High-Efficient NUV Emission with Excellent Thermal Stability in Cd/Pb-Free AS-ZnO QDs by ALD without Surface Passivation

Abstract

Recently, near-ultraviolet (NUV)-emitting quantum dots (QDs) have attracted considerable attention for their potential in next-generation NUV-driven white-lightemitting diodes (with color rendering index >90). However, their widespread application in lighting products is greatly hindered by the inferior NUV-emitting efficiency, poor temperature stability, and toxicity of currently available Cd-/ Pb-containing QD systems. In the present work, we report the synthesis of ecofriendly Al 2 O 3 -surrounded (AS)-ZnO QDs using a novel technique based on atomic layer deposition, which shows a superior quantum yield (QY) of 97.3% in the NUV band (395 nm), in contrast to <63% in literature results. Furthermore, the AS-ZnO QDs possess extraordinary thermal stability, which demonstrates little efficiency degradation or structure deterioration above room temperature, retaining a QY of 87% at 500 K. The high-efficient and thermally stable NUV emission is revealed to originate from the surface states of ZnO QDs, instead of the intrinsic emission. Under an ultrastrong quantum confinement effect, the energetic level of surface states in AS-ZnO QDs is highly enhanced from 2.1 eV to 3.14 eV (with reference to the valence band), close to the intrinsic bandgap of ZnO, and the ratio of radiative to nonradiative recombination on the surface states from ∼5% to above 97%, making them very efficient NUV emitters. The underlying mechanism has been discussed in detail. The outstanding optical merits, combined with facile synthesis and nontoxicity, promise AS-ZnO QDs great potential in lighting applications.

Atomic-Layer-Deposited ZnO/Al2O3 Nanolaminates for White-Light-Emitting Diodes

“…When the applied voltage is higher than V i , the device starts to emit light and a nonzero J L value will soon become the main current (i.e., J L > J R ) as the applied voltage further increases in a p–i–n lighting junction. It was found in our previous work , that a size reduction below the Bohr radius greatly enhances the quantum confinement effect, which eventually enhances the light emission. The ZSTs are 0.85, 1.02, and 1.19 nm in [5Z/10A] 11 , [6Z/10A] 10 , and [7Z/10A] 9 , respectively.…”

“…In the past, interface states were reported to often act as the quenching centers for the emissions, which led to nonradiative recombination, causing the system to be unfit for LEDs and solar cells . Recently, it has been found that visible photoluminescence (PL) intensity is dramatically enhanced by reducing the sublayer thickness of nanolaminates to be as small as the Bohr radius, attributed to the synergism of quantum confinement effects and interface states. ,, In compared with the photon-driven PL of nanolaminates, the electrically driven self-emissive nanolaminates are more feasible for realizing lighting LEDs as well as integrating into flexible electronic devices and microelectronic devices. It is well-known that LEDs require a multilayered structure with a charge injection layer, a charge transport layer, and a light-emitting layer to realize electroluminescence. , The gentle preparation conditions of light-emitting nanolaminates by atomic layer deposition (ALD) (e.g., low vacuum, low treatment temperature, and no postannealing process) are not only well compatible with the preparation process of the other layers, but also supply a great number of advantages for LEDs, such as low cost, good repeatability, flexibility, and nontoxicity.…”

Toward controlling the Al2O3/ZnO interface properties by in situ ALD preparation

“…3). Alternatively, for Al 2 O 3 -surrounded ZnO quantum dots Li et al 45 reported an interface-related peroxide state Zn* that was shifted by 1.4 eV toward higher BE with respect to the Zn-O component; however, this finding is…”

“…Al 2 O 3 /ZnO bilayers (nanolaminates) and Al 2 O 3 -surrounded ZnO quantum dots prepared exclusively by thermal ALD (TALD) were studied by Chaaya et al 43 and Viter et al 44 as well as by Li et al 45 While these studies mainly addressed the structural and optical properties, we here focus preferentially on the interface electronic properties resulting from a mixed TALD and PEALD process.…”

Tumor microenvironment enhanced NIR II fluorescence imaging for tumor precise surgery navigation via tetrasulfide mesoporous silica-coated Nd-based rare-earth nanocrystals

“…Commonly used NIR-II probes include quantum dots (QDs) [ 19 ], organic dyes [ 20 ], single-walled carbon nanotubes (SWCNTs) [ 21 ], were often unsuitable for long-term in vivo fluorescent imaging, on accounting of inherent toxicity, short blood half-lives or poor biocompatibility. Furthermore, they also face challenges of photo-bleaching and reticuloendothelial system (RES) capture for the widespread adoption in clinical [ [22] , [23] , [24] ].…”