ISO 27911:2011

INTERNATIONAL ISO
STANDARD 27911
First edition
2011-08-01
Surface chemical analysis — Scanning-
probe microscopy — Definition and
calibration of the lateral resolution of a
near-field optical microscope
Analyse chimique des surfaces — Microscopie à sonde à balayage —
Définition et étalonnage de la résolution latérale d'un microscope
optique en champ proche
Reference number
©
ISO 2011
©  ISO 2011
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ii © ISO 2011 – All rights reserved

Contents Page
Foreword .iv
Introduction.v
1 Scope.1
2 Normative references.1
3 Terms and definitions .1
4 Symbols and abbreviated terms .1
5 General information .2
5.1 Background information.2
5.2 Types of NSOM operation.2
5.3 Methods of measuring the lateral resolution of an NSOM .3
5.4 Parameters that affect the lateral resolution .3
6 Measurement of lateral resolution by imaging a very small object .5
6.1 Background information.5
6.2 Selection of the specimen and specimen requirements.6
6.3 Setting the parameters before the operation of the instrument.7
6.4 Data collection and analysis .7
6.5 Recording of data .8
Annex A (informative) Examples using a line-profile and a CdSe/ZnS quantum dot as specimen.9
Annex B (informative) Example of a procedure for preparing standard NSOM specimens .15
Bibliography.17

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
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International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 27911 was prepared by Technical Committee ISO/TC 201, Surface chemical analysis, Subcommittee
SC 9, Scanning probe microscopy.
iv © ISO 2011 – All rights reserved

Introduction
The near-field scanning optical microscope (NSOM or SNOM) is a form of scanning-probe microscope (SPM)
that uses an optical source but achieves, through the use of the near field, a spatial resolution significantly
superior to that defined by the Abbe diffraction limit. NSOM instruments are mainly either apertured, when the
resolution is governed by the aperture size, or apertureless, when the resolution is more complex. In
apertureless NSOMs, a very sharp scannable tip is used to probe the surface, or molecules on the surface,
through local scattering of light from the test specimen surface or the tip apex. The spatial resolution for
scattering NSOMs is a complex phenomenon and is less easily characterized in terms of an instrumental
property, and so this International Standard focuses on, and is limited to, the lateral spatial resolution of
apertured NSOM instruments.
Although the term spatial resolution has a clear meaning, it is often characterized in different ways. In this
International Standard, one convenient and effective method for measuring the spatial resolution of an
apertured NSOM instrument is presented, suitable for use by non-expert operators.

INTERNATIONAL STANDARD ISO 27911:2011(E)

Surface chemical analysis — Scanning-probe microscopy —
Definition and calibration of the lateral resolution of a near-field
optical microscope
1 Scope
This International Standard describes a method for determining the spatial (lateral) resolution of an apertured
near-field scanning optical microscope (NSOM) by imaging an object with a size much smaller than the
expected resolution. It is applicable to aperture-type NSOMs operated in the transmission, reflection,
collection or illumination/collection mode.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 18115-2, Surface chemical analysis — Vocabulary — Part 2: Terms used in scanning-probe microscopy
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 18115-2 and the following apply.
3.1
far field
electromagnetic field at a distance from a light source significantly greater than the wavelength of the light
3.2
point spread function
response of an imaging system to a point source or point object
4 Symbols and abbreviated terms
APD avalanche photodiode
FWHM full width at half maximum
NA numerical aperture
PMT photomultiplier tube
PSF point spread function
QD quantum dot
δ FWHM of the PSF of the NSOM, i.e. the lateral resolution of the NSOM instrument
5 General information
5.1 Background information
The NSOM is a form of scanning-probe microscope with a probe that has an optical aperture that can
illuminate, and/or collect the light from, the surface of a test specimen in the distance within a fraction of the
wavelength of the light, this region being called the near field. A two-dimensional NSOM image consists of
pixels that contain optical information (normally, the light intensity or photon counts obtained at each pixel
position). For an apertured NSOM, an open optical aperture of subwavelength diameter is located at the apex
of a sharp probe, and light is emitted and/or collected by it. The NSOM probe is scanned over the specimen
surface in the near field. Because the aperture is so close to the surface, the size of the spot illuminated on
the surface (or from which light is collected) is determined not by the light wavelength but mostly by the
aperture size. Since the aperture size can be made as small as a few tens of nanometres, spatial resolution
far better than the theoretical resolution limit of the conventional far-field optical microscope can be achieved
by an NSOM. The spatial resolution achievable by reducing the size of the aperture is limited by the skin
depth of the metal coating of the NSOM probe, which defines the aperture, and by the fact that optical
throughput decreases rapidly with decreasing aperture diameter, going beyond the limits of practical detection.
5.2 Types of NSOM operation
5.2.1 General
Below we describe different modes of NSOM operation. This International Standard is concerned with
apertured NSOMs operated in the illumination, collection or illumination/collection mode. Control of the gap
between the specimen and the probe is achieved by shear-force detection using optical or electrical
transduction for a straight-fibre probe, and by cantilever deflection using optical transduction for a bent or
cantilevered probe.
5.2.2 Classification
5.2.2.1 NSOMs can be classified on the basis of how the light is transmitted to/collected from the
specimen:
a) Illumination mode: The light emanates from the aperture and is collected with a lens in the far field.
b) Collection mode: The specimen is illuminated by light from a far-field source or excited to emit light by
another means and light is detected (collected) using the NSOM aperture.
c) Illumination/collection mode: The NSOM aperture is used for both illumination and collection.
5.2.2.2 NSOMs can also be classified on the basis of the position of the collection optics with respect to
the illumination optics:
a) Reflection mode: Both illumination and collection are carried out on the same side of the specimen in any
of the three modes defined above.
b) Transmission mode: The collection and the illumination optics are located on opposite sides of the
specimen. In most cases, including the reflection mode a) above, a high-NA lens is used for high
collection efficiency.
2 © ISO 2011 – All rights reserved

5.2.3 Control of gap between probe and specimen surface
The gap between the NSOM probe and the surface is typically controlled in one of two ways, depending on
the type of probe:
a) Shear-force detection type: The NSOM probe is attached to a piezo tube or tuning fork and vibrated
laterally to the surface with an amplitude of a few nanometres. Feedback is provided to keep the
amplitude, phase or frequency of the vibration constant. For homogeneous surfaces, this would provide a
constant gap; for most materials with a structured surface, the situation is more complicated, but often the
constant-gap approximation holds.
b) Cantilever type: The NSOM probe is cantilevered so that various ways of controlling atomic-force
microscope tips can be used. In particular, the deflection of a laser beam off the end of the cantilever can
be used to sense the surface topography and maintain a constant gap distance.
[1]
NOTE Care is required to ensure the correct way of doing this.
5.3 Methods of measuring the lateral resolution of an NSOM
The spatial resolution of an NSOM is mainly determined by the size of the aperture probe, its distance from
the surface, and the contrast mechanism. In addition, the nature of the specimen, pixilation and signal-to-
noise issues can affect resolution. Therefore the spatial resolution of the NSOM can be defined only for a
particular instrument and a particular specimen and, accordingly, any claim of spatial resolution should specify
[2]
the details of the experimental conditions , such as the properties of the specimen, the type of imaging mode,
the height regulation mechanism, the type of NSOM probe and other factors that could affect the
measurement of the spatial resolution.
Measurement of the spatial resolution of an NSOM instrument has been estimated by several methods,
[3]
including measurement of the size of the smallest feature appearing in the NSOM image , imaging small
[4] to [7] [8]
objects in a fluorescent mode and imaging a specimen having an abrupt optical-contrast edge .
[9]
The method chosen here is the imaging of a small object. It is based on the concept of the PSF , which is a
critical concept that determines the spatial resolution of an optical microscope. In using this method, the
following limitations of the method should be noted:
a) It is recognized that, with NSOMs, the resolution is a result of near-field interactions between a specimen
and a probe. The intensity profile in the near field of an aperture, even for the simplest possible case of
an aperture in an infinite plane, and in the absence of interactions with a specimen, is not a simple
[10]
Gaussian one . In general, the field shape varies with the aperture shape, the condition of the outer
metal coating and the polarization of the input light, etc.
b) Topography-induced artefacts that appear in the optical images produced by NSOMs are sometimes
[11]
mistaken for optical contrast . If the optical contrast of the specimen is low compared to the background
signal, which is not specific to the optical characteristics of the specimen, the contrast appearing in the
NSOM optical image could originate totally or in part from topographic change in the specimen surface.
To minimize the in
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