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Nanoparticles by accident
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20/01/2003
11:52
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RESEARCH NEWS
Nanoparticles by accident NANOTECHNOLOGY
Nanoparticles forming on a Ge surface over a 500 minute period. (Courtesy of Lon A. Porter.)
A simple method for depositing precious metal nanoparticles on a surface “came about by accident,” said Lon A. Porter of Purdue University at the MRS Fall meeting. Porter has observed the growth of Au, Pd,
and Pt nanoparticles on the surface of Ge and GaAs wafers while simply bathing the wafers in dilute aqueous solutions of the metal salts HAuCl4, Na2PdCl4, and Na2PtCl4. The deposited particles show excellent adhesion, and are not removed by sonication or Scotch® tape. X-ray photoelectron spectroscopy shows the metal is in the zero oxidation state. Au, Pd, and Pt are indispensable to electronics device fabrication, catalysis, support substrates, and sensor elements. This new electroless deposition method, in contrast to complex and expensive vacuum deposition methods, is a simple technique for the interfacing of metal nanostructures with semiconductor surfaces. Control of particle size is possible through varying the soaking time, temperature, and solution concentration. Particles grow by an island mechanism, and after an hour form a continuous film. Porter has also been able to pattern the nanoparticle deposition using photolithography, microcontact printing, and dip-pen nanolithography. Porter is confident that this simple control of a very straightforward technique has the potential to be developed for applications and added functionality. He is investigating the chemisorption of thiols to the very high surface area given by films of Au nanoparticles. He hopes it will be possible to develop this process for biosensor arrays.
Single molecule testing MOLECULAR ELECTRONICS The development, and acceptance, of molecular electronics has been limited by the inability to test the properties of single molecules in a reliable, defined, and simple manner. It remains an open question as to whether the reported measurements from test devices represent the true electronic properties of a single molecule or are the result of collective intermolecular characteristics. An additional problem is that the current method of evaporating a metal contact onto molecular devices can result in asymmetries of the metal/molecule interface, which could skew measurements. Now, however, researchers from Motorola Labs and Arizona State University claim to have come up with an answer. The team has developed a nonlithographic method for the electrical testing of single or small bundles of molecules [Appl. Phys.
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February 2003
Lett. (2002) 81 (16), 3043]. The new approach uses an Au-coated tip of a conducting atomic force microscope (cAFM) to contact an Au nanoparticle, which is attached to the molecule(s) in question. The Au nanoparticle is attached to the unreacted top thiol terminus of a dithiolated molecule, which has been by chemically inserted into the natural defect sites of an insulating, self-assembled monolayer formed on top of an epitaxial Au substrate. To demonstrate the capabilities of the new approach, the researchers made transport measurements on two test molecules. The results show qualitative agreement with previously reported data for similar thousand-molecule arrays. This also indicates, say the researchers, that such data is a molecular and not intermolecular phenomenon.
Bringing particles into line COLLOIDS Using non-uniform AC electric fields to assemble micro- and nanoscale particles into ordered structures may provide a general method of fabricating nanodevices, say researchers at the University at Buffalo, New York. They have demonstrated the controlled organization of colloidal microparticles into one-, two-, and three-dimensional structures over length scales much bigger than the particle size [Electrophoresis (2002) 23, 2174]. Non-uniform electric fields can be used to manipulate micron-sized polarizable objects. The motion of the objects (dielectrophoresis) depends on a number of factors including electrode configuration, voltage and AC frequency, particle size and concentration, and the difference in dielectric properties between the particles and the surrounding medium. Aristides Docoslis and Paschalis Alexandridis show that this motion can be predicted for different electrode, particle, and media combinations, and used to direct assembly of structures. “This process enables you to guide particles to where you want them to go and then scale them up into ordered structures with desired electrical, optical, or mechanical properties,” explains Alexandridis. Once the structure has been assembled between the electrodes, it can be fixed permanently by crosslinking the particles, or through noncovalent interactions. The great advantage of this method is its general applicability to almost any particle. “Because of this flexibility, there’s no limit to the applications of this process,” says Alexandridis. He is working to extend the technique to nanoscale particles, which could benefit the manufacture of sensors and photonic devices.
20/01/2003
11:52
Page 12
RESEARCH NEWS
Nanoparticles by accident NANOTECHNOLOGY
Nanoparticles forming on a Ge surface over a 500 minute period. (Courtesy of Lon A. Porter.)
A simple method for depositing precious metal nanoparticles on a surface “came about by accident,” said Lon A. Porter of Purdue University at the MRS Fall meeting. Porter has observed the growth of Au, Pd,
and Pt nanoparticles on the surface of Ge and GaAs wafers while simply bathing the wafers in dilute aqueous solutions of the metal salts HAuCl4, Na2PdCl4, and Na2PtCl4. The deposited particles show excellent adhesion, and are not removed by sonication or Scotch® tape. X-ray photoelectron spectroscopy shows the metal is in the zero oxidation state. Au, Pd, and Pt are indispensable to electronics device fabrication, catalysis, support substrates, and sensor elements. This new electroless deposition method, in contrast to complex and expensive vacuum deposition methods, is a simple technique for the interfacing of metal nanostructures with semiconductor surfaces. Control of particle size is possible through varying the soaking time, temperature, and solution concentration. Particles grow by an island mechanism, and after an hour form a continuous film. Porter has also been able to pattern the nanoparticle deposition using photolithography, microcontact printing, and dip-pen nanolithography. Porter is confident that this simple control of a very straightforward technique has the potential to be developed for applications and added functionality. He is investigating the chemisorption of thiols to the very high surface area given by films of Au nanoparticles. He hopes it will be possible to develop this process for biosensor arrays.
Single molecule testing MOLECULAR ELECTRONICS The development, and acceptance, of molecular electronics has been limited by the inability to test the properties of single molecules in a reliable, defined, and simple manner. It remains an open question as to whether the reported measurements from test devices represent the true electronic properties of a single molecule or are the result of collective intermolecular characteristics. An additional problem is that the current method of evaporating a metal contact onto molecular devices can result in asymmetries of the metal/molecule interface, which could skew measurements. Now, however, researchers from Motorola Labs and Arizona State University claim to have come up with an answer. The team has developed a nonlithographic method for the electrical testing of single or small bundles of molecules [Appl. Phys.
12
February 2003
Lett. (2002) 81 (16), 3043]. The new approach uses an Au-coated tip of a conducting atomic force microscope (cAFM) to contact an Au nanoparticle, which is attached to the molecule(s) in question. The Au nanoparticle is attached to the unreacted top thiol terminus of a dithiolated molecule, which has been by chemically inserted into the natural defect sites of an insulating, self-assembled monolayer formed on top of an epitaxial Au substrate. To demonstrate the capabilities of the new approach, the researchers made transport measurements on two test molecules. The results show qualitative agreement with previously reported data for similar thousand-molecule arrays. This also indicates, say the researchers, that such data is a molecular and not intermolecular phenomenon.
Bringing particles into line COLLOIDS Using non-uniform AC electric fields to assemble micro- and nanoscale particles into ordered structures may provide a general method of fabricating nanodevices, say researchers at the University at Buffalo, New York. They have demonstrated the controlled organization of colloidal microparticles into one-, two-, and three-dimensional structures over length scales much bigger than the particle size [Electrophoresis (2002) 23, 2174]. Non-uniform electric fields can be used to manipulate micron-sized polarizable objects. The motion of the objects (dielectrophoresis) depends on a number of factors including electrode configuration, voltage and AC frequency, particle size and concentration, and the difference in dielectric properties between the particles and the surrounding medium. Aristides Docoslis and Paschalis Alexandridis show that this motion can be predicted for different electrode, particle, and media combinations, and used to direct assembly of structures. “This process enables you to guide particles to where you want them to go and then scale them up into ordered structures with desired electrical, optical, or mechanical properties,” explains Alexandridis. Once the structure has been assembled between the electrodes, it can be fixed permanently by crosslinking the particles, or through noncovalent interactions. The great advantage of this method is its general applicability to almost any particle. “Because of this flexibility, there’s no limit to the applications of this process,” says Alexandridis. He is working to extend the technique to nanoscale particles, which could benefit the manufacture of sensors and photonic devices.