Research in the Zhu lab investigates the electronic properties of low dimensional materials and nanostructures. Our current projects focus on layered materials including graphene and transitional metal dichalcogenides such as (Mo/W)(S/Se)2. These atomically thin materials possess unique electronic structures that lead to novel collective phenomena in two dimensions and opportunities to create new device concepts that are based on bottom-up, layer-by-layer assembly techniques. We seek to understand the properties of pristine materials and exploit the role of substrate, defect, interface, stacking and heterojunctions to create new physics and functionalities, combining synthesis, nanofabrication, low-temperature transport and a suite of powerful characterization techniques.

Dilute fluorinated graphene

Graphene with a very dilute concentration of covalently bonded fluorine adatoms (F/C ratio on the order of 0.1%) behaves in a completely different way from pristine graphene. Adatoms may introduce magnetic moments and/or spin-orbit coupling to pristine graphene. In our dilute fluorinated graphene, we find that carriers become strongly localized at low densities and exhibit an unexpected, colossal negative magnetoresistance. The resistance at zero-field drops by a factor of 40 at 9T! At high carrier densities, weak localization measurements reveal phase breaking lengths much shorter than those in pristine graphene, suggesting the presence of fluorine-induced magnetic impurities. We are currently investigating the properties of dilute fluorinated bilayer graphene to further understand the effect of the bilayer bands and the breaking of the layer symmetry using adatoms and a perpendicular electric field.

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)

Electric field-dependent transport in bilayer and trilayer graphene

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 can break this symmetry and lead to the opening of a gate-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 bilayer. Our lab uses dual-gated field effect devices to probe the properties of bilayer and trilayer graphene. By using thin films of ALD-deposited HfO2 as the gate dielectric, we are able to generate a perpendicular electric field with D up to 6 V/nm and observe the saturation of the band gap in trilayer graphene. Temperature-dependent resistance measurement shows that because of disorder induced potential fluctuation, charge transport in gapped bilayer graphene consists of three channels: thermal activation, nearest neighbor hopping and variable-range hopping in different temperature range. We are currently investigating properties of gated bilayer junctions using h-BN as gate dielectrics.

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

Effective mass in bilayer graphene and the effect of electron-electron interaction

This project aims to provide an accurate description of the electron and hole bands in bilayer graphene through effective mass measurement as a function of carrier density. Analyzing the temperature dependence of high-quality Shubnikov-de Haas oscillations, we have determined the electron and hole mass in the 1012/cm2 regime precisely. The results demonstrate the hyperbolic nature of the bilayer bands. They also show a pronounced electron-hole asymmetry, where the hole mass is about 20-30% heavier than the electron mass at the same carrier density. This asymmetry enables us to accurately determine the interlayer hopping parameter to be 0.063. We also find that the mass data can only be described by an intra-layer nearest neighbor hopping to be 3.4 eV. The value reflects the importance of electron-electron interaction. We are currently studying the lower carrier density regime to further investigate the effect of electron-electron interaction on the effective mass.

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)

CVD synthesis and high-quality graphene devices

Large-area graphene sheets can be synthesized on copper using chemical vapor deposition (CVD). We follow the low-pressure growth method pioneered in the Ruoff lab, using methane as the carbon source and hydrogen forming gas to modulate the growth. By controlling the flow rate and sequence of the gases, we are able to grow high-quality monolayer graphene sheets free of multi-layer islands. Careful transfer, cleaning and fabrication processes lead to average carrier mobility ~ 10,000 cm2/Vs in field effect devices made from synthesized graphene. We are currently investigating transport in p-n junctions.

J. Wang, et al. manuscript in preparation

Transport in transition metal dichalcogenides WS2/WSe2

Layered transitional metal dichalcogenides MX2 (M=Mo, W, X=S, Se, Te) are van der Waals crystals similar to graphite in construction and in the lattice symmetry. Their electronic band structure, however, complements that of graphene in many aspects. Graphene is gapless. MX2 is direct/indirect gap semiconductor, where the gap size varies from 1-2 eV depending on the chalcogen element. The spin-orbit coupling strength is very weak in graphene but strong in MX2 materials. Carriers in graphene are light but heavy in MX2 compounds. Electron phonon interaction is weak in graphene and strong in MX2. The field of MX2 materials is at its infancy. What novel physics might be discovered? Can we make in-plane and vertical heterostructures of 2D layered materials to realize new functionalities non-existent in individual components? We are working hard to find out.

J. Wang, et al. manuscript in preparation

Heavily fluorinated graphene: opening a band gap

Prinstine graphene is gapless. Fluorination converts the sp2 bonds of graphene into sp3 bonds and a band gap opens. This project investigates the synthesis and properties of heavily fluorinated graphene (FG) and examines its potential applications as gate dielectric and tunnel barrier in graphene electronics. We have taken two complementary paths to the synthesis of FG. In the first approach, we synthesize bulk graphite fluoride and obtain FG through the exfoliation of high-quality stoichiometric graphite monofluoride crystals. In the second approach, we fluorinate graphene sheets synthesized via chemical vapor deposition using CF4 plasma. We have studied the properties of FG produced using both methods using microscopic and spectroscopic tools. Heavily fluorinated graphene is highly insulating. Upon photo-excitation, stoichiometric CF emits fluorescence in six modes in the visible spectrum. These emissions are likely due to defect states. In FG synthesized from CVD graphene, we find that the structural features of CVD graphene (wrinkles, folds, multi-layer patches) play a critical role in the spatial distribution of fluorine and dominate the charge transport of FG. XPS studies reveal the complexity and evolution of the carbon-fluorine bonds, where the defects and grain boundaries of CVD graphene are important.

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)

Oxide-on-graphene bio-ready field effect sensors

Nanoelectronics-based detection schemes offer promising sensitive and label-free alternatives to bioanalysis. This project investigates the design, fabrication, and operation of novel oxide-on-graphene, bio-ready field effect sensors using graphene sheets synthesized by chemical vapor deposition. Our design uses thin layers of HfO2 and SiO2 films to isolate the graphene channel and electrodes from electrolyte and uses the top SiO2 surface for detection and further chemical functionalization. This design preserves the excellent transport characteristics of the graphene transducer while taking advantage of the well-established surface chemistry of SiO2 in facilitating specific biomolecular binding. The graphene transducer channel operates in solution with high stability and high carrier mobility of approximately 5000 cm2/Vs. By applying the solution gate voltage in pulse, we eliminate hysteresis in the transfer curve of the graphene channel, which is critical to achieving a high detection resolution of the sensor. We demonstrate the silanization of the SiO2 surface with aminopropyl-trimethoxysilane (APTMS), which can be further linked to biomolecular probes and targets. The pH sensitivity of the bare and APTMS-functionalized SiO2 is measured to be 46 mV/pH and 43 mV/pH respectively, in good agreement with literature results. With suitable linking chemistry, these graphene sensors can potentially be useful in the detection of biological events such as DNA hybridization, thus opening a new avenue for biosensing using nanoscale electronics.

B. Wang, et al. manuscript in preparation

Remote Surface Optical Phonon Scattering from Gate Oxides in Graphene Transistors: Mobility Limit and Current Saturation

In conventional semiconductors, scattering between electrons and the optical phonons of the material is a major mobility-limiting factor at room temperature. In graphene, this interaction is negligible. Longitudinal phonons of graphene scatter electrons but the interaction is so weak that a room temperature mobility of ~105 cm2/Vs is expected. Experimentally much lower mobility is observed in graphene transistors supported on substrates and covered by top-gate oxides. The reason is twofold: at low temperatures, the scattering from charged impurities dominates (see the next subject). At high temperature, remote surface optical phonon scattering from the adjacent gate oxides becomes crucially important. This work quantitatively evaluates the magnitude of remote surface optical phonon scattering from both gate oxides in a dual-oxide HfO2/graphene/SiO2 structruce. The low-energy surface optical phonon modes of HfO2 (centered at 54 meV) along limit the electron mobility to roughly 20,000cm2/Vs at room temperature.

In addition to limiting carrier mobility, the emission of remote surface optical phonons is the primary channel of heat dissipation in graphene transistors operating at high source-drain bias. Joint with the Jain group (link), we combine experiment and Boltzmann theory, with no adjustable parameters, to show how this process lowers the electron temperature and leads to the saturation of the electron drift velocity at high source-drain bias.

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)

Quantum Scattering Time and the Scattering Sources in Graphene

The electron mobility in graphene devices supported on a substrate is limited to 20,000 cm2/Vs even at very low temperature due to extrinsic scattering sources, among which charged impurities are shown to play a dominant role. Where are the charges? We address this question by simultaneously measuring the transport and quantum scattering times in graphene. We show that the scattering charges are located extremely close to the graphene plane. Combining other evidences indicating the chemical reactivity of the hydroxyl groups on the SiO2 surface and its doping effect (Eklund and Sofo experiment), this experiment shows that charges transferred from the SiO2 substrate are the major scattering sources. Does this make sense? We think so. Recent experiments in the Kim group (link) show that the electron mobility can go up to 60,000 cm2/Vs by simply placing graphene on BN substrates.

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)

Integration of Graphene and Ferroelectric Substrate PZT

Using high-quality single-crystalline Pb(Zr,Ti)O3(PZT) substrate grown in the Ahn lab(Yale University, link), we demonstrate electron mobility up to 1.4 X 105cm2/Vs in few-layer graphene field effect transistors fabricated on PZT. This is a ten-fold increase over mobility in SiO2-gated graphene devices. What's more, PZT-gated graphene exhibits an unusual resistance hysteresis, which is reproducible, robust and has a time constant of approximately 6 hours at 300 K. With further improvement on the relaxation time, this mechanism can potentially be used to construct graphene non-volatile memory devices.

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)

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)

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Last Updated: August, 2013

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