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Effect of an internal built-in electric field on the non-radiative recombination process of strain-balanced InGaAs/GaAsP multiple quantum wells (MQWs) inserted into a GaAs p–i–n solar cell structure was investigated using the piezoelectric photo–thermal technique. From the experimental data obtained from two types of p–i–n solar cell structures with partly charge-neutral and uniform-gradient potentials, the decrease in the built-in electric field contributed to the decrease in the non-radiative carrier recombination in the MQW. This phenomenon was especially pronounced in the higher photon energy of the incident light and in large quantum well stack number. With the increase in the photon energy, the penetration length shortened, and most carriers were generated in the charge-neutral area in the i-region. The present experimental results could be understood by the increase in the radiative recombination processes within the MQW because of the wave function overlap is essentially unity.

We report on the optical and magnetic characteristics of the Co and Mn doped ZnO submicrometer crystals (SMCs) synthesised by organic thermal decomposition method. The doped ZnO SMCs are hexagon cone in shape with size ranging from 100 to 400 nm. X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy measurements indicate that the Mn2+ and Co2+ are doped into the ZnO lattice with concentration of 5.4 and 4.85%, respectively. Optical absorption spectra of the doped SMCs display additional peaks associated with d–d crystal-field transitions due to the presence of tetrahedral Co2+ or Mn2+ ions. Photoluminescence of the samples shows suppressed band-gap emission, indicating that the Mn or Co ions in the ZnO lattice increase the non-irradiative recombination processes. Magnetic property measurements reveal that the Mn:ZnO SMCs are paramagnetic while the Co:ZnO exhibit weak ferromagnetism at 300 K.

Transient femtosecond absorption spectroscopy and time-correlating single photon counting (TCSPC) photoluminescence (PL) were employed to study InS nanowires (NWs) grown by chemical vapor deposition (CVD) and determine the relaxation mechanisms in these nanostructures. Intensity dependent measurements revealed that Auger recombination plays an important role in the relaxation of photogenerated carriers at fluences larger than 0.4 x 10 15 photons/cm 2 . Calculations provided an estimated of the Auger recombination coefficient to be 1.1 ±0.5 x 10 -31 cm 6 /s. At the low fluence regime TCSPC PL revealed three relaxation mechanisms with time constants ranging from ps to nanosecond providing evidence of the importance of non-radiative decay channels associated with defect/trap states within the NWs.

With this paper we analyze the correlation between the degradation of InGaN-based laser diodes (LDs) and the increase in the non-radiative recombination rate in the active region. Several 405 nm MOCVD LDs have been submitted to CW stress, for 2000 h (stress current in the range 40-100 mA, case temperature = 75 °C). During stress, we extensively evaluated the optical characteristics of the LDs: a technique for the evaluation of the non-radiative recombination lifetime (τ nr ) in the active material was developed and used for the analysis of the stress effects. We demonstrate the following: (1) degradation determines the increase in LDs threshold current (I th ) and the decrease in the τ nr ; (2) degradation of I th and τ nr have similar kinetics; and (3) the degradation rate of the LDs is almost linearly related to the stress current level. The degradation process is therefore ascribed to the decrease of internal quantum efficiency caused by the increase of the non-radiative recombination rate in the active region.

Epitaxial ZnO and Al-doped ZnO (AZO) thin films were grown on (0001)-sapphire substrates using pulsed laser deposition. The photoluminescence spectrum of the highly conductive (1.3 × 103 S cm−1), as-grown AZO shows a poor near band edge (NBE) emission (3.30 eV) and no deep level emission at room temperature. In addition, the peak (3.386 eV) for the free excitons of AZO showed thermal quenching behavior with two activation energies (38.2 and 10.0 meV). The poor NBE emission is attributed to the nonradiative recombination center created by Al doping. Highly conductive (6.0 × 102 S cm−1) and intense NBE emitting AZO films could be achieved by the reduction of the nonradiative recombination centers through hydrogen annealing.

Experimental evidences are presented for the first time that singlet oxygen generated ex situ acts as an inhibitor of nonradiative recombination in porous silicon (PSi). This effect is observed on a pristine PSi as well as on degraded porous layers quenched by ozone adsorption. A photoluminescence (PL) enhancement produced by singlet oxygen is accompanied with only slight oxidation of a PSi. We assume that the observed effect on PL efficiency is due to gentle selective oxidation of single defects on silicon nanocrystal surface.

We studied the nonthermal mechanism of quantum efficiency (QE) roll-off in InGaN-based multiple-quantum-well LEDs on sapphire and bulk GaN substrates with In content ranging from 1% to 28%. Both the efficiency evolution and peak energy shift appear to be strongly dependent on the In content, but nearly independent of the dislocation density in the active region. The QE of the green LED peaks at a current density as low as 1.4 A/cm 2 , and decreases dramatically as current is increased, whereas the UV LED has a nearly constant QE at currents up to 1 kA/cm 2 . In contrast to minimal current-induced peak shift and spectral broadening in the near-UV and UV LED, the green LED has a monotonic peak blueshift of a total amount of ∼ 110 meV and a spectral broadening by a factor of 2. These results lead to a conclusion that carrier delocalization followed by nonradiative recombination at misfit defects is the major nonthermal mechanism of the efficiency roll-off in InGaN-based visible LEDs.

In this paper we will review our recent results from optical characterization studies of GaInNP. We will show that N incorporation in these alloys affects their structural and defect properties, as well as the electronic structure. The main structural changes include (i) increasing carrier localization due to strong compositional fluctuations, which is typical for all dilute nitrides, and (ii) N-induced long range ordering effects, specific for GaInNP. The observed degradation of radiative efficiency of the alloys upon increasing N content is attributed to formation of several defects acting as centres of efficient non-radiative recombination. One of the defects is identified as a complex involving a Ga interstitial atom. N incorporation is also found to change the band line up from the type I in the GaInP/GaAs structures to the type II in the GaInNP/GaAs heterojunctions with [N]>0.5%. For the range of N compositions studied ([N]≤2%), a conduction band offset at the GaInNP/GaAs interface is found to nearly linearly depend on [N] at 0.10 eV/%, whereas the valence band offset remains unaffected.

In this work we present a reliability study of low dislocation density InGaN laser diodes fabricated on high-pressure grown GaN monocrystaline substrates. The aging process was performed under pulse current conditions. Degradation of these devices manifests primarily in the increase of the threshold current. Interestingly, the differential efficiency of lasers remains stable at all times. The aging time dependence of the increase of the threshold current precisely follows a square root dependence. These observations suggest that the degradation results from the enhancement of the nonradiative recombination within the device active layers and is related to point defect diffusion.

In this work we present degradation studies of GaN based blue-violet laser diodes grown on different substrates. By replacing the SiC substrate by GaN substrate, we change from hetero to homo epitaxy, while the threading dislocation density (TDD) is reduced by 3 orders of magnitude. A detailed analysis of small signal I-V curves shows an increase of non radiative (NR) recombination centers during aging for laser diodes on SiC substrate. This was not observed for lasers on GaN substrate due to the reduced TDD and therefore reduced number of diffusion channels for Mg-atoms, acting as NR recombination centers in the active region. With an improved epitaxial structure on GaN substrate, we increased the lifetime of our lasers by a factor of 10.

Time-resolved pump-probe transmission measurements were used to determine the temperature dependence of carrier lifetime for InN epilayers with unintentionally doped levels from 10 18 to 10 19 cm -3 . The observed decay time at 20 K is well explained by a dominating radiative interband recombination, while at room temperature it is attributed to a defect related nonradiative recombination channel. The temperature dependence of the radiative lifetime is deduced from the measurements of both differential transmission decay time and PL intensity. For the best quality sample, we find the radiative lifetime increases proportionally to T 3/2 , as theory predicts when a k-selection rule holds, which suggests that the radiative band-to-band transition accounts for the observed infrared photoluminescence over the entire temperature.

The effects on structural crystalline quality and luminescence properties of deflecting nitrogen (N) ions away from the substrate were investigated for GaPN layers grown on GaP substrates by RF-plasma MBE. The GaPN layers were grown with and without use of an ion collector for N ions deflection. The Pendellosung fringes in the X-ray diffraction spectrum were more distinct in samples grown with than without the ion collector. This result indicates that the structural and/or interface quality were improved by preventing ion-induced damage. The sample grown with the ion collector indicated higher crystalline quality by the more linear excitation power dependence of photoluminescence intensity. On the other hand, the sample grown without the ion collector appears to contain a high concentration of non-radiative recombination centers related to the ion-induced damage. The ion-induced damage cannot be completely healed by rapid thermal annealing. To summarize, eliminating ion exposure during growth improves the crystalline quality and radiative recombination properties of the GaPN layer.

Temperature-dependent photoluminescence (PL) was exploited to investigate the mechanism of luminescence of MCM (Mobil Composition of Matter)-41 and MCM-48 nanotubes. The PL intensity has a maximum around 40 K. Localization of the carriers involved in the radiative recombination was deduced from the PL decay profiles at various energies. A model based on competition between radiative recombination of localized carriers and nonradiative recombination is suggested to explain the temperature-dependence of PL intensity. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of the non-radiative recombination center at or around the dislocations on the performance of the AlGaN-based UV light emitting diode (LED) is discussed. For high-efficiency emission, it is necessary to reduce the density of dislocations to less than 5 × 10 7 cm -2 .

The photoacoustic technique under heat transmission configuration is used to determine the effect of doping on both the thermal and transport properties of p- and n-type GaAs epitaxial layers grown on GaAs substrate by the molecular beam epitaxial method. Analysis of the data is made on the basis of the theoretical model of Rosencwaig and Gersho. Thermal and transport properties of the epitaxial layers are found by fitting the phase of the experimentally obtained photoacoustic signal with that of the theoretical model. It is observed that both the thermal and transport properties, i.e. thermal diffusivity, diffusion coefficient, surface recombination velocity and nonradiative recombination time, depend on the type of doping in the epitaxial layer. The results clearly show that the photoacoustic technique using heat transmission configuration is an excellent tool to study the thermal and transport properties of epitaxial layers under different doping conditions.

This paper presents failure modes observed in long-term aging of high-brightness GaN/InGaN LEDs. The blue LEDs submitted to DC aging test present large decrease of emitted optical power and increase of diode reverse leakage current. Increase of parasitic series resistance, suggesting contact degradation, has also been found in stressed sample, together with apparent carrier density increases and reduction of the junction depletion width. Furthermore stressed LEDs present modification of a specific trap property: trap activation energy decreases from 340 meV in the virgin sample down to 75 meV in the stressed sample. Generation of non-radiative recombination centers seems to be one of the dominant failure mechanisms responsible for the observed electrical and optical LED degradations.

The influence of nonradiative recombination processes on the output power P of InGaN light emitting devices (LEDs) with emission wavelength between 380 and 430 nm grown on sapphire substrates have been investigated. The electroluminescence intensity was studied as a function of the forward current I and the device temperature. The P-I curves of all LEDs show a super-linear behaviour at low currents (between 80 μA and 1 mA) and a nearly linear characteristic around the operating forward current of 20 mA (equivalent to a current density of 44 A/cm 2 ). The super-linear behaviour is attributed to the saturation of nonradiative processes which also affect the output power at operating forward current. The reduction of the output power at 20 mA with increasing temperature is correlated with the super-linear behaviour at low currents indicating that nonradiative processes are thermally activated.

Room temperature photoluminescence (PL) and piezoelectric photothermal (PPT) spectroscopy were successfully performed on two different ZnS epitaxial layers grown by pyrolytic MOVPE on GaAs(100) by using diethyldisulphide (Et 2 S 2 ) as sulphur precursor at 390 °C and 435 °C. PL spectra allowed to identify several radiative recombination channels [namely, self-activated (SA) bands] associated to deep centres. Intense SA band occurred in PL and in PPT spectra recorded from ZnS grown at low temperature, indicating that RT radiative and non-radiative transitions both increases with decreasing the growth temperature below 400°C.

Temperature and injection current dependence of electroluminescence (EL) spectral intensity of the super-bright InGaN single-quantum-well (SQW) light-emitting diodes (LEDs) has been carefully investigated over a wide temperature range (T=15-300 K) and as a function of injection current level (0.1-10 mA). It is found that, when Tis slightly decreased from 300 to 140 K, the EL intensity efficiently increases due to reduced non-radiative recombination processes. However, with further decrease of T below 100 K, it drastically decreases due to the reduced carrier capturing by the localized radiative recombination centers and shows a clear trend of saturation with current, accompanying decreases of the EL differential efficiency. These results are analyzed based on a rate equation model, assuming a finite number of radiative recombination centers.

In this contribution we present a theoretical analysis obtained by coupling a self-consistent solution of Schrodinger and Poisson equations to determine the electronic states in nanostructures with a rate equation model to account for time dependent effects of charge rearrangement. We demonstrate that PL emission is influenced by charge accumulation in the well, and loss of carriers from the ground level induced by both radiative and non-radiative recombination processes. We have investigated the effects of charge accumulation under different condition for PL excitation.

We investigated the photoluminescence intensity and material gain values of GaN based laser structures with various well widths in the active layer by temperature dependent photoluminescence and optical gain measurements. The existence of two different nonradiative recombination channels can be demonstrated by temperature dependent photoluminescence measurements at low excitation densities. This results in two thermal activation energies which can be attributed to distinct activation processes. The influence of the nonradiative recombination is also reflected by the temperature dependence of the required material gain values.

The nature and role of electronic defects in amorphous diamond-like carbons (DLC) is described. The defect has recently been shown by electron spin resonance to be an unpaired spin on a small sp 2 -bonded cluster. The defect density is large in DLC so defect states form continuous distribution of states with tail states. Defects and tail states can act as non-radiative recombination centres to quench photoluminescence, depending on the position of the demarcation levels.

We present a study of optical properties in GaN based structures and devices. Our approach is based on a rate equation model, where the role of carrier screening on recombination rates is self-consistently included. We demonstrate that Time-Resolved Photoluminescence (TR-PL) in gallium-nitride quantum wells is influenced by two main effects: (i) the charge accumulation in the well, caused by the separation of the wavefunctions and by the increase of the radiative recombination time induced by the built-in field, and (ii) the loss of carriers from the ground level induced by the non-radiative recombination processes.

Time-resolved optical characterization is an indispensable tool to study the recombination mechanisms of excitons and/or carriers based on radiative, non-radiative, localization and many-body processes. In this paper, we review the instrumentation of various spectroscopic techniques for the assessment of In x Ga 1 -x N-based semiconductors such as time-resolved photoluminescence (TRPL), time-resolved electroluminescence (TREL), transient grating (TG) method to probe photothermal processes, microscopic TRPL using optical microscope, submicroscopic TRPL using scanning near field optical microscopy (SNOM) and pump-and-probe spectroscopy for the measurement of transient absorption/gain spectra. The obtained results are cited in the references.

The ultrafast nonradiative bimolecular recombination of charge carriers was investigated in amorphous and microcrystalline silicon. The photoelectric method based on measurement of extraction time of the charge carrier reservoir in the space-charge-limited photocurrent mode was used. A series of models (Auger, diffusion-controlled, charge-carrier-fluctuation dominated) were experimentally tested but were not able to explain all features of bimolecular recombination in a-Si:H. The model of diffusion limited recombination taking into account the intrinsic random potential seems most suitable for a description of the observed suppressed bimolecular recombination.

The effect of locally introduced dislocations on time-resolved electroluminescence (EL) in GaAlAs, GaAsP and GaP p-n structures is studied. The data indicate that the local nonradiative recombination results in non-exponential EL decay. A one-dimensional model of this effect, involving back-diffusion of injected electrons, their extraction into the n-region and local recombination at dislocations and the surface, is proposed. An analysis of EL decay, based on this model, enables to estimate the bulk lifetime of minority carriers and some parameters of local nonradiative recombination.

Photoluminescence of lead tungstate and its time evolution at different temperatures and excitation conditions were investigated to identify the mechanisms of radiative and nonradiative recombination in this scintillation crystal. Two centers responsible for short-wavelength emission were revealed and characterized. The results indicated the energy transfer between different centers to be of great importance. Red luminescence was shown to be useful for quality control of comparatively perfect crystals. [Russian Text Ignored].

The effect of hydrogenation and annealing upon photoluminescence of MBE-grown AlGaAs: Si epitaxial layers is examined in detail. Nonradiative recombination centers passivated by hydrogen are found to be reactivated in a wider temperature range in comparison with Si donors or DX centers. This process cannot be described by first-order kinetics. [Russian Text Ignore]

To study the effect of damage induced during the Zn diffusion on the properties of the material, p-n junctions, obtained by zinc diffusion, are investigated by electron beam induced current (EBIC) at different temperatures. At helium temperature, the EBIC decay on the p-side is flattened and non exponential. This effect is attributed to the defects induced by the Zn-diffusion that act as non-radiative centers. The room temperature EBIC signal has a symmetric exponential decay on both sides of the junction. The EBIC measurements for different temperatures and electron beam voltages indicate that the surface recombinations affect the carrier transport at room temperature hindering the non-exponential EBIC decay observed at helium temperature. The recombination dynamic is affected stronger by the as grown (001) surface than by the cleaved (110) surface.

The recombination properties of minority carriers are determined from EBIC and CL measurements versus the primary electron beam energy on step cooling LPE grown n-type GaAs:Sn (3 × 1016 cm-3 ≦ n0 ≦ 1.2 × 1018 cm−3) in the temperature range 85 K ≦ T ≦ 300 K. The diffusion length and the averaged absorption coefficient decrease with decreasing temperature. As a consequence the increase in external luminescence strength versus diminished temperature is caused by the lowered self absorption and by the rising internal quantum efficiency. With the help of diffusion length and internal quantum yield analysis as a function of temperature, the non-radiative recombination due to the deep Sn-acceptor is shown on strongly doped samples (n0 ≧ 1018 cm−3) at T≦ 200 K. An n-leitendem, durch einen ‘step-cooling’ LPE-Prozes hergestellten GaAs:Sn (3×l016 cm−3 ≦ n0 ≦ 1,2 × 1018cm−3) werden die Rekombinationseigenschaften der Minoritatsladungstrager mit-tels energieabhangiger EBIC- und KL-Messungen im Temperaturbereich 85K ≦ T≦ 300 K unter-sucht. Die Diffusionslange und der mittlere Absorptionskoeffizient nehmen mit fallender Temperatur ab, so das die Zunahme der externen Lumineszenzintensitat durch die verringerte Selbstabsorptiorptin und den anwachsenden internen Quantenwirkungsgrad hervorgerufen wird. Unter Nutzungder Temperatur-abhangigkeit der Diffusionslange und des Quantenwirkungsgrades wird die Effektivitat des nicht strahlenden Rekombinationskanals unter Einschlus des tiefen Zinnakzeptors an starker dotierten Proben (n0 ≧ 1018cm−3) unterhalb 200 K nachgewiesen.

Degradation processes in AlGaAs/GaAs double heterostructure light emitting diodes (LEDs) are investigated by measuring of C—U and I—U characteristics, light output, DLTS and electroluminescence (EL) spectra. It is found that the DLTS peak, the p—n junction capacitance, and light output decrease when the LEDs are operated at elevated temperatures. A decrease of acceptor concentration is proved from the correlated change of the DLTS peak and capacitance. Mittels Messungen der C—U- und I—U-Charakteristiken, Lichtausgangsleistung, DLTS- und Elektrolumineszenz (EL)-Spektren werden Degradationsprozesse in lichtemittierenden AlGaAs/GaAs-Doppel-heterostrukturdioden (LEDs) untersucht. Es wird gefunden, das das DLTS-Maximum, p—n-ubergangskapazitat und Lichtausgangsleistung abnehmen, wenn die LEDs bei erhohten Temperaturen betrieben werden. Aus den korrelierten anderungen von DLTS-Maximum und Kapazitat wird eine Abnahme der Akzeptorkonzentration nachgewiesen.

Free and bound excitons and (D, A) recombination related luminescence of selectively doped MBE grown GaAs structures, consisting of GaAs: Si layers sandwiched between undoped buffer and cap layers, are studied in dependence on the doped layer thickness. The observed changes in luminescence intensity are explained by a model assuming the dominant contribution of the Si-doped layers of effective thickness exceeding several times their nominal thickness. The systematic shift in position of the (e, A) line with increasing thickness of the doped layer is explained by the shift of the Fermi energy due to the Si donors. [Russian Text Ignored.]

Comparaison entre les spectres de luminescence de As 2 Se 3 synthetise sous une atmosphere d'argon tres pur et sous atmosphere ambiante. Les intensites et les formes des deux spectres en presentent pas de differences significatives, soulignant ainsi que les impuretes incorporees au cours de la synthese des cristaux n'ont pas d'influence sur leur photoluminescence

Based on experimental evidence models are tentatively suggested to identify deep-level defects in Gap related to the phosphorus vacancy Vp. Especially, the complex (ZnGa, Vp) is proposed to be a dominant nonradiative recombination centre on the n-side of non-perfectly diffused junctions. During operation of light-emitting diodes, a specific mode of degradation may occur due to the recombination-assisted generation of the phosphorus vacancy. Moreover, defect reactions are studied associated with Vp related deep-level defects in Gap. Es wird auf die Natur von Storstellen in GaP eingegangen, die die Phosphorleerstelle enthalten. Insbesondere wird der (ZnGa, Vp)-Komplex untersucht, der ein dominierendes strahlungsloses Rekombinationszentrum auf der n-Spite von unzureichend diffundierten p-n-Ubergangen darstellt. In derartigen Dioden kann ein spezifischer Degradationsprozefs auftreten, der durch die rekom-binationsunterstutzte Generation von Phosphorleerstellen verursacht wird. Daneben findrn wahrend der Degradation Defektreaktionen statt, die mit der Phosphorleerstelle gekoppelt sind.

A detailed analysis of the excitation and recombination processes of the charge carriers in CdIn2S4 crystals is discussed. The luminescence spectra and the luminescence excitation spectra at temperatures ranging from liquid helium temperature to room temperature are presented; the effect of a secondary low energy radiation on luminescence is also discussed. Models are proposed for the excitation and recombination processes and a scheme of the levels localized in the gap of CdIn2S4. The two emission bands observed are attributed to an interimpurity recombination and to an electron transition between a donor level and the valence band, respectively; the presence of a non radiative recombination process is also proposed. Dans ce papier nous discutons un'analyse detaillee des processus d'excitation et de recombinaison des porteurs de charge dans le CdIn2S4. Nous presentons les spectres de luminescence et leur dependence de la temperature dans la rangee 2 a 300 K et les spectres d'excitation de la luminescence dans la měme rangee; finalement nous discutons l'effet sur les spectres d'emission d'une illumination secondaire a baisse energie. Nous proposons un modele des processus d'excitation et de recombinaison et un schema des niveaux localizes dans le gap de CdIn2S4. Nous pouvons attribuer les deux bandes d'emission observees l'une a une recombinaison donneur-accepteur et l'autre a une transition d'electrons entre un niveau donneur et la bande de valence respectivement. Nous proposons finalement la presence d'un processus de recombinaison non radiative.

The intensity of photoluminescence (PL) emission bands in quenched CdS:Cu:Cl powder samples is measured and it turns out to be a power function of the concentrations of Cl, NCl, and Cu, NCu, in the material. The simplifications in which case the quasichemical theory of defect formation in solids gives such a simple dependence are clarified. The method to determine the structure of recombination centres in AII BVI compounds, based on the comparison of experimental and theoretical graphs, is proposed. The method is used for clarifying the nature of r- and k-PL centres (λmax=1030 nm and λmax=780 nm, respectively) and s-centres of non-radiative recombination, and also predominant defects Cl and Cu in CdS:Cu:Cl. The r-, k- and s-centres may be identified with the following defects of crystal lattice of CdS:r-centre – (CuCdClS), k-centre – (CuCd)2; s-centre – (CuCdCui)2 at NCu>NCl and ClS at NCu

The move toward lower cost sources of solar silicon has intensified the efforts to investigate the possibilities of passivating or reducing the recombination activity caused by deep states associated with transition metals. This is particularly important for the case of the slow diffusing metals early in the periodic sequence which are not removed by conventional gettering. In this paper, we examine reactions between hydrogen and transition metals and discuss the possibility of such reactions during cell processing. We analyse the case of hydrogenation of iron in p-type Si and show that FeH can form under non-equilibrium conditions. We consider the electrical activity of the slow diffusing metals Ti, V, and Mo, how this is affected in the presence of hydrogen, and the stability of TM-H complexes formed. Finally, we discuss recent experiments which indicate that re-siting of some transition metals from the interstitial to substitutional site is possible in the presence of excess vacancies, leading to a reduction in recombination activity. Periodic ordering of the 3d, 4d, and 5d transition metals with adjacent columns of the periodic table. The metals with hydrogen complexes which have had their electronic properties reported in the literature or in this paper are highlighted in green.