Jim Mara MSc Presentation AFM Bias Induced Electrochemistry Redox Processes at the Solid Liquid Interface

11 Sep‘14

AFM Bias Induced Electrochemistry: Redox

Processes at the Solid Liquid Interface

Jim Mara B.Eng, CA, CTA, PDA

Motivation

Nanowire Based Biosensors •Nanometer-scale metal particles possess properties which differ significantly from macroscopic metal phases. •detection and quantification of biological and chemical species •critical to many areas of health care and the life sciences•diagnosing disease, discovery & screening of new drug molecules

Figure. Elements and selected components of a typical biosensor1

AFM- Redox Processes

The approach we used to fabricate the nanostructures was AFM based voltage induced electrochemical reduction of aqueous silver cations, Ag+ in a meniscus

of aqueous AgNO3

Schematic of AFM set up2

“Dip-pen” AFM used a tiny water meniscus as a nanometer-sized electrochemical cell

in which metal salts are dissolved, reduced into metals electrochemically, and

deposited on the surface.3

Schematic diagrams of nanostructures on the HOPG surface produced in ambient air by the AFM tip under an external electric field.4

AFM Imaging Modes

Contact AFM

AFM Imaging Modes

Non-Contact AFM

Materials

HOPG is an allotrope of carbon and part of the graphite family. Pyrolytic graphite is a graphite material with a high degree of preferred crystallographic orientation of the c-axes perpendicular to the surface of the substrate. HOPG is a highly-ordered form of high-purity pyrolytic graphite

Schematic representation of the structure of the bulk hexagonal graphite crystal.5

Materials

The compound silver nitrate (AgNO3) is a colourless, soluble salt made by dissolving silver in nitric acid. It is used to

cauterize sores and wounds. Silver Nitrate is easily converted to metallic silver by contact with organic matter, such as skin or

cloth, and is for this reason used in making indelible ink.6

Results- Deposition

Ag deposition on HOPG, applied bias of -3.75V for 5.5 seconds, 6.8nN tip-HOPG contact force.

Mechanism

Ag++ e- = Ag

•reduction and nucleation of Ag+ at the tip/substrate interface.

•micron deposits are initiated by nano deposits-nano deposits in the time frame ≈5 ms to ≈50ms.

•capacitance of the freshly cleaved graphite fluctuated by 10-20%.

•nucleated silver interact weakly with the graphite surface- AM-AFM used.

•size disparity relates to diffusional transport of Ag+ step edges (cylindrical symmetry), basal plane (more efficient hemispherical diffusional flux).7

•Factors: humidity, voltage, scan speed, duration7

H2O + e- = OH- + H OH- + Cx = CxO + H-

•stationary electric field induces electron-hole concentration leading to anion adsorption on the HOPG surface.

•formation of the pits:

-initially dissociative adsorption of water and oxygen induced by the voltage hole concentration

-subsequent defect-assisted oxidation of HOPG strains the HOPG lattice eventually leading to C-C bond rupture, i.e. pit formation.8

•Factors: amplitude and duration of the voltage pulse

Results- Pits

Mechanism

Bias applied -3.80V for 3.0 seconds in contact mode, 4.5nN tip-HOPG contact force.

Results- Table

Acknowledgment

I would like to express my gratitude to Dr. Brian Rodriguez for his patience and insightful help over the course of this work.

I would like, in general, to thank all members of the UCD Nanoscale Function Group who were always very generous with their time and knowledge, with special mention for Craig Carville, Liam Collins and Bart Lukasz.

Also, my family and friends, especially Mam and Da, who always supported me even when I least deserved it.

Finally, a word of mention for the unknown passer-by who smiled at me and looked away at just the right moment.

1. Patolsky, Fernando, Gengfeng Zheng, and Charles M. Lieber. "Nanowire-based biosensors." Analytical Chemistry 78.13 (2006): 4260-4269.

2. Grieshaber, Dorothee, et al. "Electrochemical biosensors-Sensor principles and architectures." Sensors 8.3 (2008): 1400-1458.

3. Li, Yan, Benjamin W. Maynor, and Jie Liu. "Electrochemical AFM “dip-pen” nanolithography." Journal of the American Chemical Society 123.9 (2001): 2105-2106.

4. Park, Jin Gyu, et al. "Nano-machining of highly oriented pyrolytic graphite using conductive atomic force microscope tips and carbon nanotubes."Nanotechnology 18.40 (2007): 405306.

5. Advanced Integrated Scanning Tools for NanoTechnology, information page HOPG, posted as of 20082014; http://nanoprobes.aist-nt.com/apps/HOPG%20info.htm

6. General Chemistry By Linus Pauling, Chap. 21, pg 7047. Zoval, Jim V., et al. "Electrochemical deposition of silver nanocrystallites on the atomically smooth graphite

basal plane." The Journal of Physical Chemistry 100.2 (1996): 837-844.8. Jiang, Yan, and Wanlin Guo. "Convex and concave nanodots and lines induced on HOPG surfaces by AFM

voltages in ambient air." Nanotechnology 19.34 (2008): 345302.

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