Nanotechnology

This review is concerned with the leading methods of bottom-up material preparation for thermal-to-electrical energy interconversion. The advantages, capabilities, and challenges from a material synthesis perspective are surveyed and the methods are discussed with respect to their potential for improvement (or possibly deterioration) of application-relevant transport properties. Solution chemistry-based synthesis approaches are re-assessed from the perspective of thermoelectric applications based on reported procedures for nanowire, quantum dot, mesoporous, hydro/solvothermal, and microwave-assisted syntheses as these techniques can effectively be exploited for industrial mass production. In terms of energy conversion efficiency, the benefit of self-assembly can occur from three paths: suppressing thermal conductivity, increasing thermopower, and boosting electrical conductivity. An ideal thermoelectric material gains from all three improvements simultaneously. Most bottom-up materials have been shown to exhibit very low values of thermal conductivity compared to their top-down (solid-state) counterparts, although the main challenge lies in improving their poor electrical properties. Recent developments in the field discussed in this review reveal that the traditional view of bottom-up thermoelectrics as inferior materials suffering from poor performance is not appropriate. Thermopower enhancement due to size and energy filtering effects, electrical conductivity enhancement, and thermal conductivity reduction mechanisms inherent in bottom-up nanoscale self-assembly syntheses are indicative of the impact that these techniques will play in future thermoelectric applications.

Titanium and titanium alloys exhibit a unique combination of strength and biocompatibility, which enables their use in medical applications and accounts for their extensive use as implant materials in the last 50 years. Currently, a large amount of research is being carried out in order to determine the optimal surface topography for use in bioapplications, and thus the emphasis is on nanotechnology for biomedical applications. It was recently shown that titanium implants with rough surface topography and free energy increase osteoblast adhesion, maturation and subsequent bone formation. Furthermore, the adhesion of different cell lines to the surface of titanium implants is influenced by the surface characteristics of titanium; namely topography, charge distribution and chemistry. The present review article focuses on the specific nanotopography of titanium, i.e. titanium dioxide (TiO ₂) nanotubes, using a simple electrochemical anodisation method of the metallic substrate and other processes such as the hydrothermal or sol-gel template. One key advantage of using TiO ₂ nanotubes in cell interactions is based on the fact that TiO ₂ nanotube morphology is correlated with cell adhesion, spreading, growth and differentiation of mesenchymal stem cells, which were shown to be maximally induced on smaller diameter nanotubes (15 nm), but hindered on larger diameter (100 nm) tubes, leading to cell death and apoptosis. Research has supported the significance of nanotopography (TiO ₂ nanotube diameter) in cell adhesion and cell growth, and suggests that the mechanics of focal adhesion formation are similar among different cell types. As such, the present review will focus on perhaps the most spectacular and surprising one-dimensional structures and their unique biomedical applications for increased osseointegration, protein interaction and antibacterial properties.

We present a novel method to trap nanoparticles in double nanohole (DNH) nanoapertures integrated on top of solid-state nanopores (ssNP). The nanoparticles were propelled by an electrophoretic force from the cis towards the trans side of the nanopore but were trapped in the process when they reached the vicinity of the DNH-ssNP interface. The self-induced back action (SIBA) plasmonic force existing between the tips of the DNH opposed the electrophoretic force and enabled simultaneous optical and electrical sensing of a single nanoparticle for seconds. The novel SIBA actuated nanopore electrophoresis (SANE) sensor was fabricated using two-beam GFIS FIB. Firstly, Ne FIB milling was used to create the DNH features and was combined with end pointing to stop milling at the metal-dielectric interface. Subsequently, He FIB was used to drill a 25 nm nanopore through the center of the DNH. Proof of principle experiments to demonstrate the potential utility of the SANE sensor were performed with 20 nm silica and Au nanoparticles. The addition of optical trapping to electrical sensing extended translocation times by four orders of magnitude. The extended electrical measurement times revealed newly observed high frequency charge transients that were attributed to bobbing of the nanoparticle driven by the competing optical and electrical forces. Frequency analysis of this bobbing behavior hinted at the possibility of distinguishing single from multi-particle trapping events. We also discuss how SANE sensor measurement characteristics differ between silica and Au nanoparticles due to differences in their physical properties and how to estimate the charge around a nanoparticle. These measurements show promise for the SANE sensor as an enabling tool for selective detection of biomolecules and quantification of their interactions.

Among two-dimensional materials, semiconducting ultrathin sheets of MoS ₂ are promising for nanoelectronics. We show how a scanning probe microscope (SPM) can be used to image the flow of electrons in a MoS ₂ Hall bar sample at 4.2 K allowing us to understand device physics at the nanoscale. The SPM tip acts as a movable gate and capacitively couples the SPM tip to the device below. By measuring the change in device conductance as the tip is raster scanned across the sample, spatial maps of the device conductance can be obtained. We present images showing the characteristic ‘bullseye’ pattern of Coulomb blockade conductance rings around a quantum dot formed in a narrow contact as the carrier density is depleted with a backgate. These images show that multiple dots are created by the disorder potential in MoS ₂. From these SPM images, we estimate the size and position of these quantum dots using a capacitive model.

Integrating layered two-dimensional (2D) materials into 3D heterostructures offers opportunities for novel material functionalities and applications in electronics and photonics. In order to build the highest quality heterostructures, it is crucial to preserve the cleanliness and morphology of 2D material surfaces that come in contact with polymers such as PDMS during transfer. Here we report that substantial residues and up to ∼0.22% compressive strain can be present in monolayer MoS ₂ transferred using PDMS. We show that a UV-ozone pre-cleaning of the PDMS surface before exfoliation significantly reduces organic residues on transferred MoS ₂ flakes. An additional 200 ^◦C vacuum anneal after transfer efficiently removes interfacial bubbles and wrinkles as well as accumulated strain, thereby restoring the surface morphology of transferred flakes to their native state. Our recipe is important for building clean heterostructures of 2D materials and increasing the reproducibility and reliability of devices based on them.

We demonstrate x-ray absorption fine structure spectroscopy (XAFS) detected by x-ray beam induced current (XBIC) in single n ⁺ -i-n ⁺ doped nanowire devices. Spatial scans with the 65 nm diameter beam show a peak of the XBIC signal in the middle segment of the nanowire. The XBIC and the x-ray fluorescence signals were detected simultaneously as a function of the excitation energy near the Ga K absorption edge at 10.37 keV. The spectra show similar oscillations around the edge, which shows that the XBIC is limited by the primary absorption. Our results reveal the feasibility of the XBIC detection mode for the XAFS investigation in nanostructured devices.

Transition metal dichalcogenides (TMDs) have attracted considerable interest for exploration of next-generation electronics and optoelectronics in recent years. Fabrication of in-plane lateral heterostructures between TMDs has opened up excellent opportunities for engineering two-dimensional materials. The creation of high quality heterostructures with a facile method is highly desirable but it still remains challenging. In this work, we demonstrate a one-step growth method for the construction of high-quality MoS ₂–WS ₂ in-plane heterostructures. The synthesis was carried out using ambient pressure chemical vapor deposition (APCVD) with the assistance of sodium chloride (NaCl). It was found that the addition of NaCl played a key role in lowering the growth temperatures, in which the Na-containing precursors could be formed and condensed on the substrates to reduce the energy of the reaction. As a result, the growth regimes of MoS ₂ and WS ₂ are better matched, leading to the formation of in-plane heterostructures in a single step. The heterostructures were proved to be of high quality with a sharp and clear interface. This newly developed strategy with the assistance of NaCl is promising for synthesizing other TMDs and their heterostructures.

MoO ₂ is used as a new source material for the growth of large area and high optical quality monolayer MoS ₂. However, a systematic study of the growth parameters is still missing and large-area growth of discreet single crystals is still challenging. Hereby, we report the shape evolution of monolayer growth of MoS ₂ and develop a methodology to achieve centimeter-scaled discrete MoS ₂ by adopting MoO ₂ as Mo source material in an atmospheric-pressure chemical vapor deposition process. Our results indicate the growth of monolayer MoS ₂ could benefit from the precise control of the introduction time of sulfur and the S/MoO ₂ ratio in experiments. Micro-Raman and photoluminescence spectra confirm the properties of the material. E-beam lithography was utilized to make contact with the as-grown MoS ₂ located at the selective area. The electrical properties of MoS ₂ with different morphologies were compared. In the end, the persistent photoconductivity properties of monolayer MoS ₂ were emphasized and the underlying mechanism was proposed. These studies demonstrate a better understanding of the growth and application of MoS ₂-based 2D materials.

Despite advances in cancer therapy, treating cancer after it has metastasized remains an unmet clinical challenge. In this study we demonstrate that 100 nm liposomes target triple-negative murine breast-cancer metastases post intravenous administration. Metastatic breast cancer was induced in BALB/c mice either experimentally, by a tail vein injection of 4T1 cells, or spontaneously, after implanting a primary tumor xenograft. To track their biodistribution in vivo the liposomes were labeled with multi-modal diagnostic agents, including indocyanine green and rhodamine for whole-animal fluorescent imaging, gadolinium for magnetic resonance imaging (MRI), and europium for a quantitative biodistribution analysis. The accumulation of liposomes in the metastases peaked at 24 h post the intravenous administration, similar to the time they peaked in the primary tumor. The efficiency of liposomal targeting to the metastatic tissue exceeded that of a non-liposomal agent by 4.5-fold. Liposomes were detected at very early stages in the metastatic progression, including metastatic lesions smaller than 2 mm in diameter. Surprisingly, while nanoparticles target breast cancer metastasis, they may also be found in elevated levels in the pre-metastatic niche, several days before metastases are visualized by MRI or histologically in the tissue. This study highlights the promise of diagnostic and therapeutic nanoparticles for treating metastatic cancer, possibly even for preventing the onset of the metastatic dissemination by targeting the pre-metastatic niche.

Here, we report the synthesis and spectral properties of ultrathin nanodiscs (NDs) of Y ₂O ₃:Eu ³⁺. It was found that the NDs of Y ₂O ₃:Eu ³⁺ with a thickness of about 1 nm can be fabricated in a reproducible, facile and self-assembling process, which does not depend on the Eu ³⁺ concentration. The thickness and morphology of these NDs were determined with small angle x-ray scattering and transmission electron microscopy. We found that the crystal field in these nanoparticles deviates from both the cubic and monoclinic characteristics, albeit the shape of the ⁵D ₀ →  ⁷F _J ( J = 0, 1, 2) transitions shows some similarity with the transitions in the monoclinic material. The Raman spectra of the non-annealed NDs manifest various vibration modes of the oleic acid molecules, which are used to stabilise the NDs. The annealed NDs show two very weak Raman lines, which may be assigned to vibrational modes of Y ₂O ₃ NDs. The concentration quenching of the Eu ³⁺ luminescence of the NDs before annealing is largely suppressed and might be explained in terms of a reduction of the phonon density of states.

Apart from conventional materials, the study of two-dimensional (2D) materials has emerged as a significant field of study for a variety of applications. Graphene-like 2D materials are important elements of potential optoelectronics applications due to their exceptional electronic and optical properties. The processing of these materials towards the realization of devices has been one of the main motivations for the recent development of photonics and optoelectronics. The recent progress in photonic devices based on graphene-like 2D materials, especially topological insulators (TIs) and transition metal dichalcogenides (TMDs) with the methodology level discussions from the viewpoint of state-of-the-art designs in device geometry and materials are detailed in this review. We have started the article with an overview of the electronic properties and continued by highlighting their linear and nonlinear optical properties. The production of TIs and TMDs by different methods is detailed. The following main applications focused towards device fabrication are elaborated: (1) photodetectors, (2) photovoltaic devices, (3) light-emitting devices, (4) flexible devices and (5) laser applications. The possibility of employing these 2D materials in different fields is also suggested based on their properties in the prospective part. This review will not only greatly complement the detailed knowledge of the device physics of these materials, but also provide contemporary perception for the researchers who wish to consider these materials for various applications by following the path of graphene.

Optical chiral metamaterials have recently attracted considerable attention because they offer new and exciting opportunities for fundamental research and practical applications. Through pragmatic designs, the chiroptical response of chiral metamaterials can be several orders of magnitude higher than that of natural chiral materials. Meanwhile, the local chiral fields can be enhanced by plasmonic resonances to drive a wide range of physical and chemical processes in both linear and nonlinear regimes. In this review, we will discuss the fundamental principles of chiral metamaterials, various optical chiral metamaterials realized by different nanofabrication approaches, and the applications and future prospects of this emerging field.

Black phosphorus (BP), the bulk counterpart of monolayer phosphorene, is a relatively stable phosphorus allotrope at room temperature. However, monolayer phosphorene and ultra-thin BP layers degrade in ambient atmosphere. In this paper, we report the investigation of BP oxidation and discuss the reaction mechanism based on the x-ray photoelectron spectroscopy (XPS) data. The kinetics of BP oxidation was examined under various well-controlled conditions, namely in 5% O ₂/Ar, 2.3% H ₂O/Ar, and 5% O ₂ and 2.3% H ₂O/Ar. At room temperature, the BP surface is demonstrated not to be oxidized at a high oxidation rate in 5% O ₂/Ar nor in 2.3% H ₂O/Ar, according to XPS, with the thickness of the oxidized phosphorus layer <5 Å for 5 h. On the other hand, in the O ₂/H ₂O mixture, a 30 Å thickness oxide layer was detected already after 2 h of the treatment. This result points to a synergetic effect of water and oxygen in the BP oxidation. The oxidation effect was also studied in applications to the electrical measurements of BP field-effect transistors (FETs) with or without passivation. The electrical performance of BP FETs with atomic layer deposition (ALD) dielectric passivation or h-BN passivation formed in a glove-box environment are also presented.

A series of WO ₃/g-C ₃N ₄ composites with different WO ₃ contents were prepared via a facile one-pot pyrolysis method, and showed notably enhanced visible-light-driven photocatalytic H ₂-evolution activities, with the highest rate of 400 μmol h ⁻¹ g _cat ⁻¹ that was 15.0 times of that for pristine g-C ₃N ₄. Contents and sizes of WO ₃ crystallites in the composites were easily adjusted by changing the molar ratios of (NH ₄) ₂WS ₄ to C ₃H ₆N ₆ in the feed reagents, thereby successfully optimizing the Z-scheme system constructed by WO ₃ and g-C ₃N ₄ and thus effectively reducing the recombination of photogenerated charge carriers in g-C ₃N ₄. Moreover, pore volumes and surface areas of the composites were gradually enlarged by introducing WO ₃ into g-C ₃N ₄ via the one-pot preparation strategy, therefore promoting the redox reactions to evolve H ₂. This work presented an effective route to simultaneously optimize the phase compositions and textural structures of photocatalysts for enhanced H ₂ evolution.

By depositing graphene circular double rings (DR) on a SiO ₂/Si/polymer substrate, the tunable Fano resonance has been theoretically investigated in the terahertz regime, including the effects of the graphene Fermi level, structural parameters and operation frequency. The results demonstrate that the obvious Fano peak can be efficiently modulated because of strong coupling between the incident waves and graphene ribbons. As the Fermi level increases, the peak amplitude of the Fano curve increases, and the resonant peak position shifts to a high frequency. The amplitude modulation depth of the Fano curves is about 30% if the Fermi level changes in the scope of 0.1–1.0 eV. The optimum gap distance between the DR is about 8–12 μm, where the value of the figure of merit shows a peak. The results are very helpful in order to develop novel graphene plasmonic devices, e.g. sensors and modulators.

Recently, DNA molecules embedded with magnetite (Fe ₃O ₄) nanoparticles (MNPs) drew much attention for their wide range of potential usage. With specific intrinsic properties such as low optical loss, high transparency, large band gap, high dielectric constant, potential for molecular recognition, and their biodegradable nature, the DNA molecule can serve as an effective template or scaffold for various functionalized nanomaterials. With the aid of cetyltrimethylammonium (CTMA) surfactant, DNA can be used in organic-based applications as well as water-based ones. Here, DNA and CTMA-DNA thin films with various concentrations of MNPs fabricated by the drop-casting method have been characterized by optical absorption, refractive index, Raman, and cathodoluminescence measurements to understand the binding, dispersion, chemical identification/functional modes, and energy transfer mechanisms, respectively. In addition, magnetization was measured as a function of either applied magnetic field or temperature in field cooling and zero field cooling. Saturation magnetization and blocking temperature demonstrate the importance of MNPs in DNA and CTMA-DNA thin films. Finally, we examine the thermal stabilities of MNP-embedded DNA and CTMA-DNA thin films through thermogravimetric analysis, derivative thermogravimetry, and differential thermal analysis. The unique optical, magnetic, and thermal characteristics of MNP-embedded DNA and CTMA-DNA thin films will prove important to fields such as spintronics, biomedicine, and function-embedded sensors and devices.

Widegap III-nitride alloys have enabled new classes of optoelectronic devices including light emitting diodes, lasers and solar cells, but it is admittedly challenging to extend their operating wavelength to the yellow–red band. This requires an increased In content x in In _x Ga _{1− x} N, prevented by the indium segregation within the miscibility gap. Beyond the known advantage of dislocation-free growth on dissimilar substrates, nanowires may help to extend the compositional range of InGaN. However, the necessary control over the material homogeneity is still lacking. Here, we present In _x Ga _{1− x} N nanowires grown by hydride vapor phase epitaxy on silicon substrates, showing rather homogeneous compositions and emitting from blue to red. The InN fraction in nanowires is tuned from x = 0.17 up to x = 0.7 by changing the growth temperature between 630 °C and 680 °C and adjusting some additional parameters. A dedicated model is presented, which attributes the wide compositional range of nanowires to the purely kinetic growth regime of self-catalyzed InGaN nanowires without macroscopic nucleation. These results may pave a new way for the controlled synthesis of indium-rich InGaN structures for optoelectronic applications in the extended spectral range.

Iron oxide nanoparticles (NPs) possessing peroxidase-like catalytic activity have been widely explored in recent decades, owing to their high stability against harsh conditions, low cost, flexibility in structure design and composition, adjustable activities and excellent biocompatibility in comparison with natural enzymes. Recently, a lot of great achievements have been made in this field of iron oxide nanozymes, however, this research has now reached a bottleneck in that the maximum activity enhancement is difficult to achieve via a material design. Hence, in this work, visible light was introduced to improve the peroxidase-like activity of Fe ₂O ₃ NPs synthesized via a combination of electrospinning technology and hydrothermal reaction. Our results showed that with the assistance of visible light, Fe ₂O ₃ NPs exhibited at least 2.2-fold higher peroxidase activity than those tested under darkness, confirming the superiorly visible light promoted peroxidase-like catalytic activity of Fe ₂O ₃ NPs. Furthermore, the affinity and maximum reaction velocity of Fe ₂O ₃ nanoflowers (bandgap = 1.78 eV) towards 3,3’,5,5’-tetramethylbenanozymeidine (TMB) were at least over 3.7 and 4.3 times greater than in Fe ₂O ₃ nanocubes (bandgap = 2.08 eV), suggesting that the reaction performance of semiconductors could be controlled by proper adjustment of the bandgap. Moreover, the Fe ₂O ₃ NPs were also successfully utilized to detect glucose. Herein, we believe that the present work exhibits a fascinating perspective for peroxidase-like catalytic fields.

Impurity addition is a crucial aspect for III–V nanowire growth. In this study, we demonstrated the effect of the Sn addition on GaAs nanowire growth by metal-organic chemical vapor deposition. With increasing the tetraethyltin flow rate, the nanowire axial growth was suppressed while the nanowire lateral growth was promoted, as well as planar defects were increased. Systematic electron microscopy characterizations suggested that the Sn addition tuned the catalyst composition, changed the vapor–solid–liquid surfaces energies and hindered the Ga atoms diffusion on nanowire sidewalls, which is responsible for the observed changes in morphology and structural quality of grown GaAs nanowires. This study contributes to understanding the role of impurity dopants on III–V nanowires growth, which will be of benefit for the design and fabrication of future nanowire-based devices.

We demonstrate a highly selective and reversible NO ₂ resistive gas sensor using vertically aligned MoS ₂ (VA-MoS ₂) flake networks. We synthesized horizontally and vertically aligned MoS ₂ flakes on SiO ₂/Si substrate using a kinetically controlled rapid growth CVD process. Uniformly interconnected MoS ₂ flakes and their orientation were confirmed by scanning electron microscopy, x-ray diffraction, Raman spectroscopy and x-ray photoelectron spectroscopy. The VA-MoS ₂ gas sensor showed two times higher response to NO ₂ compared to horizontally aligned MoS ₂ at room temperature. Moreover, the sensors exhibited a dramatically improved complete recovery upon NO ₂ exposure at its low optimum operating temperatures (100 °C). In addition, the sensing performance of the sensors was investigated with exposure to various gases such as NH ₃, CO ₂, H ₂, CH ₄ and H ₂S. It was observed that high response to gas directly correlates with the strong interaction of gas molecules on edge sites of the VA-MoS ₂. The VA-MoS ₂ gas sensor exhibited high response with good reversibility and selectivity towards NO ₂ as a result of the high aspect ratio as well as high adsorption energy on exposed edge sites.

The rapid development of advanced nanotechnology has continuously changed many aspects of society. One important nanostructured material, magnetic nanoparticles (NPs), has applications in many areas including clean energy, biology and engineering because of their special magnetic properties. The synthesis of magnetic nanomaterials with desired sizes and morphology has attracted great attention. Nanomaterials with different properties can be combined to construct multifunctional nanoplatforms through systematic surface engineering. The surface modification of magnetic NPs presents the opportunity for them to be used in many practical applications. Functionalized magnetic NPs have been successfully applied in catalysis, as thermoelectric materials, for drug delivery, as imaging agents in nuclear magnetic resonance and in biosensors. In this review, synthetic methods for magnetic NPs and some of their important properties are described. Then the latest progress of the application of magnetic NPs in energy and biology has been summarized and discussed. Finally, we discuss some issues that still need to be solved and the prospects for magnetic NPs.

The need for increasingly powerful computing hardware has spawned many ideas stipulating, primarily, the replacement of traditional transistors with alternate ‘switches’ that dissipate miniscule amounts of energy when they switch and provide additional functionality that are beneficial for information processing. An interesting idea that has emerged recently is the notion of using two-phase (piezoelectric/magnetostrictive) multiferroic nanomagnets with bistable (or multi-stable) magnetization states to encode digital information (bits), and switching the magnetization between these states with small voltages (that strain the nanomagnets) to carry out digital information processing. The switching delay is ∼1 ns and the energy dissipated in the switching operation can be few to tens of aJ, which is comparable to, or smaller than, the energy dissipated in switching a modern-day transistor. Unlike a transistor, a nanomagnet is ‘non-volatile’, so a nanomagnetic processing unit can store the result of a computation locally without refresh cycles, thereby allowing it to double as both logic and memory. These dual-role elements promise new, robust, energy-efficient, high-speed computing and signal processing architectures (usually non-Boolean and often non-von-Neumann) that can be more powerful, architecturally superior (fewer circuit elements needed to implement a given function) and sometimes faster than their traditional transistor-based counterparts. This topical review covers the important advances in computing and information processing with nanomagnets, with emphasis on strain-switched multiferroic nanomagnets acting as non-volatile and energy-efficient switches—a field known as ‘straintronics’. It also outlines key challenges in straintronics.

This review is concerned with the leading methods of bottom-up material preparation for thermal-to-electrical energy interconversion. The advantages, capabilities, and challenges from a material synthesis perspective are surveyed and the methods are discussed with respect to their potential for improvement (or possibly deterioration) of application-relevant transport properties. Solution chemistry-based synthesis approaches are re-assessed from the perspective of thermoelectric applications based on reported procedures for nanowire, quantum dot, mesoporous, hydro/solvothermal, and microwave-assisted syntheses as these techniques can effectively be exploited for industrial mass production. In terms of energy conversion efficiency, the benefit of self-assembly can occur from three paths: suppressing thermal conductivity, increasing thermopower, and boosting electrical conductivity. An ideal thermoelectric material gains from all three improvements simultaneously. Most bottom-up materials have been shown to exhibit very low values of thermal conductivity compared to their top-down (solid-state) counterparts, although the main challenge lies in improving their poor electrical properties. Recent developments in the field discussed in this review reveal that the traditional view of bottom-up thermoelectrics as inferior materials suffering from poor performance is not appropriate. Thermopower enhancement due to size and energy filtering effects, electrical conductivity enhancement, and thermal conductivity reduction mechanisms inherent in bottom-up nanoscale self-assembly syntheses are indicative of the impact that these techniques will play in future thermoelectric applications.

A metal-free photocatalyst, graphitic carbon nitride (GCN) with a moderate band gap catering for visible-light excitation, shows amazing potential in various photocatalytic applications. Carbon dioxide reduction using diversified photocatalysts has been attracting increasing public attention and the extensively studied GCN is one of the most promising photocatalysts. However, because of the low concentration and high recombination rate of photogenerated carriers, and some other disadvantages of the pristine GCN photocatalyst, the solar-to-fuel conversion efficiency is too low for practical use. Modifications or optimizations of GCN are therefore important to enhance its CO ₂ photocatalytic conversion performance. This review summarizes the research progress made during the past five years on GCN-based photocatalysts in two main areas, which includes pristine GCN and its molecular modifications, and heterostructure composite photocatalysts based on GCN, for CO ₂ reduction. It is expected that this review may benefit the development of GCN-based photocatalysts for CO ₂-to-fuel conversion.

Surface plasmons (SPs), in which the free electrons are collectively excited on the metal surface, have been successfully used in chemical analysis and signal detection. Generally, SPs possess two types of decay channels. SPs decay either nonradiatively via the generation of hot electrons or radiatively through re-emitted photons, which can trigger surface chemical reactions when the molecules are adsorbed on the surface of metal nanoparticles. An excitation light with a special wavelength is irradiated on the surface of the plasmonic nanostructure, the strong coupling interaction between electrons and light will then occur on this, and this is followed by the development of a series of unique properties. 2D materials have been a hot topic of research for more than a decade, since graphene was found in 2004. Recently, the combination of graphene with metal NPs has been shown to possess many supernormal advantages, such as high stability and catalytic activity, which have been successfully applied in plasmon–exciton co-driven chemical reactions.