Temperature-dependent efficiency droop behaviors of GaN-based green light-emitting diodes

The efficiency droop behaviors of GaN-based green light-emitting diodes (LEDs) are studied as a function of temperature from 300 K to 480 K. The overall quantum efficiency of the green LEDs is found to degrade as temperature increases, which is mainly caused by activation of new non-radiative recombination centers within the LED active layer. Meanwhile, the external quantum efficiency of the green LEDs starts to decrease at low injection current level (<1 A/cm2 ) with a temperature-insensitive peak-efficiency-current. In contrast, the peak-efficiency-current of a control GaN-based blue LED shows continuous up-shift at higher temperatures. Around the onset point of efficiency droop, the electroluminescence spectra of the green LEDs also exhibit a monotonic blue-shift of peak energy and a reduction of full width at half maximum as injection current increases. Carrier delocalization is believed to play an important role in causing the efficiency droop in GaN-based green LEDs.     

以4，4''-二(1-咪唑基)苯砜或4，4''-二(1-咪唑基)苯硫醚为配体的配位聚合物的合成、晶体结构及性质

通过水热法合成并表征了3个基于V型配体构筑的新颖配位聚合物{[Cd（BIDPS）（PA）（H₂O）]·CH₃OH}_n（1）、{[Zn（BIDPS）（p-bdc）]·H₂O}_n（2）和[Mn（BIDPT）（NBA）]_n（3）（BIDPS=4,4''-二（1-咪唑基）苯砜,H₂PA=帕莫酸,p-H₂bdc=对苯二甲酸,BIDPT=4,4''-二（1-咪唑基）苯硫醚,H₂NBA=4,4''-氮杂二基二苯甲酸）。单晶结构分析表明化合物1和2具有二维网络结构,化合物1进一步通过倾斜穿插形成三维结构,化合物2通过分子间氢键形成了三维网络结构。化合物3属于3-连接六边形的蜂窝状结构,通过C—H …π作用形成三维超分子结构。化合物1和2在水中具有很好的稳定性和荧光性质,可作为高灵敏度、高选择性荧光探针检测水溶液中的Fe³⁺和Cr₂O₇^2-,并能抵抗多种竞争离子的干扰。同时研究了其对Fe³和Cr₂O₇^2-的荧光猝灭机理。     

几类图的拉普拉斯特征值的前三项和的上界

设G是一个顶点集为V(G),边集为E(G))的简单图.S_k(G)表示图G的拉普拉斯特征值的前k项部分和.Brouwer et al.给出如下猜想:S_k(G)≤e(G)+((k+1)/2),1≤k≤n.证明了当k=3时,对边数不少于n~2/4-n/4的图及有完美匹配或有6-匹配的图,猜想是正确的.     

对一道课本应用题的研究

题目　某地的出租车按如下方法收费:起步价10元,可行3km (不含3km ) ;3km到7km (不含7km)按1.6元/km计价(不足1km ,按1km计算) ;7km以后都按2 .4元/km计价(不足1km ,按1km计算) ,试写出以行车里程为自变量,车费为函数值的函数解析式,并画出这个函数的图象.这是高中数学新教材(必修)第一册(上)第5 6页的一道应用题.教师用书所给解答为:y =10　　　　　(0 相似文献