Abstract
Obtaining monodisperse nanocrystals, and determining its composition to the atomic level and its atomic structure is highly desirable, but is generally lacking. Here, we report the discovery and comprehensive characterization of a 3-nm plasmonic nanocrystal with a composition of Au940±20(SCH2CH2Ph)160±4, which is, the largest mass spectrometrically characterized gold thiolate nanoparticle produced to date. The compositional assignment has been made using electrospray ionization (ESI) and matrix assisted laser desorption ionization (MALDI) mass spectrometry (MS). The MS results show an unprecedented size monodispersity, where the number of Au atoms vary by only 40 atoms (940 ± 20). The mass spectrometrically-determined size and composition are supported by aberration-corrected scanning transmission electron microscopy (STEM) and synchrotron-based methods such as atomic pair distribution function (PDF) and small angle X-ray scattering (SAXS). Lower resolution STEM images show an ensemble of particles – 1000’s per frame – visually demonstrating monodispersity. Modelling of SAXS on statistically significant nanoparticle population – approximately 1012 individual nanoparticles - shows that the diameter is 3.0±0.2nm, supporting mass spectrometry and electron microscopy results on monodispersity. Atomic PDF based on high energy X-ray diffraction experiments show decent match with either a Marks decahedral or truncated octrahedral structure. Atomic resolution STEM images of single particles and its FFT suggest face-centered cubic (fcc) arrangement. UV-visible spectroscopy data shows that the 940-atom size supports a surface plasmon resonance peak at 505 nm. These monodisperse plasmonic nanoparticles minimize averaging effects and has potential application in solar cells, nano-optical devices, catalysis and drug delivery.
