Directional Elastic Peak Electron Spectroscopy Research Articles

Properties of Au film on Ru(0001) revealed by AES, LEED, and DEPES

In this work the application of the surface sensitive experimental methods such as Auger electron spectroscopy, low energy electron diffraction, and directional elastic peak electron spectroscopy is shown to characterize the growth mode, the long and short range order of Au layers on the Ru(0001) surface. The adsorption was carried out at different substrate temperatures from room temperature till 460 °C. Auger signal from the substrate and the adsorbate recorded during the continuous Au adsorption indicates the formation of the first and then the second gold wetting layer at elevated substrate temperatures. At room temperature the formation of the second layer just before the completion of the first monolayer occurs. Further adsorption leads to the growth of three dimensional gold islands on 2 ML of Au on Ru(0001). At elevated temperatures the reduction of the Auger signal drop rate from the substrate and the signal rise rate from the adsorbate after the formation of 2 ML of Au can be rationalized by the coalescence of three dimensional Au islands and/or the decrease of the sticking coefficient of gold. Above 460 °C the saturation of Auger signals corresponding to 2 ML of Au is observed. The LEED pattern observations reveal the long range order within the clean and covered by about 5 ML of gold Ru(0001). The diffraction spots of the adsorbate reflect the Au(111) arrangement. The application of the DEPES method to the clean and covered by gold Ru(0001) surface reveals the presence of A and B type terraces on the Ru(0001) surface, as well as mutually rotated by 180° adsorbate domains, which exhibit the 12¯1111Au//01¯100001Ru and 1¯21¯111Au//01¯100001Ru orientations with respect to the crystalline substrate. The detailed R-factor analysis of experimental and theoretical DEPES distributions, the latter obtained with the use of the multiple scattering calculations, reveals equal population of A and B type Ru(0001) terraces, as well as unrotated and rotated Au(111) domains, which results in the six-fold symmetry DEPES intensity distributions for clean and covered substrate. The discussion of DEPES results concerns the adsorbate and substrate structures, inelastic mean free paths, and scattering properties of individual Au and Ru atoms described by the scattering factors and differential scattering cross sections.

Determining the structure of a layer under 2D-cover: the case of Pb underneath epitaxial graphene on Ru(0001)

Intercalation of solid and molecular species underneath 2D materials is one of the hottest topics in physics and chemistry nowadays – being important from the point of view of well-defined 2D materials-metals contacts formation, as well as performing catalytic reactions under cover. The confined space available for the intercalated layer’s formation often favours unique structural arrangements which are, however, difficult to study directly using conventional surface science techniques. We used directional elastic peak electron spectroscopy (DEPES) in combination with low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) to determine the structure of a Pb layer underneath epitaxial graphene (Gr) on Ru(0001). The results reveal that Pb forms a (√7×√7)R19.1° structure underneath Gr, which is similar to that formed by Pb on clean Ru(0001). As DEPES measurements utilize conventional LEED/AES optics commonly available in surface science laboratories, our approach can be adapted by other groups for the determination of the structure of various layers under 2D-covers.

Surface termination revealed by directional elastic peak electron spectroscopy

In this work the results of investigations concerning the termination of the Ru(0001) surface are presented. All experiments were carried out with the use of a retarding field analyser. The cleanness of the sample was monitored by Auger electron spectroscopy. After the cleaning procedure the long range order of surface atoms was proved by the observation of sharp diffraction spots with the use of low energy electron diffraction. The short range order of atoms within the surface-near region was investigated with the use of directional elastic peak electron spectroscopy. The experimental intensity distributions presented as stereographic projections were recorded for different energies of the primary electron beam Ep. The six-fold symmetry intensity pattern observed in all intensity plots reflected the contribution of A and B terminated terraces of the Ru(0001) surface. The quantitative analysis of the experimental data, presented in the form of stereographic distributions and azimuthal profiles, and performed by means of contrast values of intensity maxima, showed the equal population of A and B terminated terraces. The data are discussed in the context of single and double steps on Ru(0001) and face centered cubic (111) samples, and the theoretical distribution for the A terminated Ru(0001) surface obtained with the use of the multiple scattering theory is presented.

Structure and energetics of ultrathin Cu adlayers on Ru(1 0 [formula omitted] 0)

Adsorption of ultrathin epitaxial layers of Cu on Ru(1 0 1¯ 0) was investigated by ab initio density functional theory (DFT) simulations combined with directional elastic peak electron spectroscopy (DEPES) measurements. In the low-coverage regime, energetics of the adsorption process indicates that the deposited Cu atoms form instantly a bilayer (BL) and leave some substrate uncovered rather than wet the substrate with a single monolayer. The bilayer-by-bilayer growth is energetically favorable up to 3 BLs, but the third BL already occurs thermodynamically metastable. The DFT-predicted optimal arrangement of Cu adatoms in substrate-lattice-continuation sites was validated by DEPES measurements. This confirms the formation of a stable ultrathin film of hcp-like copper on the Ru(1 0 1¯ 0) substrate.

Properties of Pt on Cu(111) revealed by AES, LEED, and DEPES

The investigations of Pt on Cu(111) were carried out to reveal the growth mode and crystalline structure of the adsorbate with the use of Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and directional elastic peak electron spectroscopy (DEPES). Auger signal recorded during the continuous Pt adsorption on Cu(111) at 330 K confirms the layer by layer growth mode. LEED patterns observed at 1ML of Pt show the formation of the compressed adlayer. At higher coverages such as 3ML and 6ML the increase of the lattice constant of the topmost Pt layer, which approaches the bulk value of Pt(111), is noted. DEPES investigations performed for 1ML of platinum indicate the nucleation of adsorbate domains characterized by A/CBA and B/CBA stacking sequences at the Pt/Cu interface, which at higher coverages lead to the nucleation of mutually rotated by 180° Pt islands. Experimental DEPES data were compared to theoretical results obtained with the use of the multiple scattering (MS) calculations for different adsorption geometries including the Pt and Cu intermixing at the interface. The quantitative analysis of the data made by R-factor calculations enabled the determination of Pt domain populations already at 1 ML and 6 ML.

Surface relaxation of Pt(1 1 1) and Cu/Pt(1 1 1) revealed by DEPES

In this paper, we report the results of surface relaxation investigations for a clean Pt(111) as well as Pt(111) covered with a pseudomorphic monolayer of Cu. The pseudomorphic (1×1) Cu overlayer on Pt(111) was formed upon continuous copper adsorption at the substrate temperature equal to 450K. Considerations include both the experimental and theoretical studies employing directional elastic peak electron spectroscopy (DEPES), multiple scattering (MS) formalism, and density functional theory (DFT). A quantitative analysis of two-dimensional DEPES intensity distributions is presented and the contribution of consecutive subsurface Pt atomic layers to the measured signal is identified. The experimental DEPES distributions are compared with theoretical maps MS-simulated for a variable separation between the outermost atomic planes. For the clean Pt(111), the best fit is obtained assuming the outward relaxation of the surface layer by +0.7% with respect to the interplanar distance in the bulk, confirming the slight expansion of the Pt(111) surface. Similar analysis performed for the Cu/Pt(111) adsorption system indicates the inward shift of the pseudomorphic Cu overlayer by −6.9% with respect to the substrate lattice continuation sites. Such DEPES-determined surface relaxations of Pt(111) and Cu/Pt(111) agree with the values predicted by the corresponding DFT computations.

Investigation of surface termination by directional elastic peak electron spectroscopy: Experiment and theory

Experimental and theoretical details concerning the directional elastic peak electron spectroscopy are presented. The application of this experimental method to the identification of a crystalline structure of surface layers is shown for Cu(111) and Ru(0001), which enables the analysis of different surface terminations associated with different sequences of atoms along the surface normal. Theoretical data are obtained by applying the multiple scattering theory and different sources of phase shifts. The quantitative analysis performed by an R-factor reveals almost the same populations of A and B terminated (0001) terraces and proves explicitly the presence of steps of atomic height on the clean Ru(0001) surface. This is not the case for the Cu(111) surface, where the terrace termination does not affect the distribution of atomic directions within the first few atomic layers. The averaging of theoretical data versus scattering volume defined by the sphere radius Rmax around the emitter site is discussed in view of the computation time optimization.

Properties of Cu films on Pt(1 1 1) revealed by AES, LEED, and DEPES

The experimental and theoretical results presented in this work concern the growth mechanism and structural investigations of the ultrathin copper layers on Pt(111) at the adsorption temperatures from 330K to 450K. The growth mode of Cu on Pt(111) and the crystalline structures up to the first four adsorbate monolayers (4ML) were studied by using Auger electron spectroscopy, low-energy electron diffraction, and (in the case of 1ML) directional elastic peak electron spectroscopy. At 330K Cu forms a misfit adsorbate layer with a supercell of Cu(13×13) on Pt(12×12). The formation of the misfit Cu overlayer characterized by the LEED pattern continues to about 4ML. At higher coverages the diffraction pattern characteristic of the Cu(111) surface was observed. At 450K the pseudomorphic growth of 1ML of adsorbate was noted. The experimental DEPES anisotropy maps compared with the theoretical data obtained by using the multiple scattering formalism confirms the misfit structure of the Cu layer at 330K and pseudomorphic overlayer with threefold hollow adsorption sites of type A at 450K.

Formation of Cu domains on [formula omitted] revealed by stereographic maps of elastically backscattered electrons

The results of investigations of the Cu/Ru101¯0 adsorption system obtained by using directional elastic peak electron spectroscopy (DEPES), Auger electron spectroscopy and low energy electron diffraction are presented. The measurements were performed for the clean and Cu covered ruthenium surface at a coverage of 7 bilayers at room temperature. At early stages of growth the formation of two flat Cu bilayers was observed. Further adsorption leads to the nucleation of Cu(111) islands of the face centered cubic (fcc) structure. However, the experimental results reveal the formation of two fcc(111) domains which are mutually rotated by 180° arranged in the 21¯1¯111Cu0001101¯0Ru and 2¯11111Cu0001101¯0Ru orientations with respect to the substrate. Moreover, the quantitative analysis of DEPES data reveals nonequivalent (45%/33%) concentrations of these domains.

Properties of ultrathin Pb layers on the Ni3Al(1 1 1) face

The atomic structure and morphology of ultrathin Pb layers deposited on the Ni3Al(111) face in ultrahigh vacuum at the substrate temperature, ranging from 200K to 950K, were investigated with the use of Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and directional elastic peak electron spectroscopy (DEPES). The analysis of AES measurements indicates that two-dimensional growth of the first Pb monolayer ‘wetting layer’ takes place for substrate temperature 200K≤T≤650K. For T=200K, lead on the Ni3Al(111) grows layer-by-layer, while for T=300K flat three atomic-layer-high islands seem to grow after the completion of the first lead monolayer. Above 350K, the Stranski–Krastanov growth mode is observed. The ordered LEED patterns corresponding to p(4×4) and p(√3×√3)R30° structures are observed. AES and LEED data indicate that lead atoms and the Ni3Al(111) face form a Pb-Ni3Al(111) surface alloy. The DEPES results show that the stacking fault abcabcBACBAC or abcabcACBACB is formed in the Ni3Al(111)–Pb interface region.

Anisotropy of attenuation revealed by angular intensity distribution of elastically backscattered electrons

The structure of the first few atomic layers of Cu(111), Pt(111), and Au(111) was reflected by the directional elastic peak electron spectroscopy (DEPES). Experimental stereographic projections were compared with the theoretical data obtained by using a multiple scattering (MS) approximation. Although the same angular distribution of the maxima was observed in recorded and computed data for all samples, the relative peak intensities were found to be significantly different. The observed discrepancies suggest the anisotropic damping of electron transport in crystalline samples with respect to the low index directions. Elastic scattering of electrons penetrating the sample along the close packed atomic rows leads to an additional defocusing effect of the primary beam.

Identification of the monolayer oxygen structure on [formula omitted] by means of directional elastic peak electron spectroscopy (DEPES)

The investigation of 1 monolayer of oxygen on the Ru(1 0 1¯ 0) surface by directional elastic peak electron spectroscopy (DEPES) reveals high sensitivity of this surface method to light gases. The presentation of the experimental data as the anisotropy DEPES pattern shows an angular orientation of the oxygen adsorbate with respect to the substrate atoms. The straightforward identification of the oxygen structure is supported by a quantitative analysis providing oxygen atom coordinates within the unit cell.

Growth and crystalline structure of Ag layers on Au(111)

The Ag/Au(111) adsorption system was investigated by Auger electron and directional elastic peak electron spectroscopies (DEPES). The change of the Auger peak heights for Au (NVV transition, 74 eV) and Ag (MNN transition, 360 eV) recorded during the continuous Ag deposition show the formation of the second layer before the completion of the first monolayer. Further evaporation leads to the nucleation of islands on the not-completed second layer. The intensity of elastically backscattered electrons was measured as a function of the incidence angle of the primary electron beam to obtain stereographic DEPES distributions at energies Ep = 1.7 keV and 2.0 keV. About 8 ML of Ag on Au(111) raises intensity contrast of the total map and of the particular peaks associated with crystallographic directions proving well ordered Ag(111) overlayer formation. The quantitative analysis of the contrasts for experimental and theoretical DEPES maps, the latter obtained by using the multiple scattering theory, suggests the coexistence of the reconstructed and nonreconstructed domains on the Au(111) surface.

Scattering effects and damping of electrons in Pt(1 1 1) and Cu(1 1 1)

In this work an analysis of experimental and theoretical data associated with the scattering and attenuation of electrons in the Pt(1 1 1) and Cu(1 1 1) crystalline samples is presented. The information about the crystalline structure of the first few atomic layers was obtained by the directional elastic peak electron spectroscopy (DEPES) at the primary electron beam energies E p from 1.5 keV to 2.0 keV. The comparison of the experimental and theoretical DEPES distributions indicates a qualitative agreement between experiment and theory. The relative signal values associated with the intensity maxima were found to be different. The latter effect suggest that the electron attenuation in the crystalline samples can have an anisotropic character. A qualitative analysis of the characteristic pattern around the [1 1 1] direction concerning the calculation of the scattering factors was performed. The collective scattering of electrons by atoms located around the threefold symmetry axis resulting in the so called ring focusing effect is discussed.

Investigations of Co/Cu(1 1 0) by AES and DEPES

Auger electron (AES) and directional elastic peak electron spectroscopies (DEPES) were used to find the growth mode of Co on Cu(110) and the crystalline structure within the first few atomic layers. The Co growth at RT is characterized by the at first linear and then exponential decay of the Auger signal from the substrate hCu (CuM2,3VV, 66eV). A detailed analysis of the Auger signal suggests that at an early stage of the Co growth the formation of the second layer occurs before the completion of the first monolayer. Further deposition leads to the simultaneous nucleation of subsequent layers before the complete covering of the substrate. The DEPES profiles recorded along the two Cu[100] and Cu1¯10 azimuths at the primary electron beam energy Ep=1.0keV for equivalent quantities of 3ML and 5ML of Co show distribution of the intensity maxima similar to that for the clean Cu(110) sample, which indicates the epitaxial growth of Co. Theoretical DEPES profiles obtained by using the multiple scattering (MS) formalism at the maximal scattering order s=3 reflect the intensity maxima associated with the 100, 110, 112 and 111 directions, as it was observed in experiment.

Scattering effects of primary electrons in Co/Cu(1 1 1) measured by Auger; elastic and loss electrons

Electron energy losses were measured as a function of the incidence angle of the primary electron beam for the Co/Cu(111) adsorption system. The measurements performed for the clean and covered substrate reveal characteristic intensity maxima associated with the close packed rows of atoms, as it was observed in the so called directional Auger and directional elastic peak electron spectroscopy profiles. The incidence angle dependent signal of electron energy losses measured for the clean (Cu 3p3/2) and covered (Co 3p3/2) substrate gives the so called directional electron energy loss spectroscopy (DEELS) profiles which contain structural as well as chemical information. The scattering of primaries and different emission processes associated with electron energy losses, Auger, and elastically backscattered electrons are discussed. A change in the hCu (Cu M2,3VV transition) Auger signal recorded during the continuous cobalt deposition shows that the growth mode is not a pure layer by layer type. The complete covering of the substrate by Co at higher coverages is confirmed by the comparison between experimental and theoretical ratios of the Auger peak heights.

Properties of Co on Cu(111) revealed by AES and DEPES

Auger electron (AES) and directional elastic peak electron (DEPES) spectroscopies were used to investigate the Co/Cu(111) interface. The change in the Auger Cu (M2,3VV transition at 66 eV) peak intensity (hCu) recorded at room temperature shows that the Co growth mechanism is not a layer by layer type. This proves that Co does not wet the Cu substrate. The recorded DEPES profiles reveal the coexistence of fcc and hcp Co structures already at an early stage of growth (0.8 ML). The Co coverage increase leads to the reduction of intensities associated with the fcc structure and significant increase of the signal from the hcp structure. The comparison between experimental and theoretical data at large coverages (43 ML) shows a major contribution of the hcp structure within the Co layers in the recorded DEPES profiles.

Crystalline structure of the Au(1 1 1) surface revealed by stereographic intensity DEPES maps

The intensity of elastically backscattered electrons at the primary electron beam energy 1.9 keV was used to obtain a stereographic map of Au(1 1 1) by means of the directional elastic peak electron spectroscopy (DEPES). An experimental result is compared with the theoretical data obtained by using multiple scattering calculations (MS) performed for both not-reconstructed and model-reconstructed clusters. The lateral lattice misfit of the first layer leads to quantitative changes of theoretical intensities showing a sensitivity of DEPES to the short atomic chain axial order. This comparison proves that a main contribution of the experimental contrast originates from a higher background level. Moreover an anisotropy of the inelastic mean free path is discussed in the paper.

Stereographic intensity projections of elastically backscattered electrons in directional elastic peak electron spectroscopy: Experiment and theory

The stereographic intensity projections of elastically backscattered electrons for Au(1 1 1) were obtained by using directional elastic peak electron spectroscopy (DEPES). An enhancement of the recorded signal is observed when primary electrons with energies ranging from 0.5 to 2.0 keV strike the crystalline sample along the close-packed rows of atoms, proving the sensitivity of DEPES to the short atomic chain axial order. Besides, the obtained data reveal the threefold symmetry of the substrate. Experimental data are compared with the results of single scattering cluster (SSC) and multiple scattering (MS) calculations by means of contrast and an R-factor analysis. A satisfactory agreement between experiment and theory is found when the third scattering order is taken into account in MS approximation.

Directional elastic peak electron spectroscopy in the investigation of order–disorder transition in the Cu 3Au(0 0 1) surface layer: Measurements and SSC calculations

We present the results of directional elastic peak electron spectroscopy (DEPES) application to investigating the order–disorder transition in the surface layer of the Cu 3Au(0 0 1) crystal. We measured the DEPES polar profiles for a few sample temperatures ranging from 300 to 1000 K and for a few electron energies ranging from 1000 to 2000 eV. The obtained results were compared with those calculated in the single scattering cluster (SSC) approximation for Cu 3Au(0 0 1) clusters characterized by different inelastic mean free paths of electrons, effective Debye temperatures and surface layer atomic structure (including ordering and rippling of gold atoms). Measured and calculated DEPES profiles were found to be sensitive to the order–disorder transition. Reasonable agreement between measurements and calculations was obtained for the effective Debye temperature equal to ( 220 ± 6 ) and ( 172 ± 4 ) K for T < T C and T > T C , respectively, and for the rippling of Au atoms in the first atomic layer decreasing during the order–disorder transition from 0.12 to 0.09 Å.