Search

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.

As the tight contact interface of the lateral PN junction enables high responsivity, specific detectivity, and fast response speed, atomic-scale two-dimensional (2D) lateral PN heterostructures are emerging as viable alternatives to silicon-based photodiodes. The optical properties of the current 2D heterostructures depend entirely on the intrinsic properties of 2D materials, which can be greatly improved by forming an ideal PN diode via the doping control of 2D heterostructures. In this study, we propose a high-performance photodiode using a doping-controlled WSe2–MoSe2 PN heterojunction. During the synthesis, the low chemical reactivity of Nb2O5 with WO3 as compared to MoO3 enables sequential growth and prevents niobium (Nb) doping during MoSe2 growth at low temperatures. Conversely, in the WSe2 growth at high temperatures, tungsten (W) to Nb is selectively substituted, resulting in the lateral heterostructure of Nb-doped WSe2–MoSe2. The Nb atoms in WSe2 change the WSe2 type from ambipolar to p-type dominant. Together with intrinsically n-type MoSe2, Nb-doped WSe2 forms a lateral PN heterostructure with a near-unity ideality factor (1.3) and a high forward/reverse current ratio of 104. Our ideal 2D PN photodiode effectively suppresses the dark current in the reverse bias region (∼100 fA at an overall VDS of 0 V to approximately −10 V) and enhances the photocurrent by the high built-in potential at the PN depletion layer (VOC = 0.52 V). Thus, our device exhibits a high Ilight/Idark ratio (105) and a corresponding ultra-high detectivity (5.78 × 1015 Jones), which are approximately 100 times higher than those of reported lateral 2D PN heterostructure photodiodes. These outstanding performances show that the doping-controlled transition metal dichalcogenide PN heterostructures are promising candidates for next-generation optoelectronics. [ABSTRACT FROM AUTHOR]

Van der Waals (vdW) layered materials are promising channel materials for next‐generation field‐effect transistors (FETs). However, in vdW layered‐material FETs, the Schottky barrier and tunnel barrier at the vdW interfaces significantly reduce the electron injection efficiencies at electrical contacts, thereby limiting the development of high‐speed and low‐power FETs. This study demonstrates that intercalated lithium (Li) ions at vdW interfaces lower the Schottky and tunnel barriers at the electrical contacts of multilayer tungsten diselenide FETs owing to the low potential energy of Li and the mild electron doping from intercalation, thus decreasing the resistance at the vdW interfaces and enhancing the current flow and field‐effect mobility. Compared with before‐Li intercalation, the multilayer WSe2 planar FET demonstrates ≈1000 times higher current (IDS = 17.8 μA) at VGS = 50 V, ≈110 times higher maximum field‐effect mobility (μFE = 97.67 cm2 V−1 s−1), and ≈1000 times higher on/off current ratio (on/off ratio = ≈107). Furthermore, in multilayer WSe2 vertical FETs with vdW heterostructures having a structural strength of approximately the same as the current density, the intercalated Li ions at the graphene/WSe2 vdW interfaces additionally improves the current density and the vertical mobility by ≈20 times (1.00 A cm−2) and 10 times (2.52 × 10−4 cm2 V−1 s−1), respectively. This study establishes a method to reduce the resistance at vdW interfaces for developing high‐speed and low‐power multilayer vdW material FETs. [ABSTRACT FROM AUTHOR]

Two-dimensional transition metal dichalcogenides (TMDs) offer numerous advantages over silicon-based application in terms of atomically thin geometry, excellent opto-electrical properties, layer-number dependence, band gap variability, and lack of dangling bonds. The production of high-quality and large-scale TMD films is required with consideration of practical technology. However, the performance of scalable devices is affected by problems such as contamination and patterning arising from device processing; this is followed by an etching step, which normally damages the TMD film. Herein, we report the direct growth of MoSe 2 films on selective pattern areas via a surface-mediated liquid-phase promoter using a solution-based approach. Our growth process utilizes the promoter on the selective pattern area by enhancing wettability, resulting in a highly uniform MoSe 2 film. Moreover, our approach can produce other TMD films such as WSe 2 films as well as control various pattern shapes, sizes, and large-scale areas, thus improving their applicability in various devices in the future. Our patterned MoSe 2 field-effect transistor device exhibits a p-type dominant conduction behavior with a high on/off current ratio of ∼10 6 . Thus, our study provides general guidance for direct selective pattern growth via a solution-based approach and the future design of integrated devices for a large-scale application.

Two-dimensional (2D) layered materials with properties such as a large surface-to-volume ratio, strong light interaction, and transparency are expected to be used in future optoelectronic applications. Many studies have focused on ways to increase absorption of 2D-layered materials for use in photodetectors. In this work, we demonstrate another strategy for improving photodetector performance using a graphene/MoS 2 heterojunction phototransistor with a short channel length and a tunable Schottky barrier. The channel length of sub-30 nm, shorter than the diffusion length, decreases carrier recombination and carrier transit time in the channel and improves phototransistor performance. Furthermore, our graphene/MoS 2 heterojunction phototransistor employed a tunable Schottky barrier that is only controlled by light and gate bias. It maintains a low dark current and an increased photocurrent. As a result, our graphene/MoS 2 heterojunction phototransistor showed ultrahigh responsivity and detectivity of 2.2 × 10 5 A/W and 3.5 × 10 13 Jones, respectively. This is a considerable improvement compared to previous pristine MoS 2 phototransistors. We confirmed an effective method to develop phototransistors based on 2D materials and obtained ultrahigh performance of our phototransistor, which is promising for high-performance optoelectronic applications.

A single-layer MoS 2 achieves excellent gate controllability within the nanoscale channel length of a field-effect transistor (FET) owing to an ultra-short screening length. However, multilayer MoS 2 (ML-MoS 2 ) is more vulnerable to short channel effects (SCEs) owing to its thickness and long screening length. We eliminated the SCEs in an ML-MoS 2 FET (thickness of 4-13 nm) at a channel length of sub-30 nm using a Schottky barrier (SB) variable graphene/ML-MoS 2 heterojunction. Although the band modulation in the ML-MoS 2 channel worsens with a decrease in the channel length, which is similar to the SCEs occurring in conventional FETs, the variable Fermi level ( E F ) of a graphene electrode along the gate voltage allows control of the SB at the graphene/MoS 2 junction and backs up the current modulation through a variable SB. Electrical measurements and a theoretical band simulation demonstrate the efficient SB modulation of our graphene nanogap (GrNG) ML-MoS 2 FET with three distinct carrier transports along V gs : a thermionic emission at a low SB, Fowler-Nordheim tunneling at a moderate SB, and direct tunneling at a high SB. Our GrNG FET shows an extremely high on-off current ratio of ∼10 8 , which is approximately three-orders of magnitude better than a previously reported metal nanogap (MeNG) FET and a self-aligned metal/graphene nanogap FET with a similar MoS 2 thickness. Our GrNG FET also exhibits a 100,000-times higher on-off ratio, 100-times lower subthreshold swing, and 10-times lower drain induced barrier.

In the past decade, intensive studies on monolayer MoS 2 -based phototransistors have been carried out to achieve further enhanced optoelectronic characteristics. However, the intrinsic optoelectronic characteristics of monolayer MoS 2 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 MoS 2 , including its internal responsivity and quantum efficiency, in fully transparent monolayer MoS 2 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 MoS 2 and lays the foundation for two-dimensional materials-based transparent phototransistors.

Piezoelectricity of transition metal dichalcogenides (TMDs) under mechanical strain has been theoretically and experimentally studied. Powerful strain sensors using Schottky barrier variation in TMD/metal junctions as a result of the strain-induced lattice distortion and associated ion-charge polarization were demonstrated. However, the nearly fixed work function of metal electrodes limits the variation range of a Schottky barrier. We demonstrate a highly sensitive strain sensor using a variable Schottky barrier in a MoS 2 /graphene heterostructure field effect transistor (FET). The low density of states near the Dirac point in graphene allows large modulation of the graphene Fermi level and corresponding Schottky barrier in a MoS 2 /graphene junction by strain-induced polarized charges of MoS 2 . Our theoretical simulations and temperature-dependent electrical measurements show that the Schottky barrier change is maximized by placing the Fermi level of the graphene at the charge neutral (Dirac) point by applying gate voltage. As a result, the maximum Schottky barrier change (ΔΦ SB ) and corresponding current change ratio under 0.17% strain reach 118 meV and 978, respectively, resulting in an ultrahigh gauge factor of 575 294, which is approximately 500 times higher than that of metal/TMD junction strain sensors (1160) and 140 times higher than the conventional strain sensors (4036). The ultrahigh sensitivity of graphene/MoS 2 heterostructure FETs can be developed for next-generation electronic and mechanical-electronic devices.