Ho Chi Minh City Institute of Physics

Le T.S., Truong T.K., Huynh V.N., Bae J., Suh D.

In various wearable energy storage devices, the shape of fiber or yarn has many advantages owing to their compatibility with the environment in which they are deployed. We present a systematic approach to maximizing the capacitance of a supercapacitor yarn by significantly increasing the yarn's surface area by growing a high density of nanorods around the yarn, followed by coating the surface with a pseudo-capacitive material. The two-step strategy is implemented using a dry-spun carbon nanotube yarn-based electrode, which is surrounded by a zinc oxide nanorod forest that is coated by a pseudo-capacitive nickel-cobalt layered double hydroxide material. The flexible as-prepared electrode exhibits a maximum capacitance of 1065 mF cm−2 (1278 F g−1) at a scan rate of 5 mV s−1 and an excellent capacitance retention of 60.5% over 7000 cycles at a current density of 30 mA cm−2. The outstanding performance of the composite yarn supercapacitor can be ascribed to the enhanced ion accessibility to the deep surface of the nickel-cobalt layered double hydroxide layer through the porous carbon nanotube yarn. Furthermore, the symmetric supercapacitor configuration demonstrated nearly 100% capacity retention at a bending angle of 150°.

Anh N.T., Dinh N.X., Pham T.N., Vinh L.K., Tung L.M., Le A.

The rational design of nanomaterials for electrochemical nanosensors from the perspective of structure-property-performance relationships is a key factor in improving the analytical performance toward residual antibiotics in food. We have investigated the effects of the crystalline phase and copper loading amount on the detection performance of Cu-MoS2 nanocomposite-based electrochemical sensors for the antibiotic chloramphenicol (CAP). The phase composition and copper loading amount on the MoS2 nanosheets can be controlled using a facile electrochemical method. Cu and Cu2O nanoparticle-based electrochemical sensors showed a higher CAP electrochemical sensing performance as compared to CuO nanoparticles due to their higher electrocatalytic activity and conductivity. Moreover, the design of Cu-MoS2 nanocomposites with appropriate copper loading amounts could significantly improve their electrochemical responses for CAP. Under optimized conditions, Cu-MoS2 nanocomposite-based electrochemical nanosensor showed a remarkable sensing performance for CAP with an electrochemical sensitivity of 1.74 μA μM-1 cm-2 and a detection limit of 0.19 μM in the detection range from 0.5-50 μM. These findings provide deeper insight into the effects of nanoelectrode designs on the analytical performance of electrochemical nanosensors.

Anh N.T., Huyen N.N., Dinh N.X., Vinh L.K., Tung L.M., Vinh N.T., Quy N.V., Lam V.D., Le A.

The effect of crystallinity, phase ratio, and heterojunction formation on the FZD sensing performance of ZnO/ZnFe2O4 nanocomposite-based electrochemical sensors was investigated.

Zuo R., Trautmann A., Wang G., Hannes W., Yang S., Song X., Meier T., Ciappina M., Duc H.T., Yang W.

High harmonic generation (HHG) from solids shows great application prospects in compact short-wavelength light sources and as a tool for imaging the dynamics in crystals with subnanometer spatial and attosecond temporal resolution. However, the underlying collision dynamics behind solid HHG is still intensively debated and no direct mapping relationship between the collision dynamics with band structure has been built. Here, we show that the electron and its associated hole can be elastically scattered by neighboring atoms when their wavelength approaches the atomic size. We reveal that the elastic scattering of electron/hole from neighboring atoms can dramatically influence the electron recombination with its left-behind hole, which turns out to be the fundamental reason for the anisotropic interband HHG observed recently in bulk crystals. Our findings link the electron/hole backward scattering with Van Hove singularities and forward scattering with critical lines in the band structure and thus build a clear mapping between the band structure and the harmonic spectrum. Our work provides a unifying picture for several seemingly unrelated experimental observations and theoretical predictions, including the anisotropic harmonic emission in MgO, the atomic-like recollision mechanism of solid HHG, and the delocalization of HHG in ZnO. This strongly improved understanding will pave the way for controlling the solid-state HHG and visualizing the structure-dependent electron dynamics in solids.

Tran L.N., Neuscamman E.

We demonstrate that, rather than resorting to high-cost dynamic correlation methods, qualitative failures in excited-state potential energy surface predictions can often be remedied at no additional cost by ensuring that optimal molecular orbitals are used for each individual excited state. This approach also avoids the weighting choices required by state-averaging and dynamic weighting and obviates their need for expensive wave function response calculations when relaxing excited-state geometries. Although multistate approaches are of course preferred near conical intersections, other features of excited-state potential energy surfaces can benefit significantly from our single-state approach. In three different systems, including a double bond dissociation, a biologically relevant amino hydrogen dissociation, and an amino-to-ring intramolecular charge transfer, we show that state-specific orbitals offer qualitative improvements over the state-averaged status quo.

Thong L.H., Ngo C., Duc H.T., Song X., Meier T.

Based on the multiband semiconductor Bloch equations a microscopic approach to high-harmonic generation in crystalline solids which is able to properly describe degenerate bands and band crossings is presented and analyzed. It is well known that numerical band structure calculations typically provide electronic wave functions with an undetermined $k$-dependent phase which results in matrix elements which contain arbitrary $k$-dependent phases. In addition, such approaches usually mix degenerate bands and bands with an energy difference smaller than the numerical precision in an arbitrary way for each point in $k$ space. These ambiguities are problematic if one considers the dynamics induced by electric fields since the matrix elements of the position operator involve a derivative of the wave functions with respect to $k$. When the light-matter interaction is described in the length gauge, the problem of arbitrary phases and degenerate subspace mixing of Bloch states is solved by adopting a smooth gauge transformation along the field direction. The results obtained within this method are validated by comparing with calculations in the velocity gauge. Although we obtain in both gauges the same overall result, the length gauge is advantageous since it converges with a smaller number of bands and thus requires significantly less numerical effort than the velocity gauge. Also a unique distinction between inter- and intraband contributions and thus an instructive physical interpretation is possible in the length gauge whereas in the velocity gauge this is unclear. The computed polarization-direction-dependent high-harmonic spectra agree well with experimental data reported for GaAs. Furthermore, it is demonstrated that, under proper conditions, the Berry curvature is largely responsible for the even-order harmonics which are polarized perpendicular to the driving field.

Pham T.N., Xuan D.N., Van T.H., Khanh V.L., Minh T.L., Nguyen V.Q., Vu D.L., Le A.

MoS2-GO composites were fabricated by an ultrasonication method at room temperature. Raman spectra, emission scanning electron microscopy (SEM), and transmission electron microscopy (TEM) images were used to study the structural characteristics, morphologies, and sizes of the synthesized materials. An MoS2-GO/SPE (screen-printed electrode) was prepared by a facile dropping method and acted as an effective electrochemical sensor toward clenbuterol (CLB) and 4-nitrophenol (4-NP) detection. Based on the obtained results, the influence of analyte molecular structure on the adsorption ability and electronic interoperability between the targeted analyte and electrode surface were investigated in detail and discussed as well, through some electrochemical kinetic parameters (electron/proton-transfer number, electron transfer rate constant (ks), charge transfer coefficient, and adsorption capacity (Γ)). In particular, it should be stressed that 4-NP molecules possess a simple molecular structure with many positive effects (electronic, conjugation, and small steric effects) and flexible functional groups, resulting in fast electron transport/charge diffusion and effective adsorption process as well as strong interactions with the electrode surface. Therefore, 4-NP molecules have been facilitated better in electrochemical reactions at the electrode surface and electrode-electrolyte interfaces, leading to improved current response and electrochemical sensing performance, compared with those of CLB.

Hoang D.V., Vu N.H., Do N.T., Pham A.T., Nguyen T.H., Kuo J., Phan T.B., Tran V.C.

This paper distinguished hydrogen roles to improve electron mobility and carrier concentration in ZnO and Al doped ZnO sputtered films. By combining experimental evidences and theoretical results, we find out that hydrogen located at oxygen vacancy sites (H O ) is the main factor gives rise to increase simultaneously mobility and carrier concentration which has not been mentioned before. Introducing appropriate hydrogen content during sputtering not only results in crystalline relaxation but also supports doping Al into ZnO, increasing carrier concentration and electron mobility in the film. First principles calculations confirmed hydrogen substitutional stability for oxygen vacancy, significantly reducing electron conductivity effective mass and hence increasing electron mobility. In particular, 0.8% hydrogen partial pressure ratio achieved 61 cm 2 V −1 s −1 maximum electron mobility, optical transmittance above 82% in visible and near-infrared regions, and 2 × 10 20 cm −3 carrier concentrations for H Al co-doped ZnO film. These values approach ideal electrical and optical properties for transparent conducting oxide films. The presence of one maximum electron mobility was attributed to competition between increasing mobility due to restoring effective electron mass and hydrogen passivation of native defects, and decreased electron mobility due to electron-phonon scattering. • Experimental and computational results show that H O is the main factor controlling electron mobility & carrier concentration. • Hydrogen supports Al 3+ ions substitution for Zn 2+ ions increasing carrier concentration. • Hydrogen at zinc vacancy sites reduces ionized scattering centers, improving crystal quality and electron mobility. • Hydrogen substitution for V O sites affects the effective electron mass m∗, controlling electron mobility. • The highest mobility obtained in this study is 61 cm 2 V −1 s −1 for Al H co-doped ZnO thin film.

Vinh N.T., Tuan L.A., Vinh L.K., Van Quy N.

Fe3O4/FeOOH nanocomposites were synthesized by a simplistic precipitation method for high-performance gas sensing applications. A spray-coating method was used to deposit a sensitive film of Fe3O4/FeOOH nanocomposites on a quartz crystal microbalance (QCM). The structural and morphological characteristics were analyzed by using X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectrometry (EDS), Fourier transforms infrared (FTIR) spectroscopy, and a vibrating sample magnetometer (VSM). The average diameter and length of the FeOOH nanorods were 30 and 100 nm, respectively. The maximum saturation magnetization of the prepared Fe3O4/FeOOH nanocomposites obtained a value ~30 emu g−1. The QCM sensor based on the Fe3O4/FeOOH nanocomposites was measured with various concentrations of toxic gases (CO, NO2, SO2) at room temperature. The gas sensing results indicated that the developed sensor had a considerable potentiality to apply for toxic gas sensors with a high detection sensitivity, repeatability, and good stability.

Tien V.M., Ong V.H., Pham T.N., Quang Hoa N., Nguyen T.L., Thang P.D., Khanh Vinh L., Trinh P.T., Thanh D.T., Tung L.M., Le A.

The electrochemical behavior and sensing performance of an electrode modified with NiFe2O4 (NFO), MoS2, and MoS2–NFO were thoroughly investigated using CV, EIS, DPV, and CA measurements, respectively.