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Micro-second force spectroscopy
RESEARCH NEWS
Flipping sandwiches MOLECULAR ELECTRONICS Molecule-sized electronics components could soon be on the menu thanks to US research that has led to a simple recipe for sandwiching organic molecules between silicon and metal layers. Fabricating test structures consisting of a molecular junction held between a layer of gold and a layer of silicon is proving to be no simple task. Now, researchers at the National Institute of Standards and Technology (NIST) and the University of Maryland, have devised an approach that could one day miniaturize the electronics industry in the extreme, with single molecules acting as electronic switches [Coll et al., J. Am. Chem. Soc. (2009) 131, 12451]. The team’s approach side-steps the main problem with making the metal layer stick to the silicon – heat. Previously, researchers have tried to fabricate sandwiches by toasting them, but that damages the molecular structures. NIST’s Mariona Coll, Christina Hacker, and their colleagues have now found that they can create an ultrasmooth gold surface on a layer of polymer. They then form the molecular layer on to the gold and then
Flip chip lamination – a junction on the road to molecular electronics. flip it over on to a silicon surface to complete the sandwich. The researchers used a raft of spectroscopic techniques and electrical current-voltage measurements to verify that their silicon flip-chip sandwich has the structure they designed. “Both vibrational and electrical data indicate that electrical contact to the monolayer is formed while preserving the integrity of the molecules without metal filaments,” Coll says. The approach could now lead the way to making high-quality molecular junctions consisting of dense single layers of molecules bonded chemically to
metal and silicon electrodes. Coll calls the technique “flip-chip lamination” and suggests that it may lead to applications beyond microelectronic chip design, including biosensors for environmental and medical diagnostics. By choosing specific molecules to sandwich between the gold and silicon layers it should be possible to endow the flip-chips with selectivity for different chemicals in a sample, for instance. Moreover, the use of so-called molecular recognition and self assembly in which complementary components of a larger molecular aggregate meet and connect could allow specific functionality to be designed into the sandwich. “Our goal is to make a molecular junction that would advance the development and metrology of organic molecular electronics,” Hacker told Materials Today, “while being an adaptable fabrication approach that could be widely applied to other technologies.” Prototypes are only a few years away but much development is needed before widespread manufacture and use, she adds.
David Bradley
Micro-second force spectroscopy CHARACTERIZATION Proteins are dynamic molecular machines having structural flexibility that allows conformational changes. Current methods for the determination of protein flexibility rely mainly on the measurement of thermal fluctuations and disorder in protein conformations and tend to be experimentally challenging. Moreover, they reflect atomic fluctuations on picoseconds timescales, whereas the large conformational changes in proteins typically happen on micro- to millisecond timescales A group of scientists have successfully determined the flexibility of bacteriorhodopsin a protein that uses the energy in light to move protons across cell membranes at the microsecond timescale by monitoring force-induced deformations across the protein structure with a technique based on atomic force microscopy. They named the technique Microsecond force spectroscopy. Recently, specially designed torsional harmonic cantilevers (THC) have been developed to perform high speed force spectroscopic measurements while scanning the surface in tapping-mode AFM. Owing to the offset location of the sharp tip the torsional
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vibrations of this cantilever are sensitive to the forces on the vertically oscillating tip. To enable operation of this method in liquids with forces gentle enough to investigate proteins, the scientists redesigned the cantilever geometry and reduced its flexural and torsional force constants. The design was given the title liquid torsional harmonic cantilever (L-THC). Vibration spectra recorded in aqueous buffer demonstrate the ability of the torsional mode to enhance harmonic signals in liquids. Multifrequency excitation and detection of cantilever vibrations have proven to improve spatial resolution of imaging in liquid environments. The advantage of the enhancement of multiple harmonic signals with the L-THC is the ability to recover the tip–sample force waveforms which provide highspeed force–distance curves and allow specific material properties to be measured with high spatial resolution. The entire period of the tip sample force waveform is approx. 130 micro seconds and the interactions span 20 micro seconds. Furthermore, the waveform exhibits an rms. force noise of approx 10 pN. This represents more than three orders of magnitude improvement
SEPTEMBER 2009 | VOLUME 12 | NUMBER 9
SEM image of a liquid torsional harmonic cantilever. in force sensitivity compared to the measurements performed with conventional cantilevers in liquid. This advance enabled a reduction in the peak interaction forces to allow investigations of various types of proteins without causing them to be denatured. In addition, owing to the microsecond duration of force loading, the mechanical properties derived from these waveforms will reflect molecular behaviour at the microsecond timescale. Work continues to further enhance this technique and develop even further its application.
Jonathan Agbenyega
Flipping sandwiches MOLECULAR ELECTRONICS Molecule-sized electronics components could soon be on the menu thanks to US research that has led to a simple recipe for sandwiching organic molecules between silicon and metal layers. Fabricating test structures consisting of a molecular junction held between a layer of gold and a layer of silicon is proving to be no simple task. Now, researchers at the National Institute of Standards and Technology (NIST) and the University of Maryland, have devised an approach that could one day miniaturize the electronics industry in the extreme, with single molecules acting as electronic switches [Coll et al., J. Am. Chem. Soc. (2009) 131, 12451]. The team’s approach side-steps the main problem with making the metal layer stick to the silicon – heat. Previously, researchers have tried to fabricate sandwiches by toasting them, but that damages the molecular structures. NIST’s Mariona Coll, Christina Hacker, and their colleagues have now found that they can create an ultrasmooth gold surface on a layer of polymer. They then form the molecular layer on to the gold and then
Flip chip lamination – a junction on the road to molecular electronics. flip it over on to a silicon surface to complete the sandwich. The researchers used a raft of spectroscopic techniques and electrical current-voltage measurements to verify that their silicon flip-chip sandwich has the structure they designed. “Both vibrational and electrical data indicate that electrical contact to the monolayer is formed while preserving the integrity of the molecules without metal filaments,” Coll says. The approach could now lead the way to making high-quality molecular junctions consisting of dense single layers of molecules bonded chemically to
metal and silicon electrodes. Coll calls the technique “flip-chip lamination” and suggests that it may lead to applications beyond microelectronic chip design, including biosensors for environmental and medical diagnostics. By choosing specific molecules to sandwich between the gold and silicon layers it should be possible to endow the flip-chips with selectivity for different chemicals in a sample, for instance. Moreover, the use of so-called molecular recognition and self assembly in which complementary components of a larger molecular aggregate meet and connect could allow specific functionality to be designed into the sandwich. “Our goal is to make a molecular junction that would advance the development and metrology of organic molecular electronics,” Hacker told Materials Today, “while being an adaptable fabrication approach that could be widely applied to other technologies.” Prototypes are only a few years away but much development is needed before widespread manufacture and use, she adds.
David Bradley
Micro-second force spectroscopy CHARACTERIZATION Proteins are dynamic molecular machines having structural flexibility that allows conformational changes. Current methods for the determination of protein flexibility rely mainly on the measurement of thermal fluctuations and disorder in protein conformations and tend to be experimentally challenging. Moreover, they reflect atomic fluctuations on picoseconds timescales, whereas the large conformational changes in proteins typically happen on micro- to millisecond timescales A group of scientists have successfully determined the flexibility of bacteriorhodopsin a protein that uses the energy in light to move protons across cell membranes at the microsecond timescale by monitoring force-induced deformations across the protein structure with a technique based on atomic force microscopy. They named the technique Microsecond force spectroscopy. Recently, specially designed torsional harmonic cantilevers (THC) have been developed to perform high speed force spectroscopic measurements while scanning the surface in tapping-mode AFM. Owing to the offset location of the sharp tip the torsional
10
vibrations of this cantilever are sensitive to the forces on the vertically oscillating tip. To enable operation of this method in liquids with forces gentle enough to investigate proteins, the scientists redesigned the cantilever geometry and reduced its flexural and torsional force constants. The design was given the title liquid torsional harmonic cantilever (L-THC). Vibration spectra recorded in aqueous buffer demonstrate the ability of the torsional mode to enhance harmonic signals in liquids. Multifrequency excitation and detection of cantilever vibrations have proven to improve spatial resolution of imaging in liquid environments. The advantage of the enhancement of multiple harmonic signals with the L-THC is the ability to recover the tip–sample force waveforms which provide highspeed force–distance curves and allow specific material properties to be measured with high spatial resolution. The entire period of the tip sample force waveform is approx. 130 micro seconds and the interactions span 20 micro seconds. Furthermore, the waveform exhibits an rms. force noise of approx 10 pN. This represents more than three orders of magnitude improvement
SEPTEMBER 2009 | VOLUME 12 | NUMBER 9
SEM image of a liquid torsional harmonic cantilever. in force sensitivity compared to the measurements performed with conventional cantilevers in liquid. This advance enabled a reduction in the peak interaction forces to allow investigations of various types of proteins without causing them to be denatured. In addition, owing to the microsecond duration of force loading, the mechanical properties derived from these waveforms will reflect molecular behaviour at the microsecond timescale. Work continues to further enhance this technique and develop even further its application.
Jonathan Agbenyega