Scanning Force Microscopy.pdf

SCANNING FORCE

MICROSCOPY

Introduction

Synthetic polymers, as well as biopolymers, exhibit a rich and very complex struc-

tural and morphological hierarchy on multiple length scales, spanning the size

range from monomer residue units comprising the polymer chains to higher order

micro- or macroscale structures, such as ﬁbers or spherulites. Since the perfor-

mance of polymeric materials is intimately related to this hierarchy, the struc-

tural and morphological analysis of polymers has played a central role in polymer

science and technology. With the continual striving toward the mesoscopic and

nanoscopic size regime in modern technology, high resolution analysis tools gain

increasing importance.

Following the Nobel prize winning invention of the scanning tunneling mi-

croscope (STM) (1,2), the well-developed and established family of microscopy

techniques in the ﬁeld of polymer science, such as polarized optical microscopy

(OM) (3) and various electron microscopies (4), was enriched by a range of newly

developed scanning proximity probe microscopy techniques in the late 1980s (5).

Among these techniques, scanning force microscopy (SFM) in particular has suc-

cessfully complemented the traditional techniques for studying simultaneously

polymer morphology and surface properties.

Compared to STM, which is inherently limited to conducting samples or ul-

trathin samples on conducting substrates, SFM is generally applicable to a much

broader range of materials, including insulators, such as polymers. SFM, which

includes the originally developed atomic force microscopy (AFM) (5), can be con-

sidered to be a unique technique in the sense that it yields topographical informa-

tion in 3-D with, in the optimized case, (sub-)nanometer spatial resolution over

lateral length scales of

100

m to nanometers. In addition, material-speciﬁc

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700 SCANNING FORCE MICROSCOPY

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contrast can also be obtained based on, eg, adhesion, friction, or elasticity dif-

ferences, thus enabling high resolution mapping of composition in heterogeneous

systems.

It is clear that SFM has been quickly established as an important research

tool in academia in the ﬁeld of polymers; at the same time its huge potential

has been realized in industry, where it ﬁnds increasing application in research

and development laboratories. The range of applications reaches from unrav-

eling structure–property relationships, as well as long-standing, unresolved is-

sues, such as polymer crystallization, polymer morphology and its relation to

processing and mechanical properties, to the analysis of novel polymer archi-

tectures and substrate-supported assemblies. It includes studies of the behav-

ior of macromolecules in conﬁnement, compositional mapping for applications

in the biomedical sector, as well as in polymer surface modiﬁcation and eluci-

dation of failure mechanisms, and more recently real-time studies of dynamic

processes at the relevant length scale. In addition to a still needed standardiza-

tion and deﬁnition of protocols in the industrial environment, the development

and improvement of new SFM modes, in particular local thermal analysis (6) and

chemically sensitive imaging (7), pose challenges for the near future. The impact

of SFM can undoubtedly be expected to further grow for polymer science and

technology.

Considering the rapid increase in the number of SFM-related publications

in the ﬁeld of polymers, it is necessary to focus on key areas and make a narrow

selection of the discussed material. The author would like to offer his apologies

to those authors who consider possible omissions as negligence by the author.

Whenever possible and appropriate, references to the many excellent reviews on

particular topics have been included.

In this contribution, the basic concepts and modes of SFM, as well as some

rudimentary theory, are introduced and only brieﬂy reviewed in order to provide

a self-supporting basis for the discussion of selected areas of application of SFM

on polymers. The study of the morphology and hierarchical ordering of polymers

by SFM is reviewed, starting from the visualization of single macromolecules,

chain packing, and surface crystal structures. The imaging of lamellar crystals,

hedrites, and spherulites by SFM approaches is discussed, followed by investiga-

tions on effects of deformation and processing, as well as studies on morphology

of latex particles and ﬁbers. A second central section is devoted to the mapping

of surface properties and composition via the spatially resolved measurement of

forces and surface mechanical properties. Finally, structure development and dy-

namic processes, including polymer crystallization, followed in real-time by SFM

are discussed.

Within the numerous proximal probe techniques developed in the years fol-

lowing the breakthrough inventions of the STM (1) and AFM (5), scanning force

microscopy (8) represents a family of scanning probe techniques that rely in their

contrast mechanism on various forces between probe tip and sample (9–12) (see

A TOMIC F ORCE M ICROSCOPY ). In order to provide a basis for an understanding

and appreciation of the SFM work on polymers (13–18), as presented in this re-

view, the basic principles of SFM, as well as selected imaging modes, are brieﬂy

discussed.

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SCANNING FORCE MICROSCOPY 701

SFM and Its Imaging Modes

and the mesoscopic and nanoscopic size regime. In most of the SFM modes, topog-

raphy and additional properties can be probed simultaneously, which allows one

to make important correlations of structure and local physical properties. Compli-

cated sample preparation, such as replication techniques needed for transmission

electron microscopy (TEM), can often be circumvented and experiments can be

carried out in a wide range of media (gas, vacuum, liquids) and experimental

conditions. For other specialized SFM techniques the reader is referred to the

corresponding literature (8–20).

Contact Mode SFM. The different SFM techniques have many basic fea-

tures in common, which will be introduced in this ﬁrst section on contact mode

SFM. In contact mode SFM a sharp probe tip, which is attached to a cantilever

∼

100

Fig. 1. ( a ) Schematic diagram of contact mode SFM and SEM images of ( b ) V-shaped SFM

cantilevers and ( c ) tip. The basic elements of transducer (piezo positioner), cantilever and

tip assembly, and ultra-sensitive force detection system (optical beam deﬂection/photodiode

detector) are common for all SFM techniques, while details may differ. Adapted with per-

mission from Ref. 21.

In scanning probe microscopy (SPM), a scanned probe is utilized to collect highly

localized information of sample surface properties via the corresponding physical

quantities. Typically, the proximal probe and the sample specimen are in (near)

contact and the corresponding information is collected, while probe and sample

are mutually displaced laterally by suitable transducers (Fig. 1). The probe mi-

croscopy techniques belonging to the family of SFM discussed below comprise,

among others, contact, intermittent contact, and force modulation modes. These

modes are of central importance to provide information on, eg, topography (quanti-

tative surface proﬁle), composition in heterogeneous systems, elasticity, and other

mechanical properties of polymeric materials on length scales between

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(typical spring constant k of 1.0–0.1 N/m), is brought into contact with the sam-

ple surface (Fig. 1) (21). While the probe is scanned across the sample surface, or

likewise the sample is scanned under this probe, the deﬂection of the cantilever

is measured.

SFM cantilevers and tips are often made of silicon or silicon nitride; SFM tips

possess radii of curvature at the apex between few and several tens to hundreds

of nanometers (Fig. 1). A piezoelectric transducer is used in order to position the

sample accurately. Depending on the scanner type (piezo tube length and design)

the maximum scan sizes vary between ca 1

0.01 nm. The cantilever de-

ﬂection is typically monitored by an optical beam deﬂection technique (Fig. 1 a ).

Other possibilities to measure the deﬂection include STM, piezoresistive, capac-

itance, or interferrometric detection schemes (22). In the optical beam deﬂection

setup, laser light is reﬂected off the end of the cantilever and is collected by a

position-sensitive photodiode. For instance, a 4-quadrant photodiode can simulta-

neously measure deﬂections in vertical (surface normal) and horizontal (lateral)

direction (Fig. 1 a ).

In the so-called constant height mode the deﬂection of the cantilever is mea-

sured in order to obtain the surface proﬁle or topography of the specimen. The

deﬂection of the cantilever is a result of the repulsive and attractive interactions

between the atoms of the tip and the atoms of the sample surface. In the more

commonly used constant force mode, a feedback loop holds a preset deﬂection of

the cantilever constant by adjusting the sample height ( z ). A graphic represen-

tation of the adjustment of specimen height z yields an isoforce surface, which

represents the sample topography. These data correspond to the true proﬁle only

if variations in local sample compliance are negligible (23,24). On the basis of

an accurate calibration, SFM thus becomes an instrument for metrology, which

includes the quantitative determination of surface roughness as well (25).

SFM experiments are often performed in ambient conditions, although the

presence of a contamination layer (typically water) gives rise to sometimes signif-

icant capillary forces (26). In addition to measurements in air, experiments can

be performed in other gases, in liquids, or under (high) vacuum. The problem of

capillary forces can be easily eliminated by performing measurements in liquids

utilizing a liquid cell (27). Contact mode SFM imaging is typically difﬁcult or im-

possible on very rough samples, samples with high adhesion, such as adhesives,

as well as most types of liquids (28). Highly viscous liquids, such as polymer melts,

can be imaged in some cases even with contact mode SFM (see section Structure

Development and Dynamic Processes in Real Time). In addition, the lateral forces

between tip and sample may lead to modiﬁcation or destruction of delicate polymer

specimens (29,30).

The true lateral resolution for imaging is limited by the shape (aspect ra-

tio) and the radius of curvature of the tip (31–34). Imaging artifacts, which are

inherent to microscopic techniques, are well known and can be identiﬁed as such

by systematic variations of relative scan angles, scan velocities (comprising scan

rate and scan size), respective imaging forces, and a thorough tip shape analysis

(35).

∼

The data, in imaging applications most importantly the surface proﬁle or to-

pography (height), can be displayed graphically using a color scale to indicate the

m and several hundred micrometers,

with an accuracy of positioning in the best cases of

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SCANNING FORCE MICROSCOPY 703

height of each point observed. A signiﬁcant difference between SFM and scanning

electron microscopy (SEM) for imaging surface topography is that SFM provides

a quantitative surface proﬁle with excellent lateral and vertical resolution. In ad-

dition, contact mode SFM has been used to measure both repulsive and attractive

forces between sample and surface with a high accuracy (force spectroscopy), as

well as (lateral) friction forces, as will be discussed in the sections under Normal

Forces and Lateral Forces, respectively.

Intermittent Contact Mode SFM. Intermittent contact mode SFM, also

called tapping mode SFM, has been developed to reduce shear forces (36), which

can cause sample deformation or even destruction of soft samples in contact mode

SFM (for a recent review see Ref. 19). In addition, this mode can be useful to probe

adhesive or surface mechanical properties, and thus achieve contrast between dif-

ferent materials. Intermittent contact mode SFM uses an oscillating cantilever/tip

assembly to minimize contact time and contact forces, and to especially eliminate

shear forces between tip and sample surface. This mode relies on a similar setup

as contact mode SFM. However, in intermittent contact mode (in air) a stiffer

cantilever is used ( k

Fig. 2. Schematic of intermittent contact mode SFM: ( a ) cantilever vibrating freely in air

vs ( b ) damped oscillation due to interactions with the surface; ( c ) amplitude vs displacement

dependence showing the steep decrease of the amplitude as function of distance; ( d ) phase

angle shift

20–80 N/m). This cantilever is excited to vibrate near its

resonance frequency by an excitation piezo. Close to a sample surface, the vibra-

tion is damped as a result of tip sample interactions. The original amplitude A 0 is

hence reduced to A , depending sharply on the distance to the surface (Fig. 2 c ). The

between excitation signal and cantilever response owing to interactions of

the tip with the surface.

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