- June 2011
Improving Nano-Scale Imaging with
By Jim McMahon, email@example.com
Traditionally, bungee cords and high-performance air tables
have been the vibration isolators most used for scanning
probe microscopy (SPM) and near-field scanning optical microscopy
The ubiquitous passive-system air tables, adequate until
a decade ago, are now being seriously challenged by the
need to meet more refined imaging requirements. Benchtop
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 0.7Hz, but because they run on electricity,
they can be negatively influenced by problems of electronic
dysfunction and power modulations, which can interrupt scanning.
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 cannot achieve.
AFM with Micro-Raman
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
lowfrequency 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 auto-focus
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.
Schematic of negative stiffness
Atomic Force Microscope
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 best 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. 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 micro-technique,
but when AFM is added, it becomes a nano-technique.
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 improvement.
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
stiffness isolators vs. air tables.
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 industrys
first NSOM-AFM cryogenic systems, integrated Raman-AFM systems,
multi-probe AFM and SEM-AFM systems.The company also holds
patents for 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 nano-photonics,
plasmonics, nano-chemical imaging and even nano-chemical deposition
based on its singular NanoFountainPen technology. The
company is focused on full integration of AFM technology with
optics, chemical imaging and other analytical tools
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.
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 SPMRaman
and other NSOM systems. In particular, improved transmissibility
of a negativestiffness 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.
NSM vertical motion isolator.
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 verticalmotion 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 1/2Hz) achieve 93 percent isolation
efficiency at 2Hz; 99 percent at 5Hz; and 99.7 percent at
"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 nano-scale level,"
explains Lewis. "Vibration isolation is absolutely
necessary for the system¡¦s successful
performance, and negativestiffness isolation has enabled
AFM and micro-Raman to function as a truly integrated
Minus K Technology, Inc. develops, manufactures and
markets state-of-the-art vibration isolation products
based on the company¡¦s patented negative-stiffness-mechanism
technology. Minus K products are used in a broad spectrum
of applications including nanotechnology, biological
sciences, semiconductors, materials research, zero-g
simulation of spacecraft, and high-end audio.
NSM horizontal motion isolator.
The company is an OEM supplier to leading manufacturers of
scanning probe microscopes,micro-hardness testers and other
vibration-sensitive instruments and equipment. Minus K customers
include private companies and more than 200 leading universities
and government laboratories in 35 countries.
|Contact: Minus K Technology,
Inc.; 460 South Hindry Ave., Unit C,
Inglewood, CA 90301 310-348-9656
fax: 310-348-9638 E-mail: firstname.lastname@example.org
||Nanonics Imaging Ltd., Manhat Technology
Jerusalem, Israel 91487 s +972-2-678-9573
fax: +972-2-648-0827 In the U.S.: s 866-220-6828
E-mail: email@example.com Web: www.nanonics.co.il
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