MXene与溶剂界面电子振动耦合现象的直接观测

作为一种新型的二维材料,MXene凭借其各种优异的物理化学性质已受到极为广泛的关注,例如其具有极其出色的光热转换效率. 然而,人们对MXene的光热转换机制仍然知之甚少. 本文通过结合飞秒可见和中红外瞬态吸收光谱技术,对分散在各种溶剂中MXene(Ti₃C₂T_x)纳米片内的电子能量耗散动力学进行了系统的研究. 结果表明,MXene的激发态寿命在很大程度上取决于周围的溶剂环境. 在MXene被超快激光泵浦后,可以直接观察到MXene纳米片与相邻溶剂分子之间的界面电子振动耦合现象. 这些结果表明界面相互作用在MXene的超快能量传输动力学中起着关键的作用. 这一发现可为二维体系光转换性能的改进提供了一条潜在可行的途径.     

Ultrasound-assisted fabrication of Ti3C2Tx MXene toward enhanced energy storage performance

Ti₃C₂Tx MXenes are normally fabricated by removal of main group element from the corresponding transition metal carbides, and they have been actively studied due to their superior energy storage performance. However, the low efficiency in removal of main group element (named as chemical etching) has significantly limited the application of MXene or MXene-related materials. Herein, we demonstrated an ultrasound-assisted approach to synthesize Ti₃C₂Tx MXene material by using Ti₃AlC₂ as the precursor. The experimental results indicate that the efficiency of chemical etching of Ti₃AlC₂ was dramatically promoted by ultrasound. The etching time was greatly shortened to 8 h while typically 24 h is sufficient in dilute hydrofluoric acid. Particularly, the high etching efficiency was achieved by using 2% hydrofluoric acid under the aid of ultrasound, which is lower in concentration than those reported in the previous literature. The specific capacitance of the 8 h sonicated sample is 155F/g, which is much higher than that of the un-sonicated sample prepared under the same experimental conditions. Additionally, the specific capacitance retention of the prepared 8 h sonicated sample was 97.5% after 20,000 cycles of charging/discharging, exhibiting an outstanding energy storage stability compared with the materials reported in previous literatures. It was proposed that removal of AlF₃ from the surface of the etched particles was significantly promoted and the hydrogen bonds between the terminations of two different adjacent layers were broken by the acoustic cavitation effect of ultrasound.     

Ultrasound-assisted synthesis of graphene@MXene hybrid: A novel and promising material for electrochemical sensing

To date, multiple graphene@MXene hybrids have been reported via various synthesis approaches, but almost all the graphene@MXene hybrids inevitably used the reduced graphene oxide that prepared by chemical oxidation/reduction method, which generally involved the complex and dangerous operation procedure, and the highly toxic chemical reagent. How to prepare graphene@MXene hybrid through a simple, safe and eco-friendly synthetic route is highly desired. Compared with traditional synthesis technology, ultrasound synthesis strategy displays the merits of simplicity, low cost and environment protection. Herein, MXene (Ti₃C₂T_x) nanoflakes coupled with graphene nanosheets (graphene@MXene) were prepared in N-methylpyrrolidone (NMP) by simple ultrasound-assisted liquid-phase exfoliation method for the first time. Besides, the effect of types of solvent with different viscocity, sonication temperature and sonication duration time on the property of graphene@MXene hybrids were systematacially investigated. It is found the liquid-phase exfoliated graphene owned excellent electron transfer ability and the MXene (Ti₃C₂T_x) nanoflakes possessed outstanding adsorption property, the as-synthesized graphene@MXene hybrid exhibited significant signal synergistic enhancement effect toward the oxidation of hazardous veterinary drug residue compound (chlorpromazine) and food additives (rhodamine B). Based on this, a novel and sensitive electrochemical sensor was fabricated, the linear detection ranges were 5 nM to 0.5 μM for chlorpromazine with sensitivity of 1090 µA μM⁻¹ cm⁻², and 10 nM to 2.5 μM for rhodamine B with sensitivity of 440 and 102.14 µA μM⁻¹ cm⁻². Besides, the detection limits were evaluated to be as low as 1.25 nM and 2.45 nM for chlorpromazine and rhodamine B, respectively.     

Large optical nonlinearity in CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> thin films

CaCu₃Ti₄O₁₂ (CCTO) thin films were successfully prepared on LaAlO₃ substrates by pulsed laser deposition technique. We measured the nonlinear optical susceptibility of the thin films using Z-scan method at a wavelength of 532 nm with pulse durations of 25 ps and 7 ns. The large values of the third-order nonlinear optical susceptibility, χ ⁽³⁾, of the CCTO film were obtained to be 2.79×10⁻⁸ esu and 3.30×10⁻⁶ esu in picosecond and nanosecond time regimes, respectively, which are among the best results of some representative nonlinear optical materials. The origin of optical nonlinearity of CCTO films was discussed. The results indicate that the CCTO films on LaAlO₃ substrates are promising candidate materials for applications in nonlinear optical devices.     

Superparamagnetic Ni:SiO<Subscript>2</Subscript>–C nanocomposites films synthesized by a polymeric precursor method

In this work, we report on the magnetic properties of nickel nanoparticles (NP) in a SiO₂–C thin film matrix, prepared by a polymeric precursor method, with Ni content x in the 0–10 wt% range. Microstructural analyses of the films showed that the Ni NP are homogenously distributed in the SiO₂–C matrix and have spherical shape with average diameter of ~10 nm. The magnetic properties reveal features of superparamagnetism with blocking temperatures T _B ~ 10 K. The average diameter of the Ni NP, estimated from magnetization measurements, was found to be ~4 nm for the x = 3 wt% Ni sample, in excellent agreement with X-ray diffraction data. M versus H hysteresis loops indicated that the Ni NP are free from a surrounding oxide layer. We have also observed that coercivity (H _C) develops appreciably below T _B, and follows the H _C ∝ [1 – (T/T _B)^0.5] relationship, a feature expected for randomly oriented and non-interacting nanoparticles. The extrapolation of H _C to 0 K indicates that coercivity decreases with increasing x, suggesting that dipolar interactions may be relevant in films with x > 3 wt% Ni.     

Harmonic Mode-Locked Er-Doped Fiber Laser Based on a Microfiber-Based PbS Nanoparticle Saturable Absorber

Lead sulfide (PbS) is a nanomaterial with excellent optical and chemical properties, such as a narrow bandgap (0.37 eV), high thermal damage threshold, and high stability. Obviously, it is appropriate as a saturable absorber (SA) device for ultrafast photonics. However, PbS nanoparticles (NPs) as the SA of ultrashort harmonic mode-locked pulse still haven't been demonstrated at present. In this paper, the PbS NPs are made into an SA-device-based microfiber by optical deposition method and connected in an integrated Erbium-doped fiber laser. And both characteristics and nonlinear optical properties of PbS NPs have been systemically investigated. A fundamental frequency mode-locked pulsed laser is proposed, whose central wavelength is 1560 nm, and the pulse width is 1 ps. In addition, high repetition rate operations are achieved, with a maximum repetition rate of 833 MHz. This is the first time that PbS NPs are used to generate 96th-order harmonic mode-locking, and the corresponding pulse duration is 987 fs. It is demonstrated that PbS NPs are a kind of SA photonic material with excellent performance. It can improve the communication capacity by applying fiber communication, and it has potential application value even in material processing and optical comb.     

Third-order nonlinear optical properties in [(C4H9)4N]2[Cu(C3S5)2]-doped PMMA thin film using Z-scan technique in picosecond pulse

An organometallic complex, [(C₄H₉)₄N]₂[Cu(C₃S₅)₂], abbreviated as BuCu, was synthesized. Then the BuCu-doped polymethylmethacrylate (PMMA) thin film with a doping concentration 1% by weight (1 wt.%) was fabricated using a spin-coating method and its third-order nonlinear optical properties were characterized using the Z-scan technique with 20 ps pulse duration at 532 and 1064 nm, respectively. The Z-scan curves have revealed that the material exhibits a self-defocusing effect at both wavelengths. Saturable absorption at 532 nm and two-photon absorption at 1064 nm were also found, respectively. Additionally, the calculated results of the material in film were compared with that of acetone solution, which indicated that the values in film were larger than that of acetone solution for about two orders in magnitude. The origins were analyzed of the difference between the two wavelengths. Our results suggest that considerable nonlinear optical properties were confirmed in BuCu-doped PMMA film. The material can easily be doped into PMMA film and forms a waveguide mode. So this material should be considered to be manufactured into devices and applied in all-optical switching, laser locking-mode, optical limiting fields etc.     

Piezoelectric activities and domain patterns of orthorhombic Ba(Zr,Ti)O3 ceramics

Ba(Zr_xTi_1?x)O₃ (0.025 ≤ x ≤ 0.065) ceramics were prepared by conventional solid-state reaction method. Crystalline structures were analyzed by X-ray diffraction. It was shown that all the Ba(Zr_xTi_1?x)O₃ (0.025 ≤ x ≤ 0.065) ceramics were of orthorhombic phase at room temperature. Piezoelectric activities and domain patterns were investigated and compared with those of BaTiO₃ ceramic. All the Ba(Zr_xTi_1?x)O₃ ceramics showed nearly the same d₃₃ values of about 265 pC/N and the same domain width of about 220 nm. By comparing the grain sizes and domain width of the Ba(Zr_xTi_1?x)O₃ ceramics with those of BaTiO₃ ceramic, it is speculated that the variation of domain width with grain sizes in orthorhombic Ba(Zr_xTi_1?x)O₃ ceramics may be different with that in tetragonal BaTiO₃ ceramic. Besides domain width, the effective inertia mass of domain wall is also considered to be a very important factor that impacts the piezoelectric activities of the Ba(Zr_xTi_1?x)O₃ ceramics.     

First-principle study on catalytic activity of functionalized Ti3C2 MXene as cathode catalyst for Li–O2 batteries

Two-dimensional layered Ti₃C₂, one representative MXene, is notable as promising cathode catalyst for rechargeable lithium-oxygen (Li–O₂) batteries. Using first-principles calculations, we construct cathode electrochemical interface catalytic model to simulate the structural evolution during discharging and charging processes, and the calculated ORR, OER and TOT overpotentials are used to quantitatively assess the catalytic activity of Ti₃C₂ MXene with and without O, F and OH functional groups. Interestingly, we find that the catalytic activity follows such a trend: Ti₃C₂O₂>Ti₃C₂F₂>Ti₃C₂(OH)₂>Ti₃C₂, which suggests that O-terminated Ti₃C₂ MXene has great advantages and potentiality for catalyzing ORR and OER in Li–O₂ batteries. This is caused by Ti₃C₂O₂ surface shows stronger oxidation capability toward O₂²⁻ compared to Ti₃C₂F₂, Ti₃C₂(OH)₂ and Ti₃C₂. The present study may provide a guideline to accelerate ORR and OER reactions of Ti₃C₂ MXene as cathode catalyst in Li–O₂ batteries, with O-terminated group being taken into consideration.     

Structure dependence of ultrafast third-order optical nonlinearity for GeS2-In2S3-CsI chalcohalide glasses

Ultrafast third-order nonlinear optical responses of GeS₂-In₂S₃-CsI chalcohalide glasses have been measured by using the femtosecond time-resolved optical Kerr effect (OKE) technique at a wavelength of 820 nm. The third-order nonlinear susceptibility was estimated to be as large as 5.12×10⁻¹³ esu. The full width at half maximum of the Kerr signal was 120 fs and its response was dominantly assigned to the ultrafast distortion of the electron cloud. The relationship between the structural units and the third-order nonlinear optical responses was analysed by Raman spectra. It is suggested that the covalent bonds of S-Ge or S-In constituting the tetrahedral units [GeS_4/2] or [InS_4−xI_x], respectively, play an important role in the ultrafast third-order nonlinear optical responses of these chalcohalide glasses.     

Investigation of the nonlinear absorption and optical limiting properties of two [Q]2[Cu(C3S5)2] compounds

The nonlinear absorption properties of two organometallic compounds, [(C₂H₅)₄N]₂[Cu(C₃S₅)₂] (DCu1) and [(C₄H₉)₄N]₂[Cu(C₃S₅)₂] (DCu2), have been investigated using an open-aperture Z-scan technique at 1064 nm with 40 ps pulse width and at 1053 nm with 18 ns pulse width. The reverse saturable absorption (RSA) which was observed in both samples with nanosecond pulse excitation was much larger than that observed with picosecond pulse excitation. The nonlinear absorption properties were analyzed theoretically by a five-level model. Optical limiting based on RSA was performed and limiting thresholds were evaluated for both samples under three conditions. DCu1 exhibited the better limiting characteristics because of its stronger RSA response.     

The effect of ultrasound on synthesis and energy storage mechanism of Ti3C2Tx MXene

Removal of aluminum (abbreviated to Al) accounts for the main step for synthesizing Ti₃C₂Tx MXene. To date, the synthesis of Ti₃C₂Tx MXene is hampered by the low removal efficiency of Al from Ti₃AlC₂. Ultrasound was therefore introduced to achieve efficient synthesis of Ti₃C₂Tx MXene by promoting the removal rate of Al from Ti₃AlC₂. It was found that ultrasonic aid can significantly boost the removal efficiency of Al. Additionally, distinct kinetics for the removal of Al was recognized as the advent of ultrasonic intervention: (i) the shrinking core model was used to describe the removal kinetics of Al in the case without ultrasound, whilst the shrinking particle model was capable for the case in presence of ultrasound; (ii) the activation energy for removal of Al with ultrasonic aid was 70.2 kJ/mol, indicating a chemical reaction-controlled process, whereas the corresponding value for the case without sonication was 28.1 kJ/mol, demonstrating a mixed kinetic feature of the removal process of Al. Morphological study showed that ultrasound can remove the surface-adhering reaction products and favors the formation of structures with flower-like morphology. The sample without sonication treatment exhibited typical capacitive behavior, whilst the contribution of diffusion-limited capacitance in addition to the capacitive behavior was readily observed for the sonication-treated sample. Surface chemistry study indicated the more prevalent oxidation of the sonication treated sample, which gave rise to a higher specific capacitance than those without sonication treatment.     

Investigation on the structural,electronic, elastic and mechanical properties of Ti2Al(C1-xOx) solid solutions: First-principles calculations

In the present work, the structural, electronic, elastic and mechanical properties of Ti₂AlC and Ti₂Al(C_1-xO_x) solid solutions were investigated using first-principles calculations for varied O content incorporation (x = 0, 0.125, 0.25, 0.375, 0.5). According to the calculation results, all Ti₂Al(C_1-xO_x) solid solutions with various x values are stable, and the bonding strength of the Ti–Al bond increases with the doping of O element. In addition, the shear modulus G and C₄₄ elastic constant of Ti₂Al(C_1-xO_x) solid solutions are both lower than the bulk modulus B, indicating that the phase has good damage tolerance. Not only that, compared with Ti₂AlC, the plasticity and toughness of Ti₂Al(C_1-xO_x) solid solutions are improved with the increase of O atom doping and doping ratio. Simultaneously, the doping of O atom is also beneficial to reduce the generalized stacking fault energy of Ti₂AlC, making the Ti₂Al(C_1-xO_x) solid solutions more prone to shear deformation, thereby further enhancing plasticity.     

CW and passive Q-switching of 1331-nm Nd:GGG laser with Co<Superscript>2+</Superscript>:LMA saturable absorber

In this paper, the output performances at 1331 nm in continuous-wave (CW) operation and the passive Q-switching regime of a Nd:Gd₃Ga₅O₁₂(Nd:GGG) laser crystal have been investigated under pumping with diode lasers. A maximum CW output power of 1.5 W was reached at an incident pump power of 7.5 W; the overall optical-to-optical efficiency and the slope efficiency with respect to the pump power were 21.5% and 19.4%, respectively. The passive Q-switching regime was achieved with Co²⁺:LaMgAl₁₁O₁₉ (Co²⁺:LMA) saturable absorber (SA) crystals. A maximum average output power of 183 mW was recorded with a Co²⁺:LMA SA with initial transmission T _i of 90%. The pulse energy was 18.7 μJ and the pulse duration was 26.1 ns, which correspond to a pulse peak power of 0.7 kW. With a Co²⁺:LMA SA with T _i=81%, the average power decreased to 131 mW. However, the pulse energy increased to 21.4 μJ, the pulse duration was 16.4 ns and the pulse peak power increased to 1.3 kW.     

Study on nonlinear optical absorption properties of [(CH3)4N]2[Cu(dmit)2] by Z-scan technique

The third-order optical nonlinearities of an organo-metallic compound, [(CH₃)₄N]₂[Cu(dmit)₂] (dmit²⁻=4,5-dithiolate-1,3-dithiole-2-thione), abbreviated as MeCu, dissolved in acetone are characterized by Z-scan technique with picosecond and nanosecond laser pulses in the near-infrared region. Two-photon absorption has been found when the sample solution is irradiated by 40 ps pulse width at 1064 nm and the two-photon absorption (TPA) coefficient β_TPA is 4×10⁻¹³ m/W. While excited by 15 ns laser pulses at 1053 nm, the Z-scan spectra reveal strong reverse saturable absorption (RSA) and the nonlinear absorption coefficient β_RSA is estimated to be as high as 7.07×10⁻¹¹ m/W which is much larger than β_TPA. An explanation for this enhancement is given. All the results suggest that MeCu may be a promising candidate for the application to optical limiting in the near-infrared region.     

Non-linear optical properties of (Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)TiO<Subscript>3</Subscript> thin films

Thin films of (Pb_1−x Sr _x )TiO₃, x=0,0.5,1.0 have been prepared on glass substrates by the chemical-solution method using the spin-coating technique. The optical nonlinearity in the visible spectral region is investigated using short (5 ns) laser pulses at the off-resonant wavelength of 532 nm employing the open aperture z-scan technique. It is found that the third order nonlinear absorption is dependent on the lead content of the films, with the compositions x=0, 0.5 exhibiting large values (β∼10⁻⁷ m/W), thereby suggesting the possible use of these materials as optical limiters. No optical nonlinearity is observed for the composition with x=1.0.     

Emerging 2D Semiconducting Materials: Tin Diselenide for Ultrafast Photonics

SnSe₂, a novel 2D semiconducting material with good photoelectric properties and tunable bandgaps structure, has been widely studied in high-performance photodetectors and superconductivity applications, etc. Herein, another excellent property, nonlinear optical absorption, is applied to ultrafast photonics. The measured nonlinear optical absorption curve of SnSe₂ with modulation depth of 5.52% is obtained. Q-switching and mode-locking in Er-doped fiber lasers based on SnSe₂ is reported. A Q-switched fiber laser can be obtained with a 0.5-m-long gain fiber and a 50:50 optical coupler. With 0.75-m-long active fiber and a 40:60 optical coupler, mode locking can be obtained. Furthermore, a third-order bound-state soliton pulse with a pulse duration of 882 fs can be achieved. The experimental results show that SnSe₂ is an excellent optical saturable absorber for the applications of ultrafast photonics.     

Structural,electrical and optical properties of boron doped ZnO thin films using LSMCD method at room temperature

Zn_1−x B _x O (0≤x≤0.04) thin films were deposited by the liquid source misted chemical vapor deposition (LSMCD) method. The thin films were polycrystalline with grain sizes of 16 nm to 22 nm. The structural, optical, and electrical properties were investigated by X-ray diffraction, UV-visible spectrophotometry, Raman spectroscopy, and Hall effect measurement. Also scanning electron (SEM) and atomic force microscopy (AFM) techniques were used in order to determine the morphological and topological characteristics of the films. The optimal result of Zn_1−x B _x O films was obtained at x=0.02, with a low resistivity of ≈10⁻² Ω cm, and a high transmittancy of 85% in the visible light spectrum (300 nm ∼ 800 nm).     

DSC and elastic moduli studies on tellurite-vanadate glasses containing antimony oxide

xSb₂O₃-40TeO₂-(60 − x) V₂O₅ glasses with 0 ≤ x ≤ 10 (in mol%) have been prepared by rapid- melt quenching method. DSC curves of these ternary glasses have been investigated. The glass transition properties that have been measured and reported in this paper, include the glass transition temperature (T _g ), glass transition width (ΔT _g ), heat capacity change at glass transition (ΔC _P ) and fragility (F). Thermal stability, Poisson’s ratio, fragility and glass forming tendency of these glasses have been estimated, to determine relationship between chemical composition and the thermal stability or to interpret the structure of glass. In addition, Makishima and Makenzie’s theory was applied for determination of Young’s modulus, bulk modulus and shear modulus, indicating a strong relation between elastic properties and structure of glass. Generally, results of this work show that glass with x = 0 has the highest shear, bulk and Young’s moduli which make it as suitable candidate for the manufacture of strong glass fibers in technological applications; but it should be mentioned that glass with x = 8 has higher handling temperature and super resistance against thermal attack.     

Structure and magnetic properties of co-sputtered Co–C thin films

We studied the structure and magnetic properties of co-sputtered Co_1−xC_x thin films using a transmission electron microscope (TEM) and a SQUID magnetometer. These properties were found to depend critically on deposition temperature, T_S, and composition, x. Generally, phase separation into metallic Co and graphite-like carbon phases proceeds with increasing T_S and decreasing x. Plan view and cross-sectional TEM images of the films prepared showed that Co grains about 10–20 nm in diameter and 30–50 nm in height are three-dimensionally separated by graphite-like carbon layers 1–2 nm thick. Optimum magnetic properties with saturation magnetization of 380 emu/cc and coercivity of 400 Oe were obtained for a film with x=0.5 and T_S=350°C.