Table of contents

We have characterized the optoelectrical properties of networks of silver nanowires as a function of nanowire dimension by measuring transmittance (T) and sheet resistance (R_s) for a large number of networks of different thicknesses fabricated from wires of different diameters (D) and lengths (L). We have analysed these data using both bulk-like and percolative models. We find the network DC conductivity to scale linearly with wire length while the optical conductivity is approximately invariant with nanowire length. The ratio of DC to optical conductivity, often taken as a figure of merit for transparent conductors, scales approximately as L/D. Interestingly, the percolative exponent, n, scales empirically as D², while the percolative figure of merit, Π, displays large values at low D. As high T and low R_s are associated with low n and high Π, these data are consistent with improved optoelectrical performance for networks of low-D wires. We predict that networks of wires with D = 25 nm could give sheet resistance as low as 25 Ω/□ for T = 90%.

A tri-stable memristive switching was demonstrated on a Pt/TiO₂/Pt device and its underlying mechanism was suggested through a series of electrical measurements. Tri-stable switching could be initiated from a device in unipolar reset status. The unipolar reset status was obtained by performing an electroforming step on a pristine cell which was then followed by unipolar reset switching. It was postulated that tri-stable switching occurred at the location where the conductive filament (initially formed by the electroforming step) was ruptured by a subsequent unipolar reset process. The mechanism of the tri-stable memristive switching presented in this article was attributed to the migration of oxygen ions through the ruptured filament region and the resulting modulation of the Schottky-like interfaces. The assertion was further supported by a comparison study performed on a Pt/TiO₂/TiO_2−x/Pt cell.

Pattern collapse of small or high aspect ratio lines during traditional wet development is a major challenge for miniaturization in nanolithography. Here we report on a new dry process which combines high resolution resist exposure with selective laser ablation to achieve high resolution with high aspect ratios. Using a low power 532 nm laser, we dry develop a normally negative tone methyl acetoxy calix(6)arene in positive tone to reveal sub-20 nm half-pitch features in a ∼100 nm film at aspect ratios unattainable with conventional development with ablation time of 1–2 s per laser pixel (∼600 nm diameter spot). We also demonstrate superior negative tone wet development by combining electron beam exposure with subsequent laser exposure at a non-ablative threshold that requires far less electron beam exposure doses than traditional wet development.

We demonstrate that a low energy focused electron beam can locally pattern graphene coated with a thin ice layer. The irradiated ice plays a crucial role in the process by providing activated species that locally remove graphene from a silicon dioxide substrate. After patterning the graphene, the ice resist is easily removed by sublimation to leave behind a clean surface with no further processing. More generally, our findings demonstrate that ice-assisted e-beam lithography can be used to pattern very thin materials deposited on substrate surfaces. The procedure is performed in situ in a modified scanning electron microscope. Desirable structures such as nanoribbons are created using the method. Defects in graphene from electrons backscattered from the bulk substrate are identified. They extend several microns from the e-beam writing location. We demonstrate that these defects can be greatly reduced and localized by using thinner substrates and/or gentle thermal annealing.

Electron beam induced deposition (EBID) has recently been developed into a method to directly write optically active three-dimensional nanostructures. For this purpose a metal–organic precursor gas (here dimethyl-gold(III)-acetylacetonate) is introduced into the vacuum chamber of a scanning electron microscope where it is cracked by the focused electron beam. Upon cracking the aforementioned precursor gas, 3D deposits are realized, consisting of gold nanocrystals embedded in a carbonaceous matrix. The carbon content in the deposits hinders direct plasmonic applications. However, it is possible to activate the deposited nanostructures for plasmonics by coating the EBID structures with a continuous silver layer of a few nanometers thickness. Within this silver layer collective motions of the free electron gas can be excited. In this way, EBID structures with their intriguing precision at the nanoscale have been arranged in arrays of free-standing dimer antenna structures with nanometer sized gaps between the antennas that face each other with an angle of 90°. These dimer antenna ensembles can constitute a reproducibly manufacturable substrate for exploiting the surface enhanced Raman effect (SERS). The achieved SERS enhancement factors are of the order of 10⁴ for the incident laser light polarized along the dimer axes. To prove the signal enhancement in a Raman experiment we used the dye methyl violet as a robust test molecule. In future applications the thickness of such a silver layer on the dimer antennas can easily be varied for tuning the plasmonic resonances of the SERS substrate to match the resonance structure of the analytes to be detected.

We report a novel nanomanufacturing technique that incorporates patterned arrays built entirely from Fe₃O₄ nanoparticles into a flexible and transparent polymer film. First, the nanoparticles are patterned using the enormous magnetic field gradients at the surface of commercial disk drive media, and then the resulting architecture is transferred to the surface of a polymer film by spin-coating and peeling. Since the particles are immobilized by the field gradients during the spin-coating process, the patterned array is preserved after peeling. To demonstrate the potential of this technology, we fabricate a 5 mm diameter all-nanoparticle diffraction grating capable of producing a white-light optical spectrum. We also demonstrate several extensions to this technology, where, by adding an external magnetic field during assembly, we create both periodic variations in topography, as well as a nanocomposite with two vertically and horizontally separated nanoparticle layers. As this technique leverages the nanometer resolution inherent in current magnetic recording technology, strong potential exists for low-cost nanomanufacturing of optical and electronic devices from a variety of nanomaterials with ∼10 nm resolution.

Two methods of direct-write printing for producing highly resolved features of a polymer impregnated with luminescent upconversion phosphors for security applications are presented. The printed polymer structures range in shape from features to text. The thin polymer features were deposited by direct-write printing of atomized material as well as by screen-printing techniques. These films contain highly luminescent lanthanide-doped, rare-earth nanocrystals, β-NaYF₄:3%Er, 17%Yb, which are capped with oleic acid. This capping agent allows the nanocrystals to disperse throughout the films for full detailing of printed features. Upconversion of deposited features was obtained using a 980 nm wavelength laser with emission of upconverted light in the visible region at both 540 and 660 nm. Features were deposited onto high bond paper, Kapton^®, and glass to demonstrate possible covert and forensic security printing applications, as they are printed in various features and invisible to 'naked-eye' viewing at low concentrations of nanocrystals.

The sodium chloride surface is one of the most common platforms for the study of catalysts, thin film growth, and atmospheric aerosols. Here we report a nanoscale periodic modulation pattern on the surface of a cleaved NaCl single crystal, revealed by non-contact atomic force microscopy with a tuning fork sensor. The surface pattern shows two orthogonal domains, extending over the entire cleavage surface. The spatial modulations exhibit a characteristic period of 5.4 nm, along 〈110〉 crystallographic directions of the NaCl. The modulations are robust in vacuum, not affected by the tip-induced electric field or gentle annealing (<300 °C); however, they are eliminated after exposure to water and an atomically flat surface can be recovered by subsequent thermal annealing after water exposure. A strong electrostatic charging is revealed on the cleavage surface which may facilitate the formation of the observed metastable surface reconstruction.

Electrochemical AFM lithography was used to directly fabricate copper nanowires. The copper ions were strongly reduced by a negative sample bias at the point where the AFM tip was localized, and copper metal wires were successfully fabricated following the direction of the electrical field of the bias. A TDA⋅HCl self-assembled monolayer (SAM) was found to play an important role as an intermediate layer for enhancing the capability of high resolution and complete development after the AFM lithographic process. The physical and electrical properties of the wires were analyzed by AFM, EFM, SEM, TEM and I–V measurement. The fabricated copper has promising potential for applications such as masks and interconnectors for nanoelectronic devices.

Dielectrophoresis has been used as a technique for the parallel localization and alignment of both semiconducting and metallic carbon nanotubes (CNTs) at junctions between electrodes. A variation of this technique known as floating potential dielectrophoresis (FPD) allows for a self-limiting number of CNTs to be localized at each junction, on a massively parallel scale. However, the smallest FPD geometries to date are restricted to conductive substrates and have a lower limit on floating electrode size. We present a geometry which eliminates this lower limit and enables FPD to be performed on non-conducting substrates. We also discuss experiments clarifying the self-limiting mechanism of CNT localization and how it can be used advantageously as devices are scaled downwards to smaller sizes.

We propose a novel solar cell structure with photonic nanocrystals coupled to quantum dots (QDs) for advanced management of photons and carriers. The photonic nanocrystals at the surface create an extra interaction between the photons and the QDs, which promotes light trapping. Photo-generated carriers can be efficiently transported by preparing vertically aligned QDs with electronic coupling. Implementation of the proposed structure was realized in crystalline Si solar cells with Ge QDs by development of a simple and practical formation method based on a wet chemical process without any lithography techniques. The wet process utilizes a periodically modulated etching rate induced by self-organized Ge QDs. The effectiveness of the proposed solar cell was demonstrated by the marked increase of the absolute conversion efficiency when compared with the control crystalline Si solar cells. It is found that light trapping by the photonic nanocrystals has a larger contribution to the efficiency improvement than the contributions from the carrier transport of the vertically aligned QDs.

A process route for the fabrication of solvent-redispersible, surfactant-free Cu₂ZnSnS₄ (CZTS) nanoparticles has been designed with the objective to have the benefit of a simple sulfide source which advantageously acts as (i) a complexing agent inhibiting crystallite growth, (ii) a surface additive providing redispersion in low ionic strength polar solvents and (iii) a transient ligand easily replaced by an carbon-free surface additive. This multifunctional use of the sulfide source has been achieved through a fine tuning of ((Cu²⁺)_a(Zn²⁺)_b(Sn⁴⁺)_c(Tu)_d(OH⁻)_e)^t+, Tu = thiourea) oligomers, leading after temperature polycondensation and S²⁻ exchange to highly concentrated (c > 100 g l⁻¹), stable, ethanolic CZTS dispersions. The good electronic properties and low-defect concentration of the sintered, crack-free CZTSe films resulting from these building blocks was shown by photoluminescence investigation, making these building blocks interesting for low-cost, high-performance CZTSe solar cells.

A novel scan trajectory for high-speed scanning probe microscopy is presented in which the probe follows a two-dimensional Lissajous pattern. The Lissajous pattern is generated by actuating the scanner with two single-tone harmonic waveforms of constant frequency and amplitude. Owing to the extremely narrow frequency spectrum, high imaging speeds can be achieved without exciting the unwanted resonant modes of the scanner and without increasing the sensitivity of the feedback loop to the measurement noise. The trajectory also enables rapid multiresolution imaging, providing a preview of the scanned area in a fraction of the overall scan time. We present a procedure for tuning the spatial and the temporal resolution of Lissajous trajectories and show experimental results obtained on a custom-built atomic force microscope (AFM). Real-time AFM imaging with a frame rate of 1 frame s⁻¹ is demonstrated.

This study demonstrates, for the first time, the manufacturing of novel nano-nonwovens that are comprised of three-dimensionally distributed carbon nanofibers within the matrices of traditional wet-laid nonwovens. The preparation of these nano-nonwovens involves dispersing and flocking carbon nanofibers, and optimizing colloidal chemistry during wet-lay formation. The distribution of nanofibers within the nano-nonwoven was verified using polydispersed aerosol filtration testing, air permeability, low surface tension liquid capillary porometry, SEM and cyclic voltammetry. All these characterization techniques indicated that nanofiber flocks did not behave as large solid clumps, but retained the 'nanoporous' structure expected from nanofibers. These nano-nonwovens showed significant enhancements in aerosol filtration performance. The reduction–oxidation reactions of the functional groups on nanofibers and the linear variation of electric double-layer capacitance with nanofiber loading were measured using cyclic voltammetry. More than 65 m² (700 ft²) of the composite were made during the demonstration of process scalability using a Fourdrinier-type continuous pilot papermaking machine. The scalability of the process with the control over pore size distribution makes these composites very promising for filtration and other nonwoven applications.

Lead selenide nanoparticles (PbSe NPs) have been obtained through an easy and low cost route using colloidal synthesis in aqueous solution. The synthesis was carried out at room temperature using Extran (Na₅P₃O₁₀, NaOH and H₂O) as surfactant. Hydrochloric acid (HCl) was used to eliminate the generated by-products. The size of PbSe NPs was varied by changing the Pb:Se molar concentration. The PbSe NPs were characterized by powder x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDAX), high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. The XRD measurements showed that the PbSe NPs have the face-centered cubic phase structure. The crystal size was found to be between 14 and 20 nm as calculated from the XRD patterns and these values were corroborated with SEM and TEM. Additionally, HRTEM micrographs showed crystalline planes at (200), (220) and (111) of the PbSe NPs, in agreement with the XRD results.

The fabrication of functional thin films and devices by direct deposition of nanoparticles from the gas phase is a promising approach enabling, for instance, the integration of complex analytical and sensing capabilities on microfabricated platforms. Aerosol-based techniques ensure large-scale nanoparticle production and they are potentially suited for this goal. However, they are not adequate in terms of fine control over the lateral resolution of the coatings, mild processing conditions (avoiding high temperature and aggressive chemicals), low contamination and compatibility with microfabrication processes.

Here we report the high-rate and efficient production of functional nanostructured films by nanoparticle assembling obtained by the combination of flame spray pyrolysis and supersonic expansion. Our approach merges the advantages of flame spray pyrolysis for bulk nanopowders such as process stability and wide material library availability with those of supersonic cluster beam deposition in terms of lateral resolution and of direct integration of nanomaterials on devices. We efficiently produced nanostructured films and devices (such as gas sensors) using metal oxide, pure noble metal and oxide-supported noble metal nanoparticles.

Single anatase TiO₂ nanowire is synthesized using the electrospinning technique with the sol–gel method and is suspended over a pre-processed 100 µm-wide TEM grid for further characterization. The diameters of the nanowires fall in the range of 250–400 nm. The transient electrothermal (TET) method is adopted to acquire the voltage–time (U–t) profile of the Ir-coated nanowire under step Joule heating. The intrinsic thermal diffusivity of single anatase TiO₂ nanowires varies from 1.3 to 4.6 × 10⁻⁶ m² s⁻¹, and the thermal conductivity changes distinctly from 1.3 to 5.6 W m⁻¹ K⁻¹, much lower than the value of the bulk counterpart: 8.5 W m⁻¹ K⁻¹. The density and thermal conductivity increase significantly with the diameter, largely because at larger diameters less secondary porosity is left by decomposition of organic composites and their escape from the wire during calcination. The density of TiO₂ nanowires is found to be much lower than that of the bulk counterpart. This is supported by the SEM image of the secondary porous surface. High secondary porosity is observed for TiO₂ nanowires, ranging from 18% to 63%. This very high secondary porosity confirms that the decomposition of PVP content may distort the fibrous matrix and leave vacancies. In addition, the transition from amorphous to anatase phase could also create a porous state due to crystal particle aggregation.

CoPt–C binary alloys have been fabricated by focused-electron-beam-induced deposition by the simultaneous use of Co₂(CO)₈ and (CH₃)₃CH₃C₅H₄Pt as precursor gases. The alloys are made of CoPt nanoparticles embedded in a carbonaceous matrix. TEM investigations show that as-grown samples are in an amorphous phase. By means of a room temperature low-energy electron irradiation treatment the CoPt nanoparticles transform into face-centered tetragonal L1₀ nanocrystallites. In parallel, the system undergoes a transition from a superparamagnetic to a ferromagnetic state at room temperature. By variation of the post-growth irradiation dose the electrical and magneto-transport properties of the alloy can be continuously tuned.

We present an in-depth study of the myriad atomically resolved patterns observed on graphite using the scanning tunnelling microscope (STM) over the past three decades. Through the use of highly resolved atomic resolution images, we demonstrate how the interactions between the different graphene layers comprising graphite affect the local surface atomic charge density and its resulting symmetry orientation, with particular emphasis on interactions that are thermodynamically unstable. Moreover, the interlayer graphene coupling is controlled experimentally by varying the tip–surface interaction, leading to associated changes in the atomic patterns. The images are corroborated by first-principles calculations, further validating our claim that surface graphene displacement, coming both from lateral and vertical displacement of the top graphene layer, forms the basis of the rich variety of atomic patterns observed in STM experiments on graphite.

Coulomb blockade behaviors in individual Au nanoparticles of 2 nm core diameter in double-barrier structures have been studied by means of noncontact atomic force spectroscopy (NC-AFS) at room temperature. The Au nanoparticles with a 1-decanethiol ligand were chemisorbed by 1,10-decanedithiol molecules of a mixed 1-octanethiol/1,10-decanedithiol self-assembled monolayer coated on a Au(111) surface; these particles were observed through NC-AFS. NC-AFS measurements of the cantilever frequency shift–sample voltage (Δf–V_S) curves were sequentially conducted on three Au nanoparticles under the same experimental conditions; the Δf–V_S curves were found to deviate from the parabolic (Δf_N) curve in the cases where no extra charge existed on the Au core. The experimental Δf_CB(=Δf − Δf_N) and Δf_CB/V curves agree well with the theoretical curves obtained using a golden-rule calculation and the same parabolic parameters. All the results, through NC-AFS, suggest Coulomb blockade behaviors in the Au nanoparticles at room temperature.