Lei Zhai

University of Central Florida, Orlando, Florida, United States

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Publications (57)165.77 Total impact

  • [show abstract] [hide abstract]
    ABSTRACT: Structural order of conjugated polymers at different length scales direct the optoelectronic properties of the corresponding materials, thus it is of critical importance to understand and control conjugated polymer morphology for successful application of these materials in organic optoelectronics. Herein, with the aim of probing the dependence of single chain folding properties on the chemical structure and rigidness of the polymer backbones, single molecule fluorescence spectroscopy was applied to four thiophene-based conjugated polymers. These include regioregular poly (3-hexylthiophene) (RR-P3HT), poly(2,5-bis(3-tetradecylthiophen-2-yl)-thieno[3,2-b]thiophene) (PBTTT-14), poly(2,5-bis(3-tetradecylthiophen-2-yl)thiophene-2-yl)thiophen-2-ylthiazolo[5,4-d]thiazole) (PTzQT-12) and poly(3,3-didodecyl-quaterthiophene)] (PQT-12). Previous work has shown that RR-P3HT and PBTTT-14 polymer chains fold in their nanostructures, while PQT-12 and PTzQT-12 do not fold in their nanostructures. At the single molecule level, it was found that RR-P3HT single chains almost exclusively fold into loosely and strongly aggregated conformations, analogous to the folding properties in nanostructures. PQT-12 displays significant chain folding as well, but only into loosely aggregated conformations, showing an absence of strongly aggregated polymer chains. PBTTT-14 exhibits a significant fraction of rigid polymer chain. The findings made for single molecules of PQT-12 and PBTTT-14 are thus in contrast with the observations made in their corresponding nanostructures. PTzQT-12 appears to be the most rigid and planar conjugated polymer of these four polymers. However, while the presumably non-folding polymers PQT-12 and PTzQT-12 exhibit less folding than RR-P3HT, there is still a significant occurrence of chain folding for these polymers at the single molecule level. These results suggest that the folding properties of conjugated polymers can be influenced by the architecture of the polymer backbones, however, other factors such as intermolecular stacking interactions, solvent environment and side chain interactions in corresponding materials should also be taken into account to predict conjugated polymer material morphology.
    The Journal of Physical Chemistry B 12/2012; · 3.61 Impact Factor
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    ABSTRACT: Electronic transport of regioregular poly(3-hexylthiophene)-block-poly styrene (rr-P3HT-b-PS) copolymer in field effect transistor (FET) geometry with different surface treatment and different temperature is investigated. The devices show p type behavior with a maximum saturation mobility of 6 × 10−3 cm2/V s and current on/off ratio of 2.6 × 104 in an OTS treated sample at room temperature, which is lower compared to the controlled P3HT sample of same molecular weight fabricated with the same surface treatment. The mobility measured at different temperatures (300–150 K) show thermally activated hopping type transport mechanism with gate bias dependent activation energy of 100–270 meV which is higher compared to the reported value of pristine P3HT FET. The higher activation energy in hopping behavior and lower mobility in this block copolymer is caused by insulating PS segments.
    Synthetic Metals 10/2012; 162(s 17–18):1531–1536. · 2.11 Impact Factor
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    ABSTRACT: Polymer-derived amorphous SiBCN ceramics are synthesized through a simple dehydrocoupling and hydroboration reaction of an oligosilazane containing amine and vinyl groups and BH3·Me2S, followed by pyrolysis. Two types of ceramics, denoted as Si2B1 and Si4B1, are produced from preceramic polymers with Si/B ratios of 2/1 and 4/1, respectively. The structural evolution of these ceramics with respect to the pyrolysis temperature and boron concentration is investigated using solid-state NMR, Raman, and EPR spectroscopy. Solid-state NMR suggests the presence of three major components in the ceramics: (i) hexagonal boron nitride (h-BN), (ii) turbostratic boron nitride (t-BN), and (iii) BN2C groups. Increasing pyrolysis temperature leads to the transformation of BN2C groups into BN3 and “free” carbon. A thermodynamic model is proposed to explain such transformation. Raman spectroscopy measurements reveal that the concentration of the “free” carbon cluster decreases with increasing pyrolysis temperature, and Si4B1 contains more “free” carbon cluster than Si2B1. EPR studies reveal that the carbon (C)-dangling bond content also decreases with increasing pyrolysis temperature. It appears that the complete decomposition of the metastable BN2C groups to the BN3 groups and the “free” carbon affects the crystallization of SiBCN, which leads to Si4B1 ceramics crystallized at 1500 °C, whereas Si2B1 ceramics crystallized at 1600 °C.
  • ACS Applied Materials & Interfaces 06/2011; · 5.01 Impact Factor
  • Jianhua Zou, Binh Tran, Lei Zhai
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    ABSTRACT: A convenient approach to fabricate metal (i.e. gold, platinum, and palladium) nanoparticles on highly dispersed pristine carbon nanotubes (CNTs) was developed using a conjugated block copolymer of poly(3-hexylthiophene)-b-poly(vinylpyrrolidone) (P3HT-b-PVP). P3HT-b-PVP not only provides a stable dispersion of pristine CNTs through the π–π interactions between P3HT block and CNTs, but also introduces PVP groups on CNT surfaces to induce the heterogeneous nucleation of metal nanoparticles and protect them from aggregating. The density of metal nanoparticles on CNT surfaces was controlled by the metal salt/CNT feed ratio. The simple processing procedure, versatility in synthesizing various metal nanoparticles, high metal nanoparticle loading capacity, and excellent dispersibility and processability of the product make this approach a promising method to fabricate metal nanoparticles on CNTs.
    International Journal of Smart and Nano Materials. 06/2011; 2(2):92-100.
  • European Journal of Engineering Education. 05/2011; 36(2):199-210.
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    ABSTRACT: Graphene, a two dimensional monoatomic thick building block of a carbon allotrope, has emerged as an exotic material of the 21st century, and received world-wide attention due to its exceptional charge transport, thermal, optical, and mechanical properties. Graphene and its derivatives are being studied in nearly every field of science and engineering. Recent progress has shown that the graphene-based materials can have a profound impact on electronic and optoelectronic devices, chemical sensors, nanocomposites and energy storage. The aim of this review article is to provide a comprehensive scientific progress of graphene to date and evaluate its future perspective. Various synthesis processes of single layer graphene, graphene nanoribbons, chemically derived graphene, and graphene-based polymer and nano particle composites are reviewed. Their structural, thermal, optical, and electrical properties were also discussed along with their potential applications. The article concludes with a brief discussion on the impact of graphene and related materials on the environment, its toxicological effects and its future prospects in this rapidly emerging field.
    Progress in Materials Science 04/2011; 56(8):1178–1271. · 23.19 Impact Factor
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    ABSTRACT: We show that the low-temperature electron transport properties of chemically functionalized graphene can be explained as sequential tunneling of charges through a two-dimensional array of graphene quantum dots (GQDs). Below 15 K, a total suppression of current due to Coulomb blockade through a GQD array was observed. Temperature-dependent current-gate voltage characteristics show Coulomb oscillations with energy scales of 6.2–10 meV corresponding to GQD sizes of 5–8 nm, while resistance data exhibit an Efros-Shklovskii variable range hopping arising from structural- and size-induced disorder.
    Physical Review B 03/2011; 83(11):115323. · 3.77 Impact Factor
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    ABSTRACT: We fabricated organic field effect transistors (OFETs) by directly growing poly (3-hexylthiophne) (P3HT) crystalline nanowires on solution processed aligned array single walled carbon nanotubes (SWNT) interdigitated electrodes by exploiting strong π-π interaction for both efficient charge injection and transport. We also compared the device properties of OFETs using SWNT electrodes with control OFETs of P3HT nanowires deposited on gold electrodes. Electron transport measurements on 28 devices showed that, compared to the OFETs with gold electrodes, the OFETs with SWNT electrodes have better mobility and better current on-off ratio with a maximum of 0.13 cm(2)/(V s) and 3.1 × 10(5), respectively. The improved device characteristics with SWNT electrodes were also demonstrated by the improved charge injection and the absence of short channel effect, which was dominant in gold electrode OFETs. The enhancement of the device performance can be attributed to the improved interfacial contact between SWNT electrodes and the crystalline P3HT nanowires as well as the improved morphology of P3HT due to one-dimensional crystalline nanowire structure.
    ACS Applied Materials & Interfaces 03/2011; 3(4):1180-5. · 5.01 Impact Factor
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    ABSTRACT: This work addresses the synthesis, integration and characterization of a nanostructure-embedded thermoresponsive surface for flow regulation. In order to create a hierarchic structure which consists of microscale texture and nanoscale sub-texture, hybrid multilayers consisting of poly(allylamine hydrochloride) (PAH), poly(acrylic acid) (PAA) and colloidal silica nanoparticles (average diameter = 22 nm and 7 nm) were used. Based on the electrostatic interactions between the polyelectrolytes and nanoparticles, a layer-by-layer deposition technique in combination with photolithography was employed to obtain a localized, conformally-coated patch in a microchannel. Grafted with the thermoresponsive polymer, poly(N-isopropylacrylamide) (PNIPAAm), wettability of the surface could be tuned upon heating or cooling. The measurement of differential pressure at various stages of device verified the working conditions of the nanostructure-embedded surface for regulating a capillary flow in the microchannel.
    Journal of Nanoscience and Nanotechnology 02/2011; 11(2):1417-20. · 1.15 Impact Factor
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    ABSTRACT: Multi-walled carbon nanotube (MWCNT)/polyaniline (PANI)/MnO2 (MPM) ternary coaxial structures are fabricated as supercapacitor electrodes via a simple wet chemical method. The electrostatic interaction between negative poly(4-styrenesulfonic acid) (PSS) molecules and positive Mn2+ ions causes the generation of MnO2 nanostructures on MWCNT surfaces while the introduction of PANI layers with appropriate thickness on MWCNT surfaces facilitates the formation of MWCNT/PANI/MnO2 ternary coaxial structures. The thickness of PANI coatings is controlled by tuning the aniline/MWCNT ratio. The effect of PANI thickness on the subsequent MnO2 nanoflakes attachment onto MWCNTs, and the MPM structures is investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and field-emission scanning electron microscopy (FESEM). The results suggest that appropriate thickness of PANI layers is important for building MPM ternary coaxial structures without the agglomeration of MnO2 nanoflakes. The MPM ternary coaxial structures provide large interaction area between the MnO2 nanoflakes and electrolyte, and improve the electrochemical utilization of the hydrous MnO2, and decrease the contact resistance between MnO2 and PANI layer coated MWCNTs, leading to intriguing electrochemical properties for the applications in supercapacitors such as a specific capacitance of 330 Fg−1 and good cycle stability.
    Journal of Power Sources 01/2011; 196(1):565-572. · 4.68 Impact Factor
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    ABSTRACT: Combining the structural characterization of solution crystals fabricated from thiophene-based conjugated polymers with different molecular structures and a theoretical investigation of the polymer conformational transformability leads to an interesting discovery of the relationship between the molecular structures and their crystallization behaviors. The chain folding or nonfolding behavior of thiophene-based conjugated polymers in crystallization, an important factor to shape polymer crystals, is determined by their molecular structures, and can be estimated by the inter-ring rotation energy barriers of the polymer backbones. A quantitative theoretical calculation is proposed to evaluate the inter-ring rotation energy barriers, and the values are correlated with the experimentally observed chain folding or nonfolding behavior. The higher percentage of type I inter-ring σ bond (CH3 and H are at 3 and 3′ position of adjacent aromatic rings, respectively) or the lower average rotation barrier in polymer backbones creates higher capability of polymer conformational transformation and higher tendency of chain folding. Our study provides a valid prediction of the crystallization behavior of thiophene-based conjugated polymers through a theoretical evaluation of conjugated polymer molecular structures, and offers an essential understand of the structure–property relationship of conjugated polymers.
    Polymer. 01/2011; 52(10):2302-2309.
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    ABSTRACT: Ultralight multiwalled carbon nanotube (MWCNT) aerogel is fabricated from a wet gel of well-dispersed pristine MWCNTs. On the basis of a theoretical prediction that increasing interaction potential between CNTs lowers their critical concentration to form an infinite percolation network, poly(3-(trimethoxysilyl) propyl methacrylate) (PTMSPMA) is used to disperse and functionalize MWCNTs where the subsequent hydrolysis and condensation of PTMSPMA introduces strong and permanent chemical bonding between MWCNTs. The interaction is both experimentally and theoretically proven to facilitate the formation of a MWCNT percolation network, which leads to the gelation of MWCNT dispersion at ultralow MWCNT concentration. After removing the liquid component from the MWCNT wet gel, the lightest ever free-standing MWCNT aerogel monolith with a density of 4 mg/cm(3) is obtained. The MWCNT aerogel has an ordered macroporous honeycomb structure with straight and parallel voids in 50-150 μm separated by less than 100 nm thick walls. The entangled MWCNTs generate mesoporous structures on the honeycomb walls, creating aerogels with a surface area of 580 m(2)/g which is much higher than that of pristine MWCNTs (241 m(2)/g). Despite the ultralow density, the MWCNT aerogels have an excellent compression recoverable property as demonstrated by the compression test. The aerogels have an electrical conductivity of 3.2 × 10(-2) S·cm(-1) that can be further increased to 0.67 S·cm(-1) by a high-current pulse method without degrading their structures. The excellent compression recoverable property, hierarchically porous structure with large surface area, and high conductivity grant the MWCNT aerogels exceptional pressure and chemical vapor sensing capabilities.
    ACS Nano 12/2010; 4(12):7293-302. · 12.06 Impact Factor
  • Applied Physics Letters 11/2010; 3(11). · 3.79 Impact Factor
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    ABSTRACT: A block copolymer containing P3HT and C(60) is synthesized and subjected to single-molecule spectroscopy (SMS) and AFM studies to investigate the impact of the C(60) on the P3HT chain conformation and the phase-separation behavior upon annealing, respectively. Both the SMS and the single-molecule polarization investigations indicate that the grafting of the C(60)-containing block to the P3HT moiety does not significantly change its conformational structure. AFM studies on thin films of the block copolymer indicate that they are stable with microphase separation, while blended films of the block-copolymer precursor (without C(60)) with PCBM undergo macroscale phase separation upon thermal annealing.
    Macromolecular Chemistry and Physics 10/2010; 211(22):2416 - 2424. · 2.39 Impact Factor
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    ABSTRACT: We elucidate on the low mobility and charge traps of the chemically reduced graphene oxide (RGO) sheets by measuring and analyzing temperature dependent current-voltage characteristics. The RGO sheets were assembled between source and drain electrodes via dielectrophoresis. At low bias voltage the conduction is Ohmic while at high bias voltage and low temperatures the conduction becomes space charge limited with an exponential distribution of traps. We estimate an average trap density of 1.75×1016 cm−3. Quantitative information about charge traps will help develop optimization strategies of passivating defects in order to fabricate high quality solution processed graphene devices.
    Applied Physics Letters 08/2010; 97(9):093105-093105-3. · 3.79 Impact Factor
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    ABSTRACT: Reduced graphene oxide (RGO) composites containing tetrasulfonate salt of copper phthalocyanine (TSCuPc) are fabricated through the reduction of graphene oxide in the presence of water-soluble TSCuPc. The interaction between RGO monolayer and TSCuPc is investigated using ultraviolet−visible (UV−vis) spectroscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. The composites of electron donor (TSCuPc) and acceptor (RGO) are successfully incorporated in devices, and their optoelectronic properties are studied. Compared with RGO films, the RGO/TSCuPc composite films have lower conductivity (dark current) but much higher photoconductivity and photoresponsivity because of the presence of donor/acceptor materials and large donor/acceptor (D/A) interfaces for charge generation. The optoelectrical properties of the composite film can be improved by thermal annealing and optimizing the TSCuPc content. The RGO/TSCuPc composite films fabricated by the solution processing method could be used as large-area optoelectronics devices.
    Journal of Physical Chemistry C - J PHYS CHEM C. 08/2010;
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    ABSTRACT: We report on the fabrication of crystalline nanoribbon network field effect transistors (FETs) using low molecular weight ( M <sub> W </sub>) poly(3-hexylthiophene) (P3HT) with different surface treatments and compare with thin film FETs cast from the same M <sub> W </sub> regioregular P3HT. Nanoribbon FET shows improved performance with a maximum mobility of 0.012 cm <sup>2</sup>/ V   s and current on/off ratios of 6.5×10<sup>4</sup> due to unique crystalline structure and morphology. With various surface treatments, the nanoribbon FETs show less variation in device mobilities, while thin film FETs show more than ten times variation in device mobilities and up to 100 times change in current on/off ratios.
    Applied Physics Letters 07/2010; · 3.79 Impact Factor
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    ABSTRACT: Polymer-derived ceramic fibers with aligned multiwalled carbon nanotubes (MWCNTs) are fabricated through the electrospinning of polyaluminasilazane solutions with well-dispersed MWCNTs followed by pyrolysis. Poly(3-hexylthiophene)-b-poly (poly (ethylene glycol) methyl ether acrylate) (P3HT-b-PPEGA), a conjugated block copolymer compatible with polyaluminasilazane, is used to functionalize MWCNT surfaces with PPEGA, providing a noninvasive approach to disperse carbon nanotubes in polyaluminasilazane chloroform solutions. The electrospinning of the MWCNT/polyaluminasilazane solutions generates polymer fibers with aligned MWCNTs where MWCNTs are oriented along the electrospun jet by a sink flow. The subsequent pyrolysis of the obtained composite fibers produces ceramic fibers with aligned MWCNTs. The study of the effect of polymer and CNT concentration on the fiber structures shows that the fiber size increases with the increment of polymer concentration, whereas higher CNT content in the polymer solutions leads to thinner fibers attributable to the increased conductivity. Both the SEM and TEM characterization of the polymer and ceramic fibers demonstrates the uniform orientation of CNTs along the fibers, suggesting excellent dispersion of CNTs and efficient CNT alignment via the electrospinning. The electrical conductivity of a ceramic fibers with 1.2% aligned MWCNTs is measured to be 1.58 x 10(-6) S/cm, which is more than 500 times higher than that of bulk ceramic (3.43 x 10(-9) S/cm). Such an approach provides a versatile method to disperse CNTs in preceramic polymer solutions and offers a new approach to integrate aligned CNTs in ceramics.
    ACS Applied Materials & Interfaces 04/2010; 2(4):1150-6. · 5.01 Impact Factor
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    ABSTRACT: We demonstrate high yield fabrication of field effect transistors (FET) using chemically reduced graphene oxide (RGO) sheets. The RGO sheets suspended in water were assembled between prefabricated gold source and drain electrodes using ac dielectrophoresis. With the application of a backgate voltage, 60% of the devices showed p-type FET behavior, while the remaining 40% showed ambipolar behavior. After mild thermal annealing at 200 degrees C, all ambipolar RGO FET remained ambipolar with increased hole and electron mobility, while 60% of the p-type RGO devices were transformed to ambipolar. The maximum hole and electron mobilities of the devices were 4.0 and 1.5 cm(2) V( - 1) s( - 1) respectively. High yield assembly of chemically derived RGO FET will have significant impact in scaled up fabrication of graphene based nanoelectronic devices.
    Nanotechnology 03/2010; 21(16):165202. · 3.84 Impact Factor

Publication Stats

321 Citations
165.77 Total Impact Points


  • 2007–2012
    • University of Central Florida
      • NanoScience Technology Center
      Orlando, Florida, United States
  • 2001–2004
    • Carnegie Mellon University
      • Department of Chemistry
      Pittsburgh, PA, United States