Electrically detected magnetic resonance

Electrically detected magnetic resonance (EDMR) is conceptually similar to ODMR, i.e. the magnetic resonance is observed through spin-dependent electrical rather than optical properties of a sample. Virtually all of the EDMR in GaN-based materials reported to date has bear performed on LEDs and so the device type will serve as a basis for the organisation of this section. Three basic device types have been studied m-i-n-n+ diodes, double heterostructures (DHs) and single quantum wells (SQWs). Some details on these structures can be found elsewhere in this volume [35] and in the original work. 

Despite the potential significance of shallow traps in the operational degradation of OLED devices, there are no established general techniques for their effective detection and measurement in such an environment. The application of methods, such as electron spin resonance [43], electrically detected magnetic resonance [44], and charge modulation spectroscopy [45,46], is hindered by their limited sensitivities and selectivities in operationally degraded OLED devices. 

Pawlik, T. D., Kondakova, M. E., Giesen, D. J. et al. 2008. Charge carriers and triplets in OLED devices studied by electrically detected magnetic resonance. SID Inti. Symp. Dig. Tech. Papers 39 613. 

Finally, the silica surface has been studied using non-conventional EPR methods including EPR scanning tunnelling microscopy (ESR-STM) and electrically detected magnetic resonance (EDMR). Both of these experimental methods were performed under UHV conditions and again demonstrate the wider applications of EPR methods in surface science. 

Electrically-detected magnetic resonance has been used with Si P to sensitively probe nuclear spins with pulsed ENDOR at high and low magnetic fields/ High magnetic fields have been used to polarize Si P electron spins and this has been transferred to nuclear spins with optical excitation and entirely electrically/ As shown in Fig. 1, EDMR has been used to show that strain is useful for tuning Si P resonance frequencies. ... 

The charged quasiparticles can be probed by electrical dc conductivity measurements (for polarons), magnetic susceptibility (for polarons and bipolarons), electron-spin resonance (ESR) (for polarons) and optical measurements (for polarons and bipolarons). As ESR is well suited for studying spin-carrying polarons, optical modification of the ESR (optically detected magnetic resonance ODMR) can be applied to link the emissive or absorbing properties of the polymer with its spin state. 

The role of the dopant potential on the stability and magnetic and optical properties of polarons and bipolarons in conducting polymers is shown with the aid of calculations of singlet and triplet states of a bipolaron [167] and by spectroelectrochemical and conductivity measurements [168-170]. The X-band optically detected magnetic resonance of PHT and PDDT shows that the distant intrachain polaron recombination is temperature-independent and identical in films and solutions. However, the triplet polaronic excitation decay is observable in films, but not in solutions [171], Electrochemical in situ conductivity and EPR measurements of PT films were performed in several solutions [172]. The results indicate that polarons merely seem to initiate the electrical conductivity. The electronic delocalization of polarons is restricted to a relatively short chain length at low potentials. As the polaron concentration increases (spin density maximum), bipolarons are generated immediately (probably too fast for the detection of polarons by EPR). Thus the bipolarons prevail in the fully conducting polymer films and as a consequence should be mainly responsible of the intrinsic conductivity [172]. Asymmetrically disub-stituted PBT display well-defined redox processes which are correlated to the consecutive formation of radical cations, dimerized radical cations, and dications [173]. 

In principle the deviation <5 can be determined by the use of usual analytical chemistry or a highly sensitive thermo-balance. These methods, however, are not suitable for very small deviations. In these cases the following methods are often applied to detect the deviation physico-chemical methods (ionic conductivity, diffusion constant, etc.), electro-chemical methods (coulometric titration, etc.), and physical methods (electric conductivity, nuclear magnetic resonance, electron spin resonance, Mossbauer effect, etc.), some of which will be described in detail. 

In this section we have described in considerable detail just one aspect of the spectroscopy of OH, namely, the measurement of zl-doubling frequencies and their nuclear hyperfine structure. This has led us to develop the theory of the fine and hyperfine levels in zero field as well as a brief discussion of the Stark effect. We should note at this point, however, that OH was the first transient gas phase free radical to be studied by pure microwave spectroscopy [121], We will describe these experiments in chapter 10. We note also that magnetic resonance investigations using microwave or far-infrared laser frequencies have also provided much of the most important and accurate information these studies are described in chapter 9, where we are also able to compare OH with the equally important radical, CH, a species which, until very recently, had not been detected and studied by either electric resonance techniques or pure microwave spectroscopy. 

We must now say more about the nature of the resonance transitions, and also describe additional measurements of the Stark effect which enable the electric dipole moment of the molecule to be determined. In both SO and NF the transitions detected are actually electric-dipole allowed, so perhaps the spectrum ought not to be described as a magnetic resonance spectrum. 

We have already discussed the high-resolution spectroscopy of the OH radical at some length. It occupies a special place in the history of the subject, being the first short-lived free radical to be detected and studied in the laboratory by microwave spectroscopy. The details of the experiment by Dousmanis, Sanders and Townes [4] were described in section 10.1. It was also the first interstellar molecule to be detected by radio-astronomy. In chapter 8 we described the molecular beam electric resonance studies of yl-doubling transitions in the lowest rotational levels, and in chapter 9 we gave a comprehensive discussion of the microwave and far-infrared magnetic resonance spectra of OH. Our quantitative analysis of the magnetic resonance spectra made use of the results of pure field-free microwave studies of the rotational transitions, which we now describe. 

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