Micro/Nano Mechanical Systems Lab Class#16

Microsystems Laboratory Micro/Nano Mechanical Systems Lab Class#16 Liwei Lin Professor, Dept. of Mechanical Engineering Co-Director, Berkeley Sensor and Actuator Center The University of California, Berkeley, CA94720 e-mail: lwlin@me.berkeley.edu http://www.me.berkeley.edu/~lwlin Liwei Lin, University of California at Berkeley 1

Outline Microsystems Laboratory The rest of the semester Near-field Electrospinning Liwei Lin, University of California at Berkeley 2

Rest of the Semester Microsystems Laboratory 3/8 Other possible MEMS/NEMS labs 3/13 MD simulation lab (no lecture) 3/15 project proposal (1-2 pages ppt) 3/20 3/22 MD simulation lab (no lecture) 4/3 project design/progress (5 pages ppt) 4/5 review for quiz 4/10 quiz 4/12 quiz solution 4/17, 4/19, 4/24, 4/26 final presentations

Outline Microsystems Laboratory Electrospinning - Overview Near-Field Electrospinning Applications PVDF Nanogenrators Liwei Lin, University of California at Berkeley 4

Electrospinning Microsystems Laboratory Mechanical spinning Electrospinning early patent in 1934 Syringe Spinning 10 30 KV Polymer solution Needle 10 50 cm Stretching Whipping Random, Continuous Nanofiber < 50nm Collector Liwei Lin, University of California at Berkeley

What are the Limitations? Microsystems Laboratory Diameter of nanofiber Thinner fibers Alignment Longer distance, Lower concentration, Higher conductivity Whipping Higher Voltage D. Li et al, 2004 A. Theron et al, 2001 J. Kameoka et al, 2004 Liwei Lin, University of California at Berkeley

Near-Field Electrospinning Electrode-to-collector distance: 500 1000 m Drive voltage: 600 1500 V Tip diameter: 25 m or smaller Microsystems Laboratory High Voltage h Probe Tip Polymer Solution Liquid Jet Collector Liwei Lin, University of California at Berkeley

Results & Limitations Microsystems Laboratory 3~7 wt % Polyethylene oxide (PEO) Nanofiber diameter: 50nm 2 m Manual control Liwei Lin, University of California at Berkeley

Potential Microsystems Laboratory Machine-controlled electrospinning Liwei Lin, University of California at Berkeley

Comparisons Microsystems Laboratory Conventional Electrospinning Near-Field Needle Spinneret Metal probe tip Several hundred µm Spinneret Diameter 25 µm or smaller Continuous supply Polymer Supply Dip pen 10 30 KV Applied Voltage As low as 600 V Very long Nanofiber Length Several cm 10 50 cm Electrode-to-collector Distance 500 1000 µm Controllability Liwei Lin, University of California at Berkeley

Microsystems Laboratory Continuously NEFS Setup Direct writing of large area patterns Steel needle with 70 m opening Modified NFES process Original Modified

Microsystems Laboratory

Continuous NEFS Results Microsystems Laboratory Direct writing of large area patterns (last week) Liwei Lin, University of California at Berkeley

Large Area Deposition Microsystems Laboratory A single nanofiber in a designed trajectory 4X4 cm2 area 15 min deposition period for a total length of 108 m, nanofiber has a diameter of 700 nm 100 m Liwei Lin, University of California at Berkeley

Application (I) - NanoSensors Direct-write suspended nanofibers on MEMS Microsystems Laboratory conductive polymer by NFES 6.00 4.00 2.00 Current [μa] 0.00-2.00 100nm Nanofiber Liwei Lin, University of California at Berkeley -4.00-6.00-14 -12-10 -8-6 -4-2 0 2 4 6 8 10 12 14 Bias [V]

Microsystems Laboratory Application (II) - NanoAssembly Direct-write patterning of nanoparticles Mixing nanoparticles in nanofibers Removing nanofibers and growing nanowires/tubes 5 & 20nm Au nanoparticles Liwei Lin, University of California at Berkeley

Microsystems Laboratory Application (III) Fluidic Connector Direct-write fluidic connector (analogy to wire bonding) h 3h Liwei Lin, University of California at Berkeley

Fabrication Process Microsystems Laboratory Conformal coating of fluidic channel material after electrospinning & remove sacrificial nanofiber by etching or evaporation A B Sacrificial PEO nanofiber Coat Hydrophilic & Channel Chip Chip Chip Chip C Open etch holes D Remove nanofiber Chip Chip Chip Chip Liwei Lin, University of California at Berkeley

Fabrication Results Microsystems Laboratory Suspended nanofiber overhanging two separated chips with Parylene coating Chip Parylene Chip Nanofiber Nanofiber Chip Suspended nanofibers Parylene coating Liwei Lin, University of California at Berkeley

Microsystems Laboratory Application (IV) Bio Scaffolds High (favorable) surface-to-volume ratio with appropriate porosity, malleability to conform to a wide variety of sizes, textures, and shapes. Murine calvaria cells (MC3T3-E1) seeded onto an electrospun PLLA + collagen SUNY Stony Brook Liwei Lin, University of California at Berkeley Ordered nanofiber by NFES might lead to preferred cell growth & differentiation, localized control, functional tissue?

Microsystems Laboratory Application (V) - Composites Embedded high-modulus fibrous materials (e.g., glass fibers, carbon fibers, carbon nanotubes) for material reinforcement Embedding sensors & drug carriers. Liwei Lin, University of California at Berkeley

Other Applications Microsystems Laboratory Filtration, sensing, Liwei Lin, University of California at Berkeley

Electrospun Piezoelectric Nanogenerators Prof. Liwei Lin Berkeley Sensor and Actuator Center Department of Mechanical Engineering University of California, Berkeley 2012 University of California

Outline Background Berkeley Sensor and Actuator Center (BSAC) What s electrospinning? Near Field Electrospinning (NFES) Process design Orderly nanofiber deposition Direct Write Piezoelectric Nanogenerator Conclusion 2012 University of California

2012 University of California

2012 University of California

How Does It Work? Piezoelectric Property of PVDF Mechanical Strain Electrical Potential PVDF exists in several forms:, and crystalline phases phase is primarily responsible for piezoelectric proper es Dipole orientation Poling process Bulk or thin film PVDF Stretching and strong electric field Electrospinning In situ poling process Electrospinning of PVDF from its solutions promoted the formation of phase. In contrast, only the and phases were detected in the spin coated samples from the same solutions Non polar phase (Before NFES) Net dipole moment Polar phase (After NFES) Carbon Fluorine Hydrogen omitted 2012 University of California

2012 University of California

What s the Challenge? Conventional electrospinning Random orientation The polarities of these nanofibers mostly cancel out each other and the net piezoelectric output is close to zero Near field electrospinning Orderly nanofiber patterns with controlled direction of polarity 2012 University of California

PVDF Nanogenerator Fabrication process Spacing between electrodes: 100~500 m Fiber diameter: 500nm~3 m A Polymer jet Syringe needle High Voltage B Poling axis High Voltage Electrode Plastic substrate (NFES direction) Substrate moving direction Substrate moving direction Experimental setup Inside a Faraday cage (Not to scale) 2012 University of California

Mechanism Piezoelectric Response V + R L q R NG C NG V i R L 1. R L i 1. Start stretching 2. Hold stretching 3. Start releasing 4. End of release 2. 3. 4. V + V R L _ + + V + V R L _ + i 2012 University of California

Effect of Strain Rate The current generated by strain in poling direction Current (na) 4 3 2 1 0-1 -2-3 -4 Fast strain rate (0.04 sec) i q d EA 33 d 33 :piezoelectric constant E: Young s modulus A: cross sectional area Current output depends on strain rate Charge depends on applied strain 0.17 sec 0.33 sec 0 1 2 3 4 Time (sec) 2012 University of California Slow strain rate (0.10 sec) 300 200 100 0-100 -200-300 Charge (pc)

Voltage (mv) Effect on Stretching Frequency Higher frequency Higher electric output 15 10 5 0-5 -10 2 Hz 3 Hz 4 Hz Stretching Release C u rren t (n A ) 2.5 2 1.5 1 0.5 0-0.5-1 2 Hz 3 Hz 4 Hz Stretching Release -15-1.5-2 -20 0 10 20 30 40-2.5 0 10 20 30 40 Time (sec) Time (sec) 2012 University of California

Validation of Piezoelectric Response Different materials under the same experimental setup Voltage (mv) 40 30 20 10 0 10 20 30 40 PVDF single nanofibers Stretching Release 0 10 20 30 40 Time (sec) Voltage (mv) 10 5 0 5 10 PEO nanofiber Stretching Release Time (sec) PVDF random nanofibers mats Voltage (mv) 10 0 10 20 30 0 10 20 30 10 5 0 5 Stretching Release Time (sec) 2012 University of California

Long Term Stability Test 0.04% strain applied at a frequency of 0.5Hz for 100 min 15 4 10 2 Voltage (mv) 5 0-5 Current (na) 0-2 -10-4 -15 0 20 40 60 80 100 Time (min) 15-6 0 20 40 60 80 100 Time (min) 4 10 2 Voltage (mv) 5 0-5 Current (na) 0-2 -10-4 -15-6 1421 1426 1431 1436 2409 2414 2419 2424 Time (sec) Time (sec) 2012 University of California

High Energy Conversion Efficiency? Voltage (mv) Energy conversion efficiency W e /W m Total electric energy generated during stretching W e = VI dt Total elastic energy applied during stretching 2.0 1.5 1.0 0.5 0.0-0.5 W m = 1/8 D 2 L 0 E 2 Voltage Current 0 0.25 0.5 0.75 1 Time (sec) 1.5 1 0.5 0-0.5 Current (na) Energy conversion efficiency (%) 25 20 15 10 2012 University of California 5 0 Fiber diameter (μm) 0 2 4 6 8 PVDF fiber Film PVDF Film fiber 0 50 100 150 Film thickness (μm)

Other Related Works in Lin Lab 1 3D Electrospinning 2012 University of California

Fabrication Results grid patterns 3D Electrospinning 2012 University of California

Cylinder & Flower Wall width 2012 University of California

Other Related Works in Lin Lab 2 Direct write graphene FET 2012 University of California

Transistors, Junctions & Devices n or p type FET & pn junction A complimentary inverter made of p-gfet and n- GFET (npn junction in this case) is constructed 2012 University of California