RESEARCH/RESEARCHERS · supported networks behave as field-effect transistors. For a dielectric...

RESEARCH/RESEARCHERS

MRS BULLETIN/NOVEMBER 2003 789

Flexible Transistors with HighCarrier Mobilities Made fromCarbon Nanotubes

Organic-based electronics are of greatinterest as a replacement for inorganicsemiconductor devices, as the former areinexpensive, lightweight, and flexible,allowing the development of large flexibledisplays and electronic paper. However,progress in integrated organic-based elec-tronic devices has been slow due to the lowcharge-carrier mobilities of these materials,which typically range from ~0.1 cm2/V s—for example, poly(3-hexylthiophene)—to a maximum of ~2 cm2/V s for the bestp-type crystalline organic semiconductors,with n-type carrier mobilities even lower.Carbon nanotubes show promise in over-coming these limitations because theircarrier mobility exceeds even commonsemiconductor materials. Not only docarbon nanotubes exhibit very highstrength, but their flexibility makes them apromising material for the development oflarge-scale flexible electronics. A team ofresearchers from Nanomix Inc. in Emery-ville, Calif., recently demonstrated carbonnanotube-based p-type transistor networkssupported on a flexible polymer substratewith an order of magnitude increase inhole-carrier mobility.

As reported in the October issue of NanoLetters, K. Bradley, J.-C.P. Gabriel, and G.Grüner of Nanomix manufactured andtested carbon nanotube network transis-tors on flexible polymer substrates. Thesepolymer-supported networks show highdurability during repeated bending and ex-hibit a hole-carrier mobility of 12 cm2/V s.The researchers used chemical vapordeposition to grow the carbon nanotubenetworks on 200-nm-thick silicon oxidelayers on a Si substrate that consist of ran-domly oriented individual nanotubesrather than bundles. Using conventionallithography, the researchers patternedTi/Au source/drain contacts onto thenanotube networks (3.5 nm Ti followed by50 nm Au; 200 µm pads separated by 50 µmgaps), resulting in a field-effect transistornetwork with on/off ratios as high as 104.The scientists transferred the semicon-ducting networks to a flexible polymersubstrate by spin-coating a 15 µm poly-imide film as the polymer support ontothe silicon substrate, followed by a HFetch step to remove the SiO2 layer toinduce device lift-off.

The devices were tested for their per-formance by placing the polyimide filmson a metal chuck, which served as thegate electrode. The resulting device trans-fer characteristics exhibited large modu-lations in conductance for voltages of lessthan 100 V, confirming that the polymer-

supported networks behave as field-effect transistors. For a dielectric film asthick as 15 µm, these switching voltagesare remarkably low, according to theresearchers. The research team said thatthe low switching voltage is due to thehigh carrier mobility of the carbon nano-tube network, with a measured holemobility of 12 cm2/V s. This mobility isan order of magnitude improvementover the best currently used organicmaterials. The flexible network devicesare remarkably resilient: A 60° bendingproduces only a 12% decrease in conduc-tance with full recovery over 12 bendingcycles performed, despite the presence ofmultiple nanotube–nanotube junctions.The fabricated polymer-supported net-work transistors exhibit electronic prop-erties that are similar to those of moreconventional, silicon-supported carbonnanotube devices; they are flexible andcould be made inexpensively.

ALFRED A. ZINN

Fluorescence from IndividualSingle-Walled Carbon NanotubesIs Not Intermittent

Potential photonic applications of single-walled carbon nanotubes (SWNTs) in-

clude nanometer-scaled integratedelectroluminescent devices. Developmentof nanodevices, by their very nature,would benefit from the determination ofspecific optical properties of individualmolecules, such as the true spectrallinewidth. Until now, only ensemble-averaged spectra of SWNTs, in whichspectral details are obscured, have beenreported. However, a research team hasreported fluorescence spectra from indi-vidual SWNTs. Their findings were pub-lished in the September 5 issue of Science.

University of Rochester and UniversitätSiegen researcher A. Hartschuh, togetherwith a team of researchers, isolated indi-vidual nanotubes by spin-coating a SWNTsuspension onto a glass coverslip. Atomicforce microscopy showed that shortSWNTs, with lengths of 200–300 nm, pre-dominated. A density of about 10–20Raman-active nanotubes per 100 µm2 wasdetermined with confocal Raman imaging.Laser excitation at 633 nm ensured thatRaman signals (between 633 nm and 770nm) were isolated from the fluorescencesignals above 850 nm.

Using simultaneous resonance Ramanand fluorescence measurements, the re-searchers found that individual nanotubes

For over 25 years the Glassman name is synonymouswith High Reliability Performance and has built it’s reputation as a market leader by it’s dedication andinvestment in technology development as well as after-sale customer support that is second to none in the power supply industry.

For Vacuum processing applications, Glassman’sFX(300W), EK(600W) and EQ(1.2kW) series powersupplies will meet your needs for low-mid powerrequirements, with typical application output voltagerange from 300V-10kV. All feature compact size,advanced arc management, low stored energy, and use air as the primary dielectric—A Glassman Innovation!

Key Features:• Proprietary Arc Sense and Arc Quench

circuitry for ultimate power supply and load protection

• High frequency technology-up to 60kHz• Tight regulation and high stability• Low stored output energy• RS-232 serial interface option

For more information on the complete line of Glassman High Voltage power supplies, visit our website at www.glassmanhv.com or call us today at 908-638-3800.

Glassman High Voltage, Inc., 124 West Main Street, PO Box 317, High Bridge, NJ 08829-0317

Phone: (908) 638-3800, FAX: (908) 638-3700www.glassmanhv.com email: [email protected]

In Europe, Glassman Europe Limited (UK) +44 1256 883007, FAX: +44 1256 883017

In Asia, Glassman Japan Limited +81 45 902 9988, FAX: +81 45 902 2268

GLASSMAN HIGH VOLTAGE INC.Designing Solutions for High Voltage and Vacuum

Processing DC Power Supply Applications

GLASSMAN HIGH VOLTAGEDC Power Supply Solutions for Vacuum Processing Applications

EQ Series 1200 watt

FX Series 300 watt

EK Series 600 watt

Check out our new websitewww.glassmanhv.com

For small DC Sputtering sources, DC bias, Ion sources and Glow DischargePlasma Requirements

For more information, see http://advertisers.mrs.org

www.mrs.org/publications/bulletin


MRS BULLETIN/NOVEMBER 2003

with identical structures—that is, identical diameters and chirali-ties—display different emission energies, although each individ-ual emission line does match transitions observed in ensemble-averaged spectra. The researchers used low-energy Raman scat-tering attributed to the nanotube’s radial breathing mode to veri-fy that the source of the fluorescence was individual nanotubes.The researchers attribute the heterogeneous emission propertiesobtained from SWNTs with identical diameter and chirality val-ues to localized defects and perturbations.

For moderate excitation energies (<70 kW cm-2), time traces ofthe fluorescence of individual SWNTs showed constant ampli-tude for 100 s. The researchers said that by contrast both molec-ular dyes and individual semiconducting quantum dots exhibitfluorescence emission intermittencies or an on/off blinkingbehavior over very long time scales. The researchers concludethat “SWNTs have the potential to provide a stable, single-mole-cule infrared photon source with extremely narrow linewidth.”

STEVEN TROHALAKI

Three-Phase Boundary Fuel-Cell Reactor Produces H2O2 at 93% Selectivity

Industrial production of hydrogen peroxide (H2O2) requiresseparation and distillation from a solution of alkylatedanthraquinones (from crude oil) in a multistep procedure that con-sumes a lot of energy. For certain applications, a costly electrolysisprocess is used, but this option is not commonplace for high-scaleproduction. Uses of H2O2 include water and wastewater treat-ment, bleaching of textiles and paper, food processing, and evenodor control. To create a cost-effective and energy-efficient systemsuitable for large-scale production of H2O2, I. Yamanaka and hiscolleagues at the Tokyo Institute of Technology have designed afuel-cell reactor with a three-phase boundary that overcomes thedisadvantages of previous designs. With a three-phase boundaryformed by gaseous O2, aqueous electrolyte, and a solid porouscathode, this reactor reduces the chance of O2 and H2 coming intocontact and exploding; it generates electricity along with H2O2.

As reported in the August 8 issue of Angewandte Chemie, boththe cathode and anode were fabricated from hot-pressed vapor-grown carbon fiber and poly(tetrafluoroethylene) powder,adding a small amount of a carbon black material to the cathodeand platinum black powder to the anode. Pure O2 and H2 arethe starting gases, and titration against KMnO4 determined theconcentration of H2O2. A cation membrane separated the elec-trolyte compartment between the cathode and the anode intotwo sections, each one filled with NaOH. Diffusion of H2O tothe cathode compartment under stagnant conditions depletedthe anode compartment and reduced the current density.Pumping NaOH continuously into the anode compartmenthelped stabilize the current density. Electrochemical measure-ments determined that the rate-controlling reaction was thereduction of O2. Using this reactor, H2O2 selectivity based on H2or current efficiency based on the two-electron reaction is 93%after 2 h, the rate of formation is 2.0 mmol/h cm2, the concentra-tion is 7.0 wt%, and the current density is 100 mA/cm2, similarto that obtained by electrolysis (80–120 mA/cm2). By substitut-ing O2 for air, the current efficiency is 88% after 3 h, the rate offormation is 1.3 mmol/h cm2, the concentration is 6.5 wt%, andthe current density is 78 mA/cm2, which is a slight decrease butstill a good performance yet less expensive, representing anadvantage for industrial-scale production.

SIARI SOSA

Single-Molecule Resistance Measured by Repeated Formation of Molecular Junctions

In the process of using individual molecules to create electroniccircuits, the resistance level of a simple molecule like an alkane



MRS BULLETIN/NOVEMBER 2003 791

chain covalently attached to two electrodesis still a subject of debate. B. Xu and N.J.Tao from Arizona State University havemeasured the resistance of single moleculesby repeatedly forming molecular junctions

between a gold scanning tunneling micro-scope (STM) tip and a gold surface, asreported in the August 29 issue of Science.

The molecular junctions were created ina liquid cell by repeatedly moving a gold

STM tip into and out of contact with thesample. In the first step, the formation of achain of Au atoms was observed in puresolvents (toluene and water). The conduc-tance of this chain decreased with increas-

Review Articles and Special IssuesAnnals of Biomedical Engineering 31 (9) (October 2003) con-

tains “Bioreactors for Cardiovascular Cell and TissueGrowth: A Review” by V. Barron, E. Lyons, C. Stenson-Cox,P.E. McHugh, and A. Pandit (p. 1017).

Applied Mechanics Reviews 56 (5) (September 2003) contains“Use of Conventional Optical Fibers and Fiber Bragg Gratingsfor Damage Detection in Advanced Composite Structures: AReview” by K.S.C. Kuang and W.J. Cantwell (p. 493) and“Recent Developments and Applications of InvariantIntegrals” by Y.-H. Chen and T. J. Lu (p. 515).

Crystallography Reports 48 (5) (September 2003) contains“Magnetoplastic Effect: Basic Properties and PhysicalMechanisms” by V.I. Alshits, E.V. Darinskaya, M.V. Koldaeva,and E.A. Petrzhik (p. 768); and “The Influence of MagneticEffects on the Mechanical Properties and Real Structure ofNonmagnetic Crystals” by A.A. Urusovskaya, V.I. Alshits,A.E. Smirnov, and N.N. Bekkauer (p. 796).

Journal of Applied Physics 94 (8) (October 15, 2003) contains“Fluorescence Intensity Ratio Technique for Optical FiberPoint Temperature Sensing” by S.A. Wade, S.F. Collins, andG.W. Baxter (p. 4743).

Journal of Energy Resources Technology 125 (3) (September2003) contains “Innovative Technologies for Managing OilField Waste” by J.A. Veil (p. 238).

Journal of Experimental and Theoretical Physics 97 (2) (August2003) contains “Superconductivity in Doped NondegenerateInsulators” by A.I. Agafonov and E.A. Manykin (p. 358) and“X-Ray Optical Activity: Applications of Sum Rules” byJ. Goulon, A. Rogalev, F. Wilhelm, C. Goulon-Ginet, P. Carra,I. Marri, and Ch. Brouder (p. 402).

Journal of Microelectromechanical Systems 12 (4) (August2003) contains “Surface Tension-Powered Self-Assembly ofMicrostructures—The State of the Art” by R.R.A. Syms,E.M. Yeatman, V.M Bright, and G.M. Whitesides (p. 387).

Journal of Vacuum Science & Technology B: Microelectronics andNanometer Structures 21 (5) (September 2003) contains “Reviewof Trench and Via Plasma Etch Issues for Copper DualDamascene in Undoped and Fluorine-Doped Silicate GlassOxide” by D.L. Keil, B.A. Helmer, and S. Lassig (p. 1969).

Physics of the Solid State 45 (10) (October 2003) contains“Band Structure and Properties of Superconducting MgB2 andRelated Compounds (A Review)” by A.L. Ivanovskii (p. 1829).

Review of Scientific Instruments 74 (10) (October 2003) con-tains “Recent Advancements in Microwave ImagingPlasma Diagnostics” by H. Park, C.C. Chang, B.H. Deng,C.W. Domier, A.J.H. Donné, K. Kawahata, C. Liang, X.P. Liang,H.J. Lu, N.C. Luhmann Jr., A. Mase, H. Matsuura, E. Mazzucato,A. Miura, K. Mizuno, T. Munsat , Y. Nagayama, M.J. van de Pol,J. Wang, Z.G. Xia, and W-K. Zhang (p. 4239).

Reviews of Modern Physics 75 (4) (October 2003) contains“Colloquium: Saturation of Electrical Resistivity” byO. Gunnarsson, M. Calandra, and J.E. Han (p. 1085); “Hamil-tonian Theories of the Fractional Quantum Hall Effect” byG. Murthy and R. Shankar (p. 1101); and “How to DetectFluctuating Stripes in the High-Temperature Superconductors”

by S.A. Kivelson, I.P. Bindloss, E. Fradkin, V. Oganesyan,J.M. Tranquada, A. Kapitulnik, and C. Howald (p. 1201).

Semiconductors 37 (10) (October 2003) contains “Modifi-cation of Hg1-xCdxTe Properties by Low-Energy Ions” byK.D. Mynbaev and V.I. Ivanov-Omskii (p. 1127).

IEEE Transactions on Semiconductor Manufacturing 16 (3)(August 2003) features a “Guest Editorial: Special Section onCompound Semiconductor Microelectronics Manufacturing:The Future is Here.”

International Journal of Mass Spectrometry 229 (1–2)(September 2003) features “Mass Spectrometry Contributionsto Nanosciences and Nanotechnology.”

Journal of Biomedical Optics 8 (3) (July 2003) features a“Special Section on Multiphoton Microscopy.”

Journal of Engineering Materials and Technology 125 (4) (October2003) features a “Special Issue on Durability and DamageTolerance of Heterogeneous Materials and Structures.”

Journal of Microlithography, Microfabrication, and Micro-systems 2 (4) (October 2003) features a “Special Section onSurface Micromachining” and a “Special Section on Micro-Optics for Photonic Networks.”

Journal of Pressure Vessel Technology 125 (3) (August 2003)features a “Special Issue on Pressure Vessels TechnologyApplied to Gun Tubes.”

Journal of Vibration and Acoustics 125 (4) (October 2003) fea-tures “Special Issue: The Contributions of Jørgen W. Lund toRotor Dynamics.”

Optical Engineering 42 (10) (October 2003) features a “SpecialSection on Advances in Optical Waveguide Engineering.”

Applied Surface Science 219 (1–2 )(October 15, 2003) contains“Proceedings of Applied Surface Modeling: Experiment,Theory and Simulations.”

Cryogenics 43 (10–11) (October–November 2003) containsProceedings of the Korea-Japan Joint Workshop 2002 onApplied Superconductivity and Cryogenics.

Infrared Physics & Technology 44 (5–6) (October–December2003) contains Proceedings of the Workshop on QuantumWell Infrared Photodetectors.

Journal of Physics and Chemistry of Solids 64 (9–10) (September2003) contains selected papers from the International Con-ference on Ternary and Multinary Compounds (ICTMC13).

Microelectronics Reliability 43 (9–11) (September–November2003) contains Proceedings of the 14th European Symposiumon Reliability of Electron Devices, Failure Physics, andAnalysis.

Optical Materials 24 (1–2) (October–November 2003) con-tains Proceedings of the Fifth French-Israeli Workshop onOptical Properties of Inorganic Materials.

Solid-State Electronics 47 (10) (October 2003) contains papersselected from the 12th Workshop on Dielectrics in Micro-electronics.

Thin Solid Films 442 (1–2) (October 1, 2003) contains select-ed papers from the 4th International Conference on Coatingson Glass.


792 MRS BULLETIN/NOVEMBER 2003

ing tip distance in a stepwise fashion, witha step value equal to the quantum of con-ductance, until the conductance valuealmost reached zero. Additional stepswith step values of approximately integralmultiples of 1/100 of the conductancequantum were observed for 1 mM 4,4’-bipyridine in a 0.1 M NaClO4 water solu-tion. The researchers attribute these addi-tional conductance steps to the formationof junctions with one, two, three, andmore molecules. In order to confirm thatthe conductance steps were caused by theformation of stable molecular junctions, asolution of 2,2’-bipyridine molecules wastested. In 2,2’-bipyridine, the positions of

two nitrogen atoms prevent the moleculefrom simultaneously binding to two elec-trodes, and no conduction steps wereobserved. The same experiments wereperformed for hexanedithiol, octanedithiol,and decanedithiol molecules in toluene.The conductance showed stepwise behav-ior, but the step size was smaller than inthe case of 4,4’-bipyridine because of thehigher resistance of N-alkanedithiol mole-cules as compared with 4,4’-bipyridine.The researchers said that the resistance ofN-alkanedithiol molecules correspondswell with the widely accepted model ofelectron tunneling through the molecule.

MAXIM NIKIFOROV

“Design Rules” of Silk ProductionUnraveled in Studies of Bombyxmori Cocoons

Traditionally, silk fibers produced bysilkworms and spiders are harvested, dis-entangled, and then woven into fabrics.H.-J. Jin and D.L. Kaplan from TuftsUniversity have begun to unravel the invitro mechanism of silk processing ininsects and spiders. As described in theAugust 28 issue of Nature, the researchersmonitored the behavior of the silk fibroinsolutions as a function of decreasingwater content. Starting with the cocoonsof the domestic silk moth Bombyx mori,the researchers “degummed” the cocoonsto generate sericin-free fibroin fibers. Thefamily of sericin proteins is hydrophilicand acts as the glue between fibroinfibers. The sericin in these aqueousfibroin solutions was substituted withpolyethylene oxide (PEO). The silkfibroin and the PEO molecules preferen-tially vie for the water molecules inwhich the protein is dissolved. As thefibroin concentration is increased and thewater content is lowered, micelles formfirst, ranging in diameter from 100 nm to200 nm; subsequently, “globules” form asthe water content is further decreased.Through scanning electron microscopyand atomic force microscopy, the re-searchers observed that these globulesarise from phase separation between thehydrophilic and hydrophobic segments.Films of the fibroin aqueous solutionswere cast and subsequently treated withmethanol, physical shear, and stretching.The characteristics of the resulting silkstructures depended critically upon thenature of the postprocessing.

The researchers describe a process thatcombines the primary sequence of thesilk proteins and the biological environ-ment of the gland during silk spinning tocreate silk fibers. These “design rules,”said the researchers, apply to all silk pro-teins in terms of processing in aqueousenvironments, which can lead to materi-als engineering in aqueous systems ofnew silk-based materials with desiredproperties for potential applications intissue engineering and biomaterials.

LARKEN E. EULISS

Carbon Nanocoil Supports SurpassPerformance of OtherNanostructured Materialsin Direct Methanol Fuel Cells

T. Hyeon of Seoul National University,Y.-E. Sung of Kwangju Institute ofScience and Technology, and colleagueshave developed carbon nanocoils as acatalyst support for the fabrication ofdirect methanol fuel-cell (DMFC) elec-For more information, see http://advertisers.mrs.org



trodes. They synthesized the carbonnanocoils by heat-treating solid compos-ites of a carbon precursor (resorcinolformaldehyde gel), silica sol, and a transi-tion-metal salt. The research team saidthat because the synthetic procedure issimple and inexpensive, it can be readilyapplied to cost-effective, large-scale pro-duction of the material. High surface areaand good crystallinity are the characteris-tics needed for the catalyst support forDMFCs. As the researchers reported inthe September 22 issue of AngewandteChemie, their synthesized carbon materialexhibited a high surface area of 318 m2/gand was composed of 5–10-nm-thickgraphitic coils. The crystallinity of thematerial was comparable to that of car-bon nanotubes. The researchers said thatthis was especially important because it isdifficult to obtain crystalline carbonmaterials with a large surface area. Forexample, they said, many activated car-bons have surface areas exceeding 1000m2/g, but they are noncrystalline andamorphous. Alternatively, graphite ishighly crystalline, but has a very smallsurface area of <10 m2/g.

Carbon is a critical material for DMFCsbecause a good support exhibiting ahomogeneous high dispersion of catalyticspecies is a key factor dominating theoverall performance and stability of fuelcells. Hyeon and colleagues identifiedhighly dispersed PtRu alloy catalysts ofup to 60 wt% on their carbon support,resulting in higher catalytic activity formethanol oxidation. In a half-cell test formethanol oxidation at 0.6 V, the electrodeusing the carbon nanocoil had six timeshigher current than that of a commercial-ized fuel-cell carbon substrate.

The carbon nanocoil performed betteras a support material for fuel cells thanother nanostructured carbon materials. Inparticular, the nanocoils exhibited a cur-rent density that was three times higherat a given voltage than that of thegraphitic carbon nanofiber, which wasknown to be one of the best catalyst sup-ports for DMFC electrodes. In nanocoilexperiments using a full fuel cell—com-posed of a cathode, anode, and polymermembrane electrolyte—the maximumpower density increased by >85% overthe standard commercial sample at 30°C.

Oxide Superlattices Containing Si Nanocrystals and Er IonsLuminesce Efficiently at 1.54 µm

M. Zacharias and colleagues from theMax Planck Institute of MicrostructurePhysics (Halle, Germany) have developeda means of controlling the size of siliconnanocrystals and custom-manufacturingthese crystals on 4 in. wafers. The tech-nique is based on a combination of super-lattices—multilayer structures with layerthicknesses of a few nanometers andvarying bandgaps—and phase separationin the ultrathin layers.

The superlattice structure of amor-phous silicon oxide layers (SiOx/SiO2)was manufactured using a standarddeposition technique. The researchersemployed a variation of this technique byevaporating the silicon oxide either in avacuum or in an oxygen-containingatmosphere. The resulting amorphousSiO/SiO2 superlattice structure was thentempered in a nitrogen-containing atmos-phere at 1100°C. Through the thermallyactivated phase separation, the SiO in theultrathin sublayers transformed into puresilicon nanocrystals and amorphous SiO2,





whereby the nanocrystals were automati-cally surrounded with an appropriatebarrier material.

The size of the crystals within therange, for example, of 2–5 nm can bedetermined by the thickness of the layer.The distance between the crystals can beadjusted by varying the thickness of theSiO2 barrier layers and the oxygen con-tent of the SiOx layers. A higher oxygencontent would automatically lead to ahigher proportion of the amorphous SiO2after the phase separation and thereby toa larger distance between the siliconnanocrystals within the sublayer. Theluminescence of the silicon increases withthe number of crystals, and the quantumefficiency of smaller crystals is higherthan that of larger crystals; therefore, pri-marily very small crystals at high density

are required for the highest possibleluminescence intensity.

The researchers said that when thenanocrystal structures are implantedwith erbium ions, an effective energytransfer occurs from the nanocrystals tothe Er3+ ions, and the luminescence shiftsto ~1.54 µm. This particular wavelengthis of technological importance, since thefiber-optic cable employed in opticaldata transmission exhibits a transmissionmaximum at 1.54 µm. Additionally,erbium-doped glass fiber is used as anamplifier for optical data transmission.Memory circuitry is another area ofapplication of silicon nanocrystals inoxide matrices. The researchers submit-ted a patent for their technique for tailor-made silicon nanocrystals on 4 in.wafers.

A Spin-Transfer Switch for Magnetic Memory Spintronics?Recent studies have demonstrated that when a spin-polarized electric current is

passed through a ferromagnet a few nanometers thick, the nanomagnet experiences alarge torque due to direct transfer of spin angular momentum. This allows the mag-netic moment of the nanomagnet to oscillate at microwave frequencies around itssymmetry axis until it reverses its orientation, thereby accomplishing a magneticswitch. D.C. Ralph of Yale University, S.I. Kiselev of Cornell University, and their co-researchers have demonstrated the nature of the physics behind this phenomenon.They reported their work in the September 25 issue of Nature.

The research team formed a 130 nm × 70 nm magnetic multilayered nanopillarwith layers of magnetic Co and nonmagnetic Cu and Pt deposited on a silicon sub-strate. The multilayered composition was 80 nm Cu/40 nm Co/10 nm Cu/3 nmCo/2 nm Cu/30 nm Pt. A dc current bias was applied through a top Cu contact, anda static magnetic field was applied in the sample plane. The 40-nm-thick Co layer isreferred to as the “fixed” layer because it is relatively insensitive to the applied mag-netic field, whereas the 3 nm Co layer is referred to as the “free” layer because it issensitive to the applied field. The electrons in the dc current were spin-polarized asthey passed through the fixed layer. This spin-polarized current was then passedthrough the free layer. This subjected the magnetic moment of this layer to the spin-transfer torque, resulting in the emission of microwaves. When the researchers meas-ured the microwaves and mapped the strength of the oscillations as a function ofelectric current and magnetic field, they revealed several types of magnetic excitation.The researchers said the only input was the dc electric current and the applied staticmagnetic field, while the output contained microwave emissions in a frequencyrange predicted by theory and prior calculations, clearly demonstrating the spin-transfer mechanism.

This study shows that the magnetic multilayered structure acts as a nanoscalemotor, although there is no mechanical motion. Energy from the dc current was con-verted into high-frequency magnetic rotations. This work has implications for spin-tronic devices, such as magnetic memory devices. The researchers suggest that nano-magnets driven by spin-polarized currents could be used as tunable nanoscalemicrowave sources or oscillators. GOPAL R. RAO

FOR MORE RESEARCH NEWS ON MATERIALS SCIENCE . . . . . . access the Materials Research Society Web site:

w w w . m r s . o r g / g a t e w a y / m a t l _ n e w s . h t m l

FOR MORE RESEARCH NEWS ON MATERIALS SCIENCE . . .

Find what youneed quickly

48,000 items plus detailed

technical data

5% discount on online orders

Metals & Materials from

© 2003 Goodfellow Corporation

Metals • Alloys • Ceramics • Polymers1-800-821-2870

fax [email protected]

FasterBetter

Cheaper

www.goodfellow.com


www.mrs.org/publications/bulletin

RESEARCH/RESEARCHERS · supported networks behave as field-effect transistors. For a dielectric...

Documents

Transcript of RESEARCH/RESEARCHERS · supported networks behave as field-effect transistors. For a dielectric...