Intertube excitonic coupling in nanotube van der Waals heterostructures

Strong intertube excitonic coupling is demonstrated in 1D van der Waals heterostructures by examining the ultrafast response of radial C/BN/MoS$_2$ core/shell/skin nanotubes to femtosecond infrared light pulses. Remarkably, infrared excitation of excitons in the semiconducting CNTs creates a prominent excitonic response in the visible range from the MoS$_2$ skin, even with infrared photons at energies well below the bandgap of MoS$_2$. Via classical analogies and a quantum model of the light-matter interaction we assign these findings to intertube excitonic correlations. Dipole-dipole Coulomb interactions in the coherent regime produce intertube biexcitons, which persist for tens of femtoseconds, while on longer timescales ($>100$\,ps) hole tunneling -- from the CNT core, through the BN tunnel barrier, to the MoS$_2$ skin -- creates intertube excitons. Charge transfer and dipole-dipole interactions thus play prominent roles on different timescales, and establish new possibilities for the multi-functional use of these new nano-scale coaxial cables

Ultrafast optoelectronic processes in 1D radial van der Waals heterostructures : carbon, boron nitride, and MoS 2 nanotubes with coexisting excitons and highly mobile charges

Heterostructures built from 2D, atomically thin crystals are bound by the van der Waals force and exhibit unique optoelectronic properties. Here, we report the structure, composition and optoelectronic properties of 1D van der Waals heterostructures comprising carbon nanotubes wrapped by atomically thin nanotubes of boron nitride and molybdenum disulfide (MoS2). The high quality of the composite was directly made evident on the atomic scale by transmission electron microscopy, and on the macroscopic scale by a study of the heterostructure’s equilibrium and ultrafast optoelectronics. Ultrafast pump−probe spectroscopy across the visible and terahertz frequency ranges identified that, in the MoS 2 nanotubes, excitons coexisted with a prominent population of free charges. The electron mobility was comparable to that found in high-quality atomically thin crystals. The high mobility of the MoS2 nanotubes highlights the potential of 1D van der Waals heterostructures for nanoscale optoelectronic devices

Tunable THz flat zone plate based on stretchable single-walled carbon nanotube thin film

Tunable optoelectronics have attracted a lot of attention in recent years because of their variety of applications in next-generation devices. Among the potential uses for tuning optical elements, those allowing consistent parameter control stand out. We present an approach for the creation of mechanically tunable zone plate lenses in the THz range. Our devices comprise single-walled carbon nanotube (SWCNT) thin films of predetermined design integrated with stretchable polymer films. These offer high-performance and in situ tunability of focal length up to 50%. We studied the focusing properties of our lenses using the backward-wave oscillator THz imaging technique, supported by numerical simulations based on the finite element frequency domain method. Our approach may further enable the integration of SWCNT films into photonic and optoelectronic applications and could be of use for the creation of a variety of flexible and stretchable THz optical elements

Applications of Pristine and Functionalized Carbon Nanotubes, Graphene, and Graphene Nanoribbons in Biomedicine

This review is dedicated to a comprehensive description of the latest achievements in the chemical functionalization routes and applications of carbon nanomaterials (CNMs), such as carbon nanotubes, graphene, and graphene nanoribbons. The review starts from the description of noncovalent and covalent exohedral modification approaches, as well as an endohedral functionalization method. After that, the methods to improve the functionalities of CNMs are highlighted. These methods include the functionalization for improving the hydrophilicity, biocompatibility, blood circulation time and tumor accumulation, and the cellular uptake and selectivity. The main part of this review includes the description of the applications of functionalized CNMs in bioimaging, drug delivery, and biosensors. Then, the toxicity studies of CNMs are highlighted. Finally, the further directions of the development of the field are presented

Ultrafast terahertz and optical spectroscopy of 1D van der Waals nanomaterials

In this thesis, we report on the experimental investigation of the optical properties of 1D van der Waals materials, such as carbon and boron nitride, transition metal dichalcogenides nanotubes, and their heterostructures. Despite numerous studies of carbon nanotubes their properties in the THz region were under debate. Recent progress in understanding the nature of the THz conductivity of carbon nanotubes motivated us to further understand their photophysical properties. By using a combination of techniques, we investigated the properties of ultrathin, highly conductive carbon nanotubes films with different thicknesses. The unique photoinduced negative THz conductivity allows us to construct a modulator based on photoinduced transparency. We showed that with an increase in the carbon nanotubes film thickness, the modulation performance increases. Moreover, it was found that the photoconductivity lifetime is short and the device can be operated with the modulation speed more than 300 GHz, and a large modulation depth, up to +80 %. Furthermore, we investigated the influence of doping the carbon nanotubes’ properties and consequently on the modulator performance. Our research reveals the increase of electrical conductivity due to the doping which results in suppression of the THz modulation performance with a simultaneous decrease in the modulation time (increase in the modulation speed). The possible physical explanation is given. Overall, it was shown that the decrease of the MD due to doping needs to be taken into account when designing THz modulators. The second part of this thesis showed the first study of the novel radial 1D van der Waals heterostructure properties. An interesting result was shown: ultrafast dynamic switch from negative to positive photoconductivity was observed and explained in terms of the different temporal dynamics for free-carrier absorption in the carbon nanotubes and MoS2 nanotubes. The combination of optical pump-THz probe and optical pump-optical probe spectroscopy shows the co-existence of free charges and excitons in MoS2 nanotubes

Applications of Pristine and Functionalized Carbon Nanotubes, Graphene, and Graphene Nanoribbons in Biomedicine

This review is dedicated to a comprehensive description of the latest achievements in the chemical functionalization routes and applications of carbon nanomaterials (CNMs), such as carbon nanotubes, graphene, and graphene nanoribbons. The review starts from the description of noncovalent and covalent exohedral modification approaches, as well as an endohedral functionalization method. After that, the methods to improve the functionalities of CNMs are highlighted. These methods include the functionalization for improving the hydrophilicity, biocompatibility, blood circulation time and tumor accumulation, and the cellular uptake and selectivity. The main part of this review includes the description of the applications of functionalized CNMs in bioimaging, drug delivery, and biosensors. Then, the toxicity studies of CNMs are highlighted. Finally, the further directions of the development of the field are presented

Synthesis, Sorting, and Applications of Single-Chirality Single-Walled Carbon Nanotubes

The synthesis of high-quality chirality-pure single-walled carbon nanotubes (SWCNTs) is vital for their applications. It is of high importance to modernize the synthesis processes to decrease the synthesis temperature and improve the quality and yield of SWCNTs. This review is dedicated to the chirality-selective synthesis, sorting of SWCNTs, and applications of chirality-pure SWCNTs. The review begins with a description of growth mechanisms of carbon nanotubes. Then, we discuss the synthesis methods of semiconducting and metallic conductivity-type and single-chirality SWCNTs, such as the epitaxial growth method of SWCNT (“cloning”) using nanocarbon seeds, the growth method using nanocarbon segments obtained by organic synthesis, and the catalyst-mediated chemical vapor deposition synthesis. Then, we discuss the separation methods of SWCNTs by conductivity type, such as electrophoresis (dielectrophoresis), density gradient ultracentrifugation (DGC), low-speed DGC, ultrahigh DGC, chromatography, two-phase separation, selective solubilization, and selective reaction methods and techniques for single-chirality separation of SWCNTs, including density gradient centrifugation, two-phase separation, and chromatography methods. Finally, the applications of separated SWCNTs, such as field-effect transistors (FETs), sensors, light emitters and photodetectors, transparent electrodes, photovoltaics (solar cells), batteries, bioimaging, and other applications, are presented

Ultrafast THz spectroscopy of carbon nanotube-graphene composites

Mixed nanomaterial composites can combine the excellent properties of well-known low-dimensional nanomaterials. Here we highlight the potential of one-dimensional single-walled carbon nanotubes interfaced with two-dimensional graphene by exploring the composite's ac conductivity and photoconductivity, and the influence of HAuCl4 doping. In the composite, the equilibrium terahertz conductivity from free carrier motion was boosted, while the localised plasmon peak shifted towards higher frequencies, which we attribute to shorter conductivity pathways in the composite. A negative terahertz photoconductivity was observed for all samples under 410 nm optical excitation and was reproduced by a simple model, where the Drude spectral weight and the momentum scattering rate were both lowered under photoexcitation. The composite had an enhanced modulation depth in comparison to reference carbon nanotube films, while retaining their characteristically fast (picosecond) response time. The results show that carbon nanotube-graphene composites offer new opportunities in devices by controlling charge carrier transport and tuning their optoelectronic properties

Ultrafast, high modulation depth terahertz modulators based on carbon nanotube thin films

The development of THz technology and communication systems is creating demand for devices that can modulate THz beams rapidly. Here we report the design and characterisation of high-performance, broadband THz modulators based on the photo-induced transparency of carbon nanotube films. Rather than operating in the standard modulation mode, where optical excitation lowers transmission, this new class of modulators exhibits an inverted modulation mode with an enhanced transmission. Under femtosecond pulsed illumination, modulation depths reaching +80% were obtained simultaneously with modulation speeds of 340 GHz. The influence of the film thickness on the insertion loss, modulation speed and modulation depth was explored over a frequency range from 400 GHz to 2.6 THz. The excellent modulation depth and high modulation speed demonstrated the significant potential of carbon nanotube thin films for ultrafast THz modulators.Peer reviewe

Giant Negative Terahertz Photoconductivity in Controllably Doped Carbon Nanotube Networks

A strong negative photoconductivity was identified in thin film networks of single-walled carbon nanotubes using optical pump, THz probe spectroscopy. The films were controllably doped, using either adsorption doping with different p-type dopant concentrations or ambipolar doping using an ionic gate. While doping enhanced the THz conductivity and increased the momentum scattering rate, interband photoexcitation lowered the spectral weight and reduced the momentum scattering rate. This negative THz photoconductivity was observed for all doping levels, regardless of the chemical potential, and decayed within a few picoseconds. The strong many-body interactions inherent to these 1D conductors led to trion formation under photoexcitation, lowering the overall conductivity of the carbon nanotube network. The large amplitude of negative THz photoconductivity and the tunability of its recovery time with doping offer promise for spectrally wide-band ultrafast devices, including THz detectors, polarizers, and modulators