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We demonstrate that [Cd(N(SCNEt2)2)2] is a CdS-containing compound that is well dissolved in a PMMA (polymethylmethacrylate) matrix. The PMMA films possessing up to 10% of these molecules are visually transparent and have no evidence of light scattering. Irradiation of these films by Light Emitting Diodes (LED) operated at a wavelength of 365 nm provides a permanent material alteration with the optical properties of the modified domains (absorption band, luminescent spectra) that are typical for the solid solution of CdS nanoclusters. We show that here the LED radiation is a more effective tool than both the forth and the third harmonics of a Nd:YAG laser and that both photochemical and light heating effects are significant for the above material processing.
© 2015 Optical Society of America
1. Introduction
Nanostructured organic-inorganic composites have unique properties making them very promising for electronic and photonic applications [1–3]. Here, the polymers with embedded metal or semiconductor nanoparticles are of great importance and have great technological prospects [4–6]. The employment of polymer matrices offers opportunities for easy and cheap synthesis techniques for bulk-like as well as thin film formation, with high solubility in most of the common solvents and high thermal and mechanical stability. They protect inorganic particles from aggregation, and preserve their chemical and physical properties as nonlinear optical response [7], photoluminescence (PL), and electroluminescence [8]. Among these materials, the so-called photoinduced nanocomposites are interesting from the point of in situ nanostructuring [9]. Laser irradiation of such samples consisting of a polymer matrix with precursors of different kinds possibly followed by annealing can result in creation of nanoparticles within the irradiated domains. As a result, a significant difference in optical properties between irradiated and non-irradiated regions is obtained, which allows for effective laser patterning of the initially homogeneous samples [10–13].
The photoinduced plasmon nanocomposites with polymer matrices have been studied for a decade [13,14]. Recently, the creation and studying of exciton-photoinduced nanocomposites has attracted the attention of investigators. In particular, the photoinduced CdS nanocrystals are intensely studied because of their excellent luminescent properties [15]. The principal point in these studies is the proper choice of the precursor species [16]. Cadmium dithiolates (RS)2Cd (R = long alkyl chain or other hydrocarbon ligand) are appropriate for CdS nanoclusters formation under the effect of ultraviolet laser pulses [17,18]. It should be noted, however, that the precursors of this type have several disadvantages. The important one is that they are virtually not soluble in such matrices as PMMA and other polymer matrices, which are promising for use in optical elements. The reason for the insolubility of dithiolates (RS)2Cd is associated with the presence of vacancies on the cadmium atoms. Intermolecular donor-acceptor interaction of sulfur-cadmium leads to the formation of a coordination polymer with high molecular mass that determines the insolubility of (RS)2Cd (Fig. 1(a)).
Fig. 1 (a) - coordination polymer chain of (RS)2Cd; (b) - schematic view of a low molecular mass bicyclic cadmium thiolate complex with two intramolecular coordination S:→Cd bonds. (c) - absorption spectra of samples: pure PMMA and [Cd(N(SCNEt2)2)2] /PMMA films with the corresponding weight fractions.
However, for most of applications, these precursors should be perfectly soluble in matrix. There have recently been several attempts to avoid the problem of insolubility of CdS precursors [19–21]. However, the search for the proper precursor continues. One of the possible ways to obtain soluble cadmium thiolates is blocking the intermolecular donor-acceptor interactions S:→Cd. This, in particular, can be achieved by synthesis of cyclic cadmium thiolates [22] (Fig. 1(b)).
In this paper, we suggest using the CdS precursor of another kind, namely, bis(1,1,5,5-tetraethyl-2,4-dithiobiureto)cadmium(II) [Cd(N(SCNEt2)2)2] in photoinduced nanocomposites. This compound was synthesized and employed in [22] for the creation of CdS film on the substrate heated to a temperature of 400 ̊C. This temperature would be fatally damaging for PMMA matrix. However, we show that this compound could be a very promising candidate as a precursor for photo-induced nanocomposites with PMMA matrix. Moreover, we show that the photo-induced formation of luminescent centers in a solid solution of these precursor molecules within the PMMA matrix can be provided by the Light Emitting Diodes (LED), which are very cheap and versatile tools.
2. Sample preparation
A cadmium dithio-biuret [Cd(N(SCNEt2)2)2] precursor prepared as described in [22] 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 glass substrate and drying at room temperature. The resulting PMMA film is characterized by a high transparency in the visible region of the spectrum (see Fig. 1(c)).
3. Experimental
The samples are irradiated in the air at room temperature. Pulse regimes are provided with a Nd:YAG laser LS-2137 («Lotis ТII», Belarus) operated at the third (355 nm) and the fourth (266 nm) harmonics. For the continuous irradiation, an UV LED with the central wavelength at 365 nm (Nichia NCSU033B, Japan) is employed. The elevation of the sample temperature while irradiating with LED is monitored with an Optris PI400 thermal imager (Optris GmbH, Germany) calibrated with organic glass at 70 ̊C.
Extinction spectra of the film are measured with a LOMO SF256 spectrophotometer. Photoluminescence is excited with a 405 nm CW laser diode. The PL spectra are collected with an Ocean Optics QE65Pro through a ET425lp long-pass filter (Chroma Technology). The spectrometer is calibrated with HL2000 lamp The PL images of samples are observed with a Zeiss LSM710 confocal microscope with excitation at 405 nm.
4. Results and discussions
The extinction spectrum of a PMMA film with a [Cd(N(SCNEt2)2)2] precursor is shown in Fig. 2. The films are transparent in the visible region demonstrating no evidence of scattering even with a 10% content of the precursor. The absorption starts in the near UV (Fig. 1(c)) providing information on the wavelengths of the laser sources suitable for generation of CdS particles within the films: pure PMMA samples are transparent in the range 300 – 375 nm, while the samples with precursor absorb in this interval.
Fig. 2 PL images of the areas irradiated by 266 nm pulses with a laser fluence of 80 mJ/cm2(a), and by 355 nm pulses with a laser fluence of 200 mJ/cm2(b). PL spectra from the region irradiated by 355 nm pulses with a repetition rate of 2 Hz (c).
The laser radiation of the fourth harmonic of a Nd:YAG laser (266nm) is highly absorbed by the films. Irradiation by this laser source results in emerging of opaque lusterless area on the surface, witnessing the film damage (see Fig. 2(a)). The laser fluence is 80 mJ/cm2 with a repetition rate of 2 Hz. The sample is irradiated for 5 minutes. The film alteration is closely localized at the very surface because of the strong light absorption at this wavelength.
The third harmonics of a Nd:YAG laser (355nm) is more reliable for processing of the films because of the smaller absorption compared with the fourth harmonic. The radiation penetrates through the material bulk. Irradiation by the nanosecond pulses at this wavelength results in appearance of luminescence when the irradiated spot is pumped by a CW laser at a wavelength of 405 nm. The irradiated domain demonstrates a slightly yellow color. Both of these features are usually connected with the formation of CdS nanoparticles within the film. However, it should be noted that irradiation at a repetition rate of 2 Hz provides a remarkable effect only in one or two hours. The result shown in Fig. 2(b) is achieved upon irradiation with a fluence of 200 mJ/cm2 and a repetition rate of 2 Hz for 60 min. An increase in the laser fluence results in the material damage.
The most exciting results were obtained when the films of different content of precursor were irradiated by a CW light source, namely a LED operated at a wavelength of 365 nm. By putting the film just near the LED source, one obtains a luminescent domain with the yellow color just in a few minutes. While the energy flux from LED is constant, the increase of temperature is a complicated function of time (Fig. 3(a)). When temperature is above 70 ̊C, the process of heating becomes more intensive. This fact can be explained by the alteration of the irradiated material and the formation of absorbing centers in the matrix. The evolutions of the luminescence signals and the transmission of the films during the irradiation are shown in Figs. 3(b) and 3(c). It is seen that irradiation results in significant darkening of the sample in the near UV region, and at the wavelength of the irradiation in particular. The measurement of the optical absorption proves that the optical density increases (see Fig. 3(c)). The absorption band moves towards the longer wavelength during the irradiation and tends to be slightly above 500 nm. This value is typical for CdS complexes. The corresponding PL spectra are shown in Fig. 3(b). The maximum is at a wavelength of 480 nm.
Fig. 3 (a) Temperature vs time curve measured with an Optris PI400 thermal imager while irradiating 1% of the [Cd(N(SCNEt2)2)2] /PMMA sample with an UV LED. PL (b) and absorption (c) spectra at different times of irradiation. The corresponding temperature can be seen in Fig. 3(a).
Limitation of the film irradiation relies on the damage of the sample surface. It should be noted that the films with different content of precursor need different irradiation time to achieve the valuable level of the LED-induced luminescence. We study samples with different weight fractions of precursor, namely, 1%, 2.5%, 5%, and 10%. The film with 1% of precursor needs irradiation for 20-25 min, the film with 5% of precursor needs 10-15 min, the film with 10% of precursor attends the appropriate level of the alteration in 2 - 3 min. It is important that the maximum of the PL signal depends on the weight fraction of the precursor. Namely, the maximum is at 480 nm for 1%, at 490 nm for 5%, and at 500nm for 10% (see Fig. 4). This may correspond to the increase of the CdS clusters in size [23].
Fig. 4 Normalized PL spectra of UV LED-irradiated samples with different weight fraction of [Cd(N(SCNEt2)2)2].
It is important that the films upon irradiation do not have any sign of scattering. The images of the irradiated sample obtained with a confocal microscope are shown in Figs. 5(a) and 5(b). PL spectra measured in points situated in different regions of the irradiated area are shown in Fig. 5(c). For comparison, in Fig. 6(a) we show a similar picture obtained after the irradiation of the PMMA film containing 3% of cadmium-(bis)-dodecylthiolate Cd(C12H25S)2 precursor by laser pulses at 266 nm. It is seen that in the latter case the luminescence domains are of island nature. This relies on the poor solubility of the (bis)-thiolate precursors in PMMA. The corresponding PL signal is presented in Fig. 6(b).
Fig. 5 Fluorescent images of [Cd(N(SCNEt2)2)2] /PMMA films with weight fraction of precursor 2.5%: (a) LED irradiated region, (b) the border between irradiated and non-irradiated regions, (c) PL spectra measured in different points. The excitation wavelength is 405 nm.
Fig. 6 Fluorescent images of Cd(C12H25S)2 /PMMA films with weight fraction of precursor 3%, irradiated with UV laser pulses at the wavelength 266 nm (a). PL spectra measured in points 1 and 2 (b). The excitation wavelength is 405 nm.
To estimate the PL efficiency of LED irradiated samples we compared it with the PL efficiency of the film made with the same matrix and of the same thickness but filled with the 9,10-bis(phenylethynyl)anthracene, which is known as a luminophore [24] with the efficiency close to 100% in solutions. The integral efficiency of LED irradiated samples appeared to be 110 times smaller. Comparative study of the luminescent efficiencies of the LED irradiated films with the laser irradiated film with the cadmium-(bis)dodecylthiolate as a precursor, shows that the corresponding values are close to each other.
In order to examine the mechanism of the above LED-induced alteration of the films, we carried out experiments on pure thermal and pure photochemical treatments of the samples. The measurements of the temperature of the film irradiated by a LED when the sample was placed just near the light emitting surface yielded values within 70-130 ̊C. The annealing of the films in a furnace at such temperatures for an hour without irradiation does not provide the permanent luminescent ability of the material samples described above. In order to study the pure photochemical effect of the LED, we irradiate the films at small power densities excluding the significant material heating by the absorbing light. We use the irradiation scheme with the transfer of the image of the LED emitting surface onto the surface of the irradiated polymer film. The measurements of the temperature of the sample show that the temperature elevation does not exceed several degrees. The irradiation time provides an exposure even greater than that when the sample is irradiated in the vicinity of the LED source. No evidence of the luminescent species has been detected even after the subsequent annealing in the furnace at temperatures of about 100 ̊C.
At the moment we cannot differentiate between the photochemical and photothermal effects in the process resulting in the CdS-related luminescent complex formation.
5. Conclusions
To conclude, we report on the main results of the study.
We suggest using [Cd(N(SCNEt2)2)2] as a precursor of CdS nanoparticles in polymer matrices. This precursor is well dissolved in PMMA, offering an opportunity to prepare visually transparent PMMA films with a precursor content up to 10%.
We show that a LED operated at 365 nm is an appropriate light source for the material alteration in a way of appearance of induced green luminescence typical for CdS nanoparticles within the polymer matrix. The process of the formation of luminescent centers under the effect of LED radiation is complicated and cannot be reduced neither to pure photochemical nor to pure photothermal effects.
Acknowledgments
The authors thank Russian Scientific Foundation (Grant No. 14-19-01702) for financial support.
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