5-1 Electronic coupling in CdSe/CdS film
Though the samples under investigation are aqueous solution, we must consider the highly applications of samples in solid state. Films of colloidal quantu dots (QDs), often called QD solids, have recently experienced a significant increase in attention for the devices applications. Normally, the QDs are electronically coupled by removing or replacing the original bulky surfactants to decrease the inter-particle distances result in overlapping wave functions and increasing the mobility of charge carriers. In the final part of experiments, we studied the absorbance spectrum and transient absorption pump-probe spectrum of the Dithiothreitol (DTT) with CdSe/CdS core/shell QDs film.
This dithiol molecule was used to further study the functions of thiol group. Figure 5-1 shows the formula of DTT. The CdSe/CdS core shell QDs were surrounded by citrate and replaced by DTT. After four sizes of CdSe QDs with DTT were drop-casting on glass, we studied the room temperature absorbance. The films composed of QDs with first exciton transitions at 2.19eV, 2.28eV, 2.35eV and 2.36eV, corresponding to QD radii of 3.5nm, 3.1nm, 2.8nm, and 2.8nm, respectively.
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Figure 5-1. Showing the formula of dithiothreitol (DTT).
Figure 5-2 shows the room temperature absorbance of the 2.35eV QD film.
Obviously, the DTT treatment causes the lowest energy transition to red shift by
~380meV compared to untreated film. A red shift can arise from exciton delocalization, dipole-dipole interactions, which would lower the transition energy, or difference in dielectric screening in the films compared to solution [50]. Besides, the spectrum of the QD solids shows significant broadening with respect to the spectrum of the dispersion, but clearly still exhibits quantum confinement. Such a broadening could be due to increased polydispersity, disorder in dielectric environment resulting in a spatial variation of polarization energies or strong electronic coupling [51]. The first exciton transition peak was fitted by single Gaussian function. Table 5-1 displays the peak position and band width of pure QDs and DTT treated film in 4 different sizes. The high peak variation (red shift up to 500meV) suggested the strongly electronic coupling. A possible reason is that DTT with two thiol group at the ends of chain thus a DTT connected to two QD particles, enhanced QDs coupling. The schematic diagram shows in Figure 5-3.
HS
SH OH
OH
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Figure 5-2. Absorbance of liquid (solid line) and solid (dotted line) for CdSe/CdS QDs (Eg = 2.35eV) in citrate (black) and in DTT (blue).
Table 5-1. The peak position and band width of pure QDs and DTT treated film in 4 different sizes with peak variation.
CdSe solid CdSe@DTT solid
Size Peak
460 480 500 520 540 560 580 600
A b s(a .u .)
wavelength (nm)
pure QD530 in liquid pure QD530 on glass DTT with QD530 in liquid DTT with QD530 on glass
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Figure 5-3. The schematic diagram shows a dithol molecule DTT connect to two QDs enhanced QDs coupling.
Following we present a study of ultrafast electron and hole dynamics in coupled CdSe/ZnS QD solids, focusing on the first few picoseconds after excitation. We distinguish two separate rate relaxation processes that occur on this time scale: hot carrier relaxation from higher levels to the 1S electron and hole levels (intraband carrier cooling) and carrier relaxation due to hopping between different QDs (referred to as spectral diffusion). Figure 5-4 presents selected absorption transients for a QD / DTT film at a pump wavelength of 400nm and several probe wavelengths in the 1Sh-1Se profile. The probe wavelength dependence of the absorption transients for QD dispersions has been studied [52, 53]. In this plot, state filling (absorption bleach) and Coulomb interactions between multiple excitons (absorption shift), determine the transient absorption features. The strongly wavelength dependence, so-called biexciton shift, occurs both for hot carriers, directly after excitation, and for thermalized carriers.
This is due to a red shift of the optical transitions upon photoexcitation. There are still many unknown and interesting part of electronic coupling of QDs film needed to be studied. The related researches could be a promising theme for future work.
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Figure 5-4. (a)Transient absorption spectra at low pump intensity <Nabs = 0.5>
and various probe wavelengths near the 1Sh - 1Se transition for a QD / DTT film.
(b) Contour plot of the same data for the QD / DTT film. The black line in (b) denotes the ground state absorption maximum.
0 1 2 3 4 5
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5-2 Summary
In summary, we have demonstrated the function of short-chain thiol-containing molecules capping on CdSe/ZnS quantum dots (QDs) in determination of their optical and electrical properties by employing time -resolved photoluminescence, temperature-dependent PL, transient absorption, and steady-state fluorescence measurements. The steady-state fluorescence spectra provided the critical concentration of thiol/thiolate or coordination-type bonds /covalent-type bonds conversion. The ultrafast transient absorption confirmed the electron dynamics and surface passivation immediately after thiol molecules are added. From temperature-dependent PL, the activation energy band gap (exciton binding energy), and surface electronic state distribution could be investigated. Finally, the time-resolved PL examined the hole dynamics after thiolate was formed. We found that thiol interact with QD only by weaker coordination-type bonds through the sulfur lone-pair electrons and passivate the surface of QDs by preventing core electron from defect sites on the surface. Another stronger covalent-type bonds are formed when thiol turn to thiolate through long time incubation, and the new hole traps would be produced.
The schematic diagram was shown in Figure 5-5.
Different thiol-containing molecules were investigated and showed different performance. 1-propanethiol (NPM) exhibits little effect on the properties of QDs even though it is a short-chain thiol-containing molecule. One of possible reason is that other tested thiol molecules, β-Mercaptoethanol(BME) and 3-Mercaptopropionic acid (MPA) have smaller thiol pKa, which determines the ability to transform into thiolate. Thus, the formation of covalent bonds of 1-propanethiol (NPM) with higher pKa and QDs may be difficult. The second important reason would be the lack of second-order oxygen of NPM. Evidently, NPM without other lone-pairs compared to BME and MPA, cannot bind to QDs with coordination-type bonds effectively. Finally, the surface passivation
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of QD happens as long as the QDs are surrounded by negative charges. For electron-rich molecules such as thiol, dithiol or gel with lone-pair, surface passivation ability has been confirmed. The schematic diagram is shown in Figure 5-6.
Figure 5-5. Schematic diagram of thiols and QD surface interaction.
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Figure 5-6. Schematic diagram of QDs surrounded by sulfur lone-pairs and suppress blinking by preventing core electrons eject to the defect sites of surface.
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