Chiwon Lee

Interfacial liquid-state surface-enhanced Raman spectroscopy

Oriented assemblies of functional nanoparticles, with the aid of external physical and chemical driving forces, have been prepared on two-dimensional solid substrates. It is challengeable, however, to achieve three-dimensional assembly directly in solution, owing to thermal fluctuations and free diffusion. Here we describe the self-orientation of gold nanorods at an immiscible liquid interface (that is, oleic acid-water) and exploit this novel phenomenon to create a substrate-free interfacial liquid-state surface-enhanced Raman spectroscopy. Dark-field imaging and Raman scattering results reveal that gold nanorods spontaneously adopt a vertical orientation at an oleic acid-water interface in a stable trapping mode, which is in good agreement with simulation results. The spontaneous vertical alignment of gold nanorods at the interface allows one to accomplish significant additional amplification of the Raman signal, which is up to three to four orders of magnitude higher than that from a solution of randomly oriented gold nanorods.

Two-dimensional Hyper-branched Gold Nanoparticles Synthesized on a Two-dimensional Oil/Water Interface

Two-dimensional (2D) gold nanoparticles can possess novel physical and chemical properties, which will greatly expand the utility of gold nanoparticles in a wide variety of applications ranging from catalysis to biomedicine. However, colloidal synthesis of such particles generally requires sophisticated synthetic techniques to carefully guide anisotropic growth. Here we report that 2D hyper-branched gold nanoparticles in the lateral size range of about 50 ~ 120 nm can be synthesized selectively on a 2D immiscible oil/water interface in a few minutes at room temperature without structure-directing agents. An oleic acid/water interface can provide diffusion-controlled growth conditions, leading to the structural evolution of a smaller gold nucleus to 2D nanodendrimer and nanourchin at the interface. Simulations based on the phase field crystal model match well with experimental observations on the 2D branching of the nucleus, which occurs at the early stage of growth. Branching results in higher surface area and stronger near-field enhancement of 2D gold nanoparticles. This interfacial synthesis can be scaled up by creating an emulsion and the recovery of oleic acid is also achievable by centrifugation.

Orientation and position of cylindrical-shaped gold nanoparticles at liquid-liquid interfaces

The equilibrium orientation and position of cylindrical-shaped gold nanoparticles (GNPs) at an oil/water interface is studied by varying its geometry and surface property and the oil/water interfacial tension. Numerical calculation reveals that most of the particles studied prefer one of two states of orientation at the interface: either 0° or 90° irrespective of the geometry, the surface property, and the oil/water interfacial tension. In the case of cylindrical-shaped GNPs having a weakly hydrophobic side surface, an increase in oil/water interfacial tension leads to a remarkable change in the orientation and position of the particles depending on the geometry.

Measurement of transverse emittance and coherence of double-gate field emitter array cathodes

Achieving small transverse beam emittance is important for high brightness cathodes for free electron lasers and electron diffraction and imaging experiments. Double-gate field emitter arrays with on-chip focussing electrode, operating with electrical switching or near infrared laser excitation, have been studied as cathodes that are competitive with photocathodes excited by ultraviolet lasers, but the experimental demonstration of the low emittance has been elusive. Here we demonstrate this for a field emitter array with an optimized double-gate structure by directly measuring the beam characteristics. Further we show the successful application of the double-gate field emitter array to observe the low-energy electron beam diffraction from suspended graphene in minimal setup. The observed low emittance and long coherence length are in good agreement with theory. These results demonstrate that our all-metal double-gate field emitters are highly promising for applications that demand extremely low-electron bunch-phase space volume and large transverse coherence.

Field emission beam characteristics of single metal nanotip cathodes with on-chip collimation gate electrode

Field-emission and beam collimation characteristics of single metal nanotip devices with double-gate electrodes are studied. Applying a previously developed method to fabricate all-metal double-gate nanotip arrays with a stacked on-chip extraction Gext and collimation Gcol gate electrodes with the large Gcol apertures, the authors produced single double-gate nanotip devices and measured their beam characteristics. Excellent beam collimation capability with minimal reduction of the emission current and the enhancements of the current density up to a factor of ∼7 was observed. The results indicate that these single nanotip devices are highly promising for electron beam applications that require extremely high brilliance and coherence.

Electron beam collimation from an all-metal double-gate 40 000 nanotip array: Improved emission current and beam uniformity upon neon gas conditioning

In this study, the authors characterized field emission for stacked-double-gate all-metal field emitter arrays (FEAs) consisting of 40 000 nanotips. After careful conditioning of the FEAs under ultrahigh vacuum and in low-pressure neon gas ambient, the authors were able to produce a highly collimated beam with a current of ∼50 μA which showed an improved beam homogeneity. The beam rms radius reduced by a factor 10 and the transverse energy spread was reduced to well below 1 eV.

Transmission low-energy electron diffraction using double-gated single nanotip field emitter

We explore the spatial coherence of double-gate single nanotip field emitters by low-energy electron diffraction experiments in transmission mode. By producing collimated field emission pulses from the single nanotip cathode and irradiating a suspended monolayer graphene film without additional optics, we observed sharper and higher resolution Bragg diffraction spots than a previous experiment using a nanotip array cathode. In particular, we found complete conservation of the size and the shape of the diffraction spots with those of the incident beam on the sample. The result indicates that the transverse coherence of a nanofabricated double-gate single-tip emitter is much larger than a few nanometers as determined by the apparent diffraction spot size and overall spatial resolution of the observed diffraction pattern.We explore the spatial coherence of double-gate single nanotip field emitters by low-energy electron diffraction experiments in transmission mode. By producing collimated field emission pulses from the single nanotip cathode and irradiating a suspended monolayer graphene film without additional optics, we observed sharper and higher resolution Bragg diffraction spots than a previous experiment using a nanotip array cathode. In particular, we found complete conservation of the size and the shape of the diffraction spots with those of the incident beam on the sample. The result indicates that the transverse coherence of a nanofabricated double-gate single-tip emitter is much larger than a few nanometers as determined by the apparent diffraction spot size and overall spatial resolution of the observed diffraction pattern.

Optical fiber-driven low energy electron gun for ultrafast streak diffraction

Here, we present an optical fiber-based electron gun designed for the ultrafast streaking of low-energy electron bunches. The temporal profile of the few tens of the picosecond long electron bunch composed of 200 electrons is well characterized using a customized streak camera. Detailed analysis reveals that the stretched optical trigger pulse owing to the dispersion effects inside the waveguide dominantly determines the temporal length of the low density electron bunch. This result illustrates the capability to control the observable time-window in the streak diffraction experiment by tailoring geometrical parameters of the fiber source and its coupling condition. With the electrostatic Einzel lens system integrated on the fiber-based cathode, we also demonstrate spatial focusing of the electron beam with the RMS spot size of 98 μm and imaging of the static low-energy electron diffraction pattern of monolayer graphene in the electron kinetic energy range of 1.0–2.0 keV.Here, we present an optical fiber-based electron gun designed for the ultrafast streaking of low-energy electron bunches. The temporal profile of the few tens of the picosecond long electron bunch composed of 200 electrons is well characterized using a customized streak camera. Detailed analysis reveals that the stretched optical trigger pulse owing to the dispersion effects inside the waveguide dominantly determines the temporal length of the low density electron bunch. This result illustrates the capability to control the observable time-window in the streak diffraction experiment by tailoring geometrical parameters of the fiber source and its coupling condition. With the electrostatic Einzel lens system integrated on the fiber-based cathode, we also demonstrate spatial focusing of the electron beam with th...

Intrinsic emittance and coherence of double-gate field emitter arrays

The intrinsic transverse emittance and coherence of double-gate field emitter array cathodes were explored experimentally including the low-energy electron diffraction and compared with theory.