Nathan Cernetic

Multifunctional phosphonic acid self-assembled monolayers on metal oxides as dielectrics, interface modification layers and semiconductors for low-voltage high-performance organic field-effect transistors

Insulating and semiconducting molecular phosphonic acid (PA) self-assembled monolayers (SAMs) have been developed for applications in organic field-effect transistors (OFETs) for low-power, low-cost flexible electronics. Multifunctional SAMs on ultrathin metal oxides, such as hafnium oxide and aluminum oxide, are shown to enable (1) low-voltage (sub 2 V) OFETs through dielectric and interface engineering on rigid and plastic substrates, (2) simultaneous one-component modification of source-drain and dielectric surfaces in bottom-contact OFETs, and (3) SAM-FETs based on molecular monolayer semiconductors. The combination of excellent dielectric and interfacial properties results in high-performance OFETs with low-subthreshold slopes down to 75 mV dec(-1), high I(on)/I(off) ratios of 10(5)-10(7), contact resistance down to 700 Ω cm, charge carrier mobilities of 0.1-4.6 cm(2) V(-1) s(-1), and general applicability to solution-processed and vacuum-deposited n-type and p-type organic and polymer semiconductors.

Spin-cast and patterned organophosphonate self-assembled monolayer dielectrics on metal-oxide-activated Si.

An efficient process is developed for modifying Si with self-assembled monolayers (SAMs) through in situ metal oxide surface activation and microcontact printing or spin-coating of phosphonic-acid-based molecules. The utility of this process is demonstrated by fabricating self-organized and solution-processed low-voltage organic thin-film transistors enabled by patterned and spin-cast phosphonate SAM/metal oxide hybrid dielectrics.

Effects of self-assembled monolayer structural order, surface homogeneity and surface energy on pentacene morphology and thin film transistor device performance

A systematic study of six phosphonic acid (PA) self-assembled monolayers (SAMs) with tailored molecular structures is performed to evaluate their effectiveness as dielectric modifying layers in organic field-effect transistors (OFETs) and determine the relationship between SAM structural order, surface homogeneity, and surface energy in dictating device performance. SAM structures and surface properties are examined by near edge X-ray absorption fine structure (NEXAFS) spectroscopy, contact angle goniometry, and atomic force microscopy (AFM). Top-contact pentacene OFET devices are fabricated on SAM modified Si with a thermally grown oxide layer as a dielectric. For less ordered methyl- and phenyl-terminated alkyl ~(CH2)12 PA SAMs of varying surface energies, pentacene OFETs show high charge carrier mobilities up to 4.1 cm2 V-1 s-1. It is hypothesized that for these SAMs, mitigation of molecular scale roughness and subsequent control of surface homogeneity allow for large pentacene grain growth leading to high performance pentacene OFET devices. PA SAMs that contain bulky terminal groups or are highly crystalline in nature do not allow for a homogenous surface at a molecular level and result in charge carrier mobilities of 1.3 cm2 V-1 s-1 or less. For all molecules used in this study, no causal relationship between SAM surface energy and charge carrier mobility in pentacene FET devices is observed.

PCBM-doped electro-optic materials: investigation of dielectric, optical and electro-optic properties for highly efficient poling

Fullerenes are ubiquitously popular in organic electronic materials and devices. The high electron affinity, electron mobility and percolated networks for electron transport of fullerene derivatives, such as PCBM, have established them as excellent electron acceptors and transport materials in organic solar cells and electronic devices. It is intriguing to utilize these electronic properties and molecular three-dimensional networks to explore their potential applications in new electronic or optical devices. In this work, PCBM was doped into organic electro-optic (EO) materials and their surface morphology, photophysical properties, dielectric properties as well as optical properties (refractive index) were systematically investigated. It was found that the dielectric constant and refractive index of the doped materials were significantly enhanced. Based on temperature-dependent dielectric constant measurements, the relation between relative microscopic dipole moment and dielectric properties was established. It revealed that, at the poling temperature, the dipole moment of chromophores in the PCBM-doped film P1/PCBM was higher than that of the conventional EO film P1. This enhanced microscopic property of chromophores in P1/PCBM well accounted for the improved poling results in electric field induced poling. A larger EO coefficient (197 pm V−1versus 133 pm V−1) and figure-of-merit n3r33 (1002.9 versus 632.2), as well as a higher order parameter (15.7% versus 10.6%) and birefringence were achieved for the PCBM-doped film P1/PCBM, demonstrating the significant potential of PCBM to be used in organic EO materials and devices.

Doping Versatile n-Type Organic Semiconductors via Room Temperature Solution-Processable Anionic Dopants

In this study, we describe a facile solution-processing method to effectively dope versatile n-type organic semiconductors, including fullerene, n-type small molecules, and graphene by commercially available ammonium and phosphonium salts via in situ anion-induced electron transfer. In addition to the Lewis basicity of anions, we unveiled that the ionic binding strength between the cation and anion of the salts is also crucial in modulating the electron transfer strength of the dopants to affect the resulting doping efficiency. Furthermore, combined with the rational design of n-type molecules, an n-doped organic semiconductor is demonstrated to be thermally and environmentally stable. This finding provides a simple and generally applicable method to make highly efficient n-doped conductors which complements the well-established p-doped organics such as PEDOT:PSS for organic electronic applications.

Solid-state densification of spun-cast self-assembled monolayers for use in ultra-thin hybrid dielectrics

Ultra-thin self-assembled monolayer (SAM)-oxide hybrid dielectrics have gained significant interest for their application in low-voltage organic thin film transistors (OTFTs). A [8-(11-phenoxy-undecyloxy)-octyl]phosphonic acid (PhO-19-PA) SAM on ultrathin AlOx (2.5 nm) has been developed to significantly enhance the dielectric performance of inorganic oxides through reduction of leakage current while maintaining similar capacitance to the underlying oxide structure. Rapid processing of this SAM in ambient conditions is achieved by spin coating, however, as-cast monolayer density is not sufficient for dielectric applications. Thermal annealing of a bulk spun-cast PhO-19-PA molecular film is explored as a mechanism for SAM densification. SAM density, or surface coverage, and order are examined as a function of annealing temperature. These SAM characteristics are probed through atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure spectroscopy (NEXAFS). It is found that at temperatures sufficient to melt the as-cast bulk molecular film, SAM densification is achieved; leading to a rapid processing technique for high performance SAM-oxide hybrid dielectric systems utilizing a single wet processing step. To demonstrate low-voltage devices based on this hybrid dielectric (with leakage current density of 7.7×10-8 A cm-2 and capacitance density of 0.62 µF cm-2 at 3 V), pentacene thin-film transistors (OTFTs) are fabricated and yield sub 2 V operation and charge carrier mobilites of up to 1.1 cm2 V-1 s-1.

Photo-induced denitrogenation of triazoline moieties for efficient photo-assisted poling of electro-optic polymers

Efficient room temperature poling of a guest–host electro-optic (EO) polymer is achieved by photo-induced denitrogenation of triazoline moieties. The host polymer (P1) with Δ2-1,2,3-triazoline groups on the backbone is synthesized by 1,3-dipolar cycloaddition polymerization between a bis-N-phenylmaleimide and an aromatic bis-azide. The guest chromophore (C1) is a push–pull compound with a strong dialkylaminophenyl donor and a CF3–TCF acceptor. By irradiating the thin films of P1 with a compact UV lamp at 365 nm, the angular and asymmetric 1,2,3-triazoline structures can be readily converted into a more straight and symmetrical aziridine structure by losing nitrogen. This photo-induced denitrogenation provides the clean conversion to form an aziridine-based polymer P2. This high efficiency conversion process is insensitive to the light-absorbing nonlinear optical chromophores, and the reaction rate can be controlled by the irradiation power and the thin film UV absorbance. Upon the photo-induced denitrogenation, considerable structural change of the polymer backbone provides sufficient rotational freedom to the chromophores for the poling of EO polymers. This has been verified by the demonstration of high field poling (up to 250 V μm−1) and large EO activity at RT for thin films containing 20–25 wt% of chromophore C1 in P1, which can be efficiently activated by photochemical denitrogenation of triazoline moieties by irradiating with low-power UV light.

Influence of self-assembled monolayer binding group on graphene transistors

Graphene transistors on self-assembled monolayer (SAM) modified dielectric substrates were fabricated and characterized in order to determine the influence SAM binding group has on device properties. It was found that silane based alkyl SAMs had little to no influence in doping graphene transistors, while phosphonic acid based ones caused n-type doping of graphene transistors with a charge neutrality point shift of over 10 V. It was also discovered that alkyl SAM packing density influenced the doping magnitude. Due to substrate surface charge trap quenching, these SAMs independent of binding group enhanced charge mobility of graphene transistors compared to ones on bare oxide substrates.