Celebrating 25 Years of Scanning Probe Microscopy and the Work of Kumar Wickramasinghe

In 1981, two research scientists from IBM’s European research facility in Rueschlikon, Switzerland, made a discovery that would help change the face of the world today. With the invention of the scanning tunneling microscope (STM),1 Gerd Binnig and Heinrich Rohrer gave birth to a family of techniques that have allowed scientists to study materials from the atomic level upward. Five years later came an even more powerful invention, the atomic force microscope (AFM).2 Binnig, now collaborating with Cal Quate and Christoph Gerber, produced the instrument that was to become the father of  scanning probe microscopy (SPM), spawning a number of metrological tools that were referred to as “the picks and shovels of nanotechnology.” Unlike the usual timelines of instrumentation development, where an idea may take 15 or more years to transition from a concept to development tool and finally to a routine analytical solution, the AFM came along at the start of the explosive growth of the semiconductor industry defined by Moore’s Law. In 1975, Gordon Moore of Intel (Santa Clara, CA) predicted that data storage capacity and processing power would double every two years. To keep up with this model, and sometimes exceed it, industry required a means to locate and measure the size and performance of chips that were being packed more and more densely onto silicon wafers. The AFM had found its first niche.

The advantage of the AFM was its ability to study all types of material in a variety of environments, from air to liquid, ambient pressure to ultrahigh vacuum. This has led to AFM now being found in the laboratories of multiple scientific disciplines. Some of the most exciting steps forward in recent years have been seen in the life sciences, where AFM techniques have been used to enhance the knowledge of molecules in vitro, with the AFM probe being able to look at individual or moleculeto- molecule properties.

There are many great scientists who have spent their lives working in SPM research, but few have had the impact in applying these technologies as Kumar Wickramasinghe. In the 1980s and ’90s, Dr. Wickramasinghe was working in the T.J. Watson Research Center (Yorktown Heights, NY), where he pioneered many applied techniques that were to benefit IBM’s work in semiconductor manufacture. Having been one of the first scientists to apply tapping mode for imaging surfaces, Dr. Wickramasinghe developed it to invent other now established techniques such as magnetic force microscopy,3 electrostatic force microscopy,4 Kelvin probe microscopy,5 scanning thermal microscopy,6 and the apertureless near-field optical microscope.7

Freed now from project work, Dr. Wickramasinghe has been able to go back to his first love, practical research. For the past few years, he has been able to focus on new areas in which to apply scanning probe techniques. Working with postdoctoral research fellow Kerem Udal, Dr. Wickramasinghe has published a paper that details his work using AFM to sort and deliver molecules at extremely high speeds.8

Rather than the AFM being applied as a microscope, it is now being used as a tool that acts like a printer, writing onto a surface. In the late 1990s, the Mirkin group at Northwestern University (Evanston, IL) first applied AFM in a technique called dip-pen nanolithography (DPN),9,10 which was developed to put down a variety of chemical compounds onto a variety of substrates. According to Dr. Wickramasinghe, DPN was analogous to a quill pen, with little control of the deposition rate since the mechanism of writing was through diffusion. Also, control was related to the speed at which the pen was moved over the substrate, and the process was stopped and started by the pen being lifted to and from the surface.

This latest approach, however, works more like an inkjet printer. A new type of probe is used that is conical in shape; where it connects to the cantilever, IBM’s design incorporates a reservoir to supply the molecular ink. Not only can the device write, but it can also remove molecules from the substrate.

Figure 1 a)- Scheme of an AFM probe used in this study with the associated electric field. b) SEM images of a modified cantilever.

Figure 2- Depending on the voltage pulse and polarity, molecules (red) may be released from the tip and deposited on the substrate.

Figure 3- The different mobilities of 5- and 16- base DNA fragments enable their separation using this AFM-based electrophoretic technique.

Figure 4- Topographic (a) and lateral force (b) images showing controlled surface patterning with 5-base-long DNA fragments. Line scans (c and d) show a mean height of 2.4 nm and line width of between 59 and 79 nm.

The technique works because of the thin film of water found on the substrate and on the probe itself (Figures 1–4 display the ultrafast molecule sorting story). By applying a field between a conducting cantilever and probe and the conducting substrate, it is possible to exploit the electrophoretic mobilities of molecules. Thus, by varying field strength and polarity, it is possible to precisely control deposition and removal of the molecules from the surface. When this is combined with the positioning capabilities of a modern AFM system, it is possible to envision a method that writes features five times smaller than today’s e-beam lithography and 10 times smaller than photolithography. It is the control of the thin film of water through control of the humidity of the experiment that makes this method work.

Understanding the chemistry of the process is also extremely important. Close control of the probe and surface chemistries is required to ensure the molecules are immobilized to prevent diffusion. In the experiments used to illustrate this inkjet concept of writing, Dr. Wickramasinghe’s team investigated single-stranded DNA fragments. Deposition was confirmed using lateral force microscopy (LFM) because it is particularly sensitive to frictional change, thus readily “seeing” the sticky molecules on the smooth substrate.

The electrophoretic AFM method was compared to traditional capillary electrophoresis. The transfer times using AFM were dramatically reduced: A 15-base-long strand of DNA could be transferred in 5 msec compared to 170 sec in a conventional 8.5-cm-long microfluidic channel.

The potential impact of this work ranks alongside some of Dr. Wickramasinghe’s other achievements. The method has enabled the acceleration of molecular separation, and transfer has been speeded up by several orders of magnitude. Dr. Wickramasinghe believes this wi l l have an impa c t on futur e research in biology and medicine. For example, DNA sequencing will be speeded up through reduction in sample size. The method is very scaleable using either multiple-tip arrays or multiple reservoirs with different molecules.

It is work like this that makes exploring the field of SPM so exciting. With nanotechnology already being part of today’s world, it is the work of scientists like Dr. Wickramasinghe that emphasizes the benefits of working on the nanoscale to enhance the quality of life today and for generations to come. So what will the next 25 years bring? The only restriction is the power of our imagination.

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