Nathan O. Weiss

Holey graphene frameworks for highly efficient capacitive energy storage

Supercapacitors represent an important strategy for electrochemical energy storage, but are usually limited by relatively low energy density. Here we report a three-dimensional holey graphene framework with a hierarchical porous structure as a high-performance binder-free supercapacitor electrode. With large ion-accessible surface area, efficient electron and ion transport pathways as well as a high packing density, the holey graphene framework electrode can deliver a gravimetric capacitance of 298 F g(-1) and a volumetric capacitance of 212 F cm(-3) in organic electrolyte. Furthermore, we show that a fully packaged device stack can deliver gravimetric and volumetric energy densities of 35 Wh kg(-1) and 49 Wh l(-1), respectively, approaching those of lead acid batteries. The achievement of such high energy density bridges the gap between traditional supercapacitors and batteries, and can open up exciting opportunities for mobile power supply in diverse applications.

Graphene: an emerging electronic material.

Graphene, a single layer of carbon atoms in a honeycomb lattice, offers a number of fundamentally superior qualities that make it a promising material for a wide range of applications, particularly in electronic devices. Its unique form factor and exceptional physical properties have the potential to enable an entirely new generation of technologies beyond the limits of conventional materials. The extraordinarily high carrier mobility and saturation velocity can enable a fast switching speed for radio-frequency analog circuits. Unadulterated graphene is a semi-metal, incapable of a true off-state, which typically precludes its applications in digital logic electronics without bandgap engineering. The versatility of graphene-based devices goes beyond conventional transistor circuits and includes flexible and transparent electronics, optoelectronics, sensors, electromechanical systems, and energy technologies. Many challenges remain before this relatively new material becomes commercially viable, but laboratory prototypes have already shown the numerous advantages and novel functionality that graphene provides.

Few-layer molybdenum disulfide transistors and circuits for high-speed flexible electronics

Two-dimensional layered materials, such as molybdenum disulfide, are emerging as an exciting material system for future electronics due to their unique electronic properties and atomically thin geometry. Here we report a systematic investigation of MoS2 transistors with optimized contact and device geometry, to achieve self-aligned devices with performance including an intrinsic gain over 30, an intrinsic cut-off frequency fT up to 42 GHz and a maximum oscillation frequency fMAX up to 50 GHz, exceeding the reported values for MoS2 transistors to date (fT~0.9 GHz, fMAX~1 GHz). Our results show that logic inverters or radio frequency amplifiers can be formed by integrating multiple MoS2 transistors on quartz or flexible substrates with voltage gain in the gigahertz regime. This study demonstrates the potential of two-dimensional layered semiconductors for high-speed flexible electronics.

Chemical vapor deposition growth of monolayer MoSe2 nanosheets

The synthesis of two-dimensional (2D) layered materials with controllable thickness is of considerable interest for diverse applications. Here we report the first chemical vapor deposition growth of single- and few-layer MoSe2 nanosheets. By using Se and MoO3 as the chemical vapor supply, we demonstrate that highly crystalline MoSe2 can be directly grown on the 300 nm SiO2/Si substrates to form optically distinguishable single- and multi-layer nanosheets, typically in triangular shaped domains with edge lengths around 30 μm, which can merge into continuous thin films upon further growth. Micro-Raman spectroscopy and imaging was used to probe the thickness-dependent vibrational properties. Photoluminescence spectroscopy demonstrates that MoSe2 monolayers exhibit strong near band edge emission at 1.55 eV, while bilayers or multi-layers exhibit much weaker emission, indicating of the transition to a direct band gap semiconductor as the thickness is reduced to a monolayer.

Large Area Growth and Electrical Properties of p-Type WSe2 Atomic Layers

Transition metal dichacogenides represent a unique class of two-dimensional layered materials that can be exfoliated into single or few atomic layers. Tungsten diselenide (WSe2) is one typical example with p-type semiconductor characteristics. Bulk WSe2 has an indirect band gap (∼1.2 eV), which transits into a direct band gap (∼1.65 eV) in monolayers. Monolayer WSe2, therefore, is of considerable interest as a new electronic material for functional electronics and optoelectronics. However, the controllable synthesis of large-area WSe2 atomic layers remains a challenge. The studies on WSe2 are largely limited by relatively small lateral size of exfoliated flakes and poor yield, which has significantly restricted the large-scale applications of the WSe2 atomic layers. Here, we report a systematic study of chemical vapor deposition approach for large area growth of atomically thin WSe2 film with the lateral dimensions up to ∼1 cm2. Microphotoluminescence mapping indicates distinct layer dependent efficiency. The monolayer area exhibits much stronger light emission than bilayer or multilayers, consistent with the expected transition to direct band gap in the monolayer limit. The transmission electron microscopy studies demonstrate excellent crystalline quality of the atomically thin WSe2. Electrical transport studies further show that the p-type WSe2 field-effect transistors exhibit excellent electronic characteristics with effective hole carrier mobility up to 100 cm2 V–1 s–1 for monolayer and up to 350 cm2 V–1 s–1 for few-layer materials at room temperature, comparable or well above that of previously reported mobility values for the synthetic WSe2 and comparable to the best exfoliated materials.

Toward barrier free contact to molybdenum disulfide using graphene electrodes.

Two-dimensional layered semiconductors such as molybdenum disulfide (MoS2) have attracted tremendous interest as a new class of electronic materials. However, there are considerable challenges in making reliable contacts to these atomically thin materials. Here we present a new strategy by using graphene as the back electrodes to achieve ohmic contact to MoS2. With a finite density of states, the Fermi level of graphene can be readily tuned by a gate potential to enable a nearly perfect band alignment with MoS2. We demonstrate for the first time a transparent contact to MoS2 with zero contact barrier and linear output behavior at cryogenic temperatures (down to 1.9 K) for both monolayer and multilayer MoS2. Benefiting from the barrier-free transparent contacts, we show that a metal-insulator transition can be observed in a two-terminal MoS2 device, a phenomenon that could be easily masked by Schottky barriers found in conventional metal-contacted MoS2 devices. With further passivation by boron nitride (BN) encapsulation, we demonstrate a record-high extrinsic (two-terminal) field effect mobility up to 1300 cm(2)/(V s) in MoS2 at low temperature.

Electric-field-induced strong enhancement of electroluminescence in multilayer molybdenum disulfide

The layered transition metal dichalcogenides have attracted considerable interest for their unique electronic and optical properties. While the monolayer MoS2 exhibits a direct bandgap, the multilayer MoS2 is an indirect bandgap semiconductor and generally optically inactive. Here we report electric-field-induced strong electroluminescence in multilayer MoS2. We show that GaN–Al2O3–MoS2 and GaN–Al2O3–MoS2–Al2O3-graphene vertical heterojunctions can be created with excellent rectification behaviour. Electroluminescence studies demonstrate prominent direct bandgap excitonic emission in multilayer MoS2 over the entire vertical junction area. Importantly, the electroluminescence efficiency observed in multilayer MoS2 is comparable to or higher than that in monolayers. This strong electroluminescence can be attributed to electric-field-induced carrier redistribution from the lowest energy points (indirect bandgap) to higher energy points (direct bandgap) in k-space. The electric-field-induced electroluminescence is general for other layered materials including WSe2 and can open up a new pathway towards transition metal dichalcogenide-based optoelectronic devices.

Metal-Organic Framework Templated Synthesis of Ultrathin, Well-Aligned Metallic Nanowires

With well-defined porous structures and dimensions, metal-organic frameworks (MOFs) can function as versatile templates for the growth of metallic nanostructures with precisely controlled shapes and sizes. Using MOFs as templates, metallic nanostructures can be grown without the need of bulky surfactants and thus preserve their intrinsic surface. Additionally, the high surface area of MOFs can also ensure that the surface of the template metallic nanostructures is readily accessible, which is critical for the proper function of catalysts or sensors. The hybrid metal@MOF structures have been demonstrated to exhibit useful properties not found in either component separately. Here we report the growth of ultrafine metallic nanowires inside one-dimensional MOF pores with well-controlled shape and size. Our study shows that solvent selection plays an important role in controlling precursor loading and the reduction rate inside the MOF pores for the formation of the nanowires. The growth of the well-aligned, ultrathin nanowires was monitored and characterized by transmission electron microscopy, X-ray diffraction, UV-vis spectroscopy, fluorescence studies, and Brunauer-Emmet-Teller surface area analysis.

High-Performance Organic Vertical Thin Film Transistor Using Graphene as a Tunable Contact.

Here we present a general strategy for the fabrication of high-performance organic vertical thin film transistors (OVTFTs) based on the heterostructure of graphene and different organic semiconductor thin films. Utilizing the unique tunable work function of graphene, we show that the vertical carrier transport across the graphene-organic semiconductor junction can be effectively modulated to achieve an ON/OFF ratio greater than 10(3). Importantly, with the OVTFT design, the channel length is determined by the organic thin film thickness rather than by lithographic resolution. It can thus readily enable transistors with ultrashort channel lengths (<200 nm) to afford a delivering current greatly exceeding that of conventional planar TFTs, thus enabling a respectable operation frequency (up to 0.4 MHz) while using low-mobility organic semiconductors and low-resolution lithography. With this vertical device architecture, the entire organic channel is sandwiched and naturally protected between the source and drain electrodes, which function as the self-passivation layer to ensure stable operation of both p- and n-type OVTFTs in ambient conditions and enable complementary circuits with voltage gain. The creation of high-performance and highly robust OVTFTs can open up exciting opportunities in large-area organic macroelectronics.

Monolayer atomic crystal molecular superlattices

Artificial superlattices, based on van der Waals heterostructures of two-dimensional atomic crystals such as graphene or molybdenum disulfide, offer technological opportunities beyond the reach of existing materials. Typical strategies for creating such artificial superlattices rely on arduous layer-by-layer exfoliation and restacking, with limited yield and reproducibility. The bottom-up approach of using chemical-vapour deposition produces high-quality heterostructures but becomes increasingly difficult for high-order superlattices. The intercalation of selected two-dimensional atomic crystals with alkali metal ions offers an alternative way to superlattice structures, but these usually have poor stability and seriously altered electronic properties. Here we report an electrochemical molecular intercalation approach to a new class of stable superlattices in which monolayer atomic crystals alternate with molecular layers. Using black phosphorus as a model system, we show that intercalation with cetyl-trimethylammonium bromide produces monolayer phosphorene molecular superlattices in which the interlayer distance is more than double that in black phosphorus, effectively isolating the phosphorene monolayers. Electrical transport studies of transistors fabricated from the monolayer phosphorene molecular superlattice show an on/off current ratio exceeding 107, along with excellent mobility and superior stability. We further show that several different two-dimensional atomic crystals, such as molybdenum disulfide and tungsten diselenide, can be intercalated with quaternary ammonium molecules of varying sizes and symmetries to produce a broad class of superlattices with tailored molecular structures, interlayer distances, phase compositions, electronic and optical properties. These studies define a versatile material platform for fundamental studies and potential technological applications.