This research was supported by the 1,000 Talents Program for Young Scientists of China, National Natural Science Foundation of China (Grant No. 51472164), the A*STAR Pharos Programme (Grant No. 152 70 00014), and facility support from the NUS Center for Advanced 2D Materials.

Caution: Some of the chemical agents and gases used in this work are toxic, carcinogenic, flammable, and explosive. Please use all appropriate safety practices when performing a vapor transport deposition including the use of engineering controls (fume hood) and personal protective equipment (safety glasses, professional protective masks, gloves, lab coat, full length pants, and closed-toe shoes).
1. Auto-Tune Function of Temperature Controller Parameters
NOTE: Before...

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Here, the combination of a vapor transport deposition method and a nitrogen etching technique in an atmospheric pressure system is first reported. In this protocol, the critical steps are the section of the fabrication of single-layer SnSe flakes.
Although the bulk samples can be etched to form a high-quality single-layer sample, the thickness of the bulk samples should be uniform and the decomposition temperature of bulk samples should be higher than the etching temperature. The resulting sam...

Research into two dimensional (2D) materials has bloomed in recent years since the successful isolation of graphene, due to the possibility of 2D materials having superior electrical, optical, and mechanical properties over their bulk counterparts1,2,3,4,5. 2D materials show promising applications in optoelectronic and electronic devices6,7, catalysis and water splitting8,9, surface-enhanced Raman scattering sensing10,11, etc. The large family of layered materials which can be exfoliated into 2D materials show great diversity, ranging from the semi-metallic graphene to the semiconducting transition-metal dichalcogenides (TMDs) and black phosphorus (BP) to the insulating hexagonal boron nitride (h-BN). These materials and their heterostructures have been well studied in recent years, and have exhibited many novel properties and applications12. Other less studied, but equally promising 2D layered materials in the IIIA-VIA (GaS, GaSe, and InSe)13,14 and IVA-VIA (GeS, GeSe, and SnS)15,16,17 families have also recently received attention.
SnSe belongs to the IVA-VIA group and crystallizes in an orthorhombic structure, with the atoms arranged in the pnma space group and buckled within the layer, like the crystal structure of phosphorene. SnSe is a narrow gap semiconductor with a band gap of 0.6 eV, but is more well known for its more unique thermoelectric properties, as it is reported to have a very high ZT (thermoelectric figure of merit) value of 2.6 at 923 K18,19, which has been attributed to its unique electronic structure and low thermal conductivity. While bulk SnSe crystals are available commercially and can be grown by known methods, such as the Bridgeman-Stockbarger method20 or the chemical vapor transport method21, growing large sized few-layer and single-layer SnSe on dielectric substrates is more challenging. There are many substrates to support 2D material growth, such as highly oriented pyrolytic graphite (HOPG), mica, SiO2, Si3N4, and glass. Low-cost SiO2 dielectrics are the most commonly used substrate, as these allow the fabrication of field-effect transistors, where the dielectrics serve as part of the electrical back gate. In our experience, unlike graphene and TMDs, it is difficult to obtain few-layer or single-layer SnSe flakes by the micromechanical exfoliation method, as bulk SnSe has a high interlayer binding energy22 of 32 meV/&#197;2, which leads to thick layers, even along the edges of the exfoliated flakes. Therefore, to study the novel electronic properties of few layer and single layer SnSe, a new, simple, and low-cost synthetic method to prepare high quality large-sized single-layer SnSe crystals on insulating substrates is required, especially since SnSe has shown great promise as a candidate for thermoelectric applications for energy conversion in the low and moderate temperature range19.
Several researchers have developed methods to synthesize high quality SnSe crystals. Liu et al.23 and Franzman et al.24 used a solution-phase method to synthesize SnSe nanocrystals of different shapes, such as quantum dots, nanoplates, single crystalline nanosheets, nanoflowers, and nanopolyhedra using SnCl2 and alkyl-phosphine-selenium or dialkyl diselenium as precursors. Baumgardner et al.25 synthesized colloidal SnSe nanoparticles by injecting bis[bis(trimethylsilyl)amino]tin(II) into hot trioctylphosphine, and they obtained nanocrystals of ~4-10 nm in diameter. Boscher et al.26 used an atmospheric pressure chemical vapor deposition technique to obtain SnSe films on glass substrates using tin tetrachloride and diethyl selenide precursors with a tin tetrachloride ratio 10 larger than diethyl selenide, and their synthesized SnSe films were about 100 nm thick and silver-black in appearance. Zhao et al.27 used vapor transport deposition in a low vacuum system and synthesized single-crystal SnSe nanoplates on mica substrates and obtained square nanoplates of 1-6 &#181;m. However, obtaining single-layer SnSe crystals are not possible using these techniques. Li et al.28 successfully synthesized single-layer single-crystal SnSe nanosheets using a one-pot synthetic method with SnCl4 and SeO2 precursors. However, they were only able to obtain a lateral size of about 300 nm for their nanosheets. We have recently published our method to grow high quality, large-sized single-layer SnSe crystals which are phase pure29. This detailed protocol is intended to help new practitioners to grow other large-sized high-quality ultrathin 2D materials using this methodology.

<table border="0" cellpadding="0" cellspacing="0" width="1083">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">SnSe powder&#160;&#160;</td>
			<td style="width:295px;">Sigma-Aldrich</td>
			<td style="width:159px;">1315-06-6</td>
			<td style="width:347px;">(99.999%) toxic, carcinogenic</td>
		</tr>
		<tr height="28">
			<td height="28" style="height:28px;width:283px;">Ar gas</td>
			<td style="width:295px;"></td>
			<td style="width:159px;"></td>
			<td>explosive</td>
		</tr>
		<tr height="32">
			<td height="32" style="height:32px;width:283px;">H2 gas</td>
			<td style="width:295px;"></td>
			<td style="width:159px;"></td>
			<td>flammable, explosive</td>
		</tr>
		<tr height="32">
			<td height="32" style="height:32px;">SiO2/Si wafer</td>
			<td style="width:295px;"></td>
			<td style="width:159px;"></td>
			<td>300 nm thick SiO2 on heavily doped Si</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Acetone</td>
			<td style="width:295px;">Sigma-Aldrich</td>
			<td>67-64-1</td>
			<td>toxic, flammable</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Isopropanol</td>
			<td style="width:295px;">Sigma-Aldrich</td>
			<td>67-63-0</td>
			<td>flammable</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Quartz tube</td>
			<td>Dongjing Quartz Company, China</td>
			<td style="width:159px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Ceramic boat</td>
			<td style="width:295px;">Dongjing Quartz Company, China</td>
			<td style="width:159px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Optical microscope</td>
			<td style="width:295px;">Olympus, BX51</td>
			<td style="width:159px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Atomic force microscopy</td>
			<td style="width:295px;">Bruker&#160;</td>
			<td style="width:159px;"></td>
			<td>Using FastScan-A probe type&#160; and ScanAsyst-air</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Scanning electron microscopy&#160;</td>
			<td style="width:295px;">JEOL JSM-6700F</td>
			<td style="width:159px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">transmission electron microscopy</td>
			<td style="width:295px;">FEI Titan</td>
			<td style="width:159px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Tube furnace</td>
			<td style="width:295px;">MTI Corporation</td>
			<td style="width:159px;"></td>
			<td></td>
		</tr>
	</tbody>
</table>

atmospheric pressure fabrication of large-sized single-layer rectangular snse flakes

Tin selenide (SnSe) belongs to the family of layered metal chalcogenide materials with a buckled structure like phosphorene, and has shown potential for applications in two-dimensional nanoelectronics devices. Although many methods to synthesize SnSe nanocrystals have been developed, a simple way to fabricate large-sized single-layer SnSe flakes remains a great challenge. Herein, we show the experimental method to directly grow large-sized single-layer rectangular SnSe flakes on commonly used SiO2/Si insulating substrates using a straightforward two-step fabrication method in an atmospheric pressure quartz tube furnace system. The single-layer rectangular SnSe flakes with an average thickness of ~6.8 &#197; and lateral dimensions of about 30 &#181;m &#215; 50 &#181;m were fabricated by a combination of vapor transport deposition technique and nitrogen etching route. We characterized the morphology, microstructure, and electrical properties of the rectangular SnSe flakes and obtained excellent crystallinity and good electronic properties. This article about the two-step fabrication method can help researchers grow other similar two-dimensional, large-sized, single-layer materials using an atmospheric pressure system.

Schematic diagrams of the experimental apparatus, optical images, atomic force microscopy (AFM) images, scanning electron microscopy (SEM) images, and transmission electron microscopy (TEM) images of the fabricated SnSe flakes are shown in Figure 1, Figure 2, and Figure 3. The optical images are performed by a traditional optical microscope. The eyepiece lens is 10X, and the objective lens is 20X, 50...

Watch this Scientific Journal Video about Atmospheric Pressure Fabrication of Large-Sized Single-Layer Rectangular SnSe Flakes at JoVE.com

Atmospheric Pressure Fabrication of Large-Sized Single-Layer Rectangular SnSe Flakes

A protocol is presented demonstrating a two-step fabrication technique to grow large-sized single-layer rectangular shaped SnSe flakes on low-cost SiO2/Si dielectrics wafers in an atmospheric pressure quartz tube furnace system.

Tin selenide (SnSe) belongs to the family of layered metal chalcogenide materials with a buckled structure like phosphorene, and has shown potential for ...

The overall goal of the synthetic method of combining a vapor transport deposition technique and nitrogen etching method in an atmospheric pressure system is to demonstrate fabrication of high-quality, large-sized single-layer materials on dielectric substrates. This method provides a general structure for the growth of two-dimensional lines the size of single-layer materials, such as as tin sulfide, germanium selenide, and indium telleride. The main advantage of this technique is that that large-size single-layer flakes can be grown on low-cost silicon dioxide silicon dielectrics with a vapor transport deposition technique and the nitrogen etching method in an atmospheric pressure system.The implications of this technique extend towards the fabrication of other two-dimensional materials because the self-limiting nitrogen etching method can be beneficial to the formation of single layers on both. I first had the idea for this method when I carried out the growth of tungsten diselenide, tin selenide, high-production under nitrogen atmosphere and found that nitrogen etches the material. First, set the target temperature of a horizontal tube furnace to 560 degrees Celsius for one hour and run the furnace.When the temperature approaches 560 degrees Celsius, press the set key for two seconds. Once the parameter HAL"pops up, press the set key for one second to go to the next parameter. Continue to press the set key.After Cont equals three"appears, set it as two. The system starts the autotune function to work out the value for Int, Pro, and LT, and then the system will go to three. When re-autotune is needed, set it as two.Position a new ceramic boat inside a new one-inch diameter quartz tube. Place the quartz tube inside the horizontal tube furnace containing a new two-inch diameter quartz tube. Ensure that both ends of the tubes are firmly fixed and supported.Close the furnace lid and heat the tube furnace to 1000 degrees Celsius over 30 minutes. After maintaining the furnace at 1000 degrees Celsius for 30 minutes, gradually move the tube furnace from one end to the other to heat the whole length of the tube for cleaning the quartz tube wall and the ceramic boat. Following this, allow the tube furnace to cool to room temperature by turning off the furnace.When the furnace is cool, open the furnace lid and take out the ceramic boat and one-inch diameter quartz tube, which can be used for subsequent experiments. Using a diamond scriber, cut a silicon dioxide silicon wafer into 1.5 by two centimeter samples, to be used as growth substrates. Clean the silicon substrates in acetone, isopropanol, and water.Then blow-dry the substrates with nitrogen. Place 0.01 grams of tin selenide powder in the clean ceramic boat. Place a clean silicon dioxide silicon substrate onto the ceramic boat, growth side facing the tin selenide powder.Then position the ceramic boat inside the clean one-inch diameter quartz tube. Place the one-inch diameter quartz tube inside the horizontal tube furnace containing a two-inch diameter quartz tube on the outside, and ensure that the ceramic boat is located upstream of the heating zone of the tube furnace. Tighten the flanges at both ends of the tube and close the vent valve to seal the two-inch diameter quartz tube.Now turn on the pump that connects to the quartz tube and pump the tube to a pressure of approximately one times 10 to the minus two millibar, to remove air and moisture in the tube. Next, open the carrier gas valves, using the gas flow meter to control the gas flows. Introduce 40 sccm argon and 10 sccm hydrogen into the quartz tube until atmospheric pressure is achieved.Then open the vent valves to allow a continuous flow of gas into the quartz tubes. Close the furnace lid and rapidly heat the tube furnace with a 35-degree Celsius per minute heating rate. When the temperature at the center of the furnace approaches 700 degrees Celsius, quickly move the tube furnace to position the tin selenide powder at the center of the furnace, to evaporate the powder and deposit bulk flakes on the silicon dioxide silicon surface.After 15 minutes growth time, open the furnace lid to quickly cool the tube furnace to room temperature. Meanwhile, maximize the flow of the argon-hydrogen carrier gas to help drive the unreacted gas and particles out of the tubes. When the growth process is completed, bulk tin selenide flakes will be obtained on the surface of the silicon dioxide silicon substrates.Place the as-grown bulk sample face up onto a new, clean ceramic boat. Position the ceramic boat inside a new, clean one-inch diameter quartz tube. Place the one-inch diameter quartz tube inside the horizontal tube furnace, containing a two-inch diameter quartz tube, with the ceramic boat located upstream of the heating zone of the tube furnace.Tighten the flanges at both ends of the tube and close the vent valve to seal the two-inch diameter quartz tube. Following this, turn on the pump that connects to the quartz tube and pump the tube to a pressure of approximately one times 10 minus two millibar, to remove air and moisture in the tube. Then turn off the pump.Open the carrier gas valves, using the gas flow meter to control the gas flows. Introduce 50 sccm nitrogen into the quartz tube until atmospheric pressure is achieved. Open the vent valves to allow a continuous flow of gas in the quartz tubes.Next, close the furnace lid and rapidly heat the tube furnace to 700 degrees Celsius in 20 minutes. When the temperature at the center of the furnace approaches 700 degrees Celsius, quickly move the tube furnace to position the bulk sample at the center of the furnace, maintaining the furnace at 700 degrees Celsius for approximately five to 20 minutes to complete the etching process. Following this, open the furnace lid to quickly cool the tube furnace to room temperature.Meanwhile, maximize the flow of nitrogen gas to help drive the unreacted gas and particles out of the tubes. Finally, observe the single-layer rectangular-shaped tin selenide flakes obtained on the surface of the silicon dioxide silicon substrates. Optical microscopy images of the bulk and single-layer tin selenide flakes are shown here.The flakes are approximately rectangular, with dimensions of 30 by 50 micrometers that grow randomly on the silicon dioxide silicon substrates. The AFM image of bulk tin selenide flake revealed a flat surface with a thickness of 54.9 nanometers. The ultra-thin rectangular flakes had a thickness of 6.8 Angstroms, which is close to the theoretical value of single-layer tin selenide.The shape and dimensions of bulk and single-layer tin selenide flakes observed by SEM are in agreement with the optical microscopy images. The EDX spectrum shows a one to 0.92 atomic ratio of tin and selenide in the bulk sample. A TEM image of the transferred tin selenide fragment is shown here.The selected area electron diffraction pattern of a single-layer tin selenide fragment exhibits an orthogonally symmetric diffraction pattern, indicating that the sample is single-crystal in nature. The high-resolution TEM image of the transferred tin selenide fragment shows two orthogonal lattice fringes from the zero negative one one and zero negative one negative one planes. The angle between the lattice fringes is approximately 86.5 degrees, which corresponds to an orthorhombic crystal structure.Once mastered, this technique can be done in about two and a half hours if it is performed properly. While attempting this procedure, it's important to remember to ensure rapid movement of the furnace to the exact location of the operating site and that the etching process is done under nitrogen atmosphere. Following this procedure, other materials like tin sulfide and gallium selenide can be fabricated in order to study the unique properties of these materials.This technique can pave the way for researchers in the field of 2-D material synthesis to explore these single-layer materials'growth and etching mechanism in an atmospheric pressure system. After watching this video, you should have a good understanding of how to fabricate single-layer materials via a vapor transport deposition technique and subsequent nitrogen etching method in an atmospheric pressure system.

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Research

JoVE Journal

Engineering

8.0K vistas.  Shenzhen University. The overall goal of the synthetic method of combining a vapor transport deposition technique and nitrogen etching method in an atmospheric pressure system is to demonstrate fabrication of high-quality, large-sized single-layer materials on dielectric substrates. This method provides a general structure for the growth of two-dimensional lines the size of single-layer materials, such as as tin sulfide, germanium selenide, and indium telleride. The main advantage of this technique is that that l...