Intel Corporation funded this work through grant number 414305, and the Oregon Nanotechnology and Microtechnology Initiative (ONAMI) provided matching funds. We gratefully acknowledge the support and advice of Dr. James Blackwell in all phases of this work. Special thanks go to Drew Beasau and Chelsea Benedict for analyzing particle positioning statistics. We thank Professor Hall for a careful reading of the manuscript and Dr. Kurt Langworthy, at the University of Oregon, Eugene, OR, for his help with E-beam lithography.

1. Derivatize and Characterize the Surface of the Silicon Wafers

<ol>
	<li>Clean the surface of wafers using Radio Corporation of America (RCA) cleaning solutions SC1 and SC2.</li>
	<li>Prepare SC1 and SC2 by volumetrically mixing the following chemicals: 
	SC1: H2O2:NH4OH:H2O = 1:1:5 v/v and SC2: H2O2:HCl:H2O = 1:1:5 v/v.
	<ol>
		<li>Immerse the wafer in SC1 for 10 min at 70 &#176;C, and then perform a deionized water wash.</li>
		<li>Follow a similar protocol for SC2 (10 min at 70 &#176;C, followed by a wash). 
		NOTE: The cleaning procedure removes organic and ionic impurities and generates silanol groups on the silicon oxide surface of the silicon wafers.</li>
	</ol>
	</li>
	<li>Derivatize the surfaces of the silicon wafers with AATMS
	<ol>
		<li>Incubate the cleaned silicon wafer in 0.05 M AATMS (prepared in dry toluene) at 80 &#176;C for 20 min. Sonicate for 5 min in a 100 W sonicator at room temperature in pure toluene for 5 min and dry in a stream of nitrogen gas.</li>
	</ol>
	</li>
	<li>Characterize the surfaces of the derivatized silicon wafers
	<ol>
		<li>Measure the contact angle using a goniometer and ImageJ software25.</li>
		<li>Determine the thickness of the films using an ellipsometer (He-Ne laser light source, &#955; = 632.8 nm, fixed incidence angle of 70&#176;)25.</li>
		<li>Estimate the surface elemental composition by X-ray photoelectron spectroscopy (XPS)25.</li>
	</ol>
	</li>
</ol>
2. E-beam Patterning
<ol>
	<li>Spin-coat photoresist (2% poly(methyl methacrylate (PMMA) in anisole) on AATMS- derivatized wafers at 4,000 rpm for 60 s.</li>
	<li>Prebake the film at 180 &#176;C for 120 s to dry the photoresist film.</li>
	<li>Measure the photoresist film thickness using an ellipsometer. Ensure that the He-Ne laser light source, &#955; = 632.8 nm, is fixed at an incidence angle of 70&#176;. Optimize the sample stage to maximize the detector signal using a reflective surface such as silicon wafer. Use the ellipsometer measurement program (GEMP) on the attached PC to measure the photoresist film thickness; it should be about 100 nm.</li>
	<li>E-beam exposure:
	<ol>
		<li>Transfer the wafer to a high-vacuum chamber of an E-beam aligner.</li>
		<li>Create a poker dot hole pattern using an electron beam (30 kV accelerating voltage, 37 pA beam current) passing through a 10-&#181;m aperture and providing a 24 &#181;C/cm3 dosage.</li>
		<li>Adjust the pattern pitch and dosage as needed to produce a poker dot-type hole-pattern of the desired hole diameter (80 nm) and pitch (200 nm).</li>
		<li>Remove the wafer from the E-beam chamber.</li>
	</ol>
	</li>
	<li>Develop the lithographic pattern:
	<ol>
		<li>Begin pattern development in a solution of methyl isobutyl ketone/isopropyl alcohol (MIBK/IPA, 1:3 (v:v)) for 70 s.</li>
		<li>Continue to develop further by immersion in IPA for 30 s. Complete the development by washing in deionized water for 30 s. Dry the wafer in a stream of nitrogen gas.</li>
	</ol>
	</li>
</ol>
3. Deposition of GNPs into E-beam-patterned Holes
NOTE: Deposition of GNPs in patterned holes employs two different methods.
<ol>
	<li>Immerse pre-patterned wafers in GNP solutions (Method 1).
	<ol>
		<li>Leave the sample in the GNP suspension medium for 24-48 h, depending on the size of the GNP and the diameters of the holes. Use a 20-nm, citrate-capped GNP suspension containing 7.0 x 1011 NPs/mL. 
		NOTE: One may employ a GNP size from 10-100 nm and a concentration range from 5.7 x 1012 - 5.7 x 109 NPs/mL, as provided by Ted Pella. Note that the deposition density obeys the diffusion law &#820;(Dt)1/2, where D and t are diffusion coefficients of the nanoparticle and deposition time, respectively (i.e., smaller particles take a shorter time for deposition, as discussed in reference 20, Figure 2c).</li>
	</ol>
	</li>
	<li>Spray-deposit GNPs on the patterned wafers (Method 2)
	<ol>
		<li>To deposit GNPs by evaporation, spray a solution of GNPs onto the patterned substrate, placed horizontally. Orient the hand-held sprayer (Method 2) so that the spray is directed perpendicularly to the substrate surface.</li>
		<li>Spray a sufficient volume of the suspension solution to wet the entire substrate surface. 
		NOTE: It may be necessary to dilute the GNP suspension by 10x to avoid forcing multiple nanoparticles into a large hole.</li>
		<li>Incubate samples on a hot plate maintained at 30-35 &#176;C to produce controlled evaporation for 10 min.</li>
	</ol>
	</li>
	<li>After deposition (by either Method 1 or Method 2), mildly ultrasonicate (100 W) the samples in deionized water for 50 s and dry in a stream of nitrogen gas.</li>
</ol>
4. Scanning Electron Microscopy Imaging
NOTE: Two types of studies involved conventional top-down and cross-sectional SEM imaging.
<ol>
	<li>For top-down SEM images, use an E-beam accelerating voltage of 5 kV at a current of ~300 &#181;A to prevent damage to the photoresist film. 
	Caution: The lowest possible voltage and current setting are necessary to reduce chain scission reactions in the photoresist. These chain scission reactions reduce the glass transition temperature of the polymer, which in turn affects the operational temperature for the resist reflow step described below.</li>
	<li>Cross-section imaging:
	<ol>
		<li>Sputter-coat 10 nm of thick gold film over PMMA photoresist to prevent ion-beam damage. Use a focused Ga ion beam operated at 30 kV and 93 pA to cut through the holes. Obtain cross-section SEM images by tilting the wafer from its normal horizontal position.</li>
	</ol>
	</li>
</ol>
5. Reflow of PMMA Photoresist around GNPs in the Patterned Holes
<ol>
	<li>Heat the patterned substrates on a hot plate at Treflow (100 &#176;C) for 3 min, which is below the pre-determined glass transition temperature, Tg (110 &#176;C), of 950,000 g/mol PMMA; the rate of reflow for the polymer was 1.7 &#177; 0.1 nm/s. 
	Caution: Significantly faster reflow occurred for the patterned areas that had previously been exposed to the E-beam during imaging with scanning electron microscopy (SEM), perhaps because of the cleavage of the polymeric backbone chain during exposure to the electron beam, leading to a reduction of Tg. This observation is supported by the studies from Keymeulen and co-workers, who noted that exposure to X-ray radiation reduced the Tg of PMMA26.</li>
</ol>
6. Dry- and Wet-etch
<ol>
	<li>Dry-etch for sufficient duration (55 s) with oxygen-plasma to expose GNPs covered with a thin film of PMMA after reflow. Carefully monitor the rate of PMMA film etching as a function of time using an ellipsometer or thin film thickness monitor. 
	NOTE: A too-short etching duration may not expose the GNPs, while etching for too long would completely remove the PMMA film. For 950 kDa PMMA, the etch rate was 1.5 nm/s, necessitating 55 s of etching time.</li>
	<li>Wet-etch GNPs at the bottom of the contact holes after reflow using a solution of iodine, containing 1.0 g of iodine crystal (I2), 4.0 g of potassium iodide (KI), and 40 mL of deionized water, for 10 min. 
	NOTE: Potassium iodide improves the solubility of the iodine in solution and facilitates gold etching. The reaction of gold with iodine (2Au + I2 &#8594; 2 AuI) produces gold iodide, which is slightly soluble in aqueous solution at room temperature.</li>
</ol>
7 . Calculation of Particle Displacement, Density, and Fill Fraction
<ol>
	<li>Locating the hole center and GNP displacement:
	<ol>
		<li>By hand, draw horizontal and vertical best-fit straight lines through the rows and columns of holes, respectively, to established hole centers at the intersections of these lines (Figure 2a and 2b). Include at least 500+ holes in the calculations.</li>
		<li>Manually determine the position, r, of each nanoparticle relative to the center of the nanohole (i.e., the displacement) where it was deposited (Figure 2b).</li>
	</ol>
	</li>
	<li>Determine the particle count versus displacement histograms using a standard spreadsheet program.</li>
	<li>Calculation of the particle density: &#961; = N (Number of Particles)/Unit Area (&#181;m2):
	<ol>
		<li>First, determine the annular area of a ring with a fixed width (&#8776;R/10, where R is the radius of the hole) and bounded by two radii (r1, r2) at a displacement of r (=(r1+r2)/2); <img alt="Equation 2" src="/files/ftp_upload/54551/eq2.jpg" />.</li>
		<li>Sum the number of particles, N, in the area from the histogram constructed in step 7.2, above.</li>
		<li>Repeat the procedure as a function of r to generate about 10 equal steps terminating at r = R, the hole radius.</li>
	</ol>
	</li>
	<li>Fit the particle density versus the displacement data to a Gaussian curve using a nonlinear least squares procedure (Figure 2a, inset)20. Extract the standard deviation of displacement (&#963;deposition) for the deposition and its fitting uncertainty.</li>
	<li>Repeat the above procedure for the SEM patterns obtained after the photoresist reflow using an SEM image shown in Figure 2c. 
	NOTE: After the reflow holes disappear, draw best-fit horizontal and vertical lines on GNP positions to determine the best-fit values for the hole centers. Calculate the particle displacements as in step 7.1 and extract the combined &#963;Total following the protocol delineated in steps 7.2-7.4. 
	NOTE: Here, combined or total refers to the overall displacement of GNP due to the deposition and reflow steps.</li>
	<li>Determine the fill fraction by estimating the ratio of the number of filled holes to the total number of holes, Figure 2b.</li>
</ol>

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The authors have nothing to disclose.

Shot-noise (SN) in lithography is a simple consequence of statistical fluctuations in the number of photons or particles (N) arriving in a given nano-region; it is inversely proportional to the square root of a number of photons/particles:
<img alt="Equation 3" src="/files/ftp_upload/54551/eq3.jpg" />
where A and r are the area and the size of the exposed region, respectively. For example, when using an ArF 193-nm (6.4-eV) excimer laser to pattern 50-nm holes, the number of photons received is approximately 1 x 106 at the exposure dose of 52 mJ/cm2. For a 50 keV E-beam source, the number of electrons delivering 50 mJ/cm2 would be about 128, implying that the effect of SN would be 100 times greater for E-beam lithography. Furthermore, the number of photons/electrons arriving in a given region decreases directly with the area of the pattern. Consequently, a large fluctuation in the size of holes that received &#34;identical&#34; exposure results occurs as the dimension of the hole to be crafted decreases. For the 35 nm holes shown in Figure 1 with the 30 keV E-beam, about 1,440 electrons deliver the requisite dose of 24 &#181;c/cm2. To pattern 20 nm holes under identical conditions, one would need about 400 electrons, which is close to the theoretical SN limit (200 electrons) given by Moreau32. Currently, the only way to suppress the effect of SN is through the use of a low-sensitivity photoresist that needs a higher electron/photon dose. It comes at the cost of lower throughput and higher costs for nano-patterning. Our method reduces the effect of size fluctuations by using monodisperse nanostructures to redefine the size of the lithographically patterned holes while maintaining the original positional registry. The registry is fixed by the strong electrostatic binding of nanostructures to the surface, guided by electrostatic funneling.
The success of the patterning in this method depends on two effects. The first is the variation in the sizes of the holes, and the second is the centering of the nanoparticles within the holes that affect the positional registry. The coefficient of variation for the diameter of holes patterned through this approach (9%, or 19 &#177;2 nm) is comparable to the coefficient of the size (8%) of the GNPs20. On the other hand, as revealed in Figure 1, the E-beam-patterned 35 nm (35 &#177;9) holes, made without using this approach, had a CV of 35%. The corresponding CV of E-beam-alone patterning would become even worse when fabricating 20 nm holes. The SN can be set to be proportional to the CV, with rough estimates of CVs for 20 nm and 80 nm holes at 61% and 15%, respectively. The method presented here provides at least a six-fold (~61%/9%) improvement in CV over E-beam-alone patterning of 20 nm holes. Even starting with 80-nm holes, almost 60% improvement in CV results (~15%/9%) occurs after this approach.
The second effect concerns particle deposition away from the geometric center of the holes (registry). Two contributing factors to this effect were extracted from SEM data collected after the deposition of GNPs in holes (Figure 2b) and after photoresist-reflow (Figure 2c). The statistical analysis of the displacement of GNPs from the center of the nanoholes in which they resided (Figure 2a) revealed that during the solution-phase deposition, the distribution of GNPs about the center of the hole was Gaussian, with a standard deviation (&#963;Deposition) of 9 nm, or roughly half the diameter of GNPs. The analysis procedure was equivalent to assuming that the particles, on average, remain at the center of holes, although individually they may be displaced randomly due to diffusion during deposition.
The second factor affecting displacement is due to the motion of GNPs during the photoresist reflow. Results, presented in Figure 4, indicate that the positively charged AATMS SAM binds so strongly to GNPs that they do not move during the reflow of the photoresist. A similar analysis of SEM images (vide supra) collected after the photoresist reflow (Figure 2c) step provided the overall standard deviation (&#963;Total) in the particle positioning (i.e., registry) after the deposition and reflow. To extract the contribution of resist reflow, we assumed that the processes of deposition and photoresist-reflow produce independent effects in displacing particles from the center of holes, such that:
<img alt="Equation 4" src="/files/ftp_upload/54551/eq4.jpg" />
Using the fitted values of &#963;Deposition (9 &#177;1 nm) and &#963;Total (11 &#177;2 nm), the extracted value of &#963;Resist-reflow was estimated to be 6 nm, which is comparable to three times the standard deviation in the 20 nm GNPs. Such a low standard deviation implies a very negligible effect on the displacement of bound particles during the photoresist reflow process. Nevertheless, the &#963;Deposition appears to vary directly with the size of the GNP; thus, it needs significant improvements. A model presented elsewhere20 suggests the optimization of the charge-density of the photoresist film and SAM to improve the positional registry. Several methods, including the use of electrostatic bias and charge on GNPs, are currently being explored. Note, as discussed above, that an SEM image analysis provides a straightforward method for the above process of optimization.
The commercial availability of almost monodisperse GNP particles dictated the choice of gold nanoparticles. In principle, other inexpensive nanomaterials, such as silica nanoparticles derivatized with appropriately charged chelant, are also suitable candidates. The primary criterion for the selection of nanoparticles is their monodispersity and their susceptibility to etching. For electrostatic funneling, appropriate complementary charges on the SAM and NP must be present. The charge on NPs depends on its zeta potential, whose sign and magnitude can be tuned either through the pH of the solution or the charge on chelants. Similarly, selecting cationic or anionic terminal end groups allows for the modification of surface charge on the SAM. If perfected, the method readily extends to pattern trench lines with nanowires33-35. The overall success of the method will depend on optimizing the placement and filling nanostructures on pre-patterned photoresist films, exploiting electrostatic, magnetic, or ligand-ligation interactions.

The exponential growth in computational power, as quantified by Moore&#39;s law1,2 (1), is a result of progressive advances in optical lithography. In this top-down patterning technique, the achievable resolution, R, is given by the well-known Raleigh theorem3:
<img alt="Equation 1" src="/files/ftp_upload/54551/eq1.jpg" />
Here, &#955; and NA are the light wavelength and numerical aperture, respectively. Note that NA = &#951;&#183;sin&#952;, where &#951; is the refractive index of the medium between the lens and the wafer; &#952; = tan-1(d/2l) for the diameter, d, of the lens, and the distance, l, between the center of the lens and the wafer. Over the last fifty years, the lithographic resolution has improved through the use of (a) light sources, including excimer lasers, with progressively smaller UV wavelengths; (b) clever optical designs employing phase-shift masks4; and (c) higher NA. For exposure in air (&#951; = 1), NA is always less than unity, but by introducing a liquid with &#951; &#62;1, such as water5, between the lens and the wafer, NA can be elevated above 1, thereby improving the resolution of immersion lithography. Currently viable paths to a 20-nm node and beyond include extreme UV sources (&#955; = 13 nm) or patterning techniques using complex double and quadruple processing of a multilayered photoresist6,7.
At nanometer-length scales, statistical fluctuations, caused by shot-noise (SN), in the number of photons arriving within a nano-region cause variation in the dimensions of lithographic patterns. These effects are more pronounced with exposure to high-energy EUV light and E-beams, systems that need orders of magnitude fewer photons/particles compared to normal optical lithography8. Supersensitive chemically amplified (with a quantum efficiency &#62;1) photoresists also introduce a chemical SN caused by a variation in the number of photoreactive molecules in exposed nanoregions9,10. Lower sensitivity photoresists that need longer exposures suppress these effects, but they also reduce throughput.
On the molecular scale, the contribution to line-edge roughness from the molecular size distribution inherent to the photoresist polymers may be reduced by using molecular resists11. An approach that is complementary to this top-down processing of nano-patterning is the use of bottom-up methods12,13 that rely specifically on the directed self-assembly (DSA) of diblock polymers14. The ability of these processes to direct nucleation and to create non-uniform spacing between desired patterns, such as holes or lines, remains challenging. The size distribution of molecular components15,16 also limits the scale and yield of fabrication17,18. Similar problems limit microcontact printing of nanoparticles in soft lithography19.
This paper presents studies of a new hybrid approach (Figure 1) that combines the classic top-down projection lithography with electrostatically directed self-assembly to reduce the effect of SN/line-edge roughness(LER)20. Positively charged amine groups on self-assembled monolayers (SAMs) of N-(2-aminoethyl)-11-amino-undecyl-methoxy-silane (AATMS) underlying the PMMA film are exposed after development. The negatively charged photoresist film of PMMA electrostatically funnels negatively charged gold nanoparticles (GNPs), capped with citrate,21-24 into SN-affected holes25. Re-flow of the PMMA photoresist engulfs predeposited nanoparticles in the film.
<img alt="Figure 1" src="/files/ftp_upload/54551/54551fig1.jpg" /> 
Figure 1: Schematic representation of the strategy to remove the effects of shot-noise and line-edge roughness for the patterning of contact holes using NPs of precise size. Here, the critical dimension (CD) is the desired diameter of the holes. The approach (step 1) begins with depositing a self-assembled monolayer (SAM) of silane molecule bearing positively charged amine groups on the oxide surface of a silicon wafer. Next, E-beam lithography is used to pattern the holes (steps 2 and 3) in PMMA photoresist film, the blue layer, which generates shot-noise, as illustrated in the inset SEM image. Lithography exposes amine groups at the bottom of the holes. Step 4 entails the aqueous phase deposition of controlled-size, citrate-capped (negatively charged) gold nanoparticles (GNPs) in lithographically patterned holes using electrostatic funneling (EF). In step 5, heating the wafer to 100 &#176;C, below the glass transition temperature of the PMMA, 110 &#176;C, causes the reflow of the photoresist around pre-deposited nanoparticles. Etching overlaid PMMA with oxygen plasma (step 6) exposes the GNPs, and subsequent wet-etching (iodine) of the exposed particles (step 7) creates holes corresponding to the size of the GNPs. When coupled with reactive-ion/wet-etching, it is possible to transfer the hole pattern in the photoresist to SiO2 (step 8)31. Reprinted with permission from reference20. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54551/54551fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
The electrostatic interaction between the oppositely charged GNPs and amine groups on the substrate prevents the displacement of the GNPs from the binding site. The reflow step maintains the relative location of the GNPs but erases the holes and the effects of SN/LER. Plasma/wet etching steps regenerate holes that have the size of the GNP. Reactive-ion etching transfers their pattern to SiO2 hard-mask layers. The method relies on using more uniformly sized nanoparticles than a patterned nanohole (NH), expressed as the standard deviation, &#963;, such that &#963;GNP &#60;&#963;NH. This report focuses on steps (4 and 5 described in Figure 1) involving the deposition of nanoparticles from dispersion and the reflow of the photoresist around them to assess the advantages and limitations of the method. Both steps are, in principle, scalable to larger substrates, requiring no extensive modification of the current flow of producing modern integrated circuits on chips.

<table border="0" cellpadding="0" cellspacing="0" width="700">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:373px;">AATMS (95%)</td>
			<td style="width:71px;">Gelest Inc.</td>
			<td style="width:89px;">SIA0595.0</td>
			<td style="width:167px;">N-(2-aminoethyl)-11-aminoundecyltrimethoxysilane&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Gold colloids (Ted Pella Inc.)</td>
			<td style="width:71px;">Ted Pella</td>
			<td style="width:89px;">15705-20</td>
			<td>Gold Naoparticles</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">hydrogen peroxide</td>
			<td style="width:71px;">Fisher Scientific&#160;</td>
			<td style="width:89px;">H325-100</td>
			<td>Analytical grade (Used to clean wafer)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:373px;">hydrochloric acid</td>
			<td style="width:71px;">Fisher Scientific&#160;</td>
			<td style="width:89px;">S25358</td>
			<td>Analytical grade</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">Ammonium hydroxide</td>
			<td style="width:71px;">Fisher Scientific&#160;</td>
			<td style="width:89px;">A669S-500SDS</td>
			<td>Analytical grade (Used to clean wafer)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">hydrogen fluoride</td>
			<td style="width:71px;">Fisher Scientific&#160;</td>
			<td style="width:89px;">AC277250250</td>
			<td>Analytical grade(used to etch SiO2)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">Toluene (anhydrous, 99.8 %)&#160;</td>
			<td style="width:71px;">Sigma Aldrich</td>
			<td style="width:89px;">244511</td>
			<td>Analytical grade (solvent used in Self Assembly of AATMS</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">&#160;Isopropyl alcohol (IPA)</td>
			<td style="width:71px;">Sigma Aldrich</td>
			<td style="width:89px;">W292907</td>
			<td>Analytical grade (Used to make developer)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">Methyl butyl ketone (MIBK)</td>
			<td style="width:71px;">Sigma Aldrich</td>
			<td style="width:89px;">29261</td>
			<td>Analytical grade(used to make developer)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">1:3 MIBK:IPA developer</td>
			<td style="width:71px;">Sigma Aldrich</td>
			<td style="width:89px;"></td>
			<td>Analytical grade (Developer)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">950 k poly(methyl methacylate (PMMA, 4 % in Anisole)</td>
			<td style="width:71px;">Sigma Aldrich</td>
			<td style="width:89px;">182265</td>
			<td>Photoresist for E-beam lithography</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:373px;">Purified Water : Barnstead Sybron Corporation water purification Unit, resistivity of 19.0 M&#937;cm</td>
			<td style="width:71px;"></td>
			<td style="width:89px;"></td>
			<td>Water for substrate cleaning</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Gaertner ellipsometer&#160;</td>
			<td>Gaertner</td>
			<td></td>
			<td>Resist and SAM thickness measurements</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:373px;">XPS, ThermoScientifc ESCALAB 250 instrument</td>
			<td style="width:71px;">Thermo-Scientific</td>
			<td style="width:89px;"></td>
			<td>Surface composition</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:373px;">An FEI Siron XL30</td>
			<td style="width:71px;">Fei Corporation</td>
			<td style="width:89px;"></td>
			<td>Characterize nanopatterns</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;">Zeiss sigma VP FEG SEM</td>
			<td style="width:71px;">Zeiss Corporation</td>
			<td style="width:89px;"></td>
			<td>E-beam exposure and patterning</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">MDS 100&#160; CCD camera</td>
			<td style="width:71px;">Kodak</td>
			<td style="width:89px;"></td>
			<td>Imaging drop shapes for contact angle measurements</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:373px;">Tegal Plasmod</td>
			<td style="width:71px;">Tegal</td>
			<td style="width:89px;"></td>
			<td>Oxygen plasma to etch photoresist</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:373px;">I2</td>
			<td style="width:71px;">Sigma Aldrich</td>
			<td style="width:89px;">451045</td>
			<td>Components for gold etch solution</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:373px;">KI</td>
			<td style="width:71px;">Sigma Aldrich</td>
			<td style="width:89px;">746428</td>
			<td>Components for gold etch solution</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Ellipsometer ( LSE Stokes model L116A);</td>
			<td style="width:71px;">Gaertner</td>
			<td style="width:89px;">L116A</td>
			<td>AATMS self assembled monolayer film thickness measurements</td>
		</tr>
	</tbody>
</table>

use of sacrificial nanoparticles to remove the effects of shot-noise in contact holes fabricated by e-beam lithography

Nano-patterns fabricated with extreme ultraviolet (EUV) or electron-beam (E-beam) lithography exhibit unexpected variations in size. This variation has been attributed to statistical fluctuations in the number of photons/electrons arriving at a given nano-region arising from shot-noise (SN). The SN varies inversely to the square root of a number of photons/electrons. For a fixed dosage, the SN is larger in EUV and E-beam lithographies than for traditional (193 nm) optical lithography. Bottom-up and top-down patterning approaches are combined to minimize the effects of shot noise in nano-hole patterning. Specifically, an amino-silane surfactant self-assembles on a silicon wafer that is subsequently spin-coated with a 100 nm film of a PMMA-based E-beam photoresist. Exposure to the E-beam and the subsequent development uncover the underlying surfactant film at the bottoms of the holes. Dipping the wafer in a suspension of negatively charged, citrate-capped, 20 nm gold nanoparticles (GNP) deposits one particle per hole. The exposed positively charged surfactant film in the hole electrostatically funnels the negatively charged nanoparticle to the center of an exposed hole, which permanently fixes the positional registry. Next, by heating near the glass transition temperature of the photoresist polymer, the photoresist film reflows and engulfs the nanoparticles. This process erases the holes affected by SN but leaves the deposited GNPs locked in place by strong electrostatic binding. Treatment with oxygen plasma exposes the GNPs by etching a thin layer of the photoresist. Wet-etching the exposed GNPs with a solution of I2/KI yields uniform holes located at the center of indentations patterned by E-beam lithography. The experiments presented show that the approach reduces the variation in the size of the holes caused by SN from 35% to below 10%. The method extends the patterning limits of transistor contact holes to below 20 nm.

Figure 2 shows an SEM image of 20-nm GNPs deposited in 80-nm diameter holes patterned in a 60-100 nm-thick PMMA film driven by electrostatic funneling. As observed by others22, the process resulted in about one particle per hole. The distribution of particles around the center of the holes was Gaussian (top right inset). Most holes (93%) contained one GNP, and 95% of these particles occurred within 20 nm of the center. Further optimization, discussed elsewhere, is needed to improve the fill-fraction and the centering of the GNPs20,25. Figure 2b and 2c shows SEM images of the deposited GNPs from larger regions of the wafer, before and after the reflow of the photoresist. The discussion section presents further quantitative analysis.
<img alt="Figure 2" src="/files/ftp_upload/54551/54551fig2.jpg" /> 
Figure 2: (a) 20-nm diameter GNPs deposited in 80 nm diameter holes separated by a pitch, P, of 200 nm (top left inset). 93% of the holes contain one nanoparticle, and 95% of the nanoparticles are within 18 nm from the center (blue circle). The top right inset shows a Gaussian distribution of the particle displacement from the center, with &#963; = 9.0 &#177; 0.1 nm. (b) SEM image of a large area after deposition, &#963; = 9 &#177; 1 nm. (c) Same as b) after the reflow of the photoresist, &#963; = 11 &#177; 2 nm. Modified with permission from reference20. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54551/54551fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
The evaporative deposition could reduce the 24-48 h required for deposition25. When dispersions of nanoparticles were allowed to evaporate on the patterned surface, GNPs deposited on the PMMA film, as well as in holes. Mild ultrasonication in a buffered solution removed weakly bound GNPs on the PMMA, leaving only the strongly bound particles in the holes. Figure 3 illustrates how multiple particles could be forced into holes using this method because of the reduced inter-particle repulsion in the progressively concentrated ionic dispersion. Such ionic screening effects should be avoided, since they allowed multiparticle occupancy and lowered efficiency of the electrostatic funneling that directs the particle to the center of the hole. Lowering the concentration of GNPs in the depositing dispersion, along with ultrasonication, would potentially allow for the deposition of one particle per hole more rapidly than the bulk solution-phase deposition. In this work, we did not optimize the necessary conditions.
<img alt="Figure 3" src="/files/ftp_upload/54551/54551fig3.jpg" /> 
Figure 3: Deposition of GNPs during an evaporation lasting more than 20 min. The bottom SEM micrographs show top-down and cross-sectional views of holes patterned by E-beam. Modified with permission from reference25. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54551/54551fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
The cross-sectional SEM profile displayed the hole sidewalls tilted towards their centers. The angle of the sidewall was less than the optimal 90&#176;, creating an increasing cross-sectional area in the well upon approaching the photoresist-wafer interface. This configuration explains the observed packing pattern (left) of particles, indicating their displacement away from the center of the hole. Better focusing of the E-beam, below the interface between air and the photoresist film, would eliminate such an artifact.
Thin, solid films of PMMA photoresist27-30 liquefy and flow near the glass transition, Tg, of 110 &#186;C. A temperature 10&#176; below the Tg of the photoresist polymer, PMMA, initiated a slow reflow of the photoresist. In the softened, liquid-like, glassy state, the surface tension of the photoresist film reduced the edge curvature and roughness, resulting in a suppression of LER effects. The advancing liquid photoresist front completely engulfed nanoholes, along with the deposited GNPs, as illustrated in Figure 4, where the number of GNPs per hole was high. Note how the photoresist reflow from the hole borders erased the pattern of nanoholes in the film. Nonetheless, strongly bound GNPs locked into the positional registry of the pattern. Remarkably, these photoresist-reflow studies revealed strong electrostatic binding of the citrate-capped GNPs to amine-terminated silanes. The similarity of the deposition pattern of GNPs before and after reflow supported this conclusion; see Figure 4c (vide infra).
<img alt="Figure 4" src="/files/ftp_upload/54551/54551fig4.jpg" /> 
Figure 4: The positions of the deposited GNPs are relatively insensitive to the reflow of the photoresist (see the text). Patterns of the deposited 60 nm GNPs in 250-nm holes: (a) before reflow, (b) after partial reflow, and (c) after complete reflow. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54551/54551fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
The overall sequence of steps and the changes occurring in the corresponding SEM images are shown in Figure 5. In Figure 5b, the holes in the film shrank during photoresist reflow, while the engulfing process was completed in less than 3 min (Figure 5c). Oxygen plasma etched the thicker film of photoresist that overlaid on top of GNPs, exposing them to air. Once exposed, these GNPs were wet-etched using gold etching solution based on KI/I2 (Figure 5d). The coefficient of variation for the diameters was 9% for the hole formed by the GNP-assisted process. Note that in these studies, the starting diameter of the holes shrank from 80 to 20 nm, which illustrates the strength of the approach. The primary limitation of the method is the introduction of uncertainty in the position of a recreated hole center due to a combination of uncertainties introduced during the GNP deposition and the resist reflow. Current ongoing work attempts to address these effects. Besides gold, other nanoparticles of different materials, such as silica (etchable in dilute HF), can be used to reduce materials and processing costs. The primary requirement is that the nanoparticle/nanostructure must be selectively etchable without affecting the resist material.
<img alt="Figure 5" src="/files/ftp_upload/54551/54551fig5.jpg" /> 
Figure 5: SEM. Images collected during the steps employed to minimize the variation of hole sizes. (a) Deposition (24 h). (b and c) Reflow of the photoresist (20 s, 3 min). (d) Etching PMMA (O2 plasma: 55 s) and GNP (wet-etch with I2/KI solution: 10 min). <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54551/54551fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Use of Sacrificial Nanoparticles to Remove the Effects of Shot-noise in Contact Holes Fabricated by E-beam Lithography at JoVE.com

Use of Sacrificial Nanoparticles to Remove the Effects of Shot-noise in Contact Holes Fabricated by E-beam Lithography

Uniformly sized nanoparticles can remove fluctuations in contact hole dimensions patterned in poly(methyl methacrylate) (PMMA) photoresist films by electron beam (E-beam) lithography. The process involves electrostatic funneling to center and deposit nanoparticles in contact holes, followed by photoresist reflow and plasma- and wet-etching steps.

Nano-patterns fabricated with extreme ultraviolet (EUV) or electron-beam (E-beam) lithography exhibit unexpected variations in size. This variation has ...

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7.2K Views.  Portland State University. Uniformly sized nanoparticles can remove fluctuations in contact hole dimensions patterned in poly(methyl methacrylate) (PMMA) photoresist films by electron beam (E-beam) lithography. The process involves electrostatic funneling to center and deposit nanoparticles in contact holes, followed by photoresist reflow and plasma- and wet-etching steps.

Video: Use of Sacrificial Nanoparticles to Remove the Effects of Shot-noise in Contact Holes Fabricated by E-beam Lithography

Micropunching Lithography for Generating Micro- and Submicron-patterns on Polymer Substrates

Conducting polymers have attracted great attention since the discovery of high conductivity in doped polyacetylene in 19771. They offer the advantages of low weight, easy tailoring of properties and a wide spectrum of applications2,3. Due to sensitivity of conducting polymers to environmental conditions (e.g., air, oxygen, moisture, high temperature and chemical solutions), lithographic techniques present significant technical challenges when working with these materials4. For example, current photolithographic methods, such as ultra-violet (UV), are unsuitable for patterning the conducting polymers due to the involvement of wet and/or dry etching processes in these methods. In addition, current micro/nanosystems mainly have a planar form5,6. One layer of structures is built on the top surfaces of another layer of fabricated features. Multiple layers of these structures are stacked together to form numerous devices on a common substrate. The sidewall surfaces of the microstructures have not been used in constructing devices. On the other hand, sidewall patterns could be used, for example, to build 3-D circuits, modify fluidic channels and direct horizontal growth of nanowires and nanotubes.
A macropunching method has been applied in the manufacturing industry to create macropatterns in a sheet metal for over a hundred years. Motivated by this approach, we have developed a micropunching lithography method (MPL) to overcome the obstacles of patterning conducting polymers and generating sidewall patterns. Like the macropunching method, the MPL also includes two operations (Fig. 1): (i) cutting; and (ii) drawing. The "cutting" operation was applied to pattern three conducting polymers4, polypyrrole (PPy), Poly(3,4-ethylenedioxythiophen)-poly(4-styrenesulphonate) (PEDOT) and polyaniline (PANI). It was also employed to create Al microstructures7. The fabricated microstructures of conducting polymers have been used as humidity8, chemical8, and glucose sensors9. Combined microstructures of Al and conducting polymers have been employed to fabricate capacitors and various heterojunctions9,10,11. The "cutting" operation was also applied to generate submicron-patterns, such as 100- and 500-nm-wide PPy lines as well as 100-nm-wide Au wires. The "drawing" operation was employed for two applications: (i) produce Au sidewall patterns on high density polyethylene (HDPE) channels which could be used for building 3D microsystems12,13,14, and (ii) fabricate polydimethylsiloxane (PDMS) micropillars on HDPE substrates to increase the contact angle of the channel15.

A micropunching lithography approach is developed to generate micro- and submicron-patterns on top, sidewall and bottom surfaces of polymer substrates. It overcomes the obstacles of patterning conducting polymers and generating sidewall patterns. This method allows rapid fabrication of multiple features and is free of aggressive chemistry.

Conducting polymers have attracted great attention since the discovery of high conductivity in doped polyacetylene in 19771. They offer the advantages of ...

Optical Trapping of Nanoparticles

Optical trapping is a technique for immobilizing and manipulating small objects in a gentle way using light, and it has been widely applied in trapping and manipulating small biological particles. Ashkin and co-workers first demonstrated optical tweezers using a single focused beam1. The single beam trap can be described accurately using the perturbative gradient force formulation in the case of small Rayleigh regime particles1. In the perturbative regime, the optical power required for trapping a particle scales as the inverse fourth power of the particle size. High optical powers can damage dielectric particles and cause heating. For instance, trapped latex spheres of 109 nm in diameter were destroyed by a 15 mW beam in 25 sec1, which has serious implications for biological matter2,3.
A self-induced back-action (SIBA) optical trapping was proposed to trap 50 nm polystyrene spheres in the non-perturbative regime4. In a non-perturbative regime, even a small particle with little permittivity contrast to the background can influence significantly the ambient electromagnetic field and induce a large optical force. As a particle enters an illuminated aperture, light transmission increases dramatically because of dielectric loading. If the particle attempts to leave the aperture, decreased transmission causes a change in momentum outwards from the hole and, by Newton's Third Law, results in a force on the particle inwards into the hole, trapping the particle. The light transmission can be monitored; hence, the trap can become a sensor. The SIBA trapping technique can be further improved by using a double-nanohole structure.
The double-nanohole structure has been shown to give a strong local field enhancement5,6. Between the two sharp tips of the double-nanohole, a small particle can cause a large change in optical transmission, thereby inducing a large optical force. As a result, smaller nanoparticles can be trapped, such as 12 nm silicate spheres7 and 3.4 nm hydrodynamic radius bovine serum albumin proteins8. In this work, the experimental configuration used for nanoparticle trapping is outlined. First, we detail the assembly of the trapping setup which is based on a Thorlabs Optical Tweezer Kit. Next, we explain the nanofabrication procedure of the double-nanohole in a metal film, the fabrication of the microfluidic chamber and the sample preparation. Finally, we detail the data acquisition procedure and provide typical results for trapping 20 nm polystyrene nanospheres.

The following setup approach details low power optical trapping of dielectric nanoparticles using a double-nanohole in metal film.

Optical trapping is a technique for immobilizing and manipulating small objects in a gentle way using light, and it has been widely applied in trapping ...

Process of Making Three-dimensional Microstructures using Vaporization of a Sacrificial Component

Vascular structures in natural systems are able to provide high mass transport through high surface areas and optimized structure. Few synthetic material fabrication techniques are able to mimic the complexity of these structures while maintaining scalability. The Vaporization of a Sacrificial Component (VaSC) process is able to do so. This process uses sacrificial fibers as a template to form hollow, cylindrical microchannels embedded within a matrix. Tin (II) oxalate (SnOx) is embedded within poly(lactic) acid (PLA) fibers which facilitates the use of this process. The SnOx catalyzes the depolymerization of the PLA fibers at lower temperatures. The lactic acid monomers are gaseous at these temperatures and can be removed from the embedded matrix at temperatures that do not damage the matrix. Here we show a method for aligning these fibers using micromachined plates and a tensioning device to create complex patterns of three-dimensionally arrayed microchannels. The process allows the exploration of virtually any arrangement of fiber topologies and structures.

The Vaporization of a Sacrificial Component (VaSC) process is used to fabricate microvascular structures. This procedure uses sacrificial poly(lactic) acid fibers to form hollow microchannels with precise 3D geometric positioning provided by laser micromachined guide plates.

Vascular structures in natural systems are able to provide high mass transport through high surface areas and optimized structure. Few synthetic material ...

Analysis of Contact Interfaces for Single GaN Nanowire Devices

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

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

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

Monolayer Contact Doping of Silicon Surfaces and Nanowires Using Organophosphorus Compounds

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

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

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

Analyzing the Movement of the Nauplius 'Artemia salina' by Optical Tracking of Plasmonic Nanoparticles

We demonstrate how optical tweezers may provide a sensitive tool to analyze the fluidic vibrations generated by the movement of small aquatic organisms. A single gold nanoparticle held by an optical tweezer is used as a sensor to quantify the rhythmic motion of a Nauplius larva (Artemia salina) in a water sample. This is achieved by monitoring the time dependent displacement of the trapped nanoparticle as a consequence of the Nauplius activity. A Fourier analysis of the nanoparticle&#39;s position then yields a frequency spectrum that is characteristic to the motion of the observed species. This experiment demonstrates the capability of this method to measure and characterize the activity of small aquatic larvae without the requirement to observe them directly and to gain information about the position of the larvae with respect to the trapped particle. Overall, this approach could give an insight on the vitality of certain species found in an aquatic ecosystem and could expand the range of conventional methods for analyzing water samples.

Analyzing the Movement of the Nauplius 'Artemia salina' by Optical Tracking of Plasmonic Nanoparticles

We use optical tracking of plasmonic nanoparticles to probe and characterize the frequency movements of aquatic organisms.

We demonstrate how optical tweezers may provide a sensitive tool to analyze the fluidic vibrations generated by the movement of small aquatic organisms. A ...

Surface Enhanced Raman Spectroscopy Detection of Biomolecules Using EBL Fabricated Nanostructured Substrates

Fabrication and characterization of conjugate nano-biological systems interfacing metallic nanostructures on solid supports with immobilized biomolecules is reported. The entire sequence of relevant experimental steps is described, involving the fabrication of nanostructured substrates using electron beam lithography, immobilization of biomolecules on the substrates, and their characterization utilizing surface-enhanced Raman spectroscopy (SERS). Three different designs of nano-biological systems are employed, including protein A, glucose binding protein, and a dopamine binding DNA aptamer. In the latter two cases, the binding of respective ligands, D-glucose and dopamine, is also included. The three kinds of biomolecules are immobilized on nanostructured substrates by different methods, and the results of SERS imaging are reported. The capabilities of SERS to detect vibrational modes from surface-immobilized proteins, as well as to capture the protein-ligand and aptamer-ligand binding are demonstrated. The results also illustrate the influence of the surface nanostructure geometry, biomolecules immobilization strategy, Raman activity of the molecules and presence or absence of the ligand binding on the SERS spectra acquired.

We describe the fabrication and characterization of nano-biological systems interfacing nanostructured substrates with immobilized proteins and aptamers. The relevant experimental steps involving lithographic fabrication of nanostructured substrates, bio-functionalization, and surface-enhanced Raman spectroscopy (SERS) characterization, are reported. SERS detection of surface-immobilized proteins, and probing of protein-ligand and aptamer-ligand binding is demonstrated.

Fabrication and characterization of conjugate nano-biological systems interfacing metallic nanostructures on solid supports with immobilized biomolecules ...

Electroactive Polymer Nanoparticles Exhibiting Photothermal Properties

A method for the synthesis of electroactive polymers is demonstrated, starting with the synthesis of extended conjugation monomers using a three-step process that finishes with Negishi coupling. Negishi coupling is a cross-coupling process in which a chemical precursor is first lithiated, followed by transmetallation with ZnCl2. The resultant organozinc compound can be coupled to a dibrominated aromatic precursor to give the conjugated monomer. Polymer films can be prepared via electropolymerization of the monomer and characterized using cyclic voltammetry and ultraviolet-visible-near infrared (UV-Vis-NIR) spectroscopy. Nanoparticles (NPs) are prepared via emulsion polymerization of the monomer using a two-surfactant system to yield an aqueous dispersion of the polymer NPs. The NPs are characterized using dynamic light scattering, electron microscopy, and UV-Vis-NIR-spectroscopy. Cytocompatibility of NPs is investigated using the cell viability assay. Finally, the NP suspensions are irradiated with a NIR laser to determine their effectiveness as potential materials for photothermal therapy (PTT).

A protocol is presented for the synthesis and preparation of nanoparticles consisting of electroactive polymers.

A method for the synthesis of electroactive polymers is demonstrated, starting with the synthesis of extended conjugation monomers using a three-step ...

Scanning SQUID Study of Vortex Manipulation by Local Contact

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

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

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

In Situ Synthesis of Gold Nanoparticles without Aggregation in the Interlayer Space of Layered Titanate Transparent Films

Combinations of metal oxide semiconductors and gold nanoparticles (AuNPs) have been investigated as new types of materials. The in situ synthesis of AuNPs within the interlayer space of semiconducting layered titania nanosheet (TNS) films was investigated here. Two types of intermediate films (i.e., TNS films containing methyl viologen (TNS/MV2+) and 2-ammoniumethanethiol (TNS/2-AET+)) were prepared. The two intermediate films were soaked in an aqueous tetrachloroauric(III) acid (HAuCl4) solution, whereby considerable amounts of Au(III) species were accommodated within the interlayer spaces of the TNS films. The two types of obtained films were then soaked in an aqueous sodium tetrahydroborate (NaBH4) solution, whereupon the color of the films immediately changed from colorless to purple, suggesting the formation of AuNPs within the TNS interlayer. When only TNS/MV2+ was used as the intermediate film, the color of the film gradually changed from metallic purple to dusty purple within 30 min, suggesting that aggregation of AuNPs had occurred. In contrast, this color change was suppressed by using the TNS/2-AET+ intermediate film, and the AuNPs were stabilized for over 4 months, as evidenced by the characteristic extinction (absorption and scattering) band from the AuNPs.

In Situ Synthesis of Gold Nanoparticles without Aggregation in the Interlayer Space of Layered Titanate Transparent Films

Here, we present a protocol for the in situ synthesis of gold nanoparticles (AuNPs) within the interlayer space of layered titanate films without the aggregation of AuNPs. No spectral change was observed even after 4 months. The synthesized material has expected applications in catalysis, photo-catalysis, and the development of cost-effective plasmonic devices.

Combinations of metal oxide semiconductors and gold nanoparticles (AuNPs) have been investigated as new types of materials. The in situ synthesis of AuNPs ...