Enhanced Emission of Nanocrystal Solids Featuring Slowly Diffusive Excitons

Abstract

Solution processing of semiconductor nanocrystal (NC) solids represents an attractive platform for the development of next-generation optoelectronic devices. In search of enhanced light-emitting performance, NC solids are typically designed to have large interparticle gaps that minimize exciton diffusion to dissociative sites. This strategy, however, reduces electrical coupling between nanoparticles in a film, making the injection of charges inefficient. Here, we demonstrate that bright emission from nanocrystal solids can be achieved without compromising their electrical conductivity. Our study shows that solids featuring a low absorption-emission spectral overlap (J) exhibit an intrinsically slower exciton diffusion to recombination centers, promoting longer exciton lifetimes. As a result, enhanced emission is achieved despite a strong electronic coupling. The observed phenomenon was found consistent with a decreased resonant energy transfer in films exhibiting a reduced J value. The inverse correlation between film luminescence and J was revealed through a comparative analysis of CdSe/CdS and ZnSe/CdS solids and further confirmed in two control systems (ZnTe/CdSe and Mn2+-doped ZnCdSe/ZnS). Exceptionally slow exciton diffusion (∼0.3 ms) and high brightness were observed for Mn2+-doped Zn1–x Cd x Se/ZnS NC films exhibiting a nearly vanishing J parameter. We expect that the demonstrated combination of electrical coupling and bright emission in nanocrystal solids featuring low J can benefit the development of nanocrystal light-emitting technologies.

Coordination-induced emission enhancement in gold-nanoclusters with solid-state quantum yields up to 40% for eco-friendly, low-reabsorption nano-phosphors

“…In general, the PL-QYs of colloidal nano-materials, such as colloidal QDs and organic dyes could be significantly reduced in the solid state due to the formation of the aggregates, in which, non-radiative FRET processes could be introduced 8 , 9 . Fortunately, such negative effects play a minor role in Zn-GSH-AuNCs due to unique small spectral overlap integral.…”

“…Fortunately, such negative effects play a minor role in Zn-GSH-AuNCs due to unique small spectral overlap integral. The FRET efficiency can be expressed by this equation, , where denote the separation distances between the samples and Forster radius 9 . The Forster radius can be calculated by this formula, , where represent orientation factor, refractive index, quantum yields and spectral overlap integral, respectively.…”

“…Among CQD aggregates, the excited-state energy could be dissipated by the non-radiative relaxation through both singlet and triplet energy transfer processes, leading to concentration/aggregation induced PL-QY self-quenching 8 . The main mechanism for singlet-state quenching is dictated by dipole-mediated Forster resonance energy transfer (FRET), which strongly depends on the spectral overlap between optical absorption of the acceptors and PL emission of the donors 9 . To avoid this problem, a solid matrix, for example, organic polymer or inorganic silica needs to be introduced to disperse CQDs from the formation of aggregation 10 , 11 .…”

Self‐Assembly of Semiconductor Quantum Dots using Organic Templates

“…For example, QD dimers show specific absorption and emission spectra that are different from those of QD monomers due to the presence of inter‐QD interactions . Furthermore, inter‐QD interactions, such as energy and charge transfer, in the solid state can be tuned by changing the constituent elements of the QDs . By exploiting the differences in the emission properties between the monomer and assembled states, Kanie and co‐workers demonstrated the switching of assembly structure‐dependent emission from CdS QDs covered with phenethyl ether‐type dendrons in the liquid‐crystal phase .…”

Challenges and Prospects of Photocatalytic Applications Utilizing Semiconductor Nanocrystals

“…Lattice incorporated Mn 2+ or Cu + can serve as hole trap sites promoting exciton localization in the bulk of the nanocrystal. In colloidal solutions, this strategy can allow extending excited state lifetimes into a microsecond range for Ag + - and Cu + - doped CdSe NCs (Kholmicheva et al, 2017 ; Nelson et al, 2017 ), or even a millisecond range for Mn 2+ -doped ZnSe/ZnS core/shell (Pu et al, 2016 ) and CdS QDs (Knowles et al, 2015 ). The photocatalytic applications of doped semiconductor nanocrystals, however, have not yet been explored.…”