Ultrahigh Spatial Resolution Infrared Spectroscopic Imaging of the Phase Behavior of Block-Copolymer Nanostructures

Reports: G7 48534-G7: Ultrahigh Spatial Resolution Infrared Spectroscopic Imaging of the Phase Behavior of Block-Copolymer Nanostructures

Markus B. Raschke, University of Washington (Seattle)

Figure 1 Schematic setup of experimental instrument. Light source is shown in red dashed box. S-SNOM is shown in blue dashed box. Axillary supporting circuit has been developed for our time synchronization detection scheme.

Introduction The investigations on the relations between composition, structure and function is of the central place in the design of block-copolymer based new functional material. Microphase and nanophase separation occurred in block-copolymers are from few microns to few tens of nanometers dimension. The identification of composition variations across nanometer scale has become vital for block copolymer design and improvement on the performance of heterogeneous material. The goal of this project has been the development of a new instrumentation to extend IR vibrational spectroscopy to the nano-scale.

Instrument development

Figure 2. (A) schematically shows the triggering voltage square wave respect to tip apex position. (B) Laser hits tip twice per tip oscillation cycle, the relative phase between laser pulse train and tip oscillation determines the position when laser hits the tip. (C) Experimental measured interferograms with different phase delay of 0 (red), 3¹/8 (black) and ¹/2 (blue), showing the extracted contrast .

To achieve a functional near-field scattering spectroscopy device, it requires innovations from two aspects: generation of tunable coherent broad-band mid Infrared radiation and development of a highly sensitive detection technique considering that the emission is confined to few 10's nm of sample material only.

In our project, we have successfully generated a tunable IR source that covers a broad range of vibrational resonances. Wavelength tunable laser is produced by a home built Optical Parametric Amplifier (OPA) producing variable repetition rate tunable mid Infrared radiation of 1mW of the wavelength range from 5 to 9 µm by subsequent different frequency generation (DFG) in AgGaS₂.

The tip scattering Scanning Nearfield Optical Microscope (s-SNOM) is implemented on a commercial Atomic Force Microscope (DI). The tapping mode tip oscillates within the frequency range of 50kHz to 200kHz is matched with the repetition rate of our laser system, to synchronize tip tapping motion with laser pulse repetition. Using lock-in detection on the tip oscillation, the

Figure 3 (A) shows the vibrational resonance that we used in the simulation (B), the polarization of tip-sample system with imaging dipole approximation. Red curve shows the tip sample polarization when tip sample distance equals to tip apex radius generated by a broad band laser. Blue curve shows the tip polarization with sample far away from tip. (C) shows the interferogram that is read out from an numeric routine that does the same function as a lock-in amplifier. (D) shows the frequency domain Fourier transform of (C) with both intensity (blue) and phase (red dashed). Note the profile similarity and difference between vibrational spectrum (A) and intensity (D blue), and the phase behavior across resonance (D red dashed).

characteristic enhancement in light scattering of near-field when tip at close proximity of the sample is then extracted and recorded versus the time delay of inteferometric reference arm to form an interferogram (details see figure 2A and 2B). By setting the phase, one can obtain maximal near-field signal extraction. The experimental result (Figure 2C) confirmed the this detection scheme by showing maximal contrast at zero phase delay and minimal contrast at ¹ /2 phase delay.

Figure 4, fs s-SNOM Measurement on block copolymer PS-b-PMMA (25%:75%) (A) shows the structures of PMMA and PS. (B) shows the topography measurement of the sample (error plot of forward background scan to show better image contrast), the "protruding" dotted area is identified as PS region and flat matrix area is PMMA. Black and blue circled region indicates the spot that s-SNOM measurement take place, the location of which correspond to PMMA and PS respectively. (C) shows measured interferograms of this sample. Plot (black) corresponds to PMMA (black spot on (B)) ; Plot (blue) corresponds to measurement to PS (blue spot on (B)). Red plot shows the measured interferogram when tip apex is retracted from the sample, which serves as background reference. (D) shows the frequency domain Fourier transformed spectrum of (C). Note that the sample used was not freshly prepared, the domain formation is slightly deformed from a typical pattern that corresponds to its composition ratio.

Numerical model We also have developed a numerical simulation method to model our technique which is based on well recognized imaging dipole approximation. One such numerical simulation result is shown in figure 3. Spectral phase information is intrinsic linked to orientation of molecular bonds, a highly valuable property in surface science study and typically hard to obtain spectroscopically. But this ultrafst s-SNOM approach can obtain it together with nanoscale resolution through tip scattering, providing the potential of obtaining orientation information with nanoscale resolution.

Block-copolymer nano-imaging We have performed ultrafast mid IR s-SNOM measurement of block copolymer. Block copolymer shows phase separation at micrometer to nanometer scale. Typical atomic force microscope alone sometimes cannot guarantee differentiating composition of microdomains, especially when there are more than two constituent monomers for block copolymer or two constituent monomers of the same ratio. We have performed a preliminary measurement on a diblock copolymer of PS-b-PMMA (polystyrene block poly methyl-methacrylate). PMMA has strong resonance of carbonyl bond vibrations around 1710cm^-1(due to C=O stretching), whereas PS does not possess resonance at that region. PS has resonances around 1450cm^-1 (due to C-H bending) and 1490 cm^-1 (due to C-C bending ) to 1500cm^-1; PMMA also have vibrational resonance around 1500cm^-1(due to C-C bending). In one of our experiment, we tuned our laser to cover 300cm^-1bandwidth centered at 1600cm^-1. The result is shown in figure 3. We can clearly see the spectral contrast of PS domain and PMMA domain from the Fourier transformed spectrum. We intend to implement the pseduoheterodyne method of s-SNOM detection that can significantly reduce background and a upgrade of the laser system is in progress. For these reasons no results have yet been published.