The Modern Student laboratory: Scanning Tunneling Microscopy Scanning Tunneling Microscopy of Silicon and Carbon Robert D. Braun University of Southwestern Louisiana, Lafayette, LA 70504-4370 Scanning tunneling microscopes (STM's) have recently become commercially available at a price that is within the grasp of many chemistry departments. They are less expensive than nuclear magnetic resonance spectrometers. Since the images of surfaces that are generated by STM's can have high (atomic) resolution, and since the instruments are relatively inexpensive, it is likely that instructional use of the instruments in undergraduate and graduate courses will increase. Essentially, scanning tunneling microscopy obtains an image by moving a sharp tip above the examined surface while the current flowing between the tip and the surface is monitored. Since the tip is not actually in contact with the surface during the scan, the current that is monitored is the tunneling current. Since a description of scanning tunneling microscopy has not previously been included in this Journal, I will describe some of the advantages and uses of the method below. Then I will describe a successful student laboratory experiment that uses STM. Advantages of using the STM Scanning tunneling microscopy has several advantages when compared to other surface analytical techniques. Under ideal circumstances, the technique can be used to visualize surface atoms. Although the method can be used to study samples in a vacuum, a major advantage is that it also can be used to study samples in air and in contact with a liquid. Surfaces in contact with either water or aqueous solutions can also be studied. Most high-resolution surface analytical techniques (e.g., scanning electron microscopy) require that the studies be performed in a vacuum.
Uses of the STM Scanning tunneling microscopy has been used for a variety of purposes, some of which are mentioned below. Other uses of the technique can be found by consulting the references. A primary use of the STM has been to study the surfaces and defects in the surfaces of semiconductorssuch as germanium, titanium dioxide, and zinc oxide ( 1 3 ) . Metals, chemical reaction products on metals, and catalysts have also heen studied (4-7). Most metals are chemically active and react to form oxide coatings that prevent current from flowing between the surface and the STM tip. Consequently, metallic surfaces generally must be cleaned or polished before being examined. Insulators cannot be directly studied, although it is often possible to study them indirectly by deposition onto a conductive surface. As a n examole.. the image of DNA has hcen observed after deposit~onon highly ordered ~vrolvtlca w h i t e (HOI'GJ 18,. In mv laborntorv. ".the STM is be& uiedto study corrosion inhktors on metallic sur-
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faces. Similarly, plymers and adsorbates have been studied using scanning tunneling microscopy (9.10). Scanning tunneliw microsco~vhas been used in several l is becom~ngcommonto types of e l & o c h c m ~ astudiek-1t use the technique to study surfaces telcetndes~while the potentials of the surfac& are controlled (11-15). The method has been used to observe electrochemical depositions and stripping (16-34). It also has been used to perform microscale fabrications (35). It can be used for nanolithography. The technique is ideally suited for studying graphite electrodes (36,371and the reactions that occur on them. Since the tip used in the STM must be highly pointed, a great deal of effort has gone into preparing tips with radii in the micrometer range (3840). Readers interested in the phenomenon of electron tunneling can refer to the renew bv Mikkelsen and Ratner (41). Renews of the theorv and applications of scanning tunneling microscopy (42-51) can be consulted for other information. Class Experiment using Carbon and Silicon The experiments that I describe here use a n STM to examine the surfaces of silicon and carbon in air. The experiments have been class-tested with students in an undergraduate instrumental analysis course and with students enrolled in undergraduate research courses. For the experiments reported here the instrument was used in the constant-current mode. In other words, as the tip is scanned across the surface, a feedback network is used to change the height of the tip to maintain the current at a constant value. Since the current usually varies exponentially with the distance between the tip and the surface, the gap between the surface and the tip remains constant during the scan. Thus. anv chanee in the vertical position of the tip reflects a variaiion in 'ihe surface under the tip. The displav is a cornouter dot of tio . height .ion the z axi;) as a functik of over the surface (on the r and y axes). Both carbon and silicon samples were examined by the students during a single laboratory period. Most of the students' time in the laboratory was spent learning the instrumental controls and learning to position the tip above the surface. Thus, it only takes slightly longer to examine two surfaces. Experimental Apparatus ANanoScope I1 STM (Digital Instruments) was used for the experiment with Nanotip (Digital Instruments) PtIr tips (0.010 x 0.25 in.) and the 0.6-pm scan head. The silicon chips that were studied were donated by Dr. Mohammad
The Operating Parameters Used To Obtain Images of Silicon and Carbon Surfaces
Element
Si
C
Zrange, nm Scan size, V Number of samples Scan rate, Hz Setpoint I, nA Integral gain Proportional gain 2D gain Bias voltage, mV lnnut filter
Figure 1. The STM lineplot image of a slice taken from a carbon rod. Obtaining the Image
R. Madani of the Department of Electrical and Computer Engineering of the University of Southwestern Louisiana. The carbon samples that we used were slices taken from a carbon rod that was originally made to be a n electrode in atomic emission spectrometry. The images obtained with the carbon rod are not as regular as those obtained with highly ordered pyrolytic graphite (HOPG). However, HOPG is expensive, and HOPG samples are easy to lose due to the high slipperiness of the HOPG. So I decided to use the inexpensive carbon rod for the experiment. The microscope bead rested on a concrete block suspended from bungi cords as recommended by the manufacturer of the instrument. Procedure
The NanoScope I1 is completely computer-controlled. Consequently, all operating parameters are entered into the computer from the keyboard. The operating parameters for the scans obtained by the students are summarized in the table.
The scan begins at the scan rate listed in the table immediately after the tip is engaged. The scanned image is displayed in real time on the view monitor. If the image on the screen is deemed adequate by the student, it is stored in one of the buffers of the computer. If the initial image is not satisfactory, the X and Y offsets can be adjusted to move the tip over a different portion of the surface. If the image is still unsatisfactory, it might be necessary to use a different tip. Further information relating to obtaining satisfactory images is available in the manual t h a t accompanies the instrument. After the image has been stored, the tip is withdrawn and secured. The captured image usually requires electronic filtering to improve its quality. On the NanoScope I1 this is accomplished by choosing the appropriate entry from the screen menu. The filtered image is stored in a second buffer of the computer. Viewing the Filtered Image After the filtered image has been obtained, it is ready for viewinp. - Software commands allow the imape to be viewed (Continued on page A92)
Positioning the lip After the proper operating parameters were entered into the computer, the sample was mounted on the microscope, and the tip was inserted in the microscope head. The bead was mounted onto the microscope base. Then t h e t i p was lowered, using the adjustment bolts on the head support, until the tip was within 0.1 mm above the surface. The telescope and light supplied with the instrument were used to view the tip as the adjustment was made. The adjustment requires practice. The tip was further lowered under computer control until the preset 1 J tunneling current was Figure 2. A topview image of a slice taken from a carbon rod. Several graphite rings are drawn on the image for achieved. clarity. Volume 69 Number 3 March 1992
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The Modern Student laboratory: Scanning Tunneling Microscopy
Figure 3. The STM lineplot image of a silicon sample. i n several ways. Generally it is best to start by looking a t a topview of the image. ARer the image appeared on the screen, the students were encouraged to alter the color, contrast, view angle, and other parameters. When the students were satisfied that they had obtained the best image possible, the computer mouse was used to measure interatomic distances bn the image and to measure the vertical distance between the top two exposedlayers of the surface. After taking several sets of measurements while viewing the image from the top. the students were instructed to magnify-a of the image and to view the image while using the other possible modes. Finally, a line plot of the image was madeon the screen, and the printer was used to obtain a hard copy of the line plot. Software has recently become available t h a t makes i t possible to obtain grayscale hard copies of the image displayed on the screen. A PostScript printer is required to obtain those copies. Results and Discussion The Graphite Images
A line plot of the image of a carbon sample is shown in Figure 1,and a topview grayscale hardcopy of the image is shown in Figure 2. Unfortunately it was impossible to obtain a more-distinct hardcopy of the topview image even though the colored image was clearly visible on the screen. he peaks in Figure lcorrespond to atoms on the surface of the sample. However, it is important to note that variations in the lwal electron density on the surface affect the image. These variations can yield images that appear to be atoms, but are not. This f a d is demonstrated while carrying out the silicon scan. &though it is not apparent from the line drawings, the too views of the carbon samoles revealed the r i m structure ofgraphite. The dark spots'in the topview image shown in Firmre 2 are the holes in the centers of the e r a ~ h i t erines. FO; clarity, a few graphite rings have been ;raced over &e image in Figure 2. The interatomic distances on the same plane of the carbon samples varied from 0.13 to 0.17 nm and averaged 0.15 nm. These values compare to the covalent diameters reported for carbon (52),which vary from 0.12 to 0.15 nm.The vertical distances averaged 0.069 nm. The distances between the holes in the centeFs of the rings averaged 0.28 nm. The Silicon Images
Silicon readily forms a nonwnductive oxide on its surface. If the oxide coating is allowed to grow sufficiently, it A92
Journal of Chemical Education
Figure 4. A magnification of a pottion of the image shown in Figure 3. can prevent tunneling. Consequently no image can be obtained. To observe an image, it was usually necessary to do one of two things: etch the sample of silicon in an aqueous 5% HF solution for about 1min; or remove the upper layer by mechanical polishiw with a fine-made polish or pawr. -1n any case, the images of the silicon sukaces were-not as gwd as those of the carbon rod. (See Figures 3 and 4.) Distinct atoms were not observed on the silicon surfaces. Interpeak distances on the same plane were usually about 0.028 nm. Vertical distances varied from about 0.02 to 0.07 nm. Although no significance can be attached to these distances, performing-the measurements did allow the students to practice using the equipment. The limitations of the ex~erimentalmethod were made amarent durine " the silicon studies. .A
Acknowledgment The author acknowledees the financial assistance of the Louisiana Board of ~ e g e z tthrough s the Louisiana Educat under contract number tion Qualitv S u ~ o o r Fund L E Q S F [ I ~ ~ ~ - ~ ~ ~ - that ENH made - ~ ~purchase of the STM possible. Literature Cited 1. Thundat, T.;Nsgahera, L A,; Lindaay. S. M. J. Yoc. Scl. l b c h d , A 1990,8(1), 53%
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