Endohedral fullerene

Tiny balls act like small cages.

They can hold things inside. These cages trap atoms in their middle. One cage can hold a single atom.

Some cages hold even more. They can hold two or three atoms. They can even hold small bits of water. These cages stay strong at high heat. They are like tiny houses for atoms. They help us study how atoms work.

Fullerenes are tiny carbon cages. Some of these cages trap things inside. We call these endohedral fullerenes.

The name means things are inside the shell. Scientists use the "at" sign to show this. For example, M@C60 means an atom (M) is inside a cage of 60 carbon atoms.

Some cages hold metals. These are called metallofullerenes. When metals go inside, they give electrons to the cage. This makes the cage very stable. These cages can stay strong even in hot air. They can handle heat up to 850 °C.

Other cages hold non-metal atoms. Some hold noble gases like helium. Noble gases are atoms that do not like to react with others. Scientists can also trap nitrogen or phosphorus. These are harder to catch because they are very active. Some cages even hold small molecules like water or methane. These tiny cages act like traps. They stay stable at room temperature. This helps scientists study how single atoms act when they are all alone.

Endohedral fullerenes are special carbon cages that hold things inside. A fullerene is a tiny shell made of carbon atoms. When an atom or a small group of atoms gets trapped inside, it is called an endohedral fullerene. Scientists use a clever way to write these names. They use the "at" sign to show what is trapped. For example, M@C60 means an atom called M is inside a cage of 60 carbon atoms.

This notation helps scientists talk about these tiny structures very clearly. It shows exactly what is inside the shell and what is outside.

There are two main ways these cages work. The first type is called endohedral metallofullerenes. These cages hold metals like scandium, yttrium, or barium. When a metal enters the cage, it gives some of its electrons to the carbon shell. This transfer of electrons makes the cage very stable. It can even stay strong in hot air up to 850 °C.

The second type holds non-metals. These might be noble gases like helium or even small molecules like water. These cages act like tiny atomic traps that stay stable at room temperature.

People have been studying these structures for a long time. The first lanthanum C60 complex was made in 1985. This was a big step in making these special cages. Later, in 1991, scientists proposed the new way to write the names using the "at" sign. In 1999, Harry Dorn and his team made a huge discovery. They successfully trapped a whole molecule fragment inside a cage. This was the first time a group of atoms was included this way. It showed that fullerenes could hold much more than just a single atom.

There are many interesting facts about these tiny traps. For example, making a helium cage is quite hard. Scientists have to use high pressure to get one helium atom into every 650,000 cages. Some cages can hold many metals at once. The Sc3N@C80 cage holds three scandium atoms and one nitrogen atom. Some cages can even hold molecules like methane or hydrogen fluoride.

These different sizes and combinations allow scientists to study many different things at once.

These tiny cages are useful for much more than just looking interesting. They help us understand how single atoms behave when they are all alone. Usually, atoms need complex tools like lasers to stay trapped. But these fullerene cages can hold them at room temperature for a very long time. This lets scientists study quantum mechanics in a very stable way. Some versions are even being used as tools for medical images. For instance, a specific type was once made to work as an MRI contrast agent.

Endohedral fullerenes, often called endofullerenes, are unique molecular structures. They consist of a fullerene cage that encloses additional atoms, ions, or even small molecular clusters. A fullerene is a hollow sphere made of carbon atoms. When something is trapped inside this sphere, it becomes an endohedral complex. These structures are important because they act as tiny, stable containers. They allow scientists to study how individual atoms or molecules behave when they are isolated from their surroundings.

To describe these structures, scientists use a specific notation. In the past, a metal atom inside a C60 fullerene was simply written as MC60. This was not very clear because it did not show if the metal was inside or outside. In 1991, a better system was proposed using the "at" sign (@). In this notation, atoms to the left of the @ sign are inside the cage. The atoms to the right represent the carbon network. For example, M@C60 means the atom M is trapped inside. A more complex version is K2(K@C59B). This describes a 60-atom cage where one boron atom has replaced a carbon. It also contains one potassium atom inside and two potassium atoms on the outside.

There are two primary categories of these complexes. The first type is known as endohedral metallofullerenes. These cages contain electropositive metals. Scientists create them using a 1 kV direct current arc reactor or through laser evaporation. During this process, electrons transfer from the metal atom to the carbon cage. This transfer usually involves between 2 and 3 charge units. However, in the La2@C80 complex, about 6 electrons can transfer. This makes the carbon cage an anion, which is a negatively charged ion. These anionic cages are incredibly stable. They can withstand temperatures between 600 and 850 °C in the air.

Metallofullerenes can vary significantly in size and composition. While many contain only one metal atom, others are much more complex. Researchers have isolated di-metal complexes and tri-metal carbide fullerenes. One famous example is Sc3C2@C80. A major breakthrough occurred in 1999 when Harry Dorn and his coworkers synthesized Sc3N@C80. This was the first time a molecule fragment was successfully included inside a fullerene cage. To make this, they used arc-vaporization at temperatures up to 1100 °C. They used graphite rods packed with scandium(III) oxide, iron nitride, and graphite powder. This process took place in a nitrogen atmosphere at 300 Torr.

The second category is non-metal doped fullerenes. These include complexes containing noble gases like helium, neon, argon, krypton, or xenon. Creating these is quite difficult. For example, to make He@C60, scientists must pressurize C60 to about 3 bar in a noble-gas atmosphere. Even then, only about one out of every 650,000 cages successfully traps a helium atom. At much higher pressures of 3 kbars, the incorporation rate can reach 0.1%. Surprisingly, nitrogen and phosphorus can also form these complexes, such as N@C60 and P@C60. Unlike the metallofullerenes, these do not involve a charge transfer between the center atom and the cage.

These molecules can also trap entire small molecules. Scientists have synthesized endofullerenes containing dihydrogen (H2@C60), water (H2O@C60), hydrogen fluoride (HF@C60), and methane (CH4@C60). These encapsulated molecules can show unusual physical properties. Theoretically, if these cages are compressed, the molecules inside might break apart. Their fragments could then react with the inside of the fullerene cage to create new substances.

Endohedral fullerenes serve as highly efficient atomic traps. Most atomic traps require complex equipment like magnetic traps or laser cooling. In contrast, fullerene cages provide a trap that is stable at room temperature for a very long time. This stability allows for the study of unique quantum mechanical phenomena. For instance, scientists can use ENDOR spectroscopy to observe the compression of an atomic wave function. The cages also act like a Faraday cage. This means the carbon shell isolates the trapped atom from external electric fields. This isolation is vital for exploring how particles interact with their environment without interference.