Luminescence of CdSe/ZnS quantum dots infiltrated into an opal matrix

The effect of the photonic band gap in the photonic crystal, the synthesized SiO₂ opal with embedded CdSe/ZnS quantum dots, on its luminescence in the visible spectral region is studied. It is shown that the position of the photonic band gap in the luminescence and reflectance spectra for the infiltrated opal depends on the diameter of the constituent nanospheres and on the angle of recording the signal. The optimal conditions for embedding the CdSe/ZnS quantum dots from the solution into the opal matrix are determined. It is found that, for the opal-CdSe/ZnS nanocomposites, the emission intensity decreases and the luminescence decay time increases in the spatial directions, in which the spectral positions of the photonic band gap and the luminescence peak of the quantum dots coincide.     

Soft‐Lithographed Up‐Converted Distributed Feedback Visible Lasers Based on CdSe–CdZnS–ZnS Quantum Dots

The development of a solution‐deposited up‐converted distributed feedback laser prototype is presented. It employs a sol–gel silica/germania soft‐lithographed microcavity and CdSe–CdZnS–ZnS quantum dot/sol–gel zirconia composites as optical gain material. Characterization of the linear and nonlinear optical properties of quantum dots establishes their high absorption cross‐sections in the one‐ and two‐ photon absorption regimes to be 1 × 10^?14 cm² and 5 × 10⁴ GM, respectively. In addition, ultrafast transient absorption dynamics measurements of the graded seal quantum dots reveal that the Auger recombination lifetime is 220 ps, a value two times higher than that of the corresponding CdSe core. These factors enable the use of such quantum dots as optically pumped gain media, operating in the one‐ and two‐photon absorption regime. The incorporation of CdSe–CdZnS–ZnS quantum dots within a zirconia host matrix affords a quantum‐dot ink that can be directly deposited on our soft‐lithographed distributed feedback grating to form an all‐solution‐processed microcavity laser.     

Enhance the performance of quantum dot-sensitized solar cell by manganese-doped ZnS films as a passivation layer

To make quantum dot-sensitized solar cells (QDSSCs) more attractive, it is necessary to achieve higher power conversion efficiency. A novel Mn-doped ZnS has been successfully fabricated on CdS/CdSe quantum dots (QDs) by simple successive ion layer adsorption and reaction (SILAR) technique. The Mn-doped ZnS is used as a passivation layer in the QDSSCs. The performance of the QDSSCs was examined in detail using sulfide/polysulfide electrolyte with a Pt or copper sulfide (CuS) counter electrode. Here we demonstrated, the fabricated Mn-doped ZnS QDs shows an improved V_oc (0.65 V) compared to bare ZnS QDs (0.60 V). The QDSSC based on a photoanode with Mn-doped ZnS (10 wt% of Zn) shows higher J_sc (15.32 mA cm⁻²) and power conversion efficiency (4.18%) compared to the bare ZnS photoanode (2.90%) under AM 1.5 G one sun illumination. We explore the reasons for this enhancement and demonstrated that it is caused by improved passivation of the ZnS surface by Mn ions, leading to a lower recombination of photo-injected electrons with the electrolyte. The effect of Cu ions in ZnS has been investigated by UV–Vis spectra and current density–voltage analysis.     

Nonvolatile Memories Using Quantum Dot (QD) Floating Gates Assembled on II–VI Tunnel Insulators

This paper presents preliminary data on quantum dot gate nonvolatile memories using nearly lattice-matched ZnS/Zn_0.95Mg_0.05S/ZnS tunnel insulators. The GeO _x -cladded Ge and SiO _x -cladded Si quantum dots (QDs) are self-assembled site-specifically on the II–VI insulator grown epitaxially over the Si channel (formed between the source and drain region). The pseudomorphic II–VI stack serves both as a tunnel insulator and a high-κ dielectric. The effect of Mg incorporation in ZnMgS is also investigated. For the control gate insulator, we have used Si₃N₄ and SiO₂ layers grown by plasma- enhanced chemical vapor deposition.     

Improved performance of CdSe/ZnS quantum dot light-emitting diodes through doping with small molecule CBP

The poor film formation of CdSe/ZnS quantum dots (QDs) during spin-coating makes a substantial impact on the device performance of quantum dot light-emitting diodes (QLEDs). This work proposes a method to improve the morphology of the quantum dot light-emitting layer (EML) by adding small organic molecular 4,4''-Bis(9H-carbazol-9-yl) biphenyl (CBP) into the layer. Its surface roughness reduces from 6.21 nm to 2.71 nm, which guarantees a good contact between hole transport layer (HTL) and EML. Consequently, the CdSe/ZnS QDs:CBP based QLED achieves maximum external quantum efficiency (EQE) of 5.86%, and maximum brightness of 10 363 cd/m2. It is demonstrated that the additive of small organic molecules could be an effective way to improve the brightness and the efficiency of QLEDs.     

Nonvolatile Silicon Memory Using GeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>-Cladded Ge Quantum Dots Self-Assembled on SiO<Subscript>2</Subscript> and Lattice-Matched II–VI Tunnel Insulator

This paper presents fabrication and characterization of a quantum dot-based floating gate nonvolatile memory device with site-specific self-assembly of germanium oxide-cladded germanium (GeO _x -Ge) quantum dots on SiO₂ and ZnS/ZnMgS/ZnS (II–VI lattice-matched high-κ dielectric) tunnel insulator material. These monodispersed and individually cladded quantum dots have the potential to store charge uniformly in the floating gate and are well suited for nonvolatile memory applications.     

Polaron effect-dependent third-order optical susceptibility in a ZnS/CdSe quantum dot quantum well

The effect of the electron-phonon interaction on the third-harmonic is investigated theoretically for electrons confined in a core-shell quantum dot. The interactions of electrons with different phonon modes in the core-shell system, including the confined longitudinal optical (LO) and the interface optical (IO) phonon modes, are investigated. We carried a detailed calculation of third-harmonic generation (THG) process on a ZnS/CdSe core-shell quantum dot as a function of pump photon energy with different incident photon energy and under different sizes. The results reveal that the polaron effects are quite important especially around the peak value of the third-order susceptibility. By increasing the size of the quantum dots, the peaks of χ_THG⁽³⁾ will shift to lower energy, and the intensities of the peaks will increase.     

Improving the Power Conversion Efficiency of Organic Solar Cell by Blending with CdSe/ZnS Core–Shell Quantum Dots

We have blended poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) with CdSe/ZnS core–shell quantum dots (QDs) as the active layer to produce organic solar cells (OSC). The size of the CdSe/ZnS core–shell QDs was determined to be about 4 nm using transmission electron microscopy. The OSC were characterized by measuring the absorption spectra, current–voltage characteristics, and external quantum efficiency (EQE) spectra. The samples doped with 0.5 wt.% CdSe/ZnS core–shell QDs exhibited higher power conversion efficiency (PCE) than samples doped with other concentrations of QDs. The PCE of the OSC increases from 2.10% to 2.38% due to an increase of the short circuit current density (J _sc) from 6.00 mA/cm² to 6.25 mA/cm². The open circuit voltage (V _oc) was kept constant when comparing OSC that were undoped and doped with 0.5 wt.% CdSe/ZnS core–shell QDs. These CdSe/ZnS core–shell QDs can increase optical absorption as well as provide extra exciton dissociation and additional electric pathways in hybrid OSC.     

CdSe/ZnS和PbSe量子点光纤及光纤放大器研究进展

近年来,纳米晶体(量子点)以及量子点光纤、量子点光纤放大器成为一个研究热点。介绍了CdSe/ZnS和PbSe量子点的光谱特性以及量子点的吸收-辐射截面,表明量子点具有强的吸收和发射。总结了低浓度和较高浓度CdSe/ZnS量子点掺杂光纤、熔融法及溶胶凝胶法制备PbSe量子点光纤材料的最新研究进展,分析了两种方法制备量子点光纤材料的优缺点,概述了PbSe量子点光纤放大器的研究近况,展望了量子点光纤的应用前景。     

Tuning the spectrometric properties of white light by surface plasmon effect using Ag nanoparticles in a colour converting light-emitting diode

We report on the spectral tunability of white light by localized surface plasmon (LSP) effect in a colour converting hybrid device made of CdSe/ZnS quantum dots (QDs) integrated on InGaN/GaN blue light-emitting diodes (LEDs). Silver (Ag) nanoparticles (NPs) are mixed with QDs for generating LSP effect. When the plasmon absorption of Ag NPs is synchronized to the QW emission at 448 nm, the NPs selectively absorb the blue light and subsequently enhance the QD emission. Using this energy transfer scheme, the (x, y) chromaticity coordinates of the hybrid white LED was tuned from (0.32, 0.17) to (0.43, 0.26), and thereby generated warm white light emission with correlated colour temperature (CCT) around 1800 K. Moreover, a 47% enhancement in the external quantum efficiency (EQE) was realized.     

Formation and characterization of self-organized CdSe quantum dots

We have investigated the formation and characteristic of self-organized CdSe quantum dots (QDs) on ZnSe(001) surfaces with the use of photoluminescence (PL) and transmission electron microscopy (TEM). Coherent CdSe QDs are naturally formed on ZnSe surfaces, when the thickness of CdSe layers is around 2 ML. The plan-view TEM images exhibit that CdSe QDs have a relatively narrow distribution of QD size, and that the density of CdSe QDs is about 10¹⁰ cm⁻². The base structure of the CdSe dot is rhombic, which has the long axis of about 20 nm in length along direction. The temperature dependence of macro-PL spectra reveals that the behavior of self-organized CdSe QDs is quite different from that of ZnCdSe quantum well (QW), resulting from characteristic features of zero-dimensional structures of QDs. Moreover, the macro-PL results suggest the existence of QW-like continuous state lying over QD states. Micro-PL measurements show several numbers of high-resolved sharp lines from individual CdSe QDs. The linewidth broadening with temperature depends on peak energy position of the QDs. The linewidths of lower energy lines, corresponding to larger size QDs, are more temperature dependent.     

Luminescent high temperature sensor based on the CdSe/ZnS quantum dot thin film

A high temperature sensor based on the multi-parameter temperature dependent characteristic of photoluminescence （PL） of quantum dot （QD） thin film is demonstrated by depositing the CdSe/ZnS core/shell QDs on the SiO2 glass substrates. The variations of the intensity, the peak wavelength and the full width at half maximum （FWHM） of PL spectra with temperature are studied experimentally and theoretically. The results indicate that the peak wavelength of the PL spectra changes linearly with temperature, while the PL intensity and FWHM vary exponentially for the tem- perature range from 30 ℃ to 180 ℃. Using the obtained temperature dependent optical parameters, the resolution of the designed sensor can reach 0.1 nm/℃.     

The effects of quantum dot coverage in InAs/(In)GaAs nanostructures for long wavelength emission

We present a study on the effects of quantum dot coverage on the properties of InAs dots embedded in GaAs and in metamorphic In_0.15Ga_0.85As confining layers grown by molecular beam epitaxy on GaAs substrates. We show that redshifted emission wavelengths exceeding 1.3 μm at room temperature were obtained by the combined use of InGaAs confining layers and high quantum dot coverage. The use of high InAs coverage, however, leads to detrimental effects on the optical and electrical properties of the structures. We relate such behaviour to the formation of extended structural defects originating from relaxed large-sized quantum dots that nucleate in accordance to thermodynamic equilibrium theories predicting the quantum dot ripening. The effect of the reduced lattice-mismatch of InGaAs metamorphic layers on quantum dot ripening is discussed in comparison with the InAs/GaAs system.     

Exciton binding energy in semiconductor quantum dots

In the adiabatic approximation in the context of the modified effective mass approach, in which the reduced exciton effective mass μ = μ(a) is a function of the radius a of the semiconductor quantum dot, an expression for the exciton binding energy E _ex(a) in the quantum dot is derived. It is found that, in the CdSe and CdS quantum dots with the radii a comparable to the Bohr exciton radii a _ex, the exciton binding energy E _ex(a) is substantially (respectively, 7.4 and 4.5 times) higher than the exciton binding energy in the CdSe and CdS single crystals.     

Exceptional Catalytic Nature of Quantum Dots for Photocatalytic Hydrogen Evolution without External Cocatalysts

The catalytic nature of semiconducting quantum dots (QDs) for photocatalytic hydrogen (H₂) evolution can be thoroughly aroused, not because of coupling with external cocatalysts, but through partially covering controlled amount of ZnS shell on the surface. Specifically, CdSe QDs, with an optimal coverage of ZnS (≈46%), can produce H₂ gas with a constant rate of ≈306.3 ± 21.1 µmol mg^?1 h^?1 during 40 h, thereby giving a turnover number of ≈(4.4 ± 0.3) × 10⁵, which is ≈110‐fold to that of unmodified CdSe QDs under identical conditions. The performance of H₂ evolution is comparable to or even better than the commonly used external cocatalysts, e.g., metal complexes, noble metals assisted photosystems. Mechanistic insights indicate that the dramatically enhanced activity and stability of bare QDs for photocatalytic H₂ production are derived from (i) inhibiting exciton annihilation at trap states, (ii) preventing the photo‐oxidation of core frameworks, and (iii) retaining tunneling efficiencies of photogenerated electrons and holes to reactive sites with partial ZnS coverage.     

Highly Efficient Quantum‐Dot‐Sensitized Solar Cell Based on Co‐Sensitization of CdS/CdSe

Cadmium sulfide (CdS) and cadmium selenide (CdSe) quantum dots (QDs) are sequentially assembled onto a nanocrystalline TiO₂ film to prepare a CdS/CdSe co‐sensitized photoelectrode for QD‐sensitized solar cell application. The results show that CdS and CdSe QDs have a complementary effect in the light harvest and the performance of a QDs co‐sensitized solar cell is strongly dependent on the order of CdS and CdSe respected to the TiO₂. In the cascade structure of TiO₂/CdS/CdSe electrode, the re‐organization of energy levels between CdS and CdSe forms a stepwise structure of band‐edge levels which is advantageous to the electron injection and hole‐recovery of CdS and CdSe QDs. An energy conversion efficiency of 4.22% is achieved using a TiO₂/CdS/CdSe/ZnS electrode, under the illumination of one sun (AM1.5,100 mW cm^?2). This efficiency is relatively higher than other QD‐sensitized solar cells previously reported in the literature.     

Glucose biosensor of ruthenium-doped TiO2 sensing electrode by co-sputtering system

In this study, the ruthenium-doped titanium dioxide (TiO₂:Ru) sensing film is fabricated by co-sputtering system. The TiO₂:Ru sensing film is treated by thermal annealing process at 600 °C for 1 h in atmosphere. We present a TiO₂:Ru-based pH sensor which can obtain the higher sensing value at 64.58 mV/pH for the super-Nernstian response. The enzyme composite solution is dropped on the TiO₂:Ru sensing film as a glucose biosensor. In the concentration ranging from 100 to 500 (mg/dL) of glucose solution, the sensitivity and linearity of TiO₂:Ru sensing film after thermal annealing are 0.320 mV(mg/dL)⁻¹ and 0.995, respectively, which is better than non-annealing TiO₂:Ru sensing film. In this study, we can describe a novel doping method to prepare the TiO₂:Ru sensing film and the glucose biosensors.     

Nonvolatile Memory Effect in Indium Gallium Arsenide-Based Metal–Oxide–Semiconductor Devices Using II–VI Tunnel Insulators

This paper reports the successful use of ZnSe/ZnS/ZnMgS/ZnS/ZnSe as a gate insulator stack for an InGaAs-based metal–oxide–semiconductor (MOS) device, and demonstrates the threshold voltage shift required in nonvolatile memory devices using a floating gate quantum dot layer. An InGaAs-based nonvolatile memory MOS device was fabricated using a high-κ II–VI tunnel insulator stack and self-assembled GeO _x -cladded Ge quantum dots as the charge storage units. A Si₃N₄ layer was used as the control gate insulator. Capacitance–voltage data showed that, after applying a positive voltage to the gate of a MOS device, charges were being stored in the quantum dots. This was shown by the shift in the flat-band/threshold voltage, simulating the write process of a nonvolatile memory device.     

Optical characteristics of Si/SiO2 multilayers prepared by magnetron sputtering

Si/SiO₂ multilayers have been successfully prepared by magnetron sputtering and subsequently thermal annealed in an Ar atmosphere at a temperature of more than 500 °C. The surface of the as-deposited films is compact and smooth, and the distribution of grain size estimated to be 20 nm is uniform. For Si/SiO₂ multilayers annealed at 1100 °C, the Si sublayer sandwiched between potential barrier SiO₂ is crystalline structure by means of the analysis of Raman spectra and XRD data. The visible PL peak accompanying to a blue-shift with the decrease of Si sublayer thickness has been observed, and the intensity of this peak enhances with the increase of annealing temperature. The visible luminescence properties of Si/SiO₂ multilayers can be ascribed to quantum confinement of electron-hole pairs in quantum wells with grain size lower than 4.5 nm. In Si/SiO₂ multilayers, not only quantum confinement but also Si-SiO₂ interface states play an important role in the optical transition. The PL peak located at 779 nm is independent of the thickness of Si sublayer, so it may be ascribed to interface mediated transition. Typical Si dangling bonds defect could be a dominating obstacle to high luminescence efficiencies.     

Charging effects in CdSe nanocrystals embedded in SiO2 matrix produced by rf magnetron sputtering

Charging effects in CdSe nanocrystals embedded in SiO₂ matrix fabricated by rf magnetron co-sputtering technique were electrically characterized by means of capacitance-voltage (C-V) combined with current-voltage (I-V). The presence of CdSe nanocrystals was demonstrated by X-ray diffraction technique. The average size of nanocrystals was found to be approximately 3 nm. The carriers transport in the CdSe/SiO₂ structure was shown to be a combination of Fowler-Nordheim tunnelling and Poole-Frenkel mechanisms. A memory effect was demonstrated and a retention time was measured.