Evidence for the Ligand-Assisted Energy Transfer from Trapped Exciton to Dopant in Mn-Doped CdS/ZnS Semiconductor Nanocrystals

Abstract

Trapping of charge carriers is the major process competing with radiative recombination or transfer of charge carriers important in the application of semiconductor nanocrystals in photonics and photocatalysis. In typical semiconductor quantum dots, trapping of charge carriers usually leads to quenching of exciton luminescence. In this study, we present evidence indicating that thiol ligands on the surface that quench exciton luminescence can have an opposite effect on sensitized dopant luminescence in doped semiconductor nanocrystals by facilitating the recovery of the trapped exciton for sensitization. Despite the increase in hole trapping by the added octanethiol to the surface of Mn-doped CdS/ZnS nanocrystals, the sensitized Mn luminescence increased by the added octanethiol and the enhancement became stronger with increasing Mn doping concentration. While the role of octanethiol as the hole trap and the enhancement of Mn luminescence may seem contradictory, the thiol-induced enhancement of Mn luminescence is possible, since thiols play dual role as the hole trap and as the facilitator of the energy transfer from the trapped exciton to Mn, in contrast to the pre-existing hole traps that inhibit the energy transfer.

Evidence in Support of Exciton to Ligand Vibrational Coupling in Colloidal Quantum Dots

“…While thiol ligands generally trap the photo-generated holes and, consequently, quench the CQDs' emission, in the reported case, these thiols enhanced the Mn +2 -dopant emission intensity by mediating an energy transfer from a trap side to the dopant excited state, via the surface-to-ligand coupling. 166 Optically detected magnetic resonance of trapped holes at a surface-ligand interface: Optically detected magnetic resonance (ODMR) spectroscopy is associated with a spin flip of either unpaired electron or hole spins. A plot of the luminescence intensity, induced by a magnetic resonance event at the excited state, versus strength of an external magnetic field, displays a spectrum that resembles a ground-state electron spin resonance spectrum.…”

Microbe‐Assisted Synthesis and Luminescence Properties of Monodispersed Tb3+‐Doped ZnS Nanocrystals

“…Rare-earth-doped wide-band-gap semiconductors have attracted considerable interest in recent years because of attempts to develop novel optoelectronic devices, which combine the unique luminescence features of rare earth ions [3][4][5]. Since Bhargava et al first reported the remarkable optical properties of Mn-doped ZnS nanocrystals prepared by chemical process at room temperature in 1994 [6,7], a large number of investigations on semiconductor nanocrystals have focused on the photoluminescence properties of Mn-doped ZnS nanocrystals [8][9][10][11][12][13][14][15][16], Cu-doped ZnS nanocrystals [9,17], Sm-doped ZnS nanocrystals [18], Tb-doped ZnS nanocrystals [18][19][20], and Eu-doped ZnS nanocrystals [21][22][23][24][25] prepared by different techniques [26]. These papers, however, do not report on ZnS:Tb 3+ nanocrystals synthesized by spray precipitation method in sulfatereducing bacterial (SRB) culture at room temperature.…”

Mn-doped CdS/ZnS/CdS QD-based fluorescent nanosensor for rapid, selective, and ultrasensitive detection of copper(ii) ion

“…A substantial increase of the intensity of the Mn emission occurs, because the CdS shell not only removes the surface trap of Mn-doped QDs, but also facilitates the energy transfer from the host to the Mn dopant. 15,20 The GSH-capped Mn-doped QD-based nanosensor used in this work was prepared through two steps, shown in Scheme 1. First, water-soluble QDs can be obtained by ligand exchange with MPA.…”

“…To improve the efficiency of Mn emission and selectivity of nanosensor, an outer CdS shell was introduced, which can assist energy transfer from the host to the Mn dopant. 15 Glutathione (GSH) caps served as recognition and bonding sites for Cu 2+ sites, thus accumulating Cu 2+ on the QD surface, resulting in the high selectivity and rapid response. The ultrasensitive sensing principle was based on PL quenching of Mn 2+ emission via Cu 2+ -induced non-radiative recombination pathways, which blocked the energy transfer from the host to the Mn dopant.…”