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Nature Materials | Article

Compact high-quality CdSe–CdS core–shell nanocrystals with narrow emission linewidths and suppressed blinking

Journal name:
Nature Materials
Volume:
12,
Pages:
445–451
Year published:
DOI:
doi:10.1038/nmat3539
Received
Accepted
Published online

Abstract

High particle uniformity, high photoluminescence quantum yields, narrow and symmetric emission spectral lineshapes and minimal single-dot emission intermittency (known as blinking) have been recognized as universal requirements for the successful use of colloidal quantum dots in nearly all optical applications. However, synthesizing samples that simultaneously meet all these four criteria has proven challenging. Here, we report the synthesis of such high-quality CdSe–CdS core–shell quantum dots in an optimized process that maintains a slow growth rate of the shell through the use of octanethiol and cadmium oleate as precursors. In contrast with previous observations, single-dot blinking is significantly suppressed with only a relatively thin shell. Furthermore, we demonstrate the elimination of the ensemble luminescence photodarkening that is an intrinsic consequence of quantum dot blinking statistical ageing. Furthermore, the small size and high photoluminescence quantum yields of these novel quantum dots render them superior in vivo imaging agents compared with conventional quantum dots. We anticipate these quantum dots will also result in significant improvement in the performance of quantum dots in other applications such as solid-state lighting and illumination.

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  1. Optical properties of new generation CdSe–CdS core–shell QDs.
    Figure 1: Optical properties of new generation CdSe–CdS core–shell QDs.

    a–d, Absorption (blue) and photoluminescence (PL) (red) spectra of four different CdSe–CdS core–shell QDs synthesized with different CdSe core diameters of 2.7 nm (a), 3.4 nm (b), 4.4 nm (c) and 5.4 nm (d). e–h, Temporal evolution of photoluminescence (PL, e), absorption (f), photoluminescence QYs (g), green square shows the original photoluminescence QY of CdSe QDs, and FWHM of the photoluminescence peak of the CdSe–CdS core–shell QDs (shown in c) during the shell growth reaction (h).

  2. Morphology, composition and crystal structure characterization of new generation QDs.
    Figure 2: Morphology, composition and crystal structure characterization of new generation QDs.

    a–d, TEM images of 4.4 nm CdSe core (a) and CdSe–CdS core–shell QDs with a CdS shell thickness of 0.8, 1.6 and 2.4 nm, respectively (b–d). e, Energy-dispersive X-ray spectrum of the final CdSe–CdS core–shell QDs shown in d. Inset shows the observed and calculated atomic percentages of Cd, Se and S atoms. f, X-ray powder diffraction pattern measured from the same sample shown in d. The stick patterns show the standard peak positions of bulk wurtzite CdSe (bottom blue sticks) and CdS (top green sticks). The inset shows a representative high-resolution TEM image of a CdSe–CdS QD. Scale bars, 50 nm in a–d and 2 nm in the inset of f.

  3. Photoluminescence spectral correlation of single-QDs and ensemble QDs obtained through S-PCFS.
    Figure 3: Photoluminescence spectral correlation of single-QDs and ensemble QDs obtained through S-PCFS.

    a,b, The spectral correlations of the single-QD (red line) and the ensemble (blue line) spectrum obtained by S-PCFS for CdSe–CdS core–shell QDs synthesized by our method (a) and a conventional method with nearly the same shell thickness (~ 7 MLs; b).

  4. Blinking behaviour of new generation CdSe–CdS core–shell QDs and ensemble photoluminescence stability test.
    Figure 4: Blinking behaviour of new generation CdSe–CdS core–shell QDs and ensemble photoluminescence stability test.

    a, Representative photoluminescence blinking trace of a single CdSe–CdS core–shell QD with a CdSe core radius of 2.2 nm and a shell thickness of 2.4 nm (~ 7 MLs; bin size is 50 ms). Histograms indicate the distribution of intensities observed in the trace. The dashed red line indicates the value chosen as the threshold between ON and OFF states in calculating the ON time fraction. b, Histogram of the blinking ON time fraction. The average ON time fraction is 0.94 with a standard deviation of ±0.06. c, Log–log plot of the probability distributions of ON and OFF times. Straight lines represent a power-law fitting using the equation Pon/off(ton/off)∝t−αon/off, where αon  =  0.85for ON times (red line) and αoff  =  2.2 for OFF times (blue line). d, Representative photoluminescence blinking trace of a single CdSe–CdS core–shell QD with a CdSe core radius of 2.2 nm and a shell thickness of 0.7 nm (~ 2 MLs; bin size is 50 ms). e,f, The normalized (Nor.) photoluminescence intensity traces obtained from a collection of QDs synthesized through our new method (e) and the conventional method (f). The inset in f shows the photoluminescence intensity recovery after an initial decay. The black arrow indicates the time point when continuous excitation was stopped.

  5. Water-soluble CdSe–CdS core–shell QDs for in vivo imaging.
    Figure 5: Water-soluble CdSe–CdS core–shell QDs for in vivo imaging.

    a–c, Photoluminescence QYs of CdSe–CdS core–shell QDs before ligand exchange in chloroform (CHCl3) and after ligand exchange in phosphate buffer saline (PBS 1X, pH 7.4). Equal amounts of these QDs (2.5 μM, 200 μl) were injected retro-orbitally into Tie2-GFP transgenic mice bearing dorsal skinfold chambers, and carried out intravital multiphoton microscopy in the skin at 30 min after injection. d–f, The multiphoton in vivo images. a,d, Conventional CdSe–CdS QDs synthesized by a literature method and ligand exchanged with methoxy-polyethylene-glycol thiol (QDconv.–SH-PEG). b,e, New generation (ng) CdSe–CdS QDs synthesized by our novel method and ligand exchanged with methoxy-polyethylene-glycol thiol (QDng–SH-PEG). c,f, New generation CdSe–CdS QDs ligand exchanged with polymeric imidazole ligands (QDng–PIL). In d–f, all the images are scaled to the same contrast, and scale bars, 100 μm.

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Author information

Affiliations

  1. Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA

    • Ou Chen,
    • Jing Zhao,
    • Jian Cui,
    • Cliff Wong,
    • Daniel K. Harris,
    • He Wei,
    • Hee-Sun Han &
    • Moungi G. Bawendi

  2. Massachusetts General Hospital and Harvard Medical School, 100 Blossom Street, Boston, Massachusetts 02114, USA

    • Vikash P. Chauhan,
    • Dai Fukumura &
    • Rakesh K. Jain

Contributions

O.C. and M.G.B. conceived and designed the project. O.C. performed the bulk of the experimental work with help from J.Z., V.P.C., J.C., C.W., D.K.H., H.W. and H-S.H. The data was analysed by O.C., J.Z., V.P.C., J.C., C.W. and M.G.B. All authors discussed the results and took part in producing the manuscript.

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The authors declare no competing financial interests.

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