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Area and Thickness Dependent Bi-Exciton Auger Recombination in Colloidal CdSe Nanoplatelets: Breaking the “Universal Volume Scaling Law" Qiuyang Li, and Tianquan Lian Nano Lett., Just Accepted Manuscript • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 18, 2017
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Area
and
Thickness
Dependent
Bi-Exciton
Auger
Recombination in Colloidal CdSe Nanoplatelets: Breaking the “Universal Volume Scaling Law” Qiuyang Li, Tianquan Lian* Department of Chemistry, Emory University, 1515 Dickey Drive, NE, Atlanta, GA, 30322, USA
ABSTRACT Colloidal nanoplatelets (NPLs) have shown great potentials for lasing applications due to their sharp absorption and emission peaks, large absorption cross-sections, large radiative decay rates, and long multiexciton lifetimes. How multiexciton lifetimes depend on material dimensions remains unknown in 2D materials, despite being a key parameter affecting optical gain threshold and many other properties. Herein, we report a study of room temperature bi-exciton Auger recombination time of CdSe NPLs as a function of thickness and lateral area. Comparison of all NPLs shows that the biexciton lifetime does not increase linearly with volume, unlike previously reported “universal volume scaling law” for quantum dots. For NPLs of the same thickness (~1.8 nm), the biexciton lifetime increase linearly with their lateral area (from 143.7±12.6 to 320.1±17.1 ps when the area increases from 90.5±21.4 to 234.2±41.9 nm2). The biexciton lifetime depends linearly on (1/Ek(e))7/2 (Ek(e) is the electron confinement energy) or nearly linearly on d7 (d is NPL thickness). The observed dependence is consistent with a model in which biexciton Auger recombination rate scales with the product of exciton binary collision frequency and Auger recombination probability in bi-exciton complexes. The linear increase of Auger lifetimes with NPL lateral areas reflects a 1/area dependence of the binary collision frequency for 2D excitons and the thickness dependent bi-exciton Auger recombination time is attributed to its strong dependence on the degree of quantum confinement. This model may be generally applicable to exciton Auger recombination in quantum confined 1D and 2D nanomaterials. 1
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KEYWORDS: colloidal nanoplatelets, 2D materials, multiple exciton, Auger recombination, biexciton lifetime, universal volume scaling law
Compared to zero-dimensional (0D) quantum dots (QDs) and one-dimensional (1D) nanorods (NRs), two-dimensional (2D) zinc-blend CdSe nanoplatelets (NPLs) have sharper absorption and emission peaks (due to a more uniform quantum confinement), larger absorption cross-section (due to larger lateral dimension), and larger radiative decay rate (due to strong dielectric confinement).1-5 Owing to these novel properties, CdSe NPLs have shown great potentials as low gain threshold lasing materials.5-10 Optical gain threshold is determined in part by the lifetime of multiple exciton states.5 Multi-excitons decay predominantly by Auger recombination, in which an electron-hole pair recombines non-radiatively by exciting a third particle (electron or hole) to a higher energy level to conserve energy and momentum.5-9, 11-14 In addition to its importance in lasing applications, Auger recombination of excitons also affects many other important properties of semiconductors, such as carrier multiplication,15-22 and multi-exciton dissociation.23-25 However, unlike Auger recombination times in QDs and NRs, which have been reported to follow “universal volume scaling law”,15, 22, 26-29 the dependence of Auger recombination time on size, thickness and volume in 2D NPLs and other 2D materials remains unclear. Further study of Auger recombination in CdSe NPLs is required for both fundamental understanding of exciton-exciton interaction in low dimensional materials and rational material engineering for better lasing and other applications. Herein, we report a systematic study of the dependence of biexciton Auger recombination lifetime on the lateral size and thickness of CdSe NPLs at room temperature via transient absorption (TA) spectroscopy. We show that the biexciton Auger lifetime increases linearly with the lateral area for NPLs of the same thickness, but depends much more strongly on the thickness d (scaling nearly linearly with d7). The biexciton Auger lifetimes do not show a simple dependence on NPL volumes, inconsistent with the “universal volume scaling law” reported for QDs. 2
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15, 22, 26-29
The
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observed dependence can be well explained by a model that accounts for both lateral size dependent collision frequency and thickness dependent Auger recombination probability.
Figure 1. TEM images of (a) 3MLb, (b) 4MLc, and (c) 5MLb CdSe NPL samples. (d) Absorption spectra of CdSe NPLs. (e) Schematic energy levels of CdSe NPLs. Sample characterization. The colloidal zinc-blend CdSe NPLs with different thicknesses and lateral sizes were synthesized following reported procedures with slight modifications.3 The thickness of CdSe NPLs was tuned by changing the amount of Cd precursors and synthesis temperature while the lateral size was mainly controlled by the reaction time. Detailed synthesis procedures are described in Supporting Information. Because of zinc-blend structure, NPLs with n monolayers (MLs) of CdSe contain n Se layers and n+1 Cd layers, with Cd as the terminating layers. NPLs with 3, 4, 5 CdSe MLs were prepared, which correspond to thicknesses of ~1.5, ~1.8 and ~2.1 nm, respectively.3, 8, 30, 31. For each thickness, NPLs with different lateral area (x) were also prepared, producing a set of samples labeled as nMLx (n=3, x=a, b; n=4, x=a-d; n=5, x=a, b). TEM images of 3MLb, 4MLc and 5MLb are shown in Figure 1a, 1b, and 1c, respectively, and TEM images of all other samples are shown in Figure S1. These images 3
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show that the NPLs have approximately rectangular shapes with length and width ranging from 11.9±2.1 to 32.6±3.5 nm, and from 4.8±1.0 to 10.6±2.9 nm, respectively (see Table S1 for details). The thickness of these samples was determined from the exciton peak positions in their UV-Vis absorption spectra (see below). The UV-Vis absorption spectra (Figure 1d) show sharp A exciton (~460, ~512, ~546 nm) and B exciton (~431, ~480, ~520 nm) peaks for 3, 4, 5 ML CdSe NPLs, respectively. These exciton bands can be attributed to electron-heavy hole (e-hh) and electron-light hole (e-lh) exciton transitions, respectively.3 These peak positions are independent on the lateral sizes of the NPLs as shown in Figure S1. The conduction (CB) and valence (VB) band edge positions of these NPLs can be estimated from these exciton peak positions, reported bulk band edge and band gap (Eg), and estimated electron (Ek(e)) and hole (Ek(h)) quantum confinement energy, as shown in Figure 1e.3, 32-34 In this estimate, we have neglected the shifts due to exciton binding energy and the self-image energy (induced by dielectric confinement effects), because they have similar values (~100 meV) but opposite signs.3, 6, 33 The ratio of electron to hole quantization energies is equal to the ratio of hole (mhh*) to electron (me*) effective masses. Using the reported bulk band gap (~1.7 eV), bulk CB flat band position (-4 eV of CB minimum and -5.7 eV of VB maximum vs vacuum) and the electron (hole) effective mass value of 0.18m0 (0.89m0),3 where m0 is the free electron mass, we calculated Ek(e) (Ek(h)) as 0.80 (0.17), 0.59 (0.12), and 0.47 (0.09) eV for 3, 4, 5 ML NPLs, respectively. The estimated CB and VB edge positions for 3ML, 4ML, and 5ML CdSe NPLs are -3.20 eV, -3.41 eV, -3.53 eV, and 5.87 eV, -5.82 eV, -5.79 eV, respectively. The light hole levels for 3ML, 4ML, and 5ML NPLs are calculated as -6.05 eV, -5.96 eV, and -5.91 eV, respectively, according to the energy difference between A and B exciton. The details of the above estimate can be found in Supporting Information.
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Figure 2. Transient absorption (TA) spectra and kinetics of 5MLb NPLs. TA spectra at (a) early delay time (2-4 ps) and (b) long delay time (800-1000 ps) at indicated pump fluences. (c) A exciton bleach kinetics at indicated pump fluences. (d) Normalized A exciton bleach amplitude at early (2-4 ps, blue circles) and long (800-1000 ps, red circles) delay times. The solid line shows a fit to the probability of excited NPLs at long delay time (800-1000 ps) according to a Poisson distribution model (see SI). The dashed line represents the initial average number of excitons per NPL as a function of pump fluence. (e) Comparison of normalized A exciton kinetics at low pump fluences (
