Light-emitting Field-effect Transistors Research Articles

One-dimensional array of point-like light sources based on gold nanoparticles and tetracene: Preparation and possible operation mechanisms

A method of preparation of a linear close-packed array of point-like light sources based on a nanocomposite of gold nanoparticles and tetracene is proposed. Ordered system of microleads to the light sources with packing density up to 1000 mm−1 consists of linear conducting chains of cobalt nanoparticles self-assembled in a magnetic field. The electroluminescence from the gold-tetracene nanocomposite occurs in the visible range typical of organic light-emitting field-effect transistors based on tetracene. A theoretical substantiation of the possibility of excitation of tetracene molecules by hot electrons emitted from the gold nanoparticles is suggested and compared with other possible physical mechanisms.

High-Mobility, Heterostructure Light-Emitting Transistors and Complementary Inverters

Light-emitting field effect transistors (LEFETs) are optoelectronic devices that can simultaneously execute light emission and the standard logic functions of a transistor in a single device architecture. In this article, we show that ambipolar LEFETs can be made in a bilayer structure using Super Yellow, a light-emitting polymer layer, and a high-mobility diketopyrrolo-pyrrole-based copolymer as an ambipolar charge-transporting layer. The LEFETs were fabricated in the bottom gate architecture with top-contact, air-stable, symmetric Au–Au electrodes. The devices show light emission in both electron and hole accumulation modes with an external quantum efficiency (EQE) of 0.1% at a brightness of 650 cd/m2 in electron accumulation mode, and an EQE of 0.001% at a brightness of 4 cd/m2 in hole accumulation mode. We have also demonstrated a light-emitting inverter by combining two LEFETs into the inverter architecture. The light-emitting inverter generates both electrical and optical signals with an electrical gain of 112.

The color change in polychromatic organic light-emitting field-effect transistors: Optical filtering versus reemission

In this contribution the color conversion process of a polychromatic organic light-emitting field-effect transistor (OLET) is revisited on the basis of an analytic device model. The device of interest consists of a color conversion layer out of rubrene on top of a monochromatic light-emitting transistor based on poly(9,9-di-n-octyl-fluorene-alt-benzothiadiazole) (F8BT). The model describes the relation of color coordinate and emission intensity – set by the applied drain and gate biases – linking the optoelectronic response of the employed monochromatic OLET to the optical processes occurring in the color conversion layer. The model shows that the color shift is rather due to partial absorption of the F8BT emission by rubrene than, as was claimed earlier, due to a color conversion process by absorption and reemission in the conversion layer. In addition to the earlier publication, it will be demonstrated that such a device allows for an independent electrical tunability of emission intensity and color coordinate within the color span of the F8BT and the rubrene spectrum being a unique feature of such a polychromatic light-emitting field-effect transistor.

Light-emitting transistor structures based on semiconducting polymers and inorganic nanoparticles

One of the new directions in organic electronics is the development of light-emitting organic field-effect transistors, which combine the light-emitting properties of organic light-emitting diodes and the switching properties of organic field-effect transistors. Optical and electronic properties of novel nanocomposite materials based on semiconducting polymers and inorganic nanoparticles and designed for applications in organic electronics devices were investigated. Light-emitting organic field-effect transistors with composite active layers based on the soluble semiconducting polymers PFO and MEH-PPV and ZnO nanoparticles and having asymmetric electrodes (Al and Au) that inject electrons into ZnO and holes into PFO and MEH-PPV were prepared and investigated. The data are interpreted in the context of the possibility of organic field-effect transistors based on PFO: ZnO and MEH-PPV: ZnO composite films to work in both the unipolar regime and the ambipolar regime. It is shown that the mobility of charge carriers in light-emitting organic field-effect transistors based on PFO: ZnO at 300 K reaches ∼0.02 for electrons and ∼0.03 cm2/(V s) for holes, increasing with an increase in the concentration of nanoparticles up to ∼2 cm2/(V s), a value that is comparable to the maximum mobility values for conducting polymers. Light-emitting organic field-effect transistors based on PFO: ZnO and MEH-PPV: ZnO emit light in the green and orange ranges of the optical spectrum, respectively, their electroluminescence intensities rising with an increase in either the negative bias or the positive bias at the source-drain and the gate as well as with an increase in the concentration of ZnO nanoparticles. The results indicate that light-emitting organic field-effect transistors based on soluble conjugated polymers and semiconducting ZnO nanoparticles are examples of multifunctional devices whose production technology is compatible with the modern ink-jet printing technology of organic electronics.

Synthesis of a Novel D-A Type Photoelectrical Material: 5,5'-(9,9-Dioctyl-9H-Fluorene-2,7-Diyl)bis(Thiophene-2-Carbaldehyde)

A donor-acceptor organic molecule based on fluorene unit as an electron donor and aldehyde group as an electron acceptor has been demonstrated in high yields over four steps. This approach offers a much milder and more efficient route to synthesize the target compound via the Vilsmeier-Haack reaction. Optical spectra show that the electron-accepting groups induce an intermolecular charge transfer, resulting in a shift of the absorbance maximum toward longer wavelength. Such D-A type intermediate compounds as organic molecules display a significantly improved property profile in photoelectrical materials for applications in organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), and organic field-effect transistors (OFETs).

Study on copper phthalocyanine and perylene-based ambipolar organic light-emitting field-effect transistors produced using neutral beam deposition method

The neutral cluster beam deposition (NCBD) method has been applied to the production and characterization of ambipolar, heterojunction-based organic light-emitting field-effect transistors (OLEFETs) with a top-contact, multi-digitated, long-channel geometry. Organic thin films of n-type N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide and p-type copper phthalocyanine were successively deposited on the hydroxyl-free polymethyl-methacrylate (PMMA)-coated SiO2 dielectrics using the NCBD method. Characterization of the morphological and structural properties of the organic active layers was performed using atomic force microscopy and X-ray diffraction. Various device parameters such as hole- and electron-carrier mobilities, threshold voltages, and electroluminescence (EL) were derived from the fits of the observed current-voltage and current-voltage-light emission characteristics of OLEFETs. The OLEFETs demonstrated good field-effect characteristics, well-balanced ambipolarity, and substantial EL under ambient conditions. The device performance, which is strongly correlated with the surface morphology and the structural properties of the organic active layers, is discussed along with the operating conduction mechanism.

Computational Evaluation of Optoelectronic Properties for Organic/Carbon Materials

CONSPECTUS: Organic optoelectronic materials are used in a variety of devices, including light-emitting diodes, field-effect transistors, photovoltaics, thermoelectrics, spintronics, and chemico- and biosensors. The processes that determine the intrinsic optoelectronic properties occur either in the photoexcited states or within the electron-pumped charged species, and computations that predict these optical and electrical properties would help researchers design new materials. In this Account, we describe recent advances in related density functional theory (DFT) methods and present case studies that examine the efficiency of light emission, carrier mobility, and thermoelectric figures of merit by calculation of the electron-vibration couplings. First we present a unified vibrational correlation function formalism to evaluate the excited-state radiative decay rate constant kr, the nonradiative decay rate constant knr, the intersystem crossing rate constant kISC, and the optical spectra. The molecular parameters that appear in the formalism, such as the electronic excited-state energy, vibrational modes, and vibronic couplings, require extensive DFT calculations. We used experiments for anthracene at both low and ambient temperatures to benchmark the calculated photophysical parameters. In the framework of Fermi's golden rule, we incorporated the non-adiabatic coupling and the spin-orbit coupling to evaluate the phosphorescence efficiency and emission spectrum. Both of these are in good agreement with experimental results for anthracene and iridium compounds. Band electron scattering and relaxation processes within Boltzmann theory can describe charge transport in two-dimensional carbon materials and closely packed organic solids. For simplicity, we considered only the acoustic phonon scattering as modeled by the deformation potential approximation coupled with extensive DFT calculations for band structures. We then related the carrier mobility to the band-edge shift associated with the lattice dilation of longitudinal waves. The calculated relaxation time was in good agreement with experimental data for the graphene sheet, which supports the methodology. We then found that the intrinsic electron mobility for a 6,6,12-graphyne sheet can be even larger than that of graphene. We extended this approach to investigate the thermoelectric transport of electrons in metal phthalocyanines, which showed reasonable Seebeck coefficients when compared with experiments. For the thermal lattice transport, we employed nonequilibrium molecular dynamics simulations. Combining both electron transport and lattice thermal conductivity, we can evaluate the thermoelectric figure of merit.

Research on Charge Accumulation and Transport in Hole-Conductor-Free Perovskite (CH3NH3PbI2Cl) -based Nanostructure Solar Cells

Organic-inorganic lead halide perovskite, initially applied as active layer in light-emitting diode (LED) and field-effect transistor ( FET) devices[1], has been gradually introduced into the development of solar cells and plays such critical role as either light absorber or hole conducting material (HCM). This kind of hybrid layer structure makes perovskite as a promising candidate in photovoltaic applications, mainly due to its attractive chemical versatility that can readily tune the electronic coupling between inorganic sheets, and eventually tailor its optical/electronic properties[2]. In order to understand the performance of a photovoltaic device especially involving such novel material for its further optimization consideration, exploring the photogenerated charge accumulation and transport is imperative to be highlighted. Here we report the work on perovskite (CH3NH3PbI2Cl[3])-based nanostructure solar cell, including the cell fabrication and characterization, and particularly provide the analysis on charge carrier properties on the basis of dynamic electrochemical measurements. In this research stage, we targets more than achieving relatively fair energy conversion efficiency, instead we hope to uncover internal mechanisms of charge carrier movement and subsequently the substantial function of perovskite in photovoltaic device.The stack structure of the mesoscopic solar cell we developed for this investigation is shown in Fig. 1(a). The nanoporous semiconductor oxide membrane was fabricated by screen-printing technique on fluorine doped tin oxide (FTO) glass substrate with terpineol-based paste made by commercial-available TiO2 powder (Aldrich, d=25nm). The work electrode coated with the TiO2 paste was sintered under airflow at 550oC for 3 h. Iodide-chloride mixed-halide perovskite was synthesized from a N,N-Dimethylformamide solution of CH3NH3I and PbCl2 (3:1 molar ratio) [3]. The precursor solution was deposited on porous TiO2 surface via spin-coating or dipping process, and then was dried at 100oC. After drying, obvious color variation indicated the formation of perovskite layer. The function of the perovskite layer in nanostructure solar cells strongly depends on the deposition method. In particular, the shape of nanocrystalline CH3NH3PbI2Cl infiltrating into TiO2 porous membrane, probably with quantum-dot structure, is confirmed by electron microscopy micrographs. One thing should be noted is that, combining the band diagram shown in Fig.1(b), the extremely thin CsSnI2.95F0.05 layer (referring our previous work for its synthesis[4]) deposited on counter electrode performs more like energy ‘staircase’ for hole transport rather than pure hole conducting material. Meanwhile, a heterojunction-like cell (called ‘blank’ cell) containing only CH3NH3PbI2Cl layer on the TiO2film is produced for comparison.In order to thoroughly understand the decisive mechanisms of charge accumulation and transport behaviors on photovoltaic conversion, a systematic characterizations including photocurrent transient, open-circuit voltage decay (OCVD) and electrochemical impedance spectroscopy (EIS) were carried out under different operation conditions. Correspondingly the effective lifetime and diffusivity of charge carriers are extracted to investigate the transport properties of photon-generated electrons. Particularly, chemical capacitance (Cμ) is examined since it assists to monitor the Fermi level of electrons and density of states (DoS) in perovskite absorber, which significantly differentiates from conventional dye (e.g. N719) sensitized solar cells on the aspect of charge generation and accumulation in light absorber material.Consequently, our research demonstrates the promising HCM-free nanostructure solar cells involving solution-processed perovskite material. Fair light absorption and charge accumulation capability make self-assembled CH3NH3PbI2Cl behave as both sensitizer and charge transport material, which helps to constitute new kind of solid-state sensitized solar cells with a much simpler configuration. Reference 1. D. B. Mitzi, K. Chondroudis, and C.R. Kagan, Organic–inorganic electronics. IBM J. Res. & Dev. , 2001. 45(1): p. 29-45.2. Etgar, L., Semiconductor Nanocrystals as Light Harvesters in Solar Cells. Materials 2013. 6: p. 445--59.3. Lee, M.M., et al., Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science, 2012. 338 ( 6107): p. 643-647.4. Sun, X., et al., Effects of calcination treatment on the morphology, crystallinity, and photoelectric properties of all-solid-state dye-sensitized solar cells assembled by TiO2 nanorod arrays. Phys. Chem. Chem. Phys., 2013. 15: p. 18716-18720.

Magnetoluminescence of light-emitting field-effect transistors based on alpha sexithiophene

We demonstrated the effect of a magnetic field on the luminous intensity and electric current of light-emitting field-effect transistors (LEFETs) based on alpha sexithiophene (α-6T). Sublimate-grade α-6T was thermally deposited on an n+-Si/300 nm-SiO2 substrate with patterned asymmetric gold-aluminum electrodes to fabricate a bottom-contact LEFET. We observed an increase in luminous intensity of approximately 1.3% under a magnetic field of 100 mT. A possible explanation for this is that the magnetic field increased the probability of singlet formation at the α-6T/Al interface. While the magneto-electroluminescence (MEL) was reported to be derived from the magneto-conductance (MC) in ordinary light emitting diodes, the MEL in LEFET was independent with MC. This indicates that the luminous efficiency can be improved by optimizing the magnetic field effect.

Light-emitting field-effect transistors combining organic and metal oxide layers with partitioned heterogeneous source and drain electrodes

We fabricated organic light-emitting field-effect transistors (OLEFETs) characterized by partitioned heterogeneous source and drain contacts along with an aluminum-doped zinc oxide (AZO) layer inserted between an organic layer and a gate insulator. We elaborated such contacts so that each contact was made of a metal(s) suitable for injecting either electrons or holes. We fabricated the devices by choosing two of three kinds of metals (Au, Al, and MgAg) and one of three organic semiconductor materials. In the devices with the Au source and MgAg drain contacts, we observed drain currents at both positive and negative drain voltages. Those currents were predominant at negative drain voltages in the devices with Al drain contacts. The most intense current-injected emissions arose from the vicinity of the electron injection contact edges near channels on the AZO layers. Taking into account the energy level consideration of the materials and the effect of the partitioned contacts, we discussed these electrical and emission properties.

Exotic optoelectronic properties of organic semiconductors with super-controlled nanoscale sizes and molecular shapes.

We present several aspects of thiophene/phenylene co-oligomers (TPCOs). TPCOs are regarded as a newly occurring class of organic semiconductors. These materials are synthesized by hybridizing thiophene and phenylene rings at the molecular level with their various mutual arrangements. These materials are characterized by the super-controlled nanoscale sizes and molecular shapes. These produce peculiar crystallographic structures and high-performance optical and electronic properties. The crystals of TPCOs were obtained through both vapor phase and liquid phase. In the TPCO crystals, the molecules take upright configuration. These cause large carrier mobilities of field-effect transistors and laser oscillations under optical excitations. Spectrally-narrowed emissions (SNEs) were also achieved under weak optical excitation using a mercury lamp. The light-emitting field-effect transistors using these crystals for an active layer have shown the current-injected SNEs when the device was combined with an optical cavity and operated by an alternating-current gate-voltage method. Thus the TPCO materials will play an important role in the future in the fields of nanoscale technology and organic semiconductor materials as well as their optoelectronic device applications.

Local Electronic and Chemical Structure of Oligo-acetylene Derivatives Formed Through Radical Cyclizations at a Surface

Semiconducting π-conjugated polymers have attracted significant interest for applications in light-emitting diodes, field-effect transistors, photovoltaics, and nonlinear optoelectronic devices. Central to the success of these functional organic materials is the facile tunability of their electrical, optical, and magnetic properties along with easy processability and the outstanding mechanical properties associated with polymeric structures. In this work we characterize the chemical and electronic structure of individual chains of oligo-(E)-1,1′-bi(indenylidene), a polyacetylene derivative that we have obtained through cooperative C1–C5 thermal enediyne cyclizations on Au(111) surfaces followed by a step-growth polymerization of the (E)-1,1′-bi(indenylidene) diradical intermediates. We have determined the combined structural and electronic properties of this class of oligomers by characterizing the atomically precise chemical structure of individual monomer building blocks and oligomer chains (via noncontact atomic force microscopy (nc-AFM)), as well as by imaging their localized and extended molecular orbitals (via scanning tunneling microscopy and spectroscopy (STM/STS)). Our combined structural and electronic measurements reveal that the energy associated with extended π-conjugated states in these oligomers is significantly lower than the energy of the corresponding localized monomer orbitals, consistent with theoretical predictions.

Light-emitting organic field-effect transistors based on highly luminescent single crystals of thiophene/phenylene co-oligomers

A series of thiophene/phenylene co-oligomers containing π-conjugated anthracene, naphthalene, and biphenyl central cores have been developed as new organic laser active materials. Light-emitting organic field-effect transistors (LE-OFETs) based on 2,6-bis(5-phenylthiophen-2-yl)anthracene (BPTA), 2,6-bis(5-phenylthiophen-2-yl)naphthalene (BPTN), and 2,6-bis(5-phenylthiophen-2-yl)-1,1′-biphenyl (BPTB) single crystals were fabricated. A clear laser oscillation was observed for BPTN and BPTB single crystals. Especially for BPTB, a low amplified spontaneous emission threshold of 1.8 ± 0.2 μJ cm−2 was achieved. In addition, all of the devices showed ambipolar transport characteristics, in which both electron and hole carriers are transported in a single FET device, and electroluminescence was clearly observed. An obvious difference in the light emission direction was observed for LE-OFET devices, which is attributed to the difference in transition dipole moment arrangement of individual molecules within the crystal.

Organic single-crystal light-emitting field-effect transistors

Growth and characterisation of single crystals constitute a major field of materials science. In this feature article we overview the characteristics of organic single-crystal light-emitting field-effect transistors (OSCLEFETs). The contents include the single crystal growth of organic semiconductors and their application to transistor devices. First, we describe various single crystal growth methods that produce different morphologies and geometries of crystals. Using these single crystals we highlight construction and performance of the devices. The single crystal approach not only allows us to study the device performance that reflects the intrinsic nature of the organic semiconductors but also is advantageous to enhancement in the steady device operation. A current-injected laser oscillation in an electronic device configuration remains as a big challenge to be achieved. In this context we briefly mention the spectrally narrowed emissions as well as the possibility of light amplification in the OSCLEFETs.

Annealing effect on light-emitting FET characteristics of π-conjugated liquid crystalline polymer

Thermal annealing effects on organic light-emitting field-effect transistor (OLEFET) characteristics of a liquid crystalline π-conjugated polymer were investigated using spin-coated films of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophenione] (F8T2). Well-balanced ambipolar carrier injection was attained in the homogeneous polycrystalline film grown from the isotropic phase at 350 °C, resulting in the highest light emission under AC-gate operation. On the other hand, the film with microcrystalline grains quenched from the nematic phase at 290 °C was less electron-transporting so that the light emission intensity was lower than the amorphous film annealed at 250 °C.

A Highly Stretchable, Fiber‐Shaped Supercapacitor

Flexible and portable devices are a mainstream direction in modern electronics and related multidisciplinary fields. To this end, they are generally required to be stretchable to satisfy various substrates. As a result, stretchable devices, such as electrochemical supercapacitors, lithium-ion batteries, organic solar cells, organic light-emitting diodes, field-effect transistors, and artificial skin sensors have been widely studied. However, these stretchable devices are made in a conventional planar format that has largely hindered their development. For the portable applications, the devices need to be lightweight and small, though it is difficult for them to be made into efficient microdevices. In particular, it is challenging or even impossible for them to be used in electronic circuits and textiles that are urgently required also in a wide variety of other fields, such as microelectronic applications. Recently, some attempts have been made to fabricate wire-shaped microdevices, such as electrochemical supercapacitors. They have been generally produced by twisting two fiber electrodes with electrolytes coated on the surface. Several examples have been also successfully shown to make fiber-shaped supercapacitors with a coaxial structure. Compared with their planar counterparts, the wire or fiber shape enables promising advantages such as being lightweight and woven into textiles. Although the wire and fiber-shaped supercapacitors are also flexible with high electrochemical performance, they are not stretchable, which is critically important for many applications. For instance, the resulting electronic textiles could easily break during the use if they were not stretchable. To the best of our knowledge, herein we have, for the first time, developed a novel family of highly stretchable, fibershaped high-performance supercapacitors. Aligned carbon nanotube (CNT) sheets that are sequentially wrapped on an elastic fiber serve as two electrodes. The use of aligned CNT sheets offers combined remarkable properties including high flexibility, tensile strength, electrical conductivity, and mechanical and thermal stability. As a result, the fibershaped supercapacitor maintains a high specific capacitance of approximately 18 F/g after stretch by 75% for 100 cycles. Spinnable CNT arrays were first synthesized by chemical vapor deposition. A scanning electron microscopy (SEM) image of the array with height of 230 mm is shown in the Supporting Information, Figure S1, and the CNT shows a multi-walled structure with diameter of about 10 nm (Supporting Information, Figure S2). Aligned CNT sheets could be then continuously drawn from the array and easily attached to various substrates. Elastic fibers were used herein to offer the stretchability in the resulting supercapactiors, and rubber fibers have been mainly studied as a demonstration. For a typical fabrication on the fiber-shaped supercapacitor (Figure 1), a rubber fiber was first coated with a thin layer of

Fluorene-based macromolecular nanostructures and nanomaterials for organic (opto)electronics

Nanotechnology not only opens up the realm of nanoelectronics and nanophotonics, but also upgrades organic thin-film electronics and optoelectronics. In this review, we introduce polymer semiconductors and plastic electronics briefly, followed by various top-down and bottom-up nano approaches to organic electronics. Subsequently, we highlight the progress in polyfluorene-based nanoparticles and nanowires (nanofibres), their tunable optoelectronic properties as well as their applications in polymer light-emitting devices, solar cells, field-effect transistors, photodetectors, lasers, optical waveguides and others. Finally, an outlook is given with regard to four-element complex devices via organic nanotechnology and molecular manufacturing that will spread to areas such as organic mechatronics in the framework of robotic-directed science and technology.

Determination of Thermal Transition Depth Profiles in Polymer Semiconductor Films with Ellipsometry

Geometric confinement and interface effects can significantly alter the thermodynamic properties of thin polymer films. Phase transition temperatures have been shown to strongly depend on film thickness below a critical thickness threshold. It has been suggested that this behavior is due to an interface-induced continuous variation in phase transition temperatures throughout the depth of the films. Here we employ variable-temperature spectroscopic ellipsometry to demonstrate the existence of these depth profiles. We examine four different polymer semiconductors that are of interest for organic light-emitting diodes, solar cells, and field-effect transistors. In contrast to insulating polymers, these light-absorbing materials provide detailed information about structural changes as a function of depth due to wavelength-dependent attenuation. This concept enables us to investigate a broad range of thermodynamic processes including the glass transition, crystallization as well as crystalline and liquid-cryst...

Unlocking the full potential of light emitting field-effect transistors by engineering charge injection layers

Light emitting field-effect transistors (LEFETs) are a class of next generation devices which combine the switching properties of field-effect transistors (FETs) with light emitting capabilities of organic light-emitting diodes (OLEDs) in a single device architecture. Current LEFET architectures suffer from inefficient charge injection of electrons and holes from the source and drain electrodes, leading to unbalanced charge transport and hence poor device performance. Here we report a simple fabrication method for LEFETs that delivers asymmetric source and drain electrodes comprised of low and high work function materials. The interdigitated low and high work function source–drain electrodes consist of combinations of organic materials, salts, metal oxides and metals. Using this method we were able to obtain a maximum EQE of up to 1.2% in a single layer device with Super Yellow as the active material.

In Situ Repair of High-Performance, Flexible Nanocrystal Electronics for Large-Area Fabrication and Operation in Air

Colloidal semiconductor nanocrystal (NC) thin films have been integrated in light-emitting diodes, solar cells, field-effect transistors (FETs), and flexible, electronic circuits. However, NC devices are typically fabricated and operated in an inert environment since the reactive surface and high surface-to-volume ratio of NC materials render them sensitive to oxygen, water, and many solvents. This sensitivity has limited device scaling and large-scale device integration achievable by conventional fabrication technologies, which generally require ambient air and wet-chemical processing. Here, we present a simple, effective route to reverse the detrimental effects of chemical and environmental exposure, by incorporating, in situ, a chemical agent, namely, indium metal, which is thermally triggered to diffuse and repair the damage. Taking advantage of the recovery process, CdSe NC FETs are processed in air, patterned using the solvents of lithography, and packaged by atomic layer deposition to form large-area and flexible high-performance NC devices that operate stably in air.