skip to main content

Title: Covalent surface modifications and superconductivity of two-dimensional metal carbide MXenes

Versatile chemical transformations of surface functional groups in two-dimensional transition-metal carbides (MXenes) open up a previously unexplored design space for this broad class of functional materials. We introduce a general strategy to install and remove surface groups by performing substitution and elimination reactions in molten inorganic salts. Successful synthesis of MXenes with oxygen, imido, sulfur, chlorine, selenium, bromine, and tellurium surface terminations, as well as bare MXenes (no surface termination), was demonstrated. These MXenes show distinctive structural and electronic properties. For example, the surface groups control interatomic distances in the MXene lattice, and Tin+1Cn(n= 1, 2) MXenes terminated with telluride (Te2−) ligands show a giant (>18%) in-plane lattice expansion compared with the unstrained titanium carbide lattice. The surface groups also control superconductivity of niobium carbide MXenes.

Authors:
 ;  ;  ;  ;  ;  ;  ;  
Award ID(s):
1729420 2004880
Publication Date:
NSF-PAR ID:
10226642
Journal Name:
Science
Volume:
369
Issue:
6506
Page Range or eLocation-ID:
p. 979-983
ISSN:
0036-8075
Publisher:
American Association for the Advancement of Science (AAAS)
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    An emerging chalcogenide perovskite, CaZrSe3, holds promise for energy conversion applications given its notable optical and electrical properties. However, knowledge of its thermal properties is extremely important, e.g. for potential thermoelectric applications, and has not been previously reported in detail. In this work, we examine and explain the lattice thermal transport mechanisms in CaZrSe3using density functional theory and Boltzmann transport calculations. We find the mean relaxation time to be extremely short corroborating an enhanced phonon–phonon scattering that annihilates phonon modes, and lowers thermal conductivity. In addition, strong anharmonicity in the perovskite crystal represented by the Grüneisen parameter predictions, andmore »low phonon number density for the acoustic modes, results in the lattice thermal conductivity to be limited to 1.17 W m−1 K−1. The average phonon mean free path in the bulk CaZrSe3sample (N → ∞) is 138.1 nm and nanostructuring CaZrSe3sample to ~10 nm diminishes the thermal conductivity to 0.23 W m−1 K−1. We also find that p-type doping yields higher predictions of thermoelectric figure of merit than n-type doping, and values ofZT~0.95–1 are found for hole concentrations in the range 1016–1017 cm−3and temperature between 600 and 700 K.

    « less
  2. Abstract

    A knowledge-based understanding of the plasma-surface-interaction with the aim to precisely control (reactive) sputtering processes for the deposition of thin films with tailored and reproducible properties is highly desired for industrial applications. In order to understand the effect of plasma parameter variations on the film properties, a single plasma parameter needs to be varied, while all other process and plasma parameters should remain constant. In this work, we use the Electrical Asymmetry Effect in a multi-frequency capacitively coupled plasma to control the ion energy at the substrate without affecting the ion-to-growth flux ratio by adjusting the relative phase betweenmore »two consecutive driving harmonics and their voltage amplitudes. Measurements of the ion energy distribution function and ion flux at the substrate by a retarding field energy analyzer combined with the determined deposition rateRdfor a reactive Ar/N2(8:1) plasma at 0.5 Pa show a possible variation of the mean ion energy at the substrateEmigwithin a range of 38 and 81 eV that allows the modification of the film characteristics at the grounded electrode, when changing the relative phase shiftθbetween the applied voltage frequencies, while the ion-to-growth flux ratio Γiggrcan be kept constant. AlN thin films are deposited and exhibit an increase in compressive film stress from −5.8 to −8.4 GPa as well as an increase in elastic modulus from 175 to 224 GPa as a function of the mean ion energy. Moreover, a transition from the preferential orientation (002) at low ion energies to the (100), (101) and (110) orientations at higher ion energies is observed. In this way, the effects of the ion energy on the growing film are identified, while other process relevant parameters remain unchanged.

    « less
  3. Recent spectroscopic, kinetic, photophysical, and thermodynamic measurements show activation of nitrogenase for N2→ 2NH3reduction involves the reductive elimination (re) of H2from two [Fe–H–Fe] bridging hydrides bound to the catalytic [7Fe–9S–Mo–C–homocitrate] FeMo-cofactor (FeMo-co). These studies rationalize the Lowe–Thorneley kinetic scheme’s proposal of mechanistically obligatory formation of one H2for each N2reduced. They also provide an overall framework for understanding the mechanism of nitrogen fixation by nitrogenase. However, they directly pose fundamental questions addressed computationally here. We here report an extensive computational investigation of the structure and energetics of possible nitrogenase intermediates using structural models for the active site with a broad rangemore »in complexity, while evaluating a diverse set of density functional theory flavors. (i) This shows that to prevent spurious disruption of FeMo-co having accumulated 4[e/H+] it is necessary to include: all residues (and water molecules) interacting directly with FeMo-co via specific H-bond interactions; nonspecific local electrostatic interactions; and steric confinement. (ii) These calculations indicate an important role of sulfide hemilability in the overall conversion ofE0to a diazene-level intermediate. (iii) Perhaps most importantly, they explain (iiia) how the enzyme mechanistically couples exothermic H2formation to endothermic cleavage of the N≡N triple bond in a nearly thermoneutralre/oxidative-addition equilibrium, (iiib) while preventing the “futile” generation of two H2without N2reduction: hydrideregenerates an H2complex, but H2is only lost when displaced by N2, to form an end-on N2complex that proceeds to a diazene-level intermediate.

    « less
  4. Abstract

    N2fixation constitutes an important new nitrogen source in the open sea. One group of filamentous N2fixing cyanobacteria (Richelia intracellularis, hereafterRichelia)form symbiosis with a few genera of diatoms. High rates of N2fixation and carbon (C) fixation have been measured in the presence of diatom-Richeliasymbioses. However, it is unknown how partners coordinate C fixation and how the symbiont sustains high rates of N2fixation. Here, both the N2and C fixation in wild diatom-Richeliapopulations are reported. Inhibitor experiments designed to inhibit host photosynthesis, resulted in lower estimated growth and depressed C and N2fixation, suggesting that despite the symbionts ability to fix their ownmore »C, they must still rely on their respective hosts for C. Single cell analysis indicated that up to 22% of assimilated C in the symbiont is derived from the host, whereas 78–91% of the host N is supplied from their symbionts. A size-dependent relationship is identified where larger cells have higher N2and C fixation, and only N2fixation was light dependent. Using the single cell measures, the N-rich phycosphere surrounding these symbioses was estimated and contributes directly and rapidly to the surface ocean rather than the mesopelagic, even at high estimated sinking velocities (<10 m d−1). Several eco-physiological parameters necessary for incorporating symbiotic N2fixing populations into larger basin scale biogeochemical models (i.e., N and C cycles) are provided.

    « less
  5. We report a molecular switching ensemble whose states may be regulated in synergistic fashion by both protonation and photoirradiation. This allows hierarchical control in both a kinetic and thermodynamic sense. These pseudorotaxane-based molecular devices exploit the so-called Texas-sized molecular box (cyclo[2]-(2,6-di(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylenebenzene); 14+, studied as its tetrakis-PF6salt) as the wheel component. Anions of azobenzene-4,4′-dicarboxylic acid (2H+•2) or 4,4′-stilbenedicarboxylic acid (2H+•3) serve as the threading rod elements. The various forms of 2 and 3 (neutral, monoprotonated, and diprotonated) interact differently with 14+, as do the photoinducedcisortransforms of these classic photoactive guests. The net result is a multimodal molecular switch that can bemore »regulated in synergistic fashion through protonation/deprotonation and photoirradiation. The degree of guest protonation is the dominating control factor, with light acting as a secondary regulatory stimulus. The present dual input strategy provides a complement to more traditional orthogonal stimulus-based approaches to molecular switching and allows for the creation of nonbinary stimulus-responsive functional materials.

    « less