Showing papers by "Jiwoong Park published in 2009"
TL;DR: In this paper, the authors observed highly efficient generation of electron-hole pairs due to impact excitation in single-walled carbon nanotube p-n junction photodiodes.
Abstract: We observed highly efficient generation of electron-hole pairs due to impact excitation in single-walled carbon nanotube p-n junction photodiodes. Optical excitation into the second electronic subband E22 leads to striking photocurrent steps in the device I-VSD characteristics that occur at voltage intervals of the band-gap energy EGAP/e. Spatially and spectrally resolved photocurrent combined with temperature-dependent studies suggest that these steps result from efficient generation of multiple electron-hole pairs from a single hot E22 carrier. This process is both of fundamental interest and relevant for applications in future ultra-efficient photovoltaic devices.
448 citations
01 Jan 2009
TL;DR: Spatially and spectrally resolved photocurrent combined with temperature-dependent studies suggest that these steps result from efficient generation of multiple electron-hole pairs from a single hot E22 carrier, which is both of fundamental interest and relevant for applications in future ultra-efficient photovoltaic devices.
Abstract: Efficient Carbon Nanotube Photodiodes A single photon absorbed in a single-walled carbon nanotube device can generate multiple unbound particles carrying an electric charge. Gabor et al. (p. 1367) report that in such a device at low temperatures, excitation with light of increasing energy leads to well-defined stepwise increases in current. Interestingly, because of the unique band structure of carbon nanotubes, this behavior is analogous to particle-antiparticle creation commonly observed in high-energy particle physics. These observations point to the promise of investigations in other nanoscale carbon systems, such as graphene, and could lead to numerous applications, including highly sensitive photon detection and ultra-efficient photovoltaics. The decay of photoexcited electrons in a carbon nanotube device creates multiple pairs of charge carriers. We observed highly efficient generation of electron-hole pairs due to impact excitation in single-walled carbon nanotube p-n junction photodiodes. Optical excitation into the second electronic subband E22 leads to striking photocurrent steps in the device I-VSD characteristics that occur at voltage intervals of the band-gap energy EGAP/e. Spatially and spectrally resolved photocurrent combined with temperature-dependent studies suggest that these steps result from efficient generation of multiple electron-hole pairs from a single hot E22 carrier. This process is both of fundamental interest and relevant for applications in future ultra-efficient photovoltaic devices.
440 citations
TL;DR: This letter uses scanning photocurrent microscopy to investigate the spatial mapping of photocurrent generation and collection in SLG in a multielectrode geometry and determines the majority carrier in graphene under different gate conditions by imaging the thermocurrent generated by laser-induced heating of electrodes.
Abstract: Unlike in linear nanostructures, photocurrent generated in single-layer graphene (SLG) is expected to display two-dimensional characteristics. This allows the investigation of carrier dynamics, in relation to several spatially varying factors (such as the location of photocurrent generation and collection) and the overall electron band configuration of the SLG. In this letter, we use scanning photocurrent microscopy to investigate the spatial mapping of photocurrent generation and collection in SLG in a multielectrode geometry. A strong electric field near metal-graphene contacts leads to efficient photocurrent generation, resulting in >30% efficiency for electron-hole separation. The polarity and magnitude of contact photocurrent are used to study the band alignment and graphene electrical potential near contacts, from which it is shown that there exist large-scale spatial variations in graphene electric potential. Our measurements with a multielectrode device configuration reveal that photocurrent is distributed with a clear directional dependence among different collector electrodes. In the same measurement scheme, we also determine the majority carrier in graphene under different gate conditions by imaging the thermocurrent generated by laser-induced heating of electrodes.
363 citations
TL;DR: The synthesis of large scale single layer graphene directly onto an evaporated copper film is reported, using a novel fabrication method to directly pattern these graphene sheets into devices by simply removing the underlying copper film.
Abstract: Full integration of graphene into conventional device circuitry would require a reproducible large scale graphene synthesis that is compatible with conventional thin film technology. We report the synthesis of large scale single layer graphene directly onto an evaporated copper film. A novel fabrication method was used to directly pattern these graphene sheets into devices by simply removing the underlying copper film. Raman and conductance measurements show that the mechanical and electrical properties of our single layer graphene are uniform over a large area, ( Ferrari, A. C. et al. Phys. Rev. Lett. 2006, 97, 187401.) which leads to a high device yield and successful fabrication of ultra long (>0.5 mm) graphene channels. Our graphene based devices present excellent electrical properties including a promising carrier mobility of 700 cm(2)/V.s and current saturation characteristics similar to devices based on exfoliated graphene ( Meric, I.. et al. Nat Nanotechnol. 2008, 3, 654-659).
333 citations
TL;DR: In this paper, the authors studied the relaxation dynamics of hot optical phonons in few-layer and multi-layer graphene films grown by epitaxy on silicon carbide substrates and by chemical vapor deposition on nickel substrates.
Abstract: Using ultrafast optical pump-probe spectroscopy, we study the relaxation dynamics of hot optical phonons in few-layer and multi-layer graphene films grown by epitaxy on silicon carbide substrates and by chemical vapor deposition on nickel substrates. In the first few hundred femtoseconds after photoexcitation, the hot carriers lose most of their energy to the generation of hot optical phonons which then present the main bottleneck to subsequent carrier cooling. Optical phonon cooling on short time scales is found to be independent of the graphene growth technique, the number of layers, and the type of the substrate. We find average phonon lifetimes in the 2.5-2.55 ps range. We model the relaxation dynamics of the coupled carrier-phonon system with rate equations and find a good agreement between the experimental data and the theory. The extracted optical phonon lifetimes agree very well with the theory based on anharmonic phonon interactions.
230 citations
TL;DR: Raman spectroscopy suggests that the graphene is not chemically modified during the release of the devices, demonstrating that the technique is a robust means of fabricating large-area suspended graphene structures.
Abstract: We report on a method to produce free-standing graphene sheets from epitaxial graphene on silicon carbide (SiC) substrate. Doubly clamped nanomechanical resonators with lengths up to 20 μm were patterned using this technique and their resonant motion was actuated and detected optically. Resonance frequencies of the order of tens of megahertz were measured for most devices, indicating that the resonators are much stiffer than expected for beams under no tension. Raman spectroscopy suggests that the graphene is not chemically modified during the release of the devices, demonstrating that the technique is a robust means of fabricating large-area suspended graphene structures.
211 citations
TL;DR: A single-molecule fluorescence study of electrocatalysis by single-walled carbon nanotubes (SWNTs) at single-reaction resolution is reported, allowing quantification of the reactivity and heterogeneity of individual reactive sites.
Abstract: We report a single-molecule fluorescence study of electrocatalysis by single-walled carbon nanotubes (SWNTs) at single-reaction resolution. Applying super-resolution optical imaging, we find that the electrocatalysis occurs at discrete, nanometer-dimension sites on SWNTs. Single-molecule kinetic analysis leads to an electrocatalytic mechanism, allowing quantification of the reactivity and heterogeneity of individual reactive sites. Combined with conductivity measurements, this approach will be powerful to interrogate how the electronic structure of SWNTs affects the electrocatalytic interfacial charge transfer, a process fundamental to photoelectrochemical cells.
99 citations
TL;DR: The differential terahertz transmission is found to be large when the field is polarized parallel to the nanowires and very small when theField is polarized perpendicular to the Nanowires, which can be explained in terms of the induced depolarization fields and the resulting magnitudes of the surface plasmon frequencies.
Abstract: We have measured the terahertz response of oriented Germanium nanowires using ultrafast optical-pump terahertz-probe spectroscopy. We present results on the time, frequency, and polarization dependence of the terahertz response. Our results indicate intraband energy relaxation times of photoexcited carriers in the 1.5-2.0 ps range, carrier density dependent interband electron-hole recombination times in the 75-125 ps range, and carrier momentum scattering rates in the 60-90 fs range. Additionally, the terahertz response of the nanowires is strongly polarization dependent despite the subwavelength dimensions of the nanowires. The differential terahertz transmission is found to be large when the field is polarized parallel to the nanowires and very small when the field is polarized perpendicular to the nanowires. This polarization dependence of the terahertz response can be explained in terms of the induced depolarization fields and the resulting magnitudes of the surface plasmon frequencies.
57 citations
TL;DR: The results establish photothermal current microscopy as an important addition to the existing suite of characterization techniques for carbon nanotubes and other linear nanostructures.
Abstract: The electrical conductance of a nanotube decreases when it is heated. This phenomenon is exploited in a new technique called photothermal current microscopy to image the conductance of individual nanotubes and groups of nanotubes.
50 citations
TL;DR: In this article, the effect of grain size and surface roughness on the deff of polarity-controlled ZnO thin films for second harmonic generation has been investigated, and the correlation among the grain sizes and surface roughens with the values of deff was found strong.
Abstract: Polarity-controlled ZnO thin films grown on different buffer layers were investigated as nonlinear optical materials for second harmonic generation. The effective nonlinear optical coefficient (deff) of ZnO grown on Cr-compound buffer layers showed a higher value than that of ZnO grown on MgO buffer layers. The correlations among the grain size and surface roughness with the values of deff were found to be strong. The deff of the ZnO film increased with decrease in the grain size and surface roughness. This relationship could be explained by the increase in reaction interface and the decrease in scattering probability at the surface as decrease in grain size and surface roughness, respectively.
17 citations