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New material puts organic transistors under water
RESEARCH NEWS
New material puts organic transistors under water SENSORS/POLYMERS
Researchers from the US and Germany have developed a new polymeric material that allows organic thin-film transistors (OTFTs) to operate stably in water [Roberts et al., Proc. Natl. Acad. Sci. (2008) doi: 10.1073/pnas.0802105105]. The advance could be a boon for low-cost, disposable chemical and biological sensors. OTFTs are attractive for sensing applications because they can be fabricated on large-area, flexible substrates and have active layers that can be tuned to detect a variety of different analytes. Exposing OTFTs to a variety of solvents in the vapor phase produces a change in the device current – which is straightforward to detect. But the high operating voltages, degradation, and delamination of OTFTs under humid or aqueous conditions have limited their use as sensors in real applications. Zhenan Bao’s team at Stanford University, together with colleagues from the Max Planck Institute for Polymer Research, have created OTFTs that operate at low voltage and is stable under water. The device relies on a new cross-linked polymer gate dielectric and a stable organic
A water droplet with trace amount of trinitrobenzene on the surface of an organic transistor. The presence of the analytes in the semiconductor channel results in a disturbance to the charge transport causing a change in output current. Plastic materials form the basis of new electronic sensors for chemical detection in air or water. (Courtesy of Stefan C. B. Mannsfeld, Mark Roberts and Zhenan Bao, Stanford University.)
semiconductor. “We successfully cross-linked poly(4-vinylphenol) or PVP with commercially
available dianhydride molecules at relatively low temperatures, yielding well-insulated films with high capacitance,” explains Bao. To test the sensing capabilities of the OTFT, the researchers then constructed an elastomeric flow cell directly on the surface. The OTFT is sensitive to trinitrobenzene down to 300 ppb, glucose down to 10 ppm, and cystine down to 100 ppb. “OTFTs can be used to detect low concentrations of chemicals in a complex environment without encapsulation,” says Bao. The researchers are now working on a variety of other interesting analytes including the explosive trinitrotoluene, a chemical warfare nerve agent, and DNA. It is a big plus for OTFTs to be able to interact chemically with many different analytes, says Ananth Dodabalapur of the University of Texas at Austin. “[The results] are very interesting in that low voltages are used to operate the organic transistor, which is very helpful in avoiding ionic currents,” he adds. “This is an important advance.” Cordelia Sealy
Graphene puffs up under pressure CARBON Graphine is a one atom thick crystal layer, a chemically stable and electrically conducting membrane exhibiting a variety of unique properties due to its novel molecular structure One of the big question still remaining unanswered was; can such membranes be impermeable to atoms, molecules and ions? Researchers at Cornell University in the US have addressed this question for gases: They successfully used micron-scale graphene sheets to create the world’s smallest balloons. The team isolated graphene sheets by mechanical exfoliation, placing them across wells that had been created in silica substrates. Van der Waals forces held the sheets in place around their circumference, forming sealed microchambers nearly five microns on a side. Both positive and negative pressure differentials were created across the atom-thick membranes by placing the microchambers under pressure or in vacuum and then allowing the pressure in the chambers to equilibrate over hours or days. The sheets were then imaged by atomic force microscopy, showing that they bulged inward or outward significantly (see image).
The approach was attempted with nitrogen, air, and helium as the high pressure gases, and with graphene thicknesses from just one to 75 layers. The team found that the timescale of the equilibration to ambient pressure was not dependent on the thickness of the graphene. Thus, any leakage was either through the glass or the interface. The result is important, says lead author Scott Bunch, because it shows that “a single sheet of graphene is impermeable to helium gas atoms and therefore free of any significant vacancy over micron size areas.” Measuring the size of the bulges above or below the substrate for given pressure differentials allowed estimates of the sheets’ elasticity, which the researchers found to be more or less equal to that of graphite. That solves a longstanding question about the use of bulk elastic constants for nanoscale materials [Bunch, et. al., Nano Lett. (2008), DOI: 10.1021/nl801457b]. The work suggests graphene sheets are applicable as incredibly sensitive pressure sensors, and selectively patterning the sheets would make them ideal for ultrafiltration, the authors say. Graphene
Pressure differentials across the atom thick membranes, imaged by atomic force microscopy. Image credits Zoom in Schematic - Victor Yu-Juei Tzen. drumheads can also offer the opportunity to probe the permeability of gases through atomic vacancies in single layers of atoms. And what next for the team? Of course, Bunch says, there’s an inevitable, irresistible desire: to pop the balloons. “By popping graphene balloons we will determine how various gases diffuse through the atomic size openings created by our ‘pops’.”
Jason Palmer
OCTOBER 2008 | VOLUME 11 | NUMBER 10
13
New material puts organic transistors under water SENSORS/POLYMERS
Researchers from the US and Germany have developed a new polymeric material that allows organic thin-film transistors (OTFTs) to operate stably in water [Roberts et al., Proc. Natl. Acad. Sci. (2008) doi: 10.1073/pnas.0802105105]. The advance could be a boon for low-cost, disposable chemical and biological sensors. OTFTs are attractive for sensing applications because they can be fabricated on large-area, flexible substrates and have active layers that can be tuned to detect a variety of different analytes. Exposing OTFTs to a variety of solvents in the vapor phase produces a change in the device current – which is straightforward to detect. But the high operating voltages, degradation, and delamination of OTFTs under humid or aqueous conditions have limited their use as sensors in real applications. Zhenan Bao’s team at Stanford University, together with colleagues from the Max Planck Institute for Polymer Research, have created OTFTs that operate at low voltage and is stable under water. The device relies on a new cross-linked polymer gate dielectric and a stable organic
A water droplet with trace amount of trinitrobenzene on the surface of an organic transistor. The presence of the analytes in the semiconductor channel results in a disturbance to the charge transport causing a change in output current. Plastic materials form the basis of new electronic sensors for chemical detection in air or water. (Courtesy of Stefan C. B. Mannsfeld, Mark Roberts and Zhenan Bao, Stanford University.)
semiconductor. “We successfully cross-linked poly(4-vinylphenol) or PVP with commercially
available dianhydride molecules at relatively low temperatures, yielding well-insulated films with high capacitance,” explains Bao. To test the sensing capabilities of the OTFT, the researchers then constructed an elastomeric flow cell directly on the surface. The OTFT is sensitive to trinitrobenzene down to 300 ppb, glucose down to 10 ppm, and cystine down to 100 ppb. “OTFTs can be used to detect low concentrations of chemicals in a complex environment without encapsulation,” says Bao. The researchers are now working on a variety of other interesting analytes including the explosive trinitrotoluene, a chemical warfare nerve agent, and DNA. It is a big plus for OTFTs to be able to interact chemically with many different analytes, says Ananth Dodabalapur of the University of Texas at Austin. “[The results] are very interesting in that low voltages are used to operate the organic transistor, which is very helpful in avoiding ionic currents,” he adds. “This is an important advance.” Cordelia Sealy
Graphene puffs up under pressure CARBON Graphine is a one atom thick crystal layer, a chemically stable and electrically conducting membrane exhibiting a variety of unique properties due to its novel molecular structure One of the big question still remaining unanswered was; can such membranes be impermeable to atoms, molecules and ions? Researchers at Cornell University in the US have addressed this question for gases: They successfully used micron-scale graphene sheets to create the world’s smallest balloons. The team isolated graphene sheets by mechanical exfoliation, placing them across wells that had been created in silica substrates. Van der Waals forces held the sheets in place around their circumference, forming sealed microchambers nearly five microns on a side. Both positive and negative pressure differentials were created across the atom-thick membranes by placing the microchambers under pressure or in vacuum and then allowing the pressure in the chambers to equilibrate over hours or days. The sheets were then imaged by atomic force microscopy, showing that they bulged inward or outward significantly (see image).
The approach was attempted with nitrogen, air, and helium as the high pressure gases, and with graphene thicknesses from just one to 75 layers. The team found that the timescale of the equilibration to ambient pressure was not dependent on the thickness of the graphene. Thus, any leakage was either through the glass or the interface. The result is important, says lead author Scott Bunch, because it shows that “a single sheet of graphene is impermeable to helium gas atoms and therefore free of any significant vacancy over micron size areas.” Measuring the size of the bulges above or below the substrate for given pressure differentials allowed estimates of the sheets’ elasticity, which the researchers found to be more or less equal to that of graphite. That solves a longstanding question about the use of bulk elastic constants for nanoscale materials [Bunch, et. al., Nano Lett. (2008), DOI: 10.1021/nl801457b]. The work suggests graphene sheets are applicable as incredibly sensitive pressure sensors, and selectively patterning the sheets would make them ideal for ultrafiltration, the authors say. Graphene
Pressure differentials across the atom thick membranes, imaged by atomic force microscopy. Image credits Zoom in Schematic - Victor Yu-Juei Tzen. drumheads can also offer the opportunity to probe the permeability of gases through atomic vacancies in single layers of atoms. And what next for the team? Of course, Bunch says, there’s an inevitable, irresistible desire: to pop the balloons. “By popping graphene balloons we will determine how various gases diffuse through the atomic size openings created by our ‘pops’.”
Jason Palmer
OCTOBER 2008 | VOLUME 11 | NUMBER 10
13