The role of Cu crystallographic orientations towards growing superclean graphene on meter-sized scale

Chemical vapor deposition(CVD)-grown graphene films on Cu foils,exhibiting fine scalability and high quality,are still suffering from the adverse impact of surf...     

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.     

Self-assembled vapor-liquid-solid growth of aligned Cu-SiO2 core-shell nanocable arrays on Cu substrates

The Cu-SiO2 core-shell nanocable arrays on the Cu wafers have been synthesized via a simple thermal evaporation of the SiO powder. The morphology and structure of the as-synthesized Cu-SiO2 core-shell nanocables are characterized by using scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray energy dispersive spectrometer. The growth of amorphous SiO2 shell follows a vapor-liquid-solid mechanism, and then molten metal Cu will be diffused into the SiO2 nanotubes, forming the Cu-SiO2 core-shell nanocable arrays. It is found that the aligned Cu-SiO2 core-shell nanocables prefer to grow along the grooves of the Cu substrate, and the density of the Cu-SiO2 core-shell nanocable arrays can be controlled by adjusting the growth temperature.     

Growth mechanism and kinetics of Cu3Sn in the interfacial reaction between liquid Sn and diversely oriented Cu substrates

Journal of Materials Science: Materials in Electronics - In this study, the effects of reflow temperature, reflow time, and substrates (polycrystalline and (001/110/111) monocrystalline Cu...     

Direct growth of special-shape graphene on different templates by remote catalyzation of Cu nanoparticles

A novel method avoiding the complex transfer process is proposed to directly grow low-defect and few-layer graphene on different insulating substrates(SiO_2, Al_2O_3, etc.) by remote catalyzation of Cu nanoparticles(NPs) using ambient pressure chemical vapor deposition(APCVD). The insulating substrates with special structure are used as templates to grow wrapped graphene sheets with special shapes.Hollow graphene species are obtained by removing the substrates. The prime feature of the proposed method is using Cu NPs as catalyst rather than metal foils. The Cu NPs play an important role in the remote catalyzation during the nucleation of graphene. This method can improve the quality and relatively decrease the growth temperature of the graphene on the insulating substrates, which displays the great potential of APCVD direct growth of graphene on dielectric substrates for electronic and photovoltaic applications.     

Single- and few-layer graphene growth on stainless steel substrates by direct thermal chemical vapor deposition

Increasing interest in graphene research in basic sciences and applications emphasizes the need for an economical means of synthesizing it. We report a method for the synthesis of graphene on commercially available stainless steel foils using direct thermal chemical vapor deposition. Our method of synthesis and the use of relatively cheap precursors such as ethanol (CH(3)CH(2)OH) as a source of carbon and SS 304 as the substrate proved to be economically viable. The presence of single- and few-layer graphene was confirmed using confocal Raman microscopy/spectroscopy. X-ray photoelectron spectroscopic measurements were further used to establish the influence of various elemental species present in stainless steel on graphene growth. The role of cooling rate on surface migration of certain chemical species (oxides of Fe, Cr and Mn) that promote or hinder the growth of graphene is probed. Such analysis of the chemical species present on the surface can be promising for graphene based catalytic research.     

Selective growth of ZnO nanorods on SiO2/Si substrates using a graphene buffer layer

A promising strategy for the selective growth of ZnO nanorods on SiO₂/Si substrates using a graphene buffer layer in a low temperature solution process is described. High densities of ZnO nanorods were grown over a large area and most ZnO nanorods were vertically well-aligned on graphene. Furthermore, selective growth of ZnO nanorods on graphene was realized by applying a simple mechanical treatment, since ZnO nanorods formed on graphene are mechanically stable on an atomic level. These results were confirmed by first principles calculations which showed that the ZnO-graphene binding has a low destabilization energy. In addition, it was found that ZnO nanorods grown on SiO₂/Si with a graphene buffer layer have better optical properties than ZnO nanorods grown on bare SiO₂/Si. The nanostructured ZnO-graphene materials have promising applications in future flexible electronic and optical devices.     

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.     

Colors of graphene and graphene-oxide multilayers on various substrates

We investigated the colors of graphene and graphene-oxide multilayers that were deposited on various dielectric layers. In particular, the effects of the material thickness, the types of dielectric layers, and the existence of a back silicon substrate were analyzed. The colors of graphene-oxide layers on a SiO2/Si substrate were found to periodically change as the material thickness increased. However, the colors of graphene layers on the same substrate became saturated without a similar periodic change. The calculated colors corresponding to the material thicknesses were verified by optical microscopy and profilometry. We believe that these results demonstrate the possibility of utilizing color as a simple tool for detecting and estimating the thicknesses of graphene and graphene-oxide multilayers.     

Solution growth of Cu2S nanowalls on Cu substrates and their characterization

A simple method of preparing Cu₂S nanowalls was presented by immersing Cu substrates into a Na₂S/HCl solution. X-ray diffraction (XRD) analysis revealed the formation of the monoclinic phase of Cu₂S nanostructures on Cu substrates, presenting a thickness of less than 100 nm and a height of several micrometers (called nanowalls) from the scanning electron microscopic (SEM) images. Moreover, the transmission electron microscopy (TEM) and X-ray energy dispersive spectrometry (EDS) characterizations indicated that these Cu₂S nanowalls were thinner near the sheet edge and had voids inside the sheets supposedly due to the etching reaction by Na₂S/HCl. Cu substrates with Cu₂S nanowalls were found to exhibit extinguished hydrophobicity (> 110°) resulting from their rough surfaces based on the water contact angle measurements. Electrical current-voltage analysis indicated the formation of a Schottky barrier at the Cu₂S/Cu interface.     

Cyclic deformation response and dislocation structure of Cu single crystals oriented for double slip

Cyclic deformation behavior of double-slip oriented Cu single crystals with a stress axis in the [034] direction was investigated under plastic strain control mode for a shear strain amplitude range of 1 × 10⁻⁴ to 5 × 10⁻³. Dislocation structures in the tested samples were observed using a transmission electronic microscope. It has been found that the effect of the operation of critical slip in these [034] crystals on cyclic responses and dislocation structures is nearly the same as that of increase in strain amplitude. The nucleation stress and number of cycles for PSB formation at each specific strain amplitude in the double-slip oriented crystals were found to be both considerably lower than those observed in single-slip oriented crystals. This observation is in a good agreement with the Kuhlmann-Wilsdorf and Laird analysis, in that the formation of PSBs is associated with glide behavior on the secondary slip system. A dislocation “cord” structure has also been observed and is believed to be caused by the operation of the cross-slip system during cyclic deformation. Labyrinth wall structures were found to form with increase in strain amplitude by the operation of critical slip and cross-slip systems. However, the formation of labyrinth structure was suppressed by the coplanar slip at high strain amplitudes.     

Shedding light on the crystallographic etching of multi-layer graphene at the atomic scale

The controlled etching of graphite and graphene by catalytic hydrogenation is potentially a key engineering route for the fabrication of graphene nanoribbons with atomic precision. The hydrogenation mechanism, though, remains poorly understood. In this study we exploit the benefits of aberration-corrected high-resolution transmission electron microscopy to gain insight to the hydrogenation reaction. The etch tracks are found to be commensurate with the graphite lattice. Catalyst particles at the head of an etch channel are shown to be faceted and the angles between facets are multiples of 30°. Thus, the angles between facets are also commensurate with the graphite lattice. In addition, the results of a post-annealing step suggest that all catalyst particles—even if they are not involved in etching—are actively forming methane during the hydrogenation reaction. Furthermore, the data point against carbon dissolution being a key mechanism during the hydrogenation process.      

Hexagonal single crystal domains of few-layer graphene on copper foils

Hexagonal-shaped single crystal domains of few layer graphene (FLG) are synthesized on copper foils using atmospheric pressure chemical vapor deposition with a high methane flow. Scanning electron microscopy reveals that the graphene domains have a hexagonal shape and are randomly orientated on the copper foil. However, the sites of graphene nucleation exhibit some correlation by forming linear rows. Transmission electron microscopy is used to examine the folded edges of individual domains and reveals they are few-layer graphene consisting of approximately 5-10 layers in the central region and thinning out toward the edges of the domain. Selected area electron diffraction of individual isolated domains reveals they are single crystals with AB Bernal stacking and free from the intrinsic rotational stacking faults that are associated with turbostratic graphite. We study the time-dependent growth dynamics of the domains and show that the final continuous FLG film is polycrystalline, consisting of randomly connected single crystal domains.     

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.     

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.     

Effect of residual gas pressure on the epitaxial growth of vacuum-deposited chromium on Cu{111} substrates

Transmission electron microscopy, electron diffraction and Auger electron spectroscopy were used to study the effect of vacuum conditions on the epitaxial growth of vacuum-deposited chromium films of thicknesses less than 40 nm on Cu{111} substrates. The chromium films were condensed (i) in ultrahigh vacuum at 10⁻⁹ Torr, (ii) at total pressures of 10⁻⁵, 10⁻⁶ and 5 × 10⁻⁸ Torr and (iii) at oxygen partial pressures of 5 × 10⁻⁹, 10⁻⁷ and 10⁻⁶ Torr. The presence of contaminants during the chromium evaporation suppressed the formation of Kurdjumov-Sachs orientations. A low oxygen partial pressure was most effective in altering the orientation and morphology of the deposits.     

Preparation and characterization of preferred oriented PZT films on amorphous substrates

The (001) preferred orientation of Nb-doped Pb(Zr_0.52Ti_0.48) O₃ (PZT) thin films was successfully realized on amorphous glass substrate with LaNiO₃(LNO) as electrode by rf-sputtering method. It was found that the LNO film greatly promotes formation of the PZT film with pervoskite phase on amorphous substrate and the preferred orientation of the PZT film depends strongly on the process of preparation. The experimental results show that the dielectric constant and loss of the PZT films with the (001) preferred orientation are 1308 and 0.042, respectively, at 1 kHz, 0.05 V. The remanent polarization (P_r), saturation polarization (P_s) and coercive field (E_c) are 34.5, 43 C/cm² and 105 kV/cm, respectively. The PZT films also show a 33 kV/cm internal bias field due to its (001) preferred orientation. The piezoelectric coefficient d₃₃ of the PZT film without the poled treatment is about 15 pC/N due to its (001) preferred orientation. The effect of the foreign stress on the piezoelectric voltage response of the PZT/LNO/glass was investigated. The results make us consider using the PZT film as an artificial skin to realize the self-diagnosis of amorphous materials under the action of stress.