Mehmet Aykol

Plasmon Resonant Enhancement of Photocatalytic Water Splitting Under Visible Illumination

We demonstrate plasmonic enhancement of photocatalytic water splitting under visible illumination by integrating strongly plasmonic Au nanoparticles with strongly catalytic TiO2. Under visible illumination, we observe enhancements of up to 66× in the photocatalytic splitting of water in TiO2 with the addition of Au nanoparticles. Above the plasmon resonance, under ultraviolet radiation we observe a 4-fold reduction in the photocatalytic activity. Electromagnetic simulations indicate that the improvement of photocatalytic activity in the visible range is caused by the local electric field enhancement near the TiO2 surface, rather than by the direct transfer of charge between the two materials. Here, the near-field optical enhancement increases the electron-hole pair generation rate at the surface of the TiO2, thus increasing the amount of photogenerated charge contributing to catalysis. This mechanism of enhancement is particularly effective because of the relatively short exciton diffusion length (or minority carrier diffusion length), which otherwise limits the photocatalytic performance. Our results suggest that enhancement factors many times larger than this are possible if this mechanism can be optimized.

Graphene-Silicon Schottky Diodes

We have fabricated graphene-silicon Schottky diodes by depositing mechanically exfoliated graphene on top of silicon substrates. The resulting current-voltage characteristics exhibit rectifying diode behavior with a barrier energy of 0.41 eV on n-type silicon and 0.45 eV on p-type silicon at the room temperature. The I-V characteristics measured at 100, 300, and 400 K indicate that temperature strongly influences the ideality factor of graphene-silicon Schottky diodes. The ideality factor, however, does not depend strongly on the number of graphene layers. The optical transparency of the thin graphene layer allows the underlying silicon substrate to absorb incident laser light and generate a photocurrent. Spatially resolved photocurrent measurements reveal the importance of inhomogeneity and series resistance in the devices.

Stacking-dependent band gap and quantum transport in trilayer graphene

Graphene is an extraordinary two-dimensional (2D) system with chiral charge carriers and fascinating electronic, mechanical and thermal properties. In multilayer graphene, stacking order provides an important yet rarely explored degree of freedom for tuning its electronic properties. For instance, Bernal-stacked trilayer graphene (B-TLG) is semi-metallic with a tunable band overlap, and rhombohedral-stacked trilayer graphene (r-TLG) is predicted to be semiconducting with a tunable band gap. These multilayer graphenes are also expected to exhibit rich novel phenomena at low charge densities owing to enhanced electronic interactions and competing symmetries. Here we demonstrate the dramatically different transport properties in TLG with different stacking orders, and the unexpected spontaneous gap opening in charge neutral r-TLG. At the Dirac point, B-TLG remains metallic, whereas r-TLG becomes insulating with an intrinsic interaction-driven gap ~6 meV. In magnetic fields, well-developed quantum Hall (QH) plateaux in r-TLG split into three branches at higher fields. Such splitting is a signature of the Lifshitz transition, a topological change in the Fermi surface, that is found only in r-TLG. Our results underscore the rich interaction-induced phenomena in trilayer graphene with different stacking orders, and its potential towards electronic applications.

Plasmon Resonant Enhancement of Carbon Monoxide Catalysis

Irradiating gold nanoparticles at their plasmon resonance frequency creates immense plasmonic charge and high temperatures, which can be used to drive catalytic reactions. By integrating strongly plasmonic nanoparticles with strongly catalytic metal oxides, significant enhancements in the catalytic activity can be achieved. Here, we study the plasmonically driven catalytic conversion of CO to CO(2) by irradiating Au nanoparticle/Fe(2)O(3) composites. The reaction rate of this composite greatly exceeds that of the Au nanoparticles or Fe(2)O(3) alone, indicating that this reaction is not driven solely by the thermal (plasmonic) heating of the gold nanoparticles but relies intimately on the interaction of these two materials. A comparison of the plasmonically driven catalytic reaction rate with that obtained under uniform heating shows an enhancement of at least 2 orders of magnitude.

Optical absorption and thermal transport of individual suspended carbon nanotube bundles

A focused laser beam is used to heat individual single-walled carbon nanotube bundles bridging two suspended microthermometers. By measurement of the temperature rise of the two thermometers, the optical absorption of 7.4-10.3 nm diameter bundles is found to be between 0.03 and 0.44% of the incident photons in the 0.4 microm diameter laser spot. The thermal conductance of the bundle is obtained with the additional measurement of the temperature rise of the nanotubes in the laser spot from shifts in the Raman G band frequency. According to the nanotube bundle diameter determined by transmission electron microscopy, the thermal conductivity is obtained.

Plasmon resonant enhancement of dye sensitized solar cells

We report an improvement in the efficiency of dye sensitized solar cells (DSSCs) by exploiting the plasmonic resonance of Au nanoparticles. By comparing the performance of DSSCs with and without Au nanoparticles, we demonstrate a 2.4-fold enhancement in the photoconversion efficiency. Enhancement in the photocurrent extends over the wavelength range from 460 nm to 730 nm. The underlying mechanism of enhancement is investigated by comparing samples with different geometries, including nanoparticles deposited on top of and embedded in the TiO2 electrode, as well as samples with the light absorbing dye molecule deposited on top of and underneath the Au nanoparticles. The mechanism of enhancement is attributed to the local electromagnetic response of the plasmonic nanoparticles, which couples light very effectively from the far field to the near field at the absorbing dye molecule monolayer, thereby increasing the local electron–hole pair (or exciton) generation rate significantly. The UV-vis absorption spectra and photocurrent spectra provide further information regarding the energy transfer between the plasmonic nanoparticles and the light absorbing dye molecules. Based on scanning electron microscope images, we perform electromagnetic simulations of these different Au nanoparticle/dye/TiO2 configurations, which corroborate the enhancement observed experimentally.

A New Lower Limit for the Ultimate Breaking Strain of Carbon Nanotubes

We apply immense strain to ultralong, suspended, single-walled carbon nanotubes while monitoring their Raman spectra. We can achieve strains up to 13.7 ± 0.3% without slippage, breakage, or defect formation based on the observation of reversible change in Raman spectra. This is more than twice that of previous observations. The rate of G band downshift with strain is found to span a wide range from -6.2 to -23.6 cm(-1)/% strain. Under these immense strains, the G band is observed to downshift by up to 157 cm(-1) (from 1592 to 1435 cm(-1)). Interestingly, under these significant lattice distortions, we observe no detectable D band Raman intensity. Also, we do not observe any broadening of the G band line width until a threshold downshift of Δω(G) > 75 cm(-1) is achieved at high strains, beyond which the fwhm of the G band increases sharply and reversibly. On the basis of a theoretical nonlinear stress-strain response, we estimate the maximum applied stress of the nanotubes in this study to be 99 GPa with a strength-to-weight ratio of almost 74,000 kN x m/kg, which is 30 times that of Kevlar and 117 times that of steel.

The effect of gas environment on electrical heating in suspended carbon nanotubes

We report micro-Raman spectroscopy measurements of the temperature distribution of current-carrying, 5 μm long, suspended carbon nanotubes in different gas environments near atmospheric pressure. At the same heating power, the measured G band phonon temperature of the nanotube is found to be significantly lower in gaseous environments than in vacuum. Theoretical analysis of these results suggests that about 50%–60% of the heat dissipated in the suspended nanotube is removed by its surrounding gas molecules, and that the thermal boundary conductance is higher in carbon dioxide than in nitrogen, argon, and helium, despite the lower thermal conductivity of carbon dioxide.

Direct observation of heat dissipation in individual suspended carbon nanotubes using a two-laser technique

A two-laser technique is used to investigate heat spreading along individual single walled carbon nanotube (SWCNT) bundles in vacuum and air environments. A 532 nm laser focused on the center of a suspended SWCNT bundle is used as a local heat source, and a 633 nm laser is used to measure the spatial temperature profile along the SWCNT bundle by monitoring the G band downshifts in the Raman spectra. A constant temperature gradient is observed when the SWCNT bundle is irradiated in vacuum, giving direct evidence of diffusive transport of the phonons probed by the Raman laser. In air, however, we observe an exponentially decaying temperature profile with a decay length of about 7 μm, due to heat dissipation from the SWCNT bundle to the surrounding gas molecules. The thermal conductivity of the suspended carbon nanotube (CNT) is determined from its electrical heating temperature profile as measured in vacuum and the nanotube bundle diameter measured via transmission electron microscopy. Based on the exponent...

Thermal emission spectra from individual suspended carbon nanotubes.

We study the thermal emission spectra of individual suspended carbon nanotubes induced by electrical heating. Semiconducting and metallic devices exhibit different spectra, based on their distinctive band structures. These spectra are compared with the ideal blackbody emission spectrum. In the visible wavelength range, the thermal emission spectra of semiconducting devices agree well with Plancks law, while the spectra of metallic devices show an additional peak between 1.5 and 1.9 eV. In the near-infrared wavelength range, the semiconducting nanotubes exhibit a peak around 1 eV. These additional peaks are attributed to the E11M and E22SC transitions that are thermally driven under these high applied bias voltages. These peaks show a strong polarization dependence, while the blackbody tail is unpolarized, which provides further evidence for electron-hole recombination in thermal emission. For semiconducting devices, the temperature of the nanotube is fit to Plancks law and compared with the temperatures obtained from the G band and 2D band Raman downshifts, as well as the anti-Stokes/Stokes intensity ratio. For devices showing thermal non-equilibrium, the electron temperature agrees well with G+ downshift but deviates from G_ downshift.