Template-assisted synthesis of shape-controlled Rh2P nanocrystals

When reacted with trioctylphosphine at approximately 360 degrees C, rhodium nanocrystals convert to rhodium phosphide Rh(2)P nanocrystals. Careful control over synthetic variables, such as temperature, stabilizing ligands, and cosolvents, can result in Rh(2)P nanocrystals with shapes that reflect the Rh nanocrystal templates. Accordingly, Rh nanocrystals with multipod, cube- and triangle-derived shapes convert to Rh(2)P nanocrystals that maintain the shape of their Rh precursors. Both dense and hollow Rh(2)P nanocrystals can be generated using a single unified chemical conversion strategy. These empirical guidelines for generating a morphologically diverse library of Rh(2)P nanocrystals provide important insights into shape conservation using nanocrystal templates and will likely be portable to other multielement systems for which rigorous shape-controlled synthesis remains challenging.     

Photoexcited Carrier Transfer in CuInS2 Nanocrystal Assembly by Suppressing Resonant-Energy Transfer

High-density assemblies or superlattice structures composed of colloidal semiconductor nanocrystals have attracted attention as key materials for next-generation photoelectric conversion devices such as quantum-dot solar cells. In these nanocrystal solids, unique transport and optical phenomena occur due to quantum coupling of localized energy states, charge-carrier hopping, and electromagnetic interactions among closely arranged nanocrystals. In particular, the photoexcited carrier dynamics in nanocrystal solids is important because it significantly affects various device parameters. In this study, we report the photoexcited carrier dynamics in a solid film of CuInS₂ nanocrystals, which is one of the potential nontoxic substitutes with Cd- and Pb-free compositions. Meanwhile, these subjects have been extensively studied in nanocrystal solids formed by CdSe and PbS systems. A carrier-hopping mechanism was confirmed using temperature-dependent photoluminescence spectroscopy, which yielded a typical value of the photoexcited carrier-transfer rate of (2.2±0.6)×10⁷ s⁻¹ by suppressing the influence of the excitation-energy transfer.     

Artificial atom solids based on metal nanocrystals: Formation and electrical properties

Metal nanocrystals can behave as “artificial atoms” due to their diameter-dependent single electron charging energy. Organically passivated nanocrystals with narrow size distributions can self-assemble into ordered arrays, offering the possibility of artificial atom solids with unique collective electronic properties, derived from both the size-dependent electronic properties of the individual nanocrystal cores and the inter-nanocrystal electronic coupling mechanisms. We review our recent progress on probing the electronic properties of artificial atom solids via variable temperature charge transport measurements on laterally contacted arrays of metal nanocrystals, together with development of combined synthesis and processing routes to manipulate these properties.     

A time-domain view of charge carriers in semiconductor nanocrystal solids

The movement of charge carriers within semiconductor nanocrystal solids is fundamental to the operation of nanocrystal devices, including solar cells, LEDs, lasers, photodetectors, and thermoelectric modules. In this perspective, we explain how recent advances in the measurement and simulation of charge carrier dynamics in nanocrystal solids have led to a more complete picture of mesoscale interactions. Specifically, we show how time-resolved optical spectroscopy and transient photocurrent techniques can be used to track both equilibrium and non-equilibrium dynamics in nanocrystal solids. We discuss the central role of energetic disorder, the impact of trap states, and how these critical parameters are influenced by chemical modification of the nanocrystal surface. Finally, we close with a forward-looking assessment of emerging nanocrystal systems, including anisotropic nanocrystals, such as nanoplatelets, and colloidal lead halide perovskites.

Time-domain spectroscopy and transient photocurrent techniques have revealed new understanding of mesoscale carrier dynamics in nanocrystal solids, including the role of energetic disorder, interactions with trap states, and nonequilibrium dynamics     

Lead Sulfide Nanocrystal-Polymer Composites for Optoelectronic Applications

Summary: Nanocomposite films were prepared by two methods in which lead sulfide (PbS) nanocrystals were contained in an organic matrix. One method used a wet chemical synthesis of the nanocrystals in the direct presence of a polymer, where the polymer controlled nanocrystal growth. The second method was gaseous deposition of nanocrystals into the organic phase. The two methods were similar in that the nanocrystals in the composites were free from surfactant capping layers that otherwise would add an interfacial region between the nanocrystal and the organic matrix. The gaseous deposition technique had several advantages over the wet chemical synthesis in that it allowed direct control over nanocrystal size and density, improved flexibility in the choice of organic phase, and was compatible with lithographic methods.     

Structural, optical, and electrical properties of PbSe nanocrystal solids treated thermally or with simple amines

We describe the structural, optical, and electrical properties of films of spin-cast, oleate-capped PbSe nanocrystals that are treated thermally or chemically in solutions of hydrazine, methylamine, or pyridine to produce electronically coupled nanocrystal solids. Postdeposition heat treatments trigger nanocrystal sintering at approximately 200 degrees C, before a substantial fraction of the oleate capping group evaporates or pyrolyzes. The sintered nanocrystal films have a large hole density and are highly conductive. Most of the amine treatments preserve the size of the nanocrystals and remove much of the oleate, decreasing the separation between nanocrystals and yielding conductive films. X-ray scattering, X-ray photoelectron and optical spectroscopy, electron microscopy, and field-effect transistor electrical measurements are used to compare the impact of these chemical treatments. We find that the concentration of amines adsorbed to the NC films is very low in all cases. Treatments in hydrazine in acetonitrile remove only 2-7% of the oleate yet result in high-mobility n-type transistors. In contrast, ethanol-based hydrazine treatments remove 85-90% of the original oleate load. Treatments in pure ethanol strip 20% of the oleate and create conductive p-type transistors. Methylamine- and pyridine-treated films are also p-type. These chemically treated films oxidize rapidly in air to yield, after short air exposures, highly conductive p-type nanocrystal solids. Our results aid in the rational development of solar cells based on colloidal nanocrystal films.     

Study of nucleation and growth in the organometallic synthesis of magnetic alloy nanocrystals: the role of nucleation rate in size control of CoPt3 nanocrystals

High quality CoPt(3) nanocrystals were synthesized via simultaneous reduction of platinum acetylacetonate and thermodecomposition of cobalt carbonyl in the presence of 1-adamantanecarboxylic acid and hexadecylamine as stabilizing agents. The high flexibility and reproducibility of the synthesis allows us to consider CoPt(3) nanocrystals as a model system for the hot organometallic synthesis of metal nanoparticles. Different experimental conditions (reaction temperature, concentration of stabilizing agents, ratio between cobalt and platinum precursors, etc.) have been investigated to reveal the processes governing the formation of the metal alloy nanocrystals. It was found that CoPt(3) nanocrystals nucleate and grow up to their final size at an early stage of the synthesis with no Ostwald ripening observed upon further heating. In this case, the nanocrystal size can be controlled only via proper balance between the rates for nucleation and for growth from the molecular precursors. Thus, the size of CoPt(3) nanocrystals can be precisely tuned from approximately 3 nm up to approximately 18 nm in a predictable and reproducible way. The mechanism of homogeneous nucleation, evolution of the nanocrystal ensemble in the absence of Ostwald ripening, nanocrystal faceting, and size-dependent magnetic properties are investigated and discussed on the example of CoPt(3) magnetic alloy nanocrystals. The developed approach was found to be applicable to other systems, e.g., FePt and CoPd(2) magnetic alloy nanocrystals.     

Luminescence in colloidal Mn-doped semiconductor nanocrystals

Recent advances in nanocrystal doping chemistries have substantially broadened the variety of photophysical properties that can be observed in colloidal Mn²⁺-doped semiconductor nanocrystals. A brief overview is provided, focusing on Mn²⁺-doped II–VI semiconductor nanocrystals prepared by direct chemical synthesis and capped with coordinating surface ligands. These Mn²⁺-doped semiconductor nanocrystals are organized into three major groups according to the location of various Mn²⁺-related excited states relative to the energy gap of the host semiconductor nanocrystals. The positioning of these excited states gives rise to three distinct relaxation scenarios following photoexcitation. A brief outlook on future research directions is provided.     

Surface modification of cotton nanocrystals with a silane agent

The research herewith aims at obtaining cellulose nanocrystals with a reduced hydrophilic surface character using a silane with isocyanate groups (isocyanatepropyltriethoxysilane), which are very reactive to hydroxyl groups and thus, are readily able to react with the low quantity of free hydroxyl groups present in the cellulose nanocrystal surfaces, therefore, promoting surface modification. Cellulose nanocrystals were obtained by hydrochloric acid hydrolysis of cotton fiber and were characterized by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR) and solid state ²⁹Si nuclear magnetic resonance (NMR) and their morphologies were investigated by scanning and transmission electron microscopy techniques. The nanocrystals presented a needle-like geometry with a 10 nm approximate diameter and a 166 nm average length. FTIR, ²⁹Si NMR and silicon mapping images showed that nanocrystal surface chemical modification was successfully achieved. Also, the results confirm that the chemical modification occurred mainly at the nanocrystal surface, keeping the morphological integrity of the nanocrystals. The applied methodology for surface modification of the cellulose nanocrystals provided nanofillers with more appropriate surface characteristics that allow the dispersion in polymeric matrices and the adhesion at filler-matrix interface to be obtained. This may result in a better performance of these nanocrystals as reinforcing agents of hydrophobic polymer matrices.     

Room-temperature Wurtzite ZnS nanocrystal growth on Zn finger-like peptide nanotubes by controlling their unfolding peptide structures

ZnS nanocrystal, a class of wide-gap semiconductors, has shown interesting optical, electrical, and optoelectric properties via quantum confinement. For those applications, phase controls of ZnS nanocrystals and nanowires were critical to tune their physical properties to the appropriate ones. The wurtzite ZnS nanocrystal growth at room temperature is the useful fabrication; however, the most stable ZnS structure in nanoscale is the zinc blende (cubic) structure, and scientists have just begun exploring the room-temperature synthesis of the wurtzite (hexagonal) structure of ZnS nanocrystals. In this report, we applied the Zn finger-like peptides as templates to control the phase of ZnS nanocrystals to the wurtzite structure at room temperature. The peptide nanotubes, consisting of a 20 amino acids (VAL-CYS-ALA-THR-CYS-GLU-GLN-ILE-ALA-ASP-SER-GLN-HIS-ARG-SER-HIS-ARG-GLN-MET-VAL, M1 peptide) synthesized based on the peptide motif of the Influenza Virus Matrix Protein M1, could grow the wurtzite ZnS nanocrystals on the nanotube templates in solution. In the M1 protein, the unfolding process of the helical peptide motif via pH change creates a linker region between N- and C-terminated helical domains that contains a Zn finger-like Cys2His2 motif. Because the higher pH increases the uptake of Zn ions in the Cys2His2 motif of the M1 peptide by unfolding more helical domains, the pH change can essentially control the size and the number of the nucleation sites in the M1 peptides to grow ZnS nanocrystals with desired phases. Here we optimized the nucleation sites in the M1 peptides by unfolding them via pH change to obtain highly monodisperse and crystalline wurtzite ZnS nanocrystals on the template nanotubes at room temperature. This type of peptide-induced biomineralization technique will provide a clean and reproducible method to produce semiconductor nanotubes due to its efficient nanocrystal formation, and the band gaps of resulting nanotubes can also be tuned simply by phase control of ZnS nanocrystal coatings via the optimization of the unfolding peptide structures.     

胶粒晶体模板法制备三维有序大孔材料

使用胶粒模板法制备三维有序大孔材料（three-dimensional ordered macroporous material,3DOM）是一个快速发展的领域。本文综述了3DOM的合成技术,包括各种填充模板的方法、模板的去除方法以及它们在光子晶体、催化材料、生物传感器以及医学等方面的应用前景。     

Facile route to Zn-based II-VI semiconductor spheres, hollow spheres, and core/shell nanocrystals and their optical properties

A convenient chemical conversion method that allows the direct preparation of nanocrystalline ZnE (E = O, S, Se) semiconductor spheres and hollow spheres as well as their core/shell structures is reported. By using monodisperse ZnO nanospheres as a starting reactant and in situ template, ZnS, ZnSe solid and hollow nanospheres, and ZnO/ZnS and ZnO/ZnSe core/shell nanostructures have been obtained through an ultrasound-assisted solution-phase conversion process. The formation mechanism of these nanocrystals is connected with the sonochemical effect of ultrasound irradiation. The photoluminescence and electrogenerated chemiluminescence properties of the as-prepared nanocrystals were investigated.     

Interplay between size, composition, and phase transition of nanocrystalline Cr(3+)-doped BaTiO3 as a path to multiferroism in perovskite-type oxides

Multiferroics, materials that exhibit coupling between spontaneous magnetic and electric dipole ordering, have significant potential for high-density memory storage and the design of complex multistate memory elements. In this work, we have demonstrated the solvent-controlled synthesis of Cr(3+)-doped BaTiO(3) nanocrystals and investigated the effects of size and doping concentration on their structure and phase transformation using X-ray diffraction and Raman spectroscopy. The magnetic properties of these nanocrystals were studied by magnetic susceptibility, magnetic circular dichroism (MCD), and X-ray magnetic circular dichroism (XMCD) measurements. We observed that a decrease in nanocrystal size and an increase in doping concentration favor the stabilization of the paraelectric cubic phase, although the ferroelectric tetragonal phase is partly retained even in ca. 7 nm nanocrystals having the doping concentration of ca. 5%. The chromium(III) doping was determined to be a dominant factor for destabilization of the tetragonal phase. A combination of magnetic and magneto-optical measurements revealed that nanocrystalline films prepared from as-synthesized paramagnetic Cr(3+)-doped BaTiO(3) nanocrystals exhibit robust ferromagnetic ordering (up to ca. 2 μ(B)/Cr(3+)), similarly to magnetically doped transparent conducting oxides. The observed ferromagnetism increases with decreasing constituent nanocrystal size because of an enhancement in the interfacial defect concentration with increasing surface-to-volume ratio. Element-specific XMCD spectra measured by scanning transmission X-ray microscopy (STXM) confirmed with high spatial resolution that magnetic ordering arises from Cr(3+) dopant exchange interactions. The results of this work suggest an approach to the design and preparation of multiferroic perovskite materials that retain the ferroelectric phase and exhibit long-range magnetic ordering by using doped colloidal nanocrystals with optimized composition and size as functional building blocks.     

Shape-controlled conversion of beta-Sn nanocrystals into intermetallic M-Sn (M=Fe, Co, Ni, Pd) nanocrystals

The ability to control the shape of metal nanocrystals is critical to applications such as catalysis, magnetism, and plasmonics. Despite significant advances in controlling the shapes of single-metal nanocrystals, rigorous shape control of multimetal nanocrystals remains challenging, and has been limited largely to alloy systems of similar metals. Here we describe a robust strategy that produces shape-controlled intermetallic nanocrystals involving elements of notably different reduction potentials, reduction kinetics, and reactivity. The approach utilizes shape- and size-controlled beta-Sn nanocrystals as reactive templates that can be converted into binary M-Sn (M=Fe, Co, Ni, Pd) intermetallic compounds by reaction with appropriate metal salt solutions under reducing conditions. The result, demonstrated in detail for the FeSn2 system, is a variety of nanostructures with morphologies that include spheres, cubes, hollow squares, U-shaped structures, nanorods, and nanorod dimers. Our experiments demonstrate a size- and shape-dependent reactivity toward the formation of hollow FeSn2 nanostructures and provide empirical guidelines for the formation of other intermetallic nanocrystals. In addition to those of FeSn2, nanocrystals of intermetallic PdSn, CoSn3, and NiSn3 can be formed using this same chemical conversion strategy.     

Self-assembly of PbTe quantum dots into nanocrystal superlattices and glassy films

Monodisperse lead telluride (PbTe) nanocrystals ranging from approximately 4 to 10 nm in diameter are synthesized to provide quantum dot building blocks for the design of novel materials for electronic applications. Two complementary synthetic approaches are developed that enable either (1) isolation of small quantities of nanocrystals of many different sizes or (2) the production of up to 10 g of a single nanocrystal size. PbTe nanocrystals are characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and optical absorption. Assembly of PbTe nanocrystals is directed to prepare nanocrystal solids that display either short-range (glassy solids) or long-range (superlattices) packing order by varying deposition conditions. Film order and average interparticle spacing are analyzed with grazing-incidence small-angle X-ray scattering (GISAXS) and high-resolution scanning electron microscopy (HRSEM). We perform the first optical and electronic studies of PbTe solids and demonstrate that chemical activation of these films enhances conductivity by approximately 9-10 orders of magnitude while preserving their quantum dot nature.     

Solvothermal synthesis of zincblende and wurtzite CuInS2 nanocrystals and their photovoltaic application

We report a simple solvothermal synthesis approach to the growth of CuInS(2) nanocrystals with zincblende- and wurtzite-phase structures. Zincblende nanocrystals with particle sizes of 10-20 nm were produced using oleylamine as the solvent. When ethylenediamine was used as the solvent, similarly sized wurtzite nanocrystals with some degree of particle aggregation were formed. Use of a mixture of these solvents gave products with mixed phases including some polyhedral nanostructures. The crystal phases of these nanocrystals were carefully determined by X-ray diffraction and transmission electron microscopy analysis. All the samples exhibit strong absorption from the entire visible light region to the near-infrared region beyond 1300 nm. Pure-phase zincblende and wurtzite CuInS(2) nanocrystals were employed as ink in the fabrication of solar cells. The spray-coated nanocrystal layer was subjected to a selenization process. A power conversion efficiency of ~0.74% and a good external quantum efficiency profile over broad wavelengths have been measured. The results demonstrate that wurtzite and zincblende CuInS(2) nanocrystals may be attractive precursors to light-absorbing materials for making efficient photovoltaic devices.     

溶剂热合成单分散硫化镉纳米晶

在双表面活性剂十八胺和油酸存在条件下, 以氯化镉和硫粉作为反应前驱物, 通过简单的溶剂热方法合成单分散性闪锌矿硫化镉纳米晶, 粒径大小在13 nm. 用X射线衍射(XRD)、透射电子显微镜(TEM)对产物的结构和形貌进行了表征, 同时对硫化镉纳米晶的紫外吸收谱和光致发光谱(PL)性能进行了表征. 实验结果表明合成的样品具有很好的发光性能, 此外溶剂热反应的温度对纳米晶的单分散性有重要影响. 并对硫化镉纳米晶的形成机理做了初步的研究.     

Electron-conducting quantum dot solids: novel materials based on colloidal semiconductor nanocrystals

We review the optical and electrical properties of solids that are composed of semiconductor nanocrystals. Crystals, with dimensions in the nanometre range, of II-VI, IV-VI and III-V compound semiconductors, can be prepared by wet-chemical methods with a remarkable control of their size and shape, and surface chemistry. In the uncharged ground state, such nanocrystals are insulators. Electrons can be added, one by one, to the conduction orbitals, forming artificial atoms strongly confined in the nanocrystal. Semiconductor nanocrystals form the building blocks for larger architectures, which self-assemble due to van der Waals interactions. The electronic structure of the quantum dot solids prepared in such a way is determined by the orbital set of the nanocrystal building blocks and the electronic coupling between them. The opto-electronic properties are dramatically altered by electron injection into the orbitals. We discuss the optical and electrical properties of quantum dot solids in which the electron occupation of the orbitals is controlled by the electrochemical potential.     

Precursor conversion kinetics and the nucleation of cadmium selenide nanocrystals

The kinetics of cadmium selenide (CdSe) nanocrystal formation was studied using UV-visible absorption spectroscopy integrated with an automated, high-throughput synthesis platform. Reaction of anhydrous cadmium octadecylphosphonate (Cd-ODPA) with alkylphosphine selenides (1, tri-n-octylphosphine selenide; 2, di-n-butylphenylphosphine selenide; 3, n-butyldiphenylphosphine selenide) in recrystallized tri-n-octylphosphine oxide was monitored by following the absorbance of CdSe at λ = 350 nm, where the extinction coefficient is independent of size, and the disappearance of the selenium precursor using {(1)H}(31)P NMR spectroscopy. Our results indicate that precursor conversion limits the rate of nanocrystal nucleation and growth. The initial precursor conversion rate (Q(o)) depends linearly on [1] (Q(o)(1) = 3.0-36 μM/s) and decreases as the number of aryl groups bound to phosphorus increases (1 > 2 > 3). Changes to Q(o) influence the final number of nanocrystals and thus control particle size. Using similar methods, we show that changing [ODPA] has a negligible influence on precursor reactivity while increasing the growth rate of nuclei, thereby decreasing the final number of nanocrystals. These results are interpreted in light of a mechanism where the precursors react in an irreversible step that supplies the reaction medium with a solute form of the semiconductor.     

Application of a microfluidic reaction system for CdSe nanocrystal preparation: their growth kinetics and photoluminescence analysis

We used a microreactor for CdSe nanocrystal preparation and explored the effects of experimental conditions on the properties of the products. The particle growth kinetics and photoluminescence properties of the nanocrystals showed identical trends to previous reports, indicating the efficiency of the current method for analysis of rapid nanocrystal synthesis as well as industrial production of CdSe nanocrystals.