Alignment, functionalization and detection of carbon nanotube (CNT) bundles are vital processes for utilizing this one-dimensional nanomaterial in electronics. Here, we report a polymer-assisted wet shearing method to acquire super-aligned crater-patterned CNT arrays by nanobubble (NB) self-assembly with a "migrate and aggregation" mechanism and use craters to controllably mold even-sized nanodisks periodically along CNT bundles with tunable densities. This green, low-cost method can be extended to diverse substrates and fabricate different nanodisks. As an example, the Ag-nanodisk-patterned CNT arrays are utilized as substrates of surface-enhanced Raman scattering (SERS) for rhodamine 6G (R6G) and methylene blue (MB) in which a linear correlation is found between the SERS intensity and the CNT bundle density due to the periodic distribution of hot spots, enabling a spectral detection of CNT bundles and their densities by conventional dye molecules. Distinguishing from routine morphological characterization, this spectral method possesses an enhanced accuracy and a detection range of 0.1–2 μm^–1, showing its uniqueness in the detection of CNT bundle density since the intensity of traditional spectral merely relates to the quantity of CNTs, exhibiting its potential in future CNT-bundle-based electronics.

Carbon nanotube-silicon (CNT-Si) solar cells represent one of the alternative photovoltaic techniques with potential for low cost and high efficiency. Here, we report a method to improve solar cell performance by depositing conventional transitional metal oxides such as WO₃ and establishing a collaborative system, in which CNTs are well-embedded within the WO₃ layer and both of them are in close contact to Si substrate. This unique collaborative system optimizes the overall energy conversion process including the light absorption (antireflection by WO₃), carrier separation (forming quasi p-n junction) and charge collection (CNT conductive network throughout the oxide layer). Combining with our previous TiO₂-coating and HNO₃-doping techniques, a solar cell efficiency of >18% at an active area of 0.09 cm ² (air mass 1.5, 100 mW/cm²) was achieved. The oxide-enhanced CNT-Si solar cells which integrate the advantages of traditional semiconductors and novel nanostructures represent a promising route toward next-generation high-performance silicon-based photovoltaics.

Graphene quantum dots (GQDs), have unique quantum confinement effects, tunable bandgap and luminescence property, with a wide range of potential applications such as optoelectronic and biomedical areas. However, GQDs usually have a strong tendency toward aggregation especially in making solid films, which will degrade their optoelectronic properties, for example, causing undesired fluorescence quenching. Here, we designed a composite film by embedding GQDs in a polyvinyl pyrrolidone (PVP) matrix through hydrogen bonding with well-preserved fluorescence, with a small addition of acid for compensating the poor conductivity of PVP. As a multifunctional solid coating on carbon nanotube/silicon (CNT/Si) solar cells, the photon down-conversion by GQDs and the PVP anti-reflection layer for visible light lead to enhanced external quantum efficiency (by 12.34% in the ultraviolet (UV) range) and cell efficiency (up to 14.94%). Such advanced optical managing enabled by low-cost, carbon-based quantum dots, as demonstrated in our results, can be applied to more versatile optoelectronic and photovoltaic devices based on perovskites, organic and other materials.

Since Akira Yoshino first proposed the usage of the carbonaceous materials as an anode of lithium ion batteries (LIBs) in 1985, carbonaceous materials such as graphite and graphene have been widely considered as LIB anodes. Here, we explored the application of novel carbonaceous LIB anodes incorporating graphene quantum dots (GQDs). We fabricated a freestanding all-carbon electrode based on a porous carbon nanotube (CNT) sponge via a facile in-situ hydrothermal deposition technique, creating coaxial structure of GQD-coated CNTs (GQD@CNTs) through electrostatic interaction and π-π stacking with tunable loading and functionalization. This hybrid structure combined conductive CNTs with highly active GQDs, in which GQDs with predesigned functional groups provided massive storage sites for Li ions and the 3D CNT frameworks avoided the agglomeration of GQDs, together contributing to a high specific capacity (700 mAh·g^-1 at 100 mA·g^-1 after 100 cycles) and rate performance. Even at a high current density of 1,000 mA·g^-1, the reversible specific capacity remained at 483 mAh·g^-1 after 350 cycles. In particular, the mechanism study demonstrated the important role of oxygen functional groups of GQDs in promoting the performance of the LIB anodes by controlled grafting of GQDs onto various porous-carbon and metal-foam based structures.

Transitional metal oxides (TMOs) are important functional materials in silicon-based and thin-film optoelectronics. Here, TMOs are applied in carbon nanotube (CNT)-Si solar cells by spin-coating solutions of metal chlorides that undergo favorable transformation in ambient conditions. An unconventional change in solar cell behavior is observed after coating two particular chlorides (MoCl₅ and WCl₆, respectively), characterized by an initial severe degradation followed by gradual recovery and then well surpassing the original performance. Detailed analysis reveals that the formation of corresponding oxides (MoO₃ and WO₃) enables two primary functions on both CNTs (p-type doping) and Si (inducing inversion layer), leading to significant improvement in open-circuit voltage and fill factor, with power conversion efficiencies up to 13.0% (MoO₃) and 13.4% (WO₃). Further combining with other chlorides to increase the short-circuit current, ultimate cells efficiencies achieve >16% with over 90% retention after 24 h, which are among the highest stable efficiencies reported for CNT-Si solar cells. The transformation of functional layers as demonstrated here has profound influence on the device characteristics, and represents a potential strategy in low-cost manufacturing of next-generation high efficiency photovoltaics.

There have been intensive and continuous research efforts in large-scale controlled assembly of one-dimensional (1D) nanomaterials, since this is the most effective and promising route toward advanced functional systems including integrated nano-circuits and flexible electronic devices. To date, numerous assembly approaches have been reported, showing considerable progresses in developing a variety of 1D nanomaterial assemblies and integrated systems with outstanding performance. However, obstacles and challenges remain ahead. Here, in this review, we summarize most widely studied assembly approaches such as Langmuir-Blodgett technique, substrate release/stretching, substrate rubbing and blown bubble films, depending on three types of external forces: compressive, tensile and shear forces. We highlight the important roles of these mechanical forces in aligning 1D nanomaterials such as semiconducting nanowires and carbon nanotubes, and discuss each approach on their effectiveness in achieving high-degree alignment, distinct characteristics and major limitations. Finally, we point out possible research directions in this field including rational control on the orientation, density and registration, toward scale-up and cost-effective manufacturing, as well as novel assembled systems based on 1D heterojunctions and hybrid structures.

The ability to tailor and enhance photoluminescence (PL) behavior in two-dimensional (2D) transition metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS₂) is significant for pursuing optoelectronic applications. To achieve this, it has been essential to obtain high-quality single-layer MoS₂ and fully explore its intrinsic PL performance. Here, we fabricate single-layer MoS₂ by a thermal vapor sulfurization method in which a pre-deposited molybdenum trioxide (MoO₃) thin film is sulfurized over a short period (for several minutes) to turn into MoS₂. These as-grown MoS₂ crystals show quite strong PL, which is about one order of magnitude higher than that of chemical- vapor-deposited MoS₂. Temperature- and power-dependent spectroscopy measurements disclose the apparent influence of sulfur (S) vacancies on the PL behavior and the noticeable free-to-bound exciton recombinations in the luminescence process. The fact that PL intensity of the sample in vacuum sharply lowered down relative to in air reveals that the high PL is facilitated by molecular adsorption on S vacancies in air. And multi-channel decay processes coupled with S vacancies are revealed in the time-resolved PL spectroscopy. In our work, single-layer MoS₂ with high PL is synthesized and its defect-induced PL features are analyzed, which is of great importance for developing advanced nano-electronics and optoelectronics based on 2D structures.

Hybridization of carbon nanotubes (CNT) with graphene provides a promising means of integrating the attributes of both materials, thereby enabling widespread application. Here, we present a method to directly assemble hybrid CNT-graphene films by a blown bubble method combined with selective substrate annealing. We use polymethylmethacrylate (PMMA) as the polymeric matrix to blow bubbles containing self-assembled multi-walled CNT arrays, and then transform the bubble film into a CNT-graphene hybrid film by thermal annealing on a Cu substrate; PMMA serves as the carbon source for growing single to few-layer graphene among the CNT network until a continuously hybridized structure is formed. Compared to the bare (non-hybridized) CNT networks, the hybrid films exhibit improved electrical conductivity and structural integrity. Our method also enables the fabrication of a multi-walled CNT-Si solar cell, which has high power conversion efficiency, through the assembly of hybrid CNT-graphene structures.

Controlled synthesis of hierarchically assembled titanium dioxide (TiO₂) nanostructures is important for practical applications in environmental purification and solar energy conversion. We present here the fabrication of interconnected TiO₂ nanotubes as a macroscopic bulk material by using a porous carbon nanotube (CNT) sponge as a template. The basic idea is to uniformly coat an amorphous titania layer onto the CNT surface by the infiltration of a TiO₂ precursor into the sponge followed by a subsequent hydrolysis process. After calcination, the CNTs are completely removed and the titania is simultaneously crystallized, which results in a porous macrostructure composed of interconnected anatase TiO₂ nanotubes. The TiO₂ nanotube macrostructures show comparable photocatalytic activities to commercial products (AEROXIDE TiO₂ P25) for the degradation of rhodamine B (RhB). Moreover, the TiO₂ nanotube macrostructures can be settled and separated from water within 12 h after photocatalysis, whereas P25 remains suspended in solution after weeks. Thus the TiO₂ nanotube macrostructures offer the advantage of easy catalyst separation and recycle and can be a promising candidate for wastewater treatment.

A carbon nanotube (CNT) sponge contains a three-dimensional conductive nanotube network, and can be used as a porous electrode for various energy devices. We present here a rational strategy to fabricate a unique CNT@polypyrrole (PPy) core–shell sponge, and demonstrate its application as a highly compressible supercapacitor electrode with high performance. A PPy layer with optimal thickness was coated uniformly on individual CNTs and inter-CNT contact points by electrochemical deposition and crosslinking of pyrrole monomers, resulting in a core–shell configuration. The PPy coating significantly improves specific capacitance of the CNT sponge to above 300 F/g, and simultaneously reinforces the porous structure to achieve better strength and fully elastic structural recovery after compression. The CNT@PPy sponge can sustain 1, 000 compression cycles at a strain of 50% while maintaining a stable capacitance (> 90% of initial value). Our CNT@PPy core–shell sponges with a highly porous network structure may serve as compressible, robust electrodes for supercapacitors and many other energy devices.