Coulomb-blockade

Fig. 9. (a) Top view of a Coulomb blockade device (single-electron tiansistoi) patterned on an AlGaAs—GaAs heteiojunction, where U= the surface gate ... 

H. Grabert and M. H. Devoret, eds.. Single Charge Tunneling. Coulomb Blockade Phenomena in Nanostructures, Plenum Press, New York, 1992. 

Coulomb blockade effects have been observed in a tunnel diode architectme consisting of an aluminum electrode covered by a six-layer LB film of eicosanoic acid, a layer of 3.8-nm CdSe nanoparticles capped with hexanethiol, and a gold electrode [166]. The LB film serves as a tunneling barrier between aluminum and the conduction band of the CdSe particles. The conductance versus applied voltage showed an onset of current flow near 0.7 V. The curve shows some small peaks as the current first rises that were attributed to surface states. The data could be fit using a tunneling model integrated between the bottom of the conduction band of the particles and the Fermi level of the aluminum electrode. 

The theory foresees the possibility of coulomb blockade phenomenon in such junctions. Averim and Likharev had investigated the conditions of vanishing for the Josephson tunneling and demonstrated the possibility of having normal electrodes in the junction. That is, no superconducting electrodes are necessary, and, therefore, coulomb blockade is possible to observe, in principle, even at room temperature. 

All of the theoretical work proposed during the past 10 years forces experimental researchers to develop real systems to observe the described phenomena. In reality, only a year after the very first work of Averim and Likharev, the first measurements of the coulomb blockade and the coulomb staircase phenomena were published. 

A few months later, the observation of the coulomb blockade appeared (Fulton and Dolan 1987). In the case of a coulomb staircase, only one additional mouth was required (Earner and Ruggiero 1987). 

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). 

However, the main research result from those years was the discovery of the room-temperature single-electron phenomenon. In the 1990s, STM experiments on liquid crystal had shown a very weak staircase (Nejoh 1991) only one year later, the clear observations of the coulomb blockade and the coulomb staircase were demonstrated on gold nanoparticles (Shonenberger et al. 1992a) and the role of system symmetry on the appearance of these two phenomena was outlined (Shonenberger et al. 1992b). 

The tip-particle distance, using (Vbias = 1 V, /tmmei = 1 nA) as tunnel parameters does not correspond to that needed to observe the Coulomb staircase. The particle-substrate distance is fixed by the coating with dodecanethiol. Hence the two tunnel junctions are characterized by fixed parameters. Similar Coulomb blockade behavior has been observed [58,59]. 

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 ... 

Thus, it is concluded that the FCC structure induces an increase in the tunneling rate i.e., the resistance decreases between particles. The tunneling between adjacent particles is a major contribution to the conduction. This inhibits the Coulomb blockade in the tunneling I V) measurements, and thus the 3D superlattices yield an increased tunneling current. 

The electron transport properties described earlier markedly differ when the particles are organized on the substrate. When particles are isolated on the substrate, the well-known Coulomb blockade behavior is observed. When particles are arranged in a close-packed hexagonal network, the electron tunneling transport between two adjacent particles competes with that of particle-substrate. This is enhanced when the number of layers made of particles increases and they form a FCC structure. Then ohmic behavior dominates, with the number of neighbor particles increasing. In the FCC structure, a direct electron tunneling process from the tip to the substrate occurs via an electrical percolation process. Hence a micro-crystal made of nanoparticles acts as a metal. 

Figure 10. Tunnelling of a single electron from an electrode into an intermediate island causing a Coulomb blockade. If the electrostatic energy is large enough, transport to the counter electrode happens.

The next smaller ligand-protected nanocluster that was investigated by scanning tunneling spectroscopy (STS) was the four-shell cluster Pt309phen 36O20 [20,21]. The diameter of the Pt core is 1.8 nm, about a tenth of the former example. However, even here a Coulomb blockade could only be observed at 4.2 K, i.e. at room temperature the particle still has metallic behaviour. Since... 

Coulomb blockades in metallic quantum dots inform on the ability to trap and to store single electrons in a distinct voltage region. Practically this means nothing but to have a single electron switch If this is the case at room... 

The STS measurements were performed at two different positions of the cluster surface, as is indicated by (a) and (b). (a) indicates a position above a phenyl ring of a PPh3 ligand molecule and the position (b) is above a non-covered area. These two measurements were necessary to eliminate a possible influence of the aromatic rings. Instead of the usual /-(/-characteristic, here dljdV was used instead of /. The Coulomb blockade then appears as a broad minimum exhibiting important details, as can be seen in Figure 15. 

Figure 13. The Coulomb blockade of Au55(PPh3)i2Cl6 at room temperature. (Reprinted from Ref [22], 1998, with permission from Springer-Verlag GmbH.)...

Alivisatos and coworkers reported on the realization of an electrode structure scaled down to the level of a single Au nanocluster [24]. They combined optical lithography and angle evaporation techniques (see previous discussion of SET-device fabrication) to define a narrow gap of a few nanometers between two Au leads on a Si substrate. The Au leads were functionalized with hexane-1,6-dithiol, which binds linearly to the Au surface. 5.8 nm Au nanoclusters were immobilized from solution between the leads via the free dithiol end, which faces the solution. Slight current steps in the I U) characteristic at 77K were reflected by the resulting device (see Figure 8). By curve fitting to classical Coulomb blockade models, the resistances are 32 MQ and 2 G 2, respectively, and the junction... 

Figure 11. Experimental and predicted differential conductance plots of the double-island device of Figure 10(b). (a) Differential conductance measured at 4.2 K four peaks are found per gate period. Above the threshold for the Coulomb blockade, the current can be described as linear with small oscillations superposed, which give the peaks in dljdVj s- The linear component corresponds to a resistance of 20 GQ. (b) Electrical modeling of the device. The silicon substrate acts as a common gate electrode for both islands, (c) Monte Carlo simulation of a stability plot for the double-island device at 4.2 K with capacitance values obtained from finite-element modeling Cq = 0.84aF (island-gate capacitance). Cm = 3.7aF (inter-island capacitance). Cl = 4.9 aF (lead-island capacitance) the left, middle and right tunnel junction resistances were, respectively, set to 0.1, 10 and 10 GQ to reproduce the experimental data. (Reprinted with permission from Ref [28], 2006, American Institute of Physics.)...

Coulomb blockade thermometry (CBT) is based on the electric conductance characteristics of tunnel junctions. This type of thermometer has been developed at Jyvaskyla University. The basic results are reported in ref. [112],... 

Fig. 9.20. Normalized conductance G/GT of a coulomb blockade thermometry sensor versus bias voltage V.

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