CHEM 146C Experiment 1 Synthesis and Electron Microscopy Studies of Semiconductor Zinc Oxide ZnO Nanowires Yat Li Department of Chemistry Biochemistry University of California Santa Cruz Objective In this laboratory experiment you will learn 1 The hydrothermal synthesis of dense and vertically aligned ZnO nanowire arrays on indium tin oxide ITO glass substrate 2 The basic principles of electron microscopy TEM and SEM 3 Gain hands on experience in using TEM and SEM to characterize nanomaterials Nanomaterials versatile building blocks Nanomaterials Nanoscale building blocks Individual devices Fundamental Properties Specialized systems Nanodevices Multifunctional integrated systems Semiconductors nanowires for functional devices Semiconductor nanowires well controlled morphology dimension and electronic properties Synthesis of semiconductors nanowire arrays Hydrothermal synthesis of ZnO nanowire arrays Our growth approach the growth of dense and vertically aligned ZnO nanowires is based on the recent reported seed method1 1 Greene L Law M Goldberger J Kim F Johnson J C Zhang Y F Saykally R J Yang P D Angew Chem Int Ed 2003 42 3031 3034 Procedure Step 1 Substrates are seeded with ZnO nanoparticles by decomposing zinc acetate at 350 C a b Blank ITO substrates c ITO substrates coated with ZnO seeds Annealing 350 C Procedure Step 2 Put the seeded substrate into a zinc nitrate solution in the presence of hexamethylenetetramine at 90 C for two hours d e f ITO substrates coated with ZnO nanowire arrays Characterization of ZnO nanowire arrays Microscopy techniques 1 Optical microscopy Conventional light microscopy Fluorescence microscopy confocal multiphoton microscopy 2 Electron microscopy Scanning electron microscopy SEM Transmission electron microscopy TEM Scanning transmission electron microscopy STEM Focus ion beam microscopy FIB 3 Scanning probe microscopy Scanning tunneling microscopy STM Atomic force microscopy AFM Near field scanning optical microscopy and others Optical microscopy This is an optical instrument containing one or more lenses that produce an enlarged image of an object placed in the focal plane of the lens 1 Transmission beam of light passes through the sample e g Polarizing or petrographic microscope Samples are usually fine powder or thin slices transparent 2 Reflection beam of light reflected off the sample surface e g Metallurgical or reflected light microscope Surface of materials especially opaque ones Resolution limit submicron particles approaches the wavelength of visible light 400 to 700nm Optical microscopy Transmitted light microscope Reflected light microscope Dark field vs Bright field Bright field normal wide field illumination method bright background low contrast Dark field an opaque disc is placed underneath the condenser lens scattered light dark background high contrast structural details BF DF Resolution limit of optical microscopy Resolution limited by wavelength of light diffraction Resolution limit of optical microscopy R 1 22 NA numerical aperture 2NAobjective NA nsin n refractive index Lens in air n of air 1 sin 1 R 1 22 2NAobjective 1 22 400nm 2 1 4 175nm NA 1 Electron microscopy Wavelength of Electron h 2meV 1 2 accelerating voltage Planck s constant mass electron charge Electron microscopy Very short wavelength depends on accelerating voltage 0 04 at 100 kV Can be deflected by magnetic field focusing Interaction of electron with samples SEM or analytical EM Conventional TEM Scanning TEM Energy analysis Configuration of Scanning Electron Microscopy Secondary electrons Low energy Topographic contrast surface texture and roughness Resolve surface structure down to 5 nm Excitation region depends on the accelerating voltage 1kV low atomic number 20kV high atomic number Back scattered electrons High energy Both Compositional and Topographic information Atomic number contrast Lateral resolution is worse than secondary electron image Secondary electron image Ni Au nanorods Backscattered electron image Ni Au nanorods Energy dispersive X ray spectroscopy Chemical information of sample Energy Dispersive X ray Spectroscopy EDS Detection area is limited by the resolution of SEM accelerating voltage of electron Transmitted electrons In the TEM we utilize the electrons that go through a very thin specimen 200nm Scattering electrons strong interaction between electrons and matter Image diffraction pattern x ray spectrum and electron energy loss spectrum Non uniform distribution of electrons contains all the structural compositional information Illumination system TEM operation using a parallel beam Imaging vs Diffraction Lattice resolved imaging and diffraction pattern Atomic resolution 0 16 nm Crystalline vs amorphous materials Lattice spacing atomic structure Single vs polycrystalline materials Interface different phases crystal structure Crystal structure and orientation Combined with computer simulation Crystal phases facet Good Luck to your first experiment See you at PSB 465 on Monday 3 30pm Tuesday 2 00pm