Observing Imperfection in Atomic Interfaces for van der Waals Heterostructures

Abstract

Abstract:Vertically stacked van der Waals heterostructures are a lucrative platform for exploring the rich electronic and optoelectronic phenomena in two-dimensional materials. Their performance will be strongly affected by impurities and defects at the interfaces. Here we present the first systematic study of interfaces in van der Waals heterostructure using cross sectional scanning transmission electron microscope (STEM) imaging. By measuring interlayer separations and comparing these to density functional theory (DFT) calculations we find that pristine interfaces exist between hBN and MoS 2 or WS 2 for stacks prepared by mechanical exfoliation in air.However, for two technologically important transition metal dichalcogenide (TMDC) systems, MoSe 2 and WSe 2, our measurement of interlayer separations provide the first evidence for impurity species being trapped at buried interfaces with hBN: interfaces which are flat at the nanometer length scale. While decreasing the thickness of encapsulated WSe 2 from bulk to monolayer we see a systematic increase in the interlayer separation. We attribute these differences to the thinnest TMDC flakes being flexible and hence able to deform mechanically around a sparse population of protruding interfacial impurities. We show that the air sensitive two dimensional (2D) crystal NbSe 2 can be fabricated into heterostructures with pristine interfaces by processing in an inert-gas environment. Finally we find that adopting glove-box transfer significantly improves the quality of interfaces for WSe 2 compared to processing in air.3 Manuscript:The field of two dimensional (2D) crystals is expanding rapidly and now includes materials with a wide range of electronic properties, offering opportunities to engineer a particular bandstructure at the atomic scale by layering together exfoliated crystals 1,2 . This concept of 'van der Waals heterostructures' has facilitated the fabrication of new optical and electronic components 3,4 including novel transistors [5][6][7][8] and photovoltaic devices 9,10 . The physical and electronic properties of these novel 2D multilayer architectures will depend critically on the structure, including interfacial impurity defects. Thus the nature and purity of interfaces is key for predicting the bandstructure 11,12 and adhesion strength 13 for such systems, and hence fundamental to understanding electronic transport data, electroluminescence characteristics, interlayer diffusion and mechanical failure mechanisms 10 . Density functional theory (DFT) is a powerful technique for predicting the ground state properties of encapsulated monolayers, bilayers or bulk van der Waals crystals. Yet there is limited experimental data regarding the quality of interfaces formed between dissimilar van der Waals materials 14 and many theoretical predictions therefore assume these interfaces to be free from impurities 15,16 . The assumption of perfect interfaces is justified by the observation of mobile surface contaminants segregating into bubbles leaving large flat area...

Ultra-thin van der Waals crystals as semiconductor quantum wells

“…The thickness of hBN was selected to be 4-3 layers to allow for large bias voltages without sample overheating. These structures have been produced using the dry-stacking method in an argon environment [20] to provide atomically clean and sharp interfaces, and to avoid InSe degradation [21,22]. The assembled stacks were deposited on an oxidized silicon wafer and contacts were defined using electron beam lithography (see supplementary information).…”

Prospects and Limitations of Transition Metal Dichalcogenide Laser Gain Materials

“…At the same time, sub-and superstrates to the active TMD monolayers have been demonstrated to enable a wide range of tuning possibilities of the electronic and optical properties of the gain medium [8,[19][20][21]. In particular, pre-structured dielectric and plasmonic environments allow to tailor the local carrier landscape similar to how the photonic landscape can be formed by structuring a photonic crystal [22][23][24]. Combined advancements in the understanding of material properties and fabrication techniques hold enticing possibilities for integrated photonics applications and light sources.…”

Atomic-Scale Deformations at the Interface of a Mixed-Dimensional van der Waals Heterostructure

“…10−14 Furthermore, the energetic tendency to maximize the contact area seems to drive self-cleaning within the vdW gap, 15 enabling atomically pure interfaces. However, it is much more challenging to obtain sufficiently clean, ordered, and thin layers of lower-dimensional structures owing to their higher chemical reactivity.…”