Scanning Tunneling Microscopes STMs

Figure 10.1: Scanning tunneling microscopy (STM) depends on ultraprecise, but rapid movement of a sharp, single atom tip over a surface. The movement is controlled by X-Y-Z direction piezoceramic arms which move fractions of nanometers in response to voltage changes. By Paul Jablonka (Schneiker and Hameroff, 1987).

Figure 10.1: Scanning tunneling microscopy (STM) depends on ultraprecise, but rapid movement of a sharp, single atom tip over a surface. The movement is controlled by X-Y-Z direction piezoceramic arms which move fractions of nanometers in response to voltage changes. By Paul Jablonka (Schneiker and Hameroff, 1987).

Figure 10.2: Nanoscale view of substrate to STM tip electron tunneling. By Paul Jablonka (Schneiker and Hameroff, 1987).

Mode Number

I

II

III

IV

Quantity held constant

i, v

h, v

h, i

h, i, v

Quantity measured

h

i

v

i/v

Table 10-1: STM operating modes; i, v, h are tunneling current, voltage, and height (tip to sample surface distance) respectively (Schneiker, 1986).

Table 10-1: STM operating modes; i, v, h are tunneling current, voltage, and height (tip to sample surface distance) respectively (Schneiker, 1986).

The 1986 Nobel Prize for Physics was split between the 50 year old cornucopia of microknowledge, the electron microscope, and a brand new, unfulfilled technology, scanning tunneling microscopy (STM). The STM half of the Prize was awarded to Gerd Binnig and Heinrich Rohrer of the IBM Zurich Research Laboratories where STM was invented in 1981 (Binnig, Rohrer, Gerber and Weibel, 1982). The principle used in STM is extremely simple (Figure 10.1). Piezoceramic materials expand or shrink extremely small distances (i.e. angstroms, or tenth nanometers) in response to applied voltage. In STM, piezoceramic holders scan an ultrasharp conducting tip (such as tungsten) over a conducting or semiconducting surface (Figure 10.1). When the STM tip is within a few angstroms of a surface, a small voltage applied between the two gives rise to a tunneling current of electrons (Figures 10.2, 10.3 and 10.4). The tunneling current depends exponentially on the tip-to-substrate separation (about an order of magnitude per angstrom or tenth nanometer). Depending on the substrate, typical tunneling currents and voltages are on the order of nano-amperes and millivolts, respectively. A servo system uses a feedback control that keeps the tip to substrate separation constant by modulating the voltage across a piezoelectric positioning system. As the tip is scanned across the surface, variations in this voltage, when plotted, correspond to surface topography. Detailed surface topography maps of various materials have been obtained which demonstrate individual atoms like cobblestones. By varying and measuring different combinations of tunneling current, voltage, and distance, different types of information may be obtained about the surface being probed.

The basic modes of operation of an STM, described by Hansma and Tersoff (1987), are summarized in Table 10.1. Here i, v, and h are the tunneling current, the voltage across the gap, and the gap size respectively. Mode I is used to measure the topography of the surface of a metal or semiconductor and is the slowest mode since the electro-mechanical servo system must follow the shape of the surface during the scanning operation.

Figure 10.3: Multi-directional movement of STM tip can be controlled by single piezo-tube scanner. By Paul Jablonka (Schneiker and Hameroff, 1987).

LRS |

3 /

^ 1 1

i o A i

II II i 11 ij i

i t i i i i

m

*

PZT

Figure 10.4: Schematic of STM from Tunneling Microscope Corporation (Doug Smith). T = tip, S = sample stage, PZT = piezoceramic tube, CAM = coarse adjust micrometer, FAM = fine adjust micrometer, LRS = lever reduction system. By Conrad Schneiker (Schneiker, 1986).

The scanning speed in Mode I is determined by the response of the servo system. Modes II and III are faster since the tip maintains only a constant average height above the surface. The scanning speed in these modes is determined by the response of the preamplifier only. Mode IV measures the joint density of states which, for a small tip, is a map of the local distribution of electron states of the substrate. For this mode, one "dithers" the tip-to-substrate bias voltage with a small alternating current signal and monitors i/v. Using this mode, Smith and Quate (1986) have done spatially resolved tunneling spectroscopy and observed charge density waves. STM can thus be used to identify the elements comprising specific atoms and to monitor molecular dynamics (Figure 10.5).

Figure 10.5: STM scan with t-axis showing evolution of a nanosystem over time. With permission from Ritter, Behm, Potschke and Wintterlin (1987), courtesy of Eckhart Ritter.

Adaptability and versatility have been shown by STMs operated in air, water, ionic solution, oil and high vacuum (Drake, Sonnenfeld, Schneir, Hansma, Solugh and Coleman, 1986; Miranda, Garcia, Baro, Garcia, Pena and Rohrer, 1985). Their scanning speed may be pushed into the real-time imaging domain (Bryant, Smith and Quate, 1986). One technique for machining STM tips is based on a simple ion milling process that can generate ultrasharp tips with single atom points; a similar technique can generate ultrasharp knife edges and other nanotool shapes as well (Dietrich, 1984). Dieter Pohl (1987) of IBM-Zurich considers STM one example of a group of "stylus" microscopic technologies. Tips are being developed as thermocouples of two different metals sensitive to extremely low changes of heat, and hollow pipettes able to administer or detect individual molecules in solution. Pohl refers to these STM capabilities as molecular "tasting and smelling." Another STM related function can "touch and feel." In "atomic force microscopy," a nanoscale lever positioned by STM piezo-technology is deflected by contact and/or movement of atoms, molecules or their surrounding ions. Van der Waals force induced lever deflection is monitored by an STM tip or by interferometry. Binnig, Quate, and Gerber (1986) propose to measure forces as small as 10-18 newtons using atomic force microscopy. Mechanical shape and structural dynamics can be probed without tunneling through the sample material using this mode, which may be particularly important in the study of biomolecules.

Figure 10.6: Formation of near-field optical nano-apertures. By Conrad Schneiker (Schneiker, 1986).

Another STM spinoff may lead to "nanovison." Resolution of "nearfield microscopy" depends on the diameter of the aperture through which the reflected light passes. Binnig (1985) described how to use an STM to make 20 nm holes in opaque metal films which, when used in scanning light microscopes, become capable of resolving features much smaller than the wave-length of the light. In general, a cone of light cannot be focused to a spot which is significantly smaller than the light's wavelength. However, light passed through an aperture that is many times smaller in diameter than its wavelength results in a narrow beam of light that may be scanned mechanically close to a sample being examined. STM technology is capable of creating nanoapertures, and also the scanning mechanism required for imaging an area (Figure 10.6). Pohl, Denk and Lanz (1984) described their "optical stethoscope" in which "details of 25-nm size can be recognized using 488-nm radiation." A significant advantage of the optical stethoscope concept is "that it allows samples to be nondestructively investigated in their native environments" (Lewis, Isaacson, Harootunian and Muray, 1984). Betzig and colleagues (1986) conclude that this technology will be "able to follow the temporal evolution of macromolecular assemblies in living cells ... to provide both kinetic information and high spatial resolution." Scanning nanoapertures could also use ultraviolet, X-ray, gamma ray, synchotron, or particle beams to "snoop" in the nanoscale if suitable materials and configurations are found (Schneiker, 1986).

Schneiker's breakthrough was to realize that STM tips can be used as ultraminiature, ultraprecise robot fingers that can both "see" and be used to directly manipulate individual atoms and molecules along the lines suggested by Feynman. The scaling down process proposed by Feynman (machines building smaller machines, and so on) can be reduced to just one step! According to Schneiker's concept, STMs can directly link up to the nanoscale to implement, construct, and evaluate Feynman machines and other nanotechnologies. He notes that many natural biomolecules such as enzymes, t-RNAs, antibodies, cytoskeleton, and many synthetic compounds provide a wide variety of potential finger tips to realize Feymans vision of molecular manipulation. Rudimentary nanomanipulation has already been described. Ringger and colleagues (1985) have described STM nanolithography: atomic scale surface etching. Becker, Golovchenko and Swartzentruber (1987) of Bell Labs have reported a single atom modification of the surface of a nearly perfect germanium crystal by the tungsten tip of an STM. They conclude that manipulation of atomic sized structures on surfaces will lead to "new frontiers in high density memories, new devices whose electrical properties are dominated by quantum-size effects, and modification of single, self replicating molecules,"—supporting the earlier predictions of Schneiker. In its first few years, STM has imaged single atoms, probed atomic forces, manipulated atoms, and won a Nobel prize. Eventually STMs and their spinoffs may enable researchers to alter and communicate with genes, viruses, proteins, cytoskeletal assemblies, and perhaps biomolecular consciousness.

Figure 10.7: Nanotechnology workstation is envisioned as a synergistic combination of multiple STM tips mounted in an optical microscope. By Paul Jablonka (Schneiker and Hameroff, 1987).
Figure 10.8: Multiple tip geometry for STM probing of nanoscale systems. By Conrad Schneiker (Schneiker, 1986).

Schneiker and Hameroff (1987) have proposed an integrated system of STM and other technologies to best take advantage of STM capabilities. The "nanotechnology workstation" (Figures 10.7 and 10.9) proposes to combine multiple STM tips, optical microscopy, nanoapertures, and fiber optic waveguides in a hopeful attempt at synergism. For example, the STM is extremely "nearsighted;" it can have difficulty finding its target ("a needle in a haystack"). If integrated into a high powered light microscope, the microscope can act as a coarse approach mode to bring the STM tip to the desired area. From there the STM can perform as a "cursor" mode for the microscope: able to expand the view of, and manipulate, atoms and molecules. In one configuration of the nanotech workstation, four STMs are placed symmetrically about a central STM and these each form a 45 degree angle to the sample plane; they may later be augmented with fiber optical interferometers for precise spatial coordinate determination and motion control (Figure 10.8). The central STM may be used for high speed scanning while the peripheral STMs are used as oblique STM scanners, nanoaperture scanners, electrodes, mechanical "fingers" or optical frequency antennas. In addition, four other micrometer-adjusted mini-arms can carry horizontal optical fibers: two for general illumination and two for experimental interfaces.

Reflecting optical microscope objectives may be positioned such that the optical axis is perpendicular to the plane of the opposite pair of STMs, and the focus is where their tips would meet when fully extended. The optics of these microscopes would permit them to peer around the fiber optic illumination system mounted on the forward axis of each microscope. The optical system would be achromatic, and able to work in far infrared to ultraviolet which should lead to increased optical resolution. Fluorescent coating for STM tips (excluding the last few nm) can aid in visualizing their location. The final imaging system should consist of solid state detectors whose output may be displayed on high resolution color monitors: STM modes, optical microscope, nanoaperture "nanovision," etc. (Figure 10.9 and 10.10).

Figure 10.9: Graphic displays, monitors and control module for nanotechnology workstation. By Paul Jablonka (Schneiker and Hameroff, 1987).
Figure 10.11: Molecular movement and construction with an STM. By Conrad Schneiker (Schneiker, 1986).

A system akin to the STM nanotech workstation may dynamically observe and manipulate nanoscale materials and systems, capabilities consistent with Feynman's notions for molecular machines.

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