Metallic probe tips

Other hand, additional signal enhancement can be expected in the case of apertureless type probes, especially when the probes are made of metal. This additional signal enhancement is specifically expected when the tip diameter becomes smaller, which corresponds to higher spatial resolution. In the next section, we discuss the mechanism of the field enhancement effect of a metallic probe tip. 

B. A Metallic Probe Tip Enhances the Nanolight Source Tip-Enhancement Effect... 

FIGURE 10.3 Quantized oscillation of electrons at the surface of a metallic probe tip. This is so called surface plasmon. As the charge distribution is confined tightly at the sharp tip end, the subsequent electric field at the tip is strongly enhanced. 

It has been shown that a metallic probe tip highly localizes and strongly amplifies the optical field throngh the resonance effect of the plasmon polaritons at the probe tip. This concept has made it possible to optically observe nanometric samples with a nanometric spatial resolntion. 

Inouye, Y, and Kawata, S. 1994. Near-field scanning optical microscope using a metallic probe tip. Opt. Lett. 19 159-61. 

Roth RM, Panoiu NC, Adams MM, Osgood RM, Neacsu CC, Raschke MB (2006) Resonant-plasmon field enhancement from asymmetrically illuminated conical metallic-probe tips. Opt Exp 14 2921... 

When this chapter was written, there were only limited fluorescence lifetime data for single chromophores available, which had been measured with a near-field probe. The systems were similar to dye molecules dispersed either on a silica surface or on a PMMA surface. Of major concern was the perturbation of the fluorescence process by the presence of the metalized probing tip, which can lead to an alteration of the emission lifetime, to fluorescence quenching, and ultimately may limit the detection sensitivity and the spatial resolution of near-field fluorescence microscopy. The results are partly controversial, and more systematic work is required to explore the sensitivity limits, the resolution limits, and to elucidate the underlying physical processes. 

Semiconductors have greatly varying densities of states and thus contributions from metal probe tips are less prominent. Metal surface state densities vary to a much smaller degree and are thus comparable to those of the tip states, making electronic spectroscopy of metals more complicated. To enable comparison between spectra obtained at various surface positions it is important that the tip structure, and thus density of states remains constant between measurements. Rearrangement of the tip apex can greatly affect the observed spectra and thus lead to spurious data interpretation. 

There are many other experiments in which surface atoms have been purposely moved, removed or chemically modified with a scanning probe tip. For example, atoms on a surface have been induced to move via interaction with the large electric field associated with an STM tip [78]. A scaiming force microscope has been used to create three-dimensional nanostructures by pushing adsorbed particles with the tip [79]. In addition, the electrons that are tunnelling from an STM tip to the sample can be used as sources of electrons for stimulated desorption [80]. The tuimelling electrons have also been used to promote dissociation of adsorbed O2 molecules on metal or semiconductor surfaces [81, 82]. 

In scanning electrochemical microscopy (SECM) a microelectrode probe (tip) is used to examine solid-liquid and liquid-liquid interfaces. SECM can provide information about the chemical nature, reactivity, and topography of phase boundaries. The earlier SECM experiments employed microdisk metal electrodes as amperometric probes [29]. This limited the applicability of the SECM to studies of processes involving electroactive (i.e., either oxidizable or reducible) species. One can apply SECM to studies of processes involving electroinactive species by using potentiometric tips [36]. However, potentio-metric tips are suitable only for collection mode measurements, whereas the amperometric feedback mode has been used for most quantitative SECM applications. 

Unlike electrostatic forces, chemical forces between the probing tip and the probed surface have been shown to profoundly affect the tunneling current from a certain onset. Owing to the advent of first-principle methods and powerful computers, it could finally be resolved by a calculation of the combined tip-sample system [ 15 ]. The point of onset for chemical bonding on metals was found to be at a distance of 4—5 A. As the tip approaches the surface, chemical forces rapidly become large enough to... 

AFM has been used to image surfaces by probing both the attractive and repulsive forces experienced by the tip as a result of its proximity to the sample surface. In both modes, the probe tip is mounted on a cantilever spring. Three main designs have been employed metal foil with a splinter of diamond, a shaped tungsten wire that acts both as spring and tip, and microfabricated tip/cantilever composites. 

Another type of break junction, the in situ or STM break junction (STM-BJ), involves pushing an STM tip through a SAM to make contact with the underlying electrode (Fig. 5). The probe tip is then quickly withdrawn, stretching a bridge of metal atoms until the bridge breaks. As the gap widens, SAM molecules can fill it, giving distinctive current flows, until the gap becomes too wide for molecules to... 

The most straightforward tool for the introduction of a sample into a mass spectrometer is called the direct inlet system. It consists of a metal probe (sample rod) with a heater on its tip. The sample is inserted into a cmcible made of glass, metal, or silica, which is secured at the heated tip. The probe is introduced into the ion source through a vacuum lock. Since the pressure in the ion source is 10-5 to 10-6 torr, while the sample may be heated up to 400°C, quite a lot of organic compounds may be vaporized and analyzed. Very often there is no need to heat the sample, as the vapor pressure of an analyte in a vacuum is sufficient to record a reasonable mass spectrum. If an analyte is too volatile the cmcible may be cooled rather than heated. There are two main disadvantages of this system. If a sample contains more than one compound with close volatilities, the recorded spectrum will be a superposition of spectra of individual compounds. This phenomenon may significantly complicate the identification (both manual and computerized). Another drawback deals with the possibility of introducing too much sample. This may lead to a drop in pressure, ion-molecule reactions, poor quality of spectra, and source contamination. 

The relative simplicity and low cost of STM instrumentation has contributed significantly to the rapid increase in the number of in situ electrochemical studies performed over the last decade. An excellent discussion of the general aspects of STM design and construction is available in a recent textbook [39], Beyond instrumentation, insightful experiments depend on the preparation of a flat, well-defined substrate and the formation of a stable tip capable of atomically resolved imaging. In this sense, the ability to reliably produce high-quality noble metal electrodes outside UHV has been central to the success of many STM studies [145-148]. In contrast, our knowledge of the structure, chemistry, and operation of the probe tip may be more aptly viewed as an art form. 

For the probe system, whatever design of horn is used, a large maximum power density can be achieved at the radiating tip. This can be of the order several hundred W cm . The working frequencies are normally of the order 20 - 40 kHz. A number of probe devices are commercially available and, up to a few years ago (before the advent of sonochemistry) were referred to as cell disrupters. The majority operate at 20 kHz and utilise a wide range of different metal probes. The advantages of the probe method of energy input are threefold ... 

When noncontacting vibration probes are furnished in aecordance with 3.4.3.1, the rotor shaft sensing areas to be observed by radial vibration probes shall be concentric with the bearing journals. All shaft sensing areas (both radial vibration and axial position) shall be free from stencil and scribe marks or any other surface discontinuity, such as an oil hole or a keyway, for a minimum distance of one probe tip diameter on each side of the probe. These areas shall not be metallized. 

When a metallic probe, which has a nanometric tip, is illuminated with an optical field, conductive free electrons collectively oscillate at the surface of the metal (Figure 10.3). The quantum of the induced oscillation is referred to as surface plas-mon polariton (SPP) (Raether 1988). The electrons (and the positive charge) are concentrated at the tip apex and strongly generate an external electric field. Photon energy is confined in the local vicinity of the tip. Therefore, the metallic tip works as a photon reservoir. 

Solid samples that have a sufficient vapour pressure at 300 °C are deposited on the tip of a heated metal probe which is then inserted into the instrument through a vacuum lock. With some ionisation methods, the solid sample is mixed with a liquid matrix (e.g. glycerol or benzoic acid). 

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