Selective Sensing of Volatile Organic Compounds Using an Electrostatically Formed Nanowire Sensor Based on Automatic Machine Learning.

With the development of Internet of Things technology, various sensors are under intense development. Electrostatically formed nanowire (EFN) gas sensors are multigate Si sensors based on CMOS technology and have the unique advantages of ultralow power consumption and very large-scale integration (VLSI) compatibility for mass production. In order to achieve selectivity, machine learning is required to accurately identify the detected gas. In this work, we introduce automatic learning technology, by which the common algorithms are sorted and applied to the EFN gas sensor. The advantages and disadvantages of the top four tree-based model algorithms are discussed, and the unilateral training models are ensembled to further improve the accuracy of the algorithm. The analyses of two groups of experiments show that the CatBoost algorithm has the highest evaluation index. In addition, the feature importance of the classification is analyzed from the physical meaning of electrostatically formed nanowire dimensions, paving the way for model fusion and mechanism exploration.

Schottky Barrier Height and Image Force Lowering in Monolayer MoS2 Field Effect Transistors.

Understanding the nature of the barrier height in a two-dimensional semiconductor/metal interface is an important step for embedding layered materials in future electronic devices. We present direct measurement of the Schottky barrier height and its lowering in the transition metal dichalcogenide (TMD)/metal interface of a field effect transistor. It is found that the barrier height at the gold/ single-layer molybdenum disulfide (MoS2) interfaces decreases with increasing drain voltage, and this lowering reaches 0.5-1 V We also show that increase of the gate voltage induces additional barrier lowering.

Gap state distribution and Fermi level pinning in monolayer to multilayer MoS2 field effect transistors.

Gap states and Fermi level pinning play an important role in all semiconductor devices, but even more in transition metal dichalcogenide-based devices due to their high surface to volume ratio and the absence of intralayer dangling bonds. Here, we measure Fermi level pinning using Kelvin probe force microscopy, extract the corresponding electronic state distribution within the band gap, and present a systematic comparison between the gap state distribution obtained for exfoliated single layer, bilayer and thick MoS2 FET samples. It is found that the gap state distribution in all cases decreases from the conduction band edge and is in the order of 1019 eV-1 cm-3 and slightly decreases with increasing channel thickness. Strong Fermi level pinning is observed near the conduction band edge, and it decreases as it approaches the middle and lower part of the bandgap.

Accurate Method To Determine the Mobility of Transition-Metal Dichalcogenides with Incomplete Gate Screening.

van der Waals layered transition-metal dichalcogenides usually exhibit high contact resistance because of the induced Schottky barriers, which occur at nonideal metal-semiconductor contacts. These barriers usually contribute to an underestimation in the determination of mobility, when extracted by standard, two-terminal methods. Furthermore, in devices based on atomically thin materials, channels with thicknesses of up to a few layers cannot completely screen the applied gate bias, resulting in an incomplete potential drop over the channel; the resulting decreased field effect causes further underestimation of the mobility. We demonstrate a method based on Kelvin probe force microscopy, which allows us to extract the accurate semiconductor mobility and eliminates the effects of contact quality and/or screening ability. Our results reveal up to a sevenfold increase in mobility in a monolayer device.

Pinch-Off Formation in Monolayer and Multilayers MoS2 Field-Effect Transistors.

The discovery of layered materials, including transition metal dichalcogenides (TMD), gives rise to a variety of novel nanoelectronic devices, including fast switching field-effect transistors (FET), assembled heterostructures, flexible electronics, etc. Molybdenum disulfide (MoS2), a transition metal dichalcogenides semiconductor, is considered an auspicious candidate for the post-silicon era due to its outstanding chemical and thermal stability. We present a Kelvin probe force microscopy (KPFM) study of a MoS2 FET device, showing direct evidence for pinch-off formation in the channel by in situ monitoring of the electrostatic potential distribution along the conducting channel of the transistor. In addition, we present a systematic comparison between a monolayer MoS2 FET and a few-layer MoS2 FET regarding gating effects, electric field distribution, depletion region, and pinch-off formation in such devices.

Electrostatic Selectivity of Volatile Organic Compounds Using Electrostatically Formed Nanowire Sensor.

For the past several decades, there is growing demand for the development of low-power gas sensing technology for the selective detection of volatile organic compounds (VOCs), important for monitoring safety, pollution, and healthcare. Here we report the selective detection of homologous alcohols and different functional groups containing VOCs using the electrostatically formed nanowire (EFN) sensor without any surface modification of the device. Selectivity toward specific VOC is achieved by training machine-learning based classifiers using the calculated changes in the threshold voltage and the drain-source on current, obtained from systematically controlled biasing of the surrounding gates (junction and back gates) of the field-effect transistors (FET). This work paves the way for a Si complementary metal-oxide-semiconductor (CMOS)-based FET device as an electrostatically selective sensor suitable for mass production and low-power sensing technology.

Control of the Intrinsic Sensor Response to Volatile Organic Compounds with Fringing Electric Fields.

The ability to control surface-analyte interaction allows tailoring chemical sensor sensitivity to specific target molecules. By adjusting the bias of the shallow p-n junctions in the electrostatically formed nanowire (EFN) chemical sensor, a multiple gate transistor with an exposed top dielectric layer allows tuning of the fringing electric field strength (from 0.5 × 107 to 2.5 × 107 V/m) above the EFN surface. Herein, we report that the magnitude and distribution of this fringing electric field correlate with the intrinsic sensor response to volatile organic compounds. The local variations of the surface electric field influence the analyte-surface interaction affecting the work function of the sensor surface, assessed by Kelvin probe force microscopy on the nanometer scale. We show that the sensitivity to fixed vapor analyte concentrations can be nullified and even reversed by varying the fringing field strength, and demonstrate selectivity between ethanol and n-butylamine at room temperature using a single transistor without any extrinsic chemical modification of the exposed SiO2 surface. The results imply an electric-field-controlled analyte reaction with a dielectric surface extremely compelling for sensitivity and selectivity enhancement in chemical sensors.

Impact of Dopant Compensation on Graded p-n Junctions in Si Nanowires.

The modulation between different doping species required to produce a diode in VLS-grown nanowires (NWs) yields a complex doping profile, both axially and radially, and a gradual junction at the interface. We present a detailed analysis of the dopant distribution around the junction. By combining surface potential measurements, performed by KPFM, with finite element simulations, we show that the highly doped (5 × 10(19) cm(-3)) shell surrounding the NW can screen the junction's built in voltage at shell thickness as low as 3 nm. By comparing NWs with high and low doping contrast at the junction, we show that dopant compensation dramatically decreases the electrostatic width of the junction and results in relatively low leakage currents.

Potential barrier height at the grain boundaries of a poly-silicon nanowire.

We present measurements of the potential barrier height and its dependence on grain size in poly-silicon nanowire (P-SiNW) arrays. Measurements conducted using Kelvin probe force microscopy coupled with electrostatic simulations, enabled us also to extract the density of the grain boundary interface states and their energy distribution. In addition it was shown that the barrier height scales with the grain size as the square of the grain radius.

Electrostatic Limit of Detection of Nanowire-Based Sensors.

Scanning gate microscopy is used to determine the electrostatic limit of detection (LOD) of a nanowire (NW) based chemical sensor with a precision of sub-elementary charge. The presented method is validated with an electrostatically formed NW whose active area and shape are tunable by biasing a multiple gate field-effect transistor (FET). By using the tip of an atomic force microscope (AFM) as a local top gate, the field effect of adsorbed molecules is emulated. The tip induced charge is quantified with an analytical electrostatic model and it is shown that the NW sensor is sensitive to about an elementary charge and that the measurements with the AFM tip are in agreement with sensing of ethanol vapor. This method is applicable to any FET-based chemical and biological sensor, provides a means to predict the absolute sensor performance limit, and suggests a standardized way to compare LODs and sensitivities of various sensors.

The electronic structure of metal oxide/organo metal halide perovskite junctions in perovskite based solar cells.

Cross-sections of a hole-conductor-free CH3NH3PbI3 perovskite solar cell were characterized with Kelvin probe force microscopy. A depletion region width of about 45 nm was determined from the measured potential profiles at the interface between CH3NH3PbI3 and nanocrystalline TiO2, whereas a negligible depletion was measured at the CH3NH3PbI3/Al2O3 interface. A complete solar cell can be realized with the CH3NH3PbI3 that functions both as light harvester and hole conductor in combination with a metal oxide. The band diagrams were estimated from the measured potential profile at the interfaces, and are critical findings for a better understanding and further improvement of perovskite based solar cells.

Room temperature observation of quantum confinement in single InAs nanowires.

Quantized conductance in nanowires can be observed at low temperature in transport measurements; however, the observation of sub-bands at room temperature is challenging due to temperature broadening. So far, conduction band splitting at room temperature has not been observed in III-V nanowires mainly due to the small energetic separations between the sub-bands. We report on the measurement of conduction sub-bands at room temperature, in single InAs nanowires, using Kelvin probe force microscopy. This method does not rely on charge transport but rather on measurement of the nanowire Fermi level position as carriers are injected into a single nanowire transistor. As there is no charge transport, electron scattering is no longer an issue, allowing the observation of the sub-bands at room temperature. We measure the energy of the sub-bands in nanowires with two different diameters, and obtain excellent agreement with theoretical calculations based on an empirical tight-binding model.

Evidence for deep acceptor centers in plant photosystem I crystals.

Dry micrometer-thick crystalline photosystem I (PSI) has been shown to generate unprecedented large photovoltage under illumination. We use variable-temperature Kelvin probe force microscopy to show that deep acceptor centers are responsible for this anomalous photovoltage. We assumed that these centers are located close to the positively charged F(B)(2+) clusters, forming a coupled center that effectively captures the photoexcited electron into a deep state. We extract the main inherent parameters of the deep centers, which are extremely important in the potential use of photosynthetic proteins in various optoelectronic devices.

Density and energy distribution of interface states in the grain boundaries of polysilicon nanowire.

Wafer-scale fabrication of semiconductor nanowire devices is readily facilitated by lithography-based top-down fabrication of polysilicon nanowire (P-SiNW) arrays. However, free carrier trapping at the grain boundaries of polycrystalline materials drastically changes their properties. We present here transport measurements of P-SiNW array devices coupled with Kelvin probe force microscopy at different applied biases. By fitting the measured P-SiNW surface potential using electrostatic simulations, we extract the longitudinal dopant distribution along the nanowires as well as the density of grain boundaries interface states and their energy distribution within the band gap.

Parallel p-n junctions across nanowires by one-step ex situ doping.

The bottom-up synthesis of nanoscale building blocks is a versatile approach for the formation of a vast array of materials with controlled structures and compositions. This approach is one of the main driving forces for the immense progress in materials science and nanotechnology witnessed over the past few decades. Despite the overwhelming advances in the bottom-up synthesis of nanoscale building blocks and the fine control of accessible compositions and structures, certain aspects are still lacking. In particular, the transformation of symmetric nanostructures to asymmetric nanostructures by highly controlled processes while preserving the modified structural orientation still poses a significant challenge. We present a one-step ex situ doping process for the transformation of undoped silicon nanowires (i-Si NWs) to p-type/n-type (p-n) parallel p-n junction configuration across NWs. The vertical p-n junctions were measured by scanning tunneling microscopy (STM) in concert with scanning tunneling spectroscopy (STS), termed STM/S, to obtain the spatial electronic properties of the junction formed across the NWs. Additionally, the parallel p-n junction configuration was characterized by off-axis electron holography in a transmission electron microscope to provide an independent verification of junction formation. The doping process was simulated to elucidate the doping mechanisms involved in the one-step p-i-n junction formation.

Why lead methylammonium tri-iodide perovskite-based solar cells require a mesoporous electron transporting scaffold (but not necessarily a hole conductor).

CH3NH3PbI3-based solar cells were characterized with electron beam-induced current (EBIC) and compared to CH3NH3PbI(3-x)Clx ones. A spatial map of charge separation efficiency in working cells shows p-i-n structures for both thin film cells. Effective diffusion lengths, LD, (from EBIC profile) show that holes are extracted significantly more efficiently than electrons in CH3NH3PbI3, explaining why CH3NH3PbI3-based cells require mesoporous electron conductors, while CH3NH3PbI(3-Clx ones, where LD values are comparable for both charge types, do not.

Barrier height measurement of metal contacts to Si nanowires using internal photoemission of hot carriers.

Barrier heights between metal contacts and silicon nanowires were measured using spectrally resolved scanning photocurrent microscopy (SPCM). Illumination of the metal-semiconductor junction with sub-bandgap photons generates a photocurrent dominated by internal photoemission of hot electrons. Analysis of the dependence of photocurrent yield on photon energy enables quantitative extraction of the barrier height. Enhanced doping near the nanowire surface, mapped quantitatively with atom probe tomography, results in a lowering of the effective barrier height. Occupied interface states produce an additional lowering that depends strongly on diameter. The doping and diameter dependencies are explained quantitatively with finite element modeling. The combined tomography, electrical characterization, and numerical modeling approach represents a significant advance in the quantitative analysis of transport mechanisms at nanoscale interfaces that can be extended to other nanoscale devices and heterostructures.

Kelvin probe force microscopy of nanocrystalline TiO2 photoelectrodes.

Dye-sensitized solar cells (DSCs) provide a promising third-generation photovoltaic concept based on the spectral sensitization of a wide-bandgap metal oxide. Although the nanocrystalline TiO2 photoelectrode of a DSC consists of sintered nanoparticles, there are few studies on the nanoscale properties. We focus on the microscopic work function and surface photovoltage (SPV) determination of TiO2 photoelectrodes using Kelvin probe force microscopy in combination with a tunable illumination system. A comparison of the surface potentials for TiO2 photoelectrodes sensitized with two different dyes, i.e., the standard dye N719 and a copper(I) bis(imine) complex, reveals an inverse orientation of the surface dipole. A higher surface potential was determined for an N719 photoelectrode. The surface potential increase due to the surface dipole correlates with a higher DSC performance. Concluding from this, microscopic surface potential variations, attributed to the complex nanostructure of the photoelectrode, influence the DSC performance. For both bare and sensitized TiO2 photoelectrodes, the measurements reveal microscopic inhomogeneities of more than 100 mV in the work function and show recombination time differences at different locations. The bandgap of 3.2 eV, determined by SPV spectroscopy, remained constant throughout the TiO2 layer. The effect of the built-in potential on the DSC performance at the TiO2/SnO2:F interface, investigated on a nanometer scale by KPFM measurements under visible light illumination, has not been resolved so far.

Spatially resolved correlation of active and total doping concentrations in VLS grown nanowires.

Controlling axial and radial dopant profiles in nanowires is of utmost importance for NW-based devices, as the formation of tightly controlled electrical junctions is crucial for optimization of device performance. Recently, inhomogeneous dopant profiles have been observed in vapor­liquid­solid grown nanowires, but the underlying mechanisms that produce these inhomogeneities have not been completely characterized. In this work, P-doping profiles of axially modulation-doped Si nanowires were studied using nanoprobe scanning Auger microscopy and Kelvin probe force microscopy in order to distinguish between vapor­liquid­solid doping and the vapor­solid doping. We find that both mechanisms result in radially inhomogeneous doping, specifically, a lightly doped core surrounded by a heavily doped shell structure. Careful design of dopant modulation enables the contributions of the two mechanisms to be distinguished, revealing a surprisingly strong reservoir effect that significantly broadens the axial doping junctions.

Built-in potential and charge distribution within single heterostructured nanorods measured by scanning Kelvin probe microscopy.

The electrostatic potential distribution across single, isolated, colloidal heterostructured nanorods (NRs) with component materials expected to form a p-n junction within each NR has been measured using scanning Kelvin probe microscopy (SKPM). We compare CdS to bicomponent CdS-CdSe, CdS-PbSe, and CdS-PbS NRs prepared via different synthetic approaches to corroborate the SKPM assignments. The CdS-PbS NRs show a sharp contrast in measured potential across the material interface. We find the measured built-in potential within an individual NR to be attenuated by long-range electrostatic forces between the sample substrate, cantilever, and the measuring tip. Surface potential images were deconvoluted to yield built-in potentials ranging from 375 to 510 meV in the heterostructured NRs. We deduce the overall built-in potential as well as the charge distribution across each segment of the heterostructured NRs by combining SKPM data with simulations of the system.