Advisory Panel J o n a t h a n W. A m y Richard A. Durst G. Phillip Hicks
INSTRUMENTATION Donald R . J o h n s o n Charles E. Klopfenstein Marvin Margoshes
Harry L. Pardue Howard J . Sloane Ralph E. Thiers
Prospects for Molecular Microscopy Recent improvements in specimen support films and image contrast will help achieve the goal of visualizing single atoms or small molecules by electron microscopy. Other possible applications in chemical identification and structure determination, including the detection of X-rays, fluorescence emission, and secondary electron emission, are likely to be exploited J. WENDELL WIGGINS and MICHAEL BEER Thomas C. Jenkins Department of Biophysics Johns Hopkins University, Baltimore, Md. 21218
/^ivER T H E LAST 25 YEARS, t h e elec-
^ ^ tron microscope has emerged a s a major device for the determination of the structure of matter. M a n y recent developments in biology and metallurgy would have been inconceivable without this powerful tool. F o r all its success, electron microscopy has been beset b y a number of fundamental limitations. Particularly, in reaching for the goal of visualizing single atoms or small molecules, there have been four substantial problems: insufficient image contrast, specimen damage, i n sufficient resolution, and irregularities in t h e specimen support film. T h e problem of resolution has been attacked with a vehemence not afforded the others, perhaps because it was t h e first recognized. This attack and i t s successes have been well reported elsewhere (1). Conventional techniques now allow 2-3 Â point-to-point resolution, sufficient for m a n y molecular structure investigations. T h e problem of specimen damage owing t o t h e electron beam is t o a great extent unsolved. Indeed, t h e extent and n a ture of t h e damage a r e just beginning t o be assessed. Primarily, we will deal with progress in t h e other two problems, specimen support films and image contrast. Advantages of Graphite Crystallite Specimen Support Films
Specimens are carried into the vacuum system of t h e electron microscope after mounting on thin support films. These must be as transparent t o electrons as possible, stable in t h e electron
beam, and sufficiently good conductors of heat and electricity t o avoid local accumulation of charges or excessive temperature during irradiation. F i nally, t h e film should b e free of irregular structure t o assure t h a t details of the specimen are not obscured b y t h e superimposed structure of t h e support. Until now t h e most successful supports were thin films of carbon produced b y evaporation in vacuo onto a smooth surface {2). Such a procedure is bound to lead to variable thickness. This is indeed found; these supports have a n irregular structure which leads to a granular image. This granularity sets t h e limit on t h e minimum mass which can be detected. Recently, it h a s been found t h a t evaporated carbon films can b e converted t o films consisting of crystallites of graphite b y subjecting t h e m t o a brief heat t r e a t m e n t a t 2700°C in a n atmosphere of argon (3). These crystallites a r e easily recognizable, as shown in Figure 1. T h e high temperatures require t h a t t h e films be mounted on carbon support grids instead of t h e more usual copper ones. On examination in t h e electron microscope, the graphite crystallites are conspicuously free of t h e disturbing granularity found in evaporated carbon films. Then t h e limiting granularity of support films seems largely eliminated in t h e graphitization. T h e decrease in granularity is indicated in Figure 2. Here, t h e intensity of t h e elastically scattered electrons is sampled repeatedly, showing a scan of a graphite film and a carbon film. Since this intensity
:.
pP Figure 1. Dark field electron micrograph of thin graphite film produced by high temperature heat treatment of carbon film Crystallites 0.1 to 0.2 μνη in size are clearly visible. 50.000X
ANALYTICAL CHEMISTRY, VOL. 4 4 , NO. 1 , JANUARY 1972
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