Higher modes of vibration increase mass sensitivity in nanomechanical microcantilevers

We evaluated the potential and limitations of resonating nanomechanical microcantilevers for the detection of mass adsorption. As a test system we used mass addition of gold layers of varying thickness. Our main findings are: (1) A linear increase in mass sensitivity with the square of the mode number-a sensitivity increase of two orders of magnitude is obtained from mode 1 to mode 7 with a minimum sensitivity of 8.6 ag Hz(-1) mu m(-2) and mass resolution of 0.43 pg at mode 7 for a 1 mu m thick cantilever. (2) The quality factor increases with the mode number, thus helping to achieve a higher sensitivity. (3) The effective spring constant of the cantilever remains constant for deposition of gold layers up to at least 4% of the cantilever thickness.