Cooperativity in Ion Hydration Tielrooij, K.J; Garcia-Araez, N; Bonn, M ...
Science (American Association for the Advancement of Science),
05/2010, Letnik:
328, Številka:
5981
Journal Article
Recenzirano
Despite prolonged scientific efforts to unravel the effects of ions on the structure and dynamics of water, many open questions remain, in particular concerning the spatial extent of this effect ...(i.e., the number of water molecules affected) and the origin of ion-specific effects. A combined terahertz and femtosecond infrared spectroscopic study of water dynamics around different ions (specifically magnesium, lithium, sodium, and cesium cations, as well as sulfate, chloride, iodide, and perchlorate anions) reveals that the effect of ions and counterions on water can be strongly interdependent and nonadditive, and in certain cases extends well beyond the first solvation shell of water molecules directly surrounding the ion.
Knowledge of the structure of interfacial water molecules at electrified solid materials is the first step toward a better understanding of important processes at such surfaces, in, e.g., ...electrochemistry, atmospheric chemistry, and membrane biophysics. As graphene is an interesting material with multiple potential applications such as in transistors or sensors, we specifically investigate the graphene–water interface. We use sum-frequency generation spectroscopy to investigate the pH- and potential-dependence of the interfacial water structure in contact with a chemical vapor deposited (CVD) grown graphene surface. Our results show that the SFG signal from the interfacial water molecules at the graphene layer is dominated by the underlying substrate and that there are water molecules between the graphene and the (hydrophilic) supporting substrate.
Finding alternative optoelectronic mechanisms that overcome the limitations of conventional semiconductor devices is paramount for detecting and harvesting low-energy photons. A highly promising ...approach is to drive a current from the thermal energy added to the free-electron bath as a result of light absorption. Successful implementation of this strategy requires a broadband absorber where carriers interact among themselves more strongly than with phonons, as well as energy-selective contacts to extract the excess electronic heat. Here we show that graphene-WSe2-graphene heterostructure devices offer this possibility through the photo-thermionic effect: the absorbed photon energy in graphene is efficiently transferred to the electron bath leading to a thermalized hot carrier distribution. Carriers with energy higher than the Schottky barrier between graphene and WSe2 can be emitted over the barrier, thus creating photocurrent. We experimentally demonstrate that the photo-thermionic effect enables detection of sub-bandgap photons, while being size-scalable, electrically tunable, broadband and ultrafast.
Two-dimensional crystals such as graphene and transition-metal dichalcogenides demonstrate a range of unique and complementary optoelectronic properties. Assembling different two-dimensional ...materials in vertical heterostructures enables the combination of these properties in one device, thus creating multifunctional optoelectronic systems with superior performance. Here, we demonstrate that graphene/WSe2/graphene heterostructures ally the high photodetection efficiency of transition-metal dichalcogenides with a picosecond photoresponse comparable to that of graphene, thereby optimizing both speed and efficiency in a single photodetector. We follow the extraction of photoexcited carriers in these devices using time-resolved photocurrent measurements and demonstrate a photoresponse time as short as 5.5 ps, which we tune by applying a bias and by varying the transition-metal dichalcogenide layer thickness. Our study provides direct insight into the physical processes governing the detection speed and quantum efficiency of these van der Waals heterostuctures, such as out-of-plane carrier drift and recombination. The observation and understanding of ultrafast and efficient photodetection demonstrate the potential of hybrid transition-metal dichalcogenide-based heterostructures as a platform for future optoelectronic devices.
One of the important factors limiting solar-cell efficiency is that incident photons generate one electron-hole pair, irrespective of the photon energy. Any excess photon energy is lost as heat. The ...possible generation of multiple charge carriers per photon (carrier multiplication) is therefore of great interest for future solar cells. Carrier multiplication is known to occur in bulk semiconductors, but has been thought to be enhanced significantly in nanocrystalline materials such as quantum dots, owing to their discrete energy levels and enhanced Coulomb interactions. Contrary to this expectation, we demonstrate here that, for a given photon energy, carrier multiplication occurs more efficiently in bulk PbS and PbSe than in quantum dots of the same materials. Measured carrier-multiplication efficiencies in bulk materials are reproduced quantitatively using tight-binding calculations, which indicate that the reduced carrier-multiplication efficiency in quantum dots can be ascribed to the reduced density of states in these structures.
The near-field interaction between fluorescent emitters and graphene exhibits rich physics associated with local dipole-induced electromagnetic fields that are strongly enhanced due to the unique ...properties of graphene. Here, we measure emitter lifetimes as a function of emitter-graphene distance d, and find agreement with a universal scaling law, governed by the fine-structure constant. The observed energy transfer rate is in agreement with a 1/d 4 dependence that is characteristic of two-dimensional lossy media. The emitter decay rate is enhanced 90 times (energy transfer efficiency of ∼99%) with respect to the decay in vacuum at distances d ≈ 5 nm. This high energy transfer rate is mainly due to the two-dimensionality and gapless character of the monatomic carbon layer. Graphene is thus shown to be an extraordinary energy sink, holding great potential for photodetection, energy harvesting, and nanophotonics.
The photoresponse of graphene at mid-infrared frequencies is of high technological interest and is governed by fundamentally different underlying physics than the photoresponse at visible ...frequencies, as the energy of the photons and substrate phonons involved have comparable energies. Here, we perform a spectrally resolved study of the graphene photoresponse for mid-infrared light by measuring spatially resolved photocurrent over a broad frequency range (1000–1600 cm–1). We unveil the different mechanisms that give rise to photocurrent generation in graphene on a polar substrate. In particular, we find an enhancement of the photoresponse when the light excites bulk or surface phonons of the SiO2 substrate. This work paves the way for the development of graphene-based mid-infrared thermal sensing technology.
The ability to control the quantum state of a single electron spin in a quantum dot is at the heart of recent developments towards a scalable spin-based quantum computer. In combination with the ...recently demonstrated controlled exchange gate between two neighbouring spins, driven coherent single spin rotations would permit universal quantum operations. Here, we report the experimental realization of single electron spin rotations in a double quantum dot. First, we apply a continuous-wave oscillating magnetic field, generated on-chip, and observe electron spin resonance in spin-dependent transport measurements through the two dots. Next, we coherently control the quantum state of the electron spin by applying short bursts of the oscillating magnetic field and observe about eight oscillations of the spin state (so-called Rabi oscillations) during a microsecond burst. These results demonstrate the feasibility of operating single-electron spins in a quantum dot as quantum bits.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We study hydrated model membranes, consisting of stacked bilayers of 1,2-dioleoyl-
sn-glycero-3-phosphocholine lipids, using terahertz time-domain spectroscopy and infrared spectroscopy. Terahertz ...spectroscopy enables the investigation of water dynamics, owing to its sensitivity to dielectric relaxation processes associated with water reorientation. By controlling the number of water molecules per lipid molecule in the system, we elucidate how the interplay between the model membrane and water molecules results in different water dynamics. For decreasing hydration levels, we observe the appearance of new types of water dynamics: the collective bulklike dynamics become less pronounced, whereas an increased amount of both very slowly reorienting (i.e., irrotational) and very rapidly reorienting (i.e., fast) water molecules appear. Temperature-dependent measurements reveal the interconversion between the three distinct types of water present in the system.
For most optoelectronic applications of graphene, a thorough understanding of the processes that govern energy relaxation of photoexcited carriers is essential. The ultrafast energy relaxation in ...graphene occurs through two competing pathways: carrier–carrier scattering, creating an elevated carrier temperature, and optical phonon emission. At present, it is not clear what determines the dominating relaxation pathway. Here we reach a unifying picture of the ultrafast energy relaxation by investigating the terahertz photoconductivity, while varying the Fermi energy, photon energy and fluence over a wide range. We find that sufficiently low fluence (≲4 μJ/cm2) in conjunction with sufficiently high Fermi energy (≳0.1 eV) gives rise to energy relaxation that is dominated by carrier–carrier scattering, which leads to efficient carrier heating. Upon increasing the fluence or decreasing the Fermi energy, the carrier heating efficiency decreases, presumably due to energy relaxation that becomes increasingly dominated by phonon emission. Carrier heating through carrier–carrier scattering accounts for the negative photoconductivity for doped graphene observed at terahertz frequencies. We present a simple model that reproduces the data for a wide range of Fermi levels and excitation energies and allows us to qualitatively assess how the branching ratio between the two distinct relaxation pathways depends on excitation fluence and Fermi energy.
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