A.T. İnce

Optical Studies of Semiconductor Quantum Dots

Optical absorption (ABS), steady-state photoluminescence (PL), resonant Raman, and photoabsorption (PA) spectroscopies are employed to study quantum-size effects in II–VI semiconductor quantum dots (QDs) grown in glass samples. We observe a size-dependent shift in the energetic position of the first exciton peak and have examined the photoinduced evolution of the differential absorption spectra. The Raman shifts of the phonon modes are employed to monitor stoichiometric changes in the composition of the QDs during growth. Two sets of glass samples were prepared from color filters doped with CdS x Se1 − x and Zn x Cd1 − x Te. We analyze the optical properties of QDs through the ABS, PL, resonant Raman, and PA spectroscopies. The glass samples were prepared from commercially available semiconductor doped filters by a two-step thermal treatment. The average size of QDs is estimated from the energetic position of the first exciton peak in the ABS spectrum. A calculation based on a quantized-state effective mass model in the strong confinement regime predicts that the average radius of QDs in the glass samples ranges from 2.9 to 4.9 nm for CdTe and from 2.2 to 9.3 nm for CdS0. 08Se0. 92. We have also studied the nonlinear optical properties of QDs by reviewing the results of size-dependent photoinduced modulations in the first exciton band of CdTe QDs studied by PA spectroscopy.

Optical and Structural Properties of Quantum Dots

We report (i) thickness dependent evolution of structural disorder, strain on crystalline planes and grain size in chemical bath deposited (CBD) CdS thin films studied through a combinative evaluation of the results of optical absorption, Raman spectroscopies, X-Ray diffraction (XRD) and Scanning Electron Microscopy (SEM). (ii) refer briefly to CdSexS1-x nanocrystals in liquid and (iii) address quantum size effect in CdSexS1-x quantum dots embedded in glass studied through steady state photoluminescence spectroscopy. The asymptotic optical absorption edge is red shifted while the long wavelength tail narrows with increasing thickness which is proportional to deposition time. We employ effective mass theory under quantum size effect to estimate average grain size from the energetic position of asymptotic optical absorption edge. The long wavelength tail optical absorption is due presumably to the micro-electric field induced by crystalline defects. The transmission probability through the potential energy barrier created by micro-electric field is calculated with the help of WKB (Wentzel, Kramers, Brillouin) approximation. We conclude that as the deposition time increases from 10 to 150 min, the average grain radius changes by 2 nm, Urbach energy and the electric micro-field decrease from 600 to 400 meV and 2240–820 kV/mm respectively. The shift in XRD pattern shows that the compressive strain decreases with growth. The Raman LO1 vibrational mode display an increase up to 22 min of deposition time and then a decrease.

Comparison of the input electrical power measurement methods for HIFU transducers

HIFU (High Intensity Focused Ultrasound) transducers are attractive tools for cancer therapy. They must be used carefully for operator and patient safety as they produce very high powers up to a few hundred watts, and characterized in many ways. The ultrasonic power of a transducer is directly proportional to the applied input electrical voltage, current and consequently electrical power. Internationally accepted methods for measurements of ultrasonic output power up to 10 W are well established in the IEC 61161 standards [1]. The same method was used for higher powers up to 150 W with an uncertainty of 4.7% for 1.1 MHz and 5.0% for 3.3 MHz in an intercomparison between national metrology institutes [8]. In this paper, input electrical power of the HIFU transducer, which is mainly converted into ultrasonic energy, was determined. 3 methods for the measurement of electrical power delivered to a reactive ultrasonic (HIFU) transducer were compared. Electrical power measurements were realized with an expanded uncertainty of 3.3 % for up to 50 W and 4.3 % for up to 100 W in the frequency range between 0.93 MHz and 3.1 MHz. Measurement devices were calibrated and verified by establishing traceability to electrical and ultrasonic standards at TÜBİTAK UME (The Scientific and Technological Research Council of Turkey, National Metrology Institute) primary laboratories. Electrical power measurement results obtained by this method were also compared with two different conventional power meters. Electrical power measurement results detailed in this paper showed an agreement with each other and conventional power meters.

An urbach study of the degree of structural disorder in cdte nanocrystals in glass matrix

CdTe nanocrystals embedded in matrixes such as, glass, plastic and, aqueous solution have been investigated extensively due to their potential applications in nonlinear optical devices, biotechnology and medicine. When the size of bulk semiconductor is reduced to nanometer scales, the crystal gains new optical and electronic properties. Energy levels are quantized as in atoms and the energy difference between levels widens with the inverse of the size squared. CdTe nanocrystals were grown in glass by solid-state phase precipitation at the above glass transition temperature. A size dependent blue shift in the asymptotic absorption edge was observed in optical absorption spectra. The crystalline composition was studied by Resonant Raman spectroscopy. The width of the optical absorption tail at long wavelengths depends on the spread of electron energy levels within the structure, called the Urbach energy, which was calculated depending on size. We observe that the Urbach tail narrows as the surface to volume ratio of the nanocrystals decreases.