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

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.