The research leading to these results has received funding from the European Research Council under the European Community&#8217;s Seventh Framework Program FP7/2007&#8211;2013 Grant Agreement no 306772 and Grant ERC-2007-StG No. 203872-COMOSYEL. This work is &#8232;also partially funded by the Agence Nationale de la Recherche (ANR) under Grant Plastips (ANR-09-BLAN-0049). A. S. thanks a postdoctoral scholarship from the R&#233;gion de Bourgogne under the PARI program. M.S. acknowledges a stipend from the Chinese Scholarship Council. D.Z. acknowledges support from the National Natural Science Foundation of China for Grants 11004182 and 61036005.

1. Synthesis of a Colloidal Solution of Silver&#160;Nanowires
Silver nanowires (Ag NWs) with smooth surface and homogeneous structure are synthesized using a modified polyols-assisted method6. In a typical synthesis of Ag NWs, the following procedures are carried out step by step:
<ol>
	<li>Pour 12 ml&#160;ethylene glycol (EG) in a 100 ml&#160;precleaned, round-bottomed capped flask.</li>
	<li>Heat the solution at 150 &#176;C for 1 hr&#160;in the oil bath (dimethylsilicone) with magnetic stirring at 260 rpm.</li>
	<li>Add 4 ml&#160;AgNO3 (0.2 M in EG solution) by dropping. During this process, the color of the reaction solution changes from colorless to transparent bright yellow, which indicates the growth of silver crystal seeds.</li>
	<li>Slowly inject 4 ml poly-(vinylpyrrolidone) (PVP) solution (the concentration is 22.2 mg/ml&#160;in EG solution) in the reaction solution with a pipette. In the reaction, EG serves as both the solvent and reducing agent, and the PVP enables the silver single crystal to grow along a certain direction.</li>
	<li>Cover the reaction flask with a cap. After 150 min, the color of the solution changes from light yellow to sage green. Finally the colloid appears turbid silver-grey.</li>
	<li>Stop the reaction by placing the flask in an ice-cold water bath to decrease the temperature of the solution.</li>
	<li>Transfer 1 ml&#160;of the colloidal solution to a centrifuge tube filled with 3 ml&#160;of acetone.</li>
	<li>Centrifuge the tube at 600 x&#160;g for 25 min.</li>
	<li>Remove the supernatant and add 4 ml&#160;of DI water.</li>
	<li>Repeat steps 1.8-1.9 at least 3x to remove the excess of EG and PVP.</li>
</ol>
2. Preparing Macroscopic Alignment Landmarks on a Glass Substrate by Electron-beam Lithography
<ol>
	<li>Clean a microscope cover glass N&#176;1 &#189; calibrated for oil immersion microscopy in a diluted concentrated soap solution [1:3] using an ultrasonic bath.</li>
	<li>Rinse first in a large volume of DI water, followed by a second wash with isopropanol. Blow-dry the cover slip with nitrogen.</li>
	<li>Pipette 160 ml&#160;of poly(methyl methacrylate) (PMMA) electron beam resist (see table of reagents) and spin coat it the NW covered substrate with the following successive coating parameters:
	<ol>
		<li>10 sec at a speed of 4 x&#160;g and an acceleration of 4 x&#160;g/sec.</li>
		<li>1 sec at a speed of 350 x&#160;g and an acceleration of 200 x&#160;g/sec.</li>
		<li>60 sec at a speed of 700 x&#160;g and an acceleration of 200 x&#160;g/sec.</li>
	</ol>
	</li>
	<li>Bake the sample at 170 &#176;C for 10 min on a hot plate.</li>
	<li>Repeat steps 2.2-2.4&#160;to produce a second layer of resist to facilitate liftoff at the end of the procedure. Use the same electron-beam resist for both layers.</li>
	<li>Sputter a thin conductive gold layer (20 nm) on the top of the sample before proceeding to electron beam lithography.</li>
	<li>Transfer the sample to an electron-beam microscope equipped for lithography. Using a Faraday cage, set the beam current to 150 pA.</li>
	<li>Expose the sample following the design layout of the grid landmarks with a step size set at 48 nm, a dwell-time per pixel at 0.0243 msec&#160;for a dose of 150 &#181;A/cm2.</li>
	<li>Develop the sample using MIBK (methyl isobutyl ketone): IPA (isopropanol) with a ratio [1:3] for 45 sec. Stop the process by dipping the sample in isopropanol for another 45 sec.</li>
	<li>Transfer the substrate in a metal evaporator operating at a base pressure of 8 x 10-8 mbar. Deposit at an evaporation rate of 0.1 nm/sec 2 nm of Cr (adhesion layer) followed by 70 nm of Au.</li>
	<li>Proceed to a chemical lift-off of the sample using acetone warmed at 70 &#176;C for about 1.5 hr.</li>
</ol>
3. Deposition of the Nanowires on the Substrate
<ol>
	<li>Dilute the nanowire solution in ethanol to obtain a nanowire density on the substrate of about one nanowire every 10 &#181;m2.</li>
	<li>Pipette 50 &#181;l&#160;of the solution and drop-cast it on the prepatterned substrate. Blow&#160;dry with nitrogen.</li>
</ol>
4. Locating the Nanowires by Optical Microscope
<ol>
	<li>Place the substrate decorated with nanowires on a standard calibrated optical microscope.</li>
	<li>Precisely locate the extremities of a series of isolated high-quality nanowires with respect to the closest alignment marks.</li>
</ol>
5. Fabrication of the Electrodes by Electron Beam Lithography, Thermal Evaporation, and Chemical Lift-off
<ol>
	<li>Use the measured positions of the nanowires to design an electrode layout. Incorporate large receiving pads (50 &#181;m x 50 &#181;m) to connect tungsten voltage probes to the electrode reaching the nanowires.</li>
	<li>Pipette 160 ml of poly(methyl methacrylate) (PMMA) electron beam resist (see table of reagents) and spin coat the nanowire-covered substrate with the following successive coating parameters:
	<ol>
		<li>10 sec at a speed of 4 x&#160;g and an acceleration of 4 x&#160;g/sec.</li>
		<li>1 sec at a speed of 350 x&#160;g and an acceleration of 200 x&#160;g/sec.</li>
		<li>60 sec at a speed of 700 x&#160;g and an acceleration of 200 x&#160;g/sec.</li>
	</ol>
	</li>
	<li>Bake the sample at 170 &#176;C for 10 min on a hot plate.</li>
	<li>Repeat steps 5.2-5.3&#160;to produce a second layer of resist to facilitate liftoff at the end of the procedure. Use the same electron-beam resist for both layers.</li>
	<li>Sputter a thin conductive gold layer (20 nm) on the top of the sample before proceeding to electron beam lithography.</li>
	<li>Transfer the sample to an electron-beam microscope equipped for lithography. Using a Faraday cage, set the beam current to 150 pA.</li>
	<li>Using the alignment marks, calibrate the coordinate system of the electron-beam microscope.</li>
	<li>Expose the sample following the design layout with a step size set at 48 nm, a dwell-time per pixel at 0.0243 msec for a dose of 150 &#181;A/cm2.</li>
	<li>Develop the sample using MIBK (methyl isobutyl ketone): IPA (isopropanol) with a ratio [1:3] for 45 sec. Stop the process by dipping the developed sample in isopropanol for another 45 sec.</li>
	<li>Transfer the substrate in a metal evaporator operating at a base pressure of 8 x 10-8 mbar. Deposit at an evaporation rate of 0.1 nm/sec, 2 nm of Cr (adhesion layer) followed by 70 nm of Au.</li>
	<li>Proceed to a chemical lift-off of the sample using acetone warmed at 70 &#176;C for about 1.5 hr.</li>
</ol>
6. Transfer to a Surface Plasmon Leakage Radiation Microscope
<ol>
	<li>Set up a surface plasmon leakage microscope equipped with a high numerical aperture objective (N.A.&#62;1) and a two-axis piezoelectric stage to adjust sample position.</li>
	<li>Prepare a collimated Gaussian beam from a near-infrared laser. Longer excitation wavelength provides longer plasmon propagation distance. Align the beam in the microscope. The diameter of the collimated beam should overfill the entrance pupil of the microscope. This insures the formation of a diffraction-limited focal spot at the glass/air interface.</li>
	<li>With the help of beam splitters and a series of relay lenses positioned at one exit port of the microscope, form two planes conjugate with the object plane and with the Fourier plane (objective back focal plane), respectively. Place two charge-coupled device (CCD) cameras at the location of these conjugate planes.</li>
	<li>Using the piezoelectric stage, align the extremity of a selected contacted nanowire to overlap the focal spot. Adjust the position to maximize light coupling into the plasmon mode.</li>
	<li>Connect a regulated power supply to the receiving pads of the electrical terminals with tungsten tips mounted on three-dimensional mechanical probers. Insert a low gain current-to-voltage converter (gain 10mA/V) or a current meter in the circuitry.</li>
	<li>Monitor the applied bias, the current flowing through the nanowire and the intensity distributions recorded by the two CCDs cameras.</li>
</ol>

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The authors declare that they have no competing financial interests.

The synthesis uses an excess of chemical surfactant remaining bound to the surface of the nanowire as illustrated in the transmission electron micrograph of Figure 9(a). This layer creates a dielectric barrier preventing the current to flow through the electrode-nanowire interface. Figures 10(a)&#160;and (b)&#160;show typical examples of the current-voltage characteristics. Sharp current steps at certain biases punctuate the curves. These steps occur when the current den...

Plasmonics aims at merging electronics and photonics in a shared physical support via the mediation of an electron density wave called a surface plasmon polariton1,2,3. Surface plasmon can travel in various waveguide geometries and interfaces. Among them, metal nanowires are especially desirable. As quasi-one dimensional structures they drastically confine the plasmon field to deep subwavelength scale while acting as electrical interconnects capable of sustaining an electron flow as depicted by the artistic drawing of Figure 1.
Both surface plasmon propagation and electron transport are sensitive to structural inhomogeneities of the nanowire (e.g. kinks, crystalline defects, etc.). Because they can grow as a single crystal with few defects, chemically-synthesized metal nanowires6 typically provide improved transport performances over amorphous metal nanowires fabricated by top-down approaches (e.g. electron beam lithography)12. The realization of a plasmonic network unit cell requires transferring the nanowires from a colloidal solution to a glass substrate. Without any specific complex prepatterning like surface functionalization13 or self-assembly techniques14, nanowires are generally randomly oriented on the substrate. This uncontrolled distribution of orientations drastically complicates the electrical connection of the nanowire to an outside power source.
In this article, randomly oriented chemically-synthesized silver nanowires are successfully contacted by source and drain electrical terminals. To this purpose optical microscopy is combined with electron-beam lithography to precisely locate the nanowire and create electrical contacts15,16. A characterization procedure evaluating the electro-plasmonic performances of the circuitry is described. After electrode fabrication, the contacted nanowires are transferred to a surface plasmon leakage radiation microscope for analyzing the effect of an electron flow on the propagation of surface plasmons. The microscope uses an inverted base equipped with a high numerical aperture oil-immersion objective and two charge-coupled device (CCD) cameras placed at the conjugate object plane and conjugate Fourier plane, respectively. These two conjugate planes provide complementary information on surface plasmon properties. Details of the propagation are directly inferred from image plane analysis, while the momentum distribution is visualized by Fourier plane imaging9.
Surface plasmons are excited in an individual nanowire by focusing a near-infrared laser beam in a diffraction-limited spot at the glass/air interface. When a nanowire extremity is aligned inside the focal region, the scattered incident laser light creates a broad distribution of wave-vectors, some of them resonant with the excitation of a surface plasmon. The propagation of this surface wave is visualized either by collecting the leakage of the mode emitted in the substrate or by observing the plasmon scattered at the nanowire distal end. The propagation length and effective index of the leaky surface plasmon mode are measured by analyzing the intensity distributions in a dual-plane leakage radiation microscopy.
Once a surface plasmon develops in the nanowire, the drain and source terminals at each extremity of the nanowire are connected to a regulated voltage supply. The CCD cameras monitor in real time the surface plasmon properties as a function of current flowing through the nanowire. For each value of the electrical transfer characteristic, the effective index and the propagation length of the surface plasmon mode are determined. This procedure enables to estimate the limitation of a nanowire-based circuitry to simultaneously sustain the transport of electrons and plasmons7.

<table border="0" cellpadding="0" cellspacing="0" width="406">
	<colgroup>
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="43">
			<td height="43" style="height:43px;width:216px;">DMEM</td>
			<td style="width:71px;">Invitrogen</td>
			<td style="width:119px;">ABCD1234</td>
		</tr>
		<tr height="106">
			<td height="106" style="height:106px;width:216px;">Ethylene glycol (EG)</td>
			<td style="width:71px;">Sinopharm Chemical Reagent Co., Ltd</td>
			<td style="width:119px;">T20111130</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:216px;">PMMA</td>
			<td style="width:71px;">Allresist</td>
			<td style="width:119px;">AR-P 679</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Acetone Analar Normapur</td>
			<td style="width:71px;">VWR Prolabo</td>
			<td style="width:119px;">20066.296</td>
		</tr>
		<tr height="43">
			<td height="43" style="height:43px;width:216px;">Isopropanol (IPA) Analar Normapur</td>
			<td style="width:71px;">VWR Prolabo</td>
			<td style="width:119px;">20842.298</td>
		</tr>
		<tr height="107">
			<td height="107" style="height:107px;width:216px;">AgNO3</td>
			<td style="width:71px;">Sinopharm Chemical Reagent Co., Ltd</td>
			<td style="width:119px;">20080826</td>
		</tr>
		<tr height="65">
			<td height="65" style="height:65px;width:216px;">Poly-(vinylpyrrolidone) (PVP)</td>
			<td style="width:71px;">Aladdin Chemistry Co., Ltd</td>
			<td style="width:119px;">1041671-31744</td>
		</tr>
		<tr height="86">
			<td height="86" style="height:86px;width:216px;">Dimethysilicone</td>
			<td style="width:71px;">Sinopharm Group Company Limited</td>
			<td style="width:119px;">H201-500</td>
		</tr>
		<tr height="65">
			<td height="65" style="height:65px;width:216px;">Propylene Glycol Methyl Ether Acetate</td>
			<td style="width:71px;">Microchemicals Gmbh</td>
			<td style="width:119px;">AZ EBR</td>
		</tr>
		<tr height="23">
			<td height="23" style="height:23px;width:216px;">Inverted optical microscope</td>
			<td style="width:71px;">Nikon</td>
			<td style="width:119px;">TE 2000</td>
		</tr>
		<tr height="43">
			<td height="43" style="height:43px;width:216px;">Microscope objective</td>
			<td style="width:71px;">Nikon</td>
			<td style="width:119px;">1.49/100X TIRF Plan-Apo</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:216px;">CCD Cameras (2x)</td>
			<td style="width:71px;">Andor</td>
			<td style="width:119px;">Luca-S</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:216px;">Regulated power supply</td>
			<td style="width:71px;">RHK</td>
			<td style="width:119px;">SPM 1000</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:216px;">Acquisition data</td>
			<td style="width:71px;">RHK</td>
			<td style="width:119px;">SPM 1000</td>
		</tr>
		<tr height="43">
			<td height="43" style="height:43px;width:216px;">Current to voltage converter</td>
			<td style="width:71px;">homemade</td>
			<td style="width:119px;">Gain 10 mA/V</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:216px;">Electron beam microscope</td>
			<td style="width:71px;">JEOL</td>
			<td style="width:119px;">FEG 6500</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:216px;">Lithography addon</td>
			<td style="width:71px;">RAITH</td>
			<td style="width:119px;">Elphy</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:216px;">Spincoater</td>
			<td style="width:71px;">PRIMUS</td>
			<td style="width:119px;">STT15</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:216px;">Thermal evaporator</td>
			<td style="width:71px;">PLASSYS</td>
			<td style="width:119px;">MEB 400</td>
		</tr>
		<tr height="43">
			<td height="43" style="height:43px;width:216px;">Micrometer probing stage (2x)</td>
			<td style="width:71px;">S&#220;SS MicroTec</td>
			<td style="width:119px;">PH110</td>
		</tr>
		<tr height="43">
			<td height="43" style="height:43px;width:216px;">Piezoelectric stage</td>
			<td style="width:71px;">Mad City Labs</td>
			<td style="width:119px;">Nano LP100</td>
		</tr>
	</tbody>
</table>

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.

Figure 2(a)&#160;shows a scanning electron micrograph of a typical 10 mm long Ag nanowire synthesized using the protocol outlined above. The width of the nanowire is here 200 nm as shown in the close up view of the left extremity in Figure&#160;2(b). The figure readily shows the five-fold symmetry of the crystal growth. There are no visible defects on the surface of the nanowire (e.g. kinks, particles, etc.). The synthesis leads to a large distribution of nanowire lengt...

Watch this Scientific Journal Video about Evaluating Plasmonic Transport in Current-carrying Silver Nanowires at JoVE.com

Evaluating Plasmonic Transport in Current-carrying Silver Nanowires

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 ...

evaluating-plasmonic-transport-in-current-carrying-silver-nanowires

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JoVE Journal

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5.2K Views.  Université de Bourgogne. 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.

Video: Evaluating Plasmonic Transport in Current-carrying Silver Nanowires

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.

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 ...

Design, Fabrication, and Experimental Characterization of Plasmonic Photoconductive Terahertz Emitters

In this video article we present a detailed demonstration of a highly efficient method for generating terahertz waves. Our technique is based on photoconduction, which has been one of the most commonly used techniques for terahertz generation 1-8. Terahertz generation in a photoconductive emitter is achieved by pumping an ultrafast photoconductor with a pulsed or heterodyned laser illumination. The induced photocurrent, which follows the envelope of the pump laser, is routed to a terahertz radiating antenna connected to the photoconductor contact electrodes to generate terahertz radiation. Although the quantum efficiency of a photoconductive emitter can theoretically reach 100%, the relatively long transport path lengths of photo-generated carriers to the contact electrodes of conventional photoconductors have severely limited their quantum efficiency. Additionally, the carrier screening effect and thermal breakdown strictly limit the maximum output power of conventional photoconductive terahertz sources. To address the quantum efficiency limitations of conventional photoconductive terahertz emitters, we have developed a new photoconductive emitter concept which incorporates a plasmonic contact electrode configuration to offer high quantum-efficiency and ultrafast operation simultaneously. By using nano-scale plasmonic contact electrodes, we significantly reduce the average photo-generated carrier transport path to photoconductor contact electrodes compared to conventional photoconductors 9. Our method also allows increasing photoconductor active area without a considerable increase in the capacitive loading to the antenna, boosting the maximum terahertz radiation power by preventing the carrier screening effect and thermal breakdown at high optical pump powers. By incorporating plasmonic contact electrodes, we demonstrate enhancing the optical-to-terahertz power conversion efficiency of a conventional photoconductive terahertz emitter by a factor of 50 10.

We describe methods for the design, fabrication, and experimental characterization of plasmonic photoconductive emitters, which offer two orders of magnitude higher terahertz power levels compared to conventional photoconductive emitters.

In this video article we present a detailed demonstration of a highly efficient method for generating terahertz waves. Our technique is based on ...

Ultrahigh Density Array of Vertically Aligned Small-molecular Organic Nanowires on Arbitrary Substrates

In recent years &pi;-conjugated organic semiconductors have emerged as the active material in a number of diverse applications including large-area, low-cost displays, photovoltaics, printable and flexible electronics and organic spin valves. Organics allow (a) low-cost, low-temperature processing and (b) molecular-level design of electronic, optical and spin transport characteristics. Such features are not readily available for mainstream inorganic semiconductors, which have enabled organics to carve a niche in the silicon-dominated electronics market. The first generation of organic-based devices has focused on thin film geometries, grown by physical vapor deposition or solution processing. However, it has been realized that organic nanostructures can be used to enhance performance of above-mentioned applications and significant effort has been invested in exploring methods for organic nanostructure fabrication.	
A particularly interesting class of organic nanostructures is the one in which vertically oriented organic nanowires, nanorods or nanotubes are organized in a well-regimented, high-density array. Such structures are highly versatile and are ideal morphological architectures for various applications such as chemical sensors, split-dipole nanoantennas, photovoltaic devices with radially heterostructured "core-shell" nanowires, and memory devices with a cross-point geometry. Such architecture is generally realized by a template-directed approach. In the past this method has been used to grow metal and inorganic semiconductor nanowire arrays. More recently &pi;-conjugated polymer nanowires have been grown within nanoporous templates. However, these approaches have had limited success in growing nanowires of technologically important &pi;-conjugated small molecular weight organics, such as tris-8-hydroxyquinoline aluminum (Alq3), rubrene and methanofullerenes, which are commonly used in diverse areas including organic displays, photovoltaics, thin film transistors and spintronics.	
Recently we have been able to address the above-mentioned issue by employing a novel "centrifugation-assisted" approach. This method therefore broadens the spectrum of organic materials that can be patterned in a vertically ordered nanowire array. Due to the technological importance of Alq3, rubrene and methanofullerenes, our method can be used to explore how the nanostructuring of these materials affects the performance of aforementioned organic devices. The purpose of this article is to describe the technical details of the above-mentioned protocol, demonstrate how this process can be extended to grow small-molecular organic nanowires on arbitrary substrates and finally, to discuss the critical steps, limitations, possible modifications, trouble-shooting and future applications.

We report a simple method for fabricating an ultrahigh density array of vertically ordered small-molecular organic nanowires. This method allows for synthesis of complex heterostructured hybrid nanowire geometries, which can be inexpensively grown on arbitrary substrates. These structures have potential applications in organic electronics, optoelectronics, chemical sensing, photovoltaics and spintronics.

In  recent years &pi;-conjugated  organic semiconductors have emerged as the active material in a number of  diverse applications including large-area, ...

Monolayer Contact Doping of Silicon Surfaces and Nanowires Using Organophosphorus Compounds

Monolayer Contact Doping (MLCD) is a simple method for doping of surfaces and nanostructures1. MLCD results in the formation of highly controlled, ultra shallow and sharp doping profiles at the nanometer scale. In MLCD process the dopant source is a monolayer containing dopant atoms.
In this article a detailed procedure for surface doping of silicon substrate as well as silicon nanowires is demonstrated. Phosphorus dopant source was formed using tetraethyl methylenediphosphonate monolayer on a silicon substrate. This monolayer containing substrate was brought to contact with a pristine intrinsic silicon target substrate and annealed while in contact. Sheet resistance of the target substrate was measured using 4 point probe. Intrinsic silicon nanowires were synthesized by chemical vapor deposition (CVD) process using a vapor-liquid-solid (VLS) mechanism; gold nanoparticles were used as catalyst for nanowire growth. The nanowires were suspended in ethanol by mild sonication. This suspension was used to dropcast the nanowires on silicon substrate with a silicon nitride dielectric top layer. These nanowires were doped with phosphorus in similar manner as used for the intrinsic silicon wafer. Standard photolithography process was used to fabricate metal electrodes for the formation of nanowire based field effect transistor (NW-FET). The electrical properties of a representative nanowire device were measured by a semiconductor device analyzer and a probe station.

A detailed procedure for surface doping of Silicon interfaces is provided. The ultra-shallow surface doping is demonstrated by using phosphorus containing monolayers and rapid annealing process. The method can be used for doping of macroscopic area surfaces as well as nanostructures.

Monolayer Contact Doping (MLCD) is a simple method for doping of surfaces and nanostructures1. MLCD results in the formation of highly controlled, ultra ...

Characterization of Thermal Transport in One-dimensional Solid Materials

The TET (transient electro-thermal) technique is an effective approach developed to measure the thermal diffusivity of solid materials, including conductive, semi-conductive or nonconductive one-dimensional structures. This technique broadens the measurement scope of materials (conductive and nonconductive) and improves the accuracy and stability. If the sample (especially biomaterials, such as human head hair, spider silk, and silkworm silk) is not conductive, it will be coated with a gold layer to make it electronically conductive. The effect of parasitic conduction and radiative losses on the thermal diffusivity can be subtracted during data processing. Then the real thermal conductivity can be calculated with the given value of volume-based specific heat (&#961;cp), which can be obtained from calibration, noncontact photo-thermal technique or measuring the density and specific heat separately. In this work, human head hair samples are used to show how to set up the experiment, process the experimental data, and subtract the effect of parasitic conduction and radiative losses.

The TET (transient electro-thermal) technique is an effective approach developed to measure the thermal diffusivity of solid materials.

The TET (transient electro-thermal) technique is an effective approach developed to measure the thermal diffusivity of solid materials, including ...

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 ...

Measurement of Scattering Nonlinearities from a Single Plasmonic Nanoparticle

Plasmonics, which are based on the collective oscillation of electrons due to light excitation, involve strongly enhanced local electric fields and thus have potential applications in nonlinear optics, which requires extraordinary optical intensity. One of the most studied nonlinearities in plasmonics is nonlinear absorption, including saturation and reverse saturation behaviors. Although scattering and absorption in nanoparticles are closely correlated by the Mie theory, there has been no report of nonlinearities in plasmonic scattering until very recently. 
Last year, not only saturation, but also reverse saturation of scattering in an isolated plasmonic particle was demonstrated for the first time. The results showed that saturable scattering exhibits clear wavelength dependence, which seems to be directly linked to the localized surface plasmon resonance (LSPR). Combined with the intensity-dependent measurements, the results suggest the possibility of a common mechanism underlying the nonlinear behaviors of scattering and absorption. These nonlinearities of scattering from a single gold nanosphere (GNS) are widely applicable, including in super-resolution microscopy and optical switches. 
In this paper, it is described in detail how to measure nonlinearity of scattering in a single GNP and how to employ the super-resolution technique to enhance the optical imaging resolution based on saturable scattering. This discovery features the first super-resolution microscopy based on nonlinear scattering, which is a novel non-bleaching contrast method that can achieve a resolution as low as l/8 and will potentially be useful in biomedicine and material studies.

Saturable and reverse saturable scattering were discovered in isolated plasmonic particles and adopted as a novel non-bleaching contrast method in super-resolution microscopy. Here the experimental procedures of detecting and extracting nonlinear scattering are explained in detail, as well as how to enhance resolution with the aid of saturated excitation microscopy.

Plasmonics, which are based on the collective oscillation of electrons due to light excitation, involve strongly enhanced local electric fields and thus ...

Resonance Raman Spectroscopy of Extreme Nanowires and Other 1D Systems

This paper briefly describes how nanowires with diameters corresponding to 1 to 5 atoms can be produced by melting a range of inorganic solids in the presence of carbon nanotubes. These nanowires are extreme in the sense that they are the limit of miniaturization of nanowires and their behavior is not always a simple extrapolation of the behavior of larger nanowires as their diameter decreases. The paper then describes the methods required to obtain Raman spectra from extreme nanowires and the fact that due to the van Hove singularities that 1D systems exhibit in their optical density of states, that determining the correct choice of photon excitation energy is critical. It describes the techniques required to determine the photon energy dependence of the resonances observed in Raman spectroscopy of 1D systems and in particular how to obtain measurements of Raman cross-sections with better than 8% noise and measure the variation in the resonance as a function of sample temperature. The paper describes the importance of ensuring that the Raman scattering is linearly proportional to the intensity of the laser excitation intensity. It also describes how to use the polarization dependence of the Raman scattering to separate Raman scattering of the encapsulated 1D systems from those of other extraneous components in any sample.

The paper describes a method for producing extreme nanowires by melt infiltration into carbon nanotubes and how 1D systems may be characterized and investigated using Resonance Raman Spectroscopy to determine vibrational and optical excitation energies.

This paper briefly describes how nanowires with diameters corresponding to 1 to 5 atoms can be produced by melting a range of inorganic solids in the ...

Multifunctional Hybrid Fe2O3-Au Nanoparticles for Efficient Plasmonic Heating

One of the most widely used methods for manufacturing colloidal gold nanospherical particles involves the reduction of chloroauric acid (HAuCl4) to neutral gold Au(0) by reducing agents, such as sodium citrate or sodium borohydride. The extension of this method to decorate iron oxide or similar nanoparticles with gold nanoparticles to create multifunctional hybrid Fe2O3-Au nanoparticles is straightforward. This approach yields fairly good control over Au nanoparticle dimensions and loading onto Fe2O3. Additionally, the Au metal size, shape, and loading can easily be tuned by changing experimental parameters (e.g., reactant concentrations, reducing agents, surfactants, etc.). An advantage of this procedure is that the reaction can be done in air or water, and, in principle, is amenable to scaling up. The use of such optically tunable Fe2O3-Au nanoparticles for hyperthermia studies is an attractive option as it capitalizes on plasmonic heating of gold nanoparticles tuned to absorb light strongly in the VIS-NIR region. In addition to its plasmonic effects, nanoscale Au provides a unique surface for interesting chemistries and catalysis. The Fe2O3 material provides additional functionality due to its magnetic property. For example, an external magnetic field could be used to collect and recycle the hybrid Fe2O3-Au nanoparticles after a catalytic experiment, or alternatively, the magnetic Fe2O3 can be used for hyperthermia studies through magnetic heat induction. The photothermal experiment described in this report measures bulk temperature change and nanoparticle solution mass loss as functions of time using infrared thermocouples and a balance, respectively. The ease of sample preparation and the use of readily available equipment are distinct advantages of this technique. A caveat is that these photothermal measurements assess the bulk solution temperature and not the surface of the nanoparticle where the heat is transduced and the temperature is likely to be higher.

Multifunctional Hybrid Fe2O3-Au Nanoparticles for Efficient Plasmonic Heating

We describe the synthesis and properties of multifunctional Fe2O3-Au nanoparticles produced by a wet chemical approach and investigate their photothermal properties using laser irradiation. The composite Fe2O3-Au nanoparticles retain the properties of both materials, creating a multifunctional structure with excellent magnetic and plasmonic properties.

One of the most widely used methods for manufacturing colloidal gold nanospherical particles involves the reduction of chloroauric acid (HAuCl4) to ...

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 ...