Pierpaolo Greco

Towards all-organic field-effect transistors by additive soft lithography.

Unconventional nanofabrication is attractive for organic electronics because of its potential impact in manufacturing low-cost electronics starting from soluble precursors that can be processed and patterned via a sustainable technology. So far, the major endeavor aimed at the development of organicbased devices has been through the design of new materials, novel synthetic procedures and purification methods, optimized conditions for thin film growth, and original methods for nanofabrication. In particular, a strong effort was devoted to the technological control of organic semiconductors in transistors. Yet, only a limited number of studies have focused on new approaches for low cost fabrication of electrodes and their integration with the organic materials. Successful examples of unconventional electrode manufacturing include stencil printing of Au nanoparticles, inkjet printing, Ag electroless plating followed by microcontact patterning, lamination, microtransfer printing of Ag nanoparticles, metal transfer printing, and soft lithography. Although inkjet printing is probably the most straightforward example of an additive process where both the electrodes and the active layers can be realized on the same platform, the fabrication of the electrodes and the active layers often relies on different processes, specifications, and platforms. Precisely, standard microfabrication approaches consisting of photolithography and/or electron-beam lithography followed by vacuum metallization are generally used for the source and drain definition, while wet methods (spin-coating, layer-by-

Conductive Sub-micrometric Wires of Platinum-Carbonyl Clusters Fabricated by Soft-Lithography

Conductive wires of sub-micrometer width made from platinum-carbonyl clusters have been fabricated by solution-infilling of microchannels as in microinject molding in capillaries (MIMIC). The process is driven by the liquid surface tension within the micrometric channels followed by the precipitation of the solute. Orientation of supramolecular crystalline domains is imparted by the solution confinement combined with unidirectional flow. The wires exhibit ohmic conductivity with a value of 0.2 S/cm that increases, after thermal decomposition of the platinum-carbonyl cluster precursor to Pt, to 35 S/cm.

Micro- and nanopatterning by lithographically controlled wetting

This protocol describes how to perform lithographically controlled wetting (LCW). LCW enables large-area patterning of microstructures and nanostructures of soluble materials, either organic or inorganic, including biological compounds in buffer solutions or compounds for cell guidance. LCW exploits the capillary forces of menisci established under the protrusions of a stamp placed in contact with a liquid film. In the space confined by each meniscus, the self-organization of the deposited solute yields highly ordered structures that replicate the motif of the stamp protrusions. The method does not require any particular infrastructure and can be accomplished by using simple tools such as compact discs or microscopy grids. Compared with other printing methods, LCW is universal for soluble materials, as it does not require chemical binding or other specific interactions between the solute and the surface. A process cycle takes from 2 to 36 h to be completed, depending on the choice of materials.

Multiscale Morphology of Organic Semiconductor Thin Films Controls the Adhesion and Viability of Human Neural Cells

We investigate how multiscale morphology of functional thin films affects the in vitro behavior of human neural astrocytoma 1321N1 cells. Pentacene thin film morphology is precisely controlled by means of the film thickness, Theta (here expressed in monolayers (ML)). Fluorescence and atomic force microscopy allow us to correlate the shape, adhesion, and proliferation of cells to the morphological properties of pentacene films controlled by saturated roughness, sigma, correlation length, xi, and fractal dimension, d(f). At early incubation times, cell adhesion exhibits a transition from higher to lower values at Theta approximately 10 ML. This is explained using a model of conformal adhesion of the cell membrane onto the growing pentacene islands. From the model fitting of the data, we show that the cell explores the surface with a deformation of the membrane whose minimum curvature radius is 90 (+/- 45) nm. The transition in the adhesion at approximately 10 ML arises from the saturation of xi accompanied by the monotonic increase of sigma, which leads to a progressive decrease of the pentacene local radius of curvature and hence to the surface area accessible to the cell. Cell proliferation is also enhanced for Theta < 10 ML, and the optimum morphology parameter ranges for cell deployment and growth are sigma <or= 6 nm, xi > 500 nm, and d(f) > 2.45. The characteristic time of cell proliferation is tau approximately 10 +/- 2 h.

Patterned conductive nanostructures from reversible self-assembly of 1D coordination polymer

In this study, the outstanding ability of the coordination polymer [Pt2(nBuCS2)4I]n (nBu = n-butyl) (1) to reversibly self-organize from solution was demonstrated. This feature allowed us to generate highly electrical conductive structures located upon demand on technologically relevant surfaces, by easy-to-handle and low cost micromolding in capillaries (MIMIC) and lithographically controlled wetting (LCW). Electrical characterization reveals a near Ohmic behaviour and a high stability of the stripes (in air). Electrodes produced by the MIMIC technique from a solution of compound 1 demonstrated that this material can be efficiently used as electrodes for organic field-effect transistors (OFETs).

Control of neuronal cell adhesion on single-walled carbon nanotube 3D patterns†

Carbon nanotubes are emerging substrates for guiding neuronal cell growth. Here we investigate the influence of multiscale 3D architecture of single-walled carbon nanotubes (SWCNT) on the adhesion of human neuronal cells (neuroblastoma SHSY5Y). 3D patterns of SWCNT were fabricated by a templating process to yield a hexagonal array of interconnected SWCNT semicapsules with controlled multiscale porosity, and integrated between Pt electrodes. Neuronal cells adhered preferentially to the SWCNT semicapsules with respect to either silicon oxide or disordered networks of SWCNT. Morphological cell features (size and shape) were evaluated upon application of an electric field across SWCNT pattern. Cell adhesion is enhanced by an electric field above 1 V cm−1, whereas it is completely depleted at 5 V cm−1. This shows the possibility to tune neuronal cell adhesion across different regimes by means of 3D pattern and voltage-biasing SWCNT.

Neural cell alignment by patterning gradients of the extracellular matrix protein laminin

Anisotropic orientation and accurate positioning of neural cells is achieved by patterning stripes of the extracellular matrix protein laminin on the surface of polystyrene tissue culture dishes by micromoulding in capillaries (MIMICs). Laminin concentration decreases from the entrance of the channels in contact with the reservoir towards the end. Immunofluorescence analysis of laminin shows a decreasing gradient of concentration along the longitudinal direction of the stripes. The explanation is the superposition of diffusion and convection of the solute, the former dominating at length scales near the entrance (characteristic length around 50 μm), the latter further away (length scale in excess of 900 μm). These length scales are independent of the channel width explored from about 15 to 45 μm. Neural cells are randomly seeded and selectively adhere to the pattern, leaving the unpatterned areas depleted even upon 6 days of incubation. Cell alignment was assessed by the orientation of the long axis of the 4′,6-diamidino-2-phenylindole-stained nuclei. Samples on patterned the laminin area exhibit a large orientational order parameter. As control, cells on the unpatterned laminin film exhibit no preferential orientation. This implies that the anisotropy of laminin stripes is an effective chemical stimulus for cell recruiting and alignment.

Facile maskless fabrication of organic field effect transistors on biodegradable substrates

Fabrication of a test pattern with interdigitated gold electrodes (channel length 12 μm) on a biodegradable substrate is achieved by direct laser ablation of a Au film using a high-precision multifunction infrared-laser scan marker. The whole process involves two solvent-free steps: Au film deposition by sublimation followed by maskless ablation. The approach is suited for fast prototyping of a variety of materials. We demonstrate the fabrication of a water-gated organic field effect transistor on the biodegradable poly(lactic-co-glycolic acid) scaffold and its operations in water.

Patterning pentacene surfaces by local oxidation nanolithography

Sequential and parallel local oxidation nanolithographies have been applied to pattern pentacene samples by creating a variety of nanostructures. The sequential local oxidation process is performed with an atomic force microscope and requires the application of a sequence of voltage pulses of 36V for 1ms. The parallel local oxidation process is performed by using a conductive and patterned stamp. Then, a voltage pulse is applied between the stamp and the pentacene surface. Patterns formed by arrays of parallel lines covering 1mm(2) regions and with a periodicity of less than 1microm have been generated in a few seconds. We also show that the patterns can be used as templates for the deposition of antibodies.

One-step substrate nanofabrication and patterning of nanoparticles by lithographically controlled etching

We propose an integrated top-down and bottom-up approach to single-step nanofabrication of complex nanostructures made of different materials. The process, termed lithographically controlled etching (LCE), starts with a drop of an etching solution cast on the surface to be patterned. By placing a polymeric mold on the substrate, the stamp protrusions come into contact with the surface, thus protecting it, whereas the surface beneath the mold recesses is exposed to a thin layer of etching solution, allowing the surface to be etched. By dispersing nanoparticles into the etching solution, these can be deposited and self-organize in the recesses on the substrate as these are excavated. We demonstrate here the fabrication of complex structures and nanowires 30 nm wide. Moreover, by exploiting capillary forces, it is possible to deposit nanoparticles at precise positions with respect to optically addressable microstructures, thus realizing a multiscale functional pattern.