AVS2013 Session GR+EM+NS+PS+SS+TF-MoM: Growth of 2D Materials

Graphene growth on Si-face SiC has been shown to have improved uniformity when synthesized in an argon environment instead of in UHV. For C-face growth, which is expected to yield graphene with superior electronic properties due to the absence of the interfacial layer, similar progress has not yet been achieved. It has been shown that growth by confinement controlled sublimation, where the C-face SiC is put in a graphite pillbox during growth, can result in improved quality graphene films. However, it remains unclear whether growth by low pressure sublimation (LPS) in a conventional furnace with an argon overpressure can achieve uniformity and thickness control similar to the Si-face. Therefore, a systematic study of SiC surface preparation by hydrogen etching and subsequent graphene growth in argon has been carried out for C-face SiC substrates in a commercially available Aixtron CVD reactor that has been modified for LPS. This reactor exhibits laminar Ar flow and a uniform temperature profile. In this system, which has the advantage of being able to subject different substrates to identical growth conditions simultaneously, it has been found that relatively small differences in the substrate, such as polytype, can have a drastic effect on the resulting graphene film. In fact, growth conditions that result in complete coverage of at least a single layer of graphene on a nominally on-axis 4H substrate have been shown to result in 60% coverage for a nominally on-axis 6H substrate. Therefore, it has been found that optimizing the growth conditions for a particular substrate is crucial if graphene thickness and uniformity are to be controlled. Additionally, it has been suggested that unintentional oxidation of the SiC substrate during growth in high pressures of argon is a potential cause for the non-uniform growth that is observed on the C-face [1]. Several experiments in which LPS grown C-face graphene was transferred to a UHV system equipped with in situ XPS have been conducted, and it has been found that UHV anneals of up to 1200 °C were necessary in order to desorb the oxygen. Following the anneal, exposure to atmospheric conditions resulted in the return of only ~20% of the original oxygen concentration. This suggests that a robust oxide which is stable at high temperature may be influencing the graphene growth process. A detailed LEED, LEEM and STM study has been initiated to determine the effect that this oxygen has on the graphene film’s properties and new results will be presented.

References:

[1] L. Srivastava, G. He et al., Phys. Rev. B 82, 235406 (2010)

Graphene is the promising material for the next-generation devices due to its excellent electronic properties. We have developed epitaxy of graphene on SiC thin films on Si substrates (GOS) toward fusion of graphene with Si-based electronics. We have found that structural and electronic properties of graphene are tuned by crystallographic orientation of the Si substrates [1,2]. This result indicates that, in combination with Si microfabrication technologies, the electronic properties of GOS may be tuned by microfaceting Si surface [3].

A Si(100) substrate was fabricated by combining electron-beam lithography [3] and alkaline etching which produces (111) and (100) microfacet on the Si(100) substrate. On the microfabricated Si(100) substrate, SiC thin films were grown by using gas-source MBE, followed by graphitization in vacuum at 1500 K. Microscopic characterization of graphene was performed by using 3D NanoESCA for microscopic XPS (micro-XPS) and low-energy electron microscope for microscopic low-energy election diffraction (micro-LEED). Vibrational and bandstructure characterization was done by Raman microspectroscopy.

The micro-LEED observation reveals that graphene is Bernal stacked on the SiC(111)/Si(111) facet, while it is non-Bernal stacked on the SiC(100)/Si(100) microfacet. The observation is in consistent with the previous result on the epitaxy of graphene on non-fabricated SiC(111)/Si(111) and SiC(100)/Si(100) [1, 2]. The variation of the stacking is explained by the micro-XPS observation. The buffer layer which works as a template for the epitaxy of graphene exists only in between graphene and the SiC(111)/Si(111) microfacet. The existence of the buffer layer is also confirmed by cross-sectional transmission electron microscopy observations. Furthermore, Raman microscopy reveals that the band dispersion (splitting) microscopically changes depending on the variation of the stacking with the microfacet.

Our work can open a new way to microscopically tune control of electronic properties of graphene, semiconductive or metallic, which can make graphene devices multi-functionalized on Si substrates.

One of the authors (H. F.) acknowledges a financial support by KAKENHI (23560003).

References: [1] H. Fukidome et al., J. Mater. Chem. 21 (2011) 17242. [2] H. Fukidome et al., Appl. Phys. Exp. 4 (2011) 115104. [3] H. Fukidome et al., Jpn. J. Appl. Phys. 51 (2012) 06FD02.

Chemical vapor deposition on a Cu surface is one of the feasible methods for industrial applications, because large-area monolayer graphene film can be achieved.¹ However, grain boundaries and wrinkle formation in the graphene film are known to reduce the electrical performance of graphene. An atomic scale understanding of the growth mechanism on a Cu substrate is therefore of great importance for the synthesis of higher-quality graphene films.

Herein, we present the direction-selective growth of epitaxial graphene (EG) on a Cu(111) surface. All EGs had an identical R0^o rotational angle with respect to the lattice direction of Cu(111), although it is distinguished from the general understanding that graphene grown on weakly-interacting metal surfaces generally shows various rotation angles.² The EG growth mechanism was also elucidated with microscopic evidence of characteristic step edge formation of the Cu layers underneath individual EGs. We also observed various kinds of Moiré patterns in EGs with an identical rotational angle. The variety of Moiré patterns, even with an identical rotational angle, cannot be explained by conventional analysis showing that different rotational angles induce the different types of Moiré patterns. We suggest a “compressive strain” effect to explain the various Moiré superstructures observed in the atomically-resolved STM images.

1. Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Science 2009, 324, (5932), 1312-1314.

2. Batzill, M. Surf. Sci. Rep. 2012, 67, (3-4), 83-115.

Graphene plasmonics has recently combined near field optics with the exotic properties of graphene to demonstrate remarkable optical, biochemical, and optoelectronic architectures capable of extreme light concentration and manipulation, highly efficient photoconversion, and single molecule detection. Graphene’s unique electronic structure and chemical stability make it an optimal platform to interface with both light and matter; however, current devices are limited by the low-throughput or non-pristine processing steps to produce either the exfoliated or CVD transferred graphene, respectively. In this regard, it would be highly useful to grow a layer of graphene directly on top of a plasmonic metal substrate.

In this work, we report the novel growth of graphene on a bare Ag(111) single crystal. Growth was accomplished by evaporating atomic carbon onto a Ag(111) surface at elevated temperatures under ultra-high vacuum (UHV) conditions. The growth was verified and examined in-situ via scanning tunneling microscopy (STM)/scanning tunneling spectroscopy (STS) and further qualified via ex-situ Raman spectroscopy, scanning electron microscopy (SEM), and x-ray photoelectron spectroscopy (XPS). While the minimal C solubility in Ag suggests similar growth behavior to other noble metals (Cu, Au), this growth mode demonstrates markedly different signatures: nanoscale dendritic features, both terrace and step nucleation, strong electronic scattering at the graphene boundaries, and highly mobile Ag adatoms interacting with the graphene growth front. Furthermore, the growth was carried out at temperatures (600°C-700°C) much lower than the temperatures commonly used for conventional chemical vapor deposition (CVD) techniques (>1000°C) and it could be extended to a variety of weakly interacting substrates, including non-metals. All the observed growth was electronically characterized as single-layer graphene, and was further supported by the narrow full-width half-maximum (FWHM) of the 2D Raman band.All the observed growth was electronically characterized as single-layer graphene, and was further supported by the narrow full-width half-maximum (FWHM) of the 2D Raman band. A 65% decrease in the Ag-O peak in the O1s spectrum reveals that the graphene layer protects the underlying silver from environmental degradation. Two predominant Moiré patterns were observed in the graphene; their periodicity was ~1.55nm and ~0.95 nm corresponding to lattice offsets of ~4.5° and ~13°, respectively. Finally, the graphene grown on Ag is weakly bound to the surface indicated by the stark contrast between the dI/dV spectra of the graphene and bare silver surface. The graphene-Ag system demonstrated in this study could immediately be applied to tip-based molecular spectroscopies and will lead to the development of more advanced hybrid graphene plasmonics.

Graphene-based materials are of interest because of their electronic and thermal transport, mechanical properties, high specific surface area, and that they can act as an atom thick layer, barrier, or membrane, among other reasons. Our micromechanical exfoliation approaches [1,2] conceived of in 1998 yielded multilayer graphene and one paper described in detail how monolayer graphene could be obtained [1]. Three main research areas of our group are: (i) Growth of large area graphene on metal substrates, characterization and physical properties, and studies of devices having such CVD-grown graphene as a central component; (ii) Generation, study, and use of chemically modified graphene ‘platelets’ (typically derived from graphite oxide) including as dispersed in liquids forming colloids, and powders derived from such colloids or separately generated by microwave or thermal treatment of graphite oxide; (iii) Generation and study of new types of carbon derived from graphene-based precursors, such as activated microwave expanded graphite oxide (‘aMEGO’)[3].

Here, I will focus on growth of graphene and h-BN from research projects underway in my group and also briefly describe what I think are some important new directions in carbon and first row element research, for the next 10-20 years, that will introduce new issues involving interfaces for these new material classes that have not yet been made experimentally [4].

Current or prior support of our work by the W. M. Keck Foundation, NSF, DARPA ‘iMINT’, DARPA ‘CERA’, ONR, SWAN NRI, ARO, AEC, DOE, and the SRC, is appreciated.

1. Lu XK, Yu MF, Huang H, and Ruoff RS, Tailoring graphite with the goal of achieving single sheets, Nanotechnology, 10, 269-272 (1999).

2. Lu XK, Huang H, Nemchuk N, and Ruoff RS, Patterning of highly oriented pyrolytic graphite by oxygen plasma etching, Applied Physics Letters, 75, 193-195 (1999).

3. Zhu, Yanwu; Murali, Shanthi; Stoller, Meryl D.; Ganesh, K. J.; Cai, Weiwei; Ferreira, Paulo J.; Pirkle, Adam; Wallace, Robert M.; Cychosz, Katie A.; Thommes, Matthias; Su, Dong; Stach, Eric A.; Ruoff, Rodney S. Carbon-Based Supercapacitors Produced by Activation of Graphene. Science 332, 1537-1541 (2011).

4. Ruoff, Rodney S. Personal perspectives on graphene: New graphene-related materials on the horizon. MRS Bulletin 37, 1314-1318 (2012).