Quantifying Geometric Strain at the PbS QD-TiO2 ... - ACS Publications

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Quantifying Geometric Strain at the PbS QD-TiO2 Anode Interface and Its Effect on Electronic Structures Orlando Trejo,† Katherine E. Roelofs,‡,§ Shicheng Xu,† Manca Logar,§,∥ Ritimukta Sarangi,⊥ Dennis Nordlund,⊥ Anup L. Dadlani,∇ Rob Kravec,‡ Neil P. Dasgupta,# Stacey F. Bent,‡ and Fritz B. Prinz*,†,§ †

Department of Mechanical Engineering, Stanford University, Stanford, California 94305, United States Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States § Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States ∥ Laboratory for Materials Chemistry, National Institute of Chemistry, Ljubljana 1000, Slovenia ⊥ Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States ∇ Department of Chemistry, Stanford University, Stanford, California 94305, United States # Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States ‡

S Supporting Information *

ABSTRACT: Quantum dots (QDs) show promise as the absorber in nanostructured thin film solar cells, but achieving high device efficiencies requires surface treatments to minimize interfacial recombination. In this work, lead sulfide (PbS) QDs are grown on a mesoporous TiO2 film with a crystalline TiO2 surface, versus one coated with an amorphous TiO2 layer by atomic layer deposition (ALD). These mesoporous TiO2 films sensitized with PbS QDs are characterized by X-ray and electron diffraction, as well as X-ray absorption spectroscopy (XAS) in order to link XAS features with structural distortions in the PbS QDs. The XAS features are further analyzed with quantum simulations to probe the geometric and electronic structure of the PbS QD-TiO2 interface. We show that the anatase TiO2 surface structure induces PbS bond angle distortions, which increases the energy gap of the PbS QDs at the interface. KEYWORDS: sensitized solar cell, X-ray absorption spectroscopy, X-ray absorption near edge structure, atomic layer deposition, density functional theory, quantum dot

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order, as opposed to detailed interfacial information. Further, inherently local techniques, such as transmission electron microscopy (TEM), scanning tunneling microscopy, and extended X-ray absorption fine structure, are limited when determining atomic structure of scattered or buried nanomaterials.8 Due to its high sensitivity to interatomic distances, geometry, and oxidation state,9 and its ability to characterize subsurface materials,10,11 X-ray absorption spectroscopy (XAS)

nterfaces are of critical importance in nanostructured lightharvesting devices,1−3 as nanostructuring creates extensive internal surface area, across which photoexcited charges must be collected. For example, in quantum dot sensitized solar cells (QDSSC), which have recently reached efficiencies of 8.6%,4 incorporation of organic or inorganic layers between metal oxide anodes and QDs have improved device performance by mitigating electron−hole recombination or improving electron/hole transport.1,2,5−7 The exact nature of such surface modifications is often difficult to determine because X-ray based techniques such as diffraction, reflectivity, and scattering only provide information about bulk crystallinity and long-range © XXXX American Chemical Society

Received: June 15, 2015 Revised: October 25, 2015

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DOI: 10.1021/acs.nanolett.5b02373 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. Schematics of the sample architectures. The sample architectures for the AN (a) and AM (b) sample sets are shown. For both sets, 10, 20, and 40 ALD cycles (10×, 20×, and 40×) of PbS were deposited on the nanoporous TiO2 substrates. The resulting PbS QDs were capped with 60 cycles of ALD TiO2, which is amorphous. Absorption measurements capture changes in the PbS QDs average size and size distribution. The 40× PbS result in larger QDs with a smaller gap as seen by the absorptions edge shifts for the AN (c) and AM (d) sets, and a broadening of the QD size distribution at 40× PbS is reflected in the broadening of the absorption edge.

multiple-scattering and density functional theory (DFT) calculations. The differences in bonding environment and the resulting interfacial electronic band structure suggest potential impacts on device performance. Atomic layer deposition (ALD) was employed to deposit the QD and metal oxide layers on the TiO2 anode, due to its selflimiting, layer-by-layer nature. The use of ALD to create highly controlled nanoscale architectures in combination with XAS allows us to probe properties of atomically engineered interfaces. We use this approach in the present work to characterize the coordination environment at the interface between PbS QDs and TiO2 nanoparticles (NPs) in a quantum-dot-sensitized solar cell architecture. Schematics of our sample architectures are shown in Figure 1. We analyze two sets of samples for comparison: the first, labeled AN for “anatase”, consists of PbS QDs of various sizes deposited by ALD directly on anatase TiO2 NPs, whereas the second, labeled AM for “amorphous”, introduced an amorphous 60-cycle ALD TiO2 film between the anatase NPs and PbS QDs. In both cases, QDs were created in separate samples with 10, 20, and 40 ALD cycles of PbS, increasing the average QD diameter from ∼2 nm to ∼10 nm.6,18 Figure 1c,d shows UV−vis absorption spectra confirming the increased size

is an indispensable tool to address atomic-scale engineering challenges. Furthermore, close analysis of the X-ray absorption near edge structure (XANES) and near-edge X-ray absorption fine structure (NEXAFS) spectral regions remains an emerging technique.12−17 In this work, we exploit electronic and geometric information contained in sulfur and oxygen K-edge XANES spectra to resolve interfacial structure of metal-sulfide nanoparticles grown on a nanostructured metal-oxide surface (i.e., PbS QDs on TiO2 nanoparticles). Of particular interest is how the TiO2 crystal structure affects the nucleation of PbS nanocrystals; accordingly, we studied PbS QD growth on a film of anatase TiO2 nanoparticles, as compared to PbS QD growth on an identical substrate precoated with a layer of amorphous TiO2. X-ray diffraction (XRD) and selected area electron diffraction (SAED) are used to determine the effects of the TiO2 surface on the crystallinity of PbS QDs of varying size. Lattice mismatch between metal-sulfide sensitizer and the TiO2 anode is found to produce bonding distortions, whereas an amorphous TiO2 layer between the anode and QD is found to help preserving the original QD crystal structure. The lattice disruption in PbS can be further quantified as bonding angle distortions through interpretation of XAS spectra with full B


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Figure 2. Diffraction and TEM characterization of PbS QDs on TiO2 NPs. XRD measurements (a) show the presence of rocksalt PbS (111) and (200) peaks in the AM set while there is no evidence of crystallinity other than the substrate in the 10× and 20× PbS samples of the AN set. SAED measurements were done on 10× PbS and 40× PbS for the AN (b,c) and AM (d,e) sets. BF-TEM images (f,g) of 10× PbS (AN) and 10× PbS (AM) show SAED regions with the embedded PbS QDs, respectively.

broader size distribution.6,18,19 For improved air stability and

and broadening of the size distribution of the PbS QDs with increased ALD cycles. From the UV−vis results and our previous work in which such UV−vis spectra are tied to TEM measurements of QD size, we conclude that the 40× PbS samples have QDs that are on average larger and possess a

passivation (Supporting Information Figure S1), the PbS QDs were subsequently encapsulated by an additional 60 ALD cycles of amorphous TiO2. C


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Nano Letters To determine the interfacial atomic structure, we performed X-ray and electron diffraction measurements on both sample sets. Figure 2a shows X-ray diffraction (XRD) data for each sample and anatase TiO2 NPs for reference. All samples from the AM group show PbS crystallinity, as seen by the appearance of (111) and (200) rocksalt PbS peaks in Figure 2a. However, in the AN samples there is no evidence of crystallinity in the QDs until 40× PbS, in agreement with the emergence of peak A4 in the S Kedge measurements that will be presented later. Selected area electron diffraction (SAED) measurements match the XRD results (Figure 2b−e). For the AM set, SAED patterns exhibit diffraction rings which are assigned to (111), (200), and (220) crystal planes of rocksalt PbS QDs. Larger diffraction spots in SAED of 40× PbS (AM) indicates increasing size of the PbS QDs in the samples formed with a higher number of ALD cycles. For the 10× PbS (AN), no crystalline phase of PbS was observed from the SAED pattern. Some crystallinity of PbS QDs is observed in the 40× PbS (AN) sample. Moreover, SAED allowed us to locally explore the possibility of an alternate crystalline phase in the AN PbS QDs. No evidence of an alternate crystal structure was found (Figure 2b,c), suggesting that the disorder is noncrystalline and amorphous (further discussion in Supporting Information). Simulated SAED patterns of anatase TiO2 (white) and rocksalt PbS (yellow) are shown for comparison. The SAED regions for the 10× PbS AN and AM samples are shown in Figure 2f and g, respectively. Overall, the diffraction results suggest that the interface between the PbS QDs and anatase TiO2 NPs prevents the formation of a consistent PbS lattice spacing; which could be due to bonding strain between the anatase TiO2 and the rocksalt PbS. To further quantify the amorphous structures as a result of strain at the interface, XAS spectra were collected. The sulfur Kedge XAS spectrum of crystalline PbS was benchmarked using reference samples consisting of commercial PbS powder, colloidal PbS QDs (5 nm), and 200× ALD PbS grown on TiO2 NPs. As shown in Figure 3a, four main features in the total fluorescence yield sulfur K-edge XAS spectrum, labeled A1-A4, are seen in each of the reference samples. Features A1 and A2, in the XANES region, correspond to electronic excitations from S 1s states to unoccupied states in the PbS conduction band, which is composed of the bonding environment between Pb 6p and S 3p orbitals.20 Features A3 and A4, in the near-edge X-ray absorption fine structure (NEXAFS) region result from shortand long-range multiple-scattering effects arising from geometric arrangements of Pb and S atoms in PbS.20−22 Spectra in Figure 3b correspond to experimental QDs from the AN and AM sets. The AN spectra deviate from reference samples in features A3 and A4 of the sulfur K-edge, suggesting that longrange order or local atomic arrangement does not correspond to rocksalt PbS. Note that a lack of feature energy shifts indicates no change in oxidation state, which remains −2. Thus, changes in spectral feature A3 and A4 do not suggest significant change in Pb and S bonding but point to a measurable structural deviation from the rocksalt octahedral coordination. At 40× PbS, the AN spectrum starts to resemble the reference samples, indicating the ability of larger QDs to overcome the interfacial effect causing strain in the PbS structure and to realign into an octahedral coordination. A linear combination fit of the 40× PbS (AN) spectrum approximates that 62% of the average QD’s structure in this sample is rocksalt (Figure 3c). In contrast to QDs grown on anatase TiO2, each sample in the AM set preserves the reference spectral features (Figure 3b, top), demonstrating that QDs deposited on an amorphous TiO2 layer

Figure 3. X-ray absorption characterization of S K and O K edges. The sulfur and oxygen XAS K-edges are shown for the reference samples and the PbS QD sample sets (a,c,d). (a) Reference samples for the S K-edge depict four features that are due to electronic transitions into unoccupied orbitals (peaks A1 and A2) and features resulting from multiple-scattering of photoelectrons (peaks A3 and A4). (b) S−K XAS for the AM set look similar to the references suggesting a rocksalt atomic environment (the features become more pronounced from 10× to 40× PbS because as the QDs grow the number of sulfur atoms found in the octahedral environment increases), whereas there are clear distinctions in the AN set, specifically in the A4 peak (starred). (c) Linear combination fit of 40× PbS (AN), using 10× PbS (AM) and 10× PbS (AN), approximates that an average QD in this sample has 62% rocksalt realignment. This fit is a first-order approximation because it does not take into account the QD size distribution. (d) O K-edge can be analyzed from orbital contributions (peaks B1−B4) and multiplescattering effects (peak B5). Broadening of the features in amorphous TiO2 are due to perturbations to Ti and O bonding interaction. The O− K XAS for the AM and AN sets have features that confirm that ALD TiO2 on anatase TiO2 and PbS QDs, respectively, is amorphous.

exhibit rocksalt crystallinity even at smaller sizes. These results are consistent with the SAED and XRD findings, discussed above. This indicates that the bonding environment of an amorphous TiO2 surface allows the energetically favorable rocksalt arrangement of Pb and S atoms. Similar trends are D


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Figure 4. Full multiple-scattering simulations of S K-edge in distorted atomic arrangements. (a) Calculated S K-edge XAS spectra are plotted for PbS in a rocksalt structure, an α-GeTe modified structure calculated from a density functional theory approach (B3LYP)30 and an α-GeTe modified structure calculated using the Hartree−Fock (HF)30 method; for comparison, the 200× ALD PbS and 10× PbS (AN) are plotted. (b−d) Local bonding environment is detailed for each structure. The starred feature (peak A4) of the S−K edge XAS is dependent on the symmetry of the local bonding environment.

nonperiodicity in amorphous TiO2 accommodates the sharp transition to periodic bonds in PbS without significant interfacial strain between TiO2 and PbS. The (101) orientation is the prevalent surface of anatase TiO2 NPs,28 which results in a significant lattice mismatch between the anatase (a = 3.78 Å, b = 9.52 Å) and rocksalt (a = 5.94 Å) structures. Thus, the interaction energy at the PbS/anatase TiO2 NP interface is greater than the energy penalty to distort the rocksalt structure in PbS. We demonstrate the nature of bonding distortions in PbS through multiple-scattering simulations of PbS clusters. Theoretical XAS spectra are simulated using the FEFF9 code,29,30 based on Green’s function multiple-scattering theory. We first obtained spectral agreement between simulated and experimental rocksalt PbS (Supporting Information Figure S4). As a basis for simulating differences observed in the S K-edge XAS spectra of the AN samples, structural data from a comprehensive theoretical study on low-energy distortions of rocksalt PbS were used.31 We found that the simulated XAS spectra for the Pb and S atoms arranged in a modified α-GeTe structure had the closest resemblance to the AN samples. Figure 4a shows the simulated XAS spectra for rocksalt PbS and PbS in α-GeTe type structure, which were computed in ref 31 with CRYSTAL under a density functional theory (DFT) approach and the Hartree−Fock method. Other atomic configurations that

observed for Pb L3 XAS in the AN and AM sets (Supporting Figure S2). To probe the bonding environment of TiO2, reference O Kedge spectra were gathered for anatase TiO2 NPs in nanostructured TiO2 film substrates, an anatase TiO2 ALD film, and an amorphous TiO2 ALD film. Figure 3d has five characteristic features of the O K-edge. Features B1 and B2 correspond to transitions from O 1s states to unoccupied hybridized orbitals containing dominantly Ti 3d and O 2p character,23 whereas hybridization between Ti 4p and O 2p creates unoccupied states for the B3 and B4 transitions.23−25 Feature B5 results from multiple scattering of the photoexcited wave and is only pronounced in the anatase samples. Broadening of all features is observed in the amorphous ALD TiO2 film, which is due to nonperiodic bonding environments.26,27 The O K-edge features for the AN and AM sets in Figure 3d are primarily probing the amorphous ALD TiO2 layers (the 60× outer-coating of ALD TiO2 deposited on each sample to protect the PbS QDs from oxidation prior to XAS), and very little variation in the spectra are observed as expected. The Ti L2,3 edge showed similar trends as the O K-edge (Supporting Figure S3). The amorphous TiO2 data indicate that O 2p/3p and Ti 3d/4p orbitals are not periodically bound as in anatase, allowing small distortions in individual Ti and O orbital mixing and bonding. Combining this result with the S K-edge XAS indicates that E


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Figure 5. Schematics depicting a simplified atomic-scale representation of the PbS/TiO2 interfaces. Shown are the interfaces created between (a) structurally distorted PbS QDs grown on a (101) anatase TiO2 surface and (b) rocksalt PbS QDs grown on an amorphous TiO2 surface. These schematics imply that the anatase TiO2 surface imposes some of its periodicity onto the bottom-up deposited PbS QDs, given the significant lattice mismatch between the anatase (a = 3.78 Å, b = 9.52 Å) and rocksalt (a = 5.94 Å) structures. Conversely, PbS QDs might be able to maintain a rocksalt structure on an amorphous surface by adjusting the underlying TiO2 bonding structure.

Figure 6. (a) HOMO−LUMO energy gap of different sized PbS QDs as a function of Pb−S−Pb bonding angle shown by (b) and (c). Distortion from the rocksalt structure is introduced as a shift in displacements of sulfur atoms in the unit cell that is used for constructing QD in the density functional theory calculations.

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PbS QDs by inducing asymmetric Pb−S bonds, whereas a nonperiodic TiO2 surface structure does not distort the rocksalt structure in PbS. Moreover, DFT calculations of the system indicate that the geometric distortions alter the electronic structure of the PbS QDs by increasing their energy gap at the interface. Our characterization approaches lay out a framework for how XAS can be exploited to provide geometric and electronic information over the interfacial bonding environment of nanomaterials for the development of nanostructured devices.

were similar to the rocksalt structure in ground state energy were considered but their differences in coordination number resulted in significantly different XAS spectra (Supporting Information Figure S5), which suggests their atomic arrangements are not present in our PbS QDs. As shown in Figure 4a−d, a larger deviation in Pb−S bond lengths and angles results in a weaker A4 intensity. This trend suggests that the decrease in the A4 peak observed for the PbS QDs grown directly on anatase TiO2, especially for the smaller QDs, stems from distortion in bond angles and asymmetry in bond lengths. For comparison, S Kedges for 200× ALD PbS and 10× PbS (AN) are plotted in Figure 4a. By using a combination of XAS data, analyzed with quantum simulations, and diffraction, it was possible to obtain a detailed atomic level understanding of the interfacial structure between PbS QDs and two types of TiO2 surfaces. The XRD and SAED analysis showed that an anatase TiO2 surface disrupts the formation of a periodic lattice in the PbS QDs, an effect that can be partially overcome in larger PbS QDs. On the other hand, an amorphous TiO2 surface does not disrupt the PbS lattice. This diffraction data was paired with XAS, where analysis with quantum simulations allowed us to conclude that the disappearance of the A4 feature in the S K-edge XANES spectra was due to the disruption of the PbS QD lattice on the crystalline TiO2 samples. Quantum simulations were used to further analyze the identified feature in the XAS spectra, to learn more about the exact nature of the disruption of the PbS QD lattice. From these simulations, we identify a possible physical cause of the disruption in PbS crystallinity. Namely, that the periodic bonds at the anatase TiO2 surface distorts the crystallinity in PbS QDs by inducing asymmetric Pb−S bonds, whereas a nonperiodic TiO2 bonding environment is flexible, allowing the growth of rocksalt PbS. This possible interpretation is illustrated schematically in Figure 5: only an asymmetric, noncrystalline PbS structure has the potential to grow on the (101) surface of anatase TiO2 NPs, whereas structural variations and flexibility in amorphous TiO2 enable the formation of crystalline PbS QDs. With these detailed structural information, a quantum simulation was performed to investigate potential effects of the bonding environment on device performance. Figure 6 illustrates the HOMO−LUMO energy gap of PbS QDs as a function of distortion in the Pb−SPb bond angle. With increased distortion, the energy gap of the QD increases, and this effect becomes more pronounced with larger-sized QDs. This is in agreement with the observation in Figure 1 that the onset of light absorption shifts to higher energy with AN samples where distortion is present. As discussed above, the lattice mismatch occurs at the PbS and TiO2 interface and the distortion becomes less pronounced as it propagates from the interface with TiO2 toward the outer surface of the PbS QDs. Accordingly, the energy gap will shrink as a function of distance from the interface where charge injection occurs. The band diagram can be modeled similarly to that of a graded band gap solar cell, but with a larger energy gap closer to the injection interface. This energy gap gradient would pose an energy barrier for electron injection, resulting in increased recombination at the interface and diminished device performance.32−36 In summary, we have demonstrated the combined use of XAS and ALD to understand and manipulate the atomic arrangement of quantum dots in a quantum-dot-sensitized solar cell architecture. From local structure measurements and quantum simulations, we found that the periodic bonds at the anatase TiO2 surface compromise the crystal structure in bottom-up deposited

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b02373. Methods of material preparation, supplementary characterizations, and additional discussion and figures are included. (PDF)

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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 The Diversifying Academia, Recruiting Excellence (DARE) Fellowship of Stanford University, partially supported this study. We would like to thank additional support at the Stanford Synchrotron Radiation Lightsource (SSRL) from J.-S. Lee, P. Pianetta, and A. Metha enabled the gathering and analysis of Xray data. K. Roelofs would like to thank the support of the Center on Nanostructuring for Efficient Energy Conversion (CNEEC) at Stanford University, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DESC0001060.

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