Alfonso Reina

Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical Vapor Deposition

In this work we present a low cost and scalable technique, via ambient pressure chemical vapor deposition (CVD) on polycrystalline Ni films, to fabricate large area ( approximately cm2) films of single- to few-layer graphene and to transfer the films to nonspecific substrates. These films consist of regions of 1 to approximately 12 graphene layers. Single- or bilayer regions can be up to 20 mum in lateral size. The films are continuous over the entire area and can be patterned lithographically or by prepatterning the underlying Ni film. The transparency, conductivity, and ambipolar transfer characteristics of the films suggest their potential as another materials candidate for electronics and opto-electronic applications.

Synthesis of Few-Layer Hexagonal Boron Nitride Thin Film by Chemical Vapor Deposition

In this contribution we demonstrate a method of synthesizing a hexagonal boron nitride (h-BN) thin film by ambient pressure chemical vapor deposition on polycrystalline Ni films. Depending on the growth conditions, the thickness of the obtained h-BN film is between ∼5 and 50 nm. The h-BN grows continuously on the entire Ni surface and the region with uniform thickness can be up to 20 μm in lateral size which is only limited by the size of the Ni single crystal grains. The hexagonal structure was confirmed by both electron and X-ray diffraction. X-ray photoelectron spectroscopy shows the B/N atomic ratio to be 1:1.12. A large optical band gap (5.92 eV) was obtained from the photoabsorption spectra which suggest the potential usage of this h-BN film in optoelectronic devices.

Controlled Formation of Sharp Zigzag and Armchair Edges in Graphitic Nanoribbons

Graphene nanoribbons can exhibit either quasi-metallic or semiconducting behavior, depending on the atomic structure of their edges. Thus, it is important to control the morphology and crystallinity of these edges for practical purposes. We demonstrated an efficient edge-reconstruction process, at the atomic scale, for graphitic nanoribbons by Joule heating. During Joule heating and electron beam irradiation, carbon atoms are vaporized, and subsequently sharp edges and step-edge arrays are stabilized, mostly with either zigzag- or armchair-edge configurations. Model calculations show that the dominant annealing mechanisms involve point defect annealing and edge reconstruction.

Work Function Engineering of Graphene Electrode via Chemical Doping

In this work, we demonstrate that graphene films synthesized by chemical vapor deposition (CVD) method can be used as thin transparent electrodes with tunable work function. By immersing the CVD-grown graphene films into AuCl(3) solution, Au particles were formed on the surface of graphene films by spontaneous reduction of metal ions. The surface potential of graphene films can be adjusted (by up to approximately 0.5 eV) by controlling the immersion time. Photovoltaic devices based on n-type silicon interfacing with graphene films were fabricated to demonstrate the benefit of an electrode with tunable work function. The maximum power conversion efficiency (PCE) achieved was approximately 0.08%, which is more than 40 times larger than the devices without chemical doping.

Enhancing the conductivity of transparent graphene films via doping

We report chemical doping (p-type) to reduce the sheet resistance of graphene films for the application of high-performance transparent conducting films. The graphene film synthesized by chemical vapor deposition was transferred to silicon oxide and quartz substrates using poly(methyl methacrylate). AuCl(3) in nitromethane was used to dope the graphene films and the sheet resistance was reduced by up to 77% depending on the doping concentration. The p-type doping behavior was confirmed by characterizing the Raman G-band of the doped graphene film. Atomic force microscope and scanning electron microscope images reveal the deposition of Au particles on the film. The sizes of the Au particles are 10-100 nm. The effect of doping was also investigated by transferring the graphene films onto quartz and poly(ethylene terephthalate) substrates. The sheet resistance reached 150 Omega/sq at 87% transmittance, which is comparable to those of indium tin oxide conducting film. The doping effect was manifested only with 1-2 layer graphene but not with multi-layer graphene. This approach advances the numerous applications of graphene films as transparent conducting electrodes.

Observation of Van Hove singularities in twisted graphene layers

When a Van Hove singularity exists near the Fermi energy of a solid’s density of states, it can cause a variety of exotic phenomena to emerge. Scanning tunnelling microscope measurements indicate that when graphite’s graphene sheets are rotated out of their usual alignment, it can generate low-energy Van Hove singularities for which the position is controlled by the angle of rotation.

Single-Layer Behavior and Its Breakdown in Twisted Graphene Layers

We report high magnetic field scanning tunneling microscopy and Landau level spectroscopy of twisted graphene layers grown by chemical vapor deposition. For twist angles exceeding ~3° the low energy carriers exhibit Landau level spectra characteristic of massless Dirac fermions. Above 20° the layers effectively decouple and the electronic properties are indistinguishable from those in single-layer graphene, while for smaller angles we observe a slowdown of the carrier velocity which is strongly angle dependent. At the smallest angles the spectra are dominated by twist-induced van Hove singularities and the Dirac fermions eventually become localized. An unexpected electron-hole asymmetry is observed which is substantially larger than the asymmetry in either single or untwisted bilayer graphene.

All graphene electromechanical switch fabricated by chemical vapor deposition

We demonstrate an electromechanical switch comprising two polycrystalline graphene films; each deposited using ambient pressure chemical vapor deposition. The top film is pulled into electrical contact with the bottom film by application of approximately 5 V between the layers. Contact is broken by mechanical restoring forces after bias is removed. The device switches several times before tearing. Demonstration of switching at low voltage confirms that graphene is an attractive material for electromechanical switches. Reliability may be improved by scaling the device area to within one crystalline domain of the graphene films.

Engineering polycrystalline Ni films to improve thickness uniformity of the chemical-vapor-deposition-grown graphene films

It has been shown that few-layer graphene films can be grown by atmospheric chemical vapor deposition using deposited Ni thin films on SiO(2)/Si substrates. In this paper we report the correlation between the thickness variations of the graphene film with the grain size of the Ni film. Further investigations were carried out to increase the grain size of a polycrystalline nickel film. It was found that the minimization of the internal stress not only promotes the growth of the grains with (111) orientation in the Ni film, but it also increases their grain size. Different types of SiO(2) substrates also affect the grain size development. Based upon these observations, an annealing method was used to promote large grain growth while maintaining the continuity of the nickel film. Graphene films grown from Ni films with large versus small grains were compared for confirmation.

In-Situ Sample Rotation as a Tool to Understand Chemical Vapor Deposition Growth of Long Aligned Carbon Nanotubes

A new tool for studying the process of carbon nanotube chemical vapor deposition (CVD) synthesis is described. By rotating the substrate in situ during the CVD process, the orientation of floating nanotubes with respect to the substrate is changed by interaction with the gas stream. Nanotubes lying on the surface of the substrate, however, will maintain their relative orientation. As a result different nanotube alignment angles are observed. By defining a time window through multiple rotation steps it is possible to study carbon nanotube behavior during CVD growth in a time-resolved manner. As an example, the settling process (i.e., the sinking of the nanotube to the substrate) is investigated. The analysis of forces acting on a floating nanotube shows that a vertical gas stream due to thermal buoyancy over the sample can keep long nanotubes floating for extended times. A stochastic process, indicated by a constant settling rate over time, forces the nanotube to make contact with the substrate, and this process is attributed to flow induced instability. Additional information on the floating and settling process are revealed from our study. The settling velocity could be inferred from curved nanotubes. The clearance between a floating nanotube and the substrate was found to be several hunded micrometers.