NANO LETTERS
Electrogenerated Chemiluminescence from PbS Quantum Dots
2009 Vol. 9, No. 2 789-793
Liangfeng Sun,*,† Lei Bao,‡,| Byung-Ryool Hyun,† Adam C. Bartnik,† Yu-Wu Zhong,‡ Jason C Reed,§ Dai-Wen Pang,| He´ctor D. Abrun˜a,‡ George G. Malliaras,§ and Frank W. Wise*,† Center for Nanoscale System, School of Applied and Engineering Physics, Department of Chemistry and Chemical Biology, and Department of Materials Science and Engineering, Cornell UniVersity, Ithaca, New York 14853, and College of Chemistry and Molecular Sciences and State Key Laboratory of Virology, Wuhan UniVersity, Wuhan, 430072, People’s Republic of China Received November 14, 2008; Revised Manuscript Received December 16, 2008
ABSTRACT We report the first observation of electrogenerated chemiluminescence (ECL) from PbS quantum dots (QDs). Different ECL intensities are observed for different ligands used to passivate the QDs, which indicates that ECL is sensitive to surface chemistry, with the potential to serve as a powerful probe of surface states and charge transfer dynamics in QDs. In particular, passivation of the QD surfaces with trioctylphosphine (TOP) increases ECL intensity by 3 orders of magnitude when compared to passivation with oleic acid alone. The observed overlap of the ECL and photoluminescence spectra suggests a significant reduction of deep surface trap states from the QDs passivated with TOP.
Lead-salt quantum dots (QDs) are efficient infrared (IR) emitting materials which can be made into size-tunable inexpensive emitters for optical fiber communications and on-chip optoelectronic integrated circuits.1 As near-IR emitters, they are also valuable for in vivo bioimaging due to reduced attenuation in living tissue.2,3 Quantum confinement opens the energy gap of a QD,4 and thus blue shifts its photoluminescence peak. However, surface states, which depend on surface chemistry, can also have a strong effect on the luminescence peak and efficiency. This is particularly true for small QDs since a significant fraction of the atoms is located at the surface. Regarding photoluminescence (PL), a recent study shows that “blinking” of a single CdSe/ZnS QD could be suppressed almost completely by capping with β-mercaptoethanol ligands, thereby making the QDs more useful in bioimaging and quantum information processing.5 PL efficiency of QDs could be significantly improved by capping them with optimal ligands, as demonstrated for PbS QDs.6 Electroluminescence from QDs is also strongly affected by capping ligands. For instance, octadecylaminecapped PbS QDs achieve about 3% electroluminescence * Corresponding authors,
[email protected] and
[email protected]. † Center for Nanoscale System, School of Applied and Engineering Physics, Cornell University. ‡ Department of Chemistry and Chemical Biology, Cornell University. § Department of Materials Science and Engineering, Cornell University. | College of Chemistry and Molecular Sciences and State Key Laboratory of Virology, Wuhan University. 10.1021/nl803459b CCC: $40.75 Published on Web 12/30/2008
2009 American Chemical Society
efficiency, while oleic acid capped QDs yield undetectable electroluminescence, even though the two capping ligands have almost the same length.7 Surface states are usually thought to be responsible for such effects. However, systematic knowledge of the surfaces of QDs is still lacking, in part because of the lack of effective experimental methods to probe the QD surface. Electrogenerated chemiluminescence (ECL) can begin to fill this lack of knowledge of the surface. In the past decade, a wide range of semiconductor nanocrystal QDs (Si,8 Ge,9 CdSe,10,11 CdSe/CdS,11 CdSe/ZnSe,12 CdTe,13 and CdS14) have been studied by ECL. A significant red shift of the ECL, relative to the PL from some of the QDs, was observed and attributed to surface states.8-10,12 ECL was demonstrated to be more sensitive than PL to surface chemistry,8 which makes it a powerful tool to study surface states of QDs. However, ECL from lead-salt QDs has not been reported to date. One reason could be the instability of lead-salt quantum dots under charge injection, e.g., decomposition, as has been observed for bulk PbS in aqueous solution.15,16 In this work, we present ECL studies from PbS QDs in nonaqueous media. After PbS QDs are capped with TOP ligands, the ECL signal increases dramatically. This allows recording of the ECL spectra of lead-salt QDs. Overlap of the ECL and PL spectra suggests a significant reduction of deep surface trap states from the QDs by the surface modification with TOP.
Figure 1. Absorption and PL spectra of PbS QDs capped by oleic acid (a) and oleic acid + TOP (b) in CH2Cl2 containing 0.1 M TBAP.
Figure 2. ECL (dot) and corresponding potential (solid line) from PbS QDs capped by oleic acid (a) and from bare oleic acid (b). PMT dark count rate ∼20 counts/s.
Colloidal PbS QDs (∼3 nm in diameter) were synthesized using organometallic precursors18,19 (see the Supporting Information for details), dispersed in toluene, and kept in the dark. Before the ECL experiments, the QD solution was concentrated and dried in vacuum, and subsequently the QDs were redispersed in 1 mL of a dichloromethane (CH2Cl2) solution containing 0.1 M tetra-n-butylammonium perchlorate (TBAP). The typical concentration of PbS QD in the solution was about 25 mg/mL. A glassy carbon disk (5 mm in diameter) served as the working electrode for ECL measurements. A platinum wire and a silver wire served as the counter and the quasi-reference electrodes, respectively. For electrochemical measurements, a voltammograph (CV-27, Bioanalytical Systems Inc.) was employed. A photomultiplier tube (PMT, Hamamatsu R955) operated in photon-counting mode was mounted facing the working electrode to detect the ECL photons emitted by the QDs. The cyclic potential signal and the corresponding current signal were digitized and recorded by a computer, which also simultaneously recorded the number of photons detected by the PMT. All ECL measurements were carried out under ambient conditions. The absorption spectrum of the PbS QDs in the electrolyte solution shows the lowest energy absorption peak at 730 nm 790
(Figure 1a), which corresponds to an energy gap of 1.7 eV. The peak of the PL is at 860 nm. Such a large Stokes shift is commonly observed with small PbS QDs.20 After QDs are capped with additional ligands, a shift of the absorption and the emission peaks is often observed. For instance, capping with additional TOP shifts both absorption and emission to the red (782 and 912 nm, respectively), as shown in Figure 1b. PbS QDs can form oxidized (R•+) and reduced (R•-) QDs during potential cycling. Two oppositely charged QDs can collide to yield an excited QD and a ground-state QD (reaction 1). The excited QD then returns to the ground state via a radiative pathway by emitting a photon (reaction 2), which is the origin of the ECL signal. R•+ + R•- f R/ + R
(1)
R/ f R + hν
(2)
To obtain the ECL signal, the oxidized and reduced forms of the QDs must be sufficiently stable so that reaction 1 can take place. ECL from PbS QDs capped with oleic acid ligands was observed, although weak (Figure 2a). The potential of the working electrode was scanned between -1.5 and +1.5 V. When the potential was scanned to values more negative than Nano Lett., Vol. 9, No. 2, 2009
Figure 3. Time-dependent PL (solid dot) and ECL (solid square) of TOP capped QDs in CH2Cl2 containing 0.1 M TBAP. The relative PL quantum efficiency was calculated by normalizing to a control sample. The solid lines are to guide the eye.
-0.8 V or more positive than +0.8 V, the ECL signal appeared, and grew more intense at -1.5 and +1.5 V. The potential difference of the onset of ECL (1.6 V) is close to the energy gap (∼1.7 eV) of the PbS QDs, which indicates injection of electrons into the LUMO (or holes into the HOMO) of the QDs. No ECL signal was observed from bare oleic acid itself in the same solution (Figure 2b), indicating that the ECL in Figure 2a originated from the QDs. Why the ECL from PbS QDs is so weak is unclear. Besides the possible instability of the oxidized and reduced forms of QDs, surface passivation could be critical for ECL generation. A recent high-resolution photoelectron spectroscopy study shows that oleic acid ligands preferentially bind to lead atoms on the QD surface, while leaving the sulfur atoms unpassivated.21 Those sulfur atoms, with dangling bonds, can act as charge traps and reduce luminescence efficiency. Passivation of surface sulfur atoms with TOP during the QD synthesis has been demonstrated to improve the PL efficiency significantly.6 Following this line of thought, we passivated PbS QDs capped with oleic acid with additional TOP ligands, and obtained dramatically enhanced ECL signals. About 50 µL of TOP were added to 1 mL of a PbS QD CH2Cl2 solution (QD ∼ 25 mg/mL, TBAP 0.1M) in a nitrogen glovebox to prevent the oxidation of TOP. The solution was kept in the dark to eliminate any photodecomposition and allow the TOP ligands to passivate the QDs. After specified time periods, the solution was loaded into an electrochemical cell for ECL measurements. A QD solution was prepared in the same way for PL quantum efficiency measurements. As shown in Figure 3, the ECL intensity increased exponentially while the PL increased linearly at the beginning, and both reached their respective maxima after about 2 weeks. The maximum ECL intensity was at least 3 orders of magnitude larger than that obtained from the QDs capped by oleic acid alone, while the maximum quantum efficiency improvement was less than a factor of 3. Given its critical dependence on charge injection, it is perhaps not surprising that ECL is much more sensitive than PL to surface chemistry. Nano Lett., Vol. 9, No. 2, 2009
Figure 4. Cyclic voltammogram and ECL intensity of PbS QDs capped with oleic acid and TOP at high (25 mg/mL) (a) and low (3 mg/mL) (c) concentrations in CH2Cl2 containing 0.1 M TBAP (on the fifth day after TOP was mixed with PbS QDs), and blank supporting electrolyte (b). Scan rate 100 mV/s.
The ECL signal is asymmetric as the potential is scanned between -1.5 and +1.5 V, with the signal at negative potentials being much larger than that at positive potentials. This suggests that the TOP ligands could make the oxidized form of the QDs more stable. The asymmetry also depends on concentration. Lower QD concentrations make the ECL signal at the positive potential even larger, as shown in Figure 4c. The cyclic voltammogram recorded at low concentration (Figure 4c) shows well-defined peaks at -1.0 V (reduction) and +0.8 V (oxidation), which coincide with the two ECL peaks and indicate electron and hole injection, respectively. At high QD concentration, the features are smeared out, as shown in Figure 4a. Although PbS QDs are more stable in nonaqueous than aqueous electrolyte solutions,17 they decompose under charge 791
the working electrode and consequently diminishes the ECL intensity, as observed in time-dependent ECL measurements (shown in Figure 5). The ECL peak that occurs at reducing potentials decreases rapidly during the first few cycles of the potential, after which it degrades more slowly. The signal recovers when a clean working electrode is used in the same solution.
Figure 5. ECL (dot) and corresponding potential (solid line) from TOP and oleic acid capped PbS QDs in CH2Cl2 containing 0.1 M TBAP.
injection. The mechanism of decomposition of bulk PbS in aqueous electrolyte was proposed as the two reactions15,16 PbS + 2H+ + 2e- f Pb + H2S
(3)
PbS f Pb2+ + S + 2e-
(4)
In our experiment, the peak at -1.0 V (Figure 4c) and the sharply increasing cathodic current suggest the cathodic reaction that causes PbS QDs to decompose into lead and sulfur or thiolate. (Cathodic reaction 3 is not strictly followed due to the lack of H+ in solution.) The sharp peak at +0.8 V (Figure 4c) and sharply rising anodic current at more positive potentials can be attributed to anodic dissolution (reaction 4). Among the products of reactions 3 and 4, lead formed a layer on the working electrode while the others dispersed in the solution. Some lead might be stripped off the working electrode, which is indicated by a sharp peak at about -0.2 V as shown clearly in Figure 4a (a less significant peak is also observed in dilute solutions as shown in Figure 4c). It most likely indicates the anodic stripping of surfaceaccumulated lead.17 The deposited lead on the working electrode was studied using X-ray photoelectron spectroscopy, which showed significant lead peaks but no sulfur peaks, which supports the mechanism proposed above. This accumulated lead layer impedes excitation of the QDs by
The ECL spectrum was recorded with a silicon avalanche photodiode (APD) and a monochromator (Supporting Information available). The PL spectrum was taken using the same setup except that a smaller slit width was used to obtain higher resolution and a laser (wavelength 632 nm) was used as the exciting source. Comparison of ECL and PL spectra can provide information of the QD surfaces since the mechanism of ECL is different from PL, as proposed by Bard et al.8 (shown in Figure 6b). In the case of PL, the QD core absorbs photons and generates excitons. ECL occurs when two oppositely charged QDs collide, in which case electrons or holes tunnel through the ligands from one QD to the other, and thus are affected by the ligands and QD surfaces. The ECL spectrum depends on where the injected electrons and holes go. If the electrons or holes relax into surface states, red-shifted ECL will be observed since surface states are usually located inside the energy gap of the core (e.g., Si,8 Ge,9 CdSe10). If there is no deep surface trap state, the injected electrons or holes may relax within the core or surface states, in which case that the ECL spectrum is expected to overlap with the PL, as observed in CdTe QDs.13 We measured ECL and PL spectra from the QD capped with TOP and oleic acid ligands, as shown in Figure 6a (so far, only the TOP-capped QDs allow measurement of ECL spectrum, others yield too weak signal to be measured). There is no spectral shift between the ECL and PL, which indicates that deep surface trap states are passivated in TOP-capped PbS QDs. Finally, the dependence of ECL on different ligands was studied. Thiol ligands (2-methyl-1-propanethiol, 1-decanethiol) of different length were added to the original solution of QDs in toluene in excess amount (the mole ratio of QDs to ligands was about 1000:1). The thiol ligands bind to the QDs by replacing oleic acid, because the thiol moiety (-SH)
Figure 6. (a) PL (dot) and ECL (square) spectrum of PbS QDs with oleic acid and TOP capping in CH2Cl2 containing 0.1 M TBAP. The lines are Gaussian fits to each data set. (b) Schematic mechanism for PL and ECL.8 792
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while longer thiol ligands do not, which can be explained qualitatively by charge carriers tunneling through the ligands. We hope that with further work these initial observations can be developed into a useful and even quantitative probe of QD surface states and nanoscale charge transfer. Acknowledgment. This material is based upon work supported by the National Science Foundation under Grant No. EEC-0646547 and by NYSTAR. We thank Jonathan Shu for his help on X-ray photoelectron spectroscopy measurement. Supporting Information Available: Methods of PbS QDs synthesis and ECL spectrum measurement. This material is available free of charge via the Internet at http://pubs.acs.org. Figure 7. ECL intensity dependence on PbS capping ligands: (a) OA, pure oleic acid; (b) OA + C3-SH, oleic acid and 2-methyl1-propanethiol; (c) OA + C8-SH, oleic acid and 1-decanethiol; (d)OA + TOP, oleic acid and TOP. All the ECL intensities shown here are before degradation.
in each ligand has a stronger binding than the carboxyl moiety (-COOH) (a method to replace carboxyl ligands with thiol ligands has been developed based on this22). Although we have no direct quantitative evidence of the ligand exchange, the red-shifting of absorption and PL peaks is qualitatively consistent with binding of thiol ligands to the QDs.23 The ECL intensity was measured about 10 min (long enough for the thiol moieties to bind to the QDs) after each ligand was mixed with the QDs, and the results are summarized in Figure 7b,c. 2-Methyl-1-propanethiol improves the ECL signal by a factor of 3, while 1-decanethiol has a negligible effect. Since 2-methyl-1-propanethiol is much shorter than 1-decanethiol, we expect that an electron or hole in one QD has a higher probability of tunneling through the ligands to an oppositely charged QD and form an exciton, thus increasing the ECL intensity. The ECL from PbS QDs capped by TOP ligands (10 min after mixing) is much stronger (shown in Figure 7d). However, the ECL improvement through TOP capping has a different mechanismspassivation of surface trap statessas discussed before. We anticipate that a better surface passivation with shorter ligands can improve the ECL even more. In summary, we have observed ECL from PbS QDs in an organic electrolyte solution. Capping the QDs with TOP ligands improves the ECL signal by several orders of magnitude, which is evidence of the sensitivity of ECL to QD surface chemistry. Shorter thiol ligands increase the ECL,
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