Advanced optoelectronics with graphene-based mixed-dimensional van der Waals heterostructures
(Nanowerk Spotlight) Beginning with the first demonstration of graphene in 2004, research into 2D layered materials has evolved into a large and thriving scientific community encompassing more than 2500 other layered, atomically thin (two-dimensional, 2D), materials – the vast majority of which has not been explored yet. These include monoelemental atomic layers (such as phosphorene, silicene, etc.), semiconducting and metallic transition metal dichalcogenides (TMDCs), insulating hexagonal boron nitride (h-BN), layered oxides, and others.
While these materials cover an amazing range of electrical, chemical, optical and mechanical properties, perhaps the most astounding discovery is that these crystals can be combined freely to create altogether new materials. For that reason, in parallel with the exploration of novel 2D materials, research on artificial van der Waals (vdWs) heterostructures has also emerged as one of the leading topics in materials science.
Scientists discovered that, when two atomically thin graphene-like materials are placed on top of each other, their properties change, and a material with novel hybrid properties emerges, paving the way for design of new materials and nano-devices. The properties of these hybrid material can be precisely controlled by twisting the two stacked atomic layers, opening the way for the use of this unique degree of freedom for the nanoscale control of composite materials and device structures.
While strong covalent bonds provide in-plane stability of 2D the crystals, these materials are called van der Waals heterostructures because the atomically thin layers are not mixed through a chemical reaction but rather attached to each other via a weak so called van der Waals interaction – similar to how a sticky tape attaches to a flat surface.
These vdWs structures provide intriguing possibilities to achieve delicate manipulation of carriers’ behavior at the atomic interface and design unique devices with specific functionality.
Among 2D materials, graphene is considered as one of the important components in many vdWs heterostructure assemblies due to its high carrier mobility, good environmental stability, tunable work function, and mature processing technique. Detailed research of graphene-based vdWs heterostructures is just beginning.
Besides the contact between different 2D atomic layers, the passivated, dangling-bond-free surfaces of 2D crystals can bond together with other dimensional materials through vdWs force. Consequently, fabrication of mixed-dimensional vdWs (MDWs) heterostructures could be carried out through hybridizing 2D crystals with 0D dots or particles, 1D nanostructures, or 3D bulk materials.
These combinations reveal much higher selectivity of materials and open up a whole new paradigm for functional nanomaterials integration to harness their synergistic advantages.
What makes graphene such an exceptional material is its exceptionally high mobility at room temperature as well as its high optical damage threshold – raising high expectations for graphene applications in photonics.
However, due to the gapless and semi-metallic feature of graphene, it is weak in terms of light absorption. This is undesirable for many high performance optoelectronics, such as photovoltaics where strong light absorption is normally required.
Since it is unrealistic that a single material can satisfy all the requests concerning modern advanced optoelectronics – fast operation speed, high efficiency, easy fabrication, and good stability – hybridization of graphene with other dimensional materials may provide a promising solution to compensate for graphene's intrinsic weakness.
Generally, the graphene-based MDWs heterostructures could be separated into three distinct categories on the basis of hybrid materials’ dimensionality, as illustrated in the figure below.
Aiming to advanced optoelectronics with fast response, high sensitivity, high gain, flexible structure, graphene-based mixed-dimensional vdWs heterostructures, including graphene-0D, graphene-1D, and graphene-3D. (Reprinted with permission from Wiley-VCH Verlag) (click on image to enlarge)
Especially, the hybridization of graphene with conventional 3D bulk materials allows combination of its novel merits with the state-of-the-art and standard semiconductor processing technologies such as lithography and integration, which may enable the large-scale production of graphene-based optoelectronics and accelerate their process of practicality.
A review in Advanced Materials("Graphene-Based Mixed-Dimensional van der Waals Heterostructures for Advanced Optoelectronics") provides an overview of representative advances in graphene-based MDWs heterostructures, ranging from assembly strategies to applications in optoelectronics. The focus of this review is mainly on optoelectronic applications in which graphene acts as one component of functional units. The remaining spectrum of 2D materials-based MDWs heterostructures is not covered.
As the authors point out, due to the lack of lattice constraint, in theory, the materials that can be used to form MDWs heterostructures with graphene are unlimited – although not all of them would make sense. "As a general principle" they write, "the integration of graphene with other non-2D materials should at least result in notable performance enhancement, novel device functionality, or new physics. In addition, the availability of controlled materials quality and feasible assembly technique are also needed to be taken into account."
The main sections of the review first briefly summarizes the current popular methods to create MDWs heterostructures: transfer-assisted assembly and direct-growth method.
Then, the authors present representative hybrid structures and functional optoelectronics from the point of materials' dimensionality. This section also highlights the advantages and new physics arising from materials hybridization.
Concluding their review, the authors highlight several challenges and recommendations:
The fundamental theory of MDWs heterostructure should be investigated systematically;
The wafer-scale MDWs integration methods should be established;
The failure and service behavior of graphene-based MDWs devices should be proposed.
Looking towards the future, the authors "believe that the emergence of MDWs heterojunctions will make graphene research a long live hotspot, which might help to find the killer application of graphene, ultimately leading to significant breakthroughs in new physics as well as commercialization of graphene devices. Meanwhile, the research of graphene-based MDWs heterostructures may provide reference for other novel 2D crystals beyond graphene and serve as a general strategy for performance enhancement of vdWs devices."