Terahertz generation measurements of multilayered GeTe-Sb2Te3 phase change materials.

Multilayered structures of GeTe and Sb2Te3 phase change material, also referred to as interfacial phase change memory (iPCM), provide superior performance for nonvolatile electrical memory technology in which the atomically controlled structure plays an important role in memory operation. Here, we report on terahertz (THz) wave generation measurements. Three- and 20-layer iPCM samples were irradiated with a femtosecond laser, and the generated THz radiation was observed. The emitted THz pulse was found to be always p polarized independent of the polarization of the excitation pulse. Based on the polarization dependence as well as the flip of the THz field from photoexcited Sb2Te3 and Bi2Te3, the THz emission process can be attributed to the surge current flow due to the built-in surface depletion layer formed in p-type semiconducting iPCM materials.

Zener Tunneling Breakdown in Phase-Change Materials Revealed by Intense Terahertz Pulses.

We have systematically investigated the spatial and temporal dynamics of crystallization that occur in the phase-change material Ge_{2}Sb_{2}Te_{5} upon irradiation with an intense terahertz (THz) pulse. THz-pump-optical-probe spectroscopy revealed that Zener tunneling induces a nonlinear increase in the conductivity of the crystalline phase. This fact causes the large enhancement of electric field associated with the THz pulses only at the edge of the crystallized area. The electric field concentrating in this area causes a temperature increase via Joule heating, which in turn leads to nanometer-scale crystal growth parallel to the field and the formation of filamentary conductive domains across the sample.

Pressure-Induced Phase Transitions in GeTe-Rich Ge-Sb-Te Alloys across the Rhombohedral-to-Cubic Transitions.

We demonstrate that pressure-induced amorphization in Ge-Sb-Te alloys across the ferroelectric-paraelectric transition can be represented as a mixture of coherently distorted rhombohedral Ge8Sb2Te11 and randomly distorted cubic Ge4Sb2Te7 and high-temperature Ge8Sb2Te11 phases. While coherent distortion in Ge8Sb2Te11 does not prevent the crystalline state from collapsing into its amorphous counterpart in a similar manner to pure GeTe, the pressure-amorphized Ge8Sb2Te11 phase begins to revert to the crystalline cubic phase at ∼9 GPa in contrast to Ge4Sb2Te7, which remains amorphous under ambient conditions when gradually decompressed from 40 GPa. Moreover, experimentally, it was observed that pressure-induced amorphization in Ge8Sb2Te11 is a temperature-dependent process. Ge8Sb2Te11 transforms into the amorphous phase at ∼27.5 and 25.2 GPa at room temperature and 408 K, respectively, and completely amorphizes at 32 GPa at 408 K, while some crystalline texture could be seen until 38 GPa (the last measurement point) at room temperature. To understand the origins of the temperature dependence of the pressure-induced amorphization process, density functional theory calculations were performed for compositions along the (GeTe)x - (Sb2Te3)1-x tie line under large hydrostatic pressures. The calculated results agreed well with the experimental data.

Giant multiferroic effects in topological GeTe-Sb2Te3 superlattices.

Multiferroics, materials in which both magnetic and electric fields can induce each other, resulting in a magnetoelectric response, have been attracting increasing attention, although the induced magnetic susceptibility and dielectric constant are usually small and have typically been reported for low temperatures. The magnetoelectric response usually depends on d-electrons of transition metals. Here we report that in [(GeTe)2(Sb2Te3) l ] m superlattice films (where l and m are integers) with topological phase transition, strong magnetoelectric response may be induced at temperatures above room temperature when the external fields are applied normal to the film surface. By ab initio computer simulations, it is revealed that the multiferroic properties are induced due to the breaking of spatial inversion symmetry when the p-electrons of Ge atoms change their bonding geometry from octahedral to tetrahedral. Finally, we demonstrate the existence in such structures of spin memory, which paves the way for a future hybrid device combining nonvolatile phase-change memory and magnetic spin memory.

Label-free methods of reporting biomolecular interactions by optical biosensors.

Reporting biomolecular interactions has become part and parcel of many applications of science towards an in-depth understanding of disease and gene regulation. Apart from that, in diagnostic applications where biomolecules (antibodies and aptamers) are vastly applied, meticulous monitoring of biomolecular interaction is vital for clear-cut diagnosis. Several currently available methods of analyzing the interaction of the ligands with the appropriate analytes are aided by labeling using fluorescence or luminescence techniques. However, labeling is cumbersome and can occupy important binding sites of interactive molecules to be labeled, which may interfere with the conformational changes of the molecules and increase non-specificity. Optical-based sensing can provide an alternative way as a label-free procedure for monitoring biomolecular interactions. Optical sensors affiliated with different operating principles, including surface plasmon changes, scattering and interferometry, can impart a huge impact for in-house and point-of-care applications. This optical-based biosensing permits real-time monitoring, obviating the use of hazardous labeling molecules such as radioactive tags. Herein, label-free ways of reporting biomolecular interactions by various optical biosensors were gleaned.

Topological insulating in GeTe/Sb2Te3 phase-change superlattice.

GeTe/Sb2Te3 superlattice phase-change memory devices demonstrated greatly improved performance over that of Ge2Sb2Te5, a prototype record media for phase-change random access memory. In this work, we show that this type of GeTe/Sb2Te3 superlattice exhibits topological insulating behavior on the basis of ab initio calculations. The analysis of the band structures and parities as well as Z2 topological invariants unravels the topological insulating nature in these artificial materials. Furthermore, the topological insulating character remains in the GeTe/Sb2Te3 superlattice under small compressive strains, whereas it is not observed as more Sb2Te3 building blocks introduced in the superlattice. The present results show that multifunctional data storages may be achieved in the GeTe/Sb2Te3 superlattice. Such kinds of artificial materials can be used in phase-change random access memory, spintronics, and quantum computing.

Single-stranded DNA (ssDNA) production in DNA aptamer generation.

The discovery that synthetic short chain nucleic acids are capable of selective binding to biological targets has made them to be widely used as molecular recognition elements. These nucleic acids, called aptamers, are comprised of two types, DNA and RNA aptamers, where the DNA aptamer is preferred over the latter due to its stability, making it widely used in a number of applications. However, the success of the DNA selection process through Systematic Evolution of Ligands by Exponential Enrichment (SELEX) experiments is very much dependent on its most critical step, which is the conversion of the dsDNA to ssDNA. There is a plethora of methods available in generating ssDNA from the corresponding dsDNA. These include asymmetric PCR, biotin-streptavidin separation, lambda exonuclease digestion and size separation on denaturing-urea PAGE. Herein, different methods of ssDNA generation following the PCR amplification step in SELEX are reviewed.

Ultrafast optical manipulation of atomic arrangements in chalcogenide alloy memory materials.

A class of chalcogenide alloy materials that shows significant changes in optical properties upon an amorphous-to-crystalline phase transition has lead to development of large data capacities in modern optical data storage. Among chalcogenide phase-change materials, Ge2Sb2Te5 (GST) is most widely used because of its reliability. We use a pair of femtosecond light pulses to demonstrate the ultrafast optical manipulation of atomic arrangements from tetrahedral (amorphous) to octahedral (crystalline) Ge-coordination in GST superlattices. Depending on the parameters of the second pump-pulse, ultrafast nonthermal phase-change occurred within only few-cycles (≈1 picosecond) of the coherent motion corresponding to a GeTe4 local vibration. Using the ultrafast switch in chalcogenide alloy memory could lead to a major paradigm shift in memory devices beyond the current generation of silicon-based flash-memory.

A BioDVD media with multilayered structure is suitable for analyzing biomolecular interactions.

A biomolecular interactive analysis with antibody-antigen and aptamer-protein was evaluated on Au-over layers deposited on the BioDVD surface. BioDVD consists of multilayered structures with Au layer on the top and it detects analytes by monitoring the changes in reflected light intensity due to analyte adsorption to the sensor surface, on which functional biomolecules are immobilized to bind specifically to the analytes. The BioDVD sensing instrument is based on a commercial digital versatile disc system, which allows the instrument to be small and inexpensive. The BioDVD platform can be fabricated utilizing mass production techniques with additional functional phase change layers that can serve both to enhance sensitivity by optimization of the interferometric cavity optical properties and also as a possible medium for the storage of test related information.

Evaluation of nucleic acid duplex formation on gold over layers in biosensor fabricated using Czochralski-grown single-crystal silicon substrate.

With a view to developing an economical and elegant biosensor chip, we compared the efficiencies of biosensors that use gold-coated single-crystal silicon and amorphous glass substrates. The reflectivity of light over a wide range of wavelengths was higher from gold layer coated single-crystal silicon substrates than from glass substrates. Furthermore, the efficiency of reflection from gold layers of two different thicknesses was examined. The thicker gold layer (100 nm) on the single-crystal silicon showed a higher reflectivity than the thinner gold film (10 nm). The formation of a nucleic acid duplex and aptamer-ligand interactions were evaluated on these gold layers, and a crystalline silicon substrate coated with the 100-nm-thick gold layer is proposed as an alternative substrate for studies of interactions of biomolecules.

A sensitive multilayered structure suitable for biosensing on the BioDVD platform.

Several technologies are currently available for the analysis of biomolecular interactions with high sensitivity and efficiency. However, these instruments are invariably expensive and, thus, are not suitable for bedside analyses. To circumvent this issue, we have previously reported a BioDVD platform that allowed us to use a DVD mechanism to monitor various biomolecular interactions [Gopinath et al., 2008, ACS Nano 2, 1885-1895]. In the present study, to improve the sensitivity of the BioDVD platform for various analyses, we have performed computer simulations to optimize the ZnS-SiO(2) layer thicknesses and determined an optimized optical interferometric response after adjusting the ZnS-SiO(2) layer thickness to 65 and 60 nm for the inner and outer layer thicknesses, respectively. Biomolecular interaction analyses performed with the optimized BioDVD disks revealed a 3-fold improvement in the sensitivity, compared to our previously reported multilayered structure. In this study, we have also shown that the BioDVD platform is suitable not only for analyzing nucleic acid hybridization and interactions between RNA-small ligands and RNA-proteins, but also for antigen-antibody interactions. Furthermore, our evaluations revealed that each sample required no more than 10 tracks of data to analyze the biomolecular interactions on the BioDVD platform, which permits a greater number of spots per BioDVD disk and also reduces the time needed to measure the biomolecular interactions.

Defining the function of beta-catenin tyrosine phosphorylation in cadherin-mediated cell-cell adhesion.

Beta-catenin is a key protein in cadherin-catenin cell adhesion complex and its tyrosine phosphorylation is believed to cause destruction of junctional apparatus. The broad spectrum of substrates for kinases and phosphatases, however, does not rule out tyrosine phosphorylation of other junctional proteins as the main culprit in reduction of cell adhesion activity. Further, the endogenous beta-catenin perturbs detailed functional analysis of phosphorylated mutant beta-catenin in living cells. To directly evaluate the effect of beta-catenin tyrosine phosphorylation in cell adhesion, we utilized F9 cells in which expression of endogenous beta-catenin and its closely related protein plakoglobin were completely shut down. We also used alpha-catenin-deficient (alphaD) cells to evaluate the role of alpha-catenin on beta-catenin tyrosine phosphorylation. We show that beta-catenin with phosphorylation mutation at 654th tyrosine forms functional cadherin-catenin complex to mediate strong cadherin-mediated cell adhesion. Moreover, we show that 64th and 86th tyrosines are mainly phosphorylated in F9 cells, especially in the absence of alpha-catenin. Phosphorylation of these tyrosine residues, however, does not affect cadherin-mediated cell adhesion activity. Our data identified a novel site phosphorylated by endogenous tyrosine kinases in beta-catenin. We also demonstrate that tyrosine phosphorylation of beta-catenin might regulate cadherin-mediated cell adhesion in a more complicated way than previously expected.

Proposal of a grating-based optical reflection switch using phase change materials.

We have proposed a novel grating-based optical reflection switch using a phase change material (PCM). The device switches on/off light or shifts the light propagation direction by switching the PCM grating between its amorphous and crystalline states. Thus, the switching status is non-volatile and the device is promising for realizing low power consumption. The device structure was designed and optimized by numerical simulations to obtain high switching efficiency. It is shown that there exists a parameter window where high efficiency is achievable. The static switching characteristics were confirmed by finite-difference time-domain (FDTD) simulations. The design scheme can also be applied to other planar dielectric gratings.

Monitoring surface-assisted biomolecular assembly by means of evanescent-field-coupled waveguide-mode nanobiosensors.

Biological self-assembly is a natural process that involves various biomolecules, and finding the missing partner in these interactions is crucial for a specific biological function. Previously, we showed that evanescent-field-coupled waveguide-mode sensor in conjunction with a SiO(2) waveguide, the surfaces which contain cylindrical nanometric holes produced by atomic bombardment, allowed us to detect efficiently the biomolecular interactions. In the present studies, we showed that the assembly of biomolecules can be monitored using the evanescent-field-coupled waveguide-mode biosensor and thus provide a methodology in monitoring assembly process in macromolecular machines while they are assembling.

Photon energy dependence of Kerr rotation in GeTe/Sb2Te3 chalcogenide superlattices.

We report on pump-probe based helicity dependent time-resolved Kerr measurements under infrared excitation of chalcogenide superlattices, consisting of alternately stacked GeTe and Sb2Te3 layers. The Kerr rotation signal consists of the specular inverse Faraday effect (SIFE) and the specular optical Kerr effect (SOKE), both of which are found to monotonically increase with decreasing photon energy over a sub-eV energy range. Although the dependence of the SIFE can be attributed to the response function of direct third-order nonlinear susceptibility, the magnitude of the SOKE reflects cascading second-order nonlinear susceptibility resulting from electronic transitions between bulk valence/conduction bands and interface-originating Dirac states of the superlattice.

A plasmonic photocatalyst consisting of silver nanoparticles embedded in titanium dioxide.

Titanium dioxide (TiO2) displays photocatalytic behavior under near-ultraviolet (UV) illumination. In another scientific field, it is well understood that the excitation of localized plasmon polaritons on the surface of silver (Ag) nanoparticles (NPs) causes a tremendous increase of the near-field amplitude at well-defined wavelengths in the near UV. The exact resonance wavelength depends on the shape and the dielectric environment of the NPs. We expected that the photocatalytic behavior of TiO2 would be greatly boosted if it gets assisted by the enhanced near-field amplitudes of localized surface plasmon (LSP). Here we show that this is true indeed. We named this new phenomenon "plasmonic photocatalysis". The key to enable plasmonic photocatalysis is to deposit TiO2 on a NP comprising an Ag core covered with a silica (SiO2) shell to prevent oxidation of Ag by direct contact with TiO2. The most appropriate diameter for Ag NPs and thickness for the SiO2 shell giving rise to LSP in the near UV were estimated from Mie scattering theory. Upon implementing a device that took these design considerations into account, the measured photocatalytic activity under near UV illumination of such a plasmonic photocatalyst, monitored by decomposition of methylene blue, was enhanced by a factor of 7. The enhancement of the photocatalytic activity increases with a decreased thickness of the SiO2 shell. The plasmonic photocatalysis will be of use as a high performance photocatalyst in nearly all current applications but will be of particular importance for applications in locations of minimal light exposure.

Influence of nanometric holes on the sensitivity of a waveguide-mode sensor: label-free nanosensor for the analysis of RNA aptamer-ligand interactions.

Evanescent-field-coupled (EFC) waveguide-mode sensors can be used to detect nucleic acids or proteins from the changes in the local index of refraction upon adsorption of the target molecule on a waveguide surface. We recently described an EFC waveguide-mode sensor in which nanometric holes on a waveguide film resulted in an improved sensitivity in the analysis of the interactions of biomolecules. In the present study, we have shown that sensitivity depends upon the diameter of the holes, where increase in diameter of holes increases spectral shift resulting in an improved sensitivity. Using this improved EFC waveguide-mode sensor, we could detect interactions between RNA and a small ligand, cyanocobalamin (vitamin B 12), and between RNA and a protein (human coagulation factor IXa). These two interactions were monitored on surfaces modified with biotin-streptavidin-biotin and N-(2-trifluoroethanesulfonatoethyl)- N-(methyl)triethoxysilylpropyl-3-amine, respectively.

Silica-based monolithic sensing plates for waveguide-mode sensors.

We developed a monolithic sensing plate for a waveguide-mode sensor. The plate consists of a SiO(2) glass substrate and a thin silicon layer the surface of which is thermally oxidized to form a SiO(2) glass waveguide. We confirmed that the sensing plate is suitable for high-sensitivity detection of molecular adsorption at the waveguide surface. In addition, a significant enhancement of the sensitivity of the sensor was achieved by perforating the waveguide with holes with diameters of a few tens of nanometers by selective etching of latent tracks created by swift heavy-ion irradiation. Possible strategies for optimizing the plate are discussed.

Study on readout durability of super-RENS disk.

Characteristics essential for the readout durability of a superresolution near-field structure (super-RENS) disk are studied experimentally by using a home-built optical measuring setup and atomic force microscope, based on a simplified PtOx super-RENS disk. The experimental results show that for a super-RENS disk with constant structure and materials, readout signals including transmittance and reflectance vary with changes in bubble shape and size, indicating that the readout durability of the disk has a strong dependence on bubble stability, which is closely related to the thickness of the cover layer, the recording power and readout power, and the mechanical properties of the dielectric layer. Based on our experimental results, the main direction for improving readout durability is also proposed.

Significant Volume Expansion as a Precursor to Ablation and Micropattern Formation in Phase Change Material Induced by Intense Terahertz Pulses.

With rapid advances occurring in terahertz (THz) radiation generation techniques, the interaction between matter and intense THz fields has become an important research topic. Among different types of THz radiation sources, the free electron laser (FEL) is a promising experimental tool that is expected to pave the way for new forms of material processing, control of phase transitions, and serve as a test bench for extreme operating conditions in high-speed small-size electrical and magnetic devices through the exploitation of strong THz electrical and magnetic fields without the presence of interband electronic excitation. In the current work, we irradiated Ge2Sb2Te5 phase change memory material with intense THz pulse trains from an FEL and observed THz-induced surface changes due to damage as a precursor to ablation and the formation of fine surface undulations whose spatial period is comparable to or slightly smaller than the wavelength of the excitation THz pulses in the material. The formation of undulations as well as the fact that no significant thermal effect was observed below the volume expansion threshold suggests that THz-induced effects mainly contributed to the observed changes. To the best of our knowledge, this is the first experimental observation of THz-induced undulations (so-called "LIPSS"), which are of potential importance for laser material processing.