This chapter addresses the transfer of electronic energy between CdTe colloidal quantum dots (QDs) using time-resolved photoluminescence (PL) spectroscopy. The efficiency of energy transfer in QDs depends not only on the spectral overlap of small dots emission and large dots absorption, but also on the inter-dot distances. The quenching of the PL intensity (lifetime) of small dots, as well as an enhancement of large dots in mixed solution and a solid film are evidence of a resonant transfer of energy due to dipolar coupling between proximal QDs. In a solid with mixed QDs, the stretching exponent β increases as the probe-energy declines, and approaches one, implying efficient energy transfer from smaller to larger QDs.
I. Introduction
As a nonradiative process, Förster resonant energy transfer (FRET) is driven by dipole-dipole coupling and transports excitation energy between semiconductor quantum dots (QDs) [1-3]. The efficiency of FRET depends on the degree of spectral overlap between the emission spectrum of the small QDs (donor) and the absorption spectrum of the large QDs (acceptor), and on the sixth power of the separation between the donor and the acceptor [1-3].
Bawendi’s group [4,5] was the first to demonstrate the FRET process between close-packed CdSe QDs. Crooker et al. elucidated FRET dynamics in monodispersed, mixed-size and layered assemblies of CdSe/ZnS QDs using time-resolved and spectrally-resolved PL [6].
They identified an enhancement in the luminescence and lifetime of large QDs, accompanied by a reduction in those of small QDs. These phenomena are direct evidence of the transfer of energy from small to large QDs. Recently, Franzl et al. investigated cascaded energy transfer
in a funnel-like structure [7] and observed a very efficient energy transfer process in layer-by-layer assemblies of oppositely charged CdTe QDs without any linkers between them [8]. Komarala et al. demonstrated a surface plasmon-enhanced energy transfer process between CdTe QDs that are close to gold nanoparticles [9].
Improved spectral overlap of emission from small dots and absorption by large dots, and reduced inter-dot distances between small and large QDs can improve the efficiency of energy transfer between QDs. This work explores the electronic energy transfer in CdTe QDs as a function of concentration of QDs. Moreover, a mixture of CdTe QDs in water and in a solid film was studied using photoluminescence (PL) and time-resolved PL (TRPL). Observations demonstrated that the mixed solid film with short inter-dot distance provides efficient dipolar coupling and electronic energy transfer.
II. Experiment
CdTe QDs with diameters of 2.3 nm (~ 1.2 × 10-4 mol．L-1) and 3.4 nm (~ 2.2 × 10-5 mol．L-1) were synthesized directly in water using mercaptopropionic acid (MPA) as a stabilizer [10]. The water-soluble CdTe QDs with diameters of 3.3, 3.7, and 4.0 nm were purchased from PlasmaChem. The QD concentration and diameter were calculated from the extinction spectra, as described elsewhere [10]. The room-temperature absorption spectra were recorded in air using a Cary 50 spectrometer (Varian). PL and TRPL were excited by a 300 ps pulsed laser diode (405 nm/2.5 MHz) at room temperature, and signals were dispersed using a Spex1403 double-grating 0.85 m spectrometer and detected using a (high-speed) photomultiplier tube. The decay traces were recorded using the time-correlated single photon counting approach (Time-Harp, PicoQuant). The overall temporal resolution was about 0.4 ns.
III. Results and Discussion
Figure 7.1(a) presents the PL spectra of 3.3 nm CdTe QDs in water at various concentrations, which ranged from 1 to 1.0 × 10-4 mol．L-1. Clearly, the emission peaks were redshifted and the asymmetrical linewidth became narrower as the concentration of the QDs increased. Similar experimental results were observed from 3.7 and 4.0 nm CdTe QDs. Figure 7.1(b) plots the PL peak energy against concentration. The energy shift over the entire concentration range is about 35 meV. The magnitude of the energy shift depends on the homogeneity of QDs and not on their size. According to Fig. 7.1(b), a marked change in the gradient of the energy redshift against concentration occurs at approximately 1.0 × 10-1 mol．
L-1. The above phenomena can be understood by considering the electronic energy transfer process, which quenches the emission from small dots, and the enhancement of emission from the large dots. The rapid redshift at high concentration occurs because the distance between the QDs dramatically falls as the density of the solution increases, promoting energy transfer.
Kagan et al. and Crooker et al. made similar experimental findings using CdSe QDs [5,6]. To confirm the energy transfer process, TRPL measurements were made. Figure 7.1(c) displays the dynamic evolution of the PL spectra for CdTe QDs (3.3 nm, 1.0 × 10-1 mol．L-1). Initially, the peak energy is at 2.20 eV; as time passes, it shifts to the red. Beyond 150 ns, the PL energy remains constant at about 2.18 eV, suggesting that the energy transfer process ceases and the probability of further transfer is suppressed.
This study also investigated the dynamics of electronic energy transfer between QDs by fabricating a mixed system of 50 % 2.3 nm (small) and 50 % 3.4 nm (large) CdTe QDs. In the mixed system, the small and large QDs acted as the donors and acceptors, respectively. Figure 7.2(a) and 7.2(b) present the room-temperature absorption and PL spectra of pure small (D) and pure large (A) QDs in water, indicating sufficient spectral overlap between the emission spectrum of donors and the absorption spectrum of acceptors, and a minor overlap between
their emission spectra. Figure 7.2(b) displays the PL spectrum of the mixed solution. The mixture exhibits an increase in the ratio of the PL intensities of the large to small QDs, which is attributable to the electronic energy transfer from the small to large QDs. The spectral overlap of two distinct QDs was excluded since it would reduce the intensity ratio. To verify the suggestion, Fig. 7.3(a) presents the TRPL spectra of pure donors and a mixture in solution monitored at peak (537 nm) energy. The decay rate of the mixture obviously increases over that of pure donors. However, in Fig. 7.3(b), the decay rate of the mixture recorded at the peak (618 nm) decreases, as compared with pure acceptors. The pronounced increase in donor decay rate and decrease in acceptor decay rate of the mixture can be observed by monitoring the blue (red) side of the donor (acceptor) emission, eliminating the PL crosstalk associated with spectral overlap. The multi-exponential PL decay for CdTe QDs results from a dispersion in trap energy levels, which originate from Te surface atoms [10,11].
To measure more quantitatively the decay dynamics, the decay curves are analyzed using Kohlrausch’s stretched exponential law [12],
( / )
( ) 0 t
I t = ⋅I e− τ β , (1) where β is the stretching exponent and τ is the exciton lifetime. The τ and β of the mixture, as obtained at 537 nm, fall from 19 to 16 ns and from 0.75 to 0.72, respectively, as compared with pure donors. Conversely, τ and β of the mixture, recorded at 618 nm, increase from 20 to 23 ns and from 0.72 to 0.77, respectively, as compared with pure acceptors. The decrease in PL lifetime and the intensity of the small QDs and the increase in those of the large QDs are direct evidence of electronic energy transfer from the small to the large QDs, eliminating the reabsorption effect, which would not increase the decay rate from small QDs [3,6].
Furthermore, the decrease in the β of donors and the increase in the β in acceptors reflect the transfer of excitons from donors to acceptors, enhancing the emission intensity of the acceptors and reducing the probability of nonradiative trapping.
Reducing the inter-dot separation would increase the efficiency of energy transfer between small and large QDs. Figure 7.2(c) presents the PL spectra of close-packed pure donors, pure acceptors, and mixed solid films. The PL peak energy of solids is redshifted from that of the mixture in solution [Fig. 7.2(b)] because of electronic energy transfer presented in Fig. 7.1. Additionally, a substantial increase in the PL intensity ratio of large to small QDs in the mixed solid over that in mixed solution is observed, revealing an efficient electronic energy transfer. To verify the proposed energy transfer process, the time-resolved and spectrally-resolved PL of mixed solid were obtained with an interval of 10 meV and a fixed period of exposure from 2.53 eV (490 nm) to 1.85 eV (670 nm). As shown in Fig. 7.4(a), the PL decay rate at the high energy side exceeds that at the low energy side. The lifetime at 2.27 eV (546 nm) of small dots is about 8 ns, which increases steadily with the dot size, reaching 27 ns at 1.95 eV (636 nm). The decay curves of the double natural logarithm versus the natural logarithm of PL, plotted in the inset of Fig. 7.4(b), yield the stretching exponent β.
A sharp increase in β from 0.57 (at 2.27 eV) to 0.93 (at 1.95 eV) is observed, reflecting efficient energy transfer from small to large QDs as well as a huge suppression of nonradiative recombination. Figure 7.4(b) plots the overall evolution of the TRPL results.
Initially after excitation, the emission peak of the acceptors is at approximately 610 nm. As time passes, the emission intensity of donors falls sharply and the emission from acceptors dominates the entire spectrum. Moreover, the emission peak of acceptors shifts dramatically towards the low energy side. Beyond 10 ns, the emission peak is roughly constant at 632 nm, revealing the suppression of further energy transfer. These experimental results are strong evidence of electronic energy transfer, which results from a dramatic decline in the inter-dot distance of the solid mixture.
Figure 7.5 shows true-color images of the pure small, pure large and mixed samples in water and solid films, excited by a 405 nm laser. The color directly reflects the spectral
changes in each sample. The images of pure small (large) QDs in both solution and solid films appear green (red). Energy transfer is responsible for the slight difference between the colors of the images of QDs in solution and those in the solid. The image of the mixed QDs in the solid is almost the same color (red) as that of the pure large QDs in the solid, but the image of the mixed QDs in solution is yellow or orange (a color between green and red), reflecting limited energy transfer. The above observations demonstrate that the electronic energy transfer efficiency of the mixed CdTe QDs in solid films exceeds that of the QDs in solution.
IV. Conclusions
In summary, this work elucidated the electronic energy transfer between CdTe QDs using time-resolved PL. As the concentration of CdTe QDs increases, a redshift in the PL peak energy is accompanied by a decrease in linewidth, because of the energy transfer from small to large QDs. The mixed CdTe QDs solid exhibits more efficient energy transfer than that in solution, because the inter-dot distance drastically declines as the mixture is dispersed in solid film. Additionally, a significant enhancement in PL intensity as well as in the stretching exponent β for large dots in mixed QDs reveals the suppression of nonradiative recombination.
References
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FIG. 7.1. (a) PL spectra of 3.3 nm CdTe QDs in water at various concentrations - 1 mol．L-1 (solid), 5.0 × 10-2 mol．L-1 (dashed), and 1.0 × 10-4 mol．L-1 (dotted). (b) PL peak intensity versus concentration of QDs. (c) Temporal evolution of PL spectra of QDs (1.0 × 10-1 mol．
L-1).
FIG. 7.2. (a) Absorption spectra of pure small (donor) and pure large (acceptor) CdTe QDs in water. PL spectra of pure donors (D), pure acceptors (A), and mixed (M) CdTe QDs (b) in water, and (c) solid.
FIG. 7.3. TRPL spectra of (a) donors in pure and mixed solution probed at 537 nm, and (b) acceptors in pure and mixed solution probed at 618 nm.
FIG. 7.4. (a) Dependence of TRPL measurements on probing energies of mixed CdTe QDs solid. (b) Time-resolved PL image of mixed CdTe QDs solid. Inset plots the TRPL data obtained at 2.27 eV (open squares) and 1.95 eV (open circles) on a double logarithmic scale.
FIG. 7.5. True-color images of pure small, pure large and mixed QDs in solution and solid.
Chapter 8 Conclusions
This thesis studied the pressure-dependent physical properties of ZnCdSe, ZnMnTe and n-type ZnSe:Cl thin films and CdTe colloidal QDs. The decay dynamics of isoelectronic traps in ZnSeTe semiconductors were also discussed. Raman scattering and PL experiments were performed to investigate the high-pressure behavior of Zn1–xCdxSe. Low-temperature Raman measurements indicate that ZnCdSe exhibits an intermediate phonon mode. The pressure-driven RRS effect was observed in samples with a high Cd concentration (x ≧ 0.18).
Both the Stokes and anti-Stokes sides of the LO phonons disappear as the crystal phase changes from the semiconductor to the metal. The pressure at the onset of the semiconductor-to-metal phase transition declined as the Cd content increased.
To examine the vibrational and crystalline characteristics of Zn1-xMnxTe, the RRS effect was induced under external pressure. The disappearance of the LO phonon, which accompanies a semiconductor-to-metal phase transition in ZnTe occurs at about 15.7 ± 0.2 GPa. As the Mn content increases from 0 to 0.26, the metallic phase transition pressure falls from 15.7 to 10.3 GPa. Based on the pressure-dependent LO and TO phonon frequencies and Grüneisen parameters (γi), an application of external pressure reduces the iconicity of the Zn1-xMnxTe compound semiconductors.
The vibrational, electronic, and crystalline properties of n-type ZnSe:Cl layers were also studied. The spectra are well-modeled by taking into account the phononlike coupled mode of the electron plasmons and the LO phonon. The Raman scattering efficiency and the dielectric function were calculated for the spectral lineshape fittings. The carrier concentrations obtained from the Hall and optical Raman measurements are mutually consistent. As the carrier concentration increases from 8.2 × 1015 to 1.8 × 1018 cm-3, the metallic phase transition
pressure declines from 13.6 to 12.5 GPa, suggesting that n-type doping tends to reduce structural stability. Moreover, high-pressure Raman measurements revealed the degradation of n-type behavior in ZnSe under compression, which is attributable to the emergence of deep donor-like states.
The decay dynamics of isoelectronic ZnSe1−xTex semiconductors were studied using time-resolved photoluminescence. The Kohlrausch law closely fits the decay curves. As the Te concentration is increased, the stretching exponent β initially decreases and then monotonically increases. This result, consistent with the increase in PL lifetime and linewidth, represents strong evidence of the transfer of excitons from shallow to deep Te trap states. On the Te-rich side, since the Se localized states lie above the conduction-band edge and the Te localized states hybridize with the valence-band states, the decay profiles of ZnSe1−xTex
exhibit short lifetimes and an increase in the stretching index β.
Finally, the electronic energy transfer between CdTe QDs was elucidated using time-resolved PL. As the concentration of CdTe QDs increases, a redshift in the PL peak energy is accompanied by a decrease in linewidth, because of the energy transfer from small to large QDs. The mixed CdTe QDs solid exhibits more efficient energy transfer than that in solution, because the inter-dot distance drastically declines as the mixture is dispersed in solid film. Additionally, a significant enhancement in PL intensity as well as in the stretching exponent β for large dots in mixed QDs reveals the suppression of nonradiative recombination.
Publication Lists
1. J. T. Ku, M. C. Kuo, J. L. Shen, K. C. Chiu, T. H. Yang, G. L. Luo, C. Y. Chang, Y. C. Lin, C. B. Fu, D. S. Chuu, C. H. Chia, and W. C. Chou, “Optical characterization of ZnSe epilayers and ZnCdSe/ZnSe quantum wells grown on Ge/Ge0.95Si0.05/Ge0.9Si0.1/Si virtual substrate”, J. Appl. Phys. 99, 063506 (2006).
2. Y. J. Lai, Y. C. Lin, C. B. Fu, C. S. Yang, C. H. Chia, D. S. Chuu, W. K. Chen, M. C. Lee, W. C. Chou, M. C. Kuo, and J. S. Wang, “Growth mode transfer of self-assembled CdSe quantum dots grown by molecular beam epitaxy”, J. Cryst. Grow. 282, 338 (2006).
3. M. C. Kuo, J. S. Hsu, J. L. Shen, K. C. Chiu, W. C. Fan, Y. C. Lin, C. H. Chia, W. C.
Chou, M. Yasar, R. Mallory, A. Petrou, and H. Luo, “Photoluminescence studies of type-II diluted magnetic semiconductor ZnMnTe/ZnSe quantum dots”, Appl. Phys. Lett.
89, 263111 (2006).
4. C. T. Yuan, Y. C. Lin, Y. N. Chen, Q. L. Chiu, W. C. Chou, D. S. Chuu, W. H. Chang, H.
S. Lin, R. C. Ruaan, and C. M. Lin, “Studies on the electronic and vibrational states of colloidal CdSe/ZnS quantum dots under high pressures”, Nanotechnology 18, 185402 (2007).
5. Y. C. Lin, C. H. Chiu, W. C. Fan, S. L. Yang, D. S. Chuu, and W. C. Chou,
“Pressure-dependent Raman scattering and photoluminescence of Zn1–xCdxSe epilayers”, J.
Appl. Phys. 101, 073507 (2007).
6. Y. C. Lin, C. H. Chiu, W. C. Fan, C. H. Chia, S. L. Yang, D. S. Chuu, M. C. Lee, W. K.
Chen, W. H. Chang, and W. C. Chou, “Raman scattering of longitudinal optical phonon-plasmon coupling in Cl-doped ZnSe under high pressure”, J. Appl. Phys. 102, 123510 (2007).
7. Y. C. Lin, W. C. Fan, C. H. Chiu, F. K. Ke, S. L. Yang, D. S. Chuu, M. C. Lee, W. K.
Chen, W. H. Chang, W. C. Chou, J. S. Hsu, and J. L Shen, “Pressure-induced metallization and resonant Raman scattering in Zn1-xMnxTe”, J. Appl. Phys. 104, 013503 (2008).
8. Y. C. Lin, W. C. Chou, W. C. Fan, J. T. Ku, F. K. Ke, W. J. Wang, S. L. Yang, W. K. Chen, W. H. Chang, and C. H. Chia, “Time-resolved photoluminescence of isoelectronic traps in ZnSe1−xTex semiconductor alloys”, Appl. Phys. Lett. 93, 241909 (2008).