Local absorption spectra of single and coupled semiconductor quantum dots

We study theoretically the local absorption spectra of single and double semiconductor quantum dots (QDs), in the linear regime. The three-dimensional confinement leads to an enhancement of the Coulomb correlations, while the spectra depend crucially on the size of the ‘local’ probe. We show that because of such Coulomb correlations the intensity of certain optical peaks as a function of the resolution can exhibit an unexpected non-monotonic behavior for spatial resolutions comparable with the excitonic Bohr radius. We finally discuss the optical near-field properties of coupled QDs for different coupling strengths.     

Inorganic nanocomposites for the next generation photovoltaics

Photovoltaic is an attractive alternative of conventional energy source, but for the limitations of present materials and technology, we need to find out cost effective and environmentally stable new materials. We have synthesized a novel nanocomposite, named titania–germanium (TiO₂–Ge). TiO₂–Ge is a thermodynamically stable material. Ge nanodots are dispersed in the TiO₂ matrix of the nanocomposites. Bohr radius of Ge is relatively large, 24.3 nm, therefore, it is easy to vary the size of Ge nanodots, and consequently the properties (structural, optical and electrical) of TiO₂–Ge can be tailored in a wide range just by varying the size and density of Ge nanodots. TiO₂–Ge with size gradient of Ge nanodots is a promising active layer of the next generation solar cells.     

Mineralization of semiconductor nanocrystals on peptide-coated bionanotubes and their pH-dependent morphology changes

While various mineralizing peptides have been applied to grow metal nanoparticles on bionanotube templates, the semiconductor nanoparticle growth on nanotubes has not extensively been explored yet. In this paper, various semiconductor nanocrystals were grown on the bionanotubes surfaces with controlled sizes. When three synthetic peptides, which recognize and selectively bind Ge, Ti, and Cu ions, respectively, were incorporated on template bionanotube surfaces, highly crystalline and monodisperse Ge, TiO2, and Cu2S nanocrystals were grown on the tube surfaces. The sizes of these nanocrystals could be tuned as a function of pH, and larger semiconductor nanocrystals were grown as the pH of growth solutions was increased. All of these nanocrystals from smaller sizes to larger sizes had the same crystallinity. This peptide-controlled nanocrystal growth technique will be very useful to prepare semiconductor nanowires as building blocks for future microelectronics, whose band gaps can be tuned by the sizes of coated semiconductor nanoparticles via their quantum confinement effect. The novelty of this approach in the electronic device fabrication is that the semiconductor nanocrystal size control can be achieved by controlling peptide configurations via pH change, and this control may tune electronic structures and band gaps of the resulting semiconductor nanowires.     

A Ge/Si heterostructure nanowire-based double quantum dot with integrated charge sensor

One proposal for a solid-state-based quantum bit (qubit) is to control coupled electron spins on adjacent semiconductor quantum dots. Most experiments have focused on quantum dots made from III-V semiconductors; however, the coherence of electron spins in these materials is limited by hyperfine interactions with nuclear spins. Ge/Si core/shell nanowires seem ideally suited to overcome this limitation, because the most abundant nuclei in Ge and Si have spin zero and the nanowires can be chemically synthesized defect-free with tunable properties. Here, we present a double quantum dot based on Ge/Si nanowires in which we can completely control the coupling between the dots and to the leads. We also demonstrate that charge on the double dot can be detected by coupling it capacitively to an adjacent nanowire quantum dot. The double quantum dot and integrated charge sensor serve as an essential building block to form a solid-state qubit free of nuclear spin.     

Spontaneous formation of Ge nanocrystals with the capping layer of Si3N4 by N2 implantation and rapid thermal annealing

We studied Ge nanocrystals (nc-Ge) formed by bombarding Ge(100) surface with N₂⁺ gas followed by rapid thermal annealing (RTA). After initial N₂⁺ implantation, near-edge x-ray absorption fine structure and x-ray photoelectron spectroscopy (XPS) data showed formation of molecule-like N₂ species and chemically metastable Ge nitrides (GeN_x). The RTA transformed these into hemispherical nc-Ge of 10-25 nm in the diameter as clearly seen in transmission electron microscope images. XPS confirmed that the surface of the nc-Ge was covered with Ge₃N₄ layer and underlying layer is also mostly likely Ge₃N₄. This simple process of forming isolated nc-Ge with Ge₃N₄ surrounding layer can be useful in non-volatile memory applications.     

Luminescence of nanostructured SnO2-SiO2 glass-ceramics prepared by sol-gel method

Nanostructured silica based glass-ceramics samples of composition (100 - x)SiO2-xSnO2, with x from 1 to 10, have been synthesized by thermal treatment of precursor sol-gel glasses. The average size of the obtained SnO2 nanocrystals, calculated by using the X-ray diffraction, can be predetermined by using well-controlled concentration of tin precursor. The mean radius ranging from 1.6 to 5.5 nm, is comparable to the exciton Bohr radius, corresponding to wide band-gap semiconductor quantum-dots in an insulator SiO2 glass. A spectroscopy study in terms of optical absorption and photoluminescence spectra has been carried out as a function of SnO2 concentration. Size-dependent red-shifts of excitation and emission bands, with increasing of tin precursor concentration, point to the quantum confinement effect. The nanocrystal sizes have been obtained and compared by using the Brus and Scherrer equations. The band gap increase is in agreement with results, based on the effective mass model. The recombination of conduction band electron with oxygen vacancies is proposed to explain the luminescence red-shift.     

500℃下利用UHV/CVD在Si衬底上直接生长近平面Si0.5Ge0.5层

利用UHV/CVD系统,在一个相对较低的温度500℃下,研究了Si1-xGex层中的Ge含量与生长条件之间的关系,此时的Si1-xGex层处于一种亚稳的状态.并直接在Si衬底上生长制备了10个周期的3.0 nm-Si0.5Ge0.5/3.4 nm-Si多量子阱.拉曼谱、高分辨显微电镜和光荧光谱对其结构和光学性能进行的表征表明这种相对较厚的Si0.5Ge0.5/Si多量子阱结构基本上仍是近平面生长的,内部没有位错,其在电学和光学器件上具有潜在的应用.     

Plasmonic photosensitization of a wide band gap semiconductor: converting plasmons to charge carriers

A fruitful paradigm in the development of low-cost and efficient photovoltaics is to dope or otherwise photosensitize wide band gap semiconductors in order to improve their light harvesting ability for light with sub-band-gap photon energies.(1-8) Here, we report significant photosensitization of TiO2 due to the direct injection by quantum tunneling of hot electrons produced in the decay of localized surface-plasmon polaritons excited in gold nanoparticles (AuNPs) embedded in the semiconductor (TiO2). Surface plasmon decay produces electron-hole pairs in the gold.(9-15) We propose that a significant fraction of these electrons tunnel into the semiconductor's conduction band resulting in a significant electron current in the TiO2 even when the device is illuminated with light with photon energies well below the semiconductor's band gap. Devices fabricated with (nonpercolating) multilayers of AuNPs in a TiO2 film produced over 1000-fold increase in photoconductance when illuminated at 600 nm over what TiO2 films devoid of AuNPs produced. The overall current resulting from illumination with visible light is ～50% of the device current measured with UV (?ω>Eg band gap) illumination. The above observations suggest that plasmonic nanostructures (which can be fabricated with absorption properties that cover the full solar spectrum) can function as a viable alternative to organic photosensitizers for photovoltaic and photodetection applications.     

Ge-SiO2薄膜的结构及其发光特性的研究

采用射频磁控溅射技术制备了Ge-SiO2薄膜，在N2气氛下进行了不同温度的退火处理，分析了样品在室温下的光致发光（PL）特性，为探讨其发光机制，对薄膜的结构进行了表征，XRD、XPS、FTIR谱分析说明样品的发光特性与其结构相对应，394nmPL由GeO缺陷引起，580nmPL与Ge纳米晶粒和基质SiO2界面处的发光光中心相联系。     

基于各向异性二维Ge Se的无偏振器偏振图像传感器

偏振是光的一个重要信息,偏振探测可以把信息量从三维(光强、光谱和空间)扩充到七维(光强、光谱、空间、偏振度、光偏振等),为成像物体提供关键的视觉信息(如表面粗糙度、几何形状或方向),因此偏振成像技术在目标检测等领域有着巨大的潜力.然而这些领域往往需要复杂的偏振编码,现有的复杂透镜系统和偏振器限制了集成成像传感器的小型化能力.本文通过二维各向异性α-Ge Se半导体,成功实现了无偏振器的偏振敏感可见-近红外光电探测器/成像仪.作为传感器系统的关键部件,该原型Au/GeSe/Au光电探测器具有灵敏度高、光谱响应宽、响应速度快(~10³A W^-1, 400–1050 nm, 22.7/49.5μs)等优点.此外,该器件在690–1050 nm光谱范围内表现出独特的偏振灵敏度,并且对沿y方向的偏振光吸收最强,这一点通过分析α-Ge Se的光跃迁行为也得到了证实.最后,将2D-Ge Se器件应用到成像系统中进行偏振成像,在808 nm近红外波段处,在不同的偏振方向上,辐射目标的对比度为3.45.这种成像仪在没有偏振器的情况下,能够在场景中感知双频偏振信号,为偏振成像传感器阵列的广泛应用奠定了基础.     

Microstructure of pulsed laser irradiated Sn and Ge-50 at.%Sn thin films on Ge single crystal substrates

The microstructures produced from pulsed laser irradiation of evaporated thin films of Sn and Ge-50 at.%Sn on single-crystal Ge substrates have been investigated as a function of laser energy density and film thickness. In general, the irradiated samples exhibit a trilayer morphology consisting of an overlayer of equiaxed β-Sn and/or -Ge in a β-Sn matrix, and epitaxial cellular and segregated-free -Ge(Sn) alloy layers. The overlayer thickness decreases and the cellular layer thickness and the cell size both increase with increasing laser energy density. The maximum Sn content in the segregation-free -Ge(Sn) alloy layers remains below 2 at.%Sn, but the maximum Sn content in the cellular -Ge(Sn) alloy layers can reach up to 55 at.%Sn, estimated from the selected area diffraction pattern.     

Ion-assisted codeposition of CaF2-rich CaF2-TiO2 composites as infrared-transmitting films

The influence of the TiO(2) concentration (相似文献   

CdSe@CdS core-shell quantum dot-polymer multilayer sensitized TiO2 for photovoltaics

Colloidal CdSe@CdS core-shell quantum dots (QDs) have been prepared and exploited as inorganic dyes to sensitize a large-band-gap TiO2 layer for QD-sensitized solar cells. The sensitized films were prepared by alternating the layer-by-layer deposition of water-soluble semiconductor QDs and polycations over mesoscopic TiO2 films. The multilayer build-up, monitored by UV-vis spectroscopy shows an increase in the film absorbance with the number of adsorbed CdSe@CdS layers. The photoluminescence (PL) and photoelectrochemical properties of the multilayers were investigated. The photovoltaic performance of QD-sensitized solar cells is strongly dependent on the film structure and component. The incorporation of the electron mediators of [Co(Phen)3]2+ during the deposition process remarkably enhanced the photocurrent intensity in comparison to that in case of QD/polyelectrolyte multilayers.     

Visible light-responsive titanium dioxide thin film prepared by reactive sputtering

TiO2 is a wide band-gap semiconductor (3.4 eV) and can only absorb about 5% of sun light in the ultraviolet light region, which largely limits its practical applications because of the lower utility of sun light and quantum yield. In order to move the absorption edge of TiO2 films to visible spectrum range, we have made the impurity level within a band-gap of TiO2 thin film by introduction of oxygen vacancy. Oxygen-defected TiO2 photo-catalyst have prepared by reactive sputtering with the partial pressure of Ar:O2 = 76.7:23.3 approximately 98.5:1.5 ratios. As a result, we could have the impurity level of about 2.75 eV on condition that oxygen partial pressure is 2.9%. And the photocatalytic activity was realized at 400 nm wavelength.     

Empirical pseudo-potential studies on electronic structure of semiconducting quantum dots

Theoretical investigations of electronic structure of quantum dots is of current interest in nanophase materials. Empirical theories such as effective mass approximation, tight binding methods and empirical pseudo-potential method are capable of explaining the experimentally observed optical properties. We employ the empirical pseudo-potential to calculate the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) as a function of shape and size of the quantum dots. Our studies explain the building up of the bulk band structure when the size of the dot is much larger than the bulk Bohr exciton radius. We present our investigations of HOMO-LUMO gap variation with size, for CdSe, ZnSe and GaAs quantum dots. The calculated excitonic energies are sensitive to the shape and size of quantum dots and are in good agreement with experimental HOMO-LUMO gaps for CdSe quantum dots. The agreement improves as experimentally observed lattice contraction is incorporated in pseudo-potential calculations for ZnSe quantum dots. Electronic structure evolution, as the size of quantum dot increases, is presented for CdSe, ZnSe and GaAs quantum dots.     

Preparation of regularly ordered Ge quantum dot lattices in amorphous matrices

We compare structural and optical properties of Ge quantum dot lattices in amorphous silica matrix obtained by two recently published techniques for the preparation of regularly ordered quantum dot lattices in amorphous matrices. The first technique is self-ordering growth of (Ge + SiO₂)/SiO₂ multilayer at an elevated substrate temperature where diffusion and surface morphology effects drive the self-ordering. The second one is irradiation of (Ge + SiO₂)/SiO₂ multilayer by oxygen ions. The multilayer used for the irradiation is grown at room temperature in this case, resulting with no Ge clusters after the deposition process. The irradiation causes clustering of Ge and ordering of Ge quantum dots in the irradiation direction. We show that the size of the dots and their arrangement can be easily manipulated by the preparation parameters. The structural properties of the films prepared by these methods affect the quantum confinement of the charge carriers which is visible in the absorption properties of the films.     

3-component low temperature solvothermal synthesis of colloidal cadmium sulfide quantum dots

We report the solvothermal synthesis of colloidal cadmium sulfide quantum dots via a three-component system affording various particle diameter ranging from 3.368 nm to 8.411 nm. The band gap, and therefore the optical property, of these nanocrystals can be tuned by varying the reaction time and temperature of the system. The results obtained show strong confinement of particles with diameter smaller than the exciton Bohr radius of CdS. The growth kinetics was found to follow a diffusion-controlled process at the initial stage then shifts to a surface-controlled incorporation of reactants to the crystallite after the particle size becomes comparable to the exciton Bohr radius.     

Photoluminescence efficiency and size distribution of self assembled ge dots on porous TiO2

For Ge nanodots approximately 20 nm in diameter grown by annealing a thin amorphous Ge layer deposited by molecular beam epitaxy on a mesoporous TiO2 layer on Si(001), photoluminescence (PL) was observed as a wide near-infrared band near 800 meV. Using a tight binding theoretical model, the energy-dependent PL spectrum was transformed into a dependence on dot size. The average dot size determined the peak energy of the PL band and its shape depended on the size distribution, including bandgap enlargement due to quantum confinement. Combining the dot sample PL with an established dependence of emission efficiency on dot diameter, it was possible to derive a dot size distribution and compare it with results obtained independently from atomic force microscopy.     

Optical absorption characteristic of highly ordered and dense two-dimensional array of silicon nanodiscs

We created a two-dimensional array of sub-10 nm Si-nanodiscs (Si-NDs), i.e. a 2D array of Si-NDs, with a highly ordered arrangement and dense NDs by using a new top-down technique comprising advanced damage-free neutral-beam (NB) etching and a bio-template (iron oxide core) as a uniform sub-10 nm etching mask. The bandgap energy (E(g)) of the fabricated 2D array of Si-NDs can be simply controlled from 2.2 to 1.3 eV by changing the ND thickness from 2 to 12 nm. Due to weak quantum confinement existing in the diameter direction resulting from the sub-10 nm Si-ND diameter, even though the thickness of the Si-ND is much larger than the Bohr radius of Si, E(g) is still larger than the 1.1 eV E(g) of bulk Si. Si-ND not only has wide controllable E(g) but also a high absorption coefficient due to quantum confinement in three dimensions. This new technique is a promising candidate for developing new nanostructures and could be integrated into the fabrication of nanoelectronic devices.     

Multiple exciton generation in colloidal silicon nanocrystals

Multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap identical with Eg = 1.20 eV) to be 2.4 +/- 0.1Eg and find an exciton-production quantum yield of 2.6 +/- 0.2 excitons per absorbed photon at 3.4Eg. While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth's crust, and poses no significant environmental problems regarding toxicity.