Session C36: 2D Materials - Heterostructures II

2D Materials and Heterostructures for Electronic, Optoelectronic, and Thermoelectric Device Applications

In this talk, I will review our recent progress in the area of 2D materials and heterostructures. We have demonstrated several approaches for converting many-layer transition metal dichalcogenides (TMDs) from indirect to direct band gap semiconductors using radiation exposure and plasma treatment techniques through an inter-layer decoupling mechanism.^1-3 In addition to converting the band structure of these materials, these material processing techniques result in substantial reduction in unintentional doping and improved charge neutrality,⁴ as well as strong circularly polarized photoluminescence.⁵ We have also investigated cross-plane transport in vertically-stacked 2D materials, including graphene/BN and graphene/MoS₂ heterostructures, for applications is thermoelectric and thermionic energy conversion,^{6, 7} as well as photodiodes.⁸

1. Dhall, et al., Direct Bandgap Transition in Many-Layer MoS₂ by Plasma-Induced Layer Decoupling. Advanced Materials, 27, 1573 (2015).
2. Wang, et al., Radiation-induced direct bandgap transition in few-layer MoS₂. Applied Physics Letters, DOI:10.1063/1.5005121 (2017).
3. Li, et al., Layer Control of WSe₂ via Selective Surface Layer Oxidation. ACS Nano, 10, 6836-6842 (2016).
4. Dhall, et al., Charge neutral MoS₂ field effect transistors through oxygen plasma treatment. Journal of Applied Physics, 120, 195702 (2016).
5. Dhall, et al., Strong Circularly Polarized Photoluminescence from Multilayer MoS2 Through Plasma Driven Direct-Gap Transition. Acs Photonics, 3, 310-314 (2016).
6. Li, et al., Cross-Plane Seebeck Coefficient Measurement of Misfit Layered Compounds (SnSe)(n)(TiSe2)(n), (n=1,3,4,5). Nano Letters, 17, 1978-1986 (2017).
7. Poudel, et al., Cross-plane Thermoelectric and Thermionic Transport across Au/h-BN/Graphene Heterostructures. Scientific Reports, DOI:10.1038/s41598-017-12704-w (2017).
8. Li, et al., Highly efficient, high speed vertical photodiodes based on fewlayer MoS₂. 2D Materials, 4, 015003 (2016).   

Interface potentials on h-BN/graphene nanoribbons

Zigzag graphene nanoribbons (ZGNRs) present a half-metallic response for an external electric field's critical value[1], with possibilities for spin-filter applications. On the other hand, embedded ZGNRs in zigzag hexagonal boron nitride (ZBNR) exhibit half-metallicity without the presence of an external field[2]. In this work, we analyze electronic properties of a mixed nanoribbon system (ZBNR/ZGNR/ZBNR), using a Hubbard model Hamiltonian within a mean field approximation. Due to different electronegativities of boron and nitrogen atoms, an electric field is induced across the ZGNR and we found a spin-dependent band structure. Optimal tight-binding parameters are found in a tentative to retrieve DFT results for the electronic bands. Here, we are interested on gap engineering and improving the robustness of the half-metallic response of the mixed system by means of fold-like strain and other imperfections such as impurities. We consider edge potentials[3] and take into account the effects of the BN/graphene interfaces to reproduce DFT results.
[1] Y.Son, M.Cohen, and S.Louie, Nature 444 (2006)
[2] S. Bgowmick, A. K. Singh, and B. I.Yakobson, J. Phys. Chem. C 115 (2011)
[3] F. Zheng, K. Sasaki, R. Saito, W. Duan and B. Gu, J. Phys. Soc. Jpn. 78 (2009)
  

Towards a Complete Understanding of the Interaction of Graphene and h-BN with Metal Supports: Influence of Substrate Crystallographic Orientation

Predicting the properties that graphene and h-BN will exhibit after their growth on a certain substrate is a major challenge. While the influence on these properties of the electron configuration of the atoms in the underlying surface is well-understood, the effects of substrate geometry still remain unclear. Herein, the structural properties of h-BN monolayers grown on the rectangularly-packed Rh(110) surface are characterized and compared to those that both this material and graphene exhibit on substrates with different crystallographic orientations. STM images acquired on h-BN monolayers grown on Rh(110) are dominated by only a single quasi-one-dimensional moiré pattern suggesting a strong interaction at the interface. Interestingly, the moiré stripes exhibit an unexpectedly small corrugation in comparison to all the strongly interacting interfaces with hexagonal substrates previously reported. This discrepancy is explained by the differences in the possible binding landscapes for differently oriented substrates. As a result, a rule is derived to predict how the corrugation at the interface will depend on the substrate symmetry.   

Modulated moiré superstructures of graphene on Ni(100): stability and reactivity

The interaction of graphene with the substrate strongly affects its electronic and chemical properties [1]. We focus in particular on graphene on Ni(100), where moiré superstructures with different modulation of the interface interaction are established, depending on the graphene/substrate misorientation angle. Stability, electronic and reactivity properties of
various moiré structures are studied combining surface science techniques (mainly scanning tunneling microscopy) and numerical simulations based on Density Functional Theory and Kinetic Monte Carlo approaches [2]. Stripe-like moiré structures offer alternate regions characterized by weak/strong graphene-surface bonding, constituting a unique environment for a chemistry "above the cover" and "under the cover" [3]. We discuss preliminary results of the interaction with small molecules of environmental importance. We found that carbide segregation at the interface upon temperature cooling may affect the graphene-surface interaction, causing for instance local detachment of the graphene overlayer in stripe-like moiré
structures.

[1] Bianchini F et al., 2014 J. Phys. Chem. Lett., 5 (3), 467-473
[2] Zhiyu Z, Carnevali V et al., to be published in Carbon.
[3] Fu Q, Bao X, 2017 Chem. Soc. Rev., 46, 1842-1874   

Understanding the Atomic Structure of a Topological Insulator–2D Material Heterostructure

Topological insulators have emerged as promising materials for efficient spin–charge conversion. One avenue to harness the promise of this new class of materials is to integrate them into heterostructures with two-dimensional materials. However, the ability to grow films with the desired properties relies on understanding the film structure and defects on the atomic scale. Here, detailed structural characterization is presented, in which aberration-corrected transmission electron microscopy of (Bi,Sb)₂Te₃ grown on h-BN elucidates details of the heterostructure. The presence of several types of grain boundaries is highlighted, in addition to the identification of an impurity phase and small asymmetries within the film thickness. Finally, a comparison is drawn between the features observed and knowledge gained from the study of topological insulator films grown on more traditional, single-crystal, bulk substrates.   

Self-contacted 2D transition metal dichalcogenides devices and their lateral heterostructures.

The accelerated exploration of transition metal dichalcogenide (TMD) monolayers proves their applicability for future digital, electronic, and optoelectronic devices. Considering all the advantages of such materials, one of the largest obstacles is the ability to controllably process the material into working devices with any scalability. Here, we report a new technique to grow self-contacted monolayer devices covering different TMDs (MoS₂, MoSe₂, WS₂, and WSe₂). In this technique, a pre-growth lithographical metallic pattern serves as a seed for the TMD material to grow around and creates an electrical contact with the 2D layers. This recent growth method improves the ability to grow high-quality monolayers on different patterns, large size devices, and different substrates. A lateral heterostructure of MoS₂/WS₂, MoSe₂/WS₂, and MoSe₂/WSe₂ as a self-connected device has been reported. Our results show excellent progress in the direction of the one-step production of multi-devices on one chip.
  

Pd2Se3 Two-Dimensional Phase Driven by Interlayer Fusion in Layered PdSe2

Two-dimensional materials are easily fabricated when their bulk form has a layered structure. The monolayer form in layered transition-metal dichalcogenides is typically the same as a single layer of the bulk material. However, PdSe₂ presents a puzzle. Its monolayer form has been theoretically shown to be stable, but there have been no reports that monolayer PdSe₂ has been fabricated. Here, combining atomic-scale imaging in a scanning transmission electron microscope and density functional theory, we demonstrate that the preferred monolayer form of this material amounts to a melding of two bulk monolayers accompanied by the emission of Se atoms so that the resulting stoichiometry is Pd₂Se₃. We further verify the interlayer melding mechanism by creating Se vacancies in situ in the layered PdSe₂ matrix using electron irradiation. The discovery that strong interlayer interactions can be induced by defects and lead to the formation of new 2D materials opens a new venue for the exploration of defect engineering and novel 2D structures.   

Direct Observation of 2D Electrostatics and Ohmic Contacts in 2D Heterojunctions

Atomically thin semiconductors such as WS₂, MoS₂, WSe₂ and MoSe₂ are attractive candidates for making ultrathin field effect transistors and optoelectronics. However, owing to the saturated bonds of van der Waals surface, achieving Ohmic contacts which can largely improve their device performance is challenging. Here we introduce a way to prepare large-area graphene contacts to WS₂ using seedless chemical vapour deposition. The template-grown graphene/WS₂ heterojunctions are intriguing blocks for making WS₂ devices with Ohmic contacts. Kelvin probe force microscopy, photoluminescence spectroscopy, and scanning tunneling microscopy characterize the doping and electrostatics in graphene–WS₂ heterojunctions as-grown on sapphire and transferred to SiO₂ with and without thermal annealing. Both p–n and n–n junctions are observed, and a flat-band condition (zero Schottky barrier height) is found, promising low-resistance Ohmic contacts. This indicates a more favorable band alignment for graphene–WS₂ than has been predicted, likely explaining the low barriers observed in transport experiments on similar heterojunctions. Electrostatic measurement and modelling demonstrate that the large depletion width of the graphene–WS₂ junction reflects the electrostatics of the one-dimensional junction between two-dimensional materials.   

Low-frequency Interlayer Modes of Few-layer 2H-SnS2

We investigated interlayer phonon modes of mechanically exfoliated few-layer 2H-SnS₂ samples obtained from a SnS₂ single crystal by using low-frequency micro-Raman spectroscopy. The atomic force microscope (AFM) was used to measure the sample thickness. Raman investigations were then performed using 632.8 nm, 532 nm, 514.4 nm and 441.6 nm lasers of power lower than 100 μW in a micro-Raman setup, in which the samples were measured in a vacuum chamber to avoid photo-oxidation. The intralayer E_g and A_1g modes were resolved at ~206 cm^-1 and ~314 cm^-1, respectively. Despite the bandgap of ~2.41 eV of mono- and few-layer SnS₂, which is close to the excitation energy of the 514.4 nm laser, among the lasers that we used, the 532 nm laser provided the strongest Raman signals of A_1g mode as well as the interlayer shear (S1, S2) and breathing (B1, B2) modes. These interlayer modes appeared to red-shift as the thickness increased. The frequency dispersion of interlayer modes provides the characteristics for thickness determination of few-layer 2H-SnS₂ by Raman spectroscopy.   

Correlation of Distinct Physical Properties in Luttinger Liquid

Quantum-confined electrons in one dimension (1D) behave as Luttinger liquid, a strongly correlated electronic matter that features suppressed spectral density at the Fermi energy and spin-charge separation. Metallic single-walled carbon nanotubes (SWNTs) provide the ideal platform to explore 1D Luttinger liquid physics. However, unambiguous demonstration of Luttinger liquid phenomena in SWNTs has been challenging because the Luttinger liquid interaction parameters are not known a priori. Here we combine electrical transport and optical nanoscopy measurements to correlate completely different physical properties in the same Luttinger liquid. The Luttinger liquid interaction parameters independently determined from the two measurements agree quantitatively with each other. Such combined electrical and optical studies can open up new opportunities in exploring 1D Luttinger liquid physics, such as non-equilibrium transport and ultrafast dynamics.   

Anomalous Coulomb drag behavior in graphene/MoS2 heterointerface

Two-dimensional (2D) heterointerface often provides extraordinary carrier transports exemplified by superconductivity or excitonic superfluidity. Recently, double-layer graphene separated by few-layered boron nitride demonstrates Coulomb drag phenomenon that carriers in active layer drag carriers in passive layer. Here, we propose a new switching device platform operated via Coulomb-drag interaction at graphene/MoS₂ (GM) heterointerface. Ideal van der Waals distance allows for strong coupling of interlayer electron-hole pairs whose recombination is prevented due to the Schottky barrier built by charge transfer at heterointerface. This GM device exhibits high carrier mobility of up to ~3,700 cm²V^-1s^-1 via electron-hole Coulomb drag even at room temperature, while retaining high on/off current ratio of ~10⁸, outperforming those of individual layers. In electron-electron drag regime, we observe graphene-like Shubnikov-de Haas oscillations in GM device at low temperature. Our Coulomb-drag transistor could be a shortcut to realize the practical applications of quantum-mechanical 2D heterostructures at room temperature.