Probing the History of Scanning Tunneling Microscopy

We present a brief history of the development of scanning tunneling microscopy (STM). These microscopes, developed in 1981 by Gerd Binnig and Heinrich Rohrer (Nobel prize 1986), are capable of imaging and manipulating at an atomic level. STMs, and the group of instruments corporately referred to as scanning probe microscopes that evolved from them, are part of the instrumentation that has enabled nanotechnology. In our history we examine how these instruments have been used (perhaps wrongly) in the “standard story” of the emergence of nanotechnology. Nanotechnology has developed in a context sometimes referred to as “postacademic”, because of the increased emphasis on aspects of commercialization. We examine how this “post-academic” context has influenced the development of these instruments. Our history of STM shows an epistemological shift that is part of postacademic science and nanotechnology policy. 1. In the Beginning was Little Big Blue Figure 1 ‘The Beginning’. Courtesy: IBM Research, Almaden Research Center. In 1990 in the journal Nature D. M. Eigler and E. K. Schweizer first published this now well known image of I.B.M.’s initials spelled out with 35 individual xenon atoms. The image now ‘hangs’ in I.B.M.’s ‘STM Image Gallery’ where it joins 15 other striking, and in many ways beautiful images of the atomic world (Eigler and Schweizer 1990). The images are made with a scanning tunneling microscope [STM], which was invented in 1981 by D. Baird & A. Shew: Probing the History of Scanning Tunneling Microscopy 146 Gerd Binnig and Heinrich Rohrer, both employed by I.B.M. Research in Zurich. Binnig and Rohrer won the 1986 Nobel Prize in physics for their invention. There is much that is remarkable about Eigler and Schweizer’s ‘IBM.’ Most immediately it is the interlocked precision technology and science allowing us to ‘see’ these individual xenon atoms that we marvel at. But we are not just seeing them; we are placing them just so. The image shows our hands and eyes reaching to an atomic level of precision. ‘An atomic level of precision’ now is more commonly called ‘nanoscale precision.’ A nanometer, one-billionth of a meter, is roughly ten hydrogen atoms side-by-side. ‘Nanotechnology’ is the study and exercise of hands and eyes with sufficient precision to ‘see,’ and in some cases manipulate, individual atoms. In I.B.M.’s STM Image Gallery, Eigler and Schweizer’s ‘IBM’ is titled, ‘The Beginning.’ It is an appropriate, if immodest, title, for ‘The Beginning’ is emblematic of the beginning of genuine atomic precision, genuine nanotechnology. There are ‘nano-visionaries’ who see in nanotechnology nothing short of a complete transformation in human life on Earth, with nanotech solutions to energy, disease, pollution, even mortality. ‘IBM’ is a crude beginning indeed. In viewing this image one also may be struck by the notion that in the beginning was a corporation, IBM. To be sure, nanotechnology is pursued in academic settings where the unfettered pursuit of truth at least is the stated ideal. IBM, along with the raft of other high tech companies that are pursuing nanotechnology, no doubt seeks truth, but not at the expense of shareholder value. Indeed, Eigler and Schweizer say of their image: Artists have almost always needed the support of patrons (scientists too!). Here, the artist, shortly after discovering how to move atoms with the STM, found a way to give something back to the corporation which gave him a job when he needed one and provided him with the tools he needed in order to be successful. (www.almaden.ibm.com/vis/stm/gallery). Nanotechnology, including the instruments that make it possible, such as the scanning tunneling microscope, is developing in a much more thoroughly integrated academic/ commercial matrix. One nanotech researcher tells us, tongue only half in cheek, that an assistant professor probably should not get tenure unless he or she has two ‘start-ups’ to show for himor herself (Tour 2002). John Ziman calls this ‘post-academic science’ (Ziman 2000, ch. 4). We are interested here in the development of scanning tunneling microscopy, and in particular how its development in a ‘post academic’ context impacts the design constraints on STMs, and the various off-shoots, generically called ‘scanning probe microscopy’ [SPM]. We argue that the epistemic needs that underlie commercial development differ from those that underlie academic development. Thus, through our examination of STM and its relation to nanotechnology, we articulate a key epistemological difference between ‘academic’ and ‘post-academic’ science. 2. Scanning Tunneling Microscopy Scanning tunneling microscopy is conceptually simple. Imaging with STM involves moving a tip over a surface to obtain topographic information about the surface. One can compare STM to Braille reading or the way the tumblers in a lock ‘read’ a key’s shape. STM relies on the phenomenon of electron tunneling to image surfaces. Tunneling is a quantummechanical phenomenon that is manifested in a current induced by a voltage differential between the scanning tip and the sample (Chen 1993). The level of the tunneling current is directly proportional to the distance between the tip and the surface. The closer the tip is to the surface the higher the current. D. Baird & A. Shew: Probing the History of Scanning Tunneling Microscopy 147 The components of an STM include a probe tip, a piezo-electric material that controls the tip’s location in all three dimensions, a voltage source, a means to measure current flow from sample to tip, and finally computing power both to transform current data into an image and to control tip movement (Chen 1993). The scanning tip, which ideally is atomically sharp, is usually made of tungsten or platinum-iridium. Typically, a topographic image is produced by running the tip back and forth over the sample surface such that, by means of an electronic feedback loop, the tip is moved up or down to keep the tunneling current – and consequently the tip’s distance above the surface – at a constant value. By taking note of the amount the tip has had to be moved up or down, a topographic image of the surface can be produced with the aid of computer imaging software (Griffith & Kochanski 1990). When all works right, we see on the computer screen an image that looks as though we were looking at the landscape of atoms on the sample surface. Although simple in concept, the researchers creating STM had to solve several difficult problems: precise control of the tip’s location and movement, control of vibration, and making a tip with the necessary atomic sharpness. The tip must come within a few nanometers of the surface. Finding a material that can move the tip without crashing the tip into the surface – or worse – was a huge problem. Piezoelectric ceramics were the answer. Piezoelectric ceramics deform only slightly when an electric voltage is passed through them. By appropriately varying the voltage in the piezoelectric positioner, an STM achieves precise control over the tip’s location over the sample. The tunneling voltage, working in conjunction with the feedback system and the piezoelectric material, allows for precise control of the tip’s height and placement over the surface. Because STM is done with such a high degree of precision, where the tip is only nanometers from a surface, external and internal vibrations can present substantial problems. Early STMs were operated at night with everyone silent. Vibration also can be reduced by building the instrument with sufficient mechanical rigidity and through an appropriate configuration of the piezoelectric transducers. Sometimes STMs are hung on a double bungee cord sling to manage vibration. Further vibration isolation systems have also been made with springs and frames (Baum 1986). Making tips remains something of a dark art. One takes a piece of tungsten or platinum-iridium wire and cuts it with wire cutters, being careful to pull away from the end that will serve as the tip. Some researchers develop a good knack at this, while others do not. While tips are usually diagramed as nice symmetrical ice-cream cone structures, in reality they are messy affairs resembling a jagged mountain range. But what is crucial is that one peak from this range be sufficiently higher than all the others and itself be atomically sharp; it then can serve as the point through which the tunneling current passes (Myrick 2002a). There was some lag between Binnig and Rohrer’s development of STM in 1981 and its acceptance. Initially surface scientists were skeptical, but when Binnig and Rohrer solved a well-known outstanding problem in surface science – the structure of so called crystalline silicon (1,1,1) 7 X 7 – they began to take notice (Mody 2004). As the 1980s progressed, Binnig and other collaborators developed the scanning tunneling microscope in a variety of directions, including atomic force microscopy (AFM). Because STM depends on a current passing from sample to tip, only conducting samples could be imaged. AFM, which Binnig, Christoph Gerber and Calvin Quate developed in 1986 (Binnig, Quate & Gerber 1986), avoids this limitation by measuring the tiny deflections that a sharp probe experiences when dragged over a surface. As the surface goes up in elevation, the probe is deflected up, and this deflection can be measured. Combining measurements from the whole surface allows researchers to produce an image of the topography of the surface. D. Baird & A. Shew: Probing the History of Scanning Tunneling Microscopy 148 3. Elements of the Commercial History to STM While STM and its early siblings, AFM and the other techniques of probe microscopy, were developed in what officially is a corporate context – IBM – the work was essentially academic research pursued in an industrial research lab. Through most of the 1980s, STM and AFM remained primarily of academic interest. It took some time for the technique to catch on. There are a variety of reasons for this. Some are disciplinary or structural. While the first arena where STM could and did make a significant contribution was surface science, neither Binnig nor Rohrer came from this academic community, and their claims for STM were not, for this reason, immediately accepted by the surface science community. There were epistemological hurdles to jump as well. The images that one can produce with a STM are very nice, but on what grounds are they to be believed to be genuine images of individual atoms? Finally there were pragmatic reasons that slowed the development and acceptance of STM. Prior to the commercialization of STM in the late 1980s, the STM probe was not integrated with a computer, and this made the instrument much more difficult and time consuming to use (Myrick 2002b). These issues – disciplinary insulation, epistemological acceptability and pragmatic ease of use – create a kind of ‘chicken and egg’ problem for the commercialization of STM and SPM more generally. Profits require a large enough market to offset the costs of research and development. Broad markets, by their nature, cross disciplinary boundaries, but they also require instruments whose results can be relied on, and which can be used by people other than those academics willing to spend hours coaxing the instrument to work. Fascination with instrumental possibility, with pushing the limits of resolution, of what it is possible to ‘see,’ makes for good academic research, but not for an instrument that serves ‘transparently’ or ‘instrumentally’ in the pursuit of other concerns with broad market appeal. At the same time, these broad markets will not develop unless there are instruments available ‘off the shelf.’ Such instruments are for people who are not themselves interested in instrumental development. Navigating this chicken and egg problem is the fundamental story of the commercialization of STM and SPM during the late 1980s and 1990s.