Selected Characterization Techniques - UNESCO · 2018-04-11 · characterization methods that could...

Selected Characterization

Techniques

Tip-enhanced Raman spectroscopy

Scanning helium ion microscope

Magnetic resonance sub-nanometer imaging

Terence Kuzma

Outline

• Tip-enhanced Raman spectroscopy (Lecture 1)

• Scanning helium ion microscope (Lecture 2)

• Magnetic resonance sub-nanometer imaging

Introduction• This presentation will cover some unique

characterization methods that could be employed to interrogate bottom up materials.

• In some cases we will review a related conventional characterization method, then detail how the method is enhanced for special applications.

• The first method under review uses light as an interrogating force, the second method uses ions, and the third method measures magnetic displacement. To fully understand these methods, we will contrast other methods, conceptually build a system model, then review the detailed physics to fully understand the method and limitations.


• Tip-enhanced Raman Spectroscopy (TERS) is a refined method of Surface Enhanced Raman Spectroscopy (SERS)

• TERS is desirable for detailed molecular information because it offers a stronger signal than SERS

• So the structure of this discussion will be:

Overview SERS

Understand the physics of the light to material interaction

Discuss how TERS is an enhancement of the SERS technique


• Let’s first look at the parent technique, then the enhanced technique. SERS is a vibrational technique similar to the acoustic interaction of a wine glass from a tuning fork. Using this analogy for Raman, the tuning fork is indirect coupling of high frequency radiation such as light, and the chemical bonds act as the wine glass to absorb and shift the radiate energy. The radiant energy shift is then correlated to the chemical makeup.


• It is important to state that Raman differs from IR absorbance techniques such as FTIR, SERS is a reflection measurement

• SERS is very sensitive to chemical bond length, strength, stress, strain, and arrangement in a material.

• SERS is often used to derive structural characterization information, more than chemical analysis

• Often used in pharmaceutical quality control, and and materials such as nanotubes.

Raman spectroscopy

• The Raman effect is relatively weak, so high power monochromatic continuous gas lasers are used to excite the bonds. This inherent weakness is a reason to desire TERS.

• Laser (photon) energy excites the chemical bond to a higher energy state

• Rayleigh scattering can occur from this interaction. Rayleigh scattering is the radiated wave is the same frequency of the input wave

• Electromagnetic radiation from the illuminated spot is collected and provides both elastic and inelastic information

Raman spectroscopy

• There are 3 major light peaks that are reflected from the laser/sample interface – providing chemical bond interaction.

• The strongest radiation is the Rayleigh line. This radiation is the elastic interaction of the laser and the bond. This line is often filtered from the signal to better analyze other interaction.

• A second lower energy interaction denotes residual vibrational energy is known as Stokes scattering. This re-radiation has a lower frequency than the input signal. It is a reflected wave that is derived from the adsorption of the laser to vibrational modes in the sample

Raman spectroscopy

• Systems can be large and complex, and also smaller instruments can be used for “macro” drug and explosive detection by law enforcement.

• Even this handheld unit features sophisticated algorithms automatically determine presence of mixed and contaminated chemicals, and an extensive substance library

Raman spectroscopy

• Vibrational energy of the chemical bond is deduced from measuring the difference between the frequency of the Raleigh line and the Stokes line. This identifies the state of the bond

• This state shows chemical bonds and the stress and strain on the sample. This information is collected as wavenumbers, and this provides a “fingerprint” of the material under test.

• Another spectral line is formed from thermal excitation of the chemical bonds. This spectrum is known as the Anti-Stokes line. It mirrors the Stokes line and has higher frequencies with reference to the Raleigh line.

Raman spectroscopy

• The Anti-Stokes line has the same distance from the reference Rayleigh line as the Stokes line because it reports on the same bonds. The difference is that the Anti-Stokes incorporates ambient thermal energy. Therefore the Anti-Stokes line mirrors the Stokes line and has higher frequencies with reference to the Raleigh line.

• In a typical powdered substance, the intensity Raman scattering is roughly 10 million times less than the intensity of Rayleigh scattered light, and therefore very sensitive instrumentation is needed for Raman spectroscopy. This gives rise of the need of TERS discussed later.

Raman spectroscopy Elastic and inelastic scattering

https://commons.wikimedia.org/wiki/File:Ramanscattering.svg

Raman spectroscopy

Schematic illustration of the Stokes

(down-conversion) and anti-Stokes (up-

conversion) shifts. Solid and dotted

lines represent absorption and

emission spectra, respectively.

http://pubs.rsc.org/en/content/articlehtml/2012/jm/c2jm33654e

Raman Spectroscopy

Energy Modes

http://www.intechopen.com/books/advances-in-agrophysical-

research/determination-of-food-quality-by-using-spectroscopic-methods

Raman spectroscopy

Jablonski diagram

http://www.photonics.com/Article.aspx?AID=29869

Raman spectroscopy

Jablonski diagram

• A Jablonski diagram illustrates the

electronic states of a molecule and

the transitions between them

• Aleksander Jablonski’s diagrams

shows a portion of the possible

consequences of applying photons

from the visible spectrum of light to a

particular molecule

http://chemwiki.ucdavis.edu/Core/Physical_Chemistry/Spectrosc

opy/Electronic_Spectroscopy/Jablonski_diagram

Raman spectroscopy

Jablonski diagram

• Within each column,

horizontal lines represent

eigenstates for that particular

molecule. Bold horizontal

lines are representations of

the limits of electronic energy

states

Raman spectroscopy

Jablonski diagram

• The transition shown is the absorbance

of a photon of a particular energy by the

molecule of interest. Electrons then

transitions to a different eigenstate

corresponding to the amount of energy

transferred.

• This transition will usually occur from the lowest (ground)

electronic state due to the statistical mechanical issue of

most electrons occupying a low lying state at reasonable

temperatures.

Raman spectroscopy

Jablonski diagram

• Once an electron is excited, there are a multiple paths

that energy may be dissipated.

• Vibrational relaxation, a non-radiative process. This is

indicated on the Jablonski diagram as a curved arrow

between vibrational levels. Vibrational relaxation is photon

energy given to other vibrational modes as kinetic energy.

• Internal conversion is identical to vibrational relaxation,

where an excited electron can transition from a vibration

level in one electronic state to another vibration level in a

lower electronic state.

Raman spectroscopy

Jablonski diagram

• This figure shows the

kinetic energy transfer in

the electron states with in

the material

Raman spectroscopy

Jablonski diagram

• Fluorescence is a pathway for

molecules to deal with photon

energy.

• Fluorescence is most often

observed between the first excited

electron state and the ground state,

because higher energies are more

likely to be dissipated through

internal conversion

Raman spectroscopy

Jablonski diagram

• Due to the large number of

vibrational levels between

electronic states,

measured emission is

usually distributed over a

range of wavelengths

Raman spectroscopy

Jablonski diagram

• Intersystem crossing is

another path a molecule may

take in the dissipation of

energy. This where the

electron changes spin

multiplicity from an excited

singlet state to an excited

triplet state.

Raman spectroscopy

Jablonski diagram

• Intersystem crossing is a

forbidden transition, that is, a

transition that based strictly on

electronic selection rules should

not happen.

• Intersystem crossing leads to

several interesting routes back

to the ground electronic state

including phosphorescence

Raman spectroscopy

• Stokes absorption is quantified as wavenumbers. The family of absorption represented by wavenumbers gives a fingerprint of the material under test.

http://www.inphotonics.com

Raman spectroscopy

• A typical system from Thermo Scientific

• Modular, able to change the pre-aligned laser, filter, and gratings without any tools

• This allows rapid analysis of unique materials without recalibrating the system

http://www.thermoscientific.com/

Raman spectroscopy • Pre-aligned laser

– Blue or green lasers can be good for inorganic materials and resonance Raman experiments (e.g., for carbon nanotubes and other carbon materials) and surface enhanced Raman scattering

– Red or near infra-red (660-830 nm) are good for fluorescence suppression

– Ultra-violet lasers for resonance Raman on bio-molecules (such as proteins, DNA, and RNA), and fluorescence suppression

• Uses corresponding filter, and gratings

• This technique of customizing a characterization chamber is quick, but also add integrity to the test. The same hardware is used every time for similar materials under test.


• Tip enhanced Raman scattering (TERS) combines surface enhanced Raman spectroscopy (SERS) with Raman-AFM analysis. TERS narrows the Raman scattering near the point of a near atomically sharp probe that is typically coated with gold.

• TERS uses a metallic (usually silver-/gold-coated AFM or STM) tip to enhance the Raman signals of molecules situated in its

vicinity.

• Spatial resolution is

approximately the size

of the tip apex (10–30 nm).

Detection of Carbon Nanotubes using Tip-Enhanced Raman Spectroscopy

By Jia Wang, Xiaobin Wu, Rui Wang and Mingqian Zhang


• TERS is intended to solve some intrinsic issues with SERS.

• The spot size or interrogation size of the area of interest with SERS is certainly greater than a few molecules. So TERS refines this method by making the interrogation area equal to the sharpness of an AFM tip. Typical tip radii could be 10nm, providing an equivalent interrogation area

• SERS has limitations because the Stokes emission light energy is very small. This is reduced even further by the desire to interrogate small areas.


• Given these issues and constraints, TERS offers an eloquent and “difficult” to physically achieve solution.

• An AFM is used to bring a sharpened probe tip in very close proximity to the sample. This is important because electric field usually decease as a function of 1/r2 . So the “sensor” needs to be in close proximity of the sample for a strong signal.

• This is still a very small signal, and fortunately the tip radius can become essentially a place to confine and integrate plasmon resonance.

• This plasmon resonance enhances the strength of the laser – material interaction


• As shown, TERS combines surface enhanced Raman spectroscopy (SERS) with AFM/plasmon reduce interrogation area with plasmon aided signal strength enhancement. analysis. TERS narrows the Raman scattering near the point of a near atomically sharp probe that is typically coated with gold.

http://www.princetoninstruments.com/userfiles/files/appnotes/TE

RS_Tip_enhanced_Raman_Scattering_APP_Note_RevA1.pdf


• Tip-enhanced resonance Raman and resonance Raman spectra of Brilliant Cresyl Blue (BCB) at surfaces

– a) TERS and RRS of BCB at Au(111)

– b) TERS and RRS of BCB at Pt(111)

• Note the enhanced signal when the probe in in close proximity of the sample

http://www.fhi-berlin.mpg.de/pc/TERS/ters_sers/Tip_enhanced/page6.html


• One monolayer of a DNA base (adenine, thymine, guanine or cytosine) is adsorbed on a Au(111) subtrate.

• When the gold tip is brought into tunneling contact, a strong enhancement of the Raman intensity is observed; if the tip is retracted, no Raman signal is detectable

http://www.fhi-berlin.mpg.de/pc/TERS/ters_sers/Tip_enhanced/page8.html

Outline

• Tip-enhanced Raman spectroscopy

• Scanning helium ion microscope



• Scanning helium ion microscope, (SHIM), is a microscope similar to a conventional SEM

• SHIM offers a sharp images with a large depth of field on a wide range of materials

• SHIMs uses the short De Broglie wavelength of the helium ions, which is inversely proportional to their momentum

= wavelength,

h = Planks constant

p = momentum


• SHIM has a very small wavelength due to the very low momentum of the helium ion

• SHIM shares the ability of ion systems that allow sample milling and adding observation levels at sub-nanometer resolution

• Compared to an SEM, the secondary electron yield is high, allowing for imaging with currents as low as 1 femtoamp

• Recall that secondary electrons provide topographic information

• The this will result in an image that shows all high and low points of the sample in great detail and focus.


• Sample damage is very, very low due to relatively light mass of the helium ion

• Detectors provide information-rich images which offer topographic, material, crystallographic, and electrical properties of the sample

http://journal.frontiersin.org/article/10.3389/fphys.2014.00395/full


• Electron beams have a large

scattering effect and interact

with a broad area. This

causes the emission of

secondary electrons from an

area that is larger than the

beam size

• The smaller the area of

surface interaction, the higher

will be the ultimate image

resolution.


• Helium ions are about 7,000

times heavier than electrons and

neon ions are 40,000 times

heavier than electrons. Because

of this, helium or neon ion beam

exhibits very little diffraction

when passed through an

aperture or across an edge


• Non-conducting samples can be probed without metallic coatings, (as compared to the SEM)

• Since the impinging ion is positive, it is possible for

positive charge to build up. This is overcome with a

effective electron flood gun.

• This means that one it is not necessary to coat the

sample with conductive material that may obscure the

image

• Helium microscopes offer a large depth of field –This

depth of field can be as much as 5-10 times as

compared to a typical high end FE-SEM


• The Zeiss Orion 30 is a commercially

available tool capable of imaging

resolution of 0.5 nm

• In addition to the imaging capabilities,

the system can perform etching,

deposition and lithography

http://www.zeiss.com/microscopy/en_us/products/multiple-ion-beam/orion-nanofab-for-materials.html#introduction


• The Zeiss system features 3 beam sources for material

processing.

• This allows adjustment of beam current, beam energy,

and importantly ion mass.

• Uses include material removal using sputtering, gas

induced etching or deposition or lithography

• The gallium FIB is used to remove relatively large

amounts of material, this beam may contaminate the

material



• Neon beam (larger mass than He) for precision

nanomachining with speed, and the helium beam can

fabricate delicate sub-10 nm structures

• Having more kinetic energy, neon can expose resist faster

than helium

• Gallium having even more mass can be used for rapid

prototyping, and larger feature sizes

• Naturally He and Ne will provide less contamination than

Ga interaction

• Speed is traded for resolution



• The Orion NanoFab features the NanoPatterning and

Visualization Engine - an integrated hardware and

software control system

• Software is capable of creating a range of fully editable

shapes including rectangles, trapezoids, polygons, lines,

polylines, ellipses and spots. Vector fill these shapes

while maintaining full control over dose variation and

patterning parameters.

Outline

• Tip-enhanced Raman spectroscopy

• Scanning helium ion microscope


Magnetic resonance

sub-nanometer imaging

• Conventional magnetic resonance imaging is limited to spatial resolution in the micrometer range

• Magnetic resonance sub-nanometer (MRFM) imaging is capable of detecting the weak magnetic forces from nuclear or electronic spins

• This method can provide information on the nanometer and possibly the atomic scale, ultimate resolution is single electron-spin sensitivity

• MRFM combines ultrasensitive force microscopy with magnetic resonance imaging techniques.

MRFM

• Principle: A sample containing nuclear spins is attached to an ultrasensitive cantilever and brought into close proximity (10-100 nm) of a nanoscale ferromagnet. The resultant magnetic force slightly deflects the cantilever, measured by a laser interferometer. Radio-frequency pulses are used to modulate the nuclear spin magnetization and to create an MRI signal. Scanning can produce 3D images of the sample.

• So basically this technique works by measuring the response of the nuclear magnetic moments – or spins –of a sample to external magnetic fields and electromagnetic radiation

MRFM

• The nitrogen-vacancy center is a point defect in the diamond lattice. It consists of a nearest-neighbor pair of a nitrogen atom, which substitutes for a carbon atom, and a lattice vacancy.

https://en.wikipedia.org/wiki/File:Model_of_nitrogen-vacancy_center_in_diamond.jpg

MRFM

• Diamond defects carry a spin that is very sensitive to magnetic fields

• the optical rate of emission depends on the defect’s spin state, we can measure small fields simply by looking at the fluorescence intensity

http://web.mit.edu/degenlab/

MRFM

• N-V− centers emit bright red light which can be conveniently excited by visible light sources, such as argon, krypton, He-Ne, and other lasers.

• At room temperature, no sharp peaks are observed because of the thermal broadening. However, cooling the N-V− centers with liquid nitrogen or liquid helium dramatically narrows the lines down to few megahertz width.

• Transitions between the triplet ground state and each of the excited states can be individually addressed via resonant optical excitation

https://en.wikipedia.org/wiki/File:Model_of_nitrogen-vacancy_center_in_diamond.jpg

MRFM

http://web.mit.edu/degenlab

MRFM

Image height is about 5 cm

MRFM

• These achievements include the detection of a single electron spin in 2004 [1] and the demonstration of nuclear magnetic resonance imaging of a nanoscale biological sample in 2009

http://www.spin.ethz.ch/research_long/mrfmlab

MRFM• Advantages include, possibility of site-specific image

contrast, absence of radiation damage, single copy of an object is required, well-suited to provide structural information of large biomolecular complexes that are known to overwhelm nuclear magnetic resonance

• Disadvantages include identifying a specific area. Issues like “gas” in proximity of the probe could be mistakenly measured. Naturally this is a detailed test to look at small areas, so it is time consuming to interrogate large features.

Outline

• Tip-enhanced Raman spectroscopy (Lecture 1)

• Scanning helium ion microscope (Lecture 2)


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