These questions are pertaining to the UV-Vis and Emission spectra analysis of Qu

Question

These questions are pertaining to the UV-Vis and Emission spectra analysis of Quantum Dots.

Synthesis and Characterization of CdSe Quantum Dot-Ward References Mailings Review Vie · A,'' 1. sin No Spac Heading 1 Heading 2 Title TitleSub Sub Quantum Dots Experiment c. Why does the absorbance at the band maximum increase with reaction time? d. Discuss why the absorption and emission bands shift to longer wavelengths with increasing reaction time e Explain why the color stops changing when the reaction is cooled ú e individual aliquots do not change color once removed) E Can you explan the observed differences in the temporal evolution of absorption and emission spectra as afunction of oleic acid concentration? g For each Cdse sample, why does the emission manmum lie at longe wavelength than the absorbance maximum? oe

Explanation / Answer

c.) The longer the reaction time, the bigger the particles.Now collabrating it with the absorbance phenomena By increasing the precursor concentration, the particle size would become larger due to higher metal ions concentration, thus allowing more nucleation and aggregation processes to take place in the formation of nanoparticles like silver.. In quantum physics, the absorption spectrum of metal nanoparticles may be described as a result of the intra-band excitations of conduction electrons from the lowest energy state to higher energy states within the conduction band of metal nanoparticles. This procedure is a necessity to ensure any absorption peaks produced from other radiochemical reactions would be discounted by the spectrophotometer system. The absorption maximum at 410-nm absorption spectrum shifted to lower wavelength with increasing dose owing to a decrease in particle size at higher doses. Consequently, the conduction band energy derived from the absorption minimum increased with increasing particle size owing to the quantum confinement effect.

d.) The absorption results indicate an increase in the absorption intensity with an increase in the reaction time and same in the case of emission spectra or in other words we can say that , this effect can be related to the increase of the particle size, which increases not only the absorbed light but also the scattered light. However, in photoluminescence spectroscopy, more scattered light means that less emitted light collected by the detector at an angle of 22.5 degree to the excitation beam. This can explain the inversion in the intensity of absorbed light and the emitted light.

e.) Thermochromism is the property of certain materials to change their own color in response to a change in temperature. Thermochromism actally lies under the umbrella of a chemical phenomenon called chromism – a process that causes color changes (usually reversible in nature, meaning they can regain their original color) in certain materials. We can simply connect these with that of the aliquots.

f.) The 1H NMR spectrum of ligands bound to a QD is different from a spectrum of the free ligands in solution. First, the linewidth of the resonances corresponding to bound ligands is broader than that of the free species. This is because the linewidth depends on the tumbling rate of a molecule in solution and, as larger molecules tumble more slowly than smaller ones, the tumbling rate of the bound ligands is slower than that of the free ligands. Moreover, due to a change on the chemical environment, resonances of the bound ligands show an increased chemical shift, compared to the resonances of the free ligands. The 1H NMR spectra of free oleic acid versus oleic acid bound to the surface of CdSe QDs, where this is clearly demonstrated.

g.) Since the occurrence of emission intermittencies (blinking) is a clear indication for the observation of a single emitter, and because blinking of quantum dots is still not fully understood, no measures to suppress or minimize blinking were done too. The single quantum dot excitation spectra can exhibit the main characteristics of a declining slope from shorter to longer wavelengths, and a peak close to the band edge transition, which we identify as the 1S(e)-2S3/2(h) transition. However, we find distinct differences in the individual spectra that can be attributed to individual photophysical properties of the analyzed single quantum dots as well as to the well-known transitions of single emitters to dark, non-emitting, states. Example: A typical example of an excitation spectrum obtained from a single quantum dot, In contrast to the ensemble spectrum, we see distinct dips and gaps in the single quantum dot excitation spectra, which in principle could either result from blinking or reflect the photophysical properties of the quantum dot.

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.