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  • Dielectric properties of hexagonal boron nitride and transition metal dichalcogenides: from monolayer to bulk
    npj 2D Mater. Appl. Pub Date : 2018-03-08
    Akash Laturia, Maarten L. Van de Put, William G. Vandenberghe

    Hexagonal boron nitride (h-BN) and semiconducting transition metal dichalcogenides (TMDs) promise greatly improved electrostatic control in future scaled electronic devices. To quantify the prospects of these materials in devices, we calculate the out-of-plane and in-plane dielectric constant from first principles for TMDs in trigonal prismatic and octahedral coordination, as well as for h-BN, with a thickness ranging from monolayer and bilayer to bulk. Both the ionic and electronic contribution to the dielectric response are computed. Our calculations show that the out-of-plane dielectric response for the transition-metal dichalcogenides is dominated by its electronic component and that the dielectric constant increases with increasing chalcogen atomic number. Overall, the out-of-plane dielectric constant of the TMDs and h-BN increases by around 15% as the number of layers is increased from monolayer to bulk, while the in-plane component remains unchanged. Our computations also reveal that for octahedrally coordinated TMDs the ionic (static) contribution to the dielectric response is very high (4.5 times the electronic contribution) in the in-plane direction. This indicates that semiconducting TMDs in the tetragonal phase will suffer from excessive polar-optical scattering thereby deteriorating their electronic transport properties.

  • Electrical devices from top-down structured platinum diselenide films
    npj 2D Mater. Appl. Pub Date : 2018-02-28
    Chanyoung Yim, Vikram Passi, Max C. Lemme, Georg S. Duesberg, Cormac Ó Coileáin, Emiliano Pallecchi, Dalal Fadil, Niall McEvoy

    Platinum diselenide (PtSe2) is an exciting new member of the two-dimensional (2D) transition metal dichalcogenide (TMD) family. It has a semimetal to semiconductor transition when approaching monolayer thickness and has already shown significant potential for use in device applications. Notably, PtSe2 can be grown at low temperature making it potentially suitable for industrial usage. Here, we address thickness-dependent transport properties and investigate electrical contacts to PtSe2, a crucial and universal element of TMD-based electronic devices. PtSe2 films have been synthesized at various thicknesses and structured to allow contact engineering and the accurate extraction of electrical properties. Contact resistivity and sheet resistance extracted from transmission line method (TLM) measurements are compared for different contact metals and different PtSe2 film thicknesses. Furthermore, the transition from semimetal to semiconductor in PtSe2 has been indirectly verified by electrical characterization in field-effect devices. Finally, the influence of edge contacts at the metal–PtSe2 interface has been studied by nanostructuring the contact area using electron beam lithography. By increasing the edge contact length, the contact resistivity was improved by up to 70% compared to devices with conventional top contacts. The results presented here represent crucial steps toward realizing high-performance nanoelectronic devices based on group-10 TMDs.

  • Structural transformation of layered double hydroxides: an in situ TEM analysis
    npj 2D Mater. Appl. Pub Date : 2018-02-21
    Christopher Hobbs, Sonia Jaskaniec, Eoin K. McCarthy, Clive Downing, Konrad Opelt, Konrad Güth, Aleksey Shmeliov, Maurice C. D. Mourad, Karl Mandel, Valeria Nicolosi

    A comprehensive nanoscale understanding of layered double hydroxide (LDH) thermal evolution is critical for their current and future applications as catalysts, flame retardants and oxygen evolution performers. In this report, we applied in situ transmission electron microscopy (TEM) to extensively characterise the thermal progressions of nickel-iron containing (Ni-Fe) LDH nanomaterials. The combinative approach of TEM and selected area electron diffraction (SAED) yielded both a morphological and crystallographic understanding of such processes. As the Ni-Fe LDH nanomaterials are heated in situ, an amorphization occurred at 250 °C, followed by a transition to a heterogeneous structure of NiO particles embedded throughout a NiFe2O4 matrix at 850 °C, confirmed by high-resolution TEM and scanning TEM. Further electron microscopy characterisation methodologies of energy-filtered TEM were utilised to directly observe these mechanistic behaviours in real time, showing an evolution and nucleation to an array of spherical NiO nanoparticles on the platelet surfaces. The versatility of this characterisation approach was verified by the analogous behaviours of Ni-Fe LDH materials heated ex situ as well as parallel in situ TEM and SAED comparisons to that of an akin magnesium-aluminium containing (Mg-Al) LDH structure. The in situ TEM work hereby discussed allows for a state-of-the-art understanding of the Ni-Fe material thermal evolution. This is an important first, which reveals pivotal information, especially when considering LDH applications as catalysts and flame retardants.

  • Device physics of van der Waals heterojunction solar cells
    npj 2D Mater. Appl. Pub Date : 2018-02-19
    Marco M. Furchi, Florian Höller, Lukas Dobusch, Dmitry K. Polyushkin, Simone Schuler, Thomas Mueller

    Heterostructures based on atomically thin semiconductors are considered a promising emerging technology for the realization of ultrathin and ultralight photovoltaic solar cells on flexible substrates. Much progress has been made in recent years on a technological level, but a clear picture of the physical processes that govern the photovoltaic response remains elusive. Here, we present a device model that is able to fully reproduce the current–voltage characteristics of type-II van der Waals heterojunctions under optical illumination, including some peculiar behaviors such as exceedingly high ideality factors or bias-dependent photocurrents. While we find the spatial charge transfer across the junction to be very efficient, we also find a considerable accumulation of photogenerated carriers in the active device region due to poor electrical transport properties, giving rise to significant carrier recombination losses. Our results are important to optimize future device architectures and increase power conversion efficiencies of atomically thin solar cells.

  • Large-grain MBE-grown GaSe on GaAs with a Mexican hat-like valence band dispersion
    npj 2D Mater. Appl. Pub Date : 2018-01-25
    Ming-Wei Chen, HoKwon Kim, Dmitry Ovchinnikov, Agnieszka Kuc, Thomas Heine, Olivier Renault, Andras Kis

    Atomically thin GaSe has been predicted to have a non-parabolic, Mexican hat-like valence band structure due to the shift of the valence band maximum (VBM) near the Γ point which is expected to give rise to novel, unique properties such as tunable magnetism, high effective mass suppressing direct tunneling in scaled transistors, and an improved thermoelectric figure of merit. However, the synthesis of atomically thin GaSe remains challenging. Here, we report on the growth of atomically thin GaSe by molecular beam epitaxy (MBE) and demonstrate the high quality of the resulting van der Waals epitaxial films. The full valence band structure of nominal bilayer GaSe is revealed by photoemission electron momentum microscopy (k-PEEM), confirming the presence of a distorted valence band near the Γ point. Our results open the way to demonstrating interesting new physical phenomena based on MBE-grown GaSe films and atomically thin monochalcogenides in general.

  • Substrate engineering of graphene reactivity: towards high-performance graphene-based catalysts
    npj 2D Mater. Appl. Pub Date : 2018-01-17
    Na Guo, Kah Meng Yam, Chun Zhang

    Graphene-based solid-state catalysis is an emerging direction in research on graphene, which opens new opportunities in graphene applications and thus has attracted enormous interests recently. A central issue in graphene-based catalysis is the lack of an effective yet practical way to activate the chemically inert graphene, which is largely due to the difficulties in the direct treatment of graphene (such as doping transition metal elements and introducing particular type of vacancies). Here we report a way to overcome these difficulties by promoting the reactivity and catalytic activity of graphene via substrate engineering. With thorough first-principles investigations, we demonstrate that when introduce a defect, either a substitutional impurity atom (e.g. Au, Cu, Ag, Zn) or a single vacancy, in the underlying Ru (0001) substrate, the reactivity of the supported graphene can be greatly enhanced, resulting in the chemical adsorption of O2 molecules on graphene. The origin of the O2 chemical adsorption is found to be the impurity- or vacancy-induced significant charge transfer from the graphene–Ru (0001) contact region to the 2π* orbital of the O2 molecule. We then further show that the charge transfer also leads to high catalytic activity of graphene for chemical reaction of CO oxidation. According to our calculations, the catalyzed CO oxidation takes place in Eley-Rideal (ER) mechanism with low reaction barriers (around 0.5 eV), suggesting that the substrate engineering is an effective way to turn the supported graphene into an excellent catalyst that has potential for large-scale industrial applications.

  • Production of monolayer-rich gold-decorated 2H–WS2 nanosheets by defect engineering
    npj 2D Mater. Appl. Pub Date : 2018-01-08
    Jeremy R. Dunklin, Paul Lafargue, Thomas M. Higgins, Gregory T. Forcherio, Mourad Benamara, Niall McEvoy, D. Keith Roper, Jonathan N. Coleman, Yana Vaynzof, Claudia Backes

    Chemical functionalization of low-dimensional nanostructures has evolved as powerful tool to tailor the materials’ properties on demand. For two-dimensional transition metal dichalcogenides, functionalization strategies are mostly limited to the metallic 1T-polytype with only few examples showing a successful derivatization of the semiconducting 2H-polytype. Here, we describe that liquid-exfoliated WS2 undergoes a spontaneous redox reaction with AuCl3. We propose that thiol groups at edges and defects sites reduce the AuCl3 to Au0 and are in turn oxidized to disulfides. As a result of the reaction, Au nanoparticles nucleate predominantly at edges with tuneable nanoparticle size and density. The drastic changes in nanosheet mass obtained after high loading with Au nanoparticles can be exploited to enrich the dispersions in laterally large, monolayered nanosheets by simple centrifugation. The optical properties (for example photoluminescence) of the monolayers remain pristine, while the electrocatalytic activity towards the hydrogen evolution reaction is significantly improved.

  • Studies of two-dimensional h-BN and MoS2 for potential diffusion barrier application in copper interconnect technology
    npj 2D Mater. Appl. Pub Date : 2017-12-08
    Chun-Li Lo, Massimo Catalano, Kirby K. H. Smithe, Luhua Wang, Shengjiao Zhang, Eric Pop, Moon J. Kim, Zhihong Chen

    Copper interconnects in modern integrated circuits require a barrier layer to prevent Cu diffusion into surrounding dielectrics. However, conventional barrier materials like TaN are highly resistive compared to Cu and will occupy a large fraction of the cross-section of ultra-scaled Cu interconnects due to their thickness scaling limits at 2–3 nm, which will significantly increase the Cu line resistance. It is well understood that ultrathin, effective diffusion barriers are required to continue the interconnect scaling. In this study, a new class of two-dimensional (2D) materials, hexagonal boron nitride (h-BN) and molybdenum disulfide (MoS2), is explored as alternative Cu diffusion barriers. Based on time-dependent dielectric breakdown measurements and scanning transmission electron microscopy imaging coupled with energy dispersive X-ray spectroscopy and electron energy loss spectroscopy characterizations, these 2D materials are shown to be promising barrier solutions for Cu interconnect technology. The predicted lifetime of devices with directly deposited 2D barriers can achieve three orders of magnitude improvement compared to control devices without barriers.

  • Identifying light impurities in transition metal dichalcogenides: the local vibrational modes of S and O in ReSe2 and MoSe2
    npj 2D Mater. Appl. Pub Date : 2017-11-22
    Lewis S. Hart, James L. Webb, Stephen Murkin, Daniel Wolverson, Der-Yuh Lin

    The transition metal dichalcogenides provide a rich field for the study of two-dimensional materials, with metals, semiconductors, superconductors and charge density wave materials being known. Members of this family are typically hexagonal, but those based on rhenium (ReSe2 and ReS2) and their ternary alloys are attracting attention due to their triclinic structure and their resulting, strong in-plane anisotropy. Here, Raman spectra of dilute ReSe2 -  x S x alloys containing low levels of sulfur (x ≤ 0.25) were obtained in order to investigate the distribution of substitutional sulfur atoms over the non-equivalent chalcogen sites of the ReSe2 unit cell. Four different Raman bands arising from the local vibrational modes of sulfur atoms were observed, corresponding to these four sites. One local vibrational mode has a substantially in-plane displacement of the sulfur atom, two are partially out-of-plane and one is completely out-of-plane. The interpretation of the experimental data is based on calculations of the lattice dynamics and non-resonant Raman tensors of a model alloy via density functional theory. For comparison, polarization-dependent Raman spectra of pure ReS2 are also presented; a dramatic increase in the Raman cross-section is found for the out-of-plane modes when the excitation polarization is normal to the layers and the light propagates in the layer plane. A similar increase in cross-section is found experimentally for the local vibrational modes of sulfur in dilute ReSe2 -x  S x alloys and is predicted for dilute sulfur-containing alloys based on MoSe2. The analogous local vibrational modes of substitutional oxygen impurities in ReSe2 were also investigated computationally.

  • Contact morphology and revisited photocurrent dynamics in monolayer MoS2
    npj 2D Mater. Appl. Pub Date : 2017-11-17
    Eric Parzinger, Martin Hetzl, Ursula Wurstbauer, Alexander W. Holleitner

    Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have emerged as promising materials for electronic, optoelectronic, and valleytronic applications. Recent work suggests drastic changes of the band gap and exciton binding energies of photo-excited TMDs with ultrafast non-radiative relaxation processes effectively heating the crystal lattice. Such phenomena have not been considered in the context of optoelectronic devices yet. We resolve corresponding ultrafast photoconductance dynamics within monolayer MoS2. The data suggest that a bolometric contribution as well as a defect-related conductance dominate the overall photoconductance. We further reveal that a focused laser illumination, as is used in many standard optoelectronic measurements of MoS2, can modify and anneal the morphology of metal contacts. We show that a junction evolves with lateral built-in electric fields, although Raman spectra and photoluminescence spectra indicate no significant changes, such as a crystal phase transition. We highlight how such optimized devices can drive ultrafast electromagnetic signals in on-chip high-frequency and THz circuits.

  • Hypochlorite degrades 2D graphene oxide sheets faster than 1D oxidised carbon nanotubes and nanohorns
    npj 2D Mater. Appl. Pub Date : 2017-11-08
    Leon Newman, Neus Lozano, Minfang Zhang, Sumio Iijima, Masako Yudasaka, Cyrill Bussy, Kostas Kostarelos

    Carbon nanostructures are currently fuelling a revolution in science and technology in areas ranging from aerospace engineering to electronics. Oxidised carbon nanomaterials, such as graphene oxide, exhibit dramatically improved water dispersibility compared to their pristine equivalents, allowing their exploration in biology and medicine. Concomitant with these potential healthcare applications, the issue of degradability has been raised and has started to be investigated. The aim of the present study was to assess the potential of hypochlorite, a naturally occurring and industrially used ion, to degrade oxidised carbon nanomaterials within a week. Our main focus was to characterise the physical and chemical changes that occur during degradation of graphene oxide compared to two other oxidised carbon nanomaterials, namely carbon nanotubes and carbon nanohorns. The kinetics of degradation were closely monitored over a week using a battery of techniques including visual observation, UV–Vis spectroscopy, Raman spectroscopy, infra-red spectroscopy, transmission electron microscopy and atomic force microscopy. Graphene oxide was rapidly degraded into a dominantly amorphous structure lacking the characteristic Raman signature and microscopic morphology. Oxidised carbon nanotubes underwent degradation via a wall exfoliation mechanism, yet maintained a large fraction of the sp2 carbon backbone, while the degradation of oxidised carbon nanohorns was somewhat intermediate. The present study shows the timeline of physical and chemical alterations of oxidised carbon nanomaterials, demonstrating a faster degradation of 2D graphene oxide sheets compared to 1D oxidised carbon nanomaterials over 7 days in the presence of an oxidising species.

  • Two-dimensional negative capacitance transistor with polyvinylidene fluoride-based ferroelectric polymer gating
    npj 2D Mater. Appl. Pub Date : 2017-11-02
    Xudong Wang, Yan Chen, Guangjian Wu, Dan Li, Luqi Tu, Shuo Sun, Hong Shen, Tie Lin, Yongguang Xiao, Minghua Tang, Weida Hu, Lei Liao, Peng Zhou, Jinglan Sun, Xiangjian Meng, Junhao Chu, Jianlu Wang

    Conventional field-effect transistors (FETs) are not expected to satisfy the requirements of future large integrated nanoelectronic circuits because of these circuits’ ultra-high power dissipation and because the conventional FETs cannot overcome the subthreshold swing (SS) limit of 60 mV/decade. In this work, the ordinary oxide of the FET is replaced only by a ferroelectric (Fe) polymer, poly(vinylidene difluoride-trifluoroethylene) (P(VDF-TrFE)). Additionally, we employ a two-dimensional (2D) semiconductor, such as MoS2 and MoSe2, as the channel. This 2D Fe-FET achieves an ultralow SS of 24.2 mV/dec over four orders of magnitude in drain current at room temperature; this sub-60 mV/dec switching is derived from the Fe negative capacitance (NC) effect during the polarization of ferroelectric domain switching. Such 2D NC-FETs, realized by integrating of 2D semiconductors and organic ferroelectrics, provide a new approach to satisfy the requirements of next-generation low-energy-consumption integrated nanoelectronic circuits as well as the requirements of future flexible electronics.

  • 3D integrated monolayer graphene–Si CMOS RF gas sensor platform
    npj 2D Mater. Appl. Pub Date : 2017-10-26
    Seyedeh Maryam Mortazavi Zanjani, Milo Holt, Mir Mohammad Sadeghi, Somayyeh Rahimi, Deji Akinwande

    Integration of a complementary metal-oxide semiconductor (CMOS) and monolayer graphene is a significant step toward realizing low-cost, low-power, heterogeneous nanoelectronic devices based on two-dimensional materials such as gas sensors capable of enabling future mobile sensor networks for the Internet of Things (IoT). But CMOS and post-CMOS process parameters such as temperature and material limits, and the low-power requirements of untethered sensors in general, pose considerable barriers to heterogeneous integration. We demonstrate the first monolithically integrated CMOS-monolayer graphene gas sensor, with a minimal number of post-CMOS processing steps, to realize a gas sensor platform that combines the superior gas sensitivity of monolayer graphene with the low power consumption and cost advantages of a silicon CMOS platform. Mature 0.18 µm CMOS technology provides the driving circuit for directly integrated graphene chemiresistive junctions in a radio frequency (RF) circuit platform. This work provides important advances in scalable and feasible RF gas sensors specifically, and toward monolithic heterogeneous graphene–CMOS integration generally.

  • Author correction: Wafer-scale two-dimensional ferromagnetic Fe3GeTe2 thin films grown by molecular beam epitaxy
    npj 2D Mater. Appl. Pub Date : 2017-10-25
    Shanshan Liu, Xiang Yuan, Yichao Zou, Yu Sheng, Ce Huang, Enze Zhang, Jiwei Ling, Yanwen Liu, Weiyi Wang, Cheng Zhang, Jin Zou, Kaiyou Wang, Faxian Xiu

    Author correction: Wafer-scale two-dimensional ferromagnetic Fe3GeTe2 thin films grown by molecular beam epitaxy Author correction: Wafer-scale two-dimensional ferromagnetic Fe3GeTe2 thin films grown by molecular beam epitaxy, Published online: 25 October 2017; doi:10.1038/s41699-017-0039-x Author correction: Wafer-scale two-dimensional ferromagnetic Fe3GeTe2 thin films grown by molecular beam epitaxy

  • Graphene-based nanolaminates as ultra-high permeation barriers
    npj 2D Mater. Appl. Pub Date : 2017-10-23
    Abhay A. Sagade, Adrianus I. Aria, Steven Edge, Paolo Melgari, Bjoern Gieseking, Bernhard C. Bayer, Jannik C. Meyer, David Bird, Paul Brewer, Stephan Hofmann

    Permeation barrier films are critical to a wide range of applications. In particular, for organic electronics and photovoltaics not only ultra-low permeation values are required but also optical transparency. A laminate structure thereby allows synergistic effects between different materials. Here, we report on a combination of chemical vapor deposition (CVD) and atomic layer deposition (ALD) to create in scalable fashion few-layer graphene/aluminium oxide-based nanolaminates. The resulting ~10 nm contiguous, flexible graphene-based films are >90% optically transparent and show water vapor transmission rates below 7 × 10−3 g/m2/day measured over areas of 5 × 5 cm2. We deploy these films to provide effective encapsulation for organic light-emitting diodes (OLEDs) with measured half-life times of 880 h in ambient.

Some contents have been Reproduced with permission of the American Chemical Society.
Some contents have been Reproduced by permission of The Royal Society of Chemistry.
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