Rodolfo Fernandez, Xiaohua Wang, Peter Rogen, and A. La RosaNear-field Microscopy Group

Department of Physics, Portland State University, P.O. Box 751; Portland, Oregon 97207

INTRODUCTION EXPERIMENTAL SETUP

GND

Ref

TF

Signal generator

Acoustic feedback

control signal

Acoustic Signal from mesoscopic

film

Piezo-Electric

tube

Lock-in #1

Lock-in #2

WORKING PRINCIPLE -1

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Shear-forceAcousto

Probe retracts from the sample Ultrasonic S

ignal (a. u.)

(Arb

. uni

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Probe displacement z (nm)

amplitude of probe's oscillations

Ultrasonicsignal

WORKING PRINCIPLE -2

Left: The lateral oscillations of the probe (few nm amplitude), while immersed into the mesoscopic film, engenders acoustic waves, which are sensitively detected by a acoustic sensor located underneath the sample. Right: The intensity of the acoustic wave depends sensitively on the probe-substrate separation distance (z). The results

shown above constitute an unprecedented detection of shear-forces via acoustic means.

~ nm

Acoustic sensor #1

Solidsubstrate

Probe’s lateral oscillations

Confined mesoscopicfluid

z

Mechanical motion driver

Listening to the nanowaves engendered at confined fluids

under shear

Exploiting the microscope frame as acoustic cavity to sense probe-sample surface interactions

We have developed a compact and versatile Shear-force/Acoustic Near-field Microscope (SANM), fully operated by acoustic sensory devices (U.S. Patent No. 11/809,196). It provides :a)An alternative method (acoustic) for characterizing surface phenomena (nanotribology, adhesion, wetting).b) a suitable characterization platform towards the development of surface related technologies ( industrial and bio-engineering lubricants,) and •c) potential capability for imaging sub-surface materials properties. Further, unique to its novel development, the SANM operates using its own acoustic-based feedback control, which allows topographic characterization of the sample.

S H E A R- F OR C E / A CO U S T I C N E A R – F I E L D M I C R O S C O P E (SANM)

S A N M L I S T E N S T O S U R F A C E I N T E R A C T I O N S at the N A N O - S C A L E

SubstrateAcoustic sensor #1

Substrate

PSU Near-field Microscopy Group

Pictorially, SANM uses a simple “audiphone”, (placed around the microscope’s frame) for listening to the probe-sample interactions, allowing the probe to “walk” safely across the sample’s surface.

Acoustic-based feedback control for scanning probe microscopy

Probe

Vx

Vy

TF

Acoustic sensor # 2

Resonant Cavity

Set point

XYZ scanner To image

processing

Topographic image

Ref

Vz Feedback

control

Sample Lock-in

SANM acoustic feedback for controlling the probe’s vertical position. SANM acoustic feedback for controlling the probe’s vertical position. The inset shows the acoustic signal decreasing monotonically as the probe approaches the sample in the last 35 nm proximity. This behavior is exploited by the SANM to implement its own probe-sample distance feedback control and, hence, image the sample’s topography. SANM is the first scanning probe microscope operated via acoustic transducer.

Principle: The probe-sample interactions affect the lateral motion of the probe. The latter, in mechanical contact

Acousticcavity

Acoustic sensor #2

XYZ scanner

Acousticwaves

Probe

Fluid film

Probe-filmInteraction region

Tic, tac,

tic

, tac

Tic, tac,

Probe

Acoustic sensor 2

Acousticresonant

cavity

SampleTic, ta

c,

Tic, tac,

Sample

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File-15_(Feb-2005)Approaching_first_interval

UST

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Probe-sample distance z (nm)

Acousticsignal #2

2 cm

Acoustic sensor

Acoustic cavity

with the cavity, sets waves that travel upwards towards the cavity where they interfere (see interference pattern). The acoustic is placed judiciously at the location of o constructive node for maximum sensitivity.