Table of contents

An easy transfer procedure to obtain graphene-based gas sensing devices operating at room temperature (RT) is presented. Starting from chemical vapor deposition-grown graphene on copper foil, we obtained single layer graphene which could be transferred onto arbitrary substrates. In particular, we placed single layer graphene on top of a SiO₂/Si substrate with pre-patterned Pt electrodes to realize a chemiresistor gas sensor able to operate at RT. The responses to ammonia (10, 20, 30 ppm) and nitrogen dioxide (1, 2, 3 ppm) are shown at different values of relative humidity, in dark and under 254 nm UV light. In order to check the sensor selectivity, gas response has also been tested towards hydrogen, ethanol, acetone and carbon oxide. Finally, a model based on linear dispersion relation characteristic of graphene, which take into account humidity and UV light effects, has been proposed.

The mechanical stability of proteins has been extensively studied using AFM as a single-molecule force spectroscopy method. While this has led to many important results, these studies have been mainly limited to fast unfolding at a high-force regime due to the rapid mechanical drift in most AFM stretching experiments. Therefore, there is a gap between the knowledge obtained at a high-force regime and the mechanical properties of proteins at a lower force regime which is often more physiologically relevant. Recent studies have demonstrated that this gap can be addressed by stretching single protein molecules using magnetic tweezers, due to the excellent mechanical stability this technology offers. Here we review magnetic tweezers technology and its current application in studies of the force-dependent stability and interactions of proteins.

Heterostructure field-effect transistors (hetero-FETs) are experimentally demonstrated, consisting of van der Waals heterostructure channels based on a 2D semiconductor. By optimally selecting the band alignment of the heterostructure channels, different output characteristics of the hetero-FETs were achieved. In atomically thin WSe₂/MoS₂ hetero-FET with staggered energy band, the oscillating transfer characteristic and negative transconductance were realized. With near-broken-gap alignment in the MoTe₂/SnSe₂ heterostructure channel, a superior reverse-biased current was obtained in the hetero-FETs, which can be analyzed as typical tunneling current. Our study on the hetero-FET-based atomically thin van der Waals heterostructure channel, provides significant inspiration and reference to novel heterostructure FETs.

MoS₂ with layered structure and distinct physical properties has attracted attention for electronic or optoelectronic devices. The photoelectric response properties of MoS₂/ZnO heterojunctions based devices fabricated by spin-coating MoS₂ nanosheets solutions on ZnO nanorod arrays (NRs) were investigated. The results revealed that MoS₂ nanosheets were vertically aligned on the surface of ZnO NRs and the devices exhibit good photoresponse stability and reproducibility under UV and red light illuminations. The vertically aligned MoS₂ nanosheets facilitate the fast photogenerated carrier separation and transport. The devices with few-layered MoS₂ nanosheets show a high responsivity and detectivity under UV and red light illuminations, which can be attributed to small contact resistance between MoS₂ nanosheets and ZnO NRs. These results provide important insights in the facile fabrication strategy and understanding electronic and optoelectronic devices based on the heterostructures with vertically aligned MoS₂.

In this work we present the effect of low dose gallium (Ga) deposition (<4 ML) performed in UHV (10⁻⁷ Pa) on the electronic doping and charge carrier scattering in graphene grown by chemical vapor deposition. In situ graphene transport measurements performed with a graphene field-effect transistor structure show that at low Ga coverages a graphene layer tends to be strongly n-doped with an efficiency of 0.64 electrons per one Ga atom, while the further deposition and Ga cluster formation results in removing electrons from graphene (less n-doping). The experimental results are supported by the density functional theory calculations and explained as a consequence of distinct interaction between graphene and Ga atoms in case of individual atoms, layers, or clusters.

This paper describes a near-field electrospinning technique combined with heat treatment process used to directly align parallel metal oxide and metal nitride fibers on silicon dioxide substrate. The effects of near-field electrospinning parameters (including collector-to-needle distance, applied voltage and the moving speed of the collector) on the morphology of the resulted fibers have been studied. Metallic salt-contained precursor fibers are individually aligned via near-field electrospinning of metallic salts and polymeric solution mixtures. After applying calcination process to these well aligned precursor fibers, patterning by metal oxide and metal nitride fibers such as ZnO, Ga₂O₃, TiO₂, GaN and TiN is successfully obtained. The optical microscope images and the scanning electron microscopy show the presence of fiber patterns, whose crystalline structure is characterized by x-ray diffraction and Raman spectroscopy measurement. The results demonstrate the potential of this approach for assembling ceramic fibers into parallel arrays with controllable orientation and position.

The fabrication of nanopatterned multilayers, as used in optical and magnetic applications, is usually achieved by two independent steps, which consist in the preparation of multilayer films and in the successive patterning by means of lithography and etching processes. Here we show that multilayer nanostructures can be fabricated by using focused electron beam induced deposition (FEBID), which allows the direct writing of nanostructures of any desired shape with nanoscale resolution. In particular, ${[{{\rm{Co}}}_{2}{\rm{Fe}}/{\rm{Si}}]}_{n}$ multilayers are prepared by the alternating deposition from the metal carbonyl precursors, ${{\rm{HFeCo}}}_{3}{({\rm{CO}})}_{12}$ and ${\rm{Fe}}{({\rm{CO}})}_{5}$, and neopentasilane, ${{\rm{Si}}}_{5}{{\rm{H}}}_{12}$. The ability to fabricate nanopatterned multilayers by FEBID is of interest for the realization of hyperbolic metamaterials and related nanodevices. In a second experiment, we treated the multilayers by low-energy electron irradiation in order to induce atomic species intermixing with the purpose to obtain ternary nanostructured compounds. Transmission electron microscopy and electrical transport measurements indicate that in thick multilayers, (n = 12), the intermixing is only partial, taking place mainly in the upper part of the structures. However, for thin multilayers, (n = 2), the intermixing is such that a transformation into the L2₁ phase of the Co₂FeSi Heusler compound takes place over the whole sample volume.

In this work quantum dot sensitized solar cells (QDSSCs) were fabricated with CdSe and Mn-doped CdSe quantum dots (QDs) using the SILAR method. QDSSCs based on Mn-doped CdSe QDs exhibited improved incident photon-to-electron conversion efficiency. Carrier transport dynamics in the QDSSCs were studied using the intensity modulated photocurrent/photovoltage spectroscopy technique, from which transport and recombination time constants could be derived. Compared to CdSe QDSSCs, Mn–CdSe QDSSCs exhibited shorter transport time constant, longer recombination time constant, longer diffusion length, and higher charge collection efficiency. These observations suggested that Mn doping in CdSe QDs could benefit the performance of solar cells based on such nanostructures.

The advancement in nanostructured powder processing has attracted great interest as a cost effective and scalable strategy for high performance thermoelectric bulk materials. However, the level of technical breakthrough realized in quantum dot supperlattices/wires has not yet been demonstrated in these materials. Here, we report the first ever study on the uniform dispersion of single wall carbon nanotubes (SWCNTs) in nanostructured Bi₂Te₃ bulk, and their effect on thermoelectric parameters above room temperature. The Bi₂Te₃ based SWCNT composites were prepared through controlled powder processing, and their thermoelectric properties were finely tuned at the nanoscale by regulating various (0.5, 0.75, 1.0 and 1.5) vol% of SWCNTs in the matrix. The flexible ropes of SWCNT, making an interconnected network through the inter/trans granular positions of Bi₂Te₃, thus substantially change the transport properties of the composites. The perfect one-dimensional (1D) conducting structure of SWCNTs acts as a source of electrical transport through a percolating network, with significantly suppressed lattice thermal conductivity, via intensified boundary scattering. The remarkable increase in power factor is ascribed to energy filtering effects and excellent electrical transport of 1D SWCNTs in the composites. Consequently, with a considerable reduction in thermal conductivity, the figure of merit culminates in a several-fold improvement, at 0.5 vol% of SWCNTs, over pristine bulk Bi₂Te₃.

Gas sensors play a vital role among a wide range of practical applications. Recently, propelled by the development of layered materials, gas sensors have gained much progress. However, the high operation temperature has restricted their further application. Herein, via a facile pulsed laser deposition (PLD) method, we demonstrate a flexible, transparent and high-performance gas sensor made of highly-crystalline indium selenide (In₂Se₃) film. Under UV–vis-NIR light or even solar energy activation, the constructed gas sensors exhibit superior properties for detecting acetylene (C₂H₂) gas at room temperature. We attribute these properties to the photo-induced charger transfer mechanism upon C₂H₂ molecule adsorption. Moreover, no apparent degradation in the device properties is observed even after 100 bending cycles. In addition, we can also fabricate this device on rigid substrates, which is also capable to detect gas molecules at room temperature. These results unambiguously distinguish In₂Se₃ as a new candidate for future application in monitoring C₂H₂ gas at room temperature and open up new opportunities for developing next generation full-spectrum activated gas sensors.

Thanks to the growing interests of metal oxide sensors in environmental and industrial uses, this study presents the sensing mechanism of methane gas (CH₄) on recently synthesized two-dimensional form of ZnO, ZnO nano sheets (ZnO-NS). The adsorption energy of CH₄ on pristine ZnO-NS, calculated by means of van der Waals corrected first-principles calculations, is found to be insufficient restricting its application as an efficient nano sensor. However, the creation of (O/Zn) vacancies and the substitution of foreign dopants into ZnO-NS considerably intensify the binding energy of CH₄. Through a comprehensive energetic analysis, it is observed that among all the substituents, boron (B), sulphur (S) and gallium (Ga) improves the binding of CH₄ to 2.75, 6.1 and 7.5 times respectively than its values on pristine ZnO-NS. In addition to the CH₄ binding energies falling ideally between physisorption and chemisorption range, a prominent variation in the electronic properties before and after CH₄ exposure indicates the promise of substituted Zn-NS as a useful nano sensors.

The localized formation of gold nanostructures with controlled size and shape on chitosan films doped with gold precursor upon electromagnetic irradiation of various types is demonstrated here. Such controlled formation is achieved by tuning the wavelength, the energy and the interaction time of the radiation with the composite films. In particular, the use of a single UV nanosecond laser pulse results in the formation of gold sub-micron platelets with specific crystal structure, while increasing the number of pulses, further precursor reduction and photofragmentation induce the formation of gold nanoparticles. Using x-ray radiation as an alternative energy source, the reduction of the gold precursor and the subsequent formation of particles follow a different pathway. Specifically, x-ray-induced photo-reduction triggers the selective formation of gold sub-micron platelets with a very well defined {111} crystal phase. In this case, the density of crystal platelets increases by increasing the irradiation time of the films, while no photofragmentation process is observed. The gold structures pre-formed by x-ray radiation can be fragmented by subsequent pulsed UV laser irradiation forming nanoparticles with much narrower size distribution compared to that obtained via exclusive UV irradiation. Thanks to the perfect coupling between the natural polymeric matrix and gold nanostructures, the bionanocomposite systems developed could find various applications in biomaterial science and in biosensors field.

We report a simple single step growth of α-MoO₃ structures and energetically suitable site specific Ag nanoparticle (NP) decorated α-MoO₃ structures on varied substrates, having almost similar morphologies and oxygen vacancies. We elucidate possible growth mechanisms in light of experimental findings and density functional theory (DFT) calculations. We experimentally establish and verified by DFT calculations that the MoO₃(010) surface is a weakly interacting and stable surface compared to other orientations. From DFT study, the binding energy is found to be higher for (100) and (001) surfaces (∼−0.98 eV), compared to the (010) surface (∼−0.15 eV) and thus it is likely that Ag NP formation is not favorable on the MoO₃(010) surface. The Ag decorated MoO₃ (Ag-MoO₃) nanostructured sample shows enhanced field emission properties with an approimately 2.1 times lower turn-on voltage of 1.67 V μm⁻¹ and one order higher field enhancement factor (β) of 8.6 × 10⁴ compared to the MoO₃ sample without Ag incorporation. From Kelvin probe force microscopy measurements, the average local work function (Φ) is found to be approximately 0.47 eV smaller for the Ag-MoO₃ sample (∼5.70 ± 0.05 eV) compared to the MoO₃ sample (∼6.17 ± 0.05 eV) and the reduction in Φ can be attributed to the shifting Fermi level of MoO₃ toward vacuum via electron injection from Ag NPs to MoO₃. The presence of oxygen vacancies together with Ag NPs lead to the highest β and lowest turn-on field among the reported values under the MoO₃ emitter category.

The anchoring of platinum quantum dots (Pt QDs) on the surface of hematite (α-Fe₂O₃) nanorods is regarded as an efficient way to promote photoelectrochemical activity. To further improve the performance of the Pt-hematite material system, the size and location of the Pt QDs is a key factor to be considered. In this work, an α-Fe₂O₃ nanorod array film was grown on a transparent conductive FTO substrate by a facile hydrothermal method. Pt QDs with a diameter of ∼2 nm were uniformly deposited on the surface of the α-Fe₂O₃ nanorod. The dispersibility of the Pt QDs was greatly improved by regulating the surface wettability of the α-Fe₂O₃ thin film. The dependence of surface wettability on the micro-/nano-structure of the α-Fe₂O₃ array was revealed. Due to the structure regulation of the α-Fe₂O₃ nanoarray and the greatly improved dispersibility of the Pt QDs, the photocurrent of the 2.7 wt% Pt QD anchored α-Fe₂O₃ nanorod array was ten times higher than that of the pure α-Fe₂O₃ nanorod array. This work points to an efficient approach for dispersing the QDs in a nanoarray thin film by adjusting its micro-/nano-structure and surface wettability.

We present novel inorganic–organic hybrid nanoparticles (HNPs) constituting inorganic NPs, NaY_0.78Er_0.02Yb_0.2F₄, and organometallic nanofiber, Tb(ASA)₃Phen (TAP). X-ray diffraction, Fourier transform infrared absorption and transmission electron microscopy analyses reveal that prepared ultrafine upconversion NPs (UCNPs (5–8 nm)) are dispersed on the surface of the TAP nanofibers. We observe that the addition of TAP in UCNPs effectively limits the surface quenching to boost the upconversion (UC) intensity and enables tuning of UC emission from the green to the red region by controlling the phonon frequency around the Er³⁺ ion. On the other hand, TAP is an excellent source of green emission under ultraviolet exposure. Therefore prepared HNPs not only give enhanced and tunable UC but also emit a strong green color in the downshifting (DS) process. To further enhance the dual-mode emission of HNPs, silver NPs (AgNPs) are introduced. The emission intensity of UC as well as DS emission is found to be strongly modulated in the presence of AgNPs. It is found that AgNPs enhance red UC emission. The possible mechanism involved in enhanced emission intensity and color output is investigated in detail. The important optical properties of these nano-hybrid materials provide a great opportunity in the fields of biological imaging, drug delivery and energy devices.

Vertical graphene nanosheets (VGN) are the material of choice for application in next-generation electronic devices. The growing demand for VGN-based flexible devices for the electronics industry brings in restriction on VGN growth temperature. The difficulty associated with the direct growth of VGN on flexible substrates can be overcome by adopting an effective strategy of transferring the well-grown VGN onto arbitrary flexible substrates through a soft chemistry route. In the present study, we report an inexpensive and scalable technique for the polymer-free transfer of VGN onto arbitrary substrates without disrupting its morphology, structure, and properties. After transfer, the morphology, chemical structure, and electrical properties are analyzed by scanning electron microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, and four-probe resistive methods, respectively. The wetting properties are studied from the water contact angle measurements. The observed results indicate the retention of morphology, surface chemistry, structure, and electronic properties. Furthermore, the storage capacity of the transferred VGN-based binder-free and current collector-free flexible symmetric supercapacitor device is studied. A very low sheet resistance of 670 Ω/□ and excellent supercapacitance of 158 μF cm⁻² with 86% retention after 10 000 cycles show the prospect of the damage-free VGN transfer approach for the fabrication of flexible nanoelectronic devices.

Twin boundaries and boundaries between zincblende (ZB) and wurtzite (WZ) segments of GaAs-related nanowires (NWs) form intrinsic heterointerfaces with essential consequences for the application of such nanomaterials in optoelectronic devices. We show that for GaAs and GaAs/(Al, Ga)As core/shell NWs, crystal twinning along the NW axis can be imaged with a spatial resolution of 10 nm using secondary electrons in a scanning electron microscope (SEM). Changes of the crystal structure from the ZB to the WZ phase have been investigated by electron backscatter diffraction. In addition to these methods, we employ spectrally and spatially resolved cathodoluminescence measurements in the same SEM to study the correlation between the structural and optical properties in single NWs. Two GaAs/AlAs/GaAs core/shell/shell NWs differing significantly in the crystal structure along their axis have been investigated combining these three techniques in order to demonstrate the strength of the employed methodology. Our experiments show that based on commonly available SEM methods, an overview of the structural properties along an entire NW and their impact on the spectral and spatial luminescence distribution can be efficiently obtained providing a quick feedback for the optimization of growth conditions.

The performance of nanostructured semiconductors is frequently limited by interface defects that trap electronic carriers. In particular, high aspect ratio geometries dramatically increase the difficulty of using typical solid-state electrical measurements (multifrequency capacitance- and conductance-voltage testing) to quantify interface trap densities (D_it). We report on electrochemical impedance spectroscopy (EIS) to characterize the energy distribution of interface traps at metal oxide/semiconductor interfaces. This method takes advantage of liquid electrolytes, which provide conformal electrical contacts. Planar Al₂O₃/p-Si and Al₂O₃/p-Si_0.55Ge_0.45 interfaces are used to benchmark the EIS data against results obtained from standard electrical testing methods. We find that the solid state and EIS data agree very well, leading to the extraction of consistent D_it energy distributions. Measurements carried out on pyramid-nanostructured p-Si obtained by KOH etching followed by deposition of a 10 nm ALD-Al₂O₃ demonstrate the application of EIS to trap characterization of a nanostructured dielectric/semiconductor interface. These results show the promise of this methodology to measure interface state densities for a broad range of semiconductor nanostructures such as nanowires, nanofins, and porous structures.

Based on molecular dynamics simulations, tensile mechanical properties and plastic deformation mechanisms of nano-twinned Cu//Ag multilayered materials are investigated in this work. Simulation results show that, due to the stronger strengthening effect of the twin boundary than both the cube-on-cube and hetero-twin interfaces between Cu and Ag layers, the strength increases with the increase of layer thickness for nano-twinned Cu//Ag multilayered materials with a constant twin spacing, while it decreases with the increase of layer thickness for twin-free ones. The strength of hetero-twin multilayered materials is higher than that of the cube-on-cube samples due to the different hetero interfacial configurations. The confined layer slip of dislocation is found to be the dominant plastic deformation mechanism for twin-free hetero-twin multilayered materials and the strength versus twin spacing in nano-twinned samples follows the conventional Hall–Petch relationship. These findings will shed light on the understanding of the plastic deformation mechanisms and the fabrication of high strength nano-twinned multilayered metallic materials.

We report experimental and theoretical investigations of the observed barrier behavior of few-layer MoS₂ against nitrogenation. Owing to its low-strength shearing, low friction coefficient, and high lubricity, MoS₂ exhibits the demeanor of a natural N-resistant coating material. Raman spectroscopy is done to determine the coating capability of MoS₂ on graphene. Surface morphology of our MoS₂/graphene heterostructure is characterized by using optical microscopy, scanning electron microscopy, and atomic force microscopy. In addition, density functional theory-based calculations are performed to understand the energy barrier performance of MoS₂ against nitrogenation. The penetration of nitrogen atoms through a defect-free MoS₂ layer is prevented by a very high vertical diffusion barrier, indicating that MoS₂ can serve as a protective layer for the nitrogenation of graphene. Our experimental and theoretical results show that MoS₂ material can be used both as an efficient nanocoating material and as a nanoscale mask for selective nitrogenation of graphene layer.

Metal enhanced ultraviolet light emission has been explored in ZnO/Ag hybrid structures prepared by hydrothermal growth of multi-angled ZnO nanorods on slanted Ag nanorods array fabricated by the thermal evaporation technique. Slanted Ag nanorods are realized to be the stacking of non-spherical Ag nanoparticles, resulting in asymmetric surface plasmon resonance spectra. The surface roughness of Ag nanorod array films significantly influences the growth mechanism of ZnO nanorods, leading to the formation of multi-angled ZnO microflowers. ZnO/Ag hybrid structures facilitate the interfacial charge transfer from Ag to ZnO with the realization of negative shift in binding energy of Ag 3d orbitals by ∼0.8 eV. These high quality ZnO nanorods in ZnO/Ag hybrid nanostructures exhibit strong ultraviolet emission in the 383–396 nm region without broad deep level emission, which can be explained by a suitable band diagram. The metal enhanced photoluminescence is witnessed mainly due to interfacial charge transfer with its dependence on surface roughness of bottom layer Ag nanorods, number density of ZnO nanorods and diversity in the interfacial area between Ag and ZnO nanorods. The existence of strong ultraviolet light with minor blue light emission and appearance of CIE shade in strong violet-blue region by ZnO/Ag hybrid structures depict exciting possibilities towards near UV-blue light emitting devices.