•The role of red quantum dots was studied to fabricate high luminous efficiency warm white LEDs.•Energy transfer and reabsorption between red and green quantum dots in LED structures were discussed.•The layered white LED showed an increase of nearly 11% in efficiency, compared to one with mixed layers.•The optimized white LED yielded a luminous efficiency of 90 lm/W and a color rendering index of 80.High performance white light emitting diodes (WLEDs) were fabricated based on green CuInS2/ZnS quantum dots (QDs) combined with red CuInS2/ZnS or CdSe/ZnS QDs. It was found that the phosphors with a narrow red emission is beneficial for achieving WLEDs with higher luminous efficiency (LE), warmer correlated color temperature (CCT) and better color rendering index (CRI), avoiding the loss of light power in red region. A stacking order configuration for WLEDs was constructed by depositing green QD layers on red ones (CdSe/CIS). The optimized CdSe/CIS QD based WLED yielded an LE of 90.64 lm/W, CCT of 3213 K, and CRI of 80.3, showing an increase of nearly 11% in power conversion efficiency, compared to one with mixed QD layers. The time-resolved PL spectra confirmed the variation in decay lifetimes of phosphor layers with different configurations. The improvement in efficiency was ascribed to less reabsorption loss of red emission when the red light passed through the green QD layer or the reduced energy transfer from green QDs to red ones that occurred in the mixed QD film. The experimental results indicated that improved performances of WLEDs from cold to warm light could be realized by use of structure regulations.Quantum dot white LEDs with layered CdSe/CIS QD films were fabricated, showing an increase of 11% in power efficiency, compared with mixed QD films.

•The self-passivated Cu:ZnInS@ZnS:Al quantum dots were synthesized successfully.•The thermal and current stability of Cu:ZnInS@ZnS:Al quantum dots were improved.•The Cu:ZnInS@ZnS:Al quantum dots had a small reduction of 20% in PL lifetime under thermal annealing from 50 °C to 250 °C.•The Cu:ZnInS@ZnS:Al quantum dot based LED had a 3% luminescence enhancement under 10 mA injection current for a long time.Cu:ZnInS/ZnS/ZnS:Al quantum dots (QDs) with a Cu-doped inner core and an Al-doped outer shell were synthesized in this work, exhibiting improved stability of luminescence, compared with Cu:ZnInS/ZnS/ZnS QDs. The X-ray diffraction patterns and photoluminescence spectra confirmed that the Al ions were doped into the ZnS shell. The stability of Cu:ZnInS/ZnS/ZnS:Al QDs under various thermal annealing temperatures was studied by steady-state and time resolved photoluminescence spectroscopy. The Cu:ZnInS/ZnS/ZnS:Al QDs had a relatively small reduction of 20% in photoluminescence lifetime from 167 ns under thermal annealing at 50 °C to 132.8 ns at 250 °C, compared to a 55% drop for the undoped ones, indicating less thermal effect on the photoluminescence of the QDs with an Al-doped shell. The Cu:ZnInS/ZnS/ZnS:Al QD-based light emitting diodes (LEDs) had a 3% luminescence enhancement under injection current of 10 mA for 30 h, compared to a 13% reduction for the undoped QDs, suggesting improved thermal stability of photoluminescence for the QDs with Al-doped shell. Finally, Cu:ZnInS/ZnS/ZnS:Al QD-based white LEDs with good color rendering index (>80) and correlated color temperature (<4500 K) were fabricated by combining with green emitting CuInS2/ZnS QDs. The self-passivated QDs show promises for enhancing thermal stability of QD-based LEDs in the conserved energy lighting industry.The normalized electroluminescence spectra of Cu:ZnInS@ZnS:Al (a) and Cu:ZnInS@ZnS quantum dot based LEDs under an injection current of 10 mA for various operation times are presented. The quantum dots were mixed with same amount of silicone resin and treated at 50 °C for 30 min.Download high-res image (203KB)Download full-size image

The luminescence properties of inorganic perovskite CsPbBr3 nanocrystals (NCs) with emissions of 492 and 517 nm under thermal annealing treatment were studied by temperature-dependent photoluminescence (PL) spectroscopy. The CsPbBr3 NCs were annealed in vacuum at various temperatures. It was found that the NCs exhibited significant thermal degradation of PL at thermal annealing temperatures above 320 K. The transmission electron microscopy, X-ray diffraction and PL spectroscopy demonstrated that the size of NCs almost kept constant at thermal annealing temperatures below 360 K while it significantly enlarged at higher thermal temperatures above 380 K. The PL intensities, peak energies and linewidths of the annealed NCs, as a function of temperature, are discussed in detail. The PL degradation of the NCs was related to the formation of nonradiative recombination centers due to the loss of ligands and growth of NCs under thermal annealing.

Hybrid MoO3/HAT-CN is employed as a hole injection layer (HIL) in green inverted colloidal quantum dot light-emitting devices (QLEDs). The hybrid HILs can be easily prepared and have been found to effectively improve the electroluminescent properties. The best performance device had an HIL of 1.5 nm-thick MoO3/2.5 nm-thick HAT-CN and showed a turn-on voltage of 1.9 V, a maximum current efficiency (CEmax) of 41.3 cd A−1, and maximum external quantum efficiency of 9.72%. Compared to the corresponding devices with the single MoO3 or HAT-CN interlayer, the CEmax of the hole-only devices was improved by 1.6 or 1.5 times, respectively. The measured electrical performance shows that hole-only devices with hybrid HILs have a smaller leakage current density at low driving voltage and much enhanced hole injection current than the devices with single interlayers. It indicates that much improved electroluminescent efficiency in green inverted QLEDs with hybrid MoO3/HAT-CN orginates from the significant enhancement of hole injection efficiency and suppression of space charge accumulation in the quantum dot-emitting region due to the improved balance of the charge carriers. The hybrid HILs can be extended to other color inverted QLEDs, which are favorable to achieve bright, highly efficient, and color saturation devices for display applications.

We report all solution-processed manufacture of high performance top-emitting (TE) quantum dot light-emitting diodes (QLEDs) with an aluminum (Al) film as bottom electrode and a homogeneous molybdenum oxide (MoO3) film as hole injection layer deposited on top of Al from water solution. With an optimal organic light outcoupling layer over the top electrode to depress the multiple light beam interference effect, the QLEDs show a maximum luminance and current efficiency of 151 000 cd m−2 and 33.7 cd A−1, respectively, attaining a nearly Lambertian light source. Especially, they exhibit a maximum external quantum efficiency of 7.4%. All solution-processed fabrication of these TE QLEDs is further implemented on some active light display samples of Arabic numerals with 15 × 15 mm2 active area. The resulting simple passive matrix quantum dot light emitting displays possess excellent photoelectric characteristics, which represents a step toward practical application.

The photoluminescence (PL) properties of the Cu:Zn–In–S core quantum dots (QDs) and core–shell QDs were systematically investigated by using steady-state and time-resolved PL spectra at temperatures ranging from 80 to 400 K. The effects of the shell structure and the host bandgap on the thermal stability of Cu dopant emissions were studied by measuring the change in the PL intensity and the lifetime. It was found that the PL intensities and lifetimes of the core and core/shell QDs with green, yellow, and red emissions almost decrease with increasing temperatures while their PL was quenched at 300 K and 400 K, respectively, indicating the shell-enhanced thermal stability of the PL. The emission wavelength of the QDs as a function of temperature was also provided. The mechanisms of Cu dopant emission and thermal quenching were discussed. Finally, the green, yellow, red, and white light emitting light emitting diodes (LEDs) were fabricated based on Cu:Zn–In–S QDs.

The ultrafast carrier dynamics and hot electron extraction in tetrapod-shaped CdSe nanocrystals was studied by femtosecond transient absorption (TA) spectroscopy. The carriers relaxation process from the higher electronic states (CB2, CB3(2), and CB4) to the lowest electronic state (CB1) was demonstrated to have a time constant of 1.04 ps, resulting from the spatial electron transfer from arms to a core. The lowest electronic state in the central core exhibited a long decay time of 5.07 ns in agreement with the reported theoretical calculation. The state filling mechanism and Coulomb blockade effect in the CdSe tetrapod were clearly observed in the pump-fluence-dependent transient absorption spectra. Hot electrons were transferred from arm states into the electron acceptor molecules before relaxation into core states.Keywords: carrier dynamics; electron transfer; nanocrystals; tetrapod; transient absorption spectroscopy;

Highly efficient red quantum dot light-emitting diodes (QD-LEDs) with a very high current efficiency of 16 cd/A were demonstrated by adopting stepwise hole-transport layers (HTLs) consisting of 4,4′-N,N′-dicarbazole-biphenyl (CBP) combined with N,N′-dicarbazolyl-3,5-benzene (mCP). The mCP layer plays two important roles in this kind of QD-LEDs. One is that it can block the electron to leak into the HTL due to its higher LUMO (LUMO = the lowest unoccupied molecular orbital) energy level than that of CBP; and the other is it can separate the carrier accumulation zone from the exciton formation interface, which is attributed to the stepwise hole-transport layer structure. Moreover, the lower HOMO (HOMO = the highest occupied molecular orbital) energy level of mCP decreases the hole-injection barrier from the HTL to the QD emitting layer, which improves the charge carrier balance injected into the QD layer, reducing the turn-on voltage of QD-LEDs fabricated with the stepwise HTL structure.Keywords: charge accumulation; leakage current; light emitting diodes; quantum dots; stepwise hole-injection layer;

Photocarrier recombination dynamics in ternary chalcogenide CuInS2 quantum dots (CIS QDs) was studied by means of femtosecond transient-absorption (TA) and nanosecond time-resolved photoluminescence (PL) spectroscopy. Under strong excitation, the TA dynamics in CIS QDs is well described by a simple rate equation including single-carrier trapping, free-to-bound recombination, and trap-assisted Auger recombination. Under weak excitation, on the other hand, the PL decays of the QDs are composed of a short-lived component caused by surface trapping and a long-lived one caused by free-to-bound recombination. It is found that the surface trapping accelerates markedly with decreasing QD size while the free-to-bound radiative recombination hardly depends on the QD size. Besides this, we observed both a decrease in the PL lifetimes and a dynamic spectral redshift, which are attributed to the surface trapping and the coexistent inhomogeneous broadening in CIS QDs. The spectral redshift becomes less pronounced in CIS/ZnS core/shell QDs because of the suppression of the fast nonradiative recombination caused by the passivation of the surface traps. These results give clear evidence that the free-to-bound model is appropriate for interpreting the optical properties of CIS QDs.

The performances and spectroscopic properties of CdSe/ZnS quantum dot light-emitting diodes (QD-LEDs) with inserting a thickness-varied 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) layer between the QD emission layer and 4,4-N,N-dicarbazole-biphenyl (CBP) hole transport layer (HTL) are studied. The significant enhancement in device peak efficiency is demonstrated for the device with a 3.5 nm TPBi interlayer. The photoluminescence lifetimes of excitons formed within QDs in different devices are also measured to understand the influence of electric field on the QD emission dynamics process and device efficiency. All the excitons on QDs at different devices have nearly the same lifetime even though at different bias. The improvement of device performance is attributed to the separation of charge carrier accumulation interface from the exciton formation zone, which suppresses exciton quenching caused by accumulated carriers.Keywords: carrier accumulation interface; exciton formation zone; exciton quenching; inverted structure; quantum dot light-emitting diodes

Studies on quantum dot light emitting devices (QD-LEDs) show that the lifetime of a QD exciton is modified by localized surface plasmon resonant (LSPR) coupling of Au nanoparticles (NPs). The efficiency roll-off of the device containing Au NPs was reduced by 15%. A significant enhancement of 116% for both luminance and current efficiency for the Au NP containing device was achieved under a high current density of 300 mA cm−2. We evaluate that it takes about 10 ns for each QD to be injected by only one electron–hole from the electrodes, which is about the same length of time as the lifetime of a QD, ∼10 ns. This result indicates that the enhanced radiative rate of QD emitters will decrease the concentration of excitons in the emission layer and reduce the nonradiative recombination caused by QD charging. Present findings demonstrate that the excellent performance of the devices is principally a result of LSPR coupling between Au NPs and QD emitters, which shortens the lifetime of the QD emitters and suppresses the Auger recombination in the QD-LEDs.

Photoinduced electron transfer (ET) processes from CuInS2/CdS core/shell quantum dots (QDs) with different core sizes and shell thicknesses to TiO2 electrodes were investigated by time-resolved photoluminescence (PL) spectroscopy. The ET rates and efficiencies from CuInS2/CdS QDs to TiO2 were superior to those of CuInS2/ZnS QDs. An enhanced ET efficiency was surprisingly observed for 2.0 nm CuInS2 core QDs after growth of the CdS shell. On the basis of the experimental and theoretical analysis, the improved performances of CuInS2/CdS QDs were attributed to the passivation of nonradiative traps by overcoating shell and enhanced delocalization of electron wave function from core to CdS shell due to lower conduction band offset. These results indicated that the electron distribution regulated by the band alignment between core and shell of QDs and the passivation of surface defect states could improve ET performance between donor and acceptor.Keywords: band alignment; CuInS2 quantum dots; defect states; electron transfer; TiO2;

•We fabricated QD-LEDs with MoO3 substituting for PEDOT:PSS as hole-injection layer.•A enhancement in luminance and efficiency in MoO3-containing device was observed.•The enhancement was originated from the stability and easy hole injection of MoO3.In this work, we fabricated quantum dots light-emitting devices with hole-injection layer, molybdenum oxide (MoO3) substituting for poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) which is hygroscopic and acidic and, therefore, a source of interface instability. A significant enhancement in luminance and current efficiency in MoO3-containing devices was observed. In addition, MoO3-containing devices were more stable in the air than those with PEDOT:PSS as the hole injection layer. The hole injection and transport of the devices were studied by the J–V characteristics of the hole-only devices. The excellent performance of the devices was principally a result of MoO3 possessing lower injection barrier for the hole and better stability than PEDOT:PSS.

•Annealing-induced enhancement of electron transfer from CdSe to TiO2 is reported.•CdSe QDs on TiO2 and SiO2 films are annealed at various temperatures.•Steady-state and time-resolved PL spectroscopy of CdSe QDs is studied.•The enhancement is related to the reduced distance between CdSe QDs and TiO2.We demonstrated the enhancement of electron transfer from CdSe/ZnS core/shell quantum dots (QDs) to TiO2 films via thermal annealing by means of steady-state and time-resolved photoluminescence (PL) spectroscopy. The significant decrease in PL intensities and lifetimes of the QDs on TiO2 films was clearly observed after thermal annealing at temperature ranging from 100 °C to 300 °C. The obtained rates of electron transfer from CdSe core/shell QDs with red, yellow, and green emissions to TiO2 films were significantly enhanced from several times to an order of magnitude (from ∼107 s−1 to ∼108 s−1). The improvement in efficiencies of electron transfer in the TiO2/CdSe QD systems was also confirmed. The enhancement could be considered to result from the thermal annealing reduced distance between CdSe QDs and TiO2 films. The experimental results revealed that thermal annealing would play an important role on improving performances of QD based optoelectronic devices.

The charge separation and recombination processes between CdSe quantum dot (QD) and graphene oxide (GO) composites with linking molecule methylene blue (MB+) were studied by femtosecond transient absorption spectroscopy. Anchoring MB+ molecules on GO results in significant changes in steady-state and transient absorption spectra, where the exciton dissociation time in the CdSe QD-MB+-GO composite was determined to be 1.8 ps. Surprisingly, the ground state bleaching signal increased for MB+-GO complex was found to be 5.2 ps, in relation with electron transfer from QD to GO. On the other hand, the strong electronic coupling between MB•-GO radical and GO prolonged charge recombination process (≥5 ns) in QD-MB+-GO composites. Charge separation and recombination processes at the interface between semiconductor QDs and graphene can thus be modulated by the functionalized dye molecules.Keywords: graphene oxide; photoinduced charge separation; quantum dots; transient absorption;

Temperature-dependent photoluminescence (PL) spectroscopy of CuInS2 core and CuInS2/ZnS core–shell quantum dots (QDs) was studied for understanding the influence of a ZnS shell on the PL mechanism. The PL quantum yield and lifetime of CuInS2 core QDs were significantly enhanced after the QD surface was coated with the ZnS shell. The temperature dependences of the PL energy, linewidth, and intensity for the core and core–shell QDs were studied in the temperature range from 92 to 287 K. The temperature-dependent shifts of 98 meV and 35 meV for the PL energies of the QDs were much larger than those of the excitons in their bulk semiconductors. It was surprisingly found that the core and core–shell QDs exhibited a similar temperature dependence of the PL intensity. The PL in the CuInS2/ZnS core–shell QDs was suggested to originate from recombination of many kinds of defect-related emission centers in the interior of the cores.Highlights► Temperature-dependent PL of CuInS2 core and core/shell QDs was studied. ► Large temperature-dependent shifts of PL peaks for the QDs were observed. ► The QDs exhibited a similar temperature dependence of the PL intensity. ► The PL was related to recombination of many kinds of emission centers in QDs.

We studied the energy transfer processes from organic charge transporting materials (CTMs) to ZnCuInS (ZCIS) quantum dots (QDs) with different emission wavelength by steady-state and time-resolved photoluminescence (PL) spectroscopy. The change in the PL excitation intensity of the ZCIS QDs and the PL decay time of the CTMs clearly demonstrated an efficient energy transfer process in the ZCIS/CTM blend films. It was found that the efficiency of Förster resonance energy transfer significantly increases with increasing the particle size and decreasing the Zn content in the QDs, which is well consistent with the estimated Förster radii (R0) varying from 3 to 5 nm. In addition, the PL quenching of the QDs related to the charge separation process was also observed in some of the samples. The energy transfer and charge separation processes in the films were well explained based on the band alignment between the ZCIS QDs and CTMs.

Photoluminescence (PL) properties of 3-mercaptopropionic acid (MPA) coated CdTe/CdS core-shell quantum dots (QDs) in aqueous solution in the presence of ZnO colloidal nanocrystals were studied by steady-state and time-resolved PL spectroscopy. The PL quenching of CdTe/CdS core-shell QDs with addition of purified ZnO nanocrystals resulted in a decrease in PL lifetime and a small red shift of the PL band. It was found that CdTe(1.5 nm)/CdS type II core-shell QDs exhibited higher efficiency of PL quenching than the CdTe(3.0 nm)/CdS type I core-shell QDs, indicating an electron transfer process from CdTe/CdS core-shell QDs to ZnO nanocrystals. The experimental results indicated that the efficient electron transfer process from CdTe/CdS core-shell QDs to ZnO nanocrystals could be controlled by changing the CdTe core size on the basis of the quantum confinement effect.Research highlights► PL quenching of CdTe/CdS core-shell QDs was observed in the presence of ZnO nanocrystals. ► The PL quenching resulted in a decrease in PL lifetime and a small red shift of the PL band of CdTe/CdS core-shell QDs. ► CdTe(1.5)/CdS core-shell QDs exhibited higher efficiency of PL quenching than the CdTe(3.0)/CdS core-shell QDs. ► Electron transfer processes from CdTe/CdS core-shell QDs to ZnO nanocrystals were explained well by type II and type I QD models.

We studied the diffusion of Mn ions in MnS/ZnS core/shell nanocrystals (NCs) from the MnS core to the ZnS shell by steady-state and time-resolved photoluminescence (PL) spectroscopy. The colloidal MnS/ZnS core/shell NCs with different thicknesses of the ZnS shell synthesized in octadecene with nucleation doping strategy were annealed at different temperatures. It was found that the PL intensity and lifetime of the MnS/ZnS NCs with a thin ZnS shell significantly decreased with increasing annealing time in the heat treatment temperature range of 220−300 °C, resulting from the diffusion of Mn ions in the MnS/ZnS core/shell NCs to the surface of the ZnS shell, while the PL intensity and lifetime of the NCs with a thick ZnS shell remained almost constant in the temperature range. Furthermore, it was noted that the MnS/ZnS NCs with a small MnS core of about 2.0 nm in diameter exhibited high PL quantum yield of greater than 40%. The experimental results indicated that highly efficient luminescent Mn-doped ZnS NCs could be obtained by controlling the diffusion of Mn ions into the ZnS shell via annealing the core/shell NCs and using small-sized MnS cores and by optimizing the thickness of the ZnS passivating layer.

We studied the energy transfer between CdSe core/shell quantum dots (QDs) and 1,3,5-tris(N-phenylbenzimidazol-2,yl) benzene (TPBI) in inorganic/organic blend films using steady-state and time-resolved photoluminescence (PL) spectroscopy. The shortening in PL lifetime of TPBI molecules and the resulting lengthening in PL lifetime of the QDs demonstrated an efficient energy transfer process from donor to acceptor. The slowest PL decays of CdSe core/shell QDs observed in the blend films with low QD concentration were considered to result from the maximum energy transfer process from the surrounding TPBI molecules of a QD to itself. The PL decay curves of the core/shell QDs with a CdS, ZnS, and CdS/ZnCdS/ZnS shells were simulated to obtain the excited state lifetimes of the surrounding TPBI molecules for understanding the effect of the shells on the energy transfer process. It was surprisingly found that the obtained energy transfer rate to a QD with a thick CdS/ZnCdS/ZnS multishell from the surrounding TPBI molecules with the maximum contribution of the energy transfer was almost the same as that to a QD with a thin ZnS monoshell and smaller than that to a QD with a CdS monoshell. The experimental results indicated the energy level alignment and the structure of shells in CdSe core/shell QDs determined the energy transfer efficiency from TPBI molecules to the core/shell QDs.

The photoinduced hole transfer dynamics from CdSe quantum dots (QDs), shelled with ZnS or CdS/CdZnS/ZnS layers, to organic hole transporting materials (HTMs) is investigated by absorption, steady-state and time-resolved photoluminescence (PL) spectroscopy. The PL intensity and lifetime of the QDs are dramatically quenched when HTMs are added into the dilute QD solution. The quenching efficiency of the QDs significantly decreases with increasing the shell thickness and increases with decreasing the oxidation potential of the HTMs. These facts are correlated with the photoinduced hole transfer from the QDs to the HTMs. The above results are helpful in understanding the photoexcitation dynamics-related phenomena of organic molecule conjugated nano-object.

Photoluminescence quenching of colloidal CdSe core/shell quantum dots (QDs) with CdS, ZnS and CdS/CdZnS/ZnS shells in the presence of hole-transporting materials (HTMs) is studied by means of steady-state and time-resolved photoluminescence spectroscopy. Static quenching is surprisingly found to play an even more important role in the PL quenching process than the dynamic one originating from a hole transfer from QDs to HTMs. The static quenching efficiency of the QDs with single CdS shells of 3 and 6 monolayers is much larger than that of the QDs with a multilayer CdS/CdZnS/ZnS shell. The experimental results are important for understanding the control of the static quenching pathways in QDs by using multishell structures.

We have studied the mechanism of photoluminescence (PL) from MnS/ZnS core/shell quantum dots (QDs) synthesized via hot solution phase chemistry using a nucleation−doping strategy. Efficient PL of the Mn2+ ions with a quantum yield (QY) of over 35% is demonstrated in the resulting QDs coated with a thick ZnS shell on the MnS core. The MnS/ZnS core/shell QDs with a thick shell exhibit PL enhancement with increasing temperature in the range between 140 and 300 K, resulting from the thermal activation of charge carriers localized at the interface between the MnS core and the ZnS shell. The PL decays of the Mn2+ ions in the core/shell QDs consist of three exponential components with time constants on the scales of 1−2 ms, hundreds of μs, and tens of μs. Surprisingly, the PL lifetimes of Mn2+ ions show a very weak dependence on the shell thickness, which is clearly different from that of the PL QY of the QDs. The experimental results indicate that the mechanism for improving the PL QY in MnS/ZnS QDs can be understood in terms of a significantly enhanced energy transfer from the ZnS shell to Mn2+ ions and a slightly decreased nonradiative relaxation rate from Mn2+ ions to surface states/traps of the ZnS shell by the surface passivation of the QDs with a thick ZnS shell.

The rapidly improved performance of LEDs based on multilayers of highly luminescent quantum dots could lead to promising applications in next-generation displays and lighting.

Photoluminescence (PL) properties of 3-mercaptopropionic acid (MPA) coated CdTe/CdS core-shell quantum dots (QDs) in aqueous solution in the presence of ZnO colloidal nanocrystals were studied by steady-state and time-resolved PL spectroscopy. The PL quenching of CdTe/CdS core-shell QDs with addition of purified ZnO nanocrystals resulted in a decrease in PL lifetime and a small red shift of the PL band. It was found that CdTe(1.5 nm)/CdS type II core-shell QDs exhibited higher efficiency of PL quenching than the CdTe(3.0 nm)/CdS type I core-shell QDs, indicating an electron transfer process from CdTe/CdS core-shell QDs to ZnO nanocrystals. The experimental results indicated that the efficient electron transfer process from CdTe/CdS core-shell QDs to ZnO nanocrystals could be controlled by changing the CdTe core size on the basis of the quantum confinement effect.Research highlights► PL quenching of CdTe/CdS core-shell QDs was observed in the presence of ZnO nanocrystals. ► The PL quenching resulted in a decrease in PL lifetime and a small red shift of the PL band of CdTe/CdS core-shell QDs. ► CdTe(1.5)/CdS core-shell QDs exhibited higher efficiency of PL quenching than the CdTe(3.0)/CdS core-shell QDs. ► Electron transfer processes from CdTe/CdS core-shell QDs to ZnO nanocrystals were explained well by type II and type I QD models.

Photocarrier recombination dynamics in ternary chalcogenide CuInS2 quantum dots (CIS QDs) was studied by means of femtosecond transient-absorption (TA) and nanosecond time-resolved photoluminescence (PL) spectroscopy. Under strong excitation, the TA dynamics in CIS QDs is well described by a simple rate equation including single-carrier trapping, free-to-bound recombination, and trap-assisted Auger recombination. Under weak excitation, on the other hand, the PL decays of the QDs are composed of a short-lived component caused by surface trapping and a long-lived one caused by free-to-bound recombination. It is found that the surface trapping accelerates markedly with decreasing QD size while the free-to-bound radiative recombination hardly depends on the QD size. Besides this, we observed both a decrease in the PL lifetimes and a dynamic spectral redshift, which are attributed to the surface trapping and the coexistent inhomogeneous broadening in CIS QDs. The spectral redshift becomes less pronounced in CIS/ZnS core/shell QDs because of the suppression of the fast nonradiative recombination caused by the passivation of the surface traps. These results give clear evidence that the free-to-bound model is appropriate for interpreting the optical properties of CIS QDs.

The photoluminescence (PL) properties of the Cu:Zn–In–S core quantum dots (QDs) and core–shell QDs were systematically investigated by using steady-state and time-resolved PL spectra at temperatures ranging from 80 to 400 K. The effects of the shell structure and the host bandgap on the thermal stability of Cu dopant emissions were studied by measuring the change in the PL intensity and the lifetime. It was found that the PL intensities and lifetimes of the core and core/shell QDs with green, yellow, and red emissions almost decrease with increasing temperatures while their PL was quenched at 300 K and 400 K, respectively, indicating the shell-enhanced thermal stability of the PL. The emission wavelength of the QDs as a function of temperature was also provided. The mechanisms of Cu dopant emission and thermal quenching were discussed. Finally, the green, yellow, red, and white light emitting light emitting diodes (LEDs) were fabricated based on Cu:Zn–In–S QDs.

Studies on quantum dot light emitting devices (QD-LEDs) show that the lifetime of a QD exciton is modified by localized surface plasmon resonant (LSPR) coupling of Au nanoparticles (NPs). The efficiency roll-off of the device containing Au NPs was reduced by 15%. A significant enhancement of 116% for both luminance and current efficiency for the Au NP containing device was achieved under a high current density of 300 mA cm−2. We evaluate that it takes about 10 ns for each QD to be injected by only one electron–hole from the electrodes, which is about the same length of time as the lifetime of a QD, ∼10 ns. This result indicates that the enhanced radiative rate of QD emitters will decrease the concentration of excitons in the emission layer and reduce the nonradiative recombination caused by QD charging. Present findings demonstrate that the excellent performance of the devices is principally a result of LSPR coupling between Au NPs and QD emitters, which shortens the lifetime of the QD emitters and suppresses the Auger recombination in the QD-LEDs.

The luminescence properties of inorganic perovskite CsPbBr3 nanocrystals (NCs) with emissions of 492 and 517 nm under thermal annealing treatment were studied by temperature-dependent photoluminescence (PL) spectroscopy. The CsPbBr3 NCs were annealed in vacuum at various temperatures. It was found that the NCs exhibited significant thermal degradation of PL at thermal annealing temperatures above 320 K. The transmission electron microscopy, X-ray diffraction and PL spectroscopy demonstrated that the size of NCs almost kept constant at thermal annealing temperatures below 360 K while it significantly enlarged at higher thermal temperatures above 380 K. The PL intensities, peak energies and linewidths of the annealed NCs, as a function of temperature, are discussed in detail. The PL degradation of the NCs was related to the formation of nonradiative recombination centers due to the loss of ligands and growth of NCs under thermal annealing.