Stability of luminescent 3C-SiC nanocrystallites in aqueous solution

We study the long-term stability of luminescent suspended 3C-SiC nanocrystallites. Polycrystalline 3C-SiC wafers were electrochemically etched in HF-ethanol electrolyte and then ultrasonically treated in water to achieve spherical crystallites with diameters of 1–7 nm. The suspended 3C-SiC nanocrystallites exhibit tunable intense emissions which follow well the quantum size effect. After storage in air for over 7 months, partial larger 3C-SiC crystallites aggregate and form precipitates, while smaller ones remain uniformly dispersed in water and retain well luminescent properties. This characteristics makes small 3C-SiC crystallites excellent micro-emitters which are especially useful in biological and medical applications.     

Si-based solid blue emitters from 3C-SiC nanocrystals

A procedure for preparing 3C-SiC/SiO₂ composite nanocrystals embedded in Si matrix that emit blue light is reported. Through electrochemical etching of polycrystalline 3C-SiC wafers followed by ultrasonic treatment in water bath, we fabricated luminescent colloidal 3C-SiC nanocrystals. Porous Si samples that have been naturally oxidized in air for 12 h were immersed in agitated aqueous suspension of 3C-SiC nanocrystals for 10 min and then dried in air, followed by annealing in argon atmosphere to form core-shell structured 3C-SiC/SiO₂ nanocrystals embedded in Si matrix. Our result shows that the luminescence of 3C-SiC/SiO₂ composite nanocrystals is very stable over time or under high temperature. As robust and stable Si-based solid blue-emitters, they have important implications for engineering photonic components in optoelectronics and photonics. PACS 78.67.Bf; 78.55.Hx; 78.66.Sq     

Experimental evidence for the quantum confinement effect in 3C-SiC nanocrystallites

Using electrochemical etching of a polycrystalline 3C-SiC target and subsequent ultrasonic treatment in water solution, we have fabricated suspensions of 3C-SiC nanocrystallites that luminesce. Transmission electron microscope observations show that the 3C-SiC nanocrystallites, which uniformly disperse in water, have sizes in the range of 1-6 nm. Photoluminescence and photoluminescence excitation spectral examinations show clear evidence for the quantum confinement of 3C-SiC nanocrystallites with the emission band maximum ranging from 440 to 560 nm. Tunable, composite polystyrene/SiC film can be made by adding polystyrene to a toluene suspension of the 3C-SiC nanocrystallites and then coating the resulting solution onto a Si wafer.     

Preparation and luminescence of water soluble poly (N-vinyl-2-pyrrolidone)/LaF3:Eu nanocrystals

Lanthanide-doped luminescent nanocrystals have great potential as biological luminescent labels, but their use has been limited because of most of these nanocrystals are hydrophobic. In this work, water soluble LaF₃:Eu³⁺ down-conversion nanocrystals were prepared by encapsulated individual nanocrystals with polyvinylpyrrolidone (PVP). Their morphology, surface structure and luminescence properties were explored in detail. The results indicate that these nanocrystals can be readily dispersed in water, forming a stable and transparent colloidal solution. The colloidal solution displayed unique red luminescence with high emission intensity under ultraviolet excitation. These results suggest that these nanocrystals have great potential as luminescent labeling materials for biological applications.     

CdSe/CdS核/壳型纳米晶的光谱特性

以巯基乙酸为稳定剂制备了CdSe/CdS核／壳型纳米晶。用光吸收谱（Abs)、光致发光谱（PL）及光致发光激发谱（PLE）研究了CdS壳层对CdSe纳米晶电子结构，从而对其吸收和发光性能的影响。根据PL和PLE的结果以及带边激子精细结构的计算结果，我们用尺寸很小的纳米晶中所形成的基激缔合物解释了PL光谱与吸收边之间较大的Stokes位移。     

A detailed study of physical properties of ZnS quantum dots synthesized by reverse micelle method

In this article, zinc sulfide nanocrystal quantum dots were synthesized by reverse micelle method using polyvinyl pyrrolidone as surfactant. The various crystallite properties of these nanocrystals such as, size, d-spacing, lattice parameter, microstrain, intrinsic stress, X-ray density, specific surface area, dislocation density, porosity, and agglomeration number have been analyzed using X-ray diffraction spectrum. The transmission electron microscopy was used to calculate the size and monitoring morphology of the nanocrystals, while the scanning electron microscopy was utilized to investigate the surface morphology of nanoclusters. The various optical properties of zinc sulfide quantum dots such as absorption coefficient, extinction coefficient, optical band gap energy, Urbach energy, and threshold wavelength have been analyzed using UV-visible data. The photoluminescence was used to study the emission spectra of produced ZnS quantum dots. Moreover, Furrier Transform-Infrared studies revealed that ZnS quantum dots are pure.     

Highly luminescent nanocrystal quantum dots fabricated by lattice-type mismatched epitaxy

Lattice-type mismatched heteroepitaxy is demonstrated as a novel concept for the fabrication of almost ideal, highly luminescent nanocrystal quantum dots that are coherently embedded in a single-crystalline matrix. In this approach, the formation of quantum dots is induced by transformation of a metastable epitaxial 2D quantum well into an array of isolated nanocrystals with-highly symmetric shape. This process is driven by the lattice-type mismatch between the constituent materials and the resulting miscibility gap. The investigated PbTe/CdTe heterosystem has a model character because it combines two compounds with different cubic lattice types but almost identical lattice constants. The obtained epitaxial nanocrystals exhibit outstanding properties such as a well-defined symmetric shape, the absence of strain, intermixing and a wetting layer, which is in contrast to the conventional Stranski–Krastanow quantum dots. The small-rhomboedric-cubo-octahedron PbTe/CdTe nanocrystals on GaAs substrates display intense room temperature mid-infrared luminescence as is crucial for device applications. Ab initio density functional theory is used to clarify the interface structure, indicating that the covalent and ionic bonding character of CdTe and PbTe is maintained across the interface.     

具有表面修饰的SnO2纳米微晶的光学性质研究

给出了裸的及包裹ＳｎＯ２纳米微晶的吸收、荧光和激发谱的实验结果。发现随着粒子尺寸的减小,包覆一层有机分子的ＳｎＯ２纳米微晶的吸收谱带边向长波方向发生位移,这与裸ＳｎＯ２纳米微晶的结果是不一致。表明纳米晶体的尺寸及其表面状态对它们的光谱性质有很大影响。从量子限域效应和介电限域效应两个方面对实验结果进行了讨论。     

Electrical and optical properties of colloidal semiconductor nanocrystals in aqueous environments

Microscale and larger semiconductor crystals have electronic and optical properties that depend on their bulk band structures. When these crystals are reduced into the nanoscale, they enter a new regime in which the electrical and optical properties are no longer influenced solely by their bulk band structures, but are influenced by the crystallite size and shape. In this paper, dimensional confinement and proximity phenomena are examined for colloidal semiconductor nanocrystals in several cases of practical importance. Specifically, we determine the effective binding potentials of selected quantum dots in aqueous environments in various colloidal semiconductor nanocrystals and correlate them with experimentally obtained absorption spectra. We also study fluorescence resonance energy transfer (FRET) between semiconductor crystals connected by short peptide chains as well as the shift in photoluminescence spectra of CdTe nanowires made from a chain of CdTe quantum dots.     

Highly luminescent nanocrystal quantum dots fabricated by lattice-type mismatched epitaxy

Lattice-type mismatched heteroepitaxy is demonstrated as a novel concept for the fabrication of almost ideal, highly luminescent nanocrystal quantum dots that are coherently embedded in a single-crystalline matrix. In this approach, the formation of quantum dots is induced by transformation of a metastable epitaxial 2D quantum well into an array of isolated nanocrystals with-highly symmetric shape. This process is driven by the lattice-type mismatch between the constituent materials and the resulting miscibility gap. The investigated PbTe/CdTe heterosystem has a model character because it combines two compounds with different cubic lattice types but almost identical lattice constants. The obtained epitaxial nanocrystals exhibit outstanding properties such as a well-defined symmetric shape, the absence of strain, intermixing and a wetting layer, which is in contrast to the conventional Stranski–Krastanow quantum dots. The small-rhomboedric-cubo-octahedron PbTe/CdTe nanocrystals on GaAs substrates display intense room temperature mid-infrared luminescence as is crucial for device applications. Ab initio density functional theory is used to clarify the interface structure, indicating that the covalent and ionic bonding character of CdTe and PbTe is maintained across the interface.     

CuInS2纳米晶的制备和发光性质

以十二硫醇为溶剂,通过选择合适的金属源制备了各种尺寸的CuInS₂量子点。观察到随着粒子的尺寸减小,其吸收和发光光谱明显蓝移,存在明显的量子尺寸效应。通过在CuInS₂纳米晶表面包覆ZnS壳层,发现随着壳层厚度增加,其发光量子效率明显提高,最大达到了48%;继续增加壳层厚度,其发光量子效率反而降低。进一步测量它们的荧光寿命,发现包覆ZnS壳层后的CuInS₂纳米晶的荧光寿命明显增加,证实表面包覆明显减少其表面的无辐射复合中心,提高了其发光效率。进一步制备了CuInS₂/ZnS核壳量子点发光二极管,并对其电致发光性质进行了研究。     

衬底钝化处理对CsPbBr3 量子点薄膜发光稳定性影响

钙钛矿量子点因具有发光谱线窄、发光效率高、发光波长可调谐等优异的光学性能,在照明、显示、激光和太阳能电池等领域得到了广泛研究。然而,钙钛矿材料的稳定性问题,一直制约着其在光电器件中的应用。其中,钙钛矿材料在空气中受潮易分解的不稳定性尤为突出,这将严重影响其发光性质。为此,研究人员采用多种手段来改善钙钛矿材料的稳定性。目前,常见的方法是将一些具有疏水性的聚合物材料（例如POSS,PMMA等）引入到钙钛矿纳米晶中,或将钙钛矿纳米晶嵌入到介孔二氧化硅材料中,避免钙钛矿纳米晶暴露于空气中破坏其结构,以此来增强钙钛矿材料的发光稳定性。此外,钝化处理钙钛矿纳米晶表面,也是改善钙钛矿发光稳定性的一种常用方法。这些方法虽然在一定程度上可以改善钙钛矿的发光稳定性,但是在与有机物合成的过程中不免会引入其他有机官能团,介孔二氧化硅的引入,其处理方式相对复杂,而对钙钛矿纳米晶表面的钝化处理会破坏材料的原有结构。以上问题,都会影响钙钛矿的发光性质,不利于其在光电器件中的应用。硅（Si）具有低成本、大尺寸、高质量、导电好等优点,常被选作钙钛矿量子点光电器件的衬底材料。但是,由于Si衬底长时间暴露于空气,其表面易形成一层具有硅烷醇基团（Si-OH）的亲水性薄膜,这将对硅基钙钛矿器件的稳定性产生影响。因此,对Si表面进行钝化处理,破坏其表面Si-OH键,可以降低衬底表面的亲水性,增强疏水性,从而提高钙钛矿材料在器件中的稳定性。本研究使用氢氟酸（HF）对Si衬底表面进行钝化处理,发现钝化处理后的Si衬底表面与水的接触角由50.4°逐渐增大至87.7°,表明Si衬底表面由亲水性逐渐转变为疏水性。利用场致发射扫描电子显微镜(FE-SEM)测试发现,钝化处理后的Si衬底表面变粗糙,并且其表面上的CsPbBr₃量子点(CsPbBr₃ QDs)相对于未处理表面的分散性较好。利用光致发光(PL)光谱研究不同钝化处理时间的Si衬底表面上的CsPbBr₃ QDs薄膜的发光性质。其中,处理与未处理的Si衬底表面上CsPbBr₃QDs薄膜的PL积分强度随功率变化拟合值分别为1.12和1.203,表明其发光机制为激子发光。温度依赖性的PL光谱分析显示,随着温度的升高（10～300 K）,由于晶格热膨胀使CsPbBr₃ QDs带隙增大,发光峰位逐渐蓝移。并且,随着衬底钝化处理时间的增加,CsPbBr₃ QDs薄膜的发光热稳定性逐渐增强,最佳热稳定性可达220 K。而时间依赖性的PL光谱则进一步说明,钝化处理后的Si衬底表面CsPbBr₃QDs薄膜发光的时间稳定性逐渐增强,最高发光时间稳定性可达15 d。因此,通过简单而有效的对Si衬底表面进行钝化处理,可以有效减少了Si表面亲水基团,提高CsPbBr₃QDs薄膜的发光稳定性,为增强钙钛矿量子点在光电器件中的稳定性应用提供了新的研究思路。     

Aqueous Synthesis of Water-Soluble L-Cysteine-Modified Cadmium Sulfide Doped with Silver Ion/Zinc Sulfide Nanocrystals

The water-soluble silver ion-doped cadmium sulfide nanocrystals were synthesized by a co-precipitation technique in aqueous solution using L-cysteine as surface modifier, and then L-cysteine-modified cadmium sulfide doped with silver ion/zinc sulfide core/shell nanocrystals were prepared by zinc sulfide epitaxial coated on surface of silver ion-doped cadmium sulfide nanocrystals. The crystal structure, morphology, and spectral properties of cadmium sulfide doped with silver ion/zinc sulfide nanocrystals were characterized by X-ray power diffraction, transmission electron microscope, infrared spectrum, and photoluminescence spectrum. The results show that the photoluminescence quantum yield of cadmium sulfide doped with silver ion/zinc sulfide nanocrystals is improved greatly after doped with silver ion and coated with zinc sulfide shell. The cysteine modified on the surface of cadmium sulfide doped with silver ion/zinc sulfide nanocrystals renders the nanocrystals water-soluble and biocompatible.     

Optical properties of nanocrystalline silver halides

A review of quantum confinement effects in nanocrystals of silver bromide (AgBr) and silver iodide (AgI) is presented. AgBr is an indirect gap semiconductor while AgI has a direct band-to-band lowest energy transition. An examination of the low-temperature optical properties of quantum confined AgBr grown using a variety of synthetic techniques will be made. The dynamics of some of the involved excitonic processes will be measured and discussed in reference to a possible breakdown in the momentum selection rules as the nanocrystals are made smaller. Other explanations for this behavior such as impurity exclusion and surface effects will also be considered, as will the dynamics associated with the trapping of excitons at intrinsic iodide impurities in AgBr. Absorption measurements on AgI nanocrystals will be discussed and compared with the exciton photophysics in AgBr. Both AgBr and AgI display an increasing blue shift of their luminescence, arising from the recombination of excitons, as the crystallite size decreases. The luminescence intensity arising from this process increases with decreasing size in AgBr but it disappears in small crystals of AgI. This leads to the conclusion that in the latter material nonradiative decay channels are opening up as the size decreases.     

Optical inter- and intra-band transitions in silicon nanocrystals: The role of surface vibrations

Silicon nanocrystals (ncs) belong to an interesting class of semiconductor nanostructures that manifest size dependent electronic properties. This well known effect of quantum confinement can explain many properties of silicon ncs. However, with decreasing size and dimension of the ncs, the role of surface phenomena becomes substantial. For example, we have shown recently that the strong luminescence from these ncs should be assigned to the exclusion of nonradiative channels rather than to the enhancement of radiative inter-band transitions. In addition, using infrared intra-band transitions spectroscopy, we were able to resolve the quantized electronic sublevels of small silicon ncs. We have found that under appropriate conditions, these electronic sublevels are resonantly coupled to surface vibrations. We suggest that this coupling mechanism is responsible for the exclusion of nonradiative channels in silicon ncs.     

B、N单掺杂3C-SiC电子结构和光学性质的第一性原理研究

采用第一性原理的密度泛函理论赝势平面波方法,计算了未掺杂与B、N单掺杂3C-SiC的电子结构和光学性质.结果表明:掺杂改变了3C-SiC费米面附近的电子结构;B掺杂使得禁带宽度减小,价带顶上移,费米能级进入价带,形成p型半导体;N掺杂使得禁带宽度减小,导带底下移,费米能级进入导带,形成n型半导体.B、N掺杂均提高了3C-SiC在低能区的折射率、消光系数和吸收系数,增强了对红外光谱的吸收.     

Low dimensional silicon structures for photonic and sensor applications

We review here our work on the photonic and sensor applications of nanostructured silicon. As we change the dimensionality of silicon very fascinating and new optical properties of the material appear. Light sources, modulators, waveguides, logical gates are a few examples of the various photonic devices which have been developed based on silicon nanocrystals. Needless to say, all these devices rely on quantum confinement and on the interplay between bulk and surface properties. Size effects and surface reconstructions are two critical issues which one has to master to employ silicon nanocrystals. Finally the use of silicon nanocrystals to develop innovative sensing mechanisms to reveal biomolecules or poisoning gasses will be discussed.     

Catalytic synthesis and photoluminescence of needle-shaped 3C-SiC nanowires

Needle-shaped 3C-SiC nanowires were grown from commercially available SiC powders in a thermal evaporation process with iron as catalyst. A strong broad photoluminescence peak located around 450 nm was observed at room temperature, which may be ascribed to quantum size effects of nanomaterials. Needle-shaped 3C-SiC nanowires may have great potential applications such as blue-green light-emitting diodes and display devices.     

3C-SiC Heteroepitaxial Growth on Silicon: The Quest for Holy Grail

For a long time now, 3C-SiC has attracted attention of the semiconductor community due to its very interesting properties. The lack of commercial 3C-SiC seeds for epitaxy has forced researchers to prospect for different host materials in order to grow heteroepitaxial thin layers. Because of the obvious economical and technical advantages, silicon is a very attractive substrate so that more than 90% of the thin 3C-SiC heteroepitaxial films are grown on such seed. However, the obstacles to overcome, mainly lattice and thermal mismatch, are challenging. This article reviews the numerous attempts for growing high quality 3C-SiC heteroepitaxial layers on silicon substrate. The various aspects of the heteroepitaxial growth, from substrate carbonization to epitaxy, are discussed as a function of growth parameters. The difficulties encountered and the proposed solutions are described. Perspectives of this heteroepitaxial system are proposed.