Electrical transport and persistent photoconductivity in monolayer MoS2 phototransistors

Efficient passivation of monolayer MoS2 by epitaxially grown 2D organic crystals

Monolayer molybdenum disulfide (MoS₂) is considered to be a promising candidate for field-effect transistors and photodetectors due to its direct bandgap and atomically thin properties. However, the MoS₂ devices are impeded by the intrinsic surface defects and environmental adsorption such as H₂O and O₂. Here, we demonstrated a highly ordered, ultrathin (<5 nm) and scalable N,N'-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13) passivation layer that can be epitaxially grown on MoS₂. The van der Waals interface between PTCDI-C13 and MoS₂ can efficiently reduce the surface traps and isolate MoS₂ from ambient. As a result, the passivated devices exhibit huge improvement in both carrier mobility (from 0.5 to 8.3 cm²/(V s)) and sub-threshold swing (from 16.7 to 1.6 V/dec). Also, the photodetector made on MoS₂ after passivation has a much faster response speed (from 3 s to 10 ms) without significant sacrifice of the responsivity. Our method provides a facile approach to realize high-performance two-dimensional electronic and optoelectronic devices.

Accurate Method To Determine the Mobility of Transition-Metal Dichalcogenides with Incomplete Gate Screening

van der Waals layered transition-metal dichalcogenides usually exhibit high contact resistance because of the induced Schottky barriers, which occur at nonideal metal-semiconductor contacts. These barriers usually contribute to an underestimation in the determination of mobility, when extracted by standard, two-terminal methods. Furthermore, in devices based on atomically thin materials, channels with thicknesses of up to a few layers cannot completely screen the applied gate bias, resulting in an incomplete potential drop over the channel; the resulting decreased field effect causes further underestimation of the mobility. We demonstrate a method based on Kelvin probe force microscopy, which allows us to extract the accurate semiconductor mobility and eliminates the effects of contact quality and/or screening ability. Our results reveal up to a sevenfold increase in mobility in a monolayer device.

Potential modulations in flatland: near-infrared sensitization of MoS2 phototransistors by a solvatochromic dye directly tethered to sulfur vacancies

Near-infrared sensitization of monolayer MoS₂ is here achieved via the covalent attachment of a novel heteroleptic nickel bis-dithiolene complex into sulfur vacancies in the MoS₂ structure. Photocurrent action spectroscopy of the sensitized films reveals a discreet contribution from the sensitizer dye centred around 1300 nm (0.95 eV), well below the bandgap of MoS₂ (2.1 eV), corresponding to the excitation of the monoanionic dithiolene complex. A mechanism of conductivity enhancement is proposed based on a photo-induced flattening of the corrugated energy landscape present at sulfur vacancy defect sites within the MoS₂ due to a dipole change within the dye molecule upon photoexcitation. This method of sensitization might be readily extended to other functional molecules that can impart a change to the dielectric environment at the MoS₂ surface under stimulation, thereby extending the breadth of detector applications for MoS₂ and other transition metal dichalcogenides.

Intrinsic Optoelectronic Characteristics of MoS2 Phototransistors via a Fully Transparent van der Waals Heterostructure

In the past decade, intensive studies on monolayer MoS₂-based phototransistors have been carried out to achieve further enhanced optoelectronic characteristics. However, the intrinsic optoelectronic characteristics of monolayer MoS₂ have still not been explored until now because of unintended interferences, such as multiple reflections of incident light originating from commonly used opaque substrates. This leads to overestimated photoresponsive characteristics inevitably due to the enhanced photogating and photoconductive effects. Here, we reveal the intrinsic photoresponsive characteristics of monolayer MoS₂, including its internal responsivity and quantum efficiency, in fully transparent monolayer MoS₂ phototransistors employing a van der Waals heterostructure. Interestingly, as opposed to the previous reports, the internal photoresponsive characteristics do not significantly depend on the wavelength of the incident light as long as the electron-hole pairs are generated in the same k-space. This study provides a deeper understanding of the photoresponsive characteristics of MoS₂ and lays the foundation for two-dimensional materials-based transparent phototransistors.

Probing the Field-Effect Transistor with Monolayer MoS2 Prepared by APCVD

The two-dimensional materials can be used as the channel material of transistor, which can further decrease the size of transistor. In this paper, the molybdenum disulfide (MoS₂) is grown on the SiO₂/Si substrate by atmospheric pressure chemical vapor deposition (APCVD), and the MoS₂ is systematically characterized by the high-resolution optical microscopy, Raman spectroscopy, photoluminescence spectroscopy, and the field emission scanning electron microscopy, which can confirm that the MoS₂ is a monolayer. Then, the monolayer MoS₂ is selected as the channel material to complete the fabrication process of the back-gate field effect transistor (FET). Finally, the electrical characteristics of the monolayer MoS₂-based FET are tested to obtain the electrical performance. The switching ratio is 10³, the field effect mobility is about 0.86 cm²/Vs, the saturation current is 2.75 × 10^-7 A/μm, and the lowest gate leakage current is 10^-12 A. Besides, the monolayer MoS₂ can form the ohmic contact with the Ti/Au metal electrode. Therefore, the electrical performances of monolayer MoS₂-based FET are relatively poor, which requires the further optimization of the monolayer MoS₂ growth process. Meanwhile, it can provide the guidance for the application of monolayer MoS₂-based FETs in the future low-power optoelectronic integrated circuits.

Ambient effects on photogating in MoS2 photodetectors

Atomically thin transition metal dichalcogenides (TMDs) are ideal candidates for ultrathin optoelectronics that are flexible and semitransparent. Photodetectors based on TMDs show remarkable performance, with responsivity and detectivity higher than 10³ AW^-1 and 10¹² Jones, respectively, but they are plagued by response times as slow as several tens of seconds. Although it is well established that gas adsorbates such as water and oxygen create charge traps and significantly increase both the responsivity and the response time, the underlying mechanism is still unclear. Here we study the influence of adsorbates on MoS₂ photodetectors under ambient conditions, vacuum and illumination at different wavelengths. We show that, for wavelengths sufficiently short to excite electron-hole pairs in the MoS₂, light illumination causes desorption of water and oxygen molecules. The change in the molecular gating provided by the physisorbed molecules is the dominant contribution to the device photoresponse in ambient conditions.

CuO/ZnO Heterojunction Nanorod Arrays Prepared by Photochemical Method with Improved UV Detecting Performance

CuO/ZnO heterojunction nanorod arrays were synthesized using a facile photochemical deposition strategy. The morphology of CuO was related to the concentration of Cu²⁺ in the Cu(NO₃)₂ solution, UV illumination time, and the air annealing temperature. A possible reaction mechanism was proposed. In the photochemical deposition process, the OH⁻ was generated in the vicinity of the ZnO nanorod arrays and reacted with Cu²⁺ and NO₃⁻ in the solution to form Cu₂(NO₃)(OH)₃/ZnO heterojunction nanorod arrays firstly, which were converted into CuO/ZnO heterojunction nanorod arrays completely after air annealing at a low temperature. The fabricated CuO/ZnO heterojunction nanorod arrays exhibits a well-defined rectifying characteristic and an improved photo-response performance compared with pure ZnO nanorod arrays.

Field Emission Characterization of MoS2 Nanoflowers

Nanostructured materials have wide potential applicability as field emitters due to their high aspect ratio. We hydrothermally synthesized MoS₂ nanoflowers on copper foil and characterized their field emission properties, by applying a tip-anode configuration in which a tungsten tip with curvature radius down to 30-100 nm has been used as the anode to measure local properties from small areas down to 1-100 µm². We demonstrate that MoS₂ nanoflowers can be competitive with other well-established field emitters. Indeed, we show that a stable field emission current can be measured with a turn-on field as low as 12 V/μm and a field enhancement factor up to 880 at 0.6 μm cathode-anode separation distance.

High Optical Response of Niobium-Doped WSe₂-Layered Crystals

The optical properties of WSe₂-layered crystals doped with 0.5% niobium (Nb) grown by the chemical vapor transport method were characterized by piezoreflectance (PzR), photoconductivity (PC) spectroscopy, frequency-dependent photocurrent, and time-resolved photoresponse. With the incorporation of 0.5% Nb, the WSe₂ crystal showed slight blue shifts in the near band edge excitonic transitions and exhibited strongly enhanced photoresponsivity. Frequency-dependent photocurrent and time-resolved photoresponse were measured to explore the kinetic decay processes of carriers. Our results show the potential application of layered crystals for photodetection devices based on Nb-doped WSe₂-layered crystals.

Sub-10 nm Monolayer MoS2 Transistors Using Single-Walled Carbon Nanotubes as an Evaporating Mask

Transition-metal dichalcogenides are promising challengers to conventional semiconductors owing to their remarkable electrical performance and suppression of short-channel effects (SCEs). In particular, monolayer molybdenum disulfide has exhibited superior suppression of SCEs owing to its atomic thickness, high effective carrier mass, and low dielectric constant. However, difficulties still remain in large-scale stable fabrication of nanometer-scale channels. Herein, a method to fabricate electrodes with sub-10 nm gaps was demonstrated using horizontally aligned single-walled carbon nanotubes as an evaporation mask. The widths of the nanogaps exhibit robust stability to various process parameters according to the statistical results. Based on these nanogaps, ultrashort-channel length monolayer MoS₂ field-effect transistors were produced. Monolayer MoS₂ devices with a 7.5 nm channel length and a 10 nm thick HfO₂ dielectric layer exhibited excellent performances with an ON/OFF ratio up to 10⁷, a mobility of 17.4 cm²/V·s, a subthreshold swing of about 120 mV/dec, and a drain-induced barrier lowering of about 140 mV/V, all of which suggest a superior suppression of SCEs. This work provides a universal and stable method for large-scale fabrication of ultrashort-channel 2D-material transistors.

Surface functionalization-induced photoresponse characteristics of monolayer MoS2 for fast flexible photodetectors

Monolayered, semiconducting molybdenum disulfide (MoS2) is of considerable interest for its potential applications in next-generation flexible, wearable, and transparent photodetectors because it has outstanding physical properties coupled with unique atomically thin dimensions. However, there is still a lack of understanding in terms of the underlying mechanisms responsible for the photoresponse dynamics, which makes it difficult to identify the appropriate device design strategy for achieving a fast photoresponse time in MoS2 photodetectors. In this study, we investigate the importance of surface functionalization on controlling the charge carrier densities in a MoS2 monolayer and in turn the corresponding behavior of the photoresponse in relation to the position of the Fermi-level and the energy band structure. We find that the p-doping and n-doping, which is achieved through the surface functionalization of the MoS2 monolayer, leads to devices with different photoresponse behavior. Specifically, the MoS2 devices with surface functional groups contributing to p-doping exhibited a faster response time as well as higher sensitivity compared to that observed for the MoS2 devices with surface functional groups contributing to n-doping. We attribute this difference to the degree of bending in the energy bands at the metal-semiconductor junction as a result of shifting in the Fermi-level position, which influences the optoelectronic transport properties as well as the recombination dynamics leading to a low dark and thus high detectivity and fast decay time. Based upon these findings, we have also demonstrated the broad applicability of surface functionalization by fabricating a flexible MoS2 photodetector that shows an outstanding decay time of 0.7 s, which is the fastest response time observed in flexible MoS2 detectors ever reported.

A WSe2 vertical field emission transistor

We report the first observation of a gate-controlled field emission current from a tungsten diselenide (WSe2) monolayer, synthesized by chemical-vapour deposition on a SiO2/Si substrate. Ni contacted WSe2 monolayer back-gated transistors, under high vacuum, exhibit n-type conduction and drain-bias dependent transfer characteristics, which are attributed to oxygen/water desorption and drain induced Schottky barrier lowering, respectively. The gate-tuned n-type conduction enables field emission, i.e. the extraction of electrons by quantum tunnelling, even from the flat part of the WSe2 monolayers. Electron emission occurs under an electric field ∼100 V μm-1 and exhibits good time stability. Remarkably, the field emission current can be modulated by the back-gate voltage. The first field-emission vertical transistor based on the WSe2 monolayer is thus demonstrated and can pave the way to further optimize new WSe2 based devices for use in vacuum electronics.

2D electric-double-layer phototransistor for photoelectronic and spatiotemporal hybrid neuromorphic integration

The hardware implementation of neuromorphic computing has attracted growing interest as a promising candidate for confronting the bottleneck of traditional von Neumann computers. However, most previous reports are focusd on emulating the synaptic behaviors by a mono-mode using an electric-driving or photo-driving approach, resulting in a big challenge to synchronously handle the natural photoelectric information. Herein, we report a multifunctional photoelectronic hybrid-integrated synaptic device based on the electric-double-layer (EDL) MoS2 phototransistor. Interestingly, the electric MoS2 synapse exhibits a potentiation filtering effect, while the photonic counterpart can implement both potentiation and depression filtering effects. Most importantly, for the first time, photoelectronic and spatio-temporal four-dimensional (4D) hybrid integration was successfully demonstrated by the synergic interplay between photonic and electric stimuli within a single MoS2 synapse. An energy band model is proposed to further understand such a photoelectronic and spatio-temporal 4D hybrid coupling mechanism. These results might provide an alternative solution for the size-scaling and intellectualization campaign of the post-Moore era, and for more sophisticated photoelectronic hybrid computing in the emerging neuromorphic nanoelectronics.

Environmental Effects on the Electrical Characteristics of Back-Gated WSe₂ Field-Effect Transistors

We study the effect of polymer coating, pressure, temperature, and light on the electrical characteristics of monolayer WSe 2 back-gated transistors with Ni / Au contacts. Our investigation shows that the removal of a layer of poly(methyl methacrylate) (PMMA) or a decrease of the pressure change the device conductivity from p- to n-type. From the temperature behavior of the transistor transfer characteristics, a gate-tunable Schottky barrier at the contacts is demonstrated and a barrier height of ~ 70 meV in the flat-band condition is measured. We also report and discuss a temperature-driven change in the mobility and the subthreshold swing that is used to estimate the trap density at the WSe 2 / SiO 2 interface. Finally, from studying the spectral photoresponse of the WSe 2 , it is proven that the device can be used as a photodetector with a responsivity of ~ 0.5   AW - 1 at 700   nm and 0.37   mW / cm 2 optical power.

Molecular chemistry approaches for tuning the properties of two-dimensional transition metal dichalcogenides

Two-dimensional (2D) semiconductors, such as ultrathin layers of transition metal dichalcogenides (TMDs), offer a unique combination of electronic, optical and mechanical properties, and hold potential to enable a host of new device applications spanning from flexible/wearable (opto)electronics to energy-harvesting and sensing technologies. A critical requirement for developing practical and reliable electronic devices based on semiconducting TMDs consists in achieving a full control over their charge-carrier polarity and doping. Inconveniently, such a challenging task cannot be accomplished by means of well-established doping techniques (e.g. ion implantation and diffusion), which unavoidably damage the 2D crystals resulting in degraded device performances. Nowadays, a number of alternatives are being investigated, including various (supra)molecular chemistry approaches relying on the combination of 2D semiconductors with electroactive donor/acceptor molecules. As yet, a large variety of molecular systems have been utilized for functionalizing 2D TMDs via both covalent and non-covalent interactions. Such research endeavours enabled not only the tuning of the charge-carrier doping but also the engineering of the optical, electronic, magnetic, thermal and sensing properties of semiconducting TMDs for specific device applications. Here, we will review the most enlightening recent advancements in experimental (supra)molecular chemistry methods for tailoring the properties of atomically-thin TMDs - in the form of substrate-supported or solution-dispersed nanosheets - and we will discuss the opportunities and the challenges towards the realization of novel hybrid materials and devices based on 2D semiconductors and molecular systems.

Highly sensitive MoS2 photodetectors with graphene contacts

Two-dimensional materials such as graphene and transition metal dichalcogenides (TMDs) are ideal candidates to create ultra-thin electronics suitable for flexible substrates. Although optoelectronic devices based on TMDs have demonstrated remarkable performance, scalability is still a significant issue. Most devices are created using techniques that are not suitable for mass production, such as mechanical exfoliation of monolayer flakes and patterning by electron-beam lithography. Here we show that large-area MoS₂ grown by chemical vapor deposition and patterned by photolithography yields highly sensitive photodetectors, with record shot-noise-limited detectivities of 8.7 × 10¹⁴ Jones in ambient condition and even higher when sealed with a protective layer. These detectivity values are higher than the highest values reported for photodetectors based on exfoliated MoS₂. We study MoS₂ devices with gold electrodes and graphene electrodes. The devices with graphene electrodes have a tunable band alignment and are especially attractive for scalable ultra-thin flexible optoelectronics.

Van der Waals Coupled Organic Molecules with Monolayer MoS2 for Fast Response Photodetectors with Gate-Tunable Responsivity

As a direct-band-gap transition metal dichalcogenide (TMD), atomic thin MoS₂ has attracted extensive attention in photodetection, whereas the hitherto unsolved persistent photoconductance (PPC) from the ungoverned charge trapping in devices has severely hindered their employment. Herein, we demonstrate the realization of ultrafast photoresponse dynamics in monolayer MoS₂ by exploiting a charge transfer interface based on surface-assembled zinc phthalocyanine (ZnPc) molecules. The formed MoS₂/ZnPc van der Waals interface is found to favorably suppress the PPC phenomenon in MoS₂ by instantly separating photogenerated holes toward the ZnPc molecules, away from the traps in MoS₂ and the dielectric interface. The derived MoS₂ detector then exhibits significantly improved photoresponse speed by more than 3 orders (from over 20 s to less than 8 ms for the decay) and a high responsivity of 430 A/W after Al₂O₃ passivation. It is also demonstrated that the device could be further tailored to be 2-10-fold more sensitive without severely sacrificing the ultrafast response dynamics using gate modulation. The strategy presented here based on surface-assembled organic molecules may thus pave the way for realizing high-performance TMD-based photodetection with ultrafast speed and high sensitivity.

Photonic Potentiation and Electric Habituation in Ultrathin Memristive Synapses Based on Monolayer MoS2

Monolayer of 2D transition metal dichalcogenides, with a thickness of less than 1 nm, paves a feasible path to the development of ultrathin memristive synapses, to fulfill the requirements for constructing large-scale high density 3D stacking neuromorphic chips. Herein, memristive devices based on monolayer n-MoS₂ on p-Si substrate with a large self-rectification ratio, exhibiting photonic potentiation and electric habituation, are successfully fabricated. Versatile synaptic neuromorphic functions, such as potentiation/habituation, short-term/long-term plasticity, and paired-pulse facilitation, are successfully mimicked based on the inherent persistent photoconductivity performance and the volatile resistive switching behavior. These findings demonstrate the potential applications of ultrathin transition metal dichalcogenides for memristive synapses. These memristive synapses with the combination of photonic and electric neuromorphic functions have prospective in the applications of synthetic retinas and optoelectronic interfaces for integrated photonic circuits based on mixed-mode electro-optical operation.

Role of Hole Trap Sites in MoS2 for Inconsistency in Optical and Electrical Phenomena

Because of strong Coulomb interaction in two-dimensional van der Waals-layered materials, the trap charges at the interface strongly influence the scattering of the majority carriers and thus often degrade their electrical properties. However, the photogenerated minority carriers can be trapped at the interface, modulate the electron-hole recombination, and eventually influence the optical properties. In this study, we report the role of the hole trap sites on the inconsistency in the electrical and optical phenomena between two systems with different interfacial trap densities, which are monolayer MoS₂-based field-effect transistors (FETs) on hexagonal boron nitride (h-BN) and SiO₂ substrates. Electronic transport measurements indicate that the use of h-BN as a gate insulator can induce a higher n-doping concentration of the monolayer MoS₂ by suppressing the free-electron transfer from the intrinsically n-doped MoS₂ to the SiO₂ gate insulator. Nevertheless, optical measurements show that the electron concentration in MoS₂/SiO₂ is heavier than that in MoS₂/h-BN, manifested by the relative red shift of the A_1g Raman peak. The inconsistency in the evaluation of the electron concentration in MoS₂ by electrical and optical measurements is explained by the trapping of the photogenerated holes in the spatially modulated valence band edge of the monolayer MoS₂ caused by the local strain from the SiO₂/Si substrate. This photoinduced electron doping in MoS₂/SiO₂ is further confirmed by the development of the trion component in the power-dependent photoluminescence spectra and negative shift of the threshold voltage of the FET after illumination.

Transport and Field Emission Properties of MoS₂ Bilayers

We report the electrical characterization and field emission properties of MoS2 bilayers deposited on a SiO2/Si substrate. Current–voltage characteristics are measured in the back-gate transistor configuration, with Ti contacts patterned by electron beam lithography. We confirm the n-type character of as-grown MoS2 and we report normally-on field-effect transistors. Local characterization of field emission is performed inside a scanning electron microscope chamber with piezo-controlled tungsten tips working as the anode and the cathode. We demonstrate that an electric field of ~200 V/μm is able to extract current from the flat part of MoS2 bilayers, which can therefore be conveniently exploited for field emission applications even in low field enhancement configurations. We show that a Fowler–Nordheim model, modified to account for electron confinement in two-dimensional (2D) materials, fully describes the emission process.

Transport properties of the top and bottom surfaces in monolayer MoS2 grown by chemical vapor deposition

The advantage of MoS₂, compared with graphene, is the direct growth on various oxide substrates by chemical vapor deposition (CVD) without utilizing catalytic metal substrates, which facilitates practical applications for electronics. The carrier mobility is, however, degraded from the intrinsic limit mainly due to short-range scattering caused by S vacancies formed during CVD growth. If the upper limit for the crystallinity of CVD-MoS₂ on oxide substrates is determined by the MoS₂/substrate interaction during growth, it will hinder the advantage. In this study, we investigated the interaction between monolayer MoS₂ and a SiO₂/Si substrate and the difference in crystallinity between the top and bottom S surfaces due to the MoS₂/substrate interaction. Raman and photoluminescence spectroscopy indicated that doping and strain were induced in MoS₂ from the substrate, but they could be removed by transferring MoS₂ to a new substrate using polymers. The newly developed polymer-transfer technique enabled selective transfer of the bottom or top surface of CVD-MoS₂ onto a new SiO₂/Si substrate. The metal-insulator transition was clearly observed for both the normal and inverse transfers, suggesting that the crystallinity of CVD-MoS₂ is high and that the crystallinity of the bottom surface interacting with the substrate was similar to that of the top free surface. These results provide positive prospects for the further improvement of the crystallinity of MoS₂ on oxide substrates by reconsidering the growth conditions.