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When faceting occurs, an initially planar surface converts to a ‘hill-and-valley’ structure, typically exposing low-Miller-index crystal faces of microscopic dimensions. The bcc W(111) and Mo(111) surfaces are morphologically unstable when covered by monolayer films of certain metals (including Rh, Pd, Ir, Pt, Au), and they become ‘nanotextured’ upon annealing at T >700 K: the surface is covered by nanoscale three-sided pyramids with mainly {211} facets. In the present work, we focus on the structure, electronic properties and reactivity of planar and faceted W(111) and W(211) covered by ultrathin films of metals (mainly Pd, Rh, Pt, Au) and non-metals (S, O). The measurements include ultrahigh-vacuum scanning tunneling microscopy, soft X-ray photoelectron spectroscopy (SXPS) using synchrotron radiation, Auger electron spectroscopy, low-energy electron diffraction, low-energy electron microscopy, and thermal desorption spectroscopy. The metal film growth and thermal stability have been investigated for coverages of 0–8 ML. The observed formation of three-sided pyramids with both {110} and {211} facets, as induced by 1 ML of overlayer metal, is predicted also by recent first-principles calculations of surface energetics. The faceting is caused by an increased anisotropy in surface free energy that occurs for the film-covered surfaces. At coverages above 1 ML, SXPS data indicate that thin film alloys are formed upon annealing films of Pt and Pd; surface alloy formation is not seen for Au films. These findings are discussed in terms of structural and electronic properties of bimetallic systems. The relevance to catalytic properties of a structure-sensitive reaction (acetylene cyclization over Pd–W) is also discussed.

The restructuring of S/W(111) surfaces has been studied using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). Dosing W(111) with a saturation coverage of H 2 S followed by annealing to >800 K causes a substrate reconstruction with (4×4) periodicity. After the formation of the (4×4) structure over the entire surface, and upon further annealing, the terrace–step configurations also restructure and form triangular terraces. Upon heating to T >1000 K, the triangular terraces coalesce and enlarge, in addition to forming multiple steps. A size-mismatch mechanism, based on charge transfer between S and W atoms, is proposed to explain the formation of (4×4) structures: strain introduced at the surface by adsorption of large sulfur anions is relieved by the (4×4) reconstruction. We have explored the consequences of a terrace growth mechanism for S(4×4)/W(111) that is based on local step structures, coupled with domain mismatch.

The faceting of Pd/W(111) to form nanoscale three-sided pyramids with {l_brace}011{r_brace} facets has been observed using a scanning tunneling microscope (STM). In contrast to the well-documented {l_brace}112{r_brace} faceting, which can transform the entire Pd/W(111) surface to a morphology composed completely of pyramidal {l_brace}112{r_brace} facets, the {l_brace}011{r_brace} faceting of Pd/W(111) occurs in coexistence with {l_brace}112{r_brace} faceting upon prolonged annealing. While {l_brace}112{r_brace} facets are found to grow in size (ranging from {approximately}3 to 15 nm) with increasing annealing temperatures, the dimensions of {l_brace}011{r_brace} facets appear to be limited to {approximately}2.5 nm. The coexistence of two types of pyramidal facets is consistent with energetic considerations based on first-principles calculations. The difference in their typical sizes is, on the other hand, interpreted as an evidence for kinetic factors in the faceting processes. {copyright} {ital 1999} {ital The American Physical Society}

The restructuring of S/W(111) surfaces has been studied using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). Dosing W(111) with a saturation coverage of H 2 S followed by annealing to >800 K causes the substrate to reconstruct, forming a structure with (4× 4) periodicity. In addition, the terrace-step configurations restructure and form triangular domains with a preferential orientation. These domains coalesce and enlarge, and also form multiple steps when the surface is heated to T>1000 K. The low reactivity of sulfided W(111) to high exposures of oxygen demonstrates that the surface is passivated by sulfur. Adsorption of S onto a faceted Pd/W(111) surface causes the facets to disappear, restoring the surface's planar form upon annealing. The resulting features are dominated by a (2× 2) structure. A size-mismatch mechanism, based on charge transfer between S/W and coadsorbed Pd/S on W, has been proposed to explain the formation of (4× 4) and (2× 2) structures, as well as the transition between these structures.

The faceting of Pd W (111) surfaces has been studied using a Scanning tunneling microscope (STM). Three-sided pyramidal facets having {211} faces with dimensions ranging from ∼3 to 15 nm can be induced by ultrathin Pd films (≥ 1 monolayer), upon annealing to 700 K or higher. From atomic-resolution STM-images of these surfaces, we obtain direct confirmation of the {211} structure on individual facets of the 3-sided pyramids. In addition, the atomic structure of the facet edges indicates that edge energy may play a role in faceting. When the as-deposited coverage of Pd is greater than the critical value (∼ 1 monolayer) for inducing faceting, the extra Pd atoms diffuse to form 3-dimensional clusters, some with discernible crystalline structures, upon annealing.

High resolution photoelectron spectroscopy using synchrotron radiation is used to probe indirectly the substrate electronic response of faceting of W(111) induced by adsorbed Pd and Pt monolayers. The W 4 f 7 2 core level is measured in this study. For clean W(111), there are three components observed in W 4 f 7 2 photoemission due to surface, subsurface and bulk atoms, respectively; the core level shift between surface and bulk atoms is 430 meV. Upon adsorption of Pd or Pt at 300 K, only a bulk-like W 4 f 7 2 peak is observed. The interface peak associated with W atoms at the PdW or PtW interface of the W(111) face coincides with the W bulk peak. Upon annealing of the metal-covered W(111) above ∼750 K, the entire surface undergoes a transition from planar to faceted; the W 4 f 7 2 core level associated with Pd- and Pt-covered W(211) facets broadens and its centroid shifts to the lower binding energy side of the W bulk peak. The centroid shift and peak broadening are attributed to W atoms at the PdW or PtW(211) interface of the facets. The relation between the interfacial energy and the binding energy difference of the interface W 4 f 7 2 peak and the surface W 4 f 7 2 peak is discussed.

We have studied ultrathin films of transition and noble metals on Mo(111) and W(111) using Auger spectroscopy, LEED, thermal desorption spectroscopy (TDS) and scanning tunneling microscopy (STM). The atomically rough, open bcc(111) surfaces are morphologically unstable when covered by films ≥ 1 monolayer thick of certain metals, i.e. they form faceted structures. For example, using a UHV STM to study Pd/W(111) , we find that the Pd-covered W(111) surface becomes completely faceted to three-sided {211} pyramids upon annealing, for Pd coverages greater than a critical coverage θc. Formation of pyramidal facets also occurs when W(111) or Mo(111) surfaces are dosed with Pt, Au, Ir, Rh, oxygen or sulfur. In contrast, monolayer films of Ti, Co, Ni, Cu, Ag and Gd do not induce massive reconstruction or faceting on W(111) and Mo(111) surfaces. The faceting appears to be thermodynamically driven but kinetically limited: faceting is caused by an increased anisotropy in surface free energy that occurs for the film-covered surfaces. An interesting correlation has been observed for both substrates: faceting occurs for overlayer elements having Pauling electronegativities greater than 2.0, suggesting that surface electronic effects are controlling the structural instability of both Mo(111) and W(111) . Structure sensitivity in a model catalytic reaction, n-butane hydrogenolysis, is observed over planar and faceted Pt/W(111) . We have also used soft x-ray photoemission spectrosocopy (SXPS) based on synchrotron radiation methods to characterize the bimetallic interface; for Pt, Pd and Au on W(111) , we find that substrate core level shift effects associated with interface formation are substantial, while those associated with faceting are rather subtle.

Measurements on YBCO/(Y-Pr)BCO trilayers show that the depression of the transition temperature in ultrathin YBCO is accompanied by a decrease in the conductance of the layers adjacent to the interfaces. The lowering of the conductance seems to be primarily the result of charge transfer across the interfaces, and perhaps to some extent also of defects. The intrinsic properties of YBCO seem to change little or not at all as the thickness decreases from bulk down to that of a single unit cell.

We have made measurements of the conductance of ultrathin oriented layers of YBa2Cu3O7−δ (YBCO) between submonolayers thicker layers of YxPr1−xBa2Cu3O7−δ [(Y-Pr)BCO] which demonstrate the stepwise progression from to bulk-like behavior as the thickness of YBCO is increased in increments as small as 1 A. The depression of the zero-resistance transition temperature TKT is correlated with the depression of the sheet conductance G. For small thicknesses the relation between TKT and G is approximately linear and is independent of x. For larger thicknesses there is a transition from this “surface dependence” to the dependence in clean bulk material.

We have studied the faceting and defaceting phase transitions of $\mathrm{Pd}∕\mathrm{W}(111)$. In apparent agreement with results of recent theoretical simulation [C. Oleksy, Surf. Sci. 549, 246 (2004)], we find the fastest way to create the largest facets is to anneal at a temperature right below the temperature that the defaceting transition occurs. On the other hand, while the paths of faceting transitions show normal retardation as the cooling rate is increased, the paths of defaceting transitions show negligible dependence on the heating rate even if increased by 64-fold. Another notable observation is a phase separation of the surface into defaceted and faceted regions after long annealing time while there is more than enough Pd remaining to induce faceting of the whole surface. This leads us to the proposal that instead of thermal disorder, the observed defaceting transition of the $\mathrm{Pd}∕\mathrm{W}(111)$ system is mainly driven by local loss of Pd, which is due to thermal desorption. Such desorption loss could be effectively replenished via surface diffusion at the vicinity of the Pd $3d$ islands. The observed independence of the defaceting transition path on the heating rate is rationalized as the consequence of a balance in between the loss and the supply of Pd, which can establish very quickly as the temperature rises.

Single-atom sharp pyramidal W tips, wrapped in a Pd overlayer, having atom-perfect wedges can be routinely and repeatedly created using a surface-science technique. This single-atom tip is thermally stable up to 1000 K because of the exceptionally large surface-energy anisotropy of the Pd covered W(111) surface. We conclude from atom-by-atom analysis that the W tip is covered with only one physical monolayer of Pd. We also investigate the mechanism and energetics of the atomic processes involved in its formation.

Conductance measurements on YBa2Cu3O7_6 (YBCO) between layers of Yi_PrBa2Cu3O7_ [(Y-Pr)BCO] show a transition from a bulk regime in the interior of the YBCO to a surface regime near the interfaces. The depression of the zero-resistance transition temperature in ultrathin YBCO is correlated with the depressed con- ductance in the surface layer. 'Ihe results indicate that the changes are related to the presence of the interfaces, primarily to charge transfer between the layers, with little, if any, indication of a change in the intrinsic propertiesof the YBCO from bulk down to the thickness of a single unit cell.Ultrathin films of high-Ta superconductors down to thicknesses of one unit cell (N=1) have been the object ofstudy ever since it was realized that the superconducting properties in these materials depend primarily on theircopper-oxide planes.' Experiments on such films have the potential of showing to what extent the two-dimensionalstructure is essential for the observed superconductivity.In the most-studied system the superconductor is YBa2Cu3O7_c (YBCO) with alternating layers of its insulatinganalog PrBCO in trilayers or multilayers. Previous studies indicate that ultrathin films are superconducting, butwith reduced values of the zero-resistance temperature T4, and the mean-field transition temperature Tmj •2—6

InN quantum dots (QDs) were grown over 2in. Si (1 1 1) wafers with a 300nm thick AlN buffer layer by atmospheric-pressure metal organic vapor phase epitaxy. When the growth temperature increased from 450to625°C, the corresponding InN QDs height increased from 16to108nm while the density of the InN QDs decreased from 1.6×109cm−2to3.3×108cm−2. Transmission electron microscopy showed the presence of a 2nm thick wetting layer between the AlN buffer layer and InN QDs. The growth mechanism was determined to be the Stranski–Krastanov mode. The presence of misfit dislocations in the QDs indicated that residual strain was introduced during InN QDs formation. From x-ray diffraction analysis, when the height of the InN QDs increased from 16to62nm, the residual strain in InN QDs reduced from 0.45% to 0.22%. The residual strain remained at 0.22% for larger heights most likely due to plastic relaxation in the QDs. The critical height of the InN QDs for releasing the strain was determined to be 62nm.

Based on the charging effect common to various kinds of electron microscopy, we have developed novel methods of determining “when” and “where” a probe starts to contact an electrically isolated surface. The touchdown of an electrically grounded probe leads to an acute change in the imaging contrast of the contacted surface, which also causes a rapid jump (ranging from a few to tens of picoamperes) of the grounding current. Thus, the detection of contact can be carried out in both qualitative and quantitative manners, providing a basis for establishing relevant standard procedures. In addition, we have achieved the spatial mapping of the contact point(s) using a specially designed lithographical pattern with two mutually vertical sets of parallel conductive lines. The precision of this mapping technique is simply determined by the pitch of parallel lines, which can be as small as the capability achievable in e-beam lithography. A possible “one-probe” version of the electrical characterization is also discussed with the same underlying principle, which may turn out to be indispensable for various studies and applications of nanostructures. Further development along this track is promising to realize an instrumentally simple version of “scanning electron spectroscopy” with various modes.

We observed by using atomic force microscope (AFM) phospholipid (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) monolayers on mica being annealed and cooled to a selection of temperatures through steps of 2-4 degrees C/min. The annealed phospholipid monolayers started to disappear at 45-50 degrees C and disappeared completely above 60-63 degrees C under AFM observation. The phospholipid monolayers reformed when the samples were cooled below 60 degrees C and developed from fractal into compact monolayer films with decreasing temperatures. Simultaneously the height of the reformed phospholipid films also increased with decreasing temperatures from 0.4 nm to the value before annealing. The observed thermal features are attributed to a phase-transition process that upon heating to above 45-50 degrees C, the lipids condensed in the monolayers transform into a low-density expanded phase in which the lipids are invisible to AFM, and the transformation continues and completes at 60-63 degrees C. The lipid densities of the expanded phase inferred from the dissociated area of the condensed phase are observed to be a function of the temperature. The behavior contrasts with a conventional first-order phase transition commonly seen in the Langmuir films. The temperature-dependent height and shape of the reformed phospholipid films during cooling are argued to arise from the adjustment of the packing and molecular tilting (with respect to the mica surface) of the phospholipids in order to accommodate more condensed phospholipids.

The reduction of Tc in ultrathin films of YBCO in multilayers has often been assumed to be the result of the absence of essential transverse coupling. Our work shows instead that the changes in Tc are correlated with changes in conductance, which are the result of extrinsic effects, such as charge transfer and defects at the interfaces. Except for these effects there seems to be no evidence for any change in the YBCO properties down to the thickness of a single unit-cell layer.

The electrical resistivity, ac susceptibility, critical current density, and Josephson tunneling of Pb/Ag multilayers have been measured in wide temperature ranges in order to study the proximity effect. A resistivity drop was found to occur atT c of Pb in ρ versusT measurement and, at low temperature, the multilayers show proximity effect. The I-V characteristic of the multilayer shows voltage steps, indicating a resistive state has occurred in the sample. The superconducting properties of the multilayer were analyzed with the bilayer theory of the proximity effect.