掃描穿隧顯微術探究鐵誘導三溴化鉻表面形貌及電子特性的影響

Abstract

三溴化鉻(CrBr3)是著名的磁性材料，雖然磁性特性已被研究許久，但相關的表面形態與電子特性尚未完備，是近期研究的新方向。因鉻原子帶有磁性，即使三溴化鉻限縮成二維尺度仍具有穩定的鐵磁性。但是此材料的居禮溫度遠低於室溫，侷限了材料在電子元件上的發展。若能在不破壞材料磁性的條件下增高居禮溫度將必廣泛應用於各領域，這表示摻雜金屬元素形成的異質結構有望改善此限制。因此本研究利用機械剝離法與乾式轉移法製備CrBr3/HOPG異質結構，並利用掃描穿隧顯微鏡(STM)技術探究鐵誘導的三溴化鉻表面形貌以及利用掃描穿隧能譜(STS)研究電性變化。研究結果顯示三溴化鉻的形貌可區分成三種:片狀、層狀與團狀，包括單層到10層的厚度。而我們發現鍍鐵後的平臺表面出現許多2~3 nm寬的不規則紋路，且原子結構變得相當清晰，掃描穿隧顯微鏡能探測到上層與底層的溴原子以及中間層的鉻原子所形成的六邊形。在電性方面，鍍鐵造成相當大的差異，三溴化鉻的能帶間隙從1.837±0.058 eV降至0.148±0.024 eV，代表鐵原子促使屬於半導體的三溴化鉻轉變成半金屬；同時，鍍鐵前後的費米能階(EF)皆偏向價帶，具有P型半導體的性質。根據實驗結果，我們的研究支持密度泛函理論對於三溴化鉻電子特性的預測，為三溴化鉻在自旋電子學領域的研究開啟新頁。Chromium Tribromide (CrBr3) is a well-known magnetic material. While its magnetic properties have been extensively studied, there is a lack of comprehensive understanding about its surface morphology and electronic properties. Due to the magnetic nature of chromium atoms, CrBr3 exhibits stable ferromagnetism even confined to a two-dimensional scale. However, its Curie temperature is lower than room temperature, which limits its potential applications in electronic devices. Increasing the Curie temperature without destroying its magnetization would be useful in various fields. One possible solution to overcome this limitation is to introduce heterostructure by incorporating metallic elements through doping. In this study, we prepared CrBr3/HOPG heterostructure by mechanical exfoliation and dry transfer techniques. We investigate the iron-induced surface morphology with Scanning Tunneling Microscopy (STM) and study its electrical characteristics with Scanning Tunneling Spectroscopy (STS). The results revealed three morphologies of CrBr3, including flakes, layers, and clusters, ranging from monolayer to 10-layer thickness. After iron deposition, we observed numerous irregular patterns with widths of 2 ~ 3 nm on the platform surface, and the atomic structure became highly resolved. We can detect both top-layer and bottom-layer bromine atoms and the hexagonal arrangements formed by intermediate-layer chromium atoms. In terms of electrical properties, iron deposition caused significant changes. The energy band gap of CrBr3 decreased abruptly from 1.837±0.058 eV to 0.148±0.024 eV, indicating that iron atoms induced a transition from semiconductor to semimetal. In addition, Fermi energy level (EF) shifted towards valance band in two situations, exhibiting properties of a p-type semiconductor. Based on the experimental results, our research supports the predictions of density functional theory in describing the electronic properties of CrBr3. This opens up new avenues for the study of CrBr3 in the field of spintronics.