Stabilization of the 2H phase in molybdenum telluride (MoTe2) during molecular beam epitaxy growth on 6H silicon carbide (SiC) (0001) is desirable to exploit its potential properties in electronic applications.
Study: More indexed moiré patterns and surface states of MoTe2/graphene heterostructure grown by molecular beam epitaxy. Image credit: NEW-LIGHT-VISUALS/Shutterstock.com
In a paper published in the journal npj 2D Materials and Applications, the highly crystalline 2H phase of MoTe2 growth on graphene was achieved by molecular beam epitaxy by altering the conditions. In this van der Waals heterostructure, electronic coupling between adjacent layers and various Moiré patterns were observed at the junction between MoTe2 and graphene layers, which was an atomically clean interface.
Furthermore, the single-layer MoTe2/graphene heterostructure grown by molecular beam epitaxy was investigated using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations.
The results revealed that the MoTe2/graphene heterostructure via molecular beam epitaxy amplified the otherwise weak Moiré potential modulations, resulting in more indexed Moiré patterns. Moiré patterns that are rich in Fourier tones showed a wide range of applied bias voltages, showing the complex electronic features of the MoTe2/graphene heterostructure.
Molecular beam epitaxy and moiré pattern
Molecular beam epitaxy is an epitaxy process used in the deposition of a single crystal of thin film to fabricate diodes and semiconductor and metal oxide field effect transistors (MOSFETs). It is also used in the laser manufacture of optical discs
Molecular beam epitaxy is mainly used for some types of organic semiconductors, in which molecules are evaporated and deposited into films instead of atoms. Molecular beam epitaxy systems are flexible for alterations as needed. Nanostructures with atomically flat heterointerfaces are a major achievement achieved by molecular beam epitaxy
Molecular beam epitaxy generally takes place under high or ultrahigh vacuum. The crucial aspect of molecular beam epitaxy is its deposition speed, which allows the epitaxial growth of films that require a better vacuum to realize impurity levels than in other deposition techniques.
Moiré patterns or moiré fringes are interference patterns formed by the superposition of two similar patterns involving opaque ruled patterns and transparent voids. The moiré interference pattern requires two identical patterns with small displacements such as rotation or alignments at slightly different steps
The MoTe2 material is a transition metal dichalcogenide (TMD) material with unique properties suitable for electronic applications. In addition, the 2H phase has a bandgap of approximately 1 electronvolt like that of Si for application in bipolar field-effect transistors. Among the various methods reported for the growth of MoTe2 on different substrates, molecular beam epitaxy is advantageous to achieve precise growth control due to the high vacuum environment, slow growth rates, and low requirements temperature
A Moiré pattern is observed between two-dimensional (2D) heterostructures due to van der Waals forces that allow their relatively easy rotation. Moiré patterns influence the physical properties of these heterostructures. Also, in proportional heterostructures with finite supercells, the supercell size appears larger than the Moiré periodicity.
Moiré patterns of the MoTe2/Graphene heterostructure grown by molecular beam epitaxy
Molecular beam epitaxy growth of MoTe2/graphene heterostructure was previously reported with good crystallinity and desired phase. This heterostructure formed Moiré patterns with different twist angles. In the present study, unusually higher indexed Moiré patterns with a provided supercell periodicity were observed in the MoTe2/graphene heterostructure by using STM at a rotation angle of about 30 degrees.
In general, in heterostructures, few properties of Moiré patterns are based on the bias voltage, which affects the amplitude of electronic corrugation and contrast inversion. However, in this study, the unique substantial change in Moiré periodicity with bias voltage was observed, suggesting the existence of higher indexed patterns, in which the bias voltage change facilitated selective exchange of patterns
To this end, the combined contribution of tellurium (Te) and Mo orbitals contributed to the charge density variability to generate different bias voltages. DFT calculations revealed that this charge variability strongly influenced the graphene substrate. Furthermore, the more indexed Moiré made the patterns complex and rich in MoTe2/graphene heterostructure grown with high crystalline quality by molecular beam epitaxy.
conclusion
In summary, a variety of Moiré structures were formed in the 1H-MoTe2/graphene heterostructure with 6H-SiC (0001)-terminated graphene under molecular beam epitaxy growth conditions as observed by STM. Various tunneling conditions revealed three different patterns in the 1H-MoTe2/graphene heterostructures. Due to the appearance of more indexed moiré patterns, clearly complex and rich moiré structures were observed, as predicted by moiré theory.
The appearance of additional Moiré hues in measurements that prevailed in the Moiré pattern at specific voltages made electronic Moiré highly variable with voltage bias in real-space STM images. Results obtained from STM images revealed a strong signature of proportional supercells around 30 degrees, which were indistinct in most heterostructures.
STM simulations and DFT calculations revealed the role of interlayer electronic coupling in the MoTe2/graphene heterostructure for strong and weak Moiré potential modulations, thus explaining the Moiré patterns. Due to the importance of Moiré patterns in twisttronics phenomena, more indexed patterns expand the possibilities of Moiré physics in van der Waals heterostructures.
reference
Pham, TT, Vancsó, P., Szendrő, M. Palotás, K., Castelino, R., Bouatou, M., Chacon, C et al. (2022) Moiré patterns and more indexed surface states of MoTe2/graphene heterostructure grown by molecular beam epitaxy. Applications npj 2D Mater. https://www.nature.com/articles/s41699-022-00321-9
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