H. P. Sun

Probing Nanoscale Ferroelectricity by Ultraviolet Raman Spectroscopy

We demonstrated that ultraviolet Raman spectroscopy is an effective technique to measure the transition temperature (Tc) in ferroelectric ultrathin films and superlattices. We showed that one-unit-cell-thick BaTiO3 layers in BaTiO3/SrTiO3 superlattices are not only ferroelectric (with Tc as high as 250 kelvin) but also polarize the quantum paraelectric SrTiO3 layers adjacent to them. Tc was tuned by ∼500 kelvin by varying the thicknesses of the BaTiO3 and SrTiO3 layers, revealing the essential roles of electrical and mechanical boundary conditions for nanoscale ferroelectricity.

Improved biocompatibility of surface functionalized dendrimer-entrapped gold nanoparticles

A general approach to modifying preformed dendrimer-entrapped Au nanoparticles with different functionalities is proved to improve their biocompatibility.

CONTROLLED LI DOPING OF SI NANOWIRES BY ELECTROCHEMICAL INSERTION METHOD

Si nanowires (NWs) were doped with large amounts of Li+ ions by an electrochemical insertion method at room temperature. Si NWs with different doping levels were obtained by controlling the discharging/charging of Li/Si NWs cell. The microstructures of Si NWs with different doses of Li+ ions were investigated by high-resolution electron microscopy. The crystalline structure of the Si NWs was destroyed gradually with the increasing of Li+ ion dose. When the Li+ ions were extracted from the amorphous Li-doped Si NWs by the same electrochemical method, local ordering of atoms occurred and recrystallization was observed. The photoluminescence peak and intensity of Li+-doped Si NWs are closely related to the doping dose.

Superconducting properties of nanocrystalline MgB2 thin films made by an in situ annealing process

We have studied the structural and superconducting properties of MgB2 thin films made by pulsed-laser deposition followed by in situ annealing. The cross-sectional transmission electron microscopy reveals a nanocrystalline mixture of textured MgO and MgB2 with very small grain sizes. A zero-resistance transition temperature (Tc0) of 34 K and a zero-field critical current density (Jc) of 1.3×106 A/cm2 were obtained. The irreversibility field was ∼8 T at low temperatures, although severe pinning instability was observed. The result is a step towards making the in situ deposition process a viable technique for MgB2 Josephson junction technologies.

Coarsening of Pt particles in a model NOx trap

The effects of temperature and atmosphere on the coarsening of Pt particles in a model NOx trap (Pt/BaO/Al2O3) were examined by XRD, TEM, and CO chemisorption. The main finding, that the most significant particle growth occurs at elevated temperatures under oxidizing conditions, is relevant to NOx trap durability.

Highly ordered CdS nanoparticle arrays on silicon substrates and photoluminescence properties

Highly ordered cadmium sulphide (CdS) nanoparticle (NP) arrays were fabricated on silicon (Si) substrates using ultrathin alumina membranes as evaporation masks. The CdS NPs are polycrystalline and are composed of ultrasmall closely packed nanocrystallites. These crystallites increase in size as the duration of the CdS evaporation process increases. When the thickness of the NPs changes from about 10 to 50 nm, the size of the crystallites increases from about 5–14 to 20–40 nm. Photoluminescence measurements on the CdS NP arrays show a strong emission spectrum with two subbands that are attributed to band-edge and surface-defect emissions. The peak position and width of the band-edge emission band are closely related to the size of the crystallites in the CdS NPs.

Microstructure and crystal defects in epitaxial ZnO film grown on Ga modified (0001) sapphire surface

Surface modification of sapphire (0001) by Ga can eliminate multiple rotation domains in ZnO films. The existence of Ga at ZnO∕sapphire interface was confirmed by x-ray energy dispersive spectroscopy in a transmission electron microscope. Atomic detail of mismatch dislocations at interface was imaged by high resolution transmission electron microscopy. Inside the ZnO film, there is a high density of stacking fault. Both pure gliding of ZnO(0001) plane and condensation of vacancies or interstatials are possible mechanisms to generate the stacking fault.

Synthesis of diamond from carbon nanotubes under high pressure and high temperature

The investigation on elemental carbon has long been of nanotubes to diamond at 4.5 GPa and 13008C using a considerable interest because of its great importance in six-anvil high pressure apparatus with the existence of both science and technology. One of the most outstanding NiMnCo catalyst. The detailed characterization conducted achievements which occurred in carbon science was the shows that carbon nanotubes transform to quasi-spherical synthesis of diamond under high-pressure–high-temperaonion-like structures first and then to diamond crystals. It ture (HPHT) conditions [1,2]. Now, man-made diamond is is different from the phase transformation behavior of commercially available and plays an indispensable role in graphite to diamond under high pressure and high temperamodern industry for abrasives, tool coatings, microelecture conditions. tronics, optics and other applications. Another important For the experiments presented here, multiwalled carbon advance in carbon science was the discovery of fullerenes nanotubes with diameters of 20–50 nm produced by and carbon nanotubes [3,4]. These novel carbon phases catalytic chemical vapor deposition (CCVD) were used as have gained high visibility because they have the potential starting material (Fig. 1). A 63600 ton six-anvil high to exhibit unique structural variety and extraordinary pressure apparatus with an electric current heating device optical, mechanical, and electronic properties [5]. Recentwas employed for the high pressure experiments. In each ly, a number of studies relating to the behaviors of experiment, about 50 mg of carbon nanotubes placed fullerenes under high pressure have been reported which between two NiMnCo alloy flakes were put into the high were focused on probing the cage structure stability and pressure cavity and a cubic body with a cylinder sample looking for new novel structures [6–8]. Moreover, it has chamber of pyrophyllite was used as pressure seal and been demonstrated that fullerenes can convert to diamond transmitting medium. Before and after high pressure runs, by applying high pressure at either room temperature [9] or the samples were monitored by transmission electron high temperature [10]. However, only a few works have microscopy (TEM; H8100), scanning electron microscopy been so far focused on the structural stability and phase (SEM; S-4200) and Raman spectroscopy (T64000). Hightransformation of carbon nanotubes at high pressure resolution transmission electron microscopy (HRTEM) [11,12]. As we know, the transformation of graphite or investigations were carried out using a JEOL2010 micrographite-like materials to diamond is of great technological scope operating at 200 kV. importance and therefore remains an exciting field in both Fig. 1 shows the representative morphology of carbon experimental and theoretical studies. Therefore, a detailed nanotubes in the starting material. The carbon nanotubes study of the behaviors of carbon nanotubes at high exhibit typical concentric graphitic shell structure with a pressure is very necessary for further understanding their hollow core. The diameters of most carbon nanotubes in structures and properties and comparing with that of our sample are around 30–40 nm uniformly. No other graphite. forms of carbon can be detected in the sample by TEM In the present work we report the conversion of carbon study.

Growth of nanoscale BaTiO 3 /SrTiO 3 superlattices by molecular-beam epitaxy

Abstract : Commensurate BaTiO3/SrTiO3 superlattices were grown by reactive molecular-beam epitaxy on four different substrates: TiO2-terminated (001) SrTiO3, (101) DyScO3, (101) GdScO3, and (101) SmScO3. With the aid of reflection high-energy electron diffraction (RHEED), precise single-monolayer doses of BaO, SrO, and TiO2 were deposited sequentially to create commensurate BaTiO3/SrTiO3 superlattices with a variety of periodicities. X-ray diffraction (XRD) measurements exhibit clear superlattice peaks at the expected positions. The rocking curve full width half-maximum of the superlattices was as narrow as 7 arc s (0.002 deg). High-resolution transmission electron microscopy reveals nearly atomically abrupt interfaces. Temperature-dependent ultraviolet Raman and XRD were used to reveal the paraelectric-to-ferroelectric transition temperature (TC). Our results demonstrate the importance of finite size and strain effects on the TC of BaTiO3/SrTiO3 superlattices. In addition to probing finite size and strain effects, these heterostructures may be relevant for novel phonon devices, including mirrors, filters, and cavities for coherent phonon generation and control.

Acoustic bragg mirrors and cavities made using piezoelectric oxides

The concept and design of acoustic Bragg mirrors and cavities made of multilayers of piezoelectric oxides with superior acoustic performance and potential applications in electronic and optical terahertz modulators are described. With these applications in mind the authors have grown phonon mirrors consisting of BaTiO3∕SrTiO3 superlattices on SrTiO3 substrates by reactive molecular-beam epitaxy and investigated their properties. Characterization of the superlattices by x-ray diffraction and high-resolution transmission electron microscopy reveals high structural quality with nearly atomically abrupt interfaces. The authors have observed folded acoustic phonons at the expected frequencies using uv Raman spectroscopy.