J. Phys. Chem. C 2007, 111, 16261-16266
16261
Upconversion Luminescence of Colloidal CdS and ZnCdS Semiconductor Quantum Dots Jianying Ouyang,† John A. Ripmeester,† Xiaohua Wu,† David Kingston,† Kui Yu,*,† Alan G. Joly,*,‡ and Wei Chen*,§ National Research Council Canada, Ottawa, ON K1A 0R6, Canada, Pacific Northwest National Laboratory, P. O. Box 999, Richland, Washington 99352, and Department of Physics, the UniVersity of Texas at Arlington, Arlington, Texas 76019 ReceiVed: June 7, 2007; In Final Form: August 2, 2007
Upconversion luminescence is observed from colloidal CdS and CdZnS quantum dots dispersed in hexanes. These binary and ternary nanocrystals were synthesized via noninjection approaches and at relatively low temperature and exhibit cubic crystal structures and narrow size distributions. The upconversion luminescence of these nanocrystal ensembles is similar to their corresponding photoluminescence emission, regarding peak shape, position, and dynamics. The upconversion luminescence yield exhibits a near-quadratic laser power dependence. Accordingly, our study indicates that the upconversion luminescence is due to two-photon excitation.
1. Introduction A great number of biological detection and imaging applications rely on conventional organic dye based fluorophores. These fluorophores are usually organic molecules which are soluble in water and also in other solvents. Some of these organic molecules display poor photostability which makes them nonideal for use in biological detection, particularly for in vivo detection.1 Most organic dyes have narrow absorption spectra, very broad emission spectra, and their luminescence decay lifetimes are nearly identical to the autofluorescence decay lifetime.2 These issues limit the use of organic dyes for practical applications in biological detection. Semiconductor nanoparticles or quantum dots (QDs) have typical dimensions of 1-100 nm. This nanoscale size can lead to the quantum confinement effect, which in turn results in interesting optical and electronic properties.3 These unique properties can be utilized for biological imaging and sensing and could overcome the limitations of conventional organic fluorophores. In comparison to conventional organic dyes, QDs exhibit size-tunable fluorescence emission, making them ideal for simultaneous detection of multiple fluorescence following excitation from a single light source. Their decay lifetimes are long (∼20-50 ns), which allows imaging of living cells without interference from background autofluorescence. Stability against photobleaching, large molar extinction coefficients, high quantum yield, and large surface-to-volume ratios make QDs superior to organic fluorophores in detection sensitivity as well as in long-term tracking of biological processes.4 Therefore, semiconductor nanoparticles or QDs are potential fluorescent probes for various biological and biomedical applications, especially cellular imaging.3 Recently, numerous publications have been dedicated to the applications of photoluminescent nanocrystals for bioimaging or bioprobing.2,5 In these reports, the images are recorded with * Corresponding authors. E-mail:
[email protected] (K.Y.);
[email protected] (A.G.J.);
[email protected] (W.C.). † National Research Council Canada. ‡ Pacific Northwest National Laboratory. § University of Texas at Arlington.
an optical microscope or a confocal microscope based on nanocrystal photoluminescence (PL). For biological applications, upconversion luminescence (UCL) techniques have several advantages over PL. Upconversion luminescence is luminescence in which the emission wavelength is shorter (higher in energy) than the excitation wavelength, in contrast to PL where the emission wavelength is lower in energy than the excitation photons. For example, autofluorescence, namely, background fluorescence from biological species such as cell proteins, can be avoided with UCL6 allowing more sensitive and higher resolution imaging or detection to be achieved. UCL of nanoparticles also has the potential to be useful in display technology, as well as in memory and lighting applications. From the application point of view, it is extremely valuable to develop novel materials as well as to understand the underlying physics of the UCL processes. In addition to biological application, UCL has many other potential applications. Recent interest in laser technology,7 threedimensional microfabrication and optical storage,8,9 displays and optical limiting,10 and imaging techniques,6 has renewed attention in UCL.11 Upconversion luminescence has been readily observed in semiconductor nanoparticles such as ZnS/Mn2+,12 CdS, CdTe,13,14 colloidal CdSe and InP,13 CdSe/ZnS core-shell quantum dots (QDs),15 and III-V QDs.16 The efficient UCL from these nanoparticles may find new applications in biological detection, diagnosis, and imaging. However, the upconversion mechanism is still an open question and appears to be somewhat material dependent. Auger recombination,16 two-photon absorption,17-19 and thermally assisted surface state processes13 have been proposed to explain UCL. The Auger process involves transfer of energy from an excited electron-hole pair upon recombination to another electron or hole creating a highly excited carrier. This carrier is then available for recombination at a higher energy than the original excitation wavelength. Two-photon absorption may occur in one of two ways: the process may proceed through an intermediate state and is then termed two-step two-photon absorption (TSTPA), or the process may proceed through a virtual intermediate state (TPA). The process occurring with a
10.1021/jp074416b CCC: $37.00 © 2007 American Chemical Society Published on Web 10/11/2007
16262 J. Phys. Chem. C, Vol. 111, No. 44, 2007 virtual intermediate state is significantly weaker and often requires higher excitation powers. Both the Auger and TPA processes are inherently nonlinear in nature, requiring the initial photon to populate an intermediate state and then further excitation to a higher excited state via either the Auger process or absorption of a second photon. Upconversion via surface state processes involves thermally populated midgap states which absorb a single photon leading to higher energy luminescence. Surface state processes show single photon power dependences and a temperature dependence characterized by increasing UCL intensity at increasing temperatures. Surface state processes are also extremely efficient, often only requiring a continuous wave source such as a He-Ne laser or a Xe lamp. In this paper, we report our investigation on the UCL of colloidal cubic CdS and ZnCdS QDs based on the measurements of the power dependence as well as decay dynamics. The nanocrystals studied were synthesized from non-hot-injection approaches developed in our laboratories recently. Our observations reveal that the UCL likely occurs by two-photon excitation. 2. Experimental Details 2.1. Synthesis of Semiconductor Nanocrystals. 2.1.1. Synthesis. Our laboratories have been actively developing both hot-injection and noninjection approaches to synthesize photoluminescent semiconductor nanocrystals.20,21 In the present study, binary CdS and ternary ZnCdS nanocrystals were synthesized by non-hot-injection one-pot approaches.21,23 Such noninjection approaches have significant advantages over conventional hot-injection syntheses, featuring ready handling, low growth temperature, high reproducibility, large-scale capability, and high quality of resulting nanocrystals with narrow size distribution and high quantum yield (QY, as measured in organic solvents such as hexanes and toluene). The present synthetic approach utilized zinc stearate (Zn(St)2, with 12.514 mol % ZnO), cadmium acetate dihydrate (Cd(OAc)2‚2H2O), and elemental sulfur as Zn, Cd, and S source compounds, respectively. The growth of these CdS and ZnCdS QDs was carried out at 240 °C in reaction flasks containing the source compounds, together with stearic acid (SA), 2,2′-dithiobisbenzothiazole (MBTS), and 1-octadecene (ODE). In a typical ZnCdS synthesis, S (0.4 mmol), MBTS (26.8 µmol), and ODE (∼5 g) were sonicated together for about 1 h; this dispersion was then transferred into a three-necked round-bottom flask containing Cd(OAc)2‚2H2O (0.4 mmol), Zn(St)2 (0.4 mmol), SA (1.2 mmol), and ODE (∼17 g) at room temperature. Afterward, the mixture was heated under vacuum to 120 °C with stirring; a clear solution was obtained after approximately 2 h. The resulting solution was further heated to 240 °C; the growth and annealing processes of ZnCdS nanocrystals were carried out at this temperature (240 °C) under a flow of purified nitrogen for a defined period of time. 2.1.2. Characterization. The resulting CdS and ZnCdS nanocrystals were intensively purified for their characterization with X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and powder X-ray diffraction (XRD).21 XPS was performed using a Kratos Axis Ultra XPS equipped with a monochromated Al X-ray source. Three measurements per sample were performed, with data collected on three local areas. For composition analyses, peak fitting was performed using CasaXPS (version 2.2.107) data processing software, with all measurements calibrated to C 1s at 285 eV. Quantification was performed using sensitivity factors provided by CasaXPS’s Scofield element library. TEM was performed on a JEOL JEM-2100F electron microscope operating at 200
Ouyang et al. kV and equipped with a Gatan UltraScan 1000 CCD camera. For measurement, 300-mesh carbon-coated TEM copper grids were used; dilute nanocrystal dispersions in chloroform were dropped on the TEM grids with subsequent drying in air. XRD patterns were recorded at room temperature on a Bruker Axs D8 X-ray diffractometer using Cu KR radiation in the θ-θ mode. XRD samples were prepared on low-background quartz plates. Optical properties of as-prepared nanoparticles were measured immediately following dispersion in hexanes. Absorption and PL spectra were collected at room temperature with 10-30 µL raw reaction aliquots dispersed in 3 mL of hexanes. Optical absorption spectra were collected with a Perkin-Elmer Lambda 45 ultraviolet-visible (UV-vis) spectrometer using a 1 nm data collection interval. Photoluminescence experiments were performed with a Fluoromax-3 spectrometer (Jobin Yvon Horiba, Instruments SA), with a 450 W Xe lamp as the excitation source, an excitation wavelength of 350 or 370 nm, and a data sampling increment of 2 nm. In the present study, the PL QY is determined with the use of quinine sulfate in 0.05 M H2SO4 (whose QY is reported to be ca. 0.546) as a reference dye. The PL spectra of a QD ensemble dispersed in hexane and the organic dye are acquired under the same setting of the Fluoromax-3 spectrometer. The optical density (OD) at the excitation wavelength of the dye and the QD sample is similar and about 0.1 in order to avoid any significant reabsorption. The PL QY of the QD ensemble is thus obtained by comparing the emission peak areas and OD of the QD sample and the dye, with corrections made for the difference in the solvent refractive indexes of the two solvents used for the dye and the QD ensemble. The upconversion emission spectra and power dependences were collected using a nanosecond optical parametric oscillator/ amplifier (Spectra-Physics MOPO-730) operating at a 10 Hz repetition rate and tunable between 220 and 1800 nm. The laser output was directed onto the particles, and emission was collected at right angles to the excitation and focused into a 1/8 m monochromator equipped with a gated intensified CCD detector. The power dependences were measured by integrating the area under the luminescence peak as a function of input power. The lifetime data was collected using the output of a femtosecond regeneratively amplified Titanium:sapphire laser system operating at 1 kHz. The 150 fs pulses of this laser at either 800 nm (for UCL) or else the second harmonic at 400 nm (for PL) were directed onto the particles, and the emission was collected at right angles and focused into a streak camera (Hamamatsu C5680). Suitable band-pass and cutoff filters were used to collect the luminescence at different wavelengths. The instrumental time resolution was determined to be about 200 ps full width at half-maximum (fwhm) using a standard scattering material. 3. Results and Discussion 3.1. Nanocrystal Characterization by TEM, XRD, and Photoluminescence with a Xe Lamp. Three different colloidal nanocrystal samples were used in the present study. The first is a binary ensemble of CdS with myristic acid as the surface capping ligands. The other samples consist of ternary ensembles of ZnCdS passivated with SA. It is noteworthy that cubic CdS nanocrystals form at relatively low growth temperature via nonhot-injection approaches,21-23 whereas hexagonal CdS QDs form at high growth temperature via hot-injection approaches.24 The atomic compositions were determined utilizing XPS to be Zn0.13-
Upconversion Luminescence of CdS and ZnCdS QDs
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Figure 1. TEM image of a CdS nanocrystal ensemble synthesized via a non-hot-injection approach. The inset displays a high-resolution image showing the lattice fringes from one of the particles.
Figure 2. XRD patterns of bulk ZnS (top), a ternary Zn0.66Cd0.34S nanocrystal ensemble (middle), and bulk cubic CdS (bottom). The dashed lines serve only to guide the eye.
Cd0.87S and Zn0.66Cd0.34S, synthesized with different feed ratios of the Zn, Cd, and S source compounds. The prepared binary CdS and ternary ZnCdS QDs are spherical in shape with narrow size distribution and cubic crystal structures, as characterized by TEM and XRD. Figure 1 displays a TEM image of CdS QDs. The inset image with high TEM resolution exhibits clear crystal fringes, indicating excellent crystallinity of the nanocrystal. Figure 2 shows the XRD patterns of ternary Zn0.66Cd0.34S, bulk cubic CdS, and bulk cubic ZnS. There is a significant shift in the direction of low-to-high Bragg angle, from CdS, to Zn0.66Cd0.34S, and to ZnS. It is necessary to point out that the ternary nanocrystals are gradient in composition rather than homogeneous, i.e., they are alloy in nature. Detailed structure characterization by solid-state nuclear magnetic resonance (NMR) to reveal the alloyed structure with Zn-rich outer layers and Cd-rich interiors will be described in our forthcoming manuscript.21 Figure 3 shows the UV-vis absorption and PL from as-prepared nanocrystals dispersed in hexanes: CdS (top), Zn0.13Cd0.87S (middle), and Zn0.66Cd0.34S (bottom). The rich absorption substructures with sharp first excitonic absorption peaks also
Figure 3. Optical absorption and photoluminescence (PL) emission spectra of CdS (top), Zn0.13Cd0.87S (middle), and Zn0.66Cd0.34S (bottom) quantum dots (QDs). The excitation was at 350 nm.
indicate narrow size distributions, as confirmed by TEM. For the three samples with an increase of Zn content from 0%, to 13%, and to 66%, the first excitonic absorption peak blue-shifts from 428, to 418, and to 396 nm, while their PL emission peak blue-shifts from 437, to 430, and to 414 nm, respectively. These three nanocrystals are similar in size, being around 4 nm as determined by TEM; therefore, the gradual blue-shift of their absorption and emission peaks suggests an increase of the band gap energy due to a compression of the crystal lattice resulting from the presence of more and more Zn atoms within the crystal lattice of CdS. Meanwhile, the band gap PL QYs for CdS, Zn0.13Cd0.87S, and Zn0.66Cd0.34S in hexane are calculated to be ca. 7.6%, 8.3%, and 7.3%, with the excitation wavelength of 350 nm. These nanocrystals exhibit similar quantum efficiencies, regardless of the incorporation of Zn atoms into the CdS nanocrystals, leading to the unique gradient structure with Zn-rich
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Figure 4. Photoluminescence (solid lines, excitation at 330 nm) and upconversion luminescence (dashed lines, excitation at 660 nm) for the CdS (top), Zn0.13Cd0.87S (middle), and Zn0.66Cd0.34S (bottom) ensembles dispersed in hexanes. For comparison, the emission intensities of the UCL and PL spectra of each QD dispersion have been normalized.
outer layers. Accordingly, as the Zn content increases, the ZnCdS QDs exhibit increased blue-shifts in their absorption and emission peak positions but retain similar quantum efficiency. For the three samples, the fwhm of their band gap photoemission spectra is 23, 21, and 20 nm, respectively. In addition to the band gap emission, each of the three samples also exhibit a broad emission at a relatively long wavelength range of 470650 nm; such emissions should originate from trap states due to the presence of various defects at both the surface and interior. 3.2. Upconversion Luminescence and Luminescence Dynamics Measured with a Laser. Figure 4 displays the UCL (excitation at 660 nm) and the PL (excitation at 330 nm) of CdS and ZnCdS QDs. The data has been normalized for clarity as, in general, the UCL emission yield is significantly lower than the PL emission yield. Clearly the UCL and PL spectra are nearly identical both in shape and peak wavelength. Such similarity indicates that in these nanoparticles the upconversion and PL likely involve similar transitions. Several recent studies reported the observation of red-shifts of UCL spectra and have attributed such red-shifts to surface state emission;15,17,19 accordingly, the similarity of our present UPL and PL suggests that the UCL may not originate from surface or midgap states. Several publications have been dedicated to the upconversion from semiconductor nanoparticles; however, the mechanism for UCL in semiconductor nanoparticles is still under debate. Often the dependence of the UCL on the input photon power dependence can yield some insight into the mechanism. A logarithmic plot displaying the excitation laser power dependences of the UCL intensity is displayed in Figure 5 for the CdS and Zn0.66Cd0.34S samples. The excitation laser power dependences were performed over 1 order of magnitude, beginning at the detection limit on the low-power end. Using the relation of
Figure 5. Upconversion luminescence power dependences for CdS (top) and Zn0.66Cd0.34S (bottom) nanocrystals.
I ∼ PK, where I is the intensity of the luminescence and P represents the input laser excitation power, the values of K are near 2 for both CdS and Zn0.66Cd0.34S samples. Therefore, the UCL excitation mechanism is through a two-photon process. Two-photon excitation can occur through a real or virtual intermediate state. Both processes would show similar quadratic laser power dependences. In TSTPA, the excitation process is determined by the combined excitation cross sections of the excitation from the ground state to the intermediate state and the excitation from the intermediate state to the final state. The overall excitation efficiency is governed by these cross sections and the lifetime of the intermediate state. Because the intermediate state lifetime can be fairly long (nanoseconds, microseconds, or even milliseconds), TSTPA can usually be accomplished with low average power continuous wave laser systems. In contrast, TPA requires high peak power because the virtual intermediate state has an extremely short lifetime (femtoseconds) and a large number of photons/s are required to achieve excitation. The CdS and ZnCdS particles reported here require nanosecond or femtosecond pulses to achieve measureable UCL. Therefore, it is reasonable to conclude that the excitation mechanism is through two-photon excitation via a virtual intermediate state, although TSTPA through a short-lived real intermediate state cannot technically be ruled out. Luminescence lifetimes of both the PL and UCL measured at or near the peak emission are displayed in Figures 6 and 7, with the results tabulated in Table 1. The PL lifetimes are multiexponential in all three samples with at least two compo-
Upconversion Luminescence of CdS and ZnCdS QDs
Figure 6. Photoluminescence lifetimes following 400 nm excitation for CdS (solid), Zn0.13Cd0.87S (dash), and Zn0.66Cd0.34S (dot) nanoparticles.
J. Phys. Chem. C, Vol. 111, No. 44, 2007 16265 CdS nanocrystals clearly results in changes to both the spectra and lifetimes. As the amount of Zn increases, the absorption, PL, and UCL spectra shift to the blue and the average luminescence lifetime decreases (Table 1). The XRD data also indicate that the average lattice spacing changes toward that of ZnS. It is important to recall that the compositional structure of these particles is not uniform, with the outer layers showing greater zinc concentration. Apparently the addition of zinc likely has a significant influence on the nature and lifetimes of the excited electronic states, in addition to an increase of the average band gap. It is likely that as the amount of zinc increases, the particles obtain more of a ZnS character, resulting in an increased band gap and decreased average lifetime. Whether the decrease in lifetime is due to energy transfer to more ZnSlike regions within the particle, or due to increased carrier trapping at the Zn-rich surface region, is not known. Our observations indicate that in the CdS and ZnCdS nanocrystals reported here, the upconversion is due to twophoton excitation. The demonstration of two-photon excitation for upconversion in nanocrystal dispersions is of significant importance for applications, particularly for biological imaging, because two-photon optical imaging has several obvious advantages over fluorescence imaging.6 Two-photon excitation minimizes tissue photodamage, phototoxicity, and photobleaching as it limits the region of photointeraction to a subfemtoliter volume at the focal point. Two-photon excitation wavelengths are typically about double the one-photon excitation wavelengths. This wide separation between excitation and emission spectra ensures that the excitation light and the Raman scattering can be rejected while filtering out a minimum of fluorescence photons. More importantly, advantages arise from the use of infrared wavelengths, thus avoiding tissue autofluorescence and increasing the tissue penetration depth.6 4. Conclusions
Figure 7. Upconversion luminescence lifetimes following 800 nm excitation for CdS (solid), Zn0.13Cd0.87S (dash), and Zn0.66Cd0.34S (dot) nanoparticles.
TABLE 1: Luminescence and Upconversion Lifetimes of CdS and ZnCdS Nanoparticles samples
PL lifetimes (ns)
UCL lifetimes (ns)
CdS Zn0.13Cd0.87S Zn0.66Cd0.34S
0.7, 7.0, 50 0.7, 7.4, 25
