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Tutorial 4 Derek Wright Wednesday, February 9th, 2005 Scanning Probe Techniques • • • • • Scanning Tunneling Microscope Scanning Force Microscope Imaging of Soft Materials Manipulation of Atoms and Molecules Chemical Reactions with the STM Scanning Probes • Atomic-sized probe is dragged across the surface • Types of measurements taken: – Current – Magnetic – Force Scanning Tunneling Microscope • Scanning: – The tip is scanned across the sample in a grid pattern • Tunneling: – There is a tunneling current between the sample and the tip which is measured • Microscope: – We can see atomic sized things with it Scanning Tunneling Microscope • Tunneling current is a quantum effect • e- aren’t points in space, they have a probability of location • This waves exist with a probability density centered around the e– The e- is “smudged” in space • If a thin barrier intersects this probability density, the e- might have a chance of “appearing” on the other side of the barrier Scanning Tunneling Microscope STM Equations • I V Ntip Nsample – Ntip, Nsample = density of states • I exp(-2keffz) – z is the distance between the tip and sample – I drops off exponentially with the distance – I drops off exponentially with keff STM Equations • keff = (2meB/h2) + |k|||2 – keff = inverse effective decay length – me = mass of electron – B = barrier height (has to do with the work functions of the tip and sample and the applied voltage) – k|| = parallel wave vector of the tunneling electrons • B = (tip + sample)/2 - |eV|/2 – (tip + sample) are the work functions of the tip and sample – V is the applied voltage STM Modes • There are two modes of operation • Constant Distance (z-position const.) – The tunneling current is plotted • Constant Current – The vertical movement of the tip is plotted – This is the usual method – Good because of the exponential nature of the tunneling current + feedback STM Constraints • The STM tip must have excellent mechanical stability – Achieved through piezoelectric actuators – Rests on heavy table with many dampers • The tip must come to a very small point – Can be achieved through electrochemical etching – Carbon nanotube can be placed on the end to improve accuracy Scanning Force Microscope • Sometimes called Atomic Force Microscope (AFM) • Setup very similar to STM except tip deflection is measured instead of tip current • Can be used where current won’t flow • Two modes of operation: – Contact – Non Contact Scanning Force Microscope • Contact Mode (z < 1 nm): – The tip is dragged across the surface and the deflection is measured optically – Deflection is due to repulsion of tip particles with surface particles – Can scratch the surface – not recommended for soft substrates • Non-contact Mode (z > 1 nm): – With the tip not actually touching the surface, dominant forces are van der Waals, electrostatic, and magnetic Scanning Force Microscope • As the tip is brought from a distance closer to the sample: – First van der Waals forces pull the tip closer – Then ionic repulsion pushes it away • The tip’s deflection can be measures using laser interferometery Scanning Force Microscope • Tip can be operated in “dynamic mode” • The tip and cantilever (beam with the tip on it) have a mechanical natural resonance • The resonance will change as external forces from the sample are exerted on it • The tip’s vibration amplitude must be much less than the distance between it and the sample to ensure linear operation – Like how a transistor amplifier is linear when the signal is much less than the supply voltage Scanning Force Microscope Magnetic SFM • Used to measure magnetic media • The tip is a piece of magnetic material and is of a single domain – All dipoles are aligned in the tip • The interaction of the tip’s magnetic field and the sample create a force • The force shows the sample’s domains and boundaries between them Electrostatic SFM • A method that plots the sample’s static surface charge • Tip is electrically isolated (cantilever is an insulator) • Two pass method: – First pass is a contact pass – Second pass occurs at a constant distance from the sample and measures the force due to the charge on the sample and the charge induced in the tip Piezoresponse Force Microscopy • The tip and cantilever can bend in two axes to give an idea of the 3D domain structure of a sample • An oscillating voltage is applied to the tip • An oscillating current occurs (due to the capacitance of the tip) which interacts with the B-field of the sample • This creates a measurable force and bends the cantilever Imaging of Soft Materials • Contact with soft samples is bad – The tip will damage the delicate sample – Contact gives better resolution, but is too harsh • Non-contact methods have been tailored for soft samples – Special feedback circuits – Special modulation frequencies – High gap impedances (large gap between tip and sample) Manipulating Atoms and Molecules • Tip is brought above a loose atom or molecule • Attractive forces between the two allow tip to pick up the atom • Tip drags the atom • Tip raises to let go of the atom Manipulating Atoms and Molecules Quantum Corrals • A ring of atoms can create a “quantum corral” – The ring forces electrons within into circular wave patterns • Doesn’t need to be a ring – any closed structure will create resonance patterns within Quantum Corrals Quantum Corrals Quantum Corrals Chemical Reactions with the STM • Since the tip can: – Manipulate atoms and molecules – Provide energy in the form of a tunneling current • It is possible to make chemical reactions occur by dragging the molecules together and form or break bonds with the tunneling current Chemical Reactions with the STM Thank You! • This presentation will be available on the web.