Exciton Tunneling Research Articles

Critical concentrations for triplet exciton tunneling in binary naphthalene crystals: the case for percolation

Reduced concentration curves for triplet exciton transport are scaled by the critical concentrations in four distinct isotopic-mixed ternary systems. These systems with varying lifetimes, sensor concentrations and guest-host energy separations, are in excellent agreement with a two-dimensional cluster model, based on long-range percolation functions, without adjustable parameters. This supports “energy percolation”.

Exciton percolation. II: Naphthalene 1B 2u supertransfer

The exciton percolation theory has been tested for the migration of the lowest singlet exciton in our model organic alloy system at low temperature (2 K): binary isotopic mixed crystals of naphthalene (C 10H 8—C 10D 8), with an added exciton sensor (supertrap) of betamethylnaphthalene (about 10 −3 mole fraction). At these relatively high sensor concentrations the system's migration dynamics are described quantitatively by the simple limiting case of supertransfer, i.e. by a dynamic percolation formulation of Hoshen and Kopelman that depends only on the concentrations and interaction topology of the ternary crystal. Experimentally, the exciton's migration dynamics is monitored by fluorescence spectra, taken under controlled conditions (crystal quality, purity, concentration, excitation and temperature). The effects of exciton tunneling (superexchange), exciton—phonon coupling, coherence, exciton delocalization and non-equilibrium chemical solubility are considered. We show that a complete concentration study of this simple energy transport model system reveals four kinds of exciton transfer regions, but only one (above the critical guest site percolation concentration) with long ranged, multistep, direct guest exciton transport. The same study, on the low concentration side, yields relative trapping efficiencies and thus eliminates the need for adjustable parameters. The substitutional randomness of the isotopic mixed crystal is confirmed, as well as the short range nature of the 1B 2u exciton interactions and the dominance of the interchange equivalent, nearest neighbor, pairwise (molecular) excitation ( 1B 2u) exchange integrals in the pure, perfect, low-temperature, naphthalene crystal. Our present exciton percolation study is also relevant to the primary energy transport process in heterogeneous photosynthetic units.

Optical investigations of the undulation spectrum of GaP:N:Zn

The undulation spectrum occurs in the luminescence of GaP when it is doped with the isoelectronic trap, nitrogen, and an acceptor---in the present case Zn. A broad emission band is observed which extends about 20 meV below the nitrogen bound-exciton energy ${E}^{A}$ and contains regular undulations. We show that the luminescence originates from the decay of excitons bound to N-Zn pairs of up to 50-\AA{} separation, and that the undulatory behavior is a result of averaging the numbers of equivalent sites in the pair-separation shells. The N-Zn interaction is investigated by time-resolved resonant excitation and temperature-dependent measurements. The 1.6-K decay lifetime of the luminescence decreases sharply with luminescence energy from \ensuremath{\sim} 1 \ensuremath{\mu}sec at ${E}^{A}$ to about 50 nsec at the low-energy limit of the undulation spectrum. The splitting of the exciton energy levels increases up to 5 meV as the N-Zn separation decreases and the dominant interaction is thought to be an internal Stark effect. Spatial energy transfer by tunneling of excitons is important and determines the temperature-dependent nonradiative recombination.

Energy transfer by exciton tunneling in GaP : N

Energy transfer by tunneling of bound excitons in GaP : N is investigated by time-resolved resonant excitation spectroscopy. Evidence for this process is observed in below band gap excitation spectra and the transfer rate is deduced from time-resolved measurements using a simplified model.

Exciton Energy Transfer in GaP: N

Energy transfer of bound excitons in GaP: N is investigated by time-resolved, resonant-excitation spectroscopy and by studies of below-band-gap excitation spectra. Exciton tunneling is found to dominate over dipole-dipole interactions because of the large spatial extent of the exciton wave function. This is the first definitive observation of exciton tunneling in a semiconductor.