Nanodevices using Nanomaterials : Carbon Nanotube Thin-Films & Nanoparticle Assembly Seung-Beck Lee Division of Electronics and Computer Engineering & Department of Nanotechnology, Hanyang University, Seoul, Korea D. G. Hasko and G. A. C. Jones Cavendish Laboratory, University of Cambridge, Cambridge, U.K.
Template Assisted Electrostatic Assembly of Colloidal Au Nanoparticles Heong-Suk Jeon, Chi-Won Cho, Bonghyun Park, and Seung-Beck Lee Division of Electronics and Computer Engineering & Department of Nanotechnology, Hanyang University, Seoul, Korea D. G. Hasko and G. A. C. Jones Cavendish Laboratory, University of Cambridge, Cambridge, U.K.
Motivation One limit of Flash Memory memory Scaling scaling Parasitic Capacitive Coupling As more memories are packed closer together, the charge on neighboring memory start to affect charge coupling in the floating gate changing the threshold voltage shift.
Motivation Si nanowire split gate nano-floating gate memory Nanowire Gate Source Gate Drain Au colloidal nanoparticles Using nanoparticles, parasitic coupling may be reduced Using metallic nanoparticles, charge density may be increased Using pre-synthesized nanoparticles, size distribution may be reduced Si nanowire geometry may increase floating gate charge induced scattering by limiting the transport channel to 1D
Outline Nanoscale Floating-Gate Memory using usingcolloidal Au Au Nanoparticles Electrostatically Assembled on onsi Si Nanowires Nanoparticle Assembly Nanoparticle Floating-Gate Characteristics Template Assisted Electrostatic Assembly Summary
Nanoparticle Assembly Surface functionalization Au nanoparticle assembly on amino functionalized oxide surfaces
Nanoparticle Assembly Surface functionalization 6 h 6 h 6 h Increased sedimentation time increases Au nanoparticle density with localized sedimentation More than 6 hours deposition time Too Slow
Nanoparticle Assembly Spin-Coating on APTS 5 nm Au nanocrystal spin-coating at 3000 rpm 30 sec AFM study 2 times : even distribution 10 times : Thickness increase Spin-coating on ATPS results in relatively even distribution However, difficult to increase density above 1010 without agglomeration
Nanoparticle Assembly Electrostatic assembly (electric field guided assembly) Si SiO 2 100 nm 2 min electrophoresis 200 nm 10 min electrophoresis Electric-field applied between the substrate and the cathode guides the charged nanoparticles towards the substrate Selective deposition is also possible The process is much faster than 2 min process produces ~ 10 11 cm -2 density convection drying or sedimentation methods
NFG Characteristics Silicon-on-Insulator wafer (SIMOX) (n~1x10 14 /cm 2 ) n-type Si : 70 nm, SiO 2 : 100 nm Reactive Ion Etching PMMA Si SiO 2 Si SiCl 4 (20 sccm) + CF 4 (20 sccm) 20 mtorr, 300 W, 40 sec Si 2.3 nm/s, SiO 2 0.8 nm/s etch rates PMMA resist spin-coating A4 (4%PMMA in Anisole) 5000 rpm for 30 sec, Thickness ~ 200 nm Dry Oxidation 1000 o C for 10 min ~15 nm thermal oxide Electron-Beam Lithography 10 nm spot size at 80 kv, 10 pa 800 μc/cm 2 exposure dosage Nanofabrication done at Microelectronics Research Centre, Cavendish Laboratory
NFG Characteristics Selective Electrostatic Assembly Si SiO 2 100 nm 200 nm Selective Electrostatic Assembly Gate Si nanowire Gate 50 nm
NFG Characteristics Nanoscale Floating-Gate Memory Characteristics 10-6 Before Au nanoparticle assembly 10-6 V DS = 2 V Measurements performed in vacuum at room temperatures I DS /W (A/μm) 10-7 10-8 10-9 I DS /W (A/μm) 10-7 10-8 10-9 V DS = 1.4 V nanoparticle density ~ 10 11 /cm 2-4 -2 0 2 4 V GS (V) -4-2 0 2 4 V GS (V) Original device characteristics shows hysteresis due to surface charge After Au nanoparticle electrostatic assembly, the threshold voltage shift is increased from 0.14 to 1.9 resulting in ΔV T ~ 1.5 V
Outline G S D Singe Nanoparticle Memory(?) Requires a method to place individual nanoparticles in predesignated locations
Template Assisted Electrostatic Assembly Template Assisted Electrostatic Assembly Si PMMA AlOx Si Capillary force Template assisted electrostatic assembly of 20 nm Au nc in 60 nm line pattern nc diameter d nc / linewidth l W = 0.33 ; nc number per column ~ 2 Selectivity to Al surface over resist surface is very high High nc density resulted in multilayer nc assembly with fluctuation in layer number
Single Nanocrystal Array assembly
Template Assisted Electrostatic Assembly Single Nanocrystal Array assembly 40 nm 1 μm Template assisted nc electrostatic assembly of 20 nm Au nc in square array pattern nc diameter d nc / linewidth l W = 0.5 ; average site density ~ 1.3 High percentage(>70%) of holes filled with a single nanocrystal Density fluctuation is reduced by limiting the template dimensions
Summary We have demonstrated electrostatic selective assembly of colloidal Au nanoparticles on Si nanowires Nanoscale Floating Gate Memory characteristics were investigated with high threshold voltage shift, however retention time needs to be increased Template assisted electrostatic assembly demonstrates that by using electrostatic assembly on patterned surfaces selective deposition of Au nanoparticles on device position may be possible 40 nm By using template assisted electrostatic assembly, it may be also be possible to use a single nanocrystal for NFGM G S D Singe Nanoparticle Memory(?)
Highly Flexible and Transparent Single Wall Carbon Nanotube Network Gas Sensors Fabricated on PDMS Substrates Chang-Seung Woo, Yoon-Sun Hwang, Chae-Hyun Lim, and Seung-Beck Lee Division of Electronics and Computer Engineering & Department of Nanotechnology, Hanyang University, Seoul, Korea D G Hasko and K B K Teo University of Cambridge, Cambridge, U.K.
Motivation Future Electronics yet fully developed RFID Mobile Sensor Network requires Wearable Electronics Smart Dust Bendable (Flexible) Electronics Transparent Electronics Low power consumption High sensitivity Mechanical stability Chemical stability Flexible, transparent substrates Flexible, transparent interconnects Compatible with Si processing Mass producible (at low cost) Mobile, Wearable, Flexible, Transparent
Motivation Carbon nanotube based Future Electronics Potential Low power consumption High sensitivity Mechanical stability Chemical stability Flexible, transparent substrates Flexible, transparent interconnects Compatible with Si processing Mass producible (at low cost)
Structure of SWCNT Energy band structure and density of states (DOS) Metallic nanotubes m-n = 0, E g =0 (small-gap semiconducting nanotubes) Zigzag metals Semiconducting nanotubes (wide gap) m-n <>3p, E g = 0.5 ~ 1 ev DOS
Outline Flexible and and Transparent SWCNT Network Gas Gas Sensors on onpdms substrates Fabrication: Flexible SWCNT thin thin film film Film Film characteristics: Transparency & Conductivity Sensor Operation :: NH NH 3 Gas 3 Gas Sensing Operation Summary
Flexible Thin Film SWCNT thin film fabrication by Vacuum Filteration Dispersion: SWCNT bundles in 0.1% Sodium Dodecylbenzene - sulfonate (SDS) solution before High power ultrasonic agitation at 30 ~70 W (10 s pulses for 1 hour) Arc-discharge SWCNT using Fe catalyst (Φ:1.3 ~ 1.7 nm, E G : 0.59 ~ 0.45 ev) Chem.Phys.Lett.373, 266 (2003) Alumina filter after 1 inch Vacuum Alumina filter surface (Whatman; 20 nm pore size) 500 nm Moore et al.,nano Lett. 3, 1379 (2003) Hu et al., Nano Lett. 4, 2513 (2004) Density Control : SWCNT density in solution and filteration volume determines the thin film density
Flexible Thin Film SWCNT Thin Film on PDMS Transfer of SWCNT thin film to PDMS substrate surface PDMS poly(dimethyl siloxane) 1. PDMS mixed and placed in a vacuum chamber to remove air bubbles 2. Filter membrane placed on the surface of the uncured PDMS 3. PDMS curing in 100 o C oven for 1 hour 4. Removal of the filter membrane Flexible & Transparent SWCNT thin film Flexibility depends on PDMS thickness Transparency depends on SWCNT density
Flexible Thin Film SWCNT Thin Film on PDMS 1 μm Due to some SWCNTs trapped in the nanopores acting as ankers during filter removal process, the SWCNT thin film transferred to the PDMS shows bundle ends lifted from the surface plain (grass like) 5 μm 0.16 mg/ml Most of the SWCNT bundles have been transferred to the PDMS surface For high density films, however, some percentage of SWCNTs remained untransferred
Film characteristics SWCNT Thin Film on PDMS : Characterization Transparency, Conductivity and Flexibility testing Transparency Transmittance (%) 90 80 70 60 50 40 30 20 Optical Transmittance 0.04 mg/ml 0.12 mg/ml 0.16 mg/ml 10 400 450 500 550 600 650 700 Wavelength (nm) Visible range optical transparency depending on SWCNT density Higher SWCNT density results in low transparency Frequency dependence is believed to be due to PDMS thickness Film Resistance Electrical resistance of the SWCNT thin film on PDMS depending on SWCNT density 5 x 5 mm 2 between Au contacts Linear I-V characteristics Not all of the bundles are making contact resulting in resistance saturation Mechanical deformation I-V characteristics during bending (thin film complete folded) Only a small decrease in conductance is observed Returns back to original state when stress is removed
Sensor Operation Flexible SWCNT Gas Sensor NH 3 gas sensitivity testing Gas Sensing Measurement System Density dependent conductance change 5 min exposure to 1% NH 3 results in ~ 10% change in film conductance The reduction in conductance shows variation on SWCNT density
Sensor Operation Flexible SWCNT Gas Sensor possible sensing mechanism Effect of NH 3 gas adsorption on SWCNT energy band Metal 1 σ tot E F Tunneling 1 = σ cnt p-type 1 + σ T Electron donation from NH 3 raises the Fermi level (direct adsorption) - reduced hole concentration Adsorbed NH 3 increases local scattering of holes (adsorption on surfactant) - reduced carrier mobility Conduction in a network depends on nanotube interface properties and SWCNT density (thermally assisted tunneling reduces the effect) SWCNT network conduction reduced SWCNT density may increase sensitivity High SWCNT density (percolation) 2D Low SWCNT density ~ quasi 1D Many conduction paths reduced scattering Few conduction paths enhanced scattering
Sensor Operation Flexible SWCNT Gas Sensor NH 3 gas sensitivity testing SWCNT density dependence Low partial pressure detection There may be a correlation between SWCNT density and gas sensitivity due to the network conduction properties of SWCNT thin films It was possible to detect 10 ppm NH 3 at 10 s exposure time ( 0.2 % conduction decrease) Bias voltage dependence within error margins
Summary Flexible SWCNT thin film on PDMS was fabricated Conductivity vs Density with Grass like surface Flexible SWCNT Gas Sensor NH 3 sensitivity Transparency (%) 90 80 70 60 50 40 30 20 Transparency vs Density 0.04 mg/ml 0.12 mg/ml 0.16 mg/ml 10 400 450 500 550 600 650 700 Wavelength (nm) Density dependence 10 ppm level detection Thank you very much!