研究ZnSe/CdS/ZnS型II纳米晶体的线性和非线性光物理性质

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"线性和非线性光物理性质的锌硒化物/硫化镉/硫化锌II型核壳壳纳米晶体" 这篇文章介绍了对一种新型的ZnSe/CdS/ZnS II型核壳壳纳米晶体的线性和非线性光物理性质的研究。这种特殊的纳米晶体结构具有独特的光电特性，其性能在多个领域如光电子学、半导体光学和量子点技术中有重要应用。通过测量随温度变化的光致发光光谱，研究者揭示了这些纳米晶体的激子特性，包括带隙变化系数、激子声子耦合强度、激子纵向光学（LO） phonons以及LO-phonon能量等关键参数。首先，文章提到了ZnSe/CdS/ZnS纳米晶体的合成，这是一种多层结构的半导体材料，由ZnSe核心、CdS中间层和ZnS外壳组成。这种设计允许类型II的能级排列，意味着电子和空穴在不同的半导体材料中分离，这可以显著提高光电器件的效率和稳定性。其次，线性光物理性质的探索主要集中在光致发光光谱上。通过改变温度，研究人员能够观察到激子的性质如何随环境的变化而变化。带隙变化系数是衡量半导体材料的带隙宽度如何随温度变化的一个重要参数，这对于理解材料的光电性能至关重要。此外，激子与声子的耦合强度反映了光子与晶格振动的相互作用，这对理解和优化纳米晶体的光发射效率有直接影响。接着，非线性光物理性质的探究通常涉及到超快瞬态吸收光谱技术。这种技术可以捕捉到纳米晶体在极短时间内（飞秒级别）的光吸收变化，揭示其非线性响应，比如二次谐波产生、电荷转移和倍频效应。这些非线性特性在高功率激光器、光开关和光学数据处理等领域有着广泛的应用潜力。最后，文章中提到的LO-phonon能量是指与晶格振动相关的光学模式的能量。在纳米晶体中，LO-phonon能量的测量可以帮助理解载流子复合过程，因为这些声子可以参与载流子冷却和重组过程，从而影响器件的整体性能。这项工作深入探讨了ZnSe/CdS/ZnS II型核壳壳纳米晶体的光物理特性，为设计和优化高性能的光电器件提供了理论基础。这些研究结果对于纳米光学、量子点技术和半导体光电子学的发展具有重要意义，未来可能推动新的光子学应用的创新。

Linear and nonlinear photophysical properties

of ZnSe/CdS/ZnS core/shell/shell type II

nanocrystals

YANG GAO,

XIN QIU,

FULI ZHAO,

2,3

SHUYU XIAO,

JUNZI LI,

XIAODONG LIN,

RUI CHEN,

2,4

AND TINGCHAO HE

1,5

College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China

Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen 518055, China

College of Arts and Science, Shanghai Dianji University, Shanghai 201306, China

e-mail: chenr@sustech.edu.cn

e-mail: tche@szu.edu.cn

Received 2 January 2020; revised 10 June 2020; accepted 26 June 2020; posted 30 June 2020 (Doc. ID 387099); published 6 August 2020

In this work, one kind of type II ZnSe/CdS/ZnS core/shell/shell nanocrystals (NCs) is synthesized, and their

linear and nonlinear photophysical properties are investigated. Through measurements of the temperature-

dependent photoluminescence spectra of NCs, their excitonic properties, including the coefficient of the bandgap

change, coupling strength of the exciton acoustic phonons , exciton longitudinal optical (LO) phonons, and

LO–phonon energy are revealed. Femtosecond transient absorption spectroscopy was employed to obtain insight

into ultrafast processes occurring at the interface of ZnSe and CdS, such as those involving the injection of photo-

induced electrons into the CdS shell, interfacial state bleaching, and charge separation time. At the end, their

multiphoton absorption spectra were determined by using the z-scan technique, which yielded a maximum two-

photon absorption cross section of 3717 GM at 820 nm and three-photon absorption cross section

up to 3.9 × 10

−77

· s

· photon

−2

at 1220 nm, respectively. The photophysical properties presented

here may be important for exploiting their relevant applications in optoelectronic devices and deep-tissue

bioimaging.

https://doi.org/10.1364/PRJ.387099

1. INTRODUCTION

Due to their unique optical and optoelectronic properties,

semiconductor colloidal nanocrystals (NCs) have very broad

application prospects, such as in new solar photovoltaic devices,

light-emitting diodes, biological imaging instruments, photo-

detectors, and nanolasers [1]. Unfortunately, the semiconduc-

tor NCs with simple structures usually have many surface

defects and serious Auger recombination effects, which will

hinder their applications in the fields of optoelectronic devices

and biological science.

Up to now, various methods have been proposed to optimize

the photophysical properties of semiconductor NCs, among

which the most important methods include organic ligand sur-

face modification and shell epitaxial growth. The first method

involves passivating surface dangling bonds of NCs with

organic ligands [2]. The second method involves epitaxially

growing one or multiple shells with a large bandgap outside

the core NCs to form a core–shell structure heterojunction,

which thus eliminates the surface dangling bonds and effec-

tively reduces the nonradiative recom bination caused by surface

defects [3]. Compared with the first method, the core–shell

heterojunction obtained by the second method shows better

compatibility and photothermal stability.

According to the energy values of conduction and valence

bands, the core–shell heterojunction can be divided into types I

and II [4]. In the type I heterojunction, the electrons and holes

are trapped inside the core NCs; in the type II heterojunction,

the holes and electrons are confined to the core or shell, respec-

tively. Spatial separation of carriers can greatly reduce the effi-

ciency of Auger recombination [4]. In addition, the type II

NCs can emit photoluminescence (PL) with a large Stokes shift

and long lifetime, which is advantageous to the application of

fluorescence imaging [5– 8]. While much progress has been

made in understanding the electronic transitions and carrier

dynamics of ZnSe/CdS dot-in-rod heterostructures [9],

(ZnSe/CdS)/CdS nanorods (NRs) [10], ZnSe/CdS/ZnSe nano-

barbells [11], and ZnSe/CdS/Pt NRs [12], the relevant carrier

dynamics of ZnSe core epitaxially grown with multiple spheri-

cal shells that may be promising for further improvement

of their optoelectronic properties has not yet been elucidated.

1416

Vol. 8, No. 9 / September 2020 / Photo nics Research

Research Article

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