Jamie Wilt

Wrapping cytochrome c around single-wall carbon nanotube: engineered nanohybrid building blocks for infrared detection at high quantum efficiency

Biomolecule cytochrome c (Cty c), a small molecule of a chain of amino acids with extraordinary electron transport, was helically wrapped around a semiconductive single-wall carbon nanotube (s-SWCNT) to form a molecular building block for uncooled infrared detection with two uniquely designed functionalities: exciton dissociation to free charge carriers at the heterojunction formed on the s-SWCNT/Cty c interface and charge transport along the electron conducting chain of Cty c (acceptor) and hole conducting channel through s-SWCNT (donor). Such a design aims at addressing the long-standing challenges in exciton dissociation and charge transport in an SWCNT network, which have bottlenecked development of photonic SWCNT-based infrared detectors. Using these building blocks, uncooled s-SWCNT/Cyt c thin film infrared detectors were synthesized and shown to have extraordinary photoresponsivity up to 0.77 A W−1 due to a high external quantum efficiency (EQE) in exceeding 90%, which represents a more than two orders of magnitude enhancement than the best previously reported on CNT-based infrared detectors with EQE of only 1.72%. From a broad perspective, this work on novel s-SWCNT/Cyt c nanohybrid infrared detectors has developed a successful platform of engineered carbon nanotube/biomolecule building blocks with superior properties for optoelectronic applications.

Controlling BaZrO3 nanostructure orientation in YBa2Cu3O films for a three-dimensional pinning landscape

The orientation phase diagram of self-assembled BaZrO3 (BZO) nanostructures in c-oriented YBa2Cu3O (YBCO) films on flat and vicinal SrTiO3 substrates was studied experimentally with different dopant concentrations and vicinal angles and theoretically using a micromechanical model based on the theory of elasticity. The organized BZO nanostructure configuration was found to be tunable, between c-axis to ab-plane alignment, by the dopant concentration in the YBCO film matrix strained via lattice mismatched substrates. The correlation between the local strain caused by the BZO doping and the global strain on the matrix provides a unique approach for controllable growth of dopant nanostructure landscapes. In particular, a mixed phase of the c-axis-aligned nanorods and the ab-plane-aligned planar nanostructures can be obtained, leading to a three-dimensional pinning landscape with single impurity doping and much improved J c in almost all directions of applied magnetic field.

Time-Resolved Measurements of Photocarrier Dynamics in TiS3 Nanoribbons

We report synthesis and time-resolved transient absorption measurements of TiS3 nanoribbons. TiS3 nanoribbons were fabricated by direct reaction of titanium and sulfur. Dynamics of the photocarriers in these samples were studied by transient absorption measurements. It was found that following ultrafast injection of nonequilibrium and hot photocarriers, the thermalization, energy relaxation, and exciton formation all occur on a subpicosecond time scale. Several key parameters describing the dynamical properties of photocarriers, including their recombination lifetime, diffusion coefficient, mobility, and diffusion length, were deduced.

Atomically Thin Al2O3 Films for Tunnel Junctions

Metal-Insulator-Metal tunnel junctions (MIMTJ) are common throughout the microelectronics industry. The industry standard AlOx tunnel barrier, formed through oxygen diffusion into an Al wetting layer, is plagued by internal defects and pinholes which prevent the realization of atomically-thin barriers demanded for enhanced quantum coherence. In this work, we employed in situ scanning tunneling spectroscopy (STS) along with molecular dynamics simulations to understand and control the growth of atomically thin Al2O3 tunnel barriers using atomic layer deposition (ALD). We found that a carefully tuned initial H2O pulse hydroxylated the Al surface and enabled the creation of an atomically-thin Al2O3 tunnel barrier with a high quality M-I interface and a significantly enhanced barrier height compared to thermal AlOx. These properties, corroborated by fabricated Josephson Junctions, show that ALD Al2O3 is a dense, leak-free tunnel barrier with a low defect density which can be a key component for the next-generation of MIMTJs.

Probing Microscopic Strain Interplay Due to Impurity Doping and Vicinal Growth and Its Effect on Pinning Landscape in YBCO Films

Vortex pinning by insertion of non-superconducting defects like BZO or BSO nanorods into the YBCO matrix is an effective means to enhance pinning since they self-assemble into columnar structures that provide strong pinning along the length of the flux-line. However, only limited control of their geometry is possible by current growth methods. To meet the requirements of applications that operate in magnetic fields of varying intensity or orientation, this work studies strain-mediated self-assembly of 3D pinning landscape through theoretical modeling as well as experimental exploration to achieve controllable growth BZO or BSO nanostructures in YBCO matrix films. The microstructure of BZO- and BSO-doped YBCO thin films was studied using transmission electron microscopy and the findings indicate that it is possible to produce a controllable defect landscape and improved critical current density with respect to different orientation of the magnetic field by manipulation of the strain relationships using vicinal substrates.

Probing the Dielectric Properties of Ultrathin Al/Al2O3/Al Trilayers Fabricated Using in Situ Sputtering and Atomic Layer Deposition

Dielectric properties of ultrathin Al2O3 (1.1-4.4 nm) in metal-insulator-metal (M-I-M) Al/Al2O3/Al trilayers fabricated in situ using an integrated sputtering and atomic layer deposition (ALD) system were investigated. An M-I interfacial layer (IL) formed during the pre-ALD sample transfer even under high vacuum has a profound effect on the dielectric properties of the Al2O3 with a significantly reduced dielectric constant (εr) of 0.5-3.3 as compared to the bulk εr ∼ 9.2. Moreover, the observed soft-type electric breakdown suggests defects in both the M-I interface and the Al2O3 film. By controlling the pre-ALD exposure to reduce the IL to a negligible level, a high εr up to 8.9 was obtained on the ALD Al2O3 films with thicknesses from 3.3 to 4.4 nm, corresponding to an effective oxide thickness (EOT) of ∼1.4-1.9 nm, respectively, which are comparable to the EOTs found in high-K dielectrics like HfO2 at 3-4 nm in thickness and further suggest that the ultrathin ALD Al2O3 produced in optimal conditions may provide a low-cost alternative gate dielectric for CMOS. While εr decreases at a smaller Al2O3 thickness, the hard-type dielectric breakdown at 32 MV/cm and in situ scanning tunneling spectroscopy revealed band gap ∼2.63 eV comparable to that of an epitaxial Al2O3 film. This suggests that the IL is unlikely a dominant reason for the reduced εr at the Al2O3 thickness of 1.1-2.2 nm but rather a consequence of the electron tunneling as confirmed in the transport measurement. This result demonstrates the critical importance in controlling the IL to achieving high-performance ultrathin dielectric in MIM structures.

Effect of an Interfacial Layer on Electron Tunneling through Atomically Thin Al2O3 Tunnel Barriers

Electron tunneling through high-quality, atomically thin dielectric films can provide a critical enabling technology for future microelectronics, bringing enhanced quantum coherent transport, fast speed, small size, and high energy efficiency. A fundamental challenge is in controlling the interface between the dielectric and device electrodes. An interfacial layer (IL) will contain defects and introduce defects in the dielectric film grown atop, preventing electron tunneling through the formation of shorts. In this work, we present the first systematic investigation of the IL in Al2O3 dielectric films of 1-6 Ås in thickness on an Al electrode. We integrated several advanced approaches: molecular dynamics to simulate IL formation, in situ high vacuum sputtering atomic layer deposition (ALD) to synthesize Al2O3 on Al films, and in situ ultrahigh vacuum scanning tunneling spectroscopy to probe the electron tunneling through the Al2O3. The IL had a profound effect on electron tunneling. We observed a reduced tunnel barrier height and soft-type dielectric breakdown which indicate that defects are present in both the IL and in the Al2O3. The IL forms primarily due to exposure of the Al to trace O2 and/or H2O during the pre-ALD heating step of fabrication. As the IL was systematically reduced, by controlling the pre-ALD sample heating, we observed an increase of the ALD Al2O3 barrier height from 0.9 to 1.5 eV along with a transition from soft to hard dielectric breakdown. This work represents a key step toward the realization of high-quality, atomically thin dielectrics with electron tunneling for the next generation of microelectronics.