J. Phys. Chem. C 2008, 112, 8153–8158
8153
Exciton Recombination and Upconverted Photoluminescence in Colloidal CdSe Quantum Dots Zygmunt J. Jakubek,* Jonathan deVries, Shuqiong Lin, John Ripmeester, and Kui Yu* Steacie Institute for Molecular Sciences, National Research Council of Canada, 100 Sussex DriVe, Ottawa, Ontario K1A0R6, Canada ReceiVed: NoVember 13, 2007; ReVised Manuscript ReceiVed: March 17, 2008
We studied photoluminescence (PL) of colloidal CdSe quantum dots (QDs) synthesized by a single-step method using cadmium oxide (CdO) and tri-n-octylphosphine selenide (TOPSe) as the Cd and Se sources, respectively, and tri-n-octylphosphine (TOP) as the reaction medium and subsequently dispersed in hexane. The QDs were excited by a narrow-band incoherent cw light source with photon energies near or below the first absorption maximum of the sample, and the resulting luminescence was dispersed and recorded. In the PL spectra, we identified two displaced by ∼16-38 meV strongly overlapping components with excitation energy dependent intensities. We also investigated lifetimes of the nonresonant PL decay at various excitation and emission energies using a frequency-domain method and observed multiexponential decay with three lifetimes approximately equal to 11.5(1.0), 41(2), and 155(15) ns. The low-energy component with the full width at half-maximum (fwhm) ranging from 135-155 meV was assigned to the charged exciton emission correlating with the shortest lifetime. The narrower high-energy component with the fwhm ranging from 80-90 meV was attributed to the band edge exciton emission with the middle lifetime. Photoexcitation of the QD sample in the onset of the absorption tail yielded blue-shifted (upconverted) luminescence. The blueshifted PL was determined to result from a single-photon excitation. Therefore, we claimed that the upconversion process was thermally assisted. Numerical modeling showed the blue shift to be consistent with room temperature thermal phonon distribution in the colloidal QDs. Introduction Blue-shifted, or upconverted, photoluminescence (UPL), that is, photoluminescence (PL) with photon energies exceeding the energy of an exciting photon, has been observed in colloidal quantum dots by several research groups.1–6 Various mechanisms of the upconversion process involving thermally assisted single photon excitation via intragap surface states have been proposed.1–5 Alternatively, observation of blue-shifted emission has been attributed to a two photon excitation via intragap virtual states.6 To understand the origins and nature of UPL in colloidal QDs and verify previously proposed mechanisms, we have undertaken comprehensive spectroscopic studies of CdSe colloidal QDs. A large set of PL spectra resulting from excitation of QDs in a wide energy range, from the onset of absorption to just above the first absorption maximum, has been recorded and analyzed. We have also studied nonresonantly excited luminescence and luminescence decay lifetimes. Interpretation of experimental observations has been aided by numerical modeling. Experimental Section The colloidal CdSe quantum dots (QDs) were synthesized by a single-step method using cadmium oxide (CdO) and trin-octylphosphine selenide (TOPSe) as the Cd and Se sources, respectively, and tri-n-octylphosphine (TOP) as the reaction medium.7–10 A TOPSe/TOP solution was injected into a CdO/ TOP solution at 300 °C, and CdSe QDs were grown at 250 °C for 5-10 min. Afterward the reaction flask was cooled down to an ambient temperature. For the investigation of optical properties, the QDs were dispersed in hexane. * To whom correspondence should be addressed. E-mail: Zygmunt.
[email protected];
[email protected] Spectroscopic measurements of the CdSe QDs in hexane reported here were conducted at room temperature, 21(1) °C, using a single sample sealed in a 1 cm quartz cuvette. An absorption spectrum was recorded with a Lambda-45 spectrometer (PerkinElmer). Emission spectra were measured with a Fluorolog Tau 3 spectrofluorometer (Horiba Jobin Yvon). Excitation and emission light passed through double-grating monochromators before reaching the sample cell and the photomultiplier tube detector (R928P, Hamamatsu Photonics), respectively. The spectra were accumulated under weak excitation conditions for up to 10-50 min each, but this relatively long integration time did not noticeably affect the measurement results as the PL properties of the sample remained stable for several weeks. The spectra were acquired in three groups with an increasing excitation bandwidth and an integration time to compensate for a decrease of emission intensity with decreasing excitation energy. The raw PL spectra were corrected for instrumental effects such as variation of the excitation lightsource intensity with excitation energy, variation of the excitation light bandwidth with excitation energy, instrumental function of the spectrofluorometer, and variation of the detector quantum efficiency with PL energy. Photoluminescence lifetimes were measured with the Fluorolog Tau 3 spectrofluorometer using a frequency-domain method. The emission vs excitation intensity relationship was derived with Fluorolog Tau 3 using a set of neutral-density filters (Optics for Research) to vary the intensity of the excitation light. Results and Discussion Figure 1a shows the absorption spectrum of the colloidal CdSe QD sample and the nonresonant photoluminescence11 (NPL) spectrum resulting from an excitation at 3.553 eV, which is over 1.3 eV above the average ensemble band gap. From the
10.1021/jp710854v CCC: $40.75 Published 2008 by the American Chemical Society Published on Web 05/13/2008
8154 J. Phys. Chem. C, Vol. 112, No. 22, 2008
Jakubek et al.
Figure 1. Absorption (dashed lines) and emission (solid lines) spectra of CdSe QDs in hexane. The emission spectra result from the excitation at (a) 3.553; (b) 2.130, 2.145, 2.160, 2.175, 2.191, 2.206, 2.222, 2.238, 2.254, and 2.271; (c) 2.087, 2.101, 2.116, and 2.130; and (d) 2.006, 2.019, 2.033, 2.046, 2.060, and 2.073 eV in the order of increasing intensity in the panels b-d.
position of the first absorption maximum, we have estimated12 the average QD diameter to be ∼3 nm. By fitting the absorption spectrum to a series of Gaussians, we have determined the center energy and the width (full width at half-maximum, fwhm) of the first absorption band to be equal to 2.262(1) eV and 118(1) meV, respectively. The NPL band is centered at 2.216(2) eV, which results in a 46(3) meV ensemble nonresonant Stokes shift.11 The width (fwhm) of the NPL band equals 112(2) meV, thus is just slightly smaller than the width of the first absorption band. A position of the PL band in the spectra excited near or just above the first absorption maximum has previously been observed in various QDs to noticeably vary with the excitation energy. Such variation has been attributed to size- and stateselective excitation.1,13 In this project, we have studied in detail resonant photoluminescence spectra11 (RPL) that are spectra resulting from an excitation of QDs in the onset region and near the first ensemble maximum of absorption, that is, in the case of our CdSe QD sample, in the 2.006-2.271 eV energy range (parts b-d of Figure 1). The following trends in the RPL spectra have been observed. First, the emission band is asymmetric and its width (fwhm) decreases and asymmetry increases with decreasing excitation energy. Second, the integrated emission intensity strongly decreases with decreasing excitation energy. Third, the emission band shifts to the blue with increasing excitation energy approaching the “normal,” or nonresonant, PL band (Figure 1a) at the excitation energies near the first absorption maximum (Figure 1b). Fourth, the emission band maximum is displaced from the excitation energy (indicated by the Rayleigh scattering peak in parts b and d of Figure 1). While at the excitation energies above 2.19 eV, the RPL band maximum is located on the red side of the Rayleigh peak, below that energy it is clearly displaced to the blue. It should be pointed out that at all excitation energies in the 2.006-2.271 eV range at least a part of the PL band extends to the blue of the excitation energy. Fifth, the emission intensity does not decrease to zero in the
red tail of the PL bands. In fact, in the lowest excitation-energy scans it increases with decreasing photon energy after reaching a minimum (Figure 1d). It is the result of a superposition of emission from a broad distribution of deep trapping states located within a band gap.14 The deep trapping states’ emission shows in the spectra recorded in this work a rather small contribution to the total emission, especially at the excitation energies near or above the average ensemble band gap. In the PL spectrum excited at 2.480 eV, which has been recorded down to 1.55 eV, the integrated intensity of the deep trapping emission band centered near 1.8 eV and with the fwhm equal to 0.329 eV amounts to
