Scanning Probe Microscopy HT10 1

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

Scanning Probe Microscopy HT10 2

Scanning Probe Microscopy HT10 3

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)

Scanning Probe Microscopy HT10 4

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

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Aperture SNOM

•  Optical fiber tips - tapered - metal coated - end aperture

•  Many configurations for aperture SNOM

•  Distance control crucial - tip shear forces

(NT-MDT design)

Scanning Probe Microscopy HT10 6

Aperture SNOM example Single dye molecules - fluorescence - 15 nm res.

Hosaka and Saiki, J. Microsc.202, 362 (2001)

Aperture: 20 nm

Illumination – collection mode

Resolution 15 nm better than fundamental limit in standard SNOM on metal surfaces (30-50 nm)

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Scattering SNOM •  Relies on field enhancement due to metallic tip - surface plasmons

•  Raman scattering - SERS effect enhances signal up to 1011 -1014

•  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

Scanning Probe Microscopy HT10 8

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)

Scanning Probe Microscopy HT10 9

Instrumentation for TERS

Example NT-MDT Ntegra Spectra

SFM

Micro-Raman spectrometer including:

•  confocal microscope

•  focused laser

•  monochromator

•  detectors

Scanning Probe Microscopy HT10 10

Scanning Capacitance Microscopy

•  SCM useful for measuring dopant profiles on the nanoscale

•  Model: MOS structure

!

1Ctot

=1

Cair+1Cox

+1CD

Scanning Probe Microscopy HT10 11

SCM

!

1Ctot

=1

Cair+1Cox

+1CD

(Note: sample bias!)

Scanning Probe Microscopy HT10 12

SCM instrumentation •  Contact-mode SFM

•  AC bias, measure dC/dV

•  Special capacitance sensor

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SCM examples

SCM of 0.6 µm n-channel MOSFET

From Edwards, APL 72, 698

SFM and SCM images of an SRAM device

Scanning Probe Microscopy HT10 14

Scanning Spreading Resistance Microscopy (SSRM)

•  Measure spreading resistance by pressing the SFM tip into the sample - large force due to contamination

•  Maxwell formula:

•  Nanoscale contact: ballistic transport => Sharvin formula:

!

R = " 4a

!

RSharvin =h2e2

"F2

# 2a2

•  Equal or better resolution than SCM

•  more simple determination of doping level, but cannot determine carrier type

•  surface damage due to indentation

Multiprobe techniques

Scanning Probe Microscopy HT10 15

Local conductivity measurements on nanostructures, nanoelectronic devices, organic layers, etc. Example: Single-electron transistor (SET)

Double-probe STM

Scanning Probe Microscopy HT10 16

Sharper tips

Scanning Probe Microscopy HT10 17

Figure 5 . Schematic diagram of minimum interprobe distance, Lmin , between two probes; a) two conventional W probes, b) two WOx nanorod probes grown at apexes of two W probes. SEM and TEM images of the WOx nanorods are shown in c) and d), respectively. An atomic-resolution STM image of a Si(111)7x7 surface obtained by using this WOx nanorod probe is shown in e).

Resistance measurements

Scanning Probe Microscopy HT10 18

Quadruple- probe

Scanning Probe Microscopy HT10 19

HOPG

V-I measurements on Si(111)4x1-In, a 1-dim. surface structure

Scanning Probe Microscopy HT10 20

Electrostatic Force Microscopy

•  Special case of DFM

•  Synonymous: Kelvin probe force microscopy

Scanning Probe Microscopy HT10 21

EFM, Kelvin probe measurements

!

Fel ="C"z

Vbias # VCPD +VAC cos$mt( )2

•  Add AC signal to DC bias

!

Fel ="C"z

Vbias # VCPD( )2 + 2VAC cos$mt Vbias # VCPD( ) +VAC2 cos2$mt[ ]

•  Force contains DC, ωm and 2ωm components

•  ωm component zero when Vbias = VCPD

•  ωm component zero detected by lock-in technique