Introduction to STM Image Processing I

Introduction to STM Image Processing I

Introduction to STM Image Processing I

Notice I:
All, or any part of the following material may be downloaded and used for personal or educational purposes as far as

no profit is gained from its use (otherwise contact the author: DcsabaS)

there is a clear indication of the © author DcsabaS ^TM, with a link to this www site.
Notice II:
All the images presented in the following were made by using the "STM for Students"^TM developed by DÓRA, Gyula.

by DARÓCZI, Csaba Sándor
motto:

Make everything important more visible !

but hide the others!

Optical Grating ( 2500 nm × 2500 nm, covered by Gold ) Optical gratings are used in equipments that utilize the wave nature of light, therefore the grating constant must be comparable to the wavelength. The fine structure of the grating is also important, because it influences the efficiency. Optical microscopes can not resolve this fine structure, but STM. However, we have to have a conducting surface therefore enters the gold as covering layer.

Light Shading On the previous image the bright domains correspond to higher areas on the sample than the dark ones. This could help us a lot in discovering different features, however, in reality it is NOT a strict rule, that "the brighter is higher". Therefore we should introduce an artificial (software) light shading, that makes the image like for example the view of a coin kept in our hands.

Line Cut 2-dimensional representation of a 2-dimensional data block is rather obvious. Even though, the less sometimes is more. With a line cut we can make more precise comparision in the height profile of the different spots of the sample.

Line Cut II If we compare this line cut to the previous one, we can discover that not only the contrast is reduced, but somehow the shapes of the two profiles are also different. 1.) The previous line cut seems to be more regular 2.) but it is not really sinusoidal. 3.) The second line cut is more noisy.

Optical Grating II ( 2500 nm × 2500 nm, covered by Gold ) The initial quality of the STM images (as we receive them from the data acquisition system) might be different. For example, this image of the optical grating (fabricated at MFA (former ATKI)) is much better than the first one. We could think that there is no need for additional image processing in cases like this...

Light Shading II However, as we have seen earlier, light shading is well applicable for more realistic representation of the acquired image. In this case the grating looks like it were 16000 times larger discarding the missing color of the covering gold layer. Color can be attributed to the image, as we will see it later.

Line Cut III Before this line cut was taken we have applied a slight smooth filtering to reduce the high-spatial-frequency noise. The line cut shows roughly sinusoidal profile, and the upper and the lower half of the line is NOT so asymmetrical than in the first case. (How can become the image profile of a sample assymetrical if the sample surface profile is symmetrical?)

Line Cut IV Another line cut (in orthogonal direction) reveals that there is an irregular fluctuation of the sample surface, which is related to the grains of the sputtered Gold layer. If we compare the interval of fluctuations (roughly 4 nm) to the previous line cut, we can see that most of the deviation from ideal sinusoidal shape might be due to this origin.

Damaged Regions Even the cleanest sample surface may contain damaged or contaminated regions. It is not always easy to determine the source of a given feature. "Strange" regions of an image occasionally might be of importance.

Median Filtering Isolated bright or dark spikes are often just consequences of too high bias voltage applied between the sample and the scanning tip. These features can be eliminated from the image by median filtering, that replaces the locally extremal points with a kind of average of the surroundings. (Compare the rectangular area to that of the previous image!)

zOOM Sometimes it is useful to cut out some part of a given image and enlarge it to the size of the original one. This kind of magnification may represent a better view, however, it doesn't "enlarge" the information contained in the original image! In other words, this method is NOT able to improve the resolution.

Step by Step ( 1000 nm × 1000 nm, covered by Gold ) The quality of a newly acquired STM image can be rushed down by several factors. It maybe possible to repaire some of them, but the order in which the processing tools should be applied is usually important. The point is at each step to eliminate a given problem without influencing other properties of the image.

Side Effect It may happen, that the fine positioning system had not reached its equilibrium state when the data equisition started. This results a side effect in the image. Of course, the best is to repeat the measuring. If we can not (or don't want to) do that, we can make a sub-image that doesn't contain the effected region - in our case the dark upper edge of the previous image. The contrast of the resulting image is better.

Tilted Image After we have successfully removed the side effect, all the other problems can be seen more enhanced. Usually the next useful thing is the subtraction of the inclination plane of the image, because this inclination is influenced by the macroscopic position of the sample, which is rarely important in microscopic investigations. After having tilted back the image we gain an additional contrast enhancement.

Median II We discovered some little bright spikes on the previous image. Now they have been removed by the help of median filtering. With this method not only "point deffects" can be repaired, but erroneous lines as well...

Comparision Here You can see a new original (as-acquired) image, and the effect of smooth OR median filtering on it, respectively.

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