Domingo Ferrer

Toward the Controlled Synthesis of Hexagonal Boron Nitride Films

Atomically smooth hexagonal boron nitride (h-BN) layers have very useful properties and thus potential applications for protective coatings, deep ultraviolet (DUV) emitters, and as a dielectric for nanoelectronics devices. In this paper, we report on the growth of h-BN by a low-pressure chemical vapor deposition (LPCVD) process using diborane and ammonia as the gas precursors. The use of LPCVD allows synthesis of h-BN with a controlled number of layers defined by the growth conditions, temperature, time, and gas partial pressure. Furthermore, few-layer h-BN was also grown by a sequential growth method, and insights into the growth mechanism are described, thus forming the basis of future growth of h-BN by atomic layer epitaxy.

CMOS-compatible synthesis of large-area, high-mobility graphene by chemical vapor deposition of acetylene on cobalt thin films.

We demonstrate the synthesis of large-area graphene on Co, a complementary metal-oxide-semiconductor (CMOS)-compatible metal, using acetylene (C(2)H(2)) as a precursor in a chemical vapor deposition (CVD)-based method. Cobalt films were deposited on SiO(2)/Si, and the influence of Co film thickness on monolayer graphene growth was studied, based on the solubility of C in Co. The surface area coverage of monolayer graphene was observed to increase with decreasing Co film thickness. A thorough Raman spectroscopic analysis reveals that graphene films, grown on an optimized Co film thickness, are principally composed of monolayer graphene. Transport properties of monolayer graphene films were investigated by fabrication of back-gated graphene field-effect transistors (GFETs), which exhibited high hole and electron mobility of ∼1600 cm(2)/V s and ∼1000 cm(2)/V s, respectively, and a low trap density of ∼1.2 × 10(11) cm(-2).

Scaling of Al2O3 dielectric for graphene field-effect transistors

We investigate the scaling of Al2O3 dielectric on graphene by atomic layer deposition (ALD) using ultra-thin, oxidized Ti and Al films as nucleation layers. We show that the nucleation layer significantly impacts the dielectric constant (k) and morphology of the ALD Al2O3, yielding k = 5.5 and k = 12.7 for Al and Ti nucleation layers, respectively. Transmission electron microscopy shows that Al2O3 grown using the Ti interface is partially crystalline, while Al2O3 grown on Al is amorphous. Using a spatially uniform 0.6 nm-thick Ti nucleation layer, we demonstrate graphene field-effect transistors with top dielectric stacks as thin as 2.6 nm.

Hybrid MnO2–disordered mesoporous carbon nanocomposites: synthesis and characterization as electrochemical pseudocapacitor electrodes

MnO2–mesoporous carbon hybrid nanocomposites were synthesized to achieve high values of redox pseudocapacitance at scan rates of 100 mV s−1. High-resolution transmission electron microscopy (HRTEM) along with energy dispersive X-ray spectroscopy (EDX) demonstrated that ∼1 nm thick MnO2 nanodomains, resembling a conformal coating, were uniformly distributed throughout the mesoporous carbon structure. HRTEM and X-ray diffraction (XRD) showed formation of MnO2 nanocrystals with lattice planes corresponding to birnessite. The electrochemical redox pseudocapacitance of these composite materials in aqueous 1 M Na2SO4 electrolyte containing as little as 2 wt% MnO2 exhibited a high gravimetric MnO2 pseudocapacitance (CMnO2) of 560 F gMnO2−1. Even for 30 wt% MnO2, a high CMnO2 of 137 F gMnO2−1 was observed at 100 mV s−1. Sodium ion diffusion coefficients on the order of 10−9 to 10−10 cm2 s−1 were measured using chronoamperometry. The controlled growth and conformal coating of redox-active MnO2–mesoporous carbon composites offer the potential for achieving high power energy storage with low cost materials.

HRTEM and molecular modeling of the MoS2–Co9S8 interface: understanding the promotion effect in bulk HDS catalysts

As environmental regulations increase, more selective transition metal sulfide (TMS) catalytic materials for hydrotreating applications are needed. Highly active TMS catalysts become more and more desirable triggering new interest for unsupported Co-promoted MoS2-based systems that have high volumetric activity as reported here. Contrary to the common observation for alumina-supported MoS2-based catalysts, we found in our previous studies with dibenzothiophene (DBT) hydrodesulfurization (HDS) that the catalytic activity is directly proportional to the increase of surface area of the sulfide phases (Co9S8 and MoS2) present in Co-promoted MoS2 unsupported catalysts. This suggests that activity is directly connected with an increase of the contact surface area between the two sulfide phases. Understanding of the nature of the possible interaction between MoS2 and Co9S8 in unsupported catalytic systems is therefore critical in order to get a more generalized overview of the causes for synergy. This has been achieved herein through the detailed characterization by XRD, XPS, and HRTEM of the highly active Co9S8/MoS2 catalyst resulting in a proposed model for a Co9S8/MoS2 interface. This model was then subjected to a DFT analysis to determine a reasonable description of the surface contact region between the two bulk phases. Modelling of the interface shows the creation of open latent vacancy sites on Mo atoms interacting with Co and formation of direct Co–Mo bonds. Strong electron donation from Co to Mo also occurs through the intermediate sulfur atom bonded to both metals while an enhanced metallic character is also found. These changes in coordination and electronic properties are expected to favor a synergetic effect between Co and Mo at the proposed localized interface region between the two bulk MoS2 and Co9S8 phases.

Atomic structure of three-layer Au/Pd nanoparticles revealed by aberration-corrected scanning transmission electron microscopy

The study of nanomaterials can be greatly improved with the use of aberration-corrected transmission electron microscopy (TEM), which provides image resolutions at the level of 1 A and lower. Sub-Angstrom image resolution can yield a new level of understanding of the behavior of matter at the nanoscale. For example, bimetallic nanoparticles are extremely important in catalysis applications; the addition of a second metal in many cases produces much-improved catalysts. In this paper, we study the structure and morphology of Au/Pd bimetallic particles using primarily the high-angle annular dark-field (HAADF) imaging mode in an aberration-corrected STEM/TEM. It is well established that, when recorded under appropriate illumination and collection geometries, incoherent HAADF-STEM images are compositionally sensitive and provide direct information on atomic positions. We matched the experimental intensities of atomic columns with theoretical models of three-layer Au/Pd nanoparticles, in different orientations. Our findings indicate that the surface layer of the nanoparticle contains kinks, terraces and steps at the nanoscale. The effect of adding a second metal induces the formation of such defects, which might very likely promote the well-known improved catalytic activity of this system.

High pseudocapacitance of MnO2 nanoparticles in graphitic disordered mesoporous carbon at high scan rates

Nanocomposites composed of MnO2 and graphitic disordered mesoporous carbon (MnO2/C) were synthesized for high total specific capacitance and redox pseudocapacitance (CMnO2) at high scan rates up to 200 mV s−1. High resolution transmission electron microscopy (HRTEM) with energy dispersive X-ray spectroscopy (EDX) demonstrated that MnO2 nanodomains were highly dispersed throughout the mesoporous carbon structure. According to HRTEM and X-ray diffraction (XRD), the MnO2 domains are shown to be primarily amorphous and less than 5 nm in size. For these composites in aqueous 1 M Na2SO4 electrolyte, CMnO2 reached 500 F/gMnO2 at 2 mV s−1 for 8.8 wt% MnO2. A capacitance fade of only 20% over a 100-fold change in scan rate was observed for a high loading of 35 wt% MnO2 with a CMnO2 of 310 F/gMnO2 at the highest scan rate of 200 mV s−1. The high electronic conductivity of the graphitic 3D disordered mesoporous carbon support in conjunction with the thin MnO2 nanodomains facilitate rapid electron and ion transport offering the potential of improved high power density energy storage pseudocapacitors.

In situ formation of bismuth nanoparticles through electron-beam irradiation in a transmission electron microscope

In this work, bismuth nanoparticles were synthesized when a precursor, sodium bismuthate, was exposed to an electron beam at room temperature in a transmission electron microscope (TEM). The irradiation effects were investigated in situ using selected-area electron diffraction, high-resolution transmission electron microscopy and x-ray energy dispersive spectroscopy. After the electron irradiation, bismuth nanoparticles with a rhombohedral structure and diameter of 6 nm were observed. The average particle size increased with the irradiation time. The electron-induced reduction is attributed to the desorption of oxygen ions. This method offers a one-step route to synthesize bismuth nanoparticles using electron irradiation, and the particle size can be controlled by the irradiation time.

Direct measurement of the fermi energy in graphene using a double-layer heterostructure

We describe a technique which allows a direct measurement of the relative Fermi energy in an electron system by employing a double-layer heterostructure. We illustrate this method by using a graphene double layer to probe the Fermi energy as a function of carrier density in monolayer graphene, at zero and in high magnetic fields. This technique allows us to determine the Fermi velocity, Landau level spacing, and Landau level broadening. We find that the N=0 Landau level broadening is larger by comparison to the broadening of upper and lower Landau levels.

Band engineered epitaxial Ge–SixGe1−x core-shell nanowire heterostructures

We report the growth of germanium (Ge)—silicon-germanium (SixGe1−x) epitaxial core-shell nanowire (NW) heterostructures, with tunable Si and Ge shell content. The Ge NWs are grown using the vapor-liquid-solid growth mechanism, and the SixGe1−x shells are grown in situ, conformally on the Ge NWs using ultrahigh vacuum chemical vapor deposition. We use transmission electron microscopy to demonstrate epitaxial shell growth, and scanning energy dispersive x-ray spectroscopy to determine the shell thickness and content. The Si and Ge shell content can be tuned depending on the SiH4 and GeH4 partial pressures during the shell growth, enabling band engineered core-shell NW heterostructures.