J. Phys. Chem. C 2010, 114, 17191–17196
17191
Electron Emission from Hypervelocity C60 Impacts† M. J. Eller,‡ S. V. Verkhoturov,‡ S. Della-Negra,§ and E. A. Schweikert*,‡ Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77842-3012, and Institut de Physique Nucle´aire, 91406 Orsay, France ReceiVed: May 3, 2010; ReVised Manuscript ReceiVed: July 6, 2010
Secondary ion mass spectrometry (SIMS) performed in the event-by-event bombardment detection mode when coupled to an electron emission microscope allows one to investigate individual nano-objects. Two groups of Au and Al oxide nano-objects were compared with their bulk counterparts based on their secondary ion and electron emission from individual C60 impacts at 15 and 30 keV total impact energy. Our results show that electron yields depend on the size and surroundings of the nano-object, and at higher impact energies, these differences in electron emission are more pronounced. A second key observation for systems of similar chemical makeup but different surface topography and size is that the emission of secondary ions and electrons is independent of each other. Introduction Several authors have observed emission of electrons from hypervelocity C601,2+ impacts on solid surfaces where kinetic electron emission from comparable velocity atomic projectiles does not occur.1,2 The phenomenon cannot be attributed to potential electron emission given the C601,2+ characteristics. The yet to be explained mechanism of emission must arise from a projectile-related collective effect. We have recently reported the yields of electrons and those of coemitted secondary ions from several targets impacted with C60 at velocities of 63 and 90 km/s or 250 and 500 eV/atom, respectively.3 The experiments were run in the event-by-event bombardment-detection mode, allowing us to measure the number of electrons and to identify the coemitted secondary ions from individual C60 impacts. As might be expected, the number of electrons emitted per impact is target-specific for flat surfaces of organic molecules, ionic compounds, and metals. In all cases, multiple electrons are emitted per C60 in the energy interval considered. A notable observation on homogeneous flat surfaces is that the electrons are emitted independently from the type and number of coemitted ions. We present here a study with C601,2+ projectiles comparing electron yields and ion-specific electron emission from nanosized objects and surface topographies with those of flat surfaces of comparable chemical composition. We have previously reported a sample-size effect in the secondary ion emission when a massive cluster impacts a nano-object too small for the full projectile energy deposition.4 Some fundamental questions arise in this context. Is the coemission of electrons and negative SIs correlated? Does the electron yield vary with the coemission of multiple SIs? Experimental Section Materials. A 500 nm Al oxide layer was deposited on a silicon wafer, to act as the bulk Al oxide sample. The 50 nm Al oxide particles were obtained from Argonide and were suspended in acetone at 25 mg/mL and sonicated. Then, 10 µL †
Part of the “D. Wayne Goodman Festschrift”. * Corresponding author. E-mail:
[email protected]. ‡ Texas A&M University. § Institut de Physique Nucle´aire.
of the aluminum solution was deposited onto a silicon wafer; the thickness of this layer is a few micrometers, which is much larger than the depth of emission for this experiment, ∼10 nm. The boehmite whiskers were prepared by a process described elsewhere and are 2 nm wide and 200 nm long.4 Three gold samples were also prepared; a 500 nm thick gold layer was deposited on a silicon wafer. Second, a multilayer of 5 nm gold nanoparticles (NPs) functionalized with dodecanethiol (Sigma Aldrich) was prepared by suspending Au NPs in hexane at 10 mg/mL and depositing 15 µL of this solution onto a silicon wafer. Similar to the 50 nm Al particles, this produced a few micrometers thick layer of Au NPs on the silicon surface. A third sample consisting of the Au NPs dispersed as a single layer on glycine was prepared.5 This was performed by vapordepositing glycine in a several micrometers thick layer (Sigma Aldrich) onto a Si wafer, then suspending the Au NP in hexane, in which glycine is insoluble, at 0.5 mg/mL and depositing 5 µL of this solution onto the glycine surface. Instrumentation. The instrumentation used is described in detail elsewhere.6 In brief, the instrument is a custom built cluster-SIMS instrument equipped with a C60 effusion source, a 1 m linear time-of-flight (ToF), and an electron emission microscope (EEM). The C60 effusion source is capable of producing C60+ and C602+ at 15 and 30 keV, respectively. These projectiles are mass-selected by a Wien filter and focused onto the sample on an area 100 µm in diameter. The EEM has a detection efficiency, τ, of 0.6 and can achieve magnifications e1000×. To achieve event-by-event bombardment-detection conditions, the experiments are performed at
