Photoluminescence Intensity Fluctuations and Temperature

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Chemical and Dynamical Processes in Solution; Polymers, Glasses, and Soft Matter

Photoluminescence Intensity Fluctuations and TemperatureDependent Decay Dynamics of Individual Carbon Nanotube sp3 Defects Younghee Kim, Kirill A Velizhanin, Xiaowei He, ibrahim Sarpkaya, Yohei Yomogida, Takeshi Tanaka, Hiromichi Kataura, Stephen K. Doorn, and Han Htoon J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03732 • Publication Date (Web): 08 Mar 2019 Downloaded from http://pubs.acs.org on March 9, 2019

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The Journal of Physical Chemistry Letters

Photoluminescence Intensity Fluctuations and Temperature-Dependent Decay Dynamics of Individual Carbon Nanotube sp3 Defects Younghee Kim1, Kirill A. Velizhanin2, Xiaowei He1, Ibrahim Sarpkaya1, Yohei Yomogida,3 Takeshi, Tanaka,3Hiromichi Kataura3, Stephen K. Doorn*,1, Han Htoon*,1

1Center

for Integrated Nanotechnologies, Materials Physics and Application Division,

Los Alamos National Laboratory, Los Alamos, New Mexico, 87545, United States

2Theoretical

Division, Los Alamos National Laboratory, Los Alamos, NM 87545, United

States

3Nanomaterials

Research Institute, National Institute of Advanced Industrial Science

and Technology (AIST), Tsukuba, Ibaraki, 305-8565, Japan

1 ACS Paragon Plus Environment

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AUTHOR INFORMATION

Corresponding Author *(S.K.D), E-mail: [email protected]

*(H.H), E-mail: [email protected]

ABSTRACT

Recent demonstration of room temperature, telecommunication wavelength single photon generation by sp3 defects of single wall carbon nanotubes established these defects as a new class of quantum materials. However their practical utilization in development of quantum light sources calls for a significant improvement in their imperfect QY (10-30%). PL intensity fluctuations observed with some defects also need to be eliminated. Aiming toward attaining fundamental understanding necessary for


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addressing these critical issues, we investigate PL intensity fluctuation and PL decay dynamics of aryl sp3 defects of (6,5), (7,5), and (10,3) single wall carbon nanotubes (SWCNTs) at temperatures ranging from 300 K to 4 K. By correlating defect-state PL intensity fluctuations with change (or lack of change) in PL decay dynamics, we identified random variations in the trapping efficiency of 𝐸11 band edge excitons (likely resulting from the existence of a fluctuating potential barrier in the vicinity of the defect) as the mechanism mainly responsible for the defect PL intensity fluctuations. Furthermore, by analyzing the temperature dependence of PL intensity and decay dynamics of individual defects based on a kinetic model involving the trapping and de-trapping of excitons by optically allowed and forbidden (bright and dark) defect states, we estimate the height of the potential barrier to be in the 3-22 meV range. Our analysis also provides further confirmation of recent DFT simulation results that the emissive sp3 defect state is accompanied by an energetically higher-lying optically forbidden (dark) exciton state.



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TOC GRAPHICS

KEYWORDS: SWCNT, sp3 defects, photoluminescence decay, random opening, nonradiative decay, bright and dark excitons

Due to their ability to mimic trapped ions, solitary defects of semiconductors and insulators have been identified as key quantum materials for enabling many transformational technologies ranging from eavesdropping-proof communication and


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ultrasensitive measurements to neuromorphic quantum computers.1, 2 While nitrogen vacancies in bulk diamond crystals (diamond NV centers) stand as one of the most prominent example of such defects,3-5 recent studies report observation of defects with similar quantum optical properties in two dimensional (2D) transition metal dichalcogenides (TMDC)6, 7 and hexagonal boron nitride (hBN)8, 9 as well as in 1D single walled carbon nanotubes (SWCNTs).10, 11 Among these newly emerging defects exhibiting desirable quantum optical properties, sp3 defects of SWCNTs are particularly exciting as they have been shown to emit almost fluctuation free single photons with 99% purity at room temperature and at telecommunication wavelengths, a feat that no other materials including diamond NV centers have yet achieved.11

Such defect states were first created via attachment of oxygen functional groups (ether and epoxy) through ozonation of SWCNTs in aqueous suspension.12-14 Later, advances in SWCNT functionalization chemistry allowed creation of defects with similar optical properties via covalent attachment of monovalent and divalent alkyl and aryl functionality with varying complexities.15, 16 While the chemical structure of these



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functional groups differs greatly from one another, low temperature PL spectroscopy studies revealed that they all emit sharp isolated spectral peaks characteristic of a quantum mechanical two-level system.14 Since the electronic structure of these defects, as revealed by quantum chemistry simulations, allows a single, isolated emissive state at energies hundreds of meV below the band edge, these states can maintain the characteristics of a quantum mechanical two-level system up to room temperature and at very high pump powers, offering distinct advantages over other telecommunication wavelength quantum emitters.14, 17, 18 InGaAs self-assembled quantum dots, for example, cannot display quantum characteristics due to loss of quantum confinement at higher temperatures and interferences from the emission of multi-exciton states at higher pump fluences.19, 20 Recent studies bring significant new understanding as well as applications of these defect states, including the nature of localization potential, multi and charged exciton processes, potential for lasing and sensing etc.3, 21-25 Despite these works, a few key understandings essential for optimizing the quantum light emission performance of these defects are still missing. Particularly, an understanding of mechanisms responsible for the PL intensity fluctuations and competition between 6 ACS Paragon Plus Environment

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radiative and non-radiative recombination channels of the defect states are very important for attaining efficient, on demand single photon generation.

Figure 1. Schematic of photo-physical processes dictating the intensity fluctuation and decay dynamics of the defect-bound exciton. Excitons created at the band-edge undergo diffusion and are then trapped by the defects (green arrow). A potential barrier (of high 𝐸𝑏) or non-radiative traps (brown dotted lines) could hinder the diffusion/trapping of the excitons and cause PL intensity fluctuations. Random opening and closing of a non-radiative decay channel (red dotted arrow) could also lead to intensity fluctuation. While the PL lifetime of the defect state will remain unchanged for PL intensity fluctuations caused by the former process, the later process will lead to PL



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lifetimes that fluctuate in correlation with PL intensity. Recent DFT calculations predict a dark state that exists above the lowest bright exciton.26 Δ represents the energy splitting between dark and bright excitons and blue and purple arrows represent the thermal detrapping of the excitons from the bright and dark states.

While our recent study has revealed that ultra-stable PL emission with shot noise limited fluctuation is possible in some individual defects, a large fraction of them (~60%) exhibit intensity fluctuations significantly larger than shot-noise.11 These fluctuations are also shown to increase with power and exposure time to laser excitation, as well as with a decrease of sample temperature. Such intensity fluctuations could result either from random variations in trapping efficiencies of the excitons created in the 𝐸11 band edge of the SWCNT (green arrow of Figure 1) or random opening of non-radiative recombination channels for the trapped excitons (red dashed arrow of Figure 1). Discerning these two possible channels is the first step toward developing chemical functionalization routes for generation of 100% ultra-stable quantum emitter ensembles.


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Recent works estimate the quantum yield (QY) of the defect states to be in the range of 10% to 28%.11, 15 While this number represents an order of magnitude improvement for the QY of the SWCNTs’ band-edge excitons (

Photoluminescence Intensity Fluctuations and Temperature

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