sputter-deposition and photolithography. As a result, we found a unique property of graphene electrode, which not only showed superior ohmic or ON current behavior to those of Au/Ti but also more enhanced OFF state behavior as well. We regard that such positive results are attributed to gate-voltage-induced work function tuning in exfoliated graphene.A scanning electron microscopy (SEM) image in Figure 1 a shows 30-µm-long MoS 2 fl ake that we placed on 285-nm-thick SiO 2 /p + -Si substrate, where two Au/Ti electrodes are deposited on MoS 2 while two graphene fl akes are placed on the same MoS 2 . The graphene electrodes are then connected to Au/Ti lead lines, as shown in the overall device scheme of Figure 1 b. Figure 1 c illustrates an initial device process step where the MoS 2 exfoliation by polydimethylsiloxane (PDMS) stamp and its transfer to SiO 2 /p + -Si substrate are performed, while another similar steps for graphene transfer are also shown in Figure 1 d where in fact we use an optical microscope (OM) to fi nd the initially-transferred MoS 2 channel fl ake and to align the graphene S/D fl akes on the MoS 2 channel fl ake (note the four arrows indicating such fl ake alignment by substrate stage movement). Since the contact needs some pressure (to red arrow direction) between graphene and device substrate, we call this contact method "direct imprint" and more details were recently introduced elsewhere. [ 38 ] After the two graphene S/D electrodes were properly arranged, Au/Ti (50 nm/25 nm) contact electrodes were patterned by photolithography and DC sputter-deposition/liff-off processes as respectively shown in Figure 1 e,f, to contact both MoS 2 and graphene electrodes once and for all. Figure 1 f is a schematic version of SEM image in Figure 1 a. Since we initially assumed that our long MoS 2 should have a uniform thickness in every location, it was necessary to experimentally measure the thickness of at least two important locations in Figure 2 a (an OM version of Figure 1 a): a central location (MoS 2 I) between graphene electrodes and another central location (MoS 2 II) between Au/Ti electrodes.According to atomic force microscopy scan results, the thicknesses of those two regions appear almost the same, to be ≈5 nm (7≈8 L) (see Figure 2 b,c). We also measured the thicknesses of two graphene S/D fl akes, which appear 15 and 12 nm for Gr1 and Gr2, respectively (see Figure 2 d,e). Figure 3 a displays the drain current-gate voltage ( I D -V G ) transfer characteristics obtained from both MoS 2 nanosheet FETs with graphene and Au/Ti S/D contacts, which have not Field-Effect TransistorsMechanically-exfoliated or chemical vapor deposited graphene has been extensively studied for any practical usages as the most well-known two dimensional (2D) nanosheet, since it was found and developed. [1][2][3][4][5][6] One of the practical applications was a source/drain (S/D) electrode [7][8][9][10][11][12][13][14][15] for such a variety of transistors as organic thin-fi lm transistors, [ 11 ] Si-based transistors, [ ...
Two-dimensional (2D) molybdenum disulfide (MoS₂) field-effect transistors (FETs) have been extensively studied, but most of the FETs with gate insulators have displayed negative threshold voltage values, which indicates the presence of interfacial traps both shallow and deep in energy level. Despite such interface trap issues, reports on trap densities in MoS₂ are quite limited. Here, we probed top-gate MoS₂ FETs with two- (2L), three- (3L), and four-layer (4L) MoS₂/dielectric interfaces to quantify deep-level interface trap densities by photo-excited charge collection spectroscopy (PECCS), and reported the result that deep-level trap densities over 10(12) cm(-2) may exist in the interface and bulk MoS₂ near the interface. Transfer curve hysteresis and PECCS measurements show that shallow traps and deep traps are not that different in density order from each other. We conclude that our PECCS analysis distinguishably provides valuable information on deep level interface/bulk trap densities in 2D-based FETs.
We have fabricated dual gate field effect transistors (FETs) with 12 nm-thin black phosphorus (BP) channel on glass substrate, where our BP FETs have a patterned-gate architecture with 30 nm-thick Al2O3 dielectrics on top and bottom of a BP channel. Top gate dielectric has simultaneously been used as device encapsulation layer, controlling the threshold voltage of FETs as well when FETs mainly operate under bottom gate bias. Bottom, top, and dual gate-controlling mobilities were estimated to be 277, 92, and 213 cm(2)/V s, respectively. Maximum ON-current was measured to be ∼5 μA at a drain voltage of -0.1 V but to be as high as ∼50 μA at -1 V, while ON/OFF current ratio appeared to be 3.6 × 10(3) V. 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.
3146 wileyonlinelibrary.com disulfi de (MoS 2 ) and tungsten dislenide (WSe 2 ) are typesetting materials showing n-and p-type dominant conductions, respectively. [7][8][9][10][11] As one of quite recent 2D TMD materials, molybdenum ditelluride (α-MoTe 2 ) has also been attracting attention due to its optical and electrical properties. Monolayer α-MoTe 2 exhibits a direct optical bandgap of 1.10 eV, while its bulk form becomes an indirect semiconductor with the band gap of 0.85-1.0 eV. [12][13][14] Interestingly, it is reported that MoTe 2 shows structural and electronic phase transition. The structural phase transition from hexagonal (2H) phase to monoclinic (distorted octahedral or 1T) phase is reversible at a high temperature. [ 15 ] According to literatures, [16][17][18][19][20] few-layered α-MoTe 2 fi eld effect transistors (FETs) showed ambipolar type conduction with broad range of mobilities in 0.2-30 cm 2 V −1 s −1 for both electrons and holes, depending on the source/drain (S/D) contact electrodes, gate dielectrics, and their process conditions. Contact resistance and dielectric/ MoTe 2 channel interface are certainly affecting factors for the electrical performances of a few-layer α-MoTe 2 FETs.In the present work, we have fabricated all-2D α-MoTe 2 -based FETs on glass, using a few tens nm-thin hexagonal boron nitride (h-BN) and a few layer-thin graphene in consideration of good dielectric/channel interface and S/D contacts, respectively. Very few but similar attempts for all 2D FETs were conducted with MoS 2 and WSe 2 nanosheets. [ 21,22 ] Here, distinguished from previous works, our all-2D FETs with α-MoTe 2 nanofl akes are dual-gated for driving higher current, using two h-BN layers for top and bottom dielectrics. Moreover, for our 2D dual gate FET fabrications on glass, all thermal annealing and lithography processes were intentionally exempted. This means that our dual gate 2D FETs may be formed in a fully non-lithographic method by using only van der Waal's forces. Our dual-gate α-MoTe 2 FET displays quite a high hole and electron mobility over ≈20 cm 2 V −1 s −1 along with ON/OFF ratio of ≈10 5 in maximum as an ambipolar FET and also demonstrates drain current as high as a few tens-to-hundred µA at a low drain voltage of −2 V, which appears enough to switch organic light emitting diodes (OLEDs) for blue light. Non-Lithographic Fabrication of All-2D α-MoTe 2 Dual Gate TransistorsKyunghee Choi , Young Tack Lee , Jin Sung Kim , Sung-Wook Min , Youngsuk Cho , Atiye Pezeshki , Do Kyung Hwang , * and Seongil Im * As one of the emerging new transition-metal dichalcogenides materials, molybdenum ditelluride (α-MoTe 2 ) is attracting much attention due to its optical and electrical properties. This study fabricates all-2D MoTe 2 -based fi eld effect transistors (FETs) on glass, using thin hexagonal boron nitride and thin graphene in consideration of good dielectric/channel interface and source/drain contacts, respectively. Distinguished from previous works, in this study, all 2D FETs with α-MoTe 2 na...
Two-dimensional heterojunction diodes with WSe 2 and MoS 2 nanoflakes respectively as p-and n-type semiconductors were fabricated on both glass and SiO 2 /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. † Electronic supplementary information (ESI) available: I-V curves of another p-n diode on a SiO 2 /p + -Si substrate, parasitic capacitor induced by SiO 2 dielectric, photo-response of the unencapsulated pristine p-n diode and ambient stability of the CYTOP-capped diode. See
High‐performance, air‐stable, p‐channel WSe2 top‐gate field‐effect transistors (FETs) using a bilayer gate dielectric composed of high‐ and low‐k dielectrics are reported. Using only a high‐k Al2O3 as the top‐gate dielectric generally degrades the electrical properties of p‐channel WSe2, therefore, a thin fluoropolymer (Cytop) as a buffer layer to protect the 2D channel from high‐k oxide forming is deposited. As a result, a top‐gate‐patterned 2D WSe2 FET is realized. The top‐gate p‐channel WSe2 FET demonstrates a high hole mobility of 100 cm2 V−1 s−1 and a ION/IOFF ratio > 107 at low gate voltages (VGS ca. −4 V) and a drain voltage (VDS) of −1 V on a glass substrate. Furthermore, the top‐gate FET shows a very good stability in ambient air with a relative humidity of 45% for 7 days after device fabrication. Our approach of creating a high‐k oxide/low‐k organic bilayer dielectric is advantageous over single‐layer high‐k dielectrics for top‐gate p‐channel WSe2 FETs, which will lead the way toward future electronic nanodevices and their integration.
Energy harvesting from human motion is regarded as a promising protocol for powering portable electronics, biomedical devices, and smart objects of the Internet of things. However, state‐of‐the‐art mechanical‐energy‐harvesting devices generally operate at frequencies (>10 Hz) well beyond human activity frequencies. Here, a hydrogel ionic diode formed by the layered structures of anionic and cationic ionomers in hydrogels is presented. As confirmed by finite element analysis, the underlying mechanism of the hydrogel ionic diode involves the formation of the depletion region by mobile cations and anions and the subsequent increase of the built‐in potential across the depletion region in response to mechanical pressure. Owing to the enhanced ionic rectification ratio by the embedded carbon nanotube and silver nanowire electrodes, the hydrogel ionic diode exhibits a power density of ≈5 mW cm−2 and a charge density of ≈4 mC cm−2 at 0.01 Hz, outperforming the current energy‐harvesting devices by several orders of magnitude. The applications of the self‐powered hydrogel ionic diode to tactile sensing, pressure imaging, and touchpads are demonstrated, with sensing limitation is as low as 0.01 kPa. This work is expected to open up new opportunities for ionic‐current‐based ionotronics in electronics and energy devices.
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