Several electron microscopy techniques are used by researchers in the Nanometer Structure Consortium to study the crystal structure and other material properties of nanowires and nanoparticles.
Scanning Electron Microscopy (SEM)
Usually, nanostructures are first investigated by the scanning electron microscopes. Two scanning electron microscopes are available at the
Nanolab, and one at
nCHREM. At Nanolab, we have a combined Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM) system, FEI Nova NanoLab600, for high-resolution SEM inspection and ion-beam patterning, and one high resolution field emission gun scanning electron microscope (FEG-SEM), JSM 6400F from JEOL. At nCHREM, we have a JSM 6700F high resolution FEG-SEM, equipped with Oxford Instruments INCA XEDS for elemental analysis.
Fig.1: Scanning Electron Microscopes: Left: FEI Nova Nanolab600, right: JEOL JSM 6400F
(Scanning) Transmission Electron Microscopy ((S) TEM)
Transmission electron microscopy is used to investigate and verify the shape and size of very small structures. This technique has been used in the calibration of fabrication instruments, such as the Differential Mobility Analyser used for
aerosol nanoparticle production [1].
High resolution transmission electron microscopy (HRTEM) is used primarily to investigate the crystal structure of nanowires and nanoparticles. Most of our research is conducted at
nCHREM using a JEOL 3000F 300 kV field-emission electron microscope with 0.17 nm point resolution in conventional mode and equipped with a 2k × 2k multi-scan CCD camera (Gatan). This instrument is also used for material identification by electron diffraction or fast-fourier transforms of high-resolution images. Fourier space filtering and back-transformation of such images allows for the association of different regions of a nanostructure with different composition and structure, on a scale which is more precise than ordinary spectroscopy techniques (XEDS and EELS). The instrument has also been used for nanowire heterostructure strain mapping [2].
Fig 2.: High-resolution analytical TEM-STEM Jeol 3000F at nChREM.
Conventional HRETM is limited mainly by the lens aberrations which distorts the fine detail in the images. By measuring how the lenses have affected an image it is possible to correct for these effects by applying filters that can restore the original, unaberrated signal. When these image reconstruction methods are applied to a series of images with different defocus the resolution can be improved from the 0.17 nm point resolution in a single image to the information limit of about 0.12 nm and at the same time improve signal to noise ratio and reduce delocalization [3].
High-angle annual dark field scanning transmission electron microscopy (HAADF-STEM) is used to identify different materials in a nanostructure by differing image contrasts. Research is conducted on the same instrument as used for high-resolution studies, but here the instrument has a point resolution of 0.14 nm in STEM mode. This technique is often combined with x-ray energy dispersive spectroscopy (XEDS; Oxford) to associate different material regions with specific elements. High-resolution HAADF-STEM allows for simultaneous material contrast and crystal lattice information.
In-situ STM and local probing of electric properties can be performed inside the TEM by a Nanofactory STM-TEM holder[4].
References
[1] “Size determination of au aerosol nanoparticles by off-line TEM/STEM observations”, L.S. Karlsson, K. Deppert, J.O. Malm, J. Nanoparticle Res. 8 (2006), 971-980, DOI:10.1007/s11051-006-9094-5
[2] "Strain mapping in free-standing heterostructured wurtzite InAs/InP nanowires", M.W. Larsson, J.B Wagner, M. Wallin, P. Håkansson, L. E. Fröberg, L. Samuelson and L. R. Wallenberg, Nanotechnology 18, 1 (2007) 015504 DOI:10.1088/0957-4484/18/1/015504
[3] M. Ek: "Image Series Reconstruction for Transmission Electron Microscopy",
Master Thesis 2009
[4] "Probing of Individual Semiconductor Nanowhiskers by TEM-STM", M. W. Larsson, L. R. Wallenberg, A. I. Persson, L. Samuelson, Microscopy And Microanalysis, 10, 1 (2004), 41-46 DOI:10.1017/S1431927604040176
