In the Laboratory
Preparation of Chemically Etched Tips for Ambient Instructional Scanning Tunneling Microscopy Margot J. Zaccardi, Kurt Winkelmann, and Joel A. Olson* Department of Chemistry, Florida Institute of Technology, Melbourne, Florida 32901 *
[email protected] Since its invention in 1982, scanning tunneling microscopy (STM) has revolutionized our view of the chemistry of surfaces (1). Shortly after its invention, commercial STMs became available, including instructional-grade instruments (2, 3). To reduce costs, instructional STMs generally operate under ambient conditions and allow only a limited number of experimental parameters to be adjusted. Because STM depends on the tunneling of electrons between the tip and the sample surface, the surfaces that may be imaged under ambient conditions are limited to those that do not form insulating oxide layers in air. The favorite surface for ambient imaging is highly oriented pyrolytic graphite (HOPG), which is relatively inexpensive and of high quality when purchased as X-ray monochromator elements. It is easy to prepare large regions of the HOPG surface that are atomically flat (4). This allows ease of imaging at atomic resolution. The most common tips used in instructional STMs are mechanically prepared from Pt-Ir alloy wire (5). Although the Pt-Ir tips are easy to prepare and often provide adequate-quality STM images, the issue of tip-preparation techniques has been neglected when teaching STM. This is unfortunate because tip quality is an important aspect of STM within the research community. Typically, electrochemically etched tips are used in ultrahigh vacuum (UHV) STMs (5). The quality of etched tips (versus mechanically prepared tips) is usually more reliable and is of great importance for UHV instruments where the exchange of a bad tip can be a time-consuming process. When samples are analyzed under vacuum, a greater variety of tip materials can be used. There are several factors that contribute to the suitability of an STM tip (6). The most important factor is that the tip material allows the electrical conductance of electrons. Other important attributes of STM tips include the aspect ratio and radius of curvature. A tip is more desirable if it possesses a small radius of curvature. However, there are both positive and negative factors for high and low aspect ratios. These topics are presented in more detail in the Supporting Information. The experiment described herein allows students to learn an effective procedure for preparing electrochemically etched STM tips from gold wire. Students perform this experiment and use their tips to image HOPG in a first-year nanotechnology laboratory course described elsewhere (7, 8). These tips may also be incorporated into other undergraduate STM experiments (9-13). A description of theoretical aspects of STM methods (14) and an outreach program to junior high school students (15) have also been published in this Journal. The reader is asked to consult these references for background information about STM and other STM laboratory activities. 308
Journal of Chemical Education
_
_
Figure 1. Experimental setup for tip etching.
The Experiment Whereas other techniques for etching Au STM tips have been reported (16), the etching process used here is similar to that described by Ren et al. (17). A schematic of the apparatus is shown in Figure 1. A piece of 0.2 mm or 0.25 mm Au wire (Alfa Aesar; Ward Hill, MA; 99.99% purity) was placed into a 23 gauge syringe needle (BD; Franklin Lakes, NJ) and suspended in a solution of 1:1 (v/v) concentrated (12 M) HCl and ethanol using an alligator clip, such that only a small length of the Au wire (2-3 mm) was under the surface. No part of the syringe needle should be immersed in the solution. A graphite electrode (recovered from a no. 2 pencil) was suspended in the solution using an alligator clip. A glass tube (Pyrex; ∼4.5 cm diam) was placed around the graphite electrode to prevent any bubbles from disturbing the meniscus at the tip wire. The two electrodes were connected to a variable dc power supply and a voltage of 2.4 V was applied. The etching of the wire proceeded preferentially near the air-solution interface (Figure 2). The strain caused by the portion of wire beneath the surface of the solution became too great for the necked-down region of the wire to mechanically support, and the lower piece detached leaving a reasonably sharp tip. The power was then manually turned off as quickly as possible, and the Au tip was removed from the solution and rinsed with Millipore water. Guidelines for washing and handling STM tips can be found in the Supporting Information.
_
Vol. 87 No. 3 March 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed800089y Published on Web 02/09/2010
In the Laboratory
Figure 3. SEM images of a tip half way through etching (left) and a fully etched tip (right). Figure 2. Preferential etching near the air-solution interface.
Each tip takes approximately 6-7 min to prepare, and 500 mL of etchant solution yields approximately 4 to 5 tips before the solution can no longer be used. The etchant solution should be stirred between tip preparations. It is recommended that each student make 4 to 5 tips to account for the difficult nature of handling tips. Making a number of tips ensures that students will have extra tips to use if they damage one. The experiment should be conducted in a location that is vibrationally quiet so as to not disturb the meniscus at the wire. For STMs in which the tip is held in place by a clip, it is necessary to mount the tip into a syringe needle and cut the end of the needle before placing the tip in the STM. This is because Au is soft and may be difficult to insert into the tip mount. To prepare the HOPG surface for imaging, press a piece of clear, adhesive tape on the surface of a HOPG X-ray monochromator element (GE Advanced Ceramics, Strongsville, OH). Peeling the tape away removes many atomic layers of graphite and leaves an atomically flat, pristine surface of graphite. Electrochemical Etching In the solution, a gold complex is formed in a reaction between the gold wire and the Cl- in solution by the redox halfreactions (18): AuðsÞ þ 4Cl - ðaqÞ f AuCl4 - ðaqÞ þ 3e -
2Hþ ðaqÞ þ 2e - f H2 ðgÞ As the tetrachloroaurate(III) ion forms near the wire, it diffuses away, thus creating a diffusion flow in the solution as shown in Figure 2. The etching takes place preferentially near the air-solution interface and can be explained by the Nernst equation: Ecell ¼ Ecell ° -
0:05915V ½products log n ½reactants
air-solution interface. As AuCl4- diffuses down the length of wire, Cl- diffuses toward the wire near the meniscus. This replenishes the chloride ions used in the etching. This technique is particularly instructive because the gold complex that is formed is yellow in color and can be seen streaming from the wire (see the Supporting Information). Hazards Ethanol is flammable and is a mild eye and respiratory system irritant. Hydrochloric acid is a corrosive material as well as an eye, skin, and respiratory irritant. The 1:1 EtOH/HCl solution should be handled with proper safety equipment including a lab coat, safety goggles, and gloves. The experiment should be performed under a fume hood or other ventilation system, such as a snorkel, to prevent inhalation of potentially harmful fumes. As in any situation where electronics are being used in the presence of liquids, GFI outlets should be used. Results Although it is not imperative for the tips to be very sharp to image HOPG, the tips resulting from this technique can be used for experiments that require a reasonably small radius of curvature (
