Low Voltage Field Emission SEM (LV FE-SEM): A Promising Imaging Approach for Graphene Samples Jining Xie Agilent Technologies May 23 rd, 2012 www.agilent.com/find/nano
Outline 1. Introduction 2. Agilent 8500 compact FE-SEM 3. Contrast mechanisms 4. Morphologies of graphene samples 5. Effects of electron beam energy 6. Effects of nonconducting substrates 7. Summary
1. Introduction Large theoretical surface area: 2630m 2 /g High intrinsic mobility: 200,000cm 2 v -1 s -1 High Young s modulus: ~1.0 TPa High thermal conductivity: ~5000 Wm -1 K -1 Good optical transmittance: ~97.7% Good electrical conductivity. (scientificamerican.com) Graphene Transistors on Glass (UCLA) Graphene sensors (U of Penn.)
Characterization Approaches (a) (c) (d) (e) (a) Optical microscopy APL, 91, 063124, 2007 (b) Raman spectroscopy Nano Letters, 9, 4359, 2009 (c) Atomic force microscopy Nature Materials, 6, 183, 2007 (d) Transmission electron microscopy Nano Letters, 8, 2442, 2008 (e) Scanning electron microscopy from Science
2. Agilent 8500 Compact FE-SEM Web Site: www.nano-tm-agilent.com Search Engine: Agilent FESEM The Only High-Performance, Compact, Field Emission SEM Innovative technologies miniaturize electron beam column with all-electrostatic lens Reliable Schottky field emitter High performance imaging <10nm at 1kV, high contrast, high image quality Compact dimension Simple operation Easy maintenance no need gas, chiller and coater Multiple imaging modes SE, BSE, Topos Low cost of ownership
38mm Agilent 8500 Compact FE-SEM Stage Module Valve Module Main Assembly UHV Module Gun Module Wafer scale lens fabrication Column Module Silicon Lenses Agilent 8500 85mm The 8500 electron beam column (field emitter, lens, and detector) Modular Architecture - Cost advantage in manufacturing
Challenges in SEM Imaging of Graphene Challenges Solutions from Agilent 8500 1. Nanoscale features - require a high spatial resolution 2. Transparent to electron beams - ultra-thin film require a low beam voltage 3. Extremely low contrast - graphene films are smooth and featureless 4. Charging from substrates - insulating substrates beneath graphene 5. Mixture of SE 1 and SE 2 signals - only SE 1 are surface sensitive; require a high performance detector 1. Field emission imaging performance with sub 10nm resolution 2. Low beam voltage operation (500V- 2000V) 3. Enhanced contrast at low beam energy and topographic imaging mode 4. Low beam penetration depth and BSE imaging mode with reduced charging 5. The multi channel plate (MCP) detector detects surface-sensitive low energy electrons
3. Contrast Mechanisms 3.1 Surface Roughness Contrast More electrons escaped from the rougher surface. At this situation, the signal intensity is related to the height of the feature.
Contrast Mechanisms 3.2 Edge Contrast Electron beam induced current recovers the high secondary electron yield from the SiO 2 surface. Both topographic contrast and EBIC contribute.
Contrast Mechanisms 3.3 Thickness Contrast Graphene in different thicknesses show different brightness. Signal intensity profile
Contrast Mechanisms Monte Carlo simulation result Simulation parameters Beam voltage: 1kV Beam spot size; 10nm Substrate: Cu Thin film: 1.4 C (~4 layer graphene) Schematic illustrating the thickness contrast A significant part of the beam/specimen interaction volume is located inside the Cu substrate. Graphene films with different layers create contrast.
Contrast Mechanisms 1. Work function mechanism E k F' E ( E k k ) : SE yield; : work function; E k : SE s energy; F : a norm. factor 2. Electron attenuation mechanism I N Ae d0 N B I N : SE intensity; d 0 : 1 layer thickness; : electron inelastic mean free path; A, B: fitting factors The detection signal intensity decreases exponentially with the increasing number of graphene layers.
4. Morphologies of Graphene Samples As grown CVD graphene film Cu grain boundaries and terraces graphene wrinkles and multi-layer domains Hexagonal bilayer domains
As grown CVD graphene film SE image Topo image The topo image reveals the Cu terrace more obviously. Graphene wrinkles are discernable in the topo image. Multi-layer domains are very difficult to tell in the topo image.
Transferred graphene film By method 1. SE image Topo image Highly corrugated surface. Low energy electrons are sensitive to the surface features. Multi-layer domains and wrinkles were not observed.
Transferred graphene film By method 2. Smooth surface without ripples. Multi-layer domains and wrinkles are preserved. Impurities including particles and polymer residue exist.
Transferred graphene film By method 3. SE image Topo image The transferred film has cracks. Bright contour lines were observed at the graphene edges. The topo image reveals ripples and surface roughness obviously.
Graphene flakes Graphene flakes on Cu substrate Graphene flakes on Al 2 O 3 substrate Partially released to form curved structures Multi-layer domains (yellow circle) and multiple stacked graphene films (green circle)
Graphene ribbons Graphene nanoribbons have unique electrical properties which can find potential applications in room temperature transistors. SE image Topo image Smooth and rather straight edges in parallel which possibly represent a welldefined zigzag or armchair structure. Impurity nanoparticles (~20-30nm) were observed. Topo image clearly reveal the folding behavior of the ribbons.
Carbon nanoscrolls Carbon nanoscroll is a spirally wrapped 2D graphene sheet into a tubular structure. BSE image Topo image The driving force for nanoscroll formation is the - interaction of the overlapped parts which leads to a decreased total free image..
5. Effect of Electron Beam Voltages Monte Carlo Simulation Results 550V 2000V 1000V Thickness of graphene: 1nm (corresponding to a trilayer) 2000V beam induces a much larger interaction volume. 550 and 1000V beams have a similar resolution with different penetration depth.
Effect of Electron Beam Voltages
6. Effect of Nonconducting Substrates Graphene on SiO 2 /Si substrates 550V 700V
Effect of Nonconducting Substrates Graphene on Al 2 O 3 substrates 625V 1000V
Effect of Nonconducting Substrates Graphene on MgO substrates Charging is severe. Rough surface with scratches. Graphene layer on substrates are not shown on topo image. Multilayer domain is more obvious at lower voltage.
7. Summary Owing to its high resolution, enhanced contrast and high surface sensitivity, low-voltage field emission scanning electron microscopy is suitable for graphene imaging. With an innovative miniature electron beam columns, Agilent 8500 compact FE-SEM offers an excellent imaging capability when investigating morphologies of graphene films. Different contrast mechanisms of SEM imaging of graphene were discussed. THANK YOU!