Publications
Terahertz imaging and spectroscopy of single-layer graphene embedded in dielectrics
We study graphene/dielectric interfaces using THz imaging and spectroscopy. Non-contacting, non-destructive THz probing reveals the local sheet conductivity of single-layer graphene deposited on a Si substrate and covered by a thin PMMA layer. © 2012 OSA.
Ultrafast photocurrent measurement of the escape time of electrons and holes from carbon nanotube p-i-n photodiodes
Ultrafast photocurrent measurements are performed on individual carbon nanotube p-i-n photodiodes. The photocurrent response to subpicosecond pulses separated by a variable time delay Δt shows strong photocurrent suppression when two pulses overlap (Δt=0). The picosecond-scale decay time of photocurrent suppression scales inversely with the applied bias V SD, and is twice as long for photon energy above the second subband E 22 as compared to lower energy. The observed photocurrent behavior is well described by an escape time model that accounts for carrier effective mass.
Terahertz imaging and time-domain spectroscopy of large-area graphene on silicon
We demonstrate THz imaging and time-domain spectroscopy of a single-layer graphene film. The large-area graphene was grown by chemical vapor deposition on Cu-foil and subsequently transferred to a Si substrate. We took a transmission image of the graphene/Si sample measured by a Si:bolometer (pixel size is 0.4-mm). The graphene film (transmission: 36 - 41%) is clearly resolved against the background of the Si substrate (average transmission: 56.6%). The strong THz absorption by the graphene layer indicates that THz carrier dynamics are dominated by intraband transitions.
Softened elastic response and unzipping in chemical vapor deposition graphene membranes
We use atomic force microscopy to image grain boundaries and ripples in graphene membranes obtained by chemical vapor deposition. Nanoindentation measurements reveal that out-of-plane ripples effectively soften graphene's in-plane stiffness. Furthermore, grain boundaries significantly decrease the breaking strength of these membranes. Molecular dynamics simulations reveal that grain boundaries are especially weakening when subnanometer voids are present in the lattice. Finally, we demonstrate that two graphene membranes brought together form membranes with higher resistance to breaking.
Plasmon resonance in individual nanogap electrodes studied using graphene nanoconstrictions as photodetectors
We achieve direct electrical readout of the wavelength and polarization dependence of the plasmon resonance in individual gold nanogap antennas by positioning a graphene nanoconstriction within the gap as a localized photodetector. The polarization sensitivities can be as large as 99%, while the plasmon-induced photocurrent enhancement is 2-100. The plasmon peak frequency, polarization sensitivity, and photocurrent enhancement all vary between devices, indicating the degree to which the plasmon resonance is sensitive to nanometer-scale irregularities. © 2011 American Chemical Society.
High, size-dependent quality factor in an array of graphene mechanical resonators
Graphene's unparalleled strength, stiffness, and low mass per unit area make it an ideal material for nanomechanical resonators, but its relatively low quality factor is an important drawback that has been difficult to overcome. Here, we use a simple procedure to fabricate circular mechanical resonators of various diameters from graphene grown by chemical vapor deposition. In addition to highly reproducible resonance frequencies and mode shapes, we observe a striking improvement of the membrane quality factor with increasing size.
Terahertz imaging and spectroscopy of largearea single-layer graphene
We demonstrate terahertz (THz) imaging and spectroscopy of a 15 × 15-mm2 single-layer graphene film on Si using broadband THz pulses. The THz images clearly map out the THz carrier dynamics of the grapheneon-Si sample, allowing us to measure sheet conductivity with sub-mm resolution without fabricating electrodes. The THz carrier dynamics are dominated by intraband transitions and the THz-induced electron motion is characterized by a flat spectral response.
Grains and grain boundaries in single-layer graphene atomic patchwork quilts
The properties of polycrystalline materials are often dominated by the size of their grains and by the atomic structure of their grain boundaries. These effects should be especially pronounced in two-dimensional materials, where even a line defect can divide and disrupt a crystal. These issues take on practical significance in graphene, which is a hexagonal, two-dimensional crystal of carbon atoms. Single-atom-thick graphene sheets can now be produced by chemical vapour deposition on scales of up to metres, making their polycrystallinity almost unavoidable.
Large-scale arrays of single-layer graphene resonators
We fabricated large arrays of suspended, single-layer graphene membrane resonators using chemical vapor deposition (CVD) growth followed by patterning and transfer. We measure the resonators using both optical and electrical actuation and detection techniques. We find that the resonators can be modeled as flat membranes under tension, and that clamping the membranes on all sides improves agreement with our model and reduces the variation in frequency between identical resonators.
Electron transport in carbon nanotubes
Over the past decade, transport measurements on individual single-wall nanotubes have played a prominent role in developing our understanding of this novel carbon conductor. These measurements have identified both metallic and semiconducting nanotubes, determined their dominant electronic scattering mechanisms, and elucidated in great detail the properties of their quantized energy spectrum. Recent technological breakthroughs in nanotube device fabrication and electronic measurement have made possible experiments of unprecedented precision that reveal new and surprising phenomena.