Magic Thicknesses Research Articles

Multivalency-Driven Formation of Te-Based Monolayer Materials: A Combined First-Principles and Experimental study.

Contemporary science is witnessing a rapid expansion of the two-dimensional (2D) materials family, each member possessing intriguing emergent properties of fundamental and practical importance. Using the particle-swarm optimization method in combination with first-principles density functional theory calculations, here we predict a new category of 2D monolayers named tellurene, composed of the metalloid element Te, with stable 1T-MoS_{2}-like (α-Te), and metastable tetragonal (β-Te) and 2H-MoS_{2}-like (γ-Te) structures. The underlying formation mechanism is inherently rooted in the multivalent nature of Te, with the central-layer Te behaving more metal-like (e.g., Mo), and the two outer layers more semiconductorlike (e.g., S). We also show that the α-Te phase can be spontaneously obtained from the magic thicknesses divisible by three layers truncated along the [001] direction of the trigonal structure of bulk Te, and both the α- and β-Te phases possess electron and hole mobilities much higher than MoS_{2}. Furthermore, we present preliminary but convincing experimental evidence for the layering behavior of Te on HOPG substrates, and predict the importance of multivalency in the layering behavior of Se. These findings effectively extend the realm of 2D materials to group-VI elements.

Solid-state dewetting with a magic thickness: Electronic dewetting

We investigate the dewetting dynamics of ultrathin solid metal films. In these films, quantum confinement of electrons is known to produce a complex wetting potential, leading to ``magic thicknesses'' which are strongly favored energetically. We introduce a kinetic Monte Carlo model which accounts for a magic thickness. When the driving forces are small enough, the dewetting proceeds from the edges of the films and we find two regimes: (i) Regime I: for stronger driving forces, no dewetting rim is formed along the edge of the film, and magic-height fingers perpendicular to the film edge invade the film. (ii) Regime II: for smaller driving forces, magic-height fingers form parallel to the edge of the film. In this regime, a magic-height dewetting rim forms, and subsequently breaks up due to the emergence of two instabilities: rim closure failing, and layer-by-layer nucleation of holes in the film behind the rim. In both regimes, the dewetting velocity and the typical wavelength observed in the simulations are found to be in good agreement with a maximum velocity principle based on a simple model accounting for diffusion limited dynamics. Finally, the labyrinthine morphology resulting from the dewetting process and its typical length scales are found to be consistent with experimental observations. For stronger driving forces, we observe homogeneous nucleation. Depending on the relative values of the magic height and the film height, the dewetting process starts with the formation of holes or with the formation of magic-height islands.

The magic thicknesses of θ′ precipitates in Sn-microalloyed Al–Cu

θ′ is an effective strengthening precipitate phase in high-strength Al alloys; unfortunately its nucleation is difficult and usually requires assistance, such as that provided by Sn in Sn-microalloyed Al–Cu. In order to clarify the mechanisms by which Sn promotes the nucleation of θ′, we investigated the structure and thickness of θ′ precipitates in a Al–1.7 at.% Cu alloy with trace additions of Sn (0.01 at.%). Scanning transmission electron microscopy imaging reveals that θ′ platelets recently nucleated at 160 and 200 °C exhibit a discrete distribution of specific, or “magic”, thicknesses, corresponding to minima in the residual volumetric and shape misfit strain. This observation is unique to the Sn-assisted nucleation of θ′: θ′ platelets that undergo growth or form in the Sn-free alloy do not display this discrete distribution, although preference for the magic thicknesses persists. Direct evidence is presented that Sn does not accommodate volumetric misfit strain. Instead, it is shown that Sn in its solute form reduces either the interfacial energy of θ′ and/or the transformation shape strain associated with thicknesses intermediate to the magic thicknesses.

GROWTH AND STABILITY OF ULTRA-THIN Pb FILMS ON Pb/Si(111)-α-√3 × √3

Ultra-thin Pb films with magic thicknesses of 2 monolayer (ML), 4 ML and 6 ML were prepared of atomically flat on the substrate of Si (111)-α-√3 × √3 (or SIC phase) at 145 K. Their surface morphologies and stability were studied by low temperature scanning tunneling microscopy and temperature-dependent angle resolved photoemission spectroscopy. We found that the well ordered SIC interface can lower the diffusion barrier and enhance the interface charge transfer, leading to different critical thickness compared to Pb / Si (111)-7 × 7 grown under same conditions. Enhanced thermal expansion coefficients were also observed in ultra-thin Pb films at low temperature.

Quantum well states, resonances and stability of metallic overlayers

The role of quantum well states (QWSs) and quantum well resonances (QWRs) in thestability of metallic overlayers is studied. When the substrate presents a confining gap, as itis the case of Cu(111), the stability is determined by QWSs and there is experimentalevidence of the existence of magic thicknesses. For overlayers which do not have QWSs, asthose grown on Al(111), we explore the existence of thicknesses of enhanced stability dueto QWRs by comparing the strength of the oscillations in the surface energy.

Thermal stability of atomically flat metal nanofilms on metallic substrates

By means of variable temperature scanning tunneling microscope we studied the morphology and electronic structure of Pb films grown on Cu(1 1 1). Due to the spatial confinement of electrons, the islands display quantized energy levels. At 300 K, Pb forms 3D nanostructures with magic heights, that correspond to islands having a quantum well state (QWS) far from the Fermi energy. Below 100 K Pb grows in a quasi-layer-by-layer fashion. The QWS that develop in the films determine their total energy and, accordingly, their thermal stability. Films of particularly magic thickness are stable upon heating to 300 K.

Real-Space Direct Visualization of the Layer-Dependent Roughening Transition in Nanometer-Thick Pb Films

By means of variable-temperature scanning tunneling microscopy and spectroscopy we studied the thickness-dependent roughening temperature of Pb films grown on Cu(111), whose electronic structure and total energy is controlled by quantum well states created by the spatial confinement of electrons. Large scale STM images are employed to quantify the layer population, i.e., the fraction of the surface area covered by different Pb thicknesses, directly in the real space as a function of temperature. The roughening temperature oscillates repeatedly with bilayer periodicity plus a longer beating period, mirroring the thickness dependence of surface energy calculations. Conditions have been found to stabilize at 300 K Pb films of particular magic thicknesses, atomically flat over microns.

Uniform Pb nanowires of magic thickness on Si(111) controlled by elastic interaction and quantum size effects

An ordered array of uniform single crystal Pb nanowires with a magic thickness of 9 monolayers (ML) was obtained on Si(111) by a two-step method. The coverage of 6.4 ML is critical for the formation of uniform nanowires at the deposition temperature of 200 K and the flux rate of 1.2 ML/min, while at other coverages elongated islands or interconnected stripes with various thicknesses are formed. The width ratio of the nanowires to the Si(111) substrate terraces is found to be a characteristic value of 0.54. In situ low-temperature scanning tunneling microscopy study reveals that the magic thickness and uniform distribution of the Pb nanowires are governed by the interplay between the electronic quantum size effects along the normal direction and the lateral elastic interaction induced by strain.

Growth and magnetism of self‐organized Co nanoplatelets on Si(111) surface

AbstractMagnetic nanodots have great potential for applications in ultrahigh‐density information storage, quantum computing and many other fields. In this work, Co nanoplatelets with uniform size, height and shape were fabricated by ultrahigh vacuum metal evaporation on the Si(111)‐(7 × 7) surface, using a two‐dimensional identical Al cluster array as the template and spacer. The Al clusters reduce Si dangling bonds significantly and prevent the reaction between Si and the ferromagnetic metal Co. Almost all Co nanoplatelets appear as equilateral triangles with a side length of 5.4 nm and a magic thickness of 2 ML, pointing in the [211] direction. In situ scanning tunneling microscopy (STM) was used to determine the atomic structure and atomistic formation process of the Co nanoplatelets. Magnetic properties of the Co platelets were measured by SQUID at 5 K. Such quantum dots exhibit unusually high perpendicular magnetic anisotropy. The present study demonstrates a promising pathway to directly integrate magnetic nanostructures with Si‐based electronic devices. Copyright © 2006 John Wiley & Sons, Ltd.

Magic layer thickness in Bi ultrathin films on Si(1 1 1) surface

By using first-principles calculations, we study the origin of the flat Bi film growth on the Si(111) surface. First, we confirm the validity of the present first principles method by calculating the α (1/3ML coverage) and β (1ML coverage) phases of Si(111)√3 × √3-Bi surface: The determined structures are found to be consistent with experimental results, i.e., the T4 site is the most stable one for the α phase, and the milkstool structure is the most stable for the β phase. Next, we study the energetics of the Bi films grown on the wetting layer on the Si(111) surface. We find that even-number layer films are prominently stable and thus conclude that the even numbers correspond to the magic thicknesses which induce experimentally observed flat film growth. We conclude that appearance of the magic thicknesses is due to large atomic relaxation which paired each two neighboring layers. Thus, the mechanism that induces the magic thicknesses is different from the quantum size effect argued in the past studies.

Quantum Size Effects in Low-Temperature Growth of Pb Islands on Si(111)7×7 Surfaces

Flat-top Pb islands with critical and magic thickness have been observed in the Pb/Si(111)7×7 system at 200 K by scanning tunneling microscopy. The growth behavior, different from that in the Stranski-Krastanov mode, arises from a quantum size effect. Quantized states are detected in the current–voltage (I–V) spectra on the Pb islands of varying thickness. Our observation of asymmetrical and oscillatory relaxation in the island thickness reveals that the charge distribution of confined electrons can influence the interlayer spacing. A simple model based on the infinite potential well can explain well all of our results.

Oscillatory energetics of flat Ag films on MgO(001)

The energetics and electronic structures of flat Ag films on the MgO(001) substrate are studied by first-principles density-functional calculations. An oscillatory variation of the film energetics showing the existence of multiple magic thicknesses for smooth growth is found. This oscillatory behavior correlates well with the quantum-well states, which themselves vary with the film thickness. The results demonstrate the importance of the confined motion of the conduction electrons in stabilizing epitaxial metal films, as emphasized in a recent ``electronic growth'' model.