(Nanowerk Spotlight) Despite the rise of graphene and other two-dimensional (2-D) materials, semiconducting single-walled carbon nanotubes are still regarded as strong candidates for the next generation of high-performance, ultra-scaled and thin-film transistors as well as for opto-electronic devices (read more: "20 years of nanotube transistors").
In the emerging fields of quantum information processing and quantum sensing, recently proposed schemes in detection and manipulation of single spins foreshadow further promising opportunities for semiconducting carbon nanotubes through the engineering of intra-nanotube quantum dots, also called artificial atoms.
Quantum dots in carbon nanotubes have been reported predominantly in the form of decoupled nanotube portions defined between engineered tunneling barriers at metal-nanotube contacts and/or by gate electrodes, or in the form of unintentional localization potentials stemming from environmental disorder. All these structures are usually operated at cryogenic temperature due to the technological challenge to achieve ultra-short quantum dots allowing operation at room temperature.
Left: 3D STM image of a semiconducting single-walled carbon nanotube lying on a gold substrate. A quantum dot (QD) is defined between two argon ion induced defects, revealing quantized states with a large level spacing in the conduction and valence bands. Middle: one dimensional model with tunneling barriers. Right: First-principle simulation of an intrananotube quantum dot defined by two double vacancies. (Image courtesy of the researchers ) (click on image to enlarge)
The team conducted their investigation by using low-temperature scanning tunneling microscopy and spectroscopy measurements at EMPA nanotech@surfaces laboratories in Switzerland in the group of Dr. Oliver Gröning. The experimental findings were supported by analytical and ab initio simulations performed by Ikerbasque Research Fellow Dr. Dario Bercioux in Donostia-San Sebastian, Spain and Dr. Leonhard Mayrhofer at the Fraunhofer IWM in Freiburg, Germany.
This work is motivated by the knowledge that certain types of defects – like double vacancies that result e.g. from irradiation with medium energy argon ions – act as strong electronic scattering centers. Thus, two consecutive scattering centers shall be able to confine carriers and thus form an intrananotube quantum dot.
Furthermore, and contrary to conventional lithography techniques, it should be possible to generate very short quantum dots of the order of a few nanometers with large energy spacing.
This is precisely what the team demonstrates with semiconducting carbon nanotubes for the first time in this work.
"Such quantum dot structures are characterized by quantized states energy level spacing well above the thermal broadening at room temperature," Dr. Gilles Buchs, today a researcher at the Centre Suisse d’Electronique et de Microtechnique (CSEM), and the paper's first author, tells Nanowerk. "Our results, combined with recent progresses in the control of 1) nanotube chirality and 2) structure and location of defects, hold a high potential for applications in the design of a broad palette of nanotube-based quantum devices operating at room temperature."
"The combination of analytical and ab-initio simulations reproduce the experimental results remarkably well and further demonstrate that highly stable vacancies constitute strong scattering centers able to simultaneously confine electrons and holes very efficiently," explain Bercioux and Mayrhofer. "These simulations allow a deeper understanding of the observed quantum dot structures in view of practical applications."
The team believes that their results will serve as a motivation for further experimental and theoretical investigations of the optical properties of such 'quantum dots with leads' building-blocks.
With regard to practical applications, the proposed technique is compatible with ultraclean, suspended semiconducting nanotube devices and might offer more engineering flexibility than recently reported room temperature single photon emitters based on encapsulated semiconducting nanotubes functionalized with covalently bound oxygen or aryl groups.
"In order to assess the potential of our quantum dot structures for designing quantum opto-electronic devices, one needs to study their optical properties for different types and configurations of defect pairs via e.g. photoluminescence combined to second order correlation measurements in order to detect and characterize non-classical (quantum) emitted light," Buchs points out.
Also, notes Dr. Oliver Gröning, a stronger expression of the quantized states resolved with scanning tunneling spectroscopy could be achieved by lowering the nanotubes-substrate interaction through e.g. the insertion of atomically thin insulating sodium chloride (NaCl) films. "This would allow to gain more insight inti the electronic structure of the quantum dots as well as in the associated scattering physics at the confining defects."
The quasi one-dimensional geometry of single-walled carbon nanotubes allows for defining tunable p-n junctions (in other words, a nano diode) induced by electrostatic doping through local gates. Combining a well-defined quantum dot within such a nano diode could constitute a crucial building-block for the realization of highly desirable electrically driven, on-demand single photon emitters operating at telecom wavelength, based e.g. on a turnstile device architecture.
"Particular challenges lie in the control and localization of artificial defects, as well as in the chirality of the nanotubes," the team concludes. "Recent progress in these areas are encouraging and might allow, in the near future, the tailoring of intrananotube quantum dots with specific optical and electronic transport properties at the atomic level."