This work was funded by the Office of Naval Research (ONR) through the Naval Research Laboratory Basic Research Program.

1. Making Donor Substrates
<ol>
	<li>Mask the edges of a glass slide with tape leaving a central region of uncovered glass.</li>
	<li>Submerge the slide in Buffered HF (6:1 ratio of 40% NH4F in water to 48% HF in water) for 3-15 min. Note: This will etch the center of the slide which is unmasked, creating a well. The depth of the well should be between 1 and 5 &#181;m, which can be determined through a stylus profilometer or AFM using manufacturer&#39;s instructions.</li>
	<li>Remove the tape mask.</li>
</ol>
2. Creating the Ink Ribbon
<ol>
	<li>Spread a small amount of Ag paste at one side of the well. Ensure that there is enough to fill the well, roughly in the 10 mg range. It is not necessary to measure the amount first, however.</li>
	<li>Firmly drag a straight metal blade across the well, spreading a thin layer of paste across its entirety. Uniformly distribute the paste without any thin spots. The final product of this process &#8212; a small well containing Ag ink&#160;&#8212; is called the &#34;ribbon&#34;.</li>
	<li>Wipe away any paste that spread outside the well with a lab wipe.</li>
</ol>
3. Drying the Ribbon
<ol>
	<li>Place the ribbon face-up in a low-humidity environment. A box filled with dry nitrogen works best.</li>
	<li>Leave the ribbon for at least 2 hr at RT. At this point, the ink viscosity should be high enough to print. 
	Note: After sufficient drying, ink ribbons can be stored for roughly a month by placing the well face-down on another glass slide and storing it in a dry-nitrogen environment. Once stored this way, it is alright to leave the ink ribbons unattended for long periods of time.</li>
</ol>
4. Printing Voxels
<ol>
	<li>Attach the receiver substrate to an XY translational stage using a vacuum chuck or double-sided tape. Note: The receiver substrate must be flat, but there are no other limitations. Silicon wafers, glass slides, or 200 &#176;C compatible polymers are all acceptable receiver substrates.</li>
	<li>Place the ink ribbon face-down upon the receiver substrate.</li>
	<li>Focus the optical setup through the back of the donor substrate, onto the back surface of the ink within the well. 
	NOTE: There are many ways to arrange the optics for this process, but the following steps/components are necessary:
	<ol>
		<li>Use a pulsed UV laser with a beam having a &#34;top-hat&#34; spatial energy distribution (as opposed to Gaussian). Use a laser capable of controllably firing individual pulses, which may require an acousto-optic modulator. The acousto-optic modulator allows the user to control the firing of individual pulses.</li>
		<li>Pass the beam through an aperture, shaping the cross section of the beam into the desired shape. Note that the shape of the aperture determines the shape of the voxel. That is to say, the aperture is essentially imaged onto the donor substrate, analogous to mask projection.</li>
		<li>Use a microscopic objective to reduce the size of the beam cross section, which determines the size of the printed voxel. For example, if a 10X objective yields square voxels with 50 &#181;m lateral dimensions, then a 50X objective will print the same shape (square) voxels with 10 &#181;m lateral dimensions.</li>
		<li>Place a video camera in line (via beam splitter) with the microscopic objective. This allows active monitoring of the ink ribbon.</li>
	</ol>
	</li>
	<li>Fire a single laser pulse onto the donor substrate. A reasonable starting value for laser fluence is in the range of 40-60 mJ/cm2. Ensure that there is a visible hole in the shape of the laser beam cross section where the voxel was ejected. If the hole is not visible, there are several possible reasons:
	<ol>
		<li>Out of Focus.
		<ol>
			<li>Adjust the height of the focusing objective. This may bring the hole into focus.</li>
		</ol>
		</li>
		<li>Low Energy.
		<ol>
			<li>Slowly increase the energy of the laser up to a fluence of 60-80 mJ/cm2. Thicker ink ribbons may require high fluence values.</li>
		</ol>
		</li>
		<li>Ink Viscosity Too Low.
		<ol>
			<li>If a voxel is ejected but the hole in the ink ribbon immediately refills, then the ink viscosity is still too low, therefore dry the ribbon for another 30 min based on the instructions in step 3 and then begin step 4 again.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Move the XY translational stage along the X and Y axes to a new spot.</li>
	<li>Fire a single laser pulse onto the donor substrate again, ejecting a voxel and leaving a sharply-defined hole where the voxel was ejected from the ink ribbon.</li>
</ol>
5. Printing Complex Structures
<ol>
	<li>Create lines by linking together adjacent voxels in the following way:
	<ol>
		<li>Transfer a voxel as described in 4.1-4.4.</li>
		<li>Move the XY translational stage one voxel length along the X or Y direction.</li>
		<li>Transfer a voxel as described in 4.1-4.4.</li>
		<li>Repeat this process until a sufficiently long line is obtained.</li>
	</ol>
	</li>
	<li>Create bridging or cantilever structures in the following way:
	<ol>
		<li>Align the beam such that the ejected voxel will bridge a geometric gap on the donor substrate OR such that a portion of the voxel will overhang past the edge of a geometric gap.</li>
		<li>Transfer a voxel as described in 4.1-4.4. Note: If the paste viscosity is too low, the voxel may conform to the features beneath it instead of creating a bridge or cantilever.</li>
	</ol>
	</li>
	<li>Create high aspect ratio structures in the following way:
	<ol>
		<li>Transfer a voxel as described in 4.1-4.4.</li>
		<li>Without moving the receiver substrate, move the donor substrate to a fresh spot on the ink ribbon.</li>
		<li>Transfer a voxel as described in 4.1-4.4.</li>
		<li>Repeat steps 5.3.2 and 5.3.3 until a feature of sufficient height is obtained. If the structure is built taller than ~3-5 &#181;m, periodically insert spacers between the donor and receiver substrate so that the voxel stack and ink ribbon do not come into direct contact. Note that the optics will need to be refocused as described in section 4.4.1 to account for the change in donor substrate height.</li>
	</ol>
	</li>
</ol>
6. Printing Complex Images via DMD Chip
<ol>
	<li>Draw or upload image of desired voxel shape. Ensure that the format of the image file is a bitmap. Note: It is crucial to use the demagnification factor of the optical system to scale the drawing to the size of the voxel to be printed. The DMD essentially replaces the aperture, so instead of imaging the beam with what is essentially mask projection, an array of micromirrors is used to shape the beam.</li>
	<li>Select appropriate laser (UV or green).</li>
	<li>Turn on DMD and open DMD software.
	<ol>
		<li>Click &#34;Open Image&#34; and load bitmap pattern. Select Load and Reset.</li>
		<li>Click &#34;Add&#34;. Name of bitmap file should appear in the right panel.</li>
		<li>Click &#34;Run Once&#34;. Bitmap pattern is now loaded onto DMD.</li>
	</ol>
	</li>
	<li>Arrange the donor and receiver substrates as outlined in steps 4.1 to 4.3. Transfer ink as outlined in steps 4.4 to 4.6.</li>
	<li>Once transfers are successful, repeat steps 6.3 to 6.4 if needed; then proceed to step 7.</li>
</ol>
7. Furnace
Once all the voxels are printed, cure them in a furnace.
<ol>
	<li>Place the receiver substrate face-up in the furnace.</li>
	<li>Leave to cure at 180 &#176;C for 2 hr.</li>
</ol>

<ol><li>Park, J. U., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((High-resolution+electrohydrodynamic+jet+printing%5BTitle%5D)+AND+%22Nature+Mater%22%5BJournal%5D)">High-resolution electrohydrodynamic jet printing.</a> Nature Mater. 6, 782-789 (2007).</li>
<li>Hon, K. K. B., Li, L., Hutchings, I. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Direct+writing+technology+-+Advances+and+developments%5BTitle%5D)+AND+%22CIRP+Ann%22%5BJournal%5D)">Direct writing technology - Advances and developments.</a> CIRP Ann. 57, 601-620 (2008).</li>
<li>Calvert, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Inkjet+Printing+for+Materials+and+Devices%5BTitle%5D)+AND+%22Chem.+Mater%22%5BJournal%5D)">Inkjet Printing for Materials and Devices.</a> Chem. Mater. 13, 3299-3305 (2001).</li>
<li>Arnold, C. B., Serra, P., Piqué, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Laser+direct-write+techniques+for+printing+of+complex+materials%5BTitle%5D)+AND+%22MRS+Bulletin%22%5BJournal%5D)">Laser direct-write techniques for printing of complex materials.</a> MRS Bulletin. 32, 23-31 (2007).</li>
<li>Park, J. U., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((High-resolution+electrohydrodynamic+jet+printing%5BTitle%5D)+AND+%22Nature+Mater%22%5BJournal%5D)">High-resolution electrohydrodynamic jet printing.</a> Nature Mater. 6, 782-789 (2007).</li>
<li>Hon, K. K. B., Li, L., Hutchings, I. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Direct+writing+technology+-+Advances+and+developments%5BTitle%5D)+AND+%22CIRP+Ann%22%5BJournal%5D)">Direct writing technology - Advances and developments.</a> CIRP Ann. 57, 601-620 (2008).</li>
<li>Calvert, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Inkjet+Printing+for+Materials+and+Devices%5BTitle%5D)+AND+%22Chem.+Mater%22%5BJournal%5D)">Inkjet Printing for Materials and Devices.</a> Chem. Mater. 13, 3299-3305 (2001).</li>
<li>Arnold, C. B., Serra, P., Piqué, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Laser+direct-write+techniques+for+printing+of+complex+materials%5BTitle%5D)+AND+%22MRS+Bulletin%22%5BJournal%5D)">Laser direct-write techniques for printing of complex materials.</a> MRS Bulletin. 32, 23-31 (2007).</li>
<li>Piqué, A., Auyeung, R. C. Y., Kim, H. K., Metkus, M., Mathews, S. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Digital+microfabrication+by+laser+decal+transfer%5BTitle%5D)+AND+%22J.+Laser.+Micro.+Nanoeng%22%5BJournal%5D)">Digital microfabrication by laser decal transfer.</a> J. Laser. Micro. Nanoeng. 3, 163-168 (2008).</li>
<li>Auyeung, R. C. Y., Kim, H., Birnbaum, A. J., Zalalutdinov, M., Mathews, S. A., Piqué, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Laser+decal+transfer+of+freestanding+microcantilevers+and+microbridges%5BTitle%5D)+AND+%22Appl.+Phys.+A%22%5BJournal%5D)">Laser decal transfer of freestanding microcantilevers and microbridges.</a> Appl. Phys. A. 97, 513-519 (2009).</li>
<li>Kim, H., Melinger, J. S., Khachatrian, A., Charipar, N. A., Auyeung, R. C. Y., Piqué, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Fabrication+of+terahertz+metamaterials+by+laser+printing%5BTitle%5D)+AND+%22Opt.+Lett%22%5BJournal%5D)">Fabrication of terahertz metamaterials by laser printing.</a> Opt. Lett. 35, 4039-4041 (2010).</li>
<li>Wang, J., Auyeung, R. C. Y., Kim, H., Charipar, N. A., Piqué, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Three-dimensional+printing+of+interconnects+by+laser+direct-write+of+silver+nanopastes%5BTitle%5D)+AND+%22Adv.+Mater%22%5BJournal%5D)">Three-dimensional printing of interconnects by laser direct-write of silver nanopastes.</a> Adv. Mater. 22, 4462-4466 (2010).</li>
<li>Mathews, S. A., Auyeung, R. C. Y., Kim, H., Charipar, N. A., Piqué, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((High-speed+video+study+of+laser-induced+forward+transfer+of+silver+nano-suspensions%5BTitle%5D)+AND+%22J.+Appl.+Phys%22%5BJournal%5D)">High-speed video study of laser-induced forward transfer of silver nano-suspensions.</a> J. Appl. Phys. 114, 064910(2013).</li>
<li>King, B. H., Dimos, D., Yang, P., Morissette, S. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Direct-write+fabrication+of+integrated%2C+multilayer+ceramic+components%5BTitle%5D)+AND+%22J.+Electroceram%22%5BJournal%5D)">Direct-write fabrication of integrated, multilayer ceramic components.</a> J. Electroceram. 3, 173-178 (1999).</li>
<li>Lewis, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Direct+ink+writing+of+3D+functional+materials%5BTitle%5D)+AND+%22Adv.+Funct.+Mater%22%5BJournal%5D)">Direct ink writing of 3D functional materials.</a> Adv. Funct. Mater. 16, 2193-2204 (2006).</li>
<li>Calvert, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Inkjet+printing+for+materials+and+devices%5BTitle%5D)+AND+%22Chem.+Mater%22%5BJournal%5D)">Inkjet printing for materials and devices.</a> Chem. Mater. 13, 3299-3305 (2001).</li>
<li>Kang, H., Soltman, D., Subramanian, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Hydrostatic+Optimization+of+Inkjet-Printed+Films%5BTitle%5D)+AND+%22Langmuir%22%5BJournal%5D)">Hydrostatic Optimization of Inkjet-Printed Films.</a> Langmuir. 26, 11568-11573 (2010).</li>
<li>Piqué, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Laser+decal+transfer+of+electronic+materials+with+thin+film+characteristics%5BTitle%5D)+AND+%22Proc.+SPIE%22%5BJournal%5D)">Laser decal transfer of electronic materials with thin film characteristics.</a> Proc. SPIE. 6879, 687911(2008).</li>
<li>Auyeung, R. C. Y., Kim, H., Birnbaum, A. J., Zalalutdinov, M., Mathews, S. A., Piqué, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Laser+decal+transfer+of+freestanding+microcantilevers+and+microbridges%5BTitle%5D)+AND+%22Appl.+Phys.+A%3A+Mater.+Sci.+Process%22%5BJournal%5D)">Laser decal transfer of freestanding microcantilevers and microbridges.</a> Appl. Phys. A: Mater. Sci. Process. 97, 513-519 (2009).</li>
<li>Soltman, D., Smith, V., Kang, H., Morris, S. J. S., Subramanian, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Methodology+for+inkjet+printing+of+partially+wetting+films%5BTitle%5D)+AND+%22Langmuir%22%5BJournal%5D)">Methodology for inkjet printing of partially wetting films.</a> Langmuir. 26, 15686-15693 (2010).</li>
<li>Auyeung, R. C. Y., Kim, H., Charipar, N., Birnbaum, A., Mathews, S., Piqué, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Laser+forward+transfer+based+on+a+spatial+light+modulator%5BTitle%5D)+AND+%22Appl.+Phys.+A%3A+Mater.+Sci.+Process%22%5BJournal%5D)">Laser forward transfer based on a spatial light modulator.</a> Appl. Phys. A: Mater. Sci. Process. 102, 21-26 (2011).</li>
<li>Duocastella, M., Fernandez-Pradas, J. M., Serra, P., Morenza, J. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Jet+formation+in+the+laser+forward+transfer+of+liquids%5BTitle%5D)+AND+%22Appl.+Phys.+A%3A+Mater.+Sci.+Process%22%5BJournal%5D)">Jet formation in the laser forward transfer of liquids.</a> Appl. Phys. A: Mater. Sci. Process. 93, 453-456 (2008).</li>
<li>Feinaueugle, M., Alloncle, A. P., Delaporte, P., Sones, C. L., Eason, R. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Time-resolved+shadowgraph+imaging+of+femtosecond+laser-induced+forward+transfer+of+solid+materials%5BTitle%5D)+AND+%22Appl.+Surf.+Science%22%5BJournal%5D)">Time-resolved shadowgraph imaging of femtosecond laser-induced forward transfer of solid materials.</a> Appl. Surf. Science. 258, 8475-8483 (2012).</li>
<li>Breckenfeld, E., Kim, H., Auyeung, R. C. Y., Charipar, N., Serra, P., Piqué, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Laser-induced+forward+transfer+of+silver+nanopaste+for+microwave+interconnects%2C+A%5BTitle%5D)+AND+%22Appl.+Surf.+Science%22%5BJournal%5D)">Laser-induced forward transfer of silver nanopaste for microwave interconnects, A.</a> Appl. Surf. Science. 331, 254-261 (2015).</li>
</ol>

The authors have nothing to disclose.

In this paper, we have demonstrated a process for non-contact, three-dimensional printing as well as single-laser-shot, large area printing (via DMD chip) of high-viscosity Ag nanopaste. Unlike other direct-write techniques, such as inkjet, the LDT technique described here allows for the printing of complex voxel shapes with one laser pulse, i.e., in one single step. While many aspects of the procedure may seem straightforward, there are multiple steps that require iterative testing in order to optimize. First, paste dryness and viscosity are the most important factors for successful transfer. Although these points have been already been emphasized repeatedly in the text, we reiterate the point here to underline the importance. If the ink viscosity is too low, then it will be impossible to print sharp, well-defined voxel shapes. A telltale sign that the ink viscosity is too low occurs when attempting to eject a voxel. When the laser pulse is fired, the voxel will appear to momentarily eject, but the ink will fill back quickly in the hole left in the donor substrate. In this case, the user should stop firing the laser and the ink should be further treated as outlined in steps 3.1 and 3.2. If the ink viscosity is too high, the voxel transfer process will appear successful on the ribbon. However, when examining the voxels on the receiver substrate, there will be significant tearing, fracturing, or debris. In this case, the user needs to dispose of the current ribbon and make a new ribbon as outlined in section 2. Optimization of ink viscosity and drying time should be accomplished by evaluating the quality of voxel transfer attempts. We do not recommend trying to measure the viscosity of the paste at any point. Second, the laser fluence is almost as important as ink viscosity and very small changes in fluence can have a significant effect on the process. It should be very clear when the energy is too low&#160;&#8212; the voxel will not eject from the donor substrate. It is recommended to begin with the fluence range suggested in step 4.4, and then very incrementally increase the value. The lowest energy that results in a full transfer is called the &#34;threshold fluence&#34;. It is often best to operate at or near the threshold fluence because higher fluence values will tend to fracture or tear the voxels. Finally, depending on the variety of laser used for the process, there could be hot spots in the laser profile. This may require an adjustment of the aperture to sample a more homogeneous region of the beam. If the shape of the ejected voxel is warped or poorly matches the shape of the beam cross section, laser hotspots or ink layer thickness or uniformity could be responsible.
Beyond troubleshooting, there are a few limitations to the technique. The final furnace curing step makes it difficult or impossible to achieve voxels with the desired functional properties on non-high-temperature compatible substrates. Generally, the Ag nanopaste used in this manuscript requires a curing temperature of at least 150 &#176;C in order to obtain reasonable conductivity values. The fabrication of the ink layer on the donor substrate needs to be further optimized to improve thickness uniformity, areal coverage and processing time. The ink layer thickness has a dramatic effect on the threshold fluence and transfer quality, and inhomogeneous thickness can make the transfer process difficult, especially when transferring voxels smaller than 20 &#181;m x 20 &#181;m. The current design for the donor substrate makes it difficult to create ribbons larger than 10s of cm, which limits large-area throughput. Thus, the development of alternate donor substrate designs, such as reel-to-reel or rotating disc, would be required for enhanced automation and larger area processing.
The strength of the LDT technique lies in the ability to transfer fluids with high viscosities that other drop-on-demand techniques cannot handle. The advantages of LDT can be separated into two situations where firstly, printing high viscosity paste offers an improvement in quality or speed over printing low viscosity paste and secondly, in situations where printing with high viscosity paste enables structures that are not accessible to low viscosity printing. Examples of advantages in the first category are: minimal voxel variability from wetting effects, high degree of control over voxel shape and size, minimal shrinkage during curing, and low laser energy compared to other LIFT processes (and thus low transfer velocity). Examples in the second category are: printing of high-aspect-ratio structures, bridging structures, cantilevers, and any other structure that requires good voxel-shape-retention. By combining the LDT process with the DMD chip, parallel printing of complex shapes and patterns is enabled, which greatly speeds up the overall process. Furthermore, the use of a DMD to shape the voxels allows designs to be updated between laser pulses, enabling rapid printing of dynamically reconfigurable voxels. Generally speaking, the refresh rate of the DMD (33 kHz) is slightly slower than the max repetition rate of the laser (100 kHz or higher), but the rate limiting factor for printing speed is the stage translation.
The primary avenues for advancement with the LDT system are the continued development of additional materials, improving the ribbon fabrication process, and continuing to scale up the process through integrating digital light processing (DLP) technology such as the DMD chip. Although metallic and insulating materials have been successfully transferred through this process, few active materials have been developed. The ability to print piezoelectric, magnetic, or optoelectronic materials with the LDT process could open up enormous technological possibilities. As it stands, the geometry of the donor substrate limits scalability. The development of reel-to-reel or rotating disc donor substrates would streamline the processes considerably. Finally, combination of LDT with DLP technology is a potentially disruptive development for the field of digital fabrication, turning a previously serial process into a highly parallel process. A key challenge toward this goal is the ability to print voxels with good feature resolution at multiple scales. That is to say, voxels with lateral dimensions on the order of 10 sec or 100 sec of &#181;m containing features on the order of 1-5 &#181;m. Taken together, these developments offer significant opportunities for large-area additive manufacturing of electronic components.

Additive printing techniques are of considerable interest for the patterning of functional materials onto a variety of substrates. These so-called &#34;direct-write&#34; processes, including micropen10, direct-write assembly11, inkjet12, and LIFT4, are well suited for the fabrication of a variety of feature sizes ranging from sub-micron to macroscale1,2. The primary advantages of these techniques are low cost, environmental friendliness, and fast turnaround from concept to prototype. Indeed, rapid prototyping is a primary use for such processes. The materials utilized by these processes typically consist of a nanoparticle suspension within a solvent, and generally require a furnace curing step after deposition in order to realize their functional properties. Although micropen and direct-write assembly are relatively simple to implement, both rely on a continuous filament contact with the receiving substrate during dispensing. Although inkjet is a simple, non-contact direct-write method, it is usually limited to the transfer of low viscosity, chemically benign nanoparticle suspensions in order to avoid clogging and/or corrosion of the dispensing nozzles. In addition, printing patterns with well-defined edge features by inkjet is very difficult given the variable behavior of fluids on different surfaces and their resulting instability due to wetting effects13. Regardless, inkjet has enjoyed the most attention from researchers thus far.
LIFT, on the other hand, is a non-contact, nozzle-free additive process which is capable of transferring high-viscosity paste with well-defined edges. In this process, controlled amounts of complex materials are transferred from a donor substrate (or &#34;ribbon&#34;) to a receiving substrate by using laser pulses4 as shown schematically in Figure 1. When using high viscosity paste, it is possible for the printed voxel to match the size and shape of the incident laser pulse cross section5. This process has been referred to as laser decal transfer (LDT), and offers a unique approach to direct-writing in which voxel shape and size are readily controllable parameters, allowing the non-lithographic generation of structures for a wide range of applications such as circuit repair14, metamaterials7, interconnects8, and free-standing structures15. The ability to deposit complex shapes in one transfer step greatly reduces the processing time and avoids problems related to the merging of multiple voxels, a common problem in most digital printing techniques. The ability to dynamically adjust the spatial profile of individual laser pulses17 serves to increase the writing speed of LDT as compared to other laser direct write (LDW) techniques. As a result of these processing advantages, we refer to the LDT process as being &#34;partially parallelized&#34; since it allows the combination of multiple serial writing steps into a single parallel one. The degree of parallelization ultimately depends on the ability to rapidly alter the laser cross-section profile, and therefore the shape of the resultant voxel, and on the speed by which the ribbon and substrate can be translated.
To help visualize the process, the behavior of a material during the LIFT process is depicted schematically in Figures 2A, 2C, and 2E for three different paste viscosities. For low viscosity inks (Figure 2A)9, the transfer process follows jetting behavior, resulting in the formation of rounded, hemispherical voxels (Figure 2B)18. Figure 2C depicts the transfer for very high viscosity suspensions, in which the ejected voxel experiences fragmentation similar to what is observed with LIFT of solid ceramic layers19. Figure 2E depicts the LDT transfer of nanopaste with a suitable, intermediate viscosity, wherein the released voxel is not subject to shape deformation due to surface tension effects and reaches the receiving substrate intact. The effect of viscosity on the shape of the transferred voxels is shown in the atomic force microscopy (AFM) images in Figures 2B, 2D, and 2F. As Figure 2F demonstrates, it is possible to obtain sharp, well-defined voxels for an appropriate range of viscosities, usually ~100 Pa&#183;sec for Ag nanopaste5.
Overall, there have been few reports of methods that combine non-contact printing with the potential for micron resolution 3D structures. The LDT method offers a freeform process capable of fabricating interconnects with ultra-fine pitch bonding capabilities. A number of applications including delicate electronic devices, organic electronics, and microelectromechanical systems (MEMS) could benefit from such a process. Here we demonstrate a process for non-contact, three-dimensional printing as well as single-laser-shot, large area printing (via DMD chip) of high-viscosity Ag nanopaste.

<table><tbody><tr><td>Silver Nano-paste for Screen Printing</td><td>Harima Chemicals Group, http://www.harima.co.jp/en/</td><td>NPS Type HP</td><td>Store at 10 &amp;#176;C, do not allow to freeze; before using, wait 1 hour for paste to reach room temperature.</td></tr><tr><td>Buffered HF Solution</td><td>http://transene.com/sio2/</td><td>BUFFER HF IMPROVED</td><td>Etch rate may vary depending on material structure</td></tr></tbody></table>

laser-induced forward transfer of ag nanopaste

Over the past decade, there has been much development of non-lithographic methods1-3 for printing metallic inks or other functional materials. Many of these processes such as inkjet3 and laser-induced forward transfer (LIFT)4 have become increasingly popular as interest in printable electronics and maskless patterning has grown. These additive manufacturing processes are inexpensive, environmentally friendly, and well suited for rapid prototyping, when compared to more traditional semiconductor processing techniques. While most direct-write processes are confined to two-dimensional structures and cannot handle materials with high viscosity (particularly inkjet), LIFT can transcend both constraints if performed properly. Congruent transfer of three dimensional pixels (called voxels), also referred to as laser decal transfer (LDT)5-9, has recently been demonstrated with the LIFT technique using highly viscous Ag nanopastes to fabricate freestanding interconnects, complex voxel shapes, and high-aspect-ratio structures. In this paper, we demonstrate a simple yet versatile process for fabricating a variety of micro- and macroscale Ag structures. Structures include simple shapes for patterning electrical contacts, bridging and cantilever structures, high-aspect-ratio structures, and single-shot, large area transfers using a commercial digital micromirror device (DMD) chip.

Figure 3 shows a representative donor substrate with a well in its center. A standard glass slide was used for the donor substrate, and the depth of the well in this case is 1 &#181;m. Note that all of the Ag nanopaste is confined to the rectangular well and the rest of the substrate is clean. It is also important to note that the coloration is uniform, indicating roughly uniform paste thickness. Regions with lighter coloration indicate thin spots, which are best avoided. Figure 4 shows a 20X optical image of the donor substrate after a 6x6 array of 20 &#181;m x 20 &#181;m square voxels have been ejected. In this ideal case, there is no paste residue in the gaps and all voxels were fully ejected from the ribbon. If the energy is insufficient or if there are significant hotspots in the beam profile, voxels will only partially detach and remain stuck to the backside of the ribbon.
Voxels ejected from pastes with different viscosities can be found in Figure 59. When the paste viscosity is low, i.e., has not been sufficiently dried, surface tension will cause the voxels to become more rounded, losing their original shape (as seen in Figure 5A and B). Note how the shapes of the voxels in Figure 5B are different from the beam shapes (displayed in the inset of Figure 5B). On the other extreme, when the paste viscosity is high, i.e., has been over-dried, voxels have a tendency to fracture when ejected as seen in Figure 5C and D. Thus, there is an intermediate viscosity range which allows transfer of unfractured voxels that retain the shape of the beam profile as seen in Figure 5E and F. We demonstrate two varieties of voxel chains which form long conductive lines. The first was a simple end-to-end chain in which 40 x 60 &#181;m2 voxels were transferred adjacent to one another (Figure 6A and B)20. Generally, this linking method was somewhat unreliable, with partially or completely broken interfaces appearing after a soft cure at 100 &#176;C (as seen in Figure 6B). The second method used notched, interlocking voxels transferred end-to-end (Figure 6C and D). The dotted lines in Figure 6C outline the original shape of the voxels, as the high quality of the interface makes it difficult to visually resolve the individual shapes. This effect is very clear in Figure 6D, where the seam between voxels is nearly invisible. The notched geometry was more reliable than the simple end-to-end, with nearly all interfaces remaining continuous after a 100 &#176;C cure. Figure 7 demonstrates various stacking geometries, patterns and aspect ratios. A single voxel traversing a 100 &#181;m wide Si trench can be found in Figure 7A. Obtaining the right viscosity is of the utmost importance for bridging or freestanding applications in order to prevent the voxel from sagging or conforming to the geometry of the receiver substrate. Complex, multi-layer structures can be seen in Figure 7B-D, including two stacked pyramids and high aspect ratio micro pillars. These geometries are important for applications requiring vertical and spanning interconnects. Finally, Figure 8A shows an alternate optical setup which uses a commercial DMD chip, referred to as a &#34;digital mirror device&#34; in the diagram. As described in step 6, large, complex images can be loaded onto the computer and transferred with a single laser pulse. A successfully printed NRL logo can be found in Figure 8B. We note that with a single shot, we can transfer a paste structure with a length of 1 mm and a feature resolution of ~20 &#181;m.
<img alt="Figure 1" src="/files/ftp_upload/53728/53728fig1.jpg" /> 
Figure 1. Schematic diagram of LDT setup. Note that the voxel shape is determined by the cross-sectional beam shape only for high-viscosity ink. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/53728/53728fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/53728/53728fig2.jpg" /> 
Figure 2. Schematic diagram voxel ejection. Diagrams illustrate evolution of transfer for (A) low viscosity, (C) high viscosity, and (E) intermediate viscosity. AFM plots of the resultant voxels are provided in (B), (D), and (F), respectively. This figure has been modified from [9]. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/53728/53728fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/53728/53728fig3.jpg" /> 
Figure 3. Picture of Ag nanopaste donor substrate. The substrate itself is a glass slide with a 1 &#181;m deep well in the center. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/53728/53728fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/53728/53728fig4.jpg" /> 
Figure 4. 20X optical image of the paste layer on the ribbon (donor substrate) after voxel transfer. Sharp, well-defined edges and lack of residue indicate sufficient paste drying and complete transfer of material from the ribbon. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/53728/53728fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/53728/53728fig5.jpg" /> 
Figure 5. Scanning electron microscopy (SEM) images of several different voxels. Beam profiles are depicted in the inset (B). Three different voxel shapes were printed from low viscosity (A,B), high viscosity (C,D), and intermediate viscosity (E,F). Note that low viscosity leads to a loss of shape and voxel sharpness while high viscosity leads to voxel fracturing. This figure has been modified from [9]. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/53728/53728fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" src="/files/ftp_upload/53728/53728fig6.jpg" /> 
Figure 6. SEM images of conjoined voxel chains. Two linking geometries are depicted: simple end-to-end (A,B) and notched-interlocking (C,D). In general, notched-interlocking geometries are found to be more reliable while simple end-to-end have a tendency to crack due to shrinkage during the furnace steps. This figure has been modified from [20]. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/53728/53728fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" src="/files/ftp_upload/53728/53728fig7.jpg" /> 
Figure 7. SEM images of multiple complex voxel structures. Geometries include: A rectangular voxel bridging a 100 &#181;m wide trench (A), a multilayer scaffold (B), a high aspect ratio pyramid (C), and several high aspect ratio micro pillars (D). This figure has been modified from [8]. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/53728/53728fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" src="/files/ftp_upload/53728/53728fig8.jpg" /> 
Figure 8. Schematic diagram and results of LDT via DMD chip. In the schematic diagram (A), the laser aperture has been replaced with the DMD chip, which is a large assembly of micro-mirrors. The pattern from an image file can be faithfully imaged onto the donor substrate, ejecting an exact replica of the pattern of voxels in a single shot. As an example, an NRL logo (B) has been transferred by a single laser shot. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/53728/53728fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Laser-induced Forward Transfer of Ag Nanopaste at JoVE.com

Laser-induced Forward Transfer of Ag Nanopaste

We demonstrate the use of the Laser-induced forward transfer technique (LIFT) for the printing of high-viscosity Ag paste. This technique offers a simple, low temperature, robust process for non-lithographically printing microscale 2D and 3D structures.

Over the past decade, there has been much development of non-lithographic methods1-3 for printing metallic inks or other functional materials. Many of ...

laser-induced-forward-transfer-of-ag-nanopaste

Universidad San Sebastian

Research

JoVE Journal

Engineering

11.3K Views.  Naval Research Laboratory. We demonstrate the use of the Laser-induced forward transfer technique (LIFT) for the printing of high-viscosity Ag paste. This technique offers a simple, low temperature, robust process for non-lithographically printing microscale 2D and 3D structures.

Video: Laser-induced Forward Transfer of Ag Nanopaste

A Method to Fabricate Disconnected Silver Nanostructures in 3D

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

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

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

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 ...

Utilization of Plasmonic and Photonic Crystal Nanostructures for Enhanced Micro- and Nanoparticle Manipulation

A method to manipulate the position and orientation of submicron particles nondestructively would be an incredibly useful tool for basic biological research. Perhaps the most widely used physical force to achieve noninvasive manipulation of small particles has been dielectrophoresis(DEP).1 However, DEP on its own lacks the versatility and precision that are desired when manipulating cells since it is traditionally done with stationary electrodes. Optical tweezers, which utilize a three dimensional electromagnetic field gradient to exert forces on small particles, achieve this desired versatility and precision.2 However, a major drawback of this approach is the high radiation intensity required to achieve the necessary force to trap a particle which can damage biological samples.3 A solution that allows trapping and sorting with lower optical intensities are optoelectronic tweezers (OET) but OET's have limitations with fine manipulation of small particles; being DEP-based technology also puts constraint on the property of the solution.4,5
This video article will describe two methods that decrease the intensity of the radiation needed for optical manipulation of living cells and also describe a method for orientation control. The first method is plasmonic tweezers which use a random gold nanoparticle (AuNP) array as a substrate for the sample as shown in Figure 1. The AuNP array converts the incident photons into localized surface plasmons (LSP) which consist of resonant dipole moments that radiate and generate a patterned radiation field with a large gradient in the cell solution. Initial work on surface plasmon enhanced trapping by Righini et al and our own modeling have shown the fields generated by the plasmonic substrate reduce the initial intensity required by enhancing the gradient field that traps the particle.6,7,8 The plasmonic approach allows for fine orientation control of ellipsoidal particles and cells with low optical intensities because of more efficient optical energy conversion into mechanical energy and a dipole-dependent radiation field. These fields are shown in figure 2 and the low trapping intensities are detailed in figures 4 and 5. The main problems with plasmonic tweezers are that the LSP's generate a considerable amount of heat and the trapping is only two dimensional. This heat generates convective flows and thermophoresis which can be powerful enough to expel submicron particles from the trap.9,10 The second approach that we will describe is utilizing periodic dielectric nanostructures to scatter incident light very efficiently into diffraction modes, as shown in figure 6.11 Ideally, one would make this structure out of a dielectric material to avoid the same heating problems experienced with the plasmonic tweezers but in our approach an aluminum-coated diffraction grating is used as a one-dimensional periodic dielectric nanostructure. Although it is not a semiconductor, it did not experience significant heating and effectively trapped small particles with low trapping intensities, as shown in figure 7. Alignment of particles with the grating substrate conceptually validates the proposition that a 2-D photonic crystal could allow precise rotation of non-spherical micron sized particles.10 The efficiencies of these optical traps are increased due to the enhanced fields produced by the nanostructures described in this paper.

Plasmonic tweezers and photonic crystal nanostructures are shown to produce useful enhancements in the efficiency and orientation control of optically trapping micro- and nano-particles.

A method to manipulate the position and orientation of submicron particles nondestructively would be an incredibly useful tool for basic biological ...

Bioengineering

Laser-induced Forward Transfer for Flip-chip Packaging of Single Dies

Flip-chip (FC) packaging is a key technology for realizing high performance, ultra-miniaturized and high-density circuits in the micro-electronics industry. In this technique the chip and/or the substrate is bumped and the two are bonded via these conductive bumps. Many bumping techniques have been developed and intensively investigated since the introduction of the FC technology in 19601 such as stencil printing, stud bumping, evaporation and electroless/electroplating2. Despite the progress that these methods have made they all suffer from one or more than one drawbacks that need to be addressed such as cost, complex processing steps, high processing temperatures, manufacturing time and most importantly the lack of flexibility. In this paper, we demonstrate a simple and cost-effective laser-based bump forming technique known as Laser-induced Forward Transfer (LIFT)3. Using the LIFT technique a wide range of bump materials can be printed in a single-step with great flexibility, high speed and accuracy at RT. In addition, LIFT enables the bumping and bonding down to chip-scale, which is critical for fabricating ultra-miniature circuitry.

We demonstrate the use of the Laser-induced Forward Transfer (LIFT) technique for flip-chip assembly of optoelectronic components. This approach provides a simple, cost-effective, low-temperature, fast and flexible solution for fine-pitch bumping and bonding on chip-scale for achieving high-density circuits for optoelectronic applications.

Flip-chip (FC) packaging is a key technology for realizing high performance, ultra-miniaturized and high-density circuits in the micro-electronics ...

Selective Area Modification of Silicon Surface Wettability by Pulsed UV Laser Irradiation in Liquid Environment

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of biosensing devices. We report on a protocol of using KrF and ArF lasers irradiating Si (001) samples immersed in a liquid environment with low number of pulses and operating at moderately low pulse fluences to induce Si wettability modification. Wafers immersed for up to 4 hr in a 0.01% H2O2/H2O solution did not show measurable change in their initial contact angle (CA) ~75&#176;. However, the 500-pulse KrF and ArF lasers irradiation of such wafers in a microchamber filled with 0.01% H2O2/H2O solution at 250 and 65 mJ/cm2, respectively, has decreased the CA to near 15&#176;, indicating the formation of a superhydrophilic surface. The formation of OH-terminated Si (001), with no measurable change of the wafer&#8217;s surface morphology, has been confirmed by X-ray photoelectron spectroscopy and atomic force microscopy measurements. The selective area irradiated samples were then immersed in a biotin-conjugated fluorescein-stained nanospheres solution for 2 hr, resulting in a successful immobilization of the nanospheres in the non-irradiated area. This illustrates the potential of&#160;the method for selective area biofunctionalization and fabrication of advanced Si-based biosensing architectures. We also describe a similar protocol of irradiation of wafers immersed in methanol (CH3OH) using ArF laser operating at pulse fluence of 65 mJ/cm2 and in situ formation of a strongly hydrophobic surface of Si (001) with the CA of 103&#176;. The XPS results indicate ArF laser induced formation of Si&#8211;(OCH3)x compounds responsible for the observed hydrophobicity. However, no such compounds were found by XPS on the Si surface irradiated by KrF laser in methanol, demonstrating the inability of the KrF laser to photodissociate methanol and create -OCH3 radicals.

We report on a process of in situ alteration of HF treated Si (001) surface into a hydrophilic or hydrophobic state by irradiating samples in microfluidic chambers filled with&#160;H2O2/H2O solution (0.01%-0.5%) or methanol solutions using pulsed UV laser of a relative low pulse fluence.

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of ...

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

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

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

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

Plasmonic Trapping and Release of Nanoparticles in a Monitoring Environment

Plasmonic tweezers use surface plasmon polaritons to confine polarizable nanoscale objects. Among the various designs of plasmonic tweezers, only a few can observe immobilized particles. Moreover, a limited number of studies have experimentally measured the exertable forces on the particles. The designs can be classified as the protruding nanodisk type or the suppressed nanohole type. For the latter, microscopic observation is extremely challenging. In this paper, a new plasmonic tweezer system is introduced to monitor particles, both in directions parallel and orthogonal to the symmetric axis of a plasmonic nanohole structure. This feature enables us to observe the movement of each particle near the rim of the nanohole. Furthermore, we can quantitatively estimate the maximal trapping forces using a new fluidic channel.

A microchip fabrication process that incorporates plasmonic tweezers is presented here. The microchip enables the imaging of a trapped particle to measure maximal trapping forces.

Plasmonic tweezers use surface plasmon polaritons to confine polarizable nanoscale objects. Among the various designs of plasmonic tweezers, only a few ...

Production of Metal Nanoparticles by Pulsed Laser-ablation in Liquids: A Tool for Studying the Antibacterial Properties of Nanoparticles

The emergence of multidrug-resistant bacteria is a global clinical concern leading some to speculate about our return to a "pre-antibiotics" era of medicine. In addition to efforts to identify novel small-molecule antimicrobial drugs, there has been great interest in the use of metal nanoparticles as coatings for medical devices, wound dressings, and consumer packaging, due to their antimicrobial properties. The wide variety of methods available for nanoparticle synthesis results in a broad spectrum of chemical and physical properties which can affect antibacterial efficacy. This manuscript describes the pulsed laser-ablation in liquids (PLAL) method to create nanoparticles. This approach allows for the fine tuning of nanoparticle size, composition, and stability using post-irradiation methods as well as the addition of surfactants or volume excluders. By controlling particle size and composition, a large range of physical and chemical properties of metal nanoparticles can be explored which may contribute to their antimicrobial efficacy thereby opening new avenues for antibacterial development.

The antimicrobial properties of metals such as copper and silver have been recognized for centuries. This protocol describes pulsed laser-ablation in liquids, a method of synthesizing metal nanoparticles that provides the ability to fine tune the properties of these nanoparticles to optimize their antimicrobial effects.

The emergence of multidrug-resistant bacteria is a global clinical concern leading some to speculate about our return to a "pre-antibiotics" era of ...

Synthesizing Nanoparticles and Nanostructures via Laser Ablation

Ultrafast Laser-Ablated Nanoparticles and Nanostructures for Surface-Enhanced Raman Scattering-Based Sensing Applications

The technique of ultrafast laser ablation in liquids has evolved and matured over the past decade, with several impending applications in various fields such as sensing, catalysis, and medicine. The exceptional feature of this technique is the formation of nanoparticles (colloids) and nanostructures (solids) in a single experiment with ultrashort laser pulses. We have been working on this technique for the past few years, investigating its potential using the surface-enhanced Raman scattering (SERS) technique in hazardous materials sensing applications. Ultrafast laser-ablated substrates (solids and colloids) could detect several analyte molecules at the trace levels/mixture form, including dyes, explosives, pesticides, and biomolecules. Here, we present some of the results achieved using the targets of Ag, Au, Ag-Au, and Si. We have optimized the nanostructures (NSs) and nanoparticles (NPs) obtained (in liquids and air) using different pulse durations, wavelengths, energies, pulse shapes, and writing geometries. Thus, various NSs and NPs were tested for their efficiency in sensing numerous analyte molecules using a simple, portable Raman spectrometer. This methodology, once optimized, paves the way for on-field sensing applications. We discuss the protocols in (a) synthesizing the NPs/NSs via laser ablation, (b) characterization of NPs/NSs, and (c) their utilization in the SERS-based sensing studies.

Author Spotlight: Advancements and Applications in Nanoparticle Synthesis Through Laser Ablation in Liquids 

Ultrafast laser ablation in liquid is a precise and versatile technique for synthesizing nanomaterials (nanoparticles [NPs] and nanostructures [NSs]) in liquid/air environments. The laser-ablated nanomaterials can be functionalized with Raman-active molecules to enhance the Raman signal of analytes placed on or near the NSs/NPs.

The technique of ultrafast laser ablation in liquids has evolved and matured over the past decade, with several impending applications in various fields ...

Gold Nanorod-assisted Optical Stimulation of Neuronal Cells

Recent studies have demonstrated that nerves can be stimulated in a variety of ways by the transient heating associated with the absorption of infrared light by water in neuronal tissue. This technique holds great potential for replacing or complementing standard stimulation techniques, due to the potential for increased localization of the stimulus and minimization of mechanical contact with the tissue. However, optical approaches are limited by the inability of visible light to penetrate deep into tissues. Moreover, thermal modelling suggests that cumulative heating effects might be potentially hazardous when multiple stimulus sites or high laser repetition rates are used. The protocol outlined below describes an enhanced approach to the infrared stimulation of neuronal cells. The underlying mechanism is based on the transient heating associated with the optical absorption of gold nanorods, which can cause triggering of neuronal cell differentiation and increased levels of intracellular calcium activity. These results demonstrate that nanoparticle absorbers can enhance and/or replace the process of infrared neural stimulation based on water absorption, with potential for future applications in neural prostheses and cell therapies.

This protocol outlines how to use the transient heating associated with the optical absorption of gold nanorods to stimulate differentiation and intracellular calcium activity in neuronal cells. These results potentially open up new applications in neural prostheses and fundamental studies in neuroscience.

Recent studies have demonstrated that nerves can be stimulated in a variety of ways by the transient heating associated with the absorption of infrared ...