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In this work, electrical Joule heating (J-H) was employed for the first time to electrically isolate ZnO nanowire FETs array for one dimensional (1D) logic applications without any physical and electrical damages. The electrical properties of the isolated nanowire FETs were found to be superior to non-isolated FETs came from the neighboring gate effect. Finally, we investigated ZnO nanowire-based NOT, NAND, and NOR logic gates with the J-H nanowire isolation technique. The isolated logic gates clearly show much lower output voltage off level than the non-isolated circuits thus resulting in more accurate and reliable 1D electronic applications.

Since transition metal dichalcogenide (TMD) semiconductors are found as two-dimensional van der Waals materials with a discrete energy bandgap, many TMD based field effect transistors (FETs) are reported as prototype devices. However, overall reports indicate that threshold voltage (Vth) of those FETs are located far away from 0 V whether the channel is p- or n-type. This definitely causes high switching voltage and unintended OFF-state leakage current. Here, a facile way to simultaneously modulate the Vth of both p- and n-channel FETs with TMDs is reported. The deposition of various organic small molecules on the channel results in charge transfer between the organic molecule and TMD channels. Especially, HAT-CN molecule is found to ideally work for both p- and n-channels, shifting their Vth toward 0 V concurrently. As a proof of concept, a complementary metal oxide semiconductor (CMOS) inverter with p-MoTe2 and n-MoS2 channels shows superior voltage gain and minimal power consumption after HAT-CN deposi...

Two-dimensional (2D) transition metal dichalcogenides (TMDs)-based van der Waals (vdW) PN junctions have been used for heterojunction diodes, which basically utilize out-of-plane current across the junction interface. In fact, the same vdW PN junction structure can be utilized for another important device application, junction field effect transistors (JFETs), where in-plane current is possible along with 2D–2D heterojunction interface. Moreover, the 2D TMD-based JFET can use both p- and n-channel for low voltage operation, which might be its unique feature. Here we report vdW JFETs as an in-plane current device with heterojunction between semiconducting p- and n-TMDs. Since this vdW JFET would have low-density traps at the vdW interface unlike 2D TMD-based metal insulator semiconductor field effect transistors (MISFETs), little hysteresis of 0.0–0.1 V and best subthreshold swing of ~100 mV/dec were achieved. Easy saturation was observed either from n-channel or p-channel JFET as another advantage over 2D MISFETs, exhibiting early pinch-off at ~1 V. Operational gate voltage for threshold was near 0 V and our highest mobility reaches to ~>500 cm2/V·s for n-channel JFET with MoS2 channel. For 1 V JFET operation, our best ON/OFF current ratio was observed to be ~104. Van der Waals heterostructures of atomically thin semiconductors enable junction field effect transistors (JFETs). A team led by Seongil Im at Yonsei University fabricated JFETs using a heterojunction of semiconducting p-MoTe2 (or p-WSe2) and n-MoS2. The p-type transition metal dichalcogenide (TMDC) works as a gate for the n-type TMDC channel, whereas the n-type TMDC operates as a gate for the p-type TMDC channel. Owing to the low density of traps at the van der Waals interface, the devices exhibit low hysteresis of 0.05–0.1 V and achieve a subthreshold swing of ~100 mV/decade. The highest mobility reaches 500 cm2/V·s for the n-channel JFET with MoS2, whereas the p-channel JFET with MoTe2 is characterized by a much lower mobility of ~13 cm2/V·s. The highest ON/OFF current ratio was observed to be >104.

Mono- and few-layer transition-metal dichalcogenides (TMDCs) provide opportunities for ideal two-dimensional semiconductors for electronic and optoelectronic devices. For electronic devices on TMDCs, it is essential to incorporate n- and/or p-type dopants which are stable in positions after patterned doping. Here we investigate hydrogen doping for TMDC (MX2 with M = Mo, W and X = S, Se, Te) nanosheets by first-principles calculations to address diffusion and doping properties. We find that adsorbed hydrogen atoms in TMDCs are energetically most stable at the interstitial site right on the Mo or W plane and have substantial energy barriers against diffusion that increase in the order of sulfides, selenides, and tellurides. Located at the most stable interstitial site on the Mo or W plane, the hydrogen atoms produce electrons in the conduction bands in the extremely high rate of one electron per hydrogen atom, without any defect state inside the band gap remarkably. We analyze the chemical bonding character around the dopant and the mechanism for such high efficiency of electron doping. We also consider properties of hydrogen molecules and Te vacancies for comparison. Our work shows that hydrogen doping is the promising pathway to development of highly integrated electronic devices on TMDCs, Comment: 10 pages, 8 figures, 4 tables

A van der Waals (vdW) Schottky junction between two-dimensional (2D) transition metal dichalcogenides (TMDs) is introduced here for both vertical and in-plane current devices: Schottky diodes and metal semiconductor field-effect transistors (MESFETs). The Schottky barrier between conducting NbS

Recently, α-MoTe

Band-like transport behavior of H-doped transition metal dichalcogenide (TMD) channels in field effect transistors (FET) is studied by conducting low-temperature electrical measurements, where MoTe2 , WSe2 , and MoS2 are chosen for channels. Doped with H atoms through atomic layer deposition, those channels show strong n-type conduction and their mobility increases without losing on-state current as the measurement temperature decreases. In contrast, the mobility of unintentionally (naturally) doped TMD FETs always drops at low temperatures whether they are p- or n-type. Density functional theory calculations show that H-doped MoTe2 , WSe2 , and MoS2 have Fermi levels above conduction band edge. It is thus concluded that the charge transport behavior in H-doped TMD channels is metallic showing band-like transport rather than thermal hopping. These results indicate that H-doped TMD FETs are practically useful even at low-temperature ranges.

We have fabricated van der Waals (vdW) Schottky junction between high work function conducting NbS2 transition metal dichalcogenide (TMD) and semiconducting n-MoS2 TMD for metal semiconductor field effect transistors (MESFETs). The Schottky barrier between NbS2 and MoS2 appeared as large as ~0.5 eV due to their work function difference, as measured by scanning Kelvin probe. Schottky effect MESFET operates with a low threshold gate voltage of -0.5 ~ -1 V exhibiting easy saturation. Since this FET contains low density traps at the vdW interface, little gate hysteresis and ideal subthreshold swing of 60~80 mV/dec are expected. In addition, source/drain (S/D) contact for n-channel MoS2 could significantly affect device mobility. In the case of the graphene S/D contact, the device shows the highest mobility of more than 800 cm2/V s, although the device shows the lowest mobility of ~15 cm2/V s in the case of Au/MoS2 contact without an annealing process. TMD-based MESFET with NbS2/n-MoS2 junction is considered novel and promising in future 2D device electronics.

Apart from ordinary solar cell applications, we have attempted different approaches of device applications. We have fabricated two types of organic photovoltaic (OPV) cells with DBP/C60 heterojunction and measured their spectral photo responses when they are in planar heterojunction (PHJ) and bulk heterojunction (BHJ) type. According to our results, blue light exhibits spectrally outstanding PV effects and the PHJ device showed higher open circuit voltage (VOC) along with ten times faster switching photo-dynamics than those performances of BHJ device, although the mixture type BHJ diode appears much superior to PHJ diode in all spectral ranges of quantum efficiency. Large junction area of mixture type OPV diode is certainly an advantage in photocurrent generation and quantum efficiency, but is regarded as a disadvantage for dynamic PV switching and VOC value because complex route and high density interface traps are unavoidable in BHJ structure. Selecting the blue light illumination on DBP/C60 PHJ diode, we achieve most efficient PV switching and PV gating in logic and amplifier circuits, respectively. Conclusively, bilayer OPV device has its own advantage over mixture type device in dynamic PV switching, and it promises future electronics by exploiting the dynamic harvest of light energy. Figure 1

Being implemented as an active component in the circuits, two dimensional (2D) transition metal dichalcogenide (TMD) transistors have two kinds of difficulties. First, regardless of carrier type, threshold voltage (Vth) of TMD 2D field effect transistors (FETs) is located far away from 0 V. Second, gate bias induced hysteresis exists in general 2D TMD transistors, causing an unstable operation of FETs. Here, we have fabricated stable n-channel MoSe2 FETs with a relatively small Vth of -5 V and minimized-hysteresis of 0.5 V. And we have checked the stable operation by integrating a smart touch sensor circuit composed of piezoelectric P(VDF-TrFE) polymer, the MoSe2 FET, and organic light emitting diode (OLED). To minimize the Vth and gate voltage hysteresis of 2D FET, we inserted ultrathin (8.46 nm) polystyrene (PS)-brush layer between MoSe2 channel and 50 nm-thin Al2O3 dielectric. Our MoSe₂ FET has showed a maximum drain-current (ID) of ~9 μA at VDS of 1 V along with a high linear mobility of 11.2 cm2/Vs. In the analogue touch sensor circuit gated by piezoelectric film of large scale Al/50 μm-thick P(VDF-TrFE)/ITO/glass, the MoSe₂ FET exhibites high ON/OFF ratio and ON-state ID which quite well demonstrated OLED pixel operation. Figure 1

Two dimensional (2D) semiconductor materials haven been extensively studied for the last decade, regarded as one kind of the most important breakthrough materials toward future device technologies. Band gap of monolayer and few layer in 2D semiconductors has been reported, measured by optical probing such as photoluminescence (PL). However, if their exfoliated thickness is as large as multilayer over 10 layers (10L), PL measurements become less effective and inaccurate because the optical transition of 2D semiconductor is changed from direct to indirect mode. At the moment, the energy band gap also becomes smaller with the semiconductir thickness. In fact, for thick multilayer 2D semiconductors, practical band gap measurement is very difficult due to lack of measurementment technique, so that density function theory (DFT)-based calculations might be only way to conjecture the bandgap. Here, we introduce another way to measure such bandgap of multilayer 2D semiconductors; that is using field effect transistor (FET) as a platform. We have thus fabricated FETs with multilayer 2D channel using graphene contact and top passivation for this study. Because graphene contact would secure ambipolar behavior and Schottky contact barrier tuning of FETs with the assitance of top passivation, bandgap measurement was successfully carried out. Our FETs with WSe2, MoTe2, BP channel, show ambipolar behavior and their maximum Schottky barrier height are measured by temperature-dependent transfer curve characteristics. Bandgap values from WSe2 FET1, WSe2 FET2, MoTe2 FET, and BP FET were 1.16,1.44, 0.92, 0.42 eV, respectively obtained by twice the maximum Schottky barrier height. The values from WSe2 FETs were compared with those of PL measurement, to be confirmed for their compatibilities and according to literature, our values seems reasonable.

Recently, 2D transition metal dichalcogenides(TMDs) MoTe2 has shown outstanding properties for future electronic devices. Such TMD structures without surface dangling bonds make the 2D MoTe2 more favorable candidate than conventional 3D Si in a few nanometer scale. The band gap of thin MoTe2 appears close to that of Si and quite smaller than those of other typical TMD semiconductors. Even though there have been a few attempts to control charge carrier polarity of MoTe2, functional devices such as complementary metal–oxide–semiconductor (CMOS) inverter have not been reported. Here we demonstrate a 2D CMOS inverter in a single MoTe2 nanosheet by straightforward selective doping technique. In a single MoTe2 flake, initially p-doped channel was selectively converted to n-doped region with high electron mobility of 18 cm2/Vs by atomic layer deposition-induced H-doping. Our ultrathin CMOS inverter exhibited a high DC voltage gain of 29, AC gain of 18 at 1 kHz, and a low static power consumption of a few nW. Our results show a great potential of MoTe2 for future electronic devices based on 2D semiconducting materials Figure 1

Black phosphorus (BP) nanosheet is two-dimensional (2D) semiconductor with distinct band gap and attracting recent attention from researches because it has some similarity to gapless 2D semiconductor graphene in the following two aspects: single element (P) for its composition and quite high mobilities depending on its fabrication conditions. Apart from several electronic applications reported with BP nanosheet, here we report for the first time BP nanosheet-ZnO nanowire 2D-1D heterojunction applications for p-n diodes and BP-gated junction field effect transistors (JFETs) with n-ZnO channel on glass. For these nanodevices, we take advantages of the mechanical flexibility of p-type conducting of BP and van der Waals junction interface between BP and ZnO. As a result, our BP-ZnO nanodimension p-n diode displays a high ON/OFF ratio of ∼10(4) in static rectification and shows kilohertz dynamic rectification as well while ZnO nanowire channel JFET operations are nicely demonstrated by BP gate switching in both electrostatics and kilohertz dynamics.

We report on an abnormal output characteristics in p-type low temperature polycrystalline silicon thin-film transistors fabricated on polyimide (PI); negative differential conductance behavior is often observed in saturation region of drain current from large width devices. To understand such abnormal output characteristics, device dimension dependence was studied in a systematic way. As a result, we found that enhanced self-heating is mainly responsible originating from the poor thermal conductivity of PI substrate. A related degradation model is also proposed.

Two-dimensional (2D) semiconductor materials with discrete bandgap become important because of their interesting physical properties and potentials toward future nanoscale electronics. Many 2D-based field effect transistors (FETs) have thus been reported. Several attempts to fabricate 2D complementary (CMOS) logic inverters have been made too. However, those CMOS devices seldom showed the most important advantage of typical CMOS: low power consumption. Here, we adopted p-WSe2 and n-MoS2 nanosheets separately for the channels of bottom-gate-patterned FETs, to fabricate 2D dichalcogenide-based hetero-CMOS inverters on the same glass substrate. Our hetero-CMOS inverters with electrically isolated FETs demonstrate novel and superior device performances of a maximum voltage gain as ∼27, sub-nanowatt power consumption, almost ideal noise margin approaching 0.5VDD (supply voltage, VDD=5 V) with a transition voltage of 2.3 V, and ∼800 μs for switching delay. Moreover, our glass-substrate CMOS device nicely performed digital logic (NOT, OR, and AND) and push-pull circuits for organic light-emitting diode switching, directly displaying the prospective of practical applications.

2D layered van der Waals (vdW) atomic crystals are an emerging class of new materials that are receiving increasing attention owing to their unique properties. In particular, the dangling-bond-free surface of 2D materials enables integration of differently dimensioned materials into mixed-dimensional vdW heterostructures. Such mixed-dimensional heterostructures herald new opportunities for conducting fundamental nanoscience studies and developing nanoscale electronic/optoelectronic applications. This study presents a 1D ZnO nanowire (n-type)–2D WSe2 nanosheet (p-type) vdW heterojunction diode for photodetection and imaging process. After amorphous fluoropolymer passivation, the ZnO–WSe2 diode shows superior performance with a much-enhanced rectification (ON/OFF) ratio of over 106 and an ideality factor of 3.4–3.6 due to the carbon–fluorine (CF) dipole effect. This heterojunction device exhibits spectral photoresponses from ultraviolet (400 nm) to near infrared (950 nm). Furthermore, a prototype visible imager is demonstrated using the ZnO–WSe2 heterojunction diode as an imaging pixel. To the best of our knowledge, this is the first demonstration of an optoelectronic device based on a 1D–2D hybrid vdW heterojunction. This approach using a 1D ZnO–2D WSe2 heterojunction paves the way for the further development of electronic/optoelectronic applications using mixed-dimensional vdW heterostructures.

Recently, α-MoTe2 , a 2D transition-metal dichalcogenide (TMD), has shown outstanding properties, aiming at future electronic devices. Such TMD structures without surface dangling bonds make the 2D α-MoTe2 a more favorable candidate than conventional 3D Si on the scale of a few nanometers. The bandgap of thin α-MoTe2 appears close to that of Si and is quite smaller than those of other typical TMD semiconductors. Even though there have been a few attempts to control the charge-carrier polarity of MoTe2 , functional devices such as p-n junction or complementary metal-oxide-semiconductor (CMOS) inverters have not been reported. Here, we demonstrate a 2D CMOS inverter and p-n junction diode in a single α-MoTe2 nanosheet by a straightforward selective doping technique. In a single α-MoTe2 flake, an initially p-doped channel is selectively converted to an n-doped region with high electron mobility of 18 cm2 V-1 s-1 by atomic-layer-deposition-induced H-doping. The ultrathin CMOS inverter exhibits a high DC voltage gain of 29, an AC gain of 18 at 1 kHz, and a low static power consumption of a few nanowatts. The results show a great potential of α-MoTe2 for future electronic devices based on 2D semiconducting materials.

DNA-based small molecules of guanine, cytosine, thymine and adenine are adopted for the charge injection layer between the Au electrodes and organic semiconductor, heptazole (C26H16N2). The heptazole-channel organic field effect transistors (OFETs) with a DNA-based small molecule charge injection layer showed higher hole mobility (maximum 0.12 cm2 V−1 s−1) than that of a pristine device (0.09 cm2 V−1 s−1). We characterized the contact resistance of each device by a transfer length method (TLM) and found that the guanine layer among all DNA-based materials performs best as a hole injection layer leading to the lowest contact resistance. Since the guanine layer is also known to be a proper channel passivation layer coupled with a thin conformal Al2O3 layer protecting the channel from bias stress and ambient molecules, we could realize ultra-stable OFETs utilizing guanine/Au contact and guanine/Al2O3 bilayer on the organic channel.

Two-dimensional (2D) semiconductor materials with discrete bandgap become important because of their interesting physical properties and potentials toward future nanoscale electronics. Semiconductor p-n junctions are essential building blocks for electronic and optoelectronics devices. Conventional lateral p-n heterojunction diode had several problems. Lateral p-n diode has quasi neural region so it’s efficiency is lower than vertical p-n diode. Here we fabricated vertical structure 2D dichalcogenide-based p-n heterojunction diode circuits for Photovoltaic-induced switching applications using p-WSe2 and n-MoS2 nanosheets. Moreover we modulate the thickness of flakes for enhancing the device performance. Our p-n diode PV cell displayed an instantaneous voltage of 0.3 V as Voc due to the difference between conduction and of MoS2 and valence band of WSe2. Furthermore based on such PV effects we successfully demonstrated PV optical switching, logic gating, and finally the gate switching of an IGZO field effect transistor. Figure 1

Transition metal dichalcogenides and monoatomic buckled crystals are two-dimensional (2D) semiconductors promising for active material toward nano-electronics and nano-optoelectronics [1]. As some of recent 2D semiconductors, α-MoTe2 and black phosphorus(BP) have been extensively studied due to their optical and electrial propeties. Many groups have theoretically predictd band gap of α-MoTe2 and black phosphorus in a wide range [2], depending on their film thickness. However, only a few experimental measurements have been done. Here, we demonstrate ambipolar α-MoTe2 and black phosphorus transistors with graphene source and drain contacts to determine the band gap of channel. Temperature-dependent transfer characteristics have been used to quantitatively analyze schottky barriers height(ФB) formed at the graphene-to-channel interface as a function of applied gate voltage. Because Fermi level of graphene is modulated to that of channel, we can determine band gap of 11 nm-thin α-MoTe2 and 6 nm-thin black phosphorus are 0.826 eV and 0.416 eV, respectively. Our estimated band gap values appear quite reasonable, proving that our analysis is very useful to estimate the band gap of 2D material. Figure 1

As promising Two-dimensional (2D) materials, transition metal dichalcogenide (TMD) nanosheets recently attracted much attention in respects of its relatively high mobility and distinct energy band gap of 1.2~1.8 eV. Black phosphorus nanosheet emerged after appearance of TMD, but with some similarity to graphene, since it contains one element, phosphorus (P), quite high mobilities which are depending on its thickness and bandgap. So, We have fabricated transistor with 12 nm-thin black phosphorus channel on glass substrate, where our black phosphorus field effect transistors (FETs) have a patterned-bottom gate structure with 30 nm-thick atomic layer deposited Al2O3 dielectrics on top and bottom of black phosphorus channel. Top gate dielectric has simultaneously been used as device encapsulation layer, slightly controlling the threshold voltage of FETs when FETs mainly operate under bottom gate bias. Bottom, top, and dual gate-controlling mobilities were respectively estimated, to be 270, 90, and 210 cm2/V s. As a result, our dual gate BP FETs demonstrate organic light emitting diode (OLED) switching for green and blue OLEDs, also demonstrating NOR logic functions by separately using top- and bottom-input. Figure 1