We report a novel and efficient method for the laser direct writing of two-dimensional silver structures. Multiphoton absorption of a small fraction of the output of a Ti:sapphire oscillator is sufficient to photoreduce silver nitrate in a thin film of polyvinylpyrrolidone that has been spin-coated on a substrate. The polymer can then be washed away, leaving a pattern consisting of highly interconnected silver nanoparticles. We report the characterization of the silver patterns using scanning electron and atomic force microscopies, and demonstrate the application of this technique in the creation of diffraction gratings.
©2005 Optical Society of America
The use of multiphoton absorption (MPA) to fabricate microstructures is experiencing rapid growth. Because MPA can be confined within the focal volume of a tightly-focused laser beam, it is possible to effect highly localized photochemical or photophysical changes in a material. By controlling the position of the focus, intricate 3-D structures can be sculpted with sub-micron resolution. While the majority of research in MPA microfabrication has involved creation of polymeric structures [1–8], it is desirable to be able to deposit other materials as well. Metals are particularly important in this regard, particularly for applications in electronics, optics and magnetics, and efforts in metal deposition via MPA have been reported by a number of groups [9–12]. However, metals such as gold and silver do not have sufficient mechanical strength to make self-supporting microstructures from them. As a result, studies have generally focused on generating metallic patterns within rigid matrices. Only one technique has been shown capable of creating continuous patterns that can be removed from the matrix used for fabrication, but this method requires the use of a specialized photoreducing agent that is not commercially available . A complementary approach would be to develop a means of depositing metallic features selectively on 3-D polymeric microstructures created using MPA. As a first step towards this goal, here we introduce a simple and efficient technique for MPA direct writing of unsupported 2-D silver features with readily available materials.
2. Experimental procedure
Our approach was inspired by previous work that demonstrated that an argon ion laser could be used to deposit silver on a substrate from silver nitrate in liquid N-methyl pyrrolidone  or in a film of polyamic acid . To prepare samples, 0.25 grams of polyvinylpyrrolidone (PVP, molecular weight 40,000) were dissolved in 10 mL of ethanol at room temperature. When the polymer was dissolved completely, 0.4 grams of silver nitrate in 2 mL of deionized water were added. The solution was stirred for 25 minutes in the dark at room temperature. During this period, the originally colorless solution assumed a dark yellow/orange color.
The surface of a 2 cm×2 cm piece of a microscope slide was modified with (3-acryloxypropyl)trimethoxysilane . The glass was covered completely in the polymer solution and then a film was spin coated at 1000 rpm for 30 seconds. The thickness of the film was approximately 1 micron, as measured by atomic force microscopy (AFM). The film was baked at 110 °C for 25 minutes. After cooling the sample was affixed to a microscope slide with tape that also served as a spacer between the film and the cover slip.
Fabrication of silver structures took place on an upright multiphoton microscope that has been described in detail elsewhere . The excitation source was a commercial Ti:sapphire laser (Coherent Mira 900-F) that produced pulses with a repetition rate of 76 MHz, a center wavelength of 780 nm, and a duration of 250 fs. Once the laser had been focused at the surface of the glass slide, metal patterns were created by movement of the computer-controlled sample stage. Following silver deposition, the sample was washed in ethanol for 30 minutes and then rinsed in methanol to remove the PVP. No deposition of metal was observed at any intensity when the laser was not modelocked, which substantiates the multiphoton nature of the deposition process.
Figure 1 shows scanning electron microscope (SEM) images and Fig. 2 shows transmitted light optical images of the results of typical experiments. The patterns were created using a 20×, 0.5-NA objective to deliver 35 mW of power to the sample. The lines were created by translating the sample at a velocity of 80 µm/sec. Silver can be deposited at considerably smaller powers if lower stage velocities are employed, and deposition is significantly more efficient when shorter pulses are employed.
Although in reflected-light microscopy the metallic features appear quite shiny, the SEM images in Fig. 1 demonstrate that the lines are composed of agglomerated silver nanoparticles ranging in diameter from tens to hundreds of nanometers. While the nanoparticles are interconnected, the deposition process does not form a completely continuous layer of metal.
The lack of continuity is further confirmed by experiments that indicate that the features have resistances in the MΩ/cm range, although oxidation of the silver may contribute to this poor conductivity as well. There is no sign of residual polymer in areas in which deposition of metal did not occur, and so it appears that the polymer matrix has been removed completely. The lines are on the order of 5 µm across, although lines that are approximately 1 µm across can be fabricated under different conditions. These feature sizes are larger than might be expected from the diffraction limit, which suggests that local heating plays an important role in the deposition process. In support of this idea, as illustrated in Fig. 2(a) it is more difficult to initiate deposition in a pristine portion of the sample than it is at the edge of a feature that has been deposited previously. Additionally, while metal is deposited across the diameter of each line, there are ridges apparent at each edge of the line where the metal is thicker.
To further characterize the morphology of deposited lines, an AFM study was performed, representative results from which are shown in Fig. 3. Once again, the line can be seen to be comprised of individual nanoparticles with diameters up to hundreds of nm. Statistics obtained from several images indicate that the centers of the metallic lines are 21±9 nm thick and the ridges at the edges of the lines are 126±17 nm thick. Because the height of the centers of the lines is considerably less than the apparent diameter of many of the nanoparticles, we can conclude that the deposition process creates particles that are significantly oblate. Furthermore, the fact that the centers of the lines in particular (but also the ridges) are considerably thinner than the original polymer film is consistent with essentially complete removal of the polymer matrix. AFM studies additionally show that the ridges are only apparent after the polymer has been washed away.
We can use these results to gain insights into the metal deposition process. Stellacci et al. introduced silver nanoparticles into a polymeric matrix and were able to grow them via MPA ; we believe that a similar process is at work in our system. Transmission electron microscopy reveals the presence of polydisperse silver nanoparticles in the PVP/silver nitrate solution that give it its yellow-orange color. One possibility is that two-photon absorption into the plasmon band of these silver nanoparticles deposits heat that drives electroless deposition, causing them to grow. As shown in Fig. 2(b), the silver features luminesce as they are deposited, which is consistent with the reported observation of strong luminescence from silver clusters composed of a few atoms . If there is enough absorption by the clusters to generate luminescence, heating would be expected to be present as well. This is consistent with the seeding of deposition by existing features, as in Fig. 2(a). In areas in which silver clusters have not been deposited, the existing concentration of clusters may not be sufficient to deposit enough heat to promote deposition without increasing the laser intensity, whereas in the areas where silver has been deposited the concentration of absorbers is high enough to facilitate deposition at lower intensities. The ridges in the deposited lines may therefore result from deformation of the polymer film due to heating in the deposition process. However, the actual photodeposition mechanism is clearly more complicated than this. Preliminary experiments on the wavelength dependence of the deposition process demonstrate that the efficiency of the process is linked more closely to the absorption spectrum of the polymer host than to that of the silver nanoparticles in the film, suggesting that it is photoreduction by PVP that is responsible for silver deposition. One possibility that is consistent with these observations is that existing silver features promote further silver deposition via local electric field enhancement by the silver particles that increases the effective two-photon absorption cross-section of the PVP. The microscopic details of the deposition process are a subject of continuing investigation.
Although the features we have deposited are not electrically conductive, they are suitable for some applications in optics, as the metal is optically dense and the deposited particles are much smaller than the wavelength of visible light. To investigate the optical properties of deposited structures, complex concentric grating structures were created (Fig. 4(a)–(c)). The spacing between adjacent lines in these patterns is 20 µm, and each structure is on the order of 1.2 mm in diameter. The lines were created at a velocity of 80 µm/sec and a power of 60 mW at the sample. A HeNe laser was used to examine diffraction in transmission mode (Fig. 4(d)–(e)). The high quality of the gratings is evident from the well-resolved diffraction spots. Comparison of the intensities of the central spot and the diffracted spots in each case indicates that the gratings are efficient. Similar results were obtained in reflection mode.
In conclusion, we have developed a new technique for the multiphoton laser direct writing of silver patterns at low laser powers. This technique uses inexpensive and readily-available materials, and the polymer matrix can be removed completely after deposition. While the deposited structures are not conductive, they do show favorable optical properties. We are currently exploring using this technique to pattern silver on 3-D microstructures and using electroless deposition to make the patterns conductive.
This work was supported by the National Science Foundation, Grants ECS-0088438 (JTF) and ECS-0210497 (MJN). JTF is a Research Corporation Cottrell Scholar and a Camille Dreyfus Teacher-Scholar.
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