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

Evolution of the UV-induced absorption within the polymer matrix possessing a highly soluble CdS precursor is studied. The initially optically transparent (polymethylmethacrylate based) samples are irradiated by a light-emitting diode operated at 365 nm for different intensities and different temperatures. In situ monitoring of the process is performed at a wavelength of 405 nm where the samples are initially transparent. The study shows that the increase in absorbance is temperature dependent, and at a fixed temperature it is determined by UV exposure rather than the intensity or irradiation time separately. TEM, HR TEM data, as well as data on absorption and luminescent spectra, allow the relation of the optical absorption evolution to the CdS nanoparticles growth process. This provides new valuable information on the kinetics of this phenomenon in UV irradiated polymer films with a soluble precursor. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement OCIS codes: (160.4236) Nanomaterials; (160.4760) Optical properties; (160.5335) Photosensitive materials; (350.3850) Materials processing; (350.4990) Particles; (350.5130) Photochemistry. References and links 1. N. M. Bityurin, “Laser Nanostructuring of Polymers,” in Fundamentals of Laser–Assisted Microand Nanotechnologies, V. P. Veiko, V. I. Konov, eds. (Springer International Publishing, Switzerland, 2014). 2. A. Alexandrov, L. Smirnova, N. Yakimovich, N. Sapogova, L. Soustov, A. Kirsanov, and N. Bityurin, “UV initiated growth of gold nanoparticles in PMMA matrix,” Appl. Surf. Sci. 248(1–4), 181–184 (2005). 3. A. Athanassiou, R. Cingolani, E. Tsiranidou, C. Fotakis, A. M. Laera, E. Piscopiello, and L. 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Introduction
Self-assembly of the species originated from the light-induced precursor destruction can result in nanoparticle formation within the polymer matrix [1][2][3][4][5][6][7].The optical properties of such photoinduced nanocomposites usually drastically differ from those of the initial materials.This provides an opportunity for patterning of the initially homogeneous material because the nanoparticles are formed only within the irradiated domains [7][8][9][10][11].Another way to perform patterning is discussed in [12].Here, the inhomogeneity in the spatial distribution of nanoparticles relies on the laser-induced inhomogeneity of the matrix properties.
The light-induced formation of gold or silver nanoparticles in materials of this kind has been studied for a decade [2,7,[13][14][15].Nanocomposites with photoinduced semiconductor nanoparticles, e.g., CdS, are also of interest because of their luminescent properties [16,17], which is promising for applications in light-emitting diode (LED) production technologies [18].At the same time, the sufficiently large light-induced CdS nanoparticles have an effective absorption cross-section of about 2•10 −18 cm 2 per CdS unit within the optical range with λ < 500 nm.This cross-section value is close to that of such well-recognized strongly absorbing photoinduced species as Ti 3+ centers in TiO 2 gel and TiO 2 gel-based organicinorganic hybrids [19][20][21][22].
Most of the previously published papers used CdS precursors, which are not soluble in the polymer matrix.In this case, the nanoparticles are formed within the precursor islands in the matrix, and the process resembles the one occurring within the grains of powder [23].Some good soluble CdS precursors have recently been reported [24][25][26].This gives rise to the problem of studying and understanding of the particle formation mechanism in this case.Diffusion of species should play a significant role in the process.In the present paper, we study UV irradiation of polymer films with bis(1,1,5,5-tetraethyl-2,4dithiobiureto)cadmium(II) [Cd(N(SCNEt 2 ) 2 ) 2 ] (TEDBCd) as a precursor.This compound is readily soluble in many organic solvents and could be a very promising candidate as a precursor for photoinduced nanocomposites with polymethylmethacrylate (PMMA) matrix, offering an opportunity to prepare visually transparent PMMA films with a precursor content of up to 10% [24].Relatively cheap and easy-to-use ultraviolet light emitting diodes (UV LEDs) with a central wavelength of 365 nm [27] were chosen as light sources for this processing [24].We employ transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) to prove the formation of CdS nanoparticles within the samples under UV irradiation.
In the present paper, we designed a special setup allowing us to irradiate the samples at controlled temperatures and light intensities, and thereby to examine the effect of UV photons and temperature separately.We obtained a somewhat unexpected result.At a given temperature, the effect of UV radiation on the sample is determined by the exposure (doze) rather than the light intensity and irradiation time separately.This result may shed light on the mechanism of nanoparticle nucleation and growth in the photoinduced nanocomposites with soluble CdS precursors.

Sample preparation and UV irradiation
Cadmium complex TEDBCd was prepared as described in [28].Then it was introduced into the PMMA matrix.Due to the high solubility of the precursor, the process of obtaining samples reduced to a simple casting of the toluene solution of the desired concentration on a fused silica substrate and drying at room temperature.In order to prevent liquid spreading on the surface and to fix the thickness of the film, the substrate was put into a specially designed fluoroplastic container.The resulting PMMA film is characterized by a high transparency in the visible region of the spectrum.
For the UV irradiation the sample was placed into a heater between two quartz windows transparent for UV light.A specially designed heater-thermostat provided a controlled temperature that was monitored with an Optris PI400 thermal imager (Optris GmbH, Germany).To make the measurement more accurate, we attached a special adhesive tape with known emissivity (Testo Inc., USA) on the window.UV irradiation was provided by LED NVSU233A (Nichia, Japan) with a central wavelength of 365 nm.UV light was turned on when the measured temperature was stabilized.The scheme of the experimental setup is presented in Fig. 1.We used the part of radiation not coming to the sample as a reference.Measuring the intensity of this reference part of the radiation with a spectrometer (QE65Pro, Ocean Optics cm −2 ).

Electron
The particle s areas were measurements instrument (C of 0.12 nm (PMMA)/TED with spin coa about 15 nm.PMMA film grid.Taking i high resolutio matrix.For th plasma using Inc., USA).T the TEM sam PMMA at a r and untreated Micrograph so For examp fraction of 5% (Fig. 2(b),(c The TEM im n, Inc., USA).crographs of th ed in Fig. 2 2(d)) and DF (Fig. 2(e)) images.First of all, the width of the distributions is relatively large and indicates that the QDs are not monodispersed.Secondly, the difference between the average particle sizes obtained from BF and DF images is obvious.This difference is caused by the fact that on the BF image we observe both isolated particles and their clusters of several pieces.From the DF image, we determine the average size of crystallites (coherently diffracting domains), and this is a more accurate characteristic of the size of CdS nanoparticles.
The high-resolution (HR) micrographs, in which the crystal lattice of the nanoparticles was clearly visible, permitted use to assess more accurately their size and conclude about the presence of a particular crystalline phase (Fig. 2(f),(g)).The observed particles have lattice constants d 1 ≈0.36 nm and d 2 ≈0.316 nm, which corresponds to lattices (100) and (101) of hexagonal CdS form.Also, from these images we can see that the apparent sizes of these crystallites are in good agreement with the sizes of the coherent scattering domains, which is observed in the DF images.

UV processing of samples and study of their optical properties
For a detailed study of the UV-induced process, we used PMMA films with a thickness of about 200 µ and a TEDBCd mass fraction of 5%.We performed UV irradiation of the film samples at three different measured temperatures, namely, 90 °C ± 2°C, 100 °C ± 2°C, and 110 °C ± 2°C.At these values, no damage or bubble formation for the sample occurred.
The UV light intensity in the LED-irradiated spot was about 400 mW cm −2 .This value slightly varied during the experiment due to the LED heating.For irradiation with reduced intensity, we employed a colored filter with a transmission of about 57% of the LED light without a sufficient transformation of its spectrum.
For in situ monitoring of the nanoparticle growth process, we measured the absorbance of the irradiated sample at a wavelength of 405 nm.The sample initially is transparent at this wavelength.
Figure 3(a) demonstrates the evolution of the absorbance at 405 nm during the UV irradiation process for different cases.It is clear from the plot that for a given temperature the optical properties of the irradiated sample mainly depend on the integrated exposure.After the irradiation, we measured the full absorbance spectrum of the samples using a Shimadzu UV-1800 spectrophotometer.It is also shown that at different intensities the same exposure provides the same absorbance spectrum (see Fig. 3(b)).It can be seen that all the absorbance vs. exposure curves are close to linear starting from some incubation exposure (see Fig. 3(c)).
The results can be qualitatively understood within the framework of a simplified model.Analysis of Mie theory expansion of the extinction cross section for small-size CdS nanoparticles shows that for nanoparticle diameters of the order of 10 nm the absorption term [29] ( ) strongly dominates the other terms, including the scattering one.Here, a is the radius of the particle, n m and ε m are the refractive index and dielectric constant of the matrix, and ε' and ε" are the real and imaginary parts of the dielectric constant of the bulk CdS at the wavelength λ.
The above formula shows that if the quantum confinement is not significant, that is, if the radii of nanoparticles are large enough, then the absorption cross section is proportional to the volume of the nanoparticle or the number of CdS units within it.This means that the input of the growing CdS nanoparticles into the absorption coefficient of the sample at a wavelength of 405 nm is approximately proportional to the number density of the CdS units aggregated within relatively large nanoparticles.
On the contrary, the sufficiently small CdS nanoparticles do not absorb at this wavelength because of the quantum confinement.The estimation [30] shows that the interband (valent zone -conduction zone) absorption at a wavelength of 405 nm is significant for nanoparticles larger than 4 nm in diameter.It takes some time for such nanoparticle to grow.The absence of the nanoparticles of this size in the beginning of the process should result in incubation (delay time between the start of the irradiation and the start of the absorption elevation) seen in Fig. 3(c).
We relate the growth of absorption at a wavelength of 400 nm and larger with the CdS units accumulation within the nanoparticles.The estimations based on the known bulk optical constants [31] see Fig. 4 and Eq. ( 2) from Appendix, yield a value about 5•10 −18 cm 2 for the effective absorption cross section per CdS unit.The destruction of the precursor molecule can be accompanied by the formation of absorbing by-products.A priori, we cannot exclude this effect.However, the existence of a delay time in the absorption increase shows that the parasitic by-product absorption does not dominate.
Luminescent spectra of the irradiated films (see Fig. 3(d)) correspond to the spectra of CdS nanoparticles.It should be noted that luminescence is very sensitive to the surface condition of a nanoparticle, so that we can speak here only about a qualitative correspondence.Nevertheless, it is expedient to assume that the maximum of the luminescent signal is close to the bandgap wavelength of the CdS bulk materials.Moreover, it should be taken in consideration that the bandgap of luminescent particles is somewhat larger than the bulk bandgap due to the quantum confinement.precursor mass fraction of 5% for three different temperatures (90 °C, 100 °C, and 110 °C) and two optical power densities: "high" (about 400 mW cm −2 ) -solid lines and "low" (57% of high passing through the glass filter) -dotted lines.(b) Absorbance spectra of the PMMA/TEDBCd film with a precursor mass fraction of 5% irradiated with an UV exposure of 1.5 kJ/cm 2 under three different temperatures, namely, 90 °C (blue lines, squares), 100 °C (green lines, circles), and 110 °C (red lines, triangles).The spectra for the samples irradiated with "high" intensity are presented as solid lines with filled symbols; for "low" intensity -dotted lines with empty symbols.The spectrum of the non-irradiated sample is shown by a gray line with star symbols.(c) Magnified plot for small exposures at high intensity.(d) Normalized PL spectra of the PMMA/TEDBCd film with a precursor mass fraction of 5% irradiated with UV exposure 1.5 kJ/cm 2 under three different temperatures (90 °C, 100 °C, and 110 °C).The spectra for the samples irradiated with "high" intensity are presented filled symbols; for "low" intensitywith empty symbols.(e) PL spectra of the PMMA/TEDBCd film with a precursor mass fraction of 5%: non-irradiated (gray curve, empty squares), irradiated for 3 min (75 J/cm 2 exposure) at 100 °C (red curve, filled circles), irradiated with an UV exposure of 1.5 kJ/cm 2 for 1 hour at 100 °C (green curve, filled triangles).The excitation wavelength is 405 nm.
Evolution of the luminescent spectra at small exposures indicates a shift of the maximum of the luminescent spectrum towards the longer wavelengths (Fig. 3(e)).This agrees with the particle growth process.
Within the frame of the simplified model of light absorption of nanoparticles of different sizes, knowing the data on absorbance spectra and on the optical constants of CdS (Fig. 4(a)), it is possible to reconstruct the size distribution of nanoparticles absorbing light within the range of 400-500nm (see Appendix).The results show that there is quite a broad distribution of nanoparticles with different sizes up to 8-10nm (see Fig. 4(b)).Qualitatively, it corresponds to the distribution of Fig. 2(e).The shift from small towards large nanoparticle sizes in Fig. 2(e) comparing to the distribution in Fig. 4(b) can be explained by the aggregation of nanoparticles occurring within the solution.
The photoinduced formation of CdS nanoparticles is a complicated process that includes a photo-mediated destruction of the precursor molecules followed by a diffusion-assisted growth of nanoclusters.The above results (see Fig. 3) can be understood as suggesting that the precursor destruction is the limiting stage here.For the description of the absorbance evolution, the approach to mathematical modeling of the bulk photochemical processes considered in, e.g., Ref. 32 and in Ch.II A of Ref. 33 can be applied with some coefficients, In what follows, we use the averaged values of the optical functions of hexagonal CdS from [31].σ B (λ) is shown in Fig. 4(a).
The boundary size between absorbing and not absorbing nanoparticles can be estimated by means of a simple formula for the size dependence of the bandgap [30] According to our model, if radius of a nanoparticle r>R(λ), then the absorption cross section of each CdS unit within a nanoparticle does not depend on the nanoparticle size and is equal to σ B (λ) (2).
If we find the boundary value R* for some particular wavelength λ*, then the relation ( 4) for the other wavelengths can be overwritten Equations ( 8) and ( 9) provide the solution of the inverse problem of finding the particle size distribution knowing the wavelength dependence of the absorbance.
In order to differentiate the scattering experimental data, we approximate D(λ) and σ B (λ) within the interval 380 nm < λ < 520 nm by polynomial functions of the fourth and the fifth orders, correspondingly.
The result of the size (diameter) distribution functions evaluation for three values of temperature is shown in Fig. 4(b).
Fig. 1.Scheme o microscopy size and the c studied by s were perfor Carl Zeiss AG, .For TEM DBCd film sol ating at ~2000 Usually the P was separated into account th on TEM measu his purpose, th Evactron 25/4 This processing mples.The pla rate of ~1 nm m d structures.T oftware (Gatan ple, TEM mic % are presente ) cases.The how the contra rradiation, on m , which are vi spondingly.Th uality of the TE micrographs of the CdS partic nd aggregation BF and DF im of the CdS na distributions fo iated with an U

Fig. 2 .
Fig. 2. TEM micrographs of the PMMA/TEDBCd thin films with a precursor mass fraction of 5% as is in (a) and irradiated with UV exposure 2.3 kJ cm −2 (2 h, 320 mW cm −2 ) at 110 °C (b,c).The nanoparticles are seen as dark spots on the BF image (b), and bright dots on DF image (c).The clusters of these nanoparticles are marked by arrows.(d,e): Histograms of the particles sizes for PMMA/TEDBCd sample obtained from BF (d) and DF (e) images.(f,g): HRTEM micrographs of CdS nanoparticles in the irradiated PMMA/TEDBCd film.The inset shows the NanoBeam electron diffraction patterns of the corresponding cluster.

Fig. 3 .
Fig.3.(a) Absorbance at 405 nm vs UV exposure for the PMMA/TEDBCd film with a precursor mass fraction of 5% for three different temperatures (90 °C, 100 °C, and 110 °C) and two optical power densities: "high" (about 400 mW cm −2 ) -solid lines and "low" (57% of high passing through the glass filter) -dotted lines.(b) Absorbance spectra of the PMMA/TEDBCd film with a precursor mass fraction of 5% irradiated with an UV exposure of 1.5 kJ/cm 2 under three different temperatures, namely, 90 °C (blue lines, squares), 100 °C (green lines, circles), and 110 °C (red lines, triangles).The spectra for the samples irradiated with "high" intensity are presented as solid lines with filled symbols; for "low" intensity -dotted lines with empty symbols.The spectrum of the non-irradiated sample is shown by a gray line with star symbols.(c) Magnified plot for small exposures at high intensity.(d) Normalized PL spectra of the PMMA/TEDBCd film with a precursor mass fraction of 5% irradiated with UV exposure 1.5 kJ/cm 2 under three different temperatures (90 °C, 100 °C, and 110 °C).The spectra for the samples irradiated with "high" intensity are presented filled symbols; for "low" intensitywith empty symbols.(e) PL spectra of the PMMA/TEDBCd film with a precursor mass fraction of 5%: non-irradiated (gray curve, empty squares), irradiated for 3 min (75 J/cm 2 exposure) at 100 °C (red curve, filled circles), irradiated with an UV exposure of 1.5 kJ/cm 2 for 1 hour at 100 °C (green curve, filled triangles).The excitation wavelength is 405 nm.

m
are the effective mass of the electrons and the holes within the bulk material, and R is the radius of the particle.For each wavelength λ there is a boundary value of the nanoparticle radius R(λ)