126 Chem. Mater. 2011, 23, 126–128 DOI:10.1021/cm1027354
Embedding Quantum Dot Monolayers in Al2O3 Using Atomic Layer Deposition Karel Lambert,† Jolien Dendooven,‡ Christophe Detavernier,‡ and Zeger Hens*,† † Physics and Chemistry of Nanostructures, Ghent University, Krijgslaan 281-S3, B-9000 Ghent, Belgium, and ‡Department of Solid State Sciences, Ghent University, Krijgslaan 281-S1, B-9000 Ghent, Belgium
Received September 22, 2010 Revised Manuscript Received November 30, 2010
Semiconductor quantum dots (QDs) are promising building blocks for electronic and optoelectronic devices such as light-emitting diodes (LEDs),1 displays,2 photodetectors,3 solar cells,4,5 and field-effect transistors.6,7 A typical processing step in the fabrication of these devices is the coating of QD layers with thin films of metal oxide semiconductors or insulators. For example, in QD displays, the optically active QD layer is sandwiched between ZnO and Al2O3 waveguiding layers in order to maximize the QD luminescence to the viewer,2 and allinorganic QD-LEDs require the embedding of a QD film between metal oxide transport layers.8 In photovoltaics, the photovoltaic behavior of solar cells containing a CdSe-PbSe bilayer is enhanced by an Al2O3 interlayer, which reduces shunt resistance.5 Common techniques to deposit such metal oxide layers on top of QDs include chemical vapor deposition9 and molecular beam epitaxy.10 An attractive alternative for these methods is atomic layer deposition (ALD).11,12 ALD is a self-limiting thin-film growth method offering excellent conformality, even on samples with complex topography. Additional benefits are good reproducibility, accurate film thickness control, *Corresponding author. E-mail:
[email protected].
(1) Rogach, A. L.; Gaponik, N.; Lupton, J. M.; Bertoni, C. Angew. Chem., Int. Ed. 2008, 47, 6538–6549. (2) Wood, V.; Panzer, M. J.; Chen, J.; Bradley, M. S.; Halpert, J. E.; Bawendi, M. G.; Bulovı´ c, V. Adv. Mater. 2009, 21, 2151–2155. (3) Konstantatos, G.; Clifford, J.; Levina, L.; Sargent, E. H. Nat. Photonics 2007, 1, 531. (4) Gur, I.; Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Science 2005, 310, 462–465. (5) Sholin, V.; Breeze, A. J.; Anderson, I. E.; Sahoo, Y.; Reddy, D.; Carter, S. A. Sol. Energy Mater. Sol. Cells 2008, 92, 1706–1711. (6) Lee, J.; Shevchenko, E. V.; Talapin, D. V. J. Am. Chem. Soc. 2008, 130, 9673–9675. (7) Talapin, D. V.; Lee, J.; Kovalenko, M. V.; Shevchenko, E. V. Chem. Rev. 2010, 110, 389–458. (8) Wood, V.; Panzer, M. J.; Halpert, J. E.; Bawendi, M. G.; Bulovic, V. ACS Nano 2009, 3, 3581–3586. (9) Hanna, M. C.; Mı´ cic, O. I.; Seong, M. J.; Ahrenkiel, S. P.; Nedeljkovic, J. M.; Nozik, A. J. Appl. Phys. Lett. 2004, 84, 780– 782. (10) Woggon, U.; Herz, E.; Schops, O.; Artemyev, M. V.; Arens, C.; Rousseau, N.; Schikora, D.; Lischka, K.; Litvinov, D.; Gerthsen, D. Nano Lett. 2005, 5, 483. (11) Ritala, M.; Leskel€a, M. Nanotechnology 1999, 10, 19–24. (12) Puurunen, R. L. J. Appl. Phys. 2005, 97, 121301.
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uniformity over large areas, and relatively low growth temperatures. To date, only a few studies of ALD on QDs exist. The embedding of single QDs13 and QD multilayers14 in ZnO thin films using thermal ALD were reported. We expand on this and study the effect of film deposition by both thermal ALD and plasma-enhanced ALD (PE-ALD) on the optical properties of QD monolayers made by Langmuir-Blodgett (LB) deposition, using CdSe QDs embedded in Al2O3 as a model system. Besides the optical characterization, the layer quality is assessed using transmission electron microscopy (TEM), scanning TEM (STEM), energy dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). We find that thermal ALD is an excellent approach to embed QD monolayers in Al2O3 thin films. It leaves the QDs intact, and with CdSe/ZnS core/shell QDs, the photoluminescence (PL) is conserved after ALD. In this study, we use oleate-capped CdSe QDs suspended in toluene, synthesized according to the procedure described by Jasieniak and co-workers.15 To produce a QD monolayer, we spread typically 20-50 μL of the colloid on a water surface. The resulting submonolayer was compressed into a monolayer16 and transferred to Si or glass substrates by LB deposition.17 From AFM analysis, the transferred layers appear smooth and contain few voids (Figure 1a). TEM images of CdSe monolayers demonstrate the formation of close-packed and locally ordered monolayers (Figure 1b). Optical absorbance spectra show that the absorbance properties of the QDs in suspension and in a monolayer are similar (Figure 1c). Both thermal ALD and PE-ALD of Al2O3 were tested on these CdSe monolayers, without any sample pretreatment. The depositions were carried out in a homemade deposition system with a base pressure of 1 10-6 mbar.18 An inductively coupled plasma source was used for PE-ALD (downstream configuration, radio frequency power of 300 W).19 The sample temperature was selected to be 200 °C using a resistive heater. The reagents used for thermal ALD of Al2O3 were trimethylaluminium (TMA) and H2O, and for PE-ALD TMA and O2 plasma. (13) Kim, S. H.; Sher, P. H.; Hahn, Y. B.; Smith, J. M. Nanotechnology 2008, 19, 365202. (14) Pourret, A.; Guyot-Sionnest, P.; Elam, J. W. Adv. Mater. 2009, 21, 232–235. (15) Jasieniak, J. J.; Bullen, C.; Van Embden, J.; Mulvaney, P. J. Phys. Chem. B 2005, 109, 20665–20668. (16) Lambert, K.; Capek, R. K.; Bodnarchuk, M. I.; Kovalenko, M. V.; Van Thourhout, D.; Heiss, W.; Hens, Z. Langmuir 2010, 26, 7732– 7736. (17) Lambert, K.; Moreels, I.; Van Thourhout, D.; Hens, Z. Langmuir 2008, 24, 5961–5966. (18) Musschoot, J.; Xie, Q.; Deduytsche, D.; Van den Berghe, S.; Van Meirhaeghe, R. L.; Detavernier, C. Microelectron. Eng. 2009, 86, 72–77. (19) Dendooven, J.; Deduytsche, D.; Musschoot, J.; Vanmeirhaeghe, R. L.; Detavernier, C. J. Electrochem. Soc. 2010, 157, G111–G116.
Published on Web 12/20/2010
r 2010 American Chemical Society
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Figure 2. (a) TEM image and (b) EDX profile of a cross-section of an Al2O3 thermal ALD layer on a CdSe monolayer on a SiO2 substrate (inset scale: 60 60 nm). (c) HR-TEM image of the Al2O3/CdSe/SiO2 interface.
Figure 1. (a) AFM and (b) TEM images of CdSe LB monolayers. Optical absorbance spectra of CdSe particles in monolayers before and after (c, bottom) thermal ALD and (d) PE-ALD. The spectrum of the same CdSe QDs in suspension are given as a reference (c, top). XPS profiles of the monolayers on SiO2 substrate after (e) thermal ALD and (f) PE-ALD.
The gases entered the deposition chamber via tube lines heated to 50 °C to prevent condensation. Needle valves were used to adjust the pressure of TMA to 2 10-3 mbar, and the pressure of H2O and O2 to 3 10-3 mbar. For thermal ALD, 100 cycles were applied using pulse and evacuation times (TMA exposure-evacuation-H2O exposure-evacuation) ranging from 2 to 6-5-12 to 4-10-5-16 s. The plasma-enhanced process used 100 cycles consisting of 4 s TMA, 10 s evacuation, 5 s O2 plasma, and 6 s evacuation. The growth per cycle obtained was ∼0.9 A˚/cycle for thermal ALD and ∼1.1 A˚/cycle for PE-ALD. The optical spectra c and d in Figure 1 show that for thermal ALD, the optical properties of the particles are conserved, whereas the spectral features are lost after PEALD. Unlike thermal ALD, PE-ALD also alters the spectrum of the blank samples, which give a similar spectrum as the monolayers after PE-ALD. Similar results were obtained using longer and shorter O2 plasma exposure times (see the Supporting Information). XPS profiling on the samples was performed using Al KR X-rays. The films were etched in steps of 6 s with a beam of Ar ions to obtain a depth profile. The initially large and then disappearing Al 2p signals suggest that both deposition methods result in the formation of an Al2O3 film (Figure 1e, f). Although PE-ALD results in a stoichiometric Al2O3 film with an Al:O ratio very close to 2:3 (2:3.05), the thermal ALD process results in a slightly oxygen-rich film (see the Supporting Information). In both cases, the Cd 3d5/2 and Se 3d signals are sandwiched between the Al and Si signal with a Cd:Se ratio of about 1:1, indicative of a
layered structure with a CdSe layer between Al2O3 and SiO2. However, as compared to thermal ALD, PE-ALD results in Cd and Se signals of significantly lower intensity. This means that PE-ALD may etch the CdSe layer away from the surface. The XPS measurements on the thermal ALD treated layers show an increased C signal in the CdSe layer (Figure 1e), suggesting that the ligands are still at least partially present around the particles. No significant C signal was found after PE-ALD. From the Cd:C ratio, we estimated that about 33% of the ligands are still present in the layer after thermal ALD (see the Supporting Information). A general conclusion from the optical absorbance and XPS results is that thermal ALD is the better method for Al2O3 deposition on CdSe QDs. Therefore, in the following experiments, only thermal ALD was used. Although the XPS data suggest the deposition of a stoichiometric Al2O3 layer on top of a CdSe QD monolayer, this provides little information on the quality of this stack. To analyze this, a CdSe monolayer was deposited on a Si substrate with a 1 μm thermal oxide layer, followed by thermal ALD (200 cycles) and a cross-section for TEM analysis was made from the resulting sample. TEM images clearly show a stack of four layers, numbered from 1 to 4 (Figure 2a). Layer 1 is simply the epoxy resin, which is used to prepare the cross-section. STEMEDX line profiles (Figure 2b) show a high Al content in layer 2, indicating that this is the Al2O3 ALD film. From the TEM images, it appears that the film is homogeneously deposited on the surface and is free from pinholes. Next, layer 3 corresponds to the CdSe monolayer (peaks for the Cd signal in the STEM-EDX profile) and layer 4 is the SiO2 layer from the substrate (high Si content). Although only a few nanometers thick, the CdSe monolayer film contrasts with the other layers because of the high atomic numbers of Cd and Se compared to Al, Si, and O. High-resolution TEM shows that although the deposited Al2O3 layer is amorphous, the crystalline structure of the QDs is maintained (Figure 2c). Besides the crystal structure, also the QD interdistance in the LB film appears unaffected by ALD.
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Figure 4. PL spectra of (a) CdSe/CdS and (b) CdSe/CdS/CdZnS/ZnS QDs in suspension, and before and after thermal ALD.
Figure 3. AFM images and corresponding height profiles of (a) CdSe/ CdS and (b) CdSe/CdS/CdZnS/ZnS LB layers on Si. (c, d) TEM images of the respective layers.
To analyze the effects of ALD on the PL properties of the QDs, uncoated CdSe QDs are not a good test system due to their low initial PL efficiency (4%) and PL loss after LB deposition. Therefore, CdSe/CdS (csA) and CdSe/CdS/Cd0.5Zn0.5S/ZnS (csB) core/shell particles with a higher PL quantum yield (35 and 43%, respectively) and better stability against oxidation were used for these experiments. CdSe core particles were coated using the Successive Ionic Layer Adsorption and Reaction (SILAR) method, described by Li et al.20 (csA) and Xie et al.21 (csB). For the csA particles, 5 CdS monolayers were grown. For the csB particles, 2 CdS layers, 3 Cd0.5Zn0.5S alloy layers, and 2 ZnS layers were grown. The particles were suspended in toluene and LB deposition occurred in the same way as with CdSe cores. AFM images of csA particles deposited on Si substrates show a smooth layer with few holes (Figure 3a). The thickness of the layer is around 8 nm, consistent with the diameter of the particles þ ligand shell. TEM images of such monolayers reveal close-packed films with longrange order (Figure 3c). For the csB particles, the AFM images demonstrate that the layers consist of monolayer islands with local bilayer areas (Figure 3b). As with CdSe particles, the csB particles in the monolayer are closepacked with local order (Figure 3d). The monolayers are highly luminescent (Figure 4) and can be stored in air for weeks without significant loss of PL intensity. In both samples, the PL peak of the monolayer is red-shifted with respect to the signal of the same particles in suspension, possibly due to energy transfer between the particles in the LB film.22 The PL peak positions do not change after thermal ALD (350 cycles), although a difference in the PL intensity occurs. The reduction in PL intensity is partially caused by total internal reflection in the Al2O3 layer (20) Li, J. J.; Wang, Y. A.; guo, W. Z.; Keay, J. C.; Mishima, T. D.; Johanson, M. B.; Peng, X. G. J. Am. Chem. Soc. 2003, 125, 12567– 12575. (21) Xie, R.; Kolb, U.; Li, J.; Basche, T.; Mews, A. J. Am. Chem. Soc. 2005, 127, 7480. (22) Achermann, M.; Petruska, M. A.; Crooker, S. A.; Klimov, V. I. J. Phys. Chem. B 2003, 107, 13782–13787.
((13%, see the Supporting Information). When we correct the measured intensities for this, we find that for the csA particles, the PL is strongly quenched (3.5% of the signal remains), whereas the csB particles possess a much higher stability against ALD (69% of the PL is conserved). The difference in PL loss between the two particle types is in line with reported ALD of ZnO on QD multilayers, where ALD caused a complete loss of the PL with CdSe/CdS particles, whereas 25% of the PL remained with CdSe/ZnS particles.14 These results indicate that electron-confining core/shell QDs such as CdSe/CdS/ ZnS can preserve their PL after being embedded in a metal oxide film by ALD. This demonstrates the practical importance of ALD, especially for the fabrication of QD lighting devices. On the other hand, the PL loss of CdSe and CdSe/CdS QDs raises the question as to what interface is formed between the QDs and a metal oxide film obtained with ALD. This is an important issue. Insight in this interface is needed to further optimize the results presented here, and it may enable the formation of films in which the opto-electronic properties of the QDs and the matrix are coupled. In summary, we used thermal ALD and PE-ALD to embed CdSe LB monolayers in Al2O3. Although the particles are not compatible with PE-ALD, thermal ALD does not alter the structure or the optical absorbance properties of the layers. The deposited layers are uniform in thickness and the deposition procedure does not require any sample pretreatment. CdSe particles coated with a ZnS shell show a high stability against the process, and allow for the production of luminescent Al2O3 coated monolayers. As ALD is a flexible method that offers an easy conformal coating with many other materials, the combination of ALD with the LB technique is a promising technique for the production of optoelectronic devices. Acknowledgment. Z.H. and C.D. thank the IWT-Vlaanderen (SBO-Metacel) and Ghent University (NB-Photonics) for funding this research. Z.H. acknowledges the FWO-Vlaanderen (G.0.144.08) and BelSPo (IAP 6.10 218 photonics@be) for funding. J.D. acknowledges the FWO-Vlaanderen for a Ph. D. grant. We are grateful to N. DeRoo for the XPS depth profiles. Supporting Information Available: Additional XPS data and data analysis, and UV-vis spectra of samples after different O2 plasma exposure times (PDF). This material is available free of charge via the Internet at http://pubs.acs.org
