Negative-stiffness vibration isolators can easily support
the heavy weight of a combined AFM/micro-Raman system and
isolate it from low frequency vibrations more effectively
than high-performance air tables or active isolation systems.
The need for precise vibration isolation with scanning probe
microscopy (SPM) and near-field scanning optical microscopy
(NSOM) systems is becoming more critical as resolutions continue
to bridge from micro to nano. Whether used in academic labs
or commercial facilities SPM and NSOM systems are extremely
susceptible to vibrations from the environment. When measuring
a very few angstroms or nanometers of displacement an absolutely
stable surface must be established for the instrument. Any
vibration coupled into the mechanical structure of the instrument
will cause vertical and/or horizontal noise and bring about
a reduction in the ability to measure high resolution features
- the vertical axis being the most sensitive for SPMs but
they can also be quite sensitive to vibrations in the horizontal
Traditionally bungee cords and high-performance air tables
have been the vibration isolators most used for SPM and NSOM
work. The ubiquitous passive-system air tables adequate until
a decade ago are now being seriously challenged by the need
for more refined imaging requirements. Bench top air systems
provide limited isolation vertically and very little isolation
horizontally. Also at a disadvantage are the active isolation
systems known as electronic force cancellation that use electronics
to sense the motion and then put in equal amounts of motion
electronically to compensate and cancel out the motion. Active
systems are somewhat adequate for applications with lasers
and optics as they can start isolating as low as 0.7 Hz but
because they run on electricity they can be negatively influenced
by problems of electronic dysfunction and power modulations
which can interrupt scanning.
Lately the introduction of integrated microscopy systems employing
multiple microscopes is enabling more complex optical measurements
but these systems are also much heavier and there has been
little vibration isolation technology available for such heavy
instrumentation. Air tables which have been liberally used
for optics applications are not ideal for these nanoscale
resolution systems because of their inability to effectively
isolate vibrations below 20 Hz. Nor can active systems be
used with these newer combination systems because of their
inability to handle heavy instrumentation. But now negative-stiffness
mechanism (NSM) vibration isolation is quickly becoming the
choice for SPM and NSOM systems. This includes applications
using atomic force microscopy (AFM) integrated with micro-Raman
spectroscopy where negative-stiffness vibration isolation
is particularly well adapted. In fact it is the application
of negative-stiffness isolation that has enabled AFMs to be
truly integrated with micro-Raman into one combined system.
Negative-stiffness isolators can handle the heavy weight of
the combined AFM/micro-Raman system as well as isolate the
equipment from low frequency vibrations a critical set of
factors that high-performance air tables and active systems
AFM with Micro-Raman Integrated
The integration of AFM with micro-Raman enables a sizable
improvement in data correlation between the two techniques
and expanded Raman measurement and resolution capabilities.
Micro-Raman is a spectroscopic NSOM technique used in condensed
matter physics and chemistry to study vibrational rotational
and other low-frequency modes in a system. It relies on scattering
of monochromatic light usually from a laser in the visible
near infrared or near ultraviolet range. The laser light interacts
with phonons or other excitations in the system resulting
in the energy of the laser photons being shifted up or down.
The shift in energy gives information about the phonon modes
in the system. Scanning samples in a micro-Raman system however
suffers from several problems. As a sample is scanned even
a very flat sample it is hard to keep the distance of the
lens to the sample constant. Thus as one goes from pixel to
pixel under the lens of a Raman a mixture of sample and air
is sampled in the voxel (volumetric picture element) that
is illuminated. This causes intensity variations in the Raman
that are unrelated to the chemical composition of the sample
and are artifactual. This is even more pronounced with rough
samples and standard methods of autofocus are simply not accurate
enough for a whole host of problems that are being investigated
today. Additionally the point spread function which determines
the resolution of the Raman image is significantly broader
where there are contributions from the out-of-focus light
and this reduces resolution.
The atomic force microscope being a very high-resolution type
of scanning probe microscope has demonstrated resolution of
fractions of a nanometer making it one of the foremost tools
for imaging measuring and manipulating matter at the nano-scale.
The information is gathered by "feeling" the surface
with a mechanical probe. Piezoelectric elements that facilitate
tiny but accurate and precise movements on electronic command
enable the very precise scanning.
The AFM consists of a micro-scale cantilever with a sharp
tip (probe) at its end that is used to scan the specimen surface.
The cantilever is typically silicon or silicon nitride with
a tip radius of curvature on the order of nanometers. When
the tip is brought into proximity of a sample surface forces
between the tip and the sample lead to a deflection of the
cantilever. Resultant characteristics such as mechanical electrostatic
magnetic chemical and other forces are then measured by the
AFM using typically a laser spot reflected from the top surface
of the cantilever into an array of photodiodes.
Most systems employing AFM in concert with Raman perform separately
executing either an AFM scan or a Raman scan independently.
The recently developed direct integration of Raman spectroscopy
with AFM technique however has opened the door to significantly
improved technique and sample analyses.
Micro-Raman is a microtechnique but when AFM is added it becomes
a nanotechnique. It allows the AFM structural data to be recorded
online and improves the resolution of the Raman information
when the nanometric feedback of the system adjusts with unprecedented
precision the position of each pixel of the sample relative
to the lens. Also the small movements of the AFM stage provide
oversampling which is a well-known technique for resolution
One integrated AFM-Raman system developed by Nanonics Imaging
Ltd. in association with major Raman manufacturers such as
Renishaw plc Horiba JY and others provides simultaneous and
very importantly on-line data from both modalities. This advantage
addresses critical problems in Raman including resolution
and intensity comparisons in Raman images while permitting
on-line functional characterization such as thermal conductivity
elasticity and adhesion electrical and other properties. It
also provides for new avenues of improved resolution including
AFM functioning without optical obstruction parallel recording
with Raman in a wide variety of scanned probe imaging modalities
enabling direct and simultaneous image comparison and analysis
and high-resolution Raman mapping.
"Until recently Raman scattering has remained separate
and removed from the proliferation of insights that the scanned
probe microscopies can give" says Aaron Lewis President
of Nanonics Imaging which was the first to see the potential
of such integration. "Without this integration of the
systems investigating a sample with scanned probe microscopy
required removing the sample from the micro-Raman spectrometer.
This meant that the exact region that was being interrogated
by Raman could not be effectively correlated with the chosen
SPM imaging technique."
"Another aspect of optical integration is that SPMs can
measure forces but they cannot measure distribution of light
in micro-lasers silicon-based wave guides fluorescently stained
biological materials etc." explains Lewis. "For
example there are many important advances occurring in the
application of photonics to silicon structures and plasmonic
metals. In the past these photonic structures were in the
micrometer range now they are nanometric."
The Nanonics platform can be used for structural and photonic
characterization as well as the structural and chemical characterization
that is available with AFM and Raman integration.
For these applications Nanonics Imaging is the innovator of
AFM and NSOM systems including dual tip/sample scanning AFM
systems the industry's first NSOM-AFM cryogenic systems integrated
Raman-AFM systems multiprobe AFM and SEM-AFM systems. The
company also holds patents to the largest range of unique
nano-probes. These probes form a NanoToolKit for its unique
characterization platforms with a variety of tasks such as
for nanophotonics plasmonics nanochemical imaging and even
nanochemical deposition based on its singular NanoFountainPen
technology. The company is focused on full integration of
AFM technology with optics chemical imaging and other analytical
The Nanonics MultiView AFM-NSOM microscope with its free optical
axis on a standard micro-Raman now makes it possible to truly
integrate the separate worlds of Raman and AFM/NSOM nanocharacterization
which has led to a new era in high-resolution Raman spectroscopy.
Facilitating this integration is not only the geometry of
the AFM/NSOM platform but also a new generation of AFM glass
probes that have very unique characteristics - such as hollow
glass probes with cantilevered nano-pippets for material deposition
probes with glass surrounding a single nanowire in the middle
for ultrasensitive electrical measurements or dual wire glass
probes for thermal conductivity and thermocouple measurements.
Glass probes are ideal for Raman integration because of their
transparency to laser light and no Raman background. They
also expand outward allowing unprecedented correlation of
Raman and AFM also permitting multiple probes to be brought
easily together which is very difficult with a standard AFM
Negative-Stiffness Vibration Isolation
- Enabling AFM and Micro-Raman to Function as an Integrated
Underlying this pioneering integration AFM with micro-Raman
is negative-stiffness vibration isolation developed my Minus
K Technology Inc. What negative-stiffness isolators provide
is really quite unique to SPM-Raman and other NSOM systems.
In particular improved transmissibility of a negative-stiffness
isolator - that is the vibrations that transmit through
the isolator relative to the input floor vibrations. Transmissibility
with negative-stiffness is substantially improved over air
systems and over active isolation systems. Negative-stiffness
isolators employ a unique - and completely mechanical -
concept in low-frequency vibration isolation. Vertical-motion
isolation is provided by a stiff spring that supports a
weight load combined with a negative-stiffness mechanism.
The net vertical stiffness is made very low without affecting
the static load-supporting capability of the spring. Beam-columns
connected in series with the vertical-motion isolator provide
horizontal-motion isolation. The horizontal stiffness of
the beam-columns is reduced by the "beam-column"
effect (A beam-column behaves as a spring combined with
a negative-stiffness mechanism). The result is a compact
passive isolator capable of very low vertical and horizontal
natural frequencies and very high internal structural frequencies.
The isolators (adjusted to 1/2 Hz) achieve 93% isolation
efficiency at 2 Hz; 99% at 5 Hz; and 99.7% at 10 Hz.
"Before negative-stiffness vibration isolation was
employed AFM used in conjunction with micro-Raman systems
could not maintain adequate imaging integrity while measuring
at the nanoscale level" said Lewis. "Vibration
isolation is absolutely necessary for the system's successful
performance and negative-stiffness isolation has enabled
AFM and micro-Raman to function as a truly integrated platform."
For more information visit: www.minusk.com
About the author: Jim McMahon writes about instrumentation
technology. His feature stories have appeared in hundreds
of industrial and high-tech publications throughout the
world and are read by more than 5 million readers monthly.
He can be reached at email@example.com