We gratefully acknowledge the financial support from National Natural Science Foundation of China (21703104), Jiangsu Science and Technology Plan (SBK2017041514) Nanjing Tech University (39837131), and SICAM Fellowship from Jiangsu National Synergetic Innovation Center for Advanced Materials.

Caution: Please check the material safety data sheets (MSDS) of the chemicals for detailed handling and storage instruction. Please be careful while handling the nanomaterials, as there may be unidentified risk. Please perform the experiments in a fume hood and wear appropriate personal protective equipment.
1. Synthesis of Seed Nanoparticles
Note: To avoid failure caused by the premature nucleation during the nanoparticle synthesis, wash the glassware and stir bar used in the synthesis with aqua regia and rinse thoroughly with water.
<ol>
	<li>Synthesis of 3-5 nm Au nanoparticles
	<ol>
		<li>Prepare a hydrogen tetrachloroaurate (III) (HAuCl4) solution by dissolving 10 mg of HAuCl4&#8729;3H2O in 1 mL of deionization&#160;water (DI water) in a 4 mL vial. Pipette 0.197 mL of the HAuCl4 solution into a 50 mL round-bottom flask.</li>
		<li>Add 19.7 mL of DI water to the flask to dilute the HAuCl4 solution.</li>
		<li>Dissolve 10 mg of sodium citrate in 1 mL of DI water in another 4 mL vial to prepare a 1% sodium citrate stock solution. Add 0.147 mL of the 1% sodium citrate solution to the diluted HAuCl4 solution prepared in step 1.1.2.</li>
		<li>Prepare a 0.1 M sodium borohydride (NaBH4) solution by dissolving 2.3 mg of NaBH4 in 0.6 mL of DI water.</li>
		<li>Quickly inject 0.6 mL of 0.1 M NaBH4 solution into the mixture from step 1.1.3 under vigorous stirring. Check for an immediate color change of the solution from pale yellow to bright orange.</li>
		<li>Stir the mixture for another 10 min. Wait for a gradual color change of the solution to reddish orange.</li>
		<li>Confirm the size of the obtained Au nanoparticles with UV-Vis spectroscopy and scanning electron microscopy (SEM).</li>
	</ol>
	</li>
	<li>Synthesis of 15 and 40 nm Au nanoparticles
	<ol>
		<li>Add 100 mL of DI water in a 250 mL round-bottom flask. Weigh 10 mg of HAuCl4&#8729;3H2O solid and dissolve it in the round-bottom flask.</li>
		<li>Add a magnetic stir bar into the flask and equip the flask with a condenser. Stir and heat the solution prepared in 1.2.1 to 100 &#176;C in an oil bath. Reflux the solution for 10 min.</li>
		<li>Weigh 40 mg of sodium citrate and dissolve it in 4 mL of DI water to prepare a 1% sodium citrate stock solution.</li>
		<li>To synthesize 15 nm Au nanoparticles, add 3 mL of the 1% sodium citrate solution from step 1.2.3 to the boiled mixture with a syringe. 
		Note: The color of the solution turns gray in 1 min, and then gradually to red.
		<ol>
			<li>To synthesize 40 nm Au nanoparticles, inject 1.5 mL of the 1% sodium citrate solution to the boiling solution from step 1.2.2 with a syringe. Keep the solution boiling until it changes to red in about 10 min. 
			Note: The color of the solution changes from transparent to dark gray, then to black, and finally to purple in about 1 min.</li>
		</ol>
		</li>
		<li>Continue to reflux the reaction solution for 30 min. Cool down the solution to room temperature at ambient conditions.</li>
		<li>Characterize the size and uniformity of the resulting Au nanoparticles with UV-Vis spectroscopy and SEM.</li>
	</ol>
	</li>
</ol>
2. Synthesis of Au Nanowires (Length = ~500 nm ) on Silicon (Si) Wafers and Various Substrates
<ol>
	<li>Prepare the substrate for seed adsorption.
	<ol>
		<li>Cut the Si wafer into 5 mm &#180; 5 mm pieces. Clean the Si wafer pieces with DI water and ethanol sequentially in an ultrasonic bath, each for 15 min.</li>
		<li>Treat the Si wafer with 29.6 W of O2 plasma (operated at 220 V) for 20 min. 
		Note: The surface of the wafer becomes hydrophilic.</li>
		<li>Prepare a 5 mM 3-aminopropyltriethoxysilane (APTES) solution by dissolving 11.1 mg of APTES to a mixture of DI water (5 mL) and ethanol (5 mL) in a 20 mL vial.</li>
		<li>Soak a piece of Si wafer in the APTES solution prepared in step 2.1.3 in a 20 mL vial for 30 min.</li>
		<li>Take out the Si wafer and wash it thoroughly with ethanol and DI water.</li>
	</ol>
	</li>
	<li>Adsorb seed nanoparticles onto the substrate.
	<ol>
		<li>Soak the Si wafer in the 3-5 nm Au seeds solution prepared in step 1.1 for 2 h.
		<ol>
			<li>To grow Au nanowires from 15 nm Au seeds, soak the Si wafer in the 15 nm Au nanoparticle solution for 2 h.</li>
			<li>To grow Au nanowires from 40 nm Au seeds, soak the Si wafer in the 40 nm Au nanoparticle solution for 2 h.</li>
		</ol>
		</li>
		<li>Take out the Si wafer and wash it with 50 mL of DI water to remove unabsorbed Au nanoparticles.</li>
	</ol>
	</li>
	<li>Grow the substrate-bound Au nanowires.
	<ol>
		<li>Prepare a 1.65 mM 4-mercaptobenzoic acid (4-MBA) solution by dissolving 2.5 mg of 4-MBA in 10 mL of ethanol.</li>
		<li>Prepare a 5.10 mM HAuCl4 solution by dissolving 20.1 mg of HAuCl4&#8729;3H2O in a mixture of 5 mL of ethanol and 5 mL of DI water.</li>
		<li>Prepare a 12.3 mM L-ascorbic acid solution by dissolving 21.7 mg of L-ascorbic acid in 10 mL of DI water.</li>
		<li>Mix 0.5 mL of 5.10 mM HAuCl4 solution with 0.5 mL of 1.65 mM 4-MBA solution in a 10 mL vial.</li>
		<li>Soak the seed-adsorbed Si wafer from step 2.2.2 in the mixed solution prepared in step 2.3.4.</li>
		<li>Add 0.5 mL of 12.3 mM L-ascorbic acid solution into the wafer-soaked mixed solution. Shake the vial gently to evenly mix the solution.</li>
		<li>Leave the wafer and the solution undisturbed for 15 min. Check the bubble formation during the growth process, and the color change of the surface of the Si wafer from shiny gray to reddish brown.</li>
		<li>Take out the Si wafer and rinse it with ethanol and DI water. Dry the Si wafer at ambient conditions and wait for the surface of the wafer to turn to gold.</li>
		<li>Characterize the morphology of the Au nanowires with the SEM.</li>
	</ol>
	</li>
	<li>For growth of Au nanowires on a glass slide, Al2O3, SrTiO3, LaAlO3, indium tin oxide (ITO) and F-doped tin oxide (FTO) substrates, follow the same procedures, including the cleaning processes.</li>
</ol>
3. Synthesis of Au Nanowires with Different Ligands
<ol>
	<li>Synthesis of Au nanowire forests with various thiolated ligands: 2-naphthalenethiol (2-NpSH), 4-mercaptophenylacetic acid (4-MPAA) and 3-mercaptobenzoic acid (3-MBA).
	<ol>
		<li>Treat the Si wafer following the same procedures in steps 2.1-2.2.</li>
		<li>Prepare a 1.65 mM 2-NpSH solution by dissolving 2.6 mg of 2-NpSH in 10 mL of ethanol.
		<ol>
			<li>Prepare a 1.65 mM 4-MPAA solution by dissolving 2.8 mg of 4-MPAA in 10 mL of ethanol.</li>
			<li>Prepare a 1.65 mM 3-MBA solution by dissolving 2.5 mg of 3-MBA in 10 mL of ethanol.</li>
		</ol>
		</li>
		<li>Prepare the 5.10 mM HAuCl4 solution and 12.3 mM L-ascorbic acid solution following the same procedure in steps 2.3.1-2.3.3.</li>
		<li>In a 10 mL vial, mix 0.5 mL of the HAuCl4 solution with 0.5 mL of 2-NpSH solution, and shake the mixture to get a homogeneous solution.
		<ol>
			<li>In a 10 mL vial, mix 0.5 mL of the HAuCl4 solution with 0.5 mL of 4-MPAA solution, and shake the mixture to get a homogeneous solution.</li>
			<li>In a 10 mL vial, mix 0.5 mL of the HAuCl4 solution with 0.5 mL of 3-MBA solution, and shake the mixture to get a homogeneous solution.</li>
		</ol>
		</li>
		<li>Add 0.5 mL of 12.3 mM L-ascorbic acid solution to the wafer-soaked mixed solution. Shake the vial gently to get an evenly mixed solution.</li>
		<li>Leave the wafer and the solution undisturbed for 15 min. Observe the surface of the Si wafer turning slowly from shiny gray to reddish brown.</li>
		<li>Take out the Si wafer and rinse it with ethanol and DI water, and dry the Si wafer at ambient conditions until the surface of the wafer turns to gold.</li>
		<li>Confirm the Au nanowires forest structure with SEM.</li>
	</ol>
	</li>
	<li>Synthesis of tapered Au nanowires with mixed ligands.
	<ol>
		<li>Synthesis of thickened Au nanowires with mixed ligands of 4-MBA and 3-mercaptopropanoic acid (3-MPA) (c4-MBA/c3-MPA = 3:1).
		<ol>
			<li>Treat the Si wafer following the same procedures in steps 2.1-2.2, except dilute the seed solution 100 times.</li>
			<li>Prepare a 3 mM 4-MBA solution by dissolving 4.6 mg of 4-MBA in 10 mL of ethanol.</li>
			<li>Prepare a 3 mM 3-MPA solution by dissolving 3.2 mg of 3-MPA in 10 mL of ethanol.</li>
			<li>Mix 0.75 mL of 3 mM 4-MBA solution with 0.25 mL of 3 mM 3-MPA solution (in a 10 mL vial. Shake gently to get a homogeneous solution.</li>
			<li>Prepare the 5.10 mM HAuCl4 solution and 12.3 mM L-ascorbic acid solution following the same procedure in steps 2.3.2-2.3.3.</li>
			<li>Add 0.5 mL of 5.10 mM HAuCl4 solution to the mixed solution in step 3.2.1.4 and shake gently to homogenize the solution.</li>
			<li>Soak the Si wafer from step 3.2.1.1 in the mixed solution in a 10 mL vial. Add 0.5 mL of 12.3 mM L-ascorbic acid solution to the mixed solution.</li>
			<li>After 10 min, take the Si wafer out and rinse with ethanol and DI water.</li>
			<li>Dry the wafer at ambient conditions and confirm the structure by SEM.</li>
		</ol>
		</li>
		<li>Synthesize tapered Au nanowires with mixed ligand of 4-MBA and 3-MPA (c4-MBA/c3-MPA = 6:4). Follow the same procedures in step 3.2.1 with 0.6 mL of 3 mM 4-MBA solution and 0.4 mL of 3 mM 3-MPA solution instead.</li>
		<li>Synthesize tapered Au nanowires with mixed ligands of 4-MBA and 3-MPA (c4-MBA/c3-MPA = 1:1). Follow the same procedures in step 3.2.1 with 0.5 mL of 3 mM 4-MBA solution and 0.5 mL of 3 mM 3-MPA solution instead.</li>
	</ol>
	</li>
</ol>
4. Synthesis of Au Nanowire-Based Complex Nanostructures
<ol>
	<li>Synthesis of segmental Au nanowires with thick-thin-thick-thin segments.
	<ol>
		<li>Treat the Si wafer following the same procedures in steps 2.1-2.2.</li>
		<li>Prepare a 1.65 mM 4-MBA solution by dissolving 2.5 mg of 4-MBA in 10 mL of ethanol.</li>
		<li>Prepare a 0.0830 mM 4-MBA solution by diluting the 1.65 mM 4-MBA solution 20 times.</li>
		<li>Prepare the HAuCl4 and L-ascorbic acid solution following the same procedures in steps 2.3.2-2.3.3.</li>
		<li>Prepare 1.5 mL of growth solution A by mixing 0.5 mL of 1.65 mM 4-MBA solution, 0.5 mL of 5.10 mM HAuCl4 solution, and 0.5 mL of 12.3 mM L-ascorbic acid solution (the final concentration: c4-MBA = 0.550 mM, cHAuCl4 = 1.70 mM, cL-ascorbic acid = 4.10 mM).</li>
		<li>Prepare 1.5 mL of growth solution B by mixing 0.5 mL of 0.0830 mM 4-MBA solution, 0.5 mL of 5.10 mM HAuCl4 solution, 0.5 mL of 12.3 mM L-ascorbic acid solution&#160;(the final concentration: c4-MBA = 0.0280 mM, cHAuCl4 = 1.70 mM, cL-ascorbic acid = 4.10 mM).</li>
		<li>Immerse the Si wafer in a 10 mL vial containing growth solution B for about 1 min.</li>
		<li>Rapidly transfer the Si wafer without drying to another 10 mL vial containing growth solution A and leave it to grow for 2 min.</li>
		<li>Repeat steps 4.1.7-4.1.8 one more time.</li>
		<li>Take the Si wafer out and rinse with 50 mL of ethanol and 50 mL of DI water.</li>
		<li>Confirm the structure of the resulting segmented Au nanowires by SEM.</li>
	</ol>
	</li>
	<li>Synthesis of the nanoflowers
	<ol>
		<li>Treat the Si wafer following the same procedures in steps 2.1-2.2, except with a 5 min O2 plasma treatment.</li>
		<li>Soak the Si wafer in a 10 mL vial containing a 10000x diluted 3-5 nm Au seeds solution for 15 min.</li>
		<li>Wash the Si wafer from step 4.2.2 thoroughly with DI water to remove unabsorbed Au nanoparticles.</li>
		<li>Prepare a growth solution containing 0.550 mM 4-MBA, 1.70 mM HAuCl4, and 4.10 mM L-ascorbic-acid, following the same procedures in step 4.1.5.</li>
		<li>Soak the wafer in a 10 mL vial containing the growth solution for 30 s.</li>
		<li>Remove the wafer from the growth solution and leave a thin layer of the solution (~ 13- 15 &#956;L) on the wafer.</li>
		<li>Quickly blow-dry the wafer at room temperature.</li>
		<li>Confirm the nanoflower structure by SEM.</li>
	</ol>
	</li>
</ol>

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The authors have nothing to disclose.

The mechanism of this active surface growth governed nanowire synthesis has been discussed comprehensively in previous work19. In addition, the effects of seed sizes and types as well as the effect of ligand types and sizes have also been investigated20,21. Generally. the nanowire growth is very different from previous reported routes. No template is required, and the asymmetric growth is induced by the differences between the ligand-capped Au surface and the Au surfaces facing the substrate. The active surface remains active throughout the whole growth process, since the newly deposited Au surface is always fresh and ligand-deficient. Here we would concentrate our discussion on the experimental operation in performing this synthesis. The reaction takes place under ambient condition; nevertheless, a few points still need to be emphasized for better control in synthesizing these nanostructures.
The synthesis of the Au nanowires begins with the preparation of seed nanoparticles. Generally, any Au nanostructures can be employed as the seed for growing nanowires. However, the density of the seeds on the substrate is important for the following Au deposition and nanowire growth process. The seed density decides the density of active site, where the Au would deposit onto. If the concentrations of the ligand, HAuCl4 and L-ascorbic acid are kept constant, the amount of Au get reduced per unit time would stay the same. As a result, the Au nanowire grown on each active site would be much faster and the obtained nanowires would be longer if the density of the seed decreases. Another possible scenario would be the appearance and expansion of the Au nanowire bundles, since too much Au deposition at one active site would cause expansion and split the active surface. The density of the seed on the substrate could be controlled by two means: the seed incubation time and the concentration of the seed solution used to incubate the substrate. It is worth mentioning that the concentrations of each Au nanoparticles solution described here are not the same, and the difference could be in orders of magnitude. Therefore, the concentration of the seed solution sometimes would be necessary. As shown in the representative result comparing the 3-5 nm and 15 nm Au nanoparticles, the Au nanowire grown from the non-concentrated 40 nm Au nanoparticles is much longer, and forms bundles. On the other hand, the seed density is purposely reduced when preparing the thick nanowires with mixed ligands. This is to avoid the fusion of the nanowires, as the lateral growth during the thick nanowire growth happens simultaneously with the longitudinal elongation. Dense seed adsorption would lead to fusion of the Au seeds to a continuous Au film at the very beginning stage, preventing the further nanowire growth.
The substrate adsorption of the seed is important for creating a systematic asymmetry on the Au seed surface. We use the amine-contained siloxane to achieve the attachment of the Au seeds. It is normally accepted that the Au seeds are adsorbed by the electrostatic interaction between the amine group and the nanoparticle surface22. In some cases where the seed particles are positively charged, the cast and dry method could also be employed. The APTES is linked to the wafer and glass chip surface through the Si-O bond after hydrolyzing in the water/ethanol solution. Theoretically, any surface that could condense with the APTES would be able to facilitate the growth of Au nanowires. In this work, we demonstrate this with several oxide surfaces. An O2 plasma treatment is necessary to partially oxidize the substrate surface and implant the -OH group that could condense with the APTES. The O2 plasma treatment is chosen due to its clean and simple operation procedure; otherwise, piranha solution could also be used to create the -OH groups. Another key point for the substrate preparation is that the surface concentration of the -NH2 may also have a significant effect on the seed adsorption. Although we could not directly characterize the result of the surface treatment, the operation procedure should be done as precisely as possible.
The washing of the substrate after each step is sometimes crucial, especially after the APTES treatment. The free APTES molecule could also adsorb on the Au nanoparticles surface. Without a thorough wash, the seed solution would aggregate significantly during the soaking process. The aggregated seeds can still adsorb on the substrate and induce the Au nanowire growth. However, since aggregation would greatly reduce the concentration of the seeds, the density of the active sites also decreases exponentially. As a result, the top of the final nanowires area would be the Au nanoparticle agglomerates, and the nanowires become spared bundles instead of forest.
Typical Au nanowire growth takes place in a mixed solvent of the water and ethanol (v/v = 1:1). Since the ligand 4-MBA is not soluble in ethanol, it should first be dissolved in the ethanol solution, and then mixed with the result of the growth solution. Apart from the solubility issue, the solvent ratio itself also plays significant role deciding the nanowire growth. The area of the active surface is decided by the ligand adsorption ratio and the deposition rate of Au. The reduction ability of the L-ascorbic acid varies in different solvents and at different pH environments23. Changing the solvent ratio would change the rate of Au reduction and deposition immediately. Increasing the water ratio too much could result in rapid Au reduction and possible homogeneous nucleation, which prevents the growth of the substrate-bound Au nanowires.
Similar to the solvent ratio, the concentration of the ligands also directly affects the nanowire formation, as it not only passivates the Au surface, but also stabilizes the Au and prevents homogeneous nucleation. An excess amount of ligand would greatly slow down the Au reduction. For instance, increasing the ligand concentration 5 times to 2.5 mM would result in no Au deposition on the substrate21. Decreasing the ligand concentration will cause the increase in the nanowire diameter and sometimes homogeneous nucleation, the latter one signified by the change of solution from colorless to greyish or reddish. The homogeneous nucleation would compete with the heterogeneous Au nanowire growth for the Au feedstock. Depending on the degree of homogeneous nucleation, the Au nanowire growth might be completely or partially shut off by the lack of Au feeding stock.
In conclusion, we demonstrate a novel method to prepare substrate-bound Au nanowires on different substrates. The Au nanowires form an array on the flat substrate surface. The width, length and density are readily tuned by changing the reaction parameters. The surface chemistry of the nanowires can be adjusted by the ligands, and thickened nanowires could be formed by mixing two different types of ligands. Furthermore, Au nanowires-derived complex Au nanostructures could be formed by combining several different growth conditions.

Typical of one dimensional nanomaterials, nanowires possess both the bulk-related properties and the unique properties originated from the quantum effects of the nanoscale structure. As a bridge between the nanoscale and the bulk scale materials, they have been widely applied in various fields of catalysis, sensing, and nanoelectronic devices, etc.1,2,3.
However, the synthesis of nanowires has long been a great challenge, as it usually requires breaking the intrinsic symmetry in the crystals. Traditionally, a template is employed to regulate the deposition of materials. For instance, template-electrodeposition has been used for the formation of various types of nanowires like Ag nanowires and CdS nanowires4,5,6,7,8,9,10. Another common approach is vapor-liquid-solid (VLS) growth, which employs a molten catalyst to induce the anisotropic growth on the substrate at an elevated temperature11. Common strategies for the synthesis of metal nanowires are the polyol methods for Ag nanowires and the oleylamine-assisted ultrathin Au nanowires12,13,14,15. Both approaches are material-specific, and the nanowire parameters are not readily tuned during the synthesis. In addition, metal nanowires can also be formed by the pressure-driven method, where the assembled metal nanoparticles are mechanically compressed and fused into nanowires16,17,18.
Recently, we reported a distinctive method to synthesis Au nanowires19. With the assist of a thiolated small molecule ligand, the nanowires could grow and form a vertically aligned array on the bulk Si wafer substrate at ambient conditions. It was found that the ligands play an important role in the symmetry-breaking growth. It binds to the surface of the substrate-adsorbed Au seeds strongly, forcing the Au to deposit selectively at the ligand-deficient interface between seeds and substrate. The interface between the newly deposited Au and the substrate remains ligand deficient, therefore, the active surface exists throughout the whole growth. By tuning the ligand concentration, the seed type and concentration as well as several other parameters, a series of Au nanowire-based nanostructures could be synthesized.
In this work, we will provide a detailed protocol for this convenient Au nanowires synthesis. The derived synthesis is also presented, including the synthesis of Au nanowires with hydrophobic surface property, Au nanowires on other substrates, tapered Au nanowires by mixing two ligands and the nanowire-based Au nanostructures formed by tuning the growth conditions.

<table><tbody><tr><td>Trisodium citrate dihydrate</td><td>Alfa Aesar</td><td>LoT: 5008F14U</td><td></td></tr><tr><td>Sodium borohydride</td><td>Fluka</td><td>LoT: STBG0330V</td><td>NaBH4</td></tr><tr><td>Hydrogen tetrachloroaurate(III) trihydrate</td><td>Alfa Aesar</td><td>LoT: T19C006</td><td>HAuCl4</td></tr><tr><td>3-aminopropyltriethoxysilane</td><td>J&amp;amp;K Scientific</td><td>LoT: LT20Q102</td><td>APTES</td></tr><tr><td>L-ascorbic acid&amp;nbsp;</td><td>Sigma-Aldrich</td><td>LoT: SLBL9227V</td><td></td></tr><tr><td>4-mercaptobenzoic acid</td><td>Sigma-Aldrich</td><td>LoT: MKBV5048V</td><td>4-MBA</td></tr><tr><td>2-Naphthalenethiol</td><td>Sigma-Aldrich</td><td>LoT: BCBP4238V</td><td>2-NpSH</td></tr><tr><td>4-Mercaptophenylacetic acid</td><td>Alfa Aesar</td><td>LoT: 10199160</td><td>4-MPAA</td></tr><tr><td>3-mercaptobenzoic acid</td><td>Aladdin</td><td>LoT: G1213027</td><td>3-MBA</td></tr><tr><td>3-Mercaptopropionic acid</td><td>Aladdin</td><td>LoT: E1618095</td><td>3-MPA</td></tr><tr><td>absolute ethanol</td><td>Sinopharm chemical Reagent</td><td>20170802</td><td></td></tr><tr><td>Silicon wafer</td><td>Zhe Jiang lijing</td><td>P</td><td>Si</td></tr><tr><td>Scanning Electron Microscope</td><td>Quanta FEG 250</td><td></td><td>SEM</td></tr><tr><td>Centrifuge&amp;nbsp;</td><td>Eppendorf</td><td>5424</td><td></td></tr><tr><td>Ultrasonic cleaner&amp;nbsp;</td><td>Kun Shan hechuang</td><td></td><td></td></tr><tr><td>Ultra-pure water system</td><td>NanJing qianyan</td><td>UP6682-10-11</td><td>for deionized water</td></tr><tr><td>Plasma cleaner</td><td>Harrick Plasma</td><td>PDC-002</td><td>for oxygen plasma</td></tr></tbody></table>

synthesis of substrate-bound au nanowires via an active surface growth mechanism

Advancing synthetic capabilities is important for the development of nanoscience and nanotechnology. The synthesis of nanowires has always been a challenge, as it requires asymmetric growth of symmetric crystals. Here, we report a distinctive synthesis of substrate-bound Au nanowires. This template-free synthesis employs thiolated ligands and substrate adsorption to achieve the continuous asymmetric deposition of Au in solution at ambient conditions. The thiolated ligand prevented the Au deposition on the exposed surface of the seeds, so the Au deposition only occurs at the interface between the Au seeds and the substrate. The side of the newly deposited Au nanowires is immediately covered with the thiolated ligand, while the bottom facing the substrate remains ligand-free and active for the next round of Au deposition. We further demonstrate that this Au nanowire growth can be induced on various substrates, and different thiolated ligands can be used to regulate the surface chemistry of the nanowires. The diameter of the nanowires can also be controlled with mixed ligands, in which another &#34;bad&#34; ligand could turn on the lateral growth. With the understanding of the mechanism, Au nanowire-based nanostructures can be designed and synthesized.

The Au nanoparticle seeds, substrate-bound Au nanowires and Au nanowire-based derivative nanostructures are characterized with SEM. Figure 1 shows the representative SEM images of the 3-5 nm Au nanoparticles, 15 nm Au Nanoparticles and 40 nm Au nanoparticles adsorbed on the Si wafer, confirming their sizes, adsorption and distribution. The Au nanowires grown from the respective seeds on the Si wafer substrate are also presented. The representative SEM images of the typical Au nanowires substrates other than the wafer substrate, i.e., glass substrate, etc. are presented in Figure 2. The representative SEM images of the thickened nanowires synthesized with various ligands and mixed ligands are presented in Figure 3. The representative SEM images of the segmental nanowires and nanoflower structure are presented in Figure 4.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/57808/57808fig1.jpg" /> 
Figure 1: SEM images of the Au nanoparticle seeds before nanowire growth: (a) 3-5 nm, (b) 15 nm, and (c) 40 nm Au nanoparticles. Au nanowires grown from the (d) 3-5 nm, (e) 15 nm and (f) 40 nm seeds on wafer substrate. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57808/57808fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/57808/57808fig2.jpg" /> 
Figure 2: SEM images of the Au nanowires grown on substrate other than Si wafer. Nanowires grown from: (a) Al2O3, (b) SrTiO3, (c) LaAlO3 (d) glass slide, (e) ITO and (f) FTO substrates. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57808/57808fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/57808/57808fig3.jpg" /> 
Figure 3: SEM images of the synthesized with other ligands. Ultrathin nanowire arrays formed with (a) MPAA, (b) 2-Naphthalenethiol, (c) 3-MBA ligands. Tapered Au nanowires formed with mixed ligands of 4-MBA and 3-MPA: (d) c4-MBA : c3-MPA = 3 : 1, (e) c4-MBA : c3-MPA = 6 : 4 and (f) c4-MBA : c3-MPA = 1 : 1. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57808/57808fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/57808/57808fig4.jpg" /> 
Figure 4: SEM images of complex Au NW-based nanostructures. (a) Segmental nanowire with thick-thin-thick-thin segments; (b) nanoflowers by drying the growth solution on the substrate. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57808/57808fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Synthesis of Substrate-Bound Au Nanowires Via an Active Surface Growth Mechanism at JoVE.com

Synthesis of Substrate-Bound Au Nanowires Via an Active Surface Growth Mechanism

We report a solution-based method to synthesize substrate-bound Au nanowires. By tuning the molecular ligands used during the synthesis, the Au nanowires can be grown from various substrates with different surface properties. Au nanowire-based nanostructures can also be synthesized by adjusting the reaction parameters.

Advancing synthetic capabilities is important for the development of nanoscience and nanotechnology. The synthesis of nanowires has always been a ...

synthesis-substrate-bound-au-nanowires-via-an-active-surface-growth

Nanyang Technological University

Research

JoVE Journal

Chemistry

7.8K Views.  Nanjing Tech University. We report a solution-based method to synthesize substrate-bound Au nanowires. By tuning the molecular ligands used during the synthesis, the Au nanowires can be grown from various substrates with different surface properties. Au nanowire-based nanostructures can also be synthesized by adjusting the reaction parameters.

Video: Synthesis of Substrate-Bound Au Nanowires Via an Active Surface Growth Mechanism

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, ...

Engineering

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 ...

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 ...

Gold Nanostar Synthesis with a Silver Seed Mediated Growth Method

The physical, chemical and optical properties of nano-scale colloids depend on their material composition, size and shape 1-5. There is a great interest in using nano-colloids for photo-thermal ablation, drug delivery and many other biomedical applications 6. Gold is particularly used because of its low toxicity 7-9. A property of metal nano-colloids is that they can have a strong surface plasmon resonance 10. The peak of the surface plasmon resonance mode depends on the structure and composition of the metal nano-colloids. Since the surface plasmon resonance mode is stimulated with light there is a need to have the peak absorbance in the near infrared where biological tissue transmissivity is maximal 11, 12.
We present a method to synthesize star shaped colloidal gold, also known as star shaped nanoparticles 13-15 or nanostars 16. This method is based on a solution containing silver seeds that are used as the nucleating agent for anisotropic growth of gold colloids 17-22. Scanning electron microscopy (SEM) analysis of the resulting gold colloid showed that 70 % of the nanostructures were nanostars. The other 30 % of the particles were amorphous clusters of decahedra and rhomboids. The absorbance peak of the nanostars was detected to be in the near infrared (840 nm). Thus, our method produces gold nanostars suitable for biomedical applications, particularly for photo-thermal ablation.

We synthesized star shaped gold nanostars using a silver seed mediated growth method. The diameter of the nanostars ranges from 200 to 300 nm and the number of tips vary from 7 to 10. The nanoparticles have a broad surface plasmon resonance mode centered in the near infrared.

The physical, chemical and optical properties of nano-scale colloids depend on their material composition, size and shape 1-5. There is a great interest ...

Bioengineering

Synthesis, Characterization, and Functionalization of Hybrid Au/CdS and Au/ZnS Core/Shell Nanoparticles

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large extinction coefficients which can be tuned across the visible spectrum. Research into the plasmonic enhancement of optical transitions has become popular, due to the possibility of altering and in some cases improving photo-absorption or emission properties of nearby chromophores such as molecular dyes or quantum dots. The electric field of the plasmon can couple with the excitation dipole of a chromophore, perturbing the electronic states involved in the transition and leading to increased absorption and emission rates. These enhancements can also be negated at close distances by energy transfer mechanism, making the spatial arrangement of the two species critical. Ultimately, enhancement of light harvesting efficiency in plasmonic solar cells could lead to thinner and, therefore, lower cost devices. The development of hybrid core/shell particles could offer a solution to this issue. The addition of a dielectric spacer between a gold nanoparticles and a chromophore is the proposed method to control the exciton plasmon coupling strength and thereby balance losses with the plasmonic gains. A detailed procedure for the coating of gold nanoparticles with CdS and ZnS semiconductor shells is presented. The nanoparticles show high uniformity with size control in both the core gold particles and shell species allowing for a more accurate investigation into the plasmonic enhancement of external chromophores.

The synthesis of uniform gold nanoparticles coated with semiconductor shells of CdS or ZnS is performed. The semiconductor coating is conducted by first depositing a silver sulfide shell and exchanging the silver cations for zinc or cadmium cations.

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large ...

Ligand-Mediated Nucleation and Growth of Palladium Metal Nanoparticles

The size, size distribution and stability of colloidal nanoparticles are greatly affected by the presence of capping ligands. Despite the key contribution of capping ligands during the synthesis reaction, their role in regulating the nucleation and growth rates of colloidal nanoparticles is not well understood. In this work, we demonstrate a mechanistic investigation of the role of trioctylphosphine (TOP) in Pd nanoparticles in different solvents (toluene and pyridine) using in situ SAXS and ligand-based kinetic modeling. Our results under different synthetic conditions reveal the overlap of nucleation and growth of Pd nanoparticles during the reaction, which contradicts the LaMer-type nucleation and growth model. The model accounts for the kinetics of Pd-TOP binding for both, the precursor and the particle surface, which is essential to capture the size evolution as well as the concentration of particles in situ. In addition, we illustrate the predictive power of our ligand-based model through designing the synthetic conditions to obtain nanoparticles with desired sizes. The proposed methodology can be applied to other synthesis systems and therefore serves as an effective strategy for predictive synthesis of colloidal nanoparticles.

The main goal of this work is to elucidate the role of capping agents in regulating the size of palladium nanoparticles by combining in situ small angle x-ray scattering (SAXS) and ligand-based kinetic modeling.

The size, size distribution and stability of colloidal nanoparticles are greatly affected by the presence of capping ligands. Despite the key contribution ...

Growth of Gold Dendritic Nanoforests on Titanium Nitride-coated Silicon Substrates

In this study, a high-power impulse magnetron sputtering system is used to coat a flat and firm titanium nitride (TiN) film on silicon (Si) wafers, and a fluoride-assisted galvanic replacement reaction (FAGRR) is employed for the rapid and easy deposition of gold dendritic nanoforests (Au DNFs) on the TiN/Si substrates. Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy patterns of TiN/Si and Au DNFs/TiN/Si samples validate that the synthesis process is accurately controlled. Under the reaction conditions in this study, the thickness of the Au DNFs increases linearly to 5.10 &#177; 0.20 &#181;m within 15 min of the reaction. Therefore, the employed synthesis procedure is a simple and rapid approach for preparing Au DNFs/TiN/Si composites.

This study presents a feasible procedure for synthesizing gold dendritic nanoforests on titanium nitride/silicon substrates. The thickness of gold dendritic nanoforests increases linearly within 15 min of a synthesis reaction.

In this study, a high-power impulse magnetron sputtering system is used to coat a flat and firm titanium nitride (TiN) film on silicon (Si) wafers, and a ...

Iron Nanowire Fabrication by Nano-Porous Anodized Aluminum and its Characterization

Magnetic nanowires possess unique properties that have attracted the interest of different fields of research, including basic physics, biomedicine, and data storage. We demonstrate a fabrication method for iron (Fe) nanowires&#160;via electrochemical deposition into anodic&#160;alumina oxide (AAO) templates. The templates are fabricated by anodization of aluminum (Al) discs, and the pore length and diameter are controlled by changing the anodizing conditions. Pores with an average diameter of around 120 nm are created using oxalic acid as the electrolyte. Using this method, cylindrical nanowires are synthesized, which are released by dissolving the alumina using a selective chemical etchant.

In this work, we describe a&#160;protocol to fabricate iron nanowires, including the formation of the porous alumina membrane that is used as the template, electrodeposition into templates using electrolyte solution, and the release of the nanowires into the solution.

Magnetic nanowires possess unique properties that have attracted the interest of different fields of research, including basic physics, biomedicine, and ...

Microscopic Visualization of Porous Nanographenes Synthesized through a Combination of Solution and On-Surface Chemistry

On-surface synthesis has recently been regarded as a promising approach for the generation of new molecular structures. It has been particularly successful in the synthesis of graphene nanoribbons, nanographenes and intrinsically reactive and instable, yet attractive species. It is based on the combination of solution chemistry aimed at preparation of appropriate molecular precursors for further ultra-high vacuum surface assisted transformations. This approach also owes its success to an incredible development of characterization techniques, such as scanning tunneling/atomic force microscopy and related methods, which allow detailed, local characterization at atomic scale. While the surface-assisted synthesis can provide molecular nanostructures with outstanding precision, down to single atoms, it suffers from basing on metallic surfaces and often limited yield. Therefore, the extension of the approach away from metals and the struggle to increase productivity seem to be significant challenges toward wider applications. Herein, we demonstrate the on-surface synthesis approach for generation of non-planar nanographenes, which are synthesized through a combination of solution chemistry and sequential surface-assisted processes, together with the detailed characterization by scanning probe microscopy methods.

A combination of solution and surface assisted synthesis opens new directions in the atomically precise synthesis of nanostructures. Scanning tunneling microscopy (STM) supplemented by non-contact atomic force microscopy (nc-AFM) enables detailed characterization of newly designed and generated carbon-based nano-objects.

On-surface synthesis has recently been regarded as a promising approach for the generation of new molecular structures. It has been particularly ...

Aqueous Synthesis of Plasmonic Gold-Tin Alloy Nanoparticles

This protocol describes the synthesis of Au nanoparticle seeds and the subsequent formation of Au-Sn bimetallic nanoparticles. These nanoparticles have potential applications in catalysis, optoelectronics, imaging, and drug delivery. Previously, methods for producing alloy nanoparticles have been time-consuming, require complex reaction conditions, and can have inconsistent results. The outlined protocol first describes the synthesis of approximately 13 nm Au nanoparticle seeds using the Turkevich method. The protocol next describes the reduction of Sn and its incorporation into the Au seeds to generate Au-Sn alloy nanoparticles. The optical and structural characterization of these nanoparticles is described. Optically, prominent localized surface plasmon resonances (LSPRs) are apparent using UV-visible spectroscopy. Structurally, powder X-ray diffraction (XRD) reflects all particles to be less than 20 nm and shows patterns for Au, Sn, and multiple Au-Sn intermetallic phases. Spherical morphology and size distribution are obtained from transmission electron microscopy (TEM) imaging. TEM reveals that after Sn incorporation, the nanoparticles grow to approximately 15 nm in diameter.

Author Spotlight: Designing Sustainable Nanomaterials for Advancing Synthesis and Element Mixing

Here, the synthesis of gold (Au) seeds is described using the Turkevich method. These seeds are then used to synthesize gold-tin alloy (Au-Sn) nanoparticles with tunable plasmonic properties.

This protocol describes the synthesis of Au nanoparticle seeds and the subsequent formation of Au-Sn bimetallic nanoparticles. These nanoparticles have ...