Other SPM Techniques Scanning Near-Field Optical Microscopy (SNOM) Scanning Capacitance Microscopy (SCM) Scanning Spreading Resistance Microscopy (SSRM) Multiprobe techniques Electrostatic Force Microscopy, Kelvin probe force microscopy 1
2
Scanning Near-Field Optical Microscopy (SNOM) Optical microscope resolution limit: 0.61λ (Abbè limit) Optical properties of nanostructures very important.. Light-emitting nanoparticles, molecules, devices.. Inelastic scattering: Raman, fluorescence SNOM uses the optical near-field, non-radiative local electric field, evanescent at surfaces Near-field interactions lead to far-field changes Detector close to the surface, resolution in the 10-50 nm range Problem: tip preparation (standard SNOM) 3
SNOM basics Dipole-dipole interaction (near field) lead to far-field changes Overview of SNOM modes: a) Aperture SNOM b) scattering (aperture-less) SNOM c)-d): Related STM techniques 4
Aperture SNOM Optical fiber tips - tapered - metal coated - end aperture Many configurations for aperture SNOM Distance control crucial - tip shear forces (NT-MDT design) 5
Aperture SNOM example Illumination collection mode Hosaka and Saiki, J. Microsc.202, 362 (2001) Single dye molecules - fluorescence - 15 nm res. Aperture: 20 nm Resolution 15 nm better than fundamental limit in standard SNOM on metal surfaces (30-50 nm) 6
Scattering SNOM Relies on field enhancement due to metallic tip - surface plasmons Raman scattering - SERS effect enhances signal up to 10 11-10 14 Raman spectroscopy -very useful for nanostructures, molecules, CNT Measures vibrational modes - fingerprint for different bonds and molecules example: radial breathing mode in CNT - Raman shift direct measure of diameter (figure: diameters 1.51 1.22 nm) CNT far-field Raman spectrum (a) near-field, (b) far-field, (c) difference 7
TERS - Tip-enhanced Raman spectroscopy Images of a CNT bundle by standard confocal Raman (left) and TERS Raman (right) Branching CNT viewed by TERS Raman (left) and SFM (right) 8
Instrumentation for TERS SFM Micro-Raman spectrometer including: confocal microscope focused laser monochromator detectors Example NT-MDT Ntegra Spectra 9
Scanning Capacitance Microscopy SCM useful for measuring dopant profiles on the nanoscale Model: MOS structure 1 C tot = 1 C air + 1 C ox + 1 C D 10
SCM 1 = 1 + 1 + 1 C tot C air C ox C D (Note: sample bias!) 11
SCM instrumentation Contact-mode SFM AC bias, measure dc/dv Special capacitance sensor 12
SCM examples SCM of 0.6 µm n-channel MOSFET From Edwards, APL 72, 698 SFM and SCM images of an SRAM device 13
Scanning Spreading Resistance Microscopy (SSRM) Measure spreading resistance by pressing the SFM tip into the sample - large force due to contamination Maxwell formula: R = " 4a Nanoscale contact: ballistic transport => Sharvin formula: h 2 " F R Sharvin = 2e 2 Equal or better resolution than SCM # 2 a 2 more simple determination of doping level, but cannot determine carrier type surface damage due to indentation 14
Multiprobe techniques Local conductivity measurements on nanostructures, nanoelectronic devices, organic layers, etc. Example: Single-electron transistor (SET) 15
Double-probe STM 16
Sharper tips Figure 5. Schematic diagram of minimum interprobe distance, L min, between two probes; a) two conventional W probes, b) two WO x nanorod probes grown at apexes of two W probes. SEM and TEM images of the WO x nanorods are shown in c) and d), respectively. An atomic-resolution STM image of a Si (111)7x7 surface obtained by using this WO x nanorod probe is shown in e). 17
Resistance measurements 18
Quadrupleprobe HOPG V-I measurements on Si(111)4x1-In, a 1-dim. surface structure 19
Electrostatic Force Microscopy Special case of DFM Synonymous: Kelvin probe force microscopy 20
EFM, Kelvin probe measurements Add AC signal to DC bias F el = "C ( "z V bias # V CPD +V AC cos$ m t ) 2 F el = "C "z [( V bias # V CPD ) 2 2 + 2V AC cos$ m t( V bias # V CPD ) +V AC cos 2 $m t] Force contains DC, ω m and 2ω m components ω m component zero when V bias = V CPD ω m component zero detected by lock-in technique 21