Scanning Probe Microscopy (SPM)

CHEM53200: Lecture 9 Scanning Probe Microscopy (SPM) Major reference: 1. Scanning Probe Microscopy and Spectroscopy Edited by D. Bonnell (2001). 2. A practical guide to scanning probe microscopy by Park Scientific Instruments. 3. Surface Analysis Edited by J. C. Vickerman (1997). 1 Scanning probe microscopes (SPMs) are a family of instruments used for studying surface properties of materials from atomic to micron level. All SPMs contain the components illustrated below. Source: Ref. 2 2

Scanning Tunneling Microscopy (STM) The scanning tunneling microscope (STM) is the ancestor of all SPMs. It was invented in 1981 by Gerd Binnig and Heinrich Rohrer at IBM Zurich. Five years later they were awarded the Nobel prize in physics for its invention. The STM was the first instrument to generate real-space images of surfaces with atomic resolution. 3 Electronic structures of solid Brief review As we move from single atom multiple atoms solids Electronic properties change from atomic orbitals molecular orbitals 4

Density of states (DOS), N(E), is the # of energy levels between E and E+dE (states ev 1 ) States may have s, p, d, f or mixed (hybrid) character depending on origin Band is effectively (i) and (ii) over solid Bands may be separated by band-gaps of energy E g Widths of band is determined by magnitude of overlap between individual orbitals Depending upon # of e s and DOS, band may be filled, partially filled or empty Top (partially-) filled band is called Bottom empty band is called 5 Principles of Electron Tunneling Source: Ref. 3 6

The tunneling effect: (a) consider the simplest system Schrödinger equation: Inside solid (x < 0) 2 2 ħ d? m 2 dx 2 = E? Outside solid (x > 0) 2 2 ħ d? m 2 dx 2 + V? = E? 7?? The solutions to these equations are: ikx ikx 2 in = Ae + Be, k = out = Ce ik' x + De ik' x, me ħ 2m( E V ) k' = ħ h ħ = 2π iθ e = cosθ + i sinθ e iθ = cosθ i sinθ The wave function Ψ out (outside the solid/inside the barrier) has an imaginary part (which rises to infinity and is thus discounted) and a real part which decays exponentially with x. For E < V, : penetration is forbidden : Ψ is non-zero, e may tunnel into potential barrier 8

(b) consider two potential wells (2 metals) close together When 2 metals are far apart, the effective overlap of the Fermi level wave functions is neglegible. When they are brought close together, with some distance d, then the overlap of the wave functions may be sufficiently great to facilitate quantum mechanical tunneling. (for φ1 = φ2, current flow equal in both direction) 9 (c) under the influence of an applied potential difference For example, with metal 1 negative with respect to metal 2 tunneling from filled states of E F in metal 1 empty states at E F in metal 2 I tunnel e 2κd κ = 2m φ ħ = 0.51 φ e (for κ in Å 1, φ in ev) - κ for a typical metal 10

Tunneling is sensitive to electronic structures: 11 When the tip is brought within ~10Å of the sample, electrons from the sample begin to "tunnel" through the 10Å gap into the tip or vice versa, depending upon the sign of the bias voltage. For a typical metal (κ 1 2 Å 1 ), I tunnel by ~ one order of magnitude for an of 1 Å in d. This exponential dependence of I tunnel on d gives STM its remarkable sensitivity. vertical resolution: sub-å lateral resolution: atomic resolution (for sharp tips) sample 12

Tip Artifacts Source: Ref. 1 13 c 14

Source: Ref. 1 15 STM Instrumentation Source: Introduction to Surface Chemistry and Catalysis by G. A. Somorjai (1994). 16

STM Tip Material: It has been proposed that higher resolution can be attained when tunneling to or from d-orbitals because electrons in these orbitals are more strongly localized than electrons in s-states. The d-band tip is more sensitive to small features, because the tunneling matrix element is enhanced by the greater charge localization. [ tip Au tip (has primarily s e s at the Fermi level) ] Fabrication: mechanical forming: e.g. cutting with wire cutters or scissors electrochemical etching ion beam milling etc. In-situ methods of tip sharpening can be useful (e.g. applying a voltage pulse between the tip and the substrate) 17 Electronics control Voltage: 0 ~ 3 V I tunnel : 1 pa ~ 10 na, typically 10 pa ~ 1 na Tip-sample separation: typically 2 ~ 5 Å resistance of gap: 10 7 ~ 10 9 Ω For a tip-sample separation of ~1 Å and bias potential of 0.1 V, E = V m 1 set point gap (vice versa) If I tunnel > 1 na, the tip-sample interaction is likely to become strong enough to surface morphology (depend on sample) set point or bias voltage (i.e. I tunnel ) tip-induced surface damage I tunnel 18

Computer controls movement of tip (x, y, z) in fashion on surface & converts each I tunnel measurement to pixel (color I tunnel ) plotted versus x, y position to generate image. 19 Scanner Material: change dimensions in response to an applied V develop electrical potential in response to mechanical P lead zirconium titanate PbZrTiO 3 ( ), with various dopants added to create specific material properties Fabrication: Scanner are made pressing together a powder, then sintering the material result in a polycrystalline solid. dipole moments are randomly aligned to align dipole moments 20

Poling process: The scanner is heated to ~200 C to free dipoles apply a voltage to the scanner within hours, most dipoles become aligned the scanner is cooled to freeze the dipoles into their aligned state. The newly poled scanner can then respond to voltages by extending and contracting. scanner scanner s polarization If the scanner is not repoled by regular use, a significant fraction of the dipoles will begin to randomize ( ) again over a period of weeks. > 150 C depoling scanner (The Curie temperature for PZT materials is ~150 C.) 21 If dipoles are oriented, material changes length in applied electrical field: 10 4 ~ 10 7 % length change per V allows < 1 Å positioning Piezoelectric scanners are critical elements in SPMs. 22

Piezoelectric scanner sub-å resolution, compactness, high-speed response nonlinearities ( : intrinsic nonlinearity, hysteresis, creep, aging, cross coupling) Maximum scan size depends on the length of the scanner tube, the diameter of the tube, its wall thickness, and the strain coefficients of the particular piezoelectric ceramic. Typically SPM scanners can scan Laterally: tens of Å ~ 100 µm Vertically: distinguish height variation from sub-å to ~10 µm 23 Scanner Nonlinearities As a first approximation, the strain in a piezoelectric scanner varies linearly with applied voltage. s: strain ( L/L, in Å /m), E: electric field (in V/m) d: strain coefficient (in Å /V) But the actual relationship between s and E diverges from ideal linear behavior. the ratio of maximum deviation y from the linear behavior to the ideal linear extension y at that voltage Typically y/y: 2% 25% 24

: the ratio of maximum divergence between the ascending and descending curves to the maximum extension that a voltage can create in the scanner: y/y max. Hysteresis can be as high as 20%. Source: Ref. 2 25 : When an abrupt change in voltage is applied, the piezoelectric material change dimensions in two steps: the first step takes place in less than a millisecond, the second on a much longer timescale. The second step is known as creep. Creep: the ratio of the 2nd dimensional change to the 1st: x c / x Typical values of creep: 1% 20%, over T cr : 10 100 s. Source: Ref. 2 26

: The strain coefficient of piezoelectric materials changes exponentially with time and use. The aging rate is the change in strain coefficient per decade of time. When a scanner is not used, the lateral strain coefficient decreases over time. When a scanner is used regularly, the deflection achieved for a given voltage increases slowly with use and time. Source: Ref. 2 27 : The term refers to the tendency of x-axis or y-axis scanner movement to have a spurious z-axis component. electric field is not uniform across scanner. Some cross talk occurs between x, y, and z electrodes. Source: Ref. 2 28

' Software correction 29 ' Hardware correction Source: Ref. 2 30

STM Imaging Modes In mode, the tip travels in a horizontal plane above the sample and the tunneling current varies depending on topography and the local surface electronic properties of the sample. The tunneling current measured at each location on the sample surface constitute the data set, the topographic image. 31 In mode, STM uses feedback to keep the tunneling current constant by adjusting the height of the scanner at each measurement point. For example, when the system detects an increase in tunneling current, it adjusts the voltage applied to the piezoelectric scanner to increase the distance between the tip and the sample. 32

Spectroscopic Modes Conventional surface-sensitive spectroscopic techniques (e.g. XPS, AES, and vibrational spectroscopies) generate data which are averaged over an entire surface. Many surface phenomena are strictly local in nature. STM can, in principle, map the electronic structure of a surface with atomic resolution. When tip is biased negative get information about electronic states on surface When tip is biased positive get information about electronic states on surface By changing bias, one can resolve location of individual electronic states. Source: Ref. 3 33 There are several spectroscopic modes of operation of STM. voltage-dependent STM imaging: acquiring conventional STM images at different biased voltages Source: Ref. 3 34

between 0.15 and 0.6 V, most current arises from dangling bond states of the twelve adatoms between 0.6 and 1.0 V, image reveals the positions of the six dangling bonds on atoms in the second layer between 1.6 and 2.0 V, image reveals regions of higher current density, corresponding to Si Si backbonds 35 Scanning tunneling spectroscopy (STS): the most common form, which provides quantitative information through the use of a constant dc bias voltage with a superimposed highfrequency sinusoidal modulation voltage between the tip and the sample enables simultaneous measurement of topography and sample Local measurement of tunneling I-V curves: allows measurement of I-V curves with atomic resolution at well defined locations, with a fixed tip-sample separation in order to correlate the surface topography with the local electronic structure 36

Lithography and Micromanipulation under influence of high electric field, polarizable molecules or atoms can be made to jump from surface to tip or vice versa vdw forces between close tip and adsorbate can be used to drag species Title : The Making of the Circular Corral Media : Fe on Cu(111) Source: http://www.almaden.ibm.com/vis/stm/ corral.html 37 Title : The Beginning Media : Xe on Ni (110) Title : Carbon Monoxide Man Media : CO on Pt (111) Title: Atom Media: Fe on Cu (111) Source: http://www.almaden.ibm.com/vis/stm/atomo.html 38

Summary Advantages: provide real space images excellent lateral (< 1 Å ) and vertical (< 0.1 Å ) resolution provide information about surface unit cell size and symmetry or electronic structure can directly image molecular orbitals some spectroscopic data (e.g. STS) may allow identification fast enough to allow some dynamic study can both initiate and probe electron-induced chemistry Disadvantages: complex and expensive instrumentation (especially for UHV) subject to noise (electrical, vibration) image is convolution between tip and surface electronic structure not truly a topographic measurement must fabricate tips dull tips or multiple tips create serious artifacts only works for conductive samples, though can tunnel through thin insulators (< 20 30 Å ) 39 Atomic Force Microscopy (AFM) Also known as cantilever tip piezoelectric scanner 40

Source: Ref. 1 41 Source: Ref. 1 42

AFM Operation Modes Source: Ref. 2 43 In contact AFM mode, also known as repulsive mode, an AFM tip makes soft "physical contact" with the sample. F(x) = kx Hooke s law typical atom-atom k: ~10 N/m, typical cantilever k: 0.1 1 N/m total force on sample: 10 6 10 8 N If k cantilever k surface sample deformed can operate in 2 modes: (a) (b) 44

Non-contact AFM (NC-AFM) is one of several vibrating cantilever techniques in which an AFM cantilever is vibrated near the surface of a sample. The spacing between the tip and the sample for NC-AFM is on the order of tens to hundreds of angstroms. This spacing is indicated on the van der Waals curve of previous figure as the non-contact regime. tip sample distance: 10 100 Å very small force on sample: ~ 10 12 N best for soft or elastic surfaces least contamination least destructive long tip life Such small forces are difficult to measure using direct beam-bounce method use AC driven oscillating cantilever (100 1000 Hz frequency, 10 100 Å amplitude) 45 resonance frequency k varies with external force gradient ν = 2π (df(x)/dx) so frequency changes with external forces electronics adjust tip-surface distance to keep resonance frequency constant constant tip force NC-AFM cantilever: with greater stiffness Intermittent-contact AFM (IC-AFM) is similar to NC-AFM, except that for IC-AFM the vibrating cantilever tip is brought closer to the sample so that at the bottom of its travel it just barely hits, or "taps" the sample. As for NC-AFM, for IC-AFM the cantilever's oscillation amplitude changes in response to tip-to-sample spacing. An image representing surface topography is obtained by monitoring these changes. useful for soft surfaces less prone to external vibration/noise than NC-AFM less destructive than contact AFM & can image rougher samples 1 k m 46

AFM Cantilever Tip Cantilevers and tips are typically made of A thin layer of Au or Al is usually coated at the back of the cantilever to enhance the reflectivity. Source: Ref. 1 47 Source: Ref. 1 l tip : tip s length r : radius of curvature q : sidewall angles 48

Tip Shape and Resolution The lateral resolution of an AFM image is determined by: Consider an image taken with 512 512 data points. For a scan 1 µm 1 µm, the step size and lateral resolution would be (1 µm 512) The sharpest tips commercially available can have radius of 50 Å typically provides lateral resolution of 10 20 Å. Thus, for AFM images > (1 µm 1 µm), the resolution is usually determined not by the tip but by the image s step size. 49 50

Source: Ref. 2 51 Source: Ref. 2 52

Force vs. Distance Curves Source: Ref. 2 53 Advantages: Summary not limited to conducting samples especially powerful for biological, organic and polymer samples simpler instrumentation than STM tips and cantilevers commercially available provides true topographic imaging related AFM techniques can measure other physical properties such as hydrophobicity, magnetism, electrostatic charge, friction, elastic modulus NC-AFM or taping mode causes minimum damage to soft/fragile samples demonstrate atomic resolution using special tips (<2 Å lateral) 54

Disadvantages: noise, vibration sensitive presence of water (capillary action) may distort image contact AFM can damage surface chemically blind chemical specificity can be rendered by tip modification (CFM) as with STM, tip shape convolutes into image typical lateral resolution ~ 50 Å 55 Other Types of SPMs Lateral Force Microscopy (LFM) Source: Ref. 2 56

Magnetic Force Microscopy (MFM) Electrostatic Force Microscopy (EFM) Source: Ref. 2 57 Source: Ref. 1 58

Conductive AFM Scanning Near-field Optical Microscopy (SNOM) 59 STM Scanning Tunneling Microscope 0.2 nm AFM Atomic Force Microscope 0.2 nm LFM Lateral Force Microscope 2 nm EFM Electrostatic Force Microscope 50 nm MFM Magnetic Force Microscope 50 nm SThM Scanning Thermal Microscope 10 nm SCM Scanning Capacitance Microscope 25 nm SNOM Scanning Near-field Optical Microscope 20 nm * 2001 p. 98 108. 60