LETTER pubs.acs.org/NanoLett
Excitation Isotropy of Single CdSe/ZnS Nanocrystals Alexey I. Chizhik, Anna M. Chizhik, Dmitry Khoptyar, Sebastian B€ar, and Alfred J. Meixner* Institute of Physical and Theoretical Chemistry, Eberhard Karls University, 72076 T€ubingen, Germany ABSTRACT: We study the dimensionality of the excitation transition dipole moment for single CdSe/ZnS core-shell nanocrystals using azimuthally and radially polarized laser modes. The comparison of measured and simulated single nanocrystal excitation patterns shows that single CdSe/ZnS quantum dots possess a spherically degenerated excitation transition dipole. We show that the dimensionality of the excitation transition dipole moment distribution is the same for all individual CdSe/ZnS nanocrystals, disregarding the difference in core size and irrespective of variations in the local environment. In contrast to the emission transition dipole moment, which is oriented in one plane, the excitation transition dipole moment of a single CdSe/ZnS quantum dots possesses an isotropy in three dimensions. KEYWORDS: CdSe/ZnS, quantum dot, nanocrystal, transition dipole moment, higher order laser modes, confocal microscopy
F
luorescent colloidal CdSe quantum dots have attracted growing attention in the past years.1-4 Their size-tunable optical properties find applications in light-emitting diodes,5 solar cells,6 lasers7 and biological imaging.8 One of the most fundamental parameters of any quantum emitter is the dimensionality of its transition dipole moment (TDM). Although the fluorescence emission and excitation of the single emitter originates from a linear TDM, its spatial distribution of the emission and excitation can be different from that of single dipole emitters with fixed dipole axis. As was theoretically predicted by Efros,9,10 in contrast to organic fluorophores with fixed linear emission TDM, CdSe nanocrystals have a degenerate emission TDM oriented in two dimensions. Such a system is characterized by a unique “dark axis”, which is oriented normal to the transition dipole plane, and which does not couple to the light field. This has been experimentally shown by Empedocles et al.11,12 using polarization microscopy and later, by several groups using defocused wide-field fluorescence imaging.13-16 Remarkably, little attention has been paid to the dimensionality of excitation TDM of CdSe nanocrystals, while this parameter strongly influences the excitation efficiency of the quantum dot, which is important for numerous applications. By rotating the excitation polarization, Empedocles et al. observed a weak excitation polarization dependence, which was attributed to simultaneous excitation of multiple overlapping electronic states, decreasing the degree of polarization.11 In this Letter, we investigate the dimensionality of the excitation TDM of single CdSe/ZnS core-shell quantum dots using higher order laser modes (see ref 17 for review). This technique has proven to be very efficient in probing the three-dimensional orientation and dimensionality of the excitation TDM of single molecules,18-20 Nile Red nanospheres,21-23 SiO2 nanoparticles,24,25 gold nanoparticles,26 and gold cones.27 r 2011 American Chemical Society
The excitation pattern of an individual quantum emitter obtained by scanning it through the focal region of an azimuthally or radially polarized laser beam (abbreviated later as APLB and RPLB, respectively) with donut-shaped intensity profile has a peculiar shape, which is characteristic for the dimensionality and/ or orientation of the emitter’s excitation TDM. By comparing the experimental pattern with the theoretical images one can determine the dimensionality and/or three-dimensional orientation of the excitation TDM of the emitter. The excitation patterns obtained for all the investigated nanocrystals reveal that the CdSe/ZnS quantum dots exhibit spherically degenerate excitation TDM (i.e., excitation isotropy) in contrast to the emission TDM, oriented in two dimensions, irrespective of the differences in their core size or the local host environment. These findings are of fundamental importance for further understanding of the exciton generation mechanisms in CdSe/ZnS nanocryastals as well as for applications where uniformly high light absorption or F€orster resonance energy transfer efficiency is required. We studied commercially available CdSe/ZnS core-shell quantum dots, soluble in toluene or in water. This includes toluene soluble quantum dots QD1 (Evidot, Evident Technologies, emission centered at 557, 580, 593, and 629 nm), QD2 (Lumidot, Sigma Aldrich, at 610 nm) and QD3 (PlasmaChem at 610 nm) and water-soluble quantum dots QD4 (Qdot Invitrogen, emission centered at 565 nm). For optical single particle measurements, toluene soluble nanocrystals were embedded in a thin polymer film covering a glass cover slide of 170 μm thickness. For this purpose, toluene soluble quantum dots were dispersed in toluene, followed by mixing with a 1% solution of Received: November 18, 2010 Revised: January 18, 2011 Published: January 27, 2011 1131
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Nano Letters poly(methyl methacrylate) (PMMA, [C5O2H8]n). A 10 μL droplet of this mixture was spin coated on a glass cover slide to obtain a 50-70 nm thick polymer film as determined by atomic force microscopy (AFM). Water-soluble quantum dots were spin coated without polymer. As a result of the spin coating procedure, the spacing between the neighboring quantum dots is of the order of several micrometers, which is enough to acquire the excitation patterns of individual nanocrystals. Reference samples with Nile Red fluorescent spheres purchased from Molecular Probes (Leiden Netherlands) were produced similarly. All optical measurements were performed at room temperature using a home-built confocal microscope (based on an Axiovert 135 TV, Zeiss) equipped with a mode conversion optical line for the generation of an APLB or RPLB.24,25 The nanocrystals were excited at λ = 488 nm (Arþ laser) in the diffraction limited laser focus of a high numerical aperture (NA = 1.25) immersion oil objective lens (Plan-Neofluar, 100 NA = 1.25 oil immersion, Zeiss). The excitation density was varied from 50 to 1000 W/cm2 in different experiments. The collected fluorescence was focused onto the active area of a spectrally integrating avalanche photo diode (APD) (SPCM 200, Perkin-Elmer). The typical image resolution was 100 100 or 200 200 pixels. For each pixel, the signal was integrated over 5 ms, resulting in an acquisition time of 50 or 200 s, respectively. Monodirectional scanning doubles this time to the value of 100 or 400 s for one fluorescence image. The WSxM software from Nanotech28 was used for image processing. Further details of the setup can be found elsewhere.24,25,29 First, we would like to consider the excitation patterns observed upon scanning the quantum emitters, possessing different orientation and dimensionality of the excitation TDM, through the focal area of an APLB and RPLB. Figure 1a,b shows the field intensity distributions as a function of the radial coordinate within the focal region of an APLB and RPLB, respectively. The distributions are calculated according to the parameters of our experimental setup. Whereas an APLB has only in-plane electric field components with a donut-shaped intensity distribution, a RPLB also possesses longitudinal electric field components, which form a distinct peak in the center of the focal area of a high NA objective lens30 (Figure 1b). The excitation of a single quantum emitter, possessing a fixed linear TDM with an APLB results in a pattern consisting of two nearby bright spots of elliptical shape, if the TDM is oriented horizontally or tilted (Figure 1f,e). Because of the absence of the longitudinal component of the field, emitters with vertically oriented TDM cannot be excited and therefore are not observable (Figure 1d). Figure 1g-i shows three calculated patterns of an individual emitter possessing fixed linear TDM excited with a RPLB. Emitters, having different TDM orientation cause different excitation patterns, which allow for precise determination of TDM’s orientation by comparing experimental and simulated patterns. Such images have been typically observed upon excitation of single dye molecules18,19 or SiO2 nanoparticles,24,25 possessing a stable linear TDM. In more rare cases, an individual molecule can have a degenerate TDM isotropically distributed in a plane (for instance, due to high symmetry of the molecular structure31) or can exhibit fast flipping between two TDMs forming an angle, close to 90° (in case of tautomerization32-34). As a result of the excitation of such a molecule with higher order laser modes, the pattern exhibits a ring shape when the TDM-plane is oriented horizontally. Vertically oriented molecules are depicted as double-lobe
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Figure 1. Panels a and b show the cross section of the field intensity distributions within the focal region of an azimuthally and a radially polarized laser beam, respectively. Blue and red curves show the intensity of the inplane and longitudinal field components, respectively. Panel c exhibits the coordinate system which was used for theoretical simulations (d-u). Images d-f and g-i show the simulated excitation patterns of a single quantum emitter with a stable linear excitation transition dipole moment, excited with an azimuthally and radially polarized laser beams, respectively. The arrows represent the orientation of the transition dipole. Images j-l and m-o demonstrate calculated excitation patterns of the emitter (excitation with an azimuthal and radial mode, respectively), possessing a degenerate excitation transition dipole moment, oriented isotropically in a plane, which can be modeled as two perpendicular linear transition dipoles. Panels p-r and s-u show the simulated excitation patterns of an emitter, possessing excitation isotropy and excited with azimuthally and radially polarized laser beam, respectively. Note that the intensity of the calculated excitation patterns is normalized to one scale in each row.
patterns upon excitation with APLB or an elongated spot in the case of a RPLB. Figure 1j-o shows characteristic examples of simulated patterns, modeled with two orthogonal linear dipoles. Typical examples of nanoparticles possessing degenerate TDMs, distributed isotropically in space are Nile Red spheres. These nanometer-sized polystyrene spheres contain a large number of fluorescent molecules (∼200), oriented randomly in space. Therefore the sphere possesses emission and excitation isotropy.21-23 In contrast to the emitters, with fixed linear or degenerate TDM-plane, such an isotropic emitter exhibits the same shape of excitation pattern, independently on its orientation. A dye-doped nanosphere, excited with an APLB, renders a ring-shape pattern due to interaction of horizontal components of the molecules’ TDM with the in-plane component of the field (Figure 1p-r). The excitation of a sphere with a RPLB gives rise to an intense spot in the center of the image caused by the strong longitudinal component and a weaker ring due to the interaction with the in-plane component of the field (Figure 1s-u). Using the knowledge of the shape of excitation patterns, typical for each TDM distribution, we study the dimensionality 1132
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Figure 2. Experimental excitation patterns created by scanning the same single CdSe/ZnS quantum dot (sample QD3, see experimental details), through the focal region of a linearly (a), azimuthally (b), and radially (c) polarized laser beams. The excitation patterns demonstrate fluorescence blinking, proving that the patterns originate from individual nanocrystal (see main text for details). Insets in panels b and c show excitation patterns of Nile Red fluorescent spheres featuring excitation isotropy, excited with an azimuthally and radially polarized laser beams, respectively.
of the excitation TDMs of single CdSe/ZnS quantum dots. In the first part of the study, we carried out imaging of CdSe/ZnS nanocrystals soluble in toluene (QD1, QD2, and QD3). According to the specification of the nanocrystals, they showed photoluminescence (PL), centered at five distinct spectral ranges. Each of the single-particle emission spectra had a single-band structure with full width at half-maximum of about 15 nm, which is typical for PL of an individual CdSe/ZnS nanocrystal at room temperature. Figure 2 shows typical examples of excitation patterns for one single CdSe/ZnS quantum dot (QD3) obtained by scanning it through the focal region of a linearly (a), azimuthally (b), and radially (c) polarized laser beam. The images demonstrate fluorescence blinking, which is caused by ionization of the nanocrystal.35,36 Such binary flickering between “on” and “off” states is evidence that the excitation pattern originates from an individual quantum system. The pattern obtained upon excitation of the nanocrystal by linearly polarized Gaussian beam has no characteristic features that enable determination of the dimensionality and orientation of the excitation TDM of the particle. However when the nanocrystal is scanned through the focal region of the APLB or the RPLB it exhibits a peculiar shape of the pattern, specific for a particular TDM orientation and dimensionality. Although strong intensity fluctuations make it difficult to determine precisely the shape of the excitation patterns, the images strongly resemble those obtained upon excitation of Nile Red fluorescent spheres (see insets of Figure 2b,c). This observation indicates that the nanocrystals possess excitation isotropy. Remarkably, all studied CdSe/ZnS nanocrystals soluble in toluene (QD1, QD2, and QD3) revealed identical shape of the excitation patterns disregarding the spectral range of the emission. Since the PL tunability of the nanocrystal is imparted from its size and, partly, composition, this result suggests that excitation isotropy is not related to a specific size or composition of the nanocrystal but is rather a common property of the CdSe/ZnS quantum dots. Although the quantum dots QD1, QD2, and QD3 were produced by different suppliers, all the toluene soluble CdSe/ ZnS nanocrystals possess a hydrophobic coating (organic molecules). To compare nanocrystals coated with different types of protection layers, we studied also water-soluble quantum dots (QD4), spin coated on the top of glass cover slides without polymer matrix. Figure 3b,e shows measured excitation patterns of an individual CdSe/ZnS nanocrystal upon scanning through the focal region of APLB and RPLB, respectively. The images exhibit
Figure 3. Cross sections through the experimental (b,e) and simulated (c,f) patterns of an individual CdSe/ZnS quantum dot (sample QD4, see experimental detail), excited with an azimuthally (a) and radially (d) polarized laser beams.
significantly shorter “off” states with respect to nanoparticles investigated in the first part of the study. Excitation patterns revealed a very good agreement with theoretical images, corresponding to a spherically degenerate excitation TDM (Figure 3c,f). Comparing the cross sections through the centers of the experimental and theoretical patterns shows excellent agreement (Figure 3a,d). This suggests that the studied single CdSe/ZnS quantum dots QD4 also have a spherically degenerate excitation TDM. As has been shown in Figure 1p-u, the shape of the excitation pattern of the emitter, possessing excitation isotropy, does not change when the emitter changes its orientation with respect to the sample surface. Analyzing a large number of nanocrystals (more than 300) deposited by spin coating on the sample surface showed that all the particles exhibited the same shape of the excitation pattern. Figure 4a,b shows the same area of the sample scanned by azimuthally and radially polarized laser modes, respectively. Each of the images contains excitation patterns of the individual water-soluble CdSe/ZnS quantum dots QD4. Whereas the brightness of the different patterns varies due to the variation of the single particle emission TDM orientation, the shape of the excitation patterns is identical. Thus, we note, that all studied single CdSe/ZnS quantum dots possess a spherically degenerate TDM (i.e., excitation isotropy), disregarding the type of coating, the core size of the nanocrystal or the different local environment. This result is in good agreement with observation of the weak excitation polarization dependence of the single CdSe/ZnS nanocrystals,11 since in both studies the nanocrystals were 1133
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is acknowledged. We thank Professor Christophe Dujardin and Dr. Anne Pillonnet for providing the samples QD4 and Karsten Potrik for editorial assistance.
’ REFERENCES Figure 4. Excitation images of several individual CdSe/ZnS nanocrystals (sample QD4, see experimental details) obtained by scanning the same sample area through the focal region of an azimuthally (a) and radially (b) polarized laser beam.
excited in a region with a relatively high density of electronic states. While a distinct individual state should have a distinct transition dipole orientation, excitation of multiple overlapping states decreases the degree of excitation polarization. In this case, the weak correlation between the polarization of the excitation light and the PL intensity can also be attributed to fluctuations of the single particle PL. It is remarkable that in contrast to CdSe/ZnS quantum dots, where the CdSe core possess hexagonal crystal structure, Si/SiO2 nanocrystals with diamond cubic crystal structure of the core demonstrate isotropy in both excitation and emission.37,38 The fundamental difference between the dimensionality of the emission and excitation TDMs of CdSe/ZnS nanocrystals shows that the type of the crystal lattice structure plays a leading role in process of generation and recombination of the exciton and, moreover, can cause different dimensionality of the emission and excitation TDMs of individual nanocrystals. The single nanocrystal excitation patterns, presented in Figures 2-4 have been acquired with excitation density near 100 W/cm2, which slightly exceeds the density used in the study carried out by Empedocles et al.11,12 (60 W/cm2). However, variation of the excitation density from 50 to 1000 W/cm2 did not lead to a change of the excitation pattern shape, which suggests that the dimensionality of the single CdSe/ZnS nanocrystal is not sensitive to the change of the excitation density within this range. In summary, using higher order laser modes we have studied the dimensionality of the excitation TDM of individual CdSe/ ZnS nanocrystals. By comparing experimental and simulated excitation patterns, we concluded that the quantum dots possess a spherically degenerate excitation TDM, in contrast to the emission TDM, oriented in two dimensions. The study of a large number of single nanocrystals with different core sizes, both deposited on the top of glass cover slide and embedded in a polymer matrix, revealed that dimensionality of CdSe/ZnS quantum dots is neither affected by the local environment, nor depends on the size of the core or the type of the coating of the nanocrystal. Excitation isotropy of individual nanocrystals proves them to be appropriate for applications where uniformly high light absorption or F€orster resonance energy transfer efficiency is required.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT Financial support by the Forschungsschwerpunktprogramm Baden-W€urttemberg and from the European Commission through the Human Potential Program (Marie-Curie Research Training Network NANOMATCH, Contract No. MRTN-CT-2006-035884)
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