STUDYING OF THE INFLUENCE OF THE PARAMETERS OF LASER-INDUCED DEPOSITION OF SILVER NANOPARTICLES ON THEIR MORPHOLOGY AND PHYSICAL PROPERTIES

STUDYING OF THE INFLUENCE OF THE PARAMETERS OF LASER-INDUCED DEPOSITION OF SILVER NANOPARTICLES ON THEIR MORPHOLOGY AND PHYSICAL PROPERTIES

Aleksander Shmalko

engineer, St. Petersburg State University,

Russia, St. Petersburg

Aleksei Zakharov

student, St. Petersburg State University,

Russia, St. Petersburg

Guliia Bikbaeva

Ph. D. student, St. Petersburg State University,

Russia, St. Petersburg

Alina Manshina

doctor of chemical sciences, Professor, St. Petersburg State University,

Russia, St. Petersburg

Introduction

Thin conductive films consisting of noble metal nanoparticles (NPs) are of particular interest for the production of advanced optical and electronic devices [1–4]. The literature has reported on the possibility of forming thin films consisting of NPs on dielectric substrates surface [3–5]. However, to ensure high conductivity of such systems, organic compounds remaining after synthesis and covering them must be removed by plasma cleaning or high-temperature annealing [3–5].

Silver is one of the most attractive metals for fabrication such thin films due to its unique electrical, optical, and catalytic properties [6–7]. Ag nanoparticles demonstrate a strong plasmonic enhancement of the electromagnetic field, which makes it possible to use them as active components in optical sensors [6].

Laser-induced methods allow to metallize various substrates at high speed and using small amounts of reagents. The most common methods include pulsed laser ablation [8], laser-induced forward transfer (LIFT) [9], and direct synthesis under an intense laser beam [10]. However, these methods are based on the use of high-intensity pulsed laser irradiation, which limits their applicability and the list of substrates that can be modified with metal nanoparticles.

Recently, the laser-induced deposition (LID) method has attracted a lot of attention. It is a straightforward technique which utilizes low-intensity CW laser radiation for photochemical decomposition of metal complexes. It allows to fabricate metal structures of a certain shape on various substrates, to control morphology, and fabricated NPs have good adhesion.

In this paper, rectangular structures composed of silver nanoparticles were synthesized on glass substrates via LID process and the effect of the parameters on the morphology and physical properties was studied.

Experimentals

Materials and Reagents

In this study, we utilized silver benzoate hydrate (Alfa Aesar) as a silver precursor and methanol (Reachem) as a solvent. All reagents were analytical grade and did not require further purification. Cover glasses (Epredia) with a thickness of 0.17 mm and dimensions of 10 mm x 10 mm were used as substrates for LID. The concentration of the silver precursor used remained constant at 0.334 mM.

Synthesis of Thin Films

The silver benzoate solution was subjected to sonication for 10 minutes to ensure complete dissolution. The absorbance spectrum of the resulting solution is shown in Figure 1. To match the characteristic absorption bands of the solution, a laser wavelength was selected for the laser-induced deposition (LID) process. In this case, a Coherent MBD266 solid-state, continuous-wave UV laser operating at 266 nm was used. For the synthesis of a monolayer of silver NPs, an unfocused laser beam with a spot diameter of 5 mm and an intensity of approximately 460 mW×cm^-2 was employed. To obtain rectangles, a cover glass mask with a 3 x 5 mm cut hole was utilized. Figure 2 illustrates the scheme of the LID process. The laser beam was directed to the cuvette, passing through two focusing lenses, reflected from an aluminum mirror, and was directed through the solution to the substrate-solution interface. The total volume of the solution employed was 3 ml. Every 20 minutes, the cuvette was rotated by 180° and the old precursor solution was drained and a new was poured. Following the process, all samples were thoroughly rinsed with methanol and dried at room temperature.

Figure 1. Absorbance spectrum of a methanol solution of silver benzoate

Figure 2. Scheme of the process of LID process

Equipment

The absorbance spectrum of the silver benzoate solution and the spectra of the samples before and after plasma purification were recorded on Shimadzu UV-3600 spectrophotometer (Japan). The morphology and elemental composition of the fabricated thin films were studied using a Zeiss Merlin scanning electron microscope (Obergochen, Germany) with an INCA X-Act energy dispersive analyzer, Oxford Instruments (United Kingdom). Plasma cleaning of silver nanostructures in an argon atmosphere was carried out on a Diener Zepto purifier (Germany). The current–voltage characteristics were measured on a Keithley 6487 picoammeter (USA).

Results and Discussion

Figure 3 illustrates SEM images of thin films composed of silver nanoparticles synthesized within 20, 40 and 60 minutes.

Figure 3. SEM images of a longitudinal section of samples obtained within 20 minutes (a), 40 minutes (b), 60 minutes (c)

It can be observed from the SEM images that spherical silver nanoparticles are deposited on the glass substrate in the form of a monolayer. The particle size in the case of synthesis within 20 minutes was 40-60 nm (Fig. 3a), 40 minutes - 90-110 nm (Fig. 3b), 60 minutes - 140-160 nm (Fig. 3c).

The sample synthesized within 20 minutes, was investigated by energy dispersive X-ray (EDX) analysis, the results are shown in Table. 1.

Table 1.

Percentage of elements in a sample obtained within 20 minutes

It was confirmed that the fabricated thin films were composed of silver. The presence of other elements in the EDX spectrum can be attributed to the substrate material.

Preliminary resistance measurements showed that all samples had a high resistance without plasma purification, which is due to the fact that the nanoparticles were covered with the organic residues left after synthesis. After cleaning with argon plasma, the resistance decreased significantly. The absorbance spectra of samples synthesized for 20 minutes before and after plasma purification are shown in Fig. 4.

Figure 4. Absorbance spectra of the samples synthesized for 20 min before and after plasma purification

The spectrum before plasma cleaning has two absorption bands with maxima at 420 and 630 nm, which can be attributed to the absorption of silver and the organic component, respectively. After plasma cleaning, only one band is observed in the spectrum, which is related to the absorption of silver.

Figure 5 illustrates the current-voltage characteristic measured for all samples after plasma cleaning.

Figure 5. Current-voltage characteristics of the samples obtained within 20 minutes (a), 40 minutes (b), 60 minutes (c)

It is worth noticing that all current–voltage curves are linear. Additionally, we calculated the resistance of the silver nanostructures according to Ohm's law, the results are summarized in Table 2.

Table 2.

Calculated resistance of thin films of silver nanoparticles from the I-V curves, normalized per unit area

Conclusion

In this article, we used a straightforward and single-stage approach to synthesis of thin films composed of silver nanoparticles based on laser-induced deposition (LID) method. The influence of various synthesis parameters on the morphology and physical properties of the fabricated samples was studied.

Scanning electron micrographs revealed that silver NPs are deposited on a glass substrate in the form of a monolayer of spherical nanoparticles. With an increase in the synthesis duration, the thickness of the resulting film increased uniformly. Nanoparticles formed a continuous film without gaps and they were in close contact with each other, which resulted in low resistance after plasma purification. It was confirmed by the EDX analysis that the deposited nanostructures consisted of silver.

Plasma cleaning was carried out to remove organic compounds covering silver NPs, afterwards the current-voltage characteristics of the pure samples were measured. The results disclosed that the resistance of the films decreased with increasing synthesis length, which is associated with an increase in the size of silver nanoparticles. This leads to an increase in the conductivity of the NP monolayer due to an increase in the available contact points.

In general, the experimental results indicate the successful synthesis of thin films of rectangular shape consisting of silver nanoparticles LID method. It offers new opportunities for the development of advanced optical and electronic devices, and also finds the application in various fields of science and industry. Further studies can be aimed at optimizing the synthesis parameters and studying other potential applications of the films obtained from silver NPs.

Acknowledgements

The authors would like to acknowledge the "RC for Optical and Laser Materials Research" and the "IRC for Nanotechnology" of St. Petersburg State University for their valuable technical support. Analysis of the samples via SEM and EDX methods was carried out at the Interdisciplinary Resource Center for Nanotechnology, Science Park of SPbU, as part of project No. АААА-А19-119091190094.

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