CHAPTER 1. INTRODUCTION
Femtosecond-resolved nanotip electron microscopy has developed as explosively growing and advanc-ing nanoscience and nanotechnology which lead new advanced techniques, strategies, and perspective to understand and investigate optical phenomena, and properties on nanometer regime. Basically this technique is a combination of the ultrafast optical spectroscopy and the electron-based microscopy techniques. Therefore, this technique can provide the high spatiotemporal resolution and facilitate the investigation such as the activation of reaction mechanism for photocatalyst, solar cell, sensor for chemicals and biomolecules, and the dynamics of surface plasmons on metallic nanostructures. Our femtosecond-resolved nanotip electron microscopy system is composed of electrochemical etching system, femtosecond laser system, shear force microscopy, and tip-enhanced electron emission microscopy. And we have characterized each of systems.
CHAPTER II. ELECTROCHEMICAL ETCHING SYSTEM
In this chapter, we have investigated the factors to influence the aspect ratio and shape of the tungsten tip in electrochemical etching technique as a function of the bias voltage because the probe tip plays the critical role in our femtosecond-resolved nanotip electron microscopy. The etching procedure for the tungsten tip has two steps and the aspect ratio of the tungsten tip can be controlled by the applied DC voltage in each of steps in the etching process. And the shape and the coverture radius of the tungsten tip are affected by the cut-off time. In our system, the cut-off time is less than 150 milliseconds. The faster the cut-off time, the sharper the tungsten tip is fabricated. We have designed the program which can control the cut-off time using LabView software. As a result, we can fabricate the reasonably fascinating tungsten tip and control the aspect ratio and shape of the tungsten tip.
CHAPTER III. FEMTOSECOND LASER SYSTEM
The femtosecond pump-probe technique is one of the powerful tools to investigate ultrafast chemical reaction and dynamics. We characterized the performance of our femtosecond laser system to create electron pulses for the femtosecond-resolved nanotip electron microscopy. Pulse durations are measured by the autocorrelation and cross-correlation using the sum-frequency mixing. Observed pulse durations are ~ 15 fs, ~ 35 fs at 816 nm and 408 nm of the central wavelength, respectively. And depopulation of the Soret band of metalloporphyrins such as H2TPP, CuIITPP, and ZnTPP was observed by the transient absorption and fluorescence up-conversion experiments. The fluorescence anisotropy is measured by ~ 150 fs for ZnTPP in toluene solvent. Especially, Soret band lifetimes of the free-base tetraphenylporphine (H2TPP) and Cu(II) tetraphenylporphine (CuIITPP) have been directly measured with the femtosecond (fs) resolution using the fluorescence up-conversion technique for the first time, giving τ = 68 ± 15 and 63 ± 15 fs, respectively, in the benzene solvent.
CHAPTER IV. SHEAR FORCE MICROSCOPY
Shear force microscopy is introduced to control the tungsten tip-sample distance with shear force inter-actions and obtain two-dimensional topographic images. The tungsten tip-sample distance affects the spatial resolution of image and it depends on the quality factor and the type of materials. The performance of our shear force microscope is characterized with nickel and gold meshes by the comparison the SFM image with the SEM image. The spatial resolution is less than 20 nm mechanically. But, the spatial resolution of SFM image is determined by the scan step and the feature of the probe tip. The images obtained by shear force microscopy will be compared with images came from the tip-enhanced electron emission microscopy.
CHAPTER V. TIP-ENHANCED ELECTRON EMISSION MICROSCOPY
We have demonstrated that the tip-enhanced electron emission microscope (TEEM) can give the two-dimensional image of the surface structure and the finite-difference in time domain calculations (FDTD) are implemented to support experimental results. The pulse duration of generated electron pulse is ~ 20 fs at 810 nm of the central wavelength. In nickel mesh study, we exhibited the turning point for generating the secondary electrons. And we have verified the local field enhancement in nanoscale imaging of the gold mesh by TEEM. The lightning rod effect and surface plasmon polaritons act as important factors to increasing the electromagnetic field enhancement. Great synergic effect will be manifested by emerging the lightning rod effect which is occurred at the apex of the tungsten tip and the surface plasmon polaritons on the surface of gold mesh. These effects induce significant increase of the count of created free electrons. And, the count is affected by the applied bias voltage, the polarization of light, and the tip-sample distance. In FDTD simulations, we can find the condition to get the maximized electromagnetic field enhancement. Further, we anticipate that intensive studies for the dynamics using the surface plasmons will be achieved by the results of ongoing experiments.
펨토초 시분해 나노탐침 전자현미경 개발
급속도로 발전하고 있는 첨단 나노과학과 나노테크놀로지를 보다 깊이 이해하고 표면에서 일어나는 광학 현상 및 성질을 연구하기 위해 펨토초 시분해 나노탐침 전자현미경 (femtosecond-resolved nanotip elelctron microscopy)을 개발하게 되었다. 기본적으로, 펨토초 시분해 나노탐침 전자현미경은 초극단 광분광학 (ultrafast optical spectroscopy) 기술과 전자를 이용하는 현미경의 결합물로서, 나노미터 수준의 실공간과 펨토초 수준의 실시간 분해능을 제공할 수 있다. 따라서, 이를 이용하면 광촉매 (photocatalyst), 태양전지 (solar cell) 등의 반응 메커니즘, 화학물질과 바이오 분자 등의 센서, 금속 나노구조체 (metallic nanostructure) 에서의 표면 플라즈몬 (surface plasmon) 의 동역학을 연구할 수 있는 기반을 마련해 줄 수 있다.
펨토초 시분해 나노탐침 전자현미경은 전기화학에칭 (electrochemical etching) 시스템, 펨토초 레이저 시스템, 전단력 현미경 (shear force microscopy, SFM), 탐침 강화 전자 방출 현미경 (tip-enhanced electron emission microscopy, TEEM) 의 네 부분으로 이루어져 있다. 전기화학에칭 시스템에서는 텅스텐 막대 (tungsten rod) 나 텅스텐 와이어 (tungsten wire) 를 이용하여 텅스텐 탐침 (tungsten tip) 을 제작하였으며, 탐침의 모양, 크기 및 종횡비 (aspect ratio) 를 제어하여 재현성있게 텅스텐 탐침을 만들 수 있었다. 펨토초 레이저 시스템에서는 금속 포르피린 (metalloporphyrin) 계열의 분자를 이용하여 펨토초 레이저 시스템의 특성과 성능을 측정하고, 펌프-탐지 (pump-probe) 방법을 이용하여 시분해 동역학을 연구할 수 있었다. 추후 펌프-탐지 방법은 펨토초 시분해 전자현미경에서 전자 펄스를 만들어 표면에서의 동역학을 연구하는데 이용된다. 전단력 현미경은 탐침과 시료간의 거리를 제어하고 시료 표면의 이미지를 얻기 위해 도입되었으며, 이를 통해 얻은 이미지는 탐침 강화 전자 방출 현미경에서 얻은 이미지와 비교 분석하게 된다. 마지막으로, 탐침 강화 전자 방출 현미경에서는 펨토초 레이저를 이용하여 전자 펄스를 발생시키고, 전자 펄스와 시료와의 상호작용의 결과물들을 검출기를 통해 표면의 구조를 2차원 이미지로 구현한다. 또한 금속 표면에서의 표면 플라즈몬과 텅스텐 탐침의 피뢰침 효과 (lighting rod effect) 를 이용하여 표면에서의 변화를 관측할 수 있었다. 또한 시간 영역법의 적용시간 영역 유한 차분 (finite difference in time domain) 방법을 이용하여 전자기장의 세기를 계산하였으며, 이 결과를 실험을 통해 얻은 결과를 뒷받침할 수 있었다. 또한 모든 데이터 획득과 처리 프로그램은 LabView 소프트웨어를 이용하여 직접 제작하였다.