Excitation Energy Dependence of the Photoluminescence Quantum

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Excitation Energy Dependence of the Photoluminescence Quantum Yield of Core/Shell CdSe/CdS Quantum Dots and Correlation with Circular Dichroism Irina V. Martynenko, Anvar S. Baimuratov, Victoria Osipova, Vera A. Kuznetsova, Finn Purcell-Milton, Ivan D. Rukhlenko, Anatoly V. Fedorov, Yurii K. Gun'ko, Ute Resch-Genger, and Alexander V. Baranov Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04478 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017

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Chemistry of Materials

Excitation Energy Dependence of the Photoluminescence Quantum Yield of Core/Shell CdSe/CdS Quantum Dots and Correlation with CirCircular Dichroism Irina V. Martynenko,†,‡ Anvar S. Baimuratov,† Victoria Osipova,† Vera A. Kuznetsova,§ Finn PurcellMilton,§ Ivan D. Rukhlenko,†+ Anatoly V. Fedorov,† Yurii K. Gun’ko,§, † Ute Resch-Genger,‡,* Alexander V. Baranov† † ITMO

University, 49 Kronverksky Pr., St. Petersburg, 197101, Russia Federal Institute for Materials Research and Testing (BAM), Division Biophotonics, Richard-Willstaetter-Strasse 11, 12489 Berlin, Germany; [email protected]

‡

§ School

of Chemistry and CRANN, Trinity College Dublin, Dublin 2, Ireland

+ Monash

University, Clayton Campus, Victoria 3800, Australia

ABSTRACT: Quantum dot (QD) based nanomaterials are very promising materials for the fabrication of optoelectronic devices like solar cells, light emitting diodes (LEDs), and photodetectors as well as as reporters for chemo- and biosensing and bioimaging. Many of these applications involve the monitoring of changes in photoluminescence intensity and energy transfer processes which can strongly depend on excitation wavelength or energy. In this work, we analyzed the excitation energy dependence (EED) of the photoluminescence quantum yields (PL QYs) and decay kinetics and the circular dichroism (CD) spectra of СdSe/CdS core/shell QDs with different thicknesses of the surface passivation shell. Our results demonstrate a strong correlation between the spectral position of local maxima observed in the EED of PL QY and the zero-crossing points of the CD profiles. Theoretical analysis of the energy band structure of the QDs with effective mass approximation suggests that these structures could correspond to exciton energy levels. This underlines the potential of CD spectroscopy for the study of electronic energy structure of chiroptically active nanocrystals which reveal quantum confinement effects.

A quantum dot (QD) is a semiconductor nanoparticle with a size in the quantum confinement region that shows unique size-tunable optical properties including a large absorption cross section and narrow photoluminescence (PL) with a high quantum yield (PL QY). Therefore, these nanostructures are very promising materials for optoelectronic devices 1 such as solar cells, LEDs, photodetectors, and even qubits in future quantum computers 2. QDs represent also interesting optical reporters for chemical sensing 3, biosensing, and bioimaging 4. For most of these applications, PL intensities are measured to monitor changes in the local QD environment 5. The interpretation of changes in PL intensity or the comparison of measurements performed at different excitation wavelengths or energies can be hampered by a possible excitation energy dependence (EED) of PL QYs of QDs, particularly for high energy excitation 6. This can principally affect applications such as the monitoring energy transfer processes using QDs as donors or acceptors 7-10 or the use of QDs as optical reporters, studies of the blinking and charging dynamics of QDs as well as measurements of PL QYs of QD samples. Although there is some evidence for an EED of the PL QY of QDs, the occurrence and origin of this effect is still debated. Moreover, it can be affected by the size of PL QY value of the respective QD sample. Some groups did not observe an EED of PL QY of spherical QDs like core-only CdTe with PL QY between about 0.25 to 0.75 11 and CdSe cores for excitation wavelengths relatively close to the band gap 12 or claimed the absence of an EED for differently sized CdSe QDs despite hints

for it 13. Moreover, only recently, a unity PL QY was reported for CdSe/CdS nanorods excited at 405 nm, where most of the absorption occurred directly into the shell, thereby demonstrating complete shell-to-core energy transfer 14. Other research groups, however, noticed an EED of PL QY for CdSe, CdSe/CdS, CdSe/ZnS, CdSe/ZnS/CdSe, CdTe, and InP QDs, particularly a decrease in PL QY for excitation energies above the effective band gap 6, 15-21. Explanations for this observation range from an overestimation of the absorption of the samples at higher energies to surface mediated non-radiative deactivation of the excitons formed upon light absorption. Possible nonradiative decay pathways are the coupling of the charge carriers to the organic ligands on the QD surface as suggested by the Loomis group 22-24 or the opening of a continuum of higher energy, non-radiative states, which reduces the efficiency for relaxation of the charge carriers to the band edge as proposed by the Alivisatos group 6. A third explanation could be the existence of trap states hindering exciton diffusion, leading to a decrease of the shell-to-core exciton localization efficiency 12, 25. The latter can be especially important for nanomaterials with thick shells as recently shown by Geißler et al. in their study of EED of PL QYof a series of CdSe/CdS dot rods of different aspect ratios.12 Some research groups observed even local maxima and minima in the EED of PL QY of, e.g., CdSe core QDs 15, 17. The Loomis group speculated that these structures appeared when QDs are excited with energies in between electronic transitions. This encouraged us to analyze systematically the

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EED of PL QY of a set of СdSe/CdS core/shell QDs with different thicknesses of the surface passivation shell, using excitation wavelengths/energies covering the first excitonic absorption maximum of the core material, the onset of aborption of the shell material, and the lower lying energy states of the core material, respectively. Subsequently, the PL data were compared with circular dichroism (CD) spectra of these QDs obtained for the same QDs after exchange of the initial long alkyl chain surface ligands remaining from the synthesis for chiral D- and L-cysteine, with the overall goal to correlate the observed effects with the excitonic band structure of the QDs. These measurements are supported by a theoretical analysis of the energy band structure using the effective mass approximation. RESULTS AND DISCUSSION OPTICAL PROPERTIES CdSe nanocrystals stabilized with a mixture of ligands as oleylamine, oleate, and Cd oleate (referred to as long alkyl chain capped QDs in the following) with a diameter of 2.83 (± 0.4) nm were synthesized in toluene following an established method 26. These nanocrystals have a wurtzite crystal structure. A different number of a CdS shells was grown on the same CdSe core using a modification of the procedure reported by Mahler et al. 27. Subsequently, we refer to our CdSe/CdS core/shell QDs with n CdS shells as CdSe/nCdS QDs with n ranging from 1 to 5. The high-resolution transmission electron microscopy (HR-TEM) images of these samples presented in Figure 1 show that the CdSe and СdSe/nCdS QDs have approximately spherical shapes and a high degree of crystallinity. The mean diameters of СdSe/nCdS QDs vary in the range from 3.27(0.5) nm to 5.2(0.8) nm. More details regarding QD preparation and physico-chemical characterization are given in the Supporting Information (SI, section S1). The absorption and PL spectra of thoroughly purified СdSe/nCdS QDs in toluene are presented in Figure 1 (lower panels). The spectral position of the first exciton absorption and PL peaks of the CdSe core are 2.26 eV (548 nm) and 2.18 eV (568 nm), respectively. Both the absorption and PL bands shift substantially and continuously to lower energies with increasing shell thickness, as can be seen from Figure 1 (lower panels) as found by other groups 28. This shift is a consequence of the weakening of the confinement of the electrons caused by delocalization in the CdS shell 27. The emission bands of the CdSe/nCdS QDs are narrow and have a full width at half-maximum (FWHM) of about 100 meV. EED OF PL QY and PL DECAY KINETICS Subsequently, the PL QY of CdSe/nCdS were measured as a function of the excitation energy (E) varying from 2.1 to 3.5 eV. PL QY of the QDs were determined relatively using Rhodamine 6G in ethanol as quantum yield standard (PL QY of 95 %) 29. For the calculation of the PL QYs values, we used absorption factors 11, 30 instead of absorbances as done by other research groups 15. This circumvents a misleading decrease of PL QY with increasing absorbance of the QD samples at higher excitation energies (see also SI, section S2 for additional details of the sample preparation and data acquisition and analyses). The results are summarized in Figure 2. This figure reveals the typically low PL QY below 0.05 for the CdSe core as reported by other groups 12 and the expected increase in PL QY with increasing shell thickness across the entire excitation energy/wavelength range investigated. For example, with increasing shell thickness from 1 to 5 CdS shells, PL QY of CdSe/nCdS QDs increases by a factor of about 3 for excitation at 3.54 eV (350 nm). This trend, which is not

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yet fully completed with a 5 CdS shells as also reported by other groups 31, reflects mainly the decreased probability of hole diffusion to trap states, i.e., surface or ligand states, with increasing shell thickness due to hole localization in the QD core. As follows from Figure 2, for all CdSe samples, we observe a clear dependence of PL QY on excitation energy. Interesting are the different maxima and minima, the spectral positions of which being seemingly also affected by shell thickness (red shift with increasing shell thickness). As a general trend, PL QY reveals a pronounced decrease with excitation energy and levels off at about 2.7 eV for all CdSe samples. There are several explanations which may account for an EED of the PL QY. This includes the formation of nanoparticles from excess shell material, scattering, and QD aggregation up to a correlation with the QD band structure and exciton energy levels as suggested by other groups 15. Particularly the latter may explain the maxima and minima in the PL QY plots. The presence of nanoparticles formed from excess shell material remaining in solution from the shelling procedure could also contribute particularly to the shell thickness-dependent decrease of PL QY at higher excitation energies. The Greytak group showed recently that the nucleation and formation of particles from the shell material, here CdS, during the shelling procedure could account for a PL peak in the spectral region of about 450 nm.21 PL spectra of our CdSe/nCdS QDs measured in the spectral region from 360 to 650 nm (see SI, Figure S2), however, do not provide a hint for a significant contribution of the emission of CdS to overall PL. Although we cannot exclude the formation of nanoparticles from the shell material, the amount of separate CdS nanoparticles does not seem to be significant. Contributions from scattered excitation light could be ruled out since the absorption spectra of the CdSe/nCdS QD samples shown in Figure 1 (bottom left panel) did not shown hints for scattering below the first observed optical transition at the bandgap energy Eg.12, 13 QD aggregation can lead to a small red shift in absorption and can particularly affect the PL decay kinetics. In the case of QD aggregation, the close proximity of the nanoparticles in the QD aggregates, which favors QD-QD interaction, can result in energy transfer from smaller sized to larger QDs 32 as even nominally monodisperse QD populations contain a distribution of differently sized QDs. This can lead to a reduction of the PL lifetimes of the higher energy QD donors and an increase in the PL lifetimes of the low-energy QD acceptors 33. We subsequently recorded PL decay curves of our CdSe/nCdS QDs at different emission wavelengths/energies for excitation at 3.06 eV (405 nm). The emission was collected at the maxima of each PL band as well as in the low and high energy tails/sides of the PL bands, see also SI (Section S4). The resulting PL decay curves and average PL lifetimes are summarized in Figure S3 and Table S3 in the SI. The average lifetimes, recorded at energies within the low energy tail of the PL spectrum, were always smaller than those recorded at energies in the high energy tail. The most pronounced variation in PL decay kinetics are observed for CdSe/1CdS QDs with average PL lifetimes decreasing from 59.8 (±3.0) ns at 2.23 eV (555 nm) to 36.7(±1.8) ns at 2.10 eV (590 nm), respectively. This observation is in good agreement with the size-selection effect predicted by the Kelley group 34 and contradicts the behavior expected in the case of QD aggregation. Thus, we can rule out QD aggregation as a source of EED of PL QY of our CdSe/nCdS QDs.


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Hence, we attribute the decrease of PL QY at higher excitation energies mainly to surface mediated non-radiative relaxation pathways in agreement with conclusions of the Alivisatos and Loomis groups. These surface-mediated pathways may include relaxation to the continuum of higher-energy, nonradiative states 6 and the transfer of photogenerated charge carriers to the ligands 15. The fact that the observed decrease of PL QY at higher excitation energies is more pronounced for QDs with thicker shells (Figure 2) could be possibly caused by a contribution from surface trap states, hindering exciton diffusion and leading to a decrease of the shell-to-core exciton localization efficiency. As previously reported for a series of elongated CdSe/CdS QD/quantum rods with different aspect ratios, where the shell can considerably contribute to the absorption of the nanomaterial particularly for large aspect rods, this can increase the PL lifetimes at excitation energies where the shell material starts to absorb 12. Although these effects are expected to be only very moderate for our QD samples with their still relatively thin shells, we thus recorded the PL decay kinetics representatively for the QD sample with the thickest shell, CdSe/5CdS, at different excitation wavelengths. This included 400 nm (3.096 eV), where the CdS shell is expected to contribute to QD absorption at any case, 500 nm (2.476 eV), the onset of CdS absorption, as well as 550 nm (2.251 eV) and 560 nm (2.211 eV), where solely the CdSe core absorbs. The results are summarized in the SI (Figure S4) and do not reveal a change in PL decay kinetics with excitation wavelength. This agrees very well with the statement of the Loomis group, that the dynamics of the charge carriers in the emitting lowest-energy state of CdSe should be independent of excitation energy 15. ABSORPTION AND CD SPECTRA In order to obtain a better understanding of the nature of the modulations observed in the EED of PL QY (see Figure 2 and Figure 3, top panels) and derive a possible correlation with the exciton energy levels of the CdSe/nCdS QDs 35, 36, each absorption spectrum was fitted with a sum of Gaussian curves, with each curve corresponding to a particular exciton level. The spectral position of the maxima of the Gaussian curves were extracted from the second derivatives of the corresponding absorption spectra. The deconvolution procedure of the absorption spectra is given in the SI (section S5). The results of this fitting procedure, the absorption spectra of the QDs samples reconstructed from a sum of four Gaussian curves, are shown in Figure 3 (bottom panels). Comparison of the maxima in the PL QY plots with the results of the fitted absorption spectra suggest a correlation between these data; yet this cannot be regarded as real proof. Therefore, as additional hint for our assumption, we used ligand-induced circular dichroism (CD) for further clarification. This technique enables the analysis of the excitonic energy level structure of QDs36. It expoits chiral ligands to render intrinsically achiral nanocrystals optically active due to the modification of their electronic structure, particularly of the shell by ligand-induced distortions of the surface layers of the nanocrystals or hybritisation of holes and chiral ligand states 36-41. If the nanocrystal itself has a highly symmetric shape, its energy states are usually degenerate or almost degenerate and chiral ligands remove this degeneracy, yielding modified states of slightly different energies, i.e., | and | states. The optical transitions from the ground state |0 to these energy states can be observed in CD spectra as a characteristic modulations in intensity of the CD signal. Since CD is

proportional to the rotatory strength of optical transitions and these strengths have opposite signs for the transitions |0 ⟶ | and |0 ⟶ |, the CD curves cross zero at energies between these transitions 42. This has been described in an increasing number of CD studies of semiconductor nanostructures 36, 43-46. As a prerequisite for CD measurements, we performed a post synthesis ligand exchange using L- and D-cysteine as chiral ligands as previously reported 40. In order to confirm that this ligand exchange does not affect the spectral features of the absorption and emission spectra, as a first step, the normalized absorption and PL spectra of the CdSe/nCdS QDs before ligand exchange (capped with long alkyl chain ligands; solvent toluene) and after ligand exchange (D- or L-cysteine; solvent water) were compared (see SI, Figure S5). Apparently, ligand exchange and subsequent phase transfer into water do not change the absorption spectra of CdSe/nCdS QDs. Although ligand exchange and phase transfer lead to a reduction in PL QY as commonly observed for QDs synthesized in organic solvents 39, the PL spectra are blue shifted by only at maximum 35 meV (~5 nm) and the FWHM do not change. Thus, we conclude that ligand exchange does not strongly affect the energy positions of the exciton energy levels of the CdSe/nCdS QDs. The resulting CD spectra of the L- and D-cysteine-stabilized CdSe/nCdS QDs are presented in the middle panels in Figure 3. This figure reveals a pronounced opposite optical dichroism in the spectral region of the intrinsic QD excitonic transitions (2.1–3.0 eV). Moreover, the resulting CD spectra are clearly affected by shell thickness, with the strongest CD signals (regarding intensity and structural resolution) resulting for CdSe/1CdS.40 A subsequent comparison of the EED of PL QY with the CD and absorption spectra of the QDs reveals a rather good match of the spectral position of local maxima in the PL QY plots, the CD zero-crossing points, and the centers of the Gaussians curves used for fitting of the absorption spectra for the QD samples. It also reflects the red shift in these features with increasing shell thickness. Therefore, we can assume that these features provide a relatively good approximation of the energy positions of the excitonic transitions in our CdSe/nCdS samples. The energy positions of the excitonic transitions extracted from the PL QY, CD, and absorption measurements are shown in Figure 4(a), depicting the extracted energies of the excitonic transitions relative to that of the first excited state. For clarity reasons, we included also the four relevant energy transitions (panel b) and the energy band structure of CdSe/CdS core/shell QD (panel c) in Figure 4. THEORETICAL ANALYSIS Owing to the fact that the sequence of energy levels for CdSe QDs is wel`l known from kp theory for nanocrystals 35, we can correlate them with our results and assign 1Se1S3/2, 1Se 2S3/2, 1Se 1S1/2, and 1Pe1P3/2 transitions. The energy positions of the first four energy levels of the CdSe core QDs obtained by us agree very well with previous reports.35 CdSe/CdS core/shell nanocrystals present semiconductors with type I structure. The lowest energy states in such structures have holes strongly localized in the core and delocalized electrons (see Figure 4(c)). Figure 4(a) shows that the position of the first three energy levels relative to that of the first excited state does not vary with shell thickness. Since the size quantization of the holes remains practically unchanged for different shell thicknesses, this indicates additionally that these levels correspond to the transitions to one electron state, i.e., 1Se, from the first levels of quantization of the holes,



namely 1S3/2, 2S3/2, and 1S1/2. In contrast, the energy difference between the fourth and the first excited states decrease with shell thickness. This is a clear hint that the fourth transition corresponds to another electron state, here 1Pe. Obviously, this level is 1Pe1P3/2. In order to better illustrate this point, we fitted the PL QY data with a sum of Gaussian curves with centers located at the position of the previously obtained optical transitions. These fits are included in Figure 3 (top panels). This revealed several peaks with decreasing amplitudes in the spectral region of 22.8 eV. The centers of the Gaussians curves with the highest amplitudes at the fundamental 1Se1S3/2 transition correspond not to a local maximum but rather to a monotonic rise and fast drop of PL QY. We assume that excitation of the fundamental transition 1Se1S3/2 yields a strong increase in PL QY, but this maximum seems to be hidden in the PL QY data due to a size selection effect. The centers of the next two Gaussian peaks in the PL QY data match well with the positions of 1Se 2S3/2, 1Se 1S1/2 transitions and local maxima in the experimental PL QY data. In the lower energy region, the PL QY profiles have maxima, that correspond to different discrete levels of hole quantization. Obviously, the PL QY values arising from resonance excitation to the 1Se1S3/2, 1Se 2S3/2, and 1Se 1S1/2 levels, reapectively, are higher than those for nonresonant excitation, since the probabilities of transitions to the fundamental radiative state 1Se1S3/2 are higher. This is associated with rather high probabilities of intraband hole transitions 2S3/2⟶1S3/2 and 1S1/2⟶1S3/2 18, 24, 47, 48. Other discrete transitions such as 1Pe1P3/2 and other higher energy transitions practically do not show up as local maxima in the PL QY plots, because the mechanism of intraband relaxation of electrons and holes to the fundamental radiative state cannot effectively compete with non-radiative trapping for at increasing energies of the photogenerated electron-hole pairs. Therefore, the PL QY profiles exbibit a monotonous drop for higher energies. Consequently, the presence of defect and ligand states in core/shell QDs opens up nonradiative channels of energy dissipation, and for off-resonance excitation these channels effectively compete with transition to the radiative state 1Se1S3/2. We also speculate that there could exist an energy threshold, above which excitation leads to a decrease and saturation, i.e., a constant value of PL QY. This threshold could reflect the energies at which holes begin to delocalize from the core. The corresponding energy could be roughly estimated from our data. It exceeds the sum of the energy of the first level of quantization of electrons, i.e. 1Se, and the hole band offset Δ . The latter is 0.39 eV for CdSe/CdS as shown in Figure 4(c). Since the energies of the first level of quantization of the holes are significantly smaller than this offset, the energy of PL QY saturation was estimated as given in equation 1 (eq. 1). ≳ + Δ ≈ / + Δ ≈ 2.6 eV (eq. 1) This expression agrees rather well with the experimentally determined energy (approximately 2.7 eV) of the PL QY saturation derived from the PL QY plots in Figure 2. CONCLUSIONS We systematically assessed the excitation energy dependence (EED) of the photoluminescence quantum yields (PL QYs) of a set of СdSe/CdS core/shell quantum dots (QDs) with different shell thicknesses ranging from 1 up to 5 CdS shells in comparison with the absorption spectra of these QDs and CD spectra obtained with these QDs after introduction of the chiral ligands D- and L-cysteine. Analysis of these data together with the theoretical analysis of the energy band structure of our

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CdSe/CdS core/shell QDs demonstrated a correlation between the local maxima of the PL QY plots and the zero-crossing points of the CD spectra, which provide the spectral position of the exciton energy levels. This underlines the potential of СD spectroscopy to obtain in depth information on the exciton energy structure of QDs in ensemble studies. Our data can be utilized for optimizing the excitation wavelength of QD PL. Moreover, we believe that the presented results will contribute to new applications of CD spectroscopy in nanoscience and a further understanding of the nature of chiroptical activity in quantum confined nanomaterials. MATERIALS AND METODS Equipment Transmission Electron Microscopy (TEM) of QDs was performed using a Titan electron microscope (FEI) without aberration correction operating at a beam voltage of 300 kV. The UV/Vis absorption spectra were recorded using a UV-Probe 3600 spectrophotometer (Shimadzu). Circular dichroism (CD) spectra of the L- or D-Cys QDs were recorded with a JASCO J1500 (JASCO) spectrometer. Room temperature steady-state PL and photoluminescence excitation (PLE) spectra were obtained with Cary Eclipse spectrofluorometer (Varian). PL decays were recorded using a time-correlated single photon counting (TCSPC) fluorescence microscope MicroTime 100 (PicoQuant, Inc.) and a FLS920 fluorescence lifetime spectrometer (Edinburgh Instruments).

ASSOCIATED CONTENT Supporting Information: QD synthesis details. Materials used. Mean diameters, shell thicknesses and HTEM images and size distribution diagrams obtained from HTEM images. Details of PL QY measurements with different excitation energies. PL spectra of CdSe/nCdS QDs in the wavelength range of 360 nm to 650 nm. PL intensity decay analysis of QDs, PL intensity decays measurements as a function of detection energy. PL intensity decays measurements with different excitation energies. Details on the fitting procedure. Ligand exchange and phase transfer procedure. Absorption spectra and PL of CdSe/nCdS QDs after ligand exchange. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

ACKNOWLEDGE ACKNOWLEDGEMENT The authors gratefully acknowledge financial support from the Science Foundation of Ireland (Grant SFI 12/IA/1300), the Irish Research Council, the Ministry of Education and Science of the Russian Federation (Grant 14.B25.31.0002), and grant RE1203/17-1 from the German Research Council (DFG; MEranet project ICENAP). A.V.B. thanks the RFBR Project #1752-50004 for financial support. I.V.M. acknowledges also support from an Adolf-Martens fellowship granted by BAM.


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Figure 1. Structural and optical characterization of a series of five CdSe/nCdS QDs capped with long alkyl chain ligands remaining from the synthesis. (a) HR-TEM images; the scale bar equals 5 nm. Room-temperature absorption (b) and PL spectra (c) in toluene.

Figure 2. EED of PL QY of long alkyl chain-capped CdSe/nCd QDs in toluene.

Figure 3. Analysis of the EED of PL QY (upper panels; QDs capped with long alkyl chain ligands; solvent toluene), CD spectra (middle panels; QDs after ligand exchange; ligand D-cysteine: black curve; ligand L-cysteine: red curve; solvent: water), and absorption spectra (bottom panels; solvent toluene) of CdSe/nCdS QDs, respectively. In the upper panels, the measured PL QY data are represented by open circles and the fits of the EED of PL QY reconstructed from a sum of four Gaussian curves are given by black solid curves. In the bottom panels, the dashed curves give the measured absorption, with the four Gaussian curves reconstructing the respective absorption spectra being provided as solid blue curves; the solid black curves equal the corresponding reconstructed absorption spectra. Figure 4. (a) Energies of the first electronic transitions in CdSe/nCdS QDs relative to their first excited state obtained from fits of the absorption spectra, − / as function of the first excited state of CdSe/nCdS QDs obtained also from a fit of the absortion spectra / . The energy positions were extracted from the maxima of the PL QY plots (see Figure 2 and Figure 3, top panels), the zero-crossing points of the CD spectra (see Figure 3, middle panels), and the centers of the Gaussians curves used for fitting of the absorption spectra (see Figure 3, lower panels). (b) First four lowest energy transitions of CdSe/CdS core/shell QDs, the positions of which correspond to the PL QY maxima and zero-crossing points of the CD signals (blue lines). (c) Energy CdSe

band structure of CdSe/CdS core/shell QDs, where g is the energy gap of bulk CdSe and Δ( and Δ) are the energy offsets for electron and holes.


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(a)

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Wavelength, nm 660600 540 480 420

0

Wavelength, nm 600 550 500

(b)

(c) PL, arb. units

Absorption, arb. units

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.0

2.4 2.8 Energy, eV

3.2

0


2.0

CdSe CdSe/1CdS CdSe/2CdS CdSe/3CdS CdSe/4CdS CdSe/5CdS

2.2 2.4 Energy, eV

2.6

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Figure 2. EED of PL QY of long alkyl chain-capped CdSe/nCdS QDs in toluene. 68x55mm (600 x 600 DPI)



Figure 3. Analysis of the EED of PL QY (upper panels; QDs capped with long alkyl chain ligands; solvent toluene), CD spectra (middle panels; QDs after ligand exchange; ligand D-cysteine: black curve; ligand Lcysteine: red curve; solvent: water), and absorption spectra (bottom panels; solvent toluene) of CdSe/nCdS QDs, respectively. In the upper panels, the measured PL QY data are represented by open circles and the fits of the EED of PL QY reconstructed from a sum of four Gaussian curves are given by black solid curves. In the bottom panels, the dashed curves give the measured absorption, with the four Gaussian curves reconstructing the respective absorption spectra being provided as solid blue curves; the solid black curves equal the corresponding reconstructed absorption spectra. 81x37mm (600 x 600 DPI)


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Excitation Energy Dependence of the Photoluminescence Quantum

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