Table of contents

We demonstrate large rectification ratios ($\gt 100$) in single-molecule junctions based on a metal-oxide cluster (polyoxometalate), using a scanning tunneling microscope (STM) both at ambient conditions and at low temperature. These rectification ratios are the largest ever observed in a single-molecule junction, and in addition these junctions sustain current densities larger than 10⁵ A cm⁻². By following the variation of the I–V characteristics with tip–molecule separation we demonstrate unambiguously that rectification is due to asymmetric coupling to the electrodes of a molecule with an asymmetric level structure. This mechanism can be implemented in other type of molecular junctions using both organic and inorganic molecules and provides a simple strategy for the rational design of molecular diodes.

The cardiac ECM has a unique 3D structure responsible for tissue morphogenesis and strong contractions. It is divided into three fiber groups with specific roles and distinct dimensions; nanoscale endomysial fibers, perimysial fibers with a diameter of 1 μm, and epimysial fibers, which have a diameter of several micrometers. We report here on our work, where distinct 3D fibrous scaffolds, each of them recapitulating the dimension scales of a single fiber population in the heart matrix, were fabricated. We have assessed the mechanical properties of these scaffolds and the contribution of each fiber population to cardiomyocyte morphogenesis, tissue assembly and function. Our results show that the nanoscale fiber scaffolds were more elastic than the microscale scaffolds, however, cardiomyocytes cultured on microscale fiber scaffolds exhibited enhanced spreading and elongation, both on the single cell and on the engineered tissue levels. In addition, lower fibroblast proliferation rates were observed on these microscale topographies. Based on the collected data we have fabricated composite scaffolds containing micro and nanoscale fibers, promoting superior tissue morphogenesis without compromising tissue contraction. Cardiac tissues, engineered within these composite scaffolds exhibited superior function, including lower excitation threshold and stronger contraction forces than tissue engineered within the single-population fiber scaffolds.

By employing united atom molecular dynamics simulation, we have investigated the effects of polymer–graphene interaction ${\varepsilon }_{np},$ volume fraction of graphene $\phi ,$ thermodynamics of polymer matrix (rubbery versus glassy), interfacial interaction in the case of the same dispersion state, shape of nanoparticles (NPs) such as ${{\rm{C}}}_{60},$ CNT and graphene at the same loading on the toughening efficiency of polymer nanocomposites. By beginning with the pure polymer, we observe that a plateau stress occurs at long chain length because entangled polymer chains in fibrils cannot become broken. We find that the work needed to dissipate during the failure increases with the increase of ${\varepsilon }_{np}$ and $\phi ,$ and the yield point in the stress–strain behavior occurs at a smaller strain for an attractive NPs filled system compared to the pure case, attributed to the more mechanically heterogeneous environment. The thermodynamics of the polymer matrix (below and above T_g) seems to have a significant effect on the toughening efficiency of graphene sheets. In the case of the same dispersion state, stronger interfacial interaction always induces long and highly orientated polymer fibrils along the deformation direction, with graphene sheets being encapsulated in these fiber-like bundles. By characterizing the interaction energy between polymer–polymer and polymer–graphene as a function of the strain, we find that the separation of polymer chains from the graphene sheets cease immediately after the yield point, followed by the continuous propagation of the cavities by excluding surrounded polymer chains and graphene sheets together. We also find that at the same attractive interfacial interaction and same loading, the toughening efficiency exhibits the following order: graphene > CNT > ${{\rm{C}}}_{60}.$ Generally, the toughening mechanism of graphene sheets results from the formation of long and highly orientated polymer fibrils to prevent the occurrence of the rupture, which can be greatly improved by the strong interfacial interaction and the large surface area compared to CNT and ${{\rm{C}}}_{60}.$ This also indicates that polymer matrices with high flexibility and mobility of polymer chains tend to be better toughened. It is hoped that this simulation work will provide rational guidance for fabricating high performance of polymer nanocomposites with excellent toughness.

As an emerging class of new materials, two-dimensional (2D) non-graphene materials, including layered and non-layered, and their heterostructures are currently attracting increasing interest due to their promising applications in electronics, optoelectronics and clean energy. In contrast to traditional semiconductors, such as Si, Ge and III–V group materials, 2D materials show significant merits of ultrathin thickness, very high surface-to-volume ratio, and high compatibility with flexible devices. Owing to these unique properties, while scaling down to ultrathin thickness, devices based on these materials as well as artificially synthetic heterostructures exhibit novel and surprising functions and performances. In this review, we aim to provide a summary on the state-of-the-art research activities on 2D non-graphene materials. The scope of the review will cover the preparation of layered and non-layered 2D materials, construction of 2D vertical van der Waals and lateral ultrathin heterostructures, and especially focus on the applications in electronics, optoelectronics and clean energy. Moreover, the review is concluded with some perspectives on the future developments in this field.

We have developed a novel method to fabricate Si nanocrystals in a silica matrix with a considerably reduced thermal budget using pulsed laser deposition. Normally, Si nanocrystals are formed through phase separation by annealing a Si-rich SiO₂ film at 1100 °C; we show Si nanocrystal formation in as-deposited films at 550 °C. We suggest the mechanism for this is through surface diffusion during deposition. We also show the ability to vary the size of these nanocrystals by adjusting the deposition conditions and can increase their size through annealing. If the nanocrystals are small they have excellent photoluminescence properties however larger nanocrystals have poor luminescence.

The absorptance in vertical nanowire (nw) arrays is a result of three optical phenomena: radial mode resonances, near-field evanescent wave coupling, and Fabry–Perot (F–P) modes. The contribution of these optical phenomena to GaAs, InP and InAs nw absorptance was simulated using the finite element method. The study compared the absorptance between finite and semi-infinite nw lengths with varying geometrical parameters, including the nw diameter, length and array period. Simulation results showed that the resonance peak wavelength of the HE₁₁ and HE₁₂ radial modes linearly red-shifted with increasing nw diameter. The absorptance and spectral width of the resonance peaks increased as the nw length increased, with an absorptance plateau for very long nws that depended on diameter and period. Near-field coupling between neighboring nws was observed to increase with decreasing period. The effect of F–P modes was more pronounced for shorter nws, with a significant enhancement of HE₁₂ over HE₁₁ absorptance. Engineering of nw arrays to take advantage of these optical phenomena for multi-spectral photodetector applications is discussed.

Rapid miniaturization of electronic devices down to the nanoscale, according to Moore's law, has led to some undesirable effects like high leakage current in transistors, which can offset additional benefits from scaling down. Development of three-dimensional transistors, by spatial extension in the third dimension, has allowed higher contact area with a gate electrode and better control over conductivity in the semiconductor channel. However, these devices do not utilize the large surface area and interfaces for new electronic functionality. Here, we demonstrate air gating and chemical gating in hollow semiconductor nanotube devices and highlight the potential for development of novel transistors that can be modulated using channel bias, gate voltage, chemical composition, and concentration. Using chemical gating, we reversibly altered the conductivity of nanoscaled semiconductor nanotubes (10–500 nm TiO₂ nanotubes) by six orders of magnitude, with a tunable rectification factor (ON/OFF ratio) ranging from 1–10⁶. While demonstrated air- and chemical-gating speeds were slow here (∼seconds) due to the mechanical-evacuation rate and size of our chamber, the small nanoscale volume of these hollow semiconductors can enable much higher switching speeds, limited by the rate of adsorption/desorption of molecules at semiconductor interfaces. These chemical-gating effects are completely reversible, additive between different chemical compositions, and can enable semiconductor nanoelectronic devices for 'chemical transistors', 'chemical diodes', and very high-efficiency sensing applications.

Photocatalytic activity (PA) of silver nanoparticles (AgNPs) induced by radio frequency (RF) oxygen plasma irradiation (OPI) is investigated in this paper. An improvement in PA by 365% and 181% has been achieved when 15 nm AgNPs irradiated by oxygen plasma for 2 s were used to degrade 10⁻⁵ M Rhodamine 6 G (R6G) under ultraviolet (UV) and visible lights, respectively. The PA caused by OPI is better than that induced by the localized surface plasma resonance (LSPR) of AgNPs. The mechanism for the improvement was explored by scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS) and UV–vis absorption spectra. The OPI-induced formation of AgO/AgNP and Ag₂O/AgNP-heterogeneous photocatalysts and electrophilic oxygen are considered to be responsible for the PA improvement. This investigation deepens our understanding of oxygen-assisted photocatalysis of AgNPs and provides a practical approach using solar light for broad spectra photocatalysis with high efficiency.

The impact of the P/In flux ratio and the deposited thickness on the faceting of InP nanostructures selectively grown by molecular beam epitaxy (MBE) is reported. Homoepitaxial growth of InP is performed inside 200 nm wide stripe openings oriented either along a [110] or [1–10] azimuth in a 10 nm thick SiO₂ film deposited on an InP(001) substrate. When varying the P/In flux ratio, no major shape differences are observed for [1–10]-oriented apertures. On the other hand, the InP nanostructure cross sections strongly evolve for [110]-oriented apertures for which (111)B facets are more prominent and (001) ones shrink for large P/In flux ratio values. These results show that the growth conditions allow tailoring the nanocrystal shape. They are discussed in the framework of the equilibrium crystal shape model using existing theoretical calculations of the surface energies of different low-index InP surfaces as a function of the phosphorus chemical potential, directly related to the P/In ratio. Experimental observations strongly suggest that the relative (111)A surface energy is probably smaller than the calculated value. We also discuss the evolution of the nanostructure shape with the InP-deposited thickness.

3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) organic nanostructures possess extraordinary electronic and optoelectronic properties. However, it remains a challenge to achieve patterned growth of PTCDA nanowire (NW) arrays for integrated device applications. Here, we demonstrated the high-density, large-area, uniform, and cross-aligned growth of single-crystalline PTCDA NW arrays by using Au nanoparticles (NPs) as the growth templates. The high surface energy of Au NPs led to the cross-aligned growth of organic NWs, enabling the growth of PTCDA NW arrays with any desirable patterns by pre-patterning the Au films on a Si substrate. The PTCDA NW arrays as field emitters show good performance with a large emission current density and high emission stability. Furthermore, photodetectors based on PTCDA NW arrays were constructed via a simple in-situ growth approach, which exhibited high sensitivity to a wideband light ranging from 400–800 nm and surpassed the individual NW-based photodetectors in terms of higher photocurrent and faster response speed. Successful applications of PTCDA NW arrays in field emission and photodetectors show a great potential application of organic NW arrays in future efficient electronic and optoelectronic devices.

Magnetricity, the magnetic equivalent of electricity, was recently verified experimentally for the first time. Indeed, like the stream of electric charges that produces electric current, emergent magnetic monopoles have been observed to roam freely in geometrically frustrated magnets known as spin ice. However, such phenomena demand extreme physical conditions, say, a single spin ice crystal has to be cooled to very low temperature, around 0.36 K. Candidates to overcome this difficulty are their artificial analogues, the so-called artificial spin ices. Here, we demonstrate that a specific unidirectional arrangement of nanoislands yields a peculiar system where magnetic monopoles emerge and are constrained to move along aligned dipoles, providing an ordered flow of magnetic charges at room temperature.

Electron beam induced current (EBIC) is a powerful technique which measures the charge collection efficiency of photovoltaics with sub-micron spatial resolution. The exciting electron beam results in a high generation rate density of electron–hole pairs, which may drive the system into nonlinear regimes. An analytic model is presented which describes the EBIC response when the total electron–hole pair generation rate exceeds the rate at which carriers are extracted by the photovoltaic cell, and charge accumulation and screening occur. The model provides a simple estimate of the onset of the high injection regime in terms of the material resistivity and thickness, and provides a straightforward way to predict the EBIC lineshape in the high injection regime. The model is verified by comparing its predictions to numerical simulations in one- and two-dimensions. Features of the experimental data, such as the magnitude and position of maximum collection efficiency versus electron beam current, are consistent with the three-dimensional model.

Inducing magnetism in phosphorene nanoribbons (PNRs) is critical for practical applications. However, edge reconstruction and Peierls distortion prevent PNRs from becoming highly magnetized. Using first-principles calculations, we find that relaxed oxygen-saturated diagonal-PNRs (O-d-PNRs) realize stable spin-polarized antiferromagnetic (AFM) coupling, and the magnetism is entirely localized at the saturated edges. The AFM state is quite stable under expansive and limited compressive strain. More importantly, not only does the irreversible Wilson transition occur when applying strain, but the nonmagnetic (NM) metal phase (a new ground state) becomes more stable than the AFM state when the compressive strain exceeds −4%. The related stability and transition mechanism are demonstrated by dual tuning of the geometric and electronic structures, which manifests as a geometric deviation from a honeycomb to an orthorhombic-like structure and formation of P-p_y bonding (P-p_z nonbonding) from P-p_z nonbonding (P-p_y antibonding) because of the increase of the proportion of the P-p_y (P-p_z) orbital.

A bottom-up fabrication of graphene via molecular self-assembly of p-Terphenyl on Ru(0001) has been investigated by scanning tunneling microcopy and density functional theory. Upon annealing of the sample at 450 °C, the intermediate stage is observed, in which the adsorbed p-Terphenyl molecules and graphitized flakes converted from the molecules coexist, implying the onset of dehydrogenation of p-Terphenyl. At the annealing temperature of 480 °C, the graphitized flakes start to convert into graphene. An adsoption energy of 5.99 eV is calculated for an individual p-Terphenyl molecule on Ru(0001), denoting a strong interaction between the adsorbate and substrate. The intermolecular interaction brings an extra adsorption energy of 0.28 eV for each molecule in the di-molecule adsorption system. During the conversion process from adsorbed molecule into graphene, the intermolecular interaction leads to the increase of the dehydrogenation barrier from 1.52 to 1.64 eV.

The study of the third-order optical nonlinear response exhibited by a composite containing gold nanoparticles and silicon quantum dots nucleated by ion implantation in a high-purity silica matrix is presented. The nanocomposites were explored as an integrated configuration containing two different ion-implanted distributions. The time-resolved optical Kerr gate and z-scan techniques were conducted using 80 fs pulses at a 825 nm wavelength; while the nanosecond response was investigated by a vectorial two-wave mixing method at 532 nm with 1 ns pulses. An ultrafast purely electronic nonlinearity was associated to the optical Kerr effect for the femtosecond experiments, while a thermal effect was identified as the main mechanism responsible for the nonlinear optical refraction induced by nanosecond pulses. Comparative experimental tests for examining the contribution of the Au and Si distributions to the total third-order optical response were carried out. We consider that the additional defects generated by consecutive ion irradiations in the preparation of ion-implanted samples do not notably modify the off-resonance electronic optical nonlinearities; but they do result in an important change for near-resonant nanosecond third-order optical phenomena exhibited by the closely spaced nanoparticle distributions.

We have fabricated a bilayer molybdenum disulphide (MoS₂) transistor on boron nitride (BN) substrate and performed Raman spectroscopy and electrical measurements with this device. The characteristic Raman peaks show an upshift about 2.5 cm⁻¹ with the layer lying on BN, and a narrower line width in comparison with those on a SiO₂ substrate. The device has a maximum drain current larger than 1 μA and a high current on/off ratio of greater than 10⁸. In the temperature range of 100 K−293 K, the two terminal gate effect mobility and the carrier density do not change significantly with temperature. Results of the Raman and electrical measurements reveal that BN is a suitable substrate for atomic layer electrical devices.

Tin oxide (SnO₂) nanocrystals (NCs) based phosphor was synthesized by a green chemistry microwave-assisted hydrothermal method at different reactor pressures. The x-ray diffraction analysis showed that a single rutile SnO₂ phase with a tetragonal lattice structure was formed. The photoluminescence emission was measured for He–Cd laser excitation at 325 nm and it showed a broad band emission from 400 to 800 nm for all the synthesized reactor pressures. The broad emission spectra were due to the creation of various oxygen and tin defects as confirmed by x-ray photoelectron spectroscopy data. The origin of the emission in the SnO₂ NCs is discussed with the help of an energy band diagram. Analysis suggests that the visible emission of SnO₂ NCs is due to a transition of an electron from a level close to the conduction band edge to a deeply trapped hole in the SnO₂ NCs. The NCs were found to be suitable for warm near white light emission device applications.

The study of charge distribution on the surface and in the bulk of dielectrics is of great scientific interest because of the information gained on the storage and transport properties of the medium. Nevertheless, the processes at the nanoscale level remain out of the scope of the commonly used diagnostic methods. Atomic force microscopy (AFM) is currently applied for both injection and imaging of charges on dielectric thin films at the nanoscale level to answer the increasing demand for characterization of miniaturized components used in microelectronics, telecommunications, electrophotography, electrets, etc. However, the mechanisms for dielectric charging by AFM are not well documented, and an analysis of the literature shows that inappropriate mechanisms are sometimes presented. It is shown here that corona discharge, frequently pointed out as a likely mechanism for dielectric charging by AFM in tip-to-sample space mode, cannot develop in such small distances. Furthermore, a review of different mechanisms surmised to be at the origin of dielectric charging at the nanoscale level is offered. Field electron emission enhanced by thermionic emission is identified as a likely mechanism for dielectric charging at the nanoscale level. Experimental validation of this mechanism is obtained for typical electric field strengths in AFM.

We investigated electrical conductivity and Vickers hardness of Ag- and Ni-based composites prepared by powder metallurgy involving spark plasma sintering. The starting composition was Ag:Ni = 61:39 vol%, which provided an electrical conductivity of 3.30 × 10⁵ S cm⁻¹ and a hardness of 1.27 GPa. The addition of bare multi-walled carbon nanotubes (MWNTs, 1.45 vol%) increased hardness (1.31 GPa) but decreased electrical conductivity (2.99 × 10⁵ S cm⁻¹) and carrier mobility (11 cm² V⁻¹ s⁻¹) due to the formation of Ni₃C in the interface between the MWNTs and Ni during spark plasma sintering. The formation of Ni₃C was prevented by coating the surface of the nanotubes with Ag (nAgMWNTs), concomitantly increasing electrical conductivity (3.43 × 10⁵ S cm⁻¹) and hardness (1.37 GPa) of the sintered specimen (Ag:Ni:nAgMWNTs = 59.55:39:1.45 vol%). The electrical contact switching time (133 357) was also increased by 30%, demonstrating excellent feasibility as electrical contact materials for electric power industries.