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PHYSICAL REVIEW B, VOLUME 65, 245327

High-energy electron irradiation effects on CdS1Àx Sex quantum dots in borosilicate glass A. V. Gomonnai, Yu. M. Azhniuk,* V. V. Lopushansky, I. G. Megela, and I. I. Turok Institute of Electron Physics, Ukrainian National Academy of Sciences, Universytetska St. 21, Uzhhorod 88000, Ukraine

M. Kranjc˘ec† Ruder Boskovic´ Institute, 54 Bijenic˘ka Cesta, P. O. Box 180, 10002 Zagreb, Croatia and Department of Geotechnical Engineering, Hallerova aleja 7, 42000 Varaz˘din, Croatia

V. O. Yukhymchuk Institute of Semiconductor Physics, Ukrainian National Academy of Sciences, Prospect Nauky 45, Kyiv 03028, Ukraine 共Received 8 February 2002; published 24 June 2002兲 The effect of 5- and 10-MeV electron irradiation and subsequent annealing upon the optical absorption of CdS1⫺x Sex quantum dots embedded in a borosilicate glass matrix is studied. Gradual smearing and vanishing of the confinement-related maxima and a blue shift of the absorption edge are observed. The transformation of the spectra depends on the quantum dot composition and size as well as on the irradiating electrons’ energy and fluence. The mechanisms responsible for the observed behavior are discussed. The radiation-induced changes can be related to ionization of the quantum dots with charge-carrier transfer between the quantum dots and the electron 共hole兲 traps in the host matrix. DOI: 10.1103/PhysRevB.65.245327

PACS number共s兲: 61.82.Rx, 78.67.Bf, 61.80.Fe

I. INTRODUCTION

Spatial confinement of charge carriers in glass-embedded CdS1⫺x Sex semiconductor quantum dots 共nanocrystals兲, resulting in the specific features in their optical spectra,1,2 has led to the extensive studies of these composite materials in the recent decade 共e.g., Refs. 3–7兲. Spectroscopic investigations enable the nanocrystal parameters to be determined and complement essentially the methods of electron microscopic studies, small-angle x-ray scattering and x-ray-absorption spectroscopy, all of them in this case possessing substantial restrictions. The properties of quantum dots embedded in a silicate-based glass matrix can be affected by both preparation conditions5–13 and external factors—temperature,4,14 pressure,4,15–18 electric field,19 and illumination by intense light.20–22 Much less investigated are irradiation-induced effects in the semiconductor-doped glasses. They have been restricted to the studies of optical absorption of x-ray irradiated CdS 共Refs. 22 and 23兲 and CdS1⫺x Sex 共Refs. 24 and 25兲 quantum dots, electron irradiation effect on the photoluminescence of CdS0.7Se0.3 nanocrystals,16 and our earlier preliminary paper26 concerning the optical absorption of electron-irradiated CdS1⫺x Sex quantum dots embedded in borosilicate glass. In optical-absorption spectra of CdS1⫺x Sex quantum dots in borosilicate glass matrix under different external effects —pressure,17,18 irradiation by intense light21 and x rays23—the absorption edge blue shift is observed. At sufficiently high pressures in CdS1⫺x Sex quantum dots even a structural phase transition from wurtzite to rocksalt phase is reported, based on x-ray diffraction, Raman scattering, and optical-absorption data.4,16 –18,27–29 The light- or x-ray irradiation-induced blue shift of the absorption edge in the quantum dots is explained by ionization of the quantum dots, the electrons leaving the microcrystals and being captured by the traps in the host matrix.21,22 As follows from the results 0163-1829/2002/65共24兲/245327共7兲/$20.00

of photoluminescence and electron spin resonance 共ESR兲 studies of x-ray irradiated CdS quantum dots in borosilicate glass, the x-ray-induced defects in the quantum dots differ from the photoinduced defects.22 Since high-energy electron irradiation can result not only in photoionization processes, but also in defects related to the displacements of CdS1⫺x Sex lattice atoms in the microcrystals, it seems interesting to perform optical studies of MeV-electron-irradiated glass-embedded CdS1⫺x Sex quantum dots. Note that in Ref. 26 only a rather limited spectral range was investigated and no data for the irradiation effects on the part of the spectrum with the confinement-related absorption maxima were presented. Therefore it is important to study the effect of high-energy particle irradiation upon the optical spectra of CdS1⫺x Sex quantum dots in a broader range in view of possible radiation-induced changes both in the quantum dots themselves and in the glass matrix. II. EXPERIMENT

CdS1⫺x Sex quantum dots were embedded in a matrix of SiO2 -B2 O3 -ZnO-K2 O-Na2 O glass by conventional solidstate precipitation technique generally similar to those described in Refs. 6,8 and 9. 10⫻10-mm2 plates were prepared for optical measurements, their thickness being varied from 0.13 to 3 mm. The samples were irradiated on a M-30 electron accelerator 共Institute of Electron Physics, Ukr. Nat. Acad. Sci., Uzhhorod, Ukraine兲 with an electron beam, the fluence density ranging from 6.3⫻108 to 6.0 ⫻109 cm⫺2 s⫺1 providing the irradiating electron fluence from 1011 to 1015 cm⫺2 . The energy of the incident electrons was 5 or 10 MeV, the beam at the sample surface being monoenergetic within 0.5 MeV. The irradiation was performed at room temperature. In order to avoid the samples heating, during the irradiation they were cooled by liquidnitrogen vapor.

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FIG. 1. Raman spectra of CdS0.4Se0.6 共1兲 and CdS0.22Se0.78 共2兲 quantum dots embedded in borosilicate glass matrix.

The interval between the irradiation and the measurements did not exceed 2 h. Optical absorption in the spectral range from 300 to 1000 nm was measured at room temperature on a LOMO MDR-23 monochromator with FEU-62 and FEU-100 phototubes. Raman measurements were carried out at room temperature on a LOMO DFS-24 monochromator with a cooled FEU-136 phototube using an Ar⫹ laser (␭ ⫽488.0 nm) as the excitation source. III. RESULTS

In order to determine the chemical composition of CdS1⫺x Sex quantum dots, embedded in the glass matrix, the Raman-scattering technique was used. Raman spectra of the investigated samples are shown in Fig. 1. Since the phonon spectra of CdS1⫺x Sex mixed crystals are characterized by the two-mode type of compositional transformation, the mixed crystal spectra contain bands, corresponding to both components of the solid solution, and, as shown in Ref. 30, their spectral position can be used for characterization of the chemical composition of the quantum dots. As seen from Fig. 1, in the investigated samples of CdS1⫺x Sex quantum dots in the glass matrix two distinct peaks of LO1 共CdSe兲 and LO2 共CdS兲 phonons are observed. Having compared their spectral position with the results of Refs. 30–32, we determined the composition of the quantum dots as x⫽0.60 ⫾0.03 and x⫽0.78⫾0.03 for the samples, corresponding to the spectra 1 and 2, respectively, in Fig. 1. Figure 2 presents the transformation of the opticalabsorption spectra of CdS0.4Se0.6 and CdS0.22Se0.78 quantum dots in the glass matrix under 10-MeV electron irradiation. In the spectra of nonirradiated samples 共solid curves兲 the additional maxima at the absorption edge, related to the quantum-size effects, are observed. These singularities in the microcrystal absorption spectra are known to arise in case

FIG. 2. Optical absorption spectra of CdS0.4Se0.6 共a兲 and CdS0.22Se0.78 共b兲 quantum dots in borosilicate glass matrix, irradiated at room temperature with 10-MeV electrons.

the semiconductor quantum dot characteristic size in the glass matrix does not exceed the exciton Bohr radius.1 Having assumed the real part of the dielectric constant of the quantum dots as well as the oscillator strength per state to be size independent,9 one can obtain the following expression for the energy position of the absorption maxima: E (n e ,l e ),(n h ,l h ) ⫽E b ⫹

ប2 2r 2

冉

␸ n e ,l e m* e

⫹

␸ n h ,l h m* h

冊

,

共1兲

where E b is the bulk energy gap, r is the quantum dot radius, ␸ n,l are spherical Bessel function roots for the corresponding quantum numbers, m e* and m h* are the effective masses of electrons and holes, respectively. In CdS1⫺x Sex the latter parameters can be determined using the known values m * e ⫽0.18m 0 and m * h ⫽0.53m 0 for x⫽0 共Ref. 33兲 and m e* 34,35 ⫽(0.11–0.13)m 0 and m * h ⫽(0.44–0.63)m 0 for x⫽1. The performed calculations enabled us to derive from the experimentally observed spectra the average radius ¯r ⫽2.8 nm for CdS0.4Se0.6 and ¯r ⫽3.1 nm for CdS0.22Se0.78 quantum dots. As seen from Fig. 2, irradiation by 10-MeV electrons results in gradual smearing and vanishing of the confinementrelated features in the spectra and the blue shift of the absorption edge in the glass-embedded CdS1⫺x Sex quantum dots. These effects are seen to increase with the fluence ⌽ of

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the irradiating electrons up to 1014 cm⫺2 . However, further increase of ⌽ up to 1015 cm⫺2 does not result in any noticeable changes of the spectra. It should be noted that while scaling the absolute values of absorption coefficient ␣ , we mean the absorbance of the composite samples 共borosilicate glass with the quantum dots embedded兲 not of the quantum dots themselves. In fact, the absorbance of the microcrystals should be much higher, since the effective thickness of the quantum dots themselves is considerably smaller than the sample thickness, taking into account the concentration of CdS1⫺x Sex in the samples which follows from the content of the relevant components in the initial mixture 共about 1%兲. As follows from the Mie scattering formalism, in the case when the particles are small compared to the wavelength of the electromagnetic probe and the particles comprise a small fraction of the total sample volume, the absorption coefficient for a dilute suspension of small spherical particles in a nonabsorbing host material is given by5

␣⫽

18␲ N s Vn 30 ⑀ 2 ␭ 关共 ⑀ 1 ⫹2n 20 兲 2 ⫹ ⑀ 22 兴

,

共2兲

where N s is the number density of spheres of volume V, ␭ is wavelength, ⑀ 1 and ⑀ 2 are the real and imaginary components of the dielectric function of the material of the spheres, and n 0 is the refractive index of the host dielectric matrix. As follows from Ref. 5, for CdS1⫺x Sex quantum dots diluted in borosilicate glass ␣ ⬇0.6f ␣ qd , where ␣ qd is the absorption coefficient of the quantum dots and f ⫽N s V is the volume fraction of the nanocrystals in the composite. Hence the actual absorption coefficient of the quantum dots is about two orders of magnitude higher than that of the composite, displayed at the ␣ axis. One more point should be taken into account, which concerns the determination of the band gap value E g in the quantum dots. Generally it is calculated from the experimental spectra by extrapolation of the ␣ 2 (h ␯ ) plot for allowed direct optical transitions. However, especially in disordered materials, when the density-of-states tails may smear the shape of the absorption edge, while studying the edge variation under different factors, often a substitutive parameter E g␣ is introduced, being determined as the energy position of the fixed absorbance value ␣ f . 36 Usually for bulk materials ␣ f is taken of the order of 102 –103 cm⫺1 . In our case, due to the above discussed difference in the absorbance of the composite samples ␣ and the actual absorbance of the quantum dots ␣ qd , we have chosen for defining E ␣g the absorbance level ␣ f ⫽25 cm⫺1 . As seen from Fig. 2, the maximal irradiationinduced blue shift of the absorption edge ⌬E ␣g in CdS1⫺x Sex quantum dots, embedded in borosilicate glass matrix, reached 0.13 eV. Contrary to CdS0.4Se0.6 , where the confinement-related absorption maxima vanish already after irradiation by ⌽ ⫽1013 cm⫺2 关Fig. 2共a兲兴, in CdS0.22Se0.78 quantum dots the maxima, though smaller in intensity, are still visible even at ⌽⫽1015 cm⫺2 关Fig. 2共b兲兴. Meanwhile, their energy positions are also affected by irradiation, which is illustrated by


FIG. 3. Dependence of the energy positions of the confinementrelated maxima in the optical-absorption spectra of CdS0.22Se0.78 quantum dots on the fluence of the irradiating 10-MeV electrons.

Fig. 3. The similar behavior was observed in the same material under x-ray irradiation.25 The edge blue shift value depends not only on the irradiation dose, proportional to the electron fluence ⌽, but also on the energy of the irradiating electrons E, which is illustrated by Fig. 4 where the absorption spectra of CdS0.4Se0.6 quantum dots in the same host matrix, irradiated by equal fluences (⌽⫽1014 cm⫺2 ) of 5- and 10-MeV electrons, are shown. Another irradiation-induced feature, which is manifested in the absorption spectra of the glass-embedded CdS0.4Se0.6 quantum dots, is a rather broad absorption band centered at 1.9 eV 共Fig. 4兲, whose intensity increases with ⌽. In silicate glasses, exposed to radiation capable of ionizing the matrix or impurities, radiation-induced color centers are known to be formed, responsible for the absorption bands in ultraviolet, visible, and near-infrared spectral range.37 Therefore one could expect the 1.9-eV band to originate from a such radiation-induced color center in the host matrix. For this purpose we have carried out the similar studies for

FIG. 4. Optical-absorption spectra of CdS0.4Se0.6 quantum dots in borosilicate glass matrix, irradiated at room temperature with the fluence of ⌽⫽1014 cm⫺2 electrons of different energy E.

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FIG. 6. Effect of isochronal 共20 min兲 annealing on the absorption spectra of CdS0.4Se0.6 quantum dots embedded in borosilicate glass matrix irradiated with 10-MeV electrons (⌽⫽1015 cm⫺2 ).

FIG. 5. 10-MeV electron irradiation effect on the opticalabsorption spectra of the alkali zinc borosilicate glass matrix 共a兲 and the irradiation-induced absorption increment 共solid line兲, approximated by three Gaussian contours 共dashed lines兲共b兲. The dotted line in 共b兲 shows the superposition of the Gaussian contours.

the SiO2 -B2 O3 -ZnO-K2 O-Na2 O glass of the same composition without quantum dots, the results being presented in Fig. 5. As seen from the figure, 10-MeV electron irradiation leads to the formation of three types of color centers in the glass, the corresponding absorption bands being centered at 1.9, 3.1, and 4.6 eV with the halfwidths of 0.4, 1.1, and 1.2 eV, respectively. Radiation-induced defects in solids can be removed by annealing. We have performed isochronal 共20 min兲 annealing studies within the temperature range 300– 650 K for the electron-irradiated (⌽⫽1015 cm⫺2 ) CdS1⫺x Sex quantum dots in the borosilicate glass matrix as well as the host matrix itself. The recovery of the initial spectrum of CdS0.4Se0.6 quantum dots in the course of annealing can be seen in Fig. 6. The dependences of the relevant parameters 共irradiationinduced decrement of the absorption intensity in the first confinement-related maximum ⌬ ␣ 1 and the absorption edge blue shift ⌬E g␣ for the sample with the quantum dots as well as the intensity of the 1.9-eV centered absorption band in the borosilicate glass兲 are shown in Fig. 7. Note that the annealing studies were carried out for the samples after storage at room temperature during 250–500 h. The post-irradiational relaxation in the samples has resulted in a slight recovery of the irradiation-induced features in the spectra 共the decrease of ⌬E ␣g due to the relaxation can be seen from Fig. 7共a兲. The

post-irradiational relaxation processes are exponential, being manifested mostly during the first 24 h after the irradiation. As seen from Fig. 7, the irradiation-induced blue shift of the absorption edge as well as the intensity of the first confinement-related maximum exhibit a temperature interval of recovery at annealing within 425– 600 K. Meanwhile, the intensities of the absorption bands due to radiation-induced color centers in the host matrix reveal the similar annealing behavior within somewhat broader temperature range 关Fig. 7共c兲兴. IV. DISCUSSION

The observed behavior of the glass-embedded CdS1⫺x Sex quantum dots absorption spectra under irradiation with MeV electrons and subsequent isochronal annealing can be generally attributed to a number of factors. The gradual smearing of the size-quantum singularities can result from the irradiation-induced ionization of the quantum dots due to electron 共hole兲 transfer between the nanocrystals and irradiation-activated electron 共hole兲 traps in the host matrix. With the increase of the irradiation dose the transferred charge carriers occupy the confinement-related levels in the quantum dots, gradually disabling the lower-energy transitions. Therefore in CdS0.22Se0.78 quantum dots the irradiation-induced changes affect the lower-energy singularities much stronger than the higher-energy maxima. In the case of CdS0.4Se0.6 nanocrystals the discussed effect results in the observed blue shift of the absorption edge. Note that a similar effect 共the absorption edge blue shift兲 was observed in glass-embedded CdS 共Ref. 22兲 and CdS1⫺x Sex 共Refs. 24 and 25兲 quantum dots under x-ray irradiation as well as in CdSe quantum dots under intense light irradiation,22 though the transformation of the confinementrelated maxima is reported only in Refs. 24 and 25 and resembles the behavior reported here under MeV-electron irradiation. However, there are at least two more factors, which can possibly be responsible for the absorption edge blue shift in the irradiated CdS1⫺x Sex nanocrystals that should be discussed. At normal conditions, CdS1⫺x Sex quantum dots in borosilicate glass matrix already sustain hydrostatic pressure

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HIGH-ENERGY ELECTRON IRRADIATION EFFECTS ON . . .

FIG. 7. Dependences of the absorption edge shift ⌬E ␣g 共a兲 and irradiation-induced decrement of absorbance in the first confinement-related maximum at 2.2 eV 共b兲 on the isochronal annealing temperature for CdS0.4Se0.6 quantum dots embedded in borosilicate glass matrix irradiated with 10-MeV electrons (⌽ ⫽1015 cm⫺2 ). Figure 7共c兲 shows the annealing behavior of the integrated intensities of the additional absorption bands in the host matrix irradiated at room temperature by the fluence ⌽ ⫽1015 cm⫺2 of 10-MeV electrons: dark triangles, the band at 1.9 eV; open squares, 3.1 eV; dark squares, 4.6 eV. The arrows denote the corresponding values for the nonirradiated 共1兲, as-irradiated 共2兲, and 250-h stored 共3兲 sample.

from the matrix which is caused by the difference in the values of thermal-expansion coefficients for the glass and CdS1⫺x Sex . 5,16 High-energy electron irradiation can result in additional hydrostatic pressure upon the quantum dots due to the known phenomenon of radiation swelling of the glass matrix resulting from mechanical strains caused by ion displacements in the vicinity of radiation defects in glass.38 The increase of the pressure up to the values of the order of 4 – 8 GPa 共depending on the composition and size of CdS1⫺x Sex quantum dots and the type of the host matrix兲 can lead to the


phase transition from the wurtzite to rocksalt crystalline structure.4,15–18,27–29 Such transition from the direct-gap to the indirect-gap semiconductor is revealed in the opticalabsorption spectra of the quantum dots as smoothing of the absorption edge and vanishing of the confinement-related maxima.28 From the studies of the absorption spectra of the identical samples of CdS1⫺x Sex quantum dots in borosilicate glass matrix under hydrostatic pressure we have determined the pressure-induced energy gap shift dE g /d p values, equal to 0.041 eV/GPa for CdS0.4Se0.6 and 0.039 eV/GPa for CdS0.22Se0.78 . 39 Hence if one assumes the irradiationinduced blue shift of the absorption edge to result solely from the hydrostatic pressure due to the radiation swelling of the host matrix, then the observed maximal blue shift value of 0.13 eV in CdS0.4Se0.6 , achieved at ⌽⫽1014 cm⫺2 of 10-MeV electrons, would correspond to the additional hydrostatic pressure of 3.2 GPa. Having taken into account that at room temperature CdS1⫺x Sex quantum dots already sustain hydrostatic pressure from the matrix of about 0.5 GPa,16,40 the overall pressure would be close to the values, required for the structural phase transitions. This seems rather impressive, since the behavior of the absorption spectra of CdS0.4Se0.6 nanocrystals at high irradiation doses 共the blue shift is observed along with the smoothing of the edge and vanishing of the confinement-related singularities兲 is very much similar to that observed at the pressure-induced phase transition from the direct-gap wurtzite phase to the indirectgap rocksalt structure.28 However, if hydrostatic pressure were the major factor, explaining the observed behavior of the CdS1⫺x Sex quantum dots absorption spectra under irradiation, this should result in the distinct blue shift not only of the absorption edge, but also of the confinement-related features, similarly to Refs. 27,29 and 39. Meanwhile, in our case no blue shift of the absorption maxima is observed. Contrary, in the CdS0.22Se0.78 sample at least some of the confinement-related maxima clearly exhibit a redshift 共see Fig. 3兲. Hence the irradiation-induced pressure can hardly be responsible for the observed effects. It should be also noted that Silvestri and Schroeder have found no considerable effect of 10-MeV irradiation upon the behavior of CdS0.7Se0.3 quantum dots in borosilicate glass matrix under hydrostatic pressure.16 Generally, the observed changes in the optical-absorption spectra of CdS1⫺x Sex quantum dots under electron irradiation could be possibly related to the formation of radiation defects inside the quantum dots themselves. This may seem reasonable, since due to the small amount of Cd, S, and Se atoms in the quantum dot even a single defect 共vacancy, interstitial, antisite兲 could provide a considerable defect concentration in the quantum dot. However, in our opinion, this possibility in our case should be neglected. First-order Raman-scattering studies of bulk CdS1⫺x Sex have shown no irradiation-induced lattice disorder features even at irradiation with ⌽ up to 1018 cm⫺2 of 10-MeV electrons, only some redistribution of intensities being observed in resonant Raman and photoluminescence spectra.41 The opticalabsorption spectra of the bulk CdS1⫺x Sex crystals also remain unchanged at ⌽⫽1017 cm⫺2 .42 Therefore, since the

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irradiation-induced defect concentration is proportional to the fluence of the irradiating electrons, and having taken into account that the effective area of the quantum dots subject to irradiation, is by about an order of magnitude smaller than the whole sample area, it is hardly believable that such considerable changes in the optical spectra as observed here could result from the radiation defects formed at ⌽ ⫽1013 –1014 cm⫺2 , even if one assumes that the probability of the radiation defect formation in the quantum dots is much higher than in the bulk crystals of the same composition. It should be also noted that, since the host borosilicate matrix in our case contains a considerable 共13 mass %兲 amount of ZnO, the blue shift of the absorption edge can also be related to the irradiation-assisted incorporation of zinc into the quantum dots where it can substitute Cd, forming quaternary Cd1⫺y Zny S1⫺x Sex mixed nanocrystals. Such possibilities are reported to occur at thermal annealing of CdS1⫺x Sex quantum dots in Zn-containing silicate glass matrix.9–11 However, the completely reversible character of the absorption spectrum variation observed at the postirradiation annealing excludes the possibility of irradiationinduced Zn diffusion from the glass to CdS1⫺x Sex quantum dots. Note that the isochronal annealing temperature in our case 共below 600 K兲 is much lower than the temperatures at which zinc diffusion into the nanocrystals is reported 关 T ⬎850 K 共Refs. 10 and 11兲兴. Hence the irradiation-induced ionization of the glassembedded CdS1⫺x Sex quantum dots is the most probable mechanism responsible for the observed effects in the absorption spectra. It should be noted that the effect of nanocrystal ionization has been reported earlier for photoexcitation of solution-based single CdSe quantum dots, observed from luminescence blinking,43,44 followed by direct observation of positively charged photoionized nanocrystals by electrostatic force microscopy.45,46 A dynamic model of tunneling between core and trapped charged states was proposed to explain the statistics of the blinking rates observed.47 The

*Electronic address : [email protected] †

Electronic address : [email protected] 1 Al.L. Efros and A.L. Efros, Fiz. Tekh. Poluprovodn. 16, 1209 共1982兲关Sov. Phys. Semicond. 16, 772 共1982兲兴. 2 A.I. Ekimov and A.A. Onushchenko, Pis’ma Zh. E´ksp. Teor. Fiz. 40, 337 共1984兲关JETP Lett. 40, 1136 共1984兲兴. 3 A.I. Ekimov, F. Hache, M.C. Schanne-Klein, D. Ricard, C. Flytzanis, I.A. Kudryavtsev, T.V. Yazeva, A.V. Rodina, and Al.L. Efros, J. Opt. Soc. Am. B 10, 100 共1993兲. 4 J. Schroeder and P.D. Persans, J. Lumin. 70, 69 共1996兲. 5 P.D. Persans, L.B. Lurio, J. Pant, H. Yu¨kselici, G. Lian, and T.M. Hayes, J. Appl. Phys. 87, 3850 共2000兲. 6 A. Ekimov, J. Lumin. 70, 1 共1996兲. 7 A. Roy and A.K. Sood, Solid State Commun. 97, 97 共1996兲. 8 B.G. Potter and J.H. Simmons, Phys. Rev. B 37, 10 838 共1988兲. 9 H. Yu¨kselici, P.D. Persans, and T.M. Hayes, Phys. Rev. B 52, 11763 共1995兲. 10 M. Rajalakshmi, T. Sakuntala, and A.K. Arora, J. Phys.: Condens. Matter 9, 9745 共1997兲. 11 P.D. Persans, L.B. Lurio, J. Pant, G. Lian, and T.M. Hayes, Phys.

correlation between the luminescence blinking and spectral diffusion in single CdSe quantum dots was found.48 The measurements, performed for capped nanocrystals, have shown different rates of the photoionization slowdown depending on the nanocrystal shell material.46,49 Ionization of solution-based CdSe quantum dots in the ZnS matrix not only by laser excitation, but also by 1– 40-keV electrons, resulting in photoluminescence and cathodoluminescence decay, was also reported.49 Therefore further studies of both photo- and electron irradiation-induced ionization of solution-based nanocrystals as well as CdS1⫺x Sex quantum dots embedded in different glass matrices seem interesting in view of deeper elucidation of the shell 共matrix兲 influence. V. CONCLUSIONS

While investigating the transformation of the opticalabsorption spectra of CdS1⫺x Sex quantum dots embedded in a borosilicate glass matrix under high-energy electron irradiation, we have observed smearing and vanishing of the confinement-related maxima, accompanied by a considerable 共up to 0.13 eV兲 blue shift of the absorption edge. The shift value and the character of the confinement-related maxima transformation are seen to depend on the nanocrystal composition and size as well as on the energy and fluence of the incident electrons. Annealing at 425–550 K results in the recovery of the initial absorption edge and the confinementrelated maxima as well as the vanishing of the radiationinduced absorption band at 1.9 eV which is revealed due to the formation of color centers in the glass matrix. The consideration of possible mechanisms, responsible for the observed behavior of the glass-embedded CdS1⫺x Sex quantum dots absorption spectra under electron irradiation, enabled us to conclude that the discussed effects can be attributed to irradiation-assisted ionization of the quantum dots with charge-carrier transfer between the quantum dots and the electron 共hole兲 traps in the host matrix. Rev. B 63, 115320 共2000兲. T.M. Hayes, L.B. Lurio, J. Pant, and P.D. Persans, Solid State Commun. 117, 627 共2001兲. 13 T.M. Hayes, L.B. Lurio, and P.D. Persans, J. Phys.: Condens. Matter 13, 425 共2001兲. 14 H. Yu¨kselici and P.D. Persans, J. Non-Cryst. Solids 203, 206 共1996兲. 15 M.R. Silvestri and J. Schroeder, Phys. Rev. B 50, 15 108 共1994兲. 16 M. Silvestri and J. Schroeder, J. Phys.: Condens. Matter 7, 8519 共1995兲. 17 T. Makino, M. Arai, K. Matsuishi, S. Onari, and T. Arai, J. Phys.: Condens. Matter 10, 10 919 共1998兲. 18 T. Makino, M. Arai, S. Onari, K. Matsuishi, and T. Arai, Phys. Status Solidi B 211, 317 共1999兲. 19 K.L. Stokes and P.D. Persans, Phys. Rev. B 54, 1892 共1996兲. 20 L. Banyai and S.W. Koch, Phys. Rev. Lett. 57, 2722 共1986兲. 21 A.I. Ekimov and A.L. Efros, Phys. Status Solidi B 150, 627 共1988兲. 22 V.Ya. Grabovskis, Ya.Ya. Dzenis, A.I. Ekimov, A.I. Kudryavtsev, M.N. Tolstoi, and U.T. Rogulis, Fiz Tverd. Tela 共Leningrad兲 31, 272 共1989兲. 12

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