The role of surface treatment on p-doped silicon substrate with

Tisdale and co-workers suggest a surface treatment for silicon wafers that improve the QD adhesion to the substrate surface, which can lead to achieving a more uniform film [Wei-dman et al., 2015]. Surface treatment was done by keeping overnight the cleaned silicon wafers in toluene containing 0.02M 3-mercaptopropyl trimethoxysilane (24 hours). The substrates were washed with toluene the following day and dried before spin coating or dip coating was performed.

To ensure that the surface treatment alters the surface of silicon wafers, the contact angle measurement was performed, that measures quantitatively the extent to which a solid is wetted by a liquid [Biolin Scientific, 2018]. Geometrically, contact angle is defined as the angle a liquid forms at the three-phase boundary of solid, liquid and gas intersection [Biolin Scientific, 2018]. Measurement was performed by placing a water droplet on the solid surface and recording the image of the drop. In order to define the static contact angle, Young-Laplace equation is fitted around the fluid droplet [Biolin Scientific, 2018].

The angle formed by the droplet on the surface, given that the three-phase boundary is still is referred to as the static angle [Biolin Scientific, 2018]. Surface rearrangements, alterations, swelling, solution impurities absorbed on the surface lead to an advancing or a receding of the angle originally formed by the liquid droplet on the surface that can be determined by the contact angle measurement [Biolin Scientific, 2018]. The measurement on silicon surfaces is therefore an indication of whether the surface has become more hydrophilic or hydrophobic.

Figure 37. Contact angle measurement of p-doped silicon after surface treatment with 3-mercaptopropyl trimethoxysilane

As shown in figure 37, there is an advancement of the contact angle from 57.2^◦ to 72.1^◦,

that corresponds to a lesser surface wetting than before the treatment. This further indi-cates that upon the surface treatment of silicon, the surface has become more hydrophobic that improves QD adhesion according to Weidman et al.. Spin coating and dip coating were then performed on treated silicon substrates and compared with the previous results in section 4.1.1 and 4.1.2.

A QD solution of concentration, 2 mg/ml and 1.5 mg/ml with 1:1 volume ratio of hex-ane/octane were used for dip and spin coating respectively. Dip coating parameters were used as 60 seconds waiting time and a withdrawal speed of 1.9 mm/s, which was found to be the appropriate from the previous dip coating results in section 4.1.2.

Figure 38. CsPbBr3film fabrication on treated silicon. (a)Dip coating (concentration - 2 mg/ml with 1:1 heaxne/octane, withdrawal speed - 1.9 mm/s, waiting time - 60 sec) (b) Spin coating (concentration - 1.5 mg/ml with 1:1 hexane/octane)

Comparison of SEM images of thin films on treated and non-treated silicon elucidate that surface treatment with 3-mercaptopropyl trimethoxysilane has a great impact on the film formation on the substrate. The ligands on the QDs tend to adhere more to the silicon surface and lead to a more uniform film and an occupied substrate. Figure 35 (b) and figure 38 (b) shows how spin coating on treated substrate has improved film fabrication, uncoated gray areas are no more visible indicating complete occupancy of substrate sur-face by QDs. Similar observation with dip coating as demonstrated by figures 36 (b) and figure 38 (a).

Atomic force microscopy measurement of the dip coated sample was performed to check the thickness of the layer to confirm that only a monolayer is present. A gentle scratch was made with a soft metallic blade (stainless steel) that allowed to fully remove the QDs layer, exposing a sharp edge, but without any evidence of damages on the Si substrate and thereby to evaluate the thickness by measuring the difference in height between the scratch and the coated area.

Figure 39.AFM measurement of dip coated sample in figure 38 (a)(a)Topography of the sample, including line where a profile was measured (dark area is the scratch).(b)Line profile.

The sample presented a clean and uniform layer of 12-14nm in thickness as can be deter-mined by the line profile in the figure above.

The experimental work for obtaining a monolayer so far has been done on 1x1 cm size silicon substrates. However, for the experiments with ALD and for optical characteriza-tion a a substrate of at least 2x2 cm size are needed that fits in the cell shown in figure 29. Additionally, studying the effect of QD concentration as well as solvent volume ratio on the bigger substrate was required using both spin and dip coating on treated silicon wafers. The following experimental work has been performed with dip and spin coating.

Table 3. Overview of dip coating on treated silicon at different concentrations of QD solution as well as with different hexane/octane volume ratios

Sample Concentration, mg/ml

Hexane% Octane%

1 1 25 75

2 1 50 50

3 1 75 25

4 2 25 75

5 2 50 50

6 2 75 25

Figure 40.SEM images of dip coated samples as in table 3(a)Sample 1(b)Sample 2(c)Sample 3(d)Sample 4(e)Sample 5(e)Sample 6

SEM images in figure 40 that correspond to the samples prepared according to data in table 3 indicates that the ideal, clear and uniform monolayer was obtained with sample 5 (figure 40 e)), which is given by a QD solution of 2 mg/ml concentration with 1:1 hexane/octane volume ratio for dilution. The film fabricated in other samples tend to be not uniform as can be observed by the light greyish stacks of QDs scattered all over the substrate randomly.

Even though sample 5 was able to fabricate a monolayer, an issue was recognized when SEM images of the samples were captured towards the edge, where they were held by the tweezers during dip coating. This is shown in figure 41. The use of a bigger substrate requires a bigger volume of QD solution that can not be used to dip coat many samples due to fluctuation of concentration over time. Furthermore, there is an imbalance and non-uniformity in fabrication towards the edges of the substrate. This can be due to the fact that, in a bigger substrate the end held by the tweezers is removed from the solution faster during withdrawal, while the other end stays in the solution for more time at slow withdrawal speeds. These issues make it important to optimize the spin coating process as it is beneficial in many ways in comparison to dip coating.

Figure 41.SEM images of dip coated sample 5 towards the edge of the 2x2 substrate

Table 4.Overview of spin coating on treated silicon at different concentrations of QD solution as well as with different hexane/octane volume ratios

Sample Concentration, mg/ml Hexane% Octane%

1 0.5 25 75

2 0.5 50 50

3 0.5 75 25

4 1.5 25 75

5 1.5 50 50

6 1.5 75 25

Figure 42. SEM images of spin coated samples as in table 4. (a) Sample 1 (b) Sample 2 (c) Sample 3(d)Sample 4(e)Sample 5(e)Sample 6

Spin coating at different conditions are given in table 4 and figure 42. According to the SEM images obtained, it was seen that a uniform monolayer was given by sample 1, that was prepared with a QD solution of 0.5 mg/ml and a hexane volume percentage of 25%. Increasing the hexane volume ratio tends to lead to a thicker and non-uniform film. A higher concentration for spin coating further has a tendency to lead to a thicker film as well. Spin coating for film fabrication thus becomes beneficial and ideal, as a low concentrated solution and a small amount of solution gives a thin film, which avoids wastage of QDs that happens during dip coating.