- Email: [email protected]
Single-step light-assisted patterning of photonic properties of chemical-bath-deposited CdSe nanocrystalline films
Thin Solid Films 480–481 (2005) 457 – 461 www.elsevier.com/locate/tsf
Single-step light-assisted patterning of photonic properties of chemical-bath-deposited CdSe nanocrystalline films M. Sˇimurdaa, P. Neˇmeca, F. Troja´neka, P. Maly´a,*, T. Miyoshib, K. Kasatanib a
Charles University in Prague, Faculty of Mathematics and Physics, Ke Karlovu 3, 121 16 Prague 2, Czech Republic b Yamaguchi University, 2-16-1 Tokiwadai, 755-8611Ube, Japan Available online 2 December 2004
Abstract We report on the effect of light illumination of nanocrystalline CdSe film during its deposition from chemical bath. We concentrate on the photoexcited carrier dynamics studied by techniques of ultrafast laser spectroscopy. Light illumination by above-band gap light during the growth leads to larger nanocrystals (NCs), which means that the single-step nanocrystal size patterning (with the spatial resolution of tens microns as given by illumination light masking) is possible. Nanocrystal size is a dominating factor, which affects the properties of the films, namely absorption, photoluminescence (PL), and the photoexcited carrier dynamics. The properties are further influenced by the interplay between the interior and surface states of nanocrystals, which is affected by the nanocrystalline size and/or by their surface quality. D 2004 Elsevier B.V. All rights reserved. Keywords: Chemical bath deposition; CdSe; Semiconductor nanocrystals; Ultrafast laser spectroscopy
1. Introduction Semiconductor nanocrystalline thin films are very promising for applications in electronics, optoelectronics, and photonics. The fabrication of II–VI semiconductor nanocrystalline films have been a rapidly growing area of research [1,2]. In particular, CdSe and CdS nanocrystalline films have been prepared by various techniques including chemical bath deposition [1]. Chemically deposited films found their sound application in photovoltaics. On the other hand, CdS, CdSe nanocrystals (NCs) are regarded as a model material for investigation of fundamental physical properties of semiconductor quantum dots as a state of matter in the transition region between molecules and solid [2]. Under certain preparation conditions, the chemically deposited films tend to be nanocrystalline and exhibit quantum-size effects (due to the confinement of carriers within individual NCs). The quantum confinement affects * Corresponding author. Tel.: +420 22191 1260; fax: +420 22191 1249. E-mail address: [email protected] (P. Maly´). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.11.026
strongly the optical properties of the films in the same way as in the case of individual NCs (e.g., CdSe NCs in glass host). This fact, slightly surprising due to very good electroconductivity of these films, is now generally accepted [1]. In addition, the initial ultrafast dynamics of photoexcited carriers in the chemical deposited films are the same as in the well-separated NCs [3,4]. The properties of NCs can be changed by varying the parameters of chemical bath deposition and/or by subsequent heat treatment of the films [1,5–7]. In particular, it has been reported that the light illumination of the growing CdSe film during the deposition leads to the changes in the size of NCs within the films [7]. In our previous paper [6], we discussed the dependence of NC size on the spectrum and intensity of illuminating light, and we proposed an explanation of the observed behaviour. In this paper, we report on the influence of the light illumination on the optoelectronic and photonic properties of the films, namely on absorption and photoluminescence (PL) spectrum as well as on photoexcited carrier dynamics.
458
M. Sˇimurda et al. / Thin Solid Films 480–481 (2005) 457–461
2. Experimental The samples of CdSe nanocrystalline films were prepared by chemical bath deposition, the details of the procedure were described previously [6]. We used the deposition solution containing 120 mM potassium nitrilotriacetate, 80 mM cadmium sulphate, and 56 mM sodium seleno sulphate. The deposition proceeded for 24 h at room temperature in glass containers. We used techniques of ultrafast laser spectroscopy to study absorption and photoluminescence (PL) dynamics. For both types of measurements, the second harmonics of the output of a commercial femtosecond Ti:sapphire laser (Spectra Physics) was used as the excitation. The dynamics of PL excited by 150 fs pulses at the wavelength of 390 nm were measured using a streak camera (Hamamatsu C4334) providing spectro-temporal resolution. In order to minimize the influence of the long-lived PL component, we used the pulse selector to reduce the pulse repetition rate to 4 MHz. The pump power was kept at the low level, where PL dynamics were independent of the pump intensity. The ultrafast dynamics of transient transmission were measured by a standard pump and probe technique, in which the transmission of the probe pulse was measured at particular delay after the pump pulse. The time delay between the pump and probe pulses was varied using an optical delay line. Both beams were focused by a single lens on the surface of the sample in which they intersected under an angle of c58. The pump beam (pulses of 70 fs, 400 nm, 6 mW) was chopped at 3 kHz, a lock-in detection was used to measure the modulated part of the probe beam. Femtosecond continuum generation in a nonlinear photonic crystal fibre was used to obtain spectrally broad probe pulses. After traversing the sample, the probe pulses were detected at particular wavelength using a monochromator and photodiode. The spectral resolution of the detection was about 7 nm, the time resolution of the set-up was limited by the probe pulse width of about 0.5 ps. The standard absorption and photoluminescence (PL) spectra were measured by a grating spectrograph and diode array (InstaSpec II-Oriel) and corrected for the spectral sensitivity of the whole setup. The absorption data shown correspond to the negative logarithm of measured transmission. All the measurements were done at room temperature.
discussing the linear optical absorption and PL spectra (3.1), we concentrate on the results of the ultrafast transient absorption (3.2), and PL dynamics (3.3) measurements. 3.1. Linear absorption and PL spectra The absorption and PL spectra of the sample are shown in Fig. 1. The spectra are typical of CdSe nanocrystals in general. The arrows point the spectral positions of the minimum of the second derivative of the absorption spectra that we use to find the absorption maxima. We attribute the energetically lowest absorption maximum E 1 to the ground state-(1S3/2, 1se) transition in NC. The band gap energy of bulk CdSe is E g=1.74 eV [8] at room temperature which means that the energy of quantum confinement, DE=E 1 E g, corresponding to this transition, is 0.35 eV (0.42 eV) for the light (dark) region, respectively. The energy of quantum confinement scales approximately as 1/ R 2 (R is NC radius, in fact we use the effective mass approximation with including the Coulomb interaction for calculation of the energy of quantum confinement [5]) as we verified comparing the optical data with TEM, XRD, and STM measurements. The red-shift of the absorption spectrum of the light region in comparison to that of the dark region of the sample is in agreement with the previous observations of an increase in the NC size due to the supraband gap light illumination [6,7]. The influence of the illumination on the growth process can be understood by photoelectrodeposition of CdSe. During the light illumination, the electron hole pairs are photoexcited in the individual NCs, and the electrons can reduce the solution at the CdSe surface in similar way as in the case of electrons supplied by an external power supply. The crystal size saturates for light intensities above certain level, which can be explained by the fact that one electron-hole pair per one NC is sufficient to influence the growth process [6], or by the rapid recombination of photoexcited carriers by Auger
3. Results and discussion Here we show the effect of the light illumination of the growing film on a particular sample, which was deposited as described above. The growing film was illuminated by a halogen lamp with intensity of 60 mW cm 2. The illuminating light was masked by an edge so that one part of the film was deposited under illumination (called as the light region in the following) and the other one in a shadow (the dark region in the following). In this section, after
Fig. 1. The absorption and PL spectra of the dark (—) and light (- - -) regions of the sample. Small arrows indicate positions of the lowest absorption maxima found using the second derivative. Inset: The energy of quantum confinement vs. space coordinate across the border of the dark and light regions.
M. Sˇimurda et al. / Thin Solid Films 480–481 (2005) 457–461
processes which are effective when more than one electron hole pair per NC is created [1]. In Fig. 1, one can see a more pronounced tail in the absorption for the illuminated part of the sample. This was observed also by Hodes, similar absorption tail is typical of electrodeposited films [1]. The tail can be caused by a presence of very large NCs, i.e., by a rather broad distribution of NC sizes, and/or by a presence of the sub-band gap (surface) states of the NCs. By masking the illuminating light, one can produce light patterns on the films providing the single-step patterning of NC sizes. The transition region between large and small NCs is sharp, given by the shadow boundary. As illustrated in the inset of Fig. 1 where energy of quantum confinement is plotted versus the space coordinate. This means that illumination of the surface of the growing film, and not the illumination of the chemical solution, is important. In the PL spectrum (see Fig. 1), we ascribe the band at longer wavelengths (so-called deep-trap PL) to the recombination of a deeply trapped hole with an electron in the NC interior state or shallow traps [2]. The recombining carriers are spatially separated, which leads to the slow nonexponential PL decay ranging up to the micro-or millisecond time scales [1,10]. The band at shorter wavelengths is generally interpreted in terms of recombination of carriers in the NC interior states or shallow traps (so-called band-edge band). The spectral width of the band-edge band is mainly due to the distribution of NC sizes. For the dark region, the band-edge and deep-trap bands are well separated, whereas in the light region, the bands overlap each other to certain extent. In addition, the Stokes shift (i.e., the energy separation between the absorption and PL maxima) is different in the light and dark regions, with the values of 75 (dark) and 120 meV (light). In accord with the timeresolved measurements discussed below, we interpret this difference in terms of rapid trapping of photoexcited carriers. 3.2. Transient absorption dynamics The results of the pump and probe measurements are shown in Fig. 2 for both light and dark regions. The excitation wavelength was 400 nm; that is, the carriers were excited to the high-energy states. Clearly, there is a substantial difference between behaviour of the two regions: the dynamics are more spectrally dependent for the case of light region. Before discussing this difference between the blightQ and the bdarkQ dynamics, we will concentrate on the spectral behaviour of the bdarkQ dynamics, see Fig. 2a. The transient transmission measurements monitor the electron dynamics [3]. For shorter wavelengths of the probe pulses, one monitors the interior (the lowest transition) electron states of small NCs and high-energy (interior/surface) states of large NCs. It is now generally accepted that there is a fast carrier relaxation in CdSe NCs not hindered by a phonon bottleneck. This relaxation appears as a rapid initial decrease in signal of transient transmission. For longer probe wavelengths, one
459
Fig. 2. Dynamics of transient transmission for the (a) dark and (b) light regions of the sample. Inset: The absorption spectra for the relevant region, arrows indicate the wavelengths at which the dynamics were measured.
probes the interior states of large NCs and sub-band gap (surface) states of small NCs. In this way, going up with the probe wavelength, the contribution of carrier cooling decreases. At the absorption maximum, there is a major contribution of the interior states of NCs. For the wavelengths above the absorption maximum, the contribution of the trapped carriers with slower dynamics is more important. What is the reason of the change in the transmission dynamics for the light region (Fig. 2b)? We suggest that this is due to the fact that the large size of NCs shifts the interior state/shallow-trap transitions energetically very near to the transitions connected with the deep traps. In this way, the dynamics can be understood as to be composed of the two contributions which can be attributed to two types of NCs (namely those with and without a deep trap). The role of the deep-trap transitions increases with increasing the probe wavelength. For the absorption maximum, the dynamics of the interior states should prevail as in the case of the bdarkQ dynamics: indeed the bdarkQ and blightQ dynamics at the corresponding absorption maxima are very similar as displayed in Fig. 3a on the longer time scale. Nevertheless, on the
460
M. Sˇimurda et al. / Thin Solid Films 480–481 (2005) 457–461
shown in Fig. 4 for selected wavelengths within the bandedge PL band. The initial rapid electron relaxation (see Fig. 2) does not show up in the PL dynamics in Fig. 4 due to the low time resolution of the streak camera (40 ps). The PL dynamics for the dark region, see Fig. 4a, are nearly independent of the detection wavelength suggesting that the interplay between the interior and surface trapping states is the same for NCs of various sizes. The different situation for the light region of the sample, see Fig. 4b, can be interpreted again by the important role of the trapped electrons for PL at longer wavelengths. At the maximum of the band-edge band, the PL dynamics are the same for the light and dark as shown in Fig. 3b. At shorter wavelengths, the PL decays faster due to the enhanced extraction of the carriers from the states at relevant energy by the efficient trapping. The PL decay at longer wavelengths is again slower due to the strong contribution of the trapped carriers. The important role of the traps in the case of light region affects also the standard PL spectrum: it leads to a large Stokes shift and to the overlap of the band-edge and deep-trap bands. As mentioned in the Introduction, the optical properties and carrier dynamics in the film can be interpreted well in
Fig. 3. Comparison of dynamics of (a) transient transmission and (b) PL of the dark (—) and light (- - -) regions at spectral positions of corresponding maxima. Insets: the dynamics on the shorter time scales are shown.
shorter time scale, shown in the inset of Fig. 3a, the blightQ dynamics are faster due to the efficient electron trapping. On the contrary, for longer wavelengths the trapped carriers start to dominate the dynamics which slow down. The energetically lower-laying states can be filled gradually by carriers relaxing from the upper states as can be seen by a slow rise of the transmission dynamics at 625 nm in Fig. 2b. The origin of the electron traps is not certain. However, the presence of a broad spectral distribution of the electron energy states connected with the Se dangling bonds on the NC surface has been calculated recently [9]. 3.3. PL dynamics The PL dynamics monitor both the electron and hole populations, whereas the transient transmission is rather due to the electrons. That is why different decay times in both types of measurements can be expected. However, the results of both transient transmission and PL dynamics show similar spectral behaviour. The PL dynamics with excitation at 390 nm for the dark and light parts of the sample are
Fig. 4. Dynamics of PL for the (a) dark region and (b) light regions of the sample. Inset: PL spectra for the relevant region, arrows indicate the wavelengths at which the dynamics were measured.
M. Sˇimurda et al. / Thin Solid Films 480–481 (2005) 457–461
terms of the processes within individual NCs. We used this approach above in explaining the role of the light illumination of growing film. In photoexcited closely spaced NCs, the Ffrster excitation energy transfer between neighbouring NCs may be important [11]. However, the relevant transfer rates published for CdSe NCs [11] are too small to be consistent with our data. We plan further experiments to study the origin of carrier trapping states and ultrafast relaxation pathways of photoexcited carriers including the possible inter-NC transfer.
4. Conclusion We have investigated the effect of light illumination of the nanocrystalline CdSe films during the deposition on their photonic properties. The light illumination of the growing film leads to a larger size of NCs. The NC size is a dominating factor affecting the properties of the films. Our optical ultrafast experiments made it possible to obtain direct information on the initial carrier relaxation dynamics in the two cases. In particular, the difference in spectral behaviour of the transient absorption and PL dynamics is substantial. Our data suggest that this is due to the changed interplay between the interior and surface states of NC. This interplay is influenced by the NC size and/or by modified surface quality of NCs. We demonstrated that light illumination of the growing film can be used to pattern the photonics properties of chemically deposited CdSe films in a single-step procedure.
461
Acknowledgements The authors acknowledge the support of the Grant Agency of the Academy of Sciences of the Czech Republic (Grant No. A1010316). The streak camera measurements were done using facilities of the Venture Business Laboratory, Yamaguchi University, Japan. P.M. gratefully acknowledges the support from the Ministry of Education, Science, Sports and Culture of Japan.
References [1] Gray Hodes, Chemical Solution Deposition of Semiconductor Films, Marcel Dekker, New York, Basel, 2003. [2] U. Woggon, Optical Properties of Semiconductor Quantum Dots, Springer-Verlag, Berlin–Heidelberg–New York, 1997. [3] V.I. Klimov, D.W. McBranch, C.A. Leatherdale, M.G. Bawendi, Phys. Rev., B 60 (1999) 13740. [4] P. Neˇmec, P. Forma´nek, D. Mikesˇ, I. Neˇmec, F. Troja´nek, P. Maly´, Phys. Status Solidi, B Basic Res. 224 (2001) 481. [5] F. Troja´nek, R. Cingolani, D. Cannoletta, D. Mikesˇ, P. Neˇmec, E. Uhlı´rˇova´, J. Rohovec, P. Maly´, J. Cryst. Growth 209 (2000) 695. [6] P. Neˇmec, D. Mikesˇ, J. Rohovec, E. Uhlı´rˇova´, F. Troja´nek, P. Maly´, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 69 (2000) 500. [7] G. Hodes, Isr. J. Chem 33 (1993) 95. [8] H. Landolt, R. Bornstein, Handbook of Physics, vol. 17b, Springer, Berlin, 1982. [9] S. Pokrant, K.B. Whaley, Phys. J. D 6 (1999) 255. [10] P. Neˇmec, P. Maly´, J. Appl. Phys. 87 (2000) 3342. [11] C.R. Kagan, C.B. Murray, M.G. Bawendi, Phys. Rev., B 54 (1996) 8633.
Single-step light-assisted patterning of photonic properties of chemical-bath-deposited CdSe nanocrystalline films M. Sˇimurdaa, P. Neˇmeca, F. Troja´neka, P. Maly´a,*, T. Miyoshib, K. Kasatanib a
Charles University in Prague, Faculty of Mathematics and Physics, Ke Karlovu 3, 121 16 Prague 2, Czech Republic b Yamaguchi University, 2-16-1 Tokiwadai, 755-8611Ube, Japan Available online 2 December 2004
Abstract We report on the effect of light illumination of nanocrystalline CdSe film during its deposition from chemical bath. We concentrate on the photoexcited carrier dynamics studied by techniques of ultrafast laser spectroscopy. Light illumination by above-band gap light during the growth leads to larger nanocrystals (NCs), which means that the single-step nanocrystal size patterning (with the spatial resolution of tens microns as given by illumination light masking) is possible. Nanocrystal size is a dominating factor, which affects the properties of the films, namely absorption, photoluminescence (PL), and the photoexcited carrier dynamics. The properties are further influenced by the interplay between the interior and surface states of nanocrystals, which is affected by the nanocrystalline size and/or by their surface quality. D 2004 Elsevier B.V. All rights reserved. Keywords: Chemical bath deposition; CdSe; Semiconductor nanocrystals; Ultrafast laser spectroscopy
1. Introduction Semiconductor nanocrystalline thin films are very promising for applications in electronics, optoelectronics, and photonics. The fabrication of II–VI semiconductor nanocrystalline films have been a rapidly growing area of research [1,2]. In particular, CdSe and CdS nanocrystalline films have been prepared by various techniques including chemical bath deposition [1]. Chemically deposited films found their sound application in photovoltaics. On the other hand, CdS, CdSe nanocrystals (NCs) are regarded as a model material for investigation of fundamental physical properties of semiconductor quantum dots as a state of matter in the transition region between molecules and solid [2]. Under certain preparation conditions, the chemically deposited films tend to be nanocrystalline and exhibit quantum-size effects (due to the confinement of carriers within individual NCs). The quantum confinement affects * Corresponding author. Tel.: +420 22191 1260; fax: +420 22191 1249. E-mail address: [email protected] (P. Maly´). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.11.026
strongly the optical properties of the films in the same way as in the case of individual NCs (e.g., CdSe NCs in glass host). This fact, slightly surprising due to very good electroconductivity of these films, is now generally accepted [1]. In addition, the initial ultrafast dynamics of photoexcited carriers in the chemical deposited films are the same as in the well-separated NCs [3,4]. The properties of NCs can be changed by varying the parameters of chemical bath deposition and/or by subsequent heat treatment of the films [1,5–7]. In particular, it has been reported that the light illumination of the growing CdSe film during the deposition leads to the changes in the size of NCs within the films [7]. In our previous paper [6], we discussed the dependence of NC size on the spectrum and intensity of illuminating light, and we proposed an explanation of the observed behaviour. In this paper, we report on the influence of the light illumination on the optoelectronic and photonic properties of the films, namely on absorption and photoluminescence (PL) spectrum as well as on photoexcited carrier dynamics.
458
M. Sˇimurda et al. / Thin Solid Films 480–481 (2005) 457–461
2. Experimental The samples of CdSe nanocrystalline films were prepared by chemical bath deposition, the details of the procedure were described previously [6]. We used the deposition solution containing 120 mM potassium nitrilotriacetate, 80 mM cadmium sulphate, and 56 mM sodium seleno sulphate. The deposition proceeded for 24 h at room temperature in glass containers. We used techniques of ultrafast laser spectroscopy to study absorption and photoluminescence (PL) dynamics. For both types of measurements, the second harmonics of the output of a commercial femtosecond Ti:sapphire laser (Spectra Physics) was used as the excitation. The dynamics of PL excited by 150 fs pulses at the wavelength of 390 nm were measured using a streak camera (Hamamatsu C4334) providing spectro-temporal resolution. In order to minimize the influence of the long-lived PL component, we used the pulse selector to reduce the pulse repetition rate to 4 MHz. The pump power was kept at the low level, where PL dynamics were independent of the pump intensity. The ultrafast dynamics of transient transmission were measured by a standard pump and probe technique, in which the transmission of the probe pulse was measured at particular delay after the pump pulse. The time delay between the pump and probe pulses was varied using an optical delay line. Both beams were focused by a single lens on the surface of the sample in which they intersected under an angle of c58. The pump beam (pulses of 70 fs, 400 nm, 6 mW) was chopped at 3 kHz, a lock-in detection was used to measure the modulated part of the probe beam. Femtosecond continuum generation in a nonlinear photonic crystal fibre was used to obtain spectrally broad probe pulses. After traversing the sample, the probe pulses were detected at particular wavelength using a monochromator and photodiode. The spectral resolution of the detection was about 7 nm, the time resolution of the set-up was limited by the probe pulse width of about 0.5 ps. The standard absorption and photoluminescence (PL) spectra were measured by a grating spectrograph and diode array (InstaSpec II-Oriel) and corrected for the spectral sensitivity of the whole setup. The absorption data shown correspond to the negative logarithm of measured transmission. All the measurements were done at room temperature.
discussing the linear optical absorption and PL spectra (3.1), we concentrate on the results of the ultrafast transient absorption (3.2), and PL dynamics (3.3) measurements. 3.1. Linear absorption and PL spectra The absorption and PL spectra of the sample are shown in Fig. 1. The spectra are typical of CdSe nanocrystals in general. The arrows point the spectral positions of the minimum of the second derivative of the absorption spectra that we use to find the absorption maxima. We attribute the energetically lowest absorption maximum E 1 to the ground state-(1S3/2, 1se) transition in NC. The band gap energy of bulk CdSe is E g=1.74 eV [8] at room temperature which means that the energy of quantum confinement, DE=E 1 E g, corresponding to this transition, is 0.35 eV (0.42 eV) for the light (dark) region, respectively. The energy of quantum confinement scales approximately as 1/ R 2 (R is NC radius, in fact we use the effective mass approximation with including the Coulomb interaction for calculation of the energy of quantum confinement [5]) as we verified comparing the optical data with TEM, XRD, and STM measurements. The red-shift of the absorption spectrum of the light region in comparison to that of the dark region of the sample is in agreement with the previous observations of an increase in the NC size due to the supraband gap light illumination [6,7]. The influence of the illumination on the growth process can be understood by photoelectrodeposition of CdSe. During the light illumination, the electron hole pairs are photoexcited in the individual NCs, and the electrons can reduce the solution at the CdSe surface in similar way as in the case of electrons supplied by an external power supply. The crystal size saturates for light intensities above certain level, which can be explained by the fact that one electron-hole pair per one NC is sufficient to influence the growth process [6], or by the rapid recombination of photoexcited carriers by Auger
3. Results and discussion Here we show the effect of the light illumination of the growing film on a particular sample, which was deposited as described above. The growing film was illuminated by a halogen lamp with intensity of 60 mW cm 2. The illuminating light was masked by an edge so that one part of the film was deposited under illumination (called as the light region in the following) and the other one in a shadow (the dark region in the following). In this section, after
Fig. 1. The absorption and PL spectra of the dark (—) and light (- - -) regions of the sample. Small arrows indicate positions of the lowest absorption maxima found using the second derivative. Inset: The energy of quantum confinement vs. space coordinate across the border of the dark and light regions.
M. Sˇimurda et al. / Thin Solid Films 480–481 (2005) 457–461
processes which are effective when more than one electron hole pair per NC is created [1]. In Fig. 1, one can see a more pronounced tail in the absorption for the illuminated part of the sample. This was observed also by Hodes, similar absorption tail is typical of electrodeposited films [1]. The tail can be caused by a presence of very large NCs, i.e., by a rather broad distribution of NC sizes, and/or by a presence of the sub-band gap (surface) states of the NCs. By masking the illuminating light, one can produce light patterns on the films providing the single-step patterning of NC sizes. The transition region between large and small NCs is sharp, given by the shadow boundary. As illustrated in the inset of Fig. 1 where energy of quantum confinement is plotted versus the space coordinate. This means that illumination of the surface of the growing film, and not the illumination of the chemical solution, is important. In the PL spectrum (see Fig. 1), we ascribe the band at longer wavelengths (so-called deep-trap PL) to the recombination of a deeply trapped hole with an electron in the NC interior state or shallow traps [2]. The recombining carriers are spatially separated, which leads to the slow nonexponential PL decay ranging up to the micro-or millisecond time scales [1,10]. The band at shorter wavelengths is generally interpreted in terms of recombination of carriers in the NC interior states or shallow traps (so-called band-edge band). The spectral width of the band-edge band is mainly due to the distribution of NC sizes. For the dark region, the band-edge and deep-trap bands are well separated, whereas in the light region, the bands overlap each other to certain extent. In addition, the Stokes shift (i.e., the energy separation between the absorption and PL maxima) is different in the light and dark regions, with the values of 75 (dark) and 120 meV (light). In accord with the timeresolved measurements discussed below, we interpret this difference in terms of rapid trapping of photoexcited carriers. 3.2. Transient absorption dynamics The results of the pump and probe measurements are shown in Fig. 2 for both light and dark regions. The excitation wavelength was 400 nm; that is, the carriers were excited to the high-energy states. Clearly, there is a substantial difference between behaviour of the two regions: the dynamics are more spectrally dependent for the case of light region. Before discussing this difference between the blightQ and the bdarkQ dynamics, we will concentrate on the spectral behaviour of the bdarkQ dynamics, see Fig. 2a. The transient transmission measurements monitor the electron dynamics [3]. For shorter wavelengths of the probe pulses, one monitors the interior (the lowest transition) electron states of small NCs and high-energy (interior/surface) states of large NCs. It is now generally accepted that there is a fast carrier relaxation in CdSe NCs not hindered by a phonon bottleneck. This relaxation appears as a rapid initial decrease in signal of transient transmission. For longer probe wavelengths, one
459
Fig. 2. Dynamics of transient transmission for the (a) dark and (b) light regions of the sample. Inset: The absorption spectra for the relevant region, arrows indicate the wavelengths at which the dynamics were measured.
probes the interior states of large NCs and sub-band gap (surface) states of small NCs. In this way, going up with the probe wavelength, the contribution of carrier cooling decreases. At the absorption maximum, there is a major contribution of the interior states of NCs. For the wavelengths above the absorption maximum, the contribution of the trapped carriers with slower dynamics is more important. What is the reason of the change in the transmission dynamics for the light region (Fig. 2b)? We suggest that this is due to the fact that the large size of NCs shifts the interior state/shallow-trap transitions energetically very near to the transitions connected with the deep traps. In this way, the dynamics can be understood as to be composed of the two contributions which can be attributed to two types of NCs (namely those with and without a deep trap). The role of the deep-trap transitions increases with increasing the probe wavelength. For the absorption maximum, the dynamics of the interior states should prevail as in the case of the bdarkQ dynamics: indeed the bdarkQ and blightQ dynamics at the corresponding absorption maxima are very similar as displayed in Fig. 3a on the longer time scale. Nevertheless, on the
460
M. Sˇimurda et al. / Thin Solid Films 480–481 (2005) 457–461
shown in Fig. 4 for selected wavelengths within the bandedge PL band. The initial rapid electron relaxation (see Fig. 2) does not show up in the PL dynamics in Fig. 4 due to the low time resolution of the streak camera (40 ps). The PL dynamics for the dark region, see Fig. 4a, are nearly independent of the detection wavelength suggesting that the interplay between the interior and surface trapping states is the same for NCs of various sizes. The different situation for the light region of the sample, see Fig. 4b, can be interpreted again by the important role of the trapped electrons for PL at longer wavelengths. At the maximum of the band-edge band, the PL dynamics are the same for the light and dark as shown in Fig. 3b. At shorter wavelengths, the PL decays faster due to the enhanced extraction of the carriers from the states at relevant energy by the efficient trapping. The PL decay at longer wavelengths is again slower due to the strong contribution of the trapped carriers. The important role of the traps in the case of light region affects also the standard PL spectrum: it leads to a large Stokes shift and to the overlap of the band-edge and deep-trap bands. As mentioned in the Introduction, the optical properties and carrier dynamics in the film can be interpreted well in
Fig. 3. Comparison of dynamics of (a) transient transmission and (b) PL of the dark (—) and light (- - -) regions at spectral positions of corresponding maxima. Insets: the dynamics on the shorter time scales are shown.
shorter time scale, shown in the inset of Fig. 3a, the blightQ dynamics are faster due to the efficient electron trapping. On the contrary, for longer wavelengths the trapped carriers start to dominate the dynamics which slow down. The energetically lower-laying states can be filled gradually by carriers relaxing from the upper states as can be seen by a slow rise of the transmission dynamics at 625 nm in Fig. 2b. The origin of the electron traps is not certain. However, the presence of a broad spectral distribution of the electron energy states connected with the Se dangling bonds on the NC surface has been calculated recently [9]. 3.3. PL dynamics The PL dynamics monitor both the electron and hole populations, whereas the transient transmission is rather due to the electrons. That is why different decay times in both types of measurements can be expected. However, the results of both transient transmission and PL dynamics show similar spectral behaviour. The PL dynamics with excitation at 390 nm for the dark and light parts of the sample are
Fig. 4. Dynamics of PL for the (a) dark region and (b) light regions of the sample. Inset: PL spectra for the relevant region, arrows indicate the wavelengths at which the dynamics were measured.
M. Sˇimurda et al. / Thin Solid Films 480–481 (2005) 457–461
terms of the processes within individual NCs. We used this approach above in explaining the role of the light illumination of growing film. In photoexcited closely spaced NCs, the Ffrster excitation energy transfer between neighbouring NCs may be important [11]. However, the relevant transfer rates published for CdSe NCs [11] are too small to be consistent with our data. We plan further experiments to study the origin of carrier trapping states and ultrafast relaxation pathways of photoexcited carriers including the possible inter-NC transfer.
4. Conclusion We have investigated the effect of light illumination of the nanocrystalline CdSe films during the deposition on their photonic properties. The light illumination of the growing film leads to a larger size of NCs. The NC size is a dominating factor affecting the properties of the films. Our optical ultrafast experiments made it possible to obtain direct information on the initial carrier relaxation dynamics in the two cases. In particular, the difference in spectral behaviour of the transient absorption and PL dynamics is substantial. Our data suggest that this is due to the changed interplay between the interior and surface states of NC. This interplay is influenced by the NC size and/or by modified surface quality of NCs. We demonstrated that light illumination of the growing film can be used to pattern the photonics properties of chemically deposited CdSe films in a single-step procedure.
461
Acknowledgements The authors acknowledge the support of the Grant Agency of the Academy of Sciences of the Czech Republic (Grant No. A1010316). The streak camera measurements were done using facilities of the Venture Business Laboratory, Yamaguchi University, Japan. P.M. gratefully acknowledges the support from the Ministry of Education, Science, Sports and Culture of Japan.
References [1] Gray Hodes, Chemical Solution Deposition of Semiconductor Films, Marcel Dekker, New York, Basel, 2003. [2] U. Woggon, Optical Properties of Semiconductor Quantum Dots, Springer-Verlag, Berlin–Heidelberg–New York, 1997. [3] V.I. Klimov, D.W. McBranch, C.A. Leatherdale, M.G. Bawendi, Phys. Rev., B 60 (1999) 13740. [4] P. Neˇmec, P. Forma´nek, D. Mikesˇ, I. Neˇmec, F. Troja´nek, P. Maly´, Phys. Status Solidi, B Basic Res. 224 (2001) 481. [5] F. Troja´nek, R. Cingolani, D. Cannoletta, D. Mikesˇ, P. Neˇmec, E. Uhlı´rˇova´, J. Rohovec, P. Maly´, J. Cryst. Growth 209 (2000) 695. [6] P. Neˇmec, D. Mikesˇ, J. Rohovec, E. Uhlı´rˇova´, F. Troja´nek, P. Maly´, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 69 (2000) 500. [7] G. Hodes, Isr. J. Chem 33 (1993) 95. [8] H. Landolt, R. Bornstein, Handbook of Physics, vol. 17b, Springer, Berlin, 1982. [9] S. Pokrant, K.B. Whaley, Phys. J. D 6 (1999) 255. [10] P. Neˇmec, P. Maly´, J. Appl. Phys. 87 (2000) 3342. [11] C.R. Kagan, C.B. Murray, M.G. Bawendi, Phys. Rev., B 54 (1996) 8633.