Mariona Coll

Formation of Silicon-Based Molecular Electronic Structures Using Flip-Chip Lamination

We report the fabrication of molecular electronic test structures consisting of Au-molecule-Si junctions by first forming omega-functionalized self-assembled monolayers on ultrasmooth Au on a flexible substrate and subsequently bonding to Si(111) with flip-chip lamination by using nanotransfer printing (nTP). Infrared spectroscopy (IRS), spectroscopic ellipsometry (SE), water contact angle (CA), and X-ray photoelectron spectroscopy (XPS) verified the monolayers self-assembled on ultrasmooth Au were dense, relatively defect-free, and the -COOH was exposed to the surface. The acid terminated monolayers were then reacted with a H-terminated Si(111) surface using moderate applied pressures to facilitate the interfacial reaction. After molecular junction formation, the monolayers were characterized with p-polarized backside reflection absorption infrared spectroscopy (pb-RAIRS) and electrical current-voltage measurements. The monolayer quality remains largely unchanged after lamination to the Si(111) surface, with the exception of changes in the COOH and Si-O vibrations indicating chemical bonding. Both vibrational and electrical data indicate that electrical contact to the monolayer is formed while preserving the integrity of the molecules without metal filaments. This approach provides a facile means to fabricate high-quality molecular junctions consisting of dense monolayers chemically bonded to metal and silicon electrodes.

Tailored interfaces for self-patterning organic thin-film transistors

Patterning organic thin-film transistors (OTFTs) is critical in achieving high electronic performance and low power consumption. We report on a high-yield, low-complexity patterning method based on exploiting the strong tendency of halogen-substituted organic semiconductors to crystallize along chemically tailored interfaces. We demonstrate that the organic semiconductor molecules self-align on the contacts, when the halogen–halogen interaction is allowed by the chemical structures and conformations of the self-assembled monolayer and organic semiconductor. The ordered films exhibit high mobilities and constrain the current paths. The regions surrounding the devices, where the interaction is inhibited, consist of randomly oriented molecules, exhibiting high-resistivity and electrically insulating neighboring devices. To identify the role of F–F interactions in the development of crystalline order, we investigate OTFTs fabricated on mono-fluorinated benzene thiol treated contacts, which allows us to isolate the interactions between the F originating from the organic semiconductor and the F in each position on the benzene ring of the thiol, and to selectively study the role of each interaction. Combining the results obtained from quantitative grazing incidence X-ray diffraction and Kelvin probe measurements, we show that the surface treatments induce structural changes in the films, but also alter the injection picture as a result of work function shifts that they introduce. We show that both effects yield variations in the field-effect transistor characteristics, and we are able to tune the field-effect mobility more than two orders of magnitude in the same material.

Effect of Processing Parameters on Performance of Spray-Deposited Organic Thin-Film Transistors

The performance of organic thin-film transistors (OTFTs) is often strongly dependent on the fabrication procedure. In this study, we fabricate OTFTs of soluble small-molecule organic semiconductors by spray-deposition and explore the effect of processing parameters on film morphology and device mobility. In particular, we report on the effect of the nature of solvent, the pressure of the carrier gas used in deposition, and the spraying distance. We investigate the surface morphology using scanning force microscopy and show that the molecules pack along the π-stacking direction, which is the preferred charge transport direction. Our results demonstrate that we can tune the field-effect mobility of spray-deposited devices two orders of magnitude, from 10−3u2009cm2/Vs to 10−1u2009cm2/Vs, by controlling fabrication parameters.

Flip chip lamination to electrically contact organic single crystals on flexible substrates

The fabrication of top metal contacts for organic devices represents a challenge and has important consequences for electrical properties of such systems. We report a robust, low-cost and nondestructive printing process, flip chip lamination, to fabricate top contacts on rubrene single crystals. The use of surface chemistry treatments with fluorinated self-assembled monolayers, combined with pliable substrates, and mild nanoimprint conditions, ensures conformal contact between ultrasmooth metal contacts and the organic crystal. Space-charge limited current measurements point to better interfacial electrical properties with the flip chip lamination-fabricated contacts compared to the analog architecture of e-beam evaporated top contacts.

Tuning the microstructure and electronic performance in organic thin-film transistors using chemical modifications at interfaces

A challenge with the study of organic thin-film transistors rests in understanding the mechanism behind microstructure formation and its effects on the charge carrier mobility. Often, increased order in the molecular packing of the organic semiconductor film results in a superior mobility. The microstructure of the organic thin-films can be tuned by chemically modifying the surfaces using treatments, such as self-assembled monolayers (SAMs). In 2,8-difluoro-5,11-bis(triethylsilylethynyl) (diF-TES ADT) devices with pentafluorobenzenethiol (PFBT) treated contacts, for example, high mobility regions are produced on the gold contacts, due to large grain growth, while low mobility regions are formed off the contacts, as a result of small grain growth. To explore the mechanism driving this effect, we selectively choose fluorinated contact treatments which are able to introduce targeted interactions at interface between organic semiconductor and SAM-treated contacts. This allows us to isolate key mechanisms behind microstructure formation. Through selecting a specific number of fluorine atoms and placing them into key locations on a benzene thiol base, we can tune the field-effect mobility from 10–3 cm2V-1s-1 to 0.1 cm2V-1s-1 in this organic semiconductor. We combine FET measurements with GIXD data to correlate thin-film microstructure with electronic properties. To investigate the mechanism responsible for these differences in device performance, we perform first-principle density functional theory calculations, which allow us to quantify the interactions at the interfaces between the organic semiconductor and the various contact treatments. We find that Fluorine-Fluorine interactions between the organic semiconductor and the Fluorinated SAM used for contact treatment induce this unique microstructure. Through this knowledge one can selectively choose processing parameters to attain desired film microstructure and improve device performance.

Metal-Molecule-Silicon Junctions Produced by Flip Chip Lamination of Dithiols: Effect of Molecular Length and Backbone

The integration of organic molecules with silicon is increasingly being studied for potential as hybrid electronic devices. Creating dense and highly ordered organic monolayers on silicon with reliable metal contacts still remains a challenge. A novel technique, flip chip lamination (FCL), has been developed to create uniform metal-molecule-semiconductor junctions. FCL uses nanotransfer printing to covalently attach self-assembled monolayers to a hydrogen-passivated Si(111) surface. Several dithiol molecules were studied to explore the role of molecular length and chemical structure on the physical and electronic properties of the molecules. The effects of the FCL process on the chemical and physical properties of the imbedded molecular layer were interrogated with polarized-backside reflectance absorption infrared spectroscopy. Electrical measurements were also performed to characterize device structure and to offer better insight into the mechanisms at play in the electronic transport.