This work was supported by the Air Force Office of Scientific Research Young Investigator Research Program (AFOSR, Grant. No. FA9550-15-1-0301).

Caution: Several chemicals (such as concentrated nitric acid (15.698 M HNO3) and hydrochloric acid (6 M HCl)) used in these procedures are hazardous. Proper gloves, eye protection and other safety equipment must be used. Please refer to the materials safety datasheets (MSDS) of all chemicals before use.
1. Nanocube Synthesis
<ol>
	<li>Preparation of reagents 
	Note: Ethylene glycol (EG) must be anhydrous. Close the EG container&#39;s cap whenever it is not used to prevent water absorption. Silver trifluoroacetate (AgC2F3O2) is very sensitive to light therefore the AgC2F3O2 solution is prepared in the last step.
	<ol>
		<li>Prepare 1.3 mM sodium hydrosulfide hydrate (NaSH) solution by dissolving 1 mg of NaSH in 13.5 ml of EG.</li>
		<li>Prepare 20 mg/ml poly vinylpyrrolidone (PVP) solution by dissolving 0.1 g of PVP in 5 ml of EG.</li>
		<li>Prepare 3 mM hydrochloric solution by mixing 2.5 &#181;l of 6.0 M liquid HCl solution with 4.9975 ml of EG.</li>
		<li>Prepare AgC2F3O2 solution by dissolving 0.1 g of AgC2F3O2 in 0.8 ml of EG.</li>
	</ol>
	</li>
	<li>Equipment setup
	<ol>
		<li>Clean the round-bottom flask (RBF) and its cap with concentrated (70%, 15.698 M) nitric acid HNO3. Fill the RBF with HNO3 and put the cap on for 30 min. Ensure the cap touches the acid.</li>
		<li>After HNO3 acid, clean the RBF and cap again with clean de-ionized (DI) water. Use clean nitrogen gas to dry the RBF and cap afterwards. The RBF and its cap must be clean and dry.</li>
		<li>Clean a magnetic stirring bar by immersing it in HNO3 for 30 min. After HNO3, clean it again with DI water and dry it with clean nitrogen gas.</li>
		<li>Prepare a heating bath. Place a silicone fluid bath (shown in Figure 1A) on top of a stirring hotplate with a well-controlled temperature. Use an external thermometer to monitor the fluid bath temperature. Set the temperature to 150 &#176;C and the stirring speed to 260 rpm.</li>
		<li>Mount the RBF with a clamp as shown in Figure 1B. Place the magnetic stirring bar (prepared in step 1.2.3) into the RBF.</li>
	</ol>
	</li>
	<li>Synthesis procedure
	<ol>
		<li>Dip the RBF into the heating bath (approximately 10 mm deep into the liquid; see Figures 1A-1B).</li>
		<li>Use a micropipette to place 10 ml of EG solution into the RBF. Put the cap on the RBF and wait for 20 min. The purpose of this step is to clean the RBF again, this time with EG.</li>
		<li>After 20 min, remove the cap and then lift the RBF out of the heating bath, pour the 10 ml of EG into a disposal container. Note: The EG solution is hot (150 &#176;C) and it is recommended to take the entire clamp out (Figure 1B). Make sure that the magnetic stirring bar (see step 1.2.5) does not fall off.</li>
		<li>Put the RBF back into the heating bath (see step 1.3.1).</li>
		<li>Use a micropipette to place 5 ml of EG into the RBF and put the cap on.</li>
		<li>Wait for 5 min.</li>
		<li>Take the RBF&#39;s cap off, use a micropipette to place 60 &#181;l of NaSH (as prepared in step 1.1.1 above) into the RBF. Put the cap back on.</li>
		<li>Wait for 2 min.</li>
		<li>Take the RBR&#39;s cap off, use a micropipette to place 500 &#181;l of the HCl solution (as prepared in step 1.1.3 above) into the RBF.</li>
		<li>Immediately after the previous step, use a micropipette to place 1.25 ml of the PVP solution (as prepared in step 1.1.2 above) into the RBF. Put the cap back on.</li>
		<li>Wait for 2 min.</li>
		<li>Take the RBF&#39;s cap off, use a micropipette to place 400 &#181;l of the AgC2F3O2 solution (as prepared in step 1.1.4 above) into the RBF. Put the cap back on.</li>
		<li>Wait for 2.5 hr. Ag nanocubes are forming during this step. If possible, during this time, reduce the room light to a minimum.</li>
		<li>After 2.5 hr, turn the heater off but leave the stirring on to avoid the fluid burning on the bottom. Use the clamp (shown in Figure 1B) to raise the RBF above the heating bath. Remove the cap.</li>
		<li>Remove the RBF from the heating bath such that it will cool off faster. After ~20 min, add 5 ml of acetone into the RBF. Vortex it in order to mix the solutions well. In the end, the total volume of the solution is 12 ml. See Figure 2A.</li>
		<li>Use a micropipette and transfer the final solution to eight smaller 1.5 ml plastic tubes.</li>
		<li>Centrifuge these eight tubes at a speed of 5150 x g for 10 min. As a result, all the Ag nanocubes will be at the bottom of the tubes. Use a micropipette to remove the top supernatant, leaving ~100 &#181;l at the bottom of each tube.</li>
		<li>Fill 1 ml of DI water into each of these tubes (obtained from step 1.3.17). Vortex and sonicate (5 min) the tubes. Nanocubes are now suspended in mainly DI water.</li>
		<li>Centrifuge again the eight tubes prepared in step 1.3.18 at 5,150 x g for 5 min. All the Ag nanocubes will be at the bottom of the tubes. Use a micropipette to remove the top supernatant, leaving around 100 &#181;l at the bottom of each tube.</li>
		<li>Fill 1 ml of DI water into each of the tubes obtained from step 1.3.19. Vortex and sonicate the tubes. Nanocubes are now suspended in DI water. The final nanocube solution obtained from this synthesis is shown in Figure 2B as an example.</li>
	</ol>
	</li>
</ol>
2. Gold Film Evaporation
Note: An electron-beam evaporator was used to deposit gold (Au) films onto purchased cleanroom cleaned slides, with chromium (Cr) acting as an adhesion layer. The evaporation process takes place inside a vacuum chamber, enabling the molecules to evaporate freely in the chamber and then sublimate on the substrate. The operation procedure is:
<ol>
	<li>Vent the chamber, by pressing &#34;Auto Vent&#34;.</li>
	<li>Open chamber door and load substrates in the dome.</li>
	<li>Close the door and pump down by pressing &#34;Auto Pump&#34;, it takes approximately 1 hr for the chamber to pump down until the pressure is below 5&#215;10-6 Torr.</li>
	<li>Edit the recipe. Layer#1: Cr, thickness: 5 nm, deposition rate: 1 &#197;/sec; layer#2: Au, thickness: 50 nm, deposition rate: 2 &#197;/sec.</li>
	<li>Upon reaching the desired vacuum level, the deposition process of the first metal will automatically start by pressing &#34;Auto Run&#34;. 
	Note: During the deposition, the high voltage module is turned on and the voltage is 10 kV. Gun rotation module is turned on, and fixture rotation is 20 rpm. After the first layer finishes, the system will automatically move to the pocket location of the second metal and start deposition.</li>
	<li>After the entire process is complete, press &#34;Auto Vent&#34; to vent the chamber and take the sample out. 
	Note: The total thickness of the Au film was 50 nm and the surface roughness was measured using an atomic force microscope (AFM) yielding a typical root mean square (RMS) of 0.7 nm. No special treatment was performed of the purchased glass substrates before Au film deposition.</li>
</ol>
3. Deposition of PE Layers
<ol>
	<li>Preparation of reagents
	<ol>
		<li>For the sodium chloride (NaCl) solution, mix 29 g of NaCl powder with 500 ml of DI water.</li>
		<li>For the polystyrene sulfonate (PSS) solution, mix 29 g of NaCl powder with 500 ml of DI water then add 1.5 ml of the PSS stock solution.</li>
		<li>For the poly(allylamine) hydrochloride (PAH) solution, mix 29 g of NaCl powder with 500 ml of DI water then add 132 mg of PAH.</li>
	</ol>
	</li>
	<li>Layer-by-layer deposition 
	Note: PAH is slightly positively charged while PSS is slightly negatively charged. As the Au film fabricated in Section 2 above is slightly negatively charged, a PAH layer will be deposited first. The steps below will demonstrate in details how to deposit five PE layers: PAH/PSS/PAH/PSS/PAH.
	<ol>
		<li>First, deposit a PAH layer by immersing the gold film (fabricated in Section 2 above) into a PAH solution (prepared in step 3.1.3) for 5 min. This results in a PAH layer on top of the Au film with a thickness of ~1 nm.</li>
		<li>After 5 min, rinse the Au film + 1 PAH layer with clean DI water.</li>
		<li>Immerse the Au film + 1 PAH layer into a NaCl solution (prepared in step 3.1.1) for 1 min.</li>
		<li>Immerse the Au film + 1 PAH layer (after step 3.2.3) into a PSS solution for 5 min. This results in a PSS layer with a thickness of ~1 nm on top of the PAH layer.</li>
		<li>After 5 min, rinse the Au film + 1 PAH layer +1 PSS layer with clean DI water.</li>
		<li>Immerse the Au film + 1 PAH layer + 1 PSS layer into the NaCl solution for 1 min.</li>
		<li>Immerse the Au film + 1 PAH layer + 1 PSS layer into the PAH solution for 5 min. This results in another PAH layer with a thickness of ~1 nm on top of the PSS layer (prepared in step 3.2.4 above).</li>
		<li>After 5 min, rinse the Au film + 1 PAH +1 PSS + 1 PAH layers with clean DI water.</li>
		<li>Immerse the Au film + 1 PAH + 1 PSS + 1 PAH layers into the NaCl solution for 1 min.</li>
		<li>Immerse the Au film + 1 PAH + 1 PSS + 1 PAH layers into the PAH solution for 5 min. This results in a second PSS layer with a thickness of ~1 nm on top of the PAH layer (which was prepared in step 3.2.7 above).</li>
		<li>After 5 min, rinse the Au film + 1 PAH +1 PSS + 1 PAH + 1 PSS layers with clean DI water.</li>
		<li>Immerse the Au film + 1 PAH + 1 PSS + 1 PAH +1 PSS layers into the NaCl solution for 1 min.</li>
		<li>Immerse the Au film + 1 PAH + 1 PSS + 1 PAH +1 PSS layers into the PAH solution for 5 min. This results in another PAH layer with a thickness of ~1 nm on top of the PSS layer (which was prepared in step 3.2.10 above).</li>
		<li>Finally, rinse the Au film + 1 PAH + 1 PSS + 1 PAH +1 PSS + 1 PAH with DI water and dry the sample using clean nitrogen gas. 
		Note: The total thickness of the five PE layers was measured in air using a spectroscopic ellipsometer at incidence angles of 65&#176;, 70&#176;, and 75&#176;, yielding a thickness of 5.0 &#177; 0.1 nm.</li>
	</ol>
	</li>
</ol>
4. Deposition of Cy5 Dye Molecules
<ol>
	<li>Prepare a 25 &#181;M Cy5 solution with DI water as the solvent.</li>
	<li>Expose the surface of the sample (which has a series of five PE layers, as described in Section 3 above) to 100 &#181;l of a 25 &#181;M Cy5 solution for 10 min. First drop cast 100 &#181;l of the Cy5 solution (prepared in step 4.1) onto the sample surface and then place a cover slip on top of the solution drop. Cy5 molecules will incorporate into the top PE layers uniformly.</li>
	<li>After 10 min, rinse the sample with DI water and dry it with clean nitrogen gas.</li>
</ol>
5. Deposition of Nanocubes to Form Nanopatch Antennas (NPAs)
<ol>
	<li>Dilute the nanocube solution obtained from Section 1 by a factor of 1/100 using DI water to enable the optical study of individual NPAs.</li>
	<li>Use a micropipette to place a drop of 20 &#181;l of diluted nanocube solution (prepared in step 5.1) onto a clean cover slip. Place the sample (prepared in Section 4) in contact with the cover slip for 2 min. This results in the Ag nanocubes to be immobilized on the top terminal PAH layer because the nanocubes synthesized here are negatively charged and the top PAH layer is positively charged.</li>
	<li>After 2 min, rinse the sample with DI water and dry with clean nitrogen gas. 
	Note: Steps 5.1 - 5.3 describe a procedure to prepare a sample for optical studies of single NPAs using a dark field microscope (dark field scattering). To prepare a sample for reflectivity measurements, a similar procedure is applied except that in step 5.1 the original nanocube solution is diluted by a factor of 1/10 instead of 1/100.</li>
</ol>
6. Optical Measurements
Note: A custom built optical bright-/dark-field microscope is used in these measurements. The NPAs are illuminated by a white light source through a long working distance bright-/dark-field objective. The reflected/scattered light from the NPAs is collected by the same objective. A pinhole aperture (50 &#956;m diameter) is used at an image plane to select signal from an individual nanoantenna. A digital camera is used to capture a color image. A spectrometer and a charge coupled device (CCD) camera are used to acquire spectral data. For fluorescence measurements, a 633 nm continuous wave HeNe laser is used for excitation and the signal was spectrally filtered by a long pass filter.
<ol>
	<li>Dark field scattering spectrum of single NPAs
	<ol>
		<li>Under white light illumination, identify single NPAs on the sample that was prepared in Section 5. Under white light illumination, individual NPAs appear as bright, red or pink colored dots as shown in Figure 4C.</li>
		<li>Align a single NPA with the pinhole aperture using a translation stage. Ensure that the dark field scattering image of the NPA is still observed after the pinhole aperture.</li>
		<li>Acquire a spectrum of the scattered light from the NPA using the spectrometer and CCD camera with a 1 sec integration time. Because the aperture area (50 &#181;m) is much bigger than the physical size of the NPA (~ 75 nm) the spectrum contains scattered light from the NPA in addition to signal from the area surrounding the NPA.</li>
		<li>Move the sample to an area without any NPAs and acquire another spectrum with a 1 s integration time. This spectrum represents scattered light from the background.</li>
		<li>Remove the sample with NPAs and place a certified reflectance standard sample in the setup. Acquire a spectrum of the scattered light with a 0.1 sec integration time in order to normalize the signal from the NPA.</li>
		<li>Close the pinhole aperture and acquire a spectrum with a 0.1 sec integration time without any input signal. This spectrum represents the CCD dark counts.</li>
		<li>Calculate the final scattering spectrum of an NPA as follows: 
		<img alt="Equation 1" src="/files/ftp_upload/53876/53876eq1.jpg" /> 
		where INPA+background, Ibackground, Iwhite light, ICCD dark are the scattering spectra measured by steps 6.1.3, 6.1.4, 6.1.5 and 6.1.6, respectively.</li>
		<li>Extract the plasmon resonance of the NPA by calculating the centroid of the scattering resonance peak.16</li>
	</ol>
	</li>
	<li>Fluorescence enhancement of Cy5 molecules by single NPAs
	<ol>
		<li>Under white light illumination, identify single NPAs from the sample prepared in Section 5. In dark field, individual NPAs appear as bright, red or pink colored dots as shown in Figure 4C.</li>
		<li>Align a single NPA with the pinhole aperture using a translation stage. Ensure that the dark field scattering image of the NPA is detected by the camera placed after the pinhole aperture.</li>
		<li>Turn off the white light illumination and turn on the 633 nm continuous wave HeNe laser used for excitation.</li>
		<li>Place a 633 nm long pass laser filter in the optical path right before the entrance to the spectrometer in order to block any scattered laser light.</li>
		<li>Acquire a fluorescence spectrum of the emission from the Cy5 molecules using a 1 sec integration time. Because the aperture area (50 &#181;m) is much larger than the physical size of the NPA (~ 75 nm) this spectrum contains emission from both molecules embedded in the NPA as well as molecules surrounding the NPA.</li>
		<li>Move the sample to an area without any NPAs and acquire another spectrum with a 1 sec integration time. This spectrum represents the emission from molecules in the background, without any NPAs.</li>
		<li>Prepare a separate sample, which will be used as a control sample, following the procedure in Sections 3 and 4 where Cy5 molecules are incorporated with PE layers on top of a glass slide (without a Au film and Ag nanocubes).</li>
		<li>Acquire a fluorescence spectrum of the emission from Cy5 molecules on the control sample prepared in the previous step using a 10 sec integration time.</li>
		<li>Determine the fluorescence enhancement factor by using the fluorescence spectra measured in steps 6.2.5, 6.2.6 and 6.2.8, taking into account the CCD dark counts, normalization by unit area and acquisition times.12,14</li>
	</ol>
	</li>
</ol>

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The authors declare that they have no competing financial interests.

Silver nanocubes were chemically synthesized using reaction conditions similar to previously reported syntheses.2,12,17-20 This synthesis enables the fabrication of nanocubes with side lengths ranging from 50 to 100 nm. For example, a typical heating time of 2.5 hr will result in nanocubes with side lengths of ~75 nm. A longer synthesis time (&#62; 3 hr) will lead to larger nanoparticles, however, this may also result in different shapes such as truncated nanocubes or octahedrons. The final solution was centrifuged and re-suspended in deionized water, and may be stored for at least one month in a fridge at 4 &#176;C without any noticeable changes in the scattering spectra of the plasmon resonances.12
The size and shape of the Ag nanocubes from the process presented in the above protocol are very sensitive to the cleaning of the RBF, its cap and the stirring bar as well as the quality of the EG solution. Nanoparticles with different shapes such as rounded or elongated nanoparticles is a sign that there is likely an issue with one of these steps in the synthesis. It is therefore advised that steps 1.1.1-1.1.4 and 1.2.1-1.2.2 are critically important.
In Figure 4b the scattering spectrum collected from a single nanopatch antenna is shown which exhibits a strong plasmon resonance at 650 nm. Such a resonance indicates an excellent mode confinement in the gap region between the Ag nanocube and Au film made possible by high-quality nanocubes. Additionally, to obtain such a spectrum, it is also required that the sample is clean, the spacer layers (PE layers) have a uniform thickness and that the underlying Au film is smooth. The strong plasmon resonance is further confirmed by the data presented in Figure 4c where individual nanopatch antennas can be observed in the dark field image and in Figure 4d where large fluorescence enhancement is observed of Cy5 molecules located in the gap region. It should also be noted that the Ag nanocubes oxidize over time despite the PVP coating when exposed to air and thus it is recommended that optical measurements should be performed on the day the sample is prepared or within 1 to 3 days. To minimize oxidation, it is recommended that the nanopatch antenna samples be stored in vacuum or nitrogen gas.
The method presented in this paper enables the fabrication of Ag nanocubes and plasmonic nanopatch antennas with well-controlled dimensions utilizing colloidal synthesis and a layer-by-layer dip-coating process. Compared with other techniques such as optical or electron beam lithography, the technique presented here offers the potential for low cost and large-scale production while producing a narrow size distribution of nanoparticles.
The plasmonic nanopatch antennas presented in this paper also hold great promise for new nanomaterials by design exhibiting unique properties which may not exist in their macroscopic counterparts. In particular, these nanoantennas have shown record-high fluorescence enhancement of embedded dye molecules exceeding 30,000;12 spontaneous emission rate enhancements of 1,000; ultrafast spontaneous emission and high quantum yield.13,14 Additionally, it has been shown that emitters coupled to these nanopatch antennas exhibit highly directional emission which is critical for applications where coupling to an external detector or single mode fiber is required. Future applications of the nanoscale patch antennas may range from ultrafast optoelectronic devices, such as light emitting diodes, to high efficiency photodetectors and photovoltaic devices, sensing and quantum information processing technologies.12-14

In recent years, colloidal synthesis of nanoparticles and the assembly of these into advanced structures have attracted great interests both in research and industrial developments.1-4 Colloidal synthesis of nanoparticles have several advantages over lithographically fabricated nanostructures including superior size homogeneity, low cost and the possibility of large-scale, parallel production.
Metal nanoparticles such as silver (Ag) and gold (Au) can support localized surface plasmon polaritons and have the ability to confine light in a volume much smaller than the diffraction limit.1,3-5 The resulting high field intensity creates an enhanced local density of states enabling light-matter interactions to be tailored at the nanoscale. Recent efforts have demonstrated procedures to synthesize Ag and Au nanoparticles in a wide range of sizes and shapes, including triangles,4,6 cages,3,4 and rods4,7,8 in addition to the nanocubes discussed here. Nanostructures composed of several Ag or Au nanocomponents have also been fabricated demonstrating tailored properties.1,9-11
Here, we demonstrate a procedure to synthesize Ag nanocubes and more importantly, to couple these Ag nanocubes with an underlying Au film in order to form plasmonic nanopatch antennas. The distance between Ag nanocubes and the Au film can be controlled with ~1 nm resolution by using a series of polyelectrolyte spacer layers. We also demonstrate how to incorporate an active medium, such as an organic dye, into the plasmonic nanopatch antennas. Due to the strongly confined electromagnetic fields in the gap region between the nanocubes and the Au film, the nanopatch antennas can be used for highly enhanced fluorescence and spontaneous emission of the embedded dye molecules.12,13 The methods presented in this paper can be generalized to other emitters, such as colloidal solid-state quantum dots14 or two-dimensional semiconductor materials,15 and the plasmon resonance can be tuned over a wide spectral range by varying the size of the nanocubes or the gap.

<table>
	<tbody>
		<tr>
			<td>Reagents</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Ethylene glycol&#160;</td>
			<td>J.T. Baker</td>
			<td>9300</td>
			<td>Must be anhydrous</td>
		</tr>
		<tr>
			<td>Sodium hydrosulfide hydrate&#160;&#160;</td>
			<td>Sigma Aldrich</td>
			<td>161527</td>
			<td></td>
		</tr>
		<tr>
			<td>Poly vinylpyrrolidone&#160;</td>
			<td>Sigma Aldrich</td>
			<td>856568</td>
			<td></td>
		</tr>
		<tr>
			<td>Hydrochroric acid BDH ARISTAR PLUS</td>
			<td>VWR International</td>
			<td>7647-01-0</td>
			<td></td>
		</tr>
		<tr>
			<td>Silver trifluoroacetate&#160;</td>
			<td>Sigma Aldrich</td>
			<td>482307</td>
			<td>Store in dark place</td>
		</tr>
		<tr>
			<td>Acetone</td>
			<td>Sigma Aldrich</td>
			<td>48358</td>
			<td></td>
		</tr>
		<tr>
			<td>Nitric acid&#160;</td>
			<td>Sigma Aldrich</td>
			<td>7697-37-2</td>
			<td>&#160;concentrated (70%), for cleaning</td>
		</tr>
		<tr>
			<td>Poly(allylamine) hydrochloride (PAH)</td>
			<td>Sigma-Aldrich</td>
			<td>283215</td>
			<td></td>
		</tr>
		<tr>
			<td>Polystyrene sulfonate&#160; (PSS)</td>
			<td>Sigma-Aldrich</td>
			<td>561223</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium Chloride&#160;</td>
			<td>Macron Inc.</td>
			<td>7647</td>
			<td></td>
		</tr>
		<tr>
			<td>Sulfo-Cyanine5 carboxylic acid (Cy5)</td>
			<td>Lumiprobe</td>
			<td>13390</td>
			<td>Fluorescent dye (molecular weight: 664.76 g/mol)</td>
		</tr>
		<tr>
			<td>Equipments</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Stirring hotplate with temperature control</td>
			<td>VWR International</td>
			<td>89000-338</td>
			<td></td>
		</tr>
		<tr>
			<td>Vortex mixers</td>
			<td>VWR International</td>
			<td>10153-834</td>
			<td></td>
		</tr>
		<tr>
			<td>Microcentrifuge</td>
			<td>Thermoscientific</td>
			<td>Model 59A</td>
			<td></td>
		</tr>
		<tr>
			<td>Silicone fluid&#160;</td>
			<td>Sigma-Aldrich</td>
			<td>63148-62-9</td>
			<td></td>
		</tr>
		<tr>
			<td>Micro-scale</td>
			<td>Mettler Toledo</td>
			<td>Model ML 104/03</td>
			<td></td>
		</tr>
		<tr>
			<td>Electron-beam metal evaporator&#160;</td>
			<td>CHA Industries</td>
			<td>E-beam evaporator</td>
			<td>Located inside a clean room</td>
		</tr>
		<tr>
			<td>Pre-cleaned glass slides</td>
			<td>Schott North America, Inc.</td>
			<td>Nexterion Glass B&#160;</td>
			<td>Clean room pre-cleaned</td>
		</tr>
		<tr>
			<td>25-mL 24/40 round-bottle flask&#160;</td>
			<td>VWR International</td>
			<td>60002-290</td>
			<td></td>
		</tr>
		<tr>
			<td>Magnetic stirring bar</td>
			<td>VWR International</td>
			<td>58948-116</td>
			<td></td>
		</tr>
		<tr>
			<td>Micropipettes (1-10mL, 10&#8211;100 mL and 100&#8211;1000 mL)</td>
			<td>VWR International</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Ultrasonic cleaning bath</td>
			<td>Branson Ultrasonic</td>
			<td>Model 1510R-DTH</td>
			<td></td>
		</tr>
		<tr>
			<td>Stopwatch</td>
			<td>VWR International</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Eppendorf centrifugation tubes (1.5 mL)</td>
			<td>VWR International</td>
			<td>22364111</td>
			<td></td>
		</tr>
		<tr>
			<td>Poly(propylene) coning tubes (50 mL)</td>
			<td>VWR International</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Home built bright/darkfield microscope</td>
			<td></td>
			<td></td>
			<td>75 W Xenon white light source, Nikon BF/DF 50x ELWD 
			0.55 NA, 8.2 mm WD objective, Nikon D90 digital camera, Acton 2300i spectrometer, Photometrics CoolSnap HQ&#160; charge coupled device (CCD) camera</td>
		</tr>
		<tr>
			<td>He Ne laser (633 nm), 5 mW</td>
			<td>New Port Co.</td>
			<td>R-30990</td>
			<td></td>
		</tr>
		<tr>
			<td>Reflectance standard</td>
			<td>Lab Sphere</td>
			<td>Model SRS-99-010</td>
			<td></td>
		</tr>
		<tr>
			<td>Laser long pass filter 633 nm</td>
			<td>Semrock</td>
			<td>LP02-633RU-25</td>
			<td></td>
		</tr>
	</tbody>
</table>

colloidal synthesis of nanopatch antennas for applications in plasmonics and nanophotonics

We present a method for colloidal synthesis of silver nanocubes and the use of these in combination with a smooth gold film, to fabricate plasmonic nanoscale patch antennas. This includes a detailed procedure for the fabrication of thin films with a well-controlled thickness over macroscopic areas using layer-by-layer deposition of polyelectrolyte polymers, namely poly(allylamine) hydrochloride (PAH) and polystyrene sulfonate (PSS). These polyelectrolyte spacer layers serve as a dielectric gap in between silver nanocubes and a gold film. By controlling the size of the nanocubes or the gap thickness, the plasmon resonance can be tuned from about 500 nm to 700 nm. Next, we demonstrate how to incorporate organic sulfo-cyanine5 carboxylic acid (Cy5) dye molecules into the dielectric polymer gap region of the nanopatch antennas. Finally, we show greatly enhanced fluorescence of the Cy5 dyes by spectrally matching the plasmon resonance with the excitation energy and the Cy5 absorption peak. The method presented here enables the fabrication of plasmonic nanopatch antennas with well-controlled dimensions utilizing colloidal synthesis and a layer-by-layer dip-coating process with the potential for low cost and large-scale production. These nanopatch antennas hold great promise for practical applications, for example in sensing, ultrafast optoelectronic devices and for high-efficiency photodetectors.

Here, we show representative results of the characteristics of the plasmonic nanopatch antennas, including SEM images of the sample structure, a reflectivity spectrum of a collection of nanopatch antennas and a scattering spectrum from a single nanopatch antenna. The energy of the plasmon resonance of the nanopatch antennas depends on the size of the nanocubes, the thickness of the dielectric gap region, i.e., the number of PE layers, as well as the dielectric material. In the procedure presented above we obtained Ag nanocubes with an average side length of 75 nm and slightly rounded corners (radius of curvature ~10 nm) coated in a PVP layer with an estimated thickness of 1-3 nm. In combination with the 5 PE layers and gold film, this result in a plasmon resonance centered at ~650 nm with a full-width-at-half-maximum (FWHM) of ~50 nm. This in turn has good spectral overlap with the absorption and emission wavelength of the Cy5 molecules which is centered at 646 and 662 nm, respectively.
Figure 3A shows a SEM image of a sample with a high concentration of nanocubes. These nanocubes were deposited on top of a Au film with 5 PE layers. Such SEM images are used to verify the overall quality of the nanocube synthesis; however, these samples are not utilized for further optical measurements as the density of the nanocubes is too high. Additionally, due to the high density, some nanocubes do not lie on the surface which is essential to form the plasmonic nanopatch antenna structure.
Figure 3B shows a SEM image of a sample of nanocubes fabricated using a nanocube solution which has been diluted by a factor of 1/10. This sample is used for measurements where the reflectivity of white light from an ensemble of nanopatch antennas was measured to determine the overall plasmon resonance. Figure 3C shows a SEM image of a sample of nanocubes fabricated using a nanocube solution which has been diluted by a factor of 1/100. This sample is used for scattering measurements of an individual nanopatch antenna. Using the diluted nanocube solution enables individual nanopatch antennas to be spatially isolated at an image plane by using a small pinhole.
Figure 4A shows a reflectivity spectrum, after normalization with the white light background, measured from a similar sample to the one shown in the SEM image in Figure 3B. Figure 4B shows a scattering spectrum from a single nanopatch antenna similar to the sample shown in the SEM image in Figure 3C.
Figure 4C shows a dark field image of a nanopatch antenna sample (prepared by a 1/100 diluted nanocube solution dispersed on a gold film with 5 PE layers) taken by a Nikon D90 digital camera. The observed bright red dots are due to scattering of white light from individual nanopatch antennas. A few spots are observed to have colors other than red, which is a result of nanocubes with different sizes or larger nanoparticles with non-cubic shapes.
Figure 4D shows two fluorescence spectra, one measured from a single nanopatch antenna (from a sample similar to the one shown in Figure 3C) and the other from a control sample consisting of a glass slide with the same number of PE layers and density of Cy5 dye molecules. The fluorescence intensity from Cy5 molecules coupled to the nanopatch antenna is much stronger than on the glass slide. This results from an enhanced excitation rate as well as a modified radiation pattern and increased quantum efficiency of the dye molecules.12 After correcting for background fluorescence and normalizing per unit area by dividing the area under the nanocube with the excitation spot size,12 we obtain an enhancement factor of ~ 12,000 from the data shown in Figure 4D. This enhancement factor is smaller compared with the previously reported value of 30,00012 likely due to the use of a Au instead of Ag film, increasing non-radiative losses.
<img alt="Figure 1" src="/files/ftp_upload/53876/53876fig1.jpg" /> 
Figure 1. Equipment setup for Ag nanocube synthesis.&#160;(A) A photograph of the equipment setup showing the heating bath on top of the stirring hotplate with temperature control. (B) A close up of the round-bottom&#160;flask (RBF) containing the nanocube solution during the synthesis. The setup is located inside a fume hood with proper air ventilation. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53876/53876fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/53876/53876fig2.jpg" /> 
Figure 2. Pictures of the nanocube solution.&#160;(A) Nanocube solution after the 2.5-hr synthesis and (B) after transfer to smaller tubes and re-suspended in de-ionized water. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53876/53876fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/53876/53876fig3highres.jpg" width="700" /> 
Figure 3. SEM characterizations of Ag nanocubes.&#160;(A) SEM image of a concentrated nanocube sample, (B) a diluted (1/10) nanocube sample, and (C) a diluted (1/100) nanocube sample. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53876/53876fig3highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/53876/53876fig4highres.jpg" width="700" /> 
Figure 4. Optical characterizations of nanopatch antennas. (A) Normalized reflection spectrum measured from an ensemble of nanopatch antennas (non-diluted nanocube solution). (B) Scattering spectrum from a single nanopatch antenna (1/100 diluted nanocube solution). (C) A dark-field image of a nanopatch antenna sample (1/100 diluted nanocube solution) taken under white light illumination. Each bright red dot corresponds to an individual plasmonic nanopatch antenna. (D) Fluorescence from Cy5 dye molecules embedded in a nanopatch antenna (red solid line) compared to that from a glass slide with an identical concentration of Cy5 dyes (dashed black line). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53876/53876fig4highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Colloidal Synthesis of Nanopatch Antennas for Applications in Plasmonics and Nanophotonics at JoVE.com

Colloidal Synthesis of Nanopatch Antennas for Applications in Plasmonics and Nanophotonics

A protocol for the colloidal synthesis of silver nanocubes and fabrication of plasmonic nanoscale patch antennas with sub-10 nm gaps is presented.

We present a method for colloidal synthesis of silver nanocubes and the use of these in combination with a smooth gold film, to fabricate plasmonic ...

colloidal-synthesis-nanopatch-antennas-for-applications-plasmonics

Nanyang Technological University

Research

JoVE Journal

Engineering

11.0K Views.  Duke University. A protocol for the colloidal synthesis of silver nanocubes and fabrication of plasmonic nanoscale patch antennas with sub-10 nm gaps is presented.

Video: Colloidal Synthesis of Nanopatch Antennas for Applications in Plasmonics and Nanophotonics

Hyperpolarized Xenon for NMR and MRI Applications

Nuclear magnetic resonance (NMR) spectroscopy and imaging (MRI) suffer from intrinsic low sensitivity because even strong external magnetic fields of ~10 T generate only a small detectable net-magnetization of the sample at room temperature 1. Hence, most NMR and MRI applications rely on the detection of molecules at relative high concentration (e.g., water for imaging of biological tissue) or require excessive acquisition times. This limits our ability to exploit the very useful molecular specificity of NMR signals for many biochemical and medical applications. However, novel approaches have emerged in the past few years: Manipulation of the detected spin species prior to detection inside the NMR/MRI magnet can dramatically increase the magnetization and therefore allows detection of molecules at much lower concentration 2.
Here, we present a method for polarization of a xenon gas mixture (2-5% Xe, 10% N2, He balance) in a compact setup with a ca. 16000-fold signal enhancement. Modern line-narrowed diode lasers allow efficient polarization 7 and immediate use of gas mixture even if the noble gas is not separated from the other components. The SEOP apparatus is explained and determination of the achieved spin polarization is demonstrated for performance control of the method.
The hyperpolarized gas can be used for void space imaging, including gas flow imaging or diffusion studies at the interfaces with other materials 8,9. Moreover, the Xe NMR signal is extremely sensitive to its molecular environment 6. This enables the option to use it as an NMR/MRI contrast agent when dissolved in aqueous solution with functionalized molecular hosts that temporarily trap the gas 10,11. Direct detection and high-sensitivity indirect detection of such constructs is demonstrated in both spectroscopic and imaging mode.

The production of hyperpolarized xenon by means of spin exchange optical pumping (SEOP) is described. This method yields a ~10000-fold enhancement of the nuclear spin polarization of Xe-129 and has applications in nuclear magnetic resonance spectroscopy and imaging. Examples of gas phase and solution state experiments are given.

Nuclear magnetic resonance (NMR) spectroscopy and imaging (MRI) suffer from intrinsic low sensitivity because even strong external magnetic fields of ~10 ...

Solid-state Graft Copolymer Electrolytes for Lithium Battery Applications

Battery safety has been a very important research area over the past decade. Commercially available lithium ion batteries employ low flash point (&lt;80 &deg;C), flammable, and volatile organic electrolytes. These organic based electrolyte systems are viable at ambient temperatures, but require a cooling system to ensure that temperatures do not exceed 80 &deg;C. These cooling systems tend to increase battery costs and can malfunction which can lead to battery malfunction and explosions, thus endangering human life. Increases in petroleum prices lead to a huge demand for safe, electric hybrid vehicles that are more economically viable to operate as oil prices continue to rise. Existing organic based electrolytes used in lithium ion batteries are not applicable to high temperature automotive applications. A safer alternative to organic electrolytes is solid polymer electrolytes. This work will highlight the synthesis for a graft copolymer electrolyte (GCE) poly(oxyethylene) methacrylate (POEM) to a block with a lower glass transition temperature (Tg) poly(oxyethylene) acrylate (POEA). The conduction mechanism has been discussed and it has been demonstrated the relationship between polymer segmental motion and ionic conductivity indeed has a Vogel-Tammann-Fulcher (VTF) dependence. Batteries containing commercially available LP30 organic (LiPF6 in ethylene carbonate (EC):dimethyl carbonate (DMC) at a 1:1 ratio) and GCE were cycled at ambient temperature. It was found that at ambient temperature, the batteries containing GCE showed a greater overpotential when compared to LP30 electrolyte. However at temperatures greater than 60 &deg;C, the GCE cell exhibited much lower overpotential due to fast polymer electrolyte conductivity and nearly the full theoretical specific capacity of 170 mAh/g was accessed.

Lithium ion batteries employ flammable and volatile organic electrolytes that are suitable for ambient temperature applications. A safer alternative to organic electrolytes are solid polymer batteries. Solid polymer batteries operate safely at high temperatures (&gt;120 &deg;C), thus making them applicable to high temperature applications such as deep oil drilling and hybrid electric vehicles. This paper will discuss (a) the polymer synthesis, (b) the polymer conduction mechanism, and (c) provide temperature cycling for both solid polymer and organic electrolytes.

Battery safety has been a very important research area over the past decade. Commercially available lithium ion batteries employ low flash point (&lt;80 ...

Synthesis and Operation of Fluorescent-core Microcavities for Refractometric Sensing

This paper discusses fluorescent core microcavity-based sensors that can operate in a microfluidic analysis setup. These structures are based on the formation of a fluorescent quantum-dot (QD) coating on the channel surface of a conventional microcapillary. Silicon QDs are especially attractive for this application, owing in part to their negligible toxicity compared to the II-VI and II-VI compound QDs, which are legislatively controlled substances in many countries. While the ensemble emission spectrum is broad and featureless, an Si-QD film on the channel wall of a capillary features a set of sharp, narrow peaks in the fluorescence spectrum, corresponding to the electromagnetic resonances for light trapped within the film. The peak wavelength of these resonances is sensitive to the external medium, thus permitting the device to function as a refractometric sensor in which the QDs never come into physical contact with the analyte. The experimental methods associated with the fabrication of the fluorescent-core microcapillaries are discussed in detail, as well as the analysis methods. Finally, a comparison is made between these structures and the more widely investigated liquid-core optical ring resonators, in terms of microfluidic sensing capabilities.

Fluorescent-core microcavity sensors employ a high-index quantum-dot coating in the channel of silica microcapillaries. Changes in the refractive index of fluids pumped into the capillary channel cause shifts in the microcavity fluorescence spectrum that can be used to analyze the channel medium.

This paper discusses  fluorescent core microcavity-based sensors that can operate in a microfluidic  analysis setup. These structures are based on the ...

Three-dimensional Particle Tracking Velocimetry for Turbulence Applications: Case of a Jet Flow

3D-PTV is a quantitative flow measurement technique that aims to track the Lagrangian paths of a set of particles in three dimensions using stereoscopic recording of image sequences. The basic components, features, constraints and optimization tips of a 3D-PTV topology consisting of a high-speed camera with a four-view splitter are described and discussed in this article. The technique is applied to the intermediate flow field (5 &#60;x/d &#60;25) of a circular jet at Re &#8776; 7,000. Lagrangian flow features and turbulence quantities in an Eulerian frame are estimated around ten diameters downstream of the jet origin and at various radial distances from the jet core. Lagrangian properties include trajectory, velocity and acceleration of selected particles as well as curvature of the flow path, which are obtained from the Frenet-Serret equation. Estimation of the 3D velocity and turbulence fields around the jet core axis at a cross-plane located at ten diameters downstream of the jet is compared with literature, and the power spectrum of the large-scale streamwise velocity motions is obtained at various radial distances from the jet core.

A three-dimensional particle tracking velocimetry (3D-PTV) system based on a high-speed camera with a four-view splitter is described here. The technique is applied to a jet flow from a circular pipe in the vicinity of ten diameters downstream at Reynolds number Re &#8776; 7,000.

3D-PTV is a quantitative flow measurement technique that aims to track the Lagrangian paths of a set of particles in three dimensions using stereoscopic ...

Creating Sub-50 Nm Nanofluidic Junctions in PDMS Microfluidic Chip via Self-Assembly Process of Colloidal Particles

Polydimethylsiloxane (PDMS) is the prevailing building material to make microfluidic devices due to its ease of molding and bonding as well as its transparency. Due to the softness of the PDMS material, however, it is challenging to use PDMS for building nanochannels. The channels tend to collapse easily during plasma bonding. In this paper, we present an evaporation-driven self-assembly method of silica colloidal nanoparticles to create nanofluidic junctions with sub-50 nm pores between two microchannels. The pore size as well as the surface charge of the nanofluidic junction is tunable simply by changing the colloidal silica bead size and surface functionalization outside of the assembled microfluidic device in a vial before the self-assembly process. Using the self-assembly of nanoparticles with a bead size of 300 nm, 500 nm, and 900 nm, it was possible to fabricate a porous membrane with a pore size of ~45 nm, ~75 nm and ~135 nm, respectively. Under electrical potential, this nanoporous membrane initiated ion concentration polarization (ICP) acting as a cation-selective membrane to concentrate DNA by ~1,700 times within 15 min. This non-lithographic nanofabrication process opens up a new opportunity to build a tunable nanofluidic junction for the study of nanoscale transport processes of ions and molecules inside a PDMS microfluidic chip.

We propose a simple self-assembly technique of silica colloidal nanoparticles to create a nanofluidic junction between two microchannels in polydimethylsiloxane (PDMS). Using this technique, a nanoporous bead membrane with a pore size down to ~45 nm was built inside a microchannel and applied to electrokinetic preconcentration of DNA samples.

Polydimethylsiloxane (PDMS) is the prevailing building material to make microfluidic devices due to its ease of molding and bonding as well as its ...

Scanning Light Scattering Profiler (SLPS) Based Methodology to Quantitatively Evaluate Forward and Backward Light Scattering from Intraocular Lenses

The scanning light scattering profiler (SLSP) methodology has been developed for the full-angle quantitative evaluation of forward and backward light scattering from intraocular lenses (IOLs) using goniophotometer principles. This protocol describes the SLSP platform and how it employs a 360&#176; rotational photodetector sensor that is scanned around an IOL sample while recording the intensity and location of scattered light as it passes through the IOL medium. The SLSP platform can be used to predict, non-clinically, the propensity for current and novel IOL designs and materials to induce light scatter. Non-clinical evaluation of light scattering properties of IOLs can significantly reduce the number of patient complaints related to unwanted glare, glistening, optical defects, poor image quality, and other phenomena associated with the unintended light scattering. Future studies should be conducted to correlate SLSP data with clinical results to help identify which measured light scatter is most problematic for patients that have undergone cataract surgery subsequent to IOL implantation.

Scanning Light Scattering Profiler SLPS Based Methodology to Quantitatively Evaluate Forward and Backward Light Scattering from Intraocular Lenses

This protocol describes the scanning light scattering profiler (SLSP) that enables the full-angle quantitative evaluation of forward and backward scattering of light from intraocular lenses (IOLs) using goniophotometer principles.

The scanning light scattering profiler (SLSP) methodology has been developed for the full-angle quantitative evaluation of forward and backward light ...

Fabrication of Polymer Microspheres for Optical Resonator and Laser Applications

This paper describes three methods of preparing fluorescent microspheres comprising &#960;-conjugated or non-conjugated polymers: vapor diffusion, interface precipitation, and mini-emulsion. In all methods, well-defined, micrometer-sized spheres are obtained from a self-assembling process in solution. The vapor diffusion method can result in spheres with the highest sphericity and surface smoothness, yet the types of the polymers able to form these spheres are limited. On the other hand, in the mini-emulsion method, microspheres can be made from various types of polymers, even from highly crystalline polymers with coplanar, &#960;-conjugated backbones. The photoluminescent (PL) properties from single isolated microspheres are unusual: the PL is confined inside the spheres, propagates at the circumference of the spheres via the total internal reflection at the polymer/air interface, and self-interferes to show sharp and periodic resonant PL lines. These resonating modes are so-called "whispering gallery modes" (WGMs). This work demonstrates how to measure WGM PL from single isolated spheres using the micro-photoluminescence (&#181;-PL) technique. In this technique, a focused laser beam irradiates a single microsphere, and the luminescence is detected by a spectrometer. A micromanipulation technique is then used to connect the microspheres one by one and to demonstrate the intersphere PL propagation and color conversion from coupled microspheres upon excitation at the perimeter of one sphere and detection of PL from the other microsphere. These techniques, &#181;-PL and micromanipulation, are useful for experiments on micro-optic application using polymer materials.

Protocols for the synthesis of microspheres from polymers, the manipulation of microspheres, and micro-photoluminescence measurements are presented.

This paper describes three methods of preparing fluorescent microspheres comprising &#960;-conjugated or non-conjugated polymers: vapor diffusion, ...

Fabrication of Periodic Gold Nanocup Arrays Using Colloidal Lithography

Within recent years, the field of plasmonics has exploded as researchers have demonstrated exciting applications related to chemical and optical sensing in combination with new nanofabrication techniques. A plasmon is a quantum of charge density oscillation that lends nanoscale metals such as gold and silver unique optical properties. In particular, gold and silver nanoparticles exhibit localized surface plasmon resonances-collective charge density oscillations on the surface of the nanoparticle-in the visible spectrum. Here, we focus on the fabrication of periodic arrays of anisotropic plasmonic nanostructures. These half-shell (or nanocup) structures can exhibit additional unique light-bending and polarization-dependent optical properties that simple isotropic nanostructures cannot. Researchers are interested in the fabrication of periodic arrays of nanocups for a wide variety of applications such as low-cost optical devices, surface-enhanced Raman scattering, and tamper indication. We present a scalable technique based on colloidal lithography in which it is possible to easily fabricate large periodic arrays of nanocups using spin-coating and self-assembled commercially available polymeric nanospheres. Electron microscopy and optical spectroscopy from the visible to near-infrared (near-IR) was performed to confirm successful nanocup fabrication. We conclude with a demonstration of the transfer of nanocups to a flexible, conformal adhesive film.

We demonstrate the fabrication of periodic gold nanocup arrays using colloidal lithographic techniques and discuss the importance of nanoplasmonic films.

Within recent years, the field of plasmonics has exploded as researchers have demonstrated exciting applications related to chemical and optical sensing ...

Fabrication of Superhydrophobic Metal Surfaces for Anti-Icing Applications

Several ways to produce superhydrophobic metal surfaces are presented in this work. Aluminum was chosen as the metal substrate due to its wide use in industry. The wettability of the produced surface was analyzed by bouncing drop experiments and the topography was analyzed by confocal microscopy. In addition, we show various methodologies to measure its durability and anti-icing properties. Superhydrophobic surfaces hold a special texture that must be preserved to keep their water-repellency. To fabricate durable surfaces, we followed two strategies to incorporate a resistant texture. The first strategy is a direct incorporation of roughness to the metal substrate by acid etching. After this surface texturization, the surface energy was decreased by silanization or fluoropolymer deposition. The second strategy is the growth of a ceria layer (after surface texturization) that should enhance the surface hardness and corrosion resistance. The surface energy was decreased with a stearic acid film.
The durability of the superhydrophobic surfaces was examined by a particle impact test, mechanical wear by lateral abrasion, and UV-ozone resistance. The anti-icing properties were explored by studying the ability to repeal subcooled water, freezing delay, and ice adhesion.

We illustrate several methodologies to produce superhydrophobic metal surfaces and to explore their durability and anti-icing properties.

Several ways to produce superhydrophobic metal surfaces are presented in this work. Aluminum was chosen as the metal substrate due to its wide use in ...

A Modular Microfluidic Technology for Systematic Studies of Colloidal Semiconductor Nanocrystals

Colloidal semiconductor nanocrystals, known as quantum dots (QDs), are a rapidly growing class of materials in commercial electronics, such as light emitting diodes (LEDs) and photovoltaics (PVs). Among this material group, inorganic/organic perovskites have demonstrated significant improvement and potential towards high-efficiency, low-cost PV fabrication due to their high charge carrier mobilities and lifetimes. Despite the opportunities for perovskite QDs in large-scale PV and LED applications, the lack of fundamental and comprehensive understanding of their growth pathways has inhibited their adaptation within continuous nanomanufacturing strategies. Traditional flask-based screening approaches are generally expensive, labor-intensive, and imprecise for effectively characterizing the broad parameter space and synthesis variety relevant to colloidal QD reactions. In this work, a fully autonomous microfluidic platform is developed to systematically study the large parameter space associated with the colloidal synthesis of nanocrystals in a continuous flow format. Through the application of a novel translating three-port flow cell and modular reactor extension units, the system may rapidly collect fluorescence and absorption spectra across reactor lengths ranging 3 - 196 cm. The adjustable reactor length not only decouples the residence time from the velocity-dependent mass transfer, it also substantially improves the sampling rates and chemical consumption due to the characterization of 40 unique spectra within a single equilibrated system. Sample rates may reach up to 30,000 unique spectra per day, and the conditions cover 4 orders of magnitude in residence times ranging 100 ms - 17 min. Further applications of this system would substantially improve the rate and precision of the material discovery and screening in future studies. Detailed within this report are the system materials and assembly protocols with a general description of the automated sampling software and offline data processing.

Detailed herein are the operation and assembly protocols of a modular microfluidic screening platform for the systematic characterization of colloidal semiconductor nanocrystal syntheses. Through fully adjustable system arrangements, highly efficient spectra collection may be carried out across 4 orders of magnitude reaction time scales within a mass transfer-controlled sampling space.

Colloidal semiconductor nanocrystals, known as quantum dots (QDs), are a rapidly growing class of materials in commercial electronics, such as light ...