Ch. Kloc

High-mobility field-effect transistors based on transition metal dichalcogenides

We report on fabrication of field-effect transistors (FETs) based on transition metal dichalcogenides. The unique structure of single crystals of these layered inorganic semiconductors enables fabrication of FETs with intrinsically low field-effect threshold and high charge carrier mobility, comparable to that in the best single-crystal Si FETs (up to 500 cm2/V s for the p-type conductivity in the WSe2-based FETs at room temperature). These FETs demonstrate ambipolar operation. Owing to mechanical flexibility, they hold potential for applications in “flexible” electronics.

Physical vapor growth of centimeter-sized crystals of α-hexathiophene

Abstract The processes in physical vapor transport: vaporization, transport and crystal growth, the regimes for transport: molecular flow path limited, diffusion-limited, convection-limited and forced-convection-limited are analyzed and the results are used to guide a systematic investigation of physical vapor transport and crystal growth of α-hexathiophene (α6T), a promising thin-film transistor organic material. Successful growth occurred when the gas pressure was such that the regime was convective and when deliberate inert-gas flow (forced convection) improved volatilization. Plate-like growth morphology and thickness differences between the high-temperature and low-temperature polymorphs is explained on the basis of differing atomic structure. Conditions for the reproducible growth of crystals of up to 1 cm in size are reported. We feel that the analyses and procedures reported here can be used to grow crystals of other organic materials.

Efficient organic photovoltaic diodes based on doped pentacene.

Recent work on solar cells based on interpenetrating polymer networks and solid-state dye-sensitized devices shows that efficient solar-energy conversion is possible using organic materials. Further, it has been demonstrated that the performance of photovoltaic devices based on small molecules can be effectively enhanced by doping the organic material with electron-accepting molecules. But as inorganic solar cells show much higher efficiencies, well above 15 per cent, the practical utility of organic-based cells will require their fabrication by lower-cost techniques, ideally on flexible substrates. Here we demonstrate efficiency enhancement by molecular doping in Schottky-type photovoltaic diodes based on pentacene—an organic semiconductor that has received much attention as a promising material for organic thin-film transistors, but relatively little attention for use in photovoltaic devices. The incorporation of the dopant improves the internal quantum efficiency by more than five orders of magnitude and yields an external energy conversion efficiency as high as 2.4 per cent for a standard solar spectrum. Thin-film devices based on doped pentacene therefore appear promising for the production of efficient ‘plastic’ solar cells.

Superconductivity in molecular crystals induced by charge injection

Progress in the field of superconductivity is often linked to the discovery of new classes of materials, with the layered copper oxides being a particularly impressive example. The superconductors known today include a wide spectrum of materials, ranging in complexity from simple elemental metals, to alloys and binary compounds of metals, to multi-component compounds of metals and chalcogens or metalloids, doped fullerenes and organic charge-transfer salts. Here we present a new class of superconductors: insulating organic molecular crystals that are made metallic through charge injection. The first examples are pentacene, tetracene and anthracene, the last having the highest transition temperature, at 4 K. We anticipate that many other organic molecular crystals can also be made superconducting by this method, which will lead to surprising findings in the vast composition space of molecular crystals.

Gate-induced superconductivity in a solution-processed organic polymer film

The electrical and optical properties of conjugated polymers have received considerable attention in the context of potentially low-cost replacements for conventional metals and inorganic semiconductors. Charge transport in these organic materials has been characterized in both the doped-metallic and the semiconducting state, but superconductivity has not hitherto been observed in these polymers. Here we report a distinct metal–insulator transition and metallic levels of conductivity in a polymer field-effect transistor. The active material is solution-cast regioregular poly(3-hexylthiophene), which forms relatively well ordered films owing to self-organization, and which yields a high charge carrier mobility (0.05–0.1 cm2 V-1 s-1) at room temperature. At temperatures below ∼2.35 K with sheet carrier densities exceeding 2.5 × 1014 cm-2, the polythiophene film becomes superconducting. The appearance of superconductivity seems to be closely related to the self-assembly properties of the polymer, as the introduction of additional disorder is found to suppress superconductivity. Our findings therefore demonstrate the feasibility of tuning the electrical properties of conjugated polymers over the largest range possible—from insulating to superconducting.

Superconductivity at 52 K in hole-doped C 60

Superconductivity in electron-doped C60 was first observed almost ten years ago. The metallic state and superconductivity result from the transfer of electrons from alkaline or alkaline-earth ions to the C60 molecule, which is known to be a strong electron acceptor. For this reason, it is very difficult to remove electrons from C60—yet one might expect to see superconductivity at higher temperatures in hole-doped than in electron-doped C60, because of the higher density of electronic states in the valence band than in the conduction band. We have used the technique of gate-induced doping in a field-effect transistor configuration to introduce significant densities of holes into C60. We observe superconductivity over an extended range of hole density, with a smoothly varying transition temperature Tc that peaks at 52 K. By comparison with the well established dependence of Tc on the lattice parameter in electron-doped C60, we anticipate that Tc values significantly in excess of 100 K should be achievable in a suitably expanded, hole-doped C60 lattice.

Preparation and properties of FeSi, α-FeSi2 and β-FeSi2 single crystals

Abstract FeSi and α-FeSi2 single crystals were prepared by the Czochralski technique. β-FeSi2 and α-FeSi2 single crystals were prepared by chemical vapour transport. FeSi single crystals 18 cm3 in size and α-FeSi2 single crystals of a few cubic centimetres were obtained. β-FeSi2 decomposes peritectically above 970 °C, therefore only needle-like crystals, each about 10 mm long, were grown from the vapour. The temperature dependence of the resistivity was measured from 4.2 K up to room temperature. α-FeSi2 shows a weak temperature dependence of resistivity. The activation energy of FeSi, calculated from ln ρ = f( 1 T ) is equal to 58.2 meV. the Hall coefficient and resistivity of β-FeSi2 single crystals was measured between 30 and 300 K. The as-grown, not intentionally doped β-FeSi2 crystals exhibit n-type conductivity, while Cr and Al doped crystals show p-type conductivity. The Hall coefficient of n-type samples depends on the magnetic field; therefore, the transport properties of β-FeSi2 were explained taking into account two types of carrier, heavy and light. At low temperature (32 K) heavy electrons show a mobility of μn = 48 cm2 V−1 s−1. An impurity band and an additional deep acceptor level are observed in p-type crystals. The activation energies of the shallow acceptor level and deep acceptor level are equal to 55 meV and 100 meV respectively. The mobility of holes μp in p-type crystals reaches a maximum of 1200 cm2 V−1 s−1 at 67 K. Magnetization and magnetic susceptibility measurements in the temperature range 4–320 K show a small positive value of susceptibility and non-linear magnetization for β-FeSi2 but no ferromagnetic phase was detected. α-FeSi2 crystals show linear magnetization and susceptibility similar to β-FeSi2. Magnetization of FeSi is linear but the susceptibility passes through a minimum at about 150–200 K.

The Hall effect in β‐FeSi2 single crystals

The results of the transport and magnetization measurements on β‐FeSi2 single crystals are presented. The magnetic field dependence of the Hall coefficient in n‐type β‐FeSi2 was observed in the temperature range of 30–300 K and explained in the limits of a two‐band model. The magnetization measurements were performed within the range 4.2–300 K. It was shown that the contribution of the anomalous Hall effect to the total Hall voltage is negligible. Parameters of charge carriers, taking part in conductivity were calculated and the separation between the bands was estimated.

Perylene: A promising organic field-effect transistor material

Field-effect transistors based on single crystalline perylene have been prepared and analyzed in the temperature range from 50 to 300 K. Room temperature electron mobilities as high as 5.5 cm2/V s have been achieved. In addition, ambipolar device operation, i.e., n- and p-channel activity, is observed. The temperature dependence of the electron and hole mobilities is discussed in the limits of hopping and band-like transport mechanisms.

A New Class of Materials with Promising Thermoelectric Properties: MNiSn (M = Ti, Zr, Hf)

TiNiSn, ZrNiSn and HfNiSn are members of a large group of intermetallic compounds which crystallize in the cubic MgAgAs-type structure. Polycrystalline samples of these compounds have been prepared and investigated for their thermoelectric properties. With thermopowers of about –200 μV/K and resistivities of a few mΩcm, power factors S 2 /ρ as high as 38 μW/K 2 cm were obtained at 700 K. These remarkably high power factors are, however, accompanied by a thermal conductivity which is too high for applications. In order to reduce the parasitic lattice thermal conductivity, solid solutions Zr l−x Hf x NiSn, Zr l−x Ti x NiSn, and Hf l−x Ti x NiSn were formed. The figure of merit of Zr 0.5 Hf 0.5 NiSn at 700 K ( ZT = 0.41) exceeds the end members ZrNiSn ( ZT = 0.26) and HfNiSn ( ZT = 0.22).