Program Operacyjny Kapitał Ludzki SCANNING PROBE TECHNIQUES - INTRODUCTION Peter Liljeroth Department of Applied Physics, Aalto University School of Science peter.liljeroth@aalto.fi Projekt współfinansowany ze środków Unii Europejskiej w ramach Europejskiego Funduszu Społecznego
Aalto University at a glance Created from the merger of three leading Finnish universities on 1 st of January 2010 Helsinki University of Technology (founded 1849) The University of Art and Design Helsinki (founded 1871) The Helsinki School of Economics (founded 1911) Composition 2010 Staff 4 500 Students 20 000 Alumni 75 000 Bachelor s degrees 1 150 Master s degrees 2 300 Doctoral degrees 184
Contents Lecture 1: Introduction to scanning probe techniques. Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) experimental requirements Lecture 2: Theories of scanning probe microscopy image formation mechanisms in STM and AFM Lecture 3/4: Applications of scanning probe techniques in nanoscience and biology different derivative techiques (MFM, EFM etc.) STM and AFM applied to modern nanoscience and biology (graphene, semiconductor nanostructures, biological surfaces) Lecture 5: Chemistry at the single-molecule level: study of decoupled (molecular) systems with STM Lecture 6: Ultrasmall amplitude non-contact AFM based on the quartz tuning fork force sensor
General principle Measure some interaction between a sharp probe and the sample Scanning: Move sample or tip while keeping this interaction constant (feedback) or while keeping height constant Spectroscopy: Change something (z, voltage, etc.) while keeping x and y constant (feedback off)
Scanning Tunnelling Microscopy Measure the current between a sharp tip and the sample Originally to study electrical properties of inhomogeneous insulators Invented at 1981 (Helv. Phys. Acta. 1982, 55, 726) at the IBM Zurich Research Laboratory Nobel Prize in Physics 1986 for Gerhard Binnig and Heinrich Rohrer
wavefunction energy Scanning tunnelling microscopy e Metal Vacuum x=0 x=s Metal E Fermi x Based on quantum mechanical tunnelling effect: Tunnelling current extremely distance dependent I(d) = I 0 e -2kd, with k 1 Ǻ -1
Bias voltage Typical bias voltage from a few mv to a few V (limited by the onset of field emission) Typical currents from a few pa to a few na (tunneling conductance << conductance quantum 1/12.9kW)
STM / AFM topographic mode Functional nanostructures at surfaces Forschungszentrum Jülich
Typical STM images Au substrate with a self-assembled monolayer: 20 nm 2 pa 0 nm 0 pa topography current image; difference between the actual current and the set-point
Si(111) 7 7 reconstruction STM topography of Si(111) 7 7 reconstruction (2V, 1nA) both atoms and defects visible First surface structure determination by STM defect
STM spectroscopic mode: Scanning Tunnelling Spectroscopy (STS) Scan over surface Stop at the desired location Disconnect the feedback loop Change voltage, measure current and di / dv
Reasons for the success of STM As a consequence of the strong distance dependence of the tunneling current, it is likely that a single atom carries the main part of the current leading to very high spatial resolution Tunneling current monotonic function of the tipsample distance, which makes feedback control simple Tunneling currents are sufficiently high to be measured without difficulty Possibility of carrying out electronic spectroscopy with atomic resolution
AFM background Invented in 1986 by G. Binnig, C. Gerber, and C. Quate (IBM Zurich) Idea is to measure a force due to a very small contact with a sample Typical forces in pn - nn range Based on detecting the bending of a cantilever Hooke s law: F = - kx G. Binnig, C.F. Quate, C. Gerber, Phys. Rev. Lett. 56, 930 (1986).
Bending of a cantilever Optical detection the most common four-field detector can measure both the normal deflection (A-B) and torsional bending (C-D), i.e. both normal and lateral force components can be measured
qplus force sensor Very stiff cantilever (k=1800 N/m) Small oscillation amplitudes (<< 1 nm, record ~0.1Å) Sensitive to short-range forces Practical advantage: all electrical force detection Simultaneous STM and AFM with uncompromised performance STM/AFM tip 2 mm F.J. Giessibl, Rev. Mod. Phys. 75, 949 (2003)
AFM principles sample Force or derivatives used as the feedback signal Both long and short range forces contribute (unlike in STM) van der Waals electrostatic forces magnetic forces capillary forces short range binding (attractive) and Pauli exclusion (repulsive)
Different modes Contact mode friction Non-contact mode (dynamic mode, frequency modulation) frequency shift damping Tapping mode (Intermittent contact, amplitude modulation) frequency fixed amplitude measured
Contact mode Tip in contact with the sample Deflection of the cantilever (force) is the feedback parameter Simplest to interpret Damage to tip or sample possible Lateral force on the sample True atomic resolution is not possible jump to contact force approach retract tip-sample separation
Example on contact mode AFM topography friction
Atomic resolution in contact mode Long-range attractive interactions cause a large repulsive force on the front atom of the tip This leads to deformation Contact area radius ~ 10 nm True atomic resolution cannot be achieved in air Atomic scale stick-slip images U. Schwarz et al. Phys. Rev. B 62, 13089 (2000).
Dynamic modes ( non-contact AFM ) sample Vibrating cantilever (externally driven) avoids jump to contact problem low Q (air or liquid) amplitude modulation high Q (vacuum) frequency modulation
oscillation amplitude Amplitude modulation (tapping mode) low Q drive at close to f 0 with a constant excitation and detect change in amplitude or phase (constant frequency) not used in vacuum due to high time constant for a change in amplitude ( AM = 2Q/f 0 is high in vacuum due to high Q) tapping-mode a derivative of this f 0 frequency
Tapping mode Amplitude of the cantilever oscillation monitored Also called intermittent contact mode closest point of cantilever oscillation is incontact, hence tapping amplitude larger than in non-contact mode works very well in ambient; the oscillation amplitude larger than the thickness of a possible contamination layer minimum lateral force on the sample mode of choice for ambient AFM Q. Zhong et al. Surf. Sci. Lett. 290, L688 (1993).
Typical AFM images Polycrystalline Au layer 15 nm 250 mv 0 nm 0 mv topography amplitude image; difference between the actual amplitude and the set-point
Phase contrast wood pulp fiber: tapping mode AFM topography phase image; phase difference between the excitation and oscillation signals
oscillation amplitude Frequency modulation high Q drive at constant amplitude, measure frequency shift or the damping typical Df = 1-100Hz, f 0 = 100-300 khz, amplitude ~ 10nm, stiff cantilevers with k 10-20 N/m (stiffer is possible) time constant scales as FM 1/f 0, which means that this is appropriate for ultrahigh vacuum. true atomic resolution with AFM possible frequency
UHV non-contact AFM (FM mode) Si(111) 7 7 reconstruction topography damping; the necessary excitation amplitude to keep the oscillation amplitude constant R. Lüthi et al. Surf. Sci. Lett. 4, 1025 (1997).
Advantages with AFM insulators in principle, atomic resolution on any surface any force can be used as feedback signal minimum lateral force (tapping mode) for delicate samples force measurements on single-molecule level etc.
Problems with AFM non-monotonic force All forces contribute both long and short range contributions jump to contact in static mode can be avoided with an oscillating cantilever with a sufficient amplitude tip has to be included in modelling which makes it difficult
Physical requirements Precise positioning piezoelectric elements Sharp tip STM tip etching or cutting AFM micromachined tips Vibration isolation rigid design eddy-current damping ambient or UHV
Scanner construction Piezoelectricity: external voltage will make a crystal expand or contract Tube scanner with piezoelectric elements enable positioning with subatomic (picometre) precision either tip or sample is scanned
Tip preparation for STM A couple of volts W wire Pt loop Tungsten not suitable for ambient due to oxidation Thin film of 2M NaOH
Alternatively; cut Pt/Ir tips Two-dimensional imaging of electronic wavefunctions in carbon nanotubes, Nature, 2001
AFM tips R > 2 nm Q: 100s to 100s of thousands k: 0.01 1000 N/m f 0 : 10-100s khz
tip-size effects: convolution
tip artefacts AFM image SEM image
Double tip features Bismuth nanocrystals PbSe/CdSe hetero-nanocrystals Sharp (both atomically and overall) Stable No extra features in spectroscopy
Substrate Something flat mica graphite silicon wafers single crystals Surface chemistry important Conductive for STM
different environments ambient under liquid ultra-high vacuum low-temperature external magnetic field illumination 25cm 15 cm 500g 1.5m 1.5m 1.5m several hundred kg Pressure variation between <10-11 mbar and several bar temperature between 300mK and 1200K external magnetic fields up 14T
Program Operacyjny Kapitał Ludzki Peter Liljeroth Department of Applied Physics, Aalto University School of Science peter.liljeroth@aalto.fi Projekt współfinansowany ze środków Unii Europejskiej w ramach Europejskiego Funduszu Społecznego