Energy levels, donor-acceptor

Figure 6.9. Ideal donor-acceptor energy level arrangement for triplet-triplet energy A transfer.

Intensive effort has been devoted to the optimization of CCP structures for improved fluorescence output of CCP-based FRET assays. The inherent optoelectronic properties of CCPs make PET one of the most detrimental processes for FRET. Before considering the parameters in the Forster equation, it is of primary concern to reduce the probability of PET. As the competition between FRET and PET is mainly determined by the energy level alignment between donor and acceptor, it can be minimized by careful choice of CCP and C. A series of cationic poly(fluorene-co-phenylene) (PFP) derivatives (IBr, 9, 10 and 11, chemical structures in Scheme 8) was synthesized to fine-tune the donor/acceptor energy levels for improved FRET [70]. FI or Tex Red (TR) labeled ssDNAg (5 -ATC TTG ACT ATG TGG GTG CT-3 ) were chosen as the energy acceptor. The emission spectra of IBr, 9, 10 and 11 are similar in shape with emission maxima at 415, 410, 414 and 410 nm, respectively. The overlap between the emission of these polymers and the absorption of FI or TR is thus similar. Their electrochemical properties were determined by cyclic voltammetry experiments. The calculated HOMO and LUMO... 

Figure 6.14 Three donor-acceptor energy level schemes for quenching by energy transfer. 1. Benzophenone (D)+naphthalene (A) ...

Dimole absorption and emission, 247 Dioxetane formation, 253 Donor-acceptor energy levels, 201 property, energies of, 289 Dipole-dipole resonance energy transfer, 192, 193... 

Figure 4.5. The effect of increasing temperature on the position of the donor/acceptor energy levels within the bandgap for various doping levels, Nd or Na- Reproduced with permission from Kasap, S. O. Principles of Electronic Materials and Devices, 3rd ed., McGraw-Hill New York, 2007. Copyright 2006 The McGraw-Hill Companies.

Fig. 2. Representation of the band edges in a semiconductor p—n junction where shallow donor, acceptor energies, and the Fermi level are labeled Ejy E, and E respectively, (a) Without external bias is the built-in potential of the p—n junction (b) under an appHed forward voltage F. ...

Cadmium Sulfide Photoconductor. CdS photoconductive films are prepared by both evaporation of bulk CdS and settHng of fine CdS powder from aqueous or organic suspension foUowed by sintering (60,61). The evaporated CdS is deposited to a thickness from 100 to 600 nm on ceramic substates. The evaporated films are polycrystaUine and are heated to 250°C in oxygen at low pressure to increase photosensitivity. Copper or silver may be diffused into the films to lower the resistivity and reduce contact rectification and noise. The copper acceptor energy level is within 0.1 eV of the valence band edge. Sulfide vacancies produce donor levels and cadmium vacancies produce deep acceptor levels. 

On the other hand, in extrinsic detectors, electrons or holes are created by incident radiation with photons of energy mnch lower than the energy gap. As can be observed from Fignre 3.11(b), the inclnsion of impnrities leads to donor and/or acceptor energy levels within the semicondnctor gap. Thns, the energy separation between these impnrity levels and the valence/condnction bands is lower than the energy... 

For quenching by energy transfer mechanism, the quencher must have suitable energy levels, singlet or triplet, near or below the energy level of the donor molecule. Such a transfer has the greatest probability if there is an approximate resonance between the donor and the acceptor energy levels. 

Short range transfer by exchange mechanism occurs when donor and aceer. electronic wavefunctions spatially overlap. The rate follows diflusion-contro",. kinetics if donor and acceptor energy levels are in near resonance. Transit forbidden by dipole-dipole mechanism may occur by exchange mechanism, e.g... 

Hwang et al. [19] reported that the acceptor energy level of the phosphorus (P) impurity was estimated to be located at 0.127 eV above the valence band on the study of the free electron to the acceptor transition at 3.310 eV from the photoluminescence spectra of P-doped p-type ZnO films grown by rf-frequency magnetron sputtering. They also suggested that the emission lines at 3.310 and 3.241 eV of the photoluminescence spectra could be attributed to a conduction band to the P-related acceptor transition and a donor to the acceptor pair transition, respectively. 

Figure 13.9 (a) (i) Donor impurity in a crystal of an extrinsic semiconductor and (ii) the associated energy-band diagram donor impurities add donor energy levels below tbe conduction band, (b) (i) Acceptor impurity in a crystal of an extrinsic semiconductor and (ii) the associated energy-band diagram acceptor impurities add acceptor energy levels above the valence band... 

All teclmologically important properties of semiconductors are detennined by defect-associated energy levels in the gap. The conductivity of pure semiconductors varies as g expf-A CgT), where is the gap. In most semiconductors with practical applications, the size of the gap, E 1-2 eV, makes the thennal excitation of electrons across the gap a relatively unimportant process. The introduction of shallow states into the gap through doping, with either donors or acceptors, allows for large changes in conductivity (figure C2.16.1). The donor and acceptor levels are typically a few meV below the CB and a few tens of meV above the VB, respectively. The depth of these levels usually scales with the size of the gap (see below). 

Shallow impurities have energy levels in the gap but very close to a band. If an impurity has an empty level close to the VB maximum, an electron can be thennally promoted from the VB into this level, leaving a hole in the VB. Such an impurity is a shallow acceptor. On the other hand, if an impurity has an occupied level very close to the CB minimum, the electron in that level can be thennally promoted into the CB where it participates in the conductivity. Such an impurity is a shallow donor. 

The impurity atoms used to form the p—n junction form well-defined energy levels within the band gap. These levels are shallow in the sense that the donor levels He close to the conduction band (Fig. lb) and the acceptor levels are close to the valence band (Fig. Ic). The thermal energy at room temperature is large enough for most of the dopant atoms contributing to the impurity levels to become ionized. Thus, in the -type region, some electrons in the valence band have sufficient thermal energy to be excited into the acceptor level and leave mobile holes in the valence band. Similar excitation occurs for electrons from the donor to conduction bands of the n-ty e material. The electrons in the conduction band of the n-ty e semiconductor and the holes in the valence band of the -type semiconductor are called majority carriers. Likewise, holes in the -type, and electrons in the -type semiconductor are called minority carriers. 

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