Meeting Scene - Day 4

	In cooperation with the European Materials Research Society (View Editorial Policy)

E-MRS 2007 Spring Meeting May 28 - June 1, 2007 | Strasbourg, France Daily dispatch from Strasbourg Day 4 Thursday, May 31, 2007

Thursday was the penultimate day of the E-MRS 2007 Spring Meeting which concluded on Friday in Strasbourg, France. With over 2100 attendees from 61 countries, the conference was clearly a success. E-MRS and the organizers are to be congratulated for the success of the meeting.

TECHNICAL SESSIONS

Symposium I: Advances in Transparent Electronics: From Materials to Devices - II

Oxide Thin Film Transistors for AM-OLED Applications
In his presentation on Tuesday, H. G. Kim demonstrated the remarkable advanced capabilities of using amorphous IGZO TFTs in active matrices for OLED displays. IGZO TFTs with different gate insulators were fabricated by conventional photo-lithography and wet-etching processes. Silicon nitride, silicon oxide and their multi-layers were used as gate insulator materials. The characteristics of IGZO TFTs were dramatically changed by varying the gate insulators. He showed a video with a display of 176 (3RGB) x 200 pixels. This is real proof that we can substitute amorphous silicon with these new multicomponent oxide semiconductors with improved performances. The door is now open!

Active Matrix Color Electronic Paper Driven by Fully Transparent Amorphous Oxide TFT Array
Manabu Ito, of the Technical Research Institute, Toppan Printing Co., Ltd described an active matrix color electrophoretic display driven by a transparent amorphous oxide TFT (thin film transistor) array. In order to avoid the precise alignment that must be employed in conventional TFT arrays, the InGaZnO TFT array is fabricated directly onto the color filter array, which makes the alignment of the color filter and TFT array very precise. The fabricated amorphous InGaZnO TFT has excellent properties and very good color images have been demonstrated. The fully transparent a-InGaZnO TFT array does not affect the visibility of the display. The energy consumption of this array is low because the pixels are switched although the switching speed is a little slow at present.

Symposium J: Interfacial Nanostructures in Ceramics: A Multiscale Approach

Four Million Equations for a Grain Boundary
Christopher T. Koch and his colleagues at the Max Planck Institute for Metals Research have developed an inline holographic approach to map the potentials of grain boundaries in ceramics. "We’re looking for a continuum, rather than an atomistic view," Koch stated at the start of his presentation. Using a TEM, inline holography was accomplished by recording images of the sample at different depths behind the lens. Essentially, they recorded Fresnel fringes. While the experiment itself was not that difficult, calculating the results was; the researchers had to develop software that solves approximately 4 million non-linear equations to reconstruct the electron wave phase shift from a series of Fresnel images. Koch was happy to report that the software is completed.

The researchers have applied this new method to the determination of potential profiles across interfaces in various ceramics, most notably SrTiO₃. The electron wave phase shift is large at grain boundaries relative to the grains themselves because grain boundaries tend to collect impurities, resulting in different sizes, densities, or charges at the interface. Separating charge effects from local mass or density effects was accomplished by tomographic holography. In this method, the scientists partitioned the sample potential in elongated voxels and recorded a potential profile at several different beam tilt angles. The result was an excellent image of a triple junction in an SrTiO₃ sample, with a potential barrier of -0.8 V, which matched expectations for this type of boundary. In another experiment, they analyzed a complex Cu/W sample that had undergone severe plastic deformation, and were able to locate the Cu and W atoms with high precision using this method.

Symposium O: Functional Organic and Inorganic Materials for Micro and Nano Bio-sensing Systems

Hard Template Synthesis of Metal Oxide Gas Sensors
"This is the dream," stated J.R. Morante of the University of Barcelona at the start of his presentation, referring to the ideal mesoporous material. The dream material would be a perfect 3D network of nanocrystals with controllable, identical pores, and controllable, identical contacts between nanocrystals. Of course, the reality does not approach these ideal parameters; crystals have too many variations in their orientation, so reactive sites are not always presented to molecules passing by; nanocrystals agglomerate; and pore sizes vary, as do the necks of contact points.

In an effort to approach the ideal, Morante and his colleagues have been investigating mesoporous ( pore sizes between 2 nm and 50 nm) metal oxides as gas-sensitive resistors for gas detection applications. These mesoporous materials make excellent nanotemplates. Starting with a silica mesoporous structure, the researchers fill the pores with a metal oxide (MOx) precursor, and then dry and calcinate the filled matrix structure. When the matrix is removed using NaOH or HF, a negative replica of the precursor material remains. This "hard template" method offers the possibility of high scalability and reproducibility in manufacturing operations.

Morante has used both the SBA-15 two dimensional hexagonal silica material and the 3D cubic KIT-6 (KIT = Korean Institute of Technology) silica material as hard templates. He has investigated various MOx precursors, such as WO₃, In₂O₃ and SnO₂, and determined their response to certain gases. In the case of SnO₂, the negative replica had an average pore size of about 9 nm; this material detected NO₂ gas better than sensors made by standard sol-gel methods. WO₃ made using KIT-6 as the hard template demonstrated a higher response rate and a lower response time, making it a more sensitive NO₂ sensor than the SnO₂ material.

Symposium P: Laser Synthesis and Processing of Advanced Materials

Transmutation of Mercury into Gold
F. Bozon-Verduraz of the University of Paris discussed work by his group in using laser irradiation of colloidal nanoparticles dispersed in heavy water to release neutrons. This helps explain experimental observations of the transmutation of Hg nanoparticles into Au when suspended in D₂O. While conceding that it is not economically feasible to turn mercury into gold because mercury is so much more expensive than gold, Bozon-Verduraz argued that there is some scientific value. Experimentally, when Hg nanoparticles enriched in 196Hg were suspended in D₂O and irradiated with a laser, atomic absorption spectroscopy revealed that large quantities of gold had formed after about 4 hours. The proposed mechanism is that irradiated Hg nanoparticles emit X-rays, causing release of a slow neutron by D₂O. This neutron is absorbed by Hg, turning it into Au. Possible applications of this procedure could be in the synthesis of isotopes, or remediation of nuclear waste by using neutron bombardment to stabilize the waste.

2D Nanostructured TiN Coatings Prevent Corrosion
In the field of coatings, "Durability supposes multifunctionality," according to P. Steyer of INSA de Lyon. This multifunctionality includes resistance to wear and to environmental degradation, such as corrosion or oxidation at high temperatures. Sometimes simple chemistry changes can yield improvements in durability. For instance, adding Al to TiN to produce TiAlN increases the oxidation temperature by 300°C. In other cases, modifying the architecture of the coating is useful. Adding Ni to a CrN to form a multilayered Ni/CrN structure produces a system much more resistant to corrosion. But modifying the nanostructure of the coating has the most effect.

Steyer and his colleagues investigated the role of a 2D nanostructure coating consisting of TiN/CrN on M2 steel in preventing corrosion. They found that corrosion protection followed the trend TiN<CrN<2D-TiN/CrN. Further investigation showed that low porosity was the key to protection. TiN had a porosity of 0.3%, CrN had 0.02% porosity, and TiN/CrN 0.008% porosity. A 2D nanostructure coating also improved wear characteristics in the TiN/CrN system. In TiN, large intergranular cracks tend to form perpendicular to the surface. In the TiN/CrN nanostructured layer system, cracks parallel to the surface were found at the two nitride interfaces due to compressive stresses there. This modified cracking mechanism was responsible for better wear. In terms of high temperature oxidation, the researchers found that adding Si to TiN to form TiSiN caused an increase of almost 200°C in the temperature at which oxidation occurred. They discovered that the addition of Si drastically reduced the kinetics of oxidation—the high refractory nature of the SiN matrix provided a shield effect that resulted in oxidation resistance.

Symposium Q: Protective Coatings and Thin Films 07

OLEDs with aluminum cathode Obtained By IBAD
R.Antony of the Faculté des Sciences, Limoges (France), described green organic light-emitting diodes based on N,N’-diphenyl-N,N’-bis(3-methylphenyl)-1,1’-diphenyl-4,4’-diamine (TPD) and on tris-(8-hydroxyquinoline) aluminium (Alq3) thin films. The TPD layer is used to inject charge carriers from the anode while the Alq3 film is the emitter layer. The cathode is an Al layer and all the depositions are performed under vacuum so that the devices are not exposed to the air during fabrication. The Al is deposited at the same time as the layers are bombarded with an Argon ion beam which densifies the layer thereby reducing the permeation of water and oxygen.

OLED optoelectronic characterizations (I=f(V), L=f(V)) and lifetime measurements are measured in air. Dark spots in the layers are reduced by densification and this is thought to be due to the reduction of Al diffusion in the densified films. However the lifetime is reduced but this can be improved by the addition of a buffer layer of Al that is not densified. The optimum conditions are 200 to 300eV ion beam energies and this limits the Argon and Al diffusion into the films.

Stress and Preferred orientation in Nitride-based PVD Thin Films and Coatings
Titanium Nitride films are widely used as protective layers. Grégory Abadias, of the Université de Poitiers, France, gave an interesting talk on how plasma vapor deposition (PVD) leads to crystallographic texture in the layers which in turn affects their properties.

In particular, underlying stress at various orientations of the crystal is important. Nanomultilayer superlattices were grown with a nanocomposite coating giving hard and superhard materials where the actual hardness is influenced by the structure. TiN is grown on Si in a cubic-on-cubic epitaxy and also grown on Al2O3 and GaN where the growth is <111>. <001> and <111> are the most often reported orientations and the parameters for growth have some process dependent and some process independent parameters. Layers starting off as <001> go to <111> as the film thickness increases and <111> and <002> cross over as the power density increases or as the growth rate is reduced. Also as the substrate temperature is increased the ion to atom ratio increases which favors the <002> orientation. There are two models to explain this: a thermodynamic and a kinetics limitation model and three kinds of stress: growth stress – any microstructural modification – fixation on substrate, thermal stress – differences in expansion coefficients, and coherency stress – seen in epitaxial layers.

The results indicate that for binary TM nitrides grown by DIBS that the stress state is triaxial with a hydrostatic stress component due to growth induced defects and this can be examined by X-ray diffraction. The texture changes are not driven by strain energy minimization. For Ternary TM the addition of a third element increases the <002> orientation and Ti-Ni-N stress is reduced with increasing Ni content whereas for Ti-Ta-N the stress is independent of Ta content. It is hoped that continuation of this work will lead to a better understanding of stress build up during growth.

Symposium S (Workshop): Science and Technology of Cultural Heritage Materials: Art Conservation and Restoration

Ion Beams Solve Mysteries of Cultural Heritage Materials
J. Salomon described the benefits of using Ion Beam Analysis (IBA) in Cultural Heritage investigations. Specifically, he showed how combined Particle Induced X-ray Emission (PIXE) and Rutherford Backscattering Spectroscopy (RBS), both IBA techniques, can give valuable elemental analysis of samples to a depth of approximately 3 microns. PIXE has a high sensitivity for trace elements and can be done at atmospheric pressure, yielding the K, L, and M X-ray lines for quick and quantitative analysis, while RBS gives an accurate depth profile.

Salomon presented three case studies based on these ion beam techniques. The first was the surface analysis of a chandelier made in 1055 for the Hildesheim Dom Cathedral. From historical records we know that pieces of the original chandelier were repaired or replaced in 1901, and that in 1948 an additional restoration was done to clean it and add a wax protective coating. Using simultaneous PIXE/RBS with a single proton shot, the researchers found that the original gilding on the chandelier was an amalgam of Hg and Ag. A portion restored in 1901 had an Au/Ag gilding. In the second case study, Salomon and his coworkers used sequential proton PIXE and alpha-particle RBS to determine the composition of the luster on samples of ceramics from Spain. The proton PIXE analysis revealed that the glaze contained 30% PbO and 10% SnO2; it also detected Cu and Ag in the red luster. RBS using alpha-particles showed that a superficial sealing layer containing lead had been applied to the surface of the ceramics. Finally, the researchers used simultaneous PIXE/RBS with a single shot of alpha-particles to analyze use-wear on stone tools. The presence of Ca and P on the edge of a stone knife proved that the tool had been used to cut bones or horns. A similar study of Egyptian pre-dynastic ceremonial knives revealed that they had been used on cereal plants. "The combination of PIXE and RBS provides a wealth of information not easily attainable with any other single method," Salomon concluded.

Dating a Minoan Eruption of Santorini: A Profound Disagreement
Archaeologists and scientists disagree on the dating of the last eruption of the volcano whose remains today are known as the island of Santorini. Walter Kutschera of the University of Vienna described the discrepancy in his talk entitled "Dating a Minoan Eruption of Santorini: A Profound Disagreement." Using historical chronologies of Egyptian dynasties and archeological excavation of ceramic wares of a known period found in the pumice residue from the eruption, archaeologists place the eruption around 1540 BC. Kutschera and others, using x-ray tomography and radiocarbon 14C dating of an olive branch that miraculously survived in the 45 meter layer of pumice from the eruption, say it occurred between 1627—1600 BC. Thus, there is an approximate 120 year shift in the chronologies produced by radiocarbon dating and by historical/archeological sources. At least one prominent archaeologist contends that the written historical record must supersede any scientific arguments made by carbon dating. As a scientist, Kutschera begged to differ on this point. Resolution of the disagreement will not likely happen soon, as the ancient Egyptian chronologies have been carefully compiled over centuries of investigation, and are considered reliable in archeological circles.

     
      Korean delegates

• Conference coverage is by Dr. Michael Driver and Tim Palucka, and this report is compiled and edited by Dr. Gopal Rao
  
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