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Molecular Spintronics

Molecular Spintronics. S. K. Nayak Department of Physics, Applied Physics, and Astronomy Rensselaer Polytechnic Institute, Troy. Collaborators. Dr. R. Pati L. Senapati M. Mailmann Y. Zhang Physics , RPI Professors P. Ajayan and G. Ramanath Mat. Sci. and Eng., RPI

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Molecular Spintronics

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  1. Molecular Spintronics S. K. Nayak Department of Physics, Applied Physics, and Astronomy Rensselaer Polytechnic Institute, Troy

  2. Collaborators Dr. R. Pati L. Senapati M. Mailmann Y. Zhang Physics, RPI Professors P. Ajayan and G. Ramanath Mat. Sci. and Eng.,RPI Professor A. M. Rao, Clemson University Y. Wu, Dr. P. Giannozzi, Professor R. Car Princeton University Professor N. Marzari,MIT Professors R. Reifenberger and Datta, Purdue

  3. Scope of the Talk Introduction Fundamental Questions Technological Applications Spintronics at the Molecular Level Experimental results Theoretical Results

  4. - - - - - Si M - - - - - - - SiO2 - - - - - - - - - - - - - - - Nanoelectronics • Moore’s Law • Device sizes halve every 5 years • This law, observed in the 60’s, still holds today • By Moore’s law, devices should reach atomic scale by 2025 • Moore’s law will come to an end by 2020.

  5. NanoScience and NanoTechnology DoD SRA http://www.nanosra.nrl.navy.mil • To achieve dramatic, innovative enhancements in the properties and performance of structures, materials, and devices that have controllable features on the nanometer scale (i.e., tens of Å). • The ability to affordably fabricate structures at the nanometer scale will enable new approaches and processes for manufacturing novel, more reliable, lower cost, higher performance and more flexible electronic, magnetic, optical, and mechanical devices.

  6. There's Plenty of Room at the Bottom • An Invitation to Enter a New Field of Physics by Richard P. Feynman December 29, 1959, APS Annual meeting • Atoms in a small world • What I want to talk about is the problem of manipulating and controlling things on a small scale. • When we get to the very, very small world---say circuits of seven atoms---we have a lot of new things that would happen that represent completely new opportunities for design.Atoms on a small scale behave like nothing on a large scale, for they satisfy the laws of quantum mechanics. So, as we go down and fiddle around with the atoms down there, we are working with different laws, and we can expect to do different things. We can manufacture in different ways. We can use, not just circuits, but some system involving the quantized energy levels, or the interactions of quantized spins, etc.

  7. Self-assembled InAs QDs on GaAs substract 30 nm 30 nm - Dot feature size 5-6 nm - + - NanoScience and NanoTechnology Materials and Phenomena at Nanometer-scale (10-100 Å) Offer the Opportunities to Realize Electronics Devices with Unprecedented Performance

  8. Nano-EnabledRevolutionary Capability • Nanoelectronics and Computer Technology • Monolithic electro optic devices that detect the entire infrared (SWIR-VLWIR) spectrum • Ultra-high performance massively parallel data processors to allow downlinking target information directly to the warfighter (e.g., molecular computers) • Novel communication devices providing unheard-of frequencies and bandwidth

  9. Single-Molecule Electron Devices • R. Metzger et al., Thin Solid Films, 327-329, 326 (1998) • Rectificatoin of current demonstrated by LB films (mono and multilayers) of g-(n-hexadecyl)quinolinium tricyanoquinodimethanide, C16H33Q-3CNQ • Collier et al., Science285, 391 (1999) • Logic operations (AND and OR) demonstrated by Rotaxane monolayer sandwitched between Ti and Al2O3 • Moresco et al., Phys. Rev. Lett.86, 672 (2000) • Current switching by STM manipulation of Cu-tetra-3,5 di-ter-butyl-phenyl porphyrin (Cu-TBPP) on Cu (211) surface: Possibly change in intra-molecular conformation • J. Chen, et al, Appl. Phys. Lett.77, 1224 (2000) • Room temperature negative differential resistance (NDR) exhibited by self-assembled monolayers of nitroamine and nitro substituted di(ethenylphenyl-benzene thiolate

  10. Single-Molecule Devices • C-Nanotube Based Electronic Devices • Explosion in the field: Andriotis et al, Phys. Rev. Lett. 87, (Aug 2001) • Bio-molecules Based Electronic Devices • Fink and Schoenenberger, Nature398, 407 (1999) • Conduction through DNA molecules • Photo-activated Molecular Devices • A. P. de Silva et al., J. Am. Chem. Soc. 122, 3965 (2000) • Fluoresecent based moleular logic and arithmatic • Nagatoshi et al., Nature401, 152 (1999) • Light-driven mono-directional molecular rotor • Bermudez, et al., Nature406, 608 (2000) • AC-field induced molecular rotor

  11. q q D A d- d+ H2N- -NO2 p-electron medium benzene ring WHY ORGANICS? Organic molecules offer a natural medium for controlled electron transport Can be used as Basic Device Elements -wire (connectors) -insulator -diode (switch, memory) -transistor para-nitroaniline (PNA)

  12. Molecular Electronics • CHALLENGES • Understanding electron transport and device physics in molecular systems • Conduction different from bulk: 1-electron, overlap of localized wavefunction, involvement of discrete energy levels, tunneling • Interface with microscopic and real world • Physics and chemistry of molecule-metal contact, assembly, fabrication, measurements and interpretation • OPPORTUNITIES • True breakthroughs: Exploration of new science ==> Engineering • Device Concepts • Traditional electronics vs. new devices based on new physical mechanisms

  13. Spintronics- (Spin Electronics): Telling the electron to remember its spin • Electron has negative charge and spin (magnetic moment: 1/2). • Electron seen by an electronician: • So far electronics industry are taking advantage of only its charge character to store and process information.

  14. Electronics Application • Primary electronic device: MOSFET • Disadvantage volatile of information limited density information reaching the fundamental limit

  15. Spin Alone Phenomena- Magnetism Electrons seen by magnetician Store information using spin Alignment of spins are important

  16. Magnetic Application • Primary magnetic Application: storage information • Disadvantage mechanical access

  17. Spin-electronics- Time for electron to take a spin Exploit the quantum nature Combine charge and spin to store information in terms of spin orientation (up/down) the spins will be attached to mobile electrons which will carry the information along with wire the information will be read at the terminal Spin coherence length is large (~nm to m)

  18. New Challenges Fundamental Questions- Injection of spins into semiconductors Spin coherence length (spin relaxation) Spin entanglement Interface effect New Phenomena: Giant Magnetic Resistance: (GMR) Tunneling Magnetic Resistance (TMR)

  19. New Technology magnetic disk heads (used in computer) magnetic random access memories (M-RAM) (non volatile) Forming Integrated Circuits- Ferromagnetic metal+semiconductor (still a challenge) Spin-transistor Spin Valve Quantum Computer Magnetic Tunneling Junction- Motorola

  20. Giant Magnetic Resistance (GMR) Baibich et al., PRL 61, 2472 (1988) Binasch et al., PRB 39, 4828 (1989)

  21. GMR Mechanism RF RAF GMR Ratio = (RAF+RF)/(RAF-RF) could be larger than 50 % Ferromagnetic Anti-Ferromagnetic

  22. Tunneling Magnetic Resistance (TMR) Moodera et al., PRL 74, 3273 (1995) Applications of TMR: magnetic random access memories (M-RAM)

  23. Injecting Spin Polarized Electrons in Semiconductor Awschalom, Nature, 397 (1999)

  24. Spin-electronics at Nanoscale Size of magnetic drive is also shrinking! Reading the data through GMR needs to go to molecular scale

  25. Spin-electronics at Nanoscale

  26. Magnetic Reading Head Size of magnetic drive is also shrinking! Reading the data through GMR needs to go to molecular scale

  27. New Questions and New Challenges Fundamental Interest: Can we inject spins into molecules Spin coherence length Heating and time scale involved Just a beginning ...

  28. Coherent Spin polarized transport through carbon nanotube Tsukagoshi Nature, 401, 572 (1999)

  29. V I H S C Ni Gold Molecules goes Spintronics Schon, J. H., Science, Published online, I:10.1126/science.1070563 (2002). Challenges- How to apply local magnetic field?

  30. First Principles Quantum Conductance Calculations of Spin Polarized Electron Transport in a Molecular Wire

  31. THEORETICAL PROCEDURE • We solve Schrödinger equation: Hψ(x1, x2, x3 …) = E ψ(x1, x2, x3 …) ψ is an N-electron wave function. • A simple but accurate way of solving the above equation is to use density functional theory. • Here we work with ρ(r) : 3N to 3 • Remarkable!

  32. DENSITY FUNCTIONAL THEORY HOHENBERG-KOHN (1964): Total energy of an interacting electron gas in presence of an external potential Vext (r): functional independent of Vext KOHN-SHAM (1965): kinetic energy exchange non interacting  correlation Local Density Approximation (LDA): Gradient Corrected Approximation (GGA):

  33. WORKING SCHEME 1-electron equations: where These equations are known as Kohn-Sham orbital equations. KS equations have the similar form as Hartree equations- but have correlation, in principle can work for all systems. In practice, the present formalism works great for systems where bonding is primarily chemical. Not successful for weakly bound systems- attempts are underway.

  34. Calculation of Current Non-equilibrium Green’s Function Methoda HMol: Molecular Hamiltonian : Self-energy function T: Transmission function aW. Tian, et al., J. Chem. Phys. 109, 2874 (1998)

  35. Ferromagnetic AF is lower in energy ! Anti-Ferromagnetic Electron Spin Density Plot for anti-parallel Spin Alignment

  36. I-V Characteristics For Different Spin Alignment Down spins majority carriers

  37. Conductance-Voltage Curve

  38. Conductance-Voltage Curve

  39. Spin Up are majority carriers- Spin valve effect is less. Conductance-Voltage Curve

  40. Oscillatory MR is Atomic Wires A. I. Yanson et al , “Formation of atomic gold wires” Nature, 395 783 (1998). H. Ohnishi et al, “Quantized Conductance through a chain of Gold atoms” Nature 395, 780 (1998).

  41. Transistor with different Oxidation State State- Experimental Results:[Co(tpy-(CH2)5-SH)2]2+ (longer molecule) Park, et. al. Nature, 417, 722, (2002) I–V curves of the single-electron transistor as gate voltage is varied: from -0.4 V (red) to -1.0 V (black) in increments of -0.15 V.

  42. Why Nanotube ? Metallic:n=m, and n-m=3i Semiconducting:n-m 3i • Unique molecular structure, Highly Stable (Thermally and Chemically) • Very small dimension (nm-mm) • Some are metallic ;J~ 1011 Electrons per Sec-nm2 (Copper Wire, J~ 106 Electrons per Sec-nm2); Some are semi-conducting (Eg~1/DNT) • Technological applications: Nanoelectronic devices. • aJ. Kong et al, Science 287 (2000) 622.

  43. Motivation Molecular and Nano-electronics * Progress Towards Miniaturization * Searching for New Device Architectures * Developing Compatible Technology Carbon Nanotubes Metallic and Semiconducting Conductivity found to be higher compared to the best metal Transconductance of the nanotube is found to be twice that of conventioanal MOSFET Arrays of nanotube transistors are shown to exhibit logic circuits. • New Devices and Geometries: Challenges • 3-D Architectures, Growth, Integration • Tailoring Nanotube Structure, Properties • Making and Characterizing Junctions, Networks

  44. Achievements of Last Two Years Fabricated aligned carbon nanotube arrays at desired locations on planar substrates using substrate templating and CVD with control over:* Nucleation & Termination Sites of Nanotubes * Surface-selectivity

  45. Effect of Molecular Adsorbate on Transport Property: Nano Sensor

  46. Theory: Effect of Molecular Adsorbate on Transport Property: Nano Sensor • Oxygen doping increases conductance. • Water decreases conductance.

  47. Endohedral Doping-Magnetic atom inside nano-tube This shows that C60 provides additional path for current. Applied Physics Letters -- February 25, 2002 -- Volume 80, Issue 8, pp. 1450-1452 K. Harihara et al. PRL,85(2000)5384 has shown that GdC82 can be encapsulated inside nanotube. Experiment done by HRTEM.

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