1. Introduction

Nanocrystals of semiconductors, or quantum dots (QDs) have attracted enormous attention for fundamental researches and practical applications due to their size-dependent photophysical and photochemical properties [1–3]. The past several decades have seen a steady increase of study on QDs preparation techniques, which allow the controlled growth of nanoparticles on colloidal solutions [4], polymers [5], as well as glasses [6]. Solutions immersed with QDs usually show high photoluminescence efficiency with narrow size dispersion, which is promising for biological label application [7]; polymers combined with QDs are usually spinned into thin films for optoelectronic devices [8]; glasses, which are thermally, chemically and mechanically stable, can protect the nanoparticles from surface oxidation, therefore are excellent hosts to design three-dimensional optical devices. QDs precipitated in glasses originated from the early 1980s, and the hosts range from borosilicate [9], phosphate [10], to silicate glasses [11], and even fused silica [12], etc. In 1990, M. Nogami et al. [13] successfully prepared CdS-doped silicate glasses through sol–gel process and studied its blue shifted absorption phenomenon compared with the bulk absorption values. J. Chang et al. [9] prepared PbSe QDs inside borosilicate glasses and investigated their absorption and photoluminescence properties in 2009.

As above-mentioned works mainly focus on the linear optical characteristics of QDs fabricated in glasses, however, with the development of laser technology, laser induced multi-photon absorption of QDs has attracted a great interest in the application fields, e.g. information technology [14-15], optical power limiting [16], laser technology [17], nonlinear gain media [18], and biological system imaging [19]. CdTe QD, due to its small band gap with strong quantum confinement effect and big Bohr exciton radius, has gained extensive attentions. Nowadays, two-photon absorption induced fluorescence of CdTe QDs synthesized in solutions has been reported [20–22]. In contrast, CdTe QDs based two-photon induced fluorescence in transparent glasses remains largely explored. In this letter, silicate glass, for its wide range optically transparent window, high hardness, strong ruggedness and thermal stability, was selected as host material. Infrared femtosecond laser was used as the excitation source and the upconversion properties of CdTe QDs were carefully investigated. By measuring the dependence of luminescence intensity on laser power, two-photon induced fluorescence was confirmed. The study about the linear and nonlinear optical properties of CdTe QDs doped in silicate glass will pave a new path to develop quantum information science and may extend the applications in functional photonic devices.

2. Experimental

Glass samples were prepared by the conventional melt-quenching technique with composition 50SiO₂-35Na₂CO₃-10ZnO-5Al₂O₃-1CdO-1ZnTe in mol% (analyzed). In the glass system, SiO₂ worked as glass network former, Al₂O₃ and ZnO acted as network intermediate, and Na₂CO₃ was network outer body. We choose CdO-ZnTe instead of CdTe in order to reduce the loss of CdTe (melting point: 1041°C) during melting process (1350°C) [23]. All the powders were mixed thoroughly in an agate mortar and melted in a closed corundum crucible for 30 minutes under ambient atmosphere of 1350°C. Melts were rapidly quenched by pouring onto a steel plate to form base glasses. The base glasses continued to be annealed at 350°C for 5h to reduce thermal stress. Subsequent heat treatment of glass was done at 480°C, which was below the glass transition temperature. Heat treatment time of 2 h, 8 h and 16 h were selected to enhance the diffusion of Cd²⁺ and Te^2- ions and promote the growth of CdTe QDs in the glass matrix. All glasses were optically polished for further studies.

The laser source was a commercial Coherent Chameleon Vision laser with pulse duration of 70 fs, operating at 800 nm, and a frequency of 80 MHz. The laser induced upconversion luminescence was recorded by a Horiba Jobin Yvon iHR 550 spectrometer. The Raman spectra, excited at 488 nm, were taken with a Renishaw inVia Raman spectrometer. Absorption spectra and excitation, emission spectra were obtained by JASCO V-570 VIS and JASCO FP-6500 spectrometer, respectively. TEM measurement was done by field emission transmission electron microscope: FEI Tecnai G2 F20 S-TWIN. All of the measurements were carried out at room temperature.

3. Results and discussion

QDs, as we know, are in quantum-confined states, where QD sizes are comparable to, or smaller than the Bohr radius of exciton, so that the electronic levels nearby Fermi level change from quasi-continuous into discrete. In QDs, the Fermi level lies between two bands, so that the edges of the bands dominate the low-energy optical and electrical behaviors. QD sizes, therefore, play a great role for allowable electronic energy levels. In the work, QDs were grown by a process of diffusion-controlled phase decomposition. High-temperature heat treatment ensures effective diffusion of Cd²⁺ and Te^2- ions in the glass matrix [24]. Through this way, CdTe QDs would coalesce when the glass sample is heated to and kept above the transformation temperature. In general, the longer heat-treated time is, the larger average size and higher concentration of QDs are. As a result, the color darkens with heat treatment time. As shown in the inset of Fig. 1, the as-made glass was slightly yellowish and transparent to the eye. With further heat-treatment time changes from 2 h, to 8 h and even 16 h, the color of glass sample turns from dark yellowish, to reddish and blackish, representing the different sizes of CdTe QDs. The absorption property of CdTe-QDs-embedded silicate glasses (hereafter labeled G-CdTe) is also illustrated in Fig. 1. For as-made glass, no obvious absorption peak was observed. However, with heat-treatment time increasing, the absolute absorbance of glass samples increased due to the absorbing transition of excitation, and the corresponding absorption bands shifted from 510 nm, to 528 nm and 560 nm. Since the lowest excited state shifts red of the bulk band gap as the QD size increased, thus providing the quantum size effect, and consequently, leading to the slight red shift in the absorption spectra.

 

Fig. 1 Absorption spectra of the as-made and heat-treated glasses. The true-color images of a series of doped glasses are shown in the right inset.

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Figure 2 shows the size dependence of the Raman spectra for G-CdTe. The background originates from the Raman signal of a silicate glass matrix and it has no significant structure in the measured frequency region. The observed peaks are ascribed to the longitudinal optical (LO) phonon peaks of CdTe QDs [25]. The LO peak of G-CdTe heat-treated from 2 h, 8 h to 16 h was measured to be about 156 cm⁻¹, 160 cm⁻¹ to 163 cm⁻¹. The peak moves towards long wavelength as a result of the growth of QDs. Meanwhile, the ratio of the intensity of the Raman signal of CdTe QDs increased due to the increasing concentration of precipitated QDs.

 

Fig. 2 Raman spectra of the as-made and heat-treated glasses.

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High-resolution transmission electron (HR-TEM) micrographs of G-CdTe heated for 2 h, 8 h and 16 h were given in Fig. 3(a)-(c). It is obvious that the QDs are evenly distributed in glass materials. As expected, the average QD-size increases with heat treatment time. And the average QD-size was estimated to be be 3.387nm, 3.648nm and 3.979nm for 2 h, 8h and 16 h heat treated glass samples, respectively. From Fig. 3(d), the lattice spacing of CdTe QDs with heat treated time of 16 h was calculated to be about 0.265 nm.

 

Fig. 3 High-resolution transmission electron (HR-TEM) micrographs of CdTe QDs doped glasses for heat treatment time of (a) 2 h, (b) 8 h and (c) 16 h, respectively. (d) is the same with (c) while with higher resolution.

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The emission properties excited by 400 nm monochromatic light and the corresponding photograph of samples were shown in Fig. 4. Under the irradiation of 400 nm light, vivid red fluorescence were seen from heat-treated glasses while the as-made glass didn’t show any visible emission. The fluorescence of heat-treated glasses are emitted due to electron-hole recombination, with central wavelength fully determined by QD-size. When basic glasses are treated at 480°C for 2 h, 8h, 16 h, QD-size increased with corresponding electronic energy level going lower. As a result, the emission peak moves to long wavelength, from 556 nm to 570 nm, and 602 nm. In addition, with heat-treated time increasing, full width at half maximum of emission spectra become wider, from 43 nm, 51 nm to 68 nm, as a result of wider distribution of QD-size.

 

Fig. 4 QD-size-dependent emission spectra of the as-made and heat-treated glasses irradiated by 400 nm lamp. The inset was the corresponding glass samples illuminated by a handheld 400 nm lamp in a dark room.

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The normalized emission spectra of G-CdTe irradiated by 400 nm light and focused 800 nm femtosecond laser were compared in Fig. 5. It is worth mentioning that the upconversion luminescence of G-CdTe heated for 2 h excited by 800 nm femtosecond laser pulses was extremely weak that it cannot be detected. In contrast, the upconversion luminescence of G-CdTe heated for 8 h and 16 h excited by 800 nm femtosecond laser (red curve) both exhibits similar spectral peak and width with the luminescence under 400 nm (double frequency) light excitation (black curve). The emission peak of G-CdTe heated for 8 h is at 570 nm, while the central wavelength for G-CdTe of 16 h heat-treatment is 602 nm. As can be seen in Fig. 1, there is no absorption peak at 800 nm, therefore it should not be a linear process. And it may be related to multi-photon absorption, which can be determined by $I\propto P^n$, where I is the intensity of upconversion luminescence, P is input laser power and n is the absorbed photon number [26].

 

Fig. 5 Normalized emission spectra of G-CdTe heat-treated for 8 h (a), 16 h (b), with the excitation of 800 nm femtosecond laser at 120mW and 400 nm lamp.

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By measuring the dependence of luminescence intensity on pump average power, the absorbed photon number n can be obtained from the linearly fitted slope of logarithmic plot of the fluorescence intensity I versus the pumping power P. As depicted in Fig. 6, the laser induced fluorescence pumped by 800 nm femtosecond laser with power ranging from 40 mW to 120 mW were recorded that the fluorescence intensity increased with pumped power. And it can be seen from the inset of Fig. 6, that the fitted slope was 1.87 which confirms the two-photon absorption of 800 nm laser pulses.

 

Fig. 6 Luminescence intensity of G-CdTe heated for 16 h at a series of femtosecond pump power. Inset: The linear fitting of the logarithmic plot of fluorescence intensity I and the pumping power P.

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As can be seen in Fig. 4(b), the emission wavelength of G-CdTe heated for 16 h was 602 nm under the irradiation of both 400 nm light and 800 nm femtosecond laser. In reverse, 602 nm was used as the detect wavelength and the corresponding excitation spectrum was obtained in Fig. 7(a). Actually, with a series of experiment, it can be seen that the fluorescence emitting with same peak and shape can be excited efficiently at any wavelength shorter than the emission peak. As depicted in Fig. 7(b), by tuning the excitation wavelength from 400 nm to 480 nm, the intensity increases but the center wavelength remains unchanged. Such an excited-wavelength independent emission was also reported in solutions immersed with QDs [27].

 

Fig. 7 (a) Excitation spectra of G-CdTe heat-treated for 16 h (λ_em = 600 nm); (b) Emission spectra of G-CdTe heat-treated for 16 h.

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Inspired by the above phenomenon, the excitation source of 960 nm femtosecond laser was also used for comparison. Figure 8(a) shows that the normalized fluorescence under both 960 nm femtosecond laser irradiation and 480 nm light excitation are almost the same, and the emission peak is still, as expected, at 602 nm. The ratio of the fluorescence intensity under 960 nm and 800 nm excitation is 0.625 t. And the slope 1.74 in the inset of Fig. 8(b) confirms that the two photons was simultaneously absorbed during the irradiation of 960 nm femtosecond laser. The verification experiment for two-photon absorption was also carried out by 960 nm femtosecond laser. The quantum yield for glass with heat treated time of 8 h and 16 h is 2.619x10⁻² and 4.293x10⁻³, respectively. However, when the excitation wavelength was tuned longer to 1000 nm, or 1050 nm (which is the longest wavelength of the Coherent laser setup), upconversion luminescence couldn’t be detected while one-photon absorption still could be excited in their double frequencies. For G-CdTe heated for 8h, upconversion luminescence even couldn’t be detected at 960 nm femtosecond laser pulses, which means that femtosecond laser induced upconversion process shows a dependence on wavelength. The mechanism needs further investigation.

 

Fig. 8 (a) Emission spectra of G-CdTe heat-treated for 16 h under focused 960 nm femtosecond laser irradiation at 65mW and 480 nm light excitation; (b) Upconversion luminescence in G-CdTe excited by 960 nm femtosecond laser pulses at different pump power. Inset: The linear fitting of log emission intensity and log pump power.

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4. Summary

In summary, a growth technique of CdTe QDs precipitated in silicate glass was developed. Raman analysis demonstrated that heat treatment does lead to the precipitation of QDs in glass matrix and the QD-size was controlled by heat-treated time. In this letter, glasses, with colors of dark yellow, red and black, were obtained through a careful adjustment with heat treatment time of 2 h, 8 h and 16 h, corresponding absorption peak of 510 nm, to 528 nm and 560 nm, Raman peak of 156 cm⁻¹, 160 cm⁻¹ and 163 cm⁻¹, and visible fluorescence at 556 nm, 570 nm, and 602 nm under 400 nm monochromic light irradiation. The red-shift effect is due to the growth of QDs, where QDs with larger size have a lower excited energy level in the quantum confinement. For the heat-treated glass, the fluorescence emitting with same peak and shape can be excited efficiently at any wavelength shorter than the emission peak. In the paper, two-photon induced luminescence of CdTe QDs in silicate glasses were confirmed using 800 nm and 960 nm femtosecond laser pulses by measuring the dependence of fluorescence intensity on laser pump power, while femtosecond laser with longer wavelength is more difficult to excite two-photon induced luminescence. The two-photon-excited luminescence of CdTe quantum dots in silicate glasses may have applications in 3D display upconversion fluorescence, etc.