Basic Principle of Scanning Probe Microscopy • •

•

Introduction to Scanning Probe Microscopy: the local probe approach Operational Principle: Atomic Force Microscope (AFM): “contact and non-contact” mode AFM cantilevers AFM tips Feedback loop Operational Principle: Scanning Tunneling Microscope (STM): topographic STM operation modes STM instrumentation STM designs mechanical vibrations electronics

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Zooming in atoms with Scanning Probe Microscope

10-3

10-6

10-9

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What is Scanning Probe Microscope (SPM) SPM provides very high resolution images of various sample properties. All of these microscopes work by measuring a local property - such as height, optical absorption, or magnetism - with a probe or "tip“ placed very close to the sample. The small probe-sample separation (on the order of the instrument's resolution) makes it possible to take measurements over a small area. To acquire an image the microscope raster-scans the probe over the sample while measuring the local property in question. The resulting image resembles an image on a television screen in that both consist of many rows or lines of information placed one above the other. Unlike traditional microscopes, scanned-probe systems do not use lenses, so the size of the probe rather than diffraction effects generally limit their resolution.

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Varieties of SPM Scanning Tunneling Microscope (STM) First introduced by G. Binnig (1) and H. Rohrer in 1981 Stylus Profilometer (SP) First introduced by J. B. P. Williamson Scanning Probe Microscope (SPM)

(2)

in 1967

Atomic Force Microscope (AFM) First introduced by G. Binnig (3) et al. in 1986

Magnetic Force Microscope (MFM) First introduced by J. A. Sidles (4) et al. in 1992 Scanning Capacitance Microscope (SCM) First introduced by J. R. Matey (5) et al. in 1985 Others Pioneering Science and Technology

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Summary of progress in electron tunneling Year

Investigator

Advancement

1928 1934 1937 1958 1960

Fowler, Nordheim Zener Muller Eisaki Gaiever

Explanation of field emission Theory of interband tunneling Field emission microscope Tunneling in p-n junctions Measurement of superc. energy gap

1961 1963 1966

Bardeen Simmons Jaklevic, Lambe

Many body effect in tunneling theory Image forces in tunneling theory Inelastic tunneling spectroscopy

1971

Young, Ward, Scire

Vacuum-tunneling in plane geometry

1982

Binnig, Rohrer, Gerber

Atomic resolution STM

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Comparison among different SPM Type

Properties used for scanning

Resolution

Used for

STM

Tunneling Current between sample and probe

Vertical resolution < 0.1 Å *Lateral resolution ~ 1 Å

=> Conductors => Solids

SP

Surface profile

Vertical resolution ~ 10 Å *Lateral resolution ~1000 Å

⇒Conductors, insulators, semiconductors => solids

AFM

Force between probe tip and sample surface (Interatomic or electromagnetic force)

Vertical resolution < 1 Å *Lateral resolution ~ 10 Å

=> Conductors, insulators, semiconductor => liquid layers, liquid crystals and solids surfaces

MFM

Magnetic force

Vertical resolution ~ 1 Å *Lateral resolution ~ 100 Å

=> Magnetic materials

SCM

Capacitance developed in the presence of tip near sample surface

Vertical resolution ~ 2 Å *Lateral resolution~ 5000 Å

=> Conductors => Solids

* Lateral resolution depends upon the resolution of mechanical XYZ stage Pioneering Science and Technology

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Operational principle: Atomic Force Microscope (AFM) • First generation AFM was the combination of Scanning tunneling Microscope (STM) and Stylus Profilometer (SP). (G. Binnig et al. 1985)(1) • The atomic force microscope measures topography with a force probe. Two modes (1) contact mode (2) non-contact mode • Laser beam deflection offers a convenient and sensitive method of measuring cantilever deflection • AFM cantilevers have high flexibility • Tube piezoceramics position the tip or sample with high resolution • Feedback is used to regulate the force on the sample • AFM has alternate imaging modes.

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The atomic force microscope measures topography with a force probe AFM operates by measuring attractive or repulsive forces between a tip and the sample.

There are two ways to scan. 1. 2.

Sample holder moves and tip is fixed. Tip moves and sample holder is fixed.

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Modes of operation

• Contact Mode: direct physical contact with the sample

•Non-contact Mode: Tip oscillating at constant distance above sample surface • Tapping mode: Intermittent contact, less damaging

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“contact” and “non-contact” mode •In “contact mode” the AFM measures hard-sphere repulsion forces between the tip and sample.

Van der Waals force versus distance

•In “non-contact mode”, the AFM derives topographic images from measurements of attractive forces; the tip does not touch the sample (Albrecht et al., 1991). •In principle, AFM resembles the record player as well as the stylus profilometer. However, AFM incorporates a number of refinements that enable it to achieve atomic-scale resolution: 1.Sensitive detection 2.Flexible cantilevers 3.Sharp tips 4. High-resolution tip-sample positioning 5.Force feedback Pioneering Science and Technology

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Tapping mode AFM

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Detection: laser beam • AFM can generally measure the vertical deflection of the cantilever with pico meter resolution by using optical lever. • The optical lever 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 positionsensitive photodetector consisting of two sideby-side photodiodes. • The difference between the two 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. Pioneering Science and Technology

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AFM cantilevers •Flexible cantilever exerts lower force to sample thus less distortion and less damage.

• AFM cantilevers have generally spring constant less than 0.1 N/m.

• Higher the resonance frequency of cantilever, faster and better the imaging.

• To obtain both low spring constant and high frequency( >2 KHz), the mass of cantilever should be very small ~ 10-10 Kg.

• Microlithography process is used to make such cantilevers. Si or Si3N4 are used for making tips. A thin gold layer is deposited to upper side of tip for good reflectivity of laser beam. Pioneering Science and Technology

Schematic illustration of the meaning of "spring constant" as applied to cantilevers. Visualizing the cantilever as a coil spring, its spring constant k directly affects the downward force exerted on the sample. Office of Science U.S. Department of Energy

Commercially available AFM tips

(a)

(b)

(c)

SEM images of three common types of AFM tip. (a) normal tip (3 µm tall); (b) supertip; (c) Ultralever (also 3 µm tall).

• Tips are generally evaluated by their “end radius” which limits the resolution of AFM. • The "normal tip" ( Albrecht et al., 1990) is a 3 µm tall pyramid with ~30 nm end radius. • The electron-beam-deposited (EBD) tip or "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. • The "Ultralever" is based on an improved microlithography process. Ultralever offers a moderately high aspect ratio and on occasion a ~10 nm end radius. Pioneering Science and Technology

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Resolution: apparent width

x 2 = (Rtip + Rsample ) − (Rtip − Rsample ) 2

2

2 2 2 2 x 2 = Rtip + 2 Rtip Rsample + Rsample − Rtip + 2 Rtip Rsample − Rsample

x = 2 Rtip Rsample

w = 2 x = 4 Rtip Rsample Pioneering Science and Technology

DNA: 2nm

tip∼ 20nm tip ∼10nm

w=25nm w=18nm

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Carbon nanotube at the end of a commercial AFM tip

Extremely tiny radius of curvature ~ 1nm Extremely robust Buckle reversibly

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Piezoceramics for tip-sample position

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Piezoceramics tubes for tip-sample position Top view

•Small area scanning •High resolution positioning

•Voltage applied to all quadrants: expansion •Opposite voltage applied to two quadrants: bending

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Feedback loop to regulate the force on the sample LASER

Detector

I Sample

Tip

PZT

E

PID

Z

I

+I -Z

E=I-Z

PID

Z Z height signal “Z” is used to form image

PZT

• PID is compensation network, P=> Proportional gain, I=> Integral gain D=>Differential gain. • A feedback circuit integrates the signal coming from photodiode and applies a feedback voltage to the z piezo (PZT) to exactly balance the cantilever bending. • Since the probe force is proportional to the cantilever bending, this is constant. • The image of the surface is built up as a series of scan lines, each displaced in the y direction from the previous one. Each individual line is a plot of the voltage applied to the z piezo as a function of the voltage applied to the x piezo. Pioneering Science and Technology

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Feedback loop and contact mode Force applied on the sample: F=-kd k: cantilever force constant d: cantilever deflection

Constant force applied on the sample: •Force applied on the sample with deflection of cantilever •Variation of deflection as tip scans •Deflection measured by laser beam shift and fed back to scanner •Scanner readjust height to keep deflection constant •Consequently force applied on sample held constant Pioneering Science and Technology

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Feedback loop and non-contact mode

Tip-sample distance held constant: •Tip held above sample surface •Cantilever oscillated at resonance frequency •Oscillation amplitude and phase measured by laser detection system and fed to scanner •Oscillation amplitude change due to Van der Waals interaction when tip reaches 10-100 Å above surface •Scanner adjust height to keep amplitude constant. Pioneering Science and Technology

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Advantages of AFM Versatile technique: wide range of resolution, 100 µm -> Å Atomic resolution on MICA Scan area: 80 Å x 80 Å

Sb2O3 deposited on graphite Scan area: 2 µm x 2 µm

Z. Wang, RHK Technology Pioneering Science and Technology

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Advantages of AFM Straightforward technique: • No sample preparation • Measurements in air, no vacuum Versatile technique: wide variety of samples • • • • •

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Conductor Semiconductor Insulator Hard material: oxides, metals Soft materials: wet cells

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AFM Resolution Z resolution •Fractional Å vertical sensitivity • Limited only by electronic noise and mechanical vibrations X,Y resolution (Atomic flat samples) • Limited by diameter of atom at the probe tip

X,Y resolution (rough surface) •Limited by tip geometry

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The Basics of Scanning Tunneling Microscopy In 1981 G. Binnig and H. Rohrer at IBM Research on Zurich invented STM and in 1986 they won the Nobel Prize The basic components of the Microscope setup are: ☯ a sample ☯ a sharp tip to be placed in a very close proximity to the sample ☯ a mechanism to control the location of the tip in x-y plane parallel to the sample surface (X-Y scan control)

Z

Feedback

Piezo X

Y

I tunnel

Computer

tip sample Y-scan

V bias

X-scan

tip

Limited to conducting samples !! sample

First instrument to generate real space images of surfaces with atomic resolution Pioneering Science and Technology

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Tunneling current

vacuum gap

tip

☯ Tunneling current is exponentially dependent upon the distance between the tip and the sample:

sample

ψs

I ∝ exp(− Ad )

ψt φ

EF

eV

ρs

ρt d

Exponential dependence

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This means that for a small change in the distance between tip and sample there is a large change in current! Extremely high z resolution Office of Science U.S. Department of Energy

Topographic STM operation modes

tip height

tunnel current position

z-voltage

I

FB on

Constant current mode

Itunnel

I

FB off

Constant height mode

An individual atom can be "seen" as an increase in the tunneling current as the tip is scanned across the surface of the sample.

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STM Instrumentation Pohl, IBM J. Res. Dev. 30, 417 (1986) Kuk and Silverman, Rev. Sci. Instrum. 60, 165 (1989)

1. Mechanical Construction 2. Electronics 3. Data acquisition

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Vibration isolation The desired resolution in STM imaging of 0.1 Å vertically and 1.0 Å laterally, imposes the requirement that noise from any source be less than 0.01 Å in z and 0.1 Å in x and y. Typical floor vibration ~ 0.1 – 1.0 µm in the range 0.1-50 Hz

Vibration isolation necessary

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Damping • low frequency (< 20 Hz, building) springs (resonance frequency ~ 1-5 Hz), air table (resonance frequency ~ 1 Hz) • medium frequency (20-200 Hz, motors, acoustic noise) mounting on heavy plates

• external vibration isolation system+ rigid STM design can reduce external vibration by a factor 10-6-10-7

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Sample-tip approach mechanisms : Inchworm and Inertial slider •Step size has to be small than the total range of z piezodrive, step resolution of 50Å and dynamical range of cm is required!

•An inchworm motor employs a piezotube and piezoclamps to move an inner shaft with a series of clamping/unclamping and extension/retraction events

•Inertial motion of a free mass can be achieved by asymmetric acceleration

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Scanner (inertial slider)

Inertial slider approach mechanism

Sample holder

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Schematic and pictures of the scanner

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Scanner (inchworm)

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STM electronics Schematic view

• the computer generates the scanning motion of the tip, overall program control and image display • the integrator, low pass filter optimizes the feedback performance and prevents the system from resonating • position control system that includes: digital-analog converters (DAC), anolog-digital converters (ADC), high voltage amplifiers to control the scanner • the z piezo is adjusted by the feedback system so that the current I stays equal to the preset value Iref Pioneering Science and Technology

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First stage amplifier R

vout

Tunneling current is typically in the range 10pA-10nA. To keep it constant via feedback is usually converted to a reference voltage ∼ 1V. This requires a current-to-voltage amplifier with a gain 108-1010 V/A and noise below the equivalent of 1 pA.

punta campione

bias

The second amplifier stage is a low pass filter with a variable cut-off frequency

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Atomic images of graphite

a = 2.46 Å Atomic resolution on graphite Scan area: 53 Å x 53 Å

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