Since nano-sized metallic powders exhibit a considerably reduced melting point compared to bulk materials, by using a proper deposition technique, for example, drop-on-demand ink jet printing, suspensions containing metallic nanoparticles (NPs) could be applied to manufacture desired conductive patterns after a low temperature thermal process. Thus, interconnects for microelectronic packaging could be prepared without the use of lithography-etching. This study investigated the interfacial behavior between thiol-stablibized Au NPs deposits and electronic substrates, Cu, Ni and Ag. After curing at a low temperature of 300°C, continuous Au films with acceptable adhesion strength could be obtained. Instead of sintering or agglomerating, curing at 300°C may result in an entire or partial melting of the deposited suspension because of the drastically reduced melting point of Au NPs, measured to be 230 °C ~270°C, due to the nano-size effect. The elemental depth profiles and the chemical shifts of binding energy examined by an X-ray photoelectron spectroscopy (XPS) demonstrated that stoichiometric intermetallic phases existed at both the Au/Cu and Au/Ni interfaces, while a miscible solid solution layer was found to emerge at the interface between Au/Ag.
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Nonvolatile Interfacial Resistive Switching on the Nanoscale
Stephen Tsui, N. Das, Y.Q. Wang, Y.Y. Xue, and C.W. Chu*
Texas Center for Superconductivity at the University of Houston, Houston, TX 77204-5002
*Hong Kong University of Science and Technology, Texas Center for Superconductivity at the University of Houston, and Lawrence Berkeley National Laboratory
A major driving force in nonvolatile memory research is the need to achieve ever decreasing device size scales. A possible candidate is the polarity-dependent field-induced resistive switch occurring at the interface between a metal electrode and an oxide material. The interfacial nanolayer created by the voltage pulses can be driven reversibly to either a nonvolatile low (on) or high (off) resistive state by a positive or a negative pulse. The switching in this nanolayer appears to be associated with modifications of the defect structures along percolative paths. Our results, therefore, indicate the possibility of locally switching individual nanoscale percolation paths, although the statistical limit based on the average interface resistivity may further restrict the device size. With better understanding of the switching mechanism and better interface control, nonvolatile resistive nanoscale memories are a possibility.
II-12
Nano-Nitrides and Applications
L. W. Tu,* Y. J. Tu, M. Chen, Y. T. Lin, C. L. Hsiao, and N. J. Ho
Department of Physics and Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung, Taiwan 80424, Republic of China
Q. Y. Chen,1 H. W. Seo,2 and W. K. Chu1
1Department of Physics and Texas Center for Superconductivity at University of Houston, Houston, Texas 77204-5002, USA
2Department of Physics, University of Arkansas, Little Rock, AR, USA
Nitride nanorod structures are grown on Si(111) substrates by plasma-assisted molecular beam epitaxy. With the possible advantage of a less defective structure, potential applications are explored. In a proper growth parameter window with a buffer layer, sparse and well developed nanorods with a hexagonal shape are formed while under a growth condition without a buffer layer, high-density nanorods can be obtained. These nanorods are all aligned along a unidirection of crystallographic c-axis. Fundamental properties are characterized through a series of measurements and analyses including high-resolution transmission electron microscopy, field-emission scanning electron microscopy, photoluminescence, cathodoluminescence, micro-Raman spectroscopy, X-ray diffraction, energy dispersive spectrometer, etc. Double heterostructures of InGaN/GaN are grown under various growth conditions with a basic p/n junction structure in the nanorods. Ni/Au contacts on p-GaN are fabricated and electroluminescence is performed. Colors in visible range from red to purple are seen.
II-13
Recent progress on growth of GaN/AlGaN quantum confined structures for THz quantum cascade laser
S. C. Wang*, Richard Soref**, and Greg Sun***
* Department of Photonics & Institute of Electro-Optical Engineering,
National Chiao Tung University, 1001 TA Hsueh Road, Hsinchu,Taiwan,30010
**Air Force Research Laboratory, AFRL/SNHC, Hanscom AFB, MA 01731 USA
*** University of Massachusetts, Physics Dept., Boston, MA 02125 USA
The quantum-cascade laser (QCL) has, since its first realization, demonstrated an impressive and rapid development, extending the emission wavelengths from mid-infrared to terahertz spectral range. However, QCLs based on GaAs/AlGaAs and AlInAs/GaInAs are not capable of emitting in the energy range around the LO-phonon energies, leaving a gap in the spectral scale between 30 and 40 µm. The QCL based on AlGaN/GaN material has been reported as a promising candidate for generating emission in this wavelength range. One of the key technical issues for realization of AlGaN/GaN based QCL is the growth of high quality GaN structures suitable for fabrication of QCLs. In particular the good surface morphology, Al compositional accuracy, and precise thickness control of the quantum well of the grown structure are very important factors for the QCL. In this report we present the recent progress on the growth of AlGaN/GaN quantum confined structure using metal organic chemical vapor deposition (MOCVD). We have established growth conditions for obtaining good surface morphology AlxGa1-xN epitaxial layer with Al composition ranging from 0 < x < 1.0. A 60-period quantum cascade GaN/AlGaN structure suitable for serving as the active region of THz QCL was grown and analyzed. The grown sample showed smooth surface morphology with sharp layer interfaces. The composition and thickness of each period of quantum well were estimated from the simulation of XRD satellite peaks and TEM measurements. Various phonon modes of the grown sample were also observed using Fourier transform infrared spectrometer. These preliminary results indicate the MOCVD should be a viable method for growth of AlGaN/GaN based THz QCL structure.
II-14
Graphene-silica Composite Thin Films
Supinda Watcharotone
Northwestern University
II-15
Silicon and metal silicide nanowires: Structure and energy decomposition analysis from first principles
Boris I. Yakobson, N. Gonzalez Szwacki, Y. Lin
ME&MS Dept., Rice University, Houston, TX 77006
Succesful synthesis of extremely thin, 1-10 nm diameter nanowires of silicon (Si) and metal-silicides (MeSi) has posed a compelling problem for theory: What energy factors contribute in the stability of such structures and what is the ground-state shape for the smallest diameters? We have developed a systematic approach which allows one to perform such evaluation. Notably, the best shape for SiNW appears to be pentagonal [1,2], for the diameters below 5-6 nm. For MeSiNW, we extend this energy-decomposition approach to the binary compounds and demonstrate its applicability by comparison with direct ab initio computations. First principle calculations using density functional theory and the gradient-corrected LSDA approximation have been performed to examine the structural, electronic, and elastic properties of Y and Ni encapsulated silicon clusters and (2,2) nanotubes. The total energy is decomposed into the bulk, surface, and edge contributions [2,3] and a simple equation proposed for the cohesive energy E(n, m) of arbitrary wire as a function of its cross-section dimension n and m.
[1] Y. Lin, N. Gonzalez Szwacki, and B.I. Yakobson, chapter in: Nanosilicon (Elsevier, 2007).
[2] Y. Zhao and B. I. Yakobson, Phys. Rev. Lett. 91, 035501-1 (2003).
[3] N. Gonzalez Szwacki and B.I. Yakobson, Phys. Rev. B 75, 035406 (2007).
A Facile Method for Preventing the Aggregation of Large Gold Nanoparticles and the Fabrication of 2-D Arrays
Shishan Zhang, Gyu Leem, La-ongnuan Srisombat, and T. Randall Lee
Departments of Chemistry and Chemical Engineering, University of Houston
4800 Calhoun Road, Houston, TX 77204-5003
Bidentate and tridentate surfactants were evaluated for their ability to stabilize large Au nanoparticles (>15 nm) in solution. Citrate-stabilized gold colloid (20-50 nm) treated with various of the multidentate surfactants were extracted from the aqueous phase and dispersed into toluene. The absence of gold colloid aggregation was confirmed visually and this observation was supported by dynamic light scattering (DLS) and UV-vis spectroscopy. The tridentate thiol (1,1,1-tris(mercaptomethyl)pentadecane) showed a superior ability in preventing large gold colloid from aggregation. For gold nanoparticles modified by these multidentate thiols, only bound thiolate (S2p3/2 binding energy of 162 eV) was detected by X-ray photoelectron spectroscopy (XPS). The large gold nanoparticles dispersed in nonpolar solvents are able to self-assembly as 2D hexagonal close-packed (hcp) arrays.
II-17
Unsymmetrical Phosphatidyl Choline Derivatives for Use as 2-D Surfactants in Monolayer Films
Zhongcheng Zhang,1 Daniel K. Schwartz,2 and T. Randall Lee1,*
1Departments of Chemistry and Chemical Engineering, University of Houston, 4800 Calhoun Road, Houston, TX 77204-5003; 2Department of Chemical & Biological Engineering, 424 UCB, University of Colorado, Boulder, CO 80309-0424
This presentation describes the synthesis and study of linactants (line-active molecules) for use as two-dimensional surfactants. Our first generation linactants are unsymmetrical phospholipids having chemically dissimilar tail groups. Specifically, intramolecular immiscibility is designed to arise from an incompatibility between dissimilar hydrophobic tails. When mixed with corresponding single-chain surfactants in a Langmuir-Blodgett architecture, linactants play a role in two-dimensional monolayers analogous to that which surfactants play in bulk three-dimensional systems. In particular, linactants can be used to stabilize lithographically-defined nanoscale structures that are the two-dimensional analogs of self-organized bulk materials that form due to amphiphilic self-assembly, such as micelles, emulsions, and lyotropic liquid crystalline phases.
