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Abstract
Direct laser writing of luminescent patterns within the material consisting of the polymer (PMMA) matrix with dissolved precursor (TEDBCd) molecules is demonstrated. The luminescence here is related to the UV induced growth of CdS nanoparticles. The irradiation was performed by the third harmonic of a Nd:YAG laser with a pulse duration of 15 ns. The irradiated polymer films were kept at ambient temperatures of about 100 °C. At the maximum scanning speed, the luminescent signal is very low. A decrease in the scanning speed results in an increase in the luminescent signal and a shift of the luminescent spectrum towards longer wavelengths. However, at some speed, the increase in the luminescent signal is changed by its descent. This suggests the existence of an optimal laser exposure for the laser-induced luminescent pattering. This observation was confirmed when recording separate spots by a focused laser beam at different laser fluences. The luminescent hexagonal pattern consisting of micron-sized spots within the material bulk is obtained by means of irradiation of a single layer of closely packed ten-micron polystyrene spheres deposited on the material surface. The shape of these spots is discussed by comparing it with the calculated laser intensity space distribution within photonic jets.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Transparent polymer-based hybrid materials with photo-induced semiconductor nanoparticles are promising for various applications in flexible/transparent electronic devices [1–3]. Here nanoparticles are synthesized in-situ by means of light irradiation. A specially introduced photo-sensitive precursor is destructed under the effect of light with the consequent formation of inorganic nanoparticles due to self-organization processes. Solid materials with embedded semiconductor nanoparticles are valued for their luminescent properties [4] and long-term stability [5].
The employment of lasers allows micropatterning of the material. Here, the laser recording of luminescent micron-sized structures is a promising task [6]. The material’s homogeneity is an important condition for the creation of the luminescent structures of good quality. For this purpose, the precursor should be soluble in polymer matrices. While photoinduced nanocomposites with semiconductor nanoparticles have been investigated for more than a decade, the well-soluble precursors have been reported only recently [7,8].
The mechanisms of the nanoparticle formation in such materials are also under study. For the precursors with low enough decomposition temperature, thermally activated reaction is considered [9]. Here, the laser irradiation provides local heating of the materials initiating the nanoparticle growth process.
Another case is the photochemical process, where temperature can just influence the reaction velocity. The theory of this process has been developed and considered in detail in [10]. In our works [11,12] bis(1,1,5,5-tetraethyl-2,4-dithiobiureto)cadmium(II) [Cd(N(SCNEt2)2)2] (TEDBCd) was employed as a good soluble precursor for CdS nanoparticles in poly(methyl methacrylate) (PMMA) matrix. It was demonstrated that the precursor destruction can be considered as a photochemical reaction with the quantum yield dependent on temperature.
In the present paper, we use the same material for UV laser patterning. Here, we demonstrate two approaches for creating the luminescent microstructures. The first one is direct writing of luminescent patterns by scanning a focused laser beam over the sample [13]. Because the effect of light is mainly photochemical, we can employ laser pulses with relatively small fluences, thus avoiding the surface damage. The enhanced temperature needed for effective nanoparticle growth was controlled by a thermostat. We demonstrate the existence of an optimal scanning speed for efficient luminescent pattering and discuss fluence-dependent phenomena that were absent in our previous isothermal experiments when additional heating of material by UV light could be neglected.
The other approach is laser irradiation via a layer of closely packed dielectric microspheres deposited on the material surface. This mask acts as an array of micro lenses generating photonic jets [14]. This offers opportunity for simultaneous recording of luminescent microstructures over a wide area. Such layers have been previously employed for surface nanopatterning (see e.g. [15] and references therein), and in photoacoustic converters [16]. In the present paper, we, for the first time, employ this approach for obtaining the hexagonal structure of well-localized micron-sized luminescent jets.
2. Experimental
PMMA/TEDBCd samples were fabricated by casting from the solution in toluene onto a fused silica substrate. The mass ratio of PMMA and TEDBCd was 95:5. This material is visually transparent with no scattering, since the taken amount of precursor is well soluble in PMMA [11]. Patterning of samples was performed using the third harmonic of a Nd:YAG laser LS-2137 (355 nm) with a pulse duration of 15 ns (LOTIS TII, Belarus). During the irradiation, the samples were placed in a thermostat at about 90–100 °C, as described in detail in [12]. We studied the laser-induced formation of luminescent patterns in “thick” samples (about 200 µm) placed between fused silica glasses without air gap, and in thin films (several µm) deposited on a fused silica substrate with an open surface (no cover glass). For laser writing of luminescent lines in the material, the thermostat with a sample was mounted onto a motorized translation stage. For microsphere-assisted laser patterning, we used a 10-µm polystyrene microsphere aqueous suspension purchased from Sigma-Aldrich, Germany. A drop of the suspension was deposited on the polymer surface and left for drying. After that, a monolayer of close-packed spheres was formed.
Large-scale microscopic images were obtained with Nikon Eclipse Ci-S microscope equipped with a high-pressure mercury lamp for photoluminescence (PL) microscopy. More detailed microscopic studies including photoluminescence spectra measurements were performed with a confocal fluorescent microscope Carl Zeiss LSM 880 under 405 nm laser excitation. For each sample, we chose PL acquisition settings and kept them constant, so that we could compare the obtained PL emission quantitatively. Additionally, series of XY-images in different focus positions on the specimen were obtained to form a three dimensional data set. This allowed getting vertical cross-sections of the luminescent structures under study (subsection 3.3). The images were acquired with a 1µm step size. The obtained data were processed with Carl Zeiss ZEN software. Surface morphology of the films irradiated via a microsphere mask was studied with a Solver Pro M atomic force microscope (NT-MDT, Russia). AFM measurements were performed in a non-contact mode under normal atmospheric conditions using I-shape cantilevers NT-MDT ETALON HA_NC with tip radius less than 10 nm.
3. Results
3.1 Linear pattern
Line structure was made in a 200 µm-thick film by scanning the sample with a focused laser beam (about 30 µm in diameter) using a translation stage. Laser fluence in the center of the beam was about 115 mJ/cm2 per single pulse, and pulse repetition rate was 5 Hz. The thermostat temperature was kept at 96 ± 2 °C. We recorded four lines with the following scanning speed values: 1.25 µm/sec (v), 0.625 µm/sec (v /2), 0.313 µm/sec (v /4), and 0.156 µm/sec (v /8). A luminescent image of the obtained pattern is presented in Fig. 1(a). The lower scanning speed means the higher exposure of the irradiated area.
Fig. 1. (a) Luminescent lines written with a focused laser beam in PMMA/TEDBCd film (200 μm thickness) with different velocities: (1) v=1.25 μm/sec, (2) v /2 = 0.625 μm/sec, (3) v /4 = 0.313 μm/sec, (4) v /8 = 0.156 μm/sec. (b)-(f): PL emission spectra measured for lines written with different scanning speeds and for non-irradiated space. The arbitrary units are the same for all PL spectra. Insets: PL images of laser-written luminescent lines in PMMA/TEDBCd, white circles mark the areas for PL spectra measurements, scale bars equal 50 μm.
It can be seen that the PL emission intensity grows with the decreasing scanning speed (lines 1–3). However, line 4 looks less bright than line 3, despite the fact that it was written twice as slow. We studied luminescent properties of the written structures in detail by using confocal luminescent microscopy. The obtained PL spectra collected from a 16 µm-diameter area in the core of each line at the depth about 10 µm under the surface of the sample are presented in Fig. 1(b)-(f). Here, the laser-written lines are arranged according to the increasing laser exposure, starting with the non-irradiated area in Fig. 1(b) and ending with the line recorded at v /8 in Fig. 1(f). The corresponding PL images are shown in the insets. One can notice the change in the emitted color with changes in the scanning speed. The maximum of PL spectra in Figs. 1(c)-(f) moves from 520 nm to 610 nm with increasing laser exposure (decreasing scanning speed). The PL maximum in Figs. 1(c) and 1(d) is at the same wavelength, but the long-wavelength shoulder (570-600 nm) is more pronounced for the v /2-line.
Thus, the above experimental evidences indicate some optimal condition for laser writing. The increase in the luminescent signal with a further increase in the laser exposure leads to suppression of the PL emission accompanied by a red shift in the luminescence spectrum.
3.2 Luminescent spot formation with a focused laser beam
To consider the above phenomena in more detail, we performed laser structuring by making spots with a focused laser beam in a 200 µm-thick film. We started with laser fluences close to those used above for linear patterning.
PL images in Fig. 2 were made for the irradiation with laser fluence 120 mJ/cm2 per single pulse (beam diameter about 25 µm) at a thermostat temperature of 96 ± 2 °C. The pulse repetition rate was 5 Hz. Due to high absorption of the irradiated material at 355 nm, the modified volume looks different at different depths, with the PL signal almost disappearing at about 100 µm. Hence the PL images were obtained at the same depth of about 10 µm under the sample surface. We followed the evolution of the irradiated area with time and collected PL spectra in the center of every spot over a 4 µm diameter area. Here one can see the weakening of the PL signal for long-time irradiation, accompanied by some changes in the PL spectra shape. After 5 min irradiation, the maximum in the PL emission is at 520-530 nm, and then moves to 600 nm with the broadening of PL spectra.
Fig. 2. PL emission spectra measured in the center of laser-irradiated spot in a 200 μm-thick PMMA/TEDBCd sample at the depth of 10 μm under the surface. Corresponding PL images of the irradiated areas are given for each case (scale bars equal 20 μm). Laser pulses at 355 nm were with fluence 120 mJ/cm2 per single pulse and repetition rate 5 Hz, thermostat temperature was 96 °C. PL spectra were collected from the area with diameter 4 μm in the center of every spot (shown in the first image for 5 min). The arbitrary units are the same for all PL spectra.
Then we repeated the experiment with a lower laser fluence. Figure 3 shows the evolution of the irradiated spot at the laser fluence of 80 mJ/cm2 per pulse (beam diameter about 25 µm), pulse repetition rate of 5 Hz, and thermostat temperature of 94 ± 2 °C. Here, the photoluminescence bleaching is also observed: the spot irradiated for 40 min looks like a luminescent ring with a PL signal weaker in the center than at the periphery. However, we do not see here the red shift in the PL spectrum as it is seen in Fig. 2.
Fig. 3. PL emission spectra measured in the center of laser-irradiated spot in a 200 µm-thick PMMA/TEDBCd sample at the depth of 10 µm under the surface. Corresponding PL images of the irradiated areas are given for each case (scale bars equal 20 µm). Laser pulses at 355 nm were with fluence 80 mJ/cm2 per single pulse and repetition rate 5 Hz, thermostat temperature was 94 °C. PL spectra were collected from the area with diameter 4 µm in the center of every spot (shown in the first image for 5 min). The arbitrary units are the same for all PL spectra.
In the third experiment with laser-irradiated spots, we performed modification of a thin PMMA/TEDBCd film (4 µm thick) deposited on a fused silica plate. The sample was processed in the thermostat at 97 ± 2 °C with a single pulse laser fluence of about 320 mJ/cm2 (beam diameter about 20 µm) and pulse repetition rate of 10 Hz. Some of the PL images of the laser-irradiated spots are presented in Fig. 4(a), with the corresponding PL spectra collected in the center of every spot over a 8 µm-diameter area. Figure 4(b) shows the intensity of PL emission at 502 nm (near the maximum in PL spectra) as function of the irradiation time. It is seen that the PL intensity increased during the first 5 minutes of irradiation, reached its maximum, and then bleaching was observed after 15-min processing.
Fig. 4. (a): PL emission spectra measured in the center of laser-irradiated spot in a 4 µm-thick PMMA/TEDBCd sample with the corresponding PL images of the irradiated areas. Scale bars equal 20 µm. Laser pulses at 355 nm were with fluence 320 mJ/cm2 per single pulse and repetition rate 10 Hz, thermostat temperature was 97 °C. PL spectra were collected from the area with diameter 8 µm in the center of every spot (shown in the first image for 2 min). The arbitrary units are the same for all PL spectra. (b): PL intensity at 522nm vs irradiation time.
In all the cases of modification considered above, there was no evidence of any mechanical damage of irradiated areas.
3.3 Patterning with microspheres array
Using polystyrene microspheres as a mask for patterning offers an opportunity to create periodic hexagonal luminescent structures in PMMA/TEDBCd films. We used 10 µm spheres that were deposited on an 8 µm-thick sample surface and self-organized into a closely packed monolayer (Fig. 5(a)). Absorption coefficient of polystyrene at 355 nm is about 8.5 cm-1 (according to our measurements of a test polystyrene film). Hence, the deposited layer is practically transparent at the wavelength of irradiation. Each microsphere in the layer acts as a lens, collecting the laser light into a small spot within the sample bulk. After the irradiation, the mask is removed, and the luminescent pattern remains in the material (Fig. 5(b)).
Fig. 5. Scheme of laser patterning with a monolayer of microspheres as a mask. After the irradiation of PMMA/TEDBCd sample via the mask (a) and washing out the microspheres, the luminescent pattern remains in the sample (b).
The UV irradiation in the first experiment was performed by an unfocused laser beam with diameter 1.7 mm at a single pulse fluence of 23 mJ/cm2 and pulse repetition rate of 5 Hz for 6 minutes. Thus, the UV exposure of the sample was about 40 J/cm2. A luminescence microimage of the obtained pattern is presented in Fig. 6(a). The luminescent spot formed under each sphere has a complex form. Figures 6(b),(c) demonstrate two lateral cross-sections of the film at different depths. The light-created spots look like rings in the upper part and dots at deeper levels. Figure 6(d) shows vertical cross-section of the film through the row of spots. The luminescent volume has a bullet-like shape with the width of 3 µm and height of about 6 µm. The spot has a conical pit on its top with the depth of 3 µm and diameter of 1.5 µm.
Fig. 6. (a): Confocal microscope PL image of the periodic structure made by laser irradiation via microsphere mask after removal of the spheres; (b),(c): Lateral cross-sections of the laser-produced luminescent structure at different depths: (b) – at depth 1 µm under the surface, (c) – at depth 5 µm under the surface; (d): Vertical cross-section of luminescent structure. Additional information in the image (in yellow): assumed film surface and the monolayer of spheres are schematically shown with dotted lines, laser irradiation incidence direction is shown with arrows. Scale bars in images (a)-(d) are equal to 20 µm.
.
For a more detailed investigation, three regions were irradiated via microspheres with nearly the same total UV exposure 120-130 J/cm2, but with different energies of laser pulses (see Table 1). Pulse repetition rate was 10 Hz, and thermostat temperature was 97 ± 2 °C.
Table 1. Laser irradiation parameters for patterning via polystyrene microsphere mask
The irradiated areas were investigated by means of AFM and confocal luminescent microscopy. The results are presented in Fig. 7. One should take into consideration that the microspheres can provide significant enhancement of light intensity (see Discussions) which can lead to laser ablation of material. That is why two accompanying processes are possible: the first is removal of the material and the second is formation of nanoparticles within the polymer matrix.
Fig. 7. PMMA/TEDBCd film after the laser irradiation with total UV exposure 120-130 J/cm2 via a monolayer of 10-μm polystyrene spheres for cases I-III at different laser fluences: I - single-pulse laser fluence 0.85 mJ/cm2, II - single-pulse laser fluence 1.7 mJ/cm2 (IIa is for spots without ablation, IIb is where ablation holes are observed), III - single-pulse laser fluence 3.6 mJ/cm2. From left to right for each case: AFM image; surface profile between points marked with arrows (the supposed location of the sphere is shown as additional information); confocal microscope image of lateral cross-section of the luminescent patterns under the surface of the sample (scale bar equals 5 μm); vertical cross-sections of the luminescent spot row (assumed film surface and the monolayer of 10-μm spheres are schematically shown with dotted lines). Confocal microscope images were obtained with the same acquisition settings.
AFM studies of the laser-processed areas after removal of the mask were performed to make conclusions about laser ablation. Small impressions with depths less than 200 nm were formed under the spheres and can be observed in AFM maps of each irradiated region. They can be fitted with the sphere surface (see the surface profile presented in panel I of Fig. 7). Deeper holes up to 1 µm in depth resulting from laser ablation can be seen in case (II) under some spheres and in case (III). The width of the laser-ablated holes is about 1 µm in area (II) and up to 1.5-2 µm in area (III).
Figure 7 also demonstrates the results of confocal microscope studies of the obtained luminescent patterns: lateral cross-sections of the structures made under the sample surface, and vertical cross-sections of the structures taken through the row of spots for each case (I)-(III). In case of laser irradiation with fluence 3.6 mJ/cm2 (III), most of the spots have dark pits about 3 µm in depth and about 1.5 µm in width. This fact corresponds to the formation of laser-ablated holes in the sample. In the lateral cross-section of the region (III) we have an array of rings. It is similar to the results demonstrated earlier in Fig. 6. In case of the weakest laser pulses (I), we obtained divergent conical spots with apexes just under the microspheres. In the lateral cross-section they appear as an array of small luminescent “dots”. Irradiation of regime (II) is close to the ablation threshold. In case (II), some of the luminescent spots have pits at the top with a depth of about 2 µm and width of 1 µm (See panels IIb of Fig. 7). Some of the luminescent spots have no pits (Panel IIa of Fig. 7). They are well localized, bright and contrasted with respect to the background and spots shown in panel I. They can be called luminescent jets.
The luminescence microimages colored according to the emission spectra of the obtained periodic patterns are presented in Fig. 8(a). The PL emission spectra given in Fig. 8(b) were collected with the same acquisition parameters from a round area with diameter 1.5 µm in the center of the luminescent spot in each image. One can notice the growth of PL intensity with increasing laser fluence. For the region (III) the PL spectrum also exhibits a shoulder near 600 nm (plot 3 in Fig. 8(b)).
Fig. 8. (a): Confocal microscope PL images of the periodic structures in regions irradiated with different parameters (I)-(III): I - irradiation with single-pulse laser fluence 0.85 mJ/cm2, II - 1.7 mJ/cm2, III - 3.6 mJ/cm2. Scale bars equal 10 μm. (b): PL emission spectra collected from luminescent spots: 0 – non-irradiated area, 1,2,3 - regions (I), (II), (III) irradiated with different parameters of laser pulses (points 0-3 are marked in PL images).
4. Discussions
The UV induced formation of CdS nanoparticles in PMMA/TEDBCd composites in isothermal conditions was carefully studied in our previous papers [11,12,17]. The particle growth process includes the diffusion of species that constitute these nanoparticles. We showed that the formation of nanoparticles successfully proceeds near the glass transition point where the diffusion is facilitated, compared to the glass state. The current work demonstrates some possibilities of the material laser patterning by a focused laser beam. Besides the needed photochemical effect, the laser irradiation provides some heating of the material. In this work, we keep the irradiated samples in a thermostat at a temperature near the glass transition. This allows us to use pulses with relatively small fluences to avoid laser-induced deformation of the material surfaces. Absorption coefficient of PMMA/TEDBCd at 355 nm is approximately α ≈ 50 cm-1. The temperature increment after a single pulse at fluence 0.1 J/cm2 gives a temperature rise of about 3 °C. The absorption coefficient may increase during the irradiation by up to an order of magnitude due to formation of nanoparticles [17]. This may lead to much higher temperature elevation, which is important for long-time laser processing. The cooling of the heated regions is an order of magnitude faster than the time between the laser pulses (repetition rate 5-10 Hz). Keeping the ambient temperature at about glass transition point allows one to maintain particle growth all the time between the laser pulses. We do not need here the average heating of the material by laser pulses. When irradiating the soft samples clamped between the fused silica glasses, we avoid any hydrodynamic deformation caused by the inhomogeneous heating of the material surface by laser beam. But even in case when the front surface is free, the microscopic observation of the irradiated spots does not show any surface distortion.
Our studies revealed some features of the laser patterning of luminescent structures in PMMA/TEDBCd composites. High quality structures need optimal conditions.
In linear pattering by means of direct laser writing, there is an optimum scan speed or exposure that provides the most contrasted luminescent structures. The excess of this exposure results in a drop of luminescence of the irradiated region.
The interesting feature of the process is that some systematic deviation in writing conditions results in modulation of the recorded luminescent structures at scanning speeds far from the optimum ones. In Fig. 1 the lines were recorded top down. The luminescent signal variation along lines 1 and 4 would be assigned to some gradual decrease in exposure during the writing process. Lines 2 and 3, which are close to the optimal scanning speed, are not affected by this deviation. Thus, the results of laser writing at the conditions close to the optimal ones are more stable with respect to small deviations in writing conditions.
The degradation of the PL efficiency can be attributed to the UV aging of the surface of the growing nanoparticles. This provides the ability to form ring-like luminescent structures as shown in Figs. 2–4. The above effects are fluence-dependent. At high fluencies, the red shift of the PL spectrum with increasing exposure was observed. It can hardly be explained by the increase of the average size of the grown nanoparticles with an increase in exposure. It can rather be attributed, in particular, to the formation of nanoparticles with surface traps [18,19]. This effect is more pronounced in thick samples where heat dissipation is less efficient than in thin films.
To understand the effect of the laser irradiation via the polystyrene microsphere monolayer on PMMA/TEDBCd, we carried out calculations using the FDTD (finite-difference time-domain) method employing a previously developed code (for details, see [20]). In Fig. 9 the distribution of the 355 nm laser intensity below a close-packed layer of polystyrene spheres with a diameter of 10 µm is presented. The refractive index of the spheres and the substrate was 1.65 and 1.51, respectively. Here we see that field enhancement might be up to 800 times due to focusing by a sphere.
Fig. 9. FDTD calculation of the electric field square enhancement near the surface of one polystyrene (n=1.65) sphere within the close-packed array. The array of 10-μm spheres is deposited on the surface (sphere is indicated with a white line, sample surface is indicated with a yellow dashed line) of the sample (n=1.51) and irradiated by the plane monochromatic wave (λ=355 nm) from the top. The field enhancement is calculated with respect to the value in the incident wave.
To obtain the contrast luminescent structures by the photonic jet shown in Fig. 9 the optimal irradiation conditions should be realized. The high laser fluences result in ablation. Here, the bright part of the jet provides an ablation crater. The small fluences result in slow contrast structures with impact of luminescence bleaching within the bright parts of the jet. The high-contrast luminescent structures are obtained with fluences close to the ablation threshold.
The processes involved in the laser-induced nanoparticle formation by focused beams are complicated and not well understood. The existing theoretical models consider only the isothermal condition [10,21]. It was experimentally shown that under isothermal conditions the CdS nanoparticle growth is an exposure–dependent process, i.e., the result is the same for different light intensities if the exposure is fixed. That is not the case in the present experiments. The investigation of the CdS nanoparticles formation in PMMA/TEDBCd composites at isothermal conditions but at different temperatures showed that a higher temperature results in a higher rate of the nanoparticles formation. This was explained by the temperature dependence (Arhenius-like) of the quantum yield of precursor TEDBCd photochemical destruction. Qualitatively, this finding could explain some experimental features of the present paper. However, under non-isothermal conditions, the structure formation in photoinduced nanocomposites is a more complicated process than that studied in [22]. We believe that the presented experimental data could stimulate both experimental and theoretical activity in this field.
5. Conclusion
The presented work demonstrates the possibility of direct laser writing of luminescent patterns within a polymer PMMA film containing a soluble CdS precursor (TEDBCd) by the third harmonic of a Nd:YAG laser and reveals some features of the luminescent behavior of the obtained photoinduced nanocomposites. Line patterns and single spots were fabricated using the direct laser writing technique. It was shown that there is a critical exposure corresponding to the maximum PL emission intensity. At small enough fluences, exceeding of the critical exposure results in a decrease in the intensity of the luminescent signal without significant changes in its spectrum. The irradiation at higher fluences may lead to a red shift in the PL spectra.
The opportunity to produce micron-sized luminescent structures by employing a mask consisting of a layer of closely packed ten-micrometer polystyrene spheres was demonstrated. Luminescent jets, the high-contrast luminescent structures without pits are obtained at fluences just below the ablation threshold.
Funding
Russian Foundation for Basic Research (19-02-00694a); Russian Science Foundation (14-19-01702, 18-79-10262); Ministry of Science and Higher Education of the Russian Federation (0035-2019-0012).
Acknowledgements
AFM studies were performed using equipment of the Center for Shared Use of the Scientific Educational Center “Physics for Solid State Nanostructures” of the Lobachevsky State University of Nizhny Novgorod.
This work was carried out under financial support of Russian Foundation for Basic Research (grant 19-02-00694a). The samples employed in the experiments were synthesized using the results obtained within the project of Russian Science Foundation (grant 14-19-01702). The code for numerical calculations was worked out within the project of Russian Science Foundation (grant 18-79-10262), the grant of Ministry of Science and Higher Education of the Russian Federation as part of the state assignment of the IAP RAS, project No. 0035-2019-0012 is also acknowledged.
Disclosures
The authors declare no conflicts of interest.
References
1. J. Loste, J.-M. Lopez-Cuesta, L. Billon, H. Garay, and M. Save, “Transparent polymer nanocomposites: an overview on their synthesis and advanced properties,” Prog. Polym. Sci. 89, 133–158 (2019). [CrossRef]
2. S. Parola, B. Julián-López, L. D. Carlos, and C. Sanchez, “Optical properties of hybrid organic-Inorganic materials and their applications,” Adv. Funct. Mater. 26(36), 6506–6544 (2016). [CrossRef]
3. S. Li, M. M. Lin, M. S. Toprak, D. K. Kim, and M. Muhammed, “Nanocomposites of polymer and inorganic nanoparticles for optical and magnetic applications,” Nano Rev. 1(1), 5214 (2010). [CrossRef]
4. J. Lee, V. C. Sundar, J. R. Heine, M. G. Bawendi, and K. F. Jensen, “Full Color Emission from II-VI Semiconductor Quantum Dot-Polymer Composites,” Adv. Mater. 12(15), 1102–1105 (2000). [CrossRef]
5. V. Wood and V. Bulović, “Colloidal quantum dot light-emitting devices,” Nano Rev. 1(1), 5202 (2010). [CrossRef]
6. X. Huang, Q. Guo, D. Yang, X. Xiao, X. Liu, Z. Xia, F. Fan, J. Qiu, and G. Dong, “Reversible 3D laser printing of perovskite quantum dots inside a transparent medium,” Nat. Photonics 14(2), 82–88 (2020). [CrossRef]
7. A. K. Bansal, F. Antolini, M. T. Sajjad, L. Stroea, R. Mazzaro, S. G. Ramkumar, K.-J. Kass, S. Allard, U. Scherf, and I. D. W. Samuel, “Photophysical and structural characterisation of in situ formed quantum dots,” Phys. Chem. Chem. Phys. 16(20), 9556–9564 (2014). [CrossRef]
8. V. Resta, A. M. Laera, E. Piscopiello, M. Schioppa, and L. Tapfer, “Highly efficient precursors for direct synthesis of tailored CdS nanocrystals in organic polymers,” J. Phys. Chem. C 114(41), 17311–17317 (2010). [CrossRef]
9. F. Antolini and L. Orazi, “Quantum dots synthesis through direct laser patterning: a review,” Front. Chem. 7, 252 (2019). [CrossRef]
10. N. Bityurin and A. A. Smirnov, “Model for UV induced growth of semiconductor nanoparticles in polymer films,” Appl. Surf. Sci. 487, 678–691 (2019). [CrossRef]
11. A. A. Smirnov, A. Afanasiev, N. Ermolaev, and N. Bityurin, “LED induced green luminescence in visually transparent PMMA films with CdS precursor,” Opt. Mater. Express 6(1), 290–295 (2016). [CrossRef]
12. A. A. Smirnov, A. Afanasiev, S. Gusev, D. Tatarskiy, N. Ermolaev, and N. Bityurin, “Exposure dependence of the UV initiated optical absorption increase in polymer films with a soluble CdS precursor and its relation to the photoinduced nanoparticle growth,” Opt. Mater. Express 8(6), 1603–1612 (2018). [CrossRef]
13. F. Antolini and L. Orazi, “Solid state synthesis of CdS quantum dots through laser direct writing,” AIP Conf. Proc. 2145(1), 020016 (2019). [CrossRef]
14. B. S. Luk’yanchuk, R. Paniagua-Domínguez, I. Minin, O. Minin, and Z. Wang, “Refractive index less than two: photonic nanojets yesterday,today and tomorrow,” Opt. Mater. Express 7(6), 1820–1847 (2017). [CrossRef]
15. N. Bityurin, A. Afanasiev, V. Bredikhin, A. Alexandrov, N. Agareva, A. Pikulin, I. Ilyakov, B. Shishkin, and R. Akhmedzhanov, “Colloidal particle lens arrays-assisted nano-patterning by harmonics of a femtosecond laser,” Opt. Express 21(18), 21485–21490 (2013). [CrossRef]
16. V. Kamensky, V. Kazakov, V. Bredikhin, A. Pikulin, and N. Bityurin, “Use of colloidal monolayers of glass spheres for the improvement of the optoacoustic ultrasound generation,” Mater. Res. Express 6(4), 045201 (2019). [CrossRef]
17. A. A. Smirnov, A. Kudryashov, N. Agareva, A. Afanasiev, S. Gusev, D. Tatarskiy, and N. Bityurin, “In-situ monitoring of the evolution of the optical properties for UV LED irradiated polymer-based photo-induced nanocomposites,” Appl. Surf. Sci. 486, 376–382 (2019). [CrossRef]
18. H. Noglik and W. J. Pietro, “Chemical funtionalization of cadmium sulfide quantum-confined microclusters,” Chem. Mater. 6(10), 1593–1595 (1994). [CrossRef]
19. A. Veamatahau, B. Jiang, T. Seifert, S. Makuta, K. Latham, M. Kanehara, T. Teranishi, and Y. Tachibana, “Origin of surface trap states in CdS quantum dots: relationship between size dependent photoluminescence and sulfur vacancy trap states,” Phys. Chem. Chem. Phys. 17(4), 2850–2858 (2015). [CrossRef]
20. A. Afanasiev, V. Bredikhin, A. Pikulin, I. Ilyakov, B. Shishkin, R. Akhmedzhanov, and N. Bityurin, “Two-color beam improvement of colloidal particle lens array assisted surface nanostructuring,” Appl. Phys. Lett. 106(18), 183102 (2015). [CrossRef]
21. A. Pikulin and N. Bityurin, “Homogeneous model for the nanoparticle growth in polymer matrices,” J. Phys. Chem. C 124(29), 16136–16142 (2020). [CrossRef]
22. A. Pikulin, A. A. Smirnov, and N. Bityurin, “Model for local generation of nanoparticles in photoinduced nanocomposites by the focused laser light,” Appl. Surf. Sci. 475, 1010–1020 (2019). [CrossRef]
