This article reviews the progress of atomic force microscopy in ultrahigh vacuum, starting with its invention and covering most of the recent developments. Today, dynamic force microscopy allows us to image surfaces of conductors and insulators in vacuum with atomic resolution. The most widely used technique for atomic-resolution force microscopy in vacuum is frequency-modulation atomic force microscopy (FM-AFM). This technique, as well as other dynamic methods, is explained in detail in this article. In the last few years many groups have expanded the empirical knowledge and deepened our theoretical understanding of frequency-modulation atomic force microscopy. Consequently spatial resolution and ease of use have been increased dramatically. Vacuum atomic force microscopy opens up new classes of experiments, ranging from imaging of insulators with true atomic resolution to the measurement of forces between individual atoms.

https://hep.physics.utoronto.ca/WilliamTrischuk/courses/phy189/AFM_revmodphys.pdf

Advances in atomic force microscopy

Biomolecular motors such as F 1 –adenosine triphosphate synthase (F 1 -ATPase) and myosin are similar in size, and they generate forces compatible with currently producible nanoengineered structures. We have engineered individual biomolecular motors and nanoscale inorganic systems, and we describe their integration in a hybrid nanomechanical device powered by a biomolecular motor. The device consisted of three components: an engineered substrate, an F 1 -ATPase biomolecular motor, and fabricated nanopropellers. Rotation of the nanopropeller was initiated with 2 mM adenosine triphosphate and inhibited by sodium azide.

https://science.sciencemag.org/content/sci/290/5496/1555.full.pdf

Powering an Inorganic Nanodevice with a Biomolecular Motor

Field emission (FE) is based on the physical phenomenon of quantum tunneling, in which electrons are injected from the surface of materials into vacuum under the influence of an applied electric field. A variety of field emission cold cathode materials have been developed to date. In this review, we shall focus on several kinds of novel cold cathode materials that have been developed in the past decade. These include materials for microfabricated field-emitter arrays, diamond and related films, carbon nanotubes, other quasi one-dimensional nanomaterials and printable composite materials. In addition, cold cathode materials have a wide range of applications such as in flat panel displays, high-power vacuum electronic devices, microwave-generation devices, vacuum microelectronic devices and vacuum nanoelectronic devices. Applications are in consumer goods, military industries and also space technology. A comprehensive overview of the various applications is presented. Recently, recognizing the strong possibility that vacuum nanoelectronic devices using quasi one-dimensional nanomaterials, such as carbon nanotubes may emit electrons with driving voltages comparable to that of a solid-state device, there is a growing interest in novel applications of such devices. With such exciting opportunities, there is now a flurry of activities to explore applications far beyond those considered for the conventional hot cathodes that operate on thermionic emission. We shall discuss the details of a number of fascinating potential applications.

Novel cold cathode materials and applications

Well-aligned arrays of ZnO nanoneedles were fabricated using a simple vapor phase growth. The diameters of the nanoneedle tips are as small as several nanometers, which is highly in favor of the field emission. Field-emission measurements using the nanoneedle arrays as cathode showed emission current density as high as 2.4 mA/cm2 under the field of 7 V/μm, and a very low turn-on field of 2.4 V/μm. Such a high emission current density is attributed to the high aspect ratio of the nanoneedles. The high emission current density, high stability, and low turn-on field make the ZnO nanoneedle arrays one of the promising candidates for field-emission displays.

Efficient field emission from ZnO nanoneedle arrays

In this review/tutorial paper, we cover the history, physics, and current status of vacuum microelectronic devices. First we overview the performance requirements of vacuum microelectronic devices necessary for them to replace, or fill voids left by, solid state devices. Next we discuss the physical characteristics of micro-field-emission sources important to device applications. These characteristics include fundamental features, such as current-voltage data and noise, in addition to engineering considerations, such as life expectancy and procedures for tube assembly. We conclude with a review of a wide variety of demonstrated and proposed devices based on vacuum microelectronic principles, including electron guns, microwave tubes, and flat-panel displays. >

Vacuum microelectronic devices

Electron emitters in vacuum microelectronic devices need sharp tips in order to permit electron emission at moderate voltages A method has been found for preparing uniform silicon tips with a radius of curvature less than 1 nm These tips are formed by oxidation of 5‐μm‐high silicon cones through exploitation of a known oxidation inhibition of silicon at regions of high curvature

Formation of silicon tips with <1 nm radius

Electron emission characteristics for the cone and wedge generic field emitter structures are described. Effects of variations in emitter geometry on electron current, spectral and temporal dispersion of electron emission, and emitter heating are calculated analytically and by computer simulation. Several guidelines for emitter design are suggested. >

Simulation and design of field emitters

A method is described for forming atomically sharp silicon tips of less than 10-15 degrees half-angle by utilizing a known oxidation inhibition at regions of high curvature; equally sharp silicon wedges are now made in a similar fashion. The sharp silicon tips serve as the starting point for forming sharp tips of W, beta -W and gold. Field emission data from silicon emitters are compared with Fowler-Nordheim modelling and emission as a function of emitter-anode distance is described. >

Atomically sharp silicon and metal field emitters

Field emission of an emitter covered with a very thin oxide layer is modeled and calculated numerically. The additional barrier due to the oxide layer is included in the tunneling problem by using the standard WKB method. The following physical parameters are considered: the oxide barrier height, the oxide conduction band-edge electron effective mass, and the oxide dielectric constant. Compared with the Fowler-Nordheim equation, which was derived for a clean metallic emitter, the calculation shows a reduction of the emission current density from an emitter covered with an oxide layer a few monolayers thick. After reaching a minimum emission current density, the emission increases when the thickness of the oxide layer increases further. Finally, the emission current density is saturated and stabilized. >

Electron field emission through a very thin oxide layer

The design criteria for two forms of field emitters (cone and wedge) for vacuum microelectronic applications are discussed. Effects of practical variations in geometry on emission current, spatial and temporal dispersion of electron emission, and emitter heating are calculated by simulation. Several design guidelines for the geometry of the emitter–anode complex are suggested. A sharp silicon tip processing method is presented, which is based on the oxidation inhibition of silicon at regions of high curvature. Methods for forming emitters of other materials are also discussed.

R. B. Marcus

Papers

Formation of silicon tips with <1 nm radius

Simulation and design of field emitters

Atomically sharp silicon and metal field emitters

Electron field emission through a very thin oxide layer

Field emitter tips for vacuum microelectronic devices