Tsung-Ta Tang

Bio: Tsung-Ta Tang is an academic researcher from University of California, Berkeley. The author has contributed to research in topics: Bilayer graphene & Exciton. The author has an hindex of 4, co-authored 4 publications receiving 3132 citations. Previous affiliations of Tsung-Ta Tang include National Chiao Tung University.

Direct observation of a widely tunable bandgap in bilayer graphene

Abstract: The electronic bandgap is an intrinsic property of semiconductors and insulators that largely determines their transport and optical properties. As such, it has a central role in modern device physics and technology and governs the operation of semiconductor devices such as p-n junctions, transistors, photodiodes and lasers. A tunable bandgap would be highly desirable because it would allow great flexibility in design and optimization of such devices, in particular if it could be tuned by applying a variable external electric field. However, in conventional materials, the bandgap is fixed by their crystalline structure, preventing such bandgap control. Here we demonstrate the realization of a widely tunable electronic bandgap in electrically gated bilayer graphene. Using a dual-gate bilayer graphene field-effect transistor (FET) and infrared microspectroscopy, we demonstrate a gate-controlled, continuously tunable bandgap of up to 250 meV. Our technique avoids uncontrolled chemical doping and provides direct evidence of a widely tunable bandgap-spanning a spectral range from zero to mid-infrared-that has eluded previous attempts. Combined with the remarkable electrical transport properties of such systems, this electrostatic bandgap control suggests novel nanoelectronic and nanophotonic device applications based on graphene.

A tunable phonon-exciton Fano system in bilayer graphene

Abstract: Fano resonances are features in absorption, scattering or transport spectra resulting from the interaction of discrete and continuum states. They have been observed in a variety of systems1,2,3,4,5,6. Here, we report a many-body Fano resonance in bilayer graphene that is continuously tunable by means of electrical gating. Discrete phonons and continuous exciton (electron–hole pair) transitions are coupled by electron–phonon interactions, yielding a new hybrid phonon–exciton excited state. It may also be possible to control the phonon–exciton coupling with an optical field. This tunable phonon–exciton system could allow novel applications such as phonon lasers. A new hybrid phonon–exciton excited state in bilayer graphene can be tuned electrically, with possible application to phonon lasers.

Graphene Device, Method of Investigating Graphene, and Method of Operating Graphene Device

Abstract: The present invention provides for a graphene device comprising: a first gate structure, a second gate structure that is transparent or semi-transparent, and a bilayer graphene coupled to the first and second gate structures, the bilayer graphene situated at least partially between the first and second gate structures. The present invention also provides for a method of investigating semiconductor properties of bilayer graphene and a method of operating the graphene device by producing a bandgap of at least 50 mV within the bilayer graphene by using the graphene device.

Optical spectroscopy of bilayer graphene

Abstract: Infrared spectroscopy reveals unusual tunable electronic structure and optical behaviour in electrically gated bilayer graphene. In a dual-gate bilayer graphene device, we were able to control the carrier doping and a semiconductor bandgap independently by using different combinations of the top and bottom gate voltages. The field-induced bandgap can be probed directly through the emerging interband transitions in infrared absorption spectra. A tunable bandgap up to 250 meV has been observed in our dual-gate bilayer graphene devices. This unique tunable bandgap can lead to many new physical phenomena. One example is an unusual phonon–exciton Fano resonance when the electronic bandgap is tuned to match the phonon vibration energy. Here (continuous) electron–hole transitions and (discrete) phonon vibrations form a coupled system described by the Fano resonance, and the infrared absorption spectra exhibit characteristic quantum interference between the phonon and exciton transitions. Remarkably, this coupled phonon–exciton Fano resonance can be continuously tuned through electrical gating in bilayer graphene, and its behaviour is described quantitatively by theory.

Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

Abstract: Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.

Single-layer MoS2 transistors

Abstract: Two-dimensional materials are attractive for use in next-generation nanoelectronic devices because, compared to one-dimensional materials, it is relatively easy to fabricate complex structures from them. The most widely studied two-dimensional material is graphene, both because of its rich physics and its high mobility. However, pristine graphene does not have a bandgap, a property that is essential for many applications, including transistors. Engineering a graphene bandgap increases fabrication complexity and either reduces mobilities to the level of strained silicon films or requires high voltages. Although single layers of MoS(2) have a large intrinsic bandgap of 1.8 eV (ref. 16), previously reported mobilities in the 0.5-3 cm(2) V(-1) s(-1) range are too low for practical devices. Here, we use a halfnium oxide gate dielectric to demonstrate a room-temperature single-layer MoS(2) mobility of at least 200 cm(2) V(-1) s(-1), similar to that of graphene nanoribbons, and demonstrate transistors with room-temperature current on/off ratios of 1 × 10(8) and ultralow standby power dissipation. Because monolayer MoS(2) has a direct bandgap, it can be used to construct interband tunnel FETs, which offer lower power consumption than classical transistors. Monolayer MoS(2) could also complement graphene in applications that require thin transparent semiconductors, such as optoelectronics and energy harvesting.

Graphene and Graphene Oxide: Synthesis, Properties, and Applications

Abstract: There is intense interest in graphene in fields such as physics, chemistry, and materials science, among others. Interest in graphene's exceptional physical properties, chemical tunability, and potential for applications has generated thousands of publications and an accelerating pace of research, making review of such research timely. Here is an overview of the synthesis, properties, and applications of graphene and related materials (primarily, graphite oxide and its colloidal suspensions and materials made from them), from a materials science perspective.

Electronic transport in two-dimensional graphene

Abstract: We provide a broad review of fundamental electronic properties of two-dimensional graphene with the emphasis on density and temperature dependent carrier transport in doped or gated graphene structures. A salient feature of our review is a critical comparison between carrier transport in graphene and in two-dimensional semiconductor systems (e.g. heterostructures, quantum wells, inversion layers) so that the unique features of graphene electronic properties arising from its gap- less, massless, chiral Dirac spectrum are highlighted. Experiment and theory as well as quantum and semi-classical transport are discussed in a synergistic manner in order to provide a unified and comprehensive perspective. Although the emphasis of the review is on those aspects of graphene transport where reasonable consensus exists in the literature, open questions are discussed as well. Various physical mechanisms controlling transport are described in depth including long- range charged impurity scattering, screening, short-range defect scattering, phonon scattering, many-body effects, Klein tunneling, minimum conductivity at the Dirac point, electron-hole puddle formation, p-n junctions, localization, percolation, quantum-classical crossover, midgap states, quantum Hall effects, and other phenomena.