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

Abstract

The adsorption of Fe(CO)(5) onto Au(111)/mica and C(4), C(8), C(12), and C(16) SAMs/Au(111)/mica surfaces has been studied using infrared spectroscopy to elucidate the coverage-dependent structures of these films and the intermolecular couplings that determine the form of the spectra. For all substrates, the first layer is 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). The axial vibrational band systematically shifts to higher frequencies with increasing surface coverage because of the effects of intermolecular coupling of the quasiparallel transition dipole moments. The strong effects of dipolar coupling are also witnessed by the trends of the band positions when the distance to the image plane is systematically varied using highly organized self-assembled organic substrates; no band shifts are observed when dilute Fe(CO)(5) is embedded in Xe matrixes under identical experimental conditions. The as-deposited films are structurally stable below 125 K on Au(111)/mica surfaces and below 100 K on the organic self-assembled monolayers. The instability of the films above these temperatures demonstrates that the as-adsorbed films do not form thermodynamically well-defined phases but are structurally metastable. The results presented herein and in the companion paper provide a consistent framework to interpret the spectroscopy of these systems that resolves outstanding issues concerning these films and provides a structural model that explains the dynamic properties of these films during exposure to low-energy electron beams.

Electron induced reactions of surface adsorbed tungsten hexacarbonyl (W(CO)6)

“…This type of phenomena is prevalent for strong IR adsorbers, such as the symmetric stretch of metal carbonyls. 39,52,53 Consistent with this argument, the frequency observed at the lowest W(CO) 6 coverages, in the present study where there are no intermolecular effects, is closest to the peak position observed for W(CO) 6 in other isolated situations (e.g. gas phase or inert gas matrices).…”

“…Based on the studies of McCash et al 51,52 as well as Rowntree et al, who conducted a detailed RAIRS study of the structure of Fe(CO) 5 on Au(111), 39 we can assign the lower frequency band to a disordered monolayer state of W(CO) 6 and the higher frequency peak that appears at higher W(CO) 6 coverages to a more ordered multilayer state representative of W(CO) 6 in the solid phase. In these previous RAIRS studies the metal carbonyls were diluted in inert gas matrices to show that the blue shifts in frequencies that occur within each one of the two bands as the W(CO) 6 coverage increases are due to repulsive intermolecular dipole-dipole coupling interactions caused by the simultaneous oscillations of the surrounding dipole moments.…”

“…Previous UHV surface science studies on electron stimulated reactions with adsorbed metal carbonyls include investigations on W(CO) 6 , Mo(CO) 6 , Ni(CO) 4 , and Fe(CO) 5 . [31][32][33][34][35][36][37][38] Key findings from these studies have included the observation of electron stimulated CO desorption and the formation of partially decarbonylated metal carbonyls. In the electron stimulated reactions of Fe(CO) 5 molecules adsorbed on Au surfaces and exposed to 0-20 eV electrons the initial step was proposed to involve the formation of Fe(CO) 4…”

“…33 These partially decarbonylated metal carbonyls were then degraded by further electron exposure. The structure of the adsorbate layer 39 was also found to be important in determining the film's sensitivity to electron induced transformations. 33 In the present study we have used an array of surface analytical techniques (X-ray Photoelectron Spectroscopy, Mass Spectrometry, and Infrared Spectroscopy) to interrogate the sequence of electron stimulated bond breaking processes that occur within nanometer thick films of W(CO) 6 adsorbed on solid substrates at low temperatures under UHV conditions.…”

Modelling focused electron beam induced deposition beyond Langmuir adsorption

“…Moreover, current continuum models restrict their range of applicability to Langmuir adsorption, where a maximum of one monolayer can be adsorbed [ 1 ]. Multilayer adsorption is, however, common in standard vacuum science studies, usually conducted at low temperatures [ 33 – 36 ], in cryogenic FEBID [ 37 ] and at higher temperatures for precursors with low volatility [ 1 ]. The breakage of the Langmuir model is also common for low adsorbate concentrations on heterogeneous substrates [ 38 ].…”