RAIRS investigations on the orientation and intermolecular interactions of adenine on Cu(1 1 0)

The adsorption of the DNA base adenine (C₅N₅H₅) on Cu(1 1 0) has been investigated as a function of coverage and temperature using reflection absorption infrared spectroscopy. These data provide important information on the nature of the local adsorbed complex and the intermolecular lateral interactions that come into play at high coverages. The RAIR spectra are consistent with an adsorption geometry in which the molecular ring is substantially tilted from the surface plane and in which the two amino hydrogens are equidistant from the surface, thus rationalising the appearance of a very strong βNH₂ scissoring mode, along with the activity of in-plane vibrational modes and the observation of the symmetric ν_symNH₂ stretch, but not the asymmetric ν_asymNH₂ stretch. In addition, coordination to the metal surface is proposed to occur at the N(9) position with a possible additional interaction through the N(3) position, both of which are the favoured coordination points in the metal complexes of adenine. This is a strong interaction and leads to a highly stable adsorbed layer. Our data also provide the first direct evidence of hydrogen bonding in the adlayer as coverage is increased, attributed to interactions between the amino group of one molecule and the N(1) and N(7) positions of a neighbouring species. When adsorption is carried out at room temperature, a very heterogeneous adlayer is created in which a diversity of molecular aggregates co-exist. However, upon annealing, a more ordered hydrogen bonded adlayer is formed in which one type of hydrogen bonding assembly is preferred. Finally, we propose that the hydrogen bonded assemblies created at a surface probably involve bent hydrogen bonds which arise from a compromise between the strong molecule-metal interactions that orientate the molecule and weaker lateral hydrogen bonding interactions that dictate the two-dimensional architecture.     

Conductance histograms and conducting channels in atomic-scale metallic contacts

Properties of the conductance and transport channels of atomic-scale contacts are discussed within the free electron model. In agreement with recent experiments, we find that, owing to the contribution of evanescent channels, even when the conductance is close to an integer number N of conductance quanta more than N channels may contribute significantly to the current. We show that the observation of peaks at integer multiples of G₀ in conductance histograms is not a signature of conductance quantization of individual contacts. However, the observed peaks can still be associated to the quantum nature of electron transport in metallic contacts.     

Lattice-controlled electron transport characteristics in quantized surface layers at low temperature

A theory of intravalley acoustic scattering of the carriers in a non-degenerate two-dimensional electron gas is developed here under the condition of low lattice temperature when the assumptions of the well-known traditional theory are not valid. The scattering rates thus obtained are then used to estimate the zero-field mobility characteristics in an n-channel Si inversion layer. It is found that the finite energy of the phonons makes the energy and lattice temperature dependence of the scattering rate, and consequently the lattice temperature dependence of the mobility, significantly different from what follows, in the light of traditional theory which assumes equipartition law for the phonon distribution and neglects the phonon energy in comparison to the carrier energy.     

Probing the properties of the (1 1 1) and (1 0 0) surfaces of LaB6 through infrared spectroscopy of adsorbed CO

The adsorption of carbon monoxide on the LaB₆(1 0 0) and LaB₆(1 1 1) surfaces was studied experimentally with the techniques of reflection absorption infrared spectroscopy and X-ray photoelectron spectroscopy. The interaction of CO with the two surfaces was also studied with density functional theory. Both surfaces adsorb CO molecularly at low temperatures but in markedly different forms. On the LaB₆(1 1 1) surface CO initially adsorbs at 90 K in a form that yields a CO stretching mode at 1502-1512 cm⁻¹. With gentle annealing to 120 K, the CO switches to a bonding environment characterized by multiple CO stretch values from 1980 to 2080 cm⁻¹, assigned to one, two, or three CO molecules terminally bonded to the B atoms of a triangular B₃ unit at the (1 1 1) surface. In contrast, on the LaB₆(1 0 0) surface only a single CO stretch is observed at 2094 cm⁻¹, which is assigned to an atop CO molecule bonded to a La atom. The maximum intensity of the CO stretch vibration on the (1 0 0) surface is higher than on the (1 1 1) surface by a factor of 5. This difference is related to the different orientations of the CO molecules on the two surfaces and to reduced screening of the CO dynamic dipole moment on the (1 0 0) surface, where the bonding occurs further from the surface plane. On LaB₆(1 0 0), XPS measurements indicate that CO dissociates on the surface at temperatures above 400 K.     

IR spectroscopy of adsorbed dinitrogen: a sensitive probe of defect sites on Pt(111)

Fourier transform infrared reflection absorption spectroscopy (FT-IRAS) has been used to probe the non-dissociative adsorption of N₂ on an atomically clean Pt(111) single crystal. In contradiction to a previous IRAS study of nitrogen adsorption on a Pt(111) foil at 120 K, no nitrogen infrared (IR) band was observed on a fully annealed Pt(111) surface at 90 K. Following Ar⁺ ion bombardment, adsorption of nitrogen at 90 K produces an intense IR band at 2222 cm⁻¹ attributed to the N---N stretching mode of molecular nitrogen adsorbed on defect sites produced by ion bombardment. Annealing the Ar⁺ ion sputtered surface to a temperature above 750 K completely suppresses the adsorption of nitrogen at 90 K. Based on these and other results, we postulate that nitrogen adsorbs at 90 K mainly on monovacancies on platinum. We suggest that this specific adsorption occurs by sigma donation from nitrogen to the base of monovacancy sites which possess a low d-electron density compared to surface Pt atoms.     

Reactivity of 3-hexyne on oxygen modified Ru(0 0 1) surfaces: Observation of oxametallacycles by RAIRS

The chemical behaviour of 3-hexyne on oxygen modified Ru(0 0 1) surfaces has been analysed under ultrahigh-vacuum, using reflection-absorption infrared spectroscopy (RAIRS). The effects of oxygen coverage, 3-hexyne exposure and adsorption temperature were studied. Two modified Ru(0 0 1) surfaces were prepared: Ru(0 0 1)-(2 × 2)-O and Ru(0 0 1)-(2 × 1)-O that correspond to oxygen coverages (θ_O) of 0.25 and 0.5 ML, respectively. The striking result is the direct bonding to an O atom when the modified surfaces are exposed to a very low dose (0.2 L) of 3-hexyne at low temperature (100 K). For θ_O = 0.25 ML, an unsaturated oxametallacycle [Ru-O-C(C₂H₅)C(C₂H₅)-Ru] is proposed, identified by RAIRS for the first time, through the νCC and νCO modes. Further decomposition at 110 K yields smaller oxygenated intermediates, such as acetyl [μ₃-η²(C,O)-CH₃CO], co-adsorbed with a small amount of carbon monoxide and non-dissociated species. The temperature at which a fraction of molecules undergoes complete C-C and C-H bond breaking is thus much lower than on clean Ru(0 0 1). The ultimate decomposition product observed by RAIRS at 220 K is methylidyne [CH]. Another key observation was that the adsorption temperature is not determinant of the reaction route, contrarily to what occurs on clean Ru(0 0 1): even when 3- hexyne strikes the surface at a rather high temperature (220 K), the multiple bond does not break completely. For θ_O = 0.5 ML, a saturated oxametallacycle [Ru-O-CH(C₂H₅)-CH(C₂H₅)-Ru] is also proposed at 100 K, identified by the ν_asO-C-C (at 1043 cm⁻¹) and ν_sO-C-C (at 897 cm⁻¹) modes, showing that some decomposition with C-H bond breaking occurs. For this oxygen coverage, the reaction temperatures are lower, and the intermediate surface species are less stable.     

The surface science of xerography

Over the past four decades xerography, the dry ink marking process developed by the photocopy industry, has grown from nothing into a $170 billion industry worldwide. This amazing commercial success is due to the fact that during this period, xerographic technology experienced constant and often-dramatic improvement created by sustained industrywide research and development. Indeed, the development of the xerographic copying and printing industry is one of the great applied surface science successes of all time. In this article we outline the story of the advances in xerographic technology during the past four decades, describe the profound dependence on these advances of the control of surface and interface properties of increasingly sophisticated multi-component materials systems, and indicate the potential impact on the industry of the continuing development of the surface and interface science of the multi-component materials packages used in xerographic technology.     

A DFT investigation of potential energy surface and vibrational properties of hydrogen adsorbed on the Rh(1 1 1) surface

Vibrational properties of hydrogen on the Rh(1 1 1) surface have been investigated theoretically. The potential energy surface for this system has been calculated within the density functional theory. The potential is found to be very anharmonic. The wave functions and their energies for the hydrogen motion on the potential energy surface (PES) have been calculated and assigned by using discrete variable representation. It was found that the vibrational wave function is localized at hollow site in the ground state for hydrogen on Rh(1 1 1). Higher excited states are of delocalized nature and mixed parallel and perpendicular character. Our results are in good agreement with the observed vibrational spectra of hydrogen on the Rh(1 1 1) surface.     

Chemical and electronic properties of sulfur-passivated InAs surfaces

Treatment with ammonium sulfide ((NH₄)₂S_x) solutions is used to produce model passivated InAs(0 0 1) surfaces with well-defined chemical and electronic properties. The passivation effectively removes oxides and contaminants, with minimal surface etching, and creates a covalently bonded sulfur layer with good short-term stability in ambient air and a variety of aqueous solutions, as characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and Hall measurements. The sulfur passivation also preserves the surface charge accumulation layer, increasing the associated downward band bending.     

Tailoring magnetism in artificially structured materials: the new frontier

The current standard of electronic devices and data storage media has reached a level such that magnetic materials have to be fabricated on a nanometer scale. In particular, the emerging concept of spintronics, which is based on fact that current carriers have not only charge but also spin, requires the assembling of nanometer-sized magnetic structures with desired magnetic properties. It is this background that motivates scientists and engineers to attempt to grow and characterize magnetic objects at smaller and smaller length scales, from 2D films and multilayers to 1D wires and eventually to 0D dots. In this article, some of the most significant progress in recent years in the effort of growing artificially structured magnetic materials are reviewed. The new structural and magnetic properties of these materials are discussed, with an emphasis on the correlation between structure and magnetism, which also serves as guidance for improving their magnetic properties. The emerging emphasis is on converting the existing knowledge into growing and studying low-dimensional complex materials, which promise to have considerably higher “tuning” ability for desired properties.     

Highly sintered nickel oxide: surface morphology and FTIR investigation of CO adsorbed at low temperature

The monolayer of CO molecules adsorbed at low temperature on highly sintered nickel oxide, gives rise to a very symmetric IR absorption band at 2136 cm⁻¹ (θ_max) with a full-width at half-maximum (FWHM) of 3.7 cm⁻¹. This band shifts to higher frequenc upon decreasing the coverage, reaching the 2152 cm⁻¹ value for θ→0. The observed shift is due to changes in the lateral interactions (dynamic and static) among the adsorbed molecules. The observed spectral simplicity implies that most of the adsorbed CO molecules occupy crystallographically identical sites with a similar environment. Moreover, the remarkably small half-width indicates that inhomogeneous broadening effects, due to surface defects, are very small and that NiO microcrystals behave as single crystals. The morphology of microcrystals has been studied by SEM, AFM and HRTEM techniques: it was concluded that the surface termination of the sample is mainly represented by the (100) and (111) faces.     

Electronic transport at semiconductor surfaces––from point-contact transistor to micro-four-point probes

The electrical properties of semiconductor surfaces have played a decisive role in one of the most important discoveries of the last century, transistors. In the 1940s, the concept of surface states––new electron energy levels characteristic of the surface atoms––was instrumental in the fabrication of the first point-contact transistors, and led to the successful fabrication of field-effect transistors. However, to this day, one property of semiconductor surface states remains poorly understood, both theoretically and experimentally. That is the conduction of electrons or holes directly through the surface states. Since these states are restricted to a region only a few atom layers thick at a crystal surface, any signal from them might be swamped by conduction through the underlying bulk semiconductor crystal, as well as greatly perturbed by steps and other defects at the surface. Yet recent results show that this type of conduction is measurable using new types of experimental probes, such as the multi-tip scanning tunnelling microscope and the micro-four-point probe. The resulting electronic transport properties are intriguing, and suggest that semiconductor surfaces should be considered in their own right as a new class of electronic nanomaterials because the surface states have their own characters different from the underlying bulk states. As microelectronic devices shrink even further, and surface-to-volume ratios increase, surfaces will play an increasingly important role. These new nanomaterials could be crucial in the design of electronic devices in the coming decades, and also could become a platform for studying the physics of a new family of low-dimensional electron systems on nanometre scales.     

A RAIRS study of methoxy and ethoxy on oxidised Cu(100)

The adsorption and reaction of methanol and ethanol on a preoxidised Cu(100) surface was studied with Reflection-Absorption Infra-Red Spectroscopy (RAIRS). Both alcohols reacted with the modified surface well below room temperature, undergoing OH bond scission to form alkoxide species. The alkoxides were stable up to 340–360 K at which stage they desorbed as aldehydes. Vibrational assignments of the observed modes were made with reference to the RAIRS spectra of the alcohols and their deuterium substituted analogues on Cu(100) together with literature values for the gaseous, liquid and solid alcohols. Application of the metal surface selection rule to the alkoxide spectra indicate that in both cases the C---O bond lies perpendicular to the surface. The methoxy species is therefore assigned to an upright C_3v configuration. Oxygen precoverage is found to govern the amount of alkoxide formation which passes through a maximum at a precoverage of about 150 L.     

The IR spectra of Mg5C(CO) complexes on the (0 0 1) surfaces of polycrystalline and single crystal MgO

The FTIR spectroscopy of carbon monoxide adsorbed on polycrystalline MgO smoke has been investigated as a function of the CO equilibrium pressure at constant temperature (60 K) (optical isotherm) and of the temperature (in the 300–60 K range) at constant CO pressure (optical isobar). In both cases the spectra fully reproduce those of CO adsorbed on the (0 0 1) surface of UHV cleaved single crystals [Heidberg et al., Surf. Sci. 331–333 (1995) 1467]. This result, never attained in previous investigations on dispersed MgO, contribute to bridging the gap which is commonly supposed to exist between surface science and the study of “real” (defective) systems. Depending on the surface coverage θ the main spectral features due to the CO/MgO smoke interaction are a single band shifting from 2157.5 (at θ→0) to 2150.2 cm⁻¹ (at θ=1/4) or a triplet, at 2151.5, 2137.2 and 2132.4 cm⁻¹ (at θ>1/4). These manifestations are due to the ν(CO) modes of Mg_5C²⁺· · · CO adducts formed on the (0 0 1) terminations of the cubic MgO smoke microcrystals. The formation of the CO monolayer is occurring in two different phases: (i) a first phase with CO oscillators perpendicularly oriented to the surface (2157–2150 cm⁻¹) and (ii) a second phase constituted by an array of coexisting perpendicular and tilted species (triplet at 2151.5, 2137.2 and 2132.4 cm⁻¹). A much weaker feature at 2167.5–2164 cm⁻¹ is assigned to Mg_4C²⁺· · · CO adducts at the edges of the microcrystals. The heat of adsorption of the perpendicular Mg_5C²⁺· · · CO complex in the first phase has been estimated from the optical isobar and results to be 11 kJ mol⁻¹.     

An infrared study of the chemistry of methyl species on Pt(111) formed by the decomposition of dimethylmercury

The chemistry of dimethyl mercury on a Pt(111) single crystal surface has been investigated by reflection-absorption infrared spectroscopy (RAIRS). Dimethyl mercury appears to be highly reactive on Pt(111) and readily decomposes on the surface at temperatures of 100 K and above. Adsorption at 100 K initially occurs in a dissociative manner to produce CH₃ and CH₃Hg species on the surface, both of which are identified as having C_3v local symmetry. At higher exposures, molecular adsorption dominates with the Hg---C---Hg axis initially oriented parallel to the surface. This preferred orientation, however, does not persist into the multilayer. Thermal treatment of the surface layer results in multilayer desorption between 130 and 135 K, and no parent molecular species are observed beyond 160 K. Adsorption at 200 and 300 K produces an overlayer consisting primarily of CH₃Hg species, which are thermally stable to about 350 K. Subsequent heating to 400 K results in the formation of ethylidyne species which are characterised by RAIRS. Adsorption at 400 K results in the direct formation of an ethylidyne layer estimated to be about 85% of saturated coverage.     

Dynamical phenomena including many body effects at metal surfaces

In this contribution, we give a brief survey of some elementary many body processes observable at surfaces. We begin the survey by discussing how the electron ground state would behave and look like in real space at surfaces. Next, we discuss how these electrons behave when they are perturbed by external fields characterized by ultrashort time scales. We follow this with a discussion of how the dynamics of electrons would then affect the motion of adsorbates on surfaces. Finally, we cite some possible technological applications utilizing this knowledge. We also discuss possible trends or directions of scientific research in this century.     

Photoexcited processes at metal and alloy surfaces: electronic structure and adsorption site

Photoexcited processes of NO and CO at photon energies ranging from 2.3 to 6.4 eV are investigated on Pt(111), Ni(111) and Pt(111)---Ge surface alloys by reflection-absorption infrared spectroscopy and resonance-enhanced multiphoton ionization. The branching between three competitive processes of desorption, recapture and dissociation upon laser irradiation is dramatically changed on the three surfaces. On Pt(111), NO is either photodesorbed or photodissociated depending on the coverage, while NO is exclusively photodissociated on Ni(111). UV-photon irradiation of NO on Pt(111)---Ge, on the other hand, induces only desorption of NO. Desorption of CO bound at the on-top site of Pt(111) is induced by laser irradiation. The electronic mechanism for photodesorption and competitive branching is discussed in terms of the electronic structure of the substrate and the adsorbate.     

The interaction of D2O with Zr(0001) at 80 K

The adsorption of D₂O on Zr(0001) at 80 K and its subsequent reactions at higher temperatures have been studied by thermal desorption spectroscopy (TDS), work-function measurements (Δф), nuclear reaction analysis (NRA), LEED, infrared reflection spectroscopy (FTIR-RAS), Auger electron spectroscopy (AES), and static secondary ion mass spectroscopy (SSIMS). D₂O adsorption on Zr(0001) at 80 K is accompanied by a Δф of −1.33 eV. The adsorbed D₂O can be characterized into three layers by TDS: a chemisorbed layer (up to 0.23 ML), a second adsorbed layer, and an ice layer. The chemisorbed D₂O dissociates into OD_ad and D_ad at 80 K (possibly also into O_ad) and no desorption products could be detected, implying that the reaction products dissolved into the zirconium at temperatures appropriate for each component. The ice layer and most of the second adsorbed layer desorb as molecular water during heating. The water adsorbed at 80 K did not form any long-range ordered structure, but a (2 × 2) LEED pattern that was formed by heating the sample to temperatures above 430 K is believed due to be an ordered oxygen superstructure.