C. F. Quate

Synthesis of individual single-walled carbon nanotubes on patterned silicon wafers

Recent progress in the synthesis of high-quality single-walled carbon nanotubes (SWNTs) has enabled the measurement of their physical and materials properties. The idea that nanotubes might be integrated with conventional microstructures to obtain new types of nanoscale devices, however, requires an ability to synthesize, isolate, manipulate and connect individual nanotubes. Here we describe a strategy for making high-quality individual SWNTs on silicon wafers patterned with micrometre-scale islands of catalytic material. We synthesize SWNTs by chemical vapour deposition of methane on the patterned substrates. Many of the synthesized nanotubes are perfect, individual SWNTs with diameters of 1–3 nm and lengths of up to tens of micrometres. The nanotubes are rooted in the islands, and are easily located, characterized and manipulated with the scanning electron microscope and atomic force microscope. Some of the SWNTs bridge two metallic islands, offering the prospect of using this approach to develop ultrafine electrical interconnects and other devices.

Microfabrication of cantilever styli for the atomic force microscope

Atomic force microscopy (AFM) is a newly developed high resolution microscopy technique which is capable of mapping forces near surfaces or, by means of these forces, the topography of the surface itself. In one mode of operation, AFM can resolve individual atoms on both conducting and insulating surfaces. A crucial component for the AFM is a flexible force‐sensing cantilever stylus, whose properties should include, among other things: a sharp tip, a low force constant, and a high mechanical resonance frequency. These requirements can be met by reducing the size of the cantilever stylus through microfabrication techniques and employing novel methods to construct a sharp tip. Presented here are a number of microfabrication processes for constructing cantilever styli with properties ideal for the AFM. These fabrication processes include (1) a method for producing thin film SiO2 or Si3N4 cantilevers without tips, (2) a method for producing Si3N4 cantilevers with integrated pyramidal tips formed by using an e...

Atomic resolution with an atomic force microscope using piezoresistive detection

A new detection scheme for atomic force microscopy (AFM) is shown to yield atomic resolution images of conducting and nonconducting layered materials. This detection scheme uses a piezoresistive strain sensor embedded in the AFM cantilever. The cantilever is batch fabricated using standard silicon micromachining techniques. The deflection of the cantilever is measured directly from the resistance of the piezoresistive strain sensor without the need for external deflection sensing elements. Using this cantilever we achieved 0.1 Arms vertical resolution in a 10 Hz–1 kHz bandwidth.

Forces in atomic force microscopy in air and water

A new atomic force microscope, which combines a microfabricated cantilever with an optical lever detection system, now makes it possible to measure the absolute force applied by a tip on a surface. This absolute force has been measured as a function of distance (=position of the surface) in air and water over a range of 600 nm. In the absolute force versus distance curves there are two transitions from touching the surface to a total release in air caused by van der Waals interaction and surface tension. One transition is due to lifting off the surface; the other is due to lifting out of an adsorbed layer on the surface. In water there is just one transition due to lifting off the surface. There is also a transition in air and water when the totally released tip is pulled down to touch the surface as the surface and tip are brought together. Based on the force versus distance curves, we propose a procedure to set the lowest possible imaging force. It can now be as low as 10−9 N or less in water and 10−7 N...

Atomic Resolution with Atomic Force Microscope

The atomic force microscope (AFM) is a promising new method for studying the surface structure of both conductors and insulators. In mapping a graphite surface with an insulating stylus, we have achieved a resolution better than 2.5 A.

Parallel atomic force microscopy using cantilevers with integrated piezoresistive sensors and integrated piezoelectric actuators

We have fabricated and operated two cantilevers in parallel in a new mode for imaging with the atomic force microscope (AFM). The cantilevers contain both an integrated piezoresistive silicon sensor and an integrated piezoelectric zinc oxide (ZnO) actuator. The integration of sensor and actuator on a single cantilever allows us to simultaneously record two independent AFM images in the constant force mode. The ZnO actuator provides over 4 μm of deflection at low frequencies (dc) and over 30 μm deflection at the first resonant frequency. The piezoresistive element is used to detect the strain and provide the feedback signal for the ZnO actuator.

An atomic force microscope tip designed to measure time-varying nanomechanical forces

Tapping-mode atomic force microscopy (AFM), in which the vibrating tip periodically approaches, interacts and retracts from the sample surface, is the most common AFM imaging method. The tip experiences attractive and repulsive forces that depend on the chemical and mechanical properties of the sample, yet conventional AFM tips are limited in their ability to resolve these time-varying forces. We have created a specially designed cantilever tip that allows these interaction forces to be measured with good (sub-microsecond) temporal resolution and material properties to be determined and mapped in detail with nanoscale spatial resolution. Mechanical measurements based on these force waveforms are provided at a rate of 4 kHz. The forces and contact areas encountered in these measurements are orders of magnitude smaller than conventional indentation and AFM-based indentation techniques that typically provide data rates around 1 Hz. We use this tool to quantify and map nanomechanical changes in a binary polymer blend in the vicinity of its glass transition.

Optical antennas: Resonators for local field enhancement

Electromagnetic field enhancement in optical antenna arrays is studied by simulation and experiment at midinfrared wavelengths. The optical antennas are designed to produce intense optical fields confined to subwavelength spatial dimensions when illuminated at the resonant wavelength. Finite difference time domain (FDTD) method simulations are made of the current, charge, and field distributions in the antennas. The influence of antenna shape, length, and sharpness upon the intensity of the optical fields produced is found. Optical antennas arrays are fabricated on transparent substrates by electron beam lithography. Far-field extinction spectroscopy carried out on the antenna arrays shows the dependence of the resonant wavelength on the antenna length and material. The FDTD calculated and experimentally measured extinction efficiencies of the optical antennas are found to be in good agreement.

Atomic resolution imaging of a nonconductor by atomic force microscopy

We have demonstrated the capability of the atomic force microscope to image the surface of an electrically insulating solid with atomic resolution. Images of highly oriented pyrolytic boron nitride taken in air show atomic corrugations with a lateral resolution better than 3 A. Low‐noise images of graphite and molybdenum disulfide are also presented.

Acoustic microscopy with mechanical scanning—A review

Acoustic waves in liquids are known to have wavelengths comparable to that of visible light if the frequency is in the gigahertz range. The phenomena of Brillouin scattering in liquids is based on such waves. In helium near 2 K acoustic waves with a wavelength of 2000 Å were studied some ten years ago at UCLA. It follows from these observations that an imaging system based on acoustic radiation with a resolving power competitive with the optical microscope is within reach if an ideal lens free from aberrations could be found. Such a lens, which was so elusive at the beginning, is now a simple device and it is the basic component in the acoustic microscope that forms the basis for this review. In this article we will establish the characteristic properties of this new instrument. We will review some of the simple properties of acoustic waves and show how a single spherical surface formed at a solid liquid interface can serve as this ideal lens free from aberrations and capable of producing diffraction limited beams. When this is incorporated into a mechanical scanning system and excited with acoustic frequencies in the microwave range images can be recorded with acoustic wavelengths equal to the wavelength of visible light. We will present images that show the elastic properties of specimens selected from the fields of material science, integrated circuits, and cell biology. The information content in these images will often exceed that of the optical micrographs. In the reflection mode we illuminate the smooth surface of a crystalline material with a highly convergent acoustic beam. The reflected field is perturbed in a unique way that is determined by the elastic properties of the reflecting surface and it shows up in the phase of the reflected acoustic field. There is a distinct and characteristic response at the output when the spacing between the object and the lens is varied. This behavior in the acoustic ieflection microscope provides a rather simple and direct means for monitoring the elastic parameters of a solid surface. It is easy to distinguish between different materials, to determine the layer thickness, and to display variations in the elastic constants on a microscopic scale. These features lead us to believe there is a promising future for the field of acoustic microscopy.