Mass and Stiffness Spectrometry of 20-nm Gold Nanoparticles by Nanoelectromechanical Systems

Nanoelectromechanical systems are highly sensitive to adhered particles, and by using frequency shifts caused by particle adhesion on the surface of the resonator, it is possible to obtain stiffness of the adsorbate in addition to its mass and position on the resonator. In order to conduct our experiments, we fabricated NEMS resonators using top-down fabrication techniques. We have measured their multimode resonance response using electrothermal actuation and piezoresistive detection. Matrix Assisted Laser Desorption and Ionization (MALDI) is implemented to deliver particles towards the resonator. We have detected and characterized, in-real time, 20-nm gold nanoparticles using the first four out-of-plane modes. Simultaneous measurements of multiple modes were accomplished using phase-locked loop circuits running in parallel. By using the frequency shifts of resonance frequencies, we propose a method in which we assume the analytes adhered on the beam are hemispherical to obtain mass and stiffness, size and positions of the analytes. The size and position values for individual nanoparticles obtained were verified with independent characterization under SEM. The measurements provided here confirms the utility of multimode NEMS detection technique with analytes down to 20 nm radius.