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Posted: Jul 27, 2011

Single-molecule magnets encapsulated inside carbon nanotubes

(Nanowerk Spotlight) Single-molecule magnets (SMM) are fascinating nanoscale structures with unique functional properties showing promise for high-density electronic data storage devices, solid state quantum computers, spintronic devices such as spin valves, and other advanced technological applications (see for instance our recent Nanowerk Spotlight: "Single-molecule nanomagnets enable novel graphene spintronics devices"). Despite a flurry of research in this area – since an individual magnetic molecule represents the ultimate size limit to storing and processing information – the main challenge related to harnessing properties of SMM remained unsolved.
A new study by a group of European researchers demonstrates that SMM can be successfully integrated inside carbon nanotubes (CNT).
Reporting their work in the July 26, 2011 online issue of Nature Communications ("Encapsulation of single-molecule magnets in carbon nanotubes"), a team led by Andrei Khlobystov an Associate Professor and Reader in Chemical Nanosciences at the University of Nottingham, reports the successful encapsulation of single-molecule magnets in carbon nanotubes, yielding a new type of hybrid nanostructure that combines all the key single-molecule magnet properties of the guest molecules with the functional properties of the host CNT.
"Carbon nanotubes can serve as a container for molecules and a physical bridge between the molecules and the macroscopic world around us" Khlobystov explains to Nanowerk. "SMM are very delicate molecules that break down easily and thus lose their properties, but we have developed a method that allows transportation of intact SMM into CNT, and demonstrated that the key magnetic properties of SMM are fully retained once the molecules are encapsulated within CNT, which is a very important result for construction of electronic devices based on these SMM-CNT hybrid materials."
"Moreover," says Maria del Carmen Giménez-López, a Postdoctoral Research Fellow in Khlobystov's group and the paper's first author, "SMM 'packaged' in CNT appear to be more stable because the nanotube acts as a shield protecting the guest-molecules from the negative effects of the environment. Our latest work on SMM-CNT systems may change the way we and other researchers study molecular magnets and opens avenues for their practical applications."
The study was carried out as a part of a research programme "Transition Metals in Carbon Nanostructures" of a Marie Curie Fellowship held by Giménez-López.
Schematic diagram of Mn12Ac single-molecule magnet encapsulation in carbon nanotube
(a) Schematic diagram of Mn12 Ac single-molecule magnet encapsulation in carbon nanotube. (b) Comparison of magnetization of SMM confined in nanotubes (red) with the control sample Mn12 Ac (black) shows that magnetic properties are fully retained. (Image: Dr. Andrei Khlobystov, University of Nottigham)
The shape of an archetypical SMM (Mn12 Ac) that the researchers used for their experiments can be described as a discoid with a diameter of 1.6 nm and a height of 1.1 nm.
"Considering the minimum van der Waals gap of 0.3 nm required between the surface of the molecule and the inner surface of a nanotube, the smallest diameter of CNT capable of hosting Mn12 Ac is 2.2 nm," explains Giménez-López. "Therefore, insertion of these and even larger SMM into single-walled carbon nanotubes (typical diameter range 1-2 nm) is not feasible, and thus wider multiwalled carbon nanotubes should be used as host-structures for SMM. Indeed, all previous attempts of combining SMM and SWNT resulted in composite structures where the molecules, adsorbed on the nanotube surface, were exposed to the detrimental influences of the environment – atmospheric oxygen and water, or ambient light – often leading to fast degradation of their magnetic properties."
Using their new technique, the team's method allows insertion of SMM exclusively into nanotubes – as compared to adsorption on nanotube surface – with the result that, for the first time, this enabled preparation of hybrid nanostructures where all molecular magnets are arranged inside nanotubes and no molecules remain on the nanotube surface.
"We were able to show that the SMM functional properties are fully retained and that the molecules undergo a large degree of orientational ordering inside the nanotube" says Khlobystov. "This ordering is advantageous for addressing purposes and for controlling electronic properties of the nanotube."
The researchers used supercritical CO2 for the transport of the SMM molecules into the nanotubes because it is an excellent carrier fluid for the encapsulation of molecules that allow it to penetrate the nanotubes without hindrance, enabling insertion of the desired guest species.
"The mild filling conditions, with temperatures not exceeding 40°C, allowed for the successful insertion of Mn12 Ac, which is far too fragile to be transported into CNTs through the gas or molten phase" says Giménez-López.
After loading the SMM into the nanotubes, the team conducted an extensive investigation of their magnetic properties, confirming that the SMM remain intact inside the nanotube, with their functional magnetic properties fully preserved.
Khlobystov points out that a fundamental challenge is to understand the exact mechanisms of coupling between the magnetic states of SMM and the electronic bands of CNT, which is essential for harnessing the functional potential of these hybrid nanostructures in electronic and spintronic devices.
"Also, there is an important technical challenge of integrating these structures into nanodevices, which requires input of our colleagues from physics and engineering," he says.
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