Chung-Seog Oh

The mechanical properties of thin polymer film for nanoimprinting lithography by nanoindentation

Two thin polymer films (mr-I-8030 and mr-I-9030) for nanoimprinting lithography are coated on 4 inches silicon wafers. Nanoindentation tests are performed to measure the mechanical properties, Youngs modulus and hardness, of these thin films having the thicknesses of around 130, 200, and 400 nm. The primary elastic modulus and the viscoelastic properties are derived from the creep. The modulus values obtained by these methods are compared. The plastic deformation of these films after indentation is analyzed by using a scanning probe microscope.

A systematic exfoliation technique for isolating large and pristine samples of 2D materials

The device conceptualization and proof-of-concept testing of two-dimensional (2D) materials are performed with their pristine forms that are obtained through the micromechanical cleaving of bulk natural crystals, i.e., the so-called Scotch tape method. However, obtaining large 2D sheets is very difficult and time consuming. We developed a systematic exfoliation technique for producing sub-millimeter-sized (the largest lateral dimension ever reported) pristine 2D sheets with high throughput. It requires the treatment of both the bulk crystal and receiving substrate. Contrary to the conventional Scotch tape technique that involves the repeated folding and unfolding of an adhesive tape, the flake is stamped onto an adhesive tape to preserve the lateral size of the bulk crystal, to improve the surface flatness, and to reduce the amount of residue on the surface of the samples. When applied to graphene, the method produced monolayer and few layer graphene samples that were several hundreds of microns in length. Surprisingly, the biggest monolayer graphene sample of 367 μm in length was easily produced. The technique was also applied to produce pristine MoS2 and phosphorene sheets of about 45 μm and 95 μm in length, respectively.

Isothermal physical aging of thin PMMA films near the glass transition temperature

The isothermal physical aging and the glass transition temperature (Tg) in PMMA thin films were investigated by means of differential scanning calorimetry (DSC). Freestanding thin film was obtained by spin coating onto a silicon wafer substrate and then releasing the coated film using a water floating technique for different molecular weights (MW = 120,000, 350,000, 996,000 g/mole) and film thicknesses (40~667 nm). The thin films were stacked in a DSC pan and isothermally aged for different aging times (ta = 1 and 12 h) and aging temperatures (Ta =105, 110 and 115 oC) below but near Tg. Enthalpy relaxation (▵HRelax) due to the isothermal physical aging vs. ▵Ta (Tg - Ta, driving force of aging) data showed that the enthalpy value increased with increasing ▵Ta, reached maximum, and then decreased as ▵Ta increases further. As film thickness decreases, ▵HRelax was rapidly reduced for samples below ~100 nm of film thickness near Tg (e.g., Ta =110 and 115 °C), indicating the suppression of physical aging. About 7~10 oC depression in Tg was observed for thinner films (~40 nm), compared to thicker films (~660 nm) in this study.