AVS2012 Session TF2-TuA: ALD for Hybrid Films and Bioapplications

Atomic layer deposition (ALD) is demonstrated as a method to fabricate NP embedded thin films, and provides new opportunities to alter the characteristic properties of nanomaterials. This work examines conducting and semiconducting ALD materials growth on nanoparticles and nanowires for opportunities to alter the optical behavior and conductive behaviors of nano-enabled materials. For example, gold nanoparticles (15 nm diameter) are embedded into dense inorganic zinc oxide nanofilms deposited by atomic layer deposition onto a fibrous textile template. By changing the dielectric field surrounding the nanoparticle with the ALD ZnO, the surface plasmon resonance is dampened, resulting in significant changes to the optical absorption behavior of the textile. The alteration of the surface plasmon resonance is examined with increasing nanoparticles concentration on the fiber surface and with increasing ALD coating thickness. For example, the absorption at 900 nm is enhanced by up to 4.8x for a 45 nm ZnO ALD coating. Minimal increase in absorption is observed with additional ZnO growth. Cathodoluminescence evaluation of ZnO ALD on Au nanoparticles -loaded nylon-6 produces a ~65% decrease in the defect luminescence and a corresponding ~80% increase in the band edge luminescence. In addition, an analysis of the electrical properties of nanoparticle and nanowire embedded ALD thin films are provided. Using externally fabricated nanomaterials and embedding them in ALD thin films offers the ability to study and understand near surface interactions that can alter the characteristics of the nanomaterials.

We recently reported the use of patterned carbon nanotube (CNT) forests as scaffolds for the microfabrication of silica-based thin-layer chromatography plates (TLC) (Advanced Functional Materials2011, 21(6), 1132 – 1139). In this fabrication, CNTs were infiltrated by low pressure chemical vapor deposition of silicon using SiH₄, which was then oxidized and hydrated. Thorough characterization by RBS, XPS, TEM, SEM, and ToF-SIMS has been performed on these materials, which has given us a considerable understanding of them, e.g., the structure of the Si/SiO₂/Al₂O₃(30 nm)/Fe(6 nm) stack deposited prior to CNT growth has been confirmed, multiwall CNTs are grown in our process, CNT growth is base (bottom up) and not tip (top down), the thickness of the Fe catalyst layer plays a key role in the fabrication of our plates – when the catalyst layer is too thick CNT structures are unstable, etc. Fast and efficient separations were demonstrated on these plates.

Nevertheless, the oxidation of silicon in these materials leads to a volume expansion of the support, which appears to affect the A-term of the van Deemter equation in our separations. That is, distortions in the material appear to adversely affect separations performed on them. Accordingly, we have now shown that TLC plates can be fabricated by (i) priming patterned nanotube forests with a few nanometers of amorphous carbon, followed by a layer of Al₂O₃ deposited by atomic layer deposition (ALD), (ii) deposition of SiO₂ in a fast (pseudo) ALD of this material, and (iii) oxidizing at a lower temperature (ca. 600 K) than was used previously to remove the CNTs. An amino bonded phase was created. The resulting TLC plates show 125,000 – 225,000 N/m in a baseline separation of four fluorescent dyes. An even more recent and newly developed microfabrication method in our laboratory of CNT-templated TLC plates also shows fast separations with 400,000 – 500,000 N/m in preliminary results. These separations rival (or exceed) those of HPLC/UPLC in both speed and efficiency. Separations of biological interest will be shown.

Alucones and zincones are metal alkoxide polymer films that can be grown using molecular layer deposition (MLD) techniques. In this study, alucones and zincones were grown using trimethlyaluminum (TMA) and diethylzinc (DEZ) as the metal precursors and hydroquinone (HQ) as the organic reactant. HQ is an aromatic diol that has a rigid structure with a central benzene ring. The benzene ring may lead to interesting electrical conductivity and the rigid structure may help to avoid “double reactions”. Quartz crystal microbalance measurements and X-ray reflectivity (XRR) studies were employed to examine film growth. Individual mass gains after the TMA and HQ exposures were consistent with a TMA:HQ stoichiometry of 4:3 in the MLD film. This TMA:HQ stoichiometry suggests the presence of Al₂O₂ dimeric core species. In comparison, individual mass gains after the DEZ and HQ exposures were consistent with a DEZ:HQ stoichiometry of 1:1 in the MLD film. XRR studies measured growth rates at 150°C of 2.6 Å/cycle for TMA/HQ and 2.7 Å/cycle for DEZ/HQ. The alucone and zincone MLD films were also annealed in air at temperatures up to 350^oC. There is evidence that the benzene rings polymerize after heating to 200°C. The films turn brown and a strong absorbance observed at ~320 nm is consistent with an expanded π-conjugated system. The polymerization of the benzene rings should change the mechanical properties of these annealed MLD films. Alucone and zincone films were also grown using tetrafluorohydroquinone to observe the effect of fluorination of the benzene ring.