Atomic Force Microscopy Analysis of Nanocrystalline Patterns

Laboratory Experiment pubs.acs.org/jchemeduc

Atomic Force Microscopy Analysis of Nanocrystalline Patterns Fabricated Using Micromolding in Capillaries Benjamin M. Lyman, Orrin J. Farmer, Ryan D. Ramsey, Samuel T. Lindsey, Stephanie Stout, Adam Robison, Holly J. Moore, and Wesley C. Sanders* Engineering Department, Salt Lake Community College, Salt Lake City, Utah 84123, United States S Supporting Information *

ABSTRACT: A cost-effective, hands-on laboratory exercise is described for demonstrating nanoscale fabrication at non-research-based educational institutions. The laboratory exercise also contains a component involving qualitative and quantitative surface characterization of student-fabricated nanoscale structures at institutions with on-site access to an atomic force microscope or access via another facility. The applicability of this laboratory exercise was demonstrated in a nanotechnology instrumentation course.

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

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micro- and nanoscale imaging capabilities on site or at another facility.

critically important skill in the field of nanotechnology is the ability to deposit nanoscale structures.1 College-level science and engineering students in particular need exposure to this skill because there is a need for fabricating micro- and nanoscale artifacts for fundamental investigations and technological applications.2 Micro- and nanoscale fabrication can be accomplished using one of two methodologies: “bottom-up” or “top-down”. Bottom-up refers to using individual atoms or molecules to pattern surfaces. In the top-down approach, fabrication begins with utilizing macroscale systems to ultimately create nanoscale patterns and structures.3 The topdown approach is inclusive of a variety of nanofabrication techniques,4 including photolithography, the most commonly used technique for fabricating integrated circuits.5,6 Meenakshi et al.3 report that the increasing importance of nanofabrication in the fields of microelectronics, optoelectronics, and biomedical applications has resulted in the creation of undergraduate curricula related to these topics. They further state that laboratory exercises demonstrating nanofabrication techniques have not been developed extensively due to the expense and expertise necessary to implement these laboratories.3 There are some undergraduate-level nanofabrication laboratory exercises involving photolithography;8,9 however, the issue of cost resurfaces because expensive materials are needed for the photolithography experiments. Another report mentions the need for an additional piece of specialty equipment, such as an optical reducer, for college-level photolithography laboratory exercises.10 The laboratory exercise described here, however, is a non-photolithographic, low-cost alternative for introducing students to top-down nanofabrication for institutions with © 2012 American Chemical Society and Division of Chemical Education, Inc.

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SOFT LITHOGRAPHY The laboratory exercise involves soft lithography, a technique that employs a patterned, elastomeric stamp created from a master to generate micro- or nanoscale structures on hard, flat substrates.6 Soft lithography is an ideal hands-on nanofabrication exercise because this technique is inexpensive, easyto-learn, and the materials are easily accessible.6 The elastomeric stamps used for soft lithography are made from polydimethylsiloxane (PDMS). Ellis and co-workers state that PDMS is formed from the cross-linking of siloxane oligomers via a curing agent (Scheme 1).11 PDMS is an optimal material for soft lithography applications because stamps made from this compound make conformal contact with hard, flat surfaces.6 An additional advantage is the low interfacial free energy of the stamp; this encourages preferential wetting of the substrate instead of the stamp.12 Two of the most common forms of soft lithography are microcontact printing and micromolding in capillaries. Microcontact printing involves the transfer of patterns of selfassembled monolayers onto the surface of substrates by making direct contact between an inked stamp and a hard, flat substrate6,7 (Figure 1). The most commonly used “ink” for microcontact printing is an ethanolic solution of alkanethiols for the printing of molecular patterns on gold. Published: January 13, 2012 401

dx.doi.org/10.1021/ed200235b | J. Chem. Educ. 2012, 89, 401−405

Journal of Chemical Education

Laboratory Experiment

the laboratory exercise described here involves qualitative and quantitative assessments of the nanoscale features produced by MIMIC using atomic force microscopy.

Scheme 1. Formation of PDMS from Siloxane Oligomers and a Curing Agent

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ATOMIC FORCE MICROSCOPY Microscopy is composed of a suite of tools frequently used to characterize nanoscale materials;13 this includes the atomic force microscope (AFM). AFM characterizes micro- and nanoscale devices by monitoring the forces that act between a tip and sample.14 The basic operation of an AFM involves a small tip attached to the end of a flexible cantilever interacting with the surface of samples (Figure 3). Attractive or repulsive

Figure 1. The microcontact printing procedure. A clean dry stamp (1) is inked with the molecules that are to be printed (2). The relief patterns of the stamp are coated with molecules (3). The stamp is then placed in conformal contact with a flat substrate (4). The stamp is removed, and printed, patterned molecules are left behind (5).

Another common form of soft lithography is micromolding in capillaries (MIMIC) (Figure 2). This technique involves Figure 3. Schematic illustrating the principle components of an AFM system.

forces generated between the tip and the sample result in bending of the cantilever. The bending of the cantilever is monitored using a reflected laser beam aimed at a photodetector, which generates a signal used to produce topographic images of micro- and nanoscale structures. Atomic force microscopy is a versatile technique due to the ability to image conductive and nonconductive samples simply by changing the nature of the tip. In addition, hard and soft samples can be imaged with atomic force microscopy by changing the operating mode. Contact mode and tapping mode are the two most common operating modes of the AFM.14 Contact mode AFM involves the tip making direct, but soft physical contact with the surface of the sample. Contact mode AFM is advantageous because it effectively scans hard, rough samples with abrupt changes in vertical topography. Tapping mode AFM involves oscillating the cantilever at or near its resonant frequency. This results in intermittent contact between the tip and sample. In addition, long-range forces between the tip and sample induce a change in the oscillation amplitude of the cantilever;14 the change in the oscillation amplitude is used to generate a topographic image of a sample. Tapping mode is advantageous over contact mode because shearing forces that normally damage sample surfaces are eliminated, and higher resolution images of soft samples can be obtained.15 There are reports that describe AFM introductory courses at educational institutions that involve characterization of biological molecules such as DNA,13 integrated circuits,16 crystalline structures,17 and atomic lattices.18 The work described in this manuscript is unique because it describes a low-cost laboratory exercise that allows students to use atomic force microscopy to both qualitatively and quantitatively characterize crystal nanostructures produced by soft lithography.

Figure 2. The micromolding (MIMIC) procedure. A clean dry stamp (1) is place in conformal contact (patterned side down) with a flat, clean solid substrate (2). A fluid, the polymer precursor, is placed at the opening of the channels and the fluid is drawn into the channels via capillary action (3). The solvent component of the fluid is allowed to dry (4) and the stamp is removed leaving behind patterned material (5).

placing a PDMS stamp with empty channels onto the surface of a flat substrate. A low-viscosity liquid is introduced to the open ends of the channel, and the liquid spontaneously fills the empty channels through capillary action.6 Over time, the liquid component is cured or the solvent is evaporated, and the solidified material confined in the channels remains. The stamp is removed, and a pattern is left on the surface of the substrate.6 There are reports that describe soft lithography laboratory exercises for undergraduate science and engineering classes.1,3 Sahar-Halbany et al. describe using a bare polycarbonate recordable compact disk (CD−R), a clamp, and an oven to transfer patterns on the CD−R to a glass microscope slide.1 This technique, however, lacks the flexibility of pattern design because the students are limited to the existing pattern on the CD−R. Meenakshi and co-workers demonstrate the success of implementing an undergraduate-level soft lithography laboratory exercise involving patterning of glass substrates with polyurethane using the MIMIC procedure.3 They also describe an analysis of the resulting patterning using atomic force microscopy. However, their analysis does not include a quantitative comparison of data between master relief structures, the PDMS replica, or resulting structures produced by soft lithography. For this reason, the second component of 402



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PDMS Stamp

EXPERIMENTAL METHODS AND RESULTS

PDMS stamp formation involved covering a cut section of CD−R (pattered side facing up) with a silicone elastomer. The silicone elastomer was prepared by mixing a siloxane precursor with a curing agent. PDMS stamp formation proceeded by pouring the mixture over the cut CD−R section (pattern side up) and allowing the mixture to cure for approximately one week. An approximate curing time of one week allowed sufficient hardening making it possible for the stamp to be peeled from the surface of the CD−R. An AFM image of the PDMS stamp formed from the CD−R master is shown in Figure 5A. An AFM image of the PDMS stamp is shown in

Execution

Six students worked individually during this laboratory exercise. First- and second-year engineering students were among those participating. The students required approximately 25 min to prepare the recordable compact disk (CD−R) for use as a master and to prepare the PDMS mixture for stamp formation. One week was required for sufficient curing of the PDMS stamp. Preparation and subsequent imaging of both nanocrystalline patterns described in the following sections required between 2 and 2.5 h. Slightly over one hour was required for silicon dioxide substrate pretreatment (cutting, cleaning, etc.), MIMIC fabrication, and drying of each pattern. Once patterning was complete, one hour per sample was dedicated to AFM imaging. At the end of the laboratory exercise, a postlab survey was administered to assess student understanding of the MIMIC concepts and their views regarding the applicability of the laboratory exercise to the micro- and nanofabrication lecture material. CD−R Master

One of the first activities in this laboratory exercise involves fabrication of a PDMS stamp created from a CD−R master. The CD−R master is a small section (∼2.5 cm × 2.5 cm) cut from a commercially available CD−R. All of the layers adhered to the CD−R are removed using concentrated nitric acid. An AFM image of a CD−R section is shown in Figure 4A. A

Figure 5. (A) AFM image of a patterned PDMS stamp. (B) A crosssectional analysis of the PDMS stamp.

Figure 5B. The raised structures on the stamp are approximately 120 nm in height. Deposition of Magnesium Sulfate Nanocrystalline Lines Using MIMIC

A diamond scribe was used to cut a small (0.5 cm × 0.5 cm) section from a six-inch silicon dioxide wafer. Silicon dioxide wafers were donated from Fairchild Semiconductor (West Jordan, UT). An AFM image of a bare silicon dioxide plate shows no discernible features (Figure 6A). The MIMIC procedure was used to fabricate nanoscale magnesium sulfate crystal patterns on the silicon dioxide plate. This process involved placing a PDMS stamp (pattern side down) on the small silicon dioxide section. An alcoholic (isopropyl alcohol) solution of magnesium sulfate (Epsom salts) was added to the stamp−silicon dioxide interfaces using a disposable plastic pipet; this allowed the solution to travel through the channels of the stamp via capillary action. After evaporation of the alcohol solvent, the stamp was removed from the silicon dioxide. The fabrication of nanoscale structures is driven by

Figure 4. (A) AFM image of the surface of a CD−R. (B) A crosssectional analysis of the surface of a CD−R.

cross-sectional analysis of the bare CD−R is shown in Figure 4B. According to the cross-sectional profile, the raised structures have approximate heights of 200 nm. 403




Figure 6. A blank silicon dioxide plate (A) and magnesium sulfate patterns (B) on silicon dioxide. A cross-sectional analysis (C) comparing topographic data of magnesium sulfate structures and bare silicon dioxide plates.

crystallization. An AFM analysis of the patterned silicon dioxide section revealed parallel, crystal structures (Figure 6B). A crosssectional analysis of the crystal structures compared against a bare silicon oxide substrate (Figure 6C) shows that the repeating crystal structures are approximately 80−110 nm tall. The linear, crystal patterns produced by MIMIC are nanoscale structures because their dimensions adhere to the definition of nanoscale stated by Ashkenaz et al.19 They assert that structures reside in the nanoscale regime when the structural dimensions are 100 nm or smaller.

Figure 7. An AFM image (A) of bare CD−R gold and (B) of magnesium sulfate structures on CD−R gold. A cross-sectional analysis of the heights (C) of the gold bars and (D) of the magnesium sulfate crystal structures.

Deposition of Magnesium Sulfate Crisscross Patterns on Corrugated Gold Using MIMIC

structures (Figure 7D) suggests they are approximately 80 nm in height, which is lower than the original gold bars with heights of 112 nm.

Gold-coated CD−Rs contain a thin layer of gold with surface corrugations as seen in the AFM analysis shown in Figure 7A. A small (1 cm × 1 cm) gold square was cut from the corrugated CD−R gold and the alignment of the corrugated gold lines was noted. A cross-sectional analysis of the bare CD−R gold (Figure 7C) suggests the gold bars are approximately 112 nm in height. A small (1 cm × 1 cm) portion of the patterned PDMS stamp was cut and imaged with AFM, and the position of the channels was noted. The PDMS stamp was placed (pattern side down) on top of the bare CD−R gold so that the channels were orientated perpendicular to the gold corrugations on the CD−R. An alcoholic magnesium sulfate solution was added to the stamp−gold interface with a pipet. When the alcohol solvent dried, AFM analysis revealed linear, magnesium sulfate crystal structures perpendicular to the gold bars (Figure 7B). The salt structures can be distinguished from the gold bars based on a qualitative examination of the pattern in Figure 7B. In a typical two-dimensional AFM image, higher features are lighter and lower features are darker. There are features in Figure 7B that are darker and perpendicular to lighter structures. The assumption can be made that the darker features are the salt structures. A cross-sectional analysis of the darker

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HAZARDS Concentrated nitric acid is highly corrosive and can cause severe burns. Use concentrated nitric acid with extreme care and only in the fume hood. Isopropyl alcohol is flammable; do not use alcohol near an open flame or a heat source. Polydimethylsiloxane and magnesium sulfate may cause irritation to skin, eyes, and respiratory tract and may be harmful if swallowed or inhaled. Students should wear rubber gloves and goggles for all the experiments described here to prevent contact with chemicals.

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SUMMARY This manuscript describes implementation of a hands-on, soft lithography laboratory exercise that explores top-down nanofabrication. This lab provides students with a low-cost opportunity to fabricate nanoscale structures and to perform qualitative and quantitative surface characterization using AFM. Data obtained from a student survey administered after completion of the lab suggest that students comprehended the MIMIC concept, understood the role of the AFM, and 404




recommended continuation of this laboratory exercise in future nanotechnology and microscopy coursework.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed directions and notes for instructors; student survey and summary; postlab questions.This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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

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Atomic Force Microscopy Analysis of Nanocrystalline Patterns

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