B1. Modeling Quantum Transport at Nanoscale Chair(s): Chun ZHANG, National University of Singapore, Singapore Session s Title (if available) Tue - 17 Jan 2017 13:00 ~ 14:30 Room 2 Session Chair: Prof. Haiping Cheng (University of Florida) 13:00 B1-1(invited) Electronic and transport properties of phosphorene nanoribbons Dr. Lei Shen National University of Singapore Abstract: By combining density functional theory and nonequilibrium Green s function, we study the electronic and transport properties of monolayer black phosphorus nanoribbons (PNRs). First, we investigate the band gap of PNRs and its modulation by the ribbon width and an external transverse electric field. Our calculations indicate a giant Stark effect in PNRs, which can switch on transport channels of semiconducting PNRs under low bias, inducing an insulator-metal transition. Next, we study the transport channels in PNRs via the calculations of the current density and local electron transmission pathway. In contrast to graphene and MoS_2
nanoribbons, the carrier transport channels under low bias are mainly located in the interior of both armchair and zigzag PNRs, and immune to a small amount of edge defects. Last, a device of the PNR-based dual-gate field-effect transistor, with high on/off ratio of 10^3, is proposed based on the giant electric-field tuning effect. 13:30 B1-2(Keynote) Modeling Quantum Transport at Nanoscale Prof. Xiaoguang Zhang University of Florida Abstract: Quantum transport theory yields the celebrated Landauer formula for the conductance of a two-terminal device at zero bias in terms of T(EF,0), the transmission coefficient T(E,V) evaluated at the Fermi energy EF and V=0. For finite biases, one must use the nonequilibrium Green s function (NEGF) method, which entails substantial difficulties. Instead of NEGF calculations, T(E,0) is often interpreted as representing transport at V=E/e. Here we show that this practice is seriously flawed. In its stead, we employ quantum transport theory to derive a finite-bias analog of the Landauer formula. The new formula expresses the differential conductance di/dv at a bias V in terms of T(μL,2V)+T(μR,2V) and reduces to the Landauer formula at V=0. This simple new formula is tested for a benzene molecular junction and is shown to yield excellent agreement
with a full NEGF calculation without the need for a self-consistent calculation of T(E,V). 14:00 B1-3(invited) Controllable negative differential resistance in molecular devices Prof. Keqiu Chen Hunan University Abstract: molecular devices have attracted increasing attention. NDR has very important application in electronic devices including switch, amplifier, and memory. In order to understand and modulate NDR, the electronic transport properties in various kinds of molecular devices have been investigated. We gave a systematic theoretic study of the mechanism of NDR in molecular systems. It is found that NDR can be realized and adjusted by changing intermoleculat interaction (APL 91, 233512 (2007)), by constructing heterojunctions (APL 91, 133511 (2007); Nature Commun. 4, 1374 (2013)), by side group (APL 92, 243303 (2008))., by the deformation (APL 92, 263304 (2008); 94, 183506 (2009); 95, 232118 (2009)), by contact geometries (APL 96,
053509 (2010) ), by hybridized interface (APL 97, 193305 (2010)), by tuning edge states (APL 101, 073101 (2012)), by breaking symmetry (APL 102, 023508 (2013); Scientific Reports 4, 5976 (2014)), and by tuning spin (J. Meter. Chem. C 1, 4014 (2013); 3, 5697 (2015); APL 104, 033104 (2014); Carbon 95, 503 (2015); Org. Electronics 28, 244 (2016)). Session s Title (if available) Wed - 18 Jan 2017 10:15 ~ 11:45 Room 8 Session Chair: Prof. Xiaoguang Zhang (University of Florida) 10:15 B1-4(Keynote) First-principles simulations of tunneling field-effect transistors Prof. Haiping Cheng University of Florida Abstract: We improvise a novel approach to carry out first-principles simulations of graphene-based tunneling field-effect transistors that consist of graphene 2D-crystal/molecule graphene junction. Within the density functional theory framework, we exploit the effective screening medium method to properly treat boundary conditions for electrostatic potentials and
investigate both single-gate voltage and dualgate configurations. The distribution of free carriers and the band structure of both top and bottom graphene layers are calculated selfconsistently. Dielectric properties of 2Dcrystal thin films sandwiched between graphene layers are computed layer-by-layer. We discuss effects of gate voltage and charge doping on interface properties such as bands, gaps, and spin-orbital couplings as well as the emergence of a resonant transmission peak in graphene-molecule-graphene systems. 10:45 B1-5(Inivte) Emergent edge modes and transport in periodically driven quantum systems Prof. Erhai Zhao George Mason University Abstract: One way to control quantum transport at the nanoscale is to couple the system to an oscillating external field, for example, to irradiate a graphene ribbon with circularly polarized light. With suitable driving, the properties of the sample can be fundamentally altered, giving rise to new spectral and transport properties that are impossible to achieve in static systems. I will use simple models to illustrate the so-called Floquet topological insulators and their emergent edge states.
Then I will comment on the theoretical framework to classify and understand different Floquet phases. Lastly, model calculations of the transport characteristics will be presented for a Floquet insulator between two leads. 11:15 B1-6(invited) Coherent electron transport through an OPE based molecular junction: An SS-DFT study Dr. Guo Na National University of Singapore Abstract: Molecular electronics, in which a single molecule works as functional units of a circuit, have attracted enormous research interests. To understand the coherent electron transport through a molecular junction is crucial for future design of devices. Here we apply steady state density functional theory (SS-DFT) to study electronic and transport properties of an OPE (oligo phenylene ethynylene) molecule based junction. Previous studies have shown that there are order-of-magnitude differences between theoretical calculations and experimental observations for such junction. We found that the van der Waals interaction between the molecule and electrodes plays an important role in determining both the stable atomic structure of contact regions and also the I-V characteristics, which points out a new direction in explaining the theory-experiment
disagreements for such molecular scale junctions Session s Title (if available) Thu - 19 Jan 2017 10:15 ~ 12:15 Room 18 Session Chair: Prof. Erhai Zhao (George Mason University) 10:15 B1-7(Inivited) The multi-functional electronic devices of organic molecule(s) between graphene nanoribbon electrodes Prof. Guanghui Zhou Hunan Normal University Abstract: The development of electronic devices based on controllable molecular conduction aims to meet the urgent demand for further device miniaturization, and the need to effectively interface organic molecule and electrode materials for nanoelectronic applications. To this goal, diverse approaches to molecular devices have been proposed and have faced the important issue of interface for molecule-electrode coupling. However, the contacts between molecular and metallic electrodes are complicated and difficult to achieve perfect atomic interface, resulting in high contact
resistance for the device. Fortunately, graphene can also be used directly as electrodes in molecular devices. In contrast to the structures of sp and sp 3, the structure of sp 2 of graphene shows more unique molecular electronic and spintronic properties. On the other hand, the electronic coupling in metal-organic interfaces do not follow simple rules but are rather the consequence of subtle local interactions. As the electrode of a molecular junction, nevertheless, the planar-structured graphene may overcome this difficulty. In this talk, we propose organic molecule(s) covalently sandwiched between graphene electrodes to form stable molecular conduction junctions. By using self-consistent ab initio calculation based on DFT+NEGF, we demonstrate that the multi-functions of switching and (dual) spin-filtering effects can be realized by suitable combination of molecule-graphene conformation for this all-carbon device. 10:45 B1-8(invited) Towards Reality in Modeling of Nano-scale Contacts
Prof. Yong-Hoon Kim Korea Advanced Institute of Science and Technology Abstract: Contacts with electrodes are the critical factor that determines the characteristics of nanoscale electronic devices, but their atomic-scale understanding and precise controlling still remains elusive. In this talk, I will present several recent works within our group that are concerned with the quantum transport through nano-scale electrode contacts. First, I will consider the metal-carbon nanotube contacts, for which we have recently shown the importance of considering the length scaling behavior of contact resistance [1]. It will be demonstrated that a similar consideration is necessary for metal-graphene contacts. Next, I will discuss molecular junctions based on the ubiquitous S-Au contacts [2], for which I will establish the correlation between the single-molecule conductance and the S-Au linkage coordination number. It will be shown that explicit ab initio junction pulling molecular dynamics (MD) simulations exhibit a continuous transformation of Au apex atoms from high- to low-coordination numbers, which cannot be captured within static junction pulling simulations. Finally, I will consider the DNA sequencing based on low-dimensional carbon nanoelectrodes [3]. Controlling the dynamics of DNA translocation is again a central issue in the emerging solid-state DNA sequencing approach. Performing large-scale force-fields MD simulations, I will demonstrate that the N doping of carbon nanoelectrdoes not only increases the sensitivity and selectivity for tunneling-current nucleobase reading but also benefits the control of DNA conformations by slowing down the translocation speed and reducing structural fluctuations of nucleobases. [1] Y.-H. Kim & H. S. Kim, Appl. Phys. Lett. 100, 213113 (2012); H. S. Kim et al., MRS Commun. 2, 91 (2012); [2] Y.-H. Kim et al., J. Chem. Phys. 122, 244703 (2005); H. S. Kim & Y.-H. Kim, Phys. Rev. B 82, 75412 (2010). [3] H. S. Kim, S. J. Lee, & Y.-H. Kim, Small 10, 774 (2014); H. S. Kim & Y.-H. Kim, Biosens. Bioelectron. 69, 186 (2015). 11:15 B1-9(Contributed)
Two-dimensional Cu2Si sheet: A promising electrode material for nanoscale electronics Kah Meng Yam National University of Singapore Abstract: Recently, a new two dimensional (2D) material, Cu 2Si, has been predicted to exist. Theoretical calculations show that unlike other 2D materials that are normally semiconductors or semimetals, 2D Cu 2Si is metallic. In this work, we employ steady-state density functional theory (SS- DFT) to study the spin-dependent electron transport through a Co-phthalocyanine molecule sandwiched between two Cu 2Si leads. We show that the junction can be used as a high-performance spin filter. The transport properties are compared with a similar graphene-co-phthalocyanine-graphene junction. Our results clearly suggest that 2D Cu 2Si is an excellent candidate for electrode material for future nanoscale electronics. 11:45 B1-10 (invited)
The Nearly Free Electron in the Two-Dimensional MXene Films Prof. Yunye Liang Shanghai Normal University Abstract: As new members of two-dimensional family, the transition metal carbides (namely MXenes) and their functionalized derivatives, exhibit many different physical and chemical properties. Recently, based on the density functional theory, we have reported the nearly free electron (NFE) states in different MXenes functionalized by -OH. Most of these MXenes films are metallic. However, in the monolayer of Sc2C(OH)2 and Y2C(OH)2 which are semiconductor, we find that the NFE becomes the conduction band and the NFE state can be modulated by the external electric fields. As a result, the band gap width can be engineered. Sc2C(OH)2 monolayer can be conductor when the applied field is about 0.25 V/ Å. Our findings indicate that Sc2C(OH)2 can be used as electric devices, which can be modulated by the gate voltages.