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J. Phys. Chem. C 2010, 114, 21138–21141
Electrochemical Switching of the Photoluminescence of Single Quantum Dots Praket P. Jha and Philippe Guyot-Sionnest* James Franck Institute, The UniVersity of Chicago, 929 E. 57th Street, Chicago, Illinois 60637, United States ReceiVed: August 8, 2010; ReVised Manuscript ReceiVed: September 24, 2010
The electrochemical response of the photoluminescence from single colloidal quantum dots is investigated. CdSe/CdS and CdSe/ZnS nanocrystals on a bare ITO electrode exhibit weak fluorescence due to efficient energy transfer. This is avoided by adding a spacer layer of ZnO nanocrystals film that enables typical photoluminescence and blinking characteristics. The photoluminescence from the single quantum dots can then be reversibly quenched by electrochemical injection of an electron with a reducing potential. Introduction Colloidal quantum dots have bright and sturdy photoluminescence (PL) properties, which can be modulated by charge injection.1-3 Previous PL studies of dots under electrochemical control were done on ensemble leading to a response averaged over size distribution and charging level. Studying a single dot in a known charged state would provide valuable information on the interaction of charge and exciton, both from the spectroscopy and dynamic point of view. Single quantum dot studies might also uncover significant variability, and help develop local probes of electrochemical potentials. A further motivation is to explore the role of charge and electrochemical potential on blinking.4 Initial attempts in our group to simply extend the electrochemistry to single dots on transparent ITO electrodes were unsuccessful.5 In this work, we propose an explanation for why ITO quenched the PL and report an electrode preparation that leads to the successful observation of a reversible electrochemical response of the PL of single CdSe/CdS and CdSe/ZnS quantum dots, paving the way for more detailed studies in the future. Methods The synthesis of core/shell nanocrystals follows earlier reports.2,6 Zinc oxide nanoparticles were synthesized as reported by Cozzoli et al.7 Typically, 1 mmol of zinc acetate, 0.22 mmol of tert-butylphosphonic acid (TBPA), and 7 g of n-hexadecylamine were degassed at 100 °C for ∼1 h. The temperature was then raised to 240 °C for 5-10 min. This synthetic procedure yields zinc oxide quantum dots with a first exciton peak at ∼320-340 nm. Films of ZnO nanocrystals were deposited on ITO coverslips (SPI Supplies, sheet resistance 70-100 Ω) from a 9:1 hexane-octane solution. The ZnO nanocrystal film was treated with 1,7-diaminoheptane to make it electrochemically active. A dilute solution of the core/shell CdSe/CdS or CdSe/ ZnS was deposited on the zinc oxide film. The ITO coated coverslip is the window of an airtight electrochemical cell with silver reference electrode and platinum counter electrode. The electrolyte used is 0.1 M tetrabutyl ammonium perchlorate (Aldrich, dried 12 h at 100 mT and 100 °C) in anhydrous propylene carbonate (Aldrich). The PL is measured with a homemade confocal microscope with cw 532 nm excitation between 0.1 and 1 kW/cm2. The PL is collected by an infinity * To whom correspondence should be addressed. E-mail: pgs@ uchicago.edu.
corrected oil immersion objective. It passes through a 532 nm notch filter and a 50:50 beam splitter for simultaneous collection on an APD and a monochromator-CCD. Results and Discussion When CdSe/CdS or CdSe/ZnS dots are deposited on an ITOcoated coverslip from a dilute solution, we noticed that much fewer nanocrystals are detectable compared to a glass surface. The spreading and drying of the hexane/octane drops on the ITO and glass seem identical. Therefore the coverage of dots is expected to be similar for both substrates. Furthermore, although there are a few bright dots, these are not responsive with potentials from +0.3 to -1.2 V. We then surmise that most dots are dim, while the few bright dots must be those that are remote from the electrode and electronically disconnected. The strong PL quench due to ITO is then consistent with a previous study of quantum dots on ITO.8 It was proposed that the dots are dim on ITO because they are in a stable negative charged state, which implies that an electrochemical potential applied to lower the ITO Fermi level would remove the electrons from the dot and render them bright. However, as noted above, we failed to observe any effect from the application of an electrochemical potential from +0.3 to -1.2 V, a range which definitely allows to add and remove electrons in films of dots. We therefore propose that the dots are dim on ITO because of an energy transfer to the dielectric rather than charge transfer. The ratio of the energy transfer to the radiative rate (bˆET )(kET/ kr)) for a dipole at a distance d from a semi-infinite solid is given by the following expression9,10
bˆET )
[ {( ) } ] ( 4
θ 2πn1 d λ
3
Im
ε2 - ε1 ε2 + ε1
)
(1)
where ε1 and n1 are the solvent dielectric constant and refractive index, ε2 is the substrate dielectric constant, d is the distance from the dipole to the dielectric, λ is the emission wavelength, and θ is an orientation factor of order unity. For small values of Im(ε2), the energy transfer is proportional to Im(ε2), and varies inversely to the third power of the distance between the dipole and the dielectric. The exact value of Im(ε2) is not known for the ITO-coated coverslips used. Depending on the deposition technique, it has been reported that the real part of the refractive index of ITO varies between 1.9 and 2.0, and the imaginary
10.1021/jp1074626 2010 American Chemical Society Published on Web 11/17/2010
Photoluminescence of Single Quantum Dots
Figure 1. Photon counts per 10 ms bin for CdSe/ZnS on a ZnO quantum dot film for consecutive scans of a sawtooth cyclic voltammetry from 0.3 to -1.2 V in 10 s and back to 0.3 V in 10 ms. The scans are offset vertically by 100 counts.
part lies between 0.01 and 0.03 at 600 nm.11,12 In our case, a conservative estimate is made by using n ) 2 and k ) 0.01, yielding ε2 ) 4 + i0.04. Taking θ ) 1, ε1 ) 2, λ ) 600 nm and d ) 3 nm, eq 1 gives bˆET ) 10. The expected PL QY is then ∼9% for the quantum dot. This would render the quantum dot barely detectable in our setup and such a low QY is also consistent with the ∼3 ns lifetime reported for dots on ITO surfaces.8 Having identified energy transfer as the main cause for the weak PL of the dots on ITO, we then searched for a conductive but more transparent substrate. Since energy transfer is mostly a near-field effect, we explored a thin spacer film of small zinc oxide nanoparticles with a UV optical gap to reduce the possibility of energy transfer. Furthermore, the reduction potential of ZnO is lower than for the CdSe quantum dots due to an a priori lower conduction band. Finally, although the zinc oxide quantum dot film is insulating when neutral, the films can be reduced electrochemically to provide mobile carriers13 when cross-linked with short linkers such as 1,7-diaminoheptane. The CdSe/CdS and CdSe/ZnS quantum dots are then dispersed on the cross-linked zinc oxide film as for the previous case on bare ITO. In contrast to the case for bare ITO, the number of bright dots observed on the ITO/ZnO coverslip is similar to that of dots on a glass slide, and most of the nanocrystals respond to the electrochemical potential. Figure 1 shows the PL response of a single CdSe/ZnS emitter to a sawtooth voltammetry where the potential is scanned from +0.3 to -1.2 V in 10 s and switched back in a few milliseconds to +0.3 V. We must note that in this work blinking is the only evidence we have for assigning the emitters to a single dot. The quantum dot in Figure 1 is never on when the applied bias is more negative than -1 V. Nearly every time the bias is brought back to +0.3 V, the PL recovers “instantly” to blink as earlier. The PL switches from 90 to 10% for most individual traces over
