Contents 1 Introduction 3 2 What is STM? 3 3 Scanning with 'easyscan' 4 4 Experiments Tip Preparation and Installation

Transcription

1 'easyscan' SCANNING TUNNELING MICROSCOPE Baris Cetin Department of Physics Purdue University, West Lafayette, In Abstract A summary of the fundemental principals in using a 'easyscan' STM "Scanning Tunneling Microcsope" will be presented.

2 Contents 1 Introduction 3 2 What is STM? 3 3 Scanning with 'easyscan' 4 4 Experiments Tip Preparation and Installation Preparing the Sample Installing the Sample Approaching the Tip Measurements Results and Observations 12 2

3 1 Introduction In this report, 'easyscan' STM will be discussed in detail. In the rst part a brief description of STM's is given. Then in the second part, our STM 'easyscan' will be described with details of software using and preparing the STM for data taken like sample preparing, sample cleaning, tip cutting etc. (*) All explanations about usage taken from the manuals of 'easyscan' STM. 2 What is STM? The scanning tunneling microscope ( STM ) was invented by Binnig and Rohrer ( 1982, 1987 ) and implemented by Binnig, Rohrer, Gerber, and Weibel ( 1982a, 1982b ). Fig.1 shows its essential elements. A probe tip, usually made of W or Pt-Ir alloy, is attached to a piezodrive, which consists of three mutually perpendicular piezoelectric transducers: x piezo, y piezo and z piezo. Upon applying a voltage, piezoelectric tranducer expands or contracts. By applying a sawtooth voltage on the x piezo and a voltage ramp on the y piezo, the tip scans on the xy plane. Using the coarse positioner and the z piezo, the and the sample are brought to within a few angstroms of each other. The electron wavefunctions in the tip overlap electron wavefunctions in the sample surface. A bias voltage, applied between the tip and the sample, causes am electric current to ow. Such a current is a quantum-mechanical phenomenon, tunneling, an electron has nonzero probability of tunneling through a potential barrier, in other words electron has nonzero probability ofbeing in the classically forbidden region. The tunneling current is amplied by the current amplier to become a voltage, which is compared with a reference value. The dierence is then amplied again to drive the z piezo. The phase of the ampliers is chosen to provide negative feedback: If the tunneling current is larger then the reference value, then the voltage applied to the z piezo tends to withdraw the tip from the sample surface and vice versa. Therefore, an equilibrium z position is established through the feedback loop. As the tip scans over the xy plane, a two-dimensional array of equilibrium z positions, representing a contour plot of the equal tunneling-current surface, is obtained and stored. This contour plot is then displayed on a computer screen. To achieve the atomic resolution, vibration isolation is essential. There are two ways to achieve a suitable solution. The rst is to make the STM unit as rigid as possible. The second is to reduce the transmission of environmental vibration to the STM unit. A commonly used vibration isolation system consists of a set of suspension springs and a damping mechanism. The STM experiments 3

4 Figure 1: Schematic diagram of the scanning tunneling microcope from Ref.1. can be performed in a variety ofambiences: in air, in inert gas, in ultrahigh vacuum, or in liquids, including insulating and cryogenic liquids, and even electrolytes. the operating temperature ranges from near absolute zero ( o C ) to a few hundred degrees centigrade [1]. 3 Scanning with 'easyscan' With the easyscan STM platinum tip is clamped between two tiny springs and a platform which can be moved in all three dimensions as shown in Fig.2. All three axes are driven very precisely in the nanometer range, by piezocrystals. The sample which is to be examined is brought close to the tip ( or approached ) to a distance of about 1 nanometer. Classical physics would prohibit the appearance of electrons in the small gap between a tip and a sample. But if a sharp tip and a surface are put under a low voltage ( U 0.1V)avery small current ( I 1nA)mayowbetween the tip and the sample: tunneling current is a quantum physics eect and the tunneling current depends exponentially on the distance between the and the sample. since 4

5 Figure 2: General view of easyscan this tunneling current is extremely on the distance between the and the sample, the movement can be accurately controlled. the tip is scanned over the sample. By keeping the current between tip and sample constant by feedback loop ( constant current mode ) the distance between tip and is surface is also kept constant and the tip follows the structure of the sample's surface as shown in Fig.3. The movements of the tip are recorded during scanning and the landscape of the atomic surface can be simultaneously drwan on the computer screen line by line. The sample can be scanned in a secon mode: turning the feedback loopoorvery slow ( P-Gain = 0, I-Gain = 2 ) the tip scans at a xed distance from the sample ( constant height mode). This time the variations in the tunneling current are measured and drawn line by line on th computer screen. I- With easyscan it is possible to do any STM experiment which can be carried out in the air. II- All functions can be carried out by computer. III- The instrument is designed to be compact, simple and comfortable to operate. 5

6 Figure 3: Tip probing the surface, top view. 4 Experiments 4.1 Tip Preparation and Installation The scanning tip is prepared and installed by the user himself. Cutting and installing should be carried out with great care as a good result relies heavily on the accuracy of that process given: 1. First thing needs to be done is to ensure that the cuting part of the wire cutters, the atnosed pliers and the tweezers heve been cleaned with ethanol. It is important tonever touch the platin wire without these tools to prevent it possible contamination. 2. Hold the end of the wire rmly with pliers and cut a piece o approximately 5 mm long. 3. Still holding this piece of wire with the pliers, place the cutters at the free end, as obiquely as possible. 4. Close the cutters conveniently then as shown in Fig.4, pull and cut at the same time. The tip needs to be torn o rather tahn cleanly cut o in order to get the required point. 5. Hold the wire with the tweezers behind the freshly cut tip. 6

7 Figure 4: Tip cutting procedure 6. Insert it carefully under the golden tip holders in the scan head without twistin them as shown in Fig.5. The freshly cut tip should be well held under the clamps and reach about 2-3 mm beyond the tip holder. Important Note: delicate and not to be twisted. Golden tip holders are in the open part of the scan head and they are very 4.2 Preparing the Sample The STM can only examine electrically conductive materials. Nevertheless the choice of material is rather small because the surface of the sample must be totally clean and mirror-like to obtain useful results. Because of that some of the samples need special preparation. Gold Thin Film: Cleaning the sample is neither posible nor necessary so it is very essential to never touch the sample with the ngers or put it upside own anywhere, this will only make it unusable faster. Graphite: This is the sample we used in the experiment. The surface of the graphite should be 7

8 Figure 5: Tip mounting procedure cleaned every few months. due to the layered structure of graphite this can easily be done by using a piece of adhesive tape: - Put the sample on the table using the pair of tweezers. - Stick a piece of adhesive tape gently to the graphite and then pull it o again: The topmost layer of the sample should stick to the tape. - Remove any loose akes with the pair of tweezers. 4.3 Installing the Sample Put the prepared sample onto the magnetic end of the sample holder using a pair of tweezers. then place the sample holder carefully in the scan head so that it doesn't touch the scanning tip. It is very important toavoid strong mechanical impact on the piezo motor while palcing the sample holder in the scan head. 8

9 4.4 Approaching the Tip To start measuring, the sample must be very close to the tip to enable a tunneling current toow. Approaching the sample without touching the tip, is a delicate operation carried out in three steps. The LED on the scan head telss about the distance between the sample and the tip: LED orange: Distance is too big, no tunneling curent isowing. LED red: Sample touched or crashed into the tip, tunneling current is too high. LED green: Sample is in the measuring area, tunneling current isowing. The approach of the sample to the tip must be done in steps: 1- Coarse approach by hand until distance betwwen them is about 1 mm. 2- Then put the transparent cover onto it. This cover reduces the air ow around the scanner to avoid severe thermal drift in measurements at atomic scale. 3- Fine approach by piezometer: Watch the distance between the tip and the sample with help of magnifying glass while moving the sample towards the tip to a distance of a fraction of milimeter by computer using the approaching panel. 4- Ensure that the folowing parameters are set correctly on the feedback control panel: - the 'setpoint' ( tunneling curent ) on approx na, - the 'gap voltage' ( tip-sample-voltage) on 0.05 V, - the 'P-gain' on 12 and the 'I-gain' on 13 ( feedback loop parameters ). 5- Use the automic approach function on the approach panel to nish the approach. With the help of piezo motor the sample holder is moved towards the scanning tip until the tunneling current dened by 'setpoint' is detected. Now the distance between the sample and the tip is controlled automatically by the electronucs. if the approach is successfully done the LED on the san head cahnges from orange to green. 4.5 Measurements The ideal scan range for the tips lies in the xy plane of the piezo scanner. But mostly the sample is tilted with respect to that ideal plane. Since sample' tilt cannot be compensated for directly, the scan coordinates haveto be adjusted accordingly. By setting the suitable values for X-slope and 9

10 Figure 6: Sample orientation before tilt adjustment Y-slope the scanner's coordinate system is tilted so that the sample's surface appears to lie in the ideal xy plane. PARAMETER FIELD VALUES: Z-Range: xes the displayed range in z-direction. For example, to be able to observe atomic features on a surface the signal in z-direction has to be amplied. This is achieved by diminishing the Z-Range. ScanRange: xes the scan sizein x and y direction [nm] whre ( x = y ). Time/Line: sets the time taken to acquire a data line. By using the next three parameters, the plane on which the tip is scanned ( scan-plane ) and the surface of the sample are aligned as shown in Fig.6. Z-Oset: raises the scan-plane in z direction [nm]. X-Slope: tilts the x-axis of the scan-plane counterclockwise. Y-Slope: tilts the y-axis of the scan-plane counterclockwise ( when viewd at 90o rotaion ). 10

11 Figure 7: Image of graphite surface taken by our STM. This image was obtained by Jon Cawley and Siarhei Spirydovich. With the help of these alignments the performance of the feedback circuit can be optimized so that pnly deviations from this inclined scan-plane have to be compensated. Rotation: rotates the scanned area clockwise by the given angle. Samples: sets the number of measured datpoints per line. By changing the X-/Y-Osets the scanned area can be shifted. The values are realtive to the centre of the entire scan range: X-Oset: sets the displacement of the measured area in x-direction [nm]. Y-Oset; sets the displacement of the measured area in y-direction [nm]. note The value of Z-Oset varies slightly during measurement. this os correct because in the menu 'Options' the option ' Auto. Adjust Z-Oset ' should be active. 11

12 5 Results and Observations In our trial we failed in producing the image of the graphite surface, and since we did not have enough time we had to stop our trials like cuting new tips, cleaning the surface of the graphite etc. On the other hand previous group manage to obtain graphite data conveniently. They got the image given in Fig.7 by using the same STM as our group used. As it can be seen from the gure that the distance between the dark points or bright points is of the order 0.25 nm which is right order for the atoms of graphite. But further investigation is required since we have tobevery careful when interpreting the position of the atoms from the black, grey and white image. In this image bright spots show high points and dark spots show low ones. References [1] C. Julian Chen. Introduction to Scanning Tunneling Microscopy. Oxford University Press,