The demands on preparing, processing and characterizing materials and structures and the need for full exploration of the parameter space has led to the development of a plethora of combinatorial approaches. Most experimental analytical combinatorial methods concentrate on probing only a single material property, such as chemical information, topography, etc. For some applications, it is desirable that multiple properties be probed simultaneously in a single experiment. Particularly, information on the chemistry and molecular organization of chemical groups on surfaces is needed to shed light on the behavior of amphiphilic surface modifiers or characterizing catalysts.
We have recently shown that information about the chemistry (including bond concentration) and molecular orientation on chemically heterogeneous surfaces can be collected by simultaneously utilizing near edge X-ray absorption fine structure (NEXAFS) spectroscopy and rastering the X-ray beam on the probed specimen. This rastering produces serially two-dimensional NEXAFS images in space and energy with sub-millimeter planar spatial resolution and sub-monolayer molecular sensitivity. We call this technique combinatorial NEXAFS.
The NEXAFS experiments were carried out at the NIST/Dow soft X-ray Materials characterization facility at the National Synchrotron Light Source at Brookhaven National Laboratory. The partial-electron-yield (PEY) signal is collected using a channeltron electron multiplier with an adjustable entrance grid bias (EGB). The monochromator energy resolution and photon energy were calibrated by comparing the transmission spectrum from gas-phase carbon monoxide with electron energy-loss reference data. To eliminate the effect of incident beam intensity fluctuations and monochromator absorption features, the PEY signal was normalized by the incident beam intensity obtained from the photo yield of a clean gold grid. The NIST/Dow materials characterization end-station is equipped with a computer controlled stepping motor actuated sample holder, which controls the orientation of the sample with respect to the polarization vector of the X-rays and enables rapid horizontal and vertical sample motion (see figure below).
Schematic illustrating the principle of combinatorial NEXAFS on probing the characteristics of a double molecular gradient. The double gradient structure is indicated pictorially by the green shading.
We demonstrate the capability of combinatorial NEXAFS by probing the chemistry and molecular orientation of semifluorinated molecules (1H,1H,2H,2H-Perfluorodecyltrichloro-silane, t-F8H2) forming molecular gradients of F8H2 in surface coverage on the flat silica substrates . We formed double molecular gradients of F8H2 on silica substrates using the methodology described in Formation and properties of molecular gradients.
The top panels of the figure above show the carbon K-edge PEY NEXFAS spectra collected from a homogeneous F8H2 SAM sample measured at θ=20, 50, and 90 deg, where θ is the angle between the sample normal and the electric field vector of the X-ray beam. The PEY NEXAFS spectra were normalized using standard procedures by adjusting the pre-edge and post-edge signals to 0 and 1, respectively. The dashed lines denote the positions of the 1s->σ* transitions for the C-F (E=292.0 eV), and C-C (E=295.4 eV) bonds. The fact that the intensities originating from these transitions change with varying angle θ (as θ increases the intensity corresponding to the 1s->σ* transitions of the C-F bond increases while that of the C-C bond decreases) indicates that the sample is well oriented. More detailed analysis presented and discussed elsewhere revealed that the F8H2 molecules stand almost perpendicular to the sample surface. The combinatorial NEXAFS experiments on the double gradient sample were carried out as previously described; the vertical motion (increments 0.5 mm) of the sample was controlled by the computer. After each vertical increment, a carbon K-edge PEY NEXAFS spectrum was taken at photon energies around the C-F (291.4 eV < E < 293.0 eV ) and C-C (294.4 eV < E < 297.0 eV) signals. In addition, PEY NEXAFS data were collected at the carbon ionization pre-edge (E=280.0 eV) and post-edge (E=320.0 eV). All PEY NEXFAS spectra were recorded at three different angular specimen orientations, θ=20, 50, and 90 deg. The corresponding normalized PEY NEXAFS intensities as a function of the incident photon energy beam and the position on the substrate are shown in the bottom panel of the above figure. The data show that the PEY NEXAFS intensities of the C-F and C-C bonds collected close to the edges (located at 0 and 50 mm) of the sample follow the same trends as those in the homogeneous SAM specimen. Thus, while at θ=20 deg the 1s->σ* for the C-C bond is higher than the 1s->σ* for the C-F bond, at θ=50 and 90 deg the situation is reversed. When moving from the two edges towards the interior of the double gradient, the 1s->σ* transition intensities for the C-F and C-C bonds decrease.