We acknowledge Paul J. L. Webster for the 3D rendering of optical data with Amira. Phil Mu&ntilde;oz and Benjamin Franta provided feedback on the manuscript throughout its development. The research described in this paper was supported by the Air Force Office of Scientific Research under grants FA9550-09-1-0546 and FA9550-10-1-0402.

1. Preparing Metal-ion Doped Polymer Film
<ol>
 <li>Measure 8 ml of water in a beaker.</li>
 <li>Add 206 mg of PVP to water. Mix using magnetic stirrer or vortex mixer until the solution is clear.</li>
 <li>Add 210 mg of AgNO3 to solution. Mix using magnetic stirrer or vortex mixer until solution is clear.</li>
 <li>Coat glass slide with solution through drop casting.</li>
 <li>Place glass slide in an oven set at 100 &deg;C. Bake sample for 30 min.</li>
 <li>Remove sample from oven and let cool for 30 min.</li>
</ol>
2. Fabrication of Disconnected Silver Structures
<ol>
 <li>Align setup depicted in Figure 1 on optical table with vibration isolators.</li>
 <li>Adjust compressor to obtain 50-fsec pulses after microscope objective.</li>
 <li>Adjust neutral density filters to obtain 3-nJ pulses after the objective.</li>
 <li>Ensure laser spot size is larger than back aperture of microscope objective.</li>
 <li>Set acousto-optic modulator to produce 10-&mu;sec exposure windows during which the sample is irradiated.</li>
 <li>Block laser beam before it reaches the microscope and place sample onto 3-axis translation stage. The beam path of the femtosecond laser pulses should pass through the imaging microscope objective and into the sample.</li>
 <li>Turn on microscope illumination source to observe the sample in-situ using CCD camera.</li>
 <li>Translate z-axis of stage to find interface between glass substrate and polymer film. Then, refocus microscope to the desired depth inside polymer for patterning bottom-most layer. Z-translation during patterning must be in the direction away from the glass-polymer interface to avoid scattering with fabricated structures.</li>
 <li>Unblock laser-beam and set motion-controller software to translate sample in x-, y- and z- directions with speed of 100 &mu;m/sec. Irradiate single voxels for 10 &mu;sec and separate neighboring voxels by at least several micrometers for clear in-situ imaging. Setting acousto-optic modulator repetition rate to 25 Hz will produce 4-&mu;m spacing. Laser exposed areas will contain silver structures.</li>
</ol>

<ol>
	<li>von Freymann, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Three-Dimensional+Nanostructures+for+Photonics.">Three-Dimensional Nanostructures for Photonics.</a> Advanced Functional Materials. 20, 1038-1052 (2010).</li><li>LaFratta, C. N., Fourkas, J. T., Baldacchini, T., Farrer, R. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Multiphoton+Fabrication.">Multiphoton Fabrication.</a> Angewandte Chemie International Edition. 46, 6238-6258 (2007).</li><li>Gattass, R. R., Mazur, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Femtosecond+laser+micromachining+in+transparent+materials.">Femtosecond laser micromachining in transparent materials.</a> Nat. Photon. 2, 219-225 (2008).</li><li>Li, L., Gattass, R. R., Gershgoren, E., Hwang, H., Fourkas, J. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Achieving+&#955;/20+Resolution+by+One-Color+Initiation+and+Deactivation+of+Polymerization.">Achieving &#955;/20 Resolution by One-Color Initiation and Deactivation of Polymerization.</a> Science. 324, 910-913 (2009).</li><li>Haske, W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=65+nm+feature+sizes+using+visible+wavelength+3-D+multiphoton+lithography.">65 nm feature sizes using visible wavelength 3-D multiphoton lithography.</a> Opt. Express. 15, 3426-3436 (2007).</li><li>Xing, J. F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Improving+spatial+resolution+of+two-photon+microfabrication+by+using+photoinitiator+with+high+initiating+efficiency.">Improving spatial resolution of two-photon microfabrication by using photoinitiator with high initiating efficiency.</a> Appl. Phys. Lett. 90, 131106 (2007).</li><li>Tan, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Reduction+in+feature+size+of+two-photon+polymerization+using+SCR500.">Reduction in feature size of two-photon polymerization using SCR500.</a> Appl. Phys. Lett. 90, 071106 (2007).</li><li>Vora, K., Kang, S., Shukla, S., Mazur, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabrication+of+disconnected+three-dimensional+silver+nanostructures+in+a+polymer+matrix.">Fabrication of disconnected three-dimensional silver nanostructures in a polymer matrix.</a> Appl. Phys. Lett. 100, 063120 (2012).</li><li>G&#252;ney, D. &. #. 2. 1. 4. ;., Koschny, T., Soukoulis, C. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Intra-connected+three-dimensionally+isotropic+bulk+negative+index+photonic+metamaterial.">Intra-connected three-dimensionally isotropic bulk negative index photonic metamaterial.</a> Opt. Express. 18, 12348-12353 (2010).</li><li>Grigorenko, A. N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nanofabricated+media+with+negative+permeability+at+visible+frequencies.">Nanofabricated media with negative permeability at visible frequencies.</a> Nat. Photon. 438, 335-338 (2005).</li><li>Grigorenko, A. N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Negative+refractive+index+in+artificial+metamaterials.">Negative refractive index in artificial metamaterials.</a> Opt. Lett. 31, 2483-2485 (2006).</li><li>Shalaev, V. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Negative+index+of+refraction+in+optical+metamaterials.">Negative index of refraction in optical metamaterials.</a> Opt. Lett. 30, 3356-3358 (2005).</li><li>Ishikawa, A., Tanaka, T., Kawata, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Magnetic+excitation+of+magnetic+resonance+in+metamaterials+at+far-infrared+frequencies.">Magnetic excitation of magnetic resonance in metamaterials at far-infrared frequencies.</a> Appl. Phys. Lett. 91, 113118 (2007).</li><li>Tanaka, T., Ishikawa, A., Kawata, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Two-photon-induced+reduction+of+metal+ions+for+fabricating+three-dimensional+electrically+conductive+metallic+microstructure.">Two-photon-induced reduction of metal ions for fabricating three-dimensional electrically conductive metallic microstructure.</a> Appl. Phys. Lett. 88, 081107 (2006).</li><li>Ishikawa, A., Tanaka, T., Kawata, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Improvement+in+the+reduction+of+silver+ions+in+aqueous+solution+using+two-photon+sensitive+dye.">Improvement in the reduction of silver ions in aqueous solution using two-photon sensitive dye.</a> Appl. Phys. Lett. 89, 113102 (2006).</li><li>Cao, Y. -. Y., Takeyasu, N., Tanaka, T., Duan, X. -. M., Kawata, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=3D+Metallic+Nanostructure+Fabrication+by+Surfactant-Assisted+Multiphoton-Induced+Reduction.">3D Metallic Nanostructure Fabrication by Surfactant-Assisted Multiphoton-Induced Reduction.</a> Small. 5, 1144-1148 (2009).</li></ol>

No conflicts of interest declared.

The key to the process is obtaining a doped dielectric matrix that allows high resolution fabrication, but does not degrade soon after preparation. A simple mixture of PVP, AgNO3 and H2O allows the creation of high-resolution silver nanostructures that are embedded inside a support matrix. Varying the PVP to AgNO3 ratio will change the laser energy needed for fabrication, and potentially other properties such as feature resolution. A low ratio leads to faster degradation of the dielectric matrix, and a high ratio leads to very low amounts of silver in fabricated features.
The minimum laser spot size-which depends on wavelength, laser beam mode parameter, and microscope objective numerical aperture (NA)-is 900 nm for our system. The nonlinear nature of the light-matter interactions can lead to silver features that are smaller than this spot size. We have demonstrated 300-nm silver features using our optical setup.8 The objective used in this experiment has an NA of 0.8 and a working distance of 3 mm, allowing the potential to pattern thick 3D samples. Stronger focusing-an NA of 1.4 is typical for femtosecond laser patterning techniques-would lead to a much smaller laser spot size with the tradeoff of a shorter working distance.
The resolution of the technique could be increased with stronger focusing optics and, potentially, by modifying the chemistry. In the opposite direction, larger features can be readily created by increasing laser energy and irradiation time. Specific shapes, such as short lines, can be obtained by scanning the laser continuously over a distance. Future applications of the technique may include negative index metamaterials, invisibility cloaks, and perfect lenses for the optical and infrared wavelength regimes.9 These applications will strongly depend on the optical properties of the silver nanostructures.

a method to fabricate disconnected silver nanostructures in 3d

The standard nanofabrication toolkit includes techniques primarily aimed at creating 2D patterns in dielectric media. Creating metal patterns on a submicron scale requires a combination of nanofabrication tools and several material processing steps. For example, steps to create planar metal structures using ultraviolet photolithography and electron-beam lithography can include sample exposure, sample development, metal deposition, and metal liftoff. To create 3D metal structures, the sequence is repeated multiple times. The complexity and difficulty of stacking and aligning multiple layers limits practical implementations of 3D metal structuring using standard nanofabrication tools. Femtosecond-laser direct-writing has emerged as a pre-eminent technique for 3D nanofabrication.1,2 Femtosecond lasers are frequently used to create 3D patterns in polymers and glasses.3-7 However, 3D metal direct-writing remains a challenge. Here, we describe a method to fabricate silver nanostructures embedded inside a polymer matrix using a femtosecond laser centered at 800 nm. The method enables the fabrication of patterns not feasible using other techniques, such as 3D arrays of disconnected silver voxels.8 Disconnected 3D metal patterns are useful for metamaterials where unit cells are not in contact with each other,9 such as coupled metal dot10,11or coupled metal rod12,13 resonators. Potential applications include negative index metamaterials, invisibility cloaks, and perfect lenses.
In femtosecond-laser direct-writing, the laser wavelength is chosen such that photons are not linearly absorbed in the target medium. When the laser pulse duration is compressed to the femtosecond time scale and the radiation is tightly focused inside the target, the extremely high intensity induces nonlinear absorption. Multiple photons are absorbed simultaneously to cause electronic transitions that lead to material modification within the focused region. Using this approach, one can form structures in the bulk of a material rather than on its surface.
Most work on 3D direct metal writing has focused on creating self-supported metal structures.14-16 The method described here yields sub-micrometer silver structures that do not need to be self-supported because they are embedded inside a matrix. A doped polymer matrix is prepared using a mixture of silver nitrate (AgNO3), polyvinylpyrrolidone (PVP) and water (H2O). Samples are then patterned by irradiation with an 11-MHz femtosecond laser producing 50-fs pulses. During irradiation, photoreduction of silver ions is induced through nonlinear absorption, creating an aggregate of silver nanoparticles in the focal region. Using this approach we create silver patterns embedded in a doped PVP matrix. Adding 3D translation of the sample extends the patterning to three dimensions.

The acousto-optic modulator and neutral density filters (Figure 1) allow one to control the amount of energy deposited into the sample. Using an exposure of 110 pulses per voxel and 3 nJ per pulse, with the stage translating at 100 &mu;m/sec, the resulting silver structures are readily visible through the in-situ optical microscope. Lower laser exposure levels (by reducing pulse energy and/or pulse number) lead to smaller silver features; we have observed features as small as 300 nm.8 It is possible to create silver structures using a broad range of pulse energies from less than one nanojoule to several nanojoules. Figure 3 shows 3D renderings of optical images taken of a fabricated sample. The pattern, consisting of an array of dots on top of another array, is shown from two angles. The data can also be visualized through videos; sequential optical microscopy images are animated in the video article. The thickness of the polymer matrix is controlled by the amount of solution used during the drop casting process. One milliliter of solution on a 2.5 cm x 2.5 cm glass slide approximately yields a 15-&mu;m thick film.
High-resolution images of fabricated silver structures can be obtained through SEM imaging. Figure 4 shows SEM images of a sample consisting of a 2D array of dots that are fabricated directly on the glass substrate. We readily obtain silver features that are sub-micrometer in size.
<img alt="Figure 1" src="/files/ftp_upload/4399/4399fig1.jpg" /> 
Figure 1. Laser fabrication setup. The primary components of our fabrication setup include a femtosecond laser, a Faraday isolator, a compressor, an acousto-optic modulator (AOM), a neutral-density (ND) filter, a microscope with camera, a high precision 3-axis translation stage, and an optical table mounted on vibration isolators. The laser produces 50-fs laser pulses centered at 800 nm with a repetition rate of 11 MHz. The compressor pre-compensates for the dispersion in the optical beam path to obtain 50-fs pulses at the sample. The AOM and ND filter function as a shutter and an attenuator to control the laser exposure of the sample. We use a 0.8-NA microscope objective to simultaneously focus the laser beam and image the sample during fabrication. The sample position is controlled by a high-precision 3-axis translation stage. The entire setup is mounted on an optical table with vibration isolation.
<img alt="Figure 2" src="/files/ftp_upload/4399/4399fig2.jpg" /> 
Figure 2. Overall schematic of the experiment. A sample is prepared by coating a glass slide with a mixture of PVP, AgNO3, and H2O. Once the sample is prepared, patterning is a single-step process.
<img alt="Figure 3" src="/files/ftp_upload/4399/4399fig3.jpg" /> 
Figure 3. 3D rendered images of a silver dot array inside a matrix. (a) 2-layer array of 18 silver dots created inside a matrix. For clarity, the two layers of dots are represented in different colors. The rendering was created by stacking sequential optical microscopy images. (b) A different view of the 3D array.
<img alt="Figure 4" src="/files/ftp_upload/4399/4399fig4.jpg" /> 
Figure 4. High-resolution SEM images of a patterned sample. Silver dots are created at the glass/polymer interface to allow SEM imaging. The polymer matrix is removed after fabrication to avoid additional silver growth driven by the electron beam.8 (a) Image of a 2D array of silver dots on a glass substrate. A) Close up view of silver dots from a 61&deg; tilt angle.

Watch this Scientific Journal Video about A Method to Fabricate Disconnected Silver Nanostructures in 3D at JoVE.com

A Method to Fabricate Disconnected Silver Nanostructures in 3D

Femtosecond-laser direct-writing is frequently used to create three-dimensional (3D) patterns in polymers and glasses. However, patterning metals in 3D remains a challenge. We describe a method for fabricating silver nanostructures embedded inside a polymer matrix using a femtosecond laser centered at 800 nm.

The standard nanofabrication toolkit includes  techniques primarily aimed at creating 2D patterns in dielectric media. Creating  metal patterns on a ...

a-method-to-fabricate-disconnected-silver-nanostructures-in-3d

Nanyang Technological University

Research

JoVE Journal

Engineering

13.7K Views.  Harvard University. Femtosecond-laser direct-writing is frequently used to create three-dimensional (3D) patterns in polymers and glasses. However, patterning metals in 3D remains a challenge. We describe a method for fabricating silver nanostructures embedded inside a polymer matrix using a femtosecond laser centered at 800 nm.

Video: A Method to Fabricate Disconnected Silver Nanostructures in 3D

High-resolution, High-speed, Three-dimensional Video Imaging with Digital Fringe Projection Techniques

Digital fringe projection (DFP) techniques provide dense 3D measurements of dynamically changing surfaces.&#160;Like the human eyes and brain, DFP uses triangulation between matching points in two views of the same scene at different angles to compute depth.&#160;However, unlike a stereo-based method, DFP uses a digital video projector to replace one of the cameras1. The projector rapidly projects a known sinusoidal pattern onto the subject, and the surface of the subject distorts these patterns in the camera&#8217;s field of view. Three distorted patterns (fringe images) from the camera can be used to compute the depth using triangulation.
Unlike other 3D measurement methods, DFP techniques lead to systems that tend to be faster, lower in equipment cost, more flexible, and easier to develop.&#160;DFP systems can also achieve the same measurement resolution as the camera.&#160;For this reason, DFP and other digital structured light techniques have recently been the focus of intense research (as summarized in1-5). Taking advantage of DFP, the graphics processing unit, and optimized algorithms, we have developed a system capable of 30&#160;Hz 3D video data acquisition, reconstruction, and display for over 300,000 measurement points per frame6,7.&#160;Binary defocusing DFP methods can achieve even greater speeds8.
Diverse applications can benefit from DFP techniques. Our collaborators have used our systems for facial function analysis9, facial animation10, cardiac mechanics studies11, and fluid surface measurements, but many other potential applications exist.&#160;This video will teach the fundamentals of DFP techniques and illustrate the design and operation of a binary defocusing DFP system.

This video describes the fundamentals of digital fringe projection techniques, which provide dense 3D measurements of dynamically changing surfaces.&#160;It also demonstrates the design and operation of a high-speed binary defocusing system based on these techniques.

Digital fringe projection (DFP) techniques provide dense 3D measurements of dynamically changing surfaces.&#160;Like the human eyes and brain, DFP uses ...

Evaluating Plasmonic Transport in Current-carrying Silver Nanowires

Plasmonics is an emerging technology capable of simultaneously transporting a plasmonic signal and an electronic signal on the same information support1,2,3. In this context, metal nanowires are especially desirable for realizing dense routing networks4. A prerequisite to operate such shared nanowire-based platform relies on our ability to electrically contact individual metal nanowires and efficiently excite surface plasmon polaritons5 in this information support. In this article, we describe a protocol to bring electrical terminals to chemically-synthesized silver nanowires6 randomly distributed on a glass substrate7. The positions of the nanowire ends with respect to predefined landmarks are precisely located using standard optical transmission microscopy before encapsulation in an electron-sensitive resist. Trenches representing the electrode layout are subsequently designed by electron-beam lithography. Metal electrodes are then fabricated by thermally evaporating a Cr/Au layer followed by a chemical lift-off. The contacted silver nanowires are finally transferred to a leakage radiation microscope for surface plasmon excitation and characterization8,9. Surface plasmons are launched in the nanowires by focusing a near infrared laser beam on a diffraction-limited spot overlapping one nanowire extremity5,9. For sufficiently large nanowires, the surface plasmon mode leaks into the glass substrate9,10. This leakage radiation is readily detected, imaged, and analyzed in the different conjugate planes in leakage radiation microscopy9,11. The electrical terminals do not affect the plasmon propagation. However, a current-induced morphological deterioration of the nanowire drastically degrades the flow of surface plasmons. The combination of surface plasmon leakage radiation microscopy with a simultaneous analysis of the nanowire electrical transport characteristics reveals the intrinsic limitations of such plasmonic circuitry.

Silver nanowires can simultaneously transport electrons and optical information in the form of surface plasmons. A procedure is described here to realize such a shared circuitry and the limitations at propagating both information carriers are evaluated.

Plasmonics is an emerging technology capable of simultaneously transporting a plasmonic signal and an electronic signal on the same information ...

Writing and Low-Temperature Characterization of Oxide Nanostructures

Oxide nanoelectronics is a rapidly growing field which seeks to develop novel materials with multifunctional behavior at nanoscale dimensions. Oxide interfaces exhibit a wide range of properties that can be controlled include conduction, piezoelectric behavior, ferromagnetism, superconductivity and nonlinear optical properties. Recently, methods for controlling these properties at extreme nanoscale dimensions have been discovered and developed. Here are described explicit step-by-step procedures for creating LaAlO3/SrTiO3 nanostructures using a reversible conductive atomic force microscopy technique. The processing steps for creating electrical contacts to the LaAlO3/SrTiO3 interface are first described. Conductive nanostructures are created by applying voltages to a conductive atomic force microscope tip and locally switching the LaAlO3/SrTiO3 interface to a conductive state. A versatile nanolithography toolkit has been developed expressly for the purpose of controlling the atomic force microscope (AFM) tip path and voltage. Then, these nanostructures are placed in a cryostat and transport measurements are performed. The procedures described here should be useful to others wishing to conduct research in oxide nanoelectronics.

Oxide nanostructures provide new opportunities for science and technology. The interfacial conductivity between LaAlO3 and SrTiO3 can be controlled with near-atomic precision using a conductive atomic force microscopy technique. The protocol for creating and measuring conductive nanostructures at LaAlO3/SrTiO3 interfaces is demonstrated.

Oxide nanoelectronics is a rapidly growing field which seeks to develop novel materials with multifunctional behavior at nanoscale dimensions.  Oxide ...

Energy Dispersive X-ray Tomography for 3D Elemental Mapping of Individual Nanoparticles

Energy dispersive X-ray spectroscopy within the scanning transmission electron microscope (STEM) provides accurate elemental analysis with high spatial resolution, and is even capable of providing atomically resolved elemental maps. In this technique, a highly focused electron beam is incident upon a thin sample and the energy of emitted X-rays is measured in order to determine the atomic species of material within the beam path. This elementally sensitive spectroscopy technique can be extended to three dimensional tomographic imaging by acquiring multiple spectrum images with the sample tilted along an axis perpendicular to the electron beam direction.
Elemental distributions within single nanoparticles are often important for determining their optical, catalytic and magnetic properties. Techniques such as X-ray tomography and slice and view energy dispersive X-ray mapping in the scanning electron microscope provide elementally sensitive three dimensional imaging but are typically limited to spatial resolutions of &gt; 20 nm. Atom probe tomography provides near atomic resolution but preparing nanoparticle samples for atom probe analysis is often challenging. Thus, elementally sensitive techniques applied within the scanning transmission electron microscope are uniquely placed to study elemental distributions within nanoparticles of dimensions 10-100 nm.
Here, energy dispersive X-ray (EDX) spectroscopy within the STEM is applied to investigate the distribution of elements in single AgAu nanoparticles. The surface segregation of both Ag and Au, at different nanoparticle compositions, has been observed.

The use of energy dispersive X-ray tomography in the scanning transmission electron microscope to characterize elemental distributions within single nanoparticles in three dimensions is described.

Energy dispersive X-ray spectroscopy within the scanning transmission electron microscope (STEM) provides accurate elemental analysis with high spatial ...

Ohmic Contact Fabrication Using a Focused-ion Beam Technique and Electrical Characterization for Layer Semiconductor Nanostructures

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor performance, which suggest a new direction for the development of next-generation ultrathin and flexible photonic and electronic devices. Enhanced luminescence quantum efficiency has been widely observed in these atomically thin 2D crystals. However, dimension effects beyond quantum confinement thicknesses or even at the micrometer scale are not expected and have rarely been observed. In this study, molybdenum diselenide (MoSe2) layer crystals with a thickness range of 6-2,700 nm were fabricated as two- or four-terminal devices. Ohmic contact formation was successfully achieved by the focused-ion beam (FIB) deposition method using platinum (Pt) as a contact metal. Layer crystals with various thicknesses were prepared through simple mechanical exfoliation by using dicing tape. Current-voltage curve measurements were performed to determine the conductivity value of the layer nanocrystals. In addition, high-resolution transmission electron microscopy, selected-area electron diffractometry, and energy-dispersive X-ray spectroscopy were used to characterize the interface of the metal&#8211;semiconductor contact of the FIB-fabricated MoSe2 devices. After applying the approaches, the substantial thickness-dependent electrical conductivity in a wide thickness range for the MoSe2-layer semiconductor was observed. The conductivity increased by over two orders of magnitude from 4.6 to 1,500 &#937;&#8722;1 cm&#8722;1, with a decrease in the thickness from 2,700 to 6 nm. In addition, the temperature-dependent conductivity indicated that the thin MoSe2 multilayers exhibited considerably weak semiconducting behavior with activation energies of 3.5-8.5 meV, which are considerably smaller than those (36-38 meV) of the bulk. Probable surface-dominant transport properties and the presence of a high surface electron concentration in MoSe2 are proposed. Similar results can be obtained for other layer semiconductor materials such as MoS2 and WS2.

We describe the approaches for the device fabrication and electrical characterization of molybdenum diselenide (MoSe2) layer semiconductor nanostructures with different thicknesses. In addition, the fabrication of ohmic contacts for MoSe2-layer nanocrystals by the focused-ion beam deposition method using platinum (Pt) as a contact metal is described.

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor ...

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly known as plasmonic effects, play an important role. Research in this field has, however, been impeded owing to the difficulty of fabricating devices containing the desired functional metal nanostructures. In order to provide a viable strategy to this issue, we herein show a transfer printing-based approach that allows the quick and low-cost integration of designed metal nanostructures with a variety of device architectures, including solar cells. Nanopillar poly(dimethylsiloxane) (PDMS) stamps were fabricated from a commercially available nanohole plastic film as a master mold. On this nanopatterned PDMS stamps, Ag films were deposited, which were then transfer-printed onto block copolymer (binding layer)-coated hydrogenated microcrystalline Si (&#181;c-Si:H) surface to afford ordered Ag nanodisk structures. It was confirmed that the resulting Ag nanodisk-incorporated &#181;c-Si:H solar cells show higher performances compared to a cell without the transfer-printed Ag nanodisks, thanks to plasmonic light trapping effect derived from the Ag nanodisks. Because of the simplicity and versatility, further device application would also be feasible thorough this approach.

A viable transfer printing-based methodology to introduce plasmonic metal nanostructures in solar cells is described. Using nanopillar poly(dimethylsiloxane) stamps, an Ag-based ordered nanodisk array was integrated with standard hydrogenated microcrystalline Si solar cells, which led to improved device performances due to plasmonic light trapping.

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly ...

Methods for Measuring the Orientation and Rotation Rate of 3D-printed Particles in Turbulence

Experimental methods are presented for measuring the rotational and translational motion of anisotropic particles in turbulent fluid flows. 3D printing technology is used to fabricate particles with slender arms connected at a common center. Shapes explored are crosses (two perpendicular rods), jacks (three perpendicular rods), triads (three rods in triangular planar symmetry), and tetrads (four arms in tetrahedral symmetry). Methods for producing on the order of 10,000 fluorescently dyed particles are described. Time-resolved measurements of their orientation and solid-body rotation rate are obtained from four synchronized videos of their motion in a turbulent flow between oscillating grids with R&#955; = 91. In this relatively low-Reynolds number flow, the advected particles are small enough that they approximate ellipsoidal tracer particles. We present results of time-resolved 3D trajectories of position and orientation of the particles as well as measurements of their rotation rates.

We use 3D printing to fabricate anisotropic particles in the shapes of jacks, crosses, tetrads, and triads, whose alignments and rotations in turbulent fluid flow can be measured from multiple simultaneous video images.

Experimental methods are presented for measuring the rotational and translational motion of anisotropic particles in turbulent fluid flows. 3D printing ...

A Fabrication Method for Highly Stretchable Conductors with Silver Nanowires

Stretchable electronics are identified as a key technology for electronic applications in the next generation. One of the challenges in fabrication of stretchable electronic devices is the preparation of stretchable conductors with great mechanical stability. In this study, we developed a simple fabrication method to chemically solder the contact points between silver nanowire (AgNW) networks. AgNW nanomesh was first deposited on a glass slide via spray coating method. A reactive ink composed of silver nanoparticle (AgNPs) precursors was applied over the spray coated AgNW thin films. After heating for 40 min, AgNPs were preferentially generated over the nanowire junctions to solder the AgNW nanomesh, and reinforced the conducting network. The chemically modified AgNW thin film was then transferred to polyurethane (PU) substrates by casting method. The soldered AgNW thin films on PU exhibited no obvious change in electrical conductivity under stretching or rolling process with elongation strains up to 120%.

A simple synthesis method is used to chemically solder silver nanowire thin film to fabricate highly stretchable and conductive metal conductors.

Stretchable electronics are identified as a key technology for electronic applications in the next generation. One of the challenges in fabrication of ...

Challenges in Rheological Characterization of Highly Concentrated Suspensions &#8212; A Case Study for Screen-printing Silver Pastes

A comprehensive rheological characterization of highly concentrated suspensions or pastes is mandatory for a targeted product development meeting the manifold requirements during processing and application of such complex fluids. In this investigation, measuring protocols for a conclusive assessment of different process relevant rheological parameters have been evaluated. This includes the determination of yield stress, viscosity, wall slip velocity, structural recovery after large deformation and elongation at break as well as tensile force during filament stretching.
The importance of concomitant video recordings during parallel-plate rotational rheometry for a significant determination of rheological quantities is demonstrated. The deformation profile and flow field at the sample edge can be determined using appropriate markers. Thus, measurement parameter settings and plate roughness values can be identified for which yield stress and viscosity measurements are possible. Slip velocity can be measured directly and measuring conditions at which plug flow, shear banding or sample spillover occur can be identified clearly. 
Video recordings further confirm that the change in shear moduli observed during three stage oscillatory shear tests with small deformation amplitude in stage I and III but large oscillation amplitude in stage II can be directly attributed to structural break down and recovery. For the pastes investigated here, the degree of irreversible, shear-induced structural change increases with increasing deformation amplitude in stage II until a saturation is reached at deformations corresponding to the crossover of G' and G'', but the irreversible damage is independent of the duration of large amplitude shear. 
A capillary breakup elongational rheometer and a tensile tester have been used to characterize deformation and breakup behavior of highly filled pastes in uniaxial elongation. Significant differences were observed in all experiments described above for two commercial screen-printing silver pastes used for front side metallization of Si-solar cells.

A protocol for a robust and application relevant rheological characterization of highly concentrated suspensions is presented. Silver pastes used for screen-printing application in solar cell production are employed as model systems.

A comprehensive rheological characterization of highly concentrated suspensions or pastes is mandatory for a targeted product development meeting the ...

A Standard and Reliable Method to Fabricate Two-Dimensional Nanoelectronics

Two-dimensional (2D) materials have attracted huge attention due to their unique properties and potential applications. Since wafer scale synthesis of 2D materials is still in nascent stages, scientists cannot fully rely on traditional semiconductor techniques for related research. Delicate processes from locating the materials to electrode definition need to be well controlled. In this article, a universal fabrication protocol required in manufacturing nanoscale electronics, such as 2D quasi-heterojunction bipolar transistors (Q-HBT), and 2D back-gated transistors are demonstrated. This protocol includes the determination of material position, electron beam lithography (EBL), metal electrode definition, et al. A step by step narrative of the fabrication procedures for these devices are also presented. Furthermore, results show that each of the fabricated devices has achieved high performance with high repeatability. This work reveals a comprehensive description of process flow for preparing 2D nano-electronics, enables the research groups to access this information, and pave the way toward future electronics.

The article aims to introduce a standard and reliable fabrication procedure for the development of future low dimensional nanoelectronics.

Two-dimensional (2D) materials have attracted huge attention due to their unique properties and potential applications. Since wafer scale synthesis of 2D ...