The systemic integration of organic molecules as functional building blocks with graphene-circuitry is central for the advancement of graphene-based molecular electronic devices. Applications based on this combination span a broad spectrum from high-speed switches, memory logic to chemical sensing. To realize these predictions requires an in-depth understanding from a fundamental and technological perspective of molecular design and the electro-chemical interactions of the functional molecules with graphene. However, the potential energy landscape of graphene is not homogeneous and is known to be strongly dependent on the layer composition (mono, bi or tri-layers), defects (both intrinsic and extrinsic), grain boundaries, edge effects and underlying substrate-induced topological defects. A quantitative analysis of the surface corrugations and electronic transport across graphene down to the atomic level can be provided by scanning tunneling microscopy (STM) based on the quantum tunneling principle. In the past a vast majority of STM-based experiments on graphene have been conducted, from observing atomic collapse states [1] to visualizing defect-induced scattering [2] at ultra-high vacuum (UHV) under cryogenic conditions. Although these experiments offers a plethora of information at atomic-length scales, the experimental conditions pose a limiting factor as graphene-based molecular electronic devices are highly likely to be fabricated under ambient conditions suitable for room-temperature operation. Hence it would be beneficial to record and augment molecular-scale phenomena on graphene under robust and practical conditions. One such alternative experimental setting is to operate the STM at the liquid-solid interface, coined the “real-interface” by co-inventor of the STM and Nobel laureate Heinrich Rohrer [3]. This interface has been previously shown to serve as an excellent platform to record in real-time molecular-scale chemical processes [4] and [5] and directly visualize atomic-scale details on solid surfaces [6] under a wide range of liquids.
We in the Materials Integration and Nanoscale Devices group at IBM Research – Zurich are investigating the electronic structure and dynamics of single molecules on ultra-flat metals and emerging two-dimensional nanomaterials by means of an STM operated in a liquid environment. The experimental setup is located in state-of-the-art noise-free laboratories [7] with exceptional temperature, humidity and vibration control. The STM tool is capable of operating in constant-current mode at high-speeds in both polar (requires appropriate tip insulation) and non-polar liquids with minimal thermal fluctuations. The liquid medium (preferably with low-vapor pressure) within the enclosed liquid-cell setup ensures stable imaging conditions and secures the tunnel gap from external contamination. Previously we have demonstrated the applicability of this in situ approach for mapping the detrimental role of nanoscale surface defects on the mobility of liquid-phase dispersed fullerene molecules and show in real-time the formation of close-packed two-dimensional architectures on defect-free metal platforms [8]. Currently we are investigating graphene electrodes known to possess reduced surface mobility with respect to gold and the structure-function dependence of tailored organic molecules on graphene in collaboration with theoretical modeling groups where atomic-scale molecular dynamics simulations [9] on these systems is performed.
The image featured on this issue's cover shows a high-resolution in situ STM image of atomically resolved monolayer graphene adsorbed on an ultra-thin insulating organic layer coated on Au(1 1 1) surface in an n-tetradecane liquid medium (tunneling conditions I = 8 pA, Vbias = 400 mV). The image shown in this article was acquired with a mechanically cut bare Au metal wire as the STM tip. The honeycomb structure with its characteristic atomic hexagons and local structural corrugations of monolayer layer graphene is clearly discernible from the in situ-STM image. The graphene flakes analyzed in this study were fabricated by spray-depositing graphene exfoliated in low boiling-point solvents onto alkyl-coated-gold thin films. The deposited graphene flakes had a mixed population of predominantly mono and bilayers with a flake size of ∼450 nm. In addition to providing real-space atomic-scale information on graphene we can perform tunneling spectroscopy to extract the unperturbed linear energy dispersion profile of graphene. The insertion of the insulating organic film (C14H30) as spacers at the graphene-metal interface ensures sufficient electronic decoupling of the graphene flakes and minimizes any metal induced screening effects.
The factors controlling atomic-scale imaging of graphene in liquids are the tunnel gap-resistance, the electronic states of the STM probe, the electro-chemical nature of the encompassing liquid medium and the stable instrumental operation in a controlled environment. This capacity to resolve atomic-scale information directly on graphene under ambient conditions creates vast opportunities to understand detailed interactions between organic molecules on graphene, which is key for the rational design of functional molecular electronic devices based on single-layer-thick nanomaterials.
Acknowledgement
We gratefully acknowledge the financial support from the Marie Curie Actions-Intra-European Fellowship (IEF-PHY) under grant agreement no. 275074 “To Come” within the 7th European Community Framework Programme.
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Further reading
[1] Y. Wang et al., Science, 340 (6133) (2013), pp. 734–737
[2] G.M. Rutter et al., Science, 317 (5835) (2007), pp. 219–222
[3] A.A. Gewirth, H. Siegenthaler (Eds.), Nanoscale Probes of the Solid/Liquid Interface (1st ed.) (1993), p. Springer p. 334
[4] B. Hulsken et al., Nat. Nano, 2 (5) (2007), pp. 285–289
[5] S. De Feyter, F.C. De Schryver, J. Phys. Chem. B, 109 (10) (2005), pp. 4290–4302
[6] R. Sonnenfeld, P.K. Hansma, Science, 232 (4747) (1986), pp. 211–213
[7] E. Lortscher, D. Widmer, B. Gotsmann, Nanoscale, 5 (21) (2013), pp. 10542–10549
[8] P.N. Nirmalraj et al., Langmuir, 29 (5) (2013), pp. 1340–1345
[9] S. O’Mahony et al., Langmuir, 29 (24) (2013), pp. 7271–7282