Thin layers (2D) of nanoparticles are formed by evaporating dispersions of nanoparticles on a solid substrate Three-dimensional assemblies are prepared by slowly diffusing a poorly coordinating solvent into the liquid dispersion of nanoparticles With Fe nanoparticles the 2D and 3D assemblies have different structural and magnetic behavior 2D Nanoparticle Arrays and 3D Nanoparticle Crystals
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2D Nanoparticle Arrays and 3D Nanoparticle Crystals
2D Nanoparticle Arrays and 3D Nanoparticle Crystals. Thin layers (2D) of nanoparticles are formed by evaporating dispersions of nanoparticles on a solid substrate - PowerPoint PPT Presentation
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Thin layers (2D) of nanoparticles are formed by evaporating dispersions of nanoparticles on a solid substrate
Three-dimensional assemblies are prepared by slowly diffusing a poorly coordinating solvent into the liquid dispersion of nanoparticles
With Fe nanoparticles the 2D and 3D assemblies have different structural and magnetic behavior
2D Nanoparticle Arrays and 3D Nanoparticle Crystals
Simulated phase contrastTEM image
Layer Stacking
Found for hexagonal close packed arrays of larger Fe nanoparticles
Not seen with nonmagnetic particles
S. Yamamuro, D. Farrell, and S. A. Majetich, Phys. Rev. B65, 224431 (2002)
Preference for an Odd Number of Layers
Dilute solutions form hexagonal monolayers
Concentrated solutions form thicker cubic or hexagonal arrays
BCC structure entropically stabilized for small diameters
Slower formation increases the coherence length
Evaporating droplet
2D Array Structure Summary
Use very slow precipitation (hours, weeks, months) by diffusion of “bad” solvent
Can make 3D array crystals up to 10 microns in size
Particles dispersed in toluene
Ethanol
Propanol
3D Nanoparticle Arrays
For standard surfactants, edge-to-edge interparticle separation
≥ 2.5 nm
Expect magnetostatic interactions to dominate
Learn about interactions from Mr(H), Mrelax(t), MZFC(T)
Dipolar Interactions
-60
-40
-20
0
20
40
60
σr
(emu/g)
-4000 -2000 0 2000 4000
H (Oe)
T = 10 K
6.7 nm Fe cores, OA/OY
Arrays, H parallel
Arrays, H perpendicular
Magnetization with H perpendicular harder to saturate, decays faster
Interactions shape anisotropy in 2D arrays
1.00
0.98
0.96
0.94
0.92
0.90
0.88
0.86
0.84
Normalized M
6543210
ln(t/t0
)
H = 0 Oe
T = 10 K
6.7 nm Fe cores
2.5 nm separation
H parallel to substrate
H perpendicular
H
H=0
Field Orientation Mr(H)
€
Φmag =μ0
4πr3
r μ •
r μ [ ]
€
Φmag =1.4kT
€
Φmag=2.4kT
Dipolar energy
per pair of particles
At T = 10 K
Vary the Particle Size
1.0
0.8
0.6
0.4
0.2
Normalized M
zfc
30025020015010050
T (K)
H= 100 Oe
2.5 nm spacing
6.7 nm Fe cores
8.5 nm cores
-1.0
-0.5
0.0
0.5
1.0
Normalized M
r
-4000 -2000 0 2000 4000
H (Oe)
T = 10 K
OA/OY
6.7 nm Fe cores
8.5 nm Fe cores
1.00
0.98
0.96
0.94
0.92
0.90
0.88
0.86
0.84
Normalized M
6543210
ln(t/t0
)
H = 0
T = 10 K
OA/OY
6.7 nm Fe cores
8.5 nm Fe cores
Larger particles have:
• slightly faster approach to saturation
• slower decay in M(t)
• higher TB and broader M ZFC(T)
Particle Size Effects
Same batch of 6.7 nm Fe particles with different surfactants