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Voltage controlled photoluminescence (PL) blinking behavior in CdSe nano-particles (NPs) is studied. The NPs are sandwiched between a p-type silicon substrate and a thin Au electrode, which serve respectively as source and drain electrodes. The blinking PL from the NPs can be controlled by the bias voltage across the two electrodes. However, luminescence diminishes when photo excitation power is weak or bias is lower than a threshold voltage. The observed PL blinking is explained by a circuit model, which involves charge tunneling, Fowler-Nordheim (F-N) emission, and charging effect. The blinking intensity is controlled by the number of F-N emitted electrons whereas the pulse interval is associated with the time required for hole accumulation in the NPs. The intensity of luminescence blinking for NP clusters is found to be much higher compared to that of blinking from isolated NPs. This is explained by a collective recombination of F-N emitted electrons and accumulated holes in the NP clusters. This study provides a simple way of controlling PL blinking.
©2010 Optical Society of America
1. Introduction
Because of high quantum efficiency and a band gap corresponding to the visible range, CdSe NPs have attracted wide interest in a variety of fields for use as: single photon light sources [1–3], light emitters [4–6], solar cells [7–9] and tags to molecular sensors [10]. However, these applications suffer from alternating bright and dark states (blinking behavior) that often takes place in NPs of small diameter. Blinking behavior is also observed in single quantum dots of different materials [11–13]. It is believed that non-radiative recombination due to the presence of surface states in NPs [14] is responsible for the dark state, but the origin of blinking remains controversial. A widely accepted explanation is the presence of charge separation in which holes are localized in the NP and the wavefunction of lighter electrons extending into the surface is trapped by the defect state of the surrounding matrix [15]. Recently, several studies [16–19] showed that hopping of light excited charges between CdSe NPs and substrates can alter the number of charges available for recombination, and alter the intensity of the PL. Hence, the PL characteristics of NPs can be altered by the availability of electrons and holes for recombination [20]. In this context, we demonstrate a unique approach in which the number of electrons and holes available for recombination in NPs is controlled by an applied source-drain bias field as well as photo-excitations. The operating mechanisms involve F-N tunneling of electrons, different tunneling rates of electrons and holes to the drain electrode, and accumulation of holes in the CdSe NPs. Together, the approach makes it possible to control blinking PL by bias voltage.
2. Method and Materials
The devices were made on 5mm × 5mm, 500µm-thick p-type silicon chips, which were covered by a 3nm-thick thermally grown SiO2 layer. The deposition of colloidal particles was carried out by immersing the chips into CdSe NP solution (1nM in toluene). The NPs had a nominal diameter of about 6nm and a bandgap of about 2eV, and were capped with a thin layer of trioctylphosphine oxide (TOPO). The density of deposited NPs on the chips was controlled by immersion time, which ranged between 30min and 2hrs. Figures 1(a) and 1(b) are scanning electron microscope (SEM) images of the NPs spread on the chips with different densities. The former contains isolated NPs whereas the latter contains both isolated NPs and NP clusters. The NP layer shown in Fig. 1(b) was then covered with a thermally-evaporated 10nm-thick Au layer, as shown in Fig. 1(c), which served as the drain electrode. The thin granular Au layer has good transparency for light of 620nm wavelength, which is consistent with an estimated skin depth of 22nm. The backside of the silicon chip was subjected to BOE-etching to remove surface oxide and then covered with a Au/Cr layer (30nm/10nm in thickness) to form the source electrode. Good electric contact between the Au/Cr electrode and the p-type low-resistance (10-20Ω-cm) chips ensured that the chip body itself could act as a good electrode. All the devices were tested at room temperature and showed good stability after being stored in ambient conditions for over a month.
Fig. 1 (a) and (b) SEM images of CdSe NPs deposited on Si-chips with immersion times of 30min and 2hrs, respectively. In (a) a very low NP density is obtained whereas in (b) some NPs aggregated into small clusters. (c) Image of chip shown in (b) after coating of Au-drain electrode through which the NP PL remains observable.
The excitation light source was a 300W Hg lamp with a 530-550nm band-pass filter. The excitation light illuminates the NPs via a 100 × oil immersion objective lens (numerical aperture = 1.25) mounted on an inverted fluorescence microscope (IX-71, OLYMPUS). The PL signal from NPs was collected by the same objective lens, filtered by a 590nm high-pass filter, and imaged by a highly sensitive CCD camera at a rate of 30 frames per second. The spectra of the excitation source and PL, as shown in Fig. 2(c) , were recorded using Ocean Optics spectrometer USB-4000.
Fig. 2 (a) PL image of bare NPs under an excitation power of 17kW/cm2. (b) PL time trace of the NP indicated by the arrow in (a). (c) Green light excitation spectrum (green curve) and NP PL spectrum (red curve) (Media 1).
3. Experimental Results
Figure 2(a) is a PL image of the bare CdSe NPs shown in Fig. 1(a) and the PL time trace from one of the bare NPs is shown in Fig. 2(b). The fluctuations in both PL amplitude and blink-time interval are attributed to Auger-assisted non-radiative recombination [21–24]. However, for the sample covered with Au-drain electrode (Fig. 1(c)), the NPs luminesce only when the device is photo-excited and biased above a threshold voltage. Conversely, no photoluminescence is observed when an excitation source with 800nm wavelength is employed. Figures 3(a) and 3(b) are PL images of one of the clusters when illuminated with photo-excitation power intensities of 10kW/cm2 and 17kW/cm2, respectively. Shown in Fig. 3(c) is the PL time trace of Fig. 3(a). Note that PL is much brighter than that for the bare isolated NPs (e.g. Fig. 2(a)), while the blinking rate is much lower and the on- and off-states are distinct. Repeated experiments reveal that the threshold bias voltage and photo-excitation power density are about 7V and 8kW/cm2, respectively. Both PL intensity and blinking rate increase with increasing bias voltage as well as excitation power density. For sandwiched isolated NPs, as shown in Fig. 3(d), the blinking intensity is relatively weak; nevertheless, the same type of voltage-controlled PL behavior was observed. An optical microscope video image showing both PL blinking of bare NPs and voltage controlled PL blinking of sandwiched NPs is given in the Supplemental Information.
Fig. 3 (a) and (b) PL images of sandwiched NP clusters. (c) PL image of sandwiched isolated NPs. The excitation power density in (a) is 10kW/cm2 and in (b) and (c) is 17kW/cm2. The images are all taken at Vb = 10V. (d) The PL time trace of (a). The PL intensity is about two orders of magnitude higher than that for the case of bare NPs shown in Fig. 2(b) (Media 1).
4. Proposed explanation
A mechanism is proposed to explain the observed electric-field controlled PL behavior. Briefly, the PL is attributed to the recombination of photo-excited electrons F-N emitted from the source electrode and the accumulated holes in the NPs. The central variable in this problem is the number of accumulated holes in the NPs, which is determined by a detailed balance between the F-N emission from the source and the electron/hole tunneling to the drain electrode. To start with, hole-accumulation is attributed to the fast rate of electrons tunneling from the NP to the Au drain compared to that of holes. In this process, the number of accumulated holes is self-limiting because hole accumulation would cause a lowering of the NP potential, making subsequent electron tunneling unfavorable. Based on this self-limiting mechanism, the number of accumulated holes could be of the order of several tens. It should be noted that lowering the NP potential would also cause tilt of the SiO2 barrier. This tile would eventually trigger F-N emission of electrons from the source electrode, and bring in the observed induced PL.
Although a precise calculation of the number of accumulated holes is not possible, in the following, we illustrate the proposed mechanism by numeric calculations with order of magnitude estimation of device parameters as well as number of electrons/holes involved. Firstly, we describe the electron/hole tunneling from the NP to Au drain electrode that results in accumulation of holes in the NP. The electron and hole tunneling rates are determined by ECB-NP and EVB-NP in respect to EF-Au. Here ECB-NP and EVB-NP are the conduction and valance band-edges of the CdSe NPs, respectively, and EF-Au is the Fermi-level of the Au drain electrode. To determine the relative height between these variables, the Fermi-level of CdSe NP, EF-NP, has to be identified first. Although this value may shift upon ionization (e.g. oxidation [25–27]) assign to it a reported value [28] of 0.14eV below the conduction band edge ECB-NP and 1.86eV above the valance band edge EVB-NP. As illustrated in Figs. 4(a) and 4(b), ECB-NP is expected to be only slightly above EF-Au at zero-bias voltage (Vb = 0), while at Vb = 10V, EVB-NP is 0.08eV below EF-Au. Since EF-Au lies inside the CdSe NP band gap, the photo-excited electron-hole pairs in the CdSe conduction band can tunnel into the Au-drain. To calculate the transmission probabilities for electrons and holes, we assume that at Vb = 0 there is a potential difference [29] of 1eV between ECB-NP and the LUMO of TOPO. And the thickness of the TOPO layer enclosing the CdSe NPs is assumed to be 0.2nm. The calculation is conducted using WKB approximation by assuming effective masses of electron and hole of 0.13 and 0.45, respectively. After taking into account the charging energy of the CdSe NPs (as described below), the calculated transmission probability as a function of Vb is shown in Fig. 4(c). From which, we found that, for Vb>3.4V (i.e. ECB-NP -EF-Au>0.74eV), the transmission probability for electrons is higher than that for holes. Therefore, above this voltage electrons tend to tunnel into the Au drain electrode while the holes remain in the valence band, leaving the NPs positively charged. The tunneling of electrons results in accumulation of holes and causes the NP potential to decrease. Consequently, the transmission probability of the subsequent electrons is decreased, and the number of accumulated holes is self-limiting.
Fig. 4 (a) and (b) the NP and Au-drain energy levels at Vb = 0 and 10V, respectively. (c) Transmission probability of an excited electron (black curve) and hole (red curve) pair as a function of Vb. The difference between ECB-NP and EF-Au calculated as a function of Vb is displayed at upper x-axis for easy viewing.
The charging energy, EC, of sandwiched single NPs is estimated to be about 14meV with the substrate-to-NP and NP-to-drain capacitance values determined using the 2-D finite element method [30]. Due to accumulation of holes, NP potential is lowered and the SiO2 barrier is tilted [31]. For an illustration, Fig. 5 depicts tilt of the SiO2 barrier for the cases of no accumulation and accumulation of 100 holes in the NP. From this figure, it is understood that at a large enough tilt the probability for the F-N emission of photo-excited electrons from the conduction band of p-type source electrode to the NPs is dramatically increased. The F-N emission is characterized by a threshold voltage Vth. At bias voltages close to Vth, emission of electrons is a stochastic process and may take place by probability. At Vb smaller but close to Vth, a small number of electrons may occasionally emit and recombine with the holes in the NPs, producing a very weak PL. At Vb slightly greater than Vth, emission of more electrons takes place more frequently, bringing in brighter and more frequent PL. However, in our experiment only PL with intensity above our detection sensitivity is observable, and we suspect that the observed weak PL from single NPs corresponds to emission and recombination of a few tens of electrons. After recombination, the energy level of NPs raises and the F-N emission ceases. Subsequent PL has to wait for new hole-accumulation and F-N emission process cycles, which explains the observed discrete PL pulses. At sufficiently high Vb (e.g. 10V), the process cycles much faster and more electrons may emit at a time, therefore the PL becomes brighter and occurs more frequently. Nevertheless, since the number of accumulated holes is limited, PL from single NPs cannot be as bright as that for NP clusters, as we discuss below.
Fig. 5 Band diagram drawn for Vb = 10V. The solid red and black lines indicate levels without hole-accumulation. The dashed red and blue lines are drawn for accumulation of 100 holes.
The clusters can be modeled as a 2D array of NPs. Due to inter-NP electrostatic coupling, the charging energy of an individual NP inside the 2D array is somewhat reduced. Moreover, this inter-NP electrostatic coupling ensures a uniform potential distribution over the entire array and hence collectively F-N emissions of many electrons may take place, bringing about spontaneous recombination with a large number of accumulated holes in the array. As an order of magnitude estimation, for a 10 × 10 NP array the number of accumulated holes in each NP would exceed about 80 at Vb = 10V and about 30 of them would be recombined with the F-N emitted electrons, producing simultaneous recombination of 3000 electron-hole pairs. This spontaneous recombination is responsible for the observed bright PL. The collective F-N emissions “charge” the cluster and cause a raise in the cluster potential, and subsequent emission can occur only when cluster potential is lowered again via the “discharge” recombination process. This charge-discharge cycle takes a longer time, and that explains the observed slow but bright blinking behavior. Finally, we note that the surface plasmon resonance of the Au film may also influent the PL of the CdSe NPs. But it is known that the surface plasmon cannot be modified by the applied DC voltage, therefore the observed bias voltage controlled blinking behavior cannot be attributed to the influence of surface plasmon.
5. Conclusion
In summary, a robust and reproducible electrical field controlling blinking behavior in single CdSe NPs as well as NP clusters was observed. The origin of this voltage controlled PL blinking behavior is different from previously reported for single NPs where the phenomenon is attributed to non-radiative Auger process. A rigorous quantitative analysis was proposed involving difference in electron- and hole-transmission probabilities, charging effect, and F-N emission. The proposed mechanism for voltage controlled PL blinking is valid for both single isolated CdSe NPs and NP clusters. The analysis suggests that a collective recombination of electrons and holes is responsible for the observed unusual bright blinking behavior. This experiment demonstrates a unique method for voltage controlled CdSe NP luminescence and proposes a model to explain their blinking behavior, which may provide a useful approach for exploring new luminescence techniques.
Acknowledgements
This research was funded by the National Science Council Nos. 98-2112-M-001-023-MY3 and NSC 97-2112-M-024-002-MY3. Technical support from NanoCore, the Core Facilities for Nanoscience and Nanotechnology at Academia Sinica is also acknowledged.
References
1. X. Brokmann, G. Messin, P. Desbiolles, E. Giacobino, M. Dahan, and J. P. Hermier, “Colloidal CdSe/ZnS quantum dots as single-photon sources,” N. J. Phys. 6(99), 1–8 (2004). [CrossRef]
2. G. Messin, J. P. Hermier, E. Giacobino, P. Desbiolles, and M. Dahan, “Bunching and antibunching in the fluorescence of semiconductor nanocrystals,” Opt. Lett. 26(23), 1891–1893 (2001). [CrossRef]
3. B. Lounis, H. A. Bechtel, D. Gerion, P. Alivisatos, and W. E. Moerner, “Photon antibunching in single CdSe/ZnS quantum dot fluorescence,” Chem. Phys. Lett. 329(5-6), 399–404 (2000). [CrossRef]
4. J. M. Caruge, J. E. Halpert, V. Wood, V. Bulovic, and M. G. Bawendi, “Colloidal quantum-dot light-emitting diodes with metal-oxide charge transport layers,” Nat. Photonics 2(4), 247–250 (2008). [CrossRef]
5. W. U. Huynh, J. J. Dittmer, and A. P. Alivisatos, “Hybrid nanorod-polymer solar cells,” Science 295(5564), 2425–2427 (2002). [CrossRef] [PubMed]
6. H. Huang, A. Dorn, V. Bulovic, and M. G. Bawendi, “Electrically driven light emission from single colloidal quantum dots at room temperature,” Appl. Phys. Lett. 90(2), 023110 (2007). [CrossRef]
7. E. Anderson, A. J. Breeze, J. D. Olson, L. Yang, Y. Sahoo, and S. A. Carter, “All-inorganic spin-cast nanoparticle solar cells with nonselective electrodes,” Appl. Phys. Lett. 94(6), 063101 (2009). [CrossRef]
8. I. Robel, V. Subramanian, M. Kuno, and P. V. Kamat, “Quantum dot solar cells. harvesting light energy with CdSe nanocrystals molecularly linked to mesoscopic TiO2 films,” J. Am. Chem. Soc. 128(7), 2385–2393 (2006). [CrossRef] [PubMed]
9. D. J. Milliron, I. Gur, and A. P. Alivisatos, “Hybrid Organic–Nanocrystal Solar Cells,” MRS Bull. 30, 41–44 (2005). [CrossRef]
10. W. J. Parak, T. Pellegrino, and C. Plank, “Labelling of cells with quantum dots,” Nanotechnology 16(2), R9–R25 (2005). [CrossRef] [PubMed]
11. M. Sugisaki, H. W. Ren, K. Nishi, and Y. Masumoto, “Fluorescence intermittency in self-assembled InP quantum dots,” Phys. Rev. Lett. 86(21), 4883–4886 (2001). [CrossRef] [PubMed]
12. J. Valenta, A. Fučíková, F. Vácha, F. Adamec, J. Humpolíčková, M. Hof, I. Pelant, K. Kůsová, K. Dohnalová, and J. Linnros, “Light-Emission Performance of Silicon Nanocrystals Deduced from Single Quantum Dot Spectroscopy,” Adv. Funct. Mater. 18(18), 2666–2672 (2008). [CrossRef]
13. S. V. Kilina, D. S. Kilin, and O. V. Prezhdo, “Breaking the Phonon Bottleneck in PbSe and CdSe Quantum Dots: Time- Domain Density Functional Theory of Charge Carrier Relaxation,” ACS Nano 3(1), 93–99 (2009). [CrossRef] [PubMed]
14. D. E. Gómez, J. van Embden, J. Jasieniak, T. A. Smith, and P. Mulvaney, “Blinking and surface chemistry of single CdSe nanocrystals,” Small 2(2), 204–208 (2006). [CrossRef] [PubMed]
15. B. O. Dabbousi, J. Rodriguez-Viejo, F. V. Mikulec, J. R. Heine, H. Mattoussi, R. Ober, K. F. Jensen, and M. G. Bawendi, “(CdSe)ZnS Core-Shell Quantum Dots: Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites,” J. Phys. Chem. B 101, 9463 (1997). [CrossRef]
16. O. Cherniavskaya, L. Chen, and L. Brus, “Imaging the photoionization of individual CdSe/CdS core-shell nanocrystals on n- and p-type silicon substrates with thin oxides,” J. Phys. Chem. B 108(16), 4946–4961 (2004). [CrossRef]
17. R. Costi, G. Cohen, A. Salant, E. Rabani, and U. Banin, “Electrostatic force microscopy study of single Au-CdSe hybrid nanodumbbells: evidence for light-induced charge separation,” Nano Lett. 9(5), 2031–2039 (2009). [CrossRef] [PubMed]
18. Z. Chen, S. Berciaud, C. Nuckolls, T. F. Heinz, and L. E. Brus, “Energy Transfer from IndividualSemiconductor Nanocrystals to Graphene,” ACS Nano 4(5), 2964–2968 (2010). [CrossRef] [PubMed]
19. S. Jin, N. Song, and T. Lian, “Suppressed blinking dynamics of single QDs on ITO,” ACS Nano 4(3), 1545–1552 (2010). [CrossRef] [PubMed]
20. W. K. Woo, K. T. Shimizu, M. V. Jarosz, R. G. Neuhauser, C. A. Leatherdale, M. A. Rubner, and M. G. Bawendi, “Bawendi, “Reseversible charging of CdSe nanocrystals in a simple solid-state device,” Adv. Mater. 14(15), 1068–1071 (2002). [CrossRef]
21. K. T. Early, K. D. McCarthy, N. I. Hammer, M. Y. Odoi, R. Tangirala, T. Emrick, and M. D. Barnes, “Blinking suppression and intensity recurrences in single CdSe-oligo(phenylene vinylene) nanostructures: experiment and kinetic model,” Nanotechnology 18(42), 424027 (2007). [CrossRef] [PubMed]
22. K.-S. Jeon, S.-D. Oh, Y. D. Suh, H. Yoshikawa, H. Masuhara, and M. Yoon, “Blinking photoluminescence properties of single TiO2 nanodiscs: interfacial electron transfer dynamics,” Phys. Chem. Chem. Phys. 11(3), 534–542 (2009). [CrossRef] [PubMed]
23. M. Pelton, G. Smith, N. F. Scherer, and R. A. Marcus, “Evidence for a diffusion-controlled mechanism for fluorescence blinking of colloidal quantum dots,” Proc. Natl. Acad. Sci. U.S.A. 104(36), 14249–14254 (2007). [CrossRef] [PubMed]
24. P. A. Frantsuzov and R. A. Marcusm, “Explanation of quantum dot blinking without the long-lived trap hypothesis,” Phys. Rev. B 72(15), 155321 (2005). [CrossRef]
25. B. Nikoobakht, C. Burda, M. Braun, M. Hun, and M. A. El-Sayed, “The quenching of CdSe quantum dots photoluminescence by gold nanoparticles in solution,” Photochem. Photobiol. 75(6), 591–597 (2002). [CrossRef] [PubMed]
26. W. G. J. H. M. van Sark, P. L. T. M. Frederix, D. J. van den Heuvel, A. A. Bol, N. J. van Lingen, C. de Mello Donega, H. C. Gerritsen, and A. Meijerink, “Time-Resolved Fluorescence Spectroscopy Study on the Photophysical Behavior of Quantum Dots,” J. Fluoresc. 12(1), 69–76 (2002). [CrossRef]
27. W. G. J. H. M. van Sark, P. L. T. M. Frederix, A. A. Bol, H. C. Gerritsen, and A. Meijerink, “Blueing, Bleaching, and Blinking of Single CdSe/ZnS Quantum Dots,” ChemPhysChem 3(10), 871–879 (2002). [CrossRef]
28. B. Carlson, K. Leschkies, E. S. Aydil, and X.-Y. Zhu, “Valence Band Alignment at Cadmium Selenide Quantum Dot and Zinc Oxide (101-10) Interfaces,” J. Phys. Chem. C 112(22), 8419–8423 (2008). [CrossRef]
29. C. A. Leatherdale, C. R. Kagan, N. Y. Morgan, S. A. Empedocles, M. A. Kastner, and M. G. Bawendi, “Photoconductivity in CdSe quantum dot solids,” Phys. Rev. B 62(4), 2669–2680 (2000). [CrossRef]
30. D. R. Koenig, E. M. Weig, and J. P. Kotthaus, “Ultrasonically driven nanomechanical single-electron shuttle,” Nat. Nanotechnol. 3(8), 482–485 (2008). [CrossRef] [PubMed]
31. R. J. Walters, J. Carreras, T. Feng, L. D. Bell, and H. A. Atwater, “Silicon Nanocrystal Field-Effect Light-Emitting Devices,” IEEE J. Sel. Top. Quantum Electron. 12(6), 1647–1656 (2006). [CrossRef]
