Xuezhe Zhou

Effect of manganese doping on Li-ion intercalation properties of V2O5 films

Mn-doped V2O5 has been prepared by sol–gel processing with H2O2 and V2O5 as precursors with Mn2+ added directly during sol preparation. Stable and homogeneous Mn-doped vanadium oxide sol was obtained and the films were fabricated by dip-coating, drying at ambient, and then annealing at 250 °C in air for 3 h. X-Ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and electrochemical analyses have been employed to characterize and analyze the crystal- and microstructures, surface morphology and Li-ion intercalation properties of both Mn-doped and undoped V2O5 films. Mn-doped V2O5 films exhibit excellent cyclic stability with a fading rate of less than 0.06% per cycle, significantly better than that of the pure V2O5 films (0.8% per cycle). Mn-doped V2O5 films have demonstrated a large discharge capacity of ∼283mAh/g with a current density of 68 mA/g, again much higher than 237 mAh/g of V2O5 films. A possible explanation for such significant enhancement in lithium ion intercalation capacity, cyclic stability, and rate performance of Mn-doped V2O5 films has been discussed.

Three dimensional architecture of carbon wrapped multilayer Na3V2O2(PO4)2F nanocubes embedded in graphene for improved sodium ion batteries

A novel Na3V2O2(PO4)2F@carbon/graphene three dimensional (3D)architecture (NVPF@C/G) is developed through a simple approach for the first time. It exhibits greatly improved rate capability and delivers a reversible capacity of 113.2 mA h g−1 at 1C.

Laser refrigeration of hydrothermal nanocrystals in physiological media

Significance Although the laser refrigeration of bulk crystals has recently shown to cool below cryogenic temperatures (∼90 K) in vacuum, to date the laser refrigeration of physiological media has not been reported. In this work, a low-cost hydrothermal synthetic approach is used to prepare nanocrystals that are capable of locally refrigerating physiological buffers (PBS, DMEM) upon near-infrared illumination. Optical tweezers are used in tandem with cold Brownian motion analysis to observe the refrigeration of individual (Yb3+)-doped nanocrystals >10 °C below ambient conditions. The ability to optically generate local refrigeration fields around individual nanocrystals promises to enable precise optical temperature control within integrated electronic/photonic/microfluidic circuits, and also thermal modulation of basic biomolecular processes, including the dynamics of motor proteins. Coherent laser radiation has enabled many scientific and technological breakthroughs including Bose–Einstein condensates, ultrafast spectroscopy, superresolution optical microscopy, photothermal therapy, and long-distance telecommunications. However, it has remained a challenge to refrigerate liquid media (including physiological buffers) during laser illumination due to significant background solvent absorption and the rapid (∼ps) nonradiative vibrational relaxation of molecular electronic excited states. Here we demonstrate that single-beam laser trapping can be used to induce and quantify the local refrigeration of physiological media by >10 °C following the emission of photoluminescence from upconverting yttrium lithium fluoride (YLF) nanocrystals. A simple, low-cost hydrothermal approach is used to synthesize polycrystalline particles with sizes ranging from <200 nm to >1 μm. A tunable, near-infrared continuous-wave laser is used to optically trap individual YLF crystals with an irradiance on the order of 1 MW/cm2. Heat is transported out of the crystal lattice (across the solid–liquid interface) by anti-Stokes (blue-shifted) photons following upconversion of Yb3+ electronic excited states mediated by the absorption of optical phonons. Temperatures are quantified through analysis of the cold Brownian dynamics of individual nanocrystals in an inhomogeneous temperature field via forward light scattering in the back focal plane. The cold Brownian motion (CBM) analysis of individual YLF crystals indicates local cooling by >21 °C below ambient conditions in D2O, suggesting a range of potential future applications including single-molecule biophysics and integrated photonic, electronic, and microfluidic devices.

Laser Refrigeration of Ytterbium‐Doped Sodium–Yttrium–Fluoride Nanowires

Sodium yttrium fluoride (β-NaYF4 ) nanowires (NWs) with a hexagonal crystal structure are synthesized using a low-cost hydrothermal process and are shown to undergo laser refrigeration based on an upconversion process leading to anti-Stokes (blueshifted) photoluminescence. Single-beam laser trapping combined with forward light scattering is used to investigate cryophotonic laser refrigeration of individual NWs through analysis of their local Brownian dynamics.

Rapid synthesis of transition metal dichalcogenide–carbon aerogel composites for supercapacitor electrodes

Transition metal dichalcogenide (TMD) materials have recently demonstrated exceptional supercapacitor properties after conversion to a metallic phase, which increases the conductivity of the network. However, freestanding, exfoliated transition metal dichalcogenide films exhibit surface areas far below their theoretical maximum (1.2 %), can fail during electrochemical operation due to poor mechanical properties, and often require pyrophoric chemicals to process. On the other hand, pyrolyzed carbon aerogels exhibit extraordinary specific surface areas for double layer capacitance, high conductivity, and a strong mechanical network of covalent chemical bonds. In this paper, we demonstrate the scalable, rapid nanomanufacturing of TMD (MoS2 and WS2) and carbon aerogel composites, favoring liquid-phase exfoliation to avoid pyrophoric chemicals. The aerogel matrix support enhances conductivity of the composite and the synthesis can complete in 30 min. We find that the addition of transition metal dichalcogenides does not impact the structure of the aerogel, which maintains a high specific surface area up to 620 m2 g−1 with peak pore radii of 10 nm. While supercapacitor tests of the aerogels yield capacitances around 80 F g−1 at the lowest applied currents, the aerogels loaded with TMD’s exhibit volumetric capacitances up to 127% greater than the unloaded aerogels. In addition, the WS2 aerogels show excellent cycling stability with no capacitance loss over 2000 cycles, as well as markedly better rate capability and lower charge transfer resistance compared to their MoS2-loaded counterparts. We hypothesize that these differences in performance stem from differences in contact resistance and in the favorability of ion adsorption on the chalcogenides.

Chitosan‐Gated Magnetic‐Responsive Nanocarrier for Dual‐Modal Optical Imaging, Switchable Drug Release, and Synergistic Therapy

A dual-layer shell hollow nanostructure as drug carrier that provides instant on/off function for drug release and contrast enhancement for multimodal imaging is reported. The on-demand drug release is triggered by irradiation of an external magnetic field. The nanocarrier also demonstrates a high drug loading capacity and synergistic magnetic-thermal and chemotherapy.

Recovery of hexagonal Si-IV nanowires from extreme GPa pressure

We use Raman spectroscopy in tandem with transmission electron microscopy and density functional theory simulations to show that extreme (GPa) pressure converts the phase of silicon nanowires from cubic (Si-I) to hexagonal (Si-IV) while preserving the nanowires cylindrical morphology. In situ Raman scattering of the longitudinal transverse optical (LTO) mode demonstrates the high-pressure Si-I to Si-II phase transition near 9 GPa. Raman signal of the LTO phonon shows a decrease in intensity in the range of 9–14 GPa. Then, at 17 GPa, it is no longer detectable, indicating a second phase change (Si-II to Si-V) in the 14–17 GPa range. Recovery of exotic phases in individual silicon nanowires from diamond anvil cell experiments reaching 17 GPa is also shown. Raman measurements indicate Si-IV as the dominant phase in pressurized nanowires after decompression. Transmission electron microscopy and electron diffraction confirm crystalline Si-IV domains in individual nanowires. Computational electromagnetic simul...

Photothermal heating of nanoribbons

Abstract. Nanoscale optical materials are of great interest for building future optoelectronic devices for information processing and sensing applications. Although heat transfer ultimately limits the maximum power at which nanoscale devices may operate, gaining a quantitative experimental measurement of photothermal heating within single nanostructures remains a challenge. Here, we measure the nonlinear optical absorption coefficient of optically trapped cadmium-sulfide nanoribbons at the level of single nanostructures through observations of their Brownian dynamics during single-beam laser trapping experiments. A general solution to the heat transfer partial differential equation is derived for nanostructures having rectilinear morphology including nanocubes and nanoribbons. Numerical electromagnetic calculations using the discrete-dipole approximation enable the simulation of the photothermal heating source function and the extraction of nonlinear optical absorption coefficients from experimental observations of single nanoribbon dynamics.

Non-contact thermometry of nanoscale materials for radiation-balanced lasers (Conference Presentation)

Recent experimental breakthroughs in the laser-refrigeration-of-solids (LRS) have demonstrated that cryogenic temperatures can now be achieved opening up a range of promising applications using compact, vibration-free optical cryocoolers. These results also have stimulated significant interest in the development of new material designs for applications in radiation balanced lasing (RBL). The development of practical host materials for RBLs requires the understanding of how both spontaneous emission rates and non-radiative decay rates change under a wide range of thermal conditions and dielectric host environments. In this work the photoluminescence lifetime of 4S3/2 transitions from Er(III) ions within co-doped Yb3+/Er3+-codoped hexagonal sodium-yttrium-fluoride (beta-NaYF4) nanostructures is presented as a rapid, low-cost, spatially resolved method of quantifying the temperature of within RBL materials. Lifetime measurements from single nanostructures are made using single-beam laser-traps, where the focal plane of the trapping laser is used to control the spacing between single nanowires and dielectric chamber surfaces that are supported by a temperature-controlled piezo-stage. The lifetime of Er(III) ions is observed to change significantly based on the distance between emitting dipoles and nearby dielectric interfaces and also as a function of chamber temperature. Lifetime measurements are also presented for measuring the temperature within polydimenthylsiloxane-polymer/nanocrystal composite materials that serve as a model system for future optical-fiber cladding materials. Lastly, ratiometric photoluminescence and lifetime measurements will be presented for Yb(III):YLiF4 microcrystals supported on cadmium sulfide nanoribbon cantilevers, indicating the potential for hybrid semiconductor/RE-fluoride composite structures for future RBL applications.

Copper- and chloride-mediated synthesis and optoelectronic trapping of ultra-high aspect ratio palladium nanowires

We present a scalable hydrothermal synthesis of one-dimensional palladium nanostructures from palladium(II) chloride precursor, mediated by the introduction of low concentrations of copper(II) ions and/or sodium chloride. Adding Cu at a molar ratio of ∼1 : 12 500 relative to Pd increases the yield of 1D nanostructures from 10% to 90%. Furthermore, NaCl enhances 1D nanostructure growth such that high yields of long Pd nanowires (PdNWs)—featuring the highest aspect ratios yet reported for solution-grown Pd nanocrystals, with diameters of 20 nm and lengths up to 7 μm—can be achieved in a third of the time required for the synthesis with Cu alone. X-ray diffraction, electron microscopy, and X-ray photoelectron spectroscopy measurements confirm that the as-synthesized nanowires are indeed metallic crystalline Pd with a 5-fold twinned cross-section. It is hypothesized that the Cu ions scavenge oxygen to suppress etching of the twinned Pd seeds that eventually grow into elongated structures, whereas NaCl improves the solubility of PdCl2 and lowers the reduction rate via formation of the PdCl42− complex, promoting diffusional growth. The high aspect ratio of the PdNWs facilitates their manipulation in an optoelectronic tweezers (OET) device, which we demonstrate as a way to enhance their applicability for catalysis and hydrogen sensing.