NANO LETTERS
Blue Luminescence and Superstructures from Magic Size Clusters of CdSe
2009 Vol. 9, No. 2 514-518
Frank S. Riehle,†,‡ Roland Bienert,§ Ralf Thomann,† Gerald A. Urban,‡,| and Michael Kru¨ger*,†,‡ Freiburg Materials Research Center (FMF), UniVersity of Freiburg, Stefan Meier Strasse 21, D-79104 Freiburg, Germany, Institute for Microsystems Technology (IMTEK), UniVersity of Freiburg, George Ko¨hler Allee 103, D-79110 Freiburg, Germany, Institute of Physical Chemistry, UniVersity of Freiburg, Albertstrasse 23a, D-79104 Freiburg, Germany, and FRIAS, School of Soft Matter Research, UniVersity of Freiburg, Albertstrasse 19, D-79104 Freiburg, Germany Received January 16, 2008; Revised Manuscript Received December 11, 2008
ABSTRACT In this letter, we present a low-temperature synthesis route revealing a new type of ultrasmall CdSe nanoparticle family with exceptional narrow blue emissions between 437 and 456 nm and full width at half-maxima below 20 nm. Transmission electron microscopy characterization shows the uniformity of the nanoparticles, which have a diameter of 1.6 nm. After surface modification, the spherical particles assemble into nanowires, demonstrating their potential as building blocks for the generation of highly ordered superstructures. They can also be used as single source precursors for the synthesis of CdSe nanocrystals.
Over the past decades, scientists have discovered new species ranging between molecules and solids with unique sizedependent physical and chemical properties.1-3 One prominent example are colloidal semiconducting nanocrystals (NCs),4 which have found their way into various applications such as LEDs,5,6 solar cells,7 and fluorescent labeling.8-10 The ongoing research into their nucleation and growth is strongly motivated by the vision to control their morphology starting from the molecular regime.11-14 However, it is still a challenge to develop new synthesis routes leading to nanoparticles with uniform size and shape. In the case of semiconducting NCs, the relatively broad size distribution impedes the correlation of structural and physical properties on the atomic scale. In an alternative approach, different molecular clusters with well-defined structures and numbers of atoms n were synthesized. Since the molecular clusters did not exhibit band gap photoluminescence (PL), quantum confinement was only resolved by absorption spectroscopy and by photoluminescent excitement (PLE) measurements at low temperature.15-21 A first common synthetic route bridging the gap between clusters and NCs has been reported by Cumberland et al. using molecular inorganic clusters as single source precursors for the synthesis of colloidal * To whom correspondence should be addressed. E-mail: Michael.Krueger@ fmf.uni-freiburg.de. † Freiburg Materials Research Center (FMF). ‡ Institute for Microsystems Technology (IMTEK). § Institute of Physical Chemistry. | FRIAS, School of Soft Matter Research. 10.1021/nl080150o CCC: $40.75 Published on Web 01/13/2009
2009 American Chemical Society
semiconducting NCs.22 Peng et al. reported for the first time on the appearance of so-called magic size clusters (MSCs) of CdSe within a typical high-temperature NC synthesis.23 The MSCs were detected by UV-vis absorption spectroscopy in the initial phase of the synthesis, and their role as potential growth material or even as nuclei has been discussed.23,24 Kudera et al. found a whole MSC family of CdSe using UV-vis absorption spectroscopy to monitor the cluster size evolution during several hours of a lowtemperature reaction.24 In a detailed structural investigation, Kasuya et al. described ultrastable and well-defined (CdSe)n clusters, which were identified by mass spectroscopy and X-ray analysis. A three-dimensional model was correlated to experimental findings, suggesting a core cage structure for the most stable (CdSe)33 and (CdSe)34 clusters.25-27 Recently Kucur et al. reported the occurrence of a narrow PL signal at around 450 nm, which they attributed to fluorescent blue-emitting ultrastable CdSe clusters. However, the origin of the PL signal was not sufficiently proven.28 Here, we present for the first time a family of highly luminescent ultrasmall CdSe nanoparticles with emission maxima between 437 and 456 nm and narrow full width at half-maxima (fwhm) below 20 nm. The ultrasmall nanoparticles were synthesized in coexistence with larger particles from cadmium stearate and trioctylphosphine selenide (TopSe) in hexadecylamine (HDA) at 100 °C (see Supporting Information). After 300 h, an equilibrium between smaller and larger particles, represented by a narrow PL signal at 456
Figure 2. The ultrasmall CdSe nanoparticles act as growth material or even as nuclei for the larger NCs after raising the temperature from 100 to 200 °C. The narrow PL signal of the ultrasmall nanoparticles decreases in intensity while the broad PL signal shifts to higher wavelengths indicating the growth of NCs. After 11 h at 200 °C, the narrow signal has completely disappeared and a broad NC peak remains at 579 nm (red graph).
Figure 1. Spectroscopic characterization of the equilibrium between smaller and larger CdSe nanoparticles. (a) The absorbance with two dominant peaks at 446 and 416 nm (solid black graph) mainly represents the smaller particles. The corresponding PL spectrum with a narrow peak at 456 nm and a broad peak at 497 nm resulting from the smaller and the larger particles, respectively, (blue graph) as well as the PLE spectrum recorded at 460 nm (black dotted graph) are shown. The sample for the PL measurement was excited at 320 nm. (b) Additional PLE graphs recorded at different detection wavelengths between 450 and 500 nm are displayed. The position of the pronounced peak at 446 nm is independent from the detection wavelength. The broad shoulder related to the larger NCs shifts between 460 and 500 nm depending on the detection wavelength.
nm and a broad PL signal at 497 nm, respectively, was observed (Figure 1a). The narrow PL signal was correlated to the absorption of the smaller particles by PLE measurements (Figure 1a, black dotted graph) and thus proving the origin of the blue luminescence. Transmission electron microscopy (TEM) investigation of the above-mentioned equilibrium confirmed the existence of spherical particles with two mean diameters of 1.6 and 1.9 nm (Supporting Information, Figure S1a,b), which is consistent with the values calculated from the spectroscopic data according to Yu et al.29 The constituents of the particles were proven to be Cd and Se by energy-dispersive X-ray (EDX) and electron energy loss spectroscopy (EELS) (see Supporting Information, Figures S2 and S3). The larger particles exhibited a broad size distribution, which is typical for a low-temperature synthesis route,21 whereas the smaller particles showed an unexpected narrow size distribution. This can be concluded from the fwhm of the corresponding PL signals. PLE spectra recorded at different detection energies revealed that the Nano Lett., Vol. 9, No. 2, 2009
signals of the larger particles shift as a result of their broad size distribution (Figure 1b: broad shoulder of the PLE signals evolving between 460 and 500 nm). In contrast, no size distribution was resolved for the smaller particles, which can be inferred from the fact that the position of the pronounced PLE peak at 446 nm remained unchanged. The independence of the peak position from the detection wavelength (Figure 1b) provides strong evidence for the existence of a well-defined uniform particle species. In the following, we will demonstrate the different chemical and photophysical behavior of this new type of ultrasmall CdSe nanoparticle compared to conventional NCs, and finally we will estimate the particle size directly from high-resolution TEM (HRTEM) images. A clear chemical evidence that the ultrasmall nanoparticles differ significantly from conventional NCs can be seen by raising the reaction temperature from 100 to 200 °C (Figure 2). Subsequently, the narrow peak of the ultrasmall nanoparticles decreases without showing any red shift while the broad peak shifts to higher wavelengths, indicating the growth of NCs at the cost of the smaller particles. After 11 h, the peak of the ultrasmall nanoparticles has completely disappeared while a broad NC peak remains at 579 nm. Similar results have been reported by Kasuya et al.25 who studied the transformation of magic size CdSe clusters into CdSe NCs by UV-vis absorption spectroscopy. Our ultrasmall CdSe nanoparticles seem to act in the same manner as growth material or even as nuclei for the synthesis of NCs. A further distinguishing feature between conventional NCs and ultrasmall nanoparticles is their different photooxidation behaviors. Photooxidation of conventional NCs leads to a continuous blue shift and a broadening of the PL signal.30-33 For the ultrasmall nanoparticles, we found a sequential blue-shift of the narrow PL signal revealing a whole family of ultrasmall nanoparticles represented by eight well-distinguishable peaks between 456 and 437 nm (Figure 3). To the best of our knowledge, this is the highest resolved quantum confinement based on band 515
Figure 3. The development of the PL signal during several days of photooxidation reveals a whole family of ultrasmall CdSe nanoparticles. The larger particles represented by the broad peak at around 500 nm diminish whereas the smaller particles represented by the narrow peaks between 437 and 456 nm exhibit quantum confinement on an ultrafine scale. The inset picture displayed in the upper right corner represents a zoom in on the peaks.
gap PL ever reported for semiconducting nanoparticles. The corresponding absorption spectra are shown in the Supporting Information (Figure S4). Whether this quantum confinement is based on the change of the number of atoms n, structural changes, or both is a matter of further research. Post synthetic photooxidation leads to a nearly complete degradative loss of the larger NCs, which can be concluded from the weak PL signal at around 500 nm (Figure 4a: blue graph). The remaining ultrasmall nanoparticles represented by a blue-shifted signal at 443 nm can be used for further spectroscopic characterization. The PL intensity, as a function of the excitation wavelength, is displayed in the inset of Figure 4a (λex graph). The position of the PL peak was found to be independent from the excitation energy. Furthermore, we observed a common tendency of the absorption signals to broaden during the photooxidation process (Figure 4a: black graph and Supporting Information, Figure S4). A similar observation has been reported by Kasuya et al.26 for CdSe clusters with a well-defined number of atoms. It was shown that the fwhm of the absorption was dependent on the synthesis method and did not necessarily correlate with the size distribution. We attribute the broadening of the absorption to long-range interactions between the ultrasmall nanoparticles, a well-known phenomenon occurring, for example, in organic dyes.36-39 A fluorescent lifetime of 3.2 ns was calculated for the ultrasmall nanoparticles from the fitted decay curve using a three exponential fit (Figure 4b: solid black line of the blue graph). In direct comparison NCs exhibited a significantly longer lifetime of 20.2 ns, which is consistent with values reported in the literature.34 More details regarding the PL lifetime measurements are described in the Supporting Information. Since the radiative lifetime of CdSe NCs at room temperature is nearly independent from their size,35 the ultrasmall nanoparticles can be clearly distinguished from conventional NCs. Ligand exchange from HDA to pyridine led to the formation of highly ordered superstructures (Figure 5 and 516
Figure 4. Spectroscopic characterization of the ultrasmall CdSe nanoparticles. The sample was measured after 96 h of post synthetic photooxidation. (a) Blue graph: PL signal of the ultrasmall nanoparticles emitting at 443 nm with a fwhm of 19 nm. Black graph: corresponding absorption spectrum. Inset: excitation spectrum λex detected in the maximum of the emission peak at 443 nm. The intensity of the PL signal is monitored as a function of the excitation wavelength. Similar PL intensities were found for as prepared NCs (Supporting Information, Figure S5). (b) The fluorescence decay of the ultrasmall CdSe nanoparticles (blue graph) and of as prepared CdSe NCs with an emission maximum at 573 nm (red graph) is shown. The fluorescent lifetimes of 3.2 ns for the ultrasmall nanoparticles and 20.2 ns for the as-prepared NCs were calculated from the fitted decay curves using a three-exponential fit (solid black lines). More details are described in the Supporting Information.
6). The smaller particles assembled into parallel aligned nanowires with lengths of 100 nm or greater, which was not observed for the larger NCs. The wire diameter of 1.6 nm was directly determined by HRTEM and confirms the ultranarrow size distribution of the smaller nanoparticles. Even crystalline layers were resolved within the wires (inset picture of Figure 6). The layer distances of 0.33 nm are in agreement with values reported for CdSe nanorods.40 Since as prepared NCs were treated in a similar manner and did not show any tendency of wire formation, we attribute this phenomenon to the higher surface reactivity of the ultrasmall nanoparticles. In summary, we presented a new type of ultrasmall CdSe nanoparticle with an exceptional blue luminescence and the ability to form superstructures. The finest resolution of quantum confinement in semiconducting nanoparticles was demonstrated by showing eight distinguishable PL signals within the very narrow range of 18 nm. This new type of nanoparticle can be used as a single source precursor for the synthesis of NCs as well as a building block for the formation of ultranarrow nanowires with a diameter of 1.6 Nano Lett., Vol. 9, No. 2, 2009
fluorescent clusters of other semiconductors such as CdTe, PbS, Bi2S3 and so on would become available. Acknowledgment. The authors are thankful to Professor Dr. P. Gra¨ber and Y. Zhou for helpful discussions regarding the lifetime measurements, Dr. habil. R. Schneider (University of Karlsruhe, Laboratory of Electron Microscopy) and the companies JEOL (Germany) GmbH and Carl Zeiss SMT AG (Germany, Oberkochen) for support regarding HRTEM measurements, as well as C. Jackson for proofreading of our manuscript. F.S.R. thanks Dr. E. Kucur and Professor Dr. T. Nann for stimulating discussions as well as the Institute for Microsystems Technology (IMTEK, Laboratory for Sensors) of the University of Freiburg for financial support.
Figure 5. Illustration of the superstructure formation from the smaller CdSe nanoparticles (blue dots) after ligand exchange from HDA to pyridine. The larger particles (green dots) do not contribute to the wire formation. Electron microscopical images are included to show the clear transformation from individual spherical particles (HRTEM image (STEM mode), upper left corner) to nanowires of high aspect ratios (TEM image in the upper right corner).
Supporting Information Available: Synthesis conditions, experimental methods, detailed discussion of the lifetime measurements, additional TEM micrographs, and characterization techniques (EELS, EDX) as well as the absorption spectra of the ultrasmall CdSe nanoparticle family and the comparison of the PL intensities between the ultrasmall CdSe nanoparticles and as prepared CdSe NCs. This material is available free of charge via the Internet at http://pubs.acs.org. References
Figure 6. TEM image of ultranarrow nanowires from the smaller CdSe nanoparticles after ligand exchange from HDA to pyridine. The inset picture (upper left corner) shows a HRTEM image revealing a wire diameter of 1.6 nm and crystalline layers with distances of 0.33 nm (the white scale bar represents 5 nm).
nm. We assume that our ultrasmall nanoparticle species has a molecular core cage structure similar to the magic size (CdSe)n clusters (n ) 33, 34) reported by Kasuya et al.25,26 but with a higher number of atoms n, which can be concluded from the first absorption maxima at higher wavelengths. The phenomenon of band gap PL seems to occur only among the larger clusters with n > 34. We attribute this band gap PL to certain cluster symmetries, which could either be related to a higher number of atoms n, internal structural changes, or both. Further research is needed to clarify the reason for the occurrence of band gap PL and to elucidate the role of magic size clusters in the crystallization process. Potential applications of this new type of fluorophor including LEDs, lasing, and biolabeling can be foreseen, especially if Nano Lett., Vol. 9, No. 2, 2009
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