High-performance multilayer WSe2 p-type field effect transistors with Pd contacts for circuit applications

WSe₂ is thought to be one of the best emerging p-type transition metal dichalcogenide (TMD) materials for potential low-power complementary metal oxide semiconductor (CMOS) circuit applications. However, the contact barrier and the interface quality hinder the performance of p-type field effect transistors (FETs) with WSe₂ films. In this work, metals with different work functions—Pd, Pt, and Ag—were systematically investigated as contacts for WSe₂ to decrease the contact resistances at source/drain electrodes and potentially improve transistor performance. Optimized p-type multilayer WSe₂ FETs with Pd contacts were successfully fabricated, and excellent electrical characteristics were obtained: a hole mobility of 36 cm²V^?1 s^?1; a high on/off ratio, over 10⁶; and a record low sub-threshold swing, SS?=?95 mV/dec, which may be attributed to the small Schottky barrier height of 295 meV between Pd and WSe₂, and strong Fermi-level pinning near the top of the valence band at the interface. Finally, a full-functional CMOS inverter was also demonstrated, consisting of a p-type WSe₂ FET together with a normal n-type MoS₂ FET. This confirmed the potential of TMD FETs in future low-power CMOS digital circuit applications.

     

Mössbauer effect and dielectric behavior of NixCu0.8−xZn0.2Fe2O4 compound

The ferrite system Ni_xCu_0.8−xZn_0.2Fe₂O₄ with 0.0 ≤ x ≤ 0.8 was synthesized. XRD measurement confirmed the presence of single-phase spinel structure. The area ratio of Fe³⁺ at the tetrahedral A- and octahedral B-sites was deduced from the spectral analysis of Mössbauer measurements. The results give evidence that Ni²⁺ replaces Cu at B-site in the present unit cell. The dielectric properties ?′, ?″, loss tangent tan(δ) and ac conductivity σ_ac have been studied for the prepared samples in the temperature range (300-600 K) and over the frequency range (10² to 10⁵ Hz). The electrical conductivity results revealed a semiconductor behavior with increasing nickel concentration with a change in the slope at the transition temperature T_c. The variation of the dielectric parameters (?′, ?″ and tan(δ)) with frequency and temperature displayed a strong dependence on nickel concentration. Dielectric anomaly at the transition temperature T_c was pronounced in the relations of ?′ and ?″ with temperature. The determined T_c was found to increase with increasing Ni content. The relation of tan(δ) with frequency at different temperatures showed two relaxation processes where the relaxation time and maximum frequency of the hopping conduction mechanism were determined. The results are explained in the light of cation-anion-cation and cation-cation interactions over the octahedral site in the spinel structure.     

High performance inkjet printed phosphorescent organic light emitting diodes based on small molecules commonly used in vacuum processes

High efficiency phosphorescent organic light emitting diodes (OLEDs) are realized by inkjet printing based on small molecules commonly used in vacuum processes in spite of the limitation of the limited solubility. The OLEDs used the inkjet printed 5 wt.% tris(2-phenylpyridine)iridium(III) (Ir(ppy)₃) doped in 4,4′-Bis(carbazol-9-yl)biphenyl (CBP) as the light emitting layer on various small molecule based hole transporting layers, which are widely used in the fabrication of OLEDs by vacuum processes. The OLEDs resulted in the high power and the external quantum efficiencies of 29.9 lm/W and 11.7%, respectively, by inkjet printing the CBP:Ir(ppy)₃ on a 40 nm thick 4,4′,4″-tris(carbazol-9-yl)triphenylamine layer. The performance was very close to a vacuum deposited device with a similar structure.     

Synthesis and characterization of Y-shape electron donor–acceptor type organic dyes for dye-sensitized solar cells

Three Y-shape organic dyes, (Z)-3-(5-(3,5-bis(4-(9H-carbazol-9-yl)styryl)-4-methoxyphenyl)thiophen-2-yl)-2-cyanoacrylic acid (OD-1), (Z)-3-(5′-(3,5-bis(4-(9H-carbazol-9-yl)styryl)-4-methoxyphenyl)-2,2′-bithiophen-5-yl)-2-cyanoacrylic acid (OD-2) and (Z)-3-(5′-(3,5-bis(4-(9H-carbazol-9-yl)styryl)-4-methoxyphenyl)-3,4′-4″-trithiophenyl-5-yl)-2-cyanoacrylic acid (OD-3) were synthesized and used as sensitizers in nanocrystalline dye-sensitized solar cells (DSSCs). The introduction of the bis(carbazolylstyryl) units as an electron donor group and oligothiophene units as a both electron donors and π-spacers increased the conjugation length of the sensitizers and thus improved their molar absorption coefficient and light harvesting efficiency. DSSCs with the configuration of SnO₂:F/TiO₂/organic dye/liquid electrolyte/Pt devices were fabricated using these OD-1, OD-2 and OD-3 as a sensitizers. Among the devices, the DSSC composed of OD-3 exhibited highest power conversion efficiency of 3.03% under AM1.5G (100 mW cm⁻²).     

Photo‐/Thermal‐Responsive Field‐Effect Transistor upon Blending Polymeric Semiconductor with Hexaarylbiimidazole toward Photonically Programmable and Thermally Erasable Memory Device

It is shown that the semiconducting performance of field‐effect transistors (FETs) with PDPP4T (poly(diketopyrrolopyrrole‐quaterthiophene)) can be reversibly tuned by UV light irradiation and thermal heating after blending with the photochromic hexaarylbiimidazole compound (p‐NO₂‐HABI). A photo‐/thermal‐responsive FET with a blend thin film of PDPP4T and p‐NO₂‐HABI is successfully fabricated. The transfer characteristics are altered significantly with current enhanced up to 10⁶‐fold at V_G = 0 V after UV light irradiation. However, further heating results in the recovery of the transfer curve. This approach can be extended to other semiconducting polymers such as P3HT (poly(3‐hexyl thiophene)), PBTTT (poly(2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b] thiophene)) and PDPPDTT (poly(diketopyrrolopyrrole‐dithienothiophene)). It is hypothesized that TPIRs (2,4,5‐triphenylimidazolyl radicals) formed from p‐NO₂‐HABI after UV light irradiation can interact with charge defects at the gate dielectric–semiconductor interface and those in the semiconducting layer to induce more hole carriers in the semiconducting channel. The application of the blend thin film of PDPP4T and p‐NO₂‐HABI is further demonstrated to fabricate the photonically programmable and thermally erasable FET‐based nonvolatile memory devices that are advantageous in terms of i) high ON/OFF current ratio, ii) nondestructive reading at low electrical bias, and iii) reasonably highly stable ON‐state and OFF‐state.     

Synthesis and magnetic properties of an iron 1,2-bisthienyl perfluorocyclopentene photochromic coordination compound

The coordination compound Fe(BM-4-PTP)₂(NCS)₂⋅2MeOH (1) including the photoisomerizable ligand BM-4-PTP (1,2-bis(2′-methyl-5′-(pyrid-4″-yl)thien-3′-yl)perfluorocyclopentene) was obtained as an orange powder. The powder turns blue upon photocyclization of the 1,2-bisthienyl photochromic ligand induced by UV light irradiation at room temperature. Photocycloreversion is obtained by visible light irradiation of the material in the solid state. The orange and blue powders were investigated over the temperature range (5-293 K) and pressure range (1 bar-12 kbar) by magnetic susceptibility measurements and variable temperature ⁵⁷Fe Mössbauer spectroscopy. The photo-induced colour change is accompanied by a distinct magnetic variation at room temperature. Potentialities of this functional optical material for display and data recording are introduced.     

Crystal structure of 1,4?-Dimethoxy-4,1′:4′,1″:4″,1?-quaterphenylene

1,4?-Dimethoxy-4,1′:4′,1″:4″,1?-quaterphenylene crystallizes in space group Pbca with a = 7.4989(5) Å, b = 6.1544(3) Å, c = 40.359(2) Å, V = 1862.6(2) ų, and Z = 4. In the crystal structure the molecules are arranged in layers, which are stacked in the direction of the crystallographic c-axis. The torsion between the molecules in a layer amounts to 69.06° resulting in a herringbone structure. The phenyl rings of the molecule are approximately planar with a torsion angle of 4.64° between the outer and the center rings. UV/Vis transmission measurements of the colorless crystal reveal a band gap of ~ 3.2 eV.     

Core-cyanated distyryl-bithiophene: Synthesis and impact on charge transport in field-effect transistors

We report on the synthesis of two new distyryl-oligothiophenes with cyano groups at different positions of the ethylene linkage: E,E-5,5′-bis((2-cyano-2-phenyl)-ethenyl)-2,2′-bithiophene 1 and E,E-5,5′-bis((1-cyano-2-phenyl)-ethenyl)-2,2′-bithiophene 2. The impacts of cyanated substitutions on molecular orbitals, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), of these new oligomers investigated by the experimental data are confronted in a theoretical study. Despite lower LUMO level, introduction of electron-withdrawing groups in conjugated cores creates a local hindrance in the π-electron delocalization. Detrimental effects are observed on thin-film morphology studied by scanning electron microscopy and the crystalline structure characterization by X-ray diffraction. Organic thin-film transistors made from 1 showed only a low p-channel transistor activity in air whereas devices made from 2 showed neither a n- or a p-channel activity. Analysis of these materials reveals a direct relationship between the molecular structure and solid state properties.     

Efficient red electrophosphorescent organic light-emitting diodes based on the new sensitized heteroleptic tris-cyclometalated Ir(III) complexes

Organic light-emitting device (OLED) was fabricated using the novel red phosphorescent heteroleptic tris-cyclometalated iridium complex, bis(2-phenylpyridine)iridium(III)[2(5′-methylphenyl)-4-diphenylquinoline] [Ir(ppy)₂(dpq-5CH₃)], based on 2-phenylpyridine (ppy) and 2(5′-methylphenyl)-4-diphenylquinoline (dpq-5CH₃) ligand. Generally, the ppy ligand in heteroleptic iridium complexes plays an important role as “sensitizer” in the efficient energy transfer from the host (CBP; 4,4,N,N′-dicarbazolebiphenyl) to the luminescent ligand (dpq-5CH₃). We demonstrated that high efficiency through the “sensitizer” can be obtained, when the T₁ of the emitting ligand is close to T₁ of the sensitizing ligand. The device containing Ir(ppy)₂(dpq-5CH₃) produced red light emission of 614 nm with maximum luminescence efficiency and power efficiency of 8.29 cd/A (at 0.09 mA/cm²) and 5.79 lm/W (at 0.09 mA/cm²), respectively.     

Influence of the B-site ordering on the magnetic properties of the new La3Co2MO9 double perovskites with M = Nb or Ta

Double perovskites La₃Co₂NbO₉ and La₃Co₂TaO₉ have been prepared by both solid state and sol-gel synthesis. The crystal structures have been studied from X-ray and neutron powder diffraction data. Rietveld refinements show that the crystal structure is monoclinic (P2₁/n), with different degrees of ordering of B′ and B″ cations, with octahedra tilted according to the Glazer notation a⁻b⁻c⁺. Occupancy refinements show that the solid state materials are more B-site ordered than the sol-gel ones. Magnetization measurements show that these perovskites show two magnetic contributions, one with spontaneous magnetization and other with linear behaviour with the magnetic field associated to antiferromagnetic correlations. In the samples synthesized by solid state the spontaneous magnetization is more important than those synthesized by the sol-gel and present T_C of 62 K for Nb and 72 K for Ta. On the other hand, materials prepared by sol-gel have T_C 20 K for Nb and 40 K for Ta, respectively and major presence of the antiferromagnetic contribution. The competition between these magnetic behaviours is interpreted, by a microscopic point of view, as to be due to the different degrees of Co²⁺ ions disorder on the B site of the double perovskite structure. This disorder affects the ratio between the antiferromagnetic Co²⁺-O-Co²⁺ and the ferromagnetic Co²⁺-O-M⁵⁺-O-Co²⁺ couplings proposed for the system.     

The fabrication and enhanced nonlinear optical properties of electrostatic self-assembled film containing water-soluble chiral polymers

An ultra-thin film containing a water-soluble chiral PPV derivative and oligo-thiophene derivative was fabricated through the electrostatic self-assembly technique. The PPV and thiophene derivatives are poly{(2,5-bis(3-bromotrimethylammoniopropoxy)-phenylene-1,4-divinylene)-alt-1,4-(2,5-bis((3-hydroxy-2-(S)-methyl)propoxy)phenylenevinylene) (BHP-PPV) and 4′,3″-dipentyl-5,2′:5′,2″:5″,2″′-quaterthiophene-2,5″′-dicarboxylic acid (QTDA), respectively. The circular dichroism (CD) spectrum of BHP-PPV cast film on quartz substrate proved the chirality of BHP-PPV. The UV–vis spectra showed a continuous deposition process of BHP-PPV and QTDA. The film structure was characterized by small angle X-ray diffraction (XRD) measurement and atomic force microscopy (AFM) images. The nonlinear optical (NLO) properties of BHP-PPV/QTDA ultra-thin film with different number of bilayers were investigated by the Z-scan technique with 8 ns laser pulse at 532 nm. The Z-scan experimental data were analyzed with the double-sided film Z-scan theory. The BHP-PPV/QTDA film exhibits enhanced reverse saturable absorption (RSA) and self-defocusing effects, which may be attributed to the conjugated strength, chirality and well-ordered film structure. The chirality may lead to the RSA of BHP-PPV/QTDA film contrary to the SA of the other electrostatic self-assembled films without chiral units. The self-defocusing effect should be due to the thermal effect.     

Efficient triplet exciton confinement of white organic light-emitting diodes using a heavily doped phosphorescent blue emitter

We demonstrated efficient white electrophosphorescence with a heavily doped phosphorescent blue emitter and a triplet exciton blocking layer (TEBL) inserted between the hole transporting layer (HTL) and the emitting layer (EML). We fabricated white organic light-emitting diodes (WOLEDs) (devices A, B, C, and D) using a phosphorescent red emitter; bis(2-phenylquinolinato)-acetylacetonate iridium III (Ir(pq)₂acac) doped in the host material; N,N′-dicarbazolyl-3,5-benzene (mCP) as the red EML and the phosphorescent blue emitter; bis(3,5-Difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl) iridium III (FIrpic) doped in the host material; p-bis(triphenylsilyly)benzene (UGH2) as the blue EML. The properties of device B, which demonstrate a maximum luminous efficiency and external quantum efficiency of 26.83 cd/A and 14.0%, respectively, were found to be superior to the other WOLED devices. It also showed white emission with CIE_x,y coordinates of (x = 0.35, y = 0.35) at 8 V. Device D, which has a layer of P-type 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA) material between the HTL and TEBL, was compared with device A to determine the 430 nm emission peak.     

Demonstration of logic AND device using a split-gate pentacene field effect transistor

We report on the fabrication and performance of pentacene-based split-gate field effect transistors (FETs) on doped Si/SiO₂ substrates. Several transistors with split gate structures were fabricated and demonstrated AND logic functionality. The transistor’s functionality was controlled by applying either 0 or − 10 V to each of the gate electrodes. When − 10 V was simultaneously applied to both gates, the transistor was conductive (ON), while any other combination of gate voltages rendered the transistor highly resistive (OFF). A significant advantage of this device is that AND logic devices with multiple inputs can be built using a single pentacene channel with multiple gates. The p-type carrier mobility of charge within the pentacene active layer of these transistors was about 10^− 5 cm²/V-s. We attribute the low value of mobility primarily to the sharp contours of the pentacene film between the drain and the source contacts and to defects in the pentacene film. The average charge density was 1.4 × 10¹² holes/cm². Despite low mobility, the devices operated at lower drain-source (V_DS) and gate-source (V_GS) voltages as compared with previously reported pentacene based FETs.     

Dielectric properties of thallium halides and their mixed crystals

Dielectric constant κ′ and dielectric loss κ″ of single crystals of TlCl, TlBr, KRS6, KRS5 and TlI + 1% CsI and polycrystalline TlF and TlI have been measured as a function of frequency from 10² - 10⁷ Hz (at room temperature to 10⁸ Hz) in the temperature range from 4.2 to 655oK. κ′ at 200oK in all these compounds is frequency-independent above 10³ Hz. κ″ decreases at l/f (where f is frequency) in the single crystals but slightly deviates in polycrystalline TlF and TlI. The influence of temperature on κ′ is complicated. At 10⁷ Hz, κ′ for TlCl decreases continuously from 37.0 (extrapolated value at absolute zero) to 29.0 at 555oK and then increases to about 60 at 655oK. At high temperatures, κ′ increase is strongly frequency-dependent. Similar behavior is exhibited by TlBr, KRS6, KRS5 and TlI + CsI. In TlF, κ′ continuously increases from 17.3 (at 0oK) to 25 at 480oK with frequency dependence above 330oK; TlI behaves similarly to TlF except at the phase transition at 455oK where κ′ increases considerably. κ″ in all the compounds is found to increase exponentially with temperature and decrease with frequency. The negative temperature coefficient of cubic thallium halides is similar to that of ferroelectrics in paraelectric state, except that they do not reach the ferroelectric transition. The κ′ as a function of T obeys perfectly the Curie-Weiss law, only the Curie points fall below 0oK. Similar behavior is shown by some other crystals. They are considered to be “precursors” of ferroelectrics.     

Growth, structure and carrier transport properties of Ga2O3 epitaxial film examined for transparent field-effect transistor

Growth conditions of heteroepitaxial thin films of tin-doped Ga₂O₃ were surveyed from the viewpoint of visible application to field-effect transistors (FETs). Films were deposited by pulsed laser deposition, and post-annealing was examined to improve film structures. Atomically flat surfaces were obtained for films grown on yttria-stabilized zirconia (111) plane and post-annealed at 1400 °C, but they were insulating. Conductive heteroepitaxial films applicable to FETs were obtained on α-Al₂O₃ (0001) at specific deposition conditions, i.e. substrate temperatures from 500 to 550 °C and oxygen pressures from 5 × 10^− 4 to 1 × 10^− 3 Pa. It was found that the resulting epitaxial films have a crystal structure different from that of β-Ga₂O₃. The crystal lattice for the films is determined to be orthorhombic with a large possibility of a higher-symmetry hexagonal or rhombohedral system. The films exhibited high transparency in the near infrared-deep ultraviolet region and had bandgap of ∼ 4.9 eV. The operation of top-gate MISFETs using the Ga₂O₃ film for the n channel was demonstrated.     

Doping-free fabrication of carbon nanotube thin-film diodes and their photovoltaic characteristics

Random networks of single-walled carbon nanotubes (SWCNTs) were have been grown by chemical vapor deposition on silicon wafers and used for fabricating field-effect transistors (FETs) using symmetric Pd contacts and diodes using asymmetrical Pd and Sc contacts. For a short channel FET or diode with a channel length of about 1 μm or less, the device works in the direct transport regime, while for a longer channel device the transport mechanism changes to percolation. Detailed electronic and photovoltaic (PV) characterizations of these carbon nanotube (CNT) thin-film devices was carried out. While as-fabricated FETs exhibited typical p-type transfer characteristics, with a large current ON/OFF ratio of more than 10⁴ when metallic CNTs were removed via a controlled breakdown, it was found that the threshold voltage for the devices was typically very large, of the order of about 10 V. This situation was greatly improved when the device was coated with a passivation layer of 12 nm HfO₂, which effectively moved the threshold voltages of both FET and diode back to center around zero or turned these device to their OFF states when no bias was applied on the gate. PV measurements were then made on the short channel diodes under infrared laser illumination. It was shown that under an illumination power density of 1.5 kW/cm², the device resulted in an open circuit voltage V _OC = 0.21 V and a short circuit current I _SC = 3.74 nA. Furthermore, we compared PV characteristics of CNT film diodes with different channel lengths, and found that the power transform efficiency decreased significantly when the device changed from the direct transport to the percolation regime.     

Effect of rare earth yttrium substitution on the structural,dielectric and electrical properties of nanosized nickel aluminate

In this communication, we have systematically investigated the effect of yttrium substitution on the structural, dielectric and electrical properties of nanosized NiAl_2−2xY_2xO₄ series (where x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.07 and 0.10). All the samples were prepared by chemical coprecipitation method. Powder X-ray diffraction (XRD) confirmed the formation of single phase cubic spinel structure in all the samples. Replacement of Al³⁺ ions by Y³⁺ ions results in a slight increase of lattice parameter. It was inferred that the substitution of yttrium suppressed the crystallite size growth. Transmission electron microscopy (TEM) validated the nanocrystalline nature of the samples. The Fourier transform infrared spectroscopy (FTIR) confirmed the preference of Y³⁺ ions at the octahedral B site. Room temperature dielectric properties, namely dielectric constant (?′), dielectric loss (tan δ), ac conductivity (σ_a.c.) and electrical modulus (M″) were studied as a function of applied frequency in the range from 1 kHz to 1 MHz. These studies indicate that all the samples show usual dielectric dispersion which is due to Maxwell–Wagner type Interfacial Polarization. The ac conductivity measurement suggests that the conduction mechanism is due to small polaron hopping. The electrical modulus results clearly indicate the presence of non-Debye type of dielectric relaxation in all the samples. The dc electrical resistivity measured in the temperature range of 303–373 K was found to increase with temperature and yttrium content.     

Electrochromic behavior of poly(3,5-bis(4-bromophenyl)dithieno[3,2-b;2′,3′-d]thiophene)

Electropolymerization of 3,5-bis(4-bromophenyl)dithieno[3,2-b;2′,3′-d]thiophene, having a mild electron-withdrawing bromine atom attached to the para position of the two phenyl groups of dithienothiophene, was performed, utilizing a potentiodynamic method. The resultant homopolymer was characterized by cyclic volyammetry, fourier transform infrared and ultraviolet-visible spectroscopy. Spectroelectrochemical and electrochromic properties were investigated and a device was constructed with polyethylenedioxythiophene to understand the characteristics in detail, which had the potential range of 0.0-2.0 V for operating the device between green and blue colors. It had a good optical contrast of 18% with a switching time of less than a second. Moreover, open circuit memory and stability of the device were investigated.     

Synthesis and characterization of electrochromic poly(amide–imide)s bearing methoxy-substituted triphenylamine units

New poly(amide–imide)s (PAIs) 3a–d and 3′a–d with methoxy-substituted triphenylamine (TPA) units were prepared by the direct polycondensation from various imide ring-preformed dicarboxylic acids 2a–d with 4,4′-diamino-2″,4″-dimethoxytriphenylamine (1) and 4,4′-diamino-4″-methoxytriphenylamine (1′), respectively, using triphenyl phosphite and pyridine as condensing agents. For the comparative study, the referenced PAIs 3″a–d without methoxy substituents on the TPA unit were also prepared from 2a–d with 4,4′-diaminotriphenylamine (1″). All the polymers were readily soluble in many organic solvents and could be solution-cast into tough and flexible polymer films. The glass-transition temperatures (T_gs) of these PAIs ranged from 196 to 298 °C and the 10% weight-loss temperatures were in excess of 445 °C in nitrogen. Cyclic voltammograms of the PAI films cast onto the indium–tin oxide (ITO)-coated glass substrate exhibit one reversible oxidation redox couples at 0.73–0.89 V vs. Ag/AgCl in an electrolyte/acetonitrile solution. The polymer films revealed good electrochemical and electrochromic stability, with coloration change from a pale yellow neutral form to a green oxidized form. After over 100 redox cycles, the polymer films still exhibited good redox and electrochromic reversibility. The 3 and 3′ series PAIs exhibited enhanced redox-stability and electrochromic performance as compared to the parent 3″ analogs without methoxy substituents on the TPA unit.     

Optical second harmonic generation measurements for investigating electron injection into a pentacene field effect transistor with Au source and drain electrodes

The pentacene field effect transistors (FETs)' operation for the injection carrier was revealed by means of the drain current-elapsed time (I_ds-t) and optical second harmonic generation (SHG) measurements. The charge carriers forming the conducting channel of pentacene FETs were mainly holes injected from the Au source electrode. Carrier injection from source and drain electrodes was followed by the carrier trapping, and the SHG signal modulated by the change in the electric field distribution between Au the source and drain electrodes was shown. In particular, at the off state of the FET, electron injection and succeeding trapping were suggested. Furthermore, hole injection assisted by trapped electrons was also suggested.