Monolayers In Height Research Articles

Highly Active Pt‒M (M=Co, Fe) Alloy Catalysts for the Hydrogen Evolution Reaction in Polymer Electrolyte Water Electrolysis

Conventional polymer electrolyte water electrolysis (PEWE) requires a large amount of noble metal catalysts such as Pt and/or Ir. Platinum (ranging from 0.5 to 2 mgPt cm−2) has been used as the cathode catalyst for the hydrogen evolution reaction (HER).1, 2 Development of highly active and durable catalysts for the PEWE is essential for the efficient production of hydrogen with reduced amounts of noble metals.3 While some Pt alloys have been examined for the HER in the PEWE, the durability was not sufficient due to dealloying.2 Recently, we have found that PtxAL–PtM (M=Fe, Co, Ni)/C (with stabilized Pt skin of 1 to 2 atomic layers, PtxAL) nanoparticles on carbon supports exhibited superlative activity for the hydrogen oxidation reaction (HOR) in H2-saturated 0.1 M HClO4 at 70 and 90ºC.4 In this presentation, we demonstrate that the PtxAL–PtM and PtM/C catalysts also exhibited higher activity for the HER, compared with that of Pt. Pt3Co/C catalyst was prepared by the nanocapsule method,5 followed by heat-treatment in 5% H2 (denoted as Pt3CoHT/C). The PtxAL–PtM/C catalysts were prepared in the same manner as that described previously.6, 7 Two commercial catalysts, c-Pt/C (d TEM =2.6±0.9 nm) and c-Pt3Co/C (d TEM =5.1±1.8 nm), were used for comparison. To measure the HER activities of the catalysts in H2-saturated 0.1 M HClO4 solution, the channel flow electrode (CFE) cell4 was used to minimize the ohmic loss by H2 bubble evolution. Each catalyst was uniformly dispersed on an Au substrate as the working electrode in the CFE cell with a constant loading of carbon support of 11 µg cm−2, which corresponds to approximately two monolayers in height of the carbon particles. Nafion film was coated on the catalyst layer with an average thickness of 0.10 µm. It was found for the Pt3CoHT/C by TEM, XRD and ICP that Pt3Co nanoparticles with narrow size-distribution (d TEM =2.9±0.5 nm) were uniformly dispersed on the carbon support. The values of electrochemically active area (ECA) of all catalysts examined accorded well with those estimated from d TEM with the assumption of spherical particles. Figure 1(a) shows iR-free polarization curves for our Pt3CoHT/C and commercial catalysts at 80ºC in H2-saturated 0.1 M HClO4 with the mean flow rate U m = 111 cm s−1. Apparent area-specific activities j S for the HER at −20 mV vs. reversible hydrogen electrode (RHE) are summarized in Fig. 1(b). The value of j S on the Pt3CoHT/C was about 1.6 times higher than that of c-Pt3Co/C and 3 times higher than that of c-Pt/C. The value of apparent mass activity MA at −20 mV on the Pt3CoHT/C was as high as 4.3 A mgPt −1 (see Fig. 1(a)), indicating the possibility of reduction of the Pt cathode loading to a level as low as 0.23 mgPt cm−2 for operation at 1 A cm−2 with an overpotential of −20 mV. Evaluation of performance and durability of PEWE cells with these cathode catalysts is now in progress. This work was supported by “Fundamental Research on Highly Efficient Polymer Electrolyte Water Electrolyzers with Low Noble Metal Electrocatalysts” from Grant-in-Aid No. 17H01229 for Scientific Research (A) from the Ministry of Education, Science, Sports and Culture, Japan. References M. Carmo, D. L. Fritz, J. Mergel, and D. Stolten, Int. J. Hydrogen Energy, 38, 4901 (2013).M. K. Debe, S. M. Hendricks, G. D. Vernstrom, M. Meyers, M. Brostrom, M. Stephens, Q. Chan, J. Willey, M. Hamden, C. K. Mittelsteadt, C. B. Capuano, K. E. Ayers, and E. B. Anderson, J. Electrochem. Soc., 159, K165 (2012).H. Ohno, S. Nohara, K. Kakinuma, M. Uchida, A. Miyake, S. Deki, and H. Uchida, J. Electrochem. Soc., 164, F944 (2017).G. Y. Shi, H. Yano, D. A. Tryk, A. Iiyama, and H. Uchida, ACS Catal., 7, 267 (2017).H. Yano, J-M. Song, H. Uchida, and M. Watanabe, J. Phys. Chem. C, 112, 8372 (2008).M. Watanabe, H. Yano, D. A. Tryk, and H. Uchida, J. Electrochem. Soc., 163, F455 (2016).M. Chiwata, H. Yano, S. Ogawa, M. Watanabe, A. Iiyama, and H. Uchida, Electrochemistry, 84, 133 (2016). Figure 1

Nonradiative Step Facets in Semiconductor Nanowires.

One of the main advantages of nanowires for functional applications is their high perfection, which results from surface image forces that act on line defects such as dislocations, rendering them unstable and driving them out of the crystal. Here we show that there is a class of step facets that are stable in nanowires, with no long-range strain field or dislocation character. In zinc-blende semiconductors, they take the form of Σ3 (112) facets with heights constrained to be a multiple of three {111} monolayers. Density functional theory calculations show that they act as nonradiative recombination centers and have deleterious effects on nanowire properties. We present experimental observations of these defects on twin boundaries and twins that terminate inside GaAsP nanowires and find that they are indeed always multiples of three monolayers in height. Strategies to use the three-monolayer rule during growth to prevent their formation are discussed.

Highly Active, CO-Tolerant and Robust Hydrogen Anode Catalysts: Pt-M (M=Fe, Co, and Ni) Alloys with Stabilized Pt Skin

For residential co-generation polymer electrolyte fuel cell (PEFC) systems operated with reformates, the mass activity for the hydrogen oxidation reaction (HOR) at the state-of-the-art commercial Pt-Ru/C anode catalyst is insufficient, especially when operated under high CO concentration (> 500 ppm) in a fuel processing system simplified for cost reduction. Recently, we have developed a novel CO-tolerant catalyst by forming two uniform atomic layers of stabilized Pt skin on Pt-Co alloy nanoparticles (Pt2AL–PtCo/C) and found its superlative CO-tolerant HOR activity together with high robustness with respect to air exposure.1 In the present research, to establish a clear strategy for designing CO-tolerant catalysts, we have examined the effect of the nonprecious metal species M (M = Fe, Co, Ni) in Pt2AL–Pt-M/C on the CO-tolerance and the robustness. The Pt2AL–PtFe/C, Pt2AL–PtCo/C, and Pt2AL–PtNi/C were prepared by the use of a high-surface-area carbon black support (specific surface area = 780 m2 g−1) in the same manner as that described previously.2, 3 Two commercial catalysts, c-Pt2Ru3/C and c-Pt/C, were used for comparison. The experimental procedure, in which the channel flow electrode cell (CFE) was used, was the same as that described in ref. 1. Each catalyst was uniformly dispersed on an Au substrate as the working electrode with a constant loading of carbon support of 11 µg cm−2, which corresponds to approximately two monolayers in height of the carbon black particles. Nafion film was coated on the catalyst layer with an average thickness of 0.075 µm. All electrode potentials were referred to the reversible hydrogen electrode (RHE). Figure 1 shows the apparent mass activities, MA app, at 20 mV vs. RHE for the HOR at various catalysts in H2-purged 0.1 M HClO4 solution at 70°C and 90°C without (t ad = 0, CO-free) and with adsorbed CO (t ad = 90 min at E = 50 mV in 1000 ppm CO/H2 saturated solution). For the HOR activity on the CO-free surface of the catalysts (t ad = 0), the Pt2AL–Pt-M/C was found to exhibit higher MA app than those of c-Pt/C and c-Pt2Ru3/C. The largest MA app was seen at Pt2AL–PtFe/C, 330 A gmetal −1 at 70°C, which was about 1.8 times higher than that of c-Pt/C and 2.5 times higher than that of c-Pt2Ru3/C. All of the catalysts showed a decrease in MA app at 90°C compared to that at 70°C, which is ascribed mainly to the decreased H2 concentration in the solution at high temperature, but the trend of the MA appvalues was unchanged. After 90-min CO poisoning (t ad = 90 min) at 70°C, as shown by the black bar for the c-Pt/C electrode, MA app decreased greatly, due to a decrease in the hydrogen adsorption sites blocked by strongly adsorbed CO. In contrast, the Pt2AL–Pt-M/C and c-Pt2Ru3/C catalysts exhibited excellent CO-tolerance, suggesting that the HOR active sites were not so rigidly blocked by CO, due to its enhanced mobility. Specifically, the CO tolerance increased in the order Pt2AL–PtNi/C < c-Pt2Ru3/C < Pt2AL–PtCo/C < Pt2AL–PtFe/C. At 90°C, the CO tolerance for all the catalysts was improved. The Pt2AL–Pt-M/C still showed higher MA app than those of c-Pt/C and c-Pt2Ru3/C, e.g., the MA app at Pt2AL–PtFe/C was 3.4 times higher than that of c-Pt/C and 2.8 time higher than that of c-Pt2Ru3/C. Thus, in the practical temperature range, the highest CO-tolerant HOR activity was demonstrated for Pt2AL–PtFe/C. The effect of particle size and the optimization of preparation methods are in progress in our laboratory. This work was supported by funds for the “SPer-FC” project from NEDO of Japan.

Chemical vapor deposition growth of a periodic array of single-layer MoS2 islands via lithographic patterning of an SiO2/Si substrate

The growth of micron-scale single-layer MoS2 islands is seeded and distributed at high fidelity by means of a regular array of micron-scale holes that extend through the oxide layer of a 300 nm SiO2/Si substrate. Low coverages exhibit individual sub-micron MoS2 islands directly adjacent to the seed positions. At moderate coverage the seed holes are encircled by merged MoS2 islands, whose overall shape and internal grain boundaries reveal coalescence out of several initial crystallites. Seeded islands are strictly monolayer in height, non-overlapping and they offer high photoluminescence as well as conventional Raman signatures.

Competing allotropes of Bi deposited on the Al13Co4(100) alloy surface

The growth and stability of Bi thin films on the Al${}_{13}$Co${}_{4}$$(100)$ surface has been investigated from the submonolayer to high-coverage regime by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) for temperatures ranging from 57 to 633 K. Initially, Bi adsorption leads to the formation of a pseudomorphic monolayer, followed by the growth of islands of different heights with increasing coverage. The in-plane structure, island height, and island morphology indicate that these islands adopt either a pseudocubic (110) or hexagonal (111) orientation normal to the surface. The (110)-oriented islands correspond to bilayer stacking (either two or four monolayers in height) while the (111)-oriented islands correspond to either three- or four-layer stacking. The in-plane orientation of (110) islands with respect to the substrate is random, while (111) islands adopt one of four possible orientations. In addition, the (111) islands show a moir\'e structure. The fact that Bi islands grow with either (110) or (111) orientation simultaneously on the same substrate relates to a subtle energy balance between both orientations according to ab initio calculations, allowing both structures to coexist. The island density dependence versus both deposition temperature and flux, their most frequent structure type, reshaping effects, and chemical reactivity of the different allotropes are also discussed in this paper.

Computationally Derived Rules for Persistence of C60 Nanowires on Recumbent Pentacene Bilayers

The tendency for C(60) nanowires to persist on two monolayers of recumbent pentacene is studied using molecular dynamics (MD) simulations. A review of existing experimental literature for the tilt angle adopted by pentacene on noble metal surfaces shows that studies cover a limited range from 55° to 90°, motivating simulation studies of essentially the entire range of tilt angles (10°-90°) to predict the optimum surface tilt angle for C(60) nanowire formation. The persistence of a 1D nanowire depends sensitively on this tilt angle, the amount of initial tensile strain, and the presence of surface step edges. At room temperature, C(60) nanowires oriented along the pentacene short axes persist for several nanoseconds and are more likely to occur if they reside between, or within, pentacene rows for ϕ ≤ ∼60°. The likelihood of this persistence increases the smaller the tilt angle. Nanowires oriented along the long axes of pentacene molecules are unlikely to form. The limit of stability of nanowires was tested by raising the temperature to 400 K. Nanowires located between pentacene rows survived this temperature rise, but those located initially within pentacene rows are only stable in the range ϕ(1) = 30°-50°. Flatter pentacene surfaces, that is, tilt angles above about 60°, are subject to disorder caused by C(60) molecules "burrowing" into the pentacene surface. An initial strain of 5% applied to the C(60) nanowires significantly decreases the likelihood of nanowire persistence. In contrast, any appreciable surface roughness, even by half a monolayer in height of a third pentacene monolayer, strongly enhances the likelihood of nanowire formation due to the strong binding energy of C(60) molecules to step edges.

Improved structural ordering in sexithiophene thick films grown on single crystal oxide substrates

We report on sexithiophene films, about 150-nm thick, grown by thermal evaporation on single-crystal oxides and, as comparison, on Si/SiO2. By heating the entire deposition chamber at 100°C we obtain standing-up oriented molecules all over the bulk thickness. Surface morphology shows step-like islands, each step being only one monolayer in height. The constant and uniform warming of the molecules obtained by heating the entire deposition chamber allows a stable diffusion-limited growth process. Therefore, the regular growth kinetics is preserved when increasing the thickness of the film. Electrical measurements on differently structured films evidence the impact of the inter-island separation region size on the main charge-transport parameters.

Atomic hydrogen induced gallium nanocluster formation on the Si(1 0 0) surface

The influence of atomic hydrogen on the Si(100) substrate with submonolayer gallium surface phases – (2×3), (2×2) and (8×1) – as well as the deposition of gallium on monohydride terminated Si(100)–(2×1)-H surface were studied by synchrotron radiation photoelectron spectroscopy (SR-PES) and low energy electron diffraction (LEED) and compared with similar metal/Si systems. It was found that gallium deposition on the Si(100)–(2×1)-H surface at elevated temperature (400°C) leads to a complete hydrogen desorption and formation of the same gallium surface phases as on the bare Si(100)–(2×1). Exposing the Si(100) substrate with (2×3)-Ga and (2×2)-Ga surface phases to atomic hydrogen results in the formation of gallium nanoclusters, surrounded by the Si(100)–(2×1)-H surface. These clusters have more than 2 monolayers in height and cover approximately 20–26% of surface, depending on the initial coverage. The results indicate that the cluster size and density may be controlled by initial Ga coverage and possibly by temperature.

10.1007/s11700-008-1017-x

The effect of vacuum annealing on the morphology of hydrogenated and oxidized Si(310) surfaces is investigated by scanning tunnel microscopy, reflection high-energy electron diffraction, and low-energy electron diffraction. It is found that after desorption of a passivating coating, the surface has a strongly developed profile formed preferentially by steps two monolayers in height. Annealing at a temperature of 900±15°C with subsequent abrupt cooling leads to faceting of the surface by (510) planes. The presence of steps two monolayers in height on the Si(310) surface allows one to use Si crystals oriented along the (310) plane as the substrate for heteroepitaxy of the II–VI compounds.

Evaporation‐Induced Self‐Assembly (EISA) at Its Limit: Ultrathin, Crystalline Patterns by Templating of Micellar Monolayers

Patterning of surfaces at the nanoscale is important in many fields of application, e.g., for electronic devices, storage media, and as a starting base for the generation of more complex structures. To exemplify the latter subject, it has been shown that a periodic surface structure with periodicity of surface energy and topology allows for the alignment of liquid crystals or the consecutive construction of channels or pillars being vertically aligned with respect to the substrate, thus making the application of strong external fields redundant. Structures in the range of 45–300 nm are predominantly fabricated by either opticalor electron-beam lithography. To reach the sub-45 nm range, self-assembled block-copolymer mesophases are employed as templates within lithographic processes. For instance, Adamson and co-workers reported on the use of block copolymers in a lithography etching process to generate dense periodic arrays of holes and dots in a silicon nitride coated silicon wafer. However, this process deals only with the structuring of a given surface, i.e., the physical properties of the final material are limited by the choice of the starting material and the restrictions of lithography. Herein, we present a straightforward approach towards a whole range of ultrathin, mesostructured metal oxide layers by sol–gel processing via a modified evaporation-induced selfassembly (EISA) approach introduced by Ogawa et al. and Brinker et al. several years ago. EISA usually provides mesostructured films with a thickness of 50–1000 nm, which therefore consist of several stacked layers (multilayers of micellar aggregates). However, it is challenging to use this well-established procedure to generate corresponding ultrathin films with ordered mesoporosity, although it is usually assumed that such fine control can only be achieved by Langmuir–Blodgett (LB) techniques. Kunitake and co-workers reported the successful preparation of ultrathin, self-supporting films of different oxides by sequential sol–gel processing using polymer underlayers and spin-coating, but the porosity was disordered and the metal oxides amorphous. In the present study, it is shown that a modified EISA procedure allows for the preparation of crack-free, ultrathin, crystalline metal oxide films with highly ordered in-plane mesostructure and a tunable film thickness on the scale of only several nanometers, even achieving the sequential formation of mesostructured metal oxides with exactly one micellar monolayer in height. Different structures (e.g., circular pores or line patterns) of the final surface layer material can be simply adjusted by varying the assembly structure of the template, and by using sufficiently small templates, the periodicity of the patterns can be downsized to about 4–5 nm. In addition, the structure is instantaneously generated on macroscopic length scales (i.e., not serial/slow). This has been beyond the reach of common lithography techniques to date. Furthermore, in contrast to physical methods such as molecular-beam epitaxy, pulsed laser deposition, and chemical vapor deposition, which are often used in combination with lithography and are known to suffer from elaborate preparation techniques, the present sol–gel approach also allows one to address more complex metal oxides (e.g., BaTiO3) by the ease of “beaker” chemistry and liquid-coating processing. For the generation of ultrathin, mesoporous metal oxide layers with periodicities in two dimensions and one dimension, respectively, we used two different types of structure-directing agents, namely a novel block copolymer of the “KLE” family (poly(ethylene-co-butylene)-block-poly(ethylene oxide)), which has already enabled the synthesis of various mesoporous, crystalline metal oxide thin films (e.g., CeO2, [10] TiO2, [11] c-Al2O3, [12] FeOx, [13] SrTiO3 ) and the well-known classical nonionic surfactant “Brij 58”. The thickness of C O M M U N IC A TI O N S

Tunnelling spectroscopy on silver islands and large deposited silver clusters on Ge(001)

Morphology and electronic properties of silver islands and deposited silver clusters on Ge(001) have been studied using scanning tunnelling microscopy (STM) and spectroscopy (STS) at low temperatures. Already the clean surface bears an interesting electronic structure, which is obvious from the STS. The tunnelling spectra exhibit strong peaks associated with dangling bond-derived surface states and an antibonding σ-state of the surface dimer. For silver islands of only few monolayers in height, complex spectra are interpreted to be dominated by metal–semiconductor interface states. These islands show energy gaps which are not observed for larger ones beyond 1 nm in height. Spectra of the larger islands contain a series of distinct peaks originating from lateral and three-dimensional electron confinement, respectively. Silver clusters – preformed in the gas phase using a cluster source – have been fabricated, size selected and deposited onto germanium(001). In tunnelling spectra dips at the Fermi level are accompanied by two maxima. These characteristics seem to be almost independent from the cluster size. Additional weak structures are found at higher bias voltages, which are understood in terms of quantized states.

Nanoscale growth of silver on prepatterned hydrogen-terminated Si(001) surfaces

Silver (Ag) growth on H-terminated Si(001) surfaces with prepatterns of silicon (Si) dangling bonds has been studied at room temperature using a scanning tunneling microscope (STM). The patterns were fabricated by extraction of hydrogen (H) atoms from a H-terminated Si(001)-(2\ifmmode\times\else\texttimes\fi{}1) surface with a STM tip. The study covers four different patterns: growth on H-terminated Si(001) surfaces with (1) an isolated single dangling bond site, (2) a dangling bond wire $(\ensuremath{\sim}1$ nm wide), (3) a dangling bond area $(20\ifmmode\times\else\texttimes\fi{}20$ ${\mathrm{nm}}^{2}),$ and (4) a dangling bond array. Ag adatoms readily diffuse on the surface and preferentially stick to patterns of dangling bonds. Ag adatoms are not spontaneously substituted for H atoms around the Ag adsorption sites. The behavior of Ag growth is different in each case. An isolated single dangling bond site can be occupied by an isolated single Ag atom or a Ag cluster. The dangling bond wire is covered by an Ag island of 1 monolayer (ML) in height. On the dangling bond area, Ag atoms preferentially stick to the edge of the area and a Ag island 1 ML in height grows in the direction perpendicular to the dimer rows. The behavior of each growth pattern is explained by the difference in binding energy of diffusing adatoms on H-terminated areas and Si dangling bond sites.

Scanning force microscopy of ion-irradiated organic single crystals of benzoyl glycine

Single crystals of the amino acid benzoyl glycine (hippuric acid) are irradiated normal to the as-grown surface by highly charged Bi ions with a kinetic energy of 2.38 GeV and a fluence of 1×1010 ions/cm2. The projectiles create circular craters with a mean diameter of 40 (10) nm on the surface of the crystal as observed by scanning force microscopy (SFM). The mean depth amounts to 4 (1) nm, this value being considered as a lower limit due to the finite radius of curvature of the force cantilever tip. Thus, on the average, each single-ion projectile seems to eject about 104 molecules. On the surface of non-irradiated crystals, SFM reveals terraces of a few monolayers in height. In water, it was possible to visualize the lattice periodicity. Terraces were also observed on the irradiated crystal surface in the presence of the craters, indicating that the crystal is still intact at the given dose.

Influence of rough substrates on the morphology evolution of epitaxial films

We present results of Monte Carlo simulation of the morphology evolution of films grown on rough substrates. The initial surfaces considered for the simulation are similar to those of substrates used for the growth of GaAs films by chemical and molecular beam epitaxy. When the growth is simulated in the absence of the Erhlich-Schwoebel effect, decreasing film roughness is observed until a stable value is reached. During this decrease we observe the formation of moundlike structures of a few monolayers in height. In some conditions the structures forming the initial rough surface (pits) present a limitation to the lateral size of these mounds. These simulation results are in qualitative agreement with experimental results for homoepitaxial GaAs films grown by chemical beam epitaxy.

Determination of the roughness of heteroboundaries from photocurrent spectra of short-period AlAs/GaAs superlattices

Photocurrent spectroscopy is used to study the nature of the roughness of heteroboundaries in (AlAs)m/(GaAs)n short-period superlattices (m=3−5, n=10−13) grown by molecularbeam epitaxy. The formation of minibands broadens the optical spectra of superlattices in comparison with isolated quantum wells; therefore to analyze the degree of perfection of the boundaries we used the decay of the minibands into a series of discrete Wannier-Stark levels in an electric field parallel to the superlattice axis. Exciton lines were observed in the photocurrent spectra in an electric field corresponding to direct and indirect (in space) transitions between the Wannier-Stark levels. Comparison of experimental data with calculation indicates that even in the better structures, in addition to monotonic variation of the thickness of the layers over area, roughnesses in the heteroboundaries one monolayer in height are present with characteristic lateral dimension not exceeding 10 nm.

Effect of step structure on ordering in GaInP

Ga0.5ln0.5P layers grown by organometallic vapor phase epitaxy at rates of between 0.1 and 4.0 μm/h on exactly (001)-oriented GaAs substrates have been studied using atomic force microscopy. The surface is found to be covered by islands several monolayers in height that are elongated in the [110] direction. The edges of the islands are formed of clearly resolved bilayer (5.9 Å) steps. Monolayer steps are rare and no steps larger than 6 Å were observed. These observations explain the nature of the order twin boundaries in ordered GaInP grown on exactly (001)-oriented substrates. The (001) domain laminae are always found to consist of an even number of monolayers. For bilayer steps, the domain thickness will be twice the number of steps moving across the surface before the direction of step motion switches due to the undulating nature of the surface. This switch in the direction of step motion at a particular location on the surface produces the order twin boundaries observed.

Scanning tunnelling microscopy observations of the evolution of small-scale topography on gold surfaces following irradiation with low-energy argon ions

Abstract Room-temperature irradiation of atomically flat gold films has been observed, using scanning tunnelling microscopy, to give rise to small faceted islands and pits (about 10 nm in lateral dimension and a few monolayers in height) which subsequently evolve by surface diffusion over a period of hours. The evolution on grains with different surface orientations is observed to be markedly different and, in particular, on what appears from symmetry consideration to be a (100) grain, the surface features are apparently in dynamic equilibrium under irradiation and subsequently thermally dissociate by the emission of small mobile clusters of atoms.

Step control on silicon surfaces by electric fields

The authors applied scanning tunneling microscopy (STM) to the real-time observation of the heating-current-induced step motion on the Si(111) surface at a temperature 2 degrees C below the (1*1)-(7*7) phase transition. Each step was a monolayer in height and had a shift in the opposite direction to the heating current direction or electric field. Step shifting was terminated at certain positions. Three monolayer steps moved around the terminated positions. When another step was getting closer to these three steps, the latter had the tendency to stop and became centers for step bunching. In order to explain these behaviours, the authors propose an electric-field-induced model together with the presence of pinning centers such as impurities, stresses, or adsorbates. This electric-field-induced step shifting can be applied to control the surface structures on an atomic scale.

Al-Ga disorder in AlxGa1−xAs alloys grown by molecular beam epitaxy

A heterostructure, consisting of two GaAs quantum wells each of 40 Å width separated by a 40-Å Al0.5Ga0.5As barrier, has been grown by molecular beam epitaxy and examined optically for Al-Ga disorder (alloy clustering). The photoluminescent and excitation spectra showed no evidence of clustering, in marked contrast to results on a similar structure grown by metal-organic chemical vapor deposition. Thus any clusters present must be less than 40 Å in diameter. The spectra also support an island-like interface with steps of ∼one monolayer in height and ≳300 Å in lateral extent.