Fabrication of [001]-oriented tungsten tips for high resolution scanning tunneling microscopy (original) (raw)
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e-Journal of Surface Science and Nanotechnology, 2007
We present a method to prepare tungsten tips for use in multi-tip scanning tunneling microscopes. The motivation behind the development comes from a requirement to make very long and conical-shape tips with controlling the cone angle. The method is based on a combination of a "drop-off" method and dynamic electrochemical etching, in which the tip is continuously and slowly drawn up from the electrolyte during etching. Its reproducibility was confirmed by scanning electron microscopy. Comparison in tip shape between the dynamic and static methods was shown.
Tip preparation for usage in an ultra-low temperature UHV scanning tunneling microscope
Science and Technology of Advanced Materials, 2007
This work deals with the preparation and characterization of tungsten tips for the use in UHV low-temperature scanning tunneling microscopy and spectroscopy (STM and STS, respectively). These specific environments require in situ facilities for tip conditioning, for further sharpening of the tips, as well as for reliable tip characterization. The implemented conditioning methods include direct resistive annealing, annealing by electron bombardment, and self-sputtering with noble gas ions. Moreover, results from in situ tip characterization by field emission and STM experiments were compared to ex situ scanning electron microscopy. Using the so-prepared tips, high resolution STM images and tunneling spectra were obtained in a temperature range from ambient down to 350 mK, partially with applied magnetic field, on a variety of materials. r
Physical Review B, 2005
Detailed knowledge of the tip apex structure is necessary for quantitative comparison between theory-based simulations and experimental observations of tip-substrate interactions in scanning probe microscopy ͑SPM͒. Here, we discuss field ion microscopy ͑FIM͒ techniques to characterize and atomically define SPM tungsten tips. The tip radius can be estimated from field emission data, while FIM imaging allows the full atomic characterization of the tip apex. We find that when FIM is applied to tips with a radius of a few nanometers ͑as is desirable for high-resolution atomic force microscopy imaging͒, limitations not apparent with less sharp tips arise; successful resolution of these limitations will extend the utility of FIM. Field evaporation can be used to atomically engineer the apex into a desired atomic configuration. Starting from a W͑111͒ wire, a tip terminating in three atoms can reproducibly be fabricated; due to its geometry and stability, this apex configuration is well suited for application as an atomically defined electrical contact in SPM experiments aimed at understanding contact mechanics at the atomic scale.
Physical Review B, 2011
Scanning tunneling microscopy using W tips functionalized with transition-metal (Cr, Fe) coatings is demonstrated to facilitate the imaging of states within a few millielectron volts (meV) of the Fermi level for epitaxial graphene on SiC(0001), which are not accessible with bare W tips. First-principles modeling of these probe tips as pyramidal structures on W(110) indicates that an apex atom is stable for the Cr/W(110) tip but not for the Fe/W(110) or W/W(110) tips. This difference in their atomic structures, together with the variation in the extent of the 3d and 5d orbitals, is found to be responsible for the capability of Cr and Fe functionalized tips to selectively image the complex electronic properties of epitaxial graphene on SiC(0001).
A scanning tunneling microscopy tip with a stable atomic structure
Metals and Materials International, 2004
A single stable adatom on a {110}-type plane of a tungsten tip is created via field-evaporation in a fieldion microscope (FIM) operating at room temperature. This single adatom has sufficient surface mobility at room temperature and migrates, in one-dimension, along a <111>-type direction toward an edge of a {110}-type plane, due to the existence of an electric field gradient. The plane edge has a higher local electric field than its center, since it has a higher local geometric curvature. This result implies that the stable position of a single adatom during a scan of a scanning tunneling microscope (STM) tip on a surface is at the edge and not at the center of a {110}-type plane at room temperature. Therefore, the electron wave function of a tip is not symmetric and this fact should be taken into account in a careful analysis of STM images. Also a tip with a dislocation emerging at a {110}-type plane is suggested as an improved STM tip configuration, as the step at the surface, created by the intersection of the dislocation with it, is a perpetual source of single adatoms.
Tip-Sample Interactions in the Scanning Tunneling Microscope for Atomic-Scale Structure Fabrication
Japanese Journal of Applied Physics, 1993
In a scanning tunneling microscope (STM) operated in ultra-high vacuum, if we place a well-prepared W tip above the Si(111)-7×7 surface at a separation of ∼1 nm and apply an appropriate voltage pulse to it, we can extract a single Si atom from a predetermined position routinely at room temperature. The extracted Si atoms are redeposited onto the surface with a certain probability, their positions always being at a fixed crystallographic site. The redeposited Si atoms can be displaced intentionally to other crystallographically equivalent sites. In case of the Si(001)-2×1 surface, usually two Si atoms forming a dimer are extracted together. For both surfaces, Si atoms at crystallographically different sites including step edges are extracted with different probabilities. The microscopic mechanisms of these processes are discussed.
Systematic investigations of nanostructuring by scanning tunneling microscopy
Journal of vacuum science & technology, 1996
Scanning tunneling microscopes allow the formation of structures, by the application of voltage pulses between tip and sample whose dimensions are in the range of some nanometers. Systematic investigations have been carried out on the Si͑111͒-7ϫ7 using W and Au tips in order to better understand the physics of the underlying deposition process. Therefore the voltage pulse, current, and z-piezo voltage were measured as a function of time. Above a threshold voltage, which depends on the pulse duration and the tunneling current, too, hills were created in the range between 5 and 25 nm. The surface structure was preserved up to the hills. Further, there is a linear dependence of the diameter on the pulse amplitude and pulse duration. A fit of the data gives a slope of ͑0.1Ϯ0.02͒ nm/s for tungsten tips and ͑0.45Ϯ0.14 nm/s͒ for gold tips, respectively. In addition, there is also a logarithmically dependence of the diameter upon the tunneling current for both tip materials. The results obtained are discussed with reference to the formation mechanisms published to date. None provide a satisfying explanation of our results.