For the fabrication of nanoparticle containing optical fibers by melt and draw technique, nanoparticle stability at high temperatures is a requirement. We report the synthesis of quantum dots at temperatures as high as 1000 °C, compatible with fiber drawing, stabilized for the first time by a prior low temperature heating step. It is observed that quantum dots formed by this two step heating leads to a better emission stability at high powers associated with a reversible phenomenon, making these nanomaterials suitable for further technological applications.
© 2013 OSA
Semiconductor devices based on quantum confinement effect are of great technological importance in optoelectronic applications like laser emitters, optical communications, switching elements, photodetectors and nanocrystalline solar cells, due to their large nonlinear optical properties and fast response times (< 1 ps) [1–4]. PbS, due to its optical band gap of 0.41 eV (at room temperature), a large Bohr excitonic radius of 18 nm, nearly equivalent electron-hole effective masses and peculiar emission range (1-3 μm), is a useful material for infrared (IR) applications [5–9].
The precipitation of semiconductor nanostructured crystallites in a glass matrix makes it possible to obtain new materials that combine the useful properties of both the glass and the crystalline phase. Moreover, incorporating these QDs in glass leads to additional advantage of chemical and mechanical stability, preventing them from agglomerating, a major drawback in other chemical synthetic routes [10–12]. Despite being an exquisitely suitable candidate for optical fiber applications, QDs in glass have not been transferred to fiber fabrication due to their temperature dependent deterioration of optical properties at high temperatures [5, 6]. Nanoparticle stability with the increase in temperature is a requirement for various applications and still remains a challenge for the scientific research community. Great advances in nanoparticle stability by colloidal methods have taken place, where metal nanoparticles have been stabilized to survive temperatures as high as 850°C for catalytic applications [13–15]. Au nanoparticles stable upto 930°C have been fabricated on Si substrates, applicable as a template for growth of other nanomaterials . To date the high temperature stability of nanoparticles in glass (for optical fiber fabrication by melt and draw technique) has not been investigated in depth.
In order to overcome this limitation we report here the fabrication of PbS quantum dots at temperatures as high as 1000°C in silicate glass matrix. The emission is stabilized by a prior low temperature (550 °C) heat treatment step. It is observed that two-step heating leads to better nanoparticle formation at high temperatures. We investigate the change in emission properties of PbS quantum dots embedded in silicate glass with the change in heating time and temperature. Also, the behavior of QDs emission with the incident laser intensity is investigated. Henceforth the reversibility and stability of the emission behavior is checked by subsequent exposure of the quantum dots to the maximum pump power (576 mW).
2. Glass fabrication
The glass specimen with a nominal composition of 50SiO2-25Na2O-10BaO-5Al2O3-8ZnO-2ZnS-0.8PbO (in mol %) was prepared by the melt-quenching method. Starting powders were mixed and melted at 1350 °C for 30 minutes in alumina crucible with cover under ambient atmosphere. The melt was poured into a brass mold to prepare the glass rod with diameter of 1 cm and length of 10 cm (Fig. 1 (a)). The brass mold was preheated at 300 °C to avoid crack formation in the glass preform. The glass rod thus made was annealed at 350 °C for 3 hours to reduce the thermal stress. A part of this preform was cut into pieces of 3 mm thickness to study their absorption and emission characteristics versus heat-treatment conditions.
3. Absorption and photoluminescence results of PbS QDs and discussion
The 3 mm thick pieces of preform were then heat treated at various temperatures for different times to precipitate PbS QDs. It was found that from ~470 - 550 °C nanoparticle formation takes place. This range of nanoparticle formation temperature is very critical and a major shift in size and optical properties is observed on a slight variation. The samples are heated gradually at these temperatures. The furnace used for heating reached the required temperature in about 45 mins to 1 hr. The temperature increased step by step to reach the required temperature. In gradual heating we keep the sample inside the furnace right at the beginning. The sample goes through the heating to reach the required temperature followed by the time required in our series of experiments. So we call this heating as gradual heating as the sample is brought to a particular temperature stepwise.
Upon the formation of nanoparticles in glass, color of glass changes from yellow to brown to black with the increase in the heating time as illustrated in Fig. 1(b) at a particular temperature, in the good range. TEM micrograph in Fig. 1(c) shows the nanoparticle formation at 470 °C for 53 hr, demonstrating a ~5 nm size with good crystallinity. This evolution is confirmed by the absorption spectra plotted in Fig. 2(a) where an excitonic feature is visible in all the samples, characteristic of quantum dots (QDs) formation. Notice that the onset of absorption is shifted to the red with increasing heating time and temperature which is attributed to an increase in the particle size from 3.3 - 7.0 nm diameter. Also a large blue shift of the absorption bands from the bulk band gap energy of PbS (0.41 eV) clearly exhibits the quantum confinement effect. The corresponding emission spectra were obtained by using a green CW-laser (maximum power used of 576 mW) coupled into these preheatedglass pieces by a × 20 objective lens. The variation in emission wavelength is in correspondence to the absorption of each sample respectively (Fig. 2(b)). The emission wavelength shifts also to the red with the increase in particle size with the increase in temperature or time. The emission wavelength was tunable from 1000 to 1600 nm which is manifested by the emission spectra.
In order to check their spectral dependence with the increase in laser power, we have studied the effect of increase in pump power on these glass samples which are heated gradually. A typical spectra for gradual heating at 490 °C - 10 hrs is shown in Fig. 3(a). It is observed that with the increase in pump power the emission intensity increases up to a certain critical power (70 mW) to reach a good photoluminescence level and then decreases. The decrease is also joined by a blue shift in the emission maximum. At the maximum power (576 mW) the emission is abnormally broad and flat.
Usually the decrease in emission is attributed to photo oxidation of the QDs, but in glasses there does not seem to be any possibility of QDs exposure to oxygen or any humid atmosphere. Consequently, photoionization and Auger processes may be the possible cause of this decrease in emission intensity . In the photoionization process the electron is ejected from the nanocrystal to the glass where it is captured by the long lived centers. Also, ionization of the QDs should result in a decrease of the quantum yield due to strengthening of the Auger process resulting from the presence of extra charged particle. The auger excited charge carriers can dissipate their excess energy in different ways, including phonon emission, electron ejection or diffusion to some long-lived deep trap states that have a substantial barrier. This process can be responsible for part of the population of the deep trap states, which results in the broad band PL spectrum at maximum powers . Also, due to heat generated at the large laser powers, electron-phonon coupling becomes dominant, giving rise to a decrease in intensity [11, 19].
A blue shift of 46 nm accompanies the darkening process. Increase in laser power is associated with an increase in temperature as well. So, the blue shift could also be attributed to the temperature dependence of band gap which is well known in PbS nanoparticles . Also, it is to be noted that the emission shape is inhomogeneous, showing a reasonable size distribution. At the maximum power (576 mW), a very broad and flat emission is observed. This may be because of a large amount of defect formation taking place in QDs as well as in host glass at such high pump powers. Due to which emission not only decreases but also becomes broad and flat.
In order to check the reversibility of these QD glass samples their spectral evolution was studied after subsequent exposure to maximum power. We illuminated the QD glass by varied laser powers after exposure to the maximum power thrice with a time interval of 10 min. Figure 3(b) shows the emission spectra at three power values for the exposure for the first and the third time. A decrease in emission intensity is observed after subsequent exposures. The percent decrease at lower powers (35 mW) is more than that at higher powers (576 mW). Moreover it is noteworthy that after regular exposures, along with the QD emission, the emission from deep trap states at higher wavelengths is also observed. These results suggest that while the first emission band reflects the intrinsic property of QDs, the higher wavelength emission originates from deep trap states. Glasses are amorphous solid solutions and therefore the formation of defect states in the strained matrix volume surrounding a QD surface can be expected. Flat emission at larger wavelengths suggests that the recombination happens from the deep trap states rather than from the electron hole recombination from the confined levels .
Furthermore, no quantum dot formation takes place after 550 °C when heated gradually for hours. So in order to fabricate QDs at high temperatures we heat the glass abruptly at a particular temperature and time. First we bring the furnace to the required temperature and then introduce the sample for a particular time. So we insert the sample in the furnace after attaining the required temperature. As the sample faces the heating temperature abruptly for a particular time we coin this as abrupt heating. By this technique QDs are formed from 550 - 750 °C for heating times strongly reduced to range between 10 - 2 min. It is to be noted that the quality of nanoparticles deteriorates for temperatures ≥ 750 °C. So in order to fabricate nanoparticles at such high temperatures maintaining their illuminating nature, two step heating was proposed. Before heating the glass samples at high temperatures they were treated at 550 °C abruptly for 10 min. This prior heat treatment leads to stabilized nanoparticles at high temperatures. The as fabricated nanoparticles show homogeneous emission, which is capable of sustaining maximum pump powers and also remains reversible after subsequent exposure to maximum power. Nanoparticles were fabricated by this two-step heating in the temperature range 600 - 1000 °C for time ranging from 5 min - 40 sec. The absorption shifts in the range ~1000-1600 nm with the increase in nanoparticle size from 4.3 to 7.3 nm. The corresponding emission spectra (Fig. 4) are observed to be homogeneous indicating uniform size distribution. Also, the emission intensity of high temperature nanoparticles increased with the increase in pump power and then after 78 mW a decrease takes place. Figure 5 shows the emission spectra for 750 °C −2 min and 1000 °C-40 sec (with a initial heating at 550 °C for 10 min) quantum dot samples with the increase in pump power. However, no abnormal broadening at the maximum power is observed. The decrease inemission associated with the increase in pump power after a critical power of 78 mW does not show any blue shift up to a heating temperature of 750 °C. While there is a blue shift by 46 nm in 1000 °C samples. It is noteworthy that the emission at maximum power is still Gaussian and is not deteriorated as in gradually low temperature (470-500°C) heated samples. The emission stability is also investigated after subsequent exposure to maximum pump power. The emission spectra after subsequent exposure to maximum illumination intensity for 750 and 1000 °C heated glass samples are shown in Fig. 6. The emission seems reversible with only a slight decrease in intensity after subsequent exposure to the maximum laser powers. The emission is due to the electron hole recombination in nanoparticles, with no extra emission tail at longer wavelengths.
The emission is reversible and comfortably sustains high laser powers. Thus making clear that good quality nanoparticles can be fabricated at higher temperature limits. From this nanoparticle behavior, where the nanoparticle quality is deteriorated in terms of its emission ability, it could be said that by directly heating the glass at high temperatures, as seen for 750 °C - 2 min abrupt heating, nucleation and growth of the nanoparticles takes place at high temperatures. High growth rate at high temperatures leads to a substantial indulgence of surface states and/or the surrounding amorphous glass defects in emission characteristics. While for a two-step heating, it seems that the good quality nanoparticles formed at 550 °C - 10 min act as nuclei for growth at further high temperature treatments. Consequently, leading to stabilized nanoparticle growth, making nanoparticles capable enough of standing temperatures as high as 1000 °C and showing stable Gaussian emission at powers as high as 576 mW even after subsequent exposures.
In conclusion, tailoring of optical absorption and emission is demonstrated in QDs in glass with heating temperature and time. By one step gradual heating process, QD emission is irreversible and is attached with some deep trap emission at higher wavelengths after exposure to maximum pump powers. While QDs fabricated at high temperatures by two step heating process manifest a reversible emission with no extra emission tail at higher wavelengths. Finally, the two step heating process seems to be the key for nanoparticle fabrication at high temperatures thus making feasible their utilization for optical fiber fabrication by melt and draw technique, which will be the subject of our next publication.
This research was supported and funded by the French National Research Agency through grant JC09-489174 and the Carnot-Xlim project.
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