Printing Silver Nanogrids on Glass - Journal of Chemical Education

Laboratory Experiment pubs.acs.org/jchemeduc

Printing Silver Nanogrids on Glass Wesley C. Sanders,*,† Ron Valcarce,‡ Peter Iles,§ James S. Smith,† Gabe Glass,† Jesus Gomez,† Glen Johnson,† Dan Johnston,† Maclaine Morham,† Elliot Befus,† Aimee Oz,† and Mohammad Tomaraei† †

Engineering Department, Salt Lake Community College, Salt Lake City, Utah 84123, United States Chemistry Department, Salt Lake Community College, Salt Lake City, Utah 84123, United States § Division of Natural Sciences, Salt Lake Community College, Salt Lake City, Utah 84123, United States ‡

S Supporting Information *

ABSTRACT: This manuscript describes a laboratory experiment that provides students with an opportunity to create conductive silver nanogrids using polymeric templates. A microcontact-printed polyvinylpyrrolidone grid directs the citrate-induced reduction of silver ions for the fabrication of silver nanogrids on glass substrates. In addition to template-directed nanofabrication, students gain experience using a sputter coater, a scanning electron microscope, and an atomic force microscope. Students are also introduced to elemental analysis using energy dispersive X-ray spectroscopy. Institutions with limited characterization capabilities can use a compound light microscope to view the structures reported in this manuscript.

KEYWORDS: Second-Year Undergraduate, Upper-Division Undergraduate, Analytical Chemistry, Materials Science, Laboratory Instruction, Hands-On Learning/Manipulatives, Nanotechnology, Surface Science, Polymer Chemistry

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INTRODUCTION There is a need for undergraduate laboratory experiments that address concepts associated with conductive polymer nanostructures. A report prepared by Nguyen and co-workers justifies this claim.1 The report states the number of publications regarding conductive polymer nanostructures increased from 500 in 2004 to 2500 in 2015; this demonstrates a rapid growth in the interest and utility of these materials.1 Currently, there is significant incorporation of conductive, organic nanostructures in commercially available electronic products.2 For instance, smart phones contain active matrix organic light emitting diode (AMOLED) components in the display.3 Utilization of conductive polymeric nanostructures in consumer electronic devices is on the rise because these materials are lightweight, flexible, easy to process, and chemically stable.4 Additionally, these materials can be tailored to optimize characteristics such as charge mobility or luminescent properties.2 The rapid increase in commercialization and research associated with conductive polymer nanostructures is why there is a need for undergraduate laboratory exercises that expose students to these materials. The authors of this manuscript aim to address this need. There are reports that describe laboratory experiments that allow students to print polymeric nanostructures.5−7 Also, two major procedures incorporated in this laboratory experiment have been reported previously. These procedures involve printing polymeric grids5 as well as the citrate reduction of silver ions.8 However, this experiment is novel because it involves localizing © XXXX American Chemical Society and Division of Chemical Education, Inc.

the reduction of metal cations using student-fabricated nanoscale polymer templates. To date, there are no laboratory experiments reported in the literature that involve creating conductive nanostructures using this method. This laboratory experiment is appropriate for general chemistry because it involves such topics such as ion−dipole interactions, polymers, and redox reactions. Silver Nanogrid Formation

One of the major components of this laboratory experiment involves using polyvinylpyrrolidone (PVP) to direct the formation of the silver nanogrid. The authors would like to note a previous report that describes the use of PVP as a capping agent to control the size of silver nanoparticles formed from the reduction of silver ions with sodium borohydride.9 The experiment reported in this manuscript does not use PVP as a capping agent. PVP directs the formation of the silver nanogrid via ion−dipole interactions. This interaction is reported in the literature. PVP has a strong tendency for complex formation with small molecules and readily interacts with metal cations in solution.10 Silver ions can coordinate along the backbone of the PVP polymer chain via the carbonyl group, as shown in Figure 1.11 Then, formation of nanoscale silver particles is encouraged in the presence of an appropriate reducing agent.11 In this experiment, the silver nanogrid is Received: September 13, 2016 Revised: January 12, 2017

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DOI: 10.1021/acs.jchemed.6b00704 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Aldrich) in 30 mL of 100% isopropyl alcohol. In addition, faculty cut glass microscope slides into several square pieces approximately 3 × 3 cm in size using a diamond scribe. The glass substrates and PDMS stamps were sonicated in 100% isopropyl alcohol for 10 min and then dried with a stream of nitrogen and stored until use. Beaded PVP structures instead of linear structures were observed when stamps and glass substrates were not sufficiently dried. It is stated that high levels of humidity can result in the formation of beaded, nonlinear PVP structures.16 Moisture on the surface of the stamps and glass substrates was removed by placing them on a clean, dry microscope slide with the pattern on the PDMS stamps facing upward. The microscope slide was then placed on a hot plate and heated using a setting of 5 for approximately 10 min prior to the experiment. Execution

This faculty-led experiment was conducted by students enrolled in a materials science internship/co-op course during the Fall 2015 and Spring 2016 semesters. A total of eight second-year STEM students participated in this experiment. Students began by assembling spin coaters using procedures described in an earlier report.5 Students assembled the spin coaters and printed PVP grids on glass substrates in approximately 15 min. Procedures associated with the microcontact printing of the PVP grid are available in the Supporting Information. Small glass plates (Figure 2a) served as substrates for the microcontact printing of the PVP grid (Figure 2b). The PVP

Figure 1. Coordination between silver ions and PVP.

formed when silver ions coordinated along the PVP template are reduced with sodium citrate. The sodium citrate reduction of silver ions procedure used in this laboratory experiment is reported by Ratyakshi et al. and described in eq 1.8 4Ag +(aq) + C6H5O7 Na3(aq) + 2H 2O(l) → 4Ag 0(s) + C6H5O7 H3(aq) + 3Na +(aq) + H+(aq) + O2(g)

(1)

Printing Polymer Nanostructures

The use of a printed polymer template to direct the reduction of metal ions is a novel approach. It is unlike other templated methods used to create conductive nanostructures.4,12 Other reports describe the creation of 1D conductive polymeric nanostructures using anodized aluminum oxide (AAO) as a hard template.4 A similar process is reported by Bentley et al. for the creation of nickel nanowires.12 The method used to print the PVP template is the same soft lithographic process described in a previous report.5 Soft lithographic processes are forms of replica molding that employ the use of a polymeric stamp with patterned relief structures on its surface to generate structures with feature sizes ranging from 30 nm to 100 μm.13 The stamps are used in conjunction with a molecular “ink” to create patterns at the micro- and nanoscale levels.13 Microcontact printing is the specific form of soft lithography used in this laboratory experiment. This technique involves “inking” the protrusions on the surface of a patterned polydimethylsiloxane (PDMS) stamp for the direct transfer of patterns.14 Microcontact printing is a cost-effective fabrication procedure that can be easily adapted for use in undergraduate laboratory investigations, as reported by Campbell and co-workers.15 They describe replicating features pressed into aluminum foil with PDMS, and using the cured, patterned PDMS stamps to transfer ink images to paper or overhead projector transparencies.15

Figure 2. Silver nanogrid fabrication starting with a bare glass substrate (a). A PVP grid is stamped on the substrate using microcontact printing (b). Copper metal is sputtered on the grid pattern (c) and exposed to a dilute silver nitrate solution/sodium citrate solution (d).

polymer used to make the grid is very water-soluble.10 Therefore, it was necessary to deposit a thin barrier over the PVP grid to prevent aqueous solutions of silver nitrate and sodium citrate from removing the pattern. A Denton Desktop V sputter coater operating at 40 mA for 30−60 s (Figure 2c) was used to deposit a thin copper layer over the polymer grid. Sputter coating should not occur for more than 1 min and with a current no greater than 40 mA. This prevents formation of a silver film across the substrate when exposed to silver nitrate solutions. The authors would like to note that a previous report describes using a reaction between sputtered copper and dilute silver nitrate solutions to make silver nanostructures.17 The creation of the silver nanogrid reported in this manuscript does not involve a reaction between sputtered copper and silver

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EXPERIMENT PDMS stamps were prepared by faculty ahead of time using commercially available CDs; specific procedures are listed in the Supporting Information section. PVP solutions were made by dissolving 0.14 g of 55,000 molecular weight PVP (SigmaB




Figure 3. SEM images at 10,000× of three silver nanogrid samples.

nitrate. The primary role of the sputtered copper layer is to act as a barrier to prevent removal of the polymer grid when immersed in aqueous solutions. After sputtering, PVP grids were exposed to a 96 °C solution containing 1 mL of 5 mM silver nitrate and 1 mL of 0.6 M sodium citrate for 5 min (Figure 2d). Afterward, the glass plates were immersed in deionized water several times for rinsing and dried with a stream of nitrogen. The entire process was completed in approximately 40−45 min. This includes additional time required to clean PDMS stamps when they failed to remain adhered to the double-sided tape during spin coating procedures. This recleaning procedure included sonicating the stamp in 100% isopropyl alcohol, drying the stamp with nitrogen, heating the stamps to remove any trapped moisture, and repeating the stamping procedure. Students could observe a diffraction pattern on the plate once the process was complete. Each student participating in this experiment created a silver grid, and with faculty guidance, they characterized the grids with an atomic force microscope (AFM) and a scanning electron microscope (SEM).

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Figure 4. EDS scans on and off the silver nanogrid pattern (a). EDS spectra of both scans (b).

is formed from the reaction between the copper layer and the silver nitrate solution.

HAZARDS Isopropyl alcohol is flammable; do not use this alcohol near an open flame or heat source.

Atomic Force Microscopy

Under faculty guidance, students utilized AFM to obtain images of the copper-coated PVP grids before (Figure 5a) and after (Figure 5b) silver ion reduction. Images were obtained using a Nanosurf Easyscan 2 AFM (Nanoscience, Inc.) operating in contact mode. Figure 5c shows cross-sectional analyses of the AFM data obtained using WXSM image processing software.19 This data suggests a height increase of over 140 nm after exposure to the silver nitrate/sodium citrate solution. It is believed that the additional increase in height is attributed to the formation of silver metal directly on the PVP template.

RESULTS

Scanning Electron Microscopy

Under faculty guidance, students imaged several nanogrid samples using a Hitachi TM 3000 Benchtop SEM (Figures 3a− c). The consistency of the pattern in each image demonstrates the reproducibility of the experiment. Energy dispersive X-ray spectroscopy (EDS) analyses were conducted using a Hitachi TM 3000 Benchtop SEM as well. An EDS analysis was conducted on and off the grid (Figure 4a). Figure 4b shows a La1 peak at 3 keV, which is consistent with EDS data of silver nanostructures reported in the literature.18 As shown in Figure 4b, the peak is prominent when the EDS analysis is conducted on the grid. Figure 4b also shows a substantial decrease in the silver La1 peak when an EDS analysis is conducted off the grid. This EDS data suggests most of the citrate reduction of silver ions occurs directly on the PVP template due to the reported association between PVP and metal ions.10,11 This data also indicates little to no silver metal

Optical Microscopy

Figure 6 shows a compound light microscope image of a silver grid. This image was obtained using an OMAX compound trinocular microscope operating in bright field mode. Although the silver nanogrids possess nanoscale thicknesses, as observed in the AFM data, the grids can be viewed with a compound light microscope. Viewing the sample with a compound light microscope is feasible due to the lateral microscale dimensions of the grids. The grids were printed using PDMS stamps fabricated with CD templates containing structures 1.2 μm wide.7 C




Figure 7. Electrical characterization of the silver nanogrid printed on glass using a digital multimeter set to measure resistance.

Figure 5. Atomic force microscope images of copper-coated PVP grid before the reaction with silver nitrate/sodium citrate solution (a) and after the reaction (b). A cross-sectional profile of both AFM scans (c).

Figure 8. Cross-sectional profiles of PVP grid without and with the copper layer.

suggests that the thickness of the copper layer is approximately 5 nm-thick. Additional control experiments were conducted to determine if the copper layer contributed in any significant way to the formation of the conductive silver nanogrid. Figures 9a−c illustrate the use of tape to allow printing of the PVP grid on half of the glass substrate while leaving the half previously covered with tape bare. Afterward, copper was sputtered over the entire substrate for 30 s at 40 mA, and the substrate was immersed in a 96 °C silver nitrate/sodium citrate solution for 5 min. This resulted in the formation of a silver grid on half of the glass substrate (Figure 10). Figures 11a and b show EDS analyses performed on the sample. EDS scans were conducted on the grid and the bare half of the substrate (Figure 11a). Figure 11b shows that the silver Lα1 peak (∼3 keV) is prominent when the EDS scan is conducted on the grid and diminishes significantly when the scan is conducted on the bare half of the substrate. This data strongly suggests that silver forms predominately on the PVP grid, and little to no silver forms when PVP is absent. Additionally, this data indicates the copper layer used to prevent removal of the PVP grid does not generate appreciable amounts of silver when exposed to the silver nitrate/sodium citrate solution. A subsequent control experiment using a bare glass substrate was conducted to test this assertion. A copper layer was sputtered on a bare glass substrate with no PVP grid. The substrate was sputtered for 30 s at 40 mA. The sputtered glass substrate was exposed to a 96 °C silver

Figure 6. Light microscope image of silver grid obtained using an 100x oil immersion objective lens.

Electrical Characterization

Faculty performed conductivity tests of several silver nanogrid samples. Testing probes fitted with alligator clips were used to connect the silver nanogrids to digital multimeters (Figure 7). Electrical resistances between 2.5−5 kΩ were obtained, indicating electrical conductivity. Control Experiments

Faculty conducted control experiments to access the thickness of the copper layer. Figure 8 shows cross-sectional data for a PVP grid with no copper layer and for the same PVP grid after addition of the copper layer. Comparison of the two profiles D




Figure 9. Tape is placed on half of a glass substrate (a). A PVP grid is printed on the entire face of the glass substrate (b). The tape is removed to reveal a PVP grid on half of the glass substrate; the other half is bare (c).

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ASSESSMENT To assess student understanding of the fundamental science involved in this experiment, students were required to work as a team and develop a poster demonstrating key concepts associated with the experiment. The students’ project demonstrated their ability to perform literature searches in efforts to find pertinent articles related to the association of metal cations with PVP. Additionally, students could see the real-world relevance of this experiment by investigating the similarities between the experiment and ongoing research in industrial and academic laboratories currently investigating various forms of template-driven fabrication of nanostructures for use in practical applications. The student poster can be found in the Supporting Information.

Figure 10. Glass substrate with half of the face covered with a conductive silver grid. The other half is bare glass.

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SUMMARY This experiment allows students to print a polymeric template that directs the reduction of silver ions, resulting in the formation of a silver nanogrid on the surface of a glass substrate. This laboratory experiment was designed to introduce students to concepts associated with templatedirected nanofabrication at community colleges and undergraduate institutions without the need of costly equipment. For institutions with access to specialized forms of microscopy such as atomic force microscopy and scanning electron microscopy, this laboratory experiment can provide an opportunity for students to determine the micro- and nanoscale dimensions of the structures and perform elemental analysis.

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ASSOCIATED CONTENT

* Supporting Information S

Figure 11. EDS scan on the grid and on the bare half of the glass substrate (a). EDS spectra of both scans (b).

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00704. Instructor notes that include a parts list for the student built spin-coater and assembly and procedures for the PDMS stamp preparation and PVP grid fabrication (PDF; DOCX) Student poster (PDF)

nitrate/sodium citrate solution for 5 min. After the sample was rinsed with deionized water and dried with nitrogen, it was tested for conductivity. This sample exhibited no signs of electrical conductivity on any resistance setting used on the digital multimeter. These results indicate the sputtered copper layer does not contain enough copper to react with the silver nitrate/sodium citrate solution in a manner that contributes to conductivity. Additional information regarding this control experiment can be found in the Supporting Information.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. E




ORCID

Nanowires with an Electric Field. Nanotechnology 2006, 17, 2378− 2380. (19) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. WSXM: A Softwware for Scanning Probe Microscopy and Tool for Nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705−1−013705−8.

Wesley C. Sanders: 0000-0003-4472-3086 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Utah Engineering Initiative for generous support. REFERENCES

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Printing Silver Nanogrids on Glass - Journal of Chemical Education

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