Extrinsic Precursor Research Articles

Binding and ordering of large organic molecules on an anisotropic metal surface: PVBA on Pd(110)

The adsorption and ordering characteristics of 4- trans-2-(pyrid-4-yl-vinyl) benzoic acid (PVBA) on a Pd(110) surface were investigated by variable temperature scanning tunneling microscopy. PVBA is a rigid organic molecule ( m=225 amu) designed for nonlinear optics. The molecules adsorb flat on the surface and bind diagonally to three neighbouring Pd-rows, which configuration is maintained at all submonolayer coverages. With the completion of the first monolayer a parquet pattern evolves, resulting from the fixed bonding geometry and space limitations. Monte Carlo simulations of the film growth confirm that the monolayer ordering is stochastic in nature, even if an extrinsic precursor mechanism is operative in the adsorption. The lateral interactions between adsorbed molecules and Pd steps or adatoms at the surface are weak.

Does the rotational state of a molecule influence trapping in a precursor? An investigation of N 2/W(100), CO/FeSi(100) and O 2/Ni(111)

We have used seeded nozzle beams to obtain molecules of identical translational energy but quite different rotational energy to investigate the adsorption dynamics for precursor adsorption in the systems N 2/W(100), CO/FeSi(100) and O 2/Ni(111). The result of the investigation is unambiguous: there is no discernible influence of the rotational state on the sticking coefficient. This is true for an intrinsic as well as for an extrinsic precursor. This leads to the conclusion that rotational energy does not have to be accommodated immediately upon the first impact to facilitate trapping. Apparently the molecule can rotate more or less freely and the rotational energy can be accommodated subsequently to trapping.

A molecular beam study of ammonia adsorption on Pt{100}

Ammonia adsorption has been investigated on the hex-R and (1 × 1) faces of the Pt{100} crystal. It is found that adsorption on the hex-R surface does not lift the surface reconstruction at 150 K, and that no dissociation of NH 3 occurs. In contrast, the (1 × 1) surface exhibits an ability to dissociate NH 3. The initial sticking probability is reported as a function of incident energy and surface temperature, and the results indicate that adsorption is direct and non-activated on the clean hex-R and (1 × 1) surfaces. Sticking probability versus NH 3 coverage measurements reveal that at finite coverages, on both surfaces, there is an additional adsorption pathway involving an extrinsic precursor.

A molecular beam investigation into the dynamics and kinetics of dissociative O2 adsorption on Pt{100}-(1×1)

The dissociative chemisorption of oxygen on Pt{100}-(1×1) has been investigated using supersonic molecular beams with incident translational energies between 0.04 and 0.90 eV over the surface temperature range 300 to 380 K. The adsorption process on the Pt{100}-(1×1) surface is found to differ from that on the reconstructed Pt{100}-hex-R surface in both magnitude and mechanism. The initial dissociative sticking probability is ≳0.2 on the (1×1) surface for all beam energies and surface temperatures investigated, whereas on the reconstructed Pt{100}-hex-R surface, under corresponding conditions, the initial sticking probability never exceeds 0.003. The initial sticking probability on the (1×1) surface at first decreases with increasing incident energy and then increases as the incident energy exceeds 0.1 eV, in a manner typical of an adsorption process that is precursor mediated at low incident energies and direct at higher incident energies. Precursor-mediated adsorption at low energies is also indicated by scattering measurements and by the angular variation of the initial sticking probability. At high incident energies the angular variation of the initial sticking probability indicates that adsorption is an activated process. Sticking probability measurements as a function of coverage indicate no extrinsic precursor involvement over the energy range studied; repulsive lateral interactions exist between adsorbed oxygen atoms. Thermal energy atom scattering measurements show an absence of island formation upon adsorption, in contrast to adsorption on the hex-R face.

Chemisorption of Carbon Monoxide on the Iridium(111) Surface: In Situ Studies of Adsorption and Desorption Kinetics via Vibrational Spectroscopy

The IR absorption peak position of the CO intramolecular stretching vibration, measured with Fourier transform infrared reflection−absorption spectroscopy, has been employed for the first time as a nonintrusive coverage probe to study the adsorption of CO on Ir(111). The observed temperature and coverage dependence of the adsorption probability is consistent with an extrinsic precursor model. Isobars were obtained under steady-state conditions for pressures ranging from 10-9 to 10-5 Torr. Isosteres were constructed from these data and used to determine both isosteric heats of adsorption and the adsorbate entropy as functions of coverage. The isosteric heat of adsorption decreased from 44 ± 2 kcal/mol at 0.05 ML to 37 ± 1 kcal/mol at 0.6 ML. A pronounced decrease in the binding energy at 0.35 ML coincides with the weakening of the ( × )R30° LEED pattern and is attributed to near-neighbor CO repulsions. The entropy of the adsorbate layer indicates two ordering regions which correspond to the formation of the ( × )R30° and (2 × 2 )R30° overlayer structures.

Theoretical and numerical studies of chemisorption on a line with precursor layer diffusion.

Models for surface adsorption with mobile precursor states have been studied since the work of Kisliuk to describe gas deposition on a metal surface. As the adsorbate coverage increases, these systems can develop nontrivial correlations that are induced by the diffusion mediated deposition mechanism. Our purpose in this paper is to investigate a model with precursors that is simple enough to be mathematically solved, at least approximately, without completely neglecting the positional correlations between the chemisorbed sites. We consider a model for random deposition of monomers on a line with extrinsic precursor states. In a numeric simulation, we study various quantities describing the evolution of the island structure. We write down the exact kinetic equations for this process and solve these equations within a simple, self-consistent theory that incorporates pair correlations. The results for the correlations, island density number, average island size, and probabilities of island nucleation, growth, and coagulation show good agremeent with the simulation data. (c) 1995 The American Physical Society

The effects of adsorbate island formation on extrinsic precursor kinetics

A simple Hamiltonian is developed describing extrinsic precursor kinetics for gas-surface adsorption systems with adsorbate island formation (i.e., clustering of adsorbate) that enables the explicit calculation of the dependence on adsorbate island area and island distribution of both the migration rate coefficient and the sticking coefficient. A model calculation is performed that illustrates the importance of island formation in determining the coverage and temperature dependences of the sticking coefficient in extrinsic precursor kinetics. Comparison of the functional forms of model and experimental results suggests that the observed sticking coefficient coverage dependence is due to the variation of overlayer structure (island formation) with coverage; the qualitative and quantitative shapes of these forms are governed by competition between desorption and migration.

Carbon-doped impurity induced layer disorder 0.98 μm lasers

We have fabricated high power carbon-doped InGaAs/AlGaAs lasers using an impurity-induced layer disordering process to define the active region. The advantage of carbon doping is that it exhibits significantly lower diffusivity compared to other p-type dopants, thereby avoiding displacement of the p-n junction, even at the high temperatures and long diffusion times required by the disordering process. Secondary ion mass spectrometry (SIMS) measurements before and after Si diffusion show the p-n junction position to be unchanged during processing. The carbon was introduced using CCl4 as an extrinsic precursor, giving improved control over doping levels and ternary growth conditions that is not available with intrinsic carbon doping. Thresholds of 20 mA and slope efficiencies of 0.44 mW/mA at 25 °C were obtained for lasers with cavity lengths of 500 μm and coated facets.

Dissociative chemisorption of oxygen on the Ru(001) surface: Spectroscopic identification of precursor intermediates at low surface temperatures

The adsorption of oxygen on the Ru(001) surface at 30 K has been investigated by high-resolution electron energy loss spectroscopy. Dissociative chemisorption occurs up to the saturation atomic coverage [ν⊥(Ru–O)=595 cm−1], after which a small amount of a molecularly chemisorbed peroxide species was isolated [ν(O–O)=790 cm−1]. After saturation of these atomic and molecularly chemisorbed states, physically adsorbed oxygen with a vibrational frequency close to the gas phase value of 1555 cm−1 was observed. The physically adsorbed oxygen was found to exhibit a resonance in the electron scattering cross section at an electron beam energy of about 5.5 eV. Heating the surface to 55 K leads to desorption of the physically adsorbed overlayer. The peroxide species, on the other hand, dissociates between 100 and 450 K. Although the initial precursors to dissociative chemisorption were not isolated, the peroxide identifies an intermediate precursor state, and the physically adsorbed oxygen identifies an extrinsic precursor state.

O 2 interaction with Pt{100}-hex-R0.7°: scattering, sticking and saturating

The interaction of oxygen with the Pt{100}-hex-R0.7° surface has been studied using supersonic molecular beams at incident translational energies from 0.06 to 0.9 eV and surface temperatures from 300 to 600 K. Scattering measurements show the existence of both intrinsic and extrinsic precursor states, and the trapping probability into these states is high at low incident energies. However, sticking probability measurements on the clean surface indicate that O 2 dissociative adsorption on Pt{100}-hex-R0.7° is a direct activated process, in contrast to that on Pt{100}-(1 × 1) or Pt{111}. Strong temperature enhancement of the initial sticking probability has been observed and accounted for partly by a dynamical barrier model. The sticking probability varies strongly with oxygen coverage, which is explained through computer simulations of island formation. The formation of small islands is demonstrated by TEAS measurements. Thermal desorption measurements show that, at high incident energies above 0.5 eV, new states are populated and higher coverages, up to a full monolayer, are reached.

Dissociative chemisorption of O 2 on Cu(110)

The initial sticking coefficient S 0 for the dissociative chemisorption of O 2 on Cu(110) has been measured as a function of translational energy, surface temperature and scattering azimuth using a molecular beam. Sticking is facile on this face, having an initial sticking coefficient S 0 = 0.2 at 0.05 eV and rising to S 0 = 0.8 at translational energies above 0.35 eV. The dissociation probability is independent of scattering azimuth and follows a normal (cos 2 θ i) energy scaling. At low beam energies the sticking coefficient shows a strong surface temperature dependence, S 0 rising to ∽ 0.6 at 100 K. For beam energies above ∽ 0.2 eV the sticking coefficient is independent of surface temperature. This is interpreted in terms of two competing mechanisms, a trapping-desorption channel which dominates at low translational energies and low surface temperature ( T s) and a direct, activated dissociative chemisorption channel which becomes important as the energy is increased above 50 meV. The trapping state is thought to be a weakly bound, physisorbed molecular state, while the weak temperature dependence at high beam energies indicates direct, activated dissociation with negligible kinetic competition due to desorption via an (unstable) molecularly chemisorbed state. The coverage dependence S(θ) changes from a linear dependence at high beam energy and surface temperature to a slower decrease with θ at low T s, characteristic of an extrinsic precursor state. The trapping channel can be modelled as a function of θ and T s using a simple kinetic model for the precursors, based on the kinetic parameters obtained for the intrinsic precursor state and assuming local ordering of the adsorbate, even at the lowest surface temperatures (100 K).

A theoretical study of the parameters affecting the kinetics of gas adsorption on solid surfaces

The nondissociative adsorption of gas phase molecules onto a spatially homogeneous square lattice was studied using a Monte Carlo computer simulation method which accounts for the existence of an extrinsic precursor state. The parameters affecting the adsorption kinetics were varied systematically in order to compare our simulations with a model for adsorption originally proposed by Kisliuk [J. Phys. Chem. Solids 3, 95 (1957)]. Our studies indicate that in the presence of precursor mobility, the sticking coefficients obtained from our simulations are consistently lower than those obtained using Kisliuk’s equation. Upon further investigation, we found two reasons for this discrepancy, namely, islanding of the adsorbates on the surface, and site revisiting in the precursor state. We found that some degree of islanding within the chemisorption layer occurs as a direct consequence of the nature of the precursor mediated adsorption process and that the extent of this clustering is controlled by the competition between mobility within the physisorbed layer and desorption; when mobility dominates, islanding formation is enhanced. Site revisiting within the precursor state was also found to affect the adsorption kinetics; we were able to derive here an analytical expression for the sticking coefficient that accounts for this factor and that correlates well with results from the corresponding simulation. Finally, we show that sticking coefficients are affected by surface geometry.

Hydrogen films at low temperatures

Hydrogen (deuterium) adsorption has been studied by the static skin effect method for W(110)H 2, D 2 and Mo(110)H 2, D 2 metal-hydrogen systems in the temperature range of 1.7–30 K. The findings indicate stabilization of mobile molecular complexes, so-called extrinsic precursor states, as well as their probable participation in the formation of two-dimensional ordered structures, Eden clusters.

A Monte Carlo simulation for the precursor model of adsorption on a spatially homogeneous surface

The kinetics for the non-dissociative adsorption of gas phase molecules on a spatially homogeneous square lattice was simulated using a Monte Carlo method which accounts for the existence of an extrinsic precursor state. We find the sticking coefficients obtained using our simulations to be lower than those calculated using the equation derived by Kisluik, and show that islanding of the adsorbates on the surface and site revisiting in the precursor state account for the observed discrepancies.

Molecular beam studies of adsorption dynamics

We have investigated the trapping dynamics of C1-C3 alkanes and Xe on Pt(111) using supersonic molecular beams and a direct technique to measure trapping probabilities. We have extended a one-dimensional model based on classical mechanics to include trapping and have found semiquantitative agreement with experimental results for the dependence of the initial trapping probability on incident translational energy at normal incidence. Our measurements of the initial trapping probability as a function of incident translational energy at normal incidence are in agreement with previous mean translational energy measurements for Xe and CH4 desorbing near the surface normal, in accordance with detailed balance. However, the angular dependence of the initial trapping probability shows deviations from normal energy scaling, demonstrating the importance of parallel momentum in the trapping process and the inadequacy of one-dimensional models. The dependence of the initial trapping probability of Xe on incident translational energy and angle is quite well fit by three-dimensional stochastic classical trajectory calculations utilizing a Morse potential. Angular distributions of the scattered molecules indicate that the trapping probability is not a sensitive function of surface temperature. The trapping probability increases with surface coverage in quantitative agreement with a modified Kisliuk model which incorporates enhanced trapping onto the monolayer. We have also used the direct technique to study trapping onto a saturated monolayer state to investigate the dynamics of extrinsic precursor adsorption and find that the initial trapping probability onto the monolayer is higher than on the clean surface. The initial trapping probability onto the monolayer scales with total energy, indicating a highly corrugated interaction potential.

The dynamics of precursor adsorption: ethane on Pt(111)

We have studied ethane trapping into a second-layer state on Pt(111) using supersonic molecular beam techniques to investigate the dynamics of extrinsic precursor adsorption. Initial trapping probabilities of ethane on an ethane covered Pt(111) surface were measured directly as a function of incident translational energy and incident angle at a surface temperature of 95 K. At all incident translational energies and angles the initial trapping probability into the second-layer state is higher than on a clean surface. In addition the initial trapping probability into the second layer decreases less with incident translational energy than the initial trapping probability onto the clean surface. In contrast to previous findings for non-dissociative weak adsorption on clean surfaces showing the initial trapping probability to increase with incident angle, the initial trapping probability into the second layer is independent of incident angle indicating “total” energy scaling. A dynamical corrugation of the adsorbed layer is postulated to rationalize this strong deviation from the “normal” energy scaling implicit in one-dimensional theories of trapping.

PH3 surface chemistry on Si(111)-(7×7): A study by Auger spectroscopy and electron stimulated desorption methods

The adsorption and decomposition of PH3 on Si(111)-(7×7) was investigated in ultrahigh vacuum by means of temperature programmed desorption, low-energy electron diffraction, Auger electron spectroscopy (AES), and electron stimulated desorption (ESD) methods. Phosphine adsorbs on Si(111)-(7×7) at T=120 K with an initial sticking coefficient of S0≂1 through a mobile (extrinsic) precursor state. Some PH3 dissociative adsorption at 120 K is observed. Thermal activation of the adsorbed species results in desorption of a molecular PH3 species up to 550 K. Further heating produces H2(g) desorption at T≂740 K and P2(g) desorption at T≂1010 K, thus indicating that PH3 decomposition has occurred. AES and ESD studies of the adsorbed species reveal that decomposition takes place by the breaking of PH bonds in PHx(a) to form SiH species on the surface for 120 K<T<700 K.

The activated adsorption of silane on nickel

The dissociative adsorption of silane (SiH 4) on Ni(100) was studied using Auger electron spectroscopy, thermal desorption mass spectroscopy, high resolution electron energy loss spectroscopy, and low-energy electron diffraction. The effects of varying the surface temperature (100–1200 K), the gas temperature (300–600 K), and deuteration (SiD 4 versus SiH 4) were examined. Adsorption occurs through an extrinsic precursor state with the reactive sticking coefficient ( S r) increasing approximately linearly with increasing surface temperature. Raising the gas temperature is also effective at increasing S r. We find no significant change in ( S r when SiD 4 is adsorbed indicating that motion of the central silicon atom is most likely involved in the transition state. At surface temperatures less than 250 K the stable species is a silyl (SiH 3) group. The dissociation of this species, the formation of Ni 2Si thin films, and the dissolution of silicon into bulk nickel are also discussed. Our results are compared to recent studies on the adsorption of CH 4 on nickel and silane on silicon.

The dynamics of NO scattering from NO- and CO-covered Ni(100)

The dynamics of NO scattering from NO- and CO-covered Ni(100) has been investigated using supersonic molecular beam techniques and laser induced fluorescence (LIF). Angular distributions for NO scattering from NO-covered Ni(100) at a surface temperature of 140 K indicate a gradual change from trapping-desorption to direct inelastic scattering with increasing incident translational energy, E trans from 90 to 1100 meV. The internal energy distributions of the scattered NO in various geometries as a function of incident translational energy are also indicative of scattering into two channels. Adsorption kinetics for NO on initially clean Ni(100) exhibit precursor behavior at incident energies up to about 300 meV consistent with trapping into an extrinsic precursor state. NO efficiently displaces CO from a CO-saturated Ni(100) surface at a temperature of T s = 140 K. Initial displacement probabilities d 0 for NO on CO-covered Ni(100) are nearly as large as initial sticking probabilities s 0 for NO on clean Ni(100) ( d 0 = 0.53, s 0=0.67 at E trans=90 meV). The decline in d 0 with increasing E trans suggests that the displacement proceeds through a trapped intermediate. A very high trapping probability for the covered surface is concluded and the role of efficient energy transfer of the kinetic energy of the incident molecule to the low energy, vibrational modes of adsorbed surface molecules is discussed.

Adsorption and dissociation of N 2 on rhenium single crystal surfaces

The interaction of N 2 with a rhenium polycrystalline foil, Re(0001) and Re(11¯20) single crystal surfaces was studied under ultrahigh vacuum conditions at the temperature range of 80–1800 K. Extrinsic precursor adsorption kinetics at 80 K on the two crystals imply that the binding energy of the second layer nitrogen molecules is only sligthly affected by the substrate's structure. Molecular nitrogen was found to adsorb on a single physisorption site on Re(0001) but at least two different binding energies were identified on Re(11¯20) and the polycrystalline foil. The sticking coefficient into the molecular adsorption sites is 0.90±0.05 on the two single crystal surfaces at 80 K. Dissociation of nitrogen occurs at all three surfaces the initial dissociative sticking probabilities ( S 0 ) measured at 218 K are (4±1)×10 −4 and (9±2)×10 −6 for the Re(11¯20) and Re(0001), respectively. The polycrystalline foil was found to be most effective for the dissociation of nitrogen with a probability of 0.25 at low coverage which decreases to 0.025 at saturation coverage. Study of the dissociative sticking probability as a function of crystal temperature implies the existence of a barrier for dissociative chemisorption of 6±1 kJ/mol and 14±2 kJ/mol on the Re(11¯20) and the Re(0001) single crystal surfaces, respectively. The reactivity for nitrogen dissociation of these surfaces is in agreement with high pressure ammonia synthesis study over the same surfaces.