Different Adsorbed States of 1,4-Cyclohexadiene on Si(001) Controlled by Substrate Temperature

Abstract

To elucidate the thermal chemical processes of 1,4-cyclohexadiene (C 6 H 8 ) on Si(001), the adsorption states were characterized by temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), and high-resolution electron energy-loss spectroscopy (HREELS), in comparison with those for benzene (C 6 H 6 ) and cyclohexene (C 6 H 10 ). Consequently, three types of adsorption states, i.e., π-complex, single di-σ bonding, and double di-σ bonding species, were identified. At 85 K, all 1,4-cyclohexadiene molecules are adsorbed as π-complex species in the first layer. With increasing substrate temperature, above 150 K, these π-complex species chemically convert to single di-σ bonding species by [2 + 2] cycloaddition with a Si dimer. Upon further heating above 300 K, most single di-σ bonding species are dehydrogenated into benzene and then the benzene molecules desorb from the surface. In contrast, double di-σ bonding species are formed preferentially in low exposure at 300 K, and are dehydrogenated into benzene above 600 K.

Modeling of 1,4-cyclohexadiene adsorption thermodynamics on Si(001)-2 × 1 surface

“…Therefore, the adsorption monolayer of simple saturated hydrocarbons on Si(001)-2 × 1 has low thermal stability and, consequently, is not interesting from the practical point of view. On the other hand, the main result of the recent studies is the finding that a hydrocarbon molecule containing several unsaturated bonds can have several adsorption configurations differing in the number of di-σ bonds formed between the adsorbed molecules and the silicon surface [5,[8][9][10][11][12]. Based on the results of the DFT calculations a stable π-precursor was supposed to be formed due to the interaction between an unsaturated hydrocarbon molecule and a silicon atom (three-membered π-complex precursor) [13].…”

“…Later, these π-precursors were observed experimentally [14]. The available experimental data [9][10][11][12][15][16][17][18][19] are ambiguous and, therefore, most of the theoretical works are focused on the quantum-chemical calculations of binding energies for the different adsorption configurations [5,[19][20][21][22][23][24].…”

Statistical Thermodynamics of Lattice Gas Models of Multisite Adsorption

“…Obviously that the adsorption of more complex (in regard geometry, the chemical structure -the presence of double/triple bonds or several functional groups) of molecules is even more nontrivial [14][15][16][17][18][19][20][21][22][23][24][25]. First of all this is manifested in that complex organic molecules (cyclic hydrocarbons, aromatic systems etc.)…”

“…Indeed, over the past ten years series of experimental works devoted to the study of organic selfassembled monolayers on metal surfaces has been published. The general conclusion of these studies is that the molecule orientation in the adlayer is a function of external parameters such as concentration, pressure, temperature, electrode potential and others [14][15][16][17][18][19][20][21][22][23][24][25]. Moreover, very interesting ordered structures have been experimentally found in some of similar systems.…”

“…In [24] phase transitions in the adlayer of acid trimezin molecules on the Au(111) surface have been investigated using the method of scanning tunneling microscopy, and one of them proved to lead to formation of the ordered structure containing the acid trimezin molecules oriented both parallel and perpendicular to the surface. Another striking example of the adsorption system in which the adsorbate molecules can have different orientations with respect to the surface is the adsorption of cyclic unsaturated hydrocarbons on the reconstructed semiconductor surface especially on the reconstructed face of the Si(001) [25][26][27]. This is connected to the fact that cycloaddition reaction [2+2] with formation of di-σ bond Si-C with the silicon surface atoms results from chemisorption of unsaturated organic molecules on the silicon surface.…”