Although I knew beforehand through the physics grapevine that both Walter Brown and Rufus Ritchie had died, their obituaries published together in the April 2018 issue of Physics Today (pages 64 and 65) caused me to recognize how our colleagues’ interactions and accomplishments affect our careers. I had the great good fortune to work with both Brown and Ritchie. I did my graduate work at Rutgers University at a time when Bell Labs shared development and operation of a new tandem accelerator facility. Many Bell employees, including Brown and Walter Gibson, had joint appointments and did research on the Rutgers–Bell tandem.When Gibson, my thesis adviser, went on sabbatical, Brown mentored me throughout my thesis work. After I graduated, my first job in the real world was in Brown’s group at Bell Labs. In addition to continuing my research on the Rutgers–Bell tandem, I assisted Brown in converting a small neutron generator for use in semiconductor research; that led us to Oak Ridge, Tennessee, in search of a multi-ion source that could modify the accelerator for ion implantation of various ion species.Oak Ridge National Laboratory (ORNL) had a long history in the development of multi-ion sources because they were used in calutron facilities that separated uranium isotopes for the first atomic bomb. But ORNL also had lots of accelerators, including a tandem one used for research similar to that pursued at the Rutgers–Bell tandem. Through interactions with scientists there and continued contacts at conferences, I got to know the ORNL group well, and my next career move was to go work with them. Several of our varied experimental research interests led us to Ritchie, who became a significant source of theoretical support and collaboration.The obituaries in Physics Today were skillfully written by colleagues whose careers were also touched by Brown and Ritchie. Hundreds of others were influenced by them directly and thousands more by their research accomplishments. As noted, Brown was an excellent and prolific experimentalist, and working in his group set high standards for planning and performing experiments. Ritchie was an experimentalist’s theorist who could listen to the puzzling trends of your measurements and come back with supporting explanations of the observations and projections of the possibilities.As co-editor-in-chief of a review journal, I am always impressed to see how knowledge in all fields builds on the accomplishments of others. These two scientists were exemplary for the numerous areas of physics their careers touched, as their obituaries show. Section:ChooseTop of page <<© 2018 American Institute of Physics.

In this paper, we report a systematic study that shows how the numerous processing parameters associated with ion implantation (II) and pulsed laser annealing (PLA) can be manipulated to control the quantity and quality of graphene (G), few-layer graphene (FLG), and other carbon nanostructures selectively synthesized in crystalline SiC (c-SiC). Controlled implantations of Si− plus C− and Au+ ions in c-SiC showed that both the thickness of the amorphous layer formed by ion damage and the doping effect of the implanted Au enhance the formation of G and FLG during PLA. The relative contributions of the amorphous and doping effects were studied separately, and thermal simulation calculations were used to estimate surface temperatures and to help understand the phase changes occurring during PLA. In addition to the amorphous layer thickness and catalytic doping effects, other enhancement effects were found to depend on other ion species, the annealing environment, PLA fluence and number of pulses, and even laser frequency. Optimum II and PLA conditions are identified and possible mechanisms for selective synthesis of G, FLG, and carbon nanostructures are discussed.

Ion beam processing of materials has a tradition at Oak Ridge National Laboratory that is as old as the laboratory itself. Consequently, when we began looking for a competitive way to participate in the excitement and new physics beginning to emerge from the fabrication and study of artificially structured materials, it was natural to look for a growth technique that incorporated ion beam processing. Our division, the Solid State Division, has a variety of ion implantation and ion beam analysis accelerators which are integrated with pulsed-laser sources into ultrahigh vacuum (UHV) surface analysis and processing chambers. These facilities allow us to do ion beam and laser processing of materials in UHV at temperatures from liquid helium to several hundred degrees centigrade and to study these alterations in situ by a variety of ion beam (ion scattering, ion channeling, nuclear reactions, etc.) and surface analysis (low energy electron diffraction, Auger, etc.) techniques. Since isotope separation has been done continually at ORNL for almost 45 years, the idea and advantages for altering this technique to do materials fabrication in UHV were immediately obvious. In the following article we will briefly review the history of the ion beam deposition (IBD) concept, describe our preliminary apparatus, and point out the inherent advantages of IBD for fabricating and studying artificially structured materials. Recent results obtained by IBD will be presented.

The authors present a focused ion beam lithography (IBL) instrument and its extension toward using different ion species beyond gallium. The base instrument utilizes a lithography architecture and an ion source and column dedicated to nanofabrication. This includes large area navigation and patterning by a laser interferometer stage, long-term beam to sample positional as well as beam current stability and automation capabilities. Since the ion type can have dramatic consequences on the resulting nanostructures, the authors have extended the gallium IBL tool's ion column and source toward the stable delivery of multiple species for a nanometer scale focused ion beam based on a liquid metal alloy ion source. The IBL system is equipped with an E x B mass filter capable of selecting different single and multiple charged ion species, simultaneously originating from the same source. The authors investigated different AuSi or AuGe based sources and in particular an ion source delivering Au, Si and Be focused ion beams regarding beam current stability, beam diameter and patterning performance. Depending on the ion species and the interaction with the sample material, the authors achieved sub-20 nm results both for beam diameter and minimum line width by direct milling. The alloy ion sources show stability comparable to gallium during measurements over 10-20 h and a lifetime of several months. Initial and potential applications in the field of selective graphene synthesis, membrane and nanopore patterning as well as photonics and plasmonics are discussed. (C) 2013 American Vacuum Society.

We investigate the electronic transport properties across the pentacene/graphene interface. Current transport across the pentacene/graphene interface is found to be strikingly different from transport across pentacene/HOPG and pentacene/Cu interfaces. At low voltages, diodes using graphene as a bottom electrode display Poole–Frenkel emission, while diodes with HOPG and Cu electrodes are dominated by thermionic emission. At high voltages conduction is dominated by Poole–Frenkel emission for all three junctions. We propose that current across these interfaces can be accurately modeled by a combination of thermionic and Poole–Frenkel emission. Results presented not only suggest that graphene provides low resistive contacts to pentacene where a flat-laying orientation of pentacene and transparent metal electrodes are desired but also provides further understanding of the physics at the organic semiconductor/graphene interface.

A technique is presented to selectively graphitize regions of SiC by ion implantation and pulsed laser annealing (PLA). Nanoscale features are patterned over large areas by multi-ion beam lithography and subsequently converted to few-layer graphene via PLA in air. Graphitization occurs only where ions have been implanted and without elevating the temperature of the surrounding substrate. Samples were characterized using Raman spectroscopy, ion scattering/channeling, SEM, and AFM, from which the degree of graphitization was determined to vary with implantation species, damage and dose, laser fluence, and pulsing. Contrasting growth regimes and graphitization mechanisms during PLA are discussed.

We report on the magnetic properties of MoS2 measured from room temperature down to 10 K and magnetic fields up to 5 T. We find that single crystals of MoS2 display ferromagnetism superimposed onto large temperature-dependent diamagnetism and have observed that ferromagnetism persists from 10 K up to room temperature. We attribute the existence of ferromagnetism partly to the presence of zigzag edges in the magnetic ground state at the grain boundaries. Since the magnetic measurements are relatively insensitive to the interlayer coupling, these results are expected to be valid in the single layer limit.

We describe a straightforward technique for selective graphene growth and nanoribbon production onto 4H- and 6H-SiC. The technique presented is as easy as ion implanting regions where graphene layers are desired followed by annealing to 100 °C below the graphitization temperature (TG) of SiC. We find that ion implantation of SiC lowers the TG, allowing selective graphene growth at temperatures below the TG of pristine SiC and above TG of implanted SiC. This results in an approach for patterning device structures ranging from a couple tens of nanometers to microns in size without using conventional lithography and chemical processing.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

We demonstrate single layer graphene/n-Si Schottky junction solar cells that under AM1.5 illumination exhibit a power conversion efficiency (PCE) of 8.6%. This performance, achieved by doping the graphene with bis(trifluoromethanesulfonyl)amide, exceeds the native (undoped) device performance by a factor of 4.5 and is the highest PCE reported for graphene-based solar cells to date. Current-voltage, capacitance-voltage, and external quantum efficiency measurements show the enhancement to be due to the doping-induced shift in the graphene chemical potential that increases the graphene carrier density (decreasing the cell series resistance) and increases the cell's built-in potential (increasing the open circuit voltage) both of which improve the solar cell fill factor.

An improved process for graphene transfer was used to demonstrate high performance graphene enabled vertical organic field effect transistors (G-VFETs). The process reduces disorder and eliminates the polymeric residue that typically plagues transferred films. The method also allows for purposely creating pores in the graphene of a controlled areal density. Transconductance observed in G-VFETs fabricated with a continuous (pore-free) graphene source electrode is attributed to modulation of the contact barrier height between the graphene and organic semiconductor due to a gate field induced Fermi level shift in the low density of electronic-states graphene electrode. Pores introduced in the graphene source electrode are shown to boost the G-VFET performance, which scales with the areal pore density taking advantage of both barrier height lowering and tunnel barrier thinning. Devices with areal pore densities of 20% exhibit on/off ratios and output current densities exceeding 10(6) and 200 mA/cm(2), respectively, at drain voltages below 5 V.

Diodes based on metal-semiconductor interfaces are common place in semiconductor electronics. What happens when the normal metal is replaced by monolayer graphene? A group of physicists at University of Florida experimentally demonstrate that graphene-semiconductor interfaces make interesting diodes for a surprisingly wide variety of semiconductors.

An attempt is made to fabricate positive-index waveguides by ion implantation of metal ions into crystals of LiNbO3. The observed structural, chemical, and optical damage is summarized and possible solutions are discussed.

Semiconductor ZnO and GaN nanowires have shown the ability to detect many types of gases, biological and chemical species of interest. In this review, we give some recent examples of using these nanowires for pH sensing, glucose detection and hydrogen detection at ppm levels. In addition, new ion beam systems with nm resolution capability are now available. This opens up the possibility of directly writing doped regions into semi-insulating materials such as undoped GaAs or GaN for creation of arrays of nm channel transistors and other active elements. Initial work in this area is discussed.