Ion implantation assisted synthesis of graphene on various dielectric substrates

Direct synthesis of high-quality graphene on dielectric substrates is of great importance for the application of graphene-based electronics and optoelectronics. However, high-quality and uniform graphene film growth on dielectric substrates has proven challenging due to limited catalytic ability of dielectric substrates. Here, by employing a Cu ion implantation assisted method, high-quality and uniform graphene can be directly formed on various dielectric substrates including SiO2/Si, quartz glass, and sapphire substrates. The growth rate of graphene on the dielectric substrates was significantly improved due to the catalysis of Cu. Moreover, during the graphene growth process, the Cu atoms gradually evaporated away without involving any metal contamination. Furthermore, an interesting growth behavior of graphene on sapphire substrate was observed, and the results show the graphene domains growth tends to grow along the sapphire flat terraces. The ion implantation assisted approach could open up a new pathway for the direct synthesis of graphene and promote the potential application of graphene in electronics.     

Infrared conductivity and carrier mobility of large scale graphene on various substrates

It is known that low-field mobility of graphene depends largely on the substrate material on which it is transferred. We measured Drude optical conductivity of graphene on various substrates and determined the carrier density and carrier scattering rate. The carrier density varies widely depending on the substrate material. However the scattering rate is almost constant, approximately 100 cm(-1), for 5 different substrates. We calculate carrier mobility of graphene using the two quantities, i.e., carrier density and scattering rate, to find that it agrees with the mobility measured from dc transport experiment. We conclude that substrate-depent mobility of graphene originates from different carrier density but not from the scattering rate.     

Temperature dependence of the Raman spectra of graphene and graphene multilayers

We investigated the temperature dependence of the frequency of G peak in the Raman spectra of graphene on Si/SiO2 substrates. The micro-Raman spectroscopy was carried out under the 488 nm laser excitation over the temperature range from -190 to +100 degrees C. The extracted value of the temperature coefficient of G mode of graphene is chi = -0.016 cm-1/ degrees C for the single layer and chi = -0.015 cm-1/ degrees C for the bilayer. The obtained results shed light on the anharmonic properties of graphene.     

Toughened and machinable glass matrix composites reinforced with graphene and graphene-oxide nano platelets

Abstract

The processing conditions for preparing well dispersed silica–graphene nanoplatelets and silica–graphene oxide nanoplatelets (GONP) composites were optimized using powder and colloidal processing routes. Fully dense silica–GONP composites with up to 2.5 vol% loading were consolidated using spark plasma sintering. The GONP aligned perpendicularly to the applied pressure during sintering. The fracture toughness of the composites increased linearly with increasing concentration of GONP and reached a value of ～0.9 MPa m^1/2 for 2.5 vol% loading. Various toughening mechanisms including GONP necking, GONP pull-out, crack bridging, crack deflection and crack branching were observed. GONP decreased the hardness and brittleness index (BI) of the composites by ～30 and ～50% respectively. The decrease in BI makes silica–GONP composites machinable compared to pure silica. When compared to silica–Carbon nanotube composites, silica–GONP composites show better process-ability and enhanced mechanical properties.     

Depositing graphene films on solid and perforated substrates

Graphene-a monolayer of graphite-has attracted vast interest recently owing to its perfect two-dimensional crystallographic nature and its potential use in a new generation of microelectronic devices. Yet, a deposition method, which results in a large coverage of monolayer thick graphite, is still lacking. By using a chemical mechanical polishing (CMP) method we were able to deposit stress-free graphene on solid and perforated substrates alike, achieving area coverage of hundreds of microns squared.     

Layer-by-layer growth of graphene layers on graphene substrates by chemical vapor deposition

We demonstrate a synthesis of graphene layers on graphene templates prepared by the mechanical exfoliation of graphite crystals using a developed chemical vapor deposition (CVD) apparatus that has a furnace with three temperature zones and can regulate the temperatures separately in each zone. This results in individual control over the decomposition reaction of the carbon feedstock and the growth of graphene layers by activated carbon species. CVD growth using multi-temperature zones provides wider temperature windows appropriate to grow graphene layers. We observed that graphene layers proceed by a layer-by-layer growth mode using an optical microscopy, an atomic force microscopy, and Raman spectroscopy. This result suggests that a graphene growth technique using the CVD apparatus is a potential approach for making graphene sheets with precise control of the layer numbers.     

Nanoporous graphene oxide thin films from nanohybrid multilayers

We report on the preparation and characterization study of new nanoporous reduced graphene oxide (rGO) film fabricated from electrostatic layer-by-layer assembly of gold nanoparticles (gold-NPs) and rGO. Well distributed gold-NPs in multilayer film means that the gold-NPs can be readily dissolved by the addition of cyanide ions, which allows for a direct and precise comparison of the porosity of multilayer thin films based on rGO under the presence and absence of the gold-NPs. In addition, the LbL assembly methods offers the big advantages of possibilities in mass production and 2-dimensional nano-objects based film prepared from large size substrates. We expect that this demonstration offers a new route to introduce nanopore into multilayer thin films and opens up the possibility of building more complex functional multicomponent nanoporous structure consisting of 2-dimensional nano-objects, which are believed to be useful for nanodevice systems.     

Morphology and performance of graphene layers on as-grown and transferred substrates

Graphene’s excellent physical, electrical, mechanical and passivating properties are revolutionizing the world of nanotechnology. In its beginning, graphene was only used as the conductive channel in metal-oxide-semiconductor field-effect transistors and as metallic electrode in capacitors, but the development of chemical vapor deposited graphene on metal catalysts, together with an ingenious process to transfer it to arbitrary substrates extended the use of graphene to many other applications. The main problem of this methodology is to get a good adhesion between the graphene and the target substrate that ensures both protection and interaction. In this paper, we analyze the capability of graphene to adapt to underlying simple and complex substrates. We observe the important adhesion differences depending on the graphene thickness and the target substrate roughness. We take advantage of graphene coatings to protect different materials from high current densities, mechanical frictions and oxidation. The findings and prototypes here designed may open the way to extend the use of graphene as protective coating.     

Optimizing the identification of mono- and bilayer graphene on multilayer substrates

This work presents an investigation and optimization of the identification of graphene mono- and bilayers on various multilayer substrates. Instead of the mere contrast between substrate and substrate/mono/bilayer systems, weighted color differences are used to obtain optimum visibility. Our approach employs a genetic algorithm that allows finding the most appropriate composition of multilayer systems in terms of materials in use and their respective thicknesses. A major benefit of our approach is the possibility to qualify appropriate layer systems with respect to their manufacturability.     

Characterization of sputter deposited Au/Ni/Al multilayers on Si substrates

Multilayered Au/Ni/Al thin film metallization deposited by DC sputtering on n⁺Si substrates has been investigated. AES depth profiling was performed to reveal the concentration depth profiles of the Au/Ni/Al multilayers before and after annealing at different temperatures in the range 623-723 K. It was found that Ni aluminide layers begin to form during heat treatments at temperatures above 623 K. In addition to this process, Ni was found to diffuse significantly through the Au layer and segregates at the surface, proportionally to the increased annealing temperature. Consequently, the Ni oxidation process was found to take place thus causing the degradation of electrical contact. On the other hand Ni diffuses faster as well toward the Si/Al interface. No contamination traces at interfaces were observed. Electrical measurements of the metallized diode forward characteristics showed minor influence of the metallization heat treatment on the series resistance. Degradations were observed only in the reverse characteristics if the annealing was performed above 723 K.     

Transformations of carbon adsorbates on graphene substrates under extreme heat

We describe new phenomena of structural reorganization of carbon adsorbates as revealed by in situ atomic-resolution transmission electron microscopy (TEM) performed on specimens at extreme temperatures. In our investigations, a graphene sheet serves as both a quasi-transparent substrate for TEM and as an in situ heater. The melting of gold nanoislands deposited on the substrate surface is used to evaluate the local temperature profile. At annealing temperatures around 1000 K, we observe the transformation of physisorbed hydrocarbon adsorbates into amorphous carbon monolayers and the initiation of crystallization. At temperatures exceeding 2000 K the transformation terminates in the formation of a completely polycrystalline graphene state. The resulting layers are bounded by free edges primarily in the armchair configuration.     

Catalyst-free growth of nanographene films on various substrates

We have developed a new method to grow uniform graphene films directly on various substrates, such as insulators, semiconductors, and even metals, without using any catalyst. The growth was carried out using a remote plasma enhancement chemical vapor deposition (r-PECVD) system at relatively low temperatures, enabling the deposition of graphene films up to 4-inch wafer scale. Scanning tunneling microscopy (STM) confirmed that the films are made up of nanocrystalline graphene particles of tens of nanometers in lateral size. The growth mechanism for the nanographene is analogous to that for diamond grown by PECVD methods, in spite of sp2 carbon atoms being formed in the case of graphene rather than sp3 carbon atoms as in diamond. This growth approach is simple, low-cost, and scalable, and might have potential applications in fields such as thin film resistors, gas sensors, electrode materials, and transparent conductive films.     

Comparison of graphene films grown on 6H-SiC and 4H-SiC substrates

Abstract

Atomic force microscopy, Kelvin-probe microscopy and Raman spectroscopy have been used to examine graphene films grown by thermal decomposition of the Si face of semi-insulating substrates of 6H-SiC and 4H-SiC polytypes in the atmosphere of argon. It was demonstrated that the quality of graphene grown on substrates of various polytypes at identical technological growth regimes is about the same. A conclusion was made that the differences in crystal structure between 6H-SiC and 4H-SiC does not lead to significant dissimilarities in the mechanism of sublimation of silicon carbide components from the surface of a crystal and in that of graphene crystallization.     

Resistive switching via the converse magnetoelectric effect in ferromagnetic multilayers on ferroelectric substrates

A voltage-controlled resistive switching is predicted for ferromagnetic multilayers and spin valves mechanically coupled to a ferroelectric substrate. The switching between low- and high-resistance states results from the strain-driven magnetization reorientations by about 90°, which are shown to occur in ferromagnetic layers with a high magnetostriction and weak cubic magnetocrystalline anisotropy. Such reorientations, not requiring external magnetic fields, can be realized experimentally by applying moderate electric field to a thick substrate (bulk or membrane type) made of a relaxor ferroelectric having ultrahigh piezoelectric coefficients. The proposed multiferroic hybrids exhibiting giant magnetoresistance may be employed as electric-write nonvolatile magnetic memory cells with nondestructive readout.     

Carbon molecular beam epitaxy on various semiconductor substrates

Direct graphene growth on semiconductor substrates is an important goal for successful integration of graphene with the existing semiconductor technology. We test the feasibility of this goal by using molecular beam epitaxy on various semiconductor substrates: group IV (Si, SiC), group III–V (GaAs, GaN, InP), and group II–VI (ZnSe, ZnO). Graphitic carbon has been formed on most substrates except Si. In general, the crystallinities of carbon layers are better on substrates of hexagonal symmetry than those on cubic substrates. The flatness of graphitic carbon grown by molecular beam epitaxy is noticeable, which may help the integration with semiconductor structures.     

Transport/magnetotransport of high-performance graphene transistors on organic molecule-functionalized substrates

In this article, we present the transport and magnetotransport of high-quality graphene transistors on conventional SiO(2)/Si substrates by modification with organic molecule octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs). Graphene devices on OTS SAM-functionalized substrates with high carrier mobility, low intrinsic doping, suppressed carrier scattering, and reduced thermal activation of resistivity at room temperature were observed. Most interestingly, the remarkable magnetotransport of graphene devices with pronounced quantum Hall effect, strong Shubnikov-de Haas oscillations, a nonzero Berry's phase, and a short carrier scattering time also confirms the high quality of graphene on this ultrasmooth organic SAM-modified platform. The high-performance graphene transistors on the solution-processable OTS SAM-functionalized SiO(2)/Si substrates are promising for the future development of large-area and low-cost fabrications of graphene-based nanoelectronics.     

Water-gated charge doping of graphene induced by mica substrates

We report on the existence of water-gated charge doping of graphene deposited on atomically flat mica substrates. Molecular films of water in units of ~0.4 nm thick bilayers were found to be present in regions of the interface of graphene/mica heterostacks prepared by micromechanical exfoliation of kish graphite. The spectral variation of the G and 2D bands, as visualized by Raman mapping, shows that mica substrates induce strong p-type doping in graphene with hole densities of (9 ± 2) × 10(12) cm(-2). The ultrathin water films, however, effectively block interfacial charge transfer, rendering graphene significantly less hole-doped. Scanning Kelvin probe microscopy independently confirmed a water-gated modulation of the Fermi level by 0.35 eV, which is in agreement with the optically determined hole density. The manipulation of the electronic properties of graphene demonstrated in this study should serve as a useful tool in realizing future graphene applications.     

Surface crystallographic structure insensitive growth of oriented graphene domains on Cu substrates

Cu-based chemical vapor deposition method can produce large-area graphene films, usually polycrystalline films with grain boundaries as the main defects. One way to reduce grain boundaries is to grow oriented graphene domains (OGDs), which can ultimately perfectly integrate. In contrast to previously reported methods of limiting OGD growth on Cu (1 1 1), we find that OGDs can grow on Cu substrates with a large surface crystallographic structure tolerance. Density functional theory calculations show that this is due to the single lowest energy state of graphene nucleation. The growth temperature is crucial. It must be high enough (1045 °C) to suppress mis-OGD nucleation, but not too high (1055 °C) to deteriorate OGD growth. Mis-OGD nucleation can also be caused by C impurity in Cu grains, which can be depleted by thermal pretreatment of the substrate in an oxidizing atmosphere. On the other hand, OGD growth is not sensitive to the atmosphere at growth stage within the range that we have tested.     

Large physisorption strain in chemical vapor deposition of graphene on copper substrates

Graphene single layers grown by chemical vapor deposition on single crystal Cu substrates are subject to nonuniform physisorption strains that depend on the orientation of the Cu surface. The strains are revealed in Raman spectra and quantitatively interpreted by molecular dynamics (MD) simulations. An average compressive strain on the order of 0.5% is determined in graphene on Cu(111). In graphene on Cu (100), MD simulations interpret the observed highly nonuniform strains.