Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat Temporal evolution of white light emitting CdS core and Cd 1-x Zn x S graded shell quantum dots fabricated using single step non-injection technique Aditya Nath Bhatt, Upendra Kumar Verma, Brijesh Kumar * Department of Electronics and Communication Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, 247667, India ARTICLE INFO Keywords: Non-injection synthesis Core/shell quantum dot Quantum dot growth time Trap state emission White light ABSTRACT White light emitting quantum dot (WQD) materials are being investigated due to their possible applications in the light emitting devices as single layer white light emitters and these have major advantage of lower re- absorption and self-absorption over multilayer emitters of different sized quantum dots (QDs). In this paper, white light emitting CdS core and Cd 1-x Zn x S graded shell based quantum dots have been synthesized using a single step non-injection technique. Detailed analysis of optical and structural properties has been carried out with QDs growth time. The photoluminescence (PL) consists a narrow band edge emission and a broad (in the range of 440–800 nm) trap state emission which results in white light emission and the quantum efficiency (QE) improves from ∼12% to 34% as growth time increases from 5 min to 30 min. The average fluorescence lifetime of band edge emission and trap state emission varies in the range of ∼2–5 ns and ∼250–550 ns respectively with QDs growth time. We have obtained a series of white light whose color coordinates are very close to the standard white light (CIE-1931); hence these QDs can be a good candidate for white light applications. 1. Introduction Semiconductor quantum dots (QD) have multiple applications in several areas such as in the light emitting devices (LEDs) [1,2], laser technology [3,4], photovoltaic [5,6], and biomedical labeling [7,8] due to their size-dependent optical tunability making them a good candi- date to be used as tunable absorbers and emitters. Recently, researchers have synthesized multicolor QDs to fabricate LEDs of different colors such as red (R), green (G), and blue (B) [9–12]. Simultaneously, efforts have also been made to get the white light by mixing RGB QDs [13] or blue and yellow QDs [14] in a proper pro- portion. Though, these methods are straightforward but may result in the lower efficiency due to reabsorption of light among different sized QDs via Fӧrster energy transfer [15]. In continuation to get white light, some research groups have investigated and reported white light emitting quantum dots that can significantly reduce the reabsorption of light and so called as ‘magic-sized’ quantum dots having very small size [16–18]. These QDs generally have a band edge emission in the blue region and a trap state emission in the green and red region. In the development phase of white light emitting quantum dots (WQDs), Michael J. Bowers II et al. [19] have fabricated CdSe based WQDs whose white color spectrum was composed of two emissions; one, a narrow band emission at 414 nm due to very small size QDs and another, a broad spectrum due to the surface states. Though these QDs were able to emit white light but quantum efficiency (QE) was very less (2–3%). Rajan Jose et al. [20] reported highly efficient (QE∼40%) CdSe WQDs synthesized at room temperature having a broad full width at half maxima (FWHM~150 nm) but their QDs do not show blue emission which is important component of white color, so the color quality is away from white light and they have not discussed about CIE color coordinates while Lei Qian et al. [15] fabricated CdSe WQDs using hot solution method and obtained QE in the range of 10–30%. Further, some reports of embedding zinc (Zn) with CdSe to get Zn x Cd 1-x Se WQDs are found in literature. One such report by Chien-Chih Shen et al. [21] details using low-temperature synthesis process and synthesizing al- loyed Zn x Cd 1-x Se WQDs with QE ∼12%. CdSe and CdS based QDs are the most studied nanomaterial and are being extensively used in several areas. The size dependent quantiza- tion in energy levels leads to tune the luminescence. Since the band gap of bulk CdSe is 1.7 eV (∼730 nm) while that of bulk CdS is 2.4 eV (∼517 nm), hence it is much easier to fabricate blue QDs with CdS than CdSe. Some research groups have done substantial work to obtain white light emission through CdS based QDs. S. Sapra et al. [22] reported white light emission through trap-rich CdS QDs and onion-like CdSe/ ZnS/CdSe/ZnS QDs and observed maximum QE of 17% and 30% re- spectively. In CdSe/ZnS/CdSe/ZnS WQDs, they obtained white color https://doi.org/10.1016/j.optmat.2019.04.008 Received 24 January 2019; Received in revised form 9 March 2019; Accepted 4 April 2019 * Corresponding author. E-mail address: brijesh@iitr.ac.in (B. Kumar). Optical Materials 92 (2019) 143–149 0925-3467/ © 2019 Elsevier B.V. All rights reserved. T