Epitaxial Growth Of Ag Research Articles

Fabrication of single-crystalline plasmonic nanostructures on transparent and flexible amorphous substrates

A new experimental technique is developed for producing a high-performance single-crystalline Ag nanostructure on transparent and flexible amorphous substrates for use in plasmonic sensors and circuit components. This technique is based on the epitaxial growth of Ag on a (001)-oriented single-crystalline NaCl substrate, which is subsequently dissolved in ultrapure water to allow the Ag film to be transferred onto a wide range of different substrates. Focused ion beam milling is then used to create an Ag nanoarray structure consisting of 200 cuboid nanoparticles with a side length of 160 nm and sharp, precise edges. This array exhibits a strong signal and a sharp peak in plasmonic properties and Raman intensity when compared with a polycrystalline Ag nanoarray.

Submonolayer Ag films on Fe(100): A first-principles analysis of energetics controlling adlayer thermodynamics and kinetics

Epitaxial growth of Ag on Fe(100) and postdeposition relaxation have been studied in several experiments. We provide a first-principles density functional theory analysis of key adatom interaction energies and diffusion barriers controlling growth and relaxation kinetics for the submonolayer regime, as these have not been assessed previously. A cluster expansion approach is used to obtain an extensive set of conventional lateral interactions between adatoms on fourfold hollow adsorption sites. We find robust oscillatory decay of pair interactions with increasing separation, and of trio interactions with increasing perimeter length. First- and second-nearest-neighbor pair interactions, as well as compact linear and bent trio interactions, dominate. The adatom terrace diffusion barrier is estimated to be ${E}_{\mathrm{d}}\ensuremath{\approx}0.39$ eV. We also provide a limited analysis of unconventional interactions for which one adatom is at the bridge-site transition state for hopping and one or more others are at fourfold hollow sites. Energy barriers for diffusion along island edges can be determined with the aid of both conventional and unconventional interactions.

MgO(100) 기판 위에 증착된 Ag/CoFeB 박막의 스퍼터링 조건에 따른 미세성장구조 변화 연구

본 연구에서는 DC 마그네트론 스퍼터링을 이용해 MgO 단결정 기판 위에 성장된 Ag/CoFeB 박막의 스퍼터링 조건에 따른 박막 미세구조의 변화를 연구하였다. Ag 박막의 결정성 및 표면 거칠기는 인가전력(sputtering power) 및 증착온도의 변화에 따라 증착온도가 증가하는 경우 (200) 방향의 결정성이 향상되는 것을 확인하였으며, 인가전력이 증가되는 경우 표면 거칠기가 감소하는 것을 확인하였다. 또한 고분해능 TEM(transmission electron microscopy) 및 XRR(X-ray reflectivity) 측정을 통해 MgO 기판 위 Ag층의 켜쌓기 성장 및 MgO 기판과 Ag층 사이에 산화층에 해당하는 계면층이 존재하는 것을 알 수 있었으며, 증착온도의 증가에 따른 Ag의 섬상구조 형성 및 intermixing 효과에 의한 Ag/CoFeB 계면층의 변화 및 자기적 특성의 변화를 연구하였다. In this study, we have systematically investigated the effect of sputtering conditions on the microstructural properties of Ag/CoFeB thin film on MgO substrate. It was found that the crystallinity and surface roughness of the Ag film strongly depends on the Ar sputtering pressure and sputtering power. Epitaxial growth of Ag(100) film on MgO(100) substrate was achieved under the sputtering conditions of high sputtering power and elevated temperature. XRR (X-ray reflectivity) and high-resolution TEM (transmission electron microscopy) measurements also revealed the interfacial roughening in the Ag/CoFeB interface due to the island structure formation and intermixing between Ag and CoFeB.

Growth of (√3 × √3)-Ag and (1 1 1) oriented Ag islands on Ge/Si(1 1 1) surfaces

We have studied the growth of Ag on Ge/Si(1 1 1) substrates. The Ge/Si(1 1 1) substrates were prepared by depositing one monolayer (ML) of Ge on Si(1 1 1)-(7 × 7) surfaces. Following Ge deposition the reflection high energy electron diffraction (RHEED) pattern changed to a (1 × 1) pattern. Ge as well as Ag deposition was carried out at 550 °C. Ag deposition on Ge/Si(1 1 1) substrates up to 10 ML has shown a prominent (√3 × √3)-R30° RHEED pattern along with a streak structure from Ag(1 1 1) surface. Scanning electron microscopy (SEM) shows the formation of Ag islands along with a large fraction of open area, which presumably has the Ag-induced (√3 × √3)-R30° structure on the Ge/Si(1 1 1) surface. X-ray diffraction (XRD) experiments show the presence of only (1 1 1) peak of Ag indicating epitaxial growth of Ag on Ge/Si(1 1 1) surfaces. The possibility of growing a strain-tuned (tensile to compressive) Ag(1 1 1) layer on Ge/Si(1 1 1) substrates is discussed.

The shape evolution of gold seeds and gold@silver core–shell nanostructures

We report the seed-dependent shape evolution of gold@silver (Au@Ag) core–shellnanostructures with various morphologies through using pre-existing Au nanocrystals asnuclei in a polyvinylpyrrolidone (PVP)-assisted polyol reduction process. Aunanocrystalline seeds with different shapes such as cube, truncated-octahedron, octahedron,twinned hexagon and triangle, five-twinned decahedron and nanorod are firstly synthesizedby refluxing a 1,5-pentanediol solution containing Au precursors in the presence of PVP.The Au seeds obtained in this way then serve as the nuclei for further epitaxial growth ofAg shells by using Ag precursors via the same route. Scanning transmission electronmicroscope (STEM) characterization of the products obtained demonstrates thatthe morphological evolution of the Ag shells depends completely on the shapesof the Au seeds that are used. We have observed that the Au@Ag core–shellnanostructures formed with various regular shapes such as cube, bi-triangle, and nanorodwith five-twinned cross section, are mostly surrounded by {100}-type Ag crystallinefacets. Our findings provide new evidence and clear evolution routines from the Aucores with well-defined shapes to the corresponding Ag shells for the Au@Agcore–shell nanostructures by the family of the PVP-assisted polyol reduction methods.

Oxygen-assisted control of surface morphology in nonepitaxial sputter growth of Ag

The surface roughness of Ag films (thickness of 50nm) is drastically reduced from 2.4to1.0nm by the presence of oxygen (pressure of 2×10−5mbar) during sputter growth, without a change (undermining) in electrical transport properties nor a formation of any Ag oxide phase. It is concluded that oxygen plays the role of a surfactant. The growth scenario (nonepitaxial sputter deposition of relatively thick films) makes this observation complementary to previous studies reporting surfactant effects in epitaxial growth of Ag. The authors consider an application of this finding in the field of multilayers involving Ag metals.

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

not Available.

Epitaxial growth of Ag on W(110)

Epitaxial growth of Ag on W(110) at room temperature was studied by scanning tunneling microscopy (STM) and polarization-dependent photoemission. At coverages far below one monolayer Ag atoms populate bcc sites of the substrate and form close-packed islands of monolayer thickness. With increasing coverage geometrical misfit between Ag(111)-like layers and W(110) generates surface stress along $\mathrm{W}[1\overline{1}0]$. This is released by formation of domain walls parallel W[001] which are observed with a distance between about $25\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and $30\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, depending on the details of the growth process. At one monolayer coverage most of the Ag atoms still reside in or very near to bcc substrate positions, but now the strain release pattern is changed: solitons aligned along $\mathrm{W}[\overline{1}12]$ are formed at an average distance between $35\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and $50\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. The details of the soliton arrangement depend critically on the degree of equilibration and the presence of holes in the monolayer film which allow an additional stress release. This is evident from a comparison with results of STM studies performed at the closed and carefully annealed Ag monolayer [Kim et al., Phys. Rev. B 67, 223401 (2003)]. Further deposition of Ag starts growth of a second monolayer by formation of islands which increase in size with coverage. At a nominal coverage of 1.5 monolayers the strain relieve pattern changes again: some corrugation lines are oriented along W[001] as in the submonolayers, but other orientations related to Ag(111) directions appear as well. This indicates that several possibilities are available at similar energy costs and that the transition from the W substrate potential to a Ag potential seen by the second layer is very soft. Finally at a nominal coverage of several monolayers, Stranski-Krastanov growth is observed producing Ag(111)-like terraces with one of the dense-packed Ag rows oriented parallel to $\mathrm{W}[1\overline{1}1]$.

Epitaxial Growth of Ag on Si(111)-4×1-In Surface Studied by RHEED, STM, and Electrical Resistance Measurements

We investigated the initial stages of Ag growth on a Si(111)-4×1-In surface at room temperature (RT) and low temperatures (LT, 70∼120K) in situ by reflection-high-energy electron diffraction (RHEED), scanning tunnelling microscopy (STM) and electrical resistance measurements in ultrahigh vacuum, together with on a Si(111)-7 × 7 clean surface for comparison. It has been revealed that the 4×1-In surface actually acts as a highly anisotropic template for Ag growth at RT, but not at LT. The Ag islands grown on the wetting layers were elongated along the 4×1-stripe at RT, while they were isotropic and round in shape at LT. This is due to a difference in migration ability of the arriving Ag adatoms. The anisotropic growth has been found to affect the formation of percolation paths for electrical conduction among the Ag islands; the growth conditions (deposition rate and temperature) do not affect the critical coverage for percolation so much on the 4×1-In surface, compared with in the case of isotropic growth on the 7×7 surface. The initial deposition of submonolayer-thick wetting layers has been found to induce some structural changes of the substrate surface; at RT, the 4×1-In structure changes into a 4×’2’ structure, while the 8×’2’-In structure at LT changes into a 4×1 structure. The former change induces a resistance increase, while the latter induces a resistance drop. These changes may be caused by the induced changes in surface states and band bending beneath the surface. [DOI: 10.1380/ejssnt.2003.72]

Epitaxial Growth of Ag(Al,Ga)S2 on GaAs(100) Substrate by Multisource Evaporation

Epitaxial growth of Ag(Al,Ga)S2 layers with the [Al]/([Al]+[Ga]) ratio less than 0.3 was demonstrated on GaAs(100) substrate through multisource evaporation. The oriented domain with the a-axis normal to the substrate disappeared by stoichiometry control of the [Ag]/([Al]+[Ga]) ratio. The 180°-rotated {112} twin crystal regions slightly appeared in the range of [Al]/([Al]+[Ga])>0. Reflectance anomaly spectra due to the free exciton were observed in the layers.

Epitaxial growth of Ag on Ni(111)

Time of flight impact-collision ion scattering spectroscopy (TOF-ICISS) have been used to investigate the heteroepitaxial growth of 3 ML of Ag on Ni(111). We found that two different of preferred epitaxial growth exist depending on the substrate temperature during Ag deposition: Ag[112̄]//Ni[112̄] and Ag[1̄1̄2]//Ni[112̄]. These experimental results strongly suggest that the growth modes can be manipulated on the heteroepitaxial system, Ag(111)/Ni(111) by controlling the substrate temperature during Ag deposition.

The epitaxial growth of Ag on Si(111)-(7 x 7) surface and its (√3x√3)-R30 surface phase transformation

Ag is adsorbed in ultra-high vacuum on to the (7x7) reconstructed Si(111) surface with submonolayer coverage control with a deposition rate of 3-3 x 1012 atoms/cm2/sec. The initial stages of growth and intermediate equilibrium phase formation are determined by using low energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD) for structural information, and auger electron spectroscopy (AES) and electron energy loss spectroscopy (EELS) for composition and interaction analyses. Room temperature (RT) adsorption results in the nearly epitaxial (1 x 1) surface phase growth in the simultaneous multi-layer growth mode. The quenching of the dangling bond states during adsorption is observed by monitoring thep-character of the Si LVV auger peak. For depositions carried out at high temperatures (HT), several plateaus in the auger uptake curve with the (√3 x √3)-R30° LEED structures are formed. It is observed that a minimum coverage of 0–33 monolayer (ML) is required for the formation of the (√3 x √3) phase and this phase causes the reappearance of thep-electron-related states that were quenched by 1.0 ML adsorption at RT. However the (√3 x √3) is observed for higher coverages (0.66 and 1.0 ML) also. The polar angle anisotropy of Si(2p) emission in XPD indicates the rearrangement of substrate Si atoms for the formation of the (√3 x √3) phase. The EELS data also shows relevant changes due to adsorption of Ag at RT and upon annealing. The results suggest the importance of controlled deposition parameters, the lack of which may have kept the determination of the nature and coverage of the (√3 x √3) surface phase unresolved in literature.

RHEED intensity oscillation during epitaxial growth of Ag on Si(111) surfaces at low temperature

Epitaxial growth of Ag onto the Si(111) surface was studied using reflection high-energy electron diffraction. To understand the mechanism of epitaxy, its dependence on the substrate temperature and deposition rate was systematically investigated. The change in the intensity oscillations when the temperature is decreased from room temperature to 160 K was seen to be identical to intensity oscillation changes observed while increasing the deposition rate from 4.02 to 60 ML/min holding substrate temperature constant. The intensity oscillations are irregular up to 6 ML and become regular above 6 ML. The regular period after the growth of 6 ML is smaller than the thickness of the Ag(111) layer. A model for Ag growth on the Si(111)-7\ifmmode\times\else\texttimes\fi{}7 surface at 160 K is proposed. The changes in periodicity are attributed to island and defect formation.

Epitaxial growth of Ag in the hexagonal structure

The technique of secondary-electron imaging is applied to investigate the structure of Ag films vapor-deposited onto a (0001) surface of Ag 2Al at room temperature. The observations show that Ag grows in the hexagonal close-packed structure up to a thickness of 6–8 atomic layers, while twinned, face-centered cubic domains develop for thicker films.

Reconstruction and growth of Ag on the Si(111)- sqrt 3 x sqrt 3 -Ag surface at low temperature.

Surface reconstruction structures and epitaxial growth of Ag on the Si(111)-\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 -Ag structure at low temperature (160-K order) have been investigated by reflection high-energy electron diffraction (RHEED). When Ag was deposited on a \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 -Ag surface at 160 K, reconstruction structures \ensuremath{\surd}21 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}21 and 6\ifmmode\times\else\texttimes\fi{}6 were observed in addition to the \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 structures. The \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 and \ensuremath{\surd}21 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}21 structures were observed for a thickness range between 0.19 and 2.6 ML, and the 6\ifmmode\times\else\texttimes\fi{}6 structures appeared and coexisted with the \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 and \ensuremath{\surd}21 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}21 structure between 0.7 and 1.7 ML. From RHEED patterns analysis, a 6\ifmmode\times\else\texttimes\fi{}6 model was proposed. Beyond Ag growth of 3 ML, three-dimensional (3D) Ag islands were formed at the temperature range from 160 K to RT. These results imply that the mean free path of Ag atoms on the \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 -Ag surface is large enough even at 160 K that 2D nuclei cannot be formed on the terrace in spite of decreasing substrate temperature.

Epitaxial growth of silver on mica as studied by AFM and STM

The epitaxial growth of Ag(111) on mica was studied using the techniques of atomic force microscopy (AFM) and scanning tunneling microscopy (STM). We examined the morphology of these films primarily with AFM as a function of the substrate growth temperature (75 to 350°C) and film thickness (50 to 300 nm) for slow metal deposition rates of 1 to 2 Å/s. In the lower temperature regime near or below 75°C, a “rolling hill” morphology is observed for all film thicknesses. In the higher temperature regime above approximately 75°C, a number of different surface morphological phases exist which depend on both the film thickness and growth temperature. As the film thickness increases, the phases develop in the following order: (1) island, (2) channel, (3) hole and (4) network. The rate of progression through these phases with increasing film thickness is slower for higher substrate temperatures. Although a number of these films appear atomically flat in small regions, holes and other surface features often disrupt the surface flatness on a larger scale. We have found, however, that it is possible to grow films which are atomically flat on the micrometer-scale using higher deposition rates ( ∼ 40 Å/s) and high substrate temperatures.

Microstructural Evolution During the Epitaxial Growth of Ag/InP (100)

The initial stages of epitaxial growth of Ag on InP(100) has been studied using in-situ and ex-situ electron microscopy techniques. Vicinal InP substrates were cleaned by heating to about 450°C in ultra-high vacuum. Silver was subsequently deposited at substrate temperatures between 350–500°C. The microstructural evolution was monitored for coverages between 0.5–8ML using high spatial resolution secondary electron microscopy and transmission electron microscopy. At sub-monolayer coverages, square islands of Ag are formed with edges aligned along InP directions. Subsequent deposition leads to secondary nucleation or growth of pre-existing islands, depending on the substrate temperature. Surfaces with a uniform distribution of steps have a higher island density than those without steps but the steps are not preferred sites for island nucleation. Coalescence of islands begins at a smaller coverage for lower temperatures and for stepped surfaces. At the early stages of coalescence, island shapes with low symmetry are formed, with diminished island heights. These subsequently transform into shapes of higher symmetry. Except at the highest temperatures studied (500°C), island shape transformations (e.g. from square to elongated) are not very significant. Plan view transmission electron microscopy shows that the islands are epitaxially aligned and that even the smallest islands are generally incoherent. Island size distributions are used to analyze the growth mechanisms.

Surface atom dynamics in epitaxial growth studied by RHEED-TRAXS

The intensity of X-ray emission during RHEED observation changes with many parameters. The glancing angle (θ g) dependence of the X-ray emission is approximated by 1 sin θ g , when the atoms emitting the X-rays are confined at the surface. With increasing θ g, the X-ray emission from the nth layer below the surface increases at first, makes a peak at θ n and then decreases according to 1 sin θ g . The peak position θ n shifts to a higher angle with increasing depth. Thus, we can measure the depths of specific atom layers by measuring the θ g dependence of the characteristic X-ray emission. The experimental results can be explained by a Monte Carlo electron trajectory simulation. It was shown that the depth resolution is better than 1 ML near the surface. Applying this new method, surface atom dynamics in the epitaxial growth of Ag, Au, Sn, Ga and In on the metal-induced Si(111) surface structures (Si(111)- n × m-M) were investigated in detail. Epitaxial growth processes observed are classified into five growth modes. Normal growth mode (N) was observed for the system of Ag/Si-Au. Substitution-atom growth mode (S) and floating-atom growth mode (F) were observed for the systems of (Au, Sn, In)/Si-Ag, Au/Si-Sn, Au/Si-In, Ag/Si-Ga. Alloying growth mode (A) was observed for the systems of (Sn, Ga, In)/Si-Au, (Ag, Ga, In)/Si-Sn, Au/Si-Ga, and (Ag, Sn, Ga)/Si-In. For the systems of Ga/Si-Ag and (Sn, In)/Si-Ga. complex growth modes (C) were observed. The latter four growth modes, S, F, A and C are newly characterized growth modes.