Nanoscale chemistry on the surface of levitated gold nanoparticles

To study the properties of nanoscale materials, we levitate electrically charged Au nanoparticles of roughly 200 nm diameter in an ion trap in high vacuum. In this talk, I will discuss the implications of our data for chemical reactions on nanoparticle surfaces. We previously developed a method of heating levitated nanoparticles with a pulsed 532 nm laser and measuring the rate of evaporation of material from the particle. By analyzing low-power CW laser light scattered from the particle between pulses, we can measure the particle’s mass and observe single-|e| changes in its electric charge Q. We observe that when the particle is heated in O₂ ambient (P~10^-6 Torr), the evaporation rate remains constant at a value that is consistent with the melting point of Au. Heating the particle in environments without added O₂, however, causes the evaporation rate to decline over repeated heating cycles. Additionally, we observe that the rate of discharge dQ/dt declines along with the evaporation rate. To investigate the cause of these phenomena, we varied several parameters, adding gases such as CO, C₂H₄, H₂, and He to the system, heating the particle in vacuum (P~10^-8 Torr) with no added gases, and varying the time between laser pulses and the power of the illumination between pulses. Our working hypothesis is that the particle’s surface is gradually being covered with an impermeable layer of some material (likely adventitious C); however, some features of our data suggest that other mechanisms are at play.