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1.
ACS Nano ; 13(12): 14262-14273, 2019 Dec 24.
Artigo em Inglês | MEDLINE | ID: mdl-31790198

RESUMO

Two-dimensional (2D) layered semiconductors have recently emerged as attractive building blocks for next-generation low-power nonvolatile memories. However, challenges remain in the controllable fabrication of bipolar resistive switching circuit components from these materials. Here, the experimental realization of lateral memtransistors from monolayer single-crystal molybdenum disulfide (MoS2) utilizing a focused helium ion beam is reported. Site-specific irradiation with the focused probe of a helium ion microscope creates a nanometer-scale defect-rich region, bisecting the MoS2 lattice. The reversible drift of these defects in the applied electric field modulates the resistance of the channel, enabling versatile memristive functionality. The device can reliably retain its resistance ratios and set/reset biases for 1180 switching cycles. Long-term potentiation and depression with sharp habituation are demonstrated. This work establishes the feasibility of ion irradiation for controllable fabrication of 2D memristive devices with promising key performance parameters, such as low power consumption. The applicability of these devices for synaptic emulation may address the demands of future neuromorphic architectures.

2.
Sci Rep ; 9(1): 11550, 2019 Aug 09.
Artigo em Inglês | MEDLINE | ID: mdl-31399603

RESUMO

Networks of metallic nanowires have the potential to meet the needs of next-generation device technologies that require flexible transparent conductors. At present, there does not exist a first principles model capable of predicting the electro-optical performance of a nanowire network. Here we combine an electrical model derived from fundamental material properties and electrical equations with an optical model based on Mie theory scattering of light by small particles. This approach enables the generation of analogues for any nanowire network and then accurately predicts, without the use of fitting factors, the optical transmittance and sheet resistance of the transparent electrode. Predictions are validated using experimental data from the literature of networks comprised of a wide range of aspect ratios (nanowire length/diameter). The separation of the contributions of the material resistance and the junction resistance allows the effectiveness of post-deposition processing methods to be evaluated and provides a benchmark for the minimum attainable sheet resistance. The predictive power of this model enables a material-by-design approach, whereby suitable systems can be prescribed for targeted technology applications.

3.
Sci Rep ; 9(1): 11738, 2019 Aug 13.
Artigo em Inglês | MEDLINE | ID: mdl-31409846

RESUMO

Considerable attention has been drawn to the lead halide perovskites (LHPs) because of their outstanding optoelectronic characteristics. LHP nanosheets (NSs) grown from single crystalline lead halide possess advantages in device applications as they provide the possibility for control over morphology, composition, and crystallinity. Here, free-standing lead bromide (PbBr2) single-crystalline NSs with sizes up to one centimeter are synthesized from solution. These NSs can be converted to LHP while maintaining the NS morphology. We demonstrate that these perovskite NSs can be processed directly for fabrication of photodetector and laser arrays on a large scale. This strategy will allow high-yield synthesis of large-size perovskite NSs for functional devices in an integrated photonics platform.

4.
Nanotechnology ; 2019 Jun 27.
Artigo em Inglês | MEDLINE | ID: mdl-31247603

RESUMO

This is to provide a correction to the acknowledgement section of the previously published paper: 
 Mills, S., Sader, J.E., Boland, J.J. "Material characterisation of nanowires with Intrinsic stress" 
 Nanotechnology 28(35), Article No: 355706, 01 September 2017. DOI: 10.1088/1361-6528/aa7c31.&#13.

5.
Nanotechnology ; 30(33): 335706, 2019 Aug 16.
Artigo em Inglês | MEDLINE | ID: mdl-31026844

RESUMO

Phase-change random access memory is a promising approach to non-volatile memory. However, the inability to secure consistent, reliable switching on a nanometre scale may limit its practical use for high density applications. Here, we report on the switching behaviour of PCRAM cells comprised of single crystalline Ge9Sb1Te5 (GST) nanowires. We show that device switching is dominated by the contacts and does not result in a resistance change within the bulk of the wire. For the devices studied, the typical contact resistance was ∼30 kΩ, whereas the resistance of the GST channel was 1.8 kΩ. The applied voltage was predominately dropped across the passivating oxide on the surface of the GST nanowires, resulting in local resistive switching at the contacts and local power dissipation, which limited the endurance of the devices produced. The optimal device must balance low resistance contacts with a more resistive channel, to facilitate phase change switching within the nanowires. These results highlight the importance of contact formation on the switching properties in phase change devices and help guide the future design of more reliable neuromorphic devices.

6.
ACS Appl Mater Interfaces ; 10(50): 44183-44189, 2018 Dec 19.
Artigo em Inglês | MEDLINE | ID: mdl-30460845

RESUMO

Nanoscale materials are frequently coated with surface stabilization layers during growth that prevent flocculation in solution and facilitate processing technologies such as ink-jet device printing. Here, we show that few-nanometer-thick stabilization layers typically used swell in the presence of certain solvents and impart significant stresses to the nanomaterial that remains even after the solvent has evaporated. Solvent swelling of the surface layer dramatically enhances nanomaterial-substrate adhesion via the collapse of the stabilization layer during solvent evaporation, preventing stress relaxation. We demonstrate the stress modulation of Ag, Au, and Si nanowires functionalised with surface polymers and surfactant layers and detect strain levels between 0.1 and 0.6% using atomic force microscopy mechanical measurement and Raman spectroscopy. Dry-transferred nanowires exhibit poor adhesion and show no evidence of incorporated stress but become stressed immediately following solvent exposure. Strain engineering is demonstrated by coating nanowires with few-nanometer-thick solvent-responsive polymer layers.

7.
Nat Commun ; 9(1): 3219, 2018 08 13.
Artigo em Inglês | MEDLINE | ID: mdl-30104665

RESUMO

Nanowire networks are promising memristive architectures for neuromorphic applications due to their connectivity and neurosynaptic-like behaviours. Here, we demonstrate a self-similar scaling of the conductance of networks and the junctions that comprise them. We show this behavior is an emergent property of any junction-dominated network. A particular class of junctions naturally leads to the emergence of conductance plateaus and a "winner-takes-all" conducting path that spans the entire network, and which we show corresponds to the lowest-energy connectivity path. The memory stored in the conductance state is distributed across the network but encoded in specific connectivity pathways, similar to that found in biological systems. These results are expected to have important implications for development of neuromorphic devices based on reservoir computing.

8.
Langmuir ; 34(26): 7698-7707, 2018 07 03.
Artigo em Inglês | MEDLINE | ID: mdl-29889539

RESUMO

We combine ambient (air) and ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS) investigations together with density functional theory (DFT) calculations to gain a subnanometer insight into the structure and dynamic of two-dimensional (2D) surface-supported molecular networks. The planar tetraferrocene-porphyrin molecules employed in this study undergo spontaneous self-assembly via the formation of hydrogen bonded networks at the gold substrate-solution interface. To mimic liquid phase ambient deposition conditions, film formation was accomplished in UHV by electro-spraying a solution of the molecule in chloroform onto an Au(111) substrate, thereby providing access to the full spectroscopic capabilities of STM that can be hardly attained under ambient conditions. We show that molecular assembly on Au (111) is identical in films prepared under the two different conditions, and in good agreement with the theoretical predictions. However, we observe the contrast found for a given STM bias condition to be different in ambient and UHV conditions despite the similarity of the structures, and we propose possible origins of the different imaging contrast. This approach could be valuable for the thorough characterization of surface systems that involve large molecules and are prepared mainly in ambient conditions.

9.
ACS Nano ; 12(5): 4727-4735, 2018 05 22.
Artigo em Inglês | MEDLINE | ID: mdl-29726674

RESUMO

Optically transparent photodetectors are crucial in next-generation optoelectronic applications including smart windows and transparent image sensors. Designing photodetectors with high transparency, photoresponsivity, and robust mechanical flexibility remains a significant challenge, as is managing the inevitable trade-off between high transparency and strong photoresponse. Here we report a scalable method to produce flexible crystalline Si nanostructured wire (NW) networks fabricated from silicon-on-insulator (SOI) with seamless junctions and highly responsive porous Si segments that combine to deliver exceptional performance. These networks show high transparency (∼92% at 550 nm), broadband photodetection (350 to 950 nm) with excellent responsivity (25 A/W), optical response time (0.58 ms), and mechanical flexibility (1000 cycles). Temperature-dependent photocurrent measurements indicate the presence of localized electronic states in the porous Si segments, which play a crucial role in light harvesting and photocarrier generation. The scalable low-cost approach based on SOI has the potential to deliver new classes of flexible optoelectronic devices, including next-generation photodetectors and solar cells.

10.
Sci Adv ; 4(3): eaao5031, 2018 03.
Artigo em Inglês | MEDLINE | ID: mdl-29511736

RESUMO

Precise tunability of electronic properties of two-dimensional (2D) nanomaterials is a key goal of current research in this field of materials science. Chemical modification of layered transition metal dichalcogenides leads to the creation of heterostructures of low-dimensional variants of these materials. In particular, the effect of oxygen-containing plasma treatment on molybdenum disulfide (MoS2) has long been thought to be detrimental to the electrical performance of the material. We show that the mobility and conductivity of MoS2 can be precisely controlled and improved by systematic exposure to oxygen/argon plasma and characterize the material using advanced spectroscopy and microscopy. Through complementary theoretical modeling, which confirms conductivity enhancement, we infer the role of a transient 2D substoichiometric phase of molybdenum trioxide (2D-MoO x ) in modulating the electronic behavior of the material. Deduction of the beneficial role of MoO x will serve to open the field to new approaches with regard to the tunability of 2D semiconductors by their low-dimensional oxides in nano-modified heterostructures.

11.
ACS Appl Mater Interfaces ; 9(44): 38959-38966, 2017 Nov 08.
Artigo em Inglês | MEDLINE | ID: mdl-29027461

RESUMO

Nonpolar resistive switching (RS), a combination of bipolar and unipolar RS, is demonstrated for the first time in a single nanowire (NW) system. Exploiting Ag@TiO2 core-shell (CS) NWs synthesized by postgrowth shell formation, the switching mode is controlled by adjusting the current compliance effectively, tailoring the electrical polarity response. We demonstrate ON/OFF ratios of 105 and 107 for bipolar and unipolar modes, respectively. In the bipolar regime, retention times could be controlled up to 103 s, and in the unipolar mode, >106 s was recorded. We show how the unique dual-mode switching behavior is enabled by the defect-rich polycrystalline material structure of the TiO2 shell and the interaction between the Ag core and the Ag electrodes. These results provide a foundation for engineering nonpolar RS behaviors for memory storage and neuromorphic applications in CSNW structures.

12.
Science ; 357(6349): 397-400, 2017 07 28.
Artigo em Inglês | MEDLINE | ID: mdl-28751608

RESUMO

We used scanning tunneling microscopy to study low-angle grain boundaries at the surface of nearly planar copper nanocrystalline (111) films. The presence of grain boundaries and their emergence at the film surface create valleys composed of dissociated edge dislocations and ridges where partial dislocations have recombined. Geometric analysis and simulations indicated that valleys and ridges were created by an out-of-plane grain rotation driven by reduction of grain boundary energy. These results suggest that in general, it is impossible to form flat two-dimensional nanocrystalline films of copper and other metals exhibiting small stacking fault energies and/or large elastic anisotropy, which induce a large anisotropy in the dislocation-line energy.

13.
Waste Manag Res ; 35(6): 618-626, 2017 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-28393635

RESUMO

In Nepal, full-fledged environmental legislation was rare before the democratic constitution of 1990. The first law covering the environment and sustainability was the Environment Protection Act 1997. While the Solid Waste Act was introduced in 1987, the problem of solid waste management still surfaces in Kathmandu. In order to understand the bedrock of this unrelenting failure in solid waste management, the manuscript digs deeper into policy implementation by dissecting solid waste rules, environmental legislations, relevant local laws, and solid waste management practices in Kathmandu, Nepal. A very rich field study that included surveys, interviews, site visits, and literature review provided the basis for the article. The study shows that volumes of new Nepalese rules are crafted without effective enforcement of their predecessors and there is a frequent power struggle between local government bodies and central authority in implementing the codes and allocating resources in solid waste management. The study concludes that Kathmandu does not require any new instrument to address solid waste problems; instead, it needs creation of local resources, execution of local codes, and commitment from central government to allow free exercise of these policies.


Assuntos
Política Ambiental , Resíduos Sólidos , Gerenciamento de Resíduos , Conservação dos Recursos Naturais , Nepal , Eliminação de Resíduos
14.
Phys Chem Chem Phys ; 18(39): 27564-27571, 2016 Oct 05.
Artigo em Inglês | MEDLINE | ID: mdl-27722404

RESUMO

Motivated by numerous technological applications, there is current interest in the study of the conductive properties of networks made of randomly dispersed nanowires. The sheet resistance of such networks is normally calculated by numerically evaluating the conductance of a system of resistors but due to disorder and with so many variables to account for, calculations of this type are computationally demanding and may lack mathematical transparency. Here we establish the equivalence between the sheet resistance of disordered networks and that of a regular ordered network. Rather than through a fitting scheme, we provide a recipe to find the effective medium network that captures how the resistance of a nanowire network depends on several different parameters such as wire density, electrode size and electrode separation. Furthermore, the effective medium approach provides a simple way to distinguish the sheet resistance contribution of the junctions from that of the nanowires themselves. The contrast between these two contributions determines the potential to optimize the network performance for a particular application.

15.
Chemistry ; 22(28): 9709-23, 2016 Jul 04.
Artigo em Inglês | MEDLINE | ID: mdl-27258206

RESUMO

Here we have investigated the influence of the antenna group position on both the formation of chiral amphiphilic Eu(III) -based self-assemblies in CH3 CN solution and, on the ability to form monolayers on the surface of quartz substrates using the Langmuir-Blodgett technique, by changing from the 1-naphthyl (2(R), 2(S)) to the 2-naphthyl (1(R), 1(S)) position. The evaluation of binding constants of the self- assemblies in CH3 CN solution was achieved using conventional techniques such as UV/Visible and luminescence spectroscopies along with more specific circular dichroism (CD) spectroscopy. The binding constants obtained for EuL, EuL2 and EuL3 species in the case of 2-naphthyl derivatives were comparable to those obtained for 1-naphthyl derivatives. The analysis of the changes in the CD spectra of 1(R) and 1(S) upon addition of Eu(III) not only allowed us to evaluate the values of the binding constants but the resulting recalculated spectra may also be used as fingerprints for assignment of the chiral self-assembly species formed in solution. The obtained monolayers were predominantly formed from EuL3 (≈85 %) with the minor species present in ≈15 % EuL2 .

16.
ACS Nano ; 10(3): 3087-92, 2016 Mar 22.
Artigo em Inglês | MEDLINE | ID: mdl-26828573

RESUMO

We demonstrate templating of functional materials with unexpected and intricate micro- and nanostructures by controlling the condensation, packing, and evaporation of water droplets on a polymer solution. Spontaneous evaporation of a polymer solution induces cooling of the liquid surface and water microdroplet condensation from the ambient vapor. These droplets pack together and act as a template to imprint an entangled polymer film. This breath figure (BF) phenomenon is an example of self-organization that involves the long-range ordering of droplets. Equilibrium-based analysis provides many insights into contact angles and drop stability of individual drops, but the BF phenomenon remains poorly understood thus far, preventing translation to real applications. Here we investigate the dynamics of this phenomenon to separate out the competing influences and then introduce a modulation scheme to ultimately manipulate the water vapor-liquid equilibrium independently from the solvent evaporation. This approach to BF control provides insights into the mechanism, a rationale for microstructure design, and evidence for the benefits of dynamical control of self-organization systems. We finally present dramatically different porous architectures from this approach reminiscent of microscale Petri dishes, conical flasks, and test tubes.

17.
ACS Nano ; 9(11): 11422-9, 2015 Nov 24.
Artigo em Inglês | MEDLINE | ID: mdl-26448205

RESUMO

Networks of silver nanowires appear set to replace expensive indium tin oxide as the transparent conducting electrode material in next generation devices. The success of this approach depends on optimizing the material conductivity, which until now has largely focused on minimizing the junction resistance between wires. However, there have been no detailed reports on what the junction resistance is, nor is there a known benchmark for the minimum attainable sheet resistance of an optimized network. In this paper, we present junction resistance measurements of individual silver nanowire junctions, producing for the first time a distribution of junction resistance values and conclusively demonstrating that the junction contribution to the overall resistance can be reduced beyond that of the wires through standard processing techniques. We find that this distribution shows the presence of a small percentage (6%) of high-resistance junctions, and we show how these may impact the performance of network-based materials. Finally, through combining experiment with a rigorous model, we demonstrate the important role played by the network skeleton and the specific connectivity of the network in determining network performance.

18.
Inorg Chem ; 54(16): 7735-41, 2015 Aug 17.
Artigo em Inglês | MEDLINE | ID: mdl-26222397

RESUMO

The tripodal terpyridine ligand, L, forms 1D helical supramolecular polymers/gels in H2O-CH3OH solution mediated through hydrogen bonding and π-π interactions. These gels further cross-link into 3D supramolecular metallogels with a range of metal ions (M) such as Fe(II), Ni(II), Cu(II), Zn(II), and Ru(III); the cross-linking resulting in the formation of colored or colorless gels. The fibrous morphology of these gels was confirmed using scanning electron microscopy (SEM); while the self-assembly processes between L and M were investigated by absorbance and emission spectroscopy from which their binding constants were determined by using a nonlinear regression analysis.


Assuntos
Metais Pesados/química , Piridinas/química , Géis , Ligantes
19.
Nanoscale ; 7(30): 13011-6, 2015 Aug 14.
Artigo em Inglês | MEDLINE | ID: mdl-26169222

RESUMO

In this work, we introduce a combined experimental and computational approach to describe the conductivity of metallic nanowire networks. Due to their highly disordered nature, these materials are typically described by simplified models in which network junctions control the overall conductivity. Here, we introduce a combined experimental and simulation approach that involves a wire-by-wire junction-by-junction simulation of an actual network. Rather than dealing with computer-generated networks, we use a computational approach that captures the precise spatial distribution of wires from an SEM analysis of a real network. In this way, we fully account for all geometric aspects of the network, i.e. for the properties of the junctions and wire segments. Our model predicts characteristic junction resistances that are smaller than those found by earlier simplified models. The model outputs characteristic values that depend on the detailed connectivity of the network, which can be used to compare the performance of different networks and to predict the optimum performance of any network and its scope for improvement.

20.
J Biol Chem ; 290(47): 28343-52, 2015 Nov 20.
Artigo em Inglês | MEDLINE | ID: mdl-26221033

RESUMO

Despite significant advances, the molecular identity of the cytotoxic species populated during in vivo amyloid formation crucial for the understanding of neurodegenerative disorders is yet to be revealed. In this study lysozyme prefibrillar oligomers and fibrils in both mature and sonicated states have been isolated through an optimized ultrafiltration/ultracentrifugation method and characterized with various optical spectroscopic techniques, atomic force microscopy, and transmission electron microscopy. We examined their level and mode of toxicity on rat pheochromocytoma (PC12) cells in both differentiated and undifferentiated states. We find that oligomers and fibrils display cytotoxic capabilities toward cultured cells in vitro, with oligomers producing elevated levels of cellular injury toward undifferentiated PC12 cells (PC12(undiff)). Furthermore, dual flow cytometry staining experiments demonstrate that the oligomers and mature fibrils induce divergent cellular death pathways (apoptosis and secondary necrosis, respectively) in these PC12 cells. We have also shown that oligomers but not sonicated mature fibrils inhibit hippocampal long term potentiation, a form of synaptic plasticity implicated in learning and memory, in vivo. We conclude that our in vitro and in vivo findings confer a level of resistance toward amyloid fibrils, and that the PC 12-based comparative cytotoxicity assay can provide insights into toxicity differences between differently aggregated protein species.


Assuntos
Amiloide/metabolismo , Biopolímeros/metabolismo , Morte Celular , Amiloide/química , Animais , Biopolímeros/química , Células PC12 , Ratos
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