Joon Seok Kim

Flexible Black Phosphorus Ambipolar Transistors, Circuits and AM Demodulator

High-mobility two-dimensional (2D) semiconductors are desirable for high-performance mechanically flexible nanoelectronics. In this work, we report the first flexible black phosphorus (BP) field-effect transistors (FETs) with electron and hole mobilities superior to what has been previously achieved with other more studied flexible layered semiconducting transistors such as MoS2 and WSe2. Encapsulated bottom-gated BP ambipolar FETs on flexible polyimide afforded maximum carrier mobility of about 310 cm(2)/V·s with field-effect current modulation exceeding 3 orders of magnitude. The device ambipolar functionality and high-mobility were employed to realize essential circuits of electronic systems for flexible technology including ambipolar digital inverter, frequency doubler, and analog amplifiers featuring voltage gain higher than other reported layered semiconductor flexible amplifiers. In addition, we demonstrate the first flexible BP amplitude-modulated (AM) demodulator, an active stage useful for radio receivers, based on a single ambipolar BP transistor, which results in audible signals when connected to a loudspeaker or earphone. Moreover, the BP transistors feature mechanical robustness up to 2% uniaxial tensile strain and up to 5000 bending cycles.

Thermal Oxidation of WSe2 Nanosheets Adhered on SiO2/Si Substrates

Because of the drastically different intralayer versus interlayer bonding strengths, the mechanical, thermal, and electrical properties of two-dimensional (2D) materials are highly anisotropic between the in-plane and out-of-plane directions. The structural anisotropy may also play a role in chemical reactions, such as oxidation, reduction, and etching. Here, the composition, structure, and electrical properties of mechanically exfoliated WSe2 nanosheets on SiO2/Si substrates were studied as a function of the extent of thermal oxidation. A major component of the oxidation, as indicated from optical and Raman data, starts from the nanosheet edges and propagates laterally toward the center. Partial oxidation also occurs in certain areas at the surface of the flakes, which are shown to be highly conductive by microwave impedance microscopy. Using secondary ion mass spectroscopy, we also observed extensive oxidation at the WSe2-SiO2 interface. The combination of multiple microcopy methods can thus provide vital information on the spatial evolution of chemical reactions on 2D materials and the nanoscale electrical properties of the reaction products.

Long-Term Stability and Reliability of Black Phosphorus Field-Effect Transistors

Black phosphorus has been recently suggested as a very promising material for use in 2D field-effect transistors. However, due to its poor stability under ambient conditions, this material has not yet received as much attention as for instance MoS2. We show that the recently demonstrated Al2O3 encapsulation leads to highly stable devices. In particular, we report our long-term study on highly stable black phosphorus field-effect transistors, which show stable device characteristics for at least eight months. This high stability allows us to perform a detailed analysis of their reliability with respect to hysteresis as well as the arguably most important reliability issue in silicon technologies, the bias-temperature instability. We find that the hysteresis in these transistors depends strongly on the sweep rate and temperature. Moreover, the hysteresis dynamics in our devices are reproducible over a long time, which underlines their high reliability. Also, by using detailed physical models for oxide traps developed for Si technologies, we are able to capture the channel electrostatics of the black phosphorus FETs and determine the position of the defect energy band. Finally, we demonstrate that both hysteresis and bias-temperature instabilities are due to thermally activated charge trapping/detrapping by oxide traps and can be reduced if the device is covered by Teflon-AF.

Electrogenerated chemiluminescence on the electrogenerated chemiluminescence (ECL) of tetrakis(pyrophosphito)diplatinate(II), Pt2(P2O5H2)44−☆

Abstract The electroreduction of Pt 2 (P 2 O 5 H 2 ) 4 3− (or A 4− ) in MeCN (0.1 M Bu 4 NBF 4 ) at a Pt electrode occurs at potentials where background reduction occurs and results in green light emission characteristic of A 4−3 . A reaction mechanism involving electron transfer between A 5− and a reduction product of Bu 4 N + is proposed. The emission that occurs at a mercury cathode shows a new band at ≈ 607 nm.

Pressure-Induced Charge Transfer Doping of Monolayer Graphene/MoS2 Heterostructure.

A unique way of achieving controllable, pressure-induced charge transfer doping in the graphene/MoS2 heterostructure is proposed. The charge transfer causes an upward shift in the Dirac point with respect to Fermi level at a rate of 15.7 meV GPa(-1) as a function of applied hydrostatic pressure, leading to heavy p-type doping in graphene. The doping was confirmed by I2D /IG measurements.

High pressure Raman study of layered Mo0.5W0.5S2ternary compound

Ternary two-dimensional (2D) transition metal dichalcogenide compounds exhibit a tunable electronic structure allowing for control of the interlayer and the intralayer atomic displacement to efficiently tune their physical and electronic properties. Using a diamond anvil cell, hydrostatic pressure was applied to Mo0.5W0.5S2 up to 40 GPa in order to study the optical phonon vibrational modes. Analysis of the high-pressure Raman spectra shows that the two in-plane E2g modes resembling that of pristine MoS2 and WS2, as well as disorder-activated longitudinal acoustic phonon mode, are hardened and suppressed as pressure increases. The two A1g modes of the ternary compound that resemble the A1g modes of pristine MoS2 and WS2, displayed similar Raman shifts to the pristine compounds as pressure increases. A Raman peak at 470 cm−1 that is close to A1g peaks emerges at ~8 GPa, which represents a disorder-activated pressure-induced out-of-plane Raman mode observed only in the ternary compound under high pressure. At pressures above ~30 GPa, a Raman peak at approximately 340 cm−1 is observed, signifying additional disorder-activated vibration mode. Our results reveal the enhanced interactions in the structural and vibrational behavior of the MoS2 and WS2 domains in the Mo0.5W0.5S2 compound under hydrostatic pressure. These results could have implications in understanding the electronic, optical, and structural properties of the new 2D ternary compound materials under extreme mechanical conditions.

Highly-stable black phosphorus field-effect transistors with low density of oxide traps

Black phosphorus is considered a very promising semiconductor for two-dimensional field-effect transistors. Initially, the main disadvantage of this material was thought to be its poor air stability. However, recent studies have shown that this problem can be solved by suitable encapsulation. As such, long-term studies of the outstanding properties of black phosphorus devices have become possible. In particular, here we examine highly-stable black phosphorus field-effect transistors and demonstrate that they can exhibit reproducible characteristics for at least 17 months. Furthermore, we notice some improvement in the performance of black phosphorus devices after this long time, i.e., positive aging. Although our black phosphorus devices are stable at room temperature, we show that their performance is affected by thermally activated charge trapping by oxide traps into the adjacent SiO2 substrate layer. Aiming to analyze the dynamics of these defects in detail, we perform an accurate mapping of oxide traps with different time constants using the ‘extended incremental hysteresis sweep method’. Our results show that at room temperature the extracted oxide trap densities are (i) few orders of magnitude lower than for MoS2/SiO2 transistors and (ii) close to those reported for more mature Si/SiO2 devices (~1017 cm−3 eV−1). Taking into account the novelty of black phosphorus and recent issues with its stability, these values must be considered unexpectedly low.Electronics: encapsulated black phosphorous enables stable, long-lasting transistorsField effect transistors made of ultra-thin black phosphorous can retain long-term stability and reproducible electrical characteristics. A team led by Prof. Deji Akinwande at UT Austin developed a conformal encapsulation method of black phosphorous transistors with a 25 nm thick Al2O3 layer. Characterization of these devices by Dr. Yury Illarionov at TU Wien has shown that encapsulation results in a substantial improvement of device stability and reliability for at least 17 months in ambient conditions. The density of oxide traps that would cause deleterious variations of the device threshold voltage, thus hindering reproducibility, is as low as 1017 cm−3/eV at room temperature. This is comparable to values obtained for commercial silicon devices. Remarkably, the subthreshold slope of the black phosphorous field-effect transistors becomes steeper after several months, a signature of performance improvement that points towards a positive aging effect, despite extended storage and operation in the laboratory environment.

(Invited) silicene and phosphorene: Progress on the intriguing case of buckled atomic sheets

Two-dimensional (2D) atomic sheets yield collective properties of mechanical flexibility, electrical control, optical transparency and high surface-to-volume ratio, which hold promise for advanced flexible nanoelectronics and sensors. This work explores two newly emerging 2D materials, silicene and phosphorene (the Si and P equivalent to graphene) and their air-stability and device study. The debut of silicene transistor confirms ambipolar transport behavior in atomically thin Si with greater gate modulation than graphene, indicating potential device reach beyond graphene. On the other hand, phosphorene exhibits high mobility and tunable direct bandgap even on plastic substrates, making it the most suitable contemporary 2D semiconductor that combines the merits of graphene and transitional metal dichalcogenides. This recent progress on silicene and phosphorene represent a renewed opportunity for future nanoscale and flexible devices.