Particle junctions

Amlani et al. [49] combined conventional photolithographic techniques with self-assembly aspects to form a metal-SAM-metal-SAM-metal junction. Au-covered electrodes with a separation of 40-100 nm were covered with SAMs of OPEs or alkanethiols and an Au nanoparticle (d = 40-100 nm) was trapped in the gap between the electrodes by applying an alternating-current bias. An illustration of the system is shown in Fig. 10.13. 

I(V) measurements on nitro-substituted OPE 84b revealed two NDR peaks at 1.3 and —1.7 V, as might be expected for two molecular junctions in a series. Analogous measurements on the unsubstituted OPE 95 showed no NDR peaks. 

A conceptually similar approach was recently developed by Kushmerick and coworkers [50], who used Au-covered magnetic particles to effect directed assembly into lithographically defined magnetic arrays that were functionalized with SAMs of molecule 91b. The magnetic particles were made by depositing first Ni and then Au on a silica microsphere. 

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A particularly important variant of the optical force, interparticle forces, turns out to be crucial for SERS. This effect is similar to the attractive van der Waals force between small particles, which is due to interactions between spontaneously fluctuating dipoles, but the optical interaction is due to coupling between the actual particle dipoles induced by the trapping laser. Due to the interparticle optical forces, metal nanoparticles aggregate in an optical tweezers and produce hotspots, i.e., particle junctions with intense local fields for SERS. Raman probes can be excited either by the trapping laser or, preferably, by a separate low power beam that does not disturb the trapping. 

Taking into account that Si2 exchanges mainly non-specific interactions [8], the variation of the surface functionality cannot explain such polymodality. But, on the other hand, it is a bulky molecule which is certainly sensitive to the surface geometry. Hence, the peak at high adsorption energy (32 kJ/mol) and the shoulder at moderate energy (30 kJ/mol) may be attributed to sites in which the probe may be more or less inserted in functions respectively between aggregates and between primary particles Junctions as depicted schematically on Fig. 10. 

At the same time, coupling nanoparticles together presents an opportunity to enhance and engineer near-fields. As an example, the assembly or aggregation of nanoparticles has been shown to result in huge SERS enhancements [35], The inter-particle junctions formed by the assembly of nanoparticles are sites for electric field... 

In these equations the electrostatic potential i might be thought to be the potential at the actual electrodes, the platinum on the left and the silver on the right. However, electrons are not the hypothetical test particles of physics, and the electrostatic potential difference at a junction between two metals is nnmeasurable. Wliat is measurable is the difference in the electrochemical potential p of the electron, which at equilibrium must be the same in any two wires that are in electrical contact. One assumes that the electrochemical potential can be written as the combination of two tenns, a chemical potential minus the electrical potential (- / because of the negative charge on the electron). Wlien two copper wires are connected to the two electrodes, the... 

An interesting example of a large specific surface which is wholly external in nature is provided by a dispersed aerosol composed of fine particles free of cracks and fissures. As soon as the aerosol settles out, of course, its particles come into contact with one another and form aggregates but if the particles are spherical, more particularly if the material is hard, the particle-to-particle contacts will be very small in area the interparticulate junctions will then be so weak that many of them will become broken apart during mechanical handling, or be prized open by the film of adsorbate during an adsorption experiment. In favourable cases the flocculated specimen may have so open a structure that it behaves, as far as its adsorptive properties are concerned, as a completely non-porous material. Solids of this kind are of importance because of their relevance to standard adsorption isotherms (cf. Section 2.12) which play a fundamental role in procedures for the evaluation of specific surface area and pore size distribution by adsorption methods. 

The explanation of low-pressure hysteresis proposed by Amell and McDermott some thirty years ago was formulated in terms of the swelling of the particles which accompanies adsorption. The swelling distorts the structure, for example by prising apart weak junctions between primary... 

Precipitators are currently used for high collection efficiency on fine particles. The use of electric discharge to suppress smoke was suggested in 1828. The principle was rediscovered in 1850, and independently in 1886 and attempts were made to apply it commercially at the Dee Bank Lead Works in Great Britain. The installation was not considered a success, probably because of the cmde electrostatic generators of the day. No further developments occurred until 1906 when Frederick Gardiner Cottrell at the University of California revived interest (U.S. Pat. 895,729) in 1908. The first practical demonstration of a Cottrell precipitator occurred in a contact sulfuric acid plant at the Du Pont Hercules Works, Pinole, California, about 1907. A second installation was made at Vallejo Junction, California, for the Selby Smelting and Lead Company. 

Samples that contain suspended matter are among the most difficult types from which to obtain accurate pH readings because of the so-called suspension effect, ie, the suspended particles produce abnormal Hquid-junction potentials at the reference electrode (16). This effect is especially noticeable with soil slurries, pastes, and other types of colloidal suspensions. In the case of a slurry that separates into two layers, pH differences of several units may result, depending on the placement of the electrodes in the layers. Internal consistency is achieved by pH measurement using carefully prescribed measurement protocols, as has been used in the determination of soil pH (17). 

The particle size of typical natural mbber latex ranges from slightly higher than 1 )Tm to as small as 20 nm, and can be destabilized by mechanical or chemical action. Machinery used to stir or maintain latex circulation should be designed to minimize fluid shear, particularly where that may be locally intensified, eg, at the junction of blade with shaft. 

An important consideration is the effect of filler and its degree of interaction with the polymer matrix. Under strain, a weak bond at the binder-filler interface often leads to dewetting of the binder from the solid particles to formation of voids and deterioration of mechanical properties. The primary objective is, therefore, to enhance the particle-matrix interaction or increase debond fracture energy. A most desirable property is a narrow gap between the maximum (e ) and ultimate elongation ch) on the stress-strain curve. The ratio, e , eh, may be considered as the interface efficiency, a ratio of unity implying perfect efficiency at the interfacial Junction. 

The formation and fracture of these junctions leads to the transfer of small fragments of material from one surface to another and finally, after many such events, the release of a year particle often in a highly oxidised state. Such a wear process is common to most sliding systems and can undoubtedly occur during fretting movements, especially if the slip amplitude is large. However, many fretting situations occur where the amplitude of slip is very small and perhaps even comparable to the dimensions of a... 

In the traditional lithography approach, researchers continued to consider the idea that modem STM (scanning tunnel Microscopy) could be the proper tool for the formation of two-junction systems when working with very small particles. This consideration had related the studies of single-electron phenomena to the concept of quantum dots (Glazman and Shechter 1989). 

Finally, it was possible to build up the first stand-alone room-temperature singleelectron junction by depositing a semiconducting particle directly onto the tip of a very sharp electrode, avoiding in this case the use of an STM microscope, and it was possible to observe the coulomb staircase in such a system (Facci et al. 1996). 

Several experimental techniques were applied to characterize these objects. It was found that CdS was formed as small particles inside the LB film with sizes in the nanometer range. Similar work was carried out that resulted in the formation of PbS, CuS, HgS, etc. The sizes of the particles produced by such approaches turned out to be rather similar to that of CdS. The observed sizes suggest that the objects could be useful for the formation of nanogranules for room-temperature single-electron junctions. 

As the second step, the STM tip was locked over the desired particle, feedback was temporally switched off, and voltage-current (V-I) characteristics were measured. The typical trend of the V-I characteristics is shown in Figure 29. Current steps are clearly observable in the presented curve, indicating that the single-electron junction was formed. It is worth mentioning that the characteristics observed in areas without particles demonstrate a normal tunneling behavior (see Fig. 30). 

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