Carbon-nanotube field-effect transistors for resolving single-molecule aptamer-ligand binding kinetics

Small molecules such as neurotransmitters are critical for biochemical functions in living systems. While conventional ultraviolet-visible spectroscopy and mass spectrometry lack portability and are unsuitable for time-resolved measurements in situ, techniques such as amperometry and traditional field-effect detection require a large ensemble of molecules to reach detectable signal levels. Here we demonstrate the potential of carbon-nanotube-based single-molecule field-effect transistors (smFETs), which can detect the charge on a single molecule, as a new platform for recognizing and assaying small molecules. smFETs are formed by the covalent attachment of a probe molecule, in our case a DNA aptamer, to a carbon nanotube. Conformation changes on binding are manifest as discrete changes in the nanotube electrical conductance. By monitoring the kinetics of conformational changes in a binding aptamer, we show that smFETs can detect and quantify serotonin at the single-molecule level, providing unique insights into the dynamics of the aptamer-ligand system. In particular, we show the involvement of G-quadruplex formation and the disruption of the native hairpin structure in the conformational changes of the serotonin-aptamer complex. The smFET is a label-free approach to analysing molecular interactions at the single-molecule level with high temporal resolution, providing additional insights into complex biological processes.

Mixed-Dimensional 1D/2D van der Waals Heterojunction Diodes and Transistors in the Atomic Limit

Inverting a semiconducting channel is the basis of all field-effect transistors. In silicon-based metal-oxide-semiconductor field-effect transistors (MOSFETs), a gate dielectric mediates this inversion. Access to inversion layers may be granted by interfacing ultrathin low-dimensional semiconductors in heterojunctions to advance device downscaling. Here we demonstrate that monolayer molybdenum disulfide (MoS₂) can directly invert a single-walled semiconducting carbon nanotube (SWCNT) transistor channel without the need for a gate dielectric. We fabricate and study this atomically thin one-dimensional/two-dimensional (1D/2D) van der Waals heterojunction and employ it as the gate of a 1D heterojunction field-effect transistor (1D-HFET) channel. Gate control is based on modulating the conductance through the channel by forming a lateral p-n junction within the CNT itself. In addition, we observe a region of operation exhibiting a negative static resistance after significant gate tunneling current passes through the junction. Technology computer-aided design (TCAD) simulations confirm the role of minority carrier drift-diffusion in enabling this behavior. The resulting van der Waals transistor architecture thus has the dual characteristics of both field-effect and tunneling transistors, and it advances the downscaling of heterostructures beyond the limits of dangling bonds and epitaxial constraints faced by III-V semiconductors.

Nonlinear Chemical Sensitivity Enhancement of Nanowires in the Ultralow Concentration Regime

Much recent attention has been focused on the development of field-effect transistors based on low-dimensional nanostructures for the detection and manipulation of molecules. Because of their extraordinarily high charge sensitivity, InAs nanowires present an excellent material system in which to probe and study the behavior of molecules on their surfaces and elucidate the underlying mechanisms dictating the sensor response. So far, chemical sensors have relied on slow, activated processes restricting their applicability to high temperatures and macroscopic adsorbate coverages. Here, we identify the transition into a highly sensitive regime of chemical sensing at ultralow concentrations (<1 ppm) via physisorption at room temperature using field-effect transistors with channels composed of several thousand InAs nanowires and ethanol as a simple analyte molecule. In this regime, the nanowire conductivity is dictated by a local gating effect from individual dipoles, leading to a nonlinear enhancement of the sensitivity. At higher concentrations (>1 ppm), the nanowire channel is globally gated by a uniform dipole layer at the nanowire surface. The former leads to a dramatic increase in sensitivity due to weakened screening and the one-dimensional geometry of the nanowire. In this regime, we detect concentrations of ethanol vapor as low as 10 ppb, 100 times below the lowest concentrations previously reported. Furthermore, we demonstrate electrostatic control of the sensitivity and dynamic range of the InAs nanowire-based sensor and construct a unified model that accurately describes and predicts the sensor response over the tested concentration range (10 ppb to 10 ppm).

Surface State Dynamics Dictating Transport in InAs Nanowires

Because of their high aspect ratio, nanostructures are particularly susceptible to effects from surfaces such as slow electron trapping by surface states. However, nonequilibrium trapping dynamics have been largely overlooked when considering transport in nanoelectronic devices. In this study, we demonstrate the profound influence of dynamic trapping processes on transport in InAs nanowires through an investigation of the hysteretic and time-dependent behavior of the transconductance. We observe large densities (∼10¹³ cm^-2) of slow surface traps and demonstrate the ability to control and permanently fix their occupation and charge through electrostatic manipulation by the gate potential followed by thermal deactivation by cryogenic cooling. Furthermore, we observe a transition from enhancement- to depletion-mode and a 400% change in field-effect mobility within the same device when the initial gate voltage and sweep rate are varied, revealing the severe impact of electrostatic history and dynamics on InAs nanowire field-effect transistors. A time-dependent model for nanowire transconductance based on nonequilibrium carrier population dynamics with thermally activated capture and emission was constructed and showed excellent agreement with experiments, confirming the effects to be a direct result of the dynamics of slow surface traps characterized by large thermal activation barriers (∼ 700 meV). This work reveals a clear and direct link between the electrical conductivity and the microscopic interactions of charged species with nanowire surfaces and highlights the necessity for considering dynamic properties of surface states in nanoelectronic devices.

Mapping the Coulomb Environment in Interference-Quenched Ballistic Nanowires

The conductance of semiconductor nanowires is strongly dependent on their electrostatic history because of the overwhelming influence of charged surface and interface states on electron confinement and scattering. We show that InAs nanowire field-effect transistor devices can be conditioned to suppress resonances that obscure quantized conduction thereby revealing as many as six sub-bands in the conductance spectra as the Fermi-level is swept across the sub-band energies. The energy level spectra extracted from conductance, coupled with detailed modeling shows the significance of the interface state charge distribution revealing the Coulomb landscape of the nanowire device. Inclusion of self-consistent Coulomb potentials, the measured geometrical shape of the nanowire, the gate geometry and nonparabolicity of the conduction band provide a quantitative and accurate description of the confinement potential and resulting energy level structure. Surfaces of the nanowire terminated by HfO₂ are shown to have their interface donor density reduced by a factor of 30 signifying the passivating role played by HfO₂.

Sensing Responses Based on Transfer Characteristics of InAs Nanowire Field-Effect Transistors

Nanowire-based field-effect transistors (FETs) have demonstrated considerable promise for a new generation of chemical and biological sensors. Indium arsenide (InAs), by virtue of its high electron mobility and intrinsic surface accumulation layer of electrons, holds properties beneficial for creating high performance sensors that can be used in applications such as point-of-care testing for patients diagnosed with chronic diseases. Here, we propose devices based on a parallel configuration of InAs nanowires and investigate sensor responses from measurements of conductance over time and FET characteristics. The devices were tested in controlled concentrations of vapour containing acetic acid, 2-butanone and methanol. After adsorption of analyte molecules, trends in the transient current and transfer curves are correlated with the nature of the surface interaction. Specifically, we observed proportionality between acetic acid concentration and relative conductance change, off current and surface charge density extracted from subthreshold behaviour. We suggest the origin of the sensing response to acetic acid as a two-part, reversible acid-base and redox reaction between acetic acid, InAs and its native oxide that forms slow, donor-like states at the nanowire surface. We further describe a simple model that is able to distinguish the occurrence of physical versus chemical adsorption by comparing the values of the extracted surface charge density. These studies demonstrate that InAs nanowires can produce a multitude of sensor responses for the purpose of developing next generation, multi-dimensional sensor applications.

Surface Properties from Transconductance in Nanoscale Systems

Because of the continued scaling of transistor dimensions and incorporation of nanostructured materials into modern electronic and optoelectronic devices, surfaces and interfaces have become a dominant factor dictating material properties and device performance. In this study, we investigate the temperature-dependent electronic transport properties of InAs nanowire field-effect transistors. A point where the nanowire conductance becomes independent of temperature is observed, known as the zero-temperature-coefficient. The distribution of surface states is determined by a spectral analysis of the conductance activation energy and used to develop a carrier transport model that explains the existence and gate voltage dependence of this point. We determine that the position of this point in gate voltage is directly related to the fixed oxide charge on the nanowire surface and demonstrate the utility of this method for studying surface passivations in nanoscale systems by characterizing (NH₄)₂S_x and H₂ plasma surface treatments on InAs nanowires.

Axial p-n-junctions in nanowires

The charge distribution and potential profile of p-n-junctions in thin semiconductor nanowires (NWs) were analyzed. The characteristics of screening in one-dimensional systems result in a specific profile with large electric field at the boundary between the n- and p- regions, and long tails with a logarithmic drop in the potential and charge density. As a result of these tails, the junction properties depend sensitively on the geometry of external contacts and its capacity has an anomalously large value and frequency dispersion. In the presence of an external voltage, electrons and holes in the NWs can not be described by constant quasi-Fermi levels, due to small values of the average electric field, mobility, and lifetime of carriers. Thus, instead of the classical Sah-Noice-Shockley theory, the junction current-voltage characteristic was described by an alternative theory suitable for fast generation-recombination and slow diffusion-drift processes. For the non-uniform electric field in the junction, this theory predicts the forward branch of the characteristic to have a non-ideality factor η several times larger than the values 1 < η < 2 from classical theory. Such values of η have been experimentally observed by a number of researchers, as well as in the present work.