2D Gas Phase Research Articles

Standard deviation of microscopy images used as indicator for growth stages

Photoelectron emission microscopy (PEEM) and low energy electron microscopy (LEEM) can easily distinguish between organic molecules adsorbed in crystallites or in the wetting layers as well as the bare metal substrate due to their different electronic properties. Already before (and during) the condensation of such solid phases (2D islands or 3D crystallites), there is a dilute 2D gas phase. Such a 2D gas phase consists of molecules, which are highly mobile and diffuse across the surface. The individual molecules are too small to be resolved in PEEM/LEEM images. Here, we discuss, how image features below and above the resolution limit of a PEEM/LEEM affect the mean electron yield and its (normalized) standard deviation. We support our findings with two experimental examples: the deposition of cobalt phthalocyanine (CoPc) on Ag(100) and of perfluoro-pentacene on Ag(110). Our results demonstrate, how a spatial and temporal analysis of image series can be used to obtain information about molecular phases, which cannot be directly resolved in microscopy images.

Growth and evolution of tetracyanoquinodimethane and potassium coadsorption phases on Ag(111)

Alkali-doping is a very efficient way of tuning the electronic properties of active molecular layers in (opto-) electronic devices based on organic semiconductors. In this context, we report on the phase formation and evolution of charge transfer salts formed by 7, 7, 8, 8-tetracyanoquinodimethane (TCNQ) in coadsorption with potassium on a Ag(111) surface. Based on an in-situ study using low energy electron microscopy and diffraction we identify the structural properties of four phases with different stoichiometries, and follow their growth and inter-phase transitions. We label these four phases α to δ, with increasing K content, the last two of which (γ and δ-phases) have not been previously reported. During TCNQ deposition on a K-precovered Ag(111) surface we find a superior stability of δ-phase islands compared to the γ-phase; continued TCNQ deposition leads to a direct transition from the δ to the β-phase when the K : TCNQ ratio corresponding to this phase regime is reached, with no intermediate γ-phase formation. When, instead, K is deposited on a surface precovered with large islands of the low density commensurate (LDC) TCNQ phase that are surrounded by a TCNQ 2D-gas, we observe two different scenarios: on the one hand, in the 2D-gas phase regions, very small α-phase islands are formed (close to the resolution limit of the microscope, 10–15 nm), which transform to β-phase islands of similar size with increasing K deposition. On the other hand, the large (micrometer-sized) TCNQ islands transform directly to similarly large single-domain β-phase islands, the formation of the intermediate α-phase being suppressed. This frustration of the LDC-to-α transition can be lifted by performing the experiment at elevated temperature. In this sense, the morphology of the pure TCNQ submonolayer is conserved during phase transitions.

STM studies of the bimolecular layer of CoPc and F16CuPc on Ag(100) with non-equal composition

The bimolecular layer composed of cobalt phthalocyanine (CoPc) and hexadecafluoro copper phthalocyanine (F16CuPc) deposited at room temperature on Ag(100) was investigated by scanning tunneling microscope. The coexistence of ordered bimolecular chessboard-like structure with 2D molecular gas phase is observed after deposition 0.7 ML of CoPc and 0.1 ML F16CuPc. The 2D gas phase contains mostly CoPc molecules. In the ordered domains, the CoPc:F16CuPc ratio is about 1.2, i.e. around five and a half times lower than the deposition ratio. The molecular ratio, higher than one, in the chessboard-like structure arises from the substitution of F16CuPc by cobalt compound. A slight increase (up to ca. 1.6) of the molecular ratio is observed close to the border of the ordered domain with CoPc rich 2D gas phase. Almost complete molecular monolayer was prepared by adding about 0.2 ML of CoPc. Thenceforth, the coexistence of chessboard-like bimolecular structure with monomolecular CoPc ordered domains is found. The addition of CoPc does not change the CoPc:F16CuPc ratio assembled in the bimolecular chessboard-like structure.

Real-time decay of fluorinated fullerene molecules on Cu(001) surface controlled by initial coverage

In this study, the evolution of C60F18 molecules on a Cu(001) surface was studied by means of scanning tunneling microscopy and density functional theory calculations. The results showed that fluorinated fullerenes (tortoise-shaped polar C60F18) decay on Cu(001) surfaces by a step-by-step detachment of F atoms from the C60 cage. The most favorable adsorption configuration was realized when the F atoms of C60F18 pointed towards the Cu surface and six F atoms were detached from it. The results also showed that a further decay of C60F12 molecules strongly depended on the initial C60F18 coverage. The detached F atoms initially formed a two-dimensional (2D) gas phase which then slowly transformed into F-induced surface structures. The degree of contact between the C60F12 molecules and the Cu(001) surface depended on the density of the 2D gas phase. Hence, the life-time of fluorinated fullerenes was determined by the density of the 2D gas phase, which was affected by the formation of new F-induced structures and the decay of C60F12 molecules.

Metastable phase of lead phthalocyanine films on graphite: Correlation between geometrical and electronic structures

The geometrical and electronic structures of a metastable phase of lead phthalocyanine (PbPc) films on graphite have been studied by combined use of low energy electron diffraction (LEED) and two-photon photoemission (2PPE) spectroscopy. In submonolayer (sub-ML) PbPc films on graphite, islands in a metastable phase are formed just after deposition, as we reported previously by use of photoelectron emission microscopy (PEEM) [I. Yamamoto, N. Matsuura, M. Mikamori, R. Yamamoto, T. Yamada, K. Miyakubo, N. Ueno, and T. Munakata, Surf. Sci. 602, 2232 (2008)]. On single crystalline graphite substrates, the metastable islands produce clearly discernible LEED spots. By comparing the unit cell with that of annealed 1 ML films, molecules in the metastable islands are standing upright with a molecular density 1.8 times higher than that in the well-ordered 1 ML films. The LEED spots for the sub-ML films disappear after annealing. The islands in the metastable phase are surrounded by areas of a two-dimensional (2D) gaslike phase composed of flat-lying molecules. The metastable islands melt into the 2D gas phase, consistent with the PEEM results. In 2PPE spectroscopy, the lowest unoccupied molecular orbital (LUMO) derived level of the metastable phase is clearly distinguishable from that of flat-lying molecules. By tracking the thermal annealing process of the films by 2PPE spectroscopy, we clarify the decay of the LUMO derived peak intensity, the work function shift, and the energy shifts of molecular states associated with the transition from the metastable phase to the 2D gas phase. With this, we demonstrate the complementary capabilities of LEED and 2PPE spectroscopy to probe phase transitions of organic films in a nondestructive manner.

Direct visualization of subcritical nuclei in molecular film growth

The growth and structure of 4,4'-biphenyldicarboxylic-acid (BDA) on Cu(001) at temperatures between 300 K and 400 K was studied by LEEM and μ-LEED. BDA is a linear molecule consisting of two phenyl rings with a carboxylic-acid group at opposite ends. During growth on Cu(001) the adsorbed BDA molecules form first a disordered 2D gas phase. Once this phase reaches a sufficiently large density, a crystalline phase nucleates, in which the molecules form a hydrogen-bonded 2D supramolecular network. By a careful analysis of the LEEM bright-field image intensity we can measure the density of the 2D gas phase, which is up to 40% of that in the crystalline phase. From the equilibrium densities at different temperatures we can construct the 2D phase diagram and extract the cohesive energy (0.35 eV). During the distinct nucleation period we can observe a fascinating phenomenon: sub-critical nuclei form, grow up to 4000 nm2in size and decay with lifetimes of several seconds. These sizes are considerably larger than what is usually seen in epitaxial growth and we explain this observation with the relatively weak intermolecular interactions.

Contribution in Semiconductor Industry of Surface Science -Clusters Observed for Adsorbate Coverages Close to the Saturation Coverage-

STM (Scanning Tunneling Microscopy) observation on structural changes of Si(111)-7×7 during K and Cs adsorption has been extended to high coverages up to the saturation coverage at room temperature. Up to the alkali-metal coverage of about 0.3 ML on Si(111)-7×7 surfaces, the ordered structures such as the 2D gas phase and the magic clusters known to turn into a disordered phase around 0.25 ML were well reproduced. For coverages beyond 0.3 ML, clusters (or islands) of identical size have been identified to appear and increase the number with coverage. K and Cs atoms are suggested to form 2D islands inside the half unit cell at coverage close to the saturation coverage.

Adsorption and two-dimensional phases of a large polar molecule: Sub-phthalocyanine on Ag(111)

The adsorption and the two-dimensional (2D) ordering of chloro[subphthalocyaninato]boron(III) (SubPc) on Ag(111) has been studied in detail by combined scanning tunneling microscopy and photoelectron spectroscopy at room temperature. SubPc is a polar, highly symmetric molecule, consisting of an extended aromatic system and a central B-Cl bond. When growing on Ag(111) an interesting phase behavior is observed for the first molecular layer of SubPc. At low coverage, below $\ensuremath{\approx}0.2$ monolayer (ML), a 2D lattice gas is present, whereas at medium coverage (on the order of 0.2--0.5 ML), 2D condensed molecular islands are observed in coexistence with the 2D lattice gas. In these condensed islands, the molecules assemble into a well-ordered honeycomb pattern. At higher coverage (approximately 0.5--0.9 ML) the molecules organize into a 2D hexagonal close-packed (hcp) pattern, in equilibrium with a dense 2D gas phase. In the honeycomb and in the hcp pattern, individual molecules are imaged with submolecular resolution, giving information on their orientation. For both the honeycomb and hcp patterns, islands with two different orientations of the superstructures with respect to the Ag(111) substrate are observed. In case of the honeycomb pattern, the two superstructures are enantiomorphic. The chirality of these layers originates in the loss of the symmetry of the metal surface upon adsorption of SubPc, while the molecules alone are intrinsically achiral. Based on different photoelectron spectroscopy experiments we conclude that the SubPc molecule is adsorbed on Ag(111) with its Cl atom towards the substrate and that the molecule remains intact. Finally, several aspects of the observed 2D condensed phases and the thermodynamic phase behavior are discussed with respect to the charge distribution and the adsorption physics and chemistry of the SubPc molecules.

Real-space observation of xenon adsorption and desorption kinetics on graphite (0 0 0 1) by photoemission electron microscopy

The growth and desorption of Xe monolayers on the basal plane of graphite has been investigated by real-space imaging using photoemission electron microscopy. Adsorption kinetics was studied at different substrate temperatures (39–65 K), corresponding to different growth modes. Coexisting phases showed up as different grey values in the image. Typical domain sizes of the 2D solid phases around 60 K are of the order of one to several μm. The domains exhibit an elongated shape with their long axis oriented preferentially parallel to step edges of the substrate. With increasing coverage the brightness of the domains increases, the 2D gas-phase regions shrink and finally disappear at high coverage, in agreement with the phase diagram of Xe films. The saturated monolayer at 60 K shows a “wave-like” structure indicating coexistence of the commensurate (√3×√3) R30° and an incommensurate 2D solid phase. Desorption kinetics has been studied at temperatures between 70 and 75 K. The close-packed monolayer breaks up into many small islands, again oval-shaped and oriented along the step edges, which in turn loose intensity, shrink and finally disappear at 74.4 K. The line scans and histograms of the grey values reveal that during desorption the islands of the commensurate phase persist down to low coverages and the incommensurate and 2D gas phase decrease in 2D Xe density.

Structure of a Langmuir film on a liquid metal surface.

The structure of organic monolayers on liquid surfaces depends sensitively on the details of the molecular interactions. The structure of a stearic acid film on a mercury surface was measured as a function of coverage with angstrom resolution. Unlike monolayers on water, the molecules were found here to undergo a transition from surface-parallel to surface-normal orientation with increasing coverage. At high coverage, two condensed hexatic phases of standing-up molecules were found. At low coverage, a two-dimensional (2D) gas phase and condensed single- and double-layered phases of flat-lying molecular dimers were revealed, exhibiting a 1D longitudinal positional order. This system should provide a broader tunability range for nanostructure construction than solid-supported self-assembled monolayers.

The structure of organic langmuir films on liquid metal surfaces

Langmuir films (LFs) on water have long been studied for their interest for basic science and their numerous applications in chemistry, physics, materials science and biology. We present here Å-resolution synchrotron X-ray studies of the structure of stearic acid LFs on a liquid mercury surface. At low coverage, ⩾110 Å 2/mol, a 2D gas phase of flat-lying molecules is observed. At high coverage, ⩽23 Å 2/mol, two different hexatic phases of standing-up molecules are observed. At intermediate coverage, 52⩽ A⩽110 Å 2/mol, novel single- and double-layered phases of flat-lying molecular dimers are found, exhibiting a 1D in-layer order. Such flat-lying phases were not hitherto observed in any LF. Measurements on LFs of fatty acids of other chain lengths indicate that this structure is generic to chain molecules on mercury, although the existence of some of the flat-lying phases, and the observed phase sequence, depend on the chain length. Organic LFs on Hg, and in particular the new flat-lying phases, should provide a broader nano-structural tunability range for molecular electronic device construction than most solid-supported self-assembled monolayers used at present.

Self-assembly of amino acids on the Cu(001) surface

The major results of a series of our recent investigations on the self-assembly of amino acids on the Cu(001) surface are reviewed in the present paper. It has been found that all eight amino acids studied here adsorb on the surface in their anionic forms and that for most of them, three distinctly different phases of the adsorbates may form, that is, the 2D gas, intermediate and solid phases. In the gas phase, the intermolecular interactions are repulsive and the molecules are “standing” on their carboxylate group “feet” and can diffuse frequently on the surface. In the intermediate phase, the intermolecular interactions are also repulsive and the molecules are also “standing” on the surface; however, the molecules align into short rows. In the solid phase, the molecules “lie” down with their main chain and, in some cases, also the side chain attached to the surface, whereas, hydrogen bonds are likely responsible for stabilization of this phase. In addition to the formation of these phases, some amino acid adsorbates may cause magnificent step faceting and bunching or even chiral restructuring of the surface morphology. These results may have applications in nano-scale materials, nano-technology and probably, also in chiral separations or enantioselective heterogeneous catalysis.

Critical Temperature and LE/G Phase Transitions in Monolayer Films of the Amphiphilic TEMPO Derivatives at the Air/Water Interface

Phase behavior of five members of a homologous series of 4-alkaneamido-2,2,6,6-tetramethyl-1-piperidinyloxy (CnTEMPO with n = 14, 16, 18, 20, 22) was investigated with classical Langmuir monolayer techniques and with Brewster angle microscopy and 2D voltammetry. Surface potential data, BAM images, and 2D voltammetry showed that only C20- and C22TEMPO monolayers have a 2D LE/G critical point above room temperature and that the remaining three compounds with shorter alkane chains form supercritical monolayers. The unusually low values of the critical temperature for these amphiphiles was explained in terms of the molecular structure of their headgroup featuring two polar groups (the nitroxy and the amide moieties) located in the opposite positions of a bulky piperidine ring. The presence of the two polar groups causes a change of orientation of the headgroup during monolayer expansion and results in a substantial increase (ca. 50−100 Å2/molecule) of the cross sectional area of these amphiphiles. As a result, in the expanded monolayers, the van der Waals attraction between the alkane chains is weakened as they can only partially align. The decreased cohesion within the monolayers leads to the observed decrease of their 2D critical temperature. Analysis of the plots of BAM reflected light intensity as a function of amphiphile's surface concentration obtained at different temperatures yielded the C20TEMPO critical temperature of 28 °C. BAM and 2D voltammetry can both be used to precisely determine the position of the C22TEMPO LE/G phase transition (98.5 Å2/molecule at 23.5 °C). The proximity of the C20TEMPO critical point to the room-temperature results in a less precise value of this phase transition (127−130 Å2/molecule at 23.5 °C). A remarkably consistent ability of fresh line microelectrodes used in 2D voltammetry to nucleate the 2D gas phase makes this method less ambiguous and thus superior to BAM in determining the position of the LE/G phase transitions.

Scanning Tunneling Microscopy Investigation of l-Lysine Adsorbed on Cu(001)

Adsorption of l-lysine (or, in absolute nomenclature, S-lysine) on Cu(001) has been studied with a UHV−STM. The binding energy is estimated to be not lower than 1.6 eV, and thus lysine is expected to be in its anionic form on the surface. The adsorbates have two phases, that is, the two-dimensional (2D) gas and solid phases. We speculate that in the 2D gas phase the molecules stand upright on the surface with their two oxygen atoms at the atop sites and can diffuse frequently, and that in the 2D solid phase the molecules lie down on the surface and connect to their neighboring molecules by hydrogen bonds so to form two slightly different superstructures, Cu(001)( )- and Cu(001)( )-l-lysine, for which models have been proposed for further investigation. Surprisingly, adsorption of l-lysine can make all steps restructuring into {3 1 17} facets with the same chirality. This phenomenon shows that chiral metal surfaces have potential in discrimination of molecular chirality.

Silver films grown on a rhenium(0001) surface: a combined TDS, XPS, and Δ Φ study

The energetics and kinetics of Ag thin film growth on Re(0001) were studied by means of temperature-programmed thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), and work function change (Δ Φ) measurements. The formation of three individual Ag layers shows up in TDS as three distinct desorption maxima β 1– β 3 appearing between 950 and 1010 K ( β 3), between 900 and 960 K( β 2), and between 870 and 970 K ( β 1). Except in the very low coverage ( Θ) range, in which the desorption is a first-order process, the Ag desorption follows zero-order kinetics. For the first two layers, activation energy of desorption ( E ∗ des ) is strongly Θ dependent: within the first layer, E ∗ des increases almost linearly with Θ from ≈250 kJ mol −1 at Θ=0.05 to about 290 kJ mol −1, reflecting attractive Ag–Ag interactions. From Θ=0.5 to 0.9, E ∗ des rises by only some 10 kJ mol −1. A similar (but much less pronounced) Θ dependence appears for the second monolayer. A detailed shape analysis of the submonolayer TD spectra reveals a phase equilibrium between Ag condensed in islands and individual, mobile Ag atoms (2D gas phase). In XPS, the absence of any energy shift of the Ag and Re core levels underlines the weakness of chemical Ag–Re interactions. Two Ag layers lower the work function of the Re(0001) surface by about 750 meV, with a shallow minimum near the second monolayer. We discuss our data in conjunction with previous STM and LEED results for the same system and compare this system with other Ag-on-metal systems.

Orientational ordering of a 2D molecular gas

How does a molecule behave in a two-dimensional (2D) gas? We present an analysis of the angular distributions obtained in resonance electron scattering by submonolayer films of O 2 physisorbed on graphite, which are able to distingish between molecules adsorbed in the co-existing solid and gas phases. The scattering analysis shows that molecules in the 2D gas phase are preferentially oriented with their axes parallel to the substrate.

Interactions and dynamics of benzene on Cu{111} at low temperature

We have used ultrahigh vacuum scanning tunneling microscopy to study the structure and dynamics of benzene on Cu{111} at low coverage and low temperature. We find that at 77 K adsorbed benzene molecules preferentially form two-dimensional (2D) solid structures along Cu substrate step edges while benzene molecules on terraces move freely across the surface as a 2D molecular gas. At the interface between the 2D solid and 2D gas phases we observe lateral diffusion, 2D adsorption, and 2D desorption. The motion of the benzene is detected by observing the partial and transient occupation of preferred adsorption sites at the interface. The outward growth of the 2D benzene solid is assisted by the modulations in the local density of states due to Cu{111} surface state electrons scattered from step edges and from the tightly bound benzene molecules. These modulations in the local density of states strongly influence the binding and dynamics of the benzene molecules and are also imaged using the scanning tunneling microscope. The relative strengths of the interactions at various step and terrace sites due to nanometer-scale variations in the surface electronic structure are discussed and the resulting benzene overlayer structures described.

Physisorption on an epitaxial NaCl(100) double-layer: SF 6 and xenon

The adsorption behaviour and initial layer growth of SF 6 and Xe on an epitaxial, (100)-oriented NaCl double-layer on Ge(100) has been characterized by means of TPD, XPS, UPS, and XAES. Adsorption as well as desorption kinetics are quite different in both cases. SF 6 on NaCl(100) shows a layer-by-layer growth for the first three layers and a two-phase desorption behaviour in the monolayer regime. This indicates the coexistence of a two-dimensional (2D-) condensed and a 2D-gas phase in the monolayer with different activation energies for desorption. The TPD results agree well with theoretical calculations by Payne and Kreuzer. For xenon on NaCl(100) a completely different, non-wetting growth mechanism is observed (Volmer-Weber growth for xenon compared to Stranski-Krastanov growth for SF 6). This is mainly derived from photoemission and Auger spectra which exhibit drastic changes of adsorbate and substrate intensities, comparing the coverage dependence of SF 6 and xenon adsorption. The different growth modes are due to the relative interaction strength between adsorbates and between adsorbate and substrate, respectively. Photoemission and Auger peaks show small energy shifts from mono- to multilayer, which indicate screening only by polarization effects.

Phases and Phase Transitions in the First Few Layers of Methane, Argon and Krypton Adsorbed on Graphite

ABSTRACTThe equilibrium phase diagrams of monolayers of many substances adsorbed on graphite have long been studied as examples of 2-dimensional (2D) matter. One typically observes 2D gas and liquid phases, and solid phases that may be commensurate or incommensurate with the substrate lattice. Many experimental techniques have been used, but thermodynamic measurements are generally the most useful for tracing out phase boundaries.Recent advances in technique have made it possible to use thermodynamic measurements to study the phase diagrams of the second and higher layers, up to the fifth or sixth. These advances include ultra high resolution scanning calorimetry, and a detailed understanding of the role of capillary condensation in corners and pores of the graphite foam substrate. We find a rich array of phases and phase transitions in multilayer methane, argon and krypton. The second and third layers typically have distinct 2D gas, liquid and solid phases evidenced by 2D triple points and critical points. We observe phase transitions between solid phases that are commensurate and incommensurate with the layer below. We also observe melting of the first layer at higher temperatures, even when one to five additional layers are adsorbed on top of it.In argon and krypton, but not in methane, a strange new phenomenon is observed at temperatures above the gas-liquid critical point of the nth layer for n> 3. Below that temperature, nthlayer gas coexists with a condensed nth layer. At some temperature above it, a new coexistence region is observed in which a partial nth layer coexists with a partial n + 1st layer. This behavior is thought to be evidence for a theoretically predicted phase transition of the bulk interface, called the preroughening transition.

Characterization of mono-, bi-, and multilayers of SF 6 on Ni(111) and O/Ni(111)

The adsorption behaviour and initial layer growth of SF 6 on clean and oxygen-precovered Ni(111) was characterized by TPD, XPS, UPS, and XAES. Three different molecular physisorbate layers can be clearly distinguished: mono-, bi- and multilayers. In the monolayer regime we found two different desorption states on clean Ni(111) with different desorption energies, a 2D condensed and a 2D gas phase, whereas only one immobile physisorbate state was observed on the oxygen-precovered surface. Adsorption as well as desorption kinetics are quite different in both cases, indicating the important role of the corrugation of the surface potential. Our monolayer data were evaluated with two different methods, isosteric and leading edge method, which are compared. Data and results agree very well with theoretical calculations by Payne and Kreuzer. The formation of a distinguishable bilayer, which surprisingly desorbs at lower temperatures than the multilayer, depends on the substrate temperature and pre-adsorption and competes with a rearrangement of the monolayer. Photoemission and major Auger peaks show small shifts from mono- to multilayers indicating the lack of charge transfer screening although low-lying affinity levels are available as screening Orbitals.