在過去的十幾年中,石墨烯(graphene)、 六方氮化硼(h-BN), 過渡金屬二硫族化物(transition metal dichalcogenides;TMD)等二維材料的研究在基礎科學和應用科技中產生了很大的風潮。當二維材料的橫向大小減小到奈米尺寸時,此材料將可轉變為零維的量子點(quantum dot)。由此種二維材料做成的量子點具有下列獨特的性質:第一,與單原子層相比,具零維的量子點其橫向的奈米尺寸將表現出強烈的量子侷限效應(quantum confinement effect),從而增加了原子層的帶隙。第二,某些二維材料量子點如二硫化鉬量子點在吸收光譜中表現出巨大的自旋谷耦合(約570 meV),比在單層中(約 400 meV)更大。第三,零維的量子點比二維單原子層有明顯的邊緣效應和大的表面體積比。第四,此種二維材料量子點在平面方向比較沒有懸鍵(dangling bond),因此比傳統的半導體量子點具有較少的缺陷。另外,二維材料量子點的表面可加以修飾,將具有良好的水溶性、生物相容性與可調螢光顏色等特點,這些特性可應用在許多光催化與太陽能電池、感應器與生物影像等領域。
一般而言,半導體的激子可用氫原子模型來表示,電子和空穴之間的距離定義為激子波爾半徑。傳統半導體的激子束縛能約在數個meV到數十個meV左右, 但過渡金屬二硫族化物的激子束縛能卻可以高達數百個meV。這是由於過渡金屬二硫族化物材料具有較弱的介電遮蔽(dielectric screening)而且有較重的粒子質量,導致它具有大的庫倫作用力與激子束縛能。此巨大的激子束縛能將減少單層的光學躍遷能量,並與外界環境產生特別的物理現象。由於激子間的強烈交互作用,使得過渡金屬二硫族化物是研究激子多體現象的理想系統。當激子數目增加到一個程度時,激子之間的波函數開始重疊,激子間交互作用的影響變得顯著。如果激子與激子間的距離與激子直徑相等時,可以視為激子的波函數發生重疊,此時的激子密度稱為Mott密度,當激子密度超過Mott密度時,出現了光吸收峰變小、半高寬變寬等現象。近年來,我們通過光學測量發現,發現二硫化鎢(WS2)量子點在室溫下就可觀察到能隙重歸一化(bandgap renormalization)。當載子密度的增加,二硫化鎢量子點的帶隙重歸一化也增加,由能帶相關的公式,可導出二硫化鎢量子點的Mott密度與Mott激子波爾半徑(~1.12 nm),這是二維材料量子點第一次估算出Mott激子的波爾半徑。研究這些多體的激發粒子的Mott密度等現象,將有助於發展實際的光電元件應用如光偵測器、單光子源與雷射等。此研究的結果,已發表在2019年npj 2D Materials and Applications 的國際期刊。




In the past ten years, graphene, hexagonal boron nitride (h-BN), transition metal dichalcogenides (TMD) and other two-dimensional materials has been studied in basic science and applied technology. When the lateral size of a two-dimensional material is reduced to the nanometer size, the material will be transformed into zero-dimensional quantum dots (QDs). QDs made from the two-dimensional materials have the following peculiar properties: First, compared with a single atomic layer, a zero-dimensional QD will exhibit a strong quantum confinement effect in its lateral dimension. Therefore, its band gap is larger than that of the atomic layer. Second, some two-dimensional material quantum dots, such as molybdenum disulfide (MoS2) QDs, exhibit huge spin-valley coupling (about 570 meV) in the absorption spectrum, which is larger than that in a single layer (about 400 meV). Third, zero-dimensional QDs have edge effects and larger surface-to-volume ratios than monoatomic layers. Fourth, the two-dimensional material QDs have relatively few dangling bonds in the plane direction, and therefore have fewer defects than traditional semiconductor QDs. In addition, the surface of the two-dimensional material QDs can be modified to have good water solubility, biocompatibility and adjustable fluorescent color. These characteristics can be applied to many photocatalysis, solar cells, sensors, biological imaging, and other fields.
Recently, our laboratory has used laser ablation method to successfully synthesize fluorescent two-dimensional material QDs. Laser ablation method is an effective method to make nanomaterials. The pulsed laser was focused to destroy the molecular bonds of the target materials, so that the target is melted and become QDs. In addition to the rapid manufacture of fluorescent two-dimensional QDs, the laser ablation method can successfully achieve the doping effect if certain molecules are also added during growth. This is a convenient and fast method to synthesize doped QDs. We have some experience in this technology and have two patents.
Generally speaking, the exciton of a semiconductor can be represented by a hydrogen atom model, and the distance between an electron and a hole is defined as the Bohr radius of the exciton. The exciton binding energy of traditional semiconductors is about several meV to tens of meV, but the exciton binding energy of transition metal dichalcogenides can be as high as hundreds of meV. This is because the transition metal dichalcogenide has a weaker dielectric screening and a heavier mass, resulting in a large Coulomb force and exciton binding energy. The huge exciton binding energy will reduce the optical transition energy of the single layer and produce peculiar physical phenomena. Due to the strong interaction between excitons, transition metal dichalcogenides are an ideal system for studying the phenomenon of many-body effects. When the number of excitons increases to a certain level, the wave functions between excitons begin to overlap, and the influence of the interaction between excitons becomes significant. If the distance between two excitons is equal to the diameter of the exciton, it can be considered that the wave functions of the exciton overlap. The exciton density at this condition is called the Mott density. When the exciton density exceeds the Mott density, the many-body effect appears. The absorption peak becomes smaller and the full width at half maximum becomes wider. In recent years, we have found through optical measurement that the bandgap renormalization of tungsten disulfide (WS2) QDs can be observed at room temperature. When the carrier density increases, the renormalization of the band gap of the tungsten disulfide QDs also increases. From the energy band-related formula, the Mott density of the tungsten disulfide QDs and the Bohr radius of the Mott exciton (~1.12 nm) can be estimated. To our knowledge, this is the Bohr radius of Mott excitons has been estimated in a two-dimensional QDs for the first time. Studying the Mott density of the many-body particles will help to develop practical optoelectronic devices such as photodetectors, single photon sources, and lasers. The results of this research have been published in the journal of npj 2D Materials and Applications in 2019.


I was primarily engaged in the investigations of traditional semiconductor materials such as III-V group and gallium nitride semiconductors. In recent years, I found that domestic and foreign academia and the industry are actively implemented in the development and research of two-dimensional novel materials, so I tried to enter the field of two-dimensional materials. Fortunately, we have successfully grown graphene quantum dots and transition metal dichalcogenide quantum dots by pulsed laser ablation. Later, many optical and electrical properties of quantum dots were modulated by doping, and some research results have been obtained in quantum dot materials. Now I feel that if I didn't invest in the two-dimensional material field a few years ago and only trapped in the original traditional semiconductor materials, there should not be so many research results. Therefore, I personally feel that, unless there are still important research topics, you can consider changing the field after a research topic has been studied for 5 to 10 years.