Yongho Seo

Comparison of frictional forces on graphene and graphite

We report on the frictional force between an SiN tip and graphene/graphite surfaces using lateral force microscopy. The cantilever we have used was made of an SiN membrane and has a low stiffness of 0.006 N m(-1). We prepared graphene flakes on a Si wafer covered with silicon oxides. The frictional force on graphene was smaller than that on the Si oxide and larger than that on graphite (multilayer of graphene). Force spectroscopy was also employed to study the van der Waals force between the graphene and the tip. Judging that the van der Waals force was also in graphite-graphene-silicon oxide order, the friction is suspected to be related to the van der Waals interactions. As the normal force acting on the surface was much weaker than the attractive force, such as the van der Waals force, the friction was independent of the normal force strength. The velocity dependency of the friction showed a logarithmic behavior which was attributed to the thermally activated stick-slip effect.

Atomic force microscopy and spectroscopy

Since it was invented by Binnig et al in 1986, atomic force microscopy (AFM) has played a crucial role in nano-scale science and technology. AFM is a microscopic technique imaging a surface topography by using attractive and repulsive interaction forces between a few atoms attached at a tip on a cantilever and a sample. In the case of attractive forces, there are three main contributions causing AFM. These are short-range chemical force, van der Waals force and electrostatic force. As the effective ranges of these forces are different, one of them is dominant depending on distance. Atomic force spectroscopy is the force-versus-distance measurement when using AFM. The atomic force can be detected by cantilever bending caused by a tip‐sample interacting force, which is called static AFM. Also, the atomic force can be detected by using the resonant properties of a cantilever, which is called dynamic AFM. Under the on-resonance condition, the frequency, amplitude or phase of the cantilever will be shifted by the interaction force. While the force can be estimated in static AFM, for dynamic AFM it requires complicated formalism to evaluate the force from measured amplitude, phase or frequency data. Recently developed techniques for ultra-high resolution imaging unveil sub-atomic features of the sample, which are facilitated by low temperature, ultra-high vacuum environments together with a stiff cantilever. In this study, progress related to theoretical and experimental imaging and force spectroscopy will be discussed. (Some figures in this article are in colour only in the electronic version)

High-mobility and air-stable single-layer WS2 field-effect transistors sandwiched between chemical vapor deposition-grown hexagonal BN films.

An emerging electronic material as one of transition metal dichalcogenides (TMDCs), tungsten disulfide (WS2) can be exfoliated as an atomically thin layer and can compensate for the drawback of graphene originating from a gapless band structure. A direct bandgap, which is obtainable in single-layer WS2, is an attractive characteristic for developing optoelectronic devices, as well as field-effect transistors. However, its relatively low mobility and electrical characteristics susceptible to environments remain obstacles for the use of device materials. Here, we demonstrate remarkable improvement in the electrical characteristics of single-layer WS2 field-effect transistor (SL-WS2 FET) using chemical vapor deposition (CVD)-grown hexagonal BN (h-BN). SL-WS2 FET sandwiched between CVD-grown h-BN films shows unprecedented high mobility of 214 cm2/Vs at room temperature. The mobility of a SL-WS2 FET has been found to be 486 cm2/Vs at 5 K. The ON/OFF ratio of output current is ~107 at room temperature. Apart from an ideal substrate for WS2 FET, CVD-grown h-BN film also provides a protection layer against unwanted influence by gas environments. The h-BN/SL-WS2/h-BN sandwich structure offers a way to develop high-quality durable single-layer TMDCs electronic devices.

Local conductance measurement of graphene layer using conductive atomic force microscopy

This paper reports the local conductivity mapping of graphene films prepared by chemical vapor deposition and mechanical exfoliation with the help of atomic force microscope where a conducting tip scanned the graphene surface with bias voltage. The surface morphology measured by field emission scanning electron microscopy confirmed that domains and wrinkles were formed on graphene samples grown by chemical vapor deposition, and the difference in the amount of current is observed on these domain boundaries and wrinkles. The percolation current path observed in current map explains that graphene grown by the chemical vapor deposition has low conductivity compared with one mechanically exfoliated. On the other hand, exfoliated graphene layer showed sign of conductivity differences on step edges and wrinkles in comparison to flat region. The resulting observations can be explained with the help of existing theories regarding graphene and by considering the effect of sample preparation conditions.

Photocurrent Response of MoS2 Field-Effect Transistor by Deep Ultraviolet Light in Atmospheric and N2 Gas Environments

Molybdenum disulfide (MoS2), which is one of the representative transition metal dichalcogenides, can be made as an atomically thin layer while preserving its semiconducting characteristics. We fabricated single-, bi-, and multilayer MoS2 field-effect transistor (FET) by the mechanical exfoliation method and studied the effect of deep ultraviolet (DUV) light illumination. The thickness of the MoS2 layers was determined using an optical microscope and further confirmed by Raman spectroscopy and atomic force microscopy. The MoS2 FETs with different number of layers were assessed for DUV-sensitive performances in various environments. The photocurrent response to DUV light becomes larger with increasing numbers of MoS2 layers and is significantly enhanced in N2 gas environment compared with that in atmospheric environment.

Electrostatic force microscopy using a quartz tuning fork

We demonstrate an electrostatic force microscopy based on a quartz tuning fork with 50 nm spatial resolution and 1 pN force sensitivity. We use a tuning fork with a spring constant of 1300 N/m and a Q factor of 3000. A sharpened nickel tip is attached to a prong of the tuning fork as well as electrically connected to the electrode of the prong. By applying a dc bias to the tip, ferroelectric domain patterns are recorded and read out on piezoelectric thin film.

Micro-to-nano-scale deformation mechanisms of a bimodal ultrafine eutectic composite

The outstading mechanical properties of bimodal ultrafine eutectic composites (BUECs) containing length scale hierarchy in eutectic structure were demonstrated by using AFM observation of surface topography with quantitative height measurements and were interpreted in light of the details of the deformation mechanisms by three different interface modes. It is possible to develop a novel strain accommodated eutectic structure for triggering three different interface-controlled deformation modes; (I) rotational boundary mode, (II) accumulated interface mode and (III) individual interface mode. A strain accommodated microstructure characterized by the surface topology gives a hint to design a novel ultrafine eutectic alloys with excellent mechanical properties.

Active Q control in tuning-fork-based atomic force microscopy

The authors present comprehensive theoretical analysis and experimental realization of active Q control for the self-oscillating quartz tuning fork (TF). It is shown that the quality factor Q can be increased (decreased) by adding the signal of any phase lag, with respect to the drive signal, in the range of θ1 to θ1+π (θ1+π to θ1+2π), where θ1 is the characteristic constant of TF. Experimentally, the nominal Q value of 4.7×103 is decreased to 1.8×103 or increased to 5.0×104 in ambient condition, where the minimum detectable force is estimated to be 4.9×10−14N at 1Hz. The novel Q control scheme demonstrated in the widely used quartz TF is expected to contribute much to scanning probe microscopy of, in particular, soft and biological materials.

Low-temperature high-resolution magnetic force microscopy using a quartz tuning fork

We have developed a low-temperature high resolution magnetic force microscope (MFM) using a quartz tuning fork that can operate in a magnetic field. A tuning fork with a spring constant of 1300N∕m mounted with a commercial MFM cantilever tip was used. We have obtained high-resolution images of the stray magnetic fields exerted from grains with a spatial resolution of 15 nm and force resolution of 2 pN at 4.2 K. Tuning fork-based magnetic force microscopes have the potential to be used at millikelvin temperatures due to their low power dissipation and high force sensitivity.

Combinatorial influence of bimodal size of B2 TiCu compounds on plasticity of Ti-Cu-Ni-Zr-Sn-Si bulk metallic glass composites

We report on the formation of Ti-Cu-Ni-Zr-Sn-Si bulk metallic glass composites containing bimodal size of B2 TiCu compounds. The small B2 TiCu compound with a size of 1 to 10 μm has a strong influence on the oscillation of the shear stress, thus causing wavy propagation of the shear bands. In contrast, the large B2 TiCu compound with a size of 70 to 150 μm dissipates the shear stress by branching and multiplication of the shear bands. By forming the bimodal size of B2 TiCu compound, it is possible to determine the harmonic influence to further enhance the plasticity of the Ti-Cu-Ni-Zr-Sn-Si bulk metallic glass composites.