单个铂酞菁分子在正负偏压激发下的电致发光特性研究

Bias-Polarity Dependent Electroluminescence from a Single Platinum Phthalocyanine Molecule

本文利用扫描隧道显微镜诱导发光技术,系统地研究了铂酞菁分子在正负偏压激发下的电致发光行为及其相关的电子激发机制,铂酞菁分子在正负偏压激发下均可以发光,但发光特征有所不同.在负偏压激发下,观察到了源于中性PtPc分子LUMOtHOMO跃迁的峰位在637 nm处的荧光峰.荧光强度在三个不同的激发区域呈阶梯状增加.当激发偏压处于区域(I)时,可以产生较强的荧光,其主导激发机制是载流子注入激发机制;当激发偏压处于区域(II)时,可以产生中等发光强度的荧光,其主导激发机制是非弹性电子散射激发机制;当激发偏压处于区域(III)时,激发电子能量小于荧光光子能量,存在上转换现象,荧光峰非常弱,其激发过程需要自旋三重态作为中继态,并由载流子注入和非弹性电子散射两种机制共同作用产生.在正偏压激发下,分子的电致发光光谱特征变得更加复杂,具有多个发光峰.911 nm处的尖锐发光峰起源于PtPc-阴离子的LUMO+1→LUMO跃迁,其激发主要是由载流子注入激发机制主导.

By using scanning tunneling microscope induced luminescence(STML)technique,we investigate systematically the bias-polarity dependent electroluminescence behavior of a single platinum phthalocyanine(PtPc)molecule and the electron excitation mechanisms behind.The molecule is found to emit light at both bias polarities but with different emission energies.At negative excitation bias,only the fluorescence at 637 nm is observed,which originates from the LUMOtHOMO transition of the neutral PtPc molecule and exhibits stepwise-like increase in emission intensities over three different excitation-voltage regions.Strong fluorescence in region(I)is excited by the carrier injection mechanism with holes injected into the HOMO state first;moderate fluorescence in region(II)is excited by the inelastic electron scattering mechanism;and weak fluorescence in region(III)is associated with an up-conversion process and excited by a combined carrier injection and inelastic electron scattering mechanism involving a spintriplet relay state.At positive excitation bias,more-than-one emission peaks are observed and the excitation and emission mechanisms become complicated.The sharp moleculespecific emission peak at〜911 nm is attributed to the anionic emission of PtPc-originated from the LUMO+1 tLUMO transition,whose excitation is dominated by a carrier injection mechanism with electrons first injected into the LUMO+1 or higher-lying empty orbitals.