Papers

AVS2012 Session GR+EM+NS+SS+TF-ThA: Beyond Graphene: BN and Other 2D Electronic Materials; 2D Heterostructures

Thursday, November 1, 2012 2:00 PM in Room 13

Thursday Afternoon

Start	Invited?	Item
2:00 PM		GR+EM+NS+SS+TF-ThA-1 X-ray Photoelectron Spectroscopy Investigation of the Valence and Conduction Band Offset at Hexagonal a-BN:H/Si Interfaces Sean King, Marc French, Jeffery Bielefeld (Intel Corporation); Milt Jaehnig (Intel Coroporation); Markus Kuhn, Benjamin French (Intel Corporation) Due to a wide band gap (~ 6 eV) and close lattice matching, hexagonal boron nitride (h-BN) is of interest as a potential gate dielectric in graphene channel transistor devices. A key property for the success of h-BN as a gate dielectric in such devices is the valence and conduction band offsets at the h-BN/graphene and h-BN/gate electrode interfaces. In many graphene channel devices, amorphous or poly-Si is a desirable gate electrode material for compatibility in standard CMOS processing. In this regard, we have utilized x-ray photoelectron spectroscopy (XPS) to determine the valence band offset present at the interface between plasma enhanced chemically vapor deposited hexagonal a-BN:H and a (100) Si substrate. Combined with Reflection Electron Energy Loss Spectroscopy measurements of the a-BN:H band gap, we have also been able to determine the conduction band offset at this interface. The combined measurements indicate a type I alignment with valence and conduction band offsets of 1.95±0.1 and 2.15±0.17 eV respectively.
2:20 PM		GR+EM+NS+SS+TF-ThA-2 Monolayer Graphene-Boron Nitride 2D Heterostructures Rocio Cortes, Jayeeta Lahiri, Eli Sutter, Peter Sutter (Brookhaven National Laboratory) Unusual electronic properties have been predicted for monolayer graphene-boron nitride heterostructures, but access to these properties depends on methods for controlling the formation of graphene-boron nitride interfaces [1]. Here we report on the growth and interface formation of monolayer graphene (MLG)-hexagonal boron nitride (h-BN) 2D heterostructures on Ru(0001), investigated by a combination of real-time low-energy electron microscopy (LEEM) and scanning tunneling microscopy (STM). LEEM observations of sequential chemical vapor deposition growth show that h-BN attaches preferentially to the edges of existing MLG domains, while nucleation of h-BN on the Ru surface away from MLG is not observed at the conditions considered here. With increasing coverage, h-BN expands anisotropically and, ultimately, the substrate is covered by a continuous 2D membrane of MLG domains embedded in h-BN. The study of the 1D interface between MLG and h-BN in these membranes by STM demonstrates that, following sequential growth at high temperatures, the interface is not abrupt, but contains an intermixed zone consisting of h-BN with embedded carbon atoms. Using quantitative LEEM, we have identified processes that eliminate this intermixing and pave the way to atomically sharp graphene-boron nitride boundaries, as confirmed by STM. The application of a similar growth procedure to terminate the edges of atomically controlled graphene nanoribbons with h-BN, embedding them in a h-BN membrane, will be considered. [1] P. Sutter, R. Cortes, J. Lahiri, and E. Sutter. Submitted (2012).
2:40 PM		GR+EM+NS+SS+TF-ThA-3 Large Area Vapor Phase Growth and Characterization of MoS₂Atomic Layers Jun Lou, Sina Najmaei, Zheng Liu, Yongjie Zhan, Pulickel Ajayan (Rice University) Monolayer Molybdenum disulfide (MoS₂), a two-dimensional crystal with a direct bandgap, is a promising candidate for 2D nanoelectronic devices complementing graphene. Unlike conductive graphene and insulating h-BN, atomic layered MoS₂ is a semiconductor material with a direct bandgap, offering possibilities of fabricating high performance devices with low power consumption in a more straight-forward manner. In this talk, we will discuss our recent efforts on the large area growth of MoS₂ atomic layers by a scalable chemical vapor deposition (CVD) method. The as-prepared samples can either be readily utilized for further device fabrication or be easily released from the growth substrate and transferred to arbitrary substrates. High resolution transmission electron microscopy and Raman spectroscopy on the as grown films of MoS₂ indicate that the number of layers range from single layer to a few layers. Our results on the direct growth of MoS₂layers on dielectric leading to facile device fabrication possibilities show the expanding set of useful 2D atomic layers, on the heels of graphene, which can be controllably synthesized and manipulated for many applications.
3:20 PM		BREAK
3:40 PM		GR+EM+NS+SS+TF-ThA-6 Formation of Silicene and 2D Si Sheets on Ag(111): Growth Mode, Structural and Electronic Properties Patrick Vogt (Technical University of Berlin, Germany); Thomas Bruhn, Andrea Resta, Benedicte Ealet (CNRS CiNaM, Marseille, France); Paola De Padova (CNR-ISM, Rome, Italy); Guy Le Lay (CNRS CiNaM, Marseille, France) Since the discovery of graphene enormous efforts have been invested to discover other similar 2-dimensional materials, like e.g. silicene. These 2D materials share similar structural, electronic and optical properties as graphene but are expected to differ in terms of their respective chemical reactivity and thus their applicability for electronic devices. In particular silicene could more easily be integrated into current Si-based electronics than graphene. Silicene has been predicted theoretically [1,2] but does not seem to exist in nature. Recently, we could synthesize silicone layers grown epitaxially by depositing Si on Ag(111) surfaces. The electronic properties of these silicene layers were shown to behave as theoretically predicted [3] and the structural and electronic properties are very similar to graphene. In STM images the hexagonal 2D silicone sheet gives rise to triangular structures situated in a honeycomb arrangement with (4×4) symmetry with respect to the Ag(111) surface. A structural model derived from the STM measurements showed a very good agreement with DFT results and exhibited a downward conical electronic dispersion resembling that of relativistic Dirac fermions at the Si K-points [3]. Depending on the growth conditions the formation of different 2D silicon arrangements can be observed: 1) Si-clusters at low deposition temperatures, 2) the formation of less ordered 2D hexagonal Si-based structures at temperatures up to 180°C, 3) the formation of the (4×4) silicene sheet around 220°C and 4) a 2D Si structure with a (√13×√13)-like periodicity at higher growth temperatures exhibiting a very regular, wide range ordered Moiré-like surface pattern in STM. Here, we will discuss the formation and epitaxial growth mode of these different 2D Si structures and the dependence on the growth parameters. We will also investigate whether these different 2D Si layers all refer to similar silicene sheets which give rise to different appearances in STM due to a varying rotation with respect to each other. Keywords: silicene, 2D materials, graphene, Dirac fermions References: [1] S. Cahangirov et al., Phys. Rev. Lett. 102, 236804 (2009) [2] G. G. Guzman-Verri and L. C. Lew Yan Voon, Phys. Rev. B 76, 75131 (2007) [3] P. Vogt et al.,Phys. Rev. Lett. 108, 155501 (2012)
4:00 PM		GR+EM+NS+SS+TF-ThA-7 Yttria-monolayer on Pt(111) Supported Graphene: A Novel Two Dimensional Heterostructure and its Affect on Charge Doping of Graphene Rafik Addou, Arjun Dahal, Matthias Batzill (University of South Florida) Yttrium oxide (Y₂O₃) is a high-k dielectric material, with promising wetting behavior of graphene [1]. In our study we grew yttria by reactive MBE on Pt(111) supported graphene to investigate the structural and electronic properties of the graphene/yttria interface. Photoemission measurements indicate that the graphene layer is covered by yttria. Scanning tunneling microscopy (STM) and low energy electron diffraction reveal that at annealing temperatures higher than 600 °C yttria forms an ordered monolayer on top of graphene. In STM, a moiré pattern is observed that is a consequence of super-positioning of a hexagonal yttria monolayer lattice with that of graphene. X-ray photoemission indicates a shift of the C1s peak by 1 eV to higher binding energy upon depositing of the yttria film. This peak shift is explained by charge doping of graphene by the underlying Pt substrate due to the change in the work function of the yttria coated graphene. ___________________________ [1] Z. Wang et al. Nano Lett. 2010, 10, 2024–2030; L. Ding et al. Nano Lett. 2009, 9, 4209–4214.
4:20 PM		GR+EM+NS+SS+TF-ThA-8 Probing the BCN-triangle by Computations—Outside the Carbon Corner Boris Yakobson (Rice University) We will discuss recent work on modeling 2D-materials “beyond graphene” [1-2]: two dimensional hexagonal h-BN, pure B polymorphs, MoS2, etc. Lessons from graphene studies remain invaluable as they offer general approach and views on the edges [3] and interface structures and energies, and especially organization of the grain boundaries [4,5]. New dislocation cores in BN (both 5/7 and 4/8 types) lead to accordingly new physical properties of emerging polar GB [6]. Similarly, we identify the dislocation cores and the grain boundary structure for more complex polar layer-material, MoS2 (X. Zou, unpublished). Our analysis of edge and cleavage energies helps to explain fracture patterns emerging in the course of synthesis. In principle, computations suggest possibility of metastable 2D-layers of GaN or ZnO or even their hybrids. Finally, it is important to mentions clear opportunities of designing 2D-circuits by combining 2D-materials in specific functional patters like proposed nanoroads and quantum dots [7-8] which now become a subject of experimental laboratory work. [1] Y. Liu et al. Nano Lett. 11, 3113 (2011). [2] E. Penev, et al, Nano Lett. 12,2441 (2012). [3] Y. Liu et al. Phys. Rev. Lett. 105, 235502 (2010). [4] BIY and F. Ding, ACS Nano 5, 1569 (2011). [5] Ajayan and BIY, Nature Mater. 10, 415 (2011). [6] Y. Liu et al. ACS Nano (2012). [7] A. Singh and BIY, Nano Lett., 9, 1540 (2009). [8] A. Singh, E. Penev, and BIY, ACS Nano, 4, 3510 (2010).
5:00 PM		GR+EM+NS+SS+TF-ThA-10 Single-layer MoS₂ Devices and Circuits Andras Kis (EPFL, Switzerland) Single layer MoS₂ is a recent addition to the family of 2D materials and is reminiscent of graphene except that it is an intrinsic direct band gap semiconductor with a 1.8 eV gap. We have exfoliated single layers 6.5 Angstrom thick from bulk crystals of semiconducting MoS₂, using the micromechanical cleavage technique commonly used for the production of graphene. Our nanolayers are mechanically and chemically stable under ambient conditions. We have fabricated transistors based on single-layer MoS₂ which demonstrate that this material has several advantages over silicon for potential applications in electronics. Our transistors have room-temperature current on/off ratios higher than 10⁸, mobility higher than 780 cm²/Vs and leakage currents in the fA range. Integrated circuits based on MoS₂ have the capability to amplify signals and perform logic operations. Finally, I will show our work on suspended MoS₂ membranes that show ripples similar to those observed in graphene. MoS₂ also has superior mechanical properties: higher stiffness than steel and 30 times its breaking strength which makes it suitable for integration in flexible electronics. References 1. B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, and A. Kis. Single-layer MoS₂ transistors. Nature Nanotechnology 6, 147, 2011. doi: 10.1038/nnano.2010.279 2. M.M. Benameur, B. Radisavljevic, J.S. Heron, S. Sahoo, H. Berger, and A. Kis. Visibility of dichalcogenide nanolayers. Nanotechnology 22, 125706, 2011. doi: 10.1088/0957-4484/22/12/125706 3. B. Radisavljevic, M.B. Whitwick, and A. Kis. Integrated circuits and logic operations based on single-layer MoS₂. ACS Nano 5, 9934, 2011. doi: 10.1021/nn203715c 4. J. Brivio, D.T.L. Alexander, and A. Kis. Ripples and Layers in Ultrathin MoS₂ Membranes. Nano Letters 11, 5148, 2011. doi: 10.1021/nl2022288 5. S. Bertolazzi, J. Brivio, and A. Kis. Stretching and Breaking of Ultrathin MoS₂. ACS Nano 5, 9703, 2011. doi: 10.1021/nn203879f