Heterostructures

/Tag: Heterostructures

Visualization of Local Conductance in MoS2/WSe2 Heterostructure Transistors. (arXiv:1902.08147v1 [physics.app-ph])

2019-02-22T02:29:21+00:00February 22nd, 2019|Categories: Publications|Tags: , , , |

The vertical stacking of van der Waals (vdW) materials introduces a new degree of freedom to the research of two-dimensional (2D) systems. The interlayer coupling strongly influences the band structure of the heterostructures, resulting in novel properties that can be utilized for electronic and optoelectronic applications. Based on microwave microscopy studies, we report quantitative electrical imaging on gated molybdenum disulfide (MoS2)/tungsten diselenide (WSe2) heterostructure devices, which exhibit an intriguing anti-ambipolar effect in the transfer characteristics. Interestingly, in the region with significant source-drain current, electrons in the n-type MoS2 and holes in the p-type WSe2 segments are nearly balanced, whereas the heterostructure area is depleted of mobile charges. The configuration is analogous to the p-i-n diode, where the injected carriers dominate in the recombination current. The spatial evolution of local conductance can be ascribed to the lateral band bending and formation of depletion regions along the line of MoS2-heterostructure-WSe2. Our work vividly demonstrates the microscopic origin of novel transport behaviors, which is important for the vibrant field of vdW heterojunction research.

Published in: "arXiv Material Science".

2D–Organic Hybrid Heterostructures for Optoelectronic Applications

2019-02-21T00:37:56+00:00February 20th, 2019|Categories: Publications|Tags: |

The hybridization of 2D materials and organic materials represents a promising domain for the realization of improved or unprecedented features in comparison to those of semiconductor devices. This comprehensive review focuses on emerging 2D–organic heterostructures (from their synthesis and fabrication to their state‐of‐the‐art optoelectronic applications) and highlights the future challenges and opportunities associated with these heterostructures. Abstract The unique properties of hybrid heterostructures have motivated the integration of two or more different types of nanomaterials into a single optoelectronic device structure. Despite the promising features of organic semiconductors, such as their acceptable optoelectronic properties, availability of low‐cost processes for their fabrication, and flexibility, further optimization of both material properties and device performances remains to be achieved. With the emergence of atomically thin 2D materials, they have been integrated with conventional organic semiconductors to form multidimensional heterostructures that overcome the present limitations and provide further opportunities in the field of optoelectronics. Herein, a comprehensive review of emerging 2D–organic heterostructures—from their synthesis and fabrication to their state‐of‐the‐art optoelectronic applications—is presented. Future challenges and opportunities associated with these heterostructures are highlighted.

Published in: "Advanced Materials".

Interface‐Driven Partial Dislocation Formation in 2D Heterostructures

2019-02-21T00:37:52+00:00February 20th, 2019|Categories: Publications|Tags: , , |

Herein, extended 1D AB–AC stacking boundaries in WS2 are fabricated by using morphological defects on graphene substrates, such as graphene wrinkles. When the perfect basal dislocation direction is more perpendicular to the wrinkles, high‐density stacking boundaries are induced, due to the anisotropic friction of wrinkles. Abstract Van der Waals (vdW) epitaxy allows the fabrication of various heterostructures with dramatically released lattice matching conditions. This study demonstrates interface‐driven stacking boundaries in WS2 using epitaxially grown tungsten disulfide (WS2) on wrinkled graphene. Graphene wrinkles function as highly reactive nucleation sites on WS2 epilayers; however, they impede lateral growth and induce additional stress in the epilayer due to anisotropic friction. Moreover, partial dislocation‐driven in‐plane strain facilitates out‐of‐plane buckling with a height of 1 nm to release in‐plane strain. Remarkably, in‐plane strain relaxation at partial dislocations restores the bandgap to that of monolayer WS2 due to reduced interlayer interaction. These findings clarify significant substrate morphology effects even in vdW epitaxy and are potentially useful for various applications involving modifying optical and electronic properties by manipulating extended 1D defects via substrate morphology control.

Published in: "Advanced Materials".

2D–Organic Hybrid Heterostructures for Optoelectronic Applications

2019-02-20T22:35:18+00:00February 20th, 2019|Categories: Publications|Tags: |

The hybridization of 2D materials and organic materials represents a promising domain for the realization of improved or unprecedented features in comparison to those of semiconductor devices. This comprehensive review focuses on emerging 2D–organic heterostructures (from their synthesis and fabrication to their state‐of‐the‐art optoelectronic applications) and highlights the future challenges and opportunities associated with these heterostructures. Abstract The unique properties of hybrid heterostructures have motivated the integration of two or more different types of nanomaterials into a single optoelectronic device structure. Despite the promising features of organic semiconductors, such as their acceptable optoelectronic properties, availability of low‐cost processes for their fabrication, and flexibility, further optimization of both material properties and device performances remains to be achieved. With the emergence of atomically thin 2D materials, they have been integrated with conventional organic semiconductors to form multidimensional heterostructures that overcome the present limitations and provide further opportunities in the field of optoelectronics. Herein, a comprehensive review of emerging 2D–organic heterostructures—from their synthesis and fabrication to their state‐of‐the‐art optoelectronic applications—is presented. Future challenges and opportunities associated with these heterostructures are highlighted.

Published in: "Advanced Materials".

Interfacial and electronic properties of heterostructures of MXene and graphene

2019-02-20T14:38:37+00:00February 20th, 2019|Categories: Publications|Tags: , , |

Author(s): Rui Li, Weiwei Sun, Cheng Zhan, Paul R. C. Kent, and De-en JiangMXene-based heterostructures have received considerable interest owing to their unique properties. Herein, we examine various heterostructures of the prototypical MXene Ti3C2T2 (T=O, OH, F; terminal groups) and graphene using density-functional theory. We find that the adhesion energy, charge transf…[Phys. Rev. B 99, 085429] Published Wed Feb 20, 2019

Published in: "Physical Review B".

Interface‐Driven Partial Dislocation Formation in 2D Heterostructures

2019-02-20T10:45:43+00:00February 20th, 2019|Categories: Publications|Tags: , , |

Herein, extended 1D AB–AC stacking boundaries in WS2 are fabricated by using morphological defects on graphene substrates, such as graphene wrinkles. When the perfect basal dislocation direction is more perpendicular to the wrinkles, high‐density stacking boundaries are induced, due to the anisotropic friction of wrinkles. Abstract Van der Waals (vdW) epitaxy allows the fabrication of various heterostructures with dramatically released lattice matching conditions. This study demonstrates interface‐driven stacking boundaries in WS2 using epitaxially grown tungsten disulfide (WS2) on wrinkled graphene. Graphene wrinkles function as highly reactive nucleation sites on WS2 epilayers; however, they impede lateral growth and induce additional stress in the epilayer due to anisotropic friction. Moreover, partial dislocation‐driven in‐plane strain facilitates out‐of‐plane buckling with a height of 1 nm to release in‐plane strain. Remarkably, in‐plane strain relaxation at partial dislocations restores the bandgap to that of monolayer WS2 due to reduced interlayer interaction. These findings clarify significant substrate morphology effects even in vdW epitaxy and are potentially useful for various applications involving modifying optical and electronic properties by manipulating extended 1D defects via substrate morphology control.

Published in: "Advanced Materials".

Opto-valleytronic imaging of atomically thin semiconductors. (arXiv:1902.06856v1 [cond-mat.mes-hall])

2019-02-20T04:30:37+00:00February 20th, 2019|Categories: Publications|Tags: |

Transition metal dichalcogenide semiconductors represent elementary components of layered heterostructures for emergent technologies beyond conventional opto-electronics. In their monolayer form they host electrons with quantized circular motion and associated valley polarization and valley coherence as key elements of opto-valleytronic functionality. Here, we introduce two-dimensional polarimetry as means of direct imaging of the valley pseudospin degree of freedom in monolayer transition metal dichalcogenides. Using MoS$_2$ as a representative material with valley-selective optical transitions, we establish quantitative image analysis for polarimetric maps of extended crystals, and identify valley polarization and valley coherence as sensitive probes of crystalline disorder. Moreover, we find site-dependent thermal and non-thermal regimes of valley-polarized excitons in perpendicular magnetic fields. Finally, we demonstrate the potential of wide-field polarimetry for rapid inspection of opto-valleytronic devices based on atomically thin semiconductors and heterostructures.

Published : "arXiv Mesoscale and Nanoscale Physics".

Hall micromagnetometry of individual two-dimensional ferromagnets. (arXiv:1902.06988v1 [cond-mat.mes-hall])

2019-02-20T04:30:28+00:00February 20th, 2019|Categories: Publications|Tags: , |

The recent advent of atomically-thin ferromagnetic crystals has allowed experimental studies of two-dimensional (2D) magnetism that not only exhibits novel behavior due to the reduced dimensionality but also often serves as a starting point for understanding of the magnetic properties of bulk materials. Here we employ ballistic Hall micromagnetometry to study magnetization of individual 2D ferromagnets. Our devices are multilayer van der Waals (vdW) heterostructures comprising of an atomically-thin ferromagnetic crystal placed on top of a Hall bar made from encapsulated graphene. 2D ferromagnets can be replaced repeatedly, making the graphene-based Hall magnetometers reusable and expanding a range of their possible applications. The technique is applied for the quantitative analysis of magnetization and its behavior in atomically thin CrBr3. The compound is found to remain ferromagnetic down to a monolayer thickness and exhibit high out-of-plane anisotropy. We report how the critical temperature changes with the number of layers and how domain walls propagate through the ultimately thin ferromagnets. The temperature dependence of magnetization varies little with thickness, in agreement with the strongly layered nature of CrBr3. The observed behavior is markedly different from that given by the simple 2D Ising model normally expected to describe 2D easy-axis ferromagnetism. Due to the increasingly common usage of vdW assembly, the reported approach offers vast possibilities for investigation of 2D magnetism and related phenomena.

Published : "arXiv Mesoscale and Nanoscale Physics".

Probing the mechanical properties of vertically-stacked ultrathin graphene/Al 2 O 3 heterostructures

2019-02-19T10:42:37+00:00February 19th, 2019|Categories: Publications|Tags: , |

The superior intrinsic mechanical properties of graphene have been widely studied and utilized to enhance the mechanical properties of various composite materials. However, it is still unclear how heterostructures incorporating graphene behave, and to what extent graphene influences their mechanical response. In this work, a series of graphene/Al 2 O 3 composite films were fabricated via atomic layer deposition of Al 2 O 3 on graphene, and their mechanical behavior was studied using an experimental-computational approach. The inclusion of monolayer chemical vapor deposited graphene between ultrathin Al 2 O 3 films (1.5–4.5 nm thickness) was found to enhance the overall stiffness by as much as 70% compared to a pure Al 2 O 3 film of similar thickness (∼150 GPa to ∼250 GPa). Here, for the first time, the combination of graphene and Al 2 O 3 in vertically-stacked heterostructures results …

Published in: "Nanotechnology".

Resolving spin, valley, and moir’e quasi-angular momentum of interlayer excitons in WSe2/WS2 heterostructures. (arXiv:1902.05887v1 [cond-mat.mes-hall])

2019-02-18T04:30:21+00:00February 18th, 2019|Categories: Publications|Tags: , , |

Moir’e superlattices provide a powerful way to engineer properties of electrons and excitons in two-dimensional van der Waals heterostructures. The moir’e effect can be especially strong for interlayer excitons, where electrons and holes reside in different layers and can be addressed separately. In particular, it was recently proposed that the moir’e superlattice potential not only localizes interlayer exciton states at different superlattice positions, but also hosts an emerging moir’e quasi-angular momentum (QAM) that periodically switches the optical selection rules for interlayer excitons at different moir’e sites. Here we report the observation of multiple interlayer exciton states coexisting in a WSe2/WS2 moir’e superlattice and unambiguously determine their spin, valley, and moir’e QAM through novel resonant optical pump-probe spectroscopy and photoluminescence excitation spectroscopy. We demonstrate that interlayer excitons localized at different moir’e sites can exhibit opposite optical selection rules due to the spatially-varying moir’e QAM. Our observation reveals new opportunities to engineer interlayer exciton states and valley physics with moir’e superlattices for optoelectronic and valleytronic applications.

Published : "arXiv Mesoscale and Nanoscale Physics".

Ferroelectric driven exciton and trion modulation in monolayer MoSe2 and WSe2. (arXiv:1902.05661v1 [cond-mat.mtrl-sci])

2019-02-18T02:29:28+00:00February 18th, 2019|Categories: Publications|Tags: , , |

In this work, we show how domain engineered lithium niobate can be used to selectively dope monolayer MoSe2 and WSe2 and demonstrate that these ferroelectric domains can significantly enhance or inhibit photoluminescence (PL) with the most dramatic modulation occurring at the heterojunction interface between two domains. A micro-PL and Raman system is used to obtain spatially resolved images of the differently doped transition metal dichalcogenides (TMDs). The domain inverted lithium niobate causes changes in the TMDs due to electrostatic doping as a result of the remnant polarization from the substrate. Moreover, the differently doped TMDs (n-type MoSe2 and p-type WSe2) exhibit opposite PL modulation. Distinct oppositely charged domains were obtained with a 9-fold PL enhancement for the same single MoSe2 sheet when adhered to the positive (P+) and negative (P-) domains. This sharp PL modulation on the ferroelectric domain results from different free electron or hole concentrations in the materials conduction band or valence band. Moreover, excitons dissociate rapidly at the interface between the P+ and P- domains due to the built-in electric field. We are able to adjust the charge on the P+ and P- domains using temperature via the pyroelectric effect and observe rapid PL quenching over a narrow temperature range illustrating the observed PL modulation is electronic in nature. This observation creates an opportunity to harness the direct bandgap TMD 2D materials as an active optical component for the lithium niobate platform using domain engineering of the lithium niobate substrate to create optically active heterostructures that could

Published in: "arXiv Material Science".

Photodetectors: Ultrahigh‐Sensitive Broadband Photodetectors Based on Dielectric Shielded MoTe2/Graphene/SnS2 p–g–n Junctions (Adv. Mater. 6/2019)

2019-02-16T22:37:25+00:00February 16th, 2019|Categories: Publications|Tags: , , , |

In article number 1805656, Rui Chen, Liyuan Zhang, Youpin Gong, and co‐workers develop an h‐BN/MoTe2/graphene/SnS2/h‐BN van der Waals heterostructure to realize an ultrahigh‐sensitivity broadband (405–1550 nm) photodetector, due to its unique advantages for high‐efficiency light absorption and exciton dissociation. Graphene plays a key role in enhancing the sensitivity and broadening the spectral range, providing a viable approach toward future ultrahigh sensitivity and broadband photodetectors.

Published in: "Advanced Materials".

Photonic Synapses: Near‐Infrared Annihilation of Conductive Filaments in Quasiplane MoSe2/Bi2Se3 Nanosheets for Mimicking Heterosynaptic Plasticity (Small 7/2019)

2019-02-16T22:37:04+00:00February 16th, 2019|Categories: Publications|Tags: , , |

In article number 1805431, Qing Yang, Ye Zhou, Su‐Ting Han, and co‐workers demonstrate modulated heterosyntic plasticity based on a resistive memory device fabricated with MoSe2/Bi2Se3 heterostructured nanosheets. By modulating the synaptic plasticity between pre‐ and post‐neurons with near‐infrared light, the synaptic system displays more complicated functions.

Published in: "Small".

Photonic Synapses: Near‐Infrared Annihilation of Conductive Filaments in Quasiplane MoSe2/Bi2Se3 Nanosheets for Mimicking Heterosynaptic Plasticity (Small 7/2019)

2019-02-16T08:47:51+00:00February 16th, 2019|Categories: Publications|Tags: , , |

In article number 1805431, Qing Yang, Ye Zhou, Su‐Ting Han, and co‐workers demonstrate modulated heterosyntic plasticity based on a resistive memory device fabricated with MoSe2/Bi2Se3 heterostructured nanosheets. By modulating the synaptic plasticity between pre‐ and post‐neurons with near‐infrared light, the synaptic system displays more complicated functions.

Published in: "Small".

Moiré patterns in van der Waals heterostructures

2019-02-15T14:37:30+00:00February 15th, 2019|Categories: Publications|Tags: , |

Author(s): Maxime Le Ster, Tobias Maerkl, Pawel J. Kowalczyk, and Simon A. BrownUsing scanning tunneling microscopy, we report the observation of moiré patterns (MPs) on van der Waals heterostructures comprised of various 2D allotropes of bismuth and antimony grown on highly ordered pyrolytic graphite and MoS2. The spatial periods of the MPs range from λ∼1 to ∼10 nm. For all th…[Phys. Rev. B 99, 075422] Published Fri Feb 15, 2019

Published in: "Physical Review B".

One-dimensional edge contacts to monolayer semiconductors. (arXiv:1902.05506v1 [cond-mat.mtrl-sci])

2019-02-15T02:30:52+00:00February 15th, 2019|Categories: Publications|Tags: , |

Integration of Ohmic contacts into van der Waals (vdW) heterostructures is critical for realizing electronic and optoelectronic functionalities. However, to date no scalable methodology for gaining electrical access to buried monolayer two-dimensional (2D) semiconductors exists. Here we report viable edge contact formation to hexagonal boron nitride (hBN) encapsulated monolayer MoS$_2$ for the first time. By combining reactive ion etching, $it in$-$it situ$ Ar$^+$ sputtering and annealing, we achieve a relatively low edge contact resistance (46 $pm$ 10 k$unicode[STIXGeneral,Times]{x3A9}cdotunicode[STIXGeneral,Times]{x3BC}$m), high mobility (up to ~30cm$^2$/Vs) and high on-current density (>50 $unicode[STIXGeneral,Times]{x3BC}$A$/unicode[STIXGeneral,Times]{x3BC}$m at $it V_{rm DS}$ = 3V), comparable to top contacts. Furthermore, the atomically smooth hBN environment also preserves the intrinsic MoS$_2$ channel quality during fabrication, leading to a steep subthreshold swing (116 mV/dec) with a negligible hysteresis. Edge contacts exhibit a higher electron transmission probability than top contacts, as revealed by our quantum transport simulations, and can be arbitrarily narrow, which opens the door to further shrinkage of 2D device footprint.

Published in: "arXiv Material Science".

Tuning the topological insulator states of artificial graphene

2019-02-14T14:41:16+00:00February 14th, 2019|Categories: Publications|Tags: , |

Author(s): H. D. Scammell and O. P. SushkovWe develop a robust, nonperturbative approach to study the band structure of artificial graphene. Artificial graphene, as considered here, is generated by imposing a superlattice structure on top of a two-dimensional hole gas in a semiconductor heterostructure, where the hole gas naturally possesses…[Phys. Rev. B 99, 085419] Published Thu Feb 14, 2019

Published in: "Physical Review B".

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