TUNA® can be applied to many researchor manufacturing areas on a wide
range of materials. However, the use ofContact Mode for topographic feedback
is a limiting factor. For samples that
require low imaging forces, in either orboth the vertical or lateral directions,Contact Mode may not be applicable. It
is therefore difﬁcult to extend TUNA®
imaging to the study conductive
polymers, organics or other soft
conducting materials or loosely boundsamples such as nanowires.
TORSIONAL RESONANCE MODE

Atomic force microscopy has achieved
tremendous beneﬁts from a variety ofoscillating tip modes of operation, mostnotably Tapping Mode®. During TappingMode® the AFM cantilever is oscillated
at its fundamental ﬂexural resonance.
This has the advantage of largely
eliminating lateral forces that tend todamage the tip and/or sample when
imaging in contact mode. The verticalinteraction force is also substantiallyreduced due to high mechanical Q of
the cantilever, permitting imaging ofdelicate soft samples.
When imaging in Tapping Mode®, theAFM tip position varies relative to thesample by tens of nanometers, andmay spend up to 99% of its oscillation

Tapping Mode

cycle in a regime having no near-ﬁeld
interaction with the surface, ﬁgure 2.
This is advantageous for eliminating
tip wear and sample damage, but isproblematic for the measurement ofproperties that demand near-ﬁeld
interactions – such as TUNA®.

The implementation of a torsional drive
system for use with AFMs from Veeco
Instruments is a simple modiﬁcation
to the cantilever holder found on all
Tapping Mode® enabled AFMs. The
holder employs two parallel actuators,
marked as piezos 1 and 2 in ﬁgure 3.

It is known however, that AFM
cantilevers can oscillate at manydifferent modes1–10, including higher
order ﬂexural and torsional or twistingmodes. These extended modes enable
the study of widely differing tip–surface
interactions. Consider the case ofimaging with the cantilever oscillatingin the ﬁrst torsional resonance mode.

To excite the torsional resonance, drive
signals of opposite phase are applied tothese two piezos. Due to the asymmetryof the tip mass about the rotation axis
this actuation will create a rotation of
the cantilever substrate about its centerpoint. Since the tip is far away fromthe center, the trajectory of the tip isalmost horizontal. As with conventional
Tapping Mode® the torsional cantileverdeﬂection is detected using a foursegment photodetector.

In this mode, lateral forces that act on
the tip cause a change in the torsional
resonant frequency, amplitude, and/or phase of the cantilever11. AFM
measurements at torsional resonances
have many advantages but the key
advantage for TUNA® is the ability toachieve low-force scanning whilst
maintaining the tip in the near-ﬁeld.
With the ability to maintain the probe
tip in the near-ﬁeld (at the boundary
between direct contact and the longrange force regime), we are once again
able to measure tunneling currents
between the tip and sample.

Torsional Resonance Mode

AFM uses the angular displacement of
the cantilever and the resulting change
in the reﬂected laser beam angle at theFigure 3. The TR probe holder employs two parallel
actuators using drive signals of opposite phase.

®

®

photo-detector. For ﬂexural bending of
a cantilever, the bend angle θf is given
approximately by the relationship:

10–100nm

Cantilever Displacement

The cantilever length serves as thelever arm that converts vertical1%

0.3–3nmwhere:z is the vertical tip deflection
L is the length of the cantilever

Near-FieldTimeFigure 2. The tip is in close proximity to the sample for approximately 1% of the oscillation cycle when using
standard Tapping Mode. In comparison the tip can be in close proximity to the surface all of the time whenusing Torsional Resonance Mode with small oscillation amplitudes

PAGE 2zUf = 32 L

Electrical testing of soft delicate samples using Torsional Resonance Mode®.

deﬂection to an angular change.
Typical values of cantilever lengths
are on the order of 100μm. In this casethe tip needs to move at least a few
nanometers to generate an angulardeﬂection signal large enough tobe detected.

The lever arm for torsional motion
is much shorter as it corresponds tothe tip height plus half the cantileverthickness (this is the distance fromthe tip apex to the center of torsionalrotation). AFM tip heights are
typically 10 to 20 times shorter thanthe cantilever length L. So torsional
detection is 10 to 20 times moresensitive to tip motion than ﬂexuraldetection. As a result it is much easier
to detect sub-nanometer tip motion
laterally with the same detection
electronics and optics.
The increased torsional detection
sensitivity translates into typical tip
oscillation amplitudes of the order of acouple of nanometers or less. Compare
this to the 30-100nm oscillation
amplitudes typically used for TappingMode® and it is easy to see why thecantilever tip remains in the near-ﬁeld
during the entire imaging process. The
remainder of the feedback system usedfor TR-Mode® imaging is identical tothat used with Tapping Mode®.
TR TUNA®

Two samples that are difﬁcult to image
in contact mode are presented here
using TR-Mode® for topographicalfeedback.
The ﬁrst sample consists of a loosely
distributed network of carbon

nanotubes that are weakly-bound to agold on silicon substrate. The substrate
was in the form of a silicon wafer witha non-conducting native oxide over
it’s surface. Gold was patterned togenerate a conducting grid on to which
was placed the network of conducting
carbon nanotubes from suspension.
Due to the loosely-bound nature of thenanotubes to the gold/silicon oxidesurface this sample is not able to be
imaged using standard contact mode
based TUNA® techniques (wherehigh lateral forces simply push thenanotubes around the surface). Witheither Tapping Mode® or TR-Mode® it ispossible to obtain stable images of
the nanowires.
Topography from this sample ispresented as ﬁgure 4 (left). Part of thegold grid can be seen in the upper righthand quadrant of the image.
The current image, ﬁgure 4 (right),
shows the tunneling current asmeasured by the TUNA® sensor. It canbe seen that only those nanotubes thathave an electrical connection to thegold grid exhibit contrast. Only those
nanowires directly connected to thegold grid, not simply overlapping eachother, form a conductive path between
the imaging probe and thecounter-electrode.

The second sample imaged with a 10μm
scan was a carbon-black ﬁlled polymer
that is also difﬁcult to image in contact
mode. When imaged in Tapping Mode®
the topography is recovered but anapplied tip-sample bias of up to 10 Voltsyielded no measurable current data.
When scanning in TR-Mode®, not onlyis the topography, shown as ﬁgure 5,measured correctly, but in addition
signiﬁcant contrast is observed in theTUNA® current. This TUNA® current is
detected with a tip-sample bias as low
as 0.5 Volts. Variations in the measured
TUNA® current for biases of –0.5, –1.0
and –2.0 Volts are given as ﬁgure 5.
There are only a few bright spots
in ﬁgure 5 (b), which indicates theconductive areas at this sample bias
voltage. With increasing sample
bias voltage, the effective barrier fortunneling is reduced. Therefore, thetotal area of the sample exhibiting ameasurable TUNA® currents is seen
to increase.
With two simple samples the uniquestrength of applying TR-Mode® toTUNA® current measurements on
soft delicate samples is clearly
demonstrated. It can be seen that theuse of TR-Mode® extends the sample
space available to TUNA® to include
those samples traditionally believed

Figure 4 Carbon nanotubes deposited from solution on to a gold grid are shown as topography (left) and current (right). The TUNA®
data shows only those nanotubes that are electrically connected directly to the gold grid and not simply connected to each other.

Electrical testing of soft delicate samples using Torsional Resonance Mode®.

PAGE 3