NTU workshop

How to identify if a material is a semiconductor or a metal based on its temperature-dependent conductivity?

• For a semiconductor, the conductivity increases with temperature, but the conductivity of metal is constant during temperature change.

• For a semiconductor, the conductivity increases with temperature, but the conductivity of metal decreases with temperature.

• For a semiconductor, the conductivity decreases with temperature, but the conductivity of metal increases with temperature.

• For a semiconductor, the conductivity decreases with temperature, but the conductivity of metal is constant during temperature change.

• For a metal, the conductivity increases with temperature, but the conductivity of semiconductor is constant during temperature change.

Compare the bandgap energies of CdSe particles with different sizes (exciton Bohr radius of CdSe = 5.4 nm)

• 20 nm > 10 nm > 5 nm > 3 nm > 1.7 nm

• 20 nm < 10 nm < 5 nm < 3 nm < 1.7 nm

• 20 nm = 10 nm > 5 nm > 3 nm > 1.7 nm

• 20 nm = 10 nm < 5 nm < 3 nm < 1.7 nm

• 20 nm > 10 nm > 5 nm = 3 nm = 1.7 nm

What is the photon energy in eV emitted by a QD sample with an emission wavelength of 500 nm?

What does this energy diagram belongs to?

• intrinsic semiconductor

• metal

• insulator

• n-type semiconductor

• p-type semiconductor

The figure show absorption spectra of 5.3 nm-CdS and 6.5 nm-CdSe QD samples. Based on these absorption spectra, what can possibly be the emission wavelength of 6.5 nm-CdSe QD sample?

• 500 nm

• 550 nm

• 600 nm

• 650 nm

An as-synthesized QD sample is dissolved in toluene. What solvent can be used to precipitate the QDs in precipitation and redissolution techniques (there can be multiple correct answers)?

• hexane

• benzene

• methanol

• acetone

• cyclohexane

Base on the report from Yu and coworkers (Yu, W. W., et. al. Chem. Mater. 2003, 15, 2854–2860.), predict the diameter of CdTe QDs sample (in nm) with the first absorption peak of 650 nm. Answer with 2 decimal places

What techniques can be used to determine size or size distribution of a QD sample?

• Powder X-ray diffraction (PXRD)

• Nuclear magnetic resonance (NMR) spectroscopy

• Thermogravimetric analysis (TGA)

• Transmission electron microscopy (TEM)

• Fourier-transform infrared (FTIR) spectroscopy

What techniques can provide information about ligand identity on QD surfaces

• Powder X-ray diffraction (PXRD)

• Thermogravimetric analysis (TGA)

• Transmission electron microscopy (TEM)

• Fourier-transform infrared (FTIR) spectroscopy

• Nuclear magnetic resonance (NMR) spectroscopy

Why are long-chain ligands replaced with short-chain ligands in QD photovoltaic cells?

• Increase charge transport rate

• Increase surface passivation of QDs

• Increase charge recombination rate

• Increase open-circuit voltage

• Increase absorption coefficient of QDs