hBN

/Tag: hBN

Visualizing Encapsulated Graphene, its Defects and its Charge Environment by Sub-Micrometer Resolution Electrical Imaging. (arXiv:1811.05912v1 [cond-mat.mes-hall])

2018-11-15T04:30:20+00:00November 15th, 2018|Categories: Publications|Tags: , , |

Devices made from two-dimensional (2D) materials such as graphene or transition metal dichalcogenides possess interesting electronic properties that can become accessible to experimental probes when the samples are protected from deleterious environmental effects by encapsulating them between hexagonal boron nitride (hBN) layers. While the encapsulated flakes can be detected through post-processing of optical images or confocal Raman mapping, these techniques lack the sub-micrometer scale resolution to identify tears, structural defects or impurities, which is crucial for the fabrication of high-quality devices. Here we demonstrate a simple method to visualize such buried flakes with sub-micrometer resolution, by combining Kelvin force probe microscopy (KPFM) with electrostatic force microscopy (EFM). KPFM, which measures surface potential fluctuations, is extremely effective in spotting charged contaminants within and on top of the heterostructure, making it possible to distinguish contaminated regions in the buried flake. When applying a tip bias larger than the surface potential fluctuations, EFM becomes extremely efficient in highlighting encapsulated flakes and their sub-micron structural defects. We show that these imaging modes, which are standard extensions of atomic force microscopy (AFM), are perfectly suited for locating encapsulated conductors, for visualizing nanometer scale defects and bubbles, and for characterizing their local charge environment.

Published : "arXiv Mesoscale and Nanoscale Physics".

Characterization of Lattice Thermal Transport in Two-Dimensional BAs, BP, and BSb: A First-Principles Study. (arXiv:1811.05597v1 [cond-mat.mtrl-sci])

2018-11-15T02:29:22+00:00November 15th, 2018|Categories: Publications|Tags: |

Ever since the high thermal conductivity in cubic boron arsenide (c-BAs) was predicted theoretically by Lindsay et. al in 2013, countless studies have zeroed in on this particular material. Most recently, c-BAs has been confirmed experimentally to have a thermal conductivity of around 1,100 W/m-K. In this study, we investigate the seldom studied two dimensional hexagonal form of boron arsenide (h-BAs) using a first-principles approach and by solving the Boltzmann Transport Equation for phonons. Traditionally, a good indicator of a high thermal conductivity material is its high Debye temperature and high phonon group velocity. However, we determine h-BAs to have a much lower Debye temperature and average phonon group velocity compared to the other monolayer boron-V compounds of boron nitride (h-BN) and boron phosphide (h-BP), yet curiously it possesses a higher thermal conductivity. Further investigation reveals that this is due to the phonon frequency gap caused by large mass imbalances, which results in a restricted Umklapp phonon-phonon scattering channel and consequently a higher thermal conductivity. We determine the intrinsic lattice thermal conductivity of monolayer h-BAs to be 362.62 W/m-K at room temperature, which is considerably higher compared to the other monolayer boron-V compounds of h-BN (231.96 W/m-K), h-BP (187.11 W/m-K), and h-BSb (87.15 W/m-K). This study opens the door for investigation into a new class of monolayer structures and the properties they possess.

Published in: "arXiv Material Science".

Toward the Identification of Atomic Defects in Hexagonal Boron Nitride: X-Ray Photoelectron Spectroscopy and First-Principles Calculations. (arXiv:1811.05924v1 [cond-mat.mtrl-sci])

2018-11-15T02:29:15+00:00November 15th, 2018|Categories: Publications|Tags: , |

Defects in hexagonal boron nitride (hBN) exhibit single-photon emission (SPE) and are thus attracting broad interest as platforms for quantum information and spintronic applications. However, the atomic structure and the specific impact of the local environment on the defect physical properties remain elusive. Here we articulate X-ray photoelectron spectroscopy (XPS) and first-principles calculations to discern the experimentally-observed point defects responsible for the quantum emission observed in hBN. XPS measurements show a broad band, which was deconvolved and then assigned to $N_{B}V_{N}$, $V_{N}$, $C_{B}$, $C_{B}V_{N}$, and $O_{2B}V_{N}$ defect structures using Density Functional Theory (DFT) core-level binding energy (BE) calculations.

Published in: "arXiv Material Science".

Valley-polarised tunnelling currents in bilayer graphene tunnelling transistors. (arXiv:1811.04687v1 [cond-mat.mes-hall])

2018-11-13T04:30:24+00:00November 13th, 2018|Categories: Publications|Tags: , |

We study theoretically the electron current across a monolayer graphene/hexagonal boron nitride/bilayer graphene tunnelling junction in an external magnetic field perpendicular to the layers. We show that change in effective tunnelling barrier width for electrons on different graphene layers of bilayer graphene, coupled with the fact that its Landau level wave functions are not equally distributed amongst the layers with a distribution that is reversed between the two valleys, lead to valley polarisation of the tunnelling current. We estimate that valley polarisation $sim 70%$ can be achieved in high quality devices at $B=1$ T. Moreover, we demonstrate that strong valley polarisation can be obtained both in the limit of strong-momentum conserving tunnelling and in lower quality devices where this constraint is lifted.

Published : "arXiv Mesoscale and Nanoscale Physics".

Growth of lateral graphene/h-BN heterostructure on copper foils by chemical vapor deposition

2018-11-12T16:34:33+00:00November 12th, 2018|Categories: Publications|Tags: , , |

The synthesis of lateral heterostructures assembled by atomically-thin materials with distinct intrinsic properties is important for future heterojunction-embedded two-dimensional (2D) devices. Here we report an etching-assisted chemical vapor deposition method to synthesize large-area continuous lateral graphene/hexagonal boron nitride (Gr/h-BN) heterostructures on carbon-containing copper foils. The h-BN film is first synthesized on the copper foil, followed by hydrogen etching, and then epitaxial graphene domains are grown to form continuous lateral heterostructures. Analyses, including Raman spectroscopy, atomic force microscopy, scanning electron microscopy, x-ray photoelectron spectroscopy, and ultraviolet-visible absorption spectroscopy, are used to characterize the coexistence of both materials and the highly continuous nature of this lateral heterostructure. This facile and scalable synthesizing method enables the potential usage of Gr/h-BN heterostructure in both funda…

Published in: "Nanotechnology".

Biomimetic Artificial Nacre: Boron Nitride Nanosheets/Gelatin Nanocomposites for Biomedical Applications

2018-11-11T04:32:23+00:00November 11th, 2018|Categories: Publications|Tags: |

Through a vacuum‐assisted self‐assembly process, boron nitride nanosheets (BNNSs) and gelatin are assembled into bioinspired nacre‐like structures with controllable mechanical and biological properties, while varying the amount of BNNS in the nanocomposite material. Furthermore, functionalization by hyperbranched polyglycerol further enhances the mechanical properties of the artificial nacre, whose performance resembles that of human cortical bone. Abstract A bioinspired boron nitride nanosheet (BNNS)/gelatin nanocomposite consisting of hierarchically aligned layered BNNSs bonded with gelatin is fabricated by using electrostatic interactions between the oppositely charged functional groups on gelatin and the BNNSs. To enhance the self‐assembly and interfacial bond strength between these entities, the BNNSs are functionalized using hyperbranched polyglycerol. The 2D alignment of the BNNSs can be controlled by increasing the amount of BNNSs, or by a functionalization process, both of which result in transformation of the nanoscale structure from a randomly oriented to a brick‐and‐mortar structure. The mechanical properties of the resulting nanocomposite material, including the strength and modulus, are controlled by changing the composition of BNNSs and gelatin in the nanocomposite and by modifying the degree of alignment of the BNNSs in the material. This tuning of the mechanical properties yields a material whose performance resembles that of human cortical bone. In vitro cell viability experiments on the BNNSs/gelatin nanocomposite reveal that this artificial nacre supports adhesion, viability, and proliferation of adipose‐derived stem cells, all of which are essential for biomedical applications. Mechanical and biological testing of the material suggests applications of artificial nacre in biomedical fields and for

Published in: "Advanced Functional Materials".

Uncovering the Conduction Behavior of van der Waals Ambipolar Semiconductors

2018-11-10T22:34:32+00:00November 10th, 2018|Categories: Publications|Tags: |

The conduction behavior of ambipolar van der Waals semiconductors (vdWS) is analytically and theoretically studied. It is found that ambipolar vdWS can be fully captured by a Schottky‐barrier field‐effect transistor (FET) model. In addition, it is demonstrated that metal/vdWS/substrate interactions play a crucial role in tuning the Schottky‐barrier heights, which finally determines the conduction behavior that ambipolar vdWS exhibit. Abstract A long‐standing puzzle about van der Waals semiconductors (vdWS) is regarding the origin(s) of the conduction behavior they exhibit. Of particular interest are those with ambipolar conduction, which may provide an alternative choice for practical applications when considering the difficulties of doping the ultrathin bodies of vdWS. Here, the conduction behavior of ambipolar vdWS is analytically and theoretically studied. Using numerical simulation, it is shown that ambipolar vdWS can be fully captured by a Schottky‐barrier FET model. Based on this, it is found that the widely observed conduction polarity transition while changing the body thickness mainly comes from the tuning of band alignment at the metal/vdWS interfaces. This transition can be suppressed/inversed by introducing an inert hBN layer between the vdWS and the substrate. Through first‐principles calculations, it is demonstrated that metal/vdWS/substrate interactions play a crucial role in tuning the Schottky‐barrier heights, which finally determines the conduction behavior that ambipolar vdWS exhibit.

Published in: "Advanced Materials".

Nonvolatile and Programmable Photodoping in MoTe2 for Photoresist‐Free Complementary Electronic Devices

2018-11-10T22:34:20+00:00November 10th, 2018|Categories: Publications|Tags: , , |

A photoresist‐free p–n junction and inverter in the MoTe2 homostructure are achieved by spatially controlling the photodoping region in the MoTe2/BN heterostructure. The MoTe2 diode demonstrates an ideality factor of ≈1.13 with a current on/off ratio of ≈1.7 × 104, and the gain of the inverter reaches ≈98, illustrating its great potential in the application of 2D logic electronics. Abstract 2D transition‐metal dichalcogenide (TMD)‐based electronic devices have been extensively explored toward the post‐Moore era. Huge efforts have been devoted to modulating the doping profile of TMDs to achieve 2D p–n junctions and inverters, the fundamental units in logic circuits. Here, photoinduced nonvolatile and programmable electron doping in MoTe2 based on a heterostructure of MoTe2 and hexagonal boron nitride (BN) is reported. The electron transport property in the MoTe2 device can be precisely controlled by modulating the magnitude of the photodoping gate exerted on BN. Through tuning the polarity of the photodoping gate exerted on BN under illumination, such a doping effect in MoTe2 can be programmed with excellent repeatability and is retained for over 14 d in the absence of an external perturbation. By spatially controlling the photodoping region in MoTe2, a photoresist‐free p–n junction and inverter in the MoTe2 homostructure are achieved. The MoTe2 diode exhibits a near‐unity ideality factor of ≈1.13 with a rectification ratio of ≈1.7 × 104. Moreover, the gain of the MoTe2 inverter reaches ≈98, which is among the highest values for 2D‐material‐based homoinverters. These findings promise photodoping as an effective method to achieve

Published in: "Advanced Materials".

Moir’e Intralayer Excitons in a MoSe$_2$/MoS$_2$ Heterostructure. (arXiv:1811.03408v1 [cond-mat.mes-hall])

2018-11-09T04:30:31+00:00November 9th, 2018|Categories: Publications|Tags: , , |

Spatially periodic structures with a long range period, referred to as moir’e pattern, can be obtained in van der Waals bilayers in the presence of a small stacking angle or of lattice mismatch between the monolayers. Theoretical predictions suggest that the resulting spatially periodic variation of the band structure modifies the optical properties of both intra and interlayer excitons of transition metal dichalcogenides heterostructures. Here, we report on the impact of the moir’e pattern formed in a MoSe$_2$/MoS$_2$ heterobilayer encapsulated in hexagonal boron nitride. The periodic in-plane potential results in a splitting of the MoSe$_2$ exciton and trion in both emission and absorption spectra. The observed energy difference between the split peaks is fully consistent with theoretical predictions.

Published : "arXiv Mesoscale and Nanoscale Physics".

Super-ideal diodes at the Schottky-Mott limit in gated graphene-WSe$_2$ heterojunctions. (arXiv:1811.02660v1 [cond-mat.mes-hall])

2018-11-08T04:30:19+00:00November 8th, 2018|Categories: Publications|Tags: , , , , |

Metal-semiconductor interfaces, known as Schottky junctions, have long been hindered by defects and impurities. Such imperfections dominate the electrical characteristics of the junction by pinning the metal Fermi energy. We report measurements on a boron nitride encapsulated graphene-tungsten diselenide (WSe$_2$) Schottky junction which exhibits ideal diode characteristics and a complete lack of Fermi-level pinning. The Schottky barrier height of the device is rigidly tuned by electrostatic gating of the WSe$_2$, enabling experimental verification of the Schottky-Mott limit in a single device. Utilizing this exceptional gate control, we realize a super-ideal gated-Schottky diode which surpasses the ideal diode limit. Our results provide a pathway for defect-free electrical contact to two-dimensional semiconductors and open up possibilities for circuits with efficient switching characteristics and higher efficiency optoelectronic devices.

Published : "arXiv Mesoscale and Nanoscale Physics".

Uncovering the Conduction Behavior of van der Waals Ambipolar Semiconductors

2018-11-07T08:35:53+00:00November 7th, 2018|Categories: Publications|Tags: |

The conduction behavior of ambipolar van der Waals semiconductors (vdWS) is analytically and theoretically studied. It is found that ambipolar vdWS can be fully captured by a Schottky‐barrier field‐effect transistor (FET) model. In addition, it is demonstrated that metal/vdWS/substrate interactions play a crucial role in tuning the Schottky‐barrier heights, which finally determines the conduction behavior that ambipolar vdWS exhibit. Abstract A long‐standing puzzle about van der Waals semiconductors (vdWS) is regarding the origin(s) of the conduction behavior they exhibit. Of particular interest are those with ambipolar conduction, which may provide an alternative choice for practical applications when considering the difficulties of doping the ultrathin bodies of vdWS. Here, the conduction behavior of ambipolar vdWS is analytically and theoretically studied. Using numerical simulation, it is shown that ambipolar vdWS can be fully captured by a Schottky‐barrier FET model. Based on this, it is found that the widely observed conduction polarity transition while changing the body thickness mainly comes from the tuning of band alignment at the metal/vdWS interfaces. This transition can be suppressed/inversed by introducing an inert hBN layer between the vdWS and the substrate. Through first‐principles calculations, it is demonstrated that metal/vdWS/substrate interactions play a crucial role in tuning the Schottky‐barrier heights, which finally determines the conduction behavior that ambipolar vdWS exhibit.

Published in: "Advanced Materials".

Nonvolatile and Programmable Photodoping in MoTe2 for Photoresist‐Free Complementary Electronic Devices

2018-11-07T08:35:35+00:00November 7th, 2018|Categories: Publications|Tags: , , |

A photoresist‐free p–n junction and inverter in the MoTe2 homostructure are achieved by spatially controlling the photodoping region in the MoTe2/BN heterostructure. The MoTe2 diode demonstrates an ideality factor of ≈1.13 with a current on/off ratio of ≈1.7 × 104, and the gain of the inverter reaches ≈98, illustrating its great potential in the application of 2D logic electronics. Abstract 2D transition‐metal dichalcogenide (TMD)‐based electronic devices have been extensively explored toward the post‐Moore era. Huge efforts have been devoted to modulating the doping profile of TMDs to achieve 2D p–n junctions and inverters, the fundamental units in logic circuits. Here, photoinduced nonvolatile and programmable electron doping in MoTe2 based on a heterostructure of MoTe2 and hexagonal boron nitride (BN) is reported. The electron transport property in the MoTe2 device can be precisely controlled by modulating the magnitude of the photodoping gate exerted on BN. Through tuning the polarity of the photodoping gate exerted on BN under illumination, such a doping effect in MoTe2 can be programmed with excellent repeatability and is retained for over 14 d in the absence of an external perturbation. By spatially controlling the photodoping region in MoTe2, a photoresist‐free p–n junction and inverter in the MoTe2 homostructure are achieved. The MoTe2 diode exhibits a near‐unity ideality factor of ≈1.13 with a rectification ratio of ≈1.7 × 104. Moreover, the gain of the MoTe2 inverter reaches ≈98, which is among the highest values for 2D‐material‐based homoinverters. These findings promise photodoping as an effective method to achieve

Published in: "Advanced Materials".

Charge Density Waves Driven by Peierls Instability at the Interface of Two‐Dimensional Lateral Heterostructures

2018-11-07T08:35:22+00:00November 7th, 2018|Categories: Publications|Tags: , , , , |

Charge density wave (CDW) formation at the one‐dimensional interface embedded in a lateral two‐dimensional (2D) heterostructure comprising blue and black phosphorene is found. The CDW formation is driven by the Peierls instability and substantially modifies the band alignment of the heterostructure. These findings are applicable to other 2D lateral heterostructures and have important implications for their application. Abstract The origin of charge density wave (CDW) observed in low‐dimensional systems is, for long, a subject of intensive debate in contemporary condensed matter physics. Specifically, a simple and well established model, namely, the Peierls instability is often (but not always) used to clearly explain CDW states in real systems. Here, first‐principles density functional theory calculations are used to show CDW formation at a one‐dimensional interface embedded in a lateral heterostructure comprising blue and black phosphorene, even at room temperature. The CDW formation is fully explained by the Peierls mechanism, including a double‐periodicity lattice distortion energy lowering and a bandgap opening. The lattice distortion also substantially modifies the band alignment of the heterostructure. Comparison with a freestanding P chain shows that the structural distortion is confined to one dimension within the heterostructures, ruling out competing non‐Peierls‐type distortions in two dimensions. In addition, similar Peierls‐type distortions for other lateral heterostructures are shown by using the example of a graphene–hexagonal boron nitride heterostructure, which may stimulate related studies in different 2D systems. These findings not only shed more light on the Peierls mechanism, but also have important implications for devices based on 2D lateral

Published in: "Small".

Uncovering the Conduction Behavior of van der Waals Ambipolar Semiconductors

2018-11-07T00:34:52+00:00November 6th, 2018|Categories: Publications|Tags: |

The conduction behavior of ambipolar van der Waals semiconductors (vdWS) is analytically and theoretically studied. It is found that ambipolar vdWS can be fully captured by a Schottky‐barrier field‐effect transistor (FET) model. In addition, it is demonstrated that metal/vdWS/substrate interactions play a crucial role in tuning the Schottky‐barrier heights, which finally determines the conduction behavior that ambipolar vdWS exhibit. Abstract A long‐standing puzzle about van der Waals semiconductors (vdWS) is regarding the origin(s) of the conduction behavior they exhibit. Of particular interest are those with ambipolar conduction, which may provide an alternative choice for practical applications when considering the difficulties of doping the ultrathin bodies of vdWS. Here, the conduction behavior of ambipolar vdWS is analytically and theoretically studied. Using numerical simulation, it is shown that ambipolar vdWS can be fully captured by a Schottky‐barrier FET model. Based on this, it is found that the widely observed conduction polarity transition while changing the body thickness mainly comes from the tuning of band alignment at the metal/vdWS interfaces. This transition can be suppressed/inversed by introducing an inert hBN layer between the vdWS and the substrate. Through first‐principles calculations, it is demonstrated that metal/vdWS/substrate interactions play a crucial role in tuning the Schottky‐barrier heights, which finally determines the conduction behavior that ambipolar vdWS exhibit.

Published in: "Advanced Materials".

Charge Density Waves Driven by Peierls Instability at the Interface of Two‐Dimensional Lateral Heterostructures

2018-11-07T00:34:10+00:00November 6th, 2018|Categories: Publications|Tags: , , , , |

Charge density wave (CDW) formation at the one‐dimensional interface embedded in a lateral two‐dimensional (2D) heterostructure comprising blue and black phosphorene is found. The CDW formation is driven by the Peierls instability and substantially modifies the band alignment of the heterostructure. These findings are applicable to other 2D lateral heterostructures and have important implications for their application. Abstract The origin of charge density wave (CDW) observed in low‐dimensional systems is, for long, a subject of intensive debate in contemporary condensed matter physics. Specifically, a simple and well established model, namely, the Peierls instability is often (but not always) used to clearly explain CDW states in real systems. Here, first‐principles density functional theory calculations are used to show CDW formation at a one‐dimensional interface embedded in a lateral heterostructure comprising blue and black phosphorene, even at room temperature. The CDW formation is fully explained by the Peierls mechanism, including a double‐periodicity lattice distortion energy lowering and a bandgap opening. The lattice distortion also substantially modifies the band alignment of the heterostructure. Comparison with a freestanding P chain shows that the structural distortion is confined to one dimension within the heterostructures, ruling out competing non‐Peierls‐type distortions in two dimensions. In addition, similar Peierls‐type distortions for other lateral heterostructures are shown by using the example of a graphene–hexagonal boron nitride heterostructure, which may stimulate related studies in different 2D systems. These findings not only shed more light on the Peierls mechanism, but also have important implications for devices based on 2D lateral

Published in: "Small".

Biomimetic Artificial Nacre: Boron Nitride Nanosheets/Gelatin Nanocomposites for Biomedical Applications

2018-11-07T00:32:42+00:00November 6th, 2018|Categories: Publications|Tags: |

Through a vacuum‐assisted self‐assembly process, boron nitride nanosheets (BNNSs) and gelatin are assembled into bioinspired nacre‐like structures with controllable mechanical and biological properties, while varying the amount of BNNS in the nanocomposite material. Furthermore, functionalization by hyperbranched polyglycerol further enhances the mechanical properties of the artificial nacre, whose performance resembles that of human cortical bone. Abstract A bioinspired boron nitride nanosheet (BNNS)/gelatin nanocomposite consisting of hierarchically aligned layered BNNSs bonded with gelatin is fabricated by using electrostatic interactions between the oppositely charged functional groups on gelatin and the BNNSs. To enhance the self‐assembly and interfacial bond strength between these entities, the BNNSs are functionalized using hyperbranched polyglycerol. The 2D alignment of the BNNSs can be controlled by increasing the amount of BNNSs, or by a functionalization process, both of which result in transformation of the nanoscale structure from a randomly oriented to a brick‐and‐mortar structure. The mechanical properties of the resulting nanocomposite material, including the strength and modulus, are controlled by changing the composition of BNNSs and gelatin in the nanocomposite and by modifying the degree of alignment of the BNNSs in the material. This tuning of the mechanical properties yields a material whose performance resembles that of human cortical bone. In vitro cell viability experiments on the BNNSs/gelatin nanocomposite reveal that this artificial nacre supports adhesion, viability, and proliferation of adipose‐derived stem cells, all of which are essential for biomedical applications. Mechanical and biological testing of the material suggests applications of artificial nacre in biomedical fields and for

Published in: "Advanced Functional Materials".

Moiré Valleytronics: Realizing Dense Arrays of Topological Helical Channels

2018-11-02T16:34:35+00:00November 2nd, 2018|Categories: Publications|Tags: , |

Author(s): Chen Hu, Vincent Michaud-Rioux, Wang Yao, and Hong GuoWe propose a general and robust platform, the moiré valleytronics, to realize high-density arrays of 1D topological helical channels in real materials at room temperature. We demonstrate the idea using a long-period 1D moiré pattern of graphene on hBN by first-principles calculation. Through calcula…[Phys. Rev. Lett. 121, 186403] Published Fri Nov 02, 2018

Published in: "Physical Review Letters".

Direct nanomechanical measurements of boron nitride nanotube—ceramic interfaces

2018-11-02T10:33:59+00:00November 2nd, 2018|Categories: Publications|Tags: , |

Boron nitride nanotubes (BNNTs) are a unique class of light and strong tubular nanostructure and are highly promising as reinforcing additives in ceramic materials. However, the mechanical strength of BNNT-ceramic interfaces remains largely unexplored. Here we report the first direct measurement of the interfacial strength by pulling out individual BNNTs from silica (silicon dioxide) matrices using in situ electron microscopy techniques. Our nanomechanical measurements show that the average interfacial shear stress reaches about 34.7 MPa, while density functional theory calculations reveal strong bonded interactions between BN and silica lattices with a binding energy of –6.98 eV nm −2 . Despite this strong BNNT-silica binding, nanotube pull-out remains the dominant failure mode without noticeable silica matrix residues on the pulled-out tube surface. The fracture toughness of BNNT-silica ceramic matrix nanocomposite is evaluated based on the measured interfacial…

Published in: "Nanotechnology".

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