Site-Selective Optical Coupling of PbSe Nanocrystals to Si-Based Photonic Crystal Microcavities Andras G. Pattantyus-Abraham, †,§,# Haijun Qiao, †,# Jingning Shan, ‡,| Keith A. Abel, ‡ Tian-Si Wang, † Frank C. J. M. van Veggel, ‡ and Jeff F. Young* ,† Department of Physics and Astronomy, UniVersity of British Columbia, VancouVer, British Columbia, V6T 1Z1 Canada, and Department of Chemistry, UniVersity of Victoria, Victoria, British Columbia, V8W 3 V6, Canada Received March 26, 2009; Revised Manuscript Received June 23, 2009 ABSTRACT A novel method for patterning optically active colloidal PbSe nanocrystals on Si surfaces is reported. Oleate-capped PbSe nanocrystals were found to adhere preferentially to H-terminated Si surfaces over oxide and alkyl-terminated Si surfaces. Scanning probe lithography was used to oxidize locally a dodecyl monolayer on the Si surface of a silicon-on-insulator wafer prepatterned with photonic crystal microcavities. Aqueous HF was then used to remove the oxide and expose H-terminated Si areas, yielding patterned PbSe nanocrystals on the Si surface after exposure to a nanocrystal solution. This patterning technique allows for the selective deposition of PbSe nanocrystals at the main antinode of the silicon-based microcavities. More than a 10-fold photoluminescence enhancement due to the cavity-nanocrystal coupling was observed. The integration of nanoscale functional elements, such as organic molecules, biomolecules, and nanoparticles, into micrometer- and submicrometer-scale devices is one of the challenges of present day science and engineering. 1-3 We describe here a method for patterned deposition of 5 nm PbSe nanocrystals with pattern feature sizes below 100 nm on Si surfaces. In particular, we demonstrate integration with Si- based photonic crystal microcavities for the purpose of forthcoming cavity quantum electrodynamics (CQED) ex- periments 4-6 in which an electronic two-level system is coupled to a silicon photonic cavity. Such systems present a route toward the realization of devices for quantum informa- tion processing. 7 Free-standing Si-based photonic crystal microcavities (Figure 1A) are readily prepared from silicon-on-insulator (SOI) substrates, and Q values in excess of 10 6 have recently been reported for such microcavities. 8 These microcavities are usually designed to work in the near-infrared between 1500 and 1600 nm to take advantage of tunable lasers and detectors operating in this range and also to avoid the water absorption band (1350-1450 nm). The SOI substrates are less expensive and toxic than the corresponding III-V substrates and allow for eventual integration with standard Si-based microelectronics. However, for CQED experiments, the high quality self-assembled quantum dots prepared by epitaxial growth on III-V substrates (e.g., InGaAs on InP, InGaAs on AlGaAs) are not available in Si, although Ge quantum dots are in development. 9 Commercial infrared-emission applications rely on devices fabricated epitaxially from direct band gap semiconductors, which cannot be grown with high quality on Si. An alternative that does allow direct integration with Si is the use of solution-grown infrared emitters such as colloidal semiconductor nanocrystals, which are prepared using simple wet chemistry. 10,11 These nanocrystals have fundamental excitonic transitions at energies higher than their bulk host material’s bandgap due to the quantum confinement effect. A number of compound semiconductors have small bulk bandgaps that can be shifted into the near-infrared, 12-14 for example, PbS, PbSe, and PbTe. Their suitability for cavity QED experiments and quantum information processing has recently been discussed, 7 and in particular the homogeneous line width of the excitonic transition as well as spectral diffusion remain issues to be addressed. Colloidal semiconductor nanocrystals have been coupled in a nonselective manner to GaAs- and Si-based photonic crystal microcavities. 15-20 However, the ideal realization would involve coupling a single nanocrystal to the micro- cavity near the maximum of the electric field intensity of * To whom correspondence should be addressed. E-mail: young@ phas.ubc.ca. † University of British Columbia. ‡ University of Victoria. § Current address: Department of Electrical and Computer Engineering, University of Toronto, Toronto, ON, Canada, M5S 3G4. | Current address: Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544. # These authors contributed equally to this work. NANO LETTERS 2009 Vol. 9, No. 8 2849-2854 10.1021/nl900961r CCC: $40.75  2009 American Chemical Society Published on Web 07/07/2009