Tunnelling through diamond-like carbon nanofilms deposited by electron-beam-induced deposition

The tunnelling properties in metal/diamond-like carbon (DLC)/semiconductor junctions and structural characteristics of thin DLC films produced using different electron beam conditions were studied. We show that under the same electron dose conditions, thicker DLC films were obtained using lower accelerating voltages (2 kV) than when using higher accelerating voltage (20 kV). However, under the settings used the thicker films showed worse insulating performance than the thinner films. We attribute this effect to the variation of tunnelling barrier height in DLC deposited using different accelerating voltages. DLC films with a tunnelling barrier height of up to 3.12 eV were obtained using a 20 kV electron-beam, while only 0.73 eV was achieved for 2 kV DLC films. The X-ray photoemission spectra of the C 1s core level in these films reveal components at 284.4 ± 0.1 eV and 285 ± 0.1 eV, which were identified as the sp² and sp³ hybrid forms of carbon. The sp³/sp² concentration ratio increased with increasing electron beam accelerating voltage. We show how this effect is responsible for the barrier height variation.     

An in-vacuum x-ray diffraction microscope for use in the 0.7-2.9 keV range

A dedicated in-vacuum coherent x-ray diffraction microscope was installed at the 2-ID-B beamline of the Advanced Photon Source for use with 0.7-2.9 keV x-rays. The instrument can accommodate three common implementations of diffractive imaging; plane wave illumination; defocused-probe (Fresnel diffractive imaging) and scanning (ptychography) using either a pinhole, focused or defocused probe. The microscope design includes active feedback to limit motion of the optics with respect to the sample. Upper bounds on the relative optics-to-sample displacement have been measured to be 5.8 nm(v) and 4.4 nm(h) rms/h using capacitance micrometry and 27 nm/h using x-ray point projection imaging. The stability of the measurement platform and in-vacuum operation allows for long exposure times, high signal-to-noise and large dynamic range two-dimensional intensity measurements to be acquired. Finally, we illustrate the microscope's stability with a recent experimental result.     

Physical optics basis for the X-ray phase imaging of low contrast samples: a primer

X-ray imaging of biological materials has, in the past, relied on the difference in absorption between the two of the main constituents of living matter (water and carbon) at certain energies. In recent years, phase techniques relying on the X-ray refractive index difference of materials have been developed that have allowed biological and other low-contrast materials to be imaged. In this primer, we aim to cover some of the terminology and methods involved in X-ray phase imaging and provide a framework for understanding phase-imaging and phase-retrieval methods. We conclude with a discussion on some of the most recent advances in this field where far-field diffraction patterns of samples can be inverted to provide very high-resolution structure leading, possibly, to the prospect of single-molecule imaging.     

A coherence approach to phase-contrast microscopy II: Experiment

We report an experimental investigation of the optical transfer functions for an X-ray microscope operated in defocus phase-contrast mode. The results are compared with a theoretical model of partially coherent image formation and are found to be in excellent agreement.     

X-ray imaging: a generalized approach using phase-space tomography

We discuss the role of coherence in x-ray imaging and consider how phase-space tomography can be used to extract information about partial coherence. We describe the application of phase-space tomography to x-ray imaging and recover the spatial coherence properties of a one-dimensional soft (1.5 keV) x-ray beam from a synchrotron undulator source. We present phase-space information from a Young's experiment and observe negative regions in the quasi-probability distribution. We show that, given knowledge of the coherence of the beam, we can use partially coherent diffraction data to recover fully coherent information, and we present some simple experimental demonstrations of this capability.     

Mid-infrared emission characteristics and energy transfer processes in doubly doped Tm, Tb: KPb2Br5 and Tm, Nd: KPb2Br5

We present spectroscopic studies on the ∼5 μm mid-infrared emission and energy transfer properties of Tb³⁺ doped KPb₂Br₅ and Nd³⁺ doped KPb₂Br₅ sensitized by Tm³⁺ ions. A series of co-doped Tm, Tb: KPb₂Br₅ and Tm, Nd: KPb₂Br₅ samples were prepared from purified starting materials of PbBr₂, KBr, and rare-earth bromides. Resonant excitation into the ³H₆ → ³F₄ absorption transition of Tm³⁺ at ∼1.76 μm resulted in an enhanced 5 μm emission from Tb³⁺ and Nd³⁺ ions in Tm, Tb: KPb₂Br₅ and Tm, Nd: KPb₂Br₅, respectively. The existence of energy transfer between Tm → Tb and Tm → Nd in KPB was further evidenced by the quenching of the emission decay times of the ³F₄ → ³H₆ transition of Tm³⁺ in doubly doped Tm, Tb: KPb₂Br₅ and Tm, Nd: KPb₂Br₅ compared to singly doped Tm: KPb₂Br₅.