11 Scanning Probe Microscopy

Vinay Gupta

   Leaning Objective

 

In this module we will study about Scanning Probe Imaging Methods. In this surface scanning technology we will study about –

  • Scanning Tunneling Microscopy – Basics of Imaging, Instrumentation and various imaging modes.
  • Atomic Force Microscopy – Basics of Imaging, Instrumentation and various imaging modes.

    Introduction

 

Scanning Probe Microscopy (SPM) is a branch of imaging techniques that scans the surfaces with a very sharp probe and whose contact diameter may vary from the size of few atoms to 10s of nanometer. This imaging technique is considerably different from TEM, SEM and other optical methods. In these imaging methods source is quite far from the specimen and it may be a electron gun (for TEM & SEM) and optical imaging it maybe a Tungsten-halogen lamp or LEDs. Therefore, it is the scattering of light and electrons produces an image of the specimen. Here it is not possible to get a surface profile of the sample that can be used for measuring surface roughness and study 3D shape and size of the micro to nanosized object.

 

Scanning Probe Microscopy can be broadly classified into Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM). In STM, a tunneling current between atomically sharp probe is used for scanning the surface. Whereas, in AFM a 100-200 micrometer long cantilever with a sharp silicon nitride or silicon crystal tip is used for scanning the specimen.

 

Let us study first study about Scanning Tunneling Microscopy. Thereafter we will study about Atomic force microscope.

 

Scanning Tunneling Microscope (STM)

 

The Scanning Tunneling Microscope was developed in early 1980s at IBM research laboratory in Switzerland. The microscope was developed by Gerd Binnig and Heinrich Rohrer for which they were awarded Nobel prize in physics in 1986. STM consists of small sharp conducting tip that scans across the sample of interest. Tip diameter is as small as few atoms. The separation distance between the tip and the surface is close to 1 nm. At this small distance a quantum mechanical tunneling current can flow between atoms at the tip and the surface. Before we move ahead it is important to understand the concept of tunneling.

 

Figure 1. Quantum Tunneling

 

From classical physics point of view for electron to travel from one state to another they must cross a barrier potential i.e. they must have sufficient energy to move across a barrier. This is similar to a person trying to carry a heavy load over a hill. In quantum physics, the same can be achieved by digging a tunnel through a hill. Hence, comparatively less amount of energy is required to move an object through a mountain or a hill. In our case starting point of the movement of electron can either be tip or sample. Here the barrier is air or vacuum i.e. conditions under which imaging of the sample is being performed.

 

Tunneling Condition

 

For electrons to move through the barrier of air or vacuum, it should satisfy energy time uncertainty relation E. t ≥ h,

 

where E is uncertainty of the energy, t is the uncertainty of time and h is the Planck constant. From the above equation one can derive it to tunneling condition i.e.

 

d2(EBarr) ≤ h2/m —–(1)

 

Where d is the width of the potential barrier, EBarr the height of the energy barrier and m is the electron mass. To further understand how tunneling condition gets satisfied let us assume tip is d distance from the substrate i.e. 10-9 m. The EBarr that is height of the energy barrier is given by work function which amounts to few eV i.e. 10-18 J. The left side of the tunneling condition has –

Figure 2. Depending on the direction of the voltage, the electrons may tunnel to the sample or in reverse direction

 

One should note that electrons may not only tunnel from tip to sample but also in reverse direction. This is demonstrated by an energy diagram in figure 2. Here it is assumed that both tip and the sample have identical work function at an applied voltage of U. The electrons of the one side of the barrier (air or vacuum) tend to have more energy than on the other. The slope of the potential is drawn with a slanting potential barrier.

 

Working Principle

 

In STM there is small metal tip that is placed at a distance of 1 nm i.e. few atomic layer above the sample. The tip is attached to a scanning device that move the in a rastering manner above the sample. This scanning device has 3 piezo x,y & z that control the mobility of the tip in the 3D. During rastering, tip scans the sample in xaxis and moves one line at a time in y- axis direction. Therefore tip scans the surface faster in x-axis and comparatively slow in y-axis direction. The z piezo controls the position of the tip above the sample.

Figure 3. (a) The principle of the Scanning Tunneling Microscope (b) Rastering motion of the tip for scanning the sample. Thicker line is fast scan axis controlled by x-piezo and thinner line is slow scan axis controlled y-piezo.

 

The basic design of the system is given in the figure 3. A small potential of UT is applied between the tip and the sample. The tunneling It (usually in picoamperes) flows through circuit. All the 3 piezos are connected to a feedloop system that controls the position of the tip over the sample in real time.

 

The magnitude of current between the sample and the tip is constantly measured. It is important to note that only electrically conductive sample can be examined with STM. To get the best resolution one should try to get a one single atom at the very end of STM tip.

 

Conduction electrons within a metal are able to move almost freely. Though, they cannot leave to the attractive force of the positively charged core. For electrons to leave the sample work must be done, which is known as work function Ø. Quantum physics allows us to estimate this movement of electron between the sample and the tip.

 

The tunneling current depends upon the distance d between the tip and the sample –

 

The tunneling current decreases exponentially with increase in distance. The constant C1 dependent on the electron densities at the tip and on the sample. The exponent contains the another constant C2 and Ø is the work of metals. If tip and the sample have different work functions, then the mean value is used here. Typical working parameter are: IT = 10-9 , UT – 100 mv , Ø = 5ev and d = 10-9 m.

Figure 4. Probing at (a) constant height and (b) constant current

 

Two Operation modes. There are two different modes to operate STM –

  1. Scanning at constant height i.e.. the tip is probing the surface in a straight line and at the same time tunneling current is recorded.
  2. Scanning in constant current i.e. tip probes the surface in way that it keeps the tunneling current constant. The change in tip height gets recorded.

The easyScan is scanning in constant current mode. For imaging the constant height mode controller should be adjusted to move slower so that it is able to follow any gradual changes due to thermal expansion effects.

 

Atomic Force Microscope (AFM)

An Atomic Force Microscope (AFM) is a highly versatile instrument, that was developed in 1986 Binning, Quate, and Gerber. It was an attempt to develop a technique that can be used for measuring non-conducting samples with atomic resolution. Earlier with STM only conducting sample could be examined but with the development of AFM polymers and other biological non-conducting specimen could be imaged with high resolution. As the name of implies this technique is dependent on the intermolecular forces (like van der Waals, electrostatic, etc) between the tip and the sample. Here AFM probe is on non-conducting stage and there is not flow of current between the tip and the specimen.

 

Operating Principle and Instrument Design

 

Various component of AFM are shown in Figure 5. The main difference in operation of AFM as compared to STM are (a) a cantilever of known force constant with silicon nitride/ silicon crystal tip is used for scanning the sample instead of a sharp metallic tip. The tip has a nominal diameter of 10-40 nm, (b) x,y,z piezos can be mounted either on the cantilever stage or sample stage where for STM piezos are preferred to be mounted to a metallic tip holder, (c) motion of the cantilever is detected by deflection of the laser from the back side of the cantilever. The movement of the reflected laser is detected by a photodiode and the net difference in signal from the 4 quadrants of photodiode is used to develop an image of the scan sample.

 

Figure 5. Optical lever Deflection Method for the detection of Cantilever bending

 

Probe. An AFM probe is a micromachined cantilever that has sharp tip at its one end, which is brought into interaction with the sample surface. Each probe has different specifications and shape. Cantilevers are either rectangular or V-shaped and provide low mechanical resistance to vertical deflection, and high resistance to lateral torsion. Cantilevers typically range from 100 to 200 μm in length (l), 10 to 40 μm in width (w), and 0.3 to 2μm in thickness (t).

 

Beam Deflection Detection. The displacement of the cantilever is detected via laser reflected off the back of the cantilever and the reflected beam of laser is collected in a photodiode. The diode is divided into four parts, as seen in Figure 5. When the laser is displaced vertically along the positions top (A1-A2) and bottom (B1-B2), there exists a bending due to topography, that can be written as

 

Vertical Deflection = (A1 + A2) – (B1+B2)

 

Whereas horizontal deflection that produces a torsion due to “friction” (lateral force) when cantilever moves across the surface can be defined as

 

Horizontal Deflection (torsion) = (A1+B1) –(A2 + B2)

 

Modes of operation

 

The 2 most common mode of operation are Contact mode and Tapping Mode. Figure 6 illustrates mechanism of action of the cantilever during the two imaging modes.

 

Figure 6. Illustration explaining imaging in contact mode and tapping mode

 

Contact Mode – In the contact mode, the tip makes soft “physical contact” with the surface of the sample. This is achieved by carefully controlling the motion of the tip as it approaches the surface. The deflection of the cantilever dx when tip comes in contact with the surface is proportional to the force acting on the tip, via Hook’s law, F=-k. x, where k is the spring constant of the cantilever.

 

There are two variants of two of contact mode imaging. Here tip either scans the sample at a constant small height above the surface or under the conditions of a constant force. In the constant height mode the height of the tip is fixed and in the constant-force mode the deflection of the cantilever is fixed. The motion of the
scanner in z- direction is recorded that give 3D profile of the surface. With contactmode AFM, even “atomic resolution” images are obtained.

 

For contact mode AFM imaging, cantilever should be soft (i.e. low force constant, less than 10 N/m) enough to be deflected by very small forces and should have high enough resonant frequency to not be vulnerable to vibrational instabilities. Silicon Nitride tips are used for contact mode. In these tips, there are four cantilevers that have different geometries, resulting in 4 different spring constants.

 

Tapping Mode (intermittent contact Mode)- The force measured by AFM can be classified into long-range forces and short-range forces. The first class dominates when sample is scanned at large distances from the surface and short-range forces can be Van der Waals force and capillary forces (due to the water layer often present in an ambient environment). When operating AFM in contact mode, the surface the shortrange
forces are very important, in particular the quantum mechanical forces (Pauli Exclusion Principle forces). In tapping mode-AFM the cantilever oscillates close to its resonance frequency.

 

An electronic feedback loop ensures that the oscillation amplitude the cantilever remains constant, such that a constant tip-sample interaction is maintained during scanning. Forces that act between the sample and the tip will not only cause a change in the oscillation amplitude, but also change in the resonant frequency and phase of the cantilever. The amplitude is used for the feedback and the vertical adjustments of the piezo-scanner are recorded as a height image. Simultaneously, the phase changes are presented in the phase image (topography).

 

The advantages of the tapping mode include elimination of large part of permanent shearing forces and causing of less damage to the sample surface, even with stiffer probes. Different components of the sample, which exhibit difference adhesive and mechanical properties, will show a phase contrast and therefore even allow a compositional analysis. For a good phase contrast, larger tip forces are of advantage, whereas minimization of this force reduces the contact area and facilitates highresolution imaging. So in applications it is necessary to choose the cantilever of appropriate force constant. For Tapping mode, force constant of cantilever is generally greater than 20-40 N/m. Silicon probes are used primarily for Tapping Mode applications.

 

Summary

 

In this module we studied about two kinds of scanning probe imaging techniques. First we studied about Scanning Tunneling Microscopy (STM), where we briefly discussed about phenomenon of Tunneling. Thereafter we studied about two imaging mode, one is constant height mode and another constant current mode.

Second technique that we studied is Atomic Force Microscopy, where we studied about basic design of the instrument along with two kinds of imaging mode, i.e. contact mode and tapping mode.

 

References :-

  1. Principles of instrumental analysis, Skoog, Douglas A., F. James Holler, and Stanley R. Crouc,. Cengage learning, Edition 2017
  2. Atomic Force Microscopy, Eaton, P. & West, P. ,Oxford University Press, 2010.
  3. Scanning probe microscopy in nanoscience and nanotechnology 2. Bhushan B, editor, Springer Science & Business Media; 2010