Field-effect Transistor Measurements Research Articles

High Electron Mobility of Amorphous Red Phosphorus Thin Films.

Black phosphorus (BP) has been gathering great attention for its electronic and optoelectronic applications due to its high electron mobility and high ION/OFF current switching ratio. The limitations of this material include its low synthetic yield and high cost. One alternative to BP is another type of phosphorus allotrope, red phosphorus (RP), which is much more affordable and easier to process. Although RP has been widely used in industry for hundreds of years and considered as an insulating material, in this study, we demonstrate through field-effect transistors (FET) measurements that amorphous red phosphorus (a-RP) films are semiconductive with a high mobility of 387 cm2 V-1 s-1 and a current switching ratio of ≈103 , which is comparable to the electronic characteristics previously reported for BP. The films were produced via a thermal evaporation method or a facile drop-casting approach onto Si/SiO2 substrates. We also report a study of the oxidation process of the films over time and a method to stabilize the films via doping a-RP with metal oxides. The doped films retain stability for one thousand I-V cycles, with no signs of degradation.

The electronic properties of Au and Pt metal contacts on quasi-one-dimensional layered TiS3(001)

The interfaces of layered trichalcogenide TiS3(001), with metals Au and Pt, were examined using X-ray photoemission spectroscopy. In spite of the fact that both Au and Pt are large work function metals, no evidence of Schottky barrier formation was found with this n-type semiconductor. Two- and four-terminal field-effect transistor measurements performed on exfoliated few-nm-thick TiS3 crystals using pure Au contacts indicate that Au forms an Ohmic contact on TiS3(001), with negligible contact resistance. The absence of appreciable Schottky barrier formation is attributed to strong interactions with sulfur at the metal-semiconductor interface.

Freely Suspended, van der Waals Bound Organic Nanometer-Thin Functional Films: Mechanical and Electronic Characterization.

Determining the electronic properties of nanoscopic, low-dimensional materials free of external influences is key to the discovery and understanding of new physical phenomena. An example is the suspension of graphene, which has allowed access to their intrinsic charge transport properties. Furthermore, suspending thin films enables their application as membranes, sensors, or resonators, as has been explored extensively. While the suspension of covalently bound, electronically active thin films is well established, semiconducting thin films composed of functional molecules only held together by van der Waals interactions could only be studied supported by a substrate. In the present work, it is shown that by utilizing a surface-crystallization method, electron conductive films with thicknesses of down to 6 nm and planar chiral optical activity can be freely suspended across several hundreds of nanometers. The suspended membranes exhibit a Young's modulus of 2-13 GPa and are electronically decoupled from the environment, as established by temperature-dependent field-effect transistor measurements.

Label-Free Enzyme Activity Measurements with Quantum-Limited Biosensors

Accurate and rapid measurements of enzyme activity are critical to the development of therapeutics that target neurological disorders. An effective label-free measurement of enzyme activity can be achieved through the detection of small changes in the solution pH during enzyme-mediated phosphorylation of substrate proteins. Here, we report the measurements of solution pH by using ionic-liquid gated 2D field-effect transistors (FETs) with 200x higher sensitivity than previously reported. The FETs are fabricated with single-monolayer 2D transition metal dichalcogenide semiconducting films and are top-gated with an ionic-liquid. The high ionic-liquid polarizability allows strong coupling between the top gate (ionic liquid) and a back-gate dielectric (substrate oxide) which enables amplification of a voltage applied to the ionic-liquid gate. This amplification is limited by the intrinsic quantum capacitance of the single-monolayer MoS2 semiconducting channel; thus, the devices operate near their theoretical limits. We leveraged the high performance of the devices to enable pH measurements with sensitivities that greatly exceed the Nernst value of 59 mV/pH (at room temperature). The dramatically improved sensitivity allows the detection of pH changes as small as 80x10∧-6. The method allowed the rapid and quantitative estimation of the kinetics of enzymes that are implicated in Alzheimer's disease. We have demonstrated the ability of ionic-liquid-gated FETs to detect small changes in the solution pH during phosphorylation of the substrate protein, histone H1. We compared these FET measurements against enzyme activity measurements obtained from a radioactively labeled ATP assay (which involves handling radioactive materials and is slow to perform). The FET-based measurements are in excellent quantitative agreement with the ATP assay, while enabling results in minutes and can lead to rapid therapeutic screening.

Synthesis and Characterization of Oxygen-Embedded Quinoidal Pentacene and Nonacene.

Extension of oxygen-embedded polycyclic aromatic hydrocarbons (PAHs), in particular with a defined topology, is synthetically challenging primarily because of limited regio-specific methods and poor solubility of PAHs. We reported herein an efficient way to construct quinoidal pentacenes and nonacenes with regular zigzag O-inserted edges. These O-embedded backbones composed of benzene, pyranyl, and cyclohexa-1,4-diene moieties provided access to a new class of longitudinally conjugated acenes with superior stability. Their structures, confirmed by single-crystal XRD analysis, indicated that they possessed rich hydrogen/halogen-bonding interactions, which likely contribute to the strengthened aggregation. In contrast to many other O-annulated PAHs generally displaying short-absorption wavelengths due to partially interrupted electron communication, the O-embedded quinoidal acene exhibited highly red-shifted absorptions (up to ∼699 nm) and narrowed energy gaps (down to ∼1.5 eV). As with more O-containing rings and quinoidal subunits in the backbone, the conjugation size was enlarged, and the molar absorption coefficients (ε) of the λmax significantly increased further, in particular, a noticeable lower-energy peak at ∼790 nm for O-doping nonacenes N1-OH/OMe. By the thin-film-based organic field-effect transistor measurements, the relatively ordered O-doping pentacene P1-OMe possessed a hole transporting efficiency (μh) of 0.00406 cm-2 V-1 s-1 in in-air fabricated devices, while O-pentacene P1-PFB with two perfluorobutoxyl substituents witnessed an improved μh up to 0.0152 cm-2 V-1 s-1. In addition, one- or two-electron oxidation of O-pentacene/nonacene generated the corresponding radical cations or dications, in which electronic properties were dependent on the number of O-containing six-membered rings and quinoidal subunits. The study provided insights into the relationships between molecule structures and optoelectronic properties for the unique class of O-embedded PAHs.

Charge Carrier Polarity Modulation in Diketopyrrolopyrrole-Based Low Band Gap Semiconductors by Terminal Functionalization.

Organic semiconductors with variable charge carrier polarity are required for optoelectronic applications. Herein, we report the synthesis of three novel diketopyrrolopyrrole (DPP)-based D-A molecules having three different terminal groups (amide, ester, and dicyano) and study their electronic properties. An increase in electron acceptor strength from amide to dicyano leads to a bathochromic shift in absorption. Photoconductivity and field effect transistor (FET) measurements confirmed that a small increase in acceptor strength can result in a large change in the charge transport properties from p-type to n-type. The molecule with an amide group, DPP-amide, exhibited a moderate p-type mobility (1.3 × 10-2 cm2 V-1 s-1), whereas good n-type mobilities were observed for molecules with an ester moiety, DPP-ester (1.5 × 10-2 cm2 V-1 s-1), and with a dicyano group, DPP-DCV (1 × 10-2 cm2 V-1 s-1). The terminal functional group modification approach presented here is a simple and efficient method to alter the charge carrier polarity of organic semiconductors.

Naphthalene Diimide Ammonium Directed Single-Crystalline Perovskites with “Atypical” Ambipolar Charge Transport Signatures in Two-Dimensional Limit

A single crystal of a lead–iodine-based 2D perovskite having naphthalene diimide ammonium (NDIA) molecules as organic layers was developed, and the charge transport property was studied using field-effect transistors (FETs) measurements. Structure determination reveals the layered alternative stacking of lead iodide sheets and NDIA bilayers. The presence of NDIA promoted the lead iodide octahedron to form the unique three-point co-planar [Pb3I10]4– unit, which then connected into the 2D network in a corner-sharing manner. The NDIA cations closely stacked into 1D chains within the bilayers that were being sandwiched between the inorganic layers. FET characteristics of the single crystal obtained at room temperature demonstrate VDS-dependent electron and hole transport behavior with mobilities reaching up to more than 5 × 10–3 cm2 V–1 s–1. The 1D stack of NDIAs contributes greatly to the performance improvement for both the charge transport and the stability.

Organic n-Channel Transistors Based on [1]Benzothieno[3,2- b]benzothiophene-Rylene Diimide Donor-Acceptor Conjugated Polymers.

Improving the charge-carrier mobility of conjugated polymers is important for developing high-performance, solution-processed optoelectronic devices. Although [1]benzothieno[3,2- b]benzothiophene (BTBT) has been frequently used as a high-performance p-type small molecular semiconductor and employed a few times as a building block for p-type conjugated polymers, it has never been explored as a donor moiety for high-performance n-type conjugated polymers. Here, BTBT has been conjugated with either n-type perylene diimide (PDI) or naphthalene diimide (NDI) units to generate a donor-acceptor copolymer backbone, for the first time. Charge-transport measurements of organic field-effect transistors show n-type dominant behaviors, with the electron mobility reaching ∼0.11 cm2 V-1 s-1 for PDI-BTBT and ∼0.050 cm2 V-1 s-1 for NDI-BTBT. The PDI-BTBT mobility value is one of the highest among the PDI-containing polymers. The high π-π stacking propensity of BTBT significantly improves the charge-carrier mobility in these polymers, as supported by atomic force microscopy and grazing incidence X-ray diffraction analyses. Phototransistor applications of these polymers in the n-type mode show highly sensitive photoresponses. Our findings demonstrate that incorporation of the BTBT donor unit within the rylene diimide acceptor-based conjugated polymers can improve the molecular ordering and electron mobility.

Photoinduced electron and energy transfer in an amphiphilic perylenetetracarboxylic diimide derivative/CdS self-assembled hybrid film

An amphiphilic perylenetetracarboxylic diimide derivative with four 4-tert-butylphenoxy substituents at the bay positions (named as TPDI) has been used as not only the functional organic materials but also the organic template for growth of CdS nanoparticles. The highly ordered TPDI/CdS hybrid film was obtained where the petals-like CdS nanocrystals with a uniform size (about 50 nm) grown on the surface of TPDI film successfully. The crystalline regions for CdS were identified to be hexagonal wurtzite. Examination by spectroscopic methods revealed TPDI molecules in both pure film and hybrid film all adopt a typical J-aggregation mode with a slight declining orientation angle with respect to the substrate from 24.8° to 21.9° after incorporation of CdS nanoparticles into the TPDI matrix. In particular, energy transfer from CdS to TPDI is established by both increased fluorescence emission intensity and fluorescence quantum yield of TPDI in the hybrid film. Furthermore, the enhanced conductivity and electron mobility are obtained for the hybrid film relative to those of the pure film based on current-voltage (I-V), field-effect transistor (FET) and photocurrent measurements. Unexpectedly, photo-induced electron-transfer from TPDI molecules to CdS nanocrystals is revealed through much more enhanced conductivity and electron mobility for the TPDI/CdS hybrid film relative to pure TPDI film upon 365 nm UV light irradiation. In only one TPDI/CdS hybrid system, the energy band alignment of TPDI and CdS nanoparticles makes evident possibility for electron transfer, while their electronic absorption and fluorescence emission features provide reasonability for both electron and energy transfer.

Electron delocalization and charge mobility as a function of reduction in a metal-organic framework.

Conductive metal-organic frameworks are an emerging class of three-dimensional architectures with degrees of modularity, synthetic flexibility and structural predictability that are unprecedented in other porous materials. However, engendering long-range charge delocalization and establishing synthetic strategies that are broadly applicable to the diverse range of structures encountered for this class of materials remain challenging. Here, we report the synthesis of K x Fe2(BDP)3 (0 ≤ x ≤ 2; BDP2- = 1,4-benzenedipyrazolate), which exhibits full charge delocalization within the parent framework and charge mobilities comparable to technologically relevant polymers and ceramics. Through a battery of spectroscopic methods, computational techniques and single-microcrystal field-effect transistor measurements, we demonstrate that fractional reduction of Fe2(BDP)3 results in a metal-organic framework that displays a nearly 10,000-fold enhancement in conductivity along a single crystallographic axis. The attainment of such properties in a K x Fe2(BDP)3 field-effect transistor represents the realization of a general synthetic strategy for the creation of new porous conductor-based devices.

Electronic properties and structure of single crystal perylene.

The transport properties of electronic devices made from single crystalline molecular semiconductors typically outperform those composed of thin-films of the same material. To further understand the superiority of these extrinsic device properties, an understanding of the intrinsic electronic structure and properties of the organic semiconductor is necessary. An investigation of the electronic structure and properties of single crystal α-phase perylene (C20H12), a five-ringed aromatic molecule, is presented using angle-resolved ultraviolet photoemission, x-ray photoelectron spectroscopy (XPS), and field-effect transistor measurements. Key aspects of the electronic structure of single crystal α-perylene critical to charge transport are determined, including the energetic location of the highest occupied molecular orbital (HOMO), the HOMO bandwidth, and surface work function. In addition, using high resolution XPS, we can distinguish between inequivalent carbon atoms within the perylene crystal and, from the shake-up satellite structure in XPS, gain insight into the intramolecular properties in α-perylene. From the device measurements, the charge carrier mobility of α-perylene is found to depend on the device structure and the choice of dielectric, with values in the range of 10-3 cm2 V-1 s-1.

Evidence for Photo-Switchable Carrier Mobilities in Blends of PbS Nanocrystals and Photochromic Dithienylcyclopentene Derivatives

Abstract We use derivatives of the photochromic molecule 1,2-bis(5′-carboxy-2′-methylthien-3′-yl)-cyclopentene in combination with semiconducting PbS nanocrystals to probe the feasibility of solid-state optical switching in hybrid nanostructured thin films. X-ray photoelectron spectroscopy is applied to differentiate between the different constituents in the blends, while field-effect transistor measurements in the dark reveal the effect of prior illumination with visible or ultra-violet light on the transport properties. By analyzing the response of the electric conductivity, the carrier concentration and mobility separately, we are able to distinguish between the effect of additional photo-excited charge carriers and photo-induced changes of the electronic structure of the semiconducting film. Our results suggest that coupling between the nanocrystals and the photochromic molecule is weak but also provide evidence that photochromism of the molecules may be partially preserved.

Local Enhancement of Exciton Emission of Monolayer MoS2 by Copper Phthalocyanine Nanoparticles

Monolayer transition-metal dichalcogenides (1L-TMDs) provide ideal platforms to study light emission using two-dimensionally confined excitons. Recent studies have shown that the exciton emissions of 1L-TMDs can be conveniently modulated by developing heterostructures with zero-dimensional nanoparticles (NPs) or quantum dots. In this study, we synthesized organic semiconducting copper phthalocyanine (CuPc) NPs with sizes in the range of 30–70 nm by a re-precipitation method and decorated the chemical vapor deposition-grown 1L-MoS2 with these NPs. This hybrid system exhibited a 6 times larger local photoluminescence (PL) at the positions of the CuPc NPs compared with the pristine 1L-MoS2 sample. The PL enhancement and spectral modification of the 1L-MoS2 decorated with CuPc NPs were attributed to the p-doping effect of the CuPc NPs, confirmed by spectral analysis and field-effect transistor measurements.

Carrier Transport Dynamics in High Speed Black Phosphorus Photodetectors

A fundamental understanding of carrier transport mechanism is imperative for efficient semiconductor electronics and optoelectronics. Here, we use high speed black phosphorus photodetectors with sub-40 ps response time to elucidate carrier transport dynamics along its armchair and zigzag directions. Here we report a direct observation of carrier transport transition dynamics from phonon scattering transport to multiple trapping and release transport mechanism along the armchair direction, resulting from the relaxation of free carriers above the band edge to the band-tail states. We identified that the suppression of phonon scattering effects, a characteristic by Hall and field effect transistor measurements, is due to carrier transport in band tail states. Along the zigzag direction, only multiple trapping and release transport in band-tail states is observed, which might be due to low carrier mobility.

Critical assessment of charge mobility extraction in FETs.

Mobility is an important charge-transport parameter in organic, inorganic and hybrid semiconductors. We outline some of the common pitfalls of mobility extraction from field-effect transistor (FET) measurements and propose practical recommendations to avoid reporting erroneous mobilities in publications.

Ultra-Easy and Fast Method for Transferring Graphene Grown on Metal Foil

Growing graphene on a large scale is the first step towards its industrial application. The most widely used large-scale graphene growth method is chemical vapor deposition (CVD) on metal foil. Transferring large-scale graphene without damaging it or degrading its performance is also very important. Generally, techniques for transferring CVD-grown graphene involve metal-foil etching. In this case, the metal etchant can chemically alter the graphene and small amounts of metal residue still remain after etching. These metal residues change the properties of the transferred graphene. In this paper, we demonstrate a new technique for transferring CVD-grown graphene films onto arbitrary substrates using Crystalbond, an off-the-shelf adhesive material. This new method is very simple, easy, and fast. It also solves the aforementioned problems associated with metal etching, by using mechanical exfoliation of graphene via the high adhesive strength of Crystalbond. We transferred a [Formula: see text]1[Formula: see text]cm size piece of graphene, which exhibited reasonable optical and electrical properties, as observed using Raman spectroscopy and field effect transistor (FET) measurements, respectively.

High-Performance Field-Effect Transistor Based on Novel Conjugated P-o-Fluoro-p-alkoxyphenyl-Substituted Polymers by Graphdiyne Doping

The novel two-dimensional graphitic material, graphdiyne (GDY), has attracted great interest due to its superior stability and natural semiconductor characteristic. We realize the obvious improvement of carrier transport characteristics of P-o-FBDTP–C8DTBTff (PFC; o-FBDTP, o-fluoro-p-alkoxyphenyl-substituted benzo[1,2-b:4,5-b′]dithiophene; C8DTBTff, 5,6-difluoro-4,7-di(4-(2-ethylhexyl)-2-thienyl)-2,1,3-benzothiadiazole) organic polymer semiconductor via adding GDY. The result of the field effect transistor measurement suggests that doping GDY can dramatically improve the on/off ratio and threshold voltage of the organic semiconductor. Especially the mobility of GDY/PFC film is 2 orders of magnitude higher than that of pristine PFC, demonstrating the important role of GDY dopant. Besides, the carrier diffusion length of GDY/PFC film is also enhanced, suggesting the great potential applications of GDY-modified polymer with donor–acceptor units in organic photoelectric devices.

La chirurgie valvulaire à cœur ouvert

Composites of single-walled carbon nanotubes (SWNT) and conjugated polymers are highly interesting as active semiconducting layers in solution-processable and flexible electronics. Understanding the percolation behavior and percolation threshold for electrical conductivity in these composites is a prerequisite for future applications. Here, we investigate the concentration and length dependence of on-currents and apparent mobilities for selectively dispersed, semiconducting (6,5) single-walled carbon nanotubes in a matrix of the dispersing polyfluorene-bipyridine copolymer (PFO-BPy) through field-effect transistor measurements. More importantly, we directly visualize the percolation paths at and above the percolation limit by near-infrared electroluminescence (EL) imaging with high spatial resolution. EL imaging reveals the various shapes of critical percolation paths at the threshold and the non-uniformity of charge transport even at high SWNT concentrations. We also find that percolation paths differ depending on the assignment of the injecting electrodes probably due to different injection barriers and variation of nanotube density at the electrodes.

1,2,5,6-Naphthalenediimide-based conjugated copolymers linked by ethynyl units

Two copolymers of P1 and P2 comprising benzothiadiazole, 1,4-bis(dodecyloxy)-benzene units were synthesized by Sonogashira coupling polymerization based on ethynyl-linked 1,2,5,6-naphthalenediimide. Their thermal, optical, electrochemical as well as charge transport properties were studied. Bottom-gate top-contact organic field-effect transistors (OFETs) measurements of P1 and P2 thin films showed different charge transport behaviors. P1 displayed pure electron transport behaviors in OFETs with electron mobility up to 10−3 cm2·V−1·s−1, while P2 exhibited hole transport features. The molecular structure analysis revealed that the structure of P1 has the acceptor-linker-acceptor′ (A-L-A′) characteristic, and P2 possesses the donor-linker-acceptor (D-L-A) structure feature. The results demonstrate that different molecular structures lead them to have distinct charge transport behaviors. In particular, the first pure electron transport copolymer in OFETs based on 1,2,5,6-naphthalenediimide is achieved.

Phase-Transfer Ligand Exchange of Lead Chalcogenide Quantum Dots for Direct Deposition of Thick, Highly Conductive Films.

The use of semiconductor nanocrystal quantum dots (QDs) in optoelectronic devices typically requires postsynthetic chemical surface treatments to enhance electronic coupling between QDs and allow for efficient charge transport in QD films. Despite their importance in solar cells and infrared (IR) light-emitting diodes and photodetectors, advances in these chemical treatments for lead chalcogenide (PbE; E = S, Se, Te) QDs have lagged behind those of, for instance, II-VI semiconductor QDs. Here, we introduce a method for fast and effective ligand exchange for PbE QDs in solution, resulting in QDs completely passivated by a wide range of small anionic ligands. Due to electrostatic stabilization, these QDs are readily dispersible in polar solvents, in which they form highly concentrated solutions that remain stable for months. QDs of all three Pb chalcogenides retain their photoluminescence, allowing for a detailed study of the effect of the surface ionic double layer on electronic passivation of QD surfaces, which we find can be explained using the hard/soft acid-base theory. Importantly, we prepare highly conductive films of PbS, PbSe, and PbTe QDs by directly casting from solution without further chemical treatment, as determined by field-effect transistor measurements. This method allows for precise control over the surface chemistry, and therefore the transport properties of deposited films. It also permits single-step deposition of films of unprecedented thickness via continuous processing techniques, as we demonstrate by preparing a dense, smooth, 5.3-μm-thick PbSe QD film via doctor-blading. As such, it offers important advantages over laborious layer-by-layer methods for solar cells and photodetectors, while opening the door to new possibilities in ionizing-radiation detectors.