Chemical mapping of individual semiconductor nanostructures

We demonstrate experimentally the power of a novel analytical tool for X-ray spectromicroscopy. This provides a minimally intrusive elemental mapping of surfaces at the nanoscale and holds the promise of remarkable versatility. We have applied our procedure to the characterization of Ge(Si) islands on Si(111) substrates, with the aim of investigating the surface stoichiometry gradients and gaining insight into the intermixing dynamics. By identifying Si-richer edges with respect to the centers, we are able to associate alloying in these islands to surface transport processes.     

Thermal properties of semiconductor zinc oxide nanostructures

A combination of two methods — laser modulation and 3ω — has been used to determine the heat capacity, heat conductivity, and heat diffusivity of zinc oxide nanostructures. A significant difference between the thermal parameters of zinc oxide nanostructures grown by different technological methods has been revealed. It has been shown that the relatively low heat conductivity and heat diffusivity values of oxide zinc nanostructures are due to both the internal defects and the contact resistance between the film and its base — the substrate.     

Hybrid nanostructures of titanium-decorated ZnO nanowires

Hybrid nanostructures of titanium (Ti)-decorated zinc oxide (ZnO) nanowire were synthesized. Various thick Ti films (6 nm, 10 nm, and 20 nm) were coated to form a titanium oxide (TiO) coating layer around ZnO nanowires. Transmission electron microscope analysis was performed to verify the crystallinity and phases of the TiO layers according to the Ti-coating thickness. Under UV illumination, a bare ZnO nanowire showed a conventional n-type conducting performances. With a Ti coating on a ZnO nanowire, it was converted to a p-type conductor due to the existence of electron-captured oxygen molecules. It discusses the fabrication of Ti-decorated ZnO nanowires including the working mechanisms with respect to UV light.     

One-dimensional semiconductor nanostructures as absorber layers in solar cells

The one-dimensional (1-D) nanostructures of cadmium chalcogenides (Il-VI: CdSe, CdTe), InP and GaAs (III-V), and the ternary chalcopyrites CulnS2, CulnSe2, and CulnTe2 (I-III-VI2) are the candidate semiconductors of interest as absorber layers in solar cells. In the confinement regime (approximately 1-10 nm) of these 1-D nanostructures, the electronic energy levels are quantized so that the oscillator strength and the resultant absorption of solar energy are enhanced. Moreover, the discrete energy levels effectively separate the electrons and holes at the two electrodes or at the interfaces with a polymer in a hybrid structure, so that an oriented and 1-D nanostructured absorber layer is expected to improve the conversion efficiency of solar cells. The intrinsic anisotropy of Il-VI and l-lll-VI2 crystal lattices and the progress in various growth processes are assessed to derive suitable morphological features of these 1-D semiconductor nanostructures. The present status of research in nanorod-based solar cells is reviewed and possible routes are identified to improve the performance of nanorod-based solar cells. Finally, the characteristics of nanorod-based solar cells are compared with the dye-sensitized and organic solar cells.     

Solvothermal synthesis of copper sulfide semiconductor micro/nanostructures

Covellite copper sulfide (CuS) micro/nanometer crystals in the shape of hierarchical doughnut-shaped, superstructured spheric-shaped and flowerlike architectures congregated from those nanoplates with the thickness of 20-100 nm have been prepared by a solvothermal method. The as-obtained CuS products were characterized by means of scanning electron microscopy (SEM), X-ray diffractometry (XRD) and energy-dispersive X-ray spectroscopy (EDS). A systematic investigation has been carried out to understand the factors influencing the evolution of CuS particle morphology which found to be predominant by solvent, surfactant, sulfur resource and copper salt. The possible formation mechanism for the nanostructure formation was also discussed. These CuS products show potential applications in solar cell, photothermal conversion and chemical sensor.     

Hybrid organic on inorganic semiconductor heterojunction

Hybrid organic on inorganic semiconductor heterojunctions with a sandwich structure have been fabricated and studied using conjugated polymers. The inorganic semiconductor was n-type silicon substrate. The conjugated polymers used include poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) containing polyhedral oligomeric silsesquioxanes (MEH-PPV POSS), regioregular poly(3-hexylthiophene) (RR-P3HT) and poly(3,4-ethylenedioxythiophene) (PEDOT). Current density–voltage and capacitance–voltage measurements were performed. All of the devices displayed a rectifying characteristic. Among these devices, the first ever reported PEDOT doped with BF₃ on n-Si heterojunction devices showed the best performance with a rectification ratio around 5.7 × 10⁵ at ± 2 V and an ideality factor of 2.3. The results showed better device performance with decreased potential barrier height at the organic–inorganic interface. Results also suggested that smaller energy level offset between the HOMO of the conjugated polymer and the work function of anode metal will improve device performance.     

Terahertz optics of semiconductor nanostructures near and far from equilibrium

In semiconductor nanostructures a few tens of nanometers wide, the separation between quantized states, or subbands, can be a few terahertz. In the past year, a consistent experimental picture of intersubband relaxation in wide, doped quantum wells has emerged. Terahertz radiation has also been found to dramatically alter the emission and absorption of semiconductor nanostructures near their band-edges.     

有机半导体/金属杂化纳米结构中的强耦合相互作用及超快动力学特性

有源表面等离光子学(active plasmonics)是目前表面等离子体光子学研究的一个重要分支,其基本思想是利用“增益”物质和纳米金属结构相结合形成杂化金属纳米结构,从而克服表面等离子体激元(surface plasmon polariton,SPP)的耗散问题以及实现对SPP光子的外部操作和调制。本文主要针对有机半导体/金属杂化纳米结构,介绍其相关研究结果。结合色散补偿的光谱相干法和超快泵浦-探测瞬态光谱测量技术,分析了J-凝聚/光栅杂化金属纳米结构的静态和瞬态光学特性,揭示了X-SPP的强耦合过程中的相干和非相干相互作用通道,杂化能态的sub/super-radiance现象,以及有机半导体染料中的激子和SPP之间的瞬态相干能量交换过程：“拉比”振荡。实现了10 fs量级的SPP光学特性的外部相干调制。

     

Study of temperature dependence of electron-phonon relaxation and dephasing in semiconductor double-dot nanostructures

This paper examines mechanisms of relaxation and dephasing of electrons in double-dot nanostructures due to interaction with acoustic phonon modes. The effect of temperature of phonon bath on decay on electron quantum evolution is obtained. Our results set the temperature ranges outside of which the quantum dynamics of electrons will be significantly suppressed.     

Highly efficient resonant coupling of optical excitations in hybrid organic/inorganic semiconductor nanostructures

The integration of organic and inorganic semiconductors on the nanoscale offers the possibility of developing new photonic devices that combine the best features of these two distinct classes of material. Such devices could, for example, benefit from the large oscillator strengths found in organic materials and the nonlinear optical properties of inorganic species. Here we describe a novel hybrid organic/inorganic nanocomposite in which alternating monolayers of J-aggregates of cyanine dye and crystalline semiconductor quantum dots are grown by a layer-by-layer self-assembly technique. We demonstrate near-field photon-mediated coupling of vastly dissimilar optical excitations in the two materials that can reach efficiencies of up to 98% at room temperature. By varying the size of the quantum dots and thus tuning their optical resonance for absorption and emission, we also show how the ability of J-aggregates to harvest light can be harnessed to increase the effective absorption cross section of the quantum dots by up to a factor of ten. Combining organic and inorganic semiconductors in this way could lead to novel nanoscale designs for light-emitting, photovoltaic and sensor applications.     

Fabrication, structural characterization and photoluminescence of Q-1D semiconductor ZnS hierarchical nanostructures

Quasi-one-dimensional semiconductor ZnS hierarchical nanostructures have been fabricated by thermal evaporation of a mixture of ZnS nanopowders and Sn powders. Sn nanoparticles are located at or close to the tips of the nanowires (or nanoneedles) and served as the catalyst for quasi-one-dimensional ZnS nanostructure growth by a vapour-liquid-solid mechanism. The morphology and microstructure of the ZnS hierarchical nanostructures were measured by scanning electron microscopy and high-resolution transmission electron microscopy. The results show that a large number of ZnS nanoneedles were formed on the outer shells of a long and straight ZnS axial nanowire. The ZnS axial nanowires grow along the [001] direction, and ZnS nanoneedles are aligned over the surface of the ZnS nanowire in the radial direction. The room temperature photoluminescence spectrum exhibits a UV weak emission centred at 337?nm and one blue emission centred at 436?nm from the as-synthesized single-crystalline semiconductor ZnS hierarchical nanostructures.     

Hybrid wavelength-division and optical time-division multiplexed multiwavelength mode-locked semiconductor laser

A multiwavelength laser source composed of a single semiconductor optical amplifier and a commercially available off-the-shelf wavelength-division multiplexed (WDM) filter is constructed and tested under actively mode-locking operation. Five independent mode-locked wavelength channels are generated simultaneously, with a wavelength spacing of 1.6 nm established by the WDM filter. In addition, to demonstrate the potential of this mixed time-frequency, or hybrid WDM-optical time-division multiplexed, signal, we demonstrate a simple parallel-to-serial wavelength conversion to increase the pulse repetition rate of the mode-locked laser by a number of output wavelengths for applications in high-performance optical sampling applications.     

Hybrid organic–inorganic porous semiconductor transducer for multi-parameters sensing

Porous silicon (PSi) non-symmetric multi-layers are modified by organic molecular beam deposition of an organic semiconductor, namely the N,N′-1H,1H-perfluorobutyldicyanoperylene-carboxydi-imide (PDIF-CN₂). Joule evaporation of PDIF-CN₂ into the PSi sponge-like matrix not only improves but also adds transducing skills, making this solid-state device a dual signal sensor for chemical monitoring. PDIF-CN₂ modified PSi optical microcavities show an increase of about five orders of magnitude in electric current with respect to the same bare device. This feature can be used to sense volatile substances. PDIF-CN₂ also improves chemical resistance of PSi against alkaline and acid corrosion.     

Material sensitive scanning probe microscopy of subsurface semiconductor nanostructures via beam exit Ar ion polishing

Whereas scanning probe microscopy (SPM) is highly appreciated for its nanometre scale resolution and sensitivity to surface properties, it generally cannot image solid state nanostructures under the immediate sample surface. Existing methods of cross-sectioning (focused ion beam milling and mechanical and Ar ion polishing) are either prohibitively slow or cannot provide a required surface quality. In this paper we present a novel method of Ar ion beam cross-section polishing via a beam exiting the sample. In this approach, a sample is tilted at a small angle with respect to the polishing beam that enters from underneath the surface of interest and exits at a glancing angle. This creates an almost perfect nanometre scale flat cross-section with close to open angle prismatic shape of the polished and pristine sample surfaces ideal for SPM imaging. Using the new method and material sensitive ultrasonic force microscopy we mapped the internal structure of an InSb/InAs quantum dot superlattice of 18 nm layer periodicity with the depth resolution of the order of 5 nm. We also report using this method to reveal details of interfaces in VLSI (very large scale of integration) low k dielectric interconnects, as well as discussing the performance of the new approach for SPM as well as for scanning electron microscopy studies of nanostructured materials and devices.     

Surface exciton-plasmon polariton enhanced light emission via integration of single semiconductor nanowires with metal nanostructures

The light emission enhancement behavior from single ZnO nanowires integrated with metallic contacts is investigated by micro-photoluminescence measurements. Apart from surface plasmon polaritons at the air/metal interface, the emission of a single ZnO nanowire can be coupled into guided modes of surface excitonplasmon polaritons (SEPPs). The out-coupling avenues of SEPP guided modes are modeled in the presence of nanostructures, such as corrugation and gratings, on the metal surface. The guided modes of SEPPs in metalcontacted ZnO nanowires are calculated using the effective index method. The enhanced light emission from single semiconductor nanowires shows promise for use in highly efficient nano-emitters and nano-lasers, as well as macroscopic solid state light sources with very high efficiency. This article is published with open access at Springerlink.com     

Coupled effects of ion beam chemistry and morphology on directed self-assembly of epitaxial semiconductor nanostructures

We study the coupled effects of ion beam chemistry and morphology on the assembly of templated epitaxial nanostructures. Using a focused ion beam (FIB) system equipped with a mass-selecting filter, we pattern Si substrates with local ion doses of Si, Ge and Ga to control subsequent Ge(x)Si(1 - x) epitaxial nanostructure assembly. This capability to employ different templating species allows us to study how different incorporated ion species in the near surface region affect the ability to localize nucleation during subsequent epitaxial growth. Our results indicate that FIB-directed self-assembly is a complex process, dependent on dose-induced morphology in addition to ion-specific chemical effects.     

High-throughput optical processors based on semiconductor nanostructures for incoherent-light optical analog computers with parallel image processing

Fast-response optical recording media based on semiconductor nanostructures (CdTe, GaAs) have been developed for image recording and processing at a speed of up to 10⁶ cps, which is 2–3 orders of magnitude higher than the speed of well-known media based on liquid crystals (MIS-LC). The new media are characterized by a photosensitivity of 10^?2 W/cm² and a spatial resolution of 5–10 lines/mm. Methods for the readout of images recorded in the nanostructures are developed and high-speed incoherent-light optical processors based on these structures are created. The possibility of using these processors for building optical analog computers and image correlators is demonstrated.     

Hybrid solar cells with prescribed nanoscale morphologies based on hyperbranched semiconductor nanocrystals

In recent years, the search to develop large-area solar cells at low cost has led to research on photovoltaic (PV) systems based on nanocomposites containing conjugated polymers. These composite films can be synthesized and processed at lower costs and with greater versatility than the solid state inorganic semiconductors that comprise today's solar cells. However, the best nanocomposite solar cells are based on a complex architecture, consisting of a fine blend of interpenetrating and percolating donor and acceptor materials. Cell performance is strongly dependent on blend morphology, and solution-based fabrication techniques often result in uncontrolled and irreproducible blends, whose composite morphologies are difficult to characterize accurately. Here we incorporate three-dimensional hyperbranched colloidal semiconductor nanocrystals in solution-processed hybrid organic-inorganic solar cells, yielding reproducible and controlled nanoscale morphology.