Elsevier

Surface Science

Volume 605, Issues 17–18, September 2011, Pages 1649-1656
Surface Science

Strain-induced pseudo-magnetic fields and charging effects on CVD-grown graphene

https://doi.org/10.1016/j.susc.2011.03.025Get rights and content

Abstract

Atomically resolved imaging and spectroscopic characteristics of graphene grown by chemical vapor deposition (CVD) on copper are investigated by means of scanning tunneling microscopy and spectroscopy (STM/STS). For CVD-grown graphene remaining on the copper substrate, the monolayer carbon structures exhibit ripples and appear strongly strained, with different regions exhibiting different lattice structures and electronic density of states (DOS). In particular, ridges appear along the boundaries of different lattice structures, which exhibit excess charging effects. Additionally, the large and non-uniform strain induces pseudo-magnetic field up to ~ 50 T, as manifested by the DOS peaks at quantized energies that correspond to pseudo-magnetic field-induced integer and fractional Landau levels. In contrast, for graphene transferred from copper to SiO2 substrates after the CVD growth, the average strain on the whole diminishes, so do the corresponding charging effects and pseudo-magnetic fields except for sample areas near topological defects. These findings suggest feasible nano-scale “strain engineering” of the electronic states of graphene by proper design of the substrates and growth conditions.

Research highlights

► Atomically resolved imaging and spectroscopy of CVD-grown graphene are studied by means of scanning tunneling microscopy ► Strain-induced giant pseudo-magnetic fields in CVD-grown graphene appear as density-of-states peaks at quantized energies ► The quantized energies correspond to pseudo-magnetic field-induced integer and fractional Landau levels in grapheme ► Nano-scale charging effects in graphene are revealed by the enhanced local density of states in highly strained regions ► Both the giant pseudo-magnetic fields and charging effects in CVD-grown graphene diminish upon the reduction of strain

Introduction

The superior physical properties of graphene [1] and its compatibility with two-dimensional lithographic processes have stimulated intense research of graphene-based electronics for “beyond Si-CMOS” technology [2], [3], [4], [5], [6], [7], [8]. One of the major challenges for realizing the graphene-based beyond Si-CMOS technology is the fabrication of high-quality large-area graphene sheets and the retention of superior electronic characteristics of graphene. The apparent degradation of carrier mobility and significant variations in the electronic characteristics when graphene comes in contact with various dielectrics [9], [10] suggest significant susceptibility of the single-layer carbon atoms to the surrounding environment. On the other hand, the strong susceptibility of graphene to external influences also provides opportunities for engineering unique properties of graphene. For instance, it has been theoretically proposed that a designed shear strain aligned along three main crystallographic directions induces strong gauge fields that effectively act as a uniform magnetic field on the Dirac electrons [11], [12]. In particular, for a finite doping level, the quantizing field results in an insulating bulk and a pair of counter-circulating edge states, which is similar to the case of topological insulators [13], [14], [15]. Moreover, strained superlattices have been shown theoretically to open significant energy gaps in the electronic spectrum of graphene [11], which could be much more effective than current approaches to the bandgap engineering of graphene by either controlling the width of graphene nano-ribbons [16], [17] or applying magnetic fields to bi-layer graphene [18], [19], [20]. Recent scanning tunneling microscopic and spectroscopic (STM/STS) studies of graphene nano-bubbles on Pt(111) single crystal substrates [21] have verified the theoretical prediction of strain-induced pseudo-magnetic fields, and significant field values in excess of 300 T have been observed.

In the context of synthesis of large area graphene, various growth techniques have been developed to date, including ultra-high-vacuum annealing to desorption of Si from SiC single crystal surfaces [22], [23], deposition of grapheneoxide films from a liquid suspension followed by chemical reduction [24], [25], and chemical vapor deposition (CVD) on transition metals such as Ru [26], [27], Ni [28], [29], Co [30], Pt [31], [32] and Cu [33]. While samples as large as centimeter squared have been achieved, and characterizations of macroscopic physical properties such as Raman scattering and mobility measurements have been reported [28], [29], [33], most large-area samples appear to exhibit thicknesses ranging from one to approximately twelve layers. Moreover, few investigations with the exception of Refs. [21], [34] have been made on correlating the local electronic properties with the microscopic structural variations of these large-area films.

In this work we report scanning tunneling microscopic and spectroscopic (STM/STS) investigations of the microscopic electronic and structural properties of CVD-grown graphene on copper and also on SiO2 after transferred from copper. We find that the CVD grown graphene is strongly strained so that both the lattice structure and the local electronic density of states (DOS) of graphene are significantly affected [34]. The non-trivial strain results in significant pseudo-magnetic fields Bs up to ~ 50 T, as manifested by the DOS peaks at energies corresponding to quantized Landau levels |En|n, where n denotes both integers and fractional numbers at 0, ± 1, ± 2, ±3, 4, 6, ± 1/3, ± 2/3, ± 5/3. In addition, significant charging effects are found along the strongest strained areas of the sample, consistent with a strain-induced scalar potential. On the other hand, for CVD-grown graphene transferred from copper to SiO2, both the pseudo-magnetic fields and charging effects diminish in most parts of the sample except for small regions containing topological ridges that cannot be relaxed by changing the substrate. The occurrence of conductance peaks at quantized integer and fractional Landau levels in strained graphene is analogous to the magnetic field-induced integer and fractional quantum Hall effects (IQHE and FQHE), suggesting that the two-dimensional Dirac electrons may be quantum confined into correlated many-body states by strain. The strained-induced charging effects and quantized states also enable new possibilities of controlling the energy gaps and doping levels of graphene through nano-scale “strain engineering”.

Section snippets

Experimental

Our experimental approach to investigating the local electronic and structural correlations of graphene on dielectric or metallic substrates is to perform STM/STS studies using a home built cryogenic STM, which was compatible with high magnetic fields and also capable of variable temperature control from room temperature to 6 K, with a vacuum level of ~ 10 10 Torr at the lowest temperatures. For studies reported in this work, the measurement conditions were at 77 K under high vacuum (< 10 7 Torr) and

Results and analysis

Systematic studies of the CVD grown graphene on copper revealed large height variations everywhere, as exemplified by the topographic images Figs. 1a–c and the corresponding height histograms in Figs. 1d–f for one of the two samples. The ripple-like height variations are consistent with previous reports [33] and are primarily associated with two physical causes. The first is the large difference in the thermal contraction coefficients of graphene and copper. Namely, upon cooling the sample from

Discussion

We have shown that significant differences between the thermal contraction coefficients of graphene and the transition-metal substrates for CVD growth graphene result in strong and non-uniform lattice distortion at the nano-scale, which provides a new ground for investigating the strain-induced charging and pseudo-magnetic field effects. In particular, the strain-induced giant pseudo-magnetic fields enable studies of the integer and fractional quantum Hall effects associated with the

Conclusions

We have conducted spatially resolved topographic and spectroscopic studies of CVD-grown graphene-on-copper and transferred graphene-on-SiO2 samples. Our investigation reveals the important influence of the substrate and strain on the carbon atomic arrangements and the electronic DOS of graphene. For CVD-grown graphene remaining on the copper substrate, the monolayer carbon structures exhibit ripples and are strongly strained, with different regions exhibiting varying lattice structures and

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      The electromechanical coupling in graphene is unique because of its massless Dirac-like electronic states that give rise to novel physics via an interplay with strain, and is realistic since it can resist a tensile strain up to 25% before structural breaking (Kim et al., 2009; Liu et al., 2007). One prominent example is that a strain pattern in a graphene lattice can induce a gauge field, which leads charge carriers in graphene to behave as if under the influence of an external magnetic field (Guinea et al., 2010a, 2010b; Kim et al., 2011; Levy et al., 2010; Liu et al., 2007; Low and Guinea, 2010; Yeh et al., 2011; Zhang et al., 2014a; Zhu et al., 2015). The strain-induced pseudo-magnetic (p-mag) field allows not only for novel manipulation of graphene electronics but also for the exploration of carrier dynamics in extreme magnetic fields unattainable in normal laboratories (Levy et al., 2010).

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    This work was supported by NSF and NRI through the Center of Science and Engineering of Materials (CSEM) at Caltech.

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