ElectronicElectronic TransportTransport inin MolecularMolecular ScaleScale DevicesDevices
Takhee Lee, Tae-Wook Kim, Hyunwook Song, Gunuk Wang
Department of Materials Science and Engineering Gwangju Institute of Science and Technology (GIST), Korea
1 Molecular NanoElectronics The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006 Laboratory I. Electronic Transport of Micro & Nano Via-hole Structures for Molecular Electronic Devices
Tae-Wook Kim, Gunuk Wang, Hyunwook Song
2 Molecular NanoElectronics The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006 Laboratory Formation of Alkanethiol Self-Assembled Monolayers
C16 Standard alkanethiol molecule C12 C8 S • Octanethiol (C8) - CH3(CH2)7SH / length = 13.3Å C • Dodecanethiol (C12) - CH3(CH2)11SH / length = 18.2Å H • Hexadecanethiol (C16) - CH3(CH2)15SH / length = 23.2Å
Conjugated molecule: Oligo(phenylene ethynylene) (OPE) OPE provided by Dr. Changjin Lee (KRICT) Self-assembled monolayer (SAM)
• A spontaneous chemisorption process
RS-H + Au → RS-Au + 0.5 H2 • Chemical bond formation of functional end groups of molecules with the substrate surface • Intermolecular interactions between the backbones
Reed & Tour, Am. Sci. June 2000
3 Molecular NanoElectronics The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006 Laboratory Fabrication of Planar-type Via-Hole Molecular Devices
Design and Fabrication of molecular electronic devices Optical lithography: ~ 2 µm Mask pattern Plan: E-beam lithography: ~ 50 nm Unit Die Devices Schematic of device
Au SiO Ti/Au
Silicon wafer
One unit Contain 16 dies Each die contains 20 devices 1 cm 2.5 mm 190 µm Total devices are 320 in a unit SEM / AFM / Optical images
2 µm
4” wafer (Collaboration with 1 cm piece Devices AFM image After Top electrode ETRI, Dr. Hyoyoung Die Lee) of a device
TW Kim, HW Song, GW Wang 4 Molecular NanoElectronics The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006 Laboratory Yield of Microscale Via-Hole Molecular Devices
10-2 1. Yield of molecular devices ? 2. How to define working molecular devices ? 10-3 C8
10-4 Device yield (micro-via hole molecular devices)
10-5 # of Working C12 Fab. fabricated short Open failure -6 device C8 C12 C16 10 Current [ A ] 84 57 60 13440 392 11744 1103 10-7 C16 201
100% 2.9% 87.4% 8.2% 1.5% 10-8
Simmons tunneling fitting results for all working devices -1.0 -0.5 0.0 0.5 1.0 Voltage [ V ]
2 -1 Alkanethiol J at 1 V (A/cm ) ФB (eV) α β (Å ) C8 78000±46000 1.29±0.49 0.76±0.09 0.87±0.16 C12 2000±400 1.26±0.08 0.72±0.04 0.83±0.04 Representative devices ± ± ± ± C16 5.2 4.7 2.67 0.28 0.52 0.04 0.87 0.05 determined by Gaussian fitting of J ! 5 Molecular NanoElectronics The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006 Laboratory Determination of Working Molecular Devices
Log (J) for C8 Log (J) for C12 Log (J) for C16
16 32 16
8
16 8 12 6 6 # of Device # of # of Device # of 8 4 # of Device 4 4 2 2
0 0 0 -2-1012345678 2.0 2.5 3.0 3.5 4.0 4.5 5.0 -10123 2 2 Log10 (Current Density @ 1 V (A/cm )) 2 Log10 (Current Density @ 1 V (A/cm )) Log10 (Current Density @ 1 V (A/cm ))
Summary of Log(J) Gaussian fitting ( 3σ: 99.7 % ) 35 C8 (x−
# of Device 10
5
m − 3σ m m + 3σ 0 99.7% -10123456 2 Log10 (Current Density @ 1 V (A/cm )) 6 Molecular NanoElectronics The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006 Laboratory II. Molecular Electronic Junctions Studied by Conducting Atomic Force Microscopy
Hyunwook Song
7 Molecular NanoElectronics The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006 Laboratory Charge Transport Study by CAFM
Au(20nm)/Cr(20nm) coated AFM Probe
I HP 4145B
Au
Physical probe contact area
C8 OPE Chemisorbed Au
PSIA, XE-100 model ambient AFM 8 Molecular NanoElectronics The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006 Laboratory Force-and Length-Dependent Tunneling
I(V) for C12 (1 to 30 nN) J @ 1 V (1 to 30 nN)
105 ) 2 4 1 10 C12 103 0.1 C16 2 C8 30 nN 10 2 Loading force Jo (A/cm ) 25 nN 0.01 1 nN 1.25×108
I (nA) 1 8 2 nN 1.11×10 20 nN 1 nN 10 8 5 nN 1.37×10 15 nN 5 nN 10 nN 1.45×108 10 nN 1E-3 0 8 10 nN 15 nN 1.30×10 10 20 nN 8.03×107 5 nN 20 nN 25 nN 7.63×107 2 nN Current density (A/cm -1 8 1E-4 30 nN 10 30 nN 1.25×10 1 nN -1.0 -0.5 0.0 0.5 1.0 12 14 16 18 20 22 24 V (V) Length (Å)
• Larger current with larger loading force Exponential dependence • J: C8 > C12 > C16 R ∝ exp (βd) • Error bar: sample-to-sample variation
9 Molecular NanoElectronics The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006 Laboratory Through-bond vs. Through-space
Tunneling distance in “through space” = d – dcctanθ + dcc CAFM Tip
(dt- δ) dcc θ dcctanθ Au
d
JACS, 119, 11910 (1997)
IIt0=−+exp(β tbM L )
LM cosθ dcc ns ! I0∑ exp(−−βθβ tb (LNd M cc tan )) ×− exp( ts Nd cc ) N =1 ()!!nNNs − J. Phys. Chem. B, 106, 7469 (2002)
The dominant transport mechanism of alkanethiol SAM: “through-bond” tunneling When tilted considerably, chain-to-chain coupling: “through-space tunneling” appears 10 Molecular NanoElectronics The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006 Laboratory C8: Thru-bond + Thru-space (Single hopping: N = 1)
-2 Net force (nN) N=2 45 62 78 94 -4 70 1.2
1.0 60
) -6 o 0.8 N=1 50 J/J Tilt angle (degree)
-8 0.6 0102030 Separation between contacts (nm)Separation between ln ( -10 Applied loading force (nN) Experiment
-12
C8 -14 20 30 40 50 60 70 Tilt angle θ (degree)
N~1 (N ≤ LMcos θ/dcc) 11 Molecular NanoElectronics The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006 Laboratory C12: Thru-bond + Thru-space (Double hopping: N = 2)
-4 Net force (nN) 50 66 83 99 N=2 -6 60 1.6 -8 50 1.4 ) o -10 Tilt angle (degree) Tilt angle J/J 1.2 N=1
40 -12 0102030
Applied loading force (nN) Separation between contacts (nm) ln ( -14 -16 -18 20 30 40 50 60 70 Tilt angle θ (degree)
N~2 (N ≤ LMcos θ/dcc) 12 Molecular NanoElectronics The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006 Laboratory C16: Thru-bond + Thru-space (Triple hopping: N = 3)
-4 Net force (nN) 56 73 89 105 N=3 2.1 55 -8 2.0 50 1.9 N=2 1.8 )
o -12 45 1.7 Tilt angle (degree) 1.6 J/J
0 102030 N=1 -16 Applied loading force (nN) Separation between contacts (nm) ln (
-20 C16 -24 20 30 40 50 60 70 Tilt angle θ (degree)
N~3 (N ≤ LMcos θ/dcc) 13 Molecular NanoElectronics The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006 Laboratory Conclusions
• Electronic transport study by CAFM - Tip-loading force effects on molecular structural properties - Thru-bond vs. thru-space transport
• Fabrication and characterization of micro-via hole molecular devices - Detail yield study ~ 1.5 % (out of > 13,000 devices measured)
• Electronic properties of various nanoscale building blocks
14 Molecular NanoElectronics The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006 Laboratory