Defect paramagnetic

The intrinsic defects include paramagnetic and diamagnetic species (24,27,28). The paramagnetic defects have received the most study because they are readily detectable by electron spin resonance (esr) spectrometry. Paramagnetic defects that have been identified by esr include the center, 6i the... 

The other signal shown in Fig. 10a was observed after adsorption of TiCU under electron bombardment and subsequent treatment with electrons on MgCl2 substrates, which show paramagnetic defect states. In this case the signal intensity is increased by more than an order of magnitude as compared to the former case. In addition, the signal is shifted up-field, now located at g = 1.93, with a peak-to-peak width of 50-90 G, depending on the preparation. 

A second type of defect is associated with boron or aluminum impurities that are present in SiCh- In porous glass Muha (129) observed a rather complex spectrum which results from hyperfine interaction with 10B and UB isotopes. The spectrum is characterized by g = 2.0100, g = 2.0023, an = 15 and a a. = 13 G for nB. The paramagnetic defect is apparently a hole trapped on an oxygen atom which is bonded to a trigonally coordinated boron atom. This center is irreversibly destroyed upon adsorption of hydrogen. 

Extensive study has been devoted to paramagnetic defects that are formed on high-surface area alkaline earth oxides, particularly magnesium oxide. The work carried out by Wertz el al. 187, 188) and Henderson and Wertz 139) on bulk defects formed in MgO single crystals has been quite valuable in the identification of the surface defects. Both the bulk and surface defects may be divided into two classes those in which an electron... 

Despite this similarity with chemical shift, the Knight shift is grouped with the electron hyperfine term in (lb) to reflect the fact that both terms arise from the influence of the spin or orbital angular momentum of unpaired electrons. The distinction between the two is that for the electron hyperfine term the electron spin (or hole, as the absence of an electron can be described, e.g., in the case of d9 Cu++) is localized on a paramagnetic defect such as a deep-level transition metal ion. 

Parallel track drug policy, 18 689 Paramagnetic defects, in vitreous silica, 22 410... 

Bogomolova, L. D., Stefanovsky, S. V., Teplyakov, Y. G. Dmitriev, S. A. 1997. Formation of paramagnetic defects in oxide glasses during the bompardment of their surface with charged particles. Materials Research Society Symposium Proceedings, 465, 657-664. 

Natural radiation, a-, (3- and y-rays from radioactive elements in the environment or intrinsic to materials, ionizes the materia] and produces paramagnetic defects or radicals. They are often quite stable and accumulate with time. The ESR signal intensity is proportional to the total dose of natural radiation, i.e., to the product between the annual radiation dose rate and the time elapsed after their formation or an event which zeroed the spin concentration. 

ZnO. - Zinc oxide is a widely used semiconductor with applications in electrochemistry, photochemistry and photocatalysis. The charge carrier states produced by thermal or radiative treatment, particularly the mobile electrons, are easily trapped as paramagnetic defects at the surface of the material, and can therefore be characterised by EPR. However, unlike the situation discussed earlier for Ti02 (section 3.1) and Zr02 (section 3.2) semiconductors, once again there have been very few EPR studies carried out on ZnO in the past two years.166-167... 

Paramagnetic defects in amorphous carbon films have been studied for many... 

An HFEPR study of polymeric and diamond-like a-C H powder samples16 observed a single resonance line at room temperature for all samples at frequencies up to 189 GHz. At X-band the 3-factor for both types of a-C H was found to be 2.0025(2). The linewidth of the polymeric (diamond-like) sample increased only from 0.9 (0.6) G to 1.65 (1.67) G between 9 and 189 GHz, a similar result to that noted above, but there was no indication of extra structure in the spectra of either type of sample at any frequency. It was suggested that the extra structure observed in films could arise from paramagnetic defects in the substrates. 

The parameter cro depends on the recombination mechanism (radiative vs. non-radiative) and is typically of the order 107 or 1015 s-1, respectively [21] (see, however, Zamaraev et al. [27], who observed cro up to 1021 s 1). The other recombination parameter, r0, is nothing but half the Bohr radius of the wave-function of an electron centre and is, for example, about 0.5 A for F and Ag° centres and 1 A for shallower Tl° centre in KC1. For paramagnetic defects this parameter could be found by means of EPR and ENDOR [28-30]. 

Reaction 13 represents the production of a paramagnetic defect by capture of a photogenerated hole. The paramagnetic centres may be anihilated by the reverse of reaction 12 by capture either of photoelectrons from the conduction band or of electrons liberated during oxygen desorption. 

Another type of paramagnetic defects that are attributed to the main intrinsic defects of silica glass, are the so-called non-bridging oxygen hole centers - the (=Si-0)3Si-0 radicals. The EPR spectrum of these radicals, stabilized on the silica surface is presented in Figure 7.16a (conclusions about their existence were made earlier based on the indirect data). The temperature dependence of the intensity of the EPR signal does not obey the Curie law, and the EPR signal can be detected only at T< 100 K. 

The following discussion concentrates on the creation of bulk dangling bond defects, which may not be the only process, but is almost certainly the dominant one. Dersch, Stuke and Beichler (1980) were the first to show that illumination causes an increase in the g = 2.0055 paramagnetic defect and concluded that the Staebler-Wronski effect was the creation of dangling bonds. The metastable defect creation and annealing is described by the potential well model shown in Fig. 6.1, except that the barrier is overcome by the recombination energy from... 

Em and Merrifield [3] found that paramagnetic defects in anthracene crystals quenched triplet excitons and changed the field dependence from a negative to a positive gradient. They interpreted these MFEs in terms of the triplet-doublet quenching expressed by... 

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