scalable-solution-processed-fabrication-strategy-for-high-performance

Scalable Solution-processed Fabrication Strategy for High-performance, Flexible, Transparent Electrodes with Embedded Metal Mesh

Here, the authors report the embedded metal-mesh transparent electrode (EMTE), a new transparent electrode (TE) with a metal mesh completely embedded in a ...

University of Hong Kong

HKU-Shenzhen Institute of Research and Innovation

Quantum lithography (QL) is a revolutionary approach, increasing the throughput and lowering the cost of scanning electron beam lithography (EBL). But it has not been pursued since its inception 17 years ago, due to the lack of a viable method for making the blanks needed. Here we propose and demonstrate a new general viable approach to QL blank fabrication, that is based on (a) nanoimprinting and (b) a new wafer-scale nanoimprint mold fabrication that uses not EBL but a unique combination of interference lithography, self-perfection, multiple nanoimprinting, and other novel nanopatterning. We fabricated QL blanks (a 2D Cr square tile array of 200 nm pitch, 9 nm gap, and sub-10 nm corners, corresponding to a 50 nm node 4 x photomask) and demonstrated that QL can greatly relax the requirements for the EBL tool, increase the throughput and reduce the cost of EBL by orders of magnitude, and is scalable to the 22 nm node.

Quantized patterning using nanoimprinted blanks.

We proposed and demonstrated a new approach to pressed self-perfection by liquefaction (P-SPEL), where a layer of SiO2 is used as a stopper on one sidewall of gratings, to self-limit the final trench width in P-SPEL to a preset stopper layer thickness, allowing a precise control of the final trench width without the need to control any pressing parameters such as pressure, temperature and the gap between the pressing plate and the substrate. We achieved 20 nm wide trenches from a 90 nm original width, reducing the original trench by 450%. We also observed improvement in the trench width uniformity. Using the fabricated resist trenches as templates, 20 nm metal lines were achieved by lift-off.

Self-limited self-perfection by liquefaction for sub-20 nm trench/line fabrication.

We designed, fabricated and demonstrated a solar-blind deep-UV pass filter, that has a measured optical performance of a 27% transmission peak at 290 nm, a pass-band width of 100 nm (from 250 to 350 nm), and a 20dB rejection ratio between deep-UV wavelength and visible wavelength. The filter consists of an aluminum nano-grid, which was made by coating 20 nm Al on a SiO(2) square grid with 190 nm pitch, 30 nm linewidth and 250 nm depth. The performances agree with a rigorous coupled wave analysis. The wavelength for the peak transmission and the pass-bandwidth can be tuned through adjusting the metal nano-grid dimensions. The filter was fabricated by nanoimprint lithography, hence is large area and low cost. Combining with Si photodetectors, the filter offers simple yet effective and low cost solar-blind deep-UV detection at either a single device or large-area complex integrated imaging array level.

Solar-blind deep-UV band-pass filter (250 - 350 nm) consisting of a metal nano-grid fabricated by nanoimprint lithography.

Typically, nanopatterning on plastic substrates has poor fidelity, poor adhesion, and low yield. Here the proposal of and the first experiment using a new fabrication method that overcomes the above obstacles and has achieved arrays of 60-nm-diameter, perfectly round metal dots over a large area on a polyethylene terephthalate (PET) substrate with high fidelity and high yield is reported. This new method is based on the use of a thin hydrogen silsesquioxane (HSQ) layer on top of PET, nanoimprint lithography, and self-perfection by liquefaction (SPEL). The HSQ layer offers excellent thermal protection to the PET substrate during SPEL, as well as good surface adhesion and etching resistance. Nanoimprinting plus a lift off created a large-area array of Cr squares (100 nm x 130 nm) on HSQ and SPEL changed each Cr square into a perfectly round Cr dot with a diameter of 60 nm, reducing the Cr footprint area by 78%. Compared to bare PET, the use of HSQ also reduced the variation in the diameter of the Cr dots from 11.3 nm (standard deviation) to 1.7 nm, an improvement of over 660%. This new technology can be scaled to much larger areas (including roll-to-roll web processing) and thus potentially has applications in various fields.

Fabrication of a 60-nm-diameter perfectly round metal-dot array over a large area on a plastic substrate using nanoimprint lithography and self-perfection by liquefaction.

We propose and demonstrate a new SERS substrate architecture that couples a dense three-dimensional (3-D) cavity nanoantenna array, through nano-gaps, with dense plasmonic nanodots; and a new nanofabrication that combines nanoimprint, guided self-assembly and self-alignment and has fabricated the architecture precisely, simply, inexpensively and over large area (4-inch wafer). We experimentally achieved not only high area-average SERS enhancement (1.2×10⁹) but also excellent uniformity (22.4% variation) at the same time over the entire large-area sample by measuring 90 points with a regular mapping distance. The best uniformity achieved is 15% variation over 1.6 mm by 1.6 mm area at slightly lower enhancement factor and is independent of the excitation laser probe size, which had an area varying from ~1 to 10,000 μm2.

Three-dimensional cavity nanoantenna coupled plasmonic nanodots for ultrahigh and uniform surface-enhanced Raman scattering over large area.

Nano-graphene ribbons are promising in many electronic applications, as their bandgaps can be opened by reducing the widths, e.g. below 20 nm. However, a high-throughput method to pattern large-area nano-graphene features is still not available. Here we report a fabrication method of sub-20 nm ribbons on graphite stamps by nanoimprint lithography and a transfer-printing of the graphene ribbons to a Si wafer using electrostatic force assisted bonding. These methods provide a path for fast and high-throughput nano-graphene device production.

Printing of sub-20 nm wide graphene ribbon arrays using nanoimprinted graphite stamps and electrostatic force assisted bonding.

We observed that when subwavelength-sized holes in an optically opaque metal film are completely covered by opaque metal disks larger than the holes, the light transmission through the holes is not reduced, but rather enhanced. Particularly we report (i) the observation of light transmission through the holes blocked by the metal disks up to 70% larger than the unblocked holes; (ii) the observation of tuning the light transmission by varying the coupling strength between the blocking disks and the hole array, or by changing the size of the disks and holes; (iii) the observation and simulation that the metal disk blocker can improve light coupling from free space to a subwavelength hole; and (iv) the simulation that shows the light transmission through subwavelength holes can be enhanced, even though the gap between the disk and the metal film is partially connected with a metal. We believe these finding should have broad and significant impacts and applications to optical systems in many fields.

Extraordinary light transmission through opaque thin metal film with subwavelength holes blocked by metal disks.

Using a new nanoplasmonic architecture and an optimized spacer, we observed the following: (a) the average fluorescence of an infrared dye (indocyanine green) is enhanced by 2970 fold uniformly (variation < 11%) over a large sample area and over a wide range of dye concentrations (380 to 380,000 molecule µm(-2)), laser excitation powers and laser beam sizes; and (b) for a single molecule placed at a 'hot spot', the fluorescence enhancement is 4.5 × 10(6) fold. The giant and uniform enhancements (orders of magnitude higher than before), plus easy and inexpensive large area fabrication ( > 4″ wafers), should open up wide applications.

Giant and uniform fluorescence enhancement over large areas using plasmonic nanodots in 3D resonant cavity nanoantenna by nanoimprinting.

Quantum lithography (QL) is a revolutionary approach to significantly increase the throughput and lower the cost of electron beam lithography in writing large-area masks with nanoscale features. A major challenge in QL is that its principle can be readily applied to positive- but not negative-tone QL. In fact, negative-tone QL, which is as indispensable as positive-tone QL in practical usage, has not been achieved. Here we propose a new method to overcome the obstacle, and report the first experimental demonstration of negative-tone QL. The new method uses a new type of nanoimprinted blank with the nanoscale tiles made of an aluminum/chromium bi-layer of metals, and a novel electrochemical process that removes only non-tagged quantized tiles of the new blank while keeping tagged ones. The demonstrated negative-tone QL has a 200 nm pitch and 30 nm gap and can be further scaled down to even smaller pitch sizes.

Nanoscale negative-tone quantized patterning by novel selective electrochemical etching of a nanoimprinted sub-200 nm bimetallic tile array.

A challenge in the fabrication of nanostructures into non-planar substrates is to form a thin, uniform resist film on non-planar surfaces. This is critical to the fabrication of nanostructures via a lithographic technique due to the subsequent pattern transfer process. Here we report a new double transfer UV-curing nanoimprint technique that can create a nanopatterned thin film with a uniform residual layer not only on flat substrates but also on highly curved surfaces. Surface relief gratings with pitches down to 200 nm are successfully imprinted on the cylindrical surface of optical fibers, and further transferred into a SiO2 matrix using reactive ion etching (RIE), demonstrating that our technique is applicable for fabricating high-resolution nanostructures on non-planar substrates.

Double transfer UV-curing nanoimprint lithography.

We theoretically study the metal-insulator-metal (MIM) structure based ultrathin broadband optical absorber which consists of a metallic substrate, a dielectric middle layer, and a nanostructured metallic top layer. It is found that, there exists an effective permittivity, εnull, for the top nanostructured metallic layer which leads to unit-absorption (zero-reflection) of the MIM structure. Importantly, this εnull exhibits abnormal dispersion behaviors. Both its real and imaginary parts increase monotonically with the wavelength. To obtain such naturally non-existing permittivity, we investigate the optical properties of two typical types of metal-dielectric nanocomposites, namely, thoroughly mingled composites using Bruggeman's effective medium theory, and more realistic Au nanosphere-in-dielectric structures using numerical permittivity retrieval techniques. We demonstrate that the εnull-type dispersions, and consequently, perfect absorption can be obtained over a broad spectral range when the filling factor of the metal component is close to the percolation threshold. The result not only explains the recently reported broadband absorbers made of randomly deposited Au nanoparticles [M. K. Hedayati, et al, Adv. Mater. 23, 5410 (2011)], but also provides theoretical guidelines for designing ultrathin broadband plasmonic absorbers for a wealthy of important applications.

Dispersion engineering of plasmonic nanocomposite for ultrathin broadband optical absorber.

Plectasin might serve as a substitute for traditional antibiotics, but its yields and antimicrobial activities warrant further investigation.

Comparison of inducible versus constitutive expression of plectasin on yields and antimicrobial activities in Pichia pastoris.

A new structure of flexible transparent electrodes is reported, featuring a metal mesh fully embedded and mechanically anchored in a flexible substrate, and a cost-effective solution-based fabrication strategy for this new transparent electrode. The embedded nature of the metal-mesh electrodes provides a series of advantages, including surface smoothness that is crucial for device fabrication, mechanical stability under high bending stress, strong adhesion to the substrate with excellent flexibility, and favorable resistance against moisture, oxygen, and chemicals. The novel fabrication process replaces vacuum-based metal deposition with an electrodeposition process and is potentially suitable for high-throughput, large-volume, and low-cost production. In particular, this strategy enables fabrication of a high-aspect-ratio (thickness to linewidth) metal mesh, substantially improving conductivity without considerably sacrificing transparency. Various prototype flexible transparent electrodes are demonstrated with transmittance higher than 90% and sheet resistance below 1 ohm sq(-1) , as well as extremely high figures of merit up to 1.5 × 10(4) , which are among the highest reported values in recent studies. Finally using our embedded metal-mesh electrode, a flexible transparent thin-film heater is demonstrated with a low power density requirement, rapid response time, and a low operating voltage.

High-Performance Flexible Transparent Electrode with an Embedded Metal Mesh Fabricated by Cost-Effective Solution Process.

A new type of embedded metal-mesh transparent electrode (EMTE) with in-situ electrodeposited catalytic platinum nanoparticles (PtNPs) is developed as a high-performance counter electrode (CE) for lightweight flexible bifacial dye-sensitized solar cells (DSSCs). The thick but narrow nickel micromesh fully embedded in a plastic film provides superior electrical conductivity, optical transmittance, and mechanical stability to the novel electrode. PtNPs decorated selectively on the nickel micromesh surface provide catalytic function with minimum material cost and without interfering with optical transparency. Facile and fully solution-processed fabrication of the novel CE is demonstrated with potential for scalable and cost-effective production. Using this PtNP-decorated nickel EMTE as the CE and titanium foil as the photoanode, unifacial flexible DSSCs are fabricated with a power conversion efficiency (PCE) of 6.91%. By replacing the titanium foil with a transparent ITO-PEN photoanode, full-plastic bifacial DSSCs are fabricated and tested, demonstrating a remarkable PCE of 4.87% under rear-side illumination, which approaches 85% of the 5.67% PCE under front-side illumination, among the highest ratio in published results. These promising results reveal the enormous potential of this hybrid transparent CE in scalable production and commercialization of low-cost and efficient flexible DSSCs.

Solution-Processed Transparent Nickel-Mesh Counter Electrode with in-Situ Electrodeposited Platinum Nanoparticles for Full-Plastic Bifacial Dye-Sensitized Solar Cells.

Article	Year
Quantized patterning using nanoimprinted blanks. Nanotechnology\| PubMed ID: 19420545	2009
Self-limited self-perfection by liquefaction for sub-20 nm trench/line fabrication. Nanotechnology\| PubMed ID: 19847024	2009
Solar-blind deep-UV band-pass filter (250 - 350 nm) consisting of a metal nano-grid fabricated by nanoimprint lithography. Optics express\| PubMed ID: 20173915	2010
Fabrication of a 60-nm-diameter perfectly round metal-dot array over a large area on a plastic substrate using nanoimprint lithography and self-perfection by liquefaction. Small (Weinheim an der Bergstrasse, Germany)\| PubMed ID: 20449853	2010
Three-dimensional cavity nanoantenna coupled plasmonic nanodots for ultrahigh and uniform surface-enhanced Raman scattering over large area. Optics express\| PubMed ID: 21369218	2011
Printing of sub-20 nm wide graphene ribbon arrays using nanoimprinted graphite stamps and electrostatic force assisted bonding. Nanotechnology\| PubMed ID: 21975519	2011
Extraordinary light transmission through opaque thin metal film with subwavelength holes blocked by metal disks. Optics express\| PubMed ID: 21997118	2011
Giant and uniform fluorescence enhancement over large areas using plasmonic nanodots in 3D resonant cavity nanoantenna by nanoimprinting. Nanotechnology\| PubMed ID: 22571971	2012
Nanoscale negative-tone quantized patterning by novel selective electrochemical etching of a nanoimprinted sub-200 nm bimetallic tile array. Nanotechnology\| PubMed ID: 22895113	2012
Double transfer UV-curing nanoimprint lithography. Nanotechnology\| PubMed ID: 24164740	2013
Dispersion engineering of plasmonic nanocomposite for ultrathin broadband optical absorber. Optics express\| PubMed ID: 25836100	2015
Comparison of inducible versus constitutive expression of plectasin on yields and antimicrobial activities in Pichia pastoris. Protein expression and purification\| PubMed ID: 26500192	2016
High-Performance Flexible Transparent Electrode with an Embedded Metal Mesh Fabricated by Cost-Effective Solution Process. Small (Weinheim an der Bergstrasse, Germany)\| PubMed ID: 27027390	2016
Solution-Processed Transparent Nickel-Mesh Counter Electrode with in-Situ Electrodeposited Platinum Nanoparticles for Full-Plastic Bifacial Dye-Sensitized Solar Cells. ACS applied materials & interfaces\| PubMed ID: 28170221	2017

Wen-Di Li

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