This document describes a study of the electrochemical etching process used to create sharp metal tips for tip-enhanced Raman spectroscopy. The researchers monitored the etching process using a tuning fork oscillator to track changes in resonance frequency and amplitude as the tip geometry changed. They found that resonance frequency decreased as mass was added during immersion in liquid, in agreement with theory. Frequency then increased as etching reduced the tip size and hydrodynamic drag. Monitoring these dynamics provides insights into how tip geometry evolves during etching and could enable feedback control of the process.

1. Current dynamics of tip etching
Theory predicts that for small chang-
es in resonant frequency (f0), f0
should decrease with mass added.
Hence, as the wire is successively im-
mersed in liquid, the effective mass of
the tip increases, leading to decrease
in f0.Then,as etching occurs,the sur-
face area exposed to the electrolyte
decreases, lessening the hold the fluid
has on the wire, leading to a decrease
in the effective mass in the tuning
fork. Similarly, when the immersed
portion of the tip dropped off, there
was a larger increase in frequency.
Challenge
Creating sharp, plasmonically-active tips (i.e., via liquid-phase electrochemical etching of
wires in strong acid or base) is a critical process which is not well understood.
wire
ring
Preparation and characterization of optically-active
metal probes for scanning chemical microscopy
Jae Cho, Isaac Riisness, and Michael J. Gordon
Department of Chemical Engineering, University of California, Santa Barbara
Motivation
Methods
We used a sensitive tuning fork oscillator as a diagnostic tool to monitor and control electro-
chemical tip-etching.
poor quality probe creates
low-resolution image
good quality probe creates
high-resolution image
Lock-in amplifier Etching circuit
Computer
shake
displacement
current
on/off
oscillation
Nanoscale characterization via
scanning probe microscopy (SPM)
relies heavily on a sharp probe with
nanoscale dimensions to produce
high-quality topographic images of
surfaces. Unfortunately, SPM, per se,
cannot be used to identify surface
chemistry. However, optical methods
such as vibrational spectroscopy can
be combined with SPM for chemical
imaging below the diffraction limit
using tip-enhanced Raman spectroscopy (TERS). In this technique, far-field laser light
is coupled to electron oscillations (plasmons) in a nanostructured gold or silver “tip” to
create a tightly-confined electromagnetic field; this intense optical field near the tip apex
can enhance Raman scattering from molecules in the tip-surface gap.
As one might imagine, the tip is the most important part of the TERS experiment. To
clarify, tip size sets the ultimate spatial resolution; whereas, the tip material, geometry,
and roughness determine the plasmonic (optical) activity, and ultimately, the magnitude
of field enhancement.
Changes in fork oscillation due to
hydrodynamic and electrostatic
forces are measured using a
lock-in amplifier.
Etching dynamics were studied by
recording etching current as a function of
time as shown in the accompanying figures.
It is generally seen that the etching current
is approximately constant for most of the
process, followed by a rapid decrease in
current when the “sacrificial” part of the
wire drops off. Current spikes due to
intermittent oxide formation and stripping
on the wire surface are also observed.These
measurements will be coupled with
changes in tuning fork oscillation dynamics
to assess how tip geometry and sharpness
depends on the etching procedure.
When the tip is immersed, hydrodynamic drag dissipates the energy used to shake the fork.
Since greater immersion means more drag, the effective amplitude of tip oscillation, with
respect to identical input energies, should decrease significantly. with immersion.Then, as the
wire is etched, less surface area comes in contact with the water. As shown above, there is a
increase in amplitude with etching.
Summary & Prospects
• An electrochemical tip-etching system was developed; improvements made to tip-gluing station.
• Sharp tips (W, Au, Ag) suitable for SPM experimentation were produced.
• Developed an automated system to test and monitor tip-etching and oscillation dynamics
simultaneously.
• Resonance behavior of tip-etching process were measured; tip dynamics in liquid were
seen to follow theoretical scaling laws for immersion depth, mass added, and hydrodynamic drag.
• Experiments will be carried out to test hypotheses about observed tuning fork dynamics.
Results
Tuning fork dynamics (e.g. quality factor, amplitude damping, and resonance frequency) in
different fluids and during etching were studied to understand how oscillation parameters might
be used for feedback in the tip-etching process.
0 200 400 600 800 1000
12000
14000
16000
18000
20000
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24000
26000
Frequency
Current
Time [sec]
Frequency[Hz]
-2
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
Current[mA]
0 200 400 600 800 1000
0
2
4
6
8
10
12
14
Amplitude
Current
Time [sec]
Amplitude
-2
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
Current[mA]
wire
tip-etching process occurs
at electrolyte-air interface
Acknowledgements
The authors gratefully acknowledge the support of UC LEADS program.
oil
water
Amplitude dampening
0
1
2
3
4
5
6
7
8
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Immersion depth (millimeters)
Tipamplitude/exciteamplitude
oil
water
Resonant frequency shift
y = -116.09x + 11157
R2
= 0.9844
y = -320.09x + 11136
R2
= 0.9931
10700
10750
10800
10850
10900
10950
11000
11050
11100
11150
11200
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Immersion depth (millimeters)
Resonantfrequency(Hz)
1 μm
5 μm