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Zinc incorporation into CdTe quantum dots in glass
Materials Chemistry and Physics 119 (2010) 218–221
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Zinc incorporation into CdTe quantum dots in glass M.H. Yükselici a,∗ , C¸. Allahverdi a , H. Athalin b a b
Yıldız Technical University, Faculty of Science and Letters, Department of Physics, 34010 Davutpas¸a–Topkapı, Istanbul, Turkey Univ Nantes, Inst Mat Jean Rouxel, CNRS, 2 Rue Houssiniere, F-44322 Nantes, France
a r t i c l e
i n f o
Article history: Received 21 March 2009 Received in revised form 15 July 2009 Accepted 25 August 2009 PACS: 78.67.Bf 78.67.−n 68.65.Hb Keywords: Quantum dots Resonant Raman spectroscopy CdTe nanocrystals Zinc incorporation
a b s t r a c t We report zinc incorporation into CdTe nanoparticles grown by two-step solid phase precipitation in commercial borosilicate glass quenched from the melt, based on a co-evaluation of the results of resonant Raman and optical absorption measurements. Resonant Raman spectra display a two-peak structure at wave-number positions corresponding to ternary compound Znx Cd1−x Te. We attribute the higher intensity peak between 190 and 195 cm−1 to the first harmonic of the zone-center longitudinal optical mode (LO1 ) and, the lower intensity peak between 157 and 160 cm−1 to the second harmonic (LO2 ) of Znx Cd1−x Te crystal. The wave-number of vibrational Raman modes indicates that zinc content varies between 39 and 50% during the growth of quantum dots. The asymptotic absorption edge against heattreatment time plot extrapolates to a bulk band gap of 1.714 eV which sets a lower limit of 31% for zinc incorporated into quantum dots which is consistent with the results of resonant Raman measurements. The energetic position of asymptotic absorption edge of 1.592 eV and an additional unresolved weak structure in Raman spectrum between 166 and 182 cm−1 observed for as-received glass might serve as a evidence for the occurrence of a different nanocrystalline phase with 13% zinc content. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The optical emission/absorption of semiconductor quantum dots (QDs) or nanocrystals is sensitive to the electronic band gap which depends on the size of the nanostructure [1–13]. This distinctive characteristic of quantum dots has been an indispensable precondition for recent technological applications in opto-electronics and biology [14–17]. The wavelength of sizeselective luminescent emission is directly proportional to the square of the radius. As a consequence, these tiny crystals can be used as fluorescent labels in biomedical systems. On the other hand, the size-dependent absorption depending on the wavelength of incident light is expected to improve the conversion efficiency of solar radiation into electricity when QDs are employed in photovoltaic cells [18]. The binary and ternary group II–VI semiconductor QDs have been grown in glass for a quarter century. The robust CdSx Se1−x (0 ≤ x ≤ 1) QDs in glass have been well studied in a model system for quantum confinement effect of the charged carriers in zero dimension [2–5,12–15]. Another member of group II–VI QDs, CdTe, is especially important because its band gap is close to the optimum wavelength for the conversion of solar radiation into electricity. Studies show that it is possible to grow CdTe QDs in glass with high quality optical properties compared to that of
∗ Corresponding author. Tel.: +90 212 383 4270; fax: +90 212 383 4106. E-mail address: [email protected] (M.H. Yükselici). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.08.057
CdSx Se1−x QDs [6,8,9,11,19,20]. The growth kinetics of CdTe QDs in glass were studied by Barbosa et al. [8] who found that a twostep heat-treatment process is required to increase the particle to volume fraction and thus to optimize the optical properties of the structure. The average radii of CdTe nanoparticles embedded in a silicate film were measured by Potter et al. [6] through TEM and by Masumoto and Sonobe [9] through X-ray scattering. Their results show that there exists a linear relation between the energetic position of the first excitation peak in the optical absorption spectrum and the reciprocal of average particle radius squared for QDs with an average radius larger than 2 nm [20] as predicted by a quantized state effective mass model for charged particles in a spherical well. Ochoa et al. [11] studied photoluminescence (PL) spectra of thin film CdTe glass composites and found that PL spectra display single broad peak that undergo large blue-shifts with decreasing size. It has been recently demonstrated through pump-probe spectroscopy that CdTe nanocrystals in glass can be employed as a non-linear optical switch with a response time below 1 ps when the pump beam is chopped at a frequency of 3.1 kHz [19]. Although no study on zinc incorporation into CdTe QDs in glass through Raman spectroscopy has been reported so far to the best of our knowledge, Raman spectroscopy has been employed to monitor S-to-Se ratio in CdSx Se1−x and the Zn content in CdS QDs in glass during precipitation. Tu and Persans [21] demonstrated that the frequency difference between CdS-like and CdSe-like Raman modes varies nearly linearly with sulfur (x) content in CdSx Se1−x QDs in glass so that Raman scattering can be used as a
M.H. Yükselici et al. / Materials Chemistry and Physics 119 (2010) 218–221
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compositional probe of II–VI ternary semiconductor QDs. In a more recent work on CdSx Se1−x QDs in glass, Bellani et al. [22] found that the sulfur content determined from Raman shift agrees well with the selected Se-to-S composition mole fraction in the initial growth batch and they concluded that Raman spectroscopy is a reliable methodology to get compositional information. Persans et al. [23] observed progressive Raman shifts up to 14 cm−1 above that of bulk CdS in a Zn- and CdS-doped glass and proposed that most of the change in Raman frequency is due to Zn incorporation in CdS particles based on their comparative evaluations of X-ray absorption fine structure, optical absorption and Raman spectroscopy. X-ray diffraction has not been an efficient method for QDs in glass systems to determine the composition because glass matrix within which QDs are embedded at a volume fraction of <1% scatters X-rays too much and therefore covers the signal from the nanocrystals. HRTEM gives overall concentration of bath materials within glass and can be employed to get direct information on size and size distribution of QDs. In this work we report zinc incorporation into CdTe QDs in glass during growth based on a co-evaluation of the results of resonant Raman and optical absorption measurements. 2. Experimental In this study, highly luminescent Znx Cd1−x Te nanocrystals were grown by a two-step heat-treatment process from commercially available RG850 Schott filter glass. Typical base glass composition of Schott color filters is given as 52% SiO2 , 20% K2 O, 20% ZnO and 5% B2 O3 [5]. A set of samples sliced from the same native (as-received) glass were melted at 1000 ◦ C, quickly quenched to room temperature and then heat-treated at 550 ◦ C for 16 h to relieve stress and to initiate nucleation (primary heat-treatment). A series of different samples were prepared through heattreatment at 590 ◦ C for different periods of time from 2 to 126 h (5.25 days) to grow different size quantum dots (secondary heat-treatment). Each sample was characterized by optical absorption and Raman spectroscopies. The details of sample preparation, optical absorption spectroscopy, temperature dependence of optical absorption edge and photo-absorption spectroscopy in which the transmittance of the sample is measured while the sample is being excited by an intense (laser) light have been published elsewhere [20,24]. Representative absorption spectra are shown in Fig. 1. We note that the onset of the optical absorption edge shifts slightly to lower energies between 48 and 126 h of heat-treatment time and stops to shift at ∼1.7 eV which is ∼200 meV above the bulk band gap of CdTe of ∼1.5 eV. The origin of this difference will be associated with zinc content based on Raman results discussed below. The straight-line portions of the optical absorption coefficient squared (˛2 ) against energy curves are extrapolated to obtain the absorption edge energies (Eedge ) as intercepts with the ˛2 = 0 line. We plot Eedge as a function of secondary heat-treatment time for heat-treated samples in Fig. 2. The diffusion-
Fig. 2. Absorption edge energy (Eedge ) plotted against secondary heat-treatment time (t). The curve through the data points () is a fit given by Eq. (1).
limited growth which acts at the earlier stages of the secondary heat-treatment process when the concentration of reactants (Cd, Te or Zn) is above the critical limit predicts square-root dependence for the average radius on time: Rave ≈ t1/2 and the Ostwald ripening at work at the later stages a cubic-root dependence: Rave ≈ t1/3 [25–30]. We note that the diameters of CdTe QDs studied in this work remain within the size range of ∼5–10 nm as calculated by relating the optical absorption edge 2 energy to average radius (Rave ), that is, Eedge (eV) = Eg (eV) + 0.376/([Rave (nm)] ), where is the reduced mass of charged particles and Eg , bulk band gap [31,32]. Since the quantized-state effective mass model in the strong confinement regime −2 , predicts a linear relation between the confinement energy E = Eedge − Eg and Rave the absorption edge energy will have contributions from two growth regimes with a time dependence tn (n = −1, −2/3)), Eedge = Eg + C1 t −1 + C2 t −2/3
(1)
where the first term represents the bulk band gap of the crystal and, C1 and C2 are constants. We fit Eedge against time plot in Fig. 2 with a curve given in Eq. (1) and find that Eedge converges to the bulk band gap of Eg = 1.714 eV as time goes to infinity. Our high resolution resonant Raman scattering measurements were performed at a regulated room temperature of ∼20 ◦ C in air, essentially by using a single frequency 514.5 nm (2.41 eV) line from an Ar+ laser. The laser beam was focused by using the 100× dry objective of a Raman microscope with a microprocessor controlled XY moving stage and the power was kept below 1 mW to avoid heating by the beam. We could spectrally resolve each selected sampling area within the objective detection volume and Raman spectra of CdTe QD’s with a 514.5 nm excitation wavelength were measured again at the same spot. The scattered light was dispersed through the JY-T64000 triple-monochromator system. The Raman spectra were collected using a liquid nitrogen cooled back-thinned charge coupled device. A SuperNotch Plus filter was positioned before the entrance slit to attenuate the Rayleigh scatter by over six orders of magnitude and allowed the collection of data at Stokes shifts as low as 100 cm−1 . Measurements with the Raman spectrometer were recorded with 0.5 cm−1 resolution. So we were able to observe the Raman spectra for CdTe QD’s resonant with 514.5 nm laser line over the entire range of QD diameters present in the sample. In Fig. 3, we show Raman spectra for 12 samples studied in this work. The Raman spectra for doubly heat-treated samples possess a two-peak structure; a lower intensity peak between 157 and 160 cm−1 and a higher intensity peak between 190 and 195 cm−1 . We note that as-received RG850 Schott filter glass displays a weaker two-peak structure at 154 cm−1 and at 195 cm−1 , and seems to have an additional unresolved weak structure between 166 and 182 cm−1 . It is not possible to discern the weak Raman peaks within that range.
3. Discussion
Fig. 1. Representative absorption spectra for as-received color filter glass samples melted at 1000 ◦ C for 15 min and quickly quenched marked ‘melted’, then heattreated at 550 ◦ C for 16 h marked ‘one step’ (primary heat-treatment) and finally at 590 ◦ C for 2, 12, 48, 126 h (secondary heat-treatment).
In Fig. 4 we plot the Raman peak positions, determined by fitting the experimental data to Gaussian line shapes and linear backgrounds, against heat-treatment time. The higher intensity peak is blue-shifted from ∼190 to ∼195 cm−1 during the first 12 h of secondary heat-treatment time and then red-shifted slowly back to its initial value of ∼190 cm−1 . The lower intensity peak shows no definite trend. Previous studies show that Raman peaks shift
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where ωLO is the LO phonon frequency of the bulk crystal and, A a parameter which depends on the difference between thermal expansion coefficient of the glass host and the nanocrystal, and on temperature. B is related to the phonon negative dispersion and the size-dependent surface tension. First of all, A is constant since the composition of the glass host and the heat-treatment temperature remain fixed for all the samples studied in this work. We do not expect the size-dependent third term (B/R2 ) produce a significant Raman shift based on previous reports for II–VI nanocrystals in glass [40,41]. In a more recent study by Yang and Li [35] the sizedependent Raman shift was calculated with the help of a model based on the size dependence of root-mean-square average amplitude of atomic thermal vibration for a series of narrow and wide band gap semiconductor QDs. They found that the Raman redshift is less than 1 cm−1 for CdSe QDs with a diameter between 5 and 10 nm. However, zinc incorporation (ejection) into (out of) the nanocrystal during precipitation can alter significantly LO phonon vibrational frequency. The dependence of LO peak positions in cm−1 on zinc content (0 ≤ x ≤ 1) for bulk Znx Cd1−x Te crystal might be described by
Fig. 3. Raman spectra for as-received color filter glass samples melted at 1000 ◦ C for 15 min and quickly quenched marked ‘melted’, then heat-treated at 550 ◦ C for 16 h marked ‘one step’ (primary heat-treatment) and finally at 590 ◦ C for from 2 to 126 h (secondary heat-treatment). The samples were excited with a laser line at 514.5 nm.
due to three mechanisms: phonon confinement [33–35], lattice contraction [36–38] and/or zinc incorporation into nanocrystals [39,23]. Phonon confinement leads to a red-shift and lattice contraction to a blue-shift in the Raman peak position. Neither phonon confinement nor lattice contraction alone can therefore explain both the blue-shift during the first 12 h and the red-shift afterward observed in Fig. 4 for the higher intensity Raman peak during the secondary heat-treatment process. The size-dependent phonon frequency including peak shift due to phonon confinement and the peak shift due to lattice contraction is given by [36] ω(R) = ωLO − A −
B R2
(2)
ωLO1 (x) = −31.322x2 + 72.169x + 166.73
(3a)
ωLO2 (x) = 13.020x2 − 25.602x + 166.73
(3b)
Eqs. 3 were obtained by digitizing the empirical phonon frequency against zinc content curves given in Ref. [42]. No zinc content leads to LO phonon frequency of 166.73 cm−1 for x = 0. Since the Raman spectra for doubly heat-treated samples in Fig. 2 display a two-peak structure at wavelength positions corresponding to ternary compound Znx Cd1−x Te, we attribute the higher intensity peak between 190 and 195 cm−1 to the first harmonic of the zonecenter longitudinal optical mode (LO1 ) and the lower intensity peak between 157 and 160 cm−1 to the second harmonic (LO2 ) of bulk Znx Cd1−x Te crystal. We single out LO1 phonon vibrational modes to calculate the zinc content (x) from Eq. (3a) because the estimated uncertainty in LO2 peak positions is greater. The peak positions of LO1 vibrational modes indicate that the zinc content (x) of the nanocrystals varies as 0.39 < x < 0.50 [42]. We presume that ternary Znx Cd1−x Te nanocrystals nucleate in glass with initial zinc content of 39%. The initial and final zinc content remains the same. It is proposed [39] that the increasing Zn content is related to the depletion of Cd in the glass which shifts the thermodynamic equilibrium so that more Zn can be incorporated into crystallites. At the earlier stages where diffusion-limited growth is at work, the LO1 band shifts by ∼5 cm−1 to greater values with longer heat-treatment time up to 12 h while the shift in LO2 band remains almost constant within the experimental errors. Since the peak position of LO1 band shifts to greater values with x, zinc incorporation into the nanoparticles increases with heat-treatment time and reaches a maximum at 12 h at the early stages of particle growth. Above 12 h of heat-treatment time, zinc content decreases monotonically at the later stages. The evolution of optical absorption spectra with heat-treatment supports the presumption of zinc incorporation into nanocrystals. Zinc incorporation competes with size-dependent red-shift in the asymptotic absorption edge (Eedge ) during the early stages of growth up to 12 h, because the bulk band gap of Znx Cd1−x Te crystal is blue-shifted with zinc according to [43] Eg (eV) = 1.510 + 0.606x + 0.139x2
−1
Fig. 4. Raman peak positions between 190 and 195 cm (LO1 mode) and between 157 and 160 cm−1 (LO2 mode) plotted against the secondary heat-treatment time. The error bars represent the estimated uncertainty in the LO positions. The lines between data points are the guide for the eye to follow.
(4)
We extract a value of x = 0.31 or 31% zinc from Eq. (4) when it is solved for the value of Eg = 1.714 eV determined from Eq. (1) as time goes to infinity. This sets a lower limit for the zinc incorporated into the nanocrystals which is consistent with the results of resonant Raman spectra.
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We find the optical absorption edge of Eedge = 1.592 eV for asreceived RG850 Schott filter glass. A maximum zinc incorporation (x) of 13% into CdTe quantum dots is determined when Eq. (4) is solved for x by inserting Eg = 1.592 eV. The unresolved weak structure between 166 and 182 cm−1 for as-received glass in Fig. 3 might be due to a nanocrystalline phase with 13% zinc content. This might seem inconsistent with the conclusion of a minimum zinc content of 31% present in CdTe QDs prepared from the native glass, as drawn from the discussion above. We bring an explanation to this as follows. First of all we note that it is misleading to compare optical absorption and Raman spectra of native glass with those of the samples originated from its melt through quenching and twostage heat-treatment process since we do not know thermal history of the native glass. The optical absorption bands for QDs broaden inhomogeneously due to size distribution and therefore overlap. The onset of the optical absorption edge energy depends on the size and stoichiometry of QDs. That is why the optical absorption band edge is sensitive to the stoichiometry of the crystalline phase with the lowest absorption edge energy for a given size. However all of the crystalline phases with different compositions will appear in the Raman spectrum since the frequency of phonon vibrations depends on composition of QDs. While the size, size distribution and stoichiometry of QDs are revealed in the optical absorption, the composition and the crystalline structure of QDs are pronounced in the Raman spectrum. Therefore, the optical absorption spectrum of as-received glass as shown in Fig. 1 might be dominated by a nanocrystalline phase with 13% zinc content which is suppressed by a different phase with a minimum zinc content of 31% in Raman spectra. Dominant phase with 31% zinc is screened by a weak phase in the optical absorption spectrum. 4. Conclusion We have observed the time evolution of the resonant Raman spectra for commercial borosilicate glass doped with Znx Cd1−x Te. The diffusion-limited growth of CdTe quantum dots begins with initial 39% zinc incorporation. Zinc gets its maximum value of 50% within first 12 h of secondary heat-treatment time and then gradually decreases back to its initial percentage. The energetic position of optical absorption edge sets a lower limit for a maximum zinc content of 31% which is consistent with the results of resonant Raman measurements. The energetic position of asymptotic absorption edge of 1.592 eV and an additional unresolved weak structure in Raman spectrum between 166 and 182 cm−1 observed for as-received glass might serve as evidence for the occurrence of a different nanocrystalline phase with 13% zinc content. Acknowledgements
References [1] [2] [3] [4] [5] [6] [7] [8]
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43]
This work is supported by Yıldız Technical University Research Projects Coordination under Project Nos. 23-01-01-01 and 24-0101-03 and in part by TÜBI˙ TAK under the Project No. TBAG-AY/378 (104T119).
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Zinc incorporation into CdTe quantum dots in glass M.H. Yükselici a,∗ , C¸. Allahverdi a , H. Athalin b a b
Yıldız Technical University, Faculty of Science and Letters, Department of Physics, 34010 Davutpas¸a–Topkapı, Istanbul, Turkey Univ Nantes, Inst Mat Jean Rouxel, CNRS, 2 Rue Houssiniere, F-44322 Nantes, France
a r t i c l e
i n f o
Article history: Received 21 March 2009 Received in revised form 15 July 2009 Accepted 25 August 2009 PACS: 78.67.Bf 78.67.−n 68.65.Hb Keywords: Quantum dots Resonant Raman spectroscopy CdTe nanocrystals Zinc incorporation
a b s t r a c t We report zinc incorporation into CdTe nanoparticles grown by two-step solid phase precipitation in commercial borosilicate glass quenched from the melt, based on a co-evaluation of the results of resonant Raman and optical absorption measurements. Resonant Raman spectra display a two-peak structure at wave-number positions corresponding to ternary compound Znx Cd1−x Te. We attribute the higher intensity peak between 190 and 195 cm−1 to the first harmonic of the zone-center longitudinal optical mode (LO1 ) and, the lower intensity peak between 157 and 160 cm−1 to the second harmonic (LO2 ) of Znx Cd1−x Te crystal. The wave-number of vibrational Raman modes indicates that zinc content varies between 39 and 50% during the growth of quantum dots. The asymptotic absorption edge against heattreatment time plot extrapolates to a bulk band gap of 1.714 eV which sets a lower limit of 31% for zinc incorporated into quantum dots which is consistent with the results of resonant Raman measurements. The energetic position of asymptotic absorption edge of 1.592 eV and an additional unresolved weak structure in Raman spectrum between 166 and 182 cm−1 observed for as-received glass might serve as a evidence for the occurrence of a different nanocrystalline phase with 13% zinc content. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The optical emission/absorption of semiconductor quantum dots (QDs) or nanocrystals is sensitive to the electronic band gap which depends on the size of the nanostructure [1–13]. This distinctive characteristic of quantum dots has been an indispensable precondition for recent technological applications in opto-electronics and biology [14–17]. The wavelength of sizeselective luminescent emission is directly proportional to the square of the radius. As a consequence, these tiny crystals can be used as fluorescent labels in biomedical systems. On the other hand, the size-dependent absorption depending on the wavelength of incident light is expected to improve the conversion efficiency of solar radiation into electricity when QDs are employed in photovoltaic cells [18]. The binary and ternary group II–VI semiconductor QDs have been grown in glass for a quarter century. The robust CdSx Se1−x (0 ≤ x ≤ 1) QDs in glass have been well studied in a model system for quantum confinement effect of the charged carriers in zero dimension [2–5,12–15]. Another member of group II–VI QDs, CdTe, is especially important because its band gap is close to the optimum wavelength for the conversion of solar radiation into electricity. Studies show that it is possible to grow CdTe QDs in glass with high quality optical properties compared to that of
∗ Corresponding author. Tel.: +90 212 383 4270; fax: +90 212 383 4106. E-mail address: [email protected] (M.H. Yükselici). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.08.057
CdSx Se1−x QDs [6,8,9,11,19,20]. The growth kinetics of CdTe QDs in glass were studied by Barbosa et al. [8] who found that a twostep heat-treatment process is required to increase the particle to volume fraction and thus to optimize the optical properties of the structure. The average radii of CdTe nanoparticles embedded in a silicate film were measured by Potter et al. [6] through TEM and by Masumoto and Sonobe [9] through X-ray scattering. Their results show that there exists a linear relation between the energetic position of the first excitation peak in the optical absorption spectrum and the reciprocal of average particle radius squared for QDs with an average radius larger than 2 nm [20] as predicted by a quantized state effective mass model for charged particles in a spherical well. Ochoa et al. [11] studied photoluminescence (PL) spectra of thin film CdTe glass composites and found that PL spectra display single broad peak that undergo large blue-shifts with decreasing size. It has been recently demonstrated through pump-probe spectroscopy that CdTe nanocrystals in glass can be employed as a non-linear optical switch with a response time below 1 ps when the pump beam is chopped at a frequency of 3.1 kHz [19]. Although no study on zinc incorporation into CdTe QDs in glass through Raman spectroscopy has been reported so far to the best of our knowledge, Raman spectroscopy has been employed to monitor S-to-Se ratio in CdSx Se1−x and the Zn content in CdS QDs in glass during precipitation. Tu and Persans [21] demonstrated that the frequency difference between CdS-like and CdSe-like Raman modes varies nearly linearly with sulfur (x) content in CdSx Se1−x QDs in glass so that Raman scattering can be used as a
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compositional probe of II–VI ternary semiconductor QDs. In a more recent work on CdSx Se1−x QDs in glass, Bellani et al. [22] found that the sulfur content determined from Raman shift agrees well with the selected Se-to-S composition mole fraction in the initial growth batch and they concluded that Raman spectroscopy is a reliable methodology to get compositional information. Persans et al. [23] observed progressive Raman shifts up to 14 cm−1 above that of bulk CdS in a Zn- and CdS-doped glass and proposed that most of the change in Raman frequency is due to Zn incorporation in CdS particles based on their comparative evaluations of X-ray absorption fine structure, optical absorption and Raman spectroscopy. X-ray diffraction has not been an efficient method for QDs in glass systems to determine the composition because glass matrix within which QDs are embedded at a volume fraction of <1% scatters X-rays too much and therefore covers the signal from the nanocrystals. HRTEM gives overall concentration of bath materials within glass and can be employed to get direct information on size and size distribution of QDs. In this work we report zinc incorporation into CdTe QDs in glass during growth based on a co-evaluation of the results of resonant Raman and optical absorption measurements. 2. Experimental In this study, highly luminescent Znx Cd1−x Te nanocrystals were grown by a two-step heat-treatment process from commercially available RG850 Schott filter glass. Typical base glass composition of Schott color filters is given as 52% SiO2 , 20% K2 O, 20% ZnO and 5% B2 O3 [5]. A set of samples sliced from the same native (as-received) glass were melted at 1000 ◦ C, quickly quenched to room temperature and then heat-treated at 550 ◦ C for 16 h to relieve stress and to initiate nucleation (primary heat-treatment). A series of different samples were prepared through heattreatment at 590 ◦ C for different periods of time from 2 to 126 h (5.25 days) to grow different size quantum dots (secondary heat-treatment). Each sample was characterized by optical absorption and Raman spectroscopies. The details of sample preparation, optical absorption spectroscopy, temperature dependence of optical absorption edge and photo-absorption spectroscopy in which the transmittance of the sample is measured while the sample is being excited by an intense (laser) light have been published elsewhere [20,24]. Representative absorption spectra are shown in Fig. 1. We note that the onset of the optical absorption edge shifts slightly to lower energies between 48 and 126 h of heat-treatment time and stops to shift at ∼1.7 eV which is ∼200 meV above the bulk band gap of CdTe of ∼1.5 eV. The origin of this difference will be associated with zinc content based on Raman results discussed below. The straight-line portions of the optical absorption coefficient squared (˛2 ) against energy curves are extrapolated to obtain the absorption edge energies (Eedge ) as intercepts with the ˛2 = 0 line. We plot Eedge as a function of secondary heat-treatment time for heat-treated samples in Fig. 2. The diffusion-
Fig. 2. Absorption edge energy (Eedge ) plotted against secondary heat-treatment time (t). The curve through the data points () is a fit given by Eq. (1).
limited growth which acts at the earlier stages of the secondary heat-treatment process when the concentration of reactants (Cd, Te or Zn) is above the critical limit predicts square-root dependence for the average radius on time: Rave ≈ t1/2 and the Ostwald ripening at work at the later stages a cubic-root dependence: Rave ≈ t1/3 [25–30]. We note that the diameters of CdTe QDs studied in this work remain within the size range of ∼5–10 nm as calculated by relating the optical absorption edge 2 energy to average radius (Rave ), that is, Eedge (eV) = Eg (eV) + 0.376/([Rave (nm)] ), where is the reduced mass of charged particles and Eg , bulk band gap [31,32]. Since the quantized-state effective mass model in the strong confinement regime −2 , predicts a linear relation between the confinement energy E = Eedge − Eg and Rave the absorption edge energy will have contributions from two growth regimes with a time dependence tn (n = −1, −2/3)), Eedge = Eg + C1 t −1 + C2 t −2/3
(1)
where the first term represents the bulk band gap of the crystal and, C1 and C2 are constants. We fit Eedge against time plot in Fig. 2 with a curve given in Eq. (1) and find that Eedge converges to the bulk band gap of Eg = 1.714 eV as time goes to infinity. Our high resolution resonant Raman scattering measurements were performed at a regulated room temperature of ∼20 ◦ C in air, essentially by using a single frequency 514.5 nm (2.41 eV) line from an Ar+ laser. The laser beam was focused by using the 100× dry objective of a Raman microscope with a microprocessor controlled XY moving stage and the power was kept below 1 mW to avoid heating by the beam. We could spectrally resolve each selected sampling area within the objective detection volume and Raman spectra of CdTe QD’s with a 514.5 nm excitation wavelength were measured again at the same spot. The scattered light was dispersed through the JY-T64000 triple-monochromator system. The Raman spectra were collected using a liquid nitrogen cooled back-thinned charge coupled device. A SuperNotch Plus filter was positioned before the entrance slit to attenuate the Rayleigh scatter by over six orders of magnitude and allowed the collection of data at Stokes shifts as low as 100 cm−1 . Measurements with the Raman spectrometer were recorded with 0.5 cm−1 resolution. So we were able to observe the Raman spectra for CdTe QD’s resonant with 514.5 nm laser line over the entire range of QD diameters present in the sample. In Fig. 3, we show Raman spectra for 12 samples studied in this work. The Raman spectra for doubly heat-treated samples possess a two-peak structure; a lower intensity peak between 157 and 160 cm−1 and a higher intensity peak between 190 and 195 cm−1 . We note that as-received RG850 Schott filter glass displays a weaker two-peak structure at 154 cm−1 and at 195 cm−1 , and seems to have an additional unresolved weak structure between 166 and 182 cm−1 . It is not possible to discern the weak Raman peaks within that range.
3. Discussion
Fig. 1. Representative absorption spectra for as-received color filter glass samples melted at 1000 ◦ C for 15 min and quickly quenched marked ‘melted’, then heattreated at 550 ◦ C for 16 h marked ‘one step’ (primary heat-treatment) and finally at 590 ◦ C for 2, 12, 48, 126 h (secondary heat-treatment).
In Fig. 4 we plot the Raman peak positions, determined by fitting the experimental data to Gaussian line shapes and linear backgrounds, against heat-treatment time. The higher intensity peak is blue-shifted from ∼190 to ∼195 cm−1 during the first 12 h of secondary heat-treatment time and then red-shifted slowly back to its initial value of ∼190 cm−1 . The lower intensity peak shows no definite trend. Previous studies show that Raman peaks shift
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where ωLO is the LO phonon frequency of the bulk crystal and, A a parameter which depends on the difference between thermal expansion coefficient of the glass host and the nanocrystal, and on temperature. B is related to the phonon negative dispersion and the size-dependent surface tension. First of all, A is constant since the composition of the glass host and the heat-treatment temperature remain fixed for all the samples studied in this work. We do not expect the size-dependent third term (B/R2 ) produce a significant Raman shift based on previous reports for II–VI nanocrystals in glass [40,41]. In a more recent study by Yang and Li [35] the sizedependent Raman shift was calculated with the help of a model based on the size dependence of root-mean-square average amplitude of atomic thermal vibration for a series of narrow and wide band gap semiconductor QDs. They found that the Raman redshift is less than 1 cm−1 for CdSe QDs with a diameter between 5 and 10 nm. However, zinc incorporation (ejection) into (out of) the nanocrystal during precipitation can alter significantly LO phonon vibrational frequency. The dependence of LO peak positions in cm−1 on zinc content (0 ≤ x ≤ 1) for bulk Znx Cd1−x Te crystal might be described by
Fig. 3. Raman spectra for as-received color filter glass samples melted at 1000 ◦ C for 15 min and quickly quenched marked ‘melted’, then heat-treated at 550 ◦ C for 16 h marked ‘one step’ (primary heat-treatment) and finally at 590 ◦ C for from 2 to 126 h (secondary heat-treatment). The samples were excited with a laser line at 514.5 nm.
due to three mechanisms: phonon confinement [33–35], lattice contraction [36–38] and/or zinc incorporation into nanocrystals [39,23]. Phonon confinement leads to a red-shift and lattice contraction to a blue-shift in the Raman peak position. Neither phonon confinement nor lattice contraction alone can therefore explain both the blue-shift during the first 12 h and the red-shift afterward observed in Fig. 4 for the higher intensity Raman peak during the secondary heat-treatment process. The size-dependent phonon frequency including peak shift due to phonon confinement and the peak shift due to lattice contraction is given by [36] ω(R) = ωLO − A −
B R2
(2)
ωLO1 (x) = −31.322x2 + 72.169x + 166.73
(3a)
ωLO2 (x) = 13.020x2 − 25.602x + 166.73
(3b)
Eqs. 3 were obtained by digitizing the empirical phonon frequency against zinc content curves given in Ref. [42]. No zinc content leads to LO phonon frequency of 166.73 cm−1 for x = 0. Since the Raman spectra for doubly heat-treated samples in Fig. 2 display a two-peak structure at wavelength positions corresponding to ternary compound Znx Cd1−x Te, we attribute the higher intensity peak between 190 and 195 cm−1 to the first harmonic of the zonecenter longitudinal optical mode (LO1 ) and the lower intensity peak between 157 and 160 cm−1 to the second harmonic (LO2 ) of bulk Znx Cd1−x Te crystal. We single out LO1 phonon vibrational modes to calculate the zinc content (x) from Eq. (3a) because the estimated uncertainty in LO2 peak positions is greater. The peak positions of LO1 vibrational modes indicate that the zinc content (x) of the nanocrystals varies as 0.39 < x < 0.50 [42]. We presume that ternary Znx Cd1−x Te nanocrystals nucleate in glass with initial zinc content of 39%. The initial and final zinc content remains the same. It is proposed [39] that the increasing Zn content is related to the depletion of Cd in the glass which shifts the thermodynamic equilibrium so that more Zn can be incorporated into crystallites. At the earlier stages where diffusion-limited growth is at work, the LO1 band shifts by ∼5 cm−1 to greater values with longer heat-treatment time up to 12 h while the shift in LO2 band remains almost constant within the experimental errors. Since the peak position of LO1 band shifts to greater values with x, zinc incorporation into the nanoparticles increases with heat-treatment time and reaches a maximum at 12 h at the early stages of particle growth. Above 12 h of heat-treatment time, zinc content decreases monotonically at the later stages. The evolution of optical absorption spectra with heat-treatment supports the presumption of zinc incorporation into nanocrystals. Zinc incorporation competes with size-dependent red-shift in the asymptotic absorption edge (Eedge ) during the early stages of growth up to 12 h, because the bulk band gap of Znx Cd1−x Te crystal is blue-shifted with zinc according to [43] Eg (eV) = 1.510 + 0.606x + 0.139x2
−1
Fig. 4. Raman peak positions between 190 and 195 cm (LO1 mode) and between 157 and 160 cm−1 (LO2 mode) plotted against the secondary heat-treatment time. The error bars represent the estimated uncertainty in the LO positions. The lines between data points are the guide for the eye to follow.
(4)
We extract a value of x = 0.31 or 31% zinc from Eq. (4) when it is solved for the value of Eg = 1.714 eV determined from Eq. (1) as time goes to infinity. This sets a lower limit for the zinc incorporated into the nanocrystals which is consistent with the results of resonant Raman spectra.
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We find the optical absorption edge of Eedge = 1.592 eV for asreceived RG850 Schott filter glass. A maximum zinc incorporation (x) of 13% into CdTe quantum dots is determined when Eq. (4) is solved for x by inserting Eg = 1.592 eV. The unresolved weak structure between 166 and 182 cm−1 for as-received glass in Fig. 3 might be due to a nanocrystalline phase with 13% zinc content. This might seem inconsistent with the conclusion of a minimum zinc content of 31% present in CdTe QDs prepared from the native glass, as drawn from the discussion above. We bring an explanation to this as follows. First of all we note that it is misleading to compare optical absorption and Raman spectra of native glass with those of the samples originated from its melt through quenching and twostage heat-treatment process since we do not know thermal history of the native glass. The optical absorption bands for QDs broaden inhomogeneously due to size distribution and therefore overlap. The onset of the optical absorption edge energy depends on the size and stoichiometry of QDs. That is why the optical absorption band edge is sensitive to the stoichiometry of the crystalline phase with the lowest absorption edge energy for a given size. However all of the crystalline phases with different compositions will appear in the Raman spectrum since the frequency of phonon vibrations depends on composition of QDs. While the size, size distribution and stoichiometry of QDs are revealed in the optical absorption, the composition and the crystalline structure of QDs are pronounced in the Raman spectrum. Therefore, the optical absorption spectrum of as-received glass as shown in Fig. 1 might be dominated by a nanocrystalline phase with 13% zinc content which is suppressed by a different phase with a minimum zinc content of 31% in Raman spectra. Dominant phase with 31% zinc is screened by a weak phase in the optical absorption spectrum. 4. Conclusion We have observed the time evolution of the resonant Raman spectra for commercial borosilicate glass doped with Znx Cd1−x Te. The diffusion-limited growth of CdTe quantum dots begins with initial 39% zinc incorporation. Zinc gets its maximum value of 50% within first 12 h of secondary heat-treatment time and then gradually decreases back to its initial percentage. The energetic position of optical absorption edge sets a lower limit for a maximum zinc content of 31% which is consistent with the results of resonant Raman measurements. The energetic position of asymptotic absorption edge of 1.592 eV and an additional unresolved weak structure in Raman spectrum between 166 and 182 cm−1 observed for as-received glass might serve as evidence for the occurrence of a different nanocrystalline phase with 13% zinc content. Acknowledgements
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This work is supported by Yıldız Technical University Research Projects Coordination under Project Nos. 23-01-01-01 and 24-0101-03 and in part by TÜBI˙ TAK under the Project No. TBAG-AY/378 (104T119).
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