Neuromorphic computational devices built from memristive materials provide a potential path toward improved power and computational efficiency in a merged biomimetic and CMOS architecture. The key to such a framework is developing materials that can be reliably engineered into neuromorphic devices and integrated with CMOS platforms. Niobium dioxide has volatile memristive properties that make it an ideal candidate for future neuromorphic electronics. In this study, we thermally crystallized and reduced thin films of amorphous niobium oxide (Nb2O5) that were deposited with atomic layer deposition. We found that doping the as-deposited niobium oxide with zinc led to a lower initial crystallization temperature, which is a necessary step toward neuromorphic integration with CMOS devices that have a strict thermal budget.

Transparent conductive meshes were fabricated by inkjet printing on flexible substrate using a percolation pattern created by random removal of conducting bonds from a regular square two-dimensional lattice. With this approach, a higher gain in optical transmittance than electrical conductivity loss is achieved above the percolation threshold. As a result of this, a figure of merit for the percolation pattern is improved with respect to a regular square mesh. The transmittance (T), sheet resistance (R), and figure of merit (F) on percentage of removed bonds for square lattices were measured. The gain of the figure of merit was observed in the range of removed bonds from 5% to 15%. Our best samples exhibit T = 84%, R = 1.3 Omega/sq, and F = 130 X 10(-3) 52(-1) (highest F value and lowest R value) and T = 93%, R = 8 Omega/sq, and F = 65 X 10(-3)Omega(-1) (highest T value). This demonstrates an excellent transparent conductive film (TCF) performance and is significantly better than any continuous TCF. The percolation meshes demonstrate good mechanical stability and the absence of a Moire effect. A distinctive feature of this method is its universality and capability of being adapted to any symmetrical or asymmetrical pattern and deposition technique.

The emergence of 2D materials has led to increased attention on correlating the structural, optical, and optoelectronic properties of atomically thin transition metal chalcogenides like MoS_2. We demonstrate the tunability of the photoluminescence (PL) properties of bulk MoS_2 via implantation of Nb ions. Raman spectroscopy is used to confirm the p-type doping. The PL intensity of MoS_2 is drastically enhanced by the adsorption of p-type dopants. X-ray photoelectron spectroscopy (XPS) is used to study the change of MoS_2 structure postimplantation. Our results provide a new route for modulating the optical properties of two-dimensional semiconductors. The strong and stable PL from defect sites of MoS_2 created by Nb ion implantation may have promising applications in optoelectronic devices.

Understanding the interaction of low energy electrons with organic thin films is important for the development of a wide range of technological applications. In this study, the interaction of 80 eV electrons with self-assembled monolayers (SAMs) of 1-decanethiol grown on Au(111) via vapor phase deposition was explored for both the lying down (striped) phase and the standing up phase. Low-energy electron diffraction measurements performed at 100 K show that the SAM loses its crystalline structure within about 3 min for the lying down phase and approximately 30 s for the standing up phase. For the standing up phase, temperature programed desorption measurements reveal two desorption features for the hydrocarbon fragments of the SAM, one centered around 130 °C and a second near 220 °C. For the lying down phase, only the higher temperature desorption feature is observed. For both phases, desorption peaks for S and H2S that are centered around 250 °C were observed, suggesting that there is a high probability for the alkane chain of the 1-decanethiol molecule to detach from the sulfur head group before desorbing from the surface. For the standing up phase, exposing the SAM to the electron beam results in a near complete attenuation of the two peaks associated with the cracking fragments of the alkane chain. However, for the lying down phase, the intensities and positions of all of the desorption peaks were similar to the unexposed SAMs, which indicates that the cross section for electron beam damage for the lying down phase is much lower than that for the standing up phase. Ex situ x-ray photoelectron spectroscopy reveals a chemical shift of almost 0.5 eV for the C-1s emission after electron exposure for the standing up phase, whereas the shift for the lying down phase was less than 0.1 eV. These results indicate that exposure of alkanethiol SAMs to 80 eV electrons results in both disordering of the SAM and decomposition of the alkanethiol molecule SAMs. For the standing up phase, the rate of decomposition is much higher than the lying down phase. The lower decomposition rate for the lying down phase is primarily attributed to the quenching of excess charge in this phase since the entire molecule is in direct contact with the metallic substrate.

The topologically protected surface states of three-dimensional (3D) topological insulators have the potential to be transformative for high-performance logic and memory devices by exploiting their specific properties such as spin-polarized current transport and defect tolerance due to suppressed backscattering. However, topological insulator based devices have been underwhelming to date primarily due to the presence of parasitic issues. An important example is the challenge of suppressing bulk conduction in Bi2Se3 and achieving Fermi levels (E-F) that reside in between the bulk valence and conduction bands so that the topologically protected surface states dominate the transport. The overwhelming majority of the Bi2Se3 studies in the literature report strongly n-type materials with EF in the bulk conduction band due to the presence of a high concentration of selenium vacancies. In contrast, here we report the growth of near-intrinsic Bi2Se3 with a minimal Se vacancy concentration providing a Fermi level near midgap with no extrinsic counter-doping required. We also demonstrate the crucial ability to tune EF from below midgap into the upper half of the gap near the conduction band edge by controlling the Se vacancy concentration using post-growth anneals. Additionally, we demonstrate the ability to maintain this Fermi level control following the careful, low-temperature removal of a protective Se cap, which allows samples to be transported in air for device fabrication. Thus, we provide detailed guidance for EF control that will finally enable researchers to fabricate high-performance devices that take advantage of transport through the topologically protected surface states of Bi2Se3.

The most common technique for producing large area graphene films is by chemical vapor deposition on Cu foil substrates. Cu is used as a substrate because the solubility of carbon in Cu at the temperatures at which the chemical vapor deposition is performed is very low. This ensures a surface-mediated growth that is self-limited to a single monolayer of graphene if low precursor pressures are used. Before performing chemical vapor deposition, the surface oxide and carbon that is on the unprocessed foil need to be removed to achieve uniform graphene growth. In addition, the roughness of the surface of the Cu foil should be reduced to help prevent defects from forming in the graphene film during growth. The goal of this research project is to determine the optimal procedure for preparation of the Cu foil substrate to produce high quality graphene. Cu foils with 99.8% and 99.999% purity were used for the experiment. The Cu substrate preparation procedure involves annealing in 1 x 10(-5) Torr of H-2 at 850 degrees C to remove the native oxide and to reduce surface roughness. This is followed by annealing in 1 x 10(-6) Torr of O-2 at 500 degrees C to remove carbon from the surface of the foil by conversion to CO2 and CO. At this temperature, the solubility of oxygen in Cu is negligible, thus preventing dissolution of oxygen into the bulk. After the oxygen anneal, the foil is annealed in 1 x 10(-5) Torr of H-2 at 850 degrees C to remove chemisorbed oxygen from the Cu surface that has formed during the anneal in O-2. The anneal durations in this study were varied to determine the optimal technique for graphene synthesis for each foil purity. The samples were characterized using X-ray photoelectron spectroscopy, scanning electron microscopy, and optical microscopy.

Intelligent engineering of graphene-based electronic devices on SiC(0001) requires a better understanding of processes used to deposit gate-dielectric materials on graphene. Recently, Al2O3 dielectrics have been shown to form conformal, pinhole-free thin films by functionalizing the top surface of the graphene with fluorine prior to atomic layer deposition (ALD) of the Al2O3 using a trimethylaluminum (TMA) precursor. In this work, the functionalization and ALD-precursor adsorption processes have been studied with angle-resolved photoelectron spectroscopy, low energy electron diffraction, and X-ray photoelectron spectroscopy. It has been found that the functionalization process has a negligible effect on the electronic structure of the graphene, and that it results in a twofold increase in the adsorption of the ALD-precursor. In situ TMA-dosing and XPS studies were also performed on three different Si(100) substrates that were terminated with H, OH, or dangling Si-bonds. This dosing experiment revealed that OH is required for TMA adsorption. Based on those data along with supportive in situ measurements that showed F-functionalization increases the amount of oxygen (in the form of adsorbed H2O) on the surface of the graphene, a model for TMA-adsorption on graphene is proposed that is based on a reaction of a TMA molecule with OH.

4:00 PM – 6:05 PM Oral Presentations 6:05 PM – 6:30 PM Break – Pizza and beverages 6:30 PM – 7:30 PM Poster presentations and networking 7:30 PM – 7:45 PM Best presentation and poster announcement 7:45 PM – 8:00 PM AVS Local Chapter Executive meeting * The presentation schedule is attached in the next page. Topical Areas Biomaterials Environmental S&T Magnetic Materials Manufacturing S&T Materials Characterization Materials Processing MEMS Microelectronic Materials Nanometer-Scale S&T Plasma S&T Surface Engineering Surface Science Thin Films Vacuum Technology

Commonly used techniques for cleaning copper substrates before graphene growth via chemical vapor deposition (CVD), such as rinsing with acetone, nitric, and acetic acid, and high temperature hydrogen annealing still leave residual adventitious carbon on the copper surface. This residual carbon promotes graphene nucleation and leads to higher nucleation density. We find that copper with an oxidized surface can act as a self-cleaning substrate for graphene growth by CVD. Under vacuum conditions, copper oxide thermally decomposes, releasing oxygen from the substrate surface. The released oxygen reacts with the carbon residues on the copper surface and forms volatile carbon monoxide and carbon dioxide, leaving a clean copper surface free of carbon for large-area graphene growth. Using oxidized electropolished copper foil leads to a reduction in graphene nucleation density by over a factor of 1000 when compared to using chemically cleaned oxygen free copper foil.

The electronic properties of graphene films depend on the number of atomic layers and the stacking sequence between the layers. One method of growing graphene films that are more than one atomic layer thick is by chemical vapor deposition on metal substrates that have non-negligible carbon solubility. This allows precipitation of carbon from the bulk during the cooling phase of the growth process. In this study, graphene films were grown on foil substrates composed of a CuNi alloy with a nominal bulk composition of 90:10 by weight. To determine the average thickness of the graphene films, angle-resolved X-ray photoelectron spectroscopy was used. For films grown at 1050°C for 5, 25, and 50min on the CuNi substrates, thicknesses of 1.06±0.14, 1.19±0.13, and 1.87±0.13 monolayers were measured, respectively. Scanning electron microscopy was used to measure the growth morphology of the graphene films and provided a method of confirming the coverages determined by the analysis of the photoemission data. Ultra-violet Raman spectroscopy measurements were also performed on the graphene films, and it was found that the G-peak intensity increases and the frequency decreases with graphene thickness.

Understanding the influence that copper substrate surface symmetry and oxygen impurities have on the growth of graphene by chemical vapor deposition is important for developing techniques for producing high-quality graphene. Therefore, we have studied the growth of graphene by catalytic decomposition of ethylene in an ultrahigh-vacuum chamber on both a clean Cu(100) surface and a Cu(100) surface predosed with a layer of chemisorbed oxygen. The crystal structure of the graphene films was characterized with in situ low energy electron diffraction. By heating the clean Cu(100) substrate from room temperature to the growth temperature in ethylene, epitaxial graphene films were formed. The crystal quality was found to depend strongly on the growth temperature. At 900 degrees C, well-ordered two-domain graphene films were formed. Predosing the Cu(100) surface with a chemisorbed layer of oxygen before graphene growth was found to adversely affect the crystal quality of the graphene overlayer by inducing a much higher degree of rotational disorder of the graphene grains with respect to the Cu(100) substrate. The growth morphology of the graphene islands during the initial stages of nucleation was monitored with ex situ scanning electron microscopy. The nucleation rate of the graphene islands was observed to drop by an order of magnitude by predosing the Cu(100) surface with a chemisorbed oxygen layer before growth. Therefore, the presence of oxygen during graphene growth affects both the relative orientation and average size of grains within the films grown on Cu(100) substrates.