This site will work and look better in a browser that supports web standards, but it is accessible to any browser or Internet device.

SEARCH CNS SITE
   

More Information

 Overview
 IRG1
 IRG2
 IRG3
 IRG4
 IRG5
 Seeds
 Nuggets

Research

IRG3, Electrons in Confined Geometries

Leader: Professor Moses Chan
IRG Faculty and Seed Faculty: James Adair, Moses Chan, Mike Chung, Vincent Crespi, Elizabeth Dickey, Peter Eklund, Sridhar Komarneni, Qi Li, Serguei Lvov, Thomas Mallouk, Theresa Mayer, Suzanne Mohney, Joan Redwing, Peter Schiffer, Jorge Sofo, Srinivas Tadigapa, David Vaughan, Jian Xu, Yaw Yeboah, Jun Zhu.

The research themes of IRG3 focus on the electronic properties of confined systems. Ongoing projects include the study of superconducting and semiconducting nanowires, ferroelectric thin films and a new effort on graphene sheets. These activities are complemented by a synthetic effort in low dimensional nanostructures. This has been a productive year for this IRG with a large number of publications including cover page articles in Nature and in Small.

Recent highlights:

Superconducting Nanowires (Chan, Mallouk and Mayer). In contrast to expectation, we found that when an array of 40 nm diameter Zn wires (ZNWs) are sandwiched between bulk superconducting Sn or In electrodes, the superconductivity of the ZNWs are suppressed either completely or partially. When the bulk electrodes are driven normal by a magnetic field, the superconductivity in the ZNWs recovers. We called this intriguing phenomenon the antiproximity effect (APE) [Phys. Rev. Lett. 95, 076802 (2005)]. Mingliang Tian gave an invited talk on this subject at the 2006 APS March Meeting, and a news article on his findings appeared in the April 1, 2006 issue of Science News. Along this line of inquiry, we have discovered a fascinating proximity effect induced by superconducting leads on magnetic (nickel) nanowires.

There have been a large number of reports on the transport and other physical properties of Bi nanowires. We found evidence, for the first time, that granular Bi nanowires fabricated electrochemically can be superconducting at either 3.9, 7.2 or 8.3 K. These three temperatures correspond to the Tc reported for the three high pressure bulk Bi phases (II, III and V) at respectively 2.55, 2.7 and 7.7GPa. Our findings on these nanowires fabricated and measured at ambient pressure indicate that nominally high pressure phases are at least metastable in nanometer scale without external high pressure. Because superconductivity is not found in single crystal wires, the high pressure phases may exist in the interfacial regions between the nanocrystal grains in the wires. Further structural studies of these interfaces are in progress.

To date, electrical contact to the nanowires in this project were made by squeezing soft (Sn, In, Pb) superconducting wires of 0.5 mm diameter directly onto the porous membrane imbedded with the nanowires. This limits us to do quasi-two lead measurements. During the past year, we have succeeded in making 4 leads electrical measurements on individual Au, Ag, Sn, Zn, Bi, Ni and AuSn nanowires with the aid of focused-ion-beam (FIB). Systematic transport and possibly tunneling measurements on these wires of different diameter are in progress.

Nanowires fabricated by various means seldom have the ideal shape, structure and configuration for proper physical measurements or the integration into useful devices. We have demonstrated that high intensity electron beam (HIEB) can be used to sculpt patterns, to drill holes of atomic dimensions on individual semiconducting and metallic nanowires. HIEB can also be used to weld together these wires, forming nanoscale junctions. Some of the results of this effort have recently been published as a cover page article in the journal Small.

Semiconductor Nanowires (Dickey, Mayer, Mohney, Redwing). One of the factors limiting the utility of nanowire field-effect transistors is high contact resistance. Heavily doping the device source/drain regions can reduce the resistantce. Using the vapor-liquid-solid (VLS) method and phosphine (PH3) as the n-type dopant source, we have fabricated highly doped (P=1019-1020 cm-3), single crystal, low resistive (less than 10-2 ohm(cm) SiNWs suitable for device integration [Nano Lett. 5, 2139 (2005)]. The study also reveals the presence of unintentional acceptors in nominally undoped SiNWs, which can compensate donors at low acceptor concentrations.

Surface properties also affect the performance of NW-FETs by giving rise to severe hysteresis in the output characteristics and subthreshold properties of unpassivated devices that are fabricated with global back-gate geometry. By using thermally oxidized and passivated Si/SiO2 core-shell structures and a geometry of both global back-gate and top-gate, we have reduced the surface effects in our novel NW-FETs (submitted to J. Vac. Sci. Technol.). Our FETs exhibited improved stability and bulk switching function due to field-induced depletion of the NW channel (IEEE Dev. Res. Conf. Tech. Digest, 2005). These devices typically have ION/IOFF ~ 105, threshold voltage ~ 1 V, subthreshold slope S ~ 180 mV/dec, transconductance gm ~ 0.1 microS, and maximum ION ~ 50 nA per NW with a back-gate voltage VBG = 0V. Also, the subthreshold slope is considerably lower than any reported previously on top-gated NW-FETs.

Currently we are working on intentional doping, studying the effects of electrostatic and in situ source/drain doping on the transport properties of SiNW FETs, and the integration of smaller ( 5-10 nm) diameter SiNW. We are also pursuing atom probe tomography for mapping dopants and impurities in nanowires at the atomic scale, and on-wafer TEM microscopy characterization to facilitate structure-transport property studies.

Nano-Microscale Piezoelectrics (Trolier-McKinstry, Jackson, Mayer, Tutwiler). The main goal of the nanoscale/microscale piezoelectrics effort is to quantify the effect of mechanical constraints on the dielectric and piezoelectric response of ferroelectric thin films. This is essential to predicting and controlling the behavior of devices as they are scaled down in dimension. We found evidence that domain wall motion is far less (an order of magnitude) important in thin ferroelectric films mechanically constrained by their substrate than in bulk ferroelectrics. We have now demonstrated a mechanism whereby non-ferroelastic domain wall motion contributes to the net piezoelectric response of thin films. This new mechanism dominates piezoelectric nonlinearity in clamped thin films, although it is a negligible contributor in bulk ceramics. The essential physics of the model entails nearly reversible motion of domain walls through a random potential that is responsible for wall pinning. Our model successfully describes the amplitude and frequency dependence of the piezoelectric coefficients as well as the first three harmonics of the strain response and their phase angles.

In prior years we prepared ferroelectric microtubes using vacuum infiltration of Si templates and a XeF2 release. This year we identified a reaction that degraded the electrical conductivity of the LaNiO3 electrodes, and optimized the processing of infiltrated Pd as an alternative. These processing routes allow us to lead the effort in making high frequency ultrasound transducers with cellular resolution and drive voltages low enough allow direct integration with CMOS electronics. We have miniaturized all of the necessary electronics for a 2 dimensional steerable array from a cart-sized unit to a 5 mm x 5 mm integrated circuit. Such a system will permit, for example, investigation of how cancer metastasizes into the bone, and the monitoring of drug reactions with individual live cells.

Synthesis of Nanostructures (Komarneni, Vaughan). Our synthetic group successfully fabricated bismuth sulfide, bismuth oxide, selenium and tellurium nanowires by a protein-assisted hydrothermal method. (Chem. Comm. 2005, 531; Langmuir 2005, 21, 6002; Chem. Mater. 2006, 18, 159) This bio-molecular-mediated method, which is economical and environmentally friendly, showed its generality for the synthesis of one-dimensional nanostructures. In addition, Rh and BaTiO3 nanowires, which may have special micromechanical and dielectric properties, were made by hydrothermal/solvothermal techniques.

Synthesis and Physics of Graphene sheets (Eklund, Tadigadapa, Crespi). Electrons in a single-layer graphene sheet form a 2-d system with a linear dispersion relation near the Fermi energy and can behave as 'massless Dirac particles'. Quantum Hall effects are also observed. The fact that graphene sheets have very high electrical conductivity provides interesting possibilities for microelectronic technologies. The fact that these single layer sheets have chemically inert surface make such system ideal gas sensors via physical adsorption. There are substantial theoretical and experimental expertise on graphite, carbon nanotubes and other novel carbon materials on the Penn State campus (Eklund, Crespi, Zhu) We also have expertise on micro and nanoelectromechanical systems (Tadigadapa) and on the physics of correlated electrons (Jain, Mahan). It is therefore natural for the MRSEC to launch a program on the physics of graphene. Although this project started only recently, we have already succeeded in produced high quality graphene sheets of 1, 2 and 3 atomic layers in thickness and we have made Raman scattering measurements on these systems.