Efficient High-Order Harmonic Generation from the van der Waals Layered Crystal Copper Indium Thiophosphate.

Layered metal thio- and selenophosphates (MTPs) are a family of van der Waals gapped materials that exhibit a multitude of functionalities in terms of magnetic, ferroelectric, and optical properties. Despite the recent progress in terms of understanding the material properties of these compounds, the potential of MTPs as a material class yet needs further scrutiny, especially in terms of nonlinear optical properties. Recent reports of efficient low-order harmonic generation and extremely high third-order nonlinear optical properties in MTPs suggest the potential application of these materials in integrated nanophotonics.

In-Plane Anisotropy in the Layered Topological Insulator TaNiTe Investigated via TEM and Polarized Raman Spectroscopy.

Layered TaMTe (M = Pd, Ni) has emerged as a platform to study 2D topological insulators, which have exotic properties such as spin-momentum locking and the presence of Dirac fermions for use in conventional and quantum-based electronics. In particular, TaNiTe has been shown to have superconductivity under pressure and is predicted to have second-order topology. Despite being an interesting material with fascinating physics, the detailed crystalline and phononic properties of this material are still unknown.

Observation of momentum-dependent charge density wave gap in a layered antiferromagnet [Formula: see text].

Charge density wave (CDW) ordering has been an important topic of study for a long time owing to its connection with other exotic phases such as superconductivity and magnetism. The [Formula: see text] (R = rare-earth elements) family of materials provides a fertile ground to study the dynamics of CDW in van der Waals layered materials, and the presence of magnetism in these materials allows to explore the interplay among CDW and long range magnetic ordering. Here, we have carried out a high-resolution angle-resolved photoemission spectroscopy (ARPES) study of a CDW material [Formula: see text], which is antiferromagnetic below [Formula: see text], along with thermodynamic, electrical transport, magnetic, and Raman measurements.

Rapid Degradation of the Electrical Properties of 2D MoS Thin Films under Long-Term Ambient Exposure.

The MoS thin film has attracted a lot of attention due to its potential applications in flexible electronics, sensors, catalysis, and heterostructures. Understanding the effect of long-term ambient exposure on the electrical properties of the thin film is important for achieving many overreaching goals of this material. Here, we report for the first time a systematic study of electrical property variation and stability of MoS thin films under ambient exposure of up to a year.

Synthesis of highly dense MoO/MoS core-shell nanoparticles via chemical vapor deposition.

Nanostructure morphologies of transition metal dichalcogenides (TMDs) are gaining much interest owing to their catalytic, sensing, and energy storage capabilities. Here, we report the synthesis of highly dense MoO/MoS core-shell nanoparticles, a new form of TMD nanostructure, via chemical vapor deposition using new growth geometry where a thin film of MoO was used as a source substrate for Mo as opposed to using MoO powder used in conventional studies. To grow the MoO/MoS core-shell nanoparticles, we precisely control the carrier gas flow rate and sulfur vapor introduction time with respect to the melting of a MoO thin film used for Mo precursor.

Scalable lateral heterojunction by chemical doping of 2D TMD thin films.

Scalable heterojunctions based on two-dimensional transitional metal dichalcogenides are of great importance for their applications in the next generation of electronic and optoelectronic devices. However, reliable techniques for the fabrication of such heterojunctions are still at its infancy. Here we demonstrate a simple technique for the scalable fabrication of lateral heterojunctions via selective chemical doping of TMD thin films.

Uniform Vapor-Pressure-Based Chemical Vapor Deposition Growth of MoS Using MoO Thin Film as a Precursor for Coevaporation.

Chemical vapor deposition (CVD) is a powerful method employed for high-quality monolayer crystal growth of 2D transition metal dichalcogenides with much effort invested toward improving the growth process. Here, we report a novel method for CVD-based growth of monolayer molybdenum disulfide (MoS) by using thermally evaporated thin films of molybdenum trioxide (MoO) as the molybdenum (Mo) source for coevaporation. Uniform evaporation rate of MoO thin films provides uniform Mo vapors which promote highly reproducible single-crystal growth of MoS throughout the substrate.

Two-dimensional lateral heterojunction through bandgap engineering of MoS2 via oxygen plasma.

The present study explores the structural, optical (photoluminescence (PL)), and electrical properties of lateral heterojunctions fabricated by selective exposure of mechanically exfoliated few layer two-dimensional (2D) molybdenum disulfide (MoS2) flakes under oxygen (O2)-plasma. Raman spectra of the plasma exposed MoS2 flakes show a significant loss in the structural quality due to lattice distortion and creation of oxygen-containing domains in comparison to the pristine part of the same flake. The PL mapping evidences the complete quenching of peak A and B consistent with a change in the exciton states of MoS2 after the plasma treatment, indicating a significant change in its band gap properties.

Centimeter Scale Patterned Growth of Vertically Stacked Few Layer Only 2D MoS2/WS2 van der Waals Heterostructure.

Two-dimensional (2D) van der Waal (vdW) heterostructures composed of vertically-stacked multiple transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) are envisioned to present unprecedented materials properties unobtainable from any other material systems. Conventional fabrications of these hybrid materials have relied on the low-yield manual exfoliation and stacking of individual 2D TMD layers, which remain impractical for scaled-up applications. Attempts to chemically synthesize these materials have been recently pursued, which are presently limited to randomly and scarcely grown 2D layers with uncontrolled layer numbers on very small areas.

Towards parallel fabrication of single electron transistors using carbon nanotubes.

Single electron transistors (SETs) are considered to be promising building blocks for post CMOS era electronic devices, however, a major bottleneck for practical realization of SET based devices is a lack of a parallel fabrication approach. Here, we have demonstrated a technique for the scalable fabrication of SETs using single-walled carbon nanotubes (SWNTs). The approach is based on the integration of solution processed individual SWNTs via dielectrophoresis (DEP) at the selected position of the circuit with a 100 nm channel length, where the metal-SWNT Schottky contact works as a tunnel barrier.

Tuning the electrical property via defect engineering of single layer MoS2 by oxygen plasma.

We have demonstrated that the electrical property of single-layer molybdenum disulfide (MoS2) can be significantly tuned from the semiconducting to the insulating regime via controlled exposure to oxygen plasma. The mobility, on-current and resistance of single-layer MoS2 devices were varied by up to four orders of magnitude by controlling the plasma exposure time. Raman spectroscopy, X-ray photoelectron spectroscopy and density functional theory studies suggest that the significant variation of electronic properties is caused by the creation of insulating MoO3-rich disordered domains in the MoS2 sheet upon oxygen plasma exposure, leading to an exponential variation of resistance and mobility as a function of plasma exposure time.

Photoluminescence quenching in gold - MoS2 hybrid nanoflakes.

Achieving tunability of two dimensional (2D) transition metal dichalcogenides (TMDs) functions calls for the introduction of hybrid 2D materials by means of localized interactions with zero dimensional (0D) materials. A metal-semiconductor interface, as in gold (Au) - molybdenum disulfide (MoS2), is of great interest from the standpoint of fundamental science as it constitutes an outstanding platform to investigate plasmonic-exciton interactions and charge transfer. The applied aspects of such systems introduce new options for electronics, photovoltaics, detectors, gas sensing, catalysis, and biosensing.

High performance semiconducting enriched carbon nanotube thin film transistors using metallic carbon nanotubes as electrodes.

High-performance solution-processed short-channel carbon nanotube (CNT) thin film transistors (TFTs) are fabricated using densely aligned arrays of metallic CNTs (m-CNTs) for the source and drain electrodes, while aligned arrays of semiconducting enriched CNTs (s-CNTs) are used as the channel material. The electrical transport measurements at room temperature show that using the m-CNT as the contact for the s-CNT array devices with a 2 μm channel length performed superior to those where the control Pd was the contact. The m-CNT contact devices exhibited a maximum (average) on-conductance of 36.

Negative differential resistance in ZnO coated peptide nanotube.

We investigate the room temperature electronic transport properties of a zinc oxide (ZnO) coated peptide nanotube contacted with Au electrodes. Current-voltage (- ) characteristics show asymmetric negative differential resistance (NDR) behavior along with current rectification. The NDR phenomenon is observed in both negative and positive voltage sweep scans, and found to be dependent on the scan rate and humidity.

Thermionic emission and tunneling at carbon nanotube-organic semiconductor interface.

We study the charge carrier injection mechanism across the carbon nanotube (CNT)-organic semiconductor interface using a densely aligned carbon nanotube array as electrode and pentacene as organic semiconductor. The current density-voltage (J-V) characteristics measured at different temperatures show a transition from a thermal emission mechanism at high temperature (above 200 K) to a tunneling mechanism at low temperature (below 200 K). A barrier height of ∼0.

A general approach for high yield fabrication of CMOS-compatible all-semiconducting carbon nanotube field effect transistors.

We report strategies to achieve both high assembly yield of carbon nanotubes at selected positions of the circuit via dielectrophoresis (DEP) and field effect transistor (FET) yield using an aqueous solution of semiconducting-enriched single-walled carbon nanotubes (s-SWNTs). When the DEP parameters were optimized for the assembly of individual s-SWNTs, 97% of the devices showed FET behavior with a maximum mobility of 210 cm2 V(-1) s(-1), on-off current ratio ∼10(6) and on-conductance up to 3 µS, but with an assembly yield of only 33%. As the DEP parameters were optimized so that one to five s-SWNTs are connected per electrode pair, the assembly yield was almost 90%, with ∼90% of these assembled devices demonstrating FET behavior.

High yield assembly and electron transport investigation of semiconducting-rich local-gated single-walled carbon nanotube field effect transistors.

We report the fabrication and electron transport investigation of individual local-gated single-walled carbon nanotube field effect transistors (SWNT-FET) with high yield using a semiconducting-rich carbon nanotube solution. The individual semiconducting nanotubes were assembled at the selected position of the circuit via dielectrophoresis. Detailed electron transport investigations on 70 devices show that 99% display good FET behavior, with an average threshold voltage of 1 V, subthreshold swing as low as 140 mV/dec, and on/off current ratio as high as 8 × 10(5).

Semiconducting enriched carbon nanotube aligned arrays of tunable density and their electrical transport properties.

We demonstrate assembly of solution-processed semiconducting enriched (99%) single-walled carbon nanotubes (s-SWNTs) in an array with varying linear density via ac dielectrophoresis (DEP) and investigate detailed electronic transport properties of the fabricated devices. We show that (i) the quality of the alignment varies with frequency of the applied voltage and that (ii) by varying the frequency and concentration of the solution, we can control the linear density of the s-SWNTs in the array from 1/μm to 25/μm. The DEP assembled s-SWNT devices provide the opportunity to investigate the transport property of the arrays in the direct transport regime.

Fabrication of organic field effect transistor by directly grown poly(3 hexylthiophene) crystalline nanowires on carbon nanotube aligned array electrode.

We fabricated organic field effect transistors (OFETs) by directly growing poly (3-hexylthiophne) (P3HT) crystalline nanowires on solution processed aligned array single walled carbon nanotubes (SWNT) interdigitated electrodes by exploiting strong π-π interaction for both efficient charge injection and transport. We also compared the device properties of OFETs using SWNT electrodes with control OFETs of P3HT nanowires deposited on gold electrodes. Electron transport measurements on 28 devices showed that, compared to the OFETs with gold electrodes, the OFETs with SWNT electrodes have better mobility and better current on-off ratio with a maximum of 0.

Ultrahigh density alignment of carbon nanotube arrays by dielectrophoresis.

We report ultrahigh density assembly of aligned single-walled carbon nanotube (SWNT) two-dimensional arrays via AC dielectrophoresis using high-quality surfactant-free and stable SWNT solutions. After optimization of frequency and trapping time, we can reproducibly control the linear density of the SWNT between prefabricated electrodes from 0.5 SWNT/μm to more than 30 SWNT/μm by tuning the concentration of the nanotubes in the solution.

Electrical transport properties of peptide nanotubes coated with gold nanoparticles via peptide-induced biomineralization.

We present temperature dependent electrical transport measurements of peptide nanotube devices coated with monodisperse arrays of gold nanoparticles (AuNP). As the temperature is lowered, the current-voltage (I-V) characteristics become increasingly nonlinear and below 20 K conduction only occurs above a threshold voltage V(T). The current follows the scaling behavior I ∝ [(V − V(T))/V(T)]α for V > V(T) with α ∼ 2.

Ultralight multiwalled carbon nanotube aerogel.

Ultralight multiwalled carbon nanotube (MWCNT) aerogel is fabricated from a wet gel of well-dispersed pristine MWCNTs. On the basis of a theoretical prediction that increasing interaction potential between CNTs lowers their critical concentration to form an infinite percolation network, poly(3-(trimethoxysilyl) propyl methacrylate) (PTMSPMA) is used to disperse and functionalize MWCNTs where the subsequent hydrolysis and condensation of PTMSPMA introduces strong and permanent chemical bonding between MWCNTs. The interaction is both experimentally and theoretically proven to facilitate the formation of a MWCNT percolation network, which leads to the gelation of MWCNT dispersion at ultralow MWCNT concentration.

Controlled electroplating and electromigration in nickel electrodes for nanogap formation.

We report the fabrication of nickel nanospaced electrodes by electroplating and electromigration for nanoelectronic devices. Using a conventional electrochemical cell, nanogaps can be obtained by controlling the plating time alone and after a careful optimization of electrodeposition parameters such as electrolyte bath, applied potential, cleaning, etc. During the process, the gap width decreases exponentially with time until the electrode gaps are completely bridged.

High performance organic phototransistor based on regioregular poly(3-hexylthiophene).

We fabricated solution processed organic phototransistors (OPT) by drop casting regioregular poly 3-hexylthiophene (P3HT) from three different solvents: p-xylene, dichlorobenzene and chloroform. The best performance was obtained for films drop-casted from p-xylene with a maximum photosensitivity (P) of 3.8 x 10(3) and responsivity (R) of 250 A W(-1) under white light illumination.