Semiconductor quantum dots do not have an absorption peak, but rather a broad absorption spectrum at short wavelengths (photon energies higher than the corresponding bandgap), therefore you can excite QDs that are emitting at different energies with the same UV laser. Basically, you need to know the absorption cross section of your quantum dots, your excitation power density and the detection efficiency of your measurement system, as well as which portion of the light emitted by your quantum dots enters your measurement system. Then you can calculate the number of absorbed and emitted photons per quantum dot, which in turn yields the quantum efficiency. If you are not working with single-dot spectroscopy, you will need a pretty accurate estimate of your quantum dot density as well.

Dear Benjamin,

Thank you so much for your information. I have also found a paper giving detailed information at the point which you have also emphasized. I don't use single-dot spectroscopy. For the QY measurements, I measure optical density of QD samples (which should be lower than 0.1) at the excitation wavelength and the corresponding integrated area under the emission peak. Afterwards, I compare the result with a standart dye such as Rhodamin 6G. Both the sample and the standart dye should possess optical density at the excitation wavelength that is used. Some researchers applied 350 nm or 400 nm for the excitation of samples and Rhodamin 6G standart dye. However, Rhodamin 6G doesn't have got any optical intensity at that wavelength. So, how did they achieved to measure the QY of the samples still remains as a question.

Soooo Benjamin is not correct in saying that QD's do not have a peak. In fact the best materials will have several, they are known as exitons. If you still have questions about their optical properties check out

Washington and Strouse JACS articles

People think that QDs have only one absorption peak that corresponds to the first excitonic peak. This fact is due to problem of the crystalline structure they achieve after the synthesis. When the synthesis has very good quality, the crystalline structure allows to detect several absorption peaks corresponding to different exciton levels. I've seen until 4-5 peaks in the absorption spectra after the synthesis of QDs made in our group (the quality of the precursors and avoiding the oxygen during the synthesis is quite important).

QDs are excited at low wavelengths due to they have a broad excitation spectra, from UV to the first excitonic peak (depending on the size and composition of your QD nanocrystals). This fact is used to excite different QDs in multicolor imaging because you can excite several different color emitting QDs at the same time.

In the paper you say they talk about the excitation wavelength has no effect on most of the organic dyes brightness but for QDs, they change the brightness depending on the excitation wavelength due to the size-dependent properties (if you pay attention to the absroption spectra, the intensity of absorption is increasing when going to lower wavelength, that's why people commonly excite them at 400 nm). Because of the typically narrow absorption bands of common quantum yield standards this approach requires the use of different standards with reliably known fluorescence quantum

yields for the different excitation wavelengths.

I hope it helped you.