Namomechanical Imaging and Nanoscale Elastic Modulus Measurements of SnO 2 Nanobelts
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Namomechanical Imaging and Nanoscale Elastic Modulus Measurements of SnO2 Nanobelts Y. Zheng and R. E. Geer College of Nanoscale Science and Engineering, University at Albany, State University of New York, Albany NY 12203 ABSTRACT The relative surface contact stiffness of SnO2 nanobelts has been investigated via ultrasonic force microscopy (UFM). The nanobelt crystal structure, as determined via transmission electron microscopy, was indexed to the tetragonal rutile structure (with lattice constants identical to those of bulk SnO2) as reported previously. The atomic Sn:O composition of the nanobelts studied was at or near 1:2. Topographic imaging studies revealed the nanobelt surface to be atomically flat with the exception of surface nanodots, assumed to be local SnO2 crystallites. Preliminary reduced modulus measurements were carried out via differential UFM on both the flat and nanodot regions of the nanobelt. Using the underlying Si substrate as a calibration standard the SnO2 modulus was estimated at 157±12 GPa, significantly lower than corresponding bulk values for any of the observed crystal orientations. We speculate this discrepancy is due in part to a combination of the aspherical probe tip and unknown adhesive properties of nanobelt. An intrinsic reduction of the SnO2 nanobelt modulus cannot be ruled out. INTRODUCTION Semiconducting oxide nanobelts have been the focus of considerable experimental activity for applications in both nanoelectronics and nanosensors, in addition to their importance in elucidating the properties of self-assembling inorganic nanostructures1-3. Recently, nanomechanical analyses of individual SnO2 nanobelts have been reported4. Nanoindentation was utilized to investigate the inelastic mechanical response of nanobelts including hardness and the load profile for fracture propagation. Such information is critical for predictive reliability analyses of nanoelectronic or nanosensor devices incorporating such nanostructures. However, a nondestructive approach is preferred to enable eventual in situ mechanical analysis of nanobeltcontaining device structures. Pursuant to these goals nanomechanical, structural, and compositional characterization of individual SnO2 nanobelts has been undertaken. The nanomechanical characterization consists of nanomechanical imaging based on ultrasonic force microscopy (UFM) and quantitative reduced modulus extraction via differential-UFM utilizing Si as a calibration standard. Structure characterization employed atomic force microscopy (AFM) for measurements of nanobelt surface topography and overall dimension, and transmission electron microscopy (TEM) for determination of nanobelt crystal structure and orientation. Auger electron spectroscopy (AES) was used for elemental compositional analysis. Elemental compositional analysis via AES confirmed the expected 2:1 ratio of O:Sn. Electron diffraction confirmed a single crystal rutile structure in agreement with prior work1,5. However, in the present work separate, single-crystal nanobelts exhibited a
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