Semiconductor particles exhibit size-dependent properties such as an increase in the energy gap and a corresponding change in optical properties. Such novel properties of quantum dots (QDs) are widely recognized as a result of semiconductor QDs' size quantization and large surface-to-volume ratio compared to bulk materials [ 14 ]. These were obtained by the reaction of cadmium and selenium precursors in the presence of stabilizers tri-n-octylphosphine oxide (TOPO), trioctylphosphine (TOP) and.
The volume of the final solution in the reactor was then increased to 200.0 mL using distilled water. To obtain the structural information and particle morphology of CdSe nanoparticles, the products were centrifuged before TEM analysis. The shape and size of the gold nanowires depend on the adsorption of PVP on the surface of the gold particles [33].
This takes place through chemical interactions with the nitrogen and/or oxygen atoms of the pyrrolidone units of PVP [36]. A high concentration of PVP results in the formation of a thick coating on the entire surface of the spherical particles, including the twin boundaries [38]. The optical properties and structural information of the produced CdSe nanoparticles were obtained from UV-visible and TEM.
Increasing the capping agent concentration resulted in the formation of concentrated short rods in the solution.
PVP-Capped Au-CdSe Hybrid Nanoparticles
They further demonstrated that seeded nanocrystal growth offers a convenient way to design CdSe/CdTe, CdSe/CdS nanoheterostructures with complex shapes and morphologies by modifying the seed crystal structure [56]. The plasmon peak of Au-CdSe hybrid particles is red-shifted by 10 nm compared to pure AuNPs, which absorb at 530 nm. This may be due to the overlap of the electronic states of the different components of the hybrid particles, which change the surface plasmon resonance [63,65].
UV-visible absorption spectrum of PVP-capped Au-CdSe hybrid nanoparticles in 4.00 x 10-3 M gold nanoparticle solution. Alternatively, the shift may reflect that the Au part of the hybrid nanoparticles is partially covered with CdSe, which possesses higher refractive index than organic capping ligands. The presence of a material with a higher index of reflection is expected to shift the Au plasmon to longer wavelengths and has been observed.
These Au-CdSe hybrid nanocrystals stabilized by PVP were also analyzed for their structural properties and size using both transmission electron microscopy (TEM) and high-resolution TEM. The HRTEM micrograph of these metal-semiconductors shows the presence of lattice fringes in different orientations as seen in Figure 4.10. This is confirmed by the X-ray powder diffraction pattern of such nanomaterials as illustrated in Figure 4.11.
The XRD data show that the Au-CdSe hybrid nanoparticles are of good crystalline quality. L-cysteine capped Au-CdSe hybrid semiconductor nanocrystals were also synthesized and reported in this section. In the final step, the two solutions were mixed at different concentrations to produce Au-CdSe hybrid nanoparticles.
Figures 4.13 and 4.14 showed photomicrographs taken in the presence of 1.60 x 10-3 M gold precursor concentration in solution taken at different parts of the copper grid used and the corresponding XRD pattern. These images revealed well-dispersed nanoparticles with the presence of shaped nanocrystals confirming the presence of the absorption shoulder at longer wavelengths in the UV-visible spectrum. The hybrid Au-CdSe particles produced at this precursor concentration are fairly monodispersed and show a combination of morphologies with an average particle size of nm.
L-Cysteine Capped Gold Nanoparticles
XRD pattern shows the diffraction peaks that can be indexed to the features of Au and CdSe. The strong (111) peak for both the Au and CdSe is clearly observed in the XRD pattern. The band in cysteine at 1600 and 1390 cm-1 corresponds to the asymmetric and symmetrical stretching of COO-.
In addition, a weak band near 2550 cm-1 practically confirms the presence of S-H group in the cysteine molecule [70,71]. It is significant that the band due to S-H was not observed in the spectrum of cysteine-coated gold nanoparticles confirming the S-Au interaction. The second solution in the UV irradiation setup was composed of the precursor HAuCl4 and L-cysteine dissolved in distilled water.
The reduced selenium and CdCO3 were simultaneously added to the solution in the UV reactor. Immediately after this addition, the UV reactor was started to reduce the precursor with light and produce the Au-CdSe L-cysteine-coated metal semiconductors. The effect of precursor concentration on the synthesized metal semiconductors was studied as detailed in Table 4.2.
Figures 4.20 and 4.21 show the TEM micrographs of L-cysteine coated Au-CdSe hybrid nanomaterials at different precursor concentrations. As the precursor concentration in solution was increased to 8.00 x 10-4 M, clearly visible spherical shaped nanoparticles were observed. The increase in the precursor concentration led to an increase in the mean particle size to nm, Figure 4.20B.
A further increase in the precursor concentration to 1.20 x 10-3 M and 1.60 x 10-3 M resulted in well-dispersed medium-particle metal-semiconductor nanomaterials.
CONCLUSIONS
In these photomicrographs, the denser Au appearing as dark spots is clearly seen, which is surrounded by CdSe semiconductors covered with cysteine. From the X-ray diffraction data, it is clear that the synthesized CdSe semiconductor nanoparticles are highly polycrystalline and have hexagonal wurtzite structure. These hybrid nanoparticles show distinct quantum size effects and their optical properties can be fine-tuned.
UV-Vis of such materials showed gold absorption features for PVP-capped Au-CdSe, while very few features were observed in the spectrum for the cysteine-capped hybrid particles. Structural analysis revealed gold as a dark spot in TEM images with an average particle size of nm for PVP-capped hybrid nanoparticles and nm for cysteine-capped. High-resolution TEM clearly showed lattice fringes, confirming the crystallinity of the formed PVP-capped Au-CdSe hybrid nanoparticles, which was also evident from XRD.
The cysteine-coated AuNPs showed little absorption characteristics in their optical properties, while the TEM showed the predominance of spherically shaped particles with an average size of nm. Further characteristics of these nanoparticles were determined by the FT-IR which showed the stretching bands due to the presence of L-cysteine. Understanding these growth processes and the differences between similar material systems will provide a basis for the rational design of other hybrid nanomaterials and their inclusion in future technologies.
SUMMARY AND FUTURE WORKSUMMARY AND FUTURE WORK
SUMMARY AND FUTURE WORK
Summary
Recommendations for Future Work
List of Publications and Presentations List of Publications and Presentations List of Publications and Presentations List of Publications and Presentations.
