Jialong Zhao

Towards efficient solid-state photoluminescence based on carbon-nanodots and starch composites

A new type of environmentally friendly phosphor based on carbon nanodots (CDs) has been developed through the dispersion of CDs by integrating the CDs with starch particles. The starch particles contain large numbers of hydroxyl groups around the surfaces, which can effectively absorb the CDs, whose surfaces are functionalized by lots of carboxyl and amide groups, through hydrogen bonding. Effective dispersion of CDs on the surfaces of starch particles can suppress the non-radiative decay processes and photoluminescence (PL) quenching induced by aggregation of CDs. The starch matrix neither competes for absorbing excitation light nor absorbs the emissions of CDs, which leads to efficient PL emitting. As a result, the starch/CD phosphors with a quantum yield of ∼50% were obtained. The starch/CD phosphors show great potential in phosphor-based light emitting diodes, temperature sensors, and patterning.

Dual emissive manganese and copper Co-doped Zn-In-S quantum dots as a single color-converter for high color rendering white-light-emitting diodes.

Novel white light emitting diodes (LEDs) with environmentally friendly dual emissive quantum dots (QDs) as single color-converters are one of the most promising high-quality solid-state lighting sources for meeting the growing global demand for resource sustainability. A facile method was developed for the synthesis of the bright green-red-emitting Mn and Cu codoped Zn-In-S QDs with an absorption bangdgap of 2.56 eV (485 nm), a large Stokes shift of 150 nm, and high emission quantum yield up to 75%, which were suitable for warm white LEDs based on blue GaN chips. The wide photoluminescence (PL) spectra composed of Cu-related green and Mn-related red emissions in the codoped QDs could be controlled by varying the doping concentrations of Mn and Cu ions. The energy transfer processes in Mn and Cu codoped QDs were proposed on the basis of the changes in PL intensity and lifetime measured by means of steady-state and time-resolved PL spectra. By integrating these bicolor QDs with commercial GaN-based blue LEDs, the as-fabricated tricolor white LEDs showed bright natural white light with a color rendering index of 95, luminous efficacy of 73.2 lm/W, and color temperature of 5092 K. These results indicated that (Mn,Cu):Zn-In-S/ZnS QDs could be used as a single color-converting material for the next generation of solid-state lighting.

High color purity ZnSe/ZnS core/shell quantum dot based blue light emitting diodes with an inverted device structure

Deep-blue, high color purity electroluminescence (EL) is demonstrated in an inverted light-emitting device using nontoxic ZnSe/ZnS core/shell quantum dots (QDs) as the emitter. The device exhibits moderate turn-on voltage (4.0 V) and color-saturated deep blue emission with a narrow full width at half maximum of ∼15 nm and emission peak at 441 nm. Their maximum luminance and current efficiency reach 1170 cd/m2 and 0.51 cd/A, respectively. The high performances are achieved through a ZnO nanoparticle based electron-transporting layer due to efficient electron injection into the ZnSe/ZnS QDs. Energy transfer processes between the ZnSe/ZnS QDs and hole-transporting materials are studied by time-resolved photoluminescence spectroscopy to understand the EL mechanism of the devices. These results provide a new guide for the fabrication of efficient deep-blue quantum dot light-emitting diodes and the realization of QD-based lighting sources and full-color panel displays.

Efficient Quantum Dot Light-Emitting Diodes by Controlling the Carrier Accumulation and Exciton Formation

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.

Shell-thickness-dependent photoinduced electron transfer from CuInS2/ZnS quantum dots to TiO2 films

We demonstrate the electron transfer (ET) processes from CuInS2/ZnS core/shell quantum dots (QDs) into porous anatase TiO2 films by time-resolved photoluminescence spectroscopy. The rate and efficiency of ET can be controlled by changing the core diameter and the shell thickness. It is found that the ET rates decrease exponentially at the decay constants of 1.1 and 1.4 nm–1 with increasing ZnS shell thickness for core diameters of 2.5 and 4.0 nm, respectively, in agreement with the electron tunneling model. This shows that optimized ET efficiency and QD stability can be realized by controlling the shell thickness.

Luminescence of Mn2+ doped ZnS nanocrystallites

Abstract Optical properties of Mn2+-doped ZnS colloids are reported. The band to band excitation energy transfer to Mn2+ is more efficient compared to Mn2+ direct excitation, which is different from the case for bulk ZnS: Mn. Aging effects and the radiation-induced luminescence enhancement (RILE) effect are reported and an explanation for this behavior is presented.

The work mechanism and sub-bandgap-voltage electroluminescence in inverted quantum dot light-emitting diodes.

Through introducing a probe layer of bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (FIrpic) between QD emission layer and 4, 4-N, N- dicarbazole-biphenyl (CBP) hole transport layer, we successfully demonstrate that the electroluminescence (EL) mechanism of the inverted quantum dot light-emitting diodes (QD-LEDs) with a ZnO nanoparticle electron injection/transport layer should be direct charge-injection from charge transport layers into the QDs. Further, the EL from QD-LEDs at sub-bandgap drive voltages is achieved, which is in contrast to the general device in which the turn-on voltage is generally equal to or greater than its bandgap voltage (the bandgap energy divided by the electron charge). This sub-bandgap EL is attributed to the Auger-assisted energy up-conversion hole-injection process at the QDs/organic interface. The high energy holes induced by Auger-assisted processes can be injected into the QDs at sub-bandgap applied voltages. These results are of important significance to deeply understand the EL mechanism in QD-LEDs and to further improve device performance.

Shell-dependent electroluminescence from colloidal CdSe quantum dots in multilayer light-emitting diodes

We report electroluminescence (EL) of colloidal CdSe/CdS, CdSe/ZnS, and CdSe/CdS/CdZnS/ZnS core/shell quantum dots (QDs) in multilayer light-emitting diodes (LEDs) fabricated by spin coating a near monolayer of the core/shell QDs on cross-linkable hole transporting layers. It is found that CdSe/CdS QD-LEDs exhibit a faster decrease in EL quantum efficiency (∼2% at a brightness of 100 cd/m2) with increasing current density and lower maximum brightness than those of CdSe/ZnS QD-LEDs. A more significant redshift and spectral broadening of the EL observed in CdSe core/shell QDs with a CdS or CdS/CdZnS/ZnS shell than with a ZnS shell indicate that the electron wave function can penetrate into the shell under electric field. The difference in device performance and EL spectra results from conduction band offsets between the CdSe cores and CdS or ZnS shells, suggesting the existence of the exciton ionization in the QD-LEDs.

Highly efficient and well-resolved Mn2+ ion emission in MnS/ZnS/CdS quantum dots

We demonstrate a strategy for the growth of Mn2+ ion doped cadmium based II–VI semiconductor quantum dots (QDs) with a designed buffer layer of ZnS (MnS/ZnS/CdS or Mn:CdS QDs), which aims to meet the challenge of obtaining highly efficient and well-resolved Mn2+ ion emission. First, small, high quality MnS cores are obtained by using thiols to replace conventional alkyl amines as capping ligands. Then a buffer layer of ZnS with a tailored thickness is introduced to the QDs before the growth of CdS shells to reduce the size mismatch between the Mn2+ (dopant) and Cd2+ (host) ions. The fabricated MnS/ZnS/CdS core/shell QDs exhibit a high PL QY of up to 68%, which is the highest ever reported for any type of Mn2+ ion doped cadmium based II–VI semiconductor QD. The photoluminescence (PL) of the QDs consists of well-resolved Mn2+ ion emission without any detectable emission from the CdS band edge or surface defects. In addition, our MnS/ZnS/CdS QDs cannot only be made water-soluble, but can also be coated by ligands with short carbon chain lengths, nearly without cost to the PL QY, which could make them strong candidates for practical applications in biology/biomedicine and opto/electronic devices.

A facile and general approach to polynary semiconductor nanocrystals via a modified two-phase method

Cu(2)ZnSnS(4) nanocrystals were synthesized through a modified two-phase method and characterized with transmission electron microscopy (TEM), powder x-ray diffraction (XRD) and UV-vis spectroscopy. Inorganic metal salts were dissolved in the polar solvent triethylene glycol (TEG) and then transferred into the non-polar solvent 1-octadecene (ODE) by forming metal complexes between metal ions and octadecylamine (ODA). Since nucleation and growth occur in the single phase of the ODE solution, nanocrystals could be produced with qualities similar to those obtained through the hot-injection route. Balancing the reactivity of the metal precursors is a key factor in producing nanocrystals of a single crystalline phase. We found that increasing the reaction temperature increases the reactivity of each of the metal precursors by differing amounts, thus providing the necessary flexibility for obtaining a balanced reactivity that produces the desired product. The versatility of this synthesis strategy was demonstrated by extending it to the production of other polynary nanocrystals such as binary (CuS), ternary (CuInS(2)) and pentanary (Cu(2 - x)Ag(x)ZnSnS(4)) nanocrystals. This method is considered as a green synthesis route due to the use of inorganic metal salts as precursors, smaller amounts of coordinating solvent, shorter reaction time and simpler post-reaction treatment.