Session Z5: Synthesis, Transport, and Devices Based on Artificially Structured Materials

Ultrasmall Silver Nanopores Fabricated by Femtosecond Laser Pulses

Ultrasmall nanopores in silver thin films with a diameter of about 2 nm have been fabricated using femtosecond laser ablation in liquid [1]. Ultrafast laser pulse ablation generates highly nonequilibrium excitated states, from which silver thin films emerge and progressively grow with the assistance of capping agent molecules. During this growth process, capping agent molecules are enclaves within the film, leaving individual ultrasmall pores in the thin film. Our first-principles calculations show that the pore size is critically determined by the dimension of the confined molecules. Furthermore, by using smaller capping agent molecules, we were able to fabricate smaller nanopores with 1.6nm diameter. Our approach advances the capability of optical methods in making nanoscale structures with potential applications in areas such as near-field aperture probes, imaging masks, magnetic plasmonic resonances, and biosensing with individual nanopores. \\[4pt] [1] F. Bian, Y. C. Tian, R. Wang, H. X. Yang, H. X. Xu, Sheng Meng, and \underline {Jimin Zhao}, \textit{Ultrasmall Silver Nanopores Fabricated by Femtosecond Laser Pulses}, Nano Lett. \textbf{11}, 3251--3257 (2011).   

Effect of Li+3 Ion Irradiation on Properties of Ta and Dy Doped Bi1.5Zn0.92Nb1.5O6.92 Pyrochlores

Pyrochlore compounds with a general formula of A2B2X7 where A and B are B cations and X is anion exhibit a variety of interesting properties which allow for a broad range of applications, such as high-permittivity dielectrics, as cathode and electrolyte materials in solid electrolytes, host materials for the immobilization of fission products, catalysis, and thermal barrier coatings and fluorescence centers. Pyrochlore oxides can accommodate a wide range of solid solutions between BO2 and A2O3 compounds. Radiation effects in a wide range of pyrochlore compositions have been extensively investigated due to the potential application of pyrochlores. In this study, Ta and Dy-doped Bi1.5Zn0.92Nb1.5O6.92 (BZN) pyrochlore compounds were produced by mixed oxide technique. After determining the solubility limit of Ta and Dy in BZN by XRD, single phase Ta and Dy-doped BZN ceramics were irradiated with different fluences of Li3+ ion irradiation. The microstructures of sintered ceramics before and after irradiation were discovered by scanning electron microscopy. The effect of irradiation on dielectric properites at different frequencies and temperatures were also investigated.   

Development of ZnO/Cu Nanolaminate Materials

Promising materials to replace cost inhibitive indium-tin oxide as transparent conductive oxide layers are ZnO based alloys. Unlike In, Zn is cheap and abundant with a stable supply. Furthermore, ZnO is easily fabricated using standard industrial scale techniques. Therefore, the development of ZnO based materials may greatly advance modern electrical devices. The undoped bandgap of ZnO is 3.4 eV, and may be degenerately doped with donor species such as Al, B, or Ga. For pure ZnO, the electron mobility is ca. 200 cm$^{2}$/V, but decreases significantly with doping due to impurity scattering. Recent studies have suggested that bilayers of doped ZnO and metal may offer low enough resistivity for industrial application. Therefore, it is a logical extension to investigate the properties of ZnO/metal nanolaminate films fabricated from multiple, thin, alternating layers of doped ZnO and a metal. We will present the preliminary results of the development of ZnO based nanolaminate materials consisting of alternating layers of ZnO and Cu fabricated by reactive DC sputter deposition. Our preliminary results suggest that these materials may have applications in photovoltaic devices as well as infrared mirrors.   

Nano-structured Au surfaces by off-normal gas cluster ion beam technique

Surface nano pattern formation has generated great interest in semiconductor, optoelectronics and bio-medical industries. For the past three decade cluster ion beam technology has developed significantly. A gas cluster ion consists thousands of atoms and usually energy per atom in the cluster is about 10eV and cluster ion surface penetration is minimum. Therefore, cluster ion impact form shallow depth modifications. When a cluster ion incident on to the surface off-normal beyond a certain angle from the normal surface target atoms gains forward momentum and creates clear surface nano-ripples between angles 40 to 60 degrees from surface normal. Even though many experiments have demonstrated cluster ion ripple formation, a mechanism of these formations has limited cluster ion beam industrial applications. We will discuss experimental results of surface evolution during the cluster ion off-normal incident irradiation and azimuthal sample rotation dependence of surface patterns, and compare results with a theoretical model.   

Diamondoids enables 10nm resolution on X-ray PEEM

Diamondoids are the smallest sp3 bonded carbon cage that can be found in different sizes and shapes, starting from single cage called adamantine that contain 10 carbons all hydrogen terminated on outside. While the electronic structure and material properties of diamondoids are based on bulk diamond, their nanostructure allows for tailoring their properties to a particular task. Diamondoids can be produced reliably and cost effective in different sizes and quantities. They exhibit a tremendous potential for real-world applications, e.g. as seed crystals in diamond growth, as robust mechanical coatings or as highly efficient electron cathodes. Here, we will focus on the latter, showing how diamondoid coating can be used to push the spatial resolution to the limit and significantly increasing the efficiency of an x-ray based cathode lens microscope (X-ray Photoemission Electron Microscope or XPEEM) by minimizing the effect of chromatic aberrations caused by the energy spread of the electrons emitted from the cathode. The unique feature of XPEEM microscopy is that it is capable of acquiring images with magnetic, chemical, structural as well as topographical contrast of surfaces and shallow interfaces. The capability to obtain such images with 10nm spatial resolution through simple, low-cost and readily available nanodiamond coating represents a major step forward towards real world application of these promising nano materials. (Presenting author: Z.X. Shen)   

Quantum-Dot Cellular Automata On A Hydrogenated Silicon Surface: An Exact Diagonalization Study

Quantum-dot cellular automata (QCA) is an alternative computing paradigm for molecular-scale quantum devices. We present results from the first detailed exact diagonalization study of QCA systems using a richly extended Hubbard model. A controlled Hilbert space truncation alleviates the scaling problem and provides access to moderate system sizes. We characterize the static signal transmission for short wires and identify suitable material and system parameters for dangling bonds on a hydrogen-terminated silicon surface, a recent experimental realization. We discuss the challenges for a realistic, working implementation.   

The chemical analysis about post annealing effect of HfO2 on Si-passivated GaAs

In order to develop a high performance MOS device, 3-5 based semiconductors as a high carrier transport semiconductors have been seriously considered. Especially, GaAs with HfO2 as gate dielectric material attract as a candidate for future MOS FET device. Since, larger trap density at HfO2/GaAs interface than for HfO2/Si interface degrade device performance, Si interfacial layer was introduced to reduce interfacial trap. Moreover, Si reduces intrinsic defects at GaAs surface by reconstruct Ga or As homo bonds. In this study, we focused on changes in the chemical and structural characteristics of HfO2/Si/GaAs film as a function of post annealing temperature. The interfacial reactions induced by post annealing were investigated by XPS, REELS, and XAS. The results show that Si layer decrease the diffusion and oxide formation of Ga and As. Also, the post nitridation significantly improve the diffusion barrier by forming the Ga-N layer. XAS result also consists with the fact that the post nitridation suppress Ga diffusion. The band offsets between GaAs and high-k gate dielectric were aligned using XPS and REELS.   

Terahertz Generation Based on Gunn Oscillations in Unipolar Nanodiodes

A pressing concern in the terahertz (THz) deployment of technology is the lack of efficient, compact, solid-state THz emitters. Recently, self-switching devices (SSDs) have been demonstrated to offer a 2D planar technology with ultra-low parasitic capacitance, which enabled detection of microwave radiation up to 2.5 THz.$^{ }$Furthermore, it is possible to integrate a large array of SSDs in parallel in order to reduce overall impedance and hence reduce thermal noise. Monte Carlo simulations showed that the SSD can also emit radiation based on Gunn oscillations. In this work, we modeled using Silvaco Atlas to provide evidence of dipole domain formation in the channel and systematically study the dependence of emission frequency and intensity as a function of channel length and width as well as interface-charge density. The results showed that the fundamental oscillation frequency can reach as high as 400 GHz, whereas higher harmonics go well beyond 1.2 THz. By constructing an array that contains different geometries of SSDs placed in parallel, we expect to achieve frequency tuning in wide or narrow bands, which may have useful implications to practical applications.   

Tuning electric and magnetic responses with structure evolution in a metamaterial

We demonstrated that in an assembly of double-layered metallic U-shaped resonators, magnetic and electric responses are realized respectively. The two different types of responses are realized at higher and lower frequency. When the U-shaped structure resonators evolve into H-shaped structure gradually, the electric and magnetic frequencies move forward to each other. In this case, when electric response and magnetic response are overlapped in frequency, negative refractive index can be realized.   

Configuration interaction in a tunable wavelength shifter

It has recently been demonstrated how hybridized plasmon-phonon collective excitations in GaAs can be blue shifted by about 20 wavenumbers (0.6 THz) relative to the unperturbed longitudonal optical lattice vibration frequency as a function of excitation beam intensity [Fluegel et al., Nat. Phot. 1, 701 (2007)]. At beam intensities greater than 10 mW/cm$^{-2}$ the wavelengh shifted mode broadens and begins to exhibit a double-peak structure. We attribute this line-shape modification to configuration interaction of the named mode with the edge of the continuous background and discuss potential implications for coupled plasmon-phonon modes generated in semiconductor hetero-structures.   

Secondary Electron Emission (SEE) Calculations

Secondary electron emission (SEE) from solids is a consequence of energy loss by charged particles such as electrons. One important energy loss mechanism to be considered involves plasmon emission by the charged particle. Plasmons create electron-hole pairs when they decay. Under certain conditions, a sufficiently energetic electron, a secondary electron, may be emitted from the solid surface. In this presentation, results from two different approaches to this process will be presented. A particle-based Monte Carlo method and a continuum method will be used for these calculations. The results from the two different methods will be compared to each other to understand the effects of the various approximations in the two methods. The methods will be illustrated by an application to titanium dioxide (rutile). --Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Dark Field Imaging of Plasmonic Resonator Arrays

We present critical coupling of electromagnetic waves to plasmonic cavity arrays fabricated on Moir\'{e} surfaces. The critical coupling condition depends on the superperiod of Moir\'{e} surface, which also defines the coupling between the cavities. Complete transfer of the incident power can be achieved for traveling wave plasmonic resonators, which have relatively short superperiod. When the superperiod of the resonators increases, the coupled resonators become isolated standing wave resonators in which complete transfer of the incident power is not possible. Dark field plasmon microscopy imaging and polarization dependent spectroscopic reflection measurements reveal the critical coupling conditions of the cavities. We image the light scattered from SPPs in the plasmonic cavities excited by a tunable light source. Tuning the excitation wavelength, we measure the localization and dispersion of the plasmonic cavity mode. Dark field imaging has been achieved in the Kretschmann configuration using a supercontinuum white light laser equipped with an acoustooptic tunable filter. Polarization dependent spectroscopic reflection and dark field imaging measurements are correlated and found to be in agreement with FDTD simulations.   

Engineered Uniform Conduction Fronts in Memristive/Memcapacitive Systems

We introduce here a novel ``memristor'' design enabling the uniform propagation of the conduction front within the device, improving performance as well as device-to-device consistency. Typically, resistive switching in memristors occurs due to the localized formation of conductive filaments. Electric fields are magnified at filament tips (due to decreased separation, E = V/d), amplifying growth rates for select filaments and producing a localized and highly non-uniform conduction front. However, we show that by incorporating specifically spaced layers with alternating ionic mobilities the electric field magnification can be counterbalanced, resulting in a uniform conduction front. The uniform conduction front lowers device-to-device variability, improves analog tuning and significantly amplifies the memcapacitive properties of the device. These multilayered engineered nanostructures have potential applications in multi-bit memory storage and neuromorphic computing architectures.