Frank Eugene Jones

How metal films de-wet substrates?identifying the kinetic pathways and energetic driving forces

We study how single-crystal chromium films of uniform thickness on W(110) substrates are converted to arrays of three-dimensional (3D) Cr islands during annealing. We use low-energy electron microscopy (LEEM) to directly observe a kinetic pathway that produces trenches that expose the wetting layer. Adjacent film steps move simultaneously uphill and downhill relative to the staircase of atomic steps on the substrate. This step motion thickens the film regions where steps advance. Where film steps retract, the film thins, eventually exposing the stable wetting layer. Since our analysis shows that thick Cr films have a lattice constant close to bulk Cr, we propose that surface and interface stress provide a possible driving force for the observed morphological instability. Atomistic simulations and analytic elastic models show that surface and interface stress can cause a dependence of film energy on thickness that leads to an instability to simultaneous thinning and thickening. We observe that de-wetting is also initiated at bunches of substrate steps in two other systems, Ag/W(110) and Ag/Ru(0001). We additionally describe how Cr films are converted into patterns of unidirectional stripes as the trenches that expose the wetting layer lengthen along the W[001] direction. Finally, we observe how 3D Cr islands form directly during film growth at elevated temperature. The Cr mesas (wedges) form as Cr film steps advance down the staircase of substrate steps, another example of the critical role that substrate steps play in 3D island formation.

Electrical conduction and photoluminescence properties of solution-grown ZnO nanowires

We report on the optical and electrical properties of zinc oxide nanorods synthesized in solution using Oswald ripening of ZnO nanodots with the addition of ethylenediamene growth directing agent. This method results in high quality, single crystalline ZnO nanorods that extend up to 3μm in length and have an average diameter of 25±7nm, compared to ∼75nm diameter for similarly prepared nanorods but without the addition of the growth directing agent. Furthermore, we find that the higher aspect ratio nanorods exhibit strong size-dependent electrical characteristics, with a critical diameter of about 27nm delimiting nonconductive and conductive behaviors. Theoretical calculations indicate that the origin of this size-dependent conductivity is the presence of surface states that deplete the carriers in the smaller diameter nanorods, and an estimate of the density of these states is provided.

Microwave dissipation in arrays of single-wall carbon nanotubes

The transmission and reflection scattering parameters of arrays of single-wall carbon nanotubes (SWCNTs) directly assembled onto coplanar waveguides (CPWs) have been measured from 0.01to50GHz at room temperature. Typical arrays consisted of roughly ∼103 SWCNTs aligned parallel to the electric field polarization of the propagating field. Scattering parameters were measured on CPWs both before and after SWCNT assembly, allowing separation of SWCNT effects from the characteristics of the bare CPWs. Additional frequency-dependent power dissipation was consistently observed after assembly of SWCNT arrays.

Robustness of nanotube electronic transport to conformational deformations

We present experimental observation and theoretical analysis of looping carbon nanotubes connecting two electrodes. The measured conductance of the nanotubes is not strongly affected by the presence of these conformational defects, a result that is confirmed by quantum transport calculations. Our work indicates that solution-based fabrication methods for carbon nanotube devices can have high conformational defect tolerance, except for defects with 5–10nanometer bending radius.

RF/Microwave properties and applications of directly assembled nanotubes and nanowires: LDRD project 102662 final report.

LDRD Project 102662 provided support to pursue experiments aimed at measuring the basic electrodynamic response and possible applications of carbon nanotubes and silicon nanowires at radiofrequency to microwave frequencies, approximately 0.01 to 50 GHz. Under this project, a method was developed to integrate these nanomaterials onto high-frequency compatible co-planar waveguides. The complex reflection and transmission coefficients of the nanomaterials was studied as a function of frequency. From these data, the high-frequency loss characteristics of the nanomaterials were deduced. These data are useful to predict frequency dependence and power dissipation characteristics in new rf/microwave devices incorporating new nanomaterials.

Transport Properties of Stretch-Oriented PPV Films

Organic semiconductors are under investigation for radiation sensors at Sandia National Laboratories. The wide band gaps, high resistivities, low dielectric constants, and high dielectric strengths of conjugated polymers suggest these materials may be suitable for solid-state particle counting detectors. A range of solution cast materials have been evaluated for this application, including polythiophenes and poly( p -phenylene vinylene)s, or PPVs. Films were prepared by novel solution casting and mechanical stretching methods. Device structures including interdigital metal electrodes on glass and thin film transistors on SiO 2 on silicon were fabricated by drop casting from solution and lamination of solid films. Transient and DC responses were recorded and analyzed. Experiments include laser stimulus for photoconductive pulse response, and field effect transistor testing. Mechanical stretching was shown to dramatically alter electrical properties of polymer films. Future work will analyze the feasibility of single particle detection and analyze various geometries for optimization. The effects of traps and methods for reduction of trapping effects will be analyzed.