Scalable Synthesis of Carbon Nanomembranes from Amorphous Molecular Layers.

Nanoporous carbon nanomembranes (CNMs) created by self-assembled monolayers ideally combine a high water flux and precise ion selectivity for molecular separation and water desalination. However, their practical implementation is often challenged by the availability of large epitaxial substrates, limiting the membrane up-scaling. Here, we report a scalable synthesis of CNMs from poly(4-vinylbiphenyl) (PVBP) spin-coated on SiO2/Si wafers. Electron irradiation of the amorphous PVBP molecular layers induces the formation of a continuous membrane with a thickness of 15 nm and a high density of subnanometer pores, providing a water permeance as high as 530 L m-2 h-1 bar-1, while repelling ions and molecules larger than 1 nm in size. A further introduction of a reinforced porous block copolymer layer enables the fabrication of centimeter-scale CNM composites that efficiently separate organic dyes from water. These results suggest a feasible route for large-scale nanomembrane fabrication.

Nanomechanics of Ultrathin Carbon Nanomembranes.

Ultrathin carbon nanomembranes (CNMs) are two-dimensional materials (2DM) of a few nm thickness with sub-nm intrinsic pores that mimic the biofiltration membranes found in nature. They enable highly selective, permeable, and energy-efficient water separation and can be produced at large scales on porous substrates with tuned properties. The present work reports the mechanical performance of such CNMs produced by p-nitrobiphenyl phosphonic acid (NBPS) or polyvinylbiphenyl (PVBP) and their composite membranes of microporous supporting substrates, which constitute indispensable information for ensuring their mechanical stability during operation. Measuring the nanomechanical properties of the ultrathin material was achieved by atomic force microscopy (AFM) on membranes both supported on flat substrates and suspended on patterned substrates ("composite membrane"). The AFM analysis showed that the CNMs presented Young's modulus in the range of 2.5-8 GPa. The composite membranes' responses were investigated by tensile testing in a micro-tensile stage as a function of substrate thickness and substrate pore density and diameter, which were found to affect the mechanical properties. Thermogravimetric analysis was used to investigate the thermal stability of composite membranes at high temperatures. The results revealed the structural integrity of CNMs, while critical parameters governing their mechanical response were identified and discussed.

Scanning transmission helium ion microscopy on carbon nanomembranes.

A dark-field scanning transmission ion microscopy detector was designed for the helium ion microscope. The detection principle is based on a secondary electron conversion holder with an exchangeable aperture strip allowing its acceptance angle to be tuned from 3 to 98 mrad. The contrast mechanism and performance were investigated using freestanding nanometer-thin carbon membranes. The results demonstrate that the detector can be optimized either for most efficient signal collection or for maximum image contrast. The designed setup allows for the imaging of thin low-density materials that otherwise provide little signal or contrast and for a clear end-point detection in the fabrication of nanopores. In addition, the detector is able to determine the thickness of membranes with sub-nanometer precision by quantitatively evaluating the image signal and comparing the results with Monte Carlo simulations. The thickness determined by the dark-field transmission detector is compared to X-ray photoelectron spectroscopy and energy-filtered transmission electron microscopy measurements.

Underpotential deposition of Cu on Au(111) from neutral chloride containing electrolyte.

The structure of a chloride terminated copper monolayer electrodeposited onto Au(111) from a CuSO4/KCl electrolyte was investigated ex situ by three complementary experimental techniques (scanning tunneling microscopy (STM), photoelectron spectroscopy (PES), X-ray standing wave (XSW) excitation) and density functional theory (DFT) calculations. STM at atomic resolution reveals a stable, highly ordered layer which exhibits a Moiré structure and is described by a (5 × 5) unit cell. The XSW/PES data yield a well-defined position of the Cu layer and the value of 2.16 Å above the topmost Au layer suggests that the atoms are adsorbed in threefold hollow sites. The chloride exhibits some distribution around a distance of 3.77 Å in agreement with the observed Moiré pattern due to a higher order commensurate lattice. This structure, a high order commensurate Cl overlayer on top of a commensurate (1 × 1) Cu layer with Cu at threefold hollow sites, is corroborated by the DFT calculations.

Hydrogel nanomembranes as templates for patterned deposition of nanoparticles on arbitrary substrates.

Patterns of nanoparticles (NPs) on solid supports are usually restricted to a particular substrate or a class of substrates. Here we present a procedure that decouples the patterning step from the target substrate, enabling the fabrication of custom designed NP assemblies on nearly any solid support, including nonflat ones. The procedure relies on a hydrogel template prepared on the primary, conductive substrate and transferred to the target support as a sacrificial nanomembrane. The template is structured by electron beam lithography (EBL) which seals predefined areas of poly(ethylene glycol) based hydrogel film, making them inert to NP deposition in contrast to pristine areas that adsorb NPs in high densities. The deposition of NPs, occurring from an aqueous solution into the transferred membrane, follows EBL generated structure, delivering the desired NP pattern on the target support after removal of the organic matrix. Efficiency and flexibility of the procedure is illustrated by creating a variety of representative submicrometer patterns of densely packed gold and silver NPs on glass, including a useful pattern of a miniaturized quick-response code. The arrangement of NPs in these patterns corresponds to the negative image of EBL generated template. This significantly reduces the exposure time for designs where large areas covered with NPs are separated by thin, NP-free stripes.

UV-mediated tuning of surface biorepulsivity in aqueous environment.

While it is well-known that oligoethylene glycol (OEG) terminated self-assembled monolayers (SAMs) can be deteriorated by UV irradiation in air, we now report that the analogous modification can also be performed in water, opening the opportunity for in situ tuning of biorepulsive properties. Surprisingly, this deterioration also takes place even in the absence of molecular oxygen, resulting in a very selective process.

Ultraflexible, freestanding nanomembranes based on poly(ethylene glycol).

Extremely elastic and highly stable nanomembranes of variable thickness (5-350 nm) made completely of poly(ethylene glycol) are prepared by a simple procedure. The membranes exhibit distinct biorepulsive and hydrogel properties. They offer new possibilities for applications such as supports in transmission electron microscopy, matrices for inorganic nanoparticles, and pressure-sensitive elements for sensors.

Modification and patterning of nanometer-thin poly(ethylene glycol) films by electron irradiation.

In this study, we analyzed the effect of electron irradiation on highly cross-linked and nanometer-thin poly(ethylene glycol) (PEG) films and, in combination with electron beam lithography (EBL), tested the possibility to prepare different patterns on their basis. Using several complementary spectroscopic techniques, we demonstrated that electron irradiation results in significant chemical modification and partial desorption of the PEG material. The initially well-defined films were progressively transformed in carbon-enriched and oxygen-depleted aliphatic layers with, presumably, still a high percentage of intermolecular cross-linking bonds. The modification of the films occurred very rapidly at low doses, slowed down at moderate doses, and exhibited a leveling off behavior at higher doses. On the basis of these results, we demonstrated the fabrication of wettability patterns and sculpturing complex 3D microstructures on the PEG basis. The swelling behavior of such morphological patterns was studied in detail, and it was shown that, in contrast to the pristine material, irradiated areas of the PEG films reveal an almost complete absence of the hydrogel-typical swelling behavior. The associated sealing of the irradiated areas allows a controlled deposition of objects dissolved in water, such as metal nanoparticles or fluorophores, into the surrounding, pristine areas, resulting in the formation of nanocomposite patterns. In contrast, due to the distinct protein-repelling properties of the PEG films, proteins are exclusively adsorbed onto the irradiated areas. This makes such films a suitable platform to prepare protein-affinity patterns in a protein-repelling background.

Novel ultrathin poly(ethylene glycol) films as flexible platform for biological applications and plasmonics.

We present a novel approach to prepare ultrathin, biocompatible films based on cross-linking of multi-functionalized, star-branched poly(ethylene glycols) (STAR-PEGs) with tunable film thicknesses of 4-200 nm. A two-component mixture of amine- and epoxy-terminated four-arm STAR-PEGs (MN=2000 g/mol) was spin-coated on a flat substrate. Gentle heating induced an extensive chemical cross-linking of the macromonomers, resulting in a stable, hydrogel-like film with a density close to that of bulk PEG material. The cross-linking process could be monitored in situ, exhibiting the expected kinetics. The films revealed pronounced swelling behavior, which was fully reversible and could be precisely controlled. Additionally, they provided a high affinity to citrate-stabilized gold nanoparticles (AuNP) that could be adsorbed with high densities into the PEG matrix from an aqueous solution. These novel PEG/AuNP composite films offer interesting and potentially useful optical properties. The adsorption could also be performed in a lithographic fashion, resulting in AuNP patterns imbedded into the PEG matrix.

Micropatterning thermoplasmonic gold nanoarrays to manipulate cell adhesion.

The ability to reversibly control the interactions between the extracellular matrix (ECM) and cell surface receptors such as integrins would allow one to investigate reciprocal signaling circuits between cells and their surrounding environment. Engineering microstructured culture substrates functionalized with switchable molecules remains the most adopted strategy to manipulate surface adhesive properties, although these systems exhibit limited reversibility and require sophisticated preparation procedures. Here, we report a straightforward protocol to fabricate biofunctionalized micropatterned gold nanoarrays that favor one-dimensional cell migration and function as plasmonic nanostoves to physically block and orient the formation of new adhesion sites. Being reversible and not restricted spatiotemporally, thermoplasmonic approaches will open new opportunities to further study the complex connections between ECM and cells.

Modification of nitrile-terminated biphenylthiol self-assembled monolayers by electron irradiation and related applications.

Here we describe the behavior of self-assembled monolayers (SAMs) of 4'-cyanobiphenyl-4-thiol (CBPT) on Au(111) upon electron irradiation. Under such a treatment, the aromatic framework of CBPT SAMs is laterally cross-linked while the nitrile groups, located at the SAM-ambience interface, are reduced to active amine moieties which can be used as docking sites for the coupling of other species. This makes CBPT monolayers as a promising system for conventional and chemical lithography as well as for nanofabrication. Along these lines, we demonstrate the preparation of complex polymer brushes, patterning of the underlying substrate, and fabrication of molecule-thin, free-standing membranes on the basis of CBPT SAMs. The balance between the application-favorable processes and defragmentation in these films is studied in detail, and comparison to the well-established (for the relevant applications) system of 4'-nitrobiphenyl-4-thiols is performed. Taking CBPT SAMs as a model system, the effect of the energy of the primary electrons on the extent of the chemical transformation and cross-linking in substituted aromatic SAMs is investigated.