AFM Scans of NPN, MgF2, and Au-coated Membranes

ByGreg Madejski

With my TEM tilt stack work, we can see a marked difference in the roughness of npMg2 and NPN. Quantifying the bulk roughness properties of the npMgF2 using AFM gives us a way to check that property.

Surface roughness has been known to effect SERS activity in other substrates, and our current hypothesis is that the roughness of Au in the pores of npMgF2 is providing the SERS activity. However, these scans will not really give us great information here, relying more on the surface properties of the nanomembranes. The fabrication methods that produce the surface roughness on the surface of the npMgF2 nanomembranes (evaporation) should produce similar features inside the pores, but this is an assumption that needs to be confirmed.

1236 NPN was fabricated into 50 nm freestanding npMgF2. The substrates were then coated with 500 s of Au using the Denton Sputterer (no rotation, 15 mA, 100 mTorr, ~0.1 nm/sec targeting 50 nm of Au).

 

AFM Methods

  • Data was collected on an Aslyum Research MFP3D
  • Samples were mounted on Carbon Tape, immersed in Distilled DI water
  • AFM tips
    • Manufactured Mar 2017, Lot 170314
    • Silicon, 10 nm Tip radius
    • 9 kHz frequency
  • Scans were 500×500 nm, at 0.1Hz, 256×256 pts
  • Data was analyzed using Gwyddion
    • Data was cropped, then plane leveled
    • Heights were set to zero
    • Horizontal Scars were removed, then a gaussian blur was applied (5 pixel radius)
    • Roughness values were extracted from line scans of the non-pore regions (ie the surface of the membrane, not the holes). These values can vary widely depending on which line is drawn (up to 100% error), so these should be considered estimates, not absolutes.

 

Images

Bulk

1236 NPN Bulk. Ra= 0.15 nm, Rq = 0.18 nm

 

1236 NPN+Au Bulk. Ra = 0.353 nm, Rq = 0.438nm

 

1236 MgF2 Bulk. Ra = 0.292 nm, Rq = 0.340 nm

 

1236 MgF2+Au Bulk. Ra = 0.49 nm, Rq = 0.64 nm

 

Windowed

1236 NPN Window, Ra = 0.21 nm, Rq = 0.25 nm

 

1236 NPN+Au Window, Ra = 0.59 nm, Rq = 0.69 nm

 

1236 MgF2 Window, Ra = 0.219 nm, Rq = 0.276 nm

Unfortunately, we could not locate the metallized MgF2 membrane under the AFM microscope; it did not have enough contrast. There does appear to be a general trend of roughness increasing as layers are added to the base 1236 NPN. The npMgF2 is rougher than the NPN, and that seems also to be preserved when adding Au on top of it.

 

Example where Au nanoparticles decorate the surface of the npMgF2 nanovolcanoes.

 

1148 NPN +15nm Au -14to+14 2deginc 130kx aligned. The nanoparticles decorate the inner surface of the nanopores.

 

1148 NPN +15nm Au 10-14 Anaglyph

 

Anaglyph Stack -14to+14 2degseparation

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