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Influence of size and surface state emission on photoluminescence of CdSe quantum dots under UV irradiation
Journal of Luminescence 177 (2016) 306–313
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Full Length Article
Influence of size and surface state emission on photoluminescence of CdSe quantum dots under UV irradiation Lian Hu a,n, Huizhen Wu b a b
School of science, Jiangnan University, 1800 Lihu Avenue, Wuxi 214122, Jiangsu, People's Republic of China Department of Physics and State Key Laboratory of Silicon Materials, Zhejiang University, Zheda road 38, Hangzhou 310027, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 July 2015 Accepted 27 April 2016 Available online 7 May 2016
Under UV irradiation, photo-oxidation occurs on the surface of CdSe quantum dots (QDs), thus modifying their QD optical properties. The influence of size and surface state (SS) emission on the photoluminescence (PL) evolution of CdSe QDs was investigated. Colorimetric study of the CdSe QD emission under UV irradiation indicates that the QD size and SS emission will influence the movement of chromaticity parameters. A photo-corrosion effect reduced the QD size and caused blue shift of the band edge (BE) peak. The irradiation-induced blue shift of QD emission always moves the corresponding chromaticity coordinate counterclockwise in the chromaticity diagram. For CdSe QDs with a dominant SS peak, the different evolution between BE and SS peak under UV irradiation will substantively influence the transformation of emission color: the chromaticity coordinate moved faster when PL increased than that when it decreased. Generally, the emission color of CdSe QDs with relatively large size and weak SS peaks is more stable in irradiation process. & 2016 Elsevier B.V. All rights reserved.
Keywords: Quantum dot UV irradiation Photo-oxidation Colorimetry Surface state
1. Introduction Colloid semiconductor QDs are well known for their unique size-dependent properties. The excellent optical properties make QDs useful in light-emitting diodes (LEDs) and in some fundamental research areas [1–5]. For small QDs, most atoms are located at the surface, and thus dangling bonds and vacancies on the surface result in many trap states [6–15]. These surface states (SSs) can lower the luminescence by drastically increasing the possibility of non-irradiative recombination for electron–hole pairs. Because the photo-induced oxidation on the QD surface will evidently modify the QD optical properties [6–13], the QDs are generally stored in the environment with as little oxygen as possible. From another point of view, UV irradiation is an available way to enhance the emission efficiency of QDs with weak luminescence (photoactivation effect). For example, under UV irradiation, CdSe QD surfaces will be passivated with oxidation layers such as SeO2 and Cd–OH by the photo-oxidation effect [6–8]. Meanwhile, as the effective size of the QDs decreases, the emission inhibits a blue shift. A moderate passivation layer can suppress the nonirradiative recombination and enhance the luminescence.
n
Corresponding author. E-mail address: [email protected] (L. Hu).
http://dx.doi.org/10.1016/j.jlumin.2016.04.049 0022-2313/& 2016 Elsevier B.V. All rights reserved.
However, excessive UV irradiation will produce new surface defects that lower the luminescence. QD luminescence will change gradually with continued UV irradiation since photo-oxidation can modify the QD surface under UV irradiation. Lockwood et al. used blue and UV light to irradiate Si QDs and gradually turned the PL intensity [10]. QD size decides the weight of surface atoms. Therefore, both the QD size and density of SSs will influence the PL evolution in photo-oxidation process. The change of QD emitting properties induced by light irradiation cannot be neglected for practical application in lighting devices and displays, and consequently the chromaticity color evolution of QD emission in irradiation processes deserves to be studied. In this paper, batches of oleic acid-capped CdSe QDs of varied sizes and SS emission were prepared. The size-dependent photooxidation effects on four batches of CdSe QDs dropped on wafers were investigated systematically. Emission evolutions of three batches QDs dispersed in polymethylmethacrylate (PMMA) were studied and the PMMA–QD luminescence patterns were prepared for demonstration. The varied QD emission under UV irradiation was studied with colorimetry. The influence of SS emission on
L. Hu, H. Wu / Journal of Luminescence 177 (2016) 306–313
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Fig. 1. (a) PL spectra of QD#1–4. The wavelengths of the BE peak are 476, 514, 536 and 558 nm, respectively. (b) PL decay spectra of QD#1–4 measured at BE positions. TEM images of the (c) 476 and (d) 558 nm CdSe QDs.
evolution of QD emission color in the irradiation process was studied.
2. Experiment section 2.1. Chemicals Selenium powder (Se, 99.999%), cadmium oxide (CdO, 99.95%), 1-octadecene (ODE, 90%), oleic acid (OA, 90%), tri-n-octyl phospine (TOP, 90%), and polymethylmethacrylate (PMMA, average MW 350000) powders were purchased from Alfa Aesar. Toluene, ethanol, acetone were used as received. 2.2. Preparation of CdSe QDs For CdSe, four batches of CdSe QD product were extracted at different reaction time. Simply, 0.2606 g CdO powder (2 mmol), 4 ml OA and 26 ml ODE were loaded in a 50 ml three neck flask and then the flask was heated at 110 °C under the Ar gas flow for 15 min to remove the air and water. Then the mixture in flask was heated at 300 °C to form the transparent Cd precursor solution. While the temperature decline to 190 °C, 1.6 ml selenium precursor TOPSe (selenium dissolved in TOP with 2 M concentration)
was rapidly injected into the flask in which Cd precursor solution was stirred by a magnetron. Three batches of the product were taken out at 5 s, 40 s and 90 s after the injection. The residual mixture in flask was continuously heated at 200 °C for 4 min and the product was collected after the flask cooled off. The CdSe QDs in ODE phase were mixed with three times the volume of ethanol then the QDs were precipitated from the mixture through centrifuge. The precipitate of OA-capped QDs was dispersed in toluene [16,17]. These QD batches were named after QD#1–4 in order of the growth time. The synthesis details of QD#A and QD#B were represented in previous works [18,19]. For QD#A, 2 mmol CdO was dissolved in 3 ml OA and 30 ml ODE, and then Se precursor (2 mmol Se dissolved in 1 ml TOP) was injected into the flask at 170 °C. The flask was cool off immediately by water bath. For QD#B, 0.6 mmol CdO was dissolved in 3 ml OA and 30 ml ODE, and then Se precursor (0.6 mmol Se dissolved in 0.5 ml TOP) was injected into the flask at 230 °C. For QD#C, 3 mmol CdO, 6 ml OA and 30 ml ODE were used. 3.3 ml Se precursor (TOPSe with 1.2 M concentration in TOP) was injected at 250 °C and the flask was cooled by water bath 5 seconds later. The ratio of Cd to Se precursor were 1, 1 and 0.76 for QD#A–C, respectively. Suppose the Cd and Se precursors reacted completely and the QDs were precipitated without loss,
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Fig. 2. (a–d) Evolved PL spectra of the QD#1–4 in the UV irradiation processing. Wavelength of BE peak (e) and (f) PL area versus irradiation time.
toluene was added to re-disperse the precipitated QD to form the QDs/toluene dispersion with 0.1 mmol/ml concentration. 2.3. Preparation of PMMA–QDs film For preparing the PMMA–QD thin film, the PMMA powders were added to the QDs/toluene dispersion with a weight-ratio of 1:30. The mixture was stored for several days and the PMMA was dissolved completely. The the blend of the PMMA–QD in toluene was spin coated on a clean silicon wafer to form a uniform PMMA– QD film. The film thickness can be altered by changing the rotation speed. For preparing the luminescence patterns, PMMA–QD film coated on Si wafer was covered with a lithography mask and then
a 365 nm UV light (20 W) was laid above the mask for selectively irradiating the PMMA–QD film. 2.4. Measurement and equipment PL spectra were measured by using an Edinburgh FLS920 PL system. For studying the evolution of the PL characters under UV irradiation, 325 nm light from a He–Cd laser (Kimmon) was used for both the PL excitation and the UV light source. For QD#1–4, each batch of QD dropped on silicon wafer was continued irradiated by the 325 nm light from a He–Cd laser in air and the PL spectra were recorded. For QD#A–C, the PMMA–QD films were prepared and the PL spectra were recorded as same as above. PL time decay spectra were measured by using a 405 nm picosecond
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Fig. 3. (a) Evolved PL spectra of QD#4 irradiated by the 405 nm laser. (b) PL decay spectra collected in the irradiation process. Inset: varied lifetime (hollow squares) and intensity (solid circles) at BE peak versus irradiation time.
laser. TEM images were scanned by a Tecnai G2 F20 S-TWIN transmission electron microscope.
The PL decay curve shown in Fig. 1b can be well fitted by twoexponential decay curves. The fitted formula is: C ¼ C 0 þ A1 exp
3. Result and discussion 3.1. PL measurement of QD#1–4 and TEM characterization Fig. 1 shows the PL spectra of the QD#1–4 batches. The band edge (BE) peaks of each QD batch are 476, 514, 536 and 558 nm, respectively. The average size of CdSe QDs can be estimated from the absorption or emission by an empirical equation [14]. The estimated sizes of QD#1–4 were 2.1, 2.4, 2.7, and 3.1 nm, respectively. The full width at half maximum (FWHMs) of each PL spectrum was about 40 nm, except that of QD#1. For the 476 nm QDs with small size, the PL spectrum showed a broad SS peak located beside the BE peak because the surface states will strongly capture the BE electron–hole pair and form the SS emission. PL spectra of QD#1–3 also have evident SS peaks. Generally, the Serich surface usually is considered a significant source of SSs [20– 22]. TEM images of the 476 and 558 nm QDs are shown in Fig. 1c and d, respectively.
ðt t 0 Þ
τ1
þ A2 exp
ðt t 0 Þ
τ2
ð1Þ
A1 and A2 are the amplitude for the time constant τ1 and τ2, respectively. The average lifetime o τ 4 is calculated by the following equation: oτ4 ¼
A1 τ21 þ A2 τ22 A1 τ1 þ A2 τ2
ð2Þ
The lifetimes of the PL decay spectra for QD#1–4 are 19.3, 13.2, 17.9, and 14.8 ns, respectively. Because the lifetime of the broad SS peak is usually longer than that of BE peak, a part of the SS peak overlaps the BE peak position, and the measured lifetime of the QD#1 476 nm peak (19.3 ns) is longer than those of others. 3.2. Size-dependent photo-oxidation effect on CdSe QDs and colorimetric study Fig. 2a–d show the evolved PL spectra of QD#1–4 in the UV irradiation process. The evolution of the PL properties in the irradiation process varied with the QD size. For QD#1–3, the PL intensity initially quickly increased and then decreased. For the
310
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Fig. 4. (a–d) Evolved chromaticity coordinates of the four QD batches in UV irradiation process. (e) Series of the four chromaticity coordinates listed in the CIE 1931 chromaticity diagram.
558 nm QD#4, the PL intensity continued to increase within the period of measurement (90 min). Fig. 2e and f show the BE peak position and normalized PL area versus the irradiation time, respectively. In Fig. 2e, the peak wavelength of QD#1–3 become shorter versus the irradiation time, which is ascribed to the photocorrosion effect reported in previous studies [3]. For QD#2 and #3 the blue shifts of the BE peak are more than 10 nm whereas for the 558 nm QD#4, the blue shift is only 2 nm. QD#1 took 1 min to reach maximum PL intensity and QD#2 take 2 min. For QD#1, as the PL intensity increased, the peak position shifted from 476 nm to 472 nm, and then to 468 nm. QD#2 took 2 min to reach the maximum PL (peak position 514 nm–508 nm) and then the PL intensity fall down (peak position 508–498 nm). QD#3 took 15 min to reach maximum PL (peak position 536–532 nm) and then the PL intensity decreased (peak position 532–522 nm). With UV irradiation, Se and Cd atoms on QD surface can be passivated by oxidation which increase the PL intensity. The time to get to the maximum PL increased with the QD size. For QD#4 with the largest surface, the PL increased to the maximum slowly in the measurement period (If the irradiation was prolonged, the PL would then decrease as expected). Fig. 2f indicates that the intensity for the small QDs increased and decreased faster than the larger QDs. For QD#1–3, the decreasing segment of the curve shown in Fig. 2f can be fitted by two-exponential decay curves. The decay time for QD#1–3 are 10, 13, and 650 min, respectively. The surface area of the small QD is smaller than that of larger QD, meaning that the surface of the smaller QDs can be passivated more quickly, and thus the PL will increase to the maximum more quickly. The Se atoms were oxidized in the irradiation process and
then the product SeO2 detached from the QD surface (photo-corrosion). When the QDs suffer excessive irradiation, excessive oxidation on the surface will reduce the effective QD size and introduce new stress defects. Also, some surface atoms will detach from the surface to form more vacancy defect states [19]. These introduced defect states will promote non-irradiative combination of the fresh excitons. This non-irradiative process will rise up with the excessive irradiation, and thus the PL intensity will continue to decrease. The PL intensity of smaller QDs decreased faster because the new defects were produced more easily on the surface of smaller QDs. For QD#1–4, the ratio of the maximum PL area to the initial PL area are 1.5, 1.9, 3.1, and 13.2, respectively (Fig. 2f). The growth extent of QD#4 is larger than that of QD#1–3. For the bigger QDs under the irradiation, the generation of new defects is less than the passivation on the surface for a relatively long time. That continued passivation resulted the large enhancement of PL. In Fig. 2e, the peak position of QD#4 had a slight red shift (from 558 nm to 562 nm) at the beginning of irradiation, which was related to the size distribution of the particles in QD#4 batch. In the irradiation process, emission of QDs with bigger size grew faster than that with smaller size. This selective enhancement caused a red shift at the beginning of irradiation. With the continued UV irradiation, the photo-corrosion dominated the shift of emission peak thus the blue shift appeared. Since the BE peak position of QD#4 was relatively stable in the irradiation process, the PL time decay spectra of this QD were measured with a 405 nm picosecond pulse laser. The PL spectra and decay spectra at the BE peak were collected at intervals. With 405 nm laser irradiation, the PL intensity of QD#4 increase
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Fig. 5. (a, c, e)PL intensity versus irradiation times of QD#A, B and C in PMMA matrix, respectively. (b, d, f) The intensity and position of BE and SS peaks, respectively, versus the irradiation time.
gradually, which is similar to the tendency shown in Fig. 2d. In Fig. 3, the decay time increased as the PL intensity increased. The measured PL decay lifetime is expressed with the radiative decay rate (Γr) and non-radiative decay rate (Γnr) as,
τ¼
1
Γ r þ Γ nr
ð3Þ
The quantum yield Q is expressed as, Q¼
Γr ¼ Γr τ Γ r þ Γ nr
ð4Þ
The PL intensity increased more distinctly than the decay time
τ, which indicates that the radiative decay rate Γr will increase as the PL increases in the irradiation process. Since both the measured PL lifetime (τ) and the radiative decay rate (Γr) increase, the decrease of non-radiative decay rate Γnr outpace the growth of Γr. At the beginning of the light-irradiation process, the QD surface states were passivated by photo-oxidation, and therefore the SS population decreased. On one hand, the decrease of SSs suppresses
the non-radiative process of QD excitons. On the other hand, the decrease of SSs induces the direct combination of the BE excitons. Therefore, the non-radiative decay rate Γnr decrease whereas the radiative decay rate Γr increased when the PL increased. Fig. 4 shows the chromaticity coordinates corresponding to the PL spectra in Fig. 2 versus irradiation times. Generally, the chromaticity coordinates turn counterclockwise versus irradiation time as a result of the blue shift of the emission (Fig. 2a–d). For QD#1 with an evident SS peak, the change of the coordinates at the beginning (1 min) is apparent, and then the chromaticity coordinate changes little during the period of decreasing PL. For QD#4, with a slight blue shift in the irradiation process, the change of the chromaticity coordinate is also very slight. QD#1 has an evident SS peak, and in the first 1 min of UV irradiation, some of the surface defect states are passivated. Therefore, the BE peak increased faster than the SS peak, and thus the ratio of the BE peak to the SS peak increased. The increased ratio of BE peak to SS peak and the blue shift of the peaks benefit the blue shift of QD emission.
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Fig. 6. (a) Chromaticity coordinates corresponding to PL spectra in Fig. 5a, c, and e listed within the CIE chromaticity diagram. (b, c, d) Luminescence patterns of QD#A, B and C in PMMA matrix prepared by selective irradiation with a 365 nm lamp. (e) The schematic diagram of the mechanism in the photo-oxidation process. Table 1 The change of the PL characters of the QDs in PMMA before and after UV irradiation. QD#A BE peak Intensity Unity-0.22 Position 480– 475 nm FWHM 34–37 nm
QD#B
QD#C
SS peak
BE peak
SS peak
BE peak
Unity-0.43 601– 566 nm 173– 190 nm
Unity-9.18 564– 540 nm 41–51 nm
Unity-2.01 688– 640 nm 162– 204 nm
Unity-7.57 560–558 nm 42–44 nm
However, during the period of decreasing PL, the BE peak declines fast than the SS peak, and thus the SS peak gradually dominated the PL spectra (Fig. 2a). This benefits the red shift of QD emission. In that period, there is competition between the decreasing ratio of the BE peak to the SS peak and blue shift of the two peaks, and so the chromaticity coordinate shifts slowly (Fig. 4a). 3.3. Influence of SS emission on the color evolution in UV irradiation process In Fig. 4a, for QD#1 with apparent SS emission, the evolution of the SS and BE emissions are different, and thus the PL spectra shape changes, which results in a relatively wide distribution of corresponding chromaticity coordinates. Generally, small QDs have evident SS emission because of the large surface/volume ratio, but large QDs also can exhibit evident SS emission if the
synthesis process is tailored. Three QD batches (QD#A, B and C) dispersed in PMMA matrix were prepared in order to study how QD size and evident SS emission influence movement of the chromaticity coordinate corresponding to PL in the UV irradiation process. QD#A is the small CdSe QDs (BE peak at 480 nm) with an SS peak located at 600 nm (inset of Fig. 5a: the PL spectrum was fitted by two Gaussian peaks). QD#B is somewhat larger CdSe QDs (BE peak at 564 nm) with an SS peak locating near 700 nm (inset of Fig. 5c). QD#C with a 560 nm BE peak and weak SS emission, was prepared for comparison. For the synthesis of QD#B, the concentration of Cd was relatively low in the precursor solution, which was considered as the main reason for the evident SSE. PMMA–QD films coated on silicon wafers were prepared with these three QD batches. Measurement of these PMMA–QD films were the same as those on QD#1–4. The irradiation time was 40 min. The evolved PL spectra of the PMMA–QD film during the UV irradiation process are shown in Fig. 5a, c, and e, and the corresponding chromaticity coordinates are listed in Fig. 6a. The intensity and position of the BE and SS peaks in Fig. 5a, c, and e versus the irradiation time are shown in Fig. 5b, d, and f. The changes of the PL characteristics before and after UV irradiation are listed in Table 1. In Fig. 5a, the PL intensity of PMMA–QD#A film decreased continually during the irradiation process. The PL of QD#1 increased in a short period at the beginning of irradiation (Fig. 2a). But in Fig. 5a, for the PMMA–QD#A film, no increase of PL intensity was observed because passivation of PMMA on the QD surface make QD#A emission close to the maximum value [9]. The small surface area and the big ratio of surface to volume make the defects form easily on QD#A surface. The BE and SS peaks decreased with analogous trends (Fig. 5b), which is similar to the PL decrease shown in Fig. 2a. The BE peak intensity decreased to 0.22 time of the initial intensity, whereas the SS peak intensity decreased to 0.43 time. In the irradiation process, the position of the BE peak shift from 480 nm to 475 nm and SS peak moves from 601 nm to 566 nm and not vice versa (Table 1). The BE peak declined faster than SS peak while the positions of the two peaks experienced blue shift. These two competitive factors result in slight variations to the chromaticity coordinate of QD#A (region A in Fig. 6a). For PMMA–QD#B, both the BE and the SS peaks increased at the beginning of irradiation process (Fig. 5c). The BE peak increased faster than the SS peak, and so the ratio of the BE peak to the SS peak increased (Fig. 5d). In the passivated period, the SSs were passivated while the radiative decay rate of SSs increased. These two competitive factors make the SS peak increased slower than BE peak. Within the irradiation period, the BE peak shifted to 538 nm from 564 nm, whereas the SS peak position shifted to 631 nm from 688 nm. The increasing ratio of BE peak to SS peak and the blue shift of the peaks benefit the blue shift of the emission, therefore changing the corresponding chromaticity coordinate of PMMA–QD#B (Fig. 6a). This is similar to the noticeable change of chromaticity coordinate in Fig. 4a within the first 1 min under UV irradiation. For PMMA–QD#C film with relatively weak SS emission, the evolutions of BE and SS peaks in Fig. 5d were similar to those of PMMA–QD#B (Fig. 5f). Table 1 also indicates that the FWHM of both the BE and SS peaks will always increase in the irradiation process, because the photo-corrosion effect can broaden the size distribution of QDs. QD#C and QD#B have a close BE position ( 560 nm), whereas the SS emission of QD#C is weaker than that of QD#B because of the relatively poor surface states. Only a 2 nm blue shift occurred in the irradiation process for QD#C because SSs can influence the photo-oxidation process. For QD#B with its Se-rich surface, the Se atoms on QD surface were oxidized to SeO2 in the irradiation process. Part of the SeO2 dissociates from QD surface. This photooxidation corrodes the QD surface, and thus more than 20 nm blue
L. Hu, H. Wu / Journal of Luminescence 177 (2016) 306–313
shift of the BE peak is observed (Fig. 5c). Comparable to QD#B, the QD#C surface have relatively rich Cd atoms, and thus the oxidized Se atoms cannot leave surface easily. The small blue shift of the PL spectra in Fig. 6a focus the chromaticity coordinates in a small region (region C in Fig. 6a). The photo-corrosion on QDs also introduces new surface defect states to lower the PL intensity, which can explain why QDs with evident SS emission will reach the maximum luminescence intensity earlier under UV irradiation (Fig. 5d and f). The PL evolution of QDs with evident SS emission in long irradiation period was discussed in previous work [19]. Fig. 6e indicates the mechanism in the photo-oxidation process. The oxidation can passivate the surface atoms (Se, Cd) which brighten the PL. The oxidation product of Se will continuous leave the QD surface, which result the blue shift of PL. Photo-corrosion speed of QDs with Se-rich surface (QD#B) is bigger than that of QD#C, thus the QD#B will experience faster blue shift of PL in photo-oxidation process. Both the size and the density of SSs QDs can influence the varied PL characteristics in the UV irradiation process. Using these three kind of QDs, bright-dark luminescence patterns can be easily prepared by selectively UV irradiating the PMMA–QD film under lithography masks (Fig. 6b, c and d). For the PMMA–QD#A film, the PL intensity of the irradiated area (uncovered area) decreased, and yet for the PMMA–QD#B and PMMA–QD#C films, the PL intensity changed in an opposite way at first (Fig. 5a, c and e). In the luminescence patterns in Fig. 6b and d, the emission color of bright region is similar to that of the dark region since UV irradiation-induced a slight change of chromaticity parameters (regions A and C in Fig. 6a). In the luminescence pattern in Fig. 6c, the emission color of the irradiated area is green, whereas that of the un-irradiated area is orange.
4. Conclusions UV irradiation on CdSe QDs can improve the oxidation of the surface atoms, thus modifying the luminescence properties. The QD size and the density of SSs will influence the photo-oxidation effect on QDs. Under the same UV irradiation, the PL spectrum of small QDs vary faster than that of large QDs, which means the PL intensity of the small QDs will take the lead in reaching a maximum value and then decay. The growth extent of PL for large QDs is big than that of small QDs. In the UV irradiation process, the shape and position of the PL spectrum for the large QDs with weak SS emission is relatively stable, and thus the emission color changes slowly. The photo-oxidation effect tends to happen on the Se-rich surface of QDs with evident SS emission. The blue shift of the BE peak for CdSe QDs with evident SS emission is more noticeable than that for CdSe QDs with weak SS emission. For QDs
313
with evident SS emission, the evolution of the BE and SS peaks are different. The intensity of the BE peak always changes faster than that of the SS peak, and yet the SS peak shifts to short wavelengths faster than the BE peak versus irradiation time. During the stage with increasing PL, the increasing ratio of BE peak to SS peak and the blue shift of the peaks benefitted the corresponding chromaticity coordinate shift. In the stage with decreasing PL, the competition between the decreasing ratio of the BE peak to the SS peak and the blue shift of peaks can slow the shift of chromaticity coordinate. For the QDs with evident SS emission, the movement of the chromaticity coordinate in the PL-increasing period is faster than that in PL-decreasing period.
Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 11504138) and Fundamental Research Funds for the Central Universities (No. JUSRP11403).
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Full Length Article
Influence of size and surface state emission on photoluminescence of CdSe quantum dots under UV irradiation Lian Hu a,n, Huizhen Wu b a b
School of science, Jiangnan University, 1800 Lihu Avenue, Wuxi 214122, Jiangsu, People's Republic of China Department of Physics and State Key Laboratory of Silicon Materials, Zhejiang University, Zheda road 38, Hangzhou 310027, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 July 2015 Accepted 27 April 2016 Available online 7 May 2016
Under UV irradiation, photo-oxidation occurs on the surface of CdSe quantum dots (QDs), thus modifying their QD optical properties. The influence of size and surface state (SS) emission on the photoluminescence (PL) evolution of CdSe QDs was investigated. Colorimetric study of the CdSe QD emission under UV irradiation indicates that the QD size and SS emission will influence the movement of chromaticity parameters. A photo-corrosion effect reduced the QD size and caused blue shift of the band edge (BE) peak. The irradiation-induced blue shift of QD emission always moves the corresponding chromaticity coordinate counterclockwise in the chromaticity diagram. For CdSe QDs with a dominant SS peak, the different evolution between BE and SS peak under UV irradiation will substantively influence the transformation of emission color: the chromaticity coordinate moved faster when PL increased than that when it decreased. Generally, the emission color of CdSe QDs with relatively large size and weak SS peaks is more stable in irradiation process. & 2016 Elsevier B.V. All rights reserved.
Keywords: Quantum dot UV irradiation Photo-oxidation Colorimetry Surface state
1. Introduction Colloid semiconductor QDs are well known for their unique size-dependent properties. The excellent optical properties make QDs useful in light-emitting diodes (LEDs) and in some fundamental research areas [1–5]. For small QDs, most atoms are located at the surface, and thus dangling bonds and vacancies on the surface result in many trap states [6–15]. These surface states (SSs) can lower the luminescence by drastically increasing the possibility of non-irradiative recombination for electron–hole pairs. Because the photo-induced oxidation on the QD surface will evidently modify the QD optical properties [6–13], the QDs are generally stored in the environment with as little oxygen as possible. From another point of view, UV irradiation is an available way to enhance the emission efficiency of QDs with weak luminescence (photoactivation effect). For example, under UV irradiation, CdSe QD surfaces will be passivated with oxidation layers such as SeO2 and Cd–OH by the photo-oxidation effect [6–8]. Meanwhile, as the effective size of the QDs decreases, the emission inhibits a blue shift. A moderate passivation layer can suppress the nonirradiative recombination and enhance the luminescence.
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Corresponding author. E-mail address: [email protected] (L. Hu).
http://dx.doi.org/10.1016/j.jlumin.2016.04.049 0022-2313/& 2016 Elsevier B.V. All rights reserved.
However, excessive UV irradiation will produce new surface defects that lower the luminescence. QD luminescence will change gradually with continued UV irradiation since photo-oxidation can modify the QD surface under UV irradiation. Lockwood et al. used blue and UV light to irradiate Si QDs and gradually turned the PL intensity [10]. QD size decides the weight of surface atoms. Therefore, both the QD size and density of SSs will influence the PL evolution in photo-oxidation process. The change of QD emitting properties induced by light irradiation cannot be neglected for practical application in lighting devices and displays, and consequently the chromaticity color evolution of QD emission in irradiation processes deserves to be studied. In this paper, batches of oleic acid-capped CdSe QDs of varied sizes and SS emission were prepared. The size-dependent photooxidation effects on four batches of CdSe QDs dropped on wafers were investigated systematically. Emission evolutions of three batches QDs dispersed in polymethylmethacrylate (PMMA) were studied and the PMMA–QD luminescence patterns were prepared for demonstration. The varied QD emission under UV irradiation was studied with colorimetry. The influence of SS emission on
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Fig. 1. (a) PL spectra of QD#1–4. The wavelengths of the BE peak are 476, 514, 536 and 558 nm, respectively. (b) PL decay spectra of QD#1–4 measured at BE positions. TEM images of the (c) 476 and (d) 558 nm CdSe QDs.
evolution of QD emission color in the irradiation process was studied.
2. Experiment section 2.1. Chemicals Selenium powder (Se, 99.999%), cadmium oxide (CdO, 99.95%), 1-octadecene (ODE, 90%), oleic acid (OA, 90%), tri-n-octyl phospine (TOP, 90%), and polymethylmethacrylate (PMMA, average MW 350000) powders were purchased from Alfa Aesar. Toluene, ethanol, acetone were used as received. 2.2. Preparation of CdSe QDs For CdSe, four batches of CdSe QD product were extracted at different reaction time. Simply, 0.2606 g CdO powder (2 mmol), 4 ml OA and 26 ml ODE were loaded in a 50 ml three neck flask and then the flask was heated at 110 °C under the Ar gas flow for 15 min to remove the air and water. Then the mixture in flask was heated at 300 °C to form the transparent Cd precursor solution. While the temperature decline to 190 °C, 1.6 ml selenium precursor TOPSe (selenium dissolved in TOP with 2 M concentration)
was rapidly injected into the flask in which Cd precursor solution was stirred by a magnetron. Three batches of the product were taken out at 5 s, 40 s and 90 s after the injection. The residual mixture in flask was continuously heated at 200 °C for 4 min and the product was collected after the flask cooled off. The CdSe QDs in ODE phase were mixed with three times the volume of ethanol then the QDs were precipitated from the mixture through centrifuge. The precipitate of OA-capped QDs was dispersed in toluene [16,17]. These QD batches were named after QD#1–4 in order of the growth time. The synthesis details of QD#A and QD#B were represented in previous works [18,19]. For QD#A, 2 mmol CdO was dissolved in 3 ml OA and 30 ml ODE, and then Se precursor (2 mmol Se dissolved in 1 ml TOP) was injected into the flask at 170 °C. The flask was cool off immediately by water bath. For QD#B, 0.6 mmol CdO was dissolved in 3 ml OA and 30 ml ODE, and then Se precursor (0.6 mmol Se dissolved in 0.5 ml TOP) was injected into the flask at 230 °C. For QD#C, 3 mmol CdO, 6 ml OA and 30 ml ODE were used. 3.3 ml Se precursor (TOPSe with 1.2 M concentration in TOP) was injected at 250 °C and the flask was cooled by water bath 5 seconds later. The ratio of Cd to Se precursor were 1, 1 and 0.76 for QD#A–C, respectively. Suppose the Cd and Se precursors reacted completely and the QDs were precipitated without loss,
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Fig. 2. (a–d) Evolved PL spectra of the QD#1–4 in the UV irradiation processing. Wavelength of BE peak (e) and (f) PL area versus irradiation time.
toluene was added to re-disperse the precipitated QD to form the QDs/toluene dispersion with 0.1 mmol/ml concentration. 2.3. Preparation of PMMA–QDs film For preparing the PMMA–QD thin film, the PMMA powders were added to the QDs/toluene dispersion with a weight-ratio of 1:30. The mixture was stored for several days and the PMMA was dissolved completely. The the blend of the PMMA–QD in toluene was spin coated on a clean silicon wafer to form a uniform PMMA– QD film. The film thickness can be altered by changing the rotation speed. For preparing the luminescence patterns, PMMA–QD film coated on Si wafer was covered with a lithography mask and then
a 365 nm UV light (20 W) was laid above the mask for selectively irradiating the PMMA–QD film. 2.4. Measurement and equipment PL spectra were measured by using an Edinburgh FLS920 PL system. For studying the evolution of the PL characters under UV irradiation, 325 nm light from a He–Cd laser (Kimmon) was used for both the PL excitation and the UV light source. For QD#1–4, each batch of QD dropped on silicon wafer was continued irradiated by the 325 nm light from a He–Cd laser in air and the PL spectra were recorded. For QD#A–C, the PMMA–QD films were prepared and the PL spectra were recorded as same as above. PL time decay spectra were measured by using a 405 nm picosecond
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Fig. 3. (a) Evolved PL spectra of QD#4 irradiated by the 405 nm laser. (b) PL decay spectra collected in the irradiation process. Inset: varied lifetime (hollow squares) and intensity (solid circles) at BE peak versus irradiation time.
laser. TEM images were scanned by a Tecnai G2 F20 S-TWIN transmission electron microscope.
The PL decay curve shown in Fig. 1b can be well fitted by twoexponential decay curves. The fitted formula is: C ¼ C 0 þ A1 exp
3. Result and discussion 3.1. PL measurement of QD#1–4 and TEM characterization Fig. 1 shows the PL spectra of the QD#1–4 batches. The band edge (BE) peaks of each QD batch are 476, 514, 536 and 558 nm, respectively. The average size of CdSe QDs can be estimated from the absorption or emission by an empirical equation [14]. The estimated sizes of QD#1–4 were 2.1, 2.4, 2.7, and 3.1 nm, respectively. The full width at half maximum (FWHMs) of each PL spectrum was about 40 nm, except that of QD#1. For the 476 nm QDs with small size, the PL spectrum showed a broad SS peak located beside the BE peak because the surface states will strongly capture the BE electron–hole pair and form the SS emission. PL spectra of QD#1–3 also have evident SS peaks. Generally, the Serich surface usually is considered a significant source of SSs [20– 22]. TEM images of the 476 and 558 nm QDs are shown in Fig. 1c and d, respectively.
ðt t 0 Þ
τ1
þ A2 exp
ðt t 0 Þ
τ2
ð1Þ
A1 and A2 are the amplitude for the time constant τ1 and τ2, respectively. The average lifetime o τ 4 is calculated by the following equation: oτ4 ¼
A1 τ21 þ A2 τ22 A1 τ1 þ A2 τ2
ð2Þ
The lifetimes of the PL decay spectra for QD#1–4 are 19.3, 13.2, 17.9, and 14.8 ns, respectively. Because the lifetime of the broad SS peak is usually longer than that of BE peak, a part of the SS peak overlaps the BE peak position, and the measured lifetime of the QD#1 476 nm peak (19.3 ns) is longer than those of others. 3.2. Size-dependent photo-oxidation effect on CdSe QDs and colorimetric study Fig. 2a–d show the evolved PL spectra of QD#1–4 in the UV irradiation process. The evolution of the PL properties in the irradiation process varied with the QD size. For QD#1–3, the PL intensity initially quickly increased and then decreased. For the
310
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Fig. 4. (a–d) Evolved chromaticity coordinates of the four QD batches in UV irradiation process. (e) Series of the four chromaticity coordinates listed in the CIE 1931 chromaticity diagram.
558 nm QD#4, the PL intensity continued to increase within the period of measurement (90 min). Fig. 2e and f show the BE peak position and normalized PL area versus the irradiation time, respectively. In Fig. 2e, the peak wavelength of QD#1–3 become shorter versus the irradiation time, which is ascribed to the photocorrosion effect reported in previous studies [3]. For QD#2 and #3 the blue shifts of the BE peak are more than 10 nm whereas for the 558 nm QD#4, the blue shift is only 2 nm. QD#1 took 1 min to reach maximum PL intensity and QD#2 take 2 min. For QD#1, as the PL intensity increased, the peak position shifted from 476 nm to 472 nm, and then to 468 nm. QD#2 took 2 min to reach the maximum PL (peak position 514 nm–508 nm) and then the PL intensity fall down (peak position 508–498 nm). QD#3 took 15 min to reach maximum PL (peak position 536–532 nm) and then the PL intensity decreased (peak position 532–522 nm). With UV irradiation, Se and Cd atoms on QD surface can be passivated by oxidation which increase the PL intensity. The time to get to the maximum PL increased with the QD size. For QD#4 with the largest surface, the PL increased to the maximum slowly in the measurement period (If the irradiation was prolonged, the PL would then decrease as expected). Fig. 2f indicates that the intensity for the small QDs increased and decreased faster than the larger QDs. For QD#1–3, the decreasing segment of the curve shown in Fig. 2f can be fitted by two-exponential decay curves. The decay time for QD#1–3 are 10, 13, and 650 min, respectively. The surface area of the small QD is smaller than that of larger QD, meaning that the surface of the smaller QDs can be passivated more quickly, and thus the PL will increase to the maximum more quickly. The Se atoms were oxidized in the irradiation process and
then the product SeO2 detached from the QD surface (photo-corrosion). When the QDs suffer excessive irradiation, excessive oxidation on the surface will reduce the effective QD size and introduce new stress defects. Also, some surface atoms will detach from the surface to form more vacancy defect states [19]. These introduced defect states will promote non-irradiative combination of the fresh excitons. This non-irradiative process will rise up with the excessive irradiation, and thus the PL intensity will continue to decrease. The PL intensity of smaller QDs decreased faster because the new defects were produced more easily on the surface of smaller QDs. For QD#1–4, the ratio of the maximum PL area to the initial PL area are 1.5, 1.9, 3.1, and 13.2, respectively (Fig. 2f). The growth extent of QD#4 is larger than that of QD#1–3. For the bigger QDs under the irradiation, the generation of new defects is less than the passivation on the surface for a relatively long time. That continued passivation resulted the large enhancement of PL. In Fig. 2e, the peak position of QD#4 had a slight red shift (from 558 nm to 562 nm) at the beginning of irradiation, which was related to the size distribution of the particles in QD#4 batch. In the irradiation process, emission of QDs with bigger size grew faster than that with smaller size. This selective enhancement caused a red shift at the beginning of irradiation. With the continued UV irradiation, the photo-corrosion dominated the shift of emission peak thus the blue shift appeared. Since the BE peak position of QD#4 was relatively stable in the irradiation process, the PL time decay spectra of this QD were measured with a 405 nm picosecond pulse laser. The PL spectra and decay spectra at the BE peak were collected at intervals. With 405 nm laser irradiation, the PL intensity of QD#4 increase
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Fig. 5. (a, c, e)PL intensity versus irradiation times of QD#A, B and C in PMMA matrix, respectively. (b, d, f) The intensity and position of BE and SS peaks, respectively, versus the irradiation time.
gradually, which is similar to the tendency shown in Fig. 2d. In Fig. 3, the decay time increased as the PL intensity increased. The measured PL decay lifetime is expressed with the radiative decay rate (Γr) and non-radiative decay rate (Γnr) as,
τ¼
1
Γ r þ Γ nr
ð3Þ
The quantum yield Q is expressed as, Q¼
Γr ¼ Γr τ Γ r þ Γ nr
ð4Þ
The PL intensity increased more distinctly than the decay time
τ, which indicates that the radiative decay rate Γr will increase as the PL increases in the irradiation process. Since both the measured PL lifetime (τ) and the radiative decay rate (Γr) increase, the decrease of non-radiative decay rate Γnr outpace the growth of Γr. At the beginning of the light-irradiation process, the QD surface states were passivated by photo-oxidation, and therefore the SS population decreased. On one hand, the decrease of SSs suppresses
the non-radiative process of QD excitons. On the other hand, the decrease of SSs induces the direct combination of the BE excitons. Therefore, the non-radiative decay rate Γnr decrease whereas the radiative decay rate Γr increased when the PL increased. Fig. 4 shows the chromaticity coordinates corresponding to the PL spectra in Fig. 2 versus irradiation times. Generally, the chromaticity coordinates turn counterclockwise versus irradiation time as a result of the blue shift of the emission (Fig. 2a–d). For QD#1 with an evident SS peak, the change of the coordinates at the beginning (1 min) is apparent, and then the chromaticity coordinate changes little during the period of decreasing PL. For QD#4, with a slight blue shift in the irradiation process, the change of the chromaticity coordinate is also very slight. QD#1 has an evident SS peak, and in the first 1 min of UV irradiation, some of the surface defect states are passivated. Therefore, the BE peak increased faster than the SS peak, and thus the ratio of the BE peak to the SS peak increased. The increased ratio of BE peak to SS peak and the blue shift of the peaks benefit the blue shift of QD emission.
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Fig. 6. (a) Chromaticity coordinates corresponding to PL spectra in Fig. 5a, c, and e listed within the CIE chromaticity diagram. (b, c, d) Luminescence patterns of QD#A, B and C in PMMA matrix prepared by selective irradiation with a 365 nm lamp. (e) The schematic diagram of the mechanism in the photo-oxidation process. Table 1 The change of the PL characters of the QDs in PMMA before and after UV irradiation. QD#A BE peak Intensity Unity-0.22 Position 480– 475 nm FWHM 34–37 nm
QD#B
QD#C
SS peak
BE peak
SS peak
BE peak
Unity-0.43 601– 566 nm 173– 190 nm
Unity-9.18 564– 540 nm 41–51 nm
Unity-2.01 688– 640 nm 162– 204 nm
Unity-7.57 560–558 nm 42–44 nm
However, during the period of decreasing PL, the BE peak declines fast than the SS peak, and thus the SS peak gradually dominated the PL spectra (Fig. 2a). This benefits the red shift of QD emission. In that period, there is competition between the decreasing ratio of the BE peak to the SS peak and blue shift of the two peaks, and so the chromaticity coordinate shifts slowly (Fig. 4a). 3.3. Influence of SS emission on the color evolution in UV irradiation process In Fig. 4a, for QD#1 with apparent SS emission, the evolution of the SS and BE emissions are different, and thus the PL spectra shape changes, which results in a relatively wide distribution of corresponding chromaticity coordinates. Generally, small QDs have evident SS emission because of the large surface/volume ratio, but large QDs also can exhibit evident SS emission if the
synthesis process is tailored. Three QD batches (QD#A, B and C) dispersed in PMMA matrix were prepared in order to study how QD size and evident SS emission influence movement of the chromaticity coordinate corresponding to PL in the UV irradiation process. QD#A is the small CdSe QDs (BE peak at 480 nm) with an SS peak located at 600 nm (inset of Fig. 5a: the PL spectrum was fitted by two Gaussian peaks). QD#B is somewhat larger CdSe QDs (BE peak at 564 nm) with an SS peak locating near 700 nm (inset of Fig. 5c). QD#C with a 560 nm BE peak and weak SS emission, was prepared for comparison. For the synthesis of QD#B, the concentration of Cd was relatively low in the precursor solution, which was considered as the main reason for the evident SSE. PMMA–QD films coated on silicon wafers were prepared with these three QD batches. Measurement of these PMMA–QD films were the same as those on QD#1–4. The irradiation time was 40 min. The evolved PL spectra of the PMMA–QD film during the UV irradiation process are shown in Fig. 5a, c, and e, and the corresponding chromaticity coordinates are listed in Fig. 6a. The intensity and position of the BE and SS peaks in Fig. 5a, c, and e versus the irradiation time are shown in Fig. 5b, d, and f. The changes of the PL characteristics before and after UV irradiation are listed in Table 1. In Fig. 5a, the PL intensity of PMMA–QD#A film decreased continually during the irradiation process. The PL of QD#1 increased in a short period at the beginning of irradiation (Fig. 2a). But in Fig. 5a, for the PMMA–QD#A film, no increase of PL intensity was observed because passivation of PMMA on the QD surface make QD#A emission close to the maximum value [9]. The small surface area and the big ratio of surface to volume make the defects form easily on QD#A surface. The BE and SS peaks decreased with analogous trends (Fig. 5b), which is similar to the PL decrease shown in Fig. 2a. The BE peak intensity decreased to 0.22 time of the initial intensity, whereas the SS peak intensity decreased to 0.43 time. In the irradiation process, the position of the BE peak shift from 480 nm to 475 nm and SS peak moves from 601 nm to 566 nm and not vice versa (Table 1). The BE peak declined faster than SS peak while the positions of the two peaks experienced blue shift. These two competitive factors result in slight variations to the chromaticity coordinate of QD#A (region A in Fig. 6a). For PMMA–QD#B, both the BE and the SS peaks increased at the beginning of irradiation process (Fig. 5c). The BE peak increased faster than the SS peak, and so the ratio of the BE peak to the SS peak increased (Fig. 5d). In the passivated period, the SSs were passivated while the radiative decay rate of SSs increased. These two competitive factors make the SS peak increased slower than BE peak. Within the irradiation period, the BE peak shifted to 538 nm from 564 nm, whereas the SS peak position shifted to 631 nm from 688 nm. The increasing ratio of BE peak to SS peak and the blue shift of the peaks benefit the blue shift of the emission, therefore changing the corresponding chromaticity coordinate of PMMA–QD#B (Fig. 6a). This is similar to the noticeable change of chromaticity coordinate in Fig. 4a within the first 1 min under UV irradiation. For PMMA–QD#C film with relatively weak SS emission, the evolutions of BE and SS peaks in Fig. 5d were similar to those of PMMA–QD#B (Fig. 5f). Table 1 also indicates that the FWHM of both the BE and SS peaks will always increase in the irradiation process, because the photo-corrosion effect can broaden the size distribution of QDs. QD#C and QD#B have a close BE position ( 560 nm), whereas the SS emission of QD#C is weaker than that of QD#B because of the relatively poor surface states. Only a 2 nm blue shift occurred in the irradiation process for QD#C because SSs can influence the photo-oxidation process. For QD#B with its Se-rich surface, the Se atoms on QD surface were oxidized to SeO2 in the irradiation process. Part of the SeO2 dissociates from QD surface. This photooxidation corrodes the QD surface, and thus more than 20 nm blue
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shift of the BE peak is observed (Fig. 5c). Comparable to QD#B, the QD#C surface have relatively rich Cd atoms, and thus the oxidized Se atoms cannot leave surface easily. The small blue shift of the PL spectra in Fig. 6a focus the chromaticity coordinates in a small region (region C in Fig. 6a). The photo-corrosion on QDs also introduces new surface defect states to lower the PL intensity, which can explain why QDs with evident SS emission will reach the maximum luminescence intensity earlier under UV irradiation (Fig. 5d and f). The PL evolution of QDs with evident SS emission in long irradiation period was discussed in previous work [19]. Fig. 6e indicates the mechanism in the photo-oxidation process. The oxidation can passivate the surface atoms (Se, Cd) which brighten the PL. The oxidation product of Se will continuous leave the QD surface, which result the blue shift of PL. Photo-corrosion speed of QDs with Se-rich surface (QD#B) is bigger than that of QD#C, thus the QD#B will experience faster blue shift of PL in photo-oxidation process. Both the size and the density of SSs QDs can influence the varied PL characteristics in the UV irradiation process. Using these three kind of QDs, bright-dark luminescence patterns can be easily prepared by selectively UV irradiating the PMMA–QD film under lithography masks (Fig. 6b, c and d). For the PMMA–QD#A film, the PL intensity of the irradiated area (uncovered area) decreased, and yet for the PMMA–QD#B and PMMA–QD#C films, the PL intensity changed in an opposite way at first (Fig. 5a, c and e). In the luminescence patterns in Fig. 6b and d, the emission color of bright region is similar to that of the dark region since UV irradiation-induced a slight change of chromaticity parameters (regions A and C in Fig. 6a). In the luminescence pattern in Fig. 6c, the emission color of the irradiated area is green, whereas that of the un-irradiated area is orange.
4. Conclusions UV irradiation on CdSe QDs can improve the oxidation of the surface atoms, thus modifying the luminescence properties. The QD size and the density of SSs will influence the photo-oxidation effect on QDs. Under the same UV irradiation, the PL spectrum of small QDs vary faster than that of large QDs, which means the PL intensity of the small QDs will take the lead in reaching a maximum value and then decay. The growth extent of PL for large QDs is big than that of small QDs. In the UV irradiation process, the shape and position of the PL spectrum for the large QDs with weak SS emission is relatively stable, and thus the emission color changes slowly. The photo-oxidation effect tends to happen on the Se-rich surface of QDs with evident SS emission. The blue shift of the BE peak for CdSe QDs with evident SS emission is more noticeable than that for CdSe QDs with weak SS emission. For QDs
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with evident SS emission, the evolution of the BE and SS peaks are different. The intensity of the BE peak always changes faster than that of the SS peak, and yet the SS peak shifts to short wavelengths faster than the BE peak versus irradiation time. During the stage with increasing PL, the increasing ratio of BE peak to SS peak and the blue shift of the peaks benefitted the corresponding chromaticity coordinate shift. In the stage with decreasing PL, the competition between the decreasing ratio of the BE peak to the SS peak and the blue shift of peaks can slow the shift of chromaticity coordinate. For the QDs with evident SS emission, the movement of the chromaticity coordinate in the PL-increasing period is faster than that in PL-decreasing period.
Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 11504138) and Fundamental Research Funds for the Central Universities (No. JUSRP11403).
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