Carbon Nanotubes Allow Next-Gen Semiconductor Synthesis

While indium selenide (In_xSe_y) semiconductors have desirable properties that offer potential applications in photovoltaic and optoelectronic devices, these materials exhibit complex polymorphism, and different phases have different physical and chemical properties. Previous research has shown that confining indium selenide to two dimensions significantly alters its physical properties and could make it suitable for high-quality semiconducting components in future devices. In₂Se₃ nanowires (measuring between 40 and 200 nm wide) maintained the intricate phase-change behaviour, high photosensitivity and rapid photoresponse, but lack some of the exciting properties of single-layer In_xSe_y. However, creating 2D sheets of indium selenide is challenging, typically requiring liquid-phase or mechanical exfoliation. In work recently published in ACS Nano, researchers from the University of Nottingham synthesised In_xSe_y in single-walled carbon nanotubes (SWCNTs), which offer a unique environment for the templated growth of ultrathin nanomaterials, and demonstrated that aberration-corrected transmission electron microscopy (AC-TEM) allows identification of the phase of the encapsulated material. Their study demonstrates a robust method for synthesising two different phases of ultrathin In_xSe_y nanoribbons, offering the potential for future production of bespoke and versatile nanoelectronic devices.

(a) Structural model of monolayer InSe in the (110) orientation. (b) Three-part composite image of an InSe nanoribbon inside a SWCNT, consisting of an AC-TEM image (left), a simulated TEM image (center), and molecular model (right). (c, d) Electron density profile maps in red, generated from the red line superimposed over the experimental AC-TEM image in (b), and in blue, generated from the blue line superimposed over the simulated TEM image in (b), with calculated interatomic distances highlighted in nm. (e–h) The same as (a–d), respectively, but for the (001) orientation of the same nanoribbon.

“Indium selenide is an incredibly versatile semiconductor,” said lead author Dr Will Cull. “Semiconductors are everywhere in computers and phones, and ensuring that we control their properties and chemistry is important for creating new devices. Indium selenide is particularly exciting because it has interesting properties such as ferromagnetism, high photoresponsivity and high electron mobility. These properties are very useful for incorporation into devices ranging from solar cells to transistors, and these properties persist when this material is shrunk down to the nanometre size. We were looking at ways to shrink it to a much smaller size using carbon nanotubes. These are ultra-small cylinders of carbon, which are about 1.5 nanometres in diameter. They offer a template to grow these nanowires and nanoribbons in a very small diameter, but they also offer a great platform for imaging and analysing these materials.”

“You have two options for synthesising these nanomaterials,” added Prof Andrei Khlobystov. “You can either take a big piece of semiconductor and start carving it into little dots or wires - that's very time-consuming, and you can only make one at a time. The other approach is to take the chemical elements of the structure and make it from the bottom up. We use carbon nanotubes as nano test tubes. We employ them as containers, to trap atoms, and once you trap the atoms, you can do chemical reactions. When our group performed the first reaction in a carbon nanotube in 2004, it ended up in the Guinness Book of World Records as the world's smallest test tube. And our record still stands today. For this research, we took indium atoms and selenium atoms, combined them in the nanotube and let them react, leading to the simultaneous growth of millions of indium selenide nanowires in the nanotubes. That's the power of bottom-up versus top-down.”

SWCNTs can be thought of as tiny sheets of graphene rolled to form a tube transparent to electrons. At ePSIC, the team collected aberration-corrected transmission electron microscopy (AC-TEM) images of the nano test tubes filled with In_xSe_y nanoribbons.

Prof Khlobystov said: “There are two reasons why ePSIC was so important. One aspect is related to the material synthesis inside nano test tube. Once you’ve made it, you need to check what you made. Indium selenide is an incredibly structurally rich material, and we need to know what exactly we made, and you need an electron microscope to do that. With the transmission electron microscope we used at ePSIC, we can see atoms, we can see where they are, and therefore we can check what type of polymorph we created. Another aspect specific to this paper is the heated stage at ePSIC, which allowed us to control the temperature of the sample in situ during AC-TEM measurements. The polymorphs have the same composition and are very similar in their structures, and they can transition from one structure to another as temperature changes. So, the advantage of ePSIC with the heated stage is that it gives you an image with atomic resolution so you can count the atoms and control the temperature very precisely. When you link up temperature with the structure you see, you get the most complete understanding of your material that can ever be achieved.”

Dr Cull added: “We wanted to check if the encapsulated indium selenide in these nanowires would undergo any thermally induced phase changes. By working with the research scientists at ePSIC, we were able to use the electron microscopes to look at an individual nanowire of indium selenide and slowly increase the temperature in centigrade steps up to a certain point where suddenly this nanowire completely changed appearance, which is indicative of a thermally induced phase change. Once we'd gathered this data, we were able to get images of this nanowire before and after it changed appearance, we were able to account for the location and the bonding of each atom in this nanowire. Being able to correlate atomic positions to structures directly was only possible using these kinds of microscopes.”

“One of the other things we’ve been working on,” Dr Cull said, “is boron nitride nanotubes. Carbon nanotubes are famous for being dark and optically opaque. While boron nitride nanotubes have the same structure, they are optically transparent. So, with the help of scientists at ePSIC to do our analysis, we’ve been recreating all of this exciting work we've been doing with making nanowires inside carbon nanotubes inside these optically transparent containers. That allows us to look at the structure of these nanowires and produce ultra-thin nanowires but also investigate their optical properties, which is vital for applications such as solar cells.”

“I think it's a very exciting time for chemistry to engage with electron microscopy,” Prof Khlobystov said. “Electron microscopy has greatly improved in the past 10-15 years. We’ve started seeing atoms. We can count them. We can see where they are, what they do. For me, this is a revolution in chemistry because it changes the way we think about chemical reactions, making materials, and atomic structures of the materials. We're becoming more direct in how we approach matter and transformations of matter, which is the key question for chemistry. So, I'm very excited about that.”

To find out more about the electron Physical Science Imaging Centre (ePSIC) or discuss potential applications, please contact Principal Electron Microscopist Chris Allen: [email protected].

Cull WJ et al. Subnanometer-wide indium selenide nanoribbons. ACS Nano 17.6 (2023): 6062-6072. DOI:10.1021/acsnano.3c00670