; A=4π(2m) 1/2 /h. exp (Fowler Nordheim Eq.) 2 const

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1 Scanning Tunneling Microscopy (STM) Brief background: In 1981, G. Binnig, H. Rohrer, Ch. Gerber and J. Weibel observed vacuum tunneling of electrons between a sharp tip and a platinum surface. The tunnel current is strongly distance, z, dependent; i.e., 2 ( AΦ 1/ z) I V exp ; A=4π(2m) 1/2 /h bias with the tip-sample applied bias voltage, V bias, and the average potential barrier height Φ. Tunneling occurs in the low bias voltage regime, i.e., ~0.1 V). At high bias voltage, i.e., V bias >Φ/e, the current flow is due to field emission (FE), i.e., 2 const I FE Vbias exp (Fowler Nordheim Eq.) Vbias Conventional STM STM Tip Tunneling Current, I Conductive Piezo Scanner Bias Voltage, V Topography Imaging: Constant current imaging (slow scan mode) The current is used as the feedback system to keep the tip-sample distance constant. On the atomic level, the local density of states (LDOS) is measured. The STM tip material affects the LDOS at the Fermi level. Table: Electronic density of state at the Fermi level for common tip materials (from Wiesendanger, SPM and Spectroscopy, Table 1.3) Material Wolfram (W) Platinum (Pt) Iridium (Ir) s state 3.1 % 0.77 % 0.94 % d state 85 % 98 % 96 % 1

2 Tunneling Spectroscopy (current-voltage, I-V, information): ' The tunnel current depends on the tip-sample distance, the barrier height, and the bias voltage. Studying the bias dependence provides important spectroscopic information on the occupied and unoccupied electronic states (-> local LDOS studies). Positive sample bias: Net tunneling current arises from electrons that tunnel from occupied states of the tip into unoccupied states of the sample. Energy Level Diagram: Φ T Φ S occupied states Tip (T) (S) Negative sample bias: Net tunneling current arises from electrons that tunnel from occupied states of the sample into unoccupied states of the tip. Energy Level Diagram: Φ S Φ T DC voltage spectroscopy: Quick and simple way to assess the electronic states between the Fermi levels of the tip and the sample. Limitation: Reveals both, geometric and electronic structure information which cannot be easily separated. AC voltage spectroscopy: A high-frequency sinusoidal modulation voltage is superimposed on the constant DC bias voltage. Spectroscopic images corresponding to the spatial variation of di/dv (measured by lock-in technique) contain electronic but also geometric structure information. Current imaging tunneling spectroscpy (CITS): There is feedback and nonefeedback gate. (a) the active feedback, a constant stabilizing voltage is applied and the tip height is held constant by keeping the tunneling current constant. (b) The feedback is switched off and the bias voltage is rapidly linearly ramped between two preselected values, which results in local I-V curves. 2

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4 Constant height imaging or variable current mode (fast scan mode) The scan frequency is fast compared to the feedback response, which keeps the tip in an average (constant) distance from the sample surface. Scanning is possible in realtime video rates that allow, for instance, the study of surface diffusion processes. Differential tunneling microscopy Tip is vibrated parallel to the surface, and the modulated current signal is recorded with lock-in technology. Tracking tunneling microscopy Scanning direction is guided by modulated current signal (e.g., steepest slope). Scanning noise microscopy Use current noise as feedback signal at zero bias. Nonlinear alternating-current tunneling microscopy Conventionally, STM is restricted to non-conducting surfaces. A high frequency AC driving force causes a small number of electrons to tunnel onto and off the surface that can be measured during alternative half-cycles (third harmonics). Other STM MODES Ballistic Electron Emission Microscopy (BEEM) Application: Determination of the Schottky barrier height in metalsemiconductor heterojunctions. Procedure: Electrons tunnel from the STM tip into the metal film. The electrons that have enough energy to overcome the Schottky barrier reach the metal-semiconductor interface and are responsible for a ballistic electron (BEEM) current. STM Tip Tunneling Current, Collector Current I t,c Conductive Surface Films (e.g., Metal) Submaterial (e.g., Semiconductor) Piezo Scanner Bias Voltage, V BEEM current, I B Reference STM: Scanning Probe Microscopy and Spectroscopy, Methods and Applications, R. Wiesendanger, Cambridge University Press, Cambridge (1994). 4

5 Scanning Force Microscopy (SFM): Conventional SFM Application: Topography measurements Force: F N = k N * z k N Ppring constant: Spring deflection: z z SFM Tip Piezo Scanner Interaction or force dampening field Contact SFM Non-Contact SFM Lateral Force Microscopy (LFM) Application: Tribological studies (friction, adhesion), surface shear mechanical, contrast enhancements, material distinction ("Chemical Force Microscopy") Procedure: The lateral force is typically measured via the torsional bending mode of the cantilever and constant load. Hysteresis analyses of lateral force loops (forces recorded in forward and reverse scan direction) provide information about the reversibility of the measured lateral forces. Irreversible lateral forces are called "friction forces", F F. Load: F N = k N * z SFM Tip Lateral Force: F L = k L * x F L 0 average F F x z x Piezo Scanner/Feedback F static F dynamic 5

6 4-Quadrant Photodiode Laser Cantilever Topography Friction Scan Input Modulation ω δ Cantilever Response δ = 0 δ > 0 fully elastic viscoelastic ω δ ω δ Spinodal Decomposition of PS/PMMA Blend 50/50 PS/PMMA blend annealed at 180 o C for 1 week PMMA PS PMMA PS SFM Topography complex flow pattern over time 10 µm SFM Lateral Force 2D spinodal decomposition different from bulk Note: The bright spots (PS phase/lateral force image) represent spinodal frustration points of PMMA. 6

7 Scanning Force Microscopy (SFM) continued: Electrostatic Force Microscopy (EFM) Application: Study of the location and lifetime of surface charges on insulating surfaces. Procedure: Long-range electrostatic Coulombic forces are measured with a mechanically modulated conductive or clean silicon cantilever tip. An AC voltage is applied between the tip and the sample with a frequency ω 2 that is smaller than the mechanical modulation frequency ω 1 but larger than the gain of the feedback response. The AC voltage causes a charge and a mirror charge on the tip and the sample, respectively. The mechanically modulating tip is experiencing a Coulombic force gradient. For an uncharged surface the force grradient will oscillate at 2ω 2, whereas for a charged surface, the force gradient will be modulated at ω 2. A charge signal can be extracted by measuring the f and 2f signal with lock-in technique. The phase of that signal correponds to the sign of the surface charge. Magnetic Force Microscopy (MFM) Application: Measuring of surface magnetic structures Procedure: Using the non-contact mode with magnetically coated cantilever tips. Rheological Force Microscopy Application: An imaging method to determine local moduli (Young's modulus shear modulus) and surface viscous properties (out of phase response). Procedure: The canitlever tip is in contact (at constant load) with the sample and sinusoidally modulated normally or laterally (either directly or indirectly; see below "distance modulation" vs. "force modulation"). For example, a small and fast (in regards to the feedback response) normal modulation can be superimposed to the piezo normal feedback voltage. Both, the input modulation signal and the response photodiode signal are fed into a dual-phase lock-in amplifier for the determination of the in-phase and out-of-phase response. Load: F N = k N * z Lateral Force: F L = k L * x Amplitude Response Modulation Signal SFM Tip Input Modulation Signal z x Piezo sinusoidally modulated either in x or z Time Delay Time 7

8 x=1 Dewetting vs. Grafting Density (x) Contrast: Friction Elasticity PEA: bright dark PEA-g-xPS: dark bright x = 1 no dewetting x=3 x = 3 slow dewetting x=5 x = 5 fast dewetting Topography Friction Elasticity Topography Imaging: Operational Modes Constant deflection (contact mode) Analog to the constant current STM mode. The deflection of the cantilever probe is used as the feedback signal and kept constant. Constant dampening (AM detection, intermittent contact mode in air or liquid) The response amplitude of sinusoidally modulated cantilevers allow feedback in the pseudo-non-contact regime (intermittent contact) due to fluid dampening. Constant frequency shift (FM detection, non-contact mode in ultrahigh vacuum) Similar to the FM radio, the frequency is measured and frequency shifts are used as feedback system. This approach works only in vacuum where fluid-dampening effects can be neglected. Variable deflection imaging (contact mode) Analog to the variable current STM (constant height) mode. Uses fast scan rates compared to the force deflection feedback (close to zero). Sensitive to local force gradients such as line defects. Improved high resolution capability (atomic resolution). 8

9 Spectroscopy (local probing): Force Spectroscopy (Force-Displacement, F(D), Curves) The normal forces acting on the cantilever are measured as function of the sample-tip displacement. Used for adhesion and force interaction studies. linearly ramped voltage applied to piezo F(D) jump in contact D = D o - vt 0 F(D) forces acting on the tip D jump out of contact 9

10 Scanning Near-field Optical Microscopy (SNOM) While optical microscopy is limited by the wavelength of the probing light (typically on the order of several hundreds of nanometer) near field optical microscopy on only limited by the aperture diameter. SNOM Principle (Pohl et al. 1984): A tiny aperture, illuminated by a laser beam from the rear side, is scanned across a samle surface, and the intensity of the light transmitted through the sample is recorded. To achieve high lateral resolution (first experiments provided already tens of nanometer resolution), the aperture had to be nanometer sized, and maintained at a scanning distance of less than 10 nm from the sample surface (i.e., within the evanescent field). The evanescent field is a surface wave that is coupled, i.e., bound, to an electromagnetic wave or mode propagating inside the waveguide. In fiber optics, the evanescent field may be used to provide coupling to another fiber. Illumination Detector Evanescent Field Regime Small aperture Illumination Objective Objective Detector Illumination Mode Reflection Mode Scanning Near-field Acoustic Microscopy (SNAM) Scanning Capacitance Microscopy (SCAM) Scanning Electrochemical Microscopy (SECM) Scanning Micropipette Microscopy (SMM) 10

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