Atomic Layer Deposition of Aluminum-Free Silica onto Patterned Carbon Nanotube Forests in the Preparation of Microfabricated Thin-Layer Chromatography Plates

JPC – Journal of Planar Chromatography – Modern TLC - We describe the direct, conformal, atomic layer deposition (ALD) of silica onto carbon nanotubes (CNTs) in the microfabrication of...     

Hydroxylation of the silica in microfabricated thin layer chromatography plates as probed by time‐of‐flight secondary ion mass spectrometry and diffuse reflectance infrared Fourier transform spectroscopy

Microfabricated silica thin layer chromatography (TLC) plates have previously been prepared on patterned carbon nanotube forests. The high temperatures used in their fabrication reduce the number of hydroxyl groups on their surfaces. Fortunately, silica can be rehydroxylated. In diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), a silanol peak below 3740 cm^?1 indicates a well‐hydroxylated silica surface that is fit for chromatography. Hydroxylations of our materials with HF are so effective that it is not possible to discern the position of this peak. In contrast, this signal is discernable when the plates are treated with NH₄OH. To find a more convenient method for studying the surfaces of TLC plates, time‐of‐flight secondary ion mass spectroscopy (ToF‐SIMS) was considered. ToF‐SIMS is advantageous because multiple microfabricated TLC plates must be scraped to obtain enough silica for one DRIFT analysis, while static SIMS can be performed on very small regions (500 × 500 µm² or less) of individual plates. Ratios of the SiOH⁺ and Si⁺ ToF‐SIMS signals for microfabricated TLC plates correlated well with ~3740 cm^?1 silanol peaks from DRIFT. Thus, SIMS allows direct analysis of all of our treated and untreated plates, including those hydroxylated with HF. The best hydroxylation condition for HF, which was better than any studied for NH₄OH, was around 150 ppm at room temperature. The best hydroxylation condition for NH₄OH was 50 °C for 72 h. ToF‐SIMS versus DRIFT results of commercial TLC plates were also obtained and evaluated. Copyright © 2014 John Wiley & Sons, Ltd.     

Multi‐instrument characterization of the surfaces and materials in microfabricated,carbon nanotube‐templated thin layer chromatography plates. An analogy to ‘The Blind Men and the Elephant’

We apply a suite of analytical tools to characterize materials created in the production of microfabricated thin layer chromatography plates. Techniques used include X‐ray photoelectron spectroscopy (XPS), valence band spectroscopy, time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) in both positive and negative ion modes, Rutherford backscattering spectroscopy (RBS), and helium ion microscopy. Materials characterized include: the Si(100) substrate with native oxide: Si/SiO₂, alumina (35 nm) deposited as a diffusion barrier on the Si/SiO₂: Si/SiO₂/Al₂O₃, iron (6 nm) thermally evaporated on the Al₂O₃: Si/SiO₂/Al₂O₃/Fe, the iron film annealed in H₂ to make Fe catalyst nanoparticles: Si/SiO₂/Al₂O₃/Fe(NP), and carbon nanotubes (CNTs) grown from the Fe nanoparticles: Si/SiO₂/Al₂O₃/Fe(NP)/CNT. The Fe films and nanoparticles appear in an oxidized state. Some of the analyses of the CNTs/CNT forests appear to be unique: (i) the CNT forest appears to exhibit an interesting ‘channeling’ phenomenon by RBS, (ii) we observe an odd–even effect in the SIMS spectra of C_n^‐ species for n = 1 – 6, with the n ≥ 6 ions showing a steady decrease in intensity, and (iii) valence band characterization of CNTs using X‐radiation is reported. Initial analysis of the CNT forest by XPS shows that it is 100 at.% carbon. After one year, only ca. 0.25 at.% oxygen is observed. The information obtained from the combination of the different analytical tools provides a more complete understanding of our materials than a single technique, which is analogous to the story of ‘The Blind Men and the Elephant’. The raw XPS and ToF‐SIMS spectra from this study will be submitted to Surface Science Spectra for archiving. Copyright © 2013 John Wiley & Sons, Ltd.