以掃描穿隧顯微術研究Cr(001)單晶表面之 苝四甲酸二酐(PTCDA)的排列、電子結構與自旋極化性質

Abstract

本論文研究Cr(001)單晶表面的苝四甲酸二酐(3,4,9,10-perylenetetracarboxylic-dianhydride, PTCDA)單層分子膜的排列、電子結構與自旋極化(spin-polarized)性質。利用超高真空-低溫-掃描穿隧顯微鏡(ultrahigh vacuum-low temperature-scanning tunneling microscope, UHV-LT-STM)進行樣品製備與表面分析。 實驗樣品的準備皆在超高真空環境下進行。藉由熱阻式方式，蒸鍍PTCDA分子至Cr(001)表面獲得PTCDA/Cr(001)樣品。本論文的第一部分使用鎢探針進行STM影像掃描與STS訊號量測，以觀察PTCDA於Cr(001)基材表面的二維排列與電子結構。STM影像顯示PTCDA在Cr(001)表面排列形式的可分為方形(square, S)和人字形(herringbone, HB)兩種。STM於穿隧迴路中連接鎖相放大器(lock-in amplifier)可獲得掃描穿隧能譜(scanning tunneling spectroscopy, STS)。dI/dV能譜顯示PTCDA的最低未占軌域(LUMO)與Cr(001)形成混成態，代表Cr(001)和PTCDA有強作用力，STM影像也確認此結果。經由STS得到兩種排列的dI/dV能譜，顯示兩種排列分子能階一致，此結果經由Atomistix ToolKit (ATK)軟體模擬PTCDA/Cr(001)的能態密度(DOS)比照，確認和實驗值有一樣的結果。 論文的第二部分使用鉻探針取得PTCDA/Cr(001)之STS能譜。於定電流模式下，使用具有磁矩之鉻探針掃描樣品，觀測到相鄰兩層Cr(001)平台測得之高度不同，可呈現樣品具有自旋極化特徵。而掃描相鄰兩層Cr(001)上方的單層PTCDA分子得到穿隧電流也有異同，於dI/dV能譜或得波峰強度也有差異，可知有機分子能被下方具有磁矩之金屬單晶誘發出自旋極化性質。

This thesis presents the investigation of PTCDA (3,4,9,10-perylenetetracarboxylic-dianhydride) on Cr(001). Monolayer structure of PTCDA was studied by scanning tunneling microscopy (STM); electronic properties of PTCDA were surveyed by scanning tunneling spectroscopy (STS); spin-polarized properties of PTCDA/Cr(001) were measured by Cr tip which possesses magnetic moments. dI/dV spectra of PTCDA on Cr(001) revealed the formation of a hybrid state, indicative of a strong chemical interaction between the adsorbate and substrate. The molecular orbital involved in the hybridization was LUMO (lowest unoccupied molecular orbital), confirmed by topographic images. Topographic images revealed two types of monolayer structures of PTCDA/Cr(001)—square (S) and herringbone (HB) structure. dI/dV spectra also indicated that two types of molecules had the same occupied and unoccupied states. This result was verified with calculation of density of state (DOS) by Atomistix ToolKit (ATK). Cr tips was applied in spin-polarized scanning tunneling microscopy (SP-STM) experiment. The topographic image of Cr(001) which measured by Cr tip that revealed the different step height of two adjacent Cr layer. This means that the spin-dependent effect was occurred near the Fermi level (EF) dI/dV spectra of PTCDA/Cr(001) unraveled that the two adjacent Cr layer had the different peak intensities in the occupied states This result showed that the organic semiconductor molecules could be induced magnetic moment by the antiferromagnetic substrate.