Spectroscopic Signature of Trap States in Assembled CdSe

Article pubs.acs.org/JPCC

Spectroscopic Signature of Trap States in Assembled CdSe Nanocrystal Hybrid Films Tersilla Virgili,*,† Inmaculada Suárez López,† Barbara Vercelli,§ Giuliano Angella,§ Gianni Zotti,§ Juan Cabanillas-Gonzalez,∥ Daniel Granados,∥ Larry Luer,∥ Reinhold Wannemacher,∥ and Francesco Tassone‡ †

Istituto di Fotonica e Nanotecnologie (IFN) CNR, Dipartimento di Fisica, Politecnico di Milano, Piazza L. Da Vinci 32, 20133 Milano, Italy ‡ CNST, IIT Via Pascoli 70, 20133 Milano, Italy § IENI, CNR Corso Stati Uniti 4, 35127 Padova, and Via Cozzi 53, 20125 Milano, Italy ∥ Instituto Madrileño de Estudios Avanzados (IMDEA-Nanociencia), Cantoblanco, 28049 Madrid, Spain ABSTRACT: We report on femtosecond to millisecond pump−probe experiments performed on a multilayered structure composed by the alternation of CdSe nanocrystals (NCs) and poly(p-styrenesulphonic acid) (PSSH). We found that, after a few picoseconds of pumping at high intensity, an intense photoinduced absorption band and a strong derivative feature emerge in the differential absorption spectrum, both overlapping with the positive bleaching bands of the filled exciton states. These two former spectral features are long-living well into the millisecond range, and we attribute them to the population of charged trap states, located close to the NC surface, and thus inducing a Stark effect through the strong electric field they generate inside the NC.

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INTRODUCTION Inorganic semiconductors are materials of choice in electronics and optoelectronics due to their unique physical properties and performance. However, their production and processing procedures are rather expensive. Recently, the need for cheap but large-area optoelectronic devices emerged both in photovoltaics and illumination, urging the introduction of low-cost processing alternatives. Among these, liquid-phase processing is very promising and applicable to semiconductors nanocrystals (NCs).1 Within the liquid phase, it is possible to achieve selfassembly of ordered and compact structures of NCs resulting in a high semiconductor material density and acceptable physical properties, even in critical issues such as charge transport.2 Indeed, optical-to-electrical power conversion with promising efficiency has been achieved in cells fabricated using organic− inorganic hybrid structures, in which inorganic semiconductor NCs are introduced to take advantage of the high electron mobility of the semiconductor crystals, and overcome chargetransport limitations associated with organic materials.3,4 However, the mobility in the resulting hybrid-semiconducting layers typically remain far from those of the high-quality single/ poly crystals used in the traditional semiconductor technology. Charge transport is both limited by the lack of continuity of the crystal structure, and by the existence of a high density of defects on surface of the semiconductor NCs. Moreover, passivation of surface defects with commonly used capping ligands bearing aliphatic tails does not improve charge transport © 2012 American Chemical Society

since the ligand effectively acts as a potential barrier in the tunneling of charges between adjacent NCs, which are spaced apart by the ligand itself. The choice of ligands has demonstrated to affect the ground-state electronic structure of colloidal quantum dots (QDs) via mixing of the frontier orbitals at the QD−ligand interface and influence the dynamics of excitonic decay by mediating charge trapping.5,6Appropriate organic molecules or polymers having the double functionality of charge transport capability, and grafting to the NC surface could therefore potentially improve the performance of the hybrid layer. However, because of the limited charge transport ability of the organic matrix, it is always important to keep the interparticle distance as low as possible. Among the several techniques known for the formation of the hybrid NC−organic molecule material, the layer-by-layer (LBL) self-assembling technique7−10 introduced for the preparation of thin polymer films and later extended to prepare CdSe, CdS-NC, or CdTe11−20 thin films offers several advantages over other solution-based techniques for the achievement of efficient charge transport in thin films. This method leads to densely packed layers with good surface regularity, also featuring really small interparticle distances. Received: April 11, 2012 Revised: July 6, 2012 Published: July 9, 2012 16259

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(OMA) as a detection system. Microsecond to millisecond pump−probe experiments were performed with a passively Qswitched Nd:YAG laser (Teem Photonics) with tripled frequency output delivering 25 μJ pulses at 355 nm of 300 ps duration with adjustable repetition rate up to about 1.5 kHz. Typical repetition rates employed in this work range from 200−600 Hz, and the pulse fluence employed was 0.24 mJ/cm2. Continuum probe light was provided by a 100 W tungsten halogen lamp filtered by a f = 1/8 m monochromator (Spectral Products) with 600 grooves/mm grating equipped with 0.6 mm slits resulting in a spectral resolution of 6 nm. The light transmitted through the sample was detected by an amplified silicon photodiode. The output signal of the photodiode was amplified again by a home-built amplifier (rise time 3 μs) before being processed by a 804Zi LeCroy digital oscilloscope, which was used to average the photoinduced transmission as a function of time at individual wavelengths. Measurements were performed under ambient conditions. TEM observations were carried out through an analytical transmission electron microscope JEM FXII (JEOL) operating at 150 kV. The LBL monolayer (PAAH-CdSe-PSSH) was deposited on an Al film evaporated on laboratory glass. The Al film with the LBL monolayer was detached from the laboratory glass and placed on a commercial carbon-coated copper grid for TEM observations.

In this work, we time-resolved the excited states dynamics of hybrid multilayered structures composed of alternated CdSe NCs and poly(p-styrenesulphonic acid) (PSSH) obtained by LBL.1 Pump−probe experiments with a wide temporal detection window extending from the picosecond down to the millisecond domain were performed in order to monitor the presence of long-lived excited states. The latter are intimately related to the nature of the NC−polymer interface. PSSH is a typical commercial ligand often used as nanoscale matrix to confine the NCs layers.21−23 Here, we will be assessing the grafting of this polymer onto the NCs, i.e., the NPs−polymer interface, using time-resolved optical pump− probe techniques, showing the presence of surface charged trap states on the NC, possibly due to the nature of the ligand or to incomplete passivation of the NC surface.

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EXPERIMENTAL METHODS For the preparation of the hybrid layers, transparent substrates were employed such as indium tin oxide (ITO)/glass electrodes (20 Ohm sq-1) obtained from Merck-Balzers. Monolayer formation of poly(acrylic) acid (PAAH) on ITO was performed as previously reported.14 The build-up of multilayers on ITO/glass electrodes was performed according to the methodology introduced by Decher and Hong,7,8 that is, by dipping the electrodes alternatively into the solutions of the two components. After each immersion step, the substrate was carefully washed and dried in air. The layer build-up was monitored by UV−vis spectroscopy. Film thickness was around 39 ± 5 nm, which corresponds to 2.6 nm/layer. The concentration of CdSe-NCs dispersions in CHCl3 used for layering was 10−3 M in CdSe units, which corresponds to 0.19 g/L of CdSe and the PLQY of these NCs in solution was around 10%.14 PSSH was used as 10−3 M of CdSe in EtOH solution (as monomeric units). Exposing time was 5 min for both CdSe−NC and PSSH. Soluble capped CdSe−NCs were produced as previously reported.14,15 The NC displayed an absorption maximum at 637 nm in CHCl3, corresponding to an average diameter of about 6.5 nm.24 The photoluminescence spectrum of the hybrid layers was obtained by exciting the samples with the second harmonic (400 nm) of a Coherent Ti:Sapphire femtosecond laser system, which provides 50 fs, 1500 nJ pulses at 800 nm at a repetition rate of 1 kHz. A fiber bundle was placed close to the sample to collect the emitted light. Detection was performed using an Oriel Instaspec IV spectrometer with 1 nm spectral resolution equipped with a charge-coupled device (CCD). The sample was analyzed by optical absorption spectroscopy using a UV/ vis/NIR spectrophotometer JASCO V-570. The experiments were carried out at room temperature. In the femtosecond pump−probe transient absorption spectroscopy experiment,25,26 pump pulses at 3.18 eV (390 nm) (second harmonic of the output of a CLARK Ti:Sapphire regenerative amplifier with a repetition rate of 1 kHz and a pulse length of 150 fs) are focused to a spot of ∼150 μm diameter, resulting in an excitation energy density of ∼2.8 mJ/ cm2 (E = 500 nJ). The probe beam is a white light continuum 1.77−2.48 eV (500−700 nm), generated by focusing a small percentage of the CLARK amplifier output energy onto a sapphire plate. The pump and probe beams are spatially overlapped on the sample. The probe beam intensity used in the experiment is kept deliberately low. The evolution of the differential transmission (ΔT/T) was recorded within the visible spectral range using a fast optical multichannel analyzer

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RESULTS In Figure 1a, we report the absorption (solid line) and the PL spectrum (line + symbols) from LBL films. The absorption

Figure 1. (a) Absorption (solid line) and PL emission for an LBL film. (b) TEM image of a monolayer LBL film.

spectrum displays a low energy absorption peak at 637 nm and higher energy shoulders between 550 and 600 nm. The PL spectrum at high pump fluence exhibits a main band at 659 nm and a smaller one at 620 nm. The morphology of the LBL structure was studied by transmission electron microscopy (TEM). In Figure 1b, a micrograph of the LBL structure on Al film reveals the CdSe NCs as closely packed small spheres distributed on the LBL layer. The borders of NCs are not welldefined because of the Al film substrate and the polymer in which the particles are immersed. Nevertheless, it can be observed that the particle size is about 5−6 nm, consistent with what is expected from the absorption peak. The interparticle distance (center to center) can be evaluated and results in an average distance of 8.1 nm with a standard deviation of 2.7 nm. These values imply an average spacing of about 2.7 nm between the NCs and a negligible amount of NCs in full contact, which is also apparent in the image. This interparticle spacing is quite small compared to what can be obtained using simpler mixing techniques for the formation of the hybrid layer. At the same 16260

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Figure 2. (a) Transient transmission spectra at 600 fs (line + solid squares) and at 20 ps (solid line) probe delay, excitation fluence of 2.8 mJ/cm2. (b) Dynamic at 600 nm at two different excitation fluences.

spectrum is strikingly similar to the one obtained at 20 ps delay (see Figure 2a), while at 0.5 ms probe delay, the bleaching signal appears to be blue-shifted by around 15 nm. This blue shift is obvious in Figure 3b, where the complete differential transmission map is reported. Thus, this is an unequivocal proof that PIA contribution at 20 ps delay must be mainly attributed to the population of intermediate states, which decay on time scales exceeding by far those of excitons. Similar broad PIA bands have been assigned in some previous studies to exciton trapping and ionization at defects located on the NC surface.31,32 It was also showed how surface-induced charge trapping processes lead to false signals from two main processes such as multiple exciton recombination (MER) and multiple exciton generation (MEG).33 Their delay formation indicates a finite rate in the filling of defect states.

time, while the NC concentration is exceptionally high, aggregation is completely avoided. Clearly, this is necessary to allow for the passivation of the NP surface defects by the organic component. Such good properties are certainly related to the use of the LBL film formation technique, in combination with a narrow size distribution of NCs as also indicated in the presence of a well-defined peak in absorption.27 A deeper understanding of the photophysics of the LBL film was obtained by studying time-resolved differential transmission spectra for the LBL film. Figure 2a shows the pump− probe spectrum for the two samples at 600 fs and 20 ps probe delay. At 600 fs probe delay, the sample shows clear positive photobleach (PB) of ground-excited state transitions and a photoinduced absorption (PIA) band at the low spectral region. This observation is in line with previous studies on the excited state dynamics in NCs reporting the presence of PIA band at the tail of PB, which is ascribed to transitions from the exciton to the biexciton band.28−30 The red-shift of biexcitonic with respect to excitonic transitions is related to the biexciton binding energy. We now focus our attention on the spectral evolution of the PIA band at longer delays. At 20 ps delay, the PIA band enhances respect to PB and becomes more broad. The formation rate of PIA is illustrated upon monitoring the dynamics at 600 nm (Figure 2b). It is clear that the rise time of PIA decreases from 20 to 2 ps as the excitation pump fluence increases from 0.84 mJ/cm2 to 2.8 mJ/cm2. The PIA band does not significantly decay over the 400 ps probe delay window. We therefore carried out microsecond to millisecond transient absorption measurements with pulsed excitation at 355 nm using the setup described above. Figure 3a shows the differential transmission spectra at 50 μs and at 0.5 ms after pump excitation. Interestingly, the 50 μs delay pump−probe

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DISCUSSION At large pump fluences, a large number of excited electron hole pairs is created in each NC, note that even if the pump photon energies for the two time-resolved measurements (fs and μs regime) are different, this does not influence the photoinduced physical processes we are looking at because, in both cases, the photon energies are far above the absorption edge of the NC.34 Here, we estimated that at the large fluence, employed in the femtosecond pump−probe experiments, well over 50 pairs are present in a single NC.35 After relaxation, a significant number of pairs are expected to remain in the NC, filling both the lowest and higher confined states, as shown in the differential transmission spectra at very short times (see Figure 2a line + symbols). However, these higher energy states have very short lifetimes usually attributed to nonradiative Auger recombination.36 The fast growth of the PIA band (few ps) in the LBL film indicates that the escape/trapping of charges to surface defects states is very fast, being the corresponding trapping time comparable to nonradiative Auger recombination. Thus, at large pump fluence, we effectively load the surface defects in the LBL film. Indeed, we could quantitatively reproduce both the differential transmission spectra at 20 ps and 0.5 ms probe delays by considering 3 main contributions: a long-lived PB band (PB(λ)) centered at 640 nm due to filling with electron− hole pairs of NCs without charged defects on their surface, a contribution S(λ) given by the first order derivative of the measured absorption spectrum, and a broad PIA band (P(λ)) due to charge surface defects, centered at around 660 nm. The results of the fit are also shown in Figure 4. We remark that the differential transmission signal at shorter wavelengths between 500 and 600 nm is well described by the S(λ) contribution

Figure 3. (a) Transient transmission spectra for the LBL film obtained at delays of 50 μs (line + symbols) and 500 μs (line) after excitation. (b) Complete differential transmission map, showing the spectral shifts of the central positive band. The excitation fluence used is 0.24 mJ/ cm2. 16261

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This is consistent with a picture of charged particles, in which electron and holes are separated, and only one type of carrier fills the lowest energy quantized state of the NCs. It suggests that the slow recombination dynamics is governed by charge diffusion and/or by a trapping/detrapping dynamics, both occurring on much longer time-scales compared to that of radiative recombination. In the case of well separated NCs, such long-term dynamics has been tentatively related to the blinking of the photoluminescence from single NCs.38

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CONCLUSIONS We studied the photophysics of multilayered structures obtained by the alternation of CdSe nanoparticles and PSSH employing the LBL technique. Using femtosecond to microsecond pump probe experiments, we found evidence of the presence of long-lived charged surface states in the LBL film. These charges produce a long-lived photoinduced absorption band and a strong Stark effect on the electronic states inside the NC. As in the case of NCs in solution capped with amine or trioctyl-phosphine,28 we may tentatively attribute the formation of such an amount of charged states at the NP−polymer interface to an incomplete passivation of the NPs surface by PSSH. However, it is well-known that some ligands such as thiols, thiophenyls, and pyridines39,40 induce strong luminescence quenching by acting as hole traps, while, even though, as far as we know, sulfonates are not luminescence quencher per se, we may not exclude at this point a role in PSSH itself. We therefore conclude that grafting of polymers to the NCs surface needs to be better explored and understood before the functionality of hybrid films might be fully exploited.

Figure 4. Different contributions in the pump−probe spectra of the LBL film at 20 ps (a) and at 0.5 ms probe delay (b): bleaching band (blue line), photoinduced absorption band (pink line), Stark effect (red line), and resulting fit (black line). The experimental data are shown by the open circles.

alone, in particular at longer times where residual photobleach from higher energy states are certainly absent. Previously, it has been reported that the presence of just one charge on the surface may produce a very large electric field inside the NCs, well up to around 600 kV/cm.37 Such a large induced electric field should result in large Stark shifts of the absorption spectrum of the NCs, thus in the appearance of first and/or second derivative spectral contributions associated to a shift in the differential transmission spectra, which is taken into account in our fits as S(λ). The part of the spectrum at wavelengths longer than about 600 nm instead shows the presence of a significant PB signal, and at wavelengths above 670 nm, it shows a dominance of the PIA contribution. Interestingly, the relative ratio of S(λ) and P(λ) stays the same both at 20 ps and at 0.5 ms. Since P(λ) is proportional to the concentration of trapped charges, while S(λ) is proportional to both the concentration and the distance of the trapped charges to the respective counter-charge (through the induced electric field), this seems to imply that there is a typical e−h distance that remains constant over the course of the experiment. This is consistent with our picture, locating the trapped charge in the organic shell of the NC in the LBL films. Instead, the relative weight of the bleaching part PB(λ) is larger at shorter times by a factor of 2 with respect to longer times, as in the following:

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS I.S.L. and T.V. thank the FONDAZIONE CARIPLO for funding the “Local micro-tailoring of conjugated polymer emission by spatially resolved nanoparticles implantation for next-generation light-emitting devices” 2009-2562 project. J.C.G. and L.L acknowledge financial support from the Spanish Ministery of Science and Innovation through Programa Ramon y Cajal, the EU(Program AMAROUT) and POLYDYE project (TEC2010-21830-C02-02). R.W., J.C.-G., D.G., and L.L. acknowledge support by Comunidad de Madrid (Project Nanobiomagnet P2009/MAT-1726) .

FIT20ps(λ) ∝ S(λ) + 2αPB(λ) + βP(λ)

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FIT500μs(λ) ∝ S(λ) + αPB(λ) + βP(λ)

The decrease of the weight of the bleaching part is related to a reduction of the fraction of filled NCs. This reduction is related to recombination of electrons and holes in these NCs by both radiative and nonradiative recombination processes. The faster decay of PB(λ) with respect to the other two contributions results in the apparent blue-shift of the pump− probe spectrum at long times, as observed in Figure 3b, as the PIA band is centered at longer wavelengths with respect to the PB band. We finally remark that the typical radiative recombination rates in the NCs are in the order of tens of nanoseconds at the most,1 while here the filling of NCs evidenced by the PB band persists up to several milliseconds.

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