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Featured researches published by Ti Wang.


ACS Nano | 2017

All-Printable ZnO Quantum Dots/Graphene van der Waals Heterostructures for Ultrasensitive Detection of Ultraviolet Light

Maogang Gong; Qingfeng Liu; Brent Cook; Bhupal Kattel; Ti Wang; Wai-Lun Chan; Dan Ewing; Matthew Casper; Alex Stramel; Judy Z. Wu

In ZnO quantum dot/graphene heterojunction photodetectors, fabricated by printing quantum dots (QDs) directly on the graphene field-effect transistor (GFET) channel, the combination of the strong quantum confinement in ZnO QDs and the high charge mobility in graphene allows extraordinary quantum efficiency (or photoconductive gain) in visible-blind ultraviolet (UV) detection. Key to the high performance is a clean van der Waals interface to facilitate an efficient charge transfer from ZnO QDs to graphene upon UV illumination. Here, we report a robust ZnO QD surface activation process and demonstrate that a transition from zero to extraordinarily high photoresponsivity of 9.9 × 108 A/W and a photoconductive gain of 3.6 × 109 can be obtained in ZnO QDs/GFET heterojunction photodetectors, as the ZnO QDs surface is systematically engineered using this process. The high figure-of-merit UV detectivity D* in exceeding 1 × 1014 Jones represents more than 1 order of magnitude improvement over the best reported previously on ZnO nanostructure-based UV detectors. This result not only sheds light on the critical role of the van der Waals interface in affecting the optoelectronic process in ZnO QDs/GFET heterojunction photodetectors but also demonstrates the viability of printing quantum devices of high performance and low cost.


Journal of the American Chemical Society | 2017

A Multidimensional View of Charge Transfer Excitons at Organic Donor–Acceptor Interfaces

Ti Wang; Tika R. Kafle; Bhupal Kattel; Wai-Lun Chan

How tightly bound charge transfer (CT) excitons dissociate at organic donor-acceptor interfaces has been a long-standing question in the organic photovoltaics community. Recently, it has been proposed that exciton delocalization reduces the exciton binding energy and promotes exciton dissociation. In order to understand this mechanism, it is critical to resolve the evolution of the excitons binding energy and coherent size with femtosecond time resolution. However, because the coherent size is just a few nanometers, it presents a major experimental challenge to capture the CT process simultaneously in the energy, spatial, and temporal domains. In this work, the challenge is overcome by using time-resolved photoemission spectroscopy. The spatial size and electronic energy of a manifold of CT states are resolved at the zinc phthalocyanine (ZnPc)-fullerene (C60) donor-acceptor interface. It is found that CT at the interface first populates delocalized CT excitons with a coherent size of 4 nm. Then, this delocalized CT exciton relaxes in energy to produce CT states with delocalization sizes in the range of 1-3 nm. While the CT process from ZnPc to C60 occurs in about 150 fs after photoexcitation, the localization and energy relaxation occur in 2 ps. The multidimensional view on how CT excitons evolve in time, space, and energy provides key information to understand the exciton dissociation mechanism and to design nanostructures for effective charge separation.


ACS Nano | 2017

Charge Transfer Exciton and Spin Flipping at Organic–Transition-Metal Dichalcogenide Interfaces

Tika R. Kafle; Bhupal Kattel; Samuel D. Lane; Ti Wang; Hui Zhao; Wai-Lun Chan

Two-dimensional transition-metal dichalcogenides (TMD) can be combined with other materials such as organic small molecules to form hybrid van der Waals heterostructures. Because of different properties possessed by these two materials, the hybrid interface can exhibit properties that cannot be found in either of the materials. In this work, the zinc phthalocyanine (ZnPc)-molybdenum disulfide (MoS2) interface is used as a model system to study the charge transfer at these interfaces. It is found that the optically excited singlet exciton in ZnPc transfers its electron to MoS2 in 80 fs after photoexcitation to form a charge transfer exciton. However, back electron transfer occurs on the time scale of ∼1-100 ps, which results in the formation of a triplet exciton in the ZnPc layer. This relatively fast singlet-triplet transition is feasible because of the large singlet-triplet splitting in organic materials and the strong spin-orbit coupling in TMD crystals. The back electron transfer would reduce the yield of free carrier generation at the heterojunction if it is not avoided. On the other hand, the spin-selective back electron transfer could be used to manipulate electron spin in hybrid electronic devices.


Scientific Reports | 2016

Growing Ultra-flat Organic Films on Graphene with a Face-on Stacking via Moderate Molecule-Substrate Interaction

Ti Wang; Tika R. Kafle; Bhupal Kattel; Qingfeng Liu; Judy Wu; Wai-Lun Chan

The electronic properties of small molecule organic crystals depend heavily on the molecular orientation. For multi-layer organic photovoltaics, it is desirable for the molecules to have a face-on orientation in order to enhance the out-of-plane transport properties. However, it is challenging to grow well-ordered and smooth films with a face-on stacking on conventional substrates such as metals and oxides. In this work, metal-phthalocyanine molecules is used as a model system to demonstrate that two-dimensional crystals such as graphene can serve as a template for growing high quality, ultra-flat organic films with a face-on orientation. Furthermore, the molecule-substrate interaction is varied systematically from strong to weak interaction regime with the interaction strength characterized by ultrafast electron transfer measurements. We find that in order to achieve the optimum orientation and morphology, the molecule-substrate interaction needs to be strong enough to ensure a face-on stacking while it needs to be weak enough to avoid film roughening.


ACS Applied Materials & Interfaces | 2017

Designing the Interface of Carbon Nanotube/Biomaterials for High-Performance Ultra-Broadband Photodetection

Youpin Gong; Puja Adhikari; Qingfeng Liu; Ti Wang; Maogang Gong; Wai-Lun Chan; W. Y. Ching; Judy Z. Wu

Inorganic/biomolecule nanohybrids can combine superior electronic and optical properties of inorganic nanostructures and biomolecules for optoelectronics with performance far surpassing that achievable in conventional materials. The key toward a high-performance inorganic/biomolecule nanohybrid is to design their interface based on the electronic structures of the constituents. A major challenge is the lack of knowledge of most biomolecules due to their complex structures and composition. Here, we first calculated the electronic structure and optical properties of one of the cytochrome c (Cyt c) macromolecules (PDB ID: 1HRC ) using ab initio OLCAO method, which was followed by experimental confirmation using ultraviolet photoemission spectroscopy. For the first time, the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of Cyt c, a well-known electron transport chain in biological systems, were obtained. On the basis of the result, pairing the Cyt c with semiconductor single-wall carbon nanotubes (s-SWCNT) was predicted to have a favorable band alignment and built-in electrical field for exciton dissociation and charge transfer across the s-SWCNT/Cyt c heterojunction interface. Excitingly, photodetectors based on the s-SWCNT/Cyt c heterojunction nanohybrids demonstrated extraordinary ultra-broadband (visible light to infrared) responsivity (46-188 A W-1) and figure-of-merit detectivity D* (1-6 × 1010 cm Hz1/2 W-1). Moreover, these devices can be fabricated on transparent flexible substrates by a low-lost nonvacuum method and are stable in air. These results suggest that the s-SWCNT/biomolecule nanohybrids may be promising for the development of CNT-based ultra-broadband photodetectors.


Carbon | 2016

Metal-catalyst-free and controllable growth of high-quality monolayer and AB-stacked bilayer graphene on silicon dioxide

Qingfeng Liu; Youpin Gong; Ti Wang; Wai-Lun Chan; Judy Wu


Journal of Physical Chemistry C | 2016

Observation of an Ultrafast Exciton Hopping Channel in Organic Semiconducting Crystals

Ti Wang; Tika R. Kafle; Bhupal Kattel; Wai-Lun Chan


Physical Review B | 2015

From two-dimensional electron gas to localized charge: Dynamics of polaron formation in organic semiconductors

Ti Wang; Claudiu Caraiani; G. William Burg; Wai-Lun Chan


Physical review applied | 2015

Effect of Interlayer Coupling on Ultrafast Charge Transfer from Semiconducting Molecules to Mono- and Bilayer Graphene

Ti Wang; Qingfeng Liu; Claudiu Caraiani; Yupeng Zhang; Judy Wu; Wai-Lun Chan


Journal of Physical Chemistry C | 2016

Hot Exciton Relaxation and Exciton Trapping in Single-Walled Carbon Nanotube Thin Films

Tika R. Kafle; Ti Wang; Bhupal Kattel; Qingfeng Liu; Youpin Gong; Judy Z. Wu; Wai-Lun Chan

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Judy Wu

University of Kansas

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Puja Adhikari

University of Missouri–Kansas City

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W. Y. Ching

University of Missouri–Kansas City

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