Epitaxial Growth Of Metals Research Articles

Symmetric and asymmetric epitaxial growth of metals (Ag, Pd, and Pt) onto Au nanotriangles: effects of reductants and plasmonic properties.

The surface plasmon resonance of noble metals can be tuned by morphology and composition, offering interesting opportunities for applications in biomedicine, optoelectronics, photocatalysis, photovoltaics, and sensing. Here, we present the results of the symmetrical and asymmetrical overgrowth of metals (Ag, Pd, and Pt) onto triangular Au nanoplates using l-ascorbic acid (AA) and/or salicylic acid (SA) as reductants. By varying the reaction conditions, various types of Au nanotriangle-metal (Au NT-M) hetero-nanostructures were easily prepared. The plasmonic properties of as-synthesized nanoparticles were investigated by a combination of optical absorbance measurements and Finite-Difference Time-Domain (FDTD) simulations. We show that specific use of these reductants enables controlled growth of different metals on Au NTs, yielding different morphologies and allowing manipulation and tuning of the plasmonic properties of bimetallic Au NT-M (Ag, Pd, and Pt) structures.

Room-temperature epitaxy of metal thin films on tungsten diselenide

The orientation of selected metals (Pd, Ni, Al, and Co) deposited on WSe2 by physical vapor deposition was examined using transmission electron microscopy and selected area electron diffraction. We discovered that Ni demonstrates room-temperature epitaxy, similarly to other face centered cubic (FCC) metals Au, Ag, and Cu. These epitaxial metals exhibit the following orientation relationship, where M stands for metal: M (111) || WSe2 (0001); M [22¯0] || WSe2 [112¯0]. Hexagonally close-packed Co, and FCC Pd and Al, were not epitaxial on deposition; however, Pd became epitaxial after annealing at 673 K for 5 h. To uncover critical variables for epitaxial growth, we correlated our experimental work and reports from the literature on Cu, Ag, and Au with density functional theory calculations of the energetics of metal atoms on the surface of WSe2 and thermodynamic calculations of metal-W-Se phase equilibria. Furthermore, we compared the findings to our previous work on metal/MoS2 systems to draw conclusions more generally applicable to epitaxial growth of metals on transition metal dichalcogenides (TMDs). We observed that epitaxy of metals on TMDs can occur when there is a match in crystallographic symmetry, even with a large lattice mismatch, and it is favored by metals exhibiting a low diffusion barrier on the TMD surface. However, reaction processes between the metal and WSe2 can prevent epitaxy even when the other factors are favorable, as occurred for Al/WSe2 with the formation of aluminum selenide, tungsten aluminide, and elemental tungsten. Consideration of crystallographic symmetry, surface diffusion barriers, and reactivity can be used to predict room-temperature epitaxy in other metal/TMD systems.

Surface nucleation and growth in the system of interacting particles

Recent experiments on epitaxial growth of metals on graphene have shown a strong dependence of island densities on coverage. These investigations cannot be explained by the standard mean-field nucleation theories. To understand them, we extend to higher coverage the former theory of rate equations developed for the initial state of nucleation, in a system where adsorbate interaction is included. We account for that, in the case of high coverage, the repulsive interaction influences both the attachment of monomers to clusters and the mobility of atoms. In our work we analyze the modification of the dependence of the island density on coverage, temperature and F/D ratio. In some regimes our theory results in the experimentally observed substantial growth of island density with coverage for a high deposited amount and a weak dependence on deposition rate F. We also find out the local maxima in temperature dependence of island density, as a consequence of long-range repulsive interactions.

Epitaxial growth by monolayer restricted galvanic displacement

The development of a new method for epitaxial growth of metals in solution by galvanic displacement of layers pre-deposited by underpotential deposition (UPD) was discussed and experimentally illustrated throughout the lecture. Cyclic voltammetry (CV) and scanning tunneling microscopy (STM) are employed to carry out and monitor a ?quasi-perfect?, two-dimensional growth of Ag on Au(111), Cu on Ag(111), and Cu on Au(111) by repetitive galvanic displacement of underpotentially deposited monolayers. A comparative study emphasizes the displacement stoichiometry as an efficient tool for thickness control during the deposition process and as a key parameter that affects the deposit morphology. The excellent quality of layers deposited by monolayer-restricted galvanic displacement is manifested by a steady UPD voltammetry and ascertained by a flat and uniform surface morphology maintained during the entire growth process.

Epitaxial Growth of Metals, Alloys and Multilayers Assisted by a Monolayer Amount of UPD Atoms

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Epitaxial Growth of Ag on Au ( 111 ) by Galvanic Displacement of Pb and Tl Monolayers

The development of a new method for epitaxial growth of metals in solution by galvanic displacement of layers predeposited by underpotential deposition (UPD) is discussed and experimentally illustrated. Cyclic voltammetry and scanning tunneling microscopy are employed to carry out and monitor a “quasi-perfect,” two-dimensional growth of up to 35 monolayers of Ag on by repetitive galvanic displacement of underpotentially deposited Tl and Pb monolayers. A complementary kinetic study of Pb and Tl UPD layer stability at open-circuit potential identifies the oxygen reduction reaction and hydrogen evolution reaction as key oxidative competitors of Ag in the proposed displacement protocol. Analysis of the morphology evolution during the growth of Ag by displacement Pb and Tl UPD layers suggests the one-to-one exchange scenario as more efficient for longer maintenance of a layer-by-layer silver deposition. The excellent quality of layers deposited by monolayer-restricted galvanic displacement is manifested by a steady UPD voltammetry and ascertained by an overall flat and uniform surface morphology maintained during the entire growth process. An X-ray photoelectron spectroscopy analysis finds no traces of Pb and Tl in the Ag deposit.

Epitaxial Growth of Ag on Au(111) by Monolayer Restricted Galvanic Displacement

The development of a new method for epitaxial growth of metals in solution by galvanic displacement of layers pre-deposited by underpotential deposition (UPD) is discussed and experimentally illustrated. Cyclic Voltammetry and Scanning Tunneling Microscopy are employed to carry out and monitor a quasi-perfect , two-dimensional growth of up to 35 monolayers (MLs) of Ag on Au(111) by repetitive galvanic displacement of underpotentially deposited Tl and Pb monolayers. A comparative study emphasizes the displacement stoichiometry as an efficient tool for thickness control during the deposition process and as a key parameter that affects the deposit morphology. The excellent quality of layers deposited by monolayer-restricted galvanic displacement is manifested by a steady UPD voltammetry and ascertained by a flat and uniform surface morphology maintained during the entire growth process. An X-ray Photoelectron Spectroscopy analysis finds no traces of Pb in the Ag deposit.

Epitaxial Growth by Monolayer-Restricted Galvanic Displacement

The development of a new method for epitaxial growth of metals in solution is discussed and illustrated by proof-of-conceptresults. Cyclic voltammetry and scanning tunneling microscopy are employed to carry out and monitor a quasi-perfect, two-dimensional growth of up to 35 layers of Ag on Au 111 by repetitive galvanic displacement of an underpotentially deposited UPD Pb monolayer. The excellent quality of the deposit is manifested by an unchanged Pb UPD voltammetry and ascertained bya flat and uniform surface morphology maintained during the entire growth process.An X-ray photoelectron spectroscopy analysisfinds no traces of Pb in the Ag deposit.© 2005 The Electrochemical Society. DOI: 10.1149/1.2063267 All rights reserved.Manuscript submitted June 7, 2005; revised manuscript received July 18, 2005. Available electronically September 16, 2005.

Effects of long-range interactions in metal epitaxial growth

The van der Waals constant ${C}_{3}$ describing the long-range attractive interaction between a Cu atom and a Cu(100) surface is calculated, along with the dominant correction ${z}_{0}$ for the shift in the reference plane, using expressions derived by Persson and Zaremba which take into account the optical response of Cu. The result ${C}_{3}\ensuremath{\simeq}2.1\mathrm{eV}{\mathrm{\AA{}}}^{3}$ is significantly smaller than predicted by a simple Lennard-Jones (LJ) Cu potential used in recent simulations, but large enough to explain experimental observations of a significant increase in mound angle in Cu/Cu(100) growth for large angles of incidence. In contrast, trajectory calculations indicate that the LJ Cu potential appears to overestimate the effects of long-range attraction. Our results also indicate that for small angles of incidence $(\ensuremath{\theta}<50\ifmmode^\circ\else\textdegree\fi{}),$ the dominant effects of attraction on the surface morphology in Cu/Cu(100) growth are related to the short-range rather than to the long-range interaction. Similar results are presented for Ag/Ag(100).

Effects of short-range attraction in metal epitaxial growth.

The effects of short-range attraction in metal epitaxial growth are studied. Our results indicate that even at normal incidence, the short-range attraction of depositing atoms to step edges can significantly increase the selected mound angle and surface roughness for typical energies used in epitaxial growth. Our results also lead to a picture of the process of deposition near step edges that is quite different from the standard downward funneling picture. Results for the dependence of the uphill current on incident atom kinetic energy and substrate temperature are also presented.

Stress relaxation during two-dimensional pseudomorphic epitaxial growth of metals

AbstractThe analysis of RHEED diffraction peaks during two-dimensional (2D) pseudomorphic epitaxial growth leads to the well known RHEED oscillations but also to diffraction peak width and in-plane lattice spacing oscillations. These behaviors are evidenced in several metallic A/B systems in this paper. As in-plane lattice spacing oscillations are assumed to be due to elastic relaxation at the edge of the 2D Islands, we try to correlate the amplitude of the detected effect with the misfit. The width oscillations are assumed to be due to the scattering coming from islands. We actually show that the full width at half maximum (FWHM) at half coverage allows us to get a correct estimation of the nucleation density. We thus show experimentally that the in-plane lattice spacing oscillations amplitude depends on the nucleation density determined using FWHM measurements. Finally, a theoretical justification allows us to show that this amplitude also depends on the Young modulus ratio of both B and A layers.

Epitaxial growth of metals with high Ehrlich-Schwoebel barriers and the effect of surfactants

Interlayer diffusion in epitaxial systems with a high energy barrier at the atomic steps – the so-called Ehrlich–Schwoebel (ES) barrier – is strongly reduced. As a consequence of this, a continuous accumulation of roughness takes place during growth. This undesirable effect can be corrected by using surfactant agents. We have studied the influence of the ES barrier on the preparation of epitaxial films on Cu(111), and the surfactant effect of a monolayer of Pb.

Epitaxial growth of metals by sputter deposition

Results are presented detailing the crystallographic quality of a number of metal films deposited by epitaxial sputter deposition. Such films have single-orientation f.c.c., h.c.p., b.c.c. and hybrid crystals structures, and include the elements Ag, Au, Co, Cr, Cu, Fe, Rh, Ru, Pd and V. The crystal structure was characterized using X-ray diffraction analysis.

Towards an understanding of surfactant action in the epitaxial growth of metals: The case of Sb on Ag (111)

We address the role of “surfactant” adsorbates in determining changes in the homoepitaxial growth mode of metals, discussing the case of Sb on Ag (111). From ab initio calculations, we extract evidence that the mechanism operative in this system is that Sb induces an irregular shape and an increase in density of the growing Ag islands, and an ensuing increase of the number of attempts for an adatom to descend to a lower terrace. This results from a combination of peculiar properties of this system: Sb is adsorbed insubstitutional surface sites, leading to the formation of a Sb−Ag surface alloy; deposited Ag has reduced mobility on Sb-covered Ag (111), from which follows a higher nucleation probability. The island shape is irregular since the surface alloy is disordered. Surface seggregation of Sb once the growing layer is completed furthers the phenomenon for many deposited Ag layers. Our explanation of the surfactant action of Sb on Ag (111) does not require a reduction of the downstep diffusion barrier, which may, however, be a concurrent factor helpful to interlayer mass transport and layerby-layer growth.

Towards an understanding of surfactant action in the epitaxial growth of metals: The case of Sb on Ag (111)

Towards an understanding of surfactant action in the epitaxial growth of metals: The case of Sb on Ag (111)

Characterization of magnetic microstructure at high spatial resolution

Advances in metal epitaxial growth techniques have produced renewed interest in magnetic materials through the unique properties of novel atomically engineered structures. Ultrathin layered and nanocrystalline, textured composites possess unique magnetic properties which are intimately connected to the physical microstructure on both the atomic and nanometer length scales. The length scales present in magnetic interactions span several orders of magnitude from the atomic length scales of the exchange interaction, to the mesoscopic length scales important in anti-ferromagnetic RKKY coupling between thin ferromagnetic films and small particles, to the macroscopic length scales determined by the long range magnetostatic interactions which are responsible for the formation of magnetic domains.Special characterization and computation techniques are required to analyze the micromagnetic and microstructural properties of these novel, low dimensional systems. In order to correlate the structure with the magnetic properties of any system, in-situ growth, structural, chemical and magnetic characterization is desirable.Most methods used for the observation of micromagnetic structure rely on a contrast mechanism derived from the magnetic fields present in ferromagnetic systems. We have implemented the conventional Fresnel, and less conventional Differential Phase Contrast modes of imaging magnetic microstructure in an HB5 STEM.

A strain-relieve transition in epitaxial growth of metals on Si(111)(7 × 7)

Abstract RHEED intensity oscillations observed during growth of several metals (Cu, Al, Ba, Sr, Pb) on Si(111)(7 × 7) substrates at 100 K show that the growth mechanism is layer-by-layer like. The growth behaviour, although rather complicated, follows the same pattern for all metals. Initially the oscillations are rather irregular and damped. At higher coverages the amplitude of the oscillations increases and the period becomes much more regular. This phenomenon is explained by assuming a transition from a rather imperfect substrate-matched to a relaxed bulk metal phase. This explanation is supported by diffraction data.

A strain-relieve transition in epitaxial growth of metals on Si(111)(7 × 7)

RHEED intensity oscillations observed during growth of several metals (Cu, Al, Ba, Sr, Pb) on Si(111)(7 × 7) substrates at 100 K show that the growth mechanism is layer-by-layer like. The growth behaviour, although rather complicated, follows the same pattern for all metals. Initially the oscillations are rather irregular and damped. At higher coverages the amplitude of the oscillations increases and the period becomes much more regular. This phenomenon is explained by assuming a transition from a rather imperfect substrate-matched to a relaxed bulk metal phase. This explanation is supported by diffraction data.

Epitaxial growth of metals on Si and Ge investigated by RHEED-TRAXS and UHV-SEM

Abstract In the study of epitaxy, it is very important to investigate the surface composition during the growth process. In RHEED experiments, strong X-rays are excited from the sample surface hit by the electron beam. The intensity of these adsorbate characteristic X-rays changes drastically with change of the take-off angle (θt) and it becomes maximum at a critical angle (θc) corresponding to total reflection of the X-rays. Applying this phenomenon, surface element analysis is possible with high sensitivity. This method is called RHEED-TRAXS (total reflection angle X-ray spectroscopy). The sensitivity of this surface element analysis is generally comparable to that for AES but for heavy atoms it is higher. When the deposited metal atoms are confined to the surface, the glancing angle (θg) dependence of the emitted X-rays is expressed by I/sin θg. Thus, by measuring the θg dependences, the method gives also good information on the in-depth composition. Applying the RHEED-TRAXS method we have studied in situ the adsorption, desorption, and epitaxial growth processes of Au, Ag and Au-Ag alloys on clean Si(111) surfaces. We have developed a new UHV-SEM which has a high resolution of about 5 A. It was found that by using the UHV-SEM we could observe directly domain contrasts from two-dimensional surface structures such as 7 × 7, 5 × 2-Au, and √3 × √3-Ag which are formed by Ag and Au deposition on Si(111). These contrasts correspond to two-dimensional element mapping at the surfaces.

Epitaxial growth of metals on Si and Ge investigated by RHEED-TRAXS and UHV-SEM

In the study of epitaxy, it is very important to investigate the surface composition during the growth process. In RHEED experiments, strong X-rays are excited from the sample surface hit by the electron beam. The intensity of these adsorbate characteristic X-rays changes drastically with change of the take-off angle (θ t) and it becomes maximum at a critical angle (θ c) corresponding to total reflection of the X-rays. Applying this phenomenon, surface element analysis is possible with high sensitivity. This method is called RHEED-TRAXS (total reflection angle X-ray spectroscopy). The sensitivity of this surface element analysis is generally comparable to that for AES but for heavy atoms it is higher. When the deposited metal atoms are confined to the surface, the glancing angle (θ g) dependence of the emitted X-rays is expressed by I/sin θ g. Thus, by measuring the θ g dependences, the method gives also good information on the in-depth composition. Applying the RHEED-TRAXS method we have studied in situ the adsorption, desorption, and epitaxial growth processes of Au, Ag and Au-Ag alloys on clean Si(111) surfaces. We have developed a new UHV-SEM which has a high resolution of about 5 Å. It was found that by using the UHV-SEM we could observe directly domain contrasts from two-dimensional surface structures such as 7 × 7, 5 × 2-Au, and √3 × √3-Ag which are formed by Ag and Au deposition on Si(111). These contrasts correspond to two-dimensional element mapping at the surfaces.