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 10
, 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.
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 Weiss, Nathan O.; Zhou, Hailong; Liao, Lei ...
Advanced materials (Weinheim),
November 14, 2012, Letnik:
24, Številka:
43
Journal Article
Recenzirano
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.
Graphene offers a number of fundamentally superior qualities that make it a promising material for a wide range of applications, particularly in electronic devices. The versatility of graphene‐based devices includes high‐speed transistors, transparent and flexible electronics, displays, optoelectronics, sensors, nanoelectromechanical systems, and energy technologies. Many challenges remain before this relatively new material becomes commercially viable, but laboratory prototypes have already demonstrated the numerous advantages and novel functionality that graphene provides.
Two-dimensional layered materials (2DLMs) have been a central focus of materials research since the discovery of graphene just over a decade ago. Each layer in 2DLMs consists of a covalently bonded, ...dangling-bond-free lattice and is weakly bound to neighbouring layers by van der Waals interactions. This makes it feasible to isolate, mix and match highly disparate atomic layers to create a wide range of van der Waals heterostructures (vdWHs) without the constraints of lattice matching and processing compatibility. Exploiting the novel properties in these vdWHs with diverse layering of metals, semiconductors or insulators, new designs of electronic devices emerge, including tunnelling transistors, barristors and flexible electronics, as well as optoelectronic devices, including photodetectors, photovoltaics and light-emitting devices with unprecedented characteristics or unique functionalities. We review the recent progress and challenges, and offer our perspective on the exploration of 2DLM-based vdWHs for future application in electronics and optoelectronics.With a dangling-bond-free surface, two dimensional layered materials (2DLMs) can enable the creation of diverse van der Waals heterostructures (vdWHs) without the conventional constraint of lattice matching or process compatibility. This Review discusses the recent advances in exploring 2DLM vdWHs for future electronics and optoelectronics.
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 cm2/(V s) in MoS2 at low temperature.
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.
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.
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 103. 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.
Two-dimensional semiconductors (2DSCs) such as molybdenum disulfide (MoS2) have attracted intense interest as an alternative electronic material in the postsilicon era. However, the ON-current ...density achieved in 2DSC transistors to date is considerably lower than that of silicon devices, and it remains an open question whether 2DSC transistors can offer competitive performance. A high current device requires simultaneous minimization of the contact resistance and channel length, which is a nontrivial challenge for atomically thin 2DSCs, since the typical low contact resistance approaches for 2DSCs either degrade the electronic properties of the channel or are incompatible with the fabrication process for short channel devices. Here, we report a new approach toward high-performance MoS2 transistors by using a physically assembled nanowire as a lift-off mask to create ultrashort channel devices with pristine MoS2 channel and self-aligned low resistance metal/graphene hybrid contact. With the optimized contact in short channel devices, we demonstrate sub-100 nm MoS2 transistor delivering a record high ON-current of 0.83 mA/μm at 300 K and 1.48 mA/μm at 20 K, which compares well with that of silicon devices. Our study, for the first time, demonstrates that the 2DSC transistors can offer comparable performance to the 2017 target for silicon transistors in International Technology Roadmap for Semiconductors (ITRS), marking an important milestone in 2DSC electronics.
The electrical conductivity measured in Shewanella and Geobacter spp. is an intriguing physical property that is the fundamental basis for possible extracellular electron transport (EET) pathways. ...There is considerable debate regarding the origins of the electrical conductivity reported in these microbial cellular structures, which is essential for deciphering the EET mechanism. Here, we report systematic on-chip nanoelectronic investigations of both Shewanella and Geobacter spp. under physiological conditions to elucidate the complex basis of electrical conductivity of both individual microbial cells and biofilms. Concurrent electrical and electrochemical measurements of living Shewanella at both few-cell and the biofilm levels indicate that the apparent electrical conductivity can be traced to electrochemical-based electron transfer at the cell/electrode interface. We further show that similar results and conclusions apply to the Geobacter spp. Taken together, our study offers important insights into previously proposed physical models regarding microbial conductivities as well as EET pathways for Shewanella and Geobacter spp.
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