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Novel Group IV Based Polymer Dielectrics
 

Novel Group IV Based Polymer Dielectrics

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    Novel Group IV Based Polymer Dielectrics Novel Group IV Based Polymer Dielectrics Presentation Transcript

    • Dielectric  Proper,es  of  Carbon,  Silicon  and     Germanium  Based  Polymers:  A  First  Principles   Study    C.  C.  Wang,  G.  Pilania,  C.  S.  Liu  and  R.  Ramprasad*     Chemical,  Materials  &  Biomolecular  Engineering   Ins7tute  of  Materials  Science   University  of  Connec7cut      
    • Energy  Storage  Technologies  
    • Materials  Requirements  •  Current  standard:  biaxially-­‐oriented  polypropylene1   q   Breakdown  field  of  700  V/micron          (for  10  micron  thick  films)   q   Low  dielectric  constant  of  2.2  •  Next  generaOon  materials  requirements   q   High  dielectric  constant   q   Large  band  gap   q   High  breakdown  strength   q   Ease  of  processability   q   High  thermal  stability   1,  E.  J.  Sarjeant,  etc.  IEEE  Trans.  Magn.  43,  223  (2007)  
    • Our  Goal             Chemical   modificaOon   of   polyethylene,   to   idenOfy   promising   candidates   with   large   dielectric   constant   and  band  gap.     q   Backbone:  C,  Si,  Ge,  etc.       q   Side  chain:  H,  F,  Cl,  etc.    
    • Computa,onal  Method        Density  Func,onal    Theory   (DFT)      q  DFT  is  an  alternaOve  formulaOon  of   quantum  mechanics    q  Many  nuclei-­‐many  electron  problem  →   one  electron  problem    q  DFT  as  implemented  in  VASP  and  FHI-­‐ aims  is  used   DFT     BOOK  CHAPTER:  R.  Ramprasad,  N.  Shi  and  C.  Tang,  "Modeling  the  physics  and  chemistry  of   interfaces   in   nanodielectrics",   in   Dielectric   Polymer   Nanocomposites   (Ed.   J.   K.   Nelson),     Springer  (2010).    
    • van  der  Waals  (vdW)  Interac,ons  q Polymer  interchain  interacOons  are   controlled  by  vdW  forces     a q Labce  parameters,  volumes  and   densiOes  predicted  incorrectly   b  q Errors  carry  over  to  dielectric   constants   c q ConvenOonal  DFT  fails  to  capture     vdW  interacOons    
    • vdW  Interac,ons  in  DFT  q   ConvenOonal  DFT  funcOonals   (LDA,  PBE)  q   vdW-­‐augmented  funcOonals   (PBE-­‐D2,  PBE-­‐TS)    q Assessment  of  funcOonals  based   on  geometry  predicOons  q We  consider  10  polymers  for   which  reliable  crystallographic   informaOon  is  available    
    • Predic,ons  of  Geometry  a b Root-­‐mean-­‐ a,  b  (Å)   Volume  (Å3)   square  error  c LDA   0.47   25.75   PBE   0.52   30.84   PBE-­‐D2   0.29   13.77   PBE-­‐TS   0.19   7.65  
    • XY2  Homopolymers   q  Nine    XY2  homopolymers                (X  =  C,  Si,  Ge,  Y=  H,  F,  Cl)     q  CH2,   CF2,   SiCl2   occur   in   the   Type  A  structure   q    GeF2   occurs   in   the   Type   B   structure     q  All   nine   homopolymers   are   considered   in   both   Type   A   and   Type   B   structures   and   include  vdW  interacOons.    
    • Dielectric  Constant   ε(total)  =  ε(electronic)    +  ε(ionic)  An   approximate   inverse   relaOonship   Systems   with   bridging   halogens   display  between   electronic   dielectric   constant   large   dielectric   constant   and   large   band  and  band  gap   gap  Ge  containing  polymers  are  iden,fied  as  par,cularly  promising,  especially  GeF2.  
    •   An  Alternate  Fast  Method   ESTIMATION  OF  CRYSTAL  DIELECTRIC   CONSTANT  FROM  SINGLE-­‐CHAIN   COMPUTATIONS   Supercell   Volume  defined  through   charge  density  cutoff   Vpolymer   Vsupercell   EffecOve  medium  Polymer   mixing  rules    Chain   q   Amenable  to  “high-­‐throughput”  computaOons  
    • Valida,on  of  Single-­‐Chain  Approach    q     In  general,  the  single-­‐chain  results  are  either  close,  or  smaller  than  the  crystal  results.    q      Overall,   the   agreement   between   the   two   methods   is   good,   given   that   interchain   interacOons  are  completely  neglected  in  the  single-­‐chain  computaOons.    
    • Summary    q   We  have  studied  nine  XY2  homopolymers  in  two  types  of  crystal  structures.    q    Systems   containing   Ge   are   idenOfied   as   parOcularly   promising     for   applicaOons  requiring  insulators  with  high  dielectric  constant.        q    It   is   desirable   to   consider   heteropolymers   composed   of   GeF2   and   CH2   units   (to  opOmize  both  the  dielectric  constant  and  band  gap).    q    In   order   to   aid   in   an   efficient   search   of   the   chemical   space   spanned   by   such  heteropolymers,   a   “high-­‐throughput”   strategy   for   the   rapid   computaOon   of   the  dielectric  constant  is  presented  and  validated.