Research is carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-SC0012704.

Caution: Please consult all relevant material safety data sheets (MSDS) before use. Nanomaterials may have additional hazards compared to their bulk counterpart. Please use all appropriate safety practices when handling nanomaterial-covered substrates, including the use of engineering controls (fume hood) and personal protective equipment (safety glasses, gloves, lab coat, full length pants, closed-toe shoes).
1. Preparatory Work
<ol>
	<li>Preparation of vapor deposition system
	<ol>
		<li>Vent the deposition chamber to atmosphere pressure and open the chamber. The venting is done by pressing the &#34;Start PC Venting&#34; button on the control software interface, which automatically starts a sequence that vent the chamber to atmosphere pressure. On reaching the atmosphere pressure open the chamber by pulling the front accessing door.</li>
		<li>Mount a tungsten evaporation boat (alumina coated) between a pair of thermal evaporation electrodes. Place 1 g bismuth pellets into the evaporation boat.</li>
		<li>Mount a vanadium sputtering target to the magnetron sputtering source. Refer to step 1.1.4) for deposition system that is not equipped with a sputtering source.</li>
		<li>(optional, for deposition system that is not equipped with a sputtering source) Mount a tungsten evaporation boat between a pair of thermal evaporation electrodes. Place 0.5 g vanadium slugs into the evaporation boat.</li>
		<li>Connect the mini-banana connectors (two for heating/cooling power and two for temperature probe) of the closed loop temperature controller to the electrical feedthrough of the deposition system.</li>
	</ol>
	</li>
	<li>Preparation of growth substrates 
	Note: The formation of bismuth nanowires is insensitive to the growth substrate of choice. Similar results have been obtained from glass slide, silicon wafer, or sheet metal. It is recommended by the authors that the substrate shall be cleaned immediately prior to the vapor deposition process, in order to achieve a consistent adhesion of the vanadium underlayer. Various substrate cleaning techniques, including plasma cleaning and wet chemical cleaning, can be applied and lead to similar results.
	<ol>
		<li>Cleaning the growth substrates by oxygen plasma
		<ol>
			<li>Place the growth substrates into a plasma cleaner and pump the chamber, by pressing the &#34;VAC ON&#34; button, to its base pressure of 10 mTorr.</li>
			<li>Open the oxygen gas valve and introduce oxygen gas to the chamber by pressing the &#34;GAS ON&#34; button on the front panel and adjust the flow rate by pressing the &#34;INCR&#34; and &#34;DECR&#34; buttons for gas flow rate control to maintain a chamber pressure of about 100 mTorr.</li>
			<li>Set the plasma power at 20 W by pressing the &#34;INCR&#34; and &#34;DECR&#34; buttons for power control and ignite the plasma by pressing the &#34;RF ON&#34; button.</li>
			<li>Wait for 5 min before turning off the plasma by pressing the &#34;RF ON&#34; button. Vent the chamber by pressing the &#34;BLEED&#34; button and retrieve the substrates.</li>
		</ol>
		</li>
		<li>Cleaning the growth substrates by wet chemical method
		<ol>
			<li>Immerse the growth substrates in acetone contained in a beaker. Place the beaker into an ultrasonicator and sonicate for 2 min at the maximum power.</li>
			<li>Remove the substrates from the beaker and rinse them with a stream of absolute alcohol from a wash bottle for 30 sec.</li>
			<li>Dry the substrates in a stream of nitrogen gas.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Substrate loading and deposition system pumping
	<ol>
		<li>Mount the substrate temperature control assembly to the substrate holder.</li>
		<li>Use spring clips to mount the growth substrates on top of the Peltier cooler/heater assembly.</li>
		<li>Mount the fully assembled substrate holder into the vapor deposition chamber, with the substrates facing the deposition sources. Connect the electrical feedthroughs to the Peltier cooler/heater assembly.</li>
		<li>Close the substrate shutter to avoid unintentional deposition to the substrate.</li>
		<li>Start pumping down the deposition chamber. The pumping is done by pressing the &#34;Start PC Pumping&#34; button on the control software interface, which automatically starts a sequence that pump the chamber to its base pressure.</li>
	</ol>
	</li>
</ol>
2. Growth of Bismuth Nanowires&#160;
Note: The experiment is not moving to the next step until the base pressure of the deposition chamber has reached 2 &#215; 10-6 Torr or below.
<ol>
	<li>The deposition of vanadium underlayer 
	Note: The best experimental reproducibility is achieved when the vanadium underlayer is deposited by the magnetron sputtering method. In the absence of a sputtering source, high reproducibility can also be still achieved by depositing the vanadium underlayer using thermal evaporation method, provided that the deposition system has a low base pressure (&#8804; 5 &#215; 10-7 Torr). Refer to step 3.1.2 for the details.
	<ol>
		<li>Vanadium deposition with a magnetron sputtering source.
		<ol>
			<li>Start argon flow in the sputtering source. Set flow rate to 40 sccm.</li>
			<li>Adjust the turbomolecular pump&#39;s revolution rate for a chamber pressure of 2.5 mTorr.</li>
			<li>While the chamber is gradually reaching its steady state pressure, set the thickness calibration factors to the QCM. For vanadium, the density is 5.96 g/cm3 and the Z-factor is 0.530.</li>
			<li>Turn on the DC sputtering source and set the power at 200-250 W. For the deposition system operated by the authors, the deposition rate is about 0.4 A/sec at this power. Without opening the substrate shutter, keep the source running for 2 min. 
			NOTE: Through this step the native oxide on the vanadium source is removed, exposing fresh vanadium surface.</li>
			<li>Open the substrate shutter to start vanadium deposition. In the meantime, reset the accumulated thickness of the QCM to zero.</li>
			<li>Continue deposition until an apparent thickness of 20 nm is accumulated, per QCM reading. Close the substrate shutter.</li>
			<li>Gradually decrease the sputter power to zero. Turn the source off.</li>
			<li>Shut off the argon flow. Return the turbomolecular pump to its full power.</li>
		</ol>
		</li>
		<li>(optional, for deposition system that is not equipped with a sputtering source) Vanadium deposition with a thermal evaporation source.
		<ol>
			<li>Due to the high melting point of vanadium (1,910 &#176;C) and its readiness to oxidation, it is recommended that its thermal evaporation to be conducted at a base pressure of 5 &#215; 10-7 Torr or lower.</li>
			<li>Set the thickness calibration factors to the QCM. For vanadium, the density is 5.96 g/cm3 and the Z-factor is 0.530.</li>
			<li>Turn on the thermal evaporation power supply to the vanadium source. Slowly increase heating power to the tungsten boat until the vanadium slugs melt.</li>
			<li>With the substrate shutter kept closed, slowly increase heating power until a deposition rate of 2 &#197;/sec is achieved, per QCM reading. Open the substrate shutter to start vanadium deposition. In the meantime, reset the accumulated thickness of the QCM to zero.</li>
			<li>Continue deposition until an apparent thickness of 50 nm is accumulated. Close the substrate shutter.</li>
			<li>Gradually decrease the thermal evaporation power to zero. Turn the source off.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>The deposition of bismuth nanowires
	<ol>
		<li>For bismuth deposition at temperature above or below RT, set the desired value to the temperature controller. Wait until the desired temperature is reached.</li>
		<li>Set the thickness calibration factors to the QCM. For bismuth, the density is 9.78 g/cm3 and the Z-factor is 0.790.</li>
		<li>Turn on the thermal evaporation power supply to the bismuth source. Slowly increase heating power to the tungsten boat until the deposition rate of 2 &#197;/sec is achieved, per QCM reading.</li>
		<li>Open the substrate shutter to start bismuth deposition. In the meantime, reset the accumulated thickness of the QCM to zero.</li>
		<li>Continue deposition until an apparent thickness of 50 nm is accumulated. Close the substrate shutter.</li>
		<li>Gradually decrease the thermal evaporation power to zero. Turn the source off.</li>
		<li>Turn off power supply to the thermal electric cooler/heater.</li>
		<li>Vent the deposition chamber to atmosphere pressure and open the chamber. Retrieve the substrate holder and collect the bismuth nanowires covered substrates.</li>
	</ol>
	</li>
</ol>

<ol>
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V., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Generation+of+single+optical+plasmons+in+metallic+nanowires+coupled+to+quantum+dots.">Generation of single optical plasmons in metallic nanowires coupled to quantum dots.</a> Nature. 450, 402-406 (2007).</li><li>Thurn-Albrecht, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultrahigh-density+nanowire+arrays+grown+in+self-assembled+diblock+copolymer+templates.">Ultrahigh-density nanowire arrays grown in self-assembled diblock copolymer templates.</a> Science. 290, 2126-2129 (2000).</li><li>Xia, Y. N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=One-dimensional+nanostructures:+Synthesis,+characterization,+and+applications.">One-dimensional nanostructures: Synthesis, characterization, and applications.</a> Adv. Mater. 15, 353-389 (2003).</li><li>Gudiksen, M. S., Lauhon, L. J., Wang, J., Smith, D. C., Lieber, C. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Growth+of+nanowire+superlattice+structures+for+nanoscale+photonics+and+electronics.">Growth of nanowire superlattice structures for nanoscale photonics and electronics.</a> Nature. 415, 617-620 (2002).</li><li>Yang, P. D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Controlled+growth+of+ZnO+nanowires+and+their+optical+properties.">Controlled growth of ZnO nanowires and their optical properties.</a> Adv. Funct. Mater. 12, 323-331 (2002).</li><li>Allen, J. E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-resolution+detection+of+Au+catalyst+atoms+in+Si+nanowires.">High-resolution detection of Au catalyst atoms in Si nanowires.</a> Nature Nanotech. 3, 168-173 (2008).</li><li>Lin, Y. M., Sun, X. Z., Dresselhaus, M. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Theoretical+investigation+of+thermoelectric+transport+properties+of+cylindrical+Bi+nanowires.">Theoretical investigation of thermoelectric transport properties of cylindrical Bi nanowires.</a> Phys. Rev. B. 62, 4610-4623 (2000).</li><li>Isaacson, R. T., Williams, G. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Alfv&#233;n-Wave+propagation+in+solid-state+plasmas.+III.+Quantum+oscillations+of+the+Fermi+surface+of+bismuth.">Alfv&#233;n-Wave propagation in solid-state plasmas. III. Quantum oscillations of the Fermi surface of bismuth.</a> Phys Rev. 185, 682-688 (1969).</li><li>Sandomirskii, V. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantum+size+effect+in+a+semimetal+film.">Quantum size effect in a semimetal film.</a> Sov. Phys. JETP. 25, 101-106 (1967).</li><li>Huber, T. E., Nikolaeva, A., Gitsu, D., Konopko, L., Graf, M. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantum+confinement+and+surface-state+effects+in+bismuth+nanowires.">Quantum confinement and surface-state effects in bismuth nanowires.</a> Physica E. 37, 194-199 (2007).</li><li>Black, M. R., Lin, Y. M., Cronin, S. B., Rabin, O., Dresselhaus, M. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Infrared+absorption+in+bismuth+nanowires+resulting+from+quantum+confinement.">Infrared absorption in bismuth nanowires resulting from quantum confinement.</a> Phys. Rev. B. 65, 2921-2930 (2002).</li><li>Hofmann, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+surfaces+of+bismuth:+Structural+and+electronic+properties.">The surfaces of bismuth: Structural and electronic properties.</a> Prog. Surf. Sci. 81, 191-245 (2006).</li><li>Huber, T. E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Confinement+effects+and+surface-induced+charge+carriers+in+Bi+quantum+wires.">Confinement effects and surface-induced charge carriers in Bi quantum wires.</a> Appl Phys Lett. 84, 1326-1328 (2004).</li><li>Huber, T. E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Surface+state+band+mobility+and+thermopower+in+semiconducting+bismuth+nanowires.">Surface state band mobility and thermopower in semiconducting bismuth nanowires.</a> Phys. Rev. B. 83, 235414-23 (2011).</li><li>Dresselhaus, M. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> 23, 129-140 (2003).</li><li>Cheng, Y. -. T., Weiner, A. M., Wong, C. A., Balogh, M. P., Lukitsch, M. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stress-induced+growth+of+bismuth+nanowires.">Stress-induced growth of bismuth nanowires.</a> Appl Phys Lett. 81, 3248-3250 (2002).</li><li>Volobuev, V. V., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+mechanism+of+Bi+nanowire+growth+from+Bi/Co+immiscible+composite+thin+films.">The mechanism of Bi nanowire growth from Bi/Co immiscible composite thin films.</a> J. Nanosci. Nanotech. 12, 8624-8629 (2012).</li><li>Shim, W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On-film+formation+of+Bi+nanowires+with+extraordinary+electron+mobility.">On-film formation of Bi nanowires with extraordinary electron mobility.</a> Nano Lett. 9, 18-22 (2009).</li><li>Berglund, S. P., Rettie, A. J. E., Hoang, S., Mullins, C. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Incorporation+of+Mo+and+W+into+nanostructured+BiVO4+films+for+efficient+photoelectrochemical+water+oxidation.">Incorporation of Mo and W into nanostructured BiVO4 films for efficient photoelectrochemical water oxidation.</a> Phys. Chem. Chem. Phys. 14, 7065-7075 (2012).</li><li>Liu, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Surface-Energy+Induced+Formation+of+Single+Crystalline+Bismuth+Nanowires+over+Vanadium+Thin+Film+at+Room+Temperature.">Surface-Energy Induced Formation of Single Crystalline Bismuth Nanowires over Vanadium Thin Film at Room Temperature.</a> Nano Lett. 14, 5630-5635 (2014).</li></ol>

Authors have nothing to disclose.

The growth of bismuth nanowires is to be conducted in a physical vapor deposition system with at least two deposition sources, one for bismuth and another for vanadium. It is recommended that one of the sources is a magnetron sputtering source, for the deposition of vanadium. High vacuum is achieved by a turbomolecular pumps backed by a dry scroll pump. The vapor deposition system is equipped with a calibrated quartz crystal microbalance (QCM) for in situ thickness monitoring. The vapor deposition system has electrical feedthroughs for closed loop temperature control of the growth substrates. A thermoelectric temperature controller provides heating/cooling to the substrate, through a Peltier-type ceramic plate thermoelectric module that is thermally glued to a heat sink. Substrate temperature is monitored by a platinum resistance temperature detector (RTD). Refer to Figure 1 for an illustration of the substrate temperature control unit.
Comparing to existing methods in literature, the present technique allows high yield (&#62;70%) formation of single crystalline bismuth nanowires. The technique is also significant for its scalability: the amount of bismuth nanowires being deposited is only limited by the substrate size. For a successful growth of bismuth nanowires, it is critically important to deposit a nanoporous vanadium thin film that is consistently free of oxidation. Vanadium is chosen for its high melting point (1,910 &#176;C), which makes it easy to form a porous film when deposited over a cold substrate. Other high melting metals, such as titanium (m.p. 1,668 &#176;C), could promote bismuth nanowires growth in a similar fashion. Shown in Figure 2 are the SEM images of vanadium thin films deposited by magnetron sputtering (a) and thermal evaporation (b) methods, which both show significant porosity. As discovered in our previous study, it is necessary for the infiltrated bismuth domains to be non-wetting to vanadium, so that they can be expelled from the porous vanadium matrix to form the nanowires.21 An oxidized vanadium surface, however, is wetted by bismuth and cannot support nanowires growth. Considering vanadium&#39;s vulnerability toward oxidation, the experiment&#39;s success relies on how efficiently the spontaneous oxidation is prevented. It is found that the required consistency is best provided by magnetron sputtering under argon plasma. If thermal evaporation is the only choice for vanadium deposition, however, it is found that the required consistency is achieved when the base pressure is 5 &#215; 10-7 Torr or lower. There are two contributing factors for the advantage of magnetron sputtering over thermal evaporation: 1) in magnetron sputtering the source is much cooler than the case of thermal evaporation, which slows down the oxidation; and 2) in magnetron sputtering the source is exposed to about 2 mTorr of argon flow, which suppresses oxygen partial pressure. In addition, the excessive radiative heat from thermal evaporation heats the deposition substrate very significantly, which makes it difficult to adjust the substrate temperature during the subsequent bismuth deposition, due to the limited power of the thermoelectric heater/cooler. If bismuth is deposited as a smooth and reflective film, it is due to the oxidation of vanadium film during its deposition. To avoid this from happening, the deposition chamber should be pumped for a longer time (such as O/N) to reach its base pressure.
As shown by the SEM images in Figure 3, the morphology of the bismuth deposits varies significantly at different substrate temperature. It is clear that at lowest temperature (273 K) no bismuth nanowire but a grainy film is deposited over the vanadium. Bismuth nanowires form at a substrate temperature as low as 285 K, but are thin (60-80 nm) and short (0.5-1 &#956;m). At RT (298 K) the nanowires grow to 90-120 nm thick and 6-8 &#956;m long. It is noticeable that the nanowire tips are faceted instead of being smoothly rounded, which is typically observed from VLS growth. The reason is that in this case the nanowire&#39;s growth front locates at the bismuth/vanadium interface, where the nanodomains of bismuth are molten. As soon as the molten bismuth erupts from the porous vanadium matrix, crystallization proceeds immediately to give the faceted appearance. The nanowires grow considerably thicker and longer at higher temperature. At 323 K, the nanowires are about 200 nm in diameter and 20-30 &#956;m in length. At 348 K, the nanowires are about 400 nm in diameter and over 100 &#956;m in length. Therefore, it is important to control the substrate temperature within a few Kelvins for consistent formation of bismuth nanowires of desired dimensions. Currently, the technique cannot be used to grow bismuth nanowires with diameter less than 60 nm. On the other hand, it seems that temperature control is not important during vanadium deposition, which is likely because that the substrate is always very cold when compared to the vanadium vapor.
The thermoelectric device illustrated in Figure 1 is the solution for temperature control. With the heat sink held at RT, the substrate can be cooled to 273 K or heated to 373 K. Silver-filled epoxy is used for the thermal contact between the thermoelectric module and the heat sink. It is important that the epoxy is fully cured and dried of any solvent, since the vapor of the solvent may contaminate the substrate surface during vapor deposition and lead to inconsistent results. For the same reason no gel-like thermal paste should be used. Similar practice is made for the contact between the thermoelectric module and the Pt RTD.
In Figure 4(a)(b) we present the transmission electron microscopy (TEM) images of the bismuth nanowires. A survey of electron diffraction patterns (insets, Figure 4(a)(b)) reveals that most of the bismuth nanowires grow along either (1102) or (1210) directions. Despite not being a seed mediated growth such as the vapor-liquid-solid (VLS) mechanism, the bismuth nanowires are single crystalline, due to the presence of a growth front located near the bismuth/vanadium interface, where the liquid-to-solid phase transition happens. The nanowire&#39;s radial cross section could be irregular instead of circular, which leads to the darkness contrast observed in the TEM image shown in Figure 4(a). Powder X-ray diffraction pattern (Figure 4(c)) also confirms that the bismuth nanowires crystalize in their bulk rhombohedral lattice (R3m). As indicated by the energy dispersive X-ray (EDX) analysis in Figure 4(d), the nanowires are pure bismuth without alloying with vanadium (Figure 4(d)). The presence of signal from copper is due to the use of a copper TEM grid.
In summary, a novel technique is demonstrated in this article for scalable and high yield growth of single crystalline bismuth nanowires, induced by surface energy at bismuth/vanadium interface. The technique is capable of growing bismuth nanowires over a wide range of dimensions, simply by tuning the temperature of the growth substrate. It is anticipated that this simple but nontraditional growth mechanism will be further developed for the growth of other material system.

Nanowires confine the transport of charge carriers and other quasiparticles, such as photons and plasmons in one dimension. Accordingly, nanowires usually exhibit novel electrical, magnetic, optical and chemical properties, which grant them nearly infinite potential for applications in micro/nano electronics, photonics, biomedical, environmental and energy-related technologies.1,2 During the past two decades, numerous top-down and bottom-up approaches have been developed to synthesis a broad range of high quality metal or semiconductor nanowires at laboratory scale.3-6 Despite of these developments, each approach relies on certain unique properties of the final product for its success. For instance, the popular vapor-liquid-solid (VLS) method is better fit for the semiconductor materials that have higher melting points and form eutectic alloy with corresponding catalytic &#34;seeds&#34;.7 As a result, the synthesis of a nanowire material of particular interest may not be covered by existing techniques.
As a semimetal with small indirect band overlap (-38 meV at 0 K) and unusually light charge carriers, bismuth is one such example. The material behaves radically different at reduced dimension when compared to its bulk, as quantum confinement could turn bismuth nanowires or thin films into a narrow band gap semiconductor.8-12 In the meantime, the surface of bismuth forms a quasi-two-dimensional metal that is significantly more metallic than its bulk.13,14 It was shown that the surface of bismuth achieves an electron mobility of 2&#215;104 cm2 V-1 sec-1 and contributes strongly to its thermoelectric power in nanowire form.15 As such, there are significant interests on studying bismuth nanowires for electronic and in particular thermoelectric applications.12-16 However, due to bismuth&#39;s very low melting point (544 K) and readiness for oxidation, it remains a challenge to synthesis high quality and single crystalline bismuth nanowires using traditional vapor phase or solution phase techniques.
Previously, it has been reported by a few groups that single crystalline bismuth nanowires grow at low yield during vacuum deposition of bismuth thin film, which is attributed to the release of stress built into the film.17-20 Most recently, we discovered a novel technique that is based on the thermal evaporation of bismuth under high vacuum and leads to the scalable formation of single crystalline bismuth nanowires at high yield.21 Comparing to previously reported methods, the most unique feature of this technique is that the growth substrate is freshly coated with a thin layer of nanoporous vanadium prior to bismuth deposition. During the latter&#39;s thermal evaporation, bismuth vapor infiltrates into the nanoporous structure of the vanadium film and condenses there as nanodomains. Since vanadium is not wetted by condensed bismuth, the infiltrated domains are subsequently expulsed from the vanadium matrix to release their surface energy. It is the continuous expulsion of the bismuth nanodomains that forms the vertical bismuth nanowires. Since the bismuth domains are only 1-2 nm in diameters, they are subject to significant melting point suppression, which makes them nearly molten at RT. As a result, the nanowires growth proceeds with the substrate held at RT. On the other hand, as the migration of the bismuth domains is thermally activated, the length and width of the nanowires can be tuned over a wide range by simply controlling the temperature of the growth substrate. This detailed video protocol is intended to help new practitioners in the field avoid various common problems associated with physical vapor deposition of thin films in a high vacuum, oxygen-free environment.

<table>
	<tbody>
		<tr>
			<td>Bismuth&#160;</td>
			<td>Sigma-Aldrich</td>
			<td>556130</td>
			<td>Granular, 99.999%</td>
		</tr>
		<tr>
			<td>Vanadium Slug</td>
			<td>Alfa Aesar</td>
			<td>42829</td>
			<td>3.175mm (0.125in) dia x 6.35mm (0.25in) length, 99.8%&#160;</td>
		</tr>
		<tr>
			<td>Vanadium Sputtering Target</td>
			<td>Kurt J. Lesker</td>
			<td>EJTVXXX253A2</td>
			<td>3.00&#34; Dia. x 0.125&#34; Thick, 99.5%</td>
		</tr>
		<tr>
			<td>Acetone</td>
			<td>Sigma-Aldrich</td>
			<td>179124</td>
			<td>&#62;99.5%</td>
		</tr>
		<tr>
			<td>Ethanol</td>
			<td>Alfa Aesar</td>
			<td>33361</td>
			<td>Anhydrous</td>
		</tr>
		<tr>
			<td>Silicon Wafer</td>
			<td>University Wafers</td>
			<td></td>
			<td>300 microns in thickness, (100) orientation</td>
		</tr>
		<tr>
			<td>Silver Filled Epoxy</td>
			<td>Circuit Works</td>
			<td>CW2400</td>
			<td>Two part conductive epoxy resin</td>
		</tr>
		<tr>
			<td>Tungsten Boat, Alumina Coated</td>
			<td>R. D. Mathis</td>
			<td>S9B-AO-W</td>
			<td>For bismuth thermal evaporation</td>
		</tr>
		<tr>
			<td>Tungsten Boat</td>
			<td>R. D. Mathis</td>
			<td>S4-.015W</td>
			<td>For vanadium thermal evaporation</td>
		</tr>
		<tr>
			<td>RIE Plasma</td>
			<td>Nordson March</td>
			<td>CS-1701</td>
			<td></td>
		</tr>
		<tr>
			<td>PVD 75 Vapor Deposition Platform</td>
			<td>Kurt J. Lesker</td>
			<td>PEDP75FTCLT001</td>
			<td>Equipped with three thermal evaporation source and one DC magnetron sputtering source</td>
		</tr>
		<tr>
			<td>Thermoelectric Temperature Controller</td>
			<td>LairdTech</td>
			<td>MTTC-1410</td>
			<td></td>
		</tr>
		<tr>
			<td>PT1000 RGD</td>
			<td>LairdTech</td>
			<td>340912-01</td>
			<td>Temperature sensor for MTTC-1410</td>
		</tr>
		<tr>
			<td>Thermoelectric Module</td>
			<td>LairdTech</td>
			<td>56910-502</td>
			<td></td>
		</tr>
		<tr>
			<td>Ultrasonicator</td>
			<td>Crest Ultrasonics</td>
			<td>Tru-Sweep 175</td>
			<td></td>
		</tr>
	</tbody>
</table>

seedless growth of bismuth nanowire array via vacuum thermal evaporation

Here a seedless and template-free technique is demonstrated to scalably grow bismuth nanowires, through thermal evaporation in high vacuum at RT. Conventionally reserved for the fabrication of metal thin films, thermal evaporation deposits bismuth into an array of vertical single crystalline nanowires over a flat thin film of vanadium held at RT, which is freshly deposited by magnetron sputtering or thermal evaporation. By controlling the temperature of the growth substrate the length and width of the nanowires can be tuned over a wide range. Responsible for this novel technique is a previously unknown nanowire growth mechanism that roots in the mild porosity of the vanadium thin film. Infiltrated into the vanadium pores, the bismuth domains (~ 1 nm) carry excessive surface energy that suppresses their melting point and continuously expels them out of the vanadium matrix to form nanowires. This discovery demonstrates the feasibility of scalable vapor phase synthesis of high purity nanomaterials without using any catalysts.

The cross-sectional SEM images of vanadium underlayers formed by magnetron sputtering and thermal evaporation methods are presented in Figure 2. Scanning electron microscopy (SEM) images are presented for bismuth nanowires formed at different substrate temperatures (Figure 3). The crystal structure of the bismuth nanowires is determined through transmission electron microscopy (TEM), selective area electron diffraction (SAED), and X-ray diffraction (XRD) studies (Figure 4). Elemental analysis by energy dispersive X-ray spectroscopy indicates that the bismuth nanowires are not alloyed with the vanadium underlayer (Figure 4).
<img alt="Figure 1" src="/files/ftp_upload/53396/53396fig1.jpg" /> 
Figure 1. Layout of the substrate temperature control unit. The unit is assembled by thermally gluing a Peltier type thermoelectric module to a heat sink using silver filled epoxy. A platinum RTD is glued to the top (working) surface of the module to monitor the working temperature. The growth substrate is fixed to the top of the thermoelectric module by spring clips (not shown). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53396/53396fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/53396/53396fig2.jpg" /> 
Figure 2. The SEM images of vanadium films freshly deposited over silicon substrate, respectively by magnetron sputtering (A) and thermal evaporation (B). As indicated by their vertical cross sections, both films feature a columnar and slightly porous structure. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53396/53396fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/53396/53396fig3.jpg" /> 
Figure 3. SEM images of bismuth deposits with the substrates held at different temperatures: (A) 273K, (B) 285 K, (C) 298 K, (D) 323 K, and (E) 348 K. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53396/53396fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/53396/53396fig4.jpg" /> 
Figure 4. Characterization of the bismuth nanowires (A, B) transmission electron microscopy (TEM), (C) X-ray diffraction (XRD), and (D) energy dispersive X-ray (EDX) spectroscopy. The insets of panels (A) and (B) respectively show the corresponding selective area electron diffraction (SAED) patterns. In panel (C) the X-ray diffraction pattern of the bismuth nanowires is shown in black line, while the vertical red lines indicate the diffraction peak locations and intensities of bulk rhombohedral bismuth, according to its standard power diffraction file (PDF#01-071-4643). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53396/53396fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Seedless Growth of Bismuth Nanowire Array via Vacuum Thermal Evaporation at JoVE.com

Seedless Growth of Bismuth Nanowire Array via Vacuum Thermal Evaporation

A protocol for seedless and high yield growth of bismuth nanowire arrays through vacuum thermal evaporation technique is presented.

Here a seedless and template-free technique is demonstrated to scalably grow bismuth nanowires, through thermal evaporation in high vacuum at RT. ...

seedless-growth-bismuth-nanowire-array-via-vacuum-thermal

Nanyang Technological University

Research

JoVE Journal

Engineering

8.3K Views.  Brookhaven National Laboratory. A protocol for seedless and high yield growth of bismuth nanowire arrays through vacuum thermal evaporation technique is presented. 

Video: Seedless Growth of Bismuth Nanowire Array via Vacuum Thermal Evaporation

Molecular Beam Mass Spectrometry With Tunable Vacuum Ultraviolet (VUV) Synchrotron Radiation

Tunable soft ionization coupled to mass spectroscopy is a powerful method to investigate isolated molecules, complexes and clusters and their spectroscopy and dynamics1-4. Fundamental studies of photoionization processes of biomolecules provide information about the electronic structure of these systems. Furthermore determinations of ionization energies and other properties of biomolecules in the gas phase are not trivial, and these experiments provide a platform to generate these data. We have developed a thermal vaporization technique coupled with supersonic molecular beams that provides a gentle way to transport these species into the gas phase. Judicious combination of source gas and temperature allows for formation of dimers and higher clusters of the DNA bases. The focus of this particular work is on the effects of non-covalent interactions, i.e., hydrogen bonding, stacking, and electrostatic interactions, on the ionization energies and proton transfer of individual biomolecules, their complexes and upon micro-hydration by water1, 5-9.
We have performed experimental and theoretical characterization of the photoionization dynamics of gas-phase uracil and 1,3-dimethyluracil dimers using molecular beams coupled with synchrotron radiation at the Chemical Dynamics Beamline10 located at the Advanced Light Source and the experimental details are visualized here. This allowed us to observe the proton transfer in 1,3-dimethyluracil dimers, a system with pi stacking geometry and with no hydrogen bonds1. Molecular beams provide a very convenient and efficient way to isolate the sample of interest from environmental perturbations which in return allows accurate comparison with electronic structure calculations11, 12. By tuning the photon energy from the synchrotron, a photoionization efficiency (PIE) curve can be plotted which informs us about the cationic electronic states. These values can then be compared to theoretical models and calculations and in turn, explain in detail the electronic structure and dynamics of the investigated species 1, 3.

Molecular Beam Mass Spectrometry With Tunable Vacuum Ultraviolet VUV Synchrotron Radiation

A molecular beam coupled to tunable vacuum ultraviolet photoionization mass spectrometer at a synchrotron provides a convenient tool to explore the electronic structure of isolated gas phase molecules and clusters. Proton transfer mechanisms in DNA base dimers were elucidated with this technique.

Tunable soft ionization coupled to mass spectroscopy is a powerful method to investigate isolated molecules, complexes and clusters and their spectroscopy ...

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 ...

Characterization of Thermal Transport in One-dimensional Solid Materials

The TET (transient electro-thermal) technique is an effective approach developed to measure the thermal diffusivity of solid materials, including conductive, semi-conductive or nonconductive one-dimensional structures. This technique broadens the measurement scope of materials (conductive and nonconductive) and improves the accuracy and stability. If the sample (especially biomaterials, such as human head hair, spider silk, and silkworm silk) is not conductive, it will be coated with a gold layer to make it electronically conductive. The effect of parasitic conduction and radiative losses on the thermal diffusivity can be subtracted during data processing. Then the real thermal conductivity can be calculated with the given value of volume-based specific heat (&#961;cp), which can be obtained from calibration, noncontact photo-thermal technique or measuring the density and specific heat separately. In this work, human head hair samples are used to show how to set up the experiment, process the experimental data, and subtract the effect of parasitic conduction and radiative losses.

The TET (transient electro-thermal) technique is an effective approach developed to measure the thermal diffusivity of solid materials.

The TET (transient electro-thermal) technique is an effective approach developed to measure the thermal diffusivity of solid materials, including ...

The Preparation of Electrohydrodynamic Bridges from Polar Dielectric Liquids

Horizontal and vertical liquid bridges are simple and powerful tools for exploring the interaction of high intensity electric fields (8-20 kV/cm) and polar dielectric liquids. These bridges are unique from capillary bridges in that they exhibit extensibility beyond a few millimeters, have complex bi-directional mass transfer patterns, and emit non-Planck infrared radiation. A number of common solvents can form such bridges as well as low conductivity solutions and colloidal suspensions. The macroscopic behavior is governed by electrohydrodynamics and provides a means of studying fluid flow phenomena without the presence of rigid walls. Prior to the onset of a liquid bridge several important phenomena can be observed including advancing meniscus height (electrowetting), bulk fluid circulation (the Sumoto effect), and the ejection of charged droplets (electrospray). The interaction between surface, polarization, and displacement forces can be directly examined by varying applied voltage and bridge length. The electric field, assisted by gravity, stabilizes the liquid bridge against Rayleigh-Plateau instabilities. Construction of basic apparatus for both vertical and horizontal orientation along with operational examples, including thermographic images, for three liquids (e.g., water, DMSO, and glycerol) is presented.

Horizontal and vertical electrohydrodynamic liquid bridges are simple and powerful tools for exploring the interaction of high intensity electric fields and polar dielectric liquids. The construction of basic apparatus and operational examples, including thermographic images, for three liquids (e.g., water, DMSO, and glycerol) is presented.

Horizontal and vertical liquid bridges are simple and powerful tools for exploring the interaction of high intensity electric fields (8-20 kV/cm) and ...

Making Record-efficiency SnS Solar Cells by Thermal Evaporation and Atomic Layer Deposition

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent thermal evaporation, and it absorbs visible light strongly. However, for a long time the record power conversion efficiency of SnS solar cells remained below 2%. Recently we demonstrated new certified record efficiencies of 4.36% using SnS deposited by atomic layer deposition, and 3.88% using thermal evaporation. Here the fabrication procedure for these record solar cells is described, and the statistical distribution of the fabrication process is reported. The standard deviation of efficiency measured on a single substrate is typically over 0.5%. All steps including substrate selection and cleaning, Mo sputtering for the rear contact (cathode), SnS deposition, annealing, surface passivation, Zn(O,S) buffer layer selection and deposition, transparent conductor (anode) deposition, and metallization are described. On each substrate we fabricate 11 individual devices, each with active area 0.25 cm2. Further, a system for high throughput measurements of current-voltage curves under simulated solar light, and external quantum efficiency measurement with variable light bias is described. With this system we are able to measure full data sets on all 11 devices in an automated manner and in minimal time. These results illustrate the value of studying large sample sets, rather than focusing narrowly on the highest performing devices. Large data sets help us to distinguish and remedy individual loss mechanisms affecting our devices.

Tin sulfide (SnS) is a candidate material for Earth-abundant, non-toxic solar cells. Here, we demonstrate the fabrication procedure of the SnS solar cells employing atomic layer deposition, which yields 4.36% certified power conversion efficiency, and thermal evaporation which yields 3.88%.

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent ...

Thermal Measurement Techniques in Analytical Microfluidic Devices

Thermal measurement techniques have been used for many applications such as thermal characterization of materials and chemical reaction detection. Micromachining techniques allow reduction of the thermal mass of fabricated structures and introduce the possibility to perform high sensitivity thermal measurements in the micro-scale and nano-scale devices. Combining thermal measurement techniques with microfluidic devices allows performing different analytical measurements with low sample consumption and reduced measurement time by integrating the miniaturized system on a single chip. The procedures of thermal measurement techniques for particle detection, material characterization, and chemical detection are introduced in this paper.

Here, we present three protocols for thermal measurements in microfluidic devices.

Thermal measurement techniques have been used for many applications such as thermal characterization of materials and chemical reaction detection. ...

A High Performance Impedance-based Platform for Evaporation Rate Detection

This paper describes the method of a novel impedance-based platform for the detection of the evaporation rate. The model compound hyaluronic acid was employed here for demonstration purposes. Multiple evaporation tests on the model compound as a humectant with various concentrations in solutions were conducted for comparison purposes. A conventional weight loss approach is known as the most straightforward, but time-consuming, measurement technique for evaporation rate detection. Yet, a clear disadvantage is that a large volume of sample is required and multiple sample tests cannot be conducted at the same time. For the first time in literature, an electrical impedance sensing chip is successfully applied to a real-time evaporation investigation in a time sharing, continuous and automatic manner. Moreover, as little as 0.5 ml of test samples is required in this impedance-based apparatus, and a large impedance variation is demonstrated among various dilute solutions. The proposed high-sensitivity and fast-response impedance sensing system is found to outperform a conventional weight loss approach in terms of evaporation rate detection.

This paper presents an impedance-based apparatus for evaporation rate detection of solutions. It offers clear advantages over a conventional weight loss approach: a fast response, high-sensitivity detection, a small sample requirement, multiple sample measurements, and easy disassembly for cleaning and reuse purposes.

This paper describes the method of a novel impedance-based platform for the detection of the evaporation rate. The model compound hyaluronic acid was ...

Flow-assisted Dielectrophoresis: A Low Cost Method for the Fabrication of High Performance Solution-processable Nanowire Devices

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of nanowires. DEP is used for nanowire analysis, characterization, and for solution-based fabrication of semiconducting devices. The method works by applying an alternating electric field between metallic electrodes. The nanowire formulation is then dropped onto the electrodes which are on an inclined surface to create a flow of the formulation using gravity. The nanowires then align along the gradient of the electric field and in the direction of the liquid flow. The frequency of the field can be adjusted to select nanowires with superior conductivity and lower trap density. 
In this work, flow-assisted DEP is used to create nanowire field effect transistors. Flow-assisted DEP has several advantages: it allows selection of nanowire electrical properties; control of nanowire length; placement of nanowires in specific areas; control of orientation of nanowires; and control of nanowire density in the device. 
The technique can be expanded to many other applications such as gas sensors and microwave switches. The technique is efficient, quick, reproducible, and it uses a minimal amount of dilute solution making it ideal for the testing of novel nanomaterials. Wafer scale assembly of nanowire devices can also be achieved using this technique, allowing large numbers of samples for testing and large-area electronic applications.

In this paper, flow assisted dielectrophoresis is demonstrated for the self-assembly of nanowire devices. The fabrication of a silicon nanowire field effect transistor is shown as an example.

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of ...

Production and Characterization of Vacuum Deposited Organic Light Emitting Diodes

A method for producing simple and efficient thermally-activated delayed fluorescence organic light-emitting diodes (OLEDs) based on guest-host or exciplex donor-acceptor emitters is presented. With a step-by-step procedure, readers will be able to repeat and produce OLED devices based on simple organic emitters. A patterning procedure allowing the creation of personalized indium tin oxide (ITO) shape is shown. This is followed by the evaporation of all layers, encapsulation and characterization of each individual device. The end goal is to present a procedure that will give the opportunity to repeat the information presented in cited publication but also using different compounds and structures in order to prepare efficient OLEDs.

A protocol for the production of simple structured organic light-emitting diodes (OLEDs) is presented.

A method for producing simple and efficient thermally-activated delayed fluorescence organic light-emitting diodes (OLEDs) based on guest-host or exciplex ...

Quantitative Analysis of Vacuum Induction Melting by Laser-induced Breakdown Spectroscopy

Vacuum induction melting is a popular method for refining high purity metal and alloys. Traditionally, standard process control in metallurgy involves several steps, include drawing samples, cooling, cutting, transport to the laboratory, and analysis. The whole analysis process requires more than 30 minutes, which hinders on-line process control. Laser-induced breakdown spectroscopy is an excellent on-line analysis method that can satisfy the requirements of vacuum induction melting because it is fast and noncontact and does not require sample preparation. The experimental facility uses a lamp-pumped Q-switched laser to ablate melted liquid steel with an output energy of 80 mJ, a frequency of 5 Hz, a FWHM pulse width of 20 ns, and a working wavelength of 1,064 nm. A multi-channel linear charge coupled device (CCD) spectrometer is used to measure the emission spectrum in real time, with a spectral range from 190 to 600 nm and a resolution of 0.06 nm at a wavelength of 200 nm. The protocol includes several steps: standard alloy sample preparation and an ingredient test, smelting of standard samples and determination of the laser breakdown spectrum, and construction of the elements concentration quantitative analysis curve of each element. To realize the concentration analysis of unknown samples, the spectrum of a sample also needs to be measured and disposed with the same process. The composition of all main elements in the melted alloy can be quantitatively analyzed with an internal standard method. The calibration curve shows that the limit of detection of most metal elements ranges from 20-250 ppm. The concentration of elements, such as Ti, Mo, Nb, V, and Cu, can be lower than 100 ppm, and the concentrations of Cr, Al, Co, Fe, Mn, C, and Si range from 100-200 ppm. The R2 of some calibration curves can exceed 0.94.

During vacuum induction melting, laser-induced breakdown spectroscopy is used to perform real-time quantitative analysis of the main-ingredient elements of a molten alloy.

Vacuum induction melting is a popular method for refining high purity metal and alloys. Traditionally, standard process control in metallurgy involves ...