Session R57: 2D Semiconductors: Structure, Growth, and Electronic Properties

Epitaxial growth of stanene and antimonene on noble metal and oxidized metal surfaces

Group-IV and group-V monoelement two-dimensional (2D) materials like stanene, antimonene or bismuthene has been theoretically proposed to be quantum spin Hall (QSH) insulator with large energy gaps for room-temperature use. However, to experimentally fabricate such ultra-thin materials with topologically nontrivial properties remains challenging. It requires accurate control over substrate-interface structures at the atomic level. We successfully grown an ultraflat stanene layer with an in-plane s-p band inversion together with a large spin-orbit-coupling-induced topological gap (~ 0.3 eV), which represents a first group-IV ultraflat graphene-like material displaying topological features in experiment. Such an in-plane s-p band inversion results in a nontrivial σ-orbital-derived QSH phase described by the Bernevig-Hughes-Zhang model and this QSH phase is more environmentally stable comparing to those π-orbital-derived QSH phases. We also successfully synthesized an antimonene film on Ag(111) surface showing an extrordinarily large tensile strain and bilayer stanene on an oxidized Cu(110) surface with large anisotropic strain.   

Effects of short-range order and interfacial interactions on the electronic structure of two-dimensional antimony-arsenic alloys

The growth of two-dimensional (2D) antimony-arsenic alloys has been recently demonstrated using solid-source molecular beam epitaxy and this provides an additional degree of freedom to tailor their basic properties. With this perspective, we propose and conduct a comprehensive first principles investigation on this 2D group-V antimony arsenide (2D As_xSb_y), in both free-standing form as well as on common substrates of Ge(111), Si(111), bilayer graphene and bilayer hexagonal boron nitride (h-BN). Structural and electronic properties of the 2D As_xSb_y are evaluated for different compositions, different types of atomic arrangements for each composition, different lattice matched interfacial configurations of the composite heterostructures for the four substrates. These systematic studies provide property benchmarks for this new class of group-V 2D materials and reveal microscopic origins of the interfacial interactions, orbital hybridization, charge transfer and the resulting electronic structures of the 2D alloy.   

La2Br2Hn: a hydrogenated two-dimensional electride material

Electrides are materials with electrons localized on interstitial sites in the crystal lattice. Some materials exhibit this behaviour at high pressure when it becomes energetically more favourable for some electrons to occupy states localized in the voids between atoms rather than atomic orbitals.

LaBr, a layered magnetic material that is weakly bound by van der Waals interactions, is an ambient pressure electride¹ and, using first-principles electronic structure calculations, we show that monolayer LaBr is potentially exfoliable, stable, retains the magnetic ordering of bulk LaBr and could also be an electride at ambient pressure.

Furthermore, in monolayer LaBr, we show that the two interstitial sites occupied by electrons have a strong tendency to bind with, e.g., hydrogen and we propose several new structures with composition of La, H and Br. We have calculated the composition phase diagram of LaBr with hydrogen, finding LaBr to be a potential promising candidate material for high-density hydrogen storage, and monolayer La₂HBr₂ to be a potential promising candidate 2D multiferroic material.

1. M. Hirayama, S. Matsuishi, H. Hosono and S. Murakami, “Electrides as a New Platform to Topological Materials”, Phys. Rev. X, 8, 031067 (2018)   

Interfacial-ReS2 facilitated formation of highly ordered ferroelectric polymer nanowires

In this work, we report the fabrication and characterization of highly ordered ferroelectric polymer nanowires on an anisotropic van der Waals material, layered 1T^'-phase ReS₂. We deposited 2 - 20 nm poly(vinylidene-fluoride-trifluorethylene) (PVDF-TrFE) thin films on mechanically exfoliated ReS₂ layers using the Langmuir-Blodgett technique. After thermal annealing, the PVDF-TrFE films recrystallize into highly ordered nanowire structures with radius of 27±4 nm. The nanowires are closely packed and well aligned along the direction perpendicular to the b-axis of ReS₂. We then characterized the polarization of the nanowires using vertical and lateral piezo-response force microscopy (PFM), which shows a uniform distribution of domain structure, suggesting the molecular chains’ orientation is highly ordered within the nanowire. Switching the polarization of the nanowires can lead to the change of conductivity in underlying few-layer ReS₂, with a switching on-off ratio of up to 10⁵. Our study provides a novel approach to fabricating high-quality ferroelectric 2D heterostructures for field effect studies.   

Quasiparticle band structure of bulk and few-layer transition-metal dichalcogenides

We studied the work function, ionization energy, and electron affinity of bulk and few-layer transition-metal dichalcogenides (TMDs) in 2H phase using the density functional theory (DFT) and the GW approximation. We obtained DFT band energies of few-layer TMDs with respect to the vacuum level. For the vacuum level of bulk TMDs, we considered a sufficiently thick slab system which has a similar band gap with bulk. We introduced the GW approximation to obtain the quasiparticle energy shift of valence band maximum and conduction band minimum and estimated the work function, band gap, ionization energy, and electron affinity as functions of the number of layers. We compare our quasiparticle band energies of TMDs with available experimental reports.   

Single-particle spectral function formulated and calculated by variational Monte Carlo method

A method to calculate the one-body Green's function for ground states of correlated electron materials is formulated by extending the variational Monte Carlo method. We benchmark against the exact diagonalization (ED) for the one- and two-dimensional Hubbard models of 16 site lattices, which proves high accuracy of the method. The application of the method to larger-sized Hubbard model on the square lattice correctly reproduces the Mott insulating behavior at half filling and gap structures of $d$-wave superconducting state in the hole doped Hubbard model, evidencing a wide applicability to strongly correlated electron systems.   

Defect-enabled high crystallinity in 2D semiconductors and heterostructures

I present two 2D material systems where their crystallinities are paradoxically improved by defects during growth. First, I demonstrate that a new class of defect complex, interlayer Frenkel pairs, enhances the orientational epitaxy of transition metal dichalcogenides on hexagonal boron nitride, leading to experiments that achieve 95% consistency in orientational order by strongly suppressing inversion domains [1,2]. I will show how the stability of the defect complex originates from its electronic structure at the density functional theory level and discuss our recent extension to accelerated GW-level computations for defects. Second, I discuss our recent joint theory and experimental work on a 2D metal-semiconductor heterostructure hosting air-stable, crystalline 2D metals. Enabled by graphene defects in an initial graphene/SiC system, p-block metal atoms such as Ga, In, and Sn intercalate under graphene and bond covalently to the SiC beneath, forming lattice-matched 2D metals, as demonstrated by their calculated phase stabilities [3,4]. Based on first-principles calculations assisted by Wannier functions, I explain the presence of superconductivity in spite of the metals' free-electron-like electronic structure.

[1] F. Zhang*, Y. Wang*, C. Erb, K. Wang, P. Moradifar, V. H. Crespi, and N. Alem, Phys. Rev. B 99, 155430 (2019).
[2] X. Zhang, F. Zhang, Y. Wang, D. S. Schulman, T. Zhang, A. Bansal, N. Alem, S. Das, V. H. Crespi, M. Terrones, and J. M. Redwing, ACS Nano 13, 3341 (2019).
[3] N. Briggs et al. (2019), arXiv:1905.09962.
[4] B. Bersch et al. (2019), arXiv:1905.09938.   

Structural and optical properties of bulk and monolayer GeSe : A Quantum Monte Carlo Study

We have performed quantum Monte Carlo simulations of the monochalcogenide GeSe to study its structural and optical properties. 2D GeSe has received a great deal of attention due to its wide range of applications in industrial devices, such as photodetector, gas sensor, and anode material. Density functional theory (DFT) studies show that the monolayer has smaller lattice parameters than the bulk system, with small band gap energy (1~2 eV). However, DFT cannot conclusive determine if the monolayer has a direct or indirect band gap because of very small differences (~ 0.02 eV) between direct and indirect gaps. Moreover, the geometry for GeSe is not clear within DFT scheme because the DFT lattice parameters and atomic coordinates vary strongly with the particular exchange-correlation functional used. Using fixed-node diffusion Monte Carlo (DMC), we reproduce accurate lattice parameters and band gaps compared to the experimental values for bulk GeSe. For the monolayer, we find that the DMC optimal lattice parameters for the monolayer are close to the bulk ones, and DFT significantly underestimates its lattice parameters. Finally, we compute accurate DMC band gap energies at the optimal geometry for the monolayer.   

Tailoring of interlayer hopping integral at K valley in transition metal dichalcogenides

Interlayer electronic coupling plays a critical role in developing novel electronic metamaterials such as unconventional superconductivity in graphene and moiré excitons in transition metal dichalcogenides (TMDs), in which the potential landscape is determined by the interlayer hopping integral. In addition to the well-known effects of stacking configuration, a largely unexplored factor is the van der Waals (vdW) gap, which impacts the hopping integral exponentially. Here, by measuring the direct optical transitions, we quantitatively determine the interlayer hopping integral of K valley as ~40 meV in Bernal-stacked MoS₂. The vdW-gap dependence was further investigated by tuning the temperature and hydrostatic pressure. We observed a 2.4-fold enhancement at a reduced vdW gap of ~7%. The experimental results were compared with the density functional theory. Our work has shed light on designing the TMD-based moiré heterostructures in the future.   

The relationship between activation energy and band gap in a disordered 2D insulator

We study how the activation energy for conductivity is related to the band gap in a 2D band insulator that experiences a disorder potential created by charged impurities. This problem can be mapped to a problem of classical percolation, since the conductivity is limited by the rate at which electrons are activated from the chemical potential to a percolating energy contour in real space (the classical mobility edge). When impurities are sparse, we find that the activation energy is the same as in the disorder-free case, even though there is significant band bending. On the other hand, when the Coulomb disorder is strong enough to induce large, closely-spaced electron puddles, the activation energy reflects the Coulomb charging energy of large puddles, and has a critical dependence on the band gap that is characteristic of 2D percolation.   

The temperature dependence of the band structures in mono-layer, few-layer and bulk black phosphorus

Black phosphorus (BP), an emerging two-dimensional material, has attracted abundant research interests. However, the study of temperature dependence of the bandgap in few-layer BP is still lacking. Here we systematically investigated the temperature dependence of the electronic structures in mono-layer, few-layer and bulk BP. We found that the temperature dependence of the electronic structure has strong layer and transition-index dependence, which is closely related to the temperature dependent interlayer interaction. Surprisingly, the band gaps of monolayer and bulk BP showed opposite temperature effect. Such diverse behavior sheds light on the importance of van der Waals coupling in defining the electronic structures of 2D materials.   

Effective Hamiltonian for Extrinsic Spin-Orbit Coupling in 2D Materials

Inversion symmetry of a two-dimensional (2D) electron system can be broken by a perpendicular external electric field which lifts the spin degeneracy while it preserves the time-reversal symmetry. The resulting spin splitting, known as the Bychkov-Rashba (BR) spin-orbit coupling, can be used for spin manipulation solely using electric fields without an external magnetic field. However, the main limitation of the phenomenological BR Hamiltonian is that it is inapplicable to two-dimensional (2D) monolayer materials in which the crystal structure is different than that of conventional zinc-blende and wurtzite semiconductors. In this work, we use the theory of invariants to derive the effective Hamiltonian for 2D materials, such as phosphorene and transition metal dichalcogenides, without resorting to any phenomenological prefactor as that in the BR model. Owing to their vertical and lateral scalability and high electron mobility, 2D materials provide an excellent platform to realize semiconducting spintronics devices. We determine the bands that contribute to the extrinsic spin-orbit coupling of conduction electrons and calculate the prefactors in terms of k.p parameters.   

Metal-dependent interfacial properties in pthallocyanine-MoS2 heterostructures

Mixed-dimensional heterostructures made by combining 2D materials with other systems of different dimensionality can exhibit unique interfacial effects and modify the properties of the individual constituent layers. Organic molecules are an attractive building block for these structures due to their synthetically tunable properties and ease of processing. Here we discuss interfacial charge transfer in metallophthalocyanine (MPc) – MoS₂ heterojunctions. These interfaces show heterojunction-specific optical absorption transitions, strong Raman enhancement, and defect emission quenching in the MoS₂, all of which depend the identity of the single metal (M) atom at the MPc core. Temperature dependent optical and electrical characterization were carried out to characterize and to understand the interfacial interactions. The metal core dependence provides a ‘knob’ for controlling opto-electronic properties of both the interface and the underlying layer, highlighting the complex and tunable nature of 0D/2D van Der Waals interfaces.