Characterization of nanometer-sized particles
Abstract
New methods for characterizing nanometer sized particles produced by the multiple expansion cluster source (MECS) were developed. The cluster beam from the MECS was analyzed directly with a time of flight mass spectrometer (TOFMS) using excimer laser photoionization. The mass spectrometer was designed and built with perpendicular molecular beam and time of flight axis to combine high resolution and high mass range to study metal clusters. The TOFMS can resolve individual clusters containing upto 100 atoms per cluster and detect heavy clusters of upto 5 nm. diameter. The mass spectra of high masses were obtained by reducing the velocity of clusters in the molecular beam before ionization. The slower initial velocities permitted easier deflection of heavy particles. The TOFMS was used to analyze the growth of Sn clusters in the MECS. Sn cluster growth was found to depend solely on the concentration of Sn atoms and the residence time in the reactor. The MECS produced a normal distribution of cluster diameters. This is in good agreement with growth prediction based on pure birth growth kinetics. The standards deviation of this normal distribution is between 0.3 and 0.5 nm when the mean particle diameter is less than 2.5 nm. A secondary growth region beyond 2.5 nm diameter cluster was observed with He gas in the MECS reactor. The clusters in this growth region show bimodal peak size distribution with the heavier peak located at twice the mass of the first peak when the clusters in the beam are first slowed down with deceleration gas. The deceleration gas slows down the clusters in the beam based on their cross section area to mass ratio. This effect is due to growth from cluster to cluster collisions forming a loosely bound aggregate. These small aggregates do not change structure since the cross sectional area to mass ratio is unchanged as observed by the bimodal peak distribution. Au clusters ranging from 1 to 10 nm diameter deposited on flat Au substrates and on highly oriented pyrolytic graphite substrates were analyzed using a scanning tunneling microscope (STM). The size distribution of the cluster deposit measured by STM agreed well with size measurement made by the deceleration curve method. Au clusters were also deposited on a clean tungsten tip and imaged with field emission microscope (FEM) and a field ion microscope (FIM). Technique for depositing only few clusters on the tip was developed jointly with Castro by observing the clusters sticking to the tip in real time with a field emission microscope positioned aligned with the cluster beam. The FEM and FIM images were used to detect the melting transition of individual clusters.
Degree
Ph.D.
Advisors
Andres, Purdue University.
Subject Area
Chemical engineering|Molecules
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