CONCLUSIONS AND FUTURE PLANS

CHARACTERIZATION BY AFM

ELECTROSTATIC FORCE MICROSCOPY

ATOMIC FORCE MICROSCOPY

Topography = (A+B)-(C+D)

INTRODUCTION

ACKNOWLEDGMENT

Topography Phase

SYNTHESIS AND AFM CHARACTERIZATION OF DESIGNED NANOSTRUCTURES OF CERIUM OXIDE Azzy L. Francis, Steve M. Deese, and Jayne C. Garno*

Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803

The authors gratefully acknowledge support from the National Science Foundation Career/PECASE award (CHE-0847291); the Camille Dreyfus Teacher-Scholar award; the Petroleum Research Fund of the American Chemical Society; and IMSD Research Scholars Program.

Test platforms of cerium oxide nanoparticles were examined to determine electrical properties such as surface potential using Electrostatic Force Microscopy (EFM). With high special resolution, the EFM is commonly used to map electrical properties on a sample surface by measuring the electrostatic force between the sample surface and a biased atomic force microscope (AFM) cantilever. EFM yields information of electrical properties of a sample while simultaneously providing topography details. Phase and amplitude images are acquired simultaneously while topography frames sensitively disclose fine details of the surface morphology. Studies with EFM enabled measurements of potential energy differences with nanoscale resolution thereby enabling tracking differences in oxidation state of the material. Our goals were to apply scanning probe characterizations of nanoparticle test platforms to investigate electrical properties at the level of individual cerium oxide nanoparticles.

2Li, G; Mao, B; Lan, F; Liu, L. Review of Scientific Instruments, 2012, 83, 113701(1) – 113701(8).

In EFM, a conductive AFM tip (coated with Pt) is biased with a dc voltage (Vdc) and an ac voltage (Vac) at a frequency (ωe). The dc and ac electrical drive on the tip causes an electrostatic force between the tip and the sample surface. The electrostatic force at the electrical driving frequency can be described as

F(ωe) = ∂C/∂z (ϕ + Vdc)Vacsin(ωet) (1)

Where C is the capacitance between tip and sample surface, and ϕ is the contact potential difference between tip and sample surface. The electrostatic force F(ωe) causes the cantilever to oscillate at a certain frequency (ωe). The amplitude of the cantilever oscillation at the frequency can be detected by a lock in amplifier. Using a feedback control to adjust the dc bias, the electrostatic force F(ωe) can be nullified thus the cantilever oscillation can be minimized. Therefore, the surface contact potential difference can be acquired from the dc bias ϕ = -Vdc.

CERIUM OXIDE NANOPATICLE SYNTHESIS

1Chen, P; Chen, I. Journal of the American Ceramic Society, 1993, 76, 1577-1583.

In the Ce-urea method, 0.5 M urea was dissolved in a 0.008 M cerium nitrate solution. The solution was then heated to 85 ± 1 °C for 1 h to effect precipitation.

T h e a t o m i c f o r c e microscope (AFM) was u s e d t o i m a g e t h e substrate surfaces. When imaging in contact mode AFM, the tip is attached to the end of a cantilever and makes soft, physical contact with the substrate surface. As the tip scans the substrate surface, the cantilever attached to the tip bends to accommodate to the changes in height on the substrate’s surface.

125 nm

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CHARACTERIZATION BY EFM

60 nm 0.23 V

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Topography Phase Surface Potential

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Electrostatic fields between tip and sample detected by monitoring the amplitude response of the cantilever at ωe.

The cantilever is oscillated at a mechanical resonant frequency ωmech. AC bias applied between the tip and sample at electrical resonant frequency ωe. Additional electrostatic forces caused by the AC bias influence the tapping amplitude of the tip.

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Surface Potential

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•  Atomic force microscopy is an ideal instrument to characterize cerium oxide nanoparticles due to its high resolution in three dimensions.

•  The Keysight 5500 AFM/EFM instrument was setup correctly as proven by the images acquired.

•  Electrostatic force microscopy served as an model method mapping out surface potential differences while simultaneously providing topography details.

•  Future plans include to drop-deposit two types of nanoparticles on a silicon surface and image with EFM. Rare earth nanoparticles having difference work functions should be distinguishable using EFM. Further investigations will be done to determine surface potential as a function of nanoparticle size.

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ght (

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O

H2N NH2

Ce(NO3)3 � 6H2O

Heat

85 °C

The recovered cerium oxide nanoparticles where drop deposited on silicon (111) substrates and oven dried at 150 °C.

Cerium oxide nanoparticles were drop deposited on silicon. Topography and phase images were acquired using tapping-mode AFM for different areas.

The cerium oxide nanoparticle selected for height measurements measure 50 – 60 nm in height. The cursor profile shows a width of ~250 nm for this particular nanoparticle.

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