Janghwan Cha

Manipulation of graphene work function using a self-assembled monolayer

We report an effective and reliable method to increase the work function of graphene to as high as 5.50 eV by applying a self-assembled monolayer on its surface. The work function of pristine graphene (4.56 eV) was increased by approximately +0.94 eV following trichlorosilane (HDF-S) self-assembly. This increase in the work function was confirmed by ab initio calculations. HDF-S self-assembled graphene exhibited no significant changes in structural, optical, or electrical characteristics compared with pristine graphene. In addition, we verified that the modified work function of HDF-S self-assembled graphene was not affected by the underlying substrates.

Enhanced thermoelectric properties of Ca1-x Sm x Mn1-y W y O3-δ for power generation

A series of Ca1-xSmxMn1-yWyO3-δ (0.05 ≤ x ≤ 0.25 and 0.05 ≤ y ≤ 0.2) was prepared by the solid-state reaction technique. The partial substitution of Sm3+ for Ca2+ and of W6+ for Mn4+ in CaMnO3-δ reduced the grain size and density. The substitution of Sm3+ and W6+ yielded a marked increase in electrical conductivity and a decrease in the absolute value of the Seebeck coefficient due to an increase in electron concentration. This gave rise to improved thermoelectric properties. A maximum power factor (2.07 × 10-4 Wm-1K-2) was obtained at 800°C for Ca0.9Sm0.1MnO3-δ. It is believed that the substitution of Sm3+ for Ca2+ is a promising approach for enhancing the thermoelectric performance of CaMnO3-δ.

Visualizing Degradation of Black Phosphorus Using Liquid Crystals

Black Phosphorus (BP) is an excellent material from the post graphene era due to its layer dependent band gap, high mobility and high Ion/Ioff. However, its poor stability in ambient poses a great challenge for its practical and long-term usage. The optical visualization of the oxidized BP is the key and the foremost step for its successful passivation from the ambience. Here, we have conducted a systematic study of the oxidation of the BP and developed a technique to optically identify the oxidation of the BP using Liquid Crystal (LC). It is interesting to note that we found that the rapid oxidation of the thin layers of the BP makes them disappear and can be envisaged by using the alignment of the LC. The molecular dynamics simulations also proved the preferential alignment of the LC on the oxidized BP. We believe that this simple technique will be effective in passivation efforts of the BP, and will enable it for exploitation of its properties in the field of electronics.Black Phosphorus (BP) is an excellent material for post graphene era due to its layer dependent band gap, high mobility and high Ion/Ioff. However, its poor stability in ambient poses a great challenge in its practical and long-term usage. Optical visualization of oxidized BP is the key and foremost step for its successful passivation from the ambience. Here, we have done a systematic study of the oxidation of BP and developed a technique to optically identify the oxidation of BP using Liquid Crystal (LC). Interestingly we found that rapid oxidation of thin layers of BP makes them disappear and can be envisaged by using the alignment of LC. The molecular dynamics simulations also proved the preferential alignment of LC on oxidized BP. We believe that this simple technique will be effective in passivation efforts of BP and will enable it for exploitation of its properties in the field of electronics.

Temperature-Dependent and Gate-Tunable Rectification in a Black Phosphorus/WS2 van der Waals Heterojunction Diode

Heterostructures comprising two-dimensional (2D) semiconductors fabricated by individual stacking exhibit interesting characteristics owing to their 2D nature and atomically sharp interface. As an emerging 2D material, black phosphorus (BP) nanosheets have drawn much attention because of their small band gap semiconductor characteristics along with high mobility. Stacking structures composed of p-type BP and n-type transition metal dichalcogenides can produce an atomically sharp interface with van der Waals interaction which leads to p-n diode functionality. In this study, for the first time, we fabricated a heterojunction p-n diode composed of BP and WS2. The rectification effects are examined for monolayer, bilayer, trilayer, and multilayer WS2 flakes in our BP/WS2 van der Waals heterojunction diodes and also verified by density function theory calculations. We report superior functionalities as compared to other van der Waals heterojunction, such as efficient gate-dependent static rectification of 2.6 × 104, temperature dependence, thickness dependence of rectification, and ideality factor of the device. The temperature dependence of Zener breakdown voltage and avalanche breakdown voltage were analyzed in the same device. Additionally, superior optoelectronic characteristics such as photoresponsivity of 500 mA/W and external quantum efficiency of 103% are achieved in the BP/WS2 van der Waals p-n diode, which is unprecedented for BP/transition metal dichalcogenides heterostructures. The BP/WS2 van der Waals p-n diodes have a profound potential to fabricate rectifiers, solar cells, and photovoltaic diodes in 2D semiconductor electronics and optoelectronics.

Strain-induced non-linear optical characteristics of pyroelectric PbVO 3 epitaxial thin films

Highly strained PbVO3 films have potential applications in pyroelectric sensors and optoelectronic devices. To understand how strain influences the nonlinear optical characteristic of PbVO3, highly distorted tetragonal PbVO3 epitaxial thin films were grown on LaAlO3 and SrTiO3 substrates. Using Raman scattering spectroscopy, theoretical and experimental studies on the phonon spectra yield insights on the strained structures of V-O apical bondings. Effects of applied strain on the linear optical properties of PbVO3 thin films were experimentally characterized by spectroscopic ellipsometry. Second-order nonlinear susceptibility tensor components of PbVO3/SrTiO3 thin films were determined to be almost twice as large as those of PbVO3/LaAlO3 thin films.