VOLUME 76, NUMBER 9 PHYSICAL REVIEW LETTERS 26 FEBRUARY 1996 Electronic Energy Transfer in CdSe Quantum Dot Solids C. R. Kagan, C. B. Murray, M. Nirmal, and M. G. Bawendi Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 (Received 10 July 1995) We demonstrate electronic energy transfer between close packed quantum dots using cw and time resolved photoluminescence. Optically clear and thin, close packed quantum dot solids were prepared from mixtures of small and large CdSe quantum dots (38.5 and 62 Å, s, 4.5%). Quenching of the luminescence (lifetime) of the small dots accompanied by enhancement of the luminescence (lifetime) of the large dots is consistent with long-range resonance transfer of electronic excitations from the more electronically confined states of the small dots to the higher excited states of the large dots. PACS numbers: 73.20.Dx, 78.55.Et Close packed quantum dot (QD) solids present op- portunities to explore both the collective physical phe- nomena that develop as proximal QDs interact and the electronic and optical properties of QD solid state ma- terials with potential device applications. Advances in the fabrication of well-defined QD structures by, for ex- ample, lithographic [1], molecular beam epitaxy [2], and wet chemical [3] methods now allow the fundamental in- teractions in these structures to be uncovered. The QD is the 0D analog of the 2D quantum well (QW), hav- ing discrete electronic transitions that shift to higher en- ergy with decreasing dot diameter [4]. Interwell couplings in QW heterostructures continue to be studied for both their fundamental physics and their importance in devices [5]. QD solids provide a convenient medium for potential novel optical and electronic devices that exploit both the unique properties of the individual dots and the coopera- tive effects in the solid. For example, layers of densely packed CdSe QDs incorporated between polymeric elec- tron and hole transport materials electroluminescence with colors characteristic of the QDs [6]. Semiconductor QDs have generated interest as nonlinear optical materials be- cause their oscillator strengths are concentrated in discrete highly polarizable excitonic states [7]. Optical nonlinear- ity should be further enhanced in a QD array as coupling of electronic excitations between dots expands the exciton coherence length, enabling it to collect oscillator strength from dots within that larger volume [8]. In this Letter we present observations and analysis of electronic energy transfer in QD solids, arising from dipole-dipole interdot interactions. We spectroscopically probe electronic energy transfer between proximal dots in a close packed solid designed from a mixture of two sizes of CdSe QDs. cw and time resolved photoluminescence (PL) and photoluminescence excitation (PLE) give us independent measures of energy transfer in the mixed QD solid. Samples of CdSe QDs 38.5 (small) and 62 Å (large) in diameter s, 4.5% were synthesized according to the method of Murray, Norris, and Bawendi [9]. This synthetic route enabled us to control the dot size and optical properties and to separate the spectral features of the dots in the mixed system. The individual CdSe QDs have been extensively characterized both structurally and optically [9,10]. Organic capping groups coordinating the QD surface sterically stabilize the dots in solution. Optically clear (nonscattering), thin solid films were deposited from solutions of small and large dots [11]. All measurements were collected for films 0.1 0.4 mm thick to minimize reabsorption of emitted photons. The outer diameter of the large dots in the mixed film was ,0.05 at the emission peak of the small dots, making direct reabsorption of the luminescence from the small dots by the large dots negligible. Small-angle x-ray scattering (SAXS) was used to char- acterize the average local structure of the QD solids [11]. We collect SAXS patterns [Figs. 1(a) and 1(b)] for dots dispersed in poly(vinyl butyral) (PVB) to obtain form fac- tors for the individual dots [12]. We fit each SAXS pat- tern (solid lines) to determine dot size and sample size dis- tribution using the form factor for a sphere and allowing FIG. 1. SAXS patterns for CdSe QDs dispersed in PVB (dotted lines) fit by form factors for spheres (solid lines) (a) 38.5 Å and (b) 62 Å in diameter each with s  4.5%. Scattered intensities for (c) 38.5 Å and (d) 62 Å dots densely packed in films (solid lines) vs that for dots dispersed in PVB (dotted lines). Radial distribution functions generated for the (e) 38.5 Å and (f) 62 Å CdSe QD solids. 0031-9007 96 76(9) 1517(4)$06.00 © 1996 The American Physical Society 1517