Study of nanotribology measuring tools

deepaksubudhi456@gmail.com

Nanotribology is a branch of tribology which studies friction phenomenon at the nanometer scale (see nanotechnology,nanomechanics). The distinction between nanotribology and tribology is primarily due to the involvement of atomic forces in the determination of the final behavior of the system. Gears, bearings, and liquid lubricants can reduce friction in the macroscopic world, but the origins of friction for small devices such as micro- or nanoelectromechanical systems (NEMS) require other solutions The surface force apparatus(SFA),atomic force & friction force microsopes(AFM &FFM) are widely used in nanotribological studies

INTRODUCTION

Surface Force ApparatusThe surface force apparatus (SFA) is a scientific instrument and technique pioneered by D. Tabor, R.H.S. Winterton, J.N. Israelachvili in the early 1970s at Cambridge University. By the mid-70's Israelachvili had adapted the original design to operate in liquids, notably aqueous solutions, while at the Australian National University.

In this instrument, two surfaces are carefully moved towards and retracted from one another, all the while measuring their interaction force. One surface is held by a cantilevered spring, and the deflection of the spring is used to calculate the force being exerted. This technique uses piezoelectric positioning elements (in addition to conventional motors for coarse adjustments), and senses the distance between the surfaces using optical interferometry. Using these sensitive elements, the device can resolve distances to within 0.1 nanometer, and forces at the 10–8 N level. This extremely sensitive technique can be used to measure electrostatic forces, elusive Vander walls forces, and even hydration or solvation forces. SFA is in some ways similar to using an atomic force microscope to measure interaction between a tip (or molecule adsorbed onto the tip) and a surface. The SFA, however, is more ideally suited to measuring surface-surface interactions, and can measure much longer-range forces more accurately. The SFA technique is quite demanding, however, and only a handful of labs worldwide have functional instruments.

Fig: Schematic diagram of SFA

Principle of the Surface Forces Apparatus (SFA):

Fig: Schematic representation of the Surface Forces Apparatus.

The Surface Forces Apparatus (SFA) is an instrument that can measure forces occurring between two curved surfaces. The two surfaces are cylindrically curved and oriented such that the cylinder axis are crossed at an angle of 90°. The shortest distance, D, between the two curved surfaces can be varied by moving the approach actuator by the amount, M. One of the two surfaces is mounted on a compliant spring with spring constant, k. When the surfaces are separated sufficiently, a motion of the actuator will result in an equal change in surface distance; i.e. M=D

The situation is different when a surface force, F(D), deflects the spring at closer distances and D and M are thus no longer equal. During an experiment one optically measures D(M) to be able to calculate the external force, F(D)=k*(D-M). A negative force means attraction and a positive force means repulsion. When the surfaces are in contact, they deform elastically to form a circular contact with a diameter of several 10µm. In such contact, D may vary only little, while the actuator continues to move. Then, the external force (or load), F is increased roughly proportional to k*M.

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For the measurement, it is of utmost importance that M can be controlled very accurately and reproducibly. A well-designed mechanical approach mechanism is necessary to meet these requirements.

Common surfaces are 2-5µm thick sheets of mica, which are manually cleaved and glued onto transparent cylinder lenses after evaporation of a typically 55 nm-thick silver layer on the reverse side. The two silver layers are mirrors comprising the white light interferometer. Using Multiple Beam Interferometry (MBI), one can determine the distance between the surfaces. To this end, it is necessary to measure an optical zero when the mica surfaces are in direct contact. The measurement of the optical zero is essentially a determination of the mica substrate thickness.

Principle of the Surface Forces Apparatus (Contd.):

crossed cylinder geometry of mica sheets used in SFA is mathematically equivalent to sphere on flat surface contact

measurement of adhesion forces and interfacial energy can be analyzed by JKR (Johnson, Kendal, Roberts) theory for large soft objects, or DMT(Derjaguin, Muller, Toporov) for small hard objects

JKR: DMT:Fadhesion=3πγR Fadhesion=4πγR

Force measurement : SFA

A selection of different SFA designs is depicted below:

Fig: Different design variants of the interferometric Surface Forces Apparatus.

A number of attachments have been developed for the SFA, which allow one to apply and measure oscillatory or linear motions in-plane and out-of-plane. The best known example is a lateral-force attachment, which can be used to study friction in the SFA.

THE EXTENDED SURFACE FORCES APPARATUS (eSFA):

Fig: the extended Surface Forces Apparatus (eSFA) with

CCD camera, spectrograph and computer control.

Many SFAs are operated manually. The extended Surface Forces Apparatus (eSFA) represents a fully automated version of the SFA3TM (Surforce, USA).

Interference spectra are recorded automatically and at high speeds. Up to 150 interference fringes are tracked by the software and the distance is calculated in real time. At maximum speed, up to three distance measurements are made per second. This allows the motion of the surfaces to be monitored during adjustments and instrumental and thermal drift to be checked during instrument equilibration.

For optimal conditions, the instrument is placed in an insulated enclosure that provides high thermal stability in a range from -10°C to +70°C. Two temperature sensors measure the temperature both inside the enclosure and inside the fluid cell of the SFA during experiments. When the cover of the enclosure is closed, a typical equilibration time of 10-16 hours is allowed before sensitive measurements are made.

eSFA characteristics and specifications

high instrumental stability (typical drift rates dD/dt<50pm/min)

precision temperature control with extended temperature range (typical stability dT/dt~2mK/h, range -10°C to +70°C)

high-precision distance measurement using Fast Spectral Correlation (FSC) Interferometry (typical distance resolution: 25pm, range 0-100µm)

full lateral scanning ability (lateral resolution ~1µm)

accurate and straight forward determination of the point of closest approach (PCA)

measurement of refractive index of very thin films using Fast Spectral Correlation (FSC) Interferometryextended dynamic window (1 pm/s to 100µm/s)

full computer control for unattended batch-processing of repetitive measurements

Working principle of eSFAAutomated measurements with the eSFA are typically performed at night, without human presence

The eSFA control software (Acquisoft) allows us to program several different experiments in advance with predetermined timing. Several loading/unloading cycles (force-run) under various conditions can thus be programmed in advance.

Already measured Acquisoft data files can be reloaded. They contain all necessary commands to exactly reproduce any experiment or sequence of experiments. The Acquisoft software has a modular architecture, which allows one to perform an unlimited variety of different experiments (e.g. drift-measurement, force-run, friction experiments...) in arbitrary sequence and under any conditions that are instrumentally accessible.

Fig: Schematic of the optics setup.

Several minimotors inside the thermal enclosure allow one to make fine adjustments of the optics while the enclosure remains closed

Light is coupled in from a 450Watt Xenon arc lamp or alternatively from a fiber optic light source (not shown)

To minimize unwanted heating inside the thermally stabilized instrument, IR components of the light are filtered out using two dichroic mirrors in series. This cold white light is then directed through a modified SFA3TM(Surforce, USA) where it is filtered by the interferometer (back-silvered mica surfaces).

The emerging light is finally collected with an imaging lens and directed into an imaging spectrograph. The spectrum is recorded with a high-resolution CCD camera (5000 pixels, 7µmx7µm).

The pixel information is transferred to the computer where the wavelengths of up to 150 fringes are determined simultaneously.

The obtained set of wavelengths is used to calculate distance and/or refractive index between the surfaces. The numerical evaluation is based on Fast Spectral Correlation (FSC).

The optical probe and the Point of Closest Approach (PCA):

Fig: Schematic comparison between the optical probes of eSFA and SFA.

An important difference between automated and manual measurement of interference fringes is that the automated measurement is a local local measurement whereas the manual measurement determines the average fringe wavelength over a cross section of the flattened contact area. To better illustrate this difference, we have added in the fig. below

Atomic force microscope (afm)The atomic force microscope (AFM) or scanning force microscope (SFM) is a very high-resolution type of scanning probe microscope, with demonstrated resolution of fractions of a nanometer, more than 1000 times better than the optical diffraction limit

The precursor to the AFM, the scanning tunneling microscope, was developed by Gerd Binnig and Heinrich Rohrer in the early 1980s, a development that earned them the Nobel Prize for Physics in 1986. Binnig, Quate and Gerber invented the first AFM in 1986.

The AFM is one of the foremost tools for imaging, measuring and manipulating matter at the nanoscale.

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.

Fig: Block Diagram of Atomic Force Microscope

Working principle of afmThe AFM consists of a microscale cantilever with a sharp tip (probe) at its end that is used to scan the specimen surface. . The cantilever is typically silicon or silicon nitride with a tipradius of curvature on the order of nanometers.

When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hooke's law .

Depending on the situation, forces that are measured in AFM include mechanical contact force,Van der Waals forces, capillary forces, chemical bonding,electrostatic forces, magnetic forces (see Magnetic force microscope (MFM)), Casimir forces, solvation forces etc. As well as force, additional quantities may simultaneously be measured through the use of specialised types of probe (seeScanning thermal microscopy, photothermal microspectroscopy, etc.)

Fig:AFM cantilever (after use) in the Scanning Electron Microscope, magnification 1,000 x (image width ~ 100 micrometers)

Fig: AFM cantilever (after use) in the Scanning Electron

Microscope, magnification 3,000 x (image width ~ 30

micrometers)Typically, the deflection is measured using a laser spot reflected from the top surface of the cantilever into an array of photodiodes. Other methods that are used include optical interferometer, capacitive sensing or piezoresistive AFM cantilevers. These cantilevers are fabricated with piezoresistive elements that act as astrain gauge. Using a Wheatstone bridge, strain in the AFM cantilever due to deflection can be measured, but this method is not as sensitive as laser deflection or interferometer

If the tip was scanned at a constant height, a risk would exist that the tip collides with the surface, causing damage. Hence, in most cases a feedback mechanism is employed to adjust the tip-to-sample distance to maintain a constant force between the tip and the sample.

Traditionally, the sample is mounted on a piezoelectric tube, that can move the sample in the z direction for maintaining a constant force, and the x and y directions for scanning the sample. Alternatively a 'tripod' configuration of three piezo crystals may be employed, with each responsible for scanning in the x,y and z directions

This eliminates some of the distortion effects seen with a tube scanner. In newer designs, the tip is mounted on a vertical piezo scanner while the sample is being scanned in X and Y using another piezo block. The resulting map of the area s = f(x,y) represents the topography of the sample.

Force measurement : AFM

AFM Operation modesimaging modecontact mode

• repulsive forces ~10-9 N

Non contact mode• attractive (van der

Waals) forces regime

Tapping mode• cantilever is

oscillated at its resonant

Frequency• repulsive force

region, but touches the

surface only for short periods of time

Imaging modes The primary modes of operation are static (contact) mode and dynamic mode. In the static mode operation, the static tip deflection is used as a feedback signal. Because the measurement of a static signal is prone to noise and drift, low stiffness cantilevers are used to boost the deflection signal. However, close to the surface of the sample, attractive forces can be quite strong, causing the tip to 'snap-in' to the surface. Thus static mode AFM is almost always done in contact where the overall force is repulsive. Consequently, this technique is typically called 'contact mode'. In contact mode, the force between the tip and the surface is kept constant during scanning by maintaining a constant deflection.

In the dynamic mode, the cantilever is externally oscillated at or close to its fundamental resonance frequency or a harmonic. The oscillation amplitude, phase and resonance frequency are modified by tip-sample interaction forces; these changes in oscillation with respect to the external reference oscillation provide information about the sample's characteristics. Schemes for dynamic mode operation include frequency modulation and the more common amplitude modulation

In frequency modulation, changes in the oscillation frequency provide information about tip-sample interactions. Frequency can be measured with very high sensitivity and thus the frequency modulation mode allows for the use of very stiff cantilevers. Stiff cantilevers provide stability very close to the surface and, as a result, this technique was the first AFM technique to provide true atomic resolution in ultra-high vacuum conditions (Giessibl).

In amplitude modulation, changes in the oscillation amplitude or phase provide the feedback signal for imaging. In amplitude modulation, changes in the phase of oscillation can be used to discriminate between different types of materials on the surface. Amplitude modulation can be operated either in the non-contact or in the intermittent contact regime. In ambient conditions, most samples develop a liquid meniscus layer. Because of this, keeping the probe tip close enough to the sample for short-range forces to become detectable while preventing the tip from sticking to the surface presents a major hurdle for the non-contact dynamic mode in ambient conditions. Dynamic contact mode (also called intermittent contact or tapping mode) was developed to bypass this problem (Zhong et al.). In dynamic contact mode, the cantilever is oscillated such that the separation distance between the cantilever tip and the sample surface is modulated.

Amplitude modulation has also been used in the non-contact regime to image with atomic resolution by using very stiff cantilevers and small amplitudes in an ultra-high vacuum environment.

Imaging modes (contd.)

Contact mode

Non contact mode

Tapping (intermittant) mode

Comparison of typical operating parameters

Operating parameter

SFA AFA/FFM

Radius of mating surface/tip

10mm 5-100 nm

Radius of contact area 10-40 micrometer 0.05-0.5 nm

Normal load 10-100mN < 0.1nN-500nN

Sliding velocity 0.001-100 micrometer/s 0.02-2 micrometer/s

Sample limitation Typically atomically smooth, optically transparent mica: opaque ceramic ,smooth surfaces can also be used

None

references•  Surface Science & Technology, Swiss Federal Institute of

Technology

• Australian National University, Research School of Physical Sciences and Engineering

• The X-ray Surface Forces Apparatus: Structure of a Thin Smectic Liquid Crystal Film Under Confinement" Science 24 June 1994: Vol. 264. no. 5167, pp. 1915 – 1918

• The x-ray surface forces apparatus for simultaneous x-ray diffraction and direct normal and lateral force measurements" Review of Scientific Instruments, Volume 73, Issue 6, pp. 2486-2488 (2002).A. D L. Humphris, M. J. Miles, J. K. Hobbs, A mechanical microscope: High-speed atomic force microscopy, Applied Physics Letters 86, 034106 (2005).

• V. J. Morris, A. R. Kirby, A. P. Gunning, Atomic Force Microscopy for Biologists. (Book) (December 1999) Imperial College Press.

• F. Giessibl, Advances in Atomic Force Microscopy, Reviews of Modern Physics 75 (3), 949-983 (2003).