Optical Enhancement of Indirect Bandgap 2D Transition Metal Dichalcogenides for Multi-Functional Optoelectronic Sensors.

The unique electrical and optical properties of transition metal dichalcogenides (TMDs) make them attractive nanomaterials for optoelectronic applications, especially optical sensors. However, the optical characteristics of these materials are dependent on the number of layers. Monolayer TMDs have a direct bandgap that provides higher photoresponsivity compared to multilayer TMDs with an indirect bandgap. Nevertheless, multilayer TMDs are more appropriate for various photodetection applications due to their high carrier density, broad spectral response from UV to near-infrared, and ease of large-scale synthesis. Therefore, this review focuses on the modification of the optical properties of devices based on indirect bandgap TMDs and their emerging applications. Several successful developments in optical devices are examined, including band structure engineering, device structure optimization, and heterostructures. Furthermore, it introduces cutting-edge techniques and future directions for optoelectronic devices based on multilayer TMDs.

Se-Vacancy Healing with Substitutional Oxygen in WSe2 for High-Mobility p-Type Field-Effect Transistors.

Transition-metal dichalcogenides possess high carrier mobility and can be scaled to sub-nanometer dimensions, making them viable alternative to Si electronics. WSe2 is capable of hole and electron carrier transport, making it a key component in CMOS logic circuits. However, since the p-type electrical performance of the WSe2-field effect transistor (FET) is still limited, various approaches are being investigated to circumvent this issue. Here, we formed a heterostructural multilayer WSe2 channel and solution-processed aluminum-doped zinc oxide (AZO) for compositional modification of WSe2 to obtain a device with excellent electrical properties. Supplying oxygen anions from AZO to the WSe2 channel eliminated subgap states through Se-deficiency healing, resulting in improved transport capacity. Se vacancies are known to cause mobility degradation due to scattering, which is mitigated through ionic compensation. Consequently, the hole mobility can reach high values, with a maximum of approximately 100 cm2/V s. Further, the transport behavior of the oxygen-doped WSe2-FET is systematically analyzed using density functional theory simulations and photoexcited charge collection spectroscopy measurements.

A Wafer-Scale Nanoporous 2D Active Pixel Image Sensor Matrix with High Uniformity, High Sensitivity, and Rapid Switching.

2D transition-metal dichalcogenides (TMDs) have been successfully developed as novel ubiquitous optoelectronics owing to their excellent electrical and optical characteristics. However, active-matrix image sensors based on TMDs have limitations owing to the difficulty of fabricating large-area integrated circuitry and achieving high optical sensitivity. Herein, a large-area uniform, highly sensitive, and robust image sensor matrix with active pixels consisting of nanoporous molybdenum disulfide (MoS2 ) phototransistors and indium-gallium-zinc oxide (IGZO) switching transistors is reported. Large-area uniform 4-inch wafer-scale bilayer MoS2 films are synthesized by radio-frequency (RF) magnetron sputtering and sulfurization processes and patterned to be a nanoporous structure consisting of an array of periodic nanopores on the MoS2 surface via block copolymer lithography. Edge exposure on the nanoporous bilayer MoS2 induces the formation of subgap states, which promotes a photogating effect to obtain an exceptionally high photoresponsivity of 5.2 × 104 A W-1 . A 4-inch-wafer-scale image mapping is successively achieved using this active-matrix image sensor by controlling the device sensing and switching states. The high-performance active-matrix image sensor is state-of-the-art in 2D material-based integrated circuitry and pixel image sensor applications.

On MoS2 Thin-Film Transistor Design Consideration for a NO2 Gas Sensor.

MoS2 thin-film transistors (TFTs) are fabricated and simulated to explore the NO2 gas sensing mechanism depending on different device structures. In particular, the role of the Al2O3 passivation layer on the MoS2 channel has been investigated. In the case of nonpassivated MoS2 TFTs, increase of off-current is observed with NO2 gas, which has been modeled with the modulation of the effective Schottky barrier height for holes because of the generation of in-gap states near the valence band as NO2 gases interact with the MoS2 channel. The nonpassivated MoS2 TFTs are simulated based on nonequilibrium Green's function method, and the simulation results do confirm this sensing mechanism. On the other hand, MoS2 TFTs with the Al2O3 passivation layer have been modeled with a pseudo-double gate structure as NO2 gases on the capping layer can act like the secondary gate inducing the positive charge state. Our quantum transport simulation shows that the significant threshold voltage shift can be achieved with NO2 gas, which matches the experimental observation, thereby exhibiting a completely different sensing mechanism of the passivated device from the nonpassivated counterpart. In addition, we also discuss competing device parameters for the passivated MoS2 TFTs by varying the main and the secondary gate dielectric, suggesting co-optimization to realize high sensitivity and low power consumption simultaneously.

Interstitial Mo-Assisted Photovoltaic Effect in Multilayer MoSe2 Phototransistors.

Thin-film transistors (TFTs) based on multilayer molybdenum diselenide (MoSe2 ) synthesized by modified atmospheric pressure chemical vapor deposition (APCVD) exhibit outstanding photoresponsivity (103.1 A W-1 ), while it is generally believed that optical response of multilayer transition metal dichalcogenides (TMDs) is significantly limited due to their indirect bandgap and inefficient photoexcitation process. Here, the fundamental origin of such a high photoresponsivity in the synthesized multilayer MoSe2 TFTs is sought. A unique structural characteristic of the APCVD-grown MoSe2 is observed, in which interstitial Mo atoms exist between basal planes, unlike usual 2H phase TMDs. Density functional theory calculations and photoinduced transfer characteristics reveal that such interstitial Mo atoms form photoreactive electronic states in the bandgap. Models indicate that huge photoamplification is attributed to trapped holes in subgap states, resulting in a significant photovoltaic effect. In this study, the fundamental origin of high responsivity with synthetic MoSe2 phototransistors is identified, suggesting a novel route to high-performance, multifunctional 2D material devices for future wearable sensor applications.

Label-Free and Recalibrated Multilayer MoS2 Biosensor for Point-of-Care Diagnostics.

Molybdenum disulfide (MoS2) field-effect transistor (FET)-based biosensors have attracted significant attention as promising candidates for highly sensitive, label-free biomolecule detection devices. In this paper, toward practical applications of biosensors, we demonstrate reliable and quantitative detection of a prostate cancer biomarker using the MoS2 FET biosensor in a nonaqueous environment by reducing nonspecific molecular binding events and realizing uniform chemisorption of anti-PSA onto the MoS2 surface. A systematic and statistical study on the capability of the proposed device is presented, and the biological binding events are directly confirmed and characterized through intensive structural and electrical analysis. Our proposed biosensor can reliably detect various PSA concentrations with a limit of 100 fg/mL. Moreover, rigorous theoretical simulations provide a comprehensive understanding of the operating mechanism of the MoS2 FET biosensors, and further suggests the enhancement of the sensitivity through engineering device design parameters.

Highly Crystalline CVD-grown Multilayer MoSe2 Thin Film Transistor for Fast Photodetector.

Hexagonal molybdenum diselenide (MoSe2) multilayers were grown by chemical vapor deposition (CVD). A relatively high pressure (>760 Torr) was used during the CVD growth to achieve multilayers by creating multiple nuclei based on the two-dimensional crystal growth model. Our CVD-grown multilayer MoSe2 thin-film transistors (TFTs) show p-type-dominant ambipolar behaviors, which are attributed to the formation of Se vacancies generated at the decomposition temperature (650 °C) after the CVD growth for 10 min. Our MoSe2 TFT with a reasonably high field-effect mobility (10 cm(2)/V · s) exhibits a high photoresponsivity (93.7 A/W) and a fast photoresponse time (τ(rise) ~ 0.4 s) under the illumination of light, which demonstrates the practical feasibility of multilayer MoSe2 TFTs for photodetector applications.

Giant photoamplification in indirect-bandgap multilayer MoS2 phototransistors with local bottom-gate structures.

Local-gate multilayer MoS2 phototransistors exhibit a photoresponsivity of up to 342.6 A W(-1) , which is higher by 3 orders of magnitude than that of global-gate multilayer MoS2 phototransistors. These simulations indicate that the gate underlap is critical for the enhancement of the photoresponsivity. These results suggest that high photoresponsivity can be achieved in indirect-bandgap multilayer MoS2 phototransistors by optimizing the optoelectronic design.

How good can monolayer MoS2 transistors be?

Monolayer molybdenum disulfide (MoS(2)), unlike its bulk form, is a direct band gap semiconductor with a band gap of 1.8 eV. Recently, field-effect transistors have been demonstrated experimentally using a mechanically exfoliated MoS(2) monolayer, showing promising potential for next generation electronics. Here we project the ultimate performance limit of MoS(2) transistors by using nonequilibrium Green's function based quantum transport simulations. Our simulation results show that the strength of MoS(2) transistors lies in large ON-OFF current ratio (>10(10)), immunity to short channel effects (drain-induced barrier lowering ∼10 mV/V), and abrupt switching (subthreshold swing as low as 60 mV/decade). Our comparison of monolayer MoS(2) transistors to the state-of-the-art III-V materials based transistors, reveals that while MoS(2) transistors may not be ideal for high-performance applications due to heavier electron effective mass (m = 0.45 m(0)) and a lower mobility, they can be an attractive alternative for low power applications thanks to the large band gap and the excellent electrostatic integrity inherent in a two-dimensional system.

N-doping of graphene through electrothermal reactions with ammonia.

Graphene is readily p-doped by adsorbates, but for device applications, it would be useful to access the n-doped material. Individual graphene nanoribbons were covalently functionalized by nitrogen species through high-power electrical joule heating in ammonia gas, leading to n-type electronic doping consistent with theory. The formation of the carbon-nitrogen bond should occur mostly at the edges of graphene where chemical reactivity is high. X-ray photoelectron spectroscopy and nanometer-scale secondary ion mass spectroscopy confirm the carbon-nitrogen species in graphene thermally annealed in ammonia. We fabricated an n-type graphene field-effect transistor that operates at room temperature.

Gate electrostatics and quantum capacitance of graphene nanoribbons.

Capacitance-voltage (C-V) characteristics are important for understanding fundamental electronic structures and device applications of nanomaterials. The C-V characteristics of graphene nanoribbons (GNRs) are examined using self-consistent atomistic simulations. The results indicate strong dependence of the GNR C-V characteristics on the edge shape. For zigzag edge GNRs, highly nonuniform charge distribution in the transverse direction due to edge states lowers the gate capacitance considerably, and the self-consistent electrostatic potential significantly alters the band structure and carrier velocity. For an armchair edge GNR, the quantum capacitance is a factor of 2 smaller than its corresponding zigzag carbon nanotube, and a multiple gate geometry is less beneficial for transistor applications. Magnetic field results in pronounced oscillations on C-V characteristics.