Vignesh Suresh

Macroscopic high density nanodisc arrays of zinc oxide fabricated by block copolymer self-assembly assisted nanoimprint lithography

We report a facile means of creating nanodisc arrays of ZnO with high densities (∼22 Gbit in−2) and narrow distributions in size, shape and periodicities (<15%) using a combination of block copolymer self-assembly and nanoimprint lithography (NIL). ZnO nanodisc arrays with sub-100 nm spatial resolutions, using high-throughput and manufacturing compatible approaches are realized. The fabrication combines benefits from the use of NIL, which is a high-throughput and repeatable tool and from the use of block copolymer self-assembly which provides for low-cost production of high-resolution NIL molds. Preliminary results of the investigation of memory performance of these arrays within MOS capacitor devices show a flat-band voltage shift of 2.53 V at a relatively low operating voltage of 10 V. A high charge trap density of 2.3 × 1018 cm−3 combined with excellent retention of ∼80% after 1000 s of discharging is observed with low tunnelling oxide thickness of 3 nm, demonstrate significant promise of the ZnO nanodiscs to act as charge storage centers in non-volatile flash memory devices.

Hierarchically Built Hetero-superstructure Arrays with Structurally Controlled Material Compositions

Hierarchical assemblies are repeatedly encountered in nature, and when replicated in synthetic patterns and materials, can enhance their functionality or impart multifunctionality. In order to assemble a hierarchical superstructure that consists of components made up of multiple nanostructures, control over placement and stoichiometry is desirable. Macroscopic arrays that present up to three levels of hierarchy are demonstrated here and are achieved using the self-assembly of soft, collapsible block copolymer nanospheres for the first two levels, followed by directed self-assembly of metal nanospheres for the third. The fabrication approach combines advantages of soft sphere self-assembly to yield non-close-packed and variable array pitch values, with the inherent chemical functionality presented by the polymer-based soft spheres; these assemblies can then be transformed into a range of different materials, including metal or semiconductor nanostructures, or further tailored with an additional level of complexity. Structural investigation shows the superstructure formation to be governed by generic design rules that can be extended across different material combinations.

Engineering 3D Nanoplasmonic Assemblies for High Performance Spectroscopic Sensing.

We demonstrate the fabrication of plasmonic sensors that comprise gold nanopillar arrays exhibiting high surface areas, and narrow gaps, through self-assembly of amphiphilic diblock copolymer micelles on silicon substrates. Silicon nanopillars with high integrity over arbitrary large areas are obtained using copolymer micelles as lithographic templates. The gaps between metal features are controlled by varying the thickness of the evaporated gold. The resulting gold metal nanopillar arrays exhibit an engineered surface topography, together with uniform and controlled separations down to sub-10 nm suitable for highly sensitive detection of molecular analytes by Surface Enhanced Raman Spectroscopy (SERS). The significance of the approach is demonstrated through the control exercised at each step, including template preparation and pattern-transfer steps. The approach is a promising means to address trade-offs between resolutions, throughput, and performance in the fabrication of nanoplasmonic assemblies for sensing applications.

Impact of molybdenum out diffusion and interface quality on the performance of sputter grown CZTS based solar cells

We have investigated the impact of Cu2ZnSnS4-Molybdenum (Mo) interface quality on the performance of sputter-grown Cu2ZnSnS4 (CZTS) solar cell. Thin film CZTS was deposited by sputter deposition technique using stoichiometry quaternary CZTS target. Formation of molybdenum sulphide (MoSx) interfacial layer is observed in sputter grown CZTS films after sulphurization. Thickness of MoSx layer is found ~142 nm when CZTS layer (550 nm thick) is sulphurized at 600 °C. Thickness of MoSx layer significantly increased to ~240 nm in case of thicker CZTS layer (650 nm) under similar sulphurization condition. We also observe that high temperature (600 °C) annealing suppress the elemental impurities (Cu, Zn, Sn) at interfacial layer. The amount of out-diffused Mo significantly varies with the change in sulphurization temperature. The out-diffused Mo into CZTS layer and reconstructed interfacial layer remarkably decreases series resistance and increases shunt resistance of the solar cell. The overall efficiency of the solar cell is improved by nearly five times when 600 °C sulphurized CZTS layer is applied in place of 500 °C sulphurized layer. Molybdenum and sulphur diffusion reconstruct the interface layer during heat treatment and play the major role in charge carrier dynamics of a photovoltaic device.

Quantitative Detection with Surface Enhanced Raman Scattering (SERS) Using Self-Assembled Gold Nanoparticle Cluster Arrays

We analysed sensitivity of high-density arrays of self-assembled gold nanoparticle clusters towards trace analyte detection and quantitative determination by surface enhanced Raman spectroscopy (SERS) employing an aromatic thiol as probe molecule. Periodic nanoscale arrays of gold nanoparticle clusters consisting of an average of 18 nanoparticles per cluster, and exhibiting mean inter-particle and inter-cluster separations below 10 nm were prepared using electrostatic self-assembly on block copolymer templates. The concentration dependent scaling of SERS intensities and the lowest detection limits on the cluster arrays on silicon substrate was probed using 1-naphthalenethiol (NT) as test molecule. The substrates show a detection limit of 10 nM along with high sensitivity to changes in NT concentration, which we attribute to high density of hot-spots uniformly organised across the surface. The capability for facile realisation of such arrays without a clean room environment or expensive tools makes the approach suitable for adoption for economic and high-performing SERS sensors.

Flexible, transparent and robust SERS tapes through a two-step block copolymer self-assembly process

Surface enhanced Raman spectroscopy (SERS) is an analytical technique that offers the capability of remote sensing, single molecule detection, and detection of trace contaminants (in parts per million) with high sensitivity and accuracy. Here, we demonstrate a simple and economical method for fabricating large area SERS-active tapes that are flexible, transparent and robust using a two-step process. The first is the fabrication of the gold nanoclusters on a flat chip using block copolymer self-assembly followed by directed electrostatic self-organization of gold nanoparticles (AuNPs). The second step involves the transfer of the resulting metal nanoclusters onto a thermal tape by a simple ‘stick and peel’ technique. Such substrates facilitate the detection and quantification of contaminants on irregular surfaces such as fruit skin, fabrics and other non-planar surfaces without the need to extract the analyte. Furthermore, the SERS measurements are highly quantitative, reproducible and the two-step fabrication process is unprecedented and has potential towards realizing large scale manufacturing of low-cost, flexible SERS-tape for on-field applications.

Hierarchically built gold nanoparticle supercluster arrays as charge storage centers for enhancing the performance of flash memory devices

Flash memory devices with high-performance levels exhibiting high charge storage capacity, good charge retention, and high write/erase speeds with lower operating voltages are widely in demand. In this direction, we demonstrate hierarchical self-assembly of gold nanoparticles based on block copolymer templates as a promising route to engineer nanoparticle assemblies with high nanoparticle densities for application in nanocrystal flash memories. The hierarchical self-assembly process allows systematic multiplication of nanoparticle densities with minimal increase in footprint, thereby increasing the charge storage density without an increase in operating voltage. The protocol involves creation of a parent template composed of gold nanoclusters that guides the self-assembly of diblock copolymer reverse micelles which in turn directs electrostatic assembly of gold nanoparticles resulting in a three-level hierarchical system. Capacitance-voltage (C-V) measurements of the hierarchical nanopatterns with a metal-insulator-semiconductor capacitor configuration reveal promising enhancement in memory window as compared to nonhierarchical nanoparticle controls. Capacitance-time (C-t) measurements show that over half the stored charges were retained when extrapolated to 10 years. The fabrication route can be readily extended to programmed density multiplication of features made of other potential charge storage materials such as platinum, palladium, or hybrid metal/metal oxides for next generation, solution-processable flash memory devices.

Ultrathin Film Broadband Terahertz Antireflection Coating Based on Impedance Matching Method

Broadband antireflection coatings for terahertz (THz) components are extremely important in the application of THz technology. We present an in-depth materials study on the broadband antireflection design based on the concept of impedance matching. Several selected conductive materials are tested and compared. They can all be used as THz broadband antireflection coatings regardless of their nature of conductivity. We provide a facile method to approximately predict the impedance matching condition for various conductive materials based on their dc real conductivity. The theoretical predications are in good agreement with experimental results and published data. Doped metal oxides or polymers are recommended as good candidates for fabricating antireflection coating in THz range. The thickness-independent dc real conductivity enables better control on individual tuning parameter-film thickness. Given a substrate, THz transmission in antireflection coating has a theoretical upper limit which is further reduced with non-negligible imaginary conductivity.

Gold nanoparticle density-multiplication by tuning block copolymer self-assembly processes toward increased charge storage

We describe a simple and versatile approach for enhanced nanoparticle density multiplication through the block copolymer self-assembly technique for application in memory devices. Templates of block copolymers with functional groups directed the selective electrostatic self-assembly of the pre-formed gold nanoparticles to form gold nanocluster arrays. By simply increasing the density of the polymer templates by manipulating the spin coating conditions, a lateral increase in the nanoparticle density is observed. The significance of the particle density multiplication was best observed when they were used as charge storage centers in flash memories. Minimization of the pitch (or maximization of the template density) resulted in a maximum memory window of about 1.63 V, with a charge trap state density of 4.93 × 1011 cm−2 in the gold nanocluster arrays. The reported approach offers exciting opportunities to fabricate multicomponent nanostructure-based memory devices tailored for enhanced memory performance. In addition, the nanoparticle density can be increased significantly further when combined systematically with the hierarchical block copolymer self-assembly approach.

In situ application of polyelectrolytes in zinc oxide nanorod synthesis: understanding the effects on the structural and optical characteristics.

We report a facile and simple means of synthesizing a macroscopic array of ZnO nanorods with high feature densities using a modified hydrothermal approach that involves the in situ introduction of polyelectrolyte. The ZnO nanorod arrays with heights of 1.5 μm and diameters of 350 nm were consistently reproducible and were bestowed with the advantage of in situ process tunability offered by employing polyethylenimine (PEI) as a surface modifying agent. The fabrication combines benefits from the hydrothermal approach in terms of process simplicity and flexibility and from the use polyelectrolyte that offers a better nanorod surface, quenched defect levels and enhancement of the UV band edge emission. Structural and elemental analysis of the PEI-modified and unmodified nanorods emphasize the fact that the intentional introduction of PEI results in a nanorod with better surface quality as evidenced by photoluminescence (PL) spectra. The tunability of the feature dimensions of the nanorods and an analysis of the bulk and surface (surface defect) responses to the PL point to significant promise of high density orthogonal nanorods in a number of optoelectronic applications. While the defects in the ZnO nanorods can point towards the application of ZnO nanorods in charge trap flash memory devices, highly crystalline, size tunable, high aspect ratio nanorods find applications as building components in solid state lighting.