Precursor chemistry of h-BN: adsorption, desorption, and decomposition of borazine on Pt(110).

Adsorption, desorption and fragmentation of borazine on Pt(110) are studied by temperature-programmed desorption, ultraviolet photoemission spectroscopy, workfunction measurements and density functional theory. Borazine adsorbs in part dissociatively, forming an upright (B3N3H5˙)ads adsorption complex. Radicals with a N-Pt bond are weakly bound and desorb recombinatively following second-order kinetics. Radicals with a B-Pt bond are similar in binding strength to the molecularly adsorbed species, which binds through dispersive forces to the (111) facets of the (1 × 2) reconstructed Pt(110). Both do not desorb but are dehydrogenated beyond T = 150 K. As T approaches 600 K the B-N ring progressively breaks down into its atomic constituents. The borazine ice multilayer is capable of trapping significant amounts of hydrogen. Previous studies of borazine adsorption on other transition metal surfaces yield a very similar pattern. Reported multiple molecular desorption peaks are artefacts. Implications for the nucleation and growth of h-BN monolayers at high temperatures are discussed.

Alpha-Carbonic Acid Revisited: Carbonic Acid Monomethyl Ester as a Solid and its Conformational Isomerism in the Gas Phase.

In this work, earlier studies reporting α-H2 CO3 are revised. The cryo-technique pioneered by Hage, Hallbrucker, and Mayer (HHM) is adapted to supposedly prepare carbonic acid from KHCO3 . In methanolic solution, methylation of the salt is found, which upon acidification transforms to the monomethyl ester of carbonic acid (CAME, HO-CO-OCH3 ). Infrared spectroscopy data both of the solid at 210 K and of the evaporated molecules trapped and isolated in argon matrix at 10 K are presented. The interpretation of the observed bands on the basis of carbonic acid [as suggested originally by HHM in their publications from 1993-1997 and taken over by Winkel et al., J. Am. Chem. Soc. 2007 and Bernard et al., Angew. Chem. Int. Ed. 2011] is inferior compared with the interpretation on the basis of CAME. The assignment relies on isotope substitution experiments, including deuteration of the OH- and CH3 - groups as well as 12 C and 13 C isotope exchange and on variation of the solvents in both preparation steps. The interpretation of the single molecule spectroscopy experiments is aided by a comprehensive calculation of high-level ab initio frequencies for gas-phase molecules and clusters in the harmonic approximation. This analysis provides evidence for the existence of not only single CAME molecules but also CAME dimers and water complexes in the argon matrix. Furthermore, different conformational CAME isomers are identified, where conformational isomerism is triggered in experiments through UV irradiation. In contrast to earlier studies, this analysis allows explanation of almost every single band of the complex spectra in the range between 4000 and 600 cm-1 .

Quasiliquid Layer Promotes Hexagonal Boron Nitride (h-BN) Single-Domain Growth: h-BN on Pt(110).

Hexagonal boron nitride (h-BN) monolayers were grown on Pt(110) using borazine as a precursor molecule. The resulting surface structure was studied by scanning tunneling microscopy, low-energy electron diffraction, and density functional theory calculations. Borazine fragments reduce the roughening temperature of pristine Pt(110) ( Tr = 1090 K); consequently, growth below T = 1100 K results in a serrated h-BN/Pt(110) surface with small terraces, defects, and domain boundaries. Surprisingly, h-BN deposition at T > 1100 K yields large terraces covered by a carpet-like single-domain h-BN monolayer. Despite the incommensurability and different symmetry, the epitaxial growth is almost perfect. The key to this counterintuitive behavior is the "soft" Pt(110) surface responding to the h-BN overlayer in two ways: First, the (1 × 2)-missing-row (m.r.) reconstruction is converted into a (1 × n)-m.r. reconstruction with a regular alternation of n = 5 and 6, yielding a superperiodicity of the Moiré pattern. Second, the remaining rows experience significant relaxations. Some Pt surface atoms are mobile underneath the h-BN monolayer, even at room temperature. Under growth conditions, the top metal layer is disordered and highly mobile, rendering the h-BN growth comparable to that on liquid gold. Such a mechanism may be of general relevance for the epitaxial growth of 2D materials. Because epitaxial deposition of Pt(110) on various substrates has been demonstrated, the present system appears scalable, and its regular 1D grooves render it a promising template for nanowire arrays.

Visualization of freezing process in situ upon cooling and warming of aqueous solutions.

The freezing of aqueous solutions and reciprocal distribution of ice and a freeze-concentrated solution (FCS) are poorly understood in spite of their importance in fields ranging from biotechnology and life sciences to geophysics and climate change. Using an optical cryo-microscope and differential scanning calorimetry, we demonstrate that upon cooling of citric acid and sucrose solutions a fast freezing process results in a continuous ice framework (IF) and two freeze-concentrated solution regions of different concentrations, FCS1 and FCS2. The FCS1 is maximally freeze-concentrated and interweaves with IF. The less concentrated FCS2 envelops the entire IF/FCS1. We find that upon further cooling, the FCS1 transforms to glass, whereas the slow freezing of FCS2 continues until it is terminated by a FCS2-glass transition. We observe the resumed slow freezing of FCS2 upon subsequent warming. The net thermal effect of the resumed freezing and a reverse glass-FCS1 transition produces the Ttr2-transition which before has only been observed upon warming of frozen hydrocarbon solutions and which nature has remained misunderstood for decades.

Adsorption and Reactions of Carbon Monoxide and Oxygen on Bare and Au-Decorated Carburized W(110).

Adsorption and coadsorption of carbon monoxide and oxygen on different types of Au clusters on R(15 × 3)C/W(110) and R(15 × 12)C/W(110), respectively, are studied with respect to the catalytic behavior for oxidation of CO as well as of surface carbon. Carburization of the W(110) surface results in a weakening of the adsorption bond for molecularly adsorbed CO. Dissociation of carbon monoxide, which occurs on W(110), is reduced on the low-carbon coverage R(15 × 12) surface and completely suppressed on the carbon-saturated R(15 × 3) phase. Deposition of gold results in a blocking of adsorption sites for molecularly adsorbed CO and reopening of the dissociation channel. Probably the latter is associated with the existence of double-layer gold clusters and islands. At room temperature the gold clusters on both carburized templates are stable in CO atmosphere as shown by in-situ STM measurements. In contrast, exposure to oxygen alters the clusters on the R(15 × 12) surface, implying dissociation of oxygen not only on the substrate but also on or in immediate vicinity of the gold clusters. On the Au-free carburized templates oxygen adsorbs dissociatively and is released as CO at temperatures beyond 800 K due to reaction with carbon atoms from the templates. Deposition of gold enhances the desorption rate of the formed CO at the low-temperature end of the recombinative CO desorption range, indicating a promoting effect of gold for oxidation of surface carbon. In contrast, low-temperature CO oxidation catalyzed by the deposited Au clusters is not observed. Two reasons could be identified: (1) weakly bound CO with desorption temperatures between 100 and 200 K (as reported for other related systems) is not observed, and (2) oxygen atoms are bonded too strongly to the templates.

Quasi-critical fluctuations: a novel state of matter?

Quasi-critical fluctuations occur close to critical points or close to continuous phase transitions. In three-dimensional systems, precision tuning is required to access the fluctuation regime. Lowering the dimensionality enhances the parameter space for quasi-critical fluctuations considerably. This enables one to make use of novel properties emerging in fluctuating systems, such as giant susceptibilities, Casimir forces or novel quasi-particle interactions. Examples are discussed ranging from simple metal-adsorbate systems to unconventional superconductivity in iron-based superconductors.

Monitoring preparation and phase transitions of carburized W(1 1 0) by reflectance difference spectroscopy.

Reflectance difference spectroscopy (RDS) is applied to follow in situ the preparation of clean and carburized W(1 1 0) surfaces and to study the temperature-induced transition between the R(15 × 3) and R(15 × 12) carbon/tungsten surface phases. RDS data for this transition are compared to data obtained from Auger-electron spectroscopy and low-energy electron diffraction. All techniques reveal that this transition, occurring around 1870 K, is reversible with a small hysteresis, indicating a first-order-like behaviour. The present results also prove a high surface sensitivity of RDS, which is attributed to the excitation of electronic p-like surface resonances of W(1 1 0).

Noble-metal nanostructures on carburized W(110).

Noble metal nanostructures of Au, Ag and Cu were prepared on two types of carbon-modified W(110) surfaces-R(15 × 12) and R(15 × 3)-and investigated by means of scanning tunneling microscopy. For all deposited metals qualitatively the same behaviour is observed: On the R(15 × 12)-template always isotropic clusters are formed. In contrast, on the R(15 × 3)-substrate the anisotropy of the nanostructures can be tuned from clusters at low temperatures via thin nanowires to thicker nanobars at high deposition temperatures. At intermediate temperatures on the R(15 × 3) the anisotropic Au nanowires arrange themselves into straight lines along domain boundaries induced by deposition of the Au metal. Similarities and differences to Au nanostructures as recently reported by Varykhalov et al. [A. Varykhalov, O. Rader, W. Gudat. Physical Review B 77, 035412 (2008).] are discussed.

Halogen-induced corrosion of platinum.

The interaction of Cl with Pt(110) was studied in UHV by scanning tunneling microscopy, low-energy electron diffraction, temperature-programmed desorption, and DFT calculations. Up to half a monolayer (ML) of Cl forms an adsorbate structure. Compression to higher local coverages leads to erosion of Pt atoms from the top layer and formation of PtCl(4) pentamers. Annealing results in healing of the Pt defects and formation of a long-range-ordered PtCl(4)/Cl/Pt(110) adlayer that is remarkably similar to the adlayer structure found after deposition of [PtCl(4)](2-) from an electrolyte onto Au(100). Coadsorption of 0.5 ML of Cl with CO initiates PtCl(4) formation, but no volatile compounds are formed under these conditions. In similar experiments with Br, the original Pt surface is left intact, even if Br is compressed to local coverages of 0.75 ML, in agreement with the expected lower corrosion activity of Br.

The surface properties of nanocrystalline diamond and nanoparticulate diamond powder and their suitability as cell growth support surfaces.

Nanocrystalline diamond (NCD) films and nanoparticulate diamond powder (DP) are the two main representatives of diamond at the nanoscale. This study was designed to investigate the suitability of these biomaterials as cell growth supports and to determine surface characteristic properties best suited to cell attachment and proliferation. Surface topography, chemical termination and wetting properties of NCD- and DP-coated borosilicate glass substrates were correlated to attachment, proliferation and differentially regulated gene expression of human renal epithelial cells (HK-2 cell line) cultured on these surfaces. Hydrogen-terminated NCD (NCD-H) surfaces were shown to inhibit cell attachment, which indicates that the lack of functional polar groups prevents adherent cells from settling on a surface, whether nanostructured or not. In contrast to NCD-H, oxygen-terminated NCD (NCD-O) as well as DP surfaces demonstrated improved cell attachment, as compared to borosilicate glass, which is a commonly used material for cell growth supports. NCD-O not only revealed an increased cell attachment, but also a markedly increased proliferation rate. Finally, none of the investigated surface modifications appeared to cause adverse cellular reactions or markedly alter cellular phenotype.

Fluctuations and phase separation in a quasi-one-dimensional system.

Phase transitions in a quasi-one-dimensional surface system on a metal substrate are investigated as a function of temperature. Upon cooling the system shows a loss of long-range order, fluctuations, and a transition into an inhomogeneous ground state due to competition of local adsorbate-adsorbate interactions with an incommensurate charge density wave. This agrees with a general phase diagram for correlated systems and high-temperature superconductors. The model surface system allows direct imaging of the fluctuations and the glassy inhomogeneous ground state by scanning tunneling microscopy.

Nanocrystalline diamond--an excellent platform for life science applications.

Nanocrystalline diamond (NCD) has recently been successfully utilized in a variety of life science applications. NCD films are favorable and salubrious substrates for cells during cultivation. Therefore NCD has also been employed in tissue engineering strategies. NCD as reported in this contribution was grown by means of a modified hot-filament chemical vapor deposition technique, which results in less than 3% sp2-hybridization and yields grain sizes of 5-20 nm. After production the NCD surface was rather hydrophobic, however it could be efficiently refined to exhibit more hydrophilic properties. Changing of the surface structure was found to be an efficient means to influence growth and differentiation capacity of a variety of cells. The particular needs for any given cell type has to be proven empirically. Yet flexible features of NCD appear to be superior to plastic surfaces which can be hardly changed in quality. Besides its molecular properties, crystal structural peculiarities of NCD appear to influence cell growth as well. In our attempt to facilitate, highly specialized applications in biomedicine, we recently discovered that growth factors can be tightly bound to NCD by mere physisorption. Hence, combination of surface functionalization together with further options to coat NCD with any kind of three-dimensional structure opens up new avenues for many more applications. In fact, high through-put protein profiling of early disease stages may become possible from serum samples, because proteins bound to NCD can now be efficiently analyzed by MALDI/TOF-MS. Given these results, it is to be presumed that the physical properties and effective electrochemical characteristics of NCD will allow tailoring devices suitable for many more diagnostic as well as therapeutic applications.

Strong binding of bioactive BMP-2 to nanocrystalline diamond by physisorption.

Nano-crystalline diamond (NCD)-coated surfaces were efficiently functionalized with bone morphogenetic protein-2 (BMP-2) by means of physisorption. Due to their randomly oriented texture, NCD-coated surfaces appear to bind complex molecules firmly. Applying various highly sensitive analytical methods, the interaction was found extremely stable. The strength of the experimentally measured adherence between BMP-2 and NCD was further corroborated by theoretical calculations. Oxygen treatment rendered NCD hydrophilic by the appearance of surface oxygen containing groups. This particular NCD surface exhibited even higher binding energies towards BMP-2 than the hydrophobic surface, and this surface was also favoured by cultured cells. Most importantly in this context, bound BMP-2 was found fully active. When cultured on BMP-2-treated NCD, osteosarcoma cells strongly up-regulated alkaline phosphatase, a specific marker for osteogenic differentiation. Hence, this simple method will allow generating highly versatile surfaces with complex biomimetic coatings, essentials for novel medical devices and implants as well as for innovative scaffolds in tissue engineering.