Query: 2D materials and van der Waals heterostructures

Candidate papers:

abstract: BACKGROUND Materials by design is an appealing idea that is very hard to realize in practice. Combining the best of different ingredients in one ultimate material is a task for which we currently have no general solution. However, we do have some successful examples to draw upon: Composite materials and III V heterostructures have revolutionized many aspects of our lives. Still, we need a general strategy to solve the problem of mixing and matching crystals with different properties, creating combinations with predetermined attributes and functionalities. ADVANCES Two dimensional (2D) materials offer a platform that allows creation of heterostructures with a variety of properties. One atom thick crystals now comprise a large family of these materials, collectively covering a very broad range of properties. The first material to be included was graphene, a zero overlap semimetal. The family of 2D crystals has grown to includes metals (e.g. NbSe2) semiconductors (e.g. MoS2) and insulators [e.g. hexagonal boron nitride (hBN) Many of these materials are stable at ambient conditions, and we have come up with strategies for handling those that are not. Surprisingly, the properties of such 2D materials are often very different from those of their 3D counterparts. Furthermore, even the study of familiar phenomena (like superconductivity or ferromagnetism) in the 2D case, where there is no long range order, raises many thought provoking questions. A plethora of opportunities appear when we start to combine several 2D crystals in one vertical stack. Held together by van der Waals forces (the same forces that hold layered materials together) such heterostructures allow a far greater number of combinations than any traditional growth method. As the family of 2D crystals is expanding day by day, so too is the complexity of the heterostructures that could be created with atomic precision. When stacking different crystals together, the synergetic effects become very important. In the first order approximation, charge redistribution might occur between the neighboring (and even more distant) crystals in the stack. Neighboring crystals can also induce structural changes in each other. Furthermore, such changes can be controlled by adjusting the relative orientation between the individual elements. Such heterostructures have already led to the observation of numerous exciting physical phenomena. Thus, spectrum reconstruction in graphene interacting with hBN allowed several groups to study the Hofstadter butterfly effect and topological currents in such a system. The possibility of positioning crystals in very close (but controlled) proximity to one another allows for the study of tunneling and drag effects. The use of semiconducting monolayers leads to the creation of optically active heterostructures. The extended range of functionalities of such heterostructures yields a range of possible applications. Now the highest mobility graphene transistors are achieved by encapsulating graphene with hBN. Photovoltaic and light emitting devices have been demonstrated by combining optically active semiconducting layers and graphene as transparent electrodes. OUTLOOK Currently, most 2D heterostructures are composed by direct stacking of individual monolayer flakes of different materials. Although this method allows ultimate flexibility, it is slow and cumbersome. Thus, techniques involving transfer of large area crystals grown by chemical vapor deposition (CVD) direct growth of heterostructures by CVD or physical epitaxy, or one step growth in solution are being developed. Currently, we are at the same level as we were with graphene 10 years ago: plenty of interesting science and unclear prospects for mass production. Given the fast progress of graphene technology over the past few years, we can expect similar advances in the production of the heterostructures, making the science and applications more achievable. Production of van der Waals heterostructures. Owing to a large number of 2D crystals available today, many functional van der Waals heterostructures can be created. What started with mechanically assembled stacks (top) has now evolved to large scale growth by CVD or physical epitaxy (bottom) The physics of two dimensional (2D) materials and heterostructures based on such crystals has been developing extremely fast. With these new materials, truly 2D physics has begun to appear (for instance, the absence of long range order, 2D excitons, commensurate incommensurate transition, etc. Novel heterostructure devices such as tunneling transistors, resonant tunneling diodes, and light emitting diodes are also starting to emerge. Composed from individual 2D crystals, such devices use the properties of those materials to create functionalities that are not accessible in other heterostructures. Here we review the properties of novel 2D crystals and examine how their properties are used in new heterostructure devices.
author_names: Kostya S Novoselov

Artem Mishchenko

Alexandra Carvalho

Antonio H Castro Neto
title: 2D materials and van der Waals heterostructures
venue: Science
year: 2016

abstract: Atomically thin two dimensional (2D) materials host a rich set of excited states that differ substantially from those of their bulk counterparts due to quantum confinement and enhanced many body effects. This Topical Review focuses on the theory and computation of excitons, plasmons and quasiparticles in 2D materials and their heterostructures. The general theory is illustrated by applications to various 2D materials including transition metal dichalcogenides, graphene, phosphorene, and hexagonal boron nitride. The weak and highly non local dielectric function of atomically thin crystals is shown to be responsible for many of the unique properties exhibited by the 2D materials such as the formation of strongly bound, non Hydrogenic excitons and the different signatures of excitons and plasmons in electron energy loss spectroscopy. Among other topics covered are spin orbit coupling, trions, interlayer excitons, exciton dissociation, and substrate induced band structure renormalisations. Technical issues associated with the application of the many body GW method and Bethe Salpeter equation to 2D materials are also discussed. A combined quantum/classical method is introduced and used throughout to account for dielectric screening and self energy effects from substrates and van der Waals heterostructures including the difficult case of incommensurate lattices.
author_names: Kristian Sommer Thygesen
title: Calculating excitons, plasmons, and quasiparticles in 2D materials and van der Waals heterostructures
year: 2017

abstract: Interfacial adhesion energy is a fundamental property of two dimensional (2D) layered materials and van der Waals heterostructures due to their intrinsic ultrahigh surface to volume ratio, making adhesion forces very strong in many processes related to fabrication, integration and performance of devices incorporating 2D crystals. However, direct quantitative characterization of adhesion behavior of fresh and aged homo/heterointerfaces at nanoscale has remained elusive. Here, we use an atomic force microscopy technique to report precise adhesion measurements in ambient air through well defined interactions of tip attached 2D crystal nanomesas with 2D crystal and SiOx substrates. We quantify how different levels of short range dispersive and long range electrostatic interactions respond to airborne contaminants and humidity upon thermal annealing. We show that a simple but very effective precooling treatment can protect 2D crystal substrates against the airborne contaminants and thus boost the adhesion level at the interface of similar and dissimilar van der Waals heterostructures. Our combined experimental and computational analysis also reveals a distinctive interfacial behavior in transition metal dichalcogenides and graphite/SiOx heterostructures beyond the widely accepted van der Waals interaction.
author_names: Hossein Rokni

Wei Dang Lu
title: Direct measurements of interfacial adhesion in 2D materials and van der Waals heterostructures in ambient air
venue: Nature communications
year: 2020

author_names: Wenjing Jie

Zhibin Yang

Gongxun Bai

Jianhua Hao
title: Luminescence in 2D Materials and van der Waals Heterostructures
year: 2018

abstract: Excitons dominate the excitation kinetics in transition metal dichalcogenides (TMDs) As cobosonic particles consisting of electrons and holes, excitons show a non trivial scattering dynamics. Here, we study exciton exciton interaction and excitation induced dephasing (EID) on microscopic footing and predict efficient scattering for momentum dark excitons because of a large electron/hole mass asymmetry. We find an unexpected temperature dependence of the EID and an orders of magnitude stronger interaction of interlayer excitons in TMD heterostructures due to their permanent dipole moment.
author_names: Daniel Erkensten

Samuel Brem

Ermin Malic
title: Excitation induced dephasing in 2D materials and van der Waals heterostructures
year: 2020

abstract: Designer heterostructures can now be assembled layer by layer with unmatched precision thanks to the recently developed deterministic placement methods to transfer two dimensional (2D) materials. This possibility constitutes the birth of a very active research field on the so called van der Waals heterostructures. Moreover, these deterministic placement methods also open the door to fabricate complex devices, which would be otherwise very difficult to achieve by conventional bottom up nanofabrication approaches, and to fabricate fully encapsulated devices with exquisite electronic properties. The integration of 2D materials with existing technologies such as photonic and superconducting waveguides and fiber optics is another exciting possibility. Here, we review the state of the art of the deterministic placement methods, describing and comparing the different alternative methods available in the literature, and we illustrate their potential to fabricate van der Waals heterostructures, to integrate 2D materials into complex devices and to fabricate artificial bilayer structures where the layers present a user defined rotational twisting angle.
author_names: Riccardo Frisenda

Efren Navarro-Moratalla

Patricia Gant

David Perez De Lara

Pablo Jarillo-Herrero

Roman V Gorbachev

Andres Castellanos-Gomez
title: Recent progress in the assembly of nanodevices and van der Waals heterostructures by deterministic placement of 2D materials.
venue: Chemical Society reviews
year: 2018

abstract: Realizing the full potential of any materials system requires understanding and controlling disorder, which can obscure intrinsic properties and hinder device performance. Here we examine both intrinsic and extrinsic disorder in two dimensional (2D) materials, in particular graphene and transition metal dichalcogenides (TMDs) Minimizing disorder is crucial for realizing desired properties in 2D materials and improving device performance and repeatability for practical applications. We discuss the progress in disorder control for graphene and TMDs, as well as in van der Waals heterostructures realized by combining these materials with hexagonal boron nitride. Furthermore, we showcase how atomic defects or disorder can also be harnessed to provide useful electronic, optical, chemical and magnetic functions.Disorder in two dimensional materials largely affects their properties and performance. This Perspective discusses disorder sources in graphene and transition metal dichalcogenides, the progress in disorder control, and possible uses of disorder.
author_names: Daniel A Rhodes

Sang Hoon Chae

Rebeca Ribeiro-Palau

James C Hone
title: Disorder in van der Waals heterostructures of 2D materials
venue: Nature Materials
year: 2019

author_names: Qinyi Li

Xing Zhang

Koji Takahashi
title: Dual mode Raman method to measure thermal transport properties of 2D materials and van der Waals heterostructures
year: 2017

abstract: The fascinating point of 2D and layered materials is that they can be assembled into van der Waals (vdW) heterostructures, in which atomic layers are integrated by vdW force. There are almost infinite potential combinations in vdW heterostructures owing to the multiple degrees of freedom, i.e. the choice of materials, stacking order, and lateral orientation angle at the interfaces. In this article, we review the fabrication technique of vdW heterostructures, which has played an essential role in the development of the 2D materials research field. First, we describe the primary technique of mechanical exfoliation to fabricate and identify high quality atomic layers. We then discuss the assembly of atomic layers into vdW heterostructures. Finally, we introduce the recent advancement of fabrication techniques using autonomous robotic assembly. We hope this article would help the readers to acquire basic knowledge of vdW assembly and motivate them to fabricate vdW heterostructures.
author_names: Momoko Onodera

Satoru Masubuchi

Rai Moriya

Tomoki Machida
title: Assembly of van der Waals heterostructures: exfoliation, searching, and stacking of 2D materials
venue: Japanese Journal of Applied Physics
year: 2020

abstract: van der Waals heterostructures, composed of vertically stacked inorganic 2D materials, represent an ideal platform to demonstrate novel device architectures and to fabricate on demand materials. The incorporation of organic molecules within these systems holds an immense potential, since, while nature offers a finite number of 2D materials, an almost unlimited variety of molecules can be designed and synthesized with predictable functionalities. The possibilities offered by systems in which continuous molecular layers are interfaced with inorganic 2D materials to form hybrid organic/inorganic van der Waals heterostructures are emphasized. Similar to their inorganic counterpart, the hybrid structures have been exploited to put forward novel device architectures, such as antiambipolar transistors and barristors. Moreover, specific molecular groups can be employed to modify intrinsic properties and confer new capabilities to 2D materials. In particular, it is highlighted how molecular self assembly at the surface of 2D materials can be mastered to achieve precise control over position and density of (molecular) functional groups, paving the way for a new class of hybrid functional materials whose final properties can be selected by careful molecular design.
author_names: Marco Gobbi

Emanuele Orgiu

Paolo Samori
title: When 2D Materials Meet Molecules: Opportunities and Challenges of Hybrid Organic/Inorganic van der Waals Heterostructures.
venue: Advanced materials
year: 2018