Supporting Information
XRD was performed at ambient temperature using two instruments: the Siemens D5000 θ/2θ
powder X-Ray diffractometer (has a Bragg Brentano reflection geometry, a Kα1 (λ=1.54056 Å)
germanium primary monochromator and a point scintillation counter) and the Bruker GADDS
combinatorial X-ray instrument (has Cu
Kα
radiation, a XYZ stage and a multi-wire Hi-Star area
detector). The calibration of the instruments was checked before each set of samples using the
NIST Corundum Standard Reference Material SR1976 and found to be correct within 0.1 degree.
Raman spectroscopy was performed using a Renishaw RM1000 with a HeNe laser, at a
wavelength of 785 nm. The Raman is coupled with a Leica microscope and has an 1800 mm-1
grating and notch filter to prevent Rayleigh scattered photons reaching the detector.
TEM was performed using a JEM 3010 operating at 300 kV. The EDX analysis of the WS2
NPs showed peaks for W and S (Figure 1). The unmarked peaks at 0.93, 8.04 and 8.9 keV are
Cu from the TEM grids.
Figure 1. EDX analysis of WS2 NPs
XPS measurements were performed using a VG Escalab 250 XPS with monochromated
aluminium K-alpha X-ray source. The spot size was 500 μm with a power of 150W. Detailed
spectra of individual peaks were taken with a pass energy of 20 eV, 0.1 eV step size and dwell
time of 50 millisecond whilst the survey scan was taken with a pass energy of 150 eV, step size
of 1 eV and dwell time of 50 msec. Etching was performed using an argon ion gun calibrated to
the sample with energy of 1 keV and a current of 0.1 μA/mm2. Binding energy was calibrated by
setting the carbon 1s peak to 285 eV. Core level spectra were normalized to Shirley background
and the peaks were fitted using mixed Gaussian-Lorentzian fits (using CASAXPS).
FIB-SIMS was performed using a dual beam Zeiss NVision40 SEM-FIB. After depositing a
layer of carbon to protect the surface, FIB cross-sections were conducted through the surface of
the tribofilm (Figure 13c) to measure the thickness. The gallium ion beam was used in
conjunction with a Hiden Analytical EQS quadrapole detector to perform dynamic SIMS depth
profiles of the tribolayers. Areas of 30 x 30 μm were sputtered using a 5kV acceleration voltage
operating at 3nA beam current, with position ion masses being recorded as a function of sputter
time for carbon, sulphur, iron and tungsten. Oxygen was not monitored during the SIMS analysis
because it is an element with background limited sensitivity.