~100x100nm

~10x10nm

~1000x1000nm ~100000x100000nm

Anders Mikkelsen e-mail: anders.mikkelsen@sljus.lu.se

Synchrotron Radiation Research, Fysicum www.sljus.lu.se

What is exciting about the nanoscale?

Nanowire – nervecell interaction

Why are surfaces important in nanoscience & technology ?

s/b of Ge Quantum dot: 3%

s/b of Carbon Nanotube: 100%

Diameter: 20nm

Diameter: 3nm

Nanostructures have very large atomic scale surface to bulk ratios!!! => Surfaces can strongly influence structure and properties.

Very few defects can change properties significantly

Diameter: 4mm 0.36nm for Carbon

Importance of the surface Surface atoms

x100=s/b of a diamond: 0.000002% Atomic scale diamond structure

Bulk (all) atoms

The Lund III-V Nanowire technology platform

Complex heterostructures Perfect Ordering

A wide variety of complex, reproducible 0D, 1D, 2D, 3D structures!

Long, crystalline needles, typically 10 - 100 nm diameter, several micrometers long and with atomic scale control.

Many III-V materials combinations possible, grown from metal seed particles

The Lund Nanowire technology platform Long, crystalline needles, typically 10 - 100 nm diameter, several micrometers long and with atomic scale control.

Nanowire/cell interaction Quantum Technologies

A great playground for science and well suited for applications

Novel electronics & photonics

Many III-V materials combinations possible, grown from metal seed particles

Novel atomic scale imaging and spectroscopy probes for 1D objects.

Growth & structure from microns to the atomic scale of low dimensional objects

Inside Onside Topside Spectroscopy

Nanowires (1D, 0D) Droplets (0D, 1D) Oxides (1D, 2D)

Surface Control vs Nanowire Function

Invited Prospective, Surf. Sci. 607 (2012) 97

Nanowires have large surface to bulk ratios => surface matters

Using 2D surface science methods on 1D structures is very challenging, but possible!

Topics of today • What is STM and AFM?

• How do they work?

• Spectromicroscopy?

• Scanning Tunneling Microscopy (STM) • Scanning Tunneling Spectroscopy (STS) • Scanning Tunneling Luminesence (STL) • Atomic Force Microscopy (AFM) • Magnetic Force Microscopy (MFM) • Electrostatic Force Microscopy (EFM) • Capacitance Microscopy (CM) • Scanning Nearfield Optical Microscopy(SNOM) • ...

Some Scanning Probe Techniques

There are many different Scanning Probe techniques We will focus on the two fundamental ones: STM & AFM

• Scanning Tunneling Microscopy (STM) Resolve individual atoms, measure electrical properties,

induce photo luminesence. Only conducting samples. • Atomic Force Microscopy (AFM) Resolution limit ~STM, but more difficult to achieve. Any sample type.

The ”fundamental” Scanning Probe Techniques

Imaging with SPM

Red blood cells DNA on lipid-bilayer Butterfly wing

Carbon nanotube III-V nanowire Magnetic domains

SPM has an incredible wide range of applications, there is a microscope for any sample

The Scanning Tunneling Microscope The expermental setup:

A metal tip is approached so close to the surface that a

tunnel current can be measured

The tip is scanned across the surface using piezo robots to

form an image from the change in current/height

Typical currents: 0.1-1nA Typical voltages: 0.1-4V

The Scanning Tunneling Microscope Two ways of forming the STM image:

a) is almost always used! The expermental setup:

a) I & V constant, feedback loop active, Z variation measured

b) Z & V constant, feedback loop idle, I variation measured

STM systems at Fysicum • UHV Omicron STM1 (in C163) High resolution • UHV Omicron XA STM (in C162) 20mm scan range, low noise, precise positioning • UHV/air Lightsource coupled STM/AFM (in C161) • UHV Omicron VT STM (in C164) Variable Temperature 15-1500K • UHV SIGMA LT STM (installation 2017) Temperature <10K Adsorption Sources available: • Ga, In and As elemental sources • Atomic Hydrogen source • Oxygen, Ammonia from 1-10mbar to 1bar

STMs are found at all/most nanoscience Departments worldwide We have 4-5 UHV STM systems here in Lund with different properties

Experimental issues STM tips made by electrochemical etching

STM stage with magnetic damping

Sharp metallic tips are necessary for STM Vibrational isolation is necessary for STM

Scanner design SPM scanners are made from a piezoelectric material PZT, Lead Zirconium Titanate. PZT expands and contracts proportionally to an applied voltage.

In tripod design with three piezos are used to move in XYZ

Piezoelectric scanner: • based on images of known surfaces calibrate the piezoelectric response to convert [Volts] to [nm] • the response is continuous down to atomic lengths scales • unfortunately, the response is highly non linear (hysteresis)

Electron tunneling in 1D

Time independent Schrödinger equation: 1D model of tunneling from one side (I) to the other (III)

Ψ(x) oscillate in region I and III Ψ(x) decay exponentially in region II outgoing current density incomming current density T=

T~exp(-2ka), k2~(V0-E)

Transmission (T) falls of exponentially with barrier width

Sensitivity of STM

Tunneling current is proportional to exp(-2Kd) ; K=(2mφ)/h For average work function φ of ~4eV, K=1Å-1 => Changing d ~0.1 nm will change current by an order of magnitude This results in that the outermost atom(s) draws the majority of the current, which is what gives us atomic resolution.

0.001

0.002

0.005

0.01

0.21 0.23 0.25 0.27 0.29 0.31 0.33 0.35 0.37 0.39

Distance between tip and sample, d in nm

log(

Nom

alize

d Tu

nneli

ng cu

rrent

)

Mophology of samples

Example of how STM can be used for measurering the atomic scale surface morphology We determine island size and heights with sub-Ångstrom precision

Nanomanipulation and electron density

waves by STM: Quantum Corrals

(Don Eigler IBM)

Single atom surface manipulation is also possible. We can pick up and put down atoms (or nanostructures) by changing the bias on the tip

What are we imaging at the atomic scale?

As long as we only measure morphology it is enough to assume that we measure geometric structure of the surface.

But in more detail we must understand that it is infact the electronic structure that we measure

Local Density of States Density of state (DOS) means the "number of states at a particular energy level", i.e. the distribution of states over energy.

LDOS (x,y,E) gives the density of electrons of a certain energy at that particular spatial location.

Bardeen’s tunneling current formalism Basic assumptions:

Fermi’s golden rule + Coupling between surface & STM tip, is very low =>

Treat tunneling as a slight perturbation of the electronic structure. Then the tunneling current can be computed as the overlap of wave functions

of the sample and the tip

Tunneling current:

Fermi function Tunneling matrix elements

ψt/s ; ρt/s are wave functions and density of states of tip/sample

This slide is only here for completeness, most important message is that the interpretation of the tunneling current has a solid physical basis. We care about the result on the next slide, not the derivation.

Further simplifications (Tersoff & Hamann)

Further assume an S-wave tip with constant density of states:

Assume low voltage:

The main conclusion: I depends on the local density of states of the sample near the fermi level

which can be calculated by standard quantum mechanical theory methods.

S-wave tip

ρs is evaluated at the center of curvature of the tip

Energy levels involved in tunneling

Density of states

Independent sample and tip

Sample and tip grounded, separated by a small vacuum gap

Positive sample bias: electron tunnel from the tip to the sample.

Negative sample bias: electron tunnel from the sample into the tip

In the simple picture of STM we probe empty electron states in sample with positive bias and filled electron states in sample with negative bias Main approximation in this picture is that the DOS of the tip is without any structure.

What are we imaging in STM?

Some general concepts can be concluded: Metals: High density of states at atoms => atoms appear as bright protrusions Insulators: No conduction possible => we crash Semiconductors and thin oxides: Complex electronic structure at fermi level => be careful!

Crystal engineered nanowires

S. Lehmann & K. Dick-Thelander, et al Nano Lett. 2013, DOI: 10.1021/nl401554w

Nanowires with tailored patterns of Wurtzite (WZ) and Zincblende (ZB) crystal phases and surface facets can be grown with high uniformity.

Typical III-V nanowire sidefacets

<111>

Zincblende (ZB) Wurtzite (WZ)

<0001> {110} {112} {11-20} {10-10}

NW

Gro

wth

dire

ctio

n

ZB & WZ different stacking (ABC vs AB), but can be perfectly combined

In STM we can identify them top layer group III or V atoms symmetry

A B

C A

B C

A B

A B

A B

ZB{110} WZ{11-20} WZ {10-10}

Atomic scale structure imaged on all different facets of the same nanowire

Twinned ZB{110} Twinned ZB{110} Twin super lattice

WZ{11-20} WZ{11-20} WZ{10-10}

With STM it is possible to image on top of even high Nanowires and move along

them in a controlled fashion

GaAs(110) – an example of different filled / empty state imaging

High density of states in conduction band (empty states) above Ga atoms High density of states in valence band (filled states) above As atoms => We can change bias to image either Ga or As atoms.

LEED pattern

dI/dV spectroscopy on nanowire surfaces

GaAs(110)

The bright feature at (II) is a Ga vacancy (we are imaging As atoms)

Changes in bandgap and electronic states can be seen locally

Carbon Nanotubes structure vs properties

C.M Lieber et al.

STM spectroscopy (current vs voltage curves) can be used to determine bandgap, normal STM

to get structure (chirality). We can see which chirality gives semiconductor or metallic tubes

Atomic Force Microscopy (AFM) or

Scanning Force Microscopy (SFM)

AFM 101:

AFM probe surface with a sharp tip

Forces between tip and cantilever deflects tip.

A detector measures the deflection

Atomic Force Microscopy (AFM) or

Scanning Force Microscopy (SFM)

Two basic operational modes:

Contact mode (repulsive regime)

Non-contact mode (attractive regime)

Contact AFM

Non-contact AFM

Atomic resolution possible

AFM detection schemes Detection scheme: Static Mode (contact): Deflection Dynamic Mode (non-contact): Amplitude change Frequency shift

Frequency shift Amplitude change

We need to measure very small force changes: Set cantilever oscillating at its resonance frequency, measure frequency shift or amplitude change => This is very sensitive to even small changes in the force on the cantilever

AFM Modes

E. Meyer, H. J. Hug, R. Bennewitz; “Scanning Probe Microscopy: The Lab on a Tip”

There are many more detection modes available varying the degree of tip contact and tip dynamics (oscillation or deflection) For example in tapping mode the tip is oscillating and touches the surface (contact) at the extreme position of the tip.

AFM image of biomolecules

Human chromosone 190 nm-long DNA strands

AFM is good for measuring bio molecules. It can operate in air, liquid or vacuum.

AFM image of blood clotting

B. Drake et al., Science 243, 1586 (1989)

AFM can also be used for measurements of dynamic processes, even in liquids.

High resolution AFM of supramolecular assembly of the photosynthetic

complexes in native membranes

Scheuring, S., Sturgis, J.N., 2005. Chromatic adaptation of photosynthetic membranes. Science 309, 484–487.

Modern imaging quality is very high, AFM is a standard tool in many diciplines now.

Forces: simple view Distance dependence typically less strong than in STM =>

more difficult to get high resolution

(some discussion on forces also in Attard 1.13) E. Meyer, H. J. Hug, R. Bennewitz; “Scanning Probe Microscopy: The Lab on a Tip”

AFM detection system in more detail

Quite simmilar to STM experimental setup!

STM on an AFM

Original AFM was based on a STM as

transducer.

Detecting Cantilever Deflection

Many different detection schemes have been tried, most popular is Beam deflection and Piezoelectric

AFM tips Three common types of AFM tips

Commercial, from silicon or silicon nitride: • Standard chip size: 1.6 x 3.6 x 0.4 mm • High reflective Au coating • Typical curvature radius of a tip: 10 nm • Cantilever length: 100 -200 μm • Cantilever width: 10 -40 μm • Cantilever thickness: 0,3 -2 μm • Available for noncontact and contact modes • Triangular (V shaped) and rectangular • Available with conductive TiN, W2C, Pt, Au and

magnetic Co coatings

Different cantilever shapes

Many different tips and cantilevers are commercially available (or you make your own). Essential to a successful AFM experiment is to choose the right tip.

Tip Artifacts

Double tip

Overestimate object size

Underestimate object size

Tip shape imaged

AFM image with Double tip

One important problem in AFM is tip induced artifacts that can distort the images. This is because the image is a convolution of the morphology of the tip and

surface

High resolution AFM in non contact mode (frequency modulated)

Si(111)-(7x7) AFM vs. STM

AFM STM

(A)The tip is approached to the surface until contact occurs. (1) Retracting the cantilever stretches the connection of the single biomolecule to the surfaces.

When the force reaches the unbinding force of the complex, the biological interaction is ruptured and

(2) The cantilever is available for a new force distance curve. (B) Loading rate dependence of the unbinding forces of the avidin-biotin system under

physiological conditions ( Single Mol. 1 (2000) 285.)

Force spectroscopy

AFM - STM overview STM AFM

• Range: 10x10nm – 10x10µm • Resolution: ~0.01nm • Typical environment: UHV • Samples: Conducting

• Topography • Geometric structure • Electronic structure • Vibrational structure • Magnetic structure

• Manipulate atoms / molecules / nanostructures

• Range 10x10nm – 100x100µm • Resolution: 1 – 0.01nm • Typical environment: Air/Liquid • Samples: All types

• Topography • Geometric structure • Friction • Adhesion • Hardness

• Manipulate molecules / nanostructures

AFM vs STM

Kubo and Nozoye. Physical Review Letters, 86(9), 1801–1804, 2001

STM NC-AFM

Low temperature STM and NC-AFM images on graphite PNAS, 100(22), 12539–12542, 2003

Imaging with both AFM and STM on the same surface can give complementary results

Scanning Probe Microscopy on operating nanowire devices

O. Persson et al Nano Lett. 15 (2015) 3684, J. Webb et al NanoResearch 7 (2014) 877

VNW

Nanowire I(V)

0 -0.2 -0.1 0.1

I NW

10-7

10-8

Real atomic resolution and spectroscopy possible during device operation

Many imaging modes available simultaneously: AFM, STM, STS and SGM

AFM STM spectroscopy

Atomically resolved imaging

Some investigative applications of SPM

• Atomic scale structure of surfaces and nanostructures • Electrical properties on the nanometer scale • Bonding and structure of individual molecules • Morphology during chemical reactions • Magnetic structure of very small objects • Electronic, vibrational and geometrical structure correlation

down to the atomic scale • Diffusion and growth studied directly on the atomic scale

Some direct analysis applications of SPM

• Texture / roughness / topography of materials and lithographic structures

• Electrical vs structural properties of chip technology • Magnetic vs structural properties of harddisk disks

TappingMode AFM phase and amplitude images of Celgard 2400 tape (membrane for Lithium batteries)

A cross-sectional scanning tunneling microscopy study of a quantum dot infrared photo-detector structure (Lund + Acreo AB)

Summary in three points

• Scanning Probe: Approach sharp tip towards surface and measure tip-surface interaction while scanning across surface (possible varying tip-surface distance in a feedback loop).

• Scanning Tunneling Microscopy (STM): Image surface with typical resolution <1 Ångström. Image geometry, but also electronic structure. only conducting samples.

• Atomic Force Microscopy (AFM): Image surface with typical resolution <1 nm. Any sample type.