Intercalation Superlattices

Electrochemical intercalation produces a broad class of superlattices between covalently-bonded atomic layers and self-assembled molecular layers
Published in Chemistry
Intercalation Superlattices

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The paper in Nature is here:

Semiconductor superlattices, in which alternative layers of semiconductor materials are grown using molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOCVD) approach with nanometer scale precision, represent the fundamental material foundation for a wide range of modern electronic and optoelectronic devices, including high mobility transistors, resonant tunneling diodes, light emitting diodes, laser diodes and quantum cascade lasers. The successful growth of semiconductor superlattice structures marked an important milestone in the design and creation of artificial materials with tailored electronic properties. However, traditional semiconductor superlattices relying on epitaxial growth can only be created from materials with highly similar lattice symmetry and lattice constants (thus similar electronic structures) due to lattice matching requirement. Materials with substantially different structure or lattice parameters cannot be epitaxially grown together without generating too much defects that could fatally degrade their electronic properties.     

The recent emergence of the layered two-dimensional atomic crystals (2DACs) has opened a new dimension for material integration. With weak van der Waals interactions, single or few-atomic layers can readily isolated, mixed, matched and combined to create a new class of heterostructures and superlattices with radically different atomic structure or electronic properties beyond the limit of traditional superlattices. It can thus open up exciting opportunities to manipulate the confinement and transportation of electrons, holes, excitons, photons, and phonons at the limit of single atom thickness; and to enable novel devices with unprecedented performance or entirely new functions beyond the reach of existing materials. However, the typical strategies to such artificial superlattices today rely on an arduous layer-by-layer exfoliation and restack approach, which is clearly not practical for technological applications. The bottom-up chemical-vapor deposition approach may produce high-quality heterostructures, but faces exponentially increasing challenges towards high-order superlattices due to the extremely delicate nature of these atomically thin materials and processing incompatibility between layers with distinct atomic compositions.

This study reports a general electrochemical molecular intercalation approach to a new class of monolayer atomic crystal molecular superlattices (MACMS) consisting of alternating layers of monolayer crystals and molecular layers. Within such superlattices, the 2DACs and molecular layers can be systematically and independently varied. It thus offers a general pathway to a vast library of superlattice structures between highly distinct atomic layers and molecular layers with tailored interlayer distances, variable structure configurations, and tunable electronic properties, yet with precise atomic/molecular structure arrangement within each layer. The formation of such superlattices can open up many exciting opportunities, including:

  • Accessing monolayer characteristics from unstable 2DACs. The intercalation with electronically passive molecules into 2DACs can largely decouple the interlayer interaction and thus allow access to monolayer characteristics without the need to actually isolate the single layers on substrate, such as monolayer phosphorene molecular superlattice (MPMS) in our study.
  • Creating bulk monolayer materials. The formation of MACMS can considerably modulate the electronic and optical properties of the 2DACs. For example, by intercalating with CTAB or THAB, an indirect to direct bandgap transformation is demonstrated in MoS2 molecular superlattice (MoS2MS) with controlled phase composition thus photoluminescence efficiency. The resulted MoS2MS with nearly ideal monolayer characteristics can be viewed as a new class of bulk monolayer materials, which are particularly attractive for efficient light-emitting devices.
  • Enabling a new class of atomic superlattices with flexible integration of radically different chemical compositions and electronic structures. A wide arrange of functional molecules with different functional substituents/electronic structures or hybrid molecules with integrated functionalities, including magnetic molecules, photosensitive molecules, thermo-sensitive molecules, and charge/energy storage molecules, may be intercalated in selected 2DACs to create a new class of superlattice structures with atomic precision yet radically variable chemical compositions and physical properties.

This study provides a general approach to 2DACs-molecular superlattices and defines a versatile material platform for both the fundamental physics studies and novel device applications. We hope it will stimulate both theoretical and experimental efforts to explore the nearly infinite choices of organic or inorganic intercalants that may produce MACMS with the designed electronic/optical/magnetic properties, and open up a new dimension to tailoring and taming the electronic, optical, and magnetic properties of 2DACs.

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