To obtain graphene-based fluorescent materials, one of the effective approaches is to convert one-dimensional (1D) graphene to 0D graphene quantum dots (GQDs), yielding an emerging nanolight with extraordinary properties due to their remarkable quantum confinement and edge effects. In this review, the state-of-the-art knowledge of GQDs is presented. The synthetic methods were summarized, with emphasis on the top-down routes which possess the advantages of abundant raw materials, large scale production and simple operation. Optical properties of GQDs are also systematically discussed ranging from the mechanism, the influencing factors to the optical tunability. The current applications are also reviewed, followed by an outlook on their future and potential development, involving the effective synthetic methods, systematic photoluminescent mechanism, bandgap engineering, in addition to the potential applications in bioimaging, sensors, etc.

Focusing on luminescent graphene quantum dots: current status and future perspectives

The bandgap in graphene-based materials can be tuned from 0 eV to that of benzene by changing size and/or surface chemistry, making it a rising carbonbased fl uorescent material. Here, the surface chemistry of small size graphene (graphene quantum dots, GQDs) is tuned programmably through modifi cation or reduction and green luminescent GQDs are changed to blue luminescent GQDs. Several tools are employed to characterize the composition and morphology of resultants. More importantly, using this system, the luminescence mechanism (the competition between both the defect state emission and intrinsic state emission) is explored in detail. Experiments demonstrate that the chemical structure changes during modifi cation or reduction suppresses non-radiative recombination of localized electron-hole pairs and/or enhances the integrity of surface π electron network. Therefore the intrinsic state emission plays a leading role, as opposed to defect state emission in GQDs. The results of time-resolved measurements are consistent with the suggested PL mechanism. Up-conversion PL of GQDs is successfully applied in near-IR excitation for bioimaging.

Surface Chemistry Routes to Modulate the Photoluminescence of Graphene Quantum Dots: From Fluorescence Mechanism to Up‐Conversion Bioimaging Applications

SnO2: A comprehensive review on structures and gas sensors

Graphene oxide (GO) is a graphene sheet modified with oxygen functional groups in the form of epoxy and hydroxy groups on the basal plane and various other types at the edges. It exhibits interesting steady-state photoluminescence (PL) properties. For example, low-energy fluorescence in red to near infrared (NIR) wavelengths (from 600– 1100 nm) has been detected for suspensions and solid thin films of as-synthesized GO. 3] In addition, broad luminescence from 400 to 800 nm from oxygen plasma-treated, mechanically exfoliated, single-layer graphene sheet has been reported. Blue fluorescence with a relatively narrow bandwidth when excited with UV irradiation has also been detected from chemically reduced GO (rGO) and graphene quantum dots. 6] Recently, chemically modified GO or rGO with n-butylamine or Mn has also demonstrated PL emission at a range of energies. 10] A detailed explanation of the origin of such variable energy PL in GO has yet to be elucidated. This is partly because the sample preparation and reduction methods varied, making it difficult to compare the results. Herein, we have prepared GO suspensions that exhibit virtually all of the PL features observed by different groups, through careful and gradual reduction of the GO. The systematic evolution of the electronic structure and comprehensive analysis of steady-state and transient PL along with photoluminescence excitation (PLE) spectroscopy measurements indicate that two different types of electronically excited states are responsible for the observed emission characteristics. GO was synthesized using the modified Hummers method, the details of which have been reported. GO usually contains a large fraction of sp hybridized carbon atoms bound to oxygen functional groups, which makes it an insulator. Reduction can be achieved chemically (e.g. hydrazine exposure) or by thermal annealing in inert environments. Photothermal reduction of GO can be achieved by exposing GO samples to a Xenon flash in ambient conditions. In this study, we prepared aqueous GO solutions and subjected them to steady-state Xe lamp irradiation (500 W) with different exposure times of up to three hours. In contrast to reduction by an instantaneous flash, this method provides a controllable, gradual transformation from GO to rGO, allowing exploration of the PL evolution and emission mechanisms from as-synthesized GO to rGO. The deoxygenation of GO after reduction was confirmed by X-ray photoelectron spectroscopy (XPS), as shown in Figure 1. The C 1s signals of the original GO can be deconvoluted into signals for the C=C bond in aromatic rings (284.6 eV), C O bond (286.1 eV), C=O bond (287.5 eV), and C(=O) OH bond (289.2 eV), in agreement with previous assignments. Increased sp carbon bonding with increased reduction time can be clearly measured, which

Tunable photoluminescence from graphene oxide.

Electromagnetic Response and Energy Conversion for Functions and Devices in Low‐Dimensional Materials

Core–shell zinc oxide microspheres mechanically vibrate and emit radiation at terahertz frequencies with a quantum emission efficiency of 33% when illuminated by a continuous-wave green laser.

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Green light stimulates terahertz emission from mesocrystal microspheres

Vacancy structures in tin oxide nanoribbons fabricated via thermal evaporation and post-processing are probed by luminescence spectroscopy, and interesting properties that bode well for oxygen sensing are observed. Besides a broad 620-nm band, the fabricated tin oxide nanoribbons show a photoluminescence band at 480 nm when the measurement temperature is <100 K. The blue band appears from nanoribbons synthesized under high oxygen pressure or annealed under oxygen. The dependence suggests that the oxygen interstitial and vacancy densities determine the electronic states that produce the blue band. Calculation of the electron structures based on the density functional theory shows that decreased oxygen vacancies or increased oxygen interstitials enhance the 480-nm band but suppress the 620-nm band. The results reported here indicate that the tin oxide nanoribbons with vacancy structures have potential applications in luminescence-sensitive oxygen sensing.

Tin oxide nanoribbons with vacancy structures in luminescence-sensitive oxygen sensing.

The photoluminescence (PL) characteristics of Mn2+-bonded reduced graphene oxide (rGO) are studied in details The Mn2+-bonded rGO is synthesized using MnO2-decorated GO as the intermediate products and ideal tunable PL is obtained by enhancing the long-wavelength (450–550 nm) emission The PL spectra excited by different wavelengths are analyzed to elucidate the mechanism, and the resonant energy transfer between Mn2+ and sp2 clusters of the rGO appears to be responsible for the enhanced long-wavelength emission To examine the effect of Mn2+ on the long-wavelength emission from the Mn2+-bonded rGO, the PL characteristics of Mn2+-bonded rGO with smaller Mn concentrations are studied and weaker emission is observed Our theoretical calculation corroborates the experimental results

Mn2+-Bonded Reduced Graphene Oxide with Strong Radiative Recombination in Broad Visible Range Caused by Resonant Energy Transfer

We have produced glycerol-bonded 3C-SiC nanocrystal (NC) films, which when excited by photons of different wavelengths, produce strong and tunable violet to blue-green (360-540 nm) emission as a result of the quantum confinement effects rendered by the 3C-SiC NCs. The emission is so intense that the emission spots are visible to the naked eyes. The light emission is very stable and even after storing in air for more than six months, no intensity degradation can be observed. X-ray photoelectron spectroscopy and absorption fine structure measurements indicate that the Si-terminated NC surfaces are completely bonded to glycerol molecules. Calculations of geometry optimization and electron structures based on the density functional theory for 3C-SiC NCs with attached glycerol molecules show that these molecules are bonded on the NCs causing strong surface structural change, while the isolated levels in the conduction band of the bare 3C-SiC NCs are replaced with quasi-continuous bands that provide continuous tunability of the emitted light by changing the frequencies of exciting laser. As an application, we demonstrate the potential of using 3C-SiC NCs to fabricate full-color emitting solid films by incorporating porous silicon.

Glycerol-Bonded 3C-SiC Nanocrystal Solid Films Exhibiting Broad and Stable Violet to Blue-Green Emission

The formation of Schottky barriers between 2D semiconductors and traditional metallic electrodes has greatly limited the application of 2D semiconductors in nanoelectronic and optoelectronic devices. In this study, metallic borophene was used as a substitute for the traditional noble metal electrode to contact with the 2D semiconductor. Theoretical calculations demonstrated that no Schottky barrier exists in the borophene/2D semiconductor heterostructure. The contact remains ohmic even with a strong electric field applied. This finding provides a way to construct 2D electronic devices and sensors with greatly enhanced performance.

S. J. Xiong

Papers

Green light stimulates terahertz emission from mesocrystal microspheres

Tin oxide nanoribbons with vacancy structures in luminescence-sensitive oxygen sensing.

Mn2+-Bonded Reduced Graphene Oxide with Strong Radiative Recombination in Broad Visible Range Caused by Resonant Energy Transfer

Glycerol-Bonded 3C-SiC Nanocrystal Solid Films Exhibiting Broad and Stable Violet to Blue-Green Emission

Monolayer borophene electrode for effective elimination of both the Schottky barrier and strong electric field effect