光電控制二硒化鉬之光激螢光研究

Abstract

作為一種新穎的直接能隙二維半導體材料，原子級厚度的過渡金屬二硫族化物近年來受到許多矚目，其光電特性的可調控性對於光電元件的發展極為重要。此研究中，我們探討由化學氣相沉積法成長的二硒化鉬與鋪上酞菁鋅或十六氟酞菁鋅後的二硒化鉬之光學特性。其中酞菁鋅的還原電位低於二硒化鉬導帶最小值，但高於二硫化鉬；十六氟酞菁鋅的還原電位低於二硒化鉬導帶最小值，但略高於二硒化鉬價帶最大值。我們發現當鋪上酞菁鋅後，二硒化鉬的光致發光強度有極為明顯的下降，但當鋪上十六氟酞菁鋅後則只有些微降低。同時，鋪上酞菁鋅後二硫化鉬則無變化。我們推測光致發光的下降是由於光誘導電荷的轉移所造成，而這項推測也於功率相依之光激螢光研究中再次驗證，鋪上酞菁鋅的二硒化鉬會從飽和曲線變為線性曲線。在溫度相依的光致螢光量測中，鋪上酞菁鋅的二硒化鉬峰值位置隨著溫度上升會有更加明顯的紅移現象。同時，鋪上酞菁鋅的二硒化鉬有著較小的半高寬，這可能是由於化學氣相沉積所成長的二硒化鉬品質較低，而帶有缺陷，鋪上酞菁鋅後，缺陷受到覆蓋導致缺陷影響降低。當酞菁鋅吸附於二硒化鉬上時，我們可以觀察到二硒化鉬激子束縛能與瓦希尼參數增加、電子與縱向聲子間交互作用減少。

Atomically thin transition metal dichalcogenides (TMDCs) have attracted significant interest because of their unique electrical, optical and chemical properties. The controllable modulation of electrical and optical properties of two-dimensional (2D) TMDCs is extremely important to enable a wide range of future optoelectronic devices. In this work, we investigated the optoelectronic properties of MoSe2 and MoS2 grown by chemical vapor deposition (CVD) which are functionalized with 2D organic molecules, zinc phthalocyanine (ZnPc) or hexadecafluorinated zinc phthalocyanine (F16ZnPc). ZnPc has the reduction potential below the conduction band minimum (CBM) of MoSe2, but higher than that of MoS2. The reduction potential of F16ZnPc is way below the CBM of MoSe2, but its low limit is slightly above the valence band maximum of MoSe2. We found that the photoluminescence (PL) of MoSe2 drastically quenches when it is functionalized with ZnPc (MoSe2-ZnPc), but it shows a relatively small decrease for MoSe2-F16ZnPc. Meanwhile, MoS2-ZnPc does not show any change in the PL emission. We surmise that the PL quench observed in MoSe2-ZnPc is due to the photoinduced charge transfer: favorable electron transfer for MoSe2-ZnPc, but forbidden for MoS2-ZnPc. It is supported by the power dependent PL response of MoSe2, which changes from a saturation curve to a linear curve after functionalized with ZnPc. In the temperature dependent PL measurement of MoSe2-ZnPc, the peak energy of PL shows cryo-blueshift followed by more significant redshift. This temperature dependence along with smaller band broadening of PL response for MoSe2-ZnPc may be due to lower crystalline quality of CVD-grown two-layer MoSe2 films with higher density of defects. Low temperature PL displaying a significant blueshift indicates that the light emission from our 2D materials is dominated by the recombination of localized excitons at local potential minima. When MPc molecules are coupled with MoSe2 flakes, the increase of exciton binding energy and the Varshini coefficient as well as the decrease of electron LO phonon coupling are observed, indicating that planar MPc molecules can delocalize the carriers in defects such as point detects.