Non-Radiative Processes in Crystals and in Nanocrystals

Abstract

This paper discusses non-radiative processes that are relevant to the luminescence characteristics of optically active ions doped into insulators or large-gap semiconductors, with particular attention to how these processes are affected as the particle size is reduced from bulk single crystals to as small as a few nanometers. The non-radiative processes discussed in this article are thermal line broadening and thermal line shifting, relaxation via phonons between excited electronic states, vibronic emission and absorption, and phonon-assisted energy transfer. Given that one of the main effects of confinement in these particles is on the phonon density of states, we pay particular attention to how these non-radiative processes are altered due to the change in the phonon density of states as particle size decreases. Inorganic insulators doped with rare-earth ions and transition metal ions represent an important class of luminescent materials for many applications, including phosphors for lighting, scintillators, solid-state laser materials, bio-markers for imaging, and nanothermometry. Following excitation by radiation, the optical ions usually undergo some degree of non-radiative relaxation releasing a part or all of its energy to the lattice. During the non-radiative relaxation, all or part of the electronic energy initially stored in the optically active ion is converted into phonons.From the perspective of energy efficiency of luminescent materials, it may seem beneficial to attempt to eliminate non-radiative processes altogether. However, the conversion of electronic energy into heat energy following excitation is, in fact, desirable to many applications. Non-radiative processes play an important role in converting light from the blue LED into red and green light for white light generation in lamp phosphors, for efficient operation of many solid-state lasers, for energy transfer and multiphonon relaxation in bio-imaging, and also for the establishment of thermal equilibrium, the principle on which thermometry is based. For these technologies to be optimized, non-radiative processes must be controlled, or at least carefully considered. Thus, understanding these processes is of great interest to the luminescence community. The specific non-radiative processes addressed in this work are thermal line broadening, thermal line shifting, decay via a phonon from one electronic level to another, vibronic transitions, and phonon-assisted energy transfer.The main focus of this work is on how non-radiative processes are affected as the particle size decreases into the nano-regime. Generally speaking, the two main effects of going from the bulk to the nano are:(1) an increase in the surface to volume ratio, and (2) a reduction in the phonon density of states. Item (1) often leads to an increase in defect sites (mostly near the surface) and surface states, which in turn tends to decrease the luminescence efficiency as one goes from the bulk to the nano. This nonradiative process is outside the scope of this article. Item (...

Nanophosphors-Based White Light Sources

“…Notably, luminescence increasing rather than decreasing with increasing dopant content was reported in nanoparticles: Y 2 O 3 doped with a Tb 3+ shows concentration quenching at concentrations above ~5% in large sized particles and luminescence intensity increasing even at the dopant amount of 50% in as small as 3 nm (“as synthesized”) sized nanoparticles [88]. In order to explain this experimental evidence, reduced probability of ion–ion energy transfer due to out of resonance conditions and the absence of phonon modes able to participate in the energy transfer process were addressed [89].…”

“…At the nanoscale regime, the density of phonon states changes from a continuous (Debye approximation) to a discrete distribution, exhibits a size-dependent cutoff frequency for phonon modes and is unchanged for high frequencies phonon modes [89,105,106,107]. Since the low-frequency phonons contribute effectively to non-radiative relaxation processes between closely spaced energy levels, a lack of low-frequency phonon reduces losses and affects the luminescence dynamics of optically active ions.…”

“…Indeed, in a nanoparticle, core and surface atoms exhibit different coordination and symmetry of the surroundings, meaning that distorted surface bonds introduce defects and energy levels shifting as compared to the ones of the same core counterpart. This condition of out of resonance between energy levels of core and surface ions implies it requires emission or absorption of at least one phonon to bridge the energy difference [89,107].…”

“…In general, homogeneous line broadening results from temperature-dependent phonon coupling (weak ion–phonon coupling) and electron–lattice interactions implies temperature-dependent line shifts. Based on theoretical considerations, size-dependent broadening and shifting of the spectral lines can be likely observed only in the limit of low temperatures (depending on the phonon density of states and occupancies of the phonon modes) and particle size much smaller than 50 nm (due to inverse relationship between the electron–phonon coupling and size) [89]. In regard to the emission decay-patterns of lanthanide-activated nanophosphors, which characterize the lifetime and decay mechanism of the optical center, deviations from the conventional exponential law of bulk phosphors and multi-exponential decay patters were observed.…”

“…Further effects of the spatial confinement and its influence on electron–phonon coupling are: (i) modified temperature-dependence (from T 7 to T 3 ) of the linewidth of characteristic transitions of Eu 3+ in Y 2 O 3 and Eu 2 O 3 nanoparticles [105,112], (ii) increased lifetime (from 220 ns to 27 μs) of a multiplet of Eu 3+ in Y 2 O 3 nanocrystals, as compared to the bulk counterpart [113], and (iii) size-dependent elimination of direct phonon relaxation in Er 3+ -doped Y 2 O 2 S nanocrystals (anomalous thermalization) [105]. In addition, the vibronic coupling strength of vibronic transitions (i.e., radiative emissions concurrent with the absorption or emission of at least one phonon) is expected to change turning from bulk phosphors to nanophosphors [89].…”