Ultrasensitive Silicon Nanowire for Real-World Gas Sensing: Noninvasive Diagnosis of Cancer from Breath Volatolome

We report on an ultrasensitive, molecularly modified silicon nanowire field effect transistor that brings together the lock-and-key and cross-reactive sensing worlds for the diagnosis of (gastric) cancer from exhaled volatolome. The sensor is able to selectively detect volatile organic compounds (VOCs) that are linked with gastric cancer conditions in exhaled breath and to discriminate them from environmental VOCs that exist in exhaled breath samples but do not relate to the gastric cancer per se. Using breath samples collected from actual patients with gastric cancer and from volunteers who do not have cancer, blind analysis validated the ability of the reported sensor to discriminate between gastric cancer and control conditions with >85% accuracy, irrespective of important confounding factors such as tobacco consumption and gender. The reported sensing approach paves the way to use the power of silicon nanowires for simple, inexpensive, portable, and noninvasive diagnosis of cancer and other disease conditions

Molecular Gating of Silicon Nanowire Field-Effect Transistors with Nonpolar Analytes

Silicon nanowire field-effect transistors (Si NW FETs) have been used as powerful sensors for chemical and biological species. The detection of polar species has been attributed to variations in the electric field at the conduction channel due to molecular gating with polar molecules. However, the detection of nonpolar analytes with Si NW FETs has not been well understood to date. In this paper, we experimentally study the detection of nonpolar species and model the detection process based on changes in the carrier mobility, voltage threshold, off-current, off-voltage, and subthreshold swing of the Si NW FET. We attribute the detection of the nonpolar species to molecular gating, due to two <i>indirect</i> effects: (i) a change in the dielectric medium close to the Si NW surface and (ii) a change in the charged surface states at the functionality of the Si NW surface. The contribution of these two effects to the overall measured sensing signal is determined and discussed. The results provide a launching pad for real-world sensing applications, such as environmental monitoring, homeland security, food quality control, and medicine

Spray-Coating Route for Highly Aligned and Large-Scale Arrays of Nanowires

Technological implementation of nanowires (NWs) requires these components to be organized with controlled orientation and density on various substrates. Here, we report on a simple and efficient route for the deposition of highly ordered and highly aligned NW arrays on a wide range of receiver substrates, including silicon, glass, metals, and flexible plastics with controlled density. The deposition approach is based on spray-coating of a NW suspension under controlled conditions of the nozzle flow rate, droplet size of the sprayed NWs suspension, spray angle, and the temperature of the receiver substrate. The dynamics of droplet generation is understood by a combined action of shear forces and capillary forces. Provided that the size of the generated droplet is comparable to the length of the single NW, the shear-driven elongation of the droplets results presumably in the alignment of the confined NW in the spraying direction. Flattening the droplets upon their impact with the substrate yields fast immobilization of the spray-aligned NWs on the surface due to van der Waals attraction. The availability of the spray-coating technique in the current microelectronics technology would ensure immediate implementation in production lines, with minimal changes in the fabrication design and/or auxiliary tools used for this purpose

Role of Silicon Nanowire Diameter for Alkyl (Chain Lengths C₁–C₁₈) Passivation Efficiency through Si–C Bonds

The effect of silicon nanowire (Si NW) diameter on the functionalization efficiency as given by covalent Si–C bond formation is studied for two distinct examples of 25 ± 5 and 50 ± 5 nm diameters (Si NW₂₅ and Si NW₅₀, respectively). A two-step chlorination/alkylation process is used to connect alkyl chains of various lengths (C₁–C₁₈) to thinner and thicker Si NWs. The shorter the alkyl chain lengths, the larger the surface coverage of the two studied Si NWs. Increasing the alkyl chain length (C₂–C₉) changes the coverage on the NWs: while for Si NW₂₅ 90 ± 10% of all surface sites are covered with Si–C bonds, only 50 ± 10% of all surface sites are covered with Si–C bonds for the Si NW₅₀ wires. Increasing the chain length further to C₁₄–C₁₈ decreases the alkyl coverage to 36 ± 6% in thin Si NW₂₅ and to 20 ± 5% in thick Si NW₅₀. These findings can be interpreted as being a result of increased steric hindrance of Si–C bond formation for longer chain lengths and higher surface energy for the thinner Si NWs. As a direct consequence of these findings, Si NW surfaces have different stabilities against oxidation: they are more stable at higher Si–C bond coverage, and the surface stability was found to be dependent on the Si–C binding energy itself. The Si–C binding energy differs according to (C_1–9)–Si NW > (C_14–18)–Si NW, i.e., the shorter the alkyl chain, the greater the Si–C binding energy. However, the oxidation resistance of the (C_2–18)–Si NW₂₅ is lower than for equivalent Si NW₅₀ surfaces as explained and experimentally substantiated based on electronic (XPS and KP) and structure (TEM and HAADF) measurements

Unveiling the Hybrid n‑Si/PEDOT:PSS Interface

We investigated the <i>buried</i> interface between monocrystalline n-type silicon (n-Si) and the highly conductive polymer poly­(3,4-ethylenedioxythiophene)-poly­(styrenesulfonate) (PEDOT:PSS), which is successfully applied as a hole selective contact in hybrid solar cells. We show that a post-treatment of the polymer films by immersion in a suitable solvent reduces the layer thickness by removal of excess material. We prove that this post-treatment does not affect the functionality of the hybrid solar cells. Through the thin layer we are probing the chemical structure at the n-Si/PEDOT:PSS interface with synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). From the HAXPES data we conclude that the Si substrate of a freshly prepared hybrid solar cell is already oxidized immediately after preparation. Moreover, we show that even when storing the sample in inert gas such as, e.g., nitrogen the n-Si/SiO_<i>x</i>/PEDOT:PSS interface continues to further oxidize. Thus, without further surface treatment, an unstable Si suboxide will always be present at the hybrid interface

Silicon Nanowire Sensors Enable Diagnosis of Patients <i>via</i> Exhaled Breath

Two of the biggest challenges in medicine today are the need to detect diseases in a noninvasive manner and to differentiate between patients using a single diagnostic tool. The current study targets these two challenges by developing a molecularly modified silicon nanowire field effect transistor (SiNW FET) and showing its use in the detection and classification of many disease breathprints (lung cancer, gastric cancer, asthma, and chronic obstructive pulmonary disease). The fabricated SiNW FETs are characterized and optimized based on a training set that correlate their sensitivity and selectivity toward volatile organic compounds (VOCs) linked with the various disease breathprints. The best sensors obtained in the training set are then examined under real-world clinical conditions, using breath samples from 374 subjects. Analysis of the clinical samples show that the optimized SiNW FETs can detect and discriminate between almost all binary comparisons of the diseases under examination with >80% accuracy. Overall, this approach has the potential to support detection of many diseases in a direct harmless way, which can reassure patients and prevent numerous unpleasant investigations

Far-Field Imaging for Direct Visualization of Light Interferences in GaAs Nanowires

The optical and electrical characterization of nanostructures is crucial for all applications in nanophotonics. Particularly important is the knowledge of the optical near-field distribution for the design of future photonic devices. A common method to determine optical near-fields is scanning near-field optical microscopy (SNOM) which is slow and might distort the near-field. Here, we present a technique that permits sensing indirectly the infrared near-field in GaAs nanowires via its second-harmonic generated (SHG) signal utilizing a nonscanning far-field microscope. Using an incident light of 820 nm and the very short mean free path (16 nm) of the SHG signal in GaAs, we demonstrate a fast surface sensitive imaging technique without using a SNOM. We observe periodic intensity patterns in untapered and tapered GaAs nanowires that are attributed to the fundamental mode of a guided wave modulating the Mie-scattered incident light. The periodicity of the interferences permits to accurately determine the nanowires’ radii by just using optical microscopy, i.e., without requiring electron microscopy