Reply to comment on “The structure of monolayer SiO 2 on Mo(1 1 2): A 2-D [Si–O–Si] network or isolated [SiO 4] units?”

Abstract

1 For a tetrahedral of [SiO4], the asymmetrical vibrations of Si–O should always be IR active. Unfortunately, these modes were assumed to be IR inactive in Refs. [5,6]. 2 In Refs. [5,6], TO modes were calculated and compared with those obtained from IRAS using p-polarized and grazing incidence IR light [3– 6]. However, it is well-established that for dielectric thin films only the TO mode is observed in transmission and reflection IR using normal incident light. On the other hand both the TO and LO modes are observed with grazing incident light, with the intensity of the LO mode increasing with an increase in the grazing angle [10,13–18]. Furthermore, the intensity of the LO mode is enhanced significantly using p-polarized light [16]. For an ultra-thin oxide film on a metallic substrate, the intensity of the LO mode has been shown to be enhanced and the TO mode concomitantly reduced First, in response to the above Comment [1] it is important to consider the following experimental observations: (i) high resolution electron energy loss spectroscopy (HREELS) [2] and infrared reflection adsorption spectroscopy (IRAS) [3–6] show intense asymmetric vibrational modes related to Si–O–Mo (1048 cm 1 in HREELS and 1056–1059 cm 1 in IRAS), whereas no active asymmetric vibrational modes related to Si–O–Si (1176 cm 1 in HREELS and 1200–1250 cm 1 in IRAS) are in evidence for SiO2(1 ML)/Mo(112), (see Fig. 1). These data indicate the absence of Si–O–Si species, i.e. the presence of isolated [SiO4] units [2], or that the axis of the bonding in the Si– O–Si moities are essentially parallel to the substrate surface. Regarding the latter point, however, off-specular HREELS [7] shows no evidence for an asymmetric vibrational feature related to Si–O–Si in SiO2(1 ML)/Mo(112); (ii) ultraviolet photoelectron spectroscopy (UPS) shows only a single feature at a binding energy of 10.5 eV, corresponding to an oxygen species in the Si–O–Mo linkage, with no evidence of a feature at or near 11.5 eV related to the oxygen species in the Si–O–Si linkage [4]; (Note that there should be two oxygen species, Si–O–Mo and Si–O–Si, in the 2-D network.) (iii) a detailed STM investigation of SiO2(1 ML)/Mo(112) [7] shows a single spot in each c(2 · 2) unit and/or four spots (one clearly distinct from the other three) with a (1 · 1) arrangement. In particular the one distinct spot in each c(2 · 2) unit is inconsistent with the proposed 2-D network model but is consistent with the isolated [SiO4] structure.