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How Do We See Nanostructures? How Many Types of Microscopes and its Parts Details

How Do We See Nanostructures?
First,  it takes  some pretty  sophisticated  instruments  to see  nano structures.

Optical  (light)  Microscopes  focus  visible  light  through “lenses”  to make  a  magnified image. They work  essentially  like  a  magnifying  glass.    Some  of  you may  even own  a  microscope, or  have  used  one  in school. Precision optical  microscopes  used in nanotechnology  can cost  up to $50,000. But  even  with the  most  precision, most  sophisticated optical  microscope, one  problem  remains—light  waves  are “big”,  at  least  on  the  scale of nanostructures.    As  the  resolution power  of  these  instruments  is  limited to about  half  of  the  wavelength of  light, they  can only  reveal  features  down to ~250 nm.   When  we talk  about  seeing  small  structures, it  is  important  to distinguish between  “resolution”  and “magnification”.    We can  “blow  up” (magnify)  an  image  (e.g.  a picture)    as  much  as  we want  –  make  it  as  big  as a  poster  on  your  wall  –  but that does  not make  the  image  any  sharper  or  increase  our  ability  to  resolve  small structures  –  i.e. to have  sharp edges  and to  distinguish  separately  closely  spaced  objects.  Blowing  up  a picture too big  just  gives  you  a  fuzzy  big  picture;  that  is  called “empty  magnification”  and it  does  us  little  good. What  is important is  the  ability  to  sharply  see structures  that  are  close to  each  other.  This  latter  is  called  resolution  and  is the  most  important  property  of  any  microscope.     Optical  microscopes  give  us  a  top-down, flat,  “airplane”  view  of  the  surface.  It is  difficult to  learn  much  about 3-D  objects  with a  high powered optical  microscope  because  they  have  very  low  “depth of  field”-  i.e  only objects  at  a  certain, very  narrow  height  will  be  in focus. For  a  high magnification optical  microscope, this “depth of  field”  can be  less  than 1 micro meter-  anything  taller  than  1  micrometer  is  out of  focus  and  blurry. With a  super  high quality  optical  microscope, we  see  and resolve  structures  down to about  250 nm.    That still leaves  a lot  that  we  can’t  see.    For  those,  we need  an  electron  microscope!


Electron  Microscopes    use electron  beams  instead  of  visible  light, enabling  resolution of  features  down to a few  nm.    Several  different  types  of  EMs  exist, including  Scanning  Electron  Microscopy    (SEM) and Transmission Electron Microscopy  (TEM)  .    Electron  Microscopes  use a beam  of  high  energy  electrons  to probe  the  sample.    Electrons  do not  suffer  the  same  resolution limits  that  light  does, so we  can  “see”  features  as small  as  0.1 nm.    This  is  the  size  of  an individual  atom.    Electronic signal  processing  is  used  to  create a picture of  what  the  sample  would look like  if  we  could see  it.    While  electron microscopy  offers  finer  resolution of features  than does  optical  microscopy, it  requires  vacuum  conditions  in order  to maintain a  focused  electron beam. This  makes  electron microscopy  inconvenient  for  examining  many  biological samples,  which  must first be  preserved and  coated  with layers  of  metal  atoms.    Another  advantage  of  electron microscopes  is  that  they have  both high magnification and high depth of  field.    We can  see objects  as  in  apparent  three dimensions.  This is  again due  to the  short  “wavelength”  of  electrons.    You  may  have seen  some really  “monster”  like  pictures  of   bugs  that highlight the  imaging  capabilities  of  the  scanning  electron  microscope.    High quality  electron microscopes  can cost  from  $250,000 to $1,000,000!  They  are  one  of  the  most  useful  instruments  in our laboratories.


Scanning  Probe Microscopes  (SPM)  of  various  types  trace surface  features  by  movement  of  a very  fine pointed tip mounted on a  flexible  arm  across  a  surface.    SPM  enables  resolution of  features  down to ~1 nm  in height,  allowing  imaging  of  single  atoms  under  ideal conditions.    Scanning Tunneling  Microscopes  (STM) measure  current  (i.e., electron flow)  between the  probe  tip and sample, essentially  acting  like  a  tiny  voltmeter. This  method  requires  that the  sample  be  electrically  conductive.    Atomic Force Microscopes  (AFM  –  sometimes call  Scanning  Force  Microscopes)  measure  interaction forces  between probe  tip and sample, providing information on the  mechanical  properties  of  surfaces.  They  can  measure forces  of  10-9 Newton.    (For comparison, the  force  exerted by  an  apple  is  ~1 N.)    AFMs  are  widely  used  to measure  surface  topography  of many  types  of  sample  and do not  require  special  conditions  such as  conductive  surfaces  or  vacuum. Scanned  probe microscopes  and  particularly  AFMs  basically  see things  by  touching.    Imagine  you  have  your right  hand in a  dark box  with a  mystery  object  and  you are  trying  to figure  out  what  the  object  is, without looking.    One  systematic  way  to  do  this  would be  to touch every  point  on a  grid , say    30 points  wide  and 30 points  deep, covering  the  entire  floor  of  the  box.    Imagine  that  with  your  left  hand,  you  record the  the  “height”  ( or  any  other  physical  property)  at  each  grid point  on a  piece  of  graph paper.    You could then make  a  3-d graph surface,  or  a 2-d plot  with colors  indicating  height. After  touching  and  recording  900 points,  you would have    a “picture” of  the object.  That  is  exactly  what  an  atomic force microscope does,  except  the AFM  uses  a very  fine point  instead of  a  finger,  and is  built  on a  mechanism  that  can reproducibly    move  the  tip less  than 0.1 nm between points.    Scanning  probe microscopes  can  actually  ‘feel” the bumps  due to  individual  atoms  and molecules ! 
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Mallikarjuna

Mallikarjuna

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