Understanding the excitation wavelength dependent spectral shift and large exciton binding energy of tungsten disulfide quantum dots and its interaction with single-walled carbon nanotubes

Abhilasha Bora; Larionette P L Mawlong; Ruma Das; P K Giri

doi:10.1016/j.jcis.2019.11.027

Understanding the excitation wavelength dependent spectral shift and large exciton binding energy of tungsten disulfide quantum dots and its interaction with single-walled carbon nanotubes

J Colloid Interface Sci. 2020 Mar 1:561:519-532. doi: 10.1016/j.jcis.2019.11.027. Epub 2019 Nov 8.

Authors

Abhilasha Bora¹, Larionette P L Mawlong², Ruma Das¹, P K Giri³

Affiliations

¹ Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India.
² Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati 781039, India.
³ Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India; Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati 781039, India. Electronic address: giri@iitg.ac.in.

PMID: 31740135
DOI: 10.1016/j.jcis.2019.11.027

Abstract

Herein, we investigate the origin of excitation wavelength dependent spectral features and high fluorescence quantum yield in fluorescent 2D tungsten disulfide (WS₂) quantum dots (QDs) of average size 2.4 nm. The as-prepared WS₂ QDs possess high optical bandgap and reasonably high fluorescence quantum yield ~15.4% in the green region without any functionalization. The broad photoluminescence (PL) spectrum consists of multiple peaks owing to emissions from excitonic transitions and surface defect-related transitions. The excitation wavelength-dependent spectral redshift and narrowing of line shape in the PL peak are analyzed carefully, and it is attributed to the selective excitation/recombination of carriers from different energy levels. The temperature-dependent PL analysis yields an exciton binding energy of ~301 meV in the QDs. Furthermore, we study the interaction between fluorescent WS₂ QDs and single-walled carbon nanotubes (SWCNTs) and explore the mechanism of systematic quenching of PL of QDs by SWCNTs. The nature of the Stern-Volmer plot is found to be linear, and the time-resolved fluorescence measurements reveal that the quenching follows primarily the static behavior. Our study further reveals that defect sites in SWCNTs primarily act as the binding sites for WS₂ QDs and form non-fluorescent complexes for effective quenching of the PL. The strong interaction between the WS₂ QDs and the SWCNTs is evidenced from the spectral shift in the X-ray photoelectron spectroscopy and Raman peaks. Our study reveals the origin of excitation wavelength dependent PL emission from WS₂ QDs and the nature of the interaction between WS₂ QDs and SWCNTs, which are important for their applications in biomedical imaging and sensing, such as surface-enhanced Raman scattering, etc.

Keywords: Carbon nanotubes; Defects; Exciton binding energy; Fluorescence quenching; WS(2) quantum dots.