Heterostructures

/Tag: Heterostructures

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".

Direct Chirality Recognition of Single‐Crystalline and Single‐Walled Transition Metal Oxide Nanotubes on Carbon Nanotube Templates

2018-11-07T10:34:06+00:00November 7th, 2018|Categories: Publications|Tags: , |

Single‐crystalline molybdenum trioxide nanotubes (MONTs) are fabricated on carbon nanotube (CNT) templates. Single‐crystalline and single‐walled MONTs are prepared by van der Waals epitaxy on CNT templates. The chiral angles of the MONTs can be directly observed under an electron microscope. The MONTs show a preferred orientation with the [001] direction along the CNT axis due to the anisotropic bending rigidity of the monolayers. Abstract Chirality is a significant structural feature for chemistry, biology, physics, and materials science, and especially determines the electrical, mechanical, and optical properties of diverse tubular structures, such as carbon nanotubes (CNTs). To recognize the chirality of nanotubes, templates are introduced as potential tools to obtain crystalline samples with visible chiral fringes under electron microscopes. However, few efforts show optimistic results, and new understanding is desired to control the sample quality with CNT templates. Here, a synthesis strategy of single‐crystalline molybdenum trioxide (α‐MoO3) nanotubes (MONTs) on CNT surfaces is reported to build a 1D van der Waals (vdW) heterostructure. The chirality of the MONTs can be directly “seen” and their structural selectivity is revealed. First, the centralized distribution of the chiral angles of the MONTs indicates a preferential orientation due to the anisotropic bending rigidity of the 2D layers. Then, the interlayer mismatching rejects the radial stacking of α‐MoO3 to maintain the single‐walled nature. These results provide a spontaneous strategy for the efficient recognition and control of chirality, and open up a new avenue for CNT‐based functional 1D vdW heterostructures.

Published in: "Advanced Materials".

Quaternary Alloys: Thermally Induced 2D Alloy‐Heterostructure Transformation in Quaternary Alloys (Adv. Mater. 45/2018)

2018-11-07T10:34:00+00:00November 7th, 2018|Categories: Publications|Tags: |

In article number 1804218, Jordan A. Hachtel, Boris I. Yakobson, Chandra Sekhar Tiwary, Pulickel M. Ajayan, and co‐workers demonstrate that simple annealing of a 2D quaternary (Mo–W–S–Se) alloy results in a thermodynamically stable atomically thin heterostructure of Mo–S–Se and W–S–Se. Transition‐metal‐dichalcogenide alloys (Mo–W–S or Mo–W–Se) are more stable due to negative entropy of mixing.

Published in: "Advanced Materials".

Heterostructures Based on 2D Materials: A Versatile Platform for Efficient Catalysis

2018-11-07T08:36:04+00:00November 7th, 2018|Categories: Publications|Tags: , |

The construction of different heterostructures based on 2D materials offers great opportunities for boosting the catalytic activity in electo(photo)chemical reactions. Starting from the theoretical background of the fundamental concepts, the progressive developments in the design and applications of heterostructures based on 2D materials are summarized. Abstract The unique structural and electronic properties of 2D materials, including the metal and metal‐free ones, have prompted intense exploration in the search for new catalysts. The construction of different heterostructures based on 2D materials offers great opportunities for boosting the catalytic activity in electo(photo)chemical reactions. Particularly, the merits resulting from the synergism of the constituent components and the fascinating properties at the interface are tremendously interesting. This scenario has now become the state‐of‐the‐art point in the development of active catalysts for assisting energy conversion reactions including water splitting and CO2 reduction. Here, starting from the theoretical background of the fundamental concepts, the progressive developments in the design and applications of heterostructures based on 2D materials are traced. Furthermore, a personal perspective on the exploration of 2D heterostructures for further potential application in catalysis is offered.

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".

Reconfigurable edge-state engineering in graphene using LaAlO$_3$/SrTiO$_3$ nanostructures. (arXiv:1811.02030v1 [cond-mat.mes-hall])

2018-11-07T04:30:19+00:00November 7th, 2018|Categories: Publications|Tags: , |

The properties of graphene depend sensitively on doping with respect to the charge-neutrality point (CNP). Tuning the CNP usually requires electrical gating or chemical doping. Here, we describe a technique to reversibly control the CNP in graphene with nanoscale precision, utilizing LaAlO$_3$/SrTiO$_3$ (LAO/STO) heterostructures and conductive atomic force microscope (c-AFM) lithography. The local electron density and resulting conductivity of the LAO/STO interface can be patterned with a conductive AFM tip, and placed within two nanometers of an active graphene device. The proximal LAO/STO nanostructures shift the position of graphene CNP by ~ $10^{12}$ cm$^{-2}$, and are also gateable. Here we use this effect to create reconfigurable edge states in graphene, which are probed using the quantum Hall effect. Quantized resistance plateaus at $h/e^2$ and $h/3e^2$ are observed in a split Hall device, demonstrating edge transport along the c-AFM written edge that depends on the polarity of both the magnetic field and direction of currents. This technique can be readily extended to other device geometries.

Published : "arXiv Mesoscale and Nanoscale Physics".

Direct Chirality Recognition of Single‐Crystalline and Single‐Walled Transition Metal Oxide Nanotubes on Carbon Nanotube Templates

2018-11-07T00:35:26+00:00November 6th, 2018|Categories: Publications|Tags: , |

Single‐crystalline molybdenum trioxide nanotubes (MONTs) are fabricated on carbon nanotube (CNT) templates. Single‐crystalline and single‐walled MONTs are prepared by van der Waals epitaxy on CNT templates. The chiral angles of the MONTs can be directly observed under an electron microscope. The MONTs show a preferred orientation with the [001] direction along the CNT axis due to the anisotropic bending rigidity of the monolayers. Abstract Chirality is a significant structural feature for chemistry, biology, physics, and materials science, and especially determines the electrical, mechanical, and optical properties of diverse tubular structures, such as carbon nanotubes (CNTs). To recognize the chirality of nanotubes, templates are introduced as potential tools to obtain crystalline samples with visible chiral fringes under electron microscopes. However, few efforts show optimistic results, and new understanding is desired to control the sample quality with CNT templates. Here, a synthesis strategy of single‐crystalline molybdenum trioxide (α‐MoO3) nanotubes (MONTs) on CNT surfaces is reported to build a 1D van der Waals (vdW) heterostructure. The chirality of the MONTs can be directly “seen” and their structural selectivity is revealed. First, the centralized distribution of the chiral angles of the MONTs indicates a preferential orientation due to the anisotropic bending rigidity of the 2D layers. Then, the interlayer mismatching rejects the radial stacking of α‐MoO3 to maintain the single‐walled nature. These results provide a spontaneous strategy for the efficient recognition and control of chirality, and open up a new avenue for CNT‐based functional 1D vdW heterostructures.

Published in: "Advanced Materials".

Quaternary Alloys: Thermally Induced 2D Alloy‐Heterostructure Transformation in Quaternary Alloys (Adv. Mater. 45/2018)

2018-11-07T00:35:12+00:00November 6th, 2018|Categories: Publications|Tags: |

In article number 1804218, Jordan A. Hachtel, Boris I. Yakobson, Chandra Sekhar Tiwary, Pulickel M. Ajayan, and co‐workers demonstrate that simple annealing of a 2D quaternary (Mo–W–S–Se) alloy results in a thermodynamically stable atomically thin heterostructure of Mo–S–Se and W–S–Se. Transition‐metal‐dichalcogenide alloys (Mo–W–S or Mo–W–Se) are more stable due to negative entropy of mixing.

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".

Charge Transfer within the F4TCNQ‐MoS2 van der Waals Interface: Toward Electrical Properties Tuning and Gas Sensing Application

2018-11-07T00:32:44+00:00November 6th, 2018|Categories: Publications|Tags: , |

Employing the organic charge transfer material 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane, the electrical properties of MoS2 field‐effect transistors including V on’s and subthreshold swings are successfully modulated with no degradation of mobility. The charge transfer mechanism is investigated by first‐principles calculation and scanning Kelvin probe microscopy. A high‐performance NH3 gas sensor fabricated from this transistor reaches more than 1000% sensitivity at 100 ppm. Abstract The development of van der Waals heterostructures in 2D materials systems has attracted considerable interests for exploring new insights of (opto‐) electrical characteristics, device physics, and novel functional applications. Utilizing organic molecular material with strong electron withdrawing ability, charge transfer van der Waals interfaces are formed between 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) and MoS2, via which the modulation of the onset voltages and optimization of subthreshold swing values in MoS2‐based field effect transistors are realized. Charge transfer process and its functionality mechanisms are further verified and investigated with first‐principles calculation, scanning Kelvin probe microscope characterization, and temperature‐dependent electrical characterization. With the charge transfer effect between reducing gas molecules and F4TCNQ, NH3 gas sensor is proposed and fabricated with the sensitivity reaching higher than 1000% at 100 ppm, much more outstanding performance than those of any reported MoS2‐based NH3 gas sensors. The F4TCNQ‐MoS2 hybrid strategy might open up a pathway for tuning and optimizing the electrical properties, in addition to novel functional units designing and fabrications in electric devices based on low‐dimensional semiconducting systems.

Published in: "Advanced Functional Materials".

Room‐Temperature Ferroelectricity in Hexagonally Layered α‐In2Se3 Nanoflakes down to the Monolayer Limit

2018-11-07T00:32:16+00:00November 6th, 2018|Categories: Publications|Tags: , |

The thinnest layered ferroelectric is demonstrated for the first time at room temperature. The semiconducting hexagonal α‐In2Se3 nanoflakes exhibit out‐of‐plane and in‐plane ferroelectricity that are closely intercorrelated. The polarization switching and hysteresis loops can be realized in the thickness as thin as ≈2.3 nm (bilayer) and ≈1.2 nm (monolayer). Two types of ferroelectric switchable devices are proposed to show the potential application in nonvolatile memories. Abstract 2D ferroelectric material has emerged as an attractive building block for high‐density data storage nanodevices. Although monolayer van der Waals ferroelectrics have been theoretically predicted, a key experimental breakthrough for such calculations is still not realized. Here, hexagonally stacking α‐In2Se3 nanoflake, a rarely studied van der Waals polymorph, is reported to exhibit out‐of‐plane (OOP) and in‐plane (IP) ferroelectricity at room temperature. Ferroelectric multidomain states in a hexagonal α‐In2Se3 nanoflake with uniform thickness can survive to 6 nm. Most strikingly, the electric‐field‐induced polarization switching and hysteresis loop are, respectively, observed down to the bilayer and monolayer (≈1.2 nm) thicknesses, which designates it as the thinnest layered ferroelectric and verifies the corresponding theoretical calculation. In addition, two types of ferroelectric nanodevices employing the OOP and IP polarizations in 2H α‐In2Se3 are developed, which are applicable for nonvolatile memories and heterostructure‐based nanoelectronics/optoelectronics.

Published in: "Advanced Functional Materials".

Electrical plasmon injection in double-layer graphene heterostructures. (arXiv:1811.00460v1 [cond-mat.mes-hall])

2018-11-02T04:30:20+00:00November 2nd, 2018|Categories: Publications|Tags: , |

It is by now well established that high-quality graphene enables a gate-tunable low-loss plasmonic platform for the efficient confinement, enhancement, and manipulation of optical fields spanning a broad range of frequencies, from the mid infrared to the Terahertz domain. While all-electrical detection of graphene plasmons has been demonstrated, electrical plasmon injection (EPI), which is crucial to operate nanoplasmonic devices without the encumbrance of a far-field optical apparatus, remains elusive. In this work, we present a theory of EPI in double-layer graphene, where a vertical tunnel current excites acoustic and optical plasmon modes. We first calculate the power delivered by the applied inter-layer voltage bias into these collective modes. We then show that this system works also as a spectrally-resolved molecular sensor.

Published : "arXiv Mesoscale and Nanoscale Physics".

Manifold Learning of Four-dimensional Scanning Transmission Electron Microscopy. (arXiv:1811.00080v1 [eess.IV])

2018-11-02T02:29:27+00:00November 2nd, 2018|Categories: Publications|Tags: , |

Four-dimensional scanning transmission electron microscopy (4D-STEM) of local atomic diffraction patterns is emerging as a powerful technique for probing intricate details of atomic structure and atomic electric fields. However, efficient processing and interpretation of large volumes of data remain challenging, especially for two-dimensional or light materials because the diffraction signal recorded on the pixelated arrays is weak. Here we employ data-driven manifold leaning approaches for straightforward visualization and exploration analysis of the 4D-STEM datasets, distilling real-space neighboring effects on atomically resolved deflection patterns from single-layer graphene, with single dopant atoms, as recorded on a pixelated detector. These extracted patterns relate to both individual atom sites and sublattice structures, effectively discriminating single dopant anomalies via multi-mode views. We believe manifold learning analysis will accelerate physics discoveries coupled between data-rich imaging mechanisms and materials such as ferroelectric, topological spin and van der Waals heterostructures.

Published in: "arXiv Material Science".

Probing the nanoscale origin of strain and doping in graphene-hBN heterostructures. (arXiv:1810.13315v1 [cond-mat.mtrl-sci])

2018-11-01T02:29:15+00:00November 1st, 2018|Categories: Publications|Tags: , , |

We use confocal Raman microscopy and modified vector analysis methods to investigate the nanoscale origin of strain and carrier concentration in exfoliated graphene-hexagonal boron nitride (hBN) heterostructures on silicon dioxide (SiO2). Two types of heterostructures are studied: graphene on SiO2 partially coved by hBN, and graphene fully encapsulated between two hBN flakes. We extend the vector analysis methods to produce spatial maps of the strain and doping variation across the heterostructures. This allows us to visualise and directly quantify the much-speculated effect of the environment on carrier concentration as well as strain in graphene. Moreover, we demonstrate that variations in strain and carrier concentration in graphene arise from nanoscale features of the heterostructures such as fractures, folds and bubbles trapped between layers. For bubbles in hBN-encapsulated graphene, hydrostatic strain is shown to be greatest at bubble centres, whereas the maximum of carrier concentration is localised at bubble edges. Raman spectroscopy is shown to be a non-invasive tool for probing strain and doping in graphene, which could prove useful for engineering of two-dimensional devices.

Published in: "arXiv Material Science".

Room temperature spin Hall effect in graphene/MoS$_2$ van der Waals heterostructures. (arXiv:1810.12481v1 [cond-mat.mes-hall])

2018-10-31T04:30:29+00:00October 31st, 2018|Categories: Publications|Tags: , |

Graphene is an excellent material for long distance spin transport but allows little spin manipulation. Transition metal dichalcogenides imprint their strong spin-orbit coupling into graphene via proximity effect, and it has been predicted that efficient spin-to-charge conversion due to spin Hall and Rashba-Edelstein effects could be achieved. Here, by combining Hall probes with ferromagnetic electrodes, we unambiguously demonstrate experimentally spin Hall effect in graphene induced by MoS$_2$ proximity and for varying temperature up to room temperature. The fact that spin transport and spin Hall effect occur in different parts of the same material gives rise to a hitherto unreported efficiency for the spin-to-charge voltage output. Remarkably for a single graphene/MoS$_2$ heterostructure-based device, we evidence a superimposed spin-to-charge current conversion that can be indistinguishably associated with either the proximity-induced Rashba-Edelstein effect in graphene or the spin Hall effect in MoS$_2$. By comparing our results to theoretical calculations, the latter scenario is found the most plausible one. Our findings pave the way towards the combination of spin information transport and spin-to-charge conversion in two-dimensional materials, opening exciting opportunities in a variety of future spintronic applications.

Published : "arXiv Mesoscale and Nanoscale Physics".

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