Official Partner of the EMS - analyticalscience.wiley.com · corder of reversals in the geomagnetic field direction as seafloor spreading takes place. However, knowledge about the

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2Official Partner of the EMS

Raman Imaging

High-Resolution TEM

Two-Photon Microscopy

Optical Coherence Tomography

http://www.witec.de

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Thomas MatzelleScientific Editor

Paleomagnetic Microscopy

Plate tectonics describes the large-scale mo-tion of parts of the Earth’s lithosphere, the rigid outermost shell of the planet. The theo-ry of plate tectonics has been widely accept-ed after a process called “seafloor spreading” was validated in the early 1960s. “Seafloor spreading” occurs by volcanic activity form-ing a new oceanic crust that moves away from ridges. Evidence of this process has been found by recording the symmetric magnetic patterns seen on the seafloor. Ob-viously the Earth’s oceanic crust acts as a re-corder of reversals in the geomagnetic field direction as seafloor spreading takes place. However, knowledge about the early time-scale of plate tectonics is still quite scarce at presence. When the first tectonic plate shifts started has been a subject of discussion for a long time.

Planetary scientists Alec Brenner and Roger Fu from Harvard University and their colleagues have shed light on this problem by analyzing the strengths and directions of magnetic fields within core samples of an ancient rock using quantum diamond microscopy. The samples were extracted from the Pilbara Craton, one of only two pristine Archaean crusts identified on Earth. “Basically, this is one piece of geological evidence to extend the record of plate tec-tonics on Earth farther back in Earth his-tory,” says Alec Brenner. The newly-devel-oped quantum diamond microscope (QDM) was used to reveal information about the latitudinal drift and the time of motion. It consists of a few electromagnetic coils around a camera, a laser and a diamond sample slide. The diamond chip is doped

with defects sensitive to minuscule magnetic fields. In principle, the visualization relies on tiny defects in the diamond created when a nitrogen atom knocks out two carbons. Thus, the void might trap electrons reveal-ing sensitive quantum states. Their energy levels and spin states may then be influ-enced by laser light, microwaves or mag-netic fields. The diamond sensor sensitively maps the magnetic fields imprinted in the small sample grain of rock. These magnetic fields are shaped by the Earth’s magnetic field at the time the mineral formed. The recent study revealed a latitudinal drift of about 2.5 cm and dated the motion to 3.3 billion years ago.

“Currently, Earth is the only known plan-etary body that has robustly established plate tectonics of any kind,” says Brenner. “It really [motivates] us as we search for planets in other solar systems to understand the whole set of processes that led to plate tectonics on Earth and what driving forces transpired to initiate it.”

QDM has shown its applicability for detecting some of the earliest movements of Earth’s tectonic plates in ancient lavas. In addition, this new method seems to be a promising approach for many other inter-esting subjects. Just recently, Fu and col-leagues studied a meteorite that probably formed behind Jupiter. QDM revealed weak remnant magnetism. The finding suggests that the protoplanetary disk of material might have had an irregular magnetic field and thus, also magnetism, not just gravita-tion may have contributed to the creation of planetary bodies.

The prospects for quantum diamond microscopy are bright and we are looking forward to many more exciting and chal-lenging applications.Find more information on interesting ap-proaches in modern microscopy at Wiley Analytical Science, our newly designed web platform.

References[1] Alec R. Brenner, Roger R. Fu, David

A.D. Evans, Aleksey V. Smirnov, Raisa Trubko, and Ian R. Rose: Paleomagnetic evidence for modern-like plate motion velocities at 3.2 Ga, Science Advances 6 (17) (2020) doi: 10.1126/sciadv.aaz8670

[2] Rebecca Pool, Diamond microscope re-veals when plate tectonics started to shift, Wiley Analytical Science (2020) doi: 10.1002/was.00020051

[3] Paul Voosen, Diamond microscope re-veals slow crawl of Earth’s ancient crust (2020) doi:10.1126/science.abc4013

Enjoy reading this issue!

Imaging & Microscopy 2/2020 • 3

Editorial

EDITORIAL 3

NEWSTICKER 6

COFFEE BREAK 8

ANNOUNCEMENT

EMC2020 Cancelled Due to Covid-19 10

NanoScientific Forum Europe on Scanning Probe Microscopy 11

RMS IN FOCUS

Life after Lockdown 12RMS Is Reorganizing Meetings

NEWS FROM EMS

EMS Newsletter #69 13

COVER STORY

Automated Raman Microscopy in 2020 14New Developments in Optimization and Flexible Operation D. Storm and T. Dieing

SCANNING PROBE MICROSCOPY

Controllable Surface Damage by AFM 16Imaging with Higher Eigenmodes and Its AdvantagesJ. Putnam and B. Eslami

Tip-Enhanced Raman Spectroscopy 20A Surface Spectroscopy Technique at the NanoscaleG. Goubert et al.

PREVIEW: ISSUE 3/2020Coming out 10th August, 2020 TrackMate My Favorite Image Analysis Tool, by Neubias MembersA. H. Klemm

The winner of Read & Win issue 1/2020 is

Chantelle V. from South Africa

Contents

4 • Imaging & Microscopy 2/2020

ELECTRON MICROSCOPY

Bronze‐Type Niobium Tungsten Oxides 22A Glimpse into the Structural Complexity of a Promising Battery Material

F. Krumeich

Fatigue of Concrete Examined on the Nanoscale 25TEM Studies of Fatigue-Induced Changes in the Cement Paste of UHPC

G. Schaan et al.

LIGHT MICROSCOPY

Second Harmonic Generation Microscopy of the Living Human Eye 28Visualizing In Vivo Human Ocular Tissues with Two-Photon Microscopy

J. M. Bueno et al.

Optical Coherence Tomography Imaging through the Spectral Ranges 31State of the Art, Advances and Limits

B. Heise and I. Zorin

PRODUCTS 34

INDEX / IMPRESSUM INSIDE BACK COVER

Welcome to the knowledge age. Wiley builds on its 200-year heritage by partnering with universities, businesses, research institutions, societies and individuals to develop digital content, learning, assessment and certification tools. Wiley continues to share and deliver the answers to the world’s challenges helping you to further your mission.

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Raman Imaging

High-Resolution TEM

Two-Photon Microscopy

Optical Coherence Tomography

14

Raman microscopy, long used by specialists in purely scientific research, is being employed as a routine analysis tool in an ever-growing range of fields. Automation has been the key to ma-king the technique easier to use while still offe-ring the full benefits of its analytical power and sensitivity. In 2020, developments that intro-duce self-alignment, modular optical compon-ents and remote-operation will change what is expected of a fully-automated Raman imaging system. The following overview describes these technologies and presents several examples of their application.

Confocal Raman Imaging Microscopy

Raman microscopy is a non-destructive and label-free technique that relies on the Raman effect, in which light scattered by molecules exhibits a distinct shift in energy due to mo-lecular vibrations. It characterizes materials by these unique shifts, which are visible in their Ra-man spectra. The method’s spatial resolution is limited only by physical law and it can be applied to very small sample volumes and low materi-al concentrations. Confocal Raman microscopy uses a beam path geometry that strongly rejects light from outside the focal plane to increase sensitivity and enable 3D measurements. Ra-man imaging acquires a complete Raman spec-trum at each pixel to visualize the distribution of sample components.

COVER STORY

Automated Raman Microscopy in 2020

New Developments in Optimization and Flexible Operation

http://www.witec.de

NEWSTICKERUltrafast MicroscopyImaging Skyrmions as Never Before

An international team of research-ers has developed a time-resolved photoelectron vector microscope to determine electric fields on sur-faces with high temporal and spa-tial resolution. They have used the

new method to measure the dy-namics of optical skyrmions in the time domain for the first time. Sky-rmions are quasi-particle magnetic spin configurations that hold huge potential for high density data stor-age. Right now, researchers world-wide are investigating whether the magnetic properties of these whirl-ing vortex-like structures features can be transferred to optics.

Original publication: doi: 10.1126/science.aba6415More information: http://bit.ly/IM-22020-a

Ultra-High Resolution STMCreating Purer Pharmaceuticals

In a breakthrough for drug design, UK-based researchers have used ultra-high resolution scanning tun-nelling microscopy to identify hal-ogen bonding. By performing STM with a CO-functionalized tip, they mapped the location of carbon rings and halogen atoms within a bromi-nated polycyclic aromatic molecule, a feat that could not be achieved with standard STM measurements. STM is commonly used to identify on-surface molecular self-assem-bled structures, but can’t always resolve supramolecular structures. With this in mind, the researchers set out to analyze the assembly of

a brominated polycyclic aromatic molecule on a gold surface with standard and ultra-high resolution STM. Standard experiments were first performed on a low tempera-ture STM under ultra-high vacuum conditions. The researchers then leaked carbon monoxide leaked into the ultra-high vacuum sys-tem, which was adsorbed onto the gold surface and picked up by the STM tip, giving the carbon mon-oxide functionalized tip necessary for ultra-high resolution analyses. Ultrahigh-resolution STM images were taken with the instrument in constant height mode. The re-searchers compared results from the standard STM to the ultrahigh resolution STM.

Original publication: doi: 10.1038/s41467-020-15898-2More information: http://bit.ly/IM-22020-c

3D NanoscopyA New View of Cells and Tissue

US-based researchers have devel-oped a 3D nanoscopy method – called in situ Point Spread Function retrieval – to visualize nanoscale structures inside whole cells and tissues. The technique overcomes the aberration issues associated with super-resolution fluorescence microscopy so users can pinpoint biomolecule location to within a few nanometers. While single-mol-ecule localization microscopy is a powerful tool to study subcellular structures in cells and tissue, inho-mogeneous refractive indices inside biological samples distort fluores-cent signals from single-molecules probes, degrading resolution at ev-er-greater depths. The new method enables the construction of an in situ 3D response of single emitters, directly from single-molecule blink-

ing datasets. The method allows the researchers to pinpoint the loca-tions of these single molecules with improved accuracy and precision, down to 3 nm, so they can resolve intra- and extra-cellular structures within whole-cell and tissue speci-mens with high resolution and un-compromised fidelity

Original publication: doi: 10.1038/s41592-020-0816-xMore information: http://bit.ly/IM-22020-b

© Tim Davis

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© Purdue University

3D Acoustic BioprintingGrowing Neural Networks with Buckyballs

Austria-and US-based researchers have unveiled a stunning image of neurons caged inside buckyballs, with neural pathways growing be-tween the cells. The image follows biofabrication research from Pro-fessor Aleksandr Ovsianikov, Insti-tute of Materials Science and Tech-nology, TU Wien, and colleagues, who hope to better control the behavior of living cells. Working with researchers at Stanford, US, the researchers first manufactured buckyball cages using two-photon polymerization, which can fab-ricate 3D objects with extremely high precision. Using 3D acoustic bioprinting, the researchers were then able to pack cells into these

buckyballs. Once the cells have suc-cessfully colonized the buckyballs, multicellular nerve tissue develops with nerve connections forming between neighboring structures to form a neural network.

Original publication: doi: 10.1088/1758-5090/ab76d9More information: http://bit.ly/IM-22020-e

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www.ahf.de · [email protected] since 1981

Optical FiltersPerfectly matched to your application.

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Print your Own MicroscopeOpen Source Design

For less than US-$ 20, or £ 15, labora-tories around the world can now 3D print precision microscopes thanks to an open-source design created at the Uni-versity of Bath, UK. The OpenFlexure Microscope is a fully automated, labo-ratory-grade instrument with motor-ized sample positioning and focus con-trol. As Dr Joel Collins from Physics at Bath highlights, the instrument is unique among 3D-printed microscopes in its ability to yield high-quality images. the microscope also operates very close to diffraction limited per-formance, over the central 50% of its field of view. The instrument has been designed to be easy to

use, with an intuitive software interface and simplified alignment procedures and is also highly customizable so it can be adapted for laboratory, school and home use. The £ 15 price covers the cost of the printed plastic, a camera and fastening hardware. A top-end version would cost a couple of hundred pounds to produce, and would include a microscope objective and an embedded Raspberry Pi computer.

Original publication: doi: 10.1364/BOE.385729More information: http://bit.ly/IM-22020-d

www.ape-berlin.de

Label-freeMicroscopy

With tunable short-pulse laser sources for

Three-photon imagingTHG imagingCARS & SRS

Image courtesy ofPark, Joo Hyun and

Lee, Sang-Wong

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X-Ray and TEMRenewable Nanocapsules to Help Cut Carbon Emissions

Finland-based researchers have developed a scalable and straightforward method to convert lignin from renewable biomass into hybrid fatty acid-lignin nanocapsules that show great prom-ise as phase change materials (PCM) for ther-mal energy storage. One of the most significant potential applications for PCMs is the heating and cooling of buildings as such material sys-tems could eventually replace energy intensive

air conditioning units that increase carbon emissions. However, the development of afford-able TES has been hampered by a lack of envi-ronmentally benign and scalable PCMs. With this in mind, the scientits set out to develop a simple, low-cost, and scalable colloidal synthe-sis method for producing a hybrid nanomate-rial entirely from renewable biomass resources. They characterized the nanoscale morphology of the capsules using small angle X-ray scat-tering (SAXS) on Diamond’s High-throughput SAXS beamline, B21, in combination with ther-moporometry-differential scanning calorimetry, transmission electron microscopy, atomic force microscopy and dynamic light scattering.

Original publication:doi: 10.1016/j.cej.2020.124711More information: http://bit.ly/IM-22020-f

© Diamond Synchrotron Science Facility UK

Extreme UV ImagingCaptures Neurons at Ultra-High Resolution

Using coherent soft-x-ray mi-croscopy with a laboratory-scale extreme ultraviolet light source, UK-based researchers have im-aged neurons to a lateral res-olution of 80 nm, close to the diffraction limit, and without damaging the cells. The pty-chography method produces ex-traordinary detail compared to traditional light microscopy images, with the lat-est images set to aid analyses of neurodegener-ative disease, including Alzheimer’s disease. One particularly robust example, ptychography, scans samples to collect diffraction patterns from

overlapping sample regions and generate high resolution images, yet issues over signal to noise ra-tios as well as light source stabil-ity and coherence, have thwarted biological imaging. With this in mind, the researchers developed. a high harmonic generation (HHG) ptychography method that overcomes these issues.

Original publication: doi: 10.1126/sciadv.aaz3025More information: http://bit.ly/IM-22020-g

© Southhampton University

Newsticker

http://www.ape-berlin.de

Crazy Science Traces of Rainforests in West Antarctica

☛ A team of researchers have now provided a new and unprecedented per-spective on the climate history of Antarctica. In a sediment core collected in the Amundsen Sea, West Antarctica, in February 2017, the team discovered pristinely preserved forest soil from the Cretaceous, including a wealth of plant pollen and spores and a dense network of roots. These plant remains confirm that, roughly 90 million years ago, the coast of West Antarctica was home to temperate, swampy rainforests where the annual mean temperature was ca. 12 °C – an exceptionally warm climate for a location near the South Pole.https://bit.ly/IM-rainforest

Predators Help Prey Adapt to an Uncertain Future

☛ What effect does extinction of species have on the evolution of surviving species? Evolutionary biologists have investigated this question by conduc-ting a field experiment with a leaf galling fly and its predatory enemies. They found that losing its natural enemies could make it more difficult for the prey to adapt to future environments.

https://bit.ly/IM-predators

How Easily an Infection Can Spread Through Contact

☛ In theory, by now all of us should understand why washing our hands, wearing masks, and keeping up sound hygiene practices is important du-ring a pandemic. But nothing really drives the point home as effectively as a good old-fashioned demonstration. to set up a buffet-style meal for 10 people. In Japan they placed a little fluorescent paint on the hand of one ‘infected’ person to simulate a cough into their hand, and then let the participants have at the buffet for the next 30 minutes. Watch the video here.https://bit.ly/IM-infection

Oink, Oink Makes the Pig

☛ In a new study, neuroscientists demonstrated that the use of gestures and pictures makes foreign language teaching in primary schools more effective and sustainable. The study was conducted with eight-year-olds at a primary

school in Leipzig. In several experiments, the children learned new English vocabulary for 20 minutes per day on five consecutive days. They learned the

words while viewing pictures (sensory enrichment), performing gestures (sensori-motor enrichment), or by listening only (no enrichment). The children’s knowledge of the new vocabulary was tested eight days, two months, and six months after the learning period.https://bit.ly/IM-language

Online Education!

Bioimage Analysis

Neubias Academy is a new initiative, aimed to provide sustainable material and activi-ties focused on Training in Bioimage Analysis. Neubias Academy capitalizes on the success of 15 Training Schools (2016-2020) that have supported over 400 trainees (Early Career Sci-entists, Facility Staff and Bioimage Analysts), but could not satisfy the high and increasing demand (almost 1000 applicants). A team of about 20 members will interact with a larger pool of hundreds of trainers, analysts and developers to bring knowledge and bleeding-edge updates to the community.

More Information

The events are recorded and many are alrea-dy available on the Youtube Neubias Chan-nel. Furthermore, a thread will be opened in the image.sc Forum to report Q&As and to welcome further questions/comments for each event.

https://neubiasacademy.org

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Coffee Break

Newsletter Tip!Register for our Microscopy Newsletter

The “Microscopy channel” on Wiley Analytical Science combines all con-tent from our magazines Imaging & Microscopy and Microscopy and Ana-lysis and more. Wiley Analytical Science is a single subject-based website hosting professional and peer-reviewed content from five well-established publication-based websites in Analytical Science. For every channel you could subscrribe to a specific newsletter. The microscopy newsletter comes out every two weeks and presents you the highlights of recent published news and articles.

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Coffee Break


On April 22nd, the Executive Board of the Eu-ropean Microscopy Society (EMS) announced that the European Microscopy Congress 2020 (emc2020) would not be held in Copenhagen in August.

The Board had been monitoring the situation in the hope that there would be some indica-tion that lockdown measures would have eased enough for emc2020 to go ahead. However, the Danish Government then announced that gath-erings of more than 500 would not be allowed until September at the earliest.

The fact that the decision was ultimately taken out of the hands of the Executive Board is a small blessing. It will make it easier when re-solving contracts with the venue and other con-tractors. This should reduce the financial impact for the organisers.

The decision is a huge disappointment for the large team, headed by the Conference Chair, Professor Klaus Qvortrup. It had dedicated sig-nificant resources to making the event a suc-cess.

“It is the correct and only decision available, but it is still hugely disappointing,” said Klaus. “Scandem has been aiming to bring a congress to Scandinavia since long before we submitted our bid in 2016. We were relishing the oppor-tunity to welcome the microscopy community to Denmark. And, from a wider viewpoint, I feel most sorry for those who entrusted us with

EMC2020 Cancelled Due to Covid-19EMS Executive Board to reveal plans in the coming months

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More information: www.emc2020.eu

their abstracts and who were looking forward to showcasing their work in August. It is also a huge blow for the exhibitors who have been so supportive since the beginning, and I thank them for that.”

Whilst Klaus sympathises with delegates and companies, his input should not be overlooked. Allison Winton is CEO of the Royal Microscop-ical Society who was contracted to organize emc202. She said, “Klaus has given so much of his time, and has been the driving force behind everything. In addition, he has done so with un-relenting enthusiasm and good humour. Klaus had a vision, and together we came very close to achieving it. If it is at all possible, it would be wonderful if he could oversee a large-scale event in Copenhagen sometime in the future.”.

The EMS Executive Board has been in ongo-ing discussions since the decision was taken, and has been considering a range of options in light of the cancellation. Venues that are capable of holding similar-sized events are fully booked years in advance, and there will be a number of events looking to reschedule.

The constitution of the EMS will require any decision to be ratified by its voting members. This vote is scheduled in the near future.

“It is a dreadful shame for everyone in-volved,” said Allison, “But we have to keep things in perspective. For all events – be they in “normal” times, or times like these - our overrid-

ing duty is to keep speakers, delegates, visitors and exhibitors safe. When this is over, the team will pick itself up, and we will be ready for what-ever the EMS decision requires of us. And, for all events, we shall resume with renewed vigour and with a Klaus-like enthusiasm.”

Whatever the decision, the European Micros-copy Congress will continue.

“We are all disappointed because micros-copy is important to us; we care about it and we wish for it to thrive,” said Klaus. “However, we can draw comfort and a degree of pride that we are part of a community that is playing an im-portant role in the fightback against Covid-19. And, we will always have a part to play in future situations like this, and - most importantly - in preventing situations like this in the future. To microscopists everywhere, I wish that you stay safe, and I look forward to seeing you at the next European Microscopy Congress, wherever it is and whenever it is.”

Details of the final decision on emc2020 will be published on the emc2020 website shortly after the vote is taken.


Announcement

After the successful first and second editions in Germany and Italy, the 3rd NanoScientific Fo-rum Europe (NSFE 2020) invites scientists and researchers working in the field of Scanning Probe Microscopy to magnificent Dublin! The scientific focus lies on energy storage and na-noscale functional materials, such as organics, organic/inorganic hybrid semiconductors, nano- and biomaterials, as well as the development of novel nanometrology methods.

The NSFE 2020 will be hosted by Prof. Kim McKelvey, Trinity College Dublin, and by Prof. Brian Rodriguez, University College Dublin. The event is co-hosted and supported by Park Sys-tems Europe and AMBER, Advanced Materials and BioEngineering Research Centre Dublin.

“The Nanoscientific Forum gives us a great opportunity to present our cutting-edge elec-trochemical scanning probe techniques. With

NanoScientific Forum Europe on Scanning Probe Microscopy

Dublin, Ireland, September 23-25, 2020

More information: https://live.parksystems.com/nsfe2020/

these new techniques we aim to improve the resolution and functionality of electrochemical scanning probe microscopy (EC-SPM), and help understand the relationships between surface structure and electrocatalytic activity in energy conversion and storage systems. I’m very excited

to have this honor to host and welcome other scientists from across the scanning probe field and hear about their exciting research.,” proudly comments Kim McKelvey, Trinity College Dublin.

The NSFE 2020 will included keynote and contributed lectures on different SPM applica-tions, poster sessions, and practical live hands-on-sessions on Park Systems AFM instruments.

Science Gallery Dublin, Venue NSFE 2020

PhotonicsViews.com is the international news platform for industry and research in�Optics�Photonics�Laser Technology

Your Daily Quantum of Photonics

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Announcement

http://www.photonicsviews.com

These are strange times. I am writing this at the end of April, and it is safe to assume that by the time that this is published in May, some parts of the world will be taking only their first tentative steps out of lockdown. However, I want to take this opportunity to assure you that the Royal Microscopical Society will be ready and raring to go as soon as we are allowed, and when we can guarantee the safety of speakers, delegates and exhibitors.

The RMS is a charity and is governed by its Council – a group of elected Fellows. They have been in regular discussion to ensure that the Society takes informed decisions to negotiate this challenging period. We were quick to pub-lish a statement on how the Society is respond-ing, and we will continue to monitor and act on advice from the UK Government regarding precautions on Covid-19 and provide updates as and when the situation changes.

As many readers will know, we pride our-selves on delivering high-quality meetings and courses, and it is our duty as a charity to do so. Hence it hurts to postpone or cancel even a sin-gle event. But, like many organisations, we have no choice, and we know that it is in the long-term interests of everyone.

Life after LockdownRMS Is Reorganizing Meetings

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We have done our best to make timely deci-sions, and we feel for those who were planning to attend meetings and courses, and especially for those who had already booked. We know how difficult it can be to secure funding, and we also know (through decades of experience) how much students and delegates take from our courses and meetings. This is why we are doing everything we can to rearrange dates and venues where possible rather than lose an event. And, for the first time in the Society’s history, we are holding virtual meetings. These include the facility meetings for Light Microscopy, Electron Microscopy and Flow Cytometry. They will each focus on safe working practices in the Covid-19 era. Whilst we would never have chosen this, it has been an interest-ing exercise, and the knowledge gained may well prove useful, and could be a blueprint for a new type of meeting in the future.

Many of our members are, and will be, in-volved in the fight against Covid-19. We obtain great satisfaction knowing that some have fur-thered their skills through our courses and have shared their knowledge at our meetings and conferences. We are proud of this, and on behalf of all our members we wish them well and that they stay safe and healthy.

The Society is approaching its 200th an-niversary. In its lifetime it has endured many challenges and setbacks, and it was in existence during the global cholera pandemic in the mid-1800s, and the last global pandemic – Spanish Flu - in 1918. The world has changed a great deal since then, and so has the Society.

There is ample evidence that teams that suc-cessfully endure hardship together come out of it stronger, and I believe that this will be the case for our staff, trustees, and members who give their time so generously.

Whether you are working in a laboratory, supplying vital consumables, or manufacturing the equipment that we rely on, we look forward to seeing you and working with you when life and microscopy return to a degree of normality.

ContactAllison WintonRoyal Microscopical SocietyLondon, [email protected]


RMS in Focus

Dear EMS members,

The current global pandemic situation caused by the outbreak of SARS-CoV-2 (also referred to as Covid-19 or simply the Corona virus) is caus-ing a great amount of uncertainty in general, but to our community in particular.

We would like to inform all our members and interested colleagues that EMC2020, to be held on 23 – 28 August 2020 in Copenhagen, Den-mark is no longer taking place this year.

We will keep you informed on the status in our future newsletters and in the emails from the EMS – as well as on our website and on the EMC webpage.

You will soon receive the 2020 EMS Yearbook. We hope you will enjoy reading the reports on the various activities of our society and members during the past year, especially those written by students who received a scholarship, some of these contain truly inspiring notions from our enthusiastic new generation of microscopists.

EMS webpage: http://bit.ly/EuroMicr

Suggestions and ideas for future newsletters: [email protected]

EMS on Facebook and Twitter:http://bit.ly/EuroMicr-FB and http://bit.ly/EuroMicr-TW

For the present year, EMS has received around 100 applications for scholarships for attending one of the EMC 2020. EMS has de-cided to provide 50 grants to support EMS students to attend this event. However, due to the actual situation, the scholarships are sus-pended.

Currently, the jury of the EMS Outstanding Paper Award is taking a decision on the best papers from 2019. Very high standard nomi-nations were received, the results will soon be announced.

Due to Covid-19, this year EMS Board meet-ing has been performed by videoconference. Many aspects of the society and its members have been discussed, and many important de-cisions were taken.

Finally, we invite you to visit the EMS web-page, which is daily updated to efficiently pro-vide you information from our community.

ContactProf. Dr. Virginie SerinEMS SecretaryCEMES- CNRS & Universite ToulouseToulouse, [email protected]

Josef Zweck, EMS President

Virginie Serin, EMS Secretary

EMS Newsletter #69May 2020

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News from EMS


News from EMS

http://bit.ly/EuroMicr

mailto:[email protected]

http://bit.ly/EuroMicr-FB

http://bit.ly/EuroMicr-TW

Raman microscopy, long used by spe-cialists in purely scientific research, is being employed as a routine analysis

tool in an ever-growing range of fields. Automation has been the key to making the technique easier to use while still of-fering the full benefits of its analytical power and sensitivity. In 2020, devel-opments that introduce self-alignment, modular optical components and remote operation will change what is expected of a fully automated Raman imaging system. The following overview describes these technologies and presents several examples of their application.

Confocal Raman Imaging Microscopy

Raman microscopy is a non-destructive and label-free technique that relies on the Raman effect, in which light scattered by molecules exhibits a distinct shift in energy due to mo-lecular vibrations. It characterizes materials

Automated Raman Microscopy in 2020New Developments in Optimization and Flexible Operation

Damon Strom1 and Thomas Dieing1

by these unique shifts, which are visible in their Raman spectra. The method’s spatial resolution is limited only by physical law and it can be applied to very small sample vol-umes and low material concentrations.

Confocal Raman microscopy uses a beam path geometry that strongly rejects light from outside the focal plane to increase sen-sitivity and enable 3D measurements. Raman imaging acquires a complete Raman spec-trum at each pixel to visualize the distribu-tion of sample components.

Self-Alignment and Self-Calibration

Advanced Raman imaging experiments of-ten include many optical elements in their beam paths. A fully automated instrument controls all the individual pieces from one integrated software suite, and records their settings with each measurement.

Developments in opto-mechanical com-ponents now enable systems to self-align

and self-calibrate. This increases the sam-ple turnover rate and optimizes perfor-mance for every experimental setup. Soft-ware-driven routines ensure the consistency and repeatability of results while also sub-stantially reducing the researcher’s workload by requiring less user input and eliminating potential sources of error.

An automatically aligned and calibrated Raman microscope should always be capa-ble of a lateral resolution under 300 nm and a depth resolution of less than 900 nm with 532 nm excitation. A spectral resolu-tion down to 0.1 cm-1 relative wavenumbers at 633 nm excitation and acquisition times faster than 1 ms per spectrum should also be regularly achievable.

Modular Optical Components

Automated Raman microscopes designed around standardized optical modules with an integrated software environment can incor-

Fig. 1: Correlative Raman imaging measure-ment of a WSe2 flake. A: white-light image. B: Raman image consisting of 10,000 spec-tra. C: smoothed version of B. D: Raman im-age consisting of 102,400 spectra. E: photo-luminescence image with visible grain boundary (white arrow). Color code for Ra-man images: single-layer (red), bi-layer (green), multi-layer (blue).

A

D

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porate new capabilities as they are introduced and be configured as an experiment requires or reconfigured as requirements evolve.

The modules necessary for self-optimi-zation as described above include a calibra-tion source that can validate and calibrate spectrometer gratings, an output coupler that maximizes signal throughput to a spec-trometer, and motorized iris diaphragms that adjust the beam path for optimum contrast and homogeneity in white-light imaging.

The latest multi-laser input coupler tech-nology can adjust the optical elements for each wavelength. It can also configure the beam path to perform measurements with methods complementary to Raman micros-copy while remaining at the same sample position. Excitation laser wavelengths can be chosen to best produce the Raman effect, or to generate or avoid photoluminescence from the sample.

Remote Operation

Remote operation through automated compo-nents allows Raman imaging measurements in environmental enclosures such as glove boxes. This has great utility in semiconductor research and life science, among other disci-plines. Raman microscopes that can align and calibrate themselves can be controlled from another location entirely. Only the mounting of the sample on the microscope stage re-

quires physical interaction. This delivers the full capability of a laboratory instrument from anywhere, including home offices.

Application Examples

The following measurements were performed with a WITec alpha300 apyron automated Ra-man imaging microscope equipped with sev-eral UHTS ultra-high throughput spectrome-ters optimized for different wavelengths. Af-ter the sample was put in place, every step of each measurement was performed completely through the Suite FIVE integrated software and EasyLink handheld controller.

Correlative Raman Measurement of Tungsten DiselenideThe speed and sensitivity of a Raman mi-croscope that has been optimized through automation is demonstrated with the anal-ysis of a tungsten diselenide (WSe2) flake (fig.1). Different layers in the flake are visi-ble in the white-light image (A). In approx-imately 2 minutes, a clear and detailed 75 x 75 µm2 Raman image of 10,000 spectra was recorded (B). The flake shown consists of single-layer (red), double-layer (green) and multi-layer (blue) areas. The same mea-surement after smoothing is shown in (C). A measurement acquired in about 17 minutes of more than 100,000 spectra produced an even sharper image (D). The increased sig-nal to noise ratio was achieved by reducing the pixel size from 750 nm (B) to 230 nm (D). The photoluminescence image (E) shows the same structures as the Raman image and even the grain boundary between the larger and the smaller flake is visible. The integra-tion time was 6 milliseconds per pixel for all measurements.

Raman Image of Moisturizing Shower GelRaman imaging microscopy can be used to visualize different phases and the boundar-ies between them in liquid samples. In this investigation, a moisturizing shower gel had its oil content increased with olive oil. Its phase boundaries were located and the chemical identities of the components in

the emulsion were identified using Raman spectroscopy. Visualizing the boundaries be-tween emulsifiers, surfactants and moistur-izers is essential in their development (fig.2).

3D Raman Image of a Hand CreamAutomated confocal Raman microscopes feature exceptional lateral and depth resolu-tion. This can be clearly seen in a 3D Raman measurement of hand cream. 15 images of 200 x 200 pixels each were acquired in rap-id succession along the z-axis to form a 3D image stack (fig.3).

Conclusion

Investigations of a semiconducting material and cosmetics samples showed the analyti-cal capability of an instrument that requires minimal user input. For the tungsten disele-nide flake, white-light, Raman, and photolu-minescence measurements were carried out at the same sample position with the beam path automatically configured for each method. The benefits of self-optimization routines were also vividly demonstrated by the spatial resolution attained in measure-ments on liquid samples.

The automation of Raman micros-copy will continue to broaden its appeal. The advantages that it initially offered, in user-friendliness and the rate at which sam-ples can be measured, will be supplemented in 2020 by new advances. Self-alignment and self-calibration, modular optical compo-nents, and the ability to be operated remotely have revised the definition of fully auto-mated Raman microscopy.

Affiliation:1WITec, Ulm, Germany

Contact Damon StromTechnical Marketing and EditingWITec GmbHUlm, [email protected]

Fig. 3: 3D Raman image of a hand cream. Blue: water; red: oil; green: one of the hand cream’s moisturizing ingredients dissolved in oil. Scan range: 40 x 40 x 15 µm3.

Fig. 2: Large-area Raman image and zoomed-in views of a mois-turizing shower gel with in-creased oil content. Cyan and blue: aqueous phases; red: oil; yellow: emulsifier. 500 x 500 µm2 (2000 x 2000 pixels), 10 ms per spectrum acquisition time. 532 nm excitation laser, 100x oil-immersion objective with NA = 1.25.

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Controllable Surface Damage by AFMImaging with Higher Eigenmodes and its Advantages

Jesse Putnam1 and Babak Eslami1

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Since introduction of bimodal atom-ic force microscopy (AFM), a wide range of studies have been performed

to provide a systematic and rigorous pro-cedure for selection of imaging parame-ters that lead to the most sensitive imag-ing condition. Such control over dynamics of the cantilever is opening a new capabil-ity in multifrequency to controllably mod-ify surfaces at micro and nano-scale. By selecting insensitive imaging conditions, this study provides the capability to ma-nipulate polymer surfaces.


Scanning Probe Microscopy

Introduction

Since invention of AFM in 1980s, a common practice has been to identify and develop new imaging techniques to in-crease the amount of informa-tion observed by the cantilever in a single pass experiment. With the same goal in bimodal AFM, introduced by Rodriguez and Garcia [1], the AFM canti-lever is simultaneously excited with two frequencies instead of the single excitation frequency in amplitude modulation AFM (AM-AFM). The first eigen-mode is controlled through a feedback loop to capture topo-graphical information while the second eigenmode is excited in an open loop being responsible for compositional contrast via its phase channel. Since there are two eigenmodes consisting of an excitation frequency and drive amplitude for each, there have been studies to provide guidelines to select the imag-ing parameters that can pro-vide the most sensitive imaging. For example, it is shown that the ratio between the second eigenmode to first eigenmode amplitude play a major role in the phase contrast observed [2]. Additionally, it is found that the excitation frequency and am-plitude can be optimized based on energy quantities (i.e., virial and dissipated power) [3,4]. All of the procedures heavily rely on the effect of tip-sample force interactions on the dynamics of the cantilever. In other words, in all of these studies it is shown that by selecting the correct im-aging conditions, one can en-hance the cantilever’s sensitivity to tip-sample forces. Therefore, bimodal AFM imaging can be performed with minimum or no damage to surfaces. It is shown that bimodal AFM has the ca-pability of capturing air na-no-bubbles on graphene surfac-es in liquid environment if the excitation frequency is selected correctly [5].

Building upon these stud-ies to have full control over the dynamics of the cantilever, this study is focused on excit-ing the cantilever so it inten-tionally damages surfaces in either elastic or plastic regions. Depending on the cantilever and imaging parameters (pri-marily the response amplitude and dynamic spring constant), the sensitivity of the higher eigenmode can be significantly decreased and cause surface damage (compression). With this capability the user can use bimodal AFM as a nanomanu-facturing tool which can indent surfaces which can be beneficial for modifying drag coefficient of surfaces in aerospace indus-try and drug delivery applica-tions in biomedical field.

Theoretical Consideration

In a conventional way of run-ning bimodal AFM measure-ment, the cantilever is excited with the first and higher eigen-mode simultaneously. The first eigenmode mode is reserved for topographical information while the higher eigenmode (i.e., mostly the second eigenmode) is used for material composition. In this work, a set of single tap-ping mode experiments are per-formed to study the effect of dy-namic spring constant of higher eigenmodes in imaging. It is through these studies that these results represent a promising capability for modifying surfac-es. Equation (1) is a non-dimen-sionalized equation of motion used to model simple harmonic oscillations given a sinusoidal excitation. In this equation, the tip-sample force interaction (Fts) shown as the last term on the right side of the equation is di-vided by the product of the dy-namic spring constant (k) and the free oscillation amplitudes (A0). The quality factor is shown by Q. The normalized tip posi-tion is shown by z_.

Equation 1

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Fig. 1: PS-PMMA polymer imaged in single tapping mode AFM using the 3rd ((a) and (b)), 2nd ((c) and (d)), and 1st ((e) and (f)) eigenmodes with constant amplitude of 45 nm for all cases, set point 60%, f1 = 48 kHz, f2 = 294 kHz and f3 = 818.5 kHz. Top row is topography. Bottom row is phase contrast.

References: http://bit.ly/IM-Eslami

It is shown by Ebeling et al. [6] that the third eigenmode can be used to visualize the sub-surface features based on the same theory. The theoretical discussion is based on the fact that as the product of kiAi-0 increases, the effect of tip-sample force interactions (Fts) on the dynamics of the cantilever de-creases. Based on the common scheme of bimodal AFM, the cantilever is simultane-ously excited at two different eigenfrequen-cies, and the response in analyzed by two separate lock-in amplifiers to determine each eigenmode’s amplitude and phase. The first or fundamental eigenmode is reserved to measure the topography and the second eigenmode provides compositional contrast.

The effect of tip-sample force interaction on the dynamics of the cantilever (modeled as a mass-spring-dashpot system) is primar-ily determined by its dynamic spring con-stant and the selected amplitude. It is shown that as the product of dynamic spring con-stant and oscillation amplitude increases the sensitivity of cantilever decreases due to diminishing effect of tip-sample force interactions on the dynamics of the cantile-ver. Therefore, for a given oscillation ampli-tude (for example 100 nm), if the cantilever is excited by the second eigenmode, it will interact with the surface with higher forces compared to the first eigenmode. This is pri-marily due to the fact that second eigenmode force constant is about 37 times greater than the first eigenmode. Based on this theory, one can use higher eigenmodes of a cantile-ver to controllably apply higher forces over the surface and modify surfaces at micro- and nano-scale.

Experimental Results

For the experimental validation a model sample has been designed consisting of a blend of two soft polymers (PS-PMMA) film fabricated by spin coating on a silicon oxide substrate. Figure 1 represents three different experiments done by single tapping mode AFM. Figure 1(a) and (b) represent the region that was scanned by the third eigenmode of

the cantilever (818.5 kHz) with oscillation amplitude of 40 nm. Figure 1 (c) and (d) rep-resent the region that was scanned by the second eigenmode of the cantilever (294 kHz) with the same oscillation amplitude of 40 nm. Figure (e) and (f) represent the region that was scanned by the first eigenmode of the cantilever (48 kHz) and oscillation am-plitude of 45 nm. By Sader’s method, the first eigenmode spring constant was found to be around 2.7 N/m. The top row of figure 1 represents the topography measurements. The bottom row of Fig 1 represents the phase images (i.e. compositional mapping). All of the images are 1 μm by 1 μm. As shown in figure 1, by using higher eigenmodes the damage to the surface was more drastic. In addition to clear topographical changes of materials, it is clear how the area that was imaged by the third eigenmode has lower phase values as well. It should be noted that lower phase values generally mean stiffer surface properties. These results can open up the field of scanning probe microscopy to a new set of capabilities. The fact that higher eigenmodes of the AFM cantilever have the capability to compress surfaces in different regions (i.e., elastic or plastic) can be useful in material science, drug delivery, and na-no-manufacturing.

Concluding Remarks

Here, a newly found advantage of imag-ing with higher eigenmodes has been de-scribed in single tapping mode AFM. Unlike

the common practice in the field to image surfaces with the most sensitive configura-tion to minimize damage, this work shows as long as the product of dynamic spring constant and oscillation amplitude is large enough, it can cause permanent changes on the surface topography and material prop-erties. This study was done on PS-PMMA polymer films. This added capability can offer significant advantages in soft matter nano-manufacturing field to modulate the surface drag coefficient by manipulating the surface roughness.

Acknowledgement

This work was supported by Faculty Devel-opment Grant and Provost Grant at Widener University.

Affiliation1Mechanical Engineering Department, Wid-ener University, Chester, USA

ContactDr. Babak EslamiMechanical Engineering DepartmentWidener University Chester, [email protected]



Microscopy on Wiley Analytical Science

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Read all contents of our journals Imaging & Microscopy and Microscopy & Analysis in the microscopy channel on Wiley Analytical Science. All issues for reading in magazine format can also be found here.

In this article, the application of TERS and the experimental challenges that limit its broad application are described. Scanning

probe microscopy (SPM) techniques such as AFM and STM can provide nanoscale topo-graphic information but are “chemically blind”. Raman spectroscopy is a vibration-al spectroscopy technique that provides chemical information, but its spatial reso-lution is limited by the diffraction of light. Tip-enhanced Raman spectroscopy (TERS) combines both, providing rich chemical in-formation with nanoscale resolution.

TERS Principle

Raman spectroscopy is a vibrational spec-troscopy technique that provides rich chem-ical information on materials and molecules. It is based on the inelastic scattering of light and only produces weak signals (compared to IR spectroscopy). This limitation has been overcome with surface-enhanced Raman spectroscopy (SERS), where the Raman signal is enhanced using the plasmonic resonance of metal nanostructures. The electromagnet-ic energy is then concentrated in “hotspots”, which are found around sharp features and in gaps between particles. SERS is surface sensitive because molecules more than a few nanometers away from the metal are not af-fected by the hotspot and it has a low detec-tion limit because the signal is enhanced by up to 8 orders of magnitude in the hotspot. SERS can be turned into a microscopy tech-nique by using the sharp apex of an AFM or STM tip as a unique hot spot (fig. 1). This tip is then scanned over the surface of a sample to record a Raman map with nanoscale reso-lution. This is known as tip-enhanced Raman spectroscopy (TERS). TERS has developed into an attractive technique for the fundamen-tal study of light–matter interaction and the analysis of 2D materials (polymers, graphene, semiconductors…). It is also promising for the study of catalysts and electrochemical pro-cesses in situ and for the understanding of biological samples at the nanoscale.

Domains of Application

TERS is particularly well-suited for the study of nanomaterials and 2D materials,

Tip-Enhanced Raman SpectroscopyA Surface Spectroscopy Technique at the Nanoscale

Guillaume Goubert1, Giovanni Luca Bartolomeo1, Renato Zenobi1

including carbon nanotubes, graphene or transition metal dichalcogenides [1]. TERS can provide important insight on the prop-erties and performance of these materials, which depend on the presence of point de-fects, dopants or grain boundaries. The sam-ple volume in a TERS experiment is a few nm3, which allows TERS to achieve a high signal-to-noise ratio for nanoscale features, a feat impossible for normal Raman spec-troscopy. TERS at ambient conditions usual-ly achieves a resolution around 3-5 nm. Ad-ditionally, Raman spectroscopy can be used in aqueous conditions, an advantage over IR spectroscopy. In situ TERS work has been focused on the study of electrochemical pro-cesses at the surface of an electrode, which is both of great fundamental and practical importance [2].

Early on, it was believed that the electro-magnetic field could not be confined more tightly than to ≈15 nm. However, in 2013 a study showed that it was possible to obtain sub-molecular resolution with TERS, demon-strating a resolution below 1 nm [3]. A res-olution as good as 0.15 nm (fig. 2) [4,5] has been demonstrated in UHV at cryogenic tem-perature, comparable with the lateral reso-lution in STM. These results have pushed theoreticians to refine the description of light-matter interactions to explain extreme light localization. The current leading expla-nation is that it is possible, at cryogenic tem-peratures, to stabilize single atomic protru-sions that concentrate the electromagnetic field beyond the classical limit.

“It’s the Tip, Stupid!”

The properties of the plasmonic tip broadly control the success of a TERS experiment. An ideal tip should provide both a high spa-tial resolution and a strong signal enhance-ment. The tip material should be (photo)chemically inert to ensure a long lifetime. The tip manufacturing procedure should be highly reproducible, in particular when it comes to signal enhancement. Unfortunate-ly, the reality is still far from this ideal sit-uation and the TERS community is invested in the improvement of tip fabrication (both AFM- and STM-TERS tips). Au and Ag are the only two tip materials used in practice. Tips made of Au have a long lifetime but are less enhancing than Ag tips, which un-fortunately only stay active for a few hours in ambient conditions. STM-TERS tips are obtained by etching/cutting a piece of met-al wire. Even though the enhancement can vary greatly from one tip to the other, the high throughput and low cost of the fabrica-tion procedure makes the trade-off accept-able for most studies.

Most of the research on tip fabrication is focused on AFM-TERS tips, which are more expensive and complex to fabricate. The standard approach consists in coating a reg-ular AFM probe with a thin layer (less than 100 nm) of a plasmonic metal using phys-ical vapor deposition (PVD) [6]. It is simple but slow and requires expensive equipment. The yield of tips fabricated via this method can be high, but extreme care needs to be

Fig. 1: a) Schematic representation of a TERS setup. b) simulation of field enhancement in a TERS experiment using a 532 nm excitation laser and an Ag probe (15 nm in diameter). Adapt-ed from reference [7] with permission from Springer Nature.



More on Raman: http://bit.ly/WAS-Raman2

References: http://bit.ly/IM-Goubert

Fig. 2: a) Schematic representation of the interaction between TERS tip, sample (a single molecule in this case) and substrate. b) Sample spectra acquired in different regions of the molecule (indi-cated in a). c) heat map of the peaks selected in b), showing the intensity of the different Raman modes in space. d) Intensity distribution of the C-H stretching mode, used to evaluate the spatial resolution (shown in e) of the dot in the image. Adapted from reference [5] under a CC BY license.

taken during every phase of the production to prevent environmental contamination [7]. The use of adhesion layers consisting of sev-eral different metals is sometimes performed. Here, some interesting alternative fabrication techniques are presented (fig. 3).

An innovative electrochemical deposition method has been proposed recently [8]. The authors demonstrated that highly enhancing and stable tips can be produced, without the need for a vacuum system, even though the preparation process is not trivial. Recently, commercial suppliers of metal coated tips for TERS have appeared. Commercial probes rep-resent an (expensive) alternative, which is a welcome development. Micro- and nanofab-rication methods have also been explored, for example, the functionalization of the tip by a Ag nanowire results in tips with a high spatial resolution and a very long lifetime [9]. Follow-ing the same direction, Hecht and coworkers showed how to precisely control the size and shape of the tip using FIB to adjust the res-onance frequency of the plasmonic structure and maximize its enhancement [10]. This pro-cess is complex and time consuming and has not been widely implemented.

The development of protection layers for metal coated tips is another important ave-nue of research. Several groups demonstrated the use of a thin dielectric material protection

Fig. 3: a) AFM-TERS tip obtained via PVD. A 20-50 nm layer of Ag is evaporated onto a regular AFM cantilever. The coating appears irregular and uneven near the tip apex. Adapted from reference [6] with permission from the American Chemical Society. b) Elec-trochemically coated AFM-TERS tip. The Ag layer is smoother than in a typical PVD-coat-ed tip. Adapted from reference [8] with per-mission from the Royal Society of Chemistry. c) Ag-nanowire functionalized AFM tip. These probes have a very high spatial resolution and are stable for a long time. Adapted from refer-ence [9] under a CC BY license. d) Fork-shaped apex of an AFM tip obtained with FIB. Adapted from reference [10] with permission from IOP Publishing.

layer (such as Al2O3 or SiO2) [11,12] or even of a thiol SAM [13] over the plasmonic metal. This method effectively increases the stabil-ity of tips but also lowers signal enhance-ment by increasing the distance between the sample and the plasmonic nanostructure. All the above-mentioned alternative fabrication methods have strengths and weaknesses, and none of them has become a standard in the TERS community. Indeed, a universal TERS tip fabrication method is yet to be found: a quick and economical method for the fabrication of stable and (only!) highly enhancing tips.

Conclusions

TERS is a unique and valuable technique, especially for the characterization of 2D materials and for fundamental studies of molecules and surfaces in vacuum. Sub-mo-lecular chemical resolution has successfully been obtained, and imaging of single bonds is at hand. Like for other SPM techniques, the tip is the most important, yet the hardest

part of the experiment to control in TERS. The development of a reliable and repro-ducible tip manufacturing will be a major make-or-break point for the broader appli-cation of the technique. The current devel-opment of TERS for in situ electrochemical studies and possible applications to biolog-ically relevant samples (ambient or in situ) show that this technique has a bright future. Its chemical information content, its resolu-tion and its compatibility with a large range of experimental conditions are unmatched.

Affiliation1Department of Chemistry and Applied Bio-sciences, ETH Zurich, Zurich, Switzerland

Contact Dr. Guillaume GoubertDepartment of Chemistry and Applied BiosciencesETH ZurichZurich, [email protected]



Recent investigations of lithium in-tercalation in niobium tungsten ox-ides have demonstrated their high

potential as electrode materials for bat-tery applications. The flexible structural chemistry of these oxides is based on the connection of metal-oxygen octahedra to a multitude of frameworks exhibiting different degrees of order. The diversity is demonstrated in this comparative electron diffraction and HRTEM study on example of the real structures of two samples with slightly differing metal:oxygen ratios, namely Nb8W9O47 and Nb7W10O47.5.

Introduction

Unravelling the relationships and struc-tures of the manifold phases and structur-al variants in the pseudo-binary system Nb2O5-WO3 had been a major research topic in solid state chemistry about 50 years ago [1]. In the WO3-rich part of this system, two well-defined phases ex-ist. Nb8W9O47 crystallizes in a threefold superstructure of the tetragonal tungsten bronzes (TTB) caused by the systematic oc-cupation of 1/3 of the pentagonal channels with metal-oxygen strings (fig. 1a) [2]. The structure of Nb4W7O31 represents an inter-growth of the TTB and the ReO3 type (fig. 1b). Remarkably, this was one of the first crystal structures ever established by in-terpretation of a HRTEM image [3]. This skillful and pioneering work of Iijima and Allpress contributed not only to the under-standing of the rich structural chemistry of these bronze-type phases but also to the advancement of the HRTEM method itself at the beginning of the 1970ies. Ever since, HRTEM has been the method of choice for the comprehensive structural characteriza-tion of these phases. The complete elucida-tion of the Nb8W9O47 structure including the oxygen positions by focus series recon-struction certainly represents an outstand-ing achievement [4]. Niobium tungsten ox-ides have recently regained high attention because of their outstanding electrochem-ical properties – in particular a revers-ible fast Li-ion exchange - that promotes them to potential battery materials [5,6]. It is the large number of empty channels

Bronze‐Type Niobium Tungsten OxidesA Glimpse into the Structural Complexity of a Promising Battery Material

Frank Krumeich

of different shape and size in these oxide frameworks that evidently enables a swift incorporation of considerable amounts of lithium [7]. The variety of the structures appearing in this system is demonstrated in the present study of a selected sample with the composition Nb7W10O47.5 which is in between the two phases Nb8W9O47 and Nb4W7O31.

Results and Discussions

The electron diffraction pattern of Nb8W9O47 shows sharp reflections corresponding to the threefold TTB superstructure (fig. 3a). HR-TEM investigations confirmed the high de-gree of order of this phase with only some twin planes and grain boundaries present between perfectly ordered areas as it had

Fig. 1: Structural models of (a) Nb8W9O47 (a = aTTB, b = 3bTTB, c = cTTB) and (b) Nb4W7O31 (a = 2aTTB, b = 2bTTB, c = cTTB). All polyhedra are connected by corner-sharing in direction of view. A cell of the tetragonal tungsten bronze type (TTB) which is marked yellow consists of four penta-gons of MO6 (M=Nb,W) octahedra arranged around a central square. Some of the pentagonal channels are occupied by additional metal-oxygen strings (filled circles). In Nb4W7O31, a block of 4x4 cornersharing octahedra (cutting of the ReO3 structure type, blue background) is coherently incorporated into the TTB framework.


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Fig. 3: SAED patterns of (a) Nb8W9O47 and (b) Nb7W10O47.5. (c) Fourier-filtered version of the HRTEM image shown in figure 2 with the domains corresponding to the Nb8W9O47 and Nb4W7O31 structures colored and (d) Fourier transforms (FT) of the whole HRTEM image and of the areas framed in (c).

Fig. 2: HRTEM image of Nb7W10O47.5 (Microscope: JEM-ARM300F Grand ARM).

already been observed before in several studies (e.g. [3]). The sample Nb7W10O47.5 was prepared by high-temperature oxidiza-tion (T = 1250°C) of Nb7W10O47. This phase is isostructural to Nb8W9O47 but contains Nb4+ be-sides Nb5+ [8]. By its oxidation, a fully oxidized sample with the composition Nb7W10O47.5 is ob-tained. According to the phase diagram, Nb7W10O47.5 is in be-tween Nb8W9O47 and Nb4W7O31 and thus a separation into these phases is expected [1,9]. In fact, the SAED patterns of many crys-tallites investigated (fig. 3b) still show sharp reflections of the threefold TTB superstructure as in Nb8W9O47. This observation indicates a rather high degree of crystallinity and the preserva-tion of the starting structure to a certain extend. However, ad-ditional diffuse scattering of cir-cular shape appears in a typical SAED pattern (fig. 3b) that points to the presence of extended re-gions with a decreased amount of order. To characterize the real structure causing this effect in the diffraction pattern, HRTEM investigations of different areas of the crystal fragment giving rise to the SAED pattern shown in figure 3b were performed. Defective domains of the three-fold TTB superstructure are still

predominant in many areas. In contrast to that, extended do-mains of both phases Nb8W9O47 and Nb4W7O31 are present in the HRTEM image of the defect-rich area reproduced in figure 2. These domains are intimately inter-grown with each other as well as with regions showing little order. The FT of this image contains the spots of both structures as well as diffuse intensity (fig. 3d). A thor-ough analysis reveals that this FT pattern basically represents an overlay of the FTs arising from different areas of the HRTEM im-age. FTs of these areas show the peaks of the threefold TTB (area1)

and of the 2x2 TTB superstruc-ture (area2), respectively, whereas that of less ordered areas exhibit a square array of spots corre-sponding to the TTB substruc-ture along with high amount of diffuse intensity (area 3). In fig-ure 3c, the domains of the two crystalline phases are colored for clarity. It is well-known that such diffuse scattering is caused in TTB-type structures by vary-ing occupations of the pentag-onal channels by metal-oxygen strings in an otherwise unaf-fected TTB framework [10,11]. This leads to short-range order, an intermediate state between a completely random arrangement and a perfectly ordered structure. Remarkably, different types of local arrangements of occupied pentagonal tunnels that resul-tantly cause diffuse scattering of different shapes occur in this system [12]. The proximity of well-ordered and disordered do-mains observed here is made possible by the close structural relationship between the TTB and the ReO3 type that facilitates their intimate intergrowth [13].

Conclusions

The results discussed here con-firm previous observations that any deviation from the ideal ratio metal:oxygen = 17:47, as

realized in the thermodynam-ically stable and well-crystal-lizing phase Nb8W9O47, leads to less ordered Nb-W oxides. The multitude of structural details present already in a single image of a ca. 200 x 200 nm2 region in Nb7W10O47.5 demonstrates the unique power of the HRTEM method to unravel local arrange-ments. In comparison to that, diffraction methods that provide information averaged over a certain area are outgunned. The comprehensive characterization of such oxides’ real structure is certainly indispensable for a deeper understanding of lithium intercalation and the mechanism of Li ion diffusion into the re-spective specimen.

Acknowledgements

Electron microscopy was done at the Scientific Center for Op-tical and Electron Microscopy (ScopeM) of ETH Zurich.

ContactDr. Frank KrumeichLaboratory of Inorganic ChemistryDepartment of Chemistry and Applied BiosciencesETH ZurichZurich, [email protected]

Electron Microscopy


Fatigue of Concrete Examined on the NanoscaleTEM Studies of Fatigue-Induced Changes in the Cement Paste of UHPC

Gunnar Schaan1,2, Sebastian Rybczynski2,3, Frank Schmidt-Döhl2, Martin Ritter1

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Ultra-high performance concrete (UHPC) is a specialized form of con-crete distinguished by its high com-

pressive strength of over 150 N/mm2, which is much higher than in ordinary concrete. This strength is achieved by using various types of high-strength, fine-grained aggre-gate optimized for packing density, such as quartz sand and quartz powder, as well as by utilizing very low water-to-cement (w/c) mass ratios as low as 0.15, as opposed to ca. 0.45 in ordinary concrete [1,2]. In ad-dition to decorative purposes as béton brut, its favorable properties are taken advan-tage of in structures such as bridges, high-rise buildings and wind-turbine towers.


Electron Microscopy

Fig. 1: Lifetime curve of a cyclic fatigue test until failure of the sample (a) and photos of sample in lifetime phases I/II (b), III (c; emerging cracks marked with arrows) and after failure (d).

Introduction

In the time since the first reports on UHPC in the early 1990s, a large amount of work has been published on the composition, prop-erties and applications of UHPC [3-5]. The priority program SPP 1182 of the German Research Foundation (2005 – 2012) created fundamental knowledge on this material, while the ongoing SPP 2020 deals with its behavior in the event of mechanical fatigue.

Although both scanning (SEM) and trans-mission electron microscopy (TEM) have been applied to cementitious materials over the course of the last 40 years [6-10], few if any reports so far focused on the effects of mechanical fatigue on the microstructure of mineral building materials. The aim of this article is to investigate these effects, down to their origins on a nanometer scale, using scanning transmission electron microscopy as well as focused ion beam (FIB) sample preparation.

Experimental Methods

The UHPC formula consists of ordinary Port-land cement with a w/c ratio of 0.24, quartz

sand and quartz powder as aggregate, na-noscale silica fume and a polycarboxylate ether superplasticizer. The concrete is cast in cylindrical samples (height 180 mm, di-ameter 60 mm) and stored under water for hardening for at least 56 days before testing.

Samples are subjected to uniaxial load-ing using a hydraulic press. A cyclic com-pressive load between upper and lower lev-els of 80 and 5%, respectively, of the short-term compressive strength is applied with a frequency of 1 Hz in a triangle waveform. The macroscopic behavior of the sample is monitored by measuring strain as a function of load cycles using strain gauges. We stop fatigue testing at desired points on the life-time curve and extract suitably sized samples for electron microscopy from the cylinders. Figure 1 displays a representative lifetime curve of a sample tested until complete fail-ure and photos of samples from stages I to II, III, and after failure, respectively.

Results

On the micrometer scale accessible with light or scanning electron microscopy, no microcracks or other structural changes

clearly related to mechanical fatigue are discernable, so these changes have to oc-cur on a nanometer scale. To achieve the necessary resolution, TEM was performed on UHPC lamellae (thickness 100 – 200 nm) prepared by FIB techniques. The lamellae were extracted on the boundary of aggre-gate and cement paste matrix to ensure all constituent components of UHPC are pres-ent in the TEM sample.

Figure 2 contrasts high-angle annular dark field (HAADF) STEM images of UHPC cement paste in a pristine state before test-ing (left) and after early-stage fatigue (phase I, 14 load cycles; right). In a pristine state, the calcium silicate hydrate (C-S-H) compounds constituting the cement paste exhibit a loosely packed, fibrous morphology known to be characteristic since the 1980s [11]. After early-stage fatigue, the cement paste already appears much more densely packed. Additionally, several needle- or lath-shaped regions have formed with a length of 150 – 250 nm and a width of 20 – 35 nm appearing darker than the surrounding material. Other components of the material, such as grains of aggregate or unhydratized cement clinker, and silica fume particles, remain unaffected.

With progressing fatigue, both phenom-ena become more and more pronounced. While the darker regions do not grow in size, they do grow in number and become increasingly dark relative to the surrounding regions of the cement paste (note the differ-ence in image contrast between cement paste and silica fume particles). Figure 3 displays HAADF images of samples in phase II (150 load cycles; left) and after failure (25,000 load cycles; right).

In principle, the decreasing HAADF image contrast of these regions may be the result of three causes: 1) a change in chemical composition, with a decreased concentra-tion of heavier atoms in the material, 2) a lower sample thickness, or 3) a decreased density of material. Ruling out the first two sources, the darker regions in the cement paste were interpreted as crack precursors: regions depleted with C-S-H compounds in an increasingly dense matrix.

Near the end of the sample’s lifetime, outright nanocracks form in strongly densi-fied regions of the cement paste. Addition-ally, multiple crack precursors with a similar spatial orientation close to the crack itself were observed. Mercury intrusion poro-simetry (MIP) performed on samples in the pristine state and after failure indicates a marked increase of pore volume in the range between 30 and 500 nm pore radius, caused by fatigue-induced structural changes. How-ever, a significant decrease in pore volume is not observed in any range of pore radius.

Electron Microscopy


Figure 4 displays an HAADF image of a nanocrack after failure of the sample and MIP data of pristine and after-failure sam-ples.

Conclusions

It has been concluded that mechanical fa-tigue of ultra-high performance concrete manifests as nanometer-scale crack precur-sors in the cement paste matrix, beginning at the very onset of fatigue. These precur-sors grow in number and increasingly de-plete with C-S-H material as fatigue pro-gresses. A pileup of these precursors can lead to the formation of nanocracks, which can in turn cause a macroscopic failure of the sample.

The structural changes described in this work can only be observed using trans-mission electron microscopy. Although this method is not commonly used in the char-acterization of mineral building materials, it can contribute greatly to the understand-ing of mechanical fatigue and deteriora-tion.

Acknowledgements

This research is funded and supported by the German Research Foundation (Deut-sche Forschungsgemeinschaft DFG): priority program SPP 2020 “Cyclic deterioration of Ultra-High Performance Concrete in an Ex-perimental-Virtual Lab”.

Affiliations1Electron Microscopy Unit, Hamburg Uni-versity of Technology, Hamburg, Germany2Institute of Materials, Physics and Chem-istry of Buildings, Hamburg University of Technology, Hamburg, Germany3Institute of Solids Process Engineering and Particle Technology, Hamburg University of Technology, Hamburg, Germany

ContactDr. Gunnar SchaanElectron Microscopy UnitHamburg University of TechnologyHamburg, [email protected]

Fig. 2: HAADF STEM images of cement paste in a pristine UHPC sample (left) and after early- stage fatigue (right; some nanocrack precursors marked with arrows). Scale markers in all STEM images equal 200 nm.

Fig. 3: HAADF STEM images of cement paste in UHPC samples after phase II fatigue (left) and af-ter failure of the sample (right; some nanocrack precursors marked with arrows). Note the increasing number and darker image contrast of nanocrack precursors with progressing fatigue.

Fig. 4: HAADF STEM images of a nanocrack in the cement paste of UHPC after fatigue-induced failure (left; some crack precursors marked with arrows) and MIP data (right) showing an increase of pore volume in the 30 – 500 nm pore radius range.

More on Material Sciences: https://bit.ly/WAS-Material

References: https://bit.ly/WAS-Schaan

Electron Microscopy


Second Harmonic Generation Microscopy of the Living Human Eye

Visualizing In Vivo Human Ocular Tissues with Two-Photon Microscopy

Juan M. Bueno1, Francisco J. Ávila2 and Pablo Artal1

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omV isualization and analysis of trans-

parent living ocular tissues at mi-croscopic scale have been a chal-

lenge during the last decades. Nowadays clinical devices only allow imaging cellular corneal structures; however, collagen fi-bers of the stroma (90 % of the corneal thickness) cannot be imaged. It has been recently reported a compact Second Har-monic Generation (SHG) microscope able to image the living human cornea and the sclera for the very first time.


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Introduction

Second Harmonic Generation (SHG) mi-croscopy is a nonlinear imaging tool that provides information on non-centrosym-metric and chiral structures [1]. The phe-nomenon is based on the interaction of two excitation infrared photons from a femtosecond laser with the sample under analysis. These turn into an emitted sin-gle photon that has always half the exci-tation wavelength (no energy loss). This is a sub-micron resolution technique with in-herent confocal properties, used to visual-ize type-I collagen tissues without labeling procedures.

The cornea and the sclera of the eye are paradigmatic examples of collagen-based tissues [2]. From a clinical point of view, the visualization of the transparent colla-gen fibers of the corneal stroma has been challenging. Actual clinical devices are only effective for cellular imaging within the dif-ferent corneal layers. To our knowledge, a commercially available instrument able to show the corneal collagen fibers at micro-metric resolution does not exit.

In 2002, Yeh and co-authors reported the use of SHG microscopy for imaging the cor-neal stroma [3]. Since then, many authors have centered their research in exploring the fiber distribution in corneas of both humans and animal models [2], not only in healthy

conditions but also under pathological ones [4]. Although SHG microscopy of the sclera can also be found in the literature, the inter-est in this part of the eye has been signifi-cantly reduced.

However, the use of this powerful tech-nique has been mainly limited to the study of ex vivo tissues and was never applied in living human eyes before. Some experi-ments tried to image the in vivo cornea of animal models, but the success was very limited. The animals were always anes-thetized and immobilized. Moreover, flu-orescent viable dyes were also used to get two-photon fluorescent signal from stro-mal cellular layers and some nerves [5]. Only Latour et al. were able to observe col-lagen fibers in rats, using an immersion objective together with an ophthalmic gel and an aplanation device to minimize eye movements [6].

At the Laboratorio de Óptica of the Uni-versidad de Murcia in Spain it has been able, for the very first time, to build a pro-totype of a SHG microscope to successfully image the collagen fibers of both the cor-nea and the sclera, in the living human eye [7]. Although our main interest was colla-gen visualization, 2-photon fluorescence images from other ocular structures were also obtained (corneal nerves, trabecular meshwork, individual cells within the jux-ta-canalicular tissue).

Fig. 1: Scheme of the SHG microscope used for in vivo human eyes.

Methods

Clinically-oriented Second Harmonic Imaging MicroscopeFor the purpose of this work, a compact proto-type of a SHG microscope has been developed. The clinical instrument was mounted on a 40x25 cm2 platform and allows measurements in living humans. Figure 1 shows a schematic diagram. A mode-locked (76 MHz repetition rate) Ti:Sapphire laser was used as illumina-tion source (λ=800 nm). The beam passes a XY scanning system and a dichroic mirror (to split incoming and outgoing pathways), be-fore reaching the eye through a long-working distance objective (20x, NA=0.5). This type of non-immersion objective avoids eye-contact operation and overpasses the limitation of us-ing aplanation devices and ophthalmic gels. The beam scans the ocular area of interest, and the emitted nonlinear signal is directed towards a spectral filter and the detection unit. A XYZ adjustable chinrest was attached to the platform to ensure the comfort for the subject, to minimize movements during assessment and to facilitate the eye’s alignment operation. During measurements the subjects were also asked to stare at a fixation target. Two addi-tional cameras were used to control for the correct position of the eye and the incidence point of the laser spot. The entire device was controlled through a data acquisition card and custom C++ software.

Light Microscopy


More on 2-photon-microscopy: http://bit.ly/WAS-2PM

References: http://bit.ly/IM-Bueno2

Safety Considerations for Experimental Imaging To ensure safe image acquisition, both expo-sure time and image size must verify certain conditions that were extensively reported in reference [7]. In brief, since safety limits for this kind of experiments were not clear-ly established, we proceed to compute the irradiance limits to protect the ocular struc-tures. Both the International Commission on Non-Ionizing Radiation Protection and the ANSI Z136.1-2000 standards were consid-ered and the maximum permissible expo-sure (MPE) determined. The worst scenario was assumed: a stationary spot instead of a dynamic (scanned) one. The final image set-tings used for the actual experiment allowed being several orders of magnitude below the MPE: 0.5 s for an image size of 300x300 µm, with an incident laser power of 20 mW at the sample’s plane.

Results

SHG images of the corneal stroma in one of the volunteers involved in the exper-iment are shown in figure 2. Locations correspond to the corneal apex (fig. 2a) and the periphery (fig. 2b). Images were acquired according to the experimental conditions referred above. The distribu-tion of collagen fibers at both locations is clearly different. Whereas at the apex cer-tain interweaving is present, this pattern turns into a more aligned distribution at peripheral areas. It can be observed that in vivo SHG images provide enough contrast to make clearly visible the collagen fibers. Moreover, the distribution seen is coherent with that previously reported in ex vivo human corneas [2].

Apart from the corneal stroma, other collagen-based ocular structures were also imaged under the same living experimen-tal conditions. SHG images of figure 3 were recorded at the corneal limbus (bor-der between the transparent cornea and opaque sclera, figure 3a) and the sclera (fig. 3b). Details of the collagen architecture of human corneal limbal area were elucidated. The limbus contains stem cells. Despite the opacity of the scleral tissue, the fibers could also be seen.

Conclusions

A clinically-oriented non-contact SHG mi-croscope has been developed to image col-lagen-based tissues in the living human eye. It provides non-invasive high-resolution structural imaging without labeling tech-

Fig. 2: SHG images from volunteer #2 corresponding to the anterior stroma of the corneal apex (a) and the peripheral cornea (b). Scale bar: 50 µm.

Fig. 3: SHG images from volunteer #1 corresponding to the corneal limbus (a) and the sclera (b). Scale bar: 50 µm.

niques. The damage thresholds for safe per-formance of the device have been accurately established and defined. Results presented here represent the first recording of in vivo SHG images of the human eye. These bring out the potential of this technique as a tool in clinical environments for early diagnosis and tracking of ocular pathologies, and sur-gery follow-ups.

Acknowledgements

The authors thank A. Gambín for the help, ideas and technical support during the dif-ferent steps of the experiment.

Affiliation1Laboratorio de Óptica, Campus de Espinar-do (CiOyN), Universidad de Murcia, Spain2Departamento de Física Aplicada, Universi-dad de Zaragoza, Spain

ContactProf. Dr. Juan M. BuenoLaboratorio de Optica (LO·UM)Centro de Investigacion en Optica y Nanofisica (CiOyN)Universidad de MurciaMurcia, [email protected]

Light Microscopy


Here applications and current de-velopments in Optical Coherence Tomography are reviewed with re-

spect to novel light sources and accessible spectral ranges. Additionally, the focus is on use cases which are beyond biomedical imaging, like non-destructive testing and optical metrology. However, the discussed developments and insights can be also of interest for imaging of novel biomaterials and implants, covered e.g. by functional ceramics (multi-) layers.

Introduction

Optical Coherence Tomography (OCT) has been established as imaging technology, since the introduction as low-coherence in-terferometric technique in the 1990s [1,2] and somewhat later also realized in its mi-croscopic version as Optical Coherence Mi-croscopy (OCM) [3].

OCT has become a well-known technique supporting visual diagnostics, especially in ophthalmology [4,5], but also in other med-ical subject areas [6,7] such as dermatology, dentistry, cardiology, etc., even implemented in endoscopic versions. With a background of biomedical applications, the near infrared spectral band (NIR) of about 800-1050 nm has proven to be one of the most suitable spectral ranges, referring to the low optical absorption values and the dispersion mini-mum of water, as main constituent in bio-logical tissues [8]. Taking into account the strongly scattering properties of tissues like skin tissue, a transfer of OCT imaging to a spectral range of about 1300 nm has been an advantageous option for OCT imaging in context of dermatology [9,10]. However, OCT has also been advanced towards the visual (VIS) spectral range due to the beneficial higher resolution here which can be achieved and exploited in case of investigating rather transparent samples or varnish layers [11,12].

The entrance of OCT to non-destructive testing (NDT) [13], in particular the testing and investigation of sub-surface structures in polymer materials, inspecting organic coatings, or ceramic layers have evoked the requirements partly for a shift of OCT imag-

Optical Coherence Tomography Imaging through the Spectral Ranges

State of the Art, Advances and Limits

Bettina Heise1 and Ivan Zorin1

Fig.1: NIR-OCT imaging for NDT application, exemplified for (a) 3D printed polymers test sam-ples with internal debonding defects. The 3D printed wedge-like structures consist of PLA matrix containing (upper): red color pigments, (lower): almost no pigments (i.e. semi-transparent poly-mer sample); and for (b) multilayer polymer foils typically used for (food) packaging, imaged (upper) at a defected region, (lower) at a welding seem region. The cross-sectional OCT images have been recorded by a commercial OCT system operating at 1300 nm central wavelength.


Light Microscopy

ing to other spectral ranges which are suited best for the investigation of such technical samples and conclusions on their associated material properties.

Methods

OCT imaging using NIR light sources (cen-tral wavelength around 1300 nm or 1500 nm) is appropriate for testing e.g. pigmented and scattering polymer materials and poly-mer composites [14]. Examples for such OCT

Fig.2: XUV-OCT imaging exemplified for three-dimensional structured sample with two gold layers buried in silicon. The layer at 330 nm depth is a tiny fraction of a photograph showing the edge of a building’s window. XCT measurements were performed on ≈1800 lateral scanning points in an area of 960 μm × 420 μm with 15 μm step size in approximately 6.5 h. (a) 3D im-age reconstructed by Fourier transformation. (b) The same in the front view. Visible are the two gold layers (Au), where the upper one contains the letters “XCT” as well as three additional lay-ers closer to the surface. Ghost layer g2 was expected and appears at a depth equal to the depth difference of the gold layers. The existence of the SiO2 layer was discovered by XCT. This layer also explains ghost layer g1. Panels (c) and (d) show the 3D image of the same data set recon-structed by a three-step phase-retrieval algorithm. The ghost structures are almost completely gone. The images in panels (e) and (f) are also reconstructed by phase retrieval but are addition-ally deconvolved by the point spread function of the Kaiser–Bessel Fourier window via a Lucy–Richardson algorithm. The depth accuracy is better than 4 nm. The axial resolution, i.e., the ability to resolve two layers, is approximately 24 nm and the lateral resolution is 23 μm due to the size of the XUV focus. Reprinted/Adapted with permission from [17], The Optical Society.

imaging and probing of polymer samples are illustrated in figure 1.

Polymer material testing often compre-hends also polarization-sensitive OCT imaging that might additionally provide information and indications of optical anisotropies and possible existing internal strain-stress states in thin polymer layers or bulk components [15].

With the emergence of novel broadband sources, in particular supercontinuum (SC) generating light sources [16], new spectral ranges have become accessible and exploit-able for OCT imaging.

Recently, interesting use cases for such novel OCT sensing and imaging in spec-tral ranges beyond NIR have been pre-sented. Towards higher optical spectral fre-quencies, i.e. for the (extreme) ultraviolet (XUV) range, a common-path OCT solution has been demonstrated by Fuchs et al. [17]. There, it has been exemplified for the visu-alization of embedded gold structures buried in silicon material, figure 2, which points to potential applications, e.g. conceivable for circuit metrology.

Towards lower optical frequencies, OCT applications are shown in the short wave-length IR range at approx. 2 μm, there applied for the investigation of cultural her-itage objects, which covers, for instances, objects such as paintings or porcelain [18,19]. Recently an extension of OCT imaging towards the 4-5 μm range has been shown, which turns out to be very useful for the inspection of ceramic layers and the detec-tion of hidden defects or embedded internal structures therein [20].

A linear optical OCT approach has been applied, combined with a pyroelectric detec-tion scheme to measure the mid-infrared (MIR) spectrally resolved interference pat-tern. A SC laser (NKT or Leukos), which emits in the NIR to MIR range (approx. from 2 to 4.5 μm), was used as the light source for investigation. Although these novel SC light sources (almost laser-like with respect to their brightness and beam characteristics and yet radiating very broadband) allow new screening and testing options, low-co-herence imaging in the extended spectral ranges often provides also new challenges. This concerns efforts in correcting optical elements best to chromatic aberration and dispersion effects and nevertheless, remain within a cost-effective framework.

Results

Comparing imaging at 1.5 μm and 4 μm, the superior properties of MIR-OCT imaging can be shown, realized in Fourier domain (FD), for the inspection and testing of ceramic layers. Internal microstructures have been found by imaging under 4 μm central wave-length range, which are not or only weakly visible by tests in lower wavelength ranges of 1.3 μm [21].

In particular, the benefits of MIR-FD-OCT for the testing of 3D printed structures, cre-ated in lithographic ceramics manufactur-ing (LCM) have been demonstrated. Here, 3D-printed ceramics, exhibiting an inter-nal LCM structuring of the substrate, can be non-destructively tested up to 0.5 to more than 1 mm imaging depth, depending on ceramics type, grain size, and porosity. First

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results are shown in figure 3, exemplified for 3D-printed or laser milled ceramics samples.

The linear MIR–FD-OCT shown here is intended to open up an interesting potential for layer-by-layer inline testing and inspec-tion in ceramics 3D printing processes.

Summary and Conclusion

The OCT technology originated from the field of biomedical applications. Since then it has emerged from its past and has estab-lished itself for non-destructive testing in the technical environment. The development and advances in novel light sources that en-able low-coherence imaging beyond VIS and NIR spectral ranges, even in extended spec-tral ranges of UV or MIR, have put new OCT applications on the NDT agenda. It is hope-fully realizable that not only new broadband (MIR-)SC light sources, but also suitable and cost-effective (MIR) detector arrays will ac-company this process. Both will encourage rising of new fields for OCT imaging and sensing. Nowadays suitable light sources with a central wavelength in a wide spectral

range (reaching from UV to MIR) are avail-able, allowing to select the most appropriate center wavelength for the inspection task and material to be examined. Adjacent applica-tions such as imaging and testing of tech-nical biomaterials are expected or implants will also benefit from the revenues generated by the expanded spectral capabilities of OCT imaging.

Acknowledgement

This work has been supported by the proj-ect “multimodal and in-situ characterization of inhomogeneous materials” (MiCi) by the federal government of Upper Austria and the European Regional Development Fund (EFRE) in the framework of the EU-program IWB2020. This work was also supported by the strategic economic- research program

“Innovative Upper Austria 2020” of the province of Upper Austria. We also acknowl-edge funding by European Union’s Horizon 2020 Research and Innovation Programme under grand agreement NO. 722380. Fur-thermore, we thank Lithoz GmbH Vienna for support and providing 3D printed ceramic specimens for testing.

Affiliation1Research Center for Non-Destructive Testing, Linz, Austria

ContactDr. Bettina HeiseHead of Optical Coherence TomographyResearch Center for Non Destructive Testing GmbHLinz, [email protected]

Fig. 3: MIR-OCT imaging for NDT application, exemplified for ceramics specimens. The OCT system has been realized as spectral domain configura-tion operating in linear optical regime and applying pyroelectric line detector [20]. (a) Measured spectrum of applied MIR supercontinuum source exhibiting an exploited probing window for OCT at 4 μm. (b), (c): 3D printed and micro-structured (tricalciumphosphate) ceramics tube and 3D OCT volume image, recorded along indicated red line; (d), (e): 3D model and enface OCT image of laser-milled alumina ceramics layer, which shows an embossed internal grating structure and two blob-like defects.

More on tomography: http://bit.ly/IM-tomo2

References: http://bit.ly/IM-Heise2

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