Virgil Elings

Review of scanning force microscopy

We present an overview of scanning force microscopy with applications to electrostatic, magnetostatic, and atomic forces operating in the contact and noncontact mode, and highlight the main achievements in this field.

Noncontact nanolithography using the atomic force microscope

We have demonstrated that the atomic force microscope (AFM) operating in air may be used to pattern narrow features in resist in a noncontact lithography mode. A micromachined AFM cantilever with an integrated silicon probe tip acts as a source of electrons. The field emission current from the tip is sensitive to the tip-to-sample spacing and is used as the feedback signal to control this spacing. Feature sizes below 30 nm have been patterned in 65-nm-thick resist and transferred through reactive ion etching into the silicon substrate. We show that the same AFM probe used for noncontact patterning can be used to image the sample. In addition to eliminating the problem of tip wear, this noncontact system is easily adapted to multiple-tip arrays where each cantilever has an integrated actuator to adjust the probe height.

Creating And Observing Surface Features With A Scanning Tunneling Microscope

Scanning Tunneling Microscopy (STM) can be used to perform lithography on a surface and to image the results of lithography performed using other techniques. We have imaged a 1 nm structure created with the STM on graphite, a 300 nm structure electrodeposited with the STM on gold, and a gold diffraction grating created using a diamond scribe. In addition we present atomic resolution images of graphite showing a surface reconstruction.

Scanning tunneling microscopy of cytoskeletal proteins: Microtubules and intermediate filaments

Direct STM observation of native biomolecules has proven feasible. In this study we have used STM to image two filamentous protein components of the intracellular cytoskeleton: microtubules (MT) and intermediate filaments (IF). MT are 25 nm diameter cylinders comprised of 13 ‘‘protofilaments’’ which are linear chains of 8 nm × 4 nm × 4 nm ‘‘dimer’’ subunits. More variable than MT, IF are 10 nm diameter coils comprised of from 4 to 8 ‘‘subfilaments’’ which are chains of ‘‘tetramer’’ subunits. MT were isolated from porcine brain and IF from cell culture by standard techniques. Preparation/stabilization factors included magnesium buffer, 0.8 molar glycerol, and 0.1% glutaraldehyde. Samples were scanned on graphite in a Nanoscope I or II STM (Digital Instruments, Santa Barbara, California). STM images of MT demonstrated flattened 25 nm diameter structures composed of 4 nm wide protofilaments. Processed inverted images showed rows of 8 nm × 4 nm subunits. STM images of IF showed flattened, parallel 10 nm filam...

Improved atomic force microscope using a laser diode interferometer

The performance of an atomic force microscope using a laser diode interferometer has been improved to the point where its resolution is comparable to that of laser beam deflection systems. We describe the structure of this microscope, present a model that takes into account the main parameters associated with its operation, and demonstrate its sensitivity by showing images of a small area scan with atomic resolution as well as a large area scan in a stand‐alone configuration.

Recent developments in profiling optical surfaces

To understand better the fabrication of optical surfaces and to be able to produce smoother, lower-scatter surfaces, we need to extend characterization techniques to shorter and longer surface spatial wavelengths beyond the conventional 1-µm to 1-mm region. Scanning probe microscopes are now available for profiling optical surfaces with height and lateral resolutions of a few atomic spacings. In this paper we report on measurements made with a Nanoscope II atomic force microscope on a variety of supersmoothoptical surfaces and compare these results with measurements made with a conventional stylus profiling instrument. Consistent results have been obtained. To cover the long-spatial-wavelength region, long-scan profilers can be used to measure surface waviness in the range from a few millimeters to a few centimeters with height sensitivities ~10 times better than conventional interferometers. The characterization of a Nanostep long-scan mechanical profiler is described, and examples of surface profiles taken on selected flat samples are given.

Scanning tunneling microscopy and atomic force microscopy visualization of the components of the skeletal muscle glycogenolytic complex

The muscle glycogenolytic complex is responsible for providing access to the reserve carbohydrate energy stores in skeletal muscle during times of vigorous exercise. The complex is a set of enzymes and regulatory factors that are bound to the carbohydrate storage polymer, glycogen. These components provide the ordered synthesis and utilization of that stored form of glucose. Glycogen and the enzyme proteins, phosphorylase and phosphorylase kinase, have been imaged by atomic force microscopy (AFM) or scanning tunneling microscopy (STM). The images of all three generally correlated well with the known features of those molecules, as measured by traditional physicochemical methods. The exception for all three polymers is that the measured height by STM is in error. In each case, the molecules appear to be only about 30% of their true thickness, as measured by height above the graphite surface. It is clear that both AFM and STM will play important roles in biomedical investigation of macromolecular structures...

Direct observations of enzymes and their complexes by scanning tunneling microscopy

Scanning tunneling microscopy (STM) has been used as a method of studying the relationships between the enzymes of muscle glycogenolysis. In skeletal muscles the activation of phosphorylase b is catalyzed by phosphorylase kinase. This interaction is believed to occur in vivo as part of a multienzyme complex. The molecular structures of phosphorylase b and phosphorylase kinase have been visualized by STM.1 Phosphorylase b can be seen in dimeric and tetrameric forms as well as linear and circular aggregates. Individual molecules of phosphorylase kinase image as planar, bilobate structures with a twofold axis of symmetry and a central depression. STM has also been used to visualize complexes between phosphorylase kinase and its substrate, phosphorylase b.

Observation of phosphorylase kinase and phosphorylase b at solid–liquid interfaces by scanning tunneling microscopy

Enzymes can be immobilized at a solid–liquid interface and observed with a scanning tunneling microscope (STM) using the inherent charge of proteins and an external potential applied to the STM substrate. Phosphorylase kinase, and dimers and oligomers of phosphorylase b have been observed at the interface of aqueous solutions and highly oriented pyrolytic graphite (HOPG). The lateral dimensions of phosphorylase kinase determined by STM at the solid–liquid interface are from 74%–78% of the dimensions determined by STM at the solid–air interface [Biochem. 28, 4939 (1989); Biophys. J. (in press)]. The phosphorylase b lateral dimensions of both enzymes are between 1.9 and 2.5 nm greater than the dimensions determined by x‐ray crystallography. The vertical dimensions determined by STM at the solid–liquid and solid–air interfaces are in reasonable agreement with each other. Mixtures of the two enzymes show aggregates in which the complexes of the two enzymes are identifiable. This technique will make it possibl...