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  • Van der Waals junction field effect transistors with both n- and p-channel transition metal dichalcogenides
    npj 2D Mater. Appl. Pub Date : 2018-11-05
    June Yeong Lim, Minju Kim, Yeonsu Jeong, Kyeong Rok Ko, Sanghyuck Yu, Hyung Gon Shin, Jae Young Moon, Young Jai Choi, Yeonjin Yi, Taekyeong Kim, Seongil Im

    Two-dimensional (2D) transition metal dichalcogenides (TMDs)-based van der Waals (vdW) PN junctions have been used for heterojunction diodes, which basically utilize out-of-plane current across the junction interface. In fact, the same vdW PN junction structure can be utilized for another important device application, junction field effect transistors (JFETs), where in-plane current is possible along with 2D–2D heterojunction interface. Moreover, the 2D TMD-based JFET can use both p- and n-channel for low voltage operation, which might be its unique feature. Here we report vdW JFETs as an in-plane current device with heterojunction between semiconducting p- and n-TMDs. Since this vdW JFET would have low-density traps at the vdW interface unlike 2D TMD-based metal insulator semiconductor field effect transistors (MISFETs), little hysteresis of 0.0–0.1 V and best subthreshold swing of ~100 mV/dec were achieved. Easy saturation was observed either from n-channel or p-channel JFET as another advantage over 2D MISFETs, exhibiting early pinch-off at ~1 V. Operational gate voltage for threshold was near 0 V and our highest mobility reaches to ~>500 cm2/V·s for n-channel JFET with MoS2 channel. For 1 V JFET operation, our best ON/OFF current ratio was observed to be ~104.

    更新日期:2018-11-05
  • Spatially controlled electrostatic doping in graphene p-i-n junction for hybrid silicon photodiode
    npj 2D Mater. Appl. Pub Date : 2018-11-02
    Tiantian Li, Dun Mao, Nick W. Petrone, Robert Grassi, Hao Hu, Yunhong Ding, Zhihong Huang, Guo-Qiang Lo, James C. Hone, Tony Low, Chee Wei Wong, Tingyi Gu

    Sufficiently large depletion region for photocarrier generation and separation is a key factor for two-dimensional material optoelectronic devices, but only a few device configurations have been explored for a deterministic control over the space charge region area in graphene with convincing scalability. Here we investigate a graphene-silicon p-i-n photodiode defined in a foundry processed planar photonic crystal waveguide structure, achieving visible—near-infrared, zero-bias, and ultrafast photodetection. Graphene is electrically contacting to the wide intrinsic region of silicon and extended to the p an n doped region, functioning as the primary photocarrier conducting channel for electronic gain. Graphene significantly improves the device speed through ultrafast out-of-plane interfacial carrier transfer and the following in-plane built-in electric field assisted carrier collection. More than 50 dB converted signal-to-noise ratio at 40 GHz has been demonstrated under zero bias voltage, the quantum efficiency could be further amplified by hot carrier gain on graphene-i Si interface and avalanche process on graphene-doped Si interface. With the device architecture fully defined by nanomanufactured substrate, this work demonstrates post-fabrication-free two-dimensional material active silicon photonic devices.

    更新日期:2018-11-05
  • Author Correction: Multiscale framework for simulation-guided growth of 2D materials
    npj 2D Mater. Appl. Pub Date : 
    Kasra Momeni, Yanzhou Ji, Kehao Zhang, Joshua A. Robinson, Long-Qing Chen

    Author Correction: Multiscale framework for simulation-guided growth of 2D materials Author Correction: Multiscale framework for simulation-guided growth of 2D materials, Published online: 20 October 2018; doi:10.1038/s41699-018-0081-3 Author Correction: Multiscale framework for simulation-guided growth of 2D materials

    更新日期:2018-10-22
  • Coating two-dimensional MoS2 with polymer creates a corrosive non-uniform interface
    npj 2D Mater. Appl. Pub Date : 2018-10-19
    Quoc Huy Thi, Hyun Kim, Jiong Zhao, Thuc Hue Ly

    Two-dimensional (2D) materials and soft materials are both susceptible to mechanical instabilities, such as buckling, wrinkling, folding and creasing, especially when located on surfaces. Here, we report that weak van der Waals interactions cause the interface between 2D molybdenum disulphide (MoS2) and a soft poly(methyl methacrylate) coating to demonstrate mechanical instability and delamination. The resulting non-uniform and buckled interface greatly hampers the ability of the coating to protect the MoS2 substrate. Also, the corrosion rate of 2D MoS2 and quench rate of intrinsic luminescence in 2D MoS2 were significantly accelerated by the soft coating. Owing to the formation of corrosive cavities at the interface, the geometry and size of the flakes became the dominating factor, and a critical size of 2D flakes for such interfacial instability was determined based on elasticity theory. Such hazardous corrosion in a 2D material caused by a soft coating raises concern for their use in electronic packaging, and for the processing of van der Waals-layered materials for future applications.

    更新日期:2018-10-19
  • Strong two-dimensional plasmon in Li-intercalated hexagonal boron-nitride film with low damping
    npj 2D Mater. Appl. Pub Date : 2018-09-19
    Ivor Lončarić, Zoran Rukelj, Vyacheslav M. Silkin, Vito Despoja

    The field of plasmonics seeks to find materials with an intensive plasmon (large plasmon pole weight) with low Landau, phonon, and other losses (small decay width). In this paper, we propose a new class of materials that show exceptionally good plasmonic properties. These materials consist of van der Waals stacked “plasmon active” layers (atomically thin metallic layers) and “supporting” layers (atomically thin wide band gap insulating layers). One such material that can be experimentally realized—lithium intercalated hexagonal boron-nitride is studied in detail. We show that its 2D plasmon intensity is superior to the intensity of well-studied Dirac plasmon in heavy doped graphene, which is hard to achieve. We also propose a method for computationally very cheap, but accurate analysis of plasmon spectra in such materials, based on one band tight-binding approach and effective background dielectric function.

    更新日期:2018-09-19
  • Electronic transport in a two-dimensional superlattice engineered via self-assembled nanostructures
    npj 2D Mater. Appl. Pub Date : 2018-09-18
    Yingjie Zhang, Youngseok Kim, Matthew J. Gilbert, Nadya Mason

    Nanoscience offers a unique opportunity to design modern materials from the bottom up via low-cost, solution processed assembly of nanoscale building blocks. These systems promise electronic band structure engineering using not only the nanoscale structural modulation, but also the mesoscale spatial patterning, although experimental realization of the latter has been challenging. Here, we design and fabricate a new type of artificial solid by stacking graphene on a self-assembled, nearly periodic array of nanospheres, and experimentally observe superlattice miniband effects. We find conductance dips at commensurate fillings of charge carriers per superlattice unit cell, which are key features of minibands that are induced by the quasi-periodic deformation of the graphene lattice. These dips become stronger when the lattice strain is larger. Using a tight-binding model, we simulate the effect of lattice deformation as a parameter affecting the inter-atomic hopping integral, and confirm the superlattice transport behavior. This 2D material-nanoparticle heterostructure enables facile band structure engineering via self-assembly, promising for large-area electronics and optoelectronics applications.

    更新日期:2018-09-18
  • Enhanced interlayer neutral excitons and trions in trilayer van der Waals heterostructures
    npj 2D Mater. Appl. Pub Date : 2018-09-17
    Chanyeol Choi, Jiahui Huang, Hung-Chieh Cheng, Hyunseok Kim, Abhinav Kumar Vinod, Sang-Hoon Bae, V. Ongun Özçelik, Roberto Grassi, Jongjae Chae, Shu-Wei Huang, Xiangfeng Duan, Kristen Kaasbjerg, Tony Low, Chee Wei Wong

    Vertically stacked van der Waals heterostructures constitute a promising platform for providing tailored band alignment with enhanced excitonic systems. Here, we report observations of neutral and charged interlayer excitons in trilayer WSe2–MoSe2–WSe2 van der Waals heterostructures and their dynamics. The addition of a WSe2 layer in the trilayer leads to significantly higher photoluminescence quantum yields and tunable spectral resonance compared to its bilayer heterostructures at cryogenic temperatures. The observed enhancement in the photoluminescence quantum yield is due to significantly larger electron–hole overlap and higher light absorbance in the trilayer heterostructure, supported via first-principles pseudopotential calculations based on spin-polarized density functional theory. We further uncover the temperature- and power-dependence, as well as time-resolved photoluminescence of the trilayer heterostructure interlayer neutral excitons and trions. Our study elucidates the prospects of manipulating light emission from interlayer excitons and designing atomic heterostructures from first-principles for optoelectronics.

    更新日期:2018-09-17
  • Multiscale framework for simulation-guided growth of 2D materials
    npj 2D Mater. Appl. Pub Date : 2018-09-14
    Kasra Momeni, Yanzhou Ji, Kehao Zhang, Joshua A. Robinson, Long-Qing Chen

    Chemical vapor deposition (CVD) is a powerful technique for synthesizing monolayer materials such as transition metal dichalcogenides. It has advantages over exfoliation techniques, including higher purity and the ability to control the chemistry of the products. However, controllable and reproducible synthesis of 2D materials using CVD is a challenge because of the complex growth process and its sensitivity to subtle changes in growth conditions, making it difficult to extend conclusions obtained in one CVD chamber to another. Here, we developed a multiscale model linking CVD control parameters to the morphology, size, and distribution of synthesized 2D materials. Its capabilities are experimentally validated via the systematic growth of MoS2. In particular, we coupled the reactor-scale governing heat and mass transport equations with the mesoscale phase-field equations for the growth morphology considering the variation of edge energies with the precursor concentration within the growth chamber. The predicted spatial distributions of 2D islands are statistically analyzed, and experiments are then performed to validate the predicted island morphology and distributions. It is shown that the model can be employed to predict and control the morphology and characteristics of synthesized 2D materials.

    更新日期:2018-09-14
  • Exciton physics and device application of two-dimensional transition metal dichalcogenide semiconductors
    npj 2D Mater. Appl. Pub Date : 2018-09-10
    Thomas Mueller, Ermin Malic

    Two-dimensional group-VI transition metal dichalcogenide semiconductors, such as MoS2, WSe2, and others, exhibit strong light-matter coupling and possess direct band gaps in the infrared and visible spectral regimes, making them potentially interesting candidates for various applications in optics and optoelectronics. Here, we review their optical and optoelectronic properties with emphasis on exciton physics and devices. As excitons are tightly bound in these materials and dominate the optical response even at room-temperature, their properties are examined in depth in the first part of this article. We discuss the remarkably versatile excitonic landscape, including bright, dark, localized and interlayer excitons. In the second part, we provide an overview on the progress in optoelectronic device applications, such as electrically driven light emitters, photovoltaic solar cells, photodetectors, and opto-valleytronic devices, again bearing in mind the prominent role of excitonic effects. We conclude with a brief discussion on challenges that remain to be addressed to exploit the full potential of transition metal dichalcogenide semiconductors in possible exciton-based applications.

    更新日期:2018-09-10
  • Liquid-interface-assisted synthesis of covalent-organic and metal-organic two-dimensional crystalline polymers
    npj 2D Mater. Appl. Pub Date : 2018-09-03
    Lihuan Wang, Hafeesudeen Sahabudeen, Tao Zhang, Renhao Dong

    The development of synthetic two-dimensional crystalline polymers (2DCPs), such as 2D covalent-organic polymers and 2D metal-organic polymers, is receiving increasing attention due to their intriguing chemistry and unique properties, as well as potential role in wide ranging applications, such as electronics, sensing, catalysis, separation, and energy storage and conversion. Complementary to the top-down exfoliation towards the preparation of 2DCPs, bottom-up interface-assisted synthesis is advantageous in the 2D dynamic arrangement of the molecules or precursors, offering the chance to generate ultra-thin structures with large lateral sizes. This article provides guidelines on the preparation of free-standing, single-layer, or multi-layer 2DCPs via liquid-interface-assisted synthesis, mainly involving polymerization at the air–water and liquid–liquid interfaces, as well as the Langmuir-Blodgett method. Insight into the advantages and challenges of synthesis strategies and chemistry methodologies are provided for the future development of interfacial synthesis of 2DCPs with diverse structural and functional control.

    更新日期:2018-09-04
  • Publisher Correction: Nitrogen-doping induces tunable magnetism in ReS2
    npj 2D Mater. Appl. Pub Date : 2018-08-28
    Qin Zhang, Zemian Ren, Nian Wu, Wenjie Wang, Yingjie Gao, Qiqi Zhang, Jing Shi, Lin Zhuang, Xiangnan Sun, Lei Fu

    Publisher Correction: Nitrogen-doping induces tunable magnetism in ReS2 Publisher Correction: Nitrogen-doping induces tunable magnetism in ReS2, Published online: 28 August 2018; doi:10.1038/s41699-018-0077-z Publisher Correction: Nitrogen-doping induces tunable magnetism in ReS2

    更新日期:2018-08-29
  • Anisotropic band splitting in monolayer NbSe2: implications for superconductivity and charge density wave
    npj 2D Mater. Appl. Pub Date : 2018-05-03
    Yuki Nakata, Katsuaki Sugawara, Satoru Ichinokura, Yoshinori Okada, Taro Hitosugi, Takashi Koretsune, Keiji Ueno, Shuji Hasegawa, Takashi Takahashi, Takafumi Sato

    Realization of unconventional physical properties in two-dimensional (2D) transition-metal dichalcogenides (TMDs) is currently one of the key challenges in condensed-matter systems. However, the electronic properties of 2D TMDs remain largely unexplored compared to those of their bulk counterparts. Here, we report the fabrication of a high-quality monolayer NbSe2 film with a trigonal prismatic structure by molecular beam epitaxy, and the study of its electronic properties by scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and electrical transport measurements, together with first-principles band-structure calculations. In addition to a charge density wave (CDW) with 3 × 3 periodicity and superconductivity below 1.5 K, we observed sizable (~0.1 eV) band splitting along the Γ-K cut in the Brillouin zone due to inversion symmetry breaking in the monolayer crystal. This splitting is highly anisotropic in k space, leading to a spin-split van-Hove singularity in the band structure. The present results suggest the importance of spin–orbit coupling and symmetry breaking for unconventional superconductivity and CDW properties in monolayer TMDs.

    更新日期:2018-05-03
  • Strain relaxation via formation of cracks in compositionally modulated two-dimensional semiconductor alloys
    npj 2D Mater. Appl. Pub Date : 2018-04-30
    Hossein Taghinejad, Ali A. Eftekhar, Philip M. Campbell, Brian Beatty, Mohammad Taghinejad, Yao Zhou, Christopher J. Perini, Hesam Moradinejad, Walter E. Henderson, Eric V. Woods, Xiang Zhang, Pulickel Ajayan, Evan J. Reed, Eric M. Vogel, Ali Adibi

    Composition modulation of two-dimensional transition-metal dichalcogenides (TMDs) has introduced an enticing prospect for the synthesis of Van der Waals alloys and lateral heterostructures with tunable optoelectronic properties. Phenomenologically, the optoelectronic properties of alloys are entangled to a strain that is intrinsic to synthesis processes. Here, we report an unprecedented biaxial strain that stems from the composition modulation of monolayer TMD alloys (e.g., MoS2xSe2(1 - x)) and inflicts fracture on the crystals. We find that the starting crystal (MoSe2) fails to adjust its lattice constant as the atoms of the host crystal (selenium) are replaced by foreign atoms (sulfur) during the alloying process. Thus, the resulting alloy forms a stretched lattice and experiences a large biaxial tensile strain. Our experiments show that the biaxial strain relaxes via formation of cracks in interior crystal domains or through less constraint bounds at the edge of the monolayer alloys. Griffith’s criterion suggests that defects combined with a sulfur-rich environment have the potential to significantly reduce the critical strain at which cracking occurs. Our calculations demonstrate a substantial reduction in fracture-inducing critical strain from 11% (in standard TMD crystals) to a range below 4% in as-synthesized alloys.

    更新日期:2018-04-30
  • Author Correction: Structural transformation of layered double hydroxides: an in situ TEM analysis
    npj 2D Mater. Appl. Pub Date : 2018-04-27
    Christopher Hobbs, Sonia Jaskaniec, Eoin K. McCarthy, Clive Downing, Konrad Opelt, Konrad Güth, Aleksey Shmeliov, Maurice C. D. Mourad, Karl Mandel, Valeria Nicolosi

    Author Correction: Structural transformation of layered double hydroxides: an in situ TEM analysisAuthor Correction: Structural transformation of layered double hydroxides: an in situ TEM analysis, Published online: 27 April 2018; doi:10.1038/s41699-018-0054-6Author Correction: Structural transformation of layered double hydroxides: an in situ TEM analysis

    更新日期:2018-04-27
  • Out-of-plane interface dipoles and anti-hysteresis in graphene-strontium titanate hybrid transistor
    npj 2D Mater. Appl. Pub Date : 2018-04-09
    Anindita Sahoo, Dhani Nafday, Tathagata Paul, Roald Ruiter, Arunesh Roy, Maxim Mostovoy, Tamalika Banerjee, Tanusri Saha-Dasgupta, Arindam Ghosh

    The out-of-plane electric polarization at the surface of SrTiO3 (STO), an archetypal perovskite oxide, may stabilize new electronic states and/or host novel device functionality. This is particularly significant in proximity to atomically thin membranes, such as graphene, although a quantitative understanding of the polarization across graphene–STO interface remains experimentally elusive. Here, we report direct observation and measurement of a large intrinsic out-of-plane polarization at the interface of single-layer graphene and TiO2-terminated STO (100) crystal. Using a unique temperature dependence of anti-hysteretic gate-transfer characteristics in dual-gated graphene-on-STO field-effect transistors, we estimate the polarization to be as large as ≈12 μC cm−2, which is also supported by the density functional theory calculations and low-frequency noise measurements. The anti-hysteretic transfer characteristics is quantitatively shown to arise from an interplay of band bending at the STO surface and electrostatic potential due to interface polarization, which may be a generic feature in hybrid electronic devices from two-dimensional materials and perovskite oxides.

    更新日期:2018-04-09
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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