Abrupt Size Partitioning of Multimodal Photoluminescence Relaxation in Monodisperse Silicon Nanocrystals Samuel L. Brown,† Joseph B. Miller,†,§ Rebecca J. Anthony,‡,⊥ Uwe R. Kortshagen,‡ Andrei Kryjevski,† and Erik K. Hobbie*,† †
North Dakota State University, Fargo, North Dakota 58108, United States University of Minnesota, Minneapolis, Minnesota 55455, United States
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S Supporting Information *
ABSTRACT: Intrinsic constraints on efficient photoluminescence (PL) from smaller alkene-capped silicon nanocrystals (SiNCs) put limits on potential applications, but the root cause of such effects remains elusive. Here, plasmasynthesized colloidal SiNCs separated into monodisperse fractions reveal an abrupt size-dependent partitioning of multilevel PL relaxation, which we study as a function of temperature. Guided by theory and simulation, we explore the potential role of resonant phonon interactions with “minigaps” that emerge in the electronic density of states (DOS) under strong quantum confinement. Such higherorder structures can be very sensitive to SiNC surface chemistry, which we suggest might explain the common implication of surface effects in both the emergence of multimodal PL relaxation and the loss of quantum yield with decreasing nanocrystal size. Our results have potentially profound implications for optimizing the radiative recombination kinetics and quantum yield of smaller ligand-passivated SiNCs. KEYWORDS: silicon nanocrystals, photoluminescence, quantum confinement, surface effects
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Indeed, in the most contemporary picture there are at least two distinct relaxation rates, fast and slow, but these are accompanied by an almost dizzying array of explanations and proposed mechanisms.20−31 When there are two distinct color bands, red and blue, the fast mode has been attributed to processes affiliated with surface-mediated direct-like emission,22 multiple charge-carrier effects,23 the size of the particle core,24 direct or quasi-direct behavior,25,26 surface defects,27 impurities,28 surface states,29,30 or surface chemistry.31 The emerging picture is one in which the nanocrystal surface, as dictated by the particular passivation scheme, plays a pivotal but seemingly nonuniversal role in determining the nature of the size dependent PL response.32 Within this broad collection of behaviors, there is a particularly intriguing subclass of SiNC systems for which the fast mode is only somewhat blue-shifted (i.e., yellow) with respect to the slower red emission.17,20,21,33 This suggests that both modes are indirect in nature, which in turn implies an as-
ulk silicon dominates the microelectronics industry but has realized somewhat limited utility in the photonic realm because of the relatively poor optical characteristics associated with the indirect band gap. However, as is the case for other semiconductors, the optical absorption and emission characteristics of nanocrystalline silicon are significantly altered by changes in electronic band structure imposed by quantum confinement, pointing toward numerous potential applications in displays, photovoltaics, sensing, and labeling. Nonetheless, the indirect nature of the transition appears to persist down to small nanocrystal sizes (around 2 nm),1,2 implying that the photoluminescence (PL) from silicon nanocrystals (SiNCs) differs significantly from that of its direct band-gap counterparts. Most notably, the finite momentum shift associated with the transition implies the need for a participating lattice vibration or phonon, which leads to line broadening and PL relaxation times on the order of microseconds.3−9 Several recent studies suggest a more complex picture, with an additional nanosecond PL relaxation mode that has been attributed to either surface effects or a transition from indirect to quasi-direct recombination at small nanocrystal sizes.10−19 © 2017 American Chemical Society
Received: October 28, 2016 Accepted: January 31, 2017 Published: January 31, 2017 1597
DOI: 10.1021/acsnano.6b07285 ACS Nano 2017, 11, 1597−1603
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Figure 1. (a) Representative PL spectra of the SiNC fractions used in this study. (b) Peak PL emission wavelength vs fraction number, where the inset shows a TEM image of a 3.5 nm diameter SiNC (0.5 nm scale). Size increases with increasing fraction number. (c) TEM image of a fraction (10 nm scale). (d) Ambient PL decay of select SiNC fractions with fits to eq 1.
Figure 2. (a) Slow/fast relaxation time and (b) slow/fast amplitude as a function of size. The slow amplitude extrapolates to unity and the fast amplitude extrapolates to zero near the Bohr exciton radius of silicon (dashed lines). (c) Size dependence of the slow amplitude in comparison to the slow lifetime (scaled by a constant) and the fraction of steady-state PL from the slow mode based on the integrated intensity. (d) Slow (red) and fast (yellow) relaxation time vs temperature for “small” (dashed) and “large” (solid) samples at either end of the size window. (e) Fast and slow amplitude as a function of temperature for the “small” and “large” samples and (f) the ratio of slow to fast amplitude vs inverse temperature for the “large” and “small” samples with exponential fits. Error bars are the size of the data markers unless otherwise indicated.
significant insight into the nature of quantum confinement and PL relaxation in colloidal SiNCs. In this contribution, we use plasma-synthesized34 colloidal SiNCs separated into monodisperse fractions35−37 to study an abrupt size-dependent partitioning of multilevel PL relaxation spanning nanosecond to microsecond time scales. To gain additional insight, we measure this partitioning as a function of temperature, from ambient down to cryogenic conditions. Several recent theoretical/computational studies examine the role of indirect and direct recombination in the radiative and nonradiative transitions of quantum-confined silicon.2,22,38−43 Here, we specifically focus on the impact of higher-order structure in the electronic density of states (DOS), as recently
yet unidentified relaxation mechanism associated with the faster decay. In the case of plasma-synthesized SiNCs, Wen et al.17 linked this transient yellow PL to ligand-related surface states, arguing that it represents a critical limit for efficient tunable emission from the nanocrystal core. The authors ruled out oxygen-related surface defects and further suggested that the faster mode is directly associated with intrinsic limits on the PL quantum yield of smaller SiNCs.6,8 In contrast, a recent study of solution-synthesized SiNCs by Yang et al.20 suggested that the fast mode is intimately linked to surface oxidation and incomplete functionalization, with little bearing on quantum yield. A reconciliation of this type of disparity would provide 1598
DOI: 10.1021/acsnano.6b07285 ACS Nano 2017, 11, 1597−1603
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ACS Nano highlighted in the ab initio density-functional theory (DFT) calculations of Hapala et al.2 By employing a DFT-based approach in a manner that accounts for excitonic and multiphonon effects, we investigate the impact of phononresonant subgaps on PL relaxation through a phonon-inclusive ab initio description of SiNC PL relaxation. Similar resonance effects have recently been linked to the Fano line shape44 of size-resolved micro-Raman spectra.45 The presence and magnitude of such “minigaps” have been shown to be very sensitive to surface chemistry,2 which we suggest explains why surface effects are commonly implicated in multimodal PL relaxation and the loss of quantum yield with decreasing size.8 Because the onset of such resonance effects puts intrinsic limits on the quantum yield (QY) of smaller SiNCs, our results have potentially profound implications for controlling and optimizing radiative recombination kinetics and efficiency.
lifetime is relatively independent of size. The size resolution within the 3 to 4 nm window is excellent. The same trends can be seen in Figure 1d, where the slow mode is suppressed for the smallest fraction. We numerically integrated the two terms in eq 1 to estimate the relative spectral weight of each contribution to the steady-state PL. Despite the fact that A2 becomes significantly larger than A1 as size decreases, the fast mode makes a negligible contribution (
