Electron Diffraction From Two Dimensional Materials

Yu Xiang’s PhD Committee:

• Dr. Gwo-Ching Wang, Chair
• Dr. Toh-Ming Lu, Co-Chair
• Dr. Humberto Terrones
• Dr. Jian Shi

Abstract of the PhD Thesis:

There has been an explosive growth in the study of two dimensional (2D) materials governed by van der Waals interaction in the last decade due to their novel electronic, electrical, optical, and magnetic properties that were not known or discovered in the past. Those properties are strongly affected by their structure and long-range order perfection. A thorough structural characterization of 2D materials is the central issue but it has been challenging due to the inherent limited amounts of atoms in 2D materials under study. Fortunately, electron interacts strongly with matters and allows structure and perfection of 2D materials to be studied. In this thesis the near surface sensitive azimuthal reflection high-energy electron diffraction (ARHEED) developed at Rensselaer Physics was applied for the first time to map out:

1. 2D reciprocal space structures of single crystal graphene on amorphous SiO2 and epitaxial Cu(111) substrates and
2. 3D reciprocal space structures of monolayer (ML) MoS2 and ML WS2 on sapphire(0001) substrates.

The surface sensitive high-resolution low-energy electron diffraction (HRLEED) was used to compliment ARHEED to study single crystal graphene on Cu(111). Two application studies using RHEED and van der Waals substrates are also presented:

1. Single crystal graphene as a buffer layer to guide the growth of van der Waals epitaxial SnS film on amorphous SiO2 substrate and
2. 3D CdTe epitaxial thin film on mica substrate.

Through quantitative analysis of the 2D reciprocal space map, one can extract real space properties of the 2D material including the symmetry, orientation domain distribution, lattice constants, interlayer spacing and average domain size. The 2D reciprocal space maps of graphene on Cu(111), homemade single crystal graphene transferred on SiO2/Si substrate, and commercial polycrystalline graphene on SiO2/Si substrate clearly show differences in their corresponding 2D maps that reveal their orientation domains and wafer scale quality. For MoS2 the corresponding 3D map from ARHEED reveals:

1. The in-plane and out-of-plane epitaxial relationships with sapphire. This is consistent with the prediction of geometrical superlattice area mismatching.
2. Monolayer MoS2 and sapphire spacing of ~3 Å. This turns out to be the spacing between MoS2 and the sulfur passivation layer formed on top of sapphire. This is supported by experimental TEM and AFM results as well as first principles density functional theory (DFT) calculations.
3. Despite the continuous monolayer coverage of MoS2 on sapphire, the electron diffraction spots are unusually broader (~four times) than the instrument response width (~0.1 Å-1). This is supported by numerical simulation of incommensurate finite size domains. This may be a generic result for transition metal dichalcogenide (TMDC) monolayer on mismatched substrate.

The HRLEED study of single crystal graphene on Cu(111) reveals that diffraction peaks have very similar full-width-at-half-maximums but the average broadening is significantly (~three times) larger than the instrument response (~0.03 Å-1). This suggests a noticeable number of defects exist within both graphene and copper surface. From the LEED IV curve after an inner potential correction, the graphene to Cu(111) surface distance is estimated to be d = 3.49 ± 0.01 Å. This value is close to that (d = 3.27 ± 0.07 Å) determined by RHEED from multilayer graphene and the interlayer spacing of 3.36 Å in graphite. This suggests that the coupling between graphene and Cu metal is similar to a pure van der Waals interaction.

For graphene buffered van der Waals epitaxial tin mono-sulfide (SnS) thin film, the RHEED patterns show the (010) orientation near the surface of the SnS film grown on the single crystal graphene and a dominant (111) orientation near the surface of the SnS film grown on the polycrystalline graphene. Additional minor orientations near the surface were observed by RHEED that X-ray diffraction (XRD) is not able to detect.

PhD Thesis Defense Presentation