Weinan Zhu

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.

Toward air-stable multilayer phosphorene thin-films and transistors

Few-layer black phosphorus (BP), also known as phosphorene, is poised to be the most attractive graphene analogue owing to its high mobility approaching that of graphene, and its thickness-tunable band gap that can be as large as that of molybdenum disulfide. In essence, phosphorene represents the much sought after high-mobility, large direct band gap two-dimensional layered crystal that is ideal for optoelectronics and flexible devices. However, its instability in air is of paramount concern for practical applications. Here, we demonstrate air-stable BP devices with dielectric and hydrophobic encapsulation. Microscopy, spectroscopy, and transport techniques were employed to elucidate the aging mechanism, which can initiate from the BP surface for bare samples, or edges for samples with thin dielectric coating, highlighting the ineffectiveness of conventional scaled dielectrics. Our months-long studies indicate that a double layer capping of Al2O3 and hydrophobic fluoropolymer affords BP devices and transistors with indefinite air-stability for the first time, overcoming a critical material challenge for applied research and development.

On the mobility and contact resistance evaluation for transistors based on MoS2 or two-dimensional semiconducting atomic crystals

Contact resistance (Rc) can substantially obscure the extracted mobility based on standard transconductance or two-point conductance measurements of field-effect devices especially for low density of states materials such as MoS2 or similar atomic crystals. Currently, there exists a pressing need for a routine technique that can decouple mobility extraction from Rc. By combining experiments and analysis, we show that the Y-function method offers a robust route for evaluating the low-field mobility, threshold voltage and Rc even when the contact is a Schottky-barrier as is common in two-dimensional transistors. In addition, an independent modified transfer length method evaluation corroborates the Y-function analysis.

Black Phosphorus Flexible Thin Film Transistors at Gighertz Frequencies

Black phosphorus (BP) has attracted rapidly growing attention for high speed and low power nanoelectronics owing to its compelling combination of tunable bandgap (0.3 to 2 eV) and high carrier mobility (up to ∼1000 cm(2)/V·s) at room temperature. In this work, we report the first radio frequency (RF) flexible top-gated (TG) BP thin-film transistors on highly bendable polyimide substrate for GHz nanoelectronic applications. Enhanced p-type charge transport with low-field mobility ∼233 cm(2)/V·s and current density of ∼100 μA/μm at VDS = -2 V were obtained from flexible BP transistor at a channel length L = 0.5 μm. Importantly, with optimized dielectric coating for air-stability during microfabrication, flexible BP RF transistors afforded intrinsic maximum oscillation frequency fMAX ∼ 14.5 GHz and unity current gain cutoff frequency fT ∼ 17.5 GHz at a channel length of 0.5 μm. Notably, the experimental fT achieved here is at least 45% higher than prior results on rigid substrate, which is attributed to the improved air-stability of fabricated BP devices. In addition, the high-frequency performance was investigated through mechanical bending test up to ∼1.5% tensile strain, which is ultimately limited by the inorganic dielectric film rather than the 2D material. Comparison of BP RF devices to other 2D semiconductors clearly indicates that BP offers the highest saturation velocity, an important metric for high-speed and RF flexible nanosystems.

Characterization and sonochemical synthesis of black phosphorus from red phosphorus

Phosphorene is a new two-dimensional material which is commonly prepared by exfoliation from black phosphorus bulk crystals that historically have been synthesized from white phosphorus under high-pressure conditions. The few layers of phosphorene have a direct band gap in the range of 0.3–2 eV and high mobility at room temperature comparable to epitaxial graphene. These characteristics can be used for the design of high speed digital circuits, radio frequency circuits, flexible and printed systems, and optoelectronic devices. In this work, we synthesized black phosphorus from red phosphorus, which is a safer solid precursor, using sonochemistry. Furthermore, via a variety of microscopy and spectroscopy techniques, we report characterization results of the sonochemically synthesized black phosphorus in addition to the commercial black phosphorus. Finally, we describe the air stability of black phosphors and the crystalline structure of the synthesized material. This is the first result of sonochemical or solution-based synthesis of black phosphorus based on readily available low-cost red phosphorus. This solution-based synthesis of black phosphorus is suitable for printable applications of nanomaterial.

High-frequency prospects of 2D nanomaterials for flexible nanoelectronics from baseband to sub-THz devices

We report on the state of the art sub-μm length (L) flexible two dimensional radio frequency thin film transistors operating in the velocity saturation regime for achieving maximum carrier transport or under high-field. We realize large-area monolayer MoS₂ on flexible polyimide with 5 GHz cut-off frequency (f_T), a record value for flexible synthesized transitional metal dichalcogenides (TMDs). For higher frequency devices, flexible black phosphorus (BP) RF TFT is demonstrated for the first time with f_T ~ 17.5 GHz for L = 0.5 μm, yielding v_sat ~ 5.5 × 10⁶ cm/s. In addition, for flexible sub-THz nanosystem front-ends, we have achieved record 100 GHz graphene TFTs (v_sat ~ 8.8 × 106 cm/s) on flexible glass, 56% higher than that of graphene TFTs on polymeric substrates.

Flexible 2D nanoelectronics from baseband to sub-THz transistors and circuits

In this work, we provide an overview of the state-of-the-art flexible two dimensional radio frequency transistors based on graphene, MoS2 and black phosphorus for nanoelectronics applications from baseband to sub THz, with outstanding electrical performance obtained fulfilling the requirements of both high speed operation and low power consumptions. Flexible RF transistors based on graphene with intrinsic cutoff frequency (fT) reaching 100GHz enable sub-THz flexible nano systems. Large scale CVD grown MoS2 based flexible RF transistor was realized for the first time with record intrinsic fT ~ 5.6GHz which can afford low power RF nano systems. In addition, black phosphorus based flexible transistors and circuits have been realized with outstanding electrical performance in both DC and high frequency applications while retaining mechanical robustness.

Emerging and Potential Opportunities for 2D Flexible Nanoelectronics

The last 10 years have seen the emergence of two-dimensional (2D) nanomaterials such as graphene, transition metal dichalcogenides (TMDs), and black phosphorus (BP) among the growing portfolio of layered van der Waals thin films. Graphene, the prototypical 2D material has advanced rapidly in device, circuit and system studies that has resulted in commercial large-area applications. In this work, we provide a perspective of the emerging and potential translational applications of 2D materials including semiconductors, semimetals, and insulators that comprise the basic material set for diverse nanosystems. Applications include RF transceivers, smart systems, the so-called internet of things, and neurotechnology. We will review the DC and RF electronic performance of graphene and BP thin film transistors. 2D materials at sub-um channel length have so far enabled cut-off frequencies from baseband to 100GHz suitable for low-power RF and sub-THz concepts.

(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.

Flexible phosphorene devices and circuits

Two-dimensional (2D) semiconductors with high carrier mobilities and sizeable bandgap are desirable for future high-speed and low power mechanically flexible nanoelectronics. In this work, we report encapsulated bottom-gated black phosphorus (BP) field-effect transistors (FETs) on flexible polyimide affording maximum carrier mobility of about 310cm2/V∙s and current on/off ratio exceeding 103. Essential circuits of flexible electronic systems enabled by the device ambipolar functionality, high-mobility and current saturation are demonstrated in this work, including digital inverter, frequency doubler, and analog amplifiers featuring a voltage gain of ~8.7, which is the state-of-the-art value for flexible 2D semiconductor based amplifiers. In addition, we demonstrate the single FET based flexible BP amplitude-modulated (AM) demodulator, an active stage in radio receivers.