Phonons Localized at Step Edges: A Route to Understanding Forces at Extended Surface Defects

Abstract

Inelastic helium atom scattering has been used to measure the phonons on a stepped metallic crystalline surface, Ni(977). When the scattering plane is oriented parallel to the step edges and perpendicular to the terraces, two branches of step-induced phonons are observed. These branches are identified as transversely polarized, step-localized modes that propagate along the step edge. Analysis reveals significant anisotropy in the force field near the step edge, with all forces near the step edge being substantially smaller than in the bulk. Such measurements provide valuable information on metallic bonding and interface stability near extended surface defects.

Vibrational dynamics and thermodynamics of Ni(977)

“…Another interesting feature in Fig. 2 is that the low frequency modes associated with the step atoms display a global shift to even lower frequencies, as compared to the modes associated with the terrace ͑and Ni͑111͒ surface at-oms͒, thereby pointing to an extra softening of relevant force constants, in agreement with suggestions of Niu et al 13 This softening maybe related to the loss of neighbors. Clearly, an apparent feature of the surface atoms is their low coordination.…”

“…[7][8][9] On the experimental side, since the pioneering work of Ibach and Bruchmann 10 on Pt͑775͒, detailed analysis of Al͑221͒, 11 Cu͑211͒ and Cu͑511͒, 12 and Ni͑977͒ 13 have been performed using He atom scattering. The investigations of the vibrational modes of Ni͑977͒ are of particular interest here as their analyses 13 raise the issue of the extent of softening of the force fields at and near the step atoms, as compared to those on Ni͑111͒. Upon adsorption of oxygen, this surface also undergoes a reconstruction leading to a step doubling.…”

Bridging the structure gap: Chemistry of nanostructured surfaces at well-defined defects

“…the recent review of Hammer [5]) and, on the other hand, to try using the steps as naturally nanostructured templates for the growth and self-assembling of ultra-thin films [6,7] and other nanosized structures for applications to nanoelectronics [8][9][10][11][12][13]. Among the aspects of HMI surfaces which captured the largest attention we mention: (a) the modification of the electronic properties due to the nanosized terrace width and to the reduced dimensionality of the system [14][15][16][17][18][19][20][21]; (b) the modification of the vibrational properties due to the additional phonons localized at the step edges [22][23][24][25][26][27][28][29]; (c) the roughening transition associated to step edge meandering [30][31][32][33][34][35][36]; (d) the modification of surface diffusion, impeded by the very presence of the steps and of the related Ehrlich-Schwoebel (ES) barriers [37][38][39][40][41][42]; (e) the intrinsic chirality of some step kinked surfaces, which may be of potential interest for heterogeneous asymmetric synthesis [43] and for the separation of enantiomers in enantio-sensitive reactions [44]…”

Surface-scattering study of the interaction potential of He atoms with the step edges of the Cu(211) and Cu(511) vicinal surfaces

“…Recently, in the search for more detailed microscopic information on special dynamical features that might result from electronic modifications at step edges, Sibener and co-workers 30,31 and Witte et al 32 have investigated the steplocalized phonons on the Ni͑997͒ and the Cu͑211͒ and the Cu͑511͒ surfaces, respectively. In the latter study the step phonon modes could be clearly identified and their dispersion in k space along the two main symmetry directions could be traced out.…”