Preventing Thin Film Dewetting via Graphene Capping.

A monolayer 2D capping layer with high Young's modulus is shown to be able to effectively suppress the dewetting of underlying thin films of small organic semiconductor molecule, polymer, and polycrystalline metal, respectively. To verify the universality of this capping layer approach, the dewetting experiments are performed for single-layer graphene transferred onto polystyrene (PS), semiconducting thienoazacoronene (EH-TAC), gold, and also MoS2 on PS. Thermodynamic modeling indicates that the exceptionally high Young's modulus and surface conformity of 2D capping layers such as graphene and MoS2 substantially suppress surface fluctuations and thus dewetting. As long as the uncovered area is smaller than the fluctuation wavelength of the thin film in a dewetting process via spinodal decomposition, the dewetting should be suppressed. The 2D monolayer-capping approach opens up exciting new possibilities to enhance the thermal stability and expands the processing parameters for thin film materials without significantly altering their physical properties.

Mechanism of an ATP-independent protein disaggregase: I. structure of a membrane protein aggregate reveals a mechanism of recognition by its chaperone.

BACKGROUND: A novel chaperone, cpSRP43, recognizes and disassembles the aggregates formed by its client proteins. RESULTS: The client proteins of cpSRP43 form stable disc-shaped aggregates with the chaperone recognition motif displayed onthe surface. CONCLUSION: The surface-exposed motif on the aggregate allows it to be recognized by its chaperone. SIGNIFICANCE: Understanding the structure and energetics of protein aggregates provides insights into the mechanism of theirDISASSEMBLY.Protein aggregation is detrimental to the maintenance of proper protein homeostasis in all cells. To overcome this problem, cells have evolved a network of molecular chaperones to prevent protein aggregation and even reverse existing protein aggregates. The most extensively studied disaggregase systems are ATP-driven macromolecular machines. Recently, we reported an alternative disaggregase system in which the 38-kDa subunit of chloroplast signal recognition particle (cpSRP43) efficiently reverses the aggregation of its substrates, the light-harvesting chlorophyll a/b-binding (LHC) proteins, in the absence of external energy input. To understand the molecular mechanism of this novel activity, here we used biophysical and biochemical methods to characterize the structure and nature of LHC protein aggregates. We show that LHC proteins form micellar, disc-shaped aggregates that are kinetically stable and detergent-resistant. Despite the nonamyloidal nature, the LHC aggregates have a defined global organization, displaying the chaperone recognition motif on its solvent-accessible surface. These findings suggest an attractive mechanism for recognition of the LHC aggregate by cpSRP43 and provide important constraints to define the capability of this chaperone.

Visualizing local doping effects of individual water clusters on gold(111)-supported graphene.

The local charge carrier density of graphene can exhibit significant and highly localized variations that arise from the interaction between graphene and the local environment, such as adsorbed water, or a supporting substrate. However, it has been difficult to correlate such spatial variations with individual impurity sites. By trapping (under graphene) nanometer-sized water clusters on the atomically well-defined Au(111) substrate, we utilize scanning tunneling microscopy and spectroscopy to characterize the local doping influence of individual water clusters on graphene. We find that water clusters, predominantly nucleated at the atomic steps of Au(111), induce strong and highly localized electron doping in graphene. A positive correlation is observed between the water cluster size and the local doping level, in support of the recently proposed electrostatic-field-mediated doping mechanism. Our findings quantitatively demonstrate the importance of substrate-adsorbed water on the electronic properties of graphene.

The microscopic structure of adsorbed water on hydrophobic surfaces under ambient conditions.

The interaction of water vapor with hydrophobic surfaces is poorly understood. We utilize graphene templating to preserve and visualize the microscopic structures of adsorbed water on hydrophobic surfaces. Three well-defined surfaces [H-Si(111), graphite, and functionalized mica] were investigated, and water was found to adsorb as nanodroplets (∼10-100 nm in size) on all three surfaces under ambient conditions. The adsorbed nanodroplets were closely associated with atomic-scale surface defects and step-edges and wetted all the hydrophobic substrates with contact angles<∼10°, resulting in total water adsorption that was similar to what is found for hydrophilic surfaces. These results point to the significant differences between surface processes at the atomic/nanometer scales and in the macroscopic world.

Atomic force microscopy characterization of room-temperature adlayers of small organic molecules through graphene templating.

We report on the use of graphene templating to investigate the room-temperature structure and dynamics of weakly bound adlayers at the interfaces between solids and vapors of small organic molecules. Monolayer graphene sheets are employed to preserve and template molecularly thin adlayers of tetrahydrofuran (THF) and cyclohexane on atomically flat mica substrates, thus permitting a structural characterization of the adlayers under ambient conditions through atomic force microscopy. We found the first two adlayers of both molecules adsorb in a layer-by-layer fashion, and atomically flat two-dimensional islands are observed for both the first and the second adlayers. THF adlayers form initially as rounded islands but, over a period of weeks, evolve into faceted islands, suggesting that the adlayers possess both liquid and solid properties at room temperature. Cyclohexane adlayers form crystal-like faceted islands and are immobile under the graphene template. The heights of the second adlayers of THF and cyclohexane are measured to be 0.44 ± 0.02 and 0.50 ± 0.02 nm, respectively, in good agreement with the layer thicknesses in the monoclinic crystal structure of THF and the Phase I "plastic crystal" structure of cyclohexane. The first adlayers appear slightly thinner for both molecules, indicative of interactions of the molecules with the mica substrate.

Graphene visualizes the first water adlayers on mica at ambient conditions.

The dynamic nature of the first water adlayers on solid surfaces at room temperature has made the direct detection of their microscopic structure challenging. We used graphene as an atomically flat coating for atomic force microscopy to determine the structure of the water adlayers on mica at room temperature as a function of relative humidity. Water adlayers grew epitaxially on the mica substrate in a layer-by-layer fashion. Submonolayers form atomically flat, faceted islands of height 0.37 +/- 0.02 nanometers, in agreement with the height of a monolayer of ice. The second adlayers, observed at higher relative humidity, also appear icelike, and thicker layers appear liquidlike. Our results also indicate nanometer-scale surface defects serve as nucleation centers for the formation of both the first and the second adlayers.

Achieving the theoretical depairing current limit in superconducting nanomesh films.

We show the theoretical depairing current limit can be achieved in a robust fashion in highly ordered superconductor nanomesh films having spatial periodicities smaller than both the superconducting coherence length and the magnetic penetration depth. For a niobium nanomesh film with 34 nm spatial periodicity, the experimental critical current density is enhanced by more than 17 times over the continuous film and is in good agreement with the depairing limit over the entire measured temperature range. The nanomesh superconductors are also less susceptible to thermal fluctuations when compared to nanowire superconductors. T(c) values similar to the bulk film are achieved, and the nanomeshes are capable of retaining superconductivity to higher fields relative to the bulk. In addition, periodic oscillations in T(c) are observed as a function of field, reflecting the highly ordered nanomesh structure.

Scanning tunneling microscopy characterization of the electrical properties of wrinkles in exfoliated graphene monolayers.

We report on the scanning tunneling microscopy study of a new class of corrugations in exfoliated monolayer graphene sheets, that is, wrinkles approximately 10 nm in width and approximately 3 nm in height. We found such corrugations to be ubiquitous in graphene and have distinctly different properties when compared to other regions of graphene. In particular, a "three-for-six" triangular pattern of atoms is exclusively and consistently observed on wrinkles, suggesting the local curvature of the wrinkle provides a sufficient perturbation to break the 6-fold symmetry of the graphene lattice. Through scanning tunneling spectroscopy, we further demonstrate that the wrinkles have lower electrical conductance and are characterized by the presence of midgap states, which is in agreement with recent theoretical predictions. The observed wrinkles are likely important for understanding the electrical properties of graphene.

Azidation of silicon(111) surfaces.

A two-step chlorination/azidation process was reported to prepare azide-modified silicon(111) surfaces. XPS and IR analyses show the covalent bonding of azide with silicon. In combination with scanning tunneling microscopy and spectroscopy analyses, different kinetic rates, azide coverages, and surface-area distributions were derived depending on the azidation solvent.

Scanning tunneling microscopy and spectroscopy of wet-chemically prepared chlorinated Si111 surfaces.

Chlorine-terminated Si(111) surfaces prepared through the wet-chemical treatment of H-terminated Si(111) surfaces with PCl5 (in chlorobenzene) were investigated using ultrahigh vacuum scanning tunneling microscopy (UHV cryo-STM) and tunneling spectroscopy. STM images, collected at 77 K, revealed an unreconstructed 1 x 1 structure for the chlorination layer, consistent with what has been observed for the gas phase chlorination of H-terminated Si(111). However, the wet-chemical chlorination is shown to generate etch pits in the Si(111) surface, with an increase in etch pit density correlating with increasing PCl5 exposure temperatures. These etch pits were assumed to stabilize the edge structure through the partial removal of the <112> step edges. Tunneling spectroscopy revealed a nonzero density of states at zero bias. This is in contrast to the cases of H-, methyl-, or ethyl-terminated Si(111), in which similar measurements have revealed the presence of a large conductance gap.

Scanning tunneling microscopy of ethylated Si(111) surfaces prepared by a chlorination/alkylation process.

Scanning tunneling microscopy (STM) and computational modeling have been used to study the structure of ethyl-terminated Si(111) surfaces. The ethyl-terminated surface was prepared by treating the H-terminated Si(111) surface with PCl5 to form a Cl-terminated Si(111) surface with subsequent exposure to C(2)H(5)MgCl in tetrahydrofuran to produce an alkylated Si(111) surface. The STM data at 77 K revealed local, close-packed, and relatively ordered regions with a nearest-neighbor spacing of 0.38 nm as well as disordered regions. The average spot density corresponded to approximately 85% of the density of Si atop sites on an unreconstructed Si(111) surface. Molecular dynamics simulations of a Si(111) surface randomly populated with ethyl groups to a total coverage of approximately 80% confirmed that the ethyl-terminated Si(111) surface, in theory, can assume reasonable packing arrangements to accommodate such a high surface coverage, which could be produced by an exoergic surface functionalization route such as the two-step chlorination/alkylation process. Hence, it is possible to consistently interpret the STM data within a model suggested by recent X-ray photoelectron spectroscopic data and infrared absorption data, which indicate that the two-step halogenation/alkylation method can provide a relatively high coverage of ethyl groups on Si(111) surfaces.

[Surface-enhanced Raman spectroscopy in studies of corrosion and inhibition on an iron surface].

Application and advanced achievement of surface-enhanced Raman spectroscopy (SERS) in corrosion and inhibition on an iron surface have been reviewed. As a technology providing the molecular information of material structure, SERS is widely applied to the studies of metal corrosion and inhibition, especially for iron. Applications of this spectroscopy technology are showed in three aspects: the SERS enhancement theory of Fe surface, the adsorbent mode of inhibitions on Fe surface, and the structure of oxidation or passivation film on a Fe electrode. Furthermore, the tendency of farther study is prospected.

Optimizing detection sensitivity on surface-enhanced Raman scattering of transition-metal electrodes with confocal Raman microscopy.

Some points on how to improve the detection sensitivity of confocal Raman microscopy for the study of surface-enhanced Raman scattering (SERS) of transition-metal electrodes are discussed, including the careful design of the spectroelectrochemical cell, proper selection of the thickness of the solution layer, the binning of charge-coupled device (CCD) pixels, and appropriate setting of the notch filter. Various roughening methods for the Pt, Rh, Fe, Co, and Ni electrode surfaces have been introduced in order to obtain SERS-active surfaces. It has been shown that the appropriate roughening procedure and the optimizing performance of the confocal Raman microscope are the two most important factors to directly generate and observe SERS on net transition-metal electrodes.

[Competitive adsorption studies of pyridine and acetonitrile on platinum electrodes in non-aqueous system by surface-enhanced Raman spectroscopy].

In the non-aqueous acetonitrile solution, recurring to the confocal Raman system, we studied the intensively chemical adsorption of pyridine(Py) on the surface of platinum electrode utilizing surface-enhanced Raman scattering (SERS). With the change in the potential, the quantity and adsorptive orientation of the adsorbate also change, interacting on the surrounding molecules. In the present paper, the adsorption of Py on the surface of platinum electrode was not only clearly observed, but also the dissociation of acetonitrile was discovered simultaneously. And the more complicated double layer in the non-aqueous solution than in the aqueous one explains the differences of the results between the non-aqueous and aqueous solutions.

[Surface-enhanced Raman spectrum studies of CO adsorbed on platinum electrodes in non-aqueous acetonitrile system].

In the non-aqueous acetonitrile solution, recurring to the confocal Raman system, we studied the catalysis and oxidation of carbon monoxide on the surface of platinum electrode utilizing surface-enhanced Raman spectrum (SERS). As the movement of the potential, the process of the catalysis and oxidation interacted with the surrounding molecular. In the present paper, the catalysis and oxidation of carbon monoxide on the surface of platinum electrode was not clearly observed, but also the dissociation of acetonitrile was discovered simultaneously. Results show that acetonitrile decomposition occurs at a certain more negative potential by the appearance of the 2,133 cm-1 bands assigned to CN stretching modes. The bands observed at ca. 502 and 2,055 cm-1 are attributed to the Pt-CO(nu Pt-c) and intramolecular C-O(nu C-O) stretching vibration respectively, suggesting linearly adsorbed CO on platinum. The main product of COads oxidation is confirmed to be carbonate due to the existence of trace water in the double-layer region as a source of oxygen for the reaction.