Session Z21: Focus Session: Graphene: Growth

Carbon growth on noble metal films

Noble metals offer an attractive combination of properties as substrates for graphene growth, including low carbon solubility and minimal environmental reactivity. Regardless of noble metal commonalities, both experimental and \textit{ab initio} results show significant variations in their structural relationship with epitaxial graphene films, including carbon-metal bond lengths and the orientation of carbon films relative to the substrate [Giovannetti, \textit{et al}., PRL \textbf{101}, 026803 (2008); Loginova, \textit{et al}., New J. Phys. \textbf{11}, 063046 (2009)]. Despite the possible richness of this experimental landscape, insufficient experimental work has been done to ascertain any systematic trends in the dependence of carbon film structure on surface chemistry. In this work we report on the form and properties of carbon deposited on noble metals under UHV conditions. Carbon morphologies on gold are found which grow both up and down the step edge gradient, suggesting a dendritic precipitation mode from the adatom gas on the surface of the substrate.   

Scanning Tunneling Microscopy of Graphene on Single Crystal Copper Surface

Graphene is a monolayer of carbon atoms tightly packed into a nearly ideal two-dimensional hexagonal lattice. Graphene is a promising electronic material because of its distinctive band structure and physical properties. Large-area synthesis of high-quality graphene is one of the main obstacles towards fabricating graphene devices. Recently, large area graphene with high electrical quality has been realized on copper foils. Copper has demonstrated its advantage in fabricating high-quality uniform graphene monolayer. In this talk, we will present our studies of graphene on single crystal copper surface by variable temperature scanning tunneling microscopy and spectroscopy. We studied the bonding configurations between copper and carbon, as well as the atomic-scale electronic structure of the graphene on the copper surface. Our results provide valuable information for understanding the growth mechanism and the electronic quality of graphene on copper.   

Thermoelectric properties of CVD grown large area graphene

The thermoelectric power (TEP) of CVD (Chemical Vapor Deposition) grown large area graphene transferred onto a Si/SiO$_{2}$ substrate was measured by simply attaching two miniature thermocouples and a resistive heater. Availability of such large area graphene facilitates straight forward TEP measurement without the use of any microfabrication processes. All investigated graphene samples showed a positive TEP $\sim $ + 30 $\mu $V/K in ambient conditions and saturated at a negative value as low as $\sim $ -75 $\mu $V/K after vacuum-annealing at 500 K in a vacuum of $\sim $10$^{-7}$ Torr. The observed p-type behavior under ambient conditions is attributed to the oxygen doping, while the n-type behavior under degassed conditions is due to electron doping from SiO$_{2}$ surface states. It was observed that the sign of the TEP switched from negative to positive for the degassed graphene when exposed to acceptor gases. Conversely, the TEP of vacuum-annealed graphene exposed to the donor gases became even more negative than the TEP of vacuum-annealed sample.   

Formation of graphene-like carbon layers on TiO2(110) and Al2O3 and their relevance for protecting EUV mirrors

Extreme ultraviolet lithography (EUVL) uses short-wavelength photons (13.5 nm) to increase patterning density in IC manufacturing. Because EUV photons are strongly absorbed, multilayer reflective optics in high vacuum must be used. An active area of research is the development of a capping layer that prevents the carbon contamination and oxidation of EUV mirrors. Here, two model capping layers, a single crystal rutile TiO$_{2}$(110) surface and an ultrathin Al$_{2}$O$_{3}$ film grown on NiAl(110), were employed to investigate the fundamentals of adsorption and photo-induced oxidation processes. Using catechol (C$_{6}$H$_{6}$O$_{2})$ as a model contaminant, the adsorption and uv-induced (248 nm) removal in various gaseous atmospheres were investigated using x-ray photoelectron spectroscopy (XPS){\&}scanning tunneling microscopy (STM). When catechol overlayer is heated to 400$^{\circ}$C, some of the molecules desorb and the XPS signature of the remaining C suggests a graphene-like overlayer. Repeated adsorption/flash cycles result in a layer with self-limited thickness of 1.6 monolayers that is inert against further adsorption. Using such a layer for protecting EUV mirrors is being explored.   

Graphene on Ir(111) surface: interplay between chemical bonding and van der Waals

Graphene is an interesting new material, which consists of carbon atoms forming a hexagonal lattice. Within graphene, carbon atoms are connected by strong chemical bonds but when graphene sheets bind to something else different binding mechanisms take place. For example, when graphene sheets bind among themselves forming graphite, bonding between them is exclusively of van der Waals type i.e. there is no formation of chemical bonds and sheets are only physisorbed one ontop of each other. A graphene sheet on top of Ir(111) surface is experimentally studied by means of STM and photoemission. In standard DFT calculations this system is not described correctly due to great importance of van der Waals binding. Employing the newly developed vdW-DF functional we have calculated this system and have shown that besides pure van der Waals binding additional chemical interaction takes place giving rise to interesting phenomena (anti-corrugation) to be observed in STM images.   

Chemical vapor deposition growth of patterned graphene on copper

Graphene possesses unique electronic properties and application potentials. However, the synthesis of high-quality, single-layer graphene on large scale remains challenging. Mechanical exfoliation from graphite crystals yields graphene of the highest quality but in an uncontrolled and non-scalable way. Epitaxial growth on SiC has made significant advances in large-scale synthesis, although the cost is relatively high. Very recently, chemical vapor deposition (CVD) is used to grow graphene on Ni and Cu surfaces and has also produced large-area graphene of reasonably high quality. Cracks and ripples, however, present considerable challenges to the CVD growth and transfer process. We report the CVD growth of single-layer graphene on patterned, micron-size copper templates. Raman spectra of the films show low D-band and relatively narrow 2D peak, suggesting high quality. We present and discuss the transport properties of graphene films transferred onto an insulating substrate.   

Synthesis of large area single- and bilayer graphene on Ni (111) by chemical vapor deposition

Graphene has been reported as a promising material due to the fascinating electronic properties of ideal two-dimensional carbon. A lot of efforts have been made on the synthesis of graphene on Ni but achieving large graphene domains with uniform thickness remains a challenge. In this talk we will present our method of single- and bilayer graphene synthesis over large area, as well as micro Raman study of obtained graphene. The graphene synthesis was achieved by using Ni (111) as substrates and a scalable technique via chemical vapor deposition. The formation of the graphene layers were confirmed by micro Raman analysis. Furthermore, we obtained the information of number of layers of as-grown graphene over a large area by micro Raman map. A clear comparison of the layers between graphene synthesized on Ni (111) substrates and polycrystalline Ni films was given by Raman spectra: Within about the same size map ($\sim $40um*40um), graphene grown on Ni (111) has a much higher percentage of single- and bilayer graphene. Our results demonstrate that Ni (111) substrates have a great advantage over polycrystalline Ni film on the synthesis of large area, single- and bilayer graphene.   

CVD graphene films and its application in organic photovoltaic cells

In this work, CVD of graphene was used as a simple, scalable and cost-efficient method to prepare single and few-layer graphene films over large areas. CVD-G was characterized by Raman spectroscopy and TEM. Back-gated thin-film transistors were used to evaluate transport properties of the synthesized films. In addition, CVD graphene films were transferred to transparent substrates for photovoltaic cell fabrication. Solar cells obtained from the synthesized graphene films showed comparable performance to those fabricated with the standard indium tin oxide film (ITO) and showed superior performance under bending conditions due to the high flexibility of graphene. CVD Graphene constitutes a significant advance towards the production of transparent conductive films of graphene at large scale and has great implications for future graphene-related electronic devices.   

Electronic properties of CVD graphene grown on copper

We report the electronic properties of graphene grown by chemical vapor deposition (CVD) on copper foils at ambient pressure. Large size graphene films (4 inch*4 inch) are synthesized and transferred to SiO2/Si substrate. Raman mapping demonstrates that the films consist primarily of monolayer graphene (up to $\sim $90{\%} area coverage). Low temperature transport measurements are performed on devices made from such CVD graphene. The ``half-integer'' quantum Hall effect, which is the hall-mark of mono-layer graphene, has been observed in these devices. We also observe the ambipolar field effect and weak localization, which allow us to extract carrier mobility $\sim $3000cm$^2$/Vs and phase coherence length $\sim $300nm at 1.5K.   

Contrast Behavior of Carbon Adatom Diffusion and Nucleation in the Initial Stage of Graphene Epitaxial Growth on Stepped Metal Surfaces

Using first-principles calculations within density functional theory, we study the energetics and kinetics of carbon adatom diffusion and nucleation on three stepped metal surfaces: Ir(111), Ru(0001) and Cu(111). We find that on the flat surfaces, two carbon atoms repel each other on Ir(111) and Ru(0001), while they prefer to form a dimer on Cu(111). Moreover, the step edges on Ir and Ru surfaces cannot effectively trap single carbon adatoms either, whereas it is strongly favorable to form carbon dimers at the step edges. The different behaviors are attributed to the competition between C-C bonding and different types of C-metal bonding, and the picture is generalized to other C/metal systems with predicted results. These findings provide an insight into the understanding of experimentally observed carbon nucleation in the initial stage of graphene epitaxial growth on metal surfaces.   

Reduced-defect growth of large area graphene by CVD on Au and Cu foils

A single layer or few-layers graphene can be formed by dissolving hydrocarbon gas on the metal surfaces at high temperatures. So called, this chemical vapor deposition (CVD) method has several advantages in graphene production such as large area, controlled layers, and low cost growth. However, it is known that the quality of CVD grown graphene is not so good compared to that of a detached graphene sample from HOPG. Especially, defects were detected at 1350 (1/cm) of Raman spectroscopy in the graphene samples grown on Au and Cu surfaces, which directly might be related to the electron mobility. As a matter of fact, there are many factors for graphene quality determination in the CVD growth process. Those are growth temperature, gas composition and flow rate, and cooling rate, etc. Surprisingly we found that defects can be reduced significantly by rapid and prompt pumping hydrogen gas after the growth under the optimum growth conditions. We believe that residue atomic hydrogen after the growth might be attached on a graphene surface and leads to deformation of honeycomb graphene structure. Recent experiments have confirmed that this kind of hydrogen absorption tilts graphene structure and makes graphene insulating.   

Catalyst-assisted thermal CVD syntheses of large area uniform mono- and few-layer graphene

Catalysts with carbon solubilities varying from low to intermediate solubilities were utilized to synthesize large area uniform mono- and few-layer graphene employing catalyst-assisted thermal CVD at both ambient and low pressures. Our results demonstrate that both APCVD and LPCVD from a low carbon solubility catalyst resulted in uniform growth of monolayer graphene. Uniform growth of multi-layer graphene was observed while employing catalysts with intermediate carbon solubility in a LPCVD process that is in contrast to previous reports using APCVD process. In an APCVD process (for low solubility catalysts such as Copper), the graphene synthesis at high temperatures ($\sim $1000 $^{\circ}$C) proceeds in mass-transport limited regime, and in a LPCVD process, proceeds in a surface-reaction limited regime. The role of kinetic factors in graphene growth using APCVD and LPCVD processes would be discussed with reference to aforementioned results.   

Graphene-covered iron layers on Ni(111): structural and electronic properties

Recently, Dekdov et al. [Appl. Phys. Lett. 93, 022509 (2008)] reported the intercalation of atomic Fe layers between graphene and a Ni surface. In the present work, we report new experimental and theoretical results on this novel nanostructure. Fe intercalation was produced by repeatedly evaporating monolayers of $^{57}$Fe on previously prepared graphene/Ni(111), and post-annealing at 320 C. In situ M\"ossbauer spectra are consistent with a single hyperfine magnetic field for the one-Fe-monolayer system, while the two- monolayer system presented two field values. By using low energy electron diffraction (LEED) the strucuture of both graphene/Ni(111) and graphene/Fe/Ni(111) was investigated. The model that best fits the experimental LEED curves corresponds to one C atom on top of Ni or Fe and the other C atom on a fcc hollow-site, consistent with the most stable systems in the first-principles calculations. Regarding the calculated electronic structure of the studied systems, the graphene/Ni structure presents a bandgap of 0.06 eV for the minority-spin electronic states near the original graphene Fermi point. The inclusion of Fe layers modify the magnitude, and even the existence, of such bandgap. For instance, in the case of the one-Fe-monolayer system, the bandgap increases to 0.63 eV.   

One-dimensional extended defects in epitaxial graphene with metallic properties

Extended one dimensional line defects with metallic electronic properties are described. These defects have been formed in epitaxial graphene on Ni(111) surfaces and are the consequence of domain boundaries between graphene-sheets occupying different registry relative to the nickel substrate. The domain boundary forms a reconstructed line defect with a repeat unit of one octagon and a pair of pentagons. All the atoms in the defect are sp$^{2}$ hybridized three-fold coordinated carbon and thus do not exhibit any dangling bonds. DFT calculations indicate that these defect lines exhibit similar flat band states at the Fermi-level as zigzag-edge states in nanoribbons. STM-imaging indicates a bright contrast surrounding these defects, which we attribute to the decaying wave function of the defect states and its associated self-doping effect in the surrounding graphene sheet. This makes this extended defect a metallic wire embedded in a perfect graphene lattice.