Aerosol-Assisted Chemical Vapour Deposition of Lead Chalcogenide Thin Films from [Pb((SeP i Pr 2 ) 2 N)(S 2 CNHexMe)]

In this study, [Pb((SeP i Pr 2 ) 2 N)(S 2 CNHexMe)], a novel complex, was synthesised by reﬂuxing [Pb(S 2 CNHexMe) 2 ] and [Pb((SeP i Pr 2 ) 2 N) 2 ] complexes for 2 hours in chloroform and precipitating with methanol. The microelemental analysis, FT-IR, and NMR spectroscopies were performed on the complex and the melting point was determined. The crystal structure of this new compound has been determined. The structure showed a distorted square pyramidal geometry with 2 sulphur and 2 selenium atoms coordinating the central atom at the base of the pyramid and a lone pair occupying the axial position of the lead atom. The complex was used to deposit lead chalcogenide thin ﬁlms by the aerosol-assisted chemical vapour deposition (AACVD) method at 300, 350, 400, and 450 ° C. The thin ﬁlms matched with the powder X-ray diﬀraction (pXRD) pattern for cubic PbSe with a (200) preferred orientation at all the deposition temperatures. The scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDAX) indicated that cubic PbSe crystals were formed.

When the nanocrystals have sizes that are about a tenth of the bulk material's Bohr exciton diameter, the electrons and holes can easily move within a thin organic film, which facilitates the transportation of charges between nanocrystals due to strong electronic coupling [4,[6][7][8][9]. To date, PbSe and PbS nanocrystals are being studied as potential Schottky solar cells [10,11]. e nanocrystals of PbSe and PbS with similar bandgaps are reported to generate large open-circuit voltage and short-circuit current, respectively [12][13][14][15][16]. PbS x Se 1−x as a ternary alloy could potentially lead to better-engineered particles to optimize both voltage and carrier transport simultaneously [17].
Ternary lead sulphide selenide (PbS x Se 1−x ) nanocrystals and thin films [4,[18][19][20] have not been studied extensively relative to PbS and PbSe [21][22][23][24][25][26][27][28]. Nanocrystalline semiconductor alloys give an alternative route to quantum confinement effects and bandgap tuning and thus produce new materials with unique properties [29,30]. e variable composition of alloys makes bandgap tuning possible while maintaining their size. e single-source precursor route to the synthesis of semiconducting nanoparticles and thin films is preferred over the dual and multisources because they are cleaner and less toxic and have the right stoichiometry because of the existing bonds in the single-source precursor [1,28]. In this work, [Pb((SeP i Pr 2 ) 2 N)(S 2 CNHexMe)] complex was synthesised and characterized and used as a single-source precursor to deposit lead chalcogenide thin films using the aerosol-assisted chemical vapour deposition (AACVD) method. In the AACVD method, the precursor is dissolved in a suitable solvent and then carried as an aerosol by means of an ultrasonic humidifier with an inert carrier gas onto the substrate. e choice of solvent is critical for the aerosol formation, however, the precursor solution does not need to be volatile [31]. e AACVD method is simple and has many advantages such as the use of nonvolatile precursors, easy deposition of doped thin films, and easy control of precursor proportions when a mixture of precursors is deposited. It also eliminates issues such as blocking of carrier gas tubes, which occurs in atmospheric pressure chemical vapour deposition (APCVD) and other CVD techniques [31]. Figure 1 shows a schematic diagram of the AACVD setup.

Materials and Methods
All the chemicals used for the syntheses were procured from Sigma-Aldrich (U.K) and used as received. e synthesis of the iPr2P(Se)NHP(Se)iPr2 ligand was done using the procedure reported by Cupertino et al. [32].

Instrumentation. A Flash 2000
ermo Scientific elemental analyser was used for the microelemental analysis. A Stuart melting point apparatus (SMP 10) was used to determine the melting point. Mettler Toledo TGA/DSC analyser was used for the thermogravimetric analysis (TGA) under nitrogen from 30-600°C at a heating rate of 10°C min −1 . e powder X-ray diffraction (pXRD) analysis was performed on a Bruker AXS D8 diffractometer with Cu-Kα radiation (λ �1.5418Å).
e samples were scanned from 10°to 85°with a step size of 0.05°and 5 s dwell time.
e surface morphologies of the samples and elemental compositions were determined with the Philips (FEG) XL 30 SEM. A Bruker 400 NMR spectrometer was used for all NMR measurements.

Crystal Structure Refinement.
A slow evaporation process was used to obtain single crystals of [Pb((SeP i Pr 2 ) 2 N (S 2 CNHexMe)] in a 1 : 2 mixture of chloroform/methanol. e crystal structure was determined with a Bruker Smart Apex diffractometer having a Mo Kα X-ray source and a CCD collector. e structure was calculated using SHELXTL package version 6.10. A full-matrix least-square on F2 was used for the refinement using direct methods. All the hydrogen atoms were allotted isotropic thermal parameters at calculated positions. Anisotropic atomic displacement parameters were used to refine other atoms.

Results and Discussion
e structural refinement and crystal data are listed in Table 1. Direct methods were employed to solve the structure.
e hydrogen atoms were placed in calculated positions with C-H lengths of 0.95 (CH), 0.99 (CH 2 ), and 0.98 (CH 3 )Å. All the Uĩso (H) values were fixed at 1.2 Ueq (C) except for CH 3 where it was 1.5 Ueq (C). e nonhydrogen atoms were refined anisotropically. Figure 2 shows the crystal structure of the complex. e lead atom has a distorted square pyramidal geometry with 2 sulphur and 2 selenium atoms coordinating the central atom at the base of the pyramid and a lone pair occupying the axial position of the  lead atom. Each selenium atom is directly bonded to a phosphorus atom just as is found in the [Pb((SeP i Pr 2 ) 2 N) 2 ] complex and the sulphur atoms are bonded to the ipso carbon just as is found in the [Pb(S 2 CNHexMe) 2 ] complex. e complex has a monoclinic crystal structure with P2 (1)/c space group. e bond lengths and bond angles selected are shown in Table 2. e bond lengths for Pb-S were found as 2.6707 and 2.9515Å. ese are significantly different from each other. It can be inferred from the bond lengths that one S atom forms a stronger bond to the Pb atom than the other. e structure also indicates that the Pb-Se bond lengths are similar to the Pb-S.
is suggests that the bond strengths of Pb-S and Pb-Se may not be significantly different from each other. Pb-Se bond lengths in the complex were similar to the Pb[(SeP i Pr 2 ) 2 N] 2 complex [33]. e average Se-Pb-Se bond angle for the complex is 97.33°which was close to that of the Pb[(SeP i Pr 2 ) 2 N] 2 complex of 96.05° [33]. is indicates that the preformed bonds in the parent compounds were not significantly affected by the formation of the new complex.

ermogravimetric Analysis.
e TGA thermograms of e onset and endset temperatures of the first decomposition step were 271.97°C and 308.81°C, respectively. e percentage weight at onset was 99.75%, having lost 0.25% as volatile substances. e weight loss at the endset of the first decomposition step was 20.38%. is may be due to the loss of the [CNHexMe] fragment and some sulphur gases [34]. e decomposition was slow at the first step with a decomposition temperature of 297.92°C.
ere was a weight loss between the first and second decomposition steps of 15.24%. is may result from the thermal instability of the complex between those temperatures. e onset and endset temperatures of the second decomposition step were 367.60°C and 382.81°C, respectively, with a corresponding weight loss of 32.13%. is was due to the loss of the [ i Pr 2 PNP i Pr 2 ] fragment. e decomposition of the second step was fast with a decomposition temperature of 381.53°C. e residue obtained at 600°C was 32.0 while the expected one for PbSe 1−x S x was 32.7%. e percentage residue was very close to the expected, indicating the suitability of the complex as a single-source precursor for the production of ternary lead chalcogenide.

3.3.
pXRD e pXRD for the thin    films deposited mainly matched with standard lead selenide peaks. e results indicate that sulphur is lost in the deposition process. is suggests that the C-S bond is relatively stronger than the Pb-S bond. As a result, upon thermal decomposition, there is a breaking of the Pb-S bond leading to the formation of the [S 2 CNHexMe] moiety, which volatilizes in the decomposition process [35]. ere was a slight shift of the peaks from the standard PbSe peaks from 300-350°C and, in some cases, at 400°C. Earlier reports on the formation of PbSe 1−x S x from SSPs were possible when the mole fraction of the sulphurcontaining complex was greater than 0.5 [19,20]. However, in this experiment, the complex contained equal fractions of sulphur and selenium. Stacked pXRD spectra of the thin films at various temperatures are shown in Figure 4.

SEM Micrographs of the in Films.
e SEM micrographs of thin films deposited at 300°C, 350°C, 400°C, and 450°C ( Figure 5) show cubic lead chalcogenide crystals for the deposits at all the temperatures. Generally, as the temperature increased from 300 to 450°C, the surface coverage of the thin films also increased. During a deposition process, certain properties of the thin films change. For example, the morphology can change depending on a range of factors which include deposition time, temperature, and solvent. For this research, all deposition parameters were constant except for temperature. erefore, the changes in morphology could be attributed to the changes in temperature. At 300 and 350°C, the particle sizes decreased from 400 nm to 200 nm and further decreased to 150 nm at 400°C. ere was an increase of particle size to 250 nm at 450°C. Although this trend is not normal, as it is expected that particle size increases with increasing temperature, it can be observed from the SEM micrographs that the particle formation was irregular at the lower temperatures and may have accounted for the irregular particle sizes. e trend, however, became normal at higher temperatures due to agglomeration and formation of regular-sized particles. At 400°C, there were some truncated cubes which could have formed from the agglomeration of smaller cubes in a nonspecific pattern. e extent of irregularities in the morphology is very prominent at 450°C. Shapes such as flakes, triangles, truncated cubes, and rectangles were observed in addition to the well-resolved cubes. e EDAX quantification on the thin films ( Figure 6) indicated the formation of PbSe (52%:48%) at all the deposition temperatures. e thin films were lead rich, which is similar to our earlier reports [24,25,34,36,37].

Conclusion
A new complex [Pb((SeP i Pr 2 ) 2 N (S 2 CNHexMe)] suitable for making lead chalcogenide nanoparticles and thin films has been successfully synthesised and characterized. e crystal structure of the complex showed a distorted square pyramidal geometry with two sulphur and two selenium atoms forming the base and a lone pair at the axial position of the central atom. e complex was used as a single-source precursor to deposit thin films by AACVD. e thin films deposited were cubic PbSe at all the deposition temperatures.

Data Availability
e corresponding author will make the supplementary data available upon request. e crystal structure has been deposited in the Cambridge Crystallographic Database with CCDC number 1918617.  Advances in Materials Science and Engineering 5

Conflicts of Interest
e authors declare that they have no conflicts of interest.