Langmuir-blodgett films of stilbazole complexes of iridium(I) and rhodium(I)

Complexes of 4-alkoxystilbazoles with iridium and rhodium form stable Langmuir layers at the air-water interface even when the alkoxy chain is relatively short (C₅–C₁₂). The surface pressure-area isotherms indicate that condensed molecular monolayers are obtained. The area per molecule of each compound in its monolayer form is typically 0.60 nm², which agrees well with the cross-sectional area of the [Ir(CO)₂CI] or [Rh(CO)₂CI] head group predicated using molecular models. This suggests that the molecules are oriented with the metal moiety close to the water surface and their alkoxystilbazole ‘rod’ protruding from the plane of the water surface. Such floating monolayers have been transferred on to solid substrates such as glass, aluminium (AI₂O₃/AI/Glass) and silicon (SiO₂/Si) at relatively high speed (10 mm min ^?1) to form Y-type LB assemblies. The UV–Visible absorption properties of these materials in solution and LB film form have been studied. LB films of these complexes yield bathochromically shifted spectra relative to the LB film spectrum of the uncomplexed stilbazole. Additionally, these spectra are often broader and hypsochromically shifted relative to their corresponding solution spectra as a result of the close molecular packing within the LB film and the associated dipole–dipole interactions. The electrically polar nature of the molecules described in this paper suggest that they may be suitable candidates for new pyroelectric materials. Thus the pyroelectric coefficient (the rate of change of electric polarisation with respect to temperature) has been measured for a polar multilayer LB film containing an iridium complex. A pyroelectric coefficient of 3.5 μCm^?2K^?1 (at 30 °C) has been measured, which is one of the highest reported valued for an LB film. Additionally, a low dielectric loss of around 0.01 has been found over the frequency range 50 Hz–1 kHz, indicating that such LB films may be usfeul materials for pyroelectric sensors.     

Electrical characteristics and structural properties of ohmic contacts to p-type 4H-SiC epitaxial layers

Epitaxial films grown by low-temperature liquid phase epitaxy on p-type 4H-SiC were used as strongly doped subcontact layers for making low-resistance contacts to the p-type material. The layers had a bulk resistivity of ∼0.02 Ω · cm and an aluminum atom concentration of ∼1.5×10²⁰ cm⁻³. The absence of polytype inclusions and the distinct crystalline quality of the strongly doped subcontact layers was confirmed by x-ray diffraction methods. Ohmic contacts with resistivities less than 10⁻⁴ Ω · cm² were prepared by depositing and then annealing multilayer metal mixtures containing Al and Ti. The structural properties and energy characteristics of the resulting ohmic contacts are discussed. Fiz. Tekh. Poluprovodn. 33, 1334–1339 (November 1999)     

Langmuir–Blodgett films of indandione-1,3 pyridinium betaine. I: Preparation,electronic structure,optical properties

The synthesis of a new polar amphiphilic derivative of indandione-1,3 pyridinium betaine (IPB) with different lengths of the aliphatic tail, containing 11 (C11IPB) and 17 (C17IPB) carbon atoms respectively, is described. The electronic structure (charge distribution, energy spectra, dipole moments in ground and excited states) of the IPB molecule has been calculated in the framework of the MO CNDO methods. Studies of compression π-A isotherms of these compounds show that C17IPB forms two stable phases of monolayer on the water surface, one in the 10–30 mN m^?1 and the other in the 35–50 nM m^?1 surface pressure region. The corresponding areas A₀ per molecule are 37 and 33 Ų respectively. Techniques for the preparation of Langmuir-Blodgett (LB) multilayers of C17IPB in the high-pressure region (π = 40 mN m^?1) on glass, quartz or sapphire substrates are described. C17IPB forms LB multilayer films of Z-type configuration with oriented dipole moments. Ellipsometric data, luminescence spectra and results of preliminary studies of second-harmonic generation in these LB films are presented.     

Langmuir-Blodgett deposition and second-order non-linear optics of several azobenzene dye polymers

The Langmuir-Blodgett (LB) film-forming properties of a number of novel azobenzene dye polymers are reported. The linear and second-order non-linear optical properties of the films have been studied using ellipsometry, visible spectroscopy and Pockels effect measurements. As monolayers, the effective electro-optic r-coefficients ranged from 1 to 8.7 pm V.⁻¹ When one of the polymers was combined in an alternate layer film (230 layers in total), an r₁₃-coefficient of 13 pm V⁻¹ was achieved. This is one of the highest values reported for a ‘thick’ LB film.     

Variable-Field Hall Measurement and Transport in LW Single-Layer n-Type MBE Hg1−x Cd x Te

Molecular beam epitaxy n-type long-wavelength infrared (LWIR) Hg_1?x Cd _x Te (MCT) has been investigated using variable-field Hall measurement in the temperature range from 50 K to 293 K. A quantitative mobility spectrum analysis technique has been used to determine the role of multicarrier transport properties with respect to epilayer growth on lattice-matched cadmium zinc telluride, as well as lattice-mismatched silicon (Si) and gallium arsenide (GaAs) buffered substrates. Overall, after postgrowth annealing, all layers were found to possess three distinct electron species, which were postulated to originate from the bulk, transitional (or higher-x-value) regions, and an interfacial/surface layer carrier. Further, the mobility and concentration with respect to temperature were analyzed for all carriers, showing the expected mobility temperature dependence and intrinsic behavior of the bulk electron. Electrons from transitional regions were seen to match expected values based on the carrier concentration of the resolved peak. At high temperature, the lowest-mobility carrier was consistent with the properties of a surface carrier, while below 125 K it was postulated that interfacial-region electrons may influence peak values. After corrections for x-value and doping density at 77 K, bulk electron mobility in excess of 10⁵ cm² V^?1 s^?1 was observed in all epilayers, in line with expected values for lightly doped n-type LWIR material. Results indicate that fundamental conduction properties of electrons in MCT layers are unchanged by choice of substrate.     

Preparation and Thermoelectric Properties of p-Type Yb-Filled Skutterudites

p-Type Yb _z Fe_4?x Co _x Sb₁₂ skutterudites were prepared by encapsulated melting and hot pressing, and the filling and doping (charge compensation) effects on the transport and thermoelectric properties were examined. The electrical conductivity of all specimens decreased slightly with increasing temperature, indicating that they were in a degenerate state due to high carrier concentrations of 10²⁰ cm^?3 to 10²¹ cm^?3. The Hall and Seebeck coefficients exhibited positive signs, indicating that the majority carriers are holes (p-type). The Seebeck coefficient increased with increasing temperature to maximum values of 100 μV/K to 150 μV/K at 823 K. The electrical and thermal conductivities were reduced by substitution of Co for Fe, which was responsible for the decreased carrier concentration. Overall, the Yb-filled Fe-rich skutterudites showed better thermoelectric performance than the Yb-filled Co-rich skutterudites.     

The interface between Hg1−xCdxTe and its native oxide

The results of a study of the electrical properties of the interface between Hg_1?xCd_xTe with x = 0.21 and its native oxide at 77°K are presented. The native oxide is formed by anodic oxidation and results in an interface with reproducible properties. The surface charge, the surface mobility and the effective lifetime are obtained from galvanomagnetic measurements and are related to the semiconductor bulk parameters, the oxide thickness and the annealing conditions. The surface state charge and the metal-semiconductors work function difference are obtained from the shift of the flat band voltage of metal-oxide-semiconductor (MOS) capacitor characteristics. The interface between Hg_1?xCd_xTe and its native anodic oxide is characterized by a density of fast surface states of the order of 5 × 10¹¹cm^?2 (eV^?1) near the middle of the bandgap. The density of states increases towards the band edges to the order of 10¹³cm^?2 (eV^?1). The measured flat band voltage is approximately ?0.5 V for an oxide thickness of 500 Å and for an n-type semiconductor with an electron carrier concentration in the range 1–3 × 10¹⁵cm^?3 at 77°K. The fixed oxide surface state charge is positive for both p-type and n-type semiconductors and is of the order of 6 × 10¹¹ charges per cm^?2. The surface properties, the significance and the reproducibility of the results are evaluated.     

Photoswitch based on bacteriorhodopsin Langmuir–Blodgett films

Langmuir–Blodgett (LB) films of bacteriorhodopsin (bR) without addition of lipids were deposited on indium tin oxide (ITO) conductive electrodes. A sandwich photocell with a juction structure of ITO/bR/electrolyte/ITO has been constructed in which the bR LB film was put into contact directly with an aqueous electrolyte immobilized in an agar gel. Under visible light irradiation a transient photocurrent due to a change in light intensity can be observedl showing the property of vision-imitative material. It can be used as a multiple optical switch, since it gives positive and negative transient photocurrnts during application and removal of light irradiation respectively and has a photoresponse repeatedly in the same direction when the light intensity increases or decreases stepwise. A photoalarm consisting of the bR photocell and an amplifier was set up by utilizing the switching function of the bR LB film.     

Electrical Properties of Halogen-Doped CdTe Layers on Si Substrates Grown by Metalorganic Vapor-Phase Epitaxy

Electrical properties of halogen-doped CdTe layers grown on Si substrates using iodine and chlorine dopants are presented. No change in electrical properties of the layers was observed with chlorine as a dopant. However, doping with iodine resulted in highly conductive n-type layers or highly resistive p-type layers depending upon the growth conditions, even though a similar amount of dopant was introduced into the growth chamber. Layers grown at 560°C, with a vapor-phase Te/Cd precursor ratio of 3.0, were p-type. The resistivity of the layers remained unchanged for low dopant supply rates, but increased abruptly when the dopant supply rate was increased beyond a certain value. On the other hand, layers grown at 325°C with Te/Cd ratios from 0.1 to 0.25 were n-type. A maximum free electron concentration of 1.3 × 10¹⁷ cm⁻³ was obtained at room temperature. The types and conductivities of the grown layers were strongly dependent on the growth conditions.     

High‐Surface‐Area Nanoporous Boron Carbon Nitrides for Hydrogen Storage

Nano‐ and mesoporous boron carbon nitrides with very high surface areas up to 1560 m² g^?1 are obtained by pyrolysis of a graphitic carbon nitride mpg‐C₃N₄ infiltrated with a borane complex. This reactive hard‐templating approach provides easy composition and texture tuning by temperature adjustment between 800 and 1400 °C. The process yields B_xC_yN_zO_vH_w materials as direct copies of the initial template with controlled compositions of 0.15 ≤ x ≤ 0.36, 0.10 ≤ y ≤ 0.12, 0.14 ≤ z ≤ 0.32, and 0.11 ≤ v ≤ 0.28. The nano and mesoporosities can also be tuned in order to provide hierarchical materials with specific surface areas ranging from 610 to 1560 m² g^?1. Such high values, coupled with resistance against air oxidation up to 700 °C, suggest potential materials for gas storage and as catalyst supports. Indeed, it is demonstrated that these compounds exhibit high and tunable H₂ uptakes from 0.55 to 1.07 wt.% at 77 K and 1 bar, thus guiding further search of materials for hydrogen storage.     

Band Structure and Photoelectric Characterization of GeSe Monolayers

Germanium selenide monolayer is promising in photoelectric applications for its natural p‐type semiconductor and complicated band structures. Basic experimental investigations of few‐to‐monolayer germanium selenide are still absent; major scientific challenge is to develop of techniques for controllably thinned monolayers. In this study laser thinned monolayer germanium selenide on SiO₂/Si substrates is demonstrated. A broad photoluminescence spectrum with eight continues peaks is observed from visible to infrared wavebands centered at ≈589, 655, 737, 830, 1034, 1178, 1314, and 1456 nm, respectively. First‐principle calculations based on density functional theory illuminate the band structures of few‐to‐monolayer germanium selenide. Photoluminescence investigation combined with first‐principle calculations indicates that the indirect to direct bandgap transition happens at few layers of N = 3. Current–voltage and photoresponse characteristics of monolayer based devices show 3.3 times the photosensitivity and much faster falling edges compared with those of the pristine nanosheet based devices. The present results provide useful insight into deep understanding of thickness dependent performances of germanium selenide monocrystalline.     

Iodine Doping of CdTe and CdMgTe for Photovoltaic Applications

Iodine-doped CdTe and Cd_1?x Mg _x Te layers were grown by molecular beam epitaxy. Secondary ion mass spectrometry characterization was used to measure dopant concentration, while Hall measurement was used for determining carrier concentration. Photoluminescence intensity and time-resolved photoluminescence techniques were used for optical characterization. Maximum n-type carrier concentrations of 7.4 × 10¹⁸ cm^?3 for CdTe and 3 × 10¹⁷ cm^?3 for Cd_0.65Mg_0.35Te were achieved. Studies suggest that electrically active doping with iodine is limited with dopant concentration much above these values. Dopant activation of about 80% was observed in most of the CdTe samples. The estimated activation energy is about 6 meV for CdTe and the value for Cd_0.65Mg_0.35Te is about 58 meV. Iodine-doped samples exhibit long lifetimes with no evidence of photoluminescence degradation with doping as high as 2 × 10¹⁸ cm^?3, while indium shows substantial non-radiative recombination at carrier concentrations above 5 × 10¹⁶ cm^?3. Iodine was shown to be thermally stable in CdTe at temperatures up to 600°C. Results suggest iodine may be a preferred n-type dopant compared to indium in achieving heavily doped n-type CdTe.     

High Hole Mobility and Thickness‐Dependent Crystal Structure in α,ω‐Dihexylsexithiophene Single‐Monolayer Field‐Effect Transistors

Monolayer‐thickness two‐dimensional layers of α,ω‐dihexylsexithiophene (α,ω‐DH6T) exhibit field‐effect hole mobility of up to 0.032 cm² V^?1 s^?1, higher than previously reported for monolayers of other small‐molecule organic semiconductors. In situ measurements during deposition show that the source‐drain current saturates rapidly after the percolation of monolayer‐high islands, indicating that the electrical properties of α,ω‐DH6T transistors are largely determined by the first molecular monolayer. The α,ω‐DH6T monolayer consists of crystalline islands in which the long axes of molecules are oriented approximately perpendicular to the plane of the substrate surface. In‐plane lattice constants measured using synchrotron grazing‐incidence diffraction are larger in monolayer‐thickness films than the in‐plane lattice constants of several‐monolayer films and of previously reported thick‐film structures. Near‐edge X‐ray absorption fine structure spectroscopy (NEXAFS) reveals that the larger in‐plane lattice constant of single‐monolayer films arises from a larger tilt of the molecular axis away from the surface normal. NEXAFS spectra at the C 1s and S 2p edges are consistent with a high degree of molecular alignment and with the local symmetry imposed by the thiophene ring. The high mobility of holes in α,ω‐DH6T monolayers can be attributed to the reduction of hole scattering associated with the isolation of the thiophene core from the interface by terminal hexyl chains.     

Solution‐Processed Monolayer Organic Crystals for High‐Performance Field‐Effect Transistors and Ultrasensitive Gas Sensors

This work innovatively develops a dual solution‐shearing method utilizing the semiconductor concentration region close to the solubility limit, which successfully generates large‐area and high‐performance semiconductor monolayer crystals on the millimeter scale. The monolayer crystals with poly(methyl methacrylate) encapsulation show the highest mobility of 10.4 cm² V^?1 s^?1 among the mobility values in the reported solution‐processed semiconductor monolayers. With similar mobility to multilayer crystals, light is shed on the charge accumulation mechanism in organic field‐effect transistors (OFETs), where the first layer on interface bears the most carrier transport task, and the other above layers work as carrier suppliers and encapsulations to the first layer. The monolayer crystals show a very low dependency on channel directions with a small anisotropic ratio of 1.3. The positive mobility–temperature correlation reveals a thermally activated carrier transport mode in the monolayer crystals, which is different from the band‐like transport mode in multilayer crystals. Furthermore, because of the direct exposure of highly conductive channels, the monolayer crystal based OFETs can sense ammonia concentrations as low as 10 ppb. The decent sensitivity indicates the monolayer crystals are potential candidates for sensor applications.     

Annealing-induced evolution of the structural and morphological properties of a multilayer nanoperiodic SiO x /ZrO2 system containing Si nanoclusters

The structural and morphological properties of nanoperiodic structures produced by the alternate vacuum evaporation of SiO and ZrO₂ followed by annealing at temperatures of 500–1100°C are studied by the transmission electron microscopy of a transverse cross section. Upon annealing at temperatures below 700°C, the layers are amorphous. Upon annealing at 900°C and 1000°C, nanocrystals separated by twinned boundaries or amorphous regions are formed in the ZrO₂ layers. The formation of Si nanocrystals in the SiO _x layers occurs upon annealing at 1000°C and 1100°C. At 1100°C, because of the reaction between SiO _x and ZrO₂, spherical Si _x Zr _y O _z -type nanocrystals are formed in place of the ZrO₂ layers; the nanocrystal diameters exceed the initial layer thickness. The annealing-induced structural evolution is consistent with the previously considered behavior of the optical and luminescence properties of the system.     

A harmonic oscillator with low harmonic distortion and stable amplitude

A generator of the harmonic signal is described for frequencies in the acoustic region with the reference stabilized amplitude, linear frequency scale and total harmonic distortion below 2 × 10^?3. The signal is produced as the solution of the corresponding differential equation where the constant amplitude is maintained by periodic control of the first derivative value [xdot](t) at the moments when x(t) crosses zero and has a positive slope.     

Thermoelectric Properties of Polyaniline Films with Different Doping Concentrations of (±)-10-Camphorsulfonic Acid

The temperature dependence of the thermoelectric properties was investigated for polyaniline (PANI) films doped with different concentrations of (±)-10-camphorsulfonic acid (CSA) with molar ratio x of CSA to two phenyl-nitrogen units of x = 1 to 0.2. All PANI-CSA films exhibit p-type conduction. The temperature dependence of the electrical conductivity of the films with low CSA concentrations is consistent with a transport mechanism of variable-range hopping. On the other hand, the Seebeck coefficient above room temperature shows a linear increase with temperature, attributed to the metallic nature of PANI-CSA. As the CSA concentration decreases, the absolute value of the Seebeck coefficient increases while the electrical conductivity extremely decreases, probably due to the changes not only in the carrier concentration but also in the degree of structural disorder. The power factor increases monotonically with increasing CSA concentration toward x = 1 (the maximum limit). The thermal conductivity value of CSA-PANI film with x = 1 is as low as about 0.20 W m^?1 K^?1 in the through-plane direction and about 0.67 W m^?1 K^?1 in the in-plane direction. The thermoelectric figure of merit ZT in the in-plane direction is estimated to be approximately 1 × 10^?3 for x = 1.     

Kinetic Studies of the Interfacial Reaction of the Ba2YCu3O6+x Superconductor with a CeO2 Buffer

Interfacial reactions between the Ba₂YCu₃O_6+x superconductor and the CeO₂ buffer layers employed in coated conductors have been modeled experimentally by investigating the kinetics of the reaction between Ba₂YCu₃O_6+x films and CeO₂ substrates. At 810°C, the Ba₂YCu₃O_6+x -CeO₂ join within the BaO-Y₂O₃-CeO₂-CuO _x quaternary system is nonbinary, thereby establishing the phase diagram topology that governs the Ba₂YCu₃O_6+x /CeO₂ reaction. At a mole ratio of Ba₂YCu₃O_6+x :CeO₂ of 40:60, a phase boundary was found to separate two four-phase regions. On the Ba₂YCu₃O_6+x -rich side of the join, the four-phase region consists of Ba₂YCu₃O_{6 +x} , Ba(Ce_1−z Y _z )O_3−x , BaY₂CuO₅, and CuO _x ; on the CeO₂ rich side, the four phases were determined to be Ba(Ce_1−z Y _z ) O_3−x , BaY₂CuO₅, CuO _x and CeO₂. The Ba₂YCu₃O_6+x /CeO₂ reaction is limited by solid-state diffusion, and the reaction kinetics obey the parabolic rule, x = Kt ^1/2, where x = thickness of the reaction layer, t = time, and K = a constant related to the rate constant; K was determined to be 1.6 × 10⁻³ μm/s^1/2 at 790°C and 4.7 × 10⁻³ μm/s^1/2 at 830°C. The activation energy for the reaction was determined to be E _act = 2.67 × 10⁵ J/mol using the Arrhenius equation.     

Highly Efficient Charge Transport in a Quasi‐Monolayer Semiconductor on Pure Polymer Dielectric

The study of monolayer organic field‐effect transistors (MOFETs) provides an effective way to investigate the intrinsic charge transport of semiconductors. To date, the research based on organic monolayers on polymeric dielectrics lays far behind that on inorganic dielectrics and the realization of a bulk‐like carrier mobility on pure polymer dielectrics is still a formidable challenge for MOFETs. Herein, a quasi‐monolayer coverage of pentacene film with orthorhombic phase is grown on the poly (amic acid) (PAA) dielectric layer. More significantly, charge density redistribution occurs at the interface between the pentacene and PAA caused by electron transfer from pentacene to the PAA dielectric layer, which is verified by theoretical simulations and experiments. As a consequence, an enhanced hole accumulation layer is formed and pentacene‐based MOFETs on pure polymer dielectrics exhibit bulk‐like carrier mobilities of up to 13.7 cm² V^?1 s^?1 from the saturation region at low V_GS, 9.1 cm² V^?1 s^?1 at high V_GS and 7.6 cm² V^?1 s^?1 from the linear region, which presents one of the best results of previously reported MOFETs so far and indicates that the monolayer semiconductor growing on pure polymer dielectric could produce highly efficient charge transport.     

Properties of Mn-doped p-type InxGa1-xAsyP1-y grown by liquid-phase epitaxy

Epitaxial layers of p-type In_xGa_1-xAs_yP_1-y doped with Mn were grown by liquid-phase epitaxy on (111)-B oriented InP substrates at a growth temperature of 635°C. The doping characteristics and electrical and luminescent properties were studied and compared with those in Zn-doped epilayers. The distribution coefficient of Mn was about 0.1–0.3. The p-type epilayers with hole concentration of up to 3 × 10¹⁸ cm^?3 could be easily obtained by Mn doping. The activation energy of the Mn acceptor was about 40 meV. Mn doping yielded a broad photoluminescent emission spectrum which probably arises from d-shell interaction of the Mn as well as strong phonon coupling. From electron beam-induced current measurements in p-n heterojunctions utilizing Mn, a value of L_n = 2 μm was obtained for the minority carrier diffusion length of electrons in the p-type region.