Hyunjung Yi

Electrical spin injection and detection in an InAs quantum well

The authors demonstrate fully electrical detection of spin injection in InAs quantum wells. A spin-polarized current is injected from a Ni81Fe19 thin film to a two-dimensional electron gas (2DEG) made of InAs based epitaxial multilayers. Injected spins accumulate and diffuse out in the 2DEG, and the spins are electrically detected by a neighboring Ni81Fe19 electrode. The observed spin diffusion length is 1.8μm at 20K. The injected spin polarization across the Ni81Fe19∕InAs interface is 1.9% at 20K and remains at 1.4% even at room temperature. Their experimental results will contribute significantly to the realization of a practical spin field effect transistor.

Lateral size effects on domain structure in epitaxial PbTiO3 thin films

Lateral size effects of ferroelastic domain structures in epitaxial PbTiO3 thin films were investigated systematically with a viewpoint of misfit strain relaxation mechanism. The epitaxial PbTiO3 thin films were patterned into discrete islands and the effects of lateral dimension were analyzed by reciprocal space mapping using synchrotron x-ray diffraction as well as finite element simulation. As the lateral two-dimensional planar size decreases in the PbTiO3 patterns on MgO(001), some of the a domains turned into c domains due to the relaxed tensile strain. In the PbTiO3 patterns on Pt(001)∕MgO(001), on the other hand, the formation of 90° domains is enhanced by the reduction in compressive misfit strain. As the pattern size decreases further to 100nm, the untilted a domains arise due to the almost completely relaxed misfit strains. Equilibrium domain structures in the epitaxial thin films and discrete islands are also analyzed by the finite element simulation and found to be consistent with the experime...

Single-carbon discrimination by selected peptides for individual detection of volatile organic compounds

Although volatile organic compounds (VOCs) are becoming increasingly recognized as harmful agents and potential biomarkers, selective detection of the organic targets remains a tremendous challenge. Among the materials being investigated for target recognition, peptides are attractive candidates because of their chemical robustness, divergence, and their homology to natural olfactory receptors. Using a combinatorial peptide library and either a graphitic surface or phenyl-terminated self-assembled monolayer as relevant target surfaces, we successfully selected three interesting peptides that differentiate a single carbon deviation among benzene and its analogues. The heterogeneity of the designed target surfaces provided peptides with varying affinity toward targeted molecules and generated a set of selective peptides that complemented each other. Microcantilever sensors conjugated with each peptide quantitated benzene, toluene and xylene to sub-ppm levels in real time. The selection of specific receptors for a group of volatile molecules will provide a strong foundation for general approach to individually monitoring VOCs.

Hydrodynamic assembly of conductive nanomesh of single-walled carbon nanotubes using biological glue

A hydrodynamic phenomenon is used to assemble a large-scale conductive nanomesh of single-walled carbon nanotubes (SWNTs) with exceptional control of the nanostructure. This is accomplished by a biological material with nanoscale features and a strong binding affinity toward SWNTs. The biological material also presents a unique glue effect for the assembly. Unprecedented material characteristics are observed for the nanomesh.

Direct Electron Transfer of Enzymes in a Biologically Assembled Conductive Nanomesh Enzyme Platform.

Nondestructive assembly of a nanostructured enzyme platform is developed in combination of the specific biomolecular attraction and electrostatic coupling for highly efficient direct electron transfer (DET) of enzymes with unprecedented applicability and versatility. The biologically assembled conductive nanomesh enzyme platform enables DET-based flexible integrated biosensors and DET of eight different enzyme with various catalytic activities.

Exchange biased spin polarizer with an embedded nano-oxide layer for a substantially lower switching current density

The authors demonstrate that the spin polarizer in the form of an exchange biased ferromagnetic lead with an embedded nano-oxide layer can greatly enhance the spin transfer torque for the current induced magnetization switching. By applying it in spin valves, the switching current density (4×106A∕cm2) is one order lower and the resistance change (2.78mΩμm2) is three times higher than those gotten by using a simple spin polarizer. This spin torque enhancement is attributed to the exchange bias pinning acting on the polarizer (the fixed layer) with effective support of the nano-oxide layer, which together lead to a much higher current spin polarization.

Biologically templated assembly of hybrid semiconducting nanomesh for high performance field effect transistors and sensors

Delicately assembled composites of semiconducting nanomaterials and biological materials provide an attractive interface for emerging applications, such as chemical/biological sensors, wearable health monitoring devices, and therapeutic agent releasing devices. The nanostructure of composites as a channel and a sensing material plays a critical role in the performance of field effect transistors (FETs). Therefore, it is highly desirable to prepare elaborate composite that can allow the fabrication of high performance FETs and also provide high sensitivity and selectivity in detecting specific chemical/biological targets. In this work, we demonstrate that high performance FETs can be fabricated with a hydrodynamically assembled composite, a semiconducting nanomesh, of semiconducting single-walled carbon nanotubes (S-SWNTs) and a genetically engineered M13 phage to show strong binding affinity toward SWNTs. The semiconducting nanomesh enables a high on/off ratio (~104) of FETs. We also show that the threshold voltage and the channel current of the nanomesh FETs are sensitive to the change of the M13 phage surface charge. This biological gate effect of the phage enables the detection of biologically important molecules such as dopamine and bisphenol A using nanomesh-based FETs. Our results provide a new insight for the preparation of composite material platform for highly controllable bio/electronics interfaces.

A Reconfigurable and Portable Highly Sensitive Biosensor Platform for ISFET and Enzyme-Based Sensors

This paper presents a portable low-cost biosensor platform for ion-sensitive field-effect transistor (ISFET) and enzyme-based sensors. To meet various demands of diagnosis, our portable platform is designed to perform cyclic voltammetry, amperometry, and linear sweep voltammetry for enzyme-based sensor and ISFET sensor. For compatibility with various sensors which require various electrical driving schemes, our system can be easily reconfigured by simple switches. In addition, with disposable printed-circuit-board packages, sensors can be easily replaced. Our platform was tested with various sensors in various measurement methods. In cyclic voltammetry test with a model analyte K3Fe(CN)6, a graph provided by our system has 4.79% relative error at a current peak compared with the data acquired by a 18 times more expensive commercial system. In cyclic voltammetry and amperometry tests with the laccase enzyme-based sensors, our system achieved sensitivities of 341 and 500 μA/mM/cm2, respectively. By running a sensitive SiNW ISFET on our platform in linear sweep voltammetry, we could build a low-cost portable pH sensor system.

Fibrous all-in-one monolith electrodes with a biological gluing layer and a membrane shell for weavable lithium-ion batteries

The increasing demand for wearable devices ultimately requires the development of energy storage devices with wide structural versatility, lightweight and high energy density. Although various flexible batteries have been developed based on two-dimensional and one-dimensional platforms, truly weavable batteries with high capacity and elongation capability have not been materialized yet. Herein, we report weavable lithium ion batteries (LIBs) with high capacity by developing fibrous all-in-one electrode threads based on nanosized hybrid active layers with a biological gluing inner layer and a membrane shell. The thread consists of four distinct concentric structures, a carbon fiber core as a current collector, a conductive biological gluing layer, nanohybrid active materials, and a porous membrane layer. Nanosized LiFePO4/C-rGO and Li4Ti5O12/rGO are used for cathode and anode threads, respectively. This unique all-in-one structure combined with an inline coating approach ensures flexibility and mechanical stability with a high linear capacity of 1.6 mA h cm−1. These features all together allow for various assembly schemes such as twisting and hierarchical weaving, enabling fabric LIBs to show 50% elongation via encoded structural deformation.

Ultrasensitive and Highly Stable Resistive Pressure Sensors with Biomaterial-Incorporated Interfacial Layers for Wearable Health-Monitoring and Human–Machine Interfaces

Flexible piezoresistive sensors have huge potential for health monitoring, human-machine interfaces, prosthetic limbs, and intelligent robotics. A variety of nanomaterials and structural schemes have been proposed for realizing ultrasensitive flexible piezoresistive sensors. However, despite the success of recent efforts, high sensitivity within narrower pressure ranges and/or the challenging adhesion and stability issues still potentially limit their broad applications. Herein, we introduce a biomaterial-based scheme for the development of flexible pressure sensors that are ultrasensitive (resistance change by 5 orders) over a broad pressure range of 0.1-100 kPa, promptly responsive (20 ms), and yet highly stable. We show that employing biomaterial-incorporated conductive networks of single-walled carbon nanotubes as interfacial layers of contact-based resistive pressure sensors significantly enhances piezoresistive response via effective modulation of the interlayer resistance and provides stable interfaces for the pressure sensors. The developed flexible sensor is capable of real-time monitoring of wrist pulse waves under external medium pressure levels and providing pressure profiles applied by a thumb and a forefinger during object manipulation at a low voltage (1 V) and power consumption (<12 μW). This work provides a new insight into the material candidates and approaches for the development of wearable health-monitoring and human-machine interfaces.