Scanning tunneling microscopy STM and atomic force microscopy AFM
Content The components of a scanning probe microscope SPM The scanner Measurement of the distance between surface and tip The cantilever Basic principles of scanning tunneling microscopy STM The measurement modes Atomic force microscopy AFM The different modes for the AFM technique Scanner motion Size of the tip and resolution The SPM device in our laboratory Examples
Motivation Digitally image a topographical surface Determine the roughness of a surface sample or to measure the thickness of a crystal growth layer Image non-conducting surfaces such as proteins and DNA Study the dynamic behavior of living and fixed cells Nanolithography manipulate individual atoms - nanoscience
The different images methods Scanning Probe Microscopy SPM Tunneling Microscopy Scanning Tunneling Microscopy STM Atomic Force Microscopy Contact Atomic Force Microscopy C-AFM Non-Contact Atomic Force Microscopy NC-AFM Intermittent Contact Atomic Force Microscopy IC-AFM
Principal components of a scanning probe microscope
Piezoelectric material lead zirconium titanate PZT changes dimensions when a voltage is applied The Scanner
Measurement of the distance between surface and tip Bending of the cantilever shifts the position of the laser on the position sensitive photodetector
The cantilever 50 µm 100-200 µm long, 10-40 µm width, 0.3-2 µm thick; spring constants f(shape, dimension, material); spring constants: n x 1000 N/m to n x 1/10 N/m
Convolution Effects One thing to keep in mind : convolution effect The smaller thing images the bigger thing The signal is always a convolution of sample topography and tip topography Tips should be as sharp as possible (10nm standard)
Theory Tunneling of electrons: the charged particles can travel from occupied states through a potential barrier to unoccupied states Klassische Physik Quantum mechanics I T ~ e -d d Apply a low potential between tip and surface
Illustration of the tunnel-tip surface junction About 10 Å: overlap of WF Potential: 0.5-10 V; current: 0.1-1 na, distance: 0.3-1 nm convention: negative tunnel voltage means electron emission from the sample
The tunneling process Tunnel current density I t (Bethe & Sommefeld): 3 e 0 2 x I t = V t exp(-2xd) 8 h π 2 d e 0 = elemental charge = 1.602 10-19 As; h = Planck constant = 1.054 10-34 Js d = distance tip to surface Å; x = 2 m 0 Φ/ h = 1/2 Φ = decay curve of a wave function in the potential barrier; Φ = average barrier height = surface potential in ev with these units: 2 x is about 1.025 Φ eff. With Φ eff = several ev, I t changes by a factor of 10 for every Å of d!
Quantum well E Vakuum Φ Austrittsarbeit Work function E Fermi Φ sehr Depends materialspezifisch on!
Determination of local work function Different metals Eisen Nickel
Contact between two metals surface tip a) E Vacuum E Vacuum Φ WF Φ tip E F E F b) E Vacuum E F Φ WF Φ tip E Vacuum E F
With external voltage (bias) sample tip a) Φ OF energy Φ tip b) U=0 U>0 U<0 sample tip sample tip sample tip
The tunnel microscope
The measurement modes Constant height mode: faster, scanner always at the same height requires very smooth surfaces Constant current mode: high precision, irregular surfaces, time consuming dangerous for partially oxidized surfaces!
First STM images by Binnig and Rohrer 1982 Planar gold surface
First STM images by Binnig and Rohrer 1982 3D image of gold surface
Si(111) 7x7 STM image scheme
Gold on Ni surface Ni Au Important: Density of state around the Fermi energy
Density of states around the Fermi energy sample tip samples tip energy Φ OF Φ tip energy Φ OF Φ tip D(E) D(E) D(E) D(E) D(E)= density of states
Density of state around the Fermi energy Bias voltage V DC Constant current contour e- Distance s e e Sample
AFM - Forces between tip and surface Van der Waals force: always present, attractive, outer electrons, long distance contact force: repulsion, chemical, core electrons capillary force: attractive, water layer! electrostatic and magnetic force friction force forces in liquids J. Israelachvili: Intermolecular and Surface Forces with Appl. to Colloidal and Biological Systems, Academic Press (1985)
AFM: Forces vs. distance AFM Tip a few Å above surface Tip 10-100 Å above surface Tip is mounted at the end of a cantilever, interaction with sample: attractive or repulsive
AFM: the different operation modes C-AFM NC-AFM Repulsive mode, soft physical contact, cantilever with low spring constant, lower than holding atoms together; total force exerted on the sample: 10-8 N to 10-6 N; cantilever is bended Attractive mode, cantilever vibrates near surface, distance to surface 10 - n x 100 Å, total force exerted on the sample: 10-12 N; frequency is kept constant during movement = constant distance tip-surface
Non-Contact Mode Uses attractive forces to interact surface with tip Operates within the van der Waal radii of the atoms Oscillates cantilever near its resonant frequency (~ 200 khz) to improve sensitivity Advantages over contact: no lateral forces, nondestructive/no contamination to sample, etc. van der Waals force curve
Contact Mode Contact mode operates in the repulsive regime of the van der Waals curve Tip attached to cantilever with low spring constant (lower than effective spring constant binding the atoms of the sample together) In ambient conditions there is also a capillary force exerted by the thin water layer present (2-50 nm thick). van der Waals force curve
Using Vibrating Tips Advantages : Feedback parameter : Amplitude No permanent tip-sample contact No shear forces Non-contact imaging possible Tapping Mode, Intermittent Contact Mode And Non-Contact Mode are the most successful methods for pure imaging
Force measurement The cantilever is designed with a very low spring constant (easy to bend) so it is very sensitive to force. The laser is focused to reflect off the cantilever and onto the sensor The position of the beam in the sensor measures the deflection of the cantilever and in turn the force between the tip and the sample.
Raster the tip: Generating an Image Scanning Tip The tip passes back and forth in a straight line across the sample (think old typewriter or CRT) In the typical imaging mode, the tipsample force is held constant by adjusting the vertical position of the tip (feedback). Raster Motion A topographic image is built up by the computer by recording the vertical position as the tip is rastered across the sample. Top Image Courtesy of Nanodevices, Inc. (www.nanodevices.com) Bottom Image Courtesy of Stefanie Roes (www.fz-borstel.de/biophysik/ de/methods/afm.html)
Scanning the sample Tip brought within nanometers of the sample (van der Waals) Radius of tip limits the accuracy of analysis/ resolution Stiffer cantilevers protect against sample damage because they deflect less in response to a small force This means a more sensitive detection scheme is needed measure change in resonance frequency and amplitude of oscillation Image courtesy of (www.pacificnanotech.com)
Scanner motion during data acquisition Area size: 10 Å - > 100 µm No signal detection Minimises line-to-line registration error due to scanner hysteresis
The size of the tip and the resolution STM I T : exponential dependence of distance tip-sample, closest atom of tip interacts with surface atomic resolution is achieved AFM Several atoms of the tip interact with the sample surface, every atom of the tip sees a shifted lattice with respect to the lattice seen by the neighbor atom
The size of the tip and the resolution Time = zero Best lateral resolution: about 10 Å
Interpretation of STM Image