Weigang Lu

Super Crystal Structures of Octahedral c-In2O3 Nanocrystals

The three-dimensional self-assembly of a nanocrystal superlattice, i.e., a super crystal, has attracted increasing attention. The small building blocks for assemblies are usually spherical nanocrystals. Recent progress indicates that it is possible to achieve a super crystal using cubic nanocrystals. We further analyze and describe two-dimensional and some three-dimensional assemblies of uniform cubic-phase In2O3 nanocrystals with an octahedral shape. In this article, we demonstrate our amazing observations on these kinds of super crystals (or superlattices) as a model system, report their scale in at least tens of microns, and show other interesting features such as steps, terraces, kinks, and vacancies which are similar to those from a single crystal. Based on electron microscopy observations, three types of well-defined octahedral nanocrystal packed structures in such super crystal systems are also identified. The investigation of octahedral super crystal systems provides an alternate direction in research that may extend the interest of superlattice study to a broad spectrum by enriching and varying the shape of elemental building blocks. This may potentially result in new concepts and more challenging applications such as soft X-ray photonics.

Sizes correction on AFM images of nanometer spherical particles

Atomic Force Microscopy (AFM) is widely used in morphology characterization of materials on nanometer and sub-micron scales. However, distortions universally exist in AFM images due to geometrical interaction between the sample surface and the limited size tip. Correction factors for AFM images are given in the paper based on a simple mathematical model. The results reveal that the correction factors are related with the distribution of the particles (compacted or dispersed). The distortions can cause bigger images than the real sizes using commercial pyramidal tips and the distortions are deflation under certain conditions as well. The distortions of the images are affected by the shape of the AFM tip and circumstance of the particles. The results are compared the experimental data.

Preparation and structural characterization of nanostructured iron oxide thin films

Abstract Nanostructured iron oxide thin films were prepared using a dip-coating technique by forced hydrolysis of a FeCl3 solution with 2% gelatin at 60°C. The structural properties were characterized by X-ray diffractometer (XRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and UV–Vis absorption measurements. XRD analysis revealed that the iron oxide in the films consisted of α-Fe2O3, AFM showed that the Fe2O3 particles in the films were relatively uniform in size and XPS showed that there was little carbon on the surface and that the Fe2O3 fully covered the Si substrate. The particle sizes can be easily controlled through the pH of the solution.

Synthesis of In2O3@SiO2 core-shell nanoparticles with enhanced deeper energy level emissions of In2O3.

In(2)O(3)@SiO(2) core-shell nanoparticles were prepared using an organic solution synthesis approach and reverse-microemulsion technique. In order to explore the availability of various silica encapsulations, a partial phase diagram for this ternary system consisting of hexane/cyclohexane (1:29 wt), surfactant (polyoxyethylene(5)nonylphenyl ether, i.e., Igepal CO-520), and aqueous solution containing ammonium hydroxide was also established. It is realized that the shell-thickness can be tuned by several parameters such as the concentration of In(2)O(3) nanocrystal suspension and the dose of the Si-precursor, tetraethyl orthosilicate. It was observed that the deeper energy level emissions of In(2)O(3) were apparently enhanced when In(2)O(3) was confined by the silica-shell in such core-shell nanoparticles. However, this enhancement could be degraded by increasing the shell-thickness.

Electrochemical deposition of polypyrrole on patterned self-assembled monolayers

Patterned self-assembled monolayers have been fabricated by the reductive desorption of mercaptoacetic acid in mixed monolayers of mercaptoacetic acid and C16SH in alkaline solution. The greater the content of C16SH in the monolayers, the more negative is the reductive desorption potential of mercaptoacetic acid, indicating that mercaptoacetic acid and C16SH did not form patched domains, but mixed well with each other. The electrochemical deposition of pyrrole has been investigated at the patterned monolayers by cyclic voltammetry and atomic force microscopy (AFM). The results show that the polymer film growth strongly depends on the integrity of such patterned monolayers. In the case of patterned monolayers of high integrity, the polypyrrole grows on the top of the monolayers. However, in the case of a large number of pinhole defects, the polymerization results in the formation of polymer nuclei on pinhole sites scattered in the insulating monolayer matrix. The nuclei act as conducting paths to make the redox of the electroactive species more facile.

Spontaneous magnetization and ferromagnetism in PbSe quantum dots

A high-temperature organic solution approach was applied to prepare crystalline PbSe quantum dots. It is diamagnetic with an atomic susceptibility of ∼−1.0×10−4emu∕molOe for bulk PbSe. The core diamagnetism of bulk PbSe was subtracted from our raw data. While transforming into the nanophase, orbital susceptibility including finite-size corrections to the Landau susceptibility has been observed. A paramagnetic zero-field peak with a large diamagnetic susceptibility in high fields exhibit in the field dependent susceptibility as characteristics of the Landau orbital susceptibility. At low temperatures and fields the paramagnetism dominates the contribution of magnetization of quantum dots while the diamagnetism dominates at high temperatures and fields. All these measurements, showing paramagnetism at low fields, indicate the existence of spontaneous magnetization in the quantum dot. In addition, we have observed profound hysteresis loops implying ferromagnetism among the quantum dots even at room temperatu...

Orbital susceptibilities of PbSe quantum dots.

Different sizes of three-dimensional PbSe quantum dots have been synthesized for the study of orbital magnetic susceptibilities. Two types of orbital susceptibilities have been found, including the Curie susceptibility and finite-size corrections to the Landau susceptibility. The Curie term of a quantum dot manifests itself in the temperature dependence of magnetic susceptibility at low temperatures, while the field dependence of differential susceptibility at high temperatures shows finite-size corrections to the Landau susceptibility. Both of the two kinds of orbital susceptibility, estimated per quantum dot, show linear dependence on the size.

Study on nucleation and growth of Ag nanoparticles prepared by radio-frequency sputtering on highly oriented pyrolytic graphite and amorphous carbon

The initial growth process of silver nanoparticles on highly oriented pyrolytic graphite (HOPG) and amorphous carbon (a-C) was studied. Scanning tunneling microscopy was used for analysis of the nanoparticles formed on HOPG. Transmission electron microscopy was employed to study the growth procedure on a-C. The number densities (Dn) of silver nanoparticles on both HOPG and a-C remain constant at a very early stage, then both decrease with deposition time and have a saturated value. The Dn decrease suggests migration followed by impingement. We found evidence of migration of 5 nm particles on a-C. Interpretations are given to explain the impingement and shape change of the silver nanoparticles.

AFM study on silver/HOPG interface sputtered by low-energy argon ions

The effect of argon ion bombardment on interface of silver and high oriented pyrogenic graphite (HOPG) was investigated by atomic force microscopy. The Ag/HOPG interface morphology has been explored as a function of irradiation time. It can be seen that the size of crystal particle on HOPG is almost unchanged with increase of irradiation time, while the size of Ag crystal particle in part of the terrace increased and became highly ordered. There is no obvious mixing between Ag and HOPG, and Ag particles in nanoscale are isolated on HOPG surface. This will result in the reduction of depth resolution of the surface analysis by ion sputtering technique.