Quantum dots (QDs) are steadily being implemented as down-conversion phosphors in market-ready display products to enhance color rendering, brightness, and energy efficiency. However, for adequate longevity, QDs must be encased in a protective barrier that separates them from ambient oxygen and humidity, and device architectures are designed to avoid significant heating of the QDs as well as direct contact between the QDs and the excitation source. In order to increase the utility of QDs in display technologies and to extend their usefulness to more demanding applications as, for example, alternative phosphors for solid-state lighting (SSL), QDs must retain their photoluminescence emission properties over extended periods of time under conditions of high temperature and high light flux. Doing so would simplify the fabrication costs for QD display technologies and enable QDs to be used as down-conversion materials in light-emitting diodes for SSL, where direct-on-chip configurations expose the emitters to temperatures approaching 100 °C and to photon fluxes from 0.1 W/mm to potentially 10 W/mm. Here, we investigate the photobleaching processes of single QDs exposed to controlled temperature and photon flux. In particular, we investigate two types of room-temperature-stable core/thick-shell QDs, known as "giant" QDs for which shell growth is conducted using either a standard layer-by-layer technique or by a continuous injection method. We determine the mechanistic pathways responsible for thermally-assisted photodegradation, distinguishing effects of hot-carrier trapping and QD charging. The findings presented here will assist in the further development of advanced QD heterostructures for maximum device lifetime stability.

Monolayer transition metal dichalcogenides (TMDCs), due to their structural similarity to graphene, emerge as a promising alternative material of integrated optoelectronic devices. Recently, intense research efforts have been devoted to the combination of atomically thin TMDCs with metallic nanostructures to enhance the light−matter interaction in TMDCs. One crucial parameter for semiconductor−metallic nanostructure hybrids is the spacer thickness between the gain media and the plasmonic resonator, which needs to be optimized to balance radiation enhancement and radiation quenching. In current investigations of TMDCs−plamonic coupling, one often adopts a spacer thickness of ∼5 nm or larger, a typical value for transitional gain media−plasmonic composites. However, it is unclear whether this typical spacer thickness represents the optimal value for TMDCs−plasmonic hybrids. Here we address this critical issue by studying the spacer thickness dependence of the luminescent efficiency in the monolayer tungsten-disulfide (WS 2 )−Ag film hybrids. Surprisingly, we discovered that the optimal thickness occurs at ∼1 nm spacer, much smaller than the typical value used previously. In a WS 2 −Ag film system, at this optimal spacer thickness, the photoluminescence (PL) is increased by more than an order of magnitude due to exciton-coupled surface plasmon polaritons (SPPs), as compared to the asgrown WS 2 on sapphire. We further explore a new composite system comprising Ag nanowires on top of a WS 2 −Ag film and observe additional enhancement of the PL (by a factor of 3) contributed by SPPs that are reflected from the end of the wires. Interestingly, in such a composite system, the additional improvement of the PL signal is observed only when the underlying Ag film is an epitaxial film instead of a commonly available thermal film. This is attributed to the reduction of propagation loss of the SPPs on atomically smooth, epitaxial films.

Semiconductor heterostructures have played a critical role as the enabler for new science and technology. The emergence of transition-metal dichalcogenides (TMDs) as atomically thin semiconductors has opened new frontiers in semiconductor heterostructures either by stacking different TMDs to form vertical heterojunctions or by stitching them laterally to form lateral heterojunctions via direct growth. In conventional semiconductor heterostructures, the design of multijunctions is critical to achieve carrier confinement. Analogously, successful synthesis of a monolayer WS /WS Se /WS multijunction lateral heterostructure via direct growth by chemical vapor deposition is reported. The grown structures are characterized by Raman, photoluminescence, and annular dark-field scanning transmission electron microscopy to determine their lateral compositional profile. More importantly, using microwave impedance microscopy, it is demonstrated that the local photoconductivity in the alloy region can be tailored and enhanced by two orders of magnitude over pure WS . Finite element analysis confirms that this effect is due to the carrier diffusion and confinement into the alloy region. This work exemplifies the technological potential of atomically thin lateral heterostructures in optoelectronic applications.