Molecular SpintronicsMolecular Spintronics
S. K. NayakS. K. NayakDepartment of Physics, Applied Physics, and AstronomyDepartment of Physics, Applied Physics, and Astronomy
Rensselaer Polytechnic Institute, TroyRensselaer Polytechnic Institute, Troy
CollaboratorsCollaborators
Dr. R. PatiDr. R. PatiL. SenapatiL. Senapati
M. MailmannM. MailmannY. ZhangY. Zhang
PhysicsPhysics, , RPIRPI
Professors P. Ajayan and G. RamanathProfessors P. Ajayan and G. RamanathMat. Sci. and Eng.,Mat. Sci. and Eng., RPIRPI
Professor A. M. Rao,Professor A. M. Rao, Clemson University Clemson University
Y. Wu, Dr. P. Giannozzi, Professor R. CarY. Wu, Dr. P. Giannozzi, Professor R. CarPrinceton UniversityPrinceton University
Professor N. Marzari,Professor N. Marzari, MITMIT
Professors R. Reifenberger and Datta, PurdueProfessors R. Reifenberger and Datta, Purdue
Scope of the TalkScope of the Talk
• IntroductionIntroduction
• Fundamental QuestionsFundamental Questions
• Technological ApplicationsTechnological Applications
• Spintronics at the Molecular LevelSpintronics at the Molecular Level
• Experimental resultsExperimental results
• Theoretical ResultsTheoretical Results
NanoelectronicsNanoelectronics Moore’s LawMoore’s Law
Device sizes halve every 5 Device sizes halve every 5 yearsyears
This law, observed in This law, observed in the 60’s, still holds the 60’s, still holds todaytoday
By Moore’s law, devices By Moore’s law, devices should reach atomic scale should reach atomic scale by 2025by 2025
Moore’s law will Moore’s law will come to an end by come to an end by 2020.2020.
- - - - - - - - - - - - - - -
- - - - - - - - - - -
M
Si
SiO2
-
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.
NanoScience and NanoScience and NanoTechnologyNanoTechnology
DoD SRA
http://www.nanosra.nrl.navy.mil
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.
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
NanoScience and NanoScience and NanoTechnologyNanoTechnology
Materials and Phenomena at Nanometer-scale (10-100 Å) Offer the Opportunities to Realize Electronics Devices with Unprecedented Performance
--
--
-- ++
Self-assembled InAs QDs on GaAs substract
30 nm
30 nm
Dot feature size 5-6 nm
Nano-EnabledNano-EnabledRevolutionary Revolutionary
CapabilityCapability Nanoelectronics and Computer TechnologyNanoelectronics and Computer Technology
Monolithic electro optic devices that Monolithic electro optic devices that detect the entire infrared (SWIR-VLWIR) detect the entire infrared (SWIR-VLWIR) spectrumspectrum
Ultra-high performance massively parallel Ultra-high performance massively parallel data processors to allow downlinking data processors to allow downlinking target information directly to the target information directly to the warfighter (e.g., molecular computers)warfighter (e.g., molecular computers)
Novel communication devices providing Novel communication devices providing unheard-of frequencies and bandwidthunheard-of frequencies and bandwidth
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 -(n-hexadecyl)quinolinium tricyanoquinodimethanide, C16H33Q-3CNQ
•Collier et al., Science 285, 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
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, Nature 398, 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., Nature 401, 152 (1999)Light-driven mono-directional molecular rotor
•Bermudez, et al., Nature 406, 608 (2000)AC-field induced molecular rotor
para-nitroaniline (PNA)
DA
-electron medium benzene ring
H2N- -NO2
+
Organic molecules offer a natural medium for controlled electron transport
WHY ORGANICS?
Can be used as
Basic Device Elements
-wire (connectors)
-insulator
-diode (switch, memory)
-transistor
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
Spintronics- (Spin Electronics): Telling the electron to Spintronics- (Spin Electronics): Telling the electron to remember its spinremember its spin
Electron has negative charge and spin (magnetic moment: 1/2).Electron has negative charge and spin (magnetic moment: 1/2).
Electron seen by an electronician:Electron seen by an electronician:
So far electronics industry are taking advantage of only its charge So far electronics industry are taking advantage of only its charge character to store and process information.character to store and process information.
Electronics ApplicationElectronics Application
Primary electronic device: MOSFET
Disadvantage
volatile of information
limited density information
reaching the fundamental limit
Spin Alone Phenomena- MagnetismSpin Alone Phenomena- Magnetism
Store information using spinStore information using spin
Alignment of spins are important Alignment of spins are important
Electrons seen by magnetician Electrons seen by magnetician
Magnetic ApplicationMagnetic Application
Primary magnetic Application: storage information
Disadvantage
mechanical access
Spin-electronics- Time for electron to take a spinSpin-electronics- Time for electron to take a spin
Exploit the quantum natureExploit the quantum nature
Combine charge and spin toCombine charge and spin to
store information in terms of spin orientation store information in terms of spin orientation (up/down)(up/down)
the spins will be attached to mobile electrons the spins will be attached to mobile electrons which will carry the information along with wirewhich will carry the information along with wire
the information will be read at the terminalthe information will be read at the terminal
Spin coherence length is large (~nm to Spin coherence length is large (~nm to m)m)
New ChallengesNew 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)
New TechnologyNew 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 Magnetic Tunneling Junction- MotorolaJunction- Motorola
Giant Magnetic Resistance (GMR)Giant Magnetic Resistance (GMR)
Baibich et al., PRL 61, 2472 (1988)Binasch et al., PRB 39, 4828 (1989)
GMR MechanismGMR Mechanism
RRFF R RAFAF
GMR Ratio = (RGMR Ratio = (RAFAF+R+RFF)/(R)/(RAFAF-R-RFF) )
could be larger than 50 %could be larger than 50 %
FerromagneticFerromagnetic
Anti-FerromagneticAnti-Ferromagnetic
Tunneling Magnetic Resistance (TMR)Tunneling Magnetic Resistance (TMR)
Moodera et al., PRL 74, 3273 (1995)
Applications of TMR: magnetic random access memories (M-RAM)
Injecting Spin Polarized Electrons in Injecting Spin Polarized Electrons in Semiconductor Semiconductor
Awschalom, Nature, 397 (1999)Awschalom, Nature, 397 (1999)
Spin-electronics at NanoscaleSpin-electronics at Nanoscale
Size of magnetic drive is Size of magnetic drive is also shrinking! also shrinking!
Reading the data through Reading the data through GMR needs to go to GMR needs to go to molecular scalemolecular scale
Spin-electronics at NanoscaleSpin-electronics at Nanoscale
Magnetic Reading HeadMagnetic Reading Head
Size of magnetic drive is also Size of magnetic drive is also shrinking! shrinking!
Reading the data through GMR needs Reading the data through GMR needs to go to molecular scaleto go to molecular scale
New Questions and New ChallengesNew Questions and New Challenges
Fundamental Interest:
Can we inject spins into molecules
Spin coherence length
Heating and time scale involved
Just a beginning ...
Coherent Spin polarized transport through carbon Coherent Spin polarized transport through carbon nanotubenanotube
Tsukagoshi Nature, 401, 572 (1999)Tsukagoshi Nature, 401, 572 (1999)
V I
Ni
SC
H
Gold
Molecules goes SpintronicsMolecules goes Spintronics
Schon, J. H., Science, Published online, I:10.1126/science.1070563 (2002).
Challenges- Challenges-
How to apply local magnetic field? How to apply local magnetic field?
First Principles Quantum Conductance Calculations of Spin Polarized Electron Transport in a Molecular Wire
THEORETICAL PROCEDURETHEORETICAL PROCEDURE
We solve Schrödinger equation:We solve Schrödinger equation: Hψ(xHψ(x11, x, x22, x, x33 …) = E ψ(x …) = E ψ(x11, x, x22, x, x33 …) …)
ψ is an N-electron wave function.ψ is an N-electron wave function.
A simple but accurate way of solving the above equation is to A simple but accurate way of solving the above equation is to use use density functional theory.density functional theory.
Here we work with ρ(r) : 3N to 3Here we work with ρ(r) : 3N to 3
Remarkable!Remarkable!
DENSITY FUNCTIONAL THEORYDENSITY FUNCTIONAL THEORY
HOHENBERG-KOHN (1964):HOHENBERG-KOHN (1964):
Total energy of an interacting electron gas in presence of an external Total energy of an interacting electron gas in presence of an external potential Vpotential Vextext (r): (r):
functional independent of Vfunctional independent of Vextext
KOHN-SHAM (1965):KOHN-SHAM (1965):
kinetic energykinetic energy exchangeexchange non interacting non interacting correlationcorrelation
Local Density Approximation (LDA): Local Density Approximation (LDA):
Gradient Corrected Approximation (GGA):Gradient Corrected Approximation (GGA):
][)()( FdrrrVEext
][||)()(
21)()(][
XCextsErdrd
rrrr
drrrVTE
drrrXC
)]([)(
drrFr )](,[)(
WORKING SCHEMEWORKING SCHEME
1-electron equations:1-electron equations:
where where
These equations are known as These equations are known as Kohn-Sham orbital equations.Kohn-Sham orbital equations.
KS equations KS equations have the similar form as Hartree equations- but have have the similar form as Hartree equations- but have correlation, in principle can work for all systems.correlation, in principle can work for all systems.
In practice, In practice, the present formalism works great for systems where the present formalism works great for systems where bonding is primarily chemical.bonding is primarily chemical.
Not successfulNot successful for weakly bound systems- attempts are underway. for weakly bound systems- attempts are underway.
)()(]},[||2
21{ r
iir
iFdrrrrZ
i i
r 2||)(
Calculation of CurrentCalculation of Current
1)( RightLeftMolHIEG
HMol: Molecular Hamiltonian
)( GGtrT nmmn
)( i
leftleftleftleft CGC rightrightrightright CGC
)]()([),(2
21
)1(
EfEfVETdEh
eI
eVE
eVE
f
f
aW. Tian, et al., J. Chem. Phys. 109, 2874 (1998)
Non-equilibrium Green’s Function Methoda
: Self-energy functionT: Transmission function
Ferromagnetic
Electron Spin Density Plot for anti-parallel Spin Alignment
Anti-Ferromagnetic
AF is lower in energy !AF is lower in energy !
I-V Characteristics For Different Spin Alignment
Down spins Down spins majority carriersmajority carriers
Conductance-Voltage Curve
0
5
10
15
20
25
30
35
40
45
50
-6 .5 -6 -5 .5 -5 -4 .5
E nergy (eV )
DO
S (1
/eV)
N i-(ll)-S p in U pN i-(ll)-S p in D ow nN i-(an ti)-S p in U pN i-(an ti)-S p in D ow n
Conductance-Voltage Curve
(A)
(B)
EF
EF
Conductance-Voltage Curve Spin Up are majority carriers- Spin Up are majority carriers- Spin valve effect is less.Spin valve effect is less.
H. Ohnishi et al, “Quantized Conductance through a chain of Gold atoms” Nature 395, 780 (1998).
Oscillatory MR is Atomic WiresOscillatory MR is Atomic Wires
A. I. Yanson et al , “Formation of atomic gold wires” Nature, 395 783 (1998).
CCo Au
V I
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 1 2 3 4 5 6
Number of C-atoms in Atomic wire
E
(eV
)
Ferro- - 1.687 Å 1.294 Å 1.294 Å 1.687 Å
Anti ferro-1.692 Å 1.293 Å 1.293 Å 1.692 Å
Ferro—1.736 Å 1.269 Å 1.317 Å 1.269 Å 1.736 Å
Anti ferro-1.82 Å 1.245 Å 1.353 Å 1.245 Å 1.82 Å
0
0.5
1
1.5
2
2.5
3
3.5
4
0 1 2 3 4 5 6
Number of C- Atoms in Atomic wire
G(E
f)
(ll)-Spin Up
(ll)-Spin Down
(anti)-Spin Up
(anti)-Spin Down
(ll)-Total
(anti)-Total
Transistor with different Oxidation State Transistor with different Oxidation State State- Experimental Results:State- Experimental Results:
[Co(tpy-(CH[Co(tpy-(CH22))55-SH)-SH)22]]2+ 2+ (longer molecule)(longer molecule)
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.
Park, et. al. Nature, 417, Park, et. al. Nature, 417, 722, (2002)722, (2002)
Why Nanotube ?
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.
21 amanc
Metallic:n=m, and n-m=3iSemiconducting:n-m 3i
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
Motivation
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
Effect of Molecular Adsorbate on Transport Property: Nano Sensor
0 200 400 600
0.98
1.00
1.02
Res
ista
nce
nor
mal
ized
(O
hm
s)
Temperature (oC)
0 200 400 6000
10
20
30
40
O2
H2O
CO2
CO
H2
Par
tial
Pre
ssu
re (
Pa*
10-6)
Temperature (oC)
Theory: Effect of Molecular Adsorbate on Transport Property: Nano Sensor
Oxygen doping increases conductance.
Water decreases conductance.
-160
0
160
-5.5 0 5.5
Voltage (V)
Cu
rren
t (
A)
Ideal (3x3)Ideal (3x3-H2O)Ideal (3x3-2H2O)Ideal (3x3-5H2O)
-200
-150
-100
-50
0
50
100
150
200
-6 -4 -2 0 2 4 6Voltage (V)
Cur
rent
( A
)
Ideal (3x3)Ideal (3x3-3O2)
0
50
100
150
200
250
300
0 1 2 3 4 5
Voltage (V)
Cu
rren
t (
A)
Ideal (5x5)
Ideal (5x5-H2O)
Ideal (5x5-5H2O)
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.
Endohedral Doping-Magnetic atom inside nano-tube
V
Transport through Peapod
17x0 nanotube with buckyball inside
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
0.00 0.20 0.40 0.60 0.80 1.00
Voltage (V)
Cur
rent
(
A)
17x0
17x0-C60
0
10
20
30
40
50
60
70
-6.53 -6.03 -5.53 -5.03 -4.53
Energy (eV)
DO
S (1
/eV
)
17x0
17x0-C60
SummarySummary
• Spintronics offer a new way of store and transmit Spintronics offer a new way of store and transmit information.information.
• Molecules, Wires, Carbon Nanotubes are good Molecules, Wires, Carbon Nanotubes are good examples of studying spin assisted transport at the examples of studying spin assisted transport at the molecular levelmolecular level
Funding:Funding:
NSFNSF
NASANASA
SRCSRC
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