Atomic Force Microscopy

ESS5855 LectureFall 2011

Scanning Probe Microscopy

• Scan a sharp solid probe in the near-field of the surface of the object to be investigated

• Near-field character– Overcome the resolution limits of far-field

techniques• The specific interaction between this probe

and the sample determines the type of the scanning probe microscope

• The first instrument capable of directly obtaining 3-D images of solid surfaces with atomic resolution

Awarded the Nobel prize for physics in 1986

Quantum tunneling refers to the quantum mechanical phenomenon where a particle tunnels through a barrier

that it classically could not surmount

Scanning Tunneling Microscopy• It consists essentially in scanning a metal tip

over the surface at constant tunnel current• The displacements of the metal tip given by the

voltages applied to the piezo-drives then yield a topographic picture of the surface

• The very high resolution of the STM rests on the strong dependence of the tunnel current on the distance between the two tunnel electrodes

Scanning Tunneling Microscopy

• The spectacular spatial resolution and relative ease of obtaining atomic resolution rests on three properties of the tunneling current:

• As a consequence of the strong distance dependence of the tunneling current

• Typical tunneling currents are in the nanoampere range - measuring currents of this magnitude can be done with a very good signal-to-noise ratio even with a simple experimental setup

• Because the tunneling current is a monotonic function of the tip-sample distance, it is easy to establish a feedback loop that controls the distance so that the current is constant

Atomic Force Microscopy

• Overcome a limitation of the STM, its inability to image insulating materials

• It consists of a sensor that responds to a forceand a detector that measures the sensor’s response

• The sensor - typically a cantilever beam - bends in the presence of attractive or repulsive forces

• The detector measures the cantilever deflection, which can be converted to force by applying Hooke’s law

• If the sample is scanned beneath a pointed tip, mounted on the end of the cantilever and in contact with the sample, a force map or image of sample topography can be generated

Principle

Constant Current (Force)

Principle

Principle

Constant Height

Mechanics of Cantilevers

3

3

4L

Ebhk

E

L

hf

20 162.0

Mechanics of Cantilevers

• Common materials used are silicon, silicon oxide, or silicon nitride

Mechanics of Cantilevers

• For example, the Young’s modulus for silicon nitride is about 250 GPa

• With a silicon nitride plate 200 um long, 20um wide, and 2 um thick, the spring constant k is about 1.25 N/m

• With a density of 3190 kg/m3, the mass of the cantilever is about 2.5x10-11 kg

• The resonance frequency can be estimated to be about 220 kHz

• With the spring constant on the order of N/m, a 10-10 N force can produce a deflection of one Angstrom

Probe

Deflection Sensing

Piezoresistive Sensing

Optical Sensing

Piezoelectric Actuation

• Piezoelectric materials change their shape in an electric field due to their anisotropic crystal structure

• Transverse piezoelectric effect•d31 = 0.262 nm/V

l Edl

l31

Piezoelectric Actuation

Piezoelectric Actuation

Piezoelectric Actuation

• Tube scanner– The outside is

contacted by four symmetric and separated electrodes for bending movement

– The inside is contacted by a single electrode for linear movement

Resolution

Force-Distance Curves

D is the actual tip-sample distance, whereas Z is the distance between the sample and the cantilever rest position. The distance controlled during the measurement is not the actual tip-sample distance D, but the distance Z between sample surface and the rest

position of the cantilever. These two distances differ because of the cantilever deflection δc, and because of the sample deformation δs

Force-Distance Curves

• The curve F(D) represents the tip-sampleinteraction and the lines 1, 2, and 3 represent the elastic force of the cantilever

• At each distance the cantilever deflects until the elastic force equals the tip-sample force and the system is in equilibrium

displacement

distance

Approach: c’ - b – b’Retraction: b’ - c - c’

Z = D + δc

Force-Distance Curves

• This graphical construction has to be made going both from right to left and from left to right

• The points A, B, B’, C, and C' correspond to the points a, b, b', c, and c' respectively

• BB’ and CC’ are two discontinuities• The origin O of axis is usually put at the inter-

section between the prolongation of the zero line and the contact line of the approach curve

• The force fc’ eventually coincides with the zero force

Force-Distance Curves

• The cantilever-sample system can be described by means of a potential Utot that is the sum of three potentials: Ucs(D), Uc(δc), and Us(δs)

• Ucs(D) is the interaction potential between the tip and the sample, e.g., the Lennard-Jones potential

• Uc(δc) is Hooke's elastic potential of the cantilever• Us(δs) is the potential that describes the sample

deformation

Surface Forces

• Unlike the tunneling current, which has a very short range, Fts has long- and short-range contributions

• We can classify the contributions by their range and strength

• In vacuum, there are short-range chemical forces (fractions of nm) and van der Waals, electrostatic, and magnetic forces with a long range (up to 100 nm)

• In ambient conditions, meniscus forces formed by adhesion layers on tip and sample (water or hydrocarbons) can also be present

Surface Forces• The Morse potential

• The Stillinger-Weber potential– nearest-neighbor contribution

– next-nearest-neighbor contribution

• The Lennard-Jones potential

Surface Forces

Contact Mode AFM

•In the repulsive regime, the tip is considered as making direct contact with the surface and the AFM is operating in contact mode•The force varies dramatically over a small distance because of the steep potential curve in the repulsive region•The typical operating range is between 10-6 to 10-7 N but it can be as small as 10-9 N

Non-Contact Mode AFM

•When the tip is kept far from the sample, the AFM is operating in non-contact mode and the force between the probe and the sample is attractive•In this regime, the tip is on the order of several tens to several hundreds of angstroms from the surface and the force is on the order of 10-12 N

Contact Mode AFM

• Advantages– High scan speeds (throughput)– Contact mode AFM is the only AFM technique which can

obtain "atomic resolution" images– Rough samples with extreme changes in vertical topography

can sometimes be scanned more easily in contact mode• Problems

– Lateral (shear) forces can distort features in the image– The forces normal to the tip-sample interaction can be high

in air due to capillary forces from the adsorbed fluid layer on the sample surface

– The combination of lateral forces and high normal forces can result in reduced spatial resolution and may damage soft samples (i.e., biological samples, polymers, silicon) due to scraping between the tip and sample

Non-Contact Mode AFM

• Advantage– No force exerted on the sample surface.

• Problems– Lower lateral resolution, limited by the tip-sample

separation– Slower scan speed than Tapping mode and Contact

mode to avoid contacting the adsorbed fluid layer which results in the tip getting stuck

– Non-contact usually only works on extremely hydrophobic samples, where the adsorbed fluid layer is at a minimum. If the fluid layer is too thick, the tip becomes trapped in the adsorbed fluid layer causing unstable feedback and scraping of the sample

Tapping Mode AFM

• The cantilever is driven to oscillate up and down at near its resonance frequency with an amplitude greater than 10 nm, typically 100 to 200 nm

• The forces acting on the cantilever cause the amplitudeof this oscillation to decrease as the tip gets closer to the sample

• An electronic servo adjusts the height to maintain a set cantilever oscillation amplitude as the cantilever is scanned over the sample

• It is gentle enough even for the visualization of supported lipid bilayers or adsorbed single polymer molecules under liquid medium, at the application of proper scanning parameters

Non-Contact AFM

• Change in dynamic properties– Resonance frequency– Oscillation amplitude– Phase

Dynamic Analysis

Frequency Shift

Frequency Modulation

AFM versus SEM

• Advantages– High resolution and true 3-D surface profile– Requires no special treatments – Work perfectly well in ambient air or even a liquid

environment• Disadvantages

– Small image size– An incorrect choice of tip or cantilever can lead to

image artifacts– Relatively low scanning rate and thermal drift – Hysteresis of the piezoelectric material and cross-talk

between the (x,y,z) axes