Heliotactic Press

In the previous post, I was asked by reader and fellow scientist Riverie to comment on my experience in finding and securing postdoctoral research scientist positions. Submitted under the category of scientific philosophy, I have chosen the following response as a separate entry.

Like most employment successes, my postdoctoral opportunities have been initiated through positive connections between my former advisor (or myself) and interested parties. My credibility for scientific research improves with each year of additional research (improved network and letters of recommendation) and with successive publications (improved CV). In my experience, one improves the odds of selection for an initial postdoctoral position by searching within the known network of one’s advisor(s), and by always requesting the advisor to make initial inquiry contact regarding a position. With successive years, you should be developing your own network, but this process takes some time to “seed”.

Although I have a second position now, I can attest to the difficulty of a “cold call” to another professor (especially outside of their field) expressing your interest in a research position. Professors have exceedingly busy schedules, and often don’t have the time to confirm your credentials without some advanced confirmation from a peer, expressing that a candidate is worthy of consideration. Given the inadequate time allotted to the selection process by the professor in need, the risk for selecting a poor candidate is high and one must expect them to be very conservative in drawing their short list. Even so, if the prospective position has no funding and you have no funding, there can be no opportunity, and professors should inform the advisor right away of such a limitation.

If you are selected as a candidate and if it is possible, visit the lab. This was not possible for me to do for my experience in France, so it was extremely fortunate that my postdoc was both highly successful in research and that I had an excellent working environment with the group and group leader. I know the lab that I am currently working in, so I was certain I would be able to adjust and work efficiently.

The truth is, you need to be at the right place and time, with the additional nod from a peer, to garner consideration for a position. As much as they may be though of in terms of inexpensive labor from the point of view of the graduate student, a postdoctoral researcher is substantially more expensive to support and is required to accomplish much more than a student. There are also expectations from the postdoctoral researcher that some mentoring will occur, as an apprenticeship to managing your own future research team (in academia or business). The collaboration is an additional time investment for the professor, and can be looked upon as a means for the postdoctoral researcher to become more efficient at what she/he already does well. So, one should be able to make a good case for an independent work initiative, and expect to perform the majority of the assigned research on your own (with critiques along the way).

Once you are in the research position, expect that a mentoring professor is there to correct your form; you must be open to listen to what they suggest, even if you want to stubbornly adhere to your own hunch. On the other hand, you should continue to express your own opinions regarding the research to spur scientific discussion. If you don’t have the data to back up your claims, be prepared to listen to suggestions and go back to the bench to find that data that clarifies the discussion. Often, you will be surprised by the results of the compromise.

Finally, never underestimate the network of people that are in the surrounding group and laboratories. These people will all be potential connections. Keep your mind open to interdisciplinary connections, even if the moment is not ripe now. And remember that you are the transient in the laboratory; more so than the graduate students, even. Respect that the laboratory and office is your temporary home, and keep the peace. A postdoctoral position is not the place for the great revolt against the machine, and you are expected to behave according to the internal rules of the lab (provided they do not breach ethical standards of scientific behavior).

Bearing all that in mind, good luck!

I have recently returned to the States from a fantastic postdoctoral research experience just outside of Paris, France. There, I was able not only to work on eta-solar cell materials research, but also to learn another culture and language, and to get outside of my comfort zone. The discussions that I have had in the Lévy-Clément research group the past year have been nothing short of brilliant, and I thank all of the researchers sincerely for their friendship and patience to enter into the “what is fire” discussions of PV.

Now, I have returned to Madison, Wisconsin to apply many of the ideas that I’d developed in France in another round of postdoctoral photovoltaic materials research. I find that I am not alone in my interests, as a PhD graduate student and a senior undergraduate in chemical engineering have begun participation in my research projects. It is always exciting to see that fresh interest in the strange but fun subject of photovoltaics. Onward! to la nouvelle vague.

How should scientific research and reasoning be carried out? Are there universally- defined rules inherent to scientific discovery, or is it more like modern philosopher Paul Feyerabend¹ suggests, as an anarchistic realm punctuated by domains of apparent organization, each with a limited range of usefulness to scientific discovery?

One of the beauties I’ve found in doing research in another country is that, if you listen closely to the talk of your foreign colleagues, you may find the underlying principles of research are going to be defined differently than your own (even alien to your own scientific reasoning). I had this very experience in France, when my colleague was speaking of a mechanism of a chemical reaction and told me that another scientist had “proven” the mechanism was correct. Stop the press…”Wait, you can’t say something is proven in fields outside of mathematics! Everybody knows that you can only disprove a hypothesis, and then only demonstrate strong support for a hypothesis that has not been struck down yet. An experiment has to be falsifiable, but cannot be proven.” In fact, my music-teacher wife still remembers the day, in 11th grade Advanced Biology, when her teacher (Mr. Hjelle, pronounced “Jelly”… yes really) told her that there is no such thing as scientific fact; everything is theory, because we cannot prove anything beyond the shadow of a doubt. We can disprove, but to say we’ve “proven” something is a very arrogant and dismissive view upon scientific research.

The response of my colleague to me was that, in France, it is quite common to refer to something as being proven, as everybody knows that proof is essentially a shorthand or map to express the conditions of the regularities in an extremely well defined experimental setting. In her view, with sufficient support, one can prove something (as an asymptotic narrowing to an accurate description of a regular event). And beyond a certain point, it becomes ridiculous to search for obscure, exhaustive hypotheses to test to disprove something that has been found to be robust from experimentation.

My colleague is the senior researcher in our group, and I admire her scientific rigor and conservative approach in assessing and presenting scientific data to the public. At the time, we had been working together for over nine months. Through her actions, my colleague has repeatedly demonstrated her abilities as a rock-solid scientist. In other words, she is so effective with her approach to science that I have no reason to suspect that her mode of reasoning is any different from my own (which I had learned from my mentors in the USA). Yet it most certainly is different, and the months following have been challenging and enlightening in terms of expanding my approach to scientific discovery. So where was the source of this schism between each of our well-accepted, yet different philosophies of science? Can there be an optimal or universal method of scientific reasoning in scientific discovery, or is there “more than one way to get there”?

As an illustration, if we assume two laboratories on separate continents have expert researchers, well accepted by peer-review and having strong experimental results in the same field, are the two researchers able to arrive at similar results if they have separate scientific philosophies? In other words, as an expert scientist, is your learned approach to scientific reasoning that much better than another’s?

I’ll take the proposition one step further: assume one researcher has an outstanding pedigree in a field of research (say a chemist, like Marie Curie), while another is a bright, talented, very well-read, but self-taught inquirer (let’s say a bookbinder-turned-scientist, like Michael Faraday). The former has been trained to use a rather robust yet strict set of principles based on her own mentor’s rubrics to arrive at well reasoned results, while the latter uses an assembly of methods that he has developed on his own thorough experience. The methods of the latter (self-discovered) are a combination of very systematic experiments (not unlike those of the former) and some rather haphazard modes of experimentation and “exploration”, that do not have a strict rules of progress, but tend to produce positive results nonetheless. Upon arriving at a successfully scientific discovery, both can communicate the results quite well, and have papers accepted into highly regarded journals in the literature. Now, which researcher was the better scientist? It would be an extreme act of prejudice to say the pedigreed scientist was better because of formal qualifications, given the assumption that the self-trained researcher in fact can have the same qualities of insight, rigor, communication and perseverance. But it would be equally biased to place an overwhelming romantic support for the self-taught individual as an underdog of research (something like Good Will Hunting). The fact is, in my illustration I assume they are both excellent scientists, and deserving of praise.

In considering the history of science in France, Germany, Austria/Hungary, England and America and the philosophies of science that developed in each country (perhaps another post), the answer is that yes, there is a difference in our scientific training and our scientific philosophies from France to the USA. Without really knowing it at the time, I had tripped over a break between a few schools of scientific reasoning. In my subsequent investigation, I found that France is influenced by the two systems of inductive reasoning inherited from Pierre-Simon Laplace, and more recent and practical conventionalism favored by Henri Poincaré^{2, 3} and Pierre Duhem³ (i.e. a hypothesis is neither truly verifiable nor falsifiable, but serves to make generalizations and predictions beyond experience). In contrast, the principle of falsifiability, argued by Karl Popper is in opposition to inductive reasoning. Falsifiability has been eagerly accepted in the USA as a core scientific reasoning tenet to separate “science” from “non-science”. Arguably, falsifiability is not really the only manner in which experimentation and inquiry occurs. And even though several modern philosophers have argued against falsifiability as a basis for scientific reasoning and have subsequently suggested alternatives that are more closely related to science in practice, many American scientists have clung to Popper’s criterion. To my honest surprise, I also found myself unquestioningly in lock-step with it in the earlier conversation with my colleague.

This led me to think, perhaps there are more important questions than asking why there are different modes of scientific reasoning; such as why was I conditioned to think that there is a fixed and ordained philosophy of science? In essence, I have trained for 15 years within the shadow of a doctrine of scientific reasoning, and in the process it has been necessary to condition (and some would argue brainwash) myself into that doctrine to pass through the hoops of academia. It is the consequence and necessity of a doctorate in science. One of the components in my education that I sought out on my own was information in the history and philosophy of science–something to add awareness to your our scientific preconceptions. As I walk away from the training of the USA and into the sphere of another doctrine of scientific reasoning, I have discovered new tools and perspectives that I believe will make me a better scientist and mentor. Not the least important result of this call to order is the reminder to encourage my future students to read as much as they can in the philosophy of scientific reasoning and the philosophy of science.

One can also find these fixed vantage points in scientific reasoning across various fields of research. In fact, I propose that discovering these differences through interdisciplinary research can hold some of the tools to make scientific breakthoughs. It’s the moment of “Huh, I never thought of it that way…” And why?, not because one doesn’t have the cognitive skills and imagination to arrive at a certain line of reasoning, but rather because one was not introduced to the extent of using an alternative scientific reasoning by training. Once you’ve defined a point of view, pick another one and see if you like the scenery. There’s plenty more where that came from!

References and suggested readings:
1. Feyerabend, P. 1975 Against Method: Outline of an Anarchistic Theory of Knowledge. London: Verso.
Discussion of Feyerabend’s “Against Method” by Paul Newall at the Galilean Library
2. Jules Henri Poincaré (1854-1912). Mauro Murzi, Internet Encyclopedia of Philosophy, July 29, 2006. (including a summary of his perspectives on Conventionalism, science and hypothesis)
3. Howard, D. 2005 Physics Today, December, 34.

We live in a digital age…but what does that mean? I don’t believe people are aware of the full extent that the coined term implies. And hey, that’s okay–there’s enough stuff out there to worry about in practical daily life before asking a pointed What is Fire? type of question. People are happy to use their tiny mobile phones and without even needing to ponder how central those magical boxes are to so many varieties of energy transfer. Energy transferrals which all achieve a common goal of information exchange. We can propose that “digital” implies microchips processing electrical pulses of a set voltage, antennae passing along microwave bursts, and fiber optics passing along pulses of light. All three passing along discrete “bits” of information just like Morse code. But what happens if we expand our sphere of thought, and change our goal from information exchange to something else? Hmm…first let’s settle on what it means to be thinking digital.

From classic studies of information transfer and exhange two terms emerged: analog and digital.^[1] In an analog system, you vary the intensity of the signal along with its position in space/time continuously. Imagine that you have an amplifier and speaker system hooked up to a record player (phonograph) and a compact disc player. Imagine the zig-zag grooves in a vinyl record (I’m imagining Zappa in particular). With the aid of a needle on a phonograph and the right speed of rotation (in the right direction too–don’t want any subversive messages from those albums just yet), the signal translates as an analog stream of information that you hear directly as sound. But in a (binary) digital system, you form the signal into discrete ON/OFF elements and place them at discrete periodic places in space/time. Now imagine the dots and dashes (“pits”) etched into a compact disc. With the aid of a laser and a microchip (converting the bit coding given a known rate of data sampling), the signal translates as audio pulses of information that you recognize as sound.

But why don’t we hear the pulses? Because, if you squeeze the pulses together close enough–the two signals are equivalent. “Peaches en Regalia” will have the same information content for both players (admittedly, without the romance of vinyl white noise). Your ears hearing the pulses will tolerate the steps in exactly the same way that your eyes tolerate the watching rapidly changing frames in the movie theatre as continuous motion. In other words, by reducing the spacing of discrete pulses beyond a certain threshold, the aural neurons in your ear cannot tell the difference between a digital source and a continuous analog source.

But almost more important than that good quality sound or picture is the fact that the dots on a compact disc are not truly discrete! Whoa, take a minute to breathe as I just changed the perspective. Are the raised areas (“pits”) on a compact disc really square, with sharp changes to ON and OFF? Well, no–but the laser reading the pits will tolerate the fuzzy edges of the steps as an acceptable level of “noise”. By making the intensity of the CD pulses above a certain threshold (deep pits), and by keeping the size and spacing of dots larger than a second threshold (broad, well spaced pits) to separate the fuzzy edges, the photodiode in your compact disc player also cannot tell the difference between a continuous analog and a true digital source either. This process even has a name and a formula, the Nyquist-Shannon sampling theorem: for a band-limited signal the sampling frequency must be greater than twice the input signal bandwidth in order to reconstruct the original perfectly from the sampled version. In other words, we can make perfect systems out of imperfect parts as long as we scale down to a certain threshold.

And that brings us to considering another form of “digital”. Digital materials assembly; used in the sense that discrete units of a compound material (primary clusters of atoms as “bits”, to extend the analogy) are self-assembled to form a new product having unique collective attributes distinct from those of the original “bit”. Something like this is being investigated on macroscopic scales at SQUID labs, and it would be really groovy if one could assemble nanoscale digital materials. The reason being, you can pack a whole lot of signal into an itty-bitty volume. The main contribution to the signal in many nanoscale materials is at the surface, and when you shrink something down to the size of molecules (<5nm), the material is almost all surface.

But wait a second, nanoscale digital materials? That rings a bell–what about DNA? Yes, DNA is an old-school nanoscale digital material (like prehistoric…like Precambrian) using four bits (the molecules Adenine and Thymine, Guanine and Cytosine) that link together in two types of pairings (A with T and G with C) in a double chain of bit-pairs with periodic spacings. Okay, so we have a “proof of concept” for nanoscale digital materials that has been road-tested for around 3.5 billion years. I think we can step up to the plate and work around this concept in research now.

This change in perspective from “analog” to a “bit-wise” assembly of materials allows the researcher to apply concepts from the Digital Revolution to materials design. From this perspective, the researcher is aware that all of the nanomaterial parts being used to assemble a new structure are imperfect and “analog” in form (just like the pits in a compact disc, the primary clusters have unfulfilled surface bonds, and distorted structures). But the nanomaterial parts are also very much digital, and the goal would be for those particles to self-assemble into a new material with unique properties different from the individual unit.

As it turns out, we have another tangible example of that as well. These materials are called photonic crystals, and the most prevalent example is the naturally occurring gemstone: opal (although researchers are actively pursuing other synthetic materials and routes of assembly). Opal is a naturally occurring colloidal crystal composed of SiO₂ (silica) spheres (hundreds of nm in diameter) packed together in an ordered 3-D array, and surrounded by molecular water. By itself, silica is optically transparent and is the basis for ordinary window glass. But packed together in hydrated spheres, the path of light is affected and distorted to reveal an unusual pastel rainbow play-of-colors. Even a thin film of this nanostructured material can be used to change the way that light propagates–and it’s nothing like glass.

So let’s return to expanding our sphere of thought about digital applications. What other goals can we achieve from nanoscale digital materials other than information transfer? Perhaps we can assemble a material of nanoscale proportions to efficiently extract electrons and holes from a light absorbing matrix and effectively shuttle them to ohmic contacts, as in a photovoltaic device? We could envision getting several-times the proverbial bang for an invested buck in these systems. Ideally, one can foresee increasing the surface-charge area, increasing the efficiency of charge separation, while also reducing the volume of material that we need to use to construct a solar cell. Maybe the same principles could be translated into building better energy storage devices (e.g. batteries and ultracapacitors) as well. It often strikes me that those would be some pretty cool advanced photovoltaics and battery technologies–kind of like switching from a coiled-filament light bulb in a flashligt to using an intensely bright laser in a pointer. To put things in perspective, would you have thought 50 years ago when the first coherent light systems were being made in research labs, that the results would lead to the pocket laser diodes today? What are the possibilities, and where could “thinking digital” take us in the next 50 years of solar cell and battery research?

Note: many of the links provided come from the HyperPhysics site at the Dept. of Physics and Astronomy at Georgia State University. HyperPhysics is a robust science education source for physical phenomena and their applications in our lives.

[1] Claude Shannon. A mathematical theory of communication. The Bell System Technical Journal, 27:370–423, 623–656, July, October 1948.

Ok, if you remember Schoolhouse Rock you’ll have a certain groovy tune in your head for the rest of the day. I think it’s best to take a moment and lay down some basic descriptions and definitions of the parts of a p-n junction for those of us who want to know about currently available PV technologies. I mention these topics in passing in the other entries, and here’s where we can establish a baseline.

There is a lot of solid, scientifically validated information out on the web dealing with classic semiconductor physics. Just doing a search on “p-n junction” yields pages of websites with information, educational tools, java applications, etc. I recommend the simple yet clear descriptions at the Georgia State science site: HyperPhysics; especially the section on Semiconductor Concepts. This is a silicon-dedicated site, but the physics apply to other materials as well.

And if you really want to get a deep abstract interpretation of these things, read more on the double layer of plasma physics. Note: I use the basis of plasma physics rather than the basis of electrochemistry in this respect, because the definition of the double layer model in electrochemistry assumes a fixed surface charge or surface charge layer. This model works well enough for an oxide particle in aqueous solution, but is limiting in trying to explain mirrored effects of charge attenuation for solid/solid semiconductor interactions.

Bulk semiconductor materials:
p-type: a semiconductor that contains impurities (adding atoms, missing atoms, replacing atoms) such that the major charge carriers are positively charged electron holes. Given that the majority carriers are so prevalent relative to the electrons, if you “add” more holes from an outside source, they will be able to diffuse freely through the p-type material with a very low probability of recombining with an electron (i.e. “hole transparent”).

i-type: an “intrinsic” semiconductor. This material contains no impurities and the populations of electrons and electron holes are the same. Electron-hole pairs (termed excitons) recombine easily because they are oppositely charged and opposites attract.

n-type: a semiconductor that contains impurities (adding atoms, missing atoms, replacing atoms) such that the major charge carriers are negatively charged electrons. Given that the majority carriers are so prevalent relative to the holes, if you “add” more electrons from an outside source, they will be able to diffuse freely through the n-type material with a very low probability of recombining with a hole (i.e. “electron transparent”).

Junction (read “interface”):
Placing two materials in atomically-close contact such that their physical properties at the interface are very different from that of the bulk material. Hence, the term junction is a materials science vehicle for talking about an electrochemical interface between solids.

Electrochemical Potential
Two things to keep in mind: each material is a reservoir for a certain dominant species of charge carriers, and the respective charge carriers are oppositely charged on either side of the junction. So you have two potentials that make the holes and electrons “want” to drift and diffuse into each other. First is an electrostatic potential ( “opposites attract”, so there is electrostatic drift of charges). Second is a high concentration gradient at the interface that drives one to pour into the other like two waterfalls (the diffusion from high chemical potentials). In a p-n junction the dominant component is the electrostatic potential. This is not true for a vast majority of electrochemical interfaces (electrochemical ultracapacitors, plant and algal photosystems, neuronal charge transfer) where chemical potential gradients prevail.

Space Charge
An excess or deficiency of electrons/holes/ions that build up in one region of a material (such as at the interface or junction). The layer of this excess/deficiency found at the interface of a junction is called the space charge layer (or double layer). In a p-n junction, this is best described as a deficiency. When a p-type semiconductor is connected to an n-type semiconductor via a junction, an electrochemical driving force or potential causes the charges to move toward one another and recombine, making a neutral zone called the depletion region.

Depletion Region
Another way to talk about the space charge layer at the junction (if we can talk about holes with respect to electrons, we can talk about depletion regions with respect to space charge layers). Opposites attract, right? Right–and then they recombine and there are no charges. As an analogy, imagine if you were to roll hot coals and ice cubes into one another–in the end you would have neither hot coals nor ice cubes (in fact, you get low energy mud). The same thing occurs for electron-hole pairs. They recombine and form a layer of no charge, which inhibits an more drift of electrons and holes across the junction. In reality this conjugate pouring tapers off (attenuates) and each material is left with a deficiency of electrons or holes (a space charge layer) that would not have existed without the junction.

Electric Field
Where you have a junction with a p-type and an n-type material (in the dark), a space charge layer (the insulating depletion region) rapidly develops. In response to this insulating layer, an electrostatic potential is maintained across the junction (0.6-0.7 V in silicon). This works just like a dam to hold back the waterfall event. When light of the appropriate energy level hits the p-n junction and is absorbed, electron-hole pairs are generated. What happens to these pairs? Well, if they have nowhere to go (no potential to separate them) they just recombine. But because the electric field is present, the charges feel a pull to pour over the “dam” and flow down the circuit.

The electric field from this potential is a cornerstone of First Generation Photovoltaics. In that particular technology, if you don’t have a “field” you cannot harvest the photovoltaic effect, because there is no driving force to separate photogenerated charges. But remember that the full scope of driving forces are described as the electrochemical potential, not just the electrostatic potential. Newer devices like Advanced Photovoltaics (and very efficient organisms called plants, algae, and photosynthetic bacteria) take advantage of chemical potentials as well.

So, do we have to define a solar cell as a p-n junction? NO, but they are really common in today’s technology.

Given America’s increasing need for efficient green energy generation and storage, we must maintain our innovative edge for materials development of photovoltaic devices using the power of the small, via nanostructured systems. I’m writing about a fresh view in environmental materials engineering: environmental premediation . An environmental materials area of research will complement the demand for CO₂-free energy systems and energy efficient technologies in our country. The advances in Third Generation photovoltaics (including inorganic-sensitized solar cells, and dye-sensitized solar cells) have provided us with new alternatives for reduced-cost solar energy conversion. And now photovoltaic cells using quantum dots as light absorbing materials are on the verge of major breakthroughs. In fact, recent results from NREL (US Dept. of Energy’s National Renewable Energy Laboratory) have demonstrated that solid-state photovoltaic composites based on light-absorbing quantum dots show promise to provide up to 65% energy conversion efficiency (3x that of current Si cell limits). The future is wide open for discovering energy solutions using the tools of nanotechnology in materials science.

1) Ellingson, R. J.; Beard, M. C.; Johnson, J. C.; Yu, P.;Milic, O. I.; Nozick, A. J.; Shabaev, A.; Efros, A. J. Nano Lett. 2005 5(5), 865.