Nanotribological surface characterization by frequency modulated torsional resonance mode AFM [Elektronische Ressource] / Ayhan Yurtsever

Ayhan Yurtsever

Sujets

• Naturwissenschaften

Informations

• Publié par: ludwig-maximilians-universitat_munchen
• Publié le: 01 janvier 2008
• Nombre de lectures: 15
• Langue: English
• Poids de l'ouvrage: 3 Mo

Extrait

Nanotribological surface characterization by frequency
modulated torsional resonance mode AFM







Dissertation
der Fakultät für Geowissenschaften
der Ludwig-Maximilians-Universität München











Ayhan Yurtsever

27. March 2008


























Disputation: 04. July 2008



Referees: PD Dr. Robert W. Stark
Prof. Dr. Wolfgang W. Schmahl






2
Abstract

The aim of this work is to develop an experimental method to measure in-plane surface
properties on the nanometer scale by torsional resonance mode atomic force microscopy and
to understand the underlying system dynamics. The invention of the atomic force microscope
(AFM) and the advances in development of new AFM based techniques have significantly
enhanced the capability to probe surface properties with nanometer resolution. However, most
of these techniques are based on a flexural oscillation of the force sensing cantilever which
are sensitive to forces perpendicular to the surface. Therefore, there is a need for highly
sensitive measurement methods for the characterization of in-plane properties. To this end,
scanning shear force measurements with an AFM provide access to surface properties such as
friction, shear stiffness, and other tribological surface properties with nanometer resolution.
Dynamic atomic force microscopy utilizes the frequency response of the cantilever-probe
assembly to reveal nanomechanical properties of the surface. The frequency response function
of a cantilever in torsional motion was investigated by using a numerical model based on the
finite element method (FEM). We demonstrated that the vibration of the cantilever in a
torsional oscillation mode is highly sensitive to lateral elastic (conservative) and visco-elastic
(non-conservative) in-plane material properties, thus, mapping of these properties is possible
in the so-called torsional resonance mode AFM (TR-mode).
The theoretical results were then validated by implementing a frequency modulation (FM)
detection technique to torsion mode AFM. This method allows for measuring both
conservative and non-conservative interactions. By monitoring changes of the resonant
frequency and the oscillation amplitude, we were able to map elastic properties and
dissipation caused by the tip-sample interaction. During approach and retract cycles, we
observed a slight negative detuning of the torsional resonance frequency, depending on the tilt
angle between the oscillation plane and the surface before contact to the HOPG surface. This
angle leads to a mixing of in-plane (horizontal) and out-of-plane (vertical) sample properties.
These findings have a significant implication for the imaging process and the adjustment of
the microscope and may not be ignored when interpreting frequency shift or energy
dissipation measurements.
To elucidate the sensitivity of the frequency modulated torsional resonance mode AFM (FM-
TR-AFM) for the energy dissipation measurement, different types of samples such as a
compliant material (block copolymer), a mineral (chlorite) and a macromolecule (DNA) were
investigated. The measurement of energy dissipation on these specimens indicated that the
3
TR-AFM images reveal a clear difference for the domains which have different mechanical
properties. Simultaneously a topographic and a chemical contrast are obtained by recording
the detuning and the dissipation signal caused by the tip-surface interaction. Using FM-TR-
AFM spectroscopically, we investigated frequency shift versus distance curves on the
homopolymer polystyrene (PS). Depending on the molecular weight, the frequency detuning
curve displayed two distinct regions. Firstly, a rather compliant surface layer was probed;
secondly, the less mobile bulk of the polymer was sensed by the oscillatory motion of the tip.
The high sensitivity of this technique to mechanical in-plane properties suggests that it can be
used to discriminate different chemical properties (e.g. wetting) of the material by
simultaneously measuring energy dissipation and surface topography.




4
Contents

1. Introduction 8

2. The Atomic Force Microscope 11

2.1. Principle of the atomic force microscope………………………………….. 11

2.1.1. Imaging Modes…………………………………………………... 11

2.2. Tip-surface interaction forces………………………………….................... 15

2.2.1. Long-range attractive interactions……………………………….. 15

2.2.2. Contact forces…………………………………………................. 16

2.3. Frequency modulation atomic force microscopy…………………………... 19

2.4. Lateral modulation methods…………………………………...................... 22

3. Torsional Resonance Mode AFM 25

3.1. Theory of the torsional resonance mode…………………………………… 29

3.2. Numerical simulation (FEM)………………………………………………. 31

3.3. Simulation Results…………………………………………………………. 35

4. Experimental Results (Abstracts of the Manuscripts) 37

4.1. Frequency modulated torsional resonance mode AFM……………………. 37

4.2. Response of a laterally vibrating nanotip to surface forces………………... 39

4.3. Frequency modulation torsional resonance mode AFM
on chlorite …………………………………………………………………. 40

4.4. Frequency modulated torsional resonance mode atomic force
microscopy on polymers…………………………………………………… 43

4.5. Acoustical force nanolithography of thin polymer films…………………... 46

4.6. Torsional mode atomic force microscopy and image processing
for the analysis of protein-DNA complex binding site……………………. 48

4.7. Amplitude modulation torsional resonance mode AFM (TR-mode)
on Graphite …………………………………………………………………50


5. Conclusion 52

5
References 54

Appendix 57

1. Manuscripts………………………………………………………………… 57

2. Acknowledgements ……………………………………………………….. 100

3. CV………………………………………………………………………….. 102



























6
List of Abbreviations

AFM Atomic Force Microscope

TR-AFM Torsional Resonance Mode AFM

TM-Mode Tapping Mode

IC-AFM Intermittent Contact Mode AFM

FM-TR-AFM Frequency Modulated Torsional Resonance
Mode AFM

UHV Ultra High Vacuum

LFM Lateral Force Microscopy

nc-AFM Non-Contact AFM

FM Frequency Modulation

FFM Friction Force Microscopy

HOPG Highly Oriented Pyrolytic Graphite

CE-Mode Constant Excitation Mode

CA-Mode Constant Amplitude Mode

KPM Kelvin Probe Microscopy

SCM Scanning Capacitance Microscopy

PS Polystyrene

AGC Automatic Gain Control

PSD Position-Sensitive Photo Detector

PI Proportional Integral

RMS Root Mean Square





7
1. Introduction

The Atomic force microscope (AFM) was invented as a tool for nano-, and atomic-scale
surface topography imaging. In addition to topographic imaging, there has been great interest
in using the AFM as a nanoscale characterization tool, probing a rich variety of information
related to mechanical properties of the surface such as elasticity, hardness and tribological
properties. The measurement of these properties provides additional information which helps
to differentiate between distinct materials. Several techniques have been developed to extract
the surface mechanical properties with nanometer resolution. Typically, an AFM tip in close
contact to the surface is used as a sensor to probe the tip-sample interaction. The tip-sample
interaction is usually measured perpendicular to the sample surface. Conventional methods
based on flexural oscillations (in vertical direction) of the AFM sensor lack information on
the in-plane properties. Usually, only information in vertical direction is measured neglecting
in-plane properties. However, many material properties are described by vectors (e.g.
magnetization) or tensors (e.g. elasticity). Thus, surface characterization methods are needed
which allow for the determination of both in-plane and out-of-plane components of the
interaction between the AFM tip and the sample.
A precise measurement of the in-plane mechanical properties depends on the mode of
operation. The AFM methods cover the entire range from contact over intermittent contact, to
non-contact modes. Researchers have explored ma