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Ultrafast relaxation dynamics of charge carriers relaxation in ZnO nanocrystalline thin films
Chemical Physics Letters 387 (2004) 176–181 www.elsevier.com/locate/cplett
Ultrafast relaxation dynamics of charge carriers relaxation in ZnO nanocrystalline thin films Christophe Bauer
*,1,
Gerrit Boschloo *, Emad Mukhtar, Anders Hagfeldt
Department of Physical Chemistry, Uppsala University, Box 579, S-751 23 Uppsala, Sweden Received 6 June 2003; in final form 8 December 2003 Published online:
Abstract Ultrafast spectroscopy has been used to study the relaxation processes of charge carriers in ZnO nanocrystalline thin films. A broad red-IR absorption band linked to shallowly trapped electrons was observed by spectroelectrochemical measurements. Femtosecond transient absorption data revealed multiexponential decays of the charge carriers with time constants ranging from 1 to 400 ps. The decay profile of the signal shows a probe wavelength dependence. This effect is assigned to the trapping (localisation) of nonequilibrium charge carriers which occurs on a time scale of 1 ps. The recombination of shallowly trapped electrons with deeply trapped holes, determined by single-photon counting, mainly occurs in 400 ps. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction A general understanding of charge carrier dynamics in nanocrystalline semiconductor material thin films is of fundamental interest and important for applications such as photocatalysis [1] and solar energy conversion [2,3]. In contrast to TiO2 [4–8], only a few ultrafast studies of charge carriers dynamics in ZnO are available. Kamat and Patrick [9] have reported the photophysical behavior of ZnO colloids in ethanol studied by picosecond laser photolysis. They observed a broad absorption in the red and near IR region after a laser pulse of 18 ps which was attributed to trapped electrons in ZnO. Later, Cavaleri et al. [10] studied the charge carriers dynamics in ZnO nanocluster solutions and found that the electron–hole recombination occurs faster in the largest nanoclusters. The population of charge carriers
decay within the first 50 ps to a diameter independent plateau. In the present Letter, we report the relaxation dynamics of photogenerated charge carriers in ZnO nanocrystalline thin films studied by femtosecond transient absorption spectroscopy and single photon counting technique. The ultrafast dynamics are explained in terms of free charge carrier trapping into localized states. Furthermore, the recombination rates of trapped charge carriers have also been measured by means of time-resolved emission spectroscopy. Finally, a schematic picture for the different relaxation processes is proposed.
2. Experimental 2.1. Preparation of nanostructured ZnO films
*
Corresponding authors. E-mail addresses: christophe.bauer@epfl.ch (C. Bauer), [email protected] (G. Boschloo). 1 Present address: Laboratory for Physical and Analytical Electrochemistry, Swiss Federal Institute of Technology of Lausanne, CH1015, Lausanne, Switzerland. Fax: +41-21-693-36-67. 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.01.106
Colloidal ZnO was prepared using a simplification of previously published methods [11]. 11 g (50 mmol) zinc acetate dihydrate (Merck) was stirred in 100 ml absolute ethanol (99.5%, Kemetyl) at room temperature to give an opaque suspension. 21 ml (50 mmol) of a 25% solution of tetramethylammonium hydroxide (TMAOH) in
C. Bauer et al. / Chemical Physics Letters 387 (2004) 176–181
methanol (Aldrich) was added. The suspension was heated and refluxed (80 °C) for 30 min. During heating the solution turned completely clear, but during reflux it became slowly white again due to aggregation of the colloids. The suspension was left to cool down and settle overnight, decanted and washed with ethanol. This procedure was used to lower the concentration of TMAacetate, essential for the preparation of good quality films. Next, the ZnO colloid was concentrated until visibly viscous using a rotary evaporator. Gel-films were obtained by doctor blading the colloidal solution onto a microscopic glass using adhesive tape (Scotch Magic) as spacer. The films were fired in a hot-air stream at 350 °C for 30 min. The film thickness was determined by profilometry and was typically 2–3 lm.
177
150 fs and an energy of 800 lJ at 800 nm. The red light from the compressor is split by a beam splitter (80/20) into a pump and a probe beam. The 400 nm pump beam (bandwidth 10 nm) is generated by frequency doubling the amplifier output in a BBO crystal. The pump light passes through a chopper and is then focused on the sample. The pump fluence for the transient absorption measurements was 1.5 mJ cm2 . White light continuum, generated in a sapphire plate is used for probing after passing through a delay line. The probe beam is then focused and overlapped with the pump beam in the sample. After the sample, the probe beam is focused onto the entrance of a monochromator and detected by a photomultiplier. The output signal is produced by a lock-in amplifier locked to chopper frequency. All the data were collected at room temperature.
2.2. Spectroelectrochemistry
2.3. Luminescence measurements
2.4. Femtosecond spectroscopy
Transmission electronic microscopy reveals that ZnO particles in the nanocrystalline thin film are nearly spherical and have an average size of 20 nm. Fig. 1 shows that the nanostructured ZnO films prepared in this study are transparent in visible region and exhibit a strong absorption for wavelengths shorter than 400 nm. A comparison of the absorption of the nanostructured ZnO thin film with literature data of single crystalline ZnO revealed that the absorption coefficients at 400 nm are nearly equal (about 103 cm1 ) [12]. Two bands are observed in the emission spectrum of the nanocrystalline ZnO films after excitation at 300 nm, one quite narrow at 385 nm and a broad one centered at 600 nm. The relatively narrow UV band is attributed to the direct recombination of photogenerated charge carriers and corresponds to radiative decay of excitons. In many studies on ZnO colloids, the UV emission band is lo-
2.5 2000 2.0 1500
1.5 1.0
1000
X 25
500
0.5 0.0
The Femtosecond Laser system consists of a 1 kHz regenerative amplifier (Quantronix) pumped by a Qswitched frequency doubled Nd:YLF laser (Quantronix) and seeded by a mode-locked Titanium:Sapphire oscillator (Mira, Coherent Radiation), the later pumped by a CW argon-ion laser (Coherent Radiation). The amplified pulses have after compression a temporal width of
Emission (a. u.)
Steady-state fluorescence measurements were performed using a SPEX-fluorologfluorimeter using front face geometry. Time-resolved luminescence measurements were performed using time correlated single photon counting. The excitation source was the tunable output of a 200 kHz, 120 fs optical parametric amplifier (OPA, Coherent Radiation) and the pump fluence was 0.4 mJ cm2 . The instrument response was 120 ps. Luminescence was observed through an interference filter with a full width at half maximum of 10 nm. A microchannel plate multiplier was used for detection. Instrumental response curves were measured by recording the scattering of the sample.
3. Results
Absorbance
Spectroelectrochemical experiments were performed using a quartz single-compartment 3-electrode cell, with a nanostructured ZnO film on conducting SnO2 :F glass as working electrode, a platinum wire counter electrode, and a Ag/AgCl/3 M KCl (aq) reference electrode. The electrolyte was an aqueous solution of 0.2 M KCl buffered with 0.02 M K2 HPO4 /KH2 PO4 (pH 6.8), deaerated by bubbling with N2 prior to experiments. The cell was incorporated in the sample compartment of a Hewlett–Packard 8453 diode array spectrophotometer and connected to an EG & G Model 273 potentiostat.
0
400
500
600 700 Wavelength (nm)
800
900
Fig. 1. Absorption (thin line) and emission (thick line) spectra of nanocrystalline ZnO films (excitation wavelength for the emission spectrum was 300 nm). The intensity of the the emission band at 385 nm is magnified 25 times in respect to the broad emission band in the visible.
C. Bauer et al. / Chemical Physics Letters 387 (2004) 176–181
5x10-3
-0.5 V Absorbance
cated at shorter wavelengths (350 nm) due to quantum size effects. The ZnO nanocrystalline thin films used in this study are composed of 20 nm particles, which is outside the quantum size regime. Therefore, the fundamental band edge absorption and the exciton emission band correspond to the bulk ZnO values. The second emission band, centered around 600 nm, is linked to the recombination of a trapped charge carrier since the energy of this band (2 eV) is well below the bandgap energy (3.2 eV). Very recently, the emission in the green spectral part has been identified to be due to the recombination between shallowly trapped electron and a deeply trapped hole, which corresponds to a doubly positively charged oxygen vacancy, VO [13]. A spectroelectrochemical experiment was performed to study the absorption spectrum of electrons in nanostructured ZnO films. A strong bleach of the excitonic absorption band is observed when a negative potential is applied to nanostructured ZnO in an electrochemical cell (Fig. 2). Such a bleach has been observed in several spectroelectrochemical studies on ZnO colloids [14] and nanostructured ZnO electrodes [15,16]. In our experiments, a very small increase in absorption can be observed in the visible-near infrared (NIR) region in addition to the bleach. The absorption is more than 2 orders of magnitude weaker than the bleach, which explains why this feature is easily overlooked. The absorption increases gradually with wavelength and exhibits an interference pattern, that is related to the film thickness. The occurrence of this pattern suggests a change in the refractive index of the nanostructured ZnO. Application of a negative potential leads to a flow of electrons into the nanostructured ZnO electrode. The observed optical changes, Drude-like absorption/reflection changes and bleach of the exciton band, suggest that the electrons are free or very shallowly trapped. The dynamics of charge carrier recombination were investigated by measuring the lifetime of the green luminescence by single photon counting spectroscopy upon excitation with a pulsed laser at 400 nm with 10 nm bandwidth. It is noted that 400 nm light is only weakly absorbed by the nanostructured ZnO film (about 5% of the incoming light is absorbed) and will create charge carriers with negligible excess energy. Luminescence decay data are shown in Fig. 3 for different emission wavelengths. For wavelengths above 500 nm, the decay kinetics are multiexponential with lifetimes of
0x10 0
+0.5 V
-5x10-3
-1x10-2 400
500
600
700
800
900
1000
Wavelength (nm) Fig. 2. Difference absorption spectra of a nanostructured ZnO electrode. The reference spectrum was recorded at +0.5 V vs. Ag/AgCl. The applied potentials are indicated in the figure.
10
Log(luminescence intensity)
178
8 6 4 2 0 0
10
20
30
40
Time (ns)
Fig. 3. Time-resolved luminescence data for ZnO films. Excitation wavelength was 400 nm. The collected light was: open circles k ¼ 640 nm, full line k ¼ 500 nm, stars k > 500 nm.
340 ps (96.6%), 2.1 ns (3.2%) and 18.8 ns (0.2%). Kamat and Patrick [9] have measured similar lifetime components except for the fastest one. The main component around 400 ps was not observed by these authors due to the limited time resolution of the instrument used in their study. The emission decay lifetime recorded with single wavelengths at 500 and 640 nm show a slight wavelength dependence for the first component, see Table 1. The rate is faster with a shorter wavelength, but
Table 1 Luminescence decay parameters obtained with an excitation wavelength at 400 nm and s the lifetime and A the corresponding amplitude k (nm)
A1 (%)
s1 (ns)
A2 (%)
s2 (ns)
A3 (%)
s3 (ns)
>500 500 640
96.6 91.6 98.5
0.341 0.273 0.330
3.2 7.5 1.4
2.11 2.05 2.08
0.2 0.9 0.1
18.8 22.8 34.1
C. Bauer et al. / Chemical Physics Letters 387 (2004) 176–181
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The transient absorption data obtained by femtosecond pump–probe spectroscopy for the ZnO films collected at 750 nm following excitation at 400 nm are shown in Fig. 4a,b with different time scale windows: 0–10, 0–45 and 0–500 ps. The decay of the signal can be well fitted with the following time constants 1.5, 12 and 400 ps. It is possible to fit the traces with a second-order kinetics, but this approach does not lead to an improvement of the fit. Furthermore, the multi-exponential decay kinetics give a better view of the different relaxation mechanisms. Such multi-exponential behavior has been previously observed in several semiconductor nanoparticle systems and can be attributed to the different charge carriers relaxation processes [18–20]. The probe wavelength dependence of the decay profile was investigated between 575 and 825 nm to obtain information about the temporal evolution of the transient spectra. Fig. 4c shows the transient absorption data obtained with different probe wavelengths. By normalising the signals, it appears clearly that the transient absorption decay profiles are probe wavelength dependent. Only the faster component (1–2 ps) is affected: its contribution becomes larger by probing with a longer wavelength, the other components remain unchanged.
4. Discussion A schematic overview of the processes occurring upon photoexcitation of nanostructured ZnO is shown in Fig. 5. Absorption of a photon of the appropriate energy leads to the creation of an electron–hole pair in ZnO semiconducting nanoparticles. In the case of ZnO, the electron–hole pair is strongly electrostatically bound, i.e., an exciton is formed. Initially delocalized charge carriers can get trapped (1a) and (1b) into shallow or deep trapped states within the band gap, these trapped charge carriers recombine subsequently via radiative or non-radiative pathways (2). Alternatively,
Fig. 4. Transient absorption data following laser pulse excitation (120 fs) at 400 nm and with a probe wavelength at 750 nm. (a) 0–10 ps window time scale. The signal is fitted with a multiexponential decay with 1, 12 and 400 ps time constants. (b) 0–45 ps window time scale. As an inset: long time scale window: 0–500 ps. (c) Probe wavelength dependence of the electron relaxation dynamics. All the signals were normalized at the maximum to facilitate the comparison. Probe wavelengths in nanometers are indicated in the figure.
the relative weight is smaller. Similar observations have been reported for colloidal ZnO [9]. This phenomenon was attributed to a strong Coulomb interaction between the charge carriers [17].
Fig. 5. Electronic relaxation processes and lifetimes: (1) charge carrier trapping s1 ¼ 1 ps; (2) recombination with a deeply trapped hole, s2 400 ps; (3) exciton recombination s3 ¼ 12 ps.
180
C. Bauer et al. / Chemical Physics Letters 387 (2004) 176–181
direct radiative recombination of excitons can take place (3). Stimulated emission is another possible mechanism, but it is not expected to occur under the conditions used here. An attempt to assign the different components extracted from transient absorption and time-resolved luminescence data is proposed below. Luminescence measurements show that recombination reactions involve both excitons and trapped charge carriers. From time-resolved measurements of the visible emission band it follows that the recombination between shallowly trapped electrons and deeply trapped holes is multi-exponential with time constants ranging from 400 to 20 ns. Hence, trapping of charge carriers has to occur within 400 ps. In the transient absorption experiment (Fig. 4), the initial fast rise within the laser pulse is attributed to the formation of the photoexcited charge carriers in the ZnO semiconductor. The overall lifetime of the charge carriers is very short. More than 50% of photoexcited charge carriers disappears in less than 10 ps. The fastest component of the multiexponential decay (1–2 ps) shows a probe-wavelength dependence and is attributed to trapping of charge carriers. It will be discussed in detail below. The 400 ps component can correspond to the recombination of trapped charge carriers, which would agree with the time-resolved luminescence. The 12 ps component is possibly linked to exciton recombination, as it falls in the range of typical direct band gap recombination times [18,20,21]. The transient absorption measurements of nanostructured ZnO films (Fig. 4c) show a probe wavelength dependence of the decay profile. This is a sign that the transient absorption spectrum is not fully developed initially. The probe wavelength dependence can be due to a temporal spectral evolution of the charge carriers in the red-NIR region during the early time from a broad to a narrower spectrum. A temporal blue shift in the transient absorption spectrum is expected if the photoexcited charge carriers evolves from a delocalised to a discrete state. Thermalisation of hot excited states of dye molecules [22,23] or localisation of charge carriers by trapping can give this effect [24]. In a spectroelectrochemical study of nanostructured TiO2 , it was shown that the the spectrum of electrons in deep traps is shifted toward the blue compared to free conduction band electrons [25]. We will discuss two possible explanations for the observed signals in the transient absorption experiments and their probe wavelength dependence: I. Electron trapping: For many semiconductor nanoparticles like CdS, PbS, Cux S, Ag2 S, AgI, PbI2 , BiI3 , TiO2 , ZnS and AgBr, a transient absorption is observed around 750 nm upon band gap excitation with a laser pulse and was attributed to conduction band or trapped electrons [18]. After photonic excitation of ZnO band gap, a broad absorption in the red-IR region is
also observed and was also assigned to conduction band electrons [9,10]. Additional evidence for absorption in the red region by electrons in nanostructured ZnO comes from the spectroelectrochemical experiments in this study. The probe wavelength dependence could reflect the trapping process of conduction band electrons at the early stage or, in other words, the evolution from a delocalized to a more discrete state. The probe wavelength dependence behavior can be a direct result of the evolution of the electron state from a nonequilibrium to a shallow trap (localised) state. II. Hole trapping: The results from the transient absorption experiments may also be explained by assuming that holes are the main absorbing species. As has been demonstrated in Fig. 2, electrons in ZnO have an absorption starting at about 600 nm and increasing with wavelength. A similar Drude-like absorption may be expected for free holes in ZnO. An estimation of the extinction coefficient of holes in ZnO can be made if the effective hole mass and the hole mobility is known. While the former is known ðmh ¼ 0:48me Þ, there is little information of the latter. In a recent publication p-type ZnO thin films obtained by co-doping with Ga and N were reported [26]. The Hall mobility of the holes was one order of magnitude smaller than the electron mobility in comparable n-type films. Based on this information, the Drude absorption of holes in ZnO is estimated to be about 3 times larger than that of electrons. A significant contribution of the holes in the transient absorption measurements presented in Fig. 4 can therefore be expected. It is interesting to compare the time of carriers trapping obtained by ultrafast pump–probe spectroscopy in our study with the dynamics reported for ZnO epitaxial thin films by using an up-conversion technique [27] or optical Kerr gate method [28]. A photon energy dependence of time-resolved photoluminescence of was observed by these authors and attributed to the cooling process of hot charge carriers which occurs in 1 ps. In our transient absorption experiment by using 400 nm light, we create charge carriers with negligible excess energy. The temporal evolution of the transient spectra is therefore ascribed to a carrier localisation process.
5. Conclusion The rates of relaxation of photoexcited charge carriers have been measured by ultrafast pump–probe transient absorption spectroscopy and single-photon counting technique in ZnO nanocrystalline thin films. Spectroelectrochemical experiments have shown that shallowly trapped electrons have an absorption in the red part of the spectrum. The decay dynamics of photoexcited charge carriers were multiexponential with the following time constants 1, 12 and 400 ps. The 1 ps
C. Bauer et al. / Chemical Physics Letters 387 (2004) 176–181
component is probe wavelength dependent and is attributed to the change of charge carrier state, likely a trapping process. The second component is thought to be due to exciton annihilation giving rise to the luminescence band at 385 nm. Time-resolved luminescence decay measurements of the green emission revealed that more than 90% of this recombination process occur in less than 400 ps. This former charge carrier relaxation step is linked to the recombination of shallowly trapped electron with a deeply trapped hole. Acknowledgements We thank G€ oran Karlsson for TEM analysis. This work was conducted within Angstr€ om Solar Center, which is supported by the Foundation for Strategic Environmental Research (MISTRA) and the Swedish National Energy Administration. References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [2] B. OÕRegan, M. Gr€atzel, Nature (London) 353 (1991) 737. [3] A. Hagfeldt, M. Gr€atzel, Acc. Chem. Res. 33 (2000) 269. [4] N. Serpone, D. Lawless, R. Khairutdinov, E. Pelizzetti, J. Phys. Chem. 99 (1995) 16655. [5] D.P. Colombo, K.A. Roussel, J. Saeh, D.S. Skinner, R.M. Bowman, Chem. Phys. Lett. 323 (1995) 207. [6] D.P. Colombo, R.M. Bowman, J. Phys. Chem. 99 (1995) 11752. [7] A. Furube, T. Asashi, H. Masuhara, H. Yamashita, M. Anpo, J. Phys. Chem. B 103 (1999) 3120.
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[8] D.E. Skinner, D.P. Colombo, J.J. Cavaleri, R.M. Bowman, J. Phys. Chem. 99 (1995) 7853. [9] P.V. Kamat, B. Patrick, J. Phys. Chem. 96 (1992) 6829. [10] J.J. Cavaleri, D.E. Skinner, D.P. Colombo, R.M. Bowman, J. Chem. Phys. 103 (1995) 5378. [11] M. Hilgendorff, L. Spanhel, C. Rothenh€ausler, G. M€ uller, J. Electrochem. Soc. 145 (1998) 3632. [12] G.E. Jellison, L.A. Boatner, Phys. Rev. B 58 (1998) 3586. [13] A. van Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, A. Meijerink, J. Phys. Chem. B 104 (2000) 1715. [14] P. Hoyer, H. Weller, Chem. Phys. Lett. 221 (1994) 379. [15] P. Hoyer, H. Weller, Ber. Bunsen. Phys. Chem. 97 (1993) 630. [16] G. Redmond, A. OÕKeefe, C. Burgees, C. MacHale, D. Fitzmaurice, J. Phys. Chem. 97 (1993) 11081. [17] N.T.D. Chestnoy, R.H. Harris, L.E. Brus, J. Phys. Chem. 90 (1986) 3393. [18] J.Z. Zhang, J. Phys. Chem. B 104 (2000) 7239. [19] T.W. Roberti, N.J. Cherepy, J.Z. Zhang, J. Chem. Phys. 108 (1998) 2143. [20] S. Logunov, T. Green, S. Marguet, M.A. El-Sayed, J. Phys. Chem. A 102 (2000) 5652. [21] V.V. Travnikov, A. Freiberg, S.F. Savikhin, J. Lumin. 47 (1990) 107. [22] N.J. Cherepy, D.B. Liston, J.A. Lovejoy, H. Deng, J.Z. Zhang, J. Phys. Chem. B 102 (1998) 770. [23] D.A. Kliner, J.C. Alfano, P.F. Barbara, J. Chem. Phys. 98 (1993) 5375. [24] C.L. Thomsen, J. Thorgesen, S.R. Keiding, J. Phys. Chem. A 102 (1998) 1062. [25] G. Boschloo, D. Fitzmaurice, J. Phys. Chem. B 103 (1999) 2228. [26] M. Joseph, H. Tabata, T. Kawai, Jpn. J. Appl. Phys. 38 (1999) L1205. [27] A. Yamamoto, T. Kido, T. Goto, Y. Chen, T. Yao, A. Kasuya, J. Cryst. Growth 214/215 (2000) 308. [28] J. Takeda, H. Jinnouchi, S. Kurita, Y.F. Chen, T. Yao, Phys. Stat. Sol. (b) 229 (2002) 877.
Ultrafast relaxation dynamics of charge carriers relaxation in ZnO nanocrystalline thin films Christophe Bauer
*,1,
Gerrit Boschloo *, Emad Mukhtar, Anders Hagfeldt
Department of Physical Chemistry, Uppsala University, Box 579, S-751 23 Uppsala, Sweden Received 6 June 2003; in final form 8 December 2003 Published online:
Abstract Ultrafast spectroscopy has been used to study the relaxation processes of charge carriers in ZnO nanocrystalline thin films. A broad red-IR absorption band linked to shallowly trapped electrons was observed by spectroelectrochemical measurements. Femtosecond transient absorption data revealed multiexponential decays of the charge carriers with time constants ranging from 1 to 400 ps. The decay profile of the signal shows a probe wavelength dependence. This effect is assigned to the trapping (localisation) of nonequilibrium charge carriers which occurs on a time scale of 1 ps. The recombination of shallowly trapped electrons with deeply trapped holes, determined by single-photon counting, mainly occurs in 400 ps. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction A general understanding of charge carrier dynamics in nanocrystalline semiconductor material thin films is of fundamental interest and important for applications such as photocatalysis [1] and solar energy conversion [2,3]. In contrast to TiO2 [4–8], only a few ultrafast studies of charge carriers dynamics in ZnO are available. Kamat and Patrick [9] have reported the photophysical behavior of ZnO colloids in ethanol studied by picosecond laser photolysis. They observed a broad absorption in the red and near IR region after a laser pulse of 18 ps which was attributed to trapped electrons in ZnO. Later, Cavaleri et al. [10] studied the charge carriers dynamics in ZnO nanocluster solutions and found that the electron–hole recombination occurs faster in the largest nanoclusters. The population of charge carriers
decay within the first 50 ps to a diameter independent plateau. In the present Letter, we report the relaxation dynamics of photogenerated charge carriers in ZnO nanocrystalline thin films studied by femtosecond transient absorption spectroscopy and single photon counting technique. The ultrafast dynamics are explained in terms of free charge carrier trapping into localized states. Furthermore, the recombination rates of trapped charge carriers have also been measured by means of time-resolved emission spectroscopy. Finally, a schematic picture for the different relaxation processes is proposed.
2. Experimental 2.1. Preparation of nanostructured ZnO films
*
Corresponding authors. E-mail addresses: christophe.bauer@epfl.ch (C. Bauer), [email protected] (G. Boschloo). 1 Present address: Laboratory for Physical and Analytical Electrochemistry, Swiss Federal Institute of Technology of Lausanne, CH1015, Lausanne, Switzerland. Fax: +41-21-693-36-67. 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.01.106
Colloidal ZnO was prepared using a simplification of previously published methods [11]. 11 g (50 mmol) zinc acetate dihydrate (Merck) was stirred in 100 ml absolute ethanol (99.5%, Kemetyl) at room temperature to give an opaque suspension. 21 ml (50 mmol) of a 25% solution of tetramethylammonium hydroxide (TMAOH) in
C. Bauer et al. / Chemical Physics Letters 387 (2004) 176–181
methanol (Aldrich) was added. The suspension was heated and refluxed (80 °C) for 30 min. During heating the solution turned completely clear, but during reflux it became slowly white again due to aggregation of the colloids. The suspension was left to cool down and settle overnight, decanted and washed with ethanol. This procedure was used to lower the concentration of TMAacetate, essential for the preparation of good quality films. Next, the ZnO colloid was concentrated until visibly viscous using a rotary evaporator. Gel-films were obtained by doctor blading the colloidal solution onto a microscopic glass using adhesive tape (Scotch Magic) as spacer. The films were fired in a hot-air stream at 350 °C for 30 min. The film thickness was determined by profilometry and was typically 2–3 lm.
177
150 fs and an energy of 800 lJ at 800 nm. The red light from the compressor is split by a beam splitter (80/20) into a pump and a probe beam. The 400 nm pump beam (bandwidth 10 nm) is generated by frequency doubling the amplifier output in a BBO crystal. The pump light passes through a chopper and is then focused on the sample. The pump fluence for the transient absorption measurements was 1.5 mJ cm2 . White light continuum, generated in a sapphire plate is used for probing after passing through a delay line. The probe beam is then focused and overlapped with the pump beam in the sample. After the sample, the probe beam is focused onto the entrance of a monochromator and detected by a photomultiplier. The output signal is produced by a lock-in amplifier locked to chopper frequency. All the data were collected at room temperature.
2.2. Spectroelectrochemistry
2.3. Luminescence measurements
2.4. Femtosecond spectroscopy
Transmission electronic microscopy reveals that ZnO particles in the nanocrystalline thin film are nearly spherical and have an average size of 20 nm. Fig. 1 shows that the nanostructured ZnO films prepared in this study are transparent in visible region and exhibit a strong absorption for wavelengths shorter than 400 nm. A comparison of the absorption of the nanostructured ZnO thin film with literature data of single crystalline ZnO revealed that the absorption coefficients at 400 nm are nearly equal (about 103 cm1 ) [12]. Two bands are observed in the emission spectrum of the nanocrystalline ZnO films after excitation at 300 nm, one quite narrow at 385 nm and a broad one centered at 600 nm. The relatively narrow UV band is attributed to the direct recombination of photogenerated charge carriers and corresponds to radiative decay of excitons. In many studies on ZnO colloids, the UV emission band is lo-
2.5 2000 2.0 1500
1.5 1.0
1000
X 25
500
0.5 0.0
The Femtosecond Laser system consists of a 1 kHz regenerative amplifier (Quantronix) pumped by a Qswitched frequency doubled Nd:YLF laser (Quantronix) and seeded by a mode-locked Titanium:Sapphire oscillator (Mira, Coherent Radiation), the later pumped by a CW argon-ion laser (Coherent Radiation). The amplified pulses have after compression a temporal width of
Emission (a. u.)
Steady-state fluorescence measurements were performed using a SPEX-fluorologfluorimeter using front face geometry. Time-resolved luminescence measurements were performed using time correlated single photon counting. The excitation source was the tunable output of a 200 kHz, 120 fs optical parametric amplifier (OPA, Coherent Radiation) and the pump fluence was 0.4 mJ cm2 . The instrument response was 120 ps. Luminescence was observed through an interference filter with a full width at half maximum of 10 nm. A microchannel plate multiplier was used for detection. Instrumental response curves were measured by recording the scattering of the sample.
3. Results
Absorbance
Spectroelectrochemical experiments were performed using a quartz single-compartment 3-electrode cell, with a nanostructured ZnO film on conducting SnO2 :F glass as working electrode, a platinum wire counter electrode, and a Ag/AgCl/3 M KCl (aq) reference electrode. The electrolyte was an aqueous solution of 0.2 M KCl buffered with 0.02 M K2 HPO4 /KH2 PO4 (pH 6.8), deaerated by bubbling with N2 prior to experiments. The cell was incorporated in the sample compartment of a Hewlett–Packard 8453 diode array spectrophotometer and connected to an EG & G Model 273 potentiostat.
0
400
500
600 700 Wavelength (nm)
800
900
Fig. 1. Absorption (thin line) and emission (thick line) spectra of nanocrystalline ZnO films (excitation wavelength for the emission spectrum was 300 nm). The intensity of the the emission band at 385 nm is magnified 25 times in respect to the broad emission band in the visible.
C. Bauer et al. / Chemical Physics Letters 387 (2004) 176–181
5x10-3
-0.5 V Absorbance
cated at shorter wavelengths (350 nm) due to quantum size effects. The ZnO nanocrystalline thin films used in this study are composed of 20 nm particles, which is outside the quantum size regime. Therefore, the fundamental band edge absorption and the exciton emission band correspond to the bulk ZnO values. The second emission band, centered around 600 nm, is linked to the recombination of a trapped charge carrier since the energy of this band (2 eV) is well below the bandgap energy (3.2 eV). Very recently, the emission in the green spectral part has been identified to be due to the recombination between shallowly trapped electron and a deeply trapped hole, which corresponds to a doubly positively charged oxygen vacancy, VO [13]. A spectroelectrochemical experiment was performed to study the absorption spectrum of electrons in nanostructured ZnO films. A strong bleach of the excitonic absorption band is observed when a negative potential is applied to nanostructured ZnO in an electrochemical cell (Fig. 2). Such a bleach has been observed in several spectroelectrochemical studies on ZnO colloids [14] and nanostructured ZnO electrodes [15,16]. In our experiments, a very small increase in absorption can be observed in the visible-near infrared (NIR) region in addition to the bleach. The absorption is more than 2 orders of magnitude weaker than the bleach, which explains why this feature is easily overlooked. The absorption increases gradually with wavelength and exhibits an interference pattern, that is related to the film thickness. The occurrence of this pattern suggests a change in the refractive index of the nanostructured ZnO. Application of a negative potential leads to a flow of electrons into the nanostructured ZnO electrode. The observed optical changes, Drude-like absorption/reflection changes and bleach of the exciton band, suggest that the electrons are free or very shallowly trapped. The dynamics of charge carrier recombination were investigated by measuring the lifetime of the green luminescence by single photon counting spectroscopy upon excitation with a pulsed laser at 400 nm with 10 nm bandwidth. It is noted that 400 nm light is only weakly absorbed by the nanostructured ZnO film (about 5% of the incoming light is absorbed) and will create charge carriers with negligible excess energy. Luminescence decay data are shown in Fig. 3 for different emission wavelengths. For wavelengths above 500 nm, the decay kinetics are multiexponential with lifetimes of
0x10 0
+0.5 V
-5x10-3
-1x10-2 400
500
600
700
800
900
1000
Wavelength (nm) Fig. 2. Difference absorption spectra of a nanostructured ZnO electrode. The reference spectrum was recorded at +0.5 V vs. Ag/AgCl. The applied potentials are indicated in the figure.
10
Log(luminescence intensity)
178
8 6 4 2 0 0
10
20
30
40
Time (ns)
Fig. 3. Time-resolved luminescence data for ZnO films. Excitation wavelength was 400 nm. The collected light was: open circles k ¼ 640 nm, full line k ¼ 500 nm, stars k > 500 nm.
340 ps (96.6%), 2.1 ns (3.2%) and 18.8 ns (0.2%). Kamat and Patrick [9] have measured similar lifetime components except for the fastest one. The main component around 400 ps was not observed by these authors due to the limited time resolution of the instrument used in their study. The emission decay lifetime recorded with single wavelengths at 500 and 640 nm show a slight wavelength dependence for the first component, see Table 1. The rate is faster with a shorter wavelength, but
Table 1 Luminescence decay parameters obtained with an excitation wavelength at 400 nm and s the lifetime and A the corresponding amplitude k (nm)
A1 (%)
s1 (ns)
A2 (%)
s2 (ns)
A3 (%)
s3 (ns)
>500 500 640
96.6 91.6 98.5
0.341 0.273 0.330
3.2 7.5 1.4
2.11 2.05 2.08
0.2 0.9 0.1
18.8 22.8 34.1
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The transient absorption data obtained by femtosecond pump–probe spectroscopy for the ZnO films collected at 750 nm following excitation at 400 nm are shown in Fig. 4a,b with different time scale windows: 0–10, 0–45 and 0–500 ps. The decay of the signal can be well fitted with the following time constants 1.5, 12 and 400 ps. It is possible to fit the traces with a second-order kinetics, but this approach does not lead to an improvement of the fit. Furthermore, the multi-exponential decay kinetics give a better view of the different relaxation mechanisms. Such multi-exponential behavior has been previously observed in several semiconductor nanoparticle systems and can be attributed to the different charge carriers relaxation processes [18–20]. The probe wavelength dependence of the decay profile was investigated between 575 and 825 nm to obtain information about the temporal evolution of the transient spectra. Fig. 4c shows the transient absorption data obtained with different probe wavelengths. By normalising the signals, it appears clearly that the transient absorption decay profiles are probe wavelength dependent. Only the faster component (1–2 ps) is affected: its contribution becomes larger by probing with a longer wavelength, the other components remain unchanged.
4. Discussion A schematic overview of the processes occurring upon photoexcitation of nanostructured ZnO is shown in Fig. 5. Absorption of a photon of the appropriate energy leads to the creation of an electron–hole pair in ZnO semiconducting nanoparticles. In the case of ZnO, the electron–hole pair is strongly electrostatically bound, i.e., an exciton is formed. Initially delocalized charge carriers can get trapped (1a) and (1b) into shallow or deep trapped states within the band gap, these trapped charge carriers recombine subsequently via radiative or non-radiative pathways (2). Alternatively,
Fig. 4. Transient absorption data following laser pulse excitation (120 fs) at 400 nm and with a probe wavelength at 750 nm. (a) 0–10 ps window time scale. The signal is fitted with a multiexponential decay with 1, 12 and 400 ps time constants. (b) 0–45 ps window time scale. As an inset: long time scale window: 0–500 ps. (c) Probe wavelength dependence of the electron relaxation dynamics. All the signals were normalized at the maximum to facilitate the comparison. Probe wavelengths in nanometers are indicated in the figure.
the relative weight is smaller. Similar observations have been reported for colloidal ZnO [9]. This phenomenon was attributed to a strong Coulomb interaction between the charge carriers [17].
Fig. 5. Electronic relaxation processes and lifetimes: (1) charge carrier trapping s1 ¼ 1 ps; (2) recombination with a deeply trapped hole, s2 400 ps; (3) exciton recombination s3 ¼ 12 ps.
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direct radiative recombination of excitons can take place (3). Stimulated emission is another possible mechanism, but it is not expected to occur under the conditions used here. An attempt to assign the different components extracted from transient absorption and time-resolved luminescence data is proposed below. Luminescence measurements show that recombination reactions involve both excitons and trapped charge carriers. From time-resolved measurements of the visible emission band it follows that the recombination between shallowly trapped electrons and deeply trapped holes is multi-exponential with time constants ranging from 400 to 20 ns. Hence, trapping of charge carriers has to occur within 400 ps. In the transient absorption experiment (Fig. 4), the initial fast rise within the laser pulse is attributed to the formation of the photoexcited charge carriers in the ZnO semiconductor. The overall lifetime of the charge carriers is very short. More than 50% of photoexcited charge carriers disappears in less than 10 ps. The fastest component of the multiexponential decay (1–2 ps) shows a probe-wavelength dependence and is attributed to trapping of charge carriers. It will be discussed in detail below. The 400 ps component can correspond to the recombination of trapped charge carriers, which would agree with the time-resolved luminescence. The 12 ps component is possibly linked to exciton recombination, as it falls in the range of typical direct band gap recombination times [18,20,21]. The transient absorption measurements of nanostructured ZnO films (Fig. 4c) show a probe wavelength dependence of the decay profile. This is a sign that the transient absorption spectrum is not fully developed initially. The probe wavelength dependence can be due to a temporal spectral evolution of the charge carriers in the red-NIR region during the early time from a broad to a narrower spectrum. A temporal blue shift in the transient absorption spectrum is expected if the photoexcited charge carriers evolves from a delocalised to a discrete state. Thermalisation of hot excited states of dye molecules [22,23] or localisation of charge carriers by trapping can give this effect [24]. In a spectroelectrochemical study of nanostructured TiO2 , it was shown that the the spectrum of electrons in deep traps is shifted toward the blue compared to free conduction band electrons [25]. We will discuss two possible explanations for the observed signals in the transient absorption experiments and their probe wavelength dependence: I. Electron trapping: For many semiconductor nanoparticles like CdS, PbS, Cux S, Ag2 S, AgI, PbI2 , BiI3 , TiO2 , ZnS and AgBr, a transient absorption is observed around 750 nm upon band gap excitation with a laser pulse and was attributed to conduction band or trapped electrons [18]. After photonic excitation of ZnO band gap, a broad absorption in the red-IR region is
also observed and was also assigned to conduction band electrons [9,10]. Additional evidence for absorption in the red region by electrons in nanostructured ZnO comes from the spectroelectrochemical experiments in this study. The probe wavelength dependence could reflect the trapping process of conduction band electrons at the early stage or, in other words, the evolution from a delocalized to a more discrete state. The probe wavelength dependence behavior can be a direct result of the evolution of the electron state from a nonequilibrium to a shallow trap (localised) state. II. Hole trapping: The results from the transient absorption experiments may also be explained by assuming that holes are the main absorbing species. As has been demonstrated in Fig. 2, electrons in ZnO have an absorption starting at about 600 nm and increasing with wavelength. A similar Drude-like absorption may be expected for free holes in ZnO. An estimation of the extinction coefficient of holes in ZnO can be made if the effective hole mass and the hole mobility is known. While the former is known ðmh ¼ 0:48me Þ, there is little information of the latter. In a recent publication p-type ZnO thin films obtained by co-doping with Ga and N were reported [26]. The Hall mobility of the holes was one order of magnitude smaller than the electron mobility in comparable n-type films. Based on this information, the Drude absorption of holes in ZnO is estimated to be about 3 times larger than that of electrons. A significant contribution of the holes in the transient absorption measurements presented in Fig. 4 can therefore be expected. It is interesting to compare the time of carriers trapping obtained by ultrafast pump–probe spectroscopy in our study with the dynamics reported for ZnO epitaxial thin films by using an up-conversion technique [27] or optical Kerr gate method [28]. A photon energy dependence of time-resolved photoluminescence of was observed by these authors and attributed to the cooling process of hot charge carriers which occurs in 1 ps. In our transient absorption experiment by using 400 nm light, we create charge carriers with negligible excess energy. The temporal evolution of the transient spectra is therefore ascribed to a carrier localisation process.
5. Conclusion The rates of relaxation of photoexcited charge carriers have been measured by ultrafast pump–probe transient absorption spectroscopy and single-photon counting technique in ZnO nanocrystalline thin films. Spectroelectrochemical experiments have shown that shallowly trapped electrons have an absorption in the red part of the spectrum. The decay dynamics of photoexcited charge carriers were multiexponential with the following time constants 1, 12 and 400 ps. The 1 ps
C. Bauer et al. / Chemical Physics Letters 387 (2004) 176–181
component is probe wavelength dependent and is attributed to the change of charge carrier state, likely a trapping process. The second component is thought to be due to exciton annihilation giving rise to the luminescence band at 385 nm. Time-resolved luminescence decay measurements of the green emission revealed that more than 90% of this recombination process occur in less than 400 ps. This former charge carrier relaxation step is linked to the recombination of shallowly trapped electron with a deeply trapped hole. Acknowledgements We thank G€ oran Karlsson for TEM analysis. This work was conducted within Angstr€ om Solar Center, which is supported by the Foundation for Strategic Environmental Research (MISTRA) and the Swedish National Energy Administration. References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [2] B. OÕRegan, M. Gr€atzel, Nature (London) 353 (1991) 737. [3] A. Hagfeldt, M. Gr€atzel, Acc. Chem. Res. 33 (2000) 269. [4] N. Serpone, D. Lawless, R. Khairutdinov, E. Pelizzetti, J. Phys. Chem. 99 (1995) 16655. [5] D.P. Colombo, K.A. Roussel, J. Saeh, D.S. Skinner, R.M. Bowman, Chem. Phys. Lett. 323 (1995) 207. [6] D.P. Colombo, R.M. Bowman, J. Phys. Chem. 99 (1995) 11752. [7] A. Furube, T. Asashi, H. Masuhara, H. Yamashita, M. Anpo, J. Phys. Chem. B 103 (1999) 3120.
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