Haptic Visions. Valerie Hanson
at the same time, moving across the sample surface. In short, the shape is more like a graph of the tip’s movements over time (Russell). One result of the mutual influence of tip, sample, and image is that the shape of the tip becomes important: Ideally, one atom at the tip end should protrude slightly (even just an Ångstrom) from the others so that the applied current can flow through that one small point (J. Foster 17).30 The sample, too, can affect the tip if the atoms of the sample strongly attract and “pull off” some of the tip’s atoms. In the space of the interaction, both tip and sample meet in the vacuum, and are equally able to affect the interaction.
The use of electron tunneling also creates the possibility of continued interaction, like MRI or PET scanning, as neither sample nor tip is damaged in collecting measurements (unlike, for example, electron microscope samples that are destroyed in the imaging process). The fact that the sample is not destroyed allows researchers to collect data repeatedly over the same space, and thus track dynamics over time; the user can experience atoms as a series of movements. A Journal of Physical Chemistry B article provides an example: S. A. Kandel and P. S. Weiss state that “by comparing sequentially recorded images, [they] can see that the size and shape of the clusters [on the sample] change over time” (8103). Kandel and Weiss explain the effect of the tip on the measurement: “the mobility shown in the rearrangement of these clusters is likely (at least in part) induced by the STM tip” (8103). The dynamic that occurs between the tip and the sample as the STM collects data is similar to the HCI interaction type of “flow,” where a user experiences a merging with the computer. In this case, of course, the interaction occurs between atoms. Although the tip-sample interaction does not directly involve the user, electron tunneling affects the user’s experience of atomic phenomena through a multi-directional affordance and the related possibilities for repeated interaction. The structured dynamic of the tip-sample interaction that also produces nanoscale measurements affects what can be shown in an image and affects experiment design, just as the use of electron tunneling allows researchers to manipulate the surface using the microscope’s probe affordances. Thus, the expectation of a certain kind of multidirectional interaction inheres at the level of collecting data.
Figure 4. “Quantum Corral” image. From Science 262, 5131 (8 Oct., 1993). Cover illustration. Image originally created by IBM Corporation. Reprinted with permission from AAAS.
Raster Scanning and Z-direction Moves
The STM tip moves in three dimensions (x, y, and z, in the Cartesian system), forms other constitutive components, and works with electron tunneling to shape the multi-directional affordance of the STM. The microscope’s design, like that of the scanning electron microscope, relies on a mobile tip. The fact that the tip moves—and must move—to survey the sample affects how the STM measures each point. The tip moves in three different directions: in the x and y directions in a raster pattern to collect data about points on the surface, and in the z (up and down) direction to create the tunneling conditions in which tip and surface electrons interact. Movements in x, y, and z directions coordinate with tip-surface tunneling interactions to collect data about the nanoscale; the x, y, and z movements also are part of broader traditions of dynamics used by other visualization technologies. In the STM, the x, y, and z directional dynamics structure interactions that encourage manipulation by the user.
The process of rastering, or scanning the surface with a back-and-forth pattern, from which the STM builds an image from data, forms a key dynamic in other visualization technologies. Derived from radar’s creation of an image from a signal, rastering is perhaps best known as the method that allowed cathode ray tubes, such as those used in non-digital televisions, to build an image from a linear signal. In microscopy, using rastering to image a sample had been considered since the early twentieth century, but use of rastering was first demonstrated in 1972 (Wickramasinghe 78). The STM-like topografiner also scanned microscopic surfaces and collected measurements to create an image of the surface, although the specific pattern the scanner makes is not described (Ward, Young, and Scire 999). The incorporation of rastering into scientific visualization instruments such as the topografiner in the early 1970s also fits into the trend of developing visualization technologies that Galison discusses. The STM, then, is partly formed by the “tradition” of dynamics of rastering.
In the STM, rastering only works in conjunction with measurement of tip-surface interactions in electron tunneling, because the dynamics involved in raster scanning are structured around the challenge of movement in relation to time. To create anything other than a blur, a sample would need to remain relatively still, and the tip would need to move relatively quickly. As Lev Manovich observes, one implication for images produced using rastering is that “It is only because the scanning is fast enough and because, sometimes, the referent remains static, that we see what looks like a static image” (100). However, atoms do not slow down enough to become referents; so, following Manovich’s explanation, rastering would not work in the STM unless rastering is combined with the tip-surface interactions that provide the “stability” of a measurement of movement. Even so, STM researchers often need to correct for “drift” (or, when the sample atoms move before the tip has finished scanning) and also, at times, slow the sample atoms down by lowering the temperature to, for example, four degrees Kelvin so that the STM can scan the atoms.31
The raster scan allows STM users to convert tip-surface interactions into a camera of sorts. In so doing, the STM does not beam electrons (like a television camera) to assemble an on-screen image; instead, the STM forms what could be called a haptic camera, a “camera haptica” (playing on “camera obscura”), as the STM converts a series of interactions between atoms in the vacuum into a series of spatially arranged data points. The use of interactions as measurements moves the visualizing process closer to the sense of touch than the sense of vision or, more accurately, in a merging of the two senses—to haptic vision—because the tip gathers data about local interactions over a series of contiguous spots, and then presents the interaction data in a two-dimensional matrix, an image form. (The concepts of “camera haptica” and haptic vision are discussed further in Chapter 2.)
The rastering movement plays a role in the design of experiments, as researchers engage in practices that rastering affords. For example, Kandel and Weiss, in the experiment mentioned above, record images “with the tip rastering quickly along different directions, and . . . see a correspondence between this ‘fast scan’ direction and the locations at which atoms in the cluster either attach or detach” (8103). Kandel and Weiss present four versions of the same atoms that have been scanned in different ways, and use these versions to explore atomic properties (8104). Kandel and Weiss’s experiment design is one example of how the tip’s movement forms an experimental tool, allowing researchers to interact with surface atoms through the STM.
The ability of the tip to move up and down in the z direction also affords manipulation, because the tip can measure the three-dimensional electronic or topographic qualities of the surface. The ability to move in the z direction has made the distance between the tip and the sample a critical component of the operation of the STM from the beginning: Soon after Binnig and Rohrer developed the STM, before they achieved atomic resolution, Binnig and Rohrer “had to struggle with resolution, because Au [gold, their sample] transferred from the surface even if [they] only touched it gently with [the] tip” (“From Birth” 398). In 1987, R. S. Becker, J. A. Golovchenko, and B. S. Swartzentruber repeated this “mistake” of touching the surface with the tip; as a consequence, the atoms moved. In a letter published in Nature, Becker, Golovchenko, and Swartzentruber reported “atomic-scale modification” of a sample surface after they applied voltage to the tip. They attributed the modification to the transfer of a tip atom to the sample surface (421). Others repeated the experiment of using voltage pulses to “pin” molecules and atoms to a surface (J. Foster 29–32). In another experiment with the STM, for example, R. C. Jaklevic used the STM to dent a piece of gold, and then re-scan the same sample to measure how quickly the dent filled itself in (six to nine atoms per minute), thus using the STM as a tool to make the event of atomic movement measurable (Jaklevic 659).
The x, y, and z directional dynamics structure what can be seen with the STM. The STM images events through measuring change and movement, much like other visualization