Photoluminescence upconversion in colloidal CdTe quantum dots

Photoluminescence ~PL! up-conversion, the observation of luminescence with energies higher than those of the excitation photons, has been widely observed in the bulk 1 and heterostructures 2‐5 of semiconductors, quantum wells, 6 porous silicon, 7 self-assembled quantum dots ~QDs!, 8 and colloidal QDs. 9,10 It was proposed that the existence of intermediate states with energies resonant with or lower than those of the excitation photons is a prerequisite for these upconverted photoluminescence ~UCPL! processes. Carriers must be excited to these intermediate states by the first excitation photon and further to the higher energy states through various underlying mechanisms such as Auger excitation, 7 two-step two-photon absorption ~TS TPA!, 1‐3,5,6 or the thermal effect. 9,10 Most of these intermediate states are ‘‘observable,’’ 2‐7 i.e., in addition to the UCPL emission, one can also see the normal PL emission from these states with energies lower than those of the excitation photons. However, in some cases of self-assembled and colloidal QDs, these intermediate states can hardly be observed since carriers cannot efficiently populate them by the above-band-gap excitation of normal PL. 8 In a way, UCPL can provide a unique method for detecting the existence of these ‘‘defect’’ states in semiconductor quantum nanostructures and investigating their properties, which are usually different from those of the band-gap states. Semiconductor colloidal QDs are characterized by a large surface-to-volume ratio due to their small particle sizes, as well as the surface complexity originating from the passivation ligands used to neutralize the surface dangling bonds. Surface states have long been invoked to be responsible for some complex phenomena that could not be explained by using the concept of band-gap states only. For example, the almost universal occurrence of a biexponential distribution in the radiative lifetime strongly implies the existence of surface states in colloidal QDs. 11,12 In this sense, time-resolved measurements can provide one, but only indirect, way to probe the surface states in colloidal QDs, and consequently the existence of them is still controversial. 13 However, if one can observe a UCPL signal from colloidal QDs, this problem will be easily resolved since the demand of intermediate states can only be fulfilled by the existence of surface states. Recently, UCPL signals were observed in colloidal InP, 9 CdSe, 9,10 and CdTe ~Ref. 10! QDs, which have provided a strong proof of the existence of surface states in colloidal QDs and opened up a new opportunity for the systematic study of these surface states. Here, we report on our optical studies of efficient UCPL in colloidal CdTe QDs with an energy gain of as high as 360 meV. Compared with the normal PL, the UCPL signal shows a redshift of about 80 meV in its peak energy, while the excitation spectroscopy of the UCPL ~UCPLE! measurement at this peak energy showed a broad excitation band with energies below those of the band gap. Time-resolved measurements were utilized to study the time-resolved dynamics of this UCPL, whose radiative lifetime now becomes nearly twice as long as that of the normal PL. Finally this UCPL is attributed to the recombination of carriers from the band-gap states and surface states mainly through a thermal excitation process.