torr (25 kPa), is equivalent to a bubbler tem perature of 150 °C for tin tetrabrom ide. Given that the vapour pressure above the precursors, the flow o f nitrogen through the bubblers and the tim e allow ed for reaction w ere all the same, the am ount o f precursor reaching the substrate in all reactions w ould be the same.

The strengths o f tin-chlorine and tin-brom ine bonds in diatom ic m olecules have previously been measured. These w ere determ ined spectroscopically from the hot gases effusing from a K nudsen cell.^^^ A tin-brom ine bond has a strength o f 47 +/- 23 kcal m ol^ and a tin-chloride bond has a strength of 75 kcal mol'^ This w ould tend to indicate that tin tetrabrom ide w ould be the m ore reactive species as its bonds are weaker. A lso supporting this are the relative heats o f form ation o f tin tetrabrom ide and tin tetrachloride - A H / (SnCU) = 545.2 kJ m o l '\ A H / (SnBr4) = 406.3 k J m o l'\

The driving force m aking the reaction of tin tetrachloride m ore vigorous than that o f tin tetrabrom ide is the energy released in the form ation o f the hydrogen halide. The heats of form ation of the two species are A H / (HCl) = -92.3 kJ mol'^ and A H / (HBr) = -36.3 kJ m o l'\ The entropy o f hydrogen brom ide is slightly higher than that of hydrogen chloride, but, with such a vast difference in enthalpy o f form ation, this is negligible. A t all tem peratures the release o f energy form ing H C l is m uch higher, and this reaction proceeds faster. Figure 3.19 shows the overall energy o f reaction for Reactions 3.1 and 3.7 as a function of tem perature.

SnCU + 2 HzS -> SnS] + 4 HCl SnBr4 + 2 H2S -> SnS] + 4 HBr E q u a tio n 3.1 E q u a tio n 3 .7 150 100 200 400 600 800 1000 1230 Equation 3.1 Equation 3.7 O) -100 -150 -200 Temperature I K

Figure 3.19 Gibbs free energy of reaction for reactions of tin tetrahalides with hydrogen sulfide to form tin disulfide.

Figure 3.19 shows that the reaction o f tin tetrabromide with hydrogen sulfide is endoergic at all temperatures. Reactions involving the reduction o f tin(IV) to tin(II) accompanied by the evolution of sulfur gas are exoergic overall, but if the first step involves the formation o f a tin(IV) sulfide species, as discussed in Section 3.3.9, the overall process would not occur. Nor would tin(IV) sulfide be deposited as the final film in any reaction.

In the case o f films deposited from tin tetrabromide, the thermodynamics of the reaction suggest that it would not occur. The equilibrium position o f Reaction 3.7 and other related reactions of tin tetrabromide would be close to the reactants. However, the high rate o f flux o f reactants to the surface, the ease o f transport o f by-products away from the surface and the fact that one product is a solid all contribute to the reaction occurring.