Radial‐Position‐Controlled Doping of CdS/ZnS Core/Shell Nanocrystals: Surface Effects and Position‐Dependent Properties

Abstract

Energy transfer in doped semiconductor nanocrystals: Mn-doped CdS/ZnS nanocrystals show interesting photoluminescence and EPR properties that are strongly dependent on the radial position of Mn dopants inside nanocrystals (see graphic). Furthermore, the results suggest a two-step mechanism for the energy transfer inside the doped nanocrystals.This paper reports a study of the surface effects and position-dependent properties of Mn-doped CdS/ZnS core/shell nanocrystals, which were prepared by using a three-step synthesis method. The Mn-doping level of these nanocrystals was determined by a combination of electron paramagnetic resonance spectroscopy and inductively coupled plasma atomic emission spectroscopy. These nanocrystals were further characterized by using transmission electron microscopy and fluorescence spectroscopy. First, we found that injecting a large excess of zinc stearate at the end of nanocrystal synthesis can sufficiently eliminate the surface-trap states from the doped CdS/ZnS core/shell nanocrystals and enhance their photoluminescence (PL) quantum yield (QY). Second, our results demonstrate that the Mn-PL QY is determined by the product of the efficiency of energy transfer from an exciton inside the CdS core to a Mn ion (Phi(ET)) and the efficiency of the emission from the Mn ion (Phi(Mn)). Third, Phi(Mn) strongly depends on the radial position of Mn ions in the doped core/shell nanocrystals. The position-dependent changes of Phi(Mn) nearly perfectly correlate to those of the linewidth of Mn EPR peaks: the higher the Phi(Mn), the narrower the linewidth of the Mn EPR peak. Fourth, the results demonstrate that Phi(ET) depends on the Mn-doping level as well as the inverse sixth power of the distance between a Mn ion and the center of its host nanocrystal. Accordingly, we propose a two-step mechanism for the energy transfer: 1) the energy transfer from an exciton inside the CdS core to a bound exciton around a Mn center, which is the rate-determining step and follows the Förster mechanism, and 2) the energy transfer from the bound exciton to the Mn center, which might follow a mechanism such as dark exciton (triplet exciton) or Auger transfer.

Tuninggfactors of core-shell nanoparticles by controlled positioning of magnetic impurities

“…We consider two such nanoparticles with R = 50Å and R = 100Å since these structures are typical of Mn radial position doped core-shell nanoparticles that have been grown experimentally. 20,[22][23][24] We refer to these two structures as the 100Å diameter and 200Å diameter core-shell nanoparticle, respectively.…”

“…[15][16][17][18][19][20][21] Chemists now have the ability to selectively dope core-shell nanoparticles with magnetic impurities at a specific and precisely defined radius. 20,[22][23][24] For instance, radial positioncontrolled Mn doping of zinc-blende CdS-ZnS core-shell nanoparticles was achieved using a three-step process which includes CdS-core growth, Mn-dopant growth in a thin spherical doping layer (inside either core or shell), and ZnS-shell growth. 20,[22][23][24] As a result, the optical properties of these nanoparticles can be tuned by varying the core and shell thicknesses and the spintronic properties can be tailored by precise positioning of the magnetic impurities.…”

“…20,[22][23][24] For instance, radial positioncontrolled Mn doping of zinc-blende CdS-ZnS core-shell nanoparticles was achieved using a three-step process which includes CdS-core growth, Mn-dopant growth in a thin spherical doping layer (inside either core or shell), and ZnS-shell growth. 20,[22][23][24] As a result, the optical properties of these nanoparticles can be tuned by varying the core and shell thicknesses and the spintronic properties can be tailored by precise positioning of the magnetic impurities. In addition to providing for control of carrier concentration and spin, dopants themselves can also serve as sensitive radial pressure gauges for measuring lattice strains due to lattice mismatch at the coreshell interface.…”

“…Motivated by breakthroughs in the growth of controllably-doped core-shell nanoparticles and the ongoing exploration of their properties, 20,[22][23][24] we developed a theoretical model to describe the electronic states, magnetic g-factors, and magneto-optical properties in systems with radial position-controlled magnetic impurity doping. In addition to providing the foundational understanding of excitation processes, we examine the role of strain and ultra-high magnetic fields.…”

“…In addition to providing the foundational understanding of excitation processes, we examine the role of strain and ultra-high magnetic fields. While we focus on the large gap CdS-ZnS system with Mn doping in an effort to compare with emerging experimental work, 20,[22][23][24] the comprehensive modeling approach outlined here can easily be extended to new materials and will be a powerful predictor of similar effects in other candidate systems.…”