The recent discovery of ferromagnetism in two-dimensional van der Waals crystals has provoked a surge of interest in the exploration of fundamental spin interaction in reduced dimensions. However, existing material candidates have several limitations, notably lacking intrinsic room-temperature ferromagnetic order and air stability. Here, motivated by the anomalously high Curie temperature observed in bulk diluted magnetic oxides, we demonstrate room-temperature ferromagnetism in Co-doped graphene-like Zinc Oxide, a chemically stable layered material in air, down to single atom thickness. Through the magneto-optic Kerr effect, superconducting quantum interference device and X-ray magnetic circular dichroism measurements, we observe clear evidences of spontaneous magnetization in such exotic material systems at room temperature and above. Transmission electron microscopy and atomic force microscopy results explicitly exclude the existence of metallic Co or cobalt oxides clusters. X-ray characterizations reveal that the substitutional Co atoms form Co2+ states in the graphitic lattice of ZnO. By varying the Co doping level, we observe transitions between paramagnetic, ferromagnetic and less ordered phases due to the interplay between impurity-band-exchange and super-exchange interactions. Our discovery opens another path to 2D ferromagnetism at room temperature with the advantage of exceptional tunability and robustness.
We report a wavelength-locked cladding-pumped ytterbium-doped fiber laser that can simultaneously emit radially and azimuthally polarized beams based on Pancharatnam-Berry phase optical elements. Multi-wavelength free running operation of the radially and azimuthally polarized laser beams can be switched to a single-wavelength one assisted by volume Bragg grating, with wavelength locked at around 1053.4 nm and spectral linewidth of 0.06 nm (FWHW). By rotating the glan-taylor polarizer, we can obtain switchable radially and azimuthally polarized beams output. The radially and azimuthally polarized beams mode purity can maintain 97.3% and 96.3% at maximum output power, and the polarization extinction ratio (PER) can reach 97.8% and 95.9% for the radially and azimuthally polarized laser, respectively.
Some of these limitations-in particular, fabrication of small features and complex geometries-have been addressed with a hybrid approach, where simple lithographic templates guide the self-assembly of finer features. [1][2][3][4][5] Directed self-assembly (DSA) of block copolymers (BCPs) has successfully produced a variety of functional nanodevices, including capacitors, memory storage, and photonic crystals. [6][7][8] DSA of curved features, however, has not achieved the same level of success, despite the technological importance of ring and concentric ring patterns. Ring-based devices include zone plates, nanolenses, and optical ring resonators; certain memory storage devices, transistors, and sensors also use nanoscale ring features. [9][10][11][12][13][14][15] Compared to assembly along linear patterns, DSA of curved nanostructures has distinct requirements. In order for a lamellar or cylindrical BCP microdomain to form a ring, the polymer chains need to compress along the microdomain's inner edge and stretch along its outer edge. [16,17] This asymmetric distortion is energetically unfavorable. In solution, spontaneous formation of rings or toroids requires that the BCP blocks be chemically modified to experience self-attraction. [18,19] BCP rings and spirals have been observed within cylindrical confinement, though only a narrow set of BCP block ratios have been studied. [20][21][22][23] The confined BCP assemblies often exhibit structural degeneracy, typical of a system trapped far away from its free energy minimum state.Polymer nanostructures typically serve as etching masks for inorganic materials with the requisite electronic, magnetic, or optical properties. Alternatively, BCPs and inorganic nanoparticles (NPs) can be blended into nanocomposites, which offer additional advantages, including single-step fabrication and nanoparticle size-and arrangement-dependent properties. [24][25][26][27][28] Indeed, ordered nanocomposites with longrange alignment across one, two, and three dimensions have been obtained with DSA. However, curved features have been notably absent. Incorporated particles increase the bending moduli of their host microdomains, and thus increase the energy penalty associated with curved features. [29,30] To assemble successfully, the polymer matrix must accommodate Ring-shaped nanostructures can focus, filter, and manipulate electromagnetic waves, but are challenging to incorporate into devices using standard nanofabrication techniques. Directed self-assembly (DSA) of block copolymers (BCPs) on lithographically patterned templates has successfully been used to fabricate concentric rings and spirals as etching masks. However, this method is limited by BCP phase behavior and material selection. Here, a straightforward approach to generate ring-shaped nanoparticle assemblies in thin films of supramolecular nanocomposites is demonstrated. DSA is used to guide the formation of concentric rings with radii spanning 150-1150 nm and ring widths spanning 30-60 nm. When plasmonic nanoparticles are us...
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