STM /AFM Studies on Molecular Interaction Properties of VOPc on Au (111) Substrate

Abstract

The scanning tunneling microscope (STM) resolves individual atoms on conducting surfaces using the tunneling current between tip and surface as a signal. For resolving individual atoms on an insulating surface, Binnig, Quate, and Gerber [1] introduced an atomic force microscope (AFM), which is similar to STM. Instead of utilizing a tunneling current as a feedback signal, AFM sense the force between tip and sample. We fabricated the qPlus sensor [2] to allow simultaneous operation of STM and AFM. We made an effort to build an STM / AFM module with high resolution to study the interaction between molecules. In order to achieve the most stable operating condition, we performed the experiments in a cryogenic ultra-high vacuum (UHV) chamber. We studied Vanadyl phthalocyanine (VOPc) molecules on the Au (111) surface using a new imaging method that combines STM and AFM. VOPc consists of the vanadyl oxide (V = O) center perpendicular to the molecular ligand (Pc) plane. Because of this non-planar structure, VOPc can have two geometric configurations when absorbed on the surface: Oxygen is pointing toward the vacuum (O-up) with the Pc lying on the surface, and oxygen is pointing toward the surface (O-down). We studied the VOPc molecule absorption configuration on the Au (111) surface. Their different electronic structures in STM measurements demonstrate the absorption configuration. As a further experiment to demonstrate the power of our STM/AFM, we measured the Hydrogen molecule's interaction with VOPc. We used a hydrogen molecule's site-dependent interaction on the VOPc to get the high-resolution images of the VOPc molecule. We studied this complex using STM combined with AFM. The scanning tunneling spectroscopy (STS) and the Random tunneling noise (RTN) measured on the VOPc demonstrate the Hydrogen molecule's excitation energy on VOPc, which is related to the interaction mechanism. The performance of AFM allowed us to image the VOPc molecule's inner structure when it is interacting with a hydrogen molecule, while we found that STM cannot resolve the molecular structure.;본 논문에서는 STM 과 AFM 정보를 동시에 얻기 위해 필요한 모듈 제작과정과 제작한 모듈로 실험한 내용을 다루고 있다. 안정적인 실험 환경을 만들기 위해 극저온 초고진공 (UHV) 조건에서 실험하였다. 우리는 제작한 모듈로 Au (111) 표면 위에서 Vanadyl phthalocyanine (VOPc) 분자가 상호작용하는 양상을 연구했다. 표면에 흡착된 VOPc 의 밀도에 따라 분자가 행동하는 방향을 분석하였고, 분자가 선호하는 흡착 범위를 계산 결과와 비교했다. VOPc 는 분자의 구조 상 표면에 흡착될 때 두가지 특성으로 흡착되는데 두가지 특성을 구분하기 위해 우리는 분자 하나 단위로 스펙트럼을 측정하였고 계산 결과와 비교했다. 또한, VOPc 분자의 고해상도 이미지를 얻기 위해 수소 분자를 사용하여 AFM 과 STM 이미지를 동시에 얻었다. AFM 측정을 통해 STM 이미지에서는 보이지 않던 VOPc 분자의 내부 구조를 이미지화 하였다. VOPc 에서 측정 된 STS (Scanning Tunneling spectroscopy) 및 RTN 물리학과 (Random Tunneling Noise)은 VOPc 분자와 수소 분자의 상호 작용 메커니즘을 설명한다.