1 Charging of Dust in a Plasma with Negative Ions Su-Hyun Kim and Bob Merlino The University of Iowa Supported by DOE 11 th Workshop on the Physics of Dusty Plasmas Williamsburg, VA June 28 – July 1, 2006

2 Positive Dust Photoelectric charging Secondary emission T i > (m i /m e ) T e If electrons are attached to heavy negative ions

3 Dust in plasmas with negative ions the plasma environment of the nighttime polar mesosphere is dominated by negative and positive ions, few electrons, and nanometer size particles microelectronic manufacturing devices using reactive gases, such as silane,SiH 4, have large concentrations of negative ions and dust particles

4 Plasma densities in the polar mesosphere M. Rapp et. al, GRL 32, L23821 (2005)

5 Mesospheric particle detector current The detector currents correspond to positive charge densities, eZN ~ 100 e/cm -3

6 Hydrogenated amorphous silicon (a- Si:H) semiconductors for liquid crystal displays and solar cells made by PECVD in rf discharges using hydrogen-diluted silane Si x H m (x  200) nm size particles also form and incorporate into the depositing film particles must be neutral or + to get out in these electronegative discharges n e <<n i effect of –’ ve ions must be considered

7 STM image of a nanoparticle on a 4.2 nm thick a-Si:H film Tannenbaum, et. al, APL 68, 1705 (1996) substrate film nanoparticle

8 Charging of dust with positive ions, negative ions and electrons [ Mamun & Shukla, PoP 10,1518 (2003)] V s > 0 V s < 0 V s = V f  V p

9 Solutions to the charging equations n + = n e + n –   = n e /n +  n – = (1–  )n +

10 Experiment negative ion plasma, with n e << n + dust particles Q machine plasma K + ions T e = T i  0.2 eV admit SF 6 gas to form negative ions Disperse hollow glass microspheres (35  m) using rotating cylinder

11 Negative ion plasmas in a Q machine very effective due to low T e N. Sato, in K Q machine

12 Langmuir curves before and after dust added P(SF 6 ) =0 4.6 x x10 -4 I0I0 II I +0 I+I+

13 Data analysis Analysis of the Langmuir probe currents can be used to determine how the charge in the plasma is divided between free +/- ions, free electrons and dust particles. Charge neutrality in dusty plasma: en + + Qn d = en e + en –  is determined from the changes in negative and positive probe currents when dust is introduced

14 if  = n e /n + <<1 (electrons attached to SF 6 ) The sign of R – – R + determines the sign of the dust charge, Q Data analysis

15 Results

16 Qn d /en o  = n e /n + Qn d /en o vs. 

17 Langmuir Probe floating potential (relative to the plasma potential) vs. P(SF 6 )

18 Summary and Conclusions dust charging in a plasma with negative ions has been studied experimentally the addition of negative ions reduces the density of electrons leading to a reduction in the (negative) charge on dust conditions have been established which cause positive charging of dust, in a plasma with light + ions and heavy - ions

19 1 x10 -3