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Organic cocrystals, which are assembled by noncovalent intermolecular interactions, have garnered intense interest due to their remarkable chemicophysical properties and practical applications. One notable feature, namely, the charge transfer (CT) interactions within the cocrystals, not only facilitates the formation of an ordered supramolecular network but also endows them with desirable semiconductor characteristics. Here, we present the intriguing ambipolar CT properties exhibited by nanosheets composed of single cocrystals of C70/ferrocene (C70/Fc). When heated to 150 °C, the initially ambipolar monoclinic C70/Fc nanosheet-based field-effect transistors (FETs) were transformed into n-type face-centered cubic (fcc) C70 nanosheet-based FETs owing to the elimination of Fc. This thermally induced alteration in the crystal structure was accompanied by an irreversible switching of the semiconducting behavior of the device; thus, the device transitions from ambipolar to unipolar. Importantly, the C70/Fc nanosheet-based FETs were also found to be much more thermally stable than the previously reported C60/Fc nanosheet-based FETs. Furthermore, we conducted visible/near-infrared diffuse reflectance and photoemission yield spectroscopies to investigate the crucial role played by Fc in modulating the CT characteristics. This study provides valuable insights into the overall functionality of these nanosheet structures. [ABSTRACT FROM AUTHOR]

The photogating effect is a photocurrent generation mechanism that leads to marked responsivity in two-dimensional (2D) semiconductor-based devices. A key step to promote the photogating effect in a 2D semiconductor is to integrate it with a high density of charge traps. Here, we show that self-limiting surface oxides on atomically thin WSe2 can serve as effective electron traps to facilitate p-type photogating. By examining the gate-bias-induced threshold voltage shift of a p-type transistor based on single-layer WSe2 with surface oxide, the electron trap density and the trap rate of the oxide are determined to be >1012 cm−2 and >1010 cm−2 s−1, respectively. White-light illumination on an oxide-covered 4-layer WSe2 transistor leads to the generation of photocurrent, the magnitude of which increases with the hole mobility. During illumination, the photocurrent evolves on a timescale of seconds, and a portion of the current persists even after illumination. These observations indicate that the photogenerated electrons are trapped deeply in the surface oxide and effectively gate the underlying WSe2. Owing to the pronounced photogating effect, the responsivity of the oxide-covered WSe2 transistor is observed to exceed 3000 A/W at an incident optical power of 1.1 nW, suggesting the effectiveness of surface oxidation in facilitating the photogating effect in 2D semiconductors. [ABSTRACT FROM AUTHOR]

We present that the electrical conduction type in carbon nanotube field-effect transistors (CNT-FETs) can be converted by induced charges in a polyvinyl alcohol (PVA) insulator. When the CNT channels are covered with pure PVA, the FET characteristics clearly change from unipolar p-type to ambipolar. The addition of ammonium ions (NH4+) in the PVA leads to further conversion to unipolar n-type conduction. The capacitance - voltage characteristics indicate that a high density of positive charges is induced at the PVA/SiO2 interface and within the bulk PVA. Electrons are electrostatically accumulated in the CNT channels due to the presence of the positive charges, and thus, stable n-type conduction of PVA-coated CNT-FETs is observed, even under ambient conditions. The mechanism for conversion of the conduction type is considered to be electrostatic doping due to the large amount of positive charges in the PVA. A blue-shift of the Raman G-band peak was observed for CNTs coated with NH4+-doped PVA, which corresponds to unipolar n-type CNT-FET behavior. These results confirm that carrier polarity engineering in CNT-FETs can be achieved with a charged PVA passivation layer. [ABSTRACT FROM AUTHOR]

Despite a large steric bulk of C60, a molecular graphene with a covalently linked C60 pendant [hexabenzocoronene (HBC)-C60; 1] self-assembles into a coaxial nanotube whose wall consists of a graphite-like π-stacked HBC array, whereas the nanotube surface is fully covered by a molecular layer of clustering C60. Because of this explicit coaxial configuration, the nanotube exhibits an ambipolar character in the field-effect transistor output [hole mobility (μh) = 9.7 × 10-7 cm² V-1 s-1; electron mobility (μe) = 1.1 × 10-5 cm² V-1 s-1) and displays a photovoltaic response upon light illumination. Successful coassembly of 1 and an HBC derivative without C60 (2) allows for tailoring the p/n heterojunction in the nanotube, so that its ambipolar carrier transport property can be optimized for enhancing the open-circuit voltage in the photovoltaic output. As evaluated by an electrodeless method called flash-photolysis time-resolved microwave conductivity technique, the intratubular hole mobility (2.0 cm² V-1 s-1) of a coassembled nanotube containing 10 mol % of HBC-C60 (1) is as large as the intersheet mobility in graphite. The homotropic nanotube of 2 blended with a soluble C60 derivative [(6,6)-phenyl C61 butyric acid methyl ester] displayed a photovoltaic response with a much different composition dependency, where the largest open-circuit voltage attained was obviously lower than that realized by the coassembly of 1 and 2. [ABSTRACT FROM AUTHOR]

We investigate the effect of the parasitic access resistance on device parameters, particularly the threshold voltage (VT) and the contact resistance (RC), of staggered organic field-effect transistors by varying the active layer thickness (ds). At low gate voltages, RC decreases as ds increases due to the free carrier density increasing in the semiconductor film. At high gate voltages, RC increases as ds increases due to increasing access resistance. These factors degrade the device parameters with increasing ds. The contribution of the change in ds on the VT shift is assessed by subtracting the contact effect from the apparent VT. [ABSTRACT FROM AUTHOR]

A bottom-up fabrication technique for the preparation of self-organized organic field-effect transistors (OFETs) on flexible plastic substrates is presented. Solution-based self-organization of OFETs is achieved by patterning the insulator surface with solution-wettable and unwettable regions. The proposed method satisfies several important requirements of printable electronics, including reduction in energy consumption, minimization of facilities, and the on-demand use of molecular materials. Self-organized OFETs display an average mobility of 0.53 cm2/(V s), on/off ratio of 109, and subthreshold slope of 0.18 V/dec, with near-zero and narrowly distributed threshold voltage. An inverter circuit prepared using these devices is demonstrated with high signal gain. [ABSTRACT FROM AUTHOR]

Semiconductor channels of organic field-effect transistors have been directly self-organized from a solution phase. The alkyl-modified surface was locally patterned by using a phenyl self-assembled monolayer (SAM) for the channels. Drop-cast small organic molecules were selectively crystallized on the phenyl SAM region. The self-organized process allows the simultaneous formation of polycrystalline transistor arrays from the patterned channels. The phenyl SAM under the channel is critical for the improvement of device stability. Further optimization of the deposition process realized direct growth of a single crystal channel from solution between prefabricated electrodes, and the single-crystal transistors exhibited excellent performance. [ABSTRACT FROM AUTHOR]

The effect of semiconductor crystal structure on the performance of organic field-effect transistors is investigated through the fabrication and characterization of devices based on single porphyrin crystals with different center metals. The field-effect mobility of the transistors is found to increase with decreasing intermolecular distance, attributable to greater overlap of π orbitals among close-packed molecules and the consequent promotion of charge transport. [ABSTRACT FROM AUTHOR]

The charge injection process in top-contact organic field-effect transistors was energetically observed with displacement of the Fermi level as a result of scanning the gate voltage. Doping of charge-transfer molecules into the metal/organic interface resulted in low interface resistance, which unveiled the bulk transport of the injected charges from the contact metal into the channel. The authors found that the bulk transport clearly obeys the Meyer-Neldel rule, according to which the exponential density of states near the band edge limits the charge injection. [ABSTRACT FROM AUTHOR]

In the electrochemical transistor of a single-walled carbon nanotube, we introduced the fourth terminal, which works as a reference electrode. This enables accurate control of change in gate voltage, i.e., potential difference between the electrolyte and the source electrode, and quantitative analyses of the gate capacitance. We found that the geometrical capacitance, which was ignored in the conventional model, makes a crucial contribution to the device characteristics, comparable to that from the chemical capacitance. [ABSTRACT FROM AUTHOR]

The contact resistance (Rcont) between nanotube and metal electrodes was directly measured in a four-terminal configuration of field-effect transistors for individual single-walled carbon nanotube (SWNT) bundles and a multiwalled carbon nanotube (MWNT). Both Rcont and the nanotube resistance (RNT) in a semiconducting SWNT device drastically changed with gate voltage, while Rcont, being more than one-order smaller than Rcont in metallic SWNTs and MWNTs, was almost constant against the gate voltage. Carriers introduced either by gate voltage or chemical doping induced a rapid decrease in Rcont compared with the resistance of semiconducting SWNTs. [ABSTRACT FROM AUTHOR]

A high-quality HCa2Nb3O10 (HCNO) nanoflake ([variant_greek_epsilon]r = 9.1) consisting of high-κ perovskite nanosheets is adopted as a gate dielectric for graphene-based electronics. A dual-gated device was physically constructed by directly dry-transferring a 22-nm-thick HCNO nanoflake as a top gate dielectric onto graphene. The fabricated graphene field-effect transistor could be operated at biases <1.5 V with a gate leakage below 1 pA. The top-gate capacitance and mobility of the dual-gated graphene device at room temperature were estimated to be 367 nF/cm2 and 2500 cm2/V · s, respectively. These results show that HCNO can be employed as an alternative dielectric for graphene-based devices. [ABSTRACT FROM AUTHOR]

Unipolar transport is demonstrated in a bilayer graphene with a series of p-n junctions and is controlled by electrostatic biasing by a comb-shaped top gate. The OFF state is induced by multiple barriers in the p-n junctions, where the band gap is generated by applying a perpendicular electric field to the bilayer graphene, and the ON state is induced by the p-p or n-n configurations of the junctions. As the number of the junction increases, current suppression in the OFF state is pronounced. The multiple p-n junctions also realize the saturation of the drain current under relatively high source-drain voltages. [ABSTRACT FROM AUTHOR]

Hexabenzocoronene (HBC) nanotubes, prepared by self-assembly of 1-4 with different oligoether chains, all show a p-type conducting profile in a field-effect transistor (FET) setup, where hole mobilities of their film samples are two orders of magnitude greater than that of a cast film composed of ill-defined, nontubular HBC aggregates. Taking into account their time-resolved microwave conductivities, the observed FET properties appear to be largely affected by the kinetics of intertubular hole hopping, which could be less facilitated as the oligoether chains on the nanotube surface are longer. [ABSTRACT FROM AUTHOR]

Abstract: Measurements of power spectral density (PSD) of low-frequency noise (LFN) in pentacene field-effect transistors reveal the preponderance of a 1/f-type PSD behavior with the amplitude varying as the squared transistor gain and increasing as the inverse of the gate surface area. Such features impose an interpretation of LFN by carrier number fluctuations model involving capture/release of charges on traps uniformly distributed over the gate surface. The surface slow trap density extracted by the noise analysis is close to the surface states density deduced independently from static I(V) data, which confirms the validity of the proposed LFN interpretation. Further, we found that the trap densities in bottom-contact (BC) devices were higher than in their top-contact (TC) counterparts, in agreement with observations of a poorer crystal structure of BC devices, in the contact regions in particular. At the highest bias the noise originating from the contact resistance is also shown to be a dominant component in the PSD, and it is well explained by the noise originating from a gate-voltage dependent contact resistance. A gate area scaling was also performed, and the good scaling and the dispersion at the highest bias confirm the validity of the applied carrier number fluctuations model and the predominant contact noise at high current intensities. [Copyright &y& Elsevier]

We present a novel, fast, and effective method for growing hexagonal-shaped C 70 nanosheets through drop-drying using C 70 dissolved in 1,1,2,2-tetrachloroethane. The synthesized C 70 nanosheets were characterized by optical microscopy, atomic force microscopy, scanning electron microscopy, Raman spectroscopy, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and photoluminescence (PL) spectroscopy. The PL intensities of the C 70 nanosheets are remarkably enhanced compared to that of the C 70 powder. Interestingly, the PL intensities substantially decreased upon increasing the heat-treatment temperature. Moreover, the C 70 nanosheets showed n-type transport characteristics when they were incorporated in bottom-gate/bottom-contact field-effect transistors, and their carrier mobility was evaluated to be6.0 × 10−5 cm2 V−1 s−1under a nitrogen atmosphere. The successful fabrication of C 70 nanosheets fosters research and exploration of other C 70 properties and applications. [Display omitted] • We fabricated porous C70 nanosheets via drop-drying method with tetrachloroethane. • PL intensities of C70 nanosheets are remarkably enhanced compared to C70 powder. • PL signal decreased upon heat treatment of C70 nanosheets up to 170 °C. • n-type transport was observed when incorporated to a field-effect transistor. • Carrier mobility reached up to 6.0 × 10−5 cm2 V−1 s−1 under a nitrogen atmosphere. [ABSTRACT FROM AUTHOR]

Two-dimensional semiconductors arestructurally ideal channel materialsfor the ultimate atomic electronics after silicon era. A long-standingpuzzle is the low carrier mobility (μ) in them as compared withcorresponding bulk structures, which constitutes the main hurdle forrealizing high-performance devices. To address this issue, we performa combined experimental and theoretical study on atomically thin MoS2field effect transistors with varying the number of MoS2layers (NLs). Experimentally, an intimate μ–NLrelation is observed with a 10-fold degradation in μ for extremelythinned monolayer channels. To accurately describe the carrier scatteringprocess and shed light on the origin of the thinning-induced mobilitydegradation, a generalized Coulomb scattering model is developed withstrictly considering device configurative conditions, that is, asymmetricdielectric environments and lopsided carrier distribution. We revealthat the carrier scattering from interfacial Coulomb impurities (e.g.,chemical residues, gaseous adsorbates, and surface dangling bonds)is greatly intensified in extremely thinned channels, resulting fromshortened interaction distance between impurities and carriers. Sucha pronounced factor may surpass lattice phonons and serve as dominantscatterers. This understanding offers new insight into the thicknessinduced scattering intensity, highlights the critical role of surfacequality in electrical transport, and would lead to rational performanceimprovement strategies for future atomic electronics. [ABSTRACT FROM AUTHOR]

Abstract: We control the growth of high-quality organic semiconducting crystals in the aim of transistor application. By finely tuning the processing parameters, both isolated crystals showing characteristic facet angles and irregular-shaped, thin crystalline domains are obtained in large sizes (>400μm). Structural investigations indicate that the various shapes of crystals are in the same crystal structure, and reveal that the irregular-shaped crystalline domains are composed by terrace of molecularly flat regions, which can be up to hundreds of microns in size. When applied in field-effect transistors, the thin crystalline domains exhibit the best performance showing μ FET up to 4.4cm2/Vs. This is an order of magnitude higher than that of the transistors made from as-spun films and thick crystals. The approach well demonstrates the importance of fine control of crystal formation and can be generally used for getting organic crystal transistors. [Copyright &y& Elsevier]

Abstract: We report one-step formation of the gate dielectric and conduction channel for enhancing the performance of organic field effect transistors (OFETs). The resulting OFET with the semiconductor/dielectric bi-layers spun in ambient conditions exhibits μ FET up to 1.6cm2/Vs and on–off ratio higher than 106, no additional treatment needed. Contact angle measurements and absorption spectra reveals that a well-defined semiconductor-top and dielectric-bottom film form after spin-coating the mixture of the two components, which is due to the surface induced self-organized phase separation. Compared to the single layer semiconductor film, the staggered film exhibits over 5 times higher mobility and nearly 90% reduced hysteresis in OFET. The higher performance is attributed to the simultaneous optimization in the dielectric interface and semiconductor crystallization. The approach is significant for the fabrication of low cost, easy processed and high performance OFETs. [Copyright &y& Elsevier]

Abstract: Large surface roughness is a major obstacle for electronic systems fabricated on paper substrates. Here, a mixture solution of organic semiconductor and polymer dielectric was spin-coated on paper substrate with a patterned wettability. This spin-coating process produced organic crystals and a very smooth semiconductor/dielectric interface with a low trap density in well-confined patterns. Despite the large roughness of the paper substrate, the fabricated transistor arrays exhibited high performance with a field-effect mobility reaching 1.3cm2/Vs and an on/off ratio of 108. The presented results offer a simple fabrication method for the current rapidly developing technology of paper electronics. [Copyright &y& Elsevier]

Abstract: Solution-processed organic crystals are important in field-effect transistors because of their highly ordered molecular packing and ease of device fabrication. For practical applications, the patterning of organic crystal transistor arrays is critical. However, uniformity, which concerns the variation in electrical performance among devices fabricated simultaneously on the same substrate, is a common consideration in the commercial applications of the solution-processed organic crystal transistor arrays. Here, a simple approach for fabricating field-effect transistor arrays based on organic plate-like crystals is reported. Through this method, a direct spin-coating process from a mixture solution of organic semiconductor and polymer dielectric can produce organic plate-like crystals. The grain size of the crystals is observed to be hundreds of micrometers. By controlling the concentrations of the active materials, the transistor arrays exhibit high uniformity and good device performance. The results presented in this work promise that this approach is a comparable technology to hydrogenated amorphous silicon-based FETs and is a great candidate for practical applications in electronic devices. [Copyright &y& Elsevier]

Abstract: Electrostatic carrier injection into insulating molecular conductors has been examined by fabricating field effect transistor structure in two kinds of procedures. One procedure employed a direct growth of the organic single crystals by electrochemical reaction on platinum electrodes deposited on the gate dielectric. In the other procedure very thin crystals were laminated on the substrate in ethanol solution. We have succeeded in observing field effects in both methods and found that the lamination method gives better results. [Copyright &y& Elsevier]