Article pubs.acs.org/JPCC
Simultaneous Sign Change of Magneto-Electroluminescence and Magneto-Conductance in Polymer/Colloidal Quantum Dot Nanocomposites Lixiang Chen, Weiyao Jia, Yingbing Chen, Jie Xiang, Dongyu Liu, and Zuhong Xiong* School of Physical Science and Technology, Southwest University, Chongqing 400715, PR China ABSTRACT: Nanocomposites of conjugated polymer and colloidal quantum dots (QDs) have attracted considerable attention for optoelectronic and photovoltaic applications. Here the effects of magnetic field on the electroluminescence and current in poly[2-(4-(3′,7′-dimethyloctyloxyphenyl)-1,4-phenylenevinylene] (P-PPV)/CdSe-CdS-ZnS QDs composites are studied. Our results show that by increasing the concentration of CdSe-CdSZnS QDs in the hybrid nanocomposites from 0 to 25 wt %, the polarities of magneto-electroluminescence (MEL) and magneto-conductance (MC) are simultaneously changed from positive to negative. In addition, the amplitudes of negative MEL and MC are enhanced as the temperature decreases, showing an abnormal temperature dependence. We attribute these magnetic field effects to CdSe-CdS-ZnS QDs-induced direction reverse in spin-mixing of loosely bound polaron pairs in P-PPV matrix prior to energy transfer to QDs. With this study we show that incorporating QDs in polymer matrix can strongly influence spin-selective interactions in the hybrid nanocomposites, which may pave the way for spin-related applications of these fascinating hybrid nanocomposites.
1. INTRODUCTION The hybrid nanocomposites consisting of conjugated polymer and colloidal quantum dots (QDs) have recently attracted considerable attention.1−5 This is because the hybrid polymer/ QD nanocomposites can incorporate the performance strengths of both the solution-processable polymers and sizetunable QDs in achieving high-quality active layer for optoelectronic and photovoltaic applications.6−11 Moreover, the electrical, optical, and mechanical properties of the nanocomposites can be easily tailored according to the application requirements by tuning either the type or the quantity of the constituents.8,11 Because of the efficient photoluminescence (PL), narrow emission band, and easily tunable emission color, colloidal QDs are very suitable for the light-emitting devices.12−15 Incorporating the QDs in polymer matrix can further improve the charge balance, promote energy transfer (ET), inhibit leakage current, and enhance structural stability of QDs in the devices.16,17 In this regard, light-emitting devices based on the polymer/QD nanocomposites have been intensively studied.2,6,16−19 Many of the studies focused on the light-emitting mechanisms (such as the ET between the polymer matrix and QDs) and relaxation dynamics in the nanocomposites.2,18−20 However, it is worth noting that the effects of QDs on the spin-selective interactions in the polymer/QD nanocomposites, such as the spin-mixing between the spin-pair species, have been rarely reported, although such spin-selective interactions are strongly related to the dynamic relaxation and light-emitting.21−23 Organic magnetic field effects (MFEs) have been proved to be a facile method to investigate the nature of the relaxation © XXXX American Chemical Society
dynamics, especially the spin-selective interactions, in organic (and also some inorganic) semiconductor devices.24−27 Thanks to the weak spin−orbit coupling in organic semiconductors, the spin−lattice relaxation time (τSL) is long enough for the spinselective interactions between the spin-pair species: singlet/ triplet polaron pairs (PPs) and excitons.27 Thus the organic MFEs are commonly observable in organic light-emitting diodes,28−37 solar cells,38−40 and other functional devices.41−45 Compared with organic semiconductors, the colloidal QDs usually possess large spin−orbit coupling caused by the heavy atoms (cadmium, Cd). The large spin−orbit coupling may largely shorten the τSL,27,40 resulting in quenching of MFEs in QDs. This would be the reason why the MFEs have been rarely reported in the pure QDs thin films. However, incorporating the QDs in polymer matrixes not only changes the spin−orbit coupling in the nanocomposite thin films but also provides an extra relaxation pathway for the excited states, which may result in different MFEs. We investigated the MFEs, including the magneto-electroluminescence (MEL) and magneto-conductance (MC), in the hybrid nanocomposites based on poly[2-(4-(3′,7′-dimethyloctyloxyphenyl)-1,4-phenylenevinylene] (P-PPV) and CdSe-CdSZnS core−shell QDs with different blend ratios. It is interestingly found that incorporating the CdSe-CdS-ZnS QDs in P-PPV matrix with a proper concentration can simultaneously change the polarities of the MEL and MC Received: December 13, 2016 Revised: February 19, 2017 Published: March 22, 2017 A
DOI: 10.1021/acs.jpcc.6b12538 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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cycle cryostat (CCS-350S, 20−300 K). During the measurements, the samples were mounted on the coldfinger of the cryostat, which was located between two poles of the electromagnet (Lakeshore EM647). The direction of the magnetic field was applied parallel to the device surface. The details of the MFEs measurements followed the procedure described elsewhere.23,36
from positive to negative: positive in pristine P-PPV polymer but negative in polymer/QD nanocomposites. Moreover, the negative MFEs are significantly enhanced with increasing the concentration of the QDs in the nanocomposites or lowering the working temperature. We proposed and demonstrated that the incorporated CdSe-CdS-ZnS QDs can promote the relaxation process of singlet excited states in P-PPV matrix, which may change the direction of spin mixing between the loosely bound PPs and then generate negative MFEs in the PPPV/CdSe-CdS-ZnS QD nanocomposites. This work provides a pathway for the spin-related applications of the polymer/ colloidal QD nanocomposites, such as magnetic sensible display panels and magnetic sensors with multiple control parameters.
3. RESULTS The absorption spectrum of CdSe-CdS-ZnS QDs and the PL spectra of P-PPV and CdSe-CdS-ZnS QDs are displayed in Figure 1c. It can be seen that the PL spectrum of CdSe-CdS-
2. EXPERIMENTAL SECTION Materials. The P-PPV was purchased from Canton OLedking Optoelectric Materials (Guangzhou, China). The CdSe-CdS-ZnS core−shell QDs, dispersed in toluene with the concentration of 10 mg/mL, were purchased from NajingTech (Hangzhou, China). The poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS, Heraeus-Clevios 4083) was provided by Xi’an p-OLED. The electron-transport layer of tris(2,4,6-trimethyl-3-(pyridin-3-yl)phenyl)borane (3TPYMB) and electron injection layer of lithium fluoride (LiF) were purchased from Luminescence Technology and Sigma-Aldrich, respectively. All of the above materials were used as received. Fabrication of the Nanocomposite Devices. The matrix polymer of P-PPV was dissolved in toluene with the same concentration (10 mg/mL) as that of CdSe-CdS-ZnS QDs. Then, the P-PPV and CdSe-CdS-ZnS solutions were mixed according to volume to form P-PPV:CdSe-CdS-ZnS QDs mixtures. The concentrations of CdSe-CdS-ZnS QDs in the mixtures were 2, 10, and 25 wt %, respectively. The P-PPV/QD nanocomposite devices were fabricated on the patterned indium tin oxide (ITO) substrates with the structure of ITO/ PEDOT:PSS/P-PPV:CdSe-CdS-ZnS QDs/3TPYMB/LiF/Al. Before the deposition of P-PPV/QD nanocomposite, the PEDOT:PSS was spin-coated on the well-cleaned ITO substrates and then dried at 120 °C for 10 min in air. Then, the substrates were transferred into the nitrogen-filled glovebox. The P-PPV:CdSe-CdS-ZnS QD mixtures were spin-coated on the top of PEDOT:PSS to form polymer/QD nanocomposite thin films. After annealed at 80 °C for 30 min, the polymer/QD nanocomposite thin films were transferred into the vacuum chamber (10−5 Pa) for sequential thermal evaporation of 3TPYMB (30 nm), LiF (1 nm), and Al (100 nm) cathode. The control devices based on the pristine P-PPV were fabricated following the same procedure except for using pristine P-PPV as the emission layer. Characterizations and MFEs Measurements. The UV− vis absorption of the CdSe-CdS-ZnS QDs was recorded by an Ocean Optics HR-4000 spectrometer. The PL spectra of PPPV and QDs were obtained using a He−Cd gas laser (325 nm) as the excitation source. Both the PL and electroluminescence (EL) spectra were analyzed with a SpectraPro2300i spectroscopy. The scanning electron microscopy (SEM) images were taken on an SEM instrument (LEO 1530 Field Emission). The current−luminescence−voltage characteristics of the devices were measured by a Keithley 2400 SourceMeter and a silicon photodetector coupled to a Keithley 2000 apparatus. The MFEs measurements were taken on in a closed-
Figure 1. (a) Schematic diagram of CdSe-CdS-ZnS QDs and (b) chemical structure of P-PPV. (c) UV−visible absorption of CdSe-CdSZnS QDs and the PL spectra of P-PPV and CdSe-CdS-ZnS QDs thin films.
ZnS QDs shows a narrow red emission with peak at ∼620 nm and full width at half-maximum of ∼38 nm, indicating excellent color purity of the CdSe-CdS-ZnS QDs. The PL spectrum of PPPV shows a much broader green emission with peak at ∼520 nm. The spectral overlap between the PL of P-PPV and absorption of CdSe-CdS-ZnS QDs may enable Förster energy transfer from P-PPV to CdSe-CdS-ZnS QDs. To evaluate the morphology and topography of the P-PPV/ CdSe-CdS-ZnS QD nanocomposite films, SEM characterizations were performed. Figure 2 shows the SEM images for the pristine CdSe-CdS-ZnS QDs, P-PPV, and P-PPV/CdSeCdS-ZnS QD nanocomposite thin films on the top of PEDOT:PSS. In the pristine CdSe-CdS-ZnS QDs film (Figure 2a), the CdSe-CdS-ZnS QDs intend to aggregate to form random porous structure, yielding poor-quality film. It can also be found that a large number of pinholes (marked as red circles) are observed in the pristine CdSe-CdS-ZnS QDs film. In contrast, the pristine P-PPV film is very flat with excellent uniformity (Figure 2b). There are no microscopically visible pinholes. When a small quantity of CdSe-CdS-ZnS QDs (≤10 wt %) were doped in P-PPV matrix, the quality of the film remains unchangedflat and uniform (Figure 2c). At high B
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Figure 2. SEM images of the (a) pristine CdSe-CdS-ZnS QDs thin film, (b) pristine P-PPV thin film, (c) P-PPV:10 wt % CdSe-CdS-ZnS QDs, and (d) P-PPV:25 wt % CdSe-CdS-ZnS QD nanocomposite thin films on the top of PEDOT:PSS.
Figure 3. (a) Energy diagram of the device. (b) Normalized EL spectra of the devices based on pristine P-PPV (0 wt %) and P-PPV/CdSe-CdS-ZnS QD nanocomposites with different blend ratios at 200 K. The corresponding (c) MEL and (d) MC responses measured at the current level of ∼2.5 mA/cm2 at 200 K.
low and high doping concentration of QDs (Figure 2c,d), evidence of uniform dispersion of CdSe-CdS-ZnS QDs in the P-PPV matrix. In the following we will show the optoelectronic properties and MFEs results of the P-PPV/CdSe-CdS-ZnS QD nanocomposite-based devices. Figure 3a shows the device structure
doping concentration of QDs (25 wt %, Figure 2d), the nanocomposite film exhibits a large number of cracks (marked as red arrows) that were likely resulted from the phase separation between the P-PPV and CdSe-CdS-ZnS QDs.16 However, it is noted that no obvious aggregations are observed in P-PPV/CdSe-CdS-ZnS QD nanocomposite films with both C
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The Journal of Physical Chemistry C used in this work. The energy levels of each functional layers referred to the literatures.15,46,47 Because the lowest unoccupied molecular orbital (LUMO) level of P-PPV and conduction band of CdSe-CdS-ZnS QDs are much lower than the LUMO level of 3TPYMB, the electrons are easily injected from the electron-transport layer of 3TPYMB into P-PPV and then CdSe-CdS-ZnS QDs (blue arrows in Figure 3a). On contrary, the highest occupied molecular orbital (HOMO) levels of PEDOT:PSS and P-PPV are 1.3 and 1.1 eV lower than the valence band of QDs, respectively. Thus the holes would be difficultly injected from either the PEDOT:PSS or P-PPV into the QDs layer due to the large barrier over 1 eV for hole injection (green arrows in Figure 3a). In this case, the excitations of the CdSe-CdS-ZnS QDs in the nanocomposites are mainly caused by the ET. Figure 3b displays the normalized EL spectra of the devices based on pristine P-PPV and P-PPV/ CdSe-CdS-ZnS QD nanocomposites with different blend ratios at 200 K. The pristine P-PPV (0 wt %) devices show a broad green EL emission, which is consistent with the previous observation.48 When 2 wt % QDs are doped in the device, the EL spectrum is slightly changed with a very weak red emission at ∼620 nm. At higher concentration of QDs (10 wt %) in the nanocomposite devices, the emission of P-PPV is significantly quenched, while the QD emission at ∼620 nm becomes dominant, indicating efficient ET from polymer matrix of PPPV to CdSe-CdS-ZnS QDs. As the concentration of CdSeCdS-ZnS QDs further increases to 25 wt % in the nanocomposites, the intensity of P-PPV emission is remarkably reduced, and the EL emission is predominantly generated from CdSe-CdS-ZnS QDs. Figure 3c,d shows the corresponding MEL and MC in these devices around the current level of ∼2.5 mA/cm2, respectively. The MEL (or MC) is defined as the relative change of EL intensity (or current) of the device in the presence and absence of an external magnetic field, that is, MEL = (EL|B − EL|0)/EL|0 × 100%. As shown in Figure 3c, the MEL of the pristine PPPV-based (0 wt %) device increases fast at low magnetic field and then tends toward a plateau, reaching 8.2% at 500 mT. When 2 wt % CdSe-CdS-ZnS QDs are doped in the devices. The MEL is suppressed to be ∼4%. The MEL further decreases to only 0.8% when the concentration of CdSe-CdS-ZnS QDs in the nanocomposite reaches 4 wt % (not shown here). Intriguingly, as the concentration of CdSe-CdS-ZnS QDs increases to 10 wt %, the polarity of the MEL changes from positive to negative, although the line shape of the MEL is maintained. The amplitude of the negative MEL can be further enlarged to 5% by increasing the concentration of QDs to 25 wt %. Similar trend versus concentration of CdSe-CdS-ZnS QDs and lineshapes was observed for MC in the devices (Figure 3d), indicating that the MC and MEL would be caused by the same reasons. It should be noted that further increasing the concentration of QDs in the nanocomposites results in very poor performance and negligible MFEs in the devices. Upon comparing the EL spectra (Figure 3b) and the MFEs results (Figure 3c,d) of the devices with different concentrations of CdSe-CdS-ZnS QDs, one can conclude that the negative MEL and MC would be closely related to emission of CdSe-CdS-ZnS QDs in the nanocomposites. The characteristic field, δB (= full width at half-maximum) values for MEL and MC were extracted and plotted as a function of concentration of QDs (Figure 4). The maximum MEL and MC values at 500 mT were also summarized in Figure 4. It is clearly seen that the δB values for both MEL and MC remain unchanged and
Figure 4. Characteristic field, δB values (right axis), and maximum amplitudes at 500 mT (left axis) for (a) MEL and (b) MC versus the concentration of CdSe-CdS-ZnS QDs in the nanocomposites.
maintain to be 4.5 mT, although MEL and MC vary and change their polarity from positive to negative as the concentration of QDs increases. These results indicate that the MEL and MC in the nanocomposites with different concentration of QDs might originate from the same process but with the opposite direction because the characteristic field, δB is an important symbol of the underlying mechanism for the MFEs.27,36,41 To further investigate the negative MFEs in the P-PPV/ CdSe-CdS-ZnS QD nanocomposites, the temperature dependence of MEL and MC was recorded for the device with a moderate concentration of 10 wt % QDs. It is found that the amplitudes of both the MEL (Figure 5a) and MC (Figure 5b) become larger as the temperature decreases to 20 K. This temperature dependence of MFEs is quite different from the previous observation in other systems in which the MFEs are thermally activated and the amplitude of MFEs decreases at low temperature.32,36 The corresponding EL spectra and current− voltage (I−V) characteristics under different temperatures are shown in Figure 5c,d, respectively. The EL spectra show the superposition of light emissions from both P-PPV and CdSeCdS-ZnS QDs under all temperatures. Moreover, the emission intensities of P-PPV and CdSe-CdS-ZnS QDs are both enhanced as the temperature decreases, suggesting higher current efficiency at low temperatures, although the I−V characteristic shifts toward higher voltages.
4. DISCUSSION Remarkably, incorporating CdSe-CdS-ZnS QDs in the P-PPV matrix can change the polarities of organic MFEs from positive to negative. Both negative MEL and MC were observed in the D
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Figure 5. Temperature dependence of (a) MEL and (b) MC (under the same current levels of ∼2.5 mA/cm2) for the nanocomposite device with a moderate concentration of 10 wt % CdSe-CdS-ZnS QDs. (c) Corresponding EL spectra at current level of ∼2.5 mA/cm2 and (d) I−V characteristics under different temperatures from 270 to 20 K.
electrons, and holes are injected into the P-PPV matrix. The PP and 3PP can further recombine into tightly bound singlet and triplet exciton (SE and TE) with rate constant of kS and kT respectively, or dissociate back to free carriers. Compared with that in the strongly bound exciton, the interaction between the electron and hole in the loosely bound PP state is relatively weak,24,25,41 which results in the degeneracy between the 1PP and three 3PP substates. In this case, the 1PP and 3PP are spinmixed, and the 1PP can be converted into 3PP via the intersystem crossing or vice versa, depending on the relative rate of kS and kT. Specifically, if kS < kT, then 1PP states can be fed into 3PP; if kS > kT, then the situation reverses.21 In general, the spin mixing between the 1PP and 3PP is suppressed by the magnetic field through removing the degeneracy of the 3PP substates.24,25,32 Thus the ratio of singlet/triplet population in the device can be definitely changed under an external magnetic field, yielding positive or negative MEL and MC based on the fact that the singlet and triplet states contribute differently to the light emission and current.21−23 In the pristine P-PPV device, both positive MEL and MC were observed. These positive MFEs have also been observed in some other polymer and small molecular-based lightemitting diodes21−23,28,29,31,36 and are usually ascribed to the magnetic field-suppressed hyperfine mixing from 1PP to 3PP states.21−23 Specifically, applying an external magnetic field would suppress the intersystem crossing from 1PP to 3PP (kS < kT), which causes more 1PP states in the device. As a result, enhanced EL intensity (positive MEL) and simultaneously enlarged current (positive MC) are expected after the 1PP states radiatively decay (through singlet excitons) or dissociate, respectively. The observed Lorentzian-type line shape with small characteristic field, δB, of 4.5 mT within the hyperfine
P-PPV/CdSe-CdS-ZnS QD nanocomposites. To the best of our knowledge, this is the first time to obtain simultaneously negative MEL and MC in the same device. In what follows we will discuss the observed polarity changes in MFEs with a focus on the mechanisms underlying the negative MEL and MC in PPPV/CdSe-CdS-ZnS QD nanocomposites. Figure 6 illustrates the dynamic processes involved in lightemitting in the P-PPV/CdSe-CdS-ZnS QD nanocomposites. Loosely bound PP states with singlet and triplet spin configuration, that is, 1PP and 3PP, are first formed after the
1
Figure 6. Schematic diagram of the dynamic processes involved in light-emitting in the P-PPV/CdSe-CdS-ZnS QD nanocomposites. Here the kS and kT are the recombination rate constant for 1PP and 3 PP, respectively. The spin mixing between the 1PP and 3PP is allowed due to the degeneracy between the 1PP and three 3PP substates, while the spin mixing between the SE and TE is not allowed due to the much larger energy gap between the SE and TE. E
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5. CONCLUSIONS We have investigated the effects of magnetic field on the EL and current in P-PPV/CdSe-CdS-ZnS QD nanocomposites. It is found that incorporating CdSe-CdS-ZnS QDs in P-PPV matrix can induce the polarity changes in MFEs from positive to negative. From the QDs concentration and temperature dependences of negative MFEs, we conclude that the incorporated CdSe-CdS-ZnS QD can promote the relaxation of singlet excited states in P-PPV matrix, reversing the direction of spin mixing between the loosely bound singlet and triplet PPs. As a result, negative MEL and MC are generated in the PPPV/CdSe-CdS-ZnS QD nanocomposites. This work therefore presents much detailed information on the relaxation process, especially the spin-selective interactions, in the hybrid polymer/ QD nanocomposites, which may pave the way for spin-related applications of these fascinating hybrid composites.
interaction (HFI) strength definitely confirms that the MEL and MC in the pristine P-PPV device (Figures 3 and 4) are caused by the field-suppressed hyperfine mixing from 1PP to 3 PP.36,41 Because the negative MEL and MC in the P-PPV/ CdSe-CdS-ZnS QD nanocomposite devices exhibit the same lineshapes and characteristic field (δB = 4.5 mT, as shown in Figure 4) as the positive MFEs, we propose that the negative MEL and MC in the P-PPV/CdSe-CdS-ZnS QD nanocomposites may be caused by the same mechanism but with opposite direction, that is, field-suppressed hyperfine mixing from 3PP to 1PP states. In P-PPV/CdSe-CdS-ZnS QDs composites, the CdSe-CdSZnS QDs act as efficient energy-accepting species. As shown in Figure 6, The SE in P-PPV can transfer their energy to CdSeCdS-ZnS QDs; then, the excited CdSe-CdS-ZnS QDs radiatively decay to yield red emission due to their excellent PL efficiency. This process can be well-confirmed by the significant quenching of the P-PPV EL emission in P-PPV/ CdSe-CdS-ZnS QDs composites (Figure 3b). In this regard, incorporating CdSe-CdS-ZnS QDs in P-PPV polymer definitely provides an extra relaxation pathway for SE, which may enhance the recombination rate of kS because the relaxation of SE is promoted. When kS is improved to be larger than kT, the spin mixing will change its direction from 1PP → 3PP to 3PP → 1 PP according to previous discussion. In this case, applying a magnetic field would suppress the spin mixing from 3PP to 1PP, which decreases the singlet population in the nanocomposites. As a result, negative MEL and MC were obtained in the PPPV/CdSe-CdS-ZnS QD nanocomposites. On the basis of this scenario, the QD concentration and temperature dependences of negative MFEs in the composites can be well understood. As shown in Figure 3b, when 2 wt % CdSe-CdS-ZnS QDs are doped in P-PPV, the EL spectrum remains nearly unchanged, indicating that such a small amount CdSe-CdS-ZnS QDs in the nanocomposite can hardly change the relaxation process of SE in P-PPV. Thus the MEL and MC maintain their polarities but with decreased magnitudes. As the concentration of QDs in the P-PPV/CdSe-CdS-ZnS QD nanocomposites increases to 10 wt %, the P-PPV EL emission is significantly quenched and the QD emission becomes dominant as a result of efficient ET from P-PPV to CdSeCdS-ZnS QDs. This result definitely indicates that incorporating 10 wt % CdSe-CdS-ZnS QDs in P-PPV can strongly alter the relaxation processes of SE in the nanocomposites, resulting in the sign change in MEL and MC. Further increasing the concentration of QDs to 25 wt % will remarkably quench the EL emission of P-PPV and enhance the recombination rate of kS. As a result, the amplitudes of negative MEL and ME are enlarged. Another character of the negative MFEs in the P-PPV/CdSeCdS-ZnS QD nanocomposites is that their amplitudes increase as the temperature decreases. At low temperatures, the EL efficiency of the nanocomposite device is largely enhanced (Figure 5c), speeding up the relaxation processes of the singlet states. In addition, the lifetime of TE become much longer at low temperatures,32,49 enlarging the triplet population in the nanocomposite device. Thus these two effects will, respectively, lead to increase in kS and decrease in kT, largely promoting the spin mixing from 3PP to 1PP. As a result, applying an external magnetic field can induce much larger change in the ratio of singlet/triplet population and then larger MEL and MC at lower temperatures.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zuhong Xiong: 0000-0003-4729-300X Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the financial support by the National Natural Science Foundation (NSF) of China (Grant No. 11374242) and the Research and Innovation Project of Graduate Students of Chongqing (CYB16056).
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REFERENCES
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DOI: 10.1021/acs.jpcc.6b12538 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jpcc.6b12538 J. Phys. Chem. C XXXX, XXX, XXX−XXX
