Subscriber access provided by University of Florida | Smathers Libraries
Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials
Reducing Optical Gain Threshold in Two-Dimensional CdSe Nanoplatelets by Giant Oscillator Strength Transition Effect Qiuyang Li, Qiliang Liu, Richard D. Schaller, and Tianquan Lian J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00759 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 23, 2019
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry Letters
Reducing Optical Gain Threshold in Two-Dimensional CdSe Nanoplatelets by Giant Oscillator Strength Transition Effect Qiuyang Li,1 Qiliang Liu,1 Richard D. Schaller,2,3 Tianquan Lian*,1 1 Department of Chemistry, Emory University, 1515 Dickey Drive, NE, Atlanta, GA, 30322, USA 2
Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439,
USA 3
Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA
Abstract 2D CdSe nanoplatelets are promising lasing materials. Their large lateral areas reduce the optical gain threshold by increasing the oscillator strength and multiexciton lifetimes, but also increases the gain threshold by requiring multiple band-edge excitons (>2) to reach optical gain. We observe that the optical gain threshold of CdSe nanoplatelets at 4K is ~4fold lower than that at room temperature. Transient absorption spectroscopy measurements indicate that the exciton center-of-mass coherent area is smaller than the lateral size at room temperature and extends to nearly the whole nanoplatelets at 4K. This suggests that the reduction of optical gain threshold at low temperature can be attributed to exciton coherent area extension which reduces the saturation number of band-edge excitons to enable biexciton gain and increases the radiative decay rate, consistent with the giant oscillator strength transition effect. This work demonstrates a new direction for lowering the optical gain threshold of nanomaterials. TOC Graphic
1
ACS Paragon Plus Environment
The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
KEYWORDS: nanoplatelets, optical gain, giant oscillator strength transition, exciton coherent area, 2D materials
Compared to conventional bulk semiconductor lasers, quantum dot (QD) lasers have many potential advantages induced by the quantum confinement along all three dimensions, including the tunable lasing wavelength through size dependent quantum confinement effects.1 Moreover, the density of states of QDs near the band-edge reduce to a Dirac delta function-like distribution so that carriers are concentrated to fewer states, which reduces the optical gain (OG) threshold as fewer injected carriers are required for population inversion.2-3 QD lasers have shown promise in recent years and continuous-wave lasing4 and direct-current electrical pumped optical gain5 in QD films have been reported. Cadmium chalcogenide (CdX, X=Se, S, Te) nanoplatelets (NPLs), with atomic-precise thickness (of ~1.2-1.8 nm) and large lateral dimension (100s nm2), are an emerging class of nanocrystals6 and, by some metrics, have shown even greater potential for lasing applications than QDs.7-10 The continuous-wave lasing of CdSe NPLs has been reported and the best-reported threshold of amplified spontaneous emission (ASE) of CdSe NPLs is as low as 6 𝜇J/cm2,7 which is over an order of magnitude lower than that in similar QDs1, 11
and nanorods (NRs).12 The low optical gain (OG) threshold of NPLs is attributed to low
multiple exciton Auger recombination rates,13-15 large absorption cross-sections,16 and narrow emission peaks (due to precise confinement energy)17 of these materials, all of which are a consequence of their unique 2D morphology. Compared to QDs, 2D NPLs have much larger absorption cross-section per unit volume,16 and biexciton Auger lifetimes in NPLs have been shown to increase with their lateral dimension,13 which would suggest that increasing lateral size as a potential approach for further reducing the OG threshold. However, the extended lateral dimension of NPLs allows spatially separated excitons, which increases the maximum number of band-edge exciton states (NS) to be >2. Previous studies have shown that in cadmium chalcogenide QDs11 and NPLs,18-20 optical gain is achieved when the band-edge exciton states are just over half-occupied. Thus, OG threshold corresponds to an average number of excitons per nanocrystal (mth) of >1 for QDs and >NS/2 for NPLs (Scheme 1a). As a result, OG in NPLs is achieved at higher order 2
ACS Paragon Plus Environment
Page 2 of 22
Page 3 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry Letters
(>2) multi-exciton states. These higher multi-exciton states are generated by the absorption of multiple photons (>2) and are much shorter-lived compared to the biexciton state, both of which increases the OG threshold intensity.
Scheme 1. Scheme of temperature-dependent optical gain mechanism. (a) Scheme of different exciton states in NPLs at room temperature with the saturation number of the band-edge exciton (NS) as 4. (b) Scheme of different exciton states in NPLs at low temperature (
