Highlights

Research and education highlights are brief, digestible summaries of recent significant research results or education activities, chosen for their potential interest to a broad audience.

Recent Research Highlights

Metal slanted columnar thin film
THz optical sensors

T. Hofmann, D. Schmidt. E. Schubert, and M. Schubert
CNFM, Nebraska

  The optical properties of sculptured thin films consisting of self-organized highly spatially coherent nanostructures differ significantly from the constituent material’s bulk form. If prepared from electrically conductive materials, sculptured thin films offer the interesting opportunity to design transparent materials with very large dielectric polarizabilities. The coupling between adjacent dipoles results in an electromagnetic response, which is strongly directional dependent and can be observed as optical anisotropy.
  CNFM researchers at the University of Nebraska explore the optical properties of metal sculptured thin films in the THz frequency domain. Even at THz frequencies the optical response of metal sculptured thin films was found to be anisotropic and quantified for the first time [Appl. Phys. Lett. 99, 081903 (2011)].

THz STF sensor concept

Figure: Schematic drawing of the polarization effects for a slanted columnar thin film if the ambient is changed from air (left) to a dielectric (right) which leads to screening of the surface charges induced by the probing electromagnetic radiation.

  Now members of the CNFM team have discovered that the anisotropic optical fingerprint of metal STFs strongly depends on the dielectric properties of the ambient surrounding the STF. This observation may provide new avenues for the design of THz sensors for the detection of minute amounts of chemicals and biological molecules.

Protein adsorption on and swelling of polyelectrolyte brushes:
A simultaneous ellipsometry-quartz crystal microbalance study

E. Bittrich, K.-J. Eichhorn, M. Stamm, and P. Uhlmann
Leibniz Institute of Polymer Research, Dresden, Germany

K. B. Rodenhausen, T. Hofmann, and M. Schubert
  CNFM, Nebraska

  “Smart surfaces” that naturally and automatically respond to environmental stimuli are of contemporary interest for controlling material surface properties, such as wettability and electrostatic charge. An understanding of the various forces that interplay at the solid-liquid interface is crucial for the development of such novel surfaces.
   Polyelectrolyte brushes, on the surface of a material that is immersed in liquid solution, can respond to temperature, pH, and salt concentration shifts by swelling or deswelling with the ambient solution. Members of an international collaboration between the Leibniz Institute of Polymer Research in Dresden, Germany, and the University of Nebraska-Lincoln CNFM applied a combinatorial measurement approach consisting of spectroscopic ellipsometry and quartz crystal microbalance with dissipation techniques to monitor the real-time swelling behavior of polyelectrolyte brushes during ambient pH and salt concentration shifts [Biointerphases 5, 159 (2010)].
  The polyelectrolyte brushes were also exposed to bovine serum albumin protein in the ambient solution, and the protein adsorption and desorption behavior during liquid environmental changes was studied.

Smart Surface

Picture: Researchers study the swelling behavior of polyelectrolyte brushes to learn about solid-liquid interface phenomena.

   The combination of optically and mechanically based instrumentation allowed the researchers to separately consider the forces that liquid and salt ions exerted on the polyelectrolyte film.
   Demonstrating a new methodology to evaluate solid-liquid interface phenomena, this work, as of June 2011, has been featured each month in the Top 20 Most Downloaded Articles by its publishing journal, Biointerphases, since January 2011.

Optical properties of cobalt slanted columnar thin films passivated by atomic layer deposition

D. Schmidt, E. Schubert, and M. Schubert
CNFM, Nebraska

  The anisotropic optical properties of three-dimensional nanostructures do not only depend on the geometry but also on the material, of course. However, almost all pure metals start to develop a native oxide layer of a few nanometers as soon as they are exposed to ambient air. This, typically self-limiting, native oxide growth preponderates if individual nanostructures have dimensions in the same order - they could entirely change composition. Therefore, in order to preserve the intrinsic physical properties of nanostructured metal thin films the surface has to be passivated to prevent oxidation in air.
   The way pursued by CNFM researchers is to use atomic layer deposition, which allows for thin conformal coatings of three-dimensional objects thereby preserving the highly porous nanostructured film characteristic. It was found, that a Al2O3 layer as thin as three nanometers is sufficient to hinder metal nanostructure oxidation and hence preserve film morphology as well as the as-deposited physical film properties over time [Appl. Phys. Lett. 100, 011912 (2012)]. These findings are based on non-destructive optical ellipsometry measurements and data analysis allows for determination of the anisotropic optical properties and quantification of fractions of individual film constituents. This is particularly interesting for ongoing investigations in the field of new detection mechanisms for minute amounts of bio-chemical substances.









ALD passivation

Picture: Schematic drawing of a core-shell Co slanted columnar thin film coated with an Al2O3 passivation layer (3 ALD cycles depicted).

 

The direct growth of graphene on Co3O4 by molecular beam epitaxy

Mi Zhou1, Frank L Pasquale1, Peter A Dowben2, Alex Boosalis3, Mathias Schubert3, Vanya Darakchieva4, Rositza Yakimova4, Lingmei Kong2, and Jeffry A. Kelber3
1 Department of Chemistry, University of North Texas
2 Department of Physics and Astronomy and Nebraska Center for Materials and Nanoscience, University of Nebraska—Lincoln
3 Department of Electrical Engineering, University of Nebraska—Lincoln, CNFM
4 Department of Physics, Chemistry and Biology (IFM), Linköping University

   In a recent article highlighted by the IOP, CNFM researchers and co-authors investigate the direct growth of graphene on dielectric substrates. Developing growth techniques using practical and scalable methods is a key step toward the industrial development of graphene electronic and spintronic devices. The electronic properties of graphene, including high room-temperature carrier mobilities, long carrier mean free paths, polarizability in proximity to a magnetic substrate and long spin diffusion lengths, have exciting potential for charge or spin-based device applications. UNL researchers and co-authors have used molecular beam epitaxy (MBE) to deposit graphene in a controlled, manner on Co3O4(111), as reported in J. Phys.: Condens. Matter 24 072201. In direct graphene growth on substrates, graphene domain size and continuity (continuous sheets as opposed to nanoflakes), interactions between graphene and the substrate, and orientations between adjacent graphene layers are all significant issues. Auger and low energy electron diffraction (LEED) data in the article show the formation of (111)-ordered sp2 carbon films—large domain graphene—on the oxide substrate with thicknesses as low as 0.4 monolayers (ML) and as high as 3 ML. Photoemission and spectroscopic ellipsometry measurements indicate that the graphene layers display electronic properties similar to those of graphene grown on SiC(0001) or transferred to SiO2, but with significant graphene-to-oxide charge transfer.









Picture: Low-energy electron diffraction (LEED) and corresponding line scan data for (a,b) 0.4 ML graphene on Co3O4(111), and (c,d) 3 ML graphene on Co3O4(111.

 

Generalized ellipsometry in-situ quantification of organic adsorbate attachment within slanted columnar thin films

 K.B. Rodenhausen, D. Schmidt, T. Kasputis, A.K. Pannier, E. Schubert, and M. Schubert
CNFM, Nebraska

Sensitivity to the adsorption of organic materials onto surfaces is important for diagnostics, contraband control, and tissue engineering applications. The CNFM is pursuing a new optical detection principle based on changes in the birefringence response of optically anisotropic surfaces upon organic adsorption. The larger area of a rough anisotropic surface also allows more organic material to attach to the surface for higher signal changes.

CNFM researchers prepared a film of slanted titanium nanocolumns onto a gold-coated quartz crystal. Simultaneous generalized ellipsometry (GE) and quartz crystal microbalance (QCM) measurements were taken of this sample in a liquid cell while a fibronectin protein solution was pumped through the liquid cell. The presence of adsorbed fibronectin caused a shift in the birefringence response that was measured by GE. Optical modeling allowed a quantitative determination of the quantity of adsorbed protein, and independent QCM results were in excellent agreement with GE. The results of this study were recently published [Opt. Express 20, 5419 (2012)].

This work provides a step toward the realization of spatially coherent three-dimensional surfaces that, after chemical functionalization with a probe molecule, would exhibit binding selectivity to analyte molecules of interest. GE measurements would allow confirmation that binding events occurred and determination of the extent of analyte binding.










Figure:  Illustrations of materials composing a porous slanted columnar thin film (SCTF) layer. (a) represents the Ti SCTF in air ambient. (b) applies after liquid replaces air as the ambient. (c) and (d) represent arrangements of an equivalent amount of organic adsorbate that displaces liquid. GE allows the determination of the volume fractions of these constituent materials.