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1.	Abay, Simon, 1980, et al. (författare) High Critical-Current Superconductor-InAs Nanowire-Superconductor Junctions 2012 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 12:11, s. 5622-5625 Tidskriftsartikel (refereegranskat)abstract We report on the fabrication of InAs nanowires coupled to superconducting leads with high critical current and widely tunable conductance. We implemented a double lift off nanofabrication Method to get very short nanowire devices with Ohmic contacts. We observe very high critical. currents of up to 800 nA in a wire with a diameter of 80 nm. The current- voltage characteristics of longer and suspended nanowires display,either. Coulomb blockade or supercurrent depending on a local gate voltage, combining different regimes of transport in a single device.
2.	Abay, Simon, 1980, et al. (författare) Quantized Conductance and Its Correlation to the Supercurrent in a Nanowire Connected to Superconductors 2013 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 13:8, s. 3614-3617 Tidskriftsartikel (refereegranskat)abstract We report conductance and supercurrent of InAs nano wires coupled to Al-superconducting electrodes with short channel lengths and good Ohmic contacts. The nanowires are suspended 15 nm above a local gate electrode. The charge density in the nanowires can be controlled by a small change in the gate voltage. For large negative gate voltages, the number of conducting channels is reduced gradually, and we observe a stepwise decrease of both conductance and critical current before the conductance vanishes completely.
3.	Abdel-Hafiez, Mahmoud, et al. (författare) From Insulator to Superconductor : A Series of Pressure-Driven Transitions in Quasi-One-Dimensional TiS3 Nanoribbons 2024 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 24:18, s. 5562-5569 Tidskriftsartikel (refereegranskat)abstract Transition metal trichalcogenides (TMTCs) offer remarkable opportunities for tuning electronic states through modifications in chemical composition, temperature, and pressure. Despite considerable interest in TMTCs, there remain significant knowledge gaps concerning the evolution of their electronic properties under compression. In this study, we employ experimental and theoretical approaches to comprehensively explore the high-pressure behavior of the electronic properties of TiS3, a quasi-one-dimensional (Q1D) semiconductor, across various temperature ranges. Through high-pressure electrical resistance and magnetic measurements at elevated pressures, we uncover a distinctive sequence of phase transitions within TiS3, encompassing a transformation from an insulating state at ambient pressure to the emergence of an incipient superconducting state above 70 GPa. Our findings provide compelling evidence that superconductivity at low temperatures of ∼2.9 K is a fundamental characteristic of TiS3, shedding new light on the intriguing high-pressure electronic properties of TiS3 and underscoring the broader implications of our discoveries for TMTCs in general.
4.	Adolfsson, Karl, et al. (författare) Fluorescent Nanowire Heterostructures as a Versatile Tool for Biology Applications 2013 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 13:10, s. 4728-4732 Tidskriftsartikel (refereegranskat)abstract Nanowires are increasingly used in biology, as sensors, as injection devices, and us model systems for toxicity studies. Currently, in situ visualization of nanowires in biological media is done using organic dyes, which a;:e prone to photobleaching, or using microscopy methods which either yield poor resolution or require a sophisticated setup. Here we show that inherently fluorescent nanowire axial heterostructnies c:an be used to localize and identify nanowires in cells and tissue; By synthesizing GaP GaInP nanowire heterostructures, with nonfluorescent GaP segments and fluorescent GaInP segments, we created a barcode labeling system enabling the distinction of the nanowire morphological and chemical properties using fluorescence microscopy. The GaInP photoluminescence stability, combined with the fact that the nanowires can be coated with different materials while retaining their fluorescence, make these nanowires promising tools for biological and nanotoxicological studies.
5.	Agthe, Michael, et al. (författare) Following in Real Time the Two-Step Assembly of Nanoparticles into Mesocrystals in Levitating Drops 2016 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 16:11, s. 6838-6843 Tidskriftsartikel (refereegranskat)abstract Mesocrystals composed of crystallographically aligned nanocrystals are present in biominerals and assembled materials which show strongly directional properties of importance for mechanical protection and functional devices. Mesocrystals are commonly formed by complex biomineralization processes and can also be generated by assembly of anisotropic nanocrystals. Here, we follow the evaporation-induced assembly of maghemite nanocubes into mesocrystals in real time in levitating drops. Analysis of time-resolved small-angle X-ray scattering data and ex situ scanning electron microscopy together with interparticle potential calculations show that the substrate-free, particle-mediated crystallization process proceeds in two stages involving the formation and rapid transformation of a dense, structurally disordered phase into ordered mesocrystals. Controlling and tailoring the particle-mediated formation of mesocrystals could be utilized to assemble designed nanoparticles into new materials with unique functions.
6.	Aili, Daniel, 1977-, et al. (författare) Assembly of Polypeptide-Functionalized Gold Nanoparticles through a Heteroassociation- and Folding-Dependent Bridging 2008 Ingår i: Nano letters (Print). - : ACS Publications. - 1530-6984 .- 1530-6992. ; 8:8, s. 2473-2478 Tidskriftsartikel (refereegranskat)abstract Gold nanoparticles were functionalized with a synthetic polypeptide, de novo-designed to associate with a charge complementary linker polypeptide in a folding-dependent manner. A heterotrimeric complex that folds into two disulphide-linked four-helix bundles is formed when the linker polypeptide associates with two of the immobilized peptides. The heterotrimer forms in between separate particles and induces a rapid and extensive aggregation with a well-defined interparticle spacing. The aggregated particles are redispersed when the disulphide bridge in the linker polypeptide is reduced.
7.	Aili, Daniel, et al. (författare) Hybrid Nanoparticle-Liposome Detection of Phospholipase Activity 2011 Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 11:4, s. 1401-1405 Tidskriftsartikel (refereegranskat)abstract A flexible nanoparticle-based phospholipase (PL) assay is demonstrated in which the enzymatic substrate is decoupled from the nanoparticle surface. Liposomes are loaded with a polypeptide that is designed to heteroassociate with a second polypeptide immobilized on gold nanoparticies. Release of this polypeptide from the liposornes, triggered by PL, induces a folding-dependent nanoparticle bridging aggregation. The colorimetric response from this aggregation enables straightforward and continuous detection of PL in the picomolar range. The speed, specificity, and flexibility of this assay make it appropriate for a range of applications, from point of care diagnostics to high throughput pharmaceutical screening.
8.	Aili, Daniel, et al. (författare) Polypeptide Folding-Mediated Tuning of the Optical and Structural Properties of Gold Nanoparticle Assemblies 2011 Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 11:12, s. 5564-5573 Tidskriftsartikel (refereegranskat)abstract Responsive hybrid nanomaterials with well-defined properties are of significant interest for the development of biosensors with additional applications in tissue engineering and drug delivery. Here, we present a detailed characterization using UV-vis spectroscopy and small angle X-ray scattering of a hybrid material comprised of polypeptide-decorated gold nanoparticles with highly controllable assembly properties. The assembly is triggered by a folding-dependent bridging of the particles mediated by the heteroassociation of immobilized helix-loop-helix polypeptides and a complementary nonlinear polypeptide present in solution. The polypeptides are de novo designed to associate and fold into a heterotrimeric complex comprised of two disulfide-linked four-helix bundles. The particles form structured assemblies with a highly defined interparticle gap (4.8 +/- 0.4 nm) that correlates to the size of the folded polypeptides. Transitions in particle aggregation dynamics, mass-fractal dimensions and ordering, as a function of particle size and the concentration of the bridging polypeptide, are observed; these have significant effects on the optical properties of the assemblies. The assembly and ordering of the particles are highly complex processes that are affected by a large number of variables including the number of polypeptides bridging the particles and the particle mobility within the aggregates. A fundamental understanding of these processes is of paramount interest for the development of novel hybrid nanomaterials with tunable structural and optical properties and for the optimization of nanoparticle-based colorimetric biodetection strategies.
9.	Amini, Shahram, et al. (författare) On the Stability of Mg Nanograins to Coarsening after Repeated Melting 2009 Ingår i: NANO LETTERS. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 9:8, s. 3082-3086 Tidskriftsartikel (refereegranskat)abstract Herein we report on the extraordinary thermal stability of similar to 35 nm Mg-nanograins that constitute the matrix of a Ti2AlC-Mg composite that has previously been shown to have excellent mechanical properties. The microstructure is so stable that heating the composite three times to 700 degrees C, which is 50 degrees C over the melting point of Mg, not only resulted in the repeated melting of the Mg, but surprisingly and within the resolution of our differential scanning calorimeter, did not lead to any coarsening. The reduction in the Mg melting point due to the nanograins was similar to 50 degrees C. X-ray diffraction and neutron spectroscopy results suggest that thin, amorphous, and/or poorly crystallized rutile, anatase, and/or magnesia layers separate the Mg nanograins and prevent them from coarsening. Clearly that layer is thin enough, and thus mechanically robust enough, to survive the melting and solidification stresses encountered during cycling. Annealing in hydrogen at 250 degrees C for 20 h, also did not seem to alter the grain size significantly.
10.	Andersson, Martin, 1974, et al. (författare) Macroscopic Alignment of Silver Nanoparticles in Liquid Crystalline Templates 2002 Ingår i: Nano Letters. - 1530-6992 .- 1530-6984. ; 2, s. 1403-1407 Tidskriftsartikel (refereegranskat)
11.	Andersson, M, et al. (författare) Macroscopic alignment of silver nanoparticles in reverse hexagonal liquid crystalline templates 2002 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 2:12, s. 1403-1407 Tidskriftsartikel (refereegranskat)abstract A flexible method of preparing and macroscopically aligning nanoparticles of crystalline silver into millimeter long fibers is presented. The approach utilizes the dual functionality of a reverse hexagonal liquid crystalline template containing a built-in reducing agent facing the aqueous domain. The method is advantageous in that its slow kinetics allows for a thorough introduction of a silver salt into the liquid crystal before the reduction takes place, allowing for an efficient loading of he template and a retained mesoscopic ordering as evidenced by SAXS. It was confirmed by H-1 NMR that the oxyethylene groups of the amphiphilic polymer reduce the silver ions while being oxidized to aldehydes. The silver nanoparticles are uniform in size and in the same size range as the diameter of the aqueous domain of the liquid crystal (3 nm), further supporting that the silver particles form inside the liquid crystal. TEM images confirm the macroscopic alignment of silver nanoparticles into fibrils and the packing of fibrils into millimeter long fibers. The diameter of the fibrils and fibers ranges from 30 nm to several hundreds of micrometers. Electron diffraction analysis of a collection of silver nanoparticles confirms their crystallinity as three diffraction rings could be indexed to the face centered cubic structure of silver. A key to the successful macroscopic alignment of the nanoparticles is that the particles are formed inside the liquid crystal, thus minimizing the need for their diffusion i to and inside the liquid crystal.
12.	Andersson, Viktor, et al. (författare) Imaging of the 3D Nanostructure of a Polymer Solar Cell by Electron Tomography 2009 Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 9:2, s. 853-855 Tidskriftsartikel (refereegranskat)abstract Electron tomography has been used for analyzing the active layer in a polymer solar cell, a bulk heterojunction of an alternating copolymer of fluorene and a derivative of fullerene. The method supplies a three-dimensional representation of the morphology of the film, where domains with different scattering properties may be distinguished. The reconstruction shows good contrast between the two phases included in the film and demonstrates that electron tomography is an adequate tool for investigations of the three-dimensional nanostructure of the amorphous materials used in polymer solar cells.
13.	Anttu, Nicklas, et al. (författare) Crystal Phase-Dependent Nanophotonic Resonances in InAs Nanowire Arrays 2014 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 14:10, s. 5650-5655 Tidskriftsartikel (refereegranskat)abstract Nanostructures have many material, electronic, and optical properties that are not found in bulk systems and that are relevant for technological applications. For example, nanowires realized from III-V semiconductors can be grown into wurtzite crystal structure. This crystal structure does not naturally exist in bulk where these materials form the zinc-blende counterpart. Being able to concomitantly grow these nanowires in the zinc-blende and/or wurtzite crystal structure prlovides an important degree of control for the design and optimization of optoelectronic applications based on these semiconductor nanostructures. However, the refractive indices of this new crystallographic phase have so far not been elucidated. This shortcoming makes it impossible to predict and utilize he full potential of these new nanostructured materials for optoelectronics applications a careful design and optimization of optical resonances by tuning the nanostrucuted geometry is needed to achieve optimal performance. Here, we report and analyze striking differeences in the optical response of nanophotonic resonances in wurtzite and zinc-blend InAs nanowire arrays. Specifically, through reflectance measurements we find that the resonance can be tuned down to lambda approximate to 380 nm in wurtzite nanowires by decreasing the nanowire diameter. In stark contrast, a similar tuning to below approximate to 500 nm is not possible in the zinc-blende nanowires. Furthermore, we find that the wurtzite nanowires can absorb twice as strongly as the zinc-blende nanowires. We attribute these strikingly large differences in resonant behavior to large differences between the refractive indices of the two crystallographic phases realized in these nanostructures. We anticipate our finding to be relevant for other III-B materials as well as for all material systems that manifest polytypism. Taken together, our results demonstrate crystal phase engineering as a potentially new design dimension for optoelectronics applications.
14.	Anttu, Nicklas, et al. (författare) Optical Far-Field Method with Subwavelength Accuracy for the Determination of Nanostructure Dimensions in Large-Area Samples. 2013 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 13:6, s. 2662-2667 Tidskriftsartikel (refereegranskat)abstract The physical, chemical, and biological properties of nanostructures depend strongly on their geometrical dimensions. Here we present a fast, noninvasive, simple-to-perform, purely optical method that is capable of characterizing nanostructure dimensions over large areas with an accuracy comparable to that of scanning electron microscopy. This far-field method is based on the analysis of unique fingerprints in experimentally measured reflectance spectra using full three-dimensional optical modeling. We demonstrate the strength of our method on large-area (millimeter-sized) arrays of vertical InP nanowires, for which we simultaneously determine the diameter and length as well as cross-sample morphological variations thereof. Explicitly, the diameter is determined with an accuracy better than 10 nm and the length with an accuracy better than 30 nm. The method is versatile and robust, and we believe that it will provide a powerful and standardized measurement technique for large-area nanostructure arrays suitable for both research and industrial applications.
15.	Aristov, Victor Yu., et al. (författare) Graphene Synthesis on Cubic SiC/Si Wafers. Perspectives for Mass Production of Graphene-Based Electronic Devices 2010 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 10:3, s. 992-995 Tidskriftsartikel (refereegranskat)abstract The outstanding properties of graphene, a single graphite layer, render it a top candidate for substituting silicon in future electronic devices, The so far exploited synthesis approaches, however, require conditions typically achieved in specialized laboratories and result in graphene sheets whose electronic properties are often altered by interactions with substrate materials. The development of graphene-based technologies requires an economical fabrication method compatible with mass production. Here we demonstrate for the fist Lime the feasibility of graphene synthesis on commercially available cubic SiC/Si substrates of >300 mm in diameter, which result in graphene flakes electronically decoupled from the substrate. After optimization of the preparation procedure, the proposed synthesis method can represent a further big step toward graphene-based electronic technologies.
16.	Askari, Sadegh, et al. (författare) Low-Loss and Tunable Localized Mid-Infrared Plasmons in Nanocrystals of Highly Degenerate InN 2018 Ingår i: Nano letters (Print). - : AMER CHEMICAL SOC. - 1530-6984 .- 1530-6992. ; 18:9, s. 5681-5687 Tidskriftsartikel (refereegranskat)abstract Plasmonic response of free charges confined in nanostructures of plasmonic materials is a powerful means for manipulating the light-material interaction at the nanoscale and hence has influence on various relevant technologies. In particular, plasmonic materials responsive in the mid-infrared range are technologically important as the mid-infrared is home to the vibrational resonance of molecules and also thermal radiation of hot objects. However, the development of the field is practically challenged with the lack of low-loss materials supporting high quality plasmons in this range of the spectrum. Here, we demonstrate that degenerately doped InN nanocrystals (NCs) support tunable and low-loss plasmon resonance spanning the entire midwave infrared range. Modulating free-carrier concentration is achieved by engineering nitrogen-vacancy defects (InN1-x, 0.017 amp;lt; x amp;lt; 0.085) in highly degenerate NCs using a nonequilibrium gas-phase growth process. Despite the significant reduction in the carrier mobility relative to intrinsic InN, the mobility in degenerate InN NCs (amp;gt;60 cm(2)/(V s)) remains considerably higher than the carrier mobility reported for other materials NCs such as doped metal oxides, chalcogenides, and noble metals. These findings demonstrate feasibility of controlled tuning of infrared plasmon resonances in a low-loss material of III-V compounds and open a gateway to further studies of these materials nanostructures for infrared plasmonic applications.
17.	Assali, S., et al. (författare) Crystal Phase Quantum Well Emission with Digital Control 2017 Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 17:10, s. 6062-6068 Tidskriftsartikel (refereegranskat)abstract One of the major challenges in the growth of quantum well and quantum dot heterostructures is the realization of atomically sharp interfaces. Nanowires provide a new opportunity to engineer the band structure as they facilitate the controlled switching of the crystal structure between the zinc-blende (ZB) and wurtzite (WZ) phases. Such a crystal phase switching results in the formation of crystal phase quantum wells (CPQWs) and quantum dots (CPQDs). For GaP CPQWs, the inherent electric fields due to the discontinuity of the spontaneous polarization at the WZ/ZB junctions lead to the confinement of both types of charge carriers at the opposite interfaces of the WZ/ZB/WZ structure. This confinement leads to a novel type of transition across a ZB flat plate barrier. Here, we show digital tuning of the visible emission of WZ/ZB/WZ CPQWs in a GaP nanowire by changing the thickness of the ZB barrier. The energy spacing between the sharp emission lines is uniform and is defined by the addition of single ZB monolayers. The controlled growth of identical quantum wells with atomically flat interfaces at predefined positions featuring digitally tunable discrete emission energies may provide a new route to further advance entangled photons in solid state quantum systems.
18.	Atalaya, Juan, 1981, et al. (författare) Continuum elastic modeling of graphene resonators 2008 Ingår i: Nano Letters. - 1530-6992 .- 1530-6984. ; 8:12, s. 4196-4200 Tidskriftsartikel (refereegranskat)abstract Starting from an atomistic approach, we have derived a hierarchy of successively more simplified continuum elasticity descriptions for modeling the mechanical properties of suspended graphene sheets. We find that already for deflections of the order of 0.5 Å a theory that correctly accounts for nonlinearities is necessary and that for many purposes a set of coupled Duffing-type equations may be used to accurately describe the dynamics of graphene membranes. The descriptions are validated by applying them to square graphene-based resonators with clamped edges and studying numerically their mechanical responses. Both static and dynamic responses are treated, and we find good agreement with recent experimental findings.
19.	Balocco, C, et al. (författare) Microwave detection at 110 GHz by nanowires with broken symmetry 2005 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 5:7, s. 1423-1427 Tidskriftsartikel (refereegranskat)abstract By using arrays of nanowires with intentionally broken symmetry, we were able to detect microwaves up to 110 GHz at room temperature. This is, to the best of our knowledge, the highest speed that has been demonstrated in different types of novel electronic nanostructures to date. Our experiments showed a rather stable detection sensitivity over a broad frequency range from 100 MHz to 110 GHz. The novel working principle enabled the nanowires to detect microwaves efficiently without a dc bias. In principle, the need for only one high-resolution lithography step and the planar architecture allow an arbitrary number of nanowires to be made by folding a linear array as many times as required over a large area, for example, a whole wafer. Our experiment on 18 parallel nanowires showed a sensitivity of approximately 75 mV dc output/mW of nominal input power of the 110 GHz signal, even though only about 0.4% of the rf power was effectively applied to the structure because of an impedance mismatch. Because this array of nanowires operates simultaneously, low detection noise was achieved, allowing us to detect -25 dBm 110 GHz microwaves at zero bias with a standard setup.
20.	Bano, Fouzia, et al. (författare) Toward multiprotein nanoarrays using nanografting and DNA directed immobilization of proteins 2009 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 9:7, s. 2614-2618 Tidskriftsartikel (refereegranskat)abstract Atomic force microscopy nanografting was utilized to prepare DNA nanopatches of different sizes (200 × 200 to 1000 × 1000 nm2) onto which DNA−protein conjugates can be anchored through DNA-directed immobilization. Height measurements were used to assess the binding of the proteins as well as their subsequent interaction with other components, such as antibodies. The results indicate that nanografted patch arrays are well suited for application in biosensing and could enable the fabrication of multifeature protein nanoarrays.
21.	Bao, Jiming, et al. (författare) Optical properties of rotationally twinned InP nanowire heterostructures 2008 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 8:3, s. 836-841 Tidskriftsartikel (refereegranskat)abstract We have developed a technique so that both transmission electron microscopy and microphotoluminescence can be performed on the same semiconductor nanowire over a large range of optical power, thus allowing us to directly correlate structural and optical properties of rotationally twinned zinc blende InP nanowires. We have constructed the energy band diagram of the resulting multiquantum well heterostructure and have performed detailed quantum mechanical calculations of the electron and hole wave functions. The excitation power dependent blue-shift of the photoluminescence can be explained in terms of the predicted staggered band alignment of the rotationally twinned zinc blende/wurzite InP heterostructure and of the concomitant diagonal transitions between localized electron and hole states responsible for radiative recombination. The ability of rotational twinning to introduce a heterostructure in a chemically homogeneous nanowire material and alter in a major way its optical properties opens new possibilities for band-structure engineering.
22.	Barauskas, Justas, et al. (författare) Self-assembled lipid superstructures: Beyond vesicles and liposomes 2005 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 5:8, s. 1615-1619 Tidskriftsartikel (refereegranskat)abstract A unique set of nanoparlicle dispersions of self-assembled lipid mesophases with distinctive reversed cubic, hexagonal, and sponge phase structures has been prepared by use of original lipid combinations and a simple, generally applicable and scalable method. All key properties, particle size distributions, shape, phase structure, and stability, are controlled predictably and reproducibly. The results suggest the cross-disciplinary use of nonlamellar particle structures in science and technology as, for instance, biomimetics, in vivo drug delivery vehicles for diagnostic and therapeutic agents, protein crystallization matrices, and soft nanoporous materials.
23.	Barrigón, E., et al. (författare) GaAs Nanowire pn-Junctions Produced by Low-Cost and High-Throughput Aerotaxy 2018 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 18:2, s. 1088-1092 Tidskriftsartikel (refereegranskat)abstract Semiconductor nanowires could significantly boost the functionality and performance of future electronics, light-emitting diodes, and solar cells. However, realizing this potential requires growth methods that enable high-throughput and low-cost production of nanowires with controlled doping. Aerotaxy is an aerosol-based method with extremely high growth rate that does not require a growth substrate, allowing mass-production of high-quality nanowires at a low cost. So far, pn-junctions, a crucial element of solar cells and light-emitting diodes, have not been realized by Aerotaxy growth. Here we report a further development of the Aerotaxy method and demonstrate the growth of GaAs nanowire pn-junctions. Our Aerotaxy system uses an aerosol generator for producing the catalytic seed particles, together with a growth reactor with multiple consecutive chambers for growth of material with different dopants. We show that the produced nanowire pn-junctions have excellent diode characteristics with a rectification ratio of >105, an ideality factor around 2, and very promising photoresponse. Using electron beam induced current and hyperspectral cathodoluminescence, we determined the location of the pn-junction and show that the grown nanowires have high doping levels, as well as electrical properties and diffusion lengths comparable to nanowires grown using metal organic vapor phase epitaxy. Our findings demonstrate that high-quality GaAs nanowire pn-junctions can be produced using a low-cost technique suitable for mass-production, paving the way for industrial-scale production of nanowire-based solar cells.
24.	Barzegar, Hamid Reza, 1977-, et al. (författare) C60/Collapsed Carbon Nanotube Hybrids : A Variant of Peapods 2015 Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 15:2, s. 829-834 Tidskriftsartikel (refereegranskat)abstract We examine a variant of so-called carbon nanotube peapods by packing C60 molecules inside the open edge ducts of collapsed carbon nanotubes. C60 insertion is accomplished through a facile single-step solution-based process. Theoretical modeling is used to evaluate favorable low-energy structural configurations. Overfilling of the collapsed tubes allows infiltration of C60 over the full cross-section of the tubes and consequent partial or complete reinflation, yielding few-wall, large diameter cylindrical nanotubes packed with crystalline C60 solid cores.
25.	Barzegar, Hamid Reza, et al. (författare) Electrostatically Driven Nanoballoon Actuator 2016 Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 16:11, s. 6787-6791 Tidskriftsartikel (refereegranskat)abstract We demonstrate an inflatable nanoballoon actuator based on geometrical transitions between the inflated (cylindrical) and collapsed (flattened) forms of a carbon nanotube. In situ transmission electron microscopy experiments employing a nanoelectromechanical manipulator show that a collapsed carbon nanotube can be reinflated by electrically charging the nanotube, thus realizing an electrostatically driven nanoballoon actuator. We find that the tube actuator can be reliably cycled with only modest control voltages (few volts) with no apparent wear or fatigue. A complementary theoretical analysis identifies critical parameters for nanotube nanoballoon actuation.
26.	Barzegar, Hamid Reza, et al. (författare) Self-assembled PCBM nanosheets : a facile route to electronic layer-on-Layer heterostructures 2018 Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 18:2, s. 1442-1447 Tidskriftsartikel (refereegranskat)abstract We report on the self-assembly of semicrystalline [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) nanosheets at the interface between a hydrophobic solvent and water, and utilize this opportunity for the realization of electronically active organic/organic molecular heterostructures. The self-assembled PCBM nanosheets can feature a lateral size of >1 cm2 and be transferred from the water surface to both hydrophobic and hydrophilic surfaces using facile transfer techniques. We employ a transferred single PCBM nanosheet as the active material in a field-effect transistor (FET) and verify semiconductor function by a measured electron mobility of 1.2 × 10–2 cm2 V–1 s–1 and an on–off ratio of ∼1 × 104. We further fabricate a planar organic/organic heterostructure with the p-type organic semiconductor poly(3-hexylthiophene-2,5-diyl) as the bottom layer and the n-type PCBM nanosheet as the top layer and demonstrate ambipolar FET operation with an electron mobility of 8.7 × 10–4 cm2 V–1 s–1 and a hole mobility of 3.1 × 10–4 cm2V–1 s–1.
27.	Bavinck, Maaike Bouwes, et al. (författare) Photon Cascade from a Single Crystal Phase Nanowire Quantum Dot 2016 Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 16:2, s. 1081-1085 Tidskriftsartikel (refereegranskat)abstract We report the first comprehensive experimental and theoretical study of the optical properties of single crystal phase quantum dots in InP nanowires. Crystal phase quantum dots are defined by a transition in the crystallographic lattice between zinc blende and wurtzite segments and therefore offer unprecedented potential to be controlled with atomic layer accuracy without random alloying. We show for the first time that crystal phase quantum dots are a source of pure single-photons and cascaded photon-pairs from type II transitions with excellent optical properties in terms of intensity and line width. We notice that the emission spectra consist often of two peaks close in energy, which we explain with a comprehensive theory showing that the symmetry of the system plays a crucial role for the hole levels forming hybridized orbitals. Our results state that crystal phase quantum dots have promising quantum optical properties for single photon application and quantum optics.
28.	Benter, S., et al. (författare) Quasi One-Dimensional Metal-Semiconductor Heterostructures 2019 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 19:6, s. 3892-3897 Tidskriftsartikel (refereegranskat)abstract The band offsets occurring at the abrupt heterointerfaces of suitable material combinations offer a powerful design tool for high performance or even new kinds of devices. Because of a large variety of applications for metal-semiconductor heterostructures and the promise of low-dimensional systems to present exceptional device characteristics, nanowire heterostructures gained particular interest over the past decade. However, compared to those achieved by mature two-dimensional processing techniques, quasi one-dimensional (1D) heterostructures often suffer from low interface and crystalline quality. For the GaAs-Au system, we demonstrate exemplarily a new approach to generate epitaxial and single crystalline metal-semiconductor nanowire heterostructures with atomically sharp interfaces using standard semiconductor processing techniques. Spatially resolved Raman measurements exclude any significant strain at the lattice mismatched metal-semiconductor heterojunction. On the basis of experimental results and simulation work, a novel self-assembled mechanism is demonstrated which yields one-step reconfiguration of a semiconductor-metal core-shell nanowire to a quasi 1D axially stacked heterostructure via flash lamp annealing. Transmission electron microscopy imaging and electrical characterization confirm the high interface quality resulting in the lowest Schottky barrier for the GaAs-Au system reported to date. Without limiting the generality, this novel approach will open up new opportunities in the syntheses of other metal-semiconductor nanowire heterostructures and thus facilitate the research of high-quality interfaces in metal-semiconductor nanocontacts.
29.	Berg, Alexander, et al. (författare) Radial Nanowire Light-Emitting Diodes in the (AlxGa1-x)yIn1-yP Material System 2016 Ingår i: Nano letters (Print). - Washington, DC : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 16:1, s. 656-662 Tidskriftsartikel (refereegranskat)abstract Nanowires have the potential to play an important role for next-generation light-emitting diodes. In this work, we present a growth scheme for radial nanowire quantum-well structures in the AlGaInP material system using a GaInP nanowire core as a template for radial growth with GaInP as the active layer for emission and AlGaInP as charge carrier barriers. The different layers were analyzed by X-ray diffraction to ensure lattice-matched radial structures. Furthermore, we evaluated the material composition and heterojunction interface sharpness by scanning transmission electron microscopy energy dispersive X-ray spectroscopy. The electro-optical properties were investigated by injection luminescence measurements. The presented results can be a valuable track toward radial nanowire light-emitting diodes in the AlGaInP material system in the red/orange/yellow color spectrum. © 2015 American Chemical Society.
30.	Berseth, P. A., et al. (författare) Carbon Nanomaterials as Catalysts for Hydrogen Uptake and Release in NaAlH4 2009 Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 9:4, s. 1501-1505 Tidskriftsartikel (refereegranskat)abstract A synergistic approach involving experiment and first-principles theory not only shows that carbon nanostructures can be used as catalysts for hydrogen uptake and release in complex metal hydrides such as sodium alanate, NaAlH4, but also provides an unambiguous understanding of how the catalysts work. Here we show that the stability of NaAlH4 originates with the charge transfer from Na to the AlH4 moiety, resulting in an ionic bond between Na+ and AlH4- and a covalent bond between Al and H. Interaction of NaAlH4 with an electronegative substrate such as carbon fullerene or nanotube affects the ability of Na to donate its charge to AlH4, consequently weakening the Al-H bond and causing hydrogen to desorb at lower temperatures as well as facilitating the absorption of H-2 to reverse the dehydrogenation reaction. In addition, based on our experimental observations and theoretical calculations it appears the curvature of the carbon nanostructure plays a role in the catalytic process. Ab initio molecular dynamics simulation further reveals the time evolution of the charge transfer process.
31.	Bi, Zhaoxia, et al. (författare) InGaN Platelets : Synthesis and Applications toward Green and Red Light-Emitting Diodes 2019 Ingår i: Nano Letters. - : American Chemical Society. - 1530-6984 .- 1530-6992. ; 19:5, s. 2832-2839 Tidskriftsartikel (refereegranskat)abstract In this work, we present a method to synthesize arrays of hexagonal InGaN submicrometer platelets with a top c-plane area having an extension of a few hundred nanometers by selective area metal-organic vapor-phase epitaxy. The InGaN platelets were made by in situ annealing of InGaN pyramids, whereby InGaN from the pyramid apex was thermally etched away, leaving a c-plane surface, while the inclined {101Ì1} planes of the pyramids were intact. The as-formed c-planes, which are rough with islands of a few tens of nanometers, can be flattened with InGaN regrowth, showing single bilayer steps and high-quality optical properties (full width at half-maximum of photoluminescence at room temperature: 107 meV for In 0.09 Ga 0.91 N and 151 meV for In 0.18 Ga 0.82 N). Such platelets offer surfaces having relaxed lattice constants, thus enabling shifting the quantum well emission from blue (as when grown on GaN) to green and red. For single InGaN quantum wells grown on the c-plane of such InGaN platelets, a sharp interface between the quantum well and the barriers was observed. The emission energy from the quantum well, grown under the same conditions, was shifted from 2.17 eV on In 0.09 Ga 0.91 N platelets to 1.95 eV on In 0.18 Ga 0.82 N platelets as a result of a thicker quantum well and a reduced indium pulling effect on In 0.18 Ga 0.82 N platelets. On the basis of this method, prototype light-emitting diodes were demonstrated with green emission on In 0.09 Ga 0.91 N platelets and red emission on In 0.18 Ga 0.82 N platelets.
32.	Bian, F., et al. (författare) Ultrasmall Silver Nanopores Fabricated by Femtosecond Laser Pulses 2011 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 11:8, s. 3251-3257 Tidskriftsartikel (refereegranskat)abstract Ultrasmall nanopores in silver thin films with a diameter of about 2 nm have been fabricated using femtosecond laser ablation in liquid. Ultrafast laser pulse ablation generates highly nonequilibrium excitated states, from which silver thin films emerge and progressively grow with the assistance of capping agent molecules. During this growth process, capping agent molecules are enclaved within the film, leaving individual ultrasmall pores in the thin film. Our first-principles calculations show that the pore size is critically determined by the dimension of the confined molecules. Our approach advances the capability of optical methods in making nanoscale structures with potential applications in areas such as near-field aperture probes, imaging masks, magnetic plasmonic resonances, and biosensing with individual nanopores.
33.	Bisht, Ankit, 1988, et al. (författare) Collective Strong Light-Matter Coupling in Hierarchical Microcavity-Plasmon-Exciton Systems 2019 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 19:1, s. 189-196 Tidskriftsartikel (refereegranskat)abstract Polaritons are compositional light-matter quasiparticles that arise as a result of strong coupling between the vacuum field of a resonant optical cavity and electronic excitations in quantum emitters. Reaching such a regime is often hard, as it requires materials possessing high oscillator strengths to interact with the relevant optical mode. Two-dimensional transition metal dichalcogenides (TMDCs) have recently emerged as promising candidates for realization of strong coupling regime at room temperature. However, these materials typically provide coupling strengths in the range of 10−40 meV, which may be insufficient for reaching strong coupling with low quality factor resonators. Here, we demonstrate a universal scheme that allows a straightforward realization of strong coupling with 2D materials and beyond. By intermixing plasmonic excitations in nanoparticle arrays with excitons in a WS2 monolayer inside a resonant metallic microcavity, we fabricate a hierarchical system with the collective microcavity−plasmon−exciton Rabi splitting exceeding ∼500 meV at room temperature. Photoluminescence measurements of the coupled systems show dominant emission from the lower polariton branch, indicating the participation of excitons in the coupling process. Strong coupling has been recently suggested to affect numerous optical- and material-related properties including chemical reactivity, exciton transport, and optical nonlinearities. With the universal scheme presented here, strong coupling across a wide spectral range is within easy reach and therefore exploration of these exciting phenomena can be further pursued in a much broader class of materials.
34.	Biswas, Deepnarayan, et al. (författare) Ultrafast Triggering of Insulator-Metal Transition in Two-Dimensional VSe2 2021 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 21:5, s. 1968-1975 Tidskriftsartikel (refereegranskat)abstract The transition-metal dichalcogenide VSe2 exhibits an increased charge density wave transition temperature and an emerging insulating phase when thinned to a single layer. Here, we investigate the interplay of electronic and lattice degrees of freedom that underpin these phases in single-layer VSe2 using ultrafast pump-probe photoemission spectroscopy. In the insulating state, we observe a light-induced closure of the energy gap, which we disentangle from the ensuing hot carrier dynamics by fitting a model spectral function to the time-dependent photoemission intensity. This procedure leads to an estimated time scale of 480 fs for the closure of the gap, which suggests that the phase transition in single-layer VSe2 is driven by electron-lattice interactions rather than by Mott-like electronic effects. The ultrafast optical switching of these interactions in SL VSe2 demonstrates the potential for controlling phase transitions in 2D materials with light.
35.	Björk, Mikael, et al. (författare) Few-electron quantum dots in nanowires 2004 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 4:9, s. 1621-1625 Tidskriftsartikel (refereegranskat)abstract We demonstrate transport spectroscopy on bottom-up grown few-electron quantum dots in semiconductor nanowires. The dots are defined by InP double barrier heterostructures in InAs nanowires catalytically grown from nanoparticles. By changing the dot size, we can design devices ranging from single-electron transistors to few-electron quantum dots. In the latter case, electrons can be added one by one to the dots from 0 to similar to50 electrons while maintaining an almost constant charging energy, with addition spectra of the devices displaying shell structures as a result of spin and orbital degeneracies. The reduced dimensionality of the nanowire emitter gives rise to pronounced resonant tunneling peaks, where a gate can be used to control the peak positions.
36.	Björk, Mikael, et al. (författare) One-dimensional steeplechase for electrons realized 2002 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 2:2, s. 87-89 Tidskriftsartikel (refereegranskat)abstract We report growth of one-dimensional semiconductor nanocrystals, nanowhiskers, in which segments of the whisker with different composition are formed, illustrated by InAs whiskers containing segments of InP. Our conditions for growth allow the formation of abrupt interfaces and heterostructure barriers of thickness from a few monolayers to 100s of nanometers, thus creating a one-dimensional landscape along which the electrons move. The crystalline perfection, the quality of the interfaces, and the variation in the lattice constant are demonstrated by high-resolution transmission electron microscopy, and the conduction band off-set of 0.6 eV is deduced from the current due to thermal excitation of electrons over an InP barrier.
37.	Björk, Per, et al. (författare) Single molecular imaging and spectroscopy of conjugated polyelectrolytes decorated on stretched aligned DNA 2005 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 5:10, s. 1948-1953 Tidskriftsartikel (refereegranskat)abstract DNA is the prototype template for building nanoelectronic devices by self-assembly. The electronic functions are made possible by coordinating electronic polymer chains to DNA. This paper demonstrates two methods for fabrication of aligned and ordered DNA nanowires complexed with conjugated polyelectrolytes (CPEs). The complex can be formed either in solution prior to stretching or after stretching of the bare DNA on a surface. Molecular combing was used to stretch the complexes on surface energy patterned surfaces, and PMMA for the bare DNA. Single molecular spectroscopy, in fluorescence, and microscopy, in atomic force microscopy, give evidence for coordination of the short CPE chains to the aligned DNA.
38.	Block, Stephan, 1978, et al. (författare) Quantification of Multivalent Interactions by Tracking Single Biological Nanoparticle Mobility on a Lipid Membrane 2016 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 16:7, s. 4382-4390 Tidskriftsartikel (refereegranskat)abstract Macromolecular association commonly occurs via dynamic engagement of multiple weak bonds referred to as multivalent interactions. The distribution of the number of bonds, combined with their strong influence on the residence time, makes it very demanding to quantify this type of interaction. To address this challenge in the context of virology, we mimicked the virion association to a cell membrane by attaching lipid vesicles (100 nm diameter) to a supported lipid bilayer via multiple, identical cholesterol based DNA linker molecules, each mimicking an individual virion receptor link. Using total internal reflection microscopy to track single attached vesicles combined with a novel filtering approach, we show that histograms of the vesicle diffusion coefficient D exhibit a spectrum of distinct peaks, which are associated with vesicles differing in the number, n, of linking DNA tethers. These peaks are only observed if vesicles with transient changes in n are excluded from the analysis. D is found to be proportional to 1/n, in excellent agreement with the free draining model, allowing to quantify transient changes of n on the single vesicle level and to extract transition rates between individual linking states. Necessary imaging conditions to extend the analysis to multivalent interactions in general are also reported.
39.	Bogan, Michael J, et al. (författare) Single particle X-ray diffractive imaging 2008 Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 8:1, s. 310-6 Tidskriftsartikel (refereegranskat)abstract In nanotechnology, strategies for the creation and manipulation of nanoparticles in the gas phase are critically important for surface modification and substrate-free characterization. Recent coherent diffractive imaging with intense femtosecond X-ray pulses has verified the capability of single-shot imaging of nanoscale objects at suboptical resolutions beyond the radiation-induced damage threshold. By intercepting electrospray-generated particles with a single 15 femtosecond soft-X-ray pulse, we demonstrate diffractive imaging of a nanoscale specimen in free flight for the first time, an important step toward imaging uncrystallized biomolecules.
40.	Bonanni, Valentina, et al. (författare) Designer Magnetoplasmonics with Nickel Nanoferromagnets 2011 Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 11:12, s. 5333-5338 Tidskriftsartikel (refereegranskat)abstract We introduce a new perspective on magnetoplasmonics in nickel nanoferromagnets by exploiting the phase tunability of the optical polarizability due to localized surface plasmons and simultaneous magneto-optical activity. We demonstrate how the concerted action of nanoplasmonics and magnetization can manipulate the sign of rotation of the reflected light's polarization (i.e., to produce Kerr rotation reversal) in ferromagnetic nanomaterials and, further, how this effect can be dynamically controlled and employed to devise conceptually new schemes for biochemosensing.
41.	Borg, Mattias, et al. (författare) InAs/GaSb Heterostructure Nanowires for Tunnel Field-Effect Transistors. 2010 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 10:Online August 24, 2010, s. 4080-4085 Tidskriftsartikel (refereegranskat)abstract InAs/GaSb nanowire heterostructures with thin GaInAs inserts were grown by MOVPE and characterized by electrical measurements and transmission electron microscopy. Down-scaling of the insert thickness was limited because of an observed sensitivity of GaSb nanowire growth to the presence of In. By employing growth interrupts in between the InAs and GaInAs growth steps it was possible to reach an insert thickness down to 25 nm. Two-terminal devices show a diode behavior, where temperature-dependent measurements indicate a heterostructure barrier height of 0.5 eV, which is identified as the valence band offset between the InAs and GaSb. Three-terminal transistor structures with a top-gate positioned at the heterointerface show clear indications of band-to-band tunnelling.
42.	Borgani, Riccardo, et al. (författare) Local Charge Injection and Extraction on Surface-Modified Al2O3Nanoparticles in LDPE 2016 Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 16:9, s. 5934-5937 Tidskriftsartikel (refereegranskat)abstract We use a recently developed scanning probe technique to image with high spatial resolution the injection and extraction of charge around individual surface-modified aluminum oxide nanoparticles embedded in a low-density polyethylene (LDPE) matrix. We find that the experimental results are consistent with a simple band structure model where localized electronic states are available in the band gap (trap states) in the vicinity of the nanoparticles. This work offers experimental support to a previously proposed mechanism for enhanced insulating properties of nanocomposite LDPE and provides a powerful experimental tool to further investigate such properties.
43.	Borglin, Johan, 1986, et al. (författare) Peptide Functionalized Gold Nanoparticles as a Stimuli Responsive Contrast Medium in Multiphoton Microscopy 2017 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 17:3, s. 2102-2108 Tidskriftsartikel (refereegranskat)abstract There is a need for biochemical contrast mediators with high signal-to-noise ratios enabling noninvasive biomedical sensing, for example, for neural sensing and protein protein interactions, in addition to cancer diagnostics. The translational challenge is to develop a biocompatible approach ensuring high biochemical contrast while avoiding a raise of the background signal. We here present a concept where gold nanoparticles (AuNPs) can be utilized as a stimuli responsive contrast medium by chemically triggering their ability to exhibit multiphoton-induced luminescence (MIL) when performing multiphoton laser scanning microscopy (MPM). Proof-of-principle is demonstrated using peptide-functionalized AuNPs sensitive to zinc ions (Zn2+). Dispersed particles are invisible in the MPM until addition of millimolar concentrations of Zn2+ upon which MIL is enabled through particle aggregation caused by specific peptide interactions and folding. The process can be reversed by removal of the Zn2+ using a chelator, thereby resuspending the AuNPs. In addition, the concept was demonstrated by exposing the particles to matrix metalloproteinase-7 (MMP-7) causing peptide digestion resulting in AuNP aggregation, significantly elevating the MIL signal from the background. The approach is based on the principle that aggregation shifts the plasmon resonance, elevating the absorption cross section in the near-infrared wavelength region enabling onset of MIL. This Letter demonstrates how biochemical sensing can be obtained in far-field MPM and should be further exploited as a future tool for noninvasive optical biosensing.
44.	Borschel, Christian, et al. (författare) A New Route toward Semiconductor Nanospintronics : Highly Mn-Doped GaAs Nanowires Realized by Ion-Implantation under Dynamic Annealing Conditions 2011 Ingår i: Nano letters (Print). - Washington : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 11:9, s. 3935-3940 Tidskriftsartikel (refereegranskat)abstract We report on highly Mn-doped GaAs nanowires (NWs) of high crystalline quality fabricated by ion beam implantation, a technique that allows doping concentrations beyond the equilibrium solubility limit. We studied two approaches for the preparation of Mn-doped GaAs NWs: First, ion implantation at room temperature with subsequent annealing resulted in polycrystalline NWs and phase segregation of MnAs and GaAs. The second approach was ion implantation at elevated temperatures. In this case, the single-crystallinity of the GaAs NWs was maintained, and crystalline, highly Mn-doped GaAs NWs were obtained. The electrical resistance of such NWs dropped with increasing temperature (activation energy about 70 meV). Corresponding magnetoresistance measurements showed a decrease at low temperatures, indicating paramagnetism. Our findings suggest possibilities for future applications where dense arrays of GaMnAs nanowires may be used as a new kind of magnetic material system.
45.	Boström, Emil Viñas, et al. (författare) Charge Separation in Donor-C60 Complexes with Real-Time Green Functions : The Importance of Nonlocal Correlations 2018 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 18:2, s. 785-792 Tidskriftsartikel (refereegranskat)abstract We use the nonequilibrium Green function (NEGF) method to perform real-time simulations of the ultrafast electron dynamics of photoexcited donor-C60 complexes modeled by a Pariser-Parr-Pople Hamiltonian. The NEGF results are compared to mean-field Hartree-Fock (HF) calculations to disentangle the role of correlations. Initial benchmarking against numerically highly accurate time-dependent density matrix renormalization group calculations verifies the accuracy of NEGF. We then find that charge-transfer (CT) excitons partially decay into charge separated (CS) states if dynamical nonlocal correlation corrections are included. This CS process occurs in ∼10 fs after photoexcitation. In contrast, the probability of exciton recombination is almost 100% in HF simulations. These results are largely unaffected by nuclear vibrations; the latter become however essential whenever level misalignment hinders the CT process. The robust nature of our findings indicates that ultrafast CS driven by correlation-induced decoherence may occur in many organic nanoscale systems, but it will only be correctly predicted by theoretical treatments that include time-nonlocal correlations.
46.	Boxberg, Fredrik, et al. (författare) Photovoltaics with Piezoelectric Core-Shell Nanowires 2010 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 10:4, s. 1108-1112 Tidskriftsartikel (refereegranskat)abstract We report on a theoretical discovery of a generic piezoelectric held in strained core shell compound semiconductor nanowires. We show, using both an analytical model and numerical simulations based on fully electroelastically coupled continuum elasticity theory, that lattice-mismatch-induced strain in an epiraxial core shell nanowire gives rise to an internal electric held along the axis of the nanowire. This piezoelectric field results predominantly from atomic layer displacements along the nanowire axis within both the core and shell materials and can appear in both zinc blende and wurtzite crystalline core-shell nanowires. The effect can be employed to separate photon-generated electron hole pairs in the core shell nanowires and thus offers a new device concept for solar energy conversion.
47.	Brem, Samuel, 1991, et al. (författare) Phonon-Assisted Photoluminescence from Indirect Excitons in Monolayers of Transition-Metal Dichalcogenides 2020 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 20:4, s. 2849-2856 Tidskriftsartikel (refereegranskat)abstract The photoluminescence (PL) spectrum of transition-metal dichalcogenides (TMDs) shows a multitude of emission peaks below the bright exciton line, and not all of them have been explained yet. Here, we study the emission traces of phonon-assisted recombinations of indirect excitons. To this end, we develop a microscopic theory describing simultaneous exciton, phonon, and photon interaction and including consistent many-particle dephasing. We explain the drastically different PL below the bright exciton in tungsten- and molybdenum-based materials as the result of different configurations of bright and momentum-dark states. In good agreement with experiments, our calculations predict that WSe2 exhibits clearly visible low-temperature PL signals stemming from the phonon-assisted recombination of momentum-dark K-K′ excitons.
48.	Brem, Samuel, 1991, et al. (författare) Terahertz Fingerprint of Monolayer Wigner Crystals 2022 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 22:3, s. 1311-1315 Tidskriftsartikel (refereegranskat)abstract The strong Coulomb interaction in monolayer semiconductors represents a unique opportunity for the realization of Wigner crystals without external magnetic fields. In this work, we predict that the formation of monolayer Wigner crystals can be detected by their terahertz response spectrum, which exhibits a characteristic sequence of internal optical transitions. We apply the density matrix formalism to derive the internal quantum structure and the optical conductivity of the Wigner crystal and to microscopically analyze the multipeak shape of the obtained terahertz spectrum. Moreover, we predict a characteristic shift of the peak position as a function of charge density for different atomically thin materials and show how our results can be generalized to an arbitrary two-dimensional system.
49.	Brem, Samuel, 1991, et al. (författare) Tunable Phases of Moiré Excitons in van der Waals Heterostructures 2020 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 20:12, s. 8534-8540 Tidskriftsartikel (refereegranskat)abstract Stacking monolayers of transition metal dichalcogenides into a heterostructure with a finite twist-angle gives rise to artificial moiré superlattices with a tunable periodicity. As a consequence, excitons experience a periodic potential, which can be exploited to tailor optoelectronic properties of these materials. Whereas recent experimental studies have confirmed twist-angle-dependent optical spectra, the microscopic origin of moiré exciton resonances has not been fully clarified yet. Here, we combine first-principles calculations with the excitonic density matrix formalism to study transitions between different moiré exciton phases and their impact on optical properties of the twisted MoSe2/WSe2 heterostructure. At angles smaller than 2°, we find flat, moiré-trapped states for inter- and intralayer excitons. This moiré exciton phase changes into completely delocalized states at 3°. We predict a linear and quadratic twist-angle dependence of excitonic resonances for the moiré-trapped and delocalized exciton phases, respectively.
50.	Brown, Keith A., et al. (författare) Machine Learning in Nanoscience : Big Data at Small Scales 2019 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 20:1, s. 2-10 Forskningsöversikt (refereegranskat)abstract Recent advances in machine learning (ML) offer new tools to extract new insights from large data sets and to acquire small data sets more effectively. Researchers in nanoscience are experimenting with these tools to tackle challenges in many fields. In addition to ML's advancement of nanoscience, nanoscience provides the foundation for neuromorphic computing hardware to expand the implementation of ML algorithms. In this Mini Review, we highlight some recent efforts to connect the ML and nanoscience communities by focusing on three types of interaction: (1) using ML to analyze and extract new insights from large nanoscience data sets, (2) applying ML to accelerate material discovery, including the use of active learning to guide experimental design, and (3) the nanoscience of memristive devices to realize hardware tailored for ML. We conclude with a discussion of challenges and opportunities for future interactions between nanoscience and ML researchers.

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