Device proves solar cell potential of high bandgap inorganic nanowire arrays

A report, published in the March 14 edition of the Journal of Materials Chemistry, announced the successful fabrication and testing of a new type solar cell using an inorganic core/shell nanowire structure.

Arrays of core/shell nanowires (described has "quantum coaxial cables") had previously been theorized as a potential structure that, while composed of chemically more stable large bandgap inorganic materials, should also be capable of absorbing the broad range of the wavelengths present in sunlight. High bandgap semiconductors are generally considered not effective at absorbing most of the available wavelengths in solar radiation by themselves. For instance, high bandgap zinc oxide (ZnO) is transparent in the visible but absorptive in the ultraviolet range, and thus is widely used in sunscreens but was not considered useful in solar cells.

In the report, a team of researchers from Xiamen University in China and the University of North Carolina at Charlotte describe successfully creating zinc oxide (ZnO) nanowires with a zinc selenide (ZnSe) coating to form a material structure known as a type-II heterojunction that has a significantly lower bandgap than either of the original materials. The team reported that arrays of the structured nanowires were subsequently able to absorb light from the visible and near-infrared wavelengths, and show the potential use of wide bandgap materials for a new kind of affordable and durable solar cell.

"High bandgap materials tend to be chemically more stable than the lower bandgap semiconductors that we currently have," noted team member Yong Zhang, a Bissell Distinguished Professor in the Department of Electrical and Computer Engineering and in the Energy Production and Infrastructure Center (EPIC) at the University of North Carolina at Charlotte.

"And these nanowire structures can be made using a very low cost technology, using a chemical vapor deposition (CVD) technique to grow the array," he added. "In comparison, solar cells using silicon and gallium arsenide require more expensive production techniques."

Based on a concept published in Nano Letters in 2007 by Zhang and collaborators Lin-Wang Wang (Lawrence Berkeley National Laboratory) and Angelo Mascarenhas (National Renewable Energy Laboratory), the array was fabricated by Zhang's current collaborators Zhiming Wu, Jinjian Zheng, Xiangan Lin, Xiaohang Chen, Binwang Huang, Huiqiong Wang, Kai Huang, Shuping Li and Junyong Kang at the Fujian Key Laboratory of Semiconductor Materials and Applications in the Department of Physics at Xiamen University, China.

Past attempts to use high band gap materials did not actually use the semiconductors to absorb light but instead involved coating them with organic molecules (dyes) that accomplished the photo absorption and simply transmitted electrons to the semiconductor material. In contrast, the team's heterojunction nanowires absorb the light directly and efficiently conduct a current through nano-sized "coaxial" wires, which separate charges by putting the excited electrons in the wires' zinc oxide cores and the "holes" in the zinc selenide shells.

"By making a special heterojunction architecture at the nanoscale, we are also making coaxial nanowires which are good for conductivity," said Zhang. "Even if you have good light absorption and you are creating electron-hole pairs, you need to be able to take them out to the circuit to get current, so we need to have good conductivity. These coaxial nanowires are similar to the coaxial cable in electrical engineering. So basically we have two conducting channels -- the electron going one way in the core and the hole going the other way in the shell."

The nanowires were created by first growing an array of six-sided zinc oxide crystal "wires" from a thin film of the same material using vapor deposition. The technique created a forest of smooth-sided needle-like zinc oxide crystals with uniform diameters (40 to 80 nanometers) along their length (approximately 1.4 micrometers). A somewhat rougher zinc selenide shell was then deposited to coat all the wires. Finally, an indium tin oxide (ITO) film was bonded to the zinc selenide coating, and an indium probe was connected to the zinc oxide film, creating contacts for any current generated by the cell.

"We measured the device and showed the photoresponse threshold to be 1.6 eV," Zhang said, noting that the cell was thus effective at absorbing light wave wavelengths from the ultraviolet to the near infrared, a range that covers most of the solar radiation reaching earth's surface.

Though the use of the nanowires for absorbing light energy is an important innovation, perhaps even more important is the researchers' success in using stable high bandgap inorganic semiconductor materials for an inexpensive but effective solar energy device.

"This is a new mechanism, since these materials were previously not considered directly useful for solar cells," Zhang said. He stressed that the applications for the concept do not end there but open the door to considering a larger number of high bandgap semiconductor materials with very desirable material properties for various solar energy related applications, such as hydrogen generation by photoelectrochemical water splitting.

"The expanded use of type II nanoscale heterostructures also extends their use for other applications as well, such as photodetectors -- IR detector in particular," he noted.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by University of North Carolina at Charlotte, via EurekAlert!, a service of AAAS.

Journal Reference:

Zhiming Wu, Yong Zhang, Jinjian Zheng, Xiangan Lin, Xiaohang Chen, Binwang Huang, Huiqiong Wang, Kai Huang, Shuping Li, Junyong Kang. An all-inorganic type-II heterojunction array with nearly full solar spectral response based on ZnO/ZnSe core/shell nanowires. Journal of Materials Chemistry, 2011; 21 (16): 6020 DOI: 10.1039/C0JM03971C

Researchers advance toward hybrid spintronic computer chips

 Researchers here have created the first electronic circuit to merge traditional inorganic semiconductors with organic "spintronics" -- devices that utilize the spin of electrons to read, write and manipulate data.

Ezekiel Johnston-Halperin, assistant professor of physics, and his team combined an inorganic semiconductor with a unique plastic material that is under development in colleague Arthur J. Epstein's lab at Ohio State University.

Last year, Epstein, Distinguished University Professor of physics and chemistry and director of the Institute for Magnetic and Electronic Polymers at Ohio State, demonstrated the first successful data storage and retrieval on a plastic spintronic device.

Now Johnston-Halperin, Epstein, and their colleagues have incorporated the plastic device into a traditional circuit based on gallium arsenide. Two of their now-former doctoral students, Lei Fang and Deniz Bozdag, had to devise a new fabrication technique to make the device.

In a paper published onlineon April 13 in the journal Physical Review Letters, they describe how they transmitted a spin-polarized electrical current from the plastic material, through the gallium arsenide, and into a light-emitting diode (LED) as proof that the organic and inorganic parts were working together.

"Hybrid structures promise functionality that no other materials, neither organic nor inorganic, can currently achieve alone," Johnston-Halperin said. "We've opened the door to linking this exciting new material to traditional electronic devices with transistor and logic functionality. In the longer term this work promises new, chemically based functionality for spintronic devices."

Normal electronics encode computer data based on a binary code of ones and zeros, depending on whether an electron is present or not within the material. But researchers have long known that electrons can be polarized to orient in particular directions, like a bar magnet. They refer to this orientation as spin -- either "spin up" or "spin down" -- and this approach, dubbed spintronics, has been applied to memory-based technologies for modern computing. For example, the terabyte drives now commercially available would not be possible without spintronic technology.

If scientists could expand spintronic technology beyond memory applications into logic and computing applications, major advances in information processing could follow, Johnston-Halperin explained. Spintronic logic would theoretically require much less power, and produce much less heat, than current electronics, while enabling computers to turn on instantly without "booting up." Hybrid and organic devices further promise computers that are lighter and more flexible, much as organic LEDs are now replacing inorganic LEDs in the production of flexible displays.

A spintronic semiconductor must be magnetic, so that the spin of electrons can be flipped for data storage and manipulation. Few typical semiconductors -- that is, inorganic semiconductors -- are magnetic. Of those that are, all require extreme cold, with operating temperatures below -150 degrees Fahrenheit or -100 degrees Celsius. That's colder than the coldest outdoor temperature ever recorded in Antarctica.

"In order to build a practical spintronic device, you need a material that is both semiconducting and magnetic at room temperature. To my knowledge, Art's organic materials are the only ones that do that," Johnston-Halperin said. The organic magnetic semiconductors were developed by Epstein and his long-standing collaborator Joel S. Miller of the University of Utah.

The biggest barrier that the researchers faced was device fabrication. Traditional inorganic devices are made at high temperatures with harsh solvents and acids that organics can't tolerate. Fang and Bozdag solved this problem by building the inorganic part in a traditional cleanroom, and then adding an organic layer in Epstein's customized organics lab -- a complex process that required a redesign of the circuitry in both parts.

"You could ask, why didn't we go with all organics, then?" Johnston-Halperin said. "Well, the reality is that industry already knows how to make devices out of inorganic materials. That expertise and equipment is already in place. If we can just get organic and inorganic materials to work together, then we can take advantage of that existing infrastructure to move spintronics forward right away."

He added that much work will need to be done before manufacturers can mass-produce hybrid spintronics. But as a demonstration of fundamental science, this first hybrid circuit lays the foundation for technologies to come.

For the demonstration, the researchers used the organic magnet, which they made from a polymer called vanadium tetracyanoethylene, to polarize the spins in an electrical current. This electrical current then passed through the gallium arsenide layer, and into an LED.

To confirm that the electrons were still polarized when they reached the LED, the researchers measured the spectrum and polarization of light shining from the LED. The light was indeed polarized, indicating the initial polarization of the incoming electrons.

The fact that they were able to measure the electrons' polarization with the LED also suggests that other researchers can use this same technique to test spin in other organic systems.

Coauthors on the paper included former doctoral student Chia-Yi Chen and former postdoctoral researcher Patrick Truitt.

This research was funded by the National Science Foundation's Materials Research Science and Engineering Centers program, Ohio State's Institute for Materials Research, and the Department of Energy.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Ohio State University. The original article was written by Pam Frost Gorder.

Journal Reference:

Lei Fang, K. Bozdag, Chia-Yi Chen, P. Truitt, A. Epstein, E. Johnston-Halperin. Electrical Spin Injection from an Organic-Based Ferrimagnet in a Hybrid Organic-Inorganic Heterostructure. Physical Review Letters, 2011; 106 (15) DOI: 10.1103/PhysRevLett.106.156602

IYC Weekly Round-up, 4/2-4/8

I made an egregious omission from last week’s round-up. I refer, of course, to the IYC Chemistry Dance from #ACSAnaheim:

Also on the video front, winners of the “It’s Elemental” video contest, sponsored by Dow Chemical and hosted by the Chemical Heritage Foundation, were announced yesterday. Eleven schools received grants from Dow, and all video submissions can be viewed from the contest page on CHF’s website.

 

Making a Case for the Overqualified

You think I’m qualified for the job? I’m delighted you think so! When do I start? What’s that? You said overqualified? Really, now, that’s quite a compliment. You’re making me blush. I’m sorry – am I missing something? You say “overqualified” like it’s a bad thing. Oh…I see. I’ll just show myself out, then.

In my current combined job search and self-discovery vision quest, I’ve been met on different fronts with the recurring theme that a wealth of experience may, in fact, be a detriment. There is no shortage of “expert” advice, online or otherwise, suggesting that you should hide or neglect to mention years of education and/or employment. If your light is too bright  or its spectrum contains too many wavelengths for the position, hide it under the nearest bushel. Okay, honestly, I do get it – target your resume and cover letter toward a specific position. Focus I understand. However, I can’t completely evade the feeling that this gamesmanship of playing hide-and-seek and cherry-picking facts seems disingenuous at best, dishonest at worst. It’s somewhat against the grain of how one is trained to think as a scientist.

Even if one hasn’t been met with this particular o-word per se, it lies not too far beneath concerns that are more openly stated.

Prospective employers are worried that so-called overqualified candidates might jump ship at the first opportunity for a better position elsewhere. They’re concerned that after going through the interview process, they won’t be able to seal the deal because their budget can’t meet the candidate’s salary requirements. They fear their new hire may soon be bored. This sort of thinking is, well, a bit risk-averse, shall we say.

A recent post by Amy Gallo on the Harvard Business Review blog makes a case for taking such a risk. A challenge is posed:

“When making hiring decisions, visionary leaders don’t just focus on the current needs, but on the future.”

So, will the final hiring decision for the position you desire be made by such a visionary leader? Does the future lurch and loom darkly before them, or will they embrace the challenges ahead? I think it’s safe to say that most people would prefer to work for someone in the latter category. A perceived benefit for a hiring manager to adopt this mindset is driven home:

“Hiring overqualified candidates can help you achieve much higher productivity, grow, and achieve opportunities that you may not even be thinking about pursuing right now.” There are other less obvious benefits too: these employees can mentor others, challenge peers to exceed current expectations, and bring in areas of expertise that are not represented at the company.

Sounds good, doesn’t it? Honestly, though, don’t most people’s jobs change over time? There are new developments in technology, best practices, knowledge within your discipline, business needs, what have you, that necessitate modifying some aspect of what you do. If you’re adamantly resistant to change, you’ll be left behind. Successful people aren’t usually like that, though. They have amassed their supply of deep, diverse experience because they want to learn all the time – that’s what has driven them from day one. They don’t wait for knowledge to be fed to them; they seek it out like it’s a special treat, and then devour it – nom nom nom nom. They evolve; curiosity and a hunger for knowledge feed their evolution. To behave otherwise invites negative consequences. The philosopher and writer of social commentary Eric Hoffer put it best: “In a time of drastic change it is the learners who inherit the future. The learned usually find themselves equipped to live in a world that no longer exists.”  This preferred path of continuous learning will reap benefits whether you’re an experienced professional, a new chemistry graduate, or anywhere in between.

Okay, prospective employers, here’s my mission statement. While I’m in your employ, you will have my full attention. I will give my all and strive to grow in the position. All I ask is a chance to do what I do best every day. I will reward your courage with my efforts to contribute and make a difference. That’s my story, and I’m sticking to it.

 

Caught red-handed: Detection of latent fingerprints through release of fluorescein from a nanofiber mat

When a forensic agent dusts a surface with powder or exposes it to the vapors of an iodine chamber, mystery fans know what is going on: This is how latent fingerprints are made visible so that they can be compared to those of a suspect. Su Chen and a team at Nanjing University of Technology have now developed a new process for especially rapid and simple detection of fingerprints. As the Chinese researchers report in the journal Angewandte Chemie, all it takes is a special nanofiber mat that is pressed onto the suspect surface and briefly treated with hot air -- the fingerprints appear as red ridge patterns.

When we touch a surface, tiny traces of perspiration and oils stay behind, mirroring the ridge patterns on our fingertips. There are now a number of different methods to make these latent fingerprints visible. The new method is significantly faster than the classic technique of dusting with powder. Unlike spectroscopic methods, it does not require complex technical instruments, and problematic chemicals like ninydrin are not needed either. In addition, it is suitable for all types of surfaces: by lightly pressing the mat onto the surface, the researchers were able to reliably transfer fingerprints from a wide variety of materials, including steel, quartz, glass, plastic, marble, and wood.

The secret of their success is the special mat, a fleece made from nanofibers of thermoplastic polyurethane and fluorescein, a dye. The mat is made in a process called electrospinning. When the mat comes into contact with a fingerprint, components of the perspiration react with the polyurethane, causing cross-linking of the polymer chains. The hot air accelerates the reaction. In the cross-linked regions, the fluorescein cannot remain within the fibers so it comes out as a powdery solid. However, the dye only fluoresces when it is very finely dispersed in the nanofibers, not when it is in small solid clumps. This causes the color of the mat to change from straw yellow to red, making the fingerprint visible within 30 seconds in daylight. The method only works with nanofibers, because only they have enough surface area to produce a visible reaction.

The mat can identify more than mere fingerprints. The researchers were able to "print" an image of a small dragon onto the mat by using an ink-jet printer. Their ink was simply water, which can also cause the cross-linking reaction. The combination of ink-jet printing and the release of a chemical from a nanofiber mat could also be used to produce miniaturized systems such as sensors, microreactors, and diagnostic chips.

More information: Su Chen, et al., A Release-Induced Response for the Rapid Recognition of Latent Fingerprints and Formation of Inkjet-Printed Patterns, Angewandte Chemie International Edition 2011, 50, No. 16, 3706–3709, Permalink to the article: http://dx.doi.org/ … ie.201006537

Provided by Wiley (news : web)

New drugs from mutant bugs

Scientists from the Universities of Birmingham and Bristol have discovered how marine bacteria join together two antibiotics they make independently to produce a potent chemical that can kill drug-resistant strains of the MRSA superbug.

Working with Japanese pharmaceutical company Daiichi-Sankyo, and funded by the UK Biotechnology and Biological Sciences Research Council (BBSRC), the researchers’ work paves the way for the creation of new hybrid antibiotics that may help to solve the growing problem of bacterial infections that are resistant to essentially all antibiotics.

The research is published online in the journal PLoS ONE.

The team, comprising microbial geneticists from Birmingham and chemists from Bristol, determined the sequence of the complete DNA content of the marine bacterium that produces the new antibiotic, thiomarinol, owned by Daiichi-Sankyo. They then identified the genes responsible for making the antibiotic on the basis of their similarity to genes that make the related but less potent antibiotic, mupirocin, which is currently used to combat MRSA (methicillin resistant Staphylococcus aureus).

They found the genes are on a relatively small, separate DNA molecule called a plasmid, which is just big enough to carry the genes for making the antibiotic plus genes to allow the plasmid to replicate autonomously in the bacterium. The plasmid thus carries genes that make both the mupirocin-like antibiotic as well a second antibiotic, holomycin, and a gene responsible for joining both antibiotics together, forming a more potent molecule.

Tests showed that by joining the antibiotics together the resulting chemical is able to inhibit the growth of MRSA strains that have become resistant to mupirocin. ‘This shows how mupirocin can be modified to make it more potent and suggests that related molecules could be used against the increasingly problematic Enterobacteriacae like Escherichia coli and Klebsiella pneumoniae,’ says University of Birmingham research lead Professor Chris Thomas.

By using mutant strains that were unable to make either the mupirocin part or the holomycin part the team was able to feed alternative compounds to the bacteria – so-called mutasynthesis - so that a family of novel molecules was created, and tests showed some of these had biological activity. ‘This provides hope that the system will allow the production of new antibiotics that may help to combat the growing problem of antibiotic resistance in pathogenic bacteria,’ adds University of Bristol research lead Professor Tom Simpson.

More information: A Natural Plasmid Uniquely Encodes Two Biosynthetic Pathways Creating a Potent Anti-MRSA Antibiotic is published in PLoS ONE. It is available online at http://dx.plos.org … pone.0018031
The detailed chemical analysis was recently published in Angewandte Chemie International Edition at http://dx.doi.org/ … ie.201007029

Provided by University of Birmingham

Windows that block heat only on hot days: New research brings us closer

New materials science research from the University at Buffalo could hasten the creation of "smart" windows that reflect heat from the sun on hot summer days but let in the heat in colder weather.

The findings concern a unique class of synthetic chemical compounds that are transparent to infrared light at lower temperatures, but undergo a phase transition to begin reflecting infrared when they heat up past a certain point.

An article detailing some of these discoveries appears today (April 7) on the cover of the Journal of Physical Chemistry Letters. Additional papers have appeared online or in print in CrystEngComm, the Journal of Materials Chemistry and Physical Review B.

In the papers, UB researchers report that they have managed to manipulate the trigger temperature for vanadium oxide, one such material. The advance is a crucial step toward making the compound useful for applications such as coatings for energy-saving windows.

By preparing vanadium oxide as a nanomaterial instead of in bulk, the scientists managed to lower the compound's trigger point from 153 degrees Fahrenheit to 90. Doping vanadium oxide nanowires with tungsten brought the temperature down further, to 7 degrees Fahrenheit. Molybdenum doping had a similar but smaller effect.

Researchers also found that they were able to induce a phase transition using an electric current instead of heat.

UB chemist Sarbajit Banerjee led the studies, collaborating with Sambandamurthy Ganapathy, an assistant professor of physics, to head the Physical Review B research on the use of the electric current.

"Definitely, we are closer than we've ever been to being able to incorporate these materials into window coatings and other systems that sense infrared light," said Banerjee, an assistant professor. "What we found is an example of how much of a difference finite size can make. You have a material like vanadium oxide, where the phase transition temperature is too high for it to be useful, and you produce it as a nanomaterial and you can then use it right away."

Banerjee and Ganapathy previously led research projects demonstrating that, in nanoscale form, two additional synthetic compounds -- copper vanadate and potassium vanadate -- exhibit phase transitions akin to those in vanadium oxide.

Banerjee's work has caught the attention of the National Renewable Energy Laboratory, which has contacted him to discuss developing window coatings that could improve the energy efficiency of buildings with heating or air conditioning systems. The technology could be particularly useful in places like Phoenix and Las Vegas that experience extreme summer temperatures.

Besides smart windows, vanadium oxide could also be useful in products including computer chips, night-vision instruments and missile guidance systems, Banerjee said.

Two major awards are funding Banerjee's research on the material: A Cottrell Scholar Award from the Research Corporation for Science Advancement, announced this year, and a National Science Foundation CAREER award, the foundation's most prestigious award for junior investigators.

More information: http://pubs.acs.or … 21/jz101640n
The research is described in a video at http://pubs.acs.or … e-video.html

Provided by University at Buffalo (news : web)