AVS 69 Session 2D2-ThA: 2D-Materials: Surface and Interface Effects

Compared with their 3D counterparts, two-dimensional (2D) van der Waals (vdW) materials exhibit quantum confinement where charge carriers are spatially confined at the physical boundaries. Particularly, when mixing 2D materials with other low-dimensional (LD) materials, they exhibit enormous potential in electrochemical energy applications due to the unique properties arising from reduced dimensionality and, more importantly, material integration synergy. In this work, 2D transition metal dichalcogenides and their mixed low-dimensional hybrids (MLDHs) are introduced with an emphasis on innovations covering 2D-based hybrid structure construction and electrochemical applications. Fundamental insight into the synergistic effect of the MLDHs integration for advancing the development of Li-ion batteries and electrocatalytic hydrogen evolution reaction will also be discussed. Leveraging the unique microreactor platform based on the 2D vdW platform, a mechanistic understanding of charge transport dynamics at the electrified interface will be highlighted. The knowledge gained on how mixed-dimensional physics and chemistry will shed light on the design principle of the electrode materials and deepen the understanding of the process-structural-property-performance (PSPP) relationship of the vdW-based hybrid structures.

Fe₃GeTe₂ (FGT) is a van der Waals (vdW) ferromagnetic metallic material with Curie temperature around 230 K. Despite the central symmetric crystal structure, magnetic skyrmions and various magnetic domain textures have been reported in FGT. The magnetic domain textures can be widely tuned with thickness, temperature and magnetic field. Several ideas regarding the origin of the wild magnetic domain textures have been proposed, including oxidized layer induced DMI, Fe defect vacancy etc. Here, by utilizing spin polarized scanning tunneling microscopy (SPSTM), we revealed that, compared to Fe vacancies, the Te vacancies have stronger effects on altering the magnetic domains in the otherwise ferromagnetic system. A theoretical model will also be discussed to explain the difference between the Te vacancies and the Fe vacancies on the magnetic domain wall pinning.

The interactions between constituent layers in heterostructures provide an opportunity to stabilize 2D compounds not found in equilibrium phase diagrams. Utilizing 2D layers of 3D structures like rock-salt structured PbSe, commonly found in thermodynamically stable heterostructures known as misfit compounds, phenomena such as charge transfer to and surface stabilization can be used to stabilize new structures. In the iron-selenide system β-FeSe exhibits high-temperature superconductivity (T_c ~ 107 K) when grown in a monolayer on a SrTiO₃ substrate. This motivation sparked our interest in investigating possible Fe-Se phases when layers of controlled composition are spatially confined between adjacent layers of PbSe. Using a computational “island” approximation, potential candidate Fe-Se structures were placed between bilayers of PbSe and were relaxed in DFT calculations to assess stability of different structures. Of the trialed candidates, (PbSe)_1+δ(FeSe₂) with Fe in octahedral coordination (1T) maintained its structure when relaxed. Using the predicted densities of the relaxed model, precursors that mimic the nanoarchitecture of the heterostructure were prepared and annealed at low temperatures to prepare the heterostructures (PbSe)_1+δ(FeSe₂)_n where n = 1, 2, and 3. To probe the effect of different adjacent layers on the stability of the 1T FeSe₂ layers, we successfully stabilized 1T-FeSe₂ in (PbSe)_1+δ(NbSe₂)(PbSe)_1+δ(FeSe₂). These systems provide insights into interfacial interactions between constituent layers in 2D heterostructures that can be used to prepare layers with structures not found in the phase diagrams of the constituent elements.

The energy-filtered photoelectron microscope NanoESCA [1,2] is a powerful tool for various application including work-function mapping, imaging XPS and in the last years more prominently for momentum microscopy on 2D materials (e.g. see [3]).

This analyzer can be used with an efficient spin filter that enables to image a 2D-distribution of the electron spin polarization by scattering the electrons at a polarizing target. We will show results from the first commercial build Au/Ir Imaging Spin Filter. Sherman functions of +68% and -58% were found at a reflectivity of more than 1% (also see literature [4]).

Spin-filtered images of magnetic domains show that along the diameter of the field of view more than 100 separate image points can be resolved. This increases the effective 2D figure-of-merit of this analyzer by nearly four orders of magnitude compared to single-channel spin detectors. We also present proof of principal measurements of an Imaging Spin Filter with oxide passivated Fe as scattering target [5]. Oxide passivated Fe allows for an easy switch of the polarization detection direction, like it is known from FERRUM detectors [6] for ARPES.

References

[1]M. Escher et al., J. Phys. Cond. Matter 17 (2005)

[2]B. Krömker et al., Rev. Sci. Instrum. 79 (2008)

[3]A. Polyakov et al., Nature Comm. 13 (2022) 2472

[4]C.Tusche et al., Ultramicroscopy 159 (2015) 520–529

[5]M. Escher et al., Ultramicroscopy 253 (2023) 113814

[6]M. Escher et al., e-J. Surf. Sci. Nanotech. Vol. 9 (2011) 340-343