Asymmetric contacts

It would be interesting to obtain actual systems with asymmetric contact barriers on polymer. It seems that heat treatment or plasma modification by chemically active gases may lead to such systems from plasma polymers. One could expect for such systems an increase of the activation energy difference for two opposite polarizations of the film. 

As a result of these factors, the universal paradigm for inorganic solar cells, the p-n junction, cannot be adapted for organic semiconductors. The contrast with inorganic semiconductors is shown schematically in Fig. 7.2. The alternative of a metal-semi-conductor-metal device structure, where photocurrent is directed by the difference in work function between the two metals, also cannot be used because the electric field created by available asymmetric contact materials is insufficient to separate the singlet exciton into electron and hole polarons. Therefore, alternative device architectures are needed. 

The ultrastructure of NA fibers of the cerebellar cortex and other parts of the rat CNS was analyzed with pre-embedding dopamine-y5-hydroxylase immunohistochemistry by Olschowka et al. (1981). Immunoreaction product was present in the axoplasm, associated with smooth endoplasmatic reticulum, Golgi apparatus, synaptic and large dense core vesicles and the outer membranes of mitochondria. Large varicosities were interconnected by narrow intervaricose axon segments. Varicosities, filled with clear, round synaptic vesicles and large dark-core vesicles, made asymmetric contacts with dendrites, but never with somata or axons. More than 50% of the labelled varicosities in the cerebellum made synaptic contacts most of them with dendritic shafts, fewer on spines. 

Inputs from the subthalamic nucleus to the pars reticulata provide an excitatory input mediated by the neurotransmitter glutamate (Kita and Kitai 1987 Nakanishi et al. 1987b). At the synaptic level these inputs form asymmetric contacts principally directed to more distal parts of the dendrites of pars reticulata neurons (Kita and Kitai 1987). Thus the distribution pattern of these afferents is similar to that of the striatal inputs. 

While it is simpler from a fabrication standpoint to deposit the same contact material for both the source and drain contacts (symmetric contacts), one may also consider choosing two different materials for each contact (asymmetric contacts). Based on energy band line-up considerations with the semiconductor HOMO and LUMO levels, depositing two different contact materials at either end of the transistor channel may facilitate more efficient hole and electron injection, respectively. At this point, it is unclear whether separately engineering distinct contacts for hole/elec-tron injection in ambipolar OFETs will prevail over opting for symmetric contacts. However, there will certainly be more reports on this exciting OFET subclass in the next few years. 

HENISCH One of the systems with asymmetric contacts discussed involved two carrier tra-n sport. In the ordinary way, this is governed by two current equations in terms of field and diffusion, two continuity relationships (incl. recombination), Poisson s equation, and boundary conditions for field, potential and carrier concentrations. These transport equations cannot be explicitly solved without simplifying assumptions. It would be interesting to see what assumptions can reasonably be made to render the problem algebraically tractable. The equations given appeared to envisage diffusion without space charge. How do (a) the local field. 

As explained previously for polymer LEDs, a semiconducting polymer with asymmetric contacts (a low-work-function metal on one side and a high-work-function metal on the opposite side) functions as a tunneling injection diode [1262]. In forward bias, tunneling injection diodes exhibit relatively high efficiency electroluminescence. In reverse bias, Yu et al. (1265,1266] reported that the devices exhibit a strong photoresponse with a quantum yield larger than 20% (electron/photon at 10-V reverse bias), which comparable to UV-sensitized Si photodiodes. 

Figure 11 shows the contact pressure distribution in the cases of normal fiber orientation. It can be seen that mostly fibers transfer the load. The frictional force induces an asymmetric contact pressure distribution on the top of the loaded fibers. The local pressure peaks appear at the rear edges of the loaded fibers located within the contact area. 

Example 4 (Fig. 5.70d) illustrates the superposition of asymmetrical contact potentials at the ionic conductor AgCl, to yield a measurable potential difference and, hence, a battery voltage. The ceU shown can also be used as a sensor for CI2. This is treated in Chapter 7. 

Chemical properties of deposited monolayers have been studied in various ways. The degree of ionization of a substituted coumarin film deposited on quartz was determined as a function of the pH of a solution in contact with the film, from which comparison with Gouy-Chapman theory (see Section V-2) could be made [151]. Several studies have been made of the UV-induced polymerization of monolayers (as well as of multilayers) of diacetylene amphiphiles (see Refs. 168, 169). Excitation energy transfer has been observed in a mixed monolayer of donor and acceptor molecules in stearic acid [170]. Electrical properties have been of interest, particularly the possibility that a suitably asymmetric film might be a unidirectional conductor, that is, a rectifier (see Refs. 171, 172). Optical properties of interest include the ability to make planar optical waveguides of thick LB films [173, 174]. 

Fibers spun by this method may be isotropic or asymmetric, with dense or porous walls, depending on the dope composition. An isotropic porous membrane results from spinning solutions at the point of incipient gelation. The dope mixture comprises a polymer, a solvent, and a nonsolvent, which are spun into an evaporative column. Because of the rapid evaporation of the solvent component, the spinning dope solidifies almost immediately upon emergence from the spinneret in contact with the gas phase. The amount of time between the solution s exit from the spinneret and its entrance into the coagulation bath has been found to be a critical variable. Asymmetric fibers result from an inherently more compatible solvent/nonsolvent composition, ie, a composition containing lower nonsolvent concentrations. The nature of the exterior skin (dense or porous) of the fiber is also controlled by the dope composition. 

Aside from the general thermal state of the body, a person may find the thermal environment unacceptable or intolerable if local influences on the body from asymmetric radiation, air velocities, vertical air temperature differences, or contact with hot or cold surfaces (floors, machinery, tools, etc.) are experienced. 

Schematic energy level diagrams of a metal/polymer/metal structure before and after the layers are in contact are shown in the top two drawings of Figure 11-6. Before contact, the metals and the polymer have relative energies determined by the metal work functions and the electron affinity and ionization potential of the polymer. After contact there is a built-in electric field in the structure due to the different Schottky energy barriers of the asymmetric metal contacts. Capacitance-voltage measurements demonstrate that the metal/polymer/metal structures are fully depleted and therefore the electric field is constant throughout the bulk of the structure [31, 35]. The built-in potential, Vhh i.e. the product of the constant built-in electric field and the layer thickness may be written...

The hole current in this LED is space charge limited and the electron current is contact limited. There are many more holes than electrons in the device and all of the injected electrons recombine in the device. The measured external quantum efficiency of the device is about 0.5% al a current density of 0.1 A/cm. The recombination current calculated from the device model is in reasonable agreement with the observed quantum efficiency. The quantum efficiency of this device is limited by the asymmetric charge injection. Most of the injected holes traverse the structure without recombining because there are few electrons available to form excilons. 

NMR is the only technique capable of assigning the cysteines ligating specific Fe atoms, since the asymmetric coupling of the three Fe + ions results in different temperature dependence for the contact shifts... 

LI ligase s large intrinsic flexibility was revealed by the recent crystal structure of the ligation product of a reduced size variant with two vastly different conformers, differing by reorientation of one of the stems by around 80 A, that were resolved in the same asymmetric cell [106], Based on the presence/absence of specific contacts between distant conserved parts including the ligation site and a totally conserved residue, U38, one of the conformers was postulated to represent the catalytically active or on conformation, the other the inactive or off one [106],... 

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