This work was supported by the Natural Science and Engineering Research Council of Canada (Discovery Grant No. 122795-2010) and the program of the Canada Research Chair in Quantum Semiconductors (JJD). The help provided by Xiaohuan Xuang, Mohamed Walid Hassen and technical assistance of Sonia Blais of the Universit&#233; de Sherbrooke Centre de caract&#233;risation de mat&#233;riaux (CCM) in collecting XPS data are greatly appreciated. NL acknowledges the Merit Scholarship Program for Foreign Student, Fonds de recherche du Qu&#233;bec - Nature et technologies, for providing a graduate student scholarship.

1. Sample Preparation
<ol>
	<li>Use a diamode scribe to cleave an n-type (P-doped) one-side polished Si wafer (resistivity 3.1~4.8 &#937;.m) which is 3 inch in diameter, 380 &#181;m thick, into samples of 12 mm x 6 mm; clean the samples in OptiClear, acetone and isopropyl alcohol (5 min&#160;for every step).</li>
	<li>Etch samples in a ~0.9% HF solution for 1 &#160;min&#160;to etch away initial oxide; rinse in DI water and&#160;dry in high-purity (99.999%) nitrogen (N2).</li>
	<li>Store prepared samples in N2 bag to curb their &#160;oxidation in&#160;air.</li>
</ol>
2. Irradiate Samples by ArF (&#955;=193 nm) and KrF (&#955;=248 nm) Lasers.
<ol>
	<li>Place samples in a 0.74 mm tall chamber and then seal the chamber with a fused silica window that has high transmission in UV (&#8805;90%). Fill&#160;the chamber with H2O2/H2O solution in the range of 0.01-0.2 % or with degassed methanol using a microfluidic channel.</li>
	<li>Irradiate samples with homogenized ArF or KrF lasers at demagnification of 2.6 and 1.8, respectively. Irradiate&#160;only &#160;2&#160;sites on each sample by increasing laser pulses from 100 to 600 in step of 100 pulses &#160;through&#160;a circular mask (4 mm&#160;in&#160;diameter). Irradiate the samples in the same way with a &#8220;maple leaf&#8221;&#160;(9 mm x 7.2 mm) mask.</li>
	<li>Rinse samples in DI water, &#160;dry with N2 flush; place&#160;the samples in a sealed container, then quickly fill this container with N2, in order to avoid exposure to air prior to further experiments.</li>
</ol>
3. Immobilization of Bio-conjugated Nanospheres
<ol>
	<li>Dilute biotin-conjugated and fluorescein stained 40-nm-diameter nanospheres in a pH 7.4 phosphate buffered saline (PBS, 1X) solution to 1012 particles/ml&#160;at RT (~25 &#176;C). Immerse ArF or KrF laser irradiated samples for 2 &#160;hr&#160;in this solution at RT.</li>
	<li>Wash samples with PBS to eliminate physically bound fluorescein stained nanospheres on the surface.</li>
</ol>
4. Surface Characterization
<ol>
	<li>Contact angle (CA) measurement
	<ol>
		<li>Carry out static CA measurements with a goniometer in an environment of RT and ambient humidity.</li>
		<li>Employ high purity DI water (resistivity 17.95 M&#8486;&#183;cm) in a micro-syringe; generate similar volume (~5 &#181;l) drops on the sample surface by lowering the micro-syringe to a similar height for every measurement.</li>
		<li>Capture and save the water drops profile images by CCD camera with software. Measure independently 4 different sites with same irradiation conditions.</li>
		<li>Estimate and average&#160;the CA values in drop analysis module from ImageJ software; load the image and change it into grayscale; launch the plugin Dropsnake; place roughly a few knots on the drop contour (~10 knots) from left to right to initialize snake; accept the curve connecting these knots and evolve the curve by pressing snake button. Note: contact angles are displayed in the image and the table.</li>
	</ol>
	</li>
	<li>XPS measurement
	<ol>
		<li>Investigate surface chemical modification with a XPS spectrometer(1x10-9 Torr base pressure) outfitted with an Al K&#945; source working at 150 W:
		<ol>
			<li>Load the samples into the vacuum chamber.</li>
			<li>Acquire&#160;the surface survey data in constant energy modes of 50 eV pass energy from an area of 220 &#181;m x 220 &#181;m.</li>
			<li>Acquire high resolution scans data from the same analyzed area at 20 eV pass energy.</li>
		</ol>
		</li>
		<li>Process XPS spectra data with XPS spectra quantification software, as referenced25,26.</li>
	</ol>
	</li>
	<li>Fluorescence microscope imaging
	<ol>
		<li>Excite samples, which were irradiated through &#8220;maple leaf&#8221; mask and exposed to fluorescein stained nanospheres, using a blue light source (&#955;=450~490 nm).</li>
		<li>Observe fluorescent images, emitting at 515 nm, with a fluorescence inverted microscope in magnification of 4X.</li>
		<li>Characterize the surface morphology of these samples with AFM, as referenced27.</li>
	</ol>
	</li>
</ol>

The authors have nothing to disclose.

We have proposed a protocol of UV laser irradiation of Si wafer in a microfluidic chamber filled with low concentration of H2O2 solution to induce a superhydrophilic Si surface, which is mainly due to the generation of Si-OH. UV laser photolysis of H2O2 was supposed to form negatively charged OH- radicals. Also, UV laser photoelectric effect leads to the formation of a positively charged surface37. Therefore, the interaction of these negative OH- radicals with a positively charged surface leads to the generation of Si-OH on the surface. So, we can increase hydrophilicity by increasing the laser pulse number and increase the concentration of OH- reacting with Si15. However, the hydrophilicity ceased to increase or even decrease at larger pulse number during the process because H2O2 is thermodynamically unstable, and its decomposition is described by 2H2O2&#8594;2H2O+O238, which results in excessively formed O2 in the near surface region of Si. Although this process would potentially lead to the formation of SiO2 to improve the surface hydrophilicity, the generation of&#160;O2 molecules could also be the cause of bubbles formation near the irradiated surface. Significantly increased bubble formation by ArF laser at 65 mJ/&#160;cm2&#160;and KrF laser at 320 mJ/cm2, is consistent with the increased possibility of thermally driven decomposition of H2O2. As the minimum CA for SiO2 coated Si is known to be near 45&#176;, the formation&#160;SiO2&#160;enriched Si could cause the increase of CA observed for the sites irradiated with a large pulse number.
The calculation of temperature induced by laser irradiation is also a critical aspect, as it is important to the&#160;oxidation&#160;of Si in the H2O2/H2O solution and the increased wettability. Using COMSOL calculations, the surface peak temperatures were estimated to be 88 and 95 &#176;C when irradiated with KrF laser pulse of 250&#160;and 320&#160;mJ/cm2, respectively. In comparison, the surface peak temperature is estimated to be 40 &#176;C, when it was irradiated by ArF laser pulse of 65 mJ/cm2. These peak temperatures dropped to the original temperature in 10-5 s. There is no heat accumulation between two consecutive pulses when KrF and ArF lasers operate at a repetition rate of 2 Hz (a case investigated in this communication). Based on the temperature calculation results, the laser parameters can be optimized in future experiments.
We also proposed using ArF laser to induce hydrophobic Si surface by irradiating Si sample in methanol solution in a similar microchamber, which is due to laser induced formation of Si-O-CH3 on the irradiated surface, as shown in Figures 5 and 6. It has been reported that UV laser light (105-200 nm) induced dissociation of methanol vapor could be described by the reaction: CH3OH&#8594;CH3O+H 39. The higher the temperature, the more CH3O adsorbs on the Si surface40. Thus, by irradiation at lower laser fluence (e.g., 30 mJ/cm2), there is no methanol boiling and no obvious wettability change due to laser induced lower temperature. Also, the KrF laser irradiation of the sample in methanol solution produces no significant CA increment due to its longer wavelength and lower cross section absorption coefficient (&#60;0.1x10-20/cm2) than ArF laser (25 x10-20/cm2)41. The absorption coefficient of KrF laser in methanol is also much lower than those of ArF (61x10-20/cm2) and KrF laser (9x10-20/cm2) in H2O242.The saturation of the CA around 103&#176; is related to the CH3 surface energy, which is dominant for the wettability15. The lower the surface energy, the higher the hydrophobicity. The lowest surface energy (CF3) was reported to have the maximum CA of 120&#176;, while for a CHx bond with higher surface energy, the CA of 110&#176; 43 is always lower.
Therefore, compared with other well-known methods of laser induced modification of Si, such as laser induced surface morphology modification, the process and steps described in this report are simpler, they do not need a high cost and high power laser systems, but are effective in in situ control of Si surface wettability. This technique can be widely used to selective area induce modification of wettability for micro/nano Si based biosensor application in future. However, there are limitations of this technique, especially for UV laser induced hydrophobicity, such as the maximum hydrophobicity (CA) is restricted by the laser photon energy and CHx surface energy. The critical steps during this techniques mainly includes storing the sample in N2 container to avoid oxidation before irradiation and controlling the bubbles generation on Si surface during laser irradiation, e.g., using microfluidic channel.

The remarkable electronic and chemical properties as well as its high mechanical strength have made silicon (Si) an ideal choice for microelectronic devices and biomedical chips1. Selective area control of the Si surface has received significant attention for applications involving microfluidic and lab-on-chip devices2,3.This is often obtained either by nano-scale modification of the surface roughness or by chemical treatment of the surface4. The surface roughening or patterning to produce disordered or ordered surface structures on the Si surface include photolithography5, ion beam lithography6 and laser techniques7. Compared with these methods,&#160;laser surface texturing process is reported to be less complicated with the potential to produce microstructures with high spatial resolution8. However, as Si has an elevated texturing threshold, requiring irradiation with pulse fluence to induce surface texturing in excess of its ablation threshold (~500 mJ/cm2)9, texturing of Si surface has frequently been assisted by employing reactive gas atmospheres, such as that of a high pressure SF6 environment4,7,8. Consequently, to modify wettability of the Si surface, numerous works have focused on chemical treatment by depositing organic10 and inorganic films2, or using plasma or electron beam surface treatment11,12. It is recognized that hydrophilicity of Si originating from the existence of singular and associated OH groups on its surface could be achieved by boiling it in a H2O2 solution at 100 &#176;C for several minutes13. However, the hydrophobic Si surface states, most of which are due to the presence of Si-H or Si-O-CH3 groups, could be achieved by wet chemical handling involving etching with&#160;HF acid solution or coating with photoresist13-15. To achieve selective area control of wettability of Si, complex patterning steps are usually required, including treatment in chemical solutions16. The high chemical reactivity of UV laser radiation has also been used to selective area process organic film coated solid substrates and modify their wettability17. However, a limited amount of data is available on laser-assisted modification of Si wettability by irradiation of samples immersed in different chemical solutions.
In our previous research, UV laser irradiation of III-V semiconductors in air18-20 and NH321 was successfully used to alter the surface chemical composition of GaAs, InGaAs and InP. We established that UV laser irradiation of III-V semiconductors&#160;in deionized (DI) water decreases surface oxides and carbides, while the water adsorbed on semiconductor surface increases22. A strongly hydrophobic Si surface (CA~103&#176;) was obtained by ArF laser irradiation of Si samples in methanol in our recent work 23. As indicated by X-ray photoelectron spectroscopy (XPS), this is&#160;primarily&#160;due to the&#160;ability&#160;of the ArF laser to photodissociate CH3OH. We have also used KrF and ArF lasers to irradiate Si (001) in a 0.01% of H2O2 in DI water. This allowed us to achieve selective area formation of superhydrophilic surface of Si (001) characterized by the CA of near 15&#176;. The XPS results suggest that this is due to generation of Si-OH bonds on the irradiated surface24.
A detailed description of this new technique using KrF and ArF lasers for selective area in situ modification of the hydrophilic/hydrophobic surface of Si surface in low concentration of H2O2/H2O and methanol solutions is demonstrated in this article. The details provided here should be sufficient to allow similar experiments to be performed by interested researchers.

<table><tbody><tr><td>fluorescein stained nanospheres</td><td>Invitrogen</td><td>F8795</td><td></td></tr><tr><td>OptiClear</td><td>National Diagnostics</td><td>OE-101</td><td></td></tr><tr><td>ArF laser (&amp;lambda;=193 nm)</td><td>Lumonics</td><td>pulse master 800</td><td></td></tr><tr><td>KrF laser (&amp;lambda;=248 nm)</td><td>Lumonics</td><td>pulse master 800</td><td></td></tr><tr><td>XPS</td><td>Kratos Analytical</td><td>AXIS Ultra DLD</td><td></td></tr><tr><td>Fluorescence microscope</td><td>Olympus</td><td>IX71</td><td></td></tr><tr><td>XPS quantitification software</td><td>CasaXPS</td><td>2.3.15</td><td></td></tr></tbody></table>

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.

These representative results have been presented in our previous published work23,24. Figure 1 shows the CA vs. N (number of pulses) on sites irradiated by KrF laser at 250 mJ/cm2 in DI H2O for different concentrations of H2O2/H2O solutions (e.g., 0.01, 0.02, 0.05 and 0.2%).&#160;The CA decreases with increasing pulse number for all the H2O2 solutions. The minimum CA (~15&#176;) for the 0.02 and 0.01% H2O2 solutions is obtained at 500 pulses. A rather larger CA has been observed for 0.05 and 0.2% H2O2 solutions at larger pulse numbers (N&#8805;500). Concurrently, it is found that the CA of the sample without irradiation (N=0) decreased by 32&#176; from 75&#176; as the H2O2 concentration increased from 0.02 to 0.2%. These results, acquired after an average 10 min exposure to H2O2 solutions, likely represent the CA saturation values obtainable at respective H2O2 concentrations. However, it is important to note that the exposure of samples to 0.01 % H2O2 solution, for up to 4 hr has not resulted in a measurable change of the CA characterizing the initial surface.
Figure 2 illustrates CA vs. pulse number for the sites after KrF (Figure&#160;2A)&#160;and ArF (Figure&#160;2B)&#160;laser irradiation in a 0.01% H2O2/H2O solution. Figure 2a demonstrates that the CA decreases continuously with the pulse number up to 600 pulses of the KrF at 183 mJ/cm2. Similar results were found on samples irradiated by ArF laser at 44 mJ/cm2, as shown in Figure 2B. When the sites were irradiated by KrF laser with 300 pulses at 320 mJ/cm2 and 500 pulses at 250 mJ/cm2, a minimum CA value of 15&#176; was obtained. When the sites were irradiated by 500 ArF laser pulses at 65 mJ/cm2, the similar CA~15&#176; was achieved.
Figure 3 shows O 1s XPS spectra of Si surface freshly etched by HF (Figure 3A),&#160; exposed to 0.01% H2O2/H2O solution for approximately 10 min&#160;without laser irradiation ((Figure3B),&#160;and exposed to 0.01% H2O2/H2O solution and 500-pulse KrF laser irradiation at 250 mJ/cm2 (Figure 3C). The peaks at 531.8&#177;0.1, 532.6&#177;0.1 and 533.7&#177;0.1 eV were assigned to SiOx, SiO2 and SiOH, respectively28,29. Figure 3B shows that the exposure to an HF solution has removed most of SiO2 and SiOx from the surface.&#160;The quantities of SiO2&#160;and SiOH on the site irradiated by KrF laser are greater (Figure 3C)&#160;than those on the non-irradiated (Figure 3B).&#160;The Si surfaces coated with SiO2 were always reported to have minimum CA values of 45&#176;-55&#176;, as referenced11, depending on the O/Si ratio. However, a superhydrophilic SiOH monolayer covered Si surface was reported with a minimum CA of 13&#176;,&#160;as referenced30. Thus, the CA=14&#176; obtained with 500 pulses is mainly due to increased surface concentration of SiOH. We also observed that the SiOH/SiO2&#160;ratio increased from 0.10 (100-pulse irradiation, data not shown) to 0.17 for the 500-pulse irradiated site. The dashed lines in the spectra represent the carbon (C) adsorbates on the surface. The quantities of these adsorbates are determined depending on fixed ratios of O/C of C-O, C=O and O-C=O bonds in the C 1s spectra31. We have found that there is more C on the non-irradiated surface exposed to the H2O2/H2O solution, than on the sample freshly etched by HF acid. Figure 3C shows that&#160;the quantities of C absorbates decreased with pulse number due to the excimer laser cleaning effect9. Since the C absorbates on the surface were reported to increase hydrophobicity of Si15, the laser induced removal of C adsorbates also improves the hydrophilic nature of the surface.
Figure 4A shows a fluorescence microscopic image of the Si surface selectively coated with fluorescein stained nanospheres. The sample was first irradiated in a H2O2/H2O solution (0.01%) by projecting a &#8220;maple leaf&#8221; mask with the KrF laser delivering 400 pulses at 250 mJ/cm2. High surface concentration of nanospheres is found on the non-irradiated portion of the sample. The result illustrates formation of a laser-induced zone of a strongly hydrophilic material that prevents binding of the nanospheres.&#160;The presence of some nanospheres observed in this zone could be related to the surface defect induced oxidation&#160;of Si and related reduction in its hydrophilicity. Figure 4B shows an AFM image of a fragment of the non-irradiated surface densely covered with immobilized nanospheres.&#160;
Figure 5 shows the CA values measured for Si samples that were immersed in methanol and irradiated with ArF laser at 30, 65 and 80 mJ/cm2. It can be seen that the CA of the sample irradiated with&#160;800&#160;pulses at&#160;65&#160;mJ/cm2 increased from its initial value of 75&#176; to 103&#176;, and it is comparable to the CA for the 1,000-pulse irradiated sample. This suggests that the laser based chemical alteration of the Si surface saturates at these laser fluences. More intense dynamics of the CA increase has been observed for 80 mJ/cm2 and low number of laser pulses (N &#60; 200), as indicated by the full circle symbols. However, the formation of bubbles on samples irradiated with N&#62;200 pulses, and a related uncontrolled modification of the sample surface morphology prevented us from collecting reliable data under such conditions. Using an approach described elsewhere22,32, we estimated that an ArF laser irradiation at 65 mJ/cm2 induces peak temperature on the surface of Si comparable to the methanol boiling point, i.e., 65 &#176;C, as referenced33. Thus, irradiation with greater laser fluences is expected to induce the formation of bubbles. Consistent with this was our inability to fabricate Si samples of satisfactory characteristics with the laser fluence of 80 mJ/cm2 and N &#62; 200 pulses. In contrast, irradiation at 30 mJ/cm2 showed only a weak increase of the CA to 78&#176; for the 1,000-pulse irradiated samples.
Figure 6 shows XPS spectra of Si 2p and O 1s for sites immersed in methanol that were non-irradiated (Figures 6A and&#160;6B),&#160;and irradiated with 500 pulses of the ArF laser at 65 mJ/cm2(Figure 6C and 6D). A weak feature in the Si 2p spectrum of the non-irradiated site (Figure 6A) can be seen around BE=102.7 eV. This feature has been reported to originate from the Si-(OCH3)x bond34. The atomic concentration of this compound has been estimated at 0.7%, which is slightly underestimated due to the relatively small (60&#176;) take-off angle (TOF) applied while collecting XPS data. However, on the irradiated site (Figure 6C), the atomic percentage of Si-(OCH3)x bond increased by 5 times to 3.5% at TOF of 60&#176;. In the O 1s spectra (Figures 6B and 6D), it can be seen that the concentration of the Si-O-CH3 peak (BE=532.6 eV) increased from 1 to 2.5 % for the non-irradiated and irradiated sites, respectively. As Si-(OCH3)x has been reported to be responsible for the formation of a hydrophobic surface of Si, as referenced15,35,36, the increase of surface concentration of Si-(OCH3)x appears to be the main reason for the observed&#160;hydrophobic characteristics&#160;of the ArF irradiated Si samples. In the O 1s spectra, besides Si-O-C and C-O, there are SiOx and OH peaks. The increase of the SiOx peak at BE=531.5&#177;0.2 eV is possibly caused by the CH3O binding to SiOx sub-oxides (SiOx+1-CH3)34. As the HF treated Si sample didn&#8217;t show presence of OH (not shown here), this OH peak is possibly from CH3OH physically absorbed to the Si surface.
<img alt="Figure 1" src="/files/ftp_upload/52720/52720fig1.jpg" /> 
Figure 1.&#160;Contact angle vs. pulse number on Si (001) surface&#160;irradiated by KrF laser at 250 mJ/cm2 in DI H2O and different concentrations H2O2/H2O solutions (e.g., 0.01, 0.02, 0.05 and 0.2%). The contact angle value standard deviation (SD) is 2.5&#176;. The figure has been modified from 24. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52720/52720fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/52720/52720fig2.jpg" /> 
Figure 2.&#160;Contact angle vs. pulse number of samples immersed in 0.01% H2O2/H2O solution and irradiated by KrF (Figure 2A) and ArF (Figure 2B) lasers. The SD of contact angle value was reported to be 2.2&#176;. The figure has been modified from 24. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52720/52720fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/52720/52720fig3.jpg" /> 
Figure 3. O 1s XPS spectra of Si surface freshly etched in HF (A),&#160;exposed to 0.01% H2O2/H2O solution for approximately 10 min&#160;without laser irradiation (B),&#160;and irradiated with 500 pulses of the KrF laser at 250 mJ/cm2 while exposed to 0.01% H2O2/H2O solution (C). The figure has been modified from 24. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52720/52720fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/52720/52720fig4.jpg" /> 
Figure 4. Fluorescence microscopic picture of a sample that was, first, irradiated with 400 pulses of a KrF laser operating at 250 mJ/cm2 and projecting a &#8220;maple leaf&#8221; mask on the surface and, second, exposed to a solution of fluorescein stained nanospheres (A). AFM image of a fragment of the non-irradiated part of the sample showing immobilized nanospheres (B). The figure has been modified from 24. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52720/52720fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/52720/52720fig5.jpg" /> 
Figure 5. Contact angle of Si (001) samples immersed in methanol and irradiated with&#160;an&#160;ArF laser at&#160;30&#160;mJ/cm2 (&#9650;), 65 mJ/cm2 (&#9632;) and 80 mJ/cm2 (&#9679;). The error bars are calculated based on the measurements of 3 independent sites. A contact angle value SD of 2.0&#176; was reported. The figure has been modified from 23. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52720/52720fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" src="/files/ftp_upload/52720/52720fig6.jpg" /> 
Figure 6. Si 2p and O 1s XPS spectra of a reference (non-irradiated) sample (A and B),&#160;and a sample irradiated by an ArF&#160;laser&#160;in methanol with 500 pulses of&#160;at 65mJ/cm2&#160;(C and D). The figure has been modified from 23. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52720/52720fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Selective Area Modification of Silicon Surface Wettability by Pulsed UV Laser Irradiation in Liquid Environment at JoVE.com

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

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 ...

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8.2K Views.  Université de Sherbrooke.  We rapporteren op een werkwijze in situ verandering van HF behandelde Si (001) oppervlak in een hydrofiel of hydrofoob toestand door bestralen monsters microfluïdische kamers gevuld met H 2 O 2 / H2O-oplossing (0,01% -0,5%) en methanol oplossingen met behulp van UV-gepulste laser van een relatief lage hartslag Fluence. 

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

A Method of Targeted Cell Isolation via Glass Surface Functionalization

One of the limiting factors to the adoption and advancement of personalized medicine is the inability to develop diagnostic tools to probe individual nuances in expression from patient to patient. Current methodologies that try to separate cells to fill this niche result in disruption of physiological expression, making the separation technique useless as a diagnostic tool. In this protocol, we describe the functionalization and optimization of a surface for the cellular capture and release. This functionalized surface integrates biotinylated antibodies with a glass surface functionalized with an aminosilane (APTES), desthiobiotin and streptavidin. Cell release is facilitated through the introduction of biotin, allowing the recollection and purification of cells captured by the surface. This release is done through the targeting of the secondary moiety desthiobiotin, which results in a much more gentle release paradigm. This reduction in harsh reagents and shear forces reduces changes in cellular expression. The functionalized surface captures up to 80% of cells in a single cell mixture and has demonstrated 50% capture in a dual-cell mixture. Applications of this technology to xenografts and cancer separation studies are investigated. Quantification techniques for surface verification such as plate reader and ImageJ analyses are described as well.

This protocol describes customizable surface functionalization of the desthiobiotin, streptavidin, and APTES system in order to isolate specific cell types of interest. In addition, this manuscript covers the applications, optimization, and verification of this process.

One of the limiting factors to the adoption and advancement of personalized medicine is the inability to develop diagnostic tools to probe individual ...

Cancer Research

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

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

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

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

Soft Lithographic Functionalization and Patterning Oxide-free Silicon and Germanium

The development of hybrid electronic devices relies in large part on the integration of (bio)organic materials and inorganic semiconductors through a stable interface that permits efficient electron transport and protects underlying substrates from oxidative degradation. Group IV semiconductors can be effectively protected with highly-ordered self-assembled monolayers (SAMs) composed of simple alkyl chains that act as impervious barriers to both organic and aqueous solutions. Simple alkyl SAMs, however, are inert and not amenable to traditional patterning techniques. The motivation for immobilizing organic molecular systems on semiconductors is to impart new functionality to the surface that can provide optical, electronic, and mechanical function, as well as chemical and biological activity. 
Microcontact printing (&mu;CP) is a soft-lithographic technique for patterning SAMs on myriad surfaces.1-9 Despite its simplicity and versatility, the approach has been largely limited to noble metal surfaces and has not been well developed for pattern transfer to technologically important substrates such as oxide-free silicon and germanium. Furthermore, because this technique relies on the ink diffusion to transfer pattern from the elastomer to substrate, the resolution of such traditional printing is essentially limited to near 1 &mu;m.10-16
In contrast to traditional printing, inkless &mu;CP patterning relies on a specific reaction between a surface-immobilized substrate and a stamp-bound catalyst. Because the technique does not rely on diffusive SAM formation, it significantly expands the diversity of patternable surfaces. In addition, the inkless technique obviates the feature size limitations imposed by molecular diffusion, facilitating replication of very small (&lt;200 nm) features.17-23 However, up till now, inkless &mu;CP has been mainly used for patterning relatively disordered molecular systems, which do not protect underlying surfaces from degradation.
Here, we report a simple, reliable high-throughput method for patterning passivated silicon and germanium with reactive organic monolayers and demonstrate selective functionalization of the patterned substrates with both small molecules and proteins. The technique utilizes a preformed NHS-reactive bilayered system on oxide-free silicon and germanium. The NHS moiety is hydrolyzed in a pattern-specific manner with a sulfonic acid-modified acrylate stamp to produce chemically distinct patterns of NHS-activated and free carboxylic acids. A significant limitation to the resolution of many &mu;CP techniques is the use of PDMS material which lacks the mechanical rigidity necessary for high fidelity transfer. To alleviate this limitation we utilized a polyurethane acrylate polymer, a relatively rigid material that can be easily functionalized with different organic moieties. Our patterning approach completely protects both silicon and germanium from chemical oxidation, provides precise control over the shape and size of the patterned features, and gives ready access to chemically discriminated patterns that can be further functionalized with both organic and biological molecules. The approach is general and applicable to other technologically-relevant surfaces.

Here we describe a simple method for patterning oxide-free silicon and germanium with reactive organic monolayers and demonstrate functionalization of the patterned substrates with small molecules and proteins. The approach completely protects surfaces from chemical oxidation, provides precise control over feature morphology, and provides ready access to chemically discriminated patterns.

The development of hybrid electronic devices relies in large part on the integration of (bio)organic materials and inorganic semiconductors through a ...

Bioengineering

Patterning via Optical Saturable Transitions - Fabrication and Characterization

This protocol describes the fabrication and characterization of nanostructures using a novel nanolithographic technique called Patterning via Optical Saturable Transitions (POST). In this technique the chemical properties of organic photochromic molecules that undergo single-photon reactions are exploited, enabling rapid top-down nanopatterning over large areas at low light intensities, thereby, allowing for the circumvention of the far-field diffraction barrier.4 Simple, cost-effective, high throughput and resolution alternatives to nanopatterning are being explored, such as, two-photon polymerization5,6, beam pen lithography (BPL)7, scanning electron beam lithography (SEBL), and focused ion beam (FIB) patterning. However, multi-photon approaches require high light intensities, which limit their potential for high throughput and offer low image contrast. Although, electron and ion beam lithographic processes offer increased resolution, the serial nature of the process is limited to slow writing speeds, which also prevents patterning of features over large areas. Beam-pen lithography is an approach towards parallel near-field optical lithography. However, the gap between the source of the beam and the surface of the photoresist needs to be controlled extremely precisely for good pattern uniformity and this is very challenging to accomplish for large arrays of beams. Patterning via Optical Saturable Transitions (POST) is an alternative optical nanopatterning technique for patterning sub-wavelength features1-3. Since this technique uses single photons instead of electrons, it is extremely fast and does not require high light intensities1-3, opening the door to massive parallelization.

We report that the diffraction limit of conventional optical lithography can be overcome by exploiting the transitions of organic photochromic derivatives induced by their photoisomerization at low light intensities.1-3 This paper outlines our fabrication technique and two locking mechanisms, namely: dissolution of one photoisomer and electrochemical oxidation.

This protocol describes the fabrication and characterization of nanostructures using a novel nanolithographic technique called Patterning via Optical ...

Light Enhanced Hydrofluoric Acid Passivation: A Sensitive Technique for Detecting Bulk Silicon Defects

A procedure to measure the bulk lifetime (&#62;100 &#181;sec) of silicon wafers by temporarily attaining a very high level of surface passivation when immersing the wafers in hydrofluoric acid (HF) is presented. By this procedure three critical steps are required to attain the bulk lifetime. Firstly, prior to immersing silicon wafers into HF, they are chemically cleaned and subsequently etched in 25% tetramethylammonium hydroxide. Secondly, the chemically treated wafers are then placed into a large plastic container filled with a mixture of HF and hydrochloric acid, and then centered over an inductive coil for photoconductance (PC) measurements. Thirdly, to inhibit surface recombination and measure the bulk lifetime, the wafers are illuminated at 0.2 suns for 1 min&#160;using a halogen lamp, the illumination is switched off, and a PC measurement is immediately taken. By this procedure, the characteristics of bulk silicon defects can be accurately determined. Furthermore, it is anticipated that a sensitive RT surface passivation technique will be imperative for examining bulk silicon defects when their concentration is low (&#60;1012 cm-3).

A RT liquid surface passivation technique to investigate the recombination activity of bulk silicon defects is described. For the technique to be successful, three critical steps are required: (i) chemical cleaning and etching of silicon, (ii) immersion of silicon in 15% hydrofluoric acid and (iii) illumination for 1 min.

A procedure to measure the bulk lifetime (&#62;100 &#181;sec) of silicon wafers by temporarily attaining a very high level of surface passivation when ...

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.

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 ...

Utilization of Microscale Silicon Cantilevers to Assess Cellular Contractile Function In Vitro

The development of more predictive and biologically relevant in vitro assays is predicated on the advancement of versatile cell culture systems which facilitate the functional assessment of the seeded cells. To that end, microscale cantilever technology offers a platform with which to measure the contractile functionality of a range of cell types, including skeletal, cardiac, and smooth muscle cells, through assessment of contraction induced substrate bending. Application of multiplexed cantilever arrays provides the means to develop moderate to high-throughput protocols for assessing drug efficacy and toxicity, disease phenotype and progression, as well as neuromuscular and other cell-cell interactions. This manuscript provides the details for fabricating reliable cantilever arrays for this purpose, and the methods required to successfully culture cells on these surfaces. Further description is provided on the steps necessary to perform functional analysis of contractile cell types maintained on such arrays using a novel laser and photo-detector system. The representative data provided highlights the precision and reproducible nature of the analysis of contractile function possible using this system, as well as the wide range of studies to which such technology can be applied. Successful widespread adoption of this system could provide investigators with the means to perform rapid, low cost functional studies in vitro, leading to more accurate predictions of tissue performance, disease development and response to novel therapeutic treatment.

Utilization of Microscale Silicon Cantilevers to Assess Cellular Contractile Function In Vitro

This protocol describes the use of microscale silicon cantilevers as pliable culture surfaces for measuring the contractility of muscle cells in vitro. Cellular contraction causes cantilever bending, which can be measured, recorded, and converted into readouts of force, providing a non-invasive and scalable system for measuring contractile function in vitro.

The development of more predictive and biologically relevant in vitro assays is predicated on the advancement of versatile cell culture systems which ...

Fabricating Reactive Surfaces with Brush-like and Crosslinked Films of Azlactone-Functionalized Block Co-Polymers

In this paper, fabrication methods that generate novel surfaces using the azlactone-based block co-polymer, poly (glycidyl methacrylate)-block-poly (vinyl dimethyl azlactone) (PGMA-b-PVDMA), are presented. Due to the high reactivity of azlactone groups towards amine, thiol, and hydroxyl groups, PGMA-b-PVDMA surfaces can be modified with secondary molecules to create chemically or biologically functionalized interfaces for a variety of applications. Previous reports of patterned PGMA-b-PVDMA interfaces have used traditional top-down patterning techniques that generate non-uniform films and poorly controlled background chemistries. Here, we describe customized patterning techniques that enable precise deposition of highly uniform PGMA-b-PVDMA films in backgrounds that are chemically inert or that have biomolecule-repellent properties. Importantly, these methods are designed to deposit PGMA-b-PVDMA films in a manner that completely preserves azlactone functionality through each processing step. Patterned films show well-controlled thicknesses that correspond to polymer brushes (~90 nm) or to highly crosslinked structures (~1-10 &#956;m). Brush patterns are generated using either the parylene lift-off or interface directed assembly methods described and are useful for precise modulation of overall chemical surface reactivity by adjusting either the PGMA-b-PVDMA pattern density or the length of the VDMA block. In contrast, the thick, crosslinked PGMA-b-PVDMA patterns are obtained using a customized micro-contact printing technique and offer the benefit of higher loading or capture of secondary material due to higher surface area to volume ratios. Detailed experimental steps, critical film characterizations, and trouble-shooting guides for each fabrication method are discussed.

Surface fabrication methods for patterned deposition of nanometer thick brushes or micron thick, crosslinked films of an azlactone block co-polymer are reported. Critical experimental steps, representative results, and limitations of each method are discussed. These methods are useful for creating functional interfaces with tailored physical features and tunable surface reactivity.

In this paper, fabrication methods that generate novel surfaces using the azlactone-based block co-polymer, poly (glycidyl methacrylate)-block-poly (vinyl ...

Photopatterning Proteins and Cells in Aqueous Environment Using TiO2 Photocatalysis

Organic contaminants adsorbed on the surface of titanium dioxide (TiO2) can be decomposed by photocatalysis under ultraviolet (UV) light. Here we describe a novel protocol employing the TiO2 photocatalysis to locally alter cell affinity of the substrate surface. For this experiment, a thin TiO2 film was sputter-coated on a glass coverslip, and the TiO2 surface was subsequently modified with an organosilane monolayer derived from octadecyltrichlorosilane (OTS), which inhibits cell adhesion. The sample was immersed in a cell culture medium, and focused UV light was irradiated to an octagonal region. When a neuronal cell line PC12 cells were plated on the sample, cells adhered only on the UV-irradiated area. We further show that this surface modification can also be performed in situ, i.e., even when cells are growing on the substrate. Proper modification of the surface required an extracellular matrix protein collagen to be present in the medium at the time of UV irradiation. The technique presented here can potentially be employed in patterning multiple cell types for constructing coculture systems or to arbitrarily manipulate cells under culture.

Photopatterning Proteins and Cells in Aqueous Environment Using TiO2 Photocatalysis

We describe a protocol for modifying cell affinity of a scaffold surface in aqueous environment. The method takes advantage of titanium dioxide photocatalysis to decompose organic film in the photo-irradiated region. We show that it can be used to create microdomains of scaffolding proteins, both ex situ and in situ.

Organic contaminants adsorbed on the surface of titanium dioxide (TiO2) can be decomposed by photocatalysis under ultraviolet (UV) light. Here we describe ...

Rendering SiO2/Si Surfaces Omniphobic by Carving Gas-Entrapping Microtextures Comprising Reentrant and Doubly Reentrant Cavities or Pillars

We present microfabrication protocols for rendering intrinsically wetting materials repellent to liquids (omniphobic) by creating gas-entrapping microtextures (GEMs) on them comprising cavities and pillars with reentrant and doubly reentrant features. Specifically, we use SiO2/Si as the model system and share protocols for two-dimensional (2D) designing, photolithography, isotropic/anisotropic etching techniques, thermal oxide growth, piranha cleaning, and storage towards achieving those microtextures. Even though the conventional wisdom indicates that roughening intrinsically wetting surfaces (&#952;o &#60; 90&#176;) renders them even more wetting (&#952;r &#60; &#952;o &#60; 90&#176;), GEMs demonstrate liquid repellence despite the intrinsic wettability of the substrate. For instance, despite the intrinsic wettability of silica &#952;o &#8776; 40&#176; for the water/air system, and &#952;o &#8776; 20&#176; for the hexadecane/air system, GEMs comprising cavities entrap air robustly on immersion in those liquids, and the apparent contact angles for the droplets are &#952;r &#62; 90&#176;. The reentrant and doubly reentrant features in the GEMs stabilize the intruding liquid meniscus thereby trapping the liquid-solid-vapor system in metastable air-filled states (Cassie states) and delaying wetting transitions to the thermodynamically-stable fully-filled state (Wenzel state) by, for instance, hours to months. Similarly, SiO2/Si surfaces with arrays of reentrant and doubly reentrant micropillars demonstrate extremely high contact angles (&#952;r &#8776; 150&#176;&#8211;160&#176;) and low contact angle hysteresis for the probe liquids, thus characterized as superomniphobic. However, on immersion in the same liquids, those surfaces dramatically lose their superomniphobicity and get fully-filled within &#60;1 s. To address this challenge, we present protocols for hybrid designs that comprise arrays of doubly reentrant pillars surrounded by walls with doubly reentrant profiles. Indeed, hybrid microtextures entrap air on immersion in the probe liquids. To summarize, the protocols described here should enable the investigation of GEMs in the context of achieving omniphobicity without chemical coatings, such as perfluorocarbons, which might unlock the scope of inexpensive common materials for applications as omniphobic materials. Silica microtextures could also serve as templates for soft materials.

Rendering SiO2/Si Surfaces Omniphobic by Carving Gas-Entrapping Microtextures Comprising Reentrant and Doubly Reentrant Cavities or Pillars

This work presents microfabrication protocols for achieving cavities and pillars with reentrant and doubly reentrant profiles on SiO2/Si wafers using photolithography and dry etching. Resulting microtextured surfaces demonstrate remarkable liquid repellence, characterized by robust long-term entrapment of air under wetting liquids, despite the intrinsic wettability of silica.

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

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