Silicon Nanocrystals: Size Matters

This paper reviews new approaches to size‐controlled silicon‐nanocrystal synthesis. These approaches allow narrowing of the size distribution of the nanocrystals compared with those obtained by conventional synthesis processes such as ion implantation into SiO₂ or phase separation of sub‐stoichiometric SiO_x layers. This size control is realized by different approaches to introducing a superlattice‐like structure into the synthesis process, by velocity selection of silicon aerosols, or by the use of electron lithography and subsequent oxidation processes. Nanocrystals between 2 and 20 nm in size with a full width at half maximum of the size distribution of 1 nm can be synthesized and area densities above 10¹² cm^–2 can be achieved. The role of surface passivation is elucidated by comparing Si/SiO₂ layers with superlattices of fully passivated silicon nanocrystals within a SiO₂ matrix. The demands on silicon nanocrystals for various applications such as non‐volatile memories or light‐emitting devices are discussed for different size‐controlled nanocrystal synthesis approaches.     

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.     

Small Silicon,Big Opportunities: The Development and Future of Colloidally‐Stable Monodisperse Silicon Nanocrystals

Nanomaterials are becoming increasingly widespread in consumer technologies, but there is global concern about the toxicity of nanomaterials to humans and the environment as they move rapidly from the research laboratory to the market place. With this in mind, it makes sense to intensify the nanochemistry community's global research effort on the synthesis and study of nanoparticles that are purportedly “green”. One potentially green nanoparticle that seems to be a most promising candidate in this context is silicon, whose appealing optical, optoelectronic, photonic, and biomedical attributes are recently gaining much attention. In this paper, we outline some of our recent contributions to the development of the growing field of silicon nanocrystals (ncSi) in order to stress the importance of continued study of ncSi as a green alternative to the archetypal semiconductor nanocrystals like CdSe, InAs, and PbS. While a variety of developments in synthetic methods, characterization techniques, and applications have been reported in recent years, the ability to prepare colloidally‐stable monodisperse ncSi samples may prove to have the largest impact on the field, as it opens the door to study and access the tunable size‐dependent properties of ncSi. Here, we summarize our recent contributions in size‐separation methods to achieve monodisperse samples, the characterization of size‐dependant property trends, the development of ncSi applications, and their potential impact on the promising future of ncSi.     

Silicon Nanocrystals: Photosensitizers for Oxygen Molecules

Molecular oxygen plays an important role in many of the chemical reactions involved in the synthesis of biological life. In this review, we explore the interaction between O₂ and silicon nanocrystals, which can be employed in the photosynthesis of singlet oxygen. We demonstrate that nanoscale Si has entirely new properties owing to morphological and quantum size effects, i.e., large accessible surface areas and excitons of variable energies and with well‐defined spin structures. These features result in new emerging functionality for nanoscale silicon: it is a very efficient spin‐flip activator of O₂, and therefore, a chemically and biologically active material. This whole effect is based on energy transfer from long‐lived electronic excitations confined in Si nanocrystals to surrounding O₂ via the exchange of single electrons of opposite spin, thus enabling the spin‐flip activation of O₂. Further, we discuss the implications of these findings for physics, chemistry, biology, and medicine.     

Photoluminescence from Silicon Nanocrystals in Encapsulating Materials

Naturally oxidized freestanding silicon nanocrystals（Si NCs） are incorporated in commonly used encapsulating materials to explore the photoluminescent application of Si NCs in device structures such as solid-state lighting light-emitting diodes（LEDs） and solar cells.The quantum yield of Si NCs before the incorporation has reached about 45%at the excitation wavelength of 370 nm without any special surface modification.It is found that medium loadings,e.g.,5 wt%of Si NCs in encapsulating materials help to obtain high external quantum efficiency（EQE） of the mixtures of Si NCs and encapsulating materials.The curing of encapsulating materials significantly reduces EQE.Among all the encapsulating materials investigated in this work,siliconeOE6551 enables the highest EQE(21%at excitation wavelengthλ_ex = 370 nm) after curing.Based on current findings,we have discussed the continuous efforts to advance the photoluminescent application of Si NCs.     

硅纳米晶体在太阳电池中的应用

硅纳米晶体的电子和光学特性使其在改善太阳电池的性能方面扮演着重要角色。目前,硅纳米晶体在太阳电池中应用的主要方式有利用纯硅纳米晶体薄膜制作太阳电池、硅纳米晶体与无机(氧化硅、氮化硅或碳化硅等)或有机(P3HT)薄膜基质结合构成复合结构太阳电池、硅纳米晶体与碳纳米结构(富勒烯或单壁碳纳米管)结合形成复合结构、硅纳米晶体与传统的染料敏化太阳电池结合、利用硅纳米晶体的减反射或下转换作用将硅纳米晶体与体硅太阳电池结合。硅纳米晶体也有可能在新概念太阳电池如多激子太阳电池、中间带太阳电池和热载流子太阳电池中得到应用。     

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.     

镶嵌纳米晶硅的氧化硅薄膜微观结构调整及其光吸收特性

采用等离子体化学气相沉积(PECVD)技术在不同N_2O流量条件下制备了镶嵌有纳米晶硅(nc-Si)的富硅氧化硅(SiOx)薄膜,利用透射电镜(TEM),X射线衍射分析(XRD),傅里叶变换红外(FTIR)和透射光谱技术研究了薄膜中的氢含量和氧含量变化及其对薄膜晶化度、薄膜键合结构和光吸收特性的影响。结果表明,薄膜由nc-Si粒子和非晶SiOx组成,为混合相结构。nc-Si的生长与氧化反应的竞争决定了薄膜微观结构、键合特性以及光吸收特性。随着N_2O流量的增加,薄膜的晶粒尺寸逐渐减小。晶界区过渡晶硅的比例减少,晶粒界面随之消失,带隙呈持续增加趋势。该实验结果为ncSi/SiOx薄膜在新型太阳电池中的应用提供了基础数据。     

Light‐Emitting Silicon Nanocrystals from Laser Pyrolysis

Crystalline Si nanoparticles with diameters between 2.5 and 20 nm are prepared by CO₂‐laser‐induced decomposition of silane in a gas flow reactor. A small portion of the products created in the reaction zone is extracted through a nozzle into a high‐acuum apparatus to form a freely propagating molecular beam of clusters and nanoparticles that can be deposited on suitable substrates. The strong visible photoluminescence (PL) of the Si nanocrystals is studied as a function of their size, and as a function of the time for which they are exposed to air. All observations can be explained on the basis of quantum confinement as the only origin of the PL. Chemical methods are exploited to modify the surface of the Si nanoparticles and to reduce their size, thus shifting their PL to shorter wavelengths. With this technique, the Si nanoparticles, collected in much larger quantities in the filter of the flow reactor, can be made strongly luminescent so that they may be used for various applications.     

The Microemulsion Synthesis of Hydrophobic and Hydrophilic Silicon Nanocrystals

Silicon nanocrystals, also called quantum dots, have unique optical properties when in the quantum‐confinement regime. These optical properties make silicon nanocrystals promising materials for a wide variety of applications ranging from optoelectronic devices to fluorophores in biological imaging. A liquid‐phase synthetic approach is reported using surfactant molecules to control particle growth, producing highly monodisperse silicon particles. The surface of the nanocrystals are capped by functional organic molecules that passivate and protect the silicon particles from oxidation, enabling the particles to be used in hydrophobic and hydrophilic applications. The use of hydrophilic silicon quantum dots as optical probes is illustrated by the imaging of Vero cells.     

Size Controlled Synthesis of Silicon Nanocrystals Using Cationic Surfactant Templates

Alkyl‐terminated silicon nanocrystals (Si NCs) are synthesized at room temperature by hydride reduction of silicon tetrachloride (SiCl₄) within inverse micelles. Highly monodisperse Si nanocrystals with average diameters ranging from 2 to 6 nm are produced by variation of the cationic quaternary ammonium salts used to form the inverse micelles. Transmission electron microscopy imaging shows that the NCs are highly crystalline, while FTIR spectra confirm that the NCs are passivated by covalent attachment of alkanes, with minimal surface oxidation. UV‐vis absorbance and photoluminescence spectroscopy show significant quantum confinement effects, with moderate absorption in the UV spectral range, and a strong blue emission with a marked dependency on excitation wavelength. The photoluminescence quantum yield (Φ) of the Si NCs exhibits an inverse relationship with the mean NC diameter, with a maximum of 12% recorded for 2 nm NCs.     

Density Functional Theory Study on the Oxidation of Hydrosilylated Silicon Nanocrystals

As a leading surface modification approach,hydrosilylation enables freestanding silicon nanocrystals(Si NCs) to be well dispersed in a desired medium.Although hydrosilylation-induced organic layers at the NC surface may somehow retard the oxidation of Si NCs,oxidation eventually occurs to Si NCs after relatively long time exposure to air.We now investigated the oxidation of hydrosilylated Si NCs in the frame work of density functional theory(DFT).Three oxygen configurations that may be introduced by the oxidation of a Si NC are considered.It is found that a hydrosilylated Si NC is less prone to oxidation than a fully H-passivated Si NC in the point of view of thermodynamics.At the ground state,backbond oxygen(BBO) and hydroxyl(OH) hardly change the gap between the highest occupied molecular orbital(HOMO) and the lowest unoccupied molecular orbital(LUMO) of a hydrosilylated Si NC.At the excited state,the decrease in the HOMO-LUMO gap induced by the introduction of doubly bonded oxygen(DBO) is more significant than that induced by the introduction of BBO or OH.We have correlated the changes in the optical absorption(emission) of a hydrosilylated Si NC after oxidation to those of the HOMO—LUMO gap at the ground state(excited state).