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PVP nanocrystals by photoactivation
Materials Chemistry and Physics 133 (2012) 655–660
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Passivation-promoted photoluminescence efficiency of CdSe/PVP nanocrystals by photoactivation Hucheng Zhang ∗ , Dong Xu, Jianji Wang School of Chemistry & Environmental Science, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, Henan Normal University, Xinxiang, Henan 453007, PR China
a r t i c l e
i n f o
Article history: Received 14 April 2011 Received in revised form 23 December 2011 Accepted 18 January 2012 Keywords: Chalcogenides Interfaces Luminescence Irradiation effects
a b s t r a c t High fluorescence efficiency is desired for the practical applications of semiconductor nanocrystals (NCs) in many fields. However, reducing photoluminescence (PL) efficiency is usually observed after ligand exchanges on nanocrystal surfaces due to the generation of surface defects. Here, we report the preparation of polyvinylpyrrolidone-capped CdSe NCs (CdSe/PVP), and show the PL enhancement of CdSe/PVP NCs under UV radiation in the air. The experimental results indicate that the PL-enhanced mechanism of the NCs in methanol results from passivation of the lyates in that they act as the hole-acceptors. By screening of hole scavenger, furthermore, it is found that ditertbutyl peroxide is a better passivators of surface defects, and promotes the PL efficiency of CdSe/PVP NCs to achieve a value 110 times higher than that of as-prepared CdSe/PVP NCs. The photopassivation in the presence of organic molecules provides a prefertial, inexpensive path to harvest NCs with high PL efficiency. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Inorganic semiconductor nanocrystals (NCs) possess superior light absorption, exceptional photoluminescence (PL) efficiency, high photostability, and small exciton binding energy. Especially, their optical properties can be fine-tuned by tailoring size, shape, surface, and compositions of individual nanocrystal [1,2]. Therefore, semiconductor NCs with the impressive properties have attracted great interest over the last decades, and the prominent research advancements have shown they have many possible applications in the broad fields, such as light-emitting diodes [3,4], solar cells [5,6], field-effect transistors [7,8], photodetectors [9] and biological labeling and imaging [10–13]. It is well-known that high-quality semiconductor NCs are usually synthesized by colloidal routes in coordinating solvents [14,15], and the as-synthesized NCs with hydrophobic groups on their surfaces are insoluble in strong polar solvents. This greatly limits their uses in many important fields. In order to realize the potential applications over the wide range, an effectual strategy is to develop ligands with polar groups that can be used to replace those with hydrophobic groups on the NC surfaces [1,16]. Although such ligand exchange process can transfer NCs into strong polar solvents,
∗ Corresponding author. Tel.: +86 3733325805; fax: +86 3733326336. E-mail address: [email protected] (H. Zhang). 0254-0584/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.01.044
it often generates surface defect sites that cause nonradiative exciton recombination [17]. As a result, PL intensity of the NCs drops significantly, and even, cannot meet the desired requirements for practical applications. A technique to passivate effectively surface defects is the overgrowth of another inorganic semiconductor shell on the nanocrystal cores, and the core/shell structure can remarkably improve PL efficiency of NCs [18]. However, the shell growth is complicated for manipulation. Polyvinylpyrrolidone (PVP) is usually served as a stabilizer of nanoparticles [19], and the resulting nanoparticles as the building blocks exhibit good compatibility with many inorganic, organic, and biological materials. When PVP segments take the places of the original ligands on semiconductor NC surfaces, however, the weak coordinations of PVP toward NCs result in a high density of midgap states. Consequently, the probability of nonradiative recombination increases dramatically, accompanying a very low PL efficiency. In this work, PVP-capped CdSe (CdSe/PVP) NCs were prepared by the ligand exchange of PVP with the original ligand of octadecylamine (ODA) on as-synthesized CdSe NC surfaces. Whereafter, the organic molecules of methanol (MeOH) and ditertbutyl peroxide (DTBP) were employed as passivators of surface defects by means of photoactivation. It is shown that CdSe/PVP NCs with high PL efficiency can be harvested under UV irradiation. In particular, DTBP can exert the pronounced effect on the PL efficiency of CdSe/PVP NCs in MeOH as irradiated above the bandgap energy, and can promote the PL efficiency up to 110 times greater than that
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of as-prepared CdSe/PVP NCs. The PL-enhanced mechanism was suggested also from results of several experimental measurements. 2. Materials and methods 2.1. Materials Polyvinylpyrrolidone (PVP, MW = 40,000), and trioctylphosphine oxide (TOPO, 98%) were obtained from Sigma–Aldrich Co.; Selenium powder (99.99%), tributylphosphine (TBP, 95%), octadecene (ODE, 90%), octadecylamine (ODA, 99%), and ditertbutyl peroxide (DTBP, 98%) were purchased from Aladdin-Reagent Co., Shanghai; CdO (99.99%), oleic acid (99%), hexane (98.5%), anhydrous methanol (99.5%), anhydrous ethanol (99.5%), 1-propanol (99.5%), 1-butanol (99.5%), 2-butanol (99.5%), tert-butanol (99.5%), chloroform (95%), N,N-dimethylformamide (99.5%), and acetonitrile (99.5%) were purchased from Tianjin Chem. Reagent Co. All reagents were used as received except that ODA and TOPO were dried under vacuum at 120 ◦ C for 4 h prior to use. 2.2. Synthesis of CdSe/ODA nanocrystals Highly luminescent ODA-capped CdSe (CdSe/ODA) NCs with quite monodispersity were prepared, using a modified method as reported the literature [20,21]. Briefly, the precursor solution containing Cd was obtained by reacting 0.077 g of CdO with 0.68 g of oleic acid at 220 ◦ C under Ar flow, and dried under vacuum. Se solution was prepared by dissolving 1.4 g of Se in 3.84 g of TBP in glove box, and diluted with 12.33 g of ODE. Cd precursor, 1.5 g of ODA, 0.5 g of TOPO, and 2 g of ODE were loaded into a three-neck flask, and oxygen was removed on the Schlenk line. When the mixture was heated to 290 ◦ C under Ar flow, 3 g of Se solution was swiftly injected into the flask. Afterward, the reaction temperature was stabilized at 240 ◦ C for the growth of CdSe/ODA NCs. To monitor the reaction proceeding, small aliquots of the reaction mixture were taken at regular time intervals and diluted in hexane for the measurements of UV–vis spectra. At the desired size, the growth of CdSe/ODA NCs was stopped by removing the heating mantle. After cooled to room temperature, the reaction mixture was extracted twice using the mixed solvent of hexane and methanol. After this, an excess amount of methanol was added into the hexane solution, and then CdSe/ODA NCs were precipitated and separated by centrifugation. 2.3. Surface ligand exchange and photoactivation In order to prepare CdSe/PVP NCs, CdSe/ODA NCs were mixed with an overdose of PVP solution in chloroform, and stirred overnight at room temperature. The CdSe/PVP NCs were isolated by adding hexane into the chloroform solution, and purified by redispersing the NCs in chloroform and then precipitating them repeatedly for three times. Finally, CdSe/PVP NCs were dispersed in different solvents to form the colloidal solutions with the absorbances at the first exciton absorption peak below 0.05 to avoid any significant reabsorption during optical measurements, and henceforth the various measurements were carried out. In a typical process of photoactivation, 6 mL of the colloidal CdSe/PVP solution was loaded into a 10 mL quartz vial, and was exposed to UV irradiation using a 250 W high-pressure fluorescent Hg lamp with the strongest emission at 365 nm (Institute of Electrical Light Source, Beijing). The sample was located 20 cm apart from the UV source in a dark chamber. The UV–vis and photoluminescence spectra were measured at every 1 h intervals, respectively. For the sake of convenience, the as-prepared CdSe/PVP NCs in MeOH is denoted as Sample A. By means of the photoactivation process, the CdSe/PVP NCs in MeOH with the best PL efficiency is
referred to as Sample B, and equally the MeOH solution of DTBPCdSe/PVP NCs with maximal PL intensity as Sample C. 2.4. Measurements The UV–vis absorption spectrum of the sample was collected on a TU-1810 spectrophotometer (Beijing Purkinje General Instrument Co., Ltd.), using a matched pair of quartz cuvettes with a path length of 1 cm. The photoluminescence spectra of the samples were measured on Cary Eclipase fluorescence spectrophotometer with a xenon flash lamp (Varian, Inc.), and recorded under the same instrumental conditions, using the excitation wavenumber of 400 nm at room temperature. The colloidal solution of CdSe NCs was allowed to dry by solvent evaporation method. The resulting solid sample was mixed with KBr, and pressed into a pellet. The infrared spectrum of sample was recorded at room temperature on a computer-interfaced Bio-Rad Digilab FTS-40 FT-IR spectrometer in the range of 4000–400 cm−1 with the resolution of 2 cm−1 . Dry N2 purging gas was used in order to exclude the infrared active H2 O and CO2 in the atmosphere from the sample chamber. The colloidal solution of CdSe/PVP NCs was filled into a quartz cuvette, and DLS data were collected using Zetasizer Nano-ZS90 (Malvern instruments). For thermogravimetric analysis, the solid sample was loaded into an Al2 O3 crucible, and the experiment was performed under N2 purging gas using STA 449C simultaneous thermal analyzer (Netzsch Groups). The colloidal solutions of CdSe/PVP NCs were directly deposited on quartz glass to form the solid films. X-ray photoelectron spectra were recorded on a Kratos Axis Ultra spectrometer with the monochromatized Al K␣ radiation source. The X-ray electron gun was operated at 15 kV and 20 mA, and the kinetic energy of the photoelectrons was analyzed in a multichannel delay-line detector (DLD). The survey and high-resolution spectra were collected using 160 and 40 eV pass energies, respectively. 3. Results and discussion 3.1. Characteristics of CdSe/PVP NCs Using CdO and Se as the precursors in the presence of TOPO and ODA, a successful technique has been developed to synthesize CdSe NCs with quite monodispersity and high PL efficiency [20,21]. The as-synthesized CdSe/ODA NCs present the half-peak width of 24 nm in PL spectrum, the first exciton absorption peak at 563 nm in UV–vis spectrum, and the mean size of 3.3 nm that is estimated from the wavelength at first exciton absorption peak [22]. Experimentally, it is indicated that ODA ligands on CdSe NC surfaces can be readily replaced by PVP segments if CdSe/ODA NCs are mixed with the excessive PVP in chloroform. As shown in Fig. 1, the FT-IR spectrum of solid CdSe/PVP NCs is significantly similar to that of PVP, but very different from that of solid CdSe/ODA NCs. Comparing the spectrum of CdSe/PVP NCs with that of PVP, the spectral changes in both shift and relative intensity suggest the strong interactions of PVP segments with the CdSe NCs on surfaces. The essential differences between FT-IR spectra of CdSe/PVP and CdSe/ODA NCs give an indication of the original ligand of ODA to be fully replaced by PVP. The thermogravimetric analysis measurements were employed to estimate the surface coverage of PVP segments on CdSe/PVP NCs. It is shown that the major weight loss of PVP and CdSe/PVP NCs occurs in the temperature range from 300 to 500 ◦ C (Fig. 2), and this can be basically attributed to the decomposition of PVP. From the data of thermogravimetry, it is worked out that the weight percentage of PVP in CdSe/PVP is 45%. With the nanocrystal mean size
PVP 3463
10h
Intensity (a. u.)
5h 3h
1600
1h 0h
2924
2853
3415
2h
3000
586
3422
2956
1653
1424
1462 1465
CdSe/ODA
1400
7h
1673
1287 1293
Transmittance (a. u.)
CdSe/PVP
1200
657
570
H. Zhang et al. / Materials Chemistry and Physics 133 (2012) 655–660
540
3500
560
580
600
620
640
Wavelength (nm)
-1
Wavenumber (cm ) Fig. 1. FT-IR spectra of solid samples from pure PVP, as-synthesized CdSe/PVP, and CdSe/ODA NCs, respectively.
of 3.3 nm and the CdSe density of 5.81 g cm−3 , the number of PVP monomer adsorbed on the surface of each NC is determined to be 522. From the measurement of dynamic light scattering measurement, in addition, it is further estimated that the CdSe/PVP NCs in MeOH have the mean apparent hydrodynamic radius of 16 nm (Fig. S1). These results give a picture of the loose distribution of PVP segments over CdSe surface due to the weak coordination of PVP toward NCs, and hence it is expected that the ligand exchange can cause the great increase in the surface defects. 3.2. PL-enhancement of CdSe/PVP NCs in dispersion media Although PVP can be used to replace efficiently the ODA ligand on NC surfaces, the weak passivation of PVP toward CdSe surely results in the very low PL efficiency. As shown in Fig. 3, the as-prepared CdSe/PVP NCs in MeOH (Sample A) exhibits very small emission intensity. However, the increase of PL efficiency is observed when the colloidal solution is irradiated by a UV source in the air. The colloidal CdSe/PVP solution with the maximum of PL efficiency (Sample B) can be achieved with UV treatment of 10 h. Hereafter, the PL intensity appears as a decrease trend with the prolonged exposure time due to the photobleaching (not shown in
Fig. 3. Temporal evolution of PL spectrum of CdSe/PVP NCs in methanol with the UV irradiation.
Fig. 3), and even the precipitate of CdSe NCs can be observed as described later. The emission intensities of Sample A (F0 ) and the colloidal CdSe/PVP solution at any irradiation time (Ft ) were determined by integrating the emission peaks over the desired spectral region, respectively, and Fig. 4 shows the change of relative emission intensity (Ft /F0 ) with the irradiation time. Evidently, Sample B has the PL efficiency 73.1 times higher than Sample A. Accompanying the gradual PL enhancement, the emission peak exhibits a blue-shift from 586 nm to 570 nm during UV radiation. At the same time, the blue-shift at the first exciton absorption peak is also detected in the UV–vis spectrum of the colloidal CdSe/PVP solution (Fig. S2), and these spectra become significantly featureless with the prolonged irradiation time, suggesting the half-peak width of first exciton absorption peak is unrelated to the emission intensity of CdSe NCs in the case. All optical phenomena give the strong indications for the size reduction of CdSe cores and the occurrence of severe chemical changes on the CdSe surfaces. The control experiments show the emission intensity increases only in the presence of O2 , implying that O2 in the air plays an important role in the PL enhancement of the colloidal CdSe/PVP solutions. It is believed that O2 can pass through the loose PVP shell, and readily accepts electrons from the conduction band to passivate the carrier traps on CdSe/PVP surfaces as the energy of 120
100
585 100 80
CdSe /PVP
40
579
Ft / F0
60
60 576 40
Emission Peak (nm)
Weight (%)
582 80
573 20
20
PVP
570 0
0
0 100
200
300
400
4
8
12
16
20
24
500
o
Time (hour)
Temperature ( C) Fig. 2. Curves of thermogravimetric analysis for pure PVP and the as-synthesized CdSe/PVP NCs, respectively.
Fig. 4. Relative emission intensity and emission peak as a function of irradiation time. Sample A (䊉, ); MeOH solution of DTBP-CdSe/PVP NCs with the molar ratio of 300 DTBP per CdSe/PVP nanocrystal (, ).
H. Zhang et al. / Materials Chemistry and Physics 133 (2012) 655–660
529.7
Sample A
51
57.2
52.4 48
54
57
1110
Sample B
Sample C
1113 1128
1022 1022
530.5
Sample B
529.0
57.3
52.4
Intensity (a. u.)
Sample A
1091
O 1S
52.4
Se 3d
Transmittance (a. u.)
658
Sample C 525
528
531
534
537
900
1000
1100
1200
1300
1400
1500
-1
Wavenumber (cm )
Binding Energy (eV) Fig. 5. XPS spectrum for solid CdSe/PVP NCs obtained from different experimental stages, respectively.
Fig. 6. FT-IR spectra for solid CdSe/PVP NCs obtained from Samples A, B, and C, respectively.
exciting light is higher than the bandgap energy of CdSe NCs. The interface reactions cause the photocorrosion on CdSe/PVP NCs, and hence result in the blue-shifts of the emission or absorption spectra [23]. More importantly, a shell structure on CdSe/PVP surfaces can be built by the photoactivation [24], and greatly reduce the probability of nonradiative recombination due to the decrease of the traps associated with surface dangling bonds, ion vacancies, or disorder. The measurements of X-ray photoelectron spectroscopy (XPS) confirm the occurrence of O2 passivation (Fig. 5). For solid Sample B, the X-ray photoelectron spectrum in Se 3d region shows two chemical states. The peak at 52.4 eV, which can be ascribed to the chemical state of Se 3d in CdSe, is identical as found in Sample A, and the other at 57.3 eV results from the passivation by the photoactivation [25]. The as-prepared CdSe/PVP NCs can be dispersed in polar organic solvents with different properties, such as alcohols, chloroform, N,N-dimethyl-formamide (DMF), and acetonitrile. As conducted in MeOH, the experiments show PL enhancement of CdSe/PVP NCs in other disperse media is a general phenomenon. Table 1 lists the relative emission intensities of the as-prepared CdSe/PVP NCs in some solvents (Forig /F0 ) and their maximum of relative emission intensity (Fmax /F0 ) during UV radiation, respectively. It is noticed that 23.3 for CdSe/PVP NCs in acetonitrile is the relative emission intensity harvested best, and 8 times in DMF are the greatest increase in relative emission intensity when compared with the as-prepared CdSe/PVP NCs. Even so, these values are far less than the PL efficiency as obtained by the photoactivation in MeOH. As far as CdSe/PVP NCs in alcohols are concerned, the maximum of relative emission intensity during UV radiation follows the order: 73.1 (MeOH) 8.88 (ethanol) > 6.52 (1-propanol) ∼ 6.52
(1-butanol) > 6.18 (2-butanol) > 1.57 (tert-butanol). This order is consistent with that of autoprotolysis constants of the amphiprotic solvents (pKauto , Table 1) [26], indicating that either lyate or lyonium is responsible for the PL enhancement. As seen from the FT-IR spectrum of solid CdSe/PVP NCs prepared from Sample B (Fig. 6), the characteristic peaks of PVP in the region of 1200–1500 cm−1 are similar to those in solid CdSe/PVP NCs prepared from Sample A. This suggests that PVP does not participate in the interface reactions, and hence effects hardly on the PL efficiency. However, the appearance of FT-IR peaks at 1022 and 1110 cm−1 for solid CdSe/PVP NCs obtained from Sample B, which can be assigned to the stretching vibrations of ether bonds [19,27], reveals that MeOH is involved in the chemical changes on CdSe surface. Furthermore, the O 1s shoulder peak at 530.5 eV in the X-ray photoelectron spectrum confirms also the occurrence of ether bonds in Sample B (Fig. 5) [19]. Presumably, when the photogenerated electrons are accepted by O2 , MeO− can clear the holes from the valence band of CdSe NCs during UV irradiation. With the photoactivation, therefore, it is the synergistic actions of O2 and MeO− that effectively passivate the surface traps, and promote the relative emission intensity of CdSe/PVP NCs in MeOH to reach a value much higher than those in other solvents. However, there is a severe drawback when MeO− acts as the hole-acceptor. With the proceeding of interface reactions, the sites of PVP segments on the CdSe surface are gradually occupied by MeO− . In this situation, MeO− can play a part of the surface ligand. However, the lyate is not a good stabilizer for the dispersion of CdSe NCs in solution, and more has the very weak interactions with PVP. Consequently, CdSe/PVP NCs in MeOH becomes unstable, and a precipitate can be observed when the irradiation time is longer than 7 h. This, in a great extent, limits CdSe/PVP NCs in MeOH to achieve the optimal PL efficiency by the photoactivation. For all that, one of the most important enlightenments from MeO− is that higher PL efficiency can be harvested by judiciously selecting holeacceptor, which is excellent in stabilizing CdSe/PVP NCs during UV irradiation.
Table 1 Forig /F0 and Fmax /F0 of the CdSe/PVP NCs in several solvents, and pKauto of the amphiprotic solvents. Solvents
Forig /F0
Fmax /F0
pKauto (25 ◦ C) [24]
MeOH Ethanol 1-Propanol 1-Butanol 2-Butanol tert-Butanol Chloroform DMF Acetonitrile
1.00 1.35 2.02 1.80 1.69 0.67 5.17 1.23 9.89
73.1 8.88 6.52 6.52 6.18 1.57 7.64 9.78 23.3
17.20 18.88 19.43 21.56 N/A 26.80 – – –
3.3. Passivation of ditertbutyl peroxide toward CdSe/PVP NCs Peroxides are usually applied to manipulate the properties of NCs by etching, or to digest NCs into solutions [8,22], and they often cause PL quenching due to the generation of lattice defects [28]. In this work, however, DTBP with very weak oxidizability was experimentally proven to be a good scavenger of photogenerated
H. Zhang et al. / Materials Chemistry and Physics 133 (2012) 655–660
UV irradiation. The control experiments show no oxidation occurs between DTBP and PVP though PVP is a weak reductant, and no PL enhancement is observed in the absence of O2 even if DTBP is mixed into the colloidal CdSe/PVP solutions. From these results, it is believed that the lone pair electrons in DTBP can act as the scavenger of the photogenerated holes from the valence band of CdSe NCs, when the electrons from the conduction band are accepted by O2 . Obviously, the interface reactions of DTBP and O2 with CdSe NCs can be induced by the photoactivation, and effectively passivate the CdSe/PVP surface traps. As a result, the PL efficiency in the presence of DTBP can be promoted up to 110 times higher than that of as-prepared CdSe/PVP NCs.
110
Fmax,DTBP / F 0
659
100
90
80
4. Conclusions 70
0
100
200
300
400
500
DTBP: CdSe NC (molar ratio) Fig. 7. Variation of the maximum of relative emission intensity with the molar ratio of DTBP to CdSe/PVP nanocrystal in MeOH. Trend line is added to guide the eyes.
holes. When DTBP was mixed with CdSe/PVP NCs in MeOH, the spectral changes with the irradiation time, including relative emission intensity, emission peak shift and UV–vis absorption spectrum, are very similar to Sample A. The maximum of relative emission intensity can also be reached at 10 h. Fig. 4 shows the changes of the relative emission intensity and emission peak with irradiation time for the MeOH solution of DTBP-CdSe/PVP NCs with the molar ratio of 300 DTBP per CdSe/PVP nanocrystal. In contrast, the significant differences are that DTBP can result in the accelerated rise of the emission intensity, and more the disappearance of precipitate before the colloidal CdSe/PVP NCs solutions reach the maximum of relative emission intensity. Using DTBP as the hole-acceptor, therefore, it is expected to harvest the CdSe/PVP NCs with optimal PL efficiency under the experimental conditions. For the MeOH solutions of the CdSe/PVP NCs in the presence of DTBP, the harvested maximum of relative emission intensity (Fmax,DTBP /F0 ) depends on the molar ratio of DTBP to CdSe/PVP nanocrystal as they undergo the UV irradiation in the air (Fig. 7). It is observed that DTBP effects hardly on Fmax,DTBP /F0 when the molar ratio is less than 100, resulting from the low DTBP concentration and hence the weak adsorption CdSe surfaces toward DTBP. With increasing DTBP concentration further, Fmax,DTBP /F0 increases rapidly due to the passivation of DTBP toward the traps on CdSe surfaces. The colloidal solution with the molar ratio of 300 DTBP per CdSe/PVP nanocrystal (Sample C) exhibits the optimal Fmax,DTBP /F0 , and has the PL efficiency 110 times higher than Sample A. Thereafter, Fmax,DTBP /F0 decreases with increasing DTBP concentration, and the photobleaching is possibly the reason that results in the PL efficiency decrease at high DTBP concentration. With extremely excessive DTBP in the colloidal solution, the oxidation of DTBP can digest also CdSe/PVP NCs into solution as done by the strong oxidizer of H2 O2 , and the colloidal solution, as a result, becomes the colorless and transparent one. In addition, it is observed that the precipitate occurs also in the MeOH solutions of DTBP-CdSe NCs after 15 h of UV treatment, although the presence of DTBP is very advantage to achieve the optical PL efficiency. The surface passivation of DTBP toward CdSe/PVP NCs can be confirmed by the measurements of XPS and FT-IR spectrum. As shown in Figs. 5 and 6 for solid CdSe/PVP NCs prepared from Sample C, the Se 3d peak at 57.3 eV in XPS reveals the occurrence of oxidation due to O2 , and the peaks at 1022, 1113 and 1128 cm−1 in FT-IR spectrum, as mentioned earlier, give the indications of the stretching vibrations of ether bonds. Furthermore, the O 1s peak appears at 530.5 eV with conspicuous relative intensity, suggesting the strong chemical adsorption of CdSe NCs toward DTBP under the
PVP segments can successfully replace ODA on CdSe nanocrystal surfaces by the ligand exchange process, and hence the CdSe/PVP NCs can be readily prepared by mixing PVP with CdSe/ODA NCs in chloroform. Although the CdSe/PVP NCs can disperse in many polar solvents, they present very weak PL efficiency because of the occurrence of surface defect sites. With the UV irradiation in the air, the CdSe/PVP NCs display the characteristics of PL enhancement in all dispersion media. In particular, MeOH exerts on the pronounced effect, and can promote PL efficiency to reach a value 73.1 times higher than the as-prepared CdSe/PVP NCs. The researches on the PL-enhanced mechanism reveal that the lyate can act as the scavenger of photogenerated holes, and play a very important role in the exceptional PL enhancement of the CdSe/PVP NCs in MeOH. However, the nanocrystal surfaces passivated by MeO− can result in the deposition of the CdSe/PVP NCs, and thereby the CdSe/PVP NCs cannot achieve the optimal PL efficiency in MeOH by the photoactivation. By screening of hole scavenger, it is found that DTBP can act as a good passivator of surface defects: (i) the lone pair electrons in DTBP can readily accept the photogenerated holes to passivate CdSe nanocrystal surfaces; (ii) the ditertbutyl groups of DTBP interact strongly with PVP segments to dispel the possibility of the deposition of CdSe/PVP NCs during the photoactivation; (iii) DTBP is well miscible with many polar solvents, and particularly with water, and hence the method of PL enhancement reported in this work can be expected to apply in aqueous media. The experimental results show the CdSe/PVP NCs with PL efficiency110 times that of the as-prepared CdSe/PVP can be harvested using DTBP as the passivator of surface traps. By means of the UV treatment, therefore, it is concluded that semiconductor NCs with high PL efficiency can be expected to obtain by reasoningly screening of organic hole passivator. Acknowledgment The authors thank the National Natural Science Foundation of China (Grant No. 21073055) for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matchemphys.2012.01.044. References [1] D.V. Talapin, J.S. Lee, M.V. Kovalenko, E.V. Shevchenko, Chem. Rev. 110 (2010) 389. [2] N. Gaponik, S.G. Hickey, D. Dorfs, A.L. Rogach, A. Eychmüller, Small 6 (2010) 1364. [3] V.L. Colvin, M.C. Schlamp, A.P. Alivisatos, Nature 370 (1994) 354. [4] S. Coe, W.-K. Woo, M. Bawendi, V. Bulovic, Nature 420 (2002) 800. [5] X. Xu, S. Giménez, I. Mora-Seró, A. Abate, J. Bisquert, G. Xu, Mater. Chem. Phys. 124 (2010) 709.
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Passivation-promoted photoluminescence efficiency of CdSe/PVP nanocrystals by photoactivation Hucheng Zhang ∗ , Dong Xu, Jianji Wang School of Chemistry & Environmental Science, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, Henan Normal University, Xinxiang, Henan 453007, PR China
a r t i c l e
i n f o
Article history: Received 14 April 2011 Received in revised form 23 December 2011 Accepted 18 January 2012 Keywords: Chalcogenides Interfaces Luminescence Irradiation effects
a b s t r a c t High fluorescence efficiency is desired for the practical applications of semiconductor nanocrystals (NCs) in many fields. However, reducing photoluminescence (PL) efficiency is usually observed after ligand exchanges on nanocrystal surfaces due to the generation of surface defects. Here, we report the preparation of polyvinylpyrrolidone-capped CdSe NCs (CdSe/PVP), and show the PL enhancement of CdSe/PVP NCs under UV radiation in the air. The experimental results indicate that the PL-enhanced mechanism of the NCs in methanol results from passivation of the lyates in that they act as the hole-acceptors. By screening of hole scavenger, furthermore, it is found that ditertbutyl peroxide is a better passivators of surface defects, and promotes the PL efficiency of CdSe/PVP NCs to achieve a value 110 times higher than that of as-prepared CdSe/PVP NCs. The photopassivation in the presence of organic molecules provides a prefertial, inexpensive path to harvest NCs with high PL efficiency. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Inorganic semiconductor nanocrystals (NCs) possess superior light absorption, exceptional photoluminescence (PL) efficiency, high photostability, and small exciton binding energy. Especially, their optical properties can be fine-tuned by tailoring size, shape, surface, and compositions of individual nanocrystal [1,2]. Therefore, semiconductor NCs with the impressive properties have attracted great interest over the last decades, and the prominent research advancements have shown they have many possible applications in the broad fields, such as light-emitting diodes [3,4], solar cells [5,6], field-effect transistors [7,8], photodetectors [9] and biological labeling and imaging [10–13]. It is well-known that high-quality semiconductor NCs are usually synthesized by colloidal routes in coordinating solvents [14,15], and the as-synthesized NCs with hydrophobic groups on their surfaces are insoluble in strong polar solvents. This greatly limits their uses in many important fields. In order to realize the potential applications over the wide range, an effectual strategy is to develop ligands with polar groups that can be used to replace those with hydrophobic groups on the NC surfaces [1,16]. Although such ligand exchange process can transfer NCs into strong polar solvents,
∗ Corresponding author. Tel.: +86 3733325805; fax: +86 3733326336. E-mail address: [email protected] (H. Zhang). 0254-0584/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.01.044
it often generates surface defect sites that cause nonradiative exciton recombination [17]. As a result, PL intensity of the NCs drops significantly, and even, cannot meet the desired requirements for practical applications. A technique to passivate effectively surface defects is the overgrowth of another inorganic semiconductor shell on the nanocrystal cores, and the core/shell structure can remarkably improve PL efficiency of NCs [18]. However, the shell growth is complicated for manipulation. Polyvinylpyrrolidone (PVP) is usually served as a stabilizer of nanoparticles [19], and the resulting nanoparticles as the building blocks exhibit good compatibility with many inorganic, organic, and biological materials. When PVP segments take the places of the original ligands on semiconductor NC surfaces, however, the weak coordinations of PVP toward NCs result in a high density of midgap states. Consequently, the probability of nonradiative recombination increases dramatically, accompanying a very low PL efficiency. In this work, PVP-capped CdSe (CdSe/PVP) NCs were prepared by the ligand exchange of PVP with the original ligand of octadecylamine (ODA) on as-synthesized CdSe NC surfaces. Whereafter, the organic molecules of methanol (MeOH) and ditertbutyl peroxide (DTBP) were employed as passivators of surface defects by means of photoactivation. It is shown that CdSe/PVP NCs with high PL efficiency can be harvested under UV irradiation. In particular, DTBP can exert the pronounced effect on the PL efficiency of CdSe/PVP NCs in MeOH as irradiated above the bandgap energy, and can promote the PL efficiency up to 110 times greater than that
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of as-prepared CdSe/PVP NCs. The PL-enhanced mechanism was suggested also from results of several experimental measurements. 2. Materials and methods 2.1. Materials Polyvinylpyrrolidone (PVP, MW = 40,000), and trioctylphosphine oxide (TOPO, 98%) were obtained from Sigma–Aldrich Co.; Selenium powder (99.99%), tributylphosphine (TBP, 95%), octadecene (ODE, 90%), octadecylamine (ODA, 99%), and ditertbutyl peroxide (DTBP, 98%) were purchased from Aladdin-Reagent Co., Shanghai; CdO (99.99%), oleic acid (99%), hexane (98.5%), anhydrous methanol (99.5%), anhydrous ethanol (99.5%), 1-propanol (99.5%), 1-butanol (99.5%), 2-butanol (99.5%), tert-butanol (99.5%), chloroform (95%), N,N-dimethylformamide (99.5%), and acetonitrile (99.5%) were purchased from Tianjin Chem. Reagent Co. All reagents were used as received except that ODA and TOPO were dried under vacuum at 120 ◦ C for 4 h prior to use. 2.2. Synthesis of CdSe/ODA nanocrystals Highly luminescent ODA-capped CdSe (CdSe/ODA) NCs with quite monodispersity were prepared, using a modified method as reported the literature [20,21]. Briefly, the precursor solution containing Cd was obtained by reacting 0.077 g of CdO with 0.68 g of oleic acid at 220 ◦ C under Ar flow, and dried under vacuum. Se solution was prepared by dissolving 1.4 g of Se in 3.84 g of TBP in glove box, and diluted with 12.33 g of ODE. Cd precursor, 1.5 g of ODA, 0.5 g of TOPO, and 2 g of ODE were loaded into a three-neck flask, and oxygen was removed on the Schlenk line. When the mixture was heated to 290 ◦ C under Ar flow, 3 g of Se solution was swiftly injected into the flask. Afterward, the reaction temperature was stabilized at 240 ◦ C for the growth of CdSe/ODA NCs. To monitor the reaction proceeding, small aliquots of the reaction mixture were taken at regular time intervals and diluted in hexane for the measurements of UV–vis spectra. At the desired size, the growth of CdSe/ODA NCs was stopped by removing the heating mantle. After cooled to room temperature, the reaction mixture was extracted twice using the mixed solvent of hexane and methanol. After this, an excess amount of methanol was added into the hexane solution, and then CdSe/ODA NCs were precipitated and separated by centrifugation. 2.3. Surface ligand exchange and photoactivation In order to prepare CdSe/PVP NCs, CdSe/ODA NCs were mixed with an overdose of PVP solution in chloroform, and stirred overnight at room temperature. The CdSe/PVP NCs were isolated by adding hexane into the chloroform solution, and purified by redispersing the NCs in chloroform and then precipitating them repeatedly for three times. Finally, CdSe/PVP NCs were dispersed in different solvents to form the colloidal solutions with the absorbances at the first exciton absorption peak below 0.05 to avoid any significant reabsorption during optical measurements, and henceforth the various measurements were carried out. In a typical process of photoactivation, 6 mL of the colloidal CdSe/PVP solution was loaded into a 10 mL quartz vial, and was exposed to UV irradiation using a 250 W high-pressure fluorescent Hg lamp with the strongest emission at 365 nm (Institute of Electrical Light Source, Beijing). The sample was located 20 cm apart from the UV source in a dark chamber. The UV–vis and photoluminescence spectra were measured at every 1 h intervals, respectively. For the sake of convenience, the as-prepared CdSe/PVP NCs in MeOH is denoted as Sample A. By means of the photoactivation process, the CdSe/PVP NCs in MeOH with the best PL efficiency is
referred to as Sample B, and equally the MeOH solution of DTBPCdSe/PVP NCs with maximal PL intensity as Sample C. 2.4. Measurements The UV–vis absorption spectrum of the sample was collected on a TU-1810 spectrophotometer (Beijing Purkinje General Instrument Co., Ltd.), using a matched pair of quartz cuvettes with a path length of 1 cm. The photoluminescence spectra of the samples were measured on Cary Eclipase fluorescence spectrophotometer with a xenon flash lamp (Varian, Inc.), and recorded under the same instrumental conditions, using the excitation wavenumber of 400 nm at room temperature. The colloidal solution of CdSe NCs was allowed to dry by solvent evaporation method. The resulting solid sample was mixed with KBr, and pressed into a pellet. The infrared spectrum of sample was recorded at room temperature on a computer-interfaced Bio-Rad Digilab FTS-40 FT-IR spectrometer in the range of 4000–400 cm−1 with the resolution of 2 cm−1 . Dry N2 purging gas was used in order to exclude the infrared active H2 O and CO2 in the atmosphere from the sample chamber. The colloidal solution of CdSe/PVP NCs was filled into a quartz cuvette, and DLS data were collected using Zetasizer Nano-ZS90 (Malvern instruments). For thermogravimetric analysis, the solid sample was loaded into an Al2 O3 crucible, and the experiment was performed under N2 purging gas using STA 449C simultaneous thermal analyzer (Netzsch Groups). The colloidal solutions of CdSe/PVP NCs were directly deposited on quartz glass to form the solid films. X-ray photoelectron spectra were recorded on a Kratos Axis Ultra spectrometer with the monochromatized Al K␣ radiation source. The X-ray electron gun was operated at 15 kV and 20 mA, and the kinetic energy of the photoelectrons was analyzed in a multichannel delay-line detector (DLD). The survey and high-resolution spectra were collected using 160 and 40 eV pass energies, respectively. 3. Results and discussion 3.1. Characteristics of CdSe/PVP NCs Using CdO and Se as the precursors in the presence of TOPO and ODA, a successful technique has been developed to synthesize CdSe NCs with quite monodispersity and high PL efficiency [20,21]. The as-synthesized CdSe/ODA NCs present the half-peak width of 24 nm in PL spectrum, the first exciton absorption peak at 563 nm in UV–vis spectrum, and the mean size of 3.3 nm that is estimated from the wavelength at first exciton absorption peak [22]. Experimentally, it is indicated that ODA ligands on CdSe NC surfaces can be readily replaced by PVP segments if CdSe/ODA NCs are mixed with the excessive PVP in chloroform. As shown in Fig. 1, the FT-IR spectrum of solid CdSe/PVP NCs is significantly similar to that of PVP, but very different from that of solid CdSe/ODA NCs. Comparing the spectrum of CdSe/PVP NCs with that of PVP, the spectral changes in both shift and relative intensity suggest the strong interactions of PVP segments with the CdSe NCs on surfaces. The essential differences between FT-IR spectra of CdSe/PVP and CdSe/ODA NCs give an indication of the original ligand of ODA to be fully replaced by PVP. The thermogravimetric analysis measurements were employed to estimate the surface coverage of PVP segments on CdSe/PVP NCs. It is shown that the major weight loss of PVP and CdSe/PVP NCs occurs in the temperature range from 300 to 500 ◦ C (Fig. 2), and this can be basically attributed to the decomposition of PVP. From the data of thermogravimetry, it is worked out that the weight percentage of PVP in CdSe/PVP is 45%. With the nanocrystal mean size
PVP 3463
10h
Intensity (a. u.)
5h 3h
1600
1h 0h
2924
2853
3415
2h
3000
586
3422
2956
1653
1424
1462 1465
CdSe/ODA
1400
7h
1673
1287 1293
Transmittance (a. u.)
CdSe/PVP
1200
657
570
H. Zhang et al. / Materials Chemistry and Physics 133 (2012) 655–660
540
3500
560
580
600
620
640
Wavelength (nm)
-1
Wavenumber (cm ) Fig. 1. FT-IR spectra of solid samples from pure PVP, as-synthesized CdSe/PVP, and CdSe/ODA NCs, respectively.
of 3.3 nm and the CdSe density of 5.81 g cm−3 , the number of PVP monomer adsorbed on the surface of each NC is determined to be 522. From the measurement of dynamic light scattering measurement, in addition, it is further estimated that the CdSe/PVP NCs in MeOH have the mean apparent hydrodynamic radius of 16 nm (Fig. S1). These results give a picture of the loose distribution of PVP segments over CdSe surface due to the weak coordination of PVP toward NCs, and hence it is expected that the ligand exchange can cause the great increase in the surface defects. 3.2. PL-enhancement of CdSe/PVP NCs in dispersion media Although PVP can be used to replace efficiently the ODA ligand on NC surfaces, the weak passivation of PVP toward CdSe surely results in the very low PL efficiency. As shown in Fig. 3, the as-prepared CdSe/PVP NCs in MeOH (Sample A) exhibits very small emission intensity. However, the increase of PL efficiency is observed when the colloidal solution is irradiated by a UV source in the air. The colloidal CdSe/PVP solution with the maximum of PL efficiency (Sample B) can be achieved with UV treatment of 10 h. Hereafter, the PL intensity appears as a decrease trend with the prolonged exposure time due to the photobleaching (not shown in
Fig. 3. Temporal evolution of PL spectrum of CdSe/PVP NCs in methanol with the UV irradiation.
Fig. 3), and even the precipitate of CdSe NCs can be observed as described later. The emission intensities of Sample A (F0 ) and the colloidal CdSe/PVP solution at any irradiation time (Ft ) were determined by integrating the emission peaks over the desired spectral region, respectively, and Fig. 4 shows the change of relative emission intensity (Ft /F0 ) with the irradiation time. Evidently, Sample B has the PL efficiency 73.1 times higher than Sample A. Accompanying the gradual PL enhancement, the emission peak exhibits a blue-shift from 586 nm to 570 nm during UV radiation. At the same time, the blue-shift at the first exciton absorption peak is also detected in the UV–vis spectrum of the colloidal CdSe/PVP solution (Fig. S2), and these spectra become significantly featureless with the prolonged irradiation time, suggesting the half-peak width of first exciton absorption peak is unrelated to the emission intensity of CdSe NCs in the case. All optical phenomena give the strong indications for the size reduction of CdSe cores and the occurrence of severe chemical changes on the CdSe surfaces. The control experiments show the emission intensity increases only in the presence of O2 , implying that O2 in the air plays an important role in the PL enhancement of the colloidal CdSe/PVP solutions. It is believed that O2 can pass through the loose PVP shell, and readily accepts electrons from the conduction band to passivate the carrier traps on CdSe/PVP surfaces as the energy of 120
100
585 100 80
CdSe /PVP
40
579
Ft / F0
60
60 576 40
Emission Peak (nm)
Weight (%)
582 80
573 20
20
PVP
570 0
0
0 100
200
300
400
4
8
12
16
20
24
500
o
Time (hour)
Temperature ( C) Fig. 2. Curves of thermogravimetric analysis for pure PVP and the as-synthesized CdSe/PVP NCs, respectively.
Fig. 4. Relative emission intensity and emission peak as a function of irradiation time. Sample A (䊉, ); MeOH solution of DTBP-CdSe/PVP NCs with the molar ratio of 300 DTBP per CdSe/PVP nanocrystal (, ).
H. Zhang et al. / Materials Chemistry and Physics 133 (2012) 655–660
529.7
Sample A
51
57.2
52.4 48
54
57
1110
Sample B
Sample C
1113 1128
1022 1022
530.5
Sample B
529.0
57.3
52.4
Intensity (a. u.)
Sample A
1091
O 1S
52.4
Se 3d
Transmittance (a. u.)
658
Sample C 525
528
531
534
537
900
1000
1100
1200
1300
1400
1500
-1
Wavenumber (cm )
Binding Energy (eV) Fig. 5. XPS spectrum for solid CdSe/PVP NCs obtained from different experimental stages, respectively.
Fig. 6. FT-IR spectra for solid CdSe/PVP NCs obtained from Samples A, B, and C, respectively.
exciting light is higher than the bandgap energy of CdSe NCs. The interface reactions cause the photocorrosion on CdSe/PVP NCs, and hence result in the blue-shifts of the emission or absorption spectra [23]. More importantly, a shell structure on CdSe/PVP surfaces can be built by the photoactivation [24], and greatly reduce the probability of nonradiative recombination due to the decrease of the traps associated with surface dangling bonds, ion vacancies, or disorder. The measurements of X-ray photoelectron spectroscopy (XPS) confirm the occurrence of O2 passivation (Fig. 5). For solid Sample B, the X-ray photoelectron spectrum in Se 3d region shows two chemical states. The peak at 52.4 eV, which can be ascribed to the chemical state of Se 3d in CdSe, is identical as found in Sample A, and the other at 57.3 eV results from the passivation by the photoactivation [25]. The as-prepared CdSe/PVP NCs can be dispersed in polar organic solvents with different properties, such as alcohols, chloroform, N,N-dimethyl-formamide (DMF), and acetonitrile. As conducted in MeOH, the experiments show PL enhancement of CdSe/PVP NCs in other disperse media is a general phenomenon. Table 1 lists the relative emission intensities of the as-prepared CdSe/PVP NCs in some solvents (Forig /F0 ) and their maximum of relative emission intensity (Fmax /F0 ) during UV radiation, respectively. It is noticed that 23.3 for CdSe/PVP NCs in acetonitrile is the relative emission intensity harvested best, and 8 times in DMF are the greatest increase in relative emission intensity when compared with the as-prepared CdSe/PVP NCs. Even so, these values are far less than the PL efficiency as obtained by the photoactivation in MeOH. As far as CdSe/PVP NCs in alcohols are concerned, the maximum of relative emission intensity during UV radiation follows the order: 73.1 (MeOH) 8.88 (ethanol) > 6.52 (1-propanol) ∼ 6.52
(1-butanol) > 6.18 (2-butanol) > 1.57 (tert-butanol). This order is consistent with that of autoprotolysis constants of the amphiprotic solvents (pKauto , Table 1) [26], indicating that either lyate or lyonium is responsible for the PL enhancement. As seen from the FT-IR spectrum of solid CdSe/PVP NCs prepared from Sample B (Fig. 6), the characteristic peaks of PVP in the region of 1200–1500 cm−1 are similar to those in solid CdSe/PVP NCs prepared from Sample A. This suggests that PVP does not participate in the interface reactions, and hence effects hardly on the PL efficiency. However, the appearance of FT-IR peaks at 1022 and 1110 cm−1 for solid CdSe/PVP NCs obtained from Sample B, which can be assigned to the stretching vibrations of ether bonds [19,27], reveals that MeOH is involved in the chemical changes on CdSe surface. Furthermore, the O 1s shoulder peak at 530.5 eV in the X-ray photoelectron spectrum confirms also the occurrence of ether bonds in Sample B (Fig. 5) [19]. Presumably, when the photogenerated electrons are accepted by O2 , MeO− can clear the holes from the valence band of CdSe NCs during UV irradiation. With the photoactivation, therefore, it is the synergistic actions of O2 and MeO− that effectively passivate the surface traps, and promote the relative emission intensity of CdSe/PVP NCs in MeOH to reach a value much higher than those in other solvents. However, there is a severe drawback when MeO− acts as the hole-acceptor. With the proceeding of interface reactions, the sites of PVP segments on the CdSe surface are gradually occupied by MeO− . In this situation, MeO− can play a part of the surface ligand. However, the lyate is not a good stabilizer for the dispersion of CdSe NCs in solution, and more has the very weak interactions with PVP. Consequently, CdSe/PVP NCs in MeOH becomes unstable, and a precipitate can be observed when the irradiation time is longer than 7 h. This, in a great extent, limits CdSe/PVP NCs in MeOH to achieve the optimal PL efficiency by the photoactivation. For all that, one of the most important enlightenments from MeO− is that higher PL efficiency can be harvested by judiciously selecting holeacceptor, which is excellent in stabilizing CdSe/PVP NCs during UV irradiation.
Table 1 Forig /F0 and Fmax /F0 of the CdSe/PVP NCs in several solvents, and pKauto of the amphiprotic solvents. Solvents
Forig /F0
Fmax /F0
pKauto (25 ◦ C) [24]
MeOH Ethanol 1-Propanol 1-Butanol 2-Butanol tert-Butanol Chloroform DMF Acetonitrile
1.00 1.35 2.02 1.80 1.69 0.67 5.17 1.23 9.89
73.1 8.88 6.52 6.52 6.18 1.57 7.64 9.78 23.3
17.20 18.88 19.43 21.56 N/A 26.80 – – –
3.3. Passivation of ditertbutyl peroxide toward CdSe/PVP NCs Peroxides are usually applied to manipulate the properties of NCs by etching, or to digest NCs into solutions [8,22], and they often cause PL quenching due to the generation of lattice defects [28]. In this work, however, DTBP with very weak oxidizability was experimentally proven to be a good scavenger of photogenerated
H. Zhang et al. / Materials Chemistry and Physics 133 (2012) 655–660
UV irradiation. The control experiments show no oxidation occurs between DTBP and PVP though PVP is a weak reductant, and no PL enhancement is observed in the absence of O2 even if DTBP is mixed into the colloidal CdSe/PVP solutions. From these results, it is believed that the lone pair electrons in DTBP can act as the scavenger of the photogenerated holes from the valence band of CdSe NCs, when the electrons from the conduction band are accepted by O2 . Obviously, the interface reactions of DTBP and O2 with CdSe NCs can be induced by the photoactivation, and effectively passivate the CdSe/PVP surface traps. As a result, the PL efficiency in the presence of DTBP can be promoted up to 110 times higher than that of as-prepared CdSe/PVP NCs.
110
Fmax,DTBP / F 0
659
100
90
80
4. Conclusions 70
0
100
200
300
400
500
DTBP: CdSe NC (molar ratio) Fig. 7. Variation of the maximum of relative emission intensity with the molar ratio of DTBP to CdSe/PVP nanocrystal in MeOH. Trend line is added to guide the eyes.
holes. When DTBP was mixed with CdSe/PVP NCs in MeOH, the spectral changes with the irradiation time, including relative emission intensity, emission peak shift and UV–vis absorption spectrum, are very similar to Sample A. The maximum of relative emission intensity can also be reached at 10 h. Fig. 4 shows the changes of the relative emission intensity and emission peak with irradiation time for the MeOH solution of DTBP-CdSe/PVP NCs with the molar ratio of 300 DTBP per CdSe/PVP nanocrystal. In contrast, the significant differences are that DTBP can result in the accelerated rise of the emission intensity, and more the disappearance of precipitate before the colloidal CdSe/PVP NCs solutions reach the maximum of relative emission intensity. Using DTBP as the hole-acceptor, therefore, it is expected to harvest the CdSe/PVP NCs with optimal PL efficiency under the experimental conditions. For the MeOH solutions of the CdSe/PVP NCs in the presence of DTBP, the harvested maximum of relative emission intensity (Fmax,DTBP /F0 ) depends on the molar ratio of DTBP to CdSe/PVP nanocrystal as they undergo the UV irradiation in the air (Fig. 7). It is observed that DTBP effects hardly on Fmax,DTBP /F0 when the molar ratio is less than 100, resulting from the low DTBP concentration and hence the weak adsorption CdSe surfaces toward DTBP. With increasing DTBP concentration further, Fmax,DTBP /F0 increases rapidly due to the passivation of DTBP toward the traps on CdSe surfaces. The colloidal solution with the molar ratio of 300 DTBP per CdSe/PVP nanocrystal (Sample C) exhibits the optimal Fmax,DTBP /F0 , and has the PL efficiency 110 times higher than Sample A. Thereafter, Fmax,DTBP /F0 decreases with increasing DTBP concentration, and the photobleaching is possibly the reason that results in the PL efficiency decrease at high DTBP concentration. With extremely excessive DTBP in the colloidal solution, the oxidation of DTBP can digest also CdSe/PVP NCs into solution as done by the strong oxidizer of H2 O2 , and the colloidal solution, as a result, becomes the colorless and transparent one. In addition, it is observed that the precipitate occurs also in the MeOH solutions of DTBP-CdSe NCs after 15 h of UV treatment, although the presence of DTBP is very advantage to achieve the optical PL efficiency. The surface passivation of DTBP toward CdSe/PVP NCs can be confirmed by the measurements of XPS and FT-IR spectrum. As shown in Figs. 5 and 6 for solid CdSe/PVP NCs prepared from Sample C, the Se 3d peak at 57.3 eV in XPS reveals the occurrence of oxidation due to O2 , and the peaks at 1022, 1113 and 1128 cm−1 in FT-IR spectrum, as mentioned earlier, give the indications of the stretching vibrations of ether bonds. Furthermore, the O 1s peak appears at 530.5 eV with conspicuous relative intensity, suggesting the strong chemical adsorption of CdSe NCs toward DTBP under the
PVP segments can successfully replace ODA on CdSe nanocrystal surfaces by the ligand exchange process, and hence the CdSe/PVP NCs can be readily prepared by mixing PVP with CdSe/ODA NCs in chloroform. Although the CdSe/PVP NCs can disperse in many polar solvents, they present very weak PL efficiency because of the occurrence of surface defect sites. With the UV irradiation in the air, the CdSe/PVP NCs display the characteristics of PL enhancement in all dispersion media. In particular, MeOH exerts on the pronounced effect, and can promote PL efficiency to reach a value 73.1 times higher than the as-prepared CdSe/PVP NCs. The researches on the PL-enhanced mechanism reveal that the lyate can act as the scavenger of photogenerated holes, and play a very important role in the exceptional PL enhancement of the CdSe/PVP NCs in MeOH. However, the nanocrystal surfaces passivated by MeO− can result in the deposition of the CdSe/PVP NCs, and thereby the CdSe/PVP NCs cannot achieve the optimal PL efficiency in MeOH by the photoactivation. By screening of hole scavenger, it is found that DTBP can act as a good passivator of surface defects: (i) the lone pair electrons in DTBP can readily accept the photogenerated holes to passivate CdSe nanocrystal surfaces; (ii) the ditertbutyl groups of DTBP interact strongly with PVP segments to dispel the possibility of the deposition of CdSe/PVP NCs during the photoactivation; (iii) DTBP is well miscible with many polar solvents, and particularly with water, and hence the method of PL enhancement reported in this work can be expected to apply in aqueous media. The experimental results show the CdSe/PVP NCs with PL efficiency110 times that of the as-prepared CdSe/PVP can be harvested using DTBP as the passivator of surface traps. By means of the UV treatment, therefore, it is concluded that semiconductor NCs with high PL efficiency can be expected to obtain by reasoningly screening of organic hole passivator. Acknowledgment The authors thank the National Natural Science Foundation of China (Grant No. 21073055) for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matchemphys.2012.01.044. References [1] D.V. Talapin, J.S. Lee, M.V. Kovalenko, E.V. Shevchenko, Chem. Rev. 110 (2010) 389. [2] N. Gaponik, S.G. Hickey, D. Dorfs, A.L. Rogach, A. Eychmüller, Small 6 (2010) 1364. [3] V.L. Colvin, M.C. Schlamp, A.P. Alivisatos, Nature 370 (1994) 354. [4] S. Coe, W.-K. Woo, M. Bawendi, V. Bulovic, Nature 420 (2002) 800. [5] X. Xu, S. Giménez, I. Mora-Seró, A. Abate, J. Bisquert, G. Xu, Mater. Chem. Phys. 124 (2010) 709.
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