Experimental

In order to prepare a defect-free NC-based photodetector, confirmation of size and shape of the prepared material is necessary. This work was executed in two parts of experiments: one is PbS NC material preparation, and the other is NC PbS thin film on a flexible substrate. In both cases, different sources of sulphur were used in order to check the best response towards the light. A PbS NC material has been prepared by ball milling followed by the solid-state reaction. PbS NC films on the flexible substrate were deposited by the dip (soak) coating method.

Ball Milling

Ball milling was used for mixing appropriate salts in the metal jar with zircon balls at room temperature. The time and speed of milling were fixed for all four samples, i.e. 20 minutes and 600 rpm, respectively. We have used lead acetate [Pb(C2H,02)2] as a lead source, and four different components were used as sulphur sources, viz. Na2S (sodium sulphide), CH4N2S (thiourea), Na2S20, (sodium thiosulfate), and C2H5NS (thioacetamide). As per the atomic stoichiometry of PbS, 1:1 ratio of lead and sulphur sources was taken for ball milling. After 20 minutes of milling, a black colour material was obtained. Then, the drying process was carried out in two steps. The first step is to dry in air at 80°C for 1 hour, and the second step is to dry in air at 200°C for 3 hours. Figure 17.1 shows the actual photographs of the prepared PbS material using four different sulphur sources, and Table 17.1 shows the sample codes of the prepared PbS material.

Visual inspection of the dried material, as shown in Figure 17.1, indicates the various forms of granular nature. The sodium sulphide and thioacetamide-based material appeared as a pure black fine-textured powder. Both were easily collected after the drying process. The sodium thiosulfate-based dried powder was light browm in colour and granular in texture. However, the thiourea-based dried powder was in black mirror-polished, shiny, and crunchy texture, and hence, it requires further manual milling. After the required filtering, the prepared materials were characterized by XRD to confirm the crystalline structure and by FTIR to assure the purity.

XRD Analysis of Ball-Milled NC PbS Material

XRD profile of the prepared PbS materials using different sulphur sources is shown in Figure 17.2. The influence of the sulphur source is clearly observed in the form of the intensity of the crystalline planes and the peak width.

Dried PbS material using different sulphur sources

FIGURE 17.1 Dried PbS material using different sulphur sources: (a) CH4N,S (thiourea), (b) Na,S (sodium sulphide), (c) C,HsNS (thioacetamide), and (d) Na,S,0, (sodium thiosulfate).

(CONTINUED) Dried PbS material using different sulphur sources

FIGURE 17.1 (CONTINUED) Dried PbS material using different sulphur sources: (a) CH4N,S (thiourea), (b) Na2S (sodium sulphide), (c) C2H5NS (thioacetamide), and (d) Na,S,0, (sodium thiosulfate).

TABLE 17.1

Sample Code for Prepared PbS Materials Using Four Different Sulphur Sources

Sample Code

Used Sulphur Source

PI

CH4N,S (thiourea)

P2

Na,S (sodium sulphide)

P3

C,H5NS (thioacetamide)

P4

Na,S,0, (sodium thiosulfate)

XRD pattern of the lead sulphide material using four sulphur sources

FIGURE 17.2 XRD pattern of the lead sulphide material using four sulphur sources: (from top to bottom) CH4N,S (thiourea), Na2S (sodium sulphide), C,HsNS (thioacetamide), and Na2S20, (sodium thiosulfate).

XRD pattern reveals the formation of PbS phase in all samples. The peaks corresponding to (111), (200), (220), (311), (222), (400), (420), and (422) planes of PbS represent the cubic phase. The interplanar distance and the lattice constant match well with the values reported in JCPDS Card No. 05-0592, which can be attributed to cubic phase and confirmation of PbS powder. The average crystalline size D was calculated by the following Scherrer formula:

where A is the wavelength of X-ray (1.54nm) used, /i is the fill width half maxima of peak, and в is the Bragg angle. The average crystalline size of PbS powder is estimated to be from 26 to 47 nm. The strain is related to a lattice “misfit” that relies on the preparation conditions of the film materials. The strain was calculated by the following formula:

where в is the Bragg angle and /) is the full width half maxima.

Furthermore, the dislocation density can be calculated by the following relation:

The length of dislocation lines per unit volume of crystal is normally defined as dislocation density. It can be attributed to the crystallographic defect present in the crystal structure. All the calculated structural parameters are given in Table 17.2.

XRD plot of sodium thiosulfate (sample Pl)-based PbS indicates a couple of unidentified peaks, which may be the feature of the remaining unwanted organic/ inorganic compound. However, XRD plot of sodium sulphide (sample P2)-based PbS doesn’t show any unwanted peaks of any organic or inorganic compound, but the crystallinity is poor. Thioacetamide (sample P3)-based PbS spectra show a similar behaviour to sample P2. In comparison with samples PI, P2, and P3, thiourea-based PbS (sample P4) has a better crystalline structure with an average crystalline size of about 38 nm.

TABLE 17.2

Structural Parameters of PbS NC Materials Using Different Sulphur Sources

Sample Code

Crystalline Size (D) nm

Micro-Strain (e) x 10-2

Dislocation Density (p) L/m2

PI

47

4.34

4.3 x 1014

P2

26

7.24

1.4 x 1015

P3

29

7.26

1.2 x 1015

P4

38

5.55

7.6 x 1014

FTIR (Fourier Transform Infrared Spectroscopy)

In order to confirm the presence of organic or inorganic molecules, FTIR was used. FTIR spectra of the prepared PbS materials using four different sulphur sources are shown in Figure 17.3. Certain functional groups in a compound absorb definite

FTIR spectra of the prepared PbS materials using four different sulphur sources

FIGURE 17.3 FTIR spectra of the prepared PbS materials using four different sulphur sources: (a) Na,S (sodium sulphide), (b) C2H5NS (thioacetamide), (c) Na,S,0? (sodium thiosulfate), and (d) CH,N,S (thiourea).

(CONTINUED) FTIR spectra of the prepared PbS material using four different sulphur sources

FIGURE 17.3 (CONTINUED) FTIR spectra of the prepared PbS material using four different sulphur sources: (a) Na,S (sodium sulphide), (b) C,HsNS (thioacetamide), (c) Na2S20, (sodium thiosulfate), and (d) CH4N2S (thiourea).

frequencies, and hence, the compound can be easily identified. In all FTIR spectra, the strong peak near 1,550 cm-1 corresponds to the long alkyl chain. The peaks found at 650 to 625 cm-1 correspond to P=S band.

 
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