Sung Wook Min

MoS2 Nanosheet Phototransistors with Thickness-Modulated Optical Energy Gap

We report on the fabrication of top-gate phototransistors based on a few-layered MoS(2) nanosheet with a transparent gate electrode. Our devices with triple MoS(2) layers exhibited excellent photodetection capabilities for red light, while those with single- and double-layers turned out to be quite useful for green light detection. The varied functionalities are attributed to energy gap modulation by the number of MoS(2) layers. The photoelectric probing on working transistors with the nanosheets demonstrates that single-layer MoS(2) has a significant energy bandgap of 1.8 eV, while those of double- and triple-layer MoS(2) reduce to 1.65 and 1.35 eV, respectively.

Low Power Consumption Complementary Inverters with n-MoS2 and p-WSe2 Dichalcogenide Nanosheets on Glass for Logic and Light-Emitting Diode Circuits

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.

Enhanced device performances of WSe2–MoS2 van der Waals junction p–n diode by fluoropolymer encapsulation

Two-dimensional heterojunction diodes with WSe2 and MoS2 nanoflakes respectively as p- and n-type semiconductors were fabricated on both glass and SiO2/p+-Si by direct imprinting. Superior electrostatic and dynamic performances were acquired from the diode on glass when an electric dipole-containing fluoropolymer was employed for encapsulation: forward and reverse current toward ideal behavior, enhanced aging/ambient stability, and improved dynamic rectification resulted.

High-gain subnanowatt power consumption hybrid complementary logic inverter with WSe2 nanosheet and ZnO nanowire transistors on glass.

A 1D-2D hybrid complementary logic inverter comprising of ZnO nanowire and WSe2 nanosheet field-effect transistors (FETs) is fabricated on glass, which shows excellent static and dynamic electrical performances with a voltage gain of ≈60, sub-nanowatt power consumption, and at least 1 kHz inverting speed.

Direct imprinting of MoS2 flakes on a patterned gate for nanosheet transistors

Nanosheet transistors based on mechanically exfoliated MoS2 and other transition metal dichalcogenide layers have already been reported demonstrating good device performances. In an approach to synthesize a large area two-dimensional (2D) sheet, chemical vapor deposition methods were reported and the transfer of those sheets onto other arbitrary substrates was also attempted, although studies on the direct imprinting of such 2D semiconductor sheets are rare. Here, we report on a direct imprinting method, the polydimethylsiloxane (PDMS)-adopting approach, that enables the fabrication of patterned bottom-gate MoS2 nanosheet field-effect transistors (FETs) on any substrate; using direct printing methods MoS2 FETs were successfully fabricated on glass. Since our FETs were also controlled to be a depletion or an enhanced mode with the modulated MoS2 thickness on a patterned bottom-gate, our imprinting approach is regarded as a meaningful advance toward 2D nanosheet electronics.

Top and back gate molybdenum disulfide transistors coupled for logic and photo-inverter operation

We demonstrate an inverter type nanodevice based on 2-dimensional semiconducting molybdenum disulfide (MoS2) nanoflakes. The inverter device was comprised of back-gate and top-gate field-effect transistors (FETs) which work respectively as a load and a driver for a logic inverter in the dark but switch their roles for photo-inverter operation. Our logic inverter shows a relatively high voltage gain of more than 12. When the back-gate FET controls the circuit as a driver to sensitively detect visible light using its open channel, the device effectively operates as a photo-inverter detecting visible photons. Our inverter based on top- and back-gate MoS2 FETs would be quite promising for both logic and photo-sensing applications due to its performance and simple device configuration as well.

Transition metal dichalcogenide heterojunction PN diode toward ultimate photovoltaic benefits

Recently, two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors as van der Waals (vdW) materials have attracted much attention from researchers. Among many 2D TMDC materials, a few layer-thin molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) have been most intensively studied respectively as 2D n- and p-type semiconductors. Here, we have fabricated vertical vdW heterojunction n-MoS2/p-WSe2 diode with a few tens nm-thick layers by using vertically-sandwiched ohmic terminals, so that no quasi neutral region may exist between two terminals. As a result, we obtained high photo responsivity at zero volt without any electric power, and it appears comparable to those of commercially-optimized Si PN diode. Photo-voltage output of 0.3 V was easily obtained from our vdW PN diode as open circuit voltage, and can be doubled up to 0.6 V by using two PN diodes. These beneficial photovoltaic results from vdW PN diode were directly applied to PV switching dynamics and transistor photo gating, for the first time. We regard that our vdW n-MoS2/p-WSe2 heterojunction diode could maximize its photovoltaic energy benefits with optimized TMDC thicknesses.

Molybdenum Disulfide Nanoflake–Zinc Oxide Nanowire Hybrid Photoinverter

We demonstrate a hybrid inverter-type nanodevice composed of a MoS2 nanoflake field-effect transistor (FET) and ZnO nanowire Schottky diode on one substrate, aiming at a one-dimensional (1D)-two-dimensional (2D) hybrid integrated electronic circuit with multifunctional capacities of low power consumption, high gain, and photodetection. In the present work, we used a nanotransfer printing method using polydimethylsiloxane for the fabrication of patterned bottom-gate MoS2 nanoflake FETs, so that they could be placed near the ZnO nanowire Schottky diodes that were initially fabricated. The ZnO nanowire Schottky diode and MoS2 FET worked respectively as load and driver for a logic inverter, which exhibits a high voltage gain of ∼50 at a supply voltage of 5 V and also shows a low power consumption of less than 50 nW. Moreover, our inverter effectively operates as a photoinverter, detecting visible photons, since MoS2 FETs appear very photosensitive, while the serially connected ZnO nanowire Schottky diode was blind to visible light. Our 1D-2D hybrid nanoinverter would be quite promising for both logic and photosensing applications due to its performance and simple device configuration as well.

MoS2 nanosheet channel and guanine DNA-base charge injection layer for high performance memory transistors

DNA polymers have been studied in the research areas of information and nanotechnology as well as biotechnology with various benefits such as natural plenitude, biodegradability and low toxicity. Here we demonstrate a charge injection type non-volatile memory field-effect transistor (FET) with one DNA-base small molecule, guanine, which is coupled with a MoS2 nanosheet channel as a trapping or charge injection layer material. Owing to the unique properties of the guanine layer and the extremely thin MoS2 nanosheet, our non-volatile memory MoS2 FETs exhibit a more than 3 V memory window under a 35 V/−15 V gate voltage pulse for Program/Erase (or trapping/detrapping), maintaining a high Program/Erase ratio of ∼103 for longer than 1000 s at least. Superior dynamic Program/Erase cycles were performed with a memory inverter composed of two memory FETs connected in series. Such non-volatile memory properties have been mostly well observed even after 45 days, since the trapped electron charges were stably stored in the guanine layer.

Simultaneous protection of organic p- and n-channels in complementary inverter from aging and bias-stress by DNA-base guanine/Al2O3 double layer.

Although organic field-effect transistors (OFETs) have various advantages of lightweight, low-cost, mechanical flexibility, and nowadays even higher mobility than amorphous Si-based FET, stability issue under bias and ambient condition critically hinder its practical application. One of the most detrimental effects on organic layer comes from penetrated atmospheric species such as oxygen and water. To solve such degradation problems, several molecular engineering tactics are introduced: forming a kinetic barrier, lowering the level of molecule orbitals, and increasing the band gap. However, direct passivation of organic channels, the most promising strategy, has not been reported as often as other methods. Here, we resolved the ambient stability issues of p-type (heptazole)/or n-type (PTCDI-C13) OFETs and their bias-stability issues at once, using DNA-base small molecule guanine (C5H5N5O)/Al2O3 bilayer. The guanine protects the organic channels as buffer/and H getter layer between the channels and capping Al2O3, whereas the oxide capping resists ambient molecules. As a result, both p-type and n-type OFETs are simultaneously protected from gate-bias stress and 30 days-long ambient aging, finally demonstrating a highly stable, high-gain complementary-type logic inverter.