Patterning at the Resolution Limit of Commercial Electron Beam Lithography.

It has been long known that low molecular weight resists can achieve a very high resolution, theoretically close to the probe diameter of the electron beam lithography (EBL) system. Despite technological improvements in EBL systems, the advances in resists have lagged behind. Here we demonstrate that a low-molecular-mass single-source precursor resist (based on cadmium(II) ethylxanthate complexed with pyridine) is capable of a achieving resolution (4 nm) that closely matches the measured probe diameter (∼3.8 nm). Energetic electrons enable the top-down radiolysis of the resist, while they provide the energy to construct the functional material from the bottom-up─unit cell by unit cell. Since this occurs only within the volume of resist exposed to primary electrons, the minimum size of the patterned features is close to the beam diameter. We speculate that angstrom-scale patterning of functional materials is possible with single-source precursor resists using an aberration-corrected electron beam writer with a spot size of ∼1 Å.

Multiphoton Upconversion Enhanced by Deep Subwavelength Near-Field Confinement.

Efficient generation of anti-Stokes emission within nanometric volumes enables the design of ultracompact, miniaturized photonic devices for a host of applications. Many subwavelength crystals, such as metal nanoparticles and two-dimensional layered semiconductors, have been coupled with plasmonic nanostructures for augmented anti-Stokes luminescence through multiple-harmonic generation. However, their upconversion process remains inefficient due to their intrinsic low absorption coefficients. Here, we demonstrate on-chip, site-specific integration of lanthanide-activated nanocrystals within gold nanotrenches of sub-25 nm gaps via bottom-up self-assembly. Coupling of upconversion nanoparticles to subwavelength gap-plasmon modes boosts 3.7-fold spontaneous emission rates and enhances upconversion by a factor of 100 000. Numerical investigations reveal that the gap-mode nanocavity confines incident excitation radiation into nanometric photonic hotspots with extremely high field intensity, accelerating multiphoton upconversion processes. The ability to design lateral gap-plasmon modes for enhanced frequency conversion may hold the potential to develop on-chip, background-free molecular sensors and low-threshold upconversion lasers.

Directed self-assembly of sub-10 nm particle clusters using topographical templates.

Directed self-assembly of nanoparticles (DSA-n) is an approach that creates suitable conditions to capture nanoparticles randomly dispersed in a liquid and position them into predefined locations on a solid template. Although DSA-n is emerging as a potential bottom-up patterning technique to build nanostructures using nanoparticles of various sizes, geometries and material compositions, there are still several outstanding challenges. In this paper, we focus on the DSA-n of sub-10 nm particles using topographical templates to guide them into 1D and 2D ordered arrays. The process mechanism leading DSA-n at sub-10 nm size scale has been reviewed and experimental evidence of the impact of the template on the positioning both individual and clusters of particles with low level of structure defects have also been demonstrated. Furthermore, by controlling the drying direction of the liquid within polygonal traps, we are also able to tune the spacing between the trapped nanoparticle clusters. This self-structuring phenomenon is of crucial importance for various applications such as plasmonics and charge transport within quantum circuits, whereby the coupling effects are highly dependent on the size of the nanoparticles and their separation.

Large Area Directed Self-Assembly of Sub-10 nm Particles with Single Particle Positioning Resolution.

Directed self-assembly of nanoparticles (DSA-n) holds great potential for device miniaturization in providing patterning resolution and throughput that exceed existing lithographic capabilities. Although nanoparticles excel at assembling into regular close-packed arrays, actual devices on the other hand are often laid out in sparse and complex configurations. Hence, the deterministic positioning of single or few particles at specific positions with low defect density is imperative. Here, we report an approach of DSA-n that satisfies these requirements with less than 1% defect density over micrometer-scale areas and at technologically relevant sub-10 nm dimensions. This technique involves a simple and robust process where a solvent film containing sub-10 nm gold nanoparticles climbs against gravity to coat a prepatterned template. Particles are placed individually into nanoscale cavities, or between nanoposts arranged in varying degrees of geometric complexity. Brownian dynamics simulations suggest a mechanism in which the particles are pushed into the template by a nanomeniscus at the drying front. This process enables particle-based self-assembly to access the sub-10 nm dimension, and for device fabrication to benefit from the wealth of chemically synthesized nanoparticles with unique material properties.

Second-Harmonic Generation from Sub-5 nm Gaps by Directed Self-Assembly of Nanoparticles onto Template-Stripped Gold Substrates.

Strong field enhancement and confinement in plasmonic nanostructures provide suitable conditions for nonlinear optics in ultracompact dimensions. Despite these enhancements, second-harmonic generation (SHG) is still inefficient due to the centrosymmetric crystal structure of the bulk metals used, e.g., Au and Ag. Taking advantage of symmetry breaking at the metal surface, one could greatly enhance SHG by engineering these metal surfaces in regions where the strong electric fields are localized. Here, we combine top-down lithography and bottom-up self-assembly to lodge single rows of 8 nm diameter Au nanoparticles into trenches in a Au film. The resultant "double gap" structures increase the surface-to-volume ratio of Au colocated with the strong fields in ∼2 nm gaps to fully exploit the surface SHG of Au. Compared to a densely packed arrangement of AuNPs on a smooth Au film, the double gaps enhance SHG emission by 4200-fold to achieve an effective second-order susceptibility χ((2)) of 6.1 pm/V, making it comparable with typical nonlinear crystals. This patterning approach also allows for the scalable fabrication of smooth gold surfaces with sub-5 nm gaps and presents opportunities for optical frequency up-conversion in applications that require extreme miniaturization.

Directed Self-Assembly of sub-10 nm Particles: Role of Driving Forces and Template Geometry in Packing and Ordering.

By comparing the magnitude of forces, a directed self-assembly mechanism has been suggested previously in which immersion capillary is the only driving force responsible for packing and ordering of nanoparticles, which occur only after the meniscus recedes. However, this mechanism is insufficient to explain vacancies formed by directed self-assembly at low particle concentrations. Utilizing experiments, and Monte Carlo and Brownian dynamics simulations, we developed a theoretical model based on a new proposed mechanism. In our proposed mechanism, the competing driving forces controlling the packing and ordering of sub-10 nm particles are (1) the repulsive component of the pair potential and (2) the attractive capillary forces, both of which apply at the contact line. The repulsive force arises from the high particle concentration, and the attractive force is caused by the surface tension at the contact line. Our theoretical model also indicates that the major part of packing and ordering of nanoparticles occurs before the meniscus recedes. Furthermore, utilizing our model, we are able to predict the various self-assembly configurations of particles as their size increases. These results lay out the interplay between driving forces during directed self-assembly, motivating a better template design now that we know the importance and the dominating driving forces in each regime of particle size.

Room temperature Coulomb blockade effects in Au nanocluster/pentacene single electron transistors.

Single-electron transistors incorporating single ∼1 nm gold nanocluster (AuNCs) and pentacene as a complex charge transport system have been used to study the quantum Coulomb blockade and its single electron tunnelling behaviour at room temperature (RT) (300 K). Monodisperse ultra-small (0.86 ± 0.30 nm) AuNCs were deposited by the tilted-target sputtering technique into 12 nm nanogaps fabricated by high-resolution e-beam lithography. Tunnelling resistance was modulated to ∼10(9) Ω by addition of a pentacene layer, allowing clear observation of quantum staircases and Coulomb oscillations with on/off current modulation ratio of ∼100 in RT current-voltage measurements. The electron addition energy and average quantized energy level spacing were found to be 282 and 80.4 meV, respectively, which are significantly larger than the thermal energy at 300 K (25.9 meV).

Template-induced structure transition in sub-10 nm self-assembling nanoparticles.

We report on the directed self-assembly of sub-10 nm gold nanoparticles confined within a template comprising channels of gradually varying widths. When the colloidal lattice parameter is mismatched with the channel width, the nanoparticles rearrange and break their natural close-packed ordering, transiting through a range of structural configurations according to the constraints imposed by the channel. While much work has been done in assembling ordered configurations, studies of the transition regime between ordered states have been limited to microparticles under applied compression. Here, with coordinated experiments and Monte Carlo simulations we show that particles transit through a more diverse set of self-assembled configurations than observed for compressed systems. The new insight from this work could lead to the control and design of complex self-assembled patterns other than periodic arrays of ordered particles.

A facile approach for screening isolated nanomagnetic behavior for bit-patterned media.

Bit-patterned media (BPM) fabricated by the direct deposition of magnetic material onto prepatterned arrays of nanopillars is a promising approach for increasing magnetic recording of areal density. One of the key challenges of this approach is to identify and control the magnetic interaction between the bits (on top of the nanopillars) and the trench material between the pillars. Using independent techniques, including magnetic force microscopy, the variable-angle magneto-optic Kerr effect, and remanence curves, we were able to determine the presence and relative intensities of exchange and dipolar interactions in Co-Pd multilayer-based BPM fabricated by direct deposition. We found that for pitches of 30 nm or less, there were negligible exchange interactions, and the bits were found to be magnetically isolated. As we move to higher densities, the absence of exchange interactions indicates that direct deposition is a promising approach to BPM fabrication.

Directed self-assembly of densely packed gold nanoparticles.

Directing the self-assembly of sub-10-nm nanoparticles has been challenging because of the simultaneous requirements to achieve a densely packed monolayer and rearrange nanoparticles to assemble within a template. We met both requirements by separating the processes into two steps by first forming a monolayer of gold nanoparticles on a suitable liquid subphase of anisole and then transferring it edgewise onto a silicon substrate with a prepatterned template comprising nanoposts and nanogratings. Doing so resulted in nanoparticles that assembled in commensuration with the template design while exhibiting appreciable template-induced strain. These dense arrays of nanostructures could either be directly applied or used as lithographic masks in applications for light collection, chemical sensing, and data storage.

Nanocavity-induced trion emission from atomically thin WSe2.

Exciton is a bosonic quasiparticle consisting of a pair of electron and hole, with promising potentials for optoelectronic device applications, such as exciton transistors, photodetectors and light emitting devices. However, the charge-neutral nature of excitons renders them challenging to manipulate using electronics. Here we present the generation of trions, a form of charged excitons, together with enhanced exciton resonance in monolayer WSe2. The excitation of the trion quasiparticles is achieved by the hot carrier transport from the integrated gold plasmonic nanocavity, formed by embedding monolayer WSe2 between gold nanoparticles and a gold film. The nanocavity-induced negatively charged trions provide a promising route for the manipulation of excitons, essential for the construction of all-exciton information processing circuits.

Thermoelectric Properties of Substoichiometric Electron Beam Patterned Bismuth Sulfide.

Direct patterning of thermoelectric metal chalcogenides can be challenging and is normally constrained to certain geometries and sizes. Here we report the synthesis, characterization, and direct writing of sub-10 nm wide bismuth sulfide (Bi2S3) using a single-source, spin-coatable, and electron-beam-sensitive bismuth(III) ethylxanthate precursor. In order to increase the intrinsically low carrier concentration of pristine Bi2S3, we developed a self-doping methodology in which sulfur vacancies are manipulated by tuning the temperature during vacuum annealing, to produce an electron-rich thermoelectric material. We report a room-temperature electrical conductivity of 6 S m-1 and a Seebeck coefficient of -21.41 µV K-1 for a directly patterned, substoichiometric Bi2S3 thin film. We expect that our demonstration of directly writable thermoelectric films, with further optimization of structure and morphology, can be useful for on-chip applications.

Room-Temperature Patterning of Nanoscale MoS2 under an Electron Beam.

Molybdenum disulfide (MoS2) is traditionally grown at a high temperature and subsequently patterned to study its electronic properties or make devices. This method imposes severe limitations on the shape and size of MoS2 crystals that can be patterned precisely at required positions. Here, we describe a method of direct nanoscale patterning of MoS2 at room temperature by exposing a molybdenum thiocubane single-source precursor to a beam of electrons. Molybdenum thiocubanes with various alkylxanthate moieties [Mo4S4(ROCS2)6, where R = alkyl] were prepared using a "self-assembly" approach. Micro-Raman and micro-FTIR spectroscopic studies suggest that exposure to a relatively smaller dose of electrons results in the breakdown of xanthate moieties, leading to the formation of MoS2. High-resolution transmission electron micrographs suggest that the growth of MoS2 most likely happens along (100) planes. An electron-beam-induced chemical transformation of a molybdenum thiocubane resist was exploited to fabricate sub-10 nm MoS2 lines and dense dots as small as 13 nm with a pitch of 33 nm. Since this technique does not require the liftoff and etching steps, patterning of MoS2 with interesting shapes, sizes, and thicknesses potentially leading to tunable band gap is possible.

Ultrasmall Designed Plasmon Resonators by Fused Colloidal Nanopatterning.

This work presents a procedure for large-area patterning of designed plasmon resonators that are much smaller than possible with conventional lithography techniques. Fused Colloidal Nanopatterning combines directed self-assembly and controlled fusing of spherical colloidal nanoparticles. The two-step approach first patterns a surface covered with hydrogen silsesquioxane, an electron beam resist, forming traps into which the colloidal gold nanoparticles self-assemble. Second, the patterned nanoparticles are controllably fused to form plasmon resonators of any 2D designed shape. The heights and widths of the plasmon resonators are determined by the diameter of the nanoparticle building blocks, which can be well below 10 nm. By performing the fusing step with UV ozone and heat exposure, we demonstrate that the process is easily scalable to cover large areas on silicon wafers with designed gold nanostructures. The procedure neither requires adhesion layers nor a lift-off process, making it ideally suited for plasmonics, in comparison with regular electron beam lithography. We use monochromated electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy and boundary element method simulations to demonstrate that the designed plasmon resonators are directly tunable via the pattern design. We foresee future expansion of this approach for applications such as plasmon-enhanced photocatalysis and for large-scale patterning where chemical, optical, or confinement properties require sub-10 nm metal lines.

Second order directed positioning of nanoparticles induced by the main terminal meniscus shape in irregular template cavities.

Directed self-assembly of nanoparticles using topographical templates has demonstrated great capabilities of ordering particles at their maximum packing fraction resulting from template confinement effects and free energy minimization. However, to self-assemble nanostructures at a lower packing fraction with a precise control over particle's positioning is challenging due to the high entropy of such a system. Here, by fabricating templates of irregular cavities together with appropriate choice of solvent, we demonstrate the positioning of 8 nm Au nanoparticles within individual cavities at a low filling factor. In addition to the first-order of ordering dictated by the template topography, there is a second-order of ordering induced by the interplay between the evaporation of the residual solvent trapped within the cavities and their intrinsic geometries. The experimental results show that the cavities shaped as equilateral triangles exhibit a random positioning of the particles at the corners; in comparison, right-angled scalene (or irregular) triangles show a more controllable positioning of the particles within the corners of the smallest angle. Finally, this technique has been successfully used to fabricate arrays of dimers with a controllable center-to-center distances at sub-5 nm length scales.

Direct Patterning of Zinc Sulfide on a Sub-10 Nanometer Scale via Electron Beam Lithography.

Nanostructures of metal sulfides are conventionally prepared via chemical techniques and patterned using self-assembly. This poses a considerable amount of challenge when arbitrary shapes and sizes of nanostructures are desired to be placed at precise locations. Here, we describe an alternative approach of nanoscale patterning of zinc sulfide (ZnS) directly using a spin-coatable and electron beam sensitive zinc butylxanthate resist without the lift-off or etching step. Time-resolved electron beam damage studies using micro-Raman and micro-FTIR spectroscopies suggest that exposure to a beam of electrons leads to quick disappearance of xanthate moieties most likely via the Chugaev elimination, and further increase of electron dose results in the appearance of ZnS, thereby making the exposed resist insoluble in organic solvents. Formation of ZnS nanocrystals was confirmed by high-resolution transmission electron microscopy and selected area electron diffraction. This property was exploited for the fabrication of ZnS lines as small as 6 nm and also enabled patterning of 10 nm dots with pitches as close as 22 nm. The ZnS patterns fabricated by this technique showed defect-induced photoluminescence related to sub-band-gap optical transitions. This method offers an easy way to generate an ensemble of functional ZnS nanostructures that can be arbitrarily patterned and placed in a precise way. Such an approach may enable programmable design of functional chalcogenide nanostructures.

Nanostructure Formation by controlled dewetting on patterned substrates: A combined theoretical, modeling and experimental study.

We perform systematic two-dimensional energetic analysis to study the stability of various nanostructures formed by dewetting solid films deposited on patterned substrates. Our analytical results show that by controlling system parameters such as the substrate surface pattern, film thickness and wetting angle, a variety of equilibrium nanostructures can be obtained. Phase diagrams are presented to show the complex relations between these system parameters and various nanostructure morphologies. We further carry out both phase field simulations and dewetting experiments to validate the analytically derived phase diagrams. Good agreements between the results from our energetic analyses and those from our phase field simulations and experiments verify our analysis. Hence, the phase diagrams presented here provide guidelines for using solid-state dewetting as a tool to achieve various nanostructures.

High aspect ratio 10-nm-scale nanoaperture arrays with template-guided metal dewetting.

We introduce an approach to fabricate ordered arrays of 10-nm-scale silica-filled apertures in a metal film without etching or liftoff. Using low temperature (<400°C) thermal dewetting of metal films guided by nano-patterned templates, apertures with aspect ratios up to 5:1 are demonstrated. Apertures form spontaneously during the thermal process without need for further processing. Although the phenomenon of dewetting has been well studied, this is the first demonstration of its use in the fabrication of nanoapertures in a spatially controllable manner. In particular, the achievement of 10-nm length-scale patterning at high aspect ratio with thermal dewetting is unprecedented. By varying the nanotemplate design, we show its strong influence over the positions and sizes of the nanoapertures. In addition, we construct a three-dimensional phase field model of metal dewetting on nano-patterned substrates. The simulation data obtained closely corroborates our experimental results and reveals new insights to template dewetting at the nanoscale. Taken together, this fabrication method and simulation model form a complete toolbox for 10-nm-scale patterning using template-guided dewetting that could be extended to a wide range of material systems and geometries.