Wolfgang Eck

Electron-induced crosslinking of aromatic self-assembled monolayers: Negative resists for nanolithography

We have explored the interaction of self-assembled monolayers of 1,1′-biphenyl-4-thiol (BPT) with low energy electrons. X-ray photoelectron, infrared, and near edge x-ray absorption fine structure spectroscopy showed that BPT forms well-ordered monolayers with the phenyl rings tilted ∼15° from the surface normal. The films were exposed to 50 eV electrons and changes were monitored in situ. Even after high (∼10 mC/cm2) exposures, the molecules maintain their preferred orientation and remain bonded on the gold substrate. An increased etching resistance and changes in the infrared spectra imply a crosslinking between neighboring phenyl groups, which suggests that BPT can be utilized as an ultrathin negative resist. This is demonstrated by the generation of patterns in the underlying gold.

Anti-CD4-targeted Gold Nanoparticles Induce Specific Contrast Enhancement of Peripheral Lymph Nodes in X-ray Computed Tomography of Live Mice

Antibody-conjugated gold nanoparticles have been applied as a biologically targeted contrast agent in live mice for one of the most widely used medical imaging methods, X-ray computed tomography. Such nanoprobes directed toward the CD4 receptor lead to distinctly enhanced X-ray contrast of peripheral lymph nodes. This study demonstrates the general feasibility of biologically specific X-ray imaging in living animals and discusses basic requirements for the use of nanoparticles as a targeted X-ray contrast agent.

Nanoscale patterning of self-assembled monolayers with electrons

We show the fabrication of gold nanostructures using self-assembled monolayers of aliphatic and aromatic thiols as positive and negative electron beam resists. We applied a simple and versatile proximity printing technique using focused ion beam structured stencil masks and low energy (300 eV) electrons. We also used conventional e-beam lithography with a beam energy of 2.5 keV and doses from 3500 to 80 000 μC/cm2. Gold patterns were generated by wet etching in KCN/KOH and characterized by atomic force microscopy and scanning electron microscopy. The width of the finest lines is ∼20 nm; their edge definition is limited by the isotropic etching process in the polycrystalline gold.

Nanostructuring of silicon by electron-beam lithography of self-assembled hydroxybiphenyl monolayers

We report the fabrication of silicon nanostructures using aromatic hydroxybiphenyl self-assembled monolayers as ultrathin (1.1 nm) negative tone electron-beam resist. The formation of the monolayer and the electron-induced crosslinking have been characterized by x-ray photoelectron spectroscopy. Nanometer size patterns were defined by electron-beam lithography in the molecular layer and transferred into silicon by wet chemical etching with potassium hydroxide. We demonstrate the fabrication of silicon line gratings with a resolution of ∼20 nm and of isolated silicon lines with linewidths down to ∼10 nm.

Novel carbon nanosheets as support for ultrahigh-resolution structural analysis of nanoparticles

The resolution in transmission electron microscopy (TEM) has reached values as low as 0.08 nm. However, these values are not accessible for very small objects in the size range of a few nanometers or lower, as they have to be placed on some support, which contributes to the overall electron-scattering signal, thereby blurring the contrast. Here, we report on the use of nanosheets made from cross-linked aromatic self-assembled monolayers as TEM sample supports. When transferred onto a copper grid, a single 1.6-nm-thick nanosheet can cover the grid and is free standing within the micron-sized openings. Despite its thinness, the sheet is stable under the impact of the electron beam. Micrographs taken from nanoclusters onto these nanosheets show highly increased contrast in comparison to the images taken from amorphous carbon supports. In scanning transmission electron microscopy with nanosheet support, a size analysis of sub-nanometer Au clusters was performed and single Au atoms were resolved.

Electron induced chemical nanolithography with self-assembled monolayers

We demonstrate a simple scheme to generate chemical surface nanostructures. Electron-beam writing is used to locally modify the terminal nitro functionality in self-assembled monolayers of 4′-nitro-1,1′-biphenyl-4-thiol to amino groups, while the underlying aromatic layer is dehydrogenated and cross linked. Using low energy electron proximity printing and conventional electron-beam lithography with a beam energy of 2.5 keV and doses from 2500 to 50 000 μC/cm2, templates of reactive amino sites with lateral dimensions down to ∼20 nm could be fabricated. The templates were used for the surface immobilization of fluorinated carboxylic acid anhydrides and rhodamine dyes. The molecular structures were then imaged and analyzed by atomic force and scanning confocal fluorescence microscopy.

Interaction of self-assembled monolayers of oligo(ethylene glycol)-terminated alkanethiols with water studied by vibrational sum-frequency generation

Vibrational sum-frequency generation (VSFG) was used to investigate the conformational changes in self-assembled monolayers (SAMs) of (1-mercaptoundec-11-yl) hexa(ethylene glycol) monomethylether (EG6-OMe) on gold when exposed to liquid water. VSFG spectra of the EG6-OMe SAMs were recorded before, during, and after exposure of the films to water and after a subsequent evacuation step. While in contact with water the entire ethylene glycol chains are found in a random, solvated state, after removal from the fluid water molecules remain absorbed only at the terminal groups of the film giving rise to distinct conformational changes. After evacuation, the structure of the EG6-OMe SAM reverts to its original state, indicating that water has been removed from the monolayer. Our findings support recent ab initio calculations and Monte Carlo simulations on the interaction of ethylene glycol-terminated monolayers with water.

Investigation of the adhesion, friction, and wear properties of biphenyl thiol self-assembled monolayers by atomic force microscopy

A comparative study of the tribological properties of self-assembled 1,1′-biphenyl-4-thiol monolayer (BPT) and cross-linked BPT monolayer (BPTC) is presented. BPT monolayers were prepared on a gold substrate by spontaneous self-assembly of biphenylthiol in solution. The cross-linking of BPT was carried out by low energy electron irradiation. Structural characterization by infrared spectra shows that chemical bonds were formed between neighboring phenyl groups after irradiation. The topography, adhesion, friction, and wear properties of BPT and BPTC were studied by atomic force microscopy with silicon nitride and diamond tips. Au(111) and Si(111) substrates were also investigated for reference. Both BPT and BPTC exhibited lower adhesion and friction than Au(111) and Si(111). Cross-linking resulted in a decrease of the adhesive force, whereas the coefficient of friction is increased. The wear resistance of BPT and BPTC slid against a diamond tip were almost identical in the tested range. It appears that cro...

Abrupt change in the structure of self-assembled monolayers upon metal evaporation

We have studied the interaction of vapor-deposited nickel with thiol-terminated self-assembled monolayers (SAMs) of oligophenylthiolates on Au, which can be considered as model systems for molecular electronics. Instead of usually observed disordering of the molecular layer, a drastic decrease in molecular tilt was observed at the initial stage of the deposition. This was attributed to the formation of Ni-thiol complexes at the SAM–ambient interface. The reorientation resulted in the appearance of transient channels for the metal diffusion into the SAM and to the SAM–substrate interface.

Nanoscale patterning of self-assembled monolayers by e-beam lithography

Abstract The resolution of e-beam lithography in standard resists is limited by the size of the molecules in the resist. High resolution e-beam resists therefore should not only show a specific sensitivity to electrons but also be thin and composed of small subunits. Self-assembled monolayers (SAMs) fulfil these criteria because they are homogeneous, highly ordered films of amphiphilic molecules with a typical thickness of 1–2 nm and an intermolecular spacing of 1–0.5 nm. We demonstrate that gold nanostructures can be fabricated using aliphatic and aromatic thiol self-assembled monolayers as positive and negative electron beam resists.