Fe(CO)₅Thin Films Adsorbed on Au(111) and on Self-Assembled Organic Monolayers: II. Thermal Transformations

Abstract

The thermal transformations of as-deposited Fe(CO)(5) films adsorbed on Au(111)/mica and C(4), C(8), C(12), and C(16) self-assembled methyl-terminated monolayer organic surfaces have been studied using infrared spectroscopy to probe how the physical restructuring influences the sensitivity of these systems to low-energy electron beams. A companion publication shows that the as-deposited monolayers are composed of molecules physisorbed with one axial and two equatorial carbonyl groups directed toward the substrate; subsequent layers are preferentially oriented with the C(3) molecular axis aligned perpendicular to the substrate (i.e., one axial carbonyl group directed toward the substrate). In this work, we show that the as-deposited films are structurally unstable above 125 K on Au(111)/mica surfaces and above 100 K on the organic self-assembled monolayers. Above these thresholds, the layered structures transform into three-dimensional aggregates, implying strongly nonwetting behavior for Fe(CO)(5) on each of these substrates; molecular desorption from this aggregate structure takes place between 140 and 160 K. The irreversibility of this temperature-induced transformation demonstrates that the as-deposited layered films do not represent a thermodynamically well-defined phase; this key feature of the as-deposited films is believed to be the cause of the discrepancies in previous attempts to understand Fe(CO)(5)/surface structures based on infrared results. Moreover, the thermally induced transformation to 3D aggregate structures is shown to decrease the apparent sensitivity of the adsorbed Fe(CO)(5) to low-energy electron-induced decarbonylation (0-10 eV) by over 3 orders of magnitude.

Role of Low-Energy Electrons (<35 eV) in the Degradation of Fe(CO)5 for Focused Electron Beam Induced Deposition Applications: Study by Electron Stimulated Desorption of Negative and Positive Ions

“…Since an increase in ESD yield is seen for all anions (except O – ) across the studied energy range, it seems probable that their increase in desorption probability is due to lower polarization energy. Thus, these results confirm that the Fe­(CO) x – fragments stay on the Pt foil with desorption of CO fragments as reported by Hauchard and Rowntree, and a Fe deposition can thus be produced by loss of CO fragments by extensive electron irradiation …”

“…This kind of behavior suggests that the production of these fragments derives from one-electron events, i.e., that a single electron can dissociate the initial molecule to produce the detected fragments. This reaction has been already proposed by Hauchard et al 39 when measuring the decrease of the CO stretching IR band of 0−10 eV electron irradiated Fe(CO) 5 films deposited on Au(111) or on self-assembled organic monolayers. The energies of resonance features seen in the yield functions of Figure 1 are recorded in Table 1, together with the threshold energies for desorption of each anion.…”

“…Thus, these results confirm that the Fe(CO) x − fragments stay on the Pt foil with desorption of CO fragments as reported by Hauchard and Rowntree, 27 and a Fe deposition can thus be produced by loss of CO fragments by extensive electron irradiation. 39 ESD measurements on mixed films of Fe(CO) 5 and Xe condensed on Pt show that the chemical environment has a significant influence on anion desorption (Figure 5). As the ratio of Fe(CO) 5 increases in the mixtures, so do the desorption signals of C − and O − , while Fe(CO) x − fragments are only detected at the lowest Fe(CO) 5 ratio (i.e., Fe(CO) 5 / Xe = 20%/80%), as no significant Fe(CO) x − signal is detected with the two other mixtures.…”

Substrate temperature and electron fluence effects on metallic films created by electron beam induced deposition

“…36 The loss of these spectral features indicates that CO can desorb at substrate temperatures >25 C. Indeed, evidence of thermal CO desorption from partially decarbonylated metal carbonyls has been observed in previous temperature programmed desorption studies of Fe(CO) 5 and Ni(CO) 4 (Refs. [37][38][39][40], where films were electron beam irradiated at lower substrate temperatures under UHV conditions and then heated. Analysis of the W(4f) region indicates that when CO desorbs from the film, there is a slight increase in the spectral intensity at %34.9 eV within the W(4f) region as well as the concentration of adsorbed carbon atoms (C ads ) and oxide species (O 2À ).…”

Fe(CO)5 Thin Films Adsorbed on Au(111) and on Self-Assembled Organic Monolayers: I. Structure

“…We have verified that under the conditions of their experiment the cross-sections for IPC degradation on Au(111)/mica are indeed very small; in an effort to understand why the dissociation dynamics are so strongly perturbed by the adsorbed state, we studied the structure of these films and explored the sensitivity of the electron-induced processes to the film structure. The structural issues for as-deposited films are described in this work, while the effects of thermal processing are explored in detail in a companion paper, herein referred to as II . The central result of II is that as-deposited films of IPC on Au(111) and alkanethiol SAMs do not form thermodynamically stable structures, despite (1) the apparently crystalline simplicity of the IRRAS results for these systems and (2) the common assumption that the as-deposited films are homogeneous .…”