Mapping Graphene s surface potential with <20nm resolution: A PeakForce KPFM study in a controlled <1ppm water and oxygen environment Greg Andreev and Aravind Vijayaraghavan 9/27/13
Webinar Outline Introduction to Graphene by Aravind Introduction to PF-KPFM Results on Graphene in Air and Glovebox Questions Potential 100mV 250nm -100mV 2
An introduction to graphene and the importance of environment Dr. Aravind Vijayaraghavan The University of Manchester Bruker Webinar "Mapping Graphene s Surface Potential with less than 20nm Resolution"
WHAT IS GRAPHENE Graphene is imensional Buckyballs Carbon Nanotubes Graphite A. K. Geim, K. S. Novoselov Nat Mater 2007, 6, 183.
HOW TO MAKE GRAPHENE 1. Micromechanical cleavage of Graphite (a) Attach a piece of graphite to sticky-tape (Cellotape) (b) Use the sticky tape to thin out the graphite (c) Place the thin graphite on a Silicon wafer, with a surface layer of Silicon Dioxide (d) Remove most layers of graphite leaving behind graphene.
HOW TO MAKE GRAPHENE 2. Chemical Vapour deposition (a)carbon atoms are deposited on the surface of a metal (b)at high temperature this forms graphene. (c) A layer of polymer is deposited on top of the graphene. (d)the polymer is removed and the graphene with it. (e)the polymer is placed on a suitable substrate (f) The polymer is dissolved away leaving the graphene behind.
HOW TO FIND GRAPHENE Seeing is believing! P. Blake, et al., Appl. Phys. Lett. 2007, 91, 063124.
ELECTRONIC STRUCTURE Electronically, monolayer, bilayers and trilayer graphene are electronically distinct materials. Beyond three layers, graphene s electronic properties tends towards that of bulk graphite. Monolayer Graphene T. Ohta, et al., Science 2006, 313, 951.
EFFECT OF ENVIRONMENT Ponomarenko, L. A.; et. al. (2009). Physical Review Letters 102(20): 206603.
EFFECT OF DOPING Schedin, F., et al (2007). Nature Materials, 6(9), 652-655. Changes in resistivity, at zero B caused by graphene s exposure to various gases diluted in concentration to 1 p.p.m. The positive (negative) sign of changes is chosen here to indicate electron (hole) doping. Region I: the device is in vacuum before its exposure; II: exposure to a 5 l volume of a diluted chemical; III: evacuation of the experimental set-up; and IV: annealing at 150 C.
Before getting into KPFM: Lift mode 2nd trace 1st trace Magnetic field source Lift Height Topography LiftMode: Separates topography from the electric/magnetic info: long range vs. short range forces, on active or passive devices. Surface Potential: measure the surface potential together with topography. 3
KPFM basic principle KPFM measures the work function difference of tip/sample. AM FM Amplitude-Modulation Frequency-Modulation Better spatial resolution Better accuracy Physical Review B 2005, 71(12) 125424 4
Many Ways of Doing KPFM FM and PeakForce scaling do not compete Tapping is limited to high k levers due to adhesive forces. PeakForce Tapping k is not. KPFM AFM Tapping PeakForce Both AM & FM KPFM improves with lower k AM FM TP-AM TP-FM PeakForce KPFM-AM PeakForce KPFM *Except TP-FM, all are done in lift-mode. 5
PeakForce KPFM combines FM with PFT Allows for higher sensitivity KKKK SSSSSSSSSSS But Tapping Mode Requires : k to be not too small Q not to be too big Tapping and KPFM scaling in conflict. Q k Peak Force Tapping Mode Allows Freedom to use: Smaller k (10x or more) Big Q (10x or more) PeakForce Tapping and KPFM scaling aligned. 6
KPFM: nanoscale measurement of Work Function, Fermi Energy & Carrier Density Previous work shows Work Function relationship to E fermi for Graphene Bilayer Single layer Y. Yu et. al. NanoLett. (9) 10 2009 KPFM can be used as a nanoscale measurement of Fermi energy 7
Energy diagram of our KPFM experiment on Graphene Vacuum level W a Approx. Values W tip W tip ~ 4.5eV W smp W 0 gr W elec ~ 4.9eV e VVVV W elec E F W 0 gr ~ 4.6eV W a ~ 0.05eV What KPFM measures E F ~±0.3eV Energy band diagram of 1L Graphene (holedoped) 8
Our sample: Single layer Graphene on a Silicon Nitride window 14.5nm AFM 1um Optical Microscope (Blue filter) 25um SiN window 1L Graphene 0nm SiN window 1L Graphene Why this sample? Largely insulating substrate KPFM should probe only Graphene, little crosstalk Roughness may create interesting inhomogeneities 9 Au
Results (in Air): PF-KPFM on Graphene shows incredible contrast AFM Linear debris clearly visible 1um Potential Linear debris does not show up! 500mV <40nm gap b/n layers barely visible in Height <40nm gap b/n layers clearly visible -200mV Hole doping of Graphene confirmed, W g ~ 4.85V Signs of incredible nanoscale contrast, is it real? 10
Lift height dependence: a way to check for topographic artificats in KPFM Example From Bruker App. note #140 11
Proof of real contrast: evolution of PF- KPFM resolution w/ lift height Lift Height 28nm 40nm 50nm 75nm 1um No sudden loss of resolution = contrast not due to tip sticking 12
Let s zoom in on an interesting region.. 1um 14.5nm 13 0nm
Zooming in: how homogeneous is Graphene? Height 250nm 11nm Adhesion 11nN 0nm 6nN Substrate residue easily picked up by Adhesion channel Will it have some effect on Graphene s work function? 14
PF-KPFM of zoomed-in region: <20nm resolution reveals inhomogeneities Potential, z = 28nm +100mV Similar result to previous work: 4.85V 250nm 15-100mV 100mV 50mV 20nm wide hump resolved! 0mV J. Martin et. al. Nature 2007 15
Bruker PF-KPFM: inhomogeneities correlate with high potential Adhesion 11nN Adhesion+High Potential 11nN 6nN 6nN PF-KPFM adhesion channel makes it easy to see the inhomogeneities! Is the doping mechanism here also related to Water + Oxygen as in previous work? scale bar = 250nm 16
What is the effect of the environment on Graphene doping? Water? Oxygen? Is it reversible? An environmentally controlled measurement is needed.. Cryogenic UHV KPFM would be great, but difficult/costly Is there an easier way to study Graphene in a H2O+O2 free environment? 17
Unique Bruker technology: Glovebox Integrated System Argon Atmosphere, <1ppm H2O, 1ppm O2 Antechamber for introducing samples, pressure below 1E-6mbar 18
Physical picture: what we expect to happen Ambient GloveBox Vacuum level W a W elec W smp W 0 gr E F W tip e VVVV E F Graphene is hole doped by Water and Oxygen Water and Oxygen removed, Graphene nearly intrinsic Potential for studying nearly-intrinsic Graphene without Cryogenics or UHV 19
Physical picture confirmed: PF-KPFM in GloveBox shows lower contrast Ambient 500mV GloveBox 150mV -200mV Contrast between Graphene and SiN greatly reduced -150mV Spatial inhomogeneities in Graphene also reduced 20
Physical picture confirmed: PF-KPFM in GloveBox shows lower contrast Ambient 500mV GloveBox (same scale) 500mV -200mV -200mV Graphene signal is nearly that of an insulator 21
Inhomogeneities greatly reduced in Glovebox Air +100mV Glovebox +100mV Potential 4.85V ~4.6V -100mV 250nm -100mV AFM 22
Another sample: larger KPFM scan Potential in Air Three vastly different potentials for Graphene? SiN Au Graphene 250-470mV 93mV Graphene SiN Au 23
KPFM Potential, in Glovebox before vacuum SiN/chuck 334mV 133mV Graphene Au Au SiN Graphene Glovebox environment reduced Graphene workfunction by ~100mV KPFM Potential, in Glovebox after vacuum 284mV 193mV W g ~ 4.7eV near Au contact, smaller farther away 24
Conclusion PF-KPFM s spatial resolution is great: 20nm resolution images are achievable on Graphene PF-KPFM is sensitive: ~10meV sensitivity allows to easily distinguish inhomogeneities Glovebox Integrated System is a convenient promising environment for reducing Graphene s work function to nearly intrinsic levels. 25
Backup slides 26
Tip Cone Contribution in KPFM FM gradient detection isolates contribution from tip Cone Contribution% 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% FM z=10 nm FM z=50 nm AM z=10 nm 0% 0% 0% 1% AM 10% z=50 nm 100% Height Inclusion (h/h)% Based on SCM-PIT Geometry: W=30um, L=225um, H=10um, Cone Angle=45 FM-KPFM: The foremost 0.3% of the tip cone accounts for half of the signal in. FM can achieve a lateral resolution better than 50nm. AM-KPFM The contribution from the tip cone never reaches 50%. Its lateral resolution is dictated by the um-scale lever. 27