Joon Hwan Lee

Structural, electrical, and terahertz transmission properties of VO2 thin films grown on c-, r-, and m-plane sapphire substrates

The structure, metal-insulator transition (MIT), and related Terahertz (THz) transmission characteristics of VO2 thin films obtained by sputtering deposition on c-, r-, and m-plane sapphire substrates were investigated by different techniques. On c-sapphire, monoclinic VO2 films were characterized to be epitaxial films with triple domain structure caused by β-angle mismatch. Monoclinic VO2 β angle of 122.2° and the two angles of V4+–V4+ chain deviating from the am axis of 4.4° and 4.3° are determined. On r-sapphire, tetragonal VO2 was determined to be epitaxially deposited with VO2 (011)T perpendicular to the growth direction, while the structural phase transformation into lower symmetric monoclinic phase results in (2¯11) and (200) orientations forming a twinned structure. VO2 on m-sapphire has several growth orientations, related with the uneven substrate surface and possible inter-diffusion between film and substrate. Measurements of the electrical properties show that the sample on r-sapphire has MIT ...

Thick YBa2Cu3O7−x+BaSnO3 films with enhanced critical current density at high magnetic fields

The thickness dependence was studied for the critical current density (Jc) of YBa2Cu3O7−x(YBCO)+BaSnO3 (BSO) nanocomposite films. These films showed a significantly reduced decline of the Jc with thickness, especially at high magnetic fields. For example, a 2 μm thick YBCO+BSO film had a Jc∼3×105 A/cm2 at 5 T as compared to a typical Jc of 2.4×103 A/cm2 at 5 T for a 300 nm thick YBCO film. The thick YBCO+BSO films maintained high Tc (>88 K) and had a high density (2.5×1011/cm2) of continuous BSO nanocolumns that likely contributed for the observed Jc enhancements.

VO2 multidomain heteroepitaxial growth and terahertz transmission modulation

We report the epitaxial relationship of VO2 thin-films on c-plane sapphire and their terahertz transmission modulation with temperature. The films exhibit a triple-domain structure caused by the lattice mismatch between monoclinic VO2 and sapphire hexagon. The epitaxial relationship is determined to be VO2[010]∥Al2O3[0001] and VO2(2¯02)∥Al2O3{112¯0}, with the in-plane lattice mismatch of 2.66% (tensile) along [2¯02] and the out-of-plane lattice mismatch of −2.19% (compressive). Terahertz measurements revealed that VO2 films have over fourfold change in transmission during the metal-insulator transition, indicating a strong potential for terahertz wave switching and modulation applications.

Direct observation of Lomer-Cottrell Locks during strain hardening in nanocrystalline nickel by in situ TEM

Strain hardening capability is critical for metallic materials to achieve high ductility during plastic deformation. A majority of nanocrystalline metals, however, have inherently low work hardening capability with few exceptions. Interpretations on work hardening mechanisms in nanocrystalline metals are still controversial due to the lack of in situ experimental evidence. Here we report, by using an in situ transmission electron microscope nanoindentation tool, the direct observation of dynamic work hardening event in nanocrystalline nickel. During strain hardening stage, abundant Lomer-Cottrell (L-C) locks formed both within nanograins and against twin boundaries. Two major mechanisms were identified during interactions between L-C locks and twin boundaries. Quantitative nanoindentation experiments recorded show an increase of yield strength from 1.64 to 2.29 GPa during multiple loading-unloading cycles. This study provides both the evidence to explain the roots of work hardening at small length scales and the insight for future design of ductile nanocrystalline metals.

Growth-controlled surface roughness in Al-doped ZnO as transparent conducting oxide

The surface morphology of Al(2)O(3)-doped ZnO (AZO, 2 wt%) thin films varies from a uniform layer to nanorod structure by simply controlling oxygen pressure during growth. All AZO films were deposited on sapphire(0001) substrates using a pulsed laser deposition (PLD) technique. In the low oxygen pressure regime (vacuum approximately 50 mTorr), AZO films grow as a smooth and uniform layer. In the high oxygen pressure regime (100-250 mTorr) AZO thin films with nanorods have formed. Detailed cross-sectional transmission electron microscopy (TEM) and x-ray diffraction (XRD) studies reveal that, besides the obvious variation in the film morphology, the in-plane d spacing of AZO film increases and the out-of-plane d spacing decreases, as oxygen pressure increases. A bilayer AZO film with a nanorod structure on top of a uniform layer was demonstrated by controlling the oxygen pressure for the two layers. Electrical resistivity and optical transmittance measurements were carried out to correlate with the microstructures obtained under different oxygen pressures. The bilayer AZO films could find applications as a transparent conducting oxide (TCO) with a unique light trapping function in thin film solar cells.

Tunable lattice strain in vertically aligned nanocomposite (BiFeO3)x:(Sm2O3)1−x thin films

Unique epitaxial two-phase vertically aligned nanocomposite (VAN) (BiFeO3)x:(Sm2O3)1−x thin films were deposited on SrTiO3(001) substrates by pulsed laser deposition. The VAN thin films exhibit a highly ordered vertical columnar structure with high epitaxial quality. We demonstrate that the strains of the two phases in both out-of-plane and in-plane directions can be tuned by the deposition parameters during growth, e.g., deposition frequency and film composition of the nanocomposite. The strain tunability is found to be directly related to the systematic variation in the column widths in the nanocomposite. The dielectric property measurement shows that increasing vertical strain control will lead to a systematic dielectric loss reduction in the VAN thin films. This study suggests a promising avenue in achieving tunable strain in functional oxide thin films by using VAN structures.

Epitaxial Superconducting δ-MoN Films Grown by a Chemical Solution Method

The synthesis of pure δ-MoN with desired superconducting properties usually requires extreme conditions, such as high temperature and high pressure, which hinders its fundamental studies and applications. Herein, by using a chemical solution method, epitaxial δ-MoN thin films have been grown on c-cut Al(2)O(3) substrates at a temperature lower than 900 °C and an ambient pressure. The films are phase pure and show a T(c) of 13.0 K with a sharp transition. In addition, the films show a high critical field and excellent current carrying capabilities, which further prove the superior quality of these chemically prepared epitaxial thin films.

Ferroelectric Properties of Vertically Aligned Nanostructured BaTiO3–CeO2 Thin Films and Their Integration on Silicon

Epitaxial (BaTiO3)0.5(CeO2)0.5 films have been deposited in vertically aligned nanocomposite form on SrTiO3/TiN buffered Si substrates to achieve high-quality ferroelectrics on Si. The thin TiN seed layer promotes the epitaxial growth of the SrTiO3 buffer on Si, which in turn is essential for the high-quality growth of the vertically aligned nanocomposite structure. X-ray diffraction and transmission electron microscopy characterization show that the films consist of distinct c-axis oriented BaTiO3 and CeO2 phases. Polarization measurements show that the BaTiO3-CeO2 films on Si are actually ferroelectric at room temperature, and the ferroelectric response is comparable to pure BaTiO3 as well as the BaTiO3-CeO2 films on SrTiO3 single-crystalline substrates. Capacitance-voltage measurements show that, instead of decreasing, the Curie temperature increases to 175 and 150 °C for the samples on SrTiO3 and Si substrates, respectively. This work is an essential step towards integrating novel nanostructured materials with advanced functionalities into Si-based devices.

Resonance Raman spectroscopy of G-line and folded phonons in twisted bilayer graphene with large rotation angles

We report the synthesis and systematic Raman study of twisted bilayer graphene (tBLG) with rotation angles from below 10° to nearly 30°. Chemical vapor deposition was used to grow hexagon-shaped tBLG with a rotation angle that can be conveniently determined by relative edge misalignment. Rotation dependent G-line resonances and folded phonons were observed by selecting suitable energies of excitation lasers. The observed phonon frequencies of the tBLG superlattices agree well with our ab initio calculation.

Size dependent strengthening mechanisms in sputtered Fe/W multilayers

We investigate size dependent strengthening mechanisms in sputtered Fe/W multilayers with individual layer thickness, h, varying from 1 to 200 nm. Microstructure analyses reveal that Fe/W has incoherent bcc/bcc interface when h is greater than 5 nm. When h decreases to 1–2.5 nm, the interface becomes semicoherent, and Fe and W show significant lattice distortions comparing to their bulk counterpart due to interface constraint. The layer thickness dependent drastic variations in x-ray diffraction profiles are simulated well by using an analytical model. Film hardness increases with decreasing h, and approaches a maximum value of 12.5 GPa when h is 1 nm. The layer thickness dependent film hardnesses are compared with analytical models. Koehler’s image force plays a major role in determining the maximum strength of composites at smaller h.