Overview of JEOL 2100F

The   electron   microscope   based   in   Nanoscale   Physics   Research   Laboratory,   University   of   Birmingham   is   a   JEOL   2100F   scanning   transmission   electron   microscope   (STEM)   with   CEOS   aberration   corrector   up   to   the   fifth   order.   The   photograph  and  schematic  diagram  of  internal  structure  of  JEOL  2100F  is  shown   in  Figure  3.6.  

Figure   3.6   Photograph   and   schematic   diagram   of   internal   structure   of   JEOL   2100F  scanning  transmission  electron  microscope  (STEM)  with  CEOS  aberration   corrector  in  NPRL,  University  of  Birmingham.  

Electron  gun  

In   the   JEOL   2100F,   electrons   are   generated   from   a   Schottky   field   emission   electron  gun  (FEG)  and  are  then  extracted  and  accelerated  to  high  energy  by  two   electrodes  in  front  of  the  gun.  The  tip  of  the  FEG  is  made  of  tungsten  with  (100)   surface  coated  with  a  layer  of  ZrO  to  reduce  the  work  function  barrier.  The  size   of  the  tip  is  in  nanometer  scale  so  that  the  electric  field  between  the  tip  and  the   first  electrode  is  strong  enough  to  extract  electrons  out  of  the  tip.  An  acceleration   voltage   of   200kV   is   applied   to   the   second   electrode   accelerating   electrons   to   about   70%   of   the   light   speed.   The   electron   gun   is   installed   in   a   high   vacuum   chamber  of  pressure  down  to  10-­‐9  _{Pa.  The  electron  gun  is  slightly  heated  to  avoid}  

contamination   and   to   promote   the   emission   efficiency.   The   focused   electron   beam  probe  is  formed  by  electrons  passing  through  3  stages  of  electron  optics   system   and   the   aberration   of   the   electron   beam   is   corrected   by   the   aberration   corrector  prior  to  the  specimen.    

Electron  optics  

The   working   principle   of   the   electron   optics   system   is   to   generate   electromagnetic  fields  by  the  lens  coils  in  the  condenser  lens  system  to  collimate   and  focus  the  electrons.  Additionally,  further  coils  are  used  to  align  the  electron   beam   with   the   sample   by   tilting   and   shifting   the   beam.   A   set   of   apertures   is   mounted   after   the   condenser   lens   system   to   remove   the   widely   scattered   electrons,  and  the  most  common  aperture  we  used  is  40μm  in  diameter.    

Aberration  corrector  

The   aberration   correction   system   is   installed   after   the   condenser   lens   and   aperture,  where  the  aberration  induced  by  the  condenser  lens  is  compensated.  In   our   JEOL   2100F   STEM,   the   aberration   corrector   used   is   CEOS   double   hexapole   spherical   aberration   corrector   consisting   of   two   sets   of   6   pole   pieces   and   two   sets  of  transfer  lenses  in  the  middle.  An  approximately  circular  field  is  generated   by   the   two   sets   of   hexapole   elements   with   the   dedicated   rotational   offset   alignment   to   form   a   negative   spherical   aberration   equivalent   to   the   positive   aberration  induced  by  condenser  lenses.  The  electron  beam  passing  through  the   aberration  corrector  is  then  focused  into  a  probe  by  the  objective  lens  prior  to  

reaching   the   plane   of   the   specimen.   The   scanning   of   the   electron   beam   probe   across   the   specimen   surface   is   enabled   by   the   scan   coils.   With   the   help   of   aberration  correct  the  resolution  of  the  STEM  is  pushed  to  0.1045nm  at  the  time   of  installation.  

3.2.2  Imaging  

Two   different   types   of   images   are   obtained   from   the   STEM   in   the   works   presented  in  this  thesis,  high  angle  annular  dark  field  (HAADF)  image  and  bright   field   (BF)   image.   The   schematic   diagram   illustrating   the   formation   of   HAADF   image  and  BF  image  are  shown  in  Figure  3.7.  The  HAADF  image  is  contributed  by   high  angle  scattered  electrons  and  collected  using  dark  field  detector  from  JEOL,   which   is   similar   to   a   donut.   While   the   BF   image   is   formed   by   electrons   with   narrow   forward   angles   and   collected   by   the   detector   from   Gatan,   which   is   a   circular  plate.  Both  detectors  are  installed  beneath  the  specimen.  

Figure   3.7   Schematic   diagram   illustrating   the   positions   of   HAADF   detector   and   BF  detector.  

The  advantages  of  HAADF  image  are  that  it  exhibits  sound  atomic  resolution  and   contains  the  quantitative  information.  HAADF  images  are  formed  by  high  angle   scattered   electrons   which   lose   the   coherence   if   the   collection   angle   is   large   enough   that   the   inner   collection   angle   is   more   than   three   times   of   the   beam   convergence  semi-­‐angle  (about  50  mrad).  In  that  case,  the  electrons  to  form  the   HAADF  image  are  not  affected  by  the  complicated  phase  change,  instead  they  are   determined   by   the   elemental   atomic   number   and   the   thickness   and   can   be   described   by   Rutherford   scattering   equation.   The   intensity   of   HAADF   STEM   image   formed   by   high   angle   scattered   incoherent   electrons   which   follow   the   Rutherfold   scattering   equation   is   proportional   to   Z2_{,   Z   is   the   atomic   number.}  

However,  in  reality  the  power  exponent  is  affected  by  the  screening  of  nuclear   charge   that   the   equation   has   to   be   modified   to   I~tZα_{,   α   is   usually   varied   with}  

camera  length  in  the  STEM,  which  determines  collection  angle  and  convergence   angle.    In  our  STEM,  the  power  exponent  α  is  calibrated  with  help  of  size  selected   nanoclusters  Au923  and  Pd923  by  ZW.  Wang  in  2011  for  the  condition  of  the  inner  

and  outer  collection  angle  of  62  and  164mrad  and  convergence  angle  of  19mrad   [12].   In   the   calibration,   average   intensities   of   size   selected   Au923   and   Pd923   are  

measured   respectively   over   large   populations.   The   power   exponent   α   is   then   obtained  based  on  the  equation  

𝐼!"

𝐼_!" = ( 𝑍!"

𝑍_!")!   that  α=1.46±0.18  [12].  

The  electrons  reaching  the  BF  detector  are  assumed  to  retain  the  coherence  as   they  are  only  be  scattered  within  very  small  angles.  Thus  the  phase  change  due   to  interactions  between  electrons  and  sample  and  fine  lattice  structural  details   can  be  revealed  using  the  BF  images.    

  Figure   3.8   HAADF   image   and   BF   image   of   size-­‐selected   Au309   cluster   deposited  

on  FLG  surface.  The  atomic  structure  of  the  Au  cluster  is  clearly  revealed  in  both   the  HAADF  image  and  the  BF  image.  However,  the  lattice  structure  of  the  FLG  is   only  visible  in  the  BF  image  as  well  as  the  defects  on  the  FLG  surface.  

Examples  of  HAADF  image  and  BF  image  of  size-­‐selected  Au309  cluster  deposited  

on   few-­‐layer   graphene   (FLG)   surface   are   shown   in   Figure   3.8.   The   atomic   structure  of  the  Au  cluster  is  clearly  revealed  in  both  the  HAADF  image  and  the   BF  image.  However,  the  lattice  structure  of  the  FLG  is  only  visible  in  the  BF  image   as  well  as  the  defects  on  the  FLG  surface.  Hydrocarbons  on  the  FLG  surface  are   also  detectable  using  BF  image  as  reported  in  chapter  4.1.  

On   the   other   hand,   HAADF   image   has   its   irreplaceable   advantage,   which   is   quantitative   information.   For   example,   the   intensity   of   the   size-­‐selected   Au309  

cluster  can  be  used  as  the  mass  balance  to  measure  the  thickness  of  the  graphene   film,  which  is  used  in  Chapter  4.1  to  determine  the  number  of  layers  of  the  FLG.   Also  in  chapter  5  and  chapter  6,  the  number  of  atoms  of  clusters  produced  in  the   matrix   assembly   cluster   source   is   measured   by   the   HAADF   intensity   of   single   atoms  and  size-­‐selected  Au923  clusters.