Heterostructures

/Tag: Heterostructures

Van der Waals Heterostructure Devices with Dynamically Controlled Conduction Polarity and Multifunctionality

2018-11-17T22:32:33+00:00November 17th, 2018|Categories: Publications|Tags: , , |

A new application of vdWHs to dynamically control and optimize the electronic and optoelectronic properties of 2D materials is demonstrated. The semivertical MoTe2/MoS2 structure allows for a desirable multifunctional integration of field effect transistorswith on/off ratio >107 and diode with rectification ratio >106. Moreover, the devices exhibit strong capability of suppressing the widely observed trap states–related negative photoresponse effect. Abstract Controlling the conduction behavior of 2D materials is an important prerequisite to achieve their electronic and optoelectronic applications. However, most of the reported approaches are aware of the shortcomings of inflexibility and complexity, which limits the possibility of multifunctional integration. Here, taking advantage of van der Waals heterostructure engineering, a simple method to achieve a dynamically controlled binary channel in a semivertical MoTe2/MoS2 field effect transistor is proposed. It is enabled by the high switchability between tunneling and thermal transports through simply changing the sign of voltage bias. In addition, the proposed system allows for multifunctional integration of transistor with on/off ratio >107 and diode with rectification ratio >106. Moreover, the devices show screen capability to negative photoresponse effect that is widely observed in ambipolar materials, hence improving the photodetection reliability and sensitivity. This study broadens the functionalities of van der Waals heterostructures and opens up more possibilities to realize multifunctional devices.

Published in: "Advanced Functional Materials".

Van der Waals Heterostructure Devices with Dynamically Controlled Conduction Polarity and Multifunctionality

2018-11-16T04:33:51+00:00November 16th, 2018|Categories: Publications|Tags: , , |

A new application of vdWHs to dynamically control and optimize the electronic and optoelectronic properties of 2D materials is demonstrated. The semivertical MoTe2/MoS2 structure allows for a desirable multifunctional integration of field effect transistorswith on/off ratio >107 and diode with rectification ratio >106. Moreover, the devices exhibit strong capability of suppressing the widely observed trap states–related negative photoresponse effect. Abstract Controlling the conduction behavior of 2D materials is an important prerequisite to achieve their electronic and optoelectronic applications. However, most of the reported approaches are aware of the shortcomings of inflexibility and complexity, which limits the possibility of multifunctional integration. Here, taking advantage of van der Waals heterostructure engineering, a simple method to achieve a dynamically controlled binary channel in a semivertical MoTe2/MoS2 field effect transistor is proposed. It is enabled by the high switchability between tunneling and thermal transports through simply changing the sign of voltage bias. In addition, the proposed system allows for multifunctional integration of transistor with on/off ratio >107 and diode with rectification ratio >106. Moreover, the devices show screen capability to negative photoresponse effect that is widely observed in ambipolar materials, hence improving the photodetection reliability and sensitivity. This study broadens the functionalities of van der Waals heterostructures and opens up more possibilities to realize multifunctional devices.

Published in: "Advanced Functional Materials".

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".

Spin transport in a graphene normal-superconductor junction in the quantum Hall regime

2018-11-13T16:33:57+00:00November 13th, 2018|Categories: Publications|Tags: , , |

Author(s): Tibor Sekera, Christoph Bruder, and Rakesh P. TiwariIn graphene-superconductor heterostructures, superconductivity and the quantum Hall effect may coexist for an experimentally accessible range of magnetic fields. When the graphene edge states are coupled to a superconductor in the presence of a Zeeman field, the charge carriers with one spin projection get transmitted while the ones with the opposite spin projection get reflected within a certain energy region. This spin-filtering effect is a consequence of the interplay between specular Andreev reflections and Andreev retro-reflections. While the edge termination of graphene and the geometrical details do matter for the charge conductance, they have little effect on the spin polarization of the charge carriers.[Phys. Rev. B 98, 195418] Published Tue Nov 13, 2018

Published in: "Physical Review B".

Laser writable high-K dielectric for van der Waals nano-electronics. (arXiv:1811.04829v1 [cond-mat.mes-hall])

2018-11-13T04:30:20+00:00November 13th, 2018|Categories: Publications|Tags: , , |

Like silicon-based semiconductor devices, van der Waals heterostructures will require integration with high-K oxides. This is needed to achieve suitable voltage scaling, improved performance as well as allowing for added functionalities. Unfortunately, commonly used high-k oxide deposition methods are not directly compatible with 2D materials. Here we demonstrate a method to embed a multi-functional few nm thick high-k oxide within van der Waals devices without degrading the properties of the neighbouring 2D materials. This is achieved by in-situ laser oxidation of embedded few layer HfS2 crystals. The resultant oxide is found to be in the amorphous phase with a dielectric constant of k~15 and break-down electric fields in the range of 0.5-0.6 V/nm. This transformation allows for the creation of a variety of fundamental nano-electronic and opto-electronic devices including, flexible Schottky barrier field effect transistors, dual gated graphene transistors as well as vertical light emitting and detecting tunnelling transistors. Furthermore, upon dielectric break-down, electrically conductive filaments are formed. This filamentation process can be used to electrically contact encapsulated conductive materials. Careful control of the filamentation process also allows for reversible switching between two resistance states. This allows for the creation of resistive switching random access memories (ReRAMs). We believe that this method of embedding a high-k oxide within complex van der Waals heterostructures could play an important role in future flexible multi-functional van der Waals devices.

Published : "arXiv Mesoscale and Nanoscale Physics".

Ultrafast photocarrier recombination dynamics in black phosphorus-molybdenum disulfide (BP/MoS2) heterostructure. (arXiv:1811.04706v1 [cond-mat.mes-hall])

2018-11-13T04:30:17+00:00November 13th, 2018|Categories: Publications|Tags: , , , |

Van der Waals (vdW) heterostructures constructed with two-dimensional (2D) materials have attracted great interests, due to their fascinating properties and potential for novel applications. While earlier efforts have advanced the understanding of the ultrafast cross-layer charge transfer process in 2D heterostructures, mechanisms for the interfacial photocarrier recombination remain, to a large extent, unclear. Here, we investigate a heterostructure comprised of black phosphorus (BP) and molybdenum disulfide (MoS2), with a type-II band alignment. Interestingly, it is found that the photo-generated electrons in MoS2 (transferred from BP) exhibit an ultrafast lifetime of about 5 ps, significantly shorter than those of the constituent materials. By corroborating with the relaxation of photo-excited holes in BP, it is revealed that the ultrafast time constant is as a result of efficient Langevin recombination, where the high hole mobility of BP facilitates a large recombination coefficient (approximately 2×10^-10 m^2/s). In addition, broadband transient absorption spectroscopy confirms that the hot electrons transferred to MoS2 distribute over a broad energy range following an ultrafast thermalization. The rate of the interlayer Langevin recombination is found to exhibit no energy level dependence. Our findings provide essential insights into the fundamental photo-physics in type-II 2D heterostructures, and also provide useful guidelines for customizing photocarrier lifetimes of BP for high-speed photo-sensitive devices.

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".

Electron quantum metamaterials in van der Waals heterostructures. (arXiv:1811.03670v1 [cond-mat.mes-hall])

2018-11-12T04:30:17+00:00November 12th, 2018|Categories: Publications|Tags: |

In recent decades, scientists have developed the means to engineer synthetic periodic arrays with feature sizes below the wavelength of light. When such features are appropriately structured, electromagnetic radiation can be manipulated in unusual ways, resulting in optical metamaterials whose function is directly controlled through nanoscale structure. Nature, too, has adopted such techniques — for example in the unique coloring of butterfly wings — to manipulate photons as they propagate through nanoscale periodic assemblies. In this Perspective, we highlight the intriguing potential of designer sub-electron wavelength (as well as wavelength-scale) structuring of electronic matter, which affords a new range of synthetic quantum metamaterials with unconventional responses. Driven by experimental developments in stacking atomically layered heterostructures — e.g., mechanical pick-up/transfer assembly — atomic scale registrations and structures can be readily tuned over distances smaller than characteristic electronic length-scales (such as electron wavelength, screening length, and electron mean free path). Yet electronic metamaterials promise far richer categories of behavior than those found in conventional optical metamaterial technologies. This is because unlike photons that scarcely interact with each other, electrons in subwavelength structured metamaterials are charged, and strongly interact. As a result, an enormous variety of emergent phenomena can be expected, and radically new classes of interacting quantum metamaterials designed.

Published : "arXiv Mesoscale and Nanoscale Physics".

Photoresponsive Devices: Ultrahigh Photoresponsive Device Based on ReS2/Graphene Heterostructure (Small 45/2018)

2018-11-11T14:34:21+00:00November 11th, 2018|Categories: Publications|Tags: , , |

In article number 1802593, Changgu Lee and co‐workers develop a photodetector based on a ReS2/graphene heterostructure, which exhibits outstanding photoresponsivity, detectivity, and ultrafast responsivity due to the excellent light absorption property of ReS2 and high carrier mobility of graphene. The interface between ReS2 and graphene facilitates a high photocurrent because of the photogating effect under the applied gate voltage.

Published in: "Small".

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

2018-11-11T04:32:25+00:00November 11th, 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

2018-11-11T04:32:19+00:00November 11th, 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".

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

2018-11-10T22:34:57+00:00November 10th, 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".

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

2018-11-10T22:34:51+00:00November 10th, 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".

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

2018-11-10T22:34:35+00:00November 10th, 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

2018-11-10T22:34:20+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".

Photoresponsive Devices: Ultrahigh Photoresponsive Device Based on ReS2/Graphene Heterostructure (Small 45/2018)

2018-11-10T00:36:38+00:00November 9th, 2018|Categories: Publications|Tags: , , |

In article number 1802593, Changgu Lee and co‐workers develop a photodetector based on a ReS2/graphene heterostructure, which exhibits outstanding photoresponsivity, detectivity, and ultrafast responsivity due to the excellent light absorption property of ReS2 and high carrier mobility of graphene. The interface between ReS2 and graphene facilitates a high photocurrent because of the photogating effect under the applied gate voltage.

Published in: "Small".

Rectifying properties in 90{deg} rotated bilayer black phosphorus nanojunction: A first principle study. (arXiv:1811.03523v1 [physics.app-ph])

2018-11-09T04:30:38+00:00November 9th, 2018|Categories: Publications|Tags: , |

We explore the possibility of using van dar Waals bonded heterostructures of stacked together 2D bilayer black phosphorus (BP) for nanoscale device applications. The electronic property of BP in AA stacking and 90{deg} twisted is studied with density functional theory. Further, we study the homogeneous nanojunction architecture of BP to use its anisotropic properties. Using the first principle simulations along with NEGF approach, we calculate quantum transport properties of the nanojunction setup. The interlayer directionally dependent current characteristics are explained in different setups. Our result revealed that 90{deg} twisted nanojucntion device would be a potential rectifier despite having no p-n junction characteristic only due to the intrinsic anisotropy of the material, making tunneling between armchair- and zigzag-directional BP sheets asymmetric.

Published : "arXiv Mesoscale and Nanoscale Physics".

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".

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