Tunnel barrier

B. D. Josephson (Cambridge) theoretical predictions of the properties of a supercurrent through a tunnel barrier, in particular those phenomena which are generally known as the Josephson effects. 

Parker [55] studied the IN properties of MEH-PPV sandwiched between various low-and high work-function materials. He proposed a model for such photodiodes, where the charge carriers are transported in a rigid band model. Electrons and holes can tunnel into or leave the polymer when the applied field tilts the polymer bands so that the tunnel barriers can be overcome. It must be noted that a rigid band model is only appropriate for very low intrinsic carrier concentrations in MEH-PPV. Capacitance-voltage measurements for these devices indicated an upper limit for the dark carrier concentration of 1014 cm"3. Further measurements of the built in fields of MEH-PPV sandwiched between metal electrodes are in agreement with the results found by Parker. Electro absorption measurements [56, 57] showed that various metals did not introduce interface states in the single-particle gap of the polymer that pins the Schottky contact. Of course this does not imply that the metal and the polymer do not interact [58, 59] but these interactions do not pin the Schottky barrier. 

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. 

As we will see later, the tunneling barriers, and hence the relaxation times of the tunneling centers, are distributed. This would lead to a time-dependent heat capacity. Ignoring this complication for now, the classical, long-time heat capacity is easy to estimate aheady (assuming it exists). Since our degrees of... 

We can now use the typical value of the barrier curvature from our tunneling argument in Section IIIA (see Fig. 10) to estimate the typical frequency of motion at the tunneling barrier top. We now express the barrier profile F(7/) as a function of the droplet s radius r = a(3Al/47t) and obtain... 

Figure 1. The tunneling of a single electron (SE) between two metal electrodes through an intermediate island (quantum dot) can be blocked of the electrostatic energy of a single excess electron trapped on the central island. In case of non-symmetric tunneling barriers (e.g. tunneling junction on the left, and ideal (infinite-resistance) capacitor on the right), this device model describes a SE box .

The variation in Ec can be caused by diverse reasons, which have to be taken into account Eq will depend on the exact way in which the cluster lies on the substrate since the clusters have different facets (squares and triangles). Additionally the ligands, which for simplification have been assumed to be a spherical dress for the cluster, may have different orientations varying from cluster to cluster with respect to the underlying substrate, thus causing a different tunnel barrier between the cluster and... 

On detailed electrical characteristics of a SET transistor utilizing charging effects on metal nanoclusters were reported by Sato et al. [26]. A self-assembled chain of colloidal gold nanoparticles was connected to metal electrodes, which were formed by electron-beam lithography. The cross-linking of the particles as well as their connection to the electrodes results from a linkage by bifunctional organic molecules, which present the tunnel barriers. 

To study the electrical transport properties of this double-barrier system Pd nanoclusters have been trapped in this gap. Figure 14 shows a typical l(U) curve. The most pronounced feature at 4.2 K is the Coulomb gap at a voltage of about 55 mV, which disappears at 295 K. Above the gap voltage, the l(U) curve is not linear, but increases exponentially, which was explained by a suppression of the effective tunnel barrier by the applied voltage. 

Figure 14. I-V curves measured at 4.2 K (open squares) and at 295 K (solid squares). The solid curves denote fits of the KN model. Fitting parameters for these curves are Fc = 55mV, Ro=11x10 Q, 0 = 0.15c (offset charge) and a = E (cj h) = 0.5. The dashed curve (a = 0) represents the conventional model, which assumes a voltage-independent tunnel barrier. (Reprinted with permission from Ref. [29], 1997, American Institute of Physics.)...

The examples cited above are of molecules which are not strictly speaking noorigid, although they have more than one well-defined equilibrium configuration. The 1,2-dichloroetbane molecule discussed above is a classic example. With the aid of computer programs that have been developed to treat this problem, it has become possible to calculate wifi) confidence the equilibrium conformations of such molecules, as well as the energy differences and the tunneling barriers between them. It is appropriate here to summarize briefly the so-called molecular mechanics method that is currently employed to obtain these results. 

Figure 5.3 Tunnel barrier pT as a function of tip-sample distance for Au(l 1 1) in 0.1 M H2S04 for three different potentials (vs. SCE). s = 0 refers to the surface plane ofAu(l 1 1). pT (s) for Au(l 1 1) in air is also shown for comparison. For details see Refs [18, 19].

Fig. 2. Schematic diagram of the tunnel gap between sample and tip, with the extension of the electric double layers indicated by the outer Helmholtz plane(OHP). (a) No tip interaction at large tip-sample separation, (b) Overlap of the electric double layers at a distance s = 0.6 nm, which can be achieved by conventional imaging conditions (e.g., Uj = 50 mV It = 2 nA Rt = 2.5 x 107 Q). Inset Dependence of the tunnel gap s on the tunnel resistance Rt for a tunnel barrier of 1.5 eV.

---