To stabilize and improve compatibility and binding with the polymer films the synthesized (yellow) CdS and CdS-ZnS core-shell QDs were functionalized with 3- mercaptopropyl-methyl dimethoxysilane. To confirm the reaction proceeded successfully and the desired results were achieved, the QDs were characterized before and after ligand exchange (LE).
FTIR spectra were recorded to identify the functional groups present on the surface of the CdS and CdS-ZnS QDs before and after ligand exchange, as shown in Figure 3 - 14. Prior to ligand exchange, the dominant peaks corresponding to TOA alkyl groups can be observed at 2954 and 2920 cm-1 corresponding to the vibration of the asymmetric
stretching of the -CH3 and -CH2 functional groups, respectively. The peak at 2853
represents the symmetric stretching vibration of the -CH2 group. The peak at 1455 cm-1 is
characteristic of the symmetric and asymmetric bending vibration of the -CH2 and -CH3
groups, respectively, and the peak at 723 cm-1 corresponds to the rocking vibration of -
CH2.46,60
Figure 3 - 14: ATR FTIR spectra of CdS QD a) before and b) after ligand exchange. 600# 1100# 1600# 2100# 2600# 3100# Wavenumber*(cm-1)* 2930 2839 1186 1080 806 2954 2920 2851 1455 1266 1096 723
a) Before ligand exchange b) After ligand
exchange
After ligand exchange it can be seen that these peaks are effectively replaced with the dominant peaks corresponding to the main functional groups of MPTMO. These include peaks at 2930 and 2839 cm-1 belonging to the asymmetric and symmetric stretching vibrations of the -CH2 and -CH3 groups, respectively, the peak at 1191 cm-1 belonging to
the rocking vibration of -CH3, and peaks at 1087 and 813 cm-1 representing the
asymmetric and symmetric stretching vibrations of the S-O-Cbonds, respectively. The peak at 690 cm-1 can be attributed to C-Si bonds.46,60,64 Similar results were found for the
CdS-ZnS QDs before and after ligand exchange.
EDX elemental analysis of the QD samples before and after LE can be seen in Figure 3 - 15.
Figure 3 - 15: SEM-EDX analysis of CdS (top) and CdS-ZnS (bottom) QDs a)before and b) after Ligand Exchange
a) Before Ligand Exchange b) After Ligand Exchange
CdS
Before ligand exchange, the EDX measurements of the nanocrystals indicate the presence of Cd, Zn (only in the core-shell structure), S, P, C and O. After ligand exchange the traces of P, attributed to TOP used during synthesis, can no longer be detected. However, the presence of Si, characteristic to MPTMO is now present.
The approximate atomic ratios demonstrating the composition of the CdS QDs before and after ligand exchange, along with those of the CdS-ZnS core-shell QDs, can be seen in Table 3 - 4. Analysis of these ratios shows an increase in Si/Cd and O/Cd with a decrease in P/Cd. These findings are consistent with the successful replacement of the TOA/TOP coating the surface of the QDs with MPTMO.46
Table 3 - 4: Relative atomic ratios of QDs before and after ligand exchange.
Elemental Ratio Before LE After LE
CdS QDS Si/Cd 0.00 ± 0.00 0.17 ± 0.01 O/Cd 0.07 ± 0.03 0.23 ± 0.02 S/Cd 0.36 ± 0.00 0.49 ± 0.01 P/Cd 0.03 ± 0.00 0.00 ± 0.00 CdS-ZnS QDs Si/Cd 0.00 ± 0.00 0.10 ± 0.00 O/Cd 0.12 ± 0.03 0.19 ± 0.02 Zn/Cd 0.16 ± 0.03 0.14 ± 0.01 S/Cd 0.05 ± 0.01 0.60 ± 0.01 P/Cd 0.04 ± 0.00 0.00 ± 0.00
The absorption and emission spectra of the bare and core-shell QDs before and after ligand exchange can be seen in Figure 3 - 16. In both samples a decrease in intensity, broadening, and a redshift can be observed for the maximum absorption peak after ligand exchange. The PL emissions also experience a decrease in intensity and a red shift.
Figure 3 - 16: Absorption (top) and Emission (bottom) Spectra of a) CdS and b)
CdS-ZnSQDs before ( ) and after ( ) ligand exchange
Kalyuyuzhny and Murray reported that the surface exchange reactions with a variety of thiols resulted in a decrease in PL quantum yield as well as shifts in the absorption maxima to lower or higher energies.70 The red shift may also be caused by the loss of smaller QD particles during the purification process of repeated separation by
centrifugation and re-dispersion in toluene.46,70 A laboratory synthesized batch of QDs consists of a polydisperse collection of particles ranging in size and crystalline structure. The resulting spectrum displayed by a sample is an average of the entire range. The loss of smaller particles would decrease the minimum energy required by the QDs resulting in a shift of the maximum absorption peak to a higher (lower energy) wavelength.41 This is further proved by the reduction in the emission ‘tail’ of the core-shell QDs present prior to LE. The more Gaussian-shaped emission spectra resulting after LE indicates a smaller particle size distribution, consistent with the loss of smaller particles. Following the
290 390 490 590 690 Ab so rb an ce (a .u ) a) CdS 290 390 490 590 690 b) CdS-‐ZnS 450 460 470 480 490 500 PL I nt en si ty Wavelength (nm) CdS 400 410 420 430 440 450 460 Wavelength (nm) CdS-‐ZnS
ligand exchange reaction some of the capping ligands are irreversibly bound, whereas some bonds are reversible. During purification and sample preparation by dilution, the loosely bound surface ligands may dissociate. During each purification step, a fraction of ligands are lost resulting in a range of effectively capped QD particles with varying ligand functionality. 70 This will affect the resulting optical properties of the QDs. The
newly available surface site vacancies can become filled with surrounding solvent molecules negatively impacting the absorption and emission intensities of the QD samples. Therefore, a lower energy shift in the maxima of PL and UV-Vis peaks may be attributed to the number of purification steps.70