J. Phys. Chem. 1996, 100, 13103-13120
13103
Scanned Probe Microscopies in Chemistry R. J. Hamers Department of Chemistry, UniVersity of WisconsinsMadison, 1101 UniVersity AVenue, Madison, Wisconsin 53706 ReceiVed: January 2, 1996; In Final Form: May 16, 1996X
The theory and applications of scanned probe microscopies in chemistry are reviewed. The review includes scanning tunneling microscopy (STM), atomic force microscopy (AFM), near-field scanning optical microscopy (NSOM), and other related techniques. Applications to chemical and biochemical imaging, molecular identification, and other systems of importance in chemistry are described.
I. Introduction Scientists and philosophers have been fascinated with the idea of “atoms” as the building blocks of matter ever since Democritus first proposed the idea of an ultimate particle. The ability to observe atoms was realized in the 1950s using the field ion microscope, invented by Erwin Muller.1 Yet, not until the invention of the scanning tunneling microscope in the 1980s did it become possible to image atoms on a more general class of flat materials. The seminal ideas behind scanned probe microscopies can be traced back at least to the early 1970s, with the invention by Russel Young and co-workers at the National Bureau of Standards of an instrument known as the “topografiner”.2-4 The topografiner consisted of a metallic tip mounted on a piezoelectric positioner; a voltage was applied to the tip, which resulted in the field emission of electrons to the sample. The electrons striking the sample generated several different types of measurable signals including secondary electron emission and photon emission, which were used as the basis for spatially resolved microscopy. The topografiner contained many of the important elements of the STM, including piezoelectric positioners and a feedback system for controlling the sample-tip spacing. However, the resolution was limited by approximately 30 Å in the vertical direction and about 4000 Å in the horizontal direction. Through an unfortunate set of occurrences, work on the topografiner was dropped in the early 1970s, and subsequent work on “scanned probe microscopies” stopped for nearly 10 years. Then Gerd Binnig joined the research staff at the IBM Zurich Research Laboratory and, with Heinrich Rohrer, Christoph Gerber, and other colleagues, began investigating how one might control the properties of small tunnel junctions using piezoelectric devices. Their success in controlling sample-tip spacings at voltages much smaller than those used in the topografiner, combined with advances in vibration isolation, led them successively closer to atomic resolution.5-8 The world took notice when, in 1983, they succeeded in imaging, with atomic resolution, approximately two unit cells of the famous (7 × 7) reconstruction of Si(111).7 Figure 1 shows this first atomic resolution image, with 12 “atoms” visible within each of the two diamond-shaped unit cells. This first image was acquired using a chart recorder and subsequently cutting out the individual traces and attaching them to a wooden model to recreate a three-dimensional perspective. Binnig and Rohrer received the 1986 Nobel Prize in physics for their invention of the STM, and their Nobel Prize address X
Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(96)00054-8 CCC: $12.00
Figure 1. First atomic-resolution STM image. This image shows a relief view of two complete unit cells of the Si(111)-(7 × 7) reconstruction. Reproduced with permission from ref 7. Copyright 1983 American Institute of Physics.
was published as an article entitled “Scanning Tunneling Microscopysfrom birth to adolescence”.9 Exactly 10 years later in this “Centennial” edition of The Journal of Physical Chemistry, I will attempt to discuss the maturation of STM, together with the birth of its scanned probe siblings: atomic force microscopy, near-field scanning optical microscopy, and many of the other new “scanned probe microscopy”, or “SXM”, techniques engendered through the pioneering work on the STM. The importance (or at least the popularity) of SXM techniques can be gauged to some extent by the rapid growth in publications using these techniques to gain atomic- or nanometer-level information. For example, Figure 2 shows the results of a computer search for all articles containing STM or AFM (or variants thereof) in their title. Note that this search only includes titles and therefore certainly underestimates the true number of publications in the field. Nevertheless, it does convincingly show the dramatic growth in the number of publications. The growth can be roughly fit with an exponential function in which the number of papers increases by on the order of 30% per year. Because the number of publications in this field numbers in excess of 5000, it is impossible in a short article to adequately describe even the most important contributions to this field since its inception. Instead, the goal of this paper will be to give a flavor for some of the already realized advances in chemistry resulting from these techniques, describe some of the pitfalls, © 1996 American Chemical Society
13104 J. Phys. Chem., Vol. 100, No. 31, 1996
Hamers
Figure 4. Schematic illustration of experimental apparatus for AFM.
Figure 2. Approximate growth in number of scanned-probe publications.
Figure 3. Schematic illustration of experimental apparatus for STM.
and perhaps set the stage for potential future development and application of these techniques to chemistry and related fields. II. Basic Operating Principles of Scanned Probe Techniques Many of the SXM techniques share similar components. All require some long-range one-dimensional translator. These are typically made from piezoelectric materials,9 but purely mechanical approaches10 are also utilized. The “long-range” translator must be able to take the STM tip and move it a distance on the order of millimeters or microns. Once the STM tip is within an accessible range for the particular measurements (
