Oxidizing Hexagonal Boron Nitride into Fluorescent Structures by Photodissociated Directional Oxygen Radical.

Modifying the wide band gap semiconductor hexagonal boron nitride (hBN) can bring new chances in photonics. By virtue of the solvothermal/hydrothermal oxidation or functionalization, hBN can be converted into fluorescent nanodots. Until now, it has been a big challenge to drily oxidize hBN and turn it into bright fluorescent structures due to its superior chemical stability. Here, we report the oxidation of multilayer hBN into fluorescent structures by ultraviolet (UV, λ = 172 nm) photodissociated directional oxygen radical [O(3P)] in a gradient magnetic field. The paramagnetic O(3P), produced in a low-pressure O2 atmosphere, drifts toward hBN and then converts it into boron nitride oxide (BNO) micro/nanometer structures constituted by BO, BO2, and O-doped hBN. For a properly oxidized BNO substance, bright and photostable wide-band photoluminescence is realized with nanosecond-scaled lifetimes under the excitation of UV and visible lights.

Active mode selection by defects in lithium niobate on insulator microdisks.

Whispering gallery mode (WGM) optical microcavities are important building blocks in photonic integrated circuits. Operation of such cavities on specific lower- or higher- order transverse modes has much interest in application perspectives. Here, we demonstrate active mode selection by introducing defects in lithium niobate on insulator microdisks. A focused ion beam is applied to precisely inscribe nano slits into the perimeter of the microdisk. The transmission spectra can be significantly thinned out without severe quality factor degradation. Either fundamental or high-order transverse WGMs can be retained by properly designing the size and location of the defects. The approach may have promising applications in single-mode lasing and nonlinear optics.

Patterning Graphene Films by H2O-Based Magnetic-Assisted UV Photolysis.

Properly cutting graphene into certain high-quality micro-/nanoscale structures in a cost-effective way has a critical role. Here, we report a novel approach to pattern graphene films by H2O-based magnetic-assisted ultraviolet (UV) photolysis under irradiation at 184.9 nm. By virtue of the paramagnetic characteristic, the photo-dissociated hydroxyl [OH(X2Π)] radicals are magnetized and have their oxidation capability highly enhanced through converting into an accelerated directional motion. Meanwhile, the precursor of H2O(X̃1A1) molecules distributes uniformly thanks to its weak diamagnetic characteristic, and there exists no instable diamagnetic intermediate to cause lateral oxidation. Possessing these unique traits, the H2O-based magnetic-assisted UV photolysis has the capability of making graphene microscale patterns with the linewidth down to 8.5 µm under a copper grid shadow mask. Furthermore, it is feasible to pattern graphene films into 40 nm-wide ribbons under ZnO nanowires and realize hybrid graphene/ZnO nanoribbon field-effect transistors with a hole mobility up to 7200 cm2·V-1·s-1. The X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry analyses reveal that OH(X2Π) radicals act as a strong oxidant and that another product of H(12S) adsorbs weakly on graphene.

UV Rewritable Hybrid Graphene/Phosphor p-n Junction Photodiode.

Graphene-based p-n junction photodiodes have a potential application prospect in photodetection due to their broadband spectral response, large operating bandwidth, and mechanical flexibility. Here, we report an ultraviolet (UV) rewritable p-n junction photodiode in a configuration of graphene coated with an amorphous phosphor of 4-bromo-1,8-naphthalic anhydride derivative polymer (poly-BrNpA). Under moderate UV irradiation, occurrence of photoisomerization reaction in the poly-BrNpA film leads to its drastically modified optical characteristics and a concurrent n-type doping in the underneath graphene. Meanwhile, the poly-BrNpA film, highly sensitive to water molecules, has a capability of restoring graphene to its initial p-type doping status by means of water adsorption. Based on these findings, a lateral graphene/poly-BrNpA p-n junction photodiode, responsive to visible light at the junction interface, can be written by UV irradiation and then erased via water adsorption. The p-n junction photodiode is rewritable upon such repetitive loops showing repeatable optoelectronic properties. This study provides a new scheme and perspective of making graphene-based rewritable p-n junction photodiodes in a flexible and controllable way, and it may contribute to expanding new families of optoelectronic devices based on two-dimensional materials.

Exploring the working mechanism of graphene patterning by magnetic-assisted UV ozonation.

When assisted with an inhomogenous vertical magnetic field, ultraviolet (UV) ozonation turns directional and is testified to be applicable to graphene patterning. Using a more cost-effective low-pressure mercury lamp, we further explore the underlying working mechanism by changing oxygen content, introducing reactive ozone or inert nitrogen molecules, and study the lateral under-oxidation impeded Dirac point shifts for a graphene field-effect transistor under UV irradiation. The paramagnetic oxygen molecule X3Σ accelerates toward graphene with the magnetic moment aligned parallel to the magnetic field. The O(3P) atoms, stemming from such a directional oxygen molecule, have a high initial velocity before being further accelerated, and therefore enhance the oxidation capability compared with those from weak diamagnetic ozone molecules. Intermolecular or atomic-molecular collisions between the high-speed oxygen molecules/atoms and the randomly moved weak diamagnetic molecules, including nitrogen and ozone, appear crucial in deteriorating graphene patterning by increasing the lateral under-oxidation. This study may shed light on our understanding of graphene patterning by magnetic-assisted UV ozonation.

Patterning Graphene Film by Magnetic-assisted UV Ozonation.

Developing an alternative method for fabricating microscale graphene patterns that overcomes the obstacles of organic contamination, linewidth resolution, and substrate damaging is paramount for applications in optoelectronics. Here we propose to pattern chemical vapor deposition grown graphene film through a stencil mask by magnetic-assisted ultraviolet (UV) ozonation under irradiation of a xenon excimer lamp. In this process, the paramagnetic oxygen molecules and photochemically generated oxygen radicals are magnetized and attracted in an inhomogenous external magnetic field. As a consequence, their random motions convert into directional, which can greatly modify or enhance the quality of graphene patterns. Using a ferromagnetic steel mask, an approximately vertical magnetic-field-assisted UV ozonation (BZ = 0.31 T, ∇BZ = 90 T · m-1) has a capability of patterning graphene microstructures with a line width of 29 µm and lateral under-oxidation less than 4 µm. Our approach is applicable to patterning graphene field-effect transistor arrays, and it can be a promising solution toward resist-free, substrate non-damaging, and cost effective microscale patterning of graphene film.