Xuan P. A. Gao

Subthreshold Regime has the Optimal Sensitivity for Nanowire FET Biosensors

Nanowire field-effect transistors (NW-FETs) are emerging as powerful sensors for detection of chemical/biological species with various attractive features including high sensitivity and direct electrical readout. Yet to date there have been limited systematic studies addressing how the fundamental factors of devices affect their sensitivity. Here we demonstrate that the sensitivity of NW-FET sensors can be exponentially enhanced in the subthreshold regime where the gating effect of molecules bound on a surface is the most effective due to the reduced screening of carriers in NWs. This principle is exemplified in both pH and protein sensing experiments where the operational mode of NW-FET biosensors was tuned by electrolyte gating. The lowest charge detectable by NW-FET sensors working under different operational modes is also estimated. Our work shows that optimization of NW-FET structure and operating conditions can provide significant enhancement and fundamental understanding for the sensitivity limits of NW-FET sensors.

FREQUENCY DOMAIN DETECTION OF BIOMOLECULES USING SILICON NANOWIRE BIOSENSORS

We demonstrate a new protein detection methodology based upon frequency domain electrical measurement using silicon nanowire field-effect transistor (SiNW FET) biosensors. The power spectral density of voltage from a current-biased SiNW FET shows 1/f-dependence in frequency domain for measurements of antibody functionalized SiNW devices in buffer solution or in the presence of protein not specific to the antibody receptor. In the presence of protein (antigen) recognized specifically by the antibody-functionalized SiNW FET, the frequency spectrum exhibits a Lorentzian shape with a characteristic frequency of several kilohertz. Frequency and conventional time domain measurements carried out with the same device as a function of antigen concentration show more than 10-fold increase in detection sensitivity in the frequency domain data. These concentration-dependent results together with studies of antibody receptor density effect further address possible origins of the Lorentzian frequency spectrum. Our results show that frequency domain measurements can be used as a complementary approach to conventional time domain measurements for ultrasensitive electrical detection of proteins and other biomolecules using nanoscale FETs.

Two-dimensional transport-induced linear magneto-resistance in topological insulator Bi2Se3 nanoribbons.

We report the study of a novel linear magneto-resistance (MR) under perpendicular magnetic fields in Bi(2)Se(3) nanoribbons. Through angular dependence magneto-transport experiments, we show that this linear MR is purely due to two-dimensional (2D) transport, in agreement with the recently discovered linear MR from 2D topological surface state in bulk Bi(2)Te(3), and the linear MR of other gapless semiconductors and graphene. We further show that the linear MR of Bi(2)Se(3) nanoribbons persists to room temperature, underscoring the potential of exploiting topological insulator nanomaterials for room-temperature magneto-electronic applications.

One-Dimensional Quantum Confinement Effect Modulated Thermoelectric Properties in InAs Nanowires

We report electrical conductance and thermopower measurements on InAs nanowires synthesized by chemical vapor deposition. Gate modulation of the thermopower of individual InAs nanowires with a diameter around 20 nm is obtained over T = 40-300 K. At low temperatures (T < ∼100 K), oscillations in the thermopower and power factor concomitant with the stepwise conductance increases are observed as the gate voltage shifts the chemical potential of electrons in InAs nanowire through quasi-one-dimensional (1D) subbands. This work experimentally shows the possibility to modulate semiconductor nanowires thermoelectric properties through 1D subband formation in the diffusive transport regime for electron, a long-sought goal in nanostructured thermoelectrics research. Moreover, we point out the scattering (or disorder) induced energy level broadening as the limiting factor in smearing out the 1D confinement enhanced thermoelectric power factor.

Intrinsic Electron Mobility Exceeding 103 cm2/(V s) in Multilayer InSe FETs

Graphene-like two-dimensional (2D) materials not only are interesting for their exotic electronic structure and fundamental electronic transport or optical properties but also hold promises for device miniaturization down to atomic thickness. As one material belonging to this category, InSe, a III-VI semiconductor, not only is a promising candidate for optoelectronic devices but also has potential for ultrathin field effect transistor (FET) with high mobility transport. In this work, various substrates such as PMMA, bare silicon oxide, passivated silicon oxide, and silicon nitride were used to fabricate multilayer InSe FET devices. Through back gating and Hall measurement in four-probe configuration, the devices field effect mobility and intrinsic Hall mobility were extracted at various temperatures to study the materials intrinsic transport behavior and the effect of dielectric substrate. The samples field effect and Hall mobilities over the range of 20-300 K fall in the range of 0.1-2.0 × 10(3) cm(2)/(V s), which are comparable or better than the state of the art FETs made of widely studied 2D transition metal dichalcogenides.

InAs Nanowire Transistors as Gas Sensor and the Response Mechanism

We report a study of the response of InAs nanowire field-effect transistor sensor devices to various gases and alcoholic vapors. It is concluded that the change in conductance of the device in response to chemical vapors is a combined result of both the charge transfer and modified electron mobility effects. In particular, we found that surface adsorption of most chemical molecules can reduce electron density in nanowires from approximately 10(4) to approximately 10(3)/microm and enhance the electron mobility greatly (from tens to a few hundred of cm(2)/(V s)) at the same time. These effects are attributed to the interactions between adsorbed molecules and the electron accumulation layer and rich surface states on the InAs nanowire surface.

Intrinsic electron mobility exceeding 1000 cm

Graphene-like two-dimensional (2D) materials not only are interesting for their exotic electronic structure and fundamental electronic transport or optical properties but also hold promises for device miniaturization down to atomic thickness. As one material belonging to this category, InSe, a III-VI semiconductor, not only is a promising candidate for optoelectronic devices but also has potential for ultrathin field effect transistor (FET) with high mobility transport. In this work, various substrates such as PMMA, bare silicon oxide, passivated silicon oxide, and silicon nitride were used to fabricate multilayer InSe FET devices. Through back gating and Hall measurement in four-probe configuration, the devices field effect mobility and intrinsic Hall mobility were extracted at various temperatures to study the materials intrinsic transport behavior and the effect of dielectric substrate. The samples field effect and Hall mobilities over the range of 20-300 K fall in the range of 0.1-2.0 × 10(3) cm(2)/(V s), which are comparable or better than the state of the art FETs made of widely studied 2D transition metal dichalcogenides.

^2

A key concept in the emerging field of spintronics is the gate voltage or electric field control of spin precession via the effective magnetic field generated by the Rashba spin-orbit interaction. Here, we demonstrate the generation and tuning of electric field induced Rashba spin-orbit interaction in InAs nanowires where a strong electric field is created by either a double gate or a solid electrolyte surrounding gate. In particular, the electrolyte gating enables 6-fold tuning of Rashba coefficient and nearly 3 orders of magnitude tuning of spin relaxation time within only 1 V of gate bias. Such a dramatic tuning of spin-orbit interaction in nanowires may have implications in nanowire-based spintronic devices.

/Vs in multilayer InSe FETs

We present an overview of the measured transport properties of the two dimensional electron fluids in high mobility semiconductor devices with low electron densities, and of some of the theories that have been proposed to account for them. Many features of the observations are not easily reconciled with a description based on the well understood physics of weakly interacting quasiparticles in a disordered medium. Rather, they reflect new physics associated with strong correlation effects, which warrant further study.

Strong Tuning of Rashba Spin–Orbit Interaction in Single InAs Nanowires

The inhomogeneity of as-grown single-walled carbon nanotubes (SWCNTs), in terms of chiral structure, is a major obstacle to integration of these novel materials in advanced electronics. While separation methods have circumvented this problem, current synthesis approaches must be refined for large-scale production of SWCNTs with uniform properties. In addition, it is highly desirable to alter the initial chirality distribution which constrains fundamental study and applications. Here, we demonstrate that semiconducting SWCNTs are selectively produced in the gas phase by engineering catalysts at the nanoscale with precise size and composition. The semiconducting content in as-grown mixtures of SWCNTs is assessed by UV-visible-NIR absorbance and micro-Raman spectroscopy and reaches a maximum purity of 90% for samples catalyzed by Ni(0.27)Fe(0.73) nanoparticles (2.0 nm mean diameter). Electrical studies are performed on thin film transistors (TFTs) fabricated from as-grown SWCNTs and reveal high on/off current ratios of 10(3).