Effect of the Nanodiamond Host on a Nitrogen‐Vacancy Color‐Centre Emission State

Control over the quantum states of individual luminescent nitrogen‐vacancy (NV) centres in nanodiamonds (NDs) is demonstrated by careful design of the crystal host: its size, surface functional groups, and interfacing substrate. By progressive etching of the ND host, the NV centres are induced to switch from latent, through continuous, to intermittent or “blinking” emission states. The blinking mechanism of the NV centre in NDs is elucidated and a qualitative model proposed to explain this phenomenon in terms of the centre electron(s) tunnelling to acceptor site(s). These measurements suggest that the substrate material and its proximity to the NV are responsible for the fluorescence intermittency.     

Coating nanodiamonds with biocompatible shells for applications in biology and medicine

Use of nanodiamonds (NDs) as nontoxic nanoparticles for biological imaging, sensing, and drug delivery is expanding rapidly. The interest in NDs is triggered by their unique combination of optical properties. ND can accommodate nitrogen-vacancy color centers which provide stable fluorescence without photobleaching or photoblinking and their electronic structure is very sensitive to magnetic and electric fields. The limited options to control ND properties during synthesis or by direct surface functionalization leave room to be improved upon by employing surface coatings engineered precisely for a particular application. The major disadvantages of unmodified NDs are their limited colloidal stability and tendency to non-specifically adsorb biomolecules. This review aims to summarize recent advances in coating NDs (namely with silica and polymer shells), which addresses these disadvantages and enables the use of NDs in biological applications such as targeting of specific cells, drug delivery, and biological imaging.     

Improving Defect‐Based Quantum Emitters in Silicon Carbide via Inorganic Passivation

Defect‐based color centers in wide‐bandgap crystalline solids are actively being explored for quantum information science, sensing, and imaging. Unfortunately, the luminescent properties of these emitters are frequently degraded by blinking and photobleaching that arise from poorly passivated host crystal surfaces. Here, a new method for stabilizing the photoluminescence and charge state of color centers based on epitaxial growth of an inorganic passivation layer is presented. Specifically, carbon antisite‐vacancy pairs (CAV centers) in 4H‐SiC, which serve as single‐photon emitters at visible wavelengths, are used as a model system to demonstrate the power of this inorganic passivation scheme. Analysis of CAV centers with scanning confocal microscopy indicates a dramatic improvement in photostability and an enhancement in emission after growth of an epitaxial AlN passivation layer. Permanent, spatially selective control of the defect charge state can also be achieved by exploiting the mismatch in spontaneous polarization at the AlN/SiC interface. These results demonstrate that epitaxial inorganic passivation of defect‐based quantum emitters provides a new method for enhancing photostability, emission, and charge state stability of these color centers.     

Cathodoluminescence study of crystalline quality of (Al, In, Ga)N heterostructures

Wurtzite InGaN/GaN and AlGaN/GaN heterostructures grown by metal organic vapor phase epitaxy were studied using cathodoluminescence (CL) combined with secondary electron microscopy (SEM) and scanning transmission electron microscopy (STEM). The surface morphology of samples containing InGaN layers is dominated by three types of defects: mesa-like hexagonal structures, hexagonal pyramids and micropipes. At the positions of pyramids the whole epilayer is thicker than at defect free positions, while at the positions of micropipes the whole epilayer is much thinner. The luminescence efficiency as well as the emission wavelength are influenced by these defects. In SL structures an increasing SL period thickness in the growth direction was observed. Panchromatic CL images show intensity inhomogeneity in both InGaN/GaN and AlGaN/GaN heterostructure, which are related to local variations of the interface quality. In AlGaN/GaN SQW structures a broad deep-level luminescence band at around 543 nm was observed, which is generally absent in InGaN/GaN heterostructures. This deep-level emission is strongly enhanced in defect positions.     

Electron Transfer in Nanoscale Contact Electrification: Photon Excitation Effect

Contact electrification (CE) (or triboelectrification) is a well‐known phenomenon, and the identity of the charge carriers and their transfer mechanism have been discussed for decades. Recently, the species of transferred charges in the CE between a metal and a ceramic was revealed as electron transfer and its subsequent release is dominated by the thermionic emission process. Here, the release of CE‐induced electrostatic charges on a dielectric surface under photon excitation is studied by varying the light intensity and wavelength, but under no significant raise in temperature. The results suggest that there exists a threshold photon energy for releasing the triboelectric charges from the surface, which is 4.1 eV (light wavelength at 300 nm) for SiO₂ and 3.4 eV (light wavelength at 360 nm) for PVC; photons with energy smaller than this cannot effectively excite the surface electrostatic charges. This process is attributed to the photoelectron emission of the charges trapped in the surface states of the dielectric material. Further, a photoelectron emission model is proposed to describe light‐induced charge decay on a dielectric surface. The findings provide an additional strong evidence about the electron transfer process in the CE between metals and dielectrics as well as polymers.     

Optical and spin coherence properties of nitrogen-vacancy centers placed in a 100 nm thick isotopically purified diamond layer

We have studied optical and spin properties of near-surface nitrogen-vacancy (NV) centers incorporated during chemical vapor phase growth of isotopically purified (12)C single-crystal diamond layers. The spectral diffusion-limited line width of zero-phonon luminescence from the NV centers is 1.2 ± 0.5 GHz, a considerable improvement over that of NV centers formed by ion implantation and annealing. Enhanced spin dephasing times (T(2)* ≈ 90 μs, T(2) ≈ 1.7 ms) due to the reduction of (13)C nuclear spins persist even for NV centers placed within 100 nm of the surface.     

Largely extended light-emission shift of ZnSe nanostructures with temperature

ZnSe nanowires and nanobelts with zinc blende structure have been synthesized. The morphology and the growth mechanisms of the ZnSe nanostructures will be discussed. From the photoluminescence (PL) of the ZnSe nanostructures, it is interesting to note that red color emission with only a single peak at the photon energy of 2 eV at room temperature is obtained while the typical bandgap transition energy of ZnSe is 2.7 eV. When the temperature is reduced to 150 K, the peak wavelength shifts to 2.3 eV with yellowish emission and then blue emission with the peak at 2.7 eV at temperature less than 50 K. The overall wavelength shift of 700 meV is obtained as compared to the conventional ZnSe of about 100 meV (i.e., sevenfold extension). The ZnSe nanostructures with enhanced wavelength shift can potentially function as visible light temperature-indicator. The color change from red to yellowish and then to blue is large enough for the nanostructures to be used for temperature-sensing applications. The details of PL spectra of ZnSe at various temperatures are studied from (i) the spectral profile, (ii) the half-width half-maximum, and (iii) the peak photon energy of each of the emission centers. The results show that the simplified configuration coordinate model can be used to describe the emission spectra, and the frequency of the local vibrational mode of the emission centers is determined.     

Single nitrogen vacancy centers in chemical vapor deposited diamond nanocrystals

Nanodiamond crystals containing single color centers have been grown by chemical vapor deposition (CVD). The fluorescence from individual crystallites was directly correlated with crystallite size using a combined atomic force and scanning confocal fluorescence microscope. Under the conditions employed, the optimal size for single optically active nitrogen-vacancy (NV) center incorporation was measured to be 60-70 nm. The findings highlight a strong dependence of NV incorporation on crystal size, particularly with crystals less than 50 nm in size.     

One-step generation of Greenberger–Horne–Zeilinger states of multi solid-state spin qubits

We propose a one-step scheme for the generation of Greenberger–Horne–Zeilinger states of multi solid-state qubits and the implementation of quantum phase gates in a system consisting of N spatially separated nitrogen-vacancy centers coupled to the whispering-gallery mode (WGM) of a microsphere cavity. The proposed scheme is based on the effective electronic dipole–dipole interaction between electron spins associated with the NV centers, which is mediated by the WGM and applying external driving laser fields. As the spontaneous emission of the excited states of the NV centers is highly suppressed and the cavity mode is only virtually excited, the scheme is insensitive to decoherence.     

Controlled synthesis of CdS nanobelts and the study of their cathodoluminescence

Cadmium sulfide nanobelts were synthesized on an Au-capped Si substrate via chemical vapor deposition by annealing CdS powder at 860 °C. Two types of nanobelts were found. While both types grew towards [101¯0], only one type had Au particle at its tip. Room-temperature cathodoluminescence (CL) was performed on the nanobelts which were prepared between 840 and 1060 °C. Each CL spectrum exhibited two peaks. The maximum band edge emission (BEE) was found at 510 nm in all spectra while that of the deep level emission (DLE) was located between 706 to 756 nm. The ratio of intensity of BEE to that of DLE increased with increasing annealing temperature. The variation of the DLE intensity in the samples was due to the variation of defect density. A blue shift up to 49 nm was detected for DLE as the annealing temperature was increased by 160 °C. Such a shift is due to the presence of more than one type of defects in these CdS nanobelts.     

Spatially resolved luminescence properties of ZnO tetrapods

ZnO tetrapods were prepared by Zn-vapour deposition at 740 °C in Argon and subsequent oxidation in air for 1–30 min. The photoluminescence (PL) and cathodoluminescence (CL) spectra were measured from ZnO particles collected at various distances from the Zn source representing decreasing dimensions. The ZnO tetrapods showed a green emission centred at 516 nm (2.40 eV) band and the exciton emission at 387 nm (3.20 eV). The measured data suggested that the green emission is strongly increased for particle sizes below 500 nm, whereas the exciton emission is dominant for particle size larger than 500 nm. Spatially resolved CL-measurement on individual tetrapod legs showed, that the green emission increases with decreasing ZnO leg diameter. To our knowledge, the local CL spectroscopic measurements were correlated with the dimensions of the individual ZnO tetrapods for the first time.     

Controlling the yellow luminescence intensity from n-GaN during cathodoluminescence

GaN grown on sapphire by hydride vapour phase epitaxy is probed by ion channeling, Raman and cathodoluminescence (CL) spectroscopy. Channeling and Raman spectroscopy indicate that the quality of GaN is very good. Spot mode CL measurement signifies intense near band edge emission as compared with yellow luminescence (YL). During scanning over an area, the YL intensity could be controlled by the electron beam dwell time (DT). The YL intensity decreases with the increase of DT and saturates beyond a threshold value due to overexposure of a given pixel leading to non-radiative emission. But for shorter dwell times repeated excitations of a given pixel increase the intensity of YL. These results may be explained invoking the decay time and density of defects responsible for YL. Control over YL intensity will be useful for assessing low defect concentrations, their origin and also to increase spatial resolution of CL measurements on nanostructures.     

Laser‐Ignited Relay‐Domino‐Like Reactions in Graphene Oxide/CL‐20 Films for High‐Temperature Pulse Preparation of Bi‐Layered Photothermal Membranes

Light‐ignited combustions have been proposed for a variety of industrial and scientific applications. They suffer, however, from ultrahigh light ignition thresholds and poor self‐propagating combustion of typical high‐energy density materials, e.g., 2,4,6,8,10,12‐(hexanitrohexaaza)cyclododecane (CL‐20). Here, reported is that both light ignition and combustion performance of CL‐20 are greatly enhanced by embedding ε‐CL‐20 particles in a graphene oxide (GO) matrix. The GO matrix yields a drastic temperature rise that is sufficient to trigger the combustion of GO/CL‐20 under low laser irradiation (35.6 mJ) with only 6 wt% of GO. The domino‐like reduction‐combustion of the GO matrix can serve as a relay and deliver the decomposition‐combustion of CL‐20 to its neighbor sites, forming a relay‐domino‐like reaction. In particular, a synergistic reaction between GO and CL‐20 occurrs, facilitating more energy release of the GO/CL‐20 composite. The novel relay‐domino‐like reaction coupled with the synergistic reaction of CL‐20 and GO results in a deflagration of the material, which generates a high‐temperature pulse (HTP) that can be guided to produce advanced functional materials. As a proof of concept, a bi‐layered photothermal membrane is prepared by HTP treatment in an extremely simple and fast way, which can serve as a model architecture for efficient interfacial water evaporation.     

Revealing Photoluminescence Modulation from Layered Halide Perovskite Microcrystals upon Cyclic Compression

Halide perovskites show promise for high‐efficiency solar energy conversion and light‐emitting diode devices owing to their bandgap, which falls within the visible optical range. However, due to their rigidity, GPa pressures are necessary to control the complex interplay between their electronic and crystallographic structure. Layered perovskites are likely to be controlled using much lower pressures by exploiting the optical anisotropy of the embedded organic molecules in the structure. This work introduces layered perovskite microplatelets and demonstrates the extreme sensitivity of their emission to cyclic mechanical loading in the range of tens of MPa. A drastic change in their emission is observed in situ, from near‐white to an enhanced blue color. This process is reversible, as is evident from a hysteresis loop in the photoluminescence (PL) intensity of the microplatelets. A combination of experimental analysis and computational modelling shows that such behavior cannot be attributed to changes in the crystallographic structure of the flakes. Instead, it suggests that, thanks to their structural anisotropy, microplate alignment and reorientation are responsible for the observed PL modulation. The possibility to tune the optical emission of layered perovskite crystals via low pressures makes them highly interesting as active materials in applications where stress sensing or light modulation is desired.     

Direct observation of enhanced cathodoluminescence emissions from ZnO nanocones compared with ZnO nanowire arrays

We report, for the first time, direct observation of enhanced cathodoluminescence (CL) emissions from ZnO nanocones (NCs) compared with ZnO nanowires (NWs). For direct and unambiguous comparison of CL emissions from NWs and nanocones, periodic arrays of ZnO NW were converted to nanocone arrays by our unique HCl [aq] etching technique, enabling us to compare the CL emissions from original NWs and final nanocones at the same location. CL measurements on NW and nanocone arrays reveal that emission intensity of the nanocone at ～ 387 nm is over two times larger than that of NW arrays. The enhancement of CL emission from nanocones has been confirmed by finite-difference time-domain simulation of enhanced light extraction from ZnO nanocones compared to ZnO NWs. The enhanced CL from nanocones is attributed to its sharp morphology, resulting in more chances of photons to be extracted at the interface between ZnO and air.     

Strong morphological dependence of luminescence efficiency and emission wavelength in hexagonal GaN crystallites directly imaged by scanning cathodoluminescence microscopy

The microstructure and morphology of hexagonal GaN crystallites grown on c-axis sapphire substrates by low pressure chemical vapor deposition is correlated with the luminescence efficiency and emission wavelength. Microscopic variation of local band gap monitored by the luminescence wavelength on the a- and c-planes of hexagonal GaN-crystallites are directly mapped by means of low temperature scanning cathodoluminescence (CL) and CL wavelength imaging (CLWI). Beside minor fluctuations from crystallite to crystallite, the a-planes show pronounced red shift of emission energy of more than 132 meV with respect to the luminescence from the c-planes. The c-plane itself shows additional inhomogeneity on a micron scale. Strongly red shifted luminescence (λ>400 nm) originates from the very center region correlated with a high dislocation density found in TEM. The CL intensity shows a reticulated structure over the c-plane visualizing the local dislocation network.     

Unraveling In Vivo Brain Transport of Protein‐Coated Fluorescent Nanodiamonds

Nanotheranostics, combining diagnostics and therapy, has the potential to revolutionize treatment of neurological disorders. But one of the major obstacles for treating central nervous system diseases is the blood–brain barrier (BBB) preventing systemic delivery of drugs and optical probes into the brain. To overcome these limitations, nanodiamonds (NDs) are investigated in this study as they are a powerful sensing and imaging platform for various biological applications and possess outstanding stable far‐red fluorescence, do not photobleach, and are highly biocompatible. Herein, fluorescent NDs encapsulated by a customized human serum albumin–based biopolymer (polyethylene glycol) coating (dcHSA‐PEG) are taken up by target brain cells. In vitro BBB models reveal transcytosis and an additional direct cell–cell transport via tunneling nanotubes. Systemic application of dcHSA‐NDs confirms their ability to cross the BBB in a mouse model. Tracking of dcHSA‐NDs is possible at the single cell level and reveals their uptake into neurons and astrocytes in vivo. This study shows for the first time systemic NDs brain delivery and suggests transport mechanisms across the BBB and direct cell–cell transport. Fluorescent NDs are envisioned as traceable transporters for in vivo brain imaging, sensing, and drug delivery.     

Real‐Time Mapping of Ultratrace Singlet Oxygen in Rat during Acute and Chronic Inflammations via a Chemiluminescent Nanosensor

Sensing nonradiation‐induced singlet oxygen (¹O₂) in whole‐animal is deemed as one of the most challenging tasks in noninvasive techniques due to the µs level lifetime of ¹O₂ and quenching by numerous reductants in tissues. Here a distinct chemiluminescent (CL) nanosensor (NTPE‐PH) that boasts ultrahigh concentrated CL units in one nanoparticle is reported. Taking advantage of the intramolecular energy transfer mechanism that promises high energy transfer efficiency and the aggregation‐induced emission behavior that guarantees high CL amplification, the NTPE‐PH sensor is sensitive to a nm level ¹O₂. Experiments demonstrate that the NTPE‐PH yields a highly selective CL response toward ¹O₂ among common reactive oxygen species. With proved low cytotoxicity and good animal compatibility, real‐time mapping of ultratrace ¹O₂ in whole‐animal during acute and chronic inflammations is first achieved. It is anticipated that the NTPE‐PH sensor can be a useful tool for monitoring ¹O₂ variation during immune response and pathological processes corresponding to different stimuli, even with drug treatment included.     

Eu3+对Na3Gd2(BO3)3∶Tb3+发光性能的影响及其能量传递

采用高温固相法制备了Na_3Gd_2(BO_3)_3∶Tb~(3+),Eu~(3+)荧光粉,并对样品的物相组成、微观形貌、发光性能和能量传递进行了分析。结果表明,Na_3Gd_(2-x)(BO_3)_3∶xTb~(3+)荧光粉在紫外和近紫外区域有较强的激发峰,在368nm波长激发下,发射光呈绿色,Tb~(3+)最佳掺杂量为x=0.04。随着在Na_3Gd_(1.96)(BO_3)_3∶0.04Tb~(3+)中掺入Eu~(3+),Tb~(3+)对Eu~(3+)产生了以电偶极-电偶极相互作用为主的能量传递,且传递效率随Eu~(3+)掺杂量的增加而逐渐增大。发射光谱中Tb~(3+)的发射峰强度逐渐减弱,而Eu~(3+)的发射峰强度逐渐增强,导致Na_3Gd_(1.96-y)(BO_3)_3∶0.04Tb~(3+),yEu~(3+)荧光粉发光颜色由绿色向橙色变化。     

Single-composition trichromatic white-emitting Ca4Y6(SiO4)6O: Ce3+/Mn2+/Tb3+ phosphor: luminescence and energy transfer

A series of Ca(4)Y(6)(SiO(4))(6)O (CYS): Ce(3+)/Mn(2+)/Tb(3+) oxyapatite phosphors were prepared via high-temperature solid-state reaction. Under UV excitation, there exist dual energy transfers (ET), i.e., Ce(3+)→Mn(2+) and Ce(3+)→Tb(3+) in the CYS: Ce(3+), Mn(2+), Tb(3+) system and their emitting colors can be adjusted from blue to orange-red via ET of Ce(3+)→Mn(2+) and from blue to green via ET of Ce(3+)→Tb(3+), respectively. Moreover, a wide-range-tunable white light emission with high quantum yields (13%-30%) were obtained by precisely controlling the contents of Ce(3+), Mn(2+) and Tb(3+) ions. On the other hand, the CL properties of CYS: Ce(3+), Mn(2+), Tb(3+) phosphors have been investigated in detail. The studied results indicate that the as-prepared CYS: Ce(3+), Mn(2+), Tb(3+) phosphors have good CL intensity and CIE color coordinate stability with a color-tunable emission crossing the whole white light region under low-voltage electron beam excitation. In general, the white light with varied hues has been obtained in Ce(3+), Mn(2+), and Tb(3+)-triactivated CYS phosphors by utilizing the principle of energy transfer and properly designed activator contents under UV (284, 358 nm) and low-voltage (1-5 kV) electron beam excitation, which make them as a potential single-composition trichromatic white-emitting phosphor.