Researchers in Japan have directly visualized the formation and evolution of quasiparticles known as excitons in carbon nanotubes for the first time. The work could aid the development of nanotube-based nanoelectronic and nanophotonic devices.
Carbon nanotubes (CNTs) are rolled-up hexagonal lattices of carbon just one atom thick. When exposed to light, they generate excitons, which are bound pairs of negatively-charged electrons and positively-charged “holes”. The behaviour of these excitons governs processes such as light absorption, emission and charge carrier transport that are crucial for CNT-based devices. However, because excitons are confined to extremely small regions in space and exist for only tens of femtoseconds (fs) before annihilating, they are very difficult to observe directly with conventional imaging techniques.
Ultrafast and highly sensitive
In the new work, a team led by Jun Nishida and Takashi Kumagai at the Institute for Molecular Science (IMS)/SOKENDAI, together with colleagues at the University of Tokyo and RIKEN, developed a technique for imaging excitons in CNTs. Known as ultrafast infrared scattering-type scanning near-field optical microscopy (IR s-SNOM), it first illuminates the CNTs with a short visible laser pulse to create excitons and then uses a time-delayed mid-infrared pulse to probe how these excitons behave.
“By scanning a sharp gold-coated atomic force microscope (AFM) tip across the surface and detecting the scattered infrared signal with high sensitivity, we can measure local changes in the optical response of the CNTs with 130-nm spatial resolution and around 150-fs precision,” explains Kumagai. “These changes correspond to where and how excitons are formed and annihilated.”
According to the researchers, the main challenge was to develop a measurement that was ultrafast and highly sensitive while also having a spatial resolution high enough to detect a signal from as few as around 10 excitons. “This required not only technical innovations in the pump-probe scheme in IR s-SNOM, but also a theoretical framework to interpret the near-field response from such small systems,” Kumagai says.
The measurements reveal that local strain and interactions between CNTs (especially in complex, bundled nanotube structures) govern how excitons are created and annihilated. Being able to visualize this behaviour in real time and real space makes the new technique a “powerful platform” for investigating ultrafast quantum dynamics at the nanoscale, Kumagai says. It also has applications in device engineering: “The ability to map where excitons are created and how they move and decay in real devices could lead to better design of CNT-based photonic and electronic systems, such as quantum light sources, photodetectors, or energy-harvesting materials,” Kumagai tells Physics World.
Extending to other low-dimensional systems
Kumagai thinks the team’s approach could be extended to other low-dimensional systems, enabling insights into local dynamics that have previously been inaccessible. Indeed, the researchers now plan to apply their technique to other 1D and 2D materials (such as semiconducting nanowires or transition metal dichalcogenides) and to explore how external stimuli like strain, doping, or electric fields affect local exciton dynamics.
“We are also working on enhancing the spatial resolution and sensitivity further, possibly toward single-exciton detection,” Kumagai says. “Ultimately, we aim to combine this capability with in operando device measurements to directly observe nanoscale exciton behaviour under realistic operating conditions.”
The technique is detailed in Science Advances.
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