Session E30: 2D Materials: Processing and Application

Paper and Circuits, only Atoms Thick

2D layered materials are like paper: they can be colored, stitched, stacked, and folded to form integrated devices with atomic thickness. In this talk, I will discuss how different 2D materials can be grown with distinct electrical and optical properties (coloring), how they can be connected laterally to form pattered circuits (stitching), and how their properties can be controlled by the interlayer rotation (twisting). We will then discuss how these atomically thin papers and circuits can be folded to generate active 3D systems.   

Enhanced Absorption and Diffusion Properties of Lithium on B,N,V$_{\mathrm{\mathbf{C}}}$\textbf{-decorated Graphene}

Systematic first-principles calculations were performed to investigate the adsorption and diffusion of Li on different graphene layers with B/N-doping and/or C-vacancy, so as to understand why\textbf{ }doping heteroatoms in graphene anode could significantly improve the performance of lithium-ion batteries. We found that the formation of single or double carbon vacancies in graphene are critical for the adsorption of Li atoms. While the N-doping facilitates the formation of vacancies, it introduces over binding issue and hinders the Li diffusion. The presence of B takes the excessive electrons from Li and N and reduces the energy barrier of Li diffusion on substrates. We perceive that these clear insights are crucial for the further development of graphene based anode materials for lithium-ion batteries.   

The role of vacancies in Li-intercalated bilayer graphene

Recently, there has been increased interest in graphene for device applications both on its own and stacked with other 2D materials. Doping graphene via adsorption of lithium can shift the Fermi energy in graphene, effectively changing it from a semimetal to a metal. As an alternative to adsorption and to prevent contamination of the rest of the device stack, lithium can be intercalated between two layers of graphene. In this work, we investigate, using density functional theory, the possibility for lithium to migrate through defects in the graphene lattice and the role that excess layers play on the migration barriers. We found that there is a significant increase in the energy barrier for the pass-through of lithium of 0.26 eV when lithium leaves a bilayer through a divacancy vs when it passes through a divacancy in a monolayer, suggesting that the presence of the second layer minimizes defect migration. Furthermore, the presence of asymmetry in the energy barrier for certain defect types suggests the possibility of using defects to fabricate thermally intercalated bilayers.   

Influence of dopants on the impermeability of graphene

The effects of N-dopants on the impermeability of few-layered graphene (FLG) grown on copper using chemical vapor deposition will be presented. The grain boundaries in FLG have minimal impact on their permeability to oxygen as they do not provide a continuous channel for gas transport due to high tortuosity. However, we experimentally show that the N-dopants in FLG display multiple configurations (viz., graphitic, pyridinic, and pyrrolic) that create structural imperfections to selectively allow gas molecules to permeate. A comprehensive array of tools including Raman spectroscopy, X-ray photoelectron spectroscopy, optically stimulated electron emission measurements, and density functional theory of N-doped FLG were used to elucidate the effects of dopant configuration on the impermeability of graphene. Oxygen was found to permeate through FLG with non-graphitic nitrogen dopants that create pores in graphene and oxidize the underlying Cu substrate while graphitic nitrogen dopants did not show any changes compared to the pristine form.   

Atomic Hydrogen Adsorption in Multi-Layer Graphene

Chemisorbed hydrogen is known to modify the electronic structure of graphene as well as induce magnetism. Here we investigate the adsorption behavior of atomic hydrogen in multi-layer graphene. X-ray reflectivity was measured on epitaxial graphene ($\sim$ 25 layers) on C-face SiC over an extended range that includes three orders of Bragg reflections from the graphene layers. Pristine samples exhibit interlayer strain and a distribution of graphene island heights, with an interfacial roughness which derives from the growth process. Raman scattering shows reversible adsorption of hydrogen and results will be presented for epitaxial graphene, CVD-grown graphene and chemically-reduced graphene. The structural and electronic consequences of hydrogen chemisorption will be discussed.   

Solution-Based Processing and Applications of Two-Dimensional Heterostructures

Two-dimensional materials have emerged as promising candidates for next-generation electronics and optoelectronics, but advances in scalable nanomanufacturing are required to exploit this potential in real-world technology. This talk will explore methods for improving the uniformity of solution-processed two-dimensional materials with an eye toward realizing dispersions and inks that can be deposited into large-area thin-films. In particular, density gradient ultracentrifugation allows the solution-based isolation of graphene, boron nitride, montmorillonite, and transition metal dichalcogenides (e.g., MoS$_{\mathrm{2}}$, WS$_{\mathrm{2}}$, ReS$_{\mathrm{2}}$, MoSe$_{\mathrm{2}}$, WSe$_{\mathrm{2}})$ with homogeneous thickness down to the atomically thin limit. Similarly, two-dimensional black phosphorus is isolated in organic solvents or deoxygenated aqueous surfactant solutions with the resulting phosphorene nanosheets showing field-effect transistor mobilities and on/off ratios that are comparable to micromechanically exfoliated flakes. By adding cellulosic polymer stabilizers to these dispersions, the rheological properties can be tuned by orders of magnitude, thereby enabling two-dimensional material inks that are compatible with a range of additive manufacturing methods including inkjet, gravure, screen, and 3D printing. The resulting solution-processed two-dimensional heterostructures show promise in several device applications including photodiodes, anti-ambipolar transistors, gate-tunable memristors, and heterojunction photovoltaics.   

Cyclotron Resonance of Graphene-Boron Nitride Heterostructures

We have constructed an apparatus for performing Fourier-transform infrared magneto-spectroscopy on microscopic samples of atomically-thin materials. The design and operation of the instrument will be presented, along with initial observations of the infrared cyclotron resonance in a 200 $\mu$m$^2$ sample of boron nitride-encapsulated monolayer graphene in magnetic fields up to 11 T. Additionally we will report on progress toward the goal of performing spectroscopy on the Hofstadter butterfly state in graphene-hBN moire superlattices.   

The origin of incommensurate and commensurate phases of graphene interacting with SiC(0001)

Recently, the properties of the first graphene layer grown on SiC(0001), commonly called the buffer layer, have surprised the research community. We show that this layer is incommensurate with the SiC substrate despite 40 years of assuming that it was commensurate. The incommensurate phase explains why the buffer is semiconducting. In this work, we go on to show that previous notions that the buffer layer structure is independent of the number of graphene layers grown above it also incorrect. Both the physical and electronic structure of the buffer depends on growth conditions and coverage. At this stage, it is unclear why the system is incommensurate and why it undergoes an incommensurate to commensurate phase transition when a graphene monolayer forms above it. These important issues are addressed through a complementary study of X-ray Standing Wave photoemission spectroscopy and surface x-ray reflectivity. We gain detailed information regarding the vertical interface structure and atomic concentration of buffer and monolayer graphene. We find substantial differences in interface structure of the two systems that provide new insights of the nature of the incommensurate and commensurate phases of the SiC-graphene interaction.   

Vacancy in graphene: relevance of Exact Exchange Interaction

There has been intense study of the vacancy in graphene in the past decade, from experimental and theoretical side, with different results concerning the magnetic moment induced by the defect. The values coming from different theoretical simulations vary from 1.04 to 2.0 $\mu_B$ [1]. We simulate the defect with cluster models and periodic boundary conditions, using the same code [2]. We use Density Functional Theory DFT based formalisms, PBE and hybrid PBE0 where a fraction of Exact Exchange is included [3]. We choose different symmetries for clusters (hydrogen-terminated graphene nanoflakes) with arm-chair and zig-zag edges, and different sizes of supercells for periodic models. We find that a serious point to be taken into account is the self-interaction error present in bare DFT, which gives rise to fractional occupation of bands for periodic conditions. When using hybrid PBE0, for all the different models we simulated, our results point to one and the same magnetic moment for the vacancy in graphene, integer 2$\mu_B$. [1] Yazyev, Helm, PRB 75, 125408 (2007); Palacios, Yndurain, PRB 85, 245443 (2012); Wang, Pantelides, PRB 86, 165438 (2012) [2] Blum et al, CPC 180, 2175 (2009) [3] Perdew, Burke, Ernzerhof PRL 77, 3865 (1996); Perdew, Ernzerhof, Burke JCP 105, 9982 (1996).   

Engineered Soliton in Bilayer Graphene from Chemical Vapor Deposition

Strain engineering is an effective methodology to study the recently reported topologically protected edge states of bilayer graphene. In this work, we demonstrate that we are able to produce high yield of Bernal-stacked graphene bilayers by tuning the composition of the growth substrates. The high-yield scalable CVD fabrication process of bilayer graphene allows us to investigate the protected edge states with a variety of characterization tools. For example, large strain at strain solitons allows them to be visible in by Raman scattering, where the strain direction difference leads to diversified polarization Raman response.~ The distribution of strain solitons can be tuned by adjusting the growth condition and post-growth treatment, which allows us to observe the topologically protected edge states with assistance from and electron transport measurements.~