Session P23: Novel 2D Semiconductors

Theoretical Study of Electronic and Defect Properties of 2D Alloys

Alloy engineering has been developed for modulating the electronic and defect properties of 2D materials. Firstly, we have developed a new concept to modulate the phase diagram of 2D alloys by epitaxial growth, which is the key step to obtain the controllable electronic properties of alloys. Secondly, we have found that homogenous 2D alloys can be applied to tune the electronic properties of 2D materials, making them suitable for various energy related applications. Finally, we suggest a new way to control the defect level positions of 2D materials by alloy engineering.   

Tunable Band Gap in Phosphorene Antidot Lattices

We used first-principles density functional theory (DFT) calculations to investigate the properties of phosphorene antidot lattices. It was found that the stability (quantified by the edge energy) decreases when the density of edges increases. This trend can be broken for H-passivated systems, where in some cases incrementing to a larger radius can increase stability. Most importantly, the band gap can be widely tuned through variation of the perforation spacing and radius. Moreover, deviations from the expected quantum confinement trends are not due to edge effects in general. Spatial distributions of the band gap are roughly bimodal with larger band gap atoms emanating from the zigzag edge, which can be explained by the presence of stronger quantum confinement effects in phosphorene nanoribbons with zigzag termination as compared to armchair termination. Transport will be favored along the armchair direction, which contains a continuous path of the lowest band gap atoms. A system with an electronic signature for metals has bands near the Fermi level that are localized to a new self-passivating 4x1 reconstruction of the zigzag edge and are flat (large effective mass), which suggest transport is not supported. The ability to tune phosphorene's band gap extends its applicability in optoelectronics.   

Electronic structure and electronic properties of two-dimensional InBi

We use first-principles calculations to investigate the structure and electronic properties of InBi. We find that InBi can be considered as 2D layered material in its ground state, and that van de Waals interaction is essential to correctly describe the separation between layers. Using hybrid functional calculations, including spin-orbit effects, we study the electronic structure of bulk and individual layer of InBi. Although InBi is a semimetal in bulk, we find that one-layer InBi has a small gap of 0.05 eV. The band inversion between the In-$s$ and Bi-$p$ bands indicates that the single-layer InBi is a topological insulator material. We also discuss the effects of biaxial and uniaxial strain on the band gap of single-layer InBi, and investigate the evolution of the electronic band structure of free standing InBi as a function of number of layers.   

2D Semiconductor Electronics: Advances, Challenges and Opportunities

Two-dimensional (2-D) semiconductors exhibit excellent device characteristics, as well as novel optical, electrical, and optoelectronic characteristics. In this talk, I will present our recent advancements in defect passivation, contact engineering, surface charge transfer doping, ultrashort transistors, and heterostructure devices of layered chalcogenides. We have developed a defect repair/passivation technique that allows for observation of near-unity quantum yield in monolayer MoS$_{\mathrm{2}}$. The work presents the first demonstration of an optoelectronically perfect monolayer. Forming Ohmic contacts for both electrons and holes is necessary in order to exploit the performance limits of enabled devices while shedding light on the intrinsic properties of a material system. In this regard, we have developed different strategies, including the use of surface charge transfer doping at the contacts to thin down the Schottky barriers, thereby, enabling efficient injection of electrons or holes. We have been able to show high performance n- and p-FETs with various 2D materials, including the demonstration of a FET with 1nm physical gate length exhibiting near ideal switching characteristics. Additionally, I will discuss the use of layered chalcogenides for various heterostructure device applications, exploiting charge transfer at the van der Waals heterointerfaces. I will also present progress towards achieving tunnel transistors using layered semiconductors.   

Electronic and optical properties of two-dimensional GaN from first-principles calculations

GaN is an important commercial semiconductor for solid-state lighting applications (2014 Nobel Prize in Physics). Extreme quantum confinement in atomically thin GaN is a promising method to shift the emission wavelength into the deep ultraviolet range for sterilization applications. Recently, two-dimensional GaN has been experimentally synthesized. We report the electronic and optical properties of two-dimensional GaN using first-principles calculations. We employ density functional theory along with quasiparticle corrections with the GW method to produce accurate band-gap values. We also determine the band structure, carrier effective masses, and optical absorption spectrum. Our results provide microscopic understanding on how the reduction of the thickness to the monolayer regime affects the overall electronic and optical characteristics. This work was supported by the NSF ECCS-CDS{\&}E program under Award No. 1607796. Computational resources were provided by the DOE NERSC facility (DE-AC02-05CH11231).   

Nanosheets of MgB$_{2}$ as a new class of 2D semiconductor

The discovery of two-dimensional semiconducting materials, a decade ago, spawned an entire sub-field within solid-state physics that is focused on the development of nanoelectronics. Here we present a new class of semiconducting two-dimensional material based on hexagonal MgB$_{2}$. Although MgB$_{2}$ is a semimetal, similar to the other well-studied transition metal diborides, we demonstrate that, unlike the transition metal diborides, thinning MgB$_{2}$, to create nanosheets, opens a band gap in the density of states. We predict that a 7 {\AA} thick MgB$_{2}$ nanosheet will have a band gap of 0.51 eV. MgB$_{2}$ nanosheets differ from other two-dimensional semiconductors in that the band gap is introduced by (001) surfaces and is opened by the quantum confinement effect. The implications of these findings are that nanostructured MgB$_{2}$ is not merely a new composition, but also has intrinsic mechanisms for tuning its electronic properties, which may facilitate the development of nanoelectronics.   

Two-dimensional Rashba Lead Chalcogenides

We study two-dimensional lead chalcogenides PbX (X=S, Se, Te) using first principle methods. Due to the heavy Pb element with strong spin-orbit coupling (SOC) and the broken inversion symmetry of its buckled monolayer structure, PbX is found to possess Rashba splitting with a relatively large Rashba coefficient $\alpha_{\rm R}\sim1$ which is comparable to other giant Rashba materials (e.g BiTeBr). The direction of the buckling (polarization) can be switched and thus several critical properties such as the band gap, the Rashba coefficient, and spin projections can be controlled through applications of strain or an external electric field; this control is essential for developing multifunctional electronic devices. We develop a new tight-binding formulation to describe the band structure. Based on these results, we propose a new novel way to control spins through photo-excitations.   

Doping and Polarization Dependence of the Thin Film C$_{60}$ Band Structure

Experimental electronic band structure studies of buckminsterfullerene (C$_{60}$) thin films derived from high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements are examined with respect to new theoretical DFT band structure calculations of the unique low-dimensional structure. The dispersive band structure, substrate interaction, and potassium-doping effects are examined and quantified. Observed patterns with photon energy and polarization dependence are explained in terms of electron interactions within and between the buckyball molecules.   

Band-gap switching and scaling of nanoperforated graphene.

A framework of \textbraceleft w1, w2, R\textbraceright classification for constructing the graphene nanomesh (GNM) of zigzag-edged hexagonal nanohole is systematically built. Three integer indexes w1, w2, and R indicate the distances between two neighboring sides of nanoholes in two directions and the nanohole size respectively, which leading to a straightforward gap opening criteria, i.e., w\textunderscore 1$+$w\textunderscore 2-R$=$3n$+$1,n$\in $Z, steered via DFT band structure calculations. The guiding rule indicates that the semimetallic and semiconducting variation is consistent with a peculiar sequence “010” and “100” (“0”/“1” represent gap closure/opening) with a period of 3 for odd and even w1 respectively. The periodic nanoperforation induced gap sizes agree with a linear fitting with a smaller $\surd $(N\textunderscore rem )$/$N\textunderscore tot ratio, while deviates from that when (w\textunderscore 1$+$w\textunderscore 2)\textless R$+$1. Particularly, the \textbraceleft p, 1, p\textbraceright and \textbraceleft 1, q, q\textbraceright structures demonstrate each unique scaling rule pertaining to the nanohole size only when n is set to zero. Furthermore, the coexistence of Dirac and flat bands is observed for \textbraceleft 1, q, q\textbraceright and \textbraceleft 1, 1, m\textbraceright structures, which is sensitive to the atomic patters.   

Controlling the morphology of MBE-grown WSe$_{\mathrm{2}}$ on epitaxial graphene/SiC(0001).

Controlling the morphology of transition metal dichalcogenides (TMDs) during molecular beam epitaxy is critical for their potential device applications. In this work, by systematically changing the substrate temperature and W/Se flux ratio, the growth of sub-monolayer to few layers WSe$_{\mathrm{2}}$ on graphene/SiC(0001) is investigated by in situ scanning tunneling microscopy, x-ray photoelectron spectroscopy, and Raman spectroscopy. The results indicate that the morphology of the WSe$_{\mathrm{2}}$ films can be controlled from fractal to compact triangular. These findings and their implication for the controlled growth of TMD heterostructures will be discussed at the meeting.   

Tailoring the Energy Gap of Hexagonal Boron Nitride Sheets Embedded with Carbon Domains of Different Shapes and Sizes

We present in this work the structure, stability and electronic properties of hexagonal boron nitride sheets embedded with carbon domains of different shapes and sizes with the goal of understanding their roles in tuning the energy gaps of $h$-BN/C composite sheets compared to pristine $h-$BN sheets. We have considered triangular, hexagonal, circular, and rectangular carbon domains embedded in $h$-BN sheets and calculated their formation and cohesive energies using a semi-empirical method [Phys. Rev. B 74, 155408 (2006)] developed at the University of Louisville. Irrespective of the shape of the domain, we find the formation energy per atom to decrease in a power law fashion as the carbon domain size increases. The energy gap behaviors of $h$-BN/C composite sheets exhibit interesting size- and shape-dependence and will be understood in terms of competing bond lengths, broken symmetry, and mid-gap states in the density of states.   

Single-layer Dual Germanene Phases on Ag(111)

Two distinct phase-separated single-layer honeycomb germanene lattices were identified for germanium growth on Ag(111). The geometric and electronic structures of these two phases, and their correlations, were characterized by STM, LEED, ARPES, and ab-initio calculations. We discovered that a stripe phase germanene, which is partially commensurate with Ag(111) and possesses significant tensile strain, exhibits the unambiguous atomic up-down buckling pattern of an ideal germanene lattice. It emerges from the de-alloying process of the known Ag2Ge surface alloy phase and covers the whole surface at 0.84 ML of Ge. Up to 1.08 ML, a new strain-relaxed germanene phase, which shows an abrupt decrease of Ge-Ge bond length to that of free-standing germanene and is fully incommensurate with Ag(111). This denser phase is quasi-freestanding-like because it preserves the electronic structure symmetrical at germanene$\bar{{K}}$point, where a dominant band observed at $-$3.5 eV, corresponding to Ge-Ge $\sigma $ bonding. In contrast, the electronic structure of the stripe-phase germanene diminishes at the germanene $\bar{{K}}$point and a new band coupled strongly to the substrate emerges at the Ag(111)$\bar{{M}}$   

Machine learnt bond order potential to investigate the low thermal conductivity of stanene nanostructures

The growth of stanene on a Bi$_{\mathrm{2}}$Te$_{\mathrm{3\thinspace }}$substrate has engendered a great deal of interest, in part due to stanene's predicted exotic properties. In particular, stanene shows promise in topological insulation, large-gap 2D quantum spin hall states, lossless electrical conduction, enhanced thermoelectricity, and topological superconductivity. However, atomistic investigations of growth mechanisms (needed to guide synthesis), phonon transport (crucial for designing thermoelectrics), and thermo-mechanical behavior of stanene are scarce. This paucity is primarily due to the lack of inter-atomic potentials that can accurately capture atomic interactions in stanene. To address this, we have developed a machine learnt bond-order potential (BOP) based on Tersoff's formalism that can accurately capture bond breaking/formation events, structure, energetics, thermodynamics, thermal conductivity, and mechanical properties of single layer tin, using a training set derived from density functional theory calculations. Finally, we employed our newly developed BOP to study anisotropy in thermal conductivity of stanene sheets, temperature induced rippling, as well as dependence of anharmonicity and thermal conductivity on temperature.