Size‐Dependent Oxidation of Monodisperse Silicon Nanocrystals with Allylphenylsulfide Surfaces

The synthesis and characterization of size‐separated silicon nanocrystals functionalized with a heteroatom‐substituted organic capping group, allylphenylsulfide, via photochemical hydrosilylation are described for the first time. These silicon nanocrystals form colloidally stable and highly photoluminescent dispersions in non‐polar organic solvents with an absolute quantum yield as high as 52% which is 20% above that of the allylbenzene analogue. Solutions of the size‐separated fractions are characterized over time to monitor the effect of aging in air by following the change of their photoluminescence and absolute quantum yields, supplemented by transmission electron microscopy.     

Direct Bandgap Silicon: Tensile‐Strained Silicon Nanocrystals

Silicon, a semiconductor underpinning the vast majority of microelectronics, is an indirect‐gap material and consequently is an inefficient light emitter. This hampers the ongoing worldwide effort towards the integration of optoelectronics on silicon wafers. Even though silicon nanocrystals are much better light emitters, they retain the indirect‐gap nature. Here, we propose a solution to this long‐standing problem: silicon nanocrystals can be transformed into a material with fundamental direct bandgap via a concerted action of quantum confinement and tensile strain. We document this transformation by DFT calculations mapping the E( k ) band‐structure of Si nanocrystals. The experimental proofs are then given firstly by a 10 000× increase in the photon emission rate of strained silicon nanocrystals together with their altered absorbance spectra, both of which point to direct dipole‐allowed transitions, secondly by single nanocrystal spectroscopy, confirming reduced phonon energies and thus the presence of tensile strain, and lastly by photoluminescence studies under external hydrostatic pressure.     

Colloidally Stable Silicon Nanocrystals with Near‐Infrared Photoluminescence for Biological Fluorescence Imaging

Luminescent silicon nanocrystals (ncSi) are showing great promise as photoluminescent tags for biological fluorescence imaging, with size‐dependent emission that can be tuned into the near‐infrared biological window and reported lack of toxicity. Here, colloidally stable ncSi with NIR photoluminescence are synthesized from (HSiO_1.5)_n sol–gel glasses and are used in biological fluorescence imaging. Modifications to the thermal processing conditions of (HSiO_1.5)_n sol–gel glasses, the development of new ncSi oxide liberation chemistry, and an appropriate alkyl surface passivation scheme lead to the formation of colloidally stable ncSi with photoluminescence centered at 955 nm. Water solubility and biocompatibility are achieved through encapsulation of the hydrophobic alkyl‐capped ncSi within PEG‐terminated solid lipid nanoparticles. Their applicability to biological imaging is demonstrated with the in‐vitro fluorescence labelling of human breast tumor cells.     

Progress in the Light Emission of Colloidal Semiconductor Nanocrystals

Some 25 years ago it was found that semiconductor nanocrystals emitted light. Since then tremendous progress has been made with respect to increasing the emission quantum yields, extending the spectral range that may be addressed, from the UV across to the near infrared, and improving the color purity. Here some major lines in these developments are reviewed, touching on milestones as well as on the principles of the most successful preparative approaches.     

Core/Shell Semiconductor Nanocrystals

Colloidal core/shell nanocrystals contain at least two semiconductor materials in an onionlike structure. The possibility to tune the basic optical properties of the core nanocrystals, for example, their fluorescence wavelength, quantum yield, and lifetime, by growing an epitaxial‐type shell of another semiconductor has fueled significant progress on the chemical synthesis of these systems. In such core/shell nanocrystals, the shell provides a physical barrier between the optically active core and the surrounding medium, thus making the nanocrystals less sensitive to environmental changes, surface chemistry, and photo‐oxidation. The shell further provides an efficient passivation of the surface trap states, giving rise to a strongly enhanced fluorescence quantum yield. This effect is a fundamental prerequisite for the use of nanocrystals in applications such as biological labeling and light‐emitting devices, which rely on their emission properties. Focusing on recent advances, this Review discusses the fundamental properties and synthesis methods of core/shell and core/multiple shell structures of II–VI, IV–VI, and III–V semiconductors.

     

Alkyl Passivation and Amphiphilic Polymer Coating of Silicon Nanocrystals for Diagnostic Imaging

A method to produce biocompatible polymer‐coated silicon nanocrystals for medical imaging is shown. Silica‐embedded Si nanocrystals are formed by HSQ thermolysis. The nanocrystals are then liberated from the oxide and terminated with Si–H bonds by HF etching, followed by alkyl monolayer passivation by thermal hydrosilylation. The Si nanocrystals have an average diameter of 2.1 nm ± 0.6 nm and photoluminesce with a peak emission wavelength of 650 nm, which lies within the transmission window of 650–900 nm that is useful for biological imaging. The hydrophobic Si nanocrystals are then coated with an amphiphilic polymer for dispersion in aqueous media with the pH ranging between 7 and 10 and an ionic strength between 30 mM and 2 M , while maintaining a bright and stable photoluminescence and a hydrodynamic radius of only 20 nm. Fluorescence imaging of polymer‐coated Si nanocrystals in biological tissue is demonstrated, showing the potential for in vivo imaging.     

ZnS纳米晶的无膦法制备及表征

以硬脂酸锌和硫脲作为锌源与硫源,无膦绿色溶剂油酸（OA）作为单一的反应溶剂和包裹剂,利用一步合成路线制备ZnS纳米晶,实验结果表明,该方法制备出的ZnS纳米晶为立方闪锌矿结构,ZnS纳米晶的形成过程是典型的Ostwald熟化过程。改变反应条件不仅可以调控纳米晶的成核和生长,而且对其光学性能也有重要影响。     

微应变诱导各向异性硅纳米晶形成及其光学特性

通过磁控溅射技术制备了非晶态富硅的碳化硅锗(Si_(1-x-y)Ge_xC_y)薄膜,经过后续高温热处理,形成各向异性硅纳米晶,其微结构和光学特性由高分辨电镜、光致发光及光吸收实验进行表征,研究了各向异性应变对硅纳米晶形态和光学特性的影响,阐述了各向异性硅纳米晶的形成机理。研究表明,在各向异性应变能的诱导下硅纳米晶沿着〈002〉、〈113〉和〈220〉取向择优生长,形成具有多形态的硅纳米晶,显著改变了其能级结构,在2.57eV和2.64eV的位置硅纳米晶存在PL发射光谱,光吸收波段明显增加,可以同时吸收从红外到紫外(2.57eV,1.89eV,1.2eV和0.96eV)的光子,且光吸收范围随硅锗(RS/G)比例可调,故有望提高光伏电池的光量子产额。     

一锅煮法制备硫化镉和碲化镉纳米晶及其性能表征

以三辛基膦为配位溶剂,采用一锅煮的方法制备出分散性较好的硫化镉和碲化镉纳米晶,通过改变反应温度和硫前驱体可制备出不同形貌和结构的硫化镉纳米晶。用此方法也可制备出碲化镉网状纳米晶。测试结果表明,这两种纳米晶都具有较好的晶型。     

Microscopic Origin of the Fast Blue‐Green Luminescence of Chemically Synthesized Non‐oxidized Silicon Quantum Dots

The microscopic origin of the bright nanosecond blue‐green photoluminescence (PL), frequently reported for synthesized organically terminated Si quantum dots (Si‐QDs), has not been fully resolved, hampering potential applications of this interesting material. Here a comprehensive study of the PL from alkyl‐terminated Si‐QDs of 2–3 nm size, prepared by wet chemical synthesis is reported. Results obtained on the ensemble and those from the single nano‐object level are compared, and they provide conclusive evidence that efficient and tunable emission arises due to radiative recombination of electron–hole pairs confined in the Si‐QDs. This understanding paves the way towards applications of chemical synthesis for the development of Si‐QDs with tunable sizes and bandgaps.     

功能氧化物纳米晶的液相法制备

对适用于制备有序结构纳米功能材料或器件的功能性氧化物纳米晶的液相法制备技术的最新研究进展进行了综述,从晶粒成核生长的物理化学原理出发,分析了各种制备方法的特点,并在此基础上指出了高分散、单一尺寸和形状可控的功能性氧化物纳米晶制备研究中仍需解决的问题和发展的方向.