Name: monolayer contact doping of silicon surfaces and nanowires using organophosphorus compounds
Uploaded: 2013-12-02T21:15:00
Description: Watch this Scientific Journal Video about Monolayer Contact Doping of Silicon Surfaces and Nanowires Using Organophosphorus Compounds at JoVE.com

This work was partially funded by the Farkas center for light-induced processes.

1. Surface Cleaning
<ol>
	<li>Prepare acidic piranha solution by mixing 1:3 hydrogen peroxide (30%) and concentrated sulfuric acid. 
	Caution: Piranha solutions are extremely strong and dangerous oxidizing agents and should be used with extreme caution. These solutions may explode in contact with organic solvents. Only qualified personnel with appropriate training and safety equipment may perform the procedure.</li>
	<li>Place substrates (later used as donor and target substrates) in appropriate h...

<ol>
	<li>Hazut, O., Agarwala, A., 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=Contact+doping+of+silicon+wafers+and+nanostructures+with+phosphine+oxide+monolayers.">Contact doping of silicon wafers and nanostructures with phosphine oxide monolayers.</a> ACS Nano. 6 (11), 10311-10318 (2012).</li><li>Hisamoto, D., Lee, W. -. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=FinFET-+A+self-aligned+double-gate+MOSFET+scalable+to+20+nm.">FinFET- A self-aligned double-gate MOSFET scalable to 20 nm.</a> IEEE Trans. Electron Devices. 47, 2320-2325 (2000).</li><li>Leung, G., Chui, C. O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Variability+impact+of+random+dopant+fluctuation+on+nanoscale+junctionless+FinFETs.">Variability impact of random dopant fluctuation on nanoscale junctionless FinFETs.</a> IEEE Electron Device Lett. 33, 767-769 (2012).</li><li>Ho, J. C., Yerushalmi, R., 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=Wafer-scale,+sub-5+nm+junction+formation+by+monolayer+doping+and+conventional+spike+annealing.">Wafer-scale, sub-5 nm junction formation by monolayer doping and conventional spike annealing.</a> Nano Lett. 9 (2), 725-730 (2009).</li><li>Peercy, P. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Drive+to+Miniaturization.">The Drive to Miniaturization.</a> Nature. 406, 1023-1026 (2000).</li><li>Lu, W., Lieber, 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=Semiconductor+Nanowires.">Semiconductor Nanowires.</a> J. Phys. D. 39, R387-R406 (2006).</li><li>Gunawan, O., Wang, K., Fallahazad, B., Zhang, Y., Tutuc, E., Guha, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High+Performance+Wire-Array+Silicon+Solar+Cells.">High Performance Wire-Array Silicon Solar Cells.</a> Prog. Photovoltaics. 19, 307-312 (2011).</li><li>Ho, J. C., Yerushalmi, R., Jacobson, Z. A., Fan, Z., Alley, R. L., Javey, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Controlled+nanoscale+doping+of+semiconductors+via+molecular+monolayers.">Controlled nanoscale doping of semiconductors via molecular monolayers.</a> Nat. Mater. 7, 62-67 (2008).</li><li>Koren, E., Rosenwaks, Y., Allen, J. E., Hemesath, E. R., Lauhon, L. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nonuniform.+Doping+distribution+along+silicon+nanowires+measured+by+kelvin+probe+force+microscopy+and+scanning+photocurrent+microscopy.">Nonuniform. Doping distribution along silicon nanowires measured by kelvin probe force microscopy and scanning photocurrent microscopy.</a> Appl. Phys. Lett. 95, 092105 (2009).</li><li>Wagner, R. S., Ellis, W. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+vapor-liquid-solid+mechanism+of+crystal+growth+and+its+application+to+silicon.">The vapor-liquid-solid mechanism of crystal growth and its application to silicon.</a> Trans. Metall. Soc. AIME. 233, 1053-1064 (1965).</li><li>Cui, Y., Lauhon, L. J., Gudiksen, M. S., Wang, J., Lieber, 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=Diameter-controlled+synthesis+of+single-crystal+silicon+nanowires.">Diameter-controlled synthesis of single-crystal silicon nanowires.</a> Appl. Phys. Lett. 78 (15), 2214-2216 (2001).</li></ol>

MLCD is a simple and reproducible method. However, attention to surface cleaning and monolayer formation must be taken. Piranha cleaning of the surfaces prior to the MLCD process is important not only for the purpose of avoiding possible impurities, but also for initialization of the surface for reproducible monolayer formation providing reproducible results between processes. The piranha treatment results in hydroxylation of surface groups which is required for binding of precursor molecules to the surface for the forma...

Controlled surface doping of semiconductor structures with macroscopic areas as well as at the nanoscale is important for advanced semiconductor device architectures such as FinFet2,3, as well as for nanostructure based devices such as nanowire-based sensors and photovoltaics4-7. We recently introduced monolayer contact doping (MLCD) for repeatable, uniform surface doping of silicon interfaces with macroscopic and nanometric dimensions with control over dopant dose and diffusion profile1. An important feature of MLCD is the restriction of monolayer formation to a substrate that is termed &#34;donor substrate&#34;. MLCD simplifies some of the process steps required for Monolayer Contact Doping (MLCD) and provides complementary surface doping capabilities8. Once the donor substrate is loaded with the dopant containing monolayer by using self-limiting surface chemistry, the donor substrate is brought to contact with the substrate intended for doping, termed &#34;target substrate&#34;, and both substrates are annealed while in contact. During the anneal process, dopant atoms diffuse to both donor and target substrates, and are activated at the elevated temperature. Since MLCD does not require high energy implantation of dopant atoms, no structural damage is caused to the semiconductor lattice during the process and no further anneal step is required. Good control over dopant diffusion is possible by controlling the rapid thermal process parameters. Ultra shallow and uniform dopant diffusion lengths down to a few nanometers are easily achieved. Separation of the monolayer from the process sequence simplifies the process, allow greater control over process parameters and open new possibilities for doping schemes that were not possible by using other methods. Achieving dopant level as high as the solubility limit of phosphorus in silicon is possible by multiple MLCD doping processes applied successively. In summary, traditional doping methods suffer from intrinsic limitations to fabricate ultra-shallow doping profiles. This is because of inherent statistical variations of source concentrations, overall dose and energy distribution, which are inherent to the low implantation energies required for ultra-shallow implantation. MLCD provides a simple means for surface doping, this is the result of the unique features of MLCD relying on the precise control of dopant dose and location at the atomic scale by utilizing robust surface chemistry for generating the dopant source with self-limiting monolayer chemistry formed exclusively at the semiconductor surface.

<table>
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr>
			<td>High purity silicon wafers</td>
			<td>Topsil</td>
			<td>-</td>
			<td></td>
		</tr>
		<tr>
			<td>50 nm Si3N4/50 nm SiO2/Si wafers</td>
			<td>Silicon Valley Microelectronics</td>
			<td>-</td>
			<td></td>
		</tr>
		<tr>
			<td>Sulfuric Acid 98%</td>
			<td>BioLab</td>
			<td>19550523</td>
			<td></td>
		</tr>
		<tr>
			<td>Hydrogen Peroxide 30%</td>
			<td>J.T. Baker</td>
			<td>2190-03</td>
			<td></td>
		</tr>
		<tr>
			<td>Ammonium Hydroxide 25%</td>
			<td>J.T. Baker</td>
			<td>6051</td>
			<td></td>
		</tr>
		<tr>
			<td>Ethanol</td>
			<td>J.T. Baker</td>
			<td>8025</td>
			<td></td>
		</tr>
		<tr>
			<td>Mesitylene</td>
			<td>Sigma</td>
			<td>M7200</td>
			<td></td>
		</tr>
		<tr>
			<td>Dichloromethane</td>
			<td>Macron</td>
			<td>4881-06</td>
			<td></td>
		</tr>
		<tr>
			<td>Tetraethyl methylenediphosphonate</td>
			<td>Aldrich</td>
			<td>359181</td>
			<td></td>
		</tr>
		<tr>
			<td>Mineral Oil</td>
			<td>Sigma</td>
			<td>M3516</td>
			<td></td>
		</tr>
		<tr>
			<td>Hydrofluoric Acid 49%</td>
			<td>J.T. Baker</td>
			<td>9564-06</td>
			<td></td>
		</tr>
		<tr>
			<td>Isopropanol</td>
			<td>J.T. Baker</td>
			<td>9079-05</td>
			<td></td>
		</tr>
		<tr>
			<td>
			N-Methyl-2-pyrrolidone
			</td>
			<td>J.T. Baker</td>
			<td>9397-05</td>
			<td></td>
		</tr>
		<tr>
			<td>AZ nLOF2020</td>
			<td>AZ Electronic Materials</td>
			<td>nLOF 2020</td>
			<td></td>
		</tr>
		<tr>
			<td>AZ 726 MIF</td>
			<td>AZ Electronic Materials</td>
			<td>726 MIF</td>
			<td></td>
		</tr>
		<tr>
			<td>Poly-L-Lysine solution</td>
			<td>Sigma</td>
			<td>P8920</td>
			<td></td>
		</tr>
		<tr>
			<td>Gold colloid solution</td>
			<td>Ted Pella</td>
			<td>82160-80</td>
			<td></td>
		</tr>
		<tr>
			<td>RTA system</td>
			<td>AnnealSys</td>
			<td>MicroAS</td>
			<td></td>
		</tr>
		<tr>
			<td>4 point probe sheet resistance measurement system</td>
			<td>Jandel</td>
			<td>RM3-AR</td>
			<td></td>
		</tr>
		<tr>
			<td>Mask aligner</td>
			<td>Suss</td>
			<td>MA06</td>
			<td></td>
		</tr>
		<tr>
			<td>e-Beam evaporator</td>
			<td>VST&#160;</td>
			<td>TFDS-141E</td>
			<td></td>
		</tr>
		<tr>
			<td>Semiconductor analyzer</td>
			<td>Agilent</td>
			<td>B1500A</td>
			<td></td>
		</tr>
		<tr>
			<td>CVD system</td>
			<td>-</td>
			<td>-</td>
			<td>Home-built</td>
		</tr>
	</tbody>
</table>

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.

Representative results for phosphorous-MLCD surface doping process are shown in Figure 1. Intrinsic silicon wafers were treated with phosphorous-MLCD, resulting in monotonic decrease in the sheet resistance values. Sheet resistance values decrease for longer anneal times and higher anneal temperatures as shown by the three traces in Figure 1. Sheet resistance values can be correlated to activated dopant concentration. Lower sheet resistance values indicate higher doping levels and vice v...

Watch this Scientific Journal Video about Monolayer Contact Doping of Silicon Surfaces and Nanowires Using Organophosphorus Compounds at JoVE.com

Monolayer Contact Doping of Silicon Surfaces and Nanowires Using Organophosphorus Compounds

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 ...

monolayer-contact-doping-silicon-surfaces-nanowires-using

Research

JoVE Journal

Engineering

Probing and Mapping Electrode Surfaces in Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are potentially the most efficient and cost-effective solution to utilization of a wide variety of fuels beyond hydrogen 1-7. The performance of SOFCs and the rates of many chemical and energy transformation processes in energy storage and conversion devices in general are limited primarily by charge and mass transfer along electrode surfaces and across interfaces. Unfortunately, the mechanistic understanding of these processes is still lacking, due largely to the difficulty of characterizing these processes under in situ conditions. This knowledge gap is a chief obstacle to SOFC commercialization. The development of tools for probing and mapping surface chemistries relevant to electrode reactions is vital to unraveling the mechanisms of surface processes and to achieving rational design of new electrode materials for more efficient energy storage and conversion2. Among the relatively few in situ surface analysis methods, Raman spectroscopy can be performed even with high temperatures and harsh atmospheres, making it ideal for characterizing chemical processes relevant to SOFC anode performance and degradation8-12. It can also be used alongside electrochemical measurements, potentially allowing direct correlation of electrochemistry to surface chemistry in an operating cell. Proper in situ Raman mapping measurements would be useful for pin-pointing important anode reaction mechanisms because of its sensitivity to the relevant species, including anode performance degradation through carbon deposition8, 10, 13, 14 ("coking") and sulfur poisoning11, 15 and the manner in which surface modifications stave off this degradation16. The current work demonstrates significant progress towards this capability. In addition, the family of scanning probe microscopy (SPM) techniques provides a special approach to interrogate the electrode surface with nanoscale resolution. Besides the surface topography that is routinely collected by AFM and STM, other properties such as local electronic states, ion diffusion coefficient and surface potential can also be investigated17-22. In this work, electrochemical measurements, Raman spectroscopy, and SPM were used in conjunction with a novel test electrode platform that consists of a Ni mesh electrode embedded in an yttria-stabilized zirconia (YSZ) electrolyte. Cell performance testing and impedance spectroscopy under fuel containing H2S was characterized, and Raman mapping was used to further elucidate the nature of sulfur poisoning. In situ Raman monitoring was used to investigate coking behavior. Finally, atomic force microscopy (AFM) and electrostatic force microscopy (EFM) were used to further visualize carbon deposition on the nanoscale. From this research, we desire to produce a more complete picture of the SOFC anode.

We present a unique platform for characterizing electrode surfaces in solid oxide fuel cells (SOFCs) that allows simultaneous performance of multiple characterization techniques (e.g. in situ Raman spectroscopy and scanning probe microscopy alongside electrochemical measurements). Complementary information from these analyses may help to advance toward a more profound understanding of electrode reaction and degradation mechanisms, providing insights into rational design of better materials for SOFCs.

Solid oxide fuel cells (SOFCs) are potentially the most efficient and cost-effective solution to utilization of a wide variety of fuels beyond hydrogen ...

Fabrication of Nano-engineered Transparent Conducting Oxides by Pulsed Laser Deposition

Nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas allows the deposition of metal oxides with tunable morphology, structure, density and stoichiometry by a proper control of the plasma plume expansion dynamics. Such versatility can be exploited to produce nanostructured films from compact and dense to nanoporous characterized by a hierarchical assembly of nano-sized clusters. In particular we describe the detailed methodology to fabricate two types of Al-doped ZnO (AZO) films as transparent electrodes in photovoltaic devices: 1) at low O2 pressure, compact films with electrical conductivity and optical transparency close to the state of the art transparent conducting oxides (TCO) can be deposited at room temperature, to be compatible with thermally sensitive materials such as polymers used in organic photovoltaics (OPVs); 2) highly light scattering hierarchical structures resembling a forest of nano-trees are produced at higher pressures. Such structures show high Haze factor (&gt;80%) and may be exploited to enhance the light trapping capability. The method here described for AZO films can be applied to other metal oxides relevant for technological applications such as TiO2, Al2O3, WO3 and Ag4O4.

We describe the experimental method to deposit nanostructured oxide thin films by nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas. By using this method Al-doped ZnO (AZO) films, from compact to hierarchically structured as nano-tree forests, can be deposited.

Nanosecond Pulsed  Laser Deposition (PLD) in the presence of a background gas allows the deposition  of metal oxides with tunable morphology, structure, ...

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, ...

Analysis of Contact Interfaces for Single GaN Nanowire Devices

Single GaN nanowire (NW) devices fabricated on SiO2 can exhibit a strong degradation after annealing due to the occurrence of void formation at the contact/SiO2 interface. This void formation can cause cracking and delamination of the metal film, which can increase the resistance or lead to a complete failure of the NW device. In order to address issues associated with void formation, a technique was developed that removes Ni/Au contact metal films from the substrates to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN NW devices. This procedure determines the degree of adhesion of the contact films to the substrate and NWs and allows for the characterization of the morphology and composition of the contact interface with the substrate and nanowires. This technique is also useful for assessing the amount of residual contamination that remains from the NW suspension and from photolithographic processes on the NW-SiO2 surface prior to metal deposition. The detailed steps of this procedure are presented for the removal of annealed Ni/Au contacts to Mg-doped GaN NWs on a SiO2 substrate.

A technique was developed that removes Ni/Au contact metal films from their substrate to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN nanowire devices.

Single GaN nanowire (NW) devices fabricated on SiO2 can exhibit  a strong degradation after annealing due to the  occurrence of void formation at the ...

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 ...

Preparation and Use of Photocatalytically Active Segmented Ag|ZnO and Coaxial TiO2-Ag Nanowires Made by Templated Electrodeposition

Photocatalytically active nanostructures require a large specific surface area with the presence of many catalytically active sites for the oxidation and reduction half reactions, and fast electron (hole) diffusion and charge separation. Nanowires present suitable architectures to meet these requirements. Axially segmented Ag|ZnO and radially segmented (coaxial) TiO2-Ag nanowires with a diameter of 200 nm and a length of 6-20 &#181;m were made by templated electrodeposition within the pores of polycarbonate track-etched (PCTE) or anodized aluminum oxide (AAO) membranes, respectively. In the photocatalytic experiments, the ZnO and TiO2 phases acted as photoanodes, and Ag as cathode. No external circuit is needed to connect both electrodes, which is a key advantage over conventional photo-electrochemical cells. For making segmented Ag|ZnO nanowires, the Ag salt electrolyte was replaced after formation of the Ag segment to form a ZnO segment attached to the Ag segment. For making coaxial TiO2-Ag nanowires, a TiO2 gel was first formed by the electrochemically induced sol-gel method. Drying and thermal annealing of the as-formed TiO2 gel resulted in the formation of crystalline TiO2 nanotubes. A subsequent Ag electrodeposition step inside the TiO2 nanotubes resulted in formation of coaxial TiO2-Ag nanowires. Due to the combination of an n-type semiconductor (ZnO or TiO2) and a metal (Ag) within the same nanowire, a Schottky barrier was created at the interface between the phases. To demonstrate the photocatalytic activity of these nanowires, the Ag|ZnO nanowires were used in a photocatalytic experiment in which H2 gas was detected upon UV illumination of the nanowires dispersed in a methanol/water mixture. After 17 min&#160;of illumination, approximately 0.2 vol% H2 gas was detected from a suspension of ~0.1 g of Ag|ZnO nanowires in a 50 ml 80 vol% aqueous methanol solution.

Preparation and Use of Photocatalytically Active Segmented Ag|ZnO and Coaxial TiO2-Ag Nanowires Made by Templated Electrodeposition

Procedures are outlined to prepare segmented and coaxial nanowires via templated electrodeposition in nanopores. As examples, segmented nanowires consisting of Ag and ZnO segments, and coaxial nanowires consisting of a TiO2 shell and a Ag core were made. The nanowires were used in photocatalytic hydrogen formation experiments.

Photocatalytically active nanostructures require a large specific surface area with the presence of many catalytically active sites for the oxidation and ...

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 ...

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 ...

Scanning SQUID Study of Vortex Manipulation by Local Contact

Local, deterministic manipulation of individual vortices in type 2 superconductors is challenging. The ability to control the position of individual vortices is necessary in order to study how vortices interact with each other, with the lattice, and with other magnetic objects. Here, we present a protocol for vortex manipulation in thin superconducting films by local contact, without applying current or magnetic field. Vortices are imaged using a scanning superconducting quantum interference device (SQUID), and vertical stress is applied to the sample by pushing the tip of a silicon chip into the sample, using a piezoelectric element. Vortices are moved by tapping the sample or sweeping it with the silicon tip. Our method allows for effective manipulation of individual vortices, without damaging the film or affecting its topography. We demonstrate how vortices were relocated to distances of up to 0.8 mm. The vortices remained stable at their new location up to five days. With this method, we can control vortices and move them to form complex configurations. This technique for vortex manipulation could also be implemented in applications such as vortex based logic devices.

We present a protocol for manipulation of individual vortices in thin superconducting films, using local mechanical contact. The method does not include applying current, magnetic field or additional fabrication steps.

Local, deterministic manipulation of individual vortices in type 2 superconductors is challenging. The ability to control the position of individual ...

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical properties. The possibility of doping was investigated by adding monovalent cation halides with similar ionic radii to Pb2+, including Cu+, Na+, and Ag+. A shift in the Fermi level and a remarkable decrease of sub-bandgap optical absorption, along with a lower energetic disorder in the perovskite, was achieved. An order-of-magnitude enhancement in the bulk hole mobility and a significant reduction of transport activation energy within an additive-based perovskite device was attained. The confluence of the aforementioned improved properties in the presence of these cations led to an enhancement in the photovoltaic parameters of the perovskite solar cell. An increase of 70 mV in open circuit voltage for AgI and a 2 mA/cm2 improvement in photocurrent density for NaI- and CuBr-based solar cells were achieved compared to the pristine device. Our work paves the way for further improvements in the optoelectronic quality of CH3NH3PbI3 perovskite and subsequent devices. It highlights a new avenue for investigations on the role of dopant impurities in crystallization and controls the electronic defect density in perovskite structures.

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we present a protocol to adjust the properties of solution-processed CH3NH3PbI3 through the incorporation of monovalent cation additives in order to achieve highly efficient perovskite solar cells.

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical ...