Theory of exciton thermal radiation in semiconducting single-walled carbon nanotubes.

Spectral control of thermal radiation is an essential strategy for highly efficient and functional utilization of thermal radiation energy. Among the various proposed methods, quantum confinement in low-dimensional materials is promising because of its inherent ability to emit narrowband thermal radiation. Here, we theoretically investigate thermal radiation from one-dimensional (1D) semiconductors characterized by the strong quantum correlation effect due to the Coulomb interaction. We derive a simple and useful formula for the emissivity, which is then used to calculate the thermal radiation spectrum of semiconducting single-walled carbon nanotubes as a representative of 1D semiconductors. The calculations show that the exciton state, which is an electron-hole pair mutually bound by the Coulomb interaction, causes enhancement of the radiation spectrum peak and significant narrowing of its linewidth in the near-infrared wavelength range. The theory developed here will be a firm foundation for exciton thermal radiation in 1D semiconductors, which is expected to lead to new energy harvesting technologies.

Engineering Valley Polarization of Monolayer WS2 : A Physical Doping Approach.

The emerging field of valleytronics has boosted intensive interests in investigating and controlling valley polarized light emission of monolayer transition metal dichalcogenides (1L TMDs). However, so far, the effective control of valley polarization degree in monolayer TMDs semiconductors is mostly achieved at liquid helium cryogenic temperature (4.2 K), with the requirements of high magnetic field and on-resonance laser, which are of high cost and unwelcome for applications. To overcome this obstacle, it is depicted that by electrostatic and optical doping, even at temperatures far above liquid helium cryogenic temperature (80 K) and under off-resonance laser excitation, a competitive valley polarization degree of monolayer WS2 can be achieved (more than threefold enhancement). The enhanced polarization is understood by a general doping dependent valley relaxation mechanism, which agrees well with the unified theory of carrier screening effects on intervalley scattering process. These results demonstrate that the tunability corresponds to an effective magnet field of ≈10 T at 4.2 K. This work not only serves as a reference to future valleytronic studies based on monolayer TMDs with various external or native carrier densities, but also provides an alternative approach toward enhanced polarization degree, which denotes an essential step toward practical valleytronic applications.

Multiple exciton generation by a single photon in single-walled carbon nanotubes.

Multiple-exciton generation in single-walled carbon nanotubes is investigated theoretically. We show that multiple excitons can be directly generated by a single photon through resonant coupling with multiexciton states. Further, the theoretically predicted threshold energy for this process is consistent with recent experimental results. Our calculations clarify the elementary processes of multiple-exciton generation in single-walled carbon nanotubes.

Brightening of triplet dark excitons by atomic hydrogen adsorption in single-walled carbon nanotubes observed by photoluminescence spectroscopy.

Adsorption and desorption of atomic hydrogen on single-walled carbon nanotubes were observed by photoluminescence spectroscopy. A satellite peak appeared at the lower energy side of the E11 photoluminescence emission peak after exposure to atomic hydrogen and then disappeared after annealing at 300 °C in vacuum. The energy difference between the satellite peak and E11 peak was 40-80 meV, depending on the tube diameter. The satellite peak was attributed to the triplet dark exciton that became optically active because of the effectively enhanced spin-orbit interaction induced by adsorbed hydrogen atoms.

Surface plasmon polaritons in thin-film Weyl semimetals.

We theoretically investigate surface plasmon polaritons propagating in the thin-film Weyl semimetals. We show how the properties of surface plasmon polaritons are affected by hybridization between plasmons localized at the two metal-dielectric interfaces. Generally, this hybridization results in new mixed plasmon modes, which are called short-range surface plasmons and long-range surface plasmons, respectively. We calculate dispersion curves of these mixed modes for three principle configurations of the axion vector describing axial anomaly in Weyl semimetals. We show that the partial lack of the dispersion and the non-reciprocity can be controlled by fine-tuning of the thickness of the Weyl semimetals, the dielectric constants of the outer insulators, and the direction of the axion vector.

Confinement Effect of Sub-nanometer Difference on Melting Point of Ice-Nanotubes Measured by Photoluminescence Spectroscopy.

Melting point is independent of size and shape in bulk materials, but it exhibits a size dependence when the material size is extremely small. In this study, we measured the melting point of water confined in single-walled carbon nanotubes (SWCNTs) with 16 different chiralities, which ranged from 0.95 to 1.26 nm in diameter, and revealed the details of the SWCNT diameter dependence on the melting points. The melting points were probed by utilizing the change of photoluminescence (PL) emission wavelength of SWCNTs, which encapsulated water, and the relation between the emission wavelength, and the water phase was confirmed by first-principles calculations. The periodicity of the melting point variation with SWCNT diameter came from the discrete change of ice-nanotube (ice-NT) diameter, and in addition, even ice-NT with an identical diameter exhibited different melting points due to the slight difference of the inner space size of the encapsulating SWCNTs. The present results agreed with those of the molecular dynamics simulation (Takaiwa, D.; et al., Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 39-43). It was elucidated that the melting point of the nanomaterial changed sensitively to the atomic structure and the confinement space size.

Evidence for line width and carrier screening effects on excitonic valley relaxation in 2D semiconductors.

Monolayers of transition metal dichalcogenides (TMDC) have recently emerged as excellent platforms for exploiting new physics and applications relying on electronic valley degrees of freedom in two-dimensional (2D) systems. Here, we demonstrate that Coulomb screening by 2D carriers plays a critical role in excitonic valley pseudospin relaxation processes in naturally carrier-doped WSe2 monolayers (1L-WSe2). The exciton valley relaxation times were examined using polarization- and time-resolved photoluminescence spectroscopy at temperatures ranging from 10 to 160 K. We show that the temperature-dependent exciton valley relaxation times in 1L-WSe2 under various exciton and carrier densities can be understood using a unified framework of intervalley exciton scattering via momentum-dependent long-range electron-hole exchange interactions screened by 2D carriers that depend on the carrier density and the exciton linewidth. Moreover, the developed framework was successfully applied to engineer the valley polarization of excitons in 1L-WSe2. These findings may facilitate the development of TMDC-based opto-valleytronic devices.

Modulation of electrical potential and conductivity in an atomic-layer semiconductor heterojunction.

Semiconductor heterojunction interfaces have been an important topic, both in modern solid state physics and in electronics and optoelectronics applications. Recently, the heterojunctions of atomically-thin transition metal dichalcogenides (TMDCs) are expected to realize one-dimensional (1D) electronic systems at their heterointerfaces due to their tunable electronic properties. Herein, we report unique conductivity enhancement and electrical potential modulation of heterojunction interfaces based on TMDC bilayers consisted of MoS2 and WS2. Scanning tunneling microscopy/spectroscopy analyses showed the formation of 1D confining potential (potential barrier) in the valence (conduction) band, as well as bandgap narrowing around the heterointerface. The modulation of electronic properties were also probed as the increase of current in conducting atomic force microscopy. Notably, the observed band bending can be explained by the presence of 1D fixed charges around the heterointerface. The present findings indicate that the atomic layer heterojunctions provide a novel approach to realizing tunable 1D electrical potential for embedded quantum wires and ultrashort barriers of electrical transport.

Enhanced chemical reactivity of graphene induced by mechanical strain.

Control over chemical reactivity is essential in the field of nanotechnology. Graphene is a two-dimensional atomic sheet of sp(2) hybridized carbon with exceptional properties that can be altered by chemical functionalization. Here, we transferred single-layer graphene onto a flexible substrate and investigated the functionalization using different aryl diazonium molecules while applying mechanical strain. We found that mechanical strain can alter the structure of graphene, and dramatically increase the reaction rate, by a factor of up to 10, as well as increase the final degree of functionalization. Furthermore, we demonstrate that mechanical strain enables functionalization of graphene for both p- and n-type dopants, where unstrained graphene showed negligible reactivity. Theoretical calculations were also performed to support the experimental findings. Our findings offer a simple approach to control the chemical reactivity of graphene through the application of mechanical strain, allowing for a tuning of the properties of graphene.