A sample of 1,4-dibromobenzene can be successfully analyzed using the AXIS Supra. This was done by freezing the sample to -100 ˚C before pumping in the sample entry chamber (SEC). The sample was then transferred to the analysis chamber, analyzed by XPS and removed to the SEC all with a sample temperature below -100 ˚C. This ensured that the sample didn’t sublime under UHV conditions. The analysis of the sample was achieved without X-ray degradation as spectra were collected in large-area, high-sensitivity mode which limited the total X-ray exposure of the sample to 15 minutes. The Br 3d spectrum indicated that there were 2 chemical states associated with the bromine.
The molecule 1,4-dibromobenzene is of interest as a precursor for the synthesis of conjugated polymers and has applications in nanoelectronics and electronic devices. While the synthesis of such conjugated polymers in solution is well-established, the reactions can result in disordered structures. This can be mitigated with the use of atomically flat single crystal substrates. The most successful approach is the metal surface catalyzed coupling of halogenated hydrocarbons. 1,4-dibromobenzene has a vapor pressure of 0.0575 mmHg at 25 ˚C which means that it will volatilize in vacuum at room temperature. It is therefore vital to cool the organic material to <100 ˚C before pumping and introduction to the analysis chamber in order to achieve XPS characterization.
During the experiment 1,4-dibromobenzene was pressed into a powder well mounting accessory on the heat/cool sample holder and placed on the appropriate position on the sample magazine. The sample entry chamber (SEC) was sealed before it was backfilled with dry nitrogen. The chamber was purged for 10 minutes before the liquid nitrogen was filled to commence sample cooling while continuing to purge the chamber. Once a sample temperature of -50 ˚C was achieved, the sample entry chamber was pumped to a base pressure in the 10-7 torr in approximately 10 minutes. The rate of cooling significantly increased once the chamber was evacuated, reaching below -100 ˚C within 20 minutes.
It should be noted that in addition to cooling the sample in the SEC, the sample stage of the analysis sample was pre-cooled. The automated sample transfer was started which transferred the sample from the sample magazine to sample stage in the analysis chamber once the sample temperature below -100 ˚C was achieved During the sample transfer there is no active cooling, however, the thermal mass of the sample holder is such that the sample remains below -100 ˚C throughout the transfer process and active cooling is resumed as soon as the sample holder is gripped in the analysis chamber.
The XP spectra were acquired by exciting Photoelectrons using monochromatic Al K X-rays (1486.6 eV). Sample charging was prevented by use of the low-energy electron-only charge neutralization system, and the base pressure of the analysis chamber was maintained below 5x10-8 Torr range during the experiment.
Figure 1. 1,4-diboromobenzene in the XPS analysis position.
The cooling of the 1,4-dibromobenzene ensured that it remained in solid form throughout the measurements. The survey spectrum acquired from the experiment showed that in addition to bromine and carbon there is <1.5 atomic % contribution from oxygen. The high-resolution spectra of C 1s and Br 3d were attained which allowed the chemistry of the material to be investigated. The peak for carbon bonded to bromine occurs at 285.8 eV, with the aromatic C–C at 284.8 eV and the model of best fit has Shake-up structure associated with the aromatic benzene ring fitted with a single component at 291.6 eV.
Figure 2. Large area (300 µm x 700 µm) survey spectrum from the 1,4-dibromobenzene.
Figure 3A. C 1s high resolution spectrum charge-corrected for the C aromatic at 284.8 eV
The Br 3d spectrum comprises of a pair of doublets with 1.05 eV separation between the Br 3d5/2 and 3d3/2 components. The sample analyzed is formed from molecular crystals which are likely stacked via π – π interactions of the aromatic rings as well as halogen bonding of the end groups, analogous to hydrogen bonding. At the solid-vacuum interface it is likely that one of two bromine atoms are closer to the surface and is under-coordinated or removed. It is therefore proposed that the higher binding energy Br 3d doublet (Br 3d5/2 at 71.4 eV) arises from the bulk ‘intact’ molecules in the crystalline bulk. On the other hand the lower binding energy doublet (Br 3d5/2 at 70.3 eV) arises from the under coordinated bromine at the solid-vacuum interface.
Figure 4. X-ray excited valence band spectrum of 1,4-dibromobenzene.
The valence band spectrum is often of interest because it is sensitive to the molecular structure of the material and can reflect the changes in the valence electron distribution. The valence band is sensitive to X-ray induced damage during the analysis and both valence band and core level spectra were acquired using the spectrometer in large area analysis mode where a combination of magnetic immersion and electrostatic lenses efficiently transfer the photoelectrons to the 165 mm mean radius hemispherical analyzer. This is to collect the spectra in a matter of minutes. It should be noted that no changes were observed in sequential valance band spectral acquisitions which is an important observation as halogenated hydrocarbon are known to be easily damaged by X-ray exposure.
Thanks to Dr Josh Lipton-Duffin at QUT, Australia for providing instrument time and interesting discussion about the results.
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This information has been sourced, reviewed and adapted from materials provided by Kratos Analytical, Ltd.
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