利用雷射削融法合成二硫化鉬量子點之光學特性

本論文利用雷射削融法來處理樣品，並成功合成出二硫化鉬量子點，其合成方法可快速製作且製備過程簡單。為了瞭解二硫化鉬量子點的形成，通過掃瞄式電子顯微鏡(SEM)探討樣品殘留物之構造，隨雷射處理的時間增加，樣品漸漸從片狀結構慢慢變成球形狀，尺寸也漸漸地從微米(μm)變成奈米(nm)等級。未摻雜二硫化鉬量子點的發光效率為0.09 %，為了提升其螢光隨後摻雜二乙烯三胺(DETA)，觀察到當DETA濃度為80 nM二硫化鉬量子點的光激螢光強度最強，與未摻雜相比提高約200倍，量子發光效率約為18.83 %，經由X-ray光電子能譜的量測觀察因DETA替換式氮摻雜使其螢光強度提升，隨後由穿透式電子顯微鏡(TEM)觀察二硫化鉬量子點的平均粒徑大小 3.0 ± 0.5 nm。我們成功將二硫化鉬量子點與過氧化氫(H2O2)混和反應後，發現二硫化鉬量子點的螢光會有粹滅的效應，最低可以偵測的過氧化氫濃度為0.1 μM。我們猜測在氧化過程中，由於Mo-S鍵親和力比Mo-O鍵親和力來的低，因此當增加H2O2的濃度，硫原子漸漸被氧原子取代，由H2O2供給的氧原子來填充硫原子的空位；因此當H2O2的濃度越高，二硫化鉬量子點螢光強度就越低。隨後也使用酸性溶劑鹽酸(HCL)以及鹼性溶劑氫氧化鈉(NaOH)來調整不同的酸鹼度，當加入鹼性溶液(氫氧化鈉)OH離子增加，會增加電子的密度導致光激螢光增強；加入酸性溶液(鹽酸)會產生大量H離子，這些H離子會捕捉電子並導致光機螢光降低。隨著溫度二硫化鉬量子點擬合出低、高能量載子行為表現，利用速率方程式對變溫下的低、高能量螢光峰值強度作擬合，可以得知低能量螢光峰值強度在180 K至210 K 之間，會獲得高能量載子轉移，使低能量在這溫度區間應光峰值強度增強，擬合出高能量載子躍遷所需之活化能為 Ee= 96 meV 。

關鍵字

二硫化鉬；量子點；雷射削融

並列摘要

In this dissertation, laser ablation method was used to synthesized molybdenum disulfide quantum dots (MoS2 QDs) successfully from molybdenum disulfide (MoS2) powder in ethanol (ETOH). The synthesis method can rapidly fabricate via simple process. In order to understand the formation of MoS2 QDs, the separated residue and supernatant were both investigated. Scanning electron microscope (SEM) was used to analyze the structure of the residues where as the laser ablation time is increased, the sample gradually changes from a sheet structure to a ball shape concurrent with gradual change in size. The change in size was observed to vary from micrometer (μm) to nanometer (nm) scale with increasing laser ablation time. The solution which is observed to contain MoS2 QDs sheets was subsequently observed displaying an average particle size of roughly 3 nm, observed using transmission electron microscope (TEM), and having a quantum yield of 0.09 %. In order to enhance the photoluminescence (PL) intensity, doping using diethylenetriamine (DETA) was utilized. It was observed that when the DETA concentration of 80 nM was added during synthesis, the doped MoS2 QDs has the highest PL intensity. The impurity ratio is about 200 times higher and the quantum yield increases to 18.83 % as compared to undoped sample. The effect of the DETA dopant was observed using X-ray photoelectron spectroscopy (XPS) measurement. Chemical effect on PL intensity was also studied, where addition of hydrogen peroxide (H2O2) on MoS2 QDs solution was utilized. We found that the MoS2 QDs displayed a PL quenching effect at the lowest detectable concentration of H2O2 at 0.1 μM. We suggest that this PL quenching effect arises from the electron transfer that occurs between the MoS2 QDs and H2O2, where H2O2 is reduced to O2. Therefore, when the concentration of H2O2 is high, there is a higher chance of electron transfer from H2O2 to MoS2 QDs that will occur. Subsequent use of acidic solution (hydrochloric acid (HCl)) and basic solvent (sodium hydroxide (NaOH)) was done to adjust the different pH levels, when added to the alkaline solution (NaOH) the OH ion increases which is consequent to the increase of the density of electrons which will result to enhancement. However, adding HCl generates a large amount of H ions, which trap electrons and cause PL intensity to decrease. Lastly, the PL spectra of doped MoS2 QDs was observed at varying temperature. As the temperature is varied, the PL intensity goes down (from 15 K to 180 K), goes up again (180 K to 210 K) and then finally drops (210 K to 300 K). The doped MoS2 QDs PL spectra can be deconvoluted into two peaks using Gaussian fit. The deconvoluted peaks are found at 2.7 eV (high energy) and 2.45 eV (low energy). The carrier behavior of high and low energy peaks against temperature were studied. The low energy peak displayed an anomalous behavior compared to the high energy. The high energy peak displays a direct PL quenching with increasing temperature which is accounted to carrier escape from QD emitting state to non radiative recombination centers. However, the low energy peak demonstrated an anomalous behavior where at 180 K to 210 K the peak abruptly increases. This anomalous behavior is suggest to arise from carrier transfer effect associated with a new energy level introduced by doping. During exposure from 180 K to 210 K the, the carriers in new energy level introduced by doping is expected to escape to QD emitting state by overcoming an energy barrier through an intersystem crossing where the energy barrier is measured to be 96 meV.