PbS量子点能级结构的尺寸和配体依赖性及其对异质结电池性能的影响

通过电化学循环伏安测试和吸收光谱测试, 确定了有机配体(油酸)和原子配体(四正丁基碘化铵, TBAI)钝化的不同粒径(2.6~4.5 nm)PbS量子点的导带和价带能级, 并研究了量子点尺寸对PbS/TiO₂异质结电池(空气气氛中制备)性能的影响。结果表明：PbS量子点的能级结构受其粒径大小和表面配体特性的影响。当PbS量子点尺寸从2.6 nm增加至4.5 nm时, 油酸包覆PbS量子点的导带底从-3.67 eV减小到-4.0 eV, 价带顶从-5.19 eV增加到-4.97 eV; 而对于TBAI配体置换的PbS量子点, 其导带底和价带顶则分别从-4.15 eV和-5.61 eV变化至-4.51 eV和-5.46 eV。粒径为3.9 nm的PbS量子点所制备的电池性能最优, 其能量转化效率达到2.32%, 这可归因于其适宜的禁带宽度、结晶质量和良好的PbS/TiO₂界面能级匹配度。     

Effect of single-walled carbon nanotube in PbS/TiO2 quantum dots-sensitized solar cells

Single-walled carbon nanotubes (SWNTs) layers formed on indium-doped tin oxide (ITO) electrodes for enhanced photoconversion efficiency of PbS/TiO₂ quantum dots (Q dots)-sensitized photoelectrochemical solar cells (PECs). The short-circuit current of Q dots-sensitized PECs with SWNTs layers increased under illumination, and the dark current of the PECs was also reduced without illumination. Furthermore, the electron lifetimes of PbS/TiO₂/SWNTs in open-circuit voltage decay is higher than that of PbS/TiO₂ films at the same voltage. As a result, the power conversion efficiency of PbS/TiO₂ on ITO increased 35.6% in the presence of SWNTs due to the improved charge-collecting efficiency and reduced recombination process.     

Synthesis of PbS/poly (vinyl-pyrrolidone) nanocomposite

A simple solution growth method for synthesis of nanocomposite of PbS nanoparticles in poly(vinyl-pyrrolidone) (PVP) polymer is described. The nanocomposite is prepared from methanolic solution of lead acetate (PbAc), thiourea (TU) and PVP at room temperature (∼27 °C). Optical absorption spectrum of PbS/PVP nanocomposite solution shows strong absorption from 300 to 650 nm with significant bands at 400 and 590 nm which is characteristic of nanoscale PbS. Spin-coated nanocomposite films on glass have an absorption edge at ∼650 nm with band gap of 2.55 eV. Fourier transform infrared (FTIR) spectroscopy of PbS/PVP nanocomposite and PVP shows strong chemical bond between PbS nanoparticles and host PVP polymer. The transmission electron microscope (TEM) images reveal that 5-10 nm PbS particles are evenly embedded in PVP polymer. The formation of PbS is confirmed by selective area electron diffraction (SAED) of a typical nanoparticle.     

Enhanced open-circuit voltage in visible quantum dot photovoltaics by engineering of carrier-collecting electrodes

Colloidal quantum dots (CQDs) enable multijunction solar cells using a single material programmed using the quantum size effect. Here we report the systematic engineering of 1.6 eV PbS CQD solar cells, optimal as the front cell responsible for visible-wavelength harvesting in tandem photovoltaics. We rationally optimize each of the device's collecting electrodes-the heterointerface with electron-accepting TiO(2) and the deep-work-function hole-collecting MoO(3) for ohmic contact-for maximum efficiency. We report an open-circuit voltage of 0.70 V, the highest observed in a colloidal quantum dot solar cell operating at room temperature. We report an AM1.5 solar power conversion efficiency of 3.5%, the highest observed in >1.5 eV bandgap CQD PV device.     

Carbon nanotube-polymer nanocomposite infrared sensor

The infrared photoresponse in the electrical conductivity of single-walled carbon nanotubes (SWNTs) is dramatically enhanced by embedding SWNTs in an electrically and thermally insulating polymer matrix. The conductivity change in a 5 wt % SWNT-polycarbonate nanocomposite is significant (4.26%) and sharp upon infrared illumination in the air at room temperature. While the thermal effect predominates in the infrared photoresponse of a pure SWNT film, the photoeffect predominates in the infrared photoresponse of SWNT-polycarbonate nanocomposites.     

Novel nano-structured glasses containing semiconductor quantum dots: controlling the photoluminescence with phonons and photons

Effects of temperatures and excitation intensities on the photoluminescence properties of PbS quantum dots precipitated in the glass were investigated. Peak wavelength of the near-infrared photoluminescence shifted towards the short wavelength side with an increase in temperature and excitation intensity. The largest shift in the peak wavelength of the photoluminescence bands was approximately 90 nm. The temperature coefficient of band gap energy (deduced from the photoluminescence wavelength) of quantum dots varied from 230 to 28 μeV/K under the excitation intensity of 50–600 mW. The integrated photoluminescence intensity also showed similar dependencies on temperature and excitation intensity. The shifts of the photoluminescence with changes in the temperature and excitation intensity were associated with the trapping and re-activation of charge carriers at defect sites located at the QDs/glass interface and inside the glass matrix.     

Fabrication and characterization of silicon quantum dots in Si-rich silicon carbide films

Amorphous Si-rich silicon carbide films were prepared by magnetron co-sputtering and subsequently annealed at 900-1100 degrees C. After annealing at 1100 degrees C, this configuration of silicon quantum dots embedded in amorphous silicon carbide formed. X-ray photoelectron spectroscopy was used to study the chemical modulation of the films. The formation and orientation of silicon quantum dots were characterized by glancing angle X-ray diffraction, which shows that the ratio of silicon and carbon significantly influences the species of quantum dots. High-resolution transmission electron microscopy investigations directly demonstrated that the formation of silicon quantum dots is heavily dependent on the annealing temperatures and the ratio of silicon and carbide. Only the temperature of about 1100 degrees C is enough for the formation of high-density and small-size silicon quantum dots due to phase separation and thermal crystallization. Deconvolution of the first order Raman spectra shows the existence of a lower frequency peak in the range 500-505 cm(-1) corresponding to silicon quantum dots with different atom ratio of silicon and carbon.     

Photocurrent response of the carbon nanotube-silicon heterojunction array

A highly ordered array of parallel, identical carbon nanotubes is grown non- lithographically in a bottom-up fabrication approach to form a heterojunction with a silicon substrate. Evidence of a space-charge separated region at the nanotube-silicon interface is present in the form of diode rectification and a closed-circuit zero-bias photocurrent in response to infrared light. Because carbon nanotubes are narrow bandgap semiconductors, their heterojunction with silicon was analysed spectrally via Fourier transform infrared photocurrent spectroscopy with the aim of investigating the suitability of this structure for infrared (IR) detector applications. IR photoresponse shows signs of temperature-dependent activation that is complex but consistent with estimates of the heterojunction barrier height. Considering the many interesting benefits and properties of carbon nanotubes, these results despite their earliness suggest that nanotube-silicon heterojunction systems could form the foundation for a new kind of infrared detection device.     

Quantum-size effects of PbS nanocrystallites in evaporated composite films

Semiconductor nanocrystallites exhibit electronic, optical and photochemical properties greatly differing from those observed in the related bulk material due to quantum size effects. In view of future applications, nanoclusters based on sulphur compounds will find great potential as photocatalysts or nonlinear optical materials. The method of interrupted island growth was applied to form PbS nanocrystallites in a dielectric SiO₂ host by a two source-evaporation technology. Depending on the deposition conditions, PbS nanocrystals can be formed with grain size down to about 1 nm, giving a significant increase in the optical band gap from 0.41 eV for the bulk material to about 5.2 eV. XPS, XRD and TEM investigations were performed to verify the formation of PbS nanocrystallites in the SiO₂ host. These nanocrystallites show an intensive photoluminescence emission band at about 435 nm, with the intensity decreasing with increasing PbS concentration.     

Intersublevel Spectroscopy on Single InAs-Quantum Dots by Terahertz Near-Field Microscopy

Using scattering-type near-field infrared microscopy in combination with a free-electron laser, intersublevel transitions in buried single InAs quantum dots are investigated. The experiments are performed at room temperature on doped self-assembled quantum dots capped with a 70 nm GaAs layer. Clear near-field contrast of single dots is observed when the photon energy of the incident beam matches intersublevel transition energies, namely the p-d and s-d transition of conduction band electrons confined in the dots. The observed room-temperature line width of 5-8 meV of these resonances in the mid-infrared range is significantly below the inhomogeneously broadened spectral lines of quantum dot ensembles. The experiment highlights the strength of near-field microspectroscopy by demonstrating signals from bound-to-bound transitions of single electrons in a probe volume of the order of (100 nm)(3).     

Three-dimensional Si/Ge quantum dot crystals

Modern nanotechnology offers routes to create new artificial materials, widening the functionality of devices in physics, chemistry, and biology. Templated self-organization has been recognized as a possible route to achieve exact positioning of quantum dots to create quantum dot arrays, molecules, and crystals. Here we employ extreme ultraviolet interference lithography (EUV-IL) at a wavelength of lambda = 13.5 nm for fast, large-area exposure of templates with perfect periodicity. Si(001) substrates have been patterned with two-dimensional hole arrays using EUV-IL and reactive ion etching. On these substrates, three-dimensionally ordered SiGe quantum dot crystals with the so far smallest quantum dot sizes and periods both in lateral and vertical directions have been grown by molecular beam epitaxy. X-ray diffractometry from a sample volume corresponding to about 3.6 x 10(7) dots and atomic force microscopy (AFM) reveal an up to now unmatched structural perfection of the quantum dot crystal and a narrow quantum dot size distribution. Intense interband photoluminescence has been observed up to room temperature, indicating a low defect density in the three-dimensional (3D) SiGe quantum dot crystals. Using the Ge concentration and dot shapes determined by X-ray and AFM measurements as input parameters for 3D band structure calculations, an excellent quantitative agreement between measured and calculated PL energies is obtained. The calculations show that the band structure of the 3D ordered quantum dot crystal is significantly modified by the artificial periodicity. A calculation of the variation of the eigenenergies based on the statistical variation in the dot dimensions as determined experimentally (+/-10% in linear dimensions) shows that the calculated electronic coupling between neighboring dots is not destroyed due to the quantum dot size variations. Thus, not only from a structural point of view but also with respect to the band structure, the 3D ordered quantum dots can be regarded as artificial crystal.     

High performance multilayered nano-crystalline silicon/silicon-oxide light-emitting diodes on glass substrates

A low-temperature hydrogenation-assisted sequential deposition and crystallization technique is reported for the preparation of nano-scale silicon quantum dots suitable for light-emitting applications. Radio-frequency plasma-enhanced deposition was used to realize multiple layers of nano-crystalline silicon while reactive ion etching was employed to create nano-scale features. The physical characteristics of the films prepared using different plasma conditions were investigated using scanning electron microscopy, transmission electron microscopy, room temperature photoluminescence and infrared spectroscopy. The formation of multilayered structures improved the photon-emission properties as observed by photoluminescence and a thin layer of silicon oxy-nitride was then used for electrical isolation between adjacent silicon layers. The preparation of light-emitting diodes directly on glass substrates has been demonstrated and the electroluminescence spectrum has been measured.     

Electronic and optical properties of <Emphasis Type="Italic">β</Emphasis>-graphyne nanotubes and their BN analogues

The electronic and optical properties of zigzag and armchair β-graphyne nanotubes (β-GNTs) and their BN analogues (labeled as β-BNyne NTs) with different tube diameters are systematically investigated by the first-principles calculations. The calculated results reveal that all zigzag and armchair β-graphyne nanotubes are direct band gap semiconductors. As for zigzag β-BNyne NTs, they are wide direct band gap semiconductors. Nevertheless, armchair β-BNyne NTs are indirect band gap semiconductor. The optical spectra of β-GNTs and BNyne NTs show remarkable anisotropic behavior. Interestingly, the static dielectric constant of β-GNTs is quite high in comparison with carbon NTs, indicating higher conductivity and carrier mobility. In addition, quite broad frequency absorption spectra, extended from the infrared to the ultraviolet (UV) region, are observed for all β-GNTs. However, the photoresponse of β-BNyne NTs is mainly located in the UV region. The β-GNTs exhibit high reflectivity in the infrared region of 0.0 to 1.5 eV for both parallel and perpendicular polarization, but all β-BNyne NTs possess very low reflectivity and are highly sensitive to the UV light. Particularly, all β-GNTs demonstrate no obvious size-dependent optical properties, however, for β-BNyne NTs, all the photoresponse intensity (static dielectric constant, reflectivity and absorption coefficient) decreases monotonically with increasing tube size.     

Crystallization behavior of silicon quantum dots in a silicon nitride matrix

Silicon quantum dot superlattice was fabricated by alternating deposition of silicon rich nitride (SRN) and Si3N4 layers using RF magnetron co-sputtering. Samples were then annealed at temperatures between 800 and 1,100 degrees C and characterized by grazing incident X-ray diffraction (GIXRD), transmission electron microscopy (TEM), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR). GIXRD and Raman analyses show that the formation of silicon quantum dots occurs with annealing above 1,100 degrees C for at least 60 minutes. As the annealing time increased the crystallization of silicon quantum dots was also increased. TEM images clearly showed SRN/Si3N4 superlattice structure and silicon quantum dots formation in SRN layers after annealing at 1,100 degrees C for more than 60 minutes. The changes in FTIR transmission spectra observed with annealing condition corresponded to the configuration of Si-N bonds. Crystallization of silicon quantum dots in a silicon nitride matrix started stabilizing after 60 minutes' annealing and approached completion after 120 minutes'. The systematic investigation of silicon quantum dots in a silicon nitride matrix and their properties for solar cell application are presented.     

Luminescence tuning of amorphous Si quantum dots prepared by plasma-enhanced chemical vapor deposition

Amorphous Si (a-Si) quantum dots (QDs) embedded in a silicon nitride film were prepared by a plasma-enhanced chemical vapor deposition (PECVD) technique using gaseous mixtures of silane, hydrogen and nitrogen. We observed that the Si QDs had an amorphous structure from the Raman spectroscopy measurement. The Fourier transform infrared (FTIR) spectra showed that the relative transmittance of the SiH bands decreased, but that of the NH bands increased, with increasing nitrogen flow rate. During the deposition of SiNx, the number of dangling bonds of silicon acting as nucleation sites increased. As the hydrogen flow rate increased the growth rate decreased, due to the reduction in the hydrogen partial pressure. The hydrogen and nitrogen gas flow rates were found to be important parameters for determining the size of the a-Si QDs. In addition, we observed that the PL peak shifted toward a higher energy with increasing hydrogen and nitrogen gas flow rates, which was attributed to the increase in the quantum confinement effect in the a-Si QDs.     

Structural,optical and electrical properties of Ag doped PbS thin films: role of Ag concentration

The present work reports on the chemical synthesizes of (0–8 at.%) silver (Ag)-doped PbS thin films with tunable opto-electrical properties. From the X-ray diffraction analyses, it was understood that the preferred growth orientation of Ag:PbS films was dependent on the Ag doping concentration. The variation in the Ag:PbS films orientation was reflected in the film morphology as observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). SEM studies revealed that the variation in Ag concentration lead to different grain shapes for different grain orientations. The AFM study showed that the RMS roughness of the undoped PbS film has been reduced considerably due to silver doping. From the optical studies, a widening in the optical band gap was revealed after Ag-doping due to the quantum confinement effect. It was obtained that 4 at.% Ag-doped PbS thin films display an optimum band gap value of 1.45 eV. As for electrical characterization result, the resistivity reduces and the carrier density improved with 4 at.% Ag concentration. Based on all the data, it was concluded that the 4 at.% Ag-doped PbS thin film showed the best morphological, optical and electrical behavior, which recommend it as an active layer for solar cell devices.     

Bright infrared quantum-dot light-emitting diodes through inter-dot spacing control

Infrared light-emitting diodes are currently fabricated from direct-gap semiconductors using epitaxy, which makes them expensive and difficult to integrate with other materials. Light-emitting diodes based on colloidal semiconductor quantum dots, on the other hand, can be solution-processed at low cost, and can be directly integrated with silicon. However, so far, exciton dissociation and recombination have not been well controlled in these devices, and this has limited their performance. Here, by tuning the distance between adjacent PbS quantum dots, we fabricate thin-film quantum-dot light-emitting diodes that operate at infrared wavelengths with radiances (6.4?W?sr(-1)?m(-2)) eight times higher and external quantum efficiencies (2.0%) two times higher than the highest values previously reported. The distance between adjacent dots is tuned over a range of 1.3?nm by varying the lengths of the linker molecules from three to eight CH(2) groups, which allows us to achieve the optimum balance between charge injection and radiative exciton recombination. The electroluminescent powers of the best devices are comparable to those produced by commercial InGaAsP light-emitting diodes. By varying the size of the quantum dots, we can tune the emission wavelengths between 800 and 1,850?nm.     

Electric field modulation of infrared absorption at room temperature in electrochemically self assembled quantum dots

We report observation of electric-field-modulated infrared absorption at room temperature in electrochemically self-assembled CdS quantum dots produced by electrodepositing the semiconductor in 50-nm pores of an anodic alumina film. The absorption is associated with photoassisted real space transfer of electrons from the CdS dots to surrounding trap sites in the alumina. Similar absorption was observed in the past [Appl. Phys. Lett. 79, 4423 (2001)] and was the basis of a room temperature near infrared photodetector. An electric field modulates this absorption by altering the overlap between the wavefunctions of electronic states in the quantum dots and the trap states in the surrounding alumina, thereby affecting the matrix element for radiative transitions, similar to the quantum confined Stark or Franz-Keldysh effect. The ability to electrically modulate absorption in these structures can result in inexpensive infrared signal processing devices operating at room temperature.     

铝掺杂单壁扶手型(6,6)硅纳米管电子结构和光电性质的第一性原理研究

采用基于密度泛函理论的第一性原理计算,研究了铝掺杂对单壁扶手型(6,6)硅纳米管电子结构和光电性质的影响。结果表明,本征态硅纳米管属于直接带隙半导体,其禁带宽度为0.42eV,而铝掺杂硅纳米管为间接带隙半导体,其禁带宽度为0.02eV。单壁扶手型(6,6)硅纳米管的价带顶主要由Si-3p态电子构成,而其导带底则主要由Si-3p态电子决定。同时通过铝掺杂,使硅纳米管的禁带宽度变窄,吸收光谱产生红移,研究结果为硅纳米管在光电器件方面的应用提供了理论基础。     

电子束再结晶法制备的纳米硅薄膜室温光致发光特性

用电子束再结晶的方法对非晶硅氢材料进行快速退火,成功地制备出纳米硅薄膜,在室温下观察到光致红、蓝发光带,峰位约在１．７ｅＶ和２．５ｅＶ处,ＸＲＤ和ＰＬ谱结果表明：晶化程度的变化引起红、蓝光带强度及在整个发光谱中所占比例的改变,在我们的实验中,电子束能量密度２．７Ｗ／ｃｍ＾２的晶化样品具有强的发光带。