Interview: Percy Zahl
2012 George T. Hayno Award Recipient
Interviewed by Paul Holloway, 2012
HOLLOWAY: My name is Paul Holloway. I’m a member of the AVS History Committee. Today is October 29, 2012. We’re at the 59th International Symposium of the AVS in Tampa, Florida. Today I have the privilege and pleasure of interviewing Percy Zahl, who is the Hanyo Award winner for 2012. His citation reads, “For outstanding leadership in the development of state of the art, open source software packages and the associated hardware for data acquisition and analysis in scanning probe microscopy and spectroscopy.” So that’s quite a mouthful. That’s quite an honor. Congratulations on the award, Percy. Could we start by you giving us your birth date and birthplace and country?
ZAHL: Okay. My name is Percy Zahl. I was born in 1971 in Hannover, Germany. It’s my great pleasure to be here to talk to you about the prize I’ve been awarded.
HOLLOWAY: Good. Let’s continue the conversation and talk a little bit about your educational background. Could you give us some details on that, please?
ZAHL: So I guess I can start when I went to university. That was after I finished my military service in ’91. I started studying physics towards getting a diploma and eventually a Ph.D. later.
HOLLOWAY: That was at which university?
ZAHL: That was at the University of Hannover, Germany. .
HOLLOWAY: Okay. You got the first degree in physics.
ZAHL: Yes. In 1995, 1996, actually I did my diploma. See, this was the former German way towards a Ph.D. I finished that in ’96, and four years later in December 2000 I got my Ph.D.
HOLLOWAY: So your initial diploma work was in scanning probe microscopy area?
ZAHL: Exactly. Well, it was the last year of my studies and lectures when I got somehow in touch with a research group where I did my diploma thesis and later Ph.D. work, in the group led by Michael Horn-von Hoegen (today you can find him at Uni Duessburg: http://www.uni-due.de/ag-hvh/index.php). He was my doctor father (Doktor Vater) as we say in German, or a Ph.D. advisor. The Institute for Solid State Physics, section for Surface Sciences at University of Hannover at that time was led by Prof. Dr. Martin Henzler , who is more famous for the invention of SPA-LEED (Spot Profile Analysis -- Low Energy Electron Diffraction) and as specialist for surface electron diffraction and surface science. That time I gained a lot of experience with SPM (Scanning Probe Microscopy) from experienced people around or nearby like Ullrich Koehler and Gehard Meyer who also did his PhD in this group, but that time already moved to Berlin. . Historically he (G. Meyer) is very well known in that field of scanning probe microscopy. He’s now at IBM Zurich. We had good connections to him. I visited him a couple of times and talked about these things (STM and related technology), and I got a lot of knowledge from these now well-known people in that field. But as I started working on this microscope before that, I got somehow as a student interested in that. I always was a big friend of building things, and controlling things via software. It was that kind of work I loved doing a lot.. That's what I love about physics: Build and new instruments to investigate nature always a notch better than before! However, it was obviously a tool for my work. I never got really any awards or anything officiall recognition for my work on this instrumentations and software beyond a few papers, and nothing really got ever awarded to this project. It was always just a way or tool to get the data. I mean that was the main purpose. At that time we created our own STM control software and interfacing hardware based on commercially available hardware. Initially because we had to cut down expenses.
HOLLOWAY: So the controls were your development?
ZAHL: Yeah. Well, I didn't make the PC interface and signal processing card. This was based on universal hardware. But I made the interfacing electronics hooked up to that, the amplifiers, and I also designed and wrote all of the low signal Digital Signal Processing (DSP) code to run the feedback mechanism, do the scanning, the data acquisition, and all the details. And as well on host PC level all the graphical control and data visualization interface. So I have pretty much put my hands on everything. If the hardware is not ready to do something we want it to do, I can make it do this -- and that is still true today. Say it this way. What is an advantage in science? Often there are some new projects. There are some new demands, some new ideas, and you’re stuck with some commercial software and you can't really do what you need to do. This is so often a problem. If you have the power to do everything you need, that is kind of convenient.
I kept always working with this project in my background. I got use to starting in our own group and soon related groups at the same institute started using the software for scanning probe. Then it kind of spread a little bit further. I kept working and leading this project. As I finished my Ph.D., there was kind of a milestone decision. Pretty much there was only one other involved into this project —One of my colleagues, Andreas Klust, finished his Ph.D. thesis the same year. He now works in industry, but at that time he was working with AFM. He was involved from the very beginning, with great ideas and forcing more flexibility to support multiple types of instruments. You need to design this in a right way, very modular, and I learned a lot in that direction. I got some books about a large scale software design and tried to do my best, and always keep working that way to make it even more modular and keep the code clean…well, and get the best out of it. Reuse some unique things like data management, data handling, but it’s always the same core. Don't throw that overboard again and start from scratch. Do it right once. Re-use that and make it flexible and adapt it to different methods. Keep it up-to-date and modern! Follow technology and hardware advances in a smart way. For my PhD I needed to operate a SPA-LEED (Spot Profile Analysis – Low Energy Electron Diffraction). Also here basically you’re scanning; scanning the angle of an electron beam as it probes a surface, then collect counts to assemble a LEED pattern electronically pixel by pixel in k-space However, it is scanning something and taking counts. That’s where the “X” of the name in GXSM comes into account. Scanning something in two dimensions, in general get an image from whatever data comes in. If it's counts of electrons or a Topo/Z or Force signalor whatever – the software can handle it. Today I expanded into more dimensions, so it could be even a multi-dimension scan like doing spectroscopy at every point or any kind of 3rd dimension parameter Plus eventual a time dimension for a movie. So it’s getting more and more flexible. It can handle pretty much infinite amounts of data. Simply said, if your hardware can acquire and the computer can handle the amount of data in memory – the sofware can do it. It is designed in a way there’s no hard limitations like 2,000 points or something funny. I’m limited to the hardware capabilities only. I mean I can't go beyond 64 bits (unless the native CPU integer may be bigger in future) for dimensions; that would be a headache. But also that would be outrageous, huge.
HOLLOWAY: Right. So this was all leading you in your educational process up to the diploma and then to the PhD. You did some post-docs as well, right?
ZAHL: Oh yeah, right. So just let that finish. I pretty much had the software developed in working condition at the end of my diploma thesis to do the experiments and take the scientific data I needed for this. But since it’s never-ending and always growing…
HOLLOWAY: It’s always developing.
ZAHL: As I completed my Ph.D and prepared to leave Hannover a solution was needed to managed this software project in future and also world wide as people would spread out., I had previous experience with other software. Someone leaves who is a main designer, things get out of control. Or it spreads out some puzzle work here and there. It’s just getting in a mess, and we decided to find a more professional approach: Within the institution and the few involved developers we agreed to make it Open Source and put it under the terms of the GPL, then put it on a the web on SourceForge so it can be maintained via the WWW and has some code versioning control system or “CVS”., You can manage source code. Multiple people can work on code and put it in a kind of database, check it out, update, commit changes and it's history is managed. You can follow up on changes and revert to any older vesion if necessay. All via internet, from any where, any time. I kept the role of the project administrator since then. But there are more people involved, and today there are even a few more.
HOLLOWAY: So people can contact you at Brookhaven National Laboratory and learn about this site.
ZAHL: Yeah. Well, not even necessarily there. Ideally can download it, can play with it first. Then if there is need for help you can ask questions via a discussion forum. That’s the best way to get in touch with us and other developers or even users so you can help out each other directly! Sure you can get in touch with me or send me an email as well
HOLLOWAY: So is there a website address that you…?
ZAHL: Yes. It can be found at http://gxsm.sf.net.
HOLLOWAY: Okay. So that brings us to the development of the GXSM software control codes. But you have applied this to a number of different situations, scientific problems. Is it equally applicable to scanning tunneling as well as atomic force microscopy?
ZAHL: Yeah, pretty much. I mean this is one of the roots of very basic methods I’m pretty much covering. Its main intent is to work with scanning tunneling microscopy was just STM. It’s mostly used as most kind of straightforward simple method, but there is also AFM with all kinds of its flavors. I can't even remember all the flavors of AFM.
HOLLOWAY: Yeah, that’s right!
ZAHL: But it doesn't really matter. If you know your method and what you need, there is a way to get this control. You can scan; you can take your data. If you use an external PLL for frequency detection or use the build in PLL, we recently can do frequency detection up to 75kHz with our most modern hardware. It supports today the tuning fork style detection scheme. But these are still things under development.. So it's always progression and with new hardware capabilities.
However, after my PhD I went to the Colorado School of Mines for my first post-doc in the group of Peter Sutter . That was in 2001-2002. There we built up…You know, it was a small budget, a small STM pretty much dedicated for silicon and silicon/germanium growth . Silicon (111) and also (100). We worked on that and built a STM ourselves – completely home built. We figured fairly quickly it’s a big advantage for us and a money saver if you’re using my own software also in that group. So I was glad to help out here with my experience and it all progressed nicely.
HOLLOWAY: This was an atmospheric microscope.
ZAHL: No, it was UHV. It was always UHV systems I worked with so far. It’s not limited to that for sure. We were running some practical courses STM/AFMs for students in air. In other institutions they’re operating it also using AFM, so it got useful to these kinds of purposes as well. Of course it’s just convenient. Even some light scattering machine for wafer quality control, a more historic thing doesn't even exist anymore, that was in the group of Prof. Dr. Martin Henzler in Hannover and was also operated with GXSM. Well, the plug-ins structure comes into game for data analysis and many specialized tasks here! It’s possible to extend it without adding bulk to the core code when you have a plug-in interface.
HOLLOWAY: So at School of Mines you applied it to what problem?
ZAHL: This was pretty much plain STM we used that time. There wasn't really a specific “new” problem but we investigated the morphology of various Si/Ge film and micro cluster growth scenarios. It turned out useful to take fairly large high resolution images – I just had the capability and we used it. There is not much in particular written about it except some articles where we used those images as basis for statistical analysis as we acquired some really huge images. Images in one shot – not stitched --, something like 2x2 micron with atomic resolution in one big image. Those were 15 to 30,000 pixels in each direction. That was in 2002, 2003. I’m not aware of anyone who ever did that in one image that time. . With our small and compact type STM we had the stability to run scans of that size over 6-10 hours. The advantage was the big consistent picture and not only random chosen key hole snap shots. We could print it on a poster, huge like wallpaper, and you could for example see nicely 7x7 and 5x5 reconstructed domains and see the distribution on it, and in one nice big picture. In contrast to diffraction where you see the coexistence of both superstructures, may be get an idea about the size and distribution of domains, but hardly see how they match up, fill in between steps and such details. . This information is hard to get otherwise. You need one big view. It was beautiful by itself and of useful.
HOLLOWAY: So all this is on silicon.
ZAHL: This was silicon that time. Yeah, silicon and silicon-germanium alloys were studied. That was just one example. We still make use of large scans occasionally today – (Today at BNL/CFN we have even more stable STM system available operating at low temperature/5K for example at our user facility, accessible for researchers all over the world. Including spectroscopic mapping and more nowadays)
HOLLOWAY: That followed on your work for the Ph.D. where you looked at strain from epitaxial layers.
ZAHL: Yes. In my PhD I developed a method named SSIOD (Surface Stress Induced Optical Deflection). I studied stress relaxation mechanisms for surfactant mediated epitaxy of germanium on silicon (111). There forms a dislocation network within interface And I monitored the surface stress and morphology in-vivo while growth. No STM involved here. This was more diffraction and then x-ray diffraction, things like that, nothing really related with this project. But I use the software for SPA-LEED data acquisitions and data analysis.
HOLLOWAY: Then after you finished your post-doc at Colorado School of Mines, you went on to IBM Zürich.
ZAHL: That’s right. I took the opportunity to work as Post Doc at the IBM Zürich Research Laboratory in the group of Gerhard Meyer. There I expanded my experience to nanopatterning and worked together with Reto Schlittler on finishing and optimizing a system we named IBM-Nanostencil. This was pretty much a kind of specialized AFM/DFM (dynamic force microscope), to scan bigger areas and look at structures we made with this Nanostencil via via shadow mask deposition. It was of advantage to use the same GXSM software to take AFM images that time also and it was convenient for me as I could simple add a few plug-ins for what we needed in addition.
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HOLLOWAY: So it was still primarily semiconductors you were working on then?
ZAHL: We worked on all kinds of surfaces, often even on silicon oxide, just for structuring and looking at patterns. But the samples often were non-conductive, so we used non-contact AFM.
HOLLOWAY: Okay. So what’s your opinion of scanning probe microscopies for analysis of structures versus Low Energy Electron Diffraction (LEED) patterns, for example?
ZAHL: Well, LEED is nice, it’s easy, it’s quick. You can do often while depositing. You don't need to deal with tips when you get some statistics of millimeter range , but you don't really get a picture. I mean you can make a guess for the structures and do a mathematical transfomation... But some details of the morphology that might be of interest, how steps exactly look like is hard to get with LEED. Then if you mix both, it’s probably ideal. But with scanning probe today you get a picture, and pictures are always appealing to people. That is quite a big advantage. Having a nice picture to support your theory, always makes a good impact. I mean just some spots in k-space? Well great, but you need really to know what you’re looking at.
HOLLOWAY: Yeah, that's right.
ZAHL: And if you do spectroscopy or spot profile analysis and more you get good “hard” numbers but may miss how it actually looks – put any “clean” sample in STM and you may see pinnings and contaminations causing effects you may be bind to other wise!
HOLLOWAY: Especially if you make pictures in color. The color conveys a lot of information to people.
ZAHL: It looks pretty—what is advantage by itself! But today I find it really amazing to look at STM images life, it's a fantastic tiny world! For example what we’re doing at the Brookhaven Lab (BNL) today, imaging molecules on surfaces, you can look at single molecules and even details of them or what you “see” of their electronic structure! I don't think any other method can get you as direct a picture of a molecule or atoms. I don't know anything. All these things are kind of indirect, but don't really get you an image, not as nicely. Today people like Gerdhard Meyer using even AFM to image molecular orbitals simply using a super sensitive and tiny local force detectior in form of just a simple atomic sharp tip – “mechanically”! You can even visualize hydrogen bonds in a molecule. I just find this purely amazing. Today there is a seemingly infinite range of scanning “probe” based methods for “AFM” going on. Developments of new tricks to look at different features, scan faster, in different environments, ….
HOLLOWAY: Let me ask you about the technique of Near-field Scanning Optical Mmicroscopy. [Right.] Can you give me a status report on that? Are you familiar with it?
ZAHL: Well, I’m not much involved in near field methods. (SNOM/NSOM) Whatever you name it. But I think that you’re kind of limited in resolution. I am not aware what they can do today. The times I looked into it, it was somewhere around 100 nanometers. Maybe you can do something like 50nm today; I don't know. To get spectroscopic information, you can shine a a laser via a small hole and analyze the light coming back out.. There are a lot of tricks today they can do. Maybe they can get to 10 nanometers. I don't know.
HOLLOWAY: It made a splash, but I haven't seen much from it lately.
ZAHL: Yeah. All I know, I’ve played with it. It was fairly tedious. But I’m not an expert in that, simply said. But however, this software would be able to take the data; no question about that. [Laughter] But I’m not working with it.
HOLLOWAY: I understand. Let me ask the question. Besides STM and AFM, there is scanning differential capacitance and there are magnetic forces. [Yes.] There are differential shear forces. How important are those in deciding what software packages to go with? Your package is versatile enough to handle all of those presumably.
ZAHL: Yes the GXSM package/project is totally capable of handling a wide range of AFM variants. In contrast to most commercial instrument/software solutions, you can buy the GXSM architecture which is designed to be versatile and adaptable to almost any instrument you may have or build and get the most out of it. Sure maybe it needs some extra hardware to do some modulations and get some capacity signal or whatever signal recovered and ready to plug into the analog to digital converter. I mean if I use my software, it usually does the scanning and it acquires up to eight channels of data simultaneously plus two counter channels. It does the feedback-can mix multiple inputs as feedback sources. It includes a digital Lock-In, digital filters for automatic bandwidth adaptations and more. I mean that’s pretty much up to you and your smartness, what you do with it. If you need some special data processing and you have some skills or are willing to have someone invest some time or want to do something new, always contact me and we find a solution if it’s possible with the hardware we currently support. In contrast to commercial systems – new developments on commercial-based systems are a little bit limited. I mean it’s not always easy to convince a company to do exactly what you need, ideally right then. I’m fairly open minded to any new idea. There were a couple of discussions in the past online via our forum with several groups around the world. They need sometimes even simple things, e.g. a special version of the auto approach or simply a new mode of spectroscopy including some trigger signals from external hardware (a laser creating a pulse to start some spectroscopy. We got that working quickly by adding this feature to a existing plug-in.
HOLLOWAY: Good. Now you’re at Brookhaven in the Center for Functional Nanomaterials. Can you give us a flavor for what sort of activities are in that group?
ZAHL: The Center for Functional Nanomaterials is one of the five new nanocenters within the U.S. The main purpose of these nanocenters is to provide actually services worldwide to scientists on a peer reviewed proposal-based method to grant access to our facility. We have users starting from BNL internal, local from NY state, but also from all over the US amd even internationally from Europe. In our Proximal Probes group we have some bigger machines not every university can afford plus provide the expertise to get the most out of it in a very efficient way. We have various STMs in our group, a low temperature (5K) STM. One designed for variable temperature and also a new system allowing operation at high pressures (reactor STM). But also provides LEEM and PEEM in combination with the NSLS (National Synchrotron Light Source) and x-ray photoelectron spectroscopy (XPS) capabilities on some systems. The idea is if a researcher has good experience with something say a new molecule A on a suitable surface and is missing some additional key information we could provide using our facility like for example some dedicated spectroscopy and high resolution imaging at 5K that would make a good starting point for a proposal. . Then they can apply for time with us, and we do our best to get them the complementary data they need. So we work mostly with users from outside and little bit of remaining time we can spend on our own internal projects mainly focusing on themes given by the Department of Energy. They ate related to energy conversion, catalysis and energy storage This includes also expanding our capabilites and thus instrument development to tackle new tasks.
HOLLOWAY: Do you sometimes have chambers that house multiple techniques like LEEM and scanning probe microscopies and LEED all in the same chamber?
ZAHL: Not in one system today, but there is a STM addition for one of our LEEMs on the drawing board... But yes, the 5K STM.system and several others are equipped with LEED capability.
HOLLOWAY: I see. So you have vacuum transfer vessels that you can move sample from one to another?
ZAHL: Mostly not as sample holders are often incompatible. Normally, if someone applies for beam time, it’s probably more focused on one system, one special method. Then they’re repeating the sample preparation from scratch. It is unusual to swap samples e.g. between LEEM and STM; mostly that’s even not feasible. Sample holders, sample sizes are different. It’s not compatible.
HOLLOWAY: Now you’ve mentioned a couple of times the cryogenic applications and cryogenic capabilities. Do you have low temperature capabilities on all of your probe stations?
ZAHL: We can cool in most of the systems using liquid nitrogen or even helium. The coldest we can go is 5K – eventually a little below if we have to. This system is from Createc and has a 5k helium cryostat inside a LN2 shield. Once the sample is inside the Cryostat and cooled down the fun beginns as you have all the time of the world to study it. Virtually zero drift, and zero contaminations any more. Beyond imaging, you can do manipulation on atomic scale and you can do spectroscopy and spectroscopic mapping. Also using the GXSM software for this system now since a couple of years. It's more transparent for us and gives me a more native control or hand's on to the nano or should better say pico world!
HOLLOWAY: Okay, good. That covers the topics that I wanted to discuss today. Did you have other topics you would like to discuss?
ZAHL: I’m not really aware of what that might be. [Laughter] Any idea what…?
HOLLOWAY: What about the future for scanning probe microscopy? Would you like to speculate as to whether it’s now peaked, saturated, or will continue to grow exponentially?
ZAHL: Oh, I don't know that it’s going exponentially, but I’m pretty sure it’s continuing with new methods. I mean AFM has seen a lot of developments lately, and this seems still just the beginning. More and more are applying dynamic force microscopy and using tuning fork detectors in cryogenic environments to achive highest resolution power to image molecular structures. I think this is still the beginning of maybe a new era in imaging. There are just a few groups around the world who can do this today. In general, AFM is still expanding into new applications. And there is “chemistry” STM imaging in solutions, electrochemistry and more. There’s almost an infinite range of different methods you can apply it to if you look at something small.
HOLLOWAY: What about bio interfaces? I know that there were some people that tried early on to image biological molecules with the scanning probe microscopies. Do you think that that’s been successful or feasible?
ZAHL: You mean image on DNA or something that was imaged long ago on some surface—Well, one issue of a scanning probe is you have to stick things usually on surfaces. Ideally you would want to look at molecules undisturbed, but that is hardly possible, but there are tricks to get close. You have to fix it to look at. That is one of the problems. And also you cannot really look inside either. It’s kind of limited here as well.
HOLLOWAY: Well, I think some people imagine that you might be able to go along a DNA and use adhesion force measurements and look at the different sites along the surface. Do you think that's feasible?
ZAHL: Adhesion. Well, that sounds like something pretty involved. I don't know. [Laughter] It sounds tricky to me, but not impossible, I guess. Not sure what you exactly want to do.
HOLLOWAY: Right. Okay. Well, I think that concludes the topics that I wanted to discuss, so let me congratulate you once again on your award. Thank you very much for doing the interview.
ZAHL: Yeah. Thanks a lot for interviewing me and inviting me to the conference here today.
HOLLOWAY: Thank you. Go to sleep and go to sleep.