From Inside Out: How the Buried Interface, Shell Defects, and Surface Chemistry Conspire to Determine Optical Performance in Nonblinking Giant Quantum Dots

Abstract

"Giant" or core/thick-shell quantum dots (gQDs) are an important class of solid-state quantum emitter characterized by strongly suppressed blinking and photobleaching under ambient conditions, and reduced nonradiative Auger processes. Together, these qualities provide distinguishing and useful functionality as single- and ensemble-photon sources. For many applications, operation at elevated temperatures and under intense photon flux is desired, but performance is strongly dependent on the synthetic method employed for thick-shell growth. Here, a comprehensive analysis of gQD structural properties "from the inside out" as a function of shell-growth method is reported: successive ionic layer adsorption and reaction (SILAR) and high-temperature continuous injection (HT-CI), or sequential combinations of the two. Key correlations across synthesis methods, structural features (interfacial alloying, stacking-fault density and surface-ligand identity), and performance metrics (quantum yield, single-gQD photoluminescence under thermal/photo stress, charging behavior and quantum-optical properties) are identified. Surprisingly, it is found that interfacial alloying is the strongest indicator of gQD stability under stress, but this parameter is not the determining factor for Auger suppression. Furthermore, quantum yield is strongly influenced by surface chemistry and can approach unity even in the case of high shell-defect density, while introduction of zinc-blende stacking faults increases the likelihood that a gQD exhibits charged-state emission.

Keywords: giant quantum dots; interfacial alloying; materials-by-design; structure–function correlations.

Figure 1

Attributes of the gQD subjects under investigation. a) Schematic illustration of the components of the core/shell QD from the inside out. b,c) HAADF‐STEM images of gQD I (b) and gQD II (c) nanocrystals. d,e) gQD population fraction as a function of on‐time fraction for gQD I (54 QDs analyzed) (d) and gQD II (64 QDs analyzed) (e) under high pump‐fluence widefield excitation and collection conditions (1 W mm⁻², 405 nm continuous‐wave excitation, room temperature). The insets show minimal photobleaching in each case over 1 h. f) Absorption (onset ≈515 nm) and absorption‐normalized emission spectra for gQD I (blue) and gQD II (orange). g) Photoluminescence intensity as a function of temperature for gQD I (blue) and gQD II (orange) under ultrahigh photon flux (15 W mm⁻²) revealing differences in extent of photobleaching and recovery upon cooling (≈2.5 h of illumination under heating/cooling stress).

Figure 2

Assessing the buried core/shell interface and the extent of Se excursion from the CdSe core at the level of individual nanocrystals and for the ensemble. a,b) STEM–EDS images of core/shell nanocrystals possessing ≈9 MLs of CdS shell from gQD I (a) and gQD II (b) syntheses. c,d) EDS line scan analyses of the QDs indicated in (a) and (b) by squares. e) Average Se penetration into the CdS shell obtained for each gQD by analysis in each case of ≈30 nanocrystals. f) Ensemble technique—energy‐dependent XPS—corroborates extensive excursion of Se into the shell for gQD II via ensemble analysis: XPS spectra for CdSe/CdS core/shell QDs possessing three different shell thicknesses (3, 6, and 9 MLs) showing the doublet peaks for S 2p and Se 3p orbitals obtained using a photon energy of either 650 eV (left) or 1080 eV (right). IMFP is shown for each energy in the respective panels.

Figure 3

Quantifying defects in the shell. a–d) Aberration‐corrected HAADF‐STEM images of core/shell nanocrystals from the gQD I synthesis possessing ≈6 ML (a) and ≈9 ML (b) CdS shells, and from the gQD II synthesis possessing ≈6 ML (c) and ≈9 ML (d) CdS shells. QDs in (a)–(d) are oriented along the [110] zone axis. e) HAADF‐STEM image of a gQD II nanocrystal oriented along the [001] zone axis, for which stacking faults cannot be visualized. f) Average number of stacking faults observed for each gQD synthesis obtained by analysis 30 and 20 nanocrystals, respectively.

Figure 4

Chemical nature of the gQD surface confirmed. a,b) High‐resolution S 2p spectra for fully “giant” nanocrystals: a) gQD I and b) gQD II. c) ¹H NMR spectra of gQD I taken in CDCl₃ before (top) and after (bottom) the addition of octanethiol (OT).

Figure 5

gQD QY as a function of excitation wavelength and shell‐growth method.

Figure 6

Characteristics of gQDs across a mixed CI‐SILAR synthesis series. a) Room‐temperature homogeneous line widths (FWHM values) obtained for single‐gQDs representing different shell‐growth methods. b) Dependence of observation of photocharging in gQDs as a function of the number of shell MLs (number out of 15; obtained by analysis of FLID plots as shown in Figure S9, Supporting Information) that are prepared using the SILAR method (with balance obtained by CI method). c–e) Response of different gQDs to temperature and ultrahigh photon flux (15 W mm⁻²): photoluminescence intensity as a function of temperature (solid circles correspond to increasing temperature, open circles to decreasing temperature) for the gQD shell‐synthesis series (colors match (b)). f,g) Long‐term photobleaching behavior under thermal (≈100 °C) and photon (1 W mm⁻²) stress for products of the shell‐growth series: f) average single‐gQD intensity (normalized) over up to 11 h and g) fraction of gQDs that remain emissive over time. Dashed lines are guides to the eye.

Figure 7

Effect of annealing on g ⁽²⁾ values for gQD I‐type nanocrystals. a) g ⁽²⁾ values pre‐ and postanneal. The CdS shell is ≈9 ML thick and grown using the continuous‐injection method. b) XPS spectra for 9 ML CdSe/CdS core/shell QDs prepared by HT‐CI pre (top) and post (bottom) anneal, showing the doublet peaks for S 2p and Se 3p orbitals obtained using a photon energy of 1080 eV.