A general synthesis of high-quality inorganic nanocrystals via a two-phase method

A two-phase method is exploited to prepare many kinds of nearly monodisperse, highly crystalline, size- and shape-controlled, surface-property-tunable inorganic nanocrystals, such as metal, semiconducting, magnetic, dielectric, and rare earth nanocrystals. The reaction of the two-phase system happens at the interface between the oil (nonpolar) and water (polar) phases and the interface is an exclusive site for both nucleation and growth. Interestingly, many solvent pairs with a clear interface can be applied to synthesize inorganic nanocrystals successfully. Generally, as-prepared nanocrystals with organic ligands are soluble in nonpolar solvents. Furthermore, exchange of ligands can also be realized readily and the final nanocrystals can be soluble in polar solvents. This two-phase method is a simple, reproducible, and general route and is becoming as powerful an approach as other solution-based synthetic approaches to high-quality inorganic nanocrystals.     

Strong correlation between the cation ordering and magnetic properties of anodically electrodeposited Mn–Co–O nanocrystals

The rock salt-to-spinel structural transformation that occurs in anodically electrodeposited Mn–Co–O nanocrystals involves a rearrangement of Mn/Co cations from octahedral interstices to tetrahedral interstices. The cation ordering process leads to distinct magnetic properties. Curie temperature (T _C) and blocking temperature (T _B) increase dramatically with annealing temperature (200–400 °C), while the corresponding change in particle size for the oxide nanocrystals is rather small. A strong correlation between the magnetic properties and the cation ordering degree in annealed Mn–Co–O nanocrystals was established. These unique magnetic properties can be attributed to the magnetic moment changes induced by Mn/Co cation ordering from octahedral interstices to tetrahedral interstices in the annealed Mn–Co oxide nanocrystals.     

Electronic ferroelectricity from charge ordering in RFe(2)O(4)

We report our recent discovery of novel ferroelectricity arising from the polar ordering of Fe(3)+ and Fe(2)+ in a mixed valence triangular lattice oxide LuFe(2)O(4), where the electric polarization is not a result of ionic displacement. The polar ordering of Fe(3)+ and Fe(2)+ was confirmed with a resonant x-ray scattering study in SPring-8. The origin of such ordering is the competitive interaction between Fe(3)+ and Fe(2)+ in the triangular lattice, i.e., the charge frustration. The polar superlattice of Fe(3)+ and Fe(2)+ develops below 350 K, where the electric polarization appears. The ferroelectricity arising from the polar charge ordering or the polar electron distribution may have great potential for the future application of ferroelectrics.     

Interface-induced nucleation, orientational alignment and symmetry transformations in nanocube superlattices

The self-assembly of colloidal nanocrystals into ordered superstructures depends critically on the shape of the nanocrystal building blocks. We investigated the self-assembly of cubic PbSe nanocrystals from colloidal suspensions in real-time using in situ synchrotron-based X-ray scattering. We combined small-angle and wide-angle scattering to investigate the translational ordering of nanocrystals and their orientational ordering in the lattice sites, respectively. We found that cubic PbSe nanocrystals assembled into a face-up (i.e., ?100? normal to the interface) configuration at the liquid/substrate interface whereas nanocubes at the liquid/air interface assume a corner-up (i.e., ?111? normal to the interface) configuration. The latter nanocrystal superlattice displays polymorphism as a function inter-NC separation distance. We explain the observed superlattice structure polymorphs in terms of the interactions directing the self-assembly. Insights into the directed self-assembly of superlattices gained from this study have important implication on the future development of nanocrystals as building blocks in artificial solids.     

Size-dependent ultrafast magnetization dynamics in iron oxide (Fe3O4) nanocrystals

Optically induced ultrafast demagnetization and its recovery in superparamagnetic colloidal iron oxide (Fe3O4) nanocrystals have been investigated via time-resolved Faraday rotation measurements. Optical excitation with near-infrared laser pulse resulted in ultrafast demagnetization in approximately 100 fs via the destruction of ferrimagnetic ordering. The degree of demagnetization increased with the excitation density, and the complete demagnetization reached at approximately 10% excitation density. The magnetization recovered on two time scales, several picoseconds and hundreds of picoseconds, which can be associated with the initial reestablishment of the ferrimagnetic ordering and the electronic relaxation back to the ground state, respectively. The amplitude of the slower recovery component increased with the size of the nanocrystals, suggesting the size-dependent ferrimagnetic ordering throughout the volume of the nanocrystal.     

Self Organization of Inorganic Nanocrystals： anew Challenge for a New Physics

2008年10月23—27日，“2008第七届中国纳米科技（武汉）研讨会”在湖北大学隆重召开。欧洲科学院院士M．P．Pileni应邀出席并做了题为“Self organization of inorganic nanocrystals：a new challenge for a new physics”的大会特邀主题报告，摘要如下：     

Large triangular single crystals formed by mild annealing of self-organized silver nanocrystals

Nanocrystal organizations represent a new generation of materials with specific properties compared with those of isolated nanocrystals and of the bulk material. Here, we present the first intrinsic crystalline growth properties of highly ordered mono- and multilayers of 5 nm silver nanocrystals. Triangular single crystals with face-centred-cubic structures are obtained by annealing the ordered nanocrystals under atmospheric pressure at 50 degrees C. The triangles are mixed with well-crystallized coalesced particles of various shapes. Their size depends on the initial nanocrystal ordering range on the substrate, which is local on amorphous carbon and highly extended on highly oriented pyrolitic graphite (HOPG). Hence, the single-crystal size is larger on HOPG than on amorphous carbon. These observations show that the crystalline growth properties of silver nanocrystals can be tailored by controlling their organization. Furthermore, on HOPG an epitaxial orientation of the triangles is observed.     

Solvothermal route to CdS nanocrystals

We developed a facile solvothermal route to various CdS nanocrystals in diethylenetriamine. Well-crystalline CdS nanowires and flower-like CdS nanocrystals were obtained at temperatures 220°C and 200°C, respectively. The nanowires have smooth surfaces and are grown along polar [0?0?0?1] direction. The flower-like CdS nanocrystals have hierarchical architectures composed of fine nanowires with lengths up to 10?µm. The effects of temperature on the morphology and structure of the final products are also investigated. The experimental observations indicates that high temperature facilitates fabricate CdS nanocrystals with regular morphology and excellent crystallisation. Moreover, it is found that organic solvent diethylenetriamine plays an important role in the growth of CdS nanocrystals. Diethylenetriamine can bind Cd²⁺ ions to form intermediate complex, resulting in final CdS nanocrystals with regular morphologies.     

Resonant coupling between surface vibrations and electronic states in silicon nanocrystals at the strong confinement regime

A striking correlation between infrared photoinduced absorption spectra and the photoluminescence from silicon nanocrystals indicates that quantized electronic sublevels of the nanocrystals are resonantly coupled to surface vibrational modes via a polarization field produced by coherent longitudinal polar vibrations. Our experimental results and model support the assumption that the mechanism responsible for the efficient photoluminescence from silicon nanocrystals should be assigned to inhibition of nonradiative channels rather than enhancement of radiative channels.     

Complete phase transfer of hydrophobic magnetic nanocrystals into aqueous phase

The effective phase transfer of hydrophobic nanocrystals synthesized in nonpolar solvents into polar solvents remains great challenge for their nanomedicinal applications. To resolve this issue, the exsiting strategies for phase transfer of nanocrystals in organic solvents use amphiphilic compounds or lipids as imperative moieties by ligand exchange and encapsulations. Ligand exchange involves by exchanging the hydrophobic molecules with bifunctional compounds and small size of molecules is coordinated with functional molecules to increase the steric repulsion forces between boundary of water and nanoparticles. However, the yield of phase transferred nanocrystals from hydrophobic nonpolar to hydrophilic polar phase is exceptionally low, and sometime irreversible desorption of replaced ligands leads to agglomeration and aggregation. Also, their intrinsic physiochemical properties are easily influenced by surrounding ion species or pH when the particles are suspended in phosphate saline buffer (PBS). Moreover, conjugation of bioactive molecules often leads to colloidal instability in PBS because of the hydrodynamic nature of the amphiphilic molecules on the surfaces of nanoparticles. Here we report a robust and simple post-synthetic surface modification procedure of hydrophobic nanoparticles to remove the original surface-bound carboxylic acid and increase the water-solubility for nanomedicinal applications.     

Comparison of carrier multiplication yields in PbS and PbSe nanocrystals: the role of competing energy-loss processes

Infrared band gap semiconductor nanocrystals are promising materials for exploring generation III photovoltaic concepts that rely on carrier multiplication or multiple exciton generation, the process in which a single high-energy photon generates more than one electron-hole pair. In this work, we present measurements of carrier multiplication yields and biexciton lifetimes for a large selection of PbS nanocrystals and compare these results to the well-studied PbSe nanocrystals. The similar bulk properties of PbS and PbSe make this an important comparison for discerning the pertinent properties that determine efficient carrier multiplication. We observe that PbS and PbSe have very similar biexciton lifetimes as a function of confinement energy. Together with the similar bulk properties, this suggests that the rates of multiexciton generation, which is the inverse of Auger recombination, are also similar. The carrier multiplication yields in PbS nanocrystals, however, are strikingly lower than those observed for PbSe nanocrystals. We suggest that this implies the rate of competing processes, such as phonon emission, is higher in PbS nanocrystals than in PbSe nanocrystals. Indeed, our estimations for phonon emission mediated by the polar Fr?hlich-type interaction indicate that the corresponding energy-loss rate is approximately twice as large in PbS than in PbSe.     

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.     

Epitaxial self-alignment: A new route to hybrid active plasmonic nanostructures

Concepts of lateral ordering of epitaxial semiconductor quantum dots (QDs) are for the first time transferred to hybrid nanostructures for active plasmonics. We review our recent research on the self-alignment of epitaxial nanocrystals of In and Ag on ordered one-dimensional In(Ga)As QD arrays and isolated QDs by molecular beam epitaxy. By changing the growth conditions the size and density of the metal nanocrystals are easily controlled and the surface plasmon resonance wavelength is tuned over a wide range in order to match the emission wavelength of the QDs. Photoluminescence measurements reveal large enhancement of the emitted light intensity due to plasmon enhanced emission and absorption down to the single QD level.     

A facile and general approach to polynary semiconductor nanocrystals via a modified two-phase method

Cu(2)ZnSnS(4) nanocrystals were synthesized through a modified two-phase method and characterized with transmission electron microscopy (TEM), powder x-ray diffraction (XRD) and UV-vis spectroscopy. Inorganic metal salts were dissolved in the polar solvent triethylene glycol (TEG) and then transferred into the non-polar solvent 1-octadecene (ODE) by forming metal complexes between metal ions and octadecylamine (ODA). Since nucleation and growth occur in the single phase of the ODE solution, nanocrystals could be produced with qualities similar to those obtained through the hot-injection route. Balancing the reactivity of the metal precursors is a key factor in producing nanocrystals of a single crystalline phase. We found that increasing the reaction temperature increases the reactivity of each of the metal precursors by differing amounts, thus providing the necessary flexibility for obtaining a balanced reactivity that produces the desired product. The versatility of this synthesis strategy was demonstrated by extending it to the production of other polynary nanocrystals such as binary (CuS), ternary (CuInS(2)) and pentanary (Cu(2 - x)Ag(x)ZnSnS(4)) nanocrystals. This method is considered as a green synthesis route due to the use of inorganic metal salts as precursors, smaller amounts of coordinating solvent, shorter reaction time and simpler post-reaction treatment.     

Surfactant-free CuInSe₂ nanocrystals transformed from In₂Se₃ nanoparticles and their application for a flexible UV photodetector

In(2)Se(3) nanoparticles were synthesized in an aqueous solution without using any surfactant and then chemically transformed into CuInSe(2) nanocrystals. The transformation was thermodynamically favorable and fast. The 93% production yield in mild reaction conditions allowed mass production of the CuInSe(2) nanocrystals. By the virtue of the surface charges, the CuInSe(2) nanocrystals were well dispersed in polar solvents. The surfactant-free nanocrystals enabled the formation of semiconducting CuInSe(2) films on a flexible polymer substrate without any thermal treatment. We took advantage of this to fabricate a flexible UV photodetector. The current and sensitivity of the devices could be improved by utilizing CuInSe(2) nanocrystals annealed at 160?°C in the reaction batch. On bending test, the detection sensitivity remained the same until the bending radius was reduced down to 4 mm. The dynamic response of the film device was stable and reproducible during light illumination (350 nm).     

Cation disordering by rapid crystal growth in olivine-phosphate nanocrystals

On the basis of Pauling's first rule for ionic bonding, the coordination number of cations with oxygen anions can be determined by comparison of their relative ionic size ratio. In contrast to simple oxides, various site occupancies by multicomponent cations with similar sizes usually occur in complex oxides, resulting in distinct physical properties. Through an unprecedented combination of in situ high-temperature high-resolution electron microscopy, crystallographic image processing, geometric phase analysis, and neutron powder diffraction, we directly demonstrate that while the initial crystallites after nucleation during crystallization have a very high degree of ordering, significant local cation disordering is induced by rapid crystal growth in Li-intercalation metal-phosphate nanocrystals. The findings in this study show that control of subsequent crystal growth during coarsening is of great importance to attain a high degree of cation ordering, emphasizing the significance of atomic-level visualization in real time.     

Electrically driven phase transition in magnetite nanostructures

Magnetite (Fe3O4), an archetypal transition-metal oxide, has been used for thousands of years, from lodestones in primitive compasses to a candidate material for magnetoelectronic devices. In 1939, Verwey found that bulk magnetite undergoes a transition at TV approximately 120 K from a high-temperature 'bad metal' conducting phase to a low-temperature insulating phase. He suggested that high-temperature conduction is through the fluctuating and correlated valences of the octahedral iron atoms, and that the transition is the onset of charge ordering on cooling. The Verwey transition mechanism and the question of charge ordering remain highly controversial. Here, we show that magnetite nanocrystals and single-crystal thin films exhibit an electrically driven phase transition below the Verwey temperature. The signature of this transition is the onset of sharp conductance switching in high electric fields, hysteretic in voltage. We demonstrate that this transition is not due to local heating, but instead is due to the breakdown of the correlated insulating state when driven out of equilibrium by electrical bias. We anticipate that further studies of this newly observed transition and its low-temperature conducting phase will shed light on how charge ordering and vibrational degrees of freedom determine the ground state of this important compound.     

Pt超晶格组装和对称性转化的原位液体TEM研究（英文）

具有精确控制结构的二维(2D)纳米晶超晶格在光子、等离子体和光电子应用中具有重要意义,并已被广泛研究,但在理解超晶格的形成机理和发展途径方面仍然存在挑战.本文采用液体池透射电镜技术原位观察了铂二维超晶格的形成和六配位到四配位的局部相转化过程.胶体纳米晶在溶液中流动时,通过纳米晶的收缩和重排或者纳米晶的附着形成长程有序的六配位超晶格.当纳米晶的形貌由截角八面体转变为立方体时,六配位超晶格重新排列为四配位立方超晶格.此外,我们的观察和定量分析结果表明,从六配位到四配位的相变主要是由垂直{100}面之间的强范德华相互作用引起的.实时追踪2D立方体超晶格的形成可以为超晶格组装和稳定机制提供独特的见解.