Y. Charles Cao

Self-assembled colloidal superparticles from nanorods

Beyond Quantum Dots Semiconducting colloidal nanoparticles—quantum dots—are of interest for their unusual properties. One current challenge is the controlled assembly of colloidal particles into larger structures, such as two-dimensional lattices on a substrate, or three-dimensional superparticles. Wang et al. (p. 358) present a two-step self-assembly of CdSe/CdS semiconductor nanorods to form mesoscopic colloidal superparticles. The particles show well-defined super-crystalline domains with dimensions ranging from hundreds of nanometers to several microns, and with the particle morphology controlled by the number of constituent rods. Films of the needle-shaped superparticles were able to act as polarizing light-emitting diodes. Colloidal rods self-assemble into semiconducting superparticles with a shape controlled by the number of rods. Colloidal superparticles are nanoparticle assemblies in the form of colloidal particles. The assembly of nanoscopic objects into mesoscopic or macroscopic complex architectures allows bottom-up fabrication of functional materials. We report that the self-assembly of cadmium selenide–cadmium sulfide (CdSe-CdS) core-shell semiconductor nanorods, mediated by shape and structural anisotropy, produces mesoscopic colloidal superparticles having multiple well-defined supercrystalline domains. Moreover, functionality-based anisotropic interactions between these CdSe-CdS nanorods can be kinetically introduced during the self-assembly and, in turn, yield single-domain, needle-like superparticles with parallel alignment of constituent nanorods. Unidirectional patterning of these mesoscopic needle-like superparticles gives rise to the lateral alignment of CdSe-CdS nanorods into macroscopic, uniform, freestanding polymer films that exhibit strong photoluminescence with a striking anisotropy, enabling their use as downconversion phosphors to create polarized light-emitting diodes.

On Doping CdS/ZnS Core/Shell Nanocrystals with Mn

This paper presents a mechanistic study on the doping of CdS/ZnS core/shell semiconductor nanocrystals with Mn based on a three-step synthesis, which includes host-particle synthesis, Mn-dopant growth, and ZnS-shell growth. We used a combination of electron paramagnetic resonance spectroscopy (EPR) and inductively coupled plasma atomic emission spectroscopy (ICP) to monitor Mn-doping level and growth yield during doping synthesis at both the dopant-growth and ZnS-shell-growth steps. First, our kinetic study shows that Mn adsorption onto the nanocrystal surface includes the formation of weakly and strongly bound Mn. The formation of weakly bound Mn is associated with a chemical equilibrium between adsorbed Mn species on the nanocrystal surface and free Mn species in growth solution, while the formation of strongly bound Mn exhibits first-order kinetics with an activation-energy barrier of 211 +/- 13 kJ/mol. Second, our results demonstrate that both weakly and strongly bound Mn can be removed from the surface of nanocrystals during ZnS-shell growth. The replacement of strongly bound Mn requires a higher temperature than that of weakly bound Mn. The yield of the replacement of strongly bound Mn is strongly dependent on the temperature of ZnS-shell growth. Third, our results show that the Mn-growth yield is not dependent on the size and crystal structure of nanocrystals. All together, these results suggest a mechanism in which nanocrystal doping is determined by the chemical kinetics of three activation-controlled processes: dopant adsorption, replacement, and ZnS-shell growth.

Shape-controlled synthesis of colloidal superparticles from nanocubes.

This communication reports a shape-controlled synthesis of colloidal superparticles (SPs) from iron oxide nanocubes. Our results show that the formation of SPs is under thermodynamic control and that their shape is determined by Gibbs free energy minimization. The resulting SPs adopt a simple-cubic superlattice structure, and their shape can be tuned between spheres and cubes by varying the relative free energy contributions from the surface and bulk free energy terms. The formation of sphere-shaped SPs from nanocubes suggests that the size-dependent hydration effect predicted by the Lum-Chandler-Weeks theory plays a very important role in the self-assembly of nano-objects. In addition, the iron oxide SPs exhibit shape-dependent therapeutic effects in magnetomechanical treatments of cancer cells in vitro.

Surface-functionalization-dependent optical properties of II-VI semiconductor nanocrystals.

We report a study of the surface-functionalization-dependent optical properties of II-VI zinc-blende semiconductor nanocrystals on the basis of ligand-exchange chemistry, isomaterial core/shell growth, optical spectroscopy, transmission electron microscopy, and X-ray powder diffraction. Our results show that the transition energy and extinction coefficient of the 2S(h3/2)1S(e) excitonic band of these nanocrystals can be strongly modified by their surface ligands as well as ligand associated surface atomic arrangement. The oleylamine exchange of oleate-capped zinc-blende II-VI nanocrystals narrows the energy gap between their first and second excitonic absorption bands, and this narrowing effect is size-dependent. The oleylamine exchange results in the quenching, subsequent recovery, and even enhancing of the photoluminescence emission of these II-VI semiconductor nanocrystals. In addition, the results from our X-ray powder diffraction measurements and simulations completely rule out the possibility that oleate-capped zinc-blende CdSe nanocrystals can undergo zinc-blende-to-wurtzite crystal transformation upon ligand exchange with oleylamine. Moreover, our theoretical modeling results suggest that the surface-functionalization-dependent optical properties of these semiconductor nanocrystals can be caused by a thin type II isomaterial shell that is created by the negatively charged ligands (e.g., oleate and octadecyl phosphonate). Taking all these results together, we provide the unambiguous identification that II-VI semiconductor nanocrystals exhibit surface-functionalization-dependent excitonic absorption features.

Deviatoric Stress Driven Formation of Large Single-Crystal PbS Nanosheet from Nanoparticles and in Situ Monitoring of Oriented Attachment

Two-dimensional single-crystal PbS nanosheets were synthesized by deviatoric stress-driven orientation and attachment of nanoparticles (NPs). In situ small- and wide-angle synchrotron X-ray scattering measurements on the same spot of the sample under pressure coupled with transmission electron microscopy enable reconstruction of the nucleation route showing how enhanced deviatoric stress causes ordering NPs into single-crystal nanosheets with a lamellar mesostructure. At the same time that deviatoric stress drives SC(110) orientation in a face-centered-cubic supercrystal (SC), rocksalt (RS) NPs rotate and align their RS(200) and RS(220) planes within the SC(110) plane. When NPs approach each other along the compression axis, enhanced deviatoric stress drives soft ligands passivated at RS(200) and RS(220) surfaces to reorient from a group of SC(110) in-planes to the interspace of SC[110]-normal planes. While the internal NP structure starts a rocksalt-to-orthorhombic transition at 7.1 GPa, NPs become aligned on RS(220) and RS(200) and thus become attached at those faces. The transition-catalyzed surface atoms accelerate the inter-NP coalescing process and the formation of low-energy structure nanosheet. Above 11.6 GPa, the nucleated single-crystal nanosheets stack into a lamellar mesostructure that has a domain size comparable to the starting supercrystal.

Cylindrical Superparticles from Semiconductor Nanorods

In this communication, we report a synthesis of anisotropic colloidal superparticles (SPs) from CdSe/CdS semiconductor nanorods. These anisotropic SPs are cylindrical disks or stacked-disk arrays. We attribute the major driving forces controlling the SP shape to interparticle interactions between nanorods and solvophobic interactions between a superparticle and its surrounding solvent. According to their sizes (or volumes), the SPs adopt either single- or multilayered structures. In addition, these SPs exhibit linearly polarized emissions, demonstrating their potential role as useful components in devices such as polarized light-emitting diodes and electrooptical modulators.

Gas-Bubble Effects on the Formation of Colloidal Iron Oxide Nanocrystals

This paper reports that gas bubbles can be used to tailor the kinetics of the nucleation and growth of inorganic-nanocrystals in a colloidal synthesis. We conducted a mechanistic study of the synthesis of colloidal iron oxide nanocrystals using gas bubbles generated by boiling solvents or artificial Ar bubbling. We identified that bubbling effects take place through absorbing local latent heat released from the exothermic reactions involved in the nucleation and growth of iron oxide nanocrystals. Our results show that gas bubbles display a stronger effect on the nucleation of iron oxide nanocrystals than on their growth. These results indicate that the nucleation and growth of iron oxide nanocrystals may rely on different types of chemical reactions between the iron-oleate decomposition products: the nucleation relies on the strongly exothermic, multiple-bond formation reactions, whereas the growth of iron oxide nanocrystals may primarily depend upon single-bond formation reactions. The identification of exothermic reactions is further consistent with our results in the synthesis of iron oxide nanocrystals with boiling solvents at reaction temperatures ranging from 290 to 365 °C, by which we determined the reaction enthalpy in the nucleation of iron oxide nanocrystals to be -142 ± 12 kJ/mol. Moreover, our results suggest that a prerequisite for effectively suppressing secondary nucleation in a colloidal synthesis is that the primary nucleation must produce a critical amount of nuclei, and this finding is important for a priori design of colloidal synthesis of monodispersed nanocrystals in general.

Formation of Heterodimer Nanocrystals: UO2/In2O3 and FePt/In2O3

This Article reports a mechanistic study on the formation of colloidal UO(2)/In(2)O(3) and FePt/In(2)O(3) heterodimer nanocrystals. These dimer nanocrystals were synthesized via the growth of In(2)O(3) as the epitaxial material onto the seed nanocrystals of UO(2) or FePt. The resulting dimer nanocrystals were characterized using X-ray powder diffraction (XRD), energy dispersion spectroscopy, transmission electron microscopy (TEM), scanning transmission electron microscopy, and high-resolution TEM (HRTEM). The results from XRD and HRTEM clearly show that lattice strains exist in both of these dimer nanocrystals. Interestingly, the lattice of In(2)O(3) expands in UO(2)/In(2)O(3) dimers, whereas FePt/In(2)O(3) dimers exhibit compressed In(2)O(3) lattices. Using HRTEM and nanocrystal structure simulations, we have identified the crystallographic orientation of the attachment of the two segments in these two types of dimers. An unconventional Miller index was introduced to describe the crystallographic orientation of these heterodimer nanocrystals. On the basis of the results herein as well as those from other researchers, we propose an empirical law for the determination of the crystallographic attachment orientation in heterodimers: instead of growth on the facet of the seed nanocrystals where lattice mismatch is minimized, the growth of an epitaxial material often chooses the crystal facets where the first atomic monolayer of this material has the strongest affinity for the seed nanocrystals.

Radial‐Position‐Controlled Doping of CdS/ZnS Core/Shell Nanocrystals: Surface Effects and Position‐Dependent Properties

Energy transfer in doped semiconductor nanocrystals: Mn-doped CdS/ZnS nanocrystals show interesting photoluminescence and EPR properties that are strongly dependent on the radial position of Mn dopants inside nanocrystals (see graphic). Furthermore, the results suggest a two-step mechanism for the energy transfer inside the doped nanocrystals.This paper reports a study of the surface effects and position-dependent properties of Mn-doped CdS/ZnS core/shell nanocrystals, which were prepared by using a three-step synthesis method. The Mn-doping level of these nanocrystals was determined by a combination of electron paramagnetic resonance spectroscopy and inductively coupled plasma atomic emission spectroscopy. These nanocrystals were further characterized by using transmission electron microscopy and fluorescence spectroscopy. First, we found that injecting a large excess of zinc stearate at the end of nanocrystal synthesis can sufficiently eliminate the surface-trap states from the doped CdS/ZnS core/shell nanocrystals and enhance their photoluminescence (PL) quantum yield (QY). Second, our results demonstrate that the Mn-PL QY is determined by the product of the efficiency of energy transfer from an exciton inside the CdS core to a Mn ion (Phi(ET)) and the efficiency of the emission from the Mn ion (Phi(Mn)). Third, Phi(Mn) strongly depends on the radial position of Mn ions in the doped core/shell nanocrystals. The position-dependent changes of Phi(Mn) nearly perfectly correlate to those of the linewidth of Mn EPR peaks: the higher the Phi(Mn), the narrower the linewidth of the Mn EPR peak. Fourth, the results demonstrate that Phi(ET) depends on the Mn-doping level as well as the inverse sixth power of the distance between a Mn ion and the center of its host nanocrystal. Accordingly, we propose a two-step mechanism for the energy transfer: 1) the energy transfer from an exciton inside the CdS core to a bound exciton around a Mn center, which is the rate-determining step and follows the Förster mechanism, and 2) the energy transfer from the bound exciton to the Mn center, which might follow a mechanism such as dark exciton (triplet exciton) or Auger transfer.

Nanomaterials for biomedical applications

Recent advances in creating nanomaterials have created new opportunities in biomedical research and clinical applications [1,2]. High-quality nanomaterials, of well-controlled size and shape, are a new class of building blocks to enable the establishment of assays for monitoring molecular signals in biological systems and living organisms. Many of these new nanoassays have higher sensitivity, selectivity and throughput than conventional bioanalytical methods. On the one hand, these nanoassays will be capable of detecting biochemical changes at the single-molecule level in living cells [3]. Conversely, these assays will lead to low-cost, point-of-care devices for rapid diagnosis of pathogenic and genetic diseases (e.g., HIV and cancer) [4]. In addition, nanomaterials have been used as advanced contrast agents for clinical imaging technologies, such as MRI, computer tomography and ultrasound [5–7]. Moreover, the use of nanomaterials will lead to the invention of ‘smart’ drug-delivery vehicles, new therapies and even new scalpel-free surgery methods. These new opportunities stem primarily from the novel nature of nanomaterials. Owing to their size-dependent effects, nanomaterials exhibit new physical and chemical properties compared with conventional bulk and molecular materials [8]. In general, nanomaterials include inorganic, organic and inorganic/organic composite nanostructures, such as nanoparticles, nanowires and nanopatterns. This special issue of Nanomedicine presents readers with current exciting developments in the use of nanoparticles and nanopatterns for biomedical diagnosis and drug delivery.