Bhushan B. Nanotribology and Nanomechanics: An Introduction
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1 Introduction – Measurement Techniques and Applications 29
includes chemical synthesis, the spontaneous “self-assembly” of molecular clusters
(molecular self-assembly) from simple reagents in solution, or biological molecules
(e.g., DNA) as building blocks to produce three dimensional nanostructures, quan-
tum dots (nanocrystals) of arbitrary diameter (about 10–10
5
atoms), molecular
beam epitaxy (MBE) and organometallic vapor phase epitaxy (OMVPE) to create
specialized crystals one atomic or molecular layer at a time, and manipulation of
individual atoms by an atomic force microscope or atom optics. The self-assembly
must be encoded, that is, one must be able to precisely assemble one object next
to another to form a designed pattern. A variety of nonequilibrium plasma chem-
istry techniques are also used to produce layered nanocomposites, nanotubes, and
nanoparticles. NEMS field, in addition to fabrication of nanosystems, has provided
impetus to development of experimental and computation tools.
Examples of NEMS include nanocomponents, nanodevices, nanosystems, and
nanomaterials such as microcantilever with integrated sharp nanotips for STM and
AFM, AFM array (Millipede) for data storage, AFM tips for nanolithography, dip-
pen nanolithography for printing molecules, biological (DNA) motors, molecular
gears, molecularly-thick films (e.g., in giant magnetioresistive or GMR heads and
magnetic media), nanoparticles (e.g., nanomagnetic particles in magnetic media),
nanowires, carbon nanotubes, quantum wires (QWRs), quantum boxes (QBs), and
quantum transistors [16]. BIONEMS include nanobiosensors – microarray of sili-
con nanowires, roughly few nm in size, to selectively bind and detect even a sin-
gle biological molecule such as DNA or protein by using nanoelectronics to de-
tect the slight electrical charge caused by such binding, or a microarray of carbon
nanotubes to electrically detect glucose, implantable drug-delivery devices – e.g.,
micro/nanoparticles with drug molecules encapsulated in functionized shells for
a site-specific targeting applications, and a silicon capsule with a nanoporousmem-
brane filled with drugs for long term delivery, nanodevices for sequencing single
molecules of DNA in the Human Genome Project, cellular growth using carbon
nanotubes for spinal cord repair, nanotubes for nanostructured materials for various
applications such as spinal fusion devices, organ growth, and growth of artificial
tissues using nanofibers.
Figure 1.20 shows AFM based nanoscale data storage system for ultrahigh den-
sity magnetic recording which experiences tribological problems [73]. The system
uses arrays of several thousand silicon microcantilevers (“Millipede”) for thermo-
mechanical recording and playback on an about 40-nm thick polymer (PMMA)
medium with a harder Si substrate. The cantilevers are integrated with integrated tip
heaters with tips of nanoscale dimensions. Thermomechanical recording is a com-
bination of applying a local force to the polymer layer and softening it by local
heating. The tip heated to about 400
◦
C is brought in contact with the polymer for
recording. Imaging and reading are done using the heater cantilever, originally used
for recording, as a thermal readback sensor by exploiting its temperature-dependent
resistance. The principle of thermal sensing is based on the fact that the thermal
conductivity between the heater and the storage substrate changes according to the
spacing between them. When the spacing between the heater and sample is reduced
30 Bharat Bhushan
Millipede concept Single tip
Approaching/ thermal sensors
32u 32 array
Chip heater
2 mm
200μm
20μm
2μm
Fig. 1.20. AFM based nanoscale data storage system with 32×32 tip array – that experiences
a tribological problem (from [16])
as the tip moves into a bit, the heater’s temperature and hence its resistance will
decrease. Thus, changes in temperature of the continuouslyheated resistor are mon-
itored while the cantilever is scanned over data bits, providing a means of detecting
the bits. Erasing for subsequent rewriting is carried out by thermal reflow of the
storage field by heating the medium to 150
◦
C for a few seconds. The smoothness
of the reflown medium allows multiple rewriting of the same storage field. Bit sizes
ranging between 10 and 50 nm have been achieved by using a 32 ×32 (1024) ar-
ray write/read chip (3mm×3 mm). It has been reported that tip wear occurs by the
contact between tip and Si substrate during writing. Tip wear is considered a major
concern for the device reliability.
1.6 Role of Micro/Nanotribology and Micro/Nanomechanics
in Magnetic Storage Devices and MEMS/NEMS
The magnetic storage devices and MEMS/NEMS are the two examples where
micro/nanotribologicaland micro/nanomechanical tools and techniques are essen-
tial for studies of micro/nano scale phenomena. Magnetic storage components con-
tinue to shrink in physical dimensions. Thicknesses of hard solid coating and liquid
lubricant coatings on the magnetic disk surface continue to decrease. Number of
1 Introduction – Measurement Techniques and Applications 31
contact recordingdevices are at variousstages of development.Surfaceroughnesses
of the storage components continue to decrease and are expected to approach to
about0.5 nmRMS or lower.Interfacestudiesof componentswith ultra-thincoatings
can be ideally performed using micro/nanotribologicaland micro/nanomechanical
tools and techniques.
In the case of MEMS/NEMS, the friction and wear problems of ultrasmall
moving components generally made of single-crystal silicon, polysilicon films or
polymers need to be addressed for high performance, long life, and reliability.
Molecularly-thin films of solid and/or liquids are used for low friction and wear in
many applications. Again, interfacial phenomena in MEMS/NEMS can be ideally
studied using micro/nanotribological and micro/nanomechanical tools and tech-
niques.
1.7 Organization of the Book
The introductory book integrates knowledge of nanotribology and nanomechan-
ics. The book starts with the definition of tribology, history of tribology and
micro/nanotribology, its industrial significance, various measurement techniques
employed, followed by various industrial applications. The remaining book is di-
vided into four parts. The first part introduces scanning probe microscopy. The sec-
ond part provides an overview of nanotechnology and nanomechanics. The third
part provides an overview of molecularly-thick films for lubrication. And the last
part focuses on nanotribologyand nanomechanics studies conducted for various in-
dustrial applications.
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Part I
Scanning Probe Microscopy
2
Scanning Probe Microscopy – Principle of Operation,
Instrumentation, and Probes
Bharat Bhushan and Othmar Marti
Summary. Since the introduction of the STM in 1981 and the AFM in 1985, many varia-
tions of probe-based microscopies, referred to as SPMs, have been developed. While the pure
imaging capabilities of SPM techniques initially dominated applications of these methods,
the physics of probe–sample interactions and quantitative analyses of tribological, electronic,
magnetic, biological, and chemical surfaces using SPMs have become of increasing inter-
est in recent years. SPMs are often associated with nanoscale science and technology, since
they allow investigation and manipulation of surfaces down to the atomic scale. As our un-
derstanding of the underlying interaction mechanisms has grown, SPMs have increasingly
found application in many fields beyond basic research fields. In addition, various derivatives
of all these methods have been developed for special applications, some of them intended for
areas other than microscopy.
This chapter presents an overview of STM and AFM and various probes (tips) used in
these instruments, followed by details on AFM instrumentation and analyses.
2.1 Introduction
The scanning tunneling microscope (STM), developed by Dr. Gerd Binnig and his
colleaguesin 1981at the IBM ZurichResearch Laboratoryin Rueschlikon(Switzer-
land), was the first instrument capable of directly obtaining three-dimensional(3-D)
images of solid surfaces with atomic resolution [1]. Binnig and Rohrer received
a Nobel Prize in Physics in 1986 for their discovery. STMs can only be used to
study surfaces which are electrically conductive to some degree. Based on their
design of the STM, in 1985, Binnig et al. developed an atomic force microscope
(AFM) to measure ultrasmall forces (less than 1 µN) between the AFM tip surface
and the sample surface [2] (also see [3]). AFMs can be used to measure any en-
gineering surface, whether it is electrically conductive or insulating. The AFM has
become a popular surface profiler for topographic and normal force measurements
on the micro- to nanoscale [4]. AFMs modified in order to measure both normal and
lateral forces are called lateral force microscopes (LFMs) or friction force micro-
scopes (FFMs) [5–11]. FFMs have been further modified to measure lateral forces
in two orthogonal directions [12–16]. A number of researchers have modified and
38 Bharat Bhushan and Othmar Marti
improved the original AFM and FFM designs, and have used these improved sys-
tems to measure the adhesion and friction of solid and liquid surfaces on micro- and
nanoscales [4,17–30]. AFMs have been used to study scratching and wear, and to
measure elastic/plastic mechanicalproperties (such as indentation hardness and the
modulus of elasticity) [4,10,11,21,23,26–29,31–36]. AFMs have been used to ma-
nipulate individual atoms of xenon [37], molecules [38], silicon surfaces [39] and
polymer surfaces [40]. STMs have been used to create nanofeatures via localized
heating or by inducing chemical reactions under the STM tip [41–43] and through
nanomachining [44]. AFMs have also been used for nanofabrication [4,10,45–47]
and nanomachining [48].
STMs and AFMs are used at extreme magnifications ranging from 10
3
to 10
9
in
the x-, y-andz-directions in order to image macro to atomic dimensions with high
resolution and for spectroscopy. These instruments can be used in any environment,
suchas ambientair [2,49],variousgases[17], liquids[50–52],vacuum[1,53],at low
temperatures (lower than about 100K) [54–58] and at high temperatures [59,60].
Imaging in liquid allows the study of live biological samples and it also eliminates
the capillary forces that are present at the tip–sample interface when imaging aque-
ous samples in ambient air. Low-temperature (liquid helium temperatures) imaging
is useful when studying biological and organic materials and low-temperature phe-
nomena such as superconductivity or charge-density waves. Low-temperature op-
eration is also advantageous for high-sensitivity force mapping due to the reduced
thermal vibration. They also have been used to image liquids such as liquid crys-
tals and lubricant molecules on graphite surfaces [61–64]. While applications of
SPM techniques initially focused on their pure imaging capabilities, research into
the physics and chemistry of probe–sample interactions and SPM-based quantita-
tive analyses of tribological, electronic, magnetic, biological, and chemical surfaces
have become increasingly popular in recent years. Nanoscale science and technol-
ogy is often tied to the use of SPMs since they allow investigation and manipulation
of surfaces down to the atomic scale. As our understanding of the underlying inter-
action mechanisms has grown, SPMs and their derivatives have found applications
in many fields beyond basic research fields and microscopy.
Families of instruments based on STMs and AFMs, called scanning probe mi-
croscopes(SPMs), have beendevelopedfor variousapplicationsof scientificand in-
dustrial interest. These include STM, AFM, FFM (or LFM), scanning electrostatic
force microscopy (SEFM) [65, 66], scanning force acoustic microscopy (SFAM)
(or atomic force acoustic microscopy (AFAM)) [21,22,36,67–69], scanning mag-
netic microscopy (SMM) (or magnetic force microscopy (MFM)) [70–73], scan-
ning near-field optical microscopy (SNOM) [74–77], scanning thermal microscopy
(SThM) [78–80], scanning electrochemical microscopy (SEcM) [81], scanning
Kelvinprobemicroscopy(SKPM) [82–86],scanningchemicalpotentialmicroscopy
(SCPM) [79], scanning ion conductance microscopy (SICM) [87,88] and scanning
capacitance microscopy (SCM) [82,89–91]. When the technique is used to measure
forces (as in AFM, FFM, SEFM, SFAM and SMM) it is also referred to as scanning
force microscopy (SFM). Although these instruments offer atomic resolution and
2 Scanning Probe Microscopy 39
Table 2.1. Comparison of various conventional microscopes with SPMs
Optical SEM/TEM Confocal SPM
Magnification 10
3
10
7
10
4
10
9
Instrument Price (U.S. $) $10k $250k $30k $100k
Technology Age 200 yrs 40 yrs 20 yrs 20 yrs
Applications Ubiquitous Science and New and Cutting
technology unfolding edge
Market 1993 $800 M $400 M $80 M $100 M
Growth Rate 10% 10% 30% 70%
are ideal for basic research, they are also used for cutting-edge industrial applica-
tions which do not require atomic resolution. The commercial production of SPMs
started with the STM in 1987 and the AFM in 1989 by Digital Instruments Inc
(Santa Barbara, CA, USA). For comparisons of SPMs with other microscopes, see
Table 2.1 (Veeco Instruments, Inc., Santa Barbara, CA, USA). Numbers of these in-
struments are equally divided between the U.S., Japan and Europe, with the follow-
ing split between industry/university and government laboratories: 50/50, 70/30,
and 30/70, respectively. It is clear that research and industrial applications of SPMs
are expanding rapidly.
2.2 Scanning Tunneling Microscope
The principle of electron tunneling was first proposed by Giaever [93]. He envi-
sioned that if a potential difference is applied to two metals separated by a thin
insulating film, a current will flow because of the ability of electrons to penetrate
a potential barrier. To be able to measure a tunneling current, the two metals must
be spaced no more than 10nm apart. Binnig et al. [1] introduced vacuum tunneling
combined with lateral scanning. The vacuum provides the ideal barrier for tunnel-
ing. The lateral scanning allows one to image surfaces with exquisite resolution
– laterally to less than 1nm and vertically to less than 0.1nm–sufficient to define
the position of single atoms. The very high vertical resolution of the STM is ob-
tained because the tunnel current varies exponentiallywith the distance between the
twoelectrodes; that is, the metal tip and the scanned surface.Typically,the tunneling
current decreases by a factor of 2 as the separation is increased by 0.2nm. Very high
lateral resolution depends upon sharp tips. Binnig et al. overcame two key obstacles
by damping external vibrations and moving the tunneling probe in close proximity
to the sample. Their instrument is called the scanning tunneling microscope (STM).
Today’s STMs can be used in ambient environments for atomic-scale imaging of
surfaces. Excellent reviews on this subject have been presented by Hansma and Ter-
soff [92], Sarid and Elings [94], Durig et al. [95]; Frommer [96], Güntherodt and