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>
	<li>Hu, J. T., Odom, T. W., 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=Chemistry+and+physics+in+one+dimension:+Synthesis+and+properties+of+nanowires+and+nanotubes.">Chemistry and physics in one dimension: Synthesis and properties of nanowires and nanotubes.</a> Acc. Chem. Res. 32, 435-445 (1999).</li><li>Akimov, A. 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</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>3.175&amp;nbsp;mm (0.125&amp;nbsp;in) dia x 6.35&amp;nbsp;mm (0.25&amp;nbsp;in) length, 99.8%</td></tr><tr><td>Vanadium Sputtering Target</td><td>Kurt J. Lesker</td><td>EJTVXXX253A2</td><td>3.00&quot; Dia. x 0.125&quot; Thick, 99.5%</td></tr><tr><td>Acetone</td><td>Sigma-Aldrich</td><td>179124</td><td>&gt;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. ...

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Nanyang Technological University

Research

JoVE Journal

Engineering

8.4K 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

Fabrication, Densification, and Replica Molding of 3D Carbon Nanotube Microstructures

The introduction of new materials and processes to microfabrication has, in large part, enabled many important advances in microsystems, lab-on-a-chip devices, and their applications. In particular, capabilities for cost-effective fabrication of polymer microstructures were transformed by the advent of soft lithography and other micromolding techniques 1, 2, and this led a revolution in applications of microfabrication to biomedical engineering and biology. Nevertheless, it remains challenging to fabricate microstructures with well-defined nanoscale surface textures, and to fabricate arbitrary 3D shapes at the micro-scale. Robustness of master molds and maintenance of shape integrity is especially important to achieve high fidelity replication of complex structures and preserving their nanoscale surface texture. The combination of hierarchical textures, and heterogeneous shapes, is a profound challenge to existing microfabrication methods that largely rely upon top-down etching using fixed mask templates. On the other hand, the bottom-up synthesis of nanostructures such as nanotubes and nanowires can offer new capabilities to microfabrication, in particular by taking advantage of the collective self-organization of nanostructures, and local control of their growth behavior with respect to microfabricated patterns. 
Our goal is to introduce vertically aligned carbon nanotubes (CNTs), which we refer to as CNT "forests", as a new microfabrication material. We present details of a suite of related methods recently developed by our group: fabrication of CNT forest microstructures by thermal CVD from lithographically patterned catalyst thin films; self-directed elastocapillary densification of CNT microstructures; and replica molding of polymer microstructures using CNT composite master molds. In particular, our work shows that self-directed capillary densification ("capillary forming"), which is performed by condensation of a solvent onto the substrate with CNT microstructures, significantly increases the packing density of CNTs. This process enables directed transformation of vertical CNT microstructures into straight, inclined, and twisted shapes, which have robust mechanical properties exceeding those of typical microfabrication polymers. This in turn enables formation of nanocomposite CNT master molds by capillary-driven infiltration of polymers. The replica structures exhibit the anisotropic nanoscale texture of the aligned CNTs, and can have walls with sub-micron thickness and aspect ratios exceeding 50:1. Integration of CNT microstructures in fabrication offers further opportunity to exploit the electrical and thermal properties of CNTs, and diverse capabilities for chemical and biochemical functionalization 3.

We present methods for fabrication of patterned microstructures of vertically aligned carbon nanotubes (CNTs), and their use as master molds for production of polymer microstructures with organized nanoscale surface texture. The CNT forests are densified by condensation of solvent onto the substrate, which significantly increases their packing density and enables self-directed formation of 3D shapes.

The introduction of new materials and processes to microfabrication has, in large part, enabled many important advances in microsystems, lab-on-a-chip ...

Dry Oxidation and Vacuum Annealing Treatments for Tuning the Wetting Properties of Carbon Nanotube Arrays

In this article, we describe a simple method to reversibly tune the wetting properties of vertically aligned carbon nanotube (CNT) arrays. Here, CNT arrays are defined as densely packed multi-walled carbon nanotubes oriented perpendicular to the growth substrate as a result of a growth process by the standard thermal chemical vapor deposition (CVD) technique.1,2 These CNT arrays are then exposed to vacuum annealing treatment to make them more hydrophobic or to dry oxidation treatment to render them more hydrophilic. The hydrophobic CNT arrays can be turned hydrophilic by exposing them to dry oxidation treatment, while the hydrophilic CNT arrays can be turned hydrophobic by exposing them to vacuum annealing treatment. Using a combination of both treatments, CNT arrays can be repeatedly switched between hydrophilic and hydrophobic.2 Therefore, such combination show a very high potential in many industrial and consumer applications, including drug delivery system and high power density supercapacitors.3-5
The key to vary the wettability of CNT arrays is to control the surface concentration of oxygen adsorbates. Basically oxygen adsorbates can be introduced by exposing the CNT arrays to any oxidation treatment. Here we use dry oxidation treatments, such as oxygen plasma and UV/ozone, to functionalize the surface of CNT with oxygenated functional groups. These oxygenated functional groups allow hydrogen bond between the surface of CNT and water molecules to form, rendering the CNT hydrophilic. To turn them hydrophobic, adsorbed oxygen must be removed from the surface of CNT. Here we employ vacuum annealing treatment to induce oxygen desorption process. CNT arrays with extremely low surface concentration of oxygen adsorbates exhibit a superhydrophobic behavior.

This article describes a simple method to fabricate vertically aligned carbon nanotube arrays by CVD and to subsequently tune their wetting properties by exposing them to vacuum annealing or dry oxidation treatment.

In this article, we describe a simple method to reversibly  tune the wetting properties of vertically aligned carbon nanotube (CNT) arrays.  Here, CNT ...

Ultrahigh Density Array of Vertically Aligned Small-molecular Organic Nanowires on Arbitrary Substrates

In recent years &pi;-conjugated organic semiconductors have emerged as the active material in a number of diverse applications including large-area, low-cost displays, photovoltaics, printable and flexible electronics and organic spin valves. Organics allow (a) low-cost, low-temperature processing and (b) molecular-level design of electronic, optical and spin transport characteristics. Such features are not readily available for mainstream inorganic semiconductors, which have enabled organics to carve a niche in the silicon-dominated electronics market. The first generation of organic-based devices has focused on thin film geometries, grown by physical vapor deposition or solution processing. However, it has been realized that organic nanostructures can be used to enhance performance of above-mentioned applications and significant effort has been invested in exploring methods for organic nanostructure fabrication.	
A particularly interesting class of organic nanostructures is the one in which vertically oriented organic nanowires, nanorods or nanotubes are organized in a well-regimented, high-density array. Such structures are highly versatile and are ideal morphological architectures for various applications such as chemical sensors, split-dipole nanoantennas, photovoltaic devices with radially heterostructured "core-shell" nanowires, and memory devices with a cross-point geometry. Such architecture is generally realized by a template-directed approach. In the past this method has been used to grow metal and inorganic semiconductor nanowire arrays. More recently &pi;-conjugated polymer nanowires have been grown within nanoporous templates. However, these approaches have had limited success in growing nanowires of technologically important &pi;-conjugated small molecular weight organics, such as tris-8-hydroxyquinoline aluminum (Alq3), rubrene and methanofullerenes, which are commonly used in diverse areas including organic displays, photovoltaics, thin film transistors and spintronics.	
Recently we have been able to address the above-mentioned issue by employing a novel "centrifugation-assisted" approach. This method therefore broadens the spectrum of organic materials that can be patterned in a vertically ordered nanowire array. Due to the technological importance of Alq3, rubrene and methanofullerenes, our method can be used to explore how the nanostructuring of these materials affects the performance of aforementioned organic devices. The purpose of this article is to describe the technical details of the above-mentioned protocol, demonstrate how this process can be extended to grow small-molecular organic nanowires on arbitrary substrates and finally, to discuss the critical steps, limitations, possible modifications, trouble-shooting and future applications.

We report a simple method for fabricating an ultrahigh density array of vertically ordered small-molecular organic nanowires. This method allows for synthesis of complex heterostructured hybrid nanowire geometries, which can be inexpensively grown on arbitrary substrates. These structures have potential applications in organic electronics, optoelectronics, chemical sensing, photovoltaics and spintronics.

In  recent years &pi;-conjugated  organic semiconductors have emerged as the active material in a number of  diverse applications including large-area, ...

Analysis of Contact Interfaces for Single GaN Nanowire Devices

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

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

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

Monolayer Contact Doping of Silicon Surfaces and Nanowires Using Organophosphorus Compounds

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

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

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

Atomically Defined Templates for Epitaxial Growth of Complex Oxide Thin Films

Atomically defined substrate surfaces are prerequisite for the epitaxial growth of complex oxide thin films. In this protocol, two approaches to obtain such surfaces are described. The first approach is the preparation of single terminated perovskite SrTiO3 (001) and DyScO3 (110) substrates. Wet etching was used to selectively remove one of the two possible surface terminations, while an annealing step was used to increase the smoothness of the surface. The resulting single terminated surfaces allow for the heteroepitaxial growth of perovskite oxide thin films with high crystalline quality and well-defined interfaces between substrate and film. In the second approach, seed layers for epitaxial film growth on arbitrary substrates were created by Langmuir-Blodgett (LB) deposition of nanosheets. As model system Ca2Nb3O10- nanosheets were used, prepared by delamination of their layered parent compound HCa2Nb3O10. A key advantage of creating seed layers with nanosheets is that relatively expensive and size-limited single crystalline substrates can be replaced by virtually any substrate material.

Various procedures are outlined to prepare atomically defined templates for epitaxial growth of complex oxide thin films. Chemical treatments of single crystalline SrTiO3 (001) and DyScO3 (110) substrates were performed to obtain atomically smooth, single terminated surfaces. Ca2 Nb3 O10- nanosheets were used to create atomically defined templates on arbitrary substrates.

Atomically defined substrate surfaces are prerequisite for the epitaxial growth of complex oxide thin films. In this protocol, two approaches to obtain ...

Chemistry

Fabrication of Carbon Nanotube High-Frequency Nanoelectronic Biosensor for Sensing in High Ionic Strength Solutions

The unique electronic properties and high surface-to-volume ratios of single-walled carbon nanotubes (SWNT) and semiconductor nanowires (NW) 1-4 make them good candidates for high sensitivity biosensors. When a charged molecule binds to such a sensor surface, it alters the carrier density5 in the sensor, resulting in changes in its DC conductance. However, in an ionic solution a charged surface also attracts counter-ions from the solution, forming an electrical double layer (EDL). This EDL effectively screens off the charge, and in physiologically relevant conditions ~100 millimolar (mM), the characteristic charge screening length (Debye length) is less than a nanometer (nm). Thus, in high ionic strength solutions, charge based (DC) detection is fundamentally impeded6-8.
We overcome charge screening effects by detecting molecular dipoles rather than charges at high frequency, by operating carbon nanotube field effect transistors as high frequency mixers9-11. At high frequencies, the AC drive force can no longer overcome the solution drag and the ions in solution do not have sufficient time to form the EDL. Further, frequency mixing technique allows us to operate at frequencies high enough to overcome ionic screening, and yet detect the sensing signals at lower frequencies11-12. Also, the high transconductance of SWNT transistors provides an internal gain for the sensing signal, which obviates the need for external signal amplifier.
Here, we describe the protocol to (a) fabricate SWNT transistors, (b) functionalize biomolecules to the nanotube13, (c) design and stamp a poly-dimethylsiloxane (PDMS) micro-fluidic chamber14 onto the device, and (d) carry out high frequency sensing in different ionic strength solutions11.

We describe the device fabrication and measurement protocol for carbon nanotube based high frequency biosensors. The high frequency sensing technique mitigates the fundamental ionic (Debye) screening effect and allows nanotube biosensor to be operated in high ionic strength solutions where conventional electronic biosensors fail. Our technology provides a unique platform for point-of-care (POC) electronic biosensors operating in physiologically relevant conditions.

The unique electronic properties and high surface-to-volume ratios of single-walled carbon nanotubes (SWNT) and semiconductor nanowires (NW) 1-4 make them ...

Bioengineering

Synthesis of Substrate-Bound Au Nanowires Via an Active Surface Growth Mechanism

Advancing synthetic capabilities is important for the development of nanoscience and nanotechnology. The synthesis of nanowires has always been a challenge, as it requires asymmetric growth of symmetric crystals. Here, we report a distinctive synthesis of substrate-bound Au nanowires. This template-free synthesis employs thiolated ligands and substrate adsorption to achieve the continuous asymmetric deposition of Au in solution at ambient conditions. The thiolated ligand prevented the Au deposition on the exposed surface of the seeds, so the Au deposition only occurs at the interface between the Au seeds and the substrate. The side of the newly deposited Au nanowires is immediately covered with the thiolated ligand, while the bottom facing the substrate remains ligand-free and active for the next round of Au deposition. We further demonstrate that this Au nanowire growth can be induced on various substrates, and different thiolated ligands can be used to regulate the surface chemistry of the nanowires. The diameter of the nanowires can also be controlled with mixed ligands, in which another &#34;bad&#34; ligand could turn on the lateral growth. With the understanding of the mechanism, Au nanowire-based nanostructures can be designed and synthesized.

We report a solution-based method to synthesize substrate-bound Au nanowires. By tuning the molecular ligands used during the synthesis, the Au nanowires can be grown from various substrates with different surface properties. Au nanowire-based nanostructures can also be synthesized by adjusting the reaction parameters.

Advancing synthetic capabilities is important for the development of nanoscience and nanotechnology. The synthesis of nanowires has always been a ...

A Standard and Reliable Method to Fabricate Two-Dimensional Nanoelectronics

Two-dimensional (2D) materials have attracted huge attention due to their unique properties and potential applications. Since wafer scale synthesis of 2D materials is still in nascent stages, scientists cannot fully rely on traditional semiconductor techniques for related research. Delicate processes from locating the materials to electrode definition need to be well controlled. In this article, a universal fabrication protocol required in manufacturing nanoscale electronics, such as 2D quasi-heterojunction bipolar transistors (Q-HBT), and 2D back-gated transistors are demonstrated. This protocol includes the determination of material position, electron beam lithography (EBL), metal electrode definition, et al. A step by step narrative of the fabrication procedures for these devices are also presented. Furthermore, results show that each of the fabricated devices has achieved high performance with high repeatability. This work reveals a comprehensive description of process flow for preparing 2D nano-electronics, enables the research groups to access this information, and pave the way toward future electronics.

The article aims to introduce a standard and reliable fabrication procedure for the development of future low dimensional nanoelectronics.

Two-dimensional (2D) materials have attracted huge attention due to their unique properties and potential applications. Since wafer scale synthesis of 2D ...

Fabrication of Schottky Diodes on Zn-polar BeMgZnO/ZnO Heterostructure Grown by Plasma-assisted Molecular Beam Epitaxy

Heterostructure field effect transistors (HFETs) utilizing a two dimensional electron gas (2DEG) channel have a great potential for high speed device applications. Zinc oxide (ZnO), a semiconductor with a wide bandgap (3.4 eV) and high electron saturation velocity has gained a great deal of attention as an attractive material for high speed devices. Efficient gate modulation, however, requires high-quality Schottky contacts on the barrier layer. In this article, we present our Schottky diode fabrication procedure on Zn-polar BeMgZnO/ZnO heterostructure with high density 2DEG which is achieved through strain modulation and incorporation of a few percent Be into the MgZnO-based barrier during growth by molecular beam epitaxy (MBE). To achieve high crystalline quality, nearly lattice-matched high-resistivity GaN templates grown by metal-organic chemical vapor deposition (MOCVD) are used as the substrate for the subsequent MBE growth of the oxide layers. To obtain the requisite Zn-polarity, careful surface treatment of GaN templates and control over the VI/II ratio during the growth of low temperature ZnO nucleation layer are utilized. Ti/Au electrodes serve as Ohmic contacts, and Ag electrodes deposited on the O2 plasma pretreated BeMgZnO surface are used for Schottky contacts.

Attainment of high-quality Schottky contacts is imperative for achieving efficient gate modulation in heterostructure field effect transistors (HFETs). We present the fabrication methodology and characteristics of Schottky diodes on Zn-polar BeMgZnO/ZnO heterostructures with high-density two dimensional electron gas (2DEG), grown by plasma-assisted molecular beam epitaxy on GaN templates.

Heterostructure field effect transistors (HFETs) utilizing a two dimensional electron gas (2DEG) channel have a great potential for high speed device ...

Seedless Growth of Bismuth Nanowire Array via Vacuum Thermal Evaporation

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