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Abstract
We offer to use optical features of surface plasmon resonance in Ag nanoparticles for jewelry application as a method for the well-controlled decoration of silver items. The novel approach of silver nanoparticles formation with sizes from 5 to 50 nm via nanosecond direct laser writing allows for controlling the reflectance spectra, thus creating a color image on precious metals with a high resolution of about 450 dpi without dyes or hazardous chemicals. Moreover, the large-scale color image can be applied in single-step processing with significant productivity of 2 cm2 per minute. This work opens a strong direction for the practical application in the jewelry industry, art, and coining.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Silver is one of the most popular and available precious metals in the world nowadays, which is widely used for jewelry making. Nowadays, there are many ways to decorate a product in the jewelry industry. Among them, we can cite creating ornaments, changing the color and shape of the product, the use of various alloys and gemstones. The product color plays, however, a crucial role for visual assessment of its attractiveness.
There are various ways of color image application on precious metals such as oxidation [1,2], electrolyte coatings [3], hot and cool color enamels [4], the creation of colored patina by immersing into various chemical agents [5,6] and others. These methods are not universal and have several limitations because of the complexity of drawing an image with a good resolution. Moreover, the obtained coatings are often nondurable and crack-sensitive, so that the available color palette is limited, and these technologies are rather expensive. Therefore, it sounds considerably more promising to control the optical properties of the material itself and to manage the behavior of color using the objective parameters (for example, color coordinates) and dependences.
The coloring of precious metals by laser treatment is promising since this method allows controlling optical response of materials with high precision. Thus, there are many perspectives of formation of high-resolution color images at the contactless action on the material, that provides guaranteed reproducibility of results in the long run. To change the optical properties and consequently the color of metals, two primary mechanisms can be usually applied: interference of light in a thin oxide layer, formed on the surface by laser heating [7–9] and diffraction of light on periodic surface structures (PSS) formed by interference of laser radiation with a surface electromagnetic wave [10–13]. The adoption of the first mechanism for precious metals is problematic due to their poor reactive capability with oxygen. Nevertheless, in our previous work [14], we have already proposed an effective method of color imprinting of silver and gold by the local laser oxidation of a titanium film sputtered on the surface. However, the implementation of this technology requires the use of additional consumables and several processing steps and installations. Changing the reflection of precious metals (by the example of platinum) through the development of PSS has also been demonstrated [15,16]. However, it is possible to get only colors iridescent under various illumination angles thus colors are not controllable.
Also, it is known that nanoparticles (NPs) of a noble metal, as well as dielectrics, possess the strong resonance interactions in the visible wave range thus bright, vibrant colors can be obtained. For example, there are several remarkable works on using dielectric NPs [17,18] and mainly silicon [19,20] for optical recording and color printing. Among various methods of NPs synthesis it is worth noting several perspective techniques which give the most notable result such as formation of NPs in a liquid by pulsed laser ablation [21–24], laser-induced dewetting of ultrathin films [25,26], electron [17,19,27–29], ion [30] and two-photon beam [31] lithography, milling [32] and hot embossing or nanoimprint lithography [33,34], nucleotide-based assemblies method, plasmon-mediated process under the green light irradiation [23]. For example, Kumar et al. [27] using electron beam lithography produced color images with resolution as high as 100,000 dots per inch. However, compared with other methods laser fabrication possesses some definite advantages such as the possibility of changing optical properties during the whole process. For instance, Zhu X. et al. [35] suggested using laser post-writing for optical tuning of plasmonic colors via reshaping of already imprinted nanostructures.
There are also several notable works related to laser induction of thin noble metal films for nanoparticle formation. For this ultrashort [26], nanosecond [25] and CW [36] lasers are shown to be prospective in different kind of applications such as coding of data storage [31,36], SERS [37], sensors [30], nanoplasmonic [36,38] and photovoltaic [39]. Besides this method makes it possible to obtain rather a narrow size distribution of NPs it requires additional technological step which is thin metal film deposition. Thus, its application for jewelry marking is irrelevant.
Recently, laser coloration of silver coins related to this effect in Ag NPs has been suggested in [24] where the coloring of silver was achieved by ultrashort laser irradiation of the material in the air. The position of the surface plasmon resonance peak and, subsequently, the color of the surface depends on the composition, shape, and size of the produced NPs.
However, the described techniques cannot provide high repeatability, productivity, and they are mostly a subject of high costs. Encouraged by J.-M. Guay et al. [24], we investigated the possibilities of nanosecond laser irradiation from the perspective of NPs structural color formation and controlling the optical properties of them. In this paper, we propose for the first time to use a widely available fiber laser with nanosecond pulse duration as a tool to control optical properties of the silver surface in visible range due to plasmon resonance in laser-induced NPs in the air. The relation between reflectance spectra of the treated silver and NPs size and distribution is under the investigation. We consider here also the behavior of plasmonic colors on CIE xy Chromaticity diagram. All optical measurements are supported by simulations in the frame of Bruggeman effective medium approximation for determined NPs size.
The result of laser treatment is highly regulated by laser processing parameters, and the use of fiber lasers allows producing large-scale images in several minutes. Such treatment can be readily adapted for manufacturing, since such fiber lasers are widely used in industry, including jewelry (for such technological operations as cutting, welding, marking, volume engraving, etc.), because of their high efficiency and reliability.
2. Experimental section
2.1 Materials
In our experiments, we used a silver substrate (0.925 silver) with dimensions of 30 × 10 × 0.5 mm. Silver plates were polished with a polishing compound for jewelry and a felt disc (Ra = 1.6 µm). Then the samples were treated with acetone and washed in an ultrasonic bath with distilled water.
2.2 Laser processing
The laser exposure was carried out in the air by using a commercially available system based on ytterbium pulsed fiber laser with a wavelength λ = 1.06 µm (Laser Center Co., Ltd., Russia). The laser generates pulses of τ = 4-200 ns duration at repetition rates in the range of f = 20–99 kHz. The spot diameter in focus is d0 = 50 μm. Intensity was in the range I = (1.2-50.9)·107 W/cm2, as demonstrated in Fig. 1.
2.3 Characterization
Spectrophotometric measurements were carried out to identify the colors obtained under different processing conditions on the silver surface. To get the reflection spectra, the Ocean Optics CHEM4-VIS-NIR USB4000 spectrophotometer and R200-12-MIXED Reflection/Backscattering Probes were used [7]. The reflection coefficients were measured relative to the reflective standard for mirrors certification with the use of the Ocean Optics ecoVis light source.
The color coordinates were calculated according to CIE 1931 standard colorimetric methods for a 2° visual field, corresponding to the area of the highest clarity and chromaticity of the image perceived by the observer. Type C light source was used as a standard illuminant, representing average daylight with a low/partly cloudy sky which are ideal conditions for the observer.
Visual analysis of the surface at the microscale was done by optical microscopy (Zeiss Axio Imager A1M). More detailed morphology and elemental analysis of the resulting structures were carried out by Zeiss Merlin scanning electron microscope (SEM) with an attachment for the X-ray microanalysis of Oxford Instruments INCA X-Act. The analysis of the treated surface was carried out using a SENTERRA T64000 Raman spectrometer, which allows the detection of Raman spectra in the 80-4500 cm−1 range with a spectral resolution of 3 cm−1. Excitation performed with 785 nm diode-pumped solid-state laser with a power of 1 mW, and all spectra were collected with an accumulation time of 80 s.
Modeling of spectral reflection of a silver substrate with Ag NPs on the surface was carried out using the Mie theory to solve the Bergemann’s equation, which showed the perfect agreement with experimental data [40,41]. Here, Ag NPs were considered to be much smaller than the wavelength of incident radiation [42]. The laser formation of silver NPs was carried out in the air, which increased the probability of their oxidation to the state of Ag2O during deposition on the surface. Therefore, a layer of thickness h = 300 ± 50 nm consisting of Ag–Ag2O NPs surrounded by the air (dielectric function εair = 1.01), was considered as an effective medium. Ag–Ag2O NPs were evenly distributed over the entire volume of the layer. Both the percentage ratio of silver to its oxide μ = VAg / VFilm and the volume fraction of the medium ν = VAir / VFilm were taken into account. Herein, VFilm is the volume of the layer and VAir is the pore volume.
It was assumed that such layer was located on a silver substrate, and a transfer of electrons from NPs to the substrate is prohibited. This is due to the formation of Ag NPs from the vapor-gas phase occurs in the air, followed by their deposition on the silver substrate. In this case, both the outer part of the NPs and irradiated silver surface were partially oxidized.
The reflection RNPs from the NPs reached layer, its transmission TNPs, and the total reflection RAll with an account of the substrate, were determined by the following equations [43]:
where c is the speed of light in vacuum, RAg is silver reflectance before laser treatment. εeff – dielectric function of the effective media, ω – frequancy of initial radiation3. Results and discussion
3.1 Development of colors
Color images were produced on the silver surface by nanosecond laser pulses. By varying the laser parameters, such as laser intensity, I, the number of pulses at the point along the x (Nx) and y (Ny) axes, we obtained different palettes of colors as shown below in Fig. 2.
Fig. 2 The color palettes developed on the surface of silver under the following modes of laser action: I = [(2.80-3.38)·108 W/cm2], A: Nx = [8–17], Ny = 4; B: Nx = 11, Ny = [2–11].
With the increase of Nx, Ny, the reflectance spectra minima are red-shifted, which can be seen in Fig. 3, the same trend persists with increasing of laser intensity. To characterize the colors of the obtained samples, reflection spectra in the visible range were measured, which can be seen in Fig. 3(a), and color coordinates in the XYy system (for the C type light source), marked on the CIE xy Chromaticity diagram in Fig. 3B, were calculated. Several colors (S1, S5, S7 and S10) were selected from the palette shown in Fig. 2(a) for the follow-up observations.
Fig. 3 Color characterization of the samples: Reflection spectra of the surface of silver samples (from S1 to S10) after laser action (A), the color coordinates of palettes shown in Fig. 2 (for the C light source), marked on CIE xy Chromaticity diagram (B).
3.2 Examination of the laser-induced structures
SEM analysis of different regions revealed that particles with sizes in the range from 10 ± 5 nm to 50 ± 8 nm were formed on the surface. Statistical analysis of SEM images performed by ImageJ software showed that different colors are characterized by well-defined particle size ranges where the size rises with the increase of Nx which is demonstrated in Fig. 4 and Table 1.
Fig. 4 The morphology of obtained colors on the samples S1 (a), S5 (b), S7 (c), S10 (d): Optical microimages, SEM images, and size distribution histograms: 2r(S1) = 15 ± 6 nm, 2r(S5) = 18 ± 7 nm, 2r(S7) = 24 ± 5 nm, 2r(S10) = 43 ± 10 nm
Table 1. Energy Dispersive X-Ray Analysis (EDX) of samples.
The formation of NPs occurs according to the following mechanism. An intense nanosecond laser pulse initiates ablation of the silver target and plasma formation. In turn, high repetition rate leads to fast temperature increase and plasma plume expansion. NPs formation occurs when small droplets and clusters of evaporated material rapidly condense and solidify. Shape and size of the synthesized NPs are related to the laser processing parameters and the characteristics of the surrounding media. Note, that for lower values of Nx (e.g., lower intensity), NPs are mostly spherical shaped, as seen in Figs. 4(a)-4(c), whereas for larger Nx values NPs tend to agglomerate, as shown in Fig. 4(d) [21].
With the rise of Nx the average size of NPs increases. When Nx reaches the value of 17, NPs start to agglomerate. It can be seen from Fig. 4(d), that sample S10 differs from other samples because of NPs aspherical shape.
The reflection spectra of the silver substrate after laser treatment were simulated for certain experimental data S1, S5, S7 and S10, as seen in Fig. 5. The volume fraction of the medium is found to be extremely low in all cases: ν(S1) = 0.10, ν(S5) = 0.127, ν(S7) = 0.15, ν(S10) = 0.13 due to high density of NPs. Such assumptions are supported by SEM data. The ratio of silver to its oxide in the layer was taken as μ(S1) = 0.83, μ(S5) = 0.65, μ(S7) = 0.28, μ(S10) = 0.02.
Fig. 5 Reflectance spectra of the silver substrate covered by NPs of radiuses r(S1) = 6.5 nm, r(S5) = 9 nm, r(S7) = 10 nm, r(S10) = 21.5 nm: dashed – experimental, solid – simulated results; vertical lines show SPR peak positions.
A broad peak (minimum) in the region of 350–550 nm is referred to SPR on Ag NPs. A broadening of the absorption peak is determined by the size of NPs and wide size distribution and non-uniform NPs shape. In a good accordance with our experimental results, the absorption peak redshifts with the increase of NPs size, and peak intensity simultaneously decreases. Finally, for the samples with large NPs (S10) reflection spectrum is relatively plane. This also interprets the nontypical shape of the reflectance spectrum of S10 as shown in Fig. 3(a).
The Energy Dispersive X-Ray Analysis showed a low oxygen content (2-7%) in the irradiated samples (Table 1), which indicates a small thickness of the oxide film.
The Raman spectrum obtained at Senterra (Bruker) Raman spectrometer under 20x (NA = 0.4) objective for S5 samples (I = 3.10·108 W/cm2, Nx = 12, Ny = 4) is demonstrated in Fig. 6.
Fig. 6 Typical Raman spectra of the silver substrate after laser exposure: raw data (red) and baseline corrected with further normalization (black).
In the low-frequency part of the Raman spectra, one can find the intensive peak with a maximum at about 250 cm−1. The peak corresponds to the oxygen adsorption in metal-oxygen bonds, i.e., Ag-O bonds [44,45]. The peaks observed in the higher frequency region are presumably associated with the vibrations of the oxygen-carbon bonds and carbon-carbon bonds and other functional groups associated with them. Peaks at 931 cm−1 and 1396 cm−1 which are seen in Fig. 5 and are attributed to the ν(C COO-) and ν(COO-) modes, respectively [46–48]. A narrow sharp peak at 1003 cm−1 and peak at 1600 cm−1 indicate the formation of benzene rings, and they can be interpreted as the breathing mode in the benzene ring and a double bond stretching mode (ν(C = C)). Peaks at 1751 cm−1 and 1786 cm−1 are attributed to ν(C = O) [47,49,50]. The peaks at 1076 cm−1, 1042 cm−1 and 836 cm−1 lie in the region which is typical for stretching vibrations of the single bonds C-C, O-O and C-O. Lower frequency peaks at 612 cm−1 and 697 cm−1 are possibly attributed to the deformation and bending vibrations in carbon and oxygen-containing groups [51].
3.4 Laser coloration of silver for jewelry and over applications
An overall understanding of the origin of produced colors and concomitant effects described above allow us to precisely control the results, which is critically essential for the introduction a technology into industry. Laser irradiation, in this case, is a perfect tool for NPs formation that widens possibilities of the technology due to the high localization of the heat affected zone and the opportunity to set the specific processing parameters. In this case, the resulting resolution of the image depends on the focal spot size and in our instrumental configuration can attain the 450 dpi. On the other hand, the maximum size of the image is limited only by the scan field of the lens thus the 100 × 100 mm2 image is easily applied in a single step. Moreover, due to high repetition rates and accessible scanning speed of nanosecond fiber laser systems the coloring is relatively rapid and the productivity of about 2 cm2 per minute can be achieved that Visualization 1 and Visualization 2 demonstrate. The images obtained by using the developed technology are shown in Fig. 7.
One can observe that colors are uniform and vivid, and the applied image has high contrast against the background of polished silver. Therefore, this method can be attractive not only for the jewelry industry but for instance for coining and fine art.
4. Conclusion
In this paper, we have performed laser coloring of silver plate, by creating NPs due to surface plasmon resonance. Spectrophotometric measurements have been carried out to identify the colors obtained under different laser treatment conditions on the silver surface.
We have also studied the chemical composition of the treated surface using Raman spectroscopy, and an SEM analysis of the investigated surfaces was carried out. An analysis of the chemical composition showed that nanostructures generally consist of the silver with the low proportion of Ag-O throughout the used laser processing parameters connected with the particles formation.
Moreover, the processing parameters strongly affect on the size of the obtained particles and, thus, the location of the SPR peak and resulting color of the surface. It is found that with the rise of intensity or Nx, Ny the average size of NPs increases that finally leads to agglomeration of NPs and changes of NPs shape. Our experimental data as well as analytical simulations demonstrate that the reflectance spectra minima are red-shifted with the increase of NPs size. Thus, optical properties of silver substrates are adjusted by laser processing parameters selection.
Therefore, such well-controlled dependence can be utilized for an industry. Moreover, the developed technology of coloring of precious metals can be widely used for industrial applications such as art, jewelry and architectural objects due to its high repeatability and commercial availability of nanosecond fiber lasers. This technology of plasmonic color printing allows applying designs on the surface using only one technological circle which minimizes the production time of 2 cm2 per minute.
Despite this work being proposed mostly for the marking and coloration applications, further development of this effect can be attractive in such areas as nanoplasmonics, photovoltaics, metamaterials fabrication and other perspective fields of research.
Funding
Ministry of Education and Science of the Russia Federation (research agreement no. 14.587.21.0037 [RFMEFI58717X0037]).
Acknowledgments
Measurements of Raman spectra were done on the base of Centre for Optical and Laser Materials Research, and SEM measurements were done on the base of Interdisciplinary Resource Centre for Nanotechnology, Research Park, St. Petersburg State University.
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