Submonolayer Coverages Research Articles

Adsorption Behavior of 9,9'-Dixanthylidene and Moiré Superstructure on Ru(0001)

This paper reports the adsorptions behavior of the 9,9'-Dixanthylidene on Ru(0001), the bottom-up fabrication of graphene was also investigated through a Scanning Tunneling Microscope (STM). We analyzed the spatial structure of 9,9'-Dixanthylidene molecules which were randomly dispersed on the Ru(0001) substrate in the sub-monolayer coverage. Then we bottom-up fabricated the graphene on Ru(0001) substrate with the 9,9'-Dixanthylidene molecules as the precursor. Three kinds of moiré superstructures with different rotation angles (6.3°, 13.9°, and 16.1°) were found after high temperature annealing in an ultrahigh vacuum. This experiment provides data support for the study of moiré superstructures on Ru(0001) substrate and lays a solid foundation for further scientific research based on graphene/Ru(0001).

Role of sodium hexametaphosphate in ultra-fine grinding of alumina-doped titanium dioxide

In wet stirred milling of Al-doped TiO2, sodium hexametaphosphate (SHMP) is used as a small molecule dispersant to improve milling performance. In both acidic and basic conditions, SHMP interacts strongly with Al-doped TiO2 and remains stable during milling. Considering the apparent and true particle breakage performance, at pH 10.3, the optimal dosing concentration was 0.16 wt% (0.33θ, sub-monolayer coverage), which is consistent for both dilute (5 vol%) and concentrated (≥ 19.1 vol%) suspensions. At higher SHMP concentration (0.77θ), the increased solution conductivity compromised milling performance by weakly destabilizing the Al-doped TiO2 particles. At the optimal dose concentration of 0.33θ SHMP, particle dispersion is maintained through electrostatic and steric forces which contribute to moderating changes in suspension viscosity (~10% increase) as the particle size is reduced.

Disordered Two-Dimensional Self-Organization of Ethyl Pyruvate Molecules on the Pt(111) Surface

Ethyl pyruvate on a pristine Pt(111) surface in submonolayer coverage neither forms regular superstructures nor a completely random distribution. Instead, a random, widely scattered pattern of a fe...

Infrared reflection spectroscopy of adsorbed intermediates in real time during chemical vapor deposition of oxides

In this paper, we develop multilayer optical theory to model the real-time (in operando) optical response of a growing dielectric film being deposited by chemical vapor deposition (CVD), with a particular emphasis on understanding the deposition mechanism through direct detection of the adsorbates responsible for film growth by infrared reflection-absorption spectroscopy (IRRAS). The model involves a four-layer stack consisting of a vacuum over a monolayer or submonolayer of molecules adsorbed on the surface of a dielectric thin film, which, in turn, is growing on a metal substrate. It is well known that, in IRRAS, the sensitivity of p-polarized light to absorption by the molecular adsorbates is a function of the incident angle of the IR beam: at high angles, the sensitivity is highest. We show that, for incident beam angles above 70° (which are typically used in IRRAS experiments), the sensitivity also depends on the thickness and refractive index of the insulating thin film; as a result, the sensitivity changes dynamically during the growth of the dielectric layer. Our analysis shows that, at incident beam angles of ∼60°–70°, the sensitivity to molecular adsorbates is somewhat lower, but is almost independent of the oxide thickness from 0 to 100 nm and also independent of the oxide refractive index from 1.0 to 2.5. Despite the loss of sensitivity relative to that achievable at higher incident beam angles, 1000 scans at an incident angle of 60° are sufficient to obtain IR spectra of the adsorbed molecules with reasonable signal-to-noise ratios even at submonolayer coverages. Because the sensitivity at this incident beam angle is not thickness dependent, it is not an issue (as it is at higher beam angles) that additional oxide grows during the time required to acquire 1000 scans. Experiments can be performed using a conventional vacuum deposition system, in which the internal beam path is tens of cm. We demonstrate the use of these smaller incident beam angles to study the mechanism of a CVD process in real time by polarization-modulation IRRAS, obtained by subtracting the s-polarized from the p-polarized infrared spectra in order to eliminate the unpolarized component due to molecules in the beam path and on windows. We explore the surface coverage of various adsorbed intermediates during CVD of HfO2 from tetrakis-(dimethylamido)-hafnium (TDMAH) and water in the presence of the consumable inhibitor magnesium N,N-dimethylamino-diboranate [Mg(DMADB)2]. We find that the addition of the Mg(DMADB)2 inhibitor causes a decrease in the IR absorption from the adsorbed TDMAH precursor that correlates with the observed decrease in the HfO2 growth rate; this result indicates that the mechanism of inhibition involves Mg(DMADB)2 acting as a dynamic site-blocker that lowers the surface coverage of TDMAH.

Morphology of Nanometric Overlayers Made of Porphyrin-Type Molecules Physisorbed on Cellulose Iβ Crystals and Nanocrystals.

Molecular dynamics simulations based on an atomistic empirical force field have been carried out to investigate structural, thermodynamic, and dynamical properties of adlayers made of porphyrin-type molecules physisorbed on surfaces of cellulose Iβ nanocrystals. The results show that low-index surfaces provide a thermally stable, weakly perturbing support for the deposition of non-hydrogen-bonded organic molecules. At submonolayer coverage, the discoidal porphyrin molecules lay flat on the surface, forming compact 2D clusters with clear elements of ordering. The adlayer grows layer-by-layer for the smallest porphyrin species on compact cellulose surfaces, while forming 3D clusters on a first relatively ordered adlayer (Stranski–Krastanov growth) in all other cases. The adsorption energy exceeds ∼1 eV per molecule, underlying the thermal stability of the adsorbate. Entropy plays a non-negligible role, destabilizing to some extent the adlayer. The in-plane dynamics of the smallest porphyrin species, i.e., porphine, on compact surfaces shows signs of superlubricity, due to the low energy and momentum exchange between the flat admolecule and the equally flat cellulose surface.

A mesoscopic model of nanoclusters self-assembly on a graphene Moiré

A continuum, post-deposition mesoscopic model of a Moiré-regulated self-assembly of metal nanoclusters on a twisted bilayer graphene is presented. Quasi-two-dimensional nanocluster-like steady-states at a low adsorbate coverage are analytically determined for Pt, Ni, and Pb adsorbates, pointing that nanoclusters self-assemble at the Moiré cell centers. This is followed by the computations of nanoclusters self-assembly dynamics. Differences in the self-assembly efficiency for three chosen metals are highlighted across three typical values of an initial submonolayer coverage and for three temperature regimes. Accounting for the adsorption potential of metal atoms onto graphene leads to a significantly faster nanoclusters self-assembly and has a transient impact on the nanocluster morphologies. A model extensions to the cases of nanoclusters self-assembly on a Moiré formed by a monolayer graphene over a metal substrate, and the electromigration-guided self-assembly on such Moiré are proposed.

Cu(111) single crystal electrodes: Modifying interfacial properties to tailor electrocatalysis

Tailoring electrocatalyst materials to the specific requirements of a certain reaction and to optimize activity or enhance selectivity is a key tactic for the development of low-temperature fuel and electrolyzer cells for clean energy production. Here, we demonstrate the modification of Cu(111) electrodes with different sub-monolayer coverages of foreign metals (Cd) and metal hydroxides (Co(OH)2 and Ni(OH)2) for application in the hydrogen evolution reaction (HER) in alkaline media. In situ electrochemical scanning tunneling microscopy (EC-STM) reveals that these modifications have a significant influence on the morphology and structure of the Cu(111) surface with its characteristics depending on both the nature and the amount of the adsorbed metal(hydroxide). Ni(OH)2 and Co(OH)2 on Cu(111) lead to a significant enhancement of the electrocatalytic activity towards the HER in alkaline electrolyte, whereas a decrease in activity is found for Cd modified Cu(111). These trends can be rationalized by considering the strength of the interfacial electric field and its influence on the specific interactions of the electrode with the water ad-layer close to the surface, as determined by laser-induced temperature jump measurements. This comparative study therefore provides valuable information on the structure-activity relation as well as insights on the interfacial characteristics of different bimetallic Cu electrocatalysts.

Adsorption and self-assembly of hexa-tert-butyl-hexa-peri-hexabenzocoronene on the si(111)-[formula omitted]-Ag surface

Self-assembled structures of hexa-tert-butyl-hexa-peri-hexabenzocoronene (HB-HBC) on the Si(111)-3×3-Ag surface have been investigated by low-energy electron diffraction and low-temperature scanning tunneling microscopy (STM). The HB-HBC molecules are assembled into a 21×21−R±10.9∘ structure with respect to the Si(111)-1×1 surface. Although the periodic structure is unchanged as a function of molecular coverage, we find that the orientations and adsorption sites of HB-HBC within the molecular islands are changed at near a full monolayer. At submonolayer coverages, the tert-butyl groups of HB-HBC are adsorbed on both the Si and Ag trimers of the Si(111)-Ag surface, and only on the Si trimers at a full monolayer. This result should be associated with a balance between substrate-molecule and inter-molecule interactions.

Thermal- and Light-Induced Spin-Crossover Characteristics of a Functional Iron(II) Complex at Submonolayer Coverage on HOPG

Thermal- and Light-Induced Spin-Crossover Characteristics of a Functional Iron(II) Complex at Submonolayer Coverage on HOPG

Influence of electrospray deposition on C60 molecular assemblies.

Maintaining clean conditions for samples during all steps of preparation and investigation is important for scanning probe studies at the atomic or molecular level. For large or fragile organic molecules, where sublimation cannot be used, high-vacuum electrospray deposition is a good alternative. However, because this method requires the introduction into vacuum of the molecules from solution, clean conditions are more difficult to be maintained. Additionally, because the presence of solvent on the surface cannot be fully eliminated, one has to take care of its possible influence. Here, we compare the high-vacuum electrospray deposition method to thermal evaporation for the preparation of C60 on different surfaces and compare, for sub-monolayer coverages, the influence of the deposition method on the formation of molecular assemblies. Whereas the island location is the main difference for metal surfaces, we observe for alkali halide and metal oxide substrates that the high-vacuum electrospray method can yield single isolated molecules accompanied by surface modifications.

Thermodynamic Considerations in the Design of Electrochemical Atomic Layer Etching of Copper

Electrochemical atomic layer etching (e-ALE) is a unique approach for etching metals one atomic layer at a time. If practiced under optimal conditions, e-ALE ensures minimal evolution of surface roughness due to the atomic layer-by-layer etching characteristics. During e-ALE of copper (Cu), the crucial first step is the formation of a cuprous sulfide (Cu2S) monolayer via the surface-limited sulfidization reaction. In this paper, we investigate the surface coverage of this sulfide layer as a function of the sulfidization potential, and show that the equilibrium coverage attained can be modeled using the Frumkin adsorption isotherm. At a potential of –0.74 V vs SHE, sulfidization provides near-complete monolayer coverage of Cu by Cu2S, which then facilitates e-ALE in a layer-by-layer etching mode thereby maintaining a smooth post-etch surface. Operation at potentials negative with respect to –0.74 V provides sub-monolayer coverage, which manifests in roughness amplification during etching. This work provides a thermodynamics-guided foundation for the selection of operating conditions during Cu e-ALE.

(Invited) Photocatalytic Water Splitting Using SWCNT/TiO2 Nanohybrids

Water splitting photocatalysts based on semiconducting single-walled carbon nanotubes (s-SWCNTs) have been attracted much attention from the viewpoint of solar fuel production.[1-7] Since s-SWCNTs have chirality dependent bandgap covering a majority of the solar spectrum (400 – 2000 nm), s-SWCNTs are expected for an efficient solar-light absorber.[8] However, upon photoirradiation, the large exciton binding energy in s-SWCNTs prevents mobile carrier generation. Then, in order to make a particulate photocatalyst consisting of s-SWCNTs, an electron-extracting material is necessary. In this context, we have developed CNT-photocatalysts that contain a s-SWCNT/C60 coaxial heterojunction fabricated by a physical modification of s-SWCNTs using fullerodendron.[4] Here, the C60 acts as the electron-extracting material to generate a charge-separated state, s-SWCNT•+/C60 •–, followed by electron transfer to a co-catalyst that produces hydrogen from water. Z-scheme photocatalyst system employing BiVO4 as an oxygen evolution photocatalyst (OEP) and s-SWCNT/C60 coaxial hybrids as a hydrogen evolution photocatalyst (HEP) produced H2 and O2 from water, of which solar-to-hydrogen conversion efficiency (STH) was 0.089%.[7] We conducted an H2 evolution reaction using s-SWCNT/C60/Ru(III) upon monochromatic irradiation at 570, 650, and 680 nm, which are E22 absorption of (6,5), (7,5), and (8,3)tubes, respectively, and estimated the external quantum yields to be 0.22% for (6,5), 0.16% for (7,5), and 0.70% for (8,3). These EQY values were considerably lower than that using 1000-nm-light (E11 absorption of (6,5) and (8,3)tubes) irradiation, 12.8%.[7] The result might be attributed to branching pathways from the higher exciton (E22 excitation) to other than the lower exciton (E11 excitation). Blackburn and co-workers reported such a branching ratio is crucial for the PL quantum yield of s-SWCNTs.[9] Cui and co-workers describe E22 excitation makes interfacial electrotransfer inhibited in the (6,5)tube/C60 heterojunction, although ultrafast electron transfer process can be seen in E11 excitation by a DFT-based nonadiabatic dynamics simulation.[10] In this context, direct electron extraction from E22 excitation, which is faster than other branching pathways, is expected to improve the photocatalytic activity of CNT-photocatalysts. Recently, Blackburn reported higher-energy second excitonic s-SWCNT transitions produce more photocurrent by the use of submonolayer coverages of s-SWCNTs on single crystal TiO2 electrode, demonstrating carrier injection rates are competitive with fast hot-exciton relaxation processes.[11] These backgrounds prompt us to investigate a CNT-photocatalyst containing the s-SWCNT/TiO2 heterojunction. This paper describes fabrication of s-SWCNT/TiO2/Pt nanohybrids and their photocatalytic activity for H2 evolution from water. Interestingly, their EQYs of H2 evolution reaction using E22 excitation, 12% for (6,5)tube/TiO2/Pt and 43% for (8,3)tube/TiO2/Pt is much higher than those of (6,5)tube/C60/Ru(III) and (8,3)tube/C60/Ru(III), 0.22% and 0.70%.

LiF Nanoparticles Enhance Targeted Degradation of Organic Material under Low Dose X-ray Irradiation

The targeted irradiation of structures by X-rays has seen application in a variety of fields. Herein, the use of 5–10 nm LiF nanoparticles to locally enhance the degradation of an organic thin film, diindenoperylene, under hard X-ray irradiation, at relatively low ionizing radiation doses, is shown. X-ray reflectivity analysis indicated that the film thickness increased 12.04 Å in air and 11.34 Å in a helium atmosphere, under a radiation dose of ∼65 J/cm2 for 3 h illumination with a bi-layer structure that contained submonolayer coverage of thermally evaporated LiF. This was accompanied by significant modification of the surface topography for the organic film, which initially formed large flat islands. Accelerated aging experiments suggested that localized heating was not a major mechanism for the observed changes, suggesting a photochemical mechanism due to the formation of reactive species from LiF under irradiation. As LiF has a tendency to form active defects under radiation across the energy spectrum, this could could open a new direction to explore the efficacy of LiF or similar optically active materials that form electrically active defects under irradiation in various applications that could benefit from enhanced activity, such as radiography or targeted X-ray irradiation therapies.

The formation of a Sn monolayer on Ge(1 0 0) studied at the atomic scale

The growth of multi-layer germanium-tin (GeSn) quantum wells offers an intriguing pathway towards the integration of lasers in a CMOS platform. An important step in growing high quality quantum well interfaces is the formation of an initial wetting layer. However, key atomic-scale details of this process have not previously been discussed. We use scanning tunneling microscopy combined with density functional theory to study the deposition of Sn on Ge(100) at room temperature over a coverage range of 0.01 to 1.24 monolayers. We demonstrate the formation of a sub-2% Ge content GeSn wetting layer from three atomic-scale characteristic ad-dimer structural components, and show that small quantities of Sn incorporate into the Ge surface forming two atomic configurations. The ratio of the ad-dimer structures changes with increasing Sn coverage, indicating a change in growth kinetics. At sub-monolayer coverage, the least densely packing ad-dimer structure is most abundant. As the layer closes, forming a two-dimensional wetting layer, the more densely packing ad-dimer structure become dominant. These results demonstrate the capability to form an atomically smooth wetting layer at room temperature, and provide critical atomic-scale insights for the optimization of growth processes of GeSn multi-quantum-wells to meet the quality requirements of optical GeSn-based devices.

Ab initio molecular dynamics investigation of Cs adsorption on Mo(0 0 1): Beyond a single monolayer coverage

Ab initio molecular dynamics simulations have been conducted to investigate the properties of cesiated surface Mo(0 0 1) at the typical operating temperature of the negative hydrogen ion sources (423 K). In the systems with sub-monolayer Cs coverages (θ), both the mobility and ionicity of the adatoms decrease with θ. The computed shape of the work function (ϕ) dependence on θ agrees with experiment, although our simulations underestimate the extent of the ϕ variation. In multilayer depositions, cesium forms two phases exhibiting distinctly different properties. The atoms laid directly on the substrate form an ordered layer with the atomic density and charges similar to those of the saturated monolayer (ML). A disordered phase with a much lower particle density is formed by the atoms stacked on the ordered layer and induces a positive shift in ϕ by 0.32 eV compared to ML. A similar behavior is observed for the Cs(0 0 1) and Cs(1 1 0) surfaces used as models for thick Cs depositions. The work function determined for multilayer Cs depositions at T = 423 K is nearly independent of the substrate face and of the presence/absence of Mo, indicating that ϕ of such systems is fully determined by the disordered surface phase.

Catalytic dehydrogenation of isobutane over supported MoOx/K-Al2O3

Potassium-promoted, alumina-supported molybdenum oxide is investigated for non-oxidative isobutane dehydrogenation, with an emphasis on sub-monolayer coverages. After pre-reduction in H2, initial isobutene turnover frequencies are largely independent of Mo loading, but deactivation rate constants increase by >100-fold as loadings increase to monolayer, leading to a >13-fold difference in reaction rates at extended time on stream. Mo oxidation state by in situ X-ray absorption spectroscopy is stable with time on stream, arguing against continued catalyst restructuring as the origin of deactivation. Across the set of loadings, isobutane dehydrogenation and coke formation are correlated with partially reduced Mo4+ and deeply reduced Moδ+ sites, respectively. Given the stable dehydrogenation activity of low-loaded Mo, in contrast to the rapid deactivation of high-loaded Mo, sub-monolayer metal oxides may warrant further investigation as light alkane dehydrogenation even when their bulk counterparts may not.

Nonsymmetrical Segregation of Solutes in Periodic Misfit Dislocations Separated Tilt Grain Boundaries.

Interfacial segregation is ubiquitous in mulit-component polycrystalline materials and plays a decisive role in material properties. So far, the discovered solute segregation patterns at special high-symmetry interfaces are usually located at the boundary lines or are distributed symmetrically at the boundaries. Here, in a model Mg-Nd-Mn alloy, we confirm that elastic strain minimization facilitated nonsymmetrical segregation of solutes in four types of linear tilt grain boundaries (TGBs) to generate ordered interfacial superstructures. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy observations indicate that the solutes selectively segregate at substitutional sites at the linear TGBs separated by periodic misfit dislocations to form such two-dimensional planar structures. These findings are totally different from the classical McLean-type segregation which has assumed the monolayer or submonolayer coverage of a grain boundary and refresh understanding on strain-driven interface segregation behaviors.

Ultrasmooth cobalt films on SiO2 by chemical vapor deposition using a nucleation promoter and a growth inhibitor

Very smooth thin films of Co are deposited on SiO2 by chemical vapor deposition from the precursor dicobalt octacarbonyl Co2(CO)8 by augmenting the process conditions in two ways: (1) pretreating the surface to promote nucleation and (2) adding a coflow of a growth inhibitor. The surface pretreatment involves brief exposure of the SiO2 surface at 70 °C to a tetrakis(dimethylamido) metal complex M(NMe2)4, where M = V, Ti, or Hf. This pretreatment affords a self-limiting, submonolayer coverage of a metal-containing intermediate. When the pretreated surface is then exposed to 0.025 mTorr of the Co2(CO)8 precursor, nucleation occurs to afford a high density (1 × 1012 cm2) of nanoscale islands; the rms surface roughness for a sample with an areal density of 100 Co atoms/nm2 is 1.0 nm. Comparisons show that the M(NMe2)4 pretreatment has three benefits: it accelerates the nucleation process, it increases the nucleation density by about a factor of 3, and it reduces the surface roughness by about a factor of 2. The rms surface roughness of a sample with ∼100 Co atoms/nm2 can be further reduced to only 0.3 nm by adding up to 4 mTorr of ammonia NH3 to the feed gas along with the Co2(CO)8 precursor. Ammonia serves as a growth inhibitor that reduces the steady-state growth rate of Co by a factor of 4. This slower growth allows additional nucleation to occur on bare areas, and the growth of these nuclei affords coalescence with a smaller roughness.

Hydrated Alkali Atoms on Copper(111): A Density Functional Theory Study

We present a systematic computational study of submonolayer coverage of alkali atoms (Na, K, Cs) on Cu(111) surface hydrated from 1 to 6 water molecules. Our calculations show that water molecules ...

Hf Deposition Stabilizes the Surface Chemistry of Perovskite Manganite Oxide.

Stable composition and catalytic activity of surfaces are among the key requirements for materials employed in energy storage and conversion devices, such as solid oxide fuel cells (SOFCs). Perovskite oxides that serve as cathode in SOFCs suffer from segregation of the aliovalent substitutional cations and the formation of an inert, non-conductive phase at the surface. Here, we demonstrate that the surface of the state-of-the-art SOFC cathode material La0.8Sr0.2MnO3 (LSM) is stabilized against the segregation of Sr at high temperature by submonolayer coverages of Hf. The Hf is vapor-deposited onto the LSM thin film surface by e-beam evaporation. Using in situ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), we analyze the surface composition of LSM thin films. Half the LSM surface was kept as-prepared, and half was Hf-modified, for a direct comparison of untreated and Hf-treated regions on the same sample. The formation of a binary SrOx surface species is quantified as descriptor for surface degradation. The onset of Sr segregation is observed at 450 °C on the bare LSM, followed by a substantial advance at 550 °C. Hf-treated regions of the same LSM surface exhibit significantly less Sr surface segregation at 450–550 °C. We interpret this stabilization imparted by Hf to arise from the suppression of the electrostatic attraction of Sr2+ cations to surface oxygen vacancies. Doping the surface layer with Hf, that has a higher affinity to oxygen, reduces this attraction by decreasing the surface oxygen vacancy concentration. In doing so, the use of physical vapor deposition highlights the direct role of the metal species in this system and excludes artifacts that could be introduced via chemical routes. The present work demonstrates this stabilizing effect of Hf on the surface of LSM, broadening the relevance of our prior findings on surface metal doping of other perovskite oxides.