Because of graphene's all-surface nature, chemically bonded adatoms on graphene provides a powerful way to introduce properties absent in its pristine form, such as magnetism and spin-orbit coupling, or the presence of a band gap. Our experimentation on fluorinated graphene explores the entire range from the dilute limit of less than 1/1000 to full fluorination. We employ a wide variety of transport, scanned probes and microscopic and spectroscopic tools to examine the properties of fluorinated graphene. Heavily fluorinated graphene is a large-gap insulator whereas in the dilute limit, fluorine adatoms act as magnetic, resonant impurities that give rise to unusual weak localization and magneto-transport properties. In bilayer graphene, an electric field effectively controls the graphene-adatom interaction.

Publications:

S. Wellnhofer, A. Stabile, D. Kochan, M. Gmitra, Y. Chuang, J. Zhu, J. Fabian, "Spin relaxation in fluorinated single and bilayer graphene", Phys. Rev. B 100, 035421 (2019)

A. A. Stabile, A. Ferreira, J. Li, N. M. R. Peres, and J. Zhu, "Electrically tunable resonant scattering in fluorinated bilayer graphene", Phys. Rev. B Rapid Comm. 92, 121411(R) (2015)

B. Wang, J. Wang, J. Zhu, "Fluorination of Graphene: A Spectroscopic and Microscopic Study", ACS Nano 8 (2), 1862–1870 (2014)

X. Hong, K. Zou, B. Wang, S. -H. Cheng, and J. Zhu,"Evidence for Spin-Flip Scattering and Local Moments in Dilute Fluorinated Graphene," Phys. Rev. Lett. 108, 226602 (2012)

X. Hong, S. Cheng, C. Herding and J. Zhu, "Colossal negative magnetoresistance in dilute fluorinated graphene," Phys. Rev. B 83, 085410 (2011)

B. Wang, J.R. Sparks, H.R. Gutierrez, F. Okino, Q. Hao, Y. Tang, V.H. Crespi, J.O. Sofo, and J. Zhu, "Photoluminescence from nanocrystalline graphite monofluoride," Appl. Phys. Lett. 97, 141915 (2010)

S. Cheng, K. Zou, F. Okino, H. Rodriguez Gutierrez, A. Gupta, N. Shen, P. C. Eklund, J. O. Sofo and J. Zhu, "Reversible fluorination of graphene: Evidence of a two-dimensional wide bandgap semiconductor," Phys. Rev. B 81, 205435 (2010)

Stacking order and layer symmetry play a key role in the band structure of few-layer graphene. The bands of Bernal (AB)-stacked bilayer graphene are gapless but an electrostatic potential difference between the two layers, which can be set up by a perpendicular electric field, leads to the opening of a field-dependent band gap. ABA- and ABC-stacked trilayer graphene respond differently to the electric field; ABA-stacked trilayer remains a metal while ABC-stacked trilayer behaves similarly to AB-stacked bilayer. We use dual-gated field effect devices to probe the band structures of bilayer and trilayer graphene. By using thin high-k HfO₂ deposited by ALD as gate dielectric, we are able to generate a perpendicular displacement field D as large as several V/nm to follow the entire evolution of the band gap as a function of D in bilayer and trilayer graphene. Temperature-dependent resistivity measurement yields the value of the band gap and reveals potential disorder-induced hopping transport. We use SdH oscillations to accurately determine the carrier masses and the relevant hopping parameters in bilayer graphene.

Publications:

J. Li, Y. Tupikov, K. Watanabe, T. Taniguchi, J. Zhu, “Effective Landau level diagram of bilayer graphene”, Phys. Rev. Lett. 120, 047701 (2018)

J. Li, L. Z. Tan, K. Zou, A. A. Stabile, D. J. Seiwell, K. Watanabe, T. Taniguchi, Steven G. Louie, and J. Zhu, "Effective mass in bilayer graphene at low carrier densities: The role of potential disorder and electron-electron interaction", Phys. Rev. B Rapid Comm. 94, 161406(R) (2016)

K. Zou, F. Zhang, C. Clapp, A. H. MacDonald, and J. Zhu, "Transport studies of dual-gated ABC and ABA trilayer graphene: band gap opening and band structure tuning in very large perpendicular electric field," Nano Lett. 13, 369 (2013)

K. Zou, X. Hong and J. Zhu, "Effective mass of electrons and holes in bilayer graphene: Electron-hole asymmetry and electron-electron interaction," Phys. Rev. B 84, 085408 (2011)

K. Zou and J. Zhu, "Transport in gapped bilayer graphene: The role of potential fluctuations," Phys. Rev. B Rapid Comm. 82, 081407 (2010)

The integration of 2D material and ferroelecric substrate offers a potential route to realize non-volatile memory devices. Using high-quality single-crystalline Pb(Zr,Ti)O₃(PZT) substrate grown in the Ahn lab at Yale University, we demonstrated electron mobility up to 1.4 x 10⁵cm²/Vs in few-layer graphene. PZT-gated graphene exhibits an unusual resistance hysteresis in the transfer curve, which is reproducible and robust. We attribute this phenomenon to surface chemistry of the PZT.

Publications:

Hong, K. Zou, A. DaSilva, C. H. Ahn and J. Zhu, "Integrating functional oxides with graphene," Solid State Commun. 152, 1365 – 1374 (2012)

X. Hong, J. Hoffman, A. Posadas, K. Zou, C. H. Ahn and J. Zhu, "Unusual resistance hysteresis in n-layer graphene field effect transistors fabricated on ferroelectric Pb(Zr0.2Ti0.8)O3," Appl. Phys. Lett. 97, 033114 (2010)

X. Hong, A. Posadas, K. Zou, C. H. Ahn, and J. Zhu, "High-Mobility Few-Layer Graphene Field Effect Transistors Fabricated on Epitaxial Ferroelectric Gate Oxides," Phys. Rev. Lett. 102, 136808 (2009)

In conventional semiconductors such as Si and GaAs, electron-phonon scattering is a major mobility-limiting factor at room temperature. In graphene, electron-phonon interaction is weak and produces a room temperature mobility limit of ~10⁵cm²/Vs, which far exceeds that in Si and GaAs. In graphene transistors supported and covered by gate oxides, however, surface optical phonon scattering from the adjacent oxides becomes crucially important. Our work quantitatively measured this "remote" optical phonon scattering amplitude in HfO₂/graphene/SiO₂ field effect devices. The low-energy surface optical phonon modes of HfO₂ (centered at 54 meV) along limit the electron mobility to roughly 20,000 cm²/Vs at room temperature. In addition to limiting carrier mobility, emission into the remote surface optical phonon modes of the gate oxides is the primary channel of energy dissipation for hot carriers in graphene transistors operating at high source-drain bias voltages. Together with the Jain group, we combine experiment and theory to show how this process leads to the drift velocity saturation of carriers.

Publications:

Hong, K. Zou, A. DaSilva, C. H. Ahn and J. Zhu, "Integrating functional oxides with graphene," Solid State Commun. 152, 1365 – 1374 (2012)

K. Zou, X. Hong, D. Keefer, and J. Zhu, "Deposition of High-Quality HfO2 on Graphene and the Effect of Remote Oxide Phonon Scattering," Phys. Rev. Lett. 105, 126601 (2010)

A. DaSilva, K. Zou, J. K. Jain and J. Zhu, " Mechanism for Current Saturation and Energy Dissipation in Graphene Transistors," Phys. Rev. Lett. 104, 236601 (2010)

X. Hong, K. Zou, and J. Zhu, "Quantum scattering time and its implications on scattering sources in graphene," Phys. Rev. B 80, 241415(R) (2009)