Zinc oxide (ZnO) is an important material for hybrid inorganic-organic devices in which the characteristics of the interface can dominate both the structural and electronic properties of the system. These characteristics can be modified through chemical functionalization of the ZnO surface. One of the possible strategies involves covalent bonding of the modifier using silane chemistry. Whereas a significant body of work has been published regarding silane attachments to glass and SiO2, there is less information about the efficacy of this method for controlling the surface of metal oxides. Here we report our investigation of molecular layers attached to polycrystalline ZnO through silane bonding, controlled by an amine catalyst. The catalyst enables us to use triethoxysilane precursors and thereby avoid undesirable multilayer formation. The polycrystalline surface is a practical material, grown by sol-gel processing, that is under active exploration for device applications. Our study included terminations with alkyl and phenyl groups. We used water contact angles, infrared spectroscopy, and X-ray photoemission spectroscopy to evaluate the modified surfaces. Alkyltriethoxysilane functionalization of ZnO produced molecular layers with submonolayer coverage and evidence of disorder. Nevertheless, a very stable hydrophobic surface with contact angles approaching 106 degrees resulted. Phenyltriethoxysilane was found to deposit in a similar manner. The resulting surface, however, exhibited significantly different wetting as a result of the nature of the end group. Molecular layers of this type, with a variety of surface terminations that use the same molecular attachment scheme, should enable interface engineering that optimizes the chemical selectivity of ZnO biosensors or the charge-transfer properties of ZnO-polymer interfaces found in oxide-organic electronics.

Zinc oxide films are deposited over the temperature range 25-120 8C by plasma-enhanced atomic layer deposition (PEALD) using dimethyl zinc (DMZ, Zn(CH 3 ) 2 ) as the metal precursor. DMZ is unreactive with molecular oxygen under process conditions, facilitating its utility for PEALD. The deposition rate is independent of plasma exposure (1-5 s) and saturates for DMZ exposures !75 mTorr s. The saturated growth per cycle increases from 1.6 to 2.9 Å per cycle as the temperature is increased from 25 to 85 8C, and remains constant in the range 85-120 8C. Over the narrow range explored, substrate temperature has a dramatic impact on film structure and properties. Polycrystalline films with a (100) texture are obtained at room temperature, with the preferred orientation evolving to the (002) direction as the temperature increases. The electrical resistivity decreases exponentially from 270 at room temperature to values of $5 V cm within the ALD window. Compositional analysis shows that films at low temperature are contaminated by carbon and hydroxyl impurities, however these defects are attenuated with temperature and not detected in films deposited within the ALD window. Room temperature photoluminescence is dominated by strong emission from ZnO defects in most films, however these signals are attenuated and strong band edge emission is observed for films deposited at 120 8C.

Hybrid organic/inorganic solar cells have not lived up to their potential because of poor interface properties. Interfacial molecular layers provide a way of adjusting these devices to improve their performance. We have studied a prototypical system involving poly(3-hexylthiophene) (P3HT) on planar zinc oxide (ZnO) films that have been modified with two types of molecules having identical 18-carbon alkyl chain termination and different surface attachments: octadecanethiol (ODT) and octadecyltriethoxysilane (OTES). We examined the functionalized surfaces using water contact angle measurements, Kelvin probe measurements, infrared absorbance spectroscopy, and atomic force microscopy. These have shown that OTES forms disordered incomplete monolayers, while ODT is prone to develop multilayered islands. Both treatments enhance polymer ordering. However, inverted solar cell devices fabricated with these treated interfaces performed very differently. ODT improves the short circuit current (J SC), open circuit voltage (V OC), and power conversion efficiency (η), while these parameters all decrease in devices constructed from OTES-treated ZnO. The differences in V OC are related to modifications of the surface dipole associated with deposition of the two types of alkyl molecules, while changes in J SC are attributed to a balance between charge transfer blocking caused by the saturated hydrocarbon and the improved hole mobility in the polymer.

The parameters for an effective laser-induced forward-transfer (LIFT) process of aluminum thin films using a femtosecond laser are studied. Deposited feature size as a function of laser fluence, donor film thickness, quality of focus, and the pulse duration are varied, providing a metric of the most desirable conditions for femtosecond LIFT with thin aluminum films.