Tuning the Quantum Stability and Superconductivity of Ultrathin Metal Alloys

Özer, Mustafa M.; Jia, Yu; Zhang, Zhenyu; Thompson, James R.; Weitering, Hanno H.

doi:10.1126/science.1142159

Cited by 108 publications

(82 citation statements)

References 24 publications

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“…0 > Δ H μ , which implies that hydrogen absorption is only possible for P>1bar regardless of the temperature range. In fact, for a given T, extremely high P is required for H absorption; for example 7 …”

Section: Hydrogen Absorption/desorption Phase Diagrammentioning

confidence: 99%

“…It is comparably easy to chemically modify these systems. Ultrathin metal films are very intriguing materials, because their chemical properties can be modified interactively by changing the thickness of the films as a result of quantum size effects [7,8]. Moreover, the low structural stabilities of ultrathin Mg films may help to reduce the required temperature for releasing absorbsed hydrogen to the system.…”

Section: Introductionmentioning

confidence: 99%

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First-principles studies of hydrogen interaction with ultrathin Mg and Mg-based alloy films

2011

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The search for technologically and economically viable storage solutions for hydrogen fuel would greatly benefit from research strategies that involve systematic property tuning of potential storage materials via atomic-level modification. Here, we use first-principles density functional theory (DFT) to theoretically investigate the structural and electronic properties of ultrathin Mg films and Mg-based alloy films and their interaction with atomic hydrogen.Additional delocalized charges are distributed over the Mg films upon alloying them with 11.1% of Al or Na atoms. These extra charges contribute to enhance the hydrogen binding strength to the films. We calculated the chemical potential of hydrogen in Mg films for different dopant species and film thickness, and included the vibrational degrees of freedom. By comparing the chemical potential with that of free hydrogen gas at finite temperature (T) and pressure (P), we construct a hydrogenation phase diagram and identify the conditions for hydrogen absorption/desorption. The formation enthalpies of metal hydrides are greatly increased in thin films, and in stark contrast to its bulk phase, the hydride state can only be stabilized at high P and T (where chemical potential of free H 2 is very high). Metal-doping increases the thermodynamic stabilities of the hydride films and thus significantly helps to reduce the required pressure condition for hydrogen absorption from H 2 gas. In particular, with Na alloying hydrogen can be absorbed/desorbed at experimentally accessible T and P conditions.

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Section: Hydrogen Absorption/desorption Phase Diagrammentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

First-principles studies of hydrogen interaction with ultrathin Mg and Mg-based alloy films

2011

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“…The scanning tunneling microscopy (STM) image reveals the smoothness and the thickness uniformity of the Pb film where the steps on the overlayer directly reflect the substrate steps. This sample system, epitaxial Pb thin film on Si (111), is the most widely studied system for the investigation of QSE, and many of its physical quantities have been shown to exhibit so-called "quantum oscillations" as a function of layer thickness (18)(19)(20). While such atomically smooth, uniform thickness metal films can be grown with near perfection, it is more convenient for direct investigation of QSE to use a different growth process that leads to the formation of large 2D flat-top mesas on the stepped region.…”

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confidence: 99%

Quantum size effects on the work function of metallic thin film nanostructures

Kim

Qin

Yao

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

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In this paper, we present the direct observation of quantum size effects (QSE) on the work function in ultrathin Pb films. By using scanning tunneling microscopy and spectroscopy, we show that the very existence of quantum well states (QWS) in these ultrathin films profoundly affects the measured tunneling decay constant κ, resulting in a very rich phenomenon of "quantum oscillations" in κ as a function of thickness, L, and bias voltage, V s . More specifically, we find that the phase of the quantum oscillations in κ vs. L depends sensitively upon the bias voltage, which often results in a total phase reversal at different biases. On the other hand, at very low sample bias (jV s j < 0.03 V) the measurement of κ vs. L accurately reflects the quantum size effect on the work function. In particular, the minima in the quantum oscillations of κ vs. L occur at the locations where QWS cross the Fermi energy, thus directly unraveling the QSE on the work function in ultrathin films, which was predicted more than three decades ago. This further clarifies several contradictions regarding the relationship between the QWS locations and the work function.STM | quantum well states | Pb T he work function, the minimum energy required to move an electron from a solid into the vacuum, is the most fundamental material parameter in surface science. It plays a key role, for example, in the photo-electric effect, one of the first phenomena through which quantum mechanics unveiled itself. The work function is the result of a complex interplay between quantum mechanics and forces on the atomic scale. Recent studies suggest the exciting possibility of controlling the work function through quantum engineering of electronic structures at the nanoscale (1-5). Such efforts are, however, still in their infancy, and there are many puzzles and contradictions in the observations so far that hinder further progress (1-5). In this report, we show direct evidences for quantum size effects on the work function in ultrathin Pb films. We further establish the direct correlation of this quantity with the behavior of quantum well states. With the ability to control the growth of metallic thin films with atomic layer precision, one can anticipate the possibility of tuning the work function of metallic thin films through the quantum size effects, thus influencing chemical processes on surfaces.It should be noted that theoretical investigations of quantum size effects (QSE) on the work function (Φ) were carried out several decades ago (6). By using a jellium model and continuously varying the film thickness, Schulte showed that the QSE leads to work function oscillations as a function of layer thickness. Moreover, it was found that the Φ oscillations are directly related to the energy locations of the quantum well states (QWS). In particular, whenever a QWS channel crosses the Fermi level (E F ), there is a negative cusp in Φ as a function of film thickness (L), which results from the sudden increase of the surface dipole due to the additional electronic ...

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“…[50][51][52] One route to solve this problem is to use Pd-based metal alloys as membranes, which the current inorganic membrane technology community mainly focuses on. [53][54][55] In the quantum film community, the fabrication of atomically flat metal alloys in the quantum regime becomes feasible even for two metal elements immiscible in the bulk form, 5 and the amount of Pd consumed will be significantly reduced compared with the conventional thick membranes. Moreover, for applications of the coating systems in hydrogen purification, especially with high temperature annealing, one disadvantage is the intermixing of the coating element and the substrate at the interface.…”

Section: E H 2 Permeance Of the Symmetric And Asymmetric Quantum Pd mentioning

confidence: 99%

Drastically enhanced H2flux through asymmetric quantum Pd films

Zhu

Wang

et al. 2012

Phys. Rev. B

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It has been established recently that the quantum mechanically confined motion of conduction electrons within ultrathin metal films can strongly modify the various atomic rate processes and chemical reactivity on top of the films with thickness-dependent stability. In this work, we investigate theoretically how such quantum size effects (QSE) can alter the energetics and kinetics as gaseous hydrogen molecules adsorb dissociatively on one side of an ultrathin Pd membrane serving as a model separating system, and desorb associatively from the other side.We first establish that these quantum Pd films possess extra (or magic) stability at certain film thicknesses, and demonstrate that the H 2 flux through such quantum films can be tuned significantly by the QSE-induced variations in the key molecular/atomic rate processes.Furthermore, when the permeate side of the films is coated with a layer of less H-reactive Cu or Ag element, the depth of the H chemisorption well is lowered substantially without introducing other larger barriers, which in turn can enhance the H 2 flux by several orders of magnitude around 500 K. These findings of fundamental nature may prove to be inspirational in future design of ideal membrane-based devices with maximal hydrogen permeability and low cost.

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Tuning the Quantum Stability and Superconductivity of Ultrathin Metal Alloys

Cited by 108 publications

References 24 publications

First-principles studies of hydrogen interaction with ultrathin Mg and Mg-based alloy films

First-principles studies of hydrogen interaction with ultrathin Mg and Mg-based alloy films

Quantum size effects on the work function of metallic thin film nanostructures

Drastically enhanced H2flux through asymmetric quantum Pd films

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