Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T51: Focus Session: Beyond Graphene: Synthesis, Defects, Structure, and Properties VIII |
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Sponsoring Units: DMP Chair: Valentino Cooper, Oak Ridge National Laboratory Room: Mile High Ballroom 1E |
Thursday, March 6, 2014 11:15AM - 11:51AM |
T51.00001: Physical foundations and future perspectives of the epitaxial silicene Invited Speaker: Alessandro Molle Silicene, a graphene-like Si monolayer, has been so far a fascinating theoretical hypothesis [1] with no experimental counterpart as due to the natural sp$^{3}$ hybridization of Si bonding. Artificially forcing the silicene lattice was firstly made possible in the epitaxial growth of a Si monolayer on Ag(111) substrates [2]. Here it is shown through an \textit{in situ} scanning tunnelling microscopy investigation [3] that unlike graphene, non-trivial atomistic arrangements (reconstructions) can coexist in multiphase silicene nanosheets as due to the balance between planar and buckled bonding [1]. This structural complexity is kinetic in character as it can be governed by tuning the thermal condition during growth or in a post-growth stage, and it is expected to bring absolutely peculiar physical properties [4]. Silicene is intrinsically limited by its intimate chemical instability. An \textit{ad hoc} engineered Al$_{2}$O$_{3}$ encapsulation is proposed for \textit{ex situ} investigations such as Raman spectroscopy [5]. Supported by \textit{ab initio} calculations, the measured Raman spectrum of the silicene phases is consistent with a prevailing a sp$^{2}$ hybridization, and it also evidences a reconstruction dependent resonant/non-resonant behavior [6]. To integrate silicene in transistor structures, decoupling from the metallic templates is highly desired. New directions in this respect are outlined which include silicene deposition on cleavable substrates (e.g. Ag(111) films on mica), and the van der Waals growth of Si nanosheets on layered templates (such as MoS$_{2}$ layers). Perspectives on the silicene ``portability'' for a device-oriented exploitation will be discussed. \\[4pt] [1] S. Cahangirov, et al., Phys. Rev. Lett. 102, 236804 (2009).\\[0pt] [2] P. Vogt, et al., Phys. Rev. Lett. 108, 155501 (2012).\\[0pt] [3] D. Chiappe, et al., Adv. Mater. 24, 37, 5088 (2012).\\[0pt] [4] M. Ezawa, Phys. Rev. Lett. 109, 055502 (2012).\\[0pt] [5] A. Molle, et al., Adv. Func. Mat. 23, 4340 (2013).\\[0pt] [6] E. Cinquanta, et al. J. Phys. Chem. C 117, 16719 (2013). [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:03PM |
T51.00002: Electronic and vibrational properties of group IV 2D materials: Planar and buckled forms of graphene, silicene and germanene David Carey, Nathanael Roome We report the ab initio electronic band structure and vibrational properties of 2D group IV elemental materials. Band structure calculations of planar graphene reveal a linear dispersion relation around the Dirac point with Fermi velocities of 8.0, 5.2 and 5.6 x 10$^{\mathrm{5}}$ m/s, respectively. The behaviour of the unoccupied $\sigma $* mode, normally ignored in planar graphene, is shown to be vary significantly with energy and cross the Fermi level in germanene. Analysis of the vibrational modes indicates that the E$_{\mathrm{2g}}$ mode at the zone centre ($\Gamma $ point) appears at 1566, 604 and 366 cm$^{\mathrm{-1}}$, respectively. Two types of buckling are shown to be present in silicene and germanene and linear dispersion is found in the low buckling configurations of both silicene and germanene. The band structure of silicene and germanene in the high buckling arrangement is shown to be much more complicated with a breakdown of the Dirac cone behaviour. The stability of the different forms of free standing layers is explored and the contribution of the different phonon modes to material stability is discussed. Electron-phonon coupling matrix elements are also calculated. [Preview Abstract] |
Thursday, March 6, 2014 12:03PM - 12:15PM |
T51.00003: Electric Field Effects on the Electronic Properties of Biaxial Strained Silicene Ryan Stein, Jia-An Yan A first-principles study of the electronic properties of biaxial strained silicene under various perpendicular electric fields are presented. Both compressed and tensile strains are considered. Interesting dependence of the electronic structure on the strain and the electric field will be presented. Effects of both strain and electric field on the electron-phonon coupling of silicene will also be discussed. [Preview Abstract] |
Thursday, March 6, 2014 12:15PM - 12:27PM |
T51.00004: Electron delocalization in gate-tunable gapless silicene Wei-Feng Tsai, Yan-Yang Zhang, Kai Chang, X.-T. An, G.-P. Zhang, X.-C. Xie, Shu-Shen Li The application of a perpendicular electric field can drive silicene into a gapless state, characterized by two nearly fully spin-polarized Dirac cones owing to both relatively large spin-orbital interactions and inversion symmetry breaking. Here we argue that since inter-valley scattering from nonmagnetic impurities is highly suppressed by time-reversal symmetry, the physics should be effectively single-Dirac-cone like. Through numerical calculations, we demonstrate that there is no significant backscattering from a single impurity that is nonmagnetic and unit-cell uniform, indicating a stable delocalized state. This conjecture is then further confirmed from a scaling of conductance for disordered systems using the same type of impurities. [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 12:39PM |
T51.00005: Optical investigation of epitaxial silicene on Ag(111) Eugenio Cinquanta, Francesco Scotognella, Daniele Chiappe, Carlo Grazianetti, Emilio Scalise, Michel Houssa, Marco Fanciulli, Caterina Vozzi, Alessandro Molle Silicene keeps on attracting the attention of the scientific community due to both its expected fascinating physical properties and its integrability in the present Si-based industry. Despite huge efforts devoted to silicene characterization, a picture of its physical properties is still lacking. The presence of degenerate superstructures together with a local Si-Ag hybridization, makes the valence band structure non-trivial. We elucidate the nature of epitaxial silicene on Ag(111) by means of optical CW and time-resolved techniques supported by ab-initio modelling. Based on Raman spectroscopy we confirm the lattice hexagonal symmetry and determine the electronic character of the different superstructures. We study the ultrafast photophysical properties of silicene via differential transmission spectroscopy. An intense feature in the spectrum, followed by a decay of one picosecond, is observed in the UV spectral region. This feature evidences to the presence of the Si layer and can be ascribed to a Si-Ag coupling, resulting in an enhancement of the Ag surface plasmon. [Preview Abstract] |
Thursday, March 6, 2014 12:39PM - 12:51PM |
T51.00006: Novel two-dimensional silicon and germanium allotropes: a first-principles study Florian Gimbert, Chi-Cheng Lee, Rainer Friedlein, Antoine Fleurence, Yukiko Yamada-Takamura, Taisuke Ozaki Graphene has been extensively studied but its integration into Si-based device technologies is difficult. It has been recently predicted by first-principles calculations that freestanding silicene and germanene, the counterparts of graphene made of Si and Ge atoms respectively, have graphene-like electronic structure with a low buckled structure [1]. So far, the models predicted by first-principles calculations were not able to describe completely the experimental results. These difficulties tend to suggest a more complex phase diagram for freestanding silicene or for silicene on a substrate than the simple buckled phase. We report for the first time a novel two-dimensional silicon and germanium allotropes, with a structure similar of that of MoS$_2$ layer [2]. After investigating a large range of lattice constants by first-principles calculations with OpenMX code, we show that this structure is the ground state for freestanding two-dimensional silicon and germanium layers instead of the usually considered low buckled silicene and germanene. \\[4pt] [1] S. Cahangirov et \textit{al.}, Phys. Rev. Lett. \textbf{102}, 236804 (2007). \newline [2] B. Radisavljevic et \textit{al.}, Nature Nano. \textbf{6}, 147 (2011). [Preview Abstract] |
Thursday, March 6, 2014 12:51PM - 1:03PM |
T51.00007: Germanene, Graphene-like Germanium: Hint of Epitaxial Growth and Electronic Properties Guy Le Lay, Andrea Resta, Maria Eugenia Davila Germanene is the germanium analogue of silicene, graphene's silicon cousin, hosting Dirac fermions [1]. It is predicted to be a robust two-dimensional topological insulator up to nearly room temperature, while the mobilities of its charge carriers might potentially exceed those of graphene. After our ground breaking demonstration that silicene has a physical existence upon realization of epitaxial single and multi-layer silicene on silver (111) substrates [2,3] we will now give indications of the two-dimensional epitaxial growth of germanium in a honeycomb arrangement, most likely, single layer germanene, a novel synthetic germanium allotrope that does not exist in nature [4]. If confirmed, this new achievement might open the way to tantalizing applications. [1]~G. Brumfiel, Nature, \textbf{495}, 153 (2013); Nature \textbf{485}, 9 (2012). [2] P. Vogt et al., Phys. Rev. Lett., \textbf{108}, 155501 (2012). [3]A. Resta et al. Scientific Reports, \textbf{3}, 2399 (2013). [4] A. Resta et al., to be published. [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T51.00008: Synthesis and Photoresponse of the Graphene-MoS2 in-plane Heterostructures Xi Ling, Yuxuan Lin, Qiong Ma, Jing Kong, Mildred Dresselhaus The heterostructures of two-dimensional materials offer a possibility to create high performance electronic and optoelectronic devices. Here, we present the construction of both the stacked and in-plane Graphene-MoS2 heterostructures directly during CVD growth. Exfoliated or patterned CVD-grown graphene were prepared on the 300 nm SiO2/Si substrate in advance. Using the seed-assisted method, different kinds of seeds were chosen to synthesize the stacked or in-plane Graphene-MoS2 heterostructures. Using the F16CuPc molecule as a seed, which can stick on the graphene surface under the growth temperature, the MoS2 monolayer was obtained on top of the graphene to achieve the construction of a stacked Graphene-MoS2 heterostructure. For an in-plane Graphene-MoS2 heterostructure, which can only be constructed by direct growth, we use the PTAS promoter as a seed, which prefers to stay on the SiO2/Si substrate out of the graphene. Then, the MoS2 monolayer was grown out from the edge of graphene to obtain the in-plane Graphene-MoS2 heterostructure, which was confirmed by AFM, Raman, PL and electric measurements. Furthermore, the photocurrent from the in-plane Graphene-MoS2 junction was measured and a high performance photoresponse device was achieved. [Preview Abstract] |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T51.00009: The nature of interactions in layered two-dimensional transition metal dichalcogenides - Anomalous frequency trends and surface effects Xin Luo, Su Ying Quek MoS$_{2}$ is a prototypical layered dichalcogenide material, with interlayer interactions dominated by weak van der Waals (vdW) interactions. Recent Raman experiments reported an anomalous blue-shift of the $E_{2g}^{1} $ mode with decreasing thickness, a trend that is not understood by simply relating frequencies to the restoring force in the system. Here, we combine experimental and theoretical studies to clarify and explain this trend.[1] We show that although interlayer interactions are weak in these materials, removing layers to form a surface in thin film MoS$_{2}$ can lead to larger Mo-S force constants at the surface (``surface effect''), which in turn accounts for the observed anomalous frequency trend. We predict the same anomalous trends for other modes in layered WSe$_{2}$, which are confirmed by experiments [2]. We find that most of the important interactions responsible for this ``surface effect'' occur within $\sim$ 1.5 {\AA} of the equilibrium interlayer distance. Our results have significant implications on the nature of interactions in vdW layered transition metal dichalcogenides. \\[4pt] [1] PRB 88, 075320 (2013);\\[0pt] [2] PRB accepted (2013) [Preview Abstract] |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T51.00010: Interlayer Physics in MoSe$_{2}$/WSe$_{2}$ Heterostructures Pasqual Rivera, Hongyi Yu, Aaron M. Jones, John Schaibley, Jason Ross, Sanfeng Wu, Grant Aivazian, Phillip Klement, Nirmal Ghimire, Jiaqiang Yan, David Mandrus, Wang Yao, Xiaodong Xu Van der Waals bound heterostructures of atomically thin 2D materials have recently been shown to possess unique properties beyond those of the individual layers. The unique phenomena arising from interactions between vertically stacked layers has generated substantial attention. With direct bandgaps ranging from 1.2 eV to 2.5 eV and strong spin-orbit coupling, monolayer transition metal dichalcogenide based heterostructures provide an intriguing platform for investigating new physics in heterostructure systems. Theoretical studies suggest the possibility of new device applications based on heterostrctures built from these materials, such as optically active bandgap engineering, vertical-tunneling field effect transistors, and new light harvesting technologies. Here, we investigate the interlayer interactions of one such heterostructure configuration, vertically stacked WSe$_{2}$ and MoSe$_{2}$ monolayers, using optoelectronic techniques. Our results suggest that interlayer interactions have significant impacts on exciton physics in the heterostructure. Progress towards understanding the nature of these effects in the MoSe$_{2}$/WSe$_{2}$ heterostructure will be presented. [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 1:51PM |
T51.00011: Synthesis and Heterostructures of Metal Dichalcogenides Monolayer Xin-Quan Zhang, Kuan-Chang Chiu, Tung-Han Yang, Jenn-Ming Wu, Yi-Hsien Lee Recently, monolayers of layered transition metal dichalcogenides (TMD), such as MX$_{2}$ (M$=$Mo, W and X$=$S, Se), have been reported to exhibit excellent optoelectronic performances. Monolayers in this class of materials offered a burgeoning field in fundamental physics, energy harvesting, electronics and optoelectronics. However, most studies to date are hindered by great challenges on the synthesis and transfer of high quality TMD monolayers. Hence, a feasible synthetic process and transfer techniques to overcome the challenges are essential. Here, we demonstrate the growth of high-quality TMD monolayers using chemical vapor deposition (CVD) with the seeding of aromatic molecules. The growth of monolayer TMD single crystals is achieved on various surfaces and its growth behavior has been discussed. We also demonstrate a robust technique in transferring the TMD monolayers to diverse surfaces, which may stimulate the progress on the class of materials and open a new route toward the synthesis of various novel hybrid structures with TMD monolayers. [Preview Abstract] |
Thursday, March 6, 2014 1:51PM - 2:03PM |
T51.00012: MoS$_{2}$-WSe$_{2}$ Hetero Bilayer: Possibility of Mechanical Strain Induced Band Gap Engineering Munish Sharma, Ashok Kumar, P.K. Ahluwalia The tunability of band gap in two-dimensional (2D) hetero-bilayers of MoS$_{2}$-WSe$_{2}$ with applied mechanical strains (in-plane and out-of-plane) in two different types of stackings (AA and AB) have been investigated in the framework of density functional theory (DFT). The in-plane biaxial tensile strain is found to reduce electronic band gap monotonically and rendered considered bilayer into metal at 6{\%} of applied strain. The transition pressure required for complete semiconductor-to-metal transition is found to be of 7.89 GPa while tensile strength of the reported hetero-bilayer has been calculated 10 GPa at 25{\%} strain. In case of vertical compression strain, 16 GPa pressure has been calculated for complete semiconductor-to-metal transition. The band-gap deformation potentials and effective masses (electron and hole) have been found to posses strong dependence on the type of applied strain. Such band gap engineering in controlled manner (internal control by composition and external control by applied strain) makes the considered hetero-bilayer as a strong candidate for the application in variety of nano scale devices. [Preview Abstract] |
Thursday, March 6, 2014 2:03PM - 2:15PM |
T51.00013: Electronic band gaps and transport in aperiodic graphene-based superlattices of Thue-Morse sequence Ligang Wang, Tianxing Ma We investigate electronic band structure and transport properties in aperiodic graphene-based superlattices of Thue-Morse (TM) sequence. The robust properties of zero-$\overline{k}$ gap are demonstrated in both mono-layer and bi-layer graphene TM sequence. The Extra Dirac points may emerge at $k_{y}\ne$0, and the electronic transport behaviors such as the conductance and the Fano factor are discussed in detail. Our results provide a flexible and effective way to control the transport properties in graphene-based superlattices. [Preview Abstract] |
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