Peter Sutter, Univ of Nebraska-Lincoln
Nasim Alem, The Pennsylvania State University
Arkady Krasheninnikov, Helmholtz-Zentrum Dresden-Rossendorf
Alexander Weber-Bargioni, Lawrence Berkeley National Laboratory
J.A. Woollam Company, Inc.
RHK Technology, Inc.
NM8.1: 2D Materials Synthesis and Processing
Tuesday PM, April 18, 2017
PCC West, 100 Level, Room 101 A
11:45 AM - *NM8.1.01
Predictive Modeling of 2D Materials, Growth and Properties
Boris Yakobson 1 Show Abstract
1 Department of Materials Science & NanoEngineering, Department of Chemistry, and the Richard E. Smalley Institute, Rice University, Houston, Texas, United States
Comprehensive tools of materials modeling are derived from the principles of physics and chemistry, empowered by high performance computing. Together, this allows one to make verifiable predictions of novel physical structures with specific, often useful or even extraordinary, properties. Examples from our work will be presented, first being growth and unusual morphology of binary compositions of metal dichalcogenides MX2 , where a combination of DFT and phase-field simulations proves useful. Second, prediction of pure mono-elemental boron 2D B and its particular structures, which culminated in recent experimental confirmations, while also promises new 2D-superconductor .
 V. Artyukhov et al. Phys. Rev. Lett. 114, 115502 (2015); V. Artyukhov, Z.Hu et al. Nano Lett. 16, 3696 (2016).
 Z. Zhang et al. Nature Chem. 8, 525 (2016); Z. Zhang et al. Angewandte Chemie Int. Ed. 54, 13022 (2015); E. Penev, A. Kutana et al. Nano Lett. 16, 2522 (2016); Z. Zhang, Nano Lett. 6, 6622 (2016); A. Brotchie, Nature Reviews, doi:10.1038/natrevmats.2016.83 (2016).
12:15 PM - NM8.1.03
Deterministic Patterned Growth of High-Mobility Large-Crystal Graphene—A Path towards Wafer Scale Integration
Vaidotas Miseikis 1 2 4 , Federica Bianco 3 , Vittorio Pellegrini 2 , Marco Romagnoli 4 , Camilla Coletti 1 2 Show Abstract
1 Center for Nanotechnology Innovation, Istituto Italiano di Tecnologia, Pisa Italy, 2 Graphene Labs, Istituto Italiano di Tecnologia, Genova, Genova, Italy, 4 , Consorzio Nazionale Interuniversitario per le Telecomunicazioni (CNIT), Pisa Italy, 3 , NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore, Pisa Italy
It is well-known that single-crystal CVD graphene can have excellent transport characteristics , however, it has very limited applicability in wafer-scale integration of graphene due to the random spatial distribution large crystals. We demonstrate a solution to this problem by seeded growth of large-crystal graphene using deterministically patterned Cu growth substrates with chromium nucleation seeds. This approach is used to synthesize well-ordered arrays with a crystal size of up to 350 µm and a periodicity of up to 1 mm. The material is characterized using scanning electron microscopy (SEM), spatially-resolved Raman spectroscopy and electrical transport measurements.
We show that by adjusting the growth parameters, we can remove the parasitic chromium particles following the successful seeding of graphene. The removal of chromium after the growth is confirmed by high-magnification SEM and the absence of Raman D-peak across the whole area of graphene crystals
When the graphene is transferred on top of hexagonal boron nitride (h-BN), spatially-resolved Raman mapping of the 2D peak reveals a remarkably low FWHM of 20-23 cm-1, indicating low strain variation within the laser spot, which is known to be one of the primary causes of charge scattering and reduced mobility in graphene devices . Room-temperature field effect measurements of Hall bar devices fabricated on non-encapsulated h-BN/graphene heterostructures confirm the high carrier mobility with values as high as 21 000 cm2/Vs, further confirming the high quality of the synthesised material.
Furthermore, we present an aligned semi-dry transfer approach allowing deterministic placement of graphene arrays on the target substrates with reduced impurities. Combined with a method of synthesis which allows the growth of large single-crystals according to the desired device architecture, this approach could provide a major advance towards the adoption of CVD graphene in wafer scale applications.
 N. Petrone et al, “Chemical vapor deposition-derived graphene with electrical performance of exfoliated graphene.,” Nano Lett., vol. 12, no. 6, pp. 2751–6, Jun. 2012. DOI: 10.1021/nl204481s
 N. J. G. Couto et al “Random strain fluctuations as dominant disorder source for high-quality on-substrate graphene devices,” Phys. Rev. X, vol. 4, no. 4, pp. 1–13, 2014. DOI: 10.1103/PhysRevX.4.041019
12:30 PM - NM8.1.04
Fabrication of Sub-30nm Period Graphene Antidot Lattices by Electron Beam Lithography
Lene Gammelgaard 1 , Bjarke Jessen 1 , David Mackenzie 1 , Jose Caridad 1 , Joachim Dahl Thomsen 1 , Timothy J. Booth 1 , Peter Boggild 1 Show Abstract
1 , Technical University of Denmark, Kgs. Lyngby Denmark
The introduction of superlattices in graphene can introduce phenomena such as bandgap opening and cloning of Dirac peaks in conductance measurements. To observe clear signs of superlattice-effects, periods of the superlattice should be kept well below 50 nm, and has so-far only been observed in samples where the superlattice was introduced due to the Moiré pattern from graphene on hexagonal boron nitride (hBN), giving rise to periods of 14 nm and below. The only way to achieve high control of lattice period and unitcell is by lithographically modifying the graphene, specifically using electron-beam lithography (EBL). However, conventional EBL has so-far been limited to superlattice periods down to 55 nm in graphene.
We demonstrate ultra-dense nano-patterning of hBN encapsulated graphene fabricated by EBL. Hexagonal and square lattices with sub-30nm periods, i.e. the center-to-center distance, are fabricated with single-shot exposures in a JEOL JBX-9500FSZ EBL system with a 100keV acceleration voltage. In the single-shot exposures the hole diameter can be continuously tuned by the dose of each shot.
The ultra-dens antidote lattices are etched into heterostructures of hBN-encapsulated graphene fabricated by the hot pick-up technique. The hBN-encapsulation increases the device quality and protects the graphene while etching the superlattices. Two etch steps are performed to etch the top hBN and graphene; the etch processes are optimized to only etch either the hBN or the graphene and only little of the resist and SiO2 substrate. The selectivity of the etch processes enable us to only etch the top hBN and graphene of the stacks preserving the bottom hBN, which act as a gate dielectric between the device channel and a graphite back gate. Electrical connections to the graphene device channel are made with one dimensional edge contacts.
The periods of the antidote lattices made here with single-shot exposures are the densest structures defined via EBL in hBN-encapsulated graphene devices.
12:45 PM - NM8.1.05
Scalable and Versatile Liquid-Phase Production and Patterning of Two-Dimensional Nanomaterials
Ethan Secor 1 , Theodore Gao 1 , Mark Hersam 1 Show Abstract
1 , Northwestern University, Evanston, Illinois, United States
Two-dimensional (2D) nanomaterials offer a wide range of benefits for printed and flexible electronics, with potential applications spanning sensors, energy conversion and storage, flexible displays, logic and memory, and smart packaging. The incorporation of these materials into large-area, low-cost electronic devices requires scalable liquid-phase production and patterning methods. Here we demonstrate a versatile platform for exfoliation, ink formulation, and patterning of 2D materials using cellulose derivatives as stabilizing agents. Applied to graphene, this methodology allows the fabrication of high conductivity and flexible patterns, which provide a critical alternative to conventional metals. For example, graphene electrodes with excellent chemical and thermal stability offer a route to reliable and stable electrical contacts for reactive materials, including advanced solution-processed semiconductors and liquid metals.
Through multiple case studies, we explore the rational choice of cellulosic binder for synergistically enhancing the properties of the 2D nanomaterials. Using the polymer nitrocellulose for graphene inks, we demonstrate a dramatic improvement in mechanical and environmental durability following polymer burn-out. In-depth characterization of the resulting films reveals polymer decomposition with amorphous carbon residue, which imparts enhanced mechanical stability. This provides fundamental insight and characterization methods for designing nanomaterial inks with polymer binders. Moreover, the reactive nature of the nitrocellulose is exploited in pulsed light annealing, in which the embedded energy leads to a self-sustaining reaction with gas evolution. This enables the straightforward fabrication of highly porous structures suitable for energy storage applications, while simultaneously enhancing process generality.
Finally, we demonstrate that this platform can translate to 2D nanomaterials beyond graphene, including hexagonal boron nitride and molybdenum disulfide. By establishing a common system for multiple 2D nanomaterial inks, rational design of hybrid and composite structures enhances the utility of the individual components, enabling novel properties not obtained from a single material.
NM8.2: Functional 2D Materials and Devices
Tuesday PM, April 18, 2017
PCC West, 100 Level, Room 101 A
2:45 PM - *NM8.2.01
Functional 2-Dimensional Materials—From Photo Detectors to Molecular and Strain Sensors
Mauricio Terrones 1 2 Show Abstract
1 , The Pennsylvania State University, University Park, Pennsylvania, United States, 2 Institute of Carbon Science and Technology, Shinshu University, Nagano City, Nagano, Japan
This talk will first discuss how monolayers of nitrogen- and boron-doped graphene sheets can be synthesized and used as efficient molecular sensors. In particular, Graphene enhanced Raman spectroscopy (GERS) will be introduced and it will showed that for Nitrogen-doped graphene, the Fermi level (EF) of graphene shifts, and if this shift aligns with the lower unoccupied molecular orbital (LUMO) of a molecule, charge transfer would be enhanced, thus significantly amplifying the molecule's vibrational Raman modes. Concentrations as low as 10-11 mol/L of different dye molecules can be detected using GERS. It will also be demonstrated that B-doped graphene can be used as effective toxic gas sensor for NH3 and NO2, detections limits of parts per billion and parts per trillion will also be introduced. The electronic performance of monolayers of MoS2, WS2 and hetero-systems operating under flexural strain will also be presented. Our findings demonstrates that it is now possible to use chalcogenide layers for the fabrication of flexible electronic devices, however, defect control is required to tailor their performance.
1. R. Lv, M. Terrones et al. (2015). "Ultrasensitive gas detection of large-area boron-doped graphene". PNAS 112, 14527-14532.
2. G. R. Bhimanapati, M. Terrones, et al. (2015). "Recent Advances in Two-Dimensional Materials beyond Graphene". ACS nano 9, 11509-11539.
3. R. Lv, M. Terrones, et al. (2015). "Two-dimensional transition metal dichalcogenides: Clusters, ribbons, sheets and more". Nano Today 10, 559-592.
4. Z. Lin, M. Terrones, et al. (2016). “Defect engineering of two-dimensional transition metal dichalcogenides”. 2D Materials 3, 022002.
5. S. Feng, Terrones, et al. (2016). “Ultrasensitive Molecular Sensor Using N-doped Graphene through Enhanced Raman Scattering”. Science Advances 2, e1600322.
3:15 PM - NM8.2.03
Photoluminescence Enhancement and Carrier Type Modulation in Monolayer Transition Metal Dichalcogenides Using Isoelectronic Substitution
Xufan Li 1 , Alexander Puretzky 1 , Xiahan Sang 1 , Santosh KC 2 , Saban M. Hus 1 , Mengkun Tian 3 , Ming-Wei Lin 1 , Kai Wang 1 , Raymond Unocic 1 , Valentino Cooper 2 , An-Ping Li 1 , Christopher Rouleau 1 , David Geohegan 1 , Kai Xiao 1 Show Abstract
1 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 3 Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, United States
Doping is one of the most effective ways to engineer band structures of semiconductors to precisely tailor their properties for desired applications. Due to spatial confinement, doping in two-dimensional (2D) transition metal dichalcogenides (TMDs) produces especially pronounced effects. Isoelectronic doping with dopant atoms electronically similar to those of the host can produce robust, stable alloys, impede the generation and multiplication of defects, and modulate electrical and optical properties. Defects formed during chemical vapor deposition (CVD) of 2D TMDs currently limit their quality and optoelectronic properties. Effective synthesis and processing strategies to suppress defects and enhance the quality of 2D TMDs are urgently needed to enable next generation optoelectronic devices. In this work, isoelectronic substitutional doping is presented as a new strategy to form stable alloys and suppress defects and enhance photoluminescence (PL) in CVD-grown TMD monolayers. The isoelectronic substitution of W atoms for Mo atoms in CVD-grown monolayers of Mo1-xWxSe2 (0 < x < 0.18) is shown to effectively suppress Se vacancies by 50% compared to those found in pristine MoSe2 monolayers, resulting in a decrease in defect-mediated nonradiative recombination, ~10 times more intense PL, and an increase in the carrier lifetime by a factor of 3. Theoretical predictions reveal that isoelectronic W alloying to form Mo1-xWxSe2 monolayers raises the energy of deep level defects in MoSe2 to enable faster quenching, which was confirmed by low temperature (4–125 K) PL from defect-related localized states. In addition to the enhancement of PL, carrier type modulation was demonstrated in 2D TMDs as n-type monolayer MoSe2 was converted to non-degenerate p-type monolayer Mo1-xWxSe2 as W concentration increases. Although the alloys are mesoscopically uniform in composition, ‘W-rich’ and ‘Mo-rich’ regions on atomic scale are observed, which could possibly be formed due to composition modulation or perturbation during the growth. The p-type conduction in monolayer Mo1-xWxSe2 appears to originate from the upshift of the VBM towards the Fermi level at highly localized ‘W-rich’ regions in the lattice. Isoelectronic substitution therefore appears to be a promising synthetic method to control the heterogeneity and adjust the functionality of 2D TMD systems for many electronic and optoelectronic applications.
3:30 PM - NM8.2.04
Controllable Doping of Ultrathin MoS2 by Conventional Ion-Implantation
Kang Xu 1 , Yuda Zhao 1 , Yang Chai 1 Show Abstract
1 , The Hong Kong Polytechnic University, Hong Kong China
In the past decade, two-dimensional (2D) materials have been demonstrated as promising building blocks for next-generation electronic circuits.1 Anagalous to conventional Si CMOS technologies, p- and n-doping of 2D materials are essential for building complementary circuits. Controllable and effective doping strategies require large doping level tunability and negligible structure damage to ultrathin 2D materials. Although several novel doping methods have been demonstrated, such as organic adsorption and growth substitution, 2, 3 it still remains an important topic if we can extend conventional doping techniques in Si CMOS technologies to emerging 2D materials? Recently, several doping strategies evolved from conventional ion-implantation technique have been explored by researchers. 4, 5
In our work, we demonstrate a feasible doping method utilizing conventional high-energy ion-implantation machine (Varian CF3000). Before implantation, we spin coated a 200 nm thick PMMA layer onto mechanically exfoliated ultrathin MoS2 flakes as a protective layer to control the implantation depth. A dose of P+ (5 × 1013 ions/cm2) was accelerated at 10 keV. As-exfoliated MoS2 samples are n-doped due to the existence of sulfur vacancies. After implantation, MoS2 flakes with higher density of sulfur vacancies were p-doped more obviously. These results suggest that the PMMA layer slows down the P+ ions and the defects sites in MoS2 flakes act as attracting centers for the decelerated positive ions. According to Raman spectra, we observe no obvious lattice distortion due to ion-bombardment. After the implantation, the PMMA layer is easily washed away by immersion in Acetone and IPA. XPS, HRTEM, Raman and PL spectra are performed to characterize the doping effects. This doping method can be further extended to various 2D materials and dopant species as well.
(1) B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Nature nanotechnology 2011, 6, 147-150.
(2) P. Zhao, D. Kiriya, A. Azcatl, C. Zhang, M. Tosun, Y.-S. Liu, M. Hettick, J. S. Kang, S. McDonnell, S. KC, ACS nano 2014, 8, 10808 - 10814.
(3) J. Suh, T.-E. Park, D.-Y. Lin, D. Fu, J. Park, H. J. Jung, Y. Chen, C. Ko, C. Jang, Y. Sun, Nano letters 2014, 14, 6976 - 6982.
(4) A. Nipane, D. Karmakar, N. Kaushik, S. Karande, S. Lodha, ACS nano 2016, 10, 2128–2137.
(5) A. Nipane, N. Kaushik, S. Karande, D. Karmakar, S. Lodha, presented at 2015 73rd Annual Device Research Conference (DRC), 2015.
3:45 PM - NM8.2.05
Engineering the Structural and Electronic Phases of MoTe2 through W Substitution
Daniel Rhodes 3 , Daniel Chenet 3 , Richard Osgood 3 , Aaron Lindenberg 5 , Pinshane Huang 4 , Abhay Pasupathy 3 , Madan Dubey 2 , James Hone 3 , Luis Balicas 1 Show Abstract
3 , Columbia University, New York, New York, United States, 5 , Stanford University, Stanford, California, United States, 4 , University of Illinois at Urbana-Champaign, Urbana-Champaign, Illinois, United States, 2 , Army Research Lab, Adelphi, Maryland, United States, 1 National High Magnetic Field Lab, Florida State University, Tallahassee, Florida, United States
MoTe2 is an exfoliable transition metal dichalcogenide (TMD) which crystallizes in three symmetries;
the semiconducting trigonal-prismatic 2H-phase, the semimetallic 1T' monoclinic phase, and the semimetallic
orthorhombic Td structure. The 2H-phase displays a band gap of ~ 1 eV making it appealing for flexible and transparent optoelectronics. The Td-phase is predicted to possess unique topological properties which might lead to topologically protected
non-dissipative transport channels. Recently, it was argued that it is possible to locally induce
phase-transformations in TMDs, through chemical doping, local heating, or electric-field to achieve ohmic contacts or to induce useful functionalities such as electronic phase-change memory elements. The combination of semiconducting and topological elements based upon the same compound, might produce a new generation of high performance, low dissipation optoelectronic elements.
Here, we show that it is possible to engineer the phases of MoTe2 through W substitution by unveiling the
phase-diagram of the Mo1-xWxTe2 solid solution which displays a semiconducting to semimetallic transition as a function of x.
We find that only ~ 8 % of W stabilizes the Td-phase at room temperature. Photoemission spectroscopy, indicates that this phase possesses a Fermi surface akin to that of WTe2.
Tuesday PM, April 18, 2017
PCC West, 100 Level, Room 101 A
4:30 PM - *NM8.3.01
Atomically Precise Graphene Nanostructures through On-Surface Synthesis
Peter Liljeroth 1 Show Abstract
1 Department of Applied Physics, Aalto University, Helsinki Finland
Graphene nanoribbons (GNRs) are a new class of materials that have promising applications in next-generation nanoelectronic, photonic and spintronic devices. GNRs have been predicted to show interesting electronic properties that depend strongly on their width and edge structure. However, the required precision cannot be achieved by top-down approaches, including e-beam lithography on a sheet of graphene or unzipping carbon nanotubes. Recently, bottom-up synthesis using molecular precursors has been shown to provide precise control over the width and edge geometry of GNRs. By changing the monomer design, fabrication of a wide range of different GNRs can be achieved with engineered chemical and electronic properties.
In the typical picture of the on-surface synthesis, the substrate does not play a big role in the chemical reaction. Using low-temperature scanning tunneling microscopy (STM) and atomic force microscopy (AFM), I will show that the substrate is not always an innocent bystander in these reactions. On Au(111) surface, the prototypical precursor dibromo-bianthryl (DBBA) polymerizes via an Ullmann route to form straight GNRs with armchair edges. However, on Cu(111), the DBBA precursor forms chiral (3,1)GNRs. In contrast, dibromo-perylene (DBP) precursors do form armchair GNRs via Ullmann coupling, in close analogy to recent results on Au(111). The reaction intermediates highlight the role of the precursor shape, molecule-molecule interactions and substrate reactivity as decisive factors in determining the reaction pathway. Our findings help to realize new routes for previously unattainable covalently bonded nanostructures.
In addition mono-component GNRs, semiconductor-semiconductor junctions embedded in a single GNR through segments of different widths or doping have been demonstrated. However, the GNR equivalent of a metal-semiconductor junction has not yet been realized. We fabricate this heterostructure by joining armchair GNRs belonging to the metallic (5-atom wide) and semiconducting (7-atom wide) families through on-surface synthesis. In addition to a single junction, we have realized more complicated structures combining several interfaces. These structures constitute the first steps towards encoding more functionality into a single GNR for electronic applications.
5:00 PM - NM8.3.02
Bottom-Up Synthesis and Self-Assembly of Atomically Precise Pristine and Nitrogen-Doped Graphene Nanoribbons
Timothy Vo 1 , Mohammad Mehdi Pour 1 , U. Gayani Perera 2 , Mikhail Shekhirev 1 , Peter Sutter 1 , Axel Enders 1 , Alexander Sinitskii 1 Show Abstract
1 , University of Nebraska–Lincoln, Lincoln, Nebraska, United States, 2 , Brookhaven National Laboratory, Upton, New York, United States
Electronic properties of graphene nanoribbons (GNRs) can be tuned by their doping with heteroatoms, such as nitrogen. This possibility has been extensively studied theoretically, but only a few experimental attempts to synthesize nitrogen-doped GNRs (N-GNRs) by bottom-up approaches have been reported. This talk will be focused on the recently developed bottom-up solution method for gram quantities of narrow GNRs  and N-GNRs  that are less than 2 nm wide and have atomically precise armchair edges. The method is based on Yamamoto coupling of presynthesized molecular precursors followed by cyclodehydrogenation using Scholl reaction [1,2]. GNRs and N-GNRs were characterized by a number of microscopic (STM, AFM, SEM, TEM) and spectroscopic (XPS, UPS/IPES, UV-vis-NIR, IR and Raman spectroscopy) techniques. GNRs and N-GNRs have large electronic bandgaps, which makes them promising for applications in field-effect transistors with high on-off ratios and photovoltaic devices.
Also discussed in this talk will be self-assembly of GNRs and N-GNRs. We demonstrate that the substitutional doping with nitrogen atoms can trigger the hierarchical self-assembly of nanoribbons into highly ordered structures . This phenomenon is observed both on metal surfaces and in an unrestricted three-dimensional (3D) solution environment. On a surface, N-doping mediates the formation of hydrogen-bonded GNR sheets. In solution, sheets of side-by-side coordinated N-GNRs can in turn assemble via van der Waals and π-stacking interactions into 3D stacks, a process that ultimately produces macroscopic crystalline structures. The optoelectronic properties of these semiconducting N-GNR crystals are determined entirely by those of the individual nanoscale constituents, which are tunable by varying their width, edge orientation, termination, and so forth. The atomically precise bottom-up synthesis of bulk quantities of basic N-GNR units and their subsequent self-assembly into crystalline structures suggests that the rapidly developing toolset of organic and polymer chemistry can be harnessed to realize families of novel carbon-based materials with engineered properties.
This work was supported by the National Science Foundation (NSF) through CHE-1455330.
 T. H. Vo, et al., Large-scale solution synthesis of narrow graphene nanoribbons. Nat. Commun. 2014, 5, 3189.
 T. H. Vo, et al., Bottom-up solution synthesis of narrow nitrogen-doped graphene nanoribbons.Chem. Commun. 2014, 50, 4172.
 T. H. Vo, et al., Nitrogen-doping induced self-assembly of graphene nanoribbon-based two-dimensional and three-dimensional metamaterials. Nano Lett. 2015, 15, 5770.
5:15 PM - NM8.3.03
Atomically Thin Nanoporous Graphene Membranes for Size Selective Membrane Applications
Piran Ravichandran Kidambi 1 , Michael Boutilier 1 , Luda Wang 1 , Doojoon Jang 1 , Rohit Karnik 1 Show Abstract
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Atomically thin (2D) membranes have recently attracted a lot of interest for filtration and separation applications. In contrast to solution-diffusion of molecules through the membrane thickness in conventional membranes, 2D materials like graphene and others, offer the theoretical minimum membrane resistance (atomic thickness) along with the opportunity to tune pore sizes at the nanometer scale, thereby enabling size selective separation.
While several methods of synthesis for 2D materials exist, chemical vapor deposition and recent advances in exfoliation of single crystalline graphene from SiC, have emerged as preferable routes towards scalable, cost effective synthesis. Here, we demonstrate selective molecular transport through precise and controllably engineered, high-density, sub nanometer diameter pores in graphene. A combination of pressure and diffusion driven transport measurements shows clear evidence for size selective transport behavior. Specifically, we show the ability to achieve selective transport of small molecules across centimeter-scale single-layer porous polycrystalline and single crystalline graphene membranes through facile creation of pores after sealing leakage via interfacial polymerization. Second, we show that, by tuning graphene synthesis parameters, it is possible to directly synthesize graphene with selective pores that permit passage of salt, but block the transport of small molecules.
The possibility of precisely tuning selectivity in atomically thin membranes through controlled creation of sub nanometer and nanometer sized pores addresses a significant challenge in the development of advanced nano-porous membranes for nanofiltration, desalination, gas and chemical separation and several biological applications.
Kidambi et al. Science Advances (submitted)
Kidambi et al. Advanced Materials (submitted)
Kidambi et al. ACS Nano (submitted)
O’Hern et al. Nano Letters (2015).
Kidambi et al. Chemistry of Materials (2014).
Boutilier et al. ACS Nano (2014).
Kidambi et al. Nano Letters (2013).
O’Hern et al. Nano Letters (2013).
O’Hern et al. ACS Nano (2012).
5:30 PM - NM8.3.04
Lateral Superlattices and Anisotropic Optoelectronic Behaviour of Monolayer Semiconducting TMDCs via Large-Scale, Heterogeneous Elastic Strain Engineering
Michael Cai Wang 1 , Juyoung Leem 1 , Satoshi Takekuma 1 , SungWoo Nam 1 Show Abstract
1 , University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Transition metal dichalcogenides (TMDCs), as semiconducting graphene analogs, are atomically thin and thus amenable to large flexural deformations due to their extremely low bending rigidities. We realize for the first time 2D TMDC lateral superlattices of well-organized, periodic, and spatially varying optoelectronic properties which are enabled by large-scale, heterogeneous elastic strain generated via thermally-activated shape-memory polymers. Through tailoring the relative substrate composition, stiffness, and thickness mismatch, three-dimensional (3D) features that span from tens of nanometers to few-microns are deterministically obtainable through wrinkling and buckling-delamination of TMDCs on the centimeter-scale without any lithographic patterning. The periodic strain gradients induce controllable, heterogeneous, and anisotropic optoelectronic and mechanical behaviour in a lateral superlattice fashion. Diffraction-limited micro-Raman and micro-Photoluminescence (PL) spectroscopy of superlattices with micron-sized features show large tunability in the local optical bandgap and strain confinement, whereas the analogous nanoscale superlattices show large polarization dependence in overall Raman and PL response. This simple approach to forming various nano/micro-structured semiconducting TMDCs via substrate transformation offers a unique avenue for creating periodic, heterogeneous, and emergent material properties across a broad range of length-scales. Such strain-induced morphologies are amenable for a variety of novel applications and phenomena including active tuning of carrier and phonon behavior, local band structure, excitonic funneling, flexoelectricity, spin/valleytronics, self-assembly, and enhanced basal plane catalysis.
5:45 PM - NM8.3.05
Growth Processes of Graphene on Ni(111) Surface
Hakim Amara 1 , Rafael Martinez Gordillo 2 , Christophe Bichara 2 , Celine Varvenne 2 Show Abstract
1 , ONERA-CNRS, Chatillon France, 2 , CINaM, Marseille France
Growing graphene on a metal surface is one possible way to obtain high quality graphene, with a controllable number of layers. The synthesis usually relies on a chemical vapor deposition of a carbon bearing gas on the surface of a metal such as Ir, Cu, or Ni. We investigate the case of graphene on Ni that is of particular interest because the role of carbon solubility in subsurface layers is both difficult to investigate experimentally and important to understand to produce high quality graphene.
To study the interaction of carbon with nickel at the atomic level, we have developed a tight binding model  implemented in a Grand Canonical Monte Carlo code. It has been used to study the nucleation and growth of carbon nanotubes in CVD processes . With the same approach, we investigate the CVD synthesis of graphene on Ni (111) and correlate our results to experimental data . We identify thermodynamic conditions (temperature and carbon chemical potential) to obtain a graphene monolayer. Moreover, depending on the growth conditions, we show that variable amounts of carbon atoms can be found in the subsurface layers, while the first subsurface layer shows a tendency for carbon depletion when graphene covers the Ni surface . Experimentally, it has also been observed that below temperatures of 460 degres, the Ni (111) surface presents a reconstruction in presence of C forming a surface carbide layer . This surface carbide is accompanied by the growth of graphene and can have an important role in the synthesis process . Based on DFT calculations and Monte Carlo simulations, the key role played by this reconstructed system has been investigated to have better insight of the mechanisms of the growth process of graphene .
1 H. Amara, J.-M. Roussel, C. Bichara, J.-P. Gaspard and F. Ducastelle Phys. Rev. B 79, 014109 (2009)
2 M. Diarra, A. Zappelli, H. Amara, F. Ducastelle and C. Bichara Phys. Rev. Lett. 109, 185501 (2012)
3 R. Weatherup, H. Amara et al., J. Am. Chem. Soc. 136, 13698 (2014)
4 J. Lahiri et al., Nano Lett. 11, 518 (2011)
5 R. Martinez Gordillo, H. Amara et al. (submitted)
NM8.4: Poster Session I: Graphene and Carbon Materials
Wednesday AM, April 19, 2017
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - NM8.4.01
Exploring Surface Diels-Alder Adducts on Silica as a Controllable Carbon Precursor for Pristine Graphene
Srinivasa Kartik Nemani 1 , Seth Duvall 1 , Rama Kishore Annavarapu 1 , Hossein Sojoudi 1 Show Abstract
1 , University of Toledo, Atlanta, Georgia, United States
A dienophile-modified SiO2 surface served as a platform for Diels-Alder mediated attachment of anthracene and 9,9’-bianthryl. The resulting monolayers were investigated by x-ray photoelectron spectroscopy (XPS) and directly used as precursor for graphene, as verified by Raman spectroscopy. 9,9’-bianthryl adduct yield the best quality graphene, which is attributed to the higher carbon precursor availability and compared to anthracene adduct and the maleimide dienophile. This study opens the door towards rationale direct growth of graphene on surface reaction mediated by copper catalyst.
9:00 PM - NM8.4.02
Simple Step Growth of Graphene Nitrogen-Doped Graphene Hybrid Bilayer System in the Hot Filament Chemical Vapor Deposition
Maried Rios 1 , Jean Hernandez 1 , Carlos Martinez 2 , Tej Limbu 2 3 , Frank Mendoza 2 3 , Brad Weiner 3 4 , Gerardo Morell 2 3 , Ernesto Espada 1 Show Abstract
1 Biology, University of Puerto Rico - Rio Piedras, San Juan, Puerto Rico, United States, 2 Physics, University of Puerto Rico, Rio Piedras, Puerto Rico, United States, 3 , Institute for Functional Nanomaterials, San Juan, Puerto Rico, United States, 4 Chemistry, University of Puerto Rico, San Juan, Puerto Rico, United States
If one of the layers of a bilayer graphene is doped with nitrogen, it causes an asymmetric doping
on the two graphene layers and a band gap is opened, which makes it a promising material for
electronic applications. Here, we report a single step fabrication of a twisted bilayer graphene
with its upper layer doped with nitrogen, in the hot filament chemical vapor deposition
(HFCVD). Methane gas was used for carbon precursor and ammonia gas for nitrogen source,
which are dissociated partly at the hot filaments prior to the dissociation and adsorption on the
heated copper substrate. Making use of this special role of filaments, we fabricated a
polycrystalline graphene nitrogen-doped graphene hybrid bilayer system of 4x4 cm
2 area. The synthesized samples were characterized by Raman, Fourier transform infrared, X-ray
photoelectron spectroscopy, and scanning tunneling microscopy. Based on the obtained results, a
bilayer graphene growth mechanism in the HFCVD is proposed.
9:00 PM - NM8.4.03
Nondestructive Optical Visualisation of Graphene Domains and Boundaries
Xingyi Wu 1 , Guofang Zhong 1 , John Robertson 1 Show Abstract
1 Department of Engineering, University of Cambridge, Cambridge United Kingdom
Domain boundaries of polycrystalline graphene produced by chemical vapour deposition (CVD) adversely influence the graphene transporting properties. Spatial visualisation of the domains and boundaries is therefore desired for controlling the boundary-associated degradation. Existing domain visualisation methods for large area graphene always cause detrimental damage or contamination[2, 3]. On the other hand, the non-invasive visualisation methods using TEM, SEM or micro-Raman mappings are limited to much smaller scale than industry demanded.
In this presentation, we demonstrate a novel nondestructive method for visualisation of the domains and boundaries of large area continuous graphene grown on Cu foils (Gr/Cu) by CVD. Using a modified optical microscope, we can directly observe novel star-like bright line sets of Gr/Cu under an enhanced dark field mode. Each set of the bright lines are identified as the ridges of one Cu surface pyramid which arises beneath one enlarging graphene domain due to slower evaporation of graphene-covered Cu than that of graphene-free Cu. Such one to one correspondence thereby enables the nondestructive visualisation. The ridge-structure-based visualisation approach is purely optical and thereby, for the first time, not only nondestructive to graphene but also applicable to large area samples. It is also cost-saving and rapid. We have further discovered for the first time various types of star-like ridge structures whose morphologies are governed by the underlying Cu crystallographic orientations as revealed by EBSD studies. This raises new phenomenon for research on the complex 2D material-metal interfacing.
1. O. V. Yazyev, et al, Nat. Mater., 2010, 9, 806–809.
2. D. L. Duong, et al, Nature, 2012, 490, 235–239.
3. D. W. Kim, et al, Nat. Nanotechnol., 2012, 7, 29–34.
4. X. Wu, et al, Nanoscale, 2016, 8, 16427-16434.
9:00 PM - NM8.4.04
Continuous Single Crystal Growth of Two Dimensional Materials—The Case of Graphene
Ivan Vlassiouk 1 , Sergei Smirnov 2 , Yijing Stehle 1 , Frederick List 1 Show Abstract
1 , Oak Ridge National Lab, Oak Ridge, Tennessee, United States, 2 , New Mexico State University, Las Cruces, New Mexico, United States
There is a demand for manufacturing of 2D materials with ultimate quality of single crystals and arbitrary size. Usually, epitaxial growth is considered the method of choice in manufacturing single crystalline thin films but it in turn requires single crystal substrates for deposition, which makes the approach cost prohibitive. Here we propose locally controlled continuous chemical vapor deposition (LC CVD) as a method for manufacturing 2D single crystals and demonstrate its utility using graphene. The method yields continuous single crystal graphene of potentially unlimited dimensions on polycrystalline substrates. Using the proposed approach, we have synthesized graphene single crystals up to a foot long. We also anticipate that LC CVD can be readily adopted for synthesis of other 2D materials and heterostructures.
9:00 PM - NM8.4.06
Modelling the Effect of Electron Beam Irradiation on the Thermal Conductivity of Graphene
Srilok Srinivasan 1 , Ganesh Balasubramanian 1 Show Abstract
1 , Iowa State University, Ames, Iowa, United States
Some of the common experimental characterization techniques involve the exposure of the sample to an electron beam irradiation. In this work, we computationally investigate the effect of electron beam irradiation on thermal conductivity and morphology of a sample of graphene. We model the irradiation process using kinetic Monte Carlo (KMC) technique to predict the generation and evolution of vacancy defects as well as the reaction of defective sites with the residual impurities. The energy barriers for each of the possible processes considered are calculated from density functional theory (DFT) and the associated rates are obtained from transition state theory. The relative equilibrium composition of vacancies, -C=O, -OH and epoxy functional groups are estimated over a range of temperatures and electron energies. At each electron beam energy, we estimate the average thermal conductivity of the equilibrium configuration using reverse non-equilibrium molecular dynamics (RNEMD) simulations. A sample set of 10 equilibrated and equivalent configurations from the KMC simulations are randomly selected for the subsequent MD simulations. We show that with increasing electron beam energies, the defect density increases resulting in poor thermal transport across the graphene nanostructure. In addition, we evaluate the phonon density of states (VDOS) of pristine graphene and graphene irradiated at different electron beam energies from MD simulations. The reduction in the thermal conductivity is explained by the corresponding changes in the VDOS curves. Our results help in understanding the reason for the wide scatter in the reported experimental thermal conductivity values, which strongly depends on the synthesis conditions and the characterization techniques used.
9:00 PM - NM8.4.07
Investigation of Spin Current Absorption through a Transparent Ferro Magnet Junction on Graphene
Serol Turkyilmaz 1 , Cengiz Ozkan 2 Show Abstract
1 , University of California Riverside, Riverside, California, United States, 2 , University of California Riverside, Riverside, California, United States
Spintronic devices are very promising for future information storage and processing and have the potential to replace current CMOS technology. Low energy magnetization switching of a nanomagnet using a pure spin current is a key toward all spin logic devices. Graphene constitutes an ideal spin channel material due its high spin diffusion length and long spin lifetime. Furthermore, graphene has high carrier mobility and tunable carrier concentration providing a very unique platform toward low energy spin transfer torque switching. Here, we study spin absorption by a Py nanomagnet grown on top of a lateral graphene spin valve channel. A pure spin current is injected into a graphene channel via electrical spin injection through a Co/Al2O3 tunnel barrier junction. The Py island in between the injector and the detector is expected to modulate the spin population at the detector via spin absorption. Depending on the magnetization of the Py island, the spin current can be absorbed differently resulting in a distinct signature of non-local magnetoresistance (MR) from the Py island magnetization and hence a modulation of the spin population. We attribute this effect to a combination of both transverse and longitudinal spin current absorptions which is caused by the micro domain formations within the Perm alloy island and hence a hysteretic behaviour in the MR signal. This hysteretic behaviour can be due to the magnetization rotation of micro domains which raise the discussion of both lateral and longitudinal spin absorption. This new effect is still being investigated and further studies are underway to unravel the origin of this new effect observed in graphene spin valves.
9:00 PM - NM8.4.08
A Novel Electrochemical Sensor Based on Gold/Reduced Graphene Oxide Hollow Microspheres Modified Glass Carbon Electrode for Sensitive Detection of Nitrite
Shifeng Hou 1 , Fuhua Zhang 1 , Hua Wang 1 Show Abstract
1 , Shandong University, Jinan, Shandong, China
Hollow microspheres with a complex of gold (Au) nanoparticles and reduced graphene oxide (rGO) were synthesized through a spray drying technique, in which, the self-assembly of Au nanoparticles and graphene oxide (GO) nanosheets homogeneously into one sphere with hollow interiors, followed by thermal reduction. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy electrochemical techniques were used to characterize the as-prepared Au-nanoparticles/graphene hollow microspheres (Au/rGO). When modified to glass carbon electrode, the electrochemical catalysis and electroanalysis properties of this Au/rGO/GCE electrode toward nitrite have been investigated using a variety of electrochemical techniques. This novel electrode materials shows potentials applications in the fabrication of novel nitrite sensor, which can be used in the environment protection, pharmaceutical and biological sample analysis, with a linearity range from 5 μM to 2600 μM, and a detection limit down to 0.5 μM (S/N = 3).
9:00 PM - NM8.4.09
Synthesis of Bernal-Stacked Multilayer Graphene on Cu Surface via Chemical Vapor Deposition
Minseok Yoo 1 , Hyo Chan Lee 1 , Kilwon Cho 1 Show Abstract
1 , POSTECH, Pohang Korea (the Republic of)
Bernal-stacked multilayer graphene has a tunable bandgap making it a promising material for optoelectronic devices. Large-area synthesis of layer-controlled multilayer graphene is a critical factor for application of this material to actual devices. Metal catalysts, especially Ni or a Cu-Ni alloy with high carbon solubility have been used for multilayer graphene synthesis via chemical vapor deposition (CVD). However, this synthesis method requires delicate control for growth parameters due to precipitated-carbon, resulting in low reproducibility. Recently, studies on multilayer graphene growth on Cu enclosure were reported. Surface-mediated reaction on Cu improves uniformity of graphene but controlling the inner growth-condition of the Cu enclosure and limited up-scaling are remaining challenges. Here, we propose an asymmetric Cu catalyst for synthesis of Bernal-stacked multilayer graphene via CVD. Thin Ni film was deposited on the back of the Cu foil to induce an asymmetric carbon solubility profile between both sides of the catalyst. As a result, precise layer-control of the graphene growth up to 6 layers with high uniformity (~97%) and low sheet resistance in wafer scale was enabled. We verified the growth mechanism and developed a generalized kinetic model for graphene growth on asymmetric catalysts.
9:00 PM - NM8.4.10
Role of Extra Cu Vapors in the Growth of Graphene on Cu via Chemical Vapor Deposition
Hyo Chan Lee 1 , Minseok Yoo 1 , Kilwon Cho 1 Show Abstract
1 , POSTECH, Pohang Korea (the Republic of)
Although chemical vapor deposition (CVD) of graphene on Cu surface using methane (CH4) has advanced over the past decade, investigation of atomistic details of graphene growth is still required to suppress formation of defects in graphene. During growth of graphene, Cu vapor is inevitably and severely produced in CVD chamber because the process temperature (~ 1000 °C) is very close to the melting point of Cu (~ 1083 °C) and thus sublimation of Cu catalyst occurs. However, up to date, only few studies have investigated the effects of Cu vapors in graphene growth. Here, we investigated the role of Cu vapors in graphene growth on Cu surface. We found that Cu vapors enlarges the size of graphene grains and enhances efficiency of defect-healing of graphene by CH4. Lastly, based on our study, we proposed a new synthetic way to grow uniform and high-quality graphene.
9:00 PM - NM8.4.11
Selective Separation of Large Graphene Oxide in Liquid Crystal Phase and Its Application on Electrochemical Catalysis
Kyungeun Lee 1 , Joonwon Lim 1 , Hyeong Min Jin 1 , Jisun Yoon 1 Show Abstract
1 , KAIST, Daejeon Korea (the Republic of)
Graphene oxide (GO) is chemically modified graphene, which shows stable discotic liquid crystallinity in various solvent. Many bulk properties such as mechanical strength and electrical conductivity of GO-based materials are limited by the small flake size of GO. Unfortunately, Top-down synthetic approach of GO from graphite generally leads to a broad size distribution from hundred nanometer to micrometer. Here, we introduce a facile size selection of large-size GO exploiting liquid crystallinity and investigate the size-dependent N-doping and oxygen reduction electrochemical catalysis. In the specific concentration of GO aqueous dispersion where both isotropic and liquid crystalline phases are equilibrated, large-size GO flakes (>20 μm) are spontaneously seperated within the bottom nematic liquid crystalline phase. subsequent N-Doping and reduction of GO exhibit that N-dopant type is highly dependent on GO flake size. Large-size GO is donimantly quaternary type nitrogen doped. In alkaline solution, quaternary dominant graphene shows lower onset potential (�0.08 V) for oxygen reduction catalysis, implying that quaternary N-dopants serve as catalytic active sites.
9:00 PM - NM8.4.12
Dynamic Observation of Atomic-Scale Evolution in Graphene Layer under High Current Density
Chun-Wei Huang 1 , Jui-Yuan Chen 1 , Wen-Wei Wu 1 Show Abstract
1 , National Chiao Tung University, Hsinchu Taiwan
Graphene has demonstrated its potential in several practical applications owing to its remarkable electronic and physical properties. In this study, we successfully fabricated a suspended graphene device with a width down to 20 nm. The morphological evolution of graphene under various electric field effects was systematically examined using an in-situ transmission electron microscope (TEM). The hourglass-shaped graphene sample instantly broke apart at 7.5 mA, indicating an impressive breakdown current density. The current-carrying capacity was calculated to be ~1.6 × 109 A/cm2, which is several orders higher than that of copper. The current-carrying capacity depended on the resistivity of graphene. In addition, atomic volume changes occurred in the multilayer graphene samples due to surface diffusion and Ostwald ripening (OR), indicating that the breakdown mechanism is well approximated by the electric field. This study not only provides a theory to explain the breakdown behavior but also presents the effects on materials contacted with a graphene layer used as the transmission path.
9:00 PM - NM8.4.13
Catalyst-Free Bottom-Up Growth of Graphene Nanofeatures along with Molecular Templates on Dielectric Substrates
Sohyeon Seo 1 2 , Hyoyoung Lee 1 2 Show Abstract
1 , Sungkyunkwan University, Suwon Korea (the Republic of), 2 , Center for Integrated Nanostructure Physics, Suwon Korea (the Republic of)
Synthesis of graphene nanostructures have been investigated to provide outstanding properties for various applications. Here we report molecular thin film-assisted growth of graphene into nanofeatures such as nanoribbons and nanoporous sheets along with predetermined molecular orientation on dielectric substrates without metal catalysts. A Langmuir-Blodgett (LB) method was used for the formation of molecularly patterned SiO2 substrates with ferric stearate layers, which acted as a template for the directional growth of the polypyrrole graphene precursor. The nanofeatures of graphene were determined by the number of ferric stearate layers (e.g., nanoribbons from multiple layers and nanoporous sheets from a single layer). The graphene nanoribbons (GNRs) containing pyrrolic N enriched edges exhibited a p-type semiconducting behavior, while the nanoporous graphene sheets containing inhomogeneous pores and graphitic N enriched basal planes exhibited the typical electronic transport of nitrogen-doped graphene. Our approaches provide two central methods of graphene synthesis such as bottom-up and direct processes for the future development of graphene nanoelectronics.
9:00 PM - NM8.4.14
Comparative Study on Graphene Growth by Chemical Vapor Deposition on Cu foil and Textured Ni-W Metal
Yijie Li 1 , Linfei Liu 1 , Wei Wang 1 , Yanjie Yao 1 , Binbin Wang 1 , Saidan Lu 1 , Xiang Wu 1 Show Abstract
1 , Shanghai Jiao Tong University, Shanghai China
Graphene is a remarkable 2D material which has distinguished physical, electrical, and optical properties such as extremely high mobility, good carrier density, and high optical transparency. Many efforts have been made to achieve large-scale and high-quality graphene growth. Chemical vapor deposition (CVD) on metal surfaces is becoming a popular method because of its scalability and cost effectiveness. However, the quality of CVD graphene on polycrystalline substrates seems to be limited by the size and uniformity of the underlying grains. In this report, we report our investigation of the graphene growth on (001) textured Ni-W metal by CVD. For comparison, we also demonstrate high-quality graphene growth on polycrystalline Cu foil. Small graphene flakes first come into being on some Cu crystalline grain, and then fully covered single layer graphene forms after twenty minutes. While on textured Ni-W metal substrate, self-assembled flower-like graphene is observed on Ni-W grain boundary, and then lager area graphene is obtained during the same period. Our results indicate that uniform graphene can be fabricated on (001) textured Ni-W metal substrates by controlling carbon segregation. This process has favorable perspectives.
9:00 PM - NM8.4.15
Viscosity Increase of Graphene Oxide Aqueous Suspension after Electrophoretic Deposition
Seong Park 1 , Junho Lee 1 , Taekyun Lee 1 , Sung Jin An 1 Show Abstract
1 Department of Advanced Materials Science and Engineering, Kumoh National Institute of Technology, Gumi Korea (the Republic of)
There are a variety of method like spray coating, dip coating, layer by layer(LBL), spin coating making graphene based material films. Spin coating of them is fast, easy uniform method. But, graphene based material film after spin coating is limited because its viscosity is usually low. Recently, graphene based material film manufactured after Electrophertic Deposition(EPD) is reported. Independently, we found viscosity increase of graphene oxide aqueous suspension after EPD process. It means to control thickness of films when viscosity of suspension increases Hence, we studied for viscosity increase of EPD-GO solution various EPD conditions. Viscosity increase of EPD-GO solution is helpful for fabricating the large area spin coated GO film due to the more adhesive property.
9:00 PM - NM8.4.16
In Situ RBS, Raman, and Ellipsometry Study of Nickel-Catalyzed Amorphous Carbon Graphitization
Daniel Janke 1 , Martin Hulman 2 , Robert Wenisch 1 , Frank Lungwitz 1 , Sibylle Gemming 1 3 , David Rafaja 4 , Matthias Krause 1 Show Abstract
1 , Helmholtz-Zentrum Dresden-Rossendorf, Dresden Germany, 2 , Slovenská akadémia vied, Bratislava Slovakia, 3 , Technische Universität Chemnitz, Chemnitz Germany, 4 , Technische Universität Bergakademie Freiberg, Freiberg Germany
Metal-induced crystallization with and without layer exchange (MIC w/o LE) is a method to decrease the crystallization temperature of amorphous group 14 elements (G14E) by up to several hundred degrees. In situ experiments are expected to provide new insights into thin film evolution and elementary process steps of MIC w/o LE and to improve existing models of this type of phase transformation. While MIC w/o LE has been widely studied for Si and Ge in contact with catalytic metals, there exist only a few studies for the crystallization of amorphous carbon. Therefore, in this contribution in situ Rutherford backscattering spectrometry (RBS), Raman spectroscopy and spectroscopic ellipsometry studies were performed during annealing of amorphous carbon/nickel (a-C/Ni) layer stacks at temperatures up to 750°C.
Due to its small lattice mismatch with the basal plane of graphite and high diffusivity of C atoms, Ni is a suitable catalyst for the growth of graphene and crystalline graphitic nanostructures. During the annealing of an a-C/Ni layer stack covalent bonds between the carbon atoms at the catalyst interface are weakened. Liberated carbon atoms can move along the interface and diffuse along the grain boundaries into the Ni layer towards the catalyst surface, where nucleation and grain growth of graphitic crystallites occur. Our in situ studies showed a change in the stacking sequence between C and Ni layers under defined experimental conditions. According to in situ Raman measurements, this mechanism occurs independent of the stacking sequence, while the velocity of the LE differs significantly. As observed in time and temperature resolved Raman spectra, the position of the G peak and the I(D)/I(G) ratio changed according to the Three-Stage-Model by Ferrari and Robertson, confirming the transformation of amorphous carbon to nc-graphite. With the in situ RBS measurements more insight into LE was given. Here peak positions of C and Ni were shifted, indicating a change of the energy of the scattered ions for both layers respectively and proving the combination of the observed graphitization process with LE during annealing. The thickness of the synthesized crystalline graphitic layer is controlled by the finite carbon source – the deposited a-C film, which is a decisive advantage of this process compared to CVD. It is demonstrated that the structure and the crystallite size of the metallic catalyst layer has a strong influence on the crystallite size and the quality of the graphitic film.
LE is potentially interesting for industrial applications, as it allows the formation of polycrystalline thin films of G14E at much lower temperatures - than during thermal annealing without the metallic catalyst. Depending on the initial stacking sequence, the crystalline graphitic film can be deposited on a suitable device-ready substrate or transferred to another substrate after the dissolution of the transition metal catalyst.
9:00 PM - NM8.4.17
Large-Area Aligned Pentagonal Graphene Domains on Copper Foils
Kailun Xia 1 , Yingying Zhang 1 Show Abstract
1 , Tsinghua University, Beijing China
Single crystal graphene domains grown by chemical vapor deposition (CVD) tends intrinsically to have a six-fold symmetry due to the intrinsic atomic structure of graphene, while the crystal structures of the underlying substrates and the CVD conditions may affect the kinetics of the graphene growth, leading to the formation of graphene with various shapes1. While many studies focused on the controlled growth of hexagonal graphene2, there are few reports on the growth of graphene domains with lower symmetry, such as pentagonal graphene. It remains a big challenge to control the orientation of the graphene domains in a large scale, which is of significance will benefit the fabrication of graphene-based devices. Here, we report the growth of large area orientated pentagonal single crystal graphene domains on Cu foils by CVD3. The pentagonal graphene domains all have the same orientation within a Cu grain and could be all oriented in areas up to several square centimeter. Besides, a sharp transition of graphene shapes from hexagon to pentagon between neighboring Cu grains was observed. Electron backscatter diffraction characterization showed that the shapes of graphene domains depended on the crystalline structure of the underlying Cu grains, and high-index Cu surfaces corresponded to pentagonal graphene. We noticed that the symmetry axis of pentagonal graphene was perpendicular to the crystalline steps on the surface, showing that the crystalline steps on the surface promote the graphene growth in their perpendicular direction and finally lead to disappearance of one edge from the growth shape and the formation of pentagonal graphene. By analyzing the growth shapes we can distinguish two well-defined families with different directional dependence of growth speed. Furthermore, we observed the evolution of graphene domains from hexagon to pentagon with increased partial pressure of hydrogen, implying an anisotropic etching behavior of hydrogen. The work provides more evidence toward a deeper understanding on the mechanism of graphene growth and realize the seamless splicing of graphene domain to form a large area and high quality graphene films.
 Artyukhov, V. I., Hao, Y., Ruoff, R. S., Yakobson, B. I., Phys. Rev. Lett 2015, 114 (11), 115502.
 Nguyen, V. L., Shin, B. G., Duong, D. L., Kim, S. T., Perello, D., Lim, Y. J., Yuan, Q. H., Ding, F., Jeong, H. Y., Shin, H. S., Adv. Mater 2015, 27 (8), 1376.
 Xia, K., Artyukhov, V. I., Sun, L., Zheng, J., Jiao, L., Yakobson, B. I., Zhang, Y., Nano. Res 2016,9，2182-2189.
9:00 PM - NM8.4.18
Lattice Transparency of Graphene
Sieun Chae 1 2 , Seunghun Jang 1 , Won Jin Choi 1 , Youn Sang Kim 2 , Hyunju Chang 1 , Tae Il Lee 3 , Jeong-O Lee 1 Show Abstract
1 , Korea Research Institute of Chemical Technology, Daejeon Korea (the Republic of), 2 Graduate School of Convergence Science and Technology, Seoul National University, Suwon Korea (the Republic of), 3 Department of BioNano Technology, Gachon University, Seongnam Korea (the Republic of)
The transparency of graphene is a unique feature of graphene that arises from its ultimate-2 dimensional nature. Other than optical transparency, it has been a great interest that graphene remains transparent to wetting property and electron transfer chemistry. However, graphene’s transparency was not fully understood at the atomic resolution up to now, thus the degree of which was still debatable. Here, we introduce another interesting transparency of graphene, which is lattice transparency. We chose the substrate growth of ZnO nanocrystal through hydrothermal method as a model system. This system allows to see how the source molecules in growth solution experience the atomic potential of a substrate plane covered with graphene, while the mild growth condition barely affecting graphene chemically and mechanically. The growth behaviors of ZnO nanocrystal were investigated on graphene supporting substrates with varying crystallinity. By comparing crystal orientations of the substrate underlying graphene and the nanoparticle nucleating above it, we propose that graphene transmits the atomic structure of a substrate plane.
9:00 PM - NM8.4.19
Dopant-Specific Unzipping of Carbon Nanotube for Intact Crystalline Graphene-Carbon Nanotube Complexes
Joonwon Lim 1 , Na-Young Kim 1 , Kyung Eun Lee 1 , Hyeong Min Jin 1 , Jisun Yoon 1 , Yong-Hyun Kim 1 , Sang Ouk Kim 1 Show Abstract
1 , KAIST, Daejeon Korea (the Republic of)
Structural transformation from tubular carbon nanotube (CNT) to planar graphene, called ‘unzipping’, is a valuable route to tailor carbon nanostructures. Complete nanoscale unzipping of CNTs or graphene may produce graphene nanoribbons with electrical energy band gap. Partial unzipping of CNTs may create unique nanostructures where CNTs and graphene nanoribbons are seamlessly connected. Since the first demonstration of longitudinal cutting of CNTs, many different unzipping mechanisms have been explored, including not only wet chemical method, such as chemical oxidation, Li ion intercalation and hydrothermal reaction, but also dry processing techniques, such as plasma etching, rapid thermal expansion and metal particle catalytic cutting. Unfortunately, harsh reaction conditions for the unzipping of robust sp2 hybridized graphene plane commonly accompanies undesired damage in the basal plane. A better controllability over unzipping mechanism is highly demanded for the minimal damage to the genuine graphene-based carbon structures.
In this work, we demonstrate dopant-specific electrochemical unzipping of CNTs as a controllable method for intact crystalline unzipping. Heteroatom dopants, such as nitrogen substitutionally incorporated into sp2 hybridized carbon framework, enable atomic-scale site-selective unzipping reaction. Although pristine CNTs are stable up to the electrochemical potential above 0.8 V, nitrogen-doped CNTs (NCNTs) are readily unzipped below 0.6 V. Detailed investigation on the reaction mechanism reveals that substitutional pyridinic nitrogen (Np)-dopant can specifically initiate CNT wall unzipping. Such a dopant-specific unzipping at moderate potential enables fine controllability of unzipping level and intact crystallinity of the unzipped structures with well-defined edge configuration. Taking advantage of these unique features, we are able to synthesize intact crystalline graphene-based nanostructures, where unzipped graphene nanoribbons are seamlessly connected to the CNT strands. This structural feature offers synergistic properties comprising large surface area and robust electrical conductivity, which are highly desirable for electrochemical applications, such as energy storage. As a representative application, we demonstrate ultrahigh-power double-layer capacitors (DLCs) for alternating current (AC) line-filtering performance.
9:00 PM - NM8.4.20
Hybrid Zero-Dimensional C60 Clusters with Graphene—Synthesis, Fabrication and Transport Characteristics
Srishti Chugh 1 , Chandan Biswas 2 , Carlos Francisco de Anda Orea 2 , Isaac Deaguero 1 , Luis Echegoyen 3 , Anupama Kaul 2 Show Abstract
1 Department of Materials, Metallurgical and Biomedical Engineering, University of Texas El Paso, El Paso, Texas, United States, 2 Department of Electrical and Computer Engineering, University of Texas El Paso, El Paso, Texas, United States, 3 Department of Chemistry, University of Texas El Paso, El Paso, Texas, United States
The understanding of the transport properties of graphene has become a topic of intense interest, not only because of the underlying fascinating physics , but also due to the promising graphene-based technologies that are leading the way toward graphene-based hybrid structures. In this work, the graphene-C60 hybrid structure was formed using an electrophoretic deposition process to study the graphene-C60 interaction. The concentration of C60 was varied in toluene/acetonitrile solvent that leads to the formation of small clusters of variable sizes . It was then integrated with chemical vapor deposition (CVD) grown-graphene that was synthesized on ultra-pure copper and transferred onto SiO2/Si substrates, to see how the zero-dimensional (0D) clusters affect the electronic, opto-electronic and structural properties of the as deposited graphene. Electronic characterization of the structure was conducted after the attachment of C60 clusters onto graphene where the devices are characterized over a range of temperatures to shed insights into the transport properties of the hybrid systems. The structural characterization of the C60 clusters onto graphene was conducted using Scanning Electron Microscopy (SEM) and Raman Spectroscopy, which reveals a uniform morphology of C60 on graphene. We expect that the hybrid structures formed using a facile technique will provide a pathway toward the realization of novel electronic/optoelectronic devices for future applications.
Keywords : C60, Graphene, Electrophoretic deposition, Raman Spectroscopy, SEM, CVD
 Geim et al., Nature Materials 6, 183 - 191 (2007)
 Tung et al.,Nano Lett., 9 (5), pp 1949–195 (2009)
 Kamat et al., J. Phys. Chem. B, 104, 4014-4017(2000)
9:00 PM - NM8.4.21
Versatile Water-Based Transfer of Large-Area Graphene Films onto Flexible Substrates
Mariia Kim 1 , Changfeng Li 1 , Jannatul Susoma 1 , Harri Lipsanen 1 , Juha Riikonen 1 Show Abstract
1 Department of Micro- and Nanosciences, Aalto University, Espoo Finland
Next-generation electronic devices are expected to demonstrate greater utility, efficiency and durability. Meanwhile, plastics such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) enable transformational advantages to device shape, flexibility, weight, transparency and recyclability. Exhibiting a combination of outstanding mechanical, electrical, optical, and chemical properties of graphene with the plastic substrates could propose ideal material for the future flexible electronics. Chemical vapor deposition (CVD) allows cost-effective fabrication of a high-quality large-area graphene films, however, the critical issue is a noninvasive transfer of the films onto a desired substrate. The water-based delamination of CVD grown graphene can be considered as a green transfer process utilizing only hot deionized water. We investigated a method requiring only two essential step: hot roll lamination of 6-inch monolayer CVD graphene onto transparent and flexible substrates, and Cu delamination in hot water. Our proposed method is fast, inexpensive, reproducible, environmentally friendly, and suitable for large-scale, high quality graphene. The transfer process demonstrated films with high uniformity, free of mechanical defects and sheet resistance as low as ~150 Ω/sq with 94 % transparency at 550 nm wavelength while withstanding high strain. Further investigation of wet-chemical doping showed considerable reduction of sheet resistance to ~70 Ω/sq. By optimizing the temperature, pressure and other transfer conditions, high quality was achieved evidenced by confocal µ-Raman spectroscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM).
 MacDonald, William A., Engineered films for display technologies, J. Mater. Chem., 2004, 14(1):4-10
 Singh V, Joung D, Zhai L, Das S, Khondaker SI, Seal S. Graphene based materials: Past, present and future, Prog. Mater. Sci., 2011 10; 56(8):1178-271
9:00 PM - NM8.4.22
Stacked Graphene as an Electrode for ITO-Free Solar Cells
Ehsan Keyvani-Someh 1 Show Abstract
1 , Northeastern University, Boston, Massachusetts, United States
As a replacement to indium-tin-oxide (ITO) electrodes in organic photovoltaics (OPVs) and organic light emitting diodes (OLEDs), graphene has been recently used to achieve power conversion efficiency (PCE) values equal to those of ITO-based devices. However, the price tag for graphene electrodes is much higher than ITO which is the main motivation behind switching to an organic electrode. Herein, we employed chemical vapor deposition (CVD) to grow graphene in an inexpensive way and transferred it on a transparent substrate. To improve the performance of the electrodes, several layers of graphene have been stacked on top of each other. Stacked graphene has been characterized by Raman and UV-Vis spectroscopy, and conductivity measurements. Solar cells fabricated with graphene(1,2,3,4 layers)/poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/poly(3-hexylthiophene-2,5-diyl):phenyl-C61-butyric acid methyl ester (P3HT:PCBM)/Calcium/Aluminum architecture showed an enhancement of PCE as a function of the number of stacked graphene layers. The highest efficiency was measured for the double transferred graphene anode because of improved conductance and optimal transmittance. This work establishes that layered graphene is a viable substitute for ITO.
9:00 PM - NM8.4.23
Graphene Moiré Pattern Ultra-High Resolution Atomic Force Microscopy
Gerald Pascual 1 , Byong Kim 1 , Keibock Lee 1 Show Abstract
1 , Park Systems Corporation, Santa Clara, California, United States
The ultra-high resolution of AFM was demonstrated in a Graphene/hexagonal Boron Nitride (hBN) sample evaluation conducted by AFM. The sample consisted of hBN substrate overlaid with a Graphene layer and was scanned under ambient air. The purpose of the evaluation was to assess the AFM ability to characterize the topography of the moiré pattern that was created when one layer was set on top of the other and offset by rotation. Using non-contact AFM mode and a standard AFM probe tip, the AFM was able to successfully image the moiré pattern super lattice constant of the sample in scans as large as 500 x 500 nm. In the higher magnification image taken at a scan size of 60 x 60 nm provides the clear evidence that not only are the super lattice constants of the moiré pattern about 15 nm  in width, but that the spacing between each striation on the moiré pattern is roughly 4-5 nm in length. Observations of such striations in Graphene/hBN systems have been previously reported . This latter distance is in line with the expected tip radius curvature values for the AFM tip used to acquire all four sets of data.
 A. Zandiatashbar, B. Kim, Y. Yoo, and K. Lee, Microscopy Today 23(06):26-31 (2015)
 P. Gallagher, M. Lee, F. Amet et.al., Nature Comm. 7 10745 (2016)
9:00 PM - NM8.4.25
Solvothermal Exfoliation of Graphite—A Greener Method to Produce Few-Layered Graphene
Paulo Duarte 1 2 , Isabel Fonseca 2 , Isabel Ferreira 1 Show Abstract
1 CENIMAT/I3N and UNINOVA, Materials Science Department, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, Caparica Portugal, 2 LAQV - REQUIMTE, Chemistry Department, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, Caparica Portugal
Since its discovery in 2004, graphene has attracted great attention due to is outstanding electronic, optical, thermal and mechanical properties, which make this material suitable for many applications in supercapacitors, batteries, solar cells, sensors, among others. One of the main drawbacks for the commercial availability of graphene is the lack of methods for large scale production of high quality graphene sheets. The most common routes to produce them are chemical vapour deposition (CVD), mechanical or liquid phase exfoliation of bulk graphite or chemical synthesis. These techniques present some limitations such as: high quantities of solvents, dangerous chemicals, low bulk productivity or expensive procedures. Due to these limitations, the development of eco-friendly and low cost methods that allow the mass production of graphene sheets are highly desirable.
In this work we developed a greener method for the exfoliation of pyrolytic graphite (a waste from the furnaces of metallurgic industry). Variations of solvents or temperatures were tested. The obtained materials show the typical Raman bands of graphene and the TEM images show few layer graphene. Other characterization techniques were used to confirm these results. This method is environmental friendly, economic and easy scalable for mass production of high quantities few-layered graphene. This new route can open the door for large scale production of commercial devices such as capacitors, batteries and sensors.
9:00 PM - NM8.4.26
Low Concentration Nanofluid of Graphene-Based Amphiphilic Janus Nanosheets for Oil Recovery—High Performance by Its Unique Interfacial Behavior
Dan Luo 1 , Feng Wang 1 , Zhifeng Ren 1 Show Abstract
1 , University of Houston, Houston, Texas, United States
Nanofluid (dispersion of nanoparticles/additives) flooding for tertiary or enhanced oil recovery has been considered as a promising alternative to traditional chemical methods from the environmental and economic perspective. However, the current simple nanofluid (containing only nanoparticles) for oil recovery is inefficient, especially when used with low concentrations. Here, we have designed and produced a nanofluid of graphene-based amphiphilic Janus nanosheets that is very effective at low concentrations. After exfoliation and oxidization of graphite, single surface conjugation of alkylamine to oxidized graphene was achieved by wax masking method and checked by AFM, FTIR, TGA and XPS, which rendered its amphiphilic property and Janus structure. The partial restoration of graphitic sp2 network was also detected by Raman and UV-Vis. The stability of nanofluid was evaluated before rock core flooding tests. Our nanosheets spontaneously approached the oil-water interface and reduced the interfacial tension in a saline environment (4 wt% NaCl and 1 wt% CaCl2), regardless of the solid surface wettability. A climbing film appeared and grew at moderate hydrodynamic condition to encapsulate the oil phase. With strong hydrodynamic power input, a solid-like interfacial film formed and was able to return to its original form even after being disturbed. The film rapidly separated oil and water phases for slug-like oil displacement. The crude oil immersion testing further demonstrates the amphiphilic nanosheets can help residual oil detach from solid surface, indicated by the obvious change of droplet shape-profile. The unique interfacial behavior of our nanosheet nanofluid tripled the best performance of conventional nanofluid flooding methods under similar conditions.
9:00 PM - NM8.4.27
Xenon Flash Lamp-Induced Multilayer Graphene Growth for Roll-to-Roll Application
Tae Hong Im 1 , Keon Jae Lee 1 Show Abstract
1 , KAIST, DaeJeon Korea (the Republic of)
Graphene, the two-dimensional (2D) carbon materials, have received lots of attention for various applications such as transparent electrode, high speed semiconductor, flexible display and snart sensor owing to its outstanding electrical, mechanical, thermal, chemical and optical properties. However, it has been main problems to make large scale and high quality graphene in short time for industrial mass production. In order to solve this kind of problem, many researchers have made great efforts to find new type of graphene synthesis method that satisfy both fast growth rate and large scale growth. Hong et al. succeeded the large-scale graphene synthesis by using chemical vapor deposition (CVD) method in 2009, however, this technique has limitation to mass production of graphene because it takes a few hours to proceed heat treatment including heating and cooling process.
In this study, we demonstrated a new synthesis method for high fast fabrication of graphene by flash lamp-induced heat treatment. Due to the light duration of our xenon flash lamp is 3 or 15ms, all thermal process containing heating, synthesis and cooling to room temperature will take place within 3 or 15ms. This process is 4 orders of magnitude faster than conventional CVD method. Flash lamp-induced graphene synthesis method would not only be one of the best ways to mass-production of graphene, but also facilitate graphene-based device applications.
9:00 PM - NM8.4.30
Growth of Graphene on FIB Patterned 3C-SiC Nanostructures by UHV Annealing
Mojtaba Amjadipour 1 , Jennifer Macleod 1 , Josh Lipton-Duffin 2 1 , Francesca Iacopi 3 , Jose Alarco 1 , Nunzio Motta 1 Show Abstract
1 School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, Queensland, Australia, 2 Central Analytical Research Facility (CARF) , Queensland University of Technology, Brisbane, Queensland, Australia, 3 Faculty of Engineering and Information Technology, University of Technology Sydney, Sydney, New South Wales, Australia
Thermal decomposition of SiC has proven to be an excellent method to grow transfer-free wafer-scale graphene . There is a growing body of literature that recognizes the potential of graphene for use in electronics . However, the fact that graphene is a semimetal with zero bandgap is a key issue which challenges its remarkable range of applications. Theoretical work suggests that a bandgap might be opened in graphene through quantum confinement, for example in graphene nanoribbons. Therefore, over the past few years, a considerable literature has grown up around the theme of producing a semiconducting graphene .
In this research we attempt to manipulate the SiC substrate dimension to grow graphene over small nanostructures with lateral sizes ranging from tens of nm to 1 µm. To date, there has been a few reports about the growth of graphene on nanometre-scale SiC mesas , and very little is known about the effect of changing the dimension and characteristic of the substrate on which graphene is grown. In order to elucidate the possibility for patterned graphene-growth in substrate-defined geometries, we have examined the effect of SiC patterning on graphene growth.
We have grown graphene by high temperature annealing in Ultra High Vacuum (UHV) on 3C-SiC nanostructures fabricated by Focused Ion Beam (FIB) on 800 nm 3C-SiC layers on Si(111). Scanning Tunneling Microscopy (STM) was used to investigate the surface condition and to identify surface reconstructions produced by the growth process.
Our results indicate that Ga ion beam disturbs the normal growth process considerably, and no graphene is growing on top of the mesas. By using a Si protecting layer before the FIB patterning, we managed to successfully grow graphene over patterned areas, as demonstrated by micro-Raman, STM and Helium Ion Microscopy.
1. Gupta, B., Notarianni, M., Mishra, N., Shafiei, M., Iacopi, F., & Motta, N., Evolution of epitaxial graphene layers on 3C SiC/Si (111) as a function of annealing temperature in UHV. Carbon, 2014. 68: p. 563-572.
2. Kusunoki, M., et al., Growth and Features of Epitaxial Graphene on SiC. Journal of the Physical Society of Japan, 2015. 84(12): p. 121014.
3. Celis, A., et al., Graphene nanoribbons: fabrication, properties and devices. Journal of Physics D: Applied Physics, 2016. 49(14): p. 143001.
9:00 PM - NM8.4.31
Boosting the Electrical Conductivity and 3D Nanostructuring of Inkjet Printed Graphene with Pulsed Laser Irradiation
Suprem Das 1 2 , Qiong Nian 4 , Gary Cheng 3 , Jonathan Claussen 5 Show Abstract
1 , Iowa State University, Ames, Iowa, United States, 2 Division on Materials Science and Engineering, Ames Laboratory, Ames, Iowa, United States, 4 Industrial Engineering, Purdue University, West Lafayette, Indiana, United States, 3 Industrial Engineering, Purdue University, West Lafayette, Indiana, United States, 5 , Iowa State University, Ames, Iowa, United States
Mechanically flexible and electrically robust printed electronics fabricated via low cost and scalable manufacturing route is projected to revolutionize many technological applications such as chemical/biological sensors, solar cells, supercapacitors, and field effect devices. Graphene based printed devices and circuits have recently provided enormous promise for its practical applications. However, as in many printed technologies, the printed device/circuit undergoes post-printing annealing process where the whole active area including the substrate material undergoes high temperature thermal annealing (typically ~ 400 0C or more). However, this process is detrimental for substrates with low melting points such as plastic, polyimide, PET and paper used for printed electronics in cheaper and/or disposable electronics. In this work, we provide a novel route of using a UV pulsed laser irradiation to spatially process the printed graphene to achieve selective anneal graphene, evaporate any trapped solvent and surfactants in the structure, and reduce the graphene oxide present in the structure. Over three orders of magnitude sheet conductivity is achieved with this technique with typical values of sheet resistance below 1 kΩ/sq. Unique to the process, the high photon power density causes a 3D nanostructuring of printed graphene with enermous vertical petal-like structures that produces very high surface area that are ideal for development of sensor devices. The process is highly scalable, constitutes room temperature process and becomes tunable to single parameter – the laser energy density. Such a route makes the UV pulsed laser processing of inkjet-printed devices/circuit very promising for future sensors and electronics.
9:00 PM - NM8.4.32
Photoresponse of a Bilayer Graphene p-n Junction Using a Combination of Electrostatic and Electrolytic Gating
Sameer Grover 1 , Anupama Joshi 1 2 , Ashwin Tulapurkar 2 , Mandar Deshmukh 1 Show Abstract
1 , Tata Institute of Fundamental Research, Mumbai India, 2 Department of Electrical Engineering, Indian Institute of Technology Bombay, Mumbai, Maharashtra, India
Electrolyic gating is useful for inducing large carrier densities in graphene and other 2D-materials . We demonstrate a technique for the formation of p-n junctions in graphene using a combination of electrostatic and electrolytic gating. This was done by patterning the negative resist hydrogen silsesquioxane (HSQ) to cover part of the graphene flake.
We performed electrical and photoresponse measurements on a bilayer graphene flake that was partially covered with HSQ. It was gated with the ionic liquid 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-Im) serving as the top gate and with degenerately doped silicon as the back gate.
The device characteristics were measured both at room temperature, where the ions are mobile, and at low temperatures, where the ionic liquid is frozen. We created p-n junctions that work at both room temperature and at low temperatures below the freezing point of the ionic liquid.
This technique is suited for studying the photoresponse of graphene p-n junctions because of the larger transparency of ionic liquids compared to metallic gates as used in previous studies . We found that the photoresponse is dominated by the photo-thermoelectric effect, characterized by a six fold pattern in the photovoltage. The photovoltage increases as the temperature decreases which is indicative of hot electron thermalization by disorder assisted supercollisions.
1. Das, A., Pisana, S., Chakraborty, B., Piscanec, S., Saha, S.K., Waghmare, U.V., Novoselov, K.S., Krishnamurthy, H.R., Geim, A.K., Ferrari, A.C. and Sood, A.K., 2008. Nature nanotechnology, 3(4), pp.210-215.
2. Gabor, N.M., Song, J.C., Ma, Q., Nair, N.L., Taychatanapat, T., Watanabe, K., Taniguchi, T., Levitov, L.S. and Jarillo-Herrero, P., 2011, Science, 334(6056), pp.648-652.
9:00 PM - NM8.4.33
Characterization of Smart Polymer-Graphene Hybrid Systems: Atomistic Insights into Adsorption and Stimuli-Responsive Behaviors
Mahdi Moshref-Javadi 1 , Nikhil Medhekar 1 Show Abstract
1 , Monash University, Clayton, Victoria, Australia
Non-covalent functionalization of graphene materials with smart polymers has been employed as an approach for synthesizing innovative hybrid nanoparticles and membranes with improved solubility and mechanical properties. Interactions, adsorption, and smart behaviors, however, are not yet understood at the atomic level. In this research, the dynamic process of physical adsorption of poly(N-isopropylacrylamide) (PNIPAM) onto graphene (G) and graphene oxide (GO) was studied, followed by examining the temperature-responsive behavior of the hybrid systems. Previous results of pure PNIPAM in aqueous solution were also reproduced and compared. PNIPAM could spontaneously anchor to the surfaces of both G and GO at low temperature in the coil form. Nevertheless, configuration of PNIPAM on G proved to be different from that on GO, which resulted in distinct responsive behaviors upon temperature rise, the origin of which was construed on the basis of the ruling interactions and the solvation behaviors. The results obtained are of paramount significance in bottom-up design and manipulation of multi-functional hybrid stimuli-responsive systems with optimized properties.
Peter Sutter, Univ of Nebraska-Lincoln
Nasim Alem, The Pennsylvania State University
Arkady Krasheninnikov, Helmholtz-Zentrum Dresden-Rossendorf
Alexander Weber-Bargioni, Lawrence Berkeley National Laboratory
J.A. Woollam Company, Inc.
RHK Technology, Inc.
NM8.5: Novel 2D Materials I
Wednesday AM, April 19, 2017
PCC West, 100 Level, Room 101 A
9:00 AM - NM8.5.01
Strain Engineering of 2D Materials via Dielectric Nanosphere Assemblies
Yingjie Zhang 1 , Youngseok Kim 1 , Blanka Janicek 1 , Pinshane Huang 1 , Harley Johnson 1 , Joseph Lyding 1 , Matthew Gilbert 1 , Nadya Mason 1 Show Abstract
1 , University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Two dimensional materials are being widely explored for various electronic, optoelectronic, and spintronic applications. Atomically thin in nature, their electronic properties are very sensitive to modulations of lattice constant. Novel effects, such as bandgap modulation, pseudo magnetic field, and photoluminescence enhancement may emerge when the 2D materials are strained. Here we present a general strategy to modulate the strain of 2D materials (graphene, transition metal dichalcogenides, etc.) via controlled corrugations of the substrate. When deposited on insulating nanosphere superlattices, these 2D membranes partially conform to the morphology of the nanospheres. Due to the small radius of curvature of this corrugated substrate, the atomically thin membranes experience significant changes in lattice constant. We experimentally quantified the local strain profiles via microscopic and spectroscopic tools, and studied the effect on electronic properties.
9:15 AM - NM8.5.02
From Liquid Metals Down to Two-Dimensional Semiconductors
Benjamin Carey 1 , Torben Daeneke 1 , Richard Kaner 2 , Kourosh Kalantar-zadeh 1 Show Abstract
1 , RMIT, Melbourne, New South Wales, Australia, 2 , University of California, Los Angeles, Los Angeles, California, United States
Different deposition methods, either chemical or physical based, for two dimensional planar crystals have been devised [1-3]. However, the high quality, large scale and consistent deposition of these materials remain as significant challenges. We introduce a novel approach for depositing large scale two dimensional (2D) post transition metal chalcogenide compounds using the self-limiting metal oxide layer of the metal precursor in liquid form.
Ga, In and Sn, which are the post transition metals, have low melting points. In an oxygen containing atmosphere, these metals quickly form an atomically thin (~0.7 nm) self-limiting oxide layer . The presence of this protective oxide layer increases the wettability of post transition liquid metals on oxygen terminated substrates by providing large van der Waals forces between the two surfaces . After placing this liquid metal with its self-limiting oxide layer on a substrate, the coating is exfoliated due to the large van der Waals forces onto its surface oxide. Using this phenomenon, we establish a process that uses low melting point Ga (29.7°C) to deposit wafer scale printable 2D gallium sulphide from its exfoliated oxide. In this process, the oxide skin of Ga is exclusively placed onto a substrate. This oxide layer is then sulfurised via a specifically designed low temperature procedure to produce large area bilayer (~1.4 nm) 2D gallium sulphide. Controlling the surface chemistry of the substrate allows for selective patterning . This facile printing method is suitable for large scale fabrication of 2D post deposition sulphide based devices, overcoming one of the major impediments of the fabrication of devices based on these 2D materials.
1. E. P. Nguyen, B. Carey, T. Daeneke, J. Z. Ou, K. Latham, S. Zhuiykov and K. Kalantar-zadeh , Chem. Mater. 27, 53–59 (2015).
2. S. Balendhran, J. Z. Ou, M. Bhaskaran, S. Sriram, S. Ippolito, Z. Vasic, E. Kats, S. Bhargava, S. Zhuiykov, and K. Kalantar-zadeh, Nanoscale 4, 461–466 (2012).
3. S. Balendhran, S. Walia, H. Nili, J. Z Ou., S. Zhuiykov, R. B. Kaner, S. Sriram, M. Bhaskaran, K. Kalantar-zadeh, Adv. Funct. Mater. 23, 3952–3970 (2013)
4. A. J. Downs, Chemistry of aluminium, gallium, indium, and thallium. (Springer Science & Business Media, 1993).
5. M. D. Dickey, ACS Appl. Mater. Interfaces 6, 18369-18379 (2014).
6. E. P. Nguyen, B. J. Carey, J. Z. Ou, J. van Embden, F. Della Gaspera, A. F. Chrimes, M. J. S. Spencer, S. Zhuiykov, K. Kalantar-zadeh and T. Daeneke Adv. Mater. 27, 6225-6229 (2015).
9:30 AM - NM8.5.03
Layer Structured Gallium Chalcogenides—Controlled Synthesis and Engineering of Their Bandgap and Optical Properties
Hui Cai 1 , Emmanuel Soignard 1 , Can Ataca 2 , Bin Chen 1 , Changhyun Ko 3 , Gang Wang 6 , Anupum Pant 1 , Toshihiro Aoki 1 , Shengxue Yang 4 , Marco Manca 6 , Xiuqing Meng 1 , Xavier Marie 6 , Bernhard Urbaszek 6 , D. Frank Ogletree 5 , Jeffrey Grossman 2 , Sefaattin Tongay 1 Show Abstract
1 , Arizona State University, Tempe, Arizona, United States, 2 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 , University of California, Berkeley, Berkeley, California, United States, 6 , Université de Toulouse, Toulouse France, 4 , Beihang University, Beijing, Beijing, China, 5 , Lawrence Berkeley National Laboratory, Berkeley, California, United States
Gallium Chalcogenides (GaS, GaSe and GaTe) are layer structured semiconductors that belong to the post transition metal chalcogenides (PTMCs). We demonstrate the synthesis of GaX on a variety of substrates including GaAs (111), Si (111) and sapphire via conventional epitaxy and van der Waals epitaxy. Our results show that material properties can be engineered at will on polar surfaces through careful control over different vdW epitaxy growth mode. The bandgap of 2D GaSe grown on Si(111) is significantly reduced from its widely accepted value at 2 eV down to 1.7 eV. By manipulating various kinetic factors, we were able to tune the supersaturation concentration to choose between screw-dislocation-driven (SDD) and layer-by-layer (LBL) growth. Depending on the growth mode, material substrate interaction (strain) differs substantially, allowing us to manipulate emission characteristics continuously in the 620-700nm range on a single flake. Interestingly, this range can only be attained in layered monochalcogenides by alloying Te with GaSe to form GaSexTe(1-x).
The synthesized monoclinic GaTe flakes possess a pseudo-one diminsional chain-like atomic structure, similar to black phosphorous, ReS2 and TiS3 that have been discovered recently. The unique structure is revealed by HRTEM for the first time and leads to highly anisotropic optical responses. The sample exhibits multiple sharp sub-band emissions that are related to localized defect states and intermediate band formation. These findings open new opportunities for further defect engineering and novel optoelectronic applications such as intermediate band solar cells based on GaTe.
9:45 AM - *NM8.5.04
Properties and Device Applications of Two-Dimensional Charge Density Wave Materials
Alexander Balandin 1 , Guanxiong Liu 1 , Tina Salguero 2 , Roger Lake 1 Show Abstract
1 , University of California, Riverside, Riverside, California, United States, 2 , University of Georgia, Athens, Georgia, United States
The charge density wave (CDW) phase is a quantum state consisting of a periodic modulation of the electronic charge density accompanied by a periodic distortion of the atomic lattice in quasi-1D or quasi-2D metallic crystals. Several layered transition metal dichalcogenides, including 1T-TaSe2, 1T-TaS2 and 1T-TiSe2 exhibit unusually high transition temperatures to different CDW symmetry-reducing phases. Bulk 1T-TaSe2 has transition temperatures of 600 and 473 K below which the material exists in the incommensurate (I-CDW) and the commensurate (C-CDW) charge-density-wave phases, respectively . Crystals of 1T-TaS2 have the transition between the nearly-commensurate (NC-CDW) and the I-CDW phases at temperature of 350 K. In this talk, I review our recent experimental and computational results, which show that the transition temperature of such materials can be effectively controlled with the thickness of quasi-2D films. The phase transition can be triggered by an applied electric bias. Using variable temperature Raman spectroscopy we established the origin of the ~154 1/cm phonon peak in 1T-TaSe2, and assigned it to the zone-folding of the phonon modes following the lattice reconstruction. In order to use CDW effects for device applications, we exploited the transition between the NC-CDW and I-CDW phases with an abrupt change in the current accompanied by hysteresis. A graphene transistor, integrated with quasi-2D 1T-TaS2 channel, provided a voltage tunable, matched, low-resistance load enabling precise voltage control of the device. The integration of three different 2D materials, such as 1T-TaS2, graphene and h-BN in a way that exploited the unique properties of each, yielded a simple, miniaturized, voltage-controlled oscillator suitable for a variety of practical applications .
This work was supported, in part, by NSF EFRI 2-DARE project: Novel Switching Phenomena in Atomic MX2 Heterostructures for Multifunctional Applications.
 R. Samnakay, D. Wickramaratne, T. R. Pope, R. K. Lake, T. T. Salguero and A. A. Balandin, Nano Letters, 15, 2965 (2015).
 G. Liu, B. Debnath, T. R. Pope, R. K. Lake, T. T. Salguero and A. A. Balandin, Nature Nanotechnology, 11, 845 (2016).
10:15 AM - NM8.5.05
Synthesis of Large Area MoS2 Few Layers by RF Sputtering Process
Taekyung Oh 2 1 , Hyeongtag Jeon 1 , Jeon Kook Lee 2 Show Abstract
2 , Korea Institute of Science and Technology, Seoul Korea (the Republic of), 1 , Hanyang University, Seoul Korea (the Republic of)
Abstract Body: Atomically thin 2D transition metal Dichalcogenides (TMDs) were expected to show rich collection of physical properties and functionalities in the area of Nano-electronics, optoelectronics, catalysis, photo-detection, photovoltaics and photo-catalysis. As a prototype of TMDs, molybdenum disulfide (MoS2) has been demonstrated to exhibit high current switching ratio, high mobility and negligible base current. This indicates that the sensitivity can be significantly improved with MoS2-based transistors (FETs). In addition to its good electronic properties, the inherent band gap (1.8 eV for monolayer and 1.2 eV for bulk), excellent mechanical and optical properties permit its applications in large-area flexible optoelectronics.
To facilitate the integration into macroscopic electronic applications, it is essential to develop a large-area growth that is compatible with current micro- or Nano-fabrication processes. Our study is based on 90 degree axis sputtering, which is easily scalable to allow growth of very thin TMD layers over
very large areas. All MoS2 thin films were grown on SiO2 and GaN using a solid poly-MoS2 target of 99.9% purity. After sputtering MoS2, we annealed samples by varying sulfur atmosphere and temperature. MoS2 few-layer samples were analyzed by Raman Spectroscopy, Atomic Force Microscopy (AFM), Optical Microscopy, Scanning Electron Microscope (SEM), and Transmission Electron Microscope (TEM).
10:30 AM - NM8.5.06
Quasi-2D Monolayers of Plasmonic Nanocrystals Cross-Linked by Phthalocyanines—A New Playing Field for Molecular Electronics
Mahdi Samadi Khoshkhoo 1 , Santanu Maiti 1 , Frank Schreiber 1 , Thomas Chasse 1 , Marcus Scheele 1 Show Abstract
1 , University of Tubingen, Tuebingen Germany
We demonstrate how plasmonic nanocrystals, such as tin-doped indium oxide or copper(I) selenide, can be surface-functionalized with semiconducting phthalocyanines and assembled into hybrid, macroscopic 2D-monolayers at the liquid/air interface. These thin films consist of repetitive units of isolated nanocrystals separated by a monolayer of the organic semiconductor. Degenerate doping renders the nanocrystals metallic and leads to a tunable plasmonic oscillation in the 2D film. The nanocrystals therefore serve as local optical antennas and metal contacts alike, which are cross-linked by monolayers of phthalocyanine molecules. We show that the organic semiconductor plays a dominate role in mediating efficient electric transport in these films, such that they can be seen as a macroscopic network of molecular junctions. A particular emphasis is put on spatially resolved Raman measurements to elucidate local inhomogeneities and molecular reorganizations under applied bias.
We will discuss the prospects of exploiting the various optical resonances (plasmonic and excitonic) in the material for optoelectronic applications and comment on the type of transport through the network. Such hybrid plasmonic/semiconducting quasi-2D materials allow to test many fundamental concepts of molecular electronics on a macroscopic scale.
NM8.6: Novel Phenomena in 2D Materials
Wednesday PM, April 19, 2017
PCC West, 100 Level, Room 101 A
11:15 AM - *NM8.6.01
Novel Quantum Phenomena in Atomically Thin Two-Dimensional Materials
Steven Louie 1 2 Show Abstract
1 Physics Department, University of California, Berkeley, Berkeley, California, United States, 2 , Lawrence Berkeley National Laboratory, Berkeley, California, United States
In this talk, we present some new physical phenomena found in recent theoretical studies of atomically thin two-dimensional (2D) materials. Because of reduced dimensionality, interaction and symmetry effects as well as environmental screening dominate many properties of these systems. Several unexpected phenomena are found. We predict that electron supercollimation, in which a wave packet is guided to move undistorted along a selected direction, is realized by 1D disorder in graphene and in related 2D Dirac materials. We show that an interesting topological insulator phase is formed in graphene nanoribbons with appropriate doping. Such graphene nanoribbons can be synthesized experimentally using bottom-up molecular precursor techniques. We discuss the discovery of excitons with light-like (massless) dispersion in monolayer transition metal dichalcogenide systems, a general characteristic of 2D semiconductors arising from the electron-hole exchange interaction in two dimensions. Finally, if time permits, we discuss the engineering of the transport, optical and plasmonic properties of 2D materials through substrate modifications.
This work was supported in part by the National Science Foundation and the U.S. Department of Energy.
11:45 AM - NM8.6.02
One-Dimensional Photonic Crystals for Touchless Finger Motion Tracking Based on 2D Nanosheets with Ultrahigh Moisture Sensitivity
Katalin Szendrei 2 1 3 , Pirmin Ganter 2 1 , Bettina Lotsch 2 1 3 Show Abstract
2 , Max Planck Institute for Solid State Research, Stuttgart Germany, 1 , LMU Munich, Munich Germany, 3 , Nanoinitiative Munich and Center for Nanosciences, Munich Germany
One-dimensional Photonic Crystals (1D PC), also referred to as Bragg stacks (BS), are bioinspired periodic multilayered nanostructures exhibiting a photonic band gap - a forbidden spectral range for photons propagating through the nanostructure. The photonic stop band can be dynamically changed by chemical, physical or biological stimuli, thus environmental changes can be directly translated into color changes, which renders these platforms promising candidates for smart optical detectors.
Integrating colloidally stable 2D nanosheets into BSs offers new possibilities both in terms of materials properties and novel optical features, including morphology-enhanced sensitivity, high selectivity to a particular analyte through chemical functionalization, and high reflectance due to an ultra-large refractive index contrast.
Herein we present 1D PCs fabricated by bottom-up assembly of colloidal metal oxide nanoparticles and 2D phosphate or sulfide nanosheets. The hydrophilic 2D nanosheets can take up large amounts of moisture in the interlayer space, resulting in layer swelling followed by a full-spectrum structural color change. While phosphate-based 1D PCs are highly selective to water vapor, we demonstrate that a range of other volatile organic compounds (VOCs), including chemically similar alcohols and even isomers, can be distinguished by means of their response times and the extent of the stop band shift.
The combination of high sensitivity and selectivity to water vapor, cycling stability and response times on the subsecond time scale allowed us to detect local humidity changes in a spatially and temporally resolved fashion. As the human finger is surrounded by a humid atmosphere, finger positions and motions can be tracked by structural color changes of the BS in realtime and true color under touchless conditions. These experiments bode well for a new touchless device architecture as viable alternative to the touchscreen technology, where common disadvantages of the contact based technology such as lack of hygiene and mechanical wastage could be avoided.
K. Szendrei, P. Ganter, O. Sanchez-Sobrado, R. Eger, A. Kuhn, B. V. Lotsch, Adv. Mater. 2015, 27, 6341–6348.
K. Szendrei, P. Ganter, O. Sanchez-Sobrado, A. Kuhn, B. V. Lotsch, European Patent Application, 2015, Nr. 15 155 532.3.
K. Szendrei, P. Ganter, B.V. Lotsch, Proc. SPIE 9885, 2016, Photonic Crystal Materials and Devices XII, 98850Z. DOI: 10.1117/12.2227431.
P. Ganter, K. Szendrei, B. V. Lotsch, Adv. Mater. 2016, 28, 7436–7442.
12:00 PM - NM8.6.03
Towards Single-Photon LEDs by FRET from Metal Nanoparticles to TMDC Monolayers
John Lupton 1 Show Abstract
1 , University of Regensburg, Regensburg Germany
150 years after the invention of the incandescent bulb the generation of light by metals through inter- and intraband transitions remains intriguing [1-3]. One of the most extraordinary phenomena is the direct conversion of an electrical current between nanostructures of a noble metal, such as silver, to light. This process is thought to arise due to the statistical nature of single-electron tunnelling, which generates an effective dipole between particles. Plasmon resonances enhance radiation to the far field so that broad-band electroluminescence (EL) occurs. We exploit this phenomenon by coupling the nanoparticle dipole transition in the near-field to a strong resonant absorber, a TMDC monolayer such as MoS2. Such FRET between an LED structure and a spectral converter is usually impossible because the distances involved are too large, but becomes achievable here because the TMDC layer is directly stamped onto the silver nanoparticles. EL can be tuned by the monolayer and is both down- and up-converted to the TMDC exciton resonance. Photon correlation spectroscopy demonstrates that emission occurs from nanoscale volumes and suggests routes to achieving single-photon generation on demand, at room temperature. The approach outlined is unique since FRET occurs from the metal nanoparticles to the acceptor, rather than from a donor to the metal as is usually the case in fluorescence quenching experiments.
 Borys, Lupton et al., Sci. Rep. 3, 2090 (2013).
 Klemm, Lupton et al., PRL 113, 266805 (2014).
 Haug, Lupton et al., PRL 115, 067403 (2015).
12:15 PM - NM8.6.04
Large Scale Commercial Fabrication of High Quality Graphene-Based Assays for Biomolecule Detection
Mitchell Lerner 1 , Yingning Gao 1 , Brett Goldsmith 1 Show Abstract
1 , Nanomedical Diagnostics, San Diego, California, United States
Large numbers of high quality graphene transistors with mobility approximately 5000 cm2/V*s were fabricated by chemical vapor deposition and packaged into ceramic carriers with an open cavity design. The ceramic carrier is compatible with standard electronics assembly, enabling the readout of graphene properties on the benchtop without large, expensive probing systems. After chemical functionalization, these sensors demonstrate sensitivity in the pM range and selectivity to many classes of biomolecules as a three terminal liquid-gated field effect transistor. High precision measurements of protein kinetics captured using this technology, commercially known as AGILE R100, are comparable and can exceed the capabilities of state-of-the-art biomolecule characterization tools.
12:30 PM - *NM8.6.05
Rational Design of 2-Dimensional Magnetic Materials for the Quantum Anomalous Hall Effect and Spintronic Applications
Liang Dong 1 , Vivek Shenoy 1 Show Abstract
1 , University of Pennsylvania, Philadelphia, Pennsylvania, United States
2-Dimensional (2D) materials that display robust ferromagnetism have been pursued intensively for exploring their novel quantum phases as well as applications in nanoscale spintronic devices, but suitable candidates have not been identified. Here we present theoretical predictions on the design of 2D covalent-organic frameworks (COFs) and ordered double-transition-metal MXene structures to achieve such a goal. Using first principles simulations, we predict the quantum anomalous Hall (QAH) state in a COF monolayer based on the newly synthesized X3(C18H12N6)2 structure where X represents 5d transition metal elements Ta, Re, and Ir. We show that the QAH state can appear by chemically engineering the exchange field and the Fermi level in the monolayer structure, resulting in non-zero Chern numbers. We also demonstrate robust ferromagnetism in Ti2MnC2Tx monolayers regardless of the surface terminations (T=O, OH, and F), as well as in Hf2MnC2O2 and Hf2VC2O2 monolayers. The high magnetic moments (3-4 μB/unit cell) and high Curie temperatures (495 K-1133 K) of these MXenes are superior to the magnetic properties of existing 2D ferromagnetic materials. Furthermore, semimetal to semiconductor and ferromagnetic to antiferromagnetic phase transitions are predicted to occur in these materials in the presence of small or moderate tensile in-plane strains (0-3 %), which can be externally applied by mechanically or can be internally induced by the choice of transition metals. The QAH COF and ferromagnetic MXene structures identified in this study can serve as platforms for 2D spintronic and magnetic applications.
NM8.7: Novel 2D Materials II
Wednesday PM, April 19, 2017
PCC West, 100 Level, Room 101 A
2:30 PM - NM8.7.01
Synthesis of 2D Chromium Carbide MXene and Its Magnetic Properties
Babak Anasori 1 2 , Mohamed Alhabeb 1 2 , Eun Ju Moon 1 , Hemant Kumar 3 , Liang Dong 3 , Eun Sang Choi 4 , Nicholas Trainor 1 2 , Bernard Haines 1 2 , Vivek Shenoy 3 , Steven May 1 , Yury Gogotsi 1 2 Show Abstract
1 Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania, United States, 2 Drexel Nanomaterials Institute, Drexel University, Philadelphia, Pennsylvania, United States, 3 Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 4 National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida, United States
The family of 2D transition metal carbides and nitrides (MXenes) have been expanding rapidly with more than 20 different MXenes synthesized to date and several more theoretically predicted. MXene family found new members by discovery of ordered double transition metals MXenes, such as Mo2TiC2, in which a layer of transition metal (e.g. Ti) is sandwiched between the layers of another one (e.g. Mo) in a 2D carbide structure. Although some chromium containing MXenes have been predicted to be ferromagnetic, they have not been fully synthesized, yet. In this study, we present complete synthesis and characterization of Cr2TiC2Tx, (Tx is the surface terminations such as –OH, –O and –F). Cr2TiC2Tx is a member of ordered double transition metal MXenes, with two CrC atomic layers sandwiching the middle Ti layer. Density functional theory (DFT) calculations suggest that for all terminations (Tx: –O, –OH, –F) ground states have magnetic moment of ~ 2-3 µB per Cr atom, localized on both atomic layers of Cr. Intralayer nearest neighbors are coupled ferromagnetically for all terminations. On the other hand, interlayer exchange interaction is antiferromagnetic for the –OH and –F termination and ferromagnetic for –O termination. Temperature dependent DC magnetization and AC susceptibility will be presented and discussed within the context of the DFT predictions.
2:45 PM - NM8.7.02
Defects in Monolayer Titanium Carbide (Ti3C2Tx) MXene
Xiahan Sang 1 , Yu Xie 1 , Ming-Wei Lin 1 , Mohamed Alhabeb 2 , Katherine Van Aken 2 , Yury Gogotsi 2 , Paul Kent 1 , Kai Xiao 1 , Raymond Unocic 1 Show Abstract
1 , Oak Ridge National Lab, Raleigh, North Carolina, United States, 2 , Drexel University, Philadelphia, Pennsylvania, United States
Mxene materials, transition metal carbides or nitrides, have recently gained interest as a developing class of 2D materials with applications geared towards energy storage, catalysis, and electronic devices. To better understand the physiochemical and electronic properties, detailed atomic resolution structural analysis of monolayer MXene was investigated using a combination of aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and density functional theory (DFT). Large area Ti3C2Tx MXene flakes, were synthesized and the type and concentration of atomic scaled defects were analyzed. Ti vacancies and Ti vacancy clusters were found to be the most prevalent defects. The edge defects, although not intrinsic to the single-layer flakes, can be created using beam irradiation. The formation energy and electronic structure of point defects and edge defects have been calculated using DFT. The influence of the defects on the conductivity is also studied using DFT. Our results thus shed light on the future nano-electronic application using 2D metallic MXene single layers.
Research supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Aberration-corrected STEM imaging was conducted as part of a user proposal at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences (CNMS), a U.S. Department of Energy Office of Science User Facility.
3:00 PM - NM8.7.03
CVD Growth of 2D Pyrite and Pyrite/Graphene Vertical Heterostructures
Zafer Mutlu 1 , Ryan Wu 2 , Shanshan Su 1 , Bishwajit Debnath 1 , Selcuk Temiz 1 , Jeffrey Bell 1 , Changling Li 1 , Krassimir Bozhilov 1 , Mihri Ozkan 1 , Roger Lake 1 , Andre Mkhoyan 2 , Cengiz Ozkan 1 Show Abstract
1 , University of California at Riverside, Riverside, California, United States, 2 , University of Minnesota, Twin Cities, Minnesota, United States
Pyrite (FeS2), commonly known as “fool's gold”, is an inexpensive, non-toxic and and earth-abundant semiconductor material. Despite great progress in synthesis of pyrite nanostructures in zero-, one- and three-dimension, it remains a considerable challenge to prepare pyrite crystals in two-dimension that may bring us surprising physical, chemical and magnetic properties. Herein, we report on the growth of highly crystalline two-dimensional (2D) pyrite crystals on SiO2 substrates via direct atmospheric pressure chemical vapor deposition (CVD) technique using iron-based and sulfur powder precursors. The effect of the growth temperature on the morphology and phase of the 2D pyrite crystals was systematically investigated. Moreover, the vertical heterostructures of pyrite and graphene with clean and atomically sharp interfaces were synthesized by growing 2D pyrite crystals directly on CVD-grown graphene transferred to SiO2 substrates. Detailed characterization of the pyrite crystals and heterostructures was performed using several microscopy and spectroscopy methods, and the results are corroborated by ab-initio density functional theory (DFT) calculations.
3:15 PM - NM8.7.04
Control of Edge and Surface Chemistry in 2D Black Phosphorus and Oxides
Kaci Kuntz 1 , Rebekah Wells 1 , Jun Hu 1 , Teng Yang 2 , Huaihong Guo 3 , Adam Woomer 1 , Daniel Druffel 1 , David Tomanek 4 , Scott Warren 1 5 Show Abstract
1 Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States, 2 Shenyang National Laboratory for Materials Science, Institute of Metal Research Chinese Academy of Sciences, Shenyang China, 3 College of Sciences, Liaoning Shihua University, Fushun China, 4 Physics and Astronomy Department, Michigan State University, East Lansing, Michigan, United States, 5 Applied Physical Sciences, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States
Black phosphorus is an anisotropic 2D semiconductor with broadly tunable optoelectronic properties in the few-layer regime. The thickness-dependent band gap spans the infrared (0.3 eV in bulk form) and visible (~2.1 eV as a monolayer) spectra, implying this material has potential to be implemented in numerous applications. While the material oxidizes in air, understanding and controlling the surface chemistry of 2D black phosphorus can open pathways to passivate and functionalize the material. Here, we study the surface composition and spatial distribution of the oxide of 2D black phosphorus. We present evidence that the surface and edge chemistry of the material can be controlled by exposure to different classes of oxidant.
We studied 2D black phosphorus and its oxides after exposure to high-purity, dry O2 (99.9999%), high-purity H2O, or H2O/O2. The surface of the oxidized 2D material was characterized by X-ray photoemission spectroscopy (XPS) and further understood through simulated phosphorus oxide intermediates. We survey phosphorus oxide species through DFT, extending previous work (Phys. Rev. B, 92, 125412 (2015)) to include basal surface oxides as well as edge and defect oxides with phosphorus oxidation states ranging +1 to +5 and bond orders of 3 or 5. With evidence supporting the existence of the phosphorus oxide intermediates in XPS, we deconvoluted the experimental spectra using calculated binding energy shifts of the oxides and determine the surface composition of 2D material after each oxidant exposure. We find that as exposure time to oxidants O2, H2O, or H2O/O2 extends, the oxide composition progresses from low to high oxidation states.
Furthermore, a time-lapsed transmission electron microscopy (TEM) study allowed us to explore the spatial distribution of the oxide. High-purity, dry O2 leads to an oxide layer on the basal surface of 2D black phosphorus, while oxidation by H2O leads to physical degradation the material through etching at defect sites, such as edges and steps.
With an improved understanding of the composition and location of the phosphorus oxide species after exposure to oxidants O2, H2O, or H2O/O2, we introduce a model to understand different mechanisms of oxidation. We find that oxidation of 2D black phosphorus by O2 through a one-electron transfer (Nat. Mater., 14, 826-832 (2015)) is thermodynamically feasible and can promote oxidation on the basal surface of phosphorus, while oxidation by H2O through a one-electron transfer mechanism is not thermodynamically favorable at non-defect sites. Instead, H2O preferentially oxidizes defect-sites on 2D black phosphorus, which are more susceptible to degradation.
With this work, we show evidence for the site-selective oxidation of non-defect or defect sites of 2D black phosphorus by choice of oxidant. Controlling the surface and edge chemistry of this material opens opportunities to site-selectively engineer properties through passivation and/or covalent functionalization.
NM8.8: 2D Heterostructures
Wednesday PM, April 19, 2017
PCC West, 100 Level, Room 101 A
4:30 PM - *NM8.8.01
Excitons in van der Waals Heterostructures
Xiaodong Xu 1 Show Abstract
1 , University of Washington, Seattle, Washington, United States
Two-dimensional materials and their van der Waals heterostructures have recently developed into a powerful platform from which to explore the science of surfaces and interfaces. Here we present our latest experimental progress in understanding the interfacial effects on excitons in two types of van der Waals heterostructures. We first discuss the interlayer excitons formed at the interface between two different monolayer semiconductors, MoSe2 and WSe2. Through photoluminescence measurements, we reveal that these excitons possess valley pseudospin properties like their intralayer counterparts, but with enhanced lifetime and intriguing relaxation dynamics. We then introduce a new van der Waals heterostructure between monolayer WSe2 and an ultrathin ferromagnetic semiconductor, CrI3. Strong interfacial magnetic interactions have a dramatic effect on the WSe2 exciton valley properties. We demonstrate that basic optical studies on this type of heterostructure can provide rich information on the spin interactions in layered magnets.
5:00 PM - *NM8.8.02
The Effect of Substrates on Optical, Thermal, and Catalytic Functionalities of 2D TMDC Materials
Linyou Cao 1 Show Abstract
1 Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
It has been well recognized that substrates may affect the properties of atomically thin two-dimensional (2D) transition metal dichacogenide (TMDC) materials. However, the mechanism of the substrate effect and how much the substrate effect could be have remained unanswered. Here we have systematically studied the effects of substrates on the multiple functionalities of 2D TMDC materials, including optical, thermal, electrical, and catalytic. We find that substrates may affect the optical functionality by doping 2D TMDC materials and facilitating defect-assisted non-radiative recombination of the excitons in the materials. Substrates may also affect the thermal and catalytic functionalities of 2D TMDC materials with certain substrates being able to substantially promote the functionalities.
5:30 PM - NM8.8.03
The Hot Pick-up Technique for Batch Assembly of van der Waals Heterostructures
Bjarke Jessen 1 , Lene Gammelgaard 1 , Filippo Pizzocchero 1 , Jose Caridad 1 , Lei Wang 2 , James Hone 2 , Peter Boggild 1 , Timothy J. Booth 1 Show Abstract
1 , Technical University of Denmark, Copenhagen Denmark, 2 Mechanical Engineering, Columbia University, New York, New York, United States
The assembly of individual two-dimensional (2D) materials into van der Waals (vdW) heterostructures enables the construction of layered three-dimensional materials with desirable electronic and optical properties. Several techniques exist to fabricate vdW heterostructures of moderate- to high-quality. Techniques involving direct contact between two-dimensional crystals and chemicals and polymers are referred to as "wet" techniques, which are of a lower quality compared to "dry" techniques where the 2D materials remain pristine throughout stacking and fabrication.
In 2013 Wang et al. were able to use the strong vdW forces to directly bind graphene on SiO2 to hBN crystals, which could subsequently be "picked up", along with the graphene. In this way, the stacking of 2D crystals is done entirely through the vdW forces between clean crystal interfaces, yielding the highest quality for vdW heterostructures to date, and the first truly dry stacking technique. However, the pick-up technique lack versatility by being highly sensitive to the specific material conditions during stacking, along with being highly sequential in nature.
We present here a technique for the rapid batch fabrication of dry stacked van der Waals heterostructures, demonstrated by the controlled production of 22 mono-, bi- and trilayer graphene stacks encapsulated in hexagonal boron nitride (hBN), all from a single batch. We find that blisters of trapped interfacial contamination commonly observed in such samples by optical and atomic force microscopy can be completely eliminated by stacking individual 2D crystals into vdWs heterostructures at elevated temperatures (typically 80-110°C) even in ambient atmosphere.
By actively tuning the interfacial adhesion and cleanliness through temperature whilst completely avoiding any contact with liquids in the stacking procedure, we are able to controllably pick-up and drop-down 2D materials, including single-layer crystals that have been pre-patterned using electron-beam lithography and exfoliated on plasma-treated SiO2. This method enables us to produce a statistically significant data set of field effect mobility measurements from 22 mono-, bi- and trilayer encapsulated graphene devices with >280 contacts. Seven of the 16 monolayer devices and 55% of the measurements display carrier mean-free paths comparable or exceeding the channel width, with carrier mean-free paths limited by boundary scattering. Bi- and trilayer devices show diffusive behaviour with average mobilities above 20,000 and 12,000 cm2v-1s-1. No annealing at high temperatures is necessary to obtain this high performance.
5:45 PM - NM8.8.04
Epitaxial Growth and Characterisation of Graphene Heterostructures on SiC
Jonathan Bradford 1 , Mahnaz Shafiei 1 2 , Josh Lipton-Duffin 1 , Jennifer Macleod 1 , Nunzio Motta 1 Show Abstract
1 Institute for Future Environments, School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, Queensland, Australia, 2 Faculty of Science, Engineering and Technology, Swinburne University, Hawthorn, Victoria, Australia
Graphene has attracted a great deal of interest due to its remarkable electronic, optical and mechanical properties. Future applications in nanoelectronics will depend critically on the development of novel approaches to introducing a bandgap while preserving carrier mobility. In this work we explore two different heterostructures of graphene grown epitaxially on 6H-SiC (0001); van der Waals epitaxy of transition metal dichalcogenides (TMDs) on graphene/SiC, and in-plane heterostructures of graphene and hexagonal boron nitride (h-BN).
Stacked layers of graphene and MoS2 have previously been demonstrated to exhibit exceptional optoelectronic properties since graphene’s high carrier mobility and broad spectrum absorption is complemented by high optical absorption of monolayer MoS2 owing to its direct bandgap . MoS2 and WS2 layers have been grown directly on epitaxial graphene/SiC by CVD from MoO3, WO3 and S precursors. This allows wafer-scale and transfer-free growth with a high quality interface on a device-ready substrate. We have conducted a systematic investigation of the growth parameters for TMDs on graphene/SiC in order to develop an understanding of the domain size, areal density and thickness of the MoS2 and WS2 domains on the surface which have been characterised by AFM, Raman spectroscopy, XPS and STM. Further research is in progress to realise device applications for TMD/graphene/SiC van der Waals epitaxy; for example, gas sensitive phototransistors for sensing applications.
In-plane heterostructures of graphene and h-BN have been predicted to allow tuning of the bandgap and carrier mobility according to the carbon concentration . Such hybrid structures have previously been synthesised by CVD on metal foils, and patterned films with controlled domain shapes and sizes have been demonstrated using photolithography/reactive ion etching followed by a second growth . In this research we propose the synthesis of lateral graphene/h-BN heterostructures on 6H-SiC (0001) using a chemical conversion method from ammonia (NH3) and boric acid (H3BO3) precursors. The reaction nucleates at a defect or functionalised carbon atom with the preferential substitution of a nitrogen atom from which the growth extends, and the concentration of h-BN can be controlled by the reaction time . This mechanism will be exploited to develop a mask-free patterning technique using a focused ion beam to pattern defects and provide control over the nucleation sites.
1. Zhang, W., et al., Ultrahigh-Gain Photodetectors Based on Atomically Thin Graphene-MoS2 Heterostructures. Scientific Reports, 2014. 4: p. 3826.
2. Wang, J., et al., Widely Tunable Carrier Mobility of Boron Nitride-Embedded Graphene. Small, 2013. 9(8): p. 1373.
3. Liu, Z., et al., In-plane heterostructures of graphene and hexagonal boron nitride with controlled domain sizes. Nature Nanotechnology, 2013. 8(2): p. 119.
4. Gong, Y., et al., Direct chemical conversion of graphene to boron- and nitrogen- and carbon-containing atomic layers. Nature Communications, 2014. 5.
NM8.9: Poster Session II: 2D Materials Beyond Graphene
Thursday AM, April 20, 2017
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - NM8.9.01
Defect-Mediated Photoluminescence Up-Conversion in Cadmium Sulfide Nanobelts
Yurii Morozov 1 , Masaru Kuno 1 Show Abstract
1 , University of Notre Dame, Notre Dame, Indiana, United States
The concept of optical cooling of solids has existed for nearly 90 years ever since Pringsheim proposed a way to cool solids through the annihilation of phonons via phonon-assisted photoluminescence (PL) up-conversion. In this process, energy is removed from the solid by the emission of photons with energies larger than those of incident photons. However, actually realizing optical cooling requires exacting parameters from the condensed phase medium such as near unity external quantum efficiencies as well as existence of a low background absorption. Until recently, laser cooling has only been successfully realized in rare earth doped solids.
In semiconductors, optical cooling has very recently been demonstrated in cadmium sulfide (CdS) nanobelts as well as in hybrid lead halide perovskites. For the former, large internal quantum efficiencies, sub-wavelength thicknesses, which decrease light trapping, and low background absorption, all make near unity external quantum yields possible. Net cooling by as much as 40 K has therefore been possible with CdS nanobelts.
In this study, we describe a detailed investigation of the nature of efficient anti-Stokes photoluminescence (ASPL) in CdS nanobelts. Temperature-dependent PL up-conversion and optical absorption studies on individual NBs together with frequency-dependent up-converted PL intensity spectroscopies suggest that ASPL in CdS nanobelts is defect-mediated through involvement of defect levels below the band gap.
9:00 PM - NM8.9.02
Stacking of CVD-Grown Single Layer MoS2 to Graphene for the Reduction of Schottky Barriers
Chun-Yu Huang 1 , Sahar Naghibi 1 , Edwin Preciado 1 , Brandon Davis 1 , Ariana E. Nguyen 1 , Ludwig Bartels 1 Show Abstract
1 , University of California, Riverside, Riverside, California, United States
We present centimeter-scale, ambient pressure chemical vapor deposited (CVD) graphene film on copper with a heat-free direct-transfer onto compatible substrates for lithographic pattening. CVD MoS2 islands are grown on top of patterned graphene to create heterostructure stacks. In a heterostructure stack, graphene acts as a semi-metal contact for injecting carriers into the MoS2 reducing the contact barrier known to exiest in MoS2-based devices. The MoS2/graphene interface provides low contact resistance and leads to superior transport measurements.
9:00 PM - NM8.9.03
Aging Effects and Environmental Stability of Anisotropic GaTe Nanomaterials
Sijie Yang 1 , Hui Cai 1 , Bin Chen 1 , Sefaattin Tongay 1 Show Abstract
1 , Arizona State University, Tempe, Arizona, United States
The emerging layered two dimensional materials have been the center of interest since their discovery. A category of these 2D materials which possesses low in-plane symmetry, such as black phosphorus and rhenium disulfide, shows strong anisotropy in their electrical and optical properties. One of these anisotropic 2D materials, gallium telluride (GaTe), has a direct bandgap at ~1.65 eV with thickness from few-layer to bulk. Its unique electrical properties, along with anisotropy, make GaTe a good candidate for possible photonic devices in which polarized light is involved. In this talk, we will present our most recent studies on their environmental stability and physical properties of GaTe under different ambient conditions. TEM, XPS, Raman, PL, and polarization resolved spectroscopy measurements will be dicussed to access unusual changes in their anisotropic properties as well as optical response. Overall, this talk will provide a valuable information on how anisotropic materials respond to environmental factors and the nature of these changes.
Key words: Layered two dimensional materials, Raman spectroscopy, anisotropy, environmental effects
9:00 PM - NM8.9.04
Synthesis of Wafer Scale with Phase-Controlled 1T’ and 2H Atomic Molybdenum Ditelluride Layers
Juhong Park 1 , Minsu Kim 2 , Jeongyong Kim 2 , Wonbong Choi 1 Show Abstract
1 , University of North Texas, Denton, Texas, United States, 2 , Sungkyunkwan University, Suwon Korea (the Republic of)
The controlled phase of wafer scale thin molybdenum ditelluride (MoTe2) with thickness variations is significant for its various applications. In this work, we employed two-step magnetron sputtering-chemical vapor deposition (CVD) methods to synthesize thickness varied both 1T’ and 2H MoTe2 films on SiO2/Si substrate. The thicknesses of MoTe2 film were controlled by sputtering time of MoTe2 target. In CVD process, relative low tellurization temperature (600 oc) is applied, and phase is determined depending on time of applying thermal energy. The surface morphologies and thicknesses of the MoTe2 films were measured by atomic force microscopy (AFM), and Raman spectroscopy was utilized to characterize the specific phase of MoTe2 films. Besides, we used photoluminescence at low temperature (77 k) to analyze optical band gap properties. The phase engineering of atomic MoTe2 films (2H and 1T’) can be used to modulate the electronic properties
9:00 PM - NM8.9.05
Two-Dimensional Materials as Reinforce Particles in Health Monitoring Composite Sensors and Various Applications
Jorge Catalan 1 , Anupama Kaul 1 Show Abstract
1 , University of Texas at El Paso, El Paso, Texas, United States
Composite materials have shown their significance in various applications including tennis racquets, car tires, furniture, and aerospace components. These “hybrids” assist in retaining the properties of two different materials when combined. Excellent structural properties can be obtained by selecting the right materials when combined in proper proportions. These properties make composite materials suitable for construction of aircraft and automobile design, which aids in reducing the cost and weight of components in these industries. Recently, two-dimensional materials have gained a lot of attention since the discovery of graphene in the early 2000s. This has opened a new window for semiconductor materials like MoS2, WS2, WSe2, among other transition metal dichalcogenides that show novel properties in their monolayer form. In this paper, we have mainly focused on graphene/graphite, MoS2, and WS2 as reinforcement material in different polymer matrixes such as: Polymethyl methacrylate (PMMA), polydimethylsiloxane (PDMS), and nanocellulose. In the case of PMMA composites, we found that graphene/graphite provide good electrical conductivity and small additions of WS2 or MoS2 improves the flexibility of the composite. These small additions of MoS2 and WS2 might lower the ductile transition temperature of the composite, reducing the temperature at which the material can be 3D printed into different types of sensors. PDMS composites allow us to design shapeable electromechanical sensors that are non-toxic, thereby permitting the fabrication of wearable components that benefit the health-monitoring field. Nanocellulose was used to achieve better stability and uniformity in the fabrication on high sensibility strain sensors.
9:00 PM - NM8.9.06
Impact of the Functionality of Perovskite-Based Nanosheets on Their Optical Properties
Sara Akbarian-Tefaghi 1 , Anamika Poduval 1 , Paul Renquet 1 , Taha Rostamzadeh 1 , Clare Davis-Wheeler 1 , John Wiley 1 Show Abstract
1 Department of Chemistry and Advanced Materials Research Institute, University of New Orleans, New Orleans, Louisiana, United States
The optical properties of a series of organically modified layered perovskites have been investigated. Initially, two-dimensional nanosheets were prepared by the exfoliation of bulk samples of double- and triple-layered Dion-Jacobson type perovskites HPrNb2O7 and HLaCaNb2MnO10, respectively. The perovskites were initially exfoliated in an aqueous solution of tetra(n-butyl)ammonium hydroxide (TBAOH) before being functionalized by a series of organics with terminal hydroxyl or amine groups. Rapid production of this series of products was possible via microwave-assisted reactions where all of the modification steps could be completed within an hour. The optical properties, absorbance and emission behavior, of the various hybrid nanosheets were then studied as a function of the organic surface groups. Here we report the variation in optical response as a function of host and surface groups for both dispersed nanosheets and reassembled nanocomposite thin films. While the dispersed systems tended to exhibit similar properties, nanocomposite films showed a wider variation as a function of substituent. Engineering the host and surface groups of such oxide nanosheets, especially when utilizing a rapid, efficient synthetic approach, will allow for the effective development of novel 2D materials with targeted optical properties.
9:00 PM - NM8.9.07
Tuning Electronic Properties of Layered Tin Dichalcogenides via Electron-Beam Induced Transformations
Mahdi Ghorbani Asl 1 , Eli Sutter 3 , Yuan Huang 4 , Hannu-Pekka Komsa 2 , Arkady Krasheninnikov 1 2 , Peter Sutter 5 Show Abstract
1 Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Dresden Germany, 3 Mechanical and Materials Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska, United States, 4 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York, United States, 2 Department of Applied Physics, Aalto University, Aalto Finland, 5 Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska, United States
Using in situ transmission electron microscopy and first-principles calculations, we studied electron beam induced structural transformations of layered tin dichalcogenides. We identified possible transformation pathways (based on defect production) for structural conversion from rhombohedral layered SnS2(Se2) to highly anisotropic orthorhombic layered SnS(Se). It has been found that all bulk structures are indirect-gap semiconductor where the gap values increase from bulk to monolayers due to the quantum confinement and become direct in the case of SnS and SnSe. The average effective electron mass is lower for SnS(Se) in comparison to SnS2(Se2) and Sn2S3(Se3) suggesting high electron mobility in this material. Our quantum transport calculations indicate strong anisotropy in the electron conductance through Sn2S3(Se3), while SnS2(Se2) has almost isotropic in-plane conductivity. The anisotropic conductivity can originate from the anisotropic atomic arrangement, which leads to different transmission pathways in the material. These findings could be helpful for the usage of tin dichalcogenides in different electronic applications.
9:00 PM - NM8.9.08
Facile Synthesis of TiO2 QDS Decorated on Monolayer WS2 Nanohybrids with Enhance Gas Sensitive for Ammonia Detection at Room Temperature
Ziyu Qin 1 , Dawen Zeng 1 Show Abstract
1 State Key Laboratory of Materials Processing and Die & Mold Technology, Huazhong University of Science and Technology (HUST), Wuhan, Hubei, China
Tungsten disulfide (WS2) nanosheets, as a representative layered transition metal dichalcogenides (TMDS), are expected as a promising candidate for high-performance NH3 sensor at room temperature. However, the lower sensitivity of monolayer WS2 nanosheet severely limits its application. Hence, a new nanohybrid of few- or mono- layer WS2 nanosheets decorated with TiO2 quantum dots (QDs) (TiO2/WS2) by S-O-Ti bonding are reported. The gas-sensing studies revealed that the nanohybrids exhibited enhanced sensitivity to NH3 at room temperature compared to the bare WS2. Under the same concentration of NH3 (500 ppm), the response intensity TiO2/WS2 nanohybrids (56.69%) extremely 18 times higher than monolayer WS2 (3.14%). Furthermore, the sensor shows a quick recovery at room temperature without any condition. When exposed to 250 ppm ammonia, its conductivity can recover to its initial states within 160 s, which is much shorter than other NH3 gas sensors based on transition-metal chalcogenides as reported. As a result, the TiO2/WS2 nanohybrids exhibit exciting high sensitivity, fast recovery, and excellent selectivity detect to ammonia gas (NH3) at room temperature. It suggests that QDS decoration significantly tunes the properties of WS2 nanosheets for various applications. We hope this work could facilitate us to explore more TMDS based gas sensors with even higher sensing performances.
9:00 PM - NM8.9.09
Rotational Superstructure in Self-Assembled C60 Monolayer on WSe2
Elton Santos 2 , Declan Scullion 2 , Ximo Chu 1 , Duo Li 1 , Nathan Guisinger 3 , Qing Hua Wang 1 Show Abstract
2 , Queen's University Belfast, Belfast United Kingdom, 1 , Arizona State University, Tempe, Arizona, United States, 3 , Argonne National Laboratory, Lemont, Illinois, United States
Hybrid systems of inorganic two-dimensional (2D) layered materials combined with organic molecules are emerging as promising components for flexible electronics and optoelectronics because they combine the 2D layers' diverse electronic properties, mechanical and chemical stability, and mechanical strength with the organic molecules' chemical tunability. There is a need to develop a fundamental understanding of how organic molecules interact with 2D materials, in particular in achieving heterostructures with high ordering and crystallinity. Here we report a combined experimental and computational study of a model system of well-ordered self-assembled monolayers of C60 molecules on the surface of WSe2. We elucidate the interfacial properties of this system using high resolution scanning tunneling microscopy (STM) and ab initio density functional theory including van der Waals (vdW) interactions. The STM images reveal that the C60 molecules self-assemble into a close-packed monolayer on the surface of WSe2, and exhibit four distinct intramolecular patterns in a 2x2 superlattice. High-throughput first-principles calculations show that only a few molecular configurations are energetically favorable for C60 arranged on WSe2 and the relative rotations of neighbouring molecules drives the different arrangements through charge reordering. The observed 2x2 superlattice is thus revealed as a rotational superstructure stabilized by the interaction of charges. Moreover, a systematic increase of the charge transfer between WSe2 and C60 is observed which points to the active role of the molecule-substrate interactions in the stabilization of the interface. These results highlight the intriguing properties of a model hybrid 2D/organic system with well-defined interfaces, thus paving the way toward vdW heterojunction-based hybrid 2D/organic devices.
9:00 PM - NM8.9.10
Spectroscopic Ellipsometry of Large-Area Tungsten Disulfide
Daniel Nezich 1 , Vladimir Liberman 1 , Steven Vitale 1 , Joseph Varghese 1 , Mordechai Rothschild 1 Show Abstract
1 Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts, United States
Variation of the optical constants of tungsten disulfide nanosheets grown by vapor phase exitaxy on sapphire is investigated using cross-wafer spectroscopic ellipsometry mapping. Regions of sub-monolayer, monolayer, and multi-layer tungsten disulfide coverage are identified through ellipsometric data analysis and verified by atomic force microscopy. The width and center wavelength of the exciton peaks extracted from ellipsometric data follow changes in the photoluminescence spectrum, with the A exciton being tunable by ~20 nm by varying growth conditions. The amplitude of the exciton peaks observed by spectroscopic ellipsometry varies only slightly, unlike the amplitude of the exciton peak observed by photoluminescence which is affected by gradients in the growth conditions across the wafer. The material models used for extraction of optical constants from spectroscopic ellipsometry data will be discussed, as well as the utility of spectroscopic ellipsometry for material characterization.
9:00 PM - NM8.9.11
Understanding the Role of Novel Mineralizers and Dopants in Improving Synthesis of Black Phosphorous Crystals
Sayan Sarkar 1 , Prashant Sarswat 1 , Michael Free 1 Show Abstract
1 , University of Utah, Salt Lake City, Utah, United States
In recent times, there has been an exponential growth of research interest in the synthesis of black phosphorus, the most stable allotrope of phosphorus and a fascinating member of the 2D materials family. Through polymorphic transformation, black phosphorus can be obtained from another form of phosphorus using gold, tin and tin-iodide as mineralizers which assist in the transport reaction but remain unreacted during the synthesis process. In our present study, we investigate the role of a new set of mineralizers (combination of metal and halogens) during the synthesis of black phosphorus. X-ray diffraction, Raman spectroscopy, and morphological examinations have been performed to evaluate the crystal quality, phase purity, and the degree of crystallinity of the black phosphorus. Initial results reflect a variable crystallinity in the XRD patterns, depending on the nature of the mineralizer and the experimental parameters. In addition, incorporation of boron doping during the synthesis results in the formation of powdered bulk black phosphorus crystals. Compared to the conventional ways of synthesizing black phosphorus like mercury catalysis, bismuth flux, and other high-pressure techniques, the new method offers a convenient, environment-friendly and relatively efficient way to produce black phosphorus.
9:00 PM - NM8.9.12
Formation and Properties of Nanoscale Origami Features on 2D Material Properties
Yi Ding 2 1 , Laurene Tetard 2 3 Show Abstract
2 NanoScience Technology Center, University of Central Florida, Orlando, Florida, United States, 1 Materials Science and Engineering, University of Central Florida, Orlando, Florida, United States, 3 Physics Department, University of Central Florida, Orlando, Florida, United States
Catalytic activation using metal-free systems is in high demand for industrial applications such as CO2 capture and conversion. To achieve sustainable and recyclable processes, heterogeneous catalysis should be possible on these metal-free systems, which remains a challenge. Monolayer hexagonal boron nitride (h-BN) and potentially other two-dimensional (2D) materials may offer interesting characteristics for catalyzing alcohol synthesis reactions. Recently, defects such as vacancies and dopants have been shown to significantly modify the electronic and chemical properties of 2D materials, to offer catalytically active sites for the conversion of synthetic gases (H2 and CO2) into higher alcohols.
Here we introduce defects created by heat treatments, resembling origami features. The phenomenon was observed in graphene, Molybdenum Disulphide (MoS2) and hexagonal boron nitride (h-BN). We will present a protocol for introducing and controlling origami features in the three materials. We will compare the characteristics of origami features formed on the three types of layers, including their structure, Raman signatures, mechanical properties and charge distribution. The measurements presented are carried out on an advanced Atomic Force Microscopy (AFM) platform including pulsed-force atomic force microscopy (PF-AFM), electrostatic force microscopy (EFM) and Lorentz contact resonance force spectroscopy (LCR) modules. Finally, we will discuss how these defects affect the catalytic activity of the materials for CO2 capture and production of higher alcohols.
9:00 PM - NM8.9.13
Intrinsic Photoconductivity of Few-Layered Transition Metal Dichalcogenides Phototransistors via Multi-Terminal Measurements
Nihar Pradhan 1 , Carlos Garcia 1 2 , Joshua Holleman 1 2 , Daniel Rhodes 1 , Saikat Talapatra 3 , Mauricio Terrones 4 , Stephen McGill 1 , Luis Balicas 1 Show Abstract
1 , National High Magnetic Field Lab, Tallahassee, Florida, United States, 2 Physics, Florida State University, Tallahassee, Florida, United States, 3 Physics, Southern Illinois University, Carbondale, Illinois, United States, 4 Physics, The Pennsylvania State University, University Park, Pennsylvania, United States
Recently, two-dimensional materials and in particular transition metal dichalcogenides (TMDs) were extensively studied because of their strong light-matter interaction and the remarkable optoelectronic response of their field-effect transistors (FETs). Here, we present the photoconductivity study from FETs built from few-layers of semiconducting transition metal dichalcogenides such as WSe2, ReS2, MoS2, MoSe2 etc. measured in a multi-terminal configuration under illumination by a 532 nm laser source. The photogenerated current was measured as a function of the incident optical power, of the drain-to-source bias and of the gate voltage. We observe a considerably larger photoconductivity when the phototransistors were measured via a four-terminal configuration when compared to a two-terminal one. For WSe2 FET, at laser power of 248 nW, we extract 18 A/W and ~4000% for the two-terminal responsivity (R) and the concomitant external quantum efficiency (EQE) respectively, when a bias voltage Vds = 1 V and a gate voltage Vbg = 10 V are applied to the sample. R and EQE are observed to increase by 370% to ~85 A/W and ~20000% respectively, when using a four-terminal configuration. Thus, we conclude that previous reports have severely underestimated the optoelectronic response of transition metal dichalcogenides, which in fact reveals a remarkable potential for photosensing applications.
9:00 PM - NM8.9.14
Precise, Layer-by-Layer Control of MoS2 Thickness and Properties via Thermal Vapor Sulfurization
John Robertson 1 , Xue Liu 1 , Jiang Wei 1 , Matt Escarra 1 Show Abstract
1 , Tulane University, New Orleans, Louisiana, United States
2D transition metal dichalcogenides (TMDCs), such as MoS2, possess high monolayer energy conversion densities and unique electronic properties that may lead to ultra-lightweight and flexible optoelectronic components for use in industries such as energy harvesting, transportation, aerospace, and consumer electronics. Although monolayer TMDC’s possess very high quantum efficiencies due to their direct band gaps, they are limited by their small absolute optical absorption and low charge carrier mobility caused by defects; these limits represent a major obstacle that hinders their technological application. New technologies are needed to navigate these constraints in order to engineer a material with optimal properties for specific applications. A thermal vapor sulfurization (TVS) method is here used to synthesize MoS2 with sufficient vertical resolution to create homogeneous, wafer-scale monolayer and few-layer films. The thickness of Mo precursor films was controlled and incremented at the angstrom scale to achieve layer-by-layer thickness control. A sharp 100-fold increase in photoluminescence at Mo thicknesses below 0.5nm has been observed, indicating an indirect-to-direct bandgap transition consistent with monolayer MoS2. A region of maximum carrier mobility also emerged between 0.7nm and 1.5nm precursor thickness. This study shows that the TVS synthesis method, if controlled properly, can yield high optical quality wafer-scale monolayer MoS2, thicker films with high optical absorption, and heterogeneous “transition” thicknesses that possess intermediate qualities. Ongoing work includes the investigation of doping, heterostructures, and hybrid structures from TVS synthesized 2D TMDC materials, in an ongoing effort to generate high quality 2D optical devices.
9:00 PM - NM8.9.15
Exfoliation of Quasi-Stratified Bi2S3 Crystals into Micron-Scale Ultrathin Corrugated Nanosheets
Rhiannon Clark 1 2 , Kourosh Kalantar-zadeh 1 , Torben Daeneke 1 Show Abstract
1 , RMIT University, Melbourne, Victoria, Australia, 2 Manufacturing, CSIRO, Melbourne, Victoria, Australia
There is ongoing interest in exploring new two-dimensional materials and exploiting their functionalities. Here, a top-down approach is used for developing a new morphology of ultrathin nanosheets from highly ordered bismuth sulphide (Bi2S3) crystals. This group V-VI semiconductor has recently attracted attention as an emerging functional material in nano morphologies. It has been shown to demonstrate a range of useful properties for applications in photodetectors, gas sensors and solar cells. The most common crystal structure of Bi2S3 is orthorhombic, with a layered structure of atomic-scale ribbons held together by van der Waals forces. This favours formation of one-dimensional nanostructures, with many previous reports of nanowires, nanorods and nanotubes. The efficient chemical delamination process, developed herein, exfoliates the bulk powder into a suspension of corrugated ultrathin nanosheets. Morphological analysis shows that the sheets can be as thin as 2.5 nm and can be as large as 20 μm across. The exfoliation process introduces sulphur vacancies into the sheets, with a resulting stoichiometry of Bi2S2.6. It is hypothesized that the nanoribbons are cross-linking during the synthesis, resulting in corrugated nanosheets. Electrical measurements found the material to be highly p-doped, which is attributed to the substoichiometry. A near-linear response to elevated temperature, as well as a selective response to NO2 gas shows that the material is suitable for use in sensing applications. This is the first report showing the possibility of exfoliating planar morphologies of metal chalcogenide compounds from crystals that constitute van der Waals forces within the fundamental planes.
9:00 PM - NM8.9.16
Self-Assembly of 2D Phage-Selected Peptide Layers on MoS2 Surfaces
Jiajun Chen 1 2 , Enbo Zhu 3 , Hyunpil Boo 3 , Yu Huang 3 , James De Yoreo 1 2 Show Abstract
1 Department of Materials Science and Engineering, University of Washington, Seattle, Washington, United States, 2 Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington, United States, 3 Department of Materials Science and Engineering, University of California, Los Angeles, Los Angeles, Los Angeles, California, United States
Two-dimensional layered materials like MoS2 represent a unique class of material systems that possess novel electronic and mechanical properties. While the synthesis of these materials has largely relied on exfoliation processes and chemical vapor deposition, which are difficult to control and scale to large areas, ligand-controlled solution-based chemical synthesis offers an alternative pathway. One approach to achieving the latter is to exploit phage display techniques through which peptides can be selected that can bind to and control the growth of nanomaterial and thus serve as a guide for the development of more stable, non-peptide-based ligands and polymers. In our study, short peptides with seven amino acids, including tyrosine and phenylalanine, were selected for their ability to bind specifically to the (0001) face of MoS2. We used in situ AFM to investigate how the peptides assemble on that face and relate the assembly pathway and structure to the peptide sequence, as well as to the underlying peptide-peptide and peptide-substrate interactions. Our results show that selected peptides form ordered structures on the MoS2 surface directly. During the assembly process, two peptides first join to form 1.3nm × 5.8nm building blocks. These small units then pack closely to generate rows with a width of 4.1nm and aligning at 30° to the densest sulfur atom packing direction of MoS2 lattice. Rows pointing in the same orientation further produce larger domains of parallel rows with uniform spacing running in three equivalent crystallographic directions. The growth rate of these domains increases dramatically and non-linearly with peptide concentration. Moreover, high concentrations lead to smaller aspect ratios of the domains due to the even greater dependence of row nucleation rate on concentration. The tendency of row nucleation to occur immediately adjacent to existing rows also implies a preference for heterogeneous nucleation. In the early stages of assembly, domains aligned along lattice sites running 30° to the dominant row directions are present, but disappear over time, thereby demonstrating the ability of the peptides to reversibly bind and coarsen, as well as the higher stability of domains exhibiting the orientation that eventually dominates. High-resolution structural images and comparison between different peptide sequences imply that the existence and position of phenyl groups play an important role in surface binding and the inter-molecular interaction leading to assembly. These findings show that the structural relationship between the peptide and substrate is highly specific and argue for a true epitaxial relationship that dictates both the local order and the final macroscopic morphology. Our results also demonstrate the importance of the phenyl rings in establishing strong binding affinity and provide a basis for the design of non-peptide-based ligands for both controlling the growth of MoS2 and patterning the surfaces.
9:00 PM - NM8.9.17
Phase Transformation in Thin Films under Surface Heating and Convective Boundary Conditions
Rahul Basu 1 2 Show Abstract
1 , VTU, Bangalore India, 2 Mechanical, Adarsha Institute of Technology, Bangalore, Ka, India
Modification of thin surface films by focused energy sources is described with appropriate
idealizations. Convective boundary conditions which are appropriate for surfaces are applied
and coupled diffusion equations obtained with suitable approximations. The problem is
approached with different Stefan numbers and solutions are obtained theoretically as well as by
perturbation methods. The non linearity in the coupled problem introduces many complexities
and hitherto exact solutions have been unavailable in the literature. This work uses certain
transformations not published before in this area to obtain tractable solutions and obtain
stability benchmarks in terms of non dimensional parameters like the Stefan, Fourier and Biot
numbers. In particular, for the spherical heat source geometry work has been attempted by
several in the past. Linear ODE’s have been obtained from the set of partial coupled
differential equations describing mass and heat diffusion, which are then analysed easily.
Evaluation of the properties of the thermal boundary layer and attenuation of fluctuations
from the surface sources shows that under certain condition that the fluctuations can be
amplified. When a sinusoidal fluctuating source as in laser heating is used, the frequency can
be attenuated or amplified depending on the values of the thermal parameters. In particular,
oscillations in the nano region are possible to be sustained.
Keywords: Moving boundary, convection, perturbation analysis, surface films, phase
9:00 PM - NM8.9.18
Atomically-Layer Precision Controlled Synthesis and Characterization of cm-Scale Hexagonal Boron Nitride
Wei-Hsiang Lin 1 , Victor Brar 2 , Deep Jariwala 1 , Michelle Sherrott 3 , Wei-Shiuan Tseng 4 , Nai-Chang Yeh 4 , Harry Atwater 1 Show Abstract
1 Applied Physics, California Institute of Technology, Pasadena, California, United States, 2 Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin, United States, 3 Material Science, California Institute of Technology, Pasadena, California, United States, 4 Department of Physics, California Institute of Technology, Pasadena, California, United States
Hexagonal boron nitride (h-BN) is the most promising two-dimensional insulator, given its large band gap and low density of charged impurities in addition to being iso-structural and iso-electronic with graphene. However, while highly controlled synthesis of graphene has been achieved on sq.ft. areas, uniform atomic layer controlled synthesis of h-BN over large areas remains a significant engineering challenge. Here, we report the synthesis and atomic-scale characterization of h-BN films grown on Cu foils using a custom-designed hybrid low-and atmospheric pressure CVD system, yielding layer thickness control from 1 to 20 monolayers over cm2 size area. This degree thickness uniformity control is achievable because of our ability to precisely control the ammonia-borane (NH3-BH3) precursor flow rate, deposition temperature and pressure.
We have characterized these large-area h-BN films at both atomic and macroscopic length scales. Raman infrared spectroscopy indicate the presence of B-N bonds and linear dependence of thickness with growth time. X-ray photoelectron spectroscopy (XPS) indicate the film stoichiometry, and quantitative analysis of the B1s and N1s spectra indicates that the B/N atom ratio in our films was 1 ± 0.6% across the range of thicknesses. To investigate the atomic scale structure and electronic homogeneity, we have performed scanning tunneling microscopy (STM) on our CVD grown monolayer h-BN. Atomic resolution tunneling microscope images of monolayer h-BN film on Au substrates display both the atomic h-BN hexagonal lattice and a moiré superlattice between h-BN and Au. Electrical current transport in metal/insulator/metal (Au/h-BN/Au) heterostrcutures indicates that our CVD-grown h-BN films can act as excellent tunnel barriers and also possess a high value of the hard-breakdown field strength.
Our results suggest that the grown material is structurally, chemically and electronically uniform over cm2 areas thereby paving the way for its application as a dielectric in nanoelectronic and nanophotonic devices. In the end, we would like to understand the physic background of electron-phonon interaction on the in-plane graphene/h-BN heterostructures.
9:00 PM - NM8.9.19
High-Performance Hybrid Capacitors Based on Graphene and Carbon Sphere/Polyaniline/MnO2 Ternary Nanocomposites
Shifeng Hou 1 Show Abstract
1 , Shandong University, Jinan China
Carbon Sphere/Polyaniline/MnO2 (GS/PANI/MnO2) ternary nanocomposites are prepared as the electrode materials for supercapacitors via a simple chemical method. Polyaniline were firstly coated on the surface of canbon sphere by the in situ polymerization, while the introduction of MnO2 nanoparticles with a uniform size on GS-PANI surface facilitates the electrolyte ion penetration. The ternary nanocomposites are investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and field-emission scanning electron microscopy (FESEM). Herein, a high-performance capacitor was fabricated based on the GS/PANI/MnO2 as the cathode electrode and graphene as the anode electrode, showing an extended operating voltage window (1.8 V), good capacitance behaviour, and high energy density. The GS/PANI/MnO2 electrode also had excellent cyclic stability and showed only 12% decay capacity after 1000 cycles. The GS/PANI/MnO2 ternary nanocomposites showed high electroactivity and cyclic stability because of synergistic effects from each component. In this composite, PANI was uniformly coated on the surface of canbon sphere and was able to form conductive networks that electrically wired canbon sphere and MnO2 nanoparticles. Therefore, the ternary nanocomposites may pave the way to next-generation large-scale production of high performance hybrid supercapacitors.
9:00 PM - NM8.9.20
Cavity Ring-Down Spectroscopy Monitoring of Photochemistry in Monolayer 2D Polymer Films
Sean Casey 1 , Alissa Murray 1 , Daniel Murray 1 , William Thompson 1 , Benjamin King 1 Show Abstract
1 , University of Nevada, Reno, Reno, Nevada, United States
Normal incidence cavity ring-down spectroscopy (NICRDS) was used to probe the formation and dissociation reactions of a two-dimensional (2-D) polymer system. Cavity ring-down spectroscopy (CRDS) is an ultra-sensitive absorption technique which has been extensively used for detection of gas-phase species and has recently been extended to studies in condensed-phases. To date, this technique had not been used to study reactions in monolayers and, more specifically, 2-D polymers. Photo polymerization and depolymerization and thermal depolymerization were monitored using NICRDS to help characterize the poly(carboxy fantrip) 2-D polymer. In addition to using NICRDS, we developed a new method of CRDS which is comprised of dual cavity substrate probing, where two wavelengths of light are simultaneously used as probes. A probe at normal incidence geometry and a probe at the Brewster angle for fused silica overlap on the sample of interest. For our experiments, the Brewster angle probe served as an indicator for changes in the ring-down times for the monlayer thin film/optical flat system unrelated to the photochemistry of the 2-D polymer occurring over the course of the experiment. It was a useful check for substrate degradation during high temperature experiments.
9:00 PM - NM8.9.21
Large-Scale High Quality Single Crystal Growth of BiSbTeSe2 by Zone Melting with Bridgman
Kyu-Bum Han 1 , Akira Nagaoka 2 1 , Sukong Chong 1 , Jeffery Aguiar 3 , Heayoung Yoon 1 , Mike Scarpulla 1 , Vikram Deshpade 1 , Taylors Sparks 1 Show Abstract
1 , University of Utah, Salt Lake City, Utah, United States, 2 , Kyoto University, Kyoto Japan, 3 , Idaho National Laboratory, Idaho Falls, Idaho, United States
Three-dimensional topological insulators have been shown to exhibit a host of interesting quantum phenomena. In order to measure such phenomena it is essential to use single high-quality crystals. However, using either the conventional melting or the Bridgman method alone leads to impurities remaining in the crystal. This study motivated that a single large-scale uniform crystal of BSTS could be formed by combining the melting and the modified Bridgman methods. Extremely pure starting materials (5N) were necessary. These were prepared in vacuum ampoule at <10-6 torr after flushing the quartz tubing with argon gas. To produce the single high-quality crystal, the sample was converted into polycrystal, which was then utilized to make the single crystal. To produce the polycrystal, the amouple was sintered in a vertical furnace at 850oC for 24 hours. Then, the single crystal was synthesized from the polycrystal using the modified Bridgman technique with zone melting at 850oC and 4 mm/hr. In order to generate a control crystal for comparison, the conventional method of crystal growth was used. The XRD demonstrated that the crystal produced by the two-step method had a narrower full-width half maximum (FWHM). The orientation and stoichiometry of sample surface were measured by TEM. The conventional crystal exhibited a defective structure in the optical image, while the two-step crystal did not. Furthermore, electrical transport demonstrated Dirac point was on zero gate voltage at low temperatures. In short, the two-step method yielded a single high-quality crystal through zone melting by eliminating impurities. This resulted in the improved electrical transport behavior.
9:00 PM - NM8.9.22
Three-Dimensional Electron Beam Microscopy of BSTS Topological Insulator
Kyu-Bum Han 1 , Eric Snyder 1 , Dean Collett 1 , Zijian Wang 1 , Jeffery Aguiar 1 2 , Brian Devener 1 , Vikram Deshpade 1 , Taylors Sparks 1 , Heayoung Yoon 1 Show Abstract
1 , University of Utah, Salt Lake City, Utah, United States, 2 , Idaho National Laboratory, Idaho Falls, Idaho, United States
Bi2-xSbxTe3-ySey (BSTS) is an interesting new topological insulator with applications ranging from THz optoelectronics to quantum computing. Recent theoretical calculations have suggested that BSTS has superior topological insulating (TI) properties. The large area crystallinity and stoichiometry of synthesized BSTS, however, presents a challenge to measure unique TI properties. In this work, we present a hybrid growth method of large BSTS single crystals (> 5 mm) and 3D characterization techniques of the materials.
The BSTS single crystals are tailored to achieve structural and stoichiometric uniformity throughout the material. These single crystals were synthesized in a vacuum ampule that started with randomly oriented polycrystalline starting BSTS materials. An optimized Bridgman and zone melting technique was then applied to the starting batch materials to grow high purity crystals at 850 °C with a translation rate of 4 mm/hour. BSTS single crystals were extracted from the different sections of the ampule to study in detail. In order to investigate the chemical and structural properties of the BSTS TIs from surface to bulk interior, we used two complementary electron beam based techniques including: 1) a series of energy dispersive X-ray (EDS) measurements on bulk crystals in a scanning electron microscope (SEM), and (2) cross-sectional EDS on BSTS lamella in a scanning transmission electron microscope (STEM). In the EDS/SEM technique, the electron beam was injected directly into the surface of the samples. As the electron beam energy increases from 5 kV to 30 kV, the probing bulb size and sampling volume also increases from 70 nm to 1.5 µm in BSTS. The EDS measurements were then compared to Monte Carlo electron beam simulations, and the results showed a significant difference in the composition of selenium (Se). In the high-resolution EDS/STEM of the cross-sectional lamella, we also measured a higher Se concentration at the surface (<50 nm) than deeper into the interior. The similarity between the EDS/SEM bulk measurements and the EDS/STEM cross-sectional measurements indicate that the BSTS material does indeed have a Se-rich surface. In parallel, we collected selected area electron beam diffraction and high-resolution atomic scale lattice images. The obtained data suggested that the surface of BSTS crystal is highly ordered. Aligning both EDS and atomic resolution imaging furthermore resolved Se and Te alternation and subsequent build-up at the interface between the surface layer and the underlying single crystals. Overall, our results clearly delineate the necessary structural and chemical insights to further tailor and innovate topological insulators for a variety of properties, including emerging growth strategies to realize class leading 2D materials.
9:00 PM - NM8.9.23
Flexible 2D Organic–Inorganic Hybrid Thin Films for Band-Selective Photodetection
Dhinesh Babu Velusamy 1 , Fan Zhang 1 , Husam Alshareef 1 Show Abstract
1 , KAUST, Jeddah Saudi Arabia
The photo-current conversions of MoS2 nanosheets are unprecedentedly impressive, making them great candidates for next-generation visible light photodetectors. However, the large dark current and limited spectral selectivity in the visible region have restricted their potential practical application in broadband photodetection. Herein, we present a method for fabricating micron-thick, flexible films consisting of inorganic MoS2 and organic g-C3N4 nanosheets for band-selective photodetection from UV to Visible region. Simple but robust solution mixing of MoS2 and g-C3N4 offers an extremely convenient route to controlling their composition in the hybrid films and thus allows for tuning the optoelectronic properties. Notably, our methodology not only decreases the dark current of the MoS2 but also broaden the spectral selectivity to UV region. Our process provides quantitative control over composition of the hybrids, without affecting the structural integrity, while allowing tuning of the photodetection characteristics. The photodetection performance of hybrid thin films with different compositions were examined at both UV and Visible light illumination. For the 5:5 ratio hybrid films, the maximum ION/IOFF ratio, responsivity and detectivities are ∼4x103, 700 mA/W and 8 × 1010 Jones, respectively, under the illumination of 532 nm light, and they increase to ∼1x104, 4 A/W and 4 × 1011 Jones, respectively, under the illumination of 365 nm light. Fabrication of the hybrid films on the mechanically flexible conventional filter papers were additionally offer promising photodetection performance under repetitive bending deformation. The initial ION/IOFF ratio values of the hybrid films were rarely altered as a function of the bending radius. The values were maintained even at the bending radius of approximately 2 mm, which demonstrate high mechanical flexibility of our hybrid thin films. The detailed charge transfer and separation processes at the interface and tunable photodetection of the hybrid films were confirmed by thorough investigation of photo-induced, carrier-relaxation dynamics elucidated with the femtosecond transient absorption spectroscopy. Flexible band-selective photodetectors described in the present study represent a promising direction toward the future low-cost and high-performance flexible optoelectronic and photosensitive thin film device applications with solution processed 2D nanosheets.
9:00 PM - NM8.9.25
Light Emission from InP/Graphene Hybrid Epitaxial Structures
Samik Mukherjee 1 , Nima Nateghi 1 , Robert Jacobberger 2 , Maria Mata 3 4 , Jordi Arbiol 3 4 , Toon Coenen 5 , Austin Way 2 , Michael Arnold 2 , Oussama Moutanabbir 1 Show Abstract
1 Department of Engineering Physics, Ecole Polytechnique-Montreal, Monteal, Quebec, Canada, 2 Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States, 3 , Institut de Ciencia de Materials de Barcelona, Bellaterra, Catalonia, Spain, 4 , Institut Catala de Nanociencia i Nanotecnologia, Bellaterra, Catalonia, Spain, 5 Center for Nanophotonics, FOM Institute AMOLF, Amsterdam Netherlands
Van der Waals epitaxy is exploited in this work to grow novel InP-Graphene hybrid structures. We present the structural and optical properties of these hybrid crystals, grown on SiO2/Si(100) or Ge(111). High resolution transmission electron microscopy in conjunction with electron energy loss spectroscopy was employed to study the composition and the structure of the as-grown crystals. The micron sized InP monocrystals, having a wide variety of shapes like flat hexagons and triangles, truncated pyramids, and even 1D nanowires, were found to have a coherent and relaxed interface with the graphene layer underneath, free of any extended defects at the heterointerface. The crystals however showed a zinc-blend wurtzite polytypism, a phenomenon which can be attributed to the ultra-fast atomic migration over the chemically inert graphene surface, resulting in extremely high growth rates. Excitonic luminescence from the quantum well, so formed due to the type-II band alignment between the closely-packed zinc-blend wurtzite structures, was recorded using room temperature cathodoluminisece. The directionality of the emitted light, investigated by angle resolved cathodoluminescent measurements provided additional optical characterization of the polytypic nature of InP/graphene crystals.
1 H. Ko, K. Takei, R. Kapadia, S. Chuang, H. Fang, P.W. Leu, K. Ganapathi, E. Plis, H.S. Kim, S. Chen, M. Madsen, A.C. Ford, Y. Chueh, S. Krishna, S. Salahuddin, and A. Javey, Nature 468, 286 (2010).
2 S. Nakamura, T. Mukai, and M. Senoh, Appl. Phys. Lett. 64, 1687 (1994).
3 S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, and Y. Sugimoto, Jpn. J. Appl. Phys. 35, L74 (1996).
4 Z. Yuan, B.E. Kardynal, R.M. Stevenson, A.J. Shields, C.J. Lobo, K. Cooper, N.S. Beattie, D.A. Ritchie, and M. Pepper, Science 295, (2002).
9:00 PM - NM8.9.26
Hierarchical Assembly of Molybdenum Trioxide 2D Sheets and Aluminum Nanoparticles
Naadaa Zakiyyan 1 , Rajagopalan Thiruvengadathan 1 , Haisheng Zheng 1 , Angi Wang 1 , Keshab Gangopadhyay 1 , Matthew Maschmann 1 , Shubhra Gangopadhyay 1 Show Abstract
1 , University of Missouri, Columbia, Missouri, United States
Growing interest in functionalization characteristics of two-dimensional (2D) materials for energy application has attracted researchers to pursue a better understanding of 2D material processing and assembly. This has led to our current work using molybdenum trioxide (MoO3) 2D sheets and aluminum nanoparticles (Al NPs) to study the nanomaterial interaction and assembling behaviors. An ultrasonication method has been developed to exfoliate layered MoO3 that produces mono- or few-layer nanoflakes to be used for the 2D assembly of the nanometer-size MoO3 sheets into micron-size structures without aggregation. Dynamic light scattering (DLS) particle size and zeta potential measure the growth of nano-size 2D sheets as a function of concentration and setting time of the nanosheets to shed light on the evolution of directed assembly kinetics and the parameters that affect the process. Our results indicate that the exfoliated MoO3 nanoflakes alone in a polar solvent have grown in the lateral direction through hydrophilic edge interaction. The weak interaction between the sheets prevents the formation of multilayer aggregates. Atomic force microscopy (AFM) data of drop-casted MoO3 nanosheets on SiO2 coated silicon substrates has shown that individual 20nm-nanoflakes of MoO3 can grow laterally by 0.5-1.0 um with the height of the clusters remains in few nanometers. When mixed with Al NPs in a polar solvent (e.g., isopropyl alcohol), the 2D MoO3 micro sheets assemble to form micron-size Al-MoO3 layered structures. Furthermore, electron microscopy has also shown that the Al-MoO3 structures can reach to 5-8 um in diameter. The functional groups at the edge of these MoO3 nanosheets are thought to play a significant role in the assembly of MoO3 nanosheets leading to potential hierarchical macroscopic structure formation. This work opens important implications for further research in the directed nanosheet assembly of 2D materials with metal nanoparticles into 3D structures which are of great interest for energy and sensor applications.
9:00 PM - NM8.9.27
Conjugated Polyelectrolyte/Graphene Heterobilayer Nanocomposites Exhibit Temperature Switchable Type of Conductivity
Viktor Brus 1 , Marc Gluba 1 , Cheng-Kang Mai 2 , Stephanie Fronk 2 , Joerg Rappich 1 , Norbert Nickel 1 , Guillermo Bazan 2 Show Abstract
1 , Helmholtz-Zentrum Berlin, Berlin Germany, 2 , University of California Santa Barbara, Santa Barbara, California, United States
Conjugated polyelectrolytes (CPEs) comprise an electronically delocalized backbone that is rendered soluble in high dielectric media through the incorporation of side groups bearing ionic functionalities . This combination of structural components yields soft materials capable of integrating the optoelectronic features of organic semiconductors with the ability of polyelectrolytes to modulate physical properties through electrostatic forces. CPEs have been used to control charge injection barriers in organic optoelectronic devices through interfacial phenomena in which electrostatic dipoles modify the effective work function of adjacent metallic electrodes . Doping preferences of the backbone can also be influenced by the choice of the charged group closest to the backbone. These doping preferences also extend to single walled carbon nanotube composites, where the cationic (anionic) CPE leads to n-type (p-type) nanocomposites, despite of using the same conjugated framework .
We observed that a sub-monolayer of CPE-PyrBIm4 on CVD-grown graphene forms a novel two-dimensional hybrid material that exhibits preferential transport of holes or electrons as a function of temperature. This effect is reversible. Our findings are supported by independent characterization techniques including Hall effect, Seebeck, field-effect transistors (FETs) and ultraviolet photoelectron spectroscopy (UPS) measurements.
The doping efficiency increases with an increase of the temperature at which the heterobilayers are annealed and with decreasing CPE-PyrBIm4 film thickness. The switching of the conductivity type of the thin CPE-PyrBIm4/graphene heterobilayer composite occurs when graphene is not strongly overcompensated. An additional field tuning was applied in order to improve the reproducibility of the switching of the conductivity type. Moreover, the conversion of the conductivity type with temperature is reversible. Doping mechanisms under consideration include charge transfer from electron rich structural units in the CPE-PyrBIm4 backbone and/or field-effect doping as a result of interfacial electrostatic effects from adjacent ionic functionalities. This effect shows the unique and complex nature of electrical properties of this novel heterobilayer hybrid organic-inorganic CPE-PyrBIm4/graphene nanocomposite material and enhances interest in further investigations both scientifically and within the context of possible technological applications.
 L. Bin, G. C. Bazan, Conjugated Polyelectrolytes: Fundamentals and Applications, Wiley, Weinheim, Germany 2013.
 F. Huang, L. Hou, H. Wu, X. Wang, H. Shen, W. Cao, W. Yang, Y. Cao, J. Am. Chem. Soc. 126 (2004) 9845.
 C.-K. Mai, B. Russ, S. L. Fronk, N. Hu, M. B. Chan-Park, J. J. Urban, R. A. Segalman, M. L. Chabinyc, G. C. Bazan, Energy Environ. Sci. 8 (2015) 2341.
9:00 PM - NM8.9.28
Understanding How Spatial Heterogeneity of Nanostructures Impacts the Optical and Electronic Properties of 2D Materials
Melinda Shearer 1 , Yi Zhang 1 , Leith Samad 1 , Robert Hamers 1 , Song Jin 1 Show Abstract
1 , University of Wisconsin-Madison, Madison, Wisconsin, United States
Understanding spatial heterogeneity in nanostructures is key for the development of 2D materials and their heterostructures for applications such as photovoltaics and optoelectronics. Size, shape, defects, and interfaces of these materials at the nanoscale can create great changes in their properties, and correlating these effects will be crucial for design of these materials for diverse applications. In this work, scanning probe microscopy techniques will be utilized and correlated with spectroscopic techniques to understand how changes in 2D material nanostructures impact their properties. Atomic force microscopy, kelvin probe force microscopy, and illuminated kelvin probe will provide nanoscale information about the topography, surface potential, and surface photovoltage of these materials. Combined with Raman spectroscopy and photoluminescence measurements, the effects of edges, interfaces, and changes in nanostructure shape on the optical and electronic properties will be discussed. By studying both individual nanostructures of 2D materials such as WSe2 and SnS2 as well as their heterostructures, we can begin to understand how even seemingly small changes in structure can greatly influence the local properties of these materials.
9:00 PM - NM8.9.29
Designing Novel 2D Materials and Heterostructures for Next-Generation Ultra Energy-Efficient Electronics
Jiahao Kang 1 , Kaustav Banerjee 1 Show Abstract
1 Department of Electrical and Computer Engineering, University of California, Santa Barbara, Santa Barbara, California, United States
Recently, there has been a flurry of activities to identify various two-dimensional (2D) layered materials with diverse electronic, optical, thermal and mechanical properties. The low dimensionality and pristine surfaces of these atomically thin 2D materials, such as graphene and transition metal dichalcogenides, makes them very promising for highly-dense and low-power electronics in the emerging paradigm of Internet of Things (IoT). However, the layered nature also gives rise to several key challenges in any 2D electronic device design, among which the interfaces with metal-contacts, dielectrics and dopants are crucial. Recently, we have identified the new physics beneath these issues and have established pathways to achieve low-resistance contacts, optimal dielectric environment and efficient doping techniques. Thereby, we have been able to exploit some of the distinctive properties of 2D layered materials and have demonstrated some uniquely designed 2D material systems and radically new ultra-energy-efficient device technologies.
For example, we have employed atomically-thin 2D materials as transistor channel materials to enable dimensional scaling of transistors without degradation of device electrostatics and overcome a fundamental power dissipation challenge. More specifically, by combining 3D bulk semiconductors and 2D semiconductors to form 3D-to-2D vertically stacked material systems, we have demonstrated a new class of tunnel transistors exhibiting sub-thermionic turn-on behavior with unprecedented lower leakage currents and power dissipation. Moreover, various new 3D-to-2D heterostructures have been experimentally identified, which can be exploited to build such tunnel transistors to achieve better device performances. On the other hand, significant improvement of tunneling current has been recently achieved by engineering the tunnel interface quality.
We have also developed unique intercalated 2D material systems with tunable electronic, thermal and mechanical properties to build new interconnects and passive devices that overcome some of the fundamental limitations of currently employed materials, such as copper. Particularly, we have demonstrated a radically new approach toward building on-chip inductors (that are crucial to all radio-frequency electronics) with unprecedented area-efficiency by uniquely exploiting the high conductivity and large carrier momentum relaxation time in graphene-based material systems, leading to extraordinary inductance density and quality factors compared to copper, which represents a significant achievement in analog and solid-state electronics.
These quantum-mechanically engineered 2D material systems are expected to lead to novel integration concepts and thus, have important implications for ultra-dense and low-power electronics for the IoT.
Peter Sutter, Univ of Nebraska-Lincoln
Nasim Alem, The Pennsylvania State University
Arkady Krasheninnikov, Helmholtz-Zentrum Dresden-Rossendorf
Alexander Weber-Bargioni, Lawrence Berkeley National Laboratory
J.A. Woollam Company, Inc.
RHK Technology, Inc.
NM8.10: Defects and Grain Boundaries in 2D Materials
Thursday AM, April 20, 2017
PCC West, 100 Level, Room 101 A
9:15 AM - NM8.10.01
Unraveling Hidden Defects and Unexpected Properties of Graphene—How Advanced TEM Contributes to Materials Development
Benjamin Butz 1 2 9 , Christian Dolle 1 2 , Florian Niekiel 1 2 , Erdmann Spiecker 1 2 , Konstantin Weber 3 , Bernd Meyer 3 , Daniel Waldmann 4 , Ferdinand Kisslinger 4 , Heiko Weber 4 , Sam Shallcross 5 , Christian Halbig 6 , Siegfried Eigler 7 6 , Colin Ophus 8 Show Abstract
1 Institute of Micro- and Nanostructure Research, Friedrich-Alexander-Universität Erlangen Nürnberg, Erlangen Germany, 2 Center for Nanoanalysis and Electron Microscopy (CENEM), Friedrich-Alexander-Universität Erlangen Nürnberg, Erlangen Germany, 9 Materials Science & Engineering, Stanford University, Stanford, California, United States, 3 Interdisciplinary Center for Molecular Materials (ICMM) & Computer Chemistry Center (CCC), Friedrich-Alexander-Universität Erlangen Nürnberg, Erlangen Germany, 4 Chair for Applied Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Germany, 5 Lehrstuhl für Theoretische Festkörperphysik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Germany, 6 Department of Chemistry and Pharmacy and Central Institute of Materials and Processes (ZMP), Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Germany, 7 Institut für Chemie und Biochemie, Organische Chemie , Freie Universität Berlin, Erlangen Germany, 8 Molecular Foundry, National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Graphene, its chemically modified derivatives as well as complex devices therefrom have widely been studied for multifarious applications for more than a decade. Nonetheless, this highly interdisciplinary field, interdigitating with research on other layered materials like the chalcogenides, bears unexpected electronic, optical, chemical and mechanical properties almost every day. To understand novel properties and prospectively tailor functionality for application, it is indispensable to correlate to the material’s local structure, chemistry and defects. The author will present astonishing examples, how state-of-the-art transmission electron microscopy (TEM), being highly sensitive to the material’s crystal and atomic structure, its chemistry and even its topography, may essentially contribute to current material and device development:
I) Linear magnetoresistance of SiC graphene[1,2]: Basal-plane partial dislocations, occasionally named “strain solitons”, introduce changes of the local stacking order in few-layer graphene and may consequently cause a unique degree of disorder like in SiC-grown graphene. Each dislocation’s strain field alters the local electronic properties. This explains the perfect linear magnetoresistance (linear dependency of in-plane resistance on magnetic field perpendicular to graphene sheet) up to field strengths of more than 60 T.
II) Nanoscale topography of freestanding graphene membranes: Novel dark-field TEM tomography allows for the non-destructive determination of the topography of huge membranes of graphene and other materials with lateral resolution down to a few nm2. Together with complementary TEM, typically employed to determine the membrane’s thickness, its crystal structure, defects like dislocations and unintended residuals, this provides comprehensive insight to understand, e.g., the mechanical properties (strain relaxation, plasticity, energy dissipation) of such thin membranes and resonators.
III) Graphene from highly intact oxo-functionalized graphene (graphene oxide): A scalable route will be presented to prepare larger amounts of huge and clean monolayer graphene flakes by wet chemistry. It bases on the synthesis of highly intact monolayer graphene oxide flakes from graphite and their gentle reduction, e.g., by an electron beam.
The authors acknowledge financial support by the German Research Foundation (DFG) within the frameworks of the SFB953 Synthetic Carbon Allotropes, grants no. BU 2875/2-1 and no. EI 938/3-1, and the Erlangen Cluster of Excellence 315 Engineering of Advanced Materials.
 B. Butz, C. Dolle, F. Niekiel, K. Weber, D. Waldmann, H.B. Weber, B. Meyer, E. Spiecker, Nature 505 (2014) 533-537
 F. Kisslinger, C. Ott, C. Heide, E. Kampert, B. Butz, E. Spiecker, S. Shallcross, H.B. Weber, Nat. Phys. 11 (2015) 650–653
 B. Butz, C. Dolle, C.E. Halbig, E. Spiecker, S. Eigler, Angew. Chem. Int. Edit. (2016), DOI: 10.1002/anie.201608377R1
9:30 AM - NM8.10.02
Chemical and Electronic Repair Mechanism of Sulfur Vacancies in MoS2 Monolayers
Sibylle Gemming 2 3 , Anja Foerster 1 4 , Gotthard Seifert 1 , David Tomanek 5 Show Abstract
2 , Helmholtz-Zentrum Dresden-Rossendorf, Dresden Germany, 3 Institute of Physics, Technische Universität Chemnitz, Chemnitz Germany, 1 Theoretical Chemistry, Technische Universität Dresden, Dresden Germany, 4 , Center for Advancing Electronics Dresden, Dresden Germany, 5 Physics and Astronomy Department, Michigan State University, East Lansing, Michigan, United States
Molybdenum disulfide (MoS2) monolayers have a high potential to be used in low-power electronics. However, the MoS2 monolayers contain sulfur vacancies that drastically influence the el. properties of MoS2 monolayers. Thiols (R-SH) possess the ability to repair these sulfur vacancies1.
Recently it was reported that instead of the sulfur vacancy repair process, a disulfide formation process occurs when thiols interact with MoS2 monolayers2. Therefore, we employ density functional theory (DFT) to analyze under which conditions the contradictory experimentally observed disulfide formation and competing sulfur vacancy repair mechanisms take place.
We find that in the presence of sulfur vacancies the vacancy repair reaction is dominant. However, if in addition to the sulfur vacancies also sulfur adatoms are present in the MoS2 monolayer then disulfide formations can be observed. The byproduct of the latter reaction is hydrogen sulfide (H2S). Because H2S is also a thiol, the second reaction step after the disulfide formation is the repair of sulfur vacancies through H2S. This means that regardless whether the formation of additional disulfides are observed, the interaction of thiols with defective MoS2 monolayers always leads to the repair of sulfur vacancies and therefore also to an electronic repair of the MoS2 monolayers.
In the second part, we study the effect of dipole induced el. fields. These dipole induced el. fields can be obtained by depositing a self-assembled monolayer (SAM) of fluoroalkyl chains on top of the 2D material. We demonstrate that at close distances to the SAM el. fields in the region of 107-1010 V/m can be reached. Therefore, SAM coating will enable to tune the charge transport in 2D materials.
Makarova et al., Phys Chem C, 2012,116, 22411-22416.
Chen et al., Angew. Chem. Int. Ed., 2016, 55, 5803-5808.
9:45 AM - *NM8.10.03
Mapping the Effect of Structural Defects in 2D Transition Metal Dichalcogenides
Sara Barja 4 6 , Sebastian Wickenburg 6 , Zhen-Fei Liu 6 , Yi Zhang 1 , Hyejin Ryu 6 , Miguel Ugeda 2 , Zhi-Xun Shen 3 , Sung-Kwan Mo 6 , Miquel Salmeron 6 5 , Feng Wang 5 , Michael Crommie 5 , D. Frank Ogletree 6 , Jeffrey Neaton 6 5 , Alexander Weber-Bargioni 6 Show Abstract
4 , Centro de Fisica de Materiales, San Sebastian Spain, 6 , Lawrence Berkeley National Lab, Berkeley, California, United States, 1 Nanjing University, National Laboratory of Solid State Microstructures, Nanjing China, 2 , CIC Nanogune, San Sebastian Spain, 3 SLAC National Accelerator Laboratory, Stanford Institute of Materials and Energy Sciences, Menlo Park, California, United States, 5 , University of California Berkeley, Berkeley, California, United States
Properties of two-dimensional transition metal dichalcogenides are highly sensitive to the presence of defects in the crystal structure. A detailed understanding of defect structure may lead to control of material properties through “defect engineering”. Here we provide direct evidence for the existence of isolated, one-dimensional charge density waves at mirror twin boundaries in single-layer MoSe2. Our low-temperature scanning tunneling microscopy/spectroscopy measurements reveal a substantial bandgap of 100 meV opening at the Fermi level in the otherwise one dimensional metallic structure. We find an energy-dependent periodic modulation in the density of states along the mirror twin boundary, with a wavelength of approximately three lattice constants. The modulations in the density of states above and below the Fermi level are spatially out of phase, consistent with charge density wave order. In addition to the electronic characterization, we determine the atomic structure and bonding configuration of the one-dimensional mirror twin boundary by means of high-resolution non-contact atomic force microscopy. Density functional theory calculations reproduce both the gap opening and the modulations of the density of states.
10:15 AM - *NM8.10.04
Pushing the Performance Limit of 2D Semiconductor Transistors
Xiangfeng Duan 1 , Yuan Liu 1 Show Abstract
1 Department of Chemistry and Biochemistry, California Nanosystems Institute, University of California, Los Angeles, Los Angeles, California, United States
Two-dimensional semiconductors (2DSCs) such as MoS2 have attracted intense interest as an alternative electronic material in the post-silicon era. However, the on-current density achieved in 2DSC transistors to date is considerably lower than that of silicon devices. It remains an open question whether 2DSC transistors can offer competitive performance. To achieve a high performance (high on-current) device requires (1) a pristine channel with high carrier mobility, (2) an optimized contact with low contact resistance and (3) a short channel length. The simultaneous optimization of these parameters is of considerable challenge for atomically thin 2DSCs since the typical low contact resistance approaches either degrade the electronic properties of the channel or are incompatible with the fabrication of short channel devices. Here I will first review different strategies that have been developed to optimize these factors, and discuss how we can combine these strategies together to achieve high performance 2DSC semiconductor transistors. In particular, we will discuss a unique approach towards high-performance MoS2 transistors using a physically assembled nanowire as a lift-off mask for creating ultra-short channel devices with pristine MoS2 channel and self-aligned low resistance metal/graphene hybrid contact. With the optimized contact in short channel devices, we demonstrate that a sub-100 nm MoS2 transistor can deliver a record a high on-current density comparing well with that of silicon devices, demonstrating the exciting potential of 2DSCs for future electronic applications.
NM8.11: Defects and Grain Boundaries
Thursday PM, April 20, 2017
PCC West, 100 Level, Room 101 A
11:15 AM - *NM8.11.01
Grain Boundaries and Dislocations in Graphene and other 2D Materials
Oleg Yazyev 1 Show Abstract
1 , Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne Switzerland
Grain boundaries and dislocations are intrinsic topological defects of polycrystalline materials, which inevitably affect their physical properties. In my talk, I will discuss the structure of topological defects in two-dimensional (2D) materials such as graphene and monolayer transition metal dichalcogenides . Special attention is devoted to the effect of topological defects on the electronic transport properties of these 2D materials.
I will first introduce a general approach for constructing dislocations in graphene characterized by arbitrary Burgers vectors and grain boundaries covering the complete range of possible misorientation angles. By means of first-principles calculations we address the thermodynamic properties of grain boundaries revealing energetically favorable large-angle configurations as well as dramatic stabilization of small-angle configurations via the out-of-plane deformation, a remarkable feature of graphene as a two-dimensional material . Both the presence of stable large-angle grain-boundary motifs and the out-of-plane deformation of small-angle configurations have recently been observed by scanning tunneling microscopy .
In the rest of my talk, I will focus on the electronic transport properties of polycrystalline 2D materials. Ballistic charge-carrier transmission across periodic grain boundaries is governed primarily by momentum conservation. Two distinct transport behaviors of such grain boundaries in graphene are predicted − either perfect reflection or high transparency with respect to low-energy charge carriers depending on the grain boundary periodicity . It is also shown that certain periodic line defect structures can be engineered and offer opportunities for generating valley polarized charge carriers . Beyond the momentum conservation picture we find that the transmission of low-energy charge carriers can be dramatically suppressed in the small-angle limit . Unlike graphene, monolayer transition metal dichalcogenides (MoS2 and alike) combine a two-valley electronic band structure with strong spin-orbit effects. The latter can be employed for creating spin-polarized currents and adds yet another conservation law in the electronic transport across regular defects such as the frequently observed inversion domain boundaries [7,8].
These results demonstrate that dislocations and grain boundaries dramatically affect the transport properties of 2D materials and can be used for engineering novel functional devices.
 O. V. Yazyev and Y. P. Chen, Nature Nanotechnol. 9, 755 (2014).
 O. V. Yazyev and S. G. Louie, Phys. Rev. B 81, 195420 (2010).
 Y. Tison et al., Nano Lett. 14, 6382 (2014).
 O. V. Yazyev and S. G. Louie, Nature Mater. 9, 806 (2010).
 J. H. Chen et al., Phys. Rev. B 89, 121407 (2014).
 F. Gargiulo and O. V. Yazyev, Nano Lett. 14, 250 (2014).
 A. Pulkin and O. V. Yazyev, Phys. Rev. B 93, 041419 (2016).
 O. Lehtinen et al., ACS Nano 9, 3274 (2015).
11:45 AM - NM8.11.02
Highly Sensitive and High-Speed Imaging of Grain Boundaries in Graphene by Transient Absorption Microscopy
Chen Yang 1 Show Abstract
1 Chemistry, Purdue University, West Lafayette, Indiana, United States
To achieve producing and characterizing 2D materials at large scale as well as controlling and exploiting unique local properties, we need methods that can quantitatively and rapidly analyze 2D materials with ability to reveal local properties, such as domain boundaries. In this presentation, we report a transient absorption (TA) imaging method for fast visualization, quantitative layer analysis of graphene and mapping of grain boundaries in ambient condition. Specifically, forward and backward imaging of graphene on various substrates was imaged with a speed of 2 µs per pixel. The TA intensity linearly increased with the layer number of graphene was used to map and analyze the layer. More significantly, Images created based on TA intensity and slow TA decay constant at each pixel both mapped the grain boundaries in graphene. We have further improved the imaging speed by line-illumination and paralleled heterodyne detection and achieved an unprecedented 1,000 frames per second imaging speed. Our method enabled time-resolved transient absorption imaging at a speed of 50 frames per second, which is 100 times faster than the state-of-the-art. Using this method, we demonstrated a real-time non-destructive characterization of graphene in conditions that mimic the roll-to-roll manufacturing process at a rolling speed of 0.3 m/min, which is unattainable otherwise. In addition, real-time TA imaging of graphene oxide in vitro with capability of quantitative analysis of intracellular concentration and ex vivo in circulating blood were demonstrated. These results suggest that TA microscopy is a valid tool for the study of graphene based materials. Our study opens new opportunities to image other 2D materials including polymer-based thin-films, in vivo medical diagnosis, as well as monitoring the dynamics of biomolecules in living systems.
12:00 PM - NM8.11.03
Joule Heating in Phase Change SnS2 Nanoflakes
Yu-Kai Wu 1 , Chiu-Yen Wang 1 Show Abstract
1 , National Taiwan University of Science and Technology, Taipei Taiwan
2D-layered metal dichalcogenides (LMDs) have received great attention due to their unique electrical and optoelectronic properties. However, seldom literatures elaborate about phase transformation induced by Joule heating on semiconductor. In this work, tin disulfide (SnS2) nanoflakes were synthesized on SiO2/Si substrates via CVD method. Various shapes such as triangle, truncated triangle, hexagon and semi-hexagon with lateral size of about 10 microns were obtained. Moreover, in-plane and vertical growth were observed simultaneously in a process. SnS2 nanoflakes with semi-hexagonal shape and vertical growth model were found to be thinner and have larger lateral size than others, which could demonstrate that both models of growth are dependent on vapor concentration. The adjustment of pressure, gas flow and amount of source verify our explanations. Composition was confirmed by energy dispersive x-ray spectroscopy and well-characterized by Raman spectrum and transmission electron microscope. SnS2 nanoflakes were further applied in back-gated field-effect transistors, which exhibited a carrier mobility of about 0.5 cm2 V-1 s-1. The electrical properties varied with temperature were also studied. Furthermore, the device was annealed with phase transformation from SnS2 to tin monosulfide (SnS) via Joule heating. The annealed nanoflakes changed its carrier type from n-type to p-type and displayed a porous surface with conductivity of 4×10-5 Ω-1cm-1. This study makes further understand on restriction of thermal effect of applied SnS2-based device.
12:15 PM - NM8.11.04
Defects by Design—Molecular Engineering for Flexible Electronics
Wenbi Lai 1 , Andrew Stroud 4 , Nicholas Glavin 3 , Abby Juhl 3 , Rajiv Berry 3 , Pedro DeRosa 4 , Emily Heckman 2 , Christopher Muratore 1 Show Abstract
1 , University of Dayton, Dayton, Ohio, United States, 4 Department of Physics, Louisiana Tech University, Ruston, Louisiana, United States, 3 , Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio, United States, 2 , Sensors Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio, United States
Synthesis of flexible semiconductors using low-cost, naturally abundant materials (e.g., MoS2) directly onto inexpensive polymeric materials promises manufacturing of flexible 2D devices at economically viable scales enabling use of their unique physics in grand challenge areas of energy, healthcare, and national security. Recently-proven approaches for low temperature 2D synthesis suitable for flexible substrates developed by the authors include growth of amorphous materials with subsequent photonic annealing to access crystalline domain sizes up to several microns on PDMS and other polymers. This approach has been demonstrated for synthesis of large area ultrathin monolithic layers as well as MoS2/WS2/BN multilayers with pristine interfaces, allowing interrogation of intrinsic properties of 2D materials and their heterostructures as they apply to diverse optoelectronic devices. Another novel synthesis approach is 3D printing using biomolecules for selective molecular absorption. Atomistic simulations are used identify biomolecules adhering to substrates such as SiO2 and also demonstrating selective adhesion to 2D MoS2 or graphene. The conformation of specific amino acids at the substrate/peptide/2D material interface are determined to guide peptide molecule selection and design. The simulations correlate well to experiment, where printed peptides demonstrate strong selective adhesion to individual 2D materials, allowing device fabrication via peptide printing. With this printing approach, crystalline domain sizes can also be tailored over the same 10-9 - 10-6 meter range of length scales independent of nozzle geometries and particle suspension viscosities as in conventional 3D printing of 2D material–based inks. Printed materials are also annealed to alter defect densities, and hence, properties and device performance.
12:30 PM - NM8.11.05
Directing Interlayer Exciton and Photocurrent Dynamics by Twisting and Stacking van der Waals Materials
Kyle Vogt 1 , Hiral Patel 1 , Feng Wang 3 , Sufei Shi 2 , Matt Graham 1 Show Abstract
1 Physics, Oregon State University, Corvallis, Oregon, United States, 3 Physics, University of California, Berkeley, Berkeley, California, United States, 2 Chemical Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
When 2D materials are stacked in lowest energy configuration, interlayer van der Waals forces provide only weak coupling making interlayer electrical conductivity and energy transfer difficult. Through single-grain time-space resolved ultrafast microscopy maps, we show how long-range interlayer electronic coupling can be selectively enhanced either by applying an E-field or twisting the layer stacking orientation. Considering first twisted bilayer graphene (tBLG), we recently discovered how stacking-angle tunable absorption resonances form strongly-bound exciton states as a consequence of the symmetrized rehybridization of constrained interlayer 2p orbitals.[1,2] Using two-photon photoluminescence and intraband-transient absorption microscopies, we now image the emission and exciton dynamics of single-grain tBLG. Upon resonant excitation, the formation of strongly-bound (450-550 meV) interlayer exciton states results in an increased carrier lifetime, and an enhanced photocurrent (PC) internal quantum efficiency (IQE). Unlike tBLG, bilayer of semiconducting 2D transition metal dichalcogenides (TMDs) has diffuse interlayer d-orbital overlap that restricts interlayer mobility and exciton transport. To enhance interlayer electronic coupling in TMDs, we apply an interlayer directed E-field, inducing electron-hole dissociation. Our lab  and others  measure that stacked WSe2 TMD devices can have both IQE >50% and fast (<50 ps) picosecond electron escape times. Using a first-principle kinetic master equation, our methods analytically extracts both the E-field-dependent interlayer escape velocity and rate-limiting exciton dissociation time. Remarkably our PC response function produces the same E-field-dependent electronic escape and dissociation rates for both the optical and PC addressed ultrafast measurements. As confirmation, the resulting ratio of the electronic rates accurately matches our overall WSe2 device IQE in the intensity limit of zero Auger recombination. Thus through time-space resolved microscopy, we now obtain a timeline selective to the interlayer electronic dynamics of TMDs and tBLG van der Waals materials. We believe that this ultrafast microscopy approach is well-suited to identify new and unexpected interlayer electronic states in emerging twisted and stacked 2D materials and devices.
 H. Patel, R. Havener, L. Brown, Y. Liang, L. Yang, J. Park, M.W. Graham, Nano Letters, 15, 5932-7 (2015)
 Y. Liang, R. Soklaski, S. Huang, M.W. Graham, R. Havener, J. Park, L. Yang, Phys Rev B, 90, 115418 (2014)
 M. Massicotte, P. Schmidt, F. Vialla, K. G. Schädler, A. Reserbat-Plantey, K. Watanabe, T. Taniguchi, K.-J. Tielrooij, F. H. L. Koppens, Nature Nanotechnology. 11, 42–46 (2016)
 K. Vogt, S. Shi, F. Wang, M. W. Graham, OSA Technical Digest, Ultrafast Phenomena, DOI:10.1364/UP.2016.UW2A.3 (2016)
12:45 PM - NM8.11.06
Long-Term Stability of Mechanically Exfoliated MoS2 Flakes
Prachi Budania 1 , David McNeill 1 , Neil Mitchell 1 , Mircea Modreanu 2 , Paul Hurley 2 Show Abstract
1 , Queen's University, Belfast United Kingdom, 2 , Tyndall National Institute, Cork Ireland
Two-dimensional semiconducting transition metal dichalcogenides (TMDs) have received attention as potential materials for fabricating ultrathin electronic devices, owing to their inherent bandgap properties which are absent in graphene. The atmospheric stability of semiconducting TMDs is a crucial parameter which could significantly affect the performance of TMD-based devices. In this work, decomposition of mechanically exfoliated MoS2 flakes is compared for storage in air and storage under vacuum. Optical microscopy and contrast difference measurements were used to identify changes in lateral dimensions and thickness of flakes. More detailed studies were carried out on selected samples using scanning electron microscopy (SEM) and Raman spectroscopy.
Significant changes in MoS2 flakes were observed for samples stored in air, whereas similar flakes underwent no change for samples stored in vacuum. The first changes in lateral dimensions for the samples stored in air were observed on ultra-thin 1-2 monolayer MoS2 flakes within 55 days of initial exfoliation. No changes were observed in the lateral dimensions of bulk MoS2 flakes (with thickness > 10 nm) on the same sample, but both bulk and thin flakes took on a speckled appearance due to the development of defective regions. The nature and evolution of these defects have been studied by SEM imaging over a number of months. Of particular interest was the concentration of the defects at the edges of flakes and observation of residual surface contamination marking the original edges of shrunken flakes. Raman spectroscopy was carried out on the affected and unaffected region of representative flakes. It was observed that the MoS2 Raman peaks at 383.4 cm-1 and 408.8 cm-1 were generally unchanged for bulk flakes but displayed a distinct shift in the affected regions of thin flakes. The combined evidence confirmed that the MoS2 flakes were decomposing over time due to air exposure, with the decomposition being fastest at the edges of the flakes.
The evident difference in results between control samples stored under vacuum and samples stored in air suggests that the storage conditions play a crucial role in the long-term stability of exfoliated MoS2 flakes. In contrast to previous reports , mechanically exfoliated MoS2 flakes in this experiment have shown faster decomposition under ambient conditions. Therefore, it is believed that along with the presence of sulphur vacancies in the TMD flakes, other significant parameters such as the substrate treatment prior to TMD transfer or the transfer technique itself may be responsible for increasing the oxidation rate of these two-dimensional TMD layers in an air ambient.
 J. Gao, B. Li, J. Tan, P. Chow, T.-M. Lu and N. Koratkar, ACS Nano, 10, 2628-2635 (2016).
NM8.12: Ion and Electron Beam Effects
P James Schuck
Thursday PM, April 20, 2017
PCC West, 100 Level, Room 101 A
2:30 PM - *NM8.12.01
Performance and First Results of the Cc/Cs Corrected SALVE Microscope for Imaging Low-Dimensional Electron-Beam-Sensitive Objects
Ute Kaiser 1 Show Abstract
1 Central Facility of Materials Science Electron Microscopy, Ulm University, Ulm Germany
We obtain structural and electronic properties of low-dimensional electron-beam-sensitive objects by analytical low-voltage aberration-corrected transmission electron microscopy following three main strategies:
(1) Theory and image processing: For exact calculation of high-resolution TEM images for low-Z materials at very low voltages (approx. below 60kV), the contribution of inelastic scattering must be taken into consideration [1,2]. Moreover we demonstrate the effect of non-linear contributions to the image contrast at low voltages.
(2) Sample preparation: We demonstrate our sample preparation methods to reduce radiation damage due to inelastic effects which involve the cleaning the substrate, sandwiching the objects in-between two graphene layers or embedding them into single-walled carbon nanotubes and/or exchange hydrogen by deuterium [3,4].
(3) Very-Low-voltage transmission electron microscope: Unfortunately, the resolution of spherical aberration-corrected transmission electron microscopy at an accelerating voltage less than 80 kV with a standard Schottky type or field-emission electron source is strongly limited by the chromatic aberration of the objective lens . We outline our new voltage-tuneable (20-80kV) spherical and chromatic aberration-corrected SALVE (sub-Angstroem low-voltage electron microscope) -TEM. We show that obviously, no damping of the contrast transfer function by chromatic aberration has to be considered in the image calculation routine, although image spread due to thermal magnetic field noise needs to be taken into account as a new source of contrast transfer damping. The envelope now results from image-spread and residual defocus . We present first results showing the exceptional electron optical performance; thus we demonstrated a resolution of only 15 times the wavelength at 40kV on the example of graphene and MoS2 allowing a more detailed insight into the defect structures . Moreover, we demonstrate the new capabilities for EFTEM imaging on the example of TiO2. We now can use an energy window of about 20 eV where the defocus changes by only about 2 nm. We show that lowering the energy of the electrons down to 20kV prevents transition metal clusters and molecule inside CNTs from electron-beam stimulated damage.
The application of our strategies in imaging and/or electron energy loss spectroscopy will be shown on different low-dimensional crystalline and amorphous objects and on polycyclic molecules .
 Z. Lee, H. Rose, O. Lehtinen, J. Biskupek, U. Kaiser, Ultramicroscopy 145 (2014), 3.
 Z. Lee, H. Rose, R. Hambach, P. Wachsmuth, U. Kaiser, Ultramicroscopy 134 (2013), 102.
 G. Algara-Siller, O. Lehtinen, A, Turchanin, U. Kaiser, Appl. Phys. Lett. 104 (2014) 153115.
 G. Algara-Siller, S. Kurasch, M. Segeti, O. Lehtinen, U. Kaiser, Appl. Phys. Lett. 103 (2013) 20310.
3:00 PM - NM8.12.02
Tuning Local Electronic Structure of Monolayer MoS2 through Defect Engineering
Shengxi Huang 1 , Yan Chen 2 3 , Xiang Ji 1 , Kiran Adepalli 3 , Xi Ling 1 , Mildred Dresselhaus 1 , Bilge Yildiz 3 , Jing Kong 1 Show Abstract
1 Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 New Energy Research Institute, South China University of Technology, Guangdong China, 3 Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Transition metal dichalcogenides (TMDs) emerge in recent years as a novel two-dimensional material, showing promising applications in electronics, photonics and energy. Among these TMD materials, molybdenum disulfide (MoS2) has shown several advantages as an electrochemical catalyst compared to the traditional Pt metal, such as natural abundance and reduced cost. In particular, the defects in MoS2 have been shown to be active sites for the hydrogen evolution reaction (HER), but the mechanism of the role that defects play in MoS2 HER reactivity has not been revealed. In this work, we perform a systematic study on the effect that MoS2 defects play on the electronic structure and electrochemical reactivity. Using chemical-vapor deposited monolayer MoS2 combined with thermal driving and ion irradiation, we fabricate monolayer MoS2 with different defect densities on various substrates. We found that the electronic state of MoS2 is sensitive to both substrates and defects, supported by our X-ray photoelectron spectroscopy, Raman and photoluminescence spectroscopies, and scanning tunneling microscopy/spectroscopy. We further found that the HER reactivity is enhanced with higher defect density for MoS2. Our findings provide useful guidance for defect engineering in MoS2 and show the potential application of such defect engineering in using MoS2 for a clean and effective energy source.
3:15 PM - NM8.12.03
Transition Metal Dichalcogenides under Ion Irradiation—From Defects to Atomic Structure Engineering
Mahdi Ghorbani Asl 1 , Silvan Kretschmer 1 , Arkady Krasheninnikov 1 Show Abstract
1 Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Dresden Germany
We study the effects of ion irradiation on suspended MoS2 monolayer (ML) by using molecular dynamics (MD) combined with density-functional theory (DFT) calculations. We systematically study the production of defects in a free-standing MoS2 ML under noble gas ions bombardment for a broad range of incident angles and ion energies and determine the probabilities of producing single Mo and S vacancies. By comparing MD trajectories and analytical models for binary collision, we identified both direct and indirect mechanisms for defect production. Our results demonstrate that a selective sputtering of S atoms from the upper or lower layer can be achieved by choosing ion energy and incidence angle. The probability of producing S vacancy from upper layer increases by tilting the ion beam from the normal direction. The results showed that the defects cross section for both S and Mo vacancy grows with ion mass while the values for S vacancy are much higher than Mo vacancy. We further show the possibility of producing stable mixed MoSX (X from group V or VII) compounds with different electronic properties using ion irradiation. These findings suggest a promising route for post-growth processing of these materials for engineering electronic devices.
3:30 PM - NM8.12.04
Nanoforging of Single Layer MoSe2 through Defect Engineering Using Focused Helium Ion Beams
Alex Belianinov 1 , Vighter Iberi 1 2 , Anton Ievlev 1 , Michael Stanford 1 , Ming-Wei Lin 1 , Xufan Li 1 , Stephen Jesse 1 , Sergei Kalinin 1 , Adam Rondinone 1 , David Joy 1 , Kai Xiao 1 , Olga Ovchinnikova 1 Show Abstract
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 , Proctor & Gamble, Mentor, Ohio, United States
With the recent advances in CVD-growth processes for high quality 2D materials, large scale fabrication has become routine. Monolayer molybdenum diselenide (MoSe2) has become a highly attractive candidate in the fabrication of functional electronic and optoelectronic devices due to its high electron mobility. In order to attain novel functionalities, it is critical to be able to directly tune and engineer defects in 2D materials with nanometer precision. Here, we demonstrate the use of a focused helium ion beam in a scanning helium ion microscope (HIM) in tailoring material functionality in MoSe2. Using correlated band excitation (BE) scanning probe microscopy, and photoluminescence (PL) spectroscopy, changes in the nanomechanical, electrical and optical properties of supported MoSe2 are highlighted. The approach to precise defect engineering demonstrated here opens up new opportunities for creating functional 2D optoelectronic devices that span the visible region.
This research was conducted at the Center for Nanophase Materials Sciences, which is a Department of Energy (DOE) Office of Science User Facility. L. L. was supported by the Eugene P. Wigner Fellowship at the Oak Ridge National Laboratory. M. M. was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division. The authors also thank David A. Cullen for his assistance during the STEM measurements.
O.S.O., V.I. and K.X. conceived and designed the experiments; X.L., M.W.L, and M.M.S. synthesized and prepared MoSe2 samples; V.I. and M.G.S performed SHIM and PL measurements; A.B. performed atom finding computation and analyzed BE data; S.J. wrote code for BE-KPFM analysis; V.I. and A.I. conducted BE experiments; L.L. and B.G.S. performed 14 DFT calculations, O.S.O., B.G.S., D.C.J., S.V.K. and K.X. supervised the project; V.I., L.L., A.B. and O.S.O. prepared the manuscript with comments from all authors.
3:45 PM - NM8.12.05
Phase Transitions in Two-Dimensional Transition Metal Dichalcogenides under Electron Beam
Silvan Kretschmer 1 , Hannu-Pekka Komsa 2 , Arkady Krasheninnikov 1 Show Abstract
1 , Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Dresden Germany, 2 Department of Applied Physics, Aalto University, Aalto Finland
Recently a phase transition from the hexagonal 1H to trigonal distorted 1T'-phase in two-dimensional (2D) MoS2 has been induced by electron irradiation . Using density functional theory calculations, we study the energetics of these stable and metastable phases when electric charge, mechanical strain and vacancies are present. Based on the results of our calculations, we propose an explanation for this phenomenon which is likely promoted by charge redistribution in the monolayer combined with vacancy formation due to electron beam and associated mechanical strain in the sample. We further show that this mechanism can be extended to other 2D transition metal dichalcogenides.
 Y.-C. Lin, D. O. Dumcenco, Y.-S. Huang, and K. Suenaga, Nature Nanotechnology 9, 391 (2014)
NM8.13: Spatially Resolved Probing of Heterogeneous Properties
Thursday PM, April 20, 2017
PCC West, 100 Level, Room 101 A
4:30 PM - *NM8.13.01
Nano-Optical Investigations of 2D Semiconductors at Length Scales That Matter
P James Schuck 1 Show Abstract
1 The Molecular Foundry, Lawrence Berkeley National Lab, Berkeley, California, United States
The emergence of two-dimensional (2D) monolayer transition metal dichalcogenides (ML-TMDC) as direct bandgap semiconductors has rapidly accelerated the advancement of room temperature, 2D optoelectronic devices. However, performance of the active ML-TMDC materials often falls below theoretical expectations. Overcoming such macroscopic limitations in lower-dimensional systems requires a nanoscale understanding of the materials. Near-field optical microscopy provides a route to explore material properties below the diffraction limit in 2D systems. However, nanoscale optical visualization and spectroscopy of inelastic light-matter interactions (such as nano-PL) in two dimensions constitutes a formidable challenge, necessitating approaches that confine optical excitation and collection without hindering spectral analysis. Here, I will describe the recent near-field imaging advances that lay groundwork for generally-applicable nano-optical studies of 2D materials. I will touch on the importance of near-field polarization in probing these materials, and will also highlight recent near-field results on TMDCs, where we and others have uncovered new optoelectronic regions and spatially-varying features that were hidden in prior optical studies. These findings have broad implications for the development of atomically thin transistors, quantum optical components, photodetectors and light-emitting devices.
5:00 PM - *NM8.13.02
Atomic Resolution Imaging and Spectroscopy of Low-Dimensional Materials with Interrupted Periodicities
Kazutomo Suenaga 1 Show Abstract
1 , AIST, Tsukuba Japan
Interrupted periodicities in low-dimensional materials largely govern their properties. Here in this presentation, we will show our progress to develop the characterization tools based on TEM/STEM to reveal the various atomic defects and their evolutions. Hetero-structures of various 2D layered materials, including boundaries, dopants, or defects, will be one of our main focused subjects. Some other examples about the defects in 1D materials will be also presented.
5:30 PM - NM8.13.03
Probing Interfaces, Hidden Charges and ns Time-Scale Nanoelectromechanics of 2D Materials via Ultrasonic SPM
Oleg Kolosov 1 , Nicholas Kay 1 , Konstantin Novoselov 2 , Benjamin Robinson 1 , Franco Dinelli 3 Show Abstract
1 , Lancaster University, Lancaster United Kingdom, 2 Department of Physics and Astronomy, Manchester University, Manchester United Kingdom, 3 , cCNR, Istituto Nazionale di Ottica (INO), Pisa Italy
Graphene and numerous other two-dimensional materials (2DM) possess unique mechanical, electronic and thermal properties making them ideal materials platform for variety of nanoelectromechanical sensors (NEMS), with static as well as high frequency time response .
Here we explore 2DM nanostructures using combination of scanning probe microscopy (SPM), ultrasonic vibrations and electrostatic interactions that reveal key nanomechanical and nanoelectromechanical properties of 2DM essential for the systems where the atomically thin layers are subjected to the flexural and normal stresses and electrical fields. We provide spatial maps of a 2DM’s buckling transition that significantly increases the sensitivity of NEMS sensors to applied stimuli, and show that it is directly linked to the local in-plane stresses in 2DM and their interaction with the substrate. Our analysis of stress in a few layer 2DM, elastically transversely isotropic material, and a complementing experimental study, indicate that stress propagation in the depth of the 2DM or it heterostructure is directly governed by the ratio of the out-of-plane Young modulus and the in-plane shear modulus. This explaining experimental observation of “ultrasonic transparency” of few layer graphene and MoS2 observed in ultrasonic force microscope (UFM) and allows to observe defects and structures under immediate surface of such materials.
We demonstrate that anisotropic properties of 2DMs allow exploration of local electrostatic interactions between the material and the substrate via nanomechanical actuation, revealing and mapping with nanoscale resolution the charges hidden under the layers of such materials . By using nonlinear detection of NEMS actuation in UFM  we then probe actuation of 2DM with pm resolution amplitude and ps time-scale sensitivity, comparing these with the theoretical analysis.
 J. S. Bunch et al, Science, 315, 490-493, (2007)
 Kay N.D et al, Nano Letters. 14(6) :3400, 2014.
 M. T. Cuberes et al, Journal of Phys D: App Phys, 33, 2347, (2000)
5:45 PM - NM8.13.04
Nanoscale Heterogeneity in Exfoliated WSe2 Probed by Correlated TERS, SKM and Photocurrent Mapping
Andrey Krayev 1 , Deep Jariwala 2 , Michelle Sherrott 2 , Harry Atwater 2 , Mauricio Terrones 3 , A. Edward Robinson 1 Show Abstract
1 , AIST-NT Inc, Novato, California, United States, 2 , California Institute of Technology, Pasadena, California, United States, 3 , Pennsylvania State University, University Park, Pennsylvania, United States
Transition metal dichalcogenides of molybdenum and tungsten have recently attracted significant attention due to their band gaps in visible part of the spectrum for optoelectronic device applications. The ability to isolate these materials down to a monolayer with direct band-gap makesTMDCs very attractive alternatives to graphene.
While a lot of investigation has been devoted to understanding of crystalline and electronic quality of TMDCs in devices, little is known about its spatial distribution. Conventional Raman spectroscopy and confocal Raman microscopy have proved to be useful tools in this regard , though the spatial resolution of these techniques is diffraction limited to a few hundred nanometers.Tip enhanced Raman scattering (TERS) provides spatial resolution down to few nanometers, along with increased sensitivity due to dramatic enhancement of the Raman signal by the plasmonic tip and is therefore a suitable technique to probe nanoscale heterogeneity in TMDC samples.
Here, we report observation of nanoscale heterogeneity in exfoliated WSe2 flakes on plasmonic Au and Ag substrates using a combination of spatial mapping with TERS, contact potential difference and conductance measurements with and without illumination. In TERS mapping of exfoliated WSe2 flakes, we observe the presence of domains with quite different Raman spectra compared to adjacent material. We also observe that while WSe2 demonstrates a resonant Raman response with 638nm excitation, the TERS spectra of these domains feature a single peak at around 250 cm-1, typical for non-resonant conditions. Distribution of these domains correlates extremely well with surface potential map, non-resonant areas being negatively charged compared to adjacent areas of WSe2 that demonstrate a resonant Raman response. We excluded the possibility of the non-resonant domains being a 1-T (metallic) phase, based on spatial conductivity measurements that show them to have lower conductivity than adjacent areas.
We further correlate the TERS maps with concurrently recorded photocurrent maps, where we observe that domains showing both resonant and non-resonant Raman response, generated significant photocurrent, but of opposite polarities. Based on this observation, we conclude that in exfoliated layers of WSe2, there exist nanoscale semiconducting domains with opposite doping types. This hitherto unobserved heterogeneity is therefore critical to optoelectronic device design and performance and presents an important finding for further consideration and investigation . Our results presented here show that cross-correlation of TERS with local conductivity, surface potential and photocurrent is a vital characterization technique for nanoscale inhomogenities in 2D semiconductors and devices.