ELECTROSTATIC FIELDS IN HYBRID HETEROJUNCTIONS: FIELD-EFFECT TRANSISTOR, TOPOLOGICAL INSULATOR, & THERMOELECTRONIC APPLICATION by Robert Matthew Ireland A dissertation submitted to Johns Hopkins University in conformity with the requirements for the degree of Doctor of Philosophy Baltimore, Maryland March, 2016 Abstract Organic semiconductors (OSC) are still surging in popularity for sustainable electronic devices, especially since they can perform as well as amorphous and polycrystalline silicon materials. Although OSCs have processing advantages that give rise to novel opportunities compared to inorganic semiconductors (ISCs), devices usually require inorganic materials for highly conductive connections or other functionality. Significantly, OSCs can be used to tune or modify the behavior of inorganic semiconductors (ISCs) by exploiting the junction between two semiconductors (a heterojunction). The possible creation of stable interfaces between ISCs and OSCs provides a practically limitless range of functionalities. Broadly, my goal is to study interfaces between OSCs and ISCs (hybrid heterojunctions) by testing devices of different configurations and altering the internal fields systematically, as well as with the aid of electron- and force-microscopy, and photoelectron spectroscopy. This thesis contains three major sections based around nascent, relevant applications: field-effect transistors, topological insulators, and thermoelectrics. First I study the effects of combining tellurium thin-films with OSC layers in field-effect transistors, where the organic acts both as a substrate modification layer and electrostatic gate. Secondly, I use electron withdrawing OSCs as gating materials for modifying Bi2Se3 in order to realize fundamental topological insulator behavior. Thirdly, I develop polymer-particle composites, including doping of the polymers and stabilization of inorganic particles with an electronic density of states that supports good thermoelectric behavior. We show that OSCs can undeniably be used to significantly ii modify properties of ISCs, namely tellurium, bismuth selenide, and organometallic compounds. I will first discuss the interfacial fields intrinsic to each heterojunction or device structure. Then I implement an additional electrostatic gate as part of the overall structure in order to further interrogate and modify device behavior. The additional gate is provided by introducing a polymer dielectric layer and embedding charges in this dielectric by corona charging method. This work will lead to improved design of hybrid devices because we can obtain benefits of both materials to achieve new performance records (ZT>0.1 in thermoelectric polymer-particle composites) and capabilities (topological insulator behavior), and also use heterojunctions as a basis to study and design materials or systems (tellurium/OSC transistors). iii Dedication Dedicated to my family & friends. Acknowledgement Infinite thanks to all my mentors, advisors, and colleagues. iv Table of Contents Abstract …………………………………………………………………………….…....ii Dedication ………………………………………………………………………….…....iv Acknowledgement ………………………………………………………………….…...iv Table of Contents ………………………………………………………………….…….v List of Tables …...……………………………………………………………………....vii List of Figures …...………………………………………………………………….…viii Chapters: I. Introduction Organic-Inorganic Semiconductor Hybrid Materials ..……………………….…….1 References …………………….………………………………….…….…7 II. Field-Effect Transistor Application Introduction ……………………………………………………...…………...…....12 References …………………….……………………………….…..….…24 Interfacial Fields in P-type Tellurium-Organic Hybrids ……………………….….26 References …………………….…………………………………........…40 Self-Assembled Monolayers on Gate Dielectric ………………………………..…41 References …………………….………………………………..……..…47 Picosecond Laser Direct-Write Patterning of FET Arrays ……………………..…49 References …………………….………………………………….……...53 Electrostatic Gating by Corona Charging of Electret Polymer ………………....…54 References …………………….…………………………………........…61 v III. Topological Insulator Application Introduction ………………………………………………………………………..62 References …………………….…………………………………………64 Interfacial Fields in N-type Bismuth Selenide-Organic Hybrids ……………….…68 References …………………….…………………………………...…….77 Electrostatic Gating by Corona Charging of Electret Polymer …………..………..78 References …………………….……………………………………....…85 IV. Thermoelectric Application Introduction ………………………………………………………………..………68 References …………………….…………………………………………94 Interfacial Fields in Polymer-Particle Composites ………………………….….…96 N-type Composite Hybrids …………………………………………………...…96 P-type Composite Hybrids …………………………………………………..…109 References ………………………….………………………………..…127 Electrostatic Gating by Corona Charging of Electret Polymer ……………….…128 References …………………….……………………………………..…142 V. Conclusion Summary & Future Work …………………….……………………………….…145 References …………………….………………………………….….…148 Appendix: Bibliography …………………………………………………………………….…….149 Curriculum Vitae ………………………………………………………………….….151 vi List of Tables Table 1. FET Parameters and Performance under Static or Pulsed Bias, and Heated Substrates. (page 23) Table 2. Thermoelectric Performance of Te, 6PTTP6, and Te/OSC Bilayers. (page 40) Table 3. Hybrid OFET Parameters and Performance with Charged Polystyrene. (page 59) Table 4. 60wt% Co(acac)3 in PQT12 – TE Parameters as a Function of Solution Concentration and Volume of Solution Drop-Cast per Substrate Area, utilizing Chlorobenzene. (page 118) Table 5. Comparing TE Parameters of PQT12 Composites – 10 mg/ml solutions, 0.18 ml/cm2. (page 126) vii List of Figures Figure 1. General schematic for trilayer OFET structure; three thin-films deposited onto 100 nm thick bare silicon dioxide are represented as materials number 1 and 2; material 1 is generally the OSC and material 2 is Te, and each semiconductor is nominally deposited to 10 nm. Gold source and drain electrodes were deposited (50 nm thick) with four different W/L ratios of 80, 53, 40, and 32. (page 17) Figure 2. Chemical structures for a) Te unit cell (a=0.445 nm, c=0.591 nm) b) 5,5’-bis(4- hexylphenyl)-2,2’bithiophene (6PTTP6) c) pentacene d) N,N’-bis(3- perfluorooctylpropyl) naphthalenetetracarboxylic diimide (8-3NTCDI) e) tetratetracontane f) N,N’-diphenyl-N,N’-bis(1-Naphthyl)-1,1’-biphenyl-4,4”-diamine (NPD) g) atactic polystyrene h) (3-aminopropyl)trimethoxysilane i) 1H,1H,2H,2H- perfluorooctyltriethoxysilane j) Disperse Red 19 with triethoxysilane groups (DR19). (page 17) Figure 3. Capacitance of hybrid gate stacks using bare oxide for a) bare SiO2, b) 6PTTP6/SiO2, c) Te/6PTTP6/SiO2, d) 6PTTP6/Te/6PTTP6/SiO2. (page 21) Figure 4. Output curves for a) tellurium, b) Te/6PTTP6 bilayer, and c) 6PTTP6/Te/6PTTP6 trilayer field-effect transistors. Bare SiO2, substrates held at 55 °C during deposition. (page 22) Figure 5. X-ray diffraction of tellurium films deposited onto a) glass, b) 6PTTP6 on glass, and tetratetracontane (C44H90) on glass. (page 23) Figure 6. Scanning electron micrographs of Te morphology when deposited at 0.3 Å s-1 onto various organic thin-films, including a) SiO2, b) 6PTTP6, c) pentacene, d) viii tetratetracontane (C44H90), e) SiO2 while substrate held at 55 ˚C, f) 6PTTP6 while substrate held at 55 ˚C, g) DR19 monolayer while substrate held at 55 ˚C, and h) NPD. (page 29) -1 Figure 7. a) SEM micrograph of Te deposited on SiO2 at 3.0 Å s . b) Te deposited on -1 -1 6PTTP6/SiO2 at 3.0 Å s . c) Te deposited on pentacene/SiO2 at 3.0 Å s . d) Te deposited -1 on SiO2 at 10.0 Å s . (page 30) Figure 8. a-b) SEM micrograph of 4 nm Te on 8-3 NTCDI, located away from the source-drain electrodes c-d) 4 nm Te on 8-3 NTCDI, located inbetween source-drain electrodes. (page 33) Figure 9. a-b) ID vs. VD plot for 8-3NTCDI/Te on SiO2 at room temperature with top 1/2 contact Te electrodes. W/L is 80. b) ID vs. VG plot. (page 35) Figure 10. 2D SKPM surface potential scans across interfaces of OSC and Te deposited on top of the OSC for 6PTTP6 vs. Te/6PTTP6 (top, from left to right), and pentacene vs. Te/pentacene (bottom, from left to right). (page 37) Figure 11. Energy band diagram of isolated intrinsic materials. (page 38) Figure 12. Plots used to calculate resistance (left) and Seebeck (right) for typical Te on 6PTTP6 sample. (page 39) Figure 13. X-ray Photoelectron Spectra of substrates modified with SAMs. a) Carbon 1s spectra. APTS samples show the peak at 284.5, corresponding to the CH2 bond. FOTS samples show a peak at 291 eV with shoulder at 293 eV, corresponding to CF2 and CF3 bonds. b) Nitrogen 1s spectra. APTS shows peak for NH2 bond and DR19 with some NO2 bonds. (page 44) ix Figure 14. a) SEM micrograph of Te morphology on APTS/SiO2 substrates b) Te/6PTTP6 on APTS/SiO2 substrates c) Te on FOTS/SiO2 d) Te/6PTTP6 on FOTS/SiO2 e) Te on DR19/SiO2 substrates and f) Te/6PTTP6 on DR19/SiO2 substrates, all done at room temperature. (page 45) Figure 15. a) ID vs. VD (static VG) plot for Te deposited on DR19/SiO2 at room 1/2 temperature with top contact Au electrodes; W/L is 80 b) ID vs. VG plot. (page 47) Figure 16. a) SEM micrograph of Te morphology on uncharged polystyrene at room temperature b) Te on polystyrene charged +50 V c) Te/6PTTP6 on uncharged polystyrene d) Te/6PTTP6 on polystyrene charged +50 V. (page 55) Figure 17. 6PTTP6 morphology on SiO2/polystyrene dielectric. a) AFM image of 6PTTP6 morphology on uncharged polystyrene dielectric. b) 6PTTP6 morphology on polystyrene charged +50 V. (page 56) Figure 18. Schematic charge distributions for the two Te/OSC bilayers, and