Understanding the Intrinsic Water Wettability of Molybdenum Disulfide

Jul 14, 2015 - Andrew Kozbial†, Xiao Gong†, Haitao Liu‡, and Lei Li†§ ... Transition metal dichalcogenides (TMDCs), e.g., MoS2, have a direct...
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Understanding  the  intrinsic  water  wettability  of  molybdenum  disulfide  (MoS2)     Andrew  Kozbiala,  Xiao  Gonga,  Haitao  Liub  and  Lei  Lia,c*   a  Department  of  Chemical  &  Petroleum  Engineering,  Swanson  School  of  Engineering,  

University  of  Pittsburgh,  Pittsburgh,  PA  15261,  USA   b  Department  of  Chemistry,  University  of  Pittsburgh,  Pittsburgh,  PA  15260,  USA   c  Department  of  Mechanical  Engineering  &  Materials  Science,  Swanson  School  of  

Engineering,  University  of  Pittsburgh,  Pittsburgh,  PA  15261,  USA     Abstract   2D  semiconductors  allow  for  unique  and  ultrasensitive  devices  to  be  fabricated  for   applications   ranging   from   clinical   diagnosis   instruments   to   low-­‐energy   light   emitting  diodes  (LEDs).    Graphene  has  championed  research  in  this  field  since  it  was   first   fabricated;   however,   its   zero   bandgap   creates   many   challenges.     Transition   metal   dichalcogenides   (TMDCs),   e.g.,   MoS2,   have   a   direct   bandgap   which   alleviates   the   challenge   of   creating   a   bandgap   in   graphene-­‐based   devices.   Water   wettability   of   MoS2  is  critical  to  device  fabrication/performance  and  MoS2  has  been  believed  to  be   hydrophobic.  Herein,  we  report  that  water  contact  angle  (WCA)  of  freshly  exfoliated   MoS2  shows  temporal  evolution  with  an  intrinsic  WCA  of  69.0±3.8°  that  increases  to   89.0±3.1°   after   1   day   exposure   to   ambient   air.     ATR-­‐FTIR   and   ellipsometry   show   that   the   fresh,   intrinsically   mildly   hydrophilic   MoS2   surface   adsorbs   hydrocarbons   from  ambient  air  and  thus  becomes  hydrophobic.        

 

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Introduction   Molybdenum  disulfide  (MoS2)  has  generated  significant  interest  in  the  past  several   years  as  limitations  of  graphene  become  apparent  due  to  its  zero  bandgap.1-­‐8  Weak   interlayer   van   der   Waals   forces   allow   MoS2   to   be   easily   exfoliated   to   form   atomic   layers  which,  like  graphene,  can  be  used  in  electronic  and  optoelectronic  devices  for   environmental,   biological,   and   clinical   applications.1,  

2,   3  

MoS2   and   other   2D  

transition   metal   dichalcogenides   (TMDCs)   such   as   MoSe2,   WS2,   and   WSe2   are   semiconductors   that   have   an   intrinsic   bandgap   which   enhances   device   sensitivity   and   allows   for   fabrication   of   unique   field-­‐effect   transistors   (FETs),   biosensors,   solar   cells,   and   light-­‐emitting   diodes   (LEDs).3,   4,   5,   6   Additionally,   the   atomic   thinness   of   TMDCs   allow   for   flexible   devices   not   possible   with   traditional   organic   semiconductors.5,  6,  7,  8   Sarkar   et  al.   demonstrated   that   MoS2-­‐based   FET   biosensors   are  over  74  times  more  sensitive  than  a  graphene-­‐based  device  and  can  be  utilized   for   ultrasensitive   protein   sensing   at   extremely   low   concentrations   of   100   femtomolar.9   Moreover,   Lee   et   al.   demonstrated   efficacy   of   a   MoS2   biosensor   for   detection  of  prostate  antigens  in  order  to  diagnose  prostate  cancer.    The  minimum   antigen  concentration  detected  by  their  MoS2-­‐based  biosensor  was  1  pg/mL  which   is   4000   times   more   sensitive   than   the   current   clinical   cut-­‐off   level.3   Jiang   et   al.   created  a  MoS2-­‐based  FET  with  a  Hg+2  detection  limit  of  30  pM  useful  for  monitoring   anthropogenic   mercury   in   drinking   water.10   These   studies   show   proof-­‐of-­‐concept   that   incredible,   ultrasensitive   devices,   which   are   not   possible   with   atomically   thin   graphene,  can  be  realized  using  TMDCs.    

 

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Understanding  wettability  of  surfaces  is  critical  for  fabricating  ultrasensitive  devices   because   small   changes   in   wettability   can   significantly   influence   adhesion   in   heterostructures  and  impact  overall  device  performance.    In  1988,  Kelebek  reported   molybdenite   to   be   hydrophobic   with   a   critical   surface   tension   of   29   mJ/m2.11     Meanwhile,  Zhang    et  al.  reported  the  water  contact  angle  (WCA)  of  sputtered  MoS2   as  85°.12    More  recent  work  has  corroborated  the  hydrophobicity  of  MoS2:  the  WCA   of   bulk   MoS2   was   reported   as   88.37°   and   75.8°.3,   13   Gaur   et   al.   showed   that   increasing   synthesis   temperature   of   MoS2   thin   films   from   550°C   to   900°C   allowed   for  controlled  diffusion  of  sulfur  atoms  through  the  Mo  film  to  create  a  well  ordered   surface   with   a   high   degree   of   crystallinity,   resulting   in   a   WCA   change   from   23.8°   (550°C)  to  91.6°  (900°C)  for  2D  MoS2  films.    The  low-­‐WCA  surface  was  attributed  to   high-­‐energy   vertically   aligned   edge   sites   due   to   low   synthesis   temperature.     Moreover,   they   reported   that   WCA   decreases   with   number   of   MoS2   monolayers   to   approach   that   of   the   bulk   (88.37°)13,   a   phenomenon   which   was   also   reported   on   graphene.14    Strano  et  al.  reported  MoS2  and  other  TMDC’s  to  have  surface  energy  of   65-­‐75   mJ/m2   while   Gaur   et   al.   reported   surface   energy   of   few   layer   MoS2   as   44.5   mJ/m2  (Neumann  method)  and  40.47  mJ/m2  (Fowkes  method).6,  13  The  discrepancy   between  surface  energy  values  could  be  due  to  different  methods  used  to  calculate   surface  energy  along  with  spontaneous  contamination  by  ambient  hydrocarbons.15,   16   However,   MoS

2   has   been   believed   to   be   hydrophobic   in   all   the   aforementioned  

articles   with   WCA   of   76-­‐92°.3,  6,  11,  12,  13     Interestingly,   Chow  et  al.   recently   reported   on   the   wetting   behavior   of   monolayer   and   few-­‐layer   MoS2   and   WS2   supported   on  

 

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silica.     They   showed   that   WCA   of   fresh   monolayer   WS2   increases   from   70°   to   83°   upon  exposure  to  ambient  air.16     Surface   contamination   is   a   serious   concern   for   any   solid   surface,   including   atomically  thin  material,  since  contaminants  can  affect  water  wettability.    A  classic   example   is   the   adsorption   of   airborne   contaminants   onto   gold   rendering   the   hydrophilic   surface   to   appear   to   be   hydrophobic.     It   took   the   surface   science   community  more  than  forty  years  to  conclude  that  the  observed  hydrophobicity  of   the   gold   is   due   to   airborne   hydrocarbon   contaminants   and   the   gold   is   intrinsically   hydrophilic.17,  18,  19  Graphite  has  been  traditionally  believed  to  be  hydrophobic  with   WCA   of   ca.   90°;   however,   recent   studies   indicated   that   graphite   is   intrinsically   mildly  hydrophilic  with  a  WCA  of  ca.  53-­‐65°  and  it  adsorbs  airborne  hydrocarbons   in  the  ambient  air  to  minimize  surface  energy,  i.e.,  appear  to  be  more  hydrophobic.20,   21,   22  

Similar   results   were   also   reported   for   monolayer   graphene   on   copper   and  

multilayer  graphene  on  nickel15,  23  and  the  surface  energy  and  wettability  of  freshly   synthesized  graphene  on  copper  was  found  to  be  dependent  upon  exposure  time  to   the   ambient   air.23   Lai   et   al.   attributed   the   change   in   graphene   wettability   to   adsorption   of   both   water   molecules   and   hydrocarbons24     and   Nioradze   et   al.   demonstrated   that   HOPG   electroactivity   is   significantly   affected   by   organic   impurities   in   water   and   air.25   Boinovich   et   al.   also   showed   that   hydrocarbon   contaminants   spontaneously   adsorb   onto   boron   nitride   nanotubes   (BNNTs)   and   render   the   BNNTs   to   be   hydrophobic,   suggesting   that   other   2D   materials   are   susceptible   to   spontaneous   hydrocarbon   contamination.26   In   light   of   these  

 

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observations,   there   is   great   importance   in   investigating   how   airborne   contamination   affects   the   wettability   of   MoS2   due   to   its   salience   as   a   2D   material   beyond  graphene.         Herein,  we  investigate  bulk  MoS2  to  elucidate  its  intrinsic  water  wettability,  which  is   the   foundation   of   that   of   the   mono   (few)-­‐layer   MoS2.     WCA   shows   temporal   evolution   with   an   “intrinsic”   value   of   69.0±3.8°   that   increases   to   89.0±3.1°   after   1   day  exposure  to  ambient  air.  Surface  energy  of  fresh  and  aged  MoS2  was  calculated   from   contact   angle   measurements   with   data   indicating   that   surface   energy   is   a   strong   function   of   exposure   time   to   ambient   air.   Attenuated   total   reflectance-­‐ Fourier  transform  infrared  spectroscopy  (ATR-­‐FTIR)  and  ellipsometry  indicate  that   hydrocarbon   contaminants   adsorb   onto   freshly   exfoliated   MoS2,   rendering   the   intrinsically   mildly   hydrophilic   surface   hydrophobic.     Investigating   wettability   of   bulk  MoS2,  as  opposed  to  mono  (few)  -­‐  layer  MoS2,  provides  valuable  insight  to  the   true   material   properties   without   interfering   effects   from   sample   synthesis,   processing,  and  substrate  interactions.       Experimental   MoS2  preparation   Bulk  MoS2  (2D  Semiconductors;  ~10x5x2  mm)  was  exfoliated  with  Scotch  brand  1-­‐ inch   tape   to   expose   a   fresh   surface.     The   tape   was   applied   to   the   upper   sample   surface   and   gently   pressed   to   remove   air   bubbles   and   ensure   contact   between   the   tape   and   MoS2.     The   tape   was   then   gently   pulled   away   causing   the   upper   MoS2   layer  

 

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to   be   removed,   thereby   revealing   a   fresh   surface   on   the   bulk   sample.     The   fresh   surface   was   used   for   experiments   only   when   exfoliation   was   clean   with   no   flakes   and  the  tape  had  a  uniform  coverage  of  removed  material.    This  ensured  that  (1)  the   sample   was   actually   exfoliated   exposing  a  fresh  surface  and  (2)  tape   residue  did  not   remain  on  the  bulk  sample.    The  fresh  MoS2  was  tested  within  10  seconds  to  obtain   results  on  the  pristine  surface.           Contact  angle   Contact   angle   measurements   were   taken   in   ambient   air   at   22-­‐25°C   and   20-­‐30%   relative   humidity.     Deionized   (DI)   water   was   provided   from   a   Millipore   Academic   A10  with