Honors & Awards

  • ATV college scholarship, St Catherine's College, Oxford University (2007~2009)
  • Bullock Career Award for internship grant, St Catherine's College, Oxford University (2009)
  • Corus Prize for best overall performance in Part I (first 3 years) practicals, Oxford University (2009)
  • Best Research Poster, Global Climate & Energy Project (GCEP) Research Symposium (2012)
  • Link Foundation Energy Fellowship, Link Foundation (2012~2014)
  • Excellence in Polymer Graduate Polymer Research, American Chemical Society (ACS) (2014)
  • Graduate Research Award, American Vacuum Society (AVS) (2014)
  • Gold Graduate Student Award, Materials Research Society (MRS) (Fall 2014)
  • National Award For Outstanding Self-Financed Students Abroad, Chinese Government (2014)

Professional Education

  • PhD, Stanford University, Materials Science and Engineering (2014)
  • BEng/MEng, Oxford University, Material Science (2010)

Stanford Advisors

All Publications

  • Diketopyrrolopyrrole (DPP)-Based Donor-Acceptor Polymers for Selective Dispersion of Large-Diameter Semiconducting Carbon Nanotubes SMALL Lei, T., Lai, Y., Hong, G., Wang, H., Hayoz, P., Weitz, R. T., Chen, C., Dai, H., Bao, Z. 2015; 11 (24): 2946-2954


    Low-bandgap diketopyrrolopyrrole (DPP)-based polymers are used for the selective dispersion of semiconducting single-walled carbon nanotubes (s-SWCNTs). Through rational molecular design to tune the polymer-SWCNT interactions, highly selective dispersions of s-SWCNTs with diameters mainly around 1.5 nm are achieved. The influences of the polymer alkyl side-chain substitution (i.e., branched vs linear side chains) on the dispersing yield and selectivity of s-SWCNTs are investigated. Introducing linear alkyl side chains allows increased polymer-SWCNT interactions through close π-π stacking and improved C-H-π interactions. This work demonstrates that polymer side-chain engineering is an effective method to modulate the polymer-SWCNT interactions and thereby affecting both critical parameters in dispersing yield and selectivity. Using these sorted s-SWCNTs, high-performance SWCNT network thin-film transistors are fabricated. The solution-deposited s-SWCNT transistors yield simultaneously high mobilities of 41.2 cm(2) V(-1) s(-1) and high on/off ratios of greater than 10(4) . In summary, low-bandgap DPP donor-acceptor polymers are a promising class of polymers for selective dispersion of large-diameter s-SWCNTs.

    View details for DOI 10.1002/smll.201403761

    View details for PubMedID 25711378

  • H-Bonded Supramolecular Polymer for the Selective Dispersion and Subsequent Release of Large-Diameter Semiconducting Single-Walled Carbon Nanotubes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Pochorovski, I., Wang, H., Feldblyum, J. I., Zhang, X., Antaris, A. L., Bao, Z. 2015; 137 (13): 4328-4331


    Semiconducting, single-walled carbon nanotubes (SWNTs) are promising candidates for applications in thin-film transistors, solar cells, and biological imaging. To harness their full potential, however, it is necessary to separate the semiconducting from the metallic SWNTs present in the as-synthesized SWNT mixture. While various polymers are able to selectively disperse semiconducting SWNTs, the subsequent removal of the polymer is challenging. However, many applications require semiconducting SWNTs in their pure form. Toward this goal, we have designed a 2-ureido-6[1H]-pyrimidinone (UPy)-based H-bonded supramolecular polymer that can selectively disperse semiconducting SWNTs. The dispersion purity is inversely related to the dispersion yield. In contrast to conventional polymers, the polymer described herein was shown to disassemble into monomeric units upon addition of an H-bond-disrupting agent, enabling isolation of dispersant-free, semiconducting SWNTs.

    View details for DOI 10.1021/jacs.5b01704

    View details for PubMedID 25815604

  • N-Type Conjugated Polymer-Enabled Selective Dispersion of Semiconducting Carbon Nanotubes for Flexible CMOS-Like Circuits ADVANCED FUNCTIONAL MATERIALS Wang, H., Li, Y., Jimenez-Oses, G., Liu, P., Fang, Y., Zhang, J., Lai, Y., Park, S., Chen, L., Houk, K. N., Bao, Z. 2015; 25 (12): 1837-1844
  • Solvent effects on polymer sorting of carbon nanotubes with applications in printed electronics. Small Wang, H., Hsieh, B., Jiménez-Osés, G., Liu, P., Tassone, C. J., Diao, Y., Lei, T., Houk, K. N., Bao, Z. 2015; 11 (1): 126-133


    Regioregular poly(3-alkylthiophene) (P3AT) polymers have been previously reported for the selective, high-yield dispersion of semiconducting single-walled carbon nanotubes (SWCNTs) in toluene. Here, five alternative solvents are investigated, namely, tetrahydrofuran, decalin, tetralin, m-xylene, and o-xylene, for the dispersion of SWCNTs by poly(3-dodecylthiophene) P3DDT. The dispersion yield could be increased to over 40% using decalin or o-xylene as the solvents while maintaining high selectivity towards semiconducting SWCNTs. Molecular dynamics (MD) simulations in explicit solvents are used to explain the improved sorting yield. In addition, a general mechanism is proposed to explain the selective dispersion of semiconducting SWCNTs by conjugated polymers. The possibility to perform selective sorting of semiconducting SWCNTs using various solvents provides a greater diversity of semiconducting SWCNT ink properties, such as boiling point, viscosity, and surface tension as well as toxicity. The efficacy of these new semiconducting SWCNT inks is demonstrated by using the high boiling point and high viscosity solvent tetralin for inkjet-printed transistors, where solvent properties are more compatible with the inkjet printing head and improved droplet formation.

    View details for DOI 10.1002/smll.201401890

    View details for PubMedID 25138541

  • Highly Stable Carbon Nanotube Top-Gate Transistors with Tunable Threshold Voltage ADVANCED MATERIALS Wang, H., Cobb, B., van Breemen, A., Gelinck, G., Bao, Z. 2014; 26 (26): 4588-?


    Carbon-nanotube top-gate transistors with fluorinated dielectrics are presented. With PTrFE as the dielectric, the devices have absent or small hysteresis at different sweep rates and excellent bias-stress stability under ambient conditions. Ambipolar single-walled carbon nanotube (SWNT) transistors are observed when P(VDF-TrFE-CTFE) is utilized as a topgate dielectric. Furthermore, continuous tuning of the threshold voltages of both unipolar and ambipolar SWNT thin-film transistors (TFTs) is demonstrated for the first time.

    View details for DOI 10.1002/adma.201400540

    View details for PubMedID 24789423

  • Tuning the threshold voltage of carbon nanotube transistors by n-type molecular doping for robust and flexible complementary circuits. Proceedings of the National Academy of Sciences of the United States of America Wang, H., Wei, P., Li, Y., Han, J., Lee, H. R., Naab, B. D., Liu, N., Wang, C., Adijanto, E., Tee, B. C., Morishita, S., Li, Q., Gao, Y., Cui, Y., Bao, Z. 2014; 111 (13): 4776-4781


    Tuning the threshold voltage of a transistor is crucial for realizing robust digital circuits. For silicon transistors, the threshold voltage can be accurately controlled by doping. However, it remains challenging to tune the threshold voltage of single-wall nanotube (SWNT) thin-film transistors. Here, we report a facile method to controllably n-dope SWNTs using 1H-benzoimidazole derivatives processed via either solution coating or vacuum deposition. The threshold voltages of our polythiophene-sorted SWNT thin-film transistors can be tuned accurately and continuously over a wide range. Photoelectron spectroscopy measurements confirmed that the SWNT Fermi level shifted to the conduction band edge with increasing doping concentration. Using this doping approach, we proceeded to fabricate SWNT complementary inverters by inkjet printing of the dopants. We observed an unprecedented noise margin of 28 V at VDD = 80 V (70% of 1/2VDD) and a gain of 85. Additionally, robust SWNT complementary metal-oxide-semiconductor inverter (noise margin 72% of 1/2VDD) and logic gates with rail-to-rail output voltage swing and subnanowatt power consumption were fabricated onto a highly flexible substrate.

    View details for DOI 10.1073/pnas.1320045111

    View details for PubMedID 24639537

  • High-yield sorting of small-diameter carbon nanotubes for solar cells and transistors. ACS nano Wang, H., Koleilat, G. I., Liu, P., Jiménez-Osés, G., Lai, Y., Vosgueritchian, M., Fang, Y., Park, S., Houk, K. N., Bao, Z. 2014; 8 (3): 2609-2617


    We describe herein a high-yield method to selectively disperse semiconducting CoMoCAT (CO disproportionation on Co-Mo catalysts) single-walled carbon nanotubes (SWNTs) with regioregular poly(3-alkylthiophenes) polymers. We observed that the dispersion yield was directly related to the length of the polymer's alkyl side chains. Molecular dynamics simulations in explicit toluene (real toluene molecules) indicate that polythiophenes with longer alkyl side chains bind strongly to SWNTs, due to the increased overall surface contact area with the nanotube. Furthermore, the sorting process selectively enriches smaller-diameter CoMoCAT SWNTs with larger bandgaps, which is ideal for solar cell applications. Compared to the larger diameter sorted HiPco (High-Pressure CO) SWNTs, solar cells fabricated using our sorted CoMoCAT SWNTs demonstrated higher open-circuit voltage (Voc) and infrared external quantum efficiency (EQE). The Voc achieved is the highest reported for solar cells based on SWNT absorbers under simulated AM1.5 solar illumination. Additionally, we employed the sorted CoMoCAT SWNTs to fabricate thin film transistors with excellent uniformity and device performance.

    View details for DOI 10.1021/nn406256y

    View details for PubMedID 24484388

  • Direct growth of aligned graphitic nanoribbons from a DNA template by chemical vapour deposition. Nature communications Sokolov, A. N., Yap, F. L., Liu, N., Kim, K., Ci, L., Johnson, O. B., Wang, H., Vosgueritchian, M., Koh, A. L., Chen, J., Park, J., Bao, Z. 2013; 4: 2402-?


    Graphene, laterally confined within narrow ribbons, exhibits a bandgap and is envisioned as a next-generation material for high-performance electronics. To take advantage of this phenomenon, there is a critical need to develop methodologies that result in graphene ribbons <10 nm in width. Here we report the use of metal salts infused within stretched DNA as catalysts to grow nanoscopic graphitic nanoribbons. The nanoribbons are termed graphitic as they have been determined to consist of regions of sp(2) and sp(3) character. The nanoscopic graphitic nanoribbons are micrometres in length, <10 nm in width, and take on the shape of the DNA template. The DNA strand is converted to a graphitic nanoribbon by utilizing chemical vapour deposition conditions. Depending on the growth conditions, metallic or semiconducting graphitic nanoribbons are formed. Improvements in the growth method have potential to lead to bottom-up synthesis of pristine single-layer graphene nanoribbons.

    View details for DOI 10.1038/ncomms3402

    View details for PubMedID 23989553

  • Scalable and Selective Dispersion of Semiconducting Arc-Discharged Carbon Nanotubes by Dithiafulvalene/Thiophene Copolymers for Thin Film Transistors ACS NANO Wang, H., Mei, J., Liu, P., Schmidt, K., Jimenez-Oses, G., Osuna, S., Fang, L., Tassone, C. J., Zoombelt, A. P., Sokolov, A. N., Houk, K. N., Toney, M. F., Bao, Z. 2013; 7 (3): 2659-2668


    We report a simple and scalable method to enrich large quantities of semiconducting arc-discharged single-walled carbon nanotubes (SWNTs) with diameters of 1.1-1.8 nm using dithiafulvalene/thiophene copolymers. Stable solutions of highly individualized and highly enriched semiconducting SWNTs were obtained after a simple sonication and centrifuge process. Molecular dynamics (MD) simulations of polymer backbone interactions with and without side chains indicated that the presence of long alkyl side chains gave rise to the selectivity toward semiconducting tubes, indicating the importance of the roles of the side chains to both solubilize and confer selectivity to the polymers. We found that, by increasing the ratio of thiophene to dithiafulvalene units in the polymer backbone (from pDTFF-1T to pDTFF-3T), we can slightly improve the selectivity toward semiconducting SWNTs. This is likely due to the more flexible backbone of pDTFF-3T that allows the favorable wrapping of SWNTs with certain chirality as characterized by small-angle X-ray scattering. However, the dispersion yield was reduced from pDTFF-1T to pDTFF-3T. MD simulations showed that the reduction is due to the smaller polymer/SWNT contact area, which reduces the dispersion ability of pDTFF-3T. These experimental and modeling results provide a better understanding for future rational design of polymers for sorting SWNTs. Finally, high on/off ratio solution-processed thin film transistors were fabricated from the sorted SWNTs to confirm the selective dispersion of semiconducting arc-discharge SWNTs.

    View details for DOI 10.1021/nn4000435

    View details for PubMedID 23402644

  • Flexible polymer transistors with high pressure sensitivity for application in electronic skin and health monitoring. Nature communications Schwartz, G., Tee, B. C., Mei, J., Appleton, A. L., Kim, D. H., Wang, H., Bao, Z. 2013; 4: 1859-?


    Flexible pressure sensors are essential parts of an electronic skin to allow future biomedical prostheses and robots to naturally interact with humans and the environment. Mobile biomonitoring in long-term medical diagnostics is another attractive application for these sensors. Here we report the fabrication of flexible pressure-sensitive organic thin film transistors with a maximum sensitivity of 8.4 kPa(-1), a fast response time of <10 ms, high stability over >15,000 cycles and a low power consumption of <1 mW. The combination of a microstructured polydimethylsiloxane dielectric and the high-mobility semiconducting polyisoindigobithiophene-siloxane in a monolithic transistor design enabled us to operate the devices in the subthreshold regime, where the capacitance change upon compression of the dielectric is strongly amplified. We demonstrate that our sensors can be used for non-invasive, high fidelity, continuous radial artery pulse wave monitoring, which may lead to the use of flexible pressure sensors in mobile health monitoring and remote diagnostics in cardiovascular medicine.

    View details for DOI 10.1038/ncomms2832

    View details for PubMedID 23673644

  • Hierarchical nanostructured conducting polymer hydrogel with high electrochemical activity PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Pan, L., Yu, G., Zhai, D., Lee, H. R., Zhao, W., Liu, N., Wang, H., Tee, B. C., Shi, Y., Cui, Y., Bao, Z. 2012; 109 (24): 9287-9292


    Conducting polymer hydrogels represent a unique class of materials that synergizes the advantageous features of hydrogels and organic conductors and have been used in many applications such as bioelectronics and energy storage devices. They are often synthesized by polymerizing conductive polymer monomer within a nonconducting hydrogel matrix, resulting in deterioration of their electrical properties. Here, we report a scalable and versatile synthesis of multifunctional polyaniline (PAni) hydrogel with excellent electronic conductivity and electrochemical properties. With high surface area and three-dimensional porous nanostructures, the PAni hydrogels demonstrated potential as high-performance supercapacitor electrodes with high specific capacitance (~480 F·g(-1)), unprecedented rate capability, and cycling stability (~83% capacitance retention after 10,000 cycles). The PAni hydrogels can also function as the active component of glucose oxidase sensors with fast response time (~0.3 s) and superior sensitivity (~16.7 μA · mM(-1)). The scalable synthesis and excellent electrode performance of the PAni hydrogel make it an attractive candidate for bioelectronics and future-generation energy storage electrodes.

    View details for DOI 10.1073/pnas.1202636109

    View details for Web of Science ID 000305511300024

    View details for PubMedID 22645374

    View details for PubMedCentralID PMC3386113

  • Highly Effective Separation of Semiconducting Carbon Nanotubes verified via Short-Channel Devices Fabricated Using Dip-Pen Nanolithography ACS NANO Park, S., Lee, H. W., Wang, H., Selvarasah, S., Dokmeci, M. R., Park, Y. J., Cha, S. N., Kim, J. M., Bao, Z. 2012; 6 (3): 2487-2496


    We have verified a highly effective separation of semiconducting single-walled carbon nanotubes (sc-SWNTs) via statistical analysis of short-channel devices fabricated using multipen dip-pen nanolithography. Our SWNT separation technique utilizes a polymer (rr-P3DDT) that selectively interacts with and disperses sc-SWNTs. Our devices had channel lengths on the order of 300-500 nm, with an average of about 3 SWNTs that directly connected the source-drain electrodes. A total of 140 SWNTs were characterized, through which we have observed that all of the SWNTs exhibited semiconducting behavior with an average on/off current ratio of ~10(6). Additionally, we have characterized 50 SWNTs after the removal of rr-P3DDT, through which we have again observed semiconducting behavior for all of the SWNTs with similar electrical characteristics. The relatively low average on-conductance of 0.0796 μS was attributed to the distribution of small diameter SWNTs in our system and due to the non-ohmic Au contacts on SWNTs. The largely positive threshold voltages were shifted toward zero after vacuum annealing, indicating that the SWNTs were doped in air. To the best of our knowledge, this is the first time numerous SWNTs were electrically characterized using short-channel devices, through which all of the measured SWNTs were determined to be semiconducting. Hence, our semiconducting single-walled carbon nanotube sorting system holds a great deal of promise in bringing forth a variety of practical applications in SWNT electronics.

    View details for DOI 10.1021/nn204875a

    View details for PubMedID 22352426

  • Selective dispersion of high purity semiconducting single-walled carbon nanotubes with regioregular poly(3-alkylthiophene)s NATURE COMMUNICATIONS Lee, H. W., Yoon, Y., Park, S., Oh, J. H., Hong, S., Liyanage, L. S., Wang, H., Morishita, S., Patil, N., Park, Y. J., Park, J. J., Spakowitz, A., Galli, G., Gygi, F., Wong, P. H., Tok, J. B., Kim, J. M., Bao, Z. 2011; 2


    Conjugated polymers, such as polyfluorene and poly(phenylene vinylene), have been used to selectively disperse semiconducting single-walled carbon nanotubes (sc-SWNTs), but these polymers have limited applications in transistors and solar cells. Regioregular poly(3-alkylthiophene)s (rr-P3ATs) are the most widely used materials for organic electronics and have been observed to wrap around SWNTs. However, no sorting of sc-SWNTs has been achieved before. Here we report the application of rr-P3ATs to sort sc-SWNTs. Through rational selection of polymers, solvent and temperature, we achieved highly selective dispersion of sc-SWNTs. Our approach enables direct film preparation after a simple centrifugation step. Using the sorted sc-SWNTs, we fabricate high-performance SWNT network transistors with observed charge-carrier mobility as high as 12 cm(2) V(-1) s(-1) and on/off ratio of >10(6). Our method offers a facile and a scalable route for separating sc-SWNTs and fabrication of electronic devices.

    View details for DOI 10.1038/ncomms1545

    View details for PubMedID 22086341

  • Enhancing the Supercapacitor Performance of Graphene/MnO2 Nanostructured Electrodes by Conductive Wrapping NANO LETTERS Yu, G., Hu, L., Liu, N., Wang, H., Vosgueritchian, M., Yang, Y., Cui, Y., Bao, Z. 2011; 11 (10): 4438-4442


    MnO2 is considered one of the most promising pseudocapactive materials for high-performance supercapacitors given its high theoretical specific capacitance, low-cost, environmental benignity, and natural abundance. However, MnO2 electrodes often suffer from poor electronic and ionic conductivities, resulting in their limited performance in power density and cycling. Here we developed a "conductive wrapping" method to greatly improve the supercapacitor performance of graphene/MnO2-based nanostructured electrodes. By three-dimensional (3D) conductive wrapping of graphene/MnO2 nanostructures with carbon nanotubes or conducting polymer, specific capacitance of the electrodes (considering total mass of active materials) has substantially increased by ∼20% and ∼45%, respectively, with values as high as ∼380 F/g achieved. Moreover, these ternary composite electrodes have also exhibited excellent cycling performance with >95% capacitance retention over 3000 cycles. This 3D conductive wrapping approach represents an exciting direction for enhancing the device performance of metal oxide-based electrochemical supercapacitors and can be generalized for designing next-generation high-performance energy storage devices.

    View details for DOI 10.1021/nl2026635

    View details for PubMedID 21942427

  • Solution-Processed Graphene/MnO2 Nanostructured Textiles for High-Performance Electrochemical Capacitors NANO LETTERS Yu, G., Hu, L., Vosgueritchian, M., Wang, H., Xie, X., McDonough, J. R., Cui, X., Cui, Y., Bao, Z. 2011; 11 (7): 2905-2911


    Large scale energy storage system with low cost, high power, and long cycle life is crucial for addressing the energy problem when connected with renewable energy production. To realize grid-scale applications of the energy storage devices, there remain several key issues including the development of low-cost, high-performance materials that are environmentally friendly and compatible with low-temperature and large-scale processing. In this report, we demonstrate that solution-exfoliated graphene nanosheets (∼5 nm thickness) can be conformably coated from solution on three-dimensional, porous textiles support structures for high loading of active electrode materials and to facilitate the access of electrolytes to those materials. With further controlled electrodeposition of pseudocapacitive MnO(2) nanomaterials, the hybrid graphene/MnO(2)-based textile yields high-capacitance performance with specific capacitance up to 315 F/g achieved. Moreover, we have successfully fabricated asymmetric electrochemical capacitors with graphene/MnO(2)-textile as the positive electrode and single-walled carbon nanotubes (SWNTs)-textile as the negative electrode in an aqueous Na(2)SO(4) electrolyte solution. These devices exhibit promising characteristics with a maximum power density of 110 kW/kg, an energy density of 12.5 Wh/kg, and excellent cycling performance of ∼95% capacitance retention over 5000 cycles. Such low-cost, high-performance energy textiles based on solution-processed graphene/MnO(2) hierarchical nanostructures offer great promise in large-scale energy storage device applications.

    View details for DOI 10.1021/nl2013828

    View details for PubMedID 21667923

  • High-Performance Field Effect Transistors from Solution Processed Carbon Nanotubes ACS NANO Wang, H., Luo, J., Robertson, A., Ito, Y., Yan, W., Lang, V., Zaka, M., Schaffel, F., Rummeli, M. H., Briggs, G. A., Warner, J. H. 2010; 4 (11): 6659-6664


    Nanoelectronic field effect transistors (FETs) are produced using solution processed individual carbon nanotubes (CNTs), synthesized by both arc discharge and laser ablation methods. We show that the performance of solution processed FETs approaches that of CVD-grown FETs if the nanotubes have minimal lattice defects and are free from surface contamination. This is achieved by treating the nanotubes to a high-temperature vacuum annealing process and using 1,2-dichloroethane for dispersion. We present CNT FETs with mobilities of up to 3546 cm(2)/(V s), transconductance of 4.22 μS, on-state conductance of 9.35 μS and on/off ratios as high as 10(6). High-resolution transmission electron microscopy is used to examine the presence of catalyst particles and amorphous carbon on the surface and Raman spectroscopy is used to examine the lattice defects, both of which lead to reduced device performance.

    View details for DOI 10.1021/nn1020743

    View details for Web of Science ID 000284438000042

    View details for PubMedID 20958015

  • Ultrahigh secondary electron emission of carbon nanotubes APPLIED PHYSICS LETTERS Luo, J., Warner, J. H., Feng, C., Yao, Y., Jin, Z., Wang, H., Pan, C., Wang, S., Yang, L., Li, Y., Zhang, J., Watt, A. A., Peng, L., Zhu, J., Briggs, G. A. 2010; 96 (21)

    View details for DOI 10.1063/1.3442491

    View details for Web of Science ID 000278183200066