Research on graphene field-effect transistors (GFETs) has mainly relied on devices fabricated using electron-beam lithography for pattern generation, a method that has known problems with polymer contaminants. GFETs fabricated via photo-lithography suffer even worse from other chemical contaminations, which may lead to strong unintentional doping of the graphene. In this letter, we report on a scalable fabrication process for reliable GFETs based on ordinary photo-lithography by eliminating the aforementioned issues. The key to making this GFET processing compatible with silicon technology lies in a two-in-one process where a gate dielectric is deposited by means of atomic layer deposition. During this deposition step, contaminants, likely unintentionally introduced during the graphene transfer and patterning, are effectively removed. The resulting GFETs exhibit current-voltage characteristics representative to that of intrinsic non-doped graphene. Fundamental aspects pertaining to the surface engineering employed in this work are investigated in the light of chemical analysis in combination with electrical characterization.
This letter reports on a systematic investigation of sputter induced damage in graphene caused by low energy Ar+ ion bombardment. The integral numbers of ions per area (dose) as well as their energies are varied in the range of a few eV's up to 200 eV. The defects in the graphene are correlated to the dose/energy and different mechanisms for the defect formation are presented. The energetic bombardment associated with the conventional sputter deposition process is typically in the investigated energy range. However, during sputter deposition on graphene, the energetic particle bombardment potentially disrupts the crystallinity and consequently deteriorates its properties. One purpose with the present study is therefore to demonstrate the limits and possibilities with sputter deposition of thin films on graphene and to identify energy levels necessary to obtain defect free graphene during the sputter deposition process. Another purpose is to disclose the fundamental mechanisms responsible for defect formation in graphene for the studied energy range.
This paper presents the use of graphene as a diffusion barrier to a eutectic Ga-In-Sn alloy, i.e., galinstan, for electrical contacts in electronics. Galinstan is known to be incompatible with many conventional metals used for electrical contacts. When galinstan is in direct contact with Al thin films, Al is readily dissolved leading to the formation of Al oxides present on the surface of the galinstan droplets. This reaction is monitored ex situ using several material analysis methods as well as in situ using a simple circuit to follow the time-dependent resistance variation. In the presence of a multilayer graphene diffusion barrier, the Al-galinstan reaction is effectively prevented for galinstan deposited by means of drop casting. When deposited by spray coating, the high-impact momentum of the galinstan droplets causes damage to the multilayer graphene and the Al-galinstan reaction is observed at some defective spots. Nonetheless, the graphene barrier is likely to block the formation of Al oxides at the Al/galinstan interface leading to a stable electrical current in the test circuit.
The currently dominant thermoelectric (TE) materials used in low to medium temperature range contain Tellurium that is rare and mild-toxic. Silicon is earth abundant and environment friendly, but it is characterized by a poor TE efficiency with a low figure of merit, ZT. In this work, we report that ZT of amorphous silicon (a-Si) thin films can be enhanced by 7 orders of magnitude, reaching similar to 0.64 +/- 0.13 at room temperature, by means of arsenic ion implantation followed by low-temperature dopant activation. The dopant introduction employed represents a highly controllable doping technique used in standard silicon technology. It is found that the significant enhancement of ZT achieved is primarily due to a significant improvement of electrical conductivity by doping without crystallization so as to maintain the thermal conductivity and Seebeck coefficient at the level determined by the amorphous state of the silicon films. Our results open up a new route towards enabling a-Si as a prominent TE material for cost-efficient and environment-friendly TE applications at room temperature.
In this letter, we report the theoretical study on phonon transport in monocrystalline silicon thin-films having unfilled or metal-filled circular holes (i.e., phononic crystals, PnCs) and show that the thermal conductivity, κ" role="presentation">κ, at 1 K can be maximally reduced by using a multiscale structure, which allows us control over the porosity of the structure. The circular scatterers are placed in the square (SQ) and hexagonal (HX) pattern with a fixed 100 nm interhole spacing, and the pit diameter is varied between 10 and 90 nm. Each of the corresponding silicon PnCs shows reduced κ" role="presentation">κ compared to the unpatterned film. The SQ-PnC having tungsten-filled pits shows the greatest reduction in κ" role="presentation">κ when we consider only the effects of coherent scattering. Furthermore, we have computed κ" role="presentation">κ for the PnC where the unit cell, of 100 nm and 500 nm sizes, comprises the Sierpinski gasket (SG) with circular holes of different diameters (depending on the fractal order) in the same cell. It is observed that the κ" role="presentation">κ for the 2nd (100 nm cell) and 3rd order (500 nm cell) SG-PnC is comparable to the SQ- and HX-PnC with a pit diameter of 90 nm. When we add the effect of the diffuse boundary scattering in κ" role="presentation">κ, there is a lowering in κ" role="presentation">κ compared to that when only the coherent effects are considered. The additional κ" role="presentation">κ-reduction due to boundary scattering for the SQ-PnC and HX-PnC (both with 90 nm diam) as well as the 2nd and 3rd order SG-PnCs is 47%, 40%, 80%, and 60%, respectively.
Significant influence on the thermal stability of polyaniline (PANI) in the presence of multi-walled carbon nanotubes (MWCNTs) is reported. By means of in-situ rapid mixing approach, water-dispersible nanofibrillar PANI and composites, consisting of MWCNTs uniformly coated with PANI in the state of emeraldine salt, with a well-defined core-shell heterogeneous structure, were prepared. The de-protonation process in PANI occurs at a lower temperature under the presence of MWCNTs on the polyaniline composite upon thermal treatment. However, it is found that the presence of MWCNTs significantly enhances the thermal stability of PANI's backbone upon exposure to laser irradiation, which can be ascribed to the core-shell heterogeneous structure of the composite of MWCNTs and PANI, and the high thermal conductivity of MWCNTs.
Carbon nanoparticles (CNPs) have been synthesized by laser ablation of polycrystalline graphite in water using a pulsed Nd:YAG laser (1064 nm) with a width of 8 ns. Structural and mesoscopic characterization of the CNPs in the supernatant by Raman spectroscopy provide evidence for the presence of mainly two ranges of particle sizes: 1-5 nm and 10-50 nm corresponding to amorphous carbon and graphite NPs, respectively. These results are corroborated by complementary characterization using atomic force microscopy (AFM) and transmission electron microscopy (TEM). In addition, large (10-100 mu m) graphite particles removed from the surface are essentially unmodified (in structure and topology) by the laser as confirmed by Raman analysis.
The tactile peripheral nervous system innervating human hands, which is essential for sensitive haptic exploration and dexterous object manipulation, features overlapped receptive fields in the skin, arborization of peripheral neurons and many-to-many synaptic connections. Inspired by the structural features of the natural system, we report a supersensitive artificial slowly adapting tactile afferent nervous system based on the triboelectric nanogenerator technology. Using tribotronic transistors in the design of mechanoreceptors, the artificial afferent nervous system exhibits the typical adapting behaviours of the biological counterpart in response to mechanical stimulations. The artificial afferent nervous system is self-powered in the transduction and event-driven in the operation. Moreover, it has inherent proficiency of neuromorphic signal processing, delivering a minimum resolvable dimension two times smaller than the inter-receptor distance which is the lower limit of the dimension that existing electronic skins can resolve. These results open up a route to scalable neuromorphic skins aiming at the level of human?s exceptional perception for neurorobotic and neuroprosthetic applications.
The unique electronic properties of graphene are exploited for field-effect sensing in both capacitor and transistor modes when operating the sensor device in electrolyte. The device is fabricated using large-area graphene thin films prepared by means of layer-by-layer stacking. Although essentially the same device, its operation in the capacitor mode is found to yield more information than in the transistor mode. The capacitor sensor can simultaneously detect the variations of surface potential and electrical-double-layer capacitance at the graphene/electrolyte interface when altering the ion concentration. The capacitor-mode operation further facilitates studies of the molecular binding-adsorption kinetics by monitoring the capacitance transient
This paper presents the design, manufacturing and measurements of antennas on paper, realized using ink-jetprinting of conductive inks based on nano-silver particles (nSPs). The extraction of the substrate characteristicssuch as the dielectric constant and dielectric loss is performed using a printed ring resonator technique. Thecharacterization of the nSPs conductive inks assesses different parameters as sintering time and temperature.Two antennas are realized corresponding to the most common needs for Wireless Sensor Networks (WSN) inTrains Environment. The first one is a patch antenna characterized by a broadside radiation pattern and suitedfor operation on metallic structures. The second one is a quasi-yagi antenna, with an end fire radiation patternand higher directivity, without requiring a metallic ground plane. Both antennas present a good matching (S11 < -20 dB and S11 < -30 dB, respectively) and acceptable efficiency (55 % and 45 %, respectively) for the papersubstrate used at the center frequency of 2.4 GHz, corresponding to the first channel of the IEEE 802.15.4 band.
This paper presents the investigation of the resistive humidity-sensing properties of multi-walled carbon nanotubes (MWCNTs). MWCNTs functionalized by acid treatment (f-MWCNTs) exhibit rather high sensitivity in resistance toward humidity, owing to the presence of carboxylic groups on the nanotube surface. By integrating the f-MWCNTs resistor into a wireless sensor platform, flexible humidity sensors for ultra-high frequency applications are investigated. The operating frequency range of the sensor is dramatically increased from 600 MHz to 2 GHz by adjusting the resistor-electrodes' configuration. This enhancement is predominately attributed to the variation in parasitic capacitance between the resistor-electrodes.
Transparent/translucent glass ceramics (GCs) have broad applications in biomedicine, armor, energy, and constructions. However, GCs with improved optical properties typically suffer from impaired mechanical properties, compared to traditional sintered full-ceramics. We present a method of obtaining high-strength, translucent GCs by preparing ZrO2-SiO2 nanocrystalline glass ceramics (NCGCs), with a microstructure of monocrystalline ZrO2 nanoparticles (NPs), embedded in an amorphous SiO2 matrix. The ZrO2-SiO2 NCGC with a composition of 65%ZrO2-35%SiO2 (molar ratio, 65Zr) achieved an average flexural strength of 1 GPa. This is one of the highest flexural strength values ever reported for GCs. ZrO2 NPs have a core-shell structure, and the shell is a thin (2–3 nm) amorphous Zr/Si interfacial layer that provides strong bonding between the ZrO2 NPs and SiO2 matrix. The diffusion of Si atoms into the ZrO2 NPs forms a Zr-O-Si superlattice. Electron tomography results show that some of the ZrO2 NPs are connected in one direction, forming in situ ZrO2 nanofibers (with length of ~500 nm), and that the ZrO2 nanofibers are stacked in an ordered way in all three dimensions. The nano-architecture of the ZrO2 nanofibers mimics the architecture of mineralized collagen fibril in cortical bone. Strong interface bonding enables efficient load transfer from the SiO2 matrix to the 3D nano-architecture built by ZrO2 nanofibers and NPs, and the 3D nano-architecture carries the majority of the external load. These two factors synergistically contribute to the high strength of the 65Zr NCGC. This study deepens our fundamental understanding of the microstructure-mechanical strength relationship, which could guide the design and manufacture of other high-strength, translucent GCs.
Almost 15 years have gone ever since the discovery of graphene as a single atom layer. Numerous papers have been published to demonstrate its high electron mobility, excellent thermal and mechanical as well as optical properties. We have recently seen more and more applications towards using graphene in commercial products. This paper is an attempt to review and summarize the current status of the research of the thermal properties of graphene and other 2D based materials including the manufacturing and characterization techniques and their applications, especially in electronics and power modules. It is obvious from the review that graphene has penetrated the market and gets more and more applications in commercial electronics thermal management context. In the paper, we also made a critical analysis of how mature the manufacturing processes are; what are the accuracies and challenges with the various characterization techniques and what are the remaining questions and issues left before we see further more applications in this exciting and fascinating field.
Electrochromic devices with unique advantages of electrical/optical bistability are highly desired for energy-saving and information storage applications. Here, we put forward a self-driven AI-ion electrochromic system, which utilizes WOx films, Cu foil, and graphite rod as electrochromic optical modulation and graph display electrodes, coloration potential supplying electrodes, and bleaching potential supplying electrodes, respectively. The inactive Cu electrode can not only realize the effective Al3+ cation intercalation into electrochromic WOx electrodes but also eliminate the problem of metal anode consumption. The electrochromic WOx electrodes cycled in Al3+ aqueous media exhibit a wide potential window (similar to 1.5 V), high coloration efficiency (36.0 cm(2)/C), and super-long-term cycle stability (>2000 cycles). The dynamic optical modulation and static graph display function can be achieved independently only by switching the electrode connection mode, thus bringing more features to this electrochromic system. For a large-area electrochromic system (10 x 10 cm(2)), the absolute transmittance value in its color-neutral state can reach about 41% (27%) at 633 nm (780 nm) by connecting the Cu and WOx electrodes for 140 s. The original transparent state can be readily recovered by replacing the Cu foil with the graphite rod. This work throws light on next-generation electrochromic applications for optical/thermal modulation, privacy protection, and information display.
With large interstitial space volumes and fast ion diffusion pathways, amorphous metal oxides as cathodic intercalation materials for electrochromic devices have attracted attention. However, these incompact thin films normally suffer from two inevitable imperfections: self-deintercalation of guest ions and poor stability of the structure, which constitute a big obstacle toward the development of high-stable commercial applications. Here, we present a low-cost, eco-friendly hybrid cation 1,2-PG-AlCl3 center dot 6H(2)O electrolyte, in which the sputter-deposited a-WO3-x thin film can exhibit both the long-desired excellent open-circuit memory (>100 h, with zero optical loss) and super-long cycling lifetime (similar to 20,000 cycles, with 80% optical modulation), benefiting from the formation of unique Al-hydroxide-based solid electrolyte interphase during electrochromic operations. In addition, the optical absorption behaviors in a-WO3-x caused by host-guest interactions were elaborated. We demonstrated that the intervalence transfers are primarily via the "corner-sharing" related path (W5+ <-> W6+) but not the "edge-sharing" related paths (W4+ <-> W6+ and/or W4+ <-> W5+), and the small polaron/electron transfers taking place at the W-O bond-breaking positions are not allowed. Our findings might provide in-depth insights into the nature of electrochromism and provide a significant step in the realization of more stable, more excellent electrochromic applications based on amorphous metal oxides.
Although monovalent lithium has been successfully used as a coloring ion in electrochromic applications, it still faces the challenges of low safety, high cost, and limited reserves. Herein, we demonstrate that the amorphous WO3 films intercalated with Al3+ ions could exhibit desired wide optical modulation (similar to 63.0%) and high coloration efficiency (similar to 72.0 cm(2) A(-1), which is >100% higher than that with Li+ or Na+), benefiting from the three-electron redox properties of aluminum. Due to the strong electrostatic force and large atomic weight, the charge exchange processes for Al3+ ions are limited only to the near-surface region and consequently bring about enhanced electrochromic stability. Our findings provide in-depth insights into the nature of electrochromism and also open up a new route toward scalable electrochromic devices using sputtering techniques and earth-abundant materials.
NiOx is recognized as the leading candidate for smart window anodes that can dynamically modulate optical absorption, thereby achieving energy efficiency in construction buildings. However, the electrochromic mechanism in NiOx is not yet clear, and the ionic species involved are sometimes ambiguous, particularly in aprotic electrolytes. We demonstrate herein that the "net coloration effect" originates from newly generated high-valence Ni3+/Ni4+ ions during anion-dependent anodization, and the Li+ intercalation/deintercalation only plays a role in modulating the oxidation state of Ni. Unambiguous evidences proving the occurrence of anodization reaction were obtained by both chronoamperometry and cyclic voltammetry. Benefiting from the irreversible polarization of Ni2+ to Ni3+/Ni4+, the quantity of voltammetric charge increases by similar to 38% under the same test conditions, enhancing the corresponding electrochromic modulation by similar to 8%. Strong linkages between the coloration, evolution, and degradation observed in this work provide in-depth insights into the electrocatalytic and electrochromic mechanisms.
Electrochromic(EC), nonstoichiometric NiOx thin films were made by reactive magnetron sputtering at low oxygen flow ratio(i.e., P = O-2/Ar + O-2 < 10%). The results of optical spectral, x-ray diffraction spectrum, and x-ray photoelectron spectroscopy analyses indicate that the samples are oxygen(nickel)-deficient as P <= %4(>=%6), resulting in sub(over)-stoichiometry films. Spectroelectrochemical measurements show that the EC effect of NiOx in nonaqueous PC - LiClO4 electrolyte is direct correlation with the nickel vacancy concentration in films, while that in the aqueous KOH solution is nearly uninfluenced upon the change in stoichiometry as P >= %4. The films deposited at P = 6% exhibit higher coloration efficiency of - 25.3 cm(2)C(-1), larger ionic diffusion coefficient of - 2.84 x 10(-14) m(2)s(-1), and broader EC modulation span of 24% in PC - LiClO4 than the other ones. Based upon these values, EC devices featuring a WO3/PMMA - PC - LiClO4/NiOx structure and excellent performances were fabricated. We demonstrated that the nickel anodization should be responsible for the initial "activation" phenomena, which decreases(increases) the number of oxygen(nickel) vacancies. Moreover, the cause of degradation resulting from Li+-ion trapping in the IS layer was also clarified. This work provides a general framework for studying and designing superior EC devices, experimentally as well as theoretically.
A room-temperature polymer-assisted transfer process is developed for large-area, single-layer graphene grown by means of chemical vapor deposition (CVD). This process leads to transferred graphene layers free of polymer contamination. The absence of polymer residues boosts the surface-enhanced Raman scattering (SERS) of the CVD graphene with gold nanoparticles (Au NPs) deposited atop by evaporation. The SERS enhancement of the CVD graphene reaches similar to 120 for the characteristic 2D peak of graphene, the highest enhancement factor achieved to date, when the Au NPs are at the threshold of percolation. Our simulation supported by experiment suggests that the polymer residues persistently present on the graphene transferred by the conventional polymer-assisted method are equivalent to an ultrathin film of less than 1 nm thickness. The presence of polymer residues drastically reduces SERS due to the separation of the Au NPs from the underlying graphene. The scalability of CVD graphene opens up for the possibility of graphene-based SERS sensors.
Sensing biomolecules in electrolytes of high ionic strength has been a difficult challenge for field-effect transistor-based sensors. Here, we present a graphene-based transistor sensor that is capable of detection of antibodies against protein p53 in electrolytes of physiological ionic strength without dilution. As these molecules are much larger than the Debye screening length at physiological ionic strengths, this paper proves the concept of detection beyond the Debye length. The measured signal associated with the expected specific binding of the antibodies to p53 is concluded to result from resistance changes at the graphene-electrolyte interface, since a sensor responding to resistance changes rather than charge variations is not limited by Debye screening. The conclusion with changes in interface resistance as the underlying phenomena that lead to the observed signal is validated by impedance spectroscopy, which indeed shows an increase of the total impedance in proportion to the amounts of bound antibodies. This finding opens up a new route for electrical detection of large-size and even neutral biomolecules for biomedical detection applications with miniaturized sensors.
As a two-dimensional material with high charge carrier mobility, graphene may offer ultrahigh sensitivity in biosensing. To realize this, the first step is to functionalize the graphene. This is commonly done by using 1-pyrenebutyric acid (PBA) as a linker for biornolecules. However, the adsorption of PBA on graphene remains poorly understood despite reports of successful biosensors functionalized via this route. Here, the PBA adsorption on graphene is characterized through a combination of Raman spectroscopy, ab initio calculations, and spectroscopic ellipsometry. The PBA molecules are found to form a self-assembled monolayer on graphene, the formation of which is self-limiting and Langmuirian. Intriguingly, in concentrated solutions, the PBA molecules are found to stand up and stack horizontally with their edges contacting the graphene surface. This morphology could facilitate a surface densely populated with carboxylic functional groups. Spectroscopic analyses show that the monolayer saturates at 5.3 PBA molecules per nm(2) and measures similar to 0.7 nm in thickness. The morphology study of this PBA monolayer sheds light on the pi-pi stacking of small-molecule systems on graphene and provides an excellent base for optimizing functionalization procedures.
A dual-bandgap photoelectrochemical (PEC) cell with two semiconductors stacked in tandem is a widely adopted concept to capture a large fraction of the solar spectrum for water splitting. While two photons are theoretically needed to produce one H2 molecule using single-bandgap PEC cells, four photons are generally required for one H2 molecule in the dual-bandgap cells because of an unavoidable charge recombination at the solid-solid interface. Here, triboelectric effects are exploited in the form of triboelectric nanogenerator (TENG) to allow for the generation of one H2 molecule at the expenses of two photons in a dual-bandgap device using an array of core/shell p-type silicon/anatase-TiO2 nanowires as photoelectrode. The TENG, that converts mechanical energy to electricity, efficiently suppresses the charge recombination at the interface and significantly increases the energy of the photo-generated carriers required for the simultaneous water reduction and oxidation. The synergy of photoexcitation and triboelectrics results in a rate of hydrogen production in a neutral Na2SO4 electrolyte around 150 times higher than that of the counterpart, i.e., the device in the absence of TENG. Furthermore, the TENG-induced enhancement in the PEC water splitting remains substantial even when the solar power density is reduced to 20 mW/cm2.
Stretchable electronics and soft robotics have shown unsurpassed features, inheriting remarkable functions from stretchable and soft materials. Electrically conductive and mechanically stretchable materials based on composites have been widely studied for stretchable electronics as electrical conductors using various combinations of materials. However, thermally tunable and stretchable materials, which have high potential in soft and stretchable thermal devices as interface or packaging materials, have not been sufficiently studied. Here, a mechanically stretchable and electrically insulating thermal elastomer composite is demonstrated, which can be easily processed for device fabrication. A liquid alloy is embedded as liquid droplet fillers in an elastomer matrix to achieve softness and stretchability. This new elastomer composite is expected useful to enhance thermal response or efficiency of soft and stretchable thermal devices or systems. The thermal elastomer composites demonstrate advantages such as thermal interface and packaging layers with thermal shrink films in transient and steady-state cases and a stretchable temperature sensor.
There is a need for thermal elastomer composites (TEC) which are stretchable, electrically insulating and easily processablefor soft and stretchable sensor or actuator systems as a thermal conductor or heat spreader at an interface or in a package.A novel TEC was made by embedding a gallium based liquid alloy (Galinstan) as a droplet in polydimethylsiloxane (PDMS,Elastosil RT 601) matrix with a high speed mechanical mixing process.
Conventional thermoelectric generators (TEGs) are normally hard, rigid, and flat. However, most objects have curvy surfaces, which require soft and even stretchable TEGs for maximizing efficiency of thermal energy harvesting. Here, soft and stretchable TEGs using conventional rigid Bi2Te3 pellets metallized with a liquid alloy is reported. The fabrication is implemented by means of a tailored layer-by-layer fabrication process. The STEGs exhibit an output power density of 40.6 mu W/cm(2) at room temperature. The STEGs are operational after being mechanically stretched-and-released more than 1000 times, thanks to the compliant contact between the liquid alloy interconnects and the rigid pellets. The demonstrated interconnect scheme will provide a new route to the development of soft and stretchable energy-harvesting avenues for a variety of emerging electronic applications.
The present work reports on the development of a class of sophisticated thin-film transistors (TFTs) based on ink-jet printing of pristine single-walled carbon nanotubes (SWCNTs) for the channel formation. The transistors are manufactured on oxidized silicon wafers and flexible plastic substrates at ambient conditions. For this purpose, ink-jet printing techniques are developed with the aim of high-throughput production of SWCNT thin-film channels shaped in long strips. Stable SWCNT inks with proper fluidic characteristics are formulated by polymer addition. The present work unveils, through Monte Carlo simulations and in light of heterogeneous percolation, the underlying physics of the superiority of long-strip channels for SWCNT TFTs. It further predicts the compatibility of such a channel structure with ink-jet printing, taking into account the minimum dimensions achievable by commercially available printers. The printed devices exhibit improved electrical performance and scalability as compared to previously reported ink-jet printed SWCNT TFTs. The present work demonstrates that ink-jet printed SWCNT TFTs of long-strip channels are promising building blocks for flexible electronics.
Hysteresis-free logic gates capable of operation at 100 kHz are fabricated basing on local-gate thin-film transistors with their channel featuring solution-processed composite films of single-walled carbon nanotubes (SWCNTs) and poly(9,9-dioctylfluorene-co-bithiophene) (F8T2). Using dip-coating for deposition of composite films, high-density SWCNTs are found to be embedded in an F8T2 layer and thus being kept from the underlying AlOx gate dielectric by a certain distance. The presence of the F8T2 interlayer effectively suppresses hysteresis although it also weakens the gate electrostatic control. The fabricated transistors are characterized by nil hysteresis, high carrier mobility, large ON/OFF current ratio, low operation voltage, small subthreshold swing, and remarkable scalability. These properties are crucial for the realization of the well-performing logic circuits.
Small-hysteresis, high-performance thin-film transistors (TFTs) are readily realized simply by dip-coating of a solution-processable composite. The composite consists of single-walled carbon nanotubes (SWCNTs) embedded in semiconducting polymer used as the channel material. The resultant TFTs simultaneously exhibit large on/off current ratio, high on-current level, high mobility in the range 10−20 cm2V−1s−1, and good uniformity and scalability.
Thin-film transistors (TFTs) based on single-walled carbon nanotubes (SWCNTs) have gained enormous attention in the community of flexible/stretchable electronics. At present, such TFTs often suffer from severe problems including giant hysteresis in their transfer characteristics. With SiO2 as the gate dielectric, extensive investigations have led to generally accepted understanding of the hysteresis as being caused by charge transfer between the SWCNTs and their surroundings including both water molecules bound on the SiO2 surface (Si≡OH) and the water/oxygen molecules in the ambient atmosphere. In order to combat the hysteresis issue, significant efforts have been made by annealing the TFTs in vacuum and separating SWCNTs from SiO2 by deposition of a self-assembled monolayer (SAM) on the SiO2 or passivating the SWCNTs with an organic or inorganic dielectric film. These methods, however, require either processing in inert environment or developing elaborated processes. In the present work, we demonstrate hysteresis-free TFTs based on SWCNT/polymer composite without any complex treatment. The composite consists of SWCNTs and poly-9,9_dioctyl-fluorene-co-bithiophene (F8T2). With the aid of polymer F8T2, SWCNTs can be efficiently dissolved in commonly used solvents thereby forming a uniform composite solution. By soaking a chip with predefined TFT structures on an oxidized Si substrate in the composite solution, direct assembly of the composite on the SiO2 occurs, leading to the formation of a composite thin film in the channel region of the TFTs. Although fabricated using a very simple process, our TFTs exhibit hysteresis-free operation under ambient conditions. It is plausible to suggest that SWCNTs are embedded in the F8T2 matrix with the latter providing an effective shield for the former against the trap sites on the SiO2 and the H2O/O2 molecules in the atmosphere. In comparison to the other reported means aiming at hysteresis reduction, the present method is simple, robust, solution processable, effective, and operable under ambient conditions. In addition, we have found F8T2 to preferentially disperse semiconducting SWCNTs rendering a selective removal of the metallic species in the solution. This selectivity is of paramount importance as it results in high-performance TFTs with both high on-state current (0.1 µA/µm @ channel length = 50 µm) and large on/off current ratio (103-105). The TFTs have also shown significantly improved uniformity and dimensional scalability with a mobility value of 10-20 cm2V-1s-1, which have allowed us to investigate the TFTs using the resultant logic circuits.
This letter presents a systematic investigation of the gate capacitance C-G of thin-film transistors (TFTs) based on randomly distributed single-walled carbon nanotubes (SWCNTs) in the channel. In order to reduce false counting of SWCNTs that do not contribute to current conduction, C-G is directly measured on the TFTs using a well-established method for MOSFETs. Frequency dispersion of C-G is observed, and it is found to depend on the percolation behavior in SWCNT networks. This dependence can be accounted for using an RC transmission line model. These results are of important implications for the determination of carrier mobility in nanoparticle-based TFTs.
An extensive investigation of carrier mobility is presented for thin-film transistors (TFTs) with single-walled carbon nanotube (SWCNT) networks as the semiconductor channel. For TFTs particularly with low-density SWCNTs in the networks, the extracted mobility using the standard method for Si metal-oxide-semiconductor field-effect transistors is erroneous, mainly resulting from use of a parallel-plate capacitor model and assumption of the source-drain current being inversely proportional to the channel length. Large hysteresis in the transfer characteristics further complicates the extraction. By properly addressing all these challenges in this letter, a comprehensive methodology is established, leading to the extraction of mobility values that are independent of geometrical parameters.
Thin-film field-effect transistors (TFTs) are readily fabricated using a semiconductor composite that is solution processed under ambient conditions for the conduction channel. The composite comprises single-walled carbon nanotubes (SWCNTs) embedded in poly-9,9' dioctyl-fluorene-co-bithiophene. Carrier mobility values approaching 10 cm(2)V(-1)s(-1) are obtained for the composite with relatively high SWCNT concentrations. When the SWCNT concentration is reduced for a large ON/OFF current ratio > 10(6), the mobility remains decent around 0.3 cm(2)V(-1)s(-1). The resultant TFTs display remarkable environmental and operational reliability. Nanotube-based composites are therefore of significance in printed electronics owing to their simplicity in device fabrication and competitiveness in device performance.
The influence of carbon nanotubes on the thermal ageing effect of the electrical conductivity of composite thin films is presented. The composite thin films comprise carbon nanotube/polyaniline nanofibers. When subject to thermal treatment, the presence of nanotubes retards the loss of dopants from the polyaniline and enhances the thermal stability in electrical conductivity of the composite thin films. Specifically, an increase in temperature for the conductivity degradation and a significant reduction in the rate of the conductivity degradation of the composite thin films are observed. Upon prolonged heating, the composite thin films exhibit relative large conductivity at high nanotube content, while the polyaniline thin films become insulating.
Nanofibrillar composite of polyaniline (PANI)/multi-walled carbon nanotubes (MWNTs) is readily synthesized by means of conventional in situ polymerization process. It is found that the MWNT loading during polymerization has a significant influence on both the micro- and macro-scale morphological properties of the composites. At low MWNT loadings, PANI/MWNTs are formed as individual nanofibers, similar to that of the neat PANI in the absence of MWNTs. With the increase in MWNT loading, the composite exhibits granular form and becomes a continuous porous matrix at higher MWNT loadings. A possible mechanism is proposed to account for the structural variation of the composites caused by MWNTs at the different loadings.
We report on an efficient and highly thermally stable doping with iodine on ink-jet printed graphene films. The films consist of pristine few-layer graphene nano-platelates (p-GNPs) that are randomly stacked. With iodine doping simply by soaking in aqueous iodine solution, the printed p-GNPs films are enhanced in electrical conductivity by up to around 2 times. The doping effect exhibits excellent thermal stability up to 500 degrees C under high vacuum condition (10(-6) mBar) evidenced by electrical and spectroscopic means. Furthermore, the doping of iodine leads to a slight increment of work function by 0.07 eV. Using depth profile measurements, it is found that iodine species diffuse deeply into the films and likely intercalate between two adjacent p-GNPs which interpret the aforementioned efficient enhancement and thermal stability of the doping effect. The reported doping scheme offers a viable low-temperature optimization method for conductive electrodes with p-GNPs in the application of printed devices.
We report an efficient inkjet printing of water-based pristine GNPs graphene ink and a facile aqueous halogen doping process that provides significant and thermally stable conductivity enhancement of printed patterns. Highly concentrated aqueous graphene ink populated by few-layer pristine graphene flakes is obtained by means of scalable shear exfoliation process with the aid of bromine intercalation. The as-printed GNP films which has been merely treated by drying at 100 degrees C exhibits DC conductivity (sigma(DC)) of similar to 1400 S/m likely due to bromine doping effect. This value is significantly increased to similar to 3 x 10(4) S/m when an additional treatment by means of dipping in aqueous iodine solution is applied prior to the drying. As contrast, sigma(DC) is increased to similar to 2.4 x 10(4) S/m when a mere annealing at elevated temperature in air is employed. When the aqueous iodine doping process and annealing at elevated temperature is combined, an unprecedented value of sigma(DC) similar to 10(5) S/m is achieved. The availability of water-based GNPs inks and low-temperature doping scheme for efficient and reliable conductivity enhancement has offered a pathway for the application of GNPs in different printed electronics devices.
Electrical interconnections are one of the main challenges in the printed electronics, to connect different functional units of an electronic device. With the progressive advancement of large area and low cost printed electronic devices on polymeric and paper substrates, the requirement for reliable interconnections with lower power consumption fabricated at low temperature is necessary. The conventional copper-based interconnections suffer severe problems in terms of cost efficiency when they are processed with photolithography technique. To replace the conventional metallic interconnections we have proposed printed interconnects with graphene inks. This is mainly motivated by two reasons. First, printing is a low-cost patterning approach which is performed at ordinary ambient condition. Printing of graphene proved to be a promising since it combines the attractive features of graphene and the cost effective printing methods (ink-jet printing, nozzle printing, spray printing) which enable additive patterning, direct writing, scalability to large area manufacturing. In order to facilitate the inkjet printing process, the graphene solution needs to be highly stable, uniform and should contain smaller sheet sizes (~ 1 micro meter) because of the limitation of the nozzle size of inkjet printing machine. In this work we have proposed a cost-effective approach for large-scale production of printable stable graphene suspension by liquid-phase shear exfoliation of graphite for printed electronics application. The process is scalable and requires shorter processing time compared to the other existing exfoliation methods. Graphene sheets have been exfoliated from graphite flakes in a solvent, cyclohexanone with ethyl cellulose as stabilizer. The graphene based solution prepared after several optimizations leads to a stable ink for more than six months without any sedimentation. The initial studies confirmed the production of graphene films with average sheet thickness of 10 to 20 nm and without any agglomeration with sheet sizes less than 1 micro meter. The rheological properties, such as, viscosity, of the produced graphene ink has been carefully tuned in order for successful inkjet printing process. Highly conductive and transparent (~70 % in the visible region) interconnections have been developed after several inkjet printing steps.
In this study, we demonstrate scalable and efficient inkjet printing of graphene flexible transparent conducting films (TCFs). The highly concentrated and stable graphene ink (3.2 mg/mL) that is dominated by 4-layer graphene flakes is achieved by means of shear exfoliation process. The printed graphene TCFs with DC conductivity of ∼4 × 104 S/m (sheet resistance 260 Ω/□ coupled with optical transparency of 86%) without intentional doping are readily obtained. Excellent flexibility and air stability of the printed graphene TCFs allow their potential applications in different flexible opto-electronics devices. Systematic investigation of the inkjet printing of graphene and the annealing effect on the graphene TCFs is presented.
A general formulation engineering method is adopted in this study to produce a highly concentrated (≈3 mg mL−1) inkjet printable starch–graphene ink in aqueous media. Photonic annealing of the starch–graphene ink is validated for rapid post-processing of printed films. The experimental results demonstrate the role of starch as dispersing agent for graphene in water and photonic pulse energy in enhancing the electrical properties of the printed graphene patterns, thus leading to an electrical conductivity of ≈2.4 × 104 S m−1. The curing mechanism is discussed based on systematic material studies. The eco-friendly and cost-efficient approach presented in this work is of technical potential for the scalable production and integration of conductive graphene inks for widespread applications in printed and flexible electronics.
In this paper, two microwave irradiation methods: (i) liquid-phase microwave irradiation (MWI) reduction of graphite oxide suspension dissolved in de-ionized water and N, N-dimethylformamide, respectively, and (ii) solid-phase MWI reduction of graphite oxide powder have been successfully carried out to reduce graphite oxide. The reduced graphene oxide products are thoroughly characterized by scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectral analysis, Raman spectroscopy, UV-Vis absorption spectral analysis, and four-point probe conductivity measurements. The results show that both methods can efficiently remove the oxygen-containing functional groups attached to the graphite layers, though the solid-phase MWI reduction method can obtain far more efficiently a higher quality-reduced graphene oxide with fewer defects. The I( D)/ I( G) ratio of the solid-phase MWI sample is as low as 0.46, which is only half of that of the liquid-phase MWI samples. The electrical conductivity of the reduced graphene oxide by the solid method reaches 747.9 S/m, which is about 25 times higher than that made by the liquid-phase method.
Silicon, a candidate as an abundant-element thermoelectric material for low-temperature thermal energy scavenging applications, generally suffers from rather low thermoelectric efficiency. One viable solution to enhancing the efficiency is to boost the power factor (PF) of amorphous silicon (a-Si) while keeping the thermal conductivity sufficiently low. In this work, we report that PF >1 m Wm−1 K−2 is achievable for boron-implanted p-type a-Si films dispersed with ultrafine crystals realized by annealing with temperatures ≤600 °C. Annealing at 550 °C initiates crystallization with sub-5-nm nanocrystals embedded in the a-Si matrix. The resultant thin films remain highly resistive and thus yield a low PF. Annealing at 600 °C approximately doubles the density of the sub-5-nm nanocrystals with a bimodal size distribution characteristic and accordingly reduces the fraction of the amorphous phase in the films. Consequently, a dramatically enhanced electrical conductivity up to 104 S/m and hence PF > 1 m Wm−1 K−2 measured at room temperature are achieved. The results show the great potential of silicon in large-scale thermoelectric applications and establish a route toward high-performance energy harvesting and cooling based on silicon thermoelectrics.
In this paper, a flexible nanogenerator based on direct piezoelectric effect using a spin-coated poly(vinytidenefluoride-co-trifluoroethylene) (PVDF-TrFE) thin film as functional layer has been fabricated on polyimide substrate. The as-prepared nanogenerator exhibits the open-circuit voltage up to 7 V and short-circuit current of 58 nA with current density of 0.56 mu A/Cm-2. The impact of the variation of strain rate on the electrical outputs of the nanogenerator has been characterized experimentally and analyzed theoretically. An analytical model that explains well the experimental results has been established. (C) 2014 Elsevier Ltd. All rights reserved.