Bias current, switching

Furthermore, it was shown the unpaired spin S = 1/2, which is delocalized over the two Pc rings, still remained in the Jt-orbitals after absorption on Au(lll). Consequently, STS measurements also provided direct observation ofthe S = 1/2 radical on the TbPc2 molecules on Au(lll) whereby the indicative Kondo-peak could be switched off by tunnelling current pulses [215]. Indeed the tunnelling conductance (dl/dV) was analysed from STS experiments of TbPc2 on Au(lll) near the Fermi level showed a zero-bias peak (ZBP) in the spectra, which could be assigned as a Kondo resonance. Clear Kondo features for the molecules with 9 = 45° were observed when the tip was positioned over one ofthe lobes of TbPc2. 

Figure 11.10. NW smart pixels, (a) Schematic of an integrated crossed NW FET and LED and the equivalent circuit, (b) Shows SEM image of a representative device, (c) Plots of current and emission intensity of the nanoLED as a function of voltage apphed to the NW gate at a fixed bias of -6V. (d) EL intensity versus time relation when a voltage applied to NW gate is switched between 0 and +4V for a fixed bias of -6V. [Reprinted with permission from Ref. 59. Copyright 2005 Wiley-VCH Verlag.]...

This is the regime of cathodic currents. The silicon atoms of the electrode do not participate in the chemical reaction in this regime. An n-type electrode is under forward bias and the current is caused by majority carriers (electrons). The fact that photogenerated minority carriers (holes) are detectable at the collector indicates that the front is under flat band or accumulation. A decrease of IBC with cathodization time is observed. As Fig. 3.2 shows, the minority carrier current at the collector after switching to a cathodic potential is identical to that at VQcp in the first moment, but then it decreases within seconds to lower values, as indicated by arrows in Fig. 3.2. This can be interpreted as an increase of the surface recombination velocity with time under cathodic potential. It can be speculated that protons, which rapidly diffuse into the bulk of the electrode, are responsible for the change of the electronic properties of the surface layer [A17]. However, any other effect sufficient to produce a surface recombination velocity in excess of 100 cm s 1 would produce similar results. 

Typical anodization curves of silicon electrodes in aqueous electrolytes are shown in Fig. 5.1 [Pa9]. The oxidation can be performed under potential control or under current control. For the potentiostatic case the current density in the first few seconds of anodization is only limited by the electrolyte conductivity [Ba2]. In this respect the oxide formation in this time interval is not truly under potentiostatic control, which may cause irreproducible results [Ba7]. In aqueous electrolytes of low resistivity the potentiostatic characteristic shows a sharp current peak when the potential is switched to a positive value at t=0. After this first current peak a second broader one is observed for potentials of 16 V and higher, as shown in Fig. 5.1a. The first sharp peak due to anodic oxidation is also observed in low concentrated HF, as shown in Fig. 4.14. In order to avoid the initial current peak, the oxidation can be performed under potentiodynamic conditions (V/f =const), as shown in Fig. 5.1b. In this case the current increases slowly near t=0, but shows a pronounced first maximum at a constant bias of about 19 V, independently of scan rate. The charge consumed between t=0 and this first maximum is in the order of 0.2 mAs cnT2. After this first maximum several other maxima at different bias are observed. 

The current-voltage curve was interpreted on the basis of the mechanism illustrated in Figure 17.15a, which is derived from the behavior of the same catenane 134+ in solution.116,117 Conformation I is the switch open state and conformation IV the switch closed state of the device. When 134+ is oxidized (+2 V), the TTF unit is ionized in state II and experiences a Coulombic repulsion inside the tetra-cationic cyclophane component, resulting in circumrotation of the crown ether and formation of conformation III (note that in solution at +2 V TTF undergoes two-electron oxidation and the dioxynaphthalene unit is also oxidized).116 When the voltage is reduced to near-zero bias, a metastable conformation IV is obtained... 

Fig. 8. The OFF-ON switching transient of a formed device , drawn from an oscilloscope trace (a) applied bias, vertical scale 2 V per division, horizontal scale 20 nsec per division (b) device current, vertical scale approximately 6 mA per division, horizontal scale 20 nsec per division.

A junction of this sort can be used as a light-sensitive switch. With a reverse bias applied (extra electrons supplied to the p side), no current would flow, as described for diodes. However, if the difference in energy between the valence band and the conduction band of a semiconductor is small enough, light of visible wavelengths is energetic... 

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