Reporton

Use of STM in Microscopy and Manipulation

INTRODUCTION

Not long ago it caused a great debate whether we could “see” let alone “manipulate” an atom. However, this debate was resolved by the invention of the STM (Scanning Tunneling Microscope) by G. Binnig and W. Rohrer at the IBM Research Laboratory in 1982, as it was used in viewing surfaces at atomic level. That’s why it was honoured by the Noble Prize in 1986.

The operation of STM is based on the quantum mechanical phenomenon “Tunneling”. It basically consists of a “tip” which when becomes within 1 nm (10 -9 m) from material surface “sample” voltage is applied between them the electrons can tunnel between the tip and the sample.

It was later noticed that the tip can cause changes in the material surface due to interaction between the tip and the sample. The scientists researched these changes in hope to be able to manipulate the matter at the atomic level. When they succeeded it was the start point of “Nanotechnology”, now the STM could be used as a “Nano manipulator” and used to build Nano materials atom by atom, more research brought up the idea of Nano assemblers (very small manipulating tools) and Nano robots which made “Nanotechnology” one of the most interesting and promising research fields.

As we said, it all started by the introduction of the STM. In this report we will study the STM going through three sections:

A brief account on STM and its working principle.

The use of STM in Microscopy.

The use of STM in Manipulation.

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1.0 HOW STM WORKS:

STM is one of the SPM (Scanning Probe Microscope) family which uses probes to see atoms and molecules at surfaces (It also includes AFM (Atomic Force Microscope), SNOM (Scanning Near-field Optical Microscope), and MFM (Magnetic Force Microscope) ). Each member of this family has its working principle, as for STM it is based on the concept of “quantum tunneling”. STM probes the density of states of a material using tunneling current.

Quantum Tunneling:

Consider a 1-D vacuum barrier between two electrodes (the tip and the sampler). Assume the both have the same work function Ф (work function is the minimum energy (usually measured in electron volts) needed to remove an electron from a solid to a point immediately outside the solid surface).

Assume Ψ is the wave function, Ef is Fermi level , E is the energy of the electron, I is the tunneling current, ρ is the density of states function, and M is thetunneling matrix element.

Figure 1. A one dimensional barrier between two metal electrodes. A bias voltage of V is applied between the electrodes.

If Ф < E → k is imaginary → Ψ propagates

If Ф > E → k is real → Ψ is attenuated (look Figure 1)

The probability of finding an electron behind the barrier of the width d is

I is proportional to the probability of finding an electron

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I=const . e−2√2mФ

ђd (*)

A more detailed expression of I was derived by Bardeen

 Note: since

(***)

2.0 USE OF STM IN MICROSCOPY:

From (*) it is obvious that the tunneling current (I) is related to distance between tip and sample (d), that means that we can deduce d from I (measured in nA).

Figure 2. relation between tunneling current (I) and distance (d)

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2.1 Components of STM :

The electronic circuit is shown in Figure 3

Figure 3. Schematic view of an STM

Probe:

It is very important that the tip is sharp for good accuracy(Look Figure 4); Tips typically are made out of tungsten, platinum or a Pt-Ir wire.A sharp tip can be produced by: 

Cutting and grinding

Electrochemical etching

Amplifier:

It amplifies the tunneling current.

Feedback (distance control) unit:

To prevent that the surface to crash into the tip, the distance between the tip and surface is kept constant during the measurement, The feedback system determines continuously the height of the needle by measuring the current between tip and sample. When this current is to large or to small the distance has to be adjusted. The feedback system readjust the distance in such a way that the current gets the right value by moving the tip up or down(Look Figure 5).

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Figure 4. Sharp and blunt tips

Figure 5. Feedback system

Piezoelectric material:

Piezo-material has the property to expand or shrink a little bit when a voltage is applied to it. For example the application of 5 V to a 5 mm cube of piezo-material gives an expansion of about 0.5 nm, it is controlled by feedback unit and the tip is attached to it.

2.2 Modes of operation:

2.2.1 Constant Current Mode

By using the feedback loop the tip is vertically adjusted in such a way that the current always stays constant. As the current is proportional to the distance to tip, the tip follows a contour parallel to the surface during scanning. The image of the surface is generated by recording the vertical position of the tip (Look in Figure 6).

2.2.2 Constant Height Mode

The feedback loop isn’t used, the tip is at constant height, The image of the surface is generated by recording the tunneling current. It is only suitable for smooth surfaces (to avoid tip crash).

2.2.3 Scanning Tunneling Spectroscopy

From differentiation of (***) we can dedicate that

The density of states can be deduced by changing bias voltage (V) and calculating tunneling current (I).

2.2.4 Local Work Function Measurement

If there is an inhomogeneous compound in the surface the work function will be inhomogeneous as well. This alters the local barrier height. By using the constant height or constant current modes described above we would get a virtual hole, so we need to measure Ф . From (*) we can dedicate that

Thus the work function can directly be measured by varying the tip-sample distance.

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Constant Current Mode

Constant Height Mode

Figure 6. Constant Height Mode Vs. Constant Current Mode

Figure 7. Examples of surfaces scanned by STM (left: Silicon carbide , T=300k, middle: Au(111), right: Bucky ball C60 at 5K)

3.0 USE OF STM IN MANIPULATION:

As discussed above, STM is very powerful in studying atomic structure and electronic properties of surfaces (imaging mode). In these studies, the tip-sample interaction is usually kept as small as possible so that the investigations don’t cause undesired changes. However, if one adjusts the parameters to increase the tip-sample interaction in a controlled way, STM can also be used to fabricate Nano-structures down to the atomic level. Various Nano -structures can be constructed by different methods, including manipulation of single atoms, scratching, oxidation, tip-induced chemical reactions, and heating, etc.

3.1 Manipulation of Single Atoms:

The first example of Manipulation was demonstrated by Eigler and Schweizer in 1990 by writing the “IBM” company logo with Xe atoms on a Ni(110) surface (the smallest logo) in the world in 1990 and pioneered the new field of manipulation of single atoms .

Manipulation of single atoms and molecules with a STM tip generally requires atomically clean surfaces and an extreme stability at the tip-sample junction. Therefore, ultra-high-vacuum (UHV) environments and low substrate temperatures are favored in most manipulation experiments. Figure 9 shows some of possible interactions between molecule/atom and STM tip.

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Figure 8. the smallest logo in the world

Figure 9. Interactions between molecule/atom and STM tip

3.1.1 Lateral Manipulation:

A typical Lateral Manipulation procedure involves three steps: 1) vertically approaching the tip towards the manipulated atom to increase the tip-atom interaction, 2) scanning the tip parallel to the surface where the atom moves under the influence of the tip, and 3) retracting the tip back to the normal image-height thereby the atom is left at the final location on the surface(Look Figure 10).

Three basic Lateral Manipulation modes, “pushing”, “pulling” and “sliding”, has been distinguished. In the “pulling” mode, the atom follows the tip due to an attractive tip-atom interaction. In the “pushing” mode, a repulsive tip-atom interaction drives the atom to move in front of the tip. In the “sliding” mode, the atom is virtually bound to or trapped under the tip and it moves smoothly across the surface together with the tip.

The tip is initially located directly above the atom and hence, only a vertical force component (F┴) exists. The tip-atom distance is carefully chosen so that the attractive interaction between them is not strong enough to overcome the desorption barrier of the atom. Since the lateral force component (F║) is negligible at this point the atom will neither transfer to the tip nor move to the next adsorption site (ad-site) on the surface. When the tip moves parallel to the surface it passes over the manipulated atom, thereby tracing a part of the atomic contour. This action increases F║ and decreases F┴ (Figure 11). When F║ overcomes the hopping barrier of the atom, the atom hops to the next adsite under the tip. This action alerts the STM feedback system to retract the tip in order to maintain the current constant causing an abrupt increase in the tip-height.

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Figure 10. Lateral Manipulation

Figure 11. (a) The drawing demonstrates the vertical and parallel force components involved in LM. (b) STM tip-height manipulation curvescorrespond to (1) pulling, (2) pushing, and (3) sliding modes.

3.1.2 Vertical Manipulation:

This process involves the transfer of single atoms or molecules between the tip and substrate and vice versa. The atom/molecule transfer process can be realized by using an electric field between the tip and sample, by exciting with inelastic tunneling electrons, or by making mechanical contact between the tip and the atom/molecule. Transfer mechanism can be explained by using a model of a double potential well.

Figure 12. Vertical manipulation (VM). (a) A schematic drawing shows the process. (b) The double-potential well model. The black (solid),dash and gray curves represent the shape of potentials (the well shapes are working assumptions) at an image-height, under an electric field, and at the tip-atom/molecule contact, respectively.

00At an imaging distance, the atom/molecule has two possible stable positions, one at the surface and one at the tip apex.Each position is represented by a potential well, and the two potential wells are more or less equal in shape, separated by a barrier.If the tip is stopped above the atom at the same distance and an electric field is

applied, then the shape of the double potential well changes.The barrier between the two wells reduces, and the potential well at the tip apex has a much lower energy level.The atom now can easily transfer to the tip.By applying a reverse polarity bias, the minimum potential well can be changed to the surface side.The atom can then transfer back to the surface.3.2 Induced Chemical Reaction at Tip:

The finely focused electron beam from STM tip can also be used to induce local chemical reaction, which provides another method to fabricate various pre-designed Nano-structures on the surface.

Example: STM tip-induced synthesis steps of a biphenyl molecule (Look Figure 14).

8Figure 13. Example on Manipulation (quantum corrals)

Figure 14. Schematic illustration of the STM tip-induced synthesis steps of a biphenyl molecule (Ullmann reaction). (a),

(b) Electron-induced selective abstraction of iodine from iodobenzene. (c) Removal of the iodine atom to a terrace site by lateral manipulation. (d) Bringing together two phenyls by lateral manipulation. (e)

Electroninduced chemical association of the phenyl couple to biphenyl. (f ) Pulling the synthesizedmolecule by its front end with the STM tip to confirm the association.

CONCLUSION

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The STM in spite of its conceptually simple operation principle has been proved to be an extremely versatile and powerful technique for many disciplines in condensed matter physics, chemistry, material science, and biology. In addition, STM can be used as a Nano-tool for Nano-scale fabrication, manipulation of individual atoms and molecules, and for building nanometer scale devices one atom/molecule at a time.

REFRENCES

1. http://www.spmlab.science.ru.nl/ 2.http://www.fkf.mpg.de/ga/research/stmtutor/stmconc.html 3.http://www.omicron-instruments.com/ 4.Nan Yao, Zhong LinWang ,Handbook Of Microscopy For Nanotechnology5.Saw-Wai Hla, STM Single Atom/Molecule Manipulation and Its Application to Nanoscience

and Technology6.E. Zupanič, R. Žitko, H.J.P. van Midden, A. Prodan, I. Muševič, Single atom manipulation and

spectroscopy using low – temperature STM

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