Meeting Scene - Day 2

DAY 2
Tuesday, June 10, 2008

The conference continued in full earnest on Tuesday with nearly all symposia conducting sessions. The exhibit and poster sessions were well attended and busy, as was the conference lunch. A special keynote talk at lunch time by Dr. WANG Zhanguo focused on semiconductor nanostructure growth and devices. A riverboat cruise was organized in the evening for participants.

KEYNOTE TALK: WANG ZHANGUO

Semiconductor Nanostructures and Quantum Devices

WANG Zhanguo, from the Chinese Academy of Sciences, Beijing, presented the lunchtime keynote talk of the day. He discussed work on the growth of InAs/GaAs quantum dots. There are three growth modes for heteroepitaxy on quantum dots: Frank van der Merwe (FM), Volmer-Weber (island), and Stranski-Krastinow (SK). The SK growth at an initial stage is layer-by layer-growth, called the wetting layer. The sample preparation involved making tapered thickness InAs layers by evaporating at an angle. The evolution of the wetting layer of InAs quantum dots was studied by reflectance difference spectroscopy (RDS). The InAs amount in the wetting layer and its segregation coefficient were determined and found to be sensitive to the growth modes. The segregation coefficient varies linearly with the InAs fraction in the wetting layer.

The formation of InAs quantum dots during the 2d-3d growth mode transition was studied by atomic force microscopy. The nucleation and growth of InAs quantum dots are continuous and both the number of quantum dots and their volume increase with deposition time according to well defined power law equations. Under optimized growth conditions it is possible to grow GaAs and InP based nanostructures free of defects, of good uniformity, and well spatially ordered.

 
The exhibit

TECHNICAL TALKS

Symposium B: Sustainable Energy Materials
CNTs for Fuel Cell and Supercapacitor Applications

Li-Chyong Chen from National Taiwan University gave the first invited talk in Tuesday’s Symposium B on energy storage, speaking about her work with carbon nanotubes (CNTs) for use in methanol fuel cells. Her work seeks to improve fuel cell efficiency, streamlining the charge transfer between catalytic Pt nanoparticles by depositing them via PVD on CNTs instead of the usual polymer membrane. For a substrate she uses a carbon fiber cloth which improves the CNT-substrate charge transfer by roughly five orders of magnitude over conventional substrates, according to electrochemical impedance spectra measurements. Chen attributes this improvement to the homogenous nature of the carbon-carbon interface. Nitrogen doping the CNTs improved the dispersion of Pt, assisting nucleation and growth of nanoparticles. Density functional calculations helped explain this, showing that Pt preferentially bonds to a site adjoining a dopant N atom on a CNT wall.

DFT Studies of Hydrogen Storage in Magnesium Based Alloys

In the afternoon session on energy storage, Shu Xia Tao from the University of Eindhoven, Netherlands, presented results from density functional theory calculations of the hydrogen storage capacity of Mg and Mg-transition metal alloys, in conjunction with researchers at Philips Research Laboratory. Calculating the formation energies as a function of H concentration, Tao finds that in fcc, hcp, and rutile Mg hydride, the lattice sites at which interstitial H is incorporated depends on the H loading. For loading fractions up to 0.5, H preferentially fills tetrahedral over octahedral lattice sites. Above 0.5, the converse is true, and at high loading the rutile phase is found to be most stable. Mg-Sc alloys have desirable H storage properties but the high cost of Sc makes this impractical for commercial applications and Tao’s work aims to find an economically viable alternative. Favorable kinetics are found in alloyed Mg-Ti, and for Mg-Ti dihydride, and in preliminary results Tao finds fluorite structure to be the most stable for Ti content over 0.2, confirming experimental studies.

Synthesis and Transport Properties of Polycrystalline Co-filled type-I Clathrate Compound (Sr,Ba)₈Ga₁₆Ge₃₀

Thermoelectrics present the alluring prospect of generating electricity from a simple heat gradient. Li Wang presented her work done at the Chinese Academy of Sciences towards improving the thermoelectric figure of merit for Ga-substituted Ge-based clathrates. These materials promise good thermoelectric properties but improved thermal conductivity is needed before they can be used for practical applications. Wang synthesized (Sr,Ba)₈Ga₁₆Ge₃₀ via a melting and sintering process, and performed characterization with XRD, backscattered electron imaging and electron probe microanalysis. She reported strong dependence of carrier concentration and thermoelectric properties on Ga doping levels, suggesting this as an approach to control and improve the figure of merit.

Symposium C: Electronic Packaging Materials
Lead-free Solders with High Strength and High Service Reliability

Fu Guo, of the Beijing University of Technology, talked about the addition of rare earth elements (La and Ce) and particulates to solders to improve their properties. The lead free solders were Sn-Ag-Bi and Sn-Ag-Cu. Rare-earth elements added to these improved their microstructure and properties The optimum amount of the rare-earths is 0.1atomic %. Less or more amounts result in inferior properties. The composite solders were prepared by two different methods - In-situ and mechanically-mixed. The in-situ Cu₆Sn₅ particles added created weakly-bonded homogeneities that caused deformation, thereby enhancing creep and TMF resistance of solder joints.

Mechanically-mixed solders were much more effective in improving the properties of the solders. The problems are that the reinforcements tend to be several microns, there is coarsening of the reinforcement particles in service, agglomeration occurs, and the distribution of the reinforcements cannot be easily controlled. Polyhedral oligomeric silsesquioxane (POSS) has been used as a surface active and inert reinforcement. It facilitates the initial bonding during reflow but leaves the inert particulates from reacting any further. This has resulted in solder joints with enhanced creep and TMF resistance. It remains to be seen how they will work with electro-migration and whisker growth.

 
More of the exhibit

Symposium D: Electronic Materials
Phosphorus Atomic Arrays Along Step=Edges on vicinal Si(111) Surfaces

Shing-Chiang Huang, of Keio University, Japan, in her talk on Monday discussed the formation and control of regular arrays of phosphorous atoms on a silicon substrate for applications to quantum computers. Step arrays were formed by cutting silicon 1 degree off the (110) direction. This gives one bilayer silicon atoms height with an average terrace width of 18nm. Steps in the substrate were also prepared by direct current heating. A phosphorous layer of 0.03 Langmuir is then deposited at 500°C followed by annealing for 5 minutes and quenching to room temperature. A regular array of bright dots is seen in the STM. These bright spots are at the corner adatoms along the step edges of the faulted half of the 7 x 7 structure. There is a 0.2A height difference between the P and Si adatoms in agreement with previously reported values. This work makes progress towards Kane’s proposal for a quantum computer that calls for phosphorus atoms to be aligned at equal spacing on a pure silicon surface.

Low Temperature Doping of ZnO Nanostructures

J. B. Cui, of the University of Arkansas at Little Rock, USA, described low temperature doping of zinc oxide nanostructures. Zinc oxide is a semiconductor with wide bandgap and a large exciton binding energy that has applications in electronic and optoelectronic devices. It can also be made into a dilute magnetic semiconductor for spintronics and offers a material that can span both the magnetic and semiconductor applications. It is also a piezoelectric material for actuators and sensors. The problem with the material is that it is n-type as grown but the doping level is difficult to control. It is also difficult to get reliable p-type doping with a low resistivity.

Cui used electrochemical growth to produce the ZnO nanowires because it is low temperature (<95°C), low cost, large scale and it can produce single crystal nanowires with low temperature doping. Nanowires are grown perpendicular to a silicon wafer surface and were doped with Ni and Co. It was demonstrated that the dopants were incorporated into the ZnO because of changes in the lattice constant. Low temperature ferromagnetism was observed in these doped nanowire arrays. The wires were also doped with Li and investigated with variable photoluminescence.

Resistive Switching: Materials, Mechanisms and Memory Devices

Z. G. Liu, of Nanjing University, gave a detailed talk on various memory devices. He talked about nonvolatile memories including phase change memories (PCRAMs) and resistive switching memories (RRAMs). The PCRAMs were based on the amorphous-crystalline phase transformations in chalcogenide alloys achieved by electrical heating or quenching. The RRAMs are based on cation migration. They consist of two types. The first is based on an electrochemical decomposition reaction of RbAg4I5, where silver ions move towards a non-reactive platinum electrode until they form conducting paths through the matrix material. Upon reversal of the voltage the ions move back towards the silver electrode thereby breaking the conducting paths.

The second uses Ag₃₀Ge₁₇Se₅₃, wherein the amorphous electrolyte decomposes into conductive Ag₂(Se,Ge) particles or nano-filaments embedded in the high resistance matrix. These devices are much more robust than the Ag device and can be switched more than 10E7 times. A third type of memory was also described where (Ba_0.5,Sr_0.5)(Zr_0.2,Ti_0.8)O₃ or BSZT films were made conductive by the injection of space-charge-limited-current into traps within the material. After the trap filled threshold in voltage has been reached then the films conduct. The author proposes the existence of asymmetric traps (oxygen vacancies), confirmed by the different high resistance boundary voltages for the positive and negative branches.

Symposium F: Functional Ceramics
Development of High Temperature Superconductors for Electric Power Applications

Dean Peterson of Los Alamos National Laboratory, USA, provided a whirlwind overview of high temperature superconductors (HTS) for use in power transmission in his keynote talk in Symposium F. He cited a 62% energy loss in electricity production and transmission in the US as clear motivation for increased efficiency in power transmission. A superconducting power grid could virtually eliminate power loss in transmission and Peterson overviewed encouraging research moving us towards that goal. He explained that superconductors must remain in the “operational envelope” defined by critical current density, Curie temperature, and magnetic field, and that maximizing performance within this envelope is the central challenge in designing HTS power lines. International companies and research institutions have made tremendous progress in engineering such systems. The working design is a tape-like geometry in which multilayers of oxide materials are deposited in thin films, each on the order of 10-100 nm in thickness, where the superconducting component is typically YBCO. Tapes of this basic design have been constructed up to km lengths with critical currents around 200-250 A/cm. Prototype systems are currently being tested at several sites in the US and Peterson left the audience with optimism regarding the future prospects of a superconducting power grid.

Symposium G: Transportation Materials
High-Temperature Materials for Extreme Environments

In the drive for higher efficiency and higher power output gas turbines for aerospace, power generation, and other applications, new materials are needed that endure higher temperatures without mechanical degradation. Sammy Tin (Illinois Institute of Technology) presented developments on nickel-based superalloys, materials that are well-suited for high-temperature operation. In particular he focused on high-refractory content single crystal Ni-base superalloys to understand manufacturing and solidification-related defects. A particular focus was on creep resistance, which at lower temperatures can be optimized by controlling crystal orientation and reducing lattice misfit. At high temperatures, the challenge is to reduce diffusion and the formation of topologically closed-packed (TCP) structures. In intermediate temperatures, dislocation activity is confined to the gamma phase, and thus overall strength can be increased by strengthening the gamma phase. Adding elements such as W and Re can achieve this strengthening, to a degree. However, defects such as “freckle formation” can occur due to segregation of Re to dendrite cores. Tin said that adding Ru (about 3%) to the mix reduced the kinetics and can reduce formation and growth of TCPs and thus improve phase stability. Ru diffusion is slow, but also it slows the diffusion of Re. The reduction of the TCP phases prevents Re from being depleted from the gamma matrix, and the presence of Re in the gamma matrix prevents coarsening of the gamma prime phase, and structural integrity is retained at high temperatures. A remaining problem, however, is its poor oxidation resistance, which is the next phase of their research focus.

 
More of the exhibit

Symposium I: Biomaterials for Medical Applications
Novel Biodegradable Polycations for Gene Transfer

X-Z. Zhang of Wuhan University, China, presented the first paper of the morning in symposium I. He described the development of novel biodegradable polycations for gene therapy applications. Gene therapy is the introduction of DNA or RNA into cells for correcting defective genes responsible for diseases. Nonviral vectors have several advantages over viral ones, and this study focused on cationic polymers for this purpose with high efficiency and low cytotoxicity. The strategy that the researchers used was the incorporation of polysaccharides.

Zhang described investigations on four different systems, including carboxymethyl dextran-g-polyethylenimines (CMD-g-PEI), dextran hexamethylenediisocyanate-g-polyethylenimines ((Dex-HMDI)-g-PEIs), N-succinyl-chitosan-g-polyethylenimine (NSC-g-PEI) and N-maleated chitosan-g-polyethylenimine (NMC-g-PEI). For each, Luciferase expression and Green Fluorescence Expression (GFP) were examined. In summary, polysaccharides (chitosan and dextran) were used for the grafting of low molecular weight PEI, and a series of nonviral gene vectors with low cytotoxicity and high transfection efficiency were prepared in this work.

Novel Thiolated Carboxymethyl Chitosan-g-ß-cyclodextrin as Mucoadhesive Hydrophobic Drug Delivery Carriers
Mucosal membranes line various body cavities that are exposed to the external environment and internal organs. Uptake of drugs through these membranes is controlled by passive diffusion and therefore it is crucial for any drug delivery system to have intimate contact with the membrane, which is achieved by mucoadhesive polymer systems. In this talk by Shaoqin "Sarah" Gong of the Univ. of Wisconsin, Milwaukee, USA (actually presented by Zhenqiang "Jack" Ma), the use of thiolated polymers (thiomers) as mucoadhesive drug delivery systems was described. In conventional mucoadhesive polymers, properties are based on non-covalent bonds, whereas thiomers can form covalent bonds with cysteine-rich subdomains of the mucus layer via a disulfide bond. This presentation described the synthesis and characterization of a novel thiolated carboxymethyl chitosan-g-ß-cyclodextrin as mucoadhesive drug carriers with controlled drug release capability. Additional details are available in the following reference: M. Prabaharan and S. Gong, Carbohydrate Polymers, 73 (2008) 117-125.

RIVERBOAT CRUISE

Conference participants relaxed on a riverboat cruise Tuesday evening. After enjoying a dinner of traditional Chinese dishes, participants toured down the Yangtze and Jialing Rivers, at the confluence of which sits the city of Chongqing. Attendees were wowed by nighttime city views, resplendent with brightly flashing lights and futuristic neon buildings, many of which were turned on solely for the benefit of the conference tour. In true Chongqing style, river traffic was a contrast of gritty industrial crafts and lavish leisure boats and the cruise provided a unique and fascinating city experience.

SCENES FROM THE CONFERENCE

Our outstanding group of student volunteers

 
Posters

 
Posters

 
Lunch

 
Quongqing

© Materials Research Society, 2008