Article pubs.acs.org/JPCC
Properties of Self-Assembled Hybrid Organic Molecule/Quantum Dot Multilayered Structures Eyal Cohen,† Michael Gruber,‡ Elisabet Romero,‡ Shira Yochelis,† Rienk van Grondelle,‡ and Yossi Paltiel*,†,§ †
Department of Applied Physics, §Center for Nano-Science and Nano-Technology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel ‡ Department of Physics and Astronomy, Faculty of Sciences, VU University Amsterdam, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands ABSTRACT: Hybrid nanostructures are attractive for future use in a variety of electronic components. Self-assembled hybrid organic/nanocrystals can couple quantum properties to semiconductor working devices and modify their functionality. For example, light absorption in some core quantum dot (QD)-based self-assembled detectors induces a large dipole. These dipole moments may change the current of a shallow field effect transistor on which the QDs’ layer is assembled. In order to improve the absorption and quantum efficiency of such devices, multiple self-assembled layers are used. In such layers the charge transfer, excitonic energy transfer, surface trap passivation, and oxidation mechanisms are not fully understood. Therefore, establishing new tools for characterizing multilayer devices is essential. In the present work we have used confocal fluorescence microscopy to examine the photophysical properties of multilayered self-assembled organic molecules and QDs. We studied the interlayer coupling and surface-related mechanisms. By changing the coupling molecules and using photoinduced processes that gradually change the system, we were able to compare different binding groups and molecules under different conditions. Importantly, we found that in layered structures the binding group greatly contribute to the electronic coupling and luminescence, a contribution stronger than the linker’s molecular length. Consequently, we propose that proper illumination of core QD-based detectors be used for activation and yield improvement.
1. INTRODUCTION Nanostructures are likely to become primary components of future electronic devices. Yet the practical realization of roomtemperature operating quantum devices faces a number of major challenges. Breakthroughs are required before a new generation of quantum electronics can be achieved. Devices based on self-assembled hybrid organic quantum dots (QDs) using specific linker molecules attached represent a possible route to accomplish this goal.1,2 This approach provides many opportunities and may open the way for developing new quantum nanoengineered devices. The organic molecules can exhibit new transport properties.3−5 For example, in selfassembled detectors with layers of QDs, these properties together with the use of QDs, generate large dipole fields after light absorption. This dipole moment can change the current of a shallow field effect transistor on which the QD layers are assembled.6 Often multiple self-assembled layers are used to improve the device’s absorption quantum efficiency.7,8 Developing new tools for characterizing multilayer devices is therefore important to better understand and exploit the architecture, composition, and charge/energy transfer properties of the hybrid systems. Optical measurements could be used to indirectly probe the charge transfer through the organic molecular layers by exciting the layers and measuring the fluorescence emission from the hybrid system. © 2014 American Chemical Society
Semiconductor quantum dots are known to exhibit changes in their photoluminescence (PL) properties under continuous, above-band gap excitation;9 under wet atmospheres, photoluminescence enhancement (PLE) is observed, followed by decay on a long time scale. A number of processes, all concerning possible ways of passivating surface traps, have been suggested as possible causes for this enhancement,10−14 and it is likely that the interplay of several processes contributes to the total PLE phenomenon. Another probable factor affecting the PLE is the “photoactivation” of an initially dark fraction of the dots.15,16 The eventual decay of the dots’ PL is caused by the photooxidation,10,17,18 which gradually degrades quantum yield (QY), as well as reduces the effective size of the dots. This size reduction also causes a blue-shift in PL, in accordance with the tighter quantum confinement of the excitons. In an ensemble of molecularly linked QDs, all the above phenomena are strongly influenced by the layer’s organization and the molecular ordering. In the present work we have used confocal fluorescence microscopy and photoinduced processes to study multilayered Received: August 3, 2014 Revised: October 10, 2014 Published: October 17, 2014 25725
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Figure 1. (a) Structures of the different organic linking molecules used. In addition, the TOPO ligand originally shelling the dots is shown. (b) Absorption and emission spectra of core-type CdSe quantum dots, in toluene solution. The peaks are at 583 and 604 nm, respectively. (c) Schematic illustration of the layered samples.
Figure 2. (a) PL spectra of CdSe dots linked by MPS under continuous illumination of 7000 W/cm2, under open air conditions. Enhancement and consecutive decrease of PL intensity are apparent, as well as blue shift of the peak, as seen from the black line representing PL central emission wavelength. (b) Total PL intensity Enhancement of CdSe dots linked by MPS under continuous illumination, for different illumination powers, under open air conditions. Intensity is normalized by the initial intensity (at t = 0) for each power.
The self-assembled molecules with the silane headgroup have a strong tendency to polymerize by forming Si−O−Si bonds.23,24 Therefore, the BTB molecule, with its two silane functional groups, seems to construct a structure composed of only a few quantum dots, isolated from each other by several layers of the linker molecule. This arrangement directly affected the collective effects, as compared with all other samples (see section 3). 2.2. Experimental Methods. A home-built confocal fluorescence microscope was used to measure the samples. The experimental setup was described in detail earlier.25 A diffraction-limited area was excited using a tunable Ti:sapphire laser (Coherent MIRA 900F) coupled to an optical parametric oscillator (Coherent MIRA OPO) to provide 200 fs pulses at a 76 MHz repetition rate. Wavelengths of 400 and 550 nm were used for excitation, over a wide range of intensities. Emission from the sample was collected by either a CCD camera for spectral measurements or an APD connected to time-correlated single photon counting electronics (PicoHarp 300) to obtain fluorescence lifetimes.
self-assembled QD interlayer coupling and surface-related mechanisms, for different organic molecules and linkers.
2. EXPERIMENTAL METHODS 2.1. Sample Preparation. The QDs were adsorbed by wetchemistry techniques using the layer-by-layer (LbL) method.8,19 More specifically, fused quartz substrates were consecutively immersed in organic molecules and in QD solution, to form a covalently bound multilayered structure of dots interconnected by several different linker molecules.20−22 Figure 1a shows a representation of the four different linker molecules used. Those included two alkyl-based molecules with a different number of moieties, namely, the shorter (3mercaptopropyl)trimethoxysilane (MPS) and the longer (11mercaptoundecyl)trimethoxysilane (MUS), and two benzenebased molecules with different head groups, 1,4-Bis(triethoxysilyl)benzene (BTB) and 1,4-benzenedimethanethiol (BDT). A sample containing the dots with their original trioctylphosphine oxide (TOPO) ligands, was also prepared. A total of 12 consecutive alternating dips in molecules/dots were done in order to obtain a dense structure of layered quantum dots. We used CdSe core QDs and CdSe/ZnS core−shell QDs (Lumidot by Sigma-Aldrich), both with a peak emission at around 610 nm (see Figure 1b). The measured samples, consisting of the 12-dot layers, had an optical density of 0.05− 0.07 at the first absorption peak. The density of the QDs was estimated to be about 1011 QDs/cm2 per layer. An exception is the sample with the BTB molecules, which had a considerably lower absorption, owing to its lower density of dots (see next). The inhomogeneity broadening was around 100 meV.
3. RESULTS AND DISCUSSION Figure 2a shows a representative example of the PL emission spectra for a selected sample, as it evolves under constant illumination for 15 min. During those time scales, laser instability fluctuations did not increase over several percents. Data are normalized to the initial intensity. The sample, consisting of multilayered core CdSe quantum dots with MPS linking molecules, was illuminated at 400 nm. The total PL from the sample as a function of illumination time is presented 25726
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in Figure 2b, for various average illumination intensities, ranging from 70 to 13150 W/cm2. During this illumination at energy higher than the QDs’ band gap, a 40-fold enhancement in emission was observed at the lowest excitation power. An increasing amount of intensity decline appears for increasingly higher excitation powers. The local PLE is observable for many hours after the enhancement illumination. Both enhancement and decay are insignificant when the sample is under dry N2 as opposed to open air atmosphere. We did not manage to observe direct evidence for PL activation of single dark dots−dots that are initially nonfluorescent.15 However, experiments on bright single quantum dots in diluted samples exhibited only a small amount of enhancement of less than 100%. This enhancement is not enough to fully explain the total enhancement factor measured on the ensemble samples. It is therefore plausible that activation of dark dots11 plays a major role in PLE in our samples. This mechanism seems to be present, in addition to processes that photopassivate surface traps. It is clear that the two processes of enhancement and decay of PL do not scale similarly with illumination power; the decay process depends more strongly on laser power. The enhancement process seems to reach saturation at high excitation powers. Passivation of surface traps could occur due to adsorbed water molecules (from a humid environment)10 or by a rearrangement of surface ligands.11,12 Both of those processes can be of a limited rate due to the dense molecular structure at the interdot space. This is supported by a comparison of the enhancement rate as a function of illumination power, for two of the linker molecules, the shorter MPS and the longer MUS − both with the same headgroup linkers (see Figure 3). While both samples show a similar rise of
Figure 4. Shift of PL peak emission from its initial energy, for CdSe dots linked by MPS under illumination at various intensities, under open air conditions. (a) Energy shift in time, for various illumination intensities. All plots are fitted by a double exponential rise (solid line). (b) The two fitted time constants, τ1 and τ2, are plotted in panel as a function of intensity.
linking molecules. The curves are well fitted by a double exponential rise, as previously reported by Cordero et al:10 ΔE = a1(1 − exp( −t /τ1)) + a 2(1 − exp(−t /τ2))
(1)
The exact fitting method influenced the fitting parameters by considerably, and the error was relatively large. Yet, several trends are clearly observed. Those two exponents have, in all cases, approximately equal weight and time constants exhibiting different dependencies on illumination intensity. This indicates that two different mechanisms are responsible for the apparent blue-shift. Similarly to the processes related to the PL intensity changes (i.e., the enhancement and decay), one of the exponents displays a shortened time constant when the intensity of the excitation beam is increased, whereas the other seems to reach a saturation level. This allowed us to associate the two bluing processes with the processes of PLE and PL decay. We suggest that one mechanism causing the blue-shift is the photooxidation of the dots,10,17,18 which effectively creates an outer oxidized shell. This causes an overall reduction of the dots’ size and a tighter exciton confinement (and, therefore, will also result in shortening the lifetime), before eventually degrading the dots’ luminescence. For single, isolated dots we have measured a blue-shift of up to 50 meV before bleaching. The blue-shift attributed to photooxidation may be associated with an effective average reduction of ∼1.5 Å in the dots’ radii (an approximation made in reference to experimental and theoretical modeling of the size dependency of the QD energy levels).27,28 The other mechanism causing bluing may be related to the photopassivation of surface traps. The charges trapped at these
Figure 3. PLE for CdSe dots with MPS (blue) and MUS (red) linking molecules, for different illumination intensities, under open air conditions. Both samples exhibit similar behavior in low excitation powers, but the PLE becomes smaller for MPS compared with the MUS for increasing excitation powers.
PL at the lowest excitation power, a greater increase is seen for the long MUS molecule for higher power. Based on previous studies,26 we suggest that for linker molecules with a longer alkyl chain the packing of the monolayer is of higher order. This is due to stronger hydrophobic forces between the alkyl chains. The dense molecular shell induces two different trends: it reduces confinement and compensates for the QDs’ dangling bonds, therefore, passivating the surface. Another major change induced by excitation is the clear shift in the PL peak toward shorter wavelengths. Figure 4 shows the peak’s energy shift for multilayered core CdSe dots with MPS 25727
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Figure 5. (a) PL enhancement of CdSe layers with different linking molecules, under a continuous illumination of 700 W/cm2, under open air conditions. Intensity is normalized to the initial intensity (at t = 0) for each sample. (b) Shift of PL peak emission from its initial energy, for CdSe dots linked by different molecules, under constant illumination of 700 W/cm2. The solid line represents a single/double exponential rise fit, as described in the text. Inset: Single exponential blue-shift of CdSe/ZnS dots under a constant illumination of 13.3 kW/cm2. For clarity, only one sample is presented (BDT).
nm (23 ± 5 meV). The other samples differ mostly on the amount and rate of this effect. We attribute this difference in the effect to the variation in the linker molecule type. The BDT molecule, which contains a benzene moiety, is characterized by π electrons that are delocalized and therefore allow for better coupling between the dots. The difference between the MPS and MUS lies in their alkyl chain length. The shorter MPS molecule is, as expected, a better coupler. Therefore, the influence of one dot on its neighbor is more significant. The drop that contains only TOPO ligands, which are bulky molecules with a shorter alkyl chain, differs from other samples. The ligands are initially well ordered and therefore protect the QDs. Yet the TOPO ligands do not link adjacent dots. Lifetime measurements further support the general picture emerging from the features described above. Figure 6 presents
sites create electric fields that induce a Stark red-shift,29 and adsorption of water molecules to the trap sites is observed, in practice, as luminescence bluing.30,31 To further substantiate our claim, we compared the above results with CdSe/ZnS core−shell samples. There, the blueshift is evident only at high excitation intensities and is about half the size of that observed in the corresponding core sample. Furthermore, all core−shell samples exhibit a single exponent blue-shift (see Figure 5b inset). Since the CdSe core of these dots is protected by ∼2 Å ZnS shell, it is likely that the small, initial oxidation we observed in the core QDs would not have such a major effect here, as it had with the core CdSe. However, the effect of the Stark red-shift will still be of relevance, though smaller, and therefore, we consider this to be the mechanism for bluing seen here. A comparison of the PL spectral evolution of layered core CdSe samples with different linker molecules, under constant illumination, is plotted in Figure 5. The PL enhancement and energy shift of the emission peak are presented for all core CdSe samples (panels a and b respectively). The four different linking molecules as well as the original capping ligands of the dots are presented in Figure 1a. They form a set of molecules with different linking head groups and different carbon backbones (alkyl chains and benzene rings). Unlike the other linking molecules, the TOPO ligands covalently bind to the dots only at one of their edges, whereas their other side is chemically inactive. From the results presented above, it is clear that the linking headgroup has a profound influence on the PL properties and dynamics. The molecular length has a smaller effect, as described earlier for the MPS versus the MUS, which is plotted in Figure 3. As discussed above, the total blue-shift is fitted by two exponents. An exception, however, is the BTB sample exhibiting a much smaller shift which is well fitted by a single exponent. As described earlier, BTB molecules, owing to their silane linkers, assemble in a multilayered aggregated structure, thereby forming a sample consisting of sparse, isolated QDs. It is therefore expected that the Stark effect caused by the electric fields, induced by charge traps on neighboring dots, would be missing here. This assumption is in good agreement with our claim that the fast exponent is attributed to the passivation of surface traps. Indeed, the BTB sample’s initial emission was blue-shifted with respect to the other samples, with about 7 ± 2
Figure 6. PL decays of CdSe linked by MPS. Decay traces were taken at different times during continuous 2 min laser excitation at 400 nm and with a power of 700 W/cm2. Solid black lines represent stretched exponential decay fits. Decay traces present both the PLE (total counts) and changes in decay constants during the illumination under aerobic conditions.
lifetime measurements carried out by time-correlated single photon counting (TCSPC) on multilayered core CdSe/MPS, as it evolves during the illumination time. The sample was excited for 120 s under 700 W/cm2 and lifetime readouts were collected during a 3 s windows. Relaxation of the PL, as displayed in those plots, follows a stretched exponential decay law, with an additional fast lifetime component: 25728
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Figure 7. (a) Decay parameter β (see text) of the different CdSe samples under a continuous illumination of 400 nm at 700 W/cm2. (b) PL mean lifetime ⟨τ⟩ of the different samples.
I(t ) = I0·{a ·exp[−(t /τ1/ e)β ] + (1 − a) ·exp( −t /τfast)}
4. CONCLUSIONS Multilayered hybrid organic/semiconductor QD structures were studied by measuring fluorescence emission and by analyzing the effects of photoinduced phenomena. The linking molecules were found to profoundly influence the PL properties. A major role was played by the linking head groups of the different linker molecules, whereas the length of the linker molecule had only a small effect. Illumination of the multilayered structures under air atmosphere was found to serve as a good tool for passivation of surface charge traps and ordering of linker molecules. Lifetime measurements serve as a good means of measuring trap passivation in the form of stretched exponent parameters. Considering all the presented results, we propose the use of short time photo activation and yield enhancement of core QD-based devices, such as detectors. Proper activation and trap passivation are possible under a wet atmosphere before the devices are capsulated.
(2)
with 0 < β < 1. The fast lifetime component was found to be of the order of tens of picoseconds and occupies a small fraction of the total counts in the decay traces (about 0.1%). It is possible that this indicates fast Auger recombination processes that should occur on those time scales.32 The stretched exponential decay of luminescence was reported earlier for several materials,33−36 its physical origin mostly attributed to disorders in the system.34,37 Several authors have indicated that the stretched exponent is caused by the presence of exciton migration between nanocrystals.38,39 However, even isolated QDs have exhibited stretched exponential decay behavior, and the occurrence of β < 1 can originate from trapping and detrapping of excitons.36,37 Figure 7a shows the values of β found for the various samples, and the average relaxation time of the stretched exponential decay, defined by40
τ1/ e ⎛ 1 ⎞ ⟨τ ⟩ = Γ⎜ ⎟ β ⎝β⎠
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
(3)
where Γ is the mathematical gamma function. All four linking molecules exhibit an increase in the value of β during excitation, in accordance with the increase in the PL (see Figure 5a). The small nuances in the β’s increase reveal the same features as those of the PLE; the longer MUS molecule exhibit a faster, more elaborate increase as compared with the shorter MPS, and BTB displays an initial fast increase followed by a much slower increase rate later on. The sample with the TOPO ligands, which exhibited an initial high QY relative to other samples but a PLE by only a small factor, displays a higher initial β value, which relatively rapidly undergoes a smaller increase before reaching a constant value. Those values, of around 0.3−0.35, are similar to the values fitted for the core−shell CdSe/ZnS samples, where no change was observed during illumination. This indicates that β is a good direct measure of the trapping in our samples. It is harder to directly correlate it with mechanisms of exciton migration between adjacent QDs because of the major effects that the induced photochemistry have on the dots. The average relaxation time given by eq 3 is plotted in Figure 7b, showing the increase occurring again from the passivation of surface traps, thereby lengthening the exciton lifetime. The additional expected effect of the lifetime being shortened due to photooxidation has a minor influence only and is not observable here.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The research leading to these results has received funding from LASERLAB-EUROPE (Grant Agreement No. 284464, EC’s Seventh Framework Programme).
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