Photoluminescence Quantum Yield in Ensembles of Si Nanocrystals

Abstract

energy exchange with defects. [2,12] The strength of the cooperative processes and their effect on the optical properties of an NC ensemble depend on the characteristics of the individual NCs themselves as well as on the ensemble properties, such as NC density and proximity, [13] confining potential of the embedding matrix [14] and its quality, etc. [6] The cooperative processes typically involve an energy barrier for their activation, and therefore will change with the excitation energy. On the other hand, it is well known that the Kasha-Vavilov rule, [15] which states that the PL QY is independent of the excitation energy, is frequently violated for colloidal semiconductor NCs, [16] since carriers that are photogenerated higher in the conduction/ valence bands experience an increased probability of capturing at defect states and/or escape to the outside of an NC, leading to its ionization [10] and a temporal loss of optical activity. In result, the PL QY of the NCs decreases typically at short pump wavelengths. In this study, we investigate the excitation energy dependence of the PL QY for Si NC layers and find that it varies strongly in different samples. We investigate and discuss possible physical mechanisms influencing the PL QY at different excitation energies and conclude on an important role of impact excitation and parasitic absorption. Sample PreparationThe samples used in this study were produced by a co-sputtering method in the form of multilayer (ML) structures, featuring multiple stacks (up to 100) of 3.5 nm thin active layers of Si NCs separated by SiO 2 barriers. In this process, two sputtering guns, with Si and SiO 2 targets, are used. By operating both guns simultaneously, an Si-rich substoichiometric SiO x "active" layer can be grown, while solely the gun with silicon dioxide is applied to develop a barrier layer of pure SiO 2 . The atomic composition of the active layer can be tuned by adjusting the power of the guns, and the layer thickness is controlled by the exposure time. For the samples investigated in this study, the silicon excess of 15%, 25%, and 30% was used in the active layer. The barrier thickness between two adjacent active layers was set at 5 nm to avoid exciton diffusion between these layers. [17] Extensive past research [18,19] has shown that, in contrast to thick homogeneous layers of Si NCs in SiO 2 , ML structures allow for a better size control and therefore yield ensembles with a more narrow size distribution.This study investigates the photoluminescence quantum yield for cosputtered solid-state dispersions of Si nanocrystals in SiO 2 with different size and density, and concludes that the absolute value of the photoluminescence quantum yield shows a varied dependence on the excitation energy. Physical mechanisms influencing the photoluminescence quantum yield at different excitation energy ranges are considered. Based on the experimental evidence, this study proposes a generalized description of the excitation dependence of photoluminescence quantum yield of Si nanocrys...

Limits of emission quantum yield determination

“…The QY is widely used also in research: In the past decade, research on quantum dot 'phosphors' has been relying on the QY methodology to show various size-1-3 , excitation- [4][5][6][7] or concentration-dependent properties 4,8 . Several guidelines exist for the QY measurements, 9,10 discussing e.g.…”

Spectral Dependencies of the Stretched Exponential Dispersion Factor and Photoluminescence Quantum Yield as a Common Feature of Nanocrystalline Si

“…Therefore, only the reduction of PL QY for very small NC sizes is a general feature of literature data. Such a broad diversity of published results can be well understood as there are many parameters that were proved to directly influence PL QY: crystallinity of NC‐core, agent of surface capping, quality of surface passivation, medium (liquid solvent or solid matrix), NC concentration, excitation intensity, and wavelength . That makes the study of NC ensembles very complex.…”

Quantum Yield Bias in Materials With Lower Absorptance

“…2); this suggests that the observed excitation and concentration QY dependences are caused by the same effect. One might argue that, for semiconductor QDs, the process of emission and absorption is more complex than that for R6G, and therefore, the validity of the Kasha-Vavilov rule [31] might be weaker or not hold at all; this has led to various discussions of interesting novel physical phenomena in the recent literature [9][10][11][12][13][14][15][16][17][18]. The striking similarity in the behavior of R6G and the two types of QD materials indicates that the origin of the underestimation is the same, suggesting that a critical assessment and caution are required when interpreting such data, especially for samples absorbing below approximately 10%-15%.…”

“…The role of these effects is often neglected in the literature, while QY is frequently used to characterize materials, such as semiconductor QDs, to study the emission efficiency dependence, e.g., on size [11,13] and density [12] of the QDs, excitation energy [9,14,15], or to show ligand instability [10]. Explicit QY dependence on various parameters has been interpreted in the past, in terms of novel effects [9][10][11][12][13][14][15][16][17][18], as a result of the size polydispersity of the QD ensembles, leading to a broadening of the emission spectrum [17], which, in turn, could lead to excitation-wavelength-dependent QY via excitation of different subsets of the QD ensemble; or one could argue that the concentration of the QD dispersions is expected to affect the QD interactions [9,14,18], which strongly depend on the interparticle distance. Our finding of a systematic bias of QY measurements indicates that care should be taken in the interpretation of these measurements and suggests plotting the QY of the QDs (or other materials) versus their single-pass absorptance to check for an underlying bias.…”

“…where N and N * denote the integrated intensities (numbers of photons) at the excitation and emission wavelengths, respectively; I (λ) is the detected spectrum; and C is the correction factor for the spectral response of the detection system. The IS methodology is considered to be the most robust and reliable [1,8] and has been employed to study a wide variety of materials and for the discovery of novel photophysical effects [9][10][11][12][13][14][15][16][17][18]. The IS method is also utilized by the lighting industry, where commercial devices based on the IS method for QY measurements can be purchased from several spectroscopic companies [19,20].…”