Current staircasing

Potential-excitation signals and voltammograms for (a) normal pulse polarography, (b) differential pulse polarography, (c) staircase polarography, and (d) square-wave polarography. See text for an explanation of the symbols. Current is sampled at the time intervals indicated by the solid circles ( ). 

In particular, considering a ballistic model for the charge transport through a dot, it was possible to demonstrate that the current through it should be represented as a series of equidistant peaks whose positions correspond to the steps in the coulomb staircase. 

On the other hand, even in particle systems the coulomb blockade (Van Bentum et al. 1988a) and the coulomb staircase (Van Bentum et al. 1988b) were observed, some nonlinear effects were observed in the current-voltage characteristics (Wilkins et al. 1989), and behavior related to the quantized energy levels inside the particles was described (Crom-mie et al. 1993, Dubois et al. 1996). 

The observed phenomena can be explained if we consider that different behaviors in the V-I characteristics of the same granule (staircase and negative resistance) are measured when different values of current are locked by the STM feedback. This fact implies, of course, that different tip-granule distances are attained in the two cases. By considering the structure as a two-barrier system, we can suggest that one barrier, namely, that between the... 

When the locked current was small (0.5 nA), giving rise to a large barrier between the granule and the tip, staircase-like V-I characteristics were registered (Fig. 37). Instead, when the value of the locked current was higher (1 nA), characteristics with negative resistance occurred at constant-voltage steps (Fig. 36). 

Other sensor applications can be considered if some sensitive biological molecules (such as antibodies or receptors) are attached to the nanogranule. If, for example, an antibody molecule is attached to it, then the granule is placed between two electrodes, and single-electron current flows between them. The step value of the coulomb staircase depends on the capacity of the junctions. When the antibody molecule binds specific antigen, the capacity value will be changed, and, therefore, the step value of the VH characteristics will also change. 

When particles are arranged in an FCC structure, as shown in Figure 3, the I V) curve shows a linear ohmic behavior (Fig. 9C). The detected current, above the site point, markedly increases compared to data obtained with a monolayer made of nanocrystals (Fig. 9C). Of course, the dIldV(Y) curve is flat (inset Fig. 9C). This shows a metallic character without Coulomb blockade or staircases. There is an ohmic connection through multilayers of nanoparticles. This effect cannot be attributed to coalescence of nanocrystals on the gold substrate, for the following reasons ... 

To overcome these problems, most voltammetric detectors have used pulsed waveforms such as staircase , squarewaveand differential pulseThe current is sampled at the end of the pulse after the charging current has decayed. In addition, because the charging current is typically the major current source, iR problems are not as severe. Last has described a coulostatic detector based on charge pulses instead of potential pulses which eliminates iR and charging current... 

Similarly if you toggle the enable pin at a certain rate in many ICs you can get to see current overshoots too. This often depends on allowing the input capacitor to discharge below the UVLO threshold, but not to the point where it hits the internal POR (power on reset) threshold. Because in that case, if you suddenly enable the IC, it has no soft-start anymore, and you will hit max duty cycle, and possibly staircase if the current limit circuitry is not well designed. 

Fig. 1.8 Scheme of the square-wave voltammetric excitation signal, st starting potential, Esv, pulse height, AE potential increment, t staircase period, to delay time and 7f and 4 denote the points where the forward and backward currents are sampled, respectively... 

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