Silicon Nanowire Field-Effect Transistor as Label-Free Detection of Hepatitis B Virus Proteins with Opposite Net Charges.

The prevalence of hepatitis B virus (HBV) is a global healthcare threat, particularly chronic hepatitis B (CHB) that might lead to hepatocellular carcinoma (HCC) should not be neglected. Although many types of HBV diagnosis detection methods are available, some technical challenges, such as the high cost or lack of practical feasibility, need to be overcome. In this study, the polycrystalline silicon nanowire field-effect transistors (pSiNWFETs) were fabricated through commercial process technology and then chemically functionalized for sensing hepatitis B virus surface antigen (HBsAg) and hepatitis B virus X protein (HBx) at the femto-molar level. These two proteins have been suggested to be related to the HCC development, while the former is also the hallmark for HBV diagnosis, and the latter is an RNA-binding protein. Interestingly, these two proteins carried opposite net charges, which could serve as complementary candidates for evaluating the charge-based sensing mechanism in the pSiNWFET. The measurements on the threshold voltage shifts of pSiNWFETs showed a consistent correspondence to the polarity of the charges on the proteins studied. We believe that this report can pave the way towards developing an approachable tool for biomedical applications.

Single-Molecule Ex Situ Atomic Force Microscopy Allows Detection of Individual Antibody-Antigen Interactions on a Semiconductor Chip Surface.

Although in situ atomic force microscopy (AFM) allows single-molecule detection of antibody-antigen binding, the practical applications of in situ AFM for disease diagnosis are greatly limited, due to its operational complexity and long operational times, including the execution time for the surface chemical/biological treatments in the equipped glass liquid cell. Herein, a method of graphically superimposed alignment that enables ex situ AFM analysis of an immobilized antibody at the same location on a semiconductor chip surface before and after incubation with its antigen is presented. All of the required chemical/biological treatments are executed feasibly using standard laboratory containers, allowing single-molecule ex situ AFM detection to be conducted with great practicality, flexibility, and versatility. As an example, the analysis of hepatitis B virus X protein (HBx) and its IgG antibody is described. Using ex situ AFM, individual information on the topographical characteristics of the immobilized single and aggregated IgG antibodies on the chip surface is extracted and the data are analyzed statistically. Furthermore, in a statistical manner, the changes in AFM-measured heights of the individual and aggregated IgG antibodies that occur as a result of changes in conformation upon formation of IgG-HBx complexes are investigated.

Probing the photovoltaic properties of Ga-doped CdS-Cu2S core-shell heterostructured nanowire devices.

In this study Ga-doped cadmium sulfide (CdS) nanowires (NWs) were grown through chemical vapor deposition. The carrier conductivities of the CdS NWs improved after the incorporation of Ga; moreover, the conductivities of the CdS NWs increased upon increasing the amount of the Ga source. Using a cation exchange method, these CdS NWs served as the source material for the preparation of Cu2S-CdS p-n heterostructured NWs. The short-circuit current, open-circuit voltage, fill factor, and power conversion efficiency of the best-performing Cu2S-CdS NW photovoltaic device were 0.152 nA, 0.245 V, 44.5%, and 0.405%, respectively, when illuminated under AM 1.5 solar light. This study demonstrates the possibility of modulating not only the properties of CdS NWs through the incorporation of dopant Ga atoms but also the photovoltaic properties of Cu2S-CdS p-n heterostructured NW devices, paving the way for the exploitation of nanostructures within optoelectronics.

Time-evolution of the electrical characteristics of MoS2 field-effect transistors after electron beam irradiation.

As the feature sizes of devices decrease to the nanoscale, electron microscopy and lithography will become increasingly essential techniques for fabrication and inspection. In this study, we probed the memory effects of MoS2 field-effect transistors (FETs) subjected to electron beam (e-beam) irradiation; after fabricating the devices on 300 nm SiO2/Si substrates, we irradiated the MoS2 FETs with various doses of irradiation from a 30 kV e-beam. The threshold voltage shifted to the negative side and the mobility increased-a so-called memory effect-upon increasing the e-beam dose. These changes resulted from positively charged oxide traps, formed upon e-beam irradiation, in the gate oxide layer. Interestingly, the electrical characteristics of the MoS2 FETs after e-beam irradiation continued to change upon aging: the threshold voltage shifted toward the positive side and the mobility decreased, suggesting that the dominant mechanism changed from the presence of positively charged oxide traps to the presence of negatively charged interface traps. Notably, the threshold voltage shifts of the MoS2 FETs could be retained for one or two days. This behavior should be useful for preparing property-adjustable nanodevices, with particular potential for applications in multi-level memory devices.

Low-temperature-grown p-n ZnO nanojunction arrays as rapid and self-driven UV photodetectors.

In this study p-type ZnO nanorod (NR) arrays were grown using a low-temperature hydrothermal method in the presence of various concentrations of Sb in the doping solution. X-ray photoelectron spectroscopy revealed the atomic percentages and chemical states of the Sb dopant atoms in the p-type ZnO NR arrays. Photoluminescence and electrical measurements confirmed the p-type characteristics of the Sb-doped ZnO NR arrays. Sequential growth of n- and p-ZnO was then implemented to form p-n ZnO nanojunction arrays. The photovoltaic properties of the p-n ZnO nanojunction devices were investigated under 365 nm UV light; the short-circuit current densities and open-circuit voltages exhibited linear and logarithmic dependence, respectively, on the power density of the UV light. In addition, the p-n ZnO nanojunction devices displayed a rapid response to UV light at zero bias, with a linear correlation between the responsivity and the incident light power. Such low-temperature growth of p-n ZnO nanojunctions appears to be a facile strategy for fabricating junctioned nanostructures with applications in energy-harvesting and self-driven photodetecting optoelectronics.

Ionic screening effect on low-frequency drain current fluctuations in liquid-gated nanowire FETs.

The ionic screening effect plays an important role in determining the fundamental surface properties within liquid-semiconductor interfaces. In this study, we investigated the characteristics of low-frequency drain current noise in liquid-gated nanowire (NW) field effect transistors (FETs) to obtain physical insight into the effect of ionic screening on low-frequency current fluctuation. When the NW FET was operated close to the gate voltage corresponding to the maximum transconductance, the magnitude of the low-frequency noise for the NW exposed to a low-ionic-strength buffer (0.001 M) was approximately 70% greater than that when exposed to a high-ionic-strength buffer (0.1 M). We propose a noise model, considering the charge coupling efficiency associated with the screening competition between the electrolyte buffer and the NW, to describe the ionic screening effect on the low-frequency drain current noise in liquid-gated NW FET systems. This report not only provides a physical understanding of the ionic screening effect behind the low-frequency current noise in liquid-gated FETs but also offers useful information for developing the technology of NW FETs with liquid-gated architectures for application in bioelectronics, nanosensors, and hybrid nanoelectronics.

Quantifying the barrier lowering of ZnO Schottky nanodevices under UV light.

In this study we measured the degrees to which the Schottky barrier heights (SBHs) are lowered in ZnO nanowire (NW) devices under illumination with UV light. We measured the I-V characteristics of ZnO nanowire devices to confirm that ZnO is an n-type semiconductor and that the on/off ratio is approximately 10(4). From temperature-dependent I-V measurements we obtained a SBH of 0.661 eV for a ZnO NW Schottky device in the dark. The photosensitivity of Schottky devices under UV illumination at a power density of 3 µW/cm(2) was 9186%. Variations in the SBH account for the superior characteristics of n-type Schottky devices under illumination with UV light. The SBH variations were due to the coupled mechanism of adsorption and desorption of O2 and the increase in the carrier density. Furthermore, through temperature-dependent I-V measurements, we determined the SBHs in the dark and under illumination with UV light at power densities of 0.5, 1, 2, and 3 µW/cm(2) to be 0.661, 0.216, 0.178, 0.125, and 0.068 eV, respectively. These findings should be applicable in the design of highly sensitive nanoscale optoelectronic devices.

Selective growth of ZnO nanorods on hydrophobic Si nanorod arrays.

In this paper we describe the selective growth of ZnO nanorods (NRs) on top of hydrophobic Si NR arrays. The periodic Si NR arrays, prepared through electroless chemical etching and HF treatment, functioned as hydrophobic substrates. Droplets containing ZnO seeds could be positioned on the Si NR arrays, causing the ZnO seeds to deposit selectively upon them, with n-ZnO NR/p-Si NR array heterojunctions ultimately forming after hydrothermal growth of ZnO NRs. Because of compensation for the difference in refractive index between air and the Si substrate, the n-ZnO NR/p-Si NR arrays exhibited excellent absorption ability in the visible range. Devices based on these n-ZnO NR/p-Si NR array heterojunctions displayed not only rectifying behavior but also photovoltaic effects when illuminated with UV light. The low temperature and low cost of this fabrication process suggest that the selective growth of n-ZnO NRs on p-Si NR arrays might allow such structures to have diverse applications in optoelectronics.

Low-frequency electrical fluctuations in metal-nanowire-metal phototransistors.

Using low-frequency noise spectroscopy to explore the physical origins of electrical fluctuations in ZnO nanowire (NW) phototransistors featuring a metal-NW-metal configuration, we have found that bulk mobility scatterings gave rise to electrical fluctuations in the low-gate voltage (V G) regime, providing values of Hooge's constant in the ranges 6.0-9.6 × 10(-3) and 1.9-2.2 × 10(-1) in the dark and under UV excitation, respectively. When moving into the higher V G regime, we assign the electrical fluctuations to an interaction process involving trapping and detrapping of channel carriers by charge traps located near the NW-dielectric interface, suggesting that the mechanism of the electrical fluctuation transitioned from bulk NW-dominated to NW/dielectric interface-dominated regimes. We have also addressed the effective density of interface traps responsible for the electrical fluctuations in the high-V G region. This report provides physical insight into the origins of electrical fluctuations in NW phototransistors.

Measurement of in-plane displacement by wavelength-modulated heterodyne speckle interferometry.

The use of wavelength-modulated light incorporated into an optical-path-difference speckle interferometer is demonstrated as a heterodyne technique for measuring the in-plane displacement of a rough object. The in-plane displacement can be determined from the measured phase variation of the heterodyne speckle signal. We also improved the optical configuration to create a high-contrast interference pattern. Experimental results reveal that the proposed method can detect displacement up to a long range of 220 µm and displacement variation down to the nanometer range. Moreover, the sensitivity can reach up to 0.8°/nm. The performance of the system is discussed.

Near UV LEDs made with in situ doped p-n homojunction ZnO nanowire arrays.

Catalyst-free p-n homojunction ZnO nanowire (NW) arrays in which the phosphorus (P) and zinc (Zn) served as p- and n-type dopants, respectively, have been synthesized for the first time by a controlled in situ doping process for fabricating efficient ultraviolet light-emitting devices. The doping transition region defined as the width for P atoms gradually occupying Zn sites along the growth direction can be narrowed down to sub-50 nm. The cathodoluminescence emission peak at 340 nm emitted from n-type ZnO:Zn NW arrays is likely due to the Burstein-Moss effect in the high electron carrier concentration regime. Further, the electroluminescence spectra from the p-n ZnO NW arrays distinctively exhibit the short-wavelength emission at 342 nm and the blue shift from 342 to 325 nm is observed as the operating voltage further increasing. The ZnO NW p-n homojunctions comprising p-type segment with high electron concentration are promising building blocks for short-wavelength lighting device and photoelectronics.

Probing the sensitivity of nanowire-based biosensors using liquid-gating.

We have used liquid-gating to investigate the sensitivity of nanowire (NW)-based biosensors for application in the field of ultrasensitive biodetection. We developed an equivalent capacitance model of the biosensor system to explore the dependence of the sensitivity on the liquid-gate voltage (V(lg)), which was influenced by capacitive competition between the NW capacitance and the thin oxide capacitance. NW biosensors with highest sensitivity were obtained when we operated the device in the subthreshold regime while applying an appropriate value of V(lg); the influence of leakage paths through the ionic solutions led, however, to significant sensitivity degradation and narrowed the operating window in the subthreshold regime.

Piezoelectric nanogenerator using p-type ZnO nanowire arrays.

Using phosphorus-doped ZnO nanowire (NW) arrays grown on silicon substrate, energy conversion using the p-type ZnO NWs has been demonstrated for the first time. The p-type ZnO NWs produce positive output voltage pulses when scanned by a conductive atomic force microscope (AFM) in contact mode. The output voltage pulse is generated when the tip contacts the stretched side (positive piezoelectric potential side) of the NW. In contrast, the n-type ZnO NW produces negative output voltage when scanned by the AFM tip, and the output voltage pulse is generated when the tip contacts the compressed side (negative potential side) of the NW. In reference to theoretical simulation, these experimentally observed phenomena have been systematically explained based on the mechanism proposed for a nanogenerator.

ZnO-ZnS heterojunction and ZnS nanowire arrays for electricity generation.

Vertically aligned ZnO-ZnS heterojunction nanowire (NW) arrays were synthesized by thermal evaporation in a tube furnace under controlled conditions. Both ZnO and ZnS are of wurtzite structure, and the axial heterojunctions are formed by epitaxial growth of ZnO on ZnS with an orientation relationship of [0001](ZnO)//[0001](ZnS). Vertical ZnS NW arrays have been obtained by selectively etching ZnO-ZnS NW arrays. Cathodoluminescence measurements of ZnO-ZnS NW arrays and ZnS NW arrays show emissions at 509 and 547 nm, respectively. Both types of aligned NW arrays have been applied to convert mechanical energy into electricity when they are deflected by a conductive AFM tip in contact mode. The received results are explained by the mechanism proposed for nanogenerator.