Rapid Degradation of the Electrical Properties of 2D MoS₂ Thin Films under Long-Term Ambient Exposure

The MoS2 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 MoS2 thin films under ambient exposure of up to a year. The MoS2 thin films were grown via the sulfurization of 6 nm thick molybdenum films. We found that the resistance of the samples increases by 114% just in 4 weeks and 430% in 4 months and they become fully insulated in a year of ambient exposure. The dual-sweep current–voltage (I–V) characteristic shows hysteretic behavior for a 4-month-old sample which further exhibits pronounced nonlinear I–V curves and hysteretic behavior after 8 months. The X-ray photoelectron spectroscopy measurements show that the MoS2 thin film gradually oxidizes and 13.1% of MoO3 and 11.8% oxide of sulfur were formed in 4 months, which further increased to 23.1 and 12.7% in a year, respectively. The oxide of the sulfur peak was not reported in any previous stability studies of exfoliated and chemical vapor deposition-grown MoS2, suggesting that the origin of this peak is related to the distinct crystallinity of the MoS2 thin film due to its smaller grain sizes, abundant grain boundaries, and exposed edges. Raman studies show the broadening of E2g1 and A1g peaks with increasing exposure time, suggesting an increase in the disorder in MoS2. It is also found that coating the MoS2 thin film with polymethylmethacrylate can effectively prevent the electrical property degradation, showing only a 6% increase in resistance in 4 months and 40% over a year of ambient exposure

Structural Evolution of Reduced Graphene Oxide of Varying Carbon sp² Fractions Investigated via Coulomb Blockade Transport

We investigate the structural evolution of reduced graphene oxide (RGO) sheets with carbon sp2 fractions varying from 55 to 80% using low-temperature Coulomb blockade (CB) transport. At 4.2 K, all RGO sheets exhibit a complete suppression of current (CB) below a threshold voltage Vt, the value of which decreased from 3.34 to 0.25 V with increasing carbon sp2 fraction. From the temperature-dependent Vt, we calculate an effective charging energy and individual graphene domain size of 160 meV and 1.34 nm at 55% carbon sp2 fractions, respectively. These values are 20 meV and 4.18 nm at 80% carbon sp2 fractions, respectively. This implies that with increasing reduction, newly formed sp2 domains increase the effective size of the graphene domain. For an applied voltage V > Vt, the current I follows a scaling law I ∼ [(V – Vt)/Vt]α where the scaling parameter α increases from 2.11 to 3.40 with increasing sp2 fraction, suggesting that increasing sp2 fraction creates more topological defects on the RGO. Our report provides a much desired insight into the structural evolution of RGO sheets

Evaluating Defects in Solution-Processed Carbon Nanotube Devices <i>via</i> Low-Temperature Transport Spectroscopy

We performed low-temperature electron transport spectroscopy to evaluate defects in individual single-walled carbon nanotube (SWNT) devices assembled via dielectrophoresis from a surfactant-free solution. At 4.2 K, the majority of the devices show periodic and well-defined Coulomb diamonds near zero gate voltage corresponding to transport through a single quantum dot, while at higher gate voltages, beating behavior is observed due to small potential fluctuations induced by the substrate. The Coulomb diamonds were further modeled using a single electron transistor simulator. Our study suggests that SWNTs derived from stable solutions in this work are free from hard defects and are relatively clean. Our observations have strong implications on the use of solution-processed SWNTs for future nanoelectronic device applications

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 (<i>J–V</i>) 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 ∼0.16 eV is calculated from the thermal emission regime, which is much lower compared to the metal/pentacene devices. At low temperatures, the <i>J–V</i> curves exhibit a direct tunneling mechanism at low bias, corresponding to a trapezoidal barrier, while at high bias the mechanism is well described by Fowler–Nordheim tunneling, which corresponds to a triangular barrier. A transition from direct tunneling to Fowler–Nordheim tunneling further signifies a small injection barrier at the CNT/pentacene interface. Our results presented here are the first direct experimental evidence of low charge carrier injection barrier between CNT electrodes and an organic semiconductor and are a significant step forward in realizing the overall goal of using CNT electrodes in organic electronics

Bandgap Engineering of MoS₂ Flakes via Oxygen Plasma: A Layer Dependent Study

The ability to modify the band structure of a semiconducting material via doping or defect engineering is of significant importance for the development of many novel applications in emerging nanoelectronics. Here, we show that the electronic transport properties of molybdenum disulfide (MoS₂) field effect transistors of various layer thicknesses (up to 8 layers) can be tailored via control exposure to oxygen plasma. We demonstrate that all the samples can be turned into complete insulators with increasing plasma exposure time and that the time required to turn the samples to complete insulators depends on the number of layers (<i>L</i>). We also found that the variation of mobility (μ) with plasma time (<i>t</i>) for all samples can be collapsed onto one curve and that μ follows a relation μ/<i>L</i> ≈ exp­(−φt/L) where φ = μ̇/μ, and μ̇ is the time derivative of μ. X-ray photoelectron spectroscopy data show that MoO₃ defected regions were created by oxygen plasma and that the amount of MoO₃ increases with plasma time. Our study suggest that the energetic oxygens from the plasma not only interacts with the surface atoms but also propagate deep inside the layers to create MoO₃ defects in the MoS₂, the transport properties of which can be described as an effective medium semiconductor with a bandgap higher than MoS₂

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. Our maximum density of 30 SWNT/μm is the highest for aligned arrays via any solution processing technique reported so far. Further increase of SWNT concentration results in a dense array with multiple layers. We discuss how the orientation and density of the nanotubes vary with concentrations and channel lengths. Electrical measurement data show that the densely packed aligned arrays have low sheet resistances. Selective removal of metallic SWNTs via controlled electrical breakdown produced field-effect transistors with high current on−off ratio. Ultrahigh density alignment reported here will have important implications in fabricating high-quality devices for digital and analog electronics

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. Room temperature electron transport measurements of the fabricated devices show that with increasing nanotube density the device mobility increases while the current on–off ratio decreases dramatically. For the dense array, the device current density was 16 μA/μm, on-conductance was 390 μS, and sheet resistance was 30 kΩ/◻. These values are the best reported so far for any semiconducting nanotube array

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 0.13 cm2/(V s) and 3.1 × 105, respectively. The improved device characteristics with SWNT electrodes were also demonstrated by the improved charge injection and the absence of short channel effect, which was dominant in gold electrode OFETs. The enhancement of the device performance can be attributed to the improved interfacial contact between SWNT electrodes and the crystalline P3HT nanowires as well as the improved morphology of P3HT due to one-dimensional crystalline nanowire structure

Controlling Poly(3-hexylthiophene) Crystal Dimension: Nanowhiskers and Nanoribbons

Controlling Poly(3-hexylthiophene) Crystal Dimension: Nanowhiskers and Nanoribbon