Designing barrier-free metal/MoS2 contacts through electrene insertion

Transition-metal dichalcogenides (TMDCs), including MoS2, have great potential in electronics applications. However, achieving low-resistance metal contacts is a challenge that impacts their performance in nanodevices due to strong Fermi-level pinning and the presence of a tunnelling barrier. As a solution, we explore a strategy of inserting monolayers of alkaline-earth sub-pnictide electrenes with a general formula of [M2X]+e- (M = Ca, Sr, Ba; X = N, P, As, Sb) between the TMDC and the metal. These electrenes possess two-dimensional sheets of charge on their surfaces that can be readily donated when interfaced with a TMDC semiconductor, thereby lowering its conduction band below the Fermi level and eliminating the Schottky and tunnelling barriers. In this work, density-functional theory (DFT) calculations were performed for metal/electrene/MoS2 heterojunctions for all stable M2X electrenes and both Au and Cu metals. To identify the material combinations that provide the most effective Ohmic contact, the charge transfer, band structure, and electrostatic potential were computed. Linear correlations were found between the charge donated to the MoS2 and both the electrene surface charge and work function. Overall, Ca2N appears to be the most promising electrene for achieving an Ohmic metal/MoS2 contact due to its high surface charge density.

Ground and excited electronic structures of electride and alkalide units: The cases of Metal-Tren, -Azacryptand, and -TriPip222 complexes

A systematic electronic structure analysis was conducted for M(L)n molecular electrides and their corresponding alkalide units M(L)n @M' (M/M' = Na, K; L = Tren, Azacryptand, TriPip222; n = 1, 2). All complexes belong to the "superalkali" category due to their low ionization potentials. The saturated molecular electrides display M+ (L)n - form with a greatly diffuse quasispherical electron cloud. They were identified as "superatoms" considering the contours of populating atomic-type molecular orbitals. The observed superatomic Aufbau order of M(Tren)2 is 1S, 1P, 1D, 1F, 2S, 2P, and 1G and it is consistent with those of M(Azacryptand) and M(TriPip222) up to the analyzed 1F level. Their excitation energies decrease gradually moving from M(Tren)2 to M(Azacryptand) and to M(TriPip222). The studied alkalide complexes carry [M(L)n ]+ @M'- ionic structure and their dissociation energies vary in the sequence of K(L)n @Na > Na(L)n @Na > K(L)n @K > Na(L)n @K. Similar to molecular electrides, the anions of alkalide units occupy electrons in diffuse Rydberg-like orbitals. In this work, excited states of [M(L)n @M']0/+/- and their trends are also analyzed.

Metallicity and chemical bonding in anti-anatase Mo2N

Here we present a detailed analysis of the structure, bonding character, and electronic structure of anti-anatase β-Mo2N using density functional theory calculations. We analyze the crystal orbital Hamilton populations, phonon band structure, and electronic structure calculations to explain its low energy transport behavior. We further examine the electronic structures of (anti-)rutile and (anti-)anatase M3-nXn (X = N,O; n = 1,2) M = Ti and Mo nitrides and oxides to show that the atomic structure of anti-anatase leads to metallic behavior independent of the metal and ligand chemistry. Finally, we assess whether these anti-anatase compounds are viable electrides using electron density maps and electron localization functions. Our work shows anti-structures of known binary compounds can expand the phase space of available metallic ceramics beyond layered, hexagonal carbides and nitrides, e.g., Mn+1An (MAX) where n = 1-4.

Low thermal expansion of layered electrides predicted by density-functional theory

Layered electrides are a unique class of materials with anionic electrons bound in interstitial regions between thin, positively charged atomic layers. While density-functional theory is the tool of choice for computational study of electrides, there has to date been no systematic comparison of density functionals or dispersion corrections for their accurate simulation. There has also been no research into the thermomechanical properties of layered electrides, with computational predictions considering only static lattices. In this work, we investigate the thermomechanical properties of five layered electrides using density-functional theory to evaluate the magnitude of thermal effects on their lattice constants and cell volumes. We also assess the accuracy of five popular dispersion corrections with both planewave and numerical atomic orbital calculations.

Ohmic contacts of the two-dimensional Ca2N/MoS2 donor-acceptor heterostructure

In the current stage, conventional silicon-based devices are suffering from the scaling limits and the Fermi level pinning effect. Therefore, looking for low-resistance metal contacts for semiconductors has become one of the most important topics, and two-dimensional (2D) metal/semiconductor contacts turn out to be highly interesting. Alternatively, the Schottky barrier and the tunneling barrier impede their practical applications. In this work, we propose a new strategy for reducing the contact potential barrier by constructing a donor-acceptor heterostructure, that is, Ca₂N/MoS₂ with Ca₂N being a 2D electrene material with a significantly small work function and a rather high carrier concentration. The quasi-bond interaction of the heterostructure avoids the formation of a Fermi level pinning effect and gives rise to high tunneling probability. An excellent n-type Ohmic contact form between Ca₂N and MoS₂ monolayers, with a 100% tunneling probability and a perfect linear I-V curve, and large lateral band bending also demonstrates the good performance of the contact. We verify a fascinating phenomenon that Ca₂N can trigger the phase transition of MoS₂ from 2H to 1T'. In addition, we also identify that Ohmic contacts can be formed between Ca₂N and other 2D transition metal dichalcogenides (TMDCs), including WS₂, MoSe₂, WSe₂, and MoTe₂.

Superatomic Chelates: The Cases of Metal Aza-Crown Ethers and Cryptands

Li, Na, and Mg⁺-coordinated hexaaza-18-crown-6 ([18]aneN₆) and 1,4,7-triazacyclononane ([9]aneN₃), Li[1.1.1]cryptand, and Na[2.2.2]cryptand species possess a diffuse electron in a quasispherical s-type orbital. They populate expanded p-, d-, f-, and g-shape orbitals in low-lying excited states and hence are identified as "superatoms". By means of quantum calculations, their superatomic shell models are revealed. The observed orbital series of M([9]aneN₃)₂ and M[18]aneN₆ (M = Li, Na, Mg⁺) are identical to the 1s, 1p, 1d, 1f, 2s, and 2p. The electronic spectra of Li[1.1.1]cryptand and Na[2.2.2]cryptand were analyzed up to the 1f¹ configuration, and their transitions were found to occur at lower energies compared to their aza-crown ethers. The introduced superatomic shell models in this work closely resemble the Aufbau principle of "solvated electrons precursors". All reported alkali metal complexes bear lower ionization potentials than any atom in the periodic table; thus, they can also be recognized as "superalkalis".

Ground and excited states analysis of alkali metal ethylenediamine and crown ether complexes

High-level electronic structure calculations are carried out to obtain optimized geometries and excitation energies of neutral lithium, sodium, and potassium complexes with two ethylenediamine and one or two crown ether molecules. Three different sizes of crowns are employed (12-crown-4, 15-crown-5, 18-crown-6). The ground state of all complexes contains an electron in an s-type orbital. For the mono-crown ether complexes, this orbital is the polarized valence s-orbital of the metal, but for the other systems this orbital is a peripheral diffuse orbital. The nature of the low-lying electronic states is found to be different for each of these species. Specifically, the metal ethylenediamine complexes follow the previously discovered shell model of metal ammonia complexes (1s, 1p, 1d, 2s, 1f), but both mono- and sandwich di-crown ether complexes bear a different shell model partially due to their lower (cylindrical) symmetry and the stabilization of the 2s-type orbital. Li(15-crown-5) is the only complex with the metal in the middle of the crown ether and adopts closely the shell model of metal ammonia complexes. Our findings suggest that the electronic band structure of electrides (metal crown ether sandwich aggregates) and expanded metals (metal ammonia aggregates) should be different despite the similar nature of these systems (bearing diffuse electrons around a metal complex).

Density-functional description of alkalides: introducing the alkalide state

Alkalides are crystalline salts in which the anion is a negatively charged alkali metal. A systematic investigation of the electronic structure of thirteen alkalides, with known crystal structures, is conducted using density-functional theory. For each alkalide, a high-lying valence state is identified that is localised on the alkali anions and is consistent with the low band gap and strong reducing power characteristic of these materials. This 'alkalide state' is compared to a similar state in the related class of electride materials, where the alkali anions are replaced by crystal voids occupied by localised, interstitial electrons. Finally, a thermodynamic cycle is constructed to examine the energy differences between the alkalides and electrides, revealing that the alkali-metal anion significantly stabilises the crystal.