8:30 AM - ES19.08.02
Subpicosecond Photoionization of Mn-Doped CdSe Quantum Dots Mediated by Spin-Exchange Auger Interactions
Rohan Singh1,Wenyong Liu1,Jaehoon Lim1,Istvan Robel1,Victor Klimov1
Los Alamos National Laboratory1
Diluted magnetic semiconductors (DMS), especially those containing Mn dopants, have been the subject of numerous studies . A key property of these materials is strong interactions between electronic states of a semiconductor host and Mn dopants mediated by spin-exchange interactions [2, 3]. An interesting aspect of these interactions is their influence on multicarrier Auger-type phenomena. In the case of Mn-doped II-VI DMSs, the spin-exchange Auger effect has been invoked to explain highly efficient excitation transfer from a semiconductor to a Mn ion leading to characteristic emission via the internal 4T1−6A1 transition of a 3d electron. This effect has been observed in both bulk and quantum dot (QD) forms of II-VI DMS materials [1, 4]. Previous studies also indicate that the reverse Auger process whereby the Mn excitation is transferred to the conduction-band electron is also highly probable . In particular, Auger de-excitation was cited to rationalize Mn-emission quenching by injected electrons . Furthermore, there are several direct observations of a hot Auger electron produced by this process [7, 8].
Despite the initial indications of a considerable strength of exchange Auger interactions in Mn-doped II-VI QDs, the quantitative understanding of this effect is still lacking. The purpose of the present study is to quantify temporal characteristics of Auger-mediated exchange of excitations between the magnetic ion and the semiconductor by conducting femtosecond transient absorption (TA) measurements of Mn:CdSe QDs. Using low-intensity (sub-single exciton) near band-edge excitation (515 nm), we are able to resolve both “forward” transfer of a band-edge exciton to a Mn ion as well as “back” transfer. We find that the direct transfer takes place on a ca. 100-fs timescale. The rate of the back transfer, on the other hand, strongly (exponentially) depends on the energy difference between the Mn 4T1−6A1 transition and the QD band gap, exhibiting a thermally activated behavior. Furthermore, in the case of above-band-gap excitation (343 nm) and high pump intensities (multiexcitonic regime), we detect the excitation of a “hot” electron from the QD to an external “vacuum” state accompanied by de-excitation of the Mn ion. This indicates an extremely strong exchange coupling of the excited Mn d-d transition to the intraband QD transition, which leads to sub-ps Auger-assisted re-excitation of the “hot” electron prior to its relaxation to the band edge. As a result, the energy transferred from the Mn ion adds up with the kinetic energy of the unrelaxed carrier, which allows it to escape from the QD.
This unusually fast intra-band Auger re-excitation has never been observed in bulk DMS materials, suggesting that it might be specific to strongly confined QDs. The above studies only scratch the surface of the fascinating subject of nanoscale exchange-type Auger interactions. Future work in this areas should deliver tremendous amount of new interesting physics and can potentially lead to novel applications such as photon upconversion, optically controlled electron emission, and transient exciton storage and on-demand release.
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