This Review summarizes recent progress of single-photon emitters based on defects in solids and highlights new research directions. The photophysical properties of single-photon emitters and efforts towards scalable system integration are also discussed.

Solid-state single-photon emitters

Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

It is estimated that the world will need to double its energy supply by 2050. Nanotechnology has opened up new frontiers in materials science and engineering to meet this challenge by creating new materials, particularly carbon nanomaterials, for efficient energy conversion and storage. Comparing to conventional energy materials, carbon nanomaterials possess unique size-/surface-dependent (e.g., morphological, electrical, optical, and mechanical) properties useful for enhancing the energy-conversion and storage performances. During the past 25 years or so, therefore, considerable efforts have been made to utilize the unique properties of carbon nanomaterials, including fullerenes, carbon nanotubes, and graphene, as energy materials, and tremendous progress has been achieved in developing high-performance energy conversion (e.g., solar cells and fuel cells) and storage (e.g., supercapacitors and batteries) devices. This article reviews progress in the research and development of carbon nanomaterials during the past twenty years or so for advanced energy conversion and storage, along with some discussions on challenges and perspectives in this exciting field.

Carbon Nanomaterials for Advanced Energy Conversion and Storage

We show that the Raman scattering technique can give complete structural information for one-dimensional systems, such as carbon nanotubes. Resonant confocal micro-Raman spectroscopy of an (n,m) individual single-wall nanotube makes it possible to assign its chirality uniquely by measuring one radial breathing mode frequency omega(RBM) and using the theory of resonant transitions. A unique chirality assignment can be made for both metallic and semiconducting nanotubes of diameter d(t), using the parameters gamma(0) = 2.9 eV and omega(RBM) = 248/d(t). For example, the strong RBM intensity observed at 156 cm(-1) for 785 nm laser excitation is assigned to the (13,10) metallic chiral nanotube on a Si/SiO2 surface.

Structural (n,m) determination of isolated single wall carbon nanotubes by resonant Raman scattering

Single-walled carbon nanotubes tend to be produced in polydisperse mixtures with different lengths, diameters and electronic properties. This review article surveys the various techniques that have been developed for producing monodisperse samples from these mixtures. Selective growth techniques are also covered.

Progress towards monodisperse single-walled carbon nanotubes

Atomic-sized fluorescent defects in diamond are widely recognized as a promising solid state platform for quantum cryptography and quantum information processing. For these applications, single photon sources with a high intensity and reproducible fabrication methods are required. In this study, we report a novel color center in diamond, composed of a germanium (Ge) and a vacancy (V) and named the GeV center, which has a sharp and strong photoluminescence band with a zero-phonon line at 602 nm at room temperature. We demonstrate this new color center works as a single photon source. Both ion implantation and chemical vapor deposition techniques enabled fabrication of GeV centers in diamond. A first-principles calculation revealed the atomic crystal structure and energy levels of the GeV center.

Germanium-Vacancy Single Color Centers in Diamond.

Tin-vacancy ($\mathrm{Sn}\text{\ensuremath{-}}V$) color centers were created in diamond via ion implantation and subsequent high-temperature annealing up to $2100\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ at 7.7 GPa. The first-principles calculation suggested that a large atom of tin can be incorporated into a diamond lattice with a split-vacancy configuration, in which a tin atom sits on an interstitial site with two neighboring vacancies. The $\mathrm{Sn}\text{\ensuremath{-}}V$ center showed a sharp zero phonon line at 619 nm at room temperature. This line split into four peaks at cryogenic temperatures, with a larger ground state splitting ($\ensuremath{\sim}850\text{ }\text{ }\mathrm{GHz}$) than that of color centers based on other group-IV elements, i.e., silicon-vacancy ($\mathrm{Si}\text{\ensuremath{-}}V$) and germanium-vacancy ($\mathrm{Ge}\text{\ensuremath{-}}V$) centers. The excited state lifetime was estimated, via Hanbury Brown--Twiss interferometry measurements on single $\mathrm{Sn}\text{\ensuremath{-}}V$ quantum emitters, to be $\ensuremath{\sim}5\text{ }\text{ }\mathrm{ns}$. The order of the experimentally obtained optical transition energies, compared with those of $\mathrm{Si}\text{\ensuremath{-}}V$ and $\mathrm{Ge}\text{\ensuremath{-}}V$ centers, was in good agreement with the theoretical calculations.

Tin-Vacancy Quantum Emitters in Diamond.

A novel point-arc microwave plasma chemical vapor deposition (CVD) apparatus was employed to grow single-walled carbon nanotubes (SWNTs) on Si substrates coated with a sandwich-like nano-layer structure of 0.7 nm Al2O3 (top)/0.5 nm Fe/5–70 nm Al2O3 by conventional high frequency sputtering. The growth of extremely dense and vertically aligned SWNTs with an almost constant growth rate of 270 µm/h within 40 min at a temperature as low as 600°C was demonstrated for the first time. The volume density of the as-grown SWNT films is as higher as 66 kg/m3.

https://iopscience.iop.org/article/10.1143/JJAP.44.1558/pdf

Low Temperature Synthesis of Extremely Dense and Vertically Aligned Single-Walled Carbon Nanotubes

Vertically aligned multiwalled carbon nanotubes (MWCNTs) were synthesized by remote plasma chemical vapor deposition at a low temperature of 390°C, which meets the requirement of the large scale integration (LSI) process. For wiring application, we measured the electrical properties of MWCNT-via structures with and without chemical mechanical polishing (CMP). The via resistances were reduced using inner shells of MWCNTs whose caps were opened due to CMP. The improved resistance after annealing at 400°C was 0.6Ω for 2μm vias. Our process is suitable for LSI because the temperature never exceeds the allowable temperature of 400°C in the Si LSI process.

Takayuki Iwasaki

Papers

Germanium-Vacancy Single Color Centers in Diamond.

Tin-Vacancy Quantum Emitters in Diamond.

Low Temperature Synthesis of Extremely Dense and Vertically Aligned Single-Walled Carbon Nanotubes

Low temperature grown carbon nanotube interconnects using inner shells by chemical mechanical polishing

Very High Yield Growth of Vertically Aligned Single‐Walled Carbon Nanotubes by Point‐Arc Microwave Plasma CVD