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See detailOn-Demand Intracellular Delivery of Single Particles in Single Cells by 3D Hollow Nanoelectrodes
Huang, Jian-An; Caprettini, Valeria; Zhao, Yingqi et al

in Nano Letters (2019), 19(2), 722-731

Delivery of molecules into intracellular compartments is one of the fundamental requirements in molecular biology. However, the possibility of delivering a precise number of nano-objects with single ... [more ▼]

Delivery of molecules into intracellular compartments is one of the fundamental requirements in molecular biology. However, the possibility of delivering a precise number of nano-objects with single-particle resolution is still an open challenge. Here we present an electrophoretic platform based on 3D hollow nanoelectrodes to enable delivery of single nanoparticles into single selected cells and monitoring of the single-particle delivery by surface-enhanced Raman scattering (SERS). The gold-coated hollow nanoelectrode capable of confinement and enhancement of electromagnetic fields upon laser illumination can distinguish the SERS signals of a single nanoparticle flowing through the nanoelectrode. Tight wrapping of cell membranes around the nanoelectrodes allows effective membrane electroporation such that single gold nanorods are delivered on demand into a living cell by electrophoresis. The capability of the 3D hollow nanoelectrodes to porate cells and reveal single emitters from the background in continuous flow is promising for the analysis of both intracellular delivery and sampling. [less ▲]

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See detailHyperbolic Meta-Antennas Enable Full Control of Scattering and Absorption of Light
Maccaferri, Nicolò UL; Zhao, Yingqi; Isoniemi, Tommi et al

in Nano Letters (2019), 19(3), 1851-1859

We introduce a novel concept of hybrid metal-dielectric meta-antenna supporting type II hyperbolic dispersion, which enables full control of absorption and scattering of light in the visible/near-infrared ... [more ▼]

We introduce a novel concept of hybrid metal-dielectric meta-antenna supporting type II hyperbolic dispersion, which enables full control of absorption and scattering of light in the visible/near-infrared spectral range. This ability lies in the different nature of the localized hyperbolic Bloch-like modes excited within the meta-antenna. The experimental evidence is corroborated by a comprehensive theoretical study. In particular, we demonstrate that two main modes, one radiative and one non-radiative, can be excited by direct coupling with the free-space radiation. We show that the scattering is the dominating electromagnetic decay channel, when an electric dipolar mode is induced in the system, whereas a strong absorption process occurs when a magnetic dipole is excited. Also, by varying the geometry of the system, the relative ratio of scattering and absorption, as well as their relative enhancement and/or quenching, can be tuned at will over a broad spectral range, thus enabling full control of the two channels. Importantly, both radiative and nonradiative modes supported by our architecture can be excited directly with far-field radiation. This is observed to occur even when the radiative channels (scattering) are almost totally suppressed, thereby making the proposed architecture suitable for practical applications. Finally, the hyperbolic meta-antennas possess both angular and polarization independent structural integrity, unlocking promising applications as hybrid meta-surfaces or as solvable nanostructures. [less ▲]

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See detailIntravalley Spin-Flip Relaxation Dynamics in Single-Layer WS2
Wang, Zilong; Molina-Sanchez, Alejandro; Altmann, Patrick et al

in NANO LETTERS (2018), 18(11), 6882-6891

In monolayer (1L) transition metal dichalcogenides (TMDs) the valence and conduction bands are spin-split because of the strong spin-orbit interaction. In tungsten-based TMDs the spin-ordering of the ... [more ▼]

In monolayer (1L) transition metal dichalcogenides (TMDs) the valence and conduction bands are spin-split because of the strong spin-orbit interaction. In tungsten-based TMDs the spin-ordering of the conduction band is such that the so-called dark excitons, consisting of electrons and holes with opposite spin orientation, have lower energy than A excitons. The transition from bright to dark excitons involves the scattering of electrons from the upper to the lower conduction band at the K point of the Brillouin zone, with detrimental effects for the optoelectronic response of 1L-TMDs, since this reduces their light emission efficiency. Here, we exploit the valley selective optical selection rules and use two-color helicity-resolved pump-probe spectroscopy to directly measure the intravalley spin-flip relaxation dynamics in 1L-WS2. This occurs on a sub-ps time scale, and it is significantly dependent on temperature, indicative of phonon-assisted relaxation. Time-dependent ab initio calculations show that intravalley spin-flip scattering occurs on significantly longer time scales only at the K point, while the occupation of states away from the minimum of the conduction band significantly reduces the scattering time. Our results shed light on the scattering processes determining the light emission efficiency in optoelectronic and photonic devices based on 1L-TMDs. [less ▲]

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See detailTailoring mechanically tunable strain fields in graphene
Goldsche, Matthias; Sonntag, Jens; Khodkov, Tymofiy et al

in Nano Letters (2018), 18(3), 1707--1713

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See detailMagnetic Control of the Chiroptical Plasmonic Surfaces
Zubritskaya, Irina; Maccaferri, Nicolò UL; Inchausti Ezeiza, Xabier et al

in Nano Letters (2018), 18(1), 302-307

A major challenge facing plasmon nanophotonics is the poor dynamic tunability. A functional nanophotonic element would feature the real-time sizable tunability of transmission, reflection of light's ... [more ▼]

A major challenge facing plasmon nanophotonics is the poor dynamic tunability. A functional nanophotonic element would feature the real-time sizable tunability of transmission, reflection of light's intensity or polarization over a broad range of wavelengths, and would be robust and easy to integrate. Several approaches have been explored so far including mechanical deformation, thermal, or refractive index effects, and all-optical switching. Here we devise an ultrathin chiroptical surface, built on two-dimensional nanoantennas, where the chiral light transmission is controlled by the externally applied magnetic field. The magnetic field-induced modulation of the far-field chiroptical response with this surface exceeds 100% in the visible and near-infrared spectral ranges, opening the route for nanometer-thin magnetoplasmonic light-modulating surfaces tuned in real time and featuring a broad spectral response. [less ▲]

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See detailAb Initio Calculations of Ultrashort Carrier Dynamics in Two-Dimensional Materials: Valley Depolarization in Single-Layer WSe2
Molina-Sanchez, Alejandro UL; Sangalli, Davide; Wirtz, Ludger UL et al

in Nano Letters (2017), 17

In single-layer WSe2, a paradigmatic semiconducting transition metal dichalcogenide, a circularly polarized laser field can selectively excite electronic transitions in one of the inequivalent K± valleys ... [more ▼]

In single-layer WSe2, a paradigmatic semiconducting transition metal dichalcogenide, a circularly polarized laser field can selectively excite electronic transitions in one of the inequivalent K± valleys. Such selective valley population corresponds to a pseudospin polarization. This can be used as a degree of freedom in a “valleytronic” device provided that the time scale for its depolarization is sufficiently large. Yet, the mechanism behind the valley depolarization still remains heavily debated. Recent time–dependent Kerr experiments have provided an accurate way to visualize the valley dynamics by measuring the rotation of a linearly polarized probe pulse applied after a circularly polarized pump pulse. We present here a clear, accurate and parameter–free description of the valley dynamics. By using an atomistic, ab initio approach we fully disclose the elemental mechanisms that dictate the depolarization effects. Our results are in excellent agreement with recent time–dependent Kerr experiments. We explain the Kerr dynamics and its temperature dependence in terms of electron–phonon me- diated processes that induce spin–flip inter–valley transitions. [less ▲]

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See detailTuning the Pseudospin Polarization of Graphene by a Pseudomagnetic Field
Georgi, Alexander; Nemes-Incze, Peter; Carrillo-Bastos, Ramon et al

in Nano Letters (2017), 17

One of the intriguing characteristics of honeycomb lattices is the appearance of a pseudomagnetic field as a result of mechanical deformation. In the case of graphene, the Landau quantization resulting ... [more ▼]

One of the intriguing characteristics of honeycomb lattices is the appearance of a pseudomagnetic field as a result of mechanical deformation. In the case of graphene, the Landau quantization resulting from this pseudomagnetic field has been measured using scanning tunneling microscopy. Here we show that a signature of the pseudomagnetic field is a local sublattice symmetry breaking observable as a redistribution of the local density of states. This can be interpreted as a polarization of graphene’s pseudospin due to a strain induced pseudomagnetic field, in analogy to the alignment of a real spin in a magnetic field. We reveal this sublattice symmetry breaking by tunably straining graphene using the tip of a scanning tunneling microscope. The tip locally lifts the graphene membrane from a SiO2 support, as visible by an increased slope of the I(z) curves. The amount of lifting is consistent with molecular dynamics calculations, which reveal a deformed graphene area under the tip in the shape of a Gaussian. The pseudomagnetic field induced by the deformation becomes visible as a sublattice symmetry breaking which scales with the lifting height of the strained deformation and therefore with the pseudomagnetic field strength. Its magnitude is quantitatively reproduced by analytic and tight-binding models, revealing fields of 1000 T. These results might be the starting point for an effective THz valley filter, as a basic element of valleytronics. [less ▲]

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See detailQuantum Interference Effects in Resonant Raman Spectroscopy of Single- and Triple-Layer MoTe2 from First-Principles
Pereira Coutada Miranda, Henrique UL; Reichardt, Sven UL; Froehlicher, Guillaume et al

in Nano Letters (2017), 17(4), 2381--2388

We present a combined experimental and theoretical study of resonant Raman spectroscopy in single- and triple-layer MoTe2. Raman intensities are computed entirely from first-principles by calculating ... [more ▼]

We present a combined experimental and theoretical study of resonant Raman spectroscopy in single- and triple-layer MoTe2. Raman intensities are computed entirely from first-principles by calculating finite differences of the dielectric susceptibility. In our analysis, we investigate the role of quantum interference effects and the electron−phonon coupling. With this method, we explain the experimentally observed intensity inversion of the A′1 vibrational modes in triple-layer MoTe2 with increasing laser photon energy. Finally, we show that a quantitative comparison with experimental data requires the proper inclusion of excitonic effects. [less ▲]

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See detailAnisotropic Nanoantenna-Based Magnetoplasmonic Crystals for Highly Enhanced and Tunable Magneto-Optical Activity
Maccaferri, Nicolò UL; Bergamini, Luca; Pancaldi, Matteo et al

in Nano Letters (2016), 16(4), 2533-2542

We present a novel concept of a magnetically tunable plasmonic crystal based on the excitation of Fano lattice surface modes in periodic arrays of magnetic and optically anisotropic nanoantennas. We show ... [more ▼]

We present a novel concept of a magnetically tunable plasmonic crystal based on the excitation of Fano lattice surface modes in periodic arrays of magnetic and optically anisotropic nanoantennas. We show how coherent diffractive far-field coupling between elliptical nickel nanoantennas is governed by the two in-plane, orthogonal and spectrally detuned plasmonic responses of the individual building block, one directly induced by the incident radiation and the other induced by the application of an external magnetic field. The consequent excitation of magnetic field-induced Fano lattice surface modes leads to highly tunable and amplified magneto-optical effects as compared to a continuous film or metasurfaces made of disordered noninteracting magnetoplasmonic anisotropic nanoantennas. The concepts presented here can be exploited to design novel magnetoplasmonic sensors based on coupled localized plasmonic resonances, and nanoscale metamaterials for precise control and magnetically driven tunability of light polarization states. [less ▲]

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See detailUnified Description of the Optical Phonon Modes in N-Layer MoTe2
Froehlicher, Guillaume; Lorchat, Etienne; Fernique, François et al

in Nano Letters (2015), 15

N-layer transition metal dichalcogenides provide a unique platform to investigate the evolution of the physical properties between the bulk (three-dimensional) and monolayer (quasi-two-dimensional) limits ... [more ▼]

N-layer transition metal dichalcogenides provide a unique platform to investigate the evolution of the physical properties between the bulk (three-dimensional) and monolayer (quasi-two-dimensional) limits. Here, using high-resolution micro-Raman spectroscopy, we report a unified experimental description of the Γ-point optical phonons in N-layer 2H-molybdenum ditelluride (MoTe2). We observe series of N-dependent low-frequency interlayer shear and breathing modes (below 40 cm–1, denoted LSM and LBM) and well-defined Davydov splittings of the mid-frequency modes (in the range 100–200 cm–1, denoted iX and oX), which solely involve displacements of the chalcogen atoms. In contrast, the high-frequency modes (in the range 200–300 cm–1, denoted iMX and oMX), arising from displacements of both the metal and chalcogen atoms, exhibit considerably reduced splittings. The manifold of phonon modes associated with the in-plane and out-of-plane displacements are quantitatively described by a force constant model, including interactions up to the second nearest neighbor and surface effects as fitting parameters. The splittings for the iX and oX modes observed in N-layer crystals are directly correlated to the corresponding bulk Davydov splittings between the E2u/E1g and B1u/A1g modes, respectively, and provide a measurement of the frequencies of the bulk silent E2u and B1u optical phonon modes. Our analysis could readily be generalized to other layered crystals [less ▲]

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See detailLow B field magneto-phonon resonances in single-layer and bilayer graphene
Neumann, Christoph; Reichardt, Sven UL; Drögeler, Marc et al

in Nano Letters (2015), 15(3), 1547--1552

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See detailActive Magnetoplasmonic Ruler
Zubritskaya, Irina; Lodewijks, Kristof; Maccaferri, Nicolò UL et al

in Nano Letters (2015), 15(5), 3204-3211

Plasmon rulers are an emerging concept in which the strong near-field coupling of plasmon nanoantenna elements is employed to obtain structural information at the nanoscale. Here, we combine ... [more ▼]

Plasmon rulers are an emerging concept in which the strong near-field coupling of plasmon nanoantenna elements is employed to obtain structural information at the nanoscale. Here, we combine nanoplasmonics and nanomagnetism to conceptualize a magnetoplasmonic dimer nanoantenna that would be able to report nanoscale distances while optimizing its own spatial orientation. The latter constitutes an active operation in which a dynamically optimized optical response per measured unit length allows for the measurement of small and large nanoscale distances with about 2 orders of magnitude higher precision than current state-of-the-art plasmon rulers. We further propose a concept to optically measure the nanoscale response to the controlled application of force with a magnetic field. [less ▲]

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See detailMagnetoplasmonic Design Rules for Active Magneto-Optics
Lodewijks, Kristof; Maccaferri, Nicolò UL; Pakizeh, Tavakol et al

in Nano Letters (2014), 14(12), 7207-7214

Light polarization rotators and nonreciprocal optical isolators are essential building blocks in photonics technology. These macroscopic passive devices are commonly based on magneto-optical Faraday and ... [more ▼]

Light polarization rotators and nonreciprocal optical isolators are essential building blocks in photonics technology. These macroscopic passive devices are commonly based on magneto-optical Faraday and Kerr polarization rotation. Magnetoplasmonics, the combination of magnetism and plasmonics, is a promising route to bring these devices to the nanoscale. We introduce design rules for highly tunable active magnetoplasmonic elements in which we can tailor the amplitude and sign of the Kerr response over a broad spectral range. [less ▲]

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See detailProbing Quantum Confinement within Single Core–Multishell Nanowires
Martínez-Criado, Gema; Homs, Alejandro; Alén, Benito et al

in Nano Letters (2012), 12(11), 5829-5834

Theoretically core–multishell nanowires under a cross-section of hexagonal geometry should exhibit peculiar confinement effects. Using a hard X-ray nanobeam, here we show experimental evidence for carrier ... [more ▼]

Theoretically core–multishell nanowires under a cross-section of hexagonal geometry should exhibit peculiar confinement effects. Using a hard X-ray nanobeam, here we show experimental evidence for carrier localization phenomena at the hexagon corners by combining synchrotron excited optical luminescence with simultaneous X-ray fluorescence spectroscopy. Applied to single coaxial n-GaN/InGaN multiquantum-well/p-GaN nanowires, our experiment narrows the gap between optical microscopy and high-resolution X-ray imaging and calls for further studies on the underlying mechanisms of optoelectronic nanodevices. [less ▲]

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See detailGraphene on Metallic Substrates: Suppression of the Kohn Anomalies in the Phonon Dispersion
Allard, Adrien; Wirtz, Ludger UL

in Nano Letters (2010), 10(11), 4335-4340

The phonon dispersion of graphene is known to display two strong Kohn Anomalies (kinks) in the highest optical branch (HOB) at the high-symmetry points Gamma and K [Piscanec, S.; et al. Phys. Rev. Lett ... [more ▼]

The phonon dispersion of graphene is known to display two strong Kohn Anomalies (kinks) in the highest optical branch (HOB) at the high-symmetry points Gamma and K [Piscanec, S.; et al. Phys. Rev. Lett. 2004, 93, 185503]. The phonon slope around the Kohn anomalies is related to the electron phonon coupling (EPC) with the graphene it bands. we show that this EPC, which has strong impact, for example, on Raman scattering and electron transport, can be strongly modified due to interaction with a metallic substrate. For graphene grown on a Ni(111) surface, a total suppression of the Kohn anomaly occurs: the HOB around Gamma and K becomes completely flat. This is due to the strong hybridization of the graphene pi-bands with the nickel d bands that lifts the linear crossing of the pi bands at K. In addition, the out-of-plane modes are also found to be strongly affected by the binding to the substrate. For other metallic substrates, where the distance between the graphene sheet and the substrate is larger, hybridization is much less pronounced and the Kohn anomaly is only weakly perturbed. From experimental phonon dispersions, one can therefore draw conclusions about the interaction strength between graphene and its different substrates. [less ▲]

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See detailDoped Graphene as Tunable Electron-Phonon Coupling Material
Attaccalite, Claudio; Wirtz, Ludger UL; Lazzeri, Michele et al

in Nano Letters (2010), 10(4), 1172-1176

we present a new way to tune the electron phonon coupling (EPC) in graphene by changing the deformation potential with electron/hole doping. We show the EPC for highest optical branch at the high symmetry ... [more ▼]

we present a new way to tune the electron phonon coupling (EPC) in graphene by changing the deformation potential with electron/hole doping. We show the EPC for highest optical branch at the high symmetry point K acquires a strong dependency on the doping level due to electron electron correlation not accounted in mean-field approaches. Such a dependency influences the dispersion (with respect to the laser energy) of the Raman D and 2D lines and the splitting of the 2D peak in multilayer graphene. Finally this doping dependence opens the possibility to construct tunable electronic devices through external control of the EPC. [less ▲]

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See detailDoping Concentration of GaN Nanowires Determined by Opto-Electrical Measurements
Richter, T. UL; Lüth, H.; Meijers, R. et al

in Nano Letters (2008), 8(9), 3056-3059

The influence of n-doping on the electrical transport properties of GaN nanowires is investigated by photoconductivity measurements on wires with different diameters. The electrical transport in nanowires ... [more ▼]

The influence of n-doping on the electrical transport properties of GaN nanowires is investigated by photoconductivity measurements on wires with different diameters. The electrical transport in nanowires is extremely sensitive to the wire diameter because of the size dependent barrier for surface recombination. This effect is used to determine the doping level of the nanowires and to complete and consolidate our previously developed surface recombination model for GaN nanowires. [less ▲]

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See detailSpatially resolved raman spectroscopy of single- and few-layer graphene
Graf, D.; Molitor, F.; Ensslin, K. et al

in Nano Letters (2007), 7(2), 238-242

We present Raman spectroscopy measurements on single- and few-layer graphene flakes. By using a scanning confocal approach, we collect spectral data with spatial resolution, which allows us to directly ... [more ▼]

We present Raman spectroscopy measurements on single- and few-layer graphene flakes. By using a scanning confocal approach, we collect spectral data with spatial resolution, which allows us to directly compare Raman images with scanning force micrographs. Single-layer graphene can be distinguished from double- and few-layer by the width of the D' line: the single peak for single-layer graphene splits into different peaks for the double-layer. These findings are explained using the double-resonant Raman model based on ab initio calculations of the electronic structure and of the phonon dispersion. We investigate the D line intensity and find no defects within the flake. A finite D line response originating from the edges can be attributed either to defects or to the breakdown of translational symmetry. [less ▲]

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See detailDefect Distribution along Single GaN Nanowhiskers
Cavallini, Anna; Polenta, Laura; Rossi, Marco et al

in Nano Letters (2006), 6(7), 1548-1551

In this letter we report on spectral photoconductivity (PC) on different sections of single MBE-grown GaN nanowhiskers of diameters ranging on the order of 100 nm. The photoconductivity spectra show ... [more ▼]

In this letter we report on spectral photoconductivity (PC) on different sections of single MBE-grown GaN nanowhiskers of diameters ranging on the order of 100 nm. The photoconductivity spectra show, besides the band-gap related transition, deep-levels corresponding to the yellow, green, and blue bands. A strong spatial localization of specific photocurrent peaks has been observed, indicating that the defects responsible for such transitions are distributed inhomogeneously along the column growth direction. [less ▲]

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See detailRaman spectroscopy of single-wall boron nitride nanotubes
Arenal, R.; Ferrari, A. C.; Reich, S. et al

in Nano Letters (2006), 6(8), 1812-1816

Single-wall boron nitride nanotubes samples synthesized by laser vaporization of a hexagonal BN target under a nitrogen atmosphere are studied by UV and visible Raman spectroscopy. We show that resonant ... [more ▼]

Single-wall boron nitride nanotubes samples synthesized by laser vaporization of a hexagonal BN target under a nitrogen atmosphere are studied by UV and visible Raman spectroscopy. We show that resonant conditions are necessary for investigating phonon modes of BNNTs. Raman excitation in the UV (229 nm) provides preresonant conditions, allowing the identification of the A(1) tangential mode at 1370 cm(-1). This is 5 cm(-1) higher than the E-2g mode in bulk h-BN. Ab initio calculations show that the lower frequency of bulk h-BN with respect to large diameter nanotubes and the single sheet of h-BN is related to a softening of the sp2 bonds in the bulk due to interlayer interaction. [less ▲]

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