Charge Separation in the Hybrid CdSe NanocrystalâOrganic Interface

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Charge Separation in the Hybrid CdSe Nanocrystal-Organic Interface: Role of the Ligands Studied by Ultrafast Spectroscopy and Density Functional Theory Tersilla Virgili, Arrigo Calzolari, Inmaculada Suarez Lopez, Barbara Vercelli, Gianni Zotti, Alessandra Catellani, Alice Ruini, and Francesco Tassone J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp311621s • Publication Date (Web): 05 Mar 2013 Downloaded from http://pubs.acs.org on March 12, 2013

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The Journal of Physical Chemistry

Charge Separation in the Hybrid CdSe NanocrystalOrganic Interface: Role of the Ligands Studied by Ultrafast Spectroscopy and Density Functional Theory Tersilla Virgili*a, Arrigo Calzolarib, Inmaculada Suárez Lópeza, Barbara Vercellic, Gianni Zottic, Alessandra Catellanib, Alice Ruinib, Francesco Tassoned a

Istituto di Fotonica e Nanotecnologie (IFN) CNR, Dipartimento di Fisica, Politecnico di

Milano, P.zza L. Da Vinci 32, 20133 Milano, Italy b

Istituto Nanoscienze CNR-NANO-S3 Via Campi 213/A, I-41125Modena, Italy

c

Istituto CNR per l'Energetica e le Interfasi Via Cozzi 53, 20125 Milano, Italy

d

Center for Nano Science and Technology @Polimi, Istituto Italiano di Tecnologia, Via Pascoli

70/3, 20133 Milano; Keywords: inorganic nanoparticles, pump-probe, self assembling, theoretical modeling

ACS Paragon Plus Environment

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ABSTRACT

We present a joint experimental and theoretical study of the early stage dynamics of photoexcited charges in a prototypical organic/inorganic interface. By using femtosecond pump-probe experiments we compared the photophysic of a layer-by-layer hybrid structure obtained by alternating CdSe nanocrystals and poly(p-styrenesulphonic acid) and the same CdSe nanocrystals capped with hexadecylamine and stearic acid diluted in solutions. While in the LBL structure it is clear the appearance of a long lived charged state, no evidence of this is instead found in the diluted solutions. Density functional calculations indicate that these states are localized close to the nanoparticle surface, and that electrons and holes are separated across the hybrid interface, pointing out the effects of surfactant capping molecules on the optoelectronic properties of the interface. Our combined approach, allowing for unique access to the photo-excited electronic structure, opens the possibility to the fine tailoring of hybrid organic/semiconducting layers for photovoltaic applications.


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Introduction Inorganic semiconductors show superior photo-physical properties, including broad absorption and good photo-stability, with good carrier mobility, at least in the bulk form. However, producing high-quality bulk inorganic semiconductors requires expensive equipment, having a strong impact on development and cost. On the opposite side, organic polymers can be processed inexpensively in the liquid phase. Colloidal semiconductor nanocrystals (NCs) are produced in solution, thus allowing for liquid processing of photo-active layers. While retaining many of the advantages of bulk inorganic semiconductors crystals, transport of photo-generated charges is problematic due to the break-up of the continuous crystalline structure of the bulk. Combining colloidal NCs with semiconducting polymers could therefore potentially overcome these problems, giving rise to hybrid layers that exhibit the advantageous properties of both components. Semiconductor NCs, grown and stabilized in solution by capping with organic ligands, are hybrid materials1. The role of the organic capping molecules is crucial in NC synthesis: in fact they regulate NC shape, size, chemical composition and morphology2-5. However, their effects on the optoelectronic properties of the functionalized nanoparticles (NPs) are not yet fully understood. For example, apart from acting as a simple insulating layer, they also have deep influence on many 'bulk'-related physical properties of the NCs, such as relaxation of hot carriers inside the dot 6, activation of non-radiative decay channels and/or charge trapping 7, possibly resulting in the intermittency of steady state photoluminescence8. Specific ligands allow the control of quantum confinement of the exciton in the NCs, up to the exchange of photoexcited electrons or holes between the NC and the ligands

9-17

. This is a crucial step for the extraction

(charge separation) or injection of charges out of or into the NC. While these processes generally


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depend on the relative alignment of the electron and hole energy levels in the organic and inorganic phase, the actual transition may involve a specific hybrid state localized on the organic-inorganic interface, which is expected to have a great impact on the efficiency of the charge separation or injection processes, and on the overall device performance. The detailed understanding of these physical processes induced by ligands at the microscopic level is fundamental for many photophysical applications of NCs, including photovoltaics, light emission devices, and bio-tagging. In this work we focus on this crucial issue, by investigating the photoexcited carrier dynamics in compact, hybrid films of CdSe NCs embedded in a polymer matrix and featuring a very high volume fraction of NCs. This hybrid layer is obtained by exchanging standard ligands (hexadecylamine, HDA) that allow for good solubility of pristine NCs, with a multi-dentate binder such as poly(p-styrenesulfonic acid) (PSSH) polymer, characterized by sulfonate binding functional groups. Regular layers are deposited by a layer-by-layer (LBL) deposition technique18, which avoids NC precipitation by keeping the polymer and the NC solutions separated19. The resulting interparticle separation of around 2 nm has a positive impact on the conductivity of the films20. In a previous work19 we reported on femtosecond to millisecond pump-probe experiments performed on this LBL film. We found that after few picoseconds pumping at high intensity, an intense PIA band and a strong derivative feature emerge in the differential transmission spectrum, both overlapping with the positive bleaching bands of the filled exciton states. These two spectral features are long-living well into the millisecond range, and so we attribute them to a population of charged trap states, localized 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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In this work we compared the differential transmission spectra of the LBL film with the one of the solution stabilized CdSe NCs. Using a combined approach based on ultrafast photophysics characterization and ab-initio simulations, we show that the nature of the ligand in the film is not that of a neutral binder, but instead it dominates the carrier dynamics by readily trapping the photo-excited holes. We will demonstrate this statement in two steps: (i) we show that the previously19 observed and well studied broad photo-induced absorption (PIA) band demonstrated to be due to carriers trapped in the NC surfaces in the LBL film19, is not present in solution HAD-capped NCs; and (ii) we identify by DFT study shallow hole traps only for the deprotonated SSH attached to the CdSe surface, and not for amine-terminated surfaces. Methods The preparation procedure of the LBL sample has been previously described19. The concentration of the solution of capped CdSe-NCs used for layer build-up was 10-3 M in CdSe units. PSSH concentration used was 10-3M in EtOH solution. Exposing time was 5 minutes for both CdSe-NC and PSSH. Soluble capped CdSe-NCs were produced as previously reported21-23. The NCs display an absorption maximum at 637 nm in CHCl3, corresponding to an average diameter size of about 6.5 nm24. Photoluminescence experiments were performed 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 fibre 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 chargecoupled device (CCD). The solution containing CdSe nanoparticles was placed into silica cuvettes with 1 mm optical path for optical characterization, and 2 mm optical path for pumpprobe experiments. All samples were characterised by optical absorption spectroscopy using a


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UV/VIS/NIR spectrophotometer JASCO V-570. The experiments were carried out at room temperature. In the femtosecond pump–probe transient absorption spectroscopy experiment25, 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 mJcm-2. The probe beam is a white light continuum 1.77–2.48 eV (500-700 nm), generated by focussing 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 (∆T/T) was recorded within the visible spectral range using a fast optical multi channel analyzer (OMA) as a detection system. First principles simulations of the structural and electronic properties of prototypical NP interfaces were performed to complement the experiments: calculations are based on Density Functional Theory (DFT) with PBE exchange-correlation functional, adopting a plane wave basis set and ultrasoft pseudopotentials, as implemented in the state-of-the-art QUANTUMESPRESSO suite26 , see also Supporting Information (SI) for numerical details. Results and Discussion Fig. 1(a) shows the absorption (solid line) and the PL spectra (line + symbols) of the LBL films (i.e. with PSSH surfactants). The absorption spectrum displays a low energy absorption peak at 637 nm, and higher energy shoulders between 550 nm and 600 nm. We also measured (Fig. 1b) the optical absorption and PL of the starting solution of NCs, capped by hexadecylamine and stearic acid. We identify the lowest energy absorption peak at 648 nm,


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along with a second peak at higher energy (620 nm) not observed in the LBL case. The presence of a well defined absorption peak in the LBL film as well as in the solution indicates that the band-edge excitations are rather confined in the NCs in both cases. Instead, the disappearance of the higher energy peak at 620 nm in the LBL film could indicate some degree of delocalization of the higher energy electrons among neighboring NCs in the LBL film. This difference together with the small blue shift of the main absorption peak and the appearance of a weak shoulder at around 675 nm, may be also related to the different nature of surface capping: for the NCs in solution, hexadecylamine (HDA) and stearic acid (SA) having amine and carboxylate groups binding to the NC surface; while in the LBL film, the anchorage to NC is made through the sulfonate group of the PSSH polymer. Finally, we observed, a drastic reduction of the photoluminescence yield in the LBL film, which is also indicative of a separation of electrons and holes after photo-excitation. A deeper understanding of the molecular-induced effects on NC surface is obtained by studying time-resolved differential transmission spectra for both a solution of capped CdSe NCs and for the LBL film. Fig 2(a) shows the pump-probe spectrum for the two samples at 600 fs probe delay. The solution of capped NCs (solid line) shows three clear photo-induced positive bands at around 520 nm, 560 nm and 634 nm, this last having a broad high-energy shoulder extending well below 620 nm. These peaks correspond to the absorption peaks (see Fig 1b) and indicate the photobleach of the corresponding transitions. A similar spectrum results for the LBL film. The two samples behave differently at longer delays, as shown for example at 200 ps delay in Fig 2(b). The spectrum related to the solution (solid line) shows a positive signal over the whole spectral region, while the one related to the LBL film shows a broad, structured photonegative band with a clear bleaching peak at around 635 nm. Previous studies on the excited


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state dynamics in NCs attributed the appearance of this negative band to the presence of trap states on the surface of the NC that become ionized. These trap states are responsible of an induced Stark effect19 and of the appearance of a PIA band19, 27-28. These PIA bands have been frequently observed in the spectral region of the bleaching peaks, and they are usually much broader and delayed compared to the latter ones. It is also well known that the intensity of this band strongly increases on the initial filling of the NC. Here it was estimated to be of around 50 pairs19. The delay of the PIA band appearance also depends on the initial filling of the NC through multi exciton processes and it is related to the transfer rate to trap states. Here, we also observed a delayed formation of this strong PIA band, with a typical delay of about 2 ps, as clearly shown in the 2-dimensional plot of the ∆T/T signal from the LBL film (Fig. 2(c)). The 2D plot displays a negative signal (deep blue) at 2 ps not present in Fig 2d, where the corresponding signal from the solution is reported. Thus we can conclude that the appearance of the strong PIA band is a clear indication of the presence of a large amount of molecular-traps in the LBL film only. At this point we cannot however discriminate whether these traps are related to the styrene-sulfonate binding, or to the introduction of other defects when the HDA and SA are stripped away from the NCs. In order to better rationalize the experimental findings and to answer to the open questions we performed first principles simulations of the structural and electronic properties of prototypical NP interfaces, based on Density Functional Theory (DFT). We focused on the formation of hybrid molecule/CdSe interfaces and on the modification of the band-gap electronic properties of the NC surface, induced by the presence of the organic ligands with different anchoring groups. In particular, our calculations allowed us to distinguish the electronic structure modifications of the CdSe surface either induced by a partial coverage of amine surfactants (see chemical


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structure in Fig 3a) or connected to functionalizing SSH molecules (Fig. 3b) . We wish to remark here that, although the DFT scheme is evidently not suitable to predict in a precise way the electronic band gap of a system, which is always underestimated (see e.g. Ref. 29), this approach offers a correct description of the ground-state electronic properties of the system, and in particular of its occupied electronic bands, which are the only ones theoretically addressed in this work. In order to get a one-to-one understanding of the molecule-surface and molecule-molecule interaction, we adopted simplified models, representative of the overall experimental system. The influence of the ligands on the NC surface passivation

30-32

, NC growth

33-35

, as well as on

the NC optical absorption and on the subsequent relaxation of optical excitations

36

- have been

previously addressed by means of first principles calculations. In most cases, very small CdSe clusters have been considered; here however, since we are dealing with relatively large NCs (~ 6 nm diameter) that exhibit bulk-like facets, we disregarded the fine details of the semiconductor nanoparticles shape and we considered flat periodic CdSe (10-10) surfaces, which are – along with the polar (0001) face - among the most frequently exposed faces in NCs of hexagonal structures. If, on one hand, this simplification does not take into account some characteristic features of the nanostructures (such as edges or kinks), it provides a controlled model system that allows for a more fundamental investigation of the mechanisms that rule the formation of molecule/semiconductor interfaces. In the clean CdSe(10-10) surface, the outermost threefold-coordinated atoms assemble in ordered rows of Cd-Se dimers along the [12-10] direction. The Cd-Se dimers exhibit a vertical relaxation with a 0.8 Å relative displacement between the outward Se and the inward Cd atoms. The surface is semiconducting and non-magnetic with a net charge accumulation around the


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selenium atoms. The presence of dimer rows of acidic metal and basic chalcogen atoms together with buckling relaxation offers a natural template for the deposition of surfactant molecules. We then studied the adsorption on the clean CdSe(10-10) surface of ethylamine (EtA) fragments and single styrene sulfonic acid (SSH) monomers as representatives of the HDA anchor groups and the PSSH polymers, respectively: the two corresponding chemisorbed configurations are shown in Figure 4a and b. Since after rinse cycles performed during the preparation of the sample part of the capping surfactants may remain on the NCs, the first issue we tackled is whether the presence/absence of residual amine groups may introduce molecular/defect levels in the CdSe bandgap. Thus, we considered a large surface with a (3x2) lateral periodicity and five EtA molecules, which saturate 5/6 of the exposed CdSe dimers of the original substrate, realizing a partially covered system. After atomic relaxation, the amine groups adsorb on the surface, forming weak Cd-N bonds (see Fig. 4a). The surface does not exhibit structural distortions, except for the adsorption sites, where the substrate partially de-relaxes, slightly reducing the original buckling of the surface dimers (residual buckling 0.5 Å). This is an indication of slight charge redistribution at the N-Cd bond, as also reported in Table 1 of the SI. However, the formation of interface bonds does not imply the presence of novel states in the band-gap as demonstrated in Fig. 4c, where it is evident that the total and projected density of states (DOS) in this energy region are essentially the same of the pristine NC surface . This also excludes that the uncapped Cd site (Cd hole) is acting as a surface defect pinning the position of the Fermi level. We conclude that the valence band top (VBT) and the conduction band minimum (CBM) of the interface, which dominate the onset of optical absorption, remain similar to the clean surface case, being unaffected by the presence/absence of the amine group. Thus the features observed in the experimental pump-probe spectra should not be ascribed to surface


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defects due to the residual presence of amine molecules. Calculations for the full surface coverage (reported in Fig. S1 , SI) confirm this conclusion. Next, we considered the adsorption of one SSH molecule (Fig. 4b) on the same (3x2) CdSe(10-10) surface, starting from two different initial configurations, which include both protonated and deprotonated molecules. For the fully protonated SSH (see Fig. S2, SI) the interaction with the surface is very weak, and the molecule is physisorbed on the surface. In the case of the deprotonated molecule, instead, we observe a direct molecule-surface interaction and the formation of a Cd-O bond, along with a net flattening of the corresponding surface dimer (Fig. 4b) and a charge transfer between the two atoms (see table 1, SI). Experimentally, the deprotonation of the SSH acid is a very frequent event, favoured by the pH of the surrounding solvent, which may easily catch the released proton. In our specific model (no solvent), we simply displaced the H atom toward a surface Se, far away from the adsorption site, although still in the simulation cell in order to maintain total charge neutrality: this does not affect the results, as shown in Fig 4d, where the contribution of Hs (dashed area) to the total DOS is neglible in the gap region. From a detailed analysis of the single particle states, we identified a set of mixed Cd-O binding orbitals derived from charge transfer from Cd dimer atoms to sulfonic oxygen, which corresponds to broad peak in the DOS at ca. -2.5 eV in the valence band (Fig. 4d). This charge transfer partially restores the 4-fold coordination of surface Cd atoms, which is responsible for the complete removal of the clean dimer relaxation, mentioned above. The distribution of Lowdin charges, reported in Table SI of the Supplementary Information, reveals strong electronic redistributions of all the NC atoms directly involved in bonding to functional groups. A novel fully occupied orbital (HSSH), lying at the top of the CdSe valence band, dominates the


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gap region of the interface. This state has a clear molecular origin, being derived from the HOMO of SSH molecule, but it also presents a partial contribution of the outermost layer of the surface (Fig. 5). Conversely, the HOMO-1 (LUMO) state of the overall system originates from the former VBT (CBM) of the clean surface, but also features a minor contribution from the molecular anchor group (Fig 5). Since the occurrence of the strong PIA band in the LBL film transient absorption spectrum is indicative of the filling of characteristic trap states on the surface of the NCs, we believe that this is related to the filling of HSSH states with photo-excited holes. The 2 ps formation time of the PIA band is then indicative of the relaxation time from the 'CdSeVBT', i.e. HOMO-1 state, to this state. In the real case of a finite size CdSe NP, we may expect slight variations in the energetic position of HSSH and in the quantitative hybridization of the SSH-HOMO with the VBT of the CdSe surface, with respect to the model system addressed here (extended CdSe surface and SSH monomer). However, we do not expect qualitative changes. In particular, in the polymer, where as the double C-bond of the vinyl unit turns into a single bond, the HSSH state wavefunction should become further localized closer to the CdSe surface. This would maintain or even reinforce the hybrid nature of the state. Conclusion In conclusion, we have shown that the combination of ultrafast optical measurements and electronic structure calculations allows us to characterize the formation of an hybrid chargeseparated state created after photo-excitation, as a function of the surfactant ligands. The relaxation of the hole into this hybrid state is a fundamental first step in the separation of the photo-excited charges, as required in any photo-voltaic process. We believe that this method can be advantageously applied for the understanding and subsequent fine-tuning of the ligand-


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molecule in hybrid photovoltaic devices. Moreover, this study opens the possibility to study the subsequent fundamental steps of diffusion of the separated charges to the polymer chain, when the simulation is extended to include appropriate conducting polymers, possibly identifying specific fingerprints of this process in the differential absorption dynamics. AUTHOR INFORMATION Corresponding Author *Dr. Tersilla Virgili Istituto di Fotonica e Nanotecnologie (IFN) CNR, Dipartimento di Fisica, Politecnico di Milano, P.zza L. Da Vinci 32, 20133 Milano, Italy Phone: +39-02-23996076 E-mail: [email protected]

Notes The authors declare no competing financial interest

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 nextgeneration light-emitting devices” 2009-2562 project. Computational resources were provided at CINECA by project IscraC_ACID_sph.


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SUPPORTING INFORMATION SI includes (S1) the computational details of the DFT simulations , (S2) the results for two auxiliary structures commented in the text: CdSe surface fully saturated with amine groups and unprotonated SSH/CdSe interface; and (S3) the Löwding charge analysis discussed in the text. This material is available free of charge via the Internet athttp://pubs.acs.org.

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(13) Lutich, A.A.; Jiang, G.; Susha, A.S.; Rogach, A.L.; Stefani, F.D.; Feldmann, J. Energy Transfer versus Charge Separation in Type-II Hybrid Organic-Inorganic Nanocomposites. Nanoletters, 2009, 9, 2636-2640 (14) Gross, D.; Susha A.S.; Klar, T.A.; Da Como, E.;Rogach, A.L.;Feldmann, J. Charge Separation in Type II Tunneling Structures of Close-Packed CdTe and CdSe Nanocrystals. Nanoletters, 2008, 8, 1482-1485 (15) Xu, Z.; Hine, C.R.; Maye, M.M.; Meng, Q.; Cotlet, M. Shell Thickness Dependent Photoinduced Hole Transfer in Hybrid Conjugated Polymer/Quantum Dot Nanocomposites: From Ensemble to Single Hybrid Level. ACS Nano, 2012, 6, 4984-4992 (16) Moule, A.J.; Chang, L.L.; Thambidurai, C.; Vidu, R.; Stroeve, P. Hybrid Solar Cells: Basic Principles and the Role of Ligands. J. Mater. Chem., 2012, 22, 2351-2368 (17) Greenham, N.C.; Peng, X.; Alivisatos, A.P. Charge Separation and Transport in Conjugated-Polymer/Semiconductor-Nanocrystal Composites studied by Photoluminescence Quenching and Photoconductivity. Phys. Rev. B, 1996, 54, 17628-17637 (18) Talapin, D.V.; Jong-Soo, L.; Kovalenko, M.V.; Shevchenko, E.V. Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications. Chem. Rev, 2010, 110, 389454. (19) Virgili, T.; Suarez-Lopez, I.; Vercelli, B.; Angella, G.; Zotti, G.; Cabanillas-Gonzalez, J.; Granados, D.; Luer, L.; Wannemacher, R.; Tassone, F. Spectroscopic Signature of Trap States in Assembled CdSe Nanocrystal Hybrid Films. J. Phys. Chem. C, 2012, 116 (30), 16259–16263.


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Figures captions: Fig.1: a) Absorption (solid line) and Photoluminescence (line + symbols) of the LBL sample.


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b) Absorption and Photoluminescence from the starting NPs capped solution. Same symbols as before. Fig. 2: ∆T/T spectra at 600 fs (a) and at 200 ps (b) probe delay for the LBL film (line+symbols) and for the starting NPs capped solution (solid line). 2-Dimensional plot for the LBL film (c) and for the starting solution (d). Fig 3: Chemical structure for the ethylamine (EtA) (a) and the styrenesulphonic acid (SSH)(b) monomers. Fig 4: Side views of EtA (a) and SSH (b) molecules adsorbed on CdSe(10-10) surface. Total DOS (black line) and projected contributions on CdSe clean surface (shaded area) and functionalised surface (thick lines) with the EtA (c) and SSH (d) molecule, extracted from the final interfaces. The zero of the energy scale is aligned to the Fermi level of each system. Hs-projected contribution in panel d is magnified (x 20) to make it visible on this scale. Fig. 5: Isocharge density plots of representative single particle states in the gap area. Labels refer to Fig. 4d.


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-3 of 26 X 1022 Page

The Journal of Physical Chemistry 700

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a)

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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

New hybrid orbitals: • Valence Band Top (VBT) of the NP has some hybridization with the monomer SSH • HOMO of the SSH has some hybridization with CdSe surface


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Charge Separation in the Hybrid CdSe NanocrystalâOrganic Interface

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Charge Separation in the Hybrid CdSe NanocrystalâOrganic Interface