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Submitted on 1 Jan 1987
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ASYMMETRIC ATOMIC IMAGES OF STM RESULTING FROM PROBING TIPS
W. Mizutani, H. Tokumoto, H. Bando, M. Shigeno, M. Tanaka, M. Ono, K.
Kajimura
To cite this version:
W. Mizutani, H. Tokumoto, H. Bando, M. Shigeno, M. Tanaka, et al.. ASYMMETRIC ATOMIC
IMAGES OF STM RESULTING FROM PROBING TIPS. Journal de Physique Colloques, 1987, 48
(C6), pp.C6-73-C6-78. �10.1051/jphyscol:1987612�. �jpa-00226815�
ASYMMETRIC ATOMIC IMAGES OF STM RESULTING F R O M PROBING TIPS
W. Mizutani, H. Tokumoto, H. Bando, M. ~hi~eno*, M. ~anaka*: M. Ono and K. Kajirnura
Electrotechnical Laboratory, Sakuramura, Ibaraki 305, Japan
Abstract.- We have constructed a very stable STM operable in air, and concentrate our study on one of the remaining problems of STM
-
the probing tips. In this paper we present an example ofdistorted atomic images of graphite and an experimental technique to extract the information of the probing tip from the images. I t became clear that the images are affected by the surface structure of the tip. The distortion originated from the sample is also mentioned.
1 Introduction
The scanning tunneling microscope (STM) has been demonstrated to be an excellent tool for surface analysis with atomic
resolution.Cl1 But strictly speaking, STM images reflect both of the tip and sample surface structure.C2-41
The tip is controlled so that the total tunneling current flowing between whole area of the tip and sample surface is kept at constant value in any position on the surface. By a usual data processing technique, we can not separate the influence of the tip and sample surface structure using only one STM image. We need more information such as the image obtained from the same position of the sample scanned by the different direction of the same tip.
To achieve this, we need a very reliable STM for which we can exchange tips and/or rotate a sample easily.
@ 2 Instrumentation
We constructed an STM which gives stable and reliable operation in air.(Fig. 1) For the easy replacement, the probing tip is screwed into the center of the X-Y PZT actuator. The
"cross bar" type X-Y PZT is designed symmetrically to compensate the thermal drift. The tip is scanned in X or Y direction by push-pull motion of the two actuators. Z-piezo is located just behind the X-Y actuator on the other side of the tip. This
*permanent address:Seiko Instruments.Inc., Matsudo, Chiba 271, Japan.
**
Permanent address:Nippon Steel Corporation, R & D - 1,
Nakahara-ku, Kawasaki 211, Japan.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987612
JOURNAL DE PHYSIQUE
Fig.
s c a n n to wh
1. Schemat i n g tunnel ich this p
ic v i e w of the ing microscope
~ a p e r pertains.
L x
0.1 nm/d iv.
Pig. 2. "Normal" STM image of a g r a p h i t e s u r f a c e ; taken at a tunneling v o l t a g e o f 70mV and a current o f 2 n A .
0.0 4
nm/l ine
stable. Moreover, we are able to record atomic images of graphite in a VTR by taking one frame in less than 0.5sec.
The probing tips are made of tungsten or platinum, by mechanical grinding or by chemical etching.
Samples are mounted on a sample holder which has an X-Y rough positioning mechanism. The sample holder is attached to the main body of the STH. The holder can be rotated around its axis of symmetry at any angle, so as to adjust the tip-scanning direction relative to the crystal orientation of the sample. The coarse approach of the tip is achieved by a differential micrometer head.
We can observe the tip-to-sample distance through an optical microscope while we handle the micrometer, or adjust the tip position by the X-Y rough positioner.
As a standard specimen, we use HOPG of Le Carbone and Union Carbid,e. To confirm the
performance of this system including the electronics and the vibration isolation, we present here
"normal" atomic image of graphite. (Fig. 2 ) There are three kinds of electronic states in the graphi te surface. The three carbon atoms among six forming a
honeycomb lattice are located at the
highest parts (A).
The other three atoms are in the middle high si tes(B), because of the
interaction with the atoms belonging to the second layer.
The lowest parts ( C ) are corresponding to the center hole of the hexagon. C51
Since the system and the sample are well identified, we shall proceed to study on the effect of the probing tips on the STM images.
Fig. 3. One example of
"asymmetric" STM image of graphite atoms;tip bias -150 mV,current 10 nA.
JOURNAL DE PHYSIQUE
@ 3 Experimental Result
As we mentioned in @ 2 , we can rotate the sample around the tip axis while we separate the tip from the sample in the
macroscopic range, and reset the tip after the rotation. The deviation of the tip position after such manipulation is caused by
the engagement between the sample holder and the main body or by the eccentricity of the tip apex. Judging from the mechanical alignment, we estimated the deviation to be less than 2 0 ,U m.
Since the grain size of the HOPG is reported to be 100 to 2 0 0 fi m, we can observe the same grain of the crystal after the rotation.
We obtained series of STM images for different sample rotation angle and confirmed that the images were rotated with the sample.
In the following discussion, we suppose that the surface structure should be similar in the same grain of the crystal.
The atomic images of graphite sometimes become asymmetric as shown in Fig. 3. We at first thought these images appeared in a certain current-bias condition, but the features were not
reproduced in the same tunneling conditions with different tips.
There are two situations which generate the asymmetric STM images as illustrated in Fig. 4. The asymmetric tip (a) and the asymmetric structure of the sample (b) generate the same STM images represented by dashed lines. We can distinguish these two situations by the sample rotation experiment as follows. If the sample is symmetric and the tip is asymmetric as shown in Fig. 4
(a), the image is unchanged after rotating the sample. On the other hand, if the envelope of the electronic wave function at the sample surface is asymmetric, the asymmetric images should be rotated with the sample rotation. (Fig. 4 (b),(c))
Fig. 4. Model illustrating the origin of asymmetry.
If the tip shape is asymmetric (a), the images has the same asymmetry after rotating the sample
.
If the sample surface is asymmetric (b), the image (dashed 1 ine) is reversed as the sample is reversed (c).manner
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right hand side of the mounds are steeper than the left.Therefore the asymmetry is most likely to be originated from the asymmetric tips.
The tip collision with the sample was carefully avoided throughout the experiment, because the collision might cause the change in shape of the tip in atomic scale.
4 Discussion
The asymmetric atomic images of STM are considered to be caused by the sample structure, the servocontrol system, or some
Fig. 5. The asymmetric STM images taken before (a) and after (b) the sample rotation by 180"
.
Both wave form images shown above have steeper right shoulders indicating that these asymmetry is caused by the tip. (Tunneling condition;50 m V , 1 nh.)JOURNAL D E PHYSIQUE
other effect concerning atomic forces, besides the asymmetric surface structure of the tip.
There remains a possibility of asymmetry of sample surface.
As shown in Fig. 2, the atoms belonging to the second layer can affect the surface electronic structure to make the three atoms of hexagon appear differently from the other three atoms. In the similar manner, the lateral displacement of the second layer might cause the asymmetric images like Fig. 2. In the two dimensional plots of Fig. 5 , we recognize another asymmetry in the location of the deep holes in honeycombs. The holes are at the upper right side in the honeycombs before rotation (a) and at the lower left side after rotation (b), i.e. they are rotated with the sample.
This fact suggests that the eccentricity really exists in this particular sample. This may have a relation with some randomness appeared in X-ray diffraction pattern in HOPG, but we cannot explain this effect any more from the images only.
With regard to the servocontrol system, a PZT actuator has its own non-linearity and this may cause unpredictable responses.
To confirm that the tip traces the surface of the sample properly, we displayed "two way" image, one was taken in a forward scan and the other in a backward scan. The asymmetric wave form is
reversed at the turning point, so the tip is controlled spatially in the same way in both scanning direction.
The strongly asymmetric signals include higher harmonics than the normal sinusoidal waves. So the servo system is hard to respond to the asymmetric signal scanned as fast as for normal images and tends to cause the overshooting or ringing. Under this condition the tip is apt to collide with the sample surface and lose the original shape. This is one of the reason that the highly asymmetric images are not so often observed, so we had to observe fairly extensive samples with different tips. We watch for the wave forms and adjust the scanning speed to the signal condition for the servocontrol system to work accurately and not to distort images.
§ 5 Conclusion
We are using a very reliable STM and observing various samples, among which the standard sample like HOPG is of great importance because of checking tips and calibrating atomic scale.
We confirmed that the distorted images as shown in Fig. 3 are caused by an asymmetric tip. This indicates that we should always check the tip by standard samples when we observe unknown samples.
§ 6 Acknowledgment
The authors wish to acknowledge the valuable comments of M-Okano, Y-Kobayashi, K-Watanabe and H.Nakajima throughout the construction of the STM.
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