Dipartimento di Scienza dei Materiali, Università degli Studi di Milano-Bicocca , via Cozzi 55, I-20125 Milano, Italy.
Nano Lett. 2014 Jul 9;14(7):3855-63. doi: 10.1021/nl501026r. Epub 2014 Jun 19.
Colloidal "dot-in-bulk" nanocrystals (DiB NCs) consist of a quantum confined core embedded into a bulklike shell of a larger energy gap. The first reported example of this class of nanostructures are CdSe/CdS DiB NCs that are capable of producing tunable two-color emission under both weak continuous-wave optical excitation and electrical charge injection. This property is a consequence of a Coulomb blockade mechanism, which slows down dramatically intraband relaxation of shell-localized holes when the core is already occupied by a hole. Here, we demonstrate electrochemical control of dual emission from DiB NCs. Spectro-electrochemical (SEC) experiments are used to tune and probe the photoluminescence (PL) intensity and branching between the core and the shell emission channels as a function of applied electrochemical potential (VEC). To interpret the SEC data we develop a model that describes the changes in the intensities of the shell and core PL bands by relating them to the occupancies of electron and hole traps. Specifically, application of negative electrochemical potentials under which the Fermi level is shifted upward in energy leads to passivation of electron traps at the surface of the CdS shell thereby increasing the total PL quantum yield by favoring the shell emission. Simultaneously, the emission color changes from red (VEC = 0) through yellow to green (VEC = -1). Time-resolved PL measurements indicate that as the Fermi level approaches the NC conduction band-edge electrons are injected into the NC quantized states, which leads to typical signatures of negative trions observed under optical excitation. Application of positive potentials leads to activation of electron traps, which quenches both core and shell PL and leads to the reduction of the overall PL quantum efficiency. A high sensitivity of emission intensity (especially pronounced for the shell band) and the apparent emission color of DiB NCs to local electrochemical environment can enable interesting applications of these novel nanostructures in areas of imaging and sensing including, for example, ratiometric probing of intracellular pH.
胶态“体相”纳米晶（DiB NCs）由一个被较大能隙体相壳包裹的量子限制核组成。该类纳米结构的首例报道是 CdSe/CdS DiB NCs，它们在弱连续波光激发和电电荷注入下都能产生可调谐的双色发射。这一特性是库仑阻塞机制的结果，当核已经被一个空穴占据时，该机制极大地减缓了壳局域空穴的带内弛豫。在这里，我们展示了 DiB NCs 双发射的电化学控制。光谱电化学（SEC）实验被用来调谐和探测光致发光（PL）强度以及核和壳发射通道之间的分支，作为施加电化学势（VEC）的函数。为了解释 SEC 数据，我们开发了一个模型，通过将其与电子和空穴陷阱的占据相关联，来描述壳和核 PL 带强度的变化。具体来说，施加负电化学势，使费米能级向上移动，会导致 CdS 壳表面的电子陷阱钝化，从而通过有利于壳发射来提高总 PL 量子产率。同时，发射颜色从红色（VEC = 0）通过黄色变为绿色（VEC = -1）。时间分辨 PL 测量表明，随着费米能级接近 NC 导带边缘，电子被注入 NC 量子化态，这导致在光激发下观察到典型的负三离子特征。施加正电势会导致电子陷阱的激活，这会猝灭核和壳 PL，并导致整体 PL 量子效率降低。DiB NCs 的发射强度（特别是壳带的强度）和明显的发射颜色对局部电化学环境的高灵敏度，可以使这些新型纳米结构在成像和传感等领域的应用变得有趣，例如，细胞内 pH 的比色探测。
