Differential capacitance of hybrid organic/inorganic CdSe/ZnS quantum dots light-emitting device

We have studied the differential capacitance of self-assembly hybrid organic/inorganic quantum dot (QD) light-emitting devices (QD-OLEDs) at room temperature in the frequency range of 1×10³ to 1×10⁶ Hz. Self-assembly CdSe/ZnS QDs monolayer was used to fabricate trilayer hybrid QD-OLEDs on indium tin oxide coated glass substrates. We observed negative differential capacitance (NDC) in both OLED and QD-OLEDs for the first time in the intermediate frequency range of 5×10³ to 5×10⁵ Hz. The onset and frequency range of NDC are strongly depended on the applied bias voltage. This behavior is described by a mathematical model. The simulated results showed that the NDC was due to the time-dependent transient current from the QDs and organic/metal interfacial states in the device. The probable mechanisms of NDC in organic devices are discussed.     

混合量子点器件电致发光的能量转移研究

在量子点的研究中,对于量子点光致发光研究报道较多,而量子点电致发光研究报道较少,特别是对于混合量子点电致发光器件中能量转移机理的研究未见报道,由于不同量子点之间的能量转移机理决定着器件的性能,为此本论文对该方面进行了研究.分别制备了单种量子点器件和混合器件,混合器件是利用红、绿、蓝三种量子点按照1:1的比例两两混合,做成结构为ITO/PEDOT:PSS/QDs/Al的器件.研究发现在一定电压范围内,单种量子点器件的发光强度随着电压增加持续上升,而混合量子点器件的发光出现了短波长下降,长波长上升的现象,表明当有外加电场时不同尺寸的量子点间产生了较高效率的能量转移.同时首次对混合量子点电致发光器件能量转移的各项参数进行了计算,得到了能量转移效率E、临界能量转移距离R0与外加电场的关系,对制备混合量子点电致发光器件具有指导意义.     

Temperature-dependent photovoltaic behavior of CdSe quantum dots/P3HT hybrid thinfilm

An organic–inorganic hybrid solar cell based on CdSe quantum dots (QDs) and poly(3-hexylthiophene) (P3HT) was fabricated. Its temperature-dependent photovoltaic behaviors, such as I–V characteristic curves and open circuit voltage (V_oc) transient response, were measured. The photovoltaic behavior of this hybrid thin film device was similar with that of organic thin film solar cells, according to analysis results based on the equivalent circuit method. The exact carrier lifetime was remarkably different between under low-temperature region and under temperature above 197 K.     

Surface plasmon-waveguide hybrid polymer light-emitting devices using hexagonal Ag dots

We investigated surface plasmon-waveguide hybrid resonances for enhancement of light emission in polymer light-emitting diodes (PLEDs). Hybrid waveguide-plasmon resonances in the visible range for waveguide mode and near IR range for surface plasmons were observed by incorporation of hexagonal Ag dot arrays. Considerable overlap between the emission wavelength of the PLEDs and the waveguide mode by an Ag dot array with a lattice constant of 500 nm was observed. Because of enhanced light extraction by Bragg scattering of waveguide modes, photoluminescence (PL) and electroluminescence (EL) were increased by 70% and 50%, respectively.     

Anisotropic emission of interface fluctuation quantum dots

We calculate energy levels, dipole moments and radiative broadening of interface fluctuation quantum dots. For optically allowed states, the dipole moment grows proportionally to the lateral quantum dot radius while the radiative broadening saturates towards the quantum well radiative broadening for large lateral quantum dot radii. This is accompanied by a change in the angular emission pattern, concentrating emission in forward and backward direction. Optically forbidden states do not couple to light propagating in the growth direction yet they may have a considerable radiative broadening due to spontaneous emission in other directions. Received 20 March 2002 Published online 25 June 2002     

Spontaneous patterning of quantum dots at the air-water interface

Nanoparticles deposited at the air-water interface are observed to form circular domains at low density and stripes at higher density. We interpret these patterns as equilibrium phenomena produced by a competition between an attraction and a longer-ranged repulsion. Computer simulations of a generic pair potential with attractive and repulsive parts of this kind, reproduce both the circular and stripe patterns. Such patterns have a potential use in nanoelectronic applications.     

Optically detected resonance spectroscopy of interface fluctuation quantum dots

We have performed optically detected resonance (ODR) spectroscopy on modulation-doped GaAs/AlGaAs quantum wells of different widths in which lateral fluctuations of the well width were purposely introduced by growth interruption at the interfaces. These monolayer fluctuations form quantum dots for which confinement and Coulomb correlation energies are comparable. By monitoring resonant changes of the dot ensemble photoluminescence induced by far-infrared (FIR) radiation in a magnetic field, we have observed cyclotron resonance (CR) of free electrons in the widest wells, as well as internal transitions of mobile and localized charged excitons. The latter, which are forbidden by magnetic translational invariance, have previously not been observed. For the narrower wells the effects of non-parabolicity and carrier localization on the CR and CR-like transitions have to be included for a proper interpretation of the measurements.     

A spin switch device based on a triple quantum dots superlattice using the Dicke effect

Spin-dependent transport in a triple quantum dots superlattice system with a bridge coupling to two leads is studied. There exists an odd-even parity oscillation of spin polarization at the central dot level εc = 0 due to the spin-dependent Fano and Dicke effects induced by the quantum interference and the Rashba spin-orbit interaction. In the case of even numbers of triple quantum dots, the device can be used as a spin switch by tuning the energy difference h between the energies of the central and the lateral dots. These results may be helpful to design and fabricate practical spintronic devices.     

Enhanced lateral photovoltaic effect observed in CdSe quantum dots embedded structure of Zn/CdSe/Si

The quantum dots (QDs) system has been intensively studied for decades owing to its huge potential for applications. In this Letter, we report a lateral photovoltaic effect (LPE) with a large sensitivity observed in CdSe QDs embedded structure of Zn/CdSe/Si. This result not only enriches applications of the QDs system but also opens a new window to study the carrier dynamics of the QDs system.     

The improvement of bulk-heterojunction order in polymer photovoltaic device

The blend morphology and vertical arrangement are critical to the performance of organic bulk-heterojunction photovoltaic devices.In the present paper,the authors proposed a new annealing method that controls the blend morphology and vertical arrangement of two materials by means of simultaneously applying external electrical field and violet irradiation on the active layer of poly(3-hexylthiophene) (P3HT) and -phenyl C61-butyric acid methyl ester(PCBM) during annealing process.By using this annealing metho...     

Investigation of populated InAs/GaAs quantum dots by photoluminescence and photoreflectance

We studied self-assembled InAs/GaAs quantum dots by contrasting photoluminescence and photoreflectance spectra from 10 K to room temperature. The photoluminescence spectral profiles comprise contributions from four equally separated energy levels of InAs quantum dots. The emission profiles involving ground state and excited states have different temperature evolution. Abnormal spectral narrowing occurred above 200 K. In the photoreflectance spectra, major features corresponding to the InAs wetting layer and GaAs layers were observed. Temperature dependences of spectral intensities of these spectral features indicate that they originate from different photon-induced modulation mechanisms. Considering interband transitions of quantum dots were observed in photoluminescence spectra and those of wetting layer were observed in photoreflectance profiles, we propose that quantum dot states of the system are occupied up to the fourth energy level which is below the wetting layer quantum state.     

生物大分子杂化处理的CdSe/ZnS核/壳量子点多层膜的超快三阶非线性研究

利用静电自组装技术，以生物大分子材料壳聚糖杂化处理具有稳定结构的CdSe/ZnS核/壳量子点，形成复合多层薄膜. 与薄膜的吸收谱线比较，在375nm飞秒激光激发下测量的量子点的光致发光谱存在Stokes位移. 采用Z扫描技术，利用790nm飞秒激光研究了其三阶非线性吸收和折射特性，发现饱和吸收信号来自CdSe/ZnS量子点，而自聚焦的折射信号则部分来自壳聚糖. 测出多层膜的三阶非线性系数分别是β＝6.5×10^-6cm/W，n₂＝1.5×10^-10cm²/W. 关键词： CdSe/ZnS量子点 非线性性能 光致发光谱     

Effects of rubrene mixing on the electronic structures of donor/acceptor interface in organic photovoltaic device

Rubrene mixing has been shown to be an effect mean for enhancing both the open circuit voltage (Voc) and the short-circuit current (Jsc) of copper-phthalocyanine (CuPc)/fullerene (C₆₀) based solar cell. While the increase in Jsc can be readily explained by the additional rubrene absorption and the introduction of a bulk heterojunction; causes for Voc increase are still not clear. The energy offset between the highest occupied molecular orbital (HOMO) level of donor and the lowest unoccupied molecular orbital (LUMO) level of acceptor (HOMO_D-LUMO_A) at the CuPc/C₆₀ interface was found to increase substantially upon rubrene mixing in either side of the interface. As the HOMO_D-LUMO_A is generally considered to limit the Voc, its increase agrees well with the device results. Energy level bending and associated built-in electric fields were also observed and their possible implications to device performance are discussed.     

Passively Q-switched green laser operation using CdTe/CdS quantum dots

The direct generation of passively Q-switched lasers at a green wavelength has rarely been investigated in the past. In this Letter, we demonstrate a passively Q-switched praseodymium-doped yttrium lithium fluoride green laser at 522 nm using CdTe/CdS quantum dots as a saturable absorber. A maximum average output power of 33.6 m W is achieved with the shortest pulse width of 840 ns. The corresponding pulse energy and peak power reached 0.18 μJ and 0.21 W, respectively. To the best of our knowledge, this is the first demonstration in regard to a quantum dots saturable absorber operating in the green spectral region.     

Modification of spontaneous emission from CdSe/CdS quantum dots in the presence of a semiconductor interface

The spontaneous-emission lifetime of CdSe/CdS core-shell quantum dots was studied as a function of the distance between the dots and a polished Si surface. The experimental results reveal a significant modification of the spontaneous-emission rate of the quantum dots by the Si surface.     

Photosensitization of multiwalled carbon nanotube scaffolds with ZnO quantum dots for photovoltaic applications

In this article, we have decorated multiwalled carbon nanotubes (MWCNTs) scaffold with ZnO quantum dots (QDs, size in the range of 2.9–4.5 nm) and investigated their prospects for photovoltaic applications. ZnO QDs, in the present study, work as photosensitizer instead of electron transporting media as used in recent conventional strategic solar cells. ZnO QDs/MWCNTs composite shows an increased visible absorbance and quenching of the broad visible emission at around ~560 nm, while only ZnO QDs exhibit a strong visible emission. An efficient electron–hole separation facilitates an increase in the short-circuit current. These results show a possibility of developing a nontoxic, ZnO QDs sensitized MWCNTs composite-based photovoltaic solar cell.     

Investigation of Fe/Al interface as a function of annealing temperature using XPS

This article describes the systemic investigation of the interface chemical and electronic properties of ultrathin Fe/Al multilayer structure (MLS) as a function of annealing temperature. For this purpose electron beam evaporated [Fe/Al]_×15 ML samples have been prepared under ultrahigh vacuum conditions. The chemical and electronic information of the interfaces at different depth has been obtained from XPS technique.The core level study show a gradual change in the nature of the electronic bonding at the interface as a result of annealing. In particular, the MLS annealed at 200 °C and 400 °C clearly show shifts in the binding energy position of Fe-2p_3/2 core line towards higher energy and Al-2p_3/2 core line towards lower energy side as compared to as-deposited sample, suggesting the formation of FeAl alloy phase at the interface. Another important finding with annealing is that the intensity of peak corresponding to pure Al-2p increases and that of Fe-2p decreases as compared to as-deposited case. The increase in intensity of Al-2p core line suggests the migration of Al atoms towards the surface owing to annealing induced inter-diffusion. The corresponding valence band spectra show appreciable changes in the Fe-3d as well as Al-3s density of states due to strong hybridization of sp-d states at the Fermi level as a result of charge transfer and also provide strong evidence for FeAl alloy formation.