Scanning probe microscopy AFM, STM. Near field Scanning Optical Microscopy(NSOM) Scanning probe fabrication

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1 Scanning probe microscopy AFM, STM Near field Scanning Optical Microscopy(NSOM) Scanning probe fabrication

2 Scanning Probe Microscopy 1986 Binning and Rohrer shared Nobel Prize in Physics for invention.stm As in SEM the tip is scanned over the surface. Many variants - AFM, MFM, SCM

3 Surface Electronic Structure Energy level of (just) free electron calledvacuum level, E V Energy require to move electron from E F to E V at the surface called workfunction, - Envisaged as attraction between departing electron and positive image state (hole) - < 2eV (alkali metals) to > 5 ev (transition metal) Workfunction varies between (i) materials (ii) crystal faces Polycrystal ø (ev) Single crystal ø(ev) Na 2.4 W(111) 4.39 Cu 4.4 W(100) 4.56 Ag 4.3 W(110) 4.68 Au 4.3 W(112) 4.69 Pt 5.3 W(poly) 4.5 Workfunction also sensitive to - adsorbates - external electric fields - reconstruction Workfunction presents a barrier to electron emission - near surface, electron behaves like a particle in a box

4 Quantum mechanics tells us that electrons have both wave and particle like properties. Tunneling is an effect of the wavelike nature. The top image shows us that when an electron (the wave) hits a barrier, the wave doesn't abruptly end, but tapers off very quickly - exponentially. For a thick barrier, the wave doesn't get past. The bottom image shows the scenario if the barrier is quite thin (about a nanometer). Part of the wave does get through, and therefore some electrons may appear on the other side of the barrier.

5 When an electron is incident on a potential barrier with potential energy larger than its kinetic energy it may tunnel through the barrier. The electron wave function at the Fermi level have a characteristic exponential inverse decay length K given by K = (2mø) 1/2 where ø is the local barrier height or the average work function of the tip and sample. For a typical metal K equals 1. When a voltage, V, is applied between the tip and the sample, the overlapped electron wave function permits quantum mechanical tunneling and a tunneling current, I, will flow across the vacuum gap.

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7 I is given by Where V is the applied voltage between the tip and specimen and d the separation between the tip and surface. Typically d is of the order of 2-5 For a typical work function of 4 ev, the tunneling current reduces by a factor of 10 for every 0.1 nm increase. Hence the sensitivity. Applied voltages - mv to V Tunneling current - pa to na Owing to the exponential decrease in tunneling current with distance - the current is dominated by the contribution from the last atom on the tip and nearest atom on the sample.

8 Two ways to operate: (1) Constant Height Mode Tip-sample distance fixed - variation in I t forms image Fast but only works for flat samples (2) Constant Current Mode Keep I t constant by moving tip up and down (feedback circuitry) - z movement becomes image Slower but works for rough surfaces Most common

9 Tip Preparation Metal with a low work function, e.g. tungsten. Electrochemical etching, ion milling (FIB sharpening), cutting with scissors UHV W, Mo, Ir Air Pt, Au (soft), Pt-Ir

10 Piezoelectric ceramics Piezoelectric material has permanent dipole moment across unit cell. If dipoles are oriented, material changes length on applying an electric field to 10-7 % length change per V allows < 1 Å positioning. Many STM have a tube geometry with four segments - if a potential is applied to all four then tip in the Z direction is moved. A typical piezoelectric material used in STMs is PbZrTiO3, PZT (Lead Zirconium Titanate). Alternatively application of a force to a piezoelectric results in charge on the surface.

11 Probing the local density of states Tunneling is sensitive to electronic structure convolution of filled DOS of metal A (-) and empty DOS of metal B (+). The tunneling conductance is proportional to the local density of states (LDOS). (Density of states is the density of energy states in an energy interval). Keeping the separation constant and changing the bias voltage enables the LDOS to be probed. Changing the bias voltage can get local occupied and unoccupied states.

12 Applications of STM - Manipulation and Chemistry Several researchers have successfully moved atoms and molecules on surfaces using one of several methods - field effect - under influence of high electric field, polarizable molecule or atom can be made to jump from surface to tip or vice versa - dragging - vdw forces between close tip and adsorbate can be used to drag species Xe on Ni(110) at 4. The first atom-by-atom manipulation See Nature 344 (1990)

13 The Quantum Corral This STM image shows the direct of standing-wave patterns in the local density of states of the Cu(111) surface. These spatial oscillations are quantum-mechanical interference patterns caused by scattering of the two-dimensional electron gas off the Fe adatoms and point defects.

14 Atomic Force Microscopy V Feedback Loop Lase r Piezo Crystal Mirror Photodiod e Tip ThermoMicroscopes Explorer AFM Substrate The system operates by reflecting a laser beam off the cantilever. Angular deflection of the cantilever causes a twofold larger angular deflection of the laser beam. The reflected laser beam strikes a position-sensitive photodetector consisting of four side-by-side photodiodes. The difference between the photodiode signals indicates the position of the laser spot on the detector and thus the angular deflection of the cantilever. Because the cantilever-to-detector distance generally measures thousands of times the length of the cantilever, the optical lever greatly magnifies motions of the tip. Can be used on insulators

15 . Contact Mode Tip is touching the surface (repulsive part of the force distance curve). Typically the force i.e. between 10-6 N< F<10-9 N. <0.5 nm tip separation If spring constant of cantilever is lessr than surface cantilever bends if greater than surface bends. Tapping mode Cantilever is driven at its resonant frequency ( Hz, 1 10 nm amplitude) and intermittently touches the surface of the sample again repulsive part of the F-d graph. Ideal for imaging poorly immobilsed or soft samples nm tip separation Electronics adjust tip surface distance to keep cantilever at resonance. Non-contact mode Probe is held above the sample and vibrated In the attractive region of the F-d curve nm tip separation Depending on the system either the sample is moved or the scanning head to keep the force constant

16 Cantilever is in direct contact with sample. Obeys Hooke Law Feedback loop maintains constant cantilever deflection (or force) during scanning Lateral forces can damage soft or fragile samples Erosion of tip.

17 Cantilever oscillates at its resonance frequency and taps sample surface where feedback loop maintains constant oscillation amplitude. Reduces normal forces and shear forces, thereby reducing damage of softer samples.

18 AFM Tips The "normal tip"is a 3 µm tall pyramid with ~30 nm end radius. The electron-beam-deposited (EBD) tip or "supertip" improves on this with an electron-beam-induced deposit of carbonaceous material made by pointing a normal tip straight into the electron beam of a scanning electron microscope. The supertip offers a higher aspect ratio (it is long and thin, good for probing pits and crevices) and sometimes a better end radius than the normal tip. Park Scientific Instruments offers the "Ultralever"based on an improved microlithography process. Ultralevers offers a moderately high aspect ratio and on occasion a ~10 nm end radius. AFM cantilevers generally have spring constants of about 0.1 N/m and lengths of 100 microns. Attachment of MWNT to the tips.

19 Liquid cells for the SPM allow operation with the tip and the sample fully submerged in liquid, providing the capability for imaging hydrated samples. Operation in liquid can also reduce the total force that the tip exerts on the sample, since the large capillary force is isotropic in liquid. A liquid environment is useful for a variety of SPM applications, including studies of biology, geologic systems, corrosion, or any surface study where a solid-liquid interface is involved.

20 Tip Effects Tip convolution arises when the radius of curvature of the tip is comparable with, or greater than, the size of the feature trying to be imaged. The diagram illustrates this problem;as the tip scans over the specimen, the sides of the tip make contact before the apex, and the microscope begins to respond to the feature. This is what we may call tip convolution. Compression occurs when the tip is over the feature trying to be imaged. It is difficult to determine in many cases how important this affect is, but studies on some soft biological polymers (such as DNA) have shown the apparent DNA width to be a function of imaging force. It should be born in mind that although the force between the tip and sample may only be nn, the pressure may be MPa. Interaction forces between the tip and sample are the reason for image contrast with the AFM. However, some changes which may be perceived as being topographical may be due to a change in force interaction. Forces due to the chemical nature of the tip are probably most important here, and selection of a particular tip for its material can be important. Double tip effect when tip has two active areas.

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22 Other AFM methods MFM Magnetic Force Microscopy TIP is made of a magnetic material or coated in a magnetic layer. As the tip is scanned over the surface it can interact with the stray fields above the sample. Spatial resolution is of the order of nm but the force resolution (sensitivity) is considerably higher (10-13 N/m compared to 10-5 N/m for AFM). First collect a non-contact topography scan (<10 nm tip-sample) and then repeat in far non-contact mode (>10 nm). LFM Lateral Force Microscopy Using the four quadrant photodiode the torsion of the cantilever is measured. Measuring the friction forces - nanotribology. Modified cantilevers. Need to acquire simultaneous AFM and LFM images to deconvolute surface roughness. Data on a magneto-optical disk - left topography and right domains

23 Scanning near field optical microscopy The operational principle behind near-field optical imaging involves illuminating a specimen through a sub-wavelength sized aperture whilst keeping the specimen within the near-field regime of the source. if the aperture-specimen separation is kept roughly less than half the diameter of the aperture, the source does not have the opportunity to diffract before it interacts with the sample and the resolution of the system is determined by the aperture diameter as oppose to the wavelength of light used. An image is built up by raster-scanning the aperture across the sample and recording the optical response of the specimen through a conventional far-field microscope objective. spatial resolution limited by the aperture diameter rather than the wavelength of light

24 Instrumentation Scanning near-field optical microscopy can be performed in many different ways of operation. Most common today is the use of aperture probes for transmission microscopy, either in illumination (a) or in collection (b). However, many samples or substrates are opaque, so that working in reflection is necessary (c). The reflected light can be collected by optics close to the tip, or by the fiber probe itself, in which case often uncoated fiber tips are used. A different approach is taken by the Photon Scanning Tunneling Microscope (PSTM), where evanescent waves are created at the sample surface by oblique far-field illumination (d). The probe tip acts as a scatterer of the evanescent field, leading to homogeneous waves which can be easily detected. Easy to operate, this mode suffers somewhat from difficulties in data interpretation.

25 The most crucial part of a near-field optical microscope is the optical probe. In most cases it is fabricated from an optical fiber which as been tapered (to reduce its size) and coated with metal from the sides (to make it opaque); leaving only a small hole (the aperture) open at its very end. The tapering is usually achieved by heating and pulling, or by chemical etching, the coating is done from the sides while the fibers are rotated along their axes.

26 It is important to keep the optical probe at constant distance from the sample, so that changes in optical signal can be attributed to varying sample properties, and not to a mere change in probe-sample distance. Usually, the damping of the horizontally vibrating SNOM probe (caused by "shear forces") is taken as a measure for this distance. This way the shear force feedbackkeeps the probe tip at constant height.

27 intensity contrast Monitoring the intensity of light gives you information about the transmittivity and reflectivity of the sample, and of changes of index of refraction. Monitoring just the intensity of the signal is particularly susceptible to topographic artifacts, and care has to be taken when interpreting the data. wavelength contrast Luminescence - samples absorb light of a specific wavelength and re-emit light of a longer wavelength (i.e. of less energy). Depending on the time scales involved, one distinguishes between fluorescence (fast decay of the excited state: 10-8 sec and less) and phosphorescence (slow decay of the excited state: µsec to hours). Image size: 5 µm x 5 µm., such as many other alkali halides, is known to form color centers when irradiated with high energy electrons. Images of LiF showing topography (left), excitation light at 458 nm in transmission (middle), and photoluminescence signal (right). All signals are taken simultaneously. The luminescence seems to stem from the rims of the grains visible in the topography image. The fact that the excitation light doesn t show this feature indicates that this appearance is not due to a topography artifact.

28 Dip Pen SPM nanolithography

29 Electrical pulses result in the transfer of groups of atoms from the probe tip onto the substrate surface. Line widths nm Local decomposition of organometallic gases. 30 nm Au and Pt dots.

30 Electrochemical processing. Anodic oxidation using surface humidity V used. Use oxide for subsequent patterning.

31 Bottoms up - self assembly Languir-Blodgett films amphiphilic molecules (small molecules with both hydrophilic and hydrophobic parts e.g. lipids) on a liquid surface are compressed by a movable barrier. Chain interactions lead to a stabilising effect such that the film can be transferred onto a solid substrate. Repeated insertion of the substrate can lead to multilayer. Limitation is large number of holes. Self assembled monolayers (SAMs) Several amphiphilic molecules also form ordered layers on solid substrates. Requirements for the formations of SAMS Head group that can react with substrate Adsorption on the surface An intramolecular mobility Intermolecular stabilising interactions Organic thiols, sulfides, phosphines Substrates nobels metals (Au, Ag, Pd)

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33 Summary AFM Measures the interaction force between the tip and surface. The tip may be dragged across the surface or vibrated as it moves. The interaction force may depend on the nature of the sample the probe tip and the distance between them. STM measures a tunneling current flowing between the tip and the sample. NSOM Scans a very small light probe very close to the sample. Detection of the light forms the image. Can provide resolution below that of a conventional light microscope.