Electron - affinity hole pair

As the size of a semiconductor crystal becomes small a regime is entered in which the electronic properties, e.g. ionization potential and electron affinity, are determined by size and shape of the crystals [113], When a quantum of light (hv) with energy exceeding the band gap falls on the surface of a semiconductor crystal there appears a bounded electron-hole pair known as an exciton... 

Because of its lower electron affinity, Te sites trap holes which can then coulombically bind an electron in or near the conduction band to form an exciton. Subsequent radiative collapse of this exciton leads to emission (10,11,12,13). In the context of the PEC, emission thus serves as a probe of electron-hole (e -h" ) pair recombination which competes with e - h+ pair separation leading to photocurrent. Except for intensity, the emitted spectral distribution is found to be independent of the presence and/or composition of polychalcogenide electrolyte, excitation wavelength (Ar ion laser lines, 457.9-514.5 nm) and intensity (<30 mW/cnZ), and applied potential (-0.3V vs. SCE to open circuit) (6,1,8,9). 

Russo N, Toscano M, Grand A (2000) Theoretical determination of electron affinity and ionization potential of DNAand RNA bases. J Comput Chem 21 1243-1250 Sagstuen E, Hole EO, Nelson WH, Close DM (1998) Radiation damage to DNA base pairs. II. Paramagnetic resonance studies of 1 -methyluracil. 9-ethyladenine complex crystals X-irradiated at 10 K. Radiat Res 149 120-127... 

Lewis FD, Letsinger RL, Wasielewski MR (2001) Dynamics of photoinduced charge transfer and hole transport in synthetic DNA hairpins. Acc Chem Res 34 159-170 Li Z, Cai Z, Sevilla MD (2001) Investigation of proton transfer within DNA base pair anion and cation radicals by density functional theory (DFT).J Phys Chem B 105 10115-10123 Li Z, Cai Z, Sevilla MD (2002) DFT calculations on the electron affinities of nucleic acid bases dealing with negative electron affinities. J Phys Chem A 106 1596-1603 Lillicrap SC, Fielden EM (1969) Luminescence kinetics following pulse irradiation. II. DNA. J Chem Phys 51 3503-3511... 

Electron transport through DNA, in which electron hopping involves radical anions of base pairs, is less likely to occur than hole hopping, involving radical cations, because the DNA bases have weak electron affinities 166 consequently, the energies of the anionic states of base pairs, in general, he well above those of conventional donor and acceptor chromophores. 

In the relevant experiment to determine electron transfer, the DNA intercalator mitoxantrone (MX) is used. MX binds well to DNA and has a high electron affinity. MX appears to intercalate randomly if used at an intercalator base-pair ratio of 1/20 or less. In a 7-M LiBr glass at 77 K, /-irradiation produces electrons that attach randomly onto the DNA and MX, and the holes are... 

Figure 2.8 shows the experimental set-up. An above-band-gap, short laser pulse is used to optically excite electron-hole pairs with excess energy. The time evolution of the electron distribution is followed by the use of a second short laser pulse with sufficient photon energy to place the excited electrons above the electron affinity of the semiconductor. The optical generation of the carriers again is not selectively at the surface region of interest—but the probed states are. 

Next, we studied the magneto-transport properties on the conjugated polymer/ fulleiene BHSC. In a BHSC, a polymer - fullerene blend is used, where fullerenes having higher electron affinity attract electrons in it and holes are left on the polymer chains. Thus, electron and hole pathways are separated givinga lower probability of e-h pairs to be formed. 

In order to facilitate efficient device action, donor and acceptor polymers must possess compatible optical, electronic, and physical properties. Efficient exciton dissociation requires sufficient differences in the electron affinity and ionization potential of the paired polymers. As a rule of thumb, a 0.4 eV difference in the lowest unoccupied molecular orbitals (LUMOs) is typically required to drive dissociation of excitons generated in the donor phase via electron transfer (see Figure 14.2 for a schematic energy level diagram). A similar offset in the highest occupied molecular orbitals (HOMOs) is required to drive dissociation of excitons generated in the acceptor phase via hole transfer. Note that the assignment of donor and... 

Here, Id is the ionization potential of the excited state of the polymer (donor). As is the electron affinity of the sensitizer molecule, and Uc denotes the Goulomb attraction between the separated radicals. Polarizability of the surrounding matrix, morphology of the polymer which introduces a large separation between the donor and the acceptor, or a potential barrier preventing the separation of the electron-hole pair are factors that may inhibit an energetically allowed GT process (8,9). 

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