JOURNAL OF APPLIED PHYSICS
VOLUME 87, NUMBER 12
15 JUNE 2000
Evidence of photo- and electrodarkening of „CdSe…ZnS quantum dot composites J. Rodrı´guez-Viejoa) Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
H. Mattoussib) and J. R. Heine Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
M. K. Kuno Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
J. Michel Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
M. G. Bawendi Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
K. F. Jensen Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
共Received 14 September 1999; accepted for publication 14 March 2000兲 We present a study of the kinetics of photoluminescence 共PL兲 and cathodoluminescence 共CL兲 degradation of semiconductor quantum dot composites, formed by highly luminescent 共CdSe兲ZnS core-shell nanocrystals embedded in a ZnS matrix. The photoluminescence and cathodoluminescence spectra indicate that both emissions originate from the same near band-edge state of the nanocrystals. We observe a strong decrease in the PL and CL intensities with time. Photoluminescence experiments carried out at high laser fluences 共0.5–10 mJ/cm2 per pulse兲 show that the PL intensity decay with time depends on the size of the nanocrystals and the nature of the surrounding matrix. For instance, close-packed films showed a much slower decay than composite films. The cathodoluminescence intensity degradation is enhanced at lower temperatures. Partial recoveries of the CL signal have been achieved after thermal annealing at temperatures around 120 °C, which indicates that activation of trapped carriers can be induced by thermal stimulation. We attribute the CL and PL decay in the composite films to photo- and electroionization of the nanocrystals, and subsequent trapping of the ejected electrons in the surrounding semiconductor matrix. © 2000 American Institute of Physics. 关S0021-8979共00兲05412-8兴
particles, improves the PL quantum yield to ⬃30%–50% for particles in the size range of 20–60 Å in radius.5,6 Semiconductor doped glasses 共SDGs兲 consisting of small crystallites embedded in a glass matrix have been studied quite extensively as nonlinear optical materials.7–11 Roussignol et al.7 were the first to observe the photodarkening of these SDGs caused by a longterm irradiation with an intense laser source. They observed a decrease in the phase conjugation efficiency by a factor of 2 when exposing the glass to a picosecond pulsed laser signal with an intensity of 15 J/cm2 共a single pulse had a fluence of 5 mJ/cm2兲, but did not measure any changes for fluences lower than 0.5 mJ/cm2. Other studies also reported darkening processes in SDGs.12–14 Different mechanisms have been proposed to explain the observed photodarkening. Ekimov and co-workers12 proposed the existence of overbarrier transitions for the electron out of the nanocrystal, into the glass matrix. They attributed the photodarkening to an Auger recombination process; followed by trapping of electrons in
I. INTRODUCTION AND BACKGROUND
Semiconductor nanocrystals 共quantum dots, QDs兲 which are small compared to the Bohr exciton radius exhibit unique optical and electronic properties due to confinement of the electronic excitations.1 In particular, photoluminescence 共PL兲 emission can be tuned across the visible spectrum by simply changing the particle radius for CdSe nanocrystals. Recent improvements in the synthesis of CdSe nanoparticles organically passivated with trioctylphosphines, allow one to obtain low polydispersity nanocrystals 共standard deviation, ⬵5% – 10%) with PL quantum yields of ⬃10%–15%.2–4 Solution chemistry growth of a thin ZnS overlayer on the surface of CdSe core, to make 共CdSe兲ZnS core-shell nanoa兲
Present address: Grupo de Fı´sica de Materials I. Dep. Fı´sica. Universitat Auto`noma de Barcelona. 08193 Bellaterra, Spain; electronic mail:
[email protected] b兲 Present address: Naval Research Laboratory, Optical Sciences Division, Washington DC 20375; electronic mail:
[email protected] 0021-8979/2000/87(12)/8526/9/$17.00
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© 2000 American Institute of Physics
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Rodrı´quez-Viejo et al.
J. Appl. Phys., Vol. 87, No. 12, 15 June 2000
the matrix.13 They used thermostimulated luminescence experiments to verify the existence of such barriers. Malhotra et al. proposed a modified three step model that involves photoassisted trapping of electrons out of the crystallites into states within the glass matrix through an intermediate surface state.14 The capture of electrons in long-living traps in the matrix was also reported by MacDonald and Lawandy.15 Sercel pointed out the presence of deep-level traps coupled to the QDs in III–V semiconductor heterostructures, and explained the capture process through multiphonon assisted tunneling.16 Ultrafast electron ejection from ZnCdS and CdS QDs in aqueous solution, after multiphoton absorption from an intense subpicosecond ultraviolet 共UV兲 laser signal, has been reported by Kaschke et al.17 On the contrary, CdSe and 共CdSe兲ZnS QDs dispersed in an organic environment do not show longterm degradation due to photodarkening, although recently Nirmal et al.18 have observed intermittence 共on/off兲 in the luminescence of single CdSe QDs, and attributed this effect to photoionization of the nanocrystal. Efros and Rosen19 have addressed the temporal dependence of the PL emission 共on/off兲 of single QDs in organic matrices theoretically, within the framework of intermittent nanocrystal ionization and Auger recombination. The incorporation of quantum dots grown by solution chemistry routes into a semiconductor matrix provides significant advantages over conventional glass matrices. Recently, Danek et al.20 developed a technique that combines electrospray 共ES兲 and organometallic chemical vapor deposition 共OMCVD兲 to incorporate CdSe nanocrystals into a ZnSe matrix. Such matrices showed optical and electronic properties characteristic of the embedded nanocrystals. The choice of the appropriate semiconductor matrix to embed the nanocrystals should facilitate the integration of the QDs into electronic or optoelectronic devices. In a recent article, we reported a preliminary study of the cathodoluminescence 共CL兲 of composite films made of 共CdSe兲ZnS nanocrystals dispersed in a ZnS matrix.21 We showed that the CL emission has the same characteristics as the photoluminescence 共i.e., near band-edge emission兲 for these structures. Furthermore, that study indicated that the CL intensity decays with time 共electrodarkening兲 upon sample irradiation with kinetic electrons. In this article, we study the time decay of the photoluminescence 共photodarkening兲 and cathodoluminescence 共electrodarkening兲 of 共CdSe兲ZnS quantum dot composites upon excitation with a high intensity light source and with a high energy electron flux, respectively. To address the question of how relevant are the offsets of the conduction and valence bands at the nanocrystal/matrix interface to the photodarkening, we also perform laser irradiation measurements on films of close-packed QDs. These experiments permitted us to probe the influence of the semiconductor matrix on the luminescence properties and to explore the time dependence of low temperature near band-edge cathodoluminescence. We also show that the partial recovery of the luminescence from quantum dot ZnS composites may be associated with neutralization of the QDs following the release of trapped electrons in the ZnS matrix.
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II. EXPERIMENTAL SECTION
Highly luminescent core-shell 共CdSe兲ZnS quantum dots with measured quantum yields of ⬃30%–50% were prepared via the pyrolisis of organometallic reagents in a coordinating solvent, as described elsewhere.6 The native capping groups 共trioctyl phosphine/trioctyl phosphine oxide: TOP/TOPO兲 can be exchanged to other groups, which permits optimization of the post synthesis processing conditions. For our study, prior to dispersion in pyridine, cap exchange from TOP/TOPO to pyridine was performed. This was carried out by dispersing a powder of TOPO/TOP capped particles in pyridine, followed by precipitation with hexane, then redispersion in pyridine. Repeating this two to three times achieves a high degree of cap exchange.22,23 Earlier experiments reported that pyridine reduces the luminescence of CdSe nanocrystals in comparison with TOP/TOPO by as much as 1 order of magnitude. However, pyridine capped 共CdSe兲ZnS nanocrystals, with ⬇2 monolayers of ZnS have quantum yields of 15%–20%. The presence of a ZnS overlayer substantially reduces the leakage of the exciton to the outer surface, minimizing the influence of the organic cap on the luminescence properties. The dot:ZnS composite thin films 共referred hereafter as quantum dot composites, QDCs兲 have been synthesized according to the technique developed by Danek et al.20 Pyridine capped nanocrystals were dispersed in a mixture of pyridine and acetonitrile 共ratio 1:2兲, which provides a more stable electrospray. H2S and diethylzinc 共ZnEth2兲, at flow rates of 25 and 2.5 mol/min, were used as precursors for the ZnS film growth. The composite films were grown on glass and Si substrates at temperatures between 80 and 250 °C, and at a background pressure of ⬃600 Torr. Film thickness grown using ES-OMCVD ranges from 0.3–1 m. The composite films are sandwiched between two thin layers 共0.05–0.1 m兲 of ZnS, to prevent oxidation of the nanocrystals. Close-packed thin films 共CPF兲 are made by spin casting from concentrated dispersions of nanocrystals in toluene 共optical density ⬇50 in a 10 mm optical path cuvettes兲 on glass or Si substrates. Appropriate spinning angular velocities were chosen to obtain film thickness of ⬃500 Å.24 Once made, the films were covered with a thin layer of ZnS 共⬇500 Å兲 grown by OMCVD at 200 °C, to protect the nanocrystals from photo-oxidation during laser irradiation. Thicker glassy films (thickness⬇1 m) of quantum dots were also made by casting 共dropwise兲 from a concentrated solution dispersed either in toluene 共referred as GFT, TOP/ TOPO capped兲 or in pyridine 共referred to as GFP, pyridine capped兲, and allowing the solvent to evaporate. This provides optically transparent, colored glass coatings. Table I lists the different samples used in this study, together with their main characteristics and preparation method. The PL was measured at room temperature using a Spex fluorolog-2 with front face collection. The room temperature PL quantum yields were obtained by comparing the integrated emission of the dots to that of rhodamine 560 or rhodamine 640 with similar optical densities at the excitation wavelength, and correcting for the difference in the refrac-
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TABLE I. List of samples used in this study along with the corresponding preparation methods.
Sample
Core radius 共Å兲
QDC14 QDC16 QDC21 QDC14A
14.5 16.5 20.8 14.5
CPF16
16.5
CPF28
27.8
GFP16
16.5
GFT16
16.5
Description Composite thin film 共crystalline matrix兲 Composite thin film 共amorphous matrix兲 Glassy closed-packed film of QDs covered by thin ZnS film
Preparation method
ES-OMCVD
Spin cast and OMCVD Cast dropwise from pyridine
FIG. 1. X-ray diffraction profile of quantum dot composites containing 16 Å core radius 共CdSe兲ZnS QDs synthesized by ES-OMCVD at different temperatures: 共a兲 100 °C, 共b兲 150 °C, 共c兲 200 °C, and 共d兲 250 °C. The zinc blende and wurtzite diffractions of ZnS are labeled 䊏 and 䊊, respectively.
Glassy thick film of QDs Cast dropwise from toluene
tion index of the matrix. Absorbance of the samples was kept below 0.3 at the first absorption peak, to minimize reabsorption of the PL emission. Absorbance measurements were carried out using an UV visible near infrared spectrophotometer Cary 5E 共from Varian兲. A background correction to subtract the contribution from the ZnS matrix was done for each film. The study of the luminescence decay with time upon photoirradiation was carried out using an UV signal at 355 nm provided by a Q-switched YAG laser 共from Continuum兲, with 10 ns pulse duration and a repetition rate of 50 Hz. The laser power ranges from 2 to 20 mW, which corresponds to an energy per pulse of ⬃1–10 mJ/cm2. Additional measurements were done using an incident signal at 415 nm, provided by a H2 Raman shifter, pumped with a signal at 355 nm. The power achieved at 415 nm is ⬃1 mW 共0.5 mJ/cm2兲. Photodarkening experiments were performed on composite and close-packed samples, i.e., QDC14, QDC21, CPF16, CPF28, and GFP16. Cathodoluminescence measurements were carried out on composite samples, QDC16 and QDC21, at room and low temperatures, using a scanning electron microscope 共SEM兲 equipped with an Oxford Mono-CL spectrometer. The electron beam was swept horizontally at a frequency of 25 Hz, and a spherical mirror was used to collect the emitted photons and focus the signal on the entrance slit of the photomultiplier tube 共PMT兲. The PMT signal was synchronously detected using a Princeton Applied Research lock-in amplifier. The electron beam energy and current range between 1–40 kV and 1–160 nA, respectively. The CL intensity was recorded at the peak every 0.3 s for times up to 20 min. X-ray diffraction spectra of the thin film QD composites were collected on a Rigaku 300 rotaflex diffractometer operating in the Bragg–Brentano configuration using the copper wavelength ␣ ⫽1.54 Å.
with a crystalline phase made of a mixture of wurtzite and zincblende structures. At higher growth temperatures, the diffraction pattern shows a predominantly zinc blende phase, with a crystal orientation in the 关111兴 direction. The absence of CdSe nanocrystal contribution to the diffraction patterns is due to the stronger background contribution from the ZnS matrix. The surface morphology is dominated by micron-size grains, which increase the surface roughness relative to neat ZnS films. For example, a 5000 Å thick QDC film has a typical surface roughness of ⬃1500 Å, compared to 500 Å for a ZnS film. Such rather large surface roughness does not adversely affect the forward luminescence emission from the sample, since it reduces light waveguiding in the film. This property makes it reasonable to compare quantum yield measurements between solution and thin films.25 The quantum yield of the composites is ⬃10%–15% 共similar to pyridine solutions兲 and it does not depend significantly on the specific processing temperature. The absorption and luminescence spectra of the thin films are characteristic of the dispersed nanocrystals, and therefore the PL emission can be tuned across the visible spectrum. Figure 2 shows the absorbance and PL spectra of 共CdSe兲ZnS nanocrystals 共core radius R 0 ⬵16.5 Å) dispersed in solution and in a ZnS matrix 共QDC16兲. The absorbance of bare CdSe particles 共TOP/TOPO capped兲 in solution is also shown for comparison. The composite film with the overcoated dots was grown at 250 °C. A small redtail in the PL of the composite films is also observed. The composite thin film shows a small blue-
III. RESULTS AND DISCUSSION A. Thin film microstructure and optical properties
Figure 1 shows the x-ray diffraction patterns of QDC16 films grown at temperatures between 100–250 °C. A growth temperature of ⬃100 °C provides partially amorphous films,
FIG. 2. Absorption and emission spectra of 共CdSe兲ZnS QDs in solution 共b兲, and incorporated into a ZnS matrix 共c兲. Absorption spectrums of the 16.5 Å radius bare CdSe QDs in solution before ZnS overcoating is shown for comparison 共a兲.
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J. Appl. Phys., Vol. 87, No. 12, 15 June 2000
FIG. 3. Photoluminescence degradation as a function of time during laser irradiation of samples 共a兲 CPF28, 共b兲 CPF16, 共c兲 QDC21, and 共d兲 QDC14. A fluence of 0.44 mJ/cm2 is used.
shift 共⬇2 nm兲 in the absorption, which can be attributed to a slight alloying at the interface produced during thin film growth. B. Laser induced degradation of composite and close-packed films of „CdSe…ZnS nanocrystals
Figure 3 shows the time dependence of the PL emission 共PL degradation兲 upon exposure to the laser irradiation for four samples, QDC14, QDC21, CPF16, and CPF28. A laser fluence of 1.0 mJ/cm2 per pulse was used. Exposure to high intensity photoexcitation produces a dark spot when the film is illuminated with an UV lamp. However, no evidence of film damage was detected when the sample was examined under an optical microscope. The close-packed films show an initial increase in the luminescence intensity and no significant longterm decay with time. A more rapid decay of the near band-edge luminescence is measured for QDs embedded in a ZnS matrix. Similar data 共not shown in the graph兲 was obtained for a glassy matrix 共QDC14A兲, which presumably has a higher density of deep traps. The effects of the matrix manifest in the appearance of a broad redshift tail in the photoluminescence spectrum, and a faster decay of the luminescence emission. Figure 4 shows the absorbance spectra of the composite film QDC21 before and after laser irradiation. No significant change in the 1 S 3/2 – 1 S e UV transition can be measured during laser irradiation, which
FIG. 4. Absorbance of sample QDC21 before 共a兲 and after 共b兲 laser irradiation at a fluence of 1.0 mJ/cm2. The inset shows the typical absorbance spectrum of a 1 m thick polycrystalline ZnS film.
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FIG. 5. Depth-dose curve vs penetration distance calculated after Everhart and Hoff 共Ref. 26兲 for ZnS at different incident beam voltages. Inset shows the saturation of the CL intensity for sample QDC16 with increasing incident electron beam energy.
indicates absence of alloying of the particles with the surrounding matrix, or possible deterioration of the QDs. The inset shows the absorption spectrum of a typical 1 m polycrystalline ZnS film for comparison. The onset of the band edge absorption for this neat film is close to 3.6 eV 共344 nm兲. To decrease absorption from the ZnS matrix we have also irradiated the QDC and CPF samples at 415 nm and at a fluence of 0.5 mJ/cm2. No significant difference is observed in the luminescence behavior of the samples at either wavelength. Emission spectra for both samples 共composite and close-packed films兲 showed that the position of the PL peak remains unchanged 共within ⫾2 nm兲 with time, independent of the nanocrystal size and laser fluence. We also measured the decay of the luminescence intensity for GFP16 at an incident laser fluence of 1.0 mJ/cm2 共and ⫽355 nm). We found that the degradation was much faster for this sample than for the close-packed ones 共CPF兲 with the ZnS protecting film layer. Along with this observation, a 7–10 nm blueshift of the luminescence maximum over a period of 15 min 共at I/I 0 ⬇0.75), and a broadening of the full width at the half maximum of the PL peak by ⬇6–8 nm was measured. C. Cathodoluminescence of thin film quantum dot composites
Cathodoluminescence originates from the interaction of incident high-energy electrons with the lattice creating secondary electrons and electron–hole pairs, which may recombine radiatively and emit photons. To understand this process it is important to know a few additional parameters, such as the electron penetration depth and subsequent electron–hole pair distribution profile. Figure 5 shows the depth-dose profile g(z), calculated from the universal empirical formulas of Everhart and Hoff,26 as a function of the electron penetration depth in ZnS. For the range of QDC thicknesses we used 共⬃0.5–1 m兲, the maximum CL conversion is anticipated for incident electron energies of ⬃10–15 kV. The inset in Fig. 5 shows the dependence of the CL spectrum 共intensity兲 on the acceleration voltage 共⬀ energy兲 of the electron beam, with a saturation occurring at ⬃15–20 kV. This result is in very good agreement with the electron absorption profile. The saturation is not due to charging in the film, since SEM
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FIG. 8. Influence of temperature on the cathodoluminescence degradation over time for the sample QDC16. 共a兲 305 K, 共b兲 250 K, 共c兲 150 K, and 共d兲 50 K. FIG. 6. CL intensity decay with time for QDC16 and QDC21 at different values of electron beam voltage and current. 共a兲 QDC16: a1-30 kV, 40 nA, a2-30 kV, 20 nA, a3-30 kV, 2 nA, a4-5 kV, 5 nA, 共b兲 QDC21: b1-30 kV, 80 nA, b2-30 kV, 20 nA, b3-30 kV, 2 nA, b4-10 kV, 20 nA. The insets in both figures show the absence of recovery after interruption of the electron irradiation for 80 s.
images showed that such effects are absent in these samples. Similar results were obtained with QDCs deposited on conducting Si共100兲 substrates. Figures 6共a兲 and 6共b兲 show the time decay of the CL signal for samples QDC16 and QDC21 when irradiated with an electron beam at several acceleration voltages and current densities. The decay of the signal is more pronounced at higher voltage and current. The curves show three steps 共or modes兲. The first is a fast exponential decay with time scales smaller than 10 s, followed by a small rise 共or stabilization兲 of the intensity. The third mode, which follows at longer time, has a slow exponential decay with time scales ranging from 100 to 1000 s. Both fast and slow decay modes 共times兲 decrease with increasing operating voltage and/or current. Interruption of the electron irradiation, over intervals up to 100 min, does not result in a noticeable recovery of the CL emission. In Fig. 7 we show a logarithmic plot of the decay time constant of the slow mode versus beam current for
FIG. 7. Decay time constants obtained from the slow CL decay mode for QDC16 and QDC21 plotted as a function of the beam current.
QDC16 and QDC21; the voltage is fixed to 30 kV. The dependence has the form of ⬃I ⫺x , with x⫽1 and 0.75 for QDC16 and QDC21, respectively. Larger time constants are measured for the film with larger nanocrystals. Figure 8 shows the decay of the CL intensity with time from room temperature to 50 K for the composite QDC16. At room temperature 共curve a兲 there is a slight increase in the CL signal, after the initial fast decay. A steeper decrease in the luminescence intensity is recorded at lower temperature, with a very strong quenching of the emission at 5 K 共not shown in the graph兲. Along with the above observations, we measured a blueshift of the QD band-edge luminescence with decreasing temperature. In an attempt to check whether a recovery of the CL signal can be thermally induced, we annealed, at 120 °C in N2 and for 3 h, two samples, QDC16 and QDC21, that had been previously irradiated at 30 kV and 40 nA for ⬃30 min. Figure 9 shows that a partial recovery of the CL intensity, i.e., 30% and 10% for QDC16 and QDC21, respectively, is measured. For both samples the characteristics of the slow
FIG. 9. Partial recovery of the CL intensity of samples 共a兲 QDC16 and 共b兲 QDC21 after laser irradiation at 30 kV, 40 nA for 30 min 共continuous line兲, and after a postannealing treatment in N2 at 120 °C for 3 hr 共dashed line兲. The horizontal line is a guide to the eye.
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Rodrı´quez-Viejo et al.
J. Appl. Phys., Vol. 87, No. 12, 15 June 2000
mode remain unchanged, i.e., similar decay times are measured before and after annealing. Thus, intensity curves before and after annealing can be superimposed when using a time translation of 250 s for QDC16 and 500 s for QDC21, respectively. A similar experiment was carried at low temperatures. A composite 共QDC16兲 was cooled down to 10 K, and irradiated for 10 min at V⫽20 kV and I⫽9 nA, heated slowly to room temperature and annealed for 10 min, then cooled back to 10 K. A substantial recovery of the CL signal at low T 共by ⬃40%兲 was measured. To complement the above experiment, we attempted to probe the dynamics of carrier trap release 共from the matrix兲 by thermal stimulation, i.e., thermoluminescence. The experiment was carried out using the same SEM/CL setup, and the signal at the peak of the CL spectrum was monitored as the sample was heated from 10 K to room temperature at a rate of ⬃4.5 K/min. No signal could be measured during the temperature ramp. D. Discussion
The present data show that incorporation of the QDs in a solid ZnS matrix using ES-OMCVD at growth temperatures between 100 and 250 °C does not deteriorate the characteristics of the band-edge luminescence. The observed small redshift 共⬇10 nm兲 of the near band-edge emission of QDC films can be attributed to energy transfer from small to large nanocrystals, caused by a slight broadening in the size distribution and aggregation of the QDs in the matrix during film growth.27 The data in Fig. 3 show that composite and close-packed films exposed to laser irradiation behave differently. The initial increase in the emission from the CPFs may be attributed to a process in which carriers are rapidly transferred to fill the traps present at the inner or outer ZnS interface of the core-shell nanocrystals. It could also be associated with a photoassisted release of previously trapped electrons on the nanocrystal surface, caused by a sustained laser irradiation. This results in nanocrystal neutralization, and a slight increase in the overall PL emission. Absence of a redshift in the PL peak position, along with the similar absorption spectra before and after irradiation, indicate that large scale alloying or shrinkage of the effective radius, due to surface photo-oxidation, is absent. Composites 共QDC14 and QDC21兲, on the contrary, show a more pronounced decrease in the luminescence with time during laser irradiation. This result cannot be attributed to thermal effects, since the anticipated temperature rise due to laser irradiation is too small to induce the observed luminescence decay. Assuming that there is no heat diffusion in the sample, the temperature rise due to photon absorption in the matrix is given by14 ⌬T⬇
␣F , c T
共1兲
where ␣ is the linear absorption coefficient, F the incident fluence, c T is the specific heat capacity at room temperature, and is the film density. Using ␣ ⬇104 cm⫺1, F ⫽0.44 mJ/cm2, c T ⬇0.2 J/gK 共Ref. 28兲, and ⫽3.98 g/cm3, we obtain ⌬T⫽5 K. Such an increase cannot cause ther-
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FIG. 10. Schematic diagram showing the conduction 共valence兲 band offsets for the quantum dot composites 共a兲 and for closed-packed films 共b兲. The higher band offsets for isolated QDs may explain the difference in the photodarkening behavior.
mally induced alterations in the material. The luminescence decay in the composite films can be better explained by a mechanism of QD photoionization, due to Auger recombination and subsequent deep-level trapping of the carriers in the ZnS matrix. In such a mechanism, a nanocrystal can absorb two 共or more兲 photons at a time 共instead of one兲, creating two 共or more兲 electron–hole 共e–h兲 pairs. The energy released after recombination of one pair is transferred to a second exciton, which results in the ejection of an electron into a long-life trap in the ZnS matrix. The Auger process creates a local electric field inside the nanocrystals, which can change the internal electronic states. Therefore, during the off period in which the nanocrystal is charged the QD does not emit light even if it absorbs a photon. Electron–hole pairs would recombine nonradiatively and transfer their energy to the remaining carriers. Efros and co-workers13 showed that the ionization time, i , in a photodarkening process of SDGs depends strongly on the conduction 共valence兲 band offsets at the interface, on the nanocrystal radius, R, with i ⬀R , 5⬍ ⬍7, and on the irradiation power, P, i ⬀ P ⫺2 . These predictions are in reasonable agreement with the QDC data of Fig. 3. Namely, a faster decay of the luminescence is measured for smaller dots, with a radius dependence of the ionization time that varies as ⬃(R) 4.3. We have also observed a significantly more pronounced photodarkening when the laser fluence is increased. In close-packed samples, the presence of larger band offsets increases the barrier to carrier ejection outside the CdSe core, thus reducing the likely event 共probability兲 of nanocrystal ionization 共see Fig. 10兲. The faster degradation in glassy films 共GFP16 or GFT16兲 and the blueshift of their near band-edge peak is attributed to a photo-oxidation caused by the absence of a protecting ZnS layer on the film surface. We now focus on the cathodoluminescence data of QDC samples, and discuss the various possible processes at the origin of the signal decay, upon irradiation with energetic electrons. The similarity between the CL and PL spectra, shown in Ref. 21, proves that the two emissions originate from the same state 共near band edge兲 of the nanocrystals,
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with no contribution from the ZnS matrix. The CL spectra have a slightly larger bandwidth 共40–50 nm兲, and show a small redshift 共⬃6 nm兲 in comparison with the PL. This shift may be attributed to a Stark effect due to the presence of an electric field within the QDCs. Thermal effects on the band gap energy, resulting from power dissipation of the electron energy can be ruled out, since no changes of the CL peak maximum were observed during irradiation. The present composites are stable at temperatures up to ⬃200–250 °C 共i.e., processing values兲. The curves in Fig. 5 show that the electron penetration depth and the rate of energy loss per unit length depend strongly on the energy of the incident electrons. At higher voltage, electron energy loss is mainly produced inside the glass substrate, because of the small composite thickness. During irradiation, the electrons lose a fraction of their energy primarily by exciting the inner atomic shells, leaving a trail of fast secondary electrons and excited ions in the film. The electrons with energies up to ⬇50 eV are at the origin of e–h pair formation in the nanocrystals. Theoretical calculations by Kane,29 show that the energy loss to e–h pair creation is larger than the one required in a photoexcitation process. The average incident kinetic energy (E 0 ) required to generate an e–h pair is approximately three times the band gap value, i.e., E 0 ⬵6 eV for the present CdSe QDs. However, due to their rather high kinetic energy, the above electrons have a high probability to directly eject valence electrons outside the cores into the matrix, thus ionizing the QDs. This process is facilitated by the small conduction band offset in the QDCs 共⬇0.7 eV兲. It provides an additional mechanism that could further decrease the CL intensity, in comparison with PL where Auger recombination seems to be the main source of decay. Increasing the CL current increases the number of secondary electrons, and the probability of direct or indirect 共Auger兲 ionization of the dots. A simple calculation shows that the density of e–h pairs created in the composite due to interaction with kinetic electrons with a primary energy E p , can be estimated from the relation30 n⫽ 共 J/1.6⫻10⫺19兲共 E p 兲 / 共 E 0 d 兲 ,
共2兲
where J is the current density of the electron beam, E p /E 0 represents the number of pairs created per incident electron, is the pair lifetime, and d is the effective penetration depth of the primary electrons. Using values of J⫽1 mA/cm2, E p ⫽20 keV, E 0 ⫽6 eV, ⫽150 ps, and d⬇1 m, we obtain n⬃3⫻1016 cm⫺3. Assuming that typical volume filled fractions of QDs in the matrix is ⬃5%–10%, we determine a QD density of ⬃1013 – 1014 cm⫺3. This value suggests that in these samples the CL process enhances the probability of electron ejection due to Auger recombination, thus producing a faster CL degradation. The data shown in Fig. 6 reflect the complexity of the CL decay during electron irradiation. The initial fast decay may be due to charging of the composite, where the injected carriers charge a large number of nanocrystals. The small increase or stabilization of the signal that follows may be thermally driven 共a new equilibrium兲, since it is not observed at low temperature. The position of the small recovery maxi-
FIG. 11. Plots of CL intensity vs the product time⫻current for the composite QDC21. The recovery maximum position is approximately constant for the different intensities. 共a兲 80 nA, 共b兲 40 nA, and 共c兲 20 nA. The voltage was fixed to 30 kV.
mum remains constant 共see Fig. 11兲 in a plot of the intensity versus the product time⫻current 共electron current normalization兲. This behavior is consistent with a process of dot ionization caused by interaction with the incident energetic electrons. An increase in the beam current would enhance the probability of the dot’s interaction with an electron, thus increasing the rate of nanocrystal ionization. This translates into a faster decay as shown in Fig. 6. Using the product time⫻current would normalize the data to equal probability 共or equal rate兲, in agreement with the behavior shown in Fig. 11. The dominance of the slow decay mode results from a sustained electroionization of the nanocrystals due to irradiation. Assuming that the nanocrystal neutralization time 共i.e., time required for an electron to return to the dot兲 does not depend on the size or on the incident beam power, the variation in the decay time constant of the slow mode could be associated with changes in the ionization time, i . The CL data does not obey the power law, ⬃ P ⫺2 , as proposed by Efros and co-workers13 for photodarkening, but instead exhibits a close to P ⫺1 dependence. We attribute the difference to the predominance of direct ionization due to interaction with energetic electrons. Nonetheless, the variation of the decay time with nanocrystal size 共see Fig. 7兲 suggests that Auger recombination contributes at least partially to the CL degradation. The ‘‘saturation’’ of the CL intensity at longer time can be attributed to filling of all the traps in the vicinity of the nanocrystals. An additional result that supports the process of QD electroionization is the influence of the ZnS crystallinity on the CL ‘‘efficiency.’’ For instance, Fig. 2 of Ref. 21 shows the difference between the CL emissions from two composites with identical QDs, grown at 100 and 250 °C. The weaker CL emission from the samples grown at 100 °C 共partially amorphous兲 can be attributed to a higher density of structural defects, which provides a larger number of electron trap in these films. The recovery of the CL signal after annealing at 120 °C 共shown in Fig. 9兲 indicates that release of electrons trapped in the matrix can be thermally induced, which confirms that the CL decay is due to nanocrystal ionization. The difference
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between the two composites, QDC16 and QDC21, may be attributed to microstructure inhomogeneities in the samples, and differences between band offsets. The low temperature data can also be attributed to ejection of electrons into deep traps 共in the matrix兲, leaving ionized dark nanocrystals 共see Fig. 8兲. The much more pronounced signal decay 共at low T兲 is due to a significant increase in the lifetime of the traps as the temperature decreases, which results in longer ionization times.13,19 The absence of CL emission during the temperature ramp when the sample is heated from 5 K to room temperature seems a priori to be in disagreement with the observed recovery signal upon annealing at 120 °C. This may be due to a few scenarios. The heating rate is too fast, and the temperature ramp interval 共from ⬃10 to ⬃300 K兲 is too modest to induce measurable signal following trap release. The ionization time is too long and the trap level is too deep to be released at low temperatures. Absence of signal during heating may also indicate that subsequent recombination of released excitons is mainly nonradiative in that temperature range. Several authors observed the presence of an energy distribution of deep and shallow traps in ZnS single crystals.28,31 Sweet and Urquhart fit the experimental thermostimulated profile by two trap levels at 18 and 22.5 meV.31 The presence of multiple traps 共deep and shallow兲 in the composite samples may explain the partial recovery of the samples and the low temperature behavior. We are presently pursuing a study of thermally stimulated emission from CdSe nanocrystals embedded in various matrices. Other experiments using various types of irradiation such as soft x-ray and more intense UV signals can also provide additional information on the trap density and levels in various environments. IV. CONCLUSIONS
We have characterized the kinetics of the photoluminescence 共PL兲 and cathodoluminescence 共CL兲 degradation of highly luminescent 共CdSe兲ZnS core-shell quantum dots, either dispersed in a ZnS matrix prepared using ES-OMCVD 共composite samples兲 or spin cast into close-packed films. We found that the PL decay is much more pronounced for composites than for close-packed films, and attributed the difference to a higher probability of nanocrystal ionization, due to an Auger recombination, when the dots are dispersed in a ZnS matrix. This is caused by the lower barrier to electron ejection into the surrounding ZnS matrix, because of the smaller band offset at the interface. The time decay of the CL signal is more complex. It has three modes: fast mode at short time, a small intermediate recovery step, and a dominant slow exponential decay mode at longer time. We found that the slow mode has linear power dependence, and it varies with nanocrystal size. This suggests that in addition to Auger recombination a rather pronounced direct ionization, due to interactions with energetic incident electrons, contribute to the luminescence decay. We measured a signal recovery after thermal annealing, which indicates that trapped electrons in the matrix can be thermally activated, thus neutralizing the quantum dots. We
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also measured a steeper decay at low temperature, and associated that with a longer ionization time, caused by a more efficient carrier trapping as the temperature is decreased. Samples with QDs embedded in an amorphous, or partially crystalline matrix, showed faster CL degradation than polycrystalline films. The above set of data prove that the degradation of the photoluminescence and cathodoluminescence signals following sustained laser and electron irradiation, respectively, originate, from ionization of the quantum dots followed by trapping in the ZnS matrix. While photoluminescence decay can be mostly attributed to an Auger ionization process, an additional high probability of direct dot ionization, following interactions with energetic electrons, characterizes the CL decay. ACKNOWLEDGMENTS
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