MEE381 NANOTECHNOLOGY LECTURE 13DR. R. PRATIBHA NALINI

11.2.2014

AFM, Contact Angle, Porosimeter, Phase Transitions

AFMAtomic Force Microscope

WHAT HAPPENS ? THE BASIC PRINCIPLE

Interaction between a sharp probe and sample are used for imaging

AFM – measures the atomic force between atoms at the surface of the sample and the tip of the needle of the cantilever.

Can be used to study non-conducting materials also

ACTIVITY : See How a STM works and what kind of materials can it be used with?

ThermoMicroscopes Explorer AFM

V

Photodiode

Mirror

Laser

Tip

PiezoCrystal

Feedback Loop

Substrate

The AFM uses a sharp tip attached to the end of a cantilever which rasters across an area whilea laser and photodiode are used to monitor the force on the surface.A feedback loop between the photodiode and the piezo-crystal maintains a constant forceduring contact mode imaging and constant amplitude during intermittent contact modeImaging.

WORKING OF AFM

The physical properties of the cantilever

Because the detection systems used in the AFM are dependentOn the phase, amplitude or frequency of vibration of the cantilever.

THIS MEANS

It is important to select a material with appropriate elastic modulus and dimensions to relate with the scale of forces the tip will experience.

The tip that is located at the end of the cantilever is typically very sharp, and has a diameter as small as 20 nm (approx.)

When scanned closely over the sample surface, the forces between the tip and the surface causes the cantilever to deflect, providing data to form the images of the sample’s surface

AFM has several modes of operation

• Contact Mode• Non- Contact Mode•Tapping mode (Intermittent mode)

Contact Mode• Short range forces dominate. • The top of the AFM is kept in contact with the sample

at a constant applied force• Measures repulsion between tip and sample• Feedback regulation keeps cantilever deflection

constant• Voltage required indicates height of sample• Problems: excessive tracking forces applied by probe to

sample. Scanning speed is slow due to the response of the feedback system.

Non-Contact Mode• Measures attractive forces between tip and sample• Tip doesn’t touch sample• Van der Waals forces between tip and sample

detected• Problems: Can’t use with samples in fluid• Used to analyze semiconductors• Doesn’t degrade or interfere with sample- better for

soft samples

Tapping (Intermittent-Contact) Mode

• Tip vertically oscillates between contacting sample surface and lifting of at frequency of 50,000 to 500,000 cycles/sec.

• Oscillation amplitude reduced as probe contacts surface due to loss of energy caused by tip contacting surface

• Advantages: overcomes problems associated with friction, adhesion, electrostatic forces• More effective for larger scan sizes

Consider a liquid drop resting on a flat, horizontal solid surface. The contact angle is defined as the angle formed by the intersection of the liquid-solid interface and the liquid-vapor interface (geometrically acquired by applying a tangent line from the contact point along the liquid-vapor interface in the droplet profile). The interface where solid, liquid, and vapor co-exist is referred to as the “threephase contact line”

CONTACT ANGLE

When Liquid spreads = SMALL CONTACT ANGLES

When liquid beads = LARGE CONTACT ANGLES

WETTING FAVOURABLEComplete wetting when contact angle is 0 degree

If greater than 150 degree, superhydrophobicity – Lotus effect

Several methods for measuring the contact angles of ultrasmall droplets have been reported thanks to the availability of advanced imaging techniques such as interference microscopy, confocal microscopy, environmental scanning electron microscopy (ESEM), and AFM to establish the droplet profiles

A number of methods have been developed to deposit micro- and nanoscale liquid droplets on surfaces. •The simplest way is to use an air sprayer, which generates ultrasmall droplets by mixing macroscale droplets with a jet of compressed air. Commercially available atomizers are able to spray ultrafine droplets with a wide range of sizes. •Using micropipettes with submicron orifices to create submicron-sized droplets. •Electrospraying to generate charged liquid droplets with diameters less than 1 μm. •Fine-emulsions and nano-emulsions formed by two immiscible phases can generate•significantly smaller droplets (between 100 and 1000 nm).A syringe pump has also been used to create microdroplets.

However, with all of the aforementioned methods, control over the size of the droplet presents a challenging problem.

WHAT HAS AFM TO DO WITH CONTACT ANGLES?

To address this problem, Meister et al. developed a nanoscale dispensing (NADIS) technique by modifying a commercially available silicon nitride (Si3N4) AFM probe tip and using it to transfer liquid from the tip to the surface by direct contact.

Here, the size of the deposited droplet is controlled by the aperture width of the hollow AFM tip. The NADIS technique is able to create droplets with controlled sizes to measure the contact angle at micro- or nanoscales.

Researchers have used the NADIS technique to deposit micro- and nanodroplets of a glycerol/water mixture on different surfaces, followed by measuring the contact diameter, thickness, and volume of the droplet with an AFM to determine the contact angles.

Fang et al have shown that tips with an aperture diameter of 35 nm were able to deposit nanodroplets of glycerol-based liquids with diameters down to 70 nm and to form regular arrays on silica surfaces with different hydrophilicities. Importantly, fine control of the droplet volume is also possible.

NADIS

Droplets formed with NADIS

In recent years, AFM has been used to study the contact angle of individualparticles by measuring the interaction forces between a spherical colloidal particleand a bubble in aqueous solution.

The contact region of the AFM force curve is used to establish the position of zero force, which gives the depth of the particle penetration into the bubble. The data can then be used to calculate the contact angle. In this way, the intrinsic hydrophobic properties of an individual particle can be revealed. It is also worth mentioning that soft surfaces can sometimes deform during AFM force measurements.

It has been also observed that the contact angle measured by AFM changes with the speedof the piezoelectric translator, showing that the measurements were generally dynamic.

Calculation of contact angle from AFM

Porosimetry is an analytical technique used to determine various quantifiable aspects of a material's porous nature, such as pore diameter, total pore volume, surface area, and bulk and absolute densities.The technique involves the intrusion of a non-wetting liquid (often mercury) at high pressure into a material through the use of a porosimeter. The pore size can be determined based on the external pressure needed to force the liquid into a pore against the opposing force of the liquid's surface tension.A force balance equation known as Washburn's equation for the above material having cylindrical pores is given as

Since the technique is usually done under vacuum, the gas pressure begins at zero. The contact angle of mercury with most solids is between 135° and 142°, so an average of 140° can be taken without much error. The surface tension of mercury at 20 °C under vacuum is 480 mN/m. With the various substitutions, the equation becomes:

As pressure increases, so does the cumulative pore volume. From the cumulative pore volume, one can find the pressure and pore diameter where 50% of the total volume has been added to give the median pore diameter

Mercury porosimetry characterizes a material’s porosity by applying various levels ofpressure to a sample immersed in mercury. The pressure required to intrude mercury into thesample’s pores is inversely proportional to the size of the pores.

MOSTLY USED TECHNIQUE – MERCURY POROSIMETRY

Mercury porosimetry is based on the capillary law governing liquid penetration into small pores.This law, in the case of a non-wetting liquid like mercury, is expressed by the Washburn equation

PHASE TRANSITIONS

• The crystal phase is stable at high P and low T.

• The liquid phase is stable at high P and high T.

• The vapour phase is stable at low P and low/high T

YOU HAVE TO TELL ME THE ANSWER !!!!!

REFER

1) Nanomaterials, Nanotechnologies and Design – An introduction to engineers and architectsBy Michael F. Ashby, Paulo J. Ferreira and Daniel L. Schodek pg 283-284 for AFM

2) Chapter 1 Contact Angle and Wetting Properties Yuehua Yuan and T. Randall Lee (pdf available freely online – to read how AFM is used for Contact Angle measurements)

3) http://www.micromeritics.com/Repository/Files/Porosimetry_brochure.pdf Mercury porosimeter

4) http://www.ncp.edu.pk/docs/iss/talks/Group_II/G2_D2_Dr_Salamat_Ali.pdf Phase transitions