Electrostatic Force Microscopy and Spectral Studies of Electron

Jun 26, 2012 - Electrostatic Force Microscopy and Spectral Studies of Electron. Attachment to ... measurements show unambiguous signatures of 2−3 ex...
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Electrostatic Force Microscopy and Spectral Studies of Electron Attachment to Single Quantum Dots on Indium Tin Oxide Substrates Sibel Ebru Yalcin,†,‡,§ Boqian Yang,†,‡ Joelle A. Labastide,† and Michael D. Barnes*,†,‡ †

Department of Chemistry and ‡Department of Physics, University of Massachusetts, Amherst, Massachusetts 01003, United States S Supporting Information *

ABSTRACT: We report electrostatic force microscopy (EFM) studies combined with wavelength-resolved photoluminescence imaging of electron attachment to individual CdSe/ZnS quantum dots (QDs) coupled to semiconducting tin-doped indium oxide (ITO) substrates. Quantitative EFM measurements show unambiguous signatures of 2−3 excess electrons on individual QDs on ITO, while the distribution of measured recombination energies of QDs coupled to ITO shows ≈ −35 meV red shift (compared to QDs drop-cast on clean glass), the signature of a second-order quantum-confined Stark effect resulting from multiple-electron attachment to the QDs. We also show that the extent of QD charging can be tuned by modulating the ITO bias: EFM measurements show that ≈4 electrons are added to QDs under −2 V applied ITO bias, whereas only ≈2 electrons can be removed from the QDs for +2 V applied bias arising from Fermi level mismatch of ITO with respect to the QDs. Voltage-correlated spectral measurements on ITO coupled QDs showed a spectral modulation in their peak fluorescence energies, which can be attributed to addition or removal of electrons from the QDs.

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INTRODUCTION Understanding charge and energy exchange processes that occur between quantum dots (QDs) and semiconducting polymers1,2 and substrates3,4 is of central importance to many QD-based optoelectronic device applications.4−10 For several years, time-resolved photoluminescence studies have provided key insights into mechanisms and time scales for energy and charge transport in QD-coupled systems.3,11−14 Recently, T. Lian and co-workers3 explored the correlation among PL decay, intensity fluctuations, and fluorescence intermittency (“blinking”).15−19 The suppressed blinking activity and reduced fluorescence intensity of single QDs on ITO, along with fluorescence lifetime shortening, were attributed to interfacial charge transfer from ITO to QDs and the formation of negatively charged QDs.3 However, the amount of the transferred charges and the associated Stark shift in the wavelength-dependent PL imaging are not known. In addition, as an alternative explanation, an energy-transfer process was proposed by Guyot-Sionnest for the observed weak fluorescence of QDs placed on ITO and the associated fluorescence intermittency.12 In the previous work from our laboratory,20 polarization-modulated spectral shifts in CdSe-oligo phenylene vinylene (CdSe-OPV) hybrid QD systems showed a decrease in recombination energy that could be associated with singleelectron Stark interactions, suggesting that wavelength-resolved imaging could serve as a sensitive probe of charge attachment in these systems. In this research, we used spectrally resolved photoluminescence (PL) imaging in combination with time-resolved measurements and electrostatic force microscopy (EFM) to © 2012 American Chemical Society

probe quantum dot charging and the connection with blinking suppression of QDs on ITO. By spectrally resolving the QD photoluminescence, we were able to probe changes in the distribution of recombination energies that can be connected to the (second-order) Stark interaction of the QD exciton with the electric field from excess surface charges,20 while EFM measurements enable quantitative charge and polarizability determination of single QDs coupled to ITO. EFM studies of QD charge states for different ITO biases (0, +2, and −2 V) showed that 2−3 electrons are transferred when QDs are placed on unbiased ITO; we show further that the extent of QD charging can be tuned by modulating the ITO bias. The occurrence of QD blinking and its relation to chargetransfer processes have attracted a great deal of theoretical and experimental interest.3,15,21,22 Zunger and co-workers investigated the photoluminescence properties of epitaxial QDs with unequal numbers of optically injected electrons and holes, showing that the spectral properties of individual dots are profoundly modified by excess charges.23 Since the first observation of blinking suppression in colloidal core−shell QDs immersed in electron-rich solvents (β-mercaptoethanol),24 there has emerged a significant body of evidence supporting the idea that QD blinking can be suppressed by the presence of excess (negative) charges,25,26 although this is not the only means by which PL intermittency is affected. Recently, the role of electrostatic potential on quantum dot blinking was explored by Galland et al.26 where two types of QD blinking Received: June 14, 2012 Published: June 26, 2012 15847

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Figure 1. (A) Representative intensity and lifetime trajectories of single QDs (a and b) on ITO and (c) on glass. (B) AFM surface height image of QDs on ITO surface, and the corresponding surface height line scan at the point of the red line is presented at the bottom.

were identified.27 In “Type A” blinking, an intradot charging event (trion formation) leads to enhanced nonradiative exciton relaxation, whereas “Type B” blinking is associated with the trapping of hot carriers in surface states; the presence of a negative electrostatic potential near the surface can inhibit hotcarrier trapping thereby “turning off” the nonradiative decay pathway. In the “Diffusive Coordinate Model (DCM)” of quantum dot blinking developed by Frantsuzov and Marcus (F-M),28 blinking suppression derives from fluctuations in the energy separation, ε, defined as the energy separation between 1Se and 1Pe electron levels. When ε exceeds the energy difference between the 1S3/2 hole energy and intraband trap states, hole trapping is inhibited by energy conservation requirements, thereby suppressing QD blinking. Thus, within the DCM picture, blinking suppression in charged QDs ultimately derives from Stark perturbation of electron (and hole) levels associated with single-electron trapping near the QD surface, which increases the energy difference between perturbed 1Se and 1Pe electron states (of magnitude ≈100 meV for 1 nm distance between the charge and QD surface).29 Because the Stark interaction also reduces the recombination energy of the QD by lowering (with respect to the Fermi level) the energies of the 1Se and 1S3/2 electron−hole states, the readily measured red shift in the emission spectrum can offer unambiguous evidence of single (or few)-electron QD charging.20 In this report, we show that the spectral statistics and fluctuations for individual QDs are strikingly different on ITO and on clean glass. Understanding the spectral and blinking properties of charged QDs is essential for photovoltaic applications, especially in an ambient environment at room temperature.

coating on ITO-coated coverslips (SPI Supplies, product no. 6465-AB, sheet resistance = 8−12 ohms). See the Supporting Information for the characterization studies using atomic force microscopy (AFM). In order to correlate our spectral and EFM results with previous studies of blinking suppression,3 fluorescence intensity and lifetime trajectories were recorded for 20 single QDs on glass and ITO using time-tagged-timeresolved (TTTR) photoluminescence spectroscopy. The QD samples were excited with 440 nm pulsed excitation with a typical duration of ∼50 ps using wide-field epi-illumination. A 1.4 NA, 100× microscope objective was used to focus excitation light onto the sample. A 435 nm dichroic mirror and a 530 nm long pass emission filter were used to filter out residual laser light and sample autofluorescence. QD fluorescence was collected through the same objective and routed to an Avalanche Photodiode (APD, id-Quantique, id100-50) for single photon counting. The laser, detector, and time-correlated single photon counting (TCSPC) system together provided an approximately Gaussian instrument response function with a full width at half-maximum of ∼180 ps. The left pane of Figure 1 shows a comparison of representative fluorescence intensity and lifetime trajectories of single QDs on ITO and on glass. Other trajectories for single QDs on ITO are shown in Figure S2 of the Supporting Information. Intensity trajectories were constructed by binning the detected photons within a 100 ms time window, and fluorescence lifetime trajectories were obtained by fitting the TCSPC histograms constructed from photons within a 0.5 s bin time. We obtain results that are qualitatively similar to those previously reported by Lian et al.3 Single QDs on ITO show two distinctive types of intensity and lifetime trajectories: most of the QDs, such as the one represented in Figure 1A, show suppression of blinking, while others (Figure 1A, pane b) show the usual telegraphic on−off blinking behavior22 similar to that seen on glass substrates. This seems to indicate local heterogeneities in contact potential which could affect QD charging (or apparent lack thereof). Figure 1B shows a typical surface height image from AFM measurements of QDs on ITO surfaces with ≈2−3 nm peak-topeak surface roughness. For all QDs on glass or ITO, an attenuation of fluorescence intensity is accompanied by the

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RESULTS AND DISCUSSION Fluorescence Intensity and Lifetime Trajectories. For all measurements, we used core−shell QDs (Evidots 607, Evident Technology, ≈4.5 nm core/1.5 nm shell, ≈6−7 nm mean outer diameter) similar to QD samples in previous studies.3,12 CdSe/ZnS QDs were diluted in toluene to ≈10−9 M and sonicated for 60 min. The samples were prepared either by drop casting diluted QD solutions on plasma-cleaned glass coverslips (Fischer Scientific, catalog no. 12-544-14) or by spin 15848

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reduction of lifetimes.3 The same behavior has been observed in numerous other systems where blinking suppression has been reported.12 Additionally, recent published reports address a different type of blinking where intensity quenching does not correlate with the charging phenomena.26 Therefore, as discussed below, we have employed several complementary techniques such as wavelength-resolved PL imaging, quantitative electrostatic force microscopy, and bias-dependent spectral measurements to provide definitive evidence of QD charging. Spectral Statistics. We examined the spectral and intensity fluctuations of QDs on ITO and glass and compared their spectral and intensity trajectories to study correlations with their suppressed blinking properties. Wavelength-dependent PL imaging on single QDs was performed using an inverted microscope coupled to a high-speed imaging charge-coupled device (CCD, Princeton Instruments PIXIS 400B) and the spectrograph (Princeton Instruments, Acton Research). In order to accurately determine the peak QD emission wavelength, we used a typical integration time of 2 s on the spectrograph CCD for samples excited at 440 nm from a continuous wave laser (Crystalaser, model BCL-005-440, ∼500 μW power, 15 μm diameter spot size). It is known that large spectral fluctuations and intensity quenching for charged QDs have been associated with the presence of excess charges.26,30 Figure 2a shows typical intensity trajectories from single QDs on ITO (red) and on glass (black) substrates. For this particular example, the blinking behavior of QDs on ITO is suppressed and

accompanied by a reduction in average photoluminescence intensity, which could be due to either interfacial electron transfer3 or energy transfer12 for QDs on ITO. It is interesting to point out that the low-frequency undulation in the QD−ITO fluorescence intensity trajectory in Figure 2a is a feature commonly seen in QD systems where blinking is suppressed.19,31 We had proposed that such long time-scale intensity recurrences could be understood within the DCM picture by replacing the Lorentzian hole-trapping line shape function28 with a softer, Gaussian-type profile.31 Such diffusional characteristics are also manifest in the time evolution of QD center frequencies: Figure 2b shows representative spectral trajectories (peak QD recombination energy as a function of time) for individual QDs on ITO and on glass. The corresponding emission energy histograms in Figure 2c show that for QDs on ITO, the mean-square fluctuations in recombination energy are nearly a factor of 2 larger (19.7 meV fwhm, red) for the QD−ITO system than for the dropcasted QDs on glass (10.5 meV fwhm, black). In order to probe changes in the recombination energy resulting from surface charging, it was necessary to measure the distribution of peak recombination energies from multiple QDs on ITO and glass. Unlike earlier experiments on epitaxial QDs, where controlled numbers of charge carriers can be optically injected,23 it is unfortunately not possible within our experimental framework to examine a single QD with and without surface charge. Figure 3 shows a comparison of the

Figure 3. Distribution of fluorescence peak photon energies for 300 QDs on ITO (red) and glass (black). The mean value of peak photon energies is 2.080 eV (standard deviation of 45 meV) for QDs on glass and 2.047 eV (31 meV) for QDs on ITO. The peak-to-peak red shift is approximately −35 meV.

distribution of peak recombination energies for 300 single QDs on ITO (red) and on glass (black) substrates. The peak of this distribution for QDs on glass is 2.080 eV (with a full width at half-maximum of 45 meV) and 2.047 eV on the floating unbiased ITO substrate (with a full width at half-maximum of 31 meV). Using the distribution for QDs on glass as a reference, we observed a peak-to-peak red shift of approximately −35 meV. Previous studies have shown that the (second-order) Stark shift on the band-edge luminescence of QDs resulting from the presence of 1−3 excess electrons causes a similar spectral shift,20,30 while higher charge states (depending on the spatial arrangement of the charges) can manifest as a blue shift in recombination energy due to a firstorder correction scaling as ⟨r2⟩ that affects electron and hole energies differently.30 Thus, the red-shifted recombination

Figure 2. (a) Representative PL intensity trajectories from two different single QDs on glass (black) and ITO (red) with 2 s timebinning. (b) Time evolution of peak-recombination energies for the same QDs. (c) Histogram of center frequencies from the trajectories in pane b showing enhanced spectral fluctuations for the ITO system. 15849

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energy distribution for QDs on ITO provides compelling evidence for charge attachment. Voltage-Correlated Spectral Measurements and Observed Spectral Modulation. Our wavelength-resolved PL measurements demonstrate a red shift in the distribution of QD center wavelengths anticipated for single (or few)-electron attachment from ITO to the QD. To further understand the correlation between electrostatic perturbations and spectral properties of QDs, we studied the effect of applied bias to ITO and observed the corresponding spectrally resolved fluorescence response of individual QDs. Several groups have reported the spectral dependence of single QDs32 or quantum rods33 to an external electric field, where the field was applied using a photolithographically patterned Au/Ti or Au/Cr electrode on a quartz substrate creating an interdigitated electrode configuration with 5 or 20 μm electrode spacing. In our case, the electric field is applied by connecting the ITO surface to a dc power supply (Agilent Technologies) and grounding the microscope stage. By applying various biases, we can shift the Fermi level of ITO with respect to the Fermi level of QDs. Applying a negative bias raises the Fermi level of ITO, thus creating an energetically favorable way for electrons to move from ITO to QDs, resulting in negatively charged QDs, while the positive bias lowers the Fermi level of ITO and creates a possible electron flow from QDs to ITO. However, due to the Fermi level mismatch between the QD and ITO, switching the bias from equal negative to positive values does not create equal amounts of electron transfer. We have observed this unequal amount of charge transfer with EFM measurements on QDs coupled to biased ITO substrates [see Electrostatic Force Microscopy (EFM) Measurements]. For consistency between voltage-dependent fluorescence experiments, only the voltages applied with respect to ground were considered. Figure 4 shows histograms of peak fluorescence wavelengths for QDs on glass (black solid line), floating (unbiased) ITO (red dashed line), and biased ITO for −3 V (gray dashed line), 0 V (green dashed line), and +3 V (yellow dashed line). All the

histograms were generated from 300 different QDs. The peak of the center wavelength distribution for QDs on floating (unbiased) ITO shows a clear red shift (−35 meV, or ≈10 nm) with respect to that of QDs on glass. On the other hand, center wavelength distributions for QDs on biased ITO substrates show bimodal behavior with significant overlap with the QD/ glass reference distribution. Notably, these experiments demonstrate that it is possible to modulate the peak emission photon energies of charged QDs (QDs on ITO substrate), with respect to that of neutral QDs (QDs on glass substrate) by changing the bias voltage to the ITO substrate at room temperature. Electrostatic Force Microscopy (EFM) Measurements. In order to definitively establish the presence of excess charges on ITO-coupled QDs, we employed electrostatic force microscopy (EFM) at various applied biases to ITO. EFM measurements were carried out using an Asylum Research MFP-3D SA atomic force microscope (AFM) with conductive probes (SCM-PIT from Bruker Nano). For the bias-dependent EFM measurements, an AFM controller was used to apply a DC voltage to the tip and the ITO at the same time. EFM allows for the quantitative extraction of the number of charges transferred as a function of ITO bias. EFM experiments were performed as described previously in our recent work30 with a modification that the glass substrate is replaced with an ITOcoated coverslip. QDs were charged by applying a bias voltage to the ITO substrate with VITO values of 0, +2, and −2 V (see Figures 5 and 6). Measurements of the phase shift were

Figure 5. EFM phase shift as a function of applied tip voltage for VITO values of 0, +2, and −2 V to ITO substrate. EFM can detect the Fermi level shift of ITO for the applied bias. The number of excess electrons on QDs for the corresponding ITO bias is given.

recorded as a function of the different tip voltages in a 1 V step while maintaining a constant lift height of 20 nm to avoid any electron tunneling during the experiment as well as enabling the measurement of long-range electrostatic forces (see Figure S4 of the Supporting Information). EFM phase images were acquired by disabling the slow scan axis. This method allows repeated scans of the same line which converts the usual y axis to a time evolution of different scans for various tip voltages (see the Supporting Information for the EFM scans).34 Figure 5 shows the observed phase shift in the cantilever under the applied bias to ITO substrate. Since the EFM tip behaves as a probe, various tip voltages allow for monitoring the changes in the Fermi level of ITO due to different applied ITO bias. However, to extract the charge information using the cantilever phase shift, total EFM voltages (VEFM), defined as the difference between the tip and the ITO voltages (VEFM = Vtip − VITO), are required. The extracted phase shift as a function of

Figure 4. Distribution of QD center wavelengths correlated with applied bias (0 and ±3 V) to an ITO substrate compared with respect to emission spectra of QDs on glass (black solid line) and on floating (unbiased) ITO (red dashed line). Each distribution has 300 QDs. Histogram distributions of QDs for voltage-applied ITO substrates show a bimodal distribution where one of the arms overlaps with the QDs on glass distribution while the other one overlaps with distribution from QDs on floating ITO. On the contrary, histogram distributions of emission peak wavelengths of QDs on floating (unbiased) ITO (red dashed line) show a clear red shift (peak to peak, −35 meV ≈ 10 nm). 15850

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charges is directly manifested as an offset in parabola origin. For example, at +2 V applied bias to ITO, the phase shift shows only a quadratic dependence due to the absence of QD charging. More negative voltages shift the Fermi level of ITO and allow charge transfer from ITO to QDs that leads to a strong contribution to the linear term of the cantilever phase shift (see Figure 5 and Table 1 for the A coefficient). Our quantitative charge−force imaging measurements on single QDs revealed that QDs on ITO substrates carry 2−3 excess electrons. Since the charging will occur at the contact region, we expect that the charge distribution will be asymmetric and result in red-shifted band-edge luminescence from the QDs. The QDs placed on ITO with zero bias voltage show on average ≈2−3 electrons on the QD surface due to the equilibration of the Fermi levels which involve transfer of negative charges to the QDs.3 For these charged QDs, we have observed an approximately −35 meV red shift in their peak fluorescence energies (see Figures 3 and 4). However, our previous calculations of induced Stark perturbation to the electron−hole wave functions due to an excess electron moving from a conjugated organic ligand (OPV) to the QD core showed an approximately −70 meV red shift in the band-edge luminescence.20 Thus, the measured (average) red shift of −35 meV for QDs on ITO is roughly 50% smaller than that previously observed in the CdSe−OPV system where it was assumed that the electrostatic interaction was derived from only a single point charge. As the EFM results point to 2−3 (average) excess negative charges on the QD through interaction with ITO, it is useful to consider the magnitude of the Stark modification of recombination and comparison to experimental spectral measurements. Consider two electrons located near the surface of a quantum dot separated by an angle Φ. In the limit where Φ ≈ 0°, the Stark energy should be approximately twice that of a single charge at a distance S from the QD surface (here S would be taken to be the ZnS capping layer thickness). With an S of ≈1.5 nm and a screening constant of 5.2 for ZnS (also ignoring additional screening interactions from localized charge vacancies in the ITO), two localized electrons would produce a change in recombination energy of ≈100 meV, roughly a factor of 3 larger than our observation. To understand this effect, we looked at a generalization of the electrostatic potential expression for multiple charges in terms of Legendre polynomials given by Wang.37 Figure 7 shows a calculation of the total Stark modified electron−hole recombination energy for 2 point charges (at a fixed distance from the QD surface) as a function of angle Φ between them. In this calculation, we considered contributions of both dipole (⟨r cos θ⟩) and quadrupole (⟨r2 (3 cos2 θ −1)⟩) terms. As Φ → 180°, the sum of ⟨r cos θ⟩ terms (mixing 1Se and 1Pe states) vanishes; the quadrupole term, ⟨r2 (3 cos2 θ −1)⟩, that mixes 1Se and 1De electron states is non-zero with an upper-bound estimate of ≈20 meV for a perfectly colinear arrangement (Φ = 180°) of two charges. From inspection of Figure 7, it appears that an angle of ≈125° between the two excess electrons provides the best agreement for the −35 meV (average) Stark modified recombination energy. Unlike the case of QDs on floating or grounded ITO, the distribution of center wavelengths for the multiply charged (6−7 excess electrons) species, implied from EFM measurements for QDs on negatively biased ITO, does not appear to be dramatically different than the glass reference distribution. We speculate that this noneffect could derive from a combination of first- and second-order Stark

Figure 6. Phase shift versus VEFM measured for single QDs on a biased ITO substrate. The red, blue, and green curves are the best fit for the polynomial law of ΔΦ = AVEFM + BVEFM2 where A = 0.059°/V and B = −0.25°/V2 for QDs on a 0 V biased ITO, A = 0.020°/V and B = −0.25°/V2 for QDs on a +2 V biased ITO, and A = 0.176°/V and B = −0.29°/V2 for QDs on a −2 V biased ITO substrate. Inset: Schematic of EFM setup for interleaved scan. Note that the phase-force response of the Asylum Research MFP-3D is opposite to commonly reported literature.

VEFM is presented in Figure 6. Note that the ITO roughness does not affect the EFM measurements since the roughness is limited to 2−3 nm which is at least an order of magnitude lower than the typical lift height of 20 nm used in our experiments. A cross section of the phase images of QDs, obtained in a lift mode, was used to extract the phase information for different tip and ITO voltages. The quantitative charge extraction was performed as discussed previously.30 The data presented in Figure 6 were fitted to a polynomial function of the form ΔΦ = AVEFM + BVEFM2 where ΔΦ is the phase shift of the resonant peak (degree),30,35 and the coefficients A and B are defined as A = [Λ/(kz2)]q and B = −(Λ/k)(3α/z4), respectively. Here Λ is the quality factor, k is the spring constant, z is the lift height, and α is the electric polarizability. For the EFM tip, Λ = 186, k = 2.8 N/m, and z = 20 nm (lift height). From determination of A and B values from fitting of ΔΦ versus VEFM, the surface charge, q, and the polarizability, α, can readily be determined30 (Table 1). An important control Table 1.

a

VITO (V)

A (deg/V)

B (deg/V2)

No. of electrons on QDs

0 +2 −2

0.059 0.020 0.176

−0.25 −0.25 −0.29

≈ 2−3 (2.2 ± 0.45) ≈ 0−1 (0.75 ± 0.36) ≈ 6−7 (6.61 ± 0.33)

a The best polynomial fit (ΔΦ = AVEFM + BVEFM2) to Figure 6 gives us the A and B coefficients for three different ITO biases (VITO = 0, +2, and −2 V). Using coefficient A obtained from the equations above, the charges on QD were computed.

for calibration of the charge signal is the electric polarizability extracted from the B term in the expression for the cantilever phase shift. Our EFM measurements yielded a numerical value for α of 2 × 10−34 C2m/N which is close to the value obtained in previous measurements of the polarizability of CdSe QDs that reported a value of ∼1 × 10−34 C2m/N.36 The presence of surface charges that were caused by QD charging is also apparent in our EFM data presented in Figure 5. Since the cantilever phase shift, ΔΦ = AVEFM + BVEFM2, carries both a linear term due to the surface charges (attractive or repulsive) and a quadratic term (always attractive) due to induced dipole−dipole interactions, the presence of surface 15851

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ASSOCIATED CONTENT

S Supporting Information *

Additional details of single-QD images on ITO and glass, single-QD fluorescence intensity and lifetime trajectories on ITO, and AFM and EFM measurements on ITO-coupled QDs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address §

Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, NM 87544. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.E.Y. and B.Y. acknowledge support from the Polymer-Based Materials for Harvesting Solar Energy (PHaSE), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Grant DE-SC0001087. The Asylum Research MFP-3D SA equipment used for EFM was purchased under PolymerBased Materials for Harvesting Solar Energy and Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Grant DE-SC0001087. J.A.L. acknowledges support from the Polymer MRSEC (NSF Grant DMR-0820506). M.D.B. gratefully acknowledges support from the U.S. Department of Energy (DE-FG02-05ER15695). The authors thank Austin Cyphersmith and Nikhil S. Malvankar for their helpful comments and discussions.

Figure 7. 2-electron Stark corrections to the QD recombination energy as a function of angle Φ between the two charges 1.5 nm from QD surface; ε1 (⊞) is proportional to |⟨r cos θ⟩|2 and vanishes for Φ = π, while ε2 (△) is proportional to |⟨r2 (3 cos θ − 1)⟩|2 (θ is an internal polar coordinate). The solid line represents the sum of ε1 and ε2.

corrections to electron and hole energies that essentially offset each other; for four or more excess negative charges, the firstorder correction is positive (by increasing the energy of the 1Se state), while second-order effects are always negative. Future work will examine this question in more detail.

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CONCLUSIONS

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Our combined wavelength-resolved imaging and EFM measurements show unambiguous signatures of charge attachment to QDs on ITO substrates. When QDs are coupled to semiconducting ITO substrates, the fluorescence from QDs is attenuated, lifetimes are shortened, the spectral diffusion width is nearly doubled, and the spectral peak emission energies show a red shift as large as approximately −35 meV with respect to that of drop-cast QDs on glass substrates. This observed spectral shift can be attributed to a perturbation on the envelope wave functions of the photogenerated electron and hole in QD due to the second-order Stark correction to the electron and hole wave functions via an asymmetric arrangement of charges on QDs in contact with the ITO. EFM measurements revealed that the QDs on ITO substrate are charged with 2−3 electrons due to the Fermi level equilibration. Applying a negative bias to ITO (−2 V) adds ≈4 electrons to QD, whereas a positive bias (+2 V) removes ≈2 electrons from QD to ITO. We also show that the Fermi level asymmetry in QD and ITO does not allow an equal number of electron transfers to and from the QDs for the same applied ITO voltages. Voltage-correlated spectral measurements revealed a spectral modulation in the band-edge luminescence of QDs. The demonstrated ability to manipulate the spectral behavior of single QDs including intermittency and emission energies in the presence of charges has applications for future tunable optoelectronic components such as electrooptical modulation devices.

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp305857d | J. Phys. Chem. C 2012, 116, 15847−15853