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Nano Letters

Shinhyun Choi, Jong Hoon Shin, Jihang Lee, Patrick Sheridan, Wei D Lu
Memristors have been considered as a leading candidate for a number of critical applications ranging from non-volatile memory to non-Von Neumann computing systems. Feature extraction, which aims to transform input data from a high dimensional space to a space with fewer dimensions, is an important technique widely used in machine learning and pattern recognition applications. Here, we experimentally demonstrate that memristor arrays can be used to perform principal component analysis (PCA), one of the most commonly-used feature extraction techniques, through online, unsupervised learning...
April 24, 2017: Nano Letters
Vini Gautam, Shagufta Naureen, Naeem Shahid, Qian Gao, Yi Wang, David R Nisbet, Chennupati Jagadish, Vincent R Daria
Identifying the specific role of physical guidance cues in the growth of neurons is crucial for understanding the fundamental biology of brain development and for designing scaffolds for tissue engineering. Here, we investigate the structural significance of nanoscale topographies as physical cues for neurite outgrowth and circuit formation by growing neurons on semiconductor nanowires. We monitored neurite growth using optical and scanning electron microscopy, and evaluated the spontaneous neuronal network activity using functional calcium imaging...
April 24, 2017: Nano Letters
Chenshuo Ma, Xiaoxue Xu, Fan Wang, Zhiguang Zhou, Deming Liu, Jiangbo Zhao, Ming Guan, Candace I Lang, Dayong Jin
Each single upconversion nanocrystal (UCNC) usually contains thousands of photon sensitizers and hundreds of photon activators to up-convert near infrared photons into visible and ultraviolet emissions. Though in principle further increasing the sensitizers' concentration will enhance the absorption efficiency to produce brighter nanocrystals, typically 20% of Yb3+ ions has been used to avoid the so-called "concentration quenching" effect. Here we report that concentration quenching effect does not limit the sensitizer concentration and NaYbF4 is the most bright host matrix...
April 24, 2017: Nano Letters
Dong Shuo, Jie Lian, Mark H Jhon, Yinthai Chan, Zhi-Heng Loh
Femtosecond optical pump-probe spectroscopy resolves hitherto unobserved coherent acoustic phonons in colloidal CdSe/CdS core/shell nanoplatelets (NPLs). With increasing pump fluence, the frequency of the in-plane acoustic mode increases from 5.2 to 10.7 cm(-1), whereas the frequency of the out-of-plane mode remains at ~20 cm(-1). Analysis of the oscillation phases suggests that the coherent acoustic phonon generation mechanism transitions from displacive excitation to sub-picosecond Auger hole trapping with increasing pump fluence...
April 24, 2017: Nano Letters
Marine Schott, Anne Bernand-Mantel, Laurent Ranno, Stéfania Pizzini, Jan Vogel, Hélène Béa, Claire Baraduc, Stéphane Auffret, Gilles Gaudin, Dominique Givord
Nanoscale magnetic skyrmions are considered as potential information carriers for future spintronics memory and logic devices. Such applications will require the control of their local creation and annihilation, which involves so far solutions that are either energy consuming or difficult to integrate. Here we demonstrate the control of skyrmion bubbles nucleation and annihilation using electric field gating, an easily integrable and potentially energetically efficient solution. We present a detailed stability diagram of the skyrmion bubbles in a Pt/Co/oxide trilayer and show that their stability can be controlled via an applied electric field...
April 24, 2017: Nano Letters
Luda Wang, Christopher M Williams, Michael S H Boutilier, Piran R Kidambi, Rohit Karnik
High mechanical strength is essential for pressure-driven membrane separations with nanoporous single-layer graphene, but its ability to withstand high pressures remains to be demonstrated. We monitored failure of centimeter-scale single-layer graphene membranes on porous supports subjected to high pressures. Consistent with theory, the membranes were found to withstand higher pressures when placed on porous supports with smaller pore diameters, but failure occurred over a surprisingly broad range of pressures, attributed to heterogeneous susceptibility to failure at wrinkles, defects, and slack in the suspended graphene...
April 24, 2017: Nano Letters
Munetaka Arita, Florian Le Roux, Mark J Holmes, Satoshi Kako, Yasuhiko Arakawa
Wide bandgap III-nitride quantum dots (QDs) are promising materials for the realization of solid-state single-photon sources, especially operating at room temperature. However, so far a large degree of inhomogeneous broadening induced by spectral diffusion has compromised their use. Here, we demonstrate the ultraclean emission from single GaN QDs formed at macro-step edges in a GaN/AlGaN quantum well. As a likely consequence of the high growth temperature and hence a reduced defect density, spectral diffusion is heavily suppressed to levels at least one order of magnitude lower than conventional GaN QDs...
April 24, 2017: Nano Letters
Dohyeong Kwon, Bong Geun Cha, Yuri Cho, Jiyoun Min, Eun-Byeol Park, Suk-Jo Kang, Jaeyun Kim
Over the past decade, mesoporous silica nanoparticles (MSNs) smaller than 200 nm with a high colloidal stability have been extensively studied for systemic drug delivery. Although small molecule delivery via MSNs has been successful, the encapsulation of large therapeutic biomolecules, such as proteins or DNA, is limited due to small pore size of the conventional MSNs obtained by soft-templating. Here, we report the synthesis of mesoporous silica nanoparticles with extra-large pores (XL-MSNs) and their application to in vivo cytokine delivery for macrophage polarization...
April 24, 2017: Nano Letters
Ajjiporn Dathbun, Youngchan Kim, Seongchan Kim, Youngjae Yoo, Moon Sung Kang, Changgu Lee, Jeong Ho Cho
We demonstrated the fabrication of large-area ReS2 transistors and logic gates composed of a chemical vapor deposition (CVD)-grown multilayer ReS2 semiconductor channel and graphene electrodes. Single-layer graphene was used as the source/drain and coplanar gate electrodes. An ion gel with an ultrahigh capacitance effectively gated the ReS2 channel at a low voltage, below 2 V, through a coplanar gate. The contact resistance of the ion gel-gated ReS2 transistors with graphene electrodes decreased dramatically compared with the SiO2-devices prepared with Cr electrodes...
April 24, 2017: Nano Letters
Ahmad Zubair, Amirhasan Nourbakhsh, Jin-Yong Hong, Meng Qi, Yi Song, Debdeep Jena, Jing Kong, Mildred Dresselhaus, Tomás Palacios
Single layer graphene is an ideal material for the base layer of hot electron transistors (HETs) for potential terahertz (THz) applications. The ultrathin body and exceptionally long mean free path maximizes the probability for ballistic transport across the base of the HET. We demonstrate for the first time the operation of a high-performance HET using a graphene/WSe2 van der Waals (vdW) heterostructure as a base-collector barrier. The resulting device with a GaN/AlN heterojunction as emitter, exhibits a current density of 50 A/cm(2), direct current gain above 3 and 75% injection efficiency, which are record values among graphene-base HETs...
April 24, 2017: Nano Letters
Yuhang Jiang, Jinhai Mao, Junxi Duan, Xinyuan Lai, Kenji Watanabe, Takashi Taniguchi, Eva Y Andrei
Graphene's remarkable properties are inherent to its two-dimensional honeycomb lattice structure. Its low dimensionality, which makes it possible to rearrange the atoms by applying an external force, offers the intriguing prospect of mechanically controlling the electronic properties. In the presence of strain, graphene develops a pseudomagnetic field (PMF) that reconstructs the band structure into pseudo Landau levels (PLLs). However, a feasible route to realizing, characterizing and controlling PMFs is still lacking...
April 24, 2017: Nano Letters
Sergey V Makarov, Mihail I Petrov, Urs Zywietz, Valentin Milichko, Dmitry Zuev, Natalia Lopanitsyna, Alexey Kuksin, Ivan Mukhin, George Zograf, Evgeniy Ubyivovk, Daria A Smirnova, Sergey Starikov, Boris N Chichkov, Yuri S Kivshar
Recent trends to employ high-index dielectric particles in nanophotonics are motivated by their reduced dissipative losses and large resonant enhancement of nonlinear effects at the nanoscale. Because silicon is a centrosymmetric material, the studies of nonlinear optical properties of silicon nanoparticles have been targeting primarily the third-harmonic generation effects. Here we demonstrate, both experimentally and theoretically, that resonantly excited nanocrystalline silicon nanoparticles fabricated by an optimized laser printing technique can exhibit strong second-harmonic generation (SHG) effects...
April 24, 2017: Nano Letters
Haowei Zhai, Pengyu Xu, Mingqiang Ning, Qian Cheng, Jyotirmoy Mandal, Yuan Yang
Replacing flammable organic liquid electrolytes with solid Li-ion conductors is a promising approach to realize safe rechargeable batteries with high energy density. Composite solid electrolytes, which are comprised of a polymer matrix with ceramic Li-ion conductors dispersed inside, are attractive, since they combine the flexibility of polymer electrolytes and high ionic conductivities of ceramic electrolytes. However, the high conductivity of ceramic fillers is largely compromised by the low conductivity of the matrix, especially when nanoparticles (NPs) are used...
April 24, 2017: Nano Letters
George P Zograf, Mihail I Petrov, Dmitry A Zuev, Pavel A Dmitriev, Valentin A Milichko, Sergey V Makarov, Pavel A Belov
We propose a novel photothermal approach based on resonant dielectric nanoparticles, which possess imaginary part of permittivity significantly smaller as compared to metal ones. We show both experimentally and theoretically that a spherical silicon nanoparticle with a magnetic quadrupolar Mie resonance converts light to heat up to 4 times more effectively than similar spherical gold nanoparticle at the same heating conditions. We observe photoinduced temperature raise up to 900 K with the silicon nanoparticle on a glass substrate at moderate intensities (<2 mW/μm(2)) and typical laser wavelength (633 nm)...
April 24, 2017: Nano Letters
Donata Passarello, Simone G Altendorf, Jaewoo Jeong, Charles Rettner, Noel Arellano, Teya Topuria, Mahesh G Samant, Stuart S P Parkin
It has recently been shown that the metal-insulator transition in vanadium dioxide epitaxial films can be suppressed and the material made metallic to low temperatures by ionic liquid gating due to migration of oxygen. The gating is only possible on certain crystal facets where volume channels along the VO2's rutile c-axis intersect the surface. Here, we fabricate bars with the c-axis in plane and oriented parallel to or perpendicular to the length of the bars. We show that only bars with the c-axis perpendicular to the bars, for which the volume channels are accessible from the sides of the bar, can be metallized by ionic liquid gating...
April 24, 2017: Nano Letters
Ming Fu, Philippe Tamarat, He Huang, Jacky Even, Andrey L Rogach, Brahim Lounis
Revealing the crystal structure of lead halide perovskite nanocrystals is essential for the optimization of stability of these emerging materials in applications such as solar cells, photodetectors, and light-emitting devices. We use magneto-photoluminescence spectroscopy of individual perovskite CsPbBr3 nanocrystals as a unique tool to determine their crystal structure, which imprints distinct signatures in the excitonic sublevels of charge complexes at low temperatures. At zero magnetic field, the identification of two classes of photoluminescence spectra, displaying either two or three sublevels in their exciton fine structure, shows evidence for the existence of two crystalline structures, namely tetragonal D4h and orthorhombic D2h phases...
April 24, 2017: Nano Letters
Cheng Pan, Yong Wu, Bin Cheng, Shi Che, Takashi Taniguchi, Kenji Watanabe, Chun Ning Lau, Marc Bockrath
We report magnetotransport measurements of graphene bilayers at large perpendicular electric displacement fields, up to ∽ 1.5 V/nm, where we observe crossings between Landau levels with different orbital quantum numbers. The displacement fields at the studied crossings are primarily determined by energy shifts originating from the Landau level layer polarizability or polarization. Despite decreasing Landau level spacing with energy, successive crossings occur at larger displacement fields, resulting from decreasing polarizability with orbital quantum number...
April 21, 2017: Nano Letters
Ashish Arora, Jonathan Noky, Matthias Drüppel, Bhakti Jariwala, Thorsten Deilmann, Robert Schneider, Robert Schmidt, Osvaldo Del Pozo-Zamudio, Torsten Stiehm, Arnab Bhattacharya, Peter Krüger, Steffen Michaelis de Vasconcellos, Michael Rohlfing, Rudolf Bratschitsch
Atomically thin materials such as graphene or MoS2 are of high in-plane symmetry. Crystals with reduced symmetry hold the promise for novel optoelectronic devices based on their anisotropy in current flow or light polarization. Here, we present polarization-resolved optical transmission and photoluminescence spectroscopy of excitons in 1T'-ReSe2. On reducing the crystal thickness from bulk to a monolayer, we observe a strong blue shift of the optical band gap from 1.37 to 1.50 eV. The excitons are strongly polarized with dipole vectors along different crystal directions, which persist from bulk down to monolayer thickness...
April 21, 2017: Nano Letters
Charlotte Herbig, Timo Knispel, Sabina Simon, Ulrike A Schröder, Antonio J Martínez-Galera, Mohammad A Arman, Christian Teichert, Jan Knudsen, Arkady V Krasheninnikov, Thomas Michely
Our scanning tunneling microscopy and X-ray photoelectron spectroscopy experiments along with first-principles calculations uncover the rich phenomenology and enable a coherent understanding of carbon vapor interaction with graphene on Ir(111). At high temperatures, carbon vapor not only permeates to the metal surface but also densifies the graphene cover. Thereby, in addition to underlayer graphene growth, upon cool down also severe wrinkling of the densified graphene cover is observed. In contrast, at low temperatures the adsorbed carbon largely remains on top and self-organizes into a regular array of fullerene-like, thermally highly stable clusters that are covalently bonded to the underlying graphene sheet...
April 25, 2017: Nano Letters
Tobias A Gellen, Jet Lem, Daniel B Turner
The finite spectral line width of an ensemble of CdSe nanocrystals arises from size and shape inhomogeneity and the single-nanocrystal spectrum itself. This line width directly limits the performance of nanocrystal-based devices, yet most optical measurements cannot resolve the underlying contributions. We use two-dimensional electronic spectroscopy (2D ES) to measure the line width of the band-edge exciton of CdSe nanocrystals as a function of radii and surface chemistry. We find that the homogeneous width decreases for increasing nanocrystal radius and that surface chemistry plays a critical role in controlling this line width...
April 20, 2017: Nano Letters
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