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Roy, Xavier

Publications and source records attributed to Roy, Xavier.

33 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Layered Antiferromagnetism Induces Large Negative Magnetoresistance in the van der Waals Semiconductor CrSBr

The recent discovery of magnetism within the family of exfoliatable van der Waals (vdW) compounds has attracted considerable interest in these materials for both fundamental research and technological applications. However, current vdW magnets are limited by their extreme sensitivity to air, low ordering temperatures, and poor charge transport properties. In this work, the magnetic and electronic properties of CrSBr are reported, an air-stable vdW antiferromagnetic semiconductor that readily cleaves perpendicular to the stacking axis. Below its Néel temperature, T N = 132 ± 1 K, CrSBr adopts an A-type antiferromagnetic structure with each individual layer ferromagnetically ordered internally and the layers coupled antiferromagnetically along the stacking direction. Scanning tunneling spectroscopy and photoluminescence (PL) reveal that the electronic gap is Δ E = 1.5 ± 0.2 eV with a corresponding PL peak centered at 1.25 ± 0.07 eV. Using magnetotransport measurements, strong coupling between magnetic order and transport properties in CrSBr is demonstrated, leading to a large negative magnetoresistance response that is unique among vdW materials. These findings establish CrSBr as a promising material platform for increasing the applicability of vdW magnets to the field of spin-based electronics.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Strong polaronic effect in a superatomic two-dimensional semiconductor

Crystalline solids assembled from superatomic building blocks are attractive functional materials due to their hierarchical structure, multifunctionality, and tunability. An interesting example is Re 6 Se 8 Cl 2 , in which the Re 6 Se 8 building blocks are covalently linked into two-dimensional (2D) sheets that are stacked into a layered van der Waals solid. It is an indirect gap semiconductor that, when heavily doped, becomes a superconductor at low temperatures. Given the finite electronic bandwidths (300–400 meV), carrier properties in this material are expected to be strongly influenced by coupling to phonons. Here, we apply angle-resolved photoemission spectroscopy to probe the valence band edge (VBE) of Re 6 Se 8 Cl 2 . We find that dispersion of the VBE is a strong function of temperature. The bandwidth is W = 120 ± 30 meV at 70 K and decreases by one order of magnitude to W ~ 10 ± 20 meV as temperature is increased to 300 K. This observation reveals the dominant polaronic effects in Re 6 Se 8 Cl 2 , consistent with the Holstein polaron model commonly used to describe molecular solids.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Ligand Effect on the Electronic Structure of Cobalt Sulfide Clusters: A Combined Experimental and Theoretical Study

Recent studies have shown that capping ligands offer a new dimension for fine-tuning the properties of clusters. In this work, we investigate this concept by measuring the anion photoelectron spectra of a series of hexanuclear cobalt sulfide clusters, Co 6 S 8 , passivated by different numbers of triethylphosphine ligands, PEt 3 . We find that the addition of PEt 3 gradually shifts the electronic spectrum of the cluster to higher energy, leading to a decrease in its electron affinity. Density functional theory calculations reveal that adding ligands demagnetizes the Co6S8 core. The decrease in electron affinity results from a monotonic increase in the energy of the cluster lowest unoccupied molecular orbitals (LUMO). This effect is attributed to the electron donation from the ligands to the cluster core, which increases the charge density in the core region.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Tuning the electronic properties of hexanuclear cobalt sulfide superatoms via ligand substitution

Molecular clusters are attractive superatomic building blocks for creating materials with tailored properties due to their unique combination of atomic precision, tunability and functionality. The ligands passivating these superatomic clusters offer an exciting opportunity to control their electronic properties while preserving their closed shells and electron counts, which is not achievable in conventional atoms. Here we demonstrate this concept by measuring the anion photoelectron spectra of a series of hexanuclear cobalt sulfide superatomic clusters with different ratios of electron-donating and electron-withdrawing ligands, Co 6 S 8 (PEt 3 ) 6-x (CO) x (x = 0–3). We find that Co 6 S 8 (PEt 3 ) 6 has a low electron affinity (EA) of 1.1 eV, and that the successive replacement of PEt3 ligands with CO gradually shifts its electronic spectrum to lower energy and increases its EA to 1.8 eV. Density functional theory calculations reveal that the increase of EA results from a monotonic lowering of the cluster highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO). Our work provides unique insights into the electronic structure and tunability of superatomic building blocks.

Chemistry↗
DOE OSTI2018

Electron Cartography in Clusters

Abstract Deconvoluting the atom‐specific electron density within polynuclear systems remains a challenge. A multiple‐wavelength anomalous diffraction study on four clusters that share the same [Co 6 Se 8 ] core was performed. Two cluster types were designed, one having a symmetric ligand sphere and the other having an asymmetric ligand sphere. It was found that in the neutral, asymmetric, CO‐bound cluster, the Co−CO site is more highly oxidized than the other five Co atoms; when an electron is removed, the hole is distributed among the Se atoms. In the neutral, symmetric cluster, the Co atoms divide by electron population into two sets of three, each set being meridional; upon removal of an electron, the hole is distributed among all the Co atoms. This ligand‐dependent tuning of the electron/hole distribution relates directly to the performance of clusters in biological and synthetic systems.

Hernández Sánchez, Raúl↗
DOE OSTI2018

Solid-state materials formed of molecular clusters and method of forming same

A solid-state material comprising a solid-state compound is provided. The solid-state compound has the formula: [Cluster1][Cluster2]n, where Cluster1 can be a metal chalcogenide molecular cluster, Cluster2 a carbon cluster, and n the number of Cluster2 clusters in the solid-state compound. A method of forming a solid-state material is also provided.

Nuckolls, Colin↗
DOE OSTI2017

Electronically Transparent Au–N Bonds for Molecular Junctions

We report a series of single-molecule transport measurements carried out in an ionic environment with oligophenylenediamine wires. These molecules exhibit three discrete conducting states accessed by electrochemically modifying the contacts. Transport in these junctions is defined by the oligophenylene backbone, but the conductance is increased by factors of ~20 and ~400 when compared to traditional dative junctions. We propose that the higher-conducting states arise from in situ electrochemical conversion of the dative Au←N bond into a new type of Au–N contact. Density functional theory-based transport calculations establish that the new contacts dramatically increase the electronic coupling of the oligophenylene backbone to the Au electrodes, consistent with experimental transport data. The resulting contact resistance is the lowest reported to date; more generally, our work demonstrates a facile method for creating electronically transparent metal–organic interfaces.

Zang, Yaping↗
DOE OSTI2017

Distinct properties of the triplet pair state from singlet fission

Singlet fission, the conversion of a singlet exciton (S 1 ) to two triplets (2 × T 1 ), may increase the solar energy conversion efficiency beyond the Shockley-Queisser limit. This process is believed to involve the correlated triplet pair state 1 (TT). Despite extensive research, the nature of the 1 (TT) state and its spectroscopic signature remain actively debated. We use an end-connected pentacene dimer (BP0) as a model system and show evidence for a tightly bound 1 (TT) state. It is characterized in the near-infrared (IR) region (~1.0 eV) by a distinct excited-state absorption (ESA) spectral feature, which closely resembles that of the S 1 state; both show vibronic progressions of the aromatic ring breathing mode. We assign these near-IR spectra to 1 (TT)→S n and S 1 →S n' transitions; S n and S n' likely come from the antisymmetric and symmetric linear combinations, respectively, of the S 2 state localized on each pentacene unit in the dimer molecule. The 1 (TT)→S n transition is an indicator of the intertriplet electronic coupling strength, because inserting a phenylene spacer or twisting the dihedral angle between the two pentacene chromophores decreases the intertriplet electronic coupling and diminishes this ESA peak. In addition to spectroscopic signature, the tightly bound 1 (TT) state also shows chemical reactivity that is distinctively different from that of an individual T 1 state. Using an electron-accepting iron oxide molecular cluster [Fe 8 O 4 ] linked to the pentacene or pentacene dimer (BP0), we show that electron transfer to the cluster occurs efficiently from an individual T 1 in pentacene but not from the tightly bound 1 (TT) state. Thus, reducing intertriplet electronic coupling in 1 (TT) via molecular design might be necessary for the efficient harvesting of triplets from intramolecular singlet fission.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗