• No se han encontrado resultados

Film  Thickness  Dependence  of  the  Fe  Catalyst  Size  on   Si

N/A
N/A
Protected

Academic year: 2020

Share "Film  Thickness  Dependence  of  the  Fe  Catalyst  Size  on   Si"

Copied!
25
0
0

Texto completo

(1)

Pablo  G.  Caceres-­‐Valencia  

(*)  The  work  was  carried  out  by  the  author  when  he  was  on-­‐leave  from  the  University   of  Puerto  Rico  at  Mayaguez,  Puerto  Rico  PR  00681  

Film  Thickness  Dependence  of  the  Fe  Catalyst  Size  on   Si

3

N

4

,  Al

2

O

3

 and  SiO

2

 Substrates  for  the  Synthesis  of  

VerCcally  Aligned  Carbon  Nanotubes:    A  High-­‐

Throughput  Approach  

(2)

Background

§ Examples  of  the  various  structural  forms  of  carbon  

§ a.) Three  graphene  sheets  forming  graphite;

§ b.) Graphene;

§ c.) Diamond  

       

 

CNT’s  History

§ Multi-walled Nanotubes  1991      S.  Iijima,  Nature  354  (1991)  56    

§ Single-walled Nanotubes 1993

       

 

!

!

(3)

Background

!

S.B. Sinnott et al., Chemical Physics Letters 315 (1999) 25–30

(4)

Background

VerVcally  aligned  carbon  nanotubes  (VACNT’s)  of   uniform  diameter  and  length  have  the  potenVal  to  be   used  as  electro-­‐chemical  sensors3,  4,  electron  field   emiZers5-­‐7,  supercapacitors8,  via  interconnects9  and   others.    

The  versaVlity  of  CNTs  is  based  on  the  tuning  of  its   properVes  to  specific  applicaVons  by  controlling  the   diameter  and  chirality  of  the  tubes.  The  former  is   directly  related  to  the  diameter  of  the  catalyst   parVcles10-­‐13.  

In  thermal  CVD,  a  transiVon  metal  catalyst  such  as  Fe,   Ni  or  Co  is  usually  deposited  onto  a  substrate  as  a   uniform  film  using  techniques  such  as  spuZering  or   thermal  evaporaVon.    A  subsequent  annealing  causes   the  catalyst  film  to  fragment  into  nearly  spherical   parVcles  with  sizes  that  depends  mainly  on  the  iniVal   thickness  of  the  deposiVng  layer,  type  of  substrate   and  the  annealing  temperature.    

(5)

Background

The  applicaVon  of  CNTs  as  interconnects  requires   the  use  of  high  CNT  densiVes  (10-­‐2  to  10-­‐1  parVcles/

nm2)22,  30.    

High  CNT  densiVes  can  only  be  achieved  with  high   catalyst  parVcle  density  by  concomitantly  increasing   the  catalyst  area  coverage  and  at  the  same  Vme   decreasing  the  parVcle  size.    

The  highest  achieved  VACNT  densiVes  to  date  has   been  reported  by  Zhong  et  al34  at  1/nm2  using   controlled  thin  film  thickness  and  a  mulVlayer   approach.    

(6)

Experimental  Techniques  

Electron  transparent  membranes  of  Si3N4  were  acquired  from  Ted  Pella  for  this  invesVgaVon.    

All  the  membranes  were  spuZered  with  an  iron  film  of  gradient  thickness.  The  maximum   thickness  at  one  end  was  4.2nm.  

Taken  from  www.tedpella.com    

(7)

a.  For  the  studies  on  the  SiO

2

 and  Al

2

O

3

 substrates,  the  Si

3

N

4

 membranes  were   spuZered  with  a  3nm  uniform  film  of  SiO

2

 or  Al

2

O

3

 prior  to  the  Fe  spuZering.  

b.  An  ion-­‐beam  spuZering  equipment  and  quartz  and  sapphire  as  target  materials   respecVvely  were  used  for  this  purpose.  

c.  The  membranes  were  subsequently  annealed  using  a  rapid  inserVon  furnace  kept   at  675

o

C  under  an  atmosphere  of  flowing  helium  (200sccm)  and  hydrogen  

(250sccm).  All  samples  were  annealed  for  16minutes.  

d.  The  annealed  samples  were  observed  under  a  Transmission  Electron  Microscope   operaVng  at  200kV.  

Experimental  Techniques  

(8)

CorrelaVon  between  the  thickness  of  the  Fe  film  and  posiVon  on  the  sample.  

A  Cameca  SX-­‐100  Electron  Probe  Microanalyzer  operaVng  at  15kV   and  the  GMR  thin  film  soeware  were  used  to  correlate  posiVon   with  x-­‐ray  intensity  and  concentraVon.    

Catalyst  thickness  determined  by  posi<on  

(9)

Transmission Electron Microscopy (TEM)

AcceleraVon  voltage:  Up  to  200  kV   Point  resoluVon:  0.19  nm  

Line  ResoluVon:  0.14nm  

Side-­‐entry  computer  controlled  eucentric   goniometer:  ±EDX  and  45º  and  ±  30º  

EELS  systems  

OperaVon  Modes:  BF,  DF,  CBED,  HRTEM,   MicrodiffracVon,  EDX  and  EELS  

(10)

50nm  

Si3N4   Al2O3   SiO2-­‐IBS  

Post-­‐heat  treatment  catalyst  size  vs.  substrate  0.3  nm  

(11)

50nm  

Si3N4   Al2O3   SiO2-­‐IBS  

Post-­‐heat  treatment  catalyst  size  vs.  substrate  0.6  nm  

(12)

50nm  

Si3N4   Al2O3   SiO2-­‐IBS  

Post-­‐heat  treatment  catalyst  size  vs.  substrate  1.2  nm  

(13)

50nm  

Si3N4   Al2O3   SiO2-­‐IBS  

Post-­‐heat  treatment  catalyst  size  vs.  substrate  2.6  nm  

(14)

50nm  

Si3N4   Al2O3   SiO2-­‐IBS  

Post-­‐heat  treatment  catalyst  size  vs.  substrate  4.2  nm  

(15)

0   2   4   6   8   10   12   14   16   18  

0   0.5   1   1.5   2   2.5   3   3.5   4   4.5  

Average  Par<cle  Diameter  (nm)  

SpuOered  Thickness  (nm)  

Si3N4   Al2O3   SiO2-­‐IBS  

In  Si3N4  substrate  the  average  parVcle  size  of  the  catalyst  is  always  large,  even  at  very  small  spuZering   film  thickness  (e.g.  at  1.2nm  of  film  thickness  a  15nm  parVcle  develops).    

In  Al2O3  or  SiO2-­‐IBS  the  average  parVcle  size  obtained  at  very  small  thickness  starts  at  3.3  and  2.6nm   respecVvely.    

The  rate  of  growth  of  the  average  diameter  with  film  thickness  for  these  substrates  (as  determined  by   the  iniVal  slope  of  the  graphs)  is  faster  in  Si3N4  and  Al2O3.    

Post-­‐heat  treatment  catalyst  size  vs.  substrate  

(16)

0   0.002   0.004   0.006   0.008   0.01   0.012   0.014   0.016   0.018  

0   0.5   1   1.5   2   2.5   3   3.5   4   4.5  

Par<cle  Density  (par<cle/nm2)  

Iron  SpuOered  Film  Thickness  (nm)  

Si3N4   Al2O3   SiO2-­‐IBS  

The  Si3N4  has  low  parVcle  densiVes  for  all  the  spuZered  film  thicknesses.    

ParVcle  densiVes  of  10  (in  10-­‐4  nm-­‐2)  are  reached  at  thickness  of  0.5nm  with  an  average  parVcle   diameter  of  14nm  and  a  wide  PSD.    

These  parVcle  density  values  are  10  Vmes  higher  than  the  values  obtained  by  Sigmore  et  al  for  ill   formed  carpets.    

M.A.  Signore,  A.  Rizzo,  R.  Rossi,  E.  Piscopiello,  T.  Di  Luccio,  L.  Capodieci,  T.  Dikonimos,  R.  Giorgi;  Role  of  iron  catalyst  par0cles  density  in  the  growth  of  forest-­‐like   carbon  nanotubes;  Diamond  &  Related  Materials  17  (2008)  1936–1942  

   

Par<cle  Number  Density  vs.  Catalyst  Thickness  

(17)

0   0.002   0.004   0.006   0.008   0.01   0.012   0.014   0.016   0.018  

0   0.5   1   1.5   2   2.5   3   3.5   4   4.5  

Par<cle  Density  (par<cle/nm2)  

Iron  SpuOered  Film  Thickness  (nm)  

Si3N4  

Al2O3  

SiO2-­‐IBS  

Par<cle  Number  Density  vs.  Catalyst  Thickness  

In  Al2O3,  ultra  high  density  CNTs  reaching  6x10-­‐3  nm-­‐2  can  be  formed  with  a  0.4nm  thickness  and  an   average  diameter  between  4-­‐6nm.    

ParVcle  densiVes  of  1.6x10-­‐2  nm-­‐2  are  reached  at  thickness  of  0.8nm  with  an  average  parVcle  diameter   of  3-­‐4nm  in  the  SiO2-­‐IBS  substrate.    

(18)

As  the  most  widely  used  SiO2  substrate  is  produced  thermally,  a  SiO2-­‐thermal  substrate  was  used  to   compare  our  TEM  results.  This  comparison  was  also  extended  to  a  solid  substrate  (non-­‐electron   transparent)  of  Si/SiO2  wafer.    

(a) 1.2nm SiO2-Thermal (b) 1.2nm SiO50nm   2-IBS

TEM  images  of  catalyst  on  the  SiO2-­‐thermal  and  SiO2-­‐IBS  were  compared  for  an  iniVal  Fe  thickness  of  1.2nm.    

The  SiO2-­‐thermal  substrate  behaves  similarly  to  the  Si3N4  substrate,  that  is,  with  a  high  iniVal  catalyst  parVcle  size.    

Catalyst  Evolu<on  on  Thermal  vs.  IBS  SiO

2  

(19)

AFM  Comparison  Between    

Thermal  SiO

2

 and  IBS-­‐  SiO

2

 substrates  

Fe-­‐1.2nm  +  675oC  for  16m  (He  +H2)   On  Si  wafer  with  a  thermally  oxidized  film  

SiO2  –  254nm)   On  Si  +  SiO2  (254nm)  +  3nm  SiO2  IBS  

(20)

0   5   10   15   20   25   30   35   40  

Up   To  0  2  To  

4   6  To   8   10  

To   12  

14   To   16  

18   To   20  

22   To   24  

26   To   28  

30   To   32  

34   To   36  

38   To   40  

Frequency  

Diameter  (nm)  

0   50   100   150   200  

Up   To  0  2  To  

4   6  To   8   10  

To   12  

14   To   16  

18   To   20  

22   To   24  

26   To   28  

30   To   32  

34   To   36  

38   To   40  

Frequency  

Diameter  (nm)  

PSD  from  AFM  micrographs  of  iron  on  SiO2-­‐thermal  and  SiO2-­‐IBS  

The  PSDs  obtained  from  the  AFM  images  indicate  that  average  catalyst  parVcles  are  14nm  for  SiO2-­‐thermal  and   4nm  for  SiO2-­‐IBS  that  is,  the  Fe  parVcles  have  grown  40  Vmes  more  on  the  thermally  oxidized  substrate.  

AFM  results  are  similar  to  the  TEM  results.    

The  AFM  results  corroborate  the  suggesVon  that  the  Fe  parVcle  growth  on  SiO2-­‐thermal  is  similar  to  the  Si3N4   substrate.  

AFM  Comparison  Between    

Thermal  SiO

2

 and  IBS-­‐  SiO

2

 substrates  

(21)

CNT  Synthesis:  Comparison  between     SiO

2

-­‐thermal  and  SiO

2

-­‐IBS  –  0.6nm  

0   2000   4000   6000   8000   10000   12000   14000  

1200   1300   1400   1500   1600   1700   1800   Raman  Shi[s  (cm-­‐1)  

0   500   1000   1500   2000   2500  

1200   1300   1400   1500   1600   1700   1800  

Raman  Shi[  (cm-­‐1)  

Non-­‐aligned  

CNTs   Aligned  CNTs  

Carpet  height  =   500microns   Aeer  annealing  at  675oC  for   16m,  synthesis  was  carried  out   at    775oC-­‐16m  

At  this  high  synthesis  temperature,   OR  takes  place  in  SiO2.  

2mm  

200nm  

100nm   4mm  

(22)

Synthesis  of  CNTs  on  Al

2

O

3

-­‐IBS  0.6nm  

0   5000   10000   15000   20000   25000   30000  

1200   1300   1400   1500   1600   1700   1800   Raman  shi[  (cm-­‐1)  

0   500   1000   1500   2000   2500   3000   3500   4000   4500   5000  

100   150   200   250   300   350   400  

Raman  shi[  (cm-­‐1)  

Aligned  CNTs   Carpet  height  =   750microns  

At  this  high  synthesis   temperature,  OR  is  small   in  Al2O3.  

40mm   100nm  

(23)

Contact  Angle  Measurements  

Substrate  

Core  Shell:   AZached  to  Substrate  

(24)

Tilting Experiments

Figure.  It  shows  the  same  area   aeer  different  VlVng  angle.  (a)  0  ;   (b)  18  ;  (c)  36  ;  (d)  45  and  (e)  54   degrees.  A  schemaVc  of  the   posiVon  of  the  metallic  core  is   given  in  (f).  

           

   

(25)

Conclusions  

a.  The  average  parVcle  size  diameter  measurements  show  two  different  linear   behaviors.  At  small  film  thickness  range  the  linear  behavior  is  dominated  by  the   catalyst-­‐support  interacVon,  while  in  the  large  film  thickness  range  their  behavior  is   independent  of  the  type  of  substrate.  

b.  The  growth  of  VACNTs  is  not  solely  controlled  by  a  minimum  areal  parVcle  density.  

The  size  of  the  parVcles  and  parVcularly  a  large  spread  of  the  PSD  will  induce   uneven  CNT  growth  and  hence  the  formaVon  of  non-­‐aligned  CNT’s.  

c.  The  catalyst-­‐substrate  interacVon  is  strongly  influenced  by  the  processing   condiVons  of  the  substrate.  When  comparing  thermal  or  IBS  produced  SiO2  

substrates,  the  laZer  showed  to  strongly  enhance  the  nucleaVon  of  the  Fe  parVcles   and  reduce  coalescence  or  the  rate  of  the  parVcle  growth  with  increasing  film  

thickness.  

d.  The  opVmum  condiVons  for  the  growth  of  ultra-­‐high  density  CNT  can  be  obtained   by  controlling  the  catalyst-­‐substrate  interacVon  and  by  determining  the  effect  of   film  thickness  on  the  parVcle  size  and  areal  parVcle  density,.    

e.  These  parameters  can  also  be  tailored  to  produce  ultra  high  density  VACNTs  with  a   large  percentage  of  SWCNTs.  For  such,  the  effect  of  the  synthesis  temperature   needs  also  to  be  tailored  for  each  type  of  substrate.    

Referencias

Documento similar

Superconducting parameters of the compounds are given in Table I, and their zero-field tunneling conductance curves are shown in Fig. Note that the critical temperatures of the

e) The behaviour of the early terminated methods and OPM has a strong dependence on the absolute error allowed to the solutions e. Permisive errors give smaller values of R min =

Supported ionic liquid phase (SILP) technology is based on an IL thin film containing a homogeneous catalyst, which is uniformly distributed in the pores and

They study the power consumption as a function of membrane size and the temperature distribution with and without silicon plug.. By adding a silicon plug of about 10 m thickness

Market size improves entry profit whereas a cost advantage for the provision of perceived quality on the side of the incumbent reduces it. Nevertheless when the size of the market

We have studied the dependence of the optical contrast with the flake thickness and the illumination wavelength of few-layer antimonene flakes on SiO 2 /Si substrates produced by

We built a parameterization of the growth index that takes into account its redshift dependence as well as the dependence on the equation of state of dark energy and its speed of

We show that using the field dependence of magnetocaloric effect, the order of the phase transition can be unambiguously determined using a quantitative criterion even for