Excitation wavelength dependence of the

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Excitation wavelength dependence of the photoluminescence quantum yield and decay behavior of CdSe/CdS quantum dot/quantum rods with different aspect ratios† D. Geißler,

‡a C. Wu ¨ rth,

‡a C. Wolter,b H. Wellerb and U. Resch-Genger*a

The excitation wavelength (lexc) dependence of the photoluminescence (PL) quantum yield (FPL) and decay behavior (tPL) of a series of CdSe/CdS quantum dot/quantum rods (QDQRs), consisting of the same spherical CdSe core and rod-shaped CdS shells, with aspect ratios ranging from 2 to 20 was characterized. lexc between 400–565 nm were chosen to cover the first excitonic absorption band of the CdSe core material, the onset of absorption of the CdS shell, and the region of predominant shell absorption. A strong lexc dependence of relative and absolutely measured FPL and tPL was found particularly for the longer QDQRs with higher aspect ratios. This is attributed to combined contributions from a length-dependent shell-to-core exciton localization efficiency, an increasing number of defect states within the shell for the longest QDQRs, and probably also the presence of absorbing, yet non-emitting shell material. Although the FPL values of the QDQRs decrease at shorter wavelength, the extremely high extinction coefficients introduced by the shell outweigh this effect, leading to Received 3rd April 2017, Accepted 21st April 2017

significantly higher brightness values at wavelengths below the absorption onset of the CdS shell

DOI: 10.1039/c7cp02142a

example for the comparability of absolutely measured FPL using an integrating sphere setup and FPL

rsc.li/pccp

values measured relative to common FPL standards, and underline the need for a correction for particle scattering for QDQRs with high aspect ratios.

compared with direct excitation of the CdSe cores. Moreover, our results present also an interesting

Introduction Semiconductor nanocrystals consisting of a spherical quantum dot (QD) core and a rod-shaped shell, so-called quantum dot/ quantum rods (QDQRs) or dot-in-rods, are increasingly being used in the life and material sciences. QDQRs are applied as fluorescent reporters e.g., in bioanalysis and microscopy,1,2 and optoelectronic components in photovoltaics, light-emitting diodes (LEDs), and liquid crystal displays (LCDs).3–6 Such QDQRs can have different aspect ratios, i.e., different ratios of their length and thickness, and are commonly covered by a coordinatively bound shell of organic ligands controlling their a

Federal Institute for Materials Research and Testing (BAM), ¨tter-Str. 11, 12489, Berlin, Germany. E-mail: [email protected] Richard-Willsta b University of Hamburg, Institute of Physical Chemistry, Grindelallee 117, 20146, Hamburg, Germany † Electronic supplementary information (ESI) available: TEM images of the cores and QDQR series A and B (Fig. S1), decay curves of QDQR series A and B at different lexc (Fig. S2 and S3, respectively), single decay time components obtained from the multi-exponential fits (Table S1) and intensity-averaged decay times of QDQR series A and B at different lexc (Table S2). See DOI: 10.1039/c7cp02142a ¨rth are equally contributing authors. ‡ D. Geißler and C. Wu

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stability and dispersibility. A typical example are CdSe/ZnS or CdSe/CdS QDQRs bearing non-polar surface ligands like hydrophobic amines, phosphonic acids, TOP/TOPO, or mixtures of these ligands.7–9 QDQRs can be made water-dispersible following similar strategies as established for spherical QDs, i.e., via ligand exchange or encapsulation with amphiphilic polymers.1,10–14 Favorable of QDQRs are their higher molar extinction coefficients and photoluminescence quantum yields (FPL) often exceeding those of QDs.15,16 In addition, their emission is anisotropic or polarized, whereas the photoluminescence (PL) from QDs is isotropic.17,18 Hence, in the last years, QDQRs have increasingly replaced QDs, and many of the commercially available II/VI semiconductor nanocrystals (SCNCs) or materials incorporated into SCNC-based products are nowadays QDQRs with typical aspect ratios of up to about five.19,20 For QDs and QDQRs, the size of the spectroscopic key parameter FPL21 is particularly relevant as this value provides also a direct measure for particle and shell quality.22–24 Low FPL of QDs and QDQRs indicate insufficient surface passivation and the formation of energy states within the band gap that can act as trapping sites for photogenerated charge carriers. Within the last decades, the question if QDs and QDQRs show an

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excitation wavelength (lexc) dependence of FPL,21 has been addressed by several groups with controversial results.25–28 For example, a strong lexc dependence of FPL was observed by Hoheisel et al. for a series of CdSe QDs with radii from 0.9 nm up to 2.6 nm including a considerably diminished PL efficiency for excitation within the absorption continuum, i.e., to higher excited states relative to the band edge.25 Measurements of FPL of CdSe and CdSe/ZnS QDs dispersed in toluene from Hoy et al. revealed also the highest FPL for excitation just above the band gap.27 Knappenberger et al. obtained a strong lexc dependence of the fluorescence-blinking statistics of CdSe/ZnS core/shell QDs for ‘‘on’’ time distributions, with a reduction in ‘‘on’’ times for excitation above the band gap, whereas the ‘‘off’’ time statistics were insensitive to lexc.29 These findings were attributed to nonemissive trap states accessed by higher photon energy excitation. Furthermore, it was noticed for CdSe/CdS core/shell systems of various particle morphology (i.e., nanocrystal spheres, rods, and tetrapods), that the generic form of the PL excitation spectrum is controlled by the physical shape and resulting morphological variations in the quantum confinement parameters.30 In contrast, Tonti et al. did not find a significant deviation between PL excitation and absorption spectra of CdSe colloids of different sizes, and hence, no intrinsic lexc dependence of FPL, in line with ultrafast intraband relaxation processes.26 Also our studies of the PL behavior of differently sized CdTe QDs31 with a calibrated spectrofluorometer32 did not provide a hint for lexc-dependent FPL. A lexc dependence of FPL can principally arise from real material-, size-, and surface chemistry-dependent effects of the sample as well as from measurement-related pitfalls as FPL measurements are still regarded as challenging and error-prone even for transparent solutions, although relative and absolute optical as well as photothermal and nanocavity methods have been used for the determination of FPL of molecular and nanoscale emitters for decades.21,33 Also other SCNC-specific factors related to the number and energetic position of surface states and their surface chemistry, morphology, and constitution,31 can affect FPL. For example, in the case of QDQRs, the competition between exciton diffusion through the elongated shell to the core and exciton trapping within the shell can lead to an exciton localization efficiency that depends on the length of the QDQR and can affect FPL.28 Moreover, colloidal systems are prone to particle-to-particle variations in core and shell size, incomplete shell growth, and different numbers and types of ligands per particle (both during synthesis and on the final QDQR).23,34 This can result in different fractions of more or less dark, yet absorbing particles in a QDQR ensemble. Especially for thick shell QDs and QDQR with high aspect ratios, also the synthesis-specific presence of different species of varying FPL including smaller clusters or non-emissive nanoparticles originating from shell material can account for a lexc dependence of FPL. QD- and QDQR-specific properties, which can affect the reliability of FPL measurements are the particularly strong absorption of QDQRs at shorter wavelengths and adsorption/ desorption equilibria of weakly bound surface ligands.10,11 This can introduce an apparent concentration-dependence of FPL due to inner filter effects at high concentrations35 and/or the

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shifting of ligand adsorption/desorption equilibria towards ligand desorption at low particle concentrations,31,36–38 resulting in decreased FPL. This can be circumvented using optical cells of different path lengths.31 Also photobrightening, especially upon UV excitation using high excitation power densities can be encountered.16 Moreover, QDQRs can reveal a considerable emission anisotropy especially for larger aspect ratios; requiring the use of polarizers (magic angle conditions) to prevent polarization-related artefacts,18 and the consideration of scattered excitation light,26,27 particularly for QDQRs of larger aspect ratios can become mandatory. Furthermore, for all data relying on comparisons of PL excitation and wavelength dependent absorption factors, the wavelength dependence of the spectral photon flux reaching the sample (excitation correction curve) and its uncertainty,31,39,40 must be properly considered. Other measurement artefacts leading to an apparent lexc dependence of FPL can arise from scattered excitation light.27 This encouraged us to assess the FPL and PL decay behavior of two CdSe/CdS QDQRs series of varying aspect ratios between ca. 2–20 at different lexc in comparison to the initially synthesized CdSe cores used for QDQR preparation. The diameters of the cores were 2.7 nm for QDQR series A and 3.6 nm for QDQR series B. lexc were chosen to cover the first excitonic absorption band of the core material, the onset of absorption of the CdS shell, and the region of predominant shell absorption. In order to assure the absence of any systematic errors arising from FPL measurements, we employed a calibrated custom-made integrating sphere setup,39,41 which enables the use of very small sample volumes to minimize inner filter effects and reabsorption, and performed relative FPL measurements using previously validated quantum yield standards.33

Experimental Materials Cadmium oxide (CdO, 99.998%, Puratronics) was obtained from Alfa Aesar; n-trioctylphosphine (TOP, 97%) and n-trioctylphosphinoxide (TOPO, 99%) were obtained from ABCR; hexylphosphonic acid (HPA, 100%) and octadecylphosphonic acid (ODPA, 100%) were obtained from PCI Synthesis; sulfur (S, 99.998%) and selenium (Se, 99.99%) were obtained from Sigma Aldrich; chloroform (99%), n-hexane (98.2%), methanol (100%), and toluene (100%) were obtained from VWR; rhodamine 6G (Rh6G) and rhodamine 101 (Rh101) were obtained from Lambda Physics; UV-spectroscopic grade ethanol and n-hexane for the spectroscopic measurements were purchased from Sigma-Aldrich. If not stated otherwise, all chemicals and solvents were applied without further purification. Methods QDQR synthesis. Two different CdSe/CdS QDQR series (series A emitting at ca. 570 nm, series B emitting at ca. 610 nm), both having various rod length in the range from 10–100 nm (i.e., different aspect ratios), were synthesized from two differently sized CdSe cores (A-core and B-core) according to the hot-injection


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seeded growth approach reported by Carbone et al.,41 yielding strongly luminescent QDQRs in n-hexane. The different sizes of the CdSe samples core-A and core-B were obtained by using different reaction times before cooling down the flask (core-A: 10 s, core-B: 45 s). The length of the CdSe/CdS QDQRs was controlled via the amount of CdSe cores applied during shell synthesis. Quantum yield measurements. FPL of the CdSe cores and CdSe/CdS QDQRs were measured both absolutely and relatively. Absolute FPL measurements were carried out with an integrating sphere setup (cf. Instrumentation) at various lexc ranging from 400 nm to the first excitonic absorption band of the QD cores and QDQRs (up to 565 nm). As an optical density (OD) of approx. 0.1 is needed for the absolute FPL determination, two different dilutions were prepared for each QDQR: a stronger diluted sample to obtain an OD of ca. 0.1 in the spectral range of the shell absorption at ca. 400–500 nm, and a less diluted sample to obtain an OD of ca. 0.1 in the spectral range of the core absorption (i.e. the first excitonic absorption peak) at ca. 500–600 nm. Relative FPL measurements were performed according to the procedure previously published by us.33 FPL of the sample (FPL,x) was calculated from the integral emission photon fluxes of sample and standard (Fx and Fst), the absorption factors of sample and standard (fx and fst), the PL quantum yield of the standard (FPL,st), and the refractive indices of the solvents used for sample and standard (nx and nst) at the mean/average emission wavelength using eqn (1).33,39 FPL;x ¼ FPL;st

Fx fst ðlexc Þ nx2 Fst fx ðlexc Þ nst 2

(1)

QDQR series A was measured relative to rhodamine 6G (Rh6G, FPL = 0.91 in ethanol33) at an excitation wavelength of lexc = 500 nm, and QDQR series B was measured relative to rhodamine 101 (Rh101, FPL = 0.915 in ethanol39) at lexc = 530 nm. The samples were diluted to an OD of approx. 0.05 at the respective lexc, to minimize reabsorption effects. FPL was calculated from the integrated blank-corrected and spectrally corrected emission spectra of the QDQR samples and FPL standards according to eqn (1) using the different refractive indices of the employed solvents (n = 1.375 for the QDQR samples in n-hexane, n = 1.364 for the FPL standards in ethanol).33 Instrumentation Transmission electron microcopy (TEM). TEM images were recorded at the University of Hamburg using a JEOL JEM-1011 TEM, LaB6, operated at 100 kV. Absorption and emission spectroscopy. Absorption spectra were recorded on a calibrated Cary 5000 UV-Vis-NIR spectrometer (Varian, Agilent Technologies) with a spectral bandwidth and step size of 1 nm. The accuracy of the intensity and wavelength scale of this instrument is regularly controlled with certified absorption standards (Hellma). Steady-state and time-resolved emission measurements were performed on a FLS920 fluorescence spectrometer (Edinburgh Instruments). So-called magic-angle conditions were applied


(excitation and emission polarizers set to 01 and 54.71, respectively) to render detected emission intensities independent of sample emission anisotropy.18,42 For the steady-state measurements (emission spectra) the device-intern xenon lamp was used for sample excitation, whereas for the time-correlated single photon counting (TPSPC) measurements (PL decays) a pulsed SC400-PP supercontinuum fiber laser (Fianium) with a pulse width of o1 ns was used as excitation light source. All presented emission spectra were corrected for blank emission (blank correction) and the wavelength dependence of the instrument’s spectral responsivity (spectral correction).33,39 The instrument response function (IRF) was measured with a non-emissive scatterer (Ludox silica particle dispersion), and the PL decay curves were fitted with a deconvolution fit using the software FAST (Edinburgh Instruments). Integrating sphere setup. The integrating sphere setup consists of an integrating sphere with a diameter of 15 cm, coated with Spectraflect (Labsphere; sphere reflectivity of about 97% in the visible to near-infrared),39 coupled via a fiber bundle to a Czerny–Turner imaging spectrograph (Shamrock 303i, Andor Technology), which in turn was attached to a Peltier cooled (183 K) thinned backside illuminated deep depletion charge-coupled device (CCD array; 1024 256 pixel, Andor Technology) for spectrally resolved detection. A 450 W xenon lamp coupled to a single monochromator was employed as excitation light source, and a reference detector was used to account for fluctuations of the incident spectral radiant flux. As diffuse scattering of emitted photons in the integrating sphere results in a complete loss of polarization information, no polarizers are necessary. Measurement conditions. All spectroscopic measurements (absorption spectra, PL spectra, decay times, and quantum yields) were performed with air-saturated solutions at T = (25 1) 1C using (10 10) mm quartz cuvettes (Hellma).

Results and discussion The two spectroscopically assessed CdSe/CdS QDQR series A and B, both having similar rod diameters and various rod length of approx. 10, 25, 50, and 100 nm, were synthesized from two differently sized CdSe cores (A-core and B-core). The sizes of the cores and QDQRs as measured with TEM are summarized in Table 1 (representative TEM images are shown in the ESI† in Fig. S1), and their absorption and emission spectra are shown in Fig. 1. Relative FPL measurements were carried out at a single lexc, as wavelength-dependent measurements relative to a FPL standard are only feasible in a narrow wavelength range (approx. from the shoulder to the maximum of the reference dye absorption band). Coverage of a broader wavelength range would have required a series of different FPL standards for each lexc, and different dilutions of samples and FPL standards for every lexc. The relatively measured FPL were corrected for scattering, which is most prominent for larger QDQRs with lengths of 50 nm and 100 nm (see scattering background at wavelengths above the first excitonic absorption band in the absorbance spectra in Fig. 1), thereby preventing an apparent increase in the measured


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Table 1 TEM data of the CdSe cores and CdSe/CdS QDQRs investigated in this work. The two series A and B have similar rod length, but vary in the size of the QD core initially used for QDQR synthesis, and thus, in their emission wavelength

QDQR

Diameter/nm

Length/nm

Aspect ratio

A-core A-10 A-25 A-50 A-100

2.7 4.6 4.6 4.6 4.5

0.3 0.4 0.4 0.5 0.4

— 14.0 23.0 43.7 85.5

1.8 2.3 6.9 11.4

— 3 5 9.5 19

B-core B-10 B-25 B-50 B-100

3.6 4.9 5.0 4.9 4.9

0.3 0.4 0.6 0.4 0.5

— 12.9 26.1 49.8 91.6

0.3 1.5 2.5 4.2

— 2.6 5 10 18.6

absorbance, and thus, an overestimation of the sample absorption factor fx (see Experimental section, eqn (1)). Particle scattering was considered by fitting theoretical scattering curves to the scattering background of the QDQR absorbance spectra at wavelengths above their first excitonic absorption band (where only scattering occurs), and by subtraction of these scattering curves from the measured absorbance spectra (see Fig. 2). The scattering curves were calculated assuming a

Rayleigh-like mln behavior, with the exponent n ranging from 4 (pure Rayleigh scattering, only valid for very small spherical particles with d { l) to 1 (for larger and/or non-spherical particles),43,44 and using the factor m to adjust the theoretical curve to the scattering background of the QDQR absorbance spectra. This scatter correction is, however, error-prone and induces additional uncertainties, as the theoretical scattering curves can only be estimated from the wavelength regions were the QDQRs do not absorb. Moreover, the scatter correction does not account for multiple scattering, which increases the effective optical path length within the particle sample, and hence, the sample absorption, in a non-linear manner.45 The resulting relatively determined FPL of both QDQR series at similar lexc are summarized in Table 2, including measured (uncorrected) FPL and scatter-corrected FPL. Table 2 contains also FPL data absolutely measured with an integrating sphere setup, which are not affected by particle scattering.45 The results from absolute FPL measurements at different lexc ranging from 400 nm to the first excitonic absorption band of the QDQRs (up to 565 nm; in 15 nm steps) of the QD cores and QDQRs are also displayed in Fig. 3. It becomes obvious from Table 2, that particle scattering strongly influences relative FPL measurements, as the deviations

Fig. 1 Absorption spectra (top) and emission spectra (bottom) of the two QDQR series A (left) and B (right) and the respective QD cores, from which the QDQRs were synthesized. The insets within the absorption spectra display a magnification of the QDQR and core absorption spectra (normalized to unity) at the first excitonic absorption bands.



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Fig. 2 Absorption spectra of the QDQR series A (solid lines), demonstrating exemplarily the increasing scattering background with QDQR size. For the FPL determination, the measured absorption spectra were corrected by the theoretical scattering curve (doted lines) to obtain the scatter-corrected absorption spectra (inset).

Table 2 Comparison of relatively and absolutely measured FPL (Frel PL and rel Fabs PL , respectively) of the two QDQR series. FPL of QDQR series A and B was measured relative to rhodamine 6G (lexc = 500 nm) and rhodamine 101 (lexc = 530 nm), respectively, and corrected for particle scattering as described in the text. Fabs PL of QDQR series A and B was measured with an integrating sphere setup at lexc = 490 nm and lexc = 535 nm, respectively (see also Fig. 3)

QDQR

Frel PL (meas.)

Frel PL (corr.)

Fabs PL

A-10 A-25 A-50 A-100

0.43 0.46 0.17 0.05

0.43 0.49 0.26 0.09

0.44 0.45 0.25 0.13

B-10 B-25 B-50 B-100

0.32 0.63 0.28 0.57

0.36 0.64 0.48 0.67

0.40 0.67 0.70 0.70

between measured and scatter-corrected FPL increase significantly with increasing QDQR length, i.e., aspect ratio, and thus, the scattering intensity of the QDQR samples. Moreover, even the scatter-corrected FPL differ notably from absolutely measured FPL. This underlines the previously mentioned problems of our scatter correction approach. As follows from Fig. 3, the CdSe cores reveal the expected low FPL below 0.05 that are clearly independent of lexc. This agrees well with the behavior of purified core-only CdTe colloids previously reported by us.31 In the case of the CdSe/CdS QDQRs, an increase of the aspect ratio seems to introduce a lexc dependence of FPL, with FPL decreasing at shorter excitation wavelengths (i.e., at wavelengths below the absorption onset of the CdS shell at around 500 nm). This lexc dependence becomes particularly pronounced for the longest QDQRs with the highest aspect ratios (A-50 and A-100 as well as B-50 and B-100). This FPL decrease may arise from an increasing number of defect states at the core/shell interface or within the shell with increasing QDQR length, favoring non-radiative deactivation of the excitons,8


Fig. 3 Excitation wavelength-dependent FPL of the two QD cores and both QDQR series measured absolutely with an integrating sphere setup. As expected, the CdSe cores reveal very low FPL without any lexc dependency. The CdSe/CdS QDQRs in turn display reasonably high FPL, yet with a decrease at shorter lexc (i.e. below the absorption onset of the shell material). This FPL decrease is especially pronounced for the QDQRs with the longest shells (i.e. largest aspect ratios).

or from the presence of non-emitting material that absorbs within the same wavelength range as the shell, e.g., CdS nucleates formed as side product during shell growth.46,47 Another possible explanation are trap states hindering exciton diffusion, leading to a decrease of the shell-to-core exciton localization efficiency with increasing QDQR length.48,49 To identify the mechanism(s) leading to the excitation energy loss, and thus, to the observed wavelength-dependent FPL, the PL decay times (tPL) of the QDQRs were determined at three different lexc. Thereby, the samples were excited at 450 nm (mainly shell absorption), 500 nm (region with similar core and shell absorption), and 550 nm (core absorption only). A non-radiative deactivation of the excitons at defect states at the core/shell interface or within the shell should decrease the measured FPL and tPL of the QDQR samples at lexc below the absorption onset of the shell. In contrast, the presence of absorbing, yet non-emitting material should decrease the measured FPL of the QDQRs at shorter lexc, but would not affect tPL. Thus, the comparison of FPL and tPL enables the distinction between differently bright QDQRs accounting for the observed lexc dependence, and its origin from dark nanocrystals or clusters. A length-dependent shell-to-core exciton localization efficiency, in turn, would lead to a decrease of FPL and an increase of tPL with increasing QDQR length.28,49 All QDQR samples display multi-exponential PL decay kinetics, as typical for most SCNCs. The measured PL decays could be sufficiently fitted with either a bi- or tri-exponential decay time function (depending on the quality of the fits), and the corresponding amplitude-weighted (averaged) decay times (htPLiamp) were calculated for better comparability. Here, htPLiamp was used instead of the intensity-weighted decay times (htPLiint), as htPLiamp is proportional to the overall emission intensity, and thus, relevant for processes that are described via


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Table 3 Amplitude-averaged decay times htPLiamp of the QDQR series A and B upon excitation at 450 nm, 500 nm and 550 nm

Sample

htPLiamp/ns (450 nm)



A-10 A-25 A-50 A-100

19.1 24.2 31.0 28.6

15.8 19.2 19.9 19.9

16.4 18.6 19.1 17.0

B-10 B-25 B-50 B-100

12.1 18.8 23.5 21.0

10.7 14.1 13.4 14.6

10.3 13.3 12.6 13.4

their (relative) emission intensities, such as the investigated energy loss mechanisms.50 The obtained htPLiamp values of the QDQR series A and B at three different lexc are summarized in Table 3. The measured decay curves (Fig. S2 and S3), the single decay time components, and fractions of the multi-exponential fits (Table S1) as well as the intensity-weighted decay times htPLiint (Table S2) are given in the ESI† for comparison. The tPL of both QDQR series reveal no significant differences upon excitation at 550 nm, at which predominantly the CdSe cores absorb, and 500 nm, the onset of CdS shell absorption, although FPL is already significantly decreased in this wavelength range (see Fig. 3), especially for the QDQRs with the highest aspect ratios. This suggests that the FPL changes between excitation at 500 nm and 550 nm are mainly caused by non-emitting species, such as CdS nucleates, or other dark nanocrystals or clusters absorbing at the same wavelengths as the shell material. For excitation at 450 nm, where predominantly the CdS shell absorbs, the QDQRs display significantly longer tPL compared with excitation at 500 nm and 550 nm. Moreover, tPL also increases with QDQR length up to 50 nm. Hence, the low FPL values at this short excitation wavelength are mainly caused by a decreased shell-to-core exciton localization efficiency, resulting from the competition between exciton diffusion through the elongated CdS shell to the CdSe core and exciton trapping within the shell.28 However, the tPL values of the longest QDQRs (A-100 and B-100) at 450 nm excitation are again slightly shorter compared to the tPL of the 50 nm QDQRs (A-50 and B-50). This is attributed to an increased number of defect states for the longer rods, as the probability of defect states increases with the shell length of the QDQRs. However, non-radiative deactivation channels can be prevented by proper QD synthesis strategies. For example, it was recently shown that a second slow-injection growth step of the CdS shell can improve QDQR performance by increasing the uniformity and crystallinity of the shell, resulting in highly effective energy transfer from the shell to the core, and thus, in near-unity FPL also upon shell excitation.51 The brightness (or brilliance) of a fluorophore, defined as product of its FPL value and its molar decadic extinction coefficient e at the excitation wavelength, determines the analytical sensitivity from the material and/or sample side, and thus, is an important criterion for the choice and comparison of fluorescent reporters. Besides the FPL decrease at shorter lexc, QDQRs can be still exceptionally bright emitters when excited at these


Fig. 4 Excitation wavelength-dependent, relative brightness values of the two QD cores and both QDQR series, calculated as product of the wavelength-dependent FPL (shown in Fig. 3) and the normalized absorbance spectra (shown in Fig. 1, top).

wavelength, if the gains resulting from their extraordinary high extinction coefficients (increasing with shell length) outweigh the losses due to the FPL decrease (increasing with shell length, as well). To test this, the relative wavelength-dependent brightness values B(l) of the QD cores and QDQR series A and B were calculated according to eqn (2). B(l) = FPL(l)A(l)norm

(2)

As the molar extinction coefficients e(l), that are typically used to calculate the (absolute) brightness values, are unknown for the given QDQR samples, the scatter-corrected, normalized absorbances A(l)norm were employed instead (normalized to unity at the first excitonic absorption peaks at around 560 nm and 600 nm for QDR series A and B, respectively). The obtained relative brightness values deviate by a constant, yet unknown factor from the absolute brightness values, and enable a relative comparison of QDQR performance. The lexc-dependence of the relative brightness values is displayed in Fig. 4. Although the measured FPL values of the QDQRs decrease at shorter wavelength, the extremely high extinction coefficients of the shell outweigh this effect, leading to significantly higher brightness values at wavelengths below the absorption onset of the CdS shell compared with direct excitation of the CdSe cores. This ‘‘FPL boost’’ is so dominant, that the longer the shells of the QDQRs, i.e. the higher the aspect ratios, the higher the brightness values at short lexc, although FPL is already significantly decreased by the absorption of non-emissive species.

Conclusion and outlook The photoluminescence quantum yields (FPL) and decay times (tPL) of CdSe/CdS quantum dot/quantum rods (QDQRs) with various aspect ratios ranging from 2 to 20 were measured at different excitation wavelengths (lexc). Absolute FPL measurements revealed a lexc dependence, with FPL decreasing by a factor of up to 3 when excited at the wavelength region of predominant shell


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absorption at ca. 400–450 nm compared with excitation at the first excitonic absorption band of the core at ca. 500–550 nm. This lexc-dependent decrease of FPL is most prominent for the longest QDQRs with the highest aspect ratios, indicating involvement of the shell material in this effect. In order to identify the origin of the wavelength-dependent FPL, the tPL were measured at 450, 500, and 550 nm, displaying a lexc-dependent behavior, as well. A comparison of FPL and tPL revealed that this lexc dependence can be attributed to three different effects: (a) the presence of absorbing, yet non-emitting shell material, which is most prominent at the onset of the shell absorption around 500 nm; (b) a QDQR length-dependent competition between exciton diffusion through the shell to the core and exciton trapping within the shell, which is most prominent for longer QDQRs at wavelength below the onset of shell absorption at 400–450 nm, and (c) an increasing number of defect states within the shells of the longest QDQRs. Our comparison of relatively and absolutely measured FPL values reveals, that for scattering particle dispersion like QDQRs of large aspect ratios, FPL should strictly be measured absolutely with an integrating sphere setup. Otherwise, uncertainties in the order of about 60% or even more can occur. In addition, the FPL of QDQRs cannot be measured simply relative to standard dyes at an arbitrary lexc, as QDQRs can display lexc-dependent FPL values due to the factors derived in the previous section. Hence, FPL measurements at different lexc are recommended, using an integrating sphere setup. Moreover, the synthesis of QDQRs should be optimized to minimize the amount of non-emitting side products and the number of shell defects during shell growth, e.g. by using a second slow-injection growth step of the CdS51 and/or by using better controllable flow reactor synthesis instead of batch synthesis.52,53 The success of these optimization procedures to reduce defect states could be monitored via electron paramagnetic resonance (EPR) and photoluminescence spectroscopy, as has been demonstrated by Erdem et al. for ZnO nanocrystals.54,55 Nevertheless, the extremely high extinction coefficients of QDQRs outweigh the decrease of FPL at shorter excitation wavelengths and considerably contribute to their strongly increased brightness.

Acknowledgements URG gratefully acknowledges financial support by the M-era-Net project ICENAP (German Research Council, DFG), DG by the EU project INNANOPART (EMPIR program) and HW and CWo by SPP 1313 Biological Responses to Nanoscale Particles (DFG). Moreover, financial support from CAN GmbH is acknowledged.

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