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2.
Nano Lett ; 19(5): 3194-3198, 2019 05 08.
Artigo em Inglês | MEDLINE | ID: mdl-30943040

RESUMO

We introduce a concept of programmable ferroelectric devices composed of two-dimensional (2D) and ferroelectric (FE) materials. It enables precise modulation of the in-plane conductivity of a 2D channel material through nanoengineering FE domains with out-of-plane polarization. The functionality of these new devices has been demonstrated using field-effect transistors (FETs) fabricated with monolayer molybdenum disulfide (MoS2) channels on the Pb(Zr,Ti)O3 substrates. Using piezoresponse force microscopy (PFM), we show that local switching of FE polarization by a conductive probe can be used to tune the conductivity of the MoS2 channel. Specifically, patterning of the nanoscale domains with downward polarization creates conductive paths in a resistive MoS2 channel so that the conductivity of an FET is determined by the number and length of the paths connecting source and drain electrodes. In addition to the device programmability, we demonstrate the device ON/OFF cyclic endurance by successive writing and erasing of conductive paths in a MoS2 channel. These findings may inspire the development of advanced energy-efficient programmable synaptic devices based on a combination of 2D and FE materials.

3.
ACS Nano ; 13(1): 803-811, 2019 Jan 22.
Artigo em Inglês | MEDLINE | ID: mdl-30586504

RESUMO

We explore the electrical characteristics of TiS3 nanowire field-effect transistor (FETs), over the wide temperature range from 3 to 350 K. These nanomaterials have a quasi-one-dimensional (1D) crystal structure and exhibit a gate-controlled metal-insulator transition (MIT) in their transfer curves. Their room-temperature mobility is ∼20-30 cm2/(V s), 2 orders of magnitude smaller than predicted previously, a result that we explain quantitatively in terms of the influence of polar-optical phonon scattering in these materials. In the insulating state (<∼220 K), the transfer curves exhibit unusual mesoscopic fluctuations and a current suppression near zero bias that is common to charge-density wave (CDW) systems. The fluctuations have a nonmonotonic temperature dependence and wash out at a temperature close to that of the bulk MIT, suggesting they may be a feature of quantum interference in the CDW state. Overall, our results demonstrate that quasi-1D TiS3 nanostructures represent a viable candidate for FET realization and that their functionality is influenced by complex phenomena.

4.
ACS Nano ; 2018 Nov 30.
Artigo em Inglês | MEDLINE | ID: mdl-30499656

RESUMO

Quasi-one-dimensional (quasi-1D) materials enjoy growing interest due to their unusual physical properties and promise for miniature electronic devices. However, the mechanical exfoliation of quasi-1D materials into thin flakes and nanoribbons received considerably less attention from researchers than the exfoliation of conventional layered crystals. In this study, we investigated the micromechanical exfoliation of representative quasi-1D crystals, TiS3 whiskers, and demonstrate that they typically split into narrow nanoribbons with very smooth, straight edges and clear signatures of 1D TiS3 chains. Theoretical calculations show that the energies required for breaking weak interactions between the two-dimensional (2D) layers and between 1D chains within the layers are comparable and, in turn, are considerably lower than those required for breaking the covalent bonds within the chains. We also emulated macroscopic exfoliation experiments on the nanoscale by applying a local shear force to TiS3 crystals in different crystallographic directions using a tip of an atomic force microscopy (AFM) probe. In the AFM experiments, it was possible to slide the 2D TiS3 layers relative to each other as well as to remove selected 1D chains from the layers. We systematically studied the exfoliated TiS3 crystals by Raman spectroscopy and identified the Raman peaks whose spectral positions were most dependent on the crystals' thickness. These results could be used to distinguish between TiS3 crystals with thickness ranging from one to about seven monolayers. The conclusions established in this study for the exfoliated TiS3 crystals can be extended to a variety of transition metal trichalcogenide materials as well as other quasi-1D crystals. The possibility of exfoliation of TiS3 into narrow (few-nm wide) crystals with smooth edges could be important for the future realization of miniature device channels with reduced edge scattering of charge carriers.

5.
Nanotechnology ; 29(50): 505707, 2018 Dec 14.
Artigo em Inglês | MEDLINE | ID: mdl-30311602

RESUMO

We report on the chemical vapor deposition synthesis of MoO2 nanoplatelets by sublimation of MoO3 and its reduction in a hydrogen atmosphere at 750 °C. When grown on Si/SiO2 substrates, the platelets primarily assume a rhomboidal shape and have thicknesses ranging from several to tens of nm. The morphology of MoO2 crystals was found to depend on the chemical nature of substrates. MoO2 platelets on Si/SiO2 were characterized by a number of microscopic and spectroscopic techniques, and the electrical measurements revealed the metallic nature of their conductivity averaging at 2400 ± 1000 S cm-1. Raman spectroscopy of MoO2 platelets on graphene indicates their strong hole injection property. Small thickness, planar morphology, high chemical stability and metallic conductivity of ultrathin MoO2 platelets make them potentially interesting for integration different other two-dimensional materials in a variety of electronic structures and devices.

6.
Nat Commun ; 9(1): 3344, 2018 08 21.
Artigo em Inglês | MEDLINE | ID: mdl-30131577

RESUMO

In the ferroelectric devices, polarization control is usually accomplished by application of an electric field. In this paper, we demonstrate optically induced polarization switching in BaTiO3-based ferroelectric heterostructures utilizing a two-dimensional narrow-gap semiconductor MoS2 as a top electrode. This effect is attributed to the redistribution of the photo-generated carriers and screening charges at the MoS2/BaTiO3 interface. Specifically, a two-step process, which involves formation of intra-layer excitons during light absorption followed by their decay into inter-layer excitons, results in the positive charge accumulation at the interface forcing the polarization reversal from the upward to the downward direction. Theoretical modeling of the MoS2 optical absorption spectra with and without the applied electric field provides quantitative support for the proposed mechanism. It is suggested that the discovered effect is of general nature and should be observable in any heterostructure comprising a ferroelectric and a narrow gap semiconductor.

7.
Sci Adv ; 4(6): eaat0491, 2018 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-29922719

RESUMO

Two-dimensional (2D) transition metal carbides and nitrides, known as MXenes, are a large class of materials that are finding numerous applications ranging from energy storage and electromagnetic interference shielding to water purification and antibacterial coatings. Yet, despite the fact that more than 20 different MXenes have been synthesized, the mechanical properties of a MXene monolayer have not been experimentally studied. We measured the elastic properties of monolayers and bilayers of the most important MXene material to date, Ti3C2T x (T x stands for surface termination). We developed a method for preparing well-strained membranes of Ti3C2T x monolayers and bilayers, and performed their nanoindentation with the tip of an atomic force microscope to record the force-displacement curves. The effective Young's modulus of a single layer of Ti3C2T x was found to be 0.33 ± 0.03 TPa, which is the highest among the mean values reported in nanoindentation experiments for other solution-processed 2D materials, including graphene oxide. This work opens a pathway for investigating the mechanical properties of monolayers and bilayers of other MXenes and extends the already broad range of MXenes' applications to structural composites, protective coatings, nanoresonators, and membranes that require materials with exceptional mechanical properties.

8.
Nat Commun ; 8(1): 820, 2017 10 10.
Artigo em Inglês | MEDLINE | ID: mdl-29018185

RESUMO

Narrow atomically precise graphene nanoribbons hold great promise for electronic and optoelectronic applications, but the previously demonstrated nanoribbon-based devices typically suffer from low currents and mobilities. In this study, we explored the idea of lateral extension of graphene nanoribbons for improving their electrical conductivity. We started with a conventional chevron graphene nanoribbon, and designed its laterally extended variant. We synthesized these new graphene nanoribbons in solution and found that the lateral extension results in decrease of their electronic bandgap and improvement in the electrical conductivity of nanoribbon-based thin films. These films were employed in gas sensors and an electronic nose system, which showed improved responsivities to low molecular weight alcohols compared to similar sensors based on benchmark graphitic materials, such as graphene and reduced graphene oxide, and a reliable analyte recognition. This study shows the methodology for designing new atomically precise graphene nanoribbons with improved properties, their bottom-up synthesis, characterization, processing and implementation in electronic devices.Atomically precise graphene nanoribbons are a promising platform for tailored electron transport, yet they suffer from low conductivity. Here, the authors devise a strategy to laterally extend conventional chevron nanoribbons, thus achieving increased electrical conductivity and improved chemical sensing capabilities.

9.
Adv Funct Mater ; 4(2)2017 Jan 23.
Artigo em Inglês | MEDLINE | ID: mdl-28507484

RESUMO

We demonstrate a technique for facile encapsulation and adhesion of micro- and nano objects on arbitrary substrates, stencils, and micro structured surfaces by ultrathin graphene oxide membranes via a simple drop casting of graphene oxide solution. A self-assembled encapsulating membrane forms during the drying process at the liquid-air and liquid-solid interfaces and consists of a water-permeable quasi-2D network of overlapping graphene oxide flakes. Upon drying and interlocking between the flakes, the encapsulating coating around the object becomes mechanically robust, chemically protective, and yet highly transparent to electrons and photons in a wide energy range, enabling microscopic and spectroscopic access to encapsulated objects. The characteristic encapsulation scenarios were demonstrated on a set of representative inorganic and organic micro and nano-objects and microstructured surfaces. Different coating regimes can be achieved by controlling the pH of the supporting solution, and the hydrophobicity and morphology of interfaces. Several specific phenomena such as compression of encased objects by contracting membranes as well as hierarchical encapsulations were observed. Finally, electron as well as optical microscopy and analysis of encapsulated objects along with the membrane effect on the image contrast formation, and signal attenuation are discussed.

10.
Nano Lett ; 17(2): 922-927, 2017 02 08.
Artigo em Inglês | MEDLINE | ID: mdl-28094991

RESUMO

Hybrid structures composed of ferroelectric thin films and functional two-dimensional (2D) materials may exhibit unique characteristics and reveal new phenomena due to the cross-interface coupling between their intrinsic properties. In this report, we demonstrate a symbiotic interplay between spontaneous polarization of the ultrathin BaTiO3 ferroelectric film and conductivity of the adjacent molybdenum disulfide (MoS2) layer, a 2D narrow-bandgap semiconductor. Polarization-induced modulation of the electronic properties of MoS2 results in a giant tunneling electroresistance effect in the hybrid MoS2-BaTiO3-SrRuO3 ferroelectric tunnel junctions (FTJs) with an OFF-to-ON resistance ratio as high as 104, a 50-fold increase in comparison with the same type of FTJs with metal electrodes. The effect stems from the reversible accumulation-depletion of the majority carriers in the MoS2 electrode in response to ferroelectric switching, which alters the barrier at the MoS2-BaTiO3 interface. Continuous tunability of resistive states realized via stable sequential domain structures in BaTiO3 adds memristive functionality to the hybrid FTJs. The use of narrow band 2D semiconductors in conjunction with ferroelectric films provides a novel pathway for development of the electronic devices with enhanced performance.

11.
ACS Appl Mater Interfaces ; 9(1): 693-700, 2017 Jan 11.
Artigo em Inglês | MEDLINE | ID: mdl-27933763

RESUMO

Because of their intriguing electronic and optical properties, atomically precise graphene nanoribbons (GNRs) are considered to be promising materials for electronics and photovoltaics. However, significant aggregation and low solubility of GNRs in conventional solvents result in their poor processability for materials characterization and device studies. In this paper, we demonstrate a new fabrication approach for large-scale uniform thin films of nonfunctionalized atomically precise chevron-type GNRs. The method is based on (1) the exceptional solubility of graphitic materials in chlorosulfonic acid and (2) the original interfacial self-assembly approach by which uniform films that are single-GNR (∼2 nm) thick can be routinely prepared. These films can be transferred to various substrates including Si/SiO2 and used for the streamlined fabrication of arrays of GNR-based devices. The described self-assembly approach should be applicable to other types of solution-synthesized atomically precise GNRs as well as large polyaromatic hydrocarbon (PAH) molecules and therefore should facilitate and streamline their device characterization.

12.
Nano Lett ; 16(10): 6460-6466, 2016 10 12.
Artigo em Inglês | MEDLINE | ID: mdl-27662071

RESUMO

Polarization switching in ferroelectric capacitors is typically realized by application of an electrical bias to the capacitor electrodes and occurs via a complex process of domain structure reorganization. As the domain evolution in real devices is governed by the distribution of the nucleation centers, obtaining a domain structure of a desired configuration by electrical pulsing is challenging, if not impossible. Recent discovery of polarization reversal via the flexoelectric effect has opened a possibility for deterministic control of polarization in ferroelectric capacitors. In this paper, we demonstrate mechanical writing of arbitrary-shaped nanoscale domains in thin-film ferroelectric capacitors with graphene electrodes facilitated by a strain gradient induced by a tip of an atomic force microscope (AFM). A phase-field modeling prediction of a strong effect of graphene thickness on the threshold load required to initiate mechanical switching has been confirmed experimentally. Deliberate voltage-free domain writing represents a viable approach for development of functional devices based on domain topology and electronic properties of the domains and domain walls.

13.
ACS Appl Mater Interfaces ; 8(28): 18334-8, 2016 Jul 20.
Artigo em Inglês | MEDLINE | ID: mdl-27362332

RESUMO

We report synthesis and time-resolved transient absorption measurements of TiS3 nanoribbons. TiS3 nanoribbons were fabricated by direct reaction of titanium and sulfur. Dynamics of the photocarriers in these samples were studied by transient absorption measurements. It was found that following ultrafast injection of nonequilibrium and hot photocarriers, the thermalization, energy relaxation, and exciton formation all occur on a subpicosecond time scale. Several key parameters describing the dynamical properties of photocarriers, including their recombination lifetime, diffusion coefficient, mobility, and diffusion length, were deduced.

14.
Adv Mater ; 27(47): 7832-8, 2015 Dec 16.
Artigo em Inglês | MEDLINE | ID: mdl-26479268

RESUMO

An electrically written domain structure formed by a biased tip, and visualized in the piezoresponse force microscopy mode, shows stable charged domain walls in the organic ferroelectric diisopropylammonium chloride microcrystal.

15.
ACS Nano ; 9(8): 8089-98, 2015 Aug 25.
Artigo em Inglês | MEDLINE | ID: mdl-26222209

RESUMO

In this study, we fabricated and tested electronic and memory properties of field-effect transistors (FETs) based on monolayer or few-layer molybdenum disulfide (MoS2) on a lead zirconium titanate (Pb(Zr,Ti)O3, PZT) substrate that was used as a gate dielectric. MoS2-PZT FETs exhibit a large hysteresis of electronic transport with high ON/OFF ratios. We demonstrate that the interplay of polarization and interfacial phenomena strongly affects the electronic behavior and memory characteristics of MoS2-PZT FETs. We further demonstrate that MoS2-PZT memories have a number of advantages and unique features compared to their graphene-based counterparts as well as commercial ferroelectric random-access memories (FeRAMs), such as nondestructive data readout, low operation voltage, wide memory window and the possibility to write and erase them both electrically and optically. This dual optoelectrical operation of these memories can simplify the device architecture and offer additional practical functionalities, such as an instant optical erase of large data arrays that is unavailable for many conventional memories.

16.
Nanoscale ; 7(29): 12291-6, 2015 Aug 07.
Artigo em Inglês | MEDLINE | ID: mdl-26129825

RESUMO

Titanium trisulfide (TiS3) is a promising layered semiconductor material. Several-mm-long TiS3 whiskers can be conveniently grown by the direct reaction of titanium and sulfur. In this study, we exfoliated these whiskers using the adhesive tape approach and fabricated few-layered TiS3 field-effect transistors (FETs). The TiS3 FETs showed an n-type electronic transport with room-temperature field-effect mobilities of 18-24 cm(2) V(-1) s(-1) and ON/OFF ratios up to 300. We demonstrate that TiS3 is compatible with the conventional atomic layer deposition (ALD) procedure for Al2O3. ALD of alumina on TiS3 FETs resulted in mobility increase up to 43 cm(2) V(-1) s(-1), ON/OFF ratios up to 7000, and much improved subthreshold swing characteristics. This study shows that TiS3 is a competitive electronic material in the family of two-dimensional (2D) transition metal chalcogenides and can be considered for emerging device applications.

17.
Biochem Biophys Res Commun ; 460(2): 267-73, 2015 May 01.
Artigo em Inglês | MEDLINE | ID: mdl-25778866

RESUMO

A few recent studies demonstrated that graphene may have cytocompatibility with several cell types. However, when assessing cell behavior on graphene, there has been no precise control over the quality of graphene, number of graphene layers, and substrate surface coverage by graphene. In this study, using well-controlled monolayer graphene film substrates we tested the cytocompatibility of graphene for human neuroblastoma (SH-SY5Y) cell culture. A large-scale monolayer graphene film grown on Cu foils by chemical vapor deposition (CVD) could be successfully transferred onto glass substrates by wet transfer technique. We observed that graphene substrate could induce enhanced neurite outgrowth, both in neurite length and number, compared with control glass substrate. Interestingly, the positive stimulatory effect by graphene was achieved even in the absence of soluble neurogenic factor, retinoic acid (RA). Key genes relevant to cell neurogenesis, e.g., neurofilament light chain (NFL), were also upregulated on graphene. Inhibitor studies suggested that the graphene stimulation of cellular neurogenesis may be achieved through focal adhesion kinase (FAK) and p38 mitogen-activated protein kinase (MAPK) cascades. Our data indicate that graphene may be exploited as a platform for neural regenerative medicine, and the suggested molecular mechanism may provide an insight into the graphene control of neural cells.


Assuntos
Grafite/química , Neurite (Inflamação) , Sequência de Bases , Diferenciação Celular , Linhagem Celular Tumoral , Primers do DNA , Proteína-Tirosina Quinases de Adesão Focal/metabolismo , Humanos , Neurônios/citologia , Neurônios/enzimologia , Reação em Cadeia da Polimerase , Proteínas Quinases p38 Ativadas por Mitógeno/metabolismo
18.
ACS Appl Mater Interfaces ; 7(5): 2987-92, 2015 Feb 11.
Artigo em Inglês | MEDLINE | ID: mdl-25594774

RESUMO

We demonstrate that graphitic coatings, which consist of multilayer disordered graphene sheets, can be used for the thermal protection of delicate metal nanostructures. We studied cobalt slanted nanopillars grown by glancing angle deposition that were shown to melt at temperatures much lower than the melting point of bulk cobalt. After graphitic coatings were conformally grown over the surfaces of Co nanopillars by chemical vapor deposition, the resulting carbon-coated Co nanostructures retained their morphology at elevated temperatures, which would damage the uncoated structures. Thermal stabilization is also demonstrated for carbon-coated Ti nanopillars. The results of this study may be extended to other metallic and possibly even nonmetallic nanostructures that need to preserve their morphology at elevated temperatures in a broad range of applications.

19.
Faraday Discuss ; 173: 105-13, 2014.
Artigo em Inglês | MEDLINE | ID: mdl-25465679

RESUMO

Graphene nanoribbons (GNRs) have received a great deal of attention due to their promise for electronic and optoelectronic applications. Several recent studies have focused on the synthesis of GNRs by the bottom-up approaches that could yield very narrow GNRs with atomically precise edges. One type of GNRs that has received a considerable attention is the chevron-like GNR with a very distinct periodic structure. Surface-assisted and solution-based synthetic approaches for the chevron-like GNRs have been developed, but their electronic properties have not been reported yet. In this work, we synthesized chevron-like GNRs in bulk by a solution-based method, characterized them by a number of spectroscopic techniques and measured their bulk conductivity. We demonstrate that solution-synthesized chevron-like GNRs are electrically conductive in bulk, which makes them a potentially promising material for applications in organic electronics and photovoltaics.

20.
Nanoscale ; 5(12): 5426-34, 2013 Jun 21.
Artigo em Inglês | MEDLINE | ID: mdl-23661278

RESUMO

The electrical properties of reduced graphene oxide (rGO) have been previously shown to be very sensitive to surface adsorbates, thus making rGO a very promising platform for highly sensitive gas sensors. However, poor selectivity of rGO-based gas sensors remains a major problem for their practical use. In this paper, we address the selectivity problem by employing an array of rGO-based integrated sensors instead of focusing on the performance of a single sensing element. Each rGO-based device in such an array has a unique sensor response due to the irregular structure of rGO films at different levels of organization, ranging from nanoscale to macroscale. The resulting rGO-based gas sensing system could reliably recognize analytes of nearly the same chemical nature. In our experiments rGO-based sensor arrays demonstrated a high selectivity that was sufficient to discriminate between different alcohols, such as methanol, ethanol and isopropanol, at a 100% success rate. We also discuss a possible sensing mechanism that provides the basis for analyte differentiation.


Assuntos
Técnicas Eletroquímicas , Gases/análise , Grafite/química , 2-Propanol/análise , Eletrodos , Etanol/análise , Metanol/análise , Óxidos/química
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