Kevin F. McCarty

The Role of Surface Oxygen in the Growth of Large Single-Crystal Graphene on Copper

Oxygen Control of Graphene Growth The growth of graphene on copper surfaces through the decomposition of hydrocarbons such as methane can result in a wide variety of crystal domain sizes and morphologies. Hao et al. (p. 720, published online 24 October; see the cover) found that the presence of surface oxygen could limit the number of nucleation sites and allowed centimeter-scale domains to grow through a diffusion-limited mechanism. The electrical conductivity of the graphene was comparable to that of exfoliated graphene. Oxygen treatment of a copper surface promoted the faster growth of compact, centimeter-scale graphene domains. The growth of high-quality single crystals of graphene by chemical vapor deposition on copper (Cu) has not always achieved control over domain size and morphology, and the results vary from lab to lab under presumably similar growth conditions. We discovered that oxygen (O) on the Cu surface substantially decreased the graphene nucleation density by passivating Cu surface active sites. Control of surface O enabled repeatable growth of centimeter-scale single-crystal graphene domains. Oxygen also accelerated graphene domain growth and shifted the growth kinetics from edge-attachment–limited to diffusion-limited. Correspondingly, the compact graphene domain shapes became dendritic. The electrical quality of the graphene films was equivalent to that of mechanically exfoliated graphene, in spite of being grown in the presence of O.

Review of advances in cubic boron nitride film synthesis

Abstract Cubic boron nitride (cBN) has a number of highly desirable mechanical, thermal, electrical, and optical properties. Because of this, there has been an extensive worldwide effort to synthesize thin films of cBN. Film synthesis is difficult in that without significant levels of ion bombardment during growth, only sp2-bonded BN forms, not sp3-bonded cBN. Recently there has been considerable progress in improving the deposition techniques and cBN film quality. In addition, progress has been made in understanding how energetic deposition conditions can lead to cBN formation. However, unanswered questions remain and process improvements are still needed. In this paper we critically and comprehensively review recent developments in cBN film synthesis and characterization. First, the structures and stability of the BN phases and characterization techniques are described. Next, the key experimental parameters controlling cBN film formation and synthesis techniques are discussed. Following a review of microstructure, the proposed mechanisms of cBN formation and the observed mechanical and electrical properties of cBN films are analyzed. We conclude by highlighting the current impediments to the practical realization of cBN-film technology.

Heteroepitaxial Growth of Two-Dimensional Hexagonal Boron Nitride Templated by Graphene Edges

Heteroepitaxy Writ Thin A common method for creating a thin single-crystal layer of a semiconductor for use in an electronic device is heteroepitaxy—growing the layer on the face of a single crystal of a different material that acts as a template for assembly. Liu et al. (p. 163) now describe a similar process in which the edge of a graphene layer that was grown on a copper surface directs the assembly of a monolayer of hexagonal boron nitride. The boron nitride grew from inside edge of holes created in the graphene layer. The interface and the relative orientation of the two layers were determined by a variety of scanning microscopy and surface diffraction techniques. The orientation of a growing boron nitride film is determined by its graphene template and not the underlying substrate. By adapting the concept of epitaxy to two-dimensional space, we show the growth of a single-atomic-layer, in-plane heterostructure of a prototypical material system—graphene and hexagonal boron nitride (h-BN). Monolayer crystalline h-BN grew from fresh edges of monolayer graphene with atomic lattice coherence, forming an abrupt one-dimensional interface, or boundary. More important, the h-BN lattice orientation is solely determined by the graphene, forgoing configurations favored by the supporting copper substrate.

Ion‐assisted pulsed laser deposition of cubic boron nitride films

Ion‐assisted pulsed laser deposition has been used to produce films containing ≳85% sp3‐bonded cubic boron nitride (c‐BN). By ablating from a target of hexagonal boron nitride (h‐BN), BN films have been deposited on heated (50–800 °C) Si(100) surfaces. The growing films are irradiated with ions from a broad beam ion source operated with Ar and N2 source gasses. Successful c‐BN synthesis has been confirmed by Fourier transform infrared (FTIR) spectroscopy, high‐resolution transmission electron microscopy (TEM), selected‐area electron diffraction, electron energy‐loss spectroscopy, and x‐ray diffraction. The films are polycrystalline and show grain sizes up to 300 A. In addition, Rutherford backscattering, elastic recoil detection, and Auger electron spectroscopies have been used to further characterize the samples. The effects of varying ion current density, substrate growth temperature, growth time, and ion energy have been investigated. It is found that stoichiometric films with a high c‐BN percentage ca...

Measuring fundamental properties in operating solid oxide electrochemical cells by using in situ X-ray photoelectron spectroscopy

Photoelectron spectroscopic measurements have the potential to provide detailed mechanistic insight by resolving chemical states, electrochemically active regions and local potentials or potential losses in operating solid oxide electrochemical cells (SOCs), such as fuel cells. However, high-vacuum requirements have limited X-ray photoelectron spectroscopy (XPS) analysis of electrochemical cells to ex situ investigations. Using a combination of ambient-pressure XPS and CeO(2-x)/YSZ/Pt single-chamber cells, we carry out in situ spectroscopy to probe oxidation states of all exposed surfaces in operational SOCs at 750 °C in 1 mbar reactant gases H(2) and H(2)O. Kinetic energy shifts of core-level photoelectron spectra provide a direct measure of the local surface potentials and a basis for calculating local overpotentials across exposed interfaces. The mixed ionic/electronic conducting CeO(2-x) electrodes undergo Ce(3+)/Ce(4+) oxidation-reduction changes with applied bias. The simultaneous measurements of local surface Ce oxidation states and electric potentials reveal the active ceria regions during H(2) electro-oxidation and H(2)O electrolysis. The active regions extend ~150 μm from the current collectors and are not limited by the three-phase-boundary interfaces associated with other SOC materials. The persistence of the Ce(3+)/Ce(4+) shifts in the ~150 μm active region suggests that the surface reaction kinetics and lateral electron transport on the thin ceria electrodes are co-limiting processes.

Intercalation Pathway in Many-Particle LiFePO4 Electrode Revealed by Nanoscale State-of-Charge Mapping

The intercalation pathway of lithium iron phosphate (LFP) in the positive electrode of a lithium-ion battery was probed at the ∼40 nm length scale using oxidation-state-sensitive X-ray microscopy. Combined with morphological observations of the same exact locations using transmission electron microscopy, we quantified the local state-of-charge of approximately 450 individual LFP particles over nearly the entire thickness of the porous electrode. With the electrode charged to 50% state-of-charge in 0.5 h, we observed that the overwhelming majority of particles were either almost completely delithiated or lithiated. Specifically, only ∼2% of individual particles were at an intermediate state-of-charge. From this small fraction of particles that were actively undergoing delithiation, we conclude that the time needed to charge a particle is ∼1/50 the time needed to charge the entire particle ensemble. Surprisingly, we observed a very weak correlation between the sequence of delithiation and the particle size, contrary to the common expectation that smaller particles delithiate before larger ones. Our quantitative results unambiguously confirm the mosaic (particle-by-particle) pathway of intercalation and suggest that the rate-limiting process of charging is initiating the phase transformation by, for example, a nucleation-like event. Therefore, strategies for further enhancing the performance of LFP electrodes should not focus on increasing the phase-boundary velocity but on the rate of phase-transformation initiation.

The relationship between the spatially resolved field emission characteristics and the raman spectra of a nanocrystalline diamond cold cathode

Spatially resolved electron field emission measurements from a nanocrystalline diamond film grown by plasma‐enhanced chemical transport deposition have been obtained using a scanning probe apparatus with micrometer resolution. Macroscopic regions with a high emission site density, and turn‐on fields below 3 V/μm, comprised approximately 1/2 of the total sample area. The emitting and the nonemitting regions of the specimen are differentiated distinctly by Raman spectra and subtly by morphologies. Both areas are largely sp3‐bonded, but only the nonemitting regions exhibit a sharp line at 1332 cm−1, a well‐known signature of diamond in larger crystallites.

Origin of the mosaicity in graphene grown on Cu(111)

We use low-energy electron microscopy to investigate how graphene grows on Cu(111). Graphene islands first nucleate at substrate defects such as step bunches and impurities. A considerable fraction of these islands can be rotationally misaligned with the substrate, generating grain boundaries upon interisland impingement. New rotational boundaries are also generated as graphene grows across substrate step bunches. Thus, rougher substrates lead to higher degrees of mosaicity than do flatter substrates. Increasing the growth temperature improves crystallographic alignment. We demonstrate that graphene growth on Cu(111) is surface diffusion limited by comparing simulations of the time evolution of island shapes with experiments. Islands are dendritic with distinct lobes, but unlike the polycrystalline, four-lobed islands observed on (100)-textured Cu foils, each island can be a single crystal. Thus, epitaxial graphene on smooth, clean Cu(111) has fewer structural defects than it does on Cu(100).

Growth from below: bilayer graphene on copper by chemical vapor deposition

We evaluate how a second graphene layer forms and grows on Cu foils during chemical vapor deposition (CVD). Low-energy electron diffraction and microscopy is used to reveal that the second layer nucleates and grows next to the substrate, i.e., under a graphene layer. This underlayer mechanism can facilitate the synthesis of uniform single-layer films but presents challenges for growing uniform bilayer films by CVD. We also show that the buried and overlying layers have the same edge termination.

Electronic structure of graphene on single-crystal copper substrates

The electronic structure of graphene on Cu(111) and Cu(100) single crystals is investigated using low energy electron microscopy, low energy electron diffraction and angle resolved photoemission spectroscopy. On both substrates the graphene is rotationally disordered and interactions between the graphene and substrate lead to a shift in the Dirac crossing of