Vapor Deposition of Bilayer 3R MoS2 with Room-Temperature Ferroelectricity.

Two-dimensional ultrathin ferroelectrics have attracted much interest due to their potential application in high-density integration of non-volatile memory devices. Recently, 2D van der Waals ferroelectric based on interlayer translation has been reported in twisted bilayer h-BN and transition metal dichalcogenides (TMDs). However, sliding ferroelectricity is not well studied in non-twisted homo-bilayer TMD grown directly by chemical vapor deposition (CVD). In this paper, for the first time, experimental observation of a room-temperature out-of-plane ferroelectric switch in semiconducting bilayer 3R MoS2 synthesized by reverse-flow CVD is reported. Piezoelectric force microscopy (PFM) hysteretic loops and first principle calculations demonstrate that the ferroelectric nature and polarization switching processes are based on interlayer sliding. The vertical Au/3R MoS2/Pt device exhibits a switchable diode effect. Polarization modulated Schottky barrier height and polarization coupling of interfacial deep states trapping/detrapping may serve in coordination to determine switchable diode effect. The room-temperature ferroelectricity of CVD-grown MoS2 will proceed with the potential wafer-scale integration of 2D TMDs in the logic circuit.

Van der Waals Encapsulation by Ultra-thin Oxide for Air-Sensitive 2D Materials.

The ambient stability is one of the focal points for applications of 2D materials, especially for those well-known air-sensitive ones such as black phosphorus (BP) and transitional metal telluride. Traditional methods of encapsulation, such as atomic layer deposition of oxides and heterogeneous integration of hexagonal boron nitride, can hardly avoid removal of encapsulation layer when the 2D materials are encapsulated for further device fabrication, which causes complexity and damage during the procedure. Here, a van der Waals encapsulation method that allows direct device fabrication without removal of encapsulation layer is introduced using Ga2O3 from liquid gallium. Taking advantage of the robust isolation ability against ambient environment of the dense native oxide of gallium, hundreds of times longer retention time of (opto)electronic properties of encapsulated BP and MoTe2 devices is realized than unencapsulated devices. Due to the ultra-thin high-κ properties of Ga2O3, top-gated devices are directly fabricated with the encapsulation layer, simultaneously as a dielectric layer. This direct device fabrication is realized by selective etching of Ga2O3, leaving the encapsulated materials intact. Encapsulated 1T' MoTe2 exhibits high conductivity even after 150 days in ambient environment. This method is therefore highlighted as a promising and distinctive one compared with traditional passivation approaches. This article is protected by copyright. All rights reserved.

Plasma-Enhanced Chemical Vapor Deposition of Two-Dimensional Materials for Applications.

ConspectusSince the rise of two-dimensional (2D) materials, synthetic methods including mechanical exfoliation, solution synthesis, and chemical vapor deposition (CVD) have been developed. Mechanical exfoliation prepares randomly shaped materials with small size. Solution synthesis introduces impurities that degrade the performances. CVD is the most successful one for low-cost scalable preparation. However, when it comes to practical applications, disadvantages such as high operating temperature (∼1000 °C), probable usage of metal catalysts, contamination, defects, and interstices introduced by postgrowth transfer are not negligible. These are the reasons why plasma-enhanced CVD (PECVD), a method that enables catalyst-free in situ preparation at low temperature, is imperatively desirable.In this Account, we summarize our recent progress on controllable preparation of 2D materials by PECVD and their applications. We found that there was a competition between etching and nucleation and deposition in PECVD, making it highly controllable to obtain desired materials. Under different equilibrium states of the competition, various 2D materials with diverse morphologies and properties were prepared including pristine or nitrogen-doped graphene crystals, graphene quantum dots, graphene nanowalls, hexagonal boron nitride (h-BN), B-C-N ternary materials (BCxN), etc. We also used mild plasma to modify or treat 2D materials (e.g., WSe2) for desired properties.PECVD has advantages such as low temperature, transfer-free process, and industrial compatibility, which enable facile, scalable, and low-cost preparation of 2D materials with clean surfaces and interfaces directly on noncatalytic substrates. These merits significantly benefit the as-prepared materials in the applications. Field-effect transistors with high motilities were directly fabricated on graphene and nitrogen-doped graphene. By use of h-BN as the dielectric interfacial layer, both mobilities and saturated power densities of the devices were improved owing to the clean, closely contacted interface and enhanced interfacial thermal dissipation. High-quality materials and interfaces also enabled promising applications of these materials in photodetectors, pressure sensors, biochemical sensors, electronic skins, Raman enhancement, etc. To demonstrate the commercial applications, several prototypical devices were studied such as distributed pressure sensor arrays, touching module on a robot hand for braille recognition, and smart gloves for recording sign language. Finally, we discuss opportunities and challenges of PECVD as a comprehensive preparation methodology of 2D materials for future applications beyond traditional CVD.

Catalyst-Free Growth of Two-Dimensional BCxN Materials on Dielectrics by Temperature-Dependent Plasma-Enhanced Chemical Vapor Deposition.

Traditional methods to prepare two-dimensional (2D) B-C-N ternary materials (BCxN), such as chemical vapor deposition (CVD), require sophisticated experimental conditions such as high temperature, delicate control of precursors, and postgrowth transfer from catalytic substrates, and the products are generally thick or bulky films without the atomically mixed phase of B-C-N, hampering practical applications of these materials. Here, for the first time, we develop a temperature-dependent plasma-enhanced chemical vapor deposition (PECVD) method to grow 2D BCxN materials directly on noncatalytic dielectrics at low temperature with high controllability. The C, N, and B compositions can be tuned by simply changing the growth temperature. Thus, the properties of the as-made materials including band gap and conductivity are modulated, which is hardly achieved by other methods. A 2D hybridized BC2N film with a mixed BC2N phase is produced, for the first time, with a band gap of about 2.3 eV. The growth temperature is 580-620 °C, much lower than that of traditional catalytic CVD for growing BCxN. The product has a p-type conducting property and can be directly applied in field-effect transistors and sensors without postgrowth transfer, showing great promise for this method in future applications.

Free radical sensors based on inner-cutting graphene field-effect transistors.

Due to ultra-high reactivity, direct determination of free radicals, especially hydroxyl radical (•OH) with ultra-short lifetime, by field-effect transistor (FET) sensors remains a challenge, which hampers evaluating the role that free radical plays in physiological and pathological processes. Here, we develop a •OH FET sensor with a graphene channel functionalized by metal ion indicators. At the electrolyte/graphene interface, highly reactive •OH cuts the cysteamine to release the metal ions, resulting in surface charge de-doping and a current response. By this inner-cutting strategy, the •OH is selectively detected with a concentration down to 10-9 M. Quantitative metal ion doping enables modulation of the device sensitivity and a quasi-quantitative detection of •OH generated in aqueous solution or from living cells. Owing to its high sensitivity, selectivity, real-time label-free response, capability for quasi-quantitative detection and user-friendly portable feature, it is valuable in biological research, human health, environmental monitoring, etc.

Conformal hexagonal-boron nitride dielectric interface for tungsten diselenide devices with improved mobility and thermal dissipation.

Relatively low mobility and thermal conductance create challenges for application of tungsten diselenide (WSe2) in high performance devices. Dielectric interface is of extremely importance for improving carrier transport and heat spreading in a semiconductor device. Here, by near-equilibrium plasma-enhanced chemical vapour deposition, we realize catalyst-free growth of poly-crystalline two-dimensional hexagonal-boron nitride (2D-BN) with domains around 20~ 200 nm directly on SiO2/Si, quartz, sapphire, silicon or SiO2/Si with three-dimensional patterns at 300 °C. Owing to the atomically-clean van-der-Walls conformal interface and the fact that 2D-BN can better bridge the vibrational spectrum across the interface and protect interfacial heat conduction against substrate roughness, both improved performance and thermal dissipation of WSe2 field-effect transistor are realized with mobility around 56~ 121 cm2 V-1 s-1 and saturated power intensity up to 4.23 × 103 W cm-2. Owing to its simplicity, conformal growth on three-dimensional surface, compatibility with microelectronic process, it has potential for application in future two-dimensional electronics.

Enhanced photoelectrical response of thermodynamically epitaxial organic crystals at the two-dimensional limit.

Owing to strong light-matter interaction, two-dimensional (2D) organic crystal is regarded as promising materials for ultrasensitive photodetectors, however it still received limited success due to degraded photoelectrical response and problems in controllable growth. Here, we find the growth of 2D organic crystal obeys Gibbs-Curie-Wulff law, and develop a seed-epitaxial drop-casting method to grow millimeter-sized 1,4-bis(4-methylstyryl)benzene 2D crystals on SiO2/Si in a thermodynamically controlled process. On SiO2/Si, a distinct 2D limit effect is observed, which remarkably enhances internal photoresponsivity compared with bulk crystals. Experiment and calculation show the molecules stack more compactly at the 2D limit, thus better molecular orbital overlap and corresponding changes in the band structure lead to efficient separation and transfer of photo-generated carriers as well as enhanced photo-gating modulation. This work provides a general insight into the growth and the dimension effect of the 2D organic crystal, which is valuable for the application in high-performance photoelectrical devices.