ARTICLE pubs.acs.org/JPCC
Photoenhanced Band-Edge Luminescence in ZnO Nanocrystals Dispersed in Ethanol Sekika Yamamoto* Department of Physics, Graduate School of Science, Hokkaido University, N10 W8, Sapporo 060-0810, Japan ABSTRACT: The photoenhancement effect of the band-edge photoluminescence in ZnO nanocrystals dispersed in ethanol is investigated by observing changes in optical properties after ultraviolet illumination. Time-resolved measurements at 20 K reveal that the band-edge luminescence is composed of two components and the enhancement occurs in the low-energy one, while the intensity for the high-energy component is significantly decreased by the illumination. The origin of the changes is ascribed to the modified electronic structures caused by the negative charging of the oxygen vacancy, and also to the filling of the conduction band by the excess electrons.
’ INTRODUCTION Semiconductor nanocrystals (NCs) have been intensively investigated because of their applications in a wide range of industrial fields, such as light-emitting diodes,1,2 lasers,3 photosensors,4 biomarkers,5 solar cells,6,7 and so on. Among them, ZnO nanocrystals attract considerable attention for application as optical devices because of their potential light-emitting ability.8 10 In addition, they are nonhazardous to the human body and easily synthesized by a chemical reaction under moderate conditions,11,12 which will be advantageous in the actual use in the industrial products. One of the most challenging problems in using nanocrystals in practical applications is their optical instabilities that originate from a large surface-to-volume ratio. Some kinds of semiconductor nanocrystals, such as CdS or ZnO, dispersed in water are known to dissolve under above-gap illumination, reflecting their high photocatalytic ability. Moreover, the surface dangling bond often acts as an efficient nonradiative recombination center for photogenerated carriers. Therefore, the passivation of the particle surface has been the main concern in the study of the nanocrystal applications. In chalcogenide nanocrystals, such as CdS and CdSe, various surface control methods have been successfully developed and high quantum efficiency and high optical stability is attained,13 16 whereas those in ZnO nanocrystals in the quantum confinement regime have not been successful.17 The surface activity, however, can be used to improve its optical properties. It is called a photoenhancement or photoactivation effect of luminescence.18 22 It has been observed that the continuous illumination on CdSe nanocrystals enhances their band-edge luminescence by 60 times,20 although the detailed mechanism of the phenomenon is still under debate. Similar observations in ZnO nanocrystals have been reported so far,23 29 for which we think that the detailed mechanism is not fully understood. Among them, van Dijken et al. attributed the enhancement of the band-edge PL to the change in the charge r 2011 American Chemical Society
state of the oxygen vacancy that removes the V•O center, which is responsible for the green PL.28 Other known phenomena induced by illumination on semiconductor nanocrystals include photoinduced instability, such as blinking and spectral diffusion of luminescence. The photoenhancement effect and these phenomena must be closely related to each other, and hence elucidating the mechanism of the photoenhancement is not only interesting as a challenging problem common to the semiconductor nanostructures but also important for the practical application to the optical devices. In this paper, we perform a detailed investigation on the optical properties of illuminated ZnO nanocrystals in the quantum confinement regime and give new possible interpretation for the phenomena.
’ EXPERIMENTAL METHODS Zinc oxide NCs are prepared as follows.29 First, 1.10 g of zinc acetate dehydrate (ZnAc2 3 2H2O) is dissolved in 50 mL of absolute ethanol by boiling, and 0.29 g of lithium hydroxide monohydrate (LiOH 3 H2O) is dissolved in 50 mL of absolute ethanol ultrasonically. Typically, 10 mL of the Zn solution is heated to 74 °C before the same amount of LiOH solution, cooled at 0 °C, is poured under vigorous stirring. The solution is kept at room temperature for particle growth. After the desired particle size is attained, 200 mg of magnesium acetate tetrahydrate (MgAc2 3 4H2O) is added into the solution and dissolved ultrasonically. The resulting solution is stored at room temperature for at least 3 days. After the addition of Mg salt, the particle growth is stopped and the suspension becomes very stable even at room temperature. No turbidity is recognized after 1 year of storage at room temperature. An X-ray diffraction measurement Received: June 30, 2011 Revised: September 12, 2011 Published: September 27, 2011 21635
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Figure 1. Absorption and PL spectra measured at room temperature for ZnO NCs with an average diameter of 3.2 nm dispersed in ethanol. Black and blue lines show those before and after the illumination, respectively. After the illumination, the absorption at the excitonic peak is strongly bleached and the band-edge PL peak is 4 times enhanced, while the broad green PL is strongly reduced.
for the dried sample shows clear evidence for the wurtzite structure of the crystal, and no other peaks, such as zinc hydroxide,30,31 are recognized. The average diameters calculated from the diffraction line width using a modified Scherrer’s formula32 is about 3.2 nm.29 High-resolution TEM measurement shows that the sample shape is spherical within the resolution of the measurement.33 For absorption measurement, a Shimazu UV-2200 UV vis spectrometer is used. Room-temperature photoluminescence (PL) is detected by a prism polychromator equipped with a cooled CCD camera. The excitation is done by a 296 line of a Hg lamp, and the excitation power is kept below 0.35 mW/cm2; the measurements are done within a few seconds to minimize the effects of the excitation light on the sample properties. For timeresolved PL measurements, third harmonics of a mode-locked Ti:Sapphire laser with a repetition rate of 80 MHz and a pulse duration of about 100 fs is used for the excitation source. Typical time-averaged laser power is 10 mW/cm2. The detection is performed with a Hamamatsu C1587 streak camera that has a time resolution of about 20 ps. To investigate the effect of the continuous illumination on the ZnO NCs, the sample liquid is put in a quartz cell with an internal dimension of 0.1 10 40 mm3 and the illumination is done by a commercial UV-fluorescent lamp. The intensity of illumination is about 2.5 mW/cm2. The size of the lamp is much larger than the sample cell, and the whole cell is expected to be uniformly illuminated. The optical density of the sample at 312 nm is about 0.15, and the illumination is also expected to be uniform in the depth direction.
’ EXPERIMENTAL RESULTS Figure 1 shows typical absorption and PL spectra for asprepared ZnO NCs and for those measured after a long UV illumination. The rising edge of the absorption spectrum for the as-prepared sample shows a large blue shift from the bulk value of 3.32 eV34 because of the quantum confinement effect. The average diameter of NCs calculated from the edge energy by applying an empirical formula12 is 3.2 nm, and it accords with the values obtained from TEM and diffraction measurements.29,33 The corresponding PL spectrum has two peaks, one at about 2.4 eV and the other at about 3.6 eV. The origin of the peak at 2.4 eV (green PL) is still controversial, but it is widely accepted that this PL is related to an oxygen vacancy.17,35 38 The peak at 3.6 eV is
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Figure 2. Absorption spectrum for various illumination times. The inset shows bleach spectra. The bleach spectrum has a peak at 3.66 eV, as shown by the vertical line. Also, the small positive peak is observed for relatively shorter illumination times, as shown by the arrow.
called a band-edge PL and is related to the confined electronic states in the conduction and valence bands of the NCs. The band-edge PL is red shifted by about 130 meV from the absorption spectrum if the shift is evaluated from the difference of the low-energy edges of the spectra. The shift has been attributed to a kind of shallow trap for electrons or holes.35,39 After the UV illumination is applied on the sample, both the absorption and the PL spectra show drastic changes. The absorption spectrum shows a strong bleach at the excitonic peak and a broad increase above 3.9 eV. The strong bleaching of the excitonic absorption can be interpreted as a result of electron injection into the NCs. When ZnO nanocrystals are illuminated by above-gap light, a part of the photogenerated holes moves to surrounding organic molecules because of their large oxidation potential, while electrons tend to stay in the nanocrystal.24,25,28,39 These injected electrons are considered to occupy the conduction band39 and prohibit further optical transition to the states, which leads to the bleaching of the absorption spectrum. The observed bleach spectrum is very stable, and it does not show any recovery after 12 h of storage in the dark if the sample is carefully isolated from the air. However, it soon recovers the original spectra after the sample liquid is exposed to oxygen gas. The observation of the recovery with oxygen gas corroborates the interpretation that the excess electrons cause the spectral change, because it is well known that the oxygen molecules attached on the ZnO surface deprive the excess electrons.40 45 The UV illumination also affects the PL properties. The broad green PL almost disappears and the band-edge PL is enhanced by about 5 times with a slight red shift, as shown in Figure 1. The measured quantum efficiency for the enhanced band-edge PL reaches 0.54%.29 In addition to these prominent changes, we find that new PL peaks grow around 3.35 and 3.75 eV after the UV illumination. We will later discuss the origin of these peaks. The PL spectrum also recovers its original shape when the sample is exposed to the oxygen gas. Figure 2 shows absorption spectra of the samples with different charging levels. A relatively shorter illumination time gives a lower charging level and smaller bleaching of the absorption. The blue line shows the maximum bleaching observed in our measurement; that is, any elongated illumination yields no change 21636
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The Journal of Physical Chemistry C from this spectrum. For the shortest illumination time (green line), the bleach peak is observed at 3.66 eV, which we attribute to the first excitonic absorption peak. The shoulder-like structure around 3.80 eV may be a higher excited state of the exciton. For shorter illumination times, a small positive peak in the bleach spectrum is observed at 3.53 eV, as indicated by an arrow in the inset of Figure 2. The increase of the absorption indicates that there must be some changes in electronic states, besides the state filling. For the increase of the absorption near the band edge, several possibilities should be considered. The first is the Stark shift caused by the electronic field from the excess electrons trapped at the surface. It is well known that the strong electric field applied on NCs causes a red shift of the absorption spectrum and leads to an enhanced absorption near the band edge.46,47 In our measurement, however, the NCs are dispersed in the polar solvent and the residual ions around the NCs will strongly screen the surface charge. Therefore, the effect of the surface charge should be insignificant. The second possibility is a trion generation. The light absorption by an NC with an extra electron in its conduction band directly generates a charged exciton called a trion, which can have a different absorption energy than the neutral exciton. This assignment is justified by the observation that this absorption peak disappears as the illumination time is increased and the 1Se state in the conduction band is expected to be filled. However, for the trion energy, only small shifts of 5 18 meV from the neutral exciton are observed or calculated so far,48 50 and it does not explain the large shift of 130 meV (from the first excited state) observed here. The third possibility is a charging of the oxygen vacancy. In addition to the change in the absorption spectrum, the introduction of an extra electron causes the strong decrease of the green PL, as shown in Figure 1. The decrease is believed to be caused by the change of the charge state of the oxygen vacancy responsible for the green PL.28 Therefore, the extra electron introduced in the NC must first occupy the localized vacancy state. This localized charge will influence the electronic states in the conduction and valence bands by a strong Coulomb interaction and can cause a red shift of the absorption energy. From these considerations, we assign this absorption to an exciton interacting with the localized charge in the oxygen vacancy. This absorption is also expected to disappear when the modified 1Se state of the conduction band is filled by further changing, in accordance with the experimental observation. We also see two broad enhancements around 4.0 and 4.4 eV. The growth of the 4.0 eV peak is concurrent with the growth of the peak at 3.53 eV, and hence this peak should also be caused by the charging at the oxygen vacancy. In contrast, the peak at 4.4 eV is only conspicuous after the peak at 3.53 eV has disappeared. Therefore, this peak may be caused by the extra electrons introduced in the conduction band of NCs, as it is shown that the extra electrons can enhance the oscillator strength of the excitonic transitions at higher energies.48 It is clear from the above discussion that the change in the absorption spectrum must be considered as a combination of the modification of the electronic states and the filling of these modified states. Therefore, it is difficult only from the absorption measurement to determine how many electrons are introduced in the NCs. Figure 3 shows typical PL spectra for different levels of charging. The black line shows a spectrum for the as-prepared sample. The red line is for the sample with a relatively shorter illumination time. This spectrum is observed even before the bleach of the absorption starts and is found to be stable for a
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Figure 3. Typical PL spectra for various charging levels. The black line is for the as-prepared sample without intentional charging. After a certain time of illumination, the green PL blue shifts and loses half of its intensity, while the band-edge PL is enhanced by about 4 times (red line) with a slight red shift. After further charging, the green PL almost disappears and the band-edge PL further red shifts. In addition, new peaks emerge at both sides of the band-edge PL (blue line).
certain range of the illumination time. The blue line is for the sample with maximum bleaching of the exciton absorption. In the red line, the green PL loses about half of the original intensity and has a slightly blue shifted peak, while the band-edge PL is enhanced by about 4 times and red shifted by 40 meV. The line width of the band-edge PL is decreased from 215 to 140 meV. When the maximum bleaching is attained, the green PL almost disappears and the band-edge PL is slightly decreased with a further red shift of about 20 meV. In addition, the new peaks at 3.35 and 3.75 eV are observed at this stage. We must note that the excitation light used in the PL measurement causes the extra charging of the NCs, although the excitation power is kept low and the measurements are done within a few seconds. In addition, although not shown here, we observed the recovery of the absorption spectrum after the illumination is turned off if the illumination time is not long enough. This happens because residual oxygen molecules or other oxidants in the solution deprive the excess electrons from the NCs.40 45 The stable bleach spectra shown in Figure 2 are only obtained after these molecules are consumed. In the measurement of the red line, the charging by the excitation light and the discharging by the oxidizing molecules are supposed to be in equilibrium, and as a result, the low-level charging is fulfilled. Therefore, the precise comparison with the bleach spectrum is difficult for the red line. Nevertheless, we think that this spectrum is of the NCs that have extra electrons only in the localized state because this spectrum is stable for the certain range of illumination time. The illumination of the sample is expected to raise the quasi-Fermi level for electrons, and while this level is lower than the conduction band, only the localized state below the conduction band can be occupied. This explains the stability of the PL spectrum. If this assignment is correct, the spectral change in the blue line can be interpreted as a result of the occupation of the band states after the quasi-Fermi level reaches the conduction band. The enhanced band-edge PL in the red line has a peak around 3.53 eV, and it is about the same to that of the enhancement peak in the absorption near the band edge. This fact strongly implies that the enhanced band-edge PL in the early stage of the charging is also from an exciton interacting with the localized charge at the oxygen vacancy. 21637
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Figure 4. Time-resolved PL at low temperature measured using a streak camera. (a) Result for the sample without illumination. (b) Low-energy component extracted from the signal in (a). (c) Result for the sample with low-level charging. The temporal behavior of the low-energy component is unchanged, while its intensity is increased by 5 times. (d) Result for the sample with maximum charging. The lifetime is shortened, and the spectrum shows a temporal red shift. (e) Spectra at delay times of 0 and 80 ps. The black, red, and blue lines correspond to (b) (d), respectively. (f) Decay curves at 3.6 eV. The colors indicate the same as those in (e). The straight lines in (f) are single exponential with decay times of 260 and 150 ps, respectively.
To investigate the enhancement of the band-edge PL in detail, we measure time-resolved PL at 20 K using the streak camera system. In the measurement, a sample aliquot is illuminated by the UV lamp at room temperature to attain the desired bleach spectrum and then cooled to 20 K using a He gas-flow cryostat equipped with a quartz window. At low temperatures, it is expected that the effects of the excitation light on the sample properties are much less than those at room temperature because the sample liquid is frozen and the chemical reaction caused by the illumination is slow, although some amount of charging is still expected as it is observed in the measurements of the luminescence blinking.51,52 The stability of the charged state in the cooling process has also been confirmed by Shim et al.53 The results of the measurement are shown in Figure 4. Figure 4a shows a result for the as-prepared sample without any intentional charging. Two components with different peak energies and different lifetimes are recognized in the figure, one at lower energy with a shorter lifetime and the other at higher energy with a much longer lifetime. The high-energy component has an intensity at the negative time region because the lifetime of this component is much longer than the repetition period of the laser system (12.5 ns in this case) and the long tails from the previous pulse events are superposed in the time window. Previously, the lifetime of this component has been estimated to be about 0.1 μs, which was much longer than that of the bulk (322 ps)54 or quantum well (j1 ns)55 at low temperature, and detailed measurements of the temperature dependence of the luminescence intensity and the lifetime suggested that this component was from the thermal equilibrium of the dark and bright states of the exciton.33 The intensity of the low-energy component relative to the high-energy one is only 7% for the result shown here (note that the high-energy component has a much longer lifetime). It is found that the relative intensity can be further decreased when the excitation power is suppressed as low as
possible, by sacrificing the signal-to-noise ratio. Therefore, the existence of the low-energy component can be interpreted as a result of unintentional charging caused by the excitation light or by an unintentional illumination during the sample preparation. Indeed, the peak energy of this component is observed at 3.595 eV, which is about 65 meV higher than the enhanced PL measured at room temperature (the red line in Figure 3), and this energy difference is explained by the thermal change of the bandgap energy of ZnO.56 As mentioned above, the high-energy component is superposed in the time window and it is almost time-invariant within the window. Therefore, we can extract the low-energy component by numerically subtracting the high-energy component as a background. The result is shown in Figure 4b. The decay curve at the peak energy is shown by a black line in Figure 4f, and it is almost single exponential with a decay time of 260 ps. The fastdecaying part observed at the high-energy side (3.60 3.75 eV) is attributed to the bright-to-dark conversion of the intrinsic exciton in the previous paper.33 Next, another aliquot is illuminated until the bleach spectrum becomes identical to the red line in Figure 2, and the timeresolved PL is measured at 20 K. The result is shown in Figure 4c. The intensity of the low-energy component is enhanced by about 5 times, and the high-energy component almost disappears. The fast decay corresponding to the dark bright conversion is also weakened. The comparison of spectral line shapes and decay curves after subtracting the high-energy time-invariant part is given by black and red lines in Figure 4e,f. It is known from these figures that the temporal behavior of the lowenergy component is intact even after the charging. This observation implies that the low-level charging only increases the number of NCs with the extra electron in the localized state, in accordance with the results obtained at room temperature. 21638
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The Journal of Physical Chemistry C After the illumination is done at room temperature until the absorption spectrum is maximally bleached, the PL at 20 K shows a significant difference, as shown in Figure 4d. The spectrum just after the excitation has a peak energy at 3.585 eV, which is almost the same as that in Figure 4b,c, but gradually red shifts with time. The lifetime of the PL measured at 3.585 eV is 150 ps, and it is significantly shorter than those with lower charging levels. The shoulder-like component at the low-energy side of the main peak observed at room temperature is also observed at 20 K. The lifetime of this component is much shorter than that of the main peak, and it fades away before 80 ps, as seen in Figure 4e. On the other hand, the peak expected at the high-energy side around 200 meV above the main peak is not observed at this temperature.
’ DISCUSSION In the time-resolved PL measurement at low temperature, the disappearance of the high-energy component and the enhancement of the low-energy component are observed as a result of the charging. The high-energy component has a very long lifetime, and it is interpreted as a dark exciton PL.33 In contrast, the lifetime of the low-energy component is 260 ps, much shorter than that of the high-energy component. This shortening can be understood as the result of mixing between dark and bright wave functions induced by the strong Coulomb interaction with localized charge at the oxygen vacancy, which also causes the large red shift of the emission energy. The shortening of the lifetime and decreased energy flow to the green PL should be the cause for the enhancement of the band-edge PL. When the ZnO NCs are further charged, the excess electrons start to fill the conduction band. The electrons in the conduction band further decrease the lifetime of the exciton through the Auger recombination. It is also observed that the further charging induces the temporal red shift of the spectrum. In NC systems with strong confinement, the PL spectrum at low temperature mostly reflects the particle size dispersion, and hence the temporal spectral shift can be interpreted as a size-dependent lifetime. Therefore, the temporal red shift of the spectrum can be also explained by Auger recombination because it is more efficient in smaller NCs. It has long been considered that the charged exciton PL in colloidal NCs is difficult to observe because the Auger recombination is faster than the radiative recombination. However, recent measurements on CdSe/CdS and CdSe/ZnS core/shell NCs shows that the Auger recombination is not effective in the NCs with sufficient surface passivation, as it is true in the epitaxial quantum dots.49,57,58 Although not shown here, we measure the temperature dependence of the enhanced band-edge PL, and the result shows that the PL intensity at 20 K is 10 times stronger than that at room temperature. It corresponds to the quantum efficiency of about 5% at 20 K, which is comparable to the efficiency measured for the trion PL in CdSe/CdS samples.58 In addition to the enhancement of the main peak of the bandedge PL, we observe a shoulder-like peak at the low-energy side of the band-edge PL from the maximally bleached sample both at room temperature and at 20 K. This luminescence may be from multiply charged excitons because a part of the NCs is expected to have more than one electron in the conduction band when the maximum bleaching is reached. Franceschetti et al. calculated the energies of various kinds of charged excitons in spherical NCs and show that a doubly charged exciton will have a large red shift from the neutral exciton, whereas a singly charged exciton shows only a little shift,48 in accordance with the observation here.
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The new peak at the high-energy side observed at room temperature can be from a higher excited state of the exciton, such as the 1Pe 1Ph pair. The multiple charging can distribute electrons at the 1Pe state in the conduction band while thermal excitation is necessary for hole occupation in the 1Ph state at the valence band. This explains why this peak is not observed at 20 K. Shim et al. suggested that the introduction of the second electron into the conduction band by UV illumination is difficult because the fast Auger recombination prevents the hole to escape from the particle.39 However, we think that the observed growth behavior of the bleach spectrum and the shortening of the lifetime, and also the observation of the new peaks in the PL spectrum, which can be interpreted as the emission from 1Pe state, together show that the multiple charging is realized in the illuminated ZnO NCs. At room temperature, the band-edge PL spectrum for the asprepared sample is much broader than the enhanced PL, as shown in Figure 3. We think that the former must be the superposition of the low- and high-energy components, as observed at low temperature. To confirm this, we performed time-resolved measurements on the PL at room temperature, but the thermal broadening, decreased intensity, and short lifetime made the precise analysis difficult. The short lifetime for the highenergy component at room temperature will be partly due to the thermal excitation from the dark to bright state, which significantly shortens the lifetime above 100 K,33 and partly due to the extended wave function of the intrinsic exciton, which makes the nonradiative recombination through the surface very efficient at room temperature.
’ CONCLUSIONS The photoenhancement effect of the band-edge PL is investigated in ZnO NCs with their size in the quantum confinement regime by observing the change in PL properties both at room temperature and at 20 K. As a result, it becomes clear that the enhancement occurs in the low-energy component of the luminescence, while the high-energy component decreases significantly by UV illumination. From the detailed investigation of the experimental results, it is suggested that the excess electrons induced by the illumination are responsible for the spectral changes. The extra electron at the localized state of the oxygen vacancy causes the enhancement of the band-edge PL by changing the dark exciton into a much brighter one, while the electron induced at the conduction band causes a shortening of the lifetime and a temporal red shift of the PL due to the size-dependent Auger recombination efficiency. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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