Local atomic structure of ultra-thin Fe films grown on Cu(100)

Ultra-thin epitaxial Fe films grown by thermal deposition on Cu(100) are analyzed by scanning tunneling microscopy. Evidence is presented that the morphological characteristics and magnetic properties are a direct consequence of FCC-to-BCC transitions reminiscent of those occurring in bulk Fe. In contrast to the assumption of a ferromagnetic FCC phase in previous models of the Fe/Cu(100) system, we observe a tightly twinned and strained BCC-like phase termed nanomartensite in films below 5 ML thickness, which encompasses almost the entire film volume of 3 ML films. In addition, the surface of 7–8 ML films reconstructs by forming non-close-packed structures with BCC-like bond angles. The formation of these BCC-like phases is the reason for the expansion of the interlayer spacing observed in these films and correlates perfectly with their ferromagnetic ordering. PACS 68.55.-a; 64.70.-p; 81.30.-t     

The structure of atomic nitrogen adsorbed on Fe(100)

Nitrogen atoms adsorbed on a Fe(100) surface cause the formation of an ordered c(2 × 2) overlayer with coverage 0.5. A structure analysis was performed by comparing experimental LEED I–V spectra with the results of multiple scattering model calculations. The N atoms were found to occupy fourfold hollow sites, with their plane 0.27 Å above the plane of the surface Fe atoms. In addition, nitrogen adsorption causes an expansion of the two topmost Fe layers by 10% (= 0.14 Å). The minimum r-factor for this structure analysis is about 0.2 for a total of 16 beams. The resulting atomic arrangement is similar to that in the (002) plane of bulk Fe₄N, thus supporting the view of a “surface nitride” and providing a consistent picture of the structural and bonding properties of this surface phase.     

Local probing of the surface chemical bond using X-ray emission spectroscopy

2 and CO on Ni(100), benzene on Ni(100) and Cu(110), and glycine adsorbed on Cu(110). New types of molecular states are observed which are directly related to the surface chemical bond. The long-accepted Blyholder model which is based on a frontier orbital concept cannot explain our results for N₂ and CO chemisorption. We find it necessary to offer a new picture where changes in the whole molecular orbital framework have to be considered. We show that both π and σ type interactions are important in describing the bonding in benzene to metal surfaces. The future prospect is illustrated by the adsorption of the simplest amino acid, glycine, on Cu(110). The adsorbate has four different atomic centers where X-ray emission spectra are obtained, providing a unique view of the local electronic structure. Accepted: 6 March 1997     

Self-consistent electronic structure calculation for nitrogen chemisorbed on copper (100)

A fully self-consistent calculation of the electronic structure of a N covered Cu slab is reported. The density of states shows changes relative to a clean Cu slab that reproduce the changes observed in photoemission spectra when N is chemisorbed on Cu(100). Thus initial state information is correctly mirrored in the photoemission spectra in this case. The changes are due to the presence of the N 2p-bands, and to significant alteration of the surface Cu 3d-bands by the chemisorption of N. The localization in space of the chemisorption bond and the importance of self-consistency are demonstrated.     

Local structure determination of a chiral adsorbate: Alanine on Cu(1 1 0)

N 1s and O 1s scanned-energy mode photoelectron diffraction (PhD) has been used to investigate the local structure of a single enantiomer of deprotonated alanine, alaninate, NH₂CH₃CHCOO-, on Cu(1 1 0) in the (3 × 2) phase. The local site is found to be similar to that of glycinate on Cu(1 1 0), with the N atoms in near-atop sites and the O atoms sites consistent with bonding to single surface Cu atoms but substantially off-atop. Unlike the Cu(1 1 0)(3 × 2)pg-glycinate phase, however, in which the two molecular species per unit mesh are mirror images of one another in identical local sites, the intrinsic chirality of l-alaninate means that the two molecules per unit mesh of the (3 × 2) surface phase occupy slightly different local sites. However, an excellent fit to the PhD data can be achieved by a minor modification of the structure found in DFT calculations by R.B. Rankin and D.S. Sholl [Surf. Sci. 574 (2005) L1] in which the heights of the N and O atoms above the surface are reduced by approximately 0.1 Å. The resulting average N-Cu and O-Cu values are 2.02 and 1.98 Å, respectively, with an estimated precision of ±0.03 Å. These bondlengths are shorter than those obtained from DFT by 0.08 and 0.10 Å, respectively.     

X-ray photoemission studies of the electronic structure of single-crystalline CdO(100)

The electronic structure of single-crystalline CdO is investigated using X-ray photoemission spectroscopy. The surface is prepared by in situ Ar⁺ ion bombardment and annealing (IBA). The core level spectra before and after the IBA are presented, and reveal a dramatic reduction in the surface contamination. The semi-core Cd 4d level and valence band region following the IBA are also shown. The surface Fermi level is pinned 1.3±0.10 eV above the valence band maximum.     

Cu(100) (2×22)R45°-O的表面结构与电子态的密度泛函研究

用第一性原理的总能计算研究了Cu(100))面的表面结构、弛豫以及氧原子的(2×22)吸附状 态.计算给出了Cu(100) (2×22)R45°-O吸附表面的结构参数，并得到了上述结构下氧吸附 的Cu(100)表面氧原子和各层Cu原子的电子态密度.计算得到的吸附表面功函数为4.58 eV ，与清洁Cu(100)表面功函数(～4.53 eV)几乎相同.吸附氧原子与最外层铜原子之间的垂直 距离约为0.02 nm，其能带结构体现出一定的金属性，同时由于Cu-O的杂化作用在费米能以 下约6.4 eV附近出现了局域的表面态.可以认为，在Cu(100) (2×22)R45°的氧吸附表面结 构下，吸附氧原子和衬底之间的结合主要来源于表面最外层铜原子与氧原子的相互作用. 关键词： Cu(100)(2×22)R45°-O表面 缺列再构 表面电子态     

The local electronic structure of tin phthalocyanine studied by resonant soft X-ray emission spectroscopies

The electronic structure of thin films of the organic semiconductor tin phthalocyanine (SnPc) has been investigated by resonant and non-resonant soft X-ray emission (RXES and XES) at the carbon and nitrogen K-edges with excitation energies determined from near edge X-ray absorption fine structure (NEXAFS) spectra. The resultant NEXAFS and RXES spectra measure the unoccupied and occupied C and N 2p projected density of states, respectively. In particular, RXES results in site-specific C 2p and N 2p local partial density of states (LPDOS) being measured. An angular dependence of C 2p and N 2p RXES spectra of SnPc was observed. The observed angular dependence, the measured LPDOS and their correspondence to the results of density functional theory calculations are discussed. Observed differences on the same site-specific features between resonant (non-ionising) and non-resonant (ionising) NXES spectra are attributed to symmetry selection and screening.     

Magnetic phase transition in Co/Cu/Ni/Cu(100) and Co/Fe/Ni/Cu(100)

Magnetic phase transitions in coupled magnetic sandwiches of Cu/Co/Cu/Ni/Cu(100) and Cu/Co/Fe/Ni/Cu(100) are investigated by photoemission electron microscopy. Element-specific magnetic domains are taken at room temperature to reveal the critical thickness at which the magnetic phase transition occurs. The results show that a coupled magnetic sandwich undergoes three types of magnetic phase transitions depending on the two ferromagnetic films' thickness. A phase diagram is constructed and explained in the process of constructing Monte Carlo simulations, which corroborate the experimental results.     

Electronic structure of graphene on Ni(111) and Ni(100) surfaces

Physics of the Solid State - A comparative investigation of graphene prepared by cracking of propylene (C3H6) on nickel surfaces with different orientations, Ni(111) and Ni(100), has been carried...     

Oxygen on Cu(100): a new type of an adsorbate induced reconstruction

High resolution electron energy loss spectroscopy (EELS), Auger- spectroscopy (AES) and low energy electron diffraction (LEED) have been used to study the adsorption of oxygen on Cu(100). With increasing oxygen exposure a c(2×2) and a

R45 diffraction pattern are observed by LEED at and above room temperature. The c(2×2) pattern however always shows weak extra-

R45 spots. This is consistent with the corresponding EELS spectrum, which shows a weak dipole-active substrate phonon at 160 cm⁻¹ which should not be dipole active for a genuine c(2×2) overlayer. These findings suggest that the

R45 structure is the only well-ordered overlayer at room temperature. By an in-situ AES comparison with a well ordered c(2×2) overlayer on Ni(100) the

R45 structure is shown to correspond, to an oxygen coverage q_o = 0.48 ± 0.05. This excludes several structural models for the

)R45 structure. The EELS spectra give evidence for the adsorption of oxygen in or close to fourfold hollow sites. This indicates that the

)R45 structure can be derived from a genuine c(2×2) structure by a displacivc transition. Two structural models are proposed.     

The structure of formate on Cu(100)

There is some controversy over the question of the correct adsorption site for formate on Cu(100). On the basis of SEXAFS and NEXAFS studies Stöhr et al. favour a bridge site with oxygen atoms near fourfold hollows, while Woodruff et al. bring photoelectron diffraction studies to bear argue for a bridge site with the oxygen atoms almost on top of Cu atoms. We calculate the total energies for the two arrangements, optimising the distance of the formate from the Cu surface, using a self-consistent Hartree-Fock scheme with a Gaussian basis. We find the Woodruff structure more stable by some 0.38 eV per formate radical. We calculate the binding energy to be 1.74 eV per formate radical.     

On the structure of CO adlayers on Cu(100) and Cu(111)

Alternative models for the high coverage compressed overlayers of CO on copper (100) and (111) are proposed in which the LEED patterns previously attributed to out-of-registry, uniformly compressed structures are re-interpreted in terms of adsorption on linear and bridged sites. The models provide a simpler explanation for the small influence of compression on the frequency of the infrared adsorption bond near 2000 cm^?1 compared with the large change of surface potential.     

Development of electronic structure of Ni thin films on Cu(0 0 1) surfaces

We have traced the development of the Ni electronic structure with thickness variation on flat and nano-structured Cu(0 0 1) surfaces by means of photoemission spectroscopy. The binding energy of the Ni 2p_3/2 main peak and satellite peak is found to have shifted monotonically in the direction opposite to each other, with the increase of Ni coverage. The reduced binding energy of the thin film’s main peak is strongly correlated to the Cu 4s/Ni 3d interfacial hybridization effect (s/d IHE) and the narrowing of the d band with the reduction of dimensions, while the increased satellite binding energy results from the combination of interface hybridization and expansion of an extended 4s-like state towards the vacuum. The center of the Ni d_xy band is predicted to shift closer to the Fermi level with increasing film thickness. Enhanced satellite intensity in thin films is observed, correlating to the narrowing of the d band with decreased film thickness. Through comparison of Ni films grown on flat versus nano-structured Cu(0 0 1) surfaces, the mixing of Cu and Ni atoms is found to be enhanced at the terrace edge region and consequently a larger s/d IHE is predicted for Ni on the nano-structured surface.     

Electronic structure of the octachlorodimolybdenum(II) anion using X-ray emission and X-ray photoelectron spectroscopies

X-ray emission (Mo Lβ_{2, 15} and Cl Kβ_{1, 3}) and X-ray photoelectron spectra from K₄ [Mo₂ Cl₈] have been combined to give a detailed picture of the electronic structure of the octachlorodimolybdenum(II) anion. Chlorine 3p orbitals form a relatively narrow band which has the same range of ionization energies as the more tightly bound orbitals with molybdenum 4d character. The molecular orbitals of the MoMo quadruple bond can be identified as well as those involved in MoCl bond formation and Cl non-bonding orbitals.