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Acceleration of electrons in vacuum directly by intense laser fields holds great promise for the generation of high-charge, ultrashort, relativistic electron bunches. While the energy gain is expected to be higher with tighter focusing, this does not account for the reduced acceleration range, which is limited by diffraction. Here, we present the results of an experimental investigation that exposed nanotips to relativistic few-cycle laser pulses. We demonstrate the vacuum laser acceleration of electron beams with 100s pC charge and 15 MeV energy. Two different focusing geometries, with normalized vector potential a0 of 9.8 and 3.8, produced comparable overall charge and electron spectra, despite a factor of almost ten difference in peak intensity. Our results are in good agreement with 3D particle-in-cell simulations, which indicate the importance of dephasing.

In this work, the morphology, surface composition, and electronic properties of porous GaN films containing hexagonal V-shaped pits were studied. The V-pits are orientated along the [0001] direction of GaN, and we observed a clear relation between the growth time with the surface composition, film thickness, and pit morphology, which in turn had a significant impact on the band gap, valence band maximum, and the work function. The effect on the position of the valence band maximum and work function is explained by the formation of superficial oxygen-rich phases such as Ga2O3 and nonstoichiometric GaNxOy as supported by X-ray photoelectron spectroscopy and density functional theory (DFT). We further show a change in the optical band gap with the thickness of the porous films explained by a change in the tensile strain caused by open-core screw dislocations that gives rise to the formation of V-pits. The correlation between strain and the band gap is supported by DFT calculations. Our study provides insights into the intricate relation between surface states and electronic properties of semiconducting materials and offers directions for designing GaN heterojunctions with specific optical and electronic properties.

Experimental synthesis of new sp3 hybridized carbon/boron nitride structures remains challenging despite that numerous sp3 structures have been proposed in theory. Here, we showed that compressed multi-walled boron nitride nanotubes (MWBNNTs) and boron nitride peapods (C60@BNNTs) with small diameters could transform into a new sp3 hybridized boron nitride allotrope (Z-BN). This strategy is considered from the topological transition point of view in boron nitride nanotubes upon compression. Due to the increased curvature in compressed small-diameter MWBNNTs, the uncommon 4- and 8-membered rings in Z-BN could be more favorably formed. And the irreversible tube collapse is proved to be a critical factor for the capture of the formed Z-BN, because of the competition between the resilience of tube before collapse and the stress limitation for the lattice stabilization of Z-BN upon decompression. In this case, Z-BN starts to form above 19.0 GPa, which is fully reversible below 45 GPa and finally becomes quenchable at 93.5 GPa. This collapse-induced capture of the high-pressure phase could also be extended to other tubular materials for quenching novel sp3 structures.

One-dimensional tellurium nanostructures can exhibit distinct electronic properties from those seen in bulk Te. The electronic properties of nanostructured Te are highly dependent on their morphology, and thus controlled synthesis processes are required. Here, highly crystalline tellurium nanowires were produced via physical vapour deposition. We used growth temperature, heating rate, flow of the carrier gas, and growth time to control the degree of supersaturation in the region where Te nanostructures are grown. The latter leads to a control in the nucleation and morphology of Te nanostructures. We observed that Te nanowires grow via the vapour–solid mechanism where a Te particle acts as a seed. Transmission electron microscopy (TEM) and electron diffraction studies revealed that Te nanowires have a trigonal crystal structure and grow along the (0001) direction. Their diameter can be tuned from 26 to 200 nm with lengths from 8.5 to 22 μm, where the highest aspect ratio of 327 was obtained for wires measuring 26 nm in diameter and 8.5 μm in length. We investigated the use of bismuth as an additive to reduce the formation of tellurium oxides, and we discuss the effect of other growth parameters.

A detailed study of the oxidation of Cu substrates was carried out under controlled conditions by regulating the pressure, atmosphere composition, process time, and temperature. By tuning the synthesis conditions, the formation of cuprous oxide (Cu2O) or cupric oxide (CuO) could be preferentially promoted. The oxidation temperature was varied from 400 to 1050 °C, and a gradual oxidation of metallic Cu to Cu2O was achieved at mild oxidation conditions (400-600 °C), while the formation of CuO was only observed at higher temperatures (≥900 °C). The surface morphology was also affected changing from a highly granular texture (400 °C) with grain sizes between 0.59 ± 0.15 μm to smooth large crystallites (≥900 °C) with a size within 2.76 ± 0.97 μm. We also show that by controlling the oxidation temperature (400-1050 °C), it is possible to tune the work function and the ionization potential of the resulting Cu2O/CuO film, properties that are important for various optoelectronic applications.

We show how structurally ordered L1₀ face-centered tetragonal (fct) FePt nanoparticles are produced by a solid-state microwave-assisted synthesis method. The structural phase as well as the incorporated Fe into the nanoparticles is confirmed by X-ray diffraction and high resolution high-angle annular dark field scanning transmission electron microscopy experiments. The prepared particles exhibit a remarkable resilience toward crystallite growth at high temperatures. Directly correlated to the L1₀ phase, the best oxygen reduction reaction (ORR) characteristics are achieved for particles with a 1:1 Fe:Pt atomic ratio and an average size of ~2.9 nm where Pt-specific evaluation provided a high mass and specific activity of ~570 A/g_Pt and ~600 μA/cm²_Pt respectively. Our results demonstrate that well-structured catalysts possessing activities vastly exceeding Pt/C (~210 A/g_Pt & ~250 μA/cm²_Pt), can be synthesized through a fast and highly eco-friendly method. We note that the achieved mass activity represent a significant leap toward the theoretical maximum for fully ordered FePt nanoparticles.

The capability of graphene to adopt a property from an adjacent material is investigated by measuring the electrochemical performance of a monolayer graphene placed on top of thin cobalt oxide (Co3O4) nanosheets. In this assembly, monolayer graphene works as an interfacial layer which inhibits the direct contact of the actual electroactive material and electrolyte during electrochemical reaction. The results show that while graphene is electrochemically inert, it behaves as an active material to catalyze oxygen evolution reaction (OER) once placed on top of Co3O4 nanosheets. The graphene-covered Co3O4 model system shows electrochemical performance similar to Co3O4 indicating complete transference of the electrochemical property of the metal oxide to the graphene. Based on density functional theory (DFT) calculations, charge transfer from graphene to Co3O4 is the key factor for turning the electrochemically inactive graphene to an apparent active material. 

We present a theoretical study on the formation of graphene nanoribbons-via polymerization of coronene molecules-inside the inner cavity of boron nitride nanotubes. We examine the electronic property of the hybrid system, and we show that the boron nitride nanotube does not significantly alter the electronic properties of the encapsulated graphene nanoribbon. Motivated by previous experimental works, we examine graphene nanoribbons with two different widths and investigate probable scenarios for defect formation and/or twisting of the resulting graphene nanoribbons and their effect on the electronic properties of the hybrid system.

We report on the self-assembly of semicrystalline [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) nanosheets at the interface between a hydrophobic solvent and water, and utilize this opportunity for the realization of electronically active organic/organic molecular heterostructures. The self-assembled PCBM nanosheets can feature a lateral size of >1 cm2 and be transferred from the water surface to both hydrophobic and hydrophilic surfaces using facile transfer techniques. We employ a transferred single PCBM nanosheet as the active material in a field-effect transistor (FET) and verify semiconductor function by a measured electron mobility of 1.2 × 10–2 cm2 V–1 s–1 and an on–off ratio of ∼1 × 104. We further fabricate a planar organic/organic heterostructure with the p-type organic semiconductor poly(3-hexylthiophene-2,5-diyl) as the bottom layer and the n-type PCBM nanosheet as the top layer and demonstrate ambipolar FET operation with an electron mobility of 8.7 × 10–4 cm2 V–1 s–1 and a hole mobility of 3.1 × 10–4 cm2V–1 s–1.

We demonstrate a method for the selective encapsulation of carbonnanotubes in thin parylene films using iron as a sacrificial lift-off layer. Theiron serves as an inhibitor of parylene deposition and prevents the parylenemolecules from linking, thus facilitating selective area coating after lift-off.