Nanotip

Fig. 25 STS curves (normalized I/V plots of LB monolayers of isomers 55 (crosses) and 56 (dots), deposited on an Au film over HOPG (highly oriented pyrolytic graphite), and scanned with a Pt/Ir nanotip. The films exhibit rectification in opposite quadrants of the plot, where the polarity is defined by the sign of the substrate electrode. Electron flow at forward bias in each case is from the acceptor to the donor [127]...

As shown in Fig. 13.7, at high temperatures, about 3200 K, the diffusion process dominates, and the surface tension favors a spherical end. At lower temperatures, about 1600-1800 K, the directional effect of the external field dominates. By conducting this process in an oxygen atmosphere, a nanotip with a single W atom at the apex is routinely produced. 

Fig. 13.7. Tip treatment by annealing in a field. Top At a temperature close to the melting point of tungsten, the shape of the tip is basically determined by diffusion process. A rather round-shaped thermal tip is formed. Bottom left, at lower temperature, the directional effect of the field dominates. A built-up tip is formed. Bottom right, in an oxygen atmosphere, the corrosion process further generates a nanotip. After Binh (1988a).

Binh and Garcia (1991, 1992) reported that, at temperatures around one third of the bulk melting temperature, by applying an even higher electric field to th tip, the metal ions move to the protrusions and emit from the ends. Sharp, pyramidal nanotips, often ending with a single atom, can be reproducibly generated. The experiment was performed on W, Au and Fe tips. In particular, for W tips, the conditions to observe metal ion emission arc electrical field, 4-6 kV, or 1.2-1.8 eV/A and temperature, 1200-1500 K. This new phenomenon is termed the atomic metallic ion emission (AMIE). 

Fig. 13.8. Atomic metallic ion emission and nanotip formation, (a) At high temperature, the atoms on a W tip becomes mobile. The tip surface is macroscopically flat but microscopically rough, (b) By applying a high field (1.2-1.8 V/A,), the W atoms move to the protrusions, (c) The apex atom has the highest probability to be ionized and leave the tip. The W ions form an image of the tip on the fluorescence screen, (d) A well-defined pyramidal protrusion, often ended with a single atom, is formed. By cooling down the tip and reversing the bias, a field-emission image is observed on the fluorescence screen. The patterns are almost identical. (Reproduced from Binh and Garcia, 1992, with permission.)...

Fig. 13.10. Mechanism of tip sharpening by an electrical field, (a) W atoms on the tip shank walk to the tip apex due to the nonuniform electrical field, (b) A nanotip is formed. (Reproduced from Chen, 1991, with permission.)...

Binh, V. T., and Garcia, N. (1992). On the electron and metallic ion emission from nanotips fabricated by field-surface-melting technique experiments on W and Au tips. Ultramicroscopy, 42-44, 80-90. 

Adding a very small solution sample cell, covering most of the STM tip with lacquer (to limit unwanted conductivity from the shaft of the nanotip), and adding a third "reference" electrode permits scanning electrochemical microscopy (SECM) this is electrochemistry practiced on a nanoscale. 

In this work, we focus on the properties of a Pts cluster absorbed on transition metal nanotips that have a variety of crystal stractures fcc(lll) for Pt, CosPt, and Ni hcp(OOOl) for Co and bcc(llO) for Fe in order to identily conditions where these stractures have a dominant role in the modification of the behavior of multi-metallic supported clusters. " ... 

Pasquier, A. D., Chen, H., and Lu, Y. (2006). Dye sensitized solar cells using well-aligned zinc oxide nanotip arrays. Appl. Phys. Lett. 89 253513. 

Imperfections that are inherent to all of the discussed test structures are minimized partially in case of application of porous anodic alumina. It is known that anodic alumina has sharp pore edges [4]. Therefore it may be used as tiplike grating. In order to combine the benefits of porous anodic alumina and a nanoscale tip array a one-step electrochemical approach is used to fabricate nanoscale alumina tips (tip-like anodic alumina) [5]. However, a curvature radius of pore edges or alumina nanotips of these structures is too high for ultrasharp SPM tip characterization. 

When performed outside the scan area of interest, a sudden increase of the bias voltage to about —1 to —10 V (at the tip) for 2-4 scan lines (on metal surfaces) leads to a controlled field emission of electrons . By this treatment some atoms may move down to the tip apex due to the non-uniform electric field and form a nanotip (Fig. 10.17) [76]. 

Figure 5.15 Scanning electron micrographs of polySi-based nanotips. (A) Lateral view of the tip of 800 pm length. (B) Enlarged front view of the slot at its outlet showing a cross-sectional area of 2 pm x 2 pm. (Adapted with permission from Ref. 25).

Diamond emitter tips (a) array, (b) with gate, (c) diamond nanotips ( Eisevier... 

Possible applications of PANI-NTs in the construction of nanotip emitters in field-emission displays and polymer-based transistors have been demonstrated [238]. Gate voltage controllable p-n junction diode behavior for polymeric-acid-doped PANI-NTs with Au contacts, configured as a FET, was reported [334]. 

Nanocell is the smallest electrochemical cell developed by Sugimura and Nakagiri [11] and further developed and utilized for ENT by BloeB et al. [10]. The nanocell consists of two electrodes distance between electrodes is generally maintained in the order of less than 1 nm. In between two electrodes, absorbed water film acts as an electrolyte whose volume is maintained by vapor pressure and ranges from 10 to 10 cm. Double layer capacitance is not formed across the solid liquid interface in the nanocell due to the much smaller inter-electrode gap and hence, generated hydrogen ion and hydroxyl ion recombine immediately. Nanotip of microtool such as tip of scanning probe microscope (SPM) or AFM tip is most suitable for the formation of electrochemical nanoceU. 

A major improvement in the STM procedure produced SPBJ histograms of STM currents (Figure 4) measured between an Au(lll) surface and an Au nanotip immersed in a 0.1-M NaC104 solution " when the gap is closed by an Au filament (Figure 4a,b) then maxima in the conductance can be seen near zero bias (V = 0.013 V), in units of the Landauer conductance quantum Gl = 2e /h (4) for one filament (Gl), two filaments in parallel (2 Gl), or three filaments in parallel (3 Gl). When the An filament is absent, but the solution is made 1 mM in 4,4-bipyridine (10), where bonds can be made between the Au electrodes and the two N atoms, then conductance maxima are seen at 0.01 Gl, 0.02 Gl, and 0.03 Gl this corresponds to a molecular resistance of 1.3 0.1MS2 per 4,4 -bipyridine molecule. As the bias is increased. 

Velmimigan J, Sun P, Mirkin MV (2(X)9) Scanning elecUochemical mitxDSCopy with gold nanotips the effect of electrode material on elecUon transfer rates. J Phys Chem C 113(l) 459-464. doi 10.1021y5p808632w... 

Nanoscale jt-conjugated organic and organometallic polymers can be used as well for sensors [339, 340], biosensors [341], electrochemical devices, single electron transistors [342], nanotips of field emission displays [343] and a recent review reports the technological advances in these important topics [344]. 

Sun P, Mirkin MV (2007) Scanning electrochemical microscopy with slightly recessed nanotips. Anal Chem 79 5809-5816... 

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