Cihan Yilmaz

A Nanoparticle Convective Directed Assembly Process for the Fabrication of Periodic Surface Enhanced Raman Spectroscopy Substrates

Surface enhanced Raman scattering was discovered over 30 years ago, when it was noted that the usually weak molecular Raman scattering cross section was increased by orders of magnitude in the vicinity of metal surfaces. [ 1 , 2 ] Despite the early reports of single-molecule detection, [ 3 ] the promise of the technique as the basis for portable chemical sensors has not been fully realized. The reason for this gap between the science and the engineering of SERS lies in the formidable nanofabrication challenges it poses, which is the need to prepare large numbers of very small yet highly controlled “hot spots” as the sensing device. The need for “hot spots” arises because the SERS enhancement is composed of an electromagnetic effect and a chemical (or resonance) enhancement, with the electromagnetic effect being responsible for the majority of the enhancement. The “hot spots” are the manifestations of this fi eld enhancement, occurring only for select plasmonic materials. [ 4 ] Modeling studies of the electromagnetic effect indicate that dimerized plasmonic metal structures offer a signifi cantly higher electromagnetic fi eld enhancement than isolated structures, with the maximum fi elds occurring in the gap between the structures. [ 5 , 6 ] The fi eld enhancement dependence on the gap size is highly nonlinear: it becomes signifi cant when the gap is less than ∼ 10 nm, and then rises steeply with decreasing gap size. The importance of the small gap size on signal strength has been confi rmed experimentally. [ 7 , 8 ] While the SERS effect may be very high in a localized volume of the order of a few nm 3 , the probability that target analyte molecules adsorb there is very small. [ 9 ] Consequently, practical SERS-based sensors require the engineering of a very large number of such “hot spots” over areas that are at least a few mm 2 . Small interparticle spacing with a large number of metal particles has been realized for SERS experiments performed in nanoparticle solutions, which have been “activated” by electrolyte-induced aggregation. [ 10 ] However, solution-based

Monopole antenna arrays for optical trapping, spectroscopy, and sensing

We introduce a nanoplasmonic platform merging multiple modalities for optical trapping, nanospectroscopy, and biosensing applications. Our platform is based on surface plasmon polariton driven monopole antenna arrays combining complementary strengths of localized and extended surface plasmons. Tailoring of spectrally narrow resonances lead to large index sensitivities (S∼675 nm/RIU) with record high figure of merits (FOM∼112.5). These monopole antennas supporting strong light localization with easily accessible near-field enhanced hotspots are suitable for vibrational nanospectroscopy and optical trapping. Strong optical forces (350 pN/W/μm2) are shown at these hotspots enabling directional control with incident light polarization.

Three-Dimensional Crystalline and Homogeneous Metallic Nanostructures Using Directed Assembly of Nanoparticles

Directed assembly of nano building blocks offers a versatile route to the creation of complex nanostructures with unique properties. Bottom-up directed assembly of nanoparticles have been considered as one of the best approaches to fabricate such functional and novel nanostructures. However, there is a dearth of studies on making crystalline, solid, and homogeneous nanostructures. This requires a fundamental understanding of the forces driving the assembly of nanoparticles and precise control of these forces to enable the formation of desired nanostructures. Here, we demonstrate that colloidal nanoparticles can be assembled and simultaneously fused into 3-D solid nanostructures in a single step using externally applied electric field. By understanding the influence of various assembly parameters, we showed the fabrication of 3-D metallic materials with complex geometries such as nanopillars, nanoboxes, and nanorings with feature sizes as small as 25 nm in less than a minute. The fabricated gold nanopillars have a polycrystalline nature, have an electrical resistivity that is lower than or equivalent to electroplated gold, and support strong plasmonic resonances. We also demonstrate that the fabrication process is versatile, as fast as electroplating, and scalable to the millimeter scale. These results indicate that the presented approach will facilitate fabrication of novel 3-D nanomaterials (homogeneous or hybrid) in an aqueous solution at room temperature and pressure, while addressing many of the manufacturing challenges in semiconductor nanoelectronics and nanophotonics.

Large-Scale Nanorods Nanomanufacturing by Electric-Field-Directed Assembly for Nanoscale Device Applications

A fast and highly scalable room-temperature nanomanufacturing process for fabricating metallic nanorods from nanoparticles for applications such as interconnects and sensors is presented. Metallic nanoparticles are precisely assembled into prefabricated vias by applying a controlled dynamic electric field between the electrodes at the bottom of the vias and a counter electrode placed far away from the vias. The nanoscale vias are fabricated employing conventional electron-beam nanolithography. The dimension of the fabricated nanorods is controlled by the size of the vias and assembly parameters such as the amplitude and frequency of the applied electric field. The mechanism of the assembly process is discussed by examining the effect of the applied voltages on the assembly process to provide a fundamental understanding for scaling down to nanoscale dimensions. This room-temperature aqueous fabrication process is environmentally friendly and can be used to make nanorods using different types of metallic particles.

Size-selective template-assisted electrophoretic assembly of nanoparticles for biosensing applications.

The precise, size-selective assembly of nanoparticles gives rise to many applications where the assembly of nano building blocks with different biological or chemical functionalizations is necessary. We introduce a simple, fast, reproducible-directed assembly technique that enables a complete sorting of nanoparticles with single-particle resolution. Nanoparticles are size-selectively assembled into prefabricated via arrays using a sequential template-directed electrophoretic assembly method. Polystyrene latex (PSL) nanoparticles with diameters ranging from 200 to 50 nm are selectively assembled into vias comparable to nanoparticle diameter. We investigate the effects of particle size and via size on the sorting efficiency. We show that complete sorting can be achieved when the size of the vias is close to the diameter of the nanoparticles and the size distribution of the chosen nanoparticles does not overlap. The results also show that it is necessary to keep the electric field on during the insertion and removal of the template. To elucidate the versatility and nil effects that the electrophoresis assembly technique has on the assembled nanoparticle characteristics, we have assembled cancer-specific monoclonal antibody-2C5-coated nanoparticles and have also shown that they can successfully measure low concentrations of the nucleosome (NS) antigen.

Adhesion of graphene sheet on nano-patterned substrates with nano-pillar array

An array of gold nano-pillars is fabricated on silicon, before chemical vapor deposited graphene is transferred to the substrate. Intrinsic intersurface attraction pulls the monolayer into intimate contact conforming to the substrate geometry, but the pillars support an array of circular blisters. A simple delamination mechanics model is constructed to extract the materials and interface properties. The graphene-gold interfacial adhesion energy is found to be γ = 450 ± 100 mJ m−2 by measuring the blister dimension. Should the ratio of pillar height to inter-pillar separation falls short of (γ/Eh)1/4 with graphene elastic modulus, E and thickness, h, the blisters stay isolated; otherwise, adjacent blisters coalesce. Critical design guidelines are set for graphene devices.

High-Rate Assembly of Nanomaterials on Insulating Surfaces Using Electro-Fluidic Directed Assembly

Conductive or semiconducting nanomaterials-based applications such as electronics and sensors often require direct placement of such nanomaterials on insulating surfaces. Most fluidic-based directed assembly techniques on insulating surfaces utilize capillary force and evaporation but are diffusion limited and slow. Electrophoretic-based assembly, on the other hand, is fast but can only be utilized for assembly on a conductive surface. Here, we present a directed assembly technique that enables rapid assembly of nanomaterials on insulating surfaces. The approach leverages and combines fluidic and electrophoretic assembly by applying the electric field through an insulating surface via a conductive film underneath. The approach (called electro-fluidic) yields an assembly process that is 2 orders of magnitude faster compared to fluidic assembly. By understanding the forces on the assembly process, we have demonstrated the controlled assembly of various types of nanomaterials that are conducting, semiconducting, and insulating including nanoparticles and single-walled carbon nanotubes on insulating rigid and flexible substrates. The presented approach shows great promise for making practical devices in miniaturized sensors and flexible electronics.

Directed assembly-based printing of homogeneous and hybrid nanorods using dielectrophoresis

Printing nano and microscale three-dimensional (3D) structures using directed assembly of nanoparticles has many potential applications in electronics, photonics and biotechnology. This paper presents a reproducible and scalable 3D dielectrophoresis assembly process for printing homogeneous silica and hybrid silica/gold nanorods from silica and gold nanoparticles. The nanoparticles are assembled into patterned vias under a dielectrophoretic force generated by an alternating current (AC) field, and then completely fused in situ to form nanorods. The assembly process is governed by the applied AC voltage amplitude and frequency, pattern geometry, and assembly time. Here, we find out that complete assembly of nanorods is not possible without applying both dielectrophoresis and electrophoresis. Therefore, a direct current offset voltage is used to add an additional electrophoretic force to the assembly process. The assembly can be precisely controlled to print silica nanorods with diameters from 20-200 nm and spacing from 500 nm to 2 μm. The assembled nanorods have good uniformity in diameter and height over a millimeter scale. Besides homogeneous silica nanorods, hybrid silica/gold nanorods are also assembled by sequentially assembling silica and gold nanoparticles. The precision of the assembly process is further demonstrated by assembling a single particle on top of each nanorod to demonstrate an additional level of functionalization. The assembled hybrid silica/gold nanorods have potential to be used for metamaterial applications that require nanoscale structures as well as for plasmonic sensors for biosensing applications.

3-D perpendicular assembly of SWNTs for CMOS interconnects

Due to their superior electrical properties such as high current density and ballistic transport, carbon nanotubes (CNT) are considered as a potential candidate for future very large scale integration (VLSI) interconnects. However, direct incorporation of CNTs into a complimentary metal oxide semiconductor (CMOS) architecture by the conventional chemical vapor deposition (CVD) growth method is problematic because it requires high temperatures that might damage insulators and doped semiconductors in the underlying CMOS circuits. In this paper, we present a directed assembly method to assemble aligned CNTs into pre-patterned vias perpendicular to the substrate. A dynamic electric field with a static offset is applied to provide the force needed for directing the SWNT assembly. It is also shown that by adjusting assembly parameters the density of the assembled CNTs can be significantly enhanced. This highly scalable directed assembly method is conducted at room temperature and pressure and is accomplished in a few minutes. I-V characterization of the assembled CNTs was conducted using a Zyvex nanomanipulator in a scanning electron microscope (SEM) and the measured value of the resistance was 270 kΩs.

Plasmonic nanopillar arrays for optical trapping, biosensing, and spectroscopy

In this work, we propose a unique plasmonic substrate that combine the strength of localized and extended surface plasmons for optical trapping, spectroscopy and biosensing, all in the same platform. The system is based on periodic arrays of gold nanopillars fabricated on a thin gold sheet. The proposed periodic structure exhibits high refractive index sensitivities, as large as 675 nm/RIU which is highly desirable for biosensing applications. The spectrally sharp resonances, we determine a figure of merit, as large as 112.5. The nanopillar array also supports easily accessible high near-field enhancements, as large as 10.000 times, for surface enhanced spectroscopy. The plasmonic hot spots with high intensity enhancement lead to large gradient forces, 350 pN/W/μm2, needed for optical trapping applications.