Direct Light‐Driven Modulation of Luminescence from Mn‐Doped ZnSe Quantum Dots

Abstract

Quantum dot (QD) nanocrystals remain at the forefront of fluorescence microscopy as they have the advantages of enhanced photostability, high quantum yield, and macromolecular size. [1][2][3] Furthermore, the ability to tune the QD fluorescence, either by changing their size [1] or by doping, [4] allows for multiplexed imaging. The range of applications extends well beyond the realm of microscopy: QDs may also play a major role in developing novel photonic devices including lasers, light-emitting diodes, and displays. [5][6][7] Despite significant advancements in nanocrystal research, the inability to directly modulate the fluorescence from QDs has precluded their implementation in several areas. In particular, emerging far-field diffraction-unlimited microscopy techniques [8] uniquely benefit from the capability to reversibly modulate/switch fluorescent ensembles from a bright "on" state to a dark "off" state. This activation must occur as a response to optical stimuli which do not contain spectral components within the excitation kernel of the fluorescent markers. With the need for optical control over QD fluorescence, indirect methods have been conceived by using hybrid QD structures [9][10][11] that incorporate a photochromic activator/quencher. Although the concept has been clearly established, hybrid QD structures suffer from inherent drawbacks, such as inadequate photostability, limited fluorescence quenching, and sensitivity to local environment/ solvent.Herein we report on the direct light-driven modulation of QD fluorescence. The mechanism for the fluorescence modulation relies only on internal electronic transitions within Mn-doped ZnSe quantum dots (Mn-QDs). It is demonstrated that the fluorescence of the QD can be reversibly depleted with efficiencies of over 90 % by using continuous-wave optical intensities of approximately 1.9 MW cm À2 . Time-domain measurements during the modulation indicate that the number of fluorescent on-off cycles exceeds 10 3 before a significant reduction in the fluorescence quantum efficiency occurs. Such robust nanometric probes having remotely controllable optical transitions are useful in many areas of research, particularly in far-field nanoscopy based on reversible saturable or switchable optical fluorescence transitions (RESOLFT).[8] Consequently, we show that implementation of Mn-QDs for imaging leads to an increase in the resolution by a factor of 4.4 over that of confocal microscopy.A schematic diagram of the electronic transitions involved in light-modulated fluorescence from Mn-QDs is shown in Figure 1 a. Initially, electrons are photoexcited from the valence band to the conduction band of the ZnSe semiconductor host. Within a short time (picosecond timescale [12][13][14] ), the excited electrons are transferred to the 4 T 1 upper florescent state of the Mn 2+ ion and decay radiatively to the 6 A 1 state within a measured fluorescence lifetime of t fluo % 90 ms (see the Supporting Information). Generally speaking, direct modulation of the fluorescence requires a...

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