The dielectric permittivity and electric breakdown strength of nanocomposites comprising poly(vinylidene fluoride-co-hexafluoro propylene) and phosphonic acid surface-modified BaTiO(3) nanoparticles have been investigated as a function of the volume fraction of nanoparticles. The mode of binding of pentafluorobenzylphosphonic acid on the BaTiO(3) particles was investigated using infrared and (31)P solid-state nuclear magnetic resonance spectroscopy, and the phosphonic acid was found to form well ordered, tightly bound monolayers. The effective permittivity of nanocomposites with low volume fractions (<50%) was in good agreement with standard theoretical models, with a maximum relative permittivity of 35. However, for nanoparticle volume fractions of greater than 50%, the effective permittivity was observed to decrease with increasing nanoparticle volume fraction, and this was correlated with an increase in porosity of the spin-coated nanocomposite films. The dielectric breakdown strength was also found to decrease with increasing volume fraction of the BaTiO(3) nanoparticles, with an abrupt decrease observed around 10% and a gradual decrease for volume fractions of 20-50%. Comparison of these results with model calculations, using statistical particle packing simulations and effective medium theory for the permittivity and breakdown strength, indicates the important roles of nanoparticle percolation and porosity of the nanocomposites on the dielectric properties. The measured energy density at a field strength of 164 V/mum, well below the breakdown strength, increased to a value of 3.2 J/cm(3) as the nanoparticle volume fraction is increased to 50%, roughly in line with the trend of the permittivity. The calculated maximum energy densities indicate maximal extractable energy (7-8 J/cm(3) at 1 kHz) for two different particle volume fractions, as a result of the interplay of the dependencies of permittivity and breakdown strength on volume fraction.

Materials with high dielectric permittivity are important in electronic components such as capacitors, gate dielectrics, memories, and power-storage devices. [1][2][3][4] Conventional highpermittivity materials such as barium titanate (BT) can be processed into thin films by using chemical solution deposition yielding a relative permittivity (e r ) of about 2500 and relatively low dielectric loss but require high-temperature sintering, which is not compatible with many substrate materials.[ [7][8][9][10] Polymer/ceramic nanocomposites in which high-e r metal oxide nanoparticles such as BT [11,12] and lead magnesium niobate-lead titanate (PMN-PT) [1,13,14] are incorporated into a polymer host are of significant current interest. The combination of high-e r nanoparticles with high-dielectric-strength polymer hosts offers the potential to obtain processable highperformance dielectric materials. Simple solution processing of BT particles in a polymer host generally results in poor film quality and inhomogeneities, which are mainly caused by agglomeration of the nanoparticles. Addition of surfactants, such as phosphate esters and oligomers thereof, can improve the dispersion of BT nanoparticles in host polymers and consequently the overall nanocomposite film quality. [1,13,15] However, in such systems, residual free surfactant can lead to high leakage current and dielectric loss. [16] Thus, approaches to bind surface modifiers to BT nanoparticles via robust chemical bonds are highly desirable. Ramesh et al. have reported on the use of trialkoxysilane surface modifiers for the dispersion of BT nanoparticles in epoxy polymer hosts resulting in nanocomposites with reasonably high e r , up to 45. [12] With the objective of identifying ligands that can form stable bonds to a BT surface through coordination or condensation, we have investigated a series of different ligand functionalities. In this Communication, we report that phosphonic acid ligands effect robust surface modification of BT and related nanoparticles and that the use of particles modified with suitable phosphonic acid ligands leads to well-dispersed BT nanocomposite films with high e r and high dielectric strength.We have investigated the binding of a variety of ligands to the surface of BT nanoparticles, as the stability of the binding on the surface is vital to effective surface modification.[17] We examined the following set of ligands, each bearing an aliphatic octyl chain with a different terminal binding group: C 8 H 17 -X, where X = PO(OH) 2 (OPA), SO 2 ONa (OSA), Si(OCH 3 ) 3 (OTMOS), and CO 2 H (OCA). Trialkoxysilanes are widely used surface modifiers for silicate, indium tin oxide, and other metal oxide surfaces. Phosphonic acids have been reported to modify TiO 2 , ZrO 2 , and indium tin oxide surfaces [18][19][20] and are thought to couple to the surface of metal oxides either by heterocondensation with surface hydroxyl groups or coordination to metal ions on the surface.[18] Carboxylic acid and sulfonic acid groups may also bind to the surface ...

Indium−tin oxide (ITO) electrodes have been modified with both fluorinated alkyl and aryl phosphonic acids [n-hexylphosphonic acid (HPA) and n-octadecylphosphonic acid (ODPA); 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl phosphonic acid (FHOPA), pentafluorobenzyl phosphonic acid (PFBPA), and tetrafluorobenzyl-1,4-diphosphonic acid (TFBdiPA)]. These are modifiers designed to control both wetting properties toward nonpolar molecular solids and to provide a wide range of tunability in effective surface work function. The molecular nature of surface attachment and changes in electronic and wetting properties were characterized by X-ray photoelectron spectroscopy (XPS), UV-photoelectron spectroscopy (UPS), photoelastic modulation infrared reflection−absorption spectroscopy (PM-IRRAS), and contact angle measurements using both water and hexadecane. Interface dipoles from the PA modifiers contribute to shifts in the low kinetic energy regions of UPS spectra (local vacuum level shifts, which translate into changes in effective surface work function). We show that for ITO surfaces modified with FHOPA, and to a lesser extent with PFBPA, the high work function obtained by oxygen plasma cleaning can be maintained after modification, while decreasing the polar component of surface energy. This approach to oxide surface modification is a strategy that may be beneficial for the modification of transparent conducting oxide surfaces in both organic light emitting diodes and in organic solar cells, where oxide/organic compatibility can affect device performance.

Transparent metal oxides, in particular, indium tin oxide (ITO), are critical transparent contact materials for applications in next-generation organic electronics, including organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs). Understanding and controlling the surface properties of ITO allows for the molecular engineering of the ITO-organic interface, resulting in fine control of the interfacial chemistries and electronics. In particular, both surface energy matching and work function compatibility at material interfaces can result in marked improvement in OLED and OPV performance. Although there are numerous ways to change the surface properties of ITO, one of the more successful surface modifications is the use of monolayers based on organic molecules with widely variable end functional groups. Phosphonic acids (PAs) are known to bind strongly to metal oxides and form robust monolayers on many different metal oxide materials. They also demonstrate several advantages over other functionalizing moieties such as silanes or carboxylic acids. Most notably, PAs can be stored in ambient conditions without degradation, and the surface modification procedures are typically robust and easy to employ. This Account focuses on our research studying PA binding to ITO, the tunable properties of the resulting surfaces, and subsequent effects on the performance of organic electronic devices. We have used surface characterization techniques such as X-ray photoelectron spectroscopy (XPS) and infrared reflection adsorption spectroscopy (IRRAS) to determine that PAs bind to ITO in a predominantly bidentate fashion (where two of three oxygen atoms from the PA are involved in surface binding). Modification of the functional R-groups on PAs allows us to control and tune the surface energy and work function of the ITO surface. In one study using fluorinated benzyl PAs, we can keep the surface energy of ITO relatively low and constant but tune the surface work function. PA modification of ITO has resulted in materials that are more stable and more compatible with subsequently deposited organic materials, an effective work function that can be tuned by over 1 eV, and energy barriers to hole injection (OLED) or hole-harvesting (OPV) that can be well matched to the frontier orbital energies of the organic active layers, leading to better overall device properties.

Benzylphosphonic acids with various fluorine substitutions are designed and synthesized. They are used to modify ITO such that the work function can be tuned over a range of 1.2 eV while keeping the surface energy relatively constant. The experimentally measured work function changes are also compared to and agree well with those estimated from DFT calculations.

Indium tin oxide (ITO) is currently the most widely used transparent electrode in organic light-emitting devices and solar cells as well as in liquid-crystal displays. The electronic and geometric structure of the interface formed between the ITO surface and the organic overlayer strongly affects the charge injection characteristics and the overall efficiency of the organic electronic devices. 1 Controlling the composition of this interface can be challenging, since there is often a complex mixture of the stoichiometric oxide, hydroxides, and even oxy-hydroxides in the near surface region, whose ratios strongly depend upon the source of the ITO, cleaning and activation procedures, and modification protocols using chemisorption of small molecules. 2 Chemical modification of an ITO surface via smallmolecule organic adsorbates provides a means for tuning interfacial charge injection and constitutes a promising route toward increasing device efficiency in both organic light emitting diodes and solar cells. 2c Among various smallmolecule compounds capable of self-assembling on OHterminated surfaces, phosphonic acids (PAs) are especially promising for surface modifications of various oxides including ITO, since they form robust monolayers without the need to resort to cross-linking, as is common, for example, in silane surface modification. 2a,b Several binding scenarios have been proposed for PA adsorption on transition metal oxide surfaces, which differ in the number of oxygen atoms bound to the surface and the involvement of hydrogen bonding. The type of adsorption mode can change the orientation of the modifier and the net surface dipole at the ITO/modifier interface, which can be important in determining both wettability and effective surface work function; therefore, it is important to be able to describe the possible adsorption modes and to differentiate among them.Typical proposed PA adsorption modes on metal oxides are shown in Scheme 1. The predominant adsorption modes depend on the type of oxide surface as well as on the reaction conditions. For example, modes (a) (monodentate) and (b) (bidentate + electrostatic) have been suggested for PA adsorption on TiO 2 , 4a Al 2 O 3 , 6a and BaTiO 3 , 7b while tridentate mode (d) has been proposed to dominate on ZrO 2 5 and SiO 2 . 8 PA adsorption on ITO has been described to occur via multiple modes, with a predominance of bidentate and tridentate modes (c) and (d), as indicated by a combination of X-ray photoelectron spectroscopy (XPS) and FT-IR studies. 3a,c There remains some uncertainty in the reported spectroscopic studies, in particular XPS studies, that have been used to discern among PA adsorption modes due to a lack of precise knowledge of the spectroscopic features specific to each binding mode.Here, we present what we believe to be the first theoretical characterization, based on density functional theory (DFT), (1) (a) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. AdV. Mater. 1999, 11, 605-625. (b) Salaneck, W.; Seki, K.; Kahn, A.; Pireaux, J.-J., Eds. Con...

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