Introduction of the MACS

5.1.1  Overview  

The  matrix  assembly  cluster  source  seeks  to  generate  clusters  via  a  completely   new   approach.   The   idea   is   to   assemble   the   clusters   through   the   ion   beam   bombardment   of   a   matrix,   which   is   formed   by   cryogenically   condensed   (solid)   inert   gas   loaded   with   metal   atoms.   In   our   work,   the   matrix   is   formed   cryogenically  by  condensing  atomic  vapor  of  the  desired  cluster  material  such  as   Ag   or   Au,   and   rare   gas   atoms   such   as   Ar   simultaneously   onto   a   matrix   condensation   support,   which   is   cooled   using   liquid   helium   (to   below   20K).   Clusters  are  then  produced  by  high  energy  Ar  ion  beam  sputtering  the  matrix.    

5.1.2  Transmission  and  reflection  mode  

In   the   MACS,   clusters   can   be   produced   both   in   transmission   and   reflection   regimes   dependent   on   the   matrix   condensation   support   employed.   The   matrix   condensation   support   is   a   sheet   of   high-­‐density   holey   membrane   (a   grid   or   mesh)  for  transmission  mode.  Copper  mesh  TEM  grids,  quantifoil  or  large  copper  

mesh  sheet  were  all  investigated  for  use  as  the  matrix  support.  In  transmission   mode,  the  matrix  forms  as  an  adlayer  on  the  bars  of  each  mesh  and  is  more  likely   to  close  the  hole  when  it  is  small  enough  (e.g.  quantifoil).  The  matrix  with  cluster   atoms  embeded  in  solid  rare  gas  is  then  sputtered  by  high-­‐energy  Ar  ions  (above   1keV).   Clusters   are   produced   during   the   sputtering   in   transmission   regime,   as   shown  in  Figure  5.1  (a).  

  Figure   5.1     Schematic   diagram   of   (a)   transmission   and   (b)   reflection   modes   in   Matrix   Assembly   Cluster   Source   (MACS).   The   matrix   is   formed   by   vaporizing   cluster  material  atoms  (eg.  Ag  or  Au)  and  rare  gas  atoms  (eg.  Ar)  condensed  onto   the   matrix   condensation   grid   (less   than   20K)   at   the   same   time.   Clusters   are   produced  by  high  energy  Ar  ions  sputtering  the  matrix.  

For  the  reflection  mode,  the  matrix  condensation  support  is  replaced  by  a  solid   plate,   for   example,   a   piece   of   copper   sheet,   instead   of   holey   membrane.   The   orientation  of  the  matrix  support  is  in  an  angle,  usually  from  10°  to  45°  to  the  

direction   of   incident   ion   beam.   Clusters   are   produced   following   the   same   procedure   just   described   but   collected   in   reflection   regime   as   shown   in   Figure   5.1(b).   In   this   chapter,   only   transmission   mode   is   used   to   demonstrate   the   principle   of   the   MACS   as   well   as   preliminary   study   of   effects   of   matrix   parameters.  Reflection  mode  will  be  discussed  in  chapter  6.    

5.1.3  Methodology  

The  production  of  clusters  in  the  MACS  is  based  on  a  high-­‐energy  (>1keV)  atomic   (e.g.   Ar+_{)   ion   beam   bombarding   a   condensed   matrix   of   rare   gas   atoms.   The}  

matrix  is  Ar  impregnated  with  atoms  of  desired  cluster  materials,  including  Ag  or   Au.  The  cluster  formation  process  is  possible  through  two  mechanisms:      

(i)   Clusters   are   preformed   during   the   condensation   of   the   matrix.   The  

matrix   is   formed   by   simultaneously   condensing   of   atoms   cluster   materials   and   rare  gas.  In  the  matrix,  cluster  material  atoms  are  driven  into  small  clusters  by   the  potential  force  to  minimize  the  energy  [2-­‐5].  This  process  happens  as  soon  as   the  cluster  material  atoms  land  in  the  matrix  and  only  lasts  around  20ps.    

(ii)   Clusters   are   aggregated   through   the   ion   impact.  Due  to  the  momentum  

delivered   into   matrix   with   high-­‐energy   ion   impact,   small   clusters   and   cluster   material   atoms   inside   the   matrix   become   mobile   and   aggregate   into   bigger   clusters.   Clusters   keep   growing   with   multiple   ion   impacts   because   of   successively  delivered  momentum  and  the  depletion  of  rare  gas  atoms  [6-­‐8].    

The  clusters  produced  in  the  MACS  are  formed  with  the  combination  of  (i)  and   (ii)   and   they   are   emitted   out   of   the   matrix   through   the   collision   cascade   and   thermal  spike  [9-­‐13].  For  the  collision  cascade,  sequence  of  recoils  are  generated   in  the  sample  after  the  original  impact,  as  shown  in  Figure  5.2(a).  Thermal  spike   happens   when   the   incoming   ion   is   heavy   and   energetic   where   the   collisions   between  ions  are  not  independent,  instead  they  are  considered  to  be  many  body   collisions,  as  shown  in  Figure  5.2(b).  The  clusters  produced  initially  might  be  a   mixture   of   cluster   atoms   and   rare   gas.   However,   rare   gas   atoms   will   later   evaporate  off  while  metal  atoms  will  not.  The  size  of  clusters  depends  on  several   parameters   such   as   metal   concentration   in   the   matrix,   matrix   temperature,   incident   beam   energy   and   details   will   be   discussed   in   the   results   section   in   chapter  5  and  chapter  6.  

  Figure   5.2   Schematic   diagrams   illustrating   collision   cascade   (a)   and   thermal   spikes  (b).  Reproduced  from  reference  [14]  

5.1.4  Promising  features  and  Potential  of  scaling-­‐up  

Based   on   the   results   obtained   so   far,   the   clusters   produced   using   the   MACS   techniques   exhibit   a   “narrow”   size   distribution   (M/ΔM>1)   without   mass   selection.   Moreover,   the   size   of   clusters   can   be   controlled   by   the   experimental   parameters  primarily  the  metal  concentration  in  the  matrix.  These  two  features   enable   the   production   of   size-­‐selected   clusters,   e.g.   for   catalysis   purpose,   using   the   MACS   techniques   without   additional   mass   selection,   which   results   in   a   higher-­‐usage  ratio  of  the  clusters.  The  aim  of  the  MACS  technology  is  to  scale  up   the  cluster  production  rate  by  ~7  order  of  magnitude,  from  0.1-­‐1nA  to  1-­‐10mA,   which   is   equivalent   to   grams   of   clusters   per   day.   In   principle,   the   cluster   production  rate  in  the  MACS  is  a  function  of  the  incident  ion  beam  current,  and   ion  beam  sources  with  output  current  up  to  10A  are  available.  The  ion  to  cluster   ratio  (how  many  incident  ions  are  required  to  produce  one  cluster)  based  on  our   current   experimental   results   is   0.05%   for   transmission   mode   and   nearly   0.5%   for   reflection   mode.   Therefore,   a   cluster   beam   current   equivalent   to   10mA   is   achievable.   Of   course   the   precondition   is   the   matrix   has   a   sufficient   replenishment  rate.  

This   chapter   concentrates   on   the   proof-­‐of-­‐principle   of   the   MACS   idea   and   preliminary  studies  of  effect  of  experimental  parameters  on  cluster  production   using  MACS  demonstration  apparatus.  In  chapter  6,  we  report  the  development   of  the  upgraded  apparatus,  MACS  1,  to  scale  up  the  cluster  production  rate  and   systematically  investigate  the  controlled  cluster  production  to  better  understand   the  methodologies.