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Prospects for Strained Type-II Nanorod Heterostructures Moonsub Shim,* Hunter McDaniel, and Nuri Oh Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois 61801, United States ABSTRACT: Recent advances allowing high-quality epitaxial heterointerfaces and delicately controlled shape anisotropy combined with versatile and scalable chemical synthesis make nanorod heterostructures (NRHs) appealing for various applications in photonics, electronics, and photovoltaics. When two distinct semiconductor materials with staggered band offsets are brought together in a NRH, the type-II heterojunction formed between them is expected to promote efficient separation of photogenerated carriers, and the shape anisotropy should provide directionality in guiding these separated charges. These characteristics of type-II NRHs are especially useful for solar energy harvesting. Here, we consider prospects of type-II NRHs for such applications, in particular, with a focus on an important yet often-overlooked feature of strain arising from the lattice mismatch in these materials.
T
he staggered band offset type-II heterojunctions between two dissimilar semiconductors are enabling elements in many technologies including solar cells,1 high-frequency bipolar transistors,2 and energy up-conversion.3 Efficient spatial separation of electrons and holes in solution-processable NRHs is especially appealing for cost-effective high-performance solar energy applications. Due to a large fraction of atoms being adjacent to the interface, heterojunctions in NRHs can alter the overall electronic properties much more significantly than in bulk materials. The electronic structure of NRHs can also be tuned through size-dependent quantum confinement as well as through interfacial strain arising from the lattice mismatch (which can also be size-dependent), providing versatility in band gap engineering. In polar crystals, lattice strain can lead to electric fields (i.e., via the piezoelectric effect) that may be exploited to further enhance separating and directing photogenerated carriers. In addition, anisotropic NRHs with both components physically accessible can facilitate directional guiding and extraction of electrons and holes. The expected efficient separation of photogenerated carriers has indeed been recently observed with charge separation times of ∼0.5 ps in type-II NRHs (Figure 1).4 6 There is also some evidence that in certain NRHs, in particular, CdSe/CdTe heterostructured tetrapods, interactions with the surface states and therefore carrier trapping may be significantly suppressed compared to their single-component counterparts.7 Photoluminescence lifetimes approaching several microseconds8 indicate that extracting charge carriers for useful work should be quite feasible. The charge-transfer transition across the heterointerface also extends the spectral range of utility of NRHs, with carriers generated by this transition costing much less in binding energy penalty for dissociation. Type-II NRHs with high-quality epitaxial heterointerfaces of different morphologies can be synthesized by simple extension of now well-established single-component nanorod synthesis.8 12 All of these observations invoke visions r 2011 American Chemical Society
Figure 1. Schematic of a type-II NRH and its staggered band offset illustrating charge separation with subsequent carrier extraction into electron- and hole-transport layers (ETLs and HTLs).
of vastly improved low-cost solar energy conversion systems utilizing type-II NRHs. Why then have we not yet seen advances such as recordbreaking solar cells made from easy-to-fabricate, solution-cast NRHs? Even in the most-studied II VI semiconductor NRHs, there are still many challenges to overcome before the aforementioned benefits can be realized in a functioning device. Improvements in synthesis, processing, and assembly/ordering are still much needed not only for NRHs but also for most nanoscale materials (e.g., good surface passivation while allowing desired interparticle electronic interactions, proper orientation on large length scales that can be maintained through device fabrication steps, etc.). There are also trade-offs that come with the benefits of NRHs. For examples, the staggered band offset of type-II NRHs will inevitably cause barriers for carrier transport. Hence, how NRHs are integrated into electronic and photovoltaic devices will have to be considered differently than what has been envisioned for their single-component counterparts. As two distinct phases are brought together within nanoscale dimensions of Received: August 16, 2011 Accepted: October 7, 2011 Published: October 07, 2011 2722
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The Journal of Physical Chemistry Letters a NRH, unexpected or unusual phenomena not encountered in the individual components can also arise. Understanding and controlling such phenomena may be necessary to realize even the expected benefits but may ultimately lead to new and/or improved means of exploiting these nanostructures. An example of an important phenomenon arising from the heterointerfaces in NRHs is the overall shape and composition distribution being dictated by the lattice strain. There have been mainly two approaches to synthesizing NRHs from colloidal nanorod seeds. The first and the more widely applicable approach exploits spatially selective or continued growth at the more reactive tips. Exclusive or near-exclusive deposition of the second material at the tips may be achieved by direct extension of the initial nanorod growth as exemplified by CdTe growth on CdSe nanorods8 13 or by exploiting higher-curvature/less perfect tips for enhanced reactivity (e.g., selective tip growth of Au,14 PbSe,15 or Co16 on CdS and/or CdSe nanorods). The second approach takes advantage of partial cation exchange.17 In this latter approach, the lattice mismatch and the associated strain can limit the cation exchange process and drive near-periodic structures to form,18 that is, lattice strain dictates the resulting structure. Lattice strain effects on NRH morphology are, in general, important to consider, even in the former case of enhanced tip growth, because no two distinct materials will have identical lattice parameters.
An example of an important phenomenon arising from the heterointerfaces in NRHs is the overall shape and composition distribution being dictated by the lattice strain. Figure 2 compares CdSe/CdTe NRHs of two different morphologies imposed by strain arising from the lattice mismatch. The transmission electron microscopy (TEM) image in Figure 2a shows the expected linear barbell-like structure where CdTe selectively grows at the more reactive tips. By controlling the growth rate (e.g., by reaction temperature and CdTe reagent concentrations), a linear rod/rod/rod (CdTe/CdSe/CdTe) geometry can also be obtained. In the linear geometry, biaxial strain parallel to the heterointerface is expected. The high-resolution TEM image shown in Figure 3a suggests that the effect of strain along the c-axis of the rod is minimal. The lattice mismatch between CdSe and CdTe is ∼7% based on bulk parameters. This is a substantial mismatch, and the difference in the lattice spacings of CdSe and CdTe can be used to readily distinguish the composition distribution in these linear NRHs. In Figure 3b and c, the fast-Fourier transform (FFT) of the high-resolution TEM image of Figure 3a clearly shows two distinct features corresponding to two different lattices. The images generated by selected inverse FFT in Figure 3d and e separate out the larger lattice CdTe from the smaller lattice CdSe, respectively. Figure 3f is a false-color composite image of Figure 3d and e overlaid on top of the original TEM image, verifying locations of CdTe being confined to the tip regions of the seed CdSe nanorod. The variation in the lattice parameter between the two different compositions can also be seen in Figure 3g and h. Figure 3g shows the spacing
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Figure 2. Transmission electron micrographs of linear barbell-like (a) and curved (b) CdSe/CdTe nanorod heterostructures. Schematics are shown in the insets, with orange representing CdTe regions and blue representing CdSe regions of the heterostructures.
of lattice fringes in the boxed region of Figure 3a, where the dashed line indicates the heterointerface. Remarkably, the lattice spacing abruptly changes from 0.375 nm for CdTe to 0.351 nm for CdSe at the interface within about two lattice planes (Figure 3h). The net result is that the effects of strain arising from the relatively large lattice mismatch between CdSe and CdTe are not obvious or appear insignificant in the linear barbell or rod/rod geometry. In contrast to the linear NRHs, the strain effect on morphology is quite distinct in the curved NRHs even at low magnification, as shown in Figure 2b. These remarkable NRHs are synthesized in the same manner as the linear structures except for the higher CdTe nucleation and growth temperature.12 In order to understand how this unexpected curvature arises, the CdTe domains on the CdSe seed need to be spatially resolved. Unlike the linear structure, the curved NRHs exhibit irregular shapes and unusual deviations from the expected lattice spacings of both CdSe and CdTe, making it difficult to identify CdTe domains from high-resolution bright-field images alone. High-angle annular dark-field scanning transmission electron microscopy (HAADFSTEM) images such as the one shown in Figure 4 can provide near-atomic-level details for this purpose of identifying CdTe regions. In HAADF-STEM, incoherently scattered electrons collected at high angles result in intensity that is approximately proportional to the square of the atomic number (Z). Highresolution HAADF-STEM imaging using aberration-corrected STEM can differentiate between Cd and Se/Te atomic columns by “Z-contrast.” The Z-contrast inversion, where the Cd column to chalcogen column intensity difference inverts, can be used to identify areas of CdTe growth. In Figure 4a, curved NRHs are imaged together with seed CdSe nanorods. The pair of parallel lines along the lattice fringes in the seed nanorod contrasts the converging pair of lines identifying the lattice deflections in the curved NRHs. Electron scattering intensities from the outer (I) and inner (II) parts of the curvature outlined in Figure 4b demonstrate the Z-contrast inversion between CdSe and CdTe regions. At this curved region, CdTe growth extends to the sides of the seed CdSe nanorods, causing a very different strain spatial profile than that in the linear barbell or rod/rod structures.12 Near the heterointerface generated on the side of the seed nanorod, near the outer part of the curvature, the larger lattice CdTe imposes tensile strain on CdSe, as expected. The surprising aspect is that on the opposite side (on the inner part of the curvature), the smaller lattice CdSe is under compressive strain. This unusual compressive strain is a consequence of the small size that makes 2723
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Figure 3. (a) High-resolution TEM image of a linear CdSe/CdTe barbell-shaped NRH. (b) FFT of (a). (c) Magnified view of the boxed region in (b). Images of CdTe (d) and CdSe (e) domains generated by inverse FFT of selected regions of the diffraction pattern in (b). (f) False-color composite image of (d) and (e) overlaid on the high-resolution image of (a). (g) Spacing of lattice fringes measured from the boxed region in (a) for CdSe and CdTe domains as indicated. (h) Abrupt change in the lattice spacing across the heterointerface indicated by the dashed line in (a).
Figure 4. (a) High-resolution high-angle annular dark-field scanning transmission electron micrograph of curved CdSe/CdTe NRHs imaged along with the seed CdSe nanorods. The parallel lines along the lattice fringes of the smaller seed nanorod contrast the lattice deflection due to curvature in the NRH. (b) Higher magnification of the boxed region in (a). (c) Electron scattering intensities from the two boxed regions in (b) showing Z-contrast inversion from the outer part (I, CdTe) to the inner part (II, CdSe) of the curvature.
Figure 5. High-resolution transmission electron micrographs of curved CdSe/CdTe nanorod heterostructures of increasing strain with decreasing diameter of seed CdSe nanorods from (a) to (c). Approximate compressive strain near the inner part of the curvature is indicated in each image. Scale bars are 5 nm.
bending to accommodate the lattice mismatch easier than generating strain-relieving defects. As seen in Figure 5, larger
compressive strain results with the decreasing diameter of seed nanorods. Compressive strain up to ∼5%, estimated based on 2724
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The Journal of Physical Chemistry Letters the measured radius of curvature, is observed in Figure 5c. Verification of this compressive strain by measuring lattice spacing changes has also been reported.13 In addition to this example of varying shape/morphology of NRHs, there are additional effects/benefits of strain arising from the lattice mismatch. Strain can cause band edge shifts and therefore vary the band gap substantially.19 Anisotropic strain can reduce crystal symmetry and lift band degeneracies, leading to splitting that can favor bands with lighter carrier effective mass for transport and reduce intervalley phonon scattering. The increase in carrier mobility arising from these and other strain-related effects has been exploited for performance enhancement in now pervasive strained Si MOSFETs20 and Si/Ge heterojunction bipolar transistors.21 Similarly, understanding lattice strain effects on the band structure, optical/electronic characteristics, and charge separation processes in NRHs may be of critical importance in many applications, including photovoltaics. A comparison of optical properties of the linear and the curved NRHs with distinct strain profiles may serve as a good starting point to gain some insights, that is, the curved NRHs as the highly strained extreme and the linear structures as the control where the lattice mismatch induced strain is confined very close to the interface with minimal structural distortions. Figure 6 shows the absorption, photoluminescence (PL), and photoluminescence excitation (PLE) spectra of linear and curved NRHs. Corresponding seed CdSe nanorod absorption and PL spectra are also shown for comparison. There are several similarities between the linear and the curved NRHs. First, the main PL feature of both types of NRHs arises from recombination across the heterointerface at energies significantly smaller than those of direct band gaps of CdSe and CdTe components. In some linear NRHs, as shown in Figure 6a, very weak CdTe PL can also be observed (see also, for example, ref 8). Second, PL quantum yields for both NRHs are quite low, much lower than those of CdSe seed nanorods. Lower yields are expected for recombination across the heterointerface compared to those for direct recombination that occurs in CdSe seed nanorods. Third, PLE spectra show that the recombination across the interface in both types of NRHs can be observed even if states within CdSe or CdTe regions are excited, supporting the expectations of efficient charge separation in these NRHs. There are also notable differences between the linear and the curved NRHs. As exemplified by Figure 6, PL yields are much lower for the curved NRHs than those for the linear NRHs. Although the larger degree of lattice strain may contribute, the lower PL yield of the curved NRHs is more likely to be associated with a larger heterointerface area and the proximity of possible surface traps to the interface. Another significant difference in the optical characteristics of the curved NRHs is the more prominent charge-transfer band (red tail extending well past the CdTe band edge absorption). The larger charge-transfer transition cross section of the curved NRHs does not necessarily imply strain effects because the overall heterointerface area is expected to be larger. However, much more pronounced broadening and shifting of the first exciton transition peak of the seed CdSe around ∼600 nm in the absorption spectrum (and the PLE spectrum) in the curved NRHs may be associated with strain. Such pronounced changes in the CdSe absorption feature can arise from the gradient of strain observed in the curved regions of the CdSe domain. A larger change in the degree of quantum confinement in the curved NRHs can also contribute to spectral shifts and broadening. That is, CdTe growing partially on the sides of the seed CdSe nanorods
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Figure 6. Absorption, photoluminescence (PL), and photoluminescence excitation (PLE) spectra of linear (a) and curved (b) CdSe/ CdTe nanorod heterostructures. Black curves are the corresponding seed CdSe nanocrystal absorption and PL spectra for comparison. PL spectra are normalized to absorbance at the excitation wavelength. PLE spectra were collected at the PL intensity maximum for both cases.
will cause radial spreading of the wave functions especially for the lighter effective mass electrons, which will then lead to a red shift in the CdSe “band edge” transition. This effect should be small or negligible in the linear NRHs where the side growth is minimal. The broadening may then be associated with size, shape, and location distribution of CdTe domains (i.e., inhomogeneous broadening). Such a broadening effect should also be less prominent in the linear NRHs. This explanation of red shift and broadening of the CdSe transition due to the radial spreading of the electron wave function should predict a smaller charge-transfer transition energy (the energy difference between the CdSe conduction band minimum and CdTe valence band maximum) and therefore a more red-shifted PL for the curved NRHs. However, linear and curved NRHs synthesized from a nearly identical average seed diameter exhibit a PL intensity maximum at very similar wavelengths. This discrepancy may suggest effects of straininduced band structure modification, but other complicating factors such as the inhomogeneous broadening from size/length/ location distribution need to be sorted out. While further experimental and theoretical work will be necessary in this nascent area of strain effects on optical properties of nanocrystal heterostructures,22 the more pronounced charge-transfer absorption band of curved NRHs is especially appealing for PV applications. It not only extends the spectral range in which these NRHs can harvest light but also leads to electrons and holes with a smaller binding energy, allowing lower-energy photons to be captured with little or no reduction in open-circuit voltage. The built-in potential/band offset from the type-II heterointerface is, of course, in general, advantageous for separating photogenerated carriers. However, other factors, including surface charge traps and inefficient carrier transport,23 28 rather than advantages of the type-II band offset, can and have been shown to limit performance in the limited number of cases 2725
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single-component CdSe nanorods and about 5 times that of CdTe nanorod devices. Although these proof-of-concept solar cells incorporating NRHs are far from being optimized, they already perform noticeably better (∼3 to 7 times larger conversion efficiencies) than equivalent single-component nanorod devices, demonstrating that the benefits of enhanced charge separation across type-II heterojunctions can be realized in versatile solution-processable NRHs. Interestingly, highly strained curved NRHs outperform linear NRHs,34 and it will be important to sort out how differences in lattice strain alter PV performance (e.g., lattice strain directly modifying band structure and charge-transfer rates versus effects such as interfacial area and size/location of CdTe domains dictated by the lattice strain).
Interestingly, highly strained curved NRHs have enhanced performance, and it will be important to sort out how differences in lattice strain alter PV performance. Figure 7. External quantum efficiency (red lines) of curved (a) and linear (b) CdSe/CdTe NRH-based hybrid solar cells. Corresponding absorption spectra (black lines) are also shown. The inset in (a) is the device schematic, where the red region represents a nearmonolayer of NRHs. ETL and HTL refer to electron- and holetransport layers, respectively. The inset in (b) shows the approximate band.
examined to date.29,30 Furthermore, conduction and valence band offsets of type-II NRHs unavoidably lead to inherent barriers and traps for transport. Hence, unlike most previous PV applications of nanocrystals, where they were used as both a light absorber and a charge-transport medium,31 33 transport through multiple NRHs will inevitably limit performance and need to be avoided. One approach to exploiting type-II NRHs is to place a monolayer of them between separate carrier transport layers such that NRHs act only as absorbers and charge separation centers.34 The inset of Figure 7a shows a simple PV design where a thin film of NRHs approaching monolayer thickness is incorporated into a hybrid solar cell consisting of a mesoporous electron-transport layer and a hole-transporting polymer layer. The approximate band diagram with ideal orientation of CdSe/CdTe NRHs is shown in the inset of Figure 7b. The photocurrent action spectra (or external quantum efficiency) of devices consisting of curved and linear NRHs are compared in the main panels of Figure 7a and b, respectively. Solution absorption spectra are also shown for comparison. Both devices show clear contribution from CdSe band edge absorption (near 600 nm) to the photocurrent. The reduced contribution of direct CdTe excitation is likely caused by electron trapping at the CdTe surface, as shown previously.29 Notable is the significantly larger contribution of the chargetransfer transition to the photocurrent in the curved NRH-based device over the linear NRH device. Our recent results show that, in the monolayer limit, both of these NRH PVs outperform single-component counterparts.34 The curved NRHs exhibit short-circuit current about 3 times that of devices consisting of
The example of CdSe/CdTe NRHs discussed here highlights the importance of lattice strain in determining the overall structure/morphology and the resulting optical properties. It will be important to elucidate whether or not charge-separation dynamics are also influenced by the interfacial strain, which may then help to explain the PV performance differences observed between the linear and the curved NRHs. With size-, shape-/topology-, and strain-tunable properties, type-II NRHs are beginning to emerge as promising materials for future PVs. However, there are many challenges, such as surface charge traps and difficulties in proper alignment, still lying ahead. Understanding what roles these NRHs are not suited for (e.g., as carrier-transport medium) is as important as what advantages they bring (e.g., as chargeseparation centers). With continued improvement over morphology control and surface chemistry and increasing availability of different types of nanocrystalline materials that will expand the repertoire of anisotropic NRHs available, novel solutions to these challenges and new directions in solar applications can be anticipated.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ BIOGRAPHIES Moonsub Shim is an Associate Professor of Materials Science and Engineering and a Willett Faculty Scholar at the University of Illinois. His research interests revolve around nanoscale materials, spanning materials synthesis to charging and charge-separation processes to applications in electronics and photovoltaics. Website: http://shimlab.matse.illinois.edu Hunter McDaniel received a B.E. in electrical engineering and a B.S. in physics at the UCSB before completing his Ph.D. in materials science at UIUC with Moonsub Shim. Currently, he is a Postdoc at LANL in CASP with Victor Klimov, where he 2726
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The Journal of Physical Chemistry Letters investigates nanocrystals and nanocrystal heterostructures for photovoltaic applications. Nuri Oh received his B.S. and M.S. in Materials Science and Engineering at Hanyang University in South Korea. He is currently a Ph.D. student working with Moonsub Shim at the University of Illinois. His research is motivated by the challenges of synthesizing and characterizing nanocrystal heterostructures for novel applications.
’ ACKNOWLEDGMENT This material is based on work supported in part by the NSF (Grant No. 09-05175) and the University of Illinois. Experiments were carried out in part in the Frederick Seitz Materials Research Laboratory Central Facilities, University of Illinois. ’ REFERENCES (1) G€unes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated PolymerBased Organic Solar Cells. Chem. Rev. 2007, 107, 1324–1338. (2) Dvorak, M. W.; Bolognesi, C. R.; Pitts, O. J.; Watkins, S. P. 300 GHz InP GaAsSb InP Double HBTs with High Current Capability and BV g 6 V. IEEE Electron Device Lett. 2001, 22, 361–363. (3) Seidel, W.; Titkov, A.; Andre, J. P.; Voisin, P.; Voos, M. HighEfficiency Energy Up-Conversion by an “Auger Fountain” at an InP AIInas Type-II Heterojunction. Phys. Rev. Lett. 1994, 73, 2356–2359. (4) Dooley, C. J.; Dimitrov, S. D.; Fiebig, T. Ultrafast Electron Transfer Dynamics in CdSe/CdTe Donor Acceptor Nanorods. J. Phys. Chem. C 2008, 112, 12074–12076. (5) Lupo, M. G.; Sala, F. D.; Carbone, L.; Zavelani-Rossi, M.; Fiore, A.; Luer, L.; Polli, D.; Cingolani, R.; Manna, L.; Lanzani, G. Ultrafast Electron Hole Dynamics in Core/Shell CdSe/CdS Dot/Rod Nanocrystals. Nano Lett. 2008, 8, 4582–4587. (6) She, C.; Demortiere, A.; Shevchenko, E. V.; Pelton, M. Using Shape to Control Photoluminescence from CdSe/CdS Core/Shell Nanorods. J. Phys. Chem. Lett. 2011, 2, 1469–1475. (7) Peng, P.; Milliron, D. J.; Hughes, S. M.; Johnson, J. C.; Alivisatos, A. P.; Saykally, R. J. Femtosecond Spectroscopy of Carrier Relaxation Dynamics in Type II CdSe/CdTe Tetrapod Heteronanostructures. Nano Lett. 2005, 5, 1809–1813. (8) Kumar, S.; Jones, M.; Lo, S. S.; Scholes, G. D. Nanorod Heterostructures Showing Photoinduced Charge Separation. Small 2007, 3, 1633–1639. (9) Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J. B.; Wang, L. W.; Alivisatos, A. P. Colloidal Nanocrystal Heterostructures with Linear and Branched Topology. Nature 2004, 430, 190–195. (10) Shieh, F.; Saunders, A. E.; Korgel, B. A. General Shape Control of Colloidal CdS, CdSe, CdTe Quantum Rods and Quantum Rod Heterostructures. J. Phys. Chem. B 2005, 109, 8538–8542. (11) Halpert, J. E.; Porter, V. J.; Zimmer, J. P.; Bawendi, M. G. Synthesis of CdSe/CdTe Nanobarbells. J. Am. Chem. Soc. 2006, 128, 12590–12591. (12) McDaniel, H.; Zuo, J. M.; Shim, M. Anisotropic Strain Induced Curvature in Type II CdSe/CdTe Nanorod Heterostructures. J. Am. Chem. Soc. 2010, 132, 3286–3288. (13) Shim, M.; McDaniel, H. Anisotropic Nanocrystal Heterostructures: Synthesis and Lattice Strain. Curr. Opin. Solid State Mater. Sci. 2010, 14, 83–94. (14) Mokari, T.; Rothenberg, E.; Popov, I.; Costi, R.; Banin, U. Selective Growth of Metal Tips onto Semiconductor Quantum Rods and Tetrapods. Science 2004, 304, 1787–1790. (15) Kudera, S.; Carbone, L.; Casula, M. F.; Cingolani, R.; Falqui, A.; Snoeck, E.; Parak, W. J.; Manna, L. Selective Growth of PbSe on One or Both Tips of Colloidal Semiconductor Nanorods. Nano Lett. 2005, 5, 445–449. (16) Maynadie, J.; Salant, A.; Falqui, A.; Respaud, M.; Shaviv, E.; Banin, U.; Soulantica, K.; Chaudret, B. Cobalt Growth on the Tips of CdSe Nanorods. Angew. Chem., Int. Ed. 2009, 48, 1814–1817.
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