Mechanical Stress Modulation of Resistance in MoS2 Junctions.

Strain engineering is a powerful strategy to control the physical properties of material-enabling devices with enhanced functionality and improved performance. Here, we investigate a modulation of the transport behavior of the two-dimensional MoS2 junctions under the mechanical stress induced by a tip of an atomic force microscope (AFM). We show that the junction resistance can be reversibly tuned by up to 4 orders of magnitude by altering a tip-induced force. Analysis of the stress-induced evolution of the I-V characteristics indicates a combined effect of the tip-induced strain and strain gradient on the energy barrier height and profile. In addition, we show that the tip-generated flexoelectric effect leads to significant enhancement of the photovoltaic effect in the MoS2 junctions. A combination of the optical and mechanical stimuli facilitates reversible photomechanical tuning of resistance of the narrow-band 2D semiconductors and development of devices with an enhanced photovoltaic response.

Effect of Au/HfS3 interfacial interactions on properties of HfS3-based devices.

X-ray photoemission spectroscopy (XPS) has been used to examine the interaction between Au and HfS3 at the Au/HfS3 interface. XPS measurements reveal dissociative chemisorption of O2, leading to the formation of an oxide of Hf at the surface of HfS3. This surface hafnium oxide, along with the weakly chemisorbed molecular species, such as O2 and H2O, are likely responsible for the observed p-type characteristics of HfS3 reported elsewhere. HfS3 devices exhibit n-type behaviour if measured in vacuum but turn p-type in air. Au thickness-dependent XPS measurements provide clear evidence of band bending as the S 2p and Hf 4f core-level peak binding energies for Au/HfS3 are found to be shifted to higher binding energies. This band bending implies formation of a Schottky-barrier at the Au/HfS3 interface, which explains the low measured charge carrier mobilities of HfS3-based devices. The transistor measurements presented herein also indicate the existence of a Schottky barrier, consistent with the XPS core-level binding energy shifts, and show that the bulk of HfS3 is n-type.

UV-Light-Tunable p-/n-Type Chemiresistive Gas Sensors Based on Quasi-1D TiS3 Nanoribbons: Detection of Isopropanol at ppm Concentrations.

The growing demand of society for gas sensors for energy-efficient environmental sensing stimulates studies of new electronic materials. Here, we investigated quasi-one-dimensional titanium trisulfide (TiS3) crystals for possible applications in chemiresistors and on-chip multisensor arrays. TiS3 nanoribbons were placed as a mat over a multielectrode chip to form an array of chemiresistive gas sensors. These sensors were exposed to isopropanol as a model analyte, which was mixed with air at low concentrations of 1-100 ppm that are below the Occupational Safety and Health Administration (OSHA) permissible exposure limit. The tests were performed at room temperature (RT), as well as with heating up to 110 °C, and under an ultraviolet (UV) radiation at λ = 345 nm. We found that the RT/UV conditions result in a n-type chemiresistive response to isopropanol, which seems to be governed by its redox reactions with chemisorbed oxygen species. In contrast, the RT conditions without a UV exposure produced a p-type response that is possibly caused by the enhancement of the electron transport scattering due to the analyte adsorption. By analyzing the vector signal from the entire on-chip multisensor array, we could distinguish isopropanol from benzene, both of which produced similar responses on individual sensors. We found that the heating up to 110 °C reduces both the sensitivity and selectivity of the sensor array.

Nanodomain Engineering for Programmable Ferroelectric Devices.

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.

Chemical vapor deposition and characterization of two-dimensional molybdenum dioxide (MoO2) nanoplatelets.

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.

Polarization-Mediated Modulation of Electronic and Transport Properties of Hybrid MoS2-BaTiO3-SrRuO3 Tunnel Junctions.

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.

Imaging and Analysis of Encapsulated Objects through Self-Assembled Electron and Optically Transparent Graphene Oxide Membranes.

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.

Nanodomain Engineering in Ferroelectric Capacitors with Graphene Electrodes.

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.

Graphene substrate for inducing neurite outgrowth.

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.

Interlayer Incorporation of A-Elements into MXenes Via Selective Etching of A' from Mn+1A'1-xA″xCn MAX Phases.

MXenes are a large family of two-dimensional materials with a general formula Mn+1XnTz, where M is a transition metal, X = C and/or N, and Tz represents surface functional groups. MXenes are synthesized by etching A-elements from layered MAX phases with a composition of Mn+1AXn. As over 20 different chemical elements were shown to form A-layers in various MAX phases, we propose that they can provide an abundant source of very diverse MXene-based materials. The general strategy for A-modified MXenes relies on the synthesis of Mn+1A'1-xA″xXn MAX phase, in which the higher reactivity of the A'-element compared to that of A″ enables its selective etching, resulting in A″-modified Mn+1XnTz. In general, the A″-element could modify the interlayer spaces of MXene flakes in a form of metallic or oxide species, depending on its chemical identity and synthetic conditions. We demonstrate this strategy by synthesizing Sn-modified Ti3C2Tz MXene from the Ti3Al0.75Sn0.25C2 MAX phase, which was used as a model system. Although the incorporation of Sn in the A-layer of Ti3AlC2 decreases the MAX phase reactivity, we developed an etching procedure to completely remove Al and produce Sn-modified Ti3C2Tz MXene. The resulting MXene sheets were of very high quality and exhibited improved environmental stability, which we attribute to the effect of a uniform Sn modification. Finally, we demonstrate a peculiar electrostatic expansion of Sn-modified Ti3C2Tz accordions, which may find interesting applications in MXene-based nano-electromechanical systems. Overall, these results demonstrate that in addition to different combinations of M and X elements in MAX phases, an A-layer also provides opportunities for the synthesis of MXene-based materials.

Chiral photocurrent in a Quasi-1D TiS3(001) phototransistor.

The presence of in-plane chiral effects, hence spin-orbit coupling, is evident in the changes in the photocurrent produced in a TiS3(001) field-effect phototransistor with left versus right circularly polarized light. The direction of the photocurrent is protected by the presence of strong spin-orbit coupling and the anisotropy of the band structure as indicated in NanoARPES measurements. Dark electronic transport measurements indicate that TiS3is n-type and has an electron mobility in the range of 1-6 cm2V-1s-1.I-Vmeasurements under laser illumination indicate the photocurrent exhibits a bias directionality dependence, reminiscent of bipolar spin diode behavior. Because the TiS3contains no heavy elements, the presence of spin-orbit coupling must be attributed to the observed loss of inversion symmetry at the TiS3(001) surface.

Complexities at the Au/ZrS3(001) interface probed by x-ray photoemission spectroscopy.

Interaction between the Au adlayers and ZrS3(001) has been examined via x-ray photoemission spectroscopy (XPS). The angle-resolved XPS measurements reveal that ZrS3(001) is disulfide (S22-) terminated and the Au thickness-dependent XPS indicates that the observed band bending, for low Au coverage, is consistent with formation of a Schottky barrier at the Au/ZrS3(001) interface. This band bending, however, appears to be suppressed as the thickness of Au adlayer is increased, indicating varying interfacial interactions at the Au/ZrS3(001) interface. Such complex interface effects between Au and ZrS3(001) may explain the observed non-ohmicI-Vcharacteristics for a ZrS3-based device, and could suppress current injection.

Printed Electronic Devices with Inks of TiS3 Quasi-One-Dimensional van der Waals Material.

We report on the fabrication and characterization of electronic devices printed with inks of quasi-one-dimensional (1D) van der Waals materials. The quasi-1D van der Waals materials are characterized by 1D motifs in their crystal structure, which allow for their exfoliation into bundles of atomic chains. The ink was prepared by the liquid-phase exfoliation of crystals of TiS3 into quasi-1D nanoribbons dispersed in a mixture of ethanol and ethylene glycol. The temperature-dependent electrical measurements indicate that the electron transport in the printed devices is dominated by the electron hopping mechanisms. The low-frequency electronic noise in the printed devices is of 1/fγ-type with γ ∼ 1 near-room temperature (f is the frequency). The abrupt changes in the temperature dependence of the noise spectral density and γ parameter can be indicative of the phase transition in individual TiS3 nanoribbons as well as modifications in the hopping transport regime. The obtained results attest to the potential of quasi-1D van der Waals materials for applications in printed electronics.

The electronic band structure of quasi-one-dimensional van der Waals semiconductors: the effective hole mass of ZrS3 compared to TiS3.

The band structure of the quasi-one-dimensional transition metal trichalcogenide ZrS3(001) was investigated using nanospot angle resolved photoemission spectroscopy (nanoARPES) and shown to have many similarities with the band structure of TiS3(001). We find that ZrS3, like TiS3, is strongly n-type with the top of the valence band ∼1.9 eV below the Fermi level, at the center of the surface Brillouin zone. The nanoARPES spectra indicate that the top of the valence band of the ZrS3(001) is located at [Formula: see text]. The band structure of both TiS3 and ZrS3 exhibit strong in-plane anisotropy, which results in a larger hole effective mass along the quasi-one-dimensional chains than perpendicular to them.

Highly Selective Gas Sensors Based on Graphene Nanoribbons Grown by Chemical Vapor Deposition.

Despite the recent advances in bottom-up synthesis of different kinds of atomically precise graphene nanoribbons (GNRs) with very diverse physical properties, the translation of these GNRs into electronic devices remains challenging. Among other factors, the electronic characterization of GNRs is hampered by their complex synthesis that often requires custom-made organic precursors and the need for their transfer to dielectric substrates compatible with the conventional device fabrication procedures. In this paper, we demonstrate that uniform electrically conductive GNR films can be grown on arbitrary high-temperature-resistant substrates, such as metals, Si/SiO2, or silica glasses, by a simple chemical vapor deposition (CVD) approach based on thermal decomposition of commercially available perylenetetracarboxylic dianhydride molecules. The results of spectroscopic and microscopic characterization of the CVD-grown films were consistent with the formation of oxygen-terminated N = 5 armchair GNRs. The CVD-grown nanoribbon films exhibited an ambipolar electric field effect and low on-off ratios, which were in agreement with the predicted metallic properties of N = 5 armchair GNRs, and remarkable gas sensing properties to a variety of volatile organic compounds (VOCs). We fabricated a GNR-based electronic nose system that could reliably recognize VOCs from different chemical classes including alcohols (methanol, ethanol, and isopropanol) and amines (n-butylamine, diethylamine, and triethylamine). The simplicity of the described CVD approach and its compatibility with the conventional device fabrication procedures, as well as the demonstrated sensitivity of the GNR devices to a variety of VOCs, warrant further investigation of CVD-grown nanoribbons for sensing applications.

Effect of Band Symmetry on Photocurrent Production in Quasi-One-Dimensional Transition-Metal Trichalcogenides.

Photocurrent production in quasi-one-dimensional (1D) transition-metal trichalcogenides, TiS3(001) and ZrS3(001), was examined using polarization-dependent scanning photocurrent microscopy. The photocurrent intensity was the strongest when the excitation source was polarized along the 1D chains with dichroic ratios of 4:1 and 1.2:1 for ZrS3 and TiS3, respectively. This behavior is explained by symmetry selection rules applicable to both valence and conduction band states. Symmetry selection rules are seen to be applicable to the experimental band structure, as is observed in polarization-dependent nanospot angle-resolved photoemission spectroscopy. Based on these band symmetry assignments, it is expected that the dichroic ratios for both materials will be maximized using excitation energies within 1 eV of their band gaps, providing versatile polarization sensitive photodetection across the visible spectrum and into the near-infrared.

Gate-Controlled Metal-Insulator Transition in TiS3 Nanowire Field-Effect Transistors.

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.

Elastic properties of 2D Ti3C2T x MXene monolayers and bilayers.

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.

Optical control of polarization in ferroelectric heterostructures.

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.

Quasi-1D TiS3 Nanoribbons: Mechanical Exfoliation and Thickness-Dependent Raman Spectroscopy.

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.