Competing surface reactions limiting the performance of ion-sensitive field-effect transistors

© 2015 Elsevier B.V. All rights reserved.Ion-sensitive field-effect transistors based on silicon nanowires are promising candidates for the detection of chemical and biochemical species. These devices have been established as pH sensors thanks to the large number of surface hydroxyl groups at the gate dielectrics which makes them intrinsically sensitive to protons. To specifically detect species other than protons, the sensor surface needs to be modified. However, the remaining hydroxyl groups after functionalization may still limit the sensor response to the targeted species. Here, we describe the influence of competing reactions on the measured response using a general site-binding model. We investigate the key features of the model with a real sensing example based on gold-coated nanoribbons functionalized with a self-assembled monolayer of calcium-sensitive molecules. We identify the residual pH response as the key parameter limiting the sensor response. The competing effect of pH or any other relevant reaction at the sensor surface has therefore to be included to quantitatively understand the sensor response and prevent misleading interpretations

Enhanced Resonant Tunneling in Symmetric 2D Semiconductor Vertical Heterostructure Transistors

Tunneling transistors with negative differential resistance have widespread appeal for both digital and analog electronics. However, most attempts to demonstrate resonant tunneling devices, including graphene–insulator–graphene structures, have resulted in low peak-to-valley ratios, limiting their application. We theoretically demonstrate that vertical heterostructures consisting of two identical monolayer 2D transition-metal dichalcogenide semiconductor electrodes and a hexagonal boron nitride barrier result in a peak-to-valley ratio several orders of magnitude higher than the best that can be achieved using graphene electrodes. The peak-to-valley ratio is large even at coherence lengths on the order of a few nanometers, making these devices appealing for nanoscale electronics

Direct, Label-Free, and Rapid Transistor-Based Immunodetection in Whole Serum

Transistor-based biosensors fulfill many requirements posed upon transducers for future point-of-care diagnostic devices such as scalable fabrication and label-free and real-time quantification of chemical and biological species with high sensitivity. However, the short Debye screening length in physiological samples (<1 nm) has been a major drawback so far, preventing direct measurements in serum. In this work, we demonstrate how tailoring the sensing surface with short specific biological receptors and a polymer polyethylene glycol (PEG) can strongly enhance the sensor response. In addition, the sensor performance can be dramatically improved if the measurements are performed at elevated temperatures (37 °C instead of 21 °C). With this novel approach, highly sensitive and selective detection of a representative immunosensing parameterhuman thyroid-stimulating hormoneis shown over a wide measuring range with subpicomolar detection limits in whole serum. To the best of our knowledge, this is the first demonstration of direct immunodetection in whole serum using transistor-based biosensors, without the need for sample pretreatment, labeling, or washing steps. The presented sensor is low-cost, can be easily integrated into portable diagnostics devices, and offers a competitive performance compared to state-of-the-art central laboratory analyzers

One-Step Microheterogeneous Formation of Rutile@Anatase Core–Shell Nanostructured Microspheres Discovered by Precise Phase Mapping

Nanostructured core–shell microspheres with a rough rutile core and a thin anatase shell are synthesized via a one-step heterogeneous templated hydrolysis process of TiCl₄ vapor on the aerosol water–air interface. The rutile-in-anatase core–shell structure has been evidenced by different electron microscopy techniques, including electron energy-loss spectroscopy and 3D electron tomography. A new mechanism for the formation of a crystalline rutile core inside the anatase shell is proposed based on a statistical evaluation of a large number of electron microscopy data. We found that the control over the TiCl₄ vapor pressure, the ratio between TiCl₄ and H₂O aerosol, and the reaction conditions plays a crucial role in the formation of the core–shell morphology and increases the yield of nanostructured microspheres

Shedding Light on Aging of N‑Doped Titania Photocatalysts

A detailed analysis of nitrogen dopant behavior in nanostructured microspheres of the TiO₂ photocatalyst obtained by the thermally assisted reactions in aqueous sprays method has been performed for the first time using electron paramagnetic resonance, X-ray photoelectron spectroscopy, and UV–vis spectroscopy and is supported by theoretical simulation of possible defect structures. The nitrogen species were found to undergo the N^• to N^– transformation during sample storage under different conditions, with an activation energy of about 0.45 eV. Three main possible evolution pathways for the dopant state were identified and discussed. It was established that the most probable transformation consists of migration of an oxygen vacancy site to an interstitial nitrogen atom followed by the formation of a nonparamagnetic substitution nitrogen center. Possible diffusion routes of oxygen vacancy and corresponding energy barriers were estimated and found to be in agreement with experimental observations

Understanding the Electrolyte Background for Biochemical Sensing with Ion-Sensitive Field-Effect Transistors

Silicon nanowire field-effect transistors have attracted substantial interest for various biochemical sensing applications, yet there remains uncertainty concerning their response to changes in the supporting electrolyte concentration. In this study, we use silicon nanowires coated with highly pH-sensitive hafnium oxide (HfO₂) and aluminum oxide (Al₂O₃) to determine their response to variations in KCl concentration at several constant pH values. We observe a nonlinear sensor response as a function of ionic strength, which is independent of the pH value. Our results suggest that the signal is caused by the adsorption of anions (Cl^–) rather than cations (K⁺) on both oxide surfaces. By comparing the data to three well-established models, we have found that none of those can explain the present data set. Finally, we propose a new model which gives excellent quantitative agreement with the data

Selective Sodium Sensing with Gold-Coated Silicon Nanowire Field-Effect Transistors in a Differential Setup

Ion-sensitive field-effect transistors based on silicon nanowires with high dielectric constant gate oxide layers (<i>e.g.</i>, Al₂O₃ or HfO₂) display hydroxyl groups which are known to be sensitive to pH variations but also to other ions present in the electrolyte at high concentration. This intrinsically nonselective sensitivity of the oxide surface greatly complicates the selective sensing of ionic species other than protons. Here, we modify individual nanowires with thin gold films as a novel approach to surface functionalization for the detection of specific analytes. We demonstrate sodium ion (Na⁺) sensing by a self-assembled monolayer (SAM) of thiol-modified crown ethers in a differential measurement setup. A selective Na⁺ response of ≈−44 mV per decade in a NaCl solution is achieved and tested in the presence of protons (H⁺), potassium (K⁺), and chloride (Cl^–) ions, by measuring the difference between a nanowire with a gold surface functionalized by the SAM (active) and a nanowire with a bare gold surface (control). We find that the functional SAM does not affect the unspecific response of gold to pH and background ionic species. This represents a clear advantage of gold compared to oxide surfaces and makes it an ideal candidate for differential measurements

Solution-Processed Doping of Trilayer WSe₂ with Redox-Active Molecules

The development of processes to controllably dope two-dimensional semiconductors is critical to achieving next-generation electronic and optoelectronic devices. In this study, n- and p-doping of highly uniform large-area trilayer WSe₂ is achieved by treatment with solutions of molecular reductants and oxidants. The sign and extent of doping can be conveniently controlled by the redox potential of the (metal−)­organic molecules, the concentration of dopant solutions, and the treatment time. Threshold voltage shifts, the direction of which depends on whether a p- or n-dopant is used, and tunable channel current are observed in doped WSe₂ field-effect transistors. Detailed physical characterization including photoemission (ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy) and Raman spectroscopy provides fundamental understanding of the underlying mechanism. The origin of the doping is the electron-transfer reactions between molecular dopants and 2D semiconductors and results in a shift of the Fermi level relative to the valence band due both to state filling/emptying and to large surface dipoles between the dopant ions and the oppositely charged WSe₂. These two effects both contribute to large work function changes of up to ±1 eV

Resonant Light-Induced Heating in Hybrid Cavity-Coupled 2D Transition-Metal Dichalcogenides

Hybrid structures based on integration of two-dimensional (2D) transition-metal dichalcogenides (TMDCs) with optical resonators have recently earned significant attention. The enhanced interaction of light with 2D materials in such hybrid structures can enable devices such as efficient light-emitting diodes and lasers. However, one of the factors affecting the performance of such devices is the effect of the optically induced heat on the optoelectronic properties of the 2D materials. In this study, we systematically investigate principal roots of heat generation in hybrid cavity-coupled few-atomic-layer-thick 2D TMDC films under optical pumping. The optical resonator exploited here is a Fabry–Perot (FP) resonator, which can enhance the light–MoS₂ interaction by a significant factor of 60 at its resonance wavelength. We have combined an accurate theoretical modeling with experimental Raman spectroscopy to determine the roots of heat generation in MoS₂ films integrated with FP resonators. Our investigations reveal that the strong modulation of light absorption in the MoS₂ film, induced by excitation of an FP cavity at its resonant frequency, plays the primary role in excess heat generation in 2D materials. Furthermore, through varying the cavity length, we show that on-resonance and off-resonance excitation of the cavity results in completely different temperature profiles in the cavity-coupled MoS₂ films. Also, by changing the resonance medium of the FP cavity (SiO₂ and air), we take into account the role of the heat sinking effect of the substrate in heat generation in MoS₂ films. In this study, the temperature-dependent red-shift of the Raman spectra is employed to monitor the local temperature of the MoS₂ films. Our results show the importance of the heating effect in such hybrid structures and represent a step forward for the design of practical hybrid optical devices based on layered semiconducting 2D materials