Roberto dos Reis

Synthesis and Characterisation of Fluorescent Carbon Nanodots Produced in Ionic Liquids by Laser Ablation

Carbon nanodots (C-dots) with an average size of 1.5 and 3.0 nm were produced by laser ablation in different imidazolium ionic liquids (ILs), namely, 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI.BF4 ), 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMI.NTf2 ) and 1-n-octyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (OMI.NTf2 ). The mean size of the nanoparticles is influenced by the imidazolium alkyl side chain but not by the nature of the anion. However, by varying the anion (BF4 vs. NTf2 ) it was possible to detect a significant modification of the fluorescence properties. The C-dots are much probably stabilised by an electrostatic layer of the IL and this interaction has played an important role with regard to the formation, stabilisation and photoluminescence properties of the nanodots. A tuneable broadband fluorescence emission from the colloidal suspension was observed under ultraviolet/visible excitation with fluorescence lifetimes fitted by a multi-exponential decay with average values around 7 ns.

Stabilization of ferroelectric phase in tungsten capped Hf0.8Zr0.2O2

We report on the stabilization of the ferroelectric phase in Hf0.8Zr0.2O2 with a tungsten capping layer. Ferroelectricity is obtained in both metal-insulator-metal (MIM) and metal-insulator-semiconductor (MIS) capacitors with highly-doped Si serving as the bottom electrode in the MIS structure. Ferroelectricity is confirmed from both the electrical polarization-voltage (P-V) measurement and X-Ray Diffraction analysis that shows the presence of an orthorhombic phase. High-resolution Transmission Electron Microscopy and Energy Dispersive X-ray spectroscopy show minimal diffusion of W into the underlying Hf0.8Zr0.2O2 after the crystallization anneal. This is in contrast to significant Ti and N diffusion observed in ferroelectric HfxZr1-xO2 commonly capped with TiN.

Crystalline Molybdenum Oxide Thin-Films for Application as Interfacial Layers in Optoelectronic Devices

The ability to control the interfacial properties in metal-oxide thin films through surface defect engineering is vital to fine-tune their optoelectronic properties and thus their integration in novel optoelectronic devices. This is exemplified in photovoltaic devices based on organic, inorganic or hybrid technologies, where precise control of the charge transport properties through the interfacial layer is highly important for improving device performance. In this work, we study the effects of in situ annealing in nearly stoichiometric MoOx (x ∼ 3.0) thin-films deposited by reactive sputtering. We report on a work function increase of almost 2 eV after inducing in situ crystallization of the films at 500 °C, resulting in the formation of a single crystalline α-MoO3 overlaid by substoichiometric and highly disordered nanoaggregates. The surface nanoaggregates possess various electronic properties, such as a work function ranging from 5.5 eV up to 6.2 eV, as determined from low-energy electron microscopy studies. The crystalline underlayer possesses a work function greater than 6.3 eV, up to 6.9 eV, characteristic of a very clean and nearly defect-free MoO3. By combining electronic spectroscopies together with structural characterizations, this work addresses a novel method for tuning, and correlating, the optoelectronic properties and microstructure of device-relevant MoOx layers.

Formation of nanoscale composites of compound semiconductors driven by charge transfer

Mixing of different materials is routinely used to create alloys or composites with new functionalities and properties tailored for specific applications. We have synthesized a uniform stoichiometric composite of CdO and SnTe, two compound semiconductors with distinctly different electrical properties and electronic band structure. The carrier concentration in the composite varies from about 1020 cm-3 electrons in CdO to about 1021 cm-3 holes in SnTe with a semi-insulating material in the intermediate composition range. The optical absorption edge shows a non-monotonic dependence on the composition. These unusual properties are explained by a nanocomposite whose formation is driven by charge transfer between the constituent compounds.

Determination of the structural phase and octahedral rotation angle in halide perovskites

A key to the unique combination of electronic and optical properties in halide perovskite materials lies in their rich structural complexity. However, their radiation sensitive nature limits nanoscale structural characterization requiring dose efficient microscopic techniques in order to determine their structures precisely. In this work, we determine the space-group and directly image the Br halide sites of CsPbBr3, a promising material for optoelectronic applications. Based on the symmetry of high-order Laue zone reflections of convergent-beam electron diffraction, we identify the tetragonal (I4/mcm) structural phase of CsPbBr3 at cryogenic temperature. Electron ptychography provides a highly sensitive phase contrast measurement of the halide positions under low electron-dose conditions, enabling imaging of the elongated Br sites originating from the out-of-phase octahedral rotation viewed along the [001] direction of I4/mcm persisting at room temperature. The measurement of these features and compariso...

Room-Temperature-Synthesized High-Mobility Transparent Amorphous CdO–Ga2O3 Alloys with Widely Tunable Electronic Bands

In this work, we have synthesized Cd1-xGaxO1+δ alloy thin films at room temperature over the entire composition range by radio frequency magnetron sputtering. We found that alloy films with high Ga contents of x > 0.3 are amorphous. Amorphous Cd1-xGaxO1+δ alloys in the composition range of 0.3 < x < 0.5 exhibit a high electron mobility of 10-20 cm2 V-1 s-1 with a resistivity in the range of 10-2 to high 10-4 Ω cm range. The resistivity of the amorphous alloys can also be controlled over 5 orders of magnitude from 7 × 10-4 to 77 Ω cm by controlling the oxygen stoichiometry. Over the entire composition range, these crystalline and amorphous alloys have a large tunable intrinsic band gap range of 2.2-4.8 eV as well as a conduction band minimum range of 5.8-4.5 eV below the vacuum level. Our results suggest that amorphous Cd1-xGaxO1+δ alloy films with 0.3 < x < 0.4 have favorable optoelectronic properties as transparent conductors on flexible and/or organic substrates, whereas the band edges and electrical conductivity of films with 0.3 < x < 0.7 can be manipulated for transparent thin-film transistors as well as electron transport layers.

Work function mapping of MoOx thin-films for application in electronic devices

The knowledge of the structural and electronic surface morphology is imperative to fully understand the charge transfer at interfaces of electronic devices, such as in photovoltaic (PV) cells. To this aim, here, we use low-energy electron microscopy to probe the unoccupied states of post-annealed MoOx thin-films grown in oxygen excess (x∼3.16) and deficient (x∼2.57) environments. 2D work function maps are correlated with the surface topography extracted by mirror electron microscopy (MEM) mode, which show homogenous surface morphology and electronic levels for the specimen with x∼2.57, while it demonstrates nanoaggregates with different work functions on top of flat surface areas for the sample grown with x∼3.16.

Unraveling the Crystal Structure of All-Inorganic Halide Perovskites using CBED and Electron Ptychography

The unique electronic properties observed in halide perovskites originate in their rich structural complexity that allows compatibility with a variety of structural motifs and compounds. Adjustments of the corner-connected BX6 octahedral network in the ABX3 structure promote a wide range of optical and electronic properties [1]. For this reason, methods to precisely identify the local symmetry are necessary to unambiguously distinguish different possible structural phases. Here, we determine the crystallography of all-inorganic CsPbBr3-xClx perovskite single crystals, grown via the BridgmanStockbarger method, by exploring information contained in electron diffraction patterns. Due to the material’s sensitivity to the electron beam, precise atomistic studies of halide perovskites by transmission electron microscopy (TEM), scanning TEM (STEM) and electron diffraction are relatively underdeveloped. Consequently, X-ray diffraction has been the technique of choice to assign crystal structures, requiring a careful study of the splitting of certain lines and of the presence of superlattice reflections. Effects due to small atomic shifts can easily be missed in X-ray diffraction, which accounts for reports of conflicting structures.

Quantitative Mapping of Strain, Polarization, and Octahedral Distortion at unit cell resolution by Scanning Electron Diffraction

Scanning diffraction methods experiments in transmission electron microscopy have undergone substantial growth in recent years, due in part to the availability of high speed pixelated detectors with reasonable dynamic range. This capability enables new experiments that combine the benefits of position averaged convergent beam electron (PACBED) [1] with accurate partitioning of diffraction data into precise unit cell bins. A recent example of this combined technique is the quantitative determination of composition at projected unit cell resolution in SrTiO3-La0.7Sr0.3MnO3 multilayers from purely elastic scattering (Figure 1) [2]. PACBED has also been shown to be sensitive to the direction of polarization [3] and degree of octahedral tilt [4]. However, the optimal experimental conditions and limits of precision for mapping these quantitates quantitatively at unit cell resolution have yet to be fully explored.

Computational Methods for Large Scale Scanning Transmission Electron Microscopy (STEM) Experiments and Simulations

1. National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, USA. 2. Department of Physics and Astronomy and California NanoSystems Institute, University of California, Los Angeles, USA. 3. Department of Materials Science and Engineering, University of California, Berkeley, USA. 4. Engineering Division, Lawrence Berkeley National Laboratory, Berkeley, USA