Desorption Of Monolayers Research Articles

Reductive desorption of alkanethiolate monolayers at gold: a measure of surface coverage

Reductive desorption of alkanethiolate monolayers at gold: a measure of surface coverage

The electrochemical desorption of n-alkanethiol monolayers from polycrystalline Au and Ag electrodes

Monolayers formed at Au and Ag surfaces by the spontaneous adsorption of n-alkanethiols were characterized voltammetrically to examine the chemistry of the bound thiol head group. Electrode reactions that correspond to the oxidative- and reductive-desorption of the adsorbed n-alkanethiol monolayer are reported for the first time. Analysis of the data indicates that upon adsorption at both Au and Ag surfaces the hydrogen of the thiol group is lost and the sulfur atom is oxidized by one electron. Based on the charge required for the reductive-desorption of the monolayer, the surface coverage of the oxidized n-alkanethiol species is 9.3×10 −10 mol/cm 2 and 7.0 ×10 −10 mol/cm 2 on Au and Ag, respectively. The value of the surface coverage at Au is slightly greater than that expected for a closest-packed overlayer commensurate with a Au(111) substrate, whereas the value at Ag is somewhat less than that expected for layers commensurate at any of the low index Ag surface planes. The low apparent coverage observed for Ag electrodes is attributed to a portion of the monolayer being electroinactive at accessible applied voltages. As a probe of the mechanism for monolayer formation, films that were deposited on Ag and Au surfaces from solutions containing sodium n-octadecanethiolate were also characterized. Examination of the resulting layers with infrared reflection spectroscopy and optical ellipsometry indicates that only at Ag substrates were the films deposited from the thiolate solution complete and structurally similar to monolayers formed from n-octadecanethiol solutions. Based on this observation, it is postulated that the thiol hydrogen of the n-alkanethiol molecule participates in the reduction reaction that is concomitant to the thiol oxidation during adsorption on Au, whereas adsorption on Ag proceeds through the reduction of the Ag(I) surface species of the native oxide. A model for the electric double layer at the monolayer-coated-electrode/solution interface is also suggested based on the observed chain-length dependence of both the capacitance and the potential for the reductive desorption.

Vibrational properties of arsenic on Si(111)

Clean silicon substrates. Si(111), were arsenic capped in a molecular beam of As4 molecules. These samples were investigated by Raman scattering after transferring them through air into another UHV chamber for analysis. The thick as deposited As layers are identified as being amorphous. With increasing exposure time to air they become partly oxidized. Heating of these layers was then performed in steps from room temperature up to 550°C. As revealed by Raman scattering the amorphous arsenic is removed at first. Around 350–400°C two peaks appear in the Raman spectra at 235 and 356 cm−1. Since at this temperature range all but one monolayer of ordered (1 × 1) As has been desorbed, these peaks are interpreted as a doubly degenerated stretching and a bending vibrational mode of the first monolayer of As on Si(111). The peaks disappear above 550°C. the temperature where the arsenic monolayer desorbs. Air exposure also causes the peaks to vanish.

Vibrational properties of arsenic on Si(111)

Abstract Clean silicon substrates. Si(111), were arsenic capped in a molecular beam of As 4 molecules. These samples were investigated by Raman scattering after transferring them through air into another UHV chamber for analysis. The thick as deposited As layers are identified as being amorphous. With increasing exposure time to air they become partly oxidized. Heating of these layers was then performed in steps from room temperature up to 550°C. As revealed by Raman scattering the amorphous arsenic is removed at first. Around 350–400°C two peaks appear in the Raman spectra at 235 and 356 cm −1 . Since at this temperature range all but one monolayer of ordered (1 × 1) As has been desorbed, these peaks are interpreted as a doubly degenerated stretching and a bending vibrational mode of the first monolayer of As on Si(111). The peaks disappear above 550°C. the temperature where the arsenic monolayer desorbs. Air exposure also causes the peaks to vanish.

Thermal Desorption of Hg Monolayers from Cu(100)

AbstractA two dimensional phase transition from one adlayer structure to another is an inherent part of the thermal desorption of one monolayer of Hg on Cu(100). The energetics of this phase transition have been studied using thermal desorption spectroscopy (TDS). The TDS spectra reflect the coexistence of the two structural phases for a range of Hg exposures. The TDS spectra have been analyzed within a Polanyi-Wigner framework modified to account for the phase transition.

The pyrolysis of trimethylgallium on a heated Si(100) substrate

Trimethylgallium (TMGa) is an important source gas for metalorganic chemical vapor deposition (MOCVD) and metalorganic molecular beam epitaxy (MOMBE). In the last three years, workers in three different laboratories have examined the pyrolysis of trimethylgallium (TMGa) on heated substrates. However, the mechanism of the decomposition process is still not understood. In this paper we consider the mechanism of the TMGa decomposition on a hot Si(100) substrate in a molecular beam system. It is found that at TMGa doses of 1 × 10 15 molecules/cm 2 or less, the decomposition process in the beam system is the same as that reported by Lee et al., Surface Sci. 216 (1989) 173, in studies where TMGa was dosed at low temperature and then the substrate was heated. However, at higher TMGa doses, the mechanism in the molecular beam system is different than the ones reported previously. It is possible to transiently populate a weakly bound state which we assign to TMGa adsorption in the second monolayer. If the sample is below 400 K, the TMGa in the second monolayer desorbs. However, at higher temperatures, the TMGa in the second monolayer can react to form a CH x group ( x = 3 or 4) and dimethylgallium. The dimethylgallium desorbs. However, some of the CH x groups remain bound to the substrate leading to extra carbon incorporation. There also is evidence for formation of traces of monomethylgallium above 500 K and an indication of methyl radical desorption above 650 K. Thus it seems that at high doses, the decomposition of TMGa on a hot substrate is somewhat different than the decomposition of a monolayer of TMGa which has been adsorbed at low temperatures and then heated. These results explain some of the apparent discrepancies in the literature.

Collisional mixing in ion beam desorption of impurity monolayers

We analyze theoretically the mixing processes involved in the desorption by ion bombardment of monolayers (adsorbed or deposited) on a substrate. For analytical relocation cross sections, we solve numerically the balance equation derived by Sigmund et al. [Nucl. Instr. and Meth. 194 (1982) 541] for the evolution with ion fluence of the composition of irradiated polyatomic targets. We also analyze the effect of the surface barrier on the mixed profiles. We compare our results with experimental data from Morita et al. [Surf. Sci. 157 (1985) L361; Nucl. Instr. and Meth. B13 (1986) 457 and B18 (1987) 407].

Real-time microfluorescence studies of Langmuir-Blodgett deposition: Monolayer adsorption and desorption

Observations of the meniscus region in a typical Langmuir-Blodgett configuration are obtained using fluorescence microscopy. Studies of the meniscus position as a function of pH demonstrate that the meniscus height increases upon charging the monolayer owing to the repulsive interaction between monolayer and substrate surfaces. Through the correspondence between meniscus height and contact angle the adhesion forces between monolayer and substrate as a function of monolayer charge are assessed. The phenomenon of desorption upon resubmerging a deposited film is described and it is shown that the meniscus height at the onset of the desorption is greater for higher pH.

Interaction of manganese with the ruthenium(001) surface and chemisorption of carbon monoxide on the manganese/ruthenium(001) interface

The interaction of vapor-deposited Mn with the Ru(001) surface and its effect on CO chemisorption were studied under ultrahigh vacuum by Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), isotopic exchange, and low-energy electron diffraction (LEED). At room temperature, Mn grows layer by layer. The first three layers are pseudomorphic with respect to the Ru substrate. A complete analysis of Mn thermal desorption data shows that there is a decrease of the heat of adsorption with increasing coverage within the first monolayer (59 to 55 kcal/mol) and zero-order desorption for higher coverages with desorption energy 54 kcal/mol. A new, higher temperature CO desorption state at 650-800 K appears at submonolayer coverages. The C-O bond of CO chemisorbed on Mn/Ru is severely weakened and probably dissociated as evidenced by a high CO desorption temperature and random isotope mixing of coadsorbed isotopes of carbon monoxide.

Sticking in the quantum regime: H2 and D2 on Cu(100).

Several questions of importance have yet to be clarified concerning the sticking of a light particle on a surface. Among these are the following: the relation between permanent "sticking" and temporary "trapping," the "scaling" of the sticking coefficient with incident angle, and the role of quantum-mechanical resonance phenomena in promoting sticking. We consider these and other questions in connection with data taken for the sticking of ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ molecular beams in the energy range 8-45 meV and for angles of incidence between 0\ifmmode^\circ\else\textdegree\fi{} and 60\ifmmode^\circ\else\textdegree\fi{} on a cold (\ensuremath{\sim}10-K) Cu(100) surface. The sticking coefficient at zero coverage, ${S}_{0}$, measured with use of partial monolayer desorption, displays a background that falls off with increasing energy on which are superposed well-defined peaks at characteristic energies that coincided with features in the specular reflectivity and with the condition for "selective adsorption." The peaks are due to the formation of quasibound states that are entered via the surface corrugation, the rotational anisotropy, or both acting together, and decay via sticking and backscattering channels. These states have been studied extensively in elastic scattering measurements. We demonstrate their decay via sticking channels and show that these channels give important contributions to the resonance widths. By comparing the dependence of the observed background sticking on energy and angle with calculations, we conclude that positive-energy trapping on initial collision is prevalent at wide incident angles. The trapping probability is governed primarily by the normal rather than total energy. However, many of the trapped particles revert to the gas phase and do not contribute to the sticking coefficient. As a result, the background sticking coefficient of ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ on Cu(100) does not display a simple scaling behavior as the angle of incidence changes.

Formation and thermal decomposition of ethyl groups on transition metal surfaces: Ethyl iodide on Pt(111)

The surface chemistry of ethyl iodide over Pt(111) surfaces has been studied by using thermal desorption and X-ray photoelectron spectroscopies. Molecular desorption of ethyl iodide takes place in two steps: desorption from a condensed multilayer below 200 K is followed by monolayer desorption at 240 K. The remaining adsorbed molecules react on the platinum surface to form ethyl groups and iodine atoms. Methane and ethane desorption is observed at about 255 K. Adsorbed ethyl dehydrogenates further yielding ethylene, presumably by β-hydrogen elimination, analogously to well established results seen in organometallic compounds. Some of the ethylene desorbs around 200 K but most of it remains on the surface and reacts to produce gaseous ethane and chemisorbed ethylidyne. Ethylidyne then decomposes starting at 400 K, until total dehydrogenation around 700 K. Hydrogen desorption is detected as product from each step in the surface reactions.

The adsorption and decomposition of BIS(Benzene)chromium on Ni(100)

We have studied the bis(benzene) chromium/Ni(100) chenusorption system using TPD, AES, XPS, UPS, and exchange of isotopically labeled molecules. Low coverages of the metal complex are completely decomposed in TPD to produce C (a), Cr (a), and H 2(g). At higher coverages, molecular benzene desorption is observed at ≅ 480 K, and, at still higher coverages, multilayer and monolayer molecular bis(benzene)chromium desorption occur at ≅ 250 and 300 K, respectively. Post-dosed C 6D 6 exchanges with C 6H 6 into the monolayer desorption, indicating along with XPS evidence that monolayer bis(benzene)chromium undergoes ligand separation below 300 K. Uncomplexed benzene desorbs at ≅ 160 K from multilayer bis(benzene) chromium; explanations for this are discussed. UPS of monolayer and multilayer benzene and bis(benzene)chromium are compared.

Trimethylgallium Decomposition on a Heated Si(100) Substrate

AbstractTrimethylgallium (TMGa) is an important source gas for metal-organic chemical vapor deposition (MOCVD) and metal-organic molecular beam epitaxy (MOMBE). In the last three years, workers in three different laboratories have examined the pyrolysis of trimethylgallium (TMGa) on heated substrates. However, the mechanism of the decomposition process is still not understood. In this paper we consider the mechanism of the TMGa decomposition on a hot Si(100) substrate in a molecular beam system. It is found that at TMGa doses of 1×1015 molecules/cm2 or less, the decomposition process in the beam system is the same as that reported by Lee et al., Surface Sci. 216, 173 (1989) in studies where TMGa was dosed at low temperature and then the substrate was heated. However, at higher TMGa doses, the mechanism in the molecular beam system is different than the ones reported previously. It is possible to transiently populate a weakly bound state which we assign to TMGa adsorption in the second monolayer. If me sample is below 400 K, the TMGa in the second monolayer desorbs. However, at higher temperatures, the TMGa in the second monolayer can react to form a CHx group (x=3 or 4) and dimethylgallium. The dimethylgallium desorbs. However, some of the CHx groups remain bound to the substrate leading to extra carbon incorporation. There also is evidence for formation of traces of monomethylgallium above 500 K and an indication of methyl radical desorption above 650 K. Thus it seems that at high doses, the decomposition of TMGa on a hot substrate is somewhat different than the decomposition of a monolayer of TMGa which has been adsorbed at low temperatures and then heated. These results explain some of the apparent discrepancies in the literature.

Adsorption and thermal decomposition of [Rh(CO) 2Cl] 2 on oxidized Al(100)

The adsorption and decomposition of [Rh(CO) 2Cl] 2 on oxidized Al(100) was studied with XPS, TPD, and AES. Al(100) was oxidized with 1700 L of O 2 at 200°C to form an 18 Å amorphous Al 2O 3 overlayer. [Rh(CO) 2Cl] 2 adsorbed molecularly on oxidized Al(100) at - 165°C to form a multilayer deposit. Multilayer [Rh(CO) 2Cl] 2 desorbed at - 22°C with zero-order desorption kinetics and no evidence of a monolayer desorption peak. Decomposition of monolayer [Rh(CO) 2Cl] 2 was indicated by CO evolution at 100°C. The decomposition process deposited 5×10 14 Rh atoms per cm 2 on the Al 2O 3 substrate. CO and H 2 chemisorption measurements indicate the formation of 120 Å Rh particles at 350°C. XPS data confirm the formation of zero valent Rh particles on the oxidized Al(100) substrate after decomposition.

Thermostability and photodesorption of Langmuir-Blodgett films

We report on thermodesorption and photodesorption studies of cadmium and magnesium arachidate Langmuir-Blodgett films using a newly developed optical reflection technique and Nomarsky light microscopy. The mechanism of thermodesorption can be described as a process where only those parts of a monolayer desorb that are exposed to air in the normal direction. However, molecules in the inner monolayers can also be removed by photodesorption. This is understood within a model of defect-mediated desorption where the action of light is twofold: it creates defects and provides energy to overcome binding. The model explains the sigmoidal desorption kinetics and the independence of this kinetics on film thickness and is supported by the observation of grain structure during the desorption process. The latter indicates that crater-like defects are generated throughout the entire depth of the film.

Surface radioactivity measurements of monomolecular films

In the last several years there has been a resurgence of interest in surface films at the air/water interface. The use of radioactive tracers in interfacial studies is attractive due to the sensitivity to detect small quantities of matter. The radiotracer method, therefore, has been utilized in various surface studies at the air/water interface including adsorption from solution [ 1,2], surface chemical reactions [ 31, monolayer desorption [ 4,5], surface diffusion [ 4-81, and surface concentration of insoluble monolayers [ 91. In this communication, we report simultaneous measurements of film loss from a sparingly soluble monolayer by radiotracer and Wilhelmy plate methods. Our findings indicate that greater caution should be exercised in the interpretation of surface radiotracer measurements of monomolecular films because of a previously unrecognized mass transport process, when a Geiger-Miiller (G-M) tube is employed, due to a possible electric field effect. Myristic [ l-‘*Cl acid (Rosechem Products, 59 mCi mmol-l) was deposited from a purified n-hexane spreading solution onto a 0.01 N HCl solution at 20-21 ‘C. The experimental apparatus is described in detail elsewhere [ 81. Compression barriers were not used in order to eliminate any possible film leakage past barriers. The monolayer was spread to an initial surface pressure of 5.5 mN m-‘, and then the surface pressure was measured as a function of time at constant area by the Wilhelmy plate method. Experimentally measured surface pressure-specific surface area (l7-A ) isotherms were then used to obtain the surface concentration dependence with time. The results were evaluated by plotting the ratio r/r, versus &, where r is the surface concentration of the myristic acid molecules, r,, is the initial surface concentration, and t is time. Simultaneously, the beta radiation emanating from the radioactive monolayer was measured by an end-window G-M tube ( Amperex 18526) and decade scaler, and the ratio of the surface concentrations, r/r,, is equal to the ratio of the monolayer activities after subtraction of the background count. Figure 1 shows the film loss measurements using the Wilhelmy plate and

Desorption of spread monolayers of [ 14C] methylated and cold BSA from the air—aqueous interface

Compression—expansion cycles on spread monolayers of BSA and [ 14C] methylated BSA at the air/phosphate buffer interface at 20°C showed that irreversible losses in area occurred after films were held at surface pressures greater than 18 mN m −1, although the compressibility was unchanged. The rates of desorption of BSA and of a 2:1 mixture with radiolabelled BSA were similar at ∏= 21, 24 and 30 mN m −1, and increased with increasing surface pressure. The surface concentration of protein obtained after successive cycling, from the activity of the labelled monolayer transferred onto plates by the Langmuir—Blodgett technique, was constant within experimental error, indicating that area losses were caused by desorption. Activity measurements of aliquots of the substrate confirmed desorption into the subphase. The calculated mass of protein in the subphase was greater than calculated for the decrease in area at 10 mN m −1; it was concluded that at least 3–5% of protein was lost to the subphase although an accepted spreading technique was used.

Desorption of benzophenone oxime monolayers at the air/water interface

The interfacial behavior of 2-hydroxy-5-nonylbenzophenone oxime (HBPO) at the air/water interface has been investigated with Langmuir trough techniques. The dissolution kinetics of HBPO followed a diffusion-controlled model from which the aqueous solubility of HBPO was estimated. Because of deprotonation of the hydroxyoxime group, the aqueous solubility of HBPO was pH dependent. At pH 2.0, the solubility was 0.1 μmole dm −3 and increased to 4.0 μmole dm −3 at pH 12. From the influence of pH on the dissolution rate, the surface p K a for HBPO was estimated to be 11.5. At surface pressures greater than the equilibrium spreading pressure of HBPO (23 mN m −1), simultaneous dissolution and monolayer collapse were observed.

Laser desorption mass spectrometry of surface-adsorbed molecules

The role of the electronic-plasma-resonance absorption of surface microstructure in the visible and UV pulsed laser desorption of adsorbates on a silver surface is examined. It is shown that the surface microstructure aids in the absorption of a significant fraction of the laser radiation and can lead to a relatively gentle thermal desorption of molecular monolayers absorbed on the metal. This substantially increases the sensitivity and selectivity of time-of-flight mass spectrometry in the analysis of adsorbates on the metal surface.

Electrochemical surface science: The study of monolayers of ad-atoms and solvent molecules at charged metal interfaces

In parallel with the spectacular advances that have been made in recent years in the field of metal/vacuum surface science, there have been corresponding interesting developments of a complementary kind in electrochemical surface science. This article describes recent advances in the latter field, and their experimental and theoretical bases. However, the molecular situation at electrode interfaces is very complex compared with that at a metal/vacuum interface, owing to adsorption and orientation of solvent molecules, and the presence of solvated ions. Hence these topics must be considered as well as the surface science of the metal electrode itself. In comparison with metal/vacuum surface science, electrochemical studies on metal interfaces can utilize the electrode potential as an additional independent variable and thus control surface charge density, coverage by Faradaically deposited ad-atoms, co-adsorption of cations or anions and solvent dipole orientation. Modulations of electrode potential in time allow the kinetic relaxation characteristics of monolayers to be followed, and the potential-modulated reflectivity and spectral absorption of species in the interphase to be studied. Electrochemically controllable sub-monolayer coverages of, e.g., metal ad-atoms have interesting catalytic effects on other continuous electrochemical reactions, such as oxygen reduction or formic acid oxidation. Experiments based on electrochemical potential pulse and sweep, and current pulse methods enable chemisorption of ad-atom monolayers to be studied in fine detail. Multiple state chemisorption in monolayers is commonly observed, even on single crystal surfaces, and fine resolution of the states can be achieved by electrochemical techniques. The kinetics of monolayer deposition and desorption can also be followed, as well as the dynamics of reconstruction processes e.g. in 0 monolayers as a function of time and potential. The electrochemical methods allow direct determination of the differential isotherm for adsorption as well as the usual integral one. The former gives details of multiple state adsorption, and provides information on interaction effects in the monolayer and kinetics of its formation. Ionicity of ad-species can also be evaluated. Recent advances in combining electrochemical systems in situ with high-vacuum LEED, Auger and ESCA apparatus are described. Since electrochemical surface science has necessarily to be able to study processes at the interfaces of metals in contact with liquid solutions, an important area of the field has been the treatment and evaluation of solvent dipole adsorption and orientation as a function of electrode potential or surface charge density.