Tip Contamination Research Articles

Subfibrillar structure of type I collagen observed by atomic force microscopy

We have imaged native rat tail and reconstituted bovine dermal type I collagen by atomic force microscopy, obtaining a level of detail comparable to that obtained on the same samples by transmission electron microscopy. The characteristic 60-70 nm D periodicity consists of ridges exhibiting high tip-sample adhesion alternating with 5-15-nm-deep grooves having low adhesion. We also observe an intraperiod or "minor" band consisting of 1-nm-deep grooves, and "microfibrils" arranged parallel to or inclined approximately 5 degrees to the fibril axis. In air collagen fibrils exhibit negligible compression under the forces exerted by the tip. When immersed in water the subfibrillar features disappear and the fibrils become softer, compressing by 5% of their height under an 11-nN force. Material on the surface of the sample sometimes accumulates on the atomic force microscope tip; contrary to expectation such tip contamination can improve as well as reduce resolution.

Characterization of atomic force microscope tips by adhesion force measurements

The resolution limit in an atomic force microscope image usually is attributed to the finite radius of the contacting probe. Here, it is shown that this assumption is valid only when adhesion forces are minimal. Relative to the tip-imposed geometrical limit, the resolution and contrast in AFM images can be degraded by increasing adhesion forces. The large adhesion forces observed for some tips at low humidity conditions are shown to be due to tip contamination or poorly formed tip apexes. Methods to determine and to reduce the extent of tip contamination are described. Cleaning carried out using UV-ozone or oxygen-plasma etching were found to significantly reduce the minimum adhesion force.

BEEM spectroscopy at interfaces of Au, Ag, Cu, Mg and Ni films with n-GaP(110)

A systematic investigation with ballistic-electron emission microscopy (BEEM) of local Schottky-barrier heights for ≅ 100 Å thick films of Au, Ag, Cu, Mg and Ni evaporated onto cleaved n-GaP(110) surfaces is presented. It is shown that the onset of the BEEM current follows a 52 power law, thermally broadened by the tip Fermi function. Schottky-barrier heights are found to be 1.41 ± 0.02 eV (Au), 1.27 ± 0.02 eV (Ag), 1.21 ± 0.03 eV (Cu), 1.03 ± 0.02 eV (Mg) and 1.06 ± 0.06 eV (Ni). In addition, the influence of varying injection geometry and tip contamination on the BEEM spectra is discussed.

High-speed, large-scale imaging with the atomic force microscope

The atomic force microscope (AFM) can be used to generate both images and height profiles of samples with micron-sized features, combining the resolution and speed of the scanning electron microscope and the height information of the stylus profilometer. We have increased the scan speed of our AFM so that these images may be taken in real time, allowing the operator to interact easily with the microscope parameters. The increased scan speed does not noticeably degrade the images. The resulting images contain complete height information, allowing height profiles to be extracted. In this paper, we present images taken in this manner as well as discuss some of the nonideal characteristics of the AFM, such as tip-shape artifacts, tip contamination, sample damage, and scanner nonlinearities.

Tip contamination effects in ambient pressure scanning tunneling microscopy imaging of graphite: T Tiedje et al, J Vac Sci Technol, A6, 1988, 372–375

Tip contamination effects in ambient pressure scanning tunneling microscopy imaging of graphite: T Tiedje et al, J Vac Sci Technol, A6, 1988, 372–375

Tip contamination effects in ambient pressure scanning tunneling microscopy imaging of graphite

We have used electrical noise measurement and transmission electron microscopy to study the condition of metal tips used to obtain contrast current mode scanning tunneling microscope (STM) images of pyrolytic graphite in air. Electrochemically and mechanically sharpened tips of W, W0.97Re0.03, Pt, and Pt0.8Ir0.2, which gave giant corrugations in graphite images, were found to have various forms of carbonaceous contamination on the tip. Contamination in the form of liquid hydrocarbon, graphite clumps, and carbon fibers were observed. Tunneling current noise spectra from these tips exhibited a characteristic 1/f behavior from 10 Hz to 10 kHz. Clean atomic-resolution tips formed by sputtering thin layers of metal (Ni, Au, Pt) on prefabricated tips were found to exhibit lower noise in the tunneling current and ≤1-Å amplitude corrugations. Our interpretation of the present experiment, while similar to earlier explanations that associate the giant corrugations with contamination, differs in that we attribute the large corrugations to deformation of relatively soft carbonaceous material attached to the tip, rather than to elastic deformation of the graphite itself. In addition, we have found that several of the structural artifacts observed in atomic-resolution STM images of graphite can be explained by a moiré effect caused by a double tip.